Photoelectrochemical water splitting with concentrated solar light

Abstract

A method of fabricating an electrode device includes providing a substrate of the electrode device, the substrate having a surface, growing an array of conductive projections on the surface of the substrate such that each conductive projection of the array of conductive projections extends outward from the surface of the substrate, each conductive projection of the array of conductive projections including a nitride material, depositing a plurality of metal catalyst nanoparticles across the array of conductive projections, and implementing a surface treatment procedure to modify the plurality of metal catalyst nanoparticles. Implementing the surface treatment procedure includes removing a subset of the plurality of metal catalyst nanoparticles, and further depositing a further plurality of metal catalyst nanoparticles after removing the subset. The removed subset includes metal catalyst nanoparticles of the plurality of metal catalyst nanoparticles that are not lattice-matched with the nitride material.

Description

PHOTOELECTROCHEMICAL WATER SPLITTING WITH CONCENTRATED SOLAR LIGHTCROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of U.S. provisional application entitled “Photoelectrochemical Water Splitting with Concentrated Solar Light,” filed February 22, 2024, and assigned Serial No. 63 / 556,535, the entire disclosure of which is hereby expressly incorporated by reference.STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] This invention was made with government support under Contract No. W911 NF-21 - 1-0337 awarded by the U.S. Army. The government has certain rights in the invention.BACKGROUND OF THE DISCLOSUREField of the Disclosure

[0003] The disclosure relates generally to photoelectrodes, photocatalytic devices, and other devices for hydrogen evolution via water splitting and / or other reactions.Brief Description of Related Technology

[0004] Hydrogen (H2) stands as a clean energy source that can be produced through solar water splitting, offering a sustainable alternative to carbon-emitting fossil fuels. Over time, numerous semiconductor photoelectrodes have harnessed solar energy for the production of green H2through photoelectrochemical (PEC) water splitting. The photoelectrodes operating under solar light offer a voltage saving when compared to electrocatalysts operating in dark. However, unlike the electrochemical reactions, the maximum photocurrent density (JPh) is inherently constrained by the quantity of photogenerated charge carriers within the semiconductors, leading to limited H2production yield.

[0005] This production issue can be addressed by illuminating concentrated solar light, which can increase the number of photogenerated charge carriers, JPh, and H2production rate. This has been demonstrated by integration of photovoltaic-electrocatalyst (PV-EC)devices, offering the added benefit of reducing costs associated with catalysts, semiconductor light absorbers, and electricity.

[0006] Photoelectrodes have been fabricated by applying cocatalysts onto semiconductor materials. To date, high-efficiency semiconductor materials (Si and lll-V semiconductors) for PEC water splitting in an aqueous solution showed poor stability due to photo-corrosion. Therefore, passivation layers of amorphous oxides such as AI2O3 or TiC>2 have been deposited on the semiconductors prior to applying cocatalysts, such as Pt nanoparticle cocatalysts.

[0007] While Pt cocatalysts show high activity for HER, their low adhesion has caused instability and limited the long-term functionality of photoelectrodes. To mitigate this concern, researchers have explored solutions such as the use of reduced graphene oxide binder or metal oxide overlayer to encapsulate the Pt nanoparticles, thereby preventing their detachment. However, this approach, while enhancing the stability of cocatalysts, tends to block active sites and hinder the mass transfer of reactants and products. In a recent study, a hydrogel protective layer was employed for Pt / TiO2 / Sb2Se3 photocathode, resulting in improvement in the stability. The hydrogel protector prevented the agglomeration and detachment of Pt nanoparticles and suppressed the photo-corrosion of the TiC>2 passivation layer. While such protective schemes have resolved stability problems to some extent, efficient and stable PEC water splitting has remained elusive, especially in conditions in which concentrated solar light is used to accelerate the H2production rate.SUMMARY OF THE DISCLOSURE

[0008] In accordance with one aspect of the disclosure, a method of fabricating an electrode device includes providing a substrate of the electrode device, the substrate having a surface, growing an array of conductive projections on the surface of the substrate such that each conductive projection of the array of conductive projections extends outward from the surface of the substrate, each conductive projection of the array of conductive projections including a nitride material, depositing a plurality of metal catalyst nanoparticles across the array of conductive projections, and implementing a surface treatment procedure to modify the plurality of metal catalyst nanoparticles. Implementing the surface treatment procedure includes removing a subset of the plurality of metal catalyst nanoparticles, and further depositing a further plurality of metal catalyst nanoparticles after removing the subset. The removed subset includes metal catalyst nanoparticles of the plurality of metal catalyst nanoparticles that are not lattice-matched with the nitride material.

[0009] In accordance with another aspect of the disclosure, a method of fabricating an electrode device includes providing a substrate of the electrode device, the substrate having a surface, growing an array of conductive projections on the surface of the substrate such that each conductive projection of the array of conductive projections extends outward from the surface of the substrate, each conductive projection of the array of conductive projections including a nitride material, depositing a plurality of metal catalyst nanoparticles across the array of conductive projections, implementing a surface treatment procedure to modify the plurality of metal catalyst nanoparticles, in which implementing the surface treatment procedure includes removing a subset of the plurality of metal catalyst nanoparticles, and further depositing a further plurality of metal catalyst nanoparticles after removing the subset, and repeating the surface treatment procedure.

[0010] In accordance with yet another aspect of the disclosure, a device includes a substrate having a surface, an array of conductive projections supported by the substrate and extending outward from the surface of the substrate, each conductive projection of the array of conductive projections including a nitride material, and a plurality of metal catalyst nanoparticles disposed over the array of conductive projections. Each metal catalyst nanoparticle of the plurality of metal catalyst nanoparticles is lattice-matched with the nitride material. Each metal catalyst nanoparticle of the plurality of metal catalyst nanoparticles has an inner void.

[0011] In connection with any one of the aforementioned aspects, the devices and / or methods described herein may alternatively or additionally include or involve any combination of one or more of the following aspects or features. The method further includes repeating the surface treatment procedure. The surface treatment procedure is repeated at least five times. Removing the subset includes illuminating the electrode device in a photoelectrochemical reaction. Illuminating the electrode device includes irradiating the electrode device with concentrated solar light. Further depositing the further plurality of metal catalyst nanoparticles includes implementing a photo-deposition procedure. The nitride material includes a nitride semiconductor. The nitride material includes a Ill-nitride semiconductor. Each metal catalyst nanoparticle of the plurality of metal catalyst nanoparticles and of the further plurality of metal catalyst nanoparticles includes platinum. Removing the subset includes dissolving a spurious layer of a surface oxide of the nitride material on each conductive projection of the array of conductive projections. The metal catalyst nanoparticles of the removed subset include nanoparticles disposed on the dissolved spurious layer and nanoparticles adsorbed on the array of conductive projections. The surface treatment procedure is repeated at least five times. Removing the subsetincludes illuminating the electrode device in a photoelectrochemical reaction. Illuminating the electrode device includes irradiating the electrode device with concentrated solar light. Further depositing the further plurality of metal catalyst nanoparticles includes implementing a photo-deposition procedure. Each conductive projection of the array of conductive projections lacks a surface layer of an oxide of the nitride material. The nitride material includes a Ill-nitride semiconductor. Each metal catalyst nanoparticle of the plurality of metal catalyst nanoparticles and of the further plurality of metal catalyst nanoparticles includes platinum. A photoelectrochemical (PEC) system includes a working electrode configured in accordance with any one of the devices described herein, and further includes a counter electrode, an electrolyte in which the working and counter electrodes are immersed, and a voltage source that applies a bias voltage between the working and counter electrodes. The bias voltage is set to a level for production of hydrogen at the working electrode. A method of operating the PEC system includes immersing the working and counter electrodes in an electrolyte, and illuminating the working electrode with concentrated solar light.BRIEF DESCRIPTION OF THE DRAWING FIGURES

[0012] For a more complete understanding of the disclosure, reference should be made to the following detailed description and accompanying drawing figures, in which like reference numerals identify like elements in the figures.

[0013] Figure 1 depicts (a) a tilt-view SEM image of Pt-loaded GaN nanowires / n+-p Si photoelectrode, as well as graphical plots of (b) saturated current density (Jsaturation), onset potential (VonSet), and saturation potential (Vsaturation) with light intensity, (c) photocurrent density at 0 VRHE (JO), and (d) VonSet for Pt / GaN / Si and GaN / Si measured as a function of time under 1 sun and 6.4 sun light.

[0014] Figure 2 depicts AR-XPS analysis of Pt / GaN / Si and GaN / Si before and after reaction for 24 h under 6.4 sun light, including graphical plots of relative surface atomic ratio of (a) Pt / GaN / Si and (b) GaN / Si, along with graphical plots of XPS spectra of (c) Ga 3d, (d) O 1 s, and (e) Pt 4f.

[0015] Figure 3 depicts (a) a low-magnification HAADF-STEM image and (b) EDS elemental map of an example Pt / GaN / Si photoelectrode before a surface treatment reaction, (c) HAADF-STEM image after 24 h reaction under 6.4 sun light, (d) a high-resolution STEM image of Pt / GaN interface and inverse Fourier-filtered image by masking Pt (111), in which dislocations in Pt nanoparticles are indicated by respective marks, (e) graphical plots of d-spacing of three Pt nanoparticles in Figure 4, part d, and (f) a schematic view of lattice alignment at the Pt / GaN interface.

[0016] Figure 4 depicts (a) a schematic illustration of surface modification during the PEC HER, in which surface adsorbed Pt nanoparticles on a GaN or GaOxsurface are removed while lattice-matched Pt nanoparticles on GaN are strongly anchored on the surface, and Pt redeposition on the HER-reacted Pt / GaN structure provides more stabilized Pt nanoparticles, as well as LSV curves of (b) pristine Pt / GaN / Si (0 - 24 h) and (c) after a fifth Pt redeposition (216 - 288 h), and graphical plots of (d) JPh at 0 VRHE and (e) VonSet of Pt / GaN / Si over reaction time, in which the stability was tested over 1500 h.

[0017] Figure 5 depicts schematic views of photoelectrodes having an array of nanostructures for hydrogen evolution via water splitting from concentrated solar light in accordance with one example.

[0018] Figure 6 is a schematic view and block diagram of an electrochemical system having a photocathode with an array of nanostructures for hydrogen evolution via water splitting from concentrated solar light in accordance with one example.

[0019] Figure 7 is a flow diagram of a method of fabricating of a photocathode with a surface treatment procedure in accordance with one example.

[0020] Figure 8 is a schematic view of an electrochemical system having a photocathode with an array of nanostructures for water splitting from concentrated solar light in accordance with one example.

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

[0022] Photoelectrochemical, photocatalytic, and other systems and devices (e.g., photocathodes or photoelectrodes) for hydrogen production and / or other chemical reactions are described. Methods of fabricating the photoelectrodes and other devices are also described. Aspects of the disclosed fabrication methods are directed to improving the stability of the photoelectrodes and other devices in harsh conditions, such as those present during illumination with concentrated solar light. The disclosed methods and devices thusaddress the limitations and challenges presented in the production of hydrogen that otherwise arise from an insufficient number of photogenerated charge carriers.

[0023] The disclosed methods, systems, and devices provide a stable and efficient procedure for the production of high-yield H2using metal (e.g., Pt) nanoparticle-decorated nanowires (e.g., GaN nanowires) grown on a substrate (e.g., a n+-p Si photoelectrode substrate). Under concentrated solar light at 6.4 sun (640 mW / cm2), the Pt / GaN / Si structures exhibited high photocurrent density at 0 V vs. reversible hydrogen electrode (VRHE) (JO) over 100 mA / cm2, but degraded within 0.5 h and stabilized thereafter. The rate of performance degradation was significantly accelerated under concentrated solar light compared to that observed under conventional 1 sun (100 mW / cm2) illumination. Surface chemical and microstructure analysis unveiled that the concentrated solar light induced rapid surface modifications and removed Pt nanoparticles. Meanwhile, some Pt cocatalysts having an epitaxial relation with the GaN nanowires were strongly anchored on the surface and remained even after vigorous H2gas evolution. This unexpected finding guided the redeposition of Pt nanoparticles on the reacted surface of the photoelectrode, where more anchoring sites for Pt nanoparticles were available, leading to enhanced HER activity and stability. The resulting devices provide examples of stable bonding of Pt nanoparticles on single crystalline GaN nanowires for efficient and durable photoelectrodes working under concentrated solar light.

[0024] Although described in connection with photoelectrochemical water splitting, the disclosed devices and systems may be used in other chemical reaction contexts and applications. For instance, the disclosed devices and systems may be useful in connection with various types of photocatalytic and / or other systems, and / or in connection with other reactions, including, for instance, N2reduction, CO2reduction to various fuels and other chemicals, NOs- reduction, and urea synthesis.

[0025] Although described in connection with photoelectrochemical systems, the disclosed systems and devices are not limited to applications in which an applied bias voltage is relied upon. The disclosed systems and devices may thus be used in photocatalytic contexts and applications in which a bias voltage is not applied to the photoelectrode device. The disclosed methods may also be used to fabricate other types of electrodes.

[0026] Although described herein in connection with electrodes having GaN-based nanowire arrays for water splitting, the disclosed devices and systems are not limited to GaN-based nanowire arrays. A wide variety of other types of nanostructures and other conductive projections may be used. In some cases, the electrodes of the disclosedsystems do not include an array of nanowires, and instead include other shaped projections. Alternative or additional Ill-nitride semiconductor materials may be used, including, for instance, InGaN. Thus, the composition, nature, construction, configuration, characteristics, shape, and other aspects of the electrodes may vary.

[0027] The disclosed photocatalytic devices are also not limited to Ill-nitride semiconductor materials. For instance, other nitride materials, such as TiNx, carbon nitride, and ScN, may be used.

[0028] The disclosed devices are also not limited to nanowires or conductive projections having a uniform semiconductor composition. For instance, the conductive projections of the photocatalytic devices may have a multi-band configuration. For example, the arrays may include monolithically integrated multiple-band InGaN nanostructures or segments configured to act as photocatalysts. Each conductive projection may thus be capable of photoexcitation via a wider range of wavelengths, including, for instance, both ultraviolet and visible portions of the solar spectra. Any number or type of segments may be included.

[0029] Although described herein in connection with electrodes having platinum nanoparticle catalysts for water splitting, the disclosed devices and methods are not limited to platinum nanoparticles. Other metals may be used. For instance, alternative or additional noble metals may be used. Still other metals may be used, including, for instance, Co and Ni. Thus, the nature, construction, configuration, characteristics, shape, and other aspects of the nanoparticle catalysts may vary.

[0030] Although described herein in connection with silicon substrates, the disclosed photoelectrodes may include substrates of other compositions. For instance, alternative or additional semiconductors or other materials may be used, including, for instance, sapphire, copper, and SiC. In these and other cases, the nanowires or other conductive projections of the disclosed devices may be configured to generate charge carriers via solar light and / or other radiation absorption.

[0031] Although described in connection with solar radiation (e.g., concentrated solar light), the disclosed devices are useful in connection with a variety of different light sources. The spectrum or other characteristics of the light source may vary accordingly. For instance, the radiation may be or otherwise include various types of artificial light. The artificial light may include any combination of infrared, visible, and / or ultraviolet wavelengths.

[0032] The epitaxial growth of the nanowires of the disclosed devices may have one or more parameters or other aspects in common with those set forth in the following publications: Kibria, M. et al., "Visible light-driven efficient overall water splitting using p-typemetal-nitride nanowire arrays," Nat. Commun. 6, 1-8 (2015); Wang, D. et al., "Wafer-level photocatalytic water splitting on GaN nanowire arrays grown by molecular beam epitaxy," Nano Lett. 11 , 2353-2357 (2011), Guan, X. et al., "Making of an industry-friendly artificial photosynthesis device," ACS Energy Lett. 3, 2230-2231 (2018), U.S. Patent Publication No. 2023 / 0017032 ("CO2 Conversion with Metal Sulfide Nanoparticles"), and International Publication No. WO / 2023239712 ("Photocatalytic CO2 Reduction with Co-Catalyst- Decorated Nanostructures"), the entire disclosures of which are hereby incorporated by reference.

[0033] Disclosed herein are examples of photoelectrode devices that are fabricated and configured in a manner that takes advantage of the alteration in surface chemical composition during reaction conditions, such as concentrated solar light irradiation. The examples include Pt nanoparticles (NPs) loaded on single crystalline GaN nanowires (NWs) grown on a n+-p Si photoelectrode. The example devices operate efficiently and stably under concentrated solar light despite a large number of Pt nanoparticles detaching during an initial reaction due to H2gas bubbling. However, some Pt nanoparticles that have an epitaxial relation with GaN nanowires remain stably anchored. In addition, the stability of the example photoelectrodes further improves by redepositing Pt nanoparticles on the reacted Pt / GaN surface, which results in maintaining onset potential greater than 0.5 V vs. reversible hydrogen electrode and photocurrent density greater than 60 mA / cm2for over 1500 hours. The heterointerface between the Pt cocatalysts and the single crystalline GaN nanostructures provides an efficient and stable photoelectrode for high-yield solar to H2conversion.

[0034] Turning to Figure 1 , a Pt / GaN / Si photoelectrode was fabricated by vertical growth of n-type GaN nanowires on planar n+-p Si wafer followed by photo-deposition of Pt nanoparticles. Each GaN nanowire in the array had a length of about 400 nm, as observed in a scanning electron microscopy (SEM) image (Figure 1 , part a). Further details regarding the fabrication of these and other example photoelectrodes are depicted and described below in connection with Figure 7.

[0035] The photoelectrode was deployed in a concentrated solar light PEC water splitting system with the Pt / GaN / Si photoelectrode in an H-type flow cell including a Pt wire counter electrode, and an Ag / AgCI reference electrode with Nation proton exchange membrane. Further details regarding example systems are depicted and described below in connection with Figures 6 and 8. A 0.5 M H2SO4 aqueous electrolyte was continuously circulated, and AM 1 .5 G-filtered solar light was irradiated on the backside of the photoelectrode during the reaction. When the light illuminated the n+-p Si substrate, the photoexcited electrons in theconduction band of the Si substrate drift toward the n-type GaN nanowires due to the built-in potential generated at the p-n junction, whereas the photogenerated holes in the valance band of p-Si move to a Cu back contact through a Gain eutectic alloy. Because there is a negligible energy barrier between the conduction bands of n-Si and n-GaN, photogenerated electrons efficiently migrate to the Pt / GaN surface and participate in the HER.

[0036] For the concentrated solar light irradiation, light intensity was controlled from 0.3 sun (30 mW / cm2) to 9.2 sun (920 mW / cm2) by varying the distance between the light source and the photoelectrode. A liquid flow system was used to provide fresh reactant and remove the gaseous H2product during the vigorous reaction. A flow rate of 4 ml / min to each cathodic and anodic compartment may be used to maintain high Jphgreater than 70 mA / cm2. Linear sweep voltammetry (LSV) curves of the Pt / GaN / Si photoelectrode exhibited a positive onset potential (VonSet) greater than 0 VRHE, an increase in Jphat the mid-potential region, and a saturated photocurrent density (Jsaturation) at large biases. Jsaturation gradually increased from 6.5 to 163.6 mA / cm2as solar power intensified from 0.3 to 9.2 sun. The linear correlation between Jsautration and light intensity indicates that the number of charge carriers generated in the photoelectrode, determined by the light intensity, is the main limiting factor for the JPh (Figure 1 , part b). The saturation potential (Vsautration) at which the current density begins to saturate also linearly increased from 0.03 to -1 .64 VRHE with the light intensity. In contrast, the Vonset revealed a value of 0.51 - 0.60 VRHE regardless of the light intensity.

[0037] With a light projection area of 0.13 cm2and 1 sun light illumination, LSV curves of Pt / GaN / Si were measured after each period of reaction time at 0 VRHE. AS the reaction time increased from 0 to 72 h, VonSet gradually shifted to negative values and Joshowed a decreasing trend. This was because Pt nanoparticles detached from the GaN surface. As the number of Pt nanoparticles on the photoelectrode decreases, the charge carrier diffusion length that the photoelectrons travel in the GaN nanowires before reaching to the Pt nanoparticles increased, resulting in recombination and decreased efficiency. In contrast, a GaN / Si structure showed a gradual positive shift of VonSet and an increase in Jowith the progress of the reaction. This self-improvement is known to be due to the partial substitution of N on the nonpolar crystal facets of GaN nanowires with O during the reaction, leading to gallium oxynitrides formation and an enhancement of the catalytic properties for H2evolution. It should be noted that the performance of Pt / GaN / Si structure rapidly degraded during the first 0.5 h reaction under 6.4 sun light and stabilized thereafter, whereas the GaN / Si structure exhibited self-improvement during the initial 20 h and then stabilized. Jo(Figure 1 , part c) and VonSet (Figure 1 , part d) were plotted as a function of reaction time. The Pt / GaN / Si structure exhibited an initial drop of Jofrom 103 to 81 mA / cm2and VonSet from 0.50to 0.35 VRHE in 0.5 h of concentrated solar light PEC HER, whereas the Pt / GaN / Si structure measured under 1 sun light revealed no degradation of Joand much slower degradation speed of VonSet. In the case of the GaN / Si structure, the self-improvement rate of Joand VonSet is also faster under concentrated solar light compared to 1 sun light. The trend of applied bias photon-to-current efficiency (ABPE) changes over time was similar to the VonSet. Overall, the in-situ surface modifications such as Pt detachment and gallium oxynitride formation were accelerated under concentrated solar light, leading to degraded performance and stability relative to conventional measurement conditions under 1 sun (100 mW / cm2).

[0038] Compared to 1 sun illumination, the saturation photocurrent density under concentrated solar light linearly increased (Figure 1 , part b). However, in the potential range above 0 VRHE, where the photoelectrode exhibits its advantages over electrocatalytic hydrogen evolution, the increase in photocurrent density does not proportionally correlate with light intensity. Therefore, when the light intensity was increased from 1 to 6.4 suns, the ABPE of Pt / GaN / Si decreased from 6.9% to 2.8%, respectively, despite the increase in photocurrent density and production yield of H2. During the reaction, the ABPE of the Pt / GaN / Si structure gradually decreased under both 1 and 6.4 sun light conditions. Meanwhile, the ABPE of the GaN / Si structure under 1 and 6.4 suns was about 0%, but gradually increased to 1 .9% and 1 .5%, respectively.

[0039] Further details regarding the change in surface chemical composition and microstructure during the reaction are now provided. To investigate the surface chemical composition of the photoelectrodes, angle-resolved X-ray photoelectron spectroscopy (AR- XPS) was performed by varying the take-off angle (TOA) of photo-emitted electrons from the samples. The relative atomic ratio of pristine Pt / GaN / Si and the sample after the reaction for 24 h was measured, and the values at TOA = 60° were compared (Figure 2, part a). The surface atomic ratio of Ga and N increased after the reaction, while those of Pt and O reduced. This indicates that during the concentrated solar light PEC HER in acidic electrolyte, some of the Pt nanoparticles detached and gallium oxide (GaOx) species were removed from the surface. Consequently, more GaN was exposed to the outermost surface with a reduced number of Pt nanoparticles. Similarly, the GaN / Si structure also displayed an increased Ga and N content and reduced O ratio after the reaction under 6.4 sun light (Figure 2, part b). This finding further confirms the dissolution of the surface oxide layer. It is worth noting that no Pt 4f signals were detected on the GaN / Si structures, indicating that the self-improvement of HER performance was not due to Pt contamination.

[0040] To gain further insight into the surface binding states, core-level XPS spectra were analyzed for both pristine and reacted Pt / GaN / Si and GaN / Si photoelectrodes, and then Ga3d, O 1 s, and Pt 4f XPS spectra at TOA = 60swere plotted (Figure 2, parts c-e). The Ga 3d XPS spectra of both pristine Pt / GaN / Si and GaN / Si showed a major peak of Ga-N bond and a minor peak of Ga-0 bond (Figure 2, part c). After the PEC HER reaction for 24 h, a new peak corresponding to Ga-O-N bond appeared, indicating the formation of gallium oxynitrides. The O 1s spectra also showed emerging peaks of O-Ga-N in both Pt / GaN / Si and GaN / Si after the reaction, further confirming the formation of gallium oxynitride (Figure 2, part d). The Pt 4f spectra of pristine and reacted Pt / GaN / Si showed similar binding states of Pt° and Pt2+(Figure 2, part e).

[0041] The microstructure of the Pt / GaN / Si photoelectrode was examined using high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). The analysis revealed the presence of numerous Pt nanoparticles with bright contrast on the top and sidewalls of the GaN nanowire before the PEC HER (Figure 3, part a). Energy-dispersive X- ray spectroscopy elemental map showed a dense and uniform distribution of Pt nanoparticles on GaN nanowires (Figure 3, part b). More interestingly, some Pt nanoparticles with (111) and (200) orientations exhibited lattice alignment with GaN (002) planes. After 24 h of reaction under concentrated solar light, a considerable number of Pt nanoparticles were detached from the surface of the GaN nanowires, resulting in a significantly reduced and more sparse distribution of smaller size of Pt nanoparticles on the GaN surface (Figure 3, part c). In order to understand the mechanism behind the stable anchoring of Pt nanoparticles to GaN nanowires despite the vigorous H2production, an analysis of the heterointerface between Pt nanoparticles and GaN nanowires was conducted at three different locations after the concentrated solar light PEC water splitting (Figure 3, part d). Single crystal GaN nanowires were grown along (002) orientation and Pt nanoparticles with (200) and (111) crystal planes were found on the surface of the GaN nanowires. It is worth noting that the lattices of the Pt nanoparticles and the GaN nanowires were aligned with epitaxial relations. Specifically, the five lattice spacings of Pt (200) were aligned to the four lattice spacings of GaN (002), with an edge dislocation propagating in Pt nanoparticle NP1 . At another location, the lattices of Pt nanoparticle NP2 and Pt nanoparticle NP3 with (111) orientation exhibited an epitaxial relationship with GaN (002) (Figure 3, part d). Pt nanoparticle NP2 had a dislocation inside, while the adjacent Pt nanoparticle NP3 did not. In the case of a larger Pt nanoparticle NP4, two dislocation lines were observed inside of the particle.

[0042] Upon comparing the lattice spacing of the three representative Pt nanoparticles, Pt nanoparticle NP1 had a d-spacing (0.199 nm) that was approximately 1 .4% elongated than the literature value of Pt (200), while Pt nanoparticle NP2 (0.230 nm) and Pt nanoparticleNP3 (0.235 nm) had d-spacing extensions of approximately 1.5% and 3.8%, respectively, compared to the literature value of Pt (1 11 ) (JCPDS 04-0802) (Figure 3, part e). Pt nanoparticles with large size can form dislocations inside to release interfacial stress, while Pt nanoparticles with small size show a preference for lattice expansion over dislocation formation (Figure 3, part f). This implies that the size of Pt nanoparticles plays a useful role in determining the strain relaxation behavior at the heterointerface. According to previous studies, it is known that tensile strain acting on Pt catalysts deteriorates their H2evolution catalytic properties. This is likely one of the reasons why the Pt / GaN / Si photoelectrode degraded despite the presence of Pt nanoparticles on the photoelectrode after the concentrated solar light experiment. Nevertheless, the Pt nanoparticles lattice-matched to GaN showed strong bonding strength, which allows them to maintain good stability even under harsh reaction conditions.

[0043] Further details regarding the improved stability of the Pt nanoparticles on the example photoelectrodes are set forth below.

[0044] There were three different types of Pt nanoparticles found on the pristine GaN nanowires: physically adsorbed Pt nanoparticles, Pt nanoparticles on surface oxides, and lattice-matched Pt nanoparticles with GaN nanowires (Figure 4, part a). However, during the concentrated solar light PEC HER, the acidic electrolyte dissolved the oxides with Pt nanoparticles on it, and H2bubbles caused mechanical detachment of the surface adsorbed Pt nanoparticles, resulting in a small number of strongly anchored Pt nanoparticles remaining on the surface and degrading the HER performance. To address this stability issue, Pt nanoparticles were re-deposited onto the reacted photoelectrodes, which have a higher likelihood of forming strong interactions between Pt nanoparticles and the GaN lattices. Furthermore, regrowth of lattice-matched Pt nanoparticles can release the tensile strain, thus further improving both the HER activity and stability.

[0045] The comparison of LSV curves between pristine and 24 h-reacted Pt / GaN / Si photoelectrodes revealed an evident degradation (Figure 4, part b). However, after the Pt redeposition, the regenerated Pt / GaN / Si photoelectrode recovered the high performance. Interestingly, after the fifth iteration (reaction and redeposition) of Pt nanoparticles, the stability got dramatically improved (Figure 4, part c). Furthermore, there was no noticeable degradation of Joafter the seventh Pt iteration (Figure 4, part d). The degradation speed of Vonset (Figure 4, part e) and ABPE also slowed down after repeated Pt redeposition, indicating that the reacted surface provided more stable anchoring sites for the Pt nanoparticles. As a result, an excellent performance and stability were achieved, with VonSetgreater than 0.5 VRHE, Jph greater than 60 mA / cm2, and high faradaic efficiency of H2greater than 97% maintained over a period of 1500 hours.

[0046] After five iterations of Pt redeposition and a 288 hour long-term stability test, there were a large number of Pt nanoparticles that remained on the GaN surface. A TEM image revealed that Pt aggregates include agglomerated Pt nanoparticles with a void inside, indicating regrowth and an increase in the size of Pt nanoparticles during the redeposition process. The lattice spacings of the Pt aggregates were found to be 0.227 nm, which is consistent with the literature value of Pt (11 1 ). These results confirm that the tensile strain in Pt nanoparticles was released during the regrowth and explain why the VonSet stabilized at a value higher than 0.5 VRHE. The extent of tensile strain resulting from alignment at the heterointerface varies depending on the size of Pt nanoparticles, with larger nanoparticles leading to the release of tensile stress and an improvement in catalytic performance. This is a useful characteristic at the heterointerface between the Pt nanoparticles and the GaN nanowires because single crystal GaN has a well-defined crystal structure that can potentially influence the behavior of the Pt nanoparticles. In contrast, previous studies have shown that Pt redeposition on an amorphous TiO2passivation layer, which lacks the well- defined crystal structure, resulted in a decrease in the JPh even after the repeated Pt redeposition. Understanding how the Pt cocatalysts adhere to the surface of conventional photoelectrodes has been challenging due to the limitations and complexities associated with the metal / amorphous oxide heterointerface. However, the examples disclosed herein provide a clear understanding of the bonding mechanism between the cocatalyst and photoelectrode via use of a single crystalline GaN nanowires and the deposition of Pt cocatalysts on them. In particular, the lattice alignment between the Pt cocatalyst and GaN nanowires can induce strong bonding strength, anchoring the Pt cocatalysts onto the photoelectrodes even under harsh concentrated solar light. Nevertheless, the utilization efficiency of Pt was still limited by the detachment of the Pt cocatalysts from the GaN nanowires due to in-situ surface modification of GaN during the initial reaction. To improve the utilization efficiency of Pt, the implementation of the surface treatment techniques of the disclosed methods will establish a strong binding at the interface between GaN and Pt, thereby minimizing the detachment of the Pt nanoparticles from the GaN surface.

[0047] In the case of irradiation with very strong solar light (40 sun), Jomay be increased to over 240 mA / cm2. These and other operating conditions may result in a considerable amount of photothermic heat. However, the rubber O-rings used in the flow cell were damaged by ultraviolet light and photothermic heat after about 52 h. Despite the stability issue caused by photo-induced heat, it is noteworthy that the photothermal effect, where theabsorption of light leads to localized heating, may further enhance the efficiency of reaction kinetics at locally elevated temperatures on the surface of photoelectrodes under concentrated solar light.

[0048] In comparison to the Si-based photoelectrodes under 1 sun light illumination, the example Pt / GaN / Si photoelectrode under concentrated solar light (6.4 sun) exhibited about 4-fold higher H2production rate and one order of magnitude higher Jothan previous photoelectrodes involving, for instance, oxides or chalcogenides semiconductors. Furthermore, the example photoelectrodes, while working under accelerated reaction conditions, will both reduce the material cost (light absorbers and cocatalysts) and also increase the hydrogen production rate per unit photoelectrode area. Still further, there is an opportunity to harness the photothermal effect on the surface of photoelectrodes. The localized heating induced by concentrated solar light can enhance reaction kinetics. The localized hearting may bring significant improvements of efficiency and stability, and realize low cost solar fuel production.

[0049] Figure 5 depicts example photoelectrodes that may be used for PEC water splitting with concentrated solar light. The example photoelectrodes include Pt nanoparticles loaded on single crystal GaN nanowires grown on n+-p Si wafers. As described herein, the example photoelectrodes exhibited high hydrogen evolution activity, productivity, and stability under concentrated solar light. Lattice alignment between the Pt nanoparticles and the GaN nanowires at the heterointerface stably anchored the Pt cocatalysts during the harsh reaction conditions presented by the concentrated solar light. As described herein, repeated redeposition of the Pt nanoparticles greatly improved the performance and stability for over 1500 hours.

[0050] A number of examples of the disclosed devices, systems, and methods are now described in connection with the schematic diagram of Figure 6 and the flow diagram of Figure 7.

[0051] Figure 6 depicts a system 100 for hydrogen evolution via water splitting in accordance with one example. The system 100 may also be configured for other reactions. The system 100 may be configured as an electrochemical system. In this example, the electrochemical system 100 is a photoelectrochemical (PEC) system in which solar and / or other radiation is used to facilitate the hydrogen evolution and water splitting. The water splitting may be assisted or unassisted. For instance, unassisted water splitting (e.g., solar- only-driven operation) may be implemented in connection with InGaN nanowires having atunnel junction. The manner in which the PEC system 100 is illuminated may vary. The wavelength and other characteristics of the radiation may vary accordingly.

[0052] The electrochemical system 100 includes one or more electrochemical cells 102. A single electrochemical cell 102 is shown for ease in illustration and description. The electrochemical cell 102 and other components of the electrochemical system 100 are depicted schematically in Figure 6 also for ease in illustration. The cell 102 contains an electrolyte solution 104. The solution 104 includes dissolved NaCI, and may be acidic, neutral or alkaline, as described herein. In some cases, a CO2 and / or other source is applied. In some cases, the electrolyte solution is saturated with CO2. Additional or alternative electrolytes may be used, as described below. Further details regarding examples of the electrochemical system 100 are provided below.

[0053] In the example of Figure 6, the electrochemical cell 102 has a three-electrode configuration. The electrochemical cell 102 includes a working electrode 108, a counter electrode 110, and a reference electrode 112, each of which is immersed in the electrolyte 104. The counter electrode 110 may be or include a metal wire, such as a platinum wire. The reference electrode 112 may be configured as a reversible hydrogen electrode (RHE) (e.g., Ag / AgCI filled with 3 M KCI). The positioning of the reference electrode 112 may vary from the example shown. For example, the reference electrode 112 may be adjacent to the counter electrode 110 in other cases. The configuration of the counter and reference electrodes 110, 112 may vary. For example, the counter electrode 110 may be configured as, or otherwise include, a photoanode at which water oxidation (4H2O 2O2 + 8e + 8H+) occurs. In some cases, the counter electrode 110 is configured as, or otherwise includes, an IrOx electrode. The configuration of the electrochemical cell may vary. For instance, in other cases, a two-electrode or other configuration may be used.

[0054] The hydrogen evolution occurs at the working electrode 108 as follows: Hydrogen evolution: 2H2O + 2e H2 + 2OH-To that end, electrons may flow from the counter electrode 110 through a circuit path external to the electrochemical cell 102 to reach the working electrode 108. The working and counter electrodes 108, 110 may thus be considered a cathode and an anode, respectively. As described above, the dissociation of water may also occur at the working electrode 108.

[0055] In the example of Figure 6, the working and counter electrodes are separated from one another by a membrane 114, e.g., a proton-exchange membrane. The construction, composition, configuration and other characteristics of the membrane 114 may vary.

[0056] In the example of 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 and counter electrodes 108, 110. The bias voltage may be used to establish a ratio of CO2 reduction to hydrogen (H2) evolution at the working electrode, and / or another reaction ratio(s). The circuit path may include additional or alternative components. For example, the circuit path may include a potentiometer in some cases. In other cases, no bias voltage is applied - e.g., in unassisted systems.

[0057] In this example, the working electrode 108 is configured as a photocathode. Light 118, such as solar radiation, may be incident upon the working electrode 108 as shown. The electrochemical cell 102 may thus be considered and configured as a photoelectrochemical cell. In such cases, illumination of the working electrode 108 may cause charge carriers to be generated in the working electrode 108. Electrons that reach the surface of the working electrode 108 may then be used in the hydrogen evolution. The photogenerated electrons may augment electrons provided via the current path.Alternatively or additionally, the electrons provided via the current path may recombine with the photogenerated holes at a backside or other contact. Further details regarding examples of photocathodes are provided below.

[0058] The working electrode 108 includes a substrate 120. The substrate 120 of the working electrode 108 may constitute a part of an architecture, a scaffolding, or other support structure, of the working electrode 108. The substrate 120 may be uniform or composite. For example, the substrate 120 may include any number of layers or other components. The substrate 120 thus may or may not be monolithic. The shape of the substrate 120 may also vary. For instance, the substrate 120 may or may not be planar or flat.

[0059] In the example of Figure 6, the substrate 120 is doped and otherwise configured to present a junction. The substrate 120 of the working electrode 108 may thus be active (functional) in connection with the photogeneration of charge carriers. Alternatively or additionally, the substrate 120 is passive (e.g., structural). The substrate 120 may be configured and act as a support structure for a catalyst arrangement of the working electrode 108, as described below. Alternatively or additionally, the substrate 120 may be composed of, or otherwise include, a material suitable for the growth or other deposition of the catalyst arrangement of the working electrode 108.

[0060] In active or functional cases, the substrate 120 may include a light absorbing material. The light absorbing material is configured to generate charge carriers upon solaror other illumination. The light absorbing material has a bandgap such that incident light generates charge carriers (electron-hole pairs) within the substrate. Some or all of the substrate 120 may be configured for photogeneration of electron-hole pairs. To that end, the substrate 120 may include a semiconductor material. In some cases, the substrate 120 is composed of, or otherwise includes, silicon. For instance, the substrate 120 may be provided as a silicon wafer.

[0061] The silicon may be doped. In the example of Figure 6, the substrate 120 includes a heavily n-type doped layer 122, a moderately or lightly p-type doped layer 123, and a heavily p-type doped layer 124. The arrangement of the layers 122-124 establishes a junction within the substrate 120. The doping arrangement may vary. For example, one or more components of the substrate 120 may be non-doped (intrinsic), or effectively non-doped.The substrate 120 may include alternative or additional layers, including, for instance, support or other structural layers. In other cases, the substrate 120 is not light absorbing.

[0062] The substrate 120 of the working electrode 108 establishes a surface at which a catalyst arrangement is provided. In some cases, catalyst support structures, or scaffolding, of the electrode 108 are provided as described below. As described below, the catalyst support structures may include an array of conductive projections extending outward from a surface of the substrate 120. In other cases, the catalyst arrangement does not include conductive projections. For instance, the catalyst arrangement may include one or more planar structures, such as one or more layers supported by the substrate 120.

[0063] In the example of Figure 6, the working electrode 100 includes an array of nanostructures 126 (or other conductive projections) supported by the substrate 120. Each nanostructure 126 is configured to extract the charge carriers (e.g., electrons) from the substrate 120. The extraction brings the electrons to external sites along the nanostructures 126 for use in the hydrogen evolution.

[0064] In some cases, each nanostructure 126 is configured as a nanowire. Each nanostructure 126 may have a semiconductor composition. In some cases, the semiconductor composition includes a semiconductor core. For instance, the core may be composed of, or otherwise includes, a Group lll-V nitride semiconductor material, such as gallium nitride (GaN). Additional or alternative semiconductor materials may be used, including, for instance, indium gallium nitride (InGaN) and / or other Ill-nitride semiconductor materials.

[0065] The core of each nanowire or other nanostructure 126 may be or include a columnar, post-shaped, or other elongated structure that extends outward (e.g., upward)from the plane of the substrate 120. The semiconductor nanowires or other nanostructures 126 may be grown or formed as described in U.S. Patent No. 8,563,395, the entire disclosure of which is hereby incorporated by reference. The nanostructures 126 may be referred to herein as nanowires with the understanding that the dimensions, size, shape, composition, and other characteristics of the nanostructures 126 or other conductive projections may vary.

[0066] The semiconductor composition of each nanostructure 126 may or may not be configured to facilitate the reaction(s) supported by the electrochemical system 100. The semiconductor composition may be configured for photo-generation of charge carriers, as described below. Alternatively or additionally, the semiconductor composition may be configured to act as a catalyst for the reaction(s). The semiconductor composition may provide other functions, including, for instance, protection of the substrate 120 as described above in connection with GaN examples. Additional or alternative semiconductor materials may be used, including, for instance, indium nitride, aluminum nitride, boron nitride, aluminum oxide, silicon, and / or their alloys.

[0067] The semiconductor composition of each nanostructure 126 may be configured to provide surface passivation and / or other protection of the photoelectrode 108. For instance, in some cases, the semiconductor composition is terminated with nitrogen along surfaces of the nanostructure 126. The nitrogen termination or other nitrogen-based aspect of the nanostructures 126 may protect the nanostructures 126 and / or other components of the electrode 108 (e.g., the substrate 120) during operation from, e.g., corrosion.

[0068] The nanostructures 126 may facilitate the hydrogen evolution and / or another chemical reaction in one or more ways. For instance, each nanostructure 126 may be configured to extract the charge carriers (e.g., electrons) generated in the substrate 120. The extraction brings the electrons to external sites along the nanostructures 126 for use in the hydrogen evolution and / or other chemical reaction. The composition of the nanostructures 122 may also form an interface well-suited for hydrogen evolution and / or another chemical reaction, as explained herein.

[0069] Each nanostructure 126 may be or include a columnar, post-shaped, or other elongated structure that extends outward (e.g., upward) from the plane of the substrate 120. The dimensions, size, shape, composition, and other characteristics of the nanostructures 126 may vary. For instance, each nanostructure 126 may or may not be elongated like a nanowire. Thus, other types of nanostructures from the substrate 120, such as various shaped nanocrystals, may be used.

[0070] In some cases, the nanostructures 126 may be configured to generate electron-hole pairs upon illumination. For instance, the nanostructures 122 may be configured to absorb light at frequencies different than other light absorbing components of the electrode 108. For example, one light absorbing component, such as the substrate 120, may be configured for absorption in the visible or infrared wavelength ranges, while another component may be configured to absorb light at ultraviolet wavelengths. In other cases, the nanostructures 126 are the only light absorbing component of the electrode 108. In still other cases, the substrate 120 is the only light absorbing component of the electrode 108.

[0071] In some cases, each nanowire 126 may include a layered or segmented arrangement of semiconductor materials. For instance, in Group Ill-nitride examples, the layers or segments of the arrangement may have differing Group III (e.g., indium and gallium) compositions. One or more layers or segments in the arrangement may be configured for absorption of a respective range of wavelengths. Each nanowire 126 may include one or more segments having a compound semiconductor composition (e.g., InGaN) configured for photogeneration of charge carriers. Other layers or segments may be directed to establishing a tunnel junction. Each nanowire 126 may include segments having a compound semiconductor composition (e.g., InGaN) configured to establish a tunnel junction. Each nanowire 126 may also include additional or alternative segments, including, for instance, a segment between the tunnel junction and the substrate 120.

[0072] In other cases, the layered arrangement of semiconductor materials is also used to establish a multi-band structure, such as a quadruple band structure. Each layer or segment of the arrangement may have a different semiconductor composition to establish a different bandgap. The different bandgaps may be useful in connection with absorbing light of differing wavelengths.

[0073] Other layered arrangements may be used. For example, further details regarding the formation and configuration of multi-band structures, including, for instance, triple-band structures, are provided in U.S. Patent No. 9,112,085 ("High efficiency broadband semiconductor nanowire devices") and U.S. Patent No. 9,240,516 ("High efficiency broadband semiconductor nanowire devices"), the entire disclosures of which are incorporated by reference.

[0074] The semiconductor composition of each nanowire 126 may be configured to improve the efficiency of the water splitting in additional ways. For instance, in some cases, the semiconductor composition of each nanowire 126 may include doping to promote charge carrier separation and extraction, as well as facilitate the establishment of a photochemicaldiode. For example, a dopant concentration of the semiconductor composition may vary laterally.

[0075] In examples involving Ill-nitride compositions, the dopant may be or include magnesium. Further details regarding the manner in which magnesium doping promotes charge carrier separation and extraction are set forth in U.S. Patent No. 10,576,447 ("Methods and systems relating to photochemical water splitting "), the entire disclosure of which is incorporated by reference. Additional or alternative dopant materials may be used, including, for instance, silicon, carbon, and beryllium, depending on the semiconductor light absorber of choice.

[0076] The semiconductor device may further include catalyst nanoparticles 136 disposed over the array of nanowires 126. The nanoparticles 136 are distributed across or along the outer surface (e.g., sidewalls) of each nanowire 126. The nanoparticles 136 are configured to facilitate or promote the proton reduction reaction. In some cases, each nanoparticle 136 includes a metal, such as platinum. Other metals or materials may be used, such as rare earth metals. Still other metals may be used, including alloys and / or other metal or metallic combinations. Further details regarding the formation, configuration, functionality, and other characteristics of nanoparticles 136 in conjunction with a nanowire array are set forth in one or more of the above-referenced U.S. patents.

[0077] The distribution of the nanoparticles 136 may be uniform or non-uniform. The nanoparticles may thus be distributed randomly across each nanowire 126. The schematic arrangement of Figure 6 is shown for ease in illustration.

[0078] As described herein, each nanoparticle 136 may be lattice-matched with the material of the nanowire 126 as a result of the surface treatment of the disclosed methods. Each nanoparticle 136 may have an inner void as also described herein. The surface treatment may also result in the nanowires 126 lacking a surface layer of an oxide material (e.g., an oxide of GaN or other Ill-nitride semiconductor material).

[0079] The nanowires 126 and the nanoparticles 136 are not shown to scale in the schematic depiction of Figure 6. The shape of the nanowires 126 and the nanoparticles may also vary from the example shown.

[0080] The nanoparticle-nanowire catalyst arrangement may be fabricated on a substrate (e.g., a silicon substrate) via nanostructure-engineering. In one example, molecular beam epitaxial (MBE) growth of the nanowires is followed by photo-deposition of the nanoparticles. The photo-deposition of the nanoparticles may be configured to selectively deposit thenanoparticles on the respective sides of the nanowire. Further details regarding example fabrication procedures are provided below, e.g., in connection with Figure 7.

[0081] The disposition of the platinum nanoclusters or other metal catalyst nanoparticles over the array of nanowires or conductive projections establishes metal / metal-nitride interfaces. As described above, the metal / metal-nitride interfaces are configured to promote hydrogen production via water splitting. For instance, in GaN-platinum examples, the Ga-Pt interface promotes the water splitting through dissociation of the water.

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

[0083] The device 100 is configured for operation in a PEC (illuminated) mode. In the example of Figure 6, the working electrode 108 includes a contact 142 coupled to the backside of the substrate 120 and to which charge carriers photogenerated in the substrate 120 in the photoelectrochemical (PEC) operational mode move. As described herein, the contact 142 may include a eutectic alloy on the backside of the substrate and the surface of the substrate, respectively. For instance, a Ga-ln eutectic may be sandwiched between a Cu back contact and the n+-p Si wafer for ohmic contact. Additional or alternative electrical connections may be used to apply the bias voltage.

[0084] Figure 7 depicts a method 700 of fabricating a photoelectrode for hydrogen production in accordance with one example. The method 700 may be used to manufacture any of the devices described herein or another device. The method 700 may include additional, fewer, or alternative acts. For instance, the method 700 may or may not include one or more acts directed to annealing the device (act 726).

[0085] The method 700 may begin with an act 702 in which a substrate is prepared or otherwise provided. The substrate may be or be formed from a silicon wafer. In one example, a 2-inch or 3-inch Si wafer was used, but other (e.g., larger) size wafers may be used. Other semiconductors and substrates may be used.

[0086] In some cases, the act 702 includes an act 704 in which the substrate is a wet or other etch procedure is implemented to define the surface (e.g., nonplanar surface). For example, the etch procedure may be or include a crystallographic etch procedure. In silicon substrate examples, the crystallographic etch procedure may be or otherwise include a KOHetch procedure. In such cases, if the substrate has a <100> orientation, the wet etch procedure establishes that the surface includes a pyramidal textured surface with faces oriented along <111 > planes, but additional or alternative facets may be present in some cases.

[0087] The act 702 may include fewer, additional, or alternative acts. For instance, in the example of Figure 7, the act 702 includes an act 706 in which the substrate is cleaned (e.g., with acetone, IPA and hydrofluoric acid), and an act 708 in which oxide is removed (e.g., via annealing at a sufficiently high temperature). The oxide removal may be implemented in the MBE reaction chamber immediately before growth.

[0088] The act 702 may still further include one or more acts directed to forming the dopant profile described herein. In one example, a n+-p Si wafer was prepared by a standard thermal diffusion process using a 2-inch (100) Si wafer. Phosphorus, an n-type dopant, was spin-coated on the front side of the polished p-type Si (100) wafer. Boron, a p-type dopant, was spin-coated on the backside of the wafer. Then, the wafer was thermally annealed at 950eC under a nitrogen atmosphere for 4 h.

[0089] The method 700 includes an act 710 in which a nanowire or other nanostructure array is grown or otherwise formed on the substrate. Each nanowire is formed on the surface of the substrate such that each nanowire extends outward from the surface of the substrate. Each nanowire has a semiconductor composition, as described herein. The nanowire growth may be achieved in an act 712 in which molecular beam epitaxy (MBE) is implemented. The MBE procedure may be implemented under nitrogen-rich conditions to promote the formation of N-rich surfaces (which are useful for prevention of photo-corrosion and oxidation). Alternatively or additionally, the substrate may be rotated during the MBE procedure such that each nanostructure is shaped as a cylindrically shaped nanostructure. Each nanowire may thus have a circular cross-sectional shape, as opposed to a plateshaped or sheet-shaped nanostructure.

[0090] In some cases, the MBE procedure may be modified to fabricate the arrangement of layers or segments of each nanowire directed to providing a multi-band structure. Various parameters may be adjusted to achieve the different composition levels of the layers. For instance, the substrate temperature may be adjusted in an act 714. Beam equivalent pressures may be also adjusted in the act 714.

[0091] The act 710 includes doping the nanowires n-type in an act 716. For example, Ga and silicon and / or other fluxes may be controlled by using thermal effusion cells. In somecases, a dopant cell temperature is adjusted in an act 718 to control the doping (e.g., n-type doping) of the nanowires.

[0092] During the act 710, nitrogen radicals may be produced from a radio-frequency nitrogen plasma source. In one example, a nitrogen flow rate of 1 .0 seem and a forward plasma power of about 350 W were used in the growth process.

[0093] In one example, plasma-assisted molecular beam epitaxy was employed for the growth of GaN nanowires on the front side of a n+-p Si wafer under nitrogen-rich conditions with an N2flow rate of 1 .0 standard cubic centimeter per minute. The substrate temperature was held at 790eC and the growth duration was about 2 h. The forward plasma power was 350 W with Ga flux beam equivalent pressure of 5x10-8Torr.

[0094] The act 710 may include additional, fewer, or alternative acts. For instance, the act 710 may include one or more acts directed to forming a seed other initial layer in preparation for growth of the nanowires. The seed layer may be configured to promote the nucleation of the nanowires. In some cases, the seed layer is composed of, or otherwise includes, Ga. Further details regarding the use of seed layers are set forth below in connection with a number of examples as well as in the above-referenced patent documents.

[0095] As shown in Figure 7, the method 700 further includes an act 720 in which the array is decorated with a catalyst arrangement. Catalysts nanoparticles are deposited across the array of nanowire. In the example of Figure 7, the act 720 include an act 722 in which the nanowires are decorated with a catalyst arrangement. The act 722 may include depositing metal nanoparticles on the nanowires. The nanoparticles may be composed of, or otherwise include, silver and / or other metals, as described herein. In some cases, the deposition of the nanoparticles includes implementation of a photo-deposition procedure in an act 724. Alternative or additional deposition procedures may be used to deposit the nanoparticles, including, for instance, an e-beam evaporation procedure. Still further or alternative procedures may be used, including, for instance, other physical vapor deposition procedures, such as sputtering, as well as atomic layer deposition procedures.

[0096] In photo-deposition cases, the array of nanowires or other conductive projections are immersed in a precursor solution, as described herein. The precursor solution may have a precursor molar concentration at a level to form nanoparticles of a desired size. For instance, the precursor molar concentration may be set to a level such that each nanoparticle of the plurality of nanoparticles has a size that falls in a range from about 10 nm to about 80 nm. The molar concentration may be sufficiently low to avoid formation of plateshaped structures on the array of conductive projections. The molar concentration level mayalso be selected to limit the plurality of nanoparticles to partial coverage of each conductive projection of the array of conductive projections.

[0097] In one example, a quarter wafer of GaN / Si was placed on a Teflon holder and placed in a glass reactor with a quartz lid. 20 pl of 0.2 M chloroplatinic acid hydrate (99.9%, Sigma Aldrich), 55 ml of deionized water, and 11 ml methanol were filled into the glass reactor. The reactor was evacuated using a rotary vacuum pump for 5 min. Then, light was irradiated on the sample through a quartz lid using a 300 W xenon lamp for 30 min. The light absorbed by the GaN nanowires generates electrons and holes in the conduction and valance bands, respectively. The photogenerated electrons migrate to the GaN surface and reduce platinum ions derived from chloroplatinic acid hydrate in the aqueous solution, leading to the growth of Pt nanoparticles on the GaN nanowires. To maintain charge balance, the photogenerated holes participate in the oxidation reaction of sacrificial methanol, which is introduced into the solution. Finally, after 30 minutes, Pt-coated GaN nanowires were obtained, rinsed with DI water, and dried by blowing air.

[0098] The method 700 may include one or more additional acts directed to forming the photocatalytic structures of the device. For instance, in some cases, the method 700 includes an act 726 in which the photocatalytic structures of the device are annealed. The parameters of the anneal process may vary. Alternatively or additionally, the device may be washed (e.g., in deionized water) and dried (e.g., at 150 degrees Celsius) in argon atmosphere before use (e.g., in photocatalytic methane reforming).

[0099] The method 700 includes an act 728 in which a surface treatment procedure is implemented to modify the plurality of metal catalyst nanoparticles. As described herein, implementing the surface treatment procedure includes removing a subset of the plurality of metal catalyst nanoparticles, and further depositing a further plurality of metal catalyst nanoparticles after removing the subset. The nanoparticles removed in the act 728 may include metal catalyst nanoparticles of the plurality of metal catalyst nanoparticles that are not lattice-matched with the Ill-nitride semiconductor material.

[0100] The act 728 may include illuminating the photoelectrode device in a photoelectrochemical reaction in an act 730. In some cases, illuminating the photoelectrode device includees irradiating the photoelectrode device with concentrated solar light. In other cases, additional or alternative radiation is used.

[0101] In one example, after completing each step of the photoelectrochemical hydrogen evolution reaction, the Pt / GaN / Si photoelectrode was removed from the reactor chamber for redeposition. The identical Pt photo-deposition process described above was then carriedout for a duration of 30 minutes in an aqueous solution composed of 20 pl of 0.2 M chloroplatinic acid hydrate, 55 ml of deionized water, and 11 ml of methanol under a 300 W Xe lamp. Subsequent to the photo-deposition, the dried samples were reintroduced into the reactor for the evaluation of stability and performance. In other cases, the re-deposition may differ from the initial deposition process in one or more ways.

[0102] Additional or alternative surface treatment methods may be used to realize the post-reaction surface. For instance, temperature annealing and / or plasma treatments may be used. These methods may be used to establish a strong binding at the interface between GaN and Pt, thereby minimizing the detachment of Pt from the surface.

[0103] As described herein, the illumination of the photoelectrode dissolves a spurious layer of a surface oxide of the Ill-nitride semiconductor material on each nanowire. The nanoparticles disposed on the dissolved spurious layer are thus removed. Other non- aligned nanoparticles may also be removed in this manner, including, for instance, adsorbed nanoparticles.

[0104] To deposit further nanoparticles, the act 728 may also include implementing a photo-deposition and / or other deposition procedure in an act 732.

[0105] In the example of Figure 7, the surface treatment procedure is repeated a number of times in an act 730. For instance, the surface treatment procedure may be repeated at least five times.

[0106] The method 700 may include a number of acts directed to forming one or more other components of the photoelectrode. For instance, one or more acts may be directed to forming a backside contact. In one example, a liquid Gain eutectic alloy was applied to the Cu back contact with a circular hole in the center for the incident light to pass through. Then, the Gain eutectic alloy was sandwiched between the Cu back contact and the backside of the n+-p Si wafer to form an ohmic contact.

[0107] The order of the above-described acts of the method 700 may differ from the example shown. For instance, the annealing of the act 728 may be implemented before or after the deposition of the nanoparticles in the act 720.

[0108] Figure 8 depicts an example system 800 that may be used for PEC water splitting with concentrated solar light. The system 800 includes a photoelectrode 802 fabricated and configured as described herein for stable and high-yield hydrogen production despite the harsh conditions presented by the concentrated solar light.

[0109] Described above are examples of photoelectrodes and other devices having a heterointerface between catalyst nanoparticles (e.g., Pt cocatalysts) and a nanostructures (e.g., single crystal GaN nanostructures) that provides efficient, stable, low-cost, and high- yield solar to H2conversion under concentrated solar light. Although there was rapid decay in the initial 0.5 h of reaction due to accelerated surface modifications, the example Pt / GaN / Si photoelectrodes maintained high performance thereafter owing to the stably anchored lattice-matched Pt nanoparticles on the GaN nanowires. Furthermore, the redeposition of Pt nanoparticles on the reacted surface significantly improved the stability of the photoelectrode, outperforming conventional photoelectrodes. The stable bonding of Pt nanoparticles on single crystalline GaN nanowires provide an efficient and durable photoelectrode for water splitting and other reactions.

[0110] The term "about" is used herein in a manner to include deviations from a specified value that would be understood by one of ordinary skill in the art to effectively be the same as the specified value due to, for instance, the absence of appreciable, detectable, or otherwise effective difference in operation, outcome, characteristic, or other aspect of the disclosed methods and devices.

[0111] The present disclosure has been described with reference to specific examples that are intended to be illustrative only and not to be limiting of the disclosure. Changes, additions and / or deletions may be made to the examples without departing from the spirit and scope of the disclosure.

[0112] The foregoing description is given for clearness of understanding only, and no unnecessary limitations should be understood therefrom.

Claims

What is Claimed is:

1. A method of fabricating an electrode device, the method comprising: providing a substrate of the electrode device, the substrate having a surface; growing an array of conductive projections on the surface of the substrate such that each conductive projection of the array of conductive projections extends outward from the surface of the substrate, each conductive projection of the array of conductive projections comprising a nitride material; depositing a plurality of metal catalyst nanoparticles across the array of conductive projections; and implementing a surface treatment procedure to modify the plurality of metal catalyst nanoparticles; wherein implementing the surface treatment procedure comprises: removing a subset of the plurality of metal catalyst nanoparticles; and further depositing a further plurality of metal catalyst nanoparticles after removing the subset; and wherein the removed subset comprises metal catalyst nanoparticles of the plurality of metal catalyst nanoparticles that are not lattice-matched with the nitride material.

2. The method of claim 1 , further comprising repeating the surface treatment procedure.

3. The method of claim 2, wherein the surface treatment procedure is repeated at least five times.

4. The method of claim 1 , wherein removing the subset comprises illuminating the electrode device in a photoelectrochemical reaction.

5. The method of claim 4, wherein illuminating the electrode device comprises irradiating the electrode device with concentrated solar light.

6. The method of claim 1 , wherein further depositing the further plurality of metal catalyst nanoparticles comprises implementing a photo-deposition procedure.

7. The method of claim 1 , wherein the nitride material comprises a nitride semiconductor.

8. The method of claim 1 , wherein the nitride material comprises a Ill-nitride semiconductor.

9. The method of claim 1 , wherein each metal catalyst nanoparticle of the plurality of metal catalyst nanoparticles and of the further plurality of metal catalyst nanoparticles comprises platinum.

10. The method of claim 1 , wherein removing the subset comprises dissolving a spurious layer of a surface oxide of the nitride material on each conductive projection of the array of conductive projections.

11. The method of claim 10, wherein the metal catalyst nanoparticles of the removed subset comprise nanoparticles disposed on the dissolved spurious layer and nanoparticles adsorbed on the array of conductive projections.

12. A method of fabricating an electrode device, the method comprising: providing a substrate of the electrode device, the substrate having a surface; growing an array of conductive projections on the surface of the substrate such that each conductive projection of the array of conductive projections extends outward from the surface of the substrate, each conductive projection of the array of conductive projections comprising a nitride material; depositing a plurality of metal catalyst nanoparticles across the array of conductive projections; implementing a surface treatment procedure to modify the plurality of metal catalyst nanoparticles, wherein implementing the surface treatment procedure comprises: removing a subset of the plurality of metal catalyst nanoparticles; and further depositing a further plurality of metal catalyst nanoparticles after removing the subset; and repeating the surface treatment procedure.

13. The method of claim 12, wherein the surface treatment procedure is repeated at least five times.

14. The method of claim 12, wherein removing the subset comprises illuminating the electrode device in a photoelectrochemical reaction.

15. The method of claim 14, wherein illuminating the electrode device comprises irradiating the electrode device with concentrated solar light.

16. The method of claim 12, wherein further depositing the further plurality of metal catalyst nanoparticles comprises implementing a photo-deposition procedure.

17. A device comprising: a substrate having a surface; an array of conductive projections supported by the substrate and extending outward from the surface of the substrate, each conductive projection of the array of conductive projections comprising a nitride material; and a plurality of metal catalyst nanoparticles disposed over the array of conductive projections; wherein: each metal catalyst nanoparticle of the plurality of metal catalyst nanoparticles is lattice-matched with the nitride material; and each metal catalyst nanoparticle of the plurality of metal catalyst nanoparticles has an inner void.

18. The device of claim 17, wherein each conductive projection of the array of conductive projections lacks a surface layer of an oxide of the nitride material.

19. The device of claim 17, wherein: the nitride material comprises a Ill-nitride semiconductor; and each metal catalyst nanoparticle of the plurality of metal catalyst nanoparticles and of the further plurality of metal catalyst nanoparticles comprises platinum.

20. A photoelectrochemical (PEC) system comprising a working electrode configured in accordance with the device of claim 17, and further comprising: a counter electrode; an electrolyte in which the working and counter electrodes are immersed; and a voltage source that applies a bias voltage between the working and counter electrodes; wherein the bias voltage is set to a level for production of hydrogen at the working electrode.

21. A method of operating the PEC system of claim 20, comprising: immersing the working and counter electrodes in an electrolyte; and illuminating the working electrode with concentrated solar light.

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