Silicon Photocathodes Research Articles

Tuning the Photoelectrocatalytic Hydrogen Evolution of Pt-Decorated Silicon Photocathodes by the Temperature and Time of Electroless Pt Deposition.

The electroless deposition of Pt nanoparticles (NPs) on hydrogen-terminated silicon (H-Si) surfaces is studied as a function of the temperature and the immersion time. It is demonstrated that isolated Pt structures can be produced at all investigated temperatures (between 22 and 75 °C) for short deposition times, typically within 1-10 min if the temperature is 45 °C or less than 5 min at 75 °C. For longer times, dendritic metal structures start to grow, ultimately leading to highly rough interconnected Pt networks. Upon increasing the temperature from 22 to 75 °C and for an immersion time of 5 min, the average size of the observed Pt NPs monotonously increases from 120 to 250 nm, and their number density calculated using scanning electron microscopy decreases from (4.5 ± 1.0) × 108 to (2.0 ± 0.5) × 108 Pt NPs cm-2. The impact of both the morphology and the distribution of the Pt NPs on the photoelectrocatalytic activity of the resulting metallized photocathodes is then analyzed. Pt deposited at 45 °C for 5 min yields photocathodes with the best electrocatalytic activity for the hydrogen evolution reaction. Under illumination at 33 mW cm-2, this optimized photoelectrode shows a fill factor of 45%, an efficiency (η) of 9.7%, and a short-circuit current density (|Jsc|) at 0 V versus a reversible hydrogen electrode of 15.5 mA cm-2.

Investigation of porous silicon photocathodes for photoelectrochemical hydrogen production

Solar energy conversion is made possible through the use of silicon as a photoelectrode material. Nanostructuring of the silicon substrate further improves solar energy conversion by virtue of its antireflective surface and tuneable band gap energies. In this work, we investigated the optimal etching conditions to fabricate porous silicon films, in terms of pore size and layer thickness. The stability of the sample was improved by passivating the porous layer with methyl groups via the electrografting of methyl iodide. A bio-inspired iron sulphur carbonyl electrocatalyst loaded on the electrografted porous silicon was studied. A photocurrent density of −2.8 mA/cm2 and 46.8 μmol/h of hydrogen gas were observed for the electrografted porous silicon coated with bio-inspired iron sulphur carbonyl electrocatalyst.

Engineering Molybdenum Disulfide Protected Silicon Photocathodes for Corrosion Resistance in Acid

Photoelectrochemical (PEC) water splitting devices offer a promising path to sustainable production of hydrogen.1 Silicon shows great promise as a small band gap photocathode in tandem photoelectrochemical (PEC) water splitting due to its appropriate band structure, energetic alignment for driving hydrogen evolution, and its band gap of 1.1 eV.2 However, some challenges remain to ensure that silicon photoelectrodes possess the required catalytic activity and stability for use in highly effect PEC water splitting devices. Molybdenum disulfide (MoS2) is a low permeability material known for its excellent stability and HER activity in acid. We have previously demonstrated MoS2 protected silicon photocathodes which operate for 100 hrs without significant loss of activity.3 Here, we further this study to design strategies, such as the utilization of a molybdenum silicide interface layer, for improved activity and over 600 hour stability with unchanged photoelectrochemical performance (onset potential, fill factor and light limited photocurrent density), which is among the best performance of any precious metal free photocathode. Critical to developing future electrodes with enhanced stability, we probe the failure mechanism of the MoS2 protected photocathodes. By XPS we observe that the MoS2 is fully absent post stability testing. Detailed failure mechanisms of the photocathodes and electrolyte are uncovered by electrochemical methods, SEM, Raman microscopy and ICP. Furthermore, we implement increased light intensity (10 suns), as well as light and dark cycling to probe accelerated durability testing protocols utilizing MoS2 protected photocathodes. From such analyses we uncover a feedback loop that enables insightful design of future corrosion resistant protection schemes. (1) Walter, M. G.; Warren, E. L.; McKone, J. R.; Boettcher, S. W.; Mi, Q. X.; Santori, E. A.; Lewis, N. S. Solar Water Splitting Cells. Chem. Rev. 2010, 110, 6446–6473. (2) Seitz, L. C.; Chen, Z.; Forman, A. J.; Pinaud, B. A.; Benck, J. D.; Jaramillo, T. F. Modeling Practical Performance Limits of Photoelectrochemical Water Splitting Based on the Current State of Materials Research. ChemSusChem 2014, 7 (5), 1372–1385. (3) Benck, J. D.; Lee, S. C.; Fong, K. D.; Kibsgaard, J.; Sinclair, R.; Jaramillo, T. F. Designing Active and Stable Silicon Photocathodes for Solar Hydrogen Production Using Molybdenum Sulfide Nanomaterials. Adv. Energy Mater. 2014, 4 (18), 1400739.

Molybdenum Disulfide As a Protection Layer and Catalyst for Gallium Indium Phosphide Solar Water Splitting Photocathodes

Gallium indium phosphide (GaInP2) is a semiconductor with a nearly ideal bandgap for solar water-splitting as the top absorber in a dual junction tandem absorber device. It has been used in conjunction with a gallium arsenide (GaAs) bottom absorber in an overall water splitting cell with 12.4% solar-to-hydrogen (STH) efficiency, one of the highest STH efficiencies for an integrated photoelectrochemical (PEC) water-splitting device reported to date. However, GaInP2 suffers from one of the biggest challenges facing the field: instability due to electrochemical corrosion in aqueous electrolytes. Molybdenum disulfide (MoS2) nanomaterials can be used to both protect GaInP2 and significantly improve its catalytic ability since it is resistant to corrosion and also possesses high activity for the hydrogen evolution reaction (HER). In this work, we demonstrate that GaInP2 photocathodes coated with thin MoS2 surface protecting layers exhibit excellent activity and stability for solar hydrogen production and we probe the details of failure mechanisms using novel flow cell microscopic and spectroscopic techniques. Our GaInP2 photocathodes demonstrated no loss in performance (photocurrent onset potential, fill factor, and light limited current density) until 60 hours of operation which represents a five-hundred fold increase in stability compared to bare p-GaInP2 samples tested in identical conditions. We believe this to be one of the first successful attempts to stabilize GaInP2 using a thin film protection layer scheme. Furthermore, as this protection scheme has previously been used successfully on silicon photocathodes, this work highlights the potential for MoS2 to be used as a thin film protection layer for many different semiconductor water splitting devices that are unstable in acid. Using a custom-designed flow cell coupled with various microscopic and spectroscopic techniques (optical, Raman, FT-IR), we gained a greater understanding of the failure mechanisms of MoS2 as a thin-film protection layer. We discovered that pinhole formation in the MoS2 layer exposes the GaInP2 substrate, which readily corrodes in the acidic conditions, ultimately leading to device degradation. The flow cell further allowed us to capture the time scale of this pinhole formation. These insights represent a deeper understanding of MoS2 as a protection layer and can be leveraged to improve the stability of thin film protected semiconductor water splitting devices. References Khaselev, O.; Turner, J. A., A Monolithic Photovoltaic-Photoelectrochemical Device for Hydrogen Production Via Water Splitting. Science 1998, 280, 425-427. Benck, J. D.; Lee, S. C.; Fong, K. D.; Kibsgaard, J.; Sinclair, R.; Jaramillo, T. F., Designing Active and Stable Silicon Photocathodes for Solar Hydrogen Production Using Molybdenum Sulfide Nanomaterials. Advanced Energy Materials 2014, 4, 1-8 Britto R.J., Benck J.D., Young J.L., Hahn C., Deutsch, T.G., Jaramillo T.F. In Review (2016) Figure 1

Proton Reduction Using a Hydrogenase-Modified Nanoporous Black Silicon Photoelectrode.

Metalloenzymes featuring earth-abundant metal-based cores exhibit rates for catalytic processes such as hydrogen evolution comparable to those of noble metals. Realizing these superb catalytic properties in artificial systems is challenging owing to the difficulty of effectively interfacing metalloenzymes with an electrode surface in a manner that supports efficient charge-transfer. Here, we demonstrate that a nanoporous "black" silicon (b-Si) photocathode provides a unique interface for binding an adsorbed [FeFe]-hydrogenase enzyme ([FeFe]-H2ase). The resulting [FeFe]-H2ase/b-Si photoelectrode displays a 280 mV more positive onset potential for hydrogen generation than bare b-Si without hydrogenase, similar to that observed for a b-Si/Pt photoelectrode at the same light intensity. Additionally, we show that this H2ase/b-Si electrode exhibits a turnover frequency of ≥1300 s(-1) and a turnover number above 10(7) and sustains current densities of at least 1 mA/cm(2) based on the actual surface area of the electrode (not the smaller projected geometric area), orders of magnitude greater than that observed for previous enzyme-catalyzed electrodes. While the long-term stability of hydrogenase on the b-Si surface remains too low for practical applications, this work extends the proof-of-concept that biologically derived metalloenzymes can be interfaced with inorganic substrates to support technologically relevant current densities.

Water oxidation-coupled, photoelectrochemical redox biocatalysis toward mimicking natural photosynthesis

Redox enzymes are industrially important for catalyzing highly complex reactions because of their excellent regio- and stereo-selectivity; however, broad application of redox enzymes has been often limited by the requirement of stoichiometric supply of cofactors such as β-nicotinamide adenine dinucleotide (NADH). Here, we report light-driven cofactor regeneration coupled with water oxidation by employing a photoelectrochemical cell platform consisted of a FeOOH/Fe2O3 photoanode and a black silicon photocathode. The FeOOH layer deposited on Fe2O3 surface decreased reaction barriers for water oxidation. The black silicon photocathode exhibited high photocurrent response and superior capacity to drive cofactor reduction. The cofactor regeneration yield in the photoelectrochemical cell was almost two-fold higher than that obtained in homogenous system, which demonstrates that photoelectrochemical cell is a promising platform for redox biocatalytic reactions using water as an electron donor.

Revealing the semiconductor–catalyst interface in buried platinum black silicon photocathodes

Burying platinum black silicon photocathodes provides an atomically clean Pt catalyst/Si interface free of oxide or other phases.

Electrocatalytic Study of Carbon Dioxide Reduction By Co(TPP)Cl Complex

Carbon dioxide (CO2) is notorious for being a greenhouse gas and is the most important cause of global warming. However, it can be converted into useful products as it is a source of carbon. Reduction of CO2is therefore an attractive research topic for many chemists. Different methods of electrocatalytic reduction of CO2have been reported previously. Since CO2is very stable, the direct electroreduction of CO2into CO requires high potential at −2.2 V versus Ag/AgCl. In this work, CO2reduction was carried out by the photoelectrocatalysis of CO2in the presence of cobalt(III)tetraphenylporphyrin [Co(TPP)Cl] at −1.85 V with a current efficiency of 71%. At illuminated p-type silicon photocathode, the reduction of CO2into CO was performed at a potential of 300 mV which is positive. However, at the same conditions, potential of −1.55 V with a current efficiency ofca65% is required for the carbon electrode.

Engineering Cobalt Phosphide (CoP) Thin Film Catalysts for Enhanced Hydrogen Evolution Activity on Silicon Photocathodes

Transition metal phosphide catalysts have recently emerged as active, earth abundant alternatives to precious metals for the hydrogen evolution reaction in acid. High performance, scalable catalysts are necessary for the successful implementation of photoelectrochemical water splitting devices, which have the potential to generate hydrogen in a sustainable manner. Herein, a general synthetic route is reported to produce transition metal phosphide thin films, which is used to fabricate cobalt phosphide (CoP) catalysts with high average turnover frequency (TOFavg), 0.48 H2 s−1 and 1.0 H2 s−1 at 100 and 120 mV overpotential, respectively. Furthermore, it is shown that CoP thin films can be applied to silicon photoabsorbers to generate one of the most active precious metal‐free crystalline silicon photocathodes to date, achieving −10 mA cm−2 at +0.345 V vs. reversible hydrogen electrode. The synthesis route presented here provides a platform for both fundamental studies of well‐defined electrocatalysts and the fabrication of high‐performance photoelectrodes.

Graphene Quantum Sheet Catalyzed Silicon Photocathode for Selective CO2 Conversion to CO

The reduction of carbon dioxide (CO2) into chemical feedstock is drawing increasing attention as a prominent method of recycling atmospheric CO2. Although many studies have been devoted in designing an efficient catalyst for CO2 conversion with noble metals, low selectivity and high energy input still remain major hurdles. One possible solution is to use the combination of an earth‐abundant electrocatalyst with a photoelectrode powered by solar energy. Herein, for the first time, a p‐type silicon nanowire with nitrogen‐doped graphene quantum sheets (N‐GQSs) as heterogeneous electrocatalyst for selective CO production is demonstrated. The photoreduction of CO2 into CO is achieved at a potential of −1.53 V versus Ag/Ag+, providing 0.15 mA cm−2 of current density, which is 130 mV higher than that of a p‐type Si nanowire decorated with well‐known Cu catalyst. The faradaic efficiency for CO is 95%, demonstrating significantly improved selectivity compared with that of bare planar Si. The density functional theory (DFT) calculations are performed, which suggest that pyridinic N acts as the active site and band alignment can be achieved for N‐GQSs larger than 3 nm. The demonstrated high efficiency of the catalytic system provides new insights for the development of nonprecious, environmentally benign CO2 utilization.

Nanostructured Silicon Photocathodes for Solar Water Splitting Patterned by the Self-Assembly of Lamellar Block Copolymers.

We studied a type of nanostructured silicon photocathode for solar water splitting, where one-dimensionally periodic lamellar nanopatterns derived from the self-assembly of symmetric poly(styrene-block-methyl methacrylate) block copolymers were incorporated on the surface of single-crystalline silicon in configurations with and without a buried metallurgical junction. The resulting nanostructured silicon photocathodes with the characteristic lamellar morphology provided suppressed front-surface reflection and increased surface area, which collectively contributed to the enhanced photocatalytic performance in the hydrogen evolution reaction. The augmented light absorption in the nanostructured silicon directly translated to the increase of the saturation current density, while the onset potential decreased with the etching depth because of the increased levels of surface recombination. The pp(+)-silicon photocathodes, compared to the n(+)pp(+)-silicon with a buried solid-state junction, exhibited a more pronounced shift of the current density-potential curves upon the introduction of the nanostructured surface owing to the corresponding increase in the liquid/silicon junction area. Systematic studies on the morphology, optical properties, and photoelectrochemical characteristics of nanostructured silicon photocathodes, in conjunction with optical modeling based on the finite-difference time-domain method, provide quantitative description and optimal design rules of lamellar-patterned silicon photocathodes for solar water splitting.

Protection of Si photocathode using TiO2 deposited by high power impulse magnetron sputtering for H2 evolution in alkaline media

Si is an excellent absorber material for use in photoelectrochemical (PEC) hydrogen production. Only a few studies have been done using Si in alkaline electrolyte for hydrogen evolution due to its poor chemical stability in high pH electrolyte, indicating that a chemically stable protection layer is essential. Here we investigate thin TiO2 films deposited by high power impulse magnetron sputtering (HiPIMS) as a protection layer for a p-type silicon photocathode for photoelectrochemical H2 evolution in a high pH electrolyte. The X-ray reflectometry analysis reveals that the HiPIMS process provides improved film density for TiO2 films (4.15g/cm3), and consequently results in a significantly less corroded Si surface. The Si photocathode protected by the HiPIMS grown TiO2 film along with Pt as co-catalyst produced a photocurrent onset potential of ~0.5V vs. RHE in 1M KOH and showed a 4% decay over 24h in KOH. In contrast, the sample with the TiO2 deposited using conventional DC sputtering technique of similar thickness shows 20% loss in photocurrent for the same time interval. Considering the fact that the experiments were carried out not in the cleanroom, much less corrosion loss can be obtained if done in dust-free condition. Hence, these results suggest the HiPIMS technique as an improved approach for the protection of photoelectrodes, which are unstable in alkaline solution.

A highly efficient BiVO4/WO3/W heterojunction photoanode for visible-light responsive dual photoelectrode photocatalytic fuel cell

A highly efficient BiVO4/WO3/W heterojunction photoanode was fabricated based on the nanoporous WO3 film, which was prepared on the tungsten plate by anodic oxidation, in order to facilitate the electrons transfer from BiVO4 to WO3 by providing a natural connection between WO3 film and W substrate. Then, a visible-light responsive dual photoelectrode photocatalytic fuel cell (PFC) consisted of BiVO4/WO3/W photoanode and Pt modified commercial buried junction silicon (Pt/BJS) photocathode was constructed. The results showed that the optimized BiVO4/WO3/W photoanode obtained a photocurrent of 2.01mA/cm2 at 0.6V (vs Ag/AgCl) in 0.1M KH2PO4 (pH 7) electrolyte under simulated AM1.5 solar light, which was 180% and 205% higher than that of bare WO3 film and bare BiVO4 film, respectively. The established dual photoelectrode PFC showed high converting performance of organics into electricity. For example, a short-circuit current density of 0.26mAcm−2, which is higher than most of the reported visible-light responsive dual photoelectrode PFC systems, was obtained with the open-circuit voltage of 0.78V and maximum power output of 2.0×10−4 Wcm−2 in 20mgL−1 tetracycline hydrochloride. Furthermore, a removal ratio of 78% after 4h was achieved with a stable output photocurrent in the degradation process.

ChemInform Abstract: Solar Hydrogen Production by Amorphous Silicon Photocathodes Coated with a Magnetron Sputter Deposited Mo2C Catalyst.

AbstractA 5 nm Mo2C film is deposited on an amorphous Si photocathode by confocal magnetron co‐sputtering.

Solar water splitting with earth-abundant materials using amorphous silicon photocathodes and Al/Ni contacts as hydrogen evolution catalyst

An all earth-abundant and precious metal-free photocathode based on a low-temperature fabricated amorphous silicon tandem junction is demonstrated to be an efficient device for solar water splitting. With a particular designed Al/Ni layer stack as photocathode/electrolyte contact an onset potential for cathodic photocurrent of 1.7V vs. RHE and a saturation photocurrent density of 7.2mA/cm2 were achieved. For a high-cost alternative with a Ag/Pt layer stack an even higher photocathode performance is demonstrated. Above all we present an approach for a dedicated photovoltaic and electrochemical development for solar water splitting.

Amorphous oxygen-rich molybdenum oxysulfide Decorated p-type silicon microwire Arrays for efficient photoelectrochemical water reduction

We report the fabrication of p-type silicon (Si) photocathodes consisting of well-ordered Si microwire (Si-MW) arrays coupled with non-precious and earth-abundant amorphous oxygen-rich molybdenum oxysulfide (MoOxSy) as both a hydrogen evolution catalyst and a passivation layer. The MoOxSy is conformally grown on the Si-MW surface through photo-assisted cyclic voltammetric (CV) deposition. By adjusting the cycle numbers of the CV deposition, Si-MW array electrodes with different MoOxSy catalyst loadings (Si-MWs@MoOxSy) have been obtained and comprehensively characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and Raman and Fourier-transform infrared spectroscopies. The photoelectrochemical performance of the Si-MWs@MoOxSy cathodes toward water reduction is investigated and compared with that of platinum nanoparticle decorated Si-MW array electrodes (Si-MWs@PtNPs). An optimized Si-MWs@MoOxSy photocathode is found to exhibit activity comparable to that of the Si-MWs@PtNPs one, with a much better stability in acidic medium. In neutral electrolyte, Si-MWs@MoOxSy outperforms Si-MWs@PtNPs in terms of both activity and stability. Given the low materials cost, easy and well-established electrode fabrication procedure, and high demonstrated photoelectrochemical performance, the Si-MWs@MoOxSy arrays reported here hold substantial promise for use as low-cost and efficient photocathodes for water reduction.

Solar Hydrogen Production by Amorphous Silicon Photocathodes Coated with a Magnetron Sputter Deposited Mo2C Catalyst.

Coupling of Earth-abundant hydrogen evolution catalysts to photoabsorbers is crucial for the production of hydrogen fuel using sunlight. In this work, we demonstrate the use of magnetron sputtering to deposit Mo2C as an efficient hydrogen evolution reaction catalyst onto surface-protected amorphous silicon (a-Si) photoabsorbers. The a-Si/Mo2C photocathode evolves hydrogen under simulated solar illumination in strongly acidic and alkaline electrolytes. Onsets of photocurrents are observed at potentials as positive as 0.85 V vs RHE. Under AM 1.5G (1 sun) illumination, the photocathodes reach current densities of -11.2 mA cm(-2) at the reversible hydrogen potential in 0.1 M H2SO4 and 1.0 M KOH. The high photovoltage and low-cost of the Mo2C/a-Si assembly make it a promising photocathode for solar hydrogen production.

N-type silicon photocathodes with Al-doped rear p+ emitter and Al2O3-coated front surface for efficient and stable H2 production

Currently, p-type silicon has been studied as a photocathode in a photoelectrochemical cell for water splitting where an n+ thin layer is usually fabricated on electrode surface in order to increase band bending at the n+p interface relative to the aqueous solution/p-Si interface. However, this leads to high Auger recombination on the reaction interface. We report herein an efficient and stable photocathode based on single-crystal n-type Si with a rear np+ junction, different from the conventional one on p-type Si with a front n+p junction. Using a thin Al2O3 surface protecting layer, it shows no loss in photoelectrochemical performance after 138 h of continuous operation, and the energy conversion efficiency can be nearly doubled to 8.68%, compared with 4.51% for the corresponding normal n+p electrode under 100 mW/cm2 simulated solar illumination and Pt catalyzing. Our np+ Si photocathodes improve the H2 production by providing: (1) high on-set potential due to the rear junction; (2) high carrier life time on the electrode surface due to the low doping level of n-type Si; and (3) excellent passivating effect of Al2O3 on the surface of n-type Si.

High Performance Silicon Photocathodes for Hydrogen Production Via Solar Water Splitting

Photoelectrochemical (PEC) water splitting could provide a sustainable means of hydrogen fuel production.1 Many recent studies have focused on developing materials suitable for application in dual-absorber PEC water splitting devices due to the high solar-to-hydrogen efficiencies this architecture could enable.2, 3 Silicon is a particularly promising small band gap absorber material for application in a tandem PEC device due to its abundance, relatively low cost, excellent charger carrier transport, and near-ideal band gap. A number of recent studies have reported promising silicon photocathode structures.4-6 However, several challenges remain to be addressed to develop silicon photocathodes that are as efficient and stable as necessary for economical water splitting. The silicon must be combined with an active catalyst to reduce the kinetic overpotential necessary to drive the hydrogen evolution reaction (HER) at the electrode surface.4-6 The silicon must also be protected to prevent corrosion or oxidation, which can destroy device performance.6 We report our recent progress in designing highly active and stable silicon photocathodes. First, we discuss strategies for protecting silicon photocathodes against corrosion. We demonstrate that silicon electrodes covered with a thin surface coating of molybdenum disulfide show no loss in performance after 100 hours of operation.7 Transmission electron microscopy measurements reveal the atomic scale features of this electrode design that result in its excellent stability. While the MoS2 surface layer provides reasonable catalytic activity, the device performance is limited due to the low density of exposed catalytic sites. To further improve this cathode’s efficiency, we incorporate a second molybdenum sulfide nanomaterial, highly catalytically active [Mo3S13]2- clusters,8resulting in precious metal-free devices with photocurrent onset potentials within ~150 mV of the best reported Pt/Si photocathodes. Next, we discuss further improvements in the design of silicon photocathodes to maximize the photocurrent onset potential and device durability. These developments include the incorporation of new, highly active HER electrocatalysts as well as photocathode designs that reduce recombination within the silicon, maximizing the current-voltage characteristics of the device. Based on our findings, we propose strategies for further improving the performance of solar water splitting photocathodes. 1. M. G. Walter, et al., Chemical Reviews, 110, 6446 (2010). 2. M. F. Weber, et al., Journal of The Electrochemical Society, 131, 1258 (1984). 3. L. C. Seitz, et al., ChemSusChem, 7, 1372 (2014). 4. J. R. McKone, et al., Energy & Environmental Science, 4, 3573 (2011). 5. S. W. Boettcher, et al., Journal of the American Chemical Society, 133, 1216 (2011). 6. B. Seger, et al., Journal of the American Chemical Society, 135, 1057 (2013). 7. J. D. Benck, et al., Advanced Energy Materials, (2014). 8. J. Kibsgaard, et al., Nat Chem, 6, 248 (2014).

Oxidatively stable nanoporous silicon photocathodes with enhanced onset voltage for photoelectrochemical proton reduction.

Stable and high-performance nanoporous "black silicon" photoelectrodes with electrolessly deposited Pt nanoparticle (NP) catalysts are made with two metal-assisted etching steps. Doubly etched samples exhibit an ∼300 mV positive shift in photocurrent onset for photoelectrochemical proton reduction compared to oxide-free planar Si with identical catalysts. We find that the photocurrent onset voltage of black Si photocathodes prepared from single-crystal planar Si wafers by an Ag-assisted etching process increases in oxidative environments (e.g., aqueous electrolyte) owing to a positive flat-band potential shift caused by surface oxidation. However, within 24 h, the surface oxide layer becomes a kinetic barrier to interfacial charge transfer that inhibits proton reduction. To mitigate this issue, we developed a novel second Pt-assisted etch process that buries the Pt NPs deep into the nanoporous Si surface. This second etch shifts the onset voltage positively, from +0.25 V to +0.4 V versus reversible hydrogen electrode, and reduces the charge-transfer resistance with no performance decrease seen for at least two months. PEC performance was stable owing to Pt NP catalysts that were buried deeply in the photoelectrode by the second etch, below a thick surface layer comprised primarily of amorphous SiO2 along with some degree of remaining crystalline Si as observed by scanning and transmission electron micrographs. Electrochemical impedance studies reveal that the second etch leads to a considerably smaller interfacial charge-transfer resistance than samples without the additional etch, suggesting that burying the Pt NPs improves the interfacial contact to the crystalline silicon surface.