EP4681249A2 - Two-electrode electrochemical system stabilization - Google Patents

Abstract

An electrochemical system includes a counter electrode and a working electrode spaced from the counter electrode. The working electrode includes a substrate, an array of conductive projections supported by the substrate and extending outwardly from the substrate, each conductive projection of the array of conductive projections having a semiconductor composition, and including a surface, the surface including nitrogen, and an oxynitride layer disposed on the surface. The counter electrode and the working electrode are arranged in a two-electrode configuration.

Description

TWO-ELECTRODE ELECTROCHEMICAL SYSTEM STABILIZATION

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of U.S. provisional application entitled “Two- Electrode Electrochemical System Stabilization,” filed March 17, 2023, and assigned Serial No. 63/452,907, the entire disclosure of which is hereby expressly incorporated by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] This invention was made with government support under Contract No. DE- EE0008086 awarded by the Department of Energy and under Contract No.

W911 NF2110337 awarded by the Army Research Office. The government has certain rights in the invention.

BACKGROUND OF THE DISCLOSURE

Field of the Disclosure

[0003] The disclosure relates generally to stabilization and activation of device surfaces during water splitting and other reactions, as well as during other types of device operation.

Brief Description of Related Technology

[0004] Photoelectrochemical (PEC) water splitting mimics plants to generate sustainable clean fuels using two most abundant resources on the earth - sunlight and water. One prevailing scheme to artificially attain this solar to chemical energy conversion is to monolithically integrate a semiconductor material with electrocatalysts in a photoelectrode. In this approach, the semiconductor efficiently harvests the solar energy, while the electrocatalyst, which sometimes also acts as a protection layer, lowers the overpotential, mediates the charge carrier transfer, and provides active sites for the chemical reaction at solid / liquid interface.

[0005] The photoelectrochemical (PEC) pathway for the generation of clean chemicals and fuels, e.g., H₂ production from solar water splitting and liquid fuel generation from CO₂ reduction, has attracted tremendous attention over the past decades. Although significant progress had been made in improving the solar-to-hydrogen (STH) efficiency of PEC devices, the stability of these devices has remained a daunting challenge, preventing any practical, large scale deployment of this promising technology.

[0006] Unfortunately, efficient photo-absorbers are not durable (e.g., Si, lll-V), whereas durable materials show poor efficiency (e.g., TiO2, SrTiOs). For instance, both Si and lll-V compound semiconductors suffer from poor stability due to chemical and photochemical corrosion. Compared to photovoltaic electrolyser (PV-EL) devices, the light absorber of PEC devices is often in direct contact with electrolyte, leading to more rapid degradation. The corrosion of semiconductors is influenced by many factors, including intensity of light illumination, biasing conditions, catalyst, surface passivation, semiconductor electronic band structure, electrolyte composition, and the interfaces of semiconductor/electrolyte as well as catalyst/electrolyte. These factors can be potentially addressed by exploring thermodynamic and kinetic protection schemes.

[0007] Recent efforts have been directed to providing a stable coating to yield stable photoelectrodes. These coatings are usually a combination of a charge transport layer combined with a catalyst, thus promoting efficient charge collection and separation and preventing corrosion. Different oxides, nitrides, selenides, and sulfides materials have been explored for protection of Si-based photocathodes in the presence of catalysts for the hydrogen evolution reaction (HER). Among these, gallium nitride (GaN) is an interesting option due to the chemical stable nature of N-rich surfaces. GaN is suitable as an efficient protective layer for silicon photocathodes, due to the almost ideal band alignment of the two materials for providing optimal electron transport.

[0008] Other protection schemes have been developed to enhance the stability of photoelectrodes. Kinetic protection for a given photoelectrode is possible by using a synergetic combination of a stable surface protection layer and a highly active co-catalyst. To further improve device stability, other photoelectrodes have employed relatively thick metal oxides, such as TO2, AI2O3, and IrOx, as passivation layers, in addition to the use of suitable co-catalysts. Although the stability of these devices has improved, one major issue is the loss of photocurrent, due to poor charge transfer and, in some cases, undesired light absorption by the protection layers. SUMMARY OF THE DISCLOSURE

[0009] In accordance with one aspect of the disclosure, an electrochemical system includes a counter electrode and a working electrode spaced from the counter electrode, the working electrode including a substrate, an array of conductive projections supported by the substrate and extending outwardly from the substrate, each conductive projection of the array of conductive projections having a semiconductor composition, and including a surface, the surface including nitrogen; and an oxynitride layer disposed on the surface. The counter electrode and the working electrode are arranged in a two-electrode configuration.

[0010] In accordance with another aspect of the disclosure, a photoelectrode includes a substrate, an array of conductive projections supported by the substrate and extending outwardly from the substrate, each conductive projection of the array of conductive projections having a semiconductor composition, and including a surface, the surface including nitrogen, and an oxynitride layer disposed on the surface. The surface is not passivated.

[0011] In accordance with yet another aspect of the disclosure, a method of fabricating a photoelectrode includes providing a substrate of the photoelectrode, forming an array of conductive projections supported by the substrate and extending outwardly from the substrate, each conductive projection of the array of conductive projections having a semiconductor composition, and including a surface, the surface including nitrogen, and implementing a photoelectrochemical procedure to form an oxynitride layer on the surface. Implementing the photoelectrochemical procedure includes irradiating the photoelectrode with radiation having an intensity that corresponds with concentrated solar light.

[0012] In connection with any one of the aforementioned aspects, the electrodes, systems, and/or methods described herein may alternatively or additionally include or involve any combination of one or more of the following aspects or features. The oxynitride layer includes localized oxynitride nanoclusters. The oxynitride layer is configured such that the surface exhibits localized metallization. The working electrode lacks a co-catalyst in addition to the array of conductive projections. The working electrode lacks a passivation layer along the surface. The working electrode is configured such that an entirety of the surface is exposed to an electrolyte during operation. The substrate includes silicon and the semiconductor composition of the structure includes gallium nitride such that the oxynitride material is gallium oxynitride. The surface is oriented along a non-polar plane of the semiconductor composition. The semiconductor composition has a Wurtzite crystal structure. The surface includes a sidewall. The oxynitride layer is disposed along the sidewall. The sidewall is nitrogen-terminated. The oxynitride layer includes localized oxynitride nanoclusters. The oxynitride layer is configured such that the surface exhibits localized metallization. The surface lacks a co-catalyst. The surface includes a sidewall. The stabilizing layer is disposed along the sidewall. The sidewall is nitrogen-terminated. The substrate includes silicon. The semiconductor composition of the structure includes gallium nitride such that the oxynitride material is gallium oxynitride. The intensity of the radiation corresponds with at least about nine-sun solar light. The photoelectrochemical procedure is implemented for a period of time on the order of hours. The photoelectrochemical procedure is implemented for a period of time on the order of minutes. Implementing the photoelectrochemical procedure includes implementing a water splitting reaction in which the device is immersed in water. Implementing the photoelectrochemical procedure includes continuously circulating an electrolyte. Forming the array of conductive projections includes implementing a molecular beam epitaxy (MBE) growth procedure such that each conductive projection of the array of conductive projections includes a respective nanowire. The MBE growth procedure is implemented under nitrogen-rich conditions such that sidewalls of each conductive projection of the array of conductive projections are nitrogen-terminated. The semiconductor composition is configured such that implementing the photoelectrochemical procedure results in partial oxygen substitution of the nitrogen.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

[0013] For a more complete understanding of the disclosure, reference should be made to the following detailed description and accompanying drawing figures, in which like reference numerals identify like elements in the figures.

[0014] Figure 1 depicts operation of example photocathodes in a three-electrode configuration with (a) a schematic illustration of photoelectrochemical H₂ evolution reaction in a three-electrode configuration, in which a GaN nanowire/Si photocathode, iridium oxide (IrOx), and Ag/AgCI were used as the working, counter, and reference electrodes, respectively, (b) linear sweep voltammetry (LSV) curves of the GaN nanowire/Si photocathode after reaction for 0 - 100 h and a Pt-loaded GaN nanowire/Si photocathode, (c) LSV curves of the GaN/Si photocathode measured under 9 sun (900 mW/cm²) solar light in a flow cell, and (d) a graphical plot of onset potential vs. reaction time under 1 (black curve) and 9 sun (red curve) illumination.

[0015] Figure 2 depicts graphical plots of the operation of example photocathodes in a two- electrode configuration, including (a) an initial 10 hours of a chronoamperometry (CA) stability test of the photocathode at -2.3 V vs. lrO_x under AM 1 ,5G one-sun illumination in 0.5 M H2SO4 with 0.2 mM Triton X-100, (b) Faradaic efficiency measurements of the first 10 hours of the CA, in which the red (light grey) dots represent the average amount of H₂ measured at different times via gas chromatography, and the black dotted line is the amount of H₂ calculated from the photocurrent over time, (c) LSV curves of the photocathode before (red or light grey curve) and after 10 hours (blue or dark grey curve) of CA under AM 1 ,5G one-sun illumination and dark (black curve) conditions, and (d) Nyquist plots of the photocathode before (red or light grey curve) and after 10 h (blue or dark grey curve) of CA under AM 1 .5 G one-sun illumination.

[0016] Figure 3 depicts graphical plots of XPS O 1s spectra of the n⁺-GaN nanowires/Si photocathode as-grown and tested under conditions described in connection with Figure 2, taken with the incident angle 6 = 60° (the definition of the angle 6 is schematically shown in part (d)), in which the as-grown photoelectrode showed (a) deconvoluted peaks at 531 .3 eV and 532.6 eV for O-Ga (red or light grey curve denoted with an arrow) and O-H (blue or dark gray curve), respectively, and after 10 hours of chronoamperometry (CA), (b) an additional deconvoluted peak at 531 .6 eV corresponding to gallium oxynitride species (violet curve denoted with an arrow) emerged (the grey curves (a,b) are the fitted curves of their respective raw data), (c) valence band maximum measurements of the photocathode before (red or light grey dots) and after 10 hours of CA (blue or dark grey dots), where the quantity EFS - Ey_S increased by AE_S of about 0.5 eV (the intersection between the background intensity flatline (at binding energy less than 2 eV) and the linear fit of the onset of the photoelectron signal intensity is the position of the surface Fermi level (EFS) relative to the surface valence band maximum (Evs)).

[0017] Figure 4 depicts graphical plots of two-electrode ultralong stability experiments and structural characterization of the n⁺-GaN nanowires/Si photocathode, including (a) a plot of chronoamperometry (CA) showing 3000 hours of stability for an example photocathode having n⁺-GaN nanowires/Si at -2.3 V vs. lrO_x under AM 1.5G one-sun illumination in 0.5 M H₂SO4 with 0.2 mM Triton X-100 (the material stability is further highlighted by the stability of the onset potential over the course of the CA), (b) linear scan voltammograms of the photocathode at 0 h (red curve denoted with an arrow), after 10 h (blue curve denoted with an arrow), after 1000 h (purple curve denoted with an arrow), and after 3000 h (cyan curve denoted with an arrow) under AM 1 ,5G one-sun illumination and dark (black curve) conditions, and (c) Faradaic efficiency measurements of the last 10 hours of the 3000 hour CA (the red or light grey dots represent the average amount of H₂ measured at different times via gas chromatography, and the black dotted line is the amount of H₂ calculated from the photocurrent over time).

[0018] Figure 5 depicts calculated electronic structures of metal oxynitride species featured on GaN m-plane, including (a) top (upper panel) and side (lower panel) views of example configurations of GaN m-plane with surface metal oxynitride nanoclusters, as well as graphical plots of (b) the density of states (DOS) of metal oxynitride species featured GaN m-plane (the contributions from the surface and bulk atoms are marked with pink and green curves, respectively), and (c) orbital-projected DOS of Ga-oxynitride, as well as a view of (d) the spatial structure of charge densities of Ga-oxynitride in the energy range of -1 .39 eV to about 0 eV as marked with purple (or light grey) dashed rectangles in parts (b) and (c), in which the top and bottom panels are respectively the top and side views of the spatial structure and the value of the isosurfaces is 0.001 eA^-3, and a graphical plot of (e) a band diagram of GaN with Ga-oxynitride induced metallic states.

[0019] Figure 6 is a schematic view and block diagram of a two-electrode electrochemical system having a photocathode with a stabilizing layer for stable activation of hydrogen evolution via water splitting in accordance with one example.

[0020] Figure 7 is a flow diagram of a method of fabricating a photocathode having a stabilizing layer in accordance with one example.

[0021] The embodiments of the disclosed devices, systems, and methods may assume various forms. Specific embodiments are illustrated in the drawing and hereafter described with the understanding that the disclosure is intended to be illustrative. The disclosure is not intended to limit the invention to the specific embodiments described and illustrated herein.

DETAILED DESCRIPTION OF THE DISCLOSURE

[0022] As a diagnostic tool, systems having a three-electrode configuration have been employed to study the stability of semiconductor photoelectrodes. The incorporation of protection schemes has significantly improved the stability of various materials in three- electrode PEC testing configurations. Three-electrode PEC testing, however, does not consider the performance of the counter electrode. Nor does three-electrode PEC testing describe the overall PEC system stability. Moreover, because the protection layers are not catalytically active for the hydrogen evolution reaction (HER), additional extrinsic catalysts have been used on the photoelectrodes. [0023] Previously reported high-efficiency photoelectrodes that have demonstrated reasonable stability in a three-electrode configuration exhibit rather poor stability performance when measured in a practical system configuration, i.e., a two-electrode configuration, due to factors such as the charge build-up within the cell, solution resistance, and the inherent chemical instability of the photoelectrode. The three-electrode measurement may thus be neither sufficient for, nor relevant to, the overall stability of the eventual commercial deployment of PEC water splitting systems. In contrast, testing under the two-electrode configuration gives the actual efficiency and durability of the entire PEC cell. To date, however, there has been no demonstration of semiconductor photoelectrodes, or suitable protection schemes, that can enable long-term stable and efficient operation in a practical two-electrode configuration.

[0024] Previous studies have been largely focused on metal-oxide, Si, and lll-V based semiconductor photoelectrodes. Recently, a new class of semiconductor photoelectrodes, including metal-nitride nanostructures, has drawn considerable attention. Metal-nitrides, especially Ill-nitrides, e.g., InGaN, have a tunable energy bandgap across the entire solar spectrum. Moreover, GaN and Si, the two most produced semiconductors in the world, can be seamlessly integrated to achieve high efficiency solar water splitting with proven manufacturability, scalability, and relatively low cost. Zeng et al. showed that GaN possesses a unique self-improving property, i.e., the PEC performance showed an improvement, instead of degradation, over the course of three-electrode chronoamperometry (CA). This quite unusual behavior was attributed to the formation of oxynitride on the nonpolar and semi-polar GaN surfaces during PEC reactions. At the same time, the measurements were performed in a three-electrode configuration where the morphology of GaN was quasi-film, for which a relatively small fraction of the surfaces in contact with the electrolyte were active nonpolar surfaces. It has remained unknown whether such unique self-improving behavior can be maintained or even enhanced and long-term stability can be achieved for GaN morphologies in which the active nonpolar surfaces dominate under practical two-electrode conditions. Moreover, the underlying mechanism for the oxynitride formation, its atomic origin and catalytic properties, and its dependence on surface polarity and configuration have remained unknown.

[0025] Described below are examples of electrochemical systems having a two-electrode configuration with a photocathode including a GaN nanowire array grown on Si photocathode in a manner that achieves self-improving behavior and long-term stability. The GaN nanowire/Si photocathode has nonpolar sidewalls that predominate as described and shown herein. The example photocathodes exhibit photoelectrochemical characteristics that are dramatically improved compared to the GaN film on Si photocathode studied before. Experiments in three-electrode configurations confirm that the GaN/electrolyte interface is useful for the self-improvement effect, the speed of which scales super-linearly with increases in photocurrent density achieved via concentrated sunlight illumination. X-ray photoelectron spectroscopy (XPS) measurements further confirmed that, during the initial hours of the stability testing, there is an in-situ formation of a new gallium oxynitride species on the nonpolar m-plane of GaN nanowires, which leads to improved J- V characteristics, including a higher photocurrent density and more positive onset potential.

[0026] The disclosed examples also demonstrated stable operation for 3000 hours without any performance degradation in two-electrode configurations, exceeding the previously best reported stability of 300 hours in a two-electrode configuration by an order of magnitude. Significantly, the measurements were performed without the incorporation of any metal catalyst protection, revealing the intrinsic stability of GaN/Si photoelectrodes. First principles density functional theory (DFT) analysis further revealed the formation mechanism, atomic origin, electronic structure, and catalytic properties of the unique GaON species. The in-situ formation of atomic scale GaON nanoclusters on N-terminated GaN nanowires takes place when O atoms partially replace the N atoms on the non-polar GaN m-plane. The incorporation of O atoms on GaN not only reduced surface band bending, but also created atomic-scale localized nanoclusters of semiconductor surface metallization, i.e., GaON species, which naturally act as reduction reaction sites. In these and other ways, the disclosed examples overcome the stability bottleneck of semiconductor photoelectrodes, thereby supporting a wide variety of applications of photoelectrochemical devices and systems for clean energy.

[0027] Although described in connection with nanowire arrays, the surface stabilization and activation schemes described herein may be applied to a variety of different device structures. Various planar catalytic surfaces may be modified to include nitrogen. For instance, a nitrogen layer (or other nitrogen-based layer) may be deposited or otherwise applied to a variety of different electrode surfaces.

[0028] Photoelectrochemical, photocatalytic, and other water splitting provided by the disclosed devices and systems may involve solar-to-hydrogen conversion. The disclosed devices and systems provide improvements in the efficiency of photoelectrochemical water splitting and/or other water splitting (e.g., photocatalytic water splitting). The disclosed devices and systems may include one or more other aspects or features directed to efficiency improvements. For instance, the structures may include a double junction configuration, such as in multi-band InGaN nanowire arrays configured to provide a second junction.

[0029] Although described in connection with photoelectrochemical water splitting, the disclosed devices and systems may be used in other chemical reaction contexts and applications. For instance, the disclosed devices and systems may be useful in connection with various types of photocatalytic and/or other systems, and/or in connection with other reactions, including, for instance, nitrogen reduction to ammonia, CO2 reduction to various fuels and other chemicals, and activation of C-H bonds for the production of various chemicals.

[0030] Although described herein in connection with electrodes having GaN-based nanowire arrays for water splitting, the disclosed devices and systems are not limited to GaN-based nanowire arrays. A wide variety of other types of nanostructures and other conductive projections may be used. Thus, the nature, composition, construction, configuration, characteristics, shape, and other aspects of electrodes through which the water splitting is implemented may vary.

[0031] GaN/Si photocathode: three-electrode configuration analysis. An example photocathode having n⁺-GaN nanowires/Si p-n junction was fabricated to analyze the stabilization provided by an oxynitride layer. Details regarding the fabrication of the photocathodes are provided below. In these examples, the nanowires have an average length of about 600 nm and diameter of about 100 nm. The photocathode was first evaluated in a three-electrode configuration with an iridium oxide ( I rO_x) counter electrode and a Ag/AgCI reference electrode under AM 1 ,5G 1-sun illumination at an angle perpendicular to the photocathode wafer (Figure 1 , part (a)). In this example, incident solar photons are absorbed by the Si wafer, and photo-generated electrons are extracted by n⁺- GaN nanowires to drive proton reduction. The chronoamperometry (CA) curve of the photocathode at -0.4 V vs. reversible hydrogen electrode (VRHE) shows a rapid increase in photocurrent density from 0.6 to about 35 mA/cm² over 40-50 hours. Linear sweep voltammetry (LSV) curves of the photocathode show in-situ gradual improvement of onset potential (V_on, voltage at 1 mA/cm²) and photocurrent density (Figure 1 , part b). After 100 hours, the measured V_on of 0.36 VRHE and photocurrent density of about 30 mA/cm² at 0 VRHE are among the best performances of a Si photocathode without the incorporation of extrinsic catalysts and are within the performance range of Si photocathodes with noble metal catalysts. This is a significant advancement from the previous work on quasi-film GaN/Si photocathodes due to the unique morphology and polarity of vertically aligned GaN nanowires for which the active m-plane sidewalls dominate (or predominate) and that enhance mass transport due to increased porosity of the nanowire array relative to a quasifilm.

[0032] In the Nyquist plots, the radius of the semicircle gradually decreased with the increase in reaction time, indicating a decrease in charge transfer resistance and an accelerated electron transfer from the photocathode to the protons. Faradaic efficiency of H₂ increased from 52% to greater than 95% during the initial 1 .5 hours of reaction at -0.4 VRHE and stabilized thereafter. The performance was not improved at open-circuit potential even under light illumination, demonstrating that the photocurrent plays a key role in enhancing the catalytic activity of GaN nanowire/Si photocathode.

[0033] In order to confirm experimentally whether the self-improvement originated from in- situ modification of GaN surface, a thin (2 nm) passivation layer of AI2O3 was coated on the GaN nanowires by atomic layer deposition. A PEC H₂ evolution reaction was then implemented at -0.4 VRHE under one-sun illumination. Interestingly, there was negligible improvement of photocurrent density and V_on even after 24 hours of reaction. In contrast, after Pt cocatalyst deposition, the Pt/AhOs/GaN nanowire/Si photocathode showed a LSV curve like that of Pt/GaN nanowire/Si. As such, the AI₂O3 passivation layer efficiently transfers the photogenerated electrons from the GaN nanowires to protons but effectively prevents the in-situ surface modification of the GaN nanowires. XPS and scanning transmission electron microscopy (STEM) energy dispersive X-ray spectroscopy (EDS) further confirmed that there was no Ir contamination on the GaN nanowires from the counter electrode (lrO_x) after 48 hours of chronoamperometry.

[0034] From these results, it is identified that there are two aspects to the self-improvement of the GaN nanowire/Si photocathode: (1 ) photocurrent and (2) exposure of the GaN surface to the electrolyte. With one-sun light intensity, a relatively prolonged duration (greater than 10 hours) was used to obtain high H₂ evolution activity (/.e., V_on greater than 0 VRHE) possibly because the rate of surface modification was limited by the small photocurrent. Boosting the photocurrent density by irradiating with the concentrated solar light shortens the time duration to achieve the saturation in self-improvement. Hence, a flow cell was used to provide rapid replacement of reactants and products at the electrode. LSV curves at intervals of reaction time under nine-sun solar light are provided in Figure 1 , part (c). Interestingly, V_on greater than 0.3 VRHE was achieved within 15 min of reaction. The plot of V_on versus reaction time shows that there was a super-linear correlation between the V_on improvement speed and light intensity, thereby resulting in about 100 times faster GaN surface modification under nine-sun illumination compared to one-sun illumination (Figure 1 , part d). The high photocurrent density (about 165 mA/cm² at -0.3 VRHE) obtained by the concentrated solar light not only boosted the speed of the photocathode self-improvement but also increased the H₂ production rate by about 9 times. Moreover, the GaN nanowire/Si operated stably for more than 500 hours with very high photocurrent density (greater than 150 mA/cm²) at -0.4 VRHE under the concentrated solar light. The produced total H₂ gas is similar to the amount of H₂ produced for 4,500 hours under one-sun illumination.

[0035] GaN/Si photocathode: two-electrode configuration. The photocathode was further tested in an electrochemical system having a two-electrode configuration under AM 1 ,5G one-sun illumination. Figure 2, part (a), shows the chronoamperometry of the photocathode for the first 10 hours at -2.3 V vs. lrO_x under AM 1 ,5G one-sun illumination in 0.5 M H₂SO4 with 0.2 mM Triton X-100 as a surfactant. Further details are provided herein. The rather large bias was chosen to ensure operation at the saturation current density for the majority of the ensuing 3000 hour long-term stability testing (discussed in detail below). The photocathode produced hydrogen as efficiently at lower biases as well. It is noted that, similar to the photocathode under three-electrode configurations, the photocurrent density also increased considerably over this initial 10 hours of the CA stability test. Figure 2, part (b), shows the Faradaic efficiency and H₂ evolution for the example between 0 hours and 10 hours of chronoamperometry at -2.3 V vs. lrO_x in 0.5M H₂SO4 under AM 1 ,5G one sun illumination. The Faradaic efficiency for this duration was 89 - 100%. Due to the formation of oxynitride species, Faradaic efficiency was initially lower than 100%, but it increased steadily and reached 100% by the end of 10 hours of chronoamperometry.

[0036] Similar to the results in three-electrode configurations, the LSV measurements (Figure 2, part c) under AM1 ,5G one-sun illumination at 0 h (red or light grey curve) and 10 h (blue or dark grey curve) clearly show an improvement in fill factor, a positive shift of about 0.5 V in V_on, and an increase in photocurrent density. Furthermore, electrochemical impedance spectroscopy measurements, in the form of Nyquist plots (Figure 2, part d) at 0 hours (red or light grey curve) and 10 hours (blue or dark grey curve), show a drastic reduction in charge transfer resistance by nearly two orders of magnitude, for the 10 hour tested sample compared to the measurements taken at 0 hours. These results point to the formation of new species on the surfaces of the GaN nanowires similar to that responsible for the self-improvement observed under a three-electrode configuration. [0037] The XPS measurements, shown in Figure 3, are taken on GaN nanowire/Si photocathode examples before and after 10 hours of stability testing. Parts (a) and (b) of Figure 3 show the XPS O 1s peaks taken at an incident angle of 60°, where m-plane surfaces of nanowires were predominantly measured. Apart from the O-Ga (red or light grey curve) and O-H (blue or dark grey curve) peaks, there is an additional deconvoluted O-Ga-N peak (at about 531 .6 eV) for the 10 hour tested sample (Figure 3, part b), compared to the pristine sample (Figure 3, part a). The O-Ga-N peak shows that a new oxynitride species formed along the non-planar (m-plane) surface of the GaN nanowires during the 10 hour stability operation. It is to be noted that the deconvoluted Ga-0 peak (for the pristine sample) is due to the exposure of as-grown GaN surface to ambient conditions before transferring to the XPS chamber. As shown in Figure 3, part (b), the relative O-Ga peak intensity has considerably reduced for the 10 hour example compared to the pristine device, likely due to the dissolution of oxide in acidic conditions and conversion of some oxide into oxynitride species. Similar conclusions can also be drawn from the Ga 3d spectra. For the pristine device, the two deconvoluted peaks correspond to Ga-N (cyan curve) and Ga-0 (magenta curve) at 20.2 eV and 21 eV, respectively. Like the O Is spectra, an additional deconvoluted Ga-N-0 peak at 20.8 eV between the Ga-N and Ga-0 peaks emerges for the Ga 3d spectrum of the 10 h tested example.

[0038] Figure 3, part (c), shows the valence spectra for both the pristine and the tested sample. In Figure 3, part (c), the valence spectrum measurement for the tested sample showed an increase of about 0.5 V in EFS - £vs compared to the pristine sample, where EFS is the surface Fermi level and Evs is the surface valence band maximum. This valence spectrum shift is consistent with the V_on shift, shown in Figure 2, part (c). Theoretical calculations, to be discussed below in greater detail, point to a reduction of the conduction band barrier height due to oxynitride formation that can contribute to the increase in the EFS - Evs value after 10 hours of chronoamperometry. The conduction band barrier reduction helps improve the charge transfer kinetics for H2 production, which is reflected in the LSV curves shown in Figure 1 , part (b), and Figure 2, part (c).

[0039] Long-term Stability. The chronoamperometry (CA) stability measurements were taken at a constant applied potential of -2.3 V vs. I rO_x under AM 1 ,5G one-sun illumination in 0.5 M H2SO4 with 0.2 mM Triton X-100. The Triton X-100 was included in the electrolyte as a surfactant for enhanced hydrogen desorption, resulting in a more stable photocurrent. The electrolyte solution was replaced afresh after every 20 - 24 h of CA. Figure 4, part (a), shows the photocurrent density throughout 3000 h for the GaN nanowire/Si photocathode. A summary of the measured V_on and J at -2.3 V vs. lrO_x at the end of integer multiples of 100 hours throughout the 3000 hour chronoamperometry is plotted in Figure 4, part (a). At the beginning of the stability experiment (0^th hour), the V_on was about -1 .8 V vs. I rO_x and the photocurrent density of the sample was about 8 mA/cm² at -2.3 V vs. lrO_x. The J-V characteristics (Figure 4, part b) for the photocathode after 10 hours (blue curve and denoted with arrow) of chronoamperometry clearly show a dramatic improvement over the 0^th hour J- V curve (red curve and denoted with arrow). As discussed earlier, this improvement is attributed to the formation of oxynitride species on the m-plane of GaN nanowires. Further continuation of the CA stability experiments under the same operational conditions shows that the photocurrent density reached its saturation value of about 30 mA/cm² after about 40 hours. It is to be noted that the J-l/characteristics after 1000 h (purple curve and denoted with arrow) and 3000 h (cyan curve and denoted with arrow in Figure 4, part b) show near-identical V_on of about -1 .35 V vs. lrO_x and fill factor compared to the 10 hour point (blue curve and denoted with arrow). It can be thus concluded that, once formed on GaN nanowires m-plane surfaces, the GaON species are robust against both the continuation of stability experiments and the exposure to air during the routine electrolyte change every 20 - 24 hours of CA. During the entirety of the 3000 hour chronoamperometry (see Figure 4, part a), the photocurrent density varied by ±10% of the average value (about 29 mA/cm²).

[0040] As best understood, the ultrahigh stability of about 3000 hours for Ga(O)N nanowires/Si is the greatest stability duration measured for any photoelectrode in a two- electrode configuration at any bias under AM 1 ,5G one-sun illumination in any electrolyte solution. This extraordinary stability of the GaN nanowire/Si photocathode is attributed to material properties of the GaN nanowires such as strong ionic bonds, free of dislocations, and a unique N-termination on both the c-plane and the m-plane. By comparing the J-V characteristics of the photocathode after 10 hours (blue curve and denoted with arrow in Figure 4, part b) and 3000 hours (cyan curve and denoted with arrow in Figure 4, part b) of CA, it can be concluded that the LSV curves remained almost identical 10 hours - 3000 hours due to the excellent stability of oxynitride species on the GaN nanowires m-plane. In addition, Faradaic efficiency for hydrogen evolution during the last 10 hours of the 3000 h chronoamperometry (Figure 4, part c) remained constant, which, together with the constant photocurrent, confirms the photocathode’s stability for the entirety of the 3000 hour CA. STEM measurements indicated that the nanowire dimensions remained unchanged with length of about 600 nm and diameter of about 100 nm after 3000 hours of chronoamperometry. In addition, the SEM image of the 3000 hour tested photoelectrode showed virtually no change in either GaN nanowires coverage on Si or nanowire morphology. Dissolved Ga in the electrolyte was analyzed using inductively coupled plasma mass spectroscopy (ICP-MS) at different stages of the entire CA stability experiments. A negligible amount of the dissolved Ga concentrations of 10 - 13 nmol was found for different runs. These results show that the nanowires remained stable throughout the CA operation, consistent with the structural characterizations.

[0041] Density Functional Theory (DFT) Calculations. To gain an atomic level understanding of metal oxynitride species on the GaN nanowires, density functional theory (DFT) analysis was performed on the formation of oxynitride species and their electronic and catalytic properties associated with water splitting. Four possible atomic configurations of GaON featured GaN m-plane were considered (marked with orange, green, red, and purple circles in the upper panel of Figure 5, part (a) respectively) based on previously reported N- rich GaN surfaces. Previous studies have suggested that the m-plane surfaces of GaN nanowires grown via plasma-assisted molecular beam epitaxy (PAMBE) under nitrogen rich conditions are characterized by the presence of nitrogen nanoclusters. This exotic surface feature of the GaN nanowires facilitates further incorporation of oxygen at the nanowire m- plane sidewalls. These nitrogen clusters are relatively isolated on the surface, which are spatially conducive to achieving the replacement of the N atom with an O atom to form Ga- O-N species. The formation energy for the four proposed GaON configurations in Figure 5, part (a), was calculated to be -4.07 eV, -4.29 eV, -3.95 eV, and -2.66 eV. The negative formation energy indicates that the introduction of the O atom to the N-rich GaN surface is a thermodynamically favored process, which also agrees with the consensus that metal-0 bonds are stronger than metal-N bonds. The stability of Ga-O-N species was therefore verified. The density of states of N-rich GaN m-plane was calculated before and after oxidation (Figure 5, part b). A fascinating characteristic was found that the Fermi level of GaON shifted upward into the conduction band (with a value of 1 .39 eV for the second configuration), rendering the oxygen-incorporated m-plane surface metallized. This metallic nature could be simply understood to come from an effective n-type doping where the O atom owns one more electron than the N atom it replaces. Because the valence band of GaN has been fully occupied before oxidation, this extra electron brought by the introduction of the O element could be filled only in the conduction band. Because that the second configuration (marked with the green circle in Figure 5, part a) was the most negative in formation energy and therefore the most thermodynamically favored among the four proposed configurations, further calculations were based on the second configuration. From the calculated orbital-projected density of states (OPDOS) and atom-projected density of states (APDOS), only the p orbital of O, N atoms, and p and s orbitals of Ga atoms in the surface region were found to be responsible for the metallic manifestation, which indicates a downward band bending (Figure 5, parts b-e) and thus benefits the reduction reaction. Meanwhile, the bandgap of GaN in the outermost surface of the nanowires is effectively narrowed from 3.19 eV to 1 .83 eV due to the emerging metallic states brought by Ga oxynitride species formation. For a more intuitive view of the metallic surface, we plotted the real-space-distributed charge densities of the conduction band around Fermi level (-1 .39 eV < E-E_F < 0 eV, marked by the purple dashed rectangle in Figure 5, parts (b)-(d). The metallic property directly corresponds to the surface GaON species, which naturally acts as an electron sink and serves as atomic-scale reduction reaction sites. Therefore, through first- principles calculations, it is determined that the surface configuration of the N-rich GaN m- plane featured with GaON nanoclusters largely enhances the stability and efficiency of the solar-powered artificial photosynthesis.

[0042] The unique advantages of in-situ formation of GaON nanoclusters on N-terminated GaN nanostructures relative to conventional nitridation schemes of lll-V photoelectrodes and Ga2Os powders to improve the stability of photoelectrochemical reactions is now addressed. Nitrogen-containing photocatalysts have stable and efficient operation in harsh solar water splitting conditions compared with traditional metal oxides and lll-V compounds. In addition, the incorporation of nitrogen species improves stability and is essential for efficient light absorption by narrowing the bandgap. Previous studies explored the metal-organic chemical vapor deposition (MOCVD) growth of GaPN epilayers, with 0.2% - 2% nitrogen, on GaP substrates for protecting the lll-V photoelectrodes. While nitrogen incorporation at these low levels improved the stability of the material against photocorrosion, further nitrogen incorporation in these structures created a huge lattice mismatch leading to surface defects and increased photocorrosion. To date, it has remained elusive to achieve long-term stability not only for nitridated lll-V photoelectrodes, but for high efficiency photoelectrodes in general. To enhance the stability of the photoelectrode, much attention has been given to the employment of oxide layers, via methods such as atomic layer deposition of Ti©2 and AI2O3, for protection against various corrosion pathways, such as photo-oxidation and reaction with electrolytes. The employment of such a foreign protection layer (on the photoelectrode) often introduces an undesirable tradeoff between preservation of hard-won photoelectrode efficiency and realistic enhancement of photoelectrode stability, because such oxide layers are by design chemically inactive on the surface and very often poor in electrical conductivity as well. As demonstrated by the disclosed examples, through the exemplary atomically thin, catalytically active GaON species on the sidewalls of the GaN nanowire/Si photoelectrode in two-electrode configurations, the in-situ formation of native surface catalysts provides a compelling answer to the dilemma. Previously, GaON has been fabricated by nitridation of Ga₂O3 that relies on annealing at high temperatures for prolonged durations. This fabrication route is energy-intensive and results in low yield and quality, leading to inferior photocatalyst performance and lower stability. The N-terminated GaN nanowires on Si disclosed herein have unique advantages of N-rich m-plane sidewalls, strong ionic bonds, nearly perfect band alignment and defect-free single-crystal wurtzite structure. These GaON nanocluster species act as catalysts to improve the charge carrier kinetics and operate efficiently for thousands of hours without the need for additional catalyst regeneration. As such, GaON nanoclusters on N-terminated GaN nanowires are an excellent platform for providing ultrahigh stability and efficient surface charge transfer kinetics under two-electrode PEC conditions.

[0043] Ultra-stable in-situ self-improvement of a GaN nanowire/Si photocathode for PEC H₂ evolution reaction has been demonstrated. Morphology and light intensity are factors that may be used for enhancing or expediting the self-improvement effect. The physical and chemical nature of the GaN nanowires establishes a great advancement compared to previous Si photocathodes. The example GaN nanowires/Si photocathodes, without any foreign co-catalysts, achieved unprecedentedly ultrahigh long-term stability of 3000 hours in two-electrode conditions under AM 1 ,5G one-sun illumination with photocurrent densities greater than 25 mA/cm² and a Faradaic efficiency of about 100%. During the stability testing, the example photocathode exhibited a self-improvement mechanism in the formation of new oxynitride species on the m-plane of GaN nanowires. Through DFT calculations, it was found that the formation of Ga-O-N species on the N-terminated GaN m-plane provided natural atomic-scale reduction reaction sites because the emerging oxynitride species exhibit metallic properties. Even better, these localized metallic surface states cause downward band bending, which further facilities the reduction reactions.

[0044] Figure 6 depicts a system 100 for hydrogen evolution via water splitting. The system 100 may also be configured for other reactions. The system 100 may be configured as an electrochemical system having a two-electrode configuration. In this example, the electrochemical system 100 is a photoelectrochemical (PEC) system in which solar and/or other radiation is used to facilitate the hydrogen evolution and water splitting. The manner in which the PEC system 100 is illuminated may vary. The wavelength and other characteristics of the radiation may vary accordingly. [0045] The electrochemical system 100 includes one or more electrochemical cells 102. A single electrochemical cell 102 is shown for ease in illustration and description. The electrochemical cell 102 and other components of the electrochemical system 100 are depicted schematically in Figure 1 also for ease in illustration. The cell 102 contains an electrolyte solution 104. In some cases, a CO2 and/or other source is applied. In some cases, the electrolyte solution is saturated with CO2. Potassium bicarbonate KHCO3 may be used as an electrolyte. Additional or alternative electrolytes may be used, as described below. Further details regarding examples of the electrochemical system 100 are provided below.

[0046] In the example of Figure 6, the electrochemical cell 102 has a two-electrode configuration. The electrochemical cell 102 includes a working electrode 108 and a counter electrode 110, each of which is immersed in the electrolyte 104. The counter electrode 1 10 may be composed of, or otherwise include, I rO_x. The configuration of the counter electrodes 1 10 may vary. For example, the counter electrode 1 10 may be configured as, or otherwise include, a photoanode at which water oxidation (2H₂O <=> O₂ + 4e + 4H⁺) occurs. The configuration of the electrochemical cell may vary.

[0047] The hydrogen evolution occurs at the working electrode 108 as follows: Hydrogen evolution: 2H⁺ + 2e^_ H2

To that end, electrons may flow from the counter electrode 1 10 through a circuit path external to the electrochemical cell 102 to reach the working electrode 108. The working and counter electrodes 108, 1 10 may thus be considered a cathode and an anode, respectively.

[0048] In the example of Figure 1 , the working and counter electrodes are separated from one another by a membrane 114, e.g., a proton-exchange membrane. The construction, composition, configuration and other characteristics of the membrane 114 may vary.

[0049] In some cases, no bias voltage is applied - e.g., in unassisted systems. In the example of Figure 6, the circuit path includes a voltage source 116 of the electrochemical system 100. The voltage source 1 16 is configured to apply a bias voltage between the working and counter electrodes 108, 110. The bias voltage may be used to establish a ratio of CO2 reduction to hydrogen (H₂) evolution at the working electrode, and/or another reaction ratio(s). The circuit path may include additional or alternative components. For example, the circuit path may include a potentiometer in some cases. [0050] In this example, the working electrode 108 is configured as a photocathode. Light 118, such as solar radiation, may be incident upon the working electrode 108 as shown. The electrochemical cell 102 may thus be considered and configured as a photoelectrochemical cell. In such cases, illumination of the working electrode 108 may cause charge carriers to be generated in the working electrode 108. Electrons that reach the surface of the working electrode 108 may then be used in the hydrogen evolution. The photogenerated electrons may augment electrons provided via the current path.

Alternatively or additionally, the electrons provided via the current path may recombine with the photogenerated holes at a backside or other contact. Further details regarding examples of photocathodes are provided below.

[0051] The working electrode 108 includes a substrate 120. The substrate 120 of the working electrode 108 may constitute a part of an architecture, a scaffolding, or other support structure, of the working electrode 108. The substrate 120 may be uniform or composite. For example, the substrate 120 may include any number of layers or other components. The substrate 120 thus may or may not be monolithic. The shape of the substrate 120 may also vary. For instance, the substrate 120 may or may not be planar or flat.

[0052] In the example of Figure 1 , the substrate 120 is doped and otherwise configured to present a junction. The substrate 120 of the working electrode 108 may thus be active (functional) in connection with the photogeneration of charge carriers. Alternatively or additionally, the substrate 120 is passive (e.g., structural). The substrate 120 may be configured and act as a support structure for a catalyst arrangement of the working electrode 108, as described below. Alternatively or additionally, the substrate 120 may be composed of, or otherwise include, a material suitable for the growth or other deposition of the catalyst arrangement of the working electrode 108.

[0053] In active or functional cases, the substrate 120 may include a light absorbing material. The light absorbing material is configured to generate charge carriers upon solar or other illumination. The light absorbing material has a bandgap such that incident light generates charge carriers (electron-hole pairs) within the substrate. Some or all of the substrate 120 may be configured for photogeneration of electron-hole pairs. To that end, the substrate 120 may include a semiconductor material. In some cases, the substrate 120 is composed of, or otherwise includes, silicon. For instance, the substrate 120 may be provided as a silicon wafer. [0054] The silicon may be doped. In the example of Figure 1 , the substrate 120 includes a heavily n-type doped layer 122, a moderately or lightly p-type doped layer 123, and a heavily p-type doped layer 124. The arrangement of the layers 122-124 establishes a junction within the substrate 120. The doping arrangement may vary. For example, one or more components of the substrate 120 may be non-doped (intrinsic), or effectively non-doped. The substrate 120 may include alternative or additional layers, including, for instance, support or other structural layers. In other cases, the substrate 120 is not light absorbing.

[0055] The substrate 120 of the working electrode 108 establishes a surface at which a number of structures are provided. Each structure may be disposed on, or otherwise supported by, the substrate 120. In this example, the structures are configured to provide a catalyst arrangement. In this case, the structures are configured to provide catalytic functionality. As described herein, the structures may be free of co-catalysts, such as catalyst nanoparticles. The structures may include an array of conductive projections extending outward from a surface of the substrate 120.

[0056] In the example of Figure 1 , the working electrode 100 includes an array of nanostructures 126 (or other conductive projections) supported by the substrate 120. Each nanostructure 126 is configured to extract the charge carriers (e.g., electrons) from the substrate 120. The extraction brings the electrons to external sites along the nanostructures 126 for use in the hydrogen evolution. In some cases, each nanostructure 126 is configured as a nanowire.

[0057] Each nanostructure 126 and/or other structure supported by the substrate 120 has a semiconductor composition. In some cases, the semiconductor composition is or otherwise includes a semiconductor core 128. For instance, the core 128 may be composed of, or otherwise include, a Group lll-V nitride semiconductor material, such as gallium nitride (GaN). Additional or alternative semiconductor materials may be used, including, for instance, indium gallium nitride (InGaN) and/or other Group lll-V nitride semiconductor materials.

[0058] The core 128 of each nanowire or other nanostructure 126 may be or include a columnar, post-shaped, or other elongated structure that extends outward (e.g., upward) from the plane of the substrate 120. The semiconductor nanowires or other nanostructures 126 may be grown or formed as described in U.S. Patent No. 8,563,395, the entire disclosure of which is hereby incorporated by reference. The nanostructures 126 may be referred to herein as nanowires with the understanding that the dimensions, size, shape, composition, and other characteristics of the nanostructures 126 or other conductive projections may vary.

[0059] The semiconductor composition of each nanostructure 126 may or may not be configured to facilitate the reaction(s) supported by the electrochemical system 100. In some cases, the semiconductor composition may be configured for photo-generation of charge carriers. Alternatively or additionally, the semiconductor composition may be configured to act as a catalyst for the reaction(s). The semiconductor composition may provide other functions, including, for instance, protection of the substrate 120. As mentioned above, the semiconductor composition may include GaN and/or InGaN. Further details regarding a number of examples involving GaN are provided below. Additional or alternative semiconductor materials may be used, including, for instance, indium nitride, aluminum nitride, boron nitride, aluminum oxide, silicon, and/or their alloys.

[0060] The semiconductor composition of each nanostructure 126 may be configured to provide or support surface passivation and/or other protection of the photoelectrode 108. As described herein, nitrogen is present at one or more surfaces 130 of each nanostructure 126. The surface 130 may thus be considered to include nitrogen. For instance, in some cases, the semiconductor composition is terminated with nitrogen. The nitrogen termination or other nitrogen-based aspect of the nanostructures 126 may protect the nanostructures 126 and/or other components of the electrode 108 (e.g., the substrate 120) during operation from, e.g., corrosion. Alternative or additional nitrogen-based protection schemes may be used in other cases. For instance, a layer including nitrogen may be deposited or otherwise disposed along the surface 130 of each nanostructure 126 and/or other element of the electrode 108. As described herein, the nitrogen may be present at the surface in various ways. In some cases, the nitrogen may be disposed below the surface (e.g., about 1 nm below), but sufficiently near the surface to be functionally active (e.g., chemically active for formation of an oxynitride layer). Accordingly, the terms "at", "present" and "presence" are used herein to include various instances in which the nitrogen is functionally present at the surface, thereby including situations in which nitrogen atoms are disposed along or sufficiently near the surface to be functional for activity at the surface (e.g., chemically functional in connection with an oxidation reaction at the surface), rather than being limited to instances in which the nitrogen disposed is disposed only on the surface.

[0061] The nanostructures 126 may facilitate the hydrogen evolution and/or another chemical reaction in one or more ways. For instance, each nanostructure 126 may be configured to extract the charge carriers (e.g., electrons) generated in the substrate 120. The extraction brings the electrons to external sites along the surface 130 of the nanostructures 126 for use in the hydrogen evolution and/or other chemical reaction. The composition of the nanostructures 122 may also form an interface well-suited for hydrogen evolution and/or another chemical reaction, as explained below.

[0062] Each nanostructure 126 may be or include a columnar, post-shaped, or other elongated structure that extends outward (e.g., upward) from the plane of the substrate 120. The dimensions, size, shape, composition, and other characteristics of the nanostructures 126 may vary. For instance, each nanostructure 126 may or may not be elongated like a nanowire. Thus, other types of nanostructures from the substrate 120, such as various shaped nanocrystals, may be used.

[0063] In some cases, the nanostructures 126 are configured to generate electron-hole pairs upon illumination. For instance, the nanostructures 122 may be configured to absorb light at frequencies different than other light absorbing components of the electrode 108. For example, one light absorbing component, such as the substrate 120, may be configured for absorption in the visible or infrared wavelength ranges, while another component may be configured to absorb light at ultraviolet wavelengths. In other cases, the nanostructures 126 are the only light absorbing component of the electrode 108. In still other cases, the substrate 120 is the only light absorbing component of the electrode 108.

[0064] Each nanostructure 126 may include a layered or segmented arrangement of semiconductor materials. For instance, in Group Ill-nitride examples, the layers or segments of the arrangement may have differing Group III (e.g., indium and gallium) compositions. One or more layers or segments in the arrangement may be configured for absorption of a respective range of wavelengths. Other layers or segments may be directed to establishing a tunnel junction. Each nanostructure 126 may also include additional or alternative segments.

[0065] In other cases, the layered arrangement of semiconductor materials is also used to establish a multi-band structure, such as a quadruple band structure. Each layer or segment of the arrangement may have a different semiconductor composition to establish a different bandgap. The different bandgaps may be useful in connection with absorbing light of differing wavelengths. Further details regarding the formation and configuration of multiband structures, including, for instance, triple-band structures, are provided in U.S. Patent No. 9,112,085 ("High efficiency broadband semiconductor nanowire devices") and U.S. Patent No. 9,240,516 ("High efficiency broadband semiconductor nanowire devices"), the entire disclosures of which are incorporated by reference. Other layered or non-layered arrangements may be used in the semiconductor composition of the structure(s) of the disclosed devices.

[0066] The semiconductor composition of each nanowire 126 may be configured to improve the efficiency of the water splitting in additional ways. For instance, in some cases, the semiconductor composition of each nanowire 126 may include doping to promote charge carrier separation and extraction, as well as facilitate the establishment of a photochemical diode. For example, a dopant concentration of the semiconductor composition may vary laterally.

[0067] In examples involving Ill-nitride compositions, the dopant may be or include magnesium. Further details regarding the manner in which magnesium doping promotes charge carrier separation and extraction are set forth in U.S. Patent No. 10,576,447 ("Methods and systems relating to photochemical water splitting "), the entire disclosure of which is incorporated by reference. Additional or alternative dopant materials may be used, including, for instance, silicon, carbon, and beryllium, depending on the semiconductor light absorber of choice.

[0068] In other cases, the working electrode 108 may further include catalyst nanoparticles disposed over the array of nanostructures 126. For instance, the nanoparticles may be distributed across or along the outer surface 130 (e.g., sidewalls) of each nanowire 126. In some cases, the nanoparticles are configured to facilitate or promote the proton reduction reaction. For instance, each nanoparticle may be composed of, or otherwise include, a metal, such as platinum. Other metals or materials may be used, including alloys, oxides, and/or other metal or metallic combinations. Further details regarding the formation, configuration, functionality, and other characteristics of nanoparticles in conjunction with a nanowire array are set forth in one or more of the above-referenced U.S. patents. Further details regarding the distribution of the nanoparticles are provided below in connection with a number of examples.

[0069] The electrode 108 also includes a stabilizing layer 132 disposed on the surface 130 of each nanostructure 126. In some cases, the stabilizing layer 132 is configured to provide both stabilization and activation functionality for the electrode 108. The stabilizing layer 132 may thus be configured as an activation layer or a stabilizing activation layer. The term "activation" is used to differentiate the activation layer from layers configured to act solely as a passivation or other protective layer, as well as other non-active layers. In this electrode example, the stabilization functionality may involve protection against corrosion and/or other passivation or protection. The activation functionality may involve defining or otherwise establishing catalytic sites along the surface 130 for the electrode 108. The nature of the stabilization and activation functionality may vary in other device contexts.

[0070] The stabilizing layer 132 may be composed of, or otherwise, include an oxynitride material. The stabilizing layer 132 may accordingly be referred to herein as an oxynitride layer.

[0071] In some cases, the stabilizing layer 132 has a uniform distribution of an oxynitride material. For instance, in cases in which the nanostructures 126 have a GaN core, the stabilizing layer 132 includes gallium oxynitride. The composition of the oxynitride layer may thus vary in accordance with the semiconductor composition of the nanostructure 126. Alternative or additional oxynitride materials may thus be used, including, for instance, aluminum oxynitride, indium oxynitride, strontium gallium oxynitride, zinc oxynitride, titanium oxynitride, chromium oxynitride, and silicon oxynitride.

[0072] The distribution of the oxynitride material may be uniform in various ways. For instance, the uniformity may establish, or otherwise relate to, a thickness of the stabilizing layer 132. Alternatively or additionally, the uniformity may establish, or otherwise relate to, a conformal nature of the stabilizing layer 132. Alternatively or additionally, the uniformity may relate to the consistency, regularity or smoothness of the surface of the stabilizing layer 132. For instance, the roughness of the surface may be decreased as a result of the uniform distribution of the oxynitride material. In some cases, the oxynitride material of the stabilizing layer 132 may be continuously distributed across the surface 130 (e.g., continuously cover the entirety of the surface 130).

[0073] Alternatively or additionally, the uniformity may relate to the presence of nitrogen atoms at the surface 130. For example, if the nitrogen atoms are randomly distributed on or otherwise at the surface, then the uniformity refers to the essentially universal or comprehensive (as opposed to local or restricted) formation of oxynitride sites around those nitrogen atoms. The phrase "uniform distribution" is thus used herein to include both continuous and discontinuous instances of the stabilizing layer 132. For instance, in some cases, adjacent nitrogen atoms may be spaced sufficiently apart so as to establish a discontinuity in the stabilizing layer 132. Thus, the stabilizing layer 132 (and accordingly the oxynitride material) may not be continuous at the surface 130 in some cases, but nonetheless still has a uniform distribution of oxynitride material in relation to the nitrogen sites at the surface 130. Alternatively or additionally, the surfaces 130 (or sections of a single surface 130) of the nanostructures 126 on which the stabilizing layer 132 is disposed may be spaced apart from one another, thereby establishing discrete sections of the stabilizing layer 132. As shown in these examples, the stabilizing layer 132 may be continuous or discontinuous, notwithstanding the uniform nature of the distribution of the oxynitride material.

[0074] The terms "presence" and "present at" are not limited to situations in which the nitrogen atoms, stabilizing layer, and/or oxynitride material are disposed on the surface. The terms are instead used to include situations in which the nitrogen atoms, the stabilizing layer, and/or the oxynitride material are below the surface, but still sufficiently near the surface to be functional for activity at the surface. The nitrogen atoms, the stabilizing layer, and/or the oxynitride material may thus be disposed slightly below the surface (e.g., about 1 nm below) in some cases, yet still sufficiently near the surface for purposes of formation of the oxynitride material and consequent stabilization of the surface.

[0075] The formation and/or presence of the oxynitride material may be derived from the nitrogen at the surface 130. In some cases, the nitrogen of the surface 130 is disposed in a compound semiconductor arrangement of the semiconductor composition. For instance, in GaN and other examples, the semiconductor composition has a Wurtzite crystal structure. In some cases, a non-polar plane of the Wurtzite crystal structure may be nitrogen- terminated. The stabilizing layer 132 may be provided or otherwise configured with partial oxygen substitution of the nitrogen along the non-polar plane (and/or other planes in other cases). The surface(s) 130 may thus correspond with those surfaces of the nanostructures 126 that is/are oriented along a non-polar plane(s) of the semiconductor composition. In other cases (e.g., in transistor examples), the surface(s) 130 is/are oriented along a polar plane(s) of the semiconductor composition.

[0076] The semiconductor composition and/or the crystal orientation at the surface 130 of the nanostructure 126 may selectively determine where the stabilizing layer 132 is disposed on each nanostructure 126. Thus, the stabilizing layer 132 may or may not cover all of the surfaces of the nanostructure 126. In the example of Figure 1 , the surface 130 of each nanostructure 126 corresponds with sidewalls 134. The nanostructure 126 also includes a top or upper face 136. In this example, the stabilizing layer 132 is disposed on the sidewalls 134, but not on the upper or top face 136. The sidewalls 134 are nitrogen-terminated, but the top face 136 is not, thereby leading to the selective or partial covering of the nanostructure 126. The surface 130 covered by the stabilizing layer 132 may vary in accordance with the crystal orientation of the semiconductor composition of the nanostructure 126. [0077] The stabilizing layer 132 may be sufficiently thin to avoid detrimental effects on charge transport or other activity along or otherwise at the surface 130. In some cases, the stabilizing layer 132 has a thickness of about 1 nm. In these and other cases, the stabilizing layer 132 may conformally cover the nanostructure 126, including any nanoparticles disposed thereon. The thickness may vary in other examples, including, for instance, thicknesses on the order of 1 nm (e.g., up to about 3 nm) or less than 1 nm. For example, the thickness may be one monolayer or on the order of one monolayer (e.g., a pair or a few monolayers). The thickness may thus vary from about 0.3 nm to about 3 nm. The thickness may not be exactly the same across the entirety of the stabilizing layer 132.

[0078] The thin nature of the stabilizing layer 132 allows the stabilization function to be provided without adversely affecting the transfer of charge carriers and other catalysis of the hydrogen evolution and/or other reaction occurring at the electrode 108. Further details regarding the functionality and other characteristics of the stabilizing layer 132 are provided below in connection with a number of examples.

[0079] In the example of Figure 1 , the surface 130 is free of catalyst nanoparticles. In other cases, a number of nanoparticles may be distributed across the surface 130. The nanoparticles may have a composition and/or otherwise be configured to act as a catalyst for the electrochemical or other reaction. The nanoparticles may have additional or alternative purposes in other cases, including, for instance, charge transport.

[0080] The nanostructures 126 and the stabilizing layer 132 are not shown to scale in the schematic depiction of Figure 1 . The shape of the nanostructures 126 may also vary from the nanowire example shown. Further details regarding the nanostructures 126 and the stabilizing layer 132, including the fabrication or formation thereof, are provided below.

[0081] The nanowire arrangement may be fabricated on a substrate (e.g., a silicon substrate) via nanostructure-engineering. In one example, molecular beam epitaxial (MBE) growth of the nanowires is followed by photo-deposition of the nanoparticles. The photodeposition of the nanoparticles may be configured to selectively deposit the nanoparticles on the respective sides of the nanowire. Further details regarding example fabrication procedures are provided below, e.g., in connection with Figure 7.

[0082] The nanostructures 126 may facilitate the water splitting in one or more ways. For instance, one or more aspects of each nanostructure 126 may be configured to extract charge carriers (e.g., electrons) generated in the substrate (e.g., as a result of light absorbed by the substrate 120). The extraction brings the charge carriers to external sites along the nanostructures 126 for use in the water splitting or other reactions. For instance, the nanostructures 126 may thus form an interface well-suited for evolution of hydrogen, the reduction of CO2, and/or other reactions.

[0083] Figure 7 depicts a method 200 of fabricating a photoelectrode or other semiconductor device for photocatalytic water splitting, PEC water splitting, or other photocatalytic reactions, in accordance with one example. The method 200 may be used to manufacture any of the photoelectrodes or other devices described herein or another device. The method 200 may include additional, fewer, or alternative acts. For instance, the method 200 may or may not include one or more acts directed to forming a backside contact of the device (act 228).

[0084] The method 200 may begin with an act 202 in which a substrate is prepared or otherwise provided. The substrate may be or be formed from a silicon wafer. In one example, a 2-inch Si wafer was used, but other (e.g., larger) size wafers may be used. Other semiconductors and substrates may be used.

[0085] The substrate may have a planar or nonplanar surface. In some cases, the act 202 includes an act 204 in which a wet or other etch procedure is implemented to define the surface. For example, the etch procedure may be or include a crystallographic etch procedure. In silicon substrate examples, the crystallographic etch procedure may be a KOH etch procedure. In some cases, if the substrate has a <100> orientation, the wet etch procedure may establish that the surface includes a pyramidal textured surface with faces oriented along <111 > planes. Other surface orientations and shapes may be used.

[0086] The act 202 may include fewer, additional, or alternative acts. For instance, in the example of Figure 1 , the act 202 includes an act 206 in which the substrate is cleaned (e.g., by acetone and/or methanol), and an act 208 in which oxide is removed (e.g., by 10% hydrofluoric acid).

[0087] In one example, double-side polished 4” p-type Si (100) wafers were first RCA cleaned and then loaded to a CMOS grade oxidation furnace to form 250 - 300 nm thick SiOx at 1100 °C. The oxide grew on both sides of the double-side polished wafers. Subsequent lithography and wet etching steps led to only one side of the wafers having SiOx, (with the other side being exposed p-Si). After another RCA cleaning, the wafers were loaded in a CMOS grade phosphorus diffusion furnace to form n-i-Si doping at 950 °C for 20 min. These wafers were then cleaned in buffered HF solution for 5 - 10 min to remove the SiOx and residual silicate built on the n-i-Si side. The 4” wafer was then diced into quarter wafers which were subsequently cleaned following standard solvent/acid protocols before being loaded into the MBE chamber for nanostructure growth.

[0088] The act 202 may include still further acts. For instance, the act 202 may include one or more doping procedures to form doped regions or layers, and thereby establish a junction, as described herein. Alternatively, the substrate is provided at the outset with a desired dopant concentration profile. In some cases, the substrate is doped to form a solar cell.

[0089] The method 200 includes an act 210 in which a number of electrode or other device structures are grown or otherwise formed on the substrate. In some cases, a nanowire or other nanostructure array is grown or otherwise formed on the substrate. Each nanowire is formed on the surface of the substrate such that each nanowire extends outwardly from the surface of the substrate. Each nanostructure may have a semiconductor composition, as described herein. In some cases, the semiconductor composition is or otherwise includes a compound semiconductor material, such as a lll-V semiconductor.

[0090] The nanostructure growth may be achieved in an act 212 in which a molecular beam epitaxy (MBE) procedure is implemented. The substrate may be rotated during the MBE procedure such that each nanostructure is shaped as a cylindrically shaped nanostructure. Each nanostructure may thus have a circular cross-sectional shape, as opposed to a plate-shaped or sheet-shaped nanostructure.

[0091] The MBE procedure may be implemented under nitrogen-rich conditions. The nitrogen-rich conditions may lead to nitrogen-terminated sidewalls and/or other surfaces, as described herein.

[0092] In some cases, the MBE procedure may be modified to fabricate the arrangement of layers or segments of each nanowire. Various parameters may be adjusted to achieve the different composition levels of the segments. For instance, the substrate temperature may be adjusted in an act 214. Beam equivalent pressures may alternatively or additionally be adjusted. In some cases, a dopant cell temperature is adjusted to control the doping (e.g., Mg doping) of the nanowires.

[0093] In one example, before beginning the growth of n⁺-GaN nanowires on Si substrates, an N-terminated thin GaN quasi-film was grown on the Si substrate for 15 min with a substrate temperature at about 735 °C, Ga beam equivalent pressure (BEP) of ~ 2.2 x 10^-7 torr, Si cell (n-type dopant) temperature at 1250 °C, and a nitrogen flow rate of 0.45 standard cubic centimeter per minute (seem). The incorporation of a thin GaN quasi-film serves to protect the Si surface in photoelectrochemical reaction. The nanowires were then grown on top of the quasi-film under nitrogen-rich conditions, leading to N-terminated surfaces. The growth conditions for nanowires included a substrate temperature of about 735 °C, Ga BEP of 6 x 10^-8 torr, Si cell at 1250 °C, nitrogen flow rate of 1 seem, forward plasma power of 350 W, and a growth duration of 4 - 5 hours. The growth parameters may vary considerably in other cases.

[0094] In other cases, the act 210 may include forming additional or alternative layers or other portions of the electrode. The layer(s) may be configured to catalyze, participate in, or otherwise enable or facilitate the reaction. The layers or other portions may be deposited on the substrate in an act 216. In some cases, the layers may establish a surface configured to catalyze or otherwise facilitate the water splitting and/or other reaction. The electrode layer(s) may be formed in alternative or additional ways, including, for instance, non- selective growth procedures.

[0095] One or more nitrogen-containing layers may be deposited or otherwise formed in an act 218. The nitrogen layer may be directed to protection of the reaction layer(s) and/or other aspects of the nanostructures. The nitrogen-containing layer(s) may alternatively or additionally be directed to supporting the formation of a stabilizing layer. In some cases, the act 218 may include implementing a reaction to form the nitrogen-containing layer(s) spontaneously. The layer(s) may be composed of, or otherwise include, a nitride or an oxynitride.

[0096] In some cases, the act 218 may be integrated with, or implemented in conjunction with, the act 216, as described herein in conjunction with examples involving GaN or other nanostructures. In other cases involving GaN nanostructures, the acts 216 and 218 may be implemented separately.

[0097] The method 200 may further include an act 220 in which oxygen is incorporated into one or more surfaces of the nanostructures or other structures of the device to form a stabilizing layer on the one or more surfaces. In some cases, the surface(s) is/are oxidized. For instance, an oxidation reaction may be implemented to form the stabilizing layer. Alternatively or additionally, oxygen is deposited on the surface(s). The deposition may be followed by an annealing procedure in some cases to fully incorporate the oxygen atoms. The stabilizing layer may be configured as an activation layer or a passivation layer. The functionality of the stabilizing layer may vary with the nature of the device structure. As described herein, the oxidation of the surface(s) (or other incorporation of oxygen into the surface(s)) may be implemented such that the stabilizing layer is composed of, or otherwise includes, a uniform distribution of an oxynitride material. For instance, the oxidation reaction may be configured such that the stabilizing layer includes the uniform distribution. In some cases, the oxygen is incorporated into the surface such that the oxynitride material is continuously distributed across the surface (e.g., continuously covers the entirety of the surface).

[0098] In some cases, the oxynitride layer is formed in an act 222 in which an electrochemical procedure is implemented. The electrochemical procedure may be implemented for a period of time on the order of hours, e.g., about 5 to about 10 hours. The duration of the procedure may be sufficiently long to establish the uniform distribution of the oxynitride material. For instance, in other cases, the oxidation may be implemented for a period of time on the order of minutes. Further details are provided in connection with a number of examples below.

[0099] In some cases, the electrochemical procedure involves or otherwise includes the electrochemical reaction that the device is configured to implement. In such cases, the act 222 may be configured as an initialization step or procedure for the device. For instance, the device may be configured or directed to implementing a water splitting or other hydrogen evolution reaction. The electrochemical procedure may then include implementing the water splitting reaction in which the device is immersed in water. The hydrogen evolution reaction may be implemented for a predetermined period of time to form an oxynitride layer of a desired thickness. In other cases, the electrochemical procedure differs from the electrochemical reaction that the device is configured to implement. For instance, the electrochemical procedure may be or include a water splitting reaction, while the device is directed to implementing another reaction, such as CO2 reduction.

[00100] Alternatively or additionally, the act 220 includes exposing the surface to oxygen in an act 224. In some cases, the exposure to oxygen occurs in an oxygen-containing reaction chamber, such as a plasma asher. Alternatively or additionally, the exposure involves the ambient environment. The exposure may last for a predetermined period of time, e.g., for a period of time on the order of minutes or on the order of hours, but other time periods may be used. The duration of the exposure may be selected to produce an oxynitride layer of a desired thickness. The duration of the exposure may be otherwise selected to establish the uniform distribution of the oxynitride material.

[00101] The incorporation of oxygen in the act 220 may alternatively or additionally include an act 226 in which an oxygen-containing material is deposited on the surface(s). For example, the oxygen-containing material may be composed of, or otherwise include, an oxide material, such as aluminum oxide, hafnium oxide, or an oxygen-containing ferroelectric layer (e.g., HfZrO2), but other materials may be used.

[00102] The act 220 may include additional acts in combination with any one or more of the above-described techniques for exposing the surface to oxygen (or otherwise incorporating oxygen into the surface). In the example of Figure 2, the act 220 includes implementation of one or more cleaning procedures or other surface treatments in an act 225. The surface is treated in the act 225 before exposure of the surface to oxygen (e.g., the act 224) or the deposition of an oxygen-containing layer (e.g., the act 226). In some cases, the act 225 includes an in-situ pre-cleaning procedure, such as a chemical treatment of the surface. Various chemicals may be used in the treatment(s), including, for instance, HF, buffered HF, and/or sulfuric acid. The act 225 may include an alternative or additional cleaning or treatment, such as an ultraviolet (UV) ozone treatment. Still other types of procedures may be used, including, for instance, other types of surface treatments that are configured to improve the interface at the surface.

[00103] The act 220 may alternatively or additionally include an act 227 in which the implementation of an anneal procedure. In some cases, the anneal may be implemented in an oxygen environment to provide a source of the oxygen. Alternatively, the anneal may follow the deposition of the oxygen-containing layer in the act 226 or other provision of oxygen to the surface(s). The anneal may be configured to incorporate the oxygen into the surface (e.g., via substitution in the lattice of the structure, etc.). The temperature of the annealing procedure may vary. For instance, the temperature may fall in a range from about 100 °C to about 500 °C, but other temperatures may be used. The time period of the anneal may vary accordingly. The temperature, duration, and/or other characteristics of the procedure may be selected to establish the uniform distribution of the oxynitride material. In some cases, the anneal procedure may not involve an oxygen environment, e.g., when the oxygen is made available in another way, such as via deposition (e.g., deposition of an oxide material).

[00104] The oxidation of the surface may be enabled, enhanced or otherwise possible due to one or more aspects of the surface. For instance, the surface may be oriented along a non-polar plane of the semiconductor composition. In other cases, the surface is oriented along a polar plane. Alternatively or additionally, the semiconductor composition may be configured such that the oxidation reaction results in partial oxygen substitution of the nitrogen. In such cases, the nitrogen of the surface may be disposed in a compound semiconductor arrangement of the semiconductor composition. The semiconductor composition may have a Wurtzite crystal structure.

[00105] In other cases, the method 200 does not include the implementation of a procedure expressly dedicated to oxidation. For instance, the oxidation may occur during implementation of a photoelectrochemical procedure used to form an oxynitride layer.

[00106] The method 200 includes an act 230 in which a photochemical procedure is implemented to form an oxynitride layer. The oxynitride layer may be formed on a non-polar surface of the structure, as described herein. Implementation of the photoelectrochemical procedure may result in partial oxygen substitution of the nitrogen.

[00107] In some cases, implementing the photochemical procedure includes implementation of a water splitting reaction in which the device is immersed in water in an act 232. The water splitting reaction or other photoelectrochemical procedure may be implemented for a period of time on the order of hours. The time period may vary. For instance, the procedure may be implemented for a time period on the order of minutes, as described above.

[00108] The act 230 may include irradiating the photoelectrode with radiation having an intensity that corresponds with concentrated solar light in an act 232. In some cases, the intensity of the radiation corresponds with at least about nine-sun solar light. Lower or higher intensities may be used in other cases.

[00109] As described herein, implementing the photoelectrochemical procedure may also include continuously circulating an electrolyte in an act 234.

[00110] The method 200 may include additional, fewer or alternative acts. For instance, the method 200 may include one or more additional acts directed to forming further structures, components, or other features of the photocathode.

[00111] As described above, the method 200 may lack an act directed to the deposition of nanoparticles or other co-catalysts on the nanowires. The method 200 may also lack an act directed to the deposition of a passivation layer on the nanowires.

[00112] Described herein are examples of photoelectrodes with long-term stable operation in a systems having a two-electrode configuration. The examples include a semiconductor photocathode having elements composed of Si and GaN, the two most produced semiconductors in the world. The examples demonstrated stable operation of 3,000 hours without any performance degradation in the two-electrode configuration. Detailed measurements in both three-electrode and two-electrode configurations indicate that the surfaces of GaN nanowires on a Si photocathode are transformed to Ga-O-N during the initial hours of reaction, which protects the photocathode against photocorrosion and enhances the hydrogen evolution reaction. Factors such as morphology and irradiation intensity contribute to the efficacy and speed of the surface modification. The formation process of Ga-O-N species was also described. The in situ formed Ga-O-N species may exhibit atomic-scale surface metallization. The disclosed photocathodes also provide examples that avoid having to rely on additional catalysts. The examples also avoid the stability bottleneck of semiconductor photoelectrodes, thereby supporting the realization of photoelectrochemical devices and systems for clean energy.

[00113] The composition of the oxynitride material may vary. For instance, the ratio of nitrogen to oxygen may vary. Thus, in some cases, the oxygen composition, may vary (e.g., decrease) as the distance from the surface increases. The oxynitride material may have a varying oxygen concentration, such as O_xNi._x. For example, in devices having a GaN device structure, the oxynitride material may be or otherwise include GaO_xNi._x. The stabilizing layer in such cases may accordingly include gallium oxynitride in combination with a very thin layer (e.g., 1 nm or less) of gallium and oxide (e.g., on top of the gallium oxynitride, and a region of Ga O_xNi._x with the oxygen concentration decreasing as the distance from the surface increases. References to the oxynitride material of the stabilizing layer may thus include materials with varying levels of oxygen content. The oxygen content level may vary with a number of factors, including, for instance, surface orientation, surrounding environment (e.g., electrolyte, metal layer, passivation layer, etc.), and/or treatment conditions. In any case, the oxynitride material of the stabilizing layer provides surface stabilization and device performance enhancement, as described below. The oxynitride material may vary in alternative or additional ways. For instance, the nitrogen and oxygen may be disordered. In some cases, the positioning of the nitrogen and oxygen may be swapped or otherwise vary.

[00114] The term "about" is used herein to include deviations from a specified value that are effectively the same as the specified value, including, for instance, deviations that do not result in a detectable or discernable change in outcome.

[00115] The present disclosure has been described with reference to specific examples that are intended to be illustrative only and not to be limiting of the disclosure. Changes, additions and/or deletions may be made to the examples without departing from the spirit and scope of the disclosure. [00116] The foregoing description is given for clearness of understanding only, and no unnecessary limitations should be understood therefrom.

Claims

What is Claimed is:

1. An electrochemical system comprising: a counter electrode; and a working electrode spaced from the counter electrode, the working electrode comprising: a substrate; an array of conductive projections supported by the substrate and extending outwardly from the substrate, each conductive projection of the array of conductive projections having a semiconductor composition, and comprising a surface, the surface comprising nitrogen; and an oxynitride layer disposed on the surface; wherein the counter electrode and the working electrode are arranged in a two- electrode configuration.

2. The electrochemical system of claim 1 , wherein the oxynitride layer comprises localized oxynitride nanoclusters.

3. The electrochemical system of claim 1 , wherein the oxynitride layer is configured such that the surface exhibits localized metallization.

4. The electrochemical system of claim 1 , wherein the working electrode lacks a cocatalyst in addition to the array of conductive projections.

5. The electrochemical system of claim 1 , wherein the working electrode lacks a passivation layer along the surface.

6. The electrochemical system of claim 1 , wherein the working electrode is configured such that an entirety of the surface is exposed to an electrolyte during operation.

7. The electrochemical system of claim 1 , wherein: the substrate comprises silicon; and the semiconductor composition of the structure comprises gallium nitride such that the oxynitride material is gallium oxynitride.

8. The electrochemical system of claim 1 , wherein the surface is oriented along a nonpolar plane of the semiconductor composition.

9. The electrochemical system of claim 1 , wherein the semiconductor composition has a Wurtzite crystal structure.

10. The electrochemical system of claim 1 , wherein: the surface comprises a sidewall; the oxynitride layer is disposed along the sidewall; and the sidewall is nitrogen-terminated.

11. A photoelectrode comprising: a substrate; an array of conductive projections supported by the substrate and extending outwardly from the substrate, each conductive projection of the array of conductive projections having a semiconductor composition, and comprising a surface, the surface comprising nitrogen; and an oxynitride layer disposed on the surface; wherein the surface is not passivated.

12. The photoelectrode of claim 11 , wherein the oxynitride layer comprises localized oxynitride nanoclusters.

13. The photoelectrode of claim 11 , wherein the oxynitride layer is configured such that the surface exhibits localized metallization.

14. The photoelectrode of claim 11 , wherein the surface lacks a co-catalyst.

15. The photoelectrode of claim 11 , wherein: the surface comprises a sidewall; the stabilizing layer is disposed along the sidewall; and the sidewall is nitrogen-terminated.

16. The photoelectrode of claim 11 , wherein: the substrate comprises silicon; and the semiconductor composition of the structure comprises gallium nitride such that the oxynitride material is gallium oxynitride.

17. A method of fabricating a photoelectrode, the method comprising: providing a substrate of the photoelectrode; forming an array of conductive projections supported by the substrate and extending outwardly from the substrate, each conductive projection of the array of conductive projections having a semiconductor composition, and comprising a surface, the surface comprising nitrogen; and implementing a photoelectrochemical procedure to form an oxynitride layer on the surface; wherein implementing the photoelectrochemical procedure comprises irradiating the photoelectrode with radiation having an intensity that corresponds with concentrated solar light.

18. The method of claim 17, wherein the intensity of the radiation corresponds with at least about nine-sun solar light.

19. The method of claim 17, wherein the photoelectrochemical procedure is implemented for a period of time on the order of hours.

20. The method of claim 17, wherein the photoelectrochemical procedure is implemented for a period of time on the order of minutes.

21. The method of claim 17, wherein implementing the photoelectrochemical procedure comprises implementing a water splitting reaction in which the device is immersed in water.

22. The method of claim 17, wherein implementing the photoelectrochemical procedure comprises continuously circulating an electrolyte.

23. The method of claim 17, wherein forming the array of conductive projections comprises implementing a molecular beam epitaxy (MBE) growth procedure such that each conductive projection of the array of conductive projections comprises a respective nanowire.

24. The method of claim 23, wherein the MBE growth procedure is implemented under nitrogen-rich conditions such that sidewalls of each conductive projection of the array of conductive projections are nitrogen-terminated.

25. The method of claim 17, wherein the semiconductor composition is configured such that implementing the photoelectrochemical procedure results in partial oxygen substitution of the nitrogen.