Fullerene-decorated PdCo nano-resistor network hydrogen sensors with sub-second response and parts-per-billion detection at room temperature

Domain: physics
Published: 2025-12-19
Editorial outcome: major_revision

Abstract
Hydrogen detection with rapid response and ultra-low detection limits remains a critical challenge for safety and energy applications. Here, we report a fullerene-decorated PdCo nano-resistor network sensor that integrates nanostructuring, alloying, and surface-engineering approaches. The C60 layer enhances sensor performance by increasing the surface-to-volume ratio, enabling fast hydrogen diffusion, relieving mechanical stress during cycling, and guiding nanostructure morphology. Our composite device (20 nm C60 / 3 nm Teflon AF / 5 nm Pd63Co37 / 30 nm Teflon AF) achieves a response time of 0.40±0.06 s across 1–100 mbar H2 and detects 40 ppb H2 with a signal-to-noise ratio of 10 at room temperature. A poly(methyl methacrylate) (PMMA) top coat further improves cycling stability and selectivity under 90% relative humidity and interfering gases. This design provides a scalable approach and opens the door to future adaptation of porous carbon-based frameworks and polymeric interlayers to realize robust, high-performance hydrogen sensors for real-world applications.

Introduction
Hydrogen plays a central role in clean energy technologies and industrial processes, yet its colorless, odorless nature and flammability require rapid and reliable detection systems1–4. The sensors must feature a response time (t90) of ≤1 s for H2 concentrations ranging from 0.1 to 10 vol.% (equivalent to H2 partial pressure of 1–100 mbar), a limit of detection (LOD) in the range of 1000 parts per million (ppm) for automotive and 10 parts per billion (ppb) for environmental monitoring applications5,6.

Recent advancements across both electrical and optical platforms in achieving ultra-fast response times and ppb-level sensitivity in hydrogen sensors have centered on nanostructured Pd hydride due to their high selectivity, excellent sensitivity, fast response times, and mechanical and chemical robustness (see Supplementary Table 1)7–13. Pd-based alloys (e.g., PdCo, PdAu) dominate high-impact designs. For instance, recent works on optical sensors by Nugroho et al. using scattering spectrum from PdAu nanoparticle@polymer nanocomposite materials7, and Luong et al. using transmission intensity through PdCo nanocap arrays10 explicitly demonstrate sub-second response with LODs around single-digit ppm. The LOD could be further improved to a few hundred ppb by making use of the particle swarm optimization algorithm to optimize the sensor array configurations11, neural-network-based data treatment to correct the baseline drift while testing in extreme humidified conditions14, or by simply stacking the sensors vertically to improve the signal-to-noise ratio (SNR)12. Compared to optical-based sensors, electrical transducing method shows great promise for achieving an ultra-sensitive LOD of a few ppb by virtue of the low noise signal readouts. Pham et al. proposed a PdCo composite hole array (CHA) nano-network with a measured LOD of 180 ppb; however, the response time fell short of the 1-s response time benchmark required for automotive applications9. Pd-doped oxides (e.g., Pd@Fe2O3 nanotubes) combine Pd’s catalytic activity with oxide heterojunctions, achieving 50 ppb LOD through synergistic surface reactions and p–n junction modulation15. Non-Pd materials, such as rGO/ZnO-SnO2 composites, also reach 50 ppb sensitivity via Pd doping and heterostructure engineering, though they lag behind Pd hydrides in speed and require high temperature operation of >300 °C16. These strategies highlight the critical role of alloying, nanostructuring, and hybrid material design tailoring for each transducing method and sensing mechanism in pushing the detection speed and sensitivity limits, with Pd-based systems remaining the gold standard for safety-critical applications. To our best knowledge, no single sensor reported to date has simultaneously achieved both ≤1-s response time and a ppb LOD operating at room temperature (Supplementary Fig. 1). Achieving ppb or sub-ppb detection limits not only benchmarks sensor performance under noise-limited conditions requiring high SNR and advanced signal processing14,17, but also enables critical applications such as trace hydrogen detection in deep-space missions18, the monitoring of low-abundance hydrogen isotopes like deuterium and tritium in nuclear environments19, and breath tests for assessing gastrointestinal health20,21.

In this study, we present a state-of-the-art hydrogen sensor based on a hexagonal nano-resistance network of PdCo alloy fabricated on a fullerene C60-coated glass substrate. This design incorporates four synergistic strategies to simultaneously achieve ultra-fast response and ppb sensitivity. First, the PdCo alloy was selected based on prior optimization studies demonstrating faster kinetics than pure Pd or Pd alloys such as PdAu and PdAg under identical structural and testing conditions10. In addition, the Co incorporation increases the strain-induced energy barrier to form the hydride effectively suppressing the hysteresis typically seen in pure Pd hydrides10,22. Second, sandwiching the C60 interlayer between the PdCo and the glass substrate, known to be inert to hydrogen chemisorption and physisorption at low pressures23,24, offers multiple functional benefits: (i) effectively doubles surface-to-volume ratio (SVR) of the sensing element thanks to the nanoporous structure that facilitates the hydrogen sorption processes through the C60 and PdCo interface25; (ii) it acts as a mechanical decoupler, enabling stress-free volumetric changes during hydrogenation cycles and enhancing device stability26,27; (iii) it serves as a morphological template, guiding the formation of the PdCo nanostructure; and (iv) it provides a chemically and mechanically robust foundation compatible with vacuum deposition28. Third, the nanostructuring strategy, combining nanosphere lithography (NSL) and glancing angle co-deposition (GLACD), produces a CHA structure with high SVR. This not only enhances hydrogen sorption kinetics29 but also amplifies resistance changes through increased electron scattering at grain boundaries and surfaces at the bottle-neck of the sensing elements9. In addition, the GLACD process further promotes alloy formation under far-from-equilibrium conditions, yielding defect-rich films with abundant active diffusion pathways30. Lastly, surface modification with Teflon AF (TAF) reduces the activation energy for hydrogen sorptions, accelerating response time while maintaining signal strength7. Together, these four integrated strategies enable a sensing platform that achieves sub-second response times and ppb-level hydrogen detection under ambient conditions.

Our results in vacuum-mode measurement demonstrate that 20 nm C60/5 nm Pd63Co37/30 nm TAF CHA sensors with a hole diameter of 450±4 nm and a 500 nm array period achieve a response time of ≤0.8 s over a 1–100 mbar H2 pressure range, with a measured LOD of 144 ppb and an extrapolated LOD of 40 ppb. Remarkably, when the PdCo sensing layer is sandwiched between two TAF layers, the sensor can detect H2 as fast as 0.40±0.06 s within the same pressure range, boasting the measured LOD down to 40 ppb with an exceptional SNR of 10. This C60/TAF/Pd63Co37/TAF stacked architecture holds promise for detecting hydrogen at single-digit ppb and sub-ppb levels. When coated with Poly(methyl methacrylate) (PMMA), the sensor demonstrates resilience against interference gases such as CO2, CH4, CO, maintains performance at up to 90% relative humidity (RH), and remains robust over hundreds of hydrogen cycling events using N2 as the carrier gas. This low-cost, lightweight, and energy-efficient platform offers a viable path toward meeting key performance metrics for automotive safety, concentration control, and environmental monitoring. These findings also highlight the broader potential of employing interlayers such as carbon-based materials, metal-organic frameworks and polymers with tunable gas permeability for designing next-generation hydrogen sensors.

Results
The fabrication scheme of a PdCo CHA on fullerene C60-decorated glass substrate is depicted in Fig. 1a9,31,32, and is described in detail in the “Methods” section. First, a sacrificial nanosphere mask was prepared, and this involved depositing a monolayer of 500±10 nm polystyrene (PS) nanospheres onto the substrates in a hexagonal close-packed arrangement. Subsequently, the size of the nanospheres in the monolayer was reduced via an oxygen plasma reactive ion etching (RIE) process by controlling the etching time tRIE. These substrates were then used as templates for C60 and PdCo alloy depositions in the later steps, and the resultant hole size of the CHA was determined by tRIE. Regarding the hydrogen sensing layer, we investigated a series of PdCo alloy compositions with palladium content ranging from 80% to 55%. Among these, the Pd63Co37 composition offered the best trade-off between response time and sensitivity12, and is therefore the primary focus of this study. A detailed discussion of this compositional optimization is provided in the later part of the article. The thicknesses of C60 and PdCo are specified in each case with the corresponding atomic force microscope (AFM) images. For brevity, we use the notations PdCoCHAtRIE and C60/PdCoCHAtRIE to refer to Pd63Co37 and C60/Pd63Co37 CHAs, with t representing the RIE etching time in seconds. Sensors coated with TAF (and PMMA) are labeled as C60/PdCo/TAF(/PMMA)CHAtRIE.

The nano-architecture of the sensor was verified by a scanning electron microscopy (SEM) image of one of the devices (20 nm C60/5 nm PdCoCHA450) (Fig. 1b) and its atomic composition Pd63Co37 was confirmed by energy-dispersive spectroscopy (EDS) elemental mapping and line scans (Fig. 1c and Supplementary Fig. 2). The measured weight percentages of C, Pd, and Co were 79.5%, 15.4%, and 5.1%, respectively, corresponding to an atomic ratio of 63:37 between Pd and Co (see conversion Supplementary Table 3). The detailed experimental setups for the hydrogen sensing characterization were described in “Methods” and Supplementary Note 3. To mitigate contact and wire resistances, a four-point probe measurement technique was used throughout the paper (Fig. 1a).

To investigate the role of the buffer layer C60, we initially assessed the sensing performance of 15 nm PdCo CHA450 with and without 50 nm C60 underneath (Fig. 2). Both sensors show linear current-voltage (I-V) characteristics, in which the measured resistances (R) of the sensors with and without C60 in vacuum are 130.8 Ω and 85.3 Ω, respectively (Fig. 2a). The difference in R can be attributed to slight morphology variations in the bottle-neck regions of the two CHAs (Supplementary Fig. 7) and the interface differences between the substrate (glass or C60) and the PdCo layer. Figure 2b shows the sensors’ response to a step-wise H2 pulse of 10 mbar. Here, the percentage change in R or sensitivity of the electrical sensor as a function of PH2 is defined as: ΔR/R (%) = (R(H2) - R(0))/R(0) (1) where R(H2) and R(0) are resistances of the device with and without the presence of H2 gas, respectively. The hydrogen uptake of pure C60 at low H2 pressure and room temperature is negligible23, indicating that the change of R during (de)hydrogenation is primarily due to hydrogen sorption by Pd (Fig. 2b). The positive ΔR/R observed in both sensors indicates that electron scattering dominates the resistance change mechanism, as compared to hydrogen-induced lattice expansion mechanism during hydrogenation, which typically reduces sensor resistance33,34. In this case, more scattering centers from the absorbed hydrogen atoms at the interstitial sites were introduced to the system, causing the sensors’ resistance to increase. To quantify detection speed, absorption time (t90) and desorption time (t10) of the sensors at varying PH2 are extracted from resistance dynamics as denoted in Fig. 2b. The commonly used t90 for the electrical sensors is defined as the elapsed time from the baseline resistance to 90% of the ΔR/R saturation, and t10 is defined as the elapsed time from the saturated level down to 10% of ΔR/R. The sorption kinetics of C60/PdCoCHA450 are noticeably accelerated as the t90 of the C60/PdCo sensor reduce by a factor of >4 over a pressure range of 1–100 mbar compared with those of PdCo sensor (Fig. 2c). In particular, t90 of C60/PdCo is only 5.6 s at PH2 = 1 mbar, versus 24.7 s of PdCo. Remarkably, it takes ≤1 s for C60/PdCo CHA to detect PH2 ≥10 mbar, which is lower than the lower limit of flammable H2 concentration in air (4 vol.% or 40 mbar). Additionally, the time taken for C60/PdCo sensor to release H2 across the same pressure range is more than twice as fast as that of the PdCo sensor (Supplementary Fig. 7). The acceleration in the sensor’s response time with the C60 interlayer may be attributed to: (i) the crystallographic structure of PdCo thin films on different substrates, or (ii) rapid H2 diffusion through the porous C60 walls in the CHA architecture, or (iii) a reduction in the activation energy barrier for H2 adsorption at the C60/PdCo interface. To gain insight into the contribution of C60 and examine the plausibility of each hypothesis, we conducted X-ray Diffraction (XRD) to analyze the crystal structure (Supplementary Fig. 10) and carried out a controlled experiment using PdCo sensors with a C60 top-coating (Supplementary Fig. 13). First, PdCo deposited on both glass and C60-coated substrates exhibits a similar crystallographic structure, with the dominant Bragg diffraction peak appearing at ~41.6° (Supplementary Fig. 10b), corresponding to the (111) orientation of the face-centered cubic (fcc) PdCo lattice35. The negligible differences in the lattice strain and lattice constant (Supplementary Table 4) make the first hypothesis unlikely. For the second hypothesis, a previous study by Samad et al. reported that the porosity of thermal-evaporated C60 thin film can exceed 50% when deposited at room temperature36. This aligns with the absence of distinct peaks in the XRD spectrum of C60 thin film (Supplementary Fig. 10a), confirming its amorphous nature and lack of long-range crystalline order of both the C60 film and the glass substrate25,37. Moreover, the nanohole structure opens up several new pathways for H2 access to PdCo sensing layer through the sidewalls of the C60. Any defect sites or metal-fullerene complexes that have micropores larger than the hydrogen molecules would facilitate the H2 permeation23. These combined properties allow hydrogen gas to access the PdCo transducer layer not only from the top surface, as in conventional sensor architectures, but also from the bottom interface via the underlying porous C60 film illustrated in Supplementary Fig. 8. This dual-sided accessibility increases effective gas exposure to the sensing material. The SVR is effectively doubled, reducing the average hydrogen diffusion path within the PdCo network and substantially improving the sensor’s response time compared to the sensor without C60 underneath. The third hypothesis is ruled out, as no enhancement in sensing performance was observed when a 20-nm C60 layer was deposited on top of a PdCo sensor (Supplementary Fig. 13). Thus, we can conclude that the enhancement of the response time is solely derived from the increase of the sensing element’s SVR by introducing a buffer layer C60. In Fig. 2c, the pressure-dependent sorption times of the C60/PdCoCHA450 sensor follow Sievert’s power-law, t90 ∝ PH2n38, as confirmed by a near-linear dependence between log(t90) and log(PH2) with a similar exponent (or the slope of the linear fit) n = −0.65±0.01 as the PdCo CHA450, indicating similar sorption behavior or energy barrier for hydrogen adsorption/desorption in the two sensors, consistent with our previous conclusion. Figure 2d illustrates the sorption isotherms which present the sensitivity ΔR/R of each sensor across five orders of magnitude of PH2, from 101 to 106 μbar, at room temperature. The hysteresis-free sensitivity of PdCo CHA450 is maintained in C60/PdCoCHA450, as confirmed by accuracy calculations (Fig. 2d inset and Supplemental Note 4). All sensors exhibit accurate readings with <3.0% uncertainty over a wide H2 pressure range thanks to the increase of the strain-induced energy barrier caused by the enhanced structural dislocation in the PdCo system7,10,12.

Next, the dependence of CHA450’s sensing response on the thickness of the C60 layer was studied (Fig. 3). In this case, 5-nm-nominal thick of Pd63Co37 was utilized as a sensing layer and was deposited on substrates coated with 5-nm, 20-nm, and 50-nm thick of C60. Figure 3a shows typical AFM images of C60/5 nm PdCoCHA450 sensors etched at tRIE = 450 s, resulting in an average hole diameter of Dhole = 350±5 nm (extracted from the line profiles of the AFM images, Fig. 3b). It is worth noting that the polystyrene remnants from the PS beads, left at the center of the holes after the lift-off process, did not interfere with the electrical signals of the devices. The baseline resistances of C60/5 nm PdCoCHA450 sensors were obtained from the linear I-V characteristics (Fig. 3c), with an average resistance of R = 1100±10 Ω. The C60/5 nm PdCoCHA450 sensors showed a slight improvement in response time and sensitivity (t90 < 3.7 s and ΔR/R > 0.8% at PH2 = 1 mbar) compared to C60/15 nm PdCoCHA sensor, by virtue of the enhancement of reaction kinetics through nanostructuring29. Interestingly, CHA450 sensors with different C60 thicknesses exhibited comparable t90 and t10 values (Fig. 3d and Supplementary Fig. 12). These findings further support our assertion that hydrogen diffusion through C60 is rapid and not rate-limited in our nano hole-array structure (with a few hundred nm of the hydrogen diffusion path in C60). It is feasible to locally probe the diffusion rate of the C60/PdCo bilayer using optical techniques39 and extract the response times at different locations on the films such as near the edges or at the center, such analysis is beyond the scope of the present study. Among the three CHA450 sensors tested, the 20 nm C60/5 nm PdCo CHA450 sensor exhibited the highest sensitivity, which is required for achieving a low LOD. As a result, in the following sensor design, we will maintain the nominal thicknesses of C60 and PdCo layers at 20 nm and 5 nm, respectively, while varying tRIE to achieve a smaller feature size and, consequently, a larger SVR – a key factor for ultra-fast sensing responses.

Hole size-dependent sensing performance
In the combined NSL and GLACD method10, the nanohole diameters (Dhole) or the sizes of the sensing network elements, especially the bottle-neck feature sizes, can be controlled by adjusting the etching time tRIE of the PS beads (Fig. 4). Figure 4a depicts the top-view AFM images of CHAs with varying tRIE from 600 s to 160 s. When tRIE < 160 s (Supplementary Fig. 14), a nanotriangle array was achieved instead of a nanohole array, resulting in a discontinuous nano-network. Additionally, an in-house GLACD simulation code with MATLAB40–42 was utilized to model the morphology and thickness of each layer (C60 and PdCo) in each CHAtRIE sensor (see Supplementary Note 7), with the results shown in Fig. 4b, c. The cross-sectional view from GLACD simulations in Fig. 4c aligns well with AFM line profiles, confirming that the C60 layer is mechanically stable enough to resist penetration by high-energy metal atoms during deposition. Moreover, C60 functions as a critical morphological template, effectively shaping the geometry of the resulting PdCo nanostructures. Morphologically, the hole diameter decreases linearly with tRIE (Fig. 4d). At smaller tRIE values (<300 s), where the bead size after RIE etching exceeds 400 nm, the shadow effect becomes more pronounced, resulting in a U-shaped indentation in the middle of the bottle-neck region, leading to a reduction in the total thickness in this region. The simulated cross-sections along and across the bottleneck region of CHAtRIE, along with the thicknesses of PdCo layer (hPdCo) projected on a flat surface, are presented in Supplementary Fig. 15. In some regions, the estimated hPdCo can be as thin as 2 nm, and the actual hPdCo might be even thinner than the simulation suggests (Fig. 4c). Moreover, the SVR of the PdCo layer is significantly enhanced as tRIE decreases, due to PdCo being deposited in narrower gaps between the PS beads and on the undulating C60 surface. The detailed SVR calculations are presented in Supplementary Note 7.2, and all parameters of 20 nm C60/5 nm PdCoCHAtRIE are summarized in Supplementary Table 5. In particular, CHA185 has a SVR of ~0.75 nm−1, which is 1.5 times larger than that of CHA450 (0.5 nm−1).

The changes in morphology significantly influence the resistances and the sensing performance of the devices, as illustrated in Fig. 4e–g. As the hole diameter increases, the feature size of the resistor network decreases, leading to higher resistance. Figure 4d, e shows that the sensor’s resistance increases slightly when Dhole increases from 310 nm to 410 nm. In this range, the thickness of the network element does not change, as depicted in Fig. 4c. Therefore, the slight increase in resistance is attributed to the reduction in sensor bottleneck width. However, when Dhole increases from 410 nm to 460 nm, a U-shaped indentation forms, causing the network’s resistance to increase by nearly three orders of magnitude. A closer inspection reveals that the thickness of the triangular regions (the ends of the resistance network element, see Supplementary Fig. 16) does not change with Dhole, keeping the resistivity nearly constant in those regions. Consequently, the total resistance of the network is dominated by the resistance at the bottleneck, Rb, due to the reduction in thickness because of the shadowing effect as seen in Fig. 4c. The response of the electric current during (de)hydrogenation primarily depends on hydrogen sorption kinetics at the bottleneck9. The measured baseline R of CHAtRIE is plotted against the bottleneck’s cross-sectional area Abottleneck of the PdCo layer (Fig. 4e). Overall, the R of the nano-resistor network significantly increases as the Abottleneck decreases. This nonlinear relationship was observed in several electrical resistance studies of ultra-thin films at a typical thickness of ~10 nm43–46, and could be explained by three thickness-dependent resistivity models: (i) the Fuchs–Sondheimer model for surface and interface scatterings47, (ii) the Mayadas–Shatzkes model for grain boundary scattering46, and (iii) the Namba model considering the ratio of surface roughness to the electron mean free path48. Given the complexity of the sensors’ structure and the ultra-thin sensing layer, all these scattering mechanisms may be present and influence electron transport. Lacy developed a more general resistivity model, incorporating scatterings from surface, grain boundaries, and surface roughness into a single parameter η, known as the thickness reduction factor49. The theoretical model best fits our experimental data when η=1.5 nm for the ultra-thin film regime (tRIE ≤ 300 s), and η=0.9 nm for the uniform thickness regime (tRIE ≥ 300 s) (see Supplementary Note 7.3 for detailed fitting). The fitting suggests that these scattering effects are more pronounced in the U-shaped region due to the smaller grain boundaries in a thinner PdCo region as shown in SEM images in Supplementary Fig. 16.

All sensors underwent sensing response measurements, with sorption times and sensitivity across PH2 ranging from 100 to 1 mbar being extracted (Supplementary Note 7.42). In order to find the optimal sensor design, t90 and ΔR/R at PH2 = 1 mbar of each sensor are depicted in Fig. 4f, g (labeled in blue filled squares). The sensitivity is proportional to the SVR of the PdCo layer, as a larger SVR increases the number of available Pd sites for hydrogen adsorption and dissociation50. Additionally, response time improves at smaller film thickness, as the hydrogen diffusion path to the interstitial sites of the small PdCo nanocrystalline domains is reduced29. Among the sensors, the CHA185 sensor exhibits the fastest response time of 2.5 s while maintaining a high sensitivity of ~2.5% at 1 mbar of H2. However, this still falls short of the 1-s response time benchmark required for automotive applications6. We explored increasing the Co composition in the alloy and reducing the thickness of the PdCo layer to improve the sensor’s response time. Nonetheless, a 5-nm Pd63Co37 layer emerged as the best compromise between high sensitivity and rapid response as a higher composition of Co (e.g., 45%) significantly reduces the hydrogen solubility of the sensing element and a thinner nominal deposited thickness (e.g., 3.5 nm) might result in discontinuous resistance network with slower response times (Supplementary Fig. 20).

Perfluorinated polymers, such as Polytetrafluoroethylene (PTFE)7,51,52 and TAF12, have been widely used as a coating layer for H2 sensors due to their proven ability to effectively boost the H2 sorption kinetics by lowering the activation energy for both H2 absorption and desorption. Therefore, 30 nm of TAF was then thermally evaporated on all 20 nm C60/5 nm PdCoCHA sensors. Compared to uncoated sensors (labeled in blue filled squares), TAF-coated sensors (labeled in pink filled triangles) display a comparable baseline resistance, a similar rise in ΔR/R, and a reduction in t90 with the increase in SVR as shown in Fig. 4e, f. Notably, the TAF coating led to a significant improvement in response time. At PH2 = 1 mbar, the t90 of each TAF-coated CHA sensor decreased by ~2 s (Fig. 4g), approaching the 1-s benchmark. In fact, the CHA185/TAF sensor surpassed this benchmark, achieving an ultra-fast t90 of <0.8 s. Interestingly, in both TAF-coated and uncoated data revealed a kink in the t90 curves at tRIE = 185 s. Although the CHA160 sensor had a higher SVR, its t90 was slower than that of CHA185 sensor, likely due to the formation of defects, such as vacancies and dislocations, in the few-nanometer-thick film, which could increase the energy barrier for hydride formation53.

The 20 nm C60/5 nm Pd63Co37/30 nm TAF CHA185 sensor was found to be optimal and its detailed sensing performance is presented in Fig. 5. In Supplementary Fig. 21a, the normalized sorption kinetics of the sensor in response to PH2 ranging from 100–1 mbar exhibit a superior SNR, with the noise of the acquired signal σ=0.006% at 12.2 Hz sampling frequency (see Supplementary Note 8), enabling reliable extraction of the response time. Remarkably, the sorption times increase at a much slower rate (with nabsorption ≈ −0.30 and ndesorption ≈ −0.10) as PH2 decreases (Fig. 5a). Particularly, t90 remains ≤0.8 s in the 1–100 mbar PH2 range, much faster than any Pd-based electrical hydrogen sensors measured under similar conditions (see Supplementary Table 2). The sensor releases H2 in less than 26 s within the tested PH2 range. In addition, the sorption isotherms of the sensor summarized in Fig. 5b show a hysteresis-free characteristic. To further examine its detection capability, the sensor was tested in diluted H2 gas in an N2 balance (Supplementary Fig. 21b). Although C60 is reported to be high permeable to N2 54, the sensor’s signal remains unaffected by the N2 pressure variation (see Supplementary Fig. 22a) and the interdiffusion of N2 with H2 has a negligible effect on the sensing performance under vacuum mode measurement within the tested PH2 range as illustrated in Supplementary Fig. 23. Thus, H2 and N2 mixtures can be utilized in vacuum mode measurement to reliable extract the sensitivity of the sensor at low partial H2 pressures. The result in Fig. 5c shows that the C60/PdCo/TAF CHA185 sensor can reliably resolve the hydrogen pressure as low as 144 μbar (≈144 ppb). By defining the LOD as the PH2 at which the detectable resistance change signal = 3σ, where σ=0.004% at 8.4 Hz sampling frequency (see Supplementary Note 8), extrapolating the sensitivity obtained from Fig. 5c, the TAF-coated CHA185 sensor is able to detect the PH2 of 0.04 μbar (≈40 ppb), approaching the DOE requirement of 10 ppb LOD for environmental monitoring applications5.

Given the significant enhancement in H2 sorption kinetics provided by the TAF top coverage, one might wonder why the C60 layer is not entirely replaced by TAF. The fabrication process of the sensor involves the lift-off of PS beads to create the hole array structure. However, TAF does not adhere to the glass substrate as well as C60 to survive the lift-off. Despite varying etching times, no conductivity was detected from the TAF/PdCo/TAF CHA sensors. This may stem from the porous nature of the TAF polymer and from the penetration of Co and Pd atoms into the ultrathin soft TAF layer, whereas PdCo forms a smoother layer on C60 (Supplementary Fig. 9). Here, we demonstrate the hydrogen sensing performance of the CHA450-based sensor only because these pinhole effects also prevent the formation of a continuous PdCo nano-network in hole array structures with smaller channels, such as CHA185 or CHA250 (Fig. 4a). All three tested devices exhibited significantly faster response and release times, with record fast t90 = 0.40±0.06 s and t10 = 16±1 s across the H2 pressure range of 1–100 mbar as shown in Fig. 6b, the fastest sensor ever reported in the literature at a similar operation conditions (see Supplementary Table 2). The improved sensing response of samples with TAF underneath can be attributed to a combination of reduced grain size observed in the XRD analysis (Supplementary Table 4), efficient gas permeation through the underlayer, and a lowered energy barrier for hydrogen sorption at the TAF/PdCo interfaces7. Figure 6c demonstrates the detection capability of one of the C60/TAF/Pd63Co37/TAF CHA450 sensors through step-wise pressure pulses of diluted H2 in an N2 balance, corresponding to partial H2 pressures ranging from 54.8 to 0.11 μbar. The sensor effectively detected the lowest PH2 = 0.11 μbar (~110 ppb) with an impressive sensitivity of 0.2%. Notably, the signal is clearly distinct from the background noise (σ=0.004% at f = 8.4 Hz) and is not affected by the N2 gas carrier pressure (Supplementary Fig. 22b), which is crucial for precise monitoring. The LOD was then confirmed by repeating the measurements for sensors #2 and #3 (Supplementary Fig. 27). The sensitivity averaged from 3 individual sensors across six orders of hydrogen pressures, as shown in Fig. 6d, suggests a reliable detection of 40 ppb with sensitivity of 0.11±0.03% and an extrapolated LOD as low as 100 part-per-trillion (ppt) (Supplementary Fig. 27d). Our sensor’s exceptional performance ranks it among the fastest and most sensitive hydrogen sensors currently available (Supplementary Note 1).

Stability and selectivity tests
In addition to response time and the detection limit, the sensor’s capability to reliably detect H2 under the influence of aging, toxic gases, humidity and temperature is crucial for practical application. While TAF-coated sensors improve hydrogen sorption kinetics, their poor selectivity leaves them vulnerable to degradation in the presence of toxic gases like CO or extreme humidified condition (see Supplementary Fig. 29)7,12. To overcome this limitation, a high H2 permeable poly(methyl methacrylate) (PMMA) has been utilized as an additional protective layer (Fig. 7)7,9,10,56. It is worth noting that a 50-nm thick PMMA coating layer shows negligible effect on the sensors’ response times thanks to its high permeability and diffusivity to H2 (Supplementary Fig. 30)57. First, we assessed the stability of both a fresh and a 3-month-old (stored in a N2 filled glovebox) 20 nm C60/5 nm PdCo/30 nm TAF/PMMA CHA450 sensor upon 100 cycles of loading/unloading 2% H2 in N2 using a flow mode set-up (see Methods and Supplementary Note 3). Overall, the sensitivity of the sensors remained stable throughout the cycling test, with a small variation of <5% (Fig. 7a, b). The 3-month-old sensor showed a comparable (de)absorption time to the fresh one (Fig. 7a inset), demonstrating resilience against mechanical stress and strain from repeated lattice expansion and contraction. It can be ascribed to the high degree of rotational disorder of the C60 molecules around one of their central axes at room temperature27,58, which allows the PdCo film to expand and contract with minimal mechanical constraint. Even if chemical bonding occurs at the PdCo/C60 interface (see X-Ray Photoelectron Spectroscopy (XPS) analysis in Supplementary Fig. 11), this molecular mobility ensures that the sensor retains mechanical flexibility and stability, mitigating the degradation due to the buckle or global detachment of the PdCo film from the substrate in clamped films59–61.

Next, we conducted tests to evaluate the sensor resistance to poisonous gases and humidity. In deactivation tests, the sensor was exposed to 5% of CO2, 5% of CH4, and 0.2% of CO, each mixed with 2% H2 in N2, respectively. As shown in Fig. 7c, d, the normalized sensitivity remained within ±4%, well below the ±5% accuracy required for hydrogen sensors6. The response time remained almost unchanged (within 1 s) before and after exposure to 5% CO2 or 5% CH4, though a larger fluctuation of ~3 s in absorption time was observed in high concentrations of CO (0.2%) (Supplementary Fig. 31). It is important to note that 0.2% of CO represents an extreme testing condition, as typical urban CO levels are around 10 ppm (~0.001%)62. We also performed humidity tests with gas carriers at 40% and 90% relative humidity (RH) compared to the dry condition (Fig. 7e, f). The sensor’s sensitivity gradually decreases after each cycle due to water vapor condensation on the sensing elements, which can hinder hydrogen adsorption and dissociation63,64. ΔR/R at 40% RH appear to converge with the signal at 90% RH, suggesting that the effect of humidity may saturate once a certain amount of water is deposited on the sensor surface. The average signal amplitudes in 40% RH and 90% RH were 91±3% and 87±1%, respectively, compared to the dry reference environment. Notably, the impact of humidity is temporary since the water molecules are weakly bound to the sensor surface and can be cleared by injecting dry air65. The sensor’s performance recovered after a few pulses of dry 2% H2 in N2 (Supplementary Fig. 32). This result places the C60/PdCo/TAF/PMMA CHA450 sensor among the top-performing hydrogen sensors under extremely humidified conditions (RH=90%)8,14. While this study focuses on idealized conditions, we acknowledge that evaluating sensor performance in air is essential for real-world applications. Some preliminary results with polyvinyl alcohol (PVOH)-coated sensors show that oxygen interference can be mitigated; however, the coating introduces trade-offs in hydrogen diffusion and limits applicability under humid conditions11,66. Further optimization of coating thickness and tandem architecture to balance selectivity, response time, and environmental robustness remains an important future direction beyond the scope of this work.

To address the temperature stability of the system and the temperature dependence of resistance, we evaluated sensor performance of one of our CHAs over a range from room temperature to 115 °C and consistently observed hydrogen response (see Supplementary Fig. 33). However, two notable effects emerged at elevated temperatures: (i) increased baseline drift, which may result from thermally induced morphological changes in the nanoparticle network such as expansion or coalescence that alter percolation pathways, and (ii) a significant drop in sensitivity, consistent with suppressed β-phase hydride formation and enhanced hydrogen desorption kinetics8. These findings indicate that room temperature offers the best balance between baseline stability and sensing performance for our PdCo nano-network structure.

Methods
Materials
Polystyrene (PS) nanospheres (Polysciences Inc., average bead diameter D=500±10 nm, CV=2%) and ethanol (Sigma-Aldrich, 98%) were used to fabricate the nanosphere monolayers. Sulfuric Acid (Lab Alley, 93%) and hydrogen peroxide (Lab Alley, 30%) were utilized for the petri dish and substrate cleaning processes. Polymethyl methacrylate (PMMA), fullerene C60 (99.9%), and acetone (≥99.5%) were purchased from Sigma-Aldrich. Teflon AF 2400 (TAF) was obtained from Dupont. Palladium (99.95%), and cobalt (99.95%) from Kurt. J Lesker Company were utilized for electron beam depositions. Deionized (DI) water (18 MΩ.cm) was used for all experiments.

Composite nanohole array (CHA) fabrication
The composite nanohole array (CHA) fabrication started with the growing of hexagonal close-packed nanosphere monolayers on glass substrates or silicon wafers using the air/water interface method31,69,70. First, a 14-cm diameter petri dish was cleaned with piranha solution (4:1 mixture of sulfuric acid and hydrogen peroxide) and subsequently filled with 22 ml of DI water. Concurrently, a separate colloidal suspension was prepared by mixing 300 μl of the stock PS nanosphere solution, 1000 μl of DI water, and 650 μl of ethanol. This suspension was continuously dispensed via a syringe pump (kdScientific, series 100) at 0.012 mL/min onto the water-filled petri dish. The nanospheres were allowed to self-assemble for ~3 h until the monolayer formed large domains covering ~75% of the water surface. Substrates were then carefully slid horizontally beneath the floating monolayer. Finally, excess water was drained and the coated substrates were then left to dry overnight. The nanosphere diameter D=500 nm was chosen since the monolayers can have the highest quality with this size. The PS monolayer, then, went under the RIE process using Trion Phantom III RIE Etcher (40 mTorr O2, ICP power=25W, RF power=10W). The etching times were adjusted from 600 s to 150 s for the bead size reduction. In the next step, fullerene C60 was thermally evaporated on top of the templates with the deposition rate of 0.26 Å/s, followed by electron beam co-deposition of Pd and Co with 0.05-nm/s total deposition rate yielding a thin Pd63Co37 alloy film. To increase the mixing uniformity between Pd and Co, the sample holder was rotated azimuthally with a constant rotation rate of 80 rpm during the deposition process. The substrate was then cut into a standard size of 10×5 mm2 followed by monolayer lift-off by using the scotch tape technique. In some cases, the device was coated by a layer of TAF and a PMMA film. For TAF coating, the TAF powder was thermally evaporated at the rate of 0.02 nm/s to form a uniform coating with a nominal thickness of ~30 nm12. For PMMA coating, PMMA powder was mixed in Acetone at 10 mg/mL concentration at 80 °C until fully dissolved. The solution was then cooled down to room temperature before being spin-casted onto top of the hole array at a speed of 5000 rpm for 120 s, followed by the final soft bake at 85 °C for 20 min. The approximate thickness of the coating PMMA layer is 50 nm10.

Morphology, composition and crystal structure characterization
The nanomorphologies of fabricated samples were measured by an AFM (Park NX10), and the AFM images were then analyzed using XEI-Image Processing and Analysis Software and Gwyddion71. SEM was performed with a SU-9000 system, Hitachi (resolution of 0.4 nm at 20 kV). EDS elemental mapping was performed with 150 mm2 Oxford XMaxN detector.

XRD patterns were measured using a powder diffractometer (XRD, Rigaku) with Cu-Kα radiation (λ=1.54056 Å, 40 kV, and 40 mA). The range of 2 theta angle from 39° to 44° with speed 0.1 degree per minute and 0.03° for the sampling width for all thin film samples on the glass substrate.

XPS spectra of thin films on Si (110) wafer were taken using Thermo Scientific Nexsa G2 XPS system with Al Kα radiation (1486.6 eV) and the linewidth of 0.25−0.30 eV.

Sensing characterization
The sample was mounted in a home-built H2 gas cell70. The resistance of the CHAs was measured by a collinear 4-point probe using a SourceMeter KEITHLEY 2635B as a constant current source. In vacuum mode measurement, H2 gas pressure was monitored by three independent pressure transducers with different ranges which cover the pressure range of 2.7E-6 to 1.1 bar (two PX409-USBH, Omega and a Baratron, MKS). By using diluted 4% or 100 ppm or 10 ppm H2 balanced in N2 we can control the partial H2 pressure as low as 40 μbar (~40 ppm) or 100 nbar (~100 ppb) or 10 nbar (~10 ppb), respectively. In flow mode measurement, 4% H2 in nitrogen mixture gases (Airgas) were diluted with ultra-high purity nitrogen gas from Airgas company to targeted concentrations by a commercial gas blender (GB-103, MCQ Instruments). The gas flow rate was kept constant at 400 ml/min at 1 atm for all measurements. All experiments were performed at room temperature if not stated otherwise.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "I have major comments for authors before publication:\n1. Clarify the specific contributions of the hexagonal PdCo nano-resistor network design compared to other state-of-the-art\nsensors. While the paper mentions the improvements in response time and detection limit, explicitly state how this design\nsurpasses existing methods, particularly in robustness and scalability.\n2. Include more details about the reproducibility of the sensor's performance across different samples. The manuscript would\nbenefit from presenting error margins or statistical analyses (e.g., standard deviations) for key metrics such as response time\n(t90) and sensitivity (ΔR/R).\n3. Provide a deeper explanation of the role of the C60 buffer layer in enhancing the sensor's performance, particularly in\nmitigating the clamping effect. Expand on how the interaction between C60 and PdCo improves hydrogen uptake and\nresponse time.\n4. While the manuscript highlights the sensor's fast response and low detection limits, include a more comprehensive table\nor figure comparing these metrics to other published hydrogen sensors. This will strengthen the claim of the sensor's\nsuperior performance.\n5. Address the sensor's long-term performance under varying environmental conditions. While aging and humidity tests are\ndiscussed, include additional details about the sensor's stability and potential degradation mechanisms over extended\nperiods or harsher conditions.\n6. The manuscript mentions trade-offs between sensitivity, response time, and material composition. Provide a more detailed\ndiscussion of these trade-offs, particularly how different PdCo compositions (e.g., Pd63Co37) were optimized for balanced\nperformance.\n7. Discuss the scalability of the fabrication process and the feasibility of deploying these sensors in real-world applications.\nInclude potential challenges in mass production and integration with existing technologies.\n8. Include additional validation for the results presented. For instance:\nMore detailed characterization of the nanostructure using advanced techniques like X-ray diffraction (XRD) or Raman\nspectroscopy to complement the SEM and AFM analyses.\nVerification of the sensor's performance using independent measurement techniques to ensure robustness.\n9. The addition of Teflon AF and PMMA layers significantly influences sensor performance. Discuss how the thickness and\nuniformity of these layers impact key metrics and whether these coatings introduce any limitations.\n10.Provide a clear section on potential future work, focusing on enhancing selectivity further and addressing interference\nfrom other gases or environmental factors.", "reviewer_2": "First, I would like to congratulate the authors for such an impressive work. I follow closely the work from the group and it is no\nsurprise that they achieve this. The sensor reported accomplishes an unprecedented combination of low limit of detection\nand high speed. The authors assigned the success to the addition of C60 to PdCo nanohole arrays, a platform that the group\ndevelop and work a lot on, that provides extra surfaces yet interact minimally with the active element, allowing it to\nexpand/contract freely during sensing. This conclusion (if true, see my comments below) will inspire other strategies aiming\nfor similar goal. The reported methods are detailed, and the figure/discussion are clear. I would love to see this published in\nNat Com, however, only if the authors can clarify the major questions/suggestions below.\n(Apology for my comments below that are random both in terms of importance and order they refer to in the text).\n• One missing, critical point in this work (that a Nat Com paper should have) is the general insight that later can be extended\nto or inspire other fields. Here, author use C60, but no fundamental design/understanding derived. So further\ncharacterization/model are required to back up the argument. Also, what other materials may be used to invoke similar\neffect?\n• Related to that note, I find the argument explaining the reasons behind the enhancement to be unjustified and at times\ncontradictory. (i) C60 is thought to increase the SVR. How? Surface here only deals with the material that interacts with H2,\nexhibits properties change, which in turn converts into signal in the sensor reading. In fact, in the calculation authors include\nthe surface of C60 but exclude its volume. Both S and V must be evaluated from the same system. Or am I wrong here?\nAnother point for my argument is since authors said C60 does not interact with H2, it is then as if it wasn’t there, which then\nshould not be included in the surface area consideration. (ii) Even if the assumption by authors is correct, how come C60 is connected to SVR even with the thin film configuration (Fig 2). Here the only access for H2 to PdCo is only through the top\n(the PdCo surface), since H2 access through the bottom of PdCo is only through the side walls of C60, which is much\nsmaller than the sample lateral area. (iii) C60 is also thought to increase sensitivity (i.e. through hydrogen loading) due to the\nminimum clamping between it and the PdCo. However, author also argues that C60 increases the PdCo wettability. These\ntwo are contrary to each other. So which one is true? I think authors should study in detail this interface and see if there is\nany chemical bonding formation. (iv) Further contrasting argument related to point (iii) is that such enhancement effect is also\npresent when there is FTA in between C60 and PdCo. So what is the role of C60 here then? Does FTA also have similar\neffect as C60? If so, why then it is thickness-dependent? And the thickness dependency also shows rather contradicting\nresults. If it really due to clamping being less, the thicker the FTA, the less the clamping effect is. However, for 50 nm, the\nthickest FTA studied, the improvement is actually the lowest. All in all there are major investigation and analysis required to\nreach any conclusion for the study.\n• Results and Discussion headers are missing.\n• Please discuss recent strategies in improving sensitivity (to ppb level) in H2 sensors (electric, optic, ect). This will benefit\nthe readers immensely.\n• In the real setting, is there a need to measure H2 down to ppt level?\n• In is mentioned that GLACD method enhances quality, morphology, and SVR. How is it so?\n• Related to above, it is said the method will get the sample to “their full potential.” This is highly speculative and unjustified.\nAlso, what are their full potential? Is there any fundamental ceiling to this?\n• Since the LOD is the result of extrapolation, I don’t think it is appropriate to use it in the title and abstract, as it may trick\nreaders. Only highlight such a number in these places if it is demonstrated, or explicitly write that it is an “extrapolated LOD”\n• Regarding the extrapolation, what is the basis for it? I think it is never mentioned. Or the authors just extend the line\nfollowing the data?\n• Again regarding extrapolation, I don’t think it is justified to claim a value from an extrapolation 3 orders of magnitude\nhigher/lower than the last experimental data point (here from 100 ppb to 0.1 ppb). Many things/trends can happen within that\nexpansive range.\n• Another main concern is the use of idealized environments throughout the study. Looking at the state of the field now, where it has progressed immensely, I would highly encourage that we start using “real environment” more, especially for a\npublication in Nat Com. Thus, I am curious to see how the sensors respond in (synthetic) air. This is important in general, but\neven more for the authors since they claim that their sensors fulfil the strictest requirement for environmental monitoring\n(which ironically comprises air as background). Achieving what the authors did is impressive, but this does not reflect the\nreal condition at all. In air, there will be many complex reactions being involved. How does the proposed concept perform in\nair? If not, how far is it from the ideal situation? As such, similar to my comment above, whenever ideal conditions are used,\nplease also have an explicit statement regarding it when mentioning the performance (especially in title and abstract).\n• Related to above, in the contamination tests, I encourage authors to instead expose the sensors to constant dose of\ncontaminations, instead of having them in cycle together with the hydrogen. This, again, is closer to the real conditions. I am\ncurious to see the response to humidity using this scenario, as based on my experience, PMMA would absorb water and\neventually deteriorates the sensor’s performance.", "reviewer_3": "The authors described how to fabricate unique hydrogen sensing element, a PdCo composite hole array on a fullerene-\ndecorated glass substrate, and evaluated the sensing times and detection limits for H2 concentrations were evaluated in\nrelation to the structure of the sensing element. By fine-tuning the structure of the PdCo composite hole array, they achieved a\nreaction time of less than 1 second and a detection limit at ppt level. It is noteworthy that a high-performance sensor\nelement can be constructed using a simple fabrication method, and the structure of this element may also be applicable to\ngas sensors other than hydrogen. However, I believe that further discussion is needed on the relationship between the\nstructure and performance of sensing elements before publication, the authors need to fully and in detail address the\nfollowing points:\n[1] The authors need to explain the crystal structures of the PdCo and C60 layers, as this information is crucial for\nunderstanding the hydrogen absorption properties of the PdCo layer and the H2 gas behavior through the C60 layer.\n[2] The effect of the C60 layer is unclear. There seem to be a contradiction between the following two considerations. In fig.\n2, the authors pointed out that the C60 layer mitigated the clamping effect on the PdCo layer, maximizing the hydrogen\nuptake and the sensitivity. Meanwhile, in fig. 6, the TAF layer inserted between the PdCo and C60 layers was useful to\nimproving sensitivity. If the TAF layer also works to mitigate the clamping effect, the effect of C60 is thought to be only due to\nits weak adhesion to the substrate.\n[3] What about temperature stability in resistance measurement? Since resistance in metals depends on temperature, it is\npossible that the baseline of resistance R(0) can shift depending on temperature. While the baseline of ΔR/R shown in figs.\nS18 and S21 of the supplementally materials shows an approximately straight line, the resistance was not shown. I kindly\nask the authors to comment on the stability of the temperature of the system during measurements and on the temperature\ndependence of the resistance for the fabricated sample.\n[4] Why were response times measured in vacuum mode? For the same hydrogen concentration, is the response time the\nsame in vacuum mode and flow mode? We believe that the response time in flow mode will be slower than that in vacuum\nmode, because the H2 diffusion time in the carrier gas (N2) may be the rate-limiting step for detection at low H2\nconcentration in mixture gas. Furthermore, when applying samples to hydrogen sensors in atmospheric environments, we\ngenerally believe that the response time in flow mode reflects reality.\n[5] In fig. 7, is the deterioration speed of the uncovered sample (20nmC60/5nmPdCo/TAF CHA450) to interference gases\nand humidity faster than that of the PMMA-covered sample? Since the data of the uncovered sample was not shown, the\neffect of the PMMA coating on interference gases and humidity was not clear in the manuscript. Please show the data of the\ncomparative sample related to fig. 7 in the supplementally materials."}, "author_response": "We thank all the reviewers for their time and insightful comments that helped improved the manuscript\nsubstantially. Below are our point-by-point responses to reviewers’ comments.\nTo facilitate clarity in our responses, we have labeled each comment from the reviewers as follows:\ncomments from Reviewer 1 are denoted as 1.x (e.g., 1.1, 1.2), those from Reviewer 2 as 2.x, and those\nfrom Reviewer 3 as 3.x, with the numbers assigned in ascending order according to the sequence of\ncomments. In cases where comments from different reviewers are similar or related, we may refer back\nand forth using these notations for cross-referencing.\nWe have revised our manuscript and supplementary information in light of these comments by explicitly\nindicating how the corresponding changes within each comment.\nResponses to Comments from Reviewer #1\nI have major comments for authors before publication:\nAuthors’ response: We are happy to have an opportunity to address the reviewer’s critical remarks,\nimportant questions, and insightful suggestions.\n1.1. Clarify the specific contributions of the hexagonal PdCo nano-resistor network design compared to\nother state-of-the-art sensors. While the paper mentions the improvements in response time and\ndetection limit, explicitly state how this design surpasses existing methods, particularly in robustness\nand scalability.\nAuthors’ Response: We thank the reviewer for this thoughtful and constructive suggestion.\nRecent advancements in hydrogen sensors (both electrical and optical flatforms) have centered on\nnanostructured Pd hydride systems, optimized through material selection and structural engineering\n(Supplementary Figure 1 and Supplementary Table 1). Our work builds on this foundation with a\nsynergistic integration of four key design strategies that collectively enable a low detection limit and ultra-\nfast response time:\n(i) Optimized PdCo alloy composition and thickness to improve response time and hysteresis\nsuppression:\nFirst, the PdCo alloy was selected based on our prior study demonstrating faster kinetics than pure\nPd and Pd alloys such as PdAu and PdAg under identical structural and testing conditions.1 In this\narticle, we utilized the PdCo with the stoichiometry of 63:37 and the optimization (including Co%\nvs. response/sensitivity trade-offs) is systematically detailed in Comment 1.6. In addition, an ultra-\nthin PdCo layer of only a few nm significantly boosts the sensors’ response to H (see\nstrain-induced energy barrier to form the hydride effectively suppressing the hysteresis typically seen\nin pure Pd hydrides.1,2 While PdCo historically exhibits reduced hydrogen sensitivity attributed to\nlower H solubility, we overcame this limitation by enhancing sensitivity through a C -decorated\n2 60\nsubstrate and nanostructuring (see the following points (ii) and (iii)).\nR1\n(ii) Hexagonal nanostructuring via NSL and GLACD to improve the sensing element’s surface-to-\nvolume ratio:\nWe combine nanosphere lithography (NSL) with glancing angle co-deposition (GLACD) to create\nthe hexagonal PdCo composite nanohole arrays (CHAs) with a high surface-to-volume (SVR), which\nis essential for enhancing hydrogen sorption kinetics.3,4 The opening in the CHAs provides more\npathways for hydrogen diffusion into the sensing layers, in the meantime to make the sensing element\neven smaller laterally.5 Additionally, due to dominant electron scattering at the narrow bottlenecks\nof the hexagonal network rather than across the entire resistor, the effective sensing area becomes\nconcentrated in these high-sensitivity regions.5 The enhanced electron scattering at grain boundaries\nand surfaces in these bottlenecks leads to greater resistance changes upon hydrogenation,\nsignificantly improving sensitivity (see main text Figure 4 and Supplementary Note 7). Notably,\nthese NSL and GLACD approaches maintain low fabrication costs and high throughput with\nscalability considerations addressed in Comment 1.7.\n(iii) Surface engineering with C -coated substrate to improve response time and hydrogen\n60\npermeability:\nThe C layer enhances the SVR and overall sensor performance in several keyways. First, the C\n60 60\namorphous film, being a robust carbon-based material with internal voids, offers relatively high\nhydrogen permeability and chemical stability.6,7 Second, because hydrogen diffuses relatively fast\non C , the effective SVR of the PdCo network nearly doubles, accelerating response time.7,8 Third,\n60\nthe C layer serves as a mechanical decoupler between the PdCo sensing network and the rigid glass\n60\nsubstrate.9,10 This allows the PdCo nanostructures to expand and contract freely during\nhydrogenation and dehydrogenation cycles, reducing mechanical stress and potentially extending\nsensor lifetime. Our experiments show consistent performance even after 500 hydrogenation cycles\n(see Comment 1.5). Finally, the C layer serves as a necessary morphological template, defining the\n60\ngeometry of the PdCo nanostructure (see main text Figure 4). Each point is further elaborated\nin Comment 1.3. For these reasons, the use of a C -decorated substrate is a central innovation of\n60\nour sensor design and is highlighted in the manuscript.\n(iv) Full-interface TAF coatings for kinetic enhancement to improve response time:\nA thin TAF polymer coating on the Pd alloy lowers the activation barrier for hydrogen adsorption,\nfurther accelerating response time while preserving signal integrity.11\nThis unique design, in turn, amplifies the resistance change upon hydrogen exposure, contributing to high\nsensitivity. When paired with the inherently low-noise characteristics of resistive transducing method, our\napproach yields a markedly improved LOD. The ultra-thin (~few nm) and undulated PdCo resistor\nelements promote rapid hydrogen adsorption and desorption dynamics, thereby enabling fast response\ntimes while preserving signal strength.\nWe hope this revised explanation more clearly communicates the novelty and effectiveness of our sensor\ndesign in advancing hydrogen sensing performance beyond current state-of-the-art technologies.\nChange made to the manuscript:\n• The Introduction (page 4, from line 11) in the manuscript has been revised accordingly.\nR2\n• References have been added to the manuscript:\nhydrogen (H<sub>2</sub>) on a fullerene (C<sub>60</sub>) surface: insights from density\nfunctional theory and molecular dynamics simulation. RSC Advances 14, 36546-36556 (2024).\nhttps://doi.org:10.1039/d4ra06171c\n286 (1997). https://doi.org:https://doi.org/10.1016/S0040-6090(96)09043-8\nmesoscopic scale. The Journal of Chemical Physics 115, 9001-9009 (2001).\nhttps://doi.org:10.1063/1.1410391\nby Coulomb blockade. Scientific Reports 9 (2019). https://doi.org:10.1038/s41598-019-52544-4\ntwo-dimensional networks explored by first-principles and machine learning. Carbon 213,\n118293 (2023). https://doi.org:10.1016/j.carbon.2023.118293\ndesign of nanostructures. 8, 1-26 (2019). https://doi.org:doi:10.1515/nanoph-2018-0105\nR3\n1.2. Include more details about the reproducibility of the sensor's performance across different samples.\nThe manuscript would benefit from presenting error margins or statistical analyses (e.g., standard\ndeviations) for key metrics such as response time (t90) and sensitivity (ΔR/R).\nAuthors’ response: We thank the reviewer for the valuable comments. Here, to demonstrate the\nreproducibility of our sensors, the error margins for response times and sensitivity averaged from 3\nindividual sensors for each key configuration are shown in Figure R1.\nFigure R1. a Average response time and b sensitivity of 20 nm C /5 nm PdCo/TAF CHA sensors. c\n60 185\nAverage response time and d sensitivity of 20 nm C /3nm TAF/5 nm PdCo/TAF CHA sensors. The\n60 450\nerror bars indicate the standard deviation from 3 sensors.\nChange made to the manuscript:\n• Figure R1 has been added to Supplementary Information as Supplementary Figure 26.\nR4\n1.3. Provide a deeper explanation of the role of the C60 buffer layer in enhancing the sensor's\nperformance, particularly in mitigating the clamping effect. Expand on how the interaction between C60\nand PdCo improves hydrogen uptake and response time.\nAuthors’ response: We thank the reviewer for the constructive suggestion. We have briefly discussed the\nrole of C in Comment 1.1, the C buffer layer serves multiple key functions:\n60 60\n(i) Enhanced hydrogen access:\nC is a molecule composed of 60 carbon atoms arranged in a soccer ball-like structure, with each\n60\nmolecule measuring approximately 0.7 nm in diameter.12 In the solid state, C can crystallize into\n60\na face-centered cubic (FCC) lattice with an effective interstitial gap of about 0.4 nm, which is\nslightly larger than the size of a hydrogen molecule (~0.3 nm). However, in our study, 20-nm thick\nC film exhibits no X-ray diffraction (XRD) peaks (see Figure R7 in Comment 1.8 for the XRD\n60\nspectra), confirming its amorphous nature and the absence of long-range crystalline order. As a\nresult, the intermolecular voids are expected to be significantly larger resulting in a high-porosity\ninterlayer C .7 Additionally, the thermally activated rotational motion of C molecules at room\n60 60\ntemperature further enhances hydrogen diffusion throughout the amorphous matrix.9 A further\nbenefit arises from the fact that hydrogen does not form strong chemical bonds with C under\n60\nambient conditions, enabling rapid molecular transport through the layer.13 Our result in\nSupplementary Fig. 13 shows that the performance of sensors without and with a 20-nm C top\n60\nlayer is essentially identical, which further supports this point.\nSupplementary Figure 13. Sensing performances of 20 nm C /5 nm PdCo CHA sensors\n60 300\nwith and without 20 nm C on top. a I-V characteristics, b response time t and c sensitivity of\n60 90\nthe sensors in response to step wise decreasing H pressure from 100 to 1 mbar. All measurements\n2\nwere performed in vacuum mode at room temperature.\nThese combined properties allow hydrogen gas to access the PdCo transducer layer not only from\nthe top surface, as in conventional sensor architectures, but also from the bottom interface via the\nunderlying amorphous C film illustrated in Figure R2 below. This dual-sided accessibility\n60\nincreases effective gas exposure to the sensing material. The SVR is effectively doubled, reducing\nthe average hydrogen diffusion path within the PdCo network and substantially improving the\nsensor’s response time compared to the sensor without C underneath (manuscript Figure 2).\nFigure R2. Schematic illustration comparing hydrogen absorption and desorption pathways in\nPdCo thin films on a bare glass substrate and b C -coated substrate.\n60\nnegligible change in response time or sensitivity among the sensors. These findings further support\nour assertion that hydrogen diffusion through C is rapid and not rate-limited in our nano hole-\n60\narray structure (with a few hundred nm of the hydrogen diffusion path in C ). It is feasible to\n60\nlocally probe the diffusion rate of the C /PdCo bilayer using optical techniques14 and extract the\n60\nresponse times at different locations on the films such as near the edges or at the center, nonetheless\nsuch analysis is beyond the scope of the present study.\nChange made to the manuscript:\n• Manuscript (page 10, from line 1) has been revised accordingly.\n• Manuscript (page 11, from line 5) has been revised accordingly.\n• References have been added to the manuscript:\nand magnetization in CoPd thin films and nanocontacts. Journal of Magnetism and Magnetic\nMaterials 325, 112-116 (2013). https://doi.org:https://doi.org/10.1016/j.jmmm.2012.07.052\nfullerene-based devices: Morphology effect. Physical Review B 87 (2013).\nhttps://doi.org:10.1103/physrevb.87.075205\nthe electrical and optical properties of C60 thin films. Journal of Applied Physics 132\n(2022). https://doi.org:10.1063/5.0099291\n• Figure R2 has been added to Supplementary Information as Supplementary Fig. 8.\n(ii) Mechanical decoupling via molecular rotation:\nIn thin C films, C molecules undergo a temperature-dependent transition between static and\n60 60\nrotational states, occurring around 259K.9 At typical sensing conditions, the molecules are in a\nfreely rotating state, allowing the PdCo film to expand and contract with minimal mechanical\nconstraint. Even if chemical bonding occurs at the PdCo/C interface, this molecular mobility\n60\nensures that the sensor retains mechanical flexibility and stability and mitigates the clamping effect.\nOur experiments show consistent performance even after 500 hydrogenation cycles (see Comment\n1.5 and Figure R3).\nR6\nChange made to the manuscript:\n• Manuscript (page 23, from line 1) has been revised accordingly.\n• References have been added to the manuscript:\nsubstrate by Coulomb blockade. Scientific Reports 9 (2019). https://doi.org:10.1038/s41598-\n019-52544-4\nPhysical Review Letters 66, 2911-2914 (1991). https://doi.org:10.1103/PhysRevLett.66.2911\nto hydrogen gas. Thin Solid Films 516, 7797-7801 (2008).\nhttps://doi.org:https://doi.org/10.1016/j.tsf.2008.03.034\nhydrogen thin films. Journal of Materials Research 6, 964-968 (1991).\nhttps://doi.org:10.1557/JMR.1991.0964\nsubstrate on the mechanical and thermal stability of Pd thin films during hydrogen\n(de)sorption. International Journal of Hydrogen Energy 46, 4137-4153 (2021).\nhttps://doi.org:https://doi.org/10.1016/j.ijhydene.2020.10.163\n(iii) Morphological tuning:\nThe presence of the C buffer layer plays a critical role in enabling the formation of optimized\n60\nstructural bottlenecks in the PdCo film, as discussed in the main text Figure 4. These bottlenecks\nenhance sensing performance by concentrating resistance changes in regions with a high SVR.\nSuch morphology can only be achieved if the buffer layer is sufficiently robust to withstand\npenetration by high-kinetic-energy metal atoms during deposition. Previous studies have shown\nthat metals such as Co can readily penetrate conventional organic materials under similar\ndeposition conditions.15 This limited resistance to metal diffusion likely explains why our sensors\nfunction reliably only when the thickness of the bottom TAF layer is very thin (~3 nm) in the main\ntext Figure 6. In contrast, C effectively resists metal intrusion uniquely enabling the formation\n60\nof the PdCo nanostructures observed in this study. As a result, C is an essential material for\n60\nachieving the desirable morphology that underpins the sensor’s enhanced performance.\nChange made to the manuscript:\n• Manuscript (page 15, line 1) has been revised accordingly.\nR7\n1.4. While the manuscript highlights the sensor's fast response and low detection limits, include a more comprehensive table or figure comparing\nthese metrics to other published hydrogen sensors. This will strengthen the claim of the sensor's superior performance.\nAuthors’ response: We thank the reviewer for your keen suggestion. We have added a table below, which compares various hydrogen sensing\nmechanisms, device structures, transducing methods, and sensing metrics for broader context.\nTable R1. Ultra-sensitive state-of-the-art H sensors with LOD < 10 ppm reported for the last 10 years.\n90 10\nRef. LOD (ppb) T (°C)\nmethod structure conditions 0.1%† 0.1%† free? range gas test\nO2, CO,\nThe U.S. DoE’s requirements for environmental monitoring <30s <30s 0% ‒ -30 -\nYes 10 ppb CO2,\napplications (600ppb) (600ppb) 98% 80°C\nhydrocarbons\nO2, CO,\n<1s <1s 0% ‒ -30 -\nThe U.S. DoE’s requirements for automotive applications Yes 0.10% CO2,\n(0.1%) (0.1%) 98% 80°C\nhydrocarbons\nThis work Electrical C /PdCo/Teflon AF/ Vacuum & 0.8 26 Yes 144 0% - 90% RT CO, CO2,\nThis work Electrical C /Teflon AF/ Vacuum <0.5 17 40\n/PMMA CHA\n450\n16Nature Electrical DPP-DTT thin film on Flow mode in 0.84 6.63 N.A. 192 15% - 20 - 120 EtOH,\nElectronics 2025 Pt electrode Air 80% Me2CO,\nMeOH,\nToluene\n17Scientific Electrical Polyaniline PANI Flow mode in 15 17 N.A. 1000 N.A. N.A. N.A.\nReports 2024 hollow nanotube N2 (1ppm) (1ppm)\nR8\n18ACS Sensors Electrical Pd-Doped α- Flow mode in 49 533 N.A. 50 (300°C) 0% - 90% 300 C3H6O, NH3,\nFe2O3 Nanotubes Air NO2, C2H6O,\n2024 (200 (200\nCO, CH4,\nppm) ppm)\nCO2, SO2,\nH2S\n19Nano Energy Magneto- PS200/Pd67Co33/TAF Vacuum & 0.4 2.8 Yes 1000 0% - 90% N.A. CO, CO2,\n2023 optical nanopatches flow mode in CH4,\nN2\n20Light: Science Opto- Platinum–silicon Flow mode in 10 (1.3 N.A. N.A. 1000* N.A. RT Air\n& Applications electrical nanojunctions Air ppm)\n2023\n21Microsystems Thermo- Pt NPs@Al2O3 on Flow mode in 1.9 1.4 N.A. 1000 N.A. 50 - 120 CH4, C2H6,\n& electric P+/N+ single- Air (120°C) Acetone,\nNanoengineering crystalline silicon Toluene, CO,\n2023 thermopiles EtOH\n22Microsystems Electrical Pd-doped rGO/ZnO- N.A. 4 8 N.A. 50 (380°C) N.A. 380 HCHO,\n& SnO2 (100ppm) (100ppm) C4H10,\nNanoengineering C7H8, CO2\n2022\n23Nature Optical Pd nanoparticles Flow mode in 40 mins 50 mins N.A. 250 N.A. RT Air\nCommunications Air (250 ppb) (250 ppb)\n@PMMA\n2022\n1Nature Optical PS500/Pd80Co20/TAF Vacuum & 0.85 N.A. Yes 2500 0% - 40% 20 - 42 CO, CO2,\nCommunications nanopatches flow mode in CH4,\n24Sensors and Electrical Pd@Ni foam Flow mode in 138 (2%) 300 (2%) N.A. 7 0% - 30 - 70 O2, He, Ar,\nActuators B: N2 99.5% CO2, Co,\nChemical 2021 NH3, CH4,\nC2H4\nR9\n5ACS Applied Electrical PdCo nanohole arrays Vacuum & 10.8 20 Yes 180 0% - 90% RT Air, CO\nNano Materials flow mode in\n25Carbon 2021 Electrical Pd NP on Y2O3/CNT Flow mode in N.A. N.A. N.A. 90 ppb (RT) 0% - 75% 100 CO, NO2,\nN2 5 ppb H2S\n(100°C)\n11Nature Optical PdAu Vacuum & 1 5 Yes 1000 (in N.A. 30 - 60 CO2, CH4,\nMaterials 2019 NP@TAF/PMMA flow mode in Ar) CO, NO2, Air\nAir 5000 (in\nAir)\n26IEEE 2015 Electrical Multiwalled Carbon Vacuum & 75 (60 N.A. N.A. 400 N.A. RT CO, NH₃,\nNanotubes flow mode in ppb) CH₄, H₂S,\nAir NO₂, and\nacetone\n27Nanosale 2015 Electrical Pd NP@CPPy Flow mode: 4.5 (20 27 (20 N.A. 100 N.A. RT N.A.\nH (abs.), 9 ppm) ppm)\nmixture (des.)\n28Scientific Electrical Pd Flow mode in 80 (10 N.A. N.A. 100 N.A. RT N.A.\nReports 2015 nanoflower/graphene N2 ppm)\n†If not specified, *Extrapolated data\nChange made to the manuscript:\n• Table R1 has been added to Supplementary Information as Supplementary Table 1.\nR10\n1.5. Address the sensor's long-term performance under varying environmental conditions. While aging\nand humidity tests are discussed, include additional details about the sensor's stability and potential\ndegradation mechanisms over extended periods or harsher conditions.\nAuthors’ response: We thank the reviewer for the thoughtful comment. We address long-term\nperformance and degradation across three environmental scenarios:\n(i) Inert and cyclic conditions:\nWe performed over 500 hydrogenation/dehydrogenation cycles in inert environments to assess durability.\nWhile the sensor retained functionality, we observed a gradual decrease in sensitivity. This degradation\nis primarily attributed to grain coarsening of PdCo nanostructures and the accumulation of residual strain\nfrom repeated lattice expansion and contraction. Residual strain from cyclic volume changes alters the\nenergy landscape for hydrogen absorption, increasing the activation barrier for future cycles.4\nFigure R3. ΔR/R responses of 3-month-old 20 nm C /3 nm TAF/5 nm PdCo/TAF/PMMA CHA\n60 450\nsample upon 500 cycles (30/60 seconds of loading/unloading) of 2% H balanced in N with a total flow\n2 2\nof 400 ml/min.\n(ii) Humid and chemically contaminated environments:\nAs shown in main text Figure 7, the PMMA coating partially mitigates interference from water vapor\nand toxic gases such as CO. However, it does not fully prevent degradation. While the humidity effect is\nreversible, with performance recovery upon dry-air purging (see Supplementary Fig. 32), long-term\nexposure to CO and CO leads to irreversible performance loss due to strong C=O bonding on the Pd\n2\nsurface.29 These findings align with prior work by Kim et al.,29 and are further supported by our XPS C\n1s spectrum in Figure R4, which shows persistent C=O bonding on PdCo surfaces stored under ambient\nlab conditions for several months.\nR11\nFigure R4. XPS spectrum in C 1s region of PdCo thin film.\n(iii) Elevated temperatures:\nTo address the temperature stability of the system and the temperature dependence of resistance, we\nconducted additional measurements at varying temperatures from room temperature (~23 °C) up to\n115 °C. The temperature was continuously monitored using a thermocouple affixed to the substrate holder\ninside the gas cell. As shown in Figure R5a, the system exhibited excellent thermal stability, with\nfluctuations remaining below 0.3°C throughout the measurement period.\nIn Figure R5b, we present the resistance response under hydrogen exposure across the tested temperature\nrange. The baseline resistance R values at each temperature are shown in Figure R5c. At lower\n0\ntemperatures, R remains highly stable, following an approximately linear increase from 1260 Ω at room\n0\ntemperature to ~1320 Ω at 100 °C, consistent with typical metallic behavior where increased electron–\nphonon scattering raises resistance. We also note that the current used for resistance measurement is\nlimited to 0.1 mA, minimizing Joule heating effects. However, at 115 °C, R slightly decreases to\n0\n~1290 Ω, which we attribute to possible thermally induced morphological changes in the nanoparticle\nnetwork such as expansion or coalescence that may alter percolation paths.\nAs illustrated in Figure R5d, the sensor’s sensitivity declines at elevated temperatures, consistent with\nsuppressed β-phase hydride formation and enhanced desorption kinetics at higher temperatures.4\nOverall, our findings suggest that room temperature provides the optimal balance between baseline\nstability and sensing performance for our PdCo nano network structure. For field applications, we propose\nintegrating a local temperature control element such as a thermoelectric cooler (TEC) into the sensor\nhousing to ensure consistent operation under ambient fluctuations.\nR12\nFigure R5. a Measured temperatures at the sensor position during the tests. b Sensing responses of 20\nnm C /5 nm PdCo CHA to stepwise decreasing H concentrations with N as the gas carrier and c\n60 450 2 2\nextracted baseline R and d extracted sensitivity. The error bars in d indicate the standard deviation from\n0\n3 pulses.\nR13\n1.6. The manuscript mentions trade-offs between sensitivity, response time, and material composition.\nProvide a more detailed discussion of these trade-offs, particularly how different PdCo compositions\n(e.g., Pd63Co37) were optimized for balanced performance.\nAuthors’ response: We thank the reviewer for your on-point comment.\nA key objective in hydrogen sensor design, especially for meeting the DOE targets, is to achieve both\nhigh sensitivity and fast response/recovery times. However, there is an inherent trade-off. Sensitivity is\noften associated with the extent of hydrogen uptake and resultant changes in material properties such as\nelectrical resistance in our case. However, materials with high hydrogen solubility and large sensitivity\n(e.g., pure Pd) tend to exhibit longer response and recovery times due to phase transitions between α- and\nβ-hydride phases.4 To overcome this challenge, alloying Pd with a second metal has been widely\nexplored,4 and PdCo is one of the candidates that offer a promising balance between sorption kinetics and\nsensing performance.5,19,30,31 On the one hand, incorporating Co introduces defects and lattice distortions\nthat facilitate faster hydrogen diffusion and improve response dynamics. On the other hand, the presence\nof Co in PdCo lattice increases the strain-induced energy barrier to form the hydride, which suppresses\nthe α-to-β phase transition and shifts it to higher H pressures,2 thus reducing overall hydrogen solubility.\ndefined as achieving sensing metrics closest to the DOE targets (Table R1, page R8)?\nTo illustrate this point, we present our Co concentration optimization in Figure R6. It is worth noting that\nthe response time differences among the four selected stoichiometries are small since we have\nnanostructure and a very thin sensing layer. In contrast, the sensitivity drops drastically as the Co\nconcentration increases from 37 to 45 atomic %. Based on this trade-off, Pd Co offers the “sweet spot”\n63 37\nwhere we can both obtain moderate hydrogen absorption capacity and fast response time. Co% vs.\nresponse/sensitivity extracted at 1 mbar and 9 mbar H have shown in Supplementary Fig. 20.\n60 185\nterms of a sensitivity R/R and b response time t in measured H pressure range of 1 – 100 mbar.\n90 2\nR14\nSupplementary Figure 20. a-b Pd:Co composition-dependent and c-d PdCo thickness-dependent\nsensing performances of 20 nm C /PdCo CHA .\n60 185\nChange made to the manuscript:\n• Manuscript (page 17, from line 12) has been revised accordingly.\n1.7. Discuss the scalability of the fabrication process and the feasibility of deploying these sensors in\nreal-world applications. Include potential challenges in mass production and integration with existing\ntechnologies.\nAuthors’ response: We sincerely thank the reviewer for the insightful comment. We have addressed the\nscalability, feasibility, and potential challenges related to the mass production and integration of our PdCo\nhexagonal nano-network sensors, as outlined below:\nThe fabrication process comprises four primary steps: (i) self-assembly of a polystyrene (PS) monolayer,\n(ii) reactive ion etching (RIE) to shape the nanosphere mask, (iii) physical vapor deposition (PVD) via\nglancing angle co-deposition (GLACD), and (iv) polymer coating via spin coating or thermal evaporation.\nThese techniques are commonly used in micro/nanofabrication, and particularly RIE and PVD are\ncompatible with standard semiconductor processing workflows. We believe our sensor design is\ninherently scalable and adaptable, making it a strong candidate for transitioning from laboratory-scale\nfabrication to industrial production. Nevertheless, we acknowledge the following potential challenges for\nreal-world deployment:\nR15\n(i) Uniformity over large areas: Achieving uniform PS monolayers and consistent polymer coatings\nacross large areas is a key concern. Several techniques including spin coating, dip coating, and\ninterfacial assembly have been employed to fabricate monolayers of PS nanospheres. Park et al.\ndemonstrated the successful fabrication of a large-area PS monolayer (500 nm bead diameter,\nidentical to our study) with nearly 95% coverage on a 5 cm × 5 cm glass substrate using spin coating\nand appropriate surfactants.32 Lulek et al. reported large-area PS monolayer coating of 4- and 6-inch\nSi wafers using the air–water interface technique.33 These self-assembly techniques are well-\nestablished and can be automated for scalable production. In addition, scalable coating methods such\nas slot-die coating, inkjet printing, and blade coating offer viable alternatives to spin coating for\nlarge-area fabrication.34 Another approach for large scale production is implementing nanoimprint\nor UV lithography methods to achieve similar nanopatterns.\n(ii) Material cost: Although Pd is an expensive material, the use of ultra-thin (~several nanometers) PdCo\nfilms reduces the total material cost to below 1 USD per sensor.\n(iii) Sensor integration: Integration into packaged sensor systems (e.g., with electrical contacts,\nmicroheaters, wireless modules, etc.) requires compatibility with existing CMOS or MEMS\ntechnologies. In our case, the sensors are compact (1.0 cm  0.5 cm) and lightweight, making them\nwell-suited for integration into any commercial packaging. Furthermore, temperature control can be\nachieved locally using thermoelectric coolers (TECs) to mitigate environmental fluctuations and\nensure consistent sensor performance.\n(iv) Long-term stability and robustness: Maintaining structural and functional and structural and functional stability under real-world\nenvironmental conditions, such as variations in humidity, temperature cycling, and exposure to\ncontaminants, is an important consideration. We are currently conducting ongoing research on\nvarious polymer encapsulation strategies,4 local heating,35 and thermal refreshing technique29 to\nenhance sensor longevity under such conditions.\nThis combination of high performance, low material cost, and scalable fabrication makes our hydrogen\nsensor highly promising for future commercialization.\n1.8. Include additional validation for the results presented. For instance:\nMore detailed characterization of the nanostructure using advanced techniques like X-ray diffraction (XRD) or Raman\nspectroscopy to complement the SEM and AFM analyses.\nVerification of the sensor's performance using independent measurement techniques to ensure robustness.\nAuthors’ response: We thank the reviewer for your insightful suggestion. We have taken XRD and XPS\nfor 15 nm Pd Co thin films as shown in Figure R7 and Figure R8, respectively.\n63 37\nThermal evaporated C thin film on non-epitaxial substrates like glass often leads to a mix of\n60\npolycrystalline and amorphous-like regions.7 Thus, no peak was observed in XRD spectrum of 20 nm C\n60\nthin film in Figure R7a, confirming the absence of long-range crystalline order.36 The C film is not\n60\nperfectly close-packed but exhibits partial ordering with voids and grain boundaries. It is supported by\nSamad et al. where the porosity of C thin films was reported to be more than 50% when the substrate\n60\nwas kept at room temperature during the deposition.37\nR16\nPdCo thin films deposited on different coated substrates exhibit the strongest Bragg diffraction peaks\nbetween 41.3° and 41.6°, which can be attributed to the (111) orientation of the face-centered cubic (fcc)\nPdCo lattice (Figure R7b). These peaks are shifted to higher angles relative to the (111) peak of pure fcc\nPd (∼40.2°), indicating that Co atoms are substitutionally incorporated into the Pd lattice. The observed\npeak positions and extracted lattice constants are consistent with prior literature report by Morgan et al.38\nAnalysis of the full width at half maximum (FWHM) values allowed for the estimation of crystallite size\nand lattice strain, as summarized in the accompanying Table R2. Notably, the C /PdCo sample exhibits\n60\na larger crystallite size and reduced lattice strain compared to PdCo grown directly on glass, likely due to\nthe smoother growth surface provided by the C layer. Although slightly larger grains typically correlate\n60\nwith slower sensor response, the C /PdCo sample demonstrated enhanced sensitivity and faster response\n60\ntimes (see main text Fig. 2), suggesting that gas diffusion through the nanoporous C interlayer plays a\n60\ncritical role. In contrast, TAF/PdCo showed the lowest 2θ value, indicative of lattice expansion due to\ntensile strain, which can be attributed to pinholes in the TAF film, as observed in AFM images\n(Supplementary Fig. 9). The improved sensing response of samples with TAF underneath appears to\nresult from a combination of reduced grain size, efficient gas permeation through the underlayer,39 and a\nlowered energy barrier for hydrogen sorption at the TAF/PdCo interfaces11.\nFigure R7. XRD spectra of a 20 nm C thin film and b 15 nm Pd Co thin films on different coated\n60 63 37\nglass substrates.\nTable R2. Crystal structure and other physical properties of 15 nm Pd Co thin films on different coated\n63 37\nglass substrates extracted from XRD.\nCrystallite Lattice Lattice\nPeak position FWHM d-spacing\nsize Strain constant\nSample\n2 (deg)  (deg) D (nm)  (%) d (Å) a (Å)\nPdCo 41.559 ± 0.002 0.93 ± 0.01 9.14 ± 0.06 1.07 ± 0.01 2.17 ± 0.01 3.76 ± 0.03\nC /PdCo 41.571 ± 0.003 0.90 ± 0.01 9.55 ± 0.07 1.02 ± 0.01 2.17 ± 0.02 3.76 ± 0.03\nR17\nFigure R8. X-ray photoelectron spectroscopy (XPS) analysis of PdCo thin films with and without\nmolecular coatings. a Full survey spectra for 15 nm PdCo (black), PdCo/2 nm C (blue), and PdCo/2 nm\nchanges in chemical states and bonding environments upon C and TAF coating. e–g Deconvoluted C 1s\n60\nspectra for bare PdCo, PdCo/C , and PdCo/TAF, respectively.\ninteractions between the PdCo layer and molecular coatings, we conducted X-ray photoelectron\nspectroscopy (XPS) analysis of PdCo thin films with and without surface functionalization, as shown in\nFigure R8. The full survey spectra (Figure R8a) reveal characteristic peaks corresponding to Pd 3d, Co\n2p, C 1s, O 1s, and F 1s. A comparative analysis of Co 2p (Figure R8b) and Pd 3d (Figure R8c) regions\nbetween the bare PdCo and PdCo/C samples shows negligible differences. No peak shifts or changes in\n60\nthe peak-to-peak intensity ratio were observed, indicating minimal impact on the Pd and Co core levels\nR18\nupon C deposition. In the PdCo/TAF spectrum, the Co 2p and Pd 3d intensities are markedly reduced,\n60\nconsistent with attenuation effects arising from the overlying TAF layer, which reduces both incident X-\nray penetration and photoelectron emission due to its insulating character.\nThe C 1s region, however, displays the most pronounced differences among the samples (Figure R8d).\nDeconvoluted C 1s spectra for PdCo, PdCo/C , and PdCo/TAF are shown in Figure R8e-g, respectively.\nbare PdCo shows typical adventitious carbon contamination, including peaks corresponding to C–C\n(~284.6 eV), C–O (~286.3 eV), and C=O (~288.2 eV), likely due to atmospheric CO or CO adsorption.29\n60\n~284.6 eV remains, consistent with the sp²-hybridized carbon bonds of C molecules.40 Additional weak\n60\nfeatures in the 288–292 eV range (Figure R8f) suggest the emergence of interfacial bonding\nenvironments such as Pd–C or Co–C.40 Combining these insights with the results presented in\nSupplementary Fig. 13 (see page R5), we note that although bonding occurs at the C /PdCo interface,\n60\nthe catalytic active Pd sites for H₂ adsorption are not fully passivated. As a result, the sensor with the C\n60\ntop layer exhibits comparable sensing performance to the uncoated counterpart. This indicates that C\n60\ndoes not significantly alter the activation energy for H adsorption. Instead, the fast diffusion of H₂\n2\nmolecules through the porous C matrix appears to be the dominant factor contributing to the preserved\n60\nsensing kinetics.\nIn the case of TAF-functionalized PdCo (Figure R8g), the PdCo/TAF spectrum exhibits a distinct F 1s\npeak at ~688 eV and its C 1s spectrum displays complex features spanning 284–293 eV. In addition to\nthe typical C–C and C=O peaks, strong peaks at ~290.7 eV and ~293.3 eV correspond to CF and CF\n2 3\nmoieties, respectively, confirming the formation Pd-CF bonds.41 As a result, TAF coating modifies the\nx\nPd surface chemistry, lowering the activation energy for H adsorption and thereby enhancing sensor\n2\nresponsiveness.11\nChange made to the manuscript:\n• XRD and XPS analysis have been added to Supplementary Note 5.\n• XRD and XPS measurements have been added to the Methods section of the manuscript (page 28,\nfrom line 11).\nR19\n1.9. The addition of Teflon AF and PMMA layers significantly influences sensor performance. Discuss\nhow the thickness and uniformity of these layers impact key metrics and whether these coatings\nintroduce any limitations.\nAuthors’ response: We thank the reviewer for the valuable comments. In this work, we mainly focus on\noptimizing the sensing element PdCo and layered structure, thus we have not performed the thickness\ndependent of the polymer layers on the final sensing performance, and the thickness of Teflon AF (TAF)\nand PMMA are chosen purely based on the previous works.1,5,19 Nevertheless, such choices are based on\nthe following reasons.\nFirst, the evaporated TAF thin film is also porous, like C . Additionally, TAF can significantly improve\n60\nthe kinetics of an H sensor by modifying surface chemical and electronic states at the TAF/Pd interface,\n2\nthus it is purely interfacial effect, not the bulk effect.11 As a result, a thick coating layer is not required as\nlong as it can cover the nanostructures, and a 30-nm TAF layer is sufficient.1,11,19 In contrast, since the\nPMMA coating is used to inhibit the permeation of disparate gases such as water and oxygen, its thickness\nis crucial. Here, we chose to coat the device with a 50-nm thick PMMA. The diffusion speeds of H gas\n2\nin PMMA and TAF were found to be ~1.5 μm/s and ~23 μm/s, respectively.39 Therefore, in principle, a\nlayer of <0.1 μm of PMMA coating and <1 μm of PTFE coating should not have a noticeable effect on\nthe response time at high H concentration regime. To illustrate this point, we present the sensing\n2\nperformance of CHA sensors with and without PMMA coating in Figure R9. At low H concentration\n450 2\nwhere the external diffusion and dissociation become the rate-determining steps, coatings of any\nthicknesses might affect the reaction kinetics.42 In this context, there is a trade-off between response time\nand selectivity when using polymers as a coating layer.\nFigure R9. Response and release time of 20 nm C /3 nm TAF/5 nm PdCo/TAF CHA with\n60 450\nand without PMMA coating.\nChange made to the manuscript:\n• Figure R9 has been added to the Supplementary Information as Supplementary Fig. 30.\n• Manuscript (page 22, from line 14) has been revised accordingly.\n• Reference has been added to the manuscript:\nR20\n50 Östergren, I. et al. Highly Permeable Fluorinated Polymer Nanocomposites for Plasmonic Hydrogen\nSensing. ACS Applied Materials &amp; Interfaces 13, 21724-21732 (2021).\nhttps://doi.org:10.1021/acsami.1c01968\n1.10. Provide a clear section on potential future work, focusing on enhancing selectivity further and\naddressing interference from other gases or environmental factors.\nAuthors’ Response: We appreciate the reviewer’s suggestion.\nIn this study, we demonstrated the performance of our PdCo nano resistor networks under both vacuum\nand flow conditions with selected interferents. As future work, we aim to enhance selectivity by\noptimizing surface coatings that serve both as protective barriers and as selective filters for interfering\nspecies. To mitigate interference from gases such as O , we plan to investigate selective polymer coatings.\n2\ndemonstrated by Nugroho et al.23 However, due to its water solubility, PVOH is not effective under humid\nconditions. This highlights the need for further studies on coating thickness and tandem layer\nconfigurations to balance selectivity and environmental robustness.\nChange made to the manuscript:\n• This paragraph has been added to the manuscript (page 24, from line 2).\n• Reference has been added to the manuscript:\nTheir Nanocomposites. Molecules 20, 22833-22847 (2015).\nR21\nResponses to Comments from Reviewer #2\nFirst, I would like to congratulate the authors for such an impressive work. I follow closely the work from\nthe group and it is no surprise that they achieve this. The sensor reported accomplishes an unprecedented\ncombination of low limit of detection and high speed. The authors assigned the success to the addition of\nC60 to PdCo nanohole arrays, a platform that the group develop and work a lot on, that provides extra\nsurfaces yet interact minimally with the active element, allowing it to expand/contract freely during\nsensing. This conclusion (if true, see my comments below) will inspire other strategies aiming for similar\ngoal. The reported methods are detailed, and the figure/discussion are clear. I would love to see this\npublished in Nat Com, however, only if the authors can clarify the major questions/suggestions below.\n(Apology for my comments below that are random both in terms of importance and order they refer to in\nthe text).\nAuthors’ response: We thank the reviewer for your kind words. We are happy to have an opportunity to\naddress the reviewer’s critical remarks, important questions, and insightful suggestions.\n2.1. One missing, critical point in this work (that a Nat Com paper should have) is the general insight\nthat later can be extended to or inspire other fields. Here, author use C60, but no fundamental\ndesign/understanding derived. So further characterization/model are required to back up the argument.\nAlso, what other materials may be used to invoke similar effect?\nAuthors’ response: We thank the reviewer for this insightful comment. We fully agree that a high-impact\nstudy should provide general design principles that can inspire future research beyond the specific\nmaterials system investigated. In our response to Reviewer 1, Comment 1.3, we elaborate on the\nmultifunctional role of the C layer and frame its function within a broader materials design strategy. In\n60\naddition, we have characterized the X-ray diffraction (XRD) to study the crystallographic structures and\nX-ray photoelectron spectroscopy (XPS) analysis to study the chemical bonding and electronic\ninteractions between the PdCo and C previously shown in Comment 1.8.\n(i) enable rapid H permeability without forming strong or irreversible bonds with H at ambient\n2 2\nconditions,\n(ii) maintain mechanical and chemical stability under typical environmental conditions, and\n(iii) be compatible with scalable, reproducible nanofabrication techniques, such as thermal evaporation.\nThese design principles point to a broader class of candidate materials beyond C . For example, porous\n60\ncarbon-based frameworks (e.g., carbon nanotubes26), certain metal–organic frameworks (MOFs) with\nweak hydrogen binding energies (e.g. Zeolite imidazole framework (ZIF)43), and thin polymer layers with\nhigh gas permeability and low surface energy (e.g. polyimide (PI)10 and PTFE or Teflon AF) could all\npotentially be engineered to deliver similar performance enhancements. We believe this framework can\nguide future efforts to tailor surface/interface layers in a wide range of chemical sensing platforms.\nChange made to the manuscript:\n• A paragraph has been added to the end of the discussion section of the manuscript (page 26, line 16).\n• References have been added to the manuscript:\nR22\nsubstrate on the mechanical and thermal stability of Pd thin films during hydrogen (de)sorption.\nInternational Journal of Hydrogen Energy 46, 4137-4153 (2021).\nhttps://doi.org:https://doi.org/10.1016/j.ijhydene.2020.10.163\nGas Sensor Using Multiwalled Carbon Nanotubes for Specific Detection of Hydrogen at Parts Per\nBillion Levels. IEEE Sensors Journal 15, 2391-2398 (2015).\nhttps://doi.org:10.1109/JSEN.2014.2369738\nwith Excellent Selectivity to H2. Sensors 21, 4069 (2021). https://doi.org:10.3390/s21124069\n2.2. Related to that note, I find the argument explaining the reasons behind the enhancement to be\nunjustified and at times contradictory.\n(i) C60 is thought to increase the SVR. How? Surface here only deals with the material that interacts with H2,\nexhibits properties change, which in turn converts into signal in the sensor reading. In\nfact, in the calculation authors include the surface of C60 but exclude its volume. Both S\nand V must be evaluated from the same system. Or am I wrong here? Another point for my argument is\nsince authors said C60 does not interact with H2, it is then as if it wasn’t there, which then should\nnot be included in the surface area consideration.\nAuthors’ response: We thank the reviewer for the opportunity to clarify our calculation.\nFor PdCo deposited directly on glass substrate, the bottom surface area of PdCo is passive to the\nsensing process. While on C -coated, H can diffuse through the voids between C molecules,\n60 2 60\nespecially if the film is amorphous or contains grain boundaries (also see Comment 1.8).8 Thus,\nthe bottom of the PdCo layer now is not completely passive to hydrogenation and should be\nconsidered in the surface area calculation.\nThe reviewer is correct that our sensing element is PdCo, thus we only need the surface and volume\nof PdCo. However, since it is deposited on C , the shape and thickness of PdCo are affected by\n60\nthe shape of its C template and the size of PS beads as illustrated on Supplementary Fig. 15.\n60\nparameters of the PdCo. For instance, top surface of C is equal to the bottom surface of PdCo,\n60\nand the volume of PdCo layer V = V – V from simulation point of view.\nPdCo total (both bilayers) C60\nAs correctly noted by the reviewer, the hydrogen absorption of C is negligible under our testing\n60\nconditions. However, we hypothesize that the C interlayer facilitates hydrogen diffusion at the\n60\neffective diffusion and reaction area of the sensing layer.\n(ii) Even if the assumption by authors is correct, how come C60 is connected to SVR even with the\nthin film configuration (Fig 2). Here the only access for H2 to PdCo is only through the top\n(the PdCo surface), since H2 access through the bottom of PdCo is only through the side walls of C60,\nwhich is much smaller than the sample lateral area.\nAuthors’ response: We thank the reviewer for this important comment, as it rightly points out that\nhydrogen diffusion speed in solid-state materials such as C solid is finite and should not be\n60\noverlooked. In addition, the thickness of the C might be critical in the thin film-based sensing\nregime. In response, we have conducted a comparative analysis using several additional thin-film\nsensor configurations and found that the hydrogen diffusion speed in C is indeed not infinite. We\n60\nalso note that the response time and sensitivity of the sensor depend not only on the SVR but also\non other factors such as the clamping effect and C morphology (see Comment 1.3). We are\n60\nactively pursuing direct measurements of hydrogen diffusion through C /Pd bilayer thin films\n60\nlocally at different positions on the films using optical techniques.14 Although these measurements\nare beyond the scope of the current study, the results will be reported in future work.\nTo improve the clarity and consistency of our manuscript, we have updated Figure 2 in the main\ntext. It now directly compares the sensing performance between glass/PdCo CHA and\nglass/C /PdCo CHA structures, rather than the thin-film structures where the hydrogen diffusion\n60\ntime in amorphous C is negligible over a diffusion path length of a few hundred nanometers. The\n60\nresults reaffirm our findings, showing significantly enhanced sensor performance with the C\n60\ninterlayer.\n(iii) C60 is also thought to increase sensitivity (i.e. through hydrogen loading) due to the minimum\nclamping between it and the PdCo. However, author also argues that C60 increases the PdCo\nwettability. These two are contrary to each other. So which one is true? I think authors should study\nin detail this interface and see if there is any chemical bonding formation.\nAuthors’ response: Thank you for this insightful comment. Please refer to Comment 1.3 for\nfurther discussion on the role of the C layer, including the clamping effect and its impact on\n60\nsensing performance.\nWe would like to clarify our original statement: PdCo exhibits improved wettability on C\n60\ncompared to glass, but this does not contradict the observation of a weaker clamping effect. This\ndistinction addresses your concern regarding the seemingly conflicting interpretations of\nwettability and mechanical decoupling.\nGlass has inherently low surface energy and is chemically inert, leading to poor adhesion and\ndiscontinuous film formation for PdCo. In our previous work, Pham et al. observed that PdCo films\nthinner than 10 nm deposited on glass tend to form isolated islands, resulting in high electrical\nresistance and even negative hydrogen sensitivity due to nanoparticle coalescence during\nhydrogenation.5 Similar behavior has been reported in the literature.44,45\nWhen deposited on C , Pd and Co atoms exhibit slightly improved wettability, which we attribute\n60\nto weak charge-transfer interactions particularly under low surface coverage or elevated\ntemperature.46 However, these interactions remain weak, allowing for a loosely bonded interface\nwith abundant free Pd at the surface, beneficial for hydrogen catalysis. Film growth still follows\nthe Volmer–Weber (island) mode, resulting in a discontinuous morphology. This interpretation is\nsupported by prior studies. For instance, Kaushik et al. report a lower percolation threshold for Co\non C (2.3–4.0 nm) than on glass, indicating better film formation without strong bonding.47\n60\nresistance Pd islands.48 These findings suggest that while PdCo spreads more readily on C than\n60\non glass, the interfacial interaction remains weak and mechanically decoupled.\nR24\nRegarding chemical bonding, direct spectroscopic evidence is subtle due to the weak nature of Pd–\nC interactions. For detailed discussion on these interactions and their detection via techniques\n60\nsuch as X-ray photoelectron spectroscopy (XPS), we refer to previous works by Weckesser et al.46\nand Anděra et al.40 Furthermore, we provide evidence of chemical bonding between C and Pd by\nIn summary, PdCo films display better morphology on C than on glass due to improved\n60\nwettability, but the chemical interactions remain weak. Since C film itself is composed of van der\n60\nforces during hydrogenation and dehydrogenation, thereby, preserving and enhancing sensor\nperformance.\n(iv) Further contrasting argument related to point (iii) is that such enhancement effect is also\npresent when there is FTA in between C60 and PdCo. So what is the role of C60 here then? Does FTA also\nhave similar effect as C60? If so, why then it is thickness-dependent? And the thickness dependency\nalso shows rather contradicting results. If it really due to clamping being less, the thicker the FTA,\nthe less the clamping effect is. However, for 50 nm, the thickest FTA studied, the improvement is\nactually the lowest. All in all there are major investigation and analysis required to reach any\nconclusion for the study.\nAuthors’ response: We thank the reviewer for this insightful comment. To clarify, we have only\nconducted thickness dependence of C and have not done so for TAF. As shown in Figure 3, the\n60\nresponse time and sensitivity of the CHAs appear independent of C thickness. This may be\n60\nbecause the CHA morphology already introduces abundant nanoscale pathways and adsorption\nsites for H , effectively masking any subtle influence from varying C thickness within the tested\n2 60\nrange.\nTAF, however, cannot fully substitute for C . Due to poor adhesion to the glass substrate, TAF\n60\nalone is completely removed during the polystyrene bead lift-off process, and PdCo might penetrate\ninto a soft TAF layer resulting in a discontinuous network. The C layer serves as a necessary\n60\nmorphological template, defining the geometry of the PdCo nanostructure. When only a few-nm\nTAF is deposited on top of the C , it provides three synergistic advantages: (i) preserving of\n60\nstructure as the underlying C geometry is retained, (ii) PdCo is surrounded by TAF, which is\n60\nknown to promote hydrogen adsorption kinetic,11 and (iii) the underlayer of TAF induces lattice\nstrain and reduces PdCo crystallite size as shown in XRD (Figure R7, page R17), which can\nenhance hydrogen sorption kinetics.\nChange made to the manuscript:\n• Figure 2 in the main text has been updated.\n• The role of C has been elaborated in the main text (page 10, from line 1; page 11, from line 5;\n60\npage 13, from line 10; page 15 from line 1) and the role of TAF has been also detailed in the\nmanuscript (page 21, from line 16).\nR25\n2.3. Results and Discussion headers are missing.\nAuthors’ response: We thank the reviewer for pointing it out. We have added the headers and updated\nthe manuscript following the journal’s format guidelines.\n2.4. Please discuss recent strategies in improving sensitivity (to ppb level) in H2 sensors (electric, optic, ect). This will benefit\nthe readers immensely.\nAuthors’ response: We thank the reviewer for your constructive suggestion. We have added Table R1\n(page R8) with state-of-the-art ultra-low LOD hydrogen sensors across all flatforms (e.g. electrical,\noptical, etc.), including the devices’ structure, transducing methods and their corresponding sensing\nmetrics for a broader comparative context. Additionally, the Introduction in the manuscript has been\nrevised accordingly to discuss this point.\nChange made to the manuscript:\n• Table R1 has been added to the Supplementary Information as Supplementary Table 1.\n• The following paragraph has been added to the introduction of the manuscript (Page 3, Line 8).\n“Recent advancements across both electrical and optical platforms in achieving ultra-fast response time\nand ppb-level sensitivity in hydrogen sensors have centered on nanostructured Pd hydride systems due to\ntheir high selectivity, excellent sensitivity, fast response times, and mechanical and chemical robustness\n(see Supplementary Table 1).4,5,11,19,31,35,49 Pd-based alloys (e.g., PdCo, PdAu) dominate high-impact\ndesigns. For instance, recent works on optical sensors by Nugroho et al. using scattering spectrum from\nPdAu nanoparticle@polymer nanocomposite materials1, and Luong et al. using transmission intensity\nthrough PdCo nanocap arrays5 explicitly demonstrate sub-second response with LODs around single-\ndigit ppm. The LOD could be further improved to a few hundreds of ppb by making use of the particle\nswarm optimization algorithm to optimize the sensor array configurations,50 neural-network-based data\ntreatment to correct the baseline drift while testing in extreme humidified conditions,51 or by simply\nstacking the sensors vertically to improve the signal-to-noise ratio (SNR).19\nIn electrical platforms, this transducing method shows great promise for achieving ultra-sensitive LOD\nof a few ppb by virtue of the low noise signal readouts. Pham et al. proposed a PdCo composite hole array\n(CHA) nano network with a measured LOD of 180 ppb, however the response time fell short of the 1-s\nresponse time benchmark required for automotive applications.5 Pd-doped oxides (e.g., Pd@Fe₂O₃\nnanotubes) combine Pd's catalytic activity with oxide heterojunctions, achieving 50 ppb LOD through\nsynergistic surface reactions and p–n junction modulation.18 Non-Pd materials, such as rGO/ZnO-SnO₂\ncomposites, also reach 50 ppb sensitivity via Pd doping and heterostructure engineering,22 though they\nlag behind Pd hydrides in speed and ambient stability. These strategies highlight the critical role\nof nanostructuring, alloying, and hybrid material design tailoring for each transducing method and\nsensing mechanism in pushing sensitivity limits, with Pd-based systems remaining the gold standard for\nsafety-critical applications.”\n• References have been added to the main text:\nR26\nlimit of detection in humid air. Nature Communications 15, 1208 (2024).\nhttps://doi.org:10.1038/s41467-024-45484-9\nDoped α-Fe2O3 Nanotubes. ACS Sensors 9, 5976-5984 (2024).\nhttps://doi.org:10.1021/acssensors.4c01829\nbased on Pd-doped rGO/ZnO-SnO2 from hydrothermal synthesis. Microsystems &amp;\nNanoengineering 8 (2022). https://doi.org:10.1038/s41378-022-00398-8\n2.5. In the real setting, is there a need to measure H2 down to ppt level?\nAuthors’ response: We thank the reviewer for raising this important point. While the U.S. Department\nof Energy (DOE) specifies a detection requirement of 10 ppb for hydrogen in environmental monitoring,\nthere are compelling reasons to pursue detection limits down to the low ppb or even ppt range.\nFirst, pushing the detection limit serves as a benchmark for optimizing sensor performance under noise-\nlimited conditions. Achieving reliable detection at these ultra-low concentrations requires a low-noise\ntransduction platform, high signal-to-noise ratios, and advanced signal processing techniques to detect\nearly signal variations.51,52\nSecond, sensors capable of ppt-level hydrogen detection are valuable for specialized applications such as\ndeep-space missions, where trace hydrogen detection is critical in planetary science and astrobiology.24\nFinally, there is growing interest in detecting hydrogen isotopes such as deuterium and tritium,\nparticularly in the context of nuclear power and waste management, where their concentrations can be\nsignificantly lower than that of molecular hydrogen in air.53 Our sensor platform holds promise for further\ndevelopment to detect these isotopes in such challenging environments.\n2.6. In is mentioned that GLACD method enhances quality, morphology, and SVR. How is it so?\nAuthors’ response: We thank the reviewer for pointing it out. We have removed that claim in the abstract\nand revised the introduction in the manuscript to clarify/highlight advantages of employing glancing angle\nco-deposition in our sensor design as follows:\nGLACD enables the formation of a mixture of two metals under far-from-equilibrium conditions,\nresulting in films with high defect densities and internal stress, these features create abundant diffusion\npaths and active sites that are advantageous for sensing applications. The porous, columnar morphology\ntypical of GLACD films further improves gas accessibility.54 Despite being structurally less ordered, such\nfilms offer tunable properties not achievable through equilibrium processing.55\nChange made to the manuscript:\n• The manuscript’s abstract and introduction (page 5, line 2) have been revised accordingly.\n• References have been added to the main text:\ndesign of nanostructures. 8, 1-26 (2019). https://doi.org:doi:10.1515/nanoph-2018-0105\nR27\n2.7. Related to above, it is said the method will get the sample to “their full potential.” This is highly\nspeculative and unjustified. Also, what are their full potential? Is there any fundamental ceiling to this?\nAuthors’ response: We thank the reviewer for this insightful comment. We agree that the original phrase\n“full potential” was overly strong and could be interpreted as speculative or unjustified. Our intention\nwas to convey that we have undertaken a systematic optimization of key parameters such as film\nthickness, material concentration, and surface modification to enhance the performance of our sensing\nplatform. In response to the reviewer’s feedback, we have revised both the abstract and main text to\nremove the phrase “full potential” and to avoid any implication of a fundamental performance limit.\nInstead, we now highlight our approach to performance optimization without making speculative claims\nabout ultimate material capabilities.\nChange made to the manuscript:\n• The term “their full potential” has been removed.\n• The abstract and introduction of the manuscript have been revised accordingly.\n2.8. Since the LOD is the result of extrapolation, I don’t think it is appropriate to use it in the title and\nabstract, as it may trick readers. Only highlight such a number in these places if it is demonstrated, or\nexplicitly write that it is an “extrapolated LOD”\nAuthors’ response: We thank the reviewer for an on-point comment. We have modified the title to claim\nppb LOD, which was a measured LOD rather than the extrapolated LOD. Please refer to Comment 2.9\nbelow for the updated LOD data.\nChange made to the manuscript:\n• The title has been modified from “parts-per-trillion” to “parts-per-billion”.\n2.9. Regarding the extrapolation, what is the basis for it? I think it is never mentioned. Or the authors\njust extend the line following the data?\nAuthors’ response: The reviewer is correct. This is the way that many papers have presented to extract\nthe ultimate LOD due to the limitation of the test equipment. Here, we have taken additional LOD\nexperiments for 3 individual devices (Figure R10) to verify the ultimate measured LOD of 40 ppb without\nany extrapolation and modified the abstract at title accordingly.\nR28\nFigure R10. LOD measurements of 20 nm C /3nm TAF/5 nm PdCo/30 nm TAF CHA sensor.\n60 450\na, b Sensors’ response at low H pressure regime. c Measured sensitivities across 6 orders of H\n2 2\npressure. d Averaged R/R plot as a function of P with the error bars indicate the standard deviation\nH2\nfrom 3 devices. The vertical blue line denotes the defined LOD of 3 = 0.012% at 8.4 Hz sampling\nfrequency (Supplementary Note 8). All measurements were performed in vacuum mode at room\ntemperature. Note that d is identical with Figure 6d in the manuscript.\nChange made to the manuscript:\n• Figure R10 has been added to Supplementary Information as Supplementary Fig. 27.\n• In the main text, Figure 6d has been replaced with Figure R10d.\n• The title and abstract have been revised accordingly.\n2.10. Again regarding extrapolation, I don’t think it is justified to claim a value from an extrapolation 3\norders of magnitude higher/lower than the last experimental data point (here from 100 ppb to 0.1 ppb).\nMany things/trends can happen within that expansive range.\nAuthors’ response: We thank the reviewer for raising this important point. We agree with the reviewer\nthat the slopes of the pressure-composition curve are not the same across the wide range of H pressure\n2\ndepending on which step is the rate-determining step. There is a need to develop a theoretical model for\nH diffusion in Pd nanoparticles to justify the extrapolation at extremely low H concentration.\n2 2\nR29\nIn the revised manuscript, we removed the extrapolated curve and only focused on highlighting the\nmeasured LOD of 40 ppb with the signal-to-noise ratio SNR  10 with a potential to detect a few ppb.\nChange made to the manuscript:\n• The title and abstract have been revised accordingly.\n2.11. Another main concern is the use of idealized environments throughout the study. Looking at the\nstate of the field now, where it has progressed immensely, I would highly encourage that we start using\n“real environment” more, especially for a publication in Nat Com. Thus, I am curious to see how the\nsensors respond in (synthetic) air. This is important in general, but even more for the authors since they\nclaim that their sensors fulfil the strictest requirement for environmental monitoring (which ironically\ncomprises air as background). Achieving what the authors did is impressive, but this does not reflect the\nreal condition at all. In air, there will be many complex reactions being involved. How does the\nproposed concept perform in air? If not, how far is it from the ideal situation? As such, similar to my\ncomment above, whenever ideal conditions are used, please also have an explicit statement regarding it\nwhen mentioning the performance (especially in title and abstract).\nAuthors’ response: We appreciate the reviewer’s thoughtful comment and fully agree that testing under\nrealistic conditions (i.e. in air) is essential for evaluating the practical utility of hydrogen sensors. The\npresence of oxygen in air can indeed alter surface chemistry and sensor performance by blocking Pd\nactive sites and promoting side reactions such as water formation through the oxygen reduction reaction\n(ORR).56 This remains an area of active and ongoing investigation. Here, we incorporated recent advances\nin surface passivation strategy proposed by Nugroho et al., which employs a tandem coating consisting\nof PMMA and only 5-nm thin PVOH (polyvinyl alcohol) layers, where the PVOH coating effectively\nmitigates the O permeability while maintaining high H diffusivity.23 Our preliminary results of 15 nm\n2 2\nPd Co thin films are presented in Figure R11.\n67 33\nComparing the signal change to 2% H in synthetic air versus in N as gas carrier (Figure R11a), the\n2 2\nbaseline drift may be attributed the faster hydrogen desorption in the presence of O in air and eventually\n2\nforming water via ORR.56 While a 50 nm PMMA layer offers limited protection (Figure R11b), PVOH\ncoatings effectively suppress the adverse effects of oxygen shown in the absence of such baseline drifts\nwhen the sensors coated with PVOH (Figure R11c-d). As demonstrated in Gaaz et al., polyvinyl alcohol\nexhibits superior oxygen barrier properties among polymers due to its dense hydrogen bonding network\nand partial crystallinity, making it an effective shielding layer to suppress O interference on active\n2\nsensing surfaces under dry conditions.57 However, even with just a few nanometers of coverage, the\nPVOH layer significantly reduces hydrogen sorption kinetics. While the PMMA-coated sensor exhibits a\nresponse time t of 1.9 ± 0.1 s (extracted from the flow in N ), the addition of 8 nm and 4 nm PVOH\n90 2\ncoatings increases t to 31.0 ± 0.5 s and 22.0 ± 0.2 s, respectively, highlighting a strong diffusion barrier\n90\neffect for H . Additionally, due to its water solubility, PVOH is not effective under humid conditions. This\n2\nhighlights the need for further studies on coating thickness and tandem layer configurations to balance\nselectivity, detection speed and environmental robustness, which is beyond the scope of this study.\nR30\nFigure R11. Sensing response of 15 nm PdCo thin film sensors with different polymer coatings to 2%\nH in N and synthetic air as gas carrier. All measurements were performed in flow mode with a\n2 2\nconstant flow rate of 400 sccm at room temperature.\nChange made to the manuscript:\n• The testing conditions have been listed explicitly in the last paragraph of the manuscript\nintroduction (page 5, lines 13 and 22).\n• The following discussion has been added to the manuscript (page 24, line 3):\n“While this study focuses on idealized conditions, we acknowledge that evaluating sensor\nperformance in synthetic air is essential for real-world applications. Some preliminary results\nwith polyvinyl alcohol (PVOH)-coated sensors show that oxygen interference can be mitigated;\nhowever, the coating introduces trade-offs in hydrogen diffusion and limits applicability under\nhumid conditions.23,57 Further optimization of coating thickness and architecture to balance\nselectivity, response time, and environmental robustness remains an important future direction\nbeyond the scope of this work.”\n• References have been added to the main text:\nTheir Nanocomposites. Molecules 20, 22833-22847 (2015).\nR31\n2.12. Related to above, in the contamination tests, I encourage authors to instead expose the sensors to\nconstant dose of contaminations, instead of having them in cycle together with the hydrogen. This,\nagain, is closer to the real conditions. I am curious to see the response to humidity using this scenario,\nas based on my experience, PMMA would absorb water and eventually deteriorates the sensor’s\nperformance.\nAuthors’ response: We appreciate the reviewer’s insightful suggestion to cycle the sensor through\nvarying levels of humidity while monitoring the response, as it would indeed better reflect real-world\nconditions. However, due to the design of our current experimental apparatus, where humidity is\nintroduced via a water bath humidifier, we are only able to maintain fixed humidity levels during the tests.\nUnfortunately, this setup does not allow cycling across multiple RH levels within a single measurement\nsequence at the moment.\nWe acknowledge this as a limitation of our current work. Nonetheless, we believe our measurements at\ntwo representative conditions provide meaningful insight into the sensor's performance. We consider\ndynamic RH cycling an important direction for future apparatus improvements.\nR32\nResponses to Comments from Reviewer #3\nThe authors described how to fabricate unique hydrogen sensing element, a PdCo composite hole array\non a fullerene-decorated glass substrate, and evaluated the sensing times and detection limits for H2\nconcentrations were evaluated in relation to the structure of the sensing element. By fine-tuning the\nstructure of the PdCo composite hole array, they achieved a reaction time of less than 1 second and a\ndetection limit at ppt level. It is noteworthy that a high-performance sensor element can be constructed\nusing a simple fabrication method, and the structure of this element may also be applicable to gas sensors\nother than hydrogen. However, I believe that further discussion is needed on the relationship between the\nstructure and performance of sensing elements before publication, the authors need to fully and in detail\naddress the following points:\nAuthors’ response: We thank the reviewer for your kind words. We are happy to have an opportunity to\naddress the reviewer’s critical remarks, important questions, and insightful suggestions.\n3.1. The authors need to explain the crystal structures of the PdCo and C60 layers, as this information is\ncrucial for understanding the hydrogen absorption properties of the PdCo layer and the H2 gas behavior\nthrough the C60 layer.\nAuthors’ response: We thank the reviewer for your insightful suggestion. We have taken XRD for 15 nm\nPd Co thin films deposited on different coated glass substrates as shown in Figure R7. XRD was taken\n63 37\nusing X-ray diffractometer (Rigaku, Cu Kα: λ = 1.54056 Å, 40 kV, and 40 mA). First, thermal evaporated\nC thin film on non-epitaxial substrates like glass often leads to a mix of polycrystalline and amorphous-\n60\nlike regions.7 Thus, no peak was observed in XRD spectrum of 20 nm C thin film in Figure R7a,\n60\nconfirming the absence of long-range crystalline order.36 The C film is not perfectly close-packed but\n60\nexhibits partial ordering with voids and grain boundaries. It is supported by Samad et al. where the\nporosity of C thin films was reported to be more than 50% when the substrate was kept at room\n60\ntemperature during the deposition.37\nSecond, PdCo thin films deposited on different coated substrates exhibit the strongest Bragg diffraction\npeaks between 41.3° and 41.6°, which can be attributed to the (111) orientation of the face-centered cubic\nPdCo lattice. These peaks are shifted to higher angles relative to the (111) peak of pure fcc Pd\n(∼40.2°), indicating that Co atoms are substitutionally incorporated into the Pd lattice. The observed peak\npositions and extracted lattice constants are consistent with prior literature report.38 Analysis of the full\nwidth at half maximum (FWHM) values allowed for the estimation of crystallite size and lattice strain,\nas summarized in the accompanying Table R2. Notably, the C /PdCo sample exhibits a larger crystallite\n60\nsize and reduced lattice strain compared to PdCo grown directly on glass, likely due to the smoother\ngrowth surface provided by the C layer. This finding supports our claim of the improvement in\n60\nwettability between Pd and the C -coated substrate. Although larger grains typically correlate with slower\n60\nsensor response, the C /PdCo sample demonstrated enhanced sensitivity and faster response times (Fig.\n60\n2 main text), suggesting that gas diffusion through the C interlayer plays a critical role. In contrast,\nattributed to pinholes in the TAF film, as observed in AFM imaging (Supplementary Fig. 9). The\nimproved sensing response of samples with TAF underneath appears to result from a combination of\nR33\nreduced grain size, efficient gas permeation through the underlayer, and a lowered energy barrier for\nhydrogen sorption at the TAF/PdCo interfaces11.\nFigure R7. XRD spectra of a 20 nm C thin film and b 15 nm Pd Co thin films on different coated\n60 63 37\nglass substrates.\nTable R2. Crystal structure and other physical properties of 15 nm Pd Co thin films on different coated\n63 37\nglass substrates extracted from XRD.\nCrystallite Lattice Lattice\nPeak position FWHM d-spacing\nsize Strain constant\nSample\n2 (deg)  (deg) D (nm)  (%) d (Å) a (Å)\nPdCo 41.559 ± 0.002 0.93 ± 0.01 9.14 ± 0.06 1.07 ± 0.01 2.17 ± 0.01 3.76 ± 0.03\nC /PdCo 41.571 ± 0.003 0.90 ± 0.01 9.55 ± 0.07 1.02 ± 0.01 2.17 ± 0.02 3.76 ± 0.03\nChange made to the manuscript:\n• XRD analysis has been added to Supplementary Note 5.\n• XRD and XPS measurements have been added to the Methods section of the manuscript (page 28,\nfrom line 11).\nR34\n3.2. The effect of the C60 layer is unclear. There seem to be a contradiction between the following two\nconsiderations. In fig. 2, the authors pointed out that the C60 layer mitigated the clamping effect on the\nPdCo layer, maximizing the hydrogen uptake and the sensitivity. Meanwhile, in fig. 6, the TAF layer\ninserted between the PdCo and C60 layers was useful to improving sensitivity. If the TAF layer also\nworks to mitigate the clamping effect, the effect of C60 is thought to be only due to its weak adhesion to\nthe substrate.\nAuthors’ response: Thank you for this insightful comment. We note that Reviewers 1 and 2 raised\nsimilar concerns; for a detailed discussion of the role of the C layer and its influence on sensing\n60\nperformance, please refer to our response to Comment 1.3. The clamping effect is addressed in Comment\n2.2 (iii) on page R24, and the role of the TAF layer is discussed in Comment 2.2 (iv) on page R25.\n3.3. What about temperature stability in resistance measurement? Since resistance in metals depends on\ntemperature, it is possible that the baseline of resistance R(0) can shift depending on temperature. While\nthe baseline of ΔR/R shown in figs. S18 and S21 of the supplementally materials shows an\napproximately straight line, the resistance was not shown. I kindly ask the authors to comment on the\nstability of the temperature of the system during measurements and on the temperature dependence of\nthe resistance for the fabricated sample.\nAuthors’ response: We thank the reviewer for this valuable comment. To address the temperature\nstability of the system and the temperature dependence of resistance, we conducted additional\nmeasurements at varying temperatures from room temperature (~23 °C) up to 115 °C. The temperature\nwas continuously monitored using a thermocouple affixed to the substrate holder inside the gas cell. As\nshown in Figure R5a, the system exhibited excellent thermal stability, with fluctuations remaining below\n0.3°C throughout the measurement period.\nIn Figure R5b, we present the resistance response under hydrogen exposure across the tested temperature\nrange. The baseline resistance R values at each temperature are shown in Figure R5c. At lower\n0\ntemperatures, R remains highly stable, following an approximately linear increase from 1260 Ω at room\n0\ntemperature to ~1320 Ω at 100 °C, consistent with typical metallic behavior where increased electron–\nphonon scattering raises resistance. We also note that the current used for resistance measurement is\nlimited to 0.1 mA, minimizing Joule heating effects. However, at 115 °C, R slightly decreases to\n0\n~1290 Ω, which we attribute to possible thermally induced morphological changes in the nanoparticle\nnetwork such as expansion or coalescence that may alter percolation paths.\nAs illustrated in Figure R5d, the sensor’s sensitivity declines at elevated temperatures, consistent with\nsuppressed β-phase hydride formation and enhanced desorption kinetics at higher temperatures.4\nOverall, our findings suggest that room temperature provides the optimal balance between baseline\nstability and sensing performance for our PdCo nano network structure. For field applications, we propose\nintegrating a local temperature control element such as a thermoelectric cooler (TEC) into the sensor\nhousing to ensure consistent operation under ambient fluctuations.\nR35\nFigure R5. a Measured temperatures at the sensor position during the tests. b Sensing responses of 20\nnm C /5 nm PdCo CHA to stepwise decreasing H concentrations with N as the gas carrier and c\n60 450 2 2\nextracted baseline R and d extracted sensitivity. The error bars in d indicate the standard deviation from\n0\n3 pulses.\n3.4. Why were response times measured in vacuum mode? For the same hydrogen concentration, is the\nresponse time the same in vacuum mode and flow mode? We believe that the response time in flow\nmode will be slower than that in vacuum mode, because the H2 diffusion time in the carrier gas (N2)\nmay be the rate-limiting step for detection at low H2 concentration in mixture gas. Furthermore, when\napplying samples to hydrogen sensors in atmospheric environments, we generally believe that the\nresponse time in flow mode reflects reality.\nAuthors’ response: We thank the reviewer for this important comment. We agree that flow mode better\nreflects real-world deployment conditions for hydrogen sensors. However, we also believe vacuum mode\nmeasurements serve a distinct and complementary purpose, particularly in evaluating and optimizing the\nintrinsic response speed of the sensing material itself.\nAs shown in the added Figure R12, response time in flow mode is significantly affected by external\nparameters such as carrier gas flow rate. For example, reducing the total flow from 400 sccm to 32 sccm\nR36\nresults in a substantial increase in response time, illustrating that external diffusion becomes the rate-\nlimiting step.42 In contrast, vacuum mode eliminates these transport limitations as the gas pressure reaches\nequilibrium in <100 ms (Supplementary Fig. 4) and allows us to isolate the sensing element’s intrinsic\nkinetics.\nTo provide a specific comparison:\n• In vacuum mode (1 mbar H , ~0.1%), the CHA sensor reaches 90% response in under 2 s (see\n2 300\nmain text Figure 4g).\n• In flow mode at 400 sccm, the same concentration yields a t of ~30 s (Figure R12b).\nare inherently fast, and that the observed delay in flow mode stems from the time required for hydrogen\nmolecules to diffuse and adsorb onto the surface of the sensing element.\nTherefore, while flow mode is essential for practical validation, vacuum mode offers valuable insights\ninto the material-level optimization and can serve as a benchmark for assessing the limitations imposed\nby sensor housing and gas delivery design. This comparison also raises important considerations for\nfuture device engineering, such as enhancing gas dynamics via integrated pumps, or flow channel\noptimization.58\nFigure R12. a Sensing response of 20 nm C /5 nm PdCo/TAF CHA sensor to 0.1% H pulses at\n60 300 2\ndifferent total flow rates and b extracted response times as a function of flow rate. All measurements\nwere performed at room temperature.\nChange made to the manuscript:\n• The discussion regarding vacuum mode and flow mode measurements have been added to the\nSupplementary Note 3.\nR37\n3.5. In fig. 7, is the deterioration speed of the uncovered sample (20nmC60/5nmPdCo/TAF CHA450) to\ninterference gases and humidity faster than that of the PMMA-covered sample? Since the data of the\nuncovered sample was not shown, the effect of the PMMA coating on interference gases and humidity\nwas not clear in the manuscript. Please show the data of the comparative sample related to fig. 7 in the\nsupplementally materials.\nAuthors’ response: We thank the reviewer for this valuable comment. In this study, we directly tested\nthe PMMA-coated samples based on our previous findings in optical sensors,1 where the uncoated PdCo\nsensing element exhibited a pronounced signal degradation upon exposure to 0.2% CO and RH = 40%.\nIn the present work using an electrical resistor network, we observed a similar decline in signal amplitudes\nfor the uncoated device under interfering gas exposure as depicted in Figure R13. This deterioration is\nattributed to the competitive adsorption of CO and H O molecules on the PdCo surface, which blocks\n2\nactive hydrogen adsorption sites. This observation is consistent with recent findings by Kim et al.,29 who\nreported that CO binds strongly to Pd surfaces due to its higher adsorption energy compared to CO and\n2\nprominent C=O bond signal at ~288 eV (see Figure R4, page R12), indicating CO adsorption.\nFigure R13. Interference tests of 20 nm C /3 nm TAF/5 nm PdCo/TAF CHA (no PMMA coating)\n60 450\nwith different a gases and b relative humidity (RH) levels. The error bars in b and d indicate the standard\ndeviation from 10 pulses. All measurements were performed at room temperature in flow mode at a\nconstant flow rate of 400 sccm and with N as gas carrier.\n• Figure R13 has been added to the Supplementary Information as Supplementary Fig. 29.\n• The manuscript has been revised accordingly (page 22, line 15).\nR38\nRebuttal references\ncontrol of nano-hydride geometry and composition. 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Nature Communications 13 (2022). https://doi.org:10.1038/s41467-022-\n33466-8\noxidized nickel foam. Sensors and Actuators B: Chemical 329, 129194 (2021).\nhttps://doi.org:https://doi.org/10.1016/j.snb.2020.129194\non semiconducting carbon nanotube film. Carbon 180, 41-47 (2021).\nhttps://doi.org:https://doi.org/10.1016/j.carbon.2021.04.076\nIonization Gas Sensor Using Multiwalled Carbon Nanotubes for Specific Detection of\nHydrogen at Parts Per Billion Levels. IEEE Sensors Journal 15, 2391-2398 (2015).\nhttps://doi.org:10.1109/JSEN.2014.2369738\nnanoparticles for ultrasensitive hydrogen sensors. Nanoscale 7, 20665-20673 (2015).\nhttps://doi.org:10.1039/C5NR06193H\nUltrasensitive and Flexible Hydrogen Gas Sensing. Scientific Reports 5, 12294 (2015).\nhttps://doi.org:10.1038/srep12294\npalladium nanowire. Nature Communications 15 (2024). https://doi.org:10.1038/s41467-024-\n53080-0\nHydrogen Sensors and Hydrogen-Mediated Devices. ACS Applied Nano Materials (2025).\nhttps://doi.org:10.1021/acsanm.5c00392\ncontrol of nano-hydride geometry and composition. Nature Communications 12, 2414 (2021).\nhttps://doi.org:10.1038/s41467-021-22697-w\nfabricated by a surfactant-assisted spin-coating method with polyoxyethylene tridecyl ether.\nScientific Reports 9 (2019). https://doi.org:10.1038/s41598-019-47990-z\nAir/Water Interface. Langmuir 40, 18039-18048 (2024).\nhttps://doi.org:10.1021/acs.langmuir.4c01622\nAdvanced Science 10 (2023). https://doi.org:10.1002/advs.202303992\nUsing Fully Suspended Pure Pd Nanowire. ACS Nano 17, 23649-23658 (2023).\nhttps://doi.org:10.1021/acsnano.3c06806\nR40\nfullerene-based devices: Morphology effect. Physical Review B 87 (2013).\nhttps://doi.org:10.1103/physrevb.87.075205\nelectrical and optical properties of C60 thin films. Journal of Applied Physics 132 (2022).\nhttps://doi.org:10.1063/5.0099291\nmagnetization in CoPd thin films and nanocontacts. Journal of Magnetism and Magnetic\nMaterials 325, 112-116 (2013). https://doi.org:https://doi.org/10.1016/j.jmmm.2012.07.052\n39 Östergren, I. et al. Highly Permeable Fluorinated Polymer Nanocomposites for Plasmonic\nHydrogen Sensing. ACS Applied Materials &amp; Interfaces 13, 21724-21732 (2021).\nhttps://doi.org:10.1021/acsami.1c01968\noverlayers with fullerene films. Czechoslovak Journal of Physics 43, 863-868 (1993).\nhttps://doi.org:10.1007/bf01595270\nImproved Kinetics in Hydrogen Sensing and Energy Storage Applications. Angewandte Chemie\nInternational Edition 53, 12081-12085 (2014). https://doi.org:10.1002/anie.201406911\nreview. Journal of Membrane Science 375, 28-45 (2011).\nhttps://doi.org:https://doi.org/10.1016/j.memsci.2011.03.057\nSensors with Excellent Selectivity to H2. Sensors 21, 4069 (2021).\nhttps://doi.org:10.3390/s21124069\nfilms. Applied Physics Letters 86, 203104 (2005). https://doi.org:10.1063/1.1929075\nSwitches from Electrodeposited Palladium Mesowire Arrays. Science 293, 2227-2231 (2001).\nhttps://doi.org:10.1126/science.1063189\nmesoscopic scale. The Journal of Chemical Physics 115, 9001-9009 (2001).\nhttps://doi.org:10.1063/1.1410391\nthin Cobalt on fullerene (C60) thin-film: in-situ investigation under UHV conditions. Materials\nScience and Engineering: B 284, 115911 (2022).\nhttps://doi.org:https://doi.org/10.1016/j.mseb.2022.115911\nC60/Pd multilayer films. Physica C: Superconductivity 282-287, 735-736 (1997).\nhttps://doi.org:https://doi.org/10.1016/S0921-4534(97)00553-4\nPd thin films during the process of absorption and desorption of hydrogen gas. International\nJournal of Hydrogen Energy 35, 6984-6991 (2010).\nhttps://doi.org:https://doi.org/10.1016/j.ijhydene.2010.04.051\nhydrogen detection. Nature Communications 13, 5737 (2022). https://doi.org:10.1038/s41467-\n022-33466-8\nlimit of detection in humid air. Nature Communications 15, 1208 (2024).\nhttps://doi.org:10.1038/s41467-024-45484-9\nR41\nPlasmonic Hydrogen Sensors via Phase Space Reconstruction and Convolutional Neural\nNetworks. ACS Sensors 9, 3877-3888 (2024). https://doi.org:10.1021/acssensors.3c02651\nhydrogen from composite nanoparticles. US patent US-10507452-B2 (2019).\ndesign of nanostructures. 8, 1-26 (2019). https://doi.org:doi:10.1515/nanoph-2018-0105\napplications of micro- and nanostructured thin films. Journal of Vacuum Science & Technology\nA 25, 1317-1335 (2007). https://doi.org:10.1116/1.2764082\ndetection on Pd using Kelvin probe technique. Journal of Solid State Electrochemistry 22, 495-\n504 (2018). https://doi.org:10.1007/s10008-017-3715-z\nTheir Nanocomposites. Molecules 20, 22833-22847 (2015).\nGas Sensing. ACS Applied Materials & Interfaces 12, 1395-1403 (2020).\nhttps://doi.org:10.1021/acsami.9b19358\nR42"}, {"round": 2, "reviewer_comments": {"reviewer_1": "Authors already addressed properly", "reviewer_2": "First I would like to thank the Authors for doing an extraordinary work for their revision. As such, I think now this is an\nexceptional work and therefore I recommend for publication. I believe this will be accepted very well by the community and\nwill be an important piece for the future development. However if I may request one more thing? There are a few valuable\ndiscussion in the Response Letter that does not end up in the final manuscript, for example, comment 1.7. I think it will be a\n\"waste\" if it just end there, and there is very likely that some readers would have similar question/concern. Maybe authors\ncan somewhat include this in the SI?\nAnyway, thanks again and congratulations!!", "reviewer_3": "In their response, the authors have adequately answered the questions, and the manuscript and supplementary materials\nhave been carefully revised. While most of the responses from the authors were well understood, the answers raised\nadditional questions. Therefore, we consider the manuscript acceptable after addressing the following comments.\nCan nitrogen molecules penetrate the C60 layer? If the authors claim that measurements in vacuum mode allow evaluation\nof the intrinsic properties of the sensing material (C60/PdCo/TAF), they should mention the diffusion of N2 and H2 gases in\nC60.\nIf N2 molecules can penetrate the C60 layer, the diffusion rate of hydrogen molecules in the C60 layer in flow mode may be\nslower than that in vacuum mode because of the existence of interdiffusion of N2 and H2 in the C60 layer. In other words, the\ndiffusion of hydrogen in the C60 layer may be the rate-limiting step for the sensing material. This behavior is intrinsic to the\nsensing material and is distinct from sensing system effects such as chamber geometry and flow rate. Therefore, we\nbelieve that the intrinsic response speed of the sensing material cannot be evaluated in vacuum mode. On the other hand, if\nthe C60 layer is impermeable to N2 gas, the C60 could act as an H2 filter and the intrinsic reaction rate of the sensing\nmaterial could be measured in vacuum mode."}, "author_response": "Response Letter 2:\nWe sincerely thank all the reviewers for their time and insightful comments that helped further improve the\nmanuscript substantially. Below are our point-by-point responses to reviewers’ comments.\nResponses to Comments from Reviewer #1\nAuthors already addressed properly\nAuthors’ response: We sincerely thank the reviewer for their positive evaluation and are glad that our\nprevious revision has addressed the concerns satisfactorily.\nResponses to Comments from Reviewer #2\nFirst I would like to thank the Authors for doing an extraordinary work for their revision. As such, I think\nnow this is an exceptional work and therefore I recommend for publication. I believe this will be accepted\nvery well by the community and will be an important piece for the future development. However if I may\nrequest one more thing? There are a few valuable discussion in the Response Letter that does not end up in\nthe final manuscript, for example, comment 1.7. I think it will be a \"waste\" if it just end there, and there is\nvery likely that some readers would have similar question/concern. Maybe authors can somewhat include\nthis in the SI?\nAnyway, thanks again and congratulations!!\nAuthors’ response: We sincerely thank the reviewer for their encouraging and thoughtful feedback. We\ngreatly appreciate the recognition of our efforts and are pleased to hear that the work is considered valuable\nfor the community.\nIn response to the suggestion, we agree that some of the discussions provided in the response letter offer\nuseful insights for readers. To ensure these points are not lost, we have incorporated the relevant discussion\ninto Supplementary Information (SI) as suggested. Thank you again for your support and constructive input.\nChanges made to the manuscript:\n• The discussion in Comment 1.7 of the first response letter regarding the scalability, feasibility, and\npotential challenges related to the mass production and integration of our PdCo hexagonal nano-\nnetwork sensors has been added to the Supplementary Note 10. The following paragraph has been\nadded to the manuscript (page 27, from line 16):\n“It is noteworthy that the high-performance sensors were fabricated using simple, scalable\nmethods that are inherently compatible with standard semiconductor processes, making the design\na strong candidate for industrial integration. A detailed discussion of the fabrication steps,\nscalability, and potential challenges related to large-area uniformity, material cost, integration, and\nlong-term stability is provided in Supplementary Note 10.”\nR1\n• The discussion in Comment 2.5 of the first response letter regarding the need for ppb-LOD sensors\nhas been added to the Introduction of the manuscript (page 4, from line 10):\n“Achieving ppb or sub-ppb detection limits not only benchmarks sensor performance under\nnoise-limited conditions requiring high signal-to-noise ratios and advanced processing,1,2 but also\nenables critical applications such as trace hydrogen detection in deep-space missions,3 the\nmonitoring of low-abundance hydrogen isotopes like deuterium and tritium in nuclear\nenvironments,4 and breath tests for assessing gastrointestinal health.5,6”\nReferences have been added to the manuscript:\nHydrogen Sensors via Phase Space Reconstruction and Convolutional Neural Networks. ACS\nSensors 9, 3877-3888 (2024). https://doi.org:10.1021/acssensors.3c02651\noxidized nickel foam. Sensors and Actuators B: Chemical 329, 129194 (2021).\nhttps://doi.org:https://doi.org/10.1016/j.snb.2020.129194\nfrom composite nanoparticles. US patent US-10507452-B2 (2019).\nChemistry 406, 3931-3939 (2014). https://doi.org:10.1007/s00216-013-7606-6\nbreath testing results influenced by oral hygiene. Scientific Reports 11 (2021).\nhttps://doi.org:10.1038/s41598-020-79554-x\n• Figure R5 in Comment 3.3 of the first response letter regarding the temperature stability of the\nnano-resistor network has been added as Supplementary Figure 33. The following paragraph has\nbeen added to the manuscript (page 25, from line 13):\n“To address the temperature stability of the system and the temperature dependence of\nresistance, we evaluated sensor performance of one of our CHAs over a range from room\ntemperature to 115 °C and consistently observed hydrogen response (see Supplementary Fig. 33).\nHowever, two notable effects emerged at elevated temperatures: (i) increased baseline drift, which\nmay result from thermally induced morphological changes in the nanoparticle network such as\nexpansion or coalescence that alter percolation pathways, and (ii) a significant drop in sensitivity,\nconsistent with suppressed β-phase hydride formation and enhanced hydrogen desorption kinetics.7\nThese findings indicate that room temperature offers the best balance between baseline stability\nand sensing performance for our PdCo nano-network structure.”\nR2\nResponses to Comments from Reviewer #3\nIn their response, the authors have adequately answered the questions, and the manuscript and\nsupplementary materials have been carefully revised. While most of the responses from the authors were\nwell understood, the answers raised additional questions. Therefore, we consider the manuscript acceptable\nafter addressing the following comments.\nAuthors’ response: We sincerely thank the reviewer for their positive evaluation of our first revision. We\nare happy to have an opportunity to further address the reviewer’s critical remarks and important questions.\nCan nitrogen molecules penetrate the C60 layer? If the authors claim that measurements in vacuum mode\nallow evaluation of the intrinsic properties of the sensing material (C60/PdCo/TAF), they should mention\nthe diffusion of N2 and H2 gases in C60.\nIf N2 molecules can penetrate the C60 layer, the diffusion rate of hydrogen molecules in the C60 layer in\nflow mode may be slower than that in vacuum mode because of the existence of interdiffusion of N2 and\nH2 in the C60 layer. In other words, the diffusion of hydrogen in the C60 layer may be the rate-limiting\nstep for the sensing material. This behavior is intrinsic to the sensing material and is distinct from sensing\nsystem effects such as chamber geometry and flow rate. Therefore, we believe that the intrinsic response\nspeed of the sensing material cannot be evaluated in vacuum mode. On the other hand, if the C60 layer is\nimpermeable to N2 gas, the C60 could act as an H2 filter and the intrinsic reaction rate of the sensing\nmaterial could be measured in vacuum mode.\nAuthors’ response: We thank the reviewer for raising and explicitly clarifying these important questions.\nFirst, regarding the permeability of N in C , we honestly cannot determine its permeability using\n2 60\nthe sensing response since N does not induce a measurable change in the signal (Supplementary Fig. 22).\n60\nobserved that the sensor baseline was distorted in the presence of O , whereas no such effect occurred in an\n2 2 60\nGiven that the kinetic diameter of O (~0.346 nm) is comparable to that of N (~0.364 nm),10 we infer that\n2 2\nthe voids in the C lattice are also permeable to N . In literature, although direct measurements on pure C\n60 2 60\nfilms are limited, Karachevtsev et al. (2007) reported that both pristine and photopolymerized C films on\n60\na polycarbonatesyloxane (PCS) allow N permeation, though reduced compared to bare PCS.8 The N\n2 2\npermeability of the C /PCS membrane is on the order of 10−5 cm3/(cm2 cbar), which corresponds to ~105\n2 2\npermeability is about 103 Barrer, as reported by Östergren et al. (2021).9 These together suggest that C is\n60\nrelatively high permeable to N .\nFigure R14. Sensing responses of 15 nm PdCo thin film with 100 nm C top coating sensor to 2% H in\n60 2\nN and in synthetic air measured in flow mode with a total flow rate of 400 sccm.\n2 60 2 2\nmolecular collisions inside the C polycrystal. This effect is expected to be more pronounced at low H\n60 2\npartial pressures, where the relative influence of N is greater. Nonetheless, the permeation inhibition by N\n2 2\nis not unique to C . It also happens in uncoated Pd membranes due to concentration polarization where N\n60 2\naccumulates near the surface and reduces the effective driving force for H permeation.11 It is worth noting\n2\nthat in our design, the C layer is positioned underneath the sensing stack, while the top surface remains\n2 2\nwe designed an additional experiment with one of CHA sensors (20 nm C /5 nm PdCo CHA ) and\n60 300\nmeasured its response under two different gas environments: pure H and H /N mixture (4% H in N ) in\n2 2 2 2 2\nvacuum mode (Figure R15). The sensitivity and response time t are nearly identical within experimental\n90\nerrors across H partial pressure range of 0.1 – 10 mbar (corresponding to 100 ppm – 1 vol.% H ) between\n2 2\nthe two gas sources, indicating consistent sensor performance regardless of the presence of N . These results\n2\nsuggest that the concentration polarization of H /N mixture has a negligible effect on the sensing\n2 2\nperformance under vacuum mode measurement within the tested P range.\nH2\nFigure R15. a Sensitivity R/R and b absorption time t of 20 nm C /5 nm PdCo CHA sensor\n90 60 300\nmeasured in vacuum mode using pure H and 4% H in N balance as the gas sources.\n2 2 2\nR4\nLast but not least, vacuum mode measurements eliminate external flow effects and allow clearer\nevaluation of internal transport through the layered structure. While some interdiffusion may still occur\nwithin the C layer, vacuum-mode testing helps isolate intrinsic diffusion behavior and may clarify the\n60\ninfluence of mixed versus pure gas feeds.\nChange made to the manuscript:\n• Figure R15 has been added to the SI as Supplementary Figure 23.\n• Supplementary Note 3 (page S11, from line 6) has been added revised accordingly where “the\nsensing element’s intrinsic kinetics” is replace by “the internal transport through layered\nstructures”:\n“In contrast, vacuum mode eliminates these transport limitations as the gas pressure\nreaches equilibrium in < 100 ms (Supplementary Fig. 4) and allows us to isolate the internal\ntransport through the layered structure.”\n• The following paragraph has been added to the manuscript (page 20, from line 6):\n“Although C is reported to be high permeable to N ,8 the sensor’s signal remains\n60 2\nunaffected by the N pressure variation (see Supplementary Fig. 22a) and the interdiffusion of N\n2 2\nwith H has a negligible effect on the sensing performance under vacuum mode measurement\n2\nwithin the tested P range as illustrated in Supplementary Fig. 23. Thus, H and N mixture can be\nH2 2 2\nutilized in vacuum mode measurement to reliable extract the sensitivity of the sensor at low partial\nH pressures.”\nO2, CH4, and He gases. Applied Surface Science 253, 3062-3065 (2007).\nhttps://doi.org:https://doi.org/10.1016/j.apsusc.2006.06.053\nR5\nRebuttal References 2\nof detection in humid air. Nature Communications 15, 1208 (2024).\nhttps://doi.org:10.1038/s41467-024-45484-9\nHydrogen Sensors via Phase Space Reconstruction and Convolutional Neural Networks. ACS\nSensors 9, 3877-3888 (2024). https://doi.org:10.1021/acssensors.3c02651\noxidized nickel foam. Sensors and Actuators B: Chemical 329, 129194 (2021).\nhttps://doi.org:https://doi.org/10.1016/j.snb.2020.129194\nfrom composite nanoparticles. US patent US-10507452-B2 (2019).\nChemistry 406, 3931-3939 (2014). https://doi.org:10.1007/s00216-013-7606-6\nbreath testing results influenced by oral hygiene. Scientific Reports 11 (2021).\nhttps://doi.org:10.1038/s41598-020-79554-x\nBased Hydrogen Sensors—Current Limitations and Strategies for Their Mitigation. ACS Sensors\n5, 3306-3327 (2020). https://doi.org:10.1021/acssensors.0c02019\nO2, CH4, and He gases. Applied Surface Science 253, 3062-3065 (2007).\nhttps://doi.org:https://doi.org/10.1016/j.apsusc.2006.06.053\n9 Östergren, I. et al. Highly Permeable Fluorinated Polymer Nanocomposites for Plasmonic\nHydrogen Sensing. ACS Applied Materials &amp; Interfaces 13, 21724-21732 (2021).\nhttps://doi.org:10.1021/acsami.1c01968\nEncyclopedia, https://en.wikipedia.org/w/index.php?title=Kinetic_diameter&oldid=1209540273 (\naccessed July 28, 2025).\nbased membranes. Separation and Purification Technology 66, 613-624 (2009).\nhttps://doi.org:https://doi.org/10.1016/j.seppur.2009.01.008\nR6"}, {"round": 3, "reviewer_comments": {"reviewer_3": "I have understood that the existence of nitrogen hardly affected the reaction time as shown in figure R15\nand the nitrogen gas diffusion in C60 layer is not important to evaluate the intrinsic material sensitivity.\nThe authors have addressed my question appropriately. I recommend that the revised manuscript is\npublished to nature communications."}, "author_response": "Response Letter 3:\nWe sincerely thank the reviewers for their time and insightful comments that helped further improve the\nmanuscript substantially. Below are our point-by-point responses to reviewers’ comments.\nResponses to Comments from Reviewer #3\nI have understood that the existence of nitrogen hardly affected the reaction time as shown in figure R15\nand the nitrogen gas diffusion in C60 layer is not important to evaluate the intrinsic material sensitivity.\nThe authors have addressed my question appropriately. I recommend that the revised manuscript is\npublished to nature communications.\nAuthors’ response: We sincerely thank the reviewer for their positive evaluation and are glad that our\nprevious revision has addressed the concerns satisfactorily.\nR1"}]

Decision letter