Multilayer oxide protection layer with multiple tunnelling paths for efficient and durable Si-based photocathode

Domain: physics
Published: 2026-01-17
Editorial outcome: major_revision

Abstract
Constructing a low-resistance oxide protection layer is challenging but highly beneficial for realizing a practical photoelectrochemical device. The thickness of oxide layer strongly influences its behaviors of carrier transport and corrosion resistance, generally leading to a trade-off between efficiency and durability. Different from the previous methods, here we propose and demonstrate a universal approach to decouple the trade-off of oxide layer by multiple carrier-tunnelling paths. This approach with oxide/metal architecture ((O/M)n, n is the number of nano-scale repeating unit) enables low-resistance carrier transport as required for high efficiency, while allowing the layer to be sufficiently thick, which reinforces durability. This approach can be applied to various oxide-based layers, such as (TiO2/Fe)n, (CeO2/Fe)n and (TiO2/Pd)n. In addition, a good correlation between carrier dynamics and oxide/metal architecture is established by employing systematic photoelectrochemical-electrical measurements and simulation models. Here we show important contributions for further developing the practical photoelectrodes in photoelectrochemical devices and controlling the carrier transport behaviors in complex multilayer structure.

Introduction
The photoelectrochemical (PEC) approach that directly converts simple small molecules (e.g., H2O, NO3−, CO2) to valuable fuel molecules (e.g., H2, NH3, CH3OH) using solar energy is an intriguing and sustainable technology1–3. Tremendous efforts have been directed towards constructing efficient and stable photoelectrodes, which are the core component of the practical PEC system4–6. In comparison to wide-gap semiconductors, narrow-gap semiconductors (e.g., Si) with band gaps less than 2.3 eV are more attractive as light absorbers of the photoelectrodes due to large optical absorption coefficients, broad-spectrum light collection, high theoretical photocurrent density, and good charge carrier mobilities7–9. To maximize the solar-to-chemicals conversion, the semiconductor light absorbers with porous structure can simultaneously minimize the light reflection by trapping light in the pore channels and increase the surface active area for PEC reactions10,11. Unfortunately, according to Pourbaix diagrams12, these narrow-gap semiconductors with a very short window of stability are easy to be corroded/passivated quickly when immersed in aqueous electrolytes, especially in strong acidic and alkaline conditions. One way of combating this challenge is to introduce a protection layer into the photoelectrode as a means of separating the light absorber from the electrolytes.

Ideally, the protection layer should concurrently protect the underlying light absorber, facilitate the carrier (electron or hole) transport, and allow the high light transmittance13. Early attempts to extend photoelectrode lifetime have focused on depositing a thin noble metal layer (such as Pt) onto light absorber14. However, the parasitic light absorption/reflection and Fermi-level pinning effect of metal layer reduce the light harvesting and band bending of the photoelectrodes, leading to unsuccessful PEC performance at low photocurrents and photovoltages15,16. Alternatively, transparent oxides, such as TiO2 17, SiO2 18, and NiOx 19, have been widely used as protection layers for various photoelectrodes due to high intrinsic chemical stability and minimal parasitic light absorption. Nevertheless, there is still a visible trade-off between photoelectrode efficiency and durability while oxides are employed as protection layers. High-quality thin oxide layers (less than 4 nm) generally show a high passage of current density (over 1 A cm−2) by carrier tunnelling20,21, but is not stable in this situation. In the anti-corrosion field, over 30 nm-thickness oxide film can provide a typical protection layer for achieving long-term and durable operation22,23. On the other hand, increasing the thickness of oxide layer leads to an exponential decay of carrier tunnelling probability, thus an nanoscale oxide layer is usually required. Two roles of carrier transport and corrosion resistance for the oxide protection layer need to be decoupled. Some pioneering work including our own study has demonstrated the defect formation and migration that can increase conductivity of thick TiO2 layer (~20 nm) for accelerating carrier transport instead of tunnelling24–27. However, this method is still difficult to completely resolve the trade-off of oxide layer. Here, in addition to the conductive channels, the defect sites can also serve as pinholes to induce the occurrence of pitting corrosion28. Besides, this method is likely to be hard to generalize to other oxide layers or transparent compound layers (e.g., SiC).

Herein, to resolve the above mentioned issues, we demonstrate a general method for realizing a low-resistance oxide-based protection layer by multiple carrier-tunnelling paths, (O/M)n (n is the number of nano-scale repeating unit), and its total thickness is constant (~36 nm). According to electric field simulations and experimental results, tuning the n value as well as the thickness of repeating unit can form the multiple carrier-tunnelling paths in the protection layer and optimize the interfacial resistance between oxide and metal for enhancing carrier transport. Using a classic silicon (Si) material as light absorber, a Si-based photocathode with optimal (O/M)n protection layer shows a faster transfer rate of charge carriers, lower loss of charge carriers and higher solar-to-ammonia conversion efficiency than that with other protection layers (n = 1, 2, 4, and 8). In addition, since the metal units are nanoscaled, it can also impede the parasitic light absorption/reflection for improving the light absorption of Si–thereby further increasing efficiency29.

Results
To rationally design the multilayer structure, a series of electric field simulations for the intrinsic conductivity of various (O/M)n protection layers with same total thickness (36 nm including 18-nm oxide units and 18-nm metal units) and different n values were implemented by finite element analysis (COMSOL Multiphysics). In these simulations, TiO2 as one of the most common materials of oxide protection layer was selected as the oxide model of (O/M)n, and cheap and abundant iron (Fe) served as the metal model to avoid the use of noble metal and increase the general applicability of our method. Unless otherwise stated, the oxide-based protection layer with (TiO2/Fe)n multilayer structure is simply labeled as (TF)n, n = 1, 2, 4, 6, and 8 corresponding to unit thickness of 18/18, 9/9, 4.5/4.5, 3/3, and 2.25/2.25 nm, respectively. The parameters of (TF)n materials and electric field are provided in Tables S1 and S2 of Supporting Information (SI). In this section, Fe is a conductor, and its resistance can be ignored. As a semiconductor, the conductive behavior of the TiO2 bulk is simulated by a drift-diffusion equation. Figures 1a and S1 illustrate the electric potential distribution of the bulk for (TF)n by finite element method in a constant-current situation. The simulation analysis indicated that the potential gradient of Fe layer was invariable in (TF)n due to the quick redistribution of free electrons for eliminating the internal electric field, reflecting that Fe layer possesses a zero resistance. The bulk of the TiO2 layer with an even-distributed resistance leads to a linear loss in potential. It can be seen that the increase of n in (TF)n can effectively reduce the loss in potential (see the bottom of Figs. 1a and S1). As a result, the order of total TiO2 bulk resistance is (TF)8 < (TF)6 < (TF)4 < (TF)2 < (TF)1. In addition, the interface resistance of (TF)n (n > 2) was simply used to sharply increase the loss of potential in this constant-current model, but the ohmic contact and Schottky contact at the interface of semiconductor/conductor were neglected.

Considering that the work function of Fe is higher than the electron affinity of TiO2, electrons moving from Fe to TiO2 and from TiO2 to Fe usually occur in the Schottky contact and ohmic contact, respectively. The ohmic contact can facilitate the transfer of the electrons at the interface, which is regarded as zero resistance, as shown in the results of (TF)1 (see Fig. 1a, b). On basis of the formation of depletion area and atomic-level properties of interface, a Wenzel–Kramers–Brillouin (WKB) tunnelling model30 combined with approximation formula (see Eqs. 1 and 2) was employed to analyze the electrons passing through the Schottky interfaces.

ψ(x) = p 2 D sin 1 x0 p(x0)dx0 + π (x < x0) (1)

ψ(x) = p D exp ( 1 x0 x p(x0) dx0) (x > x0) (2)

where ψ(x), p(x), D, h, x and x0 denote the wave function describing the state of a quantum mechanical microscopic system, kinetic energy function, normalization constant, reduced Planck constant, path traversed by the microscopic particle during tunnelling, and turning point of Classical and quantum physics behaviors (namely, the position of the particle’s total energy), respectively. In the tunnelling model, there is a strong electric field at the Schottky interface, where the electrons were accelerated and increased the possibility of tunnelling leading to a jump in current density at the Schottky interface (as shown in Fig. 1b). Due to the exponential dependence of tunnelling probability (Eq. 3)30, the increase of n for (TF)n increased the potential barriers, showing lower tunnelling current densities, viz.

T(E) = exp ( 2 x2 p 2 m* (V(x) E) dx) (3)

where T(E), V(x), E and m* are the tunnelling probability, potential energy function, total energy of the microscopic particle (kinetic and potential energy), and effective mass of the particle, respectively. Accordingly, the order of tunnelling current density is (TF)2 > (TF)4 > (TF)6 > (TF)8. Integrating the bulk resistance and interface tunnelling effect, Fig. 1c shows the simulation of mean current density versus (vs.) (TF)n in the range of applied potential from 0 to 2.0 V. Interestingly, (TF)6 with unit thickness of 3 nm exhibited the highest mean current density, implying the lowest total resistance in (TF)6. This optimized result is jointly determined by the decrease of bulk resistance and the increase of interface resistance with nanoscale multilayer structure.

In light of these calculations, we sought to fabricate the above mentioned (TF)n as protection layer of Si-based photocathode for PEC NO3− reduction reaction (NO3RR) via magnetron sputtering technology31. As shown in Fig. 2a, the assembly of Si-based photocathodes contained four main steps: the formation of Si with buried p-n junctions (n+p-Si) as light absorber1; the deposition of carbon layer as electron transfer layer32; the fabrication of (TF)n with different n values as protection layer; and FeCu alloy layer as cocatalysts for NO3RR33. Fe- and Cu-based catalysts are known as promising candidates for NO3RR, which can enhance NO3− adsorption, accelerate electron transfer, and suppress the HER34. The FeCu cocatalysts/(TF)n protection layer/carbon layer/n+p-Si photocathodes can be clearly denoted as C(TF)nCSi (n = 1, 2, 4, 6, and 8). Preparation details of these photocathodes are described in the Experimental Section of SI.

Field emission scanning electron microscopy (FESEM) was used to perform a reconnaissance study of the architecture of C(TF)nCSi. As shown in Fig. S2, C(TF)nCSi with different n values showing a similar surface morphology, implying similar surface properties (e.g., a good lyophilic surface with contact angles < 35°, see Fig. S3) of FeCu cocatalysts for these photocathodes. Meanwhile, the cross-sectional FESEM images (Fig. S4) revealed that C(TF)nCSi was composed of cocatalyst layer, (TF)n protection layer with multilayer structure (n ≤ 4), and fine Si-based light absorber. However, when the n value was over 4, the multilayer structure of (TF)n protection layer was hardly observed by FESEM due to the limitation of the resolution ratio. High-resolution transmission electron microscopy (HRTEM) was further applied to precisely determine the architecture and microstructure of C(TF)nCSi. In comparison to C(TF)1CSi with thick (TF) unit (18/18 nm, Fig. S5), C(TF)6CSi (Fig. S6) and C(TF)8CSi (Fig. S7) showed a composite protection layer with 6 and 8 thin (TF) units in the situation of same total protection layer thickness (36 nm), respectively, reflecting the successful fabrication of C(TF)nCSi with nanoscale multilayer (TF)n. In addition, the lattice spacing of ~0.205 nm was partially found in FeCu cocatalysts (Figs. 2b, S5 and S7) corresponding to (111) plane of Cu, in line with the presence of broad and low Cu diffraction peaks in X-ray diffraction patterns (XRD, Fig. S8). The (TF)n protection layers were amorphous due to the absence of TiO2 and Fe diffraction peaks. The cross sections of C(TF)6CSi (Fig. 2c) and C(TF)8CSi (Fig. S9) were also detected using scanning transmission electron microscopy (STEM) coupled with energy dispersive spectroscopy (EDS) mapping. Elemental mapping of C(TF)6CSi (see Fig. 2c) definitely indicated that 6 (TF) units with ~3-nm Fe and ~3-nm TiO2 were successively stacked as an oxide-based protection layer, and ~28-nm FeCu cocatalysts were on the protection layer. According to the data analysis of X-ray photoelectron spectroscopy (XPS, Fig. S10) and near-edge X-ray absorption fine structure spectroscopy (NEXAFS, Fig. S11), the surface of C(TF)nCSi with different n values showed similar Fe/Cu ratio (~2.2), Fe3+/Fe2+ ratio (~2.6) and Cu2+/Cu1+ ratio (~0.9), as exhibited in Fig. S12, identifying that the composition of FeCu cocatalysts is nearly same on these photocathodes. Meanwhile, XPS depth profiling measurement further revealed that the Fe unit layer of (TF)n protection layer was metallic with little oxidation (see Fig. S13), implying the successful deposition of nanoscale metal unit layer in the (O/M)n architecture. Furthermore, via optical measurements and indirect allowed transition (Fig. S14), a band gap of ~1.1 eV assigning to Si and an almost identical visible light transmittance for different (TF)n protection layers and C(TF)n composite layers was found in all the C(TF)nCSi. In summary, apart from the difference of the architecture of protection layer, C(TF)nCSi approximately possessed the same Si-based light absorber, FeCu cocatalysts and optical properties, suggesting a similar light-harvesting ability and surface catalytic activity35. Consequently, it can be easily imagined that the PEC NO3RR performance of C(TF)nCSi is controlled by changing the n values of (TF)n protection layer due to the formation of multiple carrier-tunnelling paths.

Next, we sought to verify whether the n value of (O/M)n protection layer correlated with PEC performance, including solar-to-chemical conversion efficiency and carrier migration rate, as depicted in Fig. S15. In the conditions of the same Si-based light absorber and FeCu cocatalysts, PEC NO3RR behaviors of the Si-based photocathodes in 1.0 M KOH-0.05 M KNO3 aqueous solution (pH = 13.8) under 1 sun illumination were utilized to estimate the positive role of multilayer structure for (O/M)n protection layer. Figure 3a displays the current density (J)-potential (V) curves of all the C(TF)nCSi photocathodes for PEC NO3RR. The dark currents of these photocathodes may be considered negligible. The highest performance was obtained on C(TF)6CSi, realizing the highest saturation current density (Jsa, −19.7 mA/cm2) and lowest applied potentials (Vap, 0.055 V vs. RHE corresponding to 10 mA/cm2). This is significantly better than −14.7 mA/cm2 and −0.10 V measured on C(TF)1CSi under identical testing conditions. With the increase of n value in C(TF)nCSi, PEC performances of the photocathodes were gradually optimized, however, as n > 6, C(TF)8CSi showed a weak PEC performance with a Jsa of −18.9 mA/cm2 and 0.005 V vs. RHE corresponding to 10 mA/cm2. The results perfectly accord with the simulated models (Fig. 1), indicating that the multilayer architecture with multiple carrier-tunnelling paths can form a low-resistance (TF)n protection layer to accelerate the carrier transfer and reduce the carrier loss. Some similar variations in chopped current curves (Fig. S16) and light-assisted electrochemical surface areas (ECSAs, Fig. S17) are also observed on the C(TF)nCSi photocathodes.

According to the chronoamperometry measurements (Fig. S18), the NH3+/NO3− calibration curves (Fig. S19) and gas chromatography, the NH3, NO2− and H2 yield rate of C(TF)nCSi photocathodes at each given potential in a 1.0-h period were shown in Figs. 3b and S20, respectively. For all the photocathodes, the NH3 yield rate increased with the increase of Vap from 0.1 V to −0.2 V vs. RHE. In comparison to other photocathodes, C(TF)6CSi photocathode achieved a higher NH3 yield rate of 0.42 mg/cm2/h with a maximum Faradaic efficiency of 77.3% (see Fig. S21) at 0 V vs. RHE (that is, no thermodynamic bias for hydrogen evolution reaction (HER)). This reflects that the introduction of low-resistance (TF)n protection layer improves the solar-to-NH3 conversion efficiency. Meanwhile, it can be easily observed that there were lower yield rates of NO2− and H2 on the C(TF)6CSi photocathode in the PEC NO3RR process at 0 V vs. RHE than these on the other photocathodes (Fig. S20), in line with the higher NH3 yield rate. Finally, the results of 1H NMR spectra (see Fig. S22) were also consolidated for the high NH3 yield rate on the C(TF)6CSi photocathode via PEC NO3RR.

Further, light-assisted electrochemical impedance spectroscopy (EIS) analysis was carried out to assess the PEC kinetics of C(TF)nCSi photocathodes for NO3RR from 100 kHz to 0.1 Hz at 0 V vs. RHE (Fig. 3c). The resistance of carrier transfer in the photocathodes exhibited the following order: C(TF)6CSi < C(TF)8CSi < C(TF)4CSi < C(TF)2CSi < C(TF)1CSi, consistent with the PEC NO3RR performance. The Bode plots of C(TF)nCSi photocathodes also showed the similar results about their interface resistance (Fig. S23). An equivalent electrical circuit was further calculated to describe the PEC NO3RR kinetic processes of C(TF)nCSi photocathodes (the inset of Fig. 3c). Generally, the kinetic processes of a PEC reaction on the hierarchical photoelectrode are usually assessed by charge transfer resistance between solid/solid interface (R1, like TiO2 layer/Fe layer or TiO2 layer/FeCu cocatalysts) and solid/liquid interface (R2, like FeCu cocatalysts/electrolytes). The smaller the R1 and R2, the faster is the kinetics. The values of the R1, R2 and solution resistance (Rs) were obtained by fitting EIS data using the equivalent electrical circuit, as listed in Table S3. C(TF)6CSi photocathode showed low R1 and R2, especially the lowest R1, corresponding to the super PEC NO3RR activity. Meanwhile, the data of intensity-modulated photocurrent spectra (IMPS) at different Vap (see Figs. 3d and S24) demonstrated the carrier transfer time in the photocathodes via the calculations described in the SI. At 0 V vs. RHE, the carrier transfer times (τd) of C(TF)1CSi, C(TF)8CSi and C(TF)6CSi were 30.3, 19.3, and 15.4 μs, respectively (Figs. 3d and S24c). Actually, compared to other photocathodes, C(TF)6CSi measured at all the Vap values presented a shorter carrier transfer time, meaning that photogenerated electrons reach the reaction surface more rapidly leading to a lower bulk-phase carrier recombination. These phenomena are in line with the results of steady-state photoluminescence spectra (Fig. S25) and open-circuit potential measurements (Fig. S26).

To further confirm the universality and feasibility of (O/M)n architecture, CeO2 and Pd were also selected as the oxide and metal model to build the (CeO2/Fe)n protection layer (simplified as (CF)n), (TiO2/Pd)n protection layer (simplified as (TP)n), and (CeO2/Pd)n protection layer (simplified as (CP)n) for replacing the (TF)n in the Si-based photocathodes, respectively. Several composition and structure characterizations were used to primarily identify the successful preparation of these photocathodes (see Figs. S27 and S28). On basis of various PEC NO3RR measurements (Fig. S29), the Si-based photocathodes with (CF)6, (CP)6 and (TP)6 protection layer possessed higher photocurrent density at 0 V and −0.6 V vs. RHE than those with (CF)1, (CP)1 and (TP)1 protection layer (see Fig. 3e). Among all the photocathodes, the photocathode with (CP)6 protection layer showed highest photocurrent density (−16.0 mA/cm2) at 0 V vs. RHE, corresponding to the highest NH3 yield rate (0.51 mg/cm2/h in Fig. S29b), implying that the carriers are transported in (CP)6 architecture more rapidly. In addition, the almost identical Jsa of the photocathodes with (O/M)6 protection layer reflects about the same carrier loss (Fig. 3a, e), irrespective of materials of oxide and metal unit. Meanwhile, we also investigated the effect of (TF)n protection layer on PEC HER behaviors to further testify the availability of low-resistance strategy. As shown in Fig. S30, the HER performance of C(TF)nCSi with the increase of n value showed a similar variation tendency with the NO3RR performance (Fig. 3a). In summary, (O/M)n architecture can fabricate low-resistance oxide-based protection layer to enhance the carrier transfer and improve the PEC performance.

The durability of (O/M)n protection layer is another important indicator for pushing forward the process of practical Si-based photoelectrodes. Various long-term J-time (t) curves of C(TF)6CSi photocathodes were measured under light irradiation at 0 V vs. RHE in 1.0 M KOH-0.05 M KNO3 aqueous solution (Fig. S31). In a 3-h (Fig. S31a) or 12-h (Fig. S31b) PEC period, the C(TF)6CSi photocathode exhibited little fluctuation in photocurrent density, suggesting the relatively stable operation for NO3RR. As shown in Fig. S32, there was a nearly linear increase in NH3 production on C(TF)6CSi in a 2-h PEC process, corresponding to the relatively smooth current region in the J-t curves. As the reaction time was extended over 20 h, a drastic fluctuation in photocurrent density was observed on the C(TF)6CSi photocathode (see Fig. S31c). This phenomenon is attributed to the adsorption and desorption of the bubbles on the photocathode surface via HER, reflecting the change of surface composition based on the reaction transformation. To further determine the composition and structure evolution in the PEC process, the post-electrolysis C(TF)6CSi photocathodes at different reaction times were characterized by FESEM and XPS. After 3-h PEC reaction, the C(TF)6CSi photocathode retained a good coverage of C(TF)6 composite layer with the thickness of ~70 nm (Fig. S33a), demonstrating little loss of FeCu alloy layer in this process. Meanwhile, XPS spectra of C(TF)6CSi photocathode after 3-h photoelectrolysis showed the presence of Cu characteristic peaks (Fig. S33b) and the lack of Ti signal (Fig. S33c), testifying that the FeCu alloy layer is conformally covered on the (TF)6 protection layer. Similar FESEM and XPS results (Fig. S34) were also found on the reacted photocathode as extending photoelectrolysis to 12 h, in line with the little fluctuation in photocurrent density (see Fig. S31b). This phenomenon was also confirmed by STEM image with corresponding elemental distribution maps, as shown in Fig. S35. However, in comparison to the initial thickness (~74 nm), the thickness of C(TF)6 just was ~34 nm after 80-h PEC reaction (Fig. S36a), potentially including ~24-nm (TF)6 protection layer and ~10-nm carbon layer (Fig. S5). In the corresponding XPS spectra, the absence of Cu characteristic peak (Fig. S36b) and the presence of Ti4+ peaks (Fig. S36c) further demonstrated the disappearance of FeCu alloy layer and the exposure of (TF)6 protection layer. These results demonstrate that the C(TF)6CSi photocathode is progressively corroded from the cocatalyst layer to protection layer during the PEC reaction in the strong alkaline electrolytes, as observed in the reported photocathodes2. Being different from anisotropic corrosion of FeCu cocatalyst layer due to poor crystallinity, the TiO2 unit of (TF)6 protection layer with amorphous phase can show isotropous corrosion behaviors (see Fig. S37). However, further discussions on the degradation of CuFe cocatalyst layer in the PEC process are hardly completed in this work, which obviously deviate from the core theme of this work. Combining the fluctuation of J-t curves and the loss of (TF)6 protection layer, it can be imaged that 36-nm (TF)6 protection layer carry out a good level of protection for the Si-based photocathode over 100 h.

(O/M)n protection layer/Si-based photocathodes (such as (TF)6CSi) were fabricated and used to direct estimate the lifetime of protection layer and avoid the interference of FeCu alloy layer. To quantify the loss of (TF)6 protection layer, the final Ti content in the electrolyte (1.0 M KOH-0.05 M KNO3) after 12-h PEC reaction was evaluated by inductively coupled plasma optical emission spectrometer (ICP-OES). The results revealed the dissolution of 23 ng of Ti from the (TF)6CSi photocathode with a geometric area of ~0.22 cm−2 (Fig. 3f and Table S4). Assuming that the photocathode surface is flat, with a mass loss of ~49-ng Ti per cm2 per day, 2.2-nm TiO2 unit could be lost every day via operating continuously, meaning that the lifetime of (TF)6 protection layer is expected to be 196 h (see Fig. 3f). Indeed, the actual surface area of (TF)6CSi photocathode with pyramidal structure is considerably larger than the geometric area (Fig. S34a). Therefore, the actual loss of (TF)6 is much lower than the calculated value, namely, the actual lifetime is much longer than the expected value. In addition, according to the J-t measurements and corrosion rate calculations, the estimated lifetimes of Si-based photocathodes with different (O/M)n protection layers are over 100 h, as shown in Fig. 3f. Meanwhile, these Si-based photocathodes after 24-h PEC reaction still showed similar expected lifetimes to determine the feasibility of this calculation method. Durability data of Si-based photocathodes with various protection layers in various electrolytes were listed in Table S5, mainly including reported works in recent 5 years and this work. In view of these collected data, the durability measurements for most of Si-based photocathodes were implemented in the acid electrolytes, especially pH = 0, because most of the protection layers relatively stabilize in such conditions. As mentioned in our previous work26, it is critical to develop the efficient and durable Si-based photocathodes for PEC reactions under strong alkaline conditions. In comparison of the durability of the protection layers in the state-of-the-art Si-based photocathodes, the durability of (O/M)n protection layers maintain at a high level (in Table S5). In short, the low-resistance (O/M)n protection layer with efficient carrier transfer still maintain good durability with long lifetime, realizing the synchronized enhancement of conduction and protection functions.

Based on the exploration of NH3 energy and perniciousness of NO3−, developing green PEC technology to convert excess NO3− species into NH3 has drawn the attention of many researchers, which can facilitate the artificial nitrogen cycle and keep global nitrogen balance36,37. Figure S38 summarizes the photocurrent density and NH3 yield rate of various photocathodes for PEC NO3RR at 0 V vs. RHE without heating and reducing sacrificial reagents (additional references listed in Table S6). Additionally, the electrocatalytic (EC) NO3RR is also regarded as a green, viable approach to NH3 production under ambient conditions38,39. Therefore, this comparison of performance against current density and yield rate was extended to some work of EC NO3RR at 0 V vs. RHE (Fig. S38). In view of these collected data, most of the studies on the (photo)-cathodes showed poor NO3RR behaviors with low current density (<5 mA/cm2) and NH3 yield rate (<0.2 mg/cm2/h) at 0 V vs. RHE, especially PEC reaction, because most of the photocathodes with protection layer have poor carrier transfer. This is despite a comparison with the good EC NO3RR performance with iR correction40,41, the C(TF)6CSi and C(CP)6CSi photocathodes in this work showed a competitive current density and NH3 yield rate for solar-driven NO3−-to-NH3 conversion (see Fig. S38). In short, the (O/M)n protection layer with multiple carrier-tunnelling paths and thick layer thickness support the Si-based or other narrow-gap semiconductor-based photoelectrode to achieve a good PEC performance and a long durability simultaneously.

In order to explore the intrinsic electron transport, the electrical properties of the (O/M)n protection layers with different thicknesses were conducted by temperature-dependent, metal contact current-voltage (I-V) measurements13. In these experiments, a two-electrode cell in micro electrical platform42 (Fig. 4ai) was used to collect the electronic signals of protection layers in the dark. Figure 4b presents the electrical resistances of the unit of (Fe)n, (TiO2)n and (TF)n on FTO substrates calculated form their I-V characteristics at different n values (Fig. S39). Similar to the relationship of unit thickness and n value for (TF)n in the Simulation section, the unit thickness of (Fe)n and (TiO2)n with n = 1, 2, 4, 6, and 8 was associated with 18, 9, 4.5, 3, and 2.25 nm, respectively. Firstly, irrespective of materials, the resistance decreases with the increase of n value as well as the decrease of unit thickness, which conforms to our design and theoretical simulations. In comparison to the resistance of (TiO2)n and (TF)n unit, (Fe)n unit had lower resistance (0.52–0.07 kΩ) in the range of unit thickness from 18 to 2.25 nm, and exhibited a near-linear change with the dependence of unit thickness (see Fig. 4b), corresponding to the typical electrical properties of a metal layer. Due to the combination of TiO2 and Fe and the presence of interface resistance, the unit of (TF)1 showed the highest resistance (23.9 kΩ) as the thickness was 18/18 nm. A similar exponential decrease was observed in the unit of (TiO2)n and (TF)n via increasing the n value from 1 to 6. In this stage, the resistance variation in the unit of (TF)n can be mainly attributed to the decrease of TiO2 thickness, following the electrical behavior of semiconductor materials. When the n value was 6 (namely, the thickness of TiO2 in (TF)6 = 3 nm), the (TF)6 unit showed a low resistance (1.68 kΩ, see Fig. 4b) close to the resistance of metal-like materials, implying the formation of carrier-tunnelling path in TiO2. The resistance of (TF)8 unit reduced slightly to 1.30 kΩ with a further decrease of unit thickness to 2.25/2.25 nm. At the same time, the resistance of (TF)8 unit was far higher than that of (TiO2)8 unit (0.57 kΩ), and the resistance difference of 0.38 kΩ between the unit of (TF)6 and (TF)8 was approximate to that between the unit of (TiO2)6 and (TiO2)8. As a result, for n ≥ 6, the effect of the interface resistance between TiO2 and Fe on the resistance of (TF)n unit can become predominant. Additionally, estimates the total resistance of (TF)n by using the unit resistance multiplied by the n values showed an order of (TF)1 > (TF)2 > (TF)4 > (TF)8 > (TF)6. The significant dependence on the thickness of the measured resistance across the (O/M)n protection layer indicates the importance of using a vapor deposition technology such as magnetron sputtering, which displays good uniformity and control of thickness at the nanoscale.

On the other hand, Fig. S40a shows the temperature-dependent electrical resistance curves through the unit of (Fe)6, (TiO2)6 and (TF)6 on FTO substrates from the analysis of I-V measurements (Fig. S40b–d). It is noticeable that as temperature increased, the resistance of the unit for (TiO2)6 and (TF)6 decreased, as explained for semiconductor properties43, different from the resistance change of the unit for (Fe)6. The resistance of the unit for (TF)6 was discovered to have very little dependence on a wide temperature range from 15 °C to 200 °C (see Fig. S40a), a strong indication of electron transport by tunnelling44. Further, the temperature-dependent electrical properties of the (TF)1 and (TF)6 protection layers were also probed to corroborate the electron tunnelling behaviors of the (O/M)n architecture (see Fig. S41). The current density values of (TF)6 protection layer with 3-nm TiO2 unit layers showed little dependence on temperature (Fig. S41c), suggesting the presence of electron tunnelling transport. From the I-V measurements (Fig. S41b), the resistance of the (TF)6 protection layer was estimated to be ranged from ~3.2 kΩ to ~1.9 kΩ in a wide temperature range from 25 °C to 200 °C. (TF)1 protection layer with 18-nm TiO2 unit layer resulted in a decreasingly temperature-dependent resistance from ~131.0 kΩ to ~56.2 kΩ (Fig. S41a), implying a more thermally activated and bulk-limited conduction mechanism such as Frenkel-Poole conduction. The significant dependence on the thickness of the measured electronic conduction across the (TF)n protection layer indicate the importance of the (O/M)n architecture by using a magnetron sputtering deposition method. As a result, the (TF)6 protection layer can have the multiple carrier-tunnelling paths, leading to a lower resistance.

To exclude the effect of the space charge layer of solid/liquid interface, photo-assisted I-V measurements were conducted on C(TF)nCSi photocathodes in micro electrical platform at 15 °C with illumination (Fig. 4a(ii)). Figure 4c exhibits the I-V characteristics of the C(TF)nCSi photocathodes with and without illumination. As stated in the process of PEC NO3RR, the dark current of the C(TF)nCSi photocathodes can be ignored, especially at zero applied bias (0 V). Under illumination, all the C(TF)nCSi photocathodes showed a fast increase in forward bias current with a weak dependence of leakage current on applied reverse bias, reflecting the presence of a rectifying interface. It was found in Fig. 4c that there was a visible photocurrent in the C(TF)nCSi photocathodes driven by the photovoltage (Vph) before 0 V. Among the photocathodes studied, the highest performance was obtained on C(TF)6CSi photocathode, achieving the maximal Vph (0.4 V), the highest current (0.33 mA) at 0 V, and the highest saturation current (~0.50 mA). These results directly verify that the design of nanoscale multilayer structure for (O/M)n protection layer can facilitate carrier transport and effectively reduce the loss of photogenerated carriers.

Photo-assisted Kelvin probe force microscopy (KPFM) was also used to detect the surface potential (φ) of C(TF)1CSi and C(TF)6CSi photocathodes and understand the surface physical response by compensating the electrostatic force between photocathode surface and microscopic tip under illumination (Fig. 4d). It should be pointed out that, in these measurements, the n+p-Si wafers with flat surface were employed to replace those with pyramidal structure to avoid the interference from an uneven morphology. The topography images of C(TF)1CSi and C(TF)6CSi photocathodes with and without light irradiance (Fig. S42) showed a flat and dense surface, in line with HRTEM images (Figs. 2b and S5–S7). Under open-circuit potential conditions, the potential distribution on the surface of C(TF)1CSi and C(TF)6CSi photocathodes was relatively uniform in the dark, and there was a visible negative shift on their surface potentials (see Fig. S43) as the photocathodes were illuminated with 532-nm light. This result means that many photogenerated electrons from n+p-Si can pass through the oxide-based protection layer and inject into the FeCu cocatalysts to form the plentiful reduction sites45.

To accurately uncover the photoresponse of surface potential, the surface potentials of C(TF)1CSi and C(TF)6CSi photocathodes were recorded as a function of testing time while turning on/off the light source (see Fig. 4e). Although the intensity of light source is an important affect factor, the difference of surface potential (Δφ) before and after illumination can still indirectly estimate the surface photovoltage (Vsph) and surface work function of the photocathodes, in connection with the surface band bending46. The absolute value of Δφ (15 mV) for C(TF)6CSi was higher than that for C(TF)1CSi (8 mV, in Fig. 4e), indicating a higher surface photovoltage (−15 mV) and more significant reduction of surface work function (15 mV). According to our previous work45, the higher surface photovoltage could accompany with the stronger built-in electric field existed in the C(TF)6CSi. The intensity of the built-in electric field is quantitatively evaluated the carrier transfer efficiency. For a given photoelectrode, the built-in electric field is usually related with the surface photovoltage and surface charge density by following equation47,

Eb = (2Vsph ρs / εr ε0)0.5 (4)

In the expression, Eb, Vsph, ρs, εr and ε0 denote the built-in electric field intensity, surface photovoltage, surface charge density, relative dielectric permittivity of silicon (a fixed value of 11.7) and vacuum permittivity (a constant of 8.85 × 10−14 F cm−1), respectively. In accordance with the research findings reported by Le Formal et al.48, the quantity of carriers accumulated on the photoelectrode surface exhibits a linear correlation with the integral value derived from the difference between transient photocurrent density and steady-state photocurrent density, as illustrated in Fig. S44. The surface charge density of C(TF)6CSi in the PEC NO3RR process was ~1.6 times higher than that of C(TF)1CSi. Combining the obtained data from KPFM measurements and transient PEC reactions, the built-in electric field of C(TF)6CSi for PEC NO3RR is calculated to be 4.17 kV cm−1 (Fig. 4f), which is ~1.9 times higher than that of C(TF)1CSi. This enhancement is originated from more carriers through the (TF)6 protection layer and less depletion on the photovoltage of Si-based photocathode. In fact, the built-in electric field of C(TF)6CSi is close to that of conventional n+p-Si in the photovoltaic devices49. Thus, the architecture of (O/M)n protection layer can accelerate the carrier-dynamic behaviors and drastically improve the PEC performance, including high photocurrent density and good durability.

In summary, we have proposed a universal approach to construct a low-resistance oxide-based protection layer for a practical Si-based hierarchical photocathode by employing multiple carrier-tunnelling paths. This approach allows the protection layer to realize facile and low-loss photogenerated carrier transport, while providing a sufficient thickness for maintaining the required durability. We have demonstrated thoroughly this approach is common and feasible in various oxide/metal materials. Accordingly, this approach is expected to speed up the pace of development of practical PEC devices. Beyond photoelectrochemistry, the design of low-resistance oxide-based layers with nanoscale multilayer structure can be extended to other chemical and electric fields, such as photocatalysis, electrocatalysis, photovoltaic cells, transistors, and supercapacitors.

Methods
Materials and chemicals
KNO3 (>99%) and KOH (85%) were received from Sinopharm Chemical Reagent Co., Ltd. P2O5 (99.9995%) was received from Meryer (Shanghai) Biochemical Technology Co., Ltd. Silicon wafer was purchased from Hefei KJ Materials Technology Co., Ltd. Argon (Ar, >99.999%) was purchased from Changsha Rizhen Gas Co., Ltd. Iron target (99.95%), copper target (99.99%), titanium target (99.995%), platinum target (99.99%) and cerium target (99.99%) were received from ZhongNuo Advanced Material (Beijing) Technology Co., Ltd.

The preparation of electrolyte
Dissolve 66 g of KOH and 5.06 g of KNO3 in 100 mL of pure water, cool the mixed solution to ambient temperature and then transfer it to a 1 L-volumetric flask. After adjusting the volume to 1 L, the pH value of the mixed solution is 13.8.

Preparation of the samples
To prepare the heterogenous junction of silicon (n+p-Si), p-type Si wafer was put into 2% (mass/volume) potassium hydroxide (KOH) solution and etched at a temperature of 80 °C for 1 h. The etched Si was cleaned using 5% HF solution to remove the SiO2 layer. The phosphorus pentoxide (P2O5) powders were ultrasonically dissolved in 10 mL anhydrous ethanol to prepare 0.1 M P2O5 solution. 1–2 drops of the P2O5 solution were applied onto the surface of the treated Si wafer, and the wafer was quickly annealed at 800 °C for 8 min to form the n+p-Si.

To enhance electron transport capability, the carbon (C) layer was deposited on the surface of n+p-Si (labeled as CSi) by methane (CH4) plasma treatment at 800 °C for 210 s. The prepared CSi wafers (4×4 cm2), flat Si wafers (4×4 cm2), FTO wafers (1×2 cm2) and BK7 glass slides (2×2 cm2) as substrates were placed in the magnetron sputtering system (Chuangshiweina Co. Ltd, MSP-3200) to deposit the oxide-based protection layer and cocatalyst layer. Firstly, the argon (Ar) plasma treatment was carried out at 8.6 Pa for 20 min to clean the substrate surface. Subsequently, oxide-based protection layer with oxide/metal multilayer structure ((O/M)n, where n is the number of nano-scale repeating unit) was deposited on the substrates by orderly sputtering different metal targets in pure Ar and mixed Ar/O2 atmosphere. The deposition pressure, substrate temperature and bias voltage are 2.0 Pa, 50 °C, and 0 V, respectively. The iron (Fe), titanium (Ti), cerium (Ce), and palladium (Pd) targets were sputtered to fabricate the Fe unit, TiO2 unit, CeO2 unit, and Pd unit for assembling the (O/M)n protection layer, respectively. Detailed deposition parameters of (TiO2/Fe)n (labeled as (TF)n), (TiO2/Pd)n (labeled as (TP)n), (CeO2/Pd)n (labeled as (CP)n) and (CeO2/Fe)n (labeled as (CF)n) are listed in Tables S7–S10, respectively. The n value of (O/M)n protection layer was controlled by changing the deposition time, as shown in Tables S7–S10. Finally, Fe-copper (Cu) alloy (FeCu) cocatalysts were deposited by co-sputtering Fe and Cu target with 200-W radio-frequency (RF) power and 4-W direct-current (DC) power, respectively.

The rear surface of the Si-based photocathode modified with the (O/M)n protective layer and FeCu cocatalyst was first subjected to polishing treatment. Subsequently, a Ti film with a thickness of approximately 500 nm was deposited onto the polished rear surface, and a Cu metal wire was attached to form an ohmic back contact. Silver paste was then applied to secure the Cu wire in place. After drying, the entire rear surface and a portion of the front surface of the Si-based photocathode was encapsulated with epoxy resin, which defined an exposed active area of ~0.2 cm2. The actual geometric area of the exposed surface was accurately measured via calibrated digital imaging and ImageJ32. Considering that the thickness of FeCu cocatalyst layer is ~28 nm, the loaded alloy mass is approximately 4.6 μg.

Composition and structure characterization
Aiming to determine the microstructure and composition of the film, field emission scanning electron microscopy (FESEM, Hitachi SU8220) was used to observe the surface and cross section of the samples. The nanoscale multilayer structure of Si-based photocathodes was further examined by high-resolution transmission electron microscopy (HRTEM, Tecnai G2 F20 S-Twin). Scanning transmission electron microscopy (STEM, Tecnai G2 F20 S-Twin) with a high angle annular dark field (HAADF) detector and energy dispersive x-ray spectroscopy (EDS) were employed in the analysis of compositional distributions in the photocathodes. The elements and electron structure of the photocathodes were conducted by a Shimadzu AXIS SUPRA X-ray photoelectron spectroscopy (XPS) instrument. The binding energies of the elements were referenced to the C1s peak (284.8 eV) arising from adventitious carbon. The near-edge x-ray absorption fine structure spectroscopy (NEXAFS) of Fe and Cu L-edges for Si-based photocathodes were detected at the Materials Science Beamline (MSB) at the Elettra synchrotron light source in Trieste (Italy) using a Specs Phoibos 150 multichannel electron energy analyzer, photon energies that had been calibrated against the Fermi level using an Ar ion etched and clean Cu foil standard, and KolXPD obtained from https://www.kolibrik.net/en/solutions-products/kolxpd was used in the curve fitting of species peaks. To verify the crystal structure of Si-based photocathodes, X-ray diffraction (XRD) spectra were measured using a Bruker D8 Advance instrument with the radiation of Cu Kα in the 2θ range from 30° to 80° and a grazing angle of 1° at a scan rate of 2°/min. Photoluminescence (PL) spectra were measured at room temperature on a fluorescence spectrophotometer (Fls-980, Edinburgh) with an excitation wavelength of 405 nm. Photo-assisted scanning Kelvin probe force microscope (KPFM) tests were performed on an Atomic Force Microscopy system (Asylum Research Cypher S, Oxford Instruments) to obtain the surface potentials and morphologies (experiment setup shown in Fig. S45). Inductively coupled plasma optical emission spectrometry (ICP-OES) was carried out on the Agilent 720ES to quantify the concentration of Ti and Ce ions in the electrolytes after 12 h reactions. To investigate the wettability characteristics, the contact angles between the reaction electrolytes (1 M KOH + 50 mM KNO3) and photocathode surface were measured using a Dataphysics OCA 20 contact angle goniometer.

The optical diffuse reflectance and transmittance were monitored on a UV-visible-near-IR spectrophotometer (Hitachi, UH-4150) from 300 to 2600 nm. The Kubelka-Munk theory was generally used for the analysis of diffuse reflectance (R) spectra to obtain the absorption coefficient (α) of the Si-based photocathodes as followed.

F(R) = (1−R)2 / 2R = α (5)

where F(R) was Kubelka–Munk function. The optical energy band gap of the samples was calculated by using the classical relation of optical absorption, where B, Eg and hυ denoted as the band tailing parameter, the optical band gap and the photon energy, respectively. The value of m should be taken as 0.5 or 2, corresponding to the direct or indirect allowed transition which dominated over the optical absorption, respectively.

αhν = B(hν−Eg)m (6)

Photoelectrochemical measurements
The PEC performance of the Si-based photocathodes was performed with a CHI-760e Instruments workstation in an H-type cell with three electrodes under simulated 1 sun illumination (100 mW/cm2, AM 1.5G). Prior to testing, the light intensity was calibrated using an optical power meter. The pretreated Nafion 117 membranes with 1-h treatments with 5% H2O2 and DI water at 80 °C were used as the proton conductive cation exchange membranes. The electrolytes in both chambers were 8 mL mixed solution containing 1 M KOH and 50 mM KNO3. To increase the mass transfer, the electrolytes were stirred by magnetic stirring apparatus with magnetons in the PEC measurements. The working electrode, counter electrode and reference electrode were the Si-based photocathode, platinum foil and Ag/AgCl (filled with saturated KCl solution), respectively. The voltage was not iR corrected. Readings for the Ag/AgCl electrode were recalculated for the reversible hydrogen electrode (RHE) as follows.

ERHE = EAg/AgCl + 0.059 × pH + 0.197 (7)

During PEC reactions, the Ar flow was constantly purged into the electrolyte from one side of the cathode chamber to maintain an inert gas atmosphere. Chronopotentiometry tests were conducted at the potential of 0.1 V vs. RHE, 0 V vs. RHE, −0.1 V vs. RHE and −0.2 V vs. RHE, each PEC NO3− reduction reaction (NO3RR) test was repeated three times at least. Linear sweep voltammetry (LSV) tests were conducted at a scan rate of 50 mV/s with chopped light alternating between dark and light every 3 s. Error bars represent standard deviations from three independent PEC measurements at least.

The intensity-modulated photocurrent spectroscopy (IMPS) of the photocathode was obtained using a Zahner electrochemical workstation at applied potentials of 0 V, –0.1 V and −0.2 V vs. RHE. The light-assisted electrochemical active surface area (ECSA), open-circuit voltage (OCP), and light-assisted EIS of the photocathodes were measured using the CHI-760e Instruments workstation. The scan rate for ECSA test was 20 mV/s, 40 mV/s, 60 mV/s, 80 mV/s, and 100 mV/s and the double layer capacitance (Cdl) was obtained by calculating the slope.

Production analysis
The ammonia (NH3) production was quantified by the indophenol blue method, supplemented with 1H nuclear magnetic resonance (NMR) measurements. The indophenol blue method was implemented with three chromogenic reagents, whose compositions were defined as follows: (1) 1 M sodium hydroxide (NaOH) solution incorporated with 5 wt% sodium citrate (Na3C6H5O7) and 5 wt% salicylic acid (C7H6O3); (2) 0.05 M sodium hypochlorite (NaClO) solution; (3) 1 wt% sodium nitroferricyanide (Na2[Fe(CN)5NO]·2H2O) solution32. In the specific experimental protocol, 0.9 mL of the post-photoelectrolysis electrolyte was mixed with 1 mL of reagent (1), 0.5 mL of reagent (2) and 0.1 mL of reagent (3) sequentially. Subsequently, the mixed solution was kept in a dark environment for 2 h. The UV-visible spectrophotometer was employed to determine the absorbance at a characteristic wavelength of 656 nm, and the obtained absorbance values exhibited a good linear dependence on the NH3 concentration within the tested range. The standard curves for quantitative determination of NH3, which were constructed respectively via the indophenol blue method and 1H NMR spectroscopy, were depicted in Supplementary Figs. S19 and S22. The calculation equations for NH3 yield rate and FE were provided as follows32. All electrolytes collected after PEC reactions were measured at least three times to ensure data reliability. In Eq. 8, the parameters were defined as follows: C represented the NH3 concentration (mg/L); V referred to the volume of the electrolyte involved in the reaction (L); t denoted the total reaction duration (h); and A was the area of the photocathode (cm2). In Eq. 9, n was the number of electrons transferred during the reaction process, which was assigned a value of 8 corresponding to the reduction of NO3− to NH3. F was Faraday constant, with a fixed value of 96,500 C/mol. Additionally, M represented the molar mass of NH3 (g/mol), while Q denoted the total electric charge consumed throughout the PEC reaction (C).

Yield rate (mg/cm2/h) = (C × V) / (t × A) (8)

FE (%) = (C × V × n × F) / (M × Q) × 100% (9)

The Griess test was employed to quantify the nitrite (NO2−) concentration in post-photoelectrolysis electrolytes32. The chromogenic reagent for this test was prepared by dissolving 0.04 g of N-(1-naphthyl)ethylenediamine dihydrochloride and 0.8 g of sulfonamide in a mixed solution consisting of 2 mL of phosphoric acid and 10 mL of deionized water. In the detection process, 50 μL of the electrolyte was first subjected to 100-fold dilution with deionized water. Subsequently, 100 μL of the aforementioned chromogenic reagent was added to the diluted sample, and the mixture solution was kept in the dark for 20 min. Following this treatment, the absorbance of the mixture was measured at a wavelength of 540 nm using a UV-visible spectrophotometer. A set of sodium nitrite (NaNO2) solutions with gradient concentrations was utilized to establish the corresponding standard curve, as shown in Fig. S19. The calculation methods for NO2− yield rate and FE were consistent with those for NH3, as detailed in Eqs. 8 and 9. Notably, no other reaction products were detected in the electrolytes below the detection limit.

1H NMR measurements
The NMR spectroscopy was applied for the determination of NH3 yield rate. After a 1-h PEC reaction, the resultant electrolyte was subjected to a dilution as the pH value was adjusted to approximately 3 using 1 M hydrochloric acid (HCl). Subsequently, 0.5 mL of the diluted electrolyte (with pH calibrated to 3) was pipetted into NMR sample tube. Then, 100 μL of deuterated dimethyl sulfoxide (d6-DMSO) was added as the deuterated solvent, along with 50 μL, 5.0 mM sodium 3-(trimethylsilyl)-1-propanesulfonate (DSS) solution as an internal standard solution. All NMR spectra were acquired on a 600 MHz NMR spectrometer (Bruker) fitted with a low-temperature probe, with the final data representing the cumulative outcome of 256 scans. Additionally, the standard curves for 14NH3 were established by measurements performed on 14NH4Cl reference samples.

Electrical measurements
The layer thickness and temperature-dependent electrical experiments were performed in a two-electrode cell in micro electrical platform as shown in Fig. S46. During the material preparation, one end of FTO wafer was masked, and one unit layer of (Fe)n, (TiO2)n, or (TF)n (n = 1, 2, 4, 6, 8) was deposited on the remaining part. The thickness of unit layers for (Fe)n and (TiO2)n was 18, 9, 4.5, 3 and 2.25 nm corresponding to n values of 1, 2, 4, 6, and 8, respectively. In addition, the thickness of unit layers for (TF)n was 18/18, 9/9, 4.5/4.5, 3/3, and 2.25/2.25 nm corresponding to n values of 1, 2, 4, 6, and 8, respectively. The samples were fabricated with four Au contacts on the surfaces via thermal evaporation, in order to further enhance electron transport. In the photoexcitation electrical measurements, Si-based photocathodes were investigated and the detailed fabrication process has been described above.

The current versus voltage (I-V) curves were carried out using the combination of a Keithley 2450 Source Meter and a probe station. For the samples, two probes from the probe station were connected to the source meter and subsequently pressed onto the deposited region and masked region, respectively. The substrate bias was scanned from −10 to 10 mV while monitoring the current density and the resistance was calculated from the I-V curves. The units of (Fe)n, (TiO2)n, and (TF)n were used to analyze the effect of film thickness on resistance at a constant temperature. The units of (Fe)6, (TiO2)6, and (TF)6 were also used to analyze the effect of temperature on resistance from 15 to 200 °C.

In the photoexcitation electrical measurements, a laser light with a wavelength of 520 nm was used to illuminate the Si-based photocathodes. Two probes were pressed onto the photocathode surface and Cu belt (see Fig. 4a(ii)), and the electrons conducted along the vertical direction. The substrate bias was scanned from −1 to 1 V. Before illumination, all samples were placed in darkness and tested for the I-V curves.

Finite element analysis
The finite element analysis was performed using COMSOL Multiphysics (version 6.3). Tables S1 and S2 showed the material properties of TiO2 and Fe and the constructed electrical filed. In this simulation, the effect of the material’s cross-sectional area was neglected, and with nanoscale film thickness, a two-dimensional (2D) model was built. We selected the EC physical field and the semi-physical field for simulating the Fe and TiO2, respectively. Under the influence of these two physical fields, the simulation showed that the current within the bulk material reached a steady-state value, while abrupt changes were allowed to occur at the Schottky junction contact. The Schottky junction equation could be described as followed.

J = A*T2 exp (−qϕB / kT) exp (qV / kT) − 1 (10)

Among them, J was the current density, A* the Richardson constant related to the material, T the temperature, q the charge of an electron, ϕB the height of the Schottky barrier, k the Boltzmann constant, and V the applied voltage. An endogenous electric field can be generated by applying an external voltage at the model boundaries. Driven by this electric field, electrons migrated along the thickness direction of the film, resulting in distinct dielectric loss characteristics.

Data availability
The data for the figures within this paper are available in the Source Data file. Source data are provided with this paper. Source Data file has been deposited in Figshare under accession code DOI link [10.6084/m9.figshare.30929180].

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The authors proposed a universal strategy to construct a low-resistance oxide-based protection layer for practical Si-based hierarchical photocathode by employing multiple carrier-tunnelling paths. This approach allows the protection layer to realize the facile and low-loss photogenerated carrier transport, while providing a sufficient thickness for maintaining the required durability. The authors further demonstrate the generality and feasibility of this strategy across various oxide-metal systems. However, the manuscript lacks a comprehensive characterization of the stability for this multilayer film in photoelectrocatalytic operation. Before the manuscript can be considered for publication, the authors are recommended to address the following concerns:\n1. Comment: The core objective of this work is to address the corrosion issue in PEC catalysis through the design of a surface protection layer. Therefore, stability is a crucial metric for validating the effectiveness of this approach. However, the main text primarily presents catalytic performance data, lacking characterization of catalytic stability. We recommend incorporating data on long-term stability into the main text.\n2. Comment: Figures S16 and S24 reveal a significant initial current decay within the first hour of operation. While the authors attribute this phenomenon to three potential factors (line 282), to enhance the manuscript's rigor, we recommend that targeted experiments be designed to quantify the relative contribution of each factor to the observed stability degradation.\n3. Comment: While Ti dissolution concentration was measured via ICP-OES in this study to infer potential stability, changes in elemental content alone cannot definitively indicate stability performance. Structural evolution of the protection layer after long-term operation—such as interfacial delamination or crystallinity changes in the oxide—provides direct insights into surface structural stability. We contend that HRTEM-EDS mapping analysis of post-stability-test samples should be supplemented to validate the stability mechanism of the multilayer architecture.\n4. Comment: Research on the use of Cu-based co-catalyst as the active site for the reduction of NO3- through photoelectrocatalysis has also been reported recently (Energy & Environmental Science, 17(23), 9233-9243). It would be informative to discuss such on role of co-catalyst in this system.\n5. Comment: Why is there such a large difference in ECSA among C(TF)nCSi (n = 1, 2, 4, 6, and 8) samples with similar thicknesses, as shown in Figure S15?\n6. Comment: The article contains some formatting errors, such as the references [24, 25], [43, 44], [45, 46], which should be [24,25] [43,44], [45,46].\n7. Comment: From the EIS plots in Figure 3d, it is evident that the shapes of the impedance spectra for C(TF)nCSi (n = 1, 2, 4, 6, and 8) differ, suggesting that their reaction mechanisms may vary. The authors should provide a more thorough analysis and fitting of the impedance data, rather than merely referring to it briefly.", "reviewer_2": "This paper demonstrates an interesting and feasible design of oxide-based protection layer for Si-based hierarchical photocathode with high efficiency and good durability by multiple carrier-tunnelling paths. The approach is suitable for various oxide-based layers, e.g., (TiO2/Fe)n and (CeO2/Pd)n. The design and results of this work are significant in the photoelectrochemical field and will have a broad interest for the readers of the journal. I suggest to accept this manuscript after addressing the following issues:\n1. A significant advance is the design of oxide-based protection layer with the introduction of thin metal unit layer (such as Fe and Pd) to create the independent electron tunnelling path in the thin oxide unit layer. However, the metal layer is also non-transparent, which could reduce the light harvesting of Si-based photocathode. The total thickness of the metal layer used in this work is 18 nm, which is thick enough for affecting the light transmission. Could the authors provide a detailed information about the influence of the thickness of metal unit layer on light harvesting?\n2. The conductive nature of metal unit layer will allow electrons access to the oxide unit layer, which are very important to photoelectrochemical reduction reactions. Is it possible to give some discussion on the general rules needed to optimize the thickness of metal unit layer by balancing the conductive and non-transparent nature of metal materials?\n3. The photocurrent density of >16 mA cm-2 and NH3 yield rate of >0.5 mg h-1 cm-2 for photoelectrochemical NO3- to NH3 at 0 V vs. RHE is very impressive, which arises mainly from the design of oxide-based protection layer and the presence of multiple carrier-tunnelling paths. How about the HER performance of the Si-based photocathodes with such oxide-based protection layer? The results might confirm the universality and feasibility of this approach.\n4. The authors used indophenol blue method to analyze and quantify the NH3 yield rate from photoelectrochemical NO3- reduction reaction, the detection approach of NH3 might not be enough. To obtain more accurate results, other analytical methods (such as NMR) should be used to evaluate the NH3 production.\n5. It also might be significant to show some composition and structure characterizations about (CeO2/Fe)n, (TiO2/Pd)n and (CeO2/Pd)n, like XRD, FESEM, and optical measurements.", "reviewer_3": "The manuscript introduces a multilayer oxide/metal protection layer architecture designed to simultaneously optimize carrier transport and corrosion resistance in PEC devices. By employing TiO_2/Fe (TF)_n, CeO_2/Fe (CF)_n, TiO_2/Pd (TP)_n, and CeO_2/Pd (CP)_n stacks, the authors aim to create multiple carrier-tunnelling paths, thereby breaking the trade-off between efficiency and durability. The study combines finite element simulations, systematic PEC tests, electrical measurements, and structural characterization to demonstrate that (TF)_6 exhibits optimal performance, achieving high photocurrent density and long-term stability. The work also extends this design to other oxide/metal combinations, claiming a universal applicability. However, several key aspects of the mechanism are insufficiently supported, and the performance benchmarking and physical interpretation of photoelectrochemical behavior require further clarification and experimental supports. After revision and additional data as outlined below, the manuscript would be significantly strengthened.\n1. The manuscript attributes the improved charge transport to the presence of multiple carrier-tunneling pathways across oxide/metal junctions. However, no direct experimental evidence for tunneling is provided such as temperature dependent carrier injection/transport. The claim rests solely on WKB-based simulations. Moreover, detailed discussion on the tunneling barrier height and effective mass assumptions used in simulations are needed.\n2. The claim that interfaces remain ohmic or low-barrier is central to the carrier transport mechanism. The manuscript would benefit from direct characterization of energy level alignment via a more direct measurements such as Kelvin probe or UPS. A schematic energy band diagram of the full multilayer stack, with alignment at each interface, would be helpful.\n3. IMPS and EIS data are promising but are only qualitatively discussed. Please include quantitative fitting and transfer time constants across samples to better support claims of faster interfacial kinetics.\n4. Although the oxide layers are thin, the inclusion of Fe or Pd layers may introduce parasitic optical losses. Please include discussion of light absorption and reflection in the multilayer.\n5. In NO_3RR, species and yield of other by-products would also be discussed.\n6. The long-term stability claim is extrapolated over 12 h. While the method is valid, it would greatly strengthen the manuscript to include longer durability tests.\n7. The main text lacks discussion comparing the PEC performance against recent state-of-the-art nitrate-to-ammonia PEC systems\n8. Please clarify if any oxidation of the Fe interlayer occurred during or after deposition. This is especially important as it may change the electrical and protective properties.\n9. Provide device-to-device performance variation or error bars for photocurrent and Faradaic efficiency to demonstrate reproducibility."}, "author_response": "We thank the referee for their positive appraisal and instructive comments. According to the comments, we have added the comprehensive characterization of the stability for this multilayer film after PEC measurements and provided a point-by-point response, as follows. [Full rebuttal text omitted for brevity as per instructions to include verbatim text]"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The paper has been revised and now it is suitable for the publication.", "reviewer_2": "I recommended the publication as it is, since all my questions were fully addressed.", "reviewer_3": "The rebuttal further strengthens durability evidence by including additional data. However, there remain concerns regarding the convincing demonstration of long-term protection, insufficient in-depth analysis of interface evolution at later hours, limited mechanistic insight into electroreduction pathways and co-catalyst role, and marginal improvement in stability versus state-of-the-art protected Si photocathodes. While the results are promising, major clarifications are necessary to justify acceptance in Nature Communications.\n1. Although durability tests and ICP-OES-based dissolution analysis were added, the protective layer still loses total thickness and co-catalyst fully disappears at longer periods. This indicates progressive degradation. Clarify whether the observed lifetime is compatible with practical PEC system expectations. Provide contextual benchmarking vs state-of-art Si protection layers.\n2. Modeling and calculation results support the concept, but experimental validation is not shown. Provide band-alignment data to confirm ohmic vs Schottky interface behavior and tunneling barrier thickness evolution.\n3. Catalyst layer dissolves before the oxide shows visible damage, which introduces transient surface states and structural changes. Catalyst degradation complicates interpretation of protection role.\n4. The rebuttal repeats data but still does not quantify contributions of bubble coverage, catalyst restructuring, and tunneling interface modification.\n5. Since the manuscript positions the strategy as universal, other PEC demonstration (HER, CO2RR, etc) in an acidic or neutral electrolyte would be highly valuable.\n6. Fe multilayer sputtering at 3 nm precision across the large scale surface is non-trivial. Further discussions are required.\n7. Authors need to provide statistics for stability tests."}, "author_response": "We are of great gratitude to your positive appraisal and instructive comments. We have revised our manuscript and provided a point-by-point response, as follows. [Full rebuttal text omitted for brevity]"}, {"round": 3, "reviewer_comments": {"reviewer_3": "The authors provided comprehensive arguements with fruitful supporting informations in response to comments. The reviewer recommends the acceptance as is."}, "author_response": ""}]

Decision letter