Decoding the gradient-distributed colour centers in electrochromic WO_3

Domain: physics
Published: 2026-01-14
Editorial outcome: major_revision

Abstract
The optical properties of electrochromic windows can be modulated by embedding electrons and ions simultaneously to realize energy-saving functions. However, the microscopic electrochromic mechanism in cathode materials remains controversial. Herein, we have discovered that the single-crystal WO3 nanosheets exhibit thickness-dependent electrochromic characteristics. The multislice electron ptychography experiments revealed that the colour centers in electrochromic WO3 are associated with lattice distortions, and the density of the colour centers decreases from surface to bulk. Moreover, the heterogeneous distribution of polarons induces a phase transition from monoclinic to tetragonal and cubic structures, which in turn causes the band gap of the WO3 nanosheets to follow a volcano-type trend. This phenomenon elucidates the colour shift of the electrode from yellow to blue or black as the thickness of the nanosheets decreases. By visualizing the actual colour centers of electrochromic materials, our findings provide an optimal framework for the development of high-performance electrochromic devices.

Introduction
The electrochromic behavior of metal oxides is achieved by applying an electric field, which modulates their optical absorption properties across the ultraviolet–visible-infrared (UV-Vis-IR) spectrum. This technology is instrumental in energy conservation and transparency regulation in applications such as smart windows, aircraft windows, and anti-counterfeiting coatings, etc1–8. Despite numerous proposed electrochromic mechanisms for smart materials, such as the classical cathode material WO3, a universally accepted theory remains elusive9–13. In 1973, Deb and colleagues proposed a “colour center” theory to explain the electrochromic mechanism within amorphous WO3-x films. According to this theory, oxygen vacancies (VO) within the host material create defect energy levels that capture carriers, leading to the absorption of visible light14. Since ions can also be injected into host materials during the electrochemical process, charge injection models were developed, and the possibility of charge transit absorption between W5+ and W6+ species was considered15,16. Meanwhile, researchers have discovered that small polarons can form when both ions and electrons are simultaneously incorporated into the crystal structure. They attributed the modulated optical properties of electrochromic materials to the hopping of electrons from W5+ to adjacent W6+ ions17–21. Considering that WO3 phases can be transformed into tungsten bronze phases with a high concentration of charge carriers upon charge injection, scientists have utilized the Drude model to elucidate that the observed increase in absorption across the visible and infrared spectrum is due to the surface plasmon resonance effect associated with these high-density free carriers22–29. However, these perspectives fail to offer a clear explanation for the blue-shifted absorption edges observed in the UV-Vis spectra of coloured WO3. The inserted electrons and ions affect the density of the free carrier and induce the structural distortion of the host materials to form various phases30. For many transition metal compounds, the symmetry of the crystal structures can be influenced by trapping and de-trapping electrons and ions3,21,31,32. As revealed by the aberration-corrected transmission electron microscopy, the bond angles and lengths of the host materials can be decreased by embedding ions and electrons under electrochemical environments, thereby promoting the orbitals’ overlap between anions and cations33. These enhanced hybridizations of the valance electrons within the ion-inserted materials will lower the band gap in the phase, resulting in the red shift of the absorption edge in UV-Vis-IR spectra34–37. Monoclinic γ-WO3 is a stable phase at room temperature, and it can be transformed into orthorhombic, tetragonal, and cubic phases by increasing the qualities of embedded ions and electrons15. Note that the symmetry of the phases can be well characterized by the degree of tilting, rotation, and distortion of the corner-sharing [WO6] octahedra38. It is believed that alterations in crystal structures, along with the insertion of electrons and ions, significantly impact the electronic structure of WO3, which in turn plays a pivotal role in its optical properties39,40. Nevertheless, the aforementioned key factors are often neglected, and there remains a lack of atomic-level understanding regarding the spatial distribution of the inserted electrons and ions in coloured WO3, the identity of the actual colour center, and how the colour center actively modifies optical characteristics. This gap in knowledge constrains the advancement of electrochromic materials and technologies.

Herein, we have chosen γ-WO3 nanosheets with various thicknesses as a model system to investigate the active mechanism of the colour center. By comparing the crystal structures and optical modulation amplitudes of coloured WO3 nanosheets of varying thicknesses, the thickness-dependent electrochromic properties of these nanosheets have been revealed. For instance, it has been observed that thinner nanosheets exhibit enhanced electrochromic behavior. By means of state-of-the-art multislice electron ptychography41,42, the three-dimensional spatial distribution characteristics of the colour center in WO3 nanosheets have been revealed at the atomic level. By integrating with theoretical calculations, we have bridged the connection between the crystal structure and colour center and found that the colour is tightly related to the density of the colour centers at different depths, from surface to bulk, during the electrochromic process. This work deepens the understanding of the electrochromic process by directly characterizing the atomic structure and depth distribution of the colour center at the microscopic level, offering a solid theoretical foundation for controlling the optical absorption performance of smart materials.

Results
In our quest to unravel the electrochromic mechanism, we have selected the typical monoclinic γ-WO3 and compared its macroscopic structures across various states, namely pristine, coloured, and bleached. The highly crystalline WO3 nanosheets prepared via a hydrothermal process, which were deposited onto the surface of F-doped tin oxide (FTO) electrodes, followed by annealing, exhibit similar-thickness films (Fig. S1) and varying grain thicknesses of 15±6.5, 30±7.4, and 60±12.0 nm (Figs. S2 and S3, details are discussed in Supporting information.). These nanosheets are confined by two basal planes, specifically the {002} planes, and are edged by four-sided facets, comprising two {200} facets and two {020} facets (Fig. S4). As shown in Fig. 1a, a constant negative voltage technique was employed to induce the electrochromic behavior of the WO3 cathode in a KCl electrolyte within a three-electrode setup. The images of the insets in Fig. 1b demonstrate that the coloured WO3 electrodes (the detailed experimental conditions were discussed in Figs. S5-S7) undergo a transition from yellow to blue, and ultimately to black, as the grain thickness decreased under the identical experimental conditions. As shown in Fig. 1b, compared with the 30 nm- and 60 nm-thick WO3 nanosheets, the UV-vis drift reflectance spectrum of the 15 nm-thick WO3 nanosheets shows maximum reflectance modulation of 96% at wavelengths of 700 nm. In-situ optical transmittance results at 410, 440, and 770 nm reveal that the 15 nm-thick WO3 nanosheets possess a faster optical response than their thicker counterparts (Fig. S8). These results indicate that the 15 nm-thick WO3 nanosheets display improved electrochromic properties than other WO3 nanosheets with a larger thickness. Furthermore, we have calculated the inserted electric charge quantities of the coloured WO3 with various thicknesses according to the following formula:

Q = ∫0t j*A dt (1)

where Q, j, A, and t parameters refer to electric charge, current density, work area, and time. As shown in the J-t plots (Fig. 1c), the 15 nm-thick WO3 nanosheets accumulate a charge of 1.2 C within 60 s, exceeding that of the 30 nm- and 60 nm-thick counterparts. This indicates that WO3 nanosheets exhibiting greater light modulation (in the coloured state) accommodate more injected electrons. X-ray photoelectron spectroscopy (XPS) spectra (Fig. S6) and energy-dispersive spectroscopy (EDS) maps (Fig. S7) of the coloured 15 nm-thick WO3 nanosheets reveal the characteristic signals of K+ as well as their uniform distribution, demonstrating that the K+ from the solution could also insert into the WO3 lattice under applied bias. Inductively coupled plasma (ICP) analysis of coloured KxWO3 nanosheets (Fig. 1d) reveals an increase in the K+ insertion ratio from 13.83±2.84% to 19.18±0.36% as the nanosheet thickness decreases from 60 nm to 15 nm. This trend correlates with enhanced optical modulation properties, confirming a positive relationship between K+ quantity and electrochromic performance. These results indicated that the electrochromic properties of the WO3 nanosheets are closely linked to the number of inserted electrons and ions. Thinner WO3 nanosheets can host greater quantities of these species, enhancing their electrochromic performance. Moreover, the optical absorption edge of the coloured KxWO3 nanosheets shows an obvious blue shift, while the counterparts of 15 nm thick nanosheets demonstrate a prominent red shift (Fig. 1b). When the coloured KxWO3 cathode was exposed to air, which can capture the embedded electrons (briefly termed as the bleached KxWO3), the bleaching process was achieved without the need for an additional positive bias potential (Figs. S9-S14), causing the blue cathode to return to pale yellow. This experiment result indicates that the intercalation of electrons is a rate-determining step in achieving the electrochromic behaviour of smart materials.

The X-ray diffraction (XRD) pattern of the pristine WO3 nanosheets displays a typical monoclinic structure as depicted in Fig. 1e. The phase structures of coloured KxWO3 nanosheets gradually become tetragonal and cubic as their thicknesses decrease, a phenomenon that is linked to lattice expansion caused by inserted ions and electrons (Fig. S15), which is also evidenced by theoretical simulation in Fig. S16. After de-intercalation of ions and electrons, the XRD patterns of the bleached WO3 nanosheets with various thicknesses exhibited monoclinic crystal structures. Compared with the Raman spectra of the pristine and bleached WO3 nanosheets (Fig. 1f), the Raman bands corresponding to the bending mode and stretching variation of W-O-W for the coloured KxWO3 nanosheets are weaker, illustrating that the corner-connected W-O-W bonds show stronger rigidity. This is because the bond angle of the corner-connected W-O-W bonds becomes larger to generate more stress and bear the bending or stretching vibration of the chemical bonds. These findings suggested that the embedding of electrons and ions leads to a phase transition and improves the symmetry of the WO3 nanosheets.

From the XPS spectra of the WO3 nanosheets at various states (Fig. 1g), we found that the high-resolution W 4f spectrum of the coloured KxWO3 nanosheets reveals the presence of the W5+ groups at 36.1 eV and 33.7 eV, along with a blue shift in the W6+ species at 37.1 eV and 35.0 eV. Concurrently, the bleached KxWO3 nanosheets no longer exhibit the W5+ groups, indicating that the electrochromic behaviour is intimately connected to inserted electrons within the cathode system. Regarding the electronic structure, it is delineated by the changes observed in the O K-edge through electron-energy loss spectroscopy. The electron transition from the O 1s orbitals to the hybridized orbitals between O 2p and W 5d/6sp orbitals accounts for the three distinct peaks (labeled A, B, and C) in the O K-edge spectrum, as depicted in Fig. 1h. The coloured KxWO3 samples exhibit reduced signal intensity from the A peaks, attributed to the insertion of electrons into the W 5d orbitals. Conversely, the bleached KxWO3 nanosheets, which lack the additional electrons, show a recovery in the intensity of the A peaks to levels comparable to those of the pristine WO3 samples43. In light of the aforementioned experimental findings, it is determined that the coupled doping of ions and electrons can modulate the crystal structures and electronic structures of WO3, forming the colour center that will enable the cathode’s electrochromic properties.

To identify the spatial distribution of the colour centers at the atomic scale, we employed multislice electron ptychography to characterize atomic positions of W and O atoms determined through Gaussian fitting (Fig. S22). Note that the local lattice spacings are increased to 3.9 Å compared with pristine monoclinic WO3 of 3.7 Å. Furthermore, the Pearson correlation coefficient values for a and b are 0.07 and 0.10 (Fig. 2e), respectively, indicating that the lattice expansion occurs both in the {200} and {020} planes, which is attributed to the insertion of K+ ions and electrons and positively correlated with the embedded K+ ions concentration.

As seen from Fig. 2f, when the depth of doped K+ ions is close to the top and bottom surfaces, the W-O-W bond angles become larger along the [200] and [020] directions, and the part of W-O-W angles tend to even rise to 180° among the slices near the surface for producing the cubic KxWO3 and cubic WO3 crystal units. This phenomenon was also observed in the NaxWO3 nanosheets (Fig. S23). These findings show that the injected ions and electrons can cause the lattice distortion of the [WO6] octahedrons, which triggers the phase transition of the WO3 nanosheets. As mentioned in the review literature21, when the alkali metal ions and electrons are injected into the host materials WO3, they are believed to form the localized small polarons and induce lattice distortions around the trapping sites. As shown in the schematic diagram (Fig. 2g), limited by the charge transfer dynamics, K+ ions in the WO3 nanosheets exhibit gradient distribution from the inside outward as companies with gradually disappearing zigzag lattice distortion, causing depth-dependent lattice distortion (monoclinic, tetragonal, and cubic) from bulk to surface. The results show that the higher amounts of K+ ions and electrons embedded, the greater the lattice distortion, which is conducive to forming different colour centers (yellow, blue, and black) for modulating the macroscopic colour of the working electrode.

To reveal the electrochromic mechanism, the effect of the lattice distortions on the changes in electronic structures after inserting ions and electrons was investigated. Figure 3a shows the relaxed structure units of the γ-WO3 (left panel, monoclinic), K0.125WO3 with an additional 0.125 e/unit cell (middle panel, tetragonal), and K0.25WO3 with an additional 0.25 e/unit cell (right panel, cubic) by employing density functional theory (DFT), exhibiting the significant structural expansion and obvious lattice distortion. Note that the intercalations of more electrons and K+ ions do not change the connected and rotated pattern of the corner-sharing [WO6] octahedrons but increase the W-O-W bond angles and bond length to 180° and 3.89 Å, respectively. This result indicates that the cooperative interaction of injected electrons and K+ ions at interstitial sites offers strong stress to relax the zigzag deformation of O atoms along [100], [010], and [001] directions in multiple areas of the samples. Predictably, the synergistic effect is the reason that the XRD patterns of coloured KxWO3 samples with different thicknesses (30 nm and 15 nm) gradually become tetragonal and cubic symmetry as the thickness decreases (Fig. 1e). Additionally, due to the distinct expansion of the W-O-W bond angles and bond length, the orbital overlap of W 5d and O 2p in the 2D charge density maps gradually reduced from the monoclinic WO3 to tetragonal K0.125WO3 and cubic K0.25WO3 (Fig. 3b). This will have a significant effect on the band structure of the coloured KxWO3 nanosheets. As from the Kubelka-Munk plots (Fig. S24) and calculated band structures (Fig. 3c) of the cathode electrochromic samples, the band gap of the 30 nm-thick coloured KxWO3 nanosheets with tetragonal symmetry is 3.0 eV (Fig. 3d), which is larger than that of the pristine WO3 nanosheets (2.8 eV). This is the cause of the blue shift of the adsorption edge in diffuse reflectance spectroscopy (DRS) spectra of 30 nm-thick tetragonal KxWO3 nanosheets (Fig. 1b), thereby resulting in the yellow WO3 electrode changing to blue. Furthermore, the band gap of the 15 nm-thick cubic KxWO3 sample is reduced to 1.6 eV (Fig. 3c and Fig. S25). This changed electronic structure gives rise to the redshift in DRS spectra of cubic KxWO3 nanosheets, making the WO3 cathode black. Accordingly, these local atomic structures with different degrees of distortion induced by various concentrations of polarons are the real colour centers of electrochromic WO3 nanosheets. Therefore, relatively few embedded K+ ions and electrons can only induce the slight lattice distortion of the 60 nm-thick WO3 nanosheets, suppressing the improvement of optical modulation amplitude performances (Fig. 1).

Inspired by the above results, we forecasted that the anode electrochromic of the WO3 nanosheets, elevating the orbital overlap areas, could be a feasible mean to obtain red nanosheets (Fig. 3e). However, the experimental results show that although the triclinic WO3 nanosheets have the reduced W-O-W bond angles and band gap (2.78 eV), they exhibit a negligible red-shift of adsorption edge because the W6+ cannot be further deeply oxidized (Fig. S26).

We have also discussed the diffusion behaviour of the embedded ions from the surface toward the bulk during the electrochromic process within WO3 nanosheets. For the γ-WO3 nanosheets, the diffusion pathway of embedded ions is preferred for transporting along the [001] directions (Figs. S27-S31). As depicted in the inset of Fig. 4a, the slab model of K+ ions penetrated the three layers of WO3 nanosheets, as viewed from the side, has been built to mimic the migratory routes from the surface to bulk. The calculation results show the diffusion barrier rises to 16.8 eV as the K+ ion penetration layers go deeper by utilizing the climbing image nudged elastic band (CI-NEB) method48, implying that plentiful K+ ions inclined to coordinate on the surface region of the WO3 electrode (Fig. 4a). Although the faster embedding of electrons can promote the inserting of K+ ions, there are still very few K+ ions that can overcome the diffusion barrier to migrate into the depths of the bulk, producing a specific intercalation depth for the embedded ions as revealed by electron microscopy. This finding suggests that the method to improve the optical modulation amplitude of work electrodes is to decrease the size of the embedded ions, increase the pore size of the host materials, and reduce the thickness of the work electrodes to lower the diffusion barrier.

To facilitate the lattice distortion behaviour of the host materials, the electrolyte with smaller radius ions to replace the K+ ions, such as H+, Li+, and Na+ ions, was employed, respectively. We found that the colour of the WO3 nanosheets inserted with H+, Li+, and Na+ also displays blue and black (Fig. S32), and the DRS spectra of the 15 nm-thick HxWO3 nanosheets, inserted with more ions and electrons, show the most obvious optical modulation amplitude properties among these ion-inserted WO3 samples at the wavelength of 700 nm (Fig. S33). As shown in Fig. 4b, the 60 nm-thick HxWO3 nanosheets show cubic structures, while the 60 nm-thick Li+ and Na+-inserted WO3 nanosheets exhibit tetragonal structures, indicating that H+ ions with smaller sizes were easily inserted into the WO3 nanosheets to trigger obvious lattice distortion. Meanwhile, plentiful embedded ions and electrons can produce high-density polarons, which can expand the W-O-W bond angle and length of the HxWO3 nanosheets (Fig. 4c) to 3.9 Å and 180° for reducing the band gap to 1.6 eV (Fig. S34). These prominent lattice distortions were also resolved in the 15 nm-thick coloured LixWO3 and NaxWO3 electrodes (Fig. 4c, Figs. S35 and S36). These results demonstrate that the colour of the WO3 nanosheets is unrelated to the ion types, but is closely correlated with the degree and range of lattice distortion, which is positive to the qualities of the embedded ions and electrons (Fig. 4d).

In summary, taking WO3 nanosheets as an example, we have revealed the thickness-dependent electrochromic performances of coloured KxWO3 and clarified that the lattice distortion of the corner-sharing [WO6] octahedra, transitioning from the monoclinic to the tetragonal and cubic phases, is responsible for the colour center of the cathode electrochromic materials. The results of the multislice electron ptychography experiments show that the concentration of polarons increases from the bulk to the surface. This is because the inserted electrons and ions show a gradually decreasing distribution from the surface to the bulk. In addition, the band gap of the pristine WO3 electrodes can be enlarged by the formation of the tetragonal phase, which occurs through slight structural relaxation, and can be reduced by severe lattice distortion in the cubic phase. This leads to a gradient change in the macroscopic colour, ranging from yellow to blue and then to black. Furthermore, the degree and extent of the lattice distortion are positively correlated with the amount of the embedded ions and electrons. This work demonstrates that the structure-dependent optical adsorption properties can be precisely controlled by modulating the degree of lattice distortion, offering a theoretical basis for the structural design of high-performance electrochromic materials, photocatalytic materials, electrode materials, etc.

Methods
Preparation of WO3 nanosheets with a thickness of 15 nm
The WO3 nanosheets with a thickness of 15 nm were synthesized by a chemical bath deposition method. 1 mmol Na2WO4·2H2O and 1 mmol oxalic acid were added into 55 mL of nitric acid solution containing 5 mL concentrated nitric acid. After 1 h of agitation, the above suspension was transferred to a bottle with a blue cover and kept at 60 °C for 8 h. Then, the products were collected via countrifying by ethanol and water several times and dried overnight.

Preparation of WO3 nanosheets with a thickness of 30 nm
The WO3 nanosheets with 30 nm thick were obtained by a hydrothermal method. 1 mmol Na2WO4·2H2O, and 1.5 mmol citric acid were sequentially added into 33 mL hydrochloric acid solution with the concentrations of 6 M under agitation. After 30 minutes of stirring, the above suspension was transferred to a polytetrafluoroethylene reactor with a volume of 50 mL. Then, the reactor was sealed with the stainless-steel outer lining and heated at 120 °C for 24 h. The sample was centrifuged by ethanol and water several times after cooling down to room temperature and dried at 60 °C for 12 h.

Preparation of WO3 nanosheets with a thickness of 60 nm
The WO3 nanosheets with a thickness of 60 nm were fabricated via a hydrothermal method according to our previous study39,49. 1.6 mL concentrated hydrochloric acid was dropped into the mixture of 1 mmol Na2WO4·2H2O, 1.5 mmol citric acid, 5 mmol glucose, and 60 mL H2O. The mixed solution was stirred for 30 min. The above suspension was then poured into a sealed stainless-steel autoclave for hydrothermal treatment at 160 °C for 8 h. After cooling naturally, the product was collected after washing by in order of water and ethanol and drying for overnight.

Characterizations
The optical properties of the as-prepared samples were studied by UV-Vis diffuse reflectance system spectrometry (UV-Vis DRS, Varian Cary 500). The phase structures of the WO3 nanosheets were investigated by powder X-ray diffraction analysis with Cu Kα1 radiation (λ=1.5406 Å), and Raman spectroscopy (Invia Reflex equipment with excitation light wavelength of 325 nm) was employed to observe the stretching vibration of the W-O-W bond. XPS measurements (Brand: Thermo ESCALAB 250 outfitted with a monochromatized Al Kα line source) were applied to detect the surface chemical states of the pristine, colored, and bleached 15 nm-thick WO3 nanosheets without etch, which can change the raw crystal structures of the WO3-based samples. The morphologies of as-prepared WO3 nanosheets were measured by scanning electron microscopy (SEM, Helios G4). The 4D datasets were collected by a probe aberration-corrected transmission electron microscopy (AC-TEM, FEI Titan Cubed Themis G2 operated at 300 kV with a pixel array detector EMPAD), while iDPC images and EELS analysis in scan transmission electron microscopy (STEM) mode were obtained to visualize the changes in crystal structures for the pristine, colored, and bleached WO3 nanosheets with various thicknesses. The electrochromic experiments were completed by a three-electrode system (BAS Epsilon electrochemical system), where WO3 nanosheets drop-casted onto the F-doped tin oxide (FTO) substrate were applied as the working electrode, the Pt electrode was used as the counter electrode, and the Ag/AgCl electrode, in 3.0 M KCl electrolyte solution, was used as the reference electrode. Before preparing the working electrode on FTO, the FTO glasses were cleaned with ethanol and acetone solutions several times, respectively, and then dried at 60 °C. Subsequently, the samples of 2 mg were dispersed in 1 mL of ethanol under ultrasound to obtain a suspension. The suspension was dropped onto a fixed area of the FTO substrate. After drying naturally, the FTO glasses with samples were heated at 400 °C for 2 h to improve the interfacial contact between the FTO glasses and samples. For the electrochemical measurements, the electronic impedance spectroscopy (EIS) curves of the as-constructed samples were performed at the region of 200 kHz–50 mHz with Nyquist plots, and cyclic voltammetry (CV) curves were also measured in the region of -1.5–1.5 V on the BAS Epsilon electrochemical workstation. In situ optical transmittance spectra were recorded in real-time by a fiber-optical instrument from Ocean Optics (QE pro, Ocean Optics, USA). The effective optical range was from 350 nm to 1600 nm. Electrochemical measurements were carried out on a CHI 660e electrochemical workstation from CHI, CH Instruments Inc., USA.

Details of multislice electron ptychography
The acquisition conditions for the 4D data used in ptychography were set with an underfocus of 20 nm, a scan step size of 0.37 Å, a screen current of 15 pA, and a dwell time of 0.5 ms for each scan point. For the acquired 4D data, we utilized the adaptive propagator ptychography method (APP) proposed by Sha et al.44. to reconstruct the object. In this process, 5 mixed states for the electron probe were chosen, and each 128×128 diffraction pattern was expanded to 200×200 in order to obtain a more refined object function.

Details of the calculations and electrical simulations
All calculations were completed using the DFT calculations implemented in the Vienna ab initio simulation package (VASP, version 5.4.4)50,51 with the Projector Augmented Wave method52. The Perdew–Burke–Ernzerhof was used to describe the function of the exchange-correlation term53. The hybrid Heyd-Scuseria-Enrzerhof (HSE06) approximation to the exchange-correlation functional was used for calculating the electronic structures. The plane wave-off cutoff energy was set at 520 eV. The DFT-D3 correction method was used to describe the van der Waals interaction54. Convergence criteria of energy and force for all geometry structures were fixed to 10-5 eV atom-1 and 0.01 eV Å-1. The k-points were generated using the Monkhorst-Pack mesh55. For the slab model, a vacuum space was fixed as 16 Å along the z direction to minimize the interface between surface slabs.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Xue and coworkers investigated the electrochromic properties of WO3 nanosheets with varying thicknesses and their lattice\nevolution during the color variation process. The color center distribution law was analyzed with the help of theoretical\ncalculations to provide theoretical guidance for the development of high-performance electrochromic\nmaterials. However, some issues were not discussed in detail enough and there are still some data that need to be supplemented. Some\nimportant issues are attached below:\n1. Statistical significance (e.g., error bars) is needed when reporting the thickness of the nanosheets.\n2. What is the electrolyte for electrochromic testing? Three ions (K, Na, Li) are labeled in the electrolyte section of Figure 1a,\nbut the authors only discuss the properties of KxWO3. What roles do the other two ions play? Why wouldn't the other two\nions be inserted in WO3?\n3. The authors claim that the electrochemical test is conducted using a three-electrode test, but the schematic in Figure 1a\nshows a two-electrodes system.\n4. Are the WO3 nanosheets positioned flat on the FTO substrate as shown in the schematic in Figure 1a? Please add SEM\nmorphology characterization.\n5. How was the coloring/fading potential determined for the electrochromic performance test? Please add cyclic voltammetry\ntests to determine the redox potential.\n6. Different coloring potential and coloring potential also affect the color change of WO3 film, what are the specific coloring\npotential and times used for spectra testing?\n7. It is recommended to add a scale bar in digital photos to accurately reflect the size of the films.\n8. The thickness of the film also plays a significant role in determining the color and electrochromic properties. Are the\nthicknesses of the three films assembled with three different thicknesses of nanosheets the same? Please include cross-\nsectional SEM images for clarity.\n9. Does the thickness of WO3 nanosheets exhibit impact on the response speed in addition to the optical modulation?\nSuggest to add color response speed characterization.\n10. What is the UV-vis drift reflection spectrum? How were the absorption spectra obtained from the reflectance spectra test?\nWhat is the formula used for the calculation?\n11. Some of the analysis in the manuscript is too simple to understand. For example, in the statement, “From the UV-vis drift\nreflection spectra (DRS, Fig. 1b), X-ray photoelectron spectroscopy spectra (XPS, Fig. S4), energy dispersive spectroscopy\nmaps (EDS, Fig. S5), J-t plots (Fig. 1c), and inductively coupled plasma spectra (ICP, Fig. 1d) of the colored WO3, we found\nthat the thinner WO3 nanosheets exhibit enhanced optical modulation properties within the visible light spectrum in just\nseconds, a result of the insertion of electrons and K+ ions.”, how were these data analyzed and the corresponding\nconclusions drawn?\n12. How did the author obtain the “just seconds” color response? Additional in situ color change spectra are suggested.\n13. What causes the blue shift of W6+ during the coloring and bleaching processes shown in Fig. 1g?\n14. Some experiments and characterization methods in the manuscript are not clearly described. For example, XPS\ninstrument model, target material, whether it is etched or not; SEM and TEM test methods; impedance test frequency and\nvoltage, etc.\n15. What is the purpose of characterizing and comparing the properties of the nanorod film in supporting information Fig.\nS24-S27?", "reviewer_2": "The work \"Decoding the Gradient-distributed Colour Centers in Electrochromic WO3\" provides an excellent study of\nelectrochromic behaviour of WO3. With first principle calculations integrated with advanced spectroscopy, the study is\nexcellent and very well written.\n- The visualisation of gd colour centers using multislice electron ptychography and DFT+CI-NEB study is compelling and\nnovel.\nHowever, there are few drawbacks, in my opinion, that the authors should address:\n- I am not sure how dependent XRD is for thickness dependent phase transition. May be a TEM analysis will be better.\n- in the plot 3c, a proper band structure is needed, instead of a schematic. The manuscript discusses both red and blue shifts\nin absorption edges with varying nanosheet thickness but lacks a clear band diagram.", "reviewer_3": "This paper aims to explain the influence of structural and doping variations upon the electrochromic spectral properties of\nWO3 nanosheets. The authors use a very comprehensive array of analytical techniques to justify their conclusions, including\ndetailed ab initio calculations and multi-slice electron ptychography. The arguments that bring together the results are\ncomplex. My main concern is that the paper might be over-interpreting the results, especially the electron ptychography\nreconstructions, in order to report coherent story. Of course, this is a very tempting thing to do. However, I would ask the\nauthors to qualify their conclusions in the light of some of the possible ambiguities in the data.\nSome of my concerns:\n1) The authors explore the behaviour of three thicknesses of specimen: 15nm, 30nm and 60nm. Each of these is prepared\nby very different chemical processes, which also include baking at 60°C, 120°C and 160°, respectively. Are we to suppose\nthese specimens are identical in all aspects except thickness, so that comparisons between them have meaning? I would\nhave thought that a controlled experiment requires all thicknesses to be manufactured by identical procedures. The\ndifference in processing temperature alone simply must affect the structure and stoichiometry (?)\n2) I note from the review by Travis et al, (Nanoscale Adv., 2021, 3, 5166-5182): “WO3 is [also] highly complex in terms of\nhow the synthesis method affects the properties of the final material. Depending on the route used and subsequent post-\nsynthesis treatments, a wide variety of different morphologies, phases, exposed facets, and defect structures are created, all\nof which must be carefully considered for the chosen application.” Can the authors please comment on this wrt their own\nwork?\n3) The SEM images the Supp material show neat cuboids, but are the surfaces atomically flat? I find this hard to believe,\nespecially in view of the solution phase method of manufacture. I note that in Fig S3, there is clear evidence of a partially\ncomplete atomic layer in the direction perpendicular to the plane of the image. When such a surface is aligned on a zone\naxis, then the atomic plane-by-plane ptychographic analysis is called into question: it might not be wrong, but the precise\nlayer-by-layer model will be inaccurate: the reader should be alerted to this.\n4) Many of the main conclusions are dependent on the multi-slice ptychographic results. First, what exactly is the\nreconstruction algorithm which is used? The main text refers the reader to ref 27 by Sha et al, but in the reference list ref 27\nis by Tong et al. Refs 42 and 44 are by Sha et al. Which is the correct reference? The former is about adjusting for tilt\nmisalignment, the latter is about depth imaging of zeolite structures. I suppose it is 42, which describes the APP algorithm.\nHowever, the sentence “In this process, 5 mixed states for the electron probe were chosen, and each 128×128 diffraction\npattern was expanded to 200×200 in order to obtain a more refined object function.” must be explained. What were the probe\nstates for: removal of partial coherence or instabilities in the probe profile? Are these defining an orthogonal basis – if so,\nwhy were they needed? Sha et. al. do not mention inelastic scattering, which for specimens this thick must be significant,\nand possibly dominant. What is meant by “expanded to 200x200 pixels” – is this some sort of up-sampling procedure, or\nsimply padding? – and why does it refine the object estimate?\n5) On the interpretation of Figs 2a-c: I can clearly see the distortion in the W-O-W bond angles in 2b. Indeed, the projected\natomic column structure is very clear in this central slice. However, I’m confused by figs 2a and 2c. Both are remarkably\nsimilar, but also rather chaotic. Consider the central atom in the 5x5 array which is surrounded by several complex features\nwhich do not lie on any crystalline site; these patches are almost identical in 2a and 2c, as are other non-crystalline features,\nbut are entirely absent in 2b. I find it improbable that ions imbed at exactly the same (or rather equivalent) sites in the top and\nbottom regions of the specimen. The reader’s attention is instead drawn to the high-lighted atom columns top left and the\ncircled columns on the right. Or am I misunderstanding something? Could the similarity between 2a and 2c be some sort of\nartefact of the inversion process?\n6) Electron ptychography, especially multislice ptychography, is still in its infancy. By the standards of conventional imaging\nmodes, 15nm is thick – 60nm is massively thick. Most depth reconstruction experiments in the literature have been\nperformed using very heavy dopant atoms within crystals or using rather ‘open’ crystal structures (e.g. the zeolite described\nby Sha). Conventional diffraction image contrast from perfect crystals is dominated by Bloch waves – something which is\ncompletely missing from the multi-slice construct. As mentioned above, inelastic scattering must become significant as\nthicker specimens are imaged – an issue which I don’t think has yet been properly addressed. These various qualifications\nshould be spelt out in the text.\n7) Other details: Fig 1h is rather deceptive. Given that the plots have arbitrary units, it looks like the three spectra could be\nscaled to be identical. The English in the Figure captions should be checked – there are many more errors than in the main\ntext. It would help the reader to have guidance on the critical features in Fig 4c – as it stands, it is very unclear how they\ndiffer.\nI would request the authors to revise the text in the light of the above comments."}, "author_response": "Point-by-Point Responses\nManuscript Number: No. NCOMMS-25-32713\nTitle: Decoding the Gradient-distributed Colour Centers in Electrochromic WO3\nWe thank all the reviewers for their professional and in-depth comments, which have\nsignificantly improved our manuscript. We accept most of them. Corresponding changes are\nmade in the revised manuscript in red text.\nReviewer #1: P2-P12\nReviewer #2: P13-P14\nReviewer #3: P15-P22\nevolution during the color variation process. The color center distribution law was analyzed with the help of theoretical\ncalculations to provide theoretical guidance for the development of high-performance electrochromic\nmaterials. However, some issues were not discussed in detail enough and there are still some\ndata that need to be supplemented. Some important issues are attached below:\nAuthor Response:\nThank you for your positive and informative feedback. It has helped us improve the\nquality of our work. In response to your remarks, we have made the necessary adjustments to\nour text and are confident that we have fully addressed all of your concerns.\nComment 1: Statistical significance (e.g., error bars) is needed when reporting the thickness\nof the nanosheets.\nAuthor Response:\nThank you for the reviewer’s nice advice. As shown in Fig. R1 (Fig. S2), statistical\nhistograms of the WO nanosheets with thicknesses of ~15 nm, 30 nm, and 60 nm are presented.\n3\n30 nm, and 60 nm, respectively. Meanwhile, the error bars of the thickness for the WO\n3\nnanosheets were 6.5 nm, 7.4 nm, and 12.0 nm, respectively. There are 70 %, 65 %, and 75 %\nratios in the whole dataset within the error region of the 15±6.5, 30±7.4, and 60±12.0 nm,\nindicating that the statistical results of the thicknesses can be compared with each other.\nFig. R1. Statistical histograms of the thickness distribution of the as-prepared WO nanosheets with (a) 15\n3\nnm, (b) 30 nm, and (c) 60 nm thick.\nChanges made:\nError bars of the WO nanosheets have been afforded as follows:\n3\n“The highly crystalline WO nanosheets prepared via a hydrothermal process, which were\n3\ndeposited onto the surface of F-doped tin oxide (FTO) electrodes followed by annealing,\nexhibit similar thicknesses of the films (Fig. S1) and varying grain thicknesses of 15±6.5,\n30±7.4, and 60±12.0 nm (Figs. S2 and S3).”\nComment 2: What is the electrolyte for electrochromic testing? Three ions (K, Na, Li) are\nlabeled in the electrolyte section of Figure 1a, but the authors only discuss the properties of\nK WO . What roles do the other two ions play? Why wouldn't the other two ions be inserted\nx 3\n2\nin WO ?\nThank you for proposing the nice comment. In this work, we have employed KCl, NaCl,\nLiCl, and HCl as the electrolytes to measure the electrochromic testing of the WO nanosheets,\n3\nrespectively. The main difference between them lies merely in the difference of ionic radius\n(K+, Na+, Li+, and H+), while their physical and chemical properties are similar. Since the K\nelement can show stronger contrast intensity in the electron microscopy images when the ions\nare inserted into the lattice structures of host materials. To precisely investigate the influence\nof the embedded ions on the lattice structure of the host material, we selected a KCl solution\nas the prime electrolyte and mainly discussed the properties of K\nx\nWO\n3\n. Furthermore, the\nNa\nx\nWO\n3\nnanosheets were also studied by the multislice electron ptychography, and similar\nresults were obtained as the K WO (Fig. S22).\nx 3\nAs correctly pointed out, however, we have investigated the effect of the ion radius on the\nlattice distortion of the host materials, as shown in Fig. 4. With the decreased ion radius from\nK+ to H+, more ions were embedded into the WO nanosheets with deeper embedded depth,\n3\nresulting in more serious lattice distortion from monoclinic to cubic. Exemplified as the HCl\nelectrolyte, the 60 nm-thick H+-inserted WO nanosheets exhibit a cubic phase. In contrast, the\n3\n60 nm-thick Li+ and Na+-inserted WO nanosheets exhibit tetragonal structures, indicating that\n3\nlattice distortion. Their corresponding electrochromic properties were also provided in the\nsupport information (Figs. S31-35).\nComment 3: The authors claim that the electrochemical test is conducted using a three-\nelectrode test, but the schematic in Figure 1a shows a two-electrode system.\nAuthor Response:\nThank you for proposing the nice comment. We apologize for the misunderstanding\narising from Fig. 1a. As suggested by the reviewer, we have revised the schematic illustration\nof the electrochemical test system to depict a three-electrode test system.\nChanges made:\nWe have modified Fig. 1a (Fig. R3) in the revised manuscript:\nelectrons and alkali ions injected into the working electrode….\nComment 4: Are the WO nanosheets positioned flat on the FTO substrate as shown in the\n3\nschematic in Figure 1a? Please add SEM morphology characterization.\nAuthor Response:\nThank you for proposing the nice comment. We have measured the SEM morphology of\nthe WO work electrode viewed along the side. As shown in Fig. R4, we found that the 60 nm-\n3\nthick WO nanosheets were lying flat on the surface of the FTO electrode, forming a film with\n3\na thickness of 380 nm. This microscopic morphology is consistent with the schematic\nillustration of Fig. 1a.\nFig. R4. SEM image of the 60 nm-thick WO nanosheets, showing that the WO nanosheets are flat on the\n3 3\nsurface of the FTO electrode.\nperformance test? Please add cyclic voltammetry tests to determine the redox potential.\nAuthor Response:\nThank you for bringing this issue to our attention. In general, cathodic electrochromic of\nWO nanosheets deposited on the FTO electrodes in KCl produced a uniform blue coloration,\n3\nwhich intensified with increasing cathodic potential. Upon anodic polarization, these blue films\nwere bleached, becoming transparent [Adv. Mater. 2006, 18, 763-766]. The integrated cathodic\ncharge density corresponds to the number of protons intercalated into the lattice structures of\nthe WO nanosheets. The coloration process of the electrochromic is usually described as a\n3\nfunction of simultaneous injection of cations and electrons inside the oxide network, as shown\nin the following equation:\nWO (raw/bleaching)+ xM+ + xe- ↔ M WO (coloring)\n3 x 3\nWe have measured the cyclic voltammetry of the 15 nm-thick WO nanosheets. As revealed\n3\nin Fig. R5 (Fig. S5), there are obvious peaks at -0.35 and 0.73 V, which correspond to the redox\npotentials for the insertion and extraction of ions and electrons, consistent with the reported\nliterature [Chem. Mater. 2011, 23, 1105-1112].\nFig. R5 (revised Fig. S5). CV curves of the 15 nm-thick WO nanosheets with a scan speed of 20 mV s-1.\nWe have added the cyclic voltammetry curves of the 15 nm-thick WO nanosheets in the\n3\nrevised supporting information.\nComment 6: Different coloring potential and coloring potential also affect the color change of\nWO film, what are the specific coloring potential and times used for spectra testing?\nThank you for your suggestion. As noted by the reviewer, coloring potential and duration\nare key factors governing the electrochromic properties. We evaluated the light modulation\namplitude of 60 nm-thick WO nanosheets deposited on FTO electrodes under various applied\n3\npotentials and coloring durations. As shown in Fig. R6, the light modulation amplitude exhibits\na positive correlation with both coloring potential and duration, increasing to ~66% and ~78%\nat -2 V (60 s) and 120 s (-1 V), respectively. To enable direct comparison of electrochromic\nproperties across WO nanosheets of different thicknesses under identical conditions, we\n3\nselected a moderate potential (-1 V) and short duration (60 s) for subsequent experiments. This\n5\navoids saturation effects that could obscure differences between samples.\nFig. R6. Diffuse reflectance spectroscopy (DRS) spectra and the corresponding light modulation amplitude\nof 60 nm-thick WO₃ nanosheets at the wavelength of 650 nm: (a) recorded under applied potentials from 0\nV to -2 V for 60 s, and (b) recorded at -1 V for durations of 0 s, 30 s, 60 s, and 120 s.\nComment 7: It is recommended to add a scale bar in digital photos to accurately reflect the\nsize of the films.\nAuthor Response:\nThank you for your suggestion. We have added a scale bar in digital photographs in Fig.\nR7 (Fig. 1b) to display the size of the WO -based work electrodes.\n3\nthicknesses (15 nm, 30 nm, and 60 nm) and their corresponding photograph in the insets of b, demonstrating\nthat thinner nanosheets exhibit a darker color.\nand electrochromic properties. Are the thicknesses of the three films assembled with three\ndifferent thicknesses of nanosheets the same? Please include cross-sectional SEM images for\nclarity.\nAuthor Response:\nThank you for this insightful question. We have carried out the cross-sectional SEM\nimages of the three different thicknesses of WO nanosheets, as shown in Fig. R8 (Fig. S1).\n3\nthe electrodes, and the obtained films exhibit 367 nm, 370 nm, and 380 nm thicknesses,\nrespectively. These films show similar thickness, which would introduce few impacts on the\nelectrochromic properties.\nFig. R8 (revised Fig. S1). Typical SEM images for the (a) 15 nm-thick, (b) 30 nm-thick, and (c) 60 nm-thick\nWO nanosheets deposited on the electrodes along the side direction, demonstrating that these films with\n3\nflat WO nanosheets displaying a similar thickness.\nWe have added the morphologies of the WO nanosheets with various on-work electrodes in\n3\nthe revised supporting information:\n“The highly crystalline WO nanosheets prepared via a hydrothermal process, which were\n3\ndeposited onto the surface of F-doped tin oxide (FTO) electrodes followed by annealing,\nexhibit similar thicknesses of the films (Fig. S1) and varying grain thicknesses of 15±6.5,\n30±7.4, and 60±12.0 nm (Figs. S2 and S3).”\nComment 9: Does the thickness of WO nanosheets exhibit impact on the response speed in\n3\naddition to the optical modulation? Suggest to add color response speed characterization.\nAuthor Response:\nThank you for raising this insightful question. As suggested by the reviewer, we have\nmeasured the color response speed properties of the WO nanosheets with various thicknesses\n3\n(15 nm, 30 nm, and 60 nm) for different durations. As revealed in Figs. R9a-c, the optical\nmodulation amplitudes of the 15 nm-, 30 nm-, and 60 nm-thick WO nanosheets are positive\n3\nto the coloring time (as marked by red arrows). Furthermore, compared with the 30 nm- and\n60 nm-thick WO nanosheets, the 15 nm-thick WO nanosheets exhibit faster color response\n3 3\nspeed (reaching 70 % optical modulation amplitude at the wavelength of 700 nm in 30 s). Note\nthat the optical modulation amplitude of the 60 nm-thick WO nanosheets could be saturated\n3\nat the wavelength of 700 nm after 10 min. When we selected a long reaction time for eluate\nelectrochromic properties, it was difficult for us to investigate the impacts of the thickness of\n7\nthe work electrodes on the performance. Thus, we have compared the optical modulation\namplitude of the 15 nm-, 30 nm-, and 60 nm-thick WO nanosheets in 60 seconds at the bias\n3\npotential of -1 V.\nFig. R9. UV-Vis DRS spectra of the (a) 15 nm-, (b) 30 nm-, and (c) 60 nm-thick WO nanosheets under\n3\ndifferent power-on times (30 s, 1 min, 10 min, and 60 min).\nComment 10: What is the UV-vis drift reflection spectrum? How were the absorption spectra\nobtained from the reflectance spectra test? What is the formula used for the calculation?\nAuthor Response:\nThank you for your nice comment. UV-vis drift reflection technique measures light\nscattered by powdered or nanostructured samples (e.g., WO nanosheets) as opposed to direct\n3\ntransmission. Raw data is expressed as % Reflectance (R%) vs. wavelength. Absorption spectra\nare derived from DRS data using the Kubelka-Munk (K-M) function:\nF(𝑅 )=\n(1-𝑅∞)2\n=\nK\n∞\n2𝑅∞ S\nWhere R , K, and S mean the absolute reflectance, absorption coefficient, and scattering\n∞\ncoefficient. However, due to the difficulty in measuring the absolute reflectance 𝑅 , in practice,\n∞\nthe comparative reflectance 𝑅 with respect to a standard powder. The K-M model accounts\n∞\nfor both absorption and scattering in diffuse-reflecting samples. F(R) is proportional to the\nabsorption coefficient (α) and enables direct band gap analysis.\nThe band gap (Eg) is determined from F(R) via a Tauc Plot:\nn\n[F(R)*hν] =A(hν-Eg)\nWhere hν, A, n, and E respond to photon energy, proportionality constant, Transition type\ng\nexponent (2 indicating indirect band gap, 1/2 indicating direct band gap), band gap energy,\nrespectively.\nComment 11: Some of the analysis in the manuscript is too simple to understand. For example,\nin the statement, “From the UV-vis drift reflection spectra (DRS, Fig. 1b), X-ray photoelectron\nspectroscopy spectra (XPS, Fig. S4), energy dispersive spectroscopy maps (EDS, Fig. S5), J-t\nplots (Fig. 1c), and inductively coupled plasma spectra (ICP, Fig. 1d) of the colored WO , we\n3\nfound that the thinner WO nanosheets exhibit enhanced optical modulation properties within\n3\nthe visible light spectrum in just seconds, a result of the insertion of electrons and K+ ions.”,\nhow were these data analyzed and the corresponding conclusions drawn?\nAuthor Response:\nThank you for your valuable comment. We are sorry about the unclear description of the\n8\nsentences. To make it readable, the paragraph has been revised in the revised manuscript as\nfollows.\nChanges made:\n“As shown in Fig. 1b, compared with the 30 nm- and 60 nm-thick WO nanosheets, the\n3\nreflectance modulation of 96% at wavelengths of 700 nm. This indicates that the 15 nm-thick\nWO nanosheets display improved electrochromic properties than other WO nanosheets with\n3 3\na larger thickness. Furthermore, we have calculated the inserted electric charge quantities of\nthe coloured WO with various thicknesses according to the following formula:\n3\nt\nQ=∫j*A dt\n0\nwhere Q, j, A, and t parameters refer to electric charge, current density, work area, and\ntime. As shown in the J-t plots (Fig. 1c), the 15 nm-thick WO nanosheets accumulate a charge\n3\nof 1.2 C within 60 s, exceeding that of the 30 nm- and 60 nm-thick counterparts. This indicates\nthat WO nanosheets exhibiting greater light modulation (in the colored state) accommodate\n3\nmore injected electrons. X-ray photoelectron spectroscopy (XPS) spectra (Fig. S6) and energy-\ndispersive spectroscopy (EDS) maps (Fig. S7) of the coloured 15 nm-thick WO nanosheets\n3\nreveal the characteristic signals of K+ as well as their uniform distribution, demonstrating that\nthe K+ from the solution could also insert into the WO lattice under applied bias. Inductively\n3\ncoupled plasma (ICP) analysis of coloured K WO nanosheets (Fig. 1d) reveals an increase in\nx 3\nthe K+ insertion ratio from 13.83±2.84% to 19.18±0.36% as nanosheet thickness decreases\nfrom 60 nm to 15 nm. This trend correlates with enhanced optical modulation properties,\nconfirming a positive relationship between K+ quantity and electrochromic performance. These\nresults indicated that the electrochromic properties of the WO nanosheets are closely linked\n3\nto the number of inserted electrons and ions. Thinner WO nanosheets can host greater\n3\nquantities of these species, enhancing their electrochromic performance.”\nComment 12: How did the author obtain the “just seconds” color response? Additional in situ\ncolor change spectra are suggested.\nAuthor Response:\nThank you for raising this suggestion. Since we found that the work electrode could fully\ncolor under -1 V external bias in ~60 s, we described that the electrochromic process happened\nin just seconds. To avoid the misunderstanding, we have revised this sentence in the manuscript.\nLimited by the experimental equipment, we have conducted the DRS spectra of the\nsamples by employing a “quasi-in-situ” measurement method in the manuscript. Following\nelectrochromic testing in the electrochemical workstation (completed within seconds), the\nworking electrode was promptly removed and dried under a high-flow N stream. DRS was\n2\nthen immediately performed using a Varian Cary 500 spectrophotometer equipped with an\nintegrating sphere. This configuration minimizes measurement artifacts arising from light\ncollection inconsistencies during reflection analysis. Physically integrating the reaction cell\n(from the electrochemical workstation's electrolyte system) with the Varian Cary 500’s\nintegrating sphere without compromising DRS measurements presents significant challenges.\nComment 13: What causes the blue shift of W6+ during the coloring and bleaching processes\nshown in Fig. 1g?\nAuthor Response:\nThank you for raising this insightful question. As shown in Fig. R10a (Fig. 1g), compared\nwith the high-resolution W 4f XPS spectra of raw WO nanosheets, the coloured 15 nm-thick\nx 3\nspecies. This is attributed to the insertion of the electrons and K+ ions (Fig. R10b), which can\nreduce the valence states of the W6+ species, and tune the strength of the corner-sharing\nconnected O-W-O bonds. As W6+ species become W5+ species, the enhanced nuclear attraction\nstrengthens the outer electrons binding, causing the XPS peak position of W6+ species to shift\ntowards higher binding energy [ACS Appl. Mater. Interfaces 2022, 14, 2194-2201].\nWhen the colored work electrode was bleached, we found that the inserted electrons had\nbeen removed while the embedded K+ ions remained in the lattice of WO nanosheets, as\n3\nevidenced by the K 2p XPS spectrum of the bleached 15 nm-thick WO nanosheets (Fig. R10c).\nx 3\nnanosheets (Fig. R10d), we also observed that the K+ ions were located at the lattice gap of the\ncorner-sharing connected [WO ] octahedron frames. This indicated that the K+ ions can interact\n6\nwith the lattice O atoms to enhance the ion polarization effect between the O atoms and the W\natoms [J. Am. Chem. Soc. 2009, 131, 6865-6873.], leading to the further blue shift of the W6+\npeaks in the bleached WO nanosheets.\n3\n3\ncolored state. (b) Enlarged and identical phase images of the colored 15 nm-thick K WO nanosheets at 1\nx 3\nnm in depth (slice 1 is defined as 1 nm in thickness from the top surface) showing that K+ ions are inserted\ninto the A position of pseudo-K WO perovskite, as boxed by red circles. (c) High-resolution K 2p XPS\nx 3\nspectrum of bleached 15 nm-thick K WO nanosheets showcasing the existence of potassium. (d) Enlarged\nx 3\nand identical phase images of the bleached 15 nm-thick K WO nanosheets at 0 nm, 1 nm, 2 nm, 13 nm, 14\nx 3\nnm, and 15 nm in depth, showing that K+ ions are located in the bleached work electrode.\nComment 14: Some experiments and characterization methods in the manuscript are not\nclearly described. For example, XPS instrument model, target material, whether it is etched or\nnot; SEM and TEM test methods; impedance test frequency and voltage, etc.\nAuthor Response:\nWe appreciate the reviewer for the careful reading. We have completed the measurement\nconditions in the revised manuscript.\nChanges made:\n“XPS measurements (Brand: Thermo ESCALAB250 outfitted with a monochromatized\nAl Kα line source) were applied to detect the surface chemical states of the pristine, colored,\nand bleached 15 nm-thick WO nanosheets without etch, which can change the raw crystal\n3\nstructures of the WO -based samples. The morphologies of as-prepared WO nanosheets were\n3 3\nmeasured by scanning electron microscopy (SEM, Helios G4). The 4D datasets were collected\n11\nby a probe aberration-corrected transmission electron microscopy (AC-TEM, FEI Titan Cubed\nThemis G2 operated at 300 kV with a pixel array detector EMPAD), while iDPC images and\nEELS analysis in scan transmission electron microscopy (STEM) mode were obtained to\nvisualize the changes in crystal structures for the pristine, colored, and bleached WO\n3\nnanosheets with various thicknesses. The electrochromic experiments were completed by a\nthree-electrode system (BAS Epsilon electrochemical system), where WO nanosheets drop-\n3\ncasted onto the F-doped tin oxide (FTO) substrate were applied as the working electrode, the\nPt electrode was used as the counter electrode, and the Ag/AgCl electrode, in 3.0 M KCl\nelectrolyte solution, was used as the reference electrode. Before preparing the working\nelectrode on FTO, the FTO glasses were cleaned with ethanol and acetone solutions several\ntimes, respectively, and then dried at 60 °C. Subsequently, the samples of 2 mg were dispersed\nin 1 mL of ethanol under ultrasound to obtain a suspension. The suspension was dropped onto\na fixed area of the FTO substrate. After drying naturally, the FTO glasses with samples were\nheated at 400 °C for 2 h to improve the interfacial contact between the FTO glasses and samples.\nFor the electrochemical measurements, the electronic impedance spectroscopy (EIS) curves of\nthe as-constructed samples were performed at the region of 200 kHz–50 mHz with Nyquist\nplots, and cyclic voltammetry (CV) curves were also measured in the region of -1.5-1.5 V on\nthe BAS Epsilon electrochemical workstation.”\nComment 15: What is the purpose of characterizing and comparing the properties of the\nnanorod film in supporting information Fig. S26-S30?\nAuthor Response:\nWe would like to thank the reviewers for their careful review. We are sorry about the\nunclear description. Our purpose of characterizing and comparing the properties of the nanorod\nfilm in Figs. S26-S30 is to explain why the diffusion pathway of embedded ions is preferred\nfor transporting along the [001] directions.\nAs reported in previous literature, the transport directions of inserted electrons and ions\nfrom the surface to the bulk were correlated to the anisotropy and orientation of the host\nmaterials. It is necessary to exploit the effect of the surface orientation of the WO host\n3\nmaterials on the electrochromic properties. For compassion, we have synthesized WO\n3\nnanosheets, dominantly enclosed by the {001} facets, and WO nanorods, mainly exposed to\n3\nthe {010} planes, to discuss the preferred transport direction from the surface to the bulk.\nAs shown in Figs. S26-S30, the WO nanosheets show lower resistance, larger\n3\nphotocurrent density, and more stable CV curves than WO nanorods, implying that embedded\n3\nions and electrons intend to be injected into the bulk of the WO samples along the [001]\n3\ndirection. This result provides experimental evidence for the calculation of the transport energy\nbarrier of the embedded K+ ions across the different layers of WO nanosheets along the [001]\n3\ndirection.\nWO \" provides an excellent study of the electrochromic behaviour of WO3. With first-principle\n3\ncalculations integrated with advanced spectroscopy, the study is excellent and very well written.\nThe visualisation of gd colour centers using multislice electron ptychography and DFT+CI-\nNEB study is compelling and novel. However, there are few drawbacks, in my opinion, that\nthe authors should address:\nAuthor Response:\nThank you to the reviewer for the positive and insightful comments, which have attempted\nto strengthen the quality of the manuscript.\nComment 1: I am not sure how dependent XRD is for thickness dependent phase transition.\nMay be a TEM analysis will be better.\nAuthor Response:\nThank you for pointing out the issue. We employed XRD, sensitive to the crystal structure\nof crystalline materials, to characterize the crystal phase transition of WO nanosheets in both\n3\ncolored and bleached states at the macroscopic scale. As shown in Fig. 1e, XRD analysis\nreveals that the phase structures of colored K WO nanosheets gradually become tetragonal\nx 3\nand cubic as their thicknesses decrease, which is also evidenced by theoretical simulation in\nFig. S15. In addition, we substituted K⁺ with smaller Li⁺ and Na⁺ ions in the electrolyte to\nenhance ion/electron intercalation into the monoclinic WO lattice. Using XRD analysis,\n3\nnanosheets, while 30 nm and 60 nm-thick nanosheets retain a tetragonal structure. This\nconfirms a critical thickness dependence for ion/electron-induced phase transitions: thinner\nnanosheets exhibit greater susceptibility to phase change under identical coloring potentials\nand durations. However, as the reviewers considered, we are not sure how dependent XRD is\non the thickness-dependent phase transition.\nComplementary, TEM analysis can resolve the crystal structure of single-particle WO\n3\nnanosheets at the microscopic scale. By means of state-of-the-art multislice electron\nptychography, we uncovered a thickness-dependent phase transition in the 15 nm-thick WO\n3\nnanosheets at the atomic scale, enabling three-dimensional visualization of lattice structures.\nThese results demonstrate that increasing concentrations of intercalated ions and electrons from\nthe surface toward the bulk correlate with progressively severe lattice distortion.\nComment 2: In plot 3c, a proper band structure is needed, instead of a schematic. The\nmanuscript discusses both red and blue shifts in absorption edges with varying nanosheet\nthickness but lacks a clear band diagram.\nAuthor Response:\nThanks for your serious-minded review. We have measured and calculated the band\nstructures of the colored WO nanosheets via DRS measurements, as shown in the supporting\n3\ninformation (Fig. S23). The K-M plots revealed the band gaps of 2.8 eV, 3.0 eV, and 1.6 eV for\nthe 60 nm-colored monoclinic WO nanosheets, 30 nm- colored tetragonal WO nanosheets,\n3 3\nand 15 nm- colored cubic WO nanosheets, respectively (Fig. S23). These results were\n3\nconsistent with the calculated band gap of the as-constructed WO superstructures, as\n3\nillustrated in Fig. R12a (Fig. 3c). We have added the calculated band structures of the WO\n3\n13\nsupercells in Fig. R12 (revised Fig. 3).\nFig. R12 (revised Fig. 3). c. The band structures of the monoclinic WO (left panel), tetragonal K WO\n3 0.125 3\nwith additional 0.125 e/unit cell (middle panel), and cubic K WO with additional 0.125 e/unit cell (right\n0.25 3\npanel). d. Band structures of these samples from theory calculations and K-M plots, demonstrating that their\ncorresponding band gaps are higher and lower than the monoclinic WO crystal counterparts. e. The\n3\nrelationship plot between the photograph colour of electrochromic materials and their lattice distortion,\nshowing that the cathode electrochromic responds to the increased band gap, while the anode electrochromic\nexhibits decreased band gap for WO nanosheets.\nCorresponding description about the band structures analysis in the text has been revised:\n“As from the Kubelka-Munk plots (Fig. S23) and calculated band structures (Fig. 3c) of the\ncathode electrochromic samples, the band gap of the 30 nm-thick coloured K WO nanosheets\nx 3\nwith tetragonal symmetry is 3.0 eV (Fig. 3d), which is larger than that of the pristine WO\n3\nnanosheets (2.8 eV).”\nthe electrochromic spectral properties of WO nanosheets. The authors use a very\n3\ncomprehensive array of analytical techniques to justify their conclusions, including detailed ab\ninitio calculations and multi-slice electron ptychography. The arguments that bring together the\nresults are complex. My main concern is that the paper might be over-interpreting the results,\nespecially the electron ptychography reconstructions, in order to report coherent story. Of\ncourse, this is a very tempting thing to do. However, I would ask the authors to qualify their\nconclusions in the light of some of the possible ambiguities in the data. Some of my concerns:\nAuthor Response:\nThank you for your positive and informative feedback. It has helped us improve the\nquality of our work. In response to your remarks, we have made the required adjustments to\nour text and are satisfied that we have completely addressed all of your issues.\nComment 1: The authors explore the behaviour of three thicknesses of specimen: 15nm, 30nm\nand 60nm. Each of these is prepared by very different chemical processes, which also include\nbaking at 60°C, 120°C and 160°, respectively. Are we to suppose these specimens are identical\nin all aspects except thickness, so that comparisons between them have meaning? I would have\nthought that a controlled experiment requires all thicknesses to be manufactured by identical\nprocedures. The difference in processing temperature alone simply must affect the structure\nand stoichiometry?\nAuthor Response:\nThank you for the nice suggestion. We strongly agree with the reviewers’ suggestion that\nvariables should be minimized to better control influencing factors. However, the hydrothermal\nmethod cannot produce WO nanosheets reaching 15 nm thickness under optimized reaction\n3\nconditions. Therefore, we employed chemical bath deposition—a process with deliberately\nslower reaction kinetics—to achieve greater thickness control.\nIn response, we further optimized the experimental parameters of the hydrothermal\nsynthesis method, including reaction temperature and duration. As shown in Figs. R13a-c, the\nSEM images of WO nanosheets synthesized at 140 °C, 160 °C, and 180 °C exhibit thicknesses\n3\nof 50 ± 12.5 nm, 60 ±11.7 nm, and 60 ±15.8 nm, respectively. The differences in thickness are\nwithin 10 nm, falling within the margin of experimental error. Therefore, we employed distinct\nsynthesis strategies to prepare WO nanosheets with controlled thicknesses of approximately\n3\n15 nm, 30 nm, and 60 nm.\nAlthough the WO nanosheets were synthesized using different methods, all samples were\n3\nannealed in air to improve the crystallinity of the powders. We also characterized the phase\nstructures and stoichiometric ratios of the as-prepared WO nanosheets with various\n3\nthicknesses. As shown in the XRD patterns (Fig. R13d), the 15 nm-, 30 nm-, and 60 nm-thick\nWO nanosheets exhibit three characteristic peaks at 23.12°, 23.59°, and 24.38°, corresponding\n3\nto the γ-WO phase. Meanwhile, the high-resolution W 4f XPS spectra of the WO nanosheets\n3 3\nwith different thicknesses (Fig. R13e) show two distinct peaks at 35.5 and 37.7 eV, which are\nattributed to the W6+ species. No additional peaks corresponding to W5+ were observed. The\natomic ratios of W to O for the 15 nm-, 30 nm-, and 60 nm-thick nanosheets are 0.26, 0.27, and\n0.30, respectively, as determined from the survey spectra (Fig. R13f). These similar values\nindicate that the three samples are comparable and thus have minimal impact on the\n15\nelectrochromic properties. Collectively, these results demonstrate that the WO nanosheets,\n3\ndespite varying in thickness, possess identical phase structures and stoichiometric ratios after\nannealing, enabling a valid comparison of thickness-dependent electrochromic performance.\nFig. R13. Low magnification SEM images of the WO nanosheets prepared at the (a) 140 °C, (b) 160 °C,\n3\nand (c) 180 °C, showcasing that they have similar sizes (~160 nm) and thicknesses (~60 nm). (d) XRD\npatterns of the 15 nm-, 30 nm-, and 60 nm-thick WO nanosheets, demonstrating that these samples exhibit\n3\nmonoclinic structures. (e) The high-resolution W 4f XPS spectra of the 15 nm-, 30 nm-, and 60 nm-thick\nWO nanosheets, and (f) the XPS survey spectra for the WO nanosheets with different thickness, displaying\n3 3\nthat they have similar stochiometric ratios and surface chemical states.\nComment 2: I note from the review by Travis et al, (Nanoscale Adv., 2021, 3, 5166-5182):\n“WO3 is [also] highly complex in terms of how the synthesis method affects the properties of\nthe final material. Depending on the route used and subsequent post-synthesis treatments, a\nwide variety of different morphologies, phases, exposed facets, and defect structures are\ncreated, all of which must be carefully considered for the chosen application.” Can the authors\nplease comment on this wrt their own work?\nAuthor Response:\nWe appreciate the reviewer for the nice comment. As noted by the reviewer, the synthesis\nmethod significantly influences the phase structures, morphologies, and exposed facets of the\nresulting WO samples. Consequently, we characterized the phase structures and\n3\nmicrostructures of WO nanosheets of varying thicknesses synthesized by different strategies.\nand 24.38°, consistent with monoclinic structures. High-resolution TEM images (Figs. R14b-\nd) resolve periodic lattice fringes, indicating well-defined crystallinity with few defects in the\nWO nanosheets obtained via the different synthesis methods. The lattice spacings of 0.369 nm\n3\nand 0.374 nm correspond to the (200) and (020) planes along the [001] zone axis.\nCorresponding selected-area electron diffraction (SAED) patterns (insets of Figs. R14b-d)\nconfirm the single-crystalline nature of the WO nanosheets. This TEM analysis indicates that\n3\n16\nthe 15 nm-, 30 nm-, and 60 nm-thick WO nanosheets are predominantly enclosed by {200},\n3\n{020}, and {002} facets, respectively.\nConsistent with the TEM observations, SEM images (Figs. R14e-g) reveal that all\nsynthesized WO samples exhibit nanosheet-like morphologies with a lateral size of ~180 nm.\nsynthesis route. These results collectively demonstrate that the WO nanosheets, despite\n3\ndifferences in thickness achieved through different synthesis methods, possess identical phase\nstructures, defect levels, morphologies, and exposed facets.\nFig. R14. (a) XRD patterns of the WO nanosheets with thicknesses of 15 nm, 30 nm, and 60 nm. High-\n3\nresolution TEM images of the (b) 15 nm-, (c) 30 nm-, and (d) 60 nm-thick WO nanosheets along the [001]\n3\nzone axis and their corresponding selected-area electron diffraction (inset). Typical SEM images of the (e)\n15 nm-, (f) 30 nm-, and (g) 60 nm-thick WO nanosheets.\natomically flat? I find this hard to believe, especially in view of the solution phase method of\nmanufacture. I note that in Fig S3, there is clear evidence of a partially complete atomic layer\nin the direction perpendicular to the plane of the image. When such a surface is aligned on a\nzone axis, then the atomic plane-by-plane ptychographic analysis is called into question: it\nmight not be wrong, but the precise layer-by-layer model will be inaccurate: the reader should\nbe alerted to this.\nAuthor Response: Thank you for the nice comment. As illustrated in Fig. S4, the rectangular\nprism represents a schematic of the WO nanosheets, highlighting the predominant exposure\n3\nfacets of the liquid-phase-synthesized sample. Achieving atomically flat surfaces is inherently\n17\nchallenging due to the formation of intrinsic surface defects (e.g., steps, point defects, kinks),\neven if the WO samples were prepared by CVD or PLD methods. To mitigate the impact of\n3\nsuch defects on multi-slice ptychography structural analysis, we employed high-temperature\npost-annealing to enhance sample crystallinity, which is also one of the reasons why we carried\nout high-temperature annealing treatment on all the samples in advance. This treatment ensures\nthe selected analysis regions are comparable and representative. Consequently, while our\noverlay analysis characterizes a portion of the sample, it accurately reflects the properties of\nthe entire specimen. Therefore, the proposed layer-by-layer model is well-supported.\nComment 4: Many of the main conclusions are dependent on the multi-slice ptychographic\nresults. First, what exactly is the reconstruction algorithm which is used? The main text refers\nthe reader to ref 27 by Sha et al, but in the reference list ref 27 is by Tong et al. Refs 42 and 44\nare by Sha et al. Which is the correct reference? The former is about adjusting for tilt\nmisalignment, the latter is about depth imaging of zeolite structures. I suppose it is 42, which\ndescribes the APP algorithm. However, the sentence “In this process, 5 mixed states for the\nelectron probe were chosen, and each 128×128 diffraction pattern was expanded to 200×200\nin order to obtain a more refined object function.” must be explained. What were the probe\nstates for: removal of partial coherence or instabilities in the probe profile? Are these defining\nan orthogonal basis – if so, why were they needed? Sha et. al. do not mention inelastic scattering,\nwhich for specimens this thick must be significant, and possibly dominant. What is meant by\n“expanded to 200x200 pixels” – is this some sort of up-sampling procedure, or simply padding?\n– and why does it refine the object estimate?\nAuthor Response:\nThank you to the reviewer for their careful review. The correct reference in the sentence\n“For the acquired 4D data, we utilized the adaptive propagator ptychography method (APP)\nproposed by Sha et al.” corresponds to reference 42. We have revised this mistake in the text.\nThe number of probe modes is primarily used to model the partial coherence of the\nexperimental electron beam [Nature 2013, 494, 68-71; Opt. Express 2016, 24, 9038-9052; Nat.\nCommun. 2020, 11, 2994; Phys. Rev. A 2016, 94, 063621]. Selecting an appropriate number\nof probe modes is crucial for successful ptychographic reconstruction. In our experiments, we\nfound that 5 probe modes are generally sufficient to effectively model the partial coherence of\nthe electron beam.\nThe pixel size in atomic ptychography (APP) can be reduced post-acquisition by zero-\npadding the diffraction patterns, artificially increasing the maximum detector coverage angle\n[Nature 2018, 559, 343-349; Sci. Adv. 2022, 8, eabn2275; Sci. Rep. 2025, 15, 24959].\nAccording to recent literature [Nature 2024, 630, 847-852], inelastic scattering,\nprimarily\nplasmonic scattering, accounts for ~20 % of the total scattered signal in a 20-nm-thick sample.\nThis contribution is lower than that of elastic scattering, and the two exhibit distinct spatial\ndistributions at the diffraction plane. Elastic scattering strongly correlates with crystalline\ninformation, whereas inelastic scattering is spatially dispersed and carries negligible structural\ninformation. Consequently, inelastic scattering contributes a relatively uniform background in\ndiffraction patterns compared to the signal from elastic scattering.\nComment 5: On the interpretation of Figs 2a-c: I can clearly see the distortion in the W-O-W\n18\nbond angles in 2b. Indeed, the projected atomic column structure is very clear in this central\nslice. However, I’m confused by figs 2a and 2c. Both are remarkably similar, but also rather\nchaotic. Consider the central atom in the 5x5 array which is surrounded by several complex\nfeatures which do not lie on any crystalline site; these patches are almost identical in 2a and 2c,\nas are other non-crystalline features, but are entirely absent in 2b. I find it improbable that ions\nimbed at exactly the same (or rather equivalent) sites in the top and bottom regions of the\nspecimen. The reader’s attention is instead drawn to the high-lighted atom columns top left and\nthe circled columns on the right. Or am I misunderstanding something? Could the similarity\nbetween 2a and 2c be some sort of artefact of the inversion process?\nAuthor Response:\nThank you sincerely for raising this insightful and professional question. You are correct\nin observing that the reconstructed upper and lower surface layers exhibit similar structural\nfeatures. This phenomenon primarily arises from the regularization factor employed in the\nptychographic reconstruction process [J. Appl. Crystallogr, 2020, 53, 574-586;\nScience, 2021, 372, 826-831]. While regularization is essential for enhancing\nreconstruction stability, suppressing noise, and accelerating convergence, it can inadvertently\nintroduce artifacts such as periodic boundary effects. These effects may lead to a degree of\nartificial correlation or symmetry between the upper and lower surfaces in the reconstructed\nimages.\nIt is important to emphasize, however, that this regularization-induced correlation does\nnot compromise the integrity of the analysis for individual depth slices. The phase contribution\nintrinsic to each layer dominates the final reconstructed result, ensuring that local structural\ninformation—such as ion distribution and lattice distortion—within each slice remains reliable\nand physically meaningful. As shown in Fig. R15, while some ions occupy positions similar to\nthose influenced by regularization, others exhibit markedly different locations and intensities\nin slice 1 compared to slice 15 (as indicated by white arrows). These discrepancies clearly\nreflect distinct layer-specific structural responses, further corroborating our analysis regarding\nthe differences between surface and bulk phase behavior.\nFig. R15. Comparison of the reconstructed 4D-STEM images of the coloured K WO nanosheets with a\nx 3\nthickness of 15 nm at (a) 1 nm and (b) 15 nm in depth.\nFurthermore, our primary focus is on comparing the differences in ion incorporation and\nstructural response between the surface and the bulk. The observed cross-talk between the top\nand bottom layers, though present, does not alter the fundamental trend in ion concentration or\nthe resulting lattice distortion from the surface inward. The degree of octahedral distortion in\n19\nthe WO units is predominantly governed by the actual distribution of embedded ions and\n6\nelectrons, which varies systematically with depth. Therefore, the core conclusion—that the\nlayer-dependent lattice distortion correlates with the evolution of the electronic band\nstructure—remains robust and unaffected by this reconstruction artifact.\nTo address this concern transparently, we have revised Fig. S16 and included a detailed\ndiscussion of this artifact and its implications in the updated Supporting Information.\nComment 6: Electron ptychography, especially multislice ptychography, is still in its infancy.\nBy the standards of conventional imaging modes, 15nm is thick – 60nm is massively thick.\nMost depth reconstruction experiments in the literature have been performed using very heavy\ndopant atoms within crystals or using rather ‘open’ crystal structures (e.g. the zeolite described\nby Sha). Conventional diffraction image contrast from perfect crystals is dominated by Bloch\nwaves – something which is completely missing from the multi-slice construct. As mentioned\nabove, inelastic scattering must become significant as thicker specimens are imaged – an issue\nwhich I don’t think has yet been properly addressed. These various qualifications should be\nspelt out in the text.\nAuthor Response:\nThank you for this valuable comment regarding the applicability and underlying\nassumptions of multislice ptychography. The multislice formulation incorporated in iterative\nptychographic reconstruction accounts for the multiple scattering of the electron beam as it\npropagates through the sample [Journal of the Optical Society of America A, 2012, 29, 1606-\n1614; Science, 2021, 372, 826-831]. This approach serves as an approximate solution to the\nBloch wave formalism—commonly realized through the multislice method [Kirkland E J.\nAdvanced computing in electron microscopy[M]. New York: Plenum Press, 1998.]—and has\nsubstantially extended the applicability of ptychography to thicker specimens. As a result, a\nsample thickness of 15 nm is not considered particularly thick in this context. During\nreconstruction, the algorithm divides the sample into a series of thin slices, each satisfying the\nprojection or phase-object approximation, thereby enabling accurate modeling of the electron-\nspecimen interaction. In fact, successful reconstructions have been demonstrated for samples\nas thick as 100 nm, as shown in recent studies on zeolites [Sha et al., Sci. Adv., 2023, 9,\neadf1151], confirming the robustness of the method for three-dimensional structural\ncharacterization of extended volumes.\nFurthermore, multislice ptychography does not rely on the assumption of perfect\ncrystallinity, making it well-suited for studying defective or disordered materials. It has been\nsuccessfully applied to reveal complex defect structures, such as dislocations in SrTiO [Nat.\n35, 2209941], demonstrating its powerful capability in resolving local structural deviations and\nnon-periodic features.\nRegarding the influence of inelastic scattering, we refer to our response in Comment 4.\nAs highlighted in a recent quantitative study [Nature 2024, 630, 847-852], inelastic\nscattering—dominated by plasmon excitations—accounts for approximately 20% of the total\nscattered signal in a 20-nm-thick sample. Importantly, this contribution is significantly lower\nthan that of elastic scattering and exhibits a distinct spatial distribution at the diffraction plane:\nwhile elastic scattering carries high-resolution structural information and forms the basis of\n20\nimage contrast, inelastic scattering is spatially diffuse and contributes negligible structural\ndetails. Consequently, it manifests primarily as a relatively uniform background in the\ndiffraction patterns and therefore has negligible influence on the final reconstruction.\nComment 7: Other details: Fig 1h is rather deceptive. Given that the plots have arbitrary units,\nit looks like the three spectra could be scaled to be identical. The English in the Figure captions\nshould be checked – there are many more errors than in the main text. It would help the reader\nto have guidance on the critical features in Fig 4c – as it stands, it is very unclear how they\ndiffer. I would request the authors to revise the text in the light of the above comments.\nAuthor Response:\nThank you for the careful review. The background intensities of the EELS spectra for the\npristine, coloured, and bleached WO nanosheets in Fig. R16 (revised Fig. 1h) have been\n3\nnormalized to compare the relative intensities of the characterized peaks. We have revised the\nhorizontal figure caption to normalized intensities.\nWe sincerely thank the reviewer for pointing out the grammatical and typographical errors\nin the figure captions. We acknowledge that the English in the captions requires careful\nattention. We have now meticulously checked and revised the English in all figure captions\nthroughout the manuscript to ensure correctness and clarity. We apologize for any\ninconvenience caused by these errors in the original submission. The revised captions are\nprovided in the updated manuscript.\nFig. R17c (revised Fig. 4c) shows iDPC images of the 15-nm-thick coloured WO\n3\nnanosheets with embedded H+, Li+, and Na+. They uncovered that the positions of the\nembedded ions were located at the lattice gap of the cubic WO to form H WO , Li WO ,\n3 x 3 x 3\nand\nNa WO products (as highlighted by red dashed circles), which were similar to the K WO\nx 3 x 3\ncounterparts. With the smaller ion radius, more weak intensities were observed at the lattice\ngap, implying that more ions were inserted into the bulk of the WO nanosheets. These\n3\nstructures show that the O-W-O bond angles of the cubic H WO , Li WO , and Na WO\nx 3 x 3 x 3\nsamples increased to 180°, indicating that the lattice of the corner-sharing connected [WO ]\n6\noctahedron can also be distorted, although the radius of the ions was smaller. These results\ndemonstrate that the electrochromic process of the WO nanosheets was closely correlated with\n3\nthe degree and range of lattice distortion, which is positive for the qualities of the embedded\nions and electrons. However, H, Li, and Na possess significantly lower atomic numbers than\nK, resulting in weak scattering intensities in iDPC-STEM images. This makes them difficult to\nresolve, particularly hydrogen. Consequently, the iDPC images show minimal discernible\ndifferences between these light elements, especially for H and Li.\nFig. R17 (revised Fig. 4) c iDPC images of the coloured H WO (left panel), Li WO (middle panel), and\nx 3 x 3\nNa WO (right panel) nanosheets with a thickness of 15 nm, demonstrating that the embedded H+, Li+, and\nx 3\nNa+ ions induce the lattice distortion of the corner-sharing connected WO structures to form polarons,\n3\nrespectively.\nAgain, we thank all the reviewers for your constructive comments that help to improve\nour manuscript!\n22"}, {"round": 2, "reviewer_comments": {"reviewer_1": "Although the manuscript was revised by the authors, there are some issues that need to be addressed as follows.\n1. In Fig. R9, the testing and description of response speed are confusing and methodologically unconvincing. How does the\nabsorption spectrum at different time points indicate response speed? What evidence supports the claim that the 15 nm\nnanosheet exhibits a faster response speed? What is the unit on the Y-axis of the absorption spectrum? Is the endpoint of\nthe coloring process 60 seconds or 60 minutes? In which section of the main text is this information described? The authors\nshould provide comprehensive experimental details and rigorously conducted in situ kinetic analyses to support their claims.\n2. The response speed testing method employed by the authors is inadequate and fails to accurately capture the dynamic\nresponse characteristics of the materials. This is particularly problematic for materials that require response times in the\nseconds range. Furthermore, the fading effect that occurs in materials with limited memory retention during the drying and\nmeasurement stages introduces additional inaccuracies. It is strongly recommended that in-situ spectroelectrochemical\nmeasurements be performed instead, by integrating the electrochemical workstation with a spectrophotometer for testing in a\ncuvette. Real-time collection of spectral data at a specific wavelength during chronoamperometry (CA) would provide direct,\naccurate insight into the response speed and intermediate states of the film under voltage application.\n3. Most of the References are 4 years ago's, the latest works focus on WO3 electrochromic films and devices recommended\nto the authors for discussion.", "reviewer_2": "The work gives a very good example of atomic-resolution mechanistic elucidation applied to a technologically important\nmaterial. It offers novel experimental and theoretical insights. I am also satisfied with the author's reply to my queries.\nThe manuscript will be of broad interest to the electrochromic, energy materials, and microscopy communities.\nI recommend this for acceptance.", "reviewer_3": "In answer to 'replies to referees':\nThe authors have refuted the issues I raised without making substantial changes to the text, instead drawing attention to\nresults in other papers, as well as a couple of new citations they have included. My own opinion is that multislice\nptychography still poses open questions, similar to the status of high-resolution imaging in the early days of TEM. Some of\nthese (e.g. the ‘Stobbs factor’) were never unambiguously resolved. However, that didn’t stop people doing TEM,\neven though certain results were later disputed.\nThe question is whether we can use the existing state-of-the art of ptychography to assert definitive scientific conclusions.\nThe authors have optimised their experiments and calculations based on current literature, and so I admit that the main\nconclusions are as reliable as possible at this time.\nI would therefore recommend publication."}, "author_response": "Point-by-Point Responses\nManuscript Number: No. NCOMMS-25-32713A\nTitle: Decoding the Gradient-distributed Colour Centers in Electrochromic WO3\nWe thank all the reviewers for their professional and in-depth comments, which have\nsignificantly improved our manuscript. We accept most of them. Corresponding changes are\nmade in the revised manuscript in red text.\nReviewer #1: P2-P3\nReviewer #2: P4\nReviewer #3: P5\nAlthough the manuscript was revised by the authors, there are some issues that need to be\naddressed as follows.\nComment 1: In Fig. R9, the testing and description of response speed are confusing and\nmethodologically unconvincing. How does the absorption spectrum at different time points\nindicate response speed? What evidence supports the claim that the 15 nm nanosheet exhibits\na faster response speed? What is the unit on the Y-axis of the absorption spectrum? Is the\nendpoint of the coloring process 60 seconds or 60 minutes? In which section of the main text\nis this information described? The authors should provide comprehensive experimental details\nand rigorously conducted in situ kinetic analyses to support their claims.\nAuthor Response:\nThank you for the reviewer’s professional comment. Because this comment is highly\nrelevant to the next comment, and we have performed the in-situ spectroelectrochemical\nmeasurements to assess the response speed. Therefore, we have removed the non-in-situ UV-\nVis DRS spectra of the 15 nm-, 30 nm-, and 60 nm-thick WO nanosheets under different\n3\npower-on times (30 s, 1 min, 10 min, and 60 min), i.e., Fig. R9 of the previous Responses\ndocument. For the relevant discussion, please refer to the next comment.\nComment 2: The response speed testing method employed by the authors is inadequate and\nfails to accurately capture the dynamic response characteristics of the materials. This is\nparticularly problematic for materials that require response times in the seconds range.\nFurthermore, the fading effect that occurs in materials with limited memory retention during\nthe drying and measurement stages introduces additional inaccuracies. It is strongly\nrecommended that in-situ spectroelectrochemical measurements be performed instead, by\nintegrating the electrochemical workstation with a spectrophotometer for testing in a\ncuvette. Real-time collection of spectral data at a specific wavelength during chronoamperometry (CA)\nwould provide direct, accurate insight into the response speed and intermediate states of the\nfilm under voltage application.\nAuthor Response:\nThank you for the reviewer’s valuable comments. As suggested, we have performed in\nsitu spectroelectrochemical measurements on the as-prepared 15 nm-, 30 nm-, and 60 nm-thick\nWO nanosheets. Specifically, the WO working electrodes were assembled in a three-electrode\n3 3\nsystem sealed in a cuvette, with a Pt counter electrode and a calomel reference electrode in a\n0.1 M KCl electrolyte. According to the results of Figs. 3c and S24, real-time optical\ntransmittance spectra at 410 nm, 440 nm, and 770 nm were recorded using a fiber-optic\nspectrometer (QEpro, Ocean Optics, USA) during chronoamperometry (CA), with an effective\nspectral range of 350–1600 nm. The electrochemical tests were conducted using a CHI660e\nelectrochemical workstation (CH Instruments Inc., USA).\nThe in-situ transmittance measurements demonstrate that the optical response kinetics of\nWO₃ nanosheets are strongly dependent on their thickness. As shown in Figs. R1a–R1d, the 15\nnm-thick nanosheet exhibits a significant and rapid response at 410 nm and 440 nm within 5\nseconds. In contrast, the transmittance changes for the 30 nm and 60 nm nanosheets are slower,\n2\ninitiating after 7 s and 10 s, respectively. Furthermore, the magnitude of the transmittance\nincrease at these wavelengths follows the order: 15 nm > 30 nm > 60 nm. This faster kinetics\nin thinner nanosheets is further corroborated at 770 nm (Figs. R1e-f), where the onset of\ntransmittance decrease occurs at 7 s for the 15 nm sample, notably earlier than for the 30 nm\n(10 s) and 60 nm (~30 s) samples. These findings demonstrate that the 15 nm-thick WO\n3\nnanosheets possess a faster optical response than their thicker counterparts.\nFig. R1 (revised Fig. S8). In-situ transmittance spectra and their corresponding magnified spectra from the\ndashed boxes of the as-prepared 15 nm-, 30 nm-, and 60 nm-thickness WO nanosheets at (a-b) 410, (c-d)\n3\n440, and (e-f) 770 nm wavelengths as a function of power-on times under a –2 V bias.\nChange mode:\n“In-situ optical transmittance results at 410, 440, and 770 nm reveal that the 15 nm-thick WO\n3\nnanosheets possess a faster optical response than their thicker counterparts (Fig. S8). These\nresults indicate that the 15 nm-thick WO nanosheets display improved electrochromic\n3\nproperties than other WO nanosheets with a larger thickness.”\n3\nelectrochromic films and devices recommended to the authors for discussion.\nAuthor Response:\nThank you for the reviewer’s suggestion. We have added some References in the 4 years in the\nintroduction as follows:\n8. Fan, Q. et al. Dynamic thermoregulatory textiles woven from scalable‐manufactured radiative\nelectrochromic fibers. Adv. Funct. Mater. 34, 2024, 2310858.\n12. Guo, J. et al. Fast-switching WO -based electrochromic devices: design, fabrication, and applications.\n13. Shao, Z. et al. Tri-band electrochromic smart window for energy savings in buildings. Nat. Sustain. 7,\n2024, 796-803.\n50. Shen, M. et al. Engineering the Atomic Configurations of Surface-Active Sites for Retuning the\n2\nThe work gives a very good example of atomic-resolution mechanistic elucidation applied to a\ntechnologically important material. It offers novel experimental and theoretical insights. I am\nalso satisfied with the author's reply to my queries.\nThe manuscript will be of broad interest to the electrochromic, energy materials, and\nmicroscopy communities.\nI recommend this for acceptance.\nAuthor Response:\nWe sincerely thank the reviewer for their constructive comments and valuable suggestions,\nwhich have significantly improved the quality of our manuscript.\nIn answer to replies to referees': The authors have refuted the issues I raised without making\nsubstantial changes to the text, instead drawing attention to results in other papers, as well as a\ncouple of new citations they have included. My own opinion is that multislice ptychography\nstill poses open questions, similar to the status of high-resolution imaging in the early days of\nTEM. Some of these (e.g. the “Stobbs factor”) were never unambiguously resolved. However,\nthat didn’t stop people doing TEM, even though certain results were later disputed. The\nquestion is whether we can use the existing state-of-the art of ptychography to assert definitive\nscientific conclusions. The authors have optimised their experiments and calculations based on\ncurrent literature, and so I admit that the main conclusions are as reliable as possible at this\ntime. I would therefore recommend publication.\nAuthor Response:\nWe thank the reviewer for their positive assessment and recommendation for acceptance.\nWe have found that multi-slice ptychography, while powerful for resolving atomic structures,\nfaces challenges such as high computational cost, reliance on simplified physical models, and\nalgorithmic complexities. In future work, we plan to address these by employing advanced\ncomputing hardware, refining the physical model, implementing more sophisticated algorithms,\nand enhancing experimental stability. Our ultimate goal remains the achievement of atomic-\nresolution imaging along the beam direction.\nAgain, we thank all the reviewers for your constructive comments that help to improve\nour manuscript!\n6"}, {"round": 3, "reviewer_comments": {"reviewer_1": "The authors did good job of responding to my comments - this paper should be published."}, "author_response": ""}]

Decision letter