Pattern-enhanced Resonant Soft X-ray Scattering for Operando monitoring of electrochemical solid-liquid interfaces

Domain: chemistry
Published: 2026-02-21
Editorial outcome: major_revision

Abstract
Unveiling interfaces at sub-nanometer scales is essential for advancing the understanding of complex chemical transformations. However, characterizing solid-liquid interfaces with high dimensional sensitivity and temporal resolution remains challenging, due to their dynamic nature and inaccessibility by conventional probes. Here we present an approach, Pattern-enhanced Resonant Soft X-ray Scattering, to overcome the challenges. Rooted in a “sample-as-optics” philosophy, this technique utilizes precisely engineered line-grating nanopatterns to modulate near-field X-ray illumination, coherently enhancing scattering signals from the line-gratings. We implement the method using Ni line-grating nanopatterns in electrochemical water oxidation. The periodic nanostructures serve as diffractive optical elements to reveal the Ni oxidation gradients and structural dynamics at the electrode-electrolyte interfaces. Finite-element simulations corroborate the observed trends by modeling variations in compositions and structures during electrocatalysis. Through integrating advanced sample design with coherent wave nature of soft X-rays, our approach opens accessible pathways to operando exploring chemical evolution and sub-nanometer dimensional variations simultaneously in electrochemical systems. This non-destructive method is efficient and element-specific, making it valuable for probing chemical and dimensional dynamics with appropriate modeling.

Introduction
Great attention has been drawn to a comprehensive understanding of solid-liquid interfaces at atomic scales in a wider range of chemical and biological systems1–4. Although modern material characterizations have seen significant advances in resolving interface and interphase properties5–7, they are largely based on improvements in probes our- ces and detectors6,8–10. In the meantime, devices are becoming smaller and faster, and interfacial structures are becoming more complex and deeply buried11,12. However, an effective characterization technique for studying interfaces with high dimensional sensitivity and temporal resolution under realistic conditions remains a formidable challenge.

For example, operando transmission electron microscopy (TEM) enables real-time visualization of structural transformations13,14, but prolonged exposure to high-energy electrons could cause severe damage to the sample, potentially altering the structures and chemistry of interests15,16. Moreover, TEM often captures localized regions that may not be statistically representative of the entire interface17,18. In contrast, synchrotron-based surface-sensitive techniques, such as modulation excitation X-ray absorption spectroscopy (XAS)19, ambient-pressure X-ray photoelectron spectroscopy20,21, and X-ray fluorescence microscopy22,23, offer robust averaged chemical and structural information. Nevertheless, these methods often difficultly isolate and monitor reaction fronts, particularly at the (sub-)nanometer dimension and (sub-)millisecond timescales24–26. These limitations highlight the urgent need for a non-destructive operando characterization technique capable of investigating solid-liquid interfaces with remarkable dimensional sensitivity, temporal resolution, statistical reliability, and element-specific sensitivity.

Here, we present a methodology, Pattern-enhanced Resonant Soft X-ray Scattering (PE-RSoXS), for achieving significant spatial and chemical sensitivity. We demonstrate that, through modulations of soft X-ray illumination in the near-field (Fresnel regime)27,28, a “sample-as-optics” philosophy of PE-RSoXS leads to an efficient detection of electrochemical interfaces under operando conditions at the sub-nanometer scales beyond the soft X-ray diffraction limit. RSoXS integrates the structural sensitivity of small-angle X-ray scattering with the chemical specificity of near-edge X-ray absorption fine structure (NEXAFS). By tuning the incident photon energy near elemental absorption edges, RSoXS selectively enhances contrast based on chemical composition, molecular orientation, or electronic states of the materials. This element- and bond-specificity makes RSoXS particularly functional for probing heterogeneous systems, where subtle chemical variations are coupled to nanostructures29,30. Inspired by traditional crystallography, where periodicity in crystals enables coherent enhancement of scattering intensities30,31, PE-RSoXS deploys engineered nanopatterns, with periods in the order of tens of nanometers, which act as optical elements manipulating the incoming wavefront in the near field (The near field here refers to the electromagnetic field that propagates through a nanostructure, calculated both within and around it, and influenced by the nanostructure itself.) and coherently enhancing scattering signals from every interface of the periodic structure, leading to sub-nanometer sensitivity. Unlike conventional approaches treating the sample as a passive object, PE-RSoXS incorporates the patterned nanostructures into the optical pathway, enabling coherent interference and resonant contrast enhancement32 (Supplementary Table 1 and Supplementary Note 1).

PE-RSoXS provides an accessible approach for monitoring the evolution of catalysts at electrochemical solid-liquid interfaces under operando conditions. Ni line-grating nanopatterns (denoted as Ni LGNPs) were designed and fabricated as optical architectures for PE-RSoXS measurements during electrochemical water oxidation, a key reaction for green hydrogen production33,34. Structural dynamics such as lattice incorporation, dissolution/redeposition, and volume changes coexist with subtle chemical variations, like oxidation state shifts of active sites, all occurring within buried and often inaccessible environments21,35,36. PE-RSoXS captures these dynamics by integrating with rigorous finite-element simulations, leveraging the wave-like properties of soft X-rays, their spatial compatibility with interfacial phenomena, and the coherent amplification enabled by periodic nanopattern designs. This approach delivers substantial sensitivity and detection efficiency simultaneously for probing complex interfaces.

Results
Working principle of PE-RSoXS
PE-RSoXS approach is conceptually related to diffraction anomalous fine structure, a method noted by Cauchois et al.37 and formalized by Stragier et al.38, which combines Bragg diffraction with anomalous scattering near absorption edges to obtain element-specific structural information39. Well-ordered line-grating nanopatterns with uniform rectangular geometry were constructed to enhance scattering sensitivity. The angular spacing of Bragg spots reflects the pitch of the nanopatterns (structure factor), while the relative intensity distribution is governed by the line width of the nanopatterns (form factor). PE-RSoXS exploits its sensitivity to the form factor to reveal the buried electrochemical solid-liquid interfaces by enabling coherent amplification of resonant scattering signals, which effectively and uniquely decouples the interfacial signals from the response of the bulk material based on the diffraction order dependency (See details in Supplementary Notes 2 and 3). This makes PE-RSoXS sensitive to structural size changes with nanometer to sub-nanometer at the reaction fronts. Furthermore, by tuning the X-ray energy near the absorption edge of target elements, PE-RSoXS features site- and valence-state-specific capabilities comparable to XAS, providing an operando approach to track chemical evolution statistically during reaction processes13,29,36,40,41.

In this study, we demonstrate the PE-RSoXS methodology by designing periodic Ni LGNPs for electrochemical water oxidation. As shown schematically in Fig. 1 and Supplementary Fig. 1, operando PE-RSoXS is performed in a customized electrochemical liquid flow cell with Ni LGNPs (see details in Methods). Through the SiN window, the Ni LGNPs are probed by incident soft X-rays with the resonant photon energy near the Ni L-edge. The scattering signal from line gratings gives rise to an array of Bragg spots perpendicular to the orientation of the line-grating plane. The scattering results from the Ni LGNPs with 50% duty cycle structures show that the even-order diffraction intensity is significantly reduced or absent, while the odd-order signal remains prominent (Supplementary Note 4). Upon applying an anodic potential, the Ni line-grating develops a core-shell structure with an increase in width. PE-RSoXS allows a direct distinguishing of oxidized Ni shell from metallic Ni core. With the exposure time at the millisecond level and an incident X-ray beam dosage below 0.01 mJ cm−2, this approach achieves the operando monitoring of structural and chemical dynamics of Ni LGNPs simultaneously at the solid-liquid interfaces when undergoing mild oxygen evolution reaction (OER) in the alkaline media. In our implementation, the interfacial region is defined as the shell layer in a Ni core-shell structure. This region evolves chemically and structurally during electrochemical OER, and its presence modifies the form factor, which is the basis for scattering contrast in PE-RSoXS. Unlike conventional scattering methods, where the signal scales linearly with the number of scatterers (N), PE-RSoXS utilizes the coherent interference between the scattered waves, resulting in the diffraction intensity scaling as N squared42. In our experimental setup, the patterned window area was measured as approximately 80 µm × 80 µm, containing about 400 periodic units (N ≈ 400). This coherent addition contributes to a five-order-of-magnitude increase in diffraction intensity compared to a single isolated scatterer (Supplementary Note 5), making PE-RSoXS well-suited for real-time monitoring of buried electrochemical interfacial dynamics with high efficiency. This signal amplification is proved by the electric effects in the near field (E-field) induced by incident soft X-rays (Fig. 1), which intensify the scattering signals and underscore the alignment between the scale of optical modulation and the physical dimensions of line-grating structures. By incorporating optical design principles directly into the electrocatalyst geometry, the sample itself becomes an active component of the measurement system, bridging structural modulation and spectroscopy to selectively probe electrochemical interfacial dynamics with high dimensional sensitivity and elemental resolution during operating conditions.

Operando monitoring of electrochemical solid-liquid interfaces by PE-RSoXS
We fabricated well-ordered Ni LGNPs on the two SiN windows of the sample E-chip using e-beam lithography, followed by Ni metal deposition and lift-off processes. These Ni line gratings exhibit 100 nm of width, 200 nm of pitch, and 50 nm of height (Fig. 2a). We employed Ni LGNPs as the WE integrated into a customized electrochemical liquid flow cell for OER in 0.1 M NaOH aqueous electrolyte. Cyclic voltammetry (CV) profiles first confirmed well-defined electrochemical behaviors of Ni LGNPs, consistent with measurements of a Ni metal foam performed in a standard H-cell (Supplementary Figs. 2 and 3). The electrochemical redox transitions of Ni/Ni(OH)2 (0/+2) and Ni(OH)2/NiOOH (+2/+3) are well understood and illustrated in the Bode diagram43,44. In our operando PE-RSoXS experiments, all the applied potentials were calibrated and converted to the reversible hydrogen electrode (RHE) based on our previous reports (Supplementary Fig. 3)13,36. All the potentials in this work were shown versus the RHE without iR correction unless otherwise noted. We applied potentiostatic scans at the open circuit potential (OCP), 1.2, and 1.6 V, respectively, which mainly aimed to monitor the chemical dynamics of electrocatalytic Ni sites before and within the OER potential range. The obtained three diffraction patterns were collected with 1 ms exposure time of soft X-rays at 852 eV, as shown in Fig. 2b. The Bragg spots corresponding to different diffraction orders were aligned, and the beam stop was positioned to block the direct beam and first orders. As shown in Fig. 2c, the scattering intensity of different orders varies at the applied potentials, extracted by summing the area around each peak over 60 pixels (px). A significant increase in scattering intensity at 1.6 V reflects the widening of the Ni line gratings, implying volumetric swelling of the Ni LGNPs under OER conditions35,45. The scattering intensities near the Ni L3-edge, ranging from 849 eV to 861 eV were extracted and integrated as a function of photon energy at OCP, 1.2 and 1.6 V, respectively, producing energy-dependent scattering profiles. While these spectra resemble XAS in that they exhibit Ni L-edge features, they are not directly equivalent. The measured diffraction intensities result from spatially modulated differences in the complex refractive index (n = 1 − δ + iβ) and reflect both absorption (β) and dispersion (δ) contributions. These scattering-derived spectra were resolved with an exposure time of 1 ms for each energy point, and the irradiation intensity of the incident X-ray beam was kept below 0.01 mJ cm−2. The spectra, derived from the diffraction intensities of the 3rd (Fig. 2d), 4th (Fig. 2e), 5th (Fig. 2f), and 6th orders (Supplementary Figs. 4–6 and Supplementary Notes 6 and 7), illustrate the distinct spectroscopic features of the Ni LGNPs46,47.

To connect the scattering spectroscopic features with the chemical and structural dynamics at the electrochemical solid-liquid interfaces, we performed the simulations to model the Ni LGNPs under realistic reaction conditions using a finite-element method (FEM) based Maxwell solver48,49. The rigorous FEM modeling comprehensively accounts for multiple scattering and near-field interactions, which are essential for analyzing sub-wavelength structures and materials. The FEM-based simulation details are provided in Methods. The structural and chemical dynamics at different potentials were clarified by performing the energy-dependent optical constant simulations of the core and shell of the Ni LGNPs and thus obtaining the simulated scattering-derived spectra to align with our experimental PE-RSoXS results. The corresponding inverse problem was addressed with a Bayesian optimizer (BO), a machine learning tool that calculates the best next sampling point by integrating the expected improvements50. The finite elements were indicated by small triangles in the models (Fig. 2g). In our simulations, we modeled the electrolyte environment as pure water, instead of the 0.1 M NaOH aqueous solution in the experiments, to simplify the computational framework. The presence of NaOH primarily influences the ionic strength and pH of the solution but has an insignificant effect on the scattering contrast at the Ni L-edge. This is because Na⁺ and OH⁻ ions have no absorption features in the energy range of Ni L-edges, nor do they significantly alter the dielectric function of the electrolyte at these photon energies32,46,51,52. Three core-shell models represent the Ni LGNPs at the three applied potentials. The models retain the same metallic Ni core, while the chemical compositions and widths of the interfacial shells vary. While our FEM-based simulations provide an agreement with experimental data across multiple diffraction orders, the models assume idealized core-shell structures with sharp, uniform interfaces. Interfacial roughness, line-edge disorder, and thickness inhomogeneity, especially common in solid-state electrochemical systems, may reduce the intensity of higher-order diffraction peaks and influence parameter extraction. These effects are partially accounted for using a Debye–Waller factor (see Methods), but residual mismatch or misfit could still arise from structural imperfections not explicitly modeled (Supplementary Fig. 7 and Supplementary Note 8).

It is clear that the scattering-derived spectra of different orders at OCP and 1.2 V are comparable, which suggests a negligible change of chemical composition at the reaction fronts without a major alteration in its overall structure by increasing the potential and keeping it before OER. By contrast, a clear transformation of the structure and chemical composition was captured at 1.6 V under the OER condition. Details of the model parameters are provided in the Methods section, and the features of Ni LGNPs at different potentials are summarized in Table 1.

To clarify the structural and chemical changes induced by different potentials, the energy-dependent optical constants (n(E) = 1 − δ(E) + iβ(E)) simulations of the core and shell of the Ni LGNP models are presented in Fig. 3. The real (δ(E)) and imaginary (β(E)) parts of the optical constants are illustrated for the core of the Ni line gratings in Fig. 3a, b, respectively, remaining largely unchanged with applied potentials and preserving the spectral fingerprint of metallic Ni in the bulk. This indicates the electrochemical stability of the line-grating core. The optical constants of the line-grating core, derived from NEXAFS and the Kramers–Kronig (KK) relation53,54, were fitted using an energy offset and density scaling. Meanwhile, δ(E) and β(E) for the shell are exhibited in Fig. 3c, d, respectively, alongside comparisons to reference spectra for NiO, Ni(OH)2, and NiOOH. For δ(E), indicating the structural changes, the results are apparently consistent, referring to the simulations of the references, suggesting the subtle width changes of the Ni LGNPs (around 2 nm) during electrolysis and thus highlighting the structure sensitivity of PE-RSoXS. For β(E), which corresponds to the absorptive component of the optical response, the simulation captures key features of Ni L-edge absorption in the shell region. These features are not direct XAS measurements but rather arise from the fitted optical constants that reproduce the energy-dependent scattering intensity across all diffraction orders. The simulated results for the shell exhibit higher absorption at around 851.5 eV but lack of the shoulder near 854 eV at OCP and 1.2 V, suggesting Ni2+ was the dominant state in the shell at both potentials. However, at 1.6 V, the optical constants resemble a mixture of Ni(OH)2 and NiOOH, as the absorption profile aligns closely with a mixture of Ni(OH)2 and NiOOH, yet it is overall lower than those at OCP and 1.2 V. When the OER was actively driven, the main peak intensity decreased at around 851.5 eV while a distinct shoulder at around 854 eV emerges, apparently matching the spectral features of Ni3+ in γ-NiOOH. This transition features the in-situ formation of a higher-valence Ni3+ phase, which is widely regarded as the catalytically active species for OER34,55,56. The evolution of the shell (interfacial) region is modeled as a change in form factor due to the oxidation-state-dependent optical constants. The observed spectral changes are not exclusive to interfaces but rather reflect the local chemical environment within the shell region defined in the form factor.

Based on the simulation results, we captured the potential-dependent evolution of a model Ni core-shell geometry, where the core remains metallic Ni, and the shell represents the dynamic interfacial layer undergoing oxidation transitions. As shown in Table 1, upon increasing the potential from OCP to 1.2 V, the Ni LGNPs exhibited a sub-nanometer lateral contraction (~0.4 nm) in total width. The spatial resolution of PE-RSoXS is fundamentally limited by the X-ray wavelength (~1.5 nm), resulting in a diffraction-limited resolution of approximately 0.75 nm. Our ability to detect smaller structural changes (e.g., 0.2 nm in shell thickness) arises not from direct imaging beyond this limit, but from the sensitivity of diffraction intensities to such variations within the periodic structure. This dimensional change could be attributed to early-stage surface oxidation and phase transitions, leading to denser surface phases and potential-induced lattice strain57,58. Meanwhile, the extracted geometric parameters reveal that the shell thickness increases from 4.1 nm at OCP to 6.4 nm at 1.6 V. This shell thickness modulation, accompanied by a progressive increase in total width and modeling residual (χ²), reflects an oxidation-driven interfacial reorganization under anodic bias35,45. Importantly, the fidelity of the simulations in reproducing experimental scattering-derived spectra across multiple diffraction orders underscore the high sensitivity of our PE-RSoXS technique to local chemical environments and subtle changes of structures during electrocatalysis. The similarity in scattering-derived spectral features between OCP and 1.2 V suggests that the initial stages of electrochemical polarization do not significantly alter the interfacial structure statistically, despite partial oxidation of surface species. In contrast, the clear evolution of spectral features at 1.6 V indicates a nontrivial transformation of the interfacial shell layer, coinciding with the onset of OER activity on Ni sites. The increase in shell width and the emergence of Ni3+ spectral signatures together point to an electrochemically driven reconfiguration of the solid-liquid interfaces that enhances catalytic functionality (Supplementary Note 9). Besides, our simulated total width changes of the Ni LGNPs align with atomic force microscopy (AFM) results from the samples after being operated at the corresponding conditions (Supplementary Fig. 8), further clarifying the fidelity of our FEM-based simulations. However, in these AFM images, clear fluctuations are observed along the line edges. Ex situ microscopy techniques cannot directly measure the oxide layer thickness with sub-nanometer sensitivity. In contrast, by exploiting resonant scattering and diffraction-order sensitivity, PE-RSoXS can distinguish shell and core contributions without relying on post-mortem assumptions, providing statistically representative results across hundreds of nanopatterns under realistic electrochemical conditions, which highlight the significance to develop PE-RSoXS.

To further assess the dimensional sensitivity of PE-RSoXS, we monitored the dynamic redox kinetics of Ni LGNPs during potential cycling (Supplementary Fig. 9). By altering the potential between 0.8 and 1.4 V, we observed dynamic oscillations with three distinct scattering peaks corresponding to oxidation and subsequent reduction in the reversible potential scans. Both α-Ni(OH)2 and γ-NiOOH crystals, which usually form under OER conditions, exhibit layered structures with interlayer water, resulting in d-spacings of approximately 8 and 7 Å, respectively. In contrast, the unhydrated β-Ni(OH)2 and β-NiOOH phases have significantly smaller interplane d-spacings of 4.6 and 4.8 Å, respectively59. The phase dynamics between α-Ni(OH)2 and β-Ni(OH)2, or between β-NiOOH and γ-NiOOH, as well as preliminary redox between β-Ni(OH)2 and γ-NiOOH, would theoretically lead to the changes of the line-grating width due to lattice swelling and shrinking. Therefore, the fluctuations in the 2nd-order intensity, compared to the stable 3rd-order intensity, could likely be attributed to the phase changes and redox processes of Ni LGNPs60.

These results unveil the distinctive capability of PE-RSoXS to operando resolve subtle structural and chemical transformations at buried solid-liquid interfaces. The method quantitatively captures sub-nanometer dimensional changes in the Ni LGNPs during OER while simultaneously mapping oxidation-state gradients between the metallic Ni core and the evolving Ni(OH)2/NiOOH shell through the FEM simulations on the diffraction-order-dependent scattering spectra. This dual sensitivity to geometry and chemistry, achieved under low X-ray doses with statistical averaging across hundreds of identical nanopattern units, enables a direct correlation between redox transitions and sub-nanometer morphological reconstruction. Such interface-specific insights, particularly the separation of bulk and interfacial signals based on the core-shell structures, are essential for understanding electrocatalytic behaviors, which is still difficult to obtain with conventional soft XAS or electron microscopy under realistic electrochemical conditions.

Theoretical FEM-based simulations of PE-RSoXS
Building on the alignment between experimental results and simulations, we further conducted theoretical FEM-based modeling of PE-RSoXS to elucidate its potential as a sensitive probe for nanostructural and compositional variations. Uniform and rectangular core-shell line-grating nanopatterns with 100 nm width and 50 nm height (Fig. 4a), where Ni metal was in the core and oxidized Ni species were in the shell, were built up on a SiN substrate to mimic the scattering behaviors. Under electrochemical conditions, our theoretical FEM-based simulations of PE-RSoXS remain robust and representative of the Ni LGNPs. The optical constants of all compositions were employed for the simulations at the corresponding incident energy range (Supplementary Fig. 10), of which β(E) as a function of photon energy, the absorption component, was calculated from NEXAFS measurements and simulations; while δ(E) as a function of photon energy, the dispersion component, was derived from the KK relations. Diffraction intensity is primarily determined by the contrast between the line gratings and the environmental liquid. We simulated the near E-field intensity induced by soft X-rays by increasing the shell width from 1 to 2 nm and mapped out the relative E-field intensity ((I2nm − I1nm)/I1nm), which exhibit the modulation of soft X-ray illumination by the engineered nanopatterns as optical elements in the Fresnel regime. As shown in Fig. 4b, a clear enhancement of the intensity contrast by the shell width variation can be observed at the interfaces. Importantly, the optical modulation scale for E-field enhancement closely aligns with the length scale of interfacial shell structures, significantly improving the measurement sensitivity to the electrochemical interfacial dynamics via PE-RSoXS.

At the resonant photon energy (i.e., 852.0 eV, the Ni L-edge energy), the diffraction intensities were obtained in the direction of the incident X-ray perpendicular to the nanopattern plane. As shown in Fig. 4c and Supplementary Fig. 11, the odd-order intensities remain unchanged. Strikingly, those of the even orders exhibit apparent enhancement induced by statistically identical interfaces of oxidized Ni with only the width variations from 1 to 2 nm by 0.2 nm of a step. Meanwhile, site-specific chemical composition was identified by order-dependent spectroscopy derived from the scattering intensities near the Ni L-edge energy. By changing the width of oxidized Ni interfaces, the odd-order spectra show the same feature of Ni metal core (Supplementary Fig. 12), which elucidates the bulk state of the Ni LGNPs. In contrast, the even-order spectra depict an increasing signature of NiO with the increase of the shell width (Fig. 4d and Supplementary Fig. 12), indicating that the even-order spectra reveal the interfacial Ni oxidation states. We further extend the chemical composition of the oxidized Ni shell from bare NiO towards the mixture of NiO and NiOOH (Fig. 4e), as well as bare NiOOH (Fig. 4f). The involvement of Ni3+ species enhances the spectral features at around 854 eV (Supplementary Fig. 12), compared to that of the Ni2+ state, highlighting the considerable chemical sensitivity of PE-RSoXS61. To address the concept of spatial resolution more rigorously, we conducted additional simulations (Supplementary Fig. 13). The oxidized Ni shell region was displaced with distances from the Ni metal core without physical contact. These simulations reveal that the diffraction signal is not only sensitive to shell thickness and composition, but also to the position of the shell within the grating period, confirming that PE-RSoXS can resolve spatial placement within the structural unit cell. By simulating unphysical scenarios in which the shell is “floating” at varying positions within the groove, we demonstrate that the diffraction intensities change depending on shell placement, indicating the system’s sensitivity to spatial localization of chemical features.

As a result, our simulations theoretically demonstrate the characteristic features of PE-RSoXS in unveiling interfaces: (i) enhanced scattering signal from buried interfaces by statistically identical nanopatterns (form factors) and decoupled the interfacial signals from the bulk material responses; (ii) high scattering sensitivity to sub-nanometer structural variations; (iii) site-specific sensitivity of elemental oxidation state by order dependency; (iv) achievable spatial resolution within the unit cell when the geometry is well-defined.

In summary, we report the development of operando PE-RSoXS, which is a powerful and non-destructive characterization methodology for probing dynamic solid-liquid interfaces with sub-nanometer structural sensitivity and millisecond temporal resolution. Deployed via a “sample-as-optics” design, PE-RSoXS incorporates nanoengineered periodic structures directly into electrochemical samples, transforming them into diffractive optical elements that coherently amplify faint resonant scattering signals from buried interfaces. This architecture leverages the length-scale match between soft X-ray wavelengths and chemical structures, offering elemental specificity and spatial precision. Using a simulation framework based on electromagnetic wave propagation through structured media, we quantitatively resolve oxidation-state gradients and subtle structural changes on Ni LGNPs under operando OER conditions. Diffraction-order-dependent contrast enables clear separation of interfacial dynamics from bulk signals, allowing real-time probing of chemical and structural evolution through form factor sensitivity to chemical distributions within the unit pattern. The approach is not fundamentally limited to perfectly periodic systems. One potential strategy is to use a patterned template and apply a porous catalyst on top, leveraging the underlying structure to enhance diffraction signals. This capability facilitates mechanistic insights into catalyst structure-function relationships, directly informing the design of next-generation electrochemical materials.

PE-RSoXS introduces an interface-specific metrology by combining resonant soft X-ray contrast with Bragg diffraction enhancement from tailored nanopatterns. The PE-RSoXS methodology is broadly applicable beyond the Ni-based system studied in this work, providing operando access to coupled structural and chemical dynamics in a wide range of catalytic materials that undergo significant restructuring during operation. By tuning to the relevant absorption edges of the focused elements, PE-RSoXS can selectively enhance scattering signals from specific chemical states, enabling real-time correlation between redox transitions and sub-nanometer structural changes and active-phase evolution. Its compatibility with identical architecture, including powder- and pattern-based systems, low-dose operation, and high sensitivity to the buried electrochemical interfaces make it a versatile tool for probing diverse catalysts under realistic electrochemical conditions, with strong potential to address key questions in catalysis, energy conversion, and materials science. Moreover, by coherently amplifying signals from statistically identical units, PE-RSoXS is compatible with broadband (pink beam) illumination and enables single-shot, full-spectrum acquisition, dramatically improving throughput and reducing acquisition time. This opens exciting opportunities for coupling with ultrafast detectors, diffraction-limited synchrotron sources, and X-ray free-electron lasers, paving the way for real-time tracking of ultrafast interfacial transformations under realistic operating conditions.

Methods
Materials
The commercial Poly(methyl methacrylate) (PMMA) photoresist was purchased from Polymer Source, Inc. Acetone (SKU No. 270725, 99.9%), Ethanol (SKU No. 459844, 99.5%), Ni metal (SKU No. 767484, 99.95%), NaOH (SKU No. 306576, 99.99%) and sulfuric acid (H2SO4, SKU No. 258105, 95-98%) were purchased from Sigma-Aldrich to be used in this work without further purification. The Ni foam (SKU No. bcnf-16m, 99.9 wt.%, 1.6 mm thickness), graphite rod electrode (SKU: EFC-CE-G), Ag/AgCl reference electrode (SKU: EFC-RE-AAC), and standard electrochemical H-cell (SKU: EC-HDC-GB-GFRS-50, 50 mL volume) were purchased from MTI corporation. Nafion N117 proton exchange membrane (177.8 µm thickness) was obtained from Ion Power, Inc. Customized electrochemical microchips (E-chips) were obtained from Protochips, Inc. as X-ray transmissive substrates with two SiN windows (240 µm in length and 80 µm in width). Milli-Q water (resistivity 18.2 MΩ cm at 25°C) was used for the preparation of the 0.1 M NaOH aqueous solution.

Nanofabrication of Ni line-grating nanopattern electrodes
Ni line-grating nanopatterns were fabricated on the SiN windows of the E-chips using e-beam lithography, followed by Ni metal deposition and lift-off processes at the Molecular Foundry’s Nanofabrication facility of Lawrence Berkeley National Laboratory. The E-chips were first rinsed with acetone, ethanol, and isopropyl alcohol sequentially, then cleaned under UV-ozone for 6 min. Then E-chips were dehydrated in an oven at 150°C for 5 min and then cooled down to room temperature for 10 min. The E-chips were then spin-coated with around 180 nm-thick PMMA photoresist at 2000 rpm, and oven-baked for another 5 min. Next, the e-beam lithography was used to expose the line gratings with a width and pitch of 100 and 200 nm, respectively, to cover the SiN windows and connect the Pt circuit of the working electrode. Kapton tape was applied to the areas outside the vicinity of the patterned working electrode region before e-beam evaporation in order to minimize the likelihood of unintentional shorts between the counter or reference electrodes and the deposited Ni metal. A 50-nm height of Ni metal was deposited by e-beam evaporation and then lifted off by stripping the PMMA with acetone. The cross-section image of Ni LGNPs was captured with SEM imaging (ZEISS) at 5 kV.

Operando PE-RSoXS measurements
A customized electrochemical liquid flow cell constructed by Protochips, Inc. was developed at the beamline 11.0.1.2 of the Advanced Light Source in Lawrence Berkeley National Laboratory62. RSoXS patterns were collected with a back-illuminated Princeton PI-MTE charge-coupled device (CCD) cooled to −45°C. Each RSoXS pattern at one energy point was acquired in a single 1 ms exposure time of soft X-rays using 1/1000 of the incident photon flux (X-ray dosage per shot: below 0.01 mJ cm−2). This ultralow-dose configuration enables highly sensitive detection of scattering signal changes of the line gratings while preserving the native state of the electrochemical interfaces. An E-chip from Protochips, Inc. with Ni LGNPs was paired with a small supporting chip with a 500-nm spacer for the encapsulation of 0.1 M NaOH aqueous electrolyte. Both the counter and reference Pt electrodes were integrated on the E-chip. 0.1 M NaOH aqueous solution was prepared freshly before every electrochemical measurement using Mili-Q water with stirring. The pH value of the electrolyte was confirmed as 12.96 ± 0.24 by a pH meter purchased from METTLER TOLEDO (Seven Direct SD20). The distance between the reference electrode (RE) and the working electrode (WE) is minimized to reduce the iR-drop within a 500-nm-thick electrolyte, while the counter electrode (CE) has an extensive surface area to facilitate rapid polarization. Electrochemical resistance measurements or iR correction were not intentionally performed in this study, as our focus lies on exploring the electrochemical reactions during water oxidation rather than optimizing the system’s electrocatalytic performance. However, to confirm the integrity of our electrochemical cell setup, the resistance was measured to be around 20 kΩ by single-frequency electrochemical impedance spectroscopy at 10,000 Hz. Then, the same electrolyte was flowed continuously, facilitated by a Harvard Apparatus PHD ULTRA Programmable Syringe Pump with a rate of 250 µL h−1, minimizing the bubble retention effect. Electrochemical measurements were performed with a Gamry potentiostat in a 3-electrode floating mode configuration. The data was collected and processed by the software, Gamry framework. Prior to constant-potential PE-RSoXS measurements at OCP, 1.2 V, and 1.6 V vs. RHE, the Ni LGNP working electrodes underwent a standardized conditioning sequence to stabilize the electrochemical interfaces. Firstly, as-fabricated Ni LGNPs were characterized by RSoXS in the absence of electrolyte to establish baseline RSoXS patterns. Next, several CV scans were performed over a potential range from OCP to 1.2 V vs. RHE, covering the Ni oxidation/reduction region, to remove surface impurities, activate the nanopatterns, and establish a stable electrochemical response. This typical method ensures reproducible electrocatalytic activity and RSoXS patterns by removing adventitious surface species and stabilizing the core-shell structure prior to operando measurements. Following conditioning, scattering profiles at the Ni L-edge were collected operando under constant-potential control at OCP, 1.2 V, and 1.6 V vs. RHE. The topographic imaging of the nanopatterns after operating at the three potentials was captured using AFM, which was conducted using the Bruker Dimension Icon 3 in ScanAsyst mode at the Molecular Foundry of Lawrence Berkeley National Laboratory.

Theoretical FEM-based simulations
For a quantified simulation of the PE-RSoXS measurements, we constructed and simulated Ni line-grating nanopatterns using a FEM-based Maxwell solver from JCMwave. The first step for the calculation was to define a model. All the shapes were polygons (especially trapezoids), and the coordinates were generated from the following parameters: side-wall angle, height, and width by Python scripts. The model contained the computational domain and defined the maximum size of the simulated cross-section based on the total height of the line gratings, the height of the SiN window, and an offset above and below the structure. The width of the line gratings was defined by the pitch. The boundary conditions of the computational domain were classified for the solver: the upper and lower boundaries (Y-direction) were transparent, while the right and left (X-direction) boundaries were periodic. The SiN window was set in a thickness of 50 nm, and a line grating with varied height, width, and side-wall angle was placed. One shell layer was added to the grating core. Additionally, the grooves of the grating were filled with water. The generation of the mesh comprised of the finite elements was completed based on the above settings. Smaller features were adapted by the chosen mesh compared to the side length constraints of 0.7 nm. All the polygons were connected to the selected materials with pre-defined optical constants (n(E) = 1 − δ(E) + iβ(E)) from the Henke database63 except for the core and shell. For the core, XAS spectra were obtained with total electron yield. The absorption coefficient β(E) in the optical constant was obtained by scaling the XAS with the atomic spectrum from the linear combination of each individual atom from Henke data, according to the chemical formula. The dispersion coefficient δ(E) was derived from β(E) via KK relations. For the shell, the preliminary calculations were conducted by optical constants derived from reference spectra (Fig. 4), retrieved in a manner similar to those of the core. In subsequent modeling, these optical constants were approximated using Gaussian profiles in Fig. 2 and Supplementary Fig. 4. For the FEM-based simulations, the X-ray source was defined: an S-polarized X-ray light source with a varied wavelength. We established an incident plane wave coming below the SiN window perpendicular to the plane of the line gratings, along with the orientation of the line width corresponding to the experimental configurations. The FEM-based Maxwell solver calculates the electric field intensity in the near field. To gain the scattering intensities of the diffraction orders, the far field is calculated with a Fourier transformation of the E-Field intensity distribution (near field). Because of the defined periodicity, the discrete scattering diffraction intensity with its q-space coordinates was retrieved as the simulation outputs. All the FEM-based calculations were conducted on the supercomputer in the National Energy Research Scientific Computing Center of Lawrence Berkeley National Laboratory by using the commercially available software package JCMsuite (Version 6.2; JCMwave GmbH, Germany).

Experimental PE-RSoXS spectra fitting via simulation
To model the experimental PE-RSoXS results using FEM-based simulations, the experimental data were first reduced from 2D detector images into spectra for each diffraction order. The first step involves performing a background correction. To achieve this, the portion of the image (200 pixels wide), which does not contain any scattering intensities, was selected. We calculated line-wise in the X-direction, which was subsequently subtracted from the rest of the image to remove background interference. Following the background correction, the peaks were identified and labeled according to their diffraction order. Initially, the two peaks with the highest intensity were located using a peak-finding algorithm. A line was drawn through these peaks, and the intensities along this line were summed in the X-direction using a width of 60 pixels. The line cut was then taken, and the peak positions were identified. These positions remained stable during a single measurement, making the process robust and consistent. Once the peaks were identified, they were integrated over a width of 23 pixels. This process was repeated for each CCD image taken during the measurement. Additionally, an I0 calibration was applied to the data to account for intensity variations in the beamline as a function of photon energy. The next phase involved parameterizing the model that fits the experimental data. The basic shape of the model was described in the previous section. The grating was represented as a rectangle, which was parameterized by its height, width, and side-wall angle. For the oxide layer, an additional thickness was incorporated in all directions onto the rectangle of the core. The optical constants of the core and shell were fitted, while the core was based on optical constants retrieved from NEXFAS and KK with a density and energy offset. The energy shift (−0.3 eV) and density scaling factor (0.55) for 1.2 V and 1.6 V were similar, while OCP showed a higher density (0.68) and a smaller energy shift (−0.2 eV). These differences were relatively minor, especially compared to those observed in the shell or oxide layer. For the shell, a more free-floating approach was chosen to account for compositions that are not able to be retrieved from other measurements (i.e., NEXFAS). Preliminary tests revealed that 8 Gaussians were required to describe the refractive index of Ni oxide, as retrieved from NEXAFS in the range of 840–865 eV.

y(E) = sum_{i=1}^{4} a_i * e^{-(E-E_i)^2 / (2 * sigma_i^2)}

a_i: amplitude of the Gaussian, E: energy, E_i: position of the Gaussian, and sigma_i: width of the Gaussian.

The energy offset accounts for small calibration differences between the incident photon energy at the RSoXS beamline and the reference NEXAFS data used to compute the optical constants. This typically ranged between −0.2 and −0.3 eV and was used to align characteristic absorption features of the core. The shell thickness was treated as a floating parameter and optimized independently for each potential condition using Bayesian optimization. It varied along with the shell optical constants, Debye–Waller roughness, and core energy offset. To account for the minimal measurable scattering intensity (dark current), a constant offset was added to the calculated intensity. Roughness was incorporated using a Debye–Waller factor correction64:

I_fit(K_x) = I_calc(K_x) * e^{-(K_x * sigma_rough)^2}

I_calc(K_x): Calculated intensity in the diffraction order (K_x) with the FEM solver, K_x: Position of the diffraction spots in K-space, and sigma_rough: Debye–Waller correction for roughness.

Roughness was incorporated using a Debye–Waller factor correction to model the attenuation of scattering intensity at higher diffraction orders due to structural disorder. However, this simplified approach does not fully account for the realistic effects such as lateral inhomogeneity, line-edge roughness, or non-uniform shell thickness. These imperfections could lead to suppression of higher-order peaks or fitting ambiguity and represent a source of model non-uniqueness. The following error model was used for the Bayesian optimization method (BO) calculations50:

sigma^2(E, q) = (a * I_exp(E, q))^2 + b^2

With a = 0.05 and b = 10^-5, which was a feasible approximation of the experimental error.

By optimizing the χ2, the parameters of the error are modeled based on the following equation:

χ2 = sum_{E, K_x} [(I_exp(E, K_x) - scale * I_fit(E, K_x))^2 / (2 * sigma^2(E, K_x))]

A total scale of 36 model parameters were optimized using the BO. Domain boundaries were determined based on expected model parameters derived from SEM images and NEXAFS spectra for the optical constants of NiO, Ni(OH)2 and NiOOH. Bayesian optimization enabled efficient parameter sampling and paired well with the computationally intensive FEM solver.

To estimate the confidence intervals, we employed the method reported previously65. This involved inverting the Hessian matrix H_ik of the error function66:

H_ik = (partial^2 chi^2(p)) / (partial p_i * partial p_k)

where p represents the model parameters used for fitting.

At the minimum of χ2, we assumed the parameters followed a Gaussian distribution. The standard deviation (STD) at the global minimum parameter set was defined as:

STD = sqrt(chi^2 / DoF)

where (DoF = N − M) represented the degrees of freedom with N as the number of measurement points and M as the number of free model parameters.

The confidence interval for each model parameter could be calculated as:

sigma_p = STD * sqrt(diag((H)^-1))

Utilizing the Gaussian process model for (χ2(p)) at its minimum allowed us to approximate the error covariance matrix, (H)^-1, and determine the confidence intervals of the parameters.

To improve smoothness and convergence, the fitting process was seeded using the optimized solution at the previous energy step as the initial guess for the next energy point. The optical constants β(E) were parameterized using a sum of Gaussian functions, and δ(E) was calculated via KK relations, ensuring that both components followed physically meaningful trends. This regularization mitigated local minimum and produced smooth, interpretable spectral features.

Standard electrochemical measurements in an H-cell
Bench-top electrochemical measurements were performed in a three-electrode H-cell using a Biologic potentiostat without iR correction. Before the electrochemical measurements, the bulk Ni foam was washed by 0.5 M H2SO4 aqueous solution (diluted from concentrated H2SO4 using Mili-Q water), ethanol, and Mili-Q water sequentially with ultrasonication for 1 min each. Then it was dried in a vacuum at room temperature for electrochemical measurements. The process of pretreating Nafion N117 proton exchange membrane beginned with cleaning the membrane by soaking it in Mili-Q water for 1–2 h to remove contaminants, followed by thorough rinsing with fresh Mili-Q water. Next, the membrane underwent acid treatment by immersing it in 0.5 M sulfuric acid solution for 1–2 h at 80°C. After acid treatment, it is imperative to rinse the membrane thoroughly with Mili-Q water to remove any residual acid. Then, the membrane was stored in Mili-Q water to prevent it from drying out. A piece of Nafion N117 membrane (1 × 1 cm2) was tailored for the H-cell. CV measurements were carried out on a 1 × 1 cm2 piece of bulk Ni foam in 0.1 M NaOH using a typical H-type cell, with 20 mL of electrolyte placed in each chamber. An Ag/AgCl reference electrode filled with saturated KCl was employed as the reference electrode (RE), while a graphite rod with a large surface area served as the counter electrode (CE). The conversion from Ag/AgCl (sat. KCl) to the RHE scale in 0.1 M NaOH was determined from the Nernst relationship, giving a potential difference of 0.9666 V. All electrochemical data were recorded and analyzed using EC-Lab software.

Data availability
The Source data underlying the figures of this study are available with the paper. All raw data generated during the current study are available from the corresponding authors upon request. Source data are provided with this paper.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "In this manuscript, Li et al. present the feasibility of the PE-RSoXS measurement technique using nickel linear grating nano-pattern electrodes; however, the study falls short of establishing the significance of this approach for the broader electrocatalysis field. The reported findings, namely oxidation state changes and oxide layer dynamics, largely confirm well-established phenomena without offering novel insights or advancing the current understanding of electrode-electrolyte interfaces. Furthermore, the requirement for specialized nanopatterned electrodes significantly limits the method’s applicability to practical electrocatalytic systems, where powder-based catalysts dominate. These critical limitations suggest that the manuscript is unlikely to attract broad interest and therefore does not align with the scope of Nature Communications. Instead, it may be more appropriate for publication in a specialized journal focused on advanced characterization techniques. The main concerns are as follows:\n1. In line 33, The authors claim that PE-RSoXS enables high spatial and temporal resolution characterization of solid-liquid interfaces appears overstated based on the presented evidence. The technique was primarily employed to monitor oxidation state changes in Ni LGNPs during OER, information that could be similarly obtained through conventional soft XAS. Furthermore, the simulation-derived estimates of nanoscale oxide layer thickness variations do not seem to offer substantial advantages over established electron microscopy techniques, either in terms of resolution or mechanistic insight.\n2. In line 69, While the authors assert that PE-RSoXS offers unprecedented spatial resolution and chemical sensitivity, the presented results fail to demonstrate any substantial improvement over existing characterization techniques. The observation of Ni oxidation to NiOOH during OER is a well-established phenomenon, which does not sufficiently showcase the unique advantages of this technique. Therefore, the authors are encouraged to further clarify what new or distinctive insights PE-RSoXS reveals beyond what is already accessible through conventional methods, in order to highlight the necessity and uniqueness of this approach.\n3. In line 149, The authors claim that the exposure time of PE-RSoXS can reach the microsecond scale; however, no dynamic processes matching this timescale were observed in the current study. The authors are therefore encouraged to investigate processes whose dynamics better align with this temporal resolution, in order to obtain more compelling and meaningful results.\n4. In line 254, the authors use simulations to reveal the potential-induced change in oxidation layer thickness of the Ni-based catalyst. However, there is a lack of direct experimental evidence to support this claim. Since the oxidation of Ni during the OER is generally irreversible, changes in the oxide layer thickness should be observable by electron microscopy. Therefore, additional experimental evidence is needed to support the PE-RSoXS observations.\n5. The authors provide a detailed discussion of PE-RSoXS principles and technical merits in the main text. While this information is valuable, moving it to the supplementary information would improve the manuscript’s focus and clarity. The main text should instead provide a concise and clear discussion that highlights the distinctive structural features revealed by PE-RSoXS and emphasizes the necessity of this technique for the study. This approach will attract greater interest from readers.\n6. The PE-RSoXS measurements require highly specialized sample preparation to produce well-ordered Ni line-grating nanopatterns. Although this approach is interesting, its broader applicability, particularly to conventional powder-form nanocatalysts, remains to be demonstrated.", "reviewer_2": "The manuscript describes an implementation of the emerging RSoXS scattering method that exploits a Bragg structure sample to gain strong sensitivity to the composition within it. The new implementation is coined “PE-RSoXS.” It is demonstrated in a Nickel electrochemical oxidation experiment, and different optical constants are pulled out that describe different shell layers obtained at different electrochemical potentials. The analysis framework features a full Maxwell simulation beyond typical Born approximation approaches.\nThe manuscript reflects important advances and should be considered for publication in Nature Communications, provided the issues identified in review are addressed. Although this reviewer will provide extensive feedback for the author team (see below), the volume of feedback does not reflect manuscript quality. There are issues in presentation and context, and some technical issues that I will highlight in detail. Provided the editors agree, I look forward to continued consideration of this article for publication in this fine journal.\n- Overarchingly, the centerpiece of the manuscript describes a technique that is usually called “diffraction anomalous fine structure,” or DAFS. This technique is actually very old. It has been described, in some form, since 1956 - Y. Cauchois (1956) — “Distribution spectrale observée dans une région d’absorption propre de divers cristaux”, C. R. Acad. Sci. Paris 242, 100–102. The idea was extended in work by other authors in the mid-70s, and re-emerged as a method named DAFS in the surface science era of the early-to-mid-90s, with a flagship publication led by Joe Woicik who is now at NIST: Stragier, H., Cross, J. O., Rehr, J. J., Sorensen, L. B., Bouldin, C. E., & Woicik, J. C. (1992). Diffraction anomalous fine structure: A new x-ray structural technique. Phys. Rev. Lett., 69, 3064–3067. doi:10.1103/PhysRevLett.69.3064.\nNone of this rich history is acknowledged in the manuscript under review, nor are there any explanations of how the current manuscript goes beyond. In this reviewer’s eyes, there are at least two claims of novelty / advance for the manuscript under review: 1) there are very few implementation of DAFS with soft X-rays on larger Bragg structures, and 2) the simulations in the manuscript under review and the method of obtaining optical constants are extremely sophisticated compared to most previous DAFS examples, and particularly well-suited to soft X-rays given their high likelihood of multiple scattering. The authors should acknowledge the rich history of DAFS (a nice 2018 review is here https://arxiv.org/pdf/cond-mat/0309624 but I don’t know if it was published outside arxiv), and then explain their advances.\n- The reviewers make multiple claims of interface sensitivity - the word is mentioned 45 times in the article! But there is nothing especially interface sensitive about RSoXS or PE-RSoXS. The RSoXS technique is FORM FACTOR sensitive. When the form factor includes an interface, we can of course be sensitive to it, and different placements of the interface within the form factor (and within the structure factor it is convoluted with) will result in different levels of sensitivity. Yes, in the implementation here, the authors demonstrate sensitivity to a shell in a core-shell structure (this is the right language, not “interface sensitive”), but there will also be sensitivity to other parts of the structure that are not interfacial. The technique will be excruciatingly sensitive to the density and composition of the solid core, for example.\nThe reason this reviewer is so adamant about the language used here is that the “interface sensitive” claims are misleading and they echo misleading claims made about RSoXS about 10-15 years ago that have (in this reviewer’s opinion) sown confusion and stunted the growth of the technique. Much early work on the RSoXS of organic electronics and organic photovoltaics (OPVs) in particular made claims about special sensitivity to interfacial composition and orientation. The language used then (and now, in the article under review!) was extremely similar to the language of nonlinear optics techniques such as sum frequency generation (SFG) (which was emerging and highly visible at the time), in which symmetry breaking at interfaces truly affects spectroscopic selection rules, allowing spectra to be observed that isolate signals from the interface. That is NOT what is happening in RSoXS – scattering sees everything, not just interfaces, and RSoXS is frequently bulk-dominated. But this article strongly implies some kind of special sensitivity to interfaces with the 45 mentions of the word “interface” and in fact these authors use “symmetry breaking” language to explain their results in lines 127-128, when what is meant is that subtle perturbations in the form factor can have strong effects on order expression. But these perturbations don’t necessarily have to be at interfaces, and RSoXS is not sensitive to symmetry breaking specifically, just the form factor. The authors should remove their misleading language about interface sensitivity and symmetry breaking. If they would like to include it, they should include the context that the technique is sensitive to interfaces *when you know there is an interface, and you know where it is, and you can model it*. That is what is required. It’s also the same way the RSoXS is sensitive to aspects of structure other than interfaces.\n- The above concern brings me to an adjacent point, which is that the authors have not demonstrated the “spatial resolution” that they mention 19 times in the manuscript. What they have demonstrated is fine sensitivity to shell thickness in a core-shell structure. Which, in this reviewer’s opinion, should certainly be enough for publication in Nat Comm if the authors are willing to soften their claims. But “spatial resolution” implies that they can identify *where* that material is within the structure. Instead, the authors have assumed that it is on the sidewall of the solid structure. That is, of course, a perfectly valid assumption given the electrochemical nature of the experiment, but it does not demonstrate spatial resolution.\nTo demonstrate spatial resolution, the authors could consider simulating what happens if the shell were “free floating” within the electrolyte-filled part of the Bragg structure, spaced by various distances from the wall. As unphysical as those models would be, that could potentially demonstrate spatial resolution. I can assert from personal experience that scattering from Bragg structures is typically far less sensitive to specific placement within the Bragg structure (“spatial resolution”) than it is sensitive to the specific volume fractions (shell thickness) and their indices (shell and core and matrix composition), but I would defer to simulation results from the authors if they showed otherwise.\n- There is a significant missed opportunity to structure the narrative here with deeper fundamentals in the presentation of the “scattering derived spectra.” These appear to be [I integrated across a peak] vs. energy, presented in Fig 2 d-e-f. On lines 190-191 these spectra are directly compared to X-ray absorption spectra data, and described as “comparable.” But it is incorrect to compare them because [I integrated across a peak] is the result of *differences between indices* of the volumes within the scattering structure, and it includes the delta parts of those indices. So, there is dispersion baked into that “spectra”, and even if you could isolate beta, it would be a difference spectrum between nickel beta and nickel oxide beta (electrolyte beta would be reasonably flat, but sloped). The authors should remove this comparison to XAS, and more properly acknowledge the contributions to I vs. energy.\nBut there is an even stronger way to do that, and that is to lean into the concept of the total scattering invariant (TSI). The TSI describes the integrated scattering across all q and in all 3 dimensions; for RSoXS patterns an approximation frequently called “integrated scattering intensity” (ISI) is used by integrating I*q^2 (sometimes q-bounded or feature-bounded, and sometimes for all measurable q) vs energy. The authors could consider presenting Fig 2 d-e-f as feature-bounded ISI vs. energy. The reason this would be valuable is that, in principle, TSI / ISI should be directly proportional to the sum of the combinatorial pairwise binary contrasts (Delta(beta)**2 + Delta(delta)**2) weighted by their volume fraction products (see Collins and Gann 2021 Eq 8 and Eq 15; it can be extended to any number of components). Quantitative agreement of the energy-dependent ISI vs. energy and expectation contrast was robustly demonstrated in Ferron, Pope, and Collins PRL 2017, and it underpins the physics of RSoXS and of the manuscript under review. The Maxwell solver will reproduce this physics but the authors should more robustly acknowledge it, as they deal with I vs. energy much earlier than they introduce n, delta, beta, etc, when they arise from a common framework.\nThe authors’ mentions of “numbers of scatterers N” on lines 155-156 and the sensitivity to N squared are a confusing and less general recapitulation of the physics I mention above. They are misleading in stating that “conventional scattering” has linear sensitivity to the number of scatterers. All SAS has Delta(rho)**2 magnitude; the number of scattering objects increases the scattering intensity linearly in the dilute form factor limit, whereas in a Bragg structure you have a *really* strong reporter of Delta(rho)**2. Perhaps that’s what the authors intended to say but the concepts are muddled here.\n- The authors should moderate their language and results that suggest spatial resolution beyond the diffraction limit, and they need to be clear about their interaction with this limit. Spatial resolution beyond the diffraction limit is implied by their language: “atomic scale resolution” is used line 73. But the wavelength here would be ~1.5 nm, with a diffraction limit of 0.75 nm. The PE-RSoXS technique – like every other SAS technique that exists - should not be capable of real spatial resolution beyond that diffraction limit. But, the authors do demonstrate major differences in the I vs. energy simulation for 0.2 nm increments in shell thickness. This does NOT demonstrate 0.2 nm spatial resolution, it goes back to my earlier point that we are exquisitely sensitive to volume fraction of the shell (or an alternative way of thinking is the effective medium index within diffraction limit volumes), and less sensitive to where the shell is placed within the structure (what we would call “spatial resolution”). If we assume or have robust proof that the shell must be attached to the sidewall, then we have an ~ersatz sub-diffraction-limit spatial resolution, but the assumption about the location is doing all the heavy lifting in that result. I strongly recommend the authors reconsider their language and directly address their interaction with the diffraction limit lest they mislead readers.\n- Aside from my points above, the introduction is otherwise very accessible and explains some key jargon pieces such as the meaning of “near field.” However, the description of the modeling – a centerpiece of the manuscript – is extremely confusing. Some questions that came up during my reading:\n. what is the “energy offset”? I’m guessing it’s to accommodate differences between the energy of the beamline at RSoXS measurement and the energies of the NEXAFS-derived library index used for the core. But this is never explicitly stated in the body or methods.\n. was the shell thickness floated in the fit? This is never explicitly stated.\n. I’m quite surprised at how nicely smooth the delta and beta DAFS extractions are. Did the authors use tricks to keep them smooth – like a thickness constraint or the use of the previous energy’s index as an initial guess for the next? These kinds of fittings are prone to local minima.\n- The authors should acknowledge some limitations around model uniqueness. Importantly, the simulations are done on structures that perfect, and the real structures are not. An imperfection that strongly affects order expression (the principal thing that is fit here) is interfacial width / roughness, which is strongly convoluted with sidewall angle in q_xy scattering. The authors assume that the electrochemically converted material and its two interfaces are sub-atomically smooth. Any line-edge roughness or shell thickness inhomogeneity (and there is usually a significant amount with solid-state electrochemistry) in the core or shell will diminish higher peak orders relative to lower ones and potentially could be misfit. I’m not suggesting the authors need more parameters for their model, but they should acknowledge this limitation and discuss its potential effects on the results.\nThe authors perhaps overemphasize odd/even effects; these are not universal in Bragg structures and are only strongly relevant near the 50% duty cycle limit. I bring this up because it oversimplifies the physics - different fill ratios will produce different harmonic effects on order expression: 33% / 66 % will suppress every 3rd order, etc. I leave it to the authors to consider this point.", "reviewer_3": "This manuscript reports the use of pattern-enhanced resonant soft X-ray scattering (PE-RSoXS) as a new probe of chemical and structural dynamics at electrochemical solid–liquid interfaces. The methodology development is technically impressive. I recommend publication after addressing the following concerns:\n1. “In situ”, instead of “operando” is more accurate to describe the present methodology.\n2. Atomic-scale resolution might be overstated while the results mainly reach sub-nanometer sensitivity.\n3. A comparison table summarizing state-of-the-art interface characterization techniques (e.g., TEM, XAS, AP-XPS, fluorescence microscopy, RSoXS) would greatly strengthen the introduction and highlight the advantages of PE-RSoXS.\n4. Please comment on the generality of the technique. Could this approach be applied to other catalytic materials (e.g., Co or Cu) that undergo significant restructuring?\n5. The method is described as “non-destructive,” but more evidence should be provided. While 1 ms exposure per energy point is reported, is this duration sufficient to fully avoid beam-induced damage? Is it a trade-off or the minimum exposure time? Would longer exposure time improve data quality and accuracy to assess the simulation?\n6. The experimental and simulated Ni L-edge spectra do not fully coincide (e.g., 3rd-order in Fig. 2d). Please clarify the origin of these discrepancies. The spectral difference for experimental vs. simulated spectra should be plotted.\n7. In Table 1, the shell thickness increases with potential, as expected from hydroxide intercalation. However, the core width is supposed to decrease as well. Could you explain this discrepancy?\n8. Is there any conditioning procedure before the electrochemical measurement? If so, would the conditioning alter the core-shell restructuring? Beyond the OCP/1.2 V/1.6 V comparison, is it possible to assess the transition from pristine Ni (air/water-exposed) to OCP?\n9.Is there any potential bubble issue, due to the beam-induced water radiolysis or generated O2 bubble at 1.6V? Would it alter the electrolyte thickness and affect the scattering interpretation?\n10. Could you provide the SEM/AFM analysis of the nanopatterns after experiments to confirm the structural integrity."}, "author_response": "Manuscript ID: NCOMMS-25-48873-T; Response letter\nReviewer: 1\nIn this manuscript, Li et al. present the feasibility of the PE-RSoXS measurement technique\nusing nickel linear grating nano-pattern electrodes; however, the study falls short of\nestablishing the significance of this approach for the broader electrocatalysis field. The reported\nfindings, namely oxidation state changes and oxide layer dynamics, largely confirm well-\nestablished phenomena without offering novel insights or advancing the current understanding\nof electrode-electrolyte interfaces. Furthermore, the requirement for specialized nanopatterned\nelectrodes significantly limits the method’s applicability to practical electrocatalytic systems,\nwhere powder-based catalysts dominate. These critical limitations suggest that the manuscript\nis unlikely to attract broad interest and therefore does not align with the scope of Nature\nCommunications. Instead, it may be more appropriate for publication in a specialized journal\nfocused on advanced characterization techniques. The main concerns are as follows:\nResponse: We sincerely appreciate the Reviewer’s evaluation of our work, which has been\ninvaluable in refining the manuscript to appeal to a broader audience across diverse fields. Here\nwe agree that powder-based catalysts remain the dominant form in practical electrocatalysis,\noffering clear advantages for large-scale synthesis and deployment [Seh et al. Science 355,\neaad4998 (2017); Zou et al. Chem. Soc. Rev. 44, 5148–5180 (2015)]. Resonant soft X-ray\nscattering (RSoXS) has been indeed successfully applied to powder electrocatalysts in recent\nhigh-impact studies [Yang et al. J. Am. Chem. Soc. 144, 8927–8931 (2022); Yang et al. Nature\n614, 262–269 (2023); Yang et al. Nat. Catal. 8, 579–594 (2025)] with broad recognition,\ndemonstrating its strong capability to operando probe chemical and structural dynamics of\nactive species in disordered systems during electrocatalysis. However, a persistent challenge in\napplying RSoXS to directly probe complex electrochemical solid-liquid interfaces lies in\nquantitatively isolating weak and buried interfacial signals from bulk contributions when the\nsystem lacks well-defined geometry.\nTo address this, we deliberately developed a Ni line-grating nanopattern (Ni LGNP) electrode\nas a model platform for RSoXS to achieve methodology innovation in X-ray spectroscopy and\nscattering. This “sample-as-optics” approach transforms the electrode into an active optical\nelement, enabling Bragg diffraction enhancement, near-field optical modulation, and\ndiffraction-order-dependent separation of interfacial signals from bulk ones in\npatternedenhanced RSoXS (PE-RSoXS), which have not been achievable with randomly\noriented powders. This controlled architecture allows us to quantitatively benchmark the\ntechnique’s sub-nanometer spatial resolution, chemical sensitivity, and operando low-dose\nperformance before extending it to more complex powder systems. While powders offer\npotentials for practical applications, our work aims to demonstrate that nanopatterns\nuniquely serve as an optical testbed that amplifies weak buried interfacial signals and ensures\nstatistical representativeness, thereby establishing the fundamental principles and performance\nlimits of PE-RSoXS for future adaptation to industrially relevant electrocatalysts.\nCompared with other conventional operando techniques, PE-RSoXS offers a unique\ncombination of element-specific sensitivity, statistical representativeness, sub-nanometer\ndimensional sensitivity, and millisecond temporal resolution under realistic electrochemical\nconditions, while operating at ultra-low X-ray doses (< 0.01 mJ·cm⁻2). For example, while\noperando transmission electron microscopy (TEM) enables real-time imaging, high-energy\nelectrons enable damage on sensitive samples and often probe only localized regions.\nSynchrotron-based surface-sensitive methods such as X-ray absorption spectroscopy (XAS),\nambient-pressure X-ray photoelectron spectroscopy (APXPS), and X-ray fluorescence\nmicroscopy (XFM) provide robust averaged chemical and structural information but generally\nlack the high spatial and/or temporal resolution required to resolve reaction fronts at the\n(sub-)nanometer and (sub-)millisecond scales. PE-RSoXS uniquely allows us to detect\noxidation-state gradients, sub-nanometer swelling/contraction, and redox-coupled\nmorphological changes at buried solid-liquid interfaces in real time, which are extremely\nchallenging to resolve simultaneously with existing probes. PE-RSoXS achieves high\nefficiency by using engineered periodic nanostructures to amplify weak resonant scattering\nsignals, allowing rapid and statistically robust operando characterization compared to\nconventional methods.\nTo assess the feasibility and sensitivity of a new methodology, it is often effective to begin with\na well-established phenomenon, which is a widely accepted scientific strategy [Collins et al.\nNat. Mater. 11, 536–543 (2012); Kolmakov et al. Nat. Nanotechnol. 6, 651-657 (2011);\nThibault et al. Nature 494, 68–71 (2013); Feijóo et al. J. Am. Chem. Soc. 145, 20208–20213\n(2023)]. The oxidation-state transitions and oxide-layer dynamics observed in this work are\nconsistent with the established oxygen evolution reaction mechanism on Ni-based\nelectrocatalysts. The ability to quantitatively resolve these processes with high spatial and\ntemporal resolution, as well as chemical precision represents a methodological advance that\nenables new classes of mechanistic studies. Furthermore, mechanistic insights gained from such\nprecisely patterned electrodes, including the coupling between oxidation-state evolution, sub-\nnanometer structural changes, and electrocatalytic function, can directly inform the rational\ndesign of practical powder-based electrocatalysts. In this way, the nanopatterned electrode\nserves as a well-controlled testbed for uncovering generalizable interfacial principles that are\ndirectly translatable to industrial catalyst systems [Lum et al. Energy Environ. Sci. 11, 2935–\n2944 (2018); Liu et al. ACS Appl. Mater. Interfaces 13, 40513–40521 (2021)].\nIn summary, while the present study employs a model nanopatterned electrode to establish and\nvalidate PE-RSoXS, the broader significance lies in the development of a new and interface-\nspecific operando X-ray methodology that overcomes longstanding barriers in\ncharacterizations of electrochemical interfaces. With its unique combination of signal\namplification, diffraction-order selectivity, and element-specific operando capability, we\nbelieve PE-RSoXS will be of strong interests to the wider electrocatalysis, materials science,\nand interfacial chemistry communities, aligning well with the multidisciplinary scope of Nature\nCommunications.\nWe have addressed each of the Reviewer’s detailed comments point-by-point below. We hope\nthat with these revisions, the manuscript will be considered suitable for publication in Nature\nCommunications.\n1. In line 33, The authors claim that PE-RSoXS enables high spatial and temporal resolution\ncharacterization of solid-liquid interfaces appears overstated based on the presented evidence.\nThe technique was primarily employed to monitor oxidation state changes in Ni LGNPs during\nOER, information that could be similarly obtained through conventional soft XAS. Furthermore,\nthe simulation-derived estimates of nanoscale oxide layer thickness variations do not seem to\noffer substantial advantages over established electron microscopy techniques, either in terms\nof resolution or mechanistic insight.\nResponse: We thank the Reviewer for raising these important points, which allow us to clarify\nthe unique capabilities of PE-RSoXS. Firstly, the high spatial and temporal resolution of PE-\nRSoXS demonstrated here is directly supported by our experimental and simulation results. We\nresolved sub-nanometer structural dynamics (shell thickness changes of Ni LGNPs) directly at\nelectrochemical solid-liquid interfaces based on our experimental and simulation results (Table\n1 in the revised manuscript), while simultaneously distinguishing oxidation-state gradients\nbetween the metallic Ni core (from 105.6 nm to 107.7 nm) and oxidized Ni shell (from 4.1 nm\nto 6.4 nm) through diffraction-order-dependent spectral separation. Temporally, PE-RSoXS\nachieves single-shot diffraction pattern acquisition with 1 ms exposure time, enabling operando\nmonitoring of dynamic processes with ultra-low X-ray doses (< 0.01 mJ·cm⁻2) that avoid beam-\ninduced chemical changes. This combination of sub-nanometer dimensional sensitivity,\nmillisecond temporal resolution, and element-specific operando capability under realistic\nelectrochemical conditions is not achievable with conventional soft XAS or electron\nmicroscopy and is central to the methodological advance established in this work.\nHere we summarize the distinction of PE-RSoXS from other conventional techniques detailly\nin Table R1. We agree with reviewer that oxidation-state and structural changes in Ni LGNPs\nduring OER could, in principle, be detected by conventional soft XAS and electron microscopy\ntechniques respectively.\nTable R1. Comparison of Characterization Techniques for Electrochemical Interfaces.\nTechnique PE-RSoXS TEM XAS APXPS XFM\nDimensional Sub- Atomic- Tens to Few Tens to\nSensitivity nanometer scale (< 0.1 hundreds of nanometers hundreds of\n(via nm) nanometers sampling nanometers\ndiffraction- imaging (limited by depths in (microprobe);\norder possible beam spot solids; lateral < 10 nm with\nsensitivity size and resolution: ptychography\nand near-field detection nanometers to\noptical geometry) micrometers\nmodulation) (limited by\nbeam spot\nsize and\ndetection\ngeometry)\nTemporal Millisecond Millisecond Seconds to Seconds to Seconds to\nResolution scale (single- to minutes minutes per minutes; minutes per\nshot (with fast spectrum milliseconds scan\ndiffraction cameras) (energy possible with\npatterns; scans) fast detectors\ncompatible\nwith ultrafast\ndetectors)\nChemical High (tunable Limited High High High\nSensitivity (tunable to (requires (direct (chemical (Element-\nspecific EELS or absorption specificity; specific via\nabsorption EDS) edge oxidation fluorescence\nedges; tuning) states and yield)\ndiffraction- bonding)\norder\nselectivity\nisolates\ninterfacial\nspecies)\nInterface High Limited Moderate Moderate High (Bulk\nAccessibility (coherent (requires (surface- (Surface and and buried\namplification sample sensitive sub-surface interfaces\nof weak thinning or but buried accessible; accessible)\nburied cross- interfacial buried solid-\ninterfacial section signals liquid\nsignals) preparation diluted by interfaces\n; interfaces the bulk challenging,\nmay be signals) limited by\naltered probing depth\nduring the\nsample\npreparation\n)\nEnvironment Fully Fully Fully Compatible Compatible\nCompatibility compatible compatible compatible with near- with air,\nwith realistic with with ambient vacuum, and\nliquid cells realistic realistic pressure liquid cells\nand aqueous liquid cells liquid cells gases; liquids\nconditions and and via microjets\naqueous aqueous or meniscus\nconditions conditions methods\nRadiation Low (can be High Moderate Moderate Generally low\nDose / lower than (beam- dose; soft dose; soft X- dose per\nDamage Risk 0.01 mJ·cm–2 induced X-rays rays more pixel;\nper frame) structural more susceptible to scanning can\nand susceptible chemical accumulate\nchemical to chemical changes dose\nchanges, changes\nespecially\nin soft\nmaterials)\nStatistical High Limited High Moderate High (can\nmeasurement (averages (small field (averages (can measure map statistical\nover hundreds of view, over large multiple areas for a\nof identical typically illuminated surface large range)\nnanopattern single volume) regions but\nunits particles or limited\ndepending on local penetration\nthe beam spot regions) depth)\nsize)\nMeasurement High Low Low Low Low\nEfficiency (diffraction (sequential (primarily (primarily (primarily\nencodes detections provides provides elemental\nstructural with chemical- chemical- mapping,\norder while structural state state structural\nresonance and information information; information\ntuning chemical ; structural structural indirect)\nprovides information evolution evolution\nchemical- with must be must be\nstate EELS/EDS measured measured\nspecificity ) separately separately\nconcurrently) with other with other\ntechniques) techniques)\nKey Interface- Direct Chemical- Direct surface High\nStrengths specific atomic- state and sub- sensitivity to\noperando scale analysis of surface trace\nprobing of imaging surfaces chemical elements;\nstructural and and and sub- state under quantitative\nchemical crystalline surface near-realistic elemental\ndynamics; structures regions conditions mapping\nlow-dose;\ndiffraction-\norder\nselectivity\nKey Requires Beam- Limited Limited Limited\nLimitations patterned induced spatial penetration chemical state\nstructures for changes; resolution; depth; information;\ncoherent limited bulk- challenging slow scan for\nenhancement detection interface for fully high-\narea; separation buried resolution\ndifficult to challenging interfaces maps\noperando\nprobe\nburied\ninterfaces\nWe also highlight that PE-RSoXS can provide several critical advantages that go beyond the\nconventional methods as follow.\n(1) Buried Interface Sensitivity and Bulk/Interface Signal Separation\nPE-RSoXS employs a “sample-as-optics” design in which engineered periodic nanopatterns\ncoherently amplify weak individual interfacial scattering signals. Spatial chemical mapping of\neach unit by diffraction intensity analysis allows us to quantitatively decouple buried interfacial\ncontributions from bulk signals. But conventional soft XAS cannot achieve it because\nabsorption spectra inherently mix bulk and interface responses without dimensional sensitivity.\nThis capability is particularly important for electrochemical systems where buried interfaces\ngovern catalytic activity but are difficult to isolate spectroscopically.\n(2) Operando Probing Structural and Chemical Dynamics Simultaneously\nUnlike conventional soft XAS, which primarily provides chemical-state information, PE-\nRSoXS simultaneously resolves sub-nanometer structural dynamics (e.g., swelling/contraction\nof line-grating features via diffraction-order intensity changes) and chemical-state evolution\n(via energy-dependent scattering spectra) in a single operando measurement. This dual\ncapability allows direct correlation between oxidation-state transitions and morphological\nchanges at buried solid-liquid interfaces with sub-nanometer precision and millisecond\ntemporal resolution, enabling mechanistic insights into structure–function relationships that are\ninaccessible when using conventional soft XAS alone [Magnussen et al. Chem. Rev. 124, 629–\n721 (2024); Chen et al. Chem. Rev. 124, 5421–5469 (2024); Timoshenko et al. Chem. Rev. 121,\n882–961 (2021)]. While TEM can achieve high-resolution imaging at atomic scales, it’s\ndifficult to probe buried interfaces in liquid environments without invasive sample preparation,\nnor can it easily provide element-specific chemical-state information without additional\ntechniques such as EELS, which often compromise temporal resolution and increase beam\ndamage. In contrast, PE-RSoXS achieves sub-nanometer dimensional sensitivity to buried\ninterfacial structural variations via coherent diffraction enhancement, while simultaneously\noperando delivering element-specific chemical information in liquid cells. The simulation-\nderived shell thickness variations are extracted from multi-order diffraction spectra,\nrepresenting statistical and quantitative interfacial measurements under realistic electrocatalytic\nconditions, where TEM’s applicability is highly constrained [Chao et al. Chem. Rev. 123, 8347–\n8394 (2023); Li et al. Chem. Rev. 123, 10728–10749 (2023)].\n(3) Statistical Representativeness under Operando and Low-Dose Conditions\nPE-RSoXS measurements average over hundreds of identical nanopattern units within the beam\nfield, ensuring statistical reliability while operating at ultra-low X-ray doses (< 0.01 mJ·cm⁻2).\nThis largely avoids beam-induced chemical changes and allows long-term operando\nmonitoring in liquid electrochemical cells. However, TEM suffers from severe beam damage\nand limited representativeness due to its inherently localized imaging [Chen et al. Adv. Mater.\n32, 1907619 (2020); Nevison-Andrews et al. Chem. Phys. Rev. 6, 031302 (2025)].\n(4) Mechanistic Insights over Ni LGNPs\nThe Ni LGNP system was chosen as a controlled model platform to rigorously validate the PE-\nRSoXS methodology, not to claim new chemistry in OER on Ni-based electrocatalysts. By\ndemonstrating the ability to correlate oxidation-state gradients and sub-nanometer dimensional\nchanges in real time, this work establishes PE-RSoXS as a generalizable approach for studying\ndynamic solid-liquid interfaces in electrocatalysis, energy storage, and other interfacial systems.\nInsights from such controlled studies can directly inform the design of practical powder-based\ncatalysts, where similar phenomena occur but are harder to measure via conventional probes.\nThese capabilities, demonstrated here for the first time in an operando electrochemical system,\nrepresent the central methodological advance of PE-RSoXS and distinguish it from both\nconventional soft XAS and electron microscopy techniques.\nRevision made: We added Table R1 as Supplementary Table 1 and the following illustration\ninto the revised Supplementary Information (SI) as Supplementary Note 1 to make a\ncomprehensive comparison of PE-RSoXS with other conventional techniques for probing\nelectrochemical interfaces with relevant references to support our demonstration.\n“Compared to other state-of-the-art interface characterization methods such as transmission\nelectron microscopy (TEM), X-ray absorption spectroscopy (XAS), ambient-pressure X-ray\nphotoelectron spectroscopy (APXPS), and X-ray fluorescence microscopy (XFM), Pattern-\nEnhanced Soft X-ray Scattering (PE-RSoXS) offers a unique combination of high statistical\nrepresentativeness, sub-nanometer dimensional sensitivity, and element-specific chemical-state\nresolution, all achievable under realistic electrochemical operating conditions. Unlike TEM and\nAPXPS, which are limited in probing fully buried solid-liquid interfaces, PE-RSoXS can access\nthese environments without compromising spatial or chemical sensitivity. While XAS provides\nstatistical chemical-state information, it lacks lateral resolution and diffraction-order selectivity\nfor directly probing interfaces, which is inherent to PE-RSoXS through coherent scattering\namplification from identical nanopatterns. Furthermore, the ultralow-dose capability of PE-\nRSoXS (< 0.01 mJ·cm⁻2 per frame) minimizes radiation-induced artifacts, enabling operando\nmonitoring of dynamic processes with millisecond temporal resolution, surpassing the typical\ntime resolution of conventional methods. These combined attributes position PE-RSoXS as a\npowerful and complementary tool for probing chemical and structural dynamics at buried\nelectrochemical solid-liquid interfaces. PE-RSoXS operates on principles analogous to protein\ncrystallography, where diffraction from periodic units enables reconstruction of real-space\nfeatures with atomic to nanometer-scale precision. In our case, the periodic nanopatterns define\na structural unit cell, and the spatial distribution of chemical species within that unit cell\nmodulates the scattering form factor in a position-sensitive manner. This coherence-based\nsensitivity to the distribution and geometry of resonant materials enables spatial resolution at\nsub-nanometer length scales, as supported by both the experimental data and the finite-element\nmodeling.”\n2. In line 69, While the authors assert that PE-RSoXS offers unprecedented spatial resolution\nand chemical sensitivity, the presented results fail to demonstrate any substantial improvement\nover existing characterization techniques. The observation of Ni oxidation to NiOOH during\nOER is a well-established phenomenon, which does not sufficiently showcase the unique\nadvantages of this technique. Therefore, the authors are encouraged to further clarify what new\nor distinctive insights PE-RSoXS reveals beyond what is already accessible through\nconventional methods, in order to highlight the necessity and uniqueness of this approach.\nResponse: We appreciate the Reviewer’s request for the clarification on the distinctive insights\nprovided by PE-RSoXS beyond what is accessible through conventional characterization\nmethods. We agree that oxidation of Ni active sites during oxygen evolution reaction (OER) is\na well-established phenomenon; this is precisely why we chose this model system to\ndemonstrate the new operando X-ray methodology. We hope to clarify that the novelty of our\nwork lies not in uncovering new chemistry of Ni-based electrocatalysts, but in establishing a\nnew operando X-ray approach that can resolve both chemical-state evolution and sub-\nnanometer dimensional dynamics simultaneously at buried electrochemical solid-liquid\ninterfaces.\nSpecifically, PE-RSoXS reveals several unique aspects that conventional techniques cannot\nachieve (Table R1).\n(1) Simultaneous correlation of chemical and structural changes\nBy extracting energy-dependent scattering spectra from multiple diffraction orders, PE-RSoXS\ntracks oxidation-state transitions (Ni2⁺ to Ni3⁺) while concurrently quantifying sub-nanometer\ndimensional variations of the nanopatterns (Fig. 3d and Table 1 in the revised manuscript).\nThis direct correlation between redox chemistry and morphological changes at buried interfaces\nis inaccessible to conventional soft XAS, which provides chemical-state information but lacks\nstructural resolution; as well as to electron microscopy, which can image structures but requires\nseparate, often beam-intensive methods for chemical mapping. By integrating nanopatterned\nelectrodes as optical elements, PE-RSoXS boosts interfacial scattering intensity by orders of\nmagnitude, delivering simultaneous structural and chemical insights with sub-nanometer\nsensitivity, millisecond exposure, and low-dose measurements. This makes it a powerful and\nhighly efficient tool for operando studies of complex interfaces.\n(2) Diffraction-order-dependent separation of bulk and interfacial signals\nThe “sample-as-optics” nanopattern design enables Bragg diffraction enhancement and order-\nspecific sensitivity, allowing us to decouple interface-specific signals (even orders) from bulk\nresponses (odd orders) based on the 50% duty cycle structure of Ni LGNPs. This selective\nseparation is not possible in conventional soft XAS, where absorption spectra inherently mix\nbulk and interface contributions; nor in TEM, where buried interfaces are difficult to probe\noperando in liquid cells.\n(3) Buried interfacial sensitivity with statistical reliability under operando conditions\nPE-RSoXS averages over hundreds of identical nanopattern units in the beam irradiation field\nstatistically, ensuring robust sampling while operating at ultra-low X-ray doses (< 0.01 mJ·cm⁻2)\nand millisecond exposure time per shot that avoid beam-induced chemical changes. This allows\nlong-term operando monitoring of buried interfaces in realistic electrochemical environments,\nwhich is highly challenging for TEM due to beam damage and limited sampling coverage.\n(4) Quantitative extraction of sub-nanometer shell thickness under realistic\nelectrochemical conditions\nThe variations of the oxide-layer thickness derived from multi-order scattering spectra\nrepresent averaged and quantitative measurements of buried interfacial geometry under\noperando liquid-cell conditions. Conventional TEM can measure similar dimensions ex situ,\nbut operando measurements in liquid are constrained by beam damage, small sampling areas,\nand the need for invasive preparation.\nThus, while the Ni transition itself is known, PE-RSoXS uniquely demonstrates the ability to\nquantitatively resolve and correlate chemical-state gradients and sub-nanometer structural\ndynamics at buried interfaces in real time, under realistic electrochemical conditions, with\nstatistical reliability and minimal beam perturbation. Establishing these capabilities in a\ncontrolled model system is a necessary step toward applying the method to more complex and\nindustrially relevant electrocatalysts and represents the core methodological advance of this\nwork.\nMoreover, we also provide several reported examples here demonstrating that using a known\nand well-established system to validate a new methodology is a standard and widely accepted\nscientific strategy, particularly when the objective is to benchmark resolution, sensitivity, and\noperando capability. Collins et al. firstly demonstrated polarized soft X-ray scattering can probe\nmolecular orientation down to size scales of 10 nm using well-known polymers [Nat. Mater.\n11, 536–543 (2012)]; Feijóo et al. used CO electroreduction on Cu nanoparticles as a typical\n2\nplatform to show the capability of high-energy-resolution fluorescence-detected XAS [J. Am.\nChem. Soc. 145, 20208–20213 (2023)]; Chu et al. simply fabricated a stacked metal structure\nto present that multibeam scattering in grazing-incident reflection geometry is sensitive to 3D\nstructures [Nat. Commun. 14, 5795 (2023)].\n3. In line 149, The authors claim that the exposure time of PE-RSoXS can reach the\nmicrosecond scale; however, no dynamic processes matching this timescale were observed in\nthe current study. The authors are therefore encouraged to investigate processes whose\ndynamics better align with this temporal resolution, in order to obtain more compelling and\nmeaningful results.\nResponse: We thank the Reviewer for this valuable observation regarding the temporal\nresolution of PE-RSoXS. Indeed, while the current study achieved single-shot acquisition of\ndiffraction patterns with 1 ms of exposure time and demonstrated operando monitoring of\nelectrochemical solid-liquid interfaces, we did not probe processes occurring on the\nmicrosecond timescale. Our statement that PE-RSoXS can reach the microsecond regime refers\nto the intrinsic capability of the technique and instrumentation. Specifically, its compatibility\nwith high-brightness synchrotron sources, diffraction-limited storage rings, and ultrafast\ndetectors (e.g., hybrid pixel array detectors), which can operate at frame rates exceeding 1 MHz.\nIn such configurations, the amplification from nanopatterned structures enables sufficient\nsignal-to-noise ratios even at the extremely short exposure time, which largely enhances the\nefficiency to operando probe interfacial information.\nThe focus of the present work was on establishing and validating PE-RSoXS as a new interface-\nspecific operando X-ray methodology in a controlled model system, rather than to target the\nfastest possible dynamics. Ni LGNPs undergoing OER were chosen because their oxidation-\nstate transitions and sub-nanometer dimensional variations are well-established and occur on\ntimescales accessible in the millisecond range or slower ones, allowing us to benchmark\ndimensional and chemical sensitivity at the buried interfaces under realistic electrochemical\nconditions. This approach follows standard practice in methodology development, where\nknown systems are used to validate core capabilities before applying them to more complex or\nfaster processes.\nIn addition, the millisecond temporal resolution of PE-RSoXS is particularly valuable to avoid\nbeam-induced chemical changes. The coherent Bragg diffraction enhancement from\nnanopatterned electrodes enables sufficient scattering signal to be captured in single-shot\nexposures as short as 1 ms, minimizing the total X-ray dose to below 0.01 mJ·cm⁻2 per frame.\nThis ultralow dose is critical for avoiding beam-induced chemical or structural changes, which\nare well-documented in soft X-ray experiments on hydrated, organic, or nanomaterials [Wang\net al. J. Phys. Chem. B 113, 1869–1876 (2009); Kubin et al. Phys. Chem. Chem. Phys. 20,\n16817–16827 (2018)]. By acquiring high-quality diffraction and spectroscopic data before any\nsignificant radiation damage accumulates, PE-RSoXS preserves the native state of the solid-\nliquid interfaces throughout the measurements. This capability ensures that the observed\noxidation-state transitions and sub-nanometer structural changes reflect the real behaviors of\nthe electrocatalysts, rather than artifacts introduced by prolonged exposure. In this way,\nmillisecond-scale PE-RSoXS not only enhances temporal resolution but also ensures the\nintegrity of sensitive systems under realistic electrochemical conditions.\nWe agree that applying PE-RSoXS to phenomena whose dynamics match its full temporal\npotential would be compelling, such as electrochemical charging/discharging in microsecond\nregimes, ultrafast photoelectrochemical reactions triggered by pulsed laser excitation, etc. Such\nstudies will benefit from the proven low-dose, buried-interface sensitivity, and diffraction-order\nselectivity established here, while leveraging the technique’s compatibility with microsecond-\nscale acquisition. We view the current work as a foundational demonstration that paves the way\nfor these future ultrafast applications.\nRevision made: We added the following illustration into the revised SI as the Supplementary\nNote 7 to make a clear clarification of the millisecond exposure time of PE-RSoXS in a single\nshot.\n“In this work, the term “non-destructive” refers to the demonstrated ability of PE-RSoXS to\nacquire high-quality diffraction and spectroscopic data before any measurable X-ray radiation-\ninduced chemical or structural changes occur in the sample. The 1 ms of single-shot exposure\ntime used here was selected as an optimal trade-off between preserving the native state of the\nsolid-liquid interfaces and ensuring sufficient signal-to-noise ratio for reliable data analysis,\nrather than as the minimum achievable exposure. PE-RSoXS was implemented with such a\nshort single-shot exposure time (1 ms) to enable operando monitoring of electrochemical solid-\nliquid interfaces while minimizing radiation dose (< 0.01 mJ·cm⁻2 per frame) and preserving\nthe native state of the nanopatterns. Although the technique and instrumentation are\nintrinsically capable of reaching as low as microsecond timescales, the present study focused\non a well-characterized Ni LGNPs/OER model system with dynamics in the millisecond regime\nor slower ones to benchmark dimensional and chemical sensitivity at the buried interfaces under\nrealistic electrocatalytic conditions. This methodological validation provides the foundation for\nfuture studies targeting ultrafast processes, such as microsecond electrochemical switching or\nlaser-triggered photoelectrochemical reactions, where the full temporal potential of PE-RSoXS\ncan be exploited.”\n4. In line 254, the authors use simulations to reveal the potential-induced change in oxidation\nlayer thickness of the Ni-based catalyst. However, there is a lack of direct experimental\nevidence to support this claim. Since the oxidation of Ni during the OER is generally\nirreversible, changes in the oxide layer thickness should be observable by electron microscopy.\nTherefore, additional experimental evidence is needed to support the PE-RSoXS observations.\nResponse: We thank the Reviewer for highlighting the importance of direct “visible”\nexperimental evidence to support the potential-induced structural changes. To address this, we\nperformed post-mortem atomic force microscopy (AFM) measurements on the Ni LGNPs after\nthe electrochemical experiments, as shown in Figure R1.\nFigure R1. Post-mortem AFM images of the Ni LGNPs on the SiN windows of the sample\nx\nchip after electrocatalysis at a, OCP, b, 1.2 V and c, 1.6 V, showing the statistical line width\nand line edge roughness (LER) at each condition.\nThe AFM results show total width changes of the nanopatterns that closely follow the trend\nsimulated by our finite-element method (FEM) based on PE-RSoXS results (OCP: 113.8 nm;\n1.2 V: 113.4 nm; 1.6 V: 120.5 nm). However, in these AFM images, clear fluctuations are\nobserved along the line edges. These fluctuations can be quantified using the concept of line\nedge roughness (LER), as described by Nealey et al. [J. Appl. Cryst. 49, 823–834 (2016)].\nMeanwhile, ex situ microscopy techniques cannot directly measure the oxide layer thickness\nwith sub-nanometer resolution. High-resolution TEM could, in principle, provide this\ninformation, but it requires to prepare cross-sections of the line gratings, which may potentially\ndamage the sample. In contrast, PE-RSoXS enables operando, low-dose, and non-destructive\nmeasurements that quantitatively track oxide layer thickness and core-shell structural changes\nduring active electrocatalysis. By exploiting resonant scattering and diffraction-order\nsensitivity, PE-RSoXS can distinguish shell and core contributions without relying on post-\nmortem assumptions, providing statistically representative results across over hundreds of\nnanopatterns under realistic electrochemical conditions, which highlights the significance to\ndevelop PE-RSoXS.\nRevision made: We added Figure R1 into the revised SI as the Supplementary Fig. 8 to\nexhibit the Ni LGNPs after electrocatalysis and supplemented the following illustration into the\nrevised manuscript to compare our simulation results and AFM results of the structural\nvariations at each condition, highlighting the significance to develop PE-RSoXS.\n“Besides, our simulated total width changes of the Ni LGNPs align with atomic force\nmicroscopy (AFM) results from the samples after being operated at the corresponding\nconditions (Supplementary Fig. 8), further clarifying the fidelity of our FEM-based simulations.\nHowever, in these AFM images, clear fluctuations are observed along the line edges. And ex\nsitu microscopy techniques cannot directly measure the oxide layer thickness with sub-\nnanometer resolution. In contrast, by exploiting resonant scattering and diffraction-order\nsensitivity, PE-RSoXS can distinguish shell and core contributions without relying on post-\nmortem assumptions, providing statistically representative results across over hundreds of\nnanopatterns under realistic electrochemical conditions, which highlights the significance to\ndevelop PE-RSoXS.”\n5. The authors provide a detailed discussion of PE-RSoXS principles and technical merits in\nthe main text. While this information is valuable, moving it to the supplementary information\nwould improve the manuscript’s focus and clarity. The main text should instead provide a\nconcise and clear discussion that highlights the distinctive structural features revealed by PE-\nRSoXS and emphasizes the necessity of this technique for the study. This approach will attract\ngreater interest from readers.\nResponse: We appreciate the Reviewer’s suggestion to improve the manuscript’s focus. We\nfully agree that a concise and clear discussion of the distinctive structural features revealed by\nPE-RSoXS, and the necessity of this technique will help engage a broader readership. We\nmoved some PE-RSoXS principles and technical merits to the revised SI. Given that the\nprimary novelty of this work lies in the development of a new operando X-ray methodology,\nwe believe it is essential to retain concise core principles and working mechanism of PE-RSoXS\nin the main text. The “sample-as-optics” design, Bragg diffraction enhancement, and\ndiffraction-order-dependent separation of bulk and interfacial signals are integral to the\ninterpretation of the experimental results and to understanding why PE-RSoXS can achieve\nsub-nanometer structural sensitivity and millisecond temporal resolution under realistic\nelectrochemical conditions. We hope to retain these contents to ensure that readers, particularly\nthose outside the RSoXS community, can fully understand how the technique operates, what\nmakes it unique, and why it enables capabilities beyond existing methods. Meanwhile, we have\ncarefully revised the manuscript to accommodate the Reviewer’s suggestion for greater clarity\nand focus. In the Results and Discussion section, we have added a concise summary paragraph\nthat explicitly highlights the distinctive structural features revealed by PE-RSoXS and\nemphasizes the necessity of this technique for the study.\nRevision made: We have modified PE-RSoXS principles and the technical merits and moved\nsome contents to the revise SI as Supplementary Note 2. We also added the following\nillustration to the end of the section “Operando monitoring of electrochemical solid-liquid\ninterfaces by PE-RSoXS” in the revised manuscript.\n“These results unveil the distinctive capability of PE-RSoXS to operando resolve subtle\nstructural and chemical transformations at buried solid-liquid interfaces. The method\nquantitatively captures sub-nanometer dimensional changes in the Ni LGNPs during OER\nwhile simultaneously mapping oxidation-state gradients between the metallic Ni core and the\nevolving Ni(OH) /NiOOH shell through the FEM simulations on the diffraction-order-\n2\ndependent scattering spectra. This dual sensitivity to geometry and chemistry, achieved under\nultra-low X-ray doses with statistical averaging across hundreds of identical nanopattern units,\nenables a direct correlation between redox transitions and sub-nanometer morphological\nreconstruction. Such interface-specific insights, particularly the separation of bulk and\ninterfacial signals based on the core-shell structures, are essential for understanding\nelectrocatalytic behaviors, which is still difficult to be obtained with conventional soft XAS or\nelectron microscopy under realistic electrochemical conditions.”\n6. The PE-RSoXS measurements require highly specialized sample preparation to produce\nwell-ordered Ni line-grating nanopatterns. Although this approach is interesting, its broader\napplicability, particularly to conventional powder-form nanocatalysts, remains to be\ndemonstrated.\nResponse: We understand the Reviewer’s concern regarding the specialized sample preparation\nfor PE-RSoXS measurements. As we summarized in the beginning, RSoXS has been indeed\nsuccessfully applied to powder electrocatalysts in recent high-impact studies [Yang et al. J. Am.\nChem. Soc. 144, 8927–8931 (2022); Yang et al. Nature 614, 262–269 (2023); Yang et al. Nat.\nCatal. 8, 579–594 (2025)]. We emphasize that the current work uses nanopatterned electrodes\nas a deliberate model platform to establish and validate the PE-RSoXS methodology’s\nresolution limits, chemical sensitivity, and ability to decouple bulk and interfacial signals based\non the working principles of RSoXS, which are difficult to benchmark quantitatively in\ndisordered powder systems. The specialized nanopatterned electrodes in this study are not a\nlimitation, but a methodological enabler, providing the controlled geometry and optical\nmodulation necessary to validate PE-RSoXS. As the Reviewer mentioned, powders remain the\ndominant form for practical electrocatalysis. We expect that mechanistic insights derived from\nthe model systems are directly translatable to conventional powder electrocatalysts, thereby\nunderscoring the wider applicability of the technique; this perspective will be incorporated into\nthe Conclusion section.\nRevision made: We have added the following statement in the “Conclusion” of the revised\nmanuscript to emphasize the generality of PE-RSoXS and outline future applications to\nrestructuring catalysts beyond Ni-based catalysts.\n“The PE-RSoXS methodology is broadly applicable beyond the Ni-based system studied in this\nwork, providing operando access to coupled structural and chemical dynamics in a wide range\nof catalytic materials that undergo significant restructuring during operation. By tuning to the\nrelevant absorption edges of the focused elements, PE-RSoXS can selectively enhance\nscattering signals from specific chemical states, enabling real-time correlation between sub-\nnanometer structural changes and active-phase evolution. Its compatibility with identical\narchitectures including powder- and pattern-based systems, low-dose operation, and high\nsensitivity to the buried electrochemical interfaces make it a versatile tool for probing diverse\ncatalysts under realistic electrochemical conditions, with strong potential to address key\nquestions in catalysis, energy conversion, and materials science.”\nThanks again for the valuable comments and suggestions from the Reviewer. We have\ncarefully addressed each point to meet the high standards expected.\nReviewer: 2\nThe manuscript describes an implementation of the emerging RSoXS scattering method that\nexploits a Bragg structure sample to gain strong sensitivity to the composition within it. The\nnew implementation is coined “PE-RSoXS.” It is demonstrated in a Nickel electrochemical\noxidation experiment, and different optical constants are pulled out that describe different shell\nlayers obtained at different electrochemical potentials. The analysis framework features a full\nMaxwell simulation beyond typical Born approximation approaches.\nThe manuscript reflects important advances and should be considered for publication in Nature\nCommunications, provided the issues identified in review are addressed. Although this\nreviewer will provide extensive feedback for the author team (see below), the volume of\nfeedback does not reflect manuscript quality. There are issues in presentation and context, and\nsome technical issues that I will highlight in detail. Provided the editors agree, I look forward\nto continued consideration of this article for publication in this fine journal.\nResponse: We sincerely thank the Reviewer for the positive and encouraging assessment of\nour work and for recognizing the advances presented in the development and implementation\nof the PE-RSoXS methodology. We are pleased that the novelty of exploiting a Bragg-\nstructured sample for enhanced compositional sensitivity, the operando demonstration in Ni\nelectrochemical oxidation, and the application of a full Maxwell simulation framework beyond\nthe typical Born approximation were well received. We also greatly appreciate the Reviewer’s\nwillingness to provide extensive feedback and understand that the volume of comments reflects\na commitment to improving the clarity, context, and technical rigor of the manuscript. We have\ncarefully considered all points raised by the Reviewer and have made substantial point-by-point\nrevisions to address issues in presentation, contextualization, and technical details below. We\nbelieve that these changes have improved the manuscript’s readability, completeness, and\nimpact, and we look forward to the continued consideration of our article for publication in\nNature Communications.\n- Overarchingly, the centerpiece of the manuscript describes a technique that is usually called\n“diffraction anomalous fine structure,” or DAFS. This technique is actually very old. It has\nbeen described, in some form, since 1956 - Y. Cauchois (1956) — “Distribution spectrale\nobservée dans une région d’absorption propre de divers cristaux”, C. R. Acad. Sci. Paris 242,\n100–102. The idea was extended in work by other authors in the mid-70s, and re-emerged as a\nmethod named DAFS in the surface science era of the early-to-mid-90s, with a flagship\npublication led by Joe Woicik who is now at NIST: Stragier, H., Cross, J. O., Rehr, J. J.,\nSorensen, L. B., Bouldin, C. E., & Woicik, J. C. (1992). Diffraction anomalous fine structure:\nA new x-ray structural technique. Phys. Rev. Lett., 69, 3064–3067.\ndoi:10.1103/PhysRevLett.69.3064.\nNone of this rich history is acknowledged in the manuscript under review, nor are there any\nexplanations of how the current manuscript goes beyond. In this reviewer’s eyes, there are at\nleast two claims of novelty / advance for the manuscript under review: 1) there are very few\nimplementation of DAFS with soft X-rays on larger Bragg structures, and 2) the simulations in\nthe manuscript under review and the method of obtaining optical constants are extremely\nsophisticated compared to most previous DAFS examples, and particularly well-suited to soft\nX-rays given their high likelihood of multiple scattering. The authors should acknowledge the\nrich history of DAFS (a nice 2018 review is here https://arxiv.org/pdf/cond-mat/0309624 but I\ndon’t know if it was published outside arxiv), and then explain their advances.\nResponse: We thank the Reviewer for pointing out the historical context and foundational\nrelevance of Diffraction Anomalous Fine Structure (DAFS) to our work. We agree that DAFS\nhas a rich history, and we appreciate the opportunity to clarify how our operando PE-RSoXS\napproach relates to, extends, and differentiates itself from traditional DAFS.\nDAFS is a synchrotron-based X-ray technique that combines Bragg diffraction with energy-\ndependent anomalous scattering near an absorption edge to extract local structural and chemical\ninformation about specific species within a periodic structure. In essence, DAFS merges the\nelement-specificity of X-ray absorption spectroscopy (XAS) with the spatial selectivity of\ndiffraction, enabling the probing of atomic environments at crystallographically distinct sites.\nHistorically, this concept was first noted by Y. Cauchois [C. R. Acad. Sci. 242, 100–102 (1956)],\nwho observed spectral variations in diffraction intensities near absorption edges. The method\nwas extended in the 1970s and formalized in the early 1990s, notably in the seminal work by\nStragier et al. [ Phys. Rev. Lett. 69, 3064–3067 (1992)], which coined the term \"DAFS\" and\ndemonstrated its potential for probing local structure via diffraction intensity modulations.\nSince then, DAFS has been applied primarily in hard X-ray regimes to study bulk crystals,\nepitaxial films, and surface reconstructions, with relatively few implementations in the soft X-\nray range due to challenges such as strong absorption, multiple scattering, and limited\npenetration depth.\nOur PE-RSoXS shares conceptual similarities with DAFS in that both techniques exploit\nenergy-dependent diffraction intensity variations near an absorption edge to obtain chemical\nand structural information. Specifically, like DAFS, PE-RSoXS derives spectroscopic profiles\nfrom diffraction intensities across a photon energy range, allowing element-specific probing of\nlocal environments. Both approaches can separate contributions from distinct structural motifs\nbased on diffraction order or reciprocal space position. However, our work introduces several\nkey conceptual and methodological advances that go beyond traditional DAFS:\n(1) “Sample-as-Optics” Architecture with Engineered Nanopatterns\nTraditional DAFS relies on naturally occurring or epitaxially grown periodic structures (e.g.,\natomic planes in a crystal). In contrast, PE-RSoXS deliberately incorporates nanoengineered\nline-grating nanopatterns into the sample design, which act as diffractive optical elements in\nthe Fresnel (near-field) regime. This allows coherent amplification of weak resonant scattering\nsignals from buried interfaces, achieving N2 scaling in diffraction intensity and enabling\noperando measurements with sub-nanometer dimensional sensitivity.\n(2) Soft X-ray Implementation for Buried Solid-Liquid Interfaces\nWhile there have been few soft X-ray DAFS implementations, they typically target large Bragg\nstructures in vacuum or surface science contexts. PE-RSoXS operates at soft X-ray energies\n(e.g., Ni L-edge at around 852 eV) under operando electrochemical conditions in a liquid flow\ncell, overcoming challenges of strong absorption and multiple scattering by leveraging the\nlength-scale match between soft X-ray wavelengths, nanopattern periodicity, and interfacial\nstructures. This enables direct, non-destructive probing of buried solid-liquid interfaces during\nactive catalysis, which is a regime largely inaccessible to traditional DAFS.\n(3) Diffraction-Order-Dependent Chemical Sensitivity\nIn PE-RSoXS, diffraction order dependency is engineered via the duty cycle and symmetry of\nthe nanopatterns to selectively suppress or enhance certain orders. This allows us to decouple\ninterfacial signals from bulk contributions using 50% duty cycle patterned structures, where\neven-order diffraction intensities are highly sensitive to subtle interfacial inhomogeneities,\nwhile odd-order signals reflect bulk properties. This level of order-selective chemical\nsensitivity is not generally exploited in conventional DAFS.\n(4) Advanced FEM-Based Simulations Coupled to Bayesian Optimization\nOur work integrates finite-element Maxwell solvers to rigorously model multiple scattering and\nnear-field effects in sub-wavelength structures, which is a necessity for soft X-ray regimes\nwhere multiple scattering is significant. We extract energy-dependent optical constants for both\ncore and shell regions, enabling quantitative reconstruction of oxidation-state gradients and\nstructural changes. This simulation–experiment synergy is far more sophisticated than most\nprevious DAFS implementations, which often rely on simplified kinematic diffraction models.\nTherefore, while PE-RSoXS builds upon the foundational principle of DAFS, it still represents\na new class of interface-specific metrology.\nRevision made: We have acknowledged DAFS in the “Working principle of PE-RSoXS” of\nthe revised manuscript, citing the foundational works [Cauchois et al. C. R. Acad. Sci. Paris\n242, 100–102 (1956); Stragier et al. Phys. Rev. Lett. 69, 3064–3067 (1992); Renevier et al.\nSynchrotron Radiat. 10, 435-444 (2003)] and explicitly positioning PE-RSoXS as a conceptual\nand technical evolution of DAFS tailored for operando soft X-ray studies of buried\nelectrochemical interfaces in the Supplementary Note 2 (see below) of the revised SI.\n“While diffraction anomalous fine structure (DAFS) has been widely applied in hard\nX-ray regimes for bulk crystals and epitaxial films19, its implementation in soft X-rays\nhas been limited due to strong absorption and multiple scattering effects. Compared to\ntraditional DAFS, PE-RSoXS extends this principle by integrating engineered periodic\nnanopatterns directly into the sample design. Well-ordered line-grating nanopatterns\nwith uniform rectangular geometry were constructed to enhance scattering sensitivity.\nWhen the incident soft X-rays are aligned perpendicularly to the grating lines, the\nperiodic structures produce a series of diffraction patterns, as known as “Bragg spots”,\nrecorded on a charge-coupled device (CCD) detector.\nA unique advantage of PE-RSoXS is the natural alignment of length scales among the\nsoft X-ray wavelength, the engineered nanopatterns, and the interfacial phenomena of\ninterests20,21. As a proof of concept, at the Ni L-edge (around 852 eV), the soft X-ray\nwavelength is around 1.5 nm, which is comparable to the length scale of buried\ninterfacial structures as usually shown in reconstructions and chemical gradients at the\nsolid-liquid boundaries during electroctalysis22-24. Meanwhile, the precisely engineered\nnanopatterns with the width size in order of tens of nanometers, provide a periodic\noptical modulation that coherently amplifies weak scattering intensity changes of the\nline-gratings (form factors), especially from the buried electrochemical interfaces25,26.\nThis length-scale alignment is a distinctive advantage in optical and infrared methods,\nwhich conventionally lack sufficient dimensional sensitivity27,28. Besides, hard X-rays\nhave much shorter wavelengths compared to soft X-rays, and they would largely reduce\nthe energy resonance and dimensionally modulated contrast29,30. PE-RSoXS bridges\nthis gap and enables operando access to structural and chemical dynamics with\nnanometer to sub-nanometer resolution and element-specific sensitivity at solid-liquid\ninterfaces, respectively.”\n- The reviewers make multiple claims of interface sensitivity - the word is mentioned 45 times\nin the article! But there is nothing especially interface sensitive about RSoXS or PE-RSoXS.\nThe RSoXS technique is FORM FACTOR sensitive. When the form factor includes an interface,\nwe can of course be sensitive to it, and different placements of the interface within the form\nfactor (and within the structure factor it is convoluted with) will result in different levels of\nsensitivity.\nYes, in the implementation here, the authors demonstrate sensitivity to a shell in a core-shell\nstructure (this is the right language, not “interface sensitive”), but there will also be sensitivity\nto other parts of the structure that are not interfacial. The technique will be excruciatingly\nsensitive to the density and composition of the solid core, for example.\nThe reason this reviewer is so adamant about the language used here is that the “interface\nsensitive” claims are misleading and they echo misleading claims made about RSoXS about\n10-15 years ago that have (in this reviewer’s opinion) sown confusion and stunted the growth\nof the technique. Much early work on the RSoXS of organic electronics and organic\nphotovoltaics (OPVs) in particular made claims about special sensitivity to interfacial\ncomposition and orientation. The language used then (and now, in the article under review!)\nwas extremely similar to the language of nonlinear optics techniques such as sum frequency\ngeneration (SFG) (which was emerging and highly visible at the time), in which symmetry\nbreaking at interfaces truly affects spectroscopic selection rules, allowing spectra to be\nobserved that isolate signals from the interface. That is NOT what is happening in RSoXS –\nscattering sees everything, not just interfaces, and RSoXS is frequently bulk-dominated. But\nthis article strongly implies some kind of special sensitivity to interfaces with the 45 mentions\nof the word “interface” and in fact these authors use “symmetry breaking” language to explain\ntheir results in lines 127-128, when what is meant is that subtle perturbations in the form factor\ncan have strong effects on order expression. But these perturbations don’t necessarily have to\nbe at interfaces, and RSoXS is not sensitive to symmetry breaking specifically, just the form\nfactor. The authors should remove their misleading language about interface sensitivity and\nsymmetry breaking. If they would like to include it, they should include the context that the\ntechnique is sensitive to interfaces *when you know there is an interface, and you know where\nit is, and you can model it*. That is what is required. It’s also the same way the RSoXS is\nsensitive to aspects of structure other than interfaces.\nResponse: We appreciate the Reviewer’s thoughtful and technically grounded critique. We\nfully agree that RSoXS and PE-RSoXS are fundamentally form factor sensitive techniques.\nAny sensitivity to interfaces arises only when an interface is explicitly modeled as a distinct\nstructural region within the sample, and its boundaries are incorporated into the form factor.\nIn our study, the interfacial region is defined as the shell in a core-shell geometry. This shell\nlies between the metallic Ni core and the surrounding electrolyte, and it is within this shell that\nelectrochemical reactions take place. Structural and chemical variations in this shell region\naffect the form factor, which in turn modulates the scattering signal. We clarified this definition\nin the revised manuscript.\nWe also understand the Reviewer’s concern regarding the use of “interface sensitivity”, which\nhas been historically misapplied in the RSoXS literature. In response, we have carefully\nreviewed all instances of this language and revised them to more precisely describe the\nmechanism of contrast generation as form factor sensitivity, particularly in relation to well-\ndefined structural subregions such as shells. Additionally, we have removed or rephrased uses\nof the term “symmetry breaking” to avoid unintended associations with nonlinear optical\nselection rules. In our revised text, we now describe these effects as deviations from structural\nsymmetry in the form factor, which modulate the scattering intensity of diffraction peaks.\nWe believe these revisions clarify the physical interpretation of PE-RSoXS and align the\nterminology with established principles in the X-ray scattering community. We are grateful for\nthe Reviewer’s historical insight and constructive suggestions, which have improved the\naccuracy and rigor of our manuscript.\nRevision made: Throughout the revised manuscript, we have replaced broad “interface\nsensitivity” phrasing with the concept of “form-factor sensitivity to interfacial regions” or\nanalogues, making clear that this sensitivity is engineered via sample design and confirmed\nthrough modeling. We have also added the following statements to the revised manuscript and\nSI respectively to clarify that PE-RSoXS is not inherently interface-specific, but our design and\nmeasurement strategy make it exceptionally sensitive to buried electrochemical solid-liquid\ninterfaces in this study.\nIn the “Abstract”: “…PE-RSoXS opens new pathways to operando exploration of chemical\nevolution and sub-nanometer dimensional variations in nanostructured electrochemical systems.\nThis method is non-destructive, highly efficient, and element-specific, allowing for the\ndetection of chemical and dimensional dynamics at reaction fronts when appropriately modeled,\ndue to its sensitivity to form factors.”\nIn the “Working principle of PE-RSoXS” section: “In our implementation, the interfacial\nregion is defined as the shell layer in a Ni core-shell structure. This region evolves chemically\nand structurally during electrochemical OER, and its presence modifies the form factor, which\nis the basis for scattering contrast in PE-RSoXS.”\nIn the “Operando monitoring of electrochemical solid-liquid interfaces by PE-RSoXS”\nsection: “The evolution of the shell (interfacial) region is modeled as a change in form factor\ndue to the oxidation-state-dependent optical constants. The observed spectral changes are not\nexclusive to interfaces but rather reflect the local chemical environment within the shell region\ndefined in the form factor.”\nSupplementary Note 3 in the revised SI “We note that RSoXS, and by extension to PE-RSoXS,\nis fundamentally form-factor sensitive. The scattering intensity arises from the electron density\ndistribution within the scattering object, convoluted with its structural periodicity. The PE-\nRSoXS technique is not inherently interface-specific, nor does it operate under spectroscopic\nselection rules like those in nonlinear optics. In our implementation, we deliberately designed\nNi line-grating nanopatterns (Ni LGNPs) for PE-RSoXS measurements and they formed a core-\nshell geometry so that the buried electrochemical solid-liquid interfaces contribute strongly to\nthe form-factor contrast. Perturbations in this shell region under operating conditions, which\nwere confirmed through FEM-simulations, selectively influence certain diffraction orders,\nenabling us to isolate and operando monitor interfacial changes. This approach could be equally\napplied to emphasize bulk or non-interfacial regions, depending on the sample design and\nmodeling strategy.”\n- The above concern brings me to an adjacent point, which is that the authors have not\ndemonstrated the “spatial resolution” that they mention 19 times in the manuscript. What they\nhave demonstrated is fine sensitivity to shell thickness in a core-shell structure. Which, in this\nreviewer’s opinion, should certainly be enough for publication in Nat Comm if the authors are\nwilling to soften their claims. But “spatial resolution” implies that they can identify *where*\nthat material is within the structure. Instead, the authors have assumed that it is on the sidewall\nof the solid structure. That is, of course, a perfectly valid assumption given the electrochemical\nnature of the experiment, but it does not demonstrate spatial resolution.\nTo demonstrate spatial resolution, the authors could consider simulating what happens if the\nshell were “free floating” within the electrolyte-filled part of the Bragg structure, spaced by\nvarious distances from the wall. As unphysical as those models would be, that could potentially\ndemonstrate spatial resolution. I can assert from personal experience that scattering from Bragg\nstructures is typically far less sensitive to specific placement within the Bragg structure (“spatial\nresolution”) than it is sensitive to the specific volume fractions (shell thickness) and their\nindices (shell and core and matrix composition), but I would defer to simulation results from\nthe authors if they showed otherwise.\nResponse: We appreciate the Reviewer’s thoughtful feedback on the distinction between\nspatial resolution and sensitivity to shell thickness, and we understand the concern regarding\nthe strength of our claims. We followed the Reviewer’s suggestion and performed simulations\nto examine how scattering profiles change when the shell is artificially displaced within the\nelectrolyte-filled groove of the nanopattern, including unphysical but informative scenarios\nwhere the shell is not in direct contact with the core.\nTo investigate the influence of material placement, we employed a simplified model of a Ni\nline-grating with periodic boundary conditions applied laterally, as shown in Figure R2.\nThe simulation was performed using an incident photon energy of 852 eV. The grating cross-section\nis rectangular, and two smaller rectangular features matching the Ni grating in height, were\nadded to represent the oxide layer. The geometric dimensions of all components were kept\nconstant throughout the study. In configurations A and B, the oxide layers are symmetrically\npositioned at distances of 15 nm and 5 nm, respectively, from the Ni grating bulk. In\nconfigurations C and D, the oxide layers retain the same placement as configuration A, while\nthe Ni grating is shifted laterally by 5 nm to the right and left, respectively. For all\nconfigurations, the resulting diffraction peak intensities are shown in Figure R2b. The\npronounced differences in intensity across configurations demonstrate that the method is highly\nsensitive to the relative placement of the constituent materials.\nImportantly, PE-RSoXS operates on principles analogous to protein crystallography, where\ndiffraction from periodic units enables reconstruction of real-space features with sub-nanometer\nto nanometer-scale precision. In our case, the periodic nanopatterns define a structural unit cell,\nand the spatial distribution of chemical species within that unit cell modulates the scattering\nform factor in a position-sensitive manner. This coherence-based sensitivity to the distribution\nand geometry of resonant materials enables spatial resolution at sub-nanometer length scales,\nas supported by both the experimental data and the FEM modeling.\nFigure R2. a, Four configurations (A to D) of a Ni metal core with oxidized Ni shells at\ndifferent placements for diffraction intensity simulation. b, The simulated diffraction peak\nintensities of the configurations A to D at 852 eV.\nRevision made: We have revised the manuscript to clarify this conceptual framework and have\nsoftened language around “spatial resolution” where appropriate. The updated text also\nemphasizes that the demonstrated resolution is possible because the location of the shell is well-\ndefined and constrained by the model geometry, and that such spatial information is resolvable\nthrough order-dependent scattering when supported by rigorous simulations. We added Figure\nR2 into the revised SI as the Supplementary Fig. 13. In the revised manuscript, we added the\nfollowing illustrations in the section “Theoretical FEM-based Simulations of PE-RSoXS” of\nthe revised manuscript.\n“To address the concept of spatial resolution more rigorously, we conducted additional\nsimulations (Supplementary Fig. 13). The oxidized Ni shell region was displaced with distances\nfrom Ni metal core without physical contact. These simulations reveal that the diffraction signal\nis not only sensitive to shell thickness and composition, but also to the position of the shell\nwithin the grating period, confirming that PE-RSoXS can resolve spatial placement within the\nstructural unit cell. By simulating unphysical scenarios in which the shell is “floating” at\nvarying positions within the groove, we demonstrate that the diffraction intensities change\ndepending on shell placement, indicating the system’s sensitivity to spatial localization of\nchemical features.\nAs a result, our simulations demonstrate the characteristic features of PE-RSoXS in unveiling\ninterfaces: (i) enhanced scattering signal from buried interfaces by statistically identical\nnanopatterns (form factors) and decoupled the interfacial signals from the bulk material\nresponses; (ii) high scattering sensitivity to sub-nanometer structural variations; (iii) site-\nspecific sensitivity of elemental oxidation state by order dependency; (iv) achievable spatial\nresolution within the unit cell when the geometry is well-defined.”\n- There is a significant missed opportunity to structure the narrative here with deeper\nfundamentals in the presentation of the “scattering derived spectra.” These appear to be [I\nintegrated across a peak] vs. energy, presented in Fig 2 d-e-f. On lines 190-191 these spectra\nare directly compared to X-ray absorption spectra data, and described as “comparable.” But it\nis incorrect to compare them because [I integrated across a peak] is the result of *differences between indices* of the volumes within the scattering structure, and it includes the delta parts\nof those indices. So, there is dispersion baked into that “spectra”, and even if you could isolate\nbeta, it would be a difference spectrum between nickel beta and nickel oxide beta (electrolyte\nbeta would be reasonably flat, but sloped). The authors should remove this comparison to XAS,\nand more properly acknowledge the contributions to I vs. energy.\nBut there is an even stronger way to do that, and that is to lean into the concept of the total\nscattering invariant (TSI). The TSI describes the integrated scattering across all q and in all 3\ndimensions; for RSoXS patterns an approximation frequently called “integrated scattering\nintensity” (ISI) is used by integrating I*q^2 (sometimes q-bounded or feature-bounded, and\nsometimes for all measurable q) vs energy. The authors could consider presenting Fig 2 d-e-f\nas feature-bounded ISI vs. energy. The reason this would be valuable is that, in principle, TSI /\nISI should be directly proportional to the sum of the combinatorial pairwise binary contrasts\n(Delta(beta)**2 + Delta(delta)**2) weighted by their volume fraction products (see Collins and\nGann 2021 Eq 8 and Eq 15; it can be extended to any number of components). Quantitative\nagreement of the energy-dependent ISI vs. energy and expectation contrast was robustly\ndemonstrated in Ferron, Pope, and Collins PRL 2017, and it underpins the physics of RSoXS\nand of the manuscript under review. The Maxwell solver will reproduce this physics but the\nauthors should more robustly acknowledge it, as they deal with I vs. energy much earlier than\nthey introduce n, delta, beta, etc, when they arise from a common framework.\nResponse: We appreciate the Reviewer’s insightful and technically rigorous comment. We\nagree that scattering-derived intensity profiles (Fig. 2d–f) are fundamentally different from\nconventional X-ray absorption spectra (XAS), and we acknowledge that describing them as\n“comparable” may be misleading without proper qualification. We emphasize that the\nscattering-derived spectra represent energy-dependent diffraction intensities that emerge from\nspatially modulated optical contrast within the periodic nanostructure. These spectra reflect the\ncombined influence of the dispersion (δ) and absorption (β) components of the complex\nrefractive index (n = 1 – δ + iβ) and cannot be interpreted as direct analogs of XAS. We have\nremoved the word “comparable” and clarified the distinction in the revised manuscript.\nWe also recognize the reviewer’s suggestion to frame the analysis more fundamentally in terms\nof the total scattering invariant (TSI) or feature-bounded integrated scattering intensity (ISI).\nWhile ISI-based analysis is highly effective in binary systems or where phase separation is well\ndefined, our system involves a graded and non-binary core-shell geometry with subtle\noxidation-state-dependent contrasts. In such a configuration, ISI collapses the energy\ndependence of scattering into a single scalar value per energy and obscures the spatial\ndistribution of contrast. Therefore, we think ISI is not ideally suited for our operando analysis.\nInstead, our approach leverages a forward-modeling framework based on FEM, where the\nenergy-dependent diffraction intensities across multiple diffraction orders are simultaneously\nfit to extract the underlying optical constants (δ and β) and geometry of the core-shell structures.\nNotably, the β component extracted via simulation (see Fig. 3d) is directly related to the\nabsorption component of the optical response and does qualitatively reproduce Ni L-edge\nfeatures, consistent with XAS reference data. This connection is established not through a scalar\nISI, but via spatially resolved modeling that reproduces the order-dependent scattering profiles\nas a function of energy.\nRevision made: We have changed the Y-axis title of all scattering profiles to “Diffraction\nIntensity” in the revised manuscript and SI and removed the previous comparison to XAS.\nWe also have clarified that these are scattering-derived signatures and added the following\ndemonstrations into the revised manuscript. We also acknowledged the foundational theoretical\nframework that underpins this physics with new references in the revised manuscript.\nPage 5 in the section “Operando monitoring of electrochemical solid-liquid interfaces by\nPE-RSoXS” of the revised manuscript:\n“...producing energy-dependent scattering profiles. While these spectra resemble XAS in that\nthey exhibit Ni L-edge features, they are not directly equivalent. The measured diffraction\nintensities result from spatially modulated differences in the complex refractive index (n = 1 –\nδ + iβ) and reflect both absorption (β) and dispersion (δ) contributions.”\nPage 6 in the section “Operando monitoring of electrochemical solid-liquid interfaces by\nPE-RSoXS” of the revised manuscript:\n“For β(E), which corresponds to the absorptive component of the optical response, the\nsimulation captures key features of Ni L-edge absorption in the shell region. These features are\nnot direct XAS measurements but rather arise from the fitted optical constants that best\nreproduce the energy-dependent scattering intensity across all diffraction orders.”\nSupplementary Note 6 in the revised SI:\n“We also considered the possibility of presenting integrated scattering intensity (ISI) curves to\nreflect the total contrast within the system. However, because our system features a graded and\nnon-binary core-shell geometry with subtle variations in oxidation state, ISI-based methods\n(which typically integrate I·q2 across all or bounded q) are not ideally suited for resolving\nlocalized compositional changes. Instead, we adopted a FEM-modeling approach, in which the\nenergy-dependent diffraction intensities across multiple orders are simultaneously fit to extract\nthe geometry and optical constants of the shell. This approach enables location-specific\nreconstruction of β(E) (the absorption component), which closely reflects the X-ray absorption\nbehavior of the oxidized Ni species. This framework is consistent with the physical basis\nreported previously4,32.”\nThe authors’ mentions of “numbers of scatterers N” on lines 155-156 and the sensitivity to N\nsquared are a confusing and less general recapitulation of the physics I mention above. They\nare misleading in stating that “conventional scattering” has linear sensitivity to the number of\nscatterers. All SAS has Delta(rho)**2 magnitude; the number of scattering objects increases\nthe scattering intensity linearly in the dilute form factor limit, whereas in a Bragg structure you\nhave a *really* strong reporter of Delta(rho)**2. Perhaps that’s what the authors intended to\nsay but the concepts are muddled here.\nResponse: We thank the Reviewer for pointing out that our current discussion of “number of\nscatterers N” and N2 sensitivity is insufficiently general and could be misleading in its\ncomparison to “conventional scattering.” We recognize that the comment may stem from a\nconflation of two different physical concepts:\n(1) the well-known contrast dependence of scattering intensity via 2 or n2, which governs\nthe magnitude of the scattering signal, and (2) the coherent enhancement due to periodicity,\nwhich leads to an intensity scaling of N2, and the number of periodic units contributes\nconstructively to phase.\nOur reference to “N squared” sensitivity was not meant to describe the typical contrast\ndependence in small-angle scattering, but rather to highlight a fundamental principle of pattern\nenhancement (Bragg Diffraction) in coherent scattering systems. Specifically, in PE-RSoXS,\nthe nanopatterned structure acts as a diffraction grating where each identical unit contributes a\nscattering amplitude A, and the total amplitude scales as N · A, when the scattered waves are\ncoherently added in phase. As a result, the intensity scales as I  [N · A]2 = N2 ·|A|2\nRevision made: We have added the following demonstration into the revised SI as the\nSupplementary Note 5.\n“In our patterned system, the coherent scattering amplitude from each identical unit adds in\nphase, resulting in a total amplitude that scales as N · A, where N is the number of the identical\nunits. Consequently, the scattered intensity scales as N2, in contrast to the linear scaling\ntypically observed in systems with randomly distributed or dilute scatterers. This coherent\nenhancement is distinct from the contrast-driven scattering magnitude (2 or n2) discussed\nin small-angle scattering and is central to the concept of PE-RSoXS introduced in this work. In\nour experiment, with approximately 400 identical Ni line-grating units, it leads to a five-order-\nof-magnitude enhancement in scattering signal, enabling ultrahigh sensitivity to interfacial\nchanges.”\n- The authors should moderate their language and results that suggest spatial resolution beyond\nthe diffraction limit, and they need to be clear about their interaction with this limit. Spatial\nresolution beyond the diffraction limit is implied by their language: “atomic scale resolution”\nis used line 73. But the wavelength here would be ~1.5 nm, with a diffraction limit of 0.75\nnm. The PE-RSoXS technique – like every other SAS technique that exists - should not be capable\nof real spatial resolution beyond that diffraction limit. But, the authors do demonstrate major\ndifferences in the I vs. energy simulation for 0.2 nm increments in shell thickness. This does\nNOT demonstrate 0.2 nm spatial resolution, it goes back to my earlier point that we are\nexquisitely sensitive to volume fraction of the shell (or an alternative way of thinking is the\neffective medium index within diffraction limit volumes), and less sensitive to where the shell\nis placed within the structure (what we would call “spatial resolution”). If we assume or have\nrobust proof that the shell must be attached to the sidewall, then we have an ~ersatz sub-\ndiffraction-limit spatial resolution, but the assumption about the location is doing all the heavy\nlifting in that result. I strongly recommend the authors reconsider their language and directly\naddress their interaction with the diffraction limit lest they mislead readers.\nResponse: We thank the Reviewer for raising this important point. We fully agree that PE-\nRSoXS, like all wave-based scattering techniques, is subject to the diffraction limit, which in\nour case is defined by the ~1.5 nm wavelength of the soft X-rays at the Ni L-edge,\ncorresponding to a theoretical resolution limit of ~0.75 nm. Our previous use of the phrase\n“atomic-scale resolution” might have implied sub-diffraction imaging capability, which was\nnot our intention.\nWe have revised this language throughout the manuscript to refer instead to “sub-nanometer\nstructural sensitivity”, which more accurately reflects the nature of the technique. Specifically,\nthe sensitivity to small variations in shell thickness (e.g., 0.2 nm increments from our FEM\nsimulations) arises not from direct spatial resolution, but from the impact these variations have\non the energy-dependent scattering form factor, as coherently probed across multiple diffraction\norders. These changes are detectable within the diffraction-limited spatial scale, and they\nmanifest through their influence on the diffraction intensity, not through direct imaging of\nfeatures below half of the wavelength of the incident X-ray.\nWe also appreciate the opportunity to clarify that this approach is fundamentally model-based,\nin line with the principles of protein crystallography, where structural features are inferred by\nanalyzing diffraction from periodic units. In both techniques, resolution is ultimately limited\nby the wavelength, but precise information about sub-nanometer features can still be extracted\nthrough coherent interference and accurate modeling of the internal structures within the unit\ncell. We have now made this analogy explicit in the revised manuscript and clarified our\ninteraction with the diffraction limit.\nRevision made: In the revised manuscript and SI, we have changed the terminology of “atomic\nscales” to “sub-nanometer” and made the following revisions.\nIn the “Introduction” section: “…detection of electrochemical interfaces under operando\nconditions at the sub-nanometer scales in the soft X-ray diffraction limit.”\nIn the “Operando monitoring of electrochemical solid-liquid interfaces by PE-RSoXS”\nsection of the revised manuscript: “The spatial resolution of PE-RSoXS is fundamentally\nlimited by the X-ray wavelength (~1.5 nm), resulting in a diffraction-limited resolution of\napproximately 0.75 nm. Our ability to detect smaller structural changes (e.g., 0.2 nm in shell\nthickness) arises not from direct imaging beyond this limit, but from the sensitivity of\ndiffraction intensities to such variations within the periodic structure.”\n- Aside from my points above, the introduction is otherwise very accessible and explains some\nkey jargon pieces such as the meaning of “near field.” However, the description of the modeling\n– a centerpiece of the manuscript – is extremely confusing. Some questions that came up during\nmy reading:\n. what is the “energy offset”? I’m guessing it’s to accommodate differences between the energy\nof the beamline at RSoXS measurement and the energies of the NEXAFS-derived library index\nused for the core. But this is never explicitly stated in the body or methods.\n. was the shell thickness floated in the fit? This is never explicitly stated.\n. I’m quite surprised at how nicely smooth the delta and beta DAFS extractions are. Did the\nauthors use tricks to keep them smooth – like a thickness constraint or the use of the previous\nenergy’s index as an initial guess for the next? These kinds of fittings are prone to local minima.\nResponse: We thank the Reviewer for these detailed questions regarding our modeling\napproach, which is indeed a centerpiece of the manuscript. We agree that the current description\nin the Methods section is insufficiently explicit on some points, and we have revised the text to\nmake our fitting procedure and assumptions fully clear.\n1. Energy Offset\nThe “energy offset” refers to a small calibration difference between the beamline energy scale\nused in PE-RSoXS experiments and the NEXAFS-derived reference data used to calculate the\noptical constants (δ and β) for the core materials. This offset accounts for slight misalignments\nin absolute photon energy (~0.2–0.3 eV) due to instrument calibration or beamline drift. The\noffset is applied during the fitting of the core’s optical constants to align features in β(E) with\nthose observed experimentally.\n2. Shell Thickness Floating in the Fit\nYes, the shell thickness was treated as a floating parameter in the Bayesian optimization used\nfor the FEM-based simulation of the experimental scattering-derived spectra. It was\nindependently optimized for each applied potential (OCP, 1.2 V, and 1.6 V), alongside other\nparameters such as the shell’s optical constants and the roughness factor.\n3. How are δ(E) and β(E) so smooth? Were constraints used?\nWe agree that fitting energy-dependent optical constants over many energies can be prone to\nlocal minima. To ensure stable and physically meaningful results, we used a multi-Gaussian\nparameterization of β(E), and derived δ(E) via Kramers–Kronig (KK) transformation.\nAdditionally, the fit was performed sequentially across photon energies using the previous\nenergy’s result as the initial guess to improve convergence and smoothness. This approach\navoids unphysical oscillations and helps guide the optimizer through a continuous solution\nspace.\nRevision made: In the revised manuscript, we have added the following descriptions to the\n“Methods” section of the revised manuscript.\nEnergy offset: “The energy offset accounts for small calibration differences between the\nincident photon energy at the RSoXS beamline and the reference NEXAFS data used to\ncompute the optical constants. This typically ranged between -0.2 and -0.3 eV and was used to\nalign characteristic absorption features of the core.”\nShell thickness floating in the fitting: “The shell thickness was treated as a floating parameter\nand optimized independently for each potential condition using Bayesian optimization. It was\nvaried along with the shell optical constants, Debye–Waller roughness, and core energy offset.”\n“To improve smoothness and convergence, the fitting process was seeded using the optimized\nsolution at the previous energy step as the initial guess for the next energy point. The optical\nconstants β(E) were parameterized using a sum of Gaussian functions, and δ(E) was calculated\nvia Kramers–Kronig relations, ensuring that both components followed physically meaningful\ntrends. This regularization mitigated local minima and produced smooth, interpretable spectral\nfeatures.”\n- The authors should acknowledge some limitations around model uniqueness. Importantly, the\nsimulations are done on structures that perfect, and the real structures are not. An imperfection\nthat strongly affects order expression (the principal thing that is fit here) is interfacial width /\nroughness, which is strongly convoluted with sidewall angle in q_xy scattering. The authors\nassume that the electrochemically converted material and its two interfaces are sub-atomically\nsmooth. Any line-edge roughness or shell thickness inhomogeneity (and there is usually a\nsignificant amount with solid-state electrochemistry) in the core or shell will diminish higher\npeak orders relative to lower ones and potentially could be misfit. I’m not suggesting the authors\nneed more parameters for their model, but they should acknowledge this limitation and discuss\nits potential effects on the results.\nResponse: We thank the Reviewer for this important observation. We fully agree that real\nnanostructures inevitably contain imperfections, including sidewall angle variability, line-edge\nroughness, shell thickness inhomogeneity, etc., which are not captured in our idealized FEM\nmodels. Our modeling framework is intentionally simplified to balance interpretability,\ncomputational feasibility, and the risk of overfitting. However, we recognize that model\nuniqueness is inherently limited by such structural assumptions, and that interfacial roughness\nor inhomogeneity could affect the expression of higher diffraction orders, potentially leading\nto biased extraction of shell thickness or optical constants\nWe included a Debye–Waller-type roughness factor to account for some degree of order\nsuppression, but we acknowledge that this is a simplified correction that cannot fully represent\ncomplex morphological variability. We have now explicitly stated this limitation in the\nMethods section and added a discussion of its possible effects in the Discussion portion of the\nrevised manuscript. We have also clarified that while our model provides a consistent and\nphysically interpretable fit to the experimental data across multiple diffraction orders, the\nabsolute values of extracted parameters should be interpreted within the context of these\nmodeling assumptions.\nRevision made: In the “Operando monitoring of electrochemical solid-liquid interfaces by\nPE-RSoXS” section of the revised manuscript, we added “While our FEM-based simulations\nprovide strong agreement with experimental data across multiple diffraction orders, the model\nassumes idealized core-shell structures with sharp, uniform interfaces. Interfacial roughness,\nline-edge disorder, and thickness inhomogeneity, especially common in solid-state\nelectrochemical systems, may reduce the intensity of higher-order diffraction peaks and\ninfluence parameter extraction. These effects are partially accounted for using a Debye–Waller\nfactor (see Methods), but residual mismatch or misfit could still arise from structural\nimperfections not explicitly modeled (Supplementary Fig. 7 and Supplementary Note 8).”\nIn the section of “Methods”, we added “Roughness was incorporated using a Debye–Waller\nfactor correction to model the attenuation of scattering intensity at higher diffraction orders due\nto structural disorder. However, this simplified approach does not fully account for the realistic\neffects such as lateral inhomogeneity, line-edge roughness, or non-uniform shell thickness, etc.\nThese imperfections could lead to suppression of higher-order peaks or fitting ambiguity and\nrepresent a source of model non-uniqueness.”\n- The authors perhaps overemphasize odd/even effects; these are not universal in Bragg\nstructures and are only strongly relevant near the 50% duty cycle limit. I bring this up because\nit oversimplifies the physics - different fill ratios will produce different harmonic effects on\norder expression: 33% / 66 % will suppress every 3rd order, etc. I leave it to the authors to\nconsider this point.\nResponse: We appreciate the Reviewer’s clarification regarding the harmonic suppression\nconditions in periodic Bragg structures and agree that the even/odd diffraction order behavior\nis not universal. As correctly noted, harmonic suppression patterns depend on the duty cycle\n(fill factor) of the grating. For instance, a 50% duty cycle suppresses even orders under ideal\nsymmetry, while 33% or 66% can suppress every third order, etc. We have now revised the\nmanuscript to clarify that these effects arise from Fourier components of the line profile, which\nvary with fill ratio, rather than being inherently tied to “odd” or “even” orders.\nImportantly, we also note that in our actual electrochemical system, achieving an exact 50%\nduty cycle is challenging, due to electrochemical volume changes and oxidation-driven\nrestructuring. In practice, deviations from ideal symmetry likely contribute to the presence of\neven-order peaks, and we use this sensitivity to infer interfacial changes. Therefore, while our\nnominal structure is designed with 50% fill factor, we think it is more accurate to frame the\nobserved diffraction features in terms of form factor perturbation from ideal periodicity, not\nstrict even/odd logic. We have revised the relevant text to reflect this more nuanced view.\nRevision made: In the revised SI, we added the following illustrations as the Supplementary\nNote 4.\n“The pronounced odd/even diffraction order contrast in this study arises from the deliberate\nfabrication of Ni line-grating nanopatterns with a 50% duty cycle. In an ideal rectangular profile\nwith sharp edges, this geometry suppresses even orders in the Fourier series representation of\nthe form factor. Subtle structural or compositional perturbations along the line-width direction\nintroduce deviations from ideal structural symmetry in the form factor, leading to a pronounced\nenhancement of the even-order diffraction signals, while odd orders remain dominated by bulk\ncore contributions31. This makes PE-RSoXS highly responsive to structural variations within\ndefined subregions, such as interfacial shells in core-shell architectures. We note that this\nsuppression pattern is specific to the 50% duty cycle case; other fill ratios produce different\nharmonic effects (e.g., 33% duty cycle structure suppresses every third order), and\nimperfections such as sidewall angle variation or interfacial roughness can alter the ideal\nsuppression behavior. Our emphasis on odd/even effects here reflects a deliberate design choice\nto enhance sensitivity to buried interfacial changes, rather than a universal property of Bragg\nstructures.”\nWe truly thank the Reviewer for the insightful comments and kind suggestions! The reply\nfor each question/comment is expected to reach the high criteria.\nReviewer: 3\nThis manuscript reports the use of pattern-enhanced resonant soft X-ray scattering (PE-RSoXS)\nas a new probe of chemical and structural dynamics at electrochemical solid-liquid interfaces.\nThe methodology development is technically impressive. I recommend publication after\naddressing the following concerns:\nResponse: We sincerely appreciate the Reviewer’s positive assessment of our work and\nrecognition of the technical significance to develop PE-RSoXS as a new operando X-ray\nmethodology for chemical and structural dynamics at electrochemical solid-liquid interfaces.\nWe have carefully addressed point-by-point on each of the raised concerns, providing\nclarifications, additional explanations and revisions to the manuscript. We believe these\nchanges improve the clarity, completeness, and impact of this work, and we hope the revised\nversion meets the Reviewer’s expectations.\n1. “In situ”, instead of “operando” is more accurate to describe the present methodology.\nResponse: We thank the Reviewer for raising this important point regarding the terminology\nused to describe our PE-RSoXS methodology. After careful consideration, we have decided to\nretain the term “operando” in the revised manuscript, as we believe it more accurately reflects\nthe nature of our performed experiments. We acknowledge that “in situ” and “operando” are\nclosely related but distinct descriptors, and we agree that clear definitions are important for\nreaders.\nIn the context of X-ray spectroscopic characterizations, “in situ” generally refers to\nmeasurements performed under relevant environmental or reaction conditions, but not\nnecessarily during the active operation of the functional process of interests. In contrast,\n“operando” indicates that the measurement is performed while the system is actively\nfunctioning in its intended operational state, allowing direct correlation between\nstructural/chemical changes and performance metrics. This distinction has been discussed in\nthe literature [Gurlo et al. Angew. Chem. Int. Ed. 46, 3826-3848 (2007); Weckhuysen, B. M.\nIn-Situ Spectroscopy of Catalysts, American Scientific Publishers (2004)].\nIn our study, PE-RSoXS measurements were conducted under fully functional electrochemical\nconditions, where Ni line-grating nanopattern (LGNP) electrodes were actively performing\nelectrocatalysis in aqueous electrolyte under applied potential, and the RSoXS patterns were\nacquired simultaneously during electrochemical measurements. This configuration allowed us\nto directly correlate oxidation-state transitions and sub-nanometer structural variations with the\nelectrocatalytic OER during the realistic conditions. Based on this reason, we believe it’s more\naccurate to use the term “operando” rather than “in situ” as the methodology was applied under\ntrue working conditions with concurrent interface monitoring, rather than simply under static\nor simulated reaction environments.\nWe hope this explanation resolves the Reviewer’s concern and makes our use of the term\n“operando” precise and justified.\nRevision made: We have further refined the “Operando PE-RSoXS measurements” section\nin the Methods as shown below to more clearly emphasize our operando PE-RSoXS method in\nthe revised manuscript.\n“A customized electrochemical liquid flow cell constructed by Protochips, Inc. was developed\nat the beamline 11.0.1.2 of the Advanced Light Source in Lawrence Berkeley National\nLaboratory62. RSoXS patterns were collected with a back-illuminated Princeton PI-MTE\ncharge coupled device (CCD) cooled to -45 °C. Each RSoXS pattern at one energy point was\nacquired in a single 1 ms exposure time of soft X-rays using a 1/1000 of the incident photon\nflux (X-ray dosage per shot: below 0.01 mJ·cm2). This ultralow-dose configuration enables\nhighly sensitive detection of scattering signal changes of the line-gratings while preserving the\nnative state of the electrochemical interfaces. An E-chip from Protochips, Inc. with Ni LGNPs\nwas paired with a small supporting chip with a 500-nm spacer for the encapsulation of 0.1 M\nNaOH aqueous electrolyte. Then, the same electrolyte was flowed continuously facilitated by\na Harvard Apparatus PHD ULTRA Programmable Syringe Pump with a rate of 250 µL·h-1,\nminimizing the bubble retention effect. Electrochemical measurements were performed with a\nGamry potentiostat in a 3-electrode floating mode configuration. Prior to constant-potential PE-\nRSoXS measurements at OCP, 1.2 V, and 1.6 V vs. RHE, the Ni LGNP working electrodes\nunderwent a standardized conditioning sequence to stabilize the electrochemical interfaces.\nFirstly, as-fabricated Ni LGNPs were characterized by RSoXS in the absence of electrolyte to\nestablish baseline RSoXS patterns. Next, several cyclic voltammetry (CV) scans were\nperformed over a potential range from OCP to 1.2 V vs. RHE, covering the Ni\noxidation/reduction region, to remove surface impurities, activate the nanopatterns, and\nestablish a stable electrochemical response. This typical method ensures reproducible\nelectrocatalytic activity and RSoXS patterns by removing adventitious surface species and\nstabilizing the core-shell structure prior to operando measurements. Following conditioning,\nscattering profiles at the Ni L-edge were collected operando under constant-potential control at\nOCP, 1.2 V, and 1.6 V vs. RHE. The topographic imaging of the nanopatterns after operating\nat the three potentials was captured using atomic force microscopy (AFM), which were\nconducted using the Bruker Dimension Icon3 in ScanAsyst mode at the Molecular Foundry of\nLawrence Berkeley National Laboratory.”\n2. Atomic-scale resolution might be overstated while the results mainly reach sub-nanometer\nsensitivity.\nResponse: We thank the Reviewer for pointing out the importance of accurately describing the\ndimensional sensitivity achieved in this work. We agree that the term “atomic-scale resolution”\ncould be interpreted as implying direct imaging or detection of individual atomic positions,\nwhich is not the case here. To ensure precise and accurate terminology, we have revised the\nmanuscript to replace “atomic-scale” terminology with “sub-nanometer” throughout the revised\nmanuscript.\nRevision made: We replaced “atomic-scale”-related phrases to “sub-nanometer” throughout\nthe revised manuscript.\n3. A comparison table summarizing state-of-the-art interface characterization techniques (e.g.,\nTEM, XAS, AP-XPS, fluorescence microscopy, RSoXS) would greatly strengthen the\nintroduction and highlight the advantages of PE-RSoXS.\nResponse: We thank the Reviewer for this constructive suggestion. We provide a\ncomprehensive comparison table here (Table R1) summarizing the key capabilities, limitations,\nand application scopes of state-of-the-art interface characterization techniques, including TEM,\nXAS, ambient-pressure X-ray photoelectron spectroscopy (APXPS), and X-ray fluorescence\nmicroscopy (XFM), and PE-RSoXS. This addition clearly contextualizes PE-RSoXS within the\nbroader landscape of interface characterization methods and highlights its unique advantages\nfor operando studies under realistic electrochemical conditions.\nTable R1. Comparison of Characterization Techniques for Electrochemical Interfaces.\nTechnique PE-RSoXS TEM XAS APXPS XFM\nDimensional Sub- Atomic- Tens to Few Tens to\nSensitivity nanometer scale (< 0.1 hundreds of nanometers hundreds of\n(via nm) nanometers sampling nanometers\ndiffraction- imaging (limited by depths in (microprobe);\norder possible beam spot solids; lateral < 10 nm with\nsensitivity size and resolution: ptychography\nand near-field detection nanometers\noptical geometry) to\nmodulation) micrometers\n(limited by\nbeam spot\nsize and\ndetection\ngeometry)\nTemporal Millisecond Millisecond Seconds to Seconds to Seconds to\nResolution scale (single- to minutes minutes per minutes; minutes per\nshot (with fast spectrum milliseconds scan\ndiffraction cameras) (energy possible with\npatterns; scans) fast detectors\ncompatible\nwith ultrafast\ndetectors)\nChemical High (tunable Limited High High High\nSensitivity (tunable to (requires (direct (chemical (Element-\nspecific EELS or absorption specificity; specific via\nabsorption EDS) edge oxidation fluorescence\nedges; tuning) states and yield)\ndiffraction- bonding)\norder\nselectivity\nisolates\ninterfacial\nspecies)\nInterface High Limited Moderate Moderate High (Bulk\nAccessibility (coherent (requires (surface- (Surface and and buried\namplification sample sensitive sub-surface interfaces\nof weak thinning or but buried accessible; accessible)\nburied cross- interfacial buried solid-\ninterfacial section signals liquid\nsignals) preparation diluted by interfaces\n; interfaces the bulk challenging,\nmay be signals) limited by\naltered probing depth\nduring the\nsample\npreparation\n)\nEnvironment Fully Fully Fully Compatible Compatible\nCompatibility compatible compatible compatible with near- with air,\nwith realistic with with ambient vacuum, and\nliquid cells realistic realistic pressure liquid cells\nand aqueous liquid cells liquid cells gases; liquids\nconditions and and via microjets\naqueous aqueous or meniscus\nconditions conditions methods\nRadiation Low (can be High Moderate Moderate Generally low\nDose / lower than (beam- dose; soft dose; soft X- dose per\nDamage Risk 0.01 mJ·cm–2 induced X-rays rays more pixel;\nper frame) structural more susceptible to scanning can\nand susceptible chemical accumulate\nchemical to chemical changes dose\nchanges, changes\nespecially\nin soft\nmaterials)\nStatistical High Limited High Moderate High (can\nmeasurement (averages (small field (averages (can measure map\nover of view, over large multiple statistical\nhundreds of typically illuminated surface areas for a\nidentical single volume) regions but large range)\nnanopattern particles or limited\nunits local penetration\ndepending on regions) depth)\nthe beam spot\nsize)\nMeasurement High Low Low Low Low\nEfficiency (diffraction (sequential (primarily (primarily (primarily\nencodes detections provides provides elemental\nstructural with chemical- chemical- mapping,\norder while structural state state structural\nresonance and information information; information\ntuning chemical ; structural structural indirect)\nprovides information evolution evolution\nchemical- with must be must be\nstate EELS/EDS measured measured\nspecificity ) separately separately\nconcurrently) with other with other\ntechniques) techniques)\nKey Interface- Direct Chemical- Direct surface High\nStrengths specific atomic- state and sub- sensitivity to\noperando scale analysis of surface trace\nprobing of imaging surfaces chemical elements;\nstructural and and and sub- state under quantitative\nchemical crystalline surface near-realistic elemental\ndynamics; structures regions conditions mapping\nlow-dose;\ndiffraction-\norder\nselectivity\nKey Requires Beam- Limited Limited Limited\nLimitations patterned induced spatial penetration chemical state\nstructures for changes; resolution; depth; information;\ncoherent limited bulk- challenging slow scan for\nenhancement detection interface for fully high-\narea; separation buried resolution\ndifficult to challenging interfaces maps\noperando\nprobe\nburied\ninterfaces\nRevision made: We added Table R1 as Supplementary Table 1 and the following illustration\ninto the revised SI as Supplementary Note 1 to make a comprehensive comparison of PE-\nRSoXS with other conventional techniques for probing electrochemical interfaces with relevant\nreferences to support our demonstration.\n“Compared to other state-of-the-art interface characterization methods such as transmission\nelectron microscopy (TEM), X-ray absorption spectroscopy (XAS), ambient-pressure X-ray\nphotoelectron spectroscopy (APXPS), and X-ray fluorescence microscopy (XFM), Pattern-\nEnhanced Soft X-ray Scattering (PE-RSoXS) offers a unique combination of high statistical\nrepresentativeness, sub-nanometer dimensional sensitivity, and element-specific chemical-state\nresolution, all achievable under realistic electrochemical operating conditions. Unlike TEM and\nAPXPS, which are limited in probing fully buried solid-liquid interfaces, PE-RSoXS can access\nthese environments without compromising spatial or chemical sensitivity. While XAS provides\nstatistical chemical-state information, it lacks lateral resolution and diffraction-order selectivity\nfor directly probing interfaces, which is inherent to PE-RSoXS through coherent scattering\namplification from identical nanopatterns. Furthermore, the ultralow-dose capability of PE-\nRSoXS (< 0.01 mJ·cm⁻2 per frame) minimizes radiation-induced artifacts, enabling operando\nmonitoring of dynamic processes with millisecond temporal resolution, surpassing the typical\ntime resolution of conventional methods. These combined attributes position PE-RSoXS as a\npowerful and complementary tool for probing chemical and structural dynamics at buried\nelectrochemical solid-liquid interfaces. PE-RSoXS operates on principles analogous to protein\ncrystallography, where diffraction from periodic units enables reconstruction of real-space\nfeatures with atomic to nanometer-scale precision. In our case, the periodic nanopatterns define\na structural unit cell, and the spatial distribution of chemical species within that unit cell\nmodulates the scattering form factor in a position-sensitive manner. This coherence-based\nsensitivity to the distribution and geometry of resonant materials enables spatial resolution at\nsub-nanometer length scales, as supported by both experimental data and the finite-element\nmodeling.”\n4. Please comment on the generality of the technique. Could this approach be applied to other\ncatalytic materials (e.g., Co or Cu) that undergo significant restructuring?\nResponse: We thank the Reviewer for this insightful question regarding the generality of the\nPE-RSoXS approach. The methodology is inherently versatile and not limited to Ni-based\nsystems. Its core principles of scattering enhancement from identical nanopatterns, chemical-\nstate specificity via resonant soft X-ray tuning, and operando compatibility with realistic\nelectrochemical environments, are broadly applicable to a wide range of catalytic materials,\nincluding transition metals such as Co and Cu, as well as their oxides, hydroxides, and hybrid-\nmetal compounds.\nFor catalytic systems like Co or Cu that undergo significant restructuring during operation (e.g.,\nredox cycles, lattice distortions, phase transitions, etc.), PE-RSoXS can be tuned to the relevant\nabsorption edges (e.g., Co L-edge at 780–795 eV; Cu L-edge at 930–950 eV) to selectively\nenhance scattering from chemically distinct states. This enables direct correlation between\nstructural changes (sub-nanometer sensitivity) and chemical-state evolution in real time. The\ntechnique’s diffraction-order selectivity and element-specific resonance contrast are\nparticularly advantageous for multi-component or heterogeneous catalysts, where isolating the\nscattering contribution from a specific active phase is challenging with non-resonant methods.\nThere are strong precedents in the electrocatalytic field for patterned architectures enabling\nmechanistic insights [Lum et al. Energy Environ. Sci. 11, 2935–2944 (2018); Liu et al. ACS\nAppl. Mater. Interfaces 13, 40513–40521 (2021)], which demonstrated that Cu–Au and Cu–Ag\npatterned electrodes could spatially separate and sequence consecutive reaction steps in CO₂\nelectroreduction, thereby enhancing selectivity of C products. Such patterned electrodes\n2+\nwould be largely applicable for PE-RSoXS, as the coherent amplification from identical\nperiodic structures would amplify scattering signal while allowing operando tracking of\nstructural and chemical evolution in the reaction zones.\nFurthermore, Prof. Peidong Yang and colleagues have shown in a series of recent studies [Yang\net al. J. Am. Chem. Soc. 144, 8927–8931 (2022); Yang et al. Nature 614, 262–269 (2023);\nYang et al. Nat. Catal. 8, 579–594 (2025)] that RSoXS can be applied to powder samples of Cu\nnanoparticles for CO electroreduction, revealing dynamic restructuring processes and surface\n2\nphase transformations. These works highlight the ability of RSoXS to uncover mechanistic\npathways in typical electrocatalytic systems, even in non-patterned morphologies. By\ncombining such resonance-based chemical sensitivity with the coherent enhancement from\nnanopatterned catalysts, we believe PE-RSoXS could further extend these mechanistic insights\nto buried interfaces under fully realistic electrochemical operations.\nTherefore, while Ni LGNPs were chosen in this study as a well-established benchmark for\nmethodology validation, the approach is readily extendable to other catalytic materials,\nincluding Co- and Cu-based systems, and more broadly to catalysts where operando tracking\nof coupled structural and chemical dynamics at buried interfaces is desired. Given the\ndemonstrated relevance of patterned and nanoparticle-based catalysts in CO electroreduction\n2\nand other electrochemical transformations, we anticipate that PE-RSoXS will attract broad\ninterests across the catalysis, energy conversion, and materials science communities.\nRevision made: We have added the following statement in the “Conclusion” of the revised\nmanuscript to emphasize the generality of PE-RSoXS and outline future applications to\nrestructuring catalysts beyond Ni-based catalysts.\n“The PE-RSoXS methodology is broadly applicable beyond the Ni-based system studied in this\nwork, providing operando access to coupled structural and chemical dynamics in a wide range\nof catalytic materials that undergo significant restructuring during operation. By tuning to the\nrelevant absorption edges of the focused elements, PE-RSoXS can selectively enhance\nscattering signals from specific chemical states, enabling real-time correlation between sub-\nnanometer structural changes and active-phase evolution. Its compatibility with identical\narchitectures including powder- and pattern-based systems, low-dose operation, and high\nsensitivity to the buried electrochemical interfaces make it a versatile tool for probing diverse\ncatalysts under realistic electrochemical conditions, with strong potential to address key\nquestions in catalysis, energy conversion, and materials science.”\n5. The method is described as “non-destructive,” but more evidence should be provided. While\n1 ms exposure per energy point is reported, is this duration sufficient to fully avoid beam-\ninduced damage? Is it a trade-off or the minimum exposure time? Would longer exposure time\nimprove data quality and accuracy to assess the simulation?\nResponse: We thank the Reviewer for raising this important point regarding the “non-\ndestructive” nature of PE-RSoXS and the relationship between exposure time, beam-induced\nchemical changes, and data quality.\nIn this work, we describe PE-RSoXS as a non-destructive methodology because the\ncombination of diffraction enhancement from identical nanopatterns and ultrafast single-shot\nacquisition allows us to achieve high-quality data with extremely low total X-ray dose.\nSpecifically, each diffraction pattern was acquired within 1 ms exposure time per energy point,\ncorresponding to a dose of < 0.01 mJ·cm⁻2 per frame. This dose is significantly below the\nthresholds reported in the literature (for a catalyst with 100 nm thickness, the critical dose is\naround 4200 mJ·cm⁻2) for radiation-induced chemical or structural changes in hydrated, organic,\nand nanostructured materials under soft X-ray illumination [e.g., Wang et al. J. Phys. Chem. B\n113, 1869–1876 (2009); Kubin et al. Phys. Chem. Chem. Phys. 20, 16817–16827 (2018)].\nFigure R3 shows the 3 to 6 orders of the scattering-derived spectra of as-prepared Ni LGNPs.\nThe orange and green curves show consecutive 2 scans in the whole energy range of Ni L-edge\nwith 1 ms of exposure time per frame. The purple curves were obtained from the as-prepared\nNi LGNPs after direct beam irradiation for 5 min, which is considerably longer than our\nstandard measurement (155 ms per scan in the whole energy range of Ni L-edge).\nThe scattering signal shows no significant change, which experimentally demonstrates that\nPE-RSoXS is a non-destructive method.\nThe choice of 1 ms exposure time was guided by three main considerations:\n(1) Minimizing beam-induced chemical changes\nShorter exposures reduce cumulative radiation dose, preserving the native state of the solid-\nliquid interfaces throughout the operando measurements.\n(2) Ensuring adequate signal-to-noise ratio\nCoherent Bragg diffraction from the identical nanopatterns provides amplified scattering\nintensity, enabling sufficient signal-to-noise ratio even at millisecond timescales without the\nneed for prolonged exposure.\n(3) Avoiding CCD detector saturation\nLimiting the exposure time prevents saturation of the CCD pixels by intense diffraction peaks,\nensuring linear detector response and preserving quantitative accuracy for subsequent data\nanalysis and simulation.\nIn our benchmarking experiments, increasing exposure time beyond 1 ms did not yield\nsignificant improvement in fitting accuracy for structural simulations, but increase the\nprobability of detectable radiation effects over the course of multi-energy scans. Therefore, we\nselected 1 ms as a balance between preserving sample integrity and achieving robust data\nquality.\nFigure R3. Scattering-derived spectra from the a, 2nd, b, 3rd, c, 4th and d, 6th orders acquired\non the as-prepared Ni LGNPs. The orange and green curves show consecutive 2 scans in the\nwhole energy range of Ni L-edge with 1 ms of exposure time per frame. The purple curves were\nobtained from the as-prepared Ni LGNPs after direct beam irradiation for 5 min.\nRevision made: In the revised manuscript, we supplemented Figure R3 as the Supplementary\nFig. 5 and the following illustration into the revised SI as the Supplementary Note 7 to\nexplicitly state the “non-destructive” description.\n“In this work, the term “non-destructive” refers to the demonstrated ability of PE-RSoXS to\nacquire high-quality diffraction and spectroscopic data before any measurable X-ray radiation-\ninduced chemical or structural changes occur in the sample. The 1 ms of single-shot exposure\ntime used here was selected as an optimal trade-off between preserving the native state of the\nsolid-liquid interfaces and ensuring sufficient signal-to-noise ratio for reliable data analysis,\nrather than as the minimum achievable exposure. PE-RSoXS was implemented with such a\nshort single-shot exposure time (1 ms) to enable operando monitoring of electrochemical solid-\nliquid interfaces while minimizing radiation dose (< 0.01 mJ·cm⁻2 per frame) and preserving\nthe native state of the nanopatterns. Although the technique and instrumentation are\nintrinsically capable of reaching as low as microsecond timescales, the present study focused\non a well-characterized Ni LGNPs/OER model system with dynamics in the millisecond regime\nor slower ones to benchmark dimensional and chemical sensitivity at the buried interfaces under\nrealistic electrocatalytic conditions. This methodological validation provides the foundation for\nfuture studies targeting ultrafast processes, such as microsecond electrochemical switching or\nlaser-triggered photoelectrochemical reactions, where the full temporal potential of PE-RSoXS\ncan be exploited.”\n6. The experimental and simulated Ni L-edge spectra do not fully coincide (e.g., 3rd-order in\nFig. 2d). Please clarify the origin of these discrepancies. The spectral difference for\nexperimental vs. simulated spectra should be plotted.\nResponse: We thank the Reviewer for this careful observation regarding the differences\nbetween the experimental and simulated Ni L-edge spectra, particularly for the 3rd-order\ndiffraction in Fig. 2d. We agree that these discrepancies merit clarification and additional\nquantitative comparison.\nOverall, the experimental and simulated Ni L-edge spectra show fully consistent spectral trends\nacross all diffraction orders, including the energy positions and relative variations of key\nfeatures. The primary difference lies in the absolute intensity, with simulations producing\nslightly lower values than the experimental results. This discrepancy is likely due to model\nsimplifications, such as the assumption of perfectly periodic structures and uniform material\nproperties, which do not fully capture the enhanced scattering arising from minor\nheterogeneities and interface roughness present in the actual nanopatterns.\nFigure R4. The residual (experimental vs. simulated) spectral intensity as a function of photon\nenergy for the scattering-derived spectra from the a, 3rd, b, 4th, c, 5th and d, 6th orders\nmeasured at OCP, 1.2 V and 1.6 V, respectively.\nIn response to the Reviewer’s suggestion, we have added difference plots for each diffraction\norder (Figure R4), showing the residual (experimental vs. simulated) spectral intensity as a\nfunction of photon energy. This quantitative comparison allows readers to directly assess both\nthe magnitude and the energy dependence of the discrepancies.\nRevision made: We added Figure R4 as the Supplementary Fig. 7 and the following\ndescriptions as the Supplementary Note 8 into the revised SI to note that the simulation\nframework is intended to capture the primary spectral trends and diffraction-order dependence,\nrather than to reproduce every fine spectral detail. The observed differences are consistent with\nthe expected influence of interfacial heterogeneity and dynamic restructuring, which are not\nfully incorporated into the current models.\n“We note that our simulations assume idealized Ni line-grating structures with smooth\ninterfaces, uniform shell thickness, and perfect sidewall angles. In practice, fabricated and\nelectrochemically evolved nanopatterns may exhibit interfacial roughness, line-edge\nirregularities, shell thickness inhomogeneity, and sidewall angle variations, etc. Such\nimperfections can diminish the relative intensity of higher diffraction orders and broaden peaks\nin q-space, effects that are strongly convoluted with shell thickness in the fits. While our current\nmodeling focuses on the dominant parameters of shell thickness and optical constants, these\nstructural deviations could, in principle, lead to misinterpretation of fitted values. We therefore\ninterpret our results within this limitation and note that future extensions to incorporate\nroughness or inhomogeneity models may help further refine the analysis. This simulation\nframework is designed to capture the primary spectral trends and diffraction-order dependence,\nrather than to reproduce every fine spectral detail. Minor discrepancies between simulated and\nexperimental spectra are expected due to interfacial heterogeneity and dynamic restructuring\nduring operando conditions, which are not fully incorporated into the current simplified\nsimulation models.”\n7. In Table 1, the shell thickness increases with potential, as expected from hydroxide\nintercalation. However, the core width is supposed to decrease as well. Could you explain this\ndiscrepancy?\nResponse: We thank the Reviewer for this thoughtful observation regarding the apparent\ndiscrepancy between the expected and simulated core width changes during OER. Indeed,\nhydroxide intercalation into the Ni LGNP shell is anticipated to cause lattice expansion in the\nshell and a concomitant contraction of the metallic core due to oxidation and strain\nredistribution. However, our simulation results show that the core width remains essentially\nconstant at 1.2 V vs. RHE and increases by ~2 nm at 1.6 V vs. RHE.\nA key contributing factor is that the simulation models for 1.2 V and 1.6 V vs. RHE were\nconstructed with lower density than the model at OCP. This adjustment was made to physically\nrepresent the increased hydration, hydroxide incorporation, and porosity in the shell region\nunder electro-oxidizing conditions. These processes reduce the bulk mass density of the\nmaterial, particularly in the shell and at the core-shell interface, due to expansion of the lattice\nand inclusion of water molecules or hydroxide ions. In the simulations, lowering the material\ndensity changes the refractive index contrast between core and shell, which can cause the fitting\nalgorithm to assign a slightly larger effective core width to reproduce the observed intensity\ndistribution, even if the actual metallic core radius has not physically increased.\nIn addition to the density effect, several other factors may also contribute to the deviation from\nthe simple expectation of core contraction:\n(1) Electrochemical restructuring beyond elastic strain\nAt high potentials (e.g., 1.6 V vs. RHE), oxidation and hydroxide formation occur not only in\nthe shell but also at the core-shell interfaces. This can lead to partial recrystallization or\ndensification of metallic domains, increasing the coherent scattering length of the core region.\n(2) Dynamic hydration and porosity changes\nHydroxide intercalation expands the shell lattice and may introduce nanoscale voids and water\nlayers. These changes alter scattering profiles and can shift the fitted boundary between core\nand shell.\n(3) Simplifications in the core-shell models\nThe current fitting assumes uniform density within each region and a sharp core–shell boundary.\nIn reality, the interface should be graded, with mixed oxidation states and density variations\nover several nanometers. Under strongly oxidizing conditions, this graded interface could be\ninterpreted by the model as a larger core width.\nRevision made: To clarify this point in the revised manuscript, we have added a note as the\nSupplementary Note 9 in the revised SI explaining that the “core width” extracted from the\nresonant scattering fits represents an effective domain size rather than a direct measurement of\nthe metallic core radius. This effective size can be influenced by changes in material density,\ninterface composition, and scattering contrast, particularly under oxidizing conditions where\ndensity is reduced to reflect structural hydration and porosity.\n“In this study, the “core width” obtained from resonant scattering fits should be interpreted as\nan effective domain size rather than a direct physical measurement of the metallic core radius.\nThis effective size is influenced by multiple factors, including changes in material density\n(especially at 1.2 V and 1.6 V vs. RHE), interface composition, and scattering contrast. Under\nelectro-oxidizing conditions, the simulation models incorporate reduced material density to\nrepresent structural hydration, hydroxide incorporation, and increased porosity in the shell\nregion. These density changes alter the core-shell contrast in the scattering profiles, which can\nlead to slight variations in fitted core width even if the actual metallic core dimension remains\nunchanged or becomes smaller.”\n8. Is there any conditioning procedure before the electrochemical measurement? If so, would\nthe conditioning alter the core-shell restructuring? Beyond the OCP/1.2 V/1.6 V comparison,\nis it possible to assess the transition from pristine Ni (air/water-exposed) to OCP?\nResponse: We thank the Reviewer for this important question regarding the electrochemical\nconditioning procedure and its potential influence on the observed core-shell restructuring.\nBefore the constant-potential measurements at OCP, 1.2 V, and 1.6 V vs. RHE, the Ni\nnanopatterned electrodes underwent cyclic voltammetry (CV) scans in the electrolyte to\nstabilize the electrochemical interface and remove surface contaminants (see our response to\ncomment 1 from this Reviewer). This conditioning is a standard step in electrocatalyst studies\nto ensure reproducible activity and scattering signals. The CV scans likely initiate partial\nsurface oxidation and hydroxide formation, which can begin the restructuring process prior to\nthe constant-potential holds. Therefore, the structures observed at OCP in this study represent\na conditioned state rather than the fully pristine metallic Ni surface.\nWe agree with the Reviewer that assessing the transition from the pristine Ni state to the OCP\ncondition would provide valuable insight into the early stages of core-shell evolution. To\naddress this, we supplement the revised manuscript with additional PE-RSoXS results for\npristine Ni nanopatterns without liquid exposure before electrochemical operations (Figure R5).\nThese results facilitate a comparison between the pristine Ni LGNPs and the OCP state\nfollowing electrochemical conditioning. The 2nd and 4th diffraction orders exhibit similar\nprofiles for both the pristine and OCP samples, indicating comparable core structural features.\nHowever, the pristine sample displays higher intensities in the 3rd and 6th orders compared to\nthe OCP state, suggesting variations in their electronic environments and potential differences\nin chemical compositions at the interfaces. The 5th-order signal reveals two distinct local\nmaxima, highlighting subtle differences in shell characteristics associated with the active\nelectrocatalytic environment. These observations indicate that while the core structures of the\npristine Ni and OCP nanopatterns share fundamental similarities, the chemical and structural\ndynamics at the interfaces undergo significant transformations during electrocatalysis. The\nvariations in intensity and the emergence of distinct maxima in the 5th order point to the\nsubstantial influence of the reaction environment on the surface characteristics of the Ni\npatterns. The pristine Ni LGNPs predominantly consist of metallic Ni with surface NiO species\ndue to the exposure to air. In contrast, following CV conditioning to the OCP stage in alkaline\nsolution, the nanopatterns feature a core of metallic Ni surrounded by oxidized Ni species (NiO\nand Ni(OH) ) within the shell. In this work, we regard these operando measurements as critical\n2\nreflections of the evolving chemistry at the interfaces and their implications for electrocatalytic\nperformance.\nFigure R5. Scattering-derived spectra from the a, 2nd, b, 3rd, c, 4th, d, 5th and e, 6th orders\nacquired on the as-prepared Ni LGNPs and Ni LGNPs operated at OCP, 1.2 V, 1.6 V vs. RHE,\nrespectively.\nRevision made: In the revised manuscript, we have explicitly refined the procedure “Operando\nPE-RSoXS measurements” in the Methods as shown in the response to the first comment from\nthis Reviewer. We also include Figure R5 as the Supplementary Fig. 6 in the revised SI.\n9. Is there any potential bubble issue, due to the beam-induced water radiolysis or generated\nO2 bubble at 1.6V? Would it alter the electrolyte thickness and affect the scattering\ninterpretation?\nResponse: We thank the Reviewer for raising this important point. To the best of our\nknowledge, there is no potential bubble issue due to the beam-induced water radiolysis or\ngenerated O bubble at 1.6V. First, the extremely low X-ray dose in our measurements\n2\n(< 0.01 mJ·cm⁻2 per frame, corresponding to orders of magnitude below typical beam-induced\nchemical change thresholds) minimizes the likelihood of significant beam-induced radiolysis.\nAt soft X-ray energies, radiolysis rates scale strongly with dose, and our exposure levels are far\nbelow those known to produce observable bubble nucleation in aqueous systems [e.g., Kubin\net al. Phys. Chem. Chem. Phys. 20, 16817–16827 (2018)]. Meanwhile, the single-shot\nacquisition at 1 ms per energy point limits cumulative dose at any given location, reducing the\nprobability of localized gas evolution from beam–water interactions. At 1.6 V vs. RHE, oxygen\nevolution is active, and O bubbles can form at the electrode surface. To mitigate this, the\n2\noperando liquid cell was designed with continuous electrolyte flow and a vertical electrode\norientation, allowing evolved gas to be rapidly carried away from the illuminated region.\nDuring the operando measurements, we monitored scattering intensity and baseline stability\nand there were no abrupt changes indicative of transient bubble interference (e.g., sudden\nintensity drops or spurious diffuse scattering). In-between the E-chip and the supporting chip,\nthere is a 500 nm space. With continuous flow, the electrolyte thickness always kept the same\nas 500 nm based on the transmission mode of RSoXS data collection. Thus, the electrolyte\nthickness remained stable and that bubble formation did not measurably distort the scattering\nprofiles. In the unlikely event that a bubble passed through the beam, it would produce a distinct,\nnon-periodic scattering signature easily distinguished from the coherent diffraction peaks used\nin the analysis.\nIn the revised manuscript, we provided a detailed description in the Methods section regarding\nthe cell design and flow conditions that effectively minimize bubble retention (see our response\nto comment 1 from this Reviewer). Consequently, both beam-induced radiolysis and\nelectrochemically generated O bubbles are unlikely to have impacted the measurements in this\n2\nstudy, attributed to the ultralow dose, short exposure times, optimized cell geometry, and\ncontinuous flow operation. Additionally, based on our recent research [Yang et al. J. Am. Chem.\nSoc. 144, 8927–8931 (2022)], nanoparticles larger than 18 nm demonstrate significant\nresistance to beam damage compared to smaller ones, enabling reliable observation of surface\nchemical dynamics. The Ni LGNPs used in this study are substantially larger than 18 nm,\nfurther reinforcing the conclusion that they are unaffected by beam-induced chemical changes.\n10. Could you provide the SEM/AFM analysis of the nanopatterns after experiments to confirm\nthe structural integrity.\nResponse: We thank the Reviewer for this. Following the Reviewer’s suggestion, we\nperformed post-mortem atomic force microscopy (AFM) measurements on the Ni LGNPs after\nthe electrochemical experiments, as shown in Figure R1.\nFigure R1. Post-mortem AFM images of the Ni LGNPs on the SiN windows of the sample\nx\nchip after electrocatalysis at a, OCP, b, 1.2 V and c, 1.6 V, showing the statistical line width\nand line edge roughness (LER) at each condition.\nThe AFM results show total width changes of the nanopatterns that closely follow the trend\nsimulated by our finite-element method (FEM) based on PE-RSoXS results (OCP: 113.8 nm;\n1.2 V: 113.4 nm; 1.6 V: 120.5 nm). However, in these AFM images, clear fluctuations are\nobserved along the line edges. These fluctuations can be quantified using the concept of line\nedge roughness (LER), as described by Nealey et al. [J. Appl. Cryst. 49, 823–834 (2016)].\nMeanwhile, ex situ microscopy techniques cannot directly measure the oxide layer thickness\nwith sub-nanometer resolution. High-resolution TEM could, in principle, provide this\ninformation, but it requires to prepare cross-sections of the line gratings, which may potentially\ndamage the sample. In contrast, PE-RSoXS enables operando, low-dose, and non-destructive\nmeasurements that quantitatively track oxide layer thickness and core-shell structural changes\nduring active electrocatalysis. By exploiting resonant scattering and diffraction-order\nsensitivity, PE-RSoXS can distinguish shell and core contributions without relying on post-\nmortem assumptions, providing statistically representative results across over hundreds of\nnanopatterns under realistic electrochemical conditions, which highlights the significance to\ndevelop PE-RSoXS.\nRevision made: We added Figure R1 into the revised SI as the Supplementary Fig. 8 to\nexhibit the Ni LGNPs after electrocatalysis and supplemented the following illustration into the\nrevised manuscript to compare our simulation results and AFM results of the structural\nvariations at each condition, highlighting the significance to develop PE-RSoXS.\n“Besides, our simulated total width changes of the Ni LGNPs align with atomic force\nmicroscopy (AFM) results from the samples after being operated at the corresponding\nconditions (Supplementary Fig. 8), further clarifying the fidelity of our FEM-based simulations.\nHowever, in these AFM images, clear fluctuations are observed along the line edges. And ex\nsitu microscopy techniques cannot directly measure the oxide layer thickness with sub-\nnanometer resolution. In contrast, by exploiting resonant scattering and diffraction-order\nsensitivity, PE-RSoXS can distinguish shell and core contributions without relying on post-\nmortem assumptions, providing statistically representative results across over hundreds of\nnanopatterns under realistic electrochemical conditions, which highlights the significance to\ndevelop PE-RSoXS.”\nWe truly thank the Reviewer for the insightful comments and kind suggestions, which are\nvery helpful to further improve our manuscript.\nReviewer: 1\nThe authors have addressed the major concerns raised in the previous review round, and the\nrevised manuscript now meets the high standards required for publication in Nature\nCommunications. Nevertheless, several points could be further improved:\nResponse: We appreciate the Reviewer’s positive assessment that the major concerns have\nbeen addressed and that the revised manuscript meets the high standards required for\npublication in Nature Communications. We have carefully considered the additional points\nraised and addressed each of the reviewer’s detailed comments point by point below. We are\ngrateful for the reviewer’s constructive suggestions, which further helps improve the quality of\nour manuscript.\n1. The distinction between “Operando” and “In situ” measurements should be used more\nrigorously. As the present experiments probe structural and chemical evolution under controlled\nelectrochemical conditions without directly correlating these changes to real-time catalytic\nperformance, the term “In situ” appears more appropriate.\nResponse: We thank the Reviewer for emphasizing the importance of precise terminology\nregarding our PE-RSoXS methodology. After careful consideration, we have decided to retain\nthe use of the term “operando” in the revised manuscript, as it more accurately captures the\nnature of our experiments.\nIn X-ray spectroscopic characterizations, “in situ” typically refers to measurements conducted\nunder relevant environmental or reaction conditions, but not necessarily during the active\noperation of the system. Conversely, “operando” specifies that the measurements are taken\nwhile the system is actively engaged in its intended operational state, allowing for direct\ncorrelations between the observed dynamics and the applied conditions. This distinction has\nbeen discussed in the literature [Gurlo et al. Angew. Chem. Int. Ed. 46, 3826-3848 (2007);\nWeckhuysen, B. M. In-Situ Spectroscopy of Catalysts, American Scientific Publishers (2004)].\nOur study employed PE-RSoXS measurements under fully functional electrochemical\nconditions. We conducted several cyclic voltammetry (CV) scans over a potential range from\nopen circuit potential (OCP) to 1.2 V vs. RHE, effectively stabilizing the core-shell structure\nand removing surface impurities prior to our operando measurements. This conditioning\nprocess ensured that the subsequent scattering profiles collected at the Ni L-edge at OCP, 1.2\nV, and 1.6 V vs. RHE reflected real-time changes directly correlated to the electrocatalytic\nperformance during the oxygen evolution reaction (OER). The operando nature of our\nmeasurements allowed us to observe oxidation-state transitions and sub-nanometer structural\nvariations in direct relation to the electrochemical performance under realistic operating\nconditions. Based on this reason, we believe it’s more accurate to use the term “operando”\nrather than “in situ” as the methodology was applied under true working conditions with\nconcurrent interface monitoring, rather than simply under static or simulated reaction\nconditions.\nWe appreciate the Reviewer’s insight and have made a point to clearly delineate our operando\nPE-RSoXS methods in the corresponding section of the Methods. We trust this clarification\naddresses the Reviewer’s concern and supports our rationale for using the term “operando” to\nreflect our methodology accurately.\n2. While the manuscript correctly attributes the detection of ~0.2 nm structural changes to\ndiffraction-intensity sensitivity rather than direct imaging beyond the diffraction limit (~0.75\nnm), phrases such as “sub-nanometer resolution” or “visualization at sub-nanometer scales”\nmay still be misleading. Consistent use of terms like “sub-nanometer sensitivity” would\nimprove clarity.\nResponse: We appreciate the Reviewer’s suggestion regarding the consistent use of the term\n“sub-nanometer sensitivity.” We have revised the manuscript accordingly to ensure consistent\nusage throughout.\nRevision made: We have revised the phrases “sub-nanometer resolution” and “visualization at\nsub-nanometer scales” to “sub-nanometer sensitivity” throughout the manuscript.\n3. The generality and limitations of the PE-RSoXS method would benefit from clearer\ndiscussion. In particular, it would be useful to clarify whether the approach fundamentally relies\non highly periodic nanostructures and to what extent it can be extended to non-periodic,\nmultiscale, or porous electrocatalyst systems relevant to practical applications.\nResponse: We thank the Reviewer for the valuable insights regarding the generality and\nlimitations of the PE-RSoXS method. In the revised manuscript, we have expanded our\ndiscussion to provide a clearer assessment.\nWe clarify that while PE-RSoXS is particularly advantageous for highly periodic\nnanostructures that generate well-defined Bragg scattering patterns, it is not inherently\nrestricted to perfectly periodic systems. This adaptability allows PE-RSoXS to be effectively\napplied to non-periodic, multiscale, or porous electrocatalyst systems that are highly relevant\nto practical applications. To facilitate this, one potential strategy involves employing a\npatterned template combined with a porous catalyst layer on top. By leveraging the underlying\nstructured template, we can also enhance diffraction signals, especially at the electrode-\nelectrolyte interfaces, thereby improving the method’s sensitivity to structural variations and\nchemical dynamics even in less ordered systems.\nFurthermore, in Supplementary Table 1, we have provided a comprehensive comparison of PE-\nRSoXS with other techniques, outlining their general applicability and inherent limitations.\nThis serves to contextualize PE-RSoXS within the broader landscape of characterization\nmethods, offering readers a clearer understanding of its strengths and the scenarios in which it\nmay excel or face challenges.\nWe appreciate the Reviewer’s suggestion and believe that these enhancements provide a more\nthorough discussion of the versatility and applicability of the PE-RSoXS method, addressing\nits potential in real-world electrocatalytic systems.\nRevision made: We added “The approach is not fundamentally limited to perfectly periodic\nsystems. One potential strategy is to use a patterned template and apply a porous catalyst on\ntop, leveraging the underlying structure to enhance diffraction signals.” in the conclusion.\nThanks again for the valuable comments and suggestions from the Reviewer. We have\ncarefully addressed each point.\nReviewer: 2\nI am satisfied that the authors not only agree with me on almost every point, but they have\nfully addressed my concerns with important and substantial revisions.\nWe can quibble about whether PE-RSoXS is really a \"new class\" of measurement or just an\nevolution of DAFS with an artificial vs. naturally-occurring Bragg structure, but the current\narticle provides sufficient context to let the reader decide.\nResponse: We truly thank the Reviewer for the insightful comments and kind suggestions\nthat have helped improve the manuscript! We deeply respect the importance and\nsignificance of DAFS, which inspired the development of PE-RSoXS.\nReviewer: 3\nThe concerns have been addressed.\nResponse: We appreciate the Reviewer’s assessment that the concerns have been\nadequately addressed. We sincerely thank the Reviewer again for the insightful comments\nand constructive suggestions provided in the previous review round.\n4"}]

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