Electrochemical systems, photoelectrodes, and methods for manufacturing photoelectrodes

Abstract

This invention provides an electrochemical system that contributes to the stabilization and activation of device surfaces during water splitting and other reactions, as well as during the operation of other types of devices. [Solution] The electrochemical system includes a counter electrode and a working electrode spaced apart from the counter electrode. The working electrode includes a substrate and an array of conductive protrusions supported by the substrate and extending outward from the substrate. Each conductive protrusion in the array of conductive protrusions has a semiconductor composition and includes a nitrogen-containing surface and an oxynitride layer disposed on the surface. The counter electrode and the working electrode are arranged in a two-electrode configuration.

Description

Cross - References to Related Applications

[0001] This application claims the benefit of U.S. Provisional Application No. 63 / 452,907, filed Mar. 17, 2023, entitled “Stabilization of Two - Electrode Electrochemical Systems,” the entire disclosure of which is hereby expressly incorporated by reference herein. 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 Contract No. W911NF2110337 awarded by the Army Research Laboratory. The government has certain rights in this invention.

Technical Field

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

Background Art

[0004] Photoelectrochemical (PEC) water splitting mimics a plant for producing sustainable clean fuels using two of the most abundant resources on Earth - sunlight and water. One common scheme for artificially achieving this conversion of solar energy to chemical energy is to integrally combine a semiconductor material with an electrode catalyst in a photoelectrode. In this approach, the semiconductor efficiently harvests solar energy, while the electrode catalyst, which may also act as a protective layer, reduces overvoltage, mediates charge - carrier transport, and provides active sites for chemical reactions at the solid / liquid interface.

[0005] The photoelectrochemical (PEC) pathway for the production of clean chemicals and fuels, such as H2 production from solar water splitting and liquid fuel production from CO2 reduction, has received significant attention over the past few decades. Notable progress has been made in improving the solar - to - hydrogen (STH) efficiency of PEC devices, but the stability of these devices remains a difficult challenge, hindering the practical large - scale deployment of this promising technology.

[0006] Unfortunately, efficient light absorbers lack durability (e.g., Si, III-V), and durable materials are inefficient (e.g., TiO2, SrTiO3). For example, both Si and III-V compound semiconductors have the drawback of low stability due to chemical and photochemical corrosion. Compared to a photovoltaicelectrolyzer (PV-EL) device, the light absorber of a PEC device often directly contacts the electrolyte, resulting in more rapid decomposition. The corrosion of semiconductors is affected by many factors, including the intensity of light irradiation, bias conditions, catalysts, surface passivation, the semiconductor electronic band structure, electrolyte composition, and the semiconductor / electrolyte as well as catalyst / electrolyte interfaces. These factors can potentially be addressed by exploring thermodynamic and kinetic protection schemes.

[0007] Recent efforts have been directed towards providing stable coatings to obtain stable photoanodes. These coatings are usually a combination of charge transport layers combined with catalysts, thus promoting efficient charge collection and separation and preventing corrosion. To protect Si-based photoanodes in the presence of a catalyst for the hydrogen evolution reaction (HER), various oxide, nitride, selenide, and sulfide materials have been investigated. Among these, gallium nitride (GaN) is an interesting option due to the chemical stability of its N-rich surface. GaN is suitable as an efficient protective layer for silicon photoanodes due to the nearly ideal band alignment of two materials for optimal electron transport.

[0008] To enhance the stability of photoanodes, other protection schemes have been developed. The dynamic protection of a given photoanode is possible by using a synergistic combination of a stable surface protection layer and a highly active cocatalyst. To further improve device stability, other photoanodes, in addition to the use of appropriate cocatalysts, use TiO2, Al2O3, and IrO as passivation layers xThese devices use relatively thick metal oxides. While the stability of these devices has improved, one major problem is the loss of photocurrent due to insufficient charge transfer, and in some cases, undesirable light absorption by the protective layer. [Overview of the Initiative]

[0009] According to one aspect of the present disclosure, an electrochemical system includes a counter electrode and a working electrode spaced apart from the counter electrode, the working electrode comprising a substrate and an array of conductive protrusions supported by the substrate and extending outward from the substrate, each conductive protrusion of the array having a semiconductor composition and including a nitrogen-containing surface, and an oxynitride layer disposed on the surface. The counter electrode and the working electrode are arranged in a two-electrode configuration.

[0010] According to another aspect of the present disclosure, the photoelectrode includes a substrate and an array of conductive protrusions supported by the substrate and extending outward from the substrate, wherein each conductive protrusion in the array of conductive protrusions has a semiconductor composition and includes a nitrogen-containing surface, and an oxynitride layer disposed on the surface. The surface is not passivated.

[0011] According to yet another aspect of the present disclosure, a method for manufacturing a photoelectrode includes providing a substrate for the photoelectrode and forming an array of conductive protrusions supported by the substrate and extending outward from the substrate. Each conductive protrusion in the array of conductive protrusions has a semiconductor composition and includes a nitrogen-containing surface. The method for manufacturing the photoelectrode includes performing a photoelectrochemical procedure to form an oxynitride layer on the surface. Performing the photoelectrochemical procedure includes irradiating the photoelectrode with radiation having an intensity corresponding to focused sunlight.

[0012] In relation to any one of the embodiments described herein, the electrodes, systems, and / or methods described herein may, as an alternative or in addition, include or be associated with any combination of one or more of the following embodiments or features: The oxynitride layer contains localized oxynitride nanoclusters. The oxynitride layer is configured so that the surface exhibits localized metallization. The working electrode does not include a cocatalyst in addition to an array of conductive protrusions. The working electrode lacks a passivation layer along its surface. The working electrode is configured so that the entire surface is exposed to the electrolyte during operation. The substrate contains silicon, the semiconductor composition of the structure contains gallium nitride, and the oxynitride material is gallium oxynitride. The surface is oriented along the non-polar plane of the semiconductor composition. The semiconductor composition has a wurtzite-type crystalline structure. The surface includes sidewalls. The oxynitride layer is positioned along the sidewalls. The sidewalls are nitrogen-terminated. The oxynitride layer contains localized oxynitride nanoclusters. The oxynitride layer is configured so that the surface exhibits localized metallization. The surface lacks a cocatalyst. The surface includes sidewalls. A stabilizing layer is positioned along the sidewalls. The sidewalls are nitrogen-terminated. The substrate contains silicon. The semiconductor composition of the structure contains gallium nitride, such that the oxynitride material is gallium oxynitride. The radiation intensity corresponds to at least about 9 times the sunlight of the sun. The photoelectrochemical procedure is performed for a period of several hours. The photoelectrochemical procedure is performed for a period of several minutes. Performing the photoelectrochemical procedure involves performing a water splitting reaction in which the device is immersed in water. Performing the photoelectrochemical procedure involves continuously circulating the electrolyte. Forming an array of conductive protrusions involves performing a molecular beam epitaxy (MBE) growth procedure such that each conductive protrusion in the array of conductive protrusions contains its own nanowire. The MBE growth procedure is performed under nitrogen-rich conditions such that the sidewalls of each conductive protrusion in the array of conductive protrusions are nitrogen-terminated. The semiconductor composition is configured such that the performance of the photoelectrochemical procedure results in partial oxygen substitution of nitrogen.

[0013] For a more complete understanding of this disclosure, the following detailed description and accompanying drawings should be referenced, where similar reference numbers identify similar elements in the drawings. [Brief explanation of the drawing]

[0014] [Figure 1] The diagram illustrates the operation of an exemplary photocathode in a three-electrode configuration: (a) is a schematic diagram of the photoelectrochemical H2 evolution reaction in a three-electrode configuration, with GaN nanowire / Si photocathode, iridium oxide (IrOx), and Ag / AgCl used as the working electrode, counter electrode, and reference electrode, respectively; (b) is the linear sweep voltammetry (LSV) curve of the GaN nanowire / Si photocathode and Pt-supported GaN nanowire / Si photocathode after 0 to 100 hours of reaction; (c) is the LSV curve of the GaN / Si photocathode measured in a flow cell under 9 sun (900 mW / cm2) of sunlight; and (d) is a graphical plot of the onset potential versus reaction time under 1 sun (black curve) and 9 sun (red curve) irradiation. [Figure 2] This figure shows a graph plot of the operation of an exemplary photocathode in a two-electrode configuration, where (a) is a triton at 0.2 mH. (b) shows the first 10 hours of a chronoamperometry (CA) stability test of a photocathode at -2.3V against IrOx under 1 sun irradiation of 1.5G AM in 0.5M H2SO4 containing X-100, where (b) shows the Faraday efficiency measurements for the first 10 hours of CA, where the red (light gray) 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) shows the LSV curves of the photocathode 10 hours before (red or light gray curve) and 10 hours after (blue or dark gray curve) of CA under 1 sun irradiation of 1.5G AM, and (d) shows the Nyquist plots of the photocathode 10 hours before (red or light gray curve) and 10 hours after (blue or dark gray curve) of CA under 1 sun irradiation of 1.5G AM. [Figure 3]Figure 2 shows a graph plot of the XPS 0 1s spectrum of n+-GaN nanowire / Si photocathodes grown and tested under the conditions described in relation to Figure 2, taken at an incident angle θ = 60° (the definition of angle θ is schematically shown in part (d)), where the as-grown photoelectrode shows (a) deconvoluted peaks at 531.3 eV and 532.6 eV for O-Ga (red or light gray curves indicated by arrows) and OH (blue or dark gray curves), respectively, as well as (b) an additional deconvolution at 531.6 eV corresponding to the emerging gallium oxynitride species (purple curve indicated by arrow) after 10 hours of chronoamperometry (CA). (c) Volatilized peaks (gray curves (a, b) are fitted curves of the respective raw data), (c) 10-hour valence band maximum measurements of the photocathode before (red or light gray dots) and after (blue or dark gray dots) CA, where the quantity EFS-EVS increased by ΔEs of approximately 0.5 eV (the intersection of the background intensity flat line (at binding energies less than 2 eV) and the linear fitting 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)). [Figure 4]The graph plots show experimental and structural characterization of the ultra-long-term stability of two electrodes of n+-GaN nanowire / Si photocathode, (a) 0.5M Triton X-100 including 0.2 mM (b) Chronoamperometry (CA) plots showing the 3000-hour stability of an exemplary photocathode having n+-GaN nanowires / Si at -2.3V relative to IrOx under 1 sun irradiation of AM1.5G in H2SO4 (material stability is further emphasized by the stability of the onset potential over the CA process), (b) linear scan voltammograms of the photocathode at 0 hours (red curve indicated by arrow), 10 hours (blue curve indicated by arrow), 1000 hours (purple curve indicated by arrow), and 3000 hours (cyan curve indicated by arrow) under 1 sun irradiation of AM1.5G and dark (black curve), as well as (c) Faraday efficiency measurements for the last 10 hours of CA at 3000 hours (red or light gray 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). [Figure 5] The calculated electronic structure of a metallic oxynitride species characterized on the m-plane of GaN is shown, (a) is a top view (upper panel) and a side view (lower panel) of an exemplary configuration of the m-plane of GaN with surface metallic oxynitride nanoclusters, (b) is a graphical plot of the density of states (DOS) of a metallic oxynitride species characterized on the m-plane of GaN (contributions from surface and bulk atoms are marked by pink and green curves, respectively), (c) is the orbital projection DOS of Ga oxynitride, (d) is a diagram of the spatial structure of the charge density of Ga oxynitride in the energy range of -1.39 eV to about 0 eV, marked by purple (or light gray) dashed rectangles in parts (b) and (c), with the upper and lower panels being top and side views of the spatial structure, respectively, and the isosurface value being 0.001 eÅ-3, and (e) is a graphical plot of the band diagram of GaN with Ga oxynitride-induced metallic states. [Figure 6]This is a schematic diagram and block diagram of a two-electrode electrochemical system having a photocathode with a stabilizing layer for stable activation of hydrogen generation by water splitting, as an example. [Figure 7] This is a flowchart illustrating a method for manufacturing a photocathode with a stabilizing layer, as an example. [Modes for carrying out the invention]

[0015] The embodiments of the disclosed devices, systems, and methods may take on various forms. Under the understanding that this disclosure is intended to be illustrative, specific embodiments are shown in the drawings and described below. This disclosure is not intended to limit the invention to the specific embodiments described and illustrated herein.

[0016] As a diagnostic tool, systems with a three-electrode configuration have been used to study the stability of semiconductor photoelectrodes. The incorporation of protective schemes has significantly improved the stability of various materials in three-electrode PEC test configurations. However, three-electrode PEC testing does not consider the performance of the counter electrode. Furthermore, three-electrode PEC testing does not describe the stability of the entire PEC system. Additionally, since the protective layer is not catalytically active for the hydrogen evolution reaction (HER), additional exogenous catalysts have been used on the photoelectrode.

[0017] Previously reported high-efficiency photoelectrodes that demonstrated reasonable stability in a three-electrode configuration exhibit considerably lower stability performance when measured in a practical system configuration, namely a two-electrode configuration, due to factors such as charge accumulation within the cell, solution resistance, and the inherent chemical instability of the photoelectrode. Therefore, three-electrode measurements are neither sufficient nor relevant for the overall stability of the final commercial deployment of a PEC water splitting system. In contrast, testing under a two-electrode configuration provides the actual efficiency and durability of the entire PEC cell. However, to date, there has been no demonstration of a semiconductor photoelectrode or appropriate protection scheme that can enable long-term stable and efficient operation in a practical two-electrode configuration.

[0018] Previous research has primarily focused on metal oxides, Si, and III-V semiconductor photoelectrodes. In recent years, a new class of semiconductor photoelectrodes, including metal nitride nanostructures, has attracted considerable attention. Metal nitrides, particularly group III nitrides such as InGaN, possess an energy bandgap that can be tuned across the entire solar spectrum. Furthermore, GaN and Si, the two most produced semiconductors in the world, can be seamlessly integrated to achieve highly efficient solar water splitting with proven manufacturability, scalability, and relatively low cost. Zeng et al. demonstrated that GaN possesses intrinsic self-improving properties; specifically, PEC performance improved rather than degraded during the three-electrode chronoamperometry (CA) process. This highly unusual behavior was attributed to the formation of oxynitrides on nonpolar and semipolar GaN surfaces during the PEC reaction. Simultaneously, measurements were performed in a three-electrode configuration where the GaN morphology was quasi-film, and a relatively small portion of the surface in contact with the electrolyte was an active nonpolar surface. For GaN morphologies where the active nonpolar surface is dominant under actual two-electrode conditions, it remains unclear whether such unique self-improving behavior can be maintained or further enhanced, and whether long-term stability can be achieved. Furthermore, the underlying mechanism of oxynitride formation, its atomic origin and catalytic properties, and its dependence on surface polarity and composition remain unknown.

[0019] An example of an electrochemical system having a two-electrode configuration with a photocathode containing a GaN nanowire array grown on a Si photocathode is described below to achieve self-improving behavior and long-term stability. The GaN nanowire / Si photocathode has predominantly nonpolar sidewalls as described and shown herein. The photocathode of the example exhibits dramatically improved photoelectrochemical properties compared to GaN films on previously studied Si photocathodes. Experiments with a three-electrode configuration confirm that the GaN / electrolyte interface is useful for the self-improving effect, and that its rate increases or decreases hyperlinearly with increasing photocurrent density achieved via focused sunlight irradiation. X-ray photoelectron spectroscopy (XPS) measurements further confirmed that new gallium oxynitride species were formed in situ on the nonpolar m-plane of the GaN nanowires during the first few hours of stability testing, which improved JV properties, including higher photocurrent density and a more positive onset potential.

[0020] The disclosed examples also demonstrate stable operation for 3000 hours without any performance degradation in a two-electrode configuration, an order of magnitude better than the previously reported best stability of 300 hours in a two-electrode configuration. Importantly, the measurements were performed without incorporating any metal catalyst protection, revealing the intrinsic stability of the GaN / Si photoelectrode. First-principles density functional theory (DFT) analysis further revealed the formation mechanism, atomic origin, electronic structure, and catalytic properties of the unique GaON species. In-situ formation of atomic-scale GaON nanoclusters on N-terminal GaN nanowires occurs when O atoms partially substitute for N atoms on the nonpolar GaN m-plane. The incorporation of O atoms onto GaN not only reduced surface band bending but also generated 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 for photoelectrochemical devices and systems for clean energy.

[0021] Although described in relation to nanowire arrays, the surface stabilization and activation schemes described herein can be applied to a variety of different device structures. Various planar catalyst surfaces can be modified to contain nitrogen. For example, a nitrogen layer (or other nitrogen-based layer) can be deposited on various different electrode surfaces or applied in other ways.

[0022] The photoelectrochemical, photocatalytic, and other water splitting methods provided by the disclosed devices and systems may involve the conversion of sunlight to hydrogen. 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 embodiments or features that address efficiency improvements. For example, the structure may include a double-junction configuration such as a multiband InGaN nanowire array configured to provide a second junction.

[0023] Although described in relation to photoelectrochemical water splitting, the disclosed devices and systems may be used in other chemical reaction situations and applications. For example, the disclosed devices and systems may be useful in relation to various types of photocatalysts and / or other systems, and / or in relation to other reactions, such as nitrogen reduction to ammonia, CO2 reduction to various fuels and other chemicals, and activation of CH bonds for the production of various chemicals.

[0024] While described herein in relation to 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 protrusions can be used. Thus, the properties, composition, structure, configuration, characteristics, shape, and other aspects of the electrode on which water splitting is carried out can vary.

[0025] GaN / Si photocathode: 3-electrode configuration analysis. n +-An exemplary photocathode having a -GaN nanowire / Si p-n junction was fabricated and the stabilization provided by the oxynitride layer was analyzed. Details regarding the fabrication of the photocathode are shown below. In these examples, the nanowires have an average length of about 600 nm and a diameter of about 100 nm. First, the photocathode was evaluated in a three-electrode configuration with an iridium oxide (IrOx) counter electrode and an Ag / AgCl reference electrode at an angle perpendicular to the photocathode wafer under AM1.5G 1-sun illumination (Figure 1, part (a)). In this example, incident solar photons are absorbed by the Si wafer, and the photo-generated electrons are extracted by the n + -GaN nanowires to drive proton reduction. The chronoamperometry (CA) curve of the photocathode at -0.4 V versus the reversible hydrogen electrode (V RHE ) shows a rapid increase in photocurrent density from 0.6 mA / cm 2 to about 35 mA / cm 2 over 40 - 50 hours. The linear sweep voltammetry (LSV) curve of the photocathode shows a progressive improvement in the on-site voltage (V ON , voltage at 1 mA / cm 2 ) and photocurrent density (Figure 1, part b). After 100 hours, the measured V RHE of 0.36 V ON and a photocurrent density of about 30 mA / cm 2 at 0 V RHE are among the best performances of Si photocathodes without incorporating an exogenous catalyst and are within the performance range of Si photocathodes with noble metal catalysts. This is a significant advancement from previous studies on quasi-film GaN / Si photocathodes. This is due to the inherent morphology and polarity of the vertically aligned GaN nanowires, where the active m-plane sidewalls are dominant (or preponderant), and the porosity of the nanowire array increases compared to the quasi-film, improving mass transport.

[0026] In the Nyquist plot, the radius of the semi-circle gradually decreases with the increase in reaction time, indicating a decrease in charge transfer resistance and accelerated electron transfer from the photocathode to protons. The Faradaic efficiency of H2 is -0.4 V RHEThe reaction increased from 52% to over 95% during the first 1.5 hours and then stabilized. The performance did not improve at the open-circuit potential even under light irradiation, indicating that the photocurrent plays a crucial role in enhancing the catalytic activity of the GaN nanowire / Si photocathode.

[0027] To experimentally confirm whether the self-improvement originates from in-situ modification of the GaN surface, a thin (2 nm) passivation layer of Al2O3 was coated onto GaN nanowires by atomic layer deposition. Subsequently, the PEC H2 generation reaction was carried out under 1 sun illumination at -0.4 V. RHE The experiment was conducted using the following method. Interestingly, even after 24 hours of reaction, the photocurrent density and V remained unchanged. ON The improvement was negligible. In contrast, after Pt cocatalyst deposition, the Pt / Al2O3 / GaN nanowire / Si photocathode showed a similar LSV curve to the Pt / GaN nanowire / Si. Therefore, the Al2O3 passivation layer efficiently transfers photogenerated electrons from the GaN nanowire to protons while effectively preventing in-situ surface modification of the GaN nanowire. XPS and scanning transmission electron microscopy (STEM) energy-dispersive X-ray spectroscopy (EDS) showed that after 48 hours of chronoamperometry, the counter electrode (IrO) x We further confirmed the absence of Ir contamination on the GaN nanowires from )

[0028] These results confirm that the self-improvement of GaN nanowires / Si photocathodes has two aspects: (1) photocurrent and (2) exposure of the GaN surface to the electrolyte. At the light intensity of one sun, using a relatively long duration (over 10 hours), high H2 generation activity (i.e., 0V) is observed. RHE Super V ON) was obtained, but this is probably because the rate of surface modification was limited by a small photocurrent. By increasing the photocurrent density by irradiating with focused sunlight, the time until self-improvement saturates can be shortened. Therefore, a flow cell was used to provide rapid replacement of reactants and products at the electrodes. The LSV curves at reaction time intervals under sunlight are shown in Figure 1(c). Interestingly, 0.3V RHE V exceeding ON This was achieved within 15 minutes of the reaction. ON The plot against reaction time is V ON A superlinear correlation was observed between the improvement rate and light intensity, resulting in approximately 100 times faster GaN surface modification under 9 sun irradiation compared to 1 sun irradiation (Figure 1, part d). The high photocurrent density (-0.3V) was achieved by focused sunlight. RHE Approximately 165mA / cm² 2 This not only increased the rate of photocathode self-improvement but also increased the H2 generation rate by approximately nine times. Furthermore, the GaN nanowire / Si under focused sunlight at -0.4V RHE This results in a very high photocurrent density (150 mA / cm²). 2 It operated stably for over 500 hours under super-high power conditions. The total amount of H2 gas produced is equivalent to the amount of H2 produced in 4500 hours under one day of sunlight.

[0029] GaN / Si photocathode: 2-electrode configuration. The photocathode was further tested in an electrochemical system having a 2-electrode configuration under AM1.5G1 solar irradiation. Part (a) of Figure 2 shows IrO2 in 0.5M H2SO4 containing 0.2 mM Triton X-100 as a surfactant under AM1.5G1 sun illumination. xThe chronoamperometry of the photocathode for the first 10 hours at -2.3V is shown. Further details are provided herein. A fairly high bias was selected to ensure operation at the saturation current density for most of the subsequent 3000-hour long-term stability test (described in detail below). The photocathode produced hydrogen just as efficiently at lower biases. Note that, as with the photocathode under a three-electrode configuration, the photocurrent density also increased considerably over this first 10 hours of the CA stability test. Part (b) of Figure 2 shows IrO in 0.5M H2SO4 under AM1.5G 1sun irradiation. x The Faraday efficiency and H2 generation for an example of chronoamperometry at -2.3V from 0 to 10 hours are shown. The Faraday efficiency during this period was 89-100%. Due to the formation of oxynitride species, the Faraday efficiency, which was initially less than 100%, steadily increased and reached 100% by the end of the 10-hour chronoamperometry.

[0030] Similar to the results with the 3-electrode configuration, LSV measurements under AM1.5G 1sun irradiation at 0 hours (red or light gray curve) and 10 hours (blue or dark gray curve) (Figure 2, part c) show improvement in the curve factor, V ON This clearly shows a positive shift of approximately 0.5 V 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 gray curve) and 10 hours (blue or dark gray curve) show a dramatic decrease of almost two orders of magnitude in charge transfer resistance for the sample tested at 10 hours compared to the measurement obtained at 0 hours. These results indicate the formation of a new species on the surface of the GaN nanowires, similar to what is responsible for the self-improvement observed under the three-electrode configuration.

[0031] The XPS measurements shown in Figure 3 are performed on an example of a GaN nanowire / Si photocathode before and after a 10-hour stability test. Parts (a) and (b) of Figure 3 show the XPS 0 1s peak taken at an incident angle of 60°, primarily measuring the m-plane surface of the nanowire. Apart from the O-Ga (red or light gray curve) and OH (blue or dark gray curve) peaks, there is an additional deconvoluted O-Ga-N peak (approximately 531.6 eV) for the sample tested for 10 hours (Figure 3, part b) compared to the original sample (Figure 3, part a). The O-Ga-N peak indicates the formation of new oxynitride species along the non-planar (m-plane) surface of the GaN nanowire during the 10-hour stability operation. Note that the deconvoluted Ga-O peak (in the case of the initial sample) is due to the exposure of the as-grown GaN surface to ambient conditions before being moved into the XPS chamber. As shown in part (b) of Figure 3, the relative O-Ga peak intensity is considerably reduced in the 10-hour example compared to the initial device, which is likely due to the dissolution of oxides under acidic conditions and the conversion of some oxides to oxynitride species. Similar conclusions can be drawn from the Ga 3d spectrum. In the original device, the two deconvoluted peaks correspond to Ga-N (cyan curve) and Ga-O (magenta curve) at 20.2 eV and 21 eV, respectively. Similar to the O 1s spectrum, an additional deconvoluted Ga-N peak at 20.8 eV between the Ga-N and Ga-O peaks appears in the Ga 3d spectrum of the 10-hour test example.

[0032] Part (c) of Figure 3 shows the valence electron spectra of both the original sample and the tested sample. In Part (c) of Figure 3, the valence electron spectral measurement of the tested sample is E compared to the initial sample. FS -E VS This showed an increase of approximately 0.5V. Here, E FS This is the surface Fermi level, E VS This is the maximum value of the surface valence band. This valence spectrum shift is shown in Figure 2(c) V ONIt coincides with the shift. The theoretical calculations, which will be explained in more detail below, are E after 10 hours of chronoamperometry. FS -E VS This shows a decrease in conduction band barrier height due to oxynitride formation, which can contribute to the increase in value. The decrease in conduction band barrier helps improve the charge transfer rate of H2 production, which is reflected in the LSV curves shown in Figures 1(b) and 2(c).

[0033] Long-term stability. Chronoamperometry (CA) stability measurements were taken in 0.5M H2SO4 containing 0.2mM Triton X-100 under AM 1.5G 1sun irradiation with IrO2. x The procedure was performed at a constant applied potential of -2.3V. Triton X-100 was included in the electrolyte as a surfactant to enhance hydrogen desorption, resulting in a more stable photocurrent. The electrolyte solution was replaced every 20–24 hours of CA. Figure 4(a) shows the photocurrent density of the GaN nanowire / Si photocathode over 3000 hours. IrO was measured at integer multiples of 100 hours throughout the 3000-hour chronoamperometry. x V measured at -2.3V ON The outlines of and J are plotted in Part (a) of Figure 4. At the start of the stability experiment (hour 0), V ON is IrO x The voltage is approximately -1.8V, and the photocurrent density of the sample is IrO x For comparison, at -2.3V, the current is approximately 8mA / cm². 2 The JV characteristics of the photocathode after 10 hours of chronoamperometry (Figure 4, part b) (shown by the blue curve and arrow) clearly show a dramatic improvement compared to the JV curve at 0 hours (shown by the red curve and arrow). As previously mentioned, this improvement is attributed to the formation of oxynitride species on the m-plane of the GaN nanowire. Further continuation of CA stability experiments under the same operating conditions showed that the photocurrent density was approximately 30 mA / cm² after about 40 hours. 2 This indicates that the saturation value has been reached. The JV characteristics at 1000 hours (purple curve, indicated by arrow) and 3000 hours (cyan curve, indicated by arrow in part b of Figure 4) are compared to the 10-hour point (blue curve, indicated by arrow) with respect to IrO xFor comparison, it is approximately -1.35V, which is almost the same voltage. ON Note that the curve factor is shown. Therefore, it can be concluded that when formed on the m-plane surface of GaN nanowires, the GaON species is robust to both the continuation of stability experiments and exposure to air during periodic electrolyte exchange of CA every 20-24 hours. During the entire 3000-hour chronoamperometry (see Figure 4, part a), the photocurrent density was averaged (approximately 29 mA / cm²). 2 It fluctuated by ±10%.

[0034] As is best understood, the ultra-high stability of Ga(O)N nanowires / Si for approximately 3000 hours is the maximum stability duration measured for any photoelectrode in a two-electrode configuration with any bias under AM1.5G 1 solar irradiation in any electrolyte solution. This exceptional stability of the GaN nanowire / Si photocathode is attributed to the material properties of the GaN nanowire, such as strong ionic bonding, the absence of dislocations, and unique N-termination on both the c-plane and m-plane. By comparing the JV properties of the photocathode after 10 hours (blue curve, indicated by arrows in part b of Figure 4) and 3000 hours (cyan curve, indicated by arrows in part b of Figure 4), it can be concluded that the LSV curves remained nearly identical from 10 hours to 3000 hours, due to the superior stability of the oxynitride species on the m-plane of the GaN nanowire. Furthermore, the Faraday efficiency for hydrogen evolution during the last 10 hours of the 3000-hour chronoamperometry (Figure 4, part c) remained constant, which, along with a constant photocurrent, confirms the stability of the photocathode for the entire 3000-hour CA. STEM measurements showed that after 3000 hours of chronoamperometry, the dimensions of the nanowire remained unchanged at approximately 600 nm in length and approximately 100 nm in diameter. In addition, SEM images of the photoelectrode tested for 3000 hours showed virtually no change in either the GaN nanowire coverage on Si or the nanowire morphology. The Ga dissolved in the electrolyte was analyzed using inductively coupled plasma mass spectrometry (ICP-MS) at different stages throughout the CA stability experiment. Negligible amounts of dissolved Ga concentrations of 10–13 nmol were found in different experiments. These results indicate that the nanowire remained stable throughout the CA operation, consistent with its structural characterization.

[0035] Density Functional Theory (DFT) calculations were performed. To gain an atomic-level understanding of metallic oxynitride species on GaN nanowires, density functional theory (DFT) analysis was performed on the formation of oxynitride species and their electronic and catalytic properties related to water splitting. Based on previously reported N-rich GaN surfaces, four possible atomic arrangements of the GaN m-plane characterized by GaON were investigated (marked by orange, green, red, and purple circles, respectively, in the upper panel of Figure 5(a)). Previous studies have suggested that the m-plane surface of GaN nanowires grown by plasma-assisted molecular beam epitaxy (PAMBE) under nitrogen-rich conditions is characterized by the presence of nitrogen nanoclusters. This unique surface feature of GaN nanowires facilitates further oxygen uptake at the m-plane sidewalls of the nanowires.

[0036] These nitrogen clusters are relatively isolated on the surface and spatially contribute to the formation of Ga-ON species by substituting N atoms with O atoms. The formation energies for the four proposed GaON configurations in Figure 5(a) were calculated to be -4.07 eV, -4.29 eV, -3.95 eV, and -2.66 eV. The negative formation energies indicate that the introduction of O atoms into the N-rich GaN surface is a thermodynamically favorable process, which is also consistent with the consensus that metal-O bonds are stronger than metal-N bonds. Thus, the stability of the Ga-ON species was verified. The density of states of the m-plane of N-rich GaN was calculated before and after oxidation (Figure 5, part b). An intriguing property was found in that the Fermi level of GaON shifts upward into the conduction band (a value of 1.39 eV in the second configuration), and the m-plane surface with incorporated oxygen is metallized. This metallic property can be simply understood as effective n-type doping, where the O atom has one more electron than the N atom it substitutes for. Since the valence band of GaN is completely occupied before oxidation, these extra electrons introduced by the O element can only fill the conduction band. The second configuration (marked by a green circle in Figure 5, part a) was the most negative in terms of formation energy and therefore the most thermodynamically favorable of the four proposed configurations, so further calculations were based on the second configuration. From the calculated orbital projected density of states (OPDOS) and atomic projected density of states (APDOS), it was found that only the p orbitals of the O and N atoms, as well as the p and s orbitals of the Ga atom, in the surface region are responsible for the metallic manifestation, which shows downward band bending (Figure 5, parts b-e) and is therefore beneficial for reduction reactions. On the other hand, the band gap of GaN at the outermost surface of the nanowire is effectively narrowed from 3.19 eV to 1.83 eV due to the emerging metallic state brought about by the formation of the Ga oxynitride species. For a more intuitive view of the metallic surface, the Fermi level (-1.39 eV) <E-E FThe real-space distributed charge density of the conduction band around <0 eV (marked by purple dashed rectangles in parts (b)-(d) of Figure 5) was plotted. The metallic properties directly correspond to surface GaON species that naturally act as electron sinks and function as atomic-scale reduction reaction sites. Thus, first-principles calculations revealed that the surface morphology of the m-plane of N-rich GaN, characterized by GaON nanoclusters, significantly improves the stability and efficiency of solar artificial photosynthesis.

[0037] The unique advantages of in-situ formation of GaON nanoclusters on N-terminated GaN nanostructures, compared to conventional nitriding schemes for III-V group photoelectrodes and Ga2O3 powder, for improving the stability of photoelectrochemical reactions, are addressed here. Nitrogen-containing photocatalysts exhibit stable and efficient operation under harsh solar water splitting conditions compared to conventional metal oxides and III-V group compounds. Furthermore, the incorporation of nitrogen species is essential for efficient light absorption by improving stability and narrowing the band gap. In previous studies, organometallic vapor deposition (MOCVD) growth of GaPN epitaxial layers containing 0.2% to 2% nitrogen on GaP substrates was investigated to protect III-V group photoelectrodes. While these low levels of nitrogen incorporation improved the material's stability against photocorrosion, further nitrogen incorporation into these structures created huge lattice mismatches, leading to increased surface defects and photocorrosion. To date, achieving long-term stability not only for nitrided III-V group photoelectrodes but also for high-efficiency photoelectrodes in general has remained challenging. To enhance the stability of photoelectrodes, much attention has been paid to the use of oxide layers, such as atomic layer deposition of TiO2 and Al2O3, for protection against various corrosion pathways, including photo-oxidation and reactions with electrolytes. The use of such heterogeneous protective layers (on photoelectrodes) often introduces an undesirable trade-off between maintaining challenging photoelectrode efficiency and achieving a realistic improvement in photoelectrode stability. This is because such oxide layers, by design, are chemically inert on the surface and, very often, poorly conductive. As demonstrated by the disclosed examples, in-situ formation of a natural surface catalyst through exemplary atomically thin catalytically active GaON species on the sidewalls of GaN nanowire / Si photoelectrodes in a two-electrode configuration offers a compelling answer to this dilemma. Previously, GaON was produced by nitridation of Ga2O3, relying on prolonged annealing at high temperatures. This production pathway is energy-intensive, resulting in low yield and quality, inferior photocatalytic performance, and lower stability. The N-terminated GaN nanowires on Si disclosed herein have unique advantages, including N-rich m-plane sidewalls, strong ionic bonding, nearly perfect band alignment, and a defect-free single-crystal wurtzite structure.These GaON nanocluster species act as catalysts, improving charge carrier dynamics and operating efficiently for thousands of hours without requiring additional catalyst regeneration. Therefore, GaON nanoclusters on N-terminal GaN nanowires are an excellent platform for providing ultra-high stability and efficient surface charge transfer rates under two-electrode PEC conditions.

[0038] The ultra-stable in-situ self-improvement of GaN nanowire / Si photocathodes for PEC H2 generation reactions has been demonstrated. Morphology and light intensity are factors that can be used to enhance or promote the self-improvement effect. The physical and chemical properties of GaN nanowires establish a significant improvement compared to previous Si photocathodes. The GaN nanowire / Si photocathodes in the examples achieved a light emission of 25 mA / cm² without the use of an external co-catalyst. 2 With a photocurrent density exceeding 100% and a Faraday efficiency of approximately 100%, unprecedented ultra-long-term stability of 3000 hours was achieved under two-electrode conditions with AM1.5G 1 solar irradiation. During stability testing, the exemplary photocathode demonstrated a self-improving mechanism in the formation of novel oxynitride species on the m-plane of the GaN nanowire. DFT calculations revealed that the formation of Ga-ON species on the N-terminal GaN m-plane provides a natural atomic-scale reduction reaction site, as the emerging oxynitride species exhibit metallic properties. Even better, these localized metallic surface states induce downward band bending, which further promotes the reduction reaction.

[0039] Figure 6 shows system 100 for hydrogen generation by water splitting. System 100 may also be configured for other reactions. 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 that uses sunlight and / or other radiation to promote hydrogen generation and water splitting. The way in which the PEC system 100 is illuminated may vary. The wavelength and other characteristics of the radiation may vary accordingly.

[0040] The electrochemical system 100 includes one or more electrochemical cells 102. For ease of illustration and explanation, a single electrochemical cell 102 is shown. The electrochemical cell 102 and other components of the electrochemical system 100 are also schematically shown in Figure 1 for ease of explanation. The cell 102 contains an electrolyte solution 104. In some cases, CO2 and / or other sources are applied. In some cases, the electrolyte solution is saturated with CO2. Potassium bicarbonate KHCO3 can be used as the electrolyte. Additional or alternative electrolytes may be used, as described below. Further details regarding an example of the electrochemical system 100 are provided below.

[0041] In the example shown in 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 an electrolyte 104. The counter electrode 110 is IrO x It may be formed as follows. The configuration of the counter electrode 110 can vary. For example, the counter electrode 110 may be formed as an oxidation of water (2H2O⇔O2+4e - +4H + The photoanode may be configured as the one in which the photoanode occurs, or it may otherwise contain a photoanode. The configuration of the electrochemical cell can vary.

[0042] Hydrogen generation occurs at the working electrode 108 as follows:

[0043] Hydrogen generation: 2H + +2e - ⇔H2 Therefore, electrons can flow from the counter electrode 110 through the external circuit path of the electrochemical cell 102 and reach the working electrode 108. Thus, the working electrode 108 and the counter electrode 110 can be considered as the cathode and anode, respectively.

[0044] In the example shown in Figure 1, the working electrode and the counter electrode are separated from each other by a membrane 114, such as a proton exchange membrane. The structure, composition, configuration, and other properties of the membrane 114 may vary.

[0045] In some cases, for example in unsupported systems, no bias voltage is applied. In the example in Figure 6, the circuit path includes a voltage source 116 of the electrochemical system 100. The voltage source 116 is configured to apply a bias voltage between the working electrode 108 and the counter electrode 110. The bias voltage may be used to establish the 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.

[0046] In this example, the working electrode 108 is configured as a photocathode. Light 118, such as solar radiation, can be incident on the working electrode 108 as shown in the figure. Thus, the electrochemical cell 102 can be considered a photoelectrochemical cell and can be configured as such. In such a case, irradiation of the working electrode 108 can generate a charge carrier within the working electrode 108. Electrons that reach the surface of the working electrode 108 can then be used for hydrogen generation. Photogenerated electrons can enhance electrons supplied through the current path. Alternatively or additionally, electrons supplied through the current path can recombine with photogenerated holes at the back surface or other contacts. Further details regarding an example of a photocathode are given below.

[0047] The working electrode 108 includes a substrate 120. The substrate 120 of the working electrode 108 can constitute part of the architecture, scaffolding, or other support structure of the working electrode 108. The substrate 120 may be uniform or composite. For example, the substrate 120 may contain any number of layers or other components. Therefore, the substrate 120 may be monolithic or not. The shape of the substrate 120 can also vary. For example, the substrate 120 may not be planar or flat.

[0048] In the example in Figure 1, the substrate 120 is doped and otherwise configured to exhibit a junction. Thus, the substrate 120 of the working electrode 108 may be active (functional) with respect to the photogeneration of charge carriers. Alternatively or additionally, the substrate 120 is passive (e.g., structural). The substrate 120 can be configured and act as a support structure for the catalytic arrangement of the working electrode 108, as described below. Alternatively or additionally, the substrate 120 may be composed of a material suitable for the growth or other deposition of the catalytic arrangement of the working electrode 108, or otherwise may include such a material.

[0049] In active or functional cases, the substrate 120 may include a light-absorbing material. The light-absorbing material is configured to generate charge carriers in the event of sunlight or other illumination. The light-absorbing material has a band gap such that incident light generates charge carriers (electron-hole pairs) within the substrate. Part or all of the substrate 120 may be configured for photogenerating electron-hole pairs. For this purpose, the substrate 120 may include a semiconductor material. In some cases, the substrate 120 is composed of or contains silicon. For example, the substrate 120 may be provided as a silicon wafer.

[0050] Silicon may be doped. In the example in Figure 1, the substrate 120 includes a highly concentrated n-type doped layer 122, a moderately or moderately concentrated p-type doped layer 123, and a highly concentrated p-type doped layer 124. The arrangement of layers 122-124 establishes a bond within the substrate 120. The doping arrangement can vary. For example, one or more components of the substrate 120 may be undoped (intrinsic) or substantially undoped. The substrate 120 may include alternative or additional layers, such as support layers or other structural layers. In other cases, the substrate 120 is not light-absorbing.

[0051] The substrate 120 of the working electrode 108 establishes a surface on which numerous structures are provided. Each structure may be placed on the substrate 120 or supported by the substrate 120. In this example, the structures are configured to provide a catalytic arrangement. In this case, the structures are configured to provide a catalytic function. As described herein, the structures do not necessarily include co-catalysts such as catalytic nanoparticles. The structures may include arrays of conductive protrusions extending outward from the surface of the substrate 120.

[0052] In the example shown in Figure 1, the working electrode 100 includes an array of nanostructures 126 (or other conductive protrusions) supported by a substrate 120. Each nanostructure 126 is configured to extract charge carriers (e.g., electrons) from the substrate 120. The extraction transports the electrons along the nanostructure 126 to an external site for use in hydrogen generation. In some cases, each nanostructure 126 is configured as a nanowire.

[0053] Each nanostructure 126 and / or other structure supported by the substrate 120 has a semiconductor composition. In some cases, the semiconductor composition is a semiconductor core 128, or otherwise comprises the semiconductor core 128. For example, the core 128 may be composed of, or comprise, a III-V nitride semiconductor material such as gallium nitride (GaN). Additional or alternative semiconductor materials may be used, for example, including indium gallium nitride (InGaN) and / or other III-V nitride semiconductor materials.

[0054] The core 128 of each nanowire or other nanostructure 126 may be or include a columnar, post-shaped, or other elongated structure extending outward (e.g., upward) from the plane of the substrate 120. 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 incorporated herein by reference. Nanostructures 126 may be referred to herein as nanowires, with the understanding that the dimensions, size, shape, composition, and other properties of the nanostructures 126 or other conductive protrusions may vary.

[0055] Each semiconductor composition of nanostructure 126 may or may not be configured to facilitate a reaction(s) supported by the electrochemical system 100. In some cases, the semiconductor composition may be configured for photogeneration of charge carriers. Alternatively or additionally, the semiconductor composition may be configured to act as a catalyst for the reaction. The semiconductor composition can provide other functions, such as protection of the substrate 120. As described above, the semiconductor composition may include GaN and / or InGaN. Further details regarding some examples including GaN are provided below. Additional or alternative semiconductor materials may be used, for example, including indium nitride, aluminum nitride, boron nitride, aluminum oxide, silicon, and / or alloys thereof.

[0056] 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 on one or more surfaces 130 of each nanostructure 126. Thus, the surface 130 can be considered to contain nitrogen. For example, in some cases, the semiconductor composition is terminated with nitrogen. Nitrogen termination or other nitrogen-based embodiments of the nanostructure 126 can protect the nanostructure 126 and / or other components of the electrode 108 (e.g., the substrate 120) from, for example, corrosion during operation. In other cases, alternative or additional nitrogen-based protection schemes can be used. For example, a nitrogen-containing layer may be deposited or otherwise arranged along the surface 130 of each nanostructure 126 and / or other elements of the electrode 108. As described herein, nitrogen can be present on the surface in a variety of ways. In some cases, nitrogen may be located below the surface (e.g., about 1 nm below) but close enough to the surface to be functionally active (e.g., chemically active for the formation of an oxynitride layer). Accordingly, the terms “at,” “present,” and “presence” are used herein to include a variety of examples in which nitrogen is functionally present on a surface, thereby including situations in which nitrogen atoms are positioned close enough to the surface to be functional along the surface or for activity at the surface (e.g., chemically functional in relation to oxidation reactions at the surface), rather than being limited to cases in which the positioned nitrogen is positioned only on the surface.

[0057] The nanostructures 126 can facilitate hydrogen generation and / or other chemical reactions in one or more ways. For example, each nanostructure 126 may be configured to extract charge carriers (e.g., electrons) generated on the substrate 120. The extraction carries the electrons to external sites along the surface 130 of the nanostructure 126 for use in hydrogen generation and / or other chemical reactions. The composition of the nanostructures 122 may also form interfaces well suited for hydrogen generation and / or other chemical reactions, as described below.

[0058] Each nanostructure 126 is or may be a columnar, post-shaped, or other elongated structure extending outward (e.g., upward) from the plane of the substrate 120. The dimensions, size, shape, composition, and other properties of the nanostructures 126 may vary. For example, each nanostructure 126 may be elongated like a nanowire or not. Therefore, other types of nanostructures from the substrate 120, such as nanocrystals of various shapes, can be used.

[0059] In some cases, the nanostructure 126 is configured to generate electron-hole pairs upon irradiation. For example, the nanostructure 122 may be configured to absorb light of a different frequency than other light-absorbing components of the electrode 108. For example, one light-absorbing component, such as the substrate 120, may be configured to absorb light in the visible or infrared wavelength range, while another component may be configured to absorb light at ultraviolet wavelengths. In other cases, the nanostructure 126 is the sole light-absorbing component of the electrode 108. In yet another case, the substrate 120 is the sole light-absorbing component of the electrode 108.

[0060] Each nanostructure 126 may include a layered or segmented arrangement of semiconductor material. For example, in the case of a group III nitride, the layers or segments of the arrangement may have different group III (e.g., indium and gallium) compositions. One or more layers or segments in the arrangement may be configured for absorption in their respective wavelength ranges. Other layers or segments may be directed to establish tunnel junctions. Each nanostructure 126 may also include additional or alternative segments.

[0061] In other cases, layered arrangements of semiconductor materials are also used to establish multiband structures, such as quadruple-band structures. Each layer or segment of the configuration may have a different semiconductor composition to establish different band gaps. Different band gaps may be useful in relation to absorbing light of different wavelengths. Further details regarding the formation and configuration of multiband structures, including, for example, 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 disclosure of which is incorporated by reference. Other layered or non-layered arrangements may be used in the semiconductor composition of the structures of the disclosed devices.

[0062] The semiconductor composition of each nanowire 126 may be configured to improve the efficiency of water splitting in additional ways. For example, the semiconductor composition of each nanowire 126 may optionally include doping to facilitate the separation and extraction of charge carriers and to facilitate the establishment of photochemical diodes. For example, the dopant concentration of the semiconductor composition may vary laterally.

[0063] In examples involving Group III nitride compositions, the dopant may be magnesium or may contain magnesium. Further details regarding methods by which magnesium doping facilitates the separation and extraction of charge carriers are described 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. Depending on the selected semiconductor light absorber, additional or alternative dopant materials, including, for example, silicon, carbon, and beryllium, may be used.

[0064] In other cases, the working electrode 108 may further include catalyst nanoparticles arranged on an array of nanostructures 126. For example, the nanoparticles may be distributed across or along the outer surface 130 (e.g., the sidewall) of each nanowire 126. In some cases, the nanoparticles are configured to promote or facilitate a proton reduction reaction. For example, each nanoparticle may be composed of a metal such as platinum, or otherwise contain one. Other metals or materials may be used, including alloys, oxides, and / or other metals or combinations of metals. Further details regarding the formation, composition, functionality, and other properties of nanoparticles in combination with the nanowire array are described in one or more of the U.S. patents referenced above. Further details regarding the distribution of nanoparticles are provided below in relation to several embodiments.

[0065] 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 stabilizing and activating functions to electrode 108. Thus, the stabilizing layer 132 may be configured as an activating layer or a stabilizing-activated layer. The term “activating” is used to distinguish an activating layer from layers configured to act only as passivation or other protective layers, as well as other inactive layers. In this example of electrode, the stabilizing function may include protection against corrosion and / or other passivation or protection. The activating function may include defining or otherwise establishing catalytic sites along the surface 130 of electrode 108. The nature of the stabilizing and activating functions may vary in the context of other devices.

[0066] The stabilizing layer 132 may be composed of an oxynitride material, or otherwise may contain an oxynitride material. Therefore, the stabilizing layer 132 can be referred to as the oxynitride layer in this specification.

[0067] In some cases, the stabilizing layer 132 has a uniform distribution of oxynitride material. For example, if the nanostructure 126 has a GaN core, the stabilizing layer 132 contains gallium oxynitride. Therefore, the composition of the oxynitride layer can vary according to the semiconductor composition of the nanostructure 126. Thus, alternative or additional oxynitride materials may be used, including, for example, aluminum oxynitride, indium oxynitride, strontium gallium oxynitride, zinc oxynitride, titanium oxynitride, chromium oxynitride, and silicon oxynitride.

[0068] The distribution of the oxynitride material can be uniform in various ways. For example, uniformity can establish the thickness of the stabilizing layer 132, or otherwise be related to the thickness of the stabilizing layer 132. Alternatively or additionally, uniformity can establish the conformability of the stabilizing layer 132, or otherwise be related to the conformability of the stabilizing layer 132. Alternatively or additionally, uniformity can be related to the consistency, regularity, or smoothness of the surface of the stabilizing layer 132. For example, surface roughness can be reduced as a result of a uniform distribution of the oxynitride material. In some cases, the oxynitride material of the stabilizing layer 132 may be continuously distributed over the surface 130 (e.g., continuously covering the entire surface 130).

[0069] Alternatively or additionally, uniformity may relate to the presence of nitrogen atoms on the surface 130. For example, if nitrogen atoms are distributed on the surface, or otherwise randomly across the surface, uniformity refers to the essentially universal or comprehensive (as opposed to local or restrictive) formation of oxynitride moieties around those nitrogen atoms. Thus, the phrase “uniform distribution” is used herein to include both continuous and discontinuous examples of the stabilizing layer 132. For example, in some cases, adjacent nitrogen atoms may be spaced far enough apart to establish discontinuity within the stabilizing layer 132. Thus, the stabilizing layer 132 (and therefore the oxynitride material) may, in some cases, not be continuous on the surface 130, but still have a uniform distribution of the oxynitride material with respect to nitrogen moieties on the surface 130. Alternatively or additionally, the surfaces 130 (or sections of a single surface 130) of the nanostructure 126 on which the stabilizing layer 132 is located may be spaced apart from each other, thereby establishing separate sections of the stabilizing layer 132. As these examples show, the stabilizing layer 132 can be continuous or discontinuous despite the uniform nature of the oxynitride material distribution.

[0070] The term “present” is not limited to situations where nitrogen atoms, stabilizing layers, and / or oxynitride materials are located on the surface. These terms are instead used to include situations where nitrogen atoms, stabilizing layers, and / or oxynitride materials are located below the surface but close enough to the surface to function for surface activity. Therefore, nitrogen atoms, stabilizing layers, and / or oxynitride materials may, in some cases, be located slightly below the surface (e.g., about 1 nm below), but may also be located close enough to the surface for the formation of the oxynitride material and the resulting surface stabilization.

[0071] The formation and / or presence of oxynitride material may originate from nitrogen on surface 130. In some cases, the nitrogen on surface 130 is located within a compound semiconductor arrangement of the semiconductor composition. For example, in GaN and other examples, the semiconductor composition has a wurtzite crystal structure. In some cases, the nonpolar surfaces of the wurtzite crystal structure may be nitrogen-terminated. The stabilizing layer 132 may be provided or constituted with partial oxygen substitution of nitrogen along the nonpolar surface (and / or other surfaces in other cases). Thus, surface 130 may correspond to the surface of a nanostructure 126 oriented along the nonpolar surface of the semiconductor composition. In other cases (e.g., the example of a transistor), surface 130 is oriented along the polar surface of the semiconductor composition.

[0072] The semiconductor composition and / or crystal orientation on the surface 130 of the nanostructure 126 can selectively determine where the stabilizing layer 132 is positioned on each nanostructure 126. Thus, the stabilizing layer 132 may or may not cover the entire surface of the nanostructure 126. In the example in Figure 1, the surface 130 of each nanostructure 126 corresponds to a side wall 134. The nanostructure 126 also includes a top surface or upper surface 136. In this example, the stabilizing layer 132 is positioned on the side wall 134 but not on the top surface or top surface 136. The side wall 134 is nitrogen-terminated, while the top surface 136 is not, thereby resulting in selective or partial covering of the nanostructure 126. The surface 130 covered by the stabilizing layer 132 may vary according to the crystal orientation of the semiconductor composition of the nanostructure 126.

[0073] The stabilizing layer 132 may be thin enough to avoid adverse effects on charge transport or other activities along or otherwise on 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 can conformally cover the nanostructure 126, which may contain any nanoparticles placed on it. The thickness may vary, in other embodiments, to include thicknesses of, for example, about 1 nm (e.g., up to about 3 nm) or less than 1 nm. For example, the thickness may be on the order of one monolayer or nearly one monolayer (e.g., one pair or several monolayers). Thus, the thickness may vary from about 0.3 nm to about 3 nm. The thickness does not have to be exactly the same throughout the stabilizing layer 132.

[0074] The thin nature of the stabilizing layer 132 allows for the provision of stabilization functionality without adversely affecting charge carrier movement, other catalytic activities of hydrogen generation, and / or other reactions occurring at electrode 108. Further details regarding the function and other properties of the stabilizing layer 132 are provided below in relation to several examples.

[0075] In the example in Figure 1, surface 130 does not contain catalyst nanoparticles. In other cases, several nanoparticles may be distributed across surface 130. The nanoparticles may have a composition such that they act as catalysts for electrochemical reactions or other reactions, and / or may not be otherwise configured. In other cases, the nanoparticles may have additional or alternative purposes, including, for example, charge transport.

[0076] The nanostructure 126 and stabilizing layer 132 are not shown to scale in the schematic diagram of Figure 1. The shape of the nanostructure 126 may differ from the illustrated example of nanowires. Further details regarding the nanostructure 126 and stabilizing layer 132, including their fabrication or formation, are provided below.

[0077] Nanowire arrays may be fabricated on a substrate (e.g., a silicon substrate) by nanostructure engineering. In one example, molecular beam epitaxial (MBE) growth of nanowires is followed by photodeposition of nanoparticles. The photodeposition of nanoparticles may be configured to selectively deposit nanoparticles on each side of the nanowire. Further details regarding exemplary fabrication procedures are provided below, for example, in reference to Figure 7.

[0078] The nanostructure 126 can facilitate water splitting in one or more ways. For example, one or more embodiments of each nanostructure 126 may be configured to extract charge carriers (e.g., electrons) generated within the substrate (e.g., as a result of light absorbed by the substrate 120). The extraction transports the charge carriers along the nanostructure 126 to external sites for use in water splitting or other reactions. Thus, for example, the nanostructure 126 can form an interface well suited for hydrogen generation, CO2 reduction, and / or other reactions.

[0079] Figure 7 shows an example of method 200 for manufacturing a photoelectrode or other semiconductor device for photocatalytic water splitting, PEC water splitting, or other photocatalytic reactions. Method 200 may be used to manufacture any of the photoelectrodes or other devices described herein, or any other device. Method 200 may include additional, fewer, or alternative operations. For example, Method 200 may include or not include one or more operations (operation 228) that concern forming back surface contacts of the device.

[0080] Method 200 can begin with operation 202, in which the substrate is prepared or otherwise provided. The substrate may be a silicon wafer or may be formed from a silicon wafer. In one example, a 2-inch Si wafer was used, but other (e.g., larger) wafer sizes may be used. Other semiconductors and substrates may be used.

[0081] The substrate may have a planar or non-planar surface. In some cases, act 202 includes act 204 in which a wet or other etching procedure is performed to define the surface. For example, the etching procedure may be or include a crystallographic etching procedure. In the example of a silicon substrate, the crystallographic etching procedure may be a KOH etching procedure. In some cases, the substrate may be <100> If orientation is present, the wet etching procedure is performed on the surface <111> It can be established that the surface includes a pyramidal texture surface having surfaces oriented along its contours. Other surface orientations and shapes may be used.

[0082] Operation 202 may include fewer, additional, or alternative operations. For example, in the example in Figure 1, operation 202 includes operation 206 in which the substrate is cleaned (e.g., by acetone and / or methanol) and operation 208 in which oxides are removed (e.g., by 10% hydrofluoric acid).

[0083] In one example, a double-sided polished 4"p-type Si(100) wafer is first RCA-cleaned, then loaded into a CMOS-grade oxidation furnace, and 250-300 nm thick SiO(100) is produced at 1100°C. x The oxide was formed. The oxide grew on both sides of the double-sided polished wafer. Subsequent lithography and wet etching steps resulted in only one side of the wafer being SiO2. x It had (the other side was exposed p-Si). After another RCA cleaning, the wafer was placed in a CMOS-grade phosphorus diffusion furnace and heated at 950°C for 20 minutes. + -Si doping was formed. Then, these wafers were washed in a buffered HF solution for 5-10 minutes, n + -SiO formed on the Si side x The residual silicates were then removed. The 4” wafers were then diced into quarter wafers, washed according to a standard solvent / acid protocol, and loaded into an MBE chamber for nanostructure growth.

[0084] Operation 202 may include further operations. For example, operation 202 may include one or more doping steps to form doped regions or layers and thereby establish a junction, as described herein. Alternatively, the substrate is first provided with a desired dopant concentration profile. In some cases, the substrate is doped to form a solar cell.

[0085] Method 200 includes an act 210 in which several electrodes or other device structures are grown or otherwise formed on a substrate. In some cases, nanowires or other nanostructure arrays are grown or otherwise formed on the substrate. Each nanowire is formed on the surface of the substrate such that each nanowire extends outward from the surface of the substrate. Each nanostructure may have a semiconductor composition as described herein. In some cases, the semiconductor composition is or includes a compound semiconductor material such as a III-V semiconductor.

[0086] The growth of nanostructures can be achieved in act 212, in which a molecular beam epitaxy (MBE) procedure is performed. The substrate may be rotated during the MBE procedure so that each nanostructure is formed as a cylindrical nanostructure. Thus, each nanostructure may have a circular cross-sectional shape, as opposed to plate-like or sheet-like nanostructures.

[0087] The MBE procedure can be carried out under nitrogen-rich conditions. Nitrogen-rich conditions may result in nitrogen-terminated sidewalls and / or other surfaces, as described herein.

[0088] In some cases, the MBE procedure can be modified to fabricate the arrangement of each nanowire layer or segment. Different composition levels of segments can be achieved by adjusting various parameters. For example, the substrate temperature can be adjusted in operation 214. The beam equivalent pressure may be adjusted alternatively or additionally. In some cases, the dopant cell temperature can be adjusted to control the doping of the nanowires (e.g., Mg doping).

[0089] In one example, n on a Si substrate + - Before starting GaN nanowire growth, the substrate temperature should be set to approximately 735°C and the Ga beam equivalent pressure (BEP) to approximately 2.2 × 10⁻⁶. -7 Torr grew an N-terminated GaN pseudo-thin film on a Si substrate for 15 minutes using a Si cell (n-type dopant) at a temperature of 1250°C and a nitrogen flow rate of 0.45 standard cubic centimeters per minute (sccm). The incorporation of a thin GaN quasi-film helps protect the Si surface during photoelectrochemical reactions. Nanowires were then grown on the quasi-film under nitrogen-rich conditions to provide an N-terminated surface. The nanowire growth conditions were a substrate temperature of approximately 735°C and 6 × 10⁻¹⁶ nanowires. -8 This included a Torr Ga BEP, a Si cell at 1250°C, a nitrogen flow rate of 1 sccm, a forward plasma power of 350 W, and a growth duration of 4-5 hours. Growth parameters can vary considerably in other cases.

[0090] In other cases, operation 210 may include forming additional or alternative layers or other parts of the electrode. The layer(s) may be configured to catalyze, participate in, or otherwise enable or facilitate a reaction. In operation 216, the layer(s) or other parts may be deposited on the substrate. In some cases, the layer(s) may establish a surface configured to catalyze or otherwise facilitate water splitting and / or other reactions. The electrode layer(s) may be formed by alternative or additional methods, for example, including non-selective growth procedures.

[0091] In operation 218, one or more nitrogen-containing layers may be deposited or otherwise formed. The nitrogen layers may be directed to protect the reaction layer(s) and / or other embodiments of the nanostructure. The nitrogen-containing layers may, alternatively or additionally, be intended to assist in the formation of a stabilizing layer. In some cases, operation 218 may include carrying out a reaction to spontaneously form the nitrogen-containing layer(s). The layers may consist of or otherwise contain nitrides or oxynitrides.

[0092] In some cases, action 218 may be integrated with or implemented in conjunction with action 216, as described herein in conjunction with embodiments involving GaN or other nanostructures. In other cases involving GaN nanostructures, actions 216 and 218 may be performed separately.

[0093] Method 200 may further include the act 220 of incorporating oxygen into one or more surfaces of a device nanostructure or other structure to form a stabilizing layer on one or more surfaces. In some cases, the surface is oxidized. For example, an oxidation reaction may be carried out to form the stabilizing layer. Alternatively or additionally, oxygen is deposited on the surface. Following deposition, an annealing procedure may be performed to fully incorporate the oxygen atoms. The stabilizing layer may be configured as an activation layer or a passivation layer. The function of the stabilizing layer may vary depending on the nature of the device structure. As described herein, the oxidation of the surface (or other incorporation of oxygen into the surface) may be carried out such that the stabilizing layer consists of or otherwise contains a uniform distribution of oxynitride material. For example, the oxidation reaction may be configured such that the stabilizing layer contains a uniform distribution. In some cases, oxygen is incorporated into the surface such that the oxynitride material is continuously distributed across the entire surface (e.g., continuously covering the entire surface).

[0094] In some cases, the oxynitride layer is formed in act 222, in which an electrochemical procedure is performed. The electrochemical procedure can be carried out for a period of several hours, for example, about 5 to about 10 hours. The duration of the procedure may be long enough to establish a uniform distribution of the oxynitride material. For example, in other cases, oxidation may be carried out for a period of several minutes. Further details are provided in relation to some of the following embodiments.

[0095] In some cases, the electrochemical procedure involves, or otherwise includes, an electrochemical reaction that the device is configured to perform. In such cases, operation 222 may be configured as an initialization step or procedure for the device. For example, the device may be configured or directed to perform water splitting or other hydrogen evolution reactions. The electrochemical procedure may then include performing a water splitting reaction in which the device is immersed in water. The hydrogen evolution reaction may be carried out for a predetermined time to form an oxynitride layer of a desired thickness. In other cases, the electrochemical procedure is different from the electrochemical reaction that the device is configured to perform. For example, the electrochemical procedure may be, or include, a water splitting reaction, while the device is CO 2 This refers to carrying out other reactions, such as reduction.

[0096] Alternatively or additionally, act 220 includes exposing the surface to oxygen in act 224. In some cases, the oxygen exposure is carried out in an oxygen-containing reaction chamber, such as a plasma asher. Alternatively or additionally, the exposure includes the ambient environment. The exposure may last for a predetermined period, e.g., on the order of minutes or hours, but other periods may be used. The duration of exposure may be selected to produce an oxynitride layer of the desired thickness. The duration of exposure may be selected to establish a uniform distribution of the oxynitride material.

[0097] The oxygen incorporation in operation 220 may alternatively or additionally include operation 226 in which an oxygen-containing material is deposited on the surface. For example, the oxygen-containing material may consist of or otherwise contain an oxide material such as aluminum oxide, hafnium oxide, or an oxygen-containing ferroelectric layer (e.g., HfZrO2), but other materials may be used.

[0098] Operation 220 may include additional operations combined with any one or more of the techniques described above for exposing the surface to oxygen (or otherwise incorporating oxygen into the surface). In the example in Figure 2, operation 220 includes performing one or more cleaning procedures or other surface treatments in operation 225. The surface is treated in operation 225 before exposure of the surface to oxygen (e.g., operation 224) or deposition of an oxygen-containing layer (e.g., operation 226). In some cases, operation 225 includes a field pre-cleaning procedure such as a chemical treatment of the surface. Various chemicals, including, for example, HF, buffered HF, and / or sulfuric acid, can be used for treatment. Operation 225 may include alternative or additional cleaning or treatments such as ultraviolet (UV) ozone treatment. Yet another type of procedure may be used, for example, including other types of surface treatments configured to improve interfaces on the surface.

[0099] Operation 220 may optionally or additionally include Operation 227 in which the annealing procedure is carried out. In some cases, the annealing may be carried out in an oxygen environment to provide an oxygen source. Alternatively, the annealing may follow the deposition of an oxygen-containing layer in Operation 226 or other provision of oxygen to the surface. The annealing may be configured to incorporate oxygen into the surface (e.g., through substitution in the lattice of the structure). The temperature of the annealing procedure may vary. For example, the temperature may be in the range of about 100°C to about 500°C, but other temperatures may be used. The duration of the annealing may vary accordingly. The temperature, duration, and / or other properties of the procedure may be selected to establish a uniform distribution of the oxynitride material. In some cases, the annealing procedure may not include an oxygen environment if oxygen is made available by another means, for example, deposition (e.g., deposition of oxide material).

[0100] Surface oxidation may be made possible, enhanced, or otherwise possible due to one or more aspects of the surface. For example, the surface may be oriented along a non-polar surface of the semiconductor composition. In other cases, the surface is oriented along a polar surface. Alternatively or additionally, the semiconductor composition may be configured such that the oxidation reaction results in partial oxygen substitution of nitrogen. In such cases, the nitrogen on the surface may be arranged in the compound semiconductor configuration of the semiconductor composition. The semiconductor composition may have a wurtzite crystal structure.

[0101] In other cases, method 200 does not explicitly involve the implementation of a procedure dedicated to oxidation. For example, oxidation may occur during the implementation of a photoelectrochemical procedure used to form an oxynitride layer.

[0102] Method 200 includes act 230 in which a photochemical procedure is carried out to form an oxynitride layer. The oxynitride layer may be formed on a nonpolar surface of the structure as described herein. The carry out of the photoelectrochemical procedure may result in partial oxygen substitution of nitrogen.

[0103] In some cases, performing a photochemical procedure may involve performing a water splitting reaction in which the device is immersed in water in act 232. The water splitting reaction or other photoelectrochemical procedure may be performed for a period of several hours. The duration may vary. For example, this procedure may be performed for a period of several minutes, as described above.

[0104] Operation 230 may include, in operation 232, irradiating the photoelectrode with radiation having an intensity corresponding to focused sunlight. In some cases, the intensity of the radiation corresponds to at least about 9 times the sunlight of the sun. In other cases, lower or higher intensities may be used.

[0105] Performing the photoelectrochemical procedure as described herein may also include continuously circulating the electrolyte in act 234.

[0106] Method 200 may include additional, fewer, or alternative operations. For example, Method 200 may include one or more additional operations that aim to form further structures, components, or other features of the photocathode.

[0107] As described above, Method 200 may lack operations targeting the deposition of nanoparticles or other co-catalysts on the nanowire. Method 200 may also lack operations targeting the deposition of a passivation layer on the nanowire.

[0108] This specification describes an example of a photoelectrode having long-term stable operation in a system having a two-electrode configuration. An example is a semiconductor photocathode having elements composed of Si and GaN, two of the world's most produced semiconductors. The example demonstrated stable operation for 3,000 hours without performance degradation in a two-electrode configuration. Detailed measurements in both three-electrode and two-electrode configurations show that the surface of the GaN nanowire on the Si photocathode is converted to Ga-ON during the first few hours of the reaction, protecting the photocathode from photocorrosion and enhancing the hydrogen evolution reaction. Factors such as morphology and irradiation intensity contribute to the effectiveness and rate of surface modification. The process of Ga-ON species formation is also described. The in-situ formed Ga-ON species may exhibit atomic-scale surface metallization. The disclosed photocathode also provides an example that avoids the need to rely on additional catalysts. The example also avoids the stability bottleneck of semiconductor photoelectrodes, thereby supporting the realization of photoelectrochemical devices and systems for clean energy.

[0109] The composition of oxynitride materials can vary. For example, the ratio of nitrogen to oxygen can change. Therefore, in some cases, the oxygen composition may change (e.g., decrease) as the distance from the surface increases. Oxynitride materials are O x N 1-x It can have various oxygen concentrations such as GaO. For example, in a device having a GaN device structure, the oxynitride material is GaO x N 1-xIt may be, or otherwise GaO x N 1-x This may include. Therefore, the stabilizing layer in such cases may include gallium oxynitride combined with a very thin layer (e.g., less than 1 nm) of gallium and oxide (e.g., GaO on top of the gallium oxynitride, where the oxygen concentration decreases as the distance from the surface increases). x N 1-x (in the region of). Therefore, references to oxynitride materials in the stabilization layer may include materials having varying levels of oxygen content. The oxygen content level may vary due to several factors, including, for example, surface orientation, the surrounding environment (e.g., electrolyte, metal layer, passivation layer, etc.), and / or processing conditions. In any case, the oxynitride material in the stabilization layer provides surface stabilization and improved device performance, as described below. The oxynitride material may be modified in alternative or additional ways. For example, nitrogen and oxygen may be disordered. In some cases, the positions of nitrogen and oxygen may be swapped or otherwise altered.

[0110] The term "approximately" is used herein to include deviations from a given value that are substantially the same as the given value, including, for example, deviations that do not result in a detectable or identifiable change in the result.

[0111] While this disclosure has been described with reference to specific embodiments, these embodiments are illustrative and not limiting. Examples may be modified, added to, and / or deleted without departing from the spirit and scope of this disclosure.

[0112] The above explanation is given solely to clarify understanding, and no unnecessary limitations should be inferred from it.

Claims

1. It is an electrochemical system, Counter electrode and The system comprises a working electrode positioned at a distance from the counter electrode, The aforementioned working electrode is circuit board and The system comprises an array of conductive protrusions supported by the substrate and extending outward from the substrate, Each conductive protrusion in the array of conductive protrusions has a semiconductor composition and a surface containing nitrogen. The working electrode further comprises an oxynitride layer disposed on the surface, The counter electrode and the working electrode are arranged in a two-electrode configuration in this electrochemical system.

2. The electrochemical system according to claim 1, wherein the oxynitride layer includes localized oxynitride nanoclusters.

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

4. The electrochemical system according to claim 1, wherein the working electrode does not include a cocatalyst in addition to the array of conductive protrusions.

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

6. The electrochemical system according to claim 1, wherein the working electrode is configured such that its entire surface is exposed to the electrolyte during operation.

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

8. The electrochemical system according to claim 1, wherein the surface is oriented along the non-polar surface of the semiconductor composition.

9. The electrochemical system according to claim 1, wherein the semiconductor composition has a wurtzite-type crystal structure.

10. The aforementioned surface includes a side wall, The oxynitride layer is arranged along the side wall, The electrochemical system according to claim 1, wherein the side wall is a nitrogen terminator.

11. A photoelectrode, circuit board and The system comprises an array of conductive protrusions supported by the substrate and extending outward from the substrate, wherein each conductive protrusion in the array of conductive protrusions has a semiconductor composition and a nitrogen-containing surface. The photoelectrode further comprises an oxynitride layer disposed on the surface, A photoelectrode in which the aforementioned surface is not passivated.

12. The photoelectrode according to claim 11, wherein the oxynitride layer includes localized oxynitride nanoclusters.

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

14. The photoelectrode according to claim 11, wherein the surface does not contain a co-catalyst.

15. The aforementioned surface includes a side wall, A stabilizing layer is arranged along the side wall, The photoelectrode according to claim 11, wherein the side wall is nitrogen-terminated.

16. The substrate contains silicon, The photoelectrode according to claim 11, wherein the semiconductor composition of the structure comprises gallium nitride such that the oxynitride material is gallium oxynitride.

17. A method for manufacturing a photoelectrode, The steps include providing the substrate for the photoelectrode, The method comprises the step of forming an array of conductive protrusions supported by the substrate and extending outward from the substrate, wherein each conductive protrusion in the array of conductive protrusions has a semiconductor composition and a nitrogen-containing surface, and further The procedure includes carrying out a photoelectrochemical procedure to form an oxynitride layer on the surface, The method for performing the aforementioned photoelectrochemical procedure includes irradiating the photoelectrode with radiation having an intensity corresponding to focused sunlight.

18. The method according to claim 17, wherein the intensity of the radiation corresponds to at least about 9 times the sunlight of the sun.

19. The method according to claim 17, wherein the photoelectrochemical procedure is carried out for a period of several hours.

20. The method according to claim 17, wherein the photoelectrochemical procedure is carried out for a period of time of several minutes.

21. The method according to claim 17, wherein carrying out the aforementioned photoelectrochemical procedure includes carrying out a water splitting reaction in which the device is immersed in water.

22. The method according to claim 17, wherein the photoelectrochemical procedure includes continuously circulating the electrolyte.

23. The method according to claim 17, wherein forming the array of conductive protrusions involves performing a molecular beam epitaxy (MBE) growth procedure such that each conductive protrusion in the array of conductive protrusions contains its own nanowire.

24. The method according to claim 23, wherein the MBE growth procedure is carried out under nitrogen-rich conditions such that the sidewalls of each conductive protrusion in the array of conductive protrusions are nitrogen-terminated.

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

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