Capping layer-based photocatalytic device protection

Abstract

A device for catalyzing a reaction includes a substrate, an array of nanostructures supported by the substrate, each nanostructure of the array of nanostructures including a sidewall surface that extends outward from the substrate and an end surface at an outer end of the nanostructure, and a protection architecture composed of a metal oxide and disposed on each nanostructure of the array of nanostructures, the protection architecture including a continuous capping layer that covers the end surface of each nanostructure and a discontinuous distribution of the metal oxide disposed on the sidewall surface of each nanostructure.

Description

Atty. Docket No. 10110-24016ACAPPING LAYER-BASED PHOTOCATALYTIC DEVICE PROTECTIONCROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of U.S. provisional application entitled “Capping Layer-Based Photocatalytic Device Protection,” filed September 20, 2024, and assigned Serial No. 63 / 697,227, the entire disclosure of which is hereby expressly incorporated by reference.BACKGROUND OF THE DISCLOSUREField of the Disclosure

[0002] The disclosure relates generally to photocatalytic devices.Brief Description of Related Technology

[0003] Hydrogen production through solar water splitting is one of the most promising approaches for solar energy conversion to chemical fuels. Along this line, unassisted photocatalytic solar water splitting at a stoichiometric H2to O2ratio of 2:1 is useful for sustainable and renewable green fuel production. The counter electrode is deposited on the photocatalyst surface as nanoparticle cocatalysts in this wireless counterpart of the photoelectrochemical water splitting process, which can substantially reduce the system cost due to its simplicity. Furthermore, photocatalytic solar water splitting can viably operate in freshwater or seawater with no additional bias or circuitry, and can thus address a number of photocatalytic stability and safety-related concerns. Despite the substantial progress made in the solar-to-hydrogen (STH) efficiency of photocatalytic devices, their long-term stability has remained a considerable challenge, which has thus far prevented the large-scale practical application of this technology.

[0004] Some of the issues limiting the long-term stability of photocatalytic reactions are photocatalytic corrosion and cocatalyst nanoparticle displacement ( / .e., agglomeration and detachment), as depicted schematically in Figure 1 , part a. Catalytic corrosion is a common phenomenon for materials reacting in aqueous media, leading to faster deactivation of the materials. Furthermore, the photocatalytic activity is directly related to the uniformity ofAtty. Docket No. 10110-24016A cocatalyst nanoparticle dispersion on the photocatalyst surface. The nanoparticles tend to migrate and aggregate on the photocatalyst surface in response to thermal or optical excitation due to formation energy differences caused by the lattice mismatch variation between the nanoparticles and different photocatalyst facets. The resulting nanoparticle agglomeration leads to the degradation of photocatalytic activity due to the reduced number of active sites on the semiconductor material surface. Moreover, cocatalyst nanoparticle detachment or dissolution during the reaction can also result in the loss of active sites, leading to an expedited deactivation of the photocatalyst materials.

[0005] Ill-nitride semiconductors such as InGaN are one of the few materials among the currently known photocatalysts with a tunable energy bandgap across the entire solar spectrum, while straddling the water redox potentials under ultraviolet (UV), visible, and near-infrared (IR) spectra. As such, these semiconductor materials are useful as efficient light absorbers for enhanced photocatalytic activity in solar water splitting. Moreover, the considerable chemical stability of metal-nitride semiconductors further makes them a viable option as photocatalysts for practical solar water splitting applications.

[0006] The inclusion of protection layers has been reported to have significantly enhanced the stability of different photoelectrode materials for PEC solar water splitting. For instance, Lyu etal., Chem. Sci 10 (2019), showed that the photo-deposition of CoOOH and TiC>2 on oxide-based photocatalysts helped reduce the cocatalyst dissolution and significantly enhanced the material durability. The work of Han et al., ACS Catal. 11 , 11049-11058 (2021), also reported improved stability of oxide photocatalysts resulting from CrOxmodification.SUMMARY OF THE DISCLOSURE

[0007] In accordance with one aspect of the disclosure, a device for catalyzing a reaction includes a substrate, an array of nanostructures supported by the substrate, each nanostructure of the array of nanostructures including a sidewall surface that extends outward from the substrate and an end surface at an outer end of the nanostructure, and a protection architecture composed of a metal oxide and disposed on each nanostructure of the array of nanostructures. The protection architecture includes a continuous capping layer that covers the end surface of each nanostructure and a discontinuous distribution of the metal oxide disposed on the sidewall surface of each nanostructure.

[0008] In accordance with another aspect of the disclosure, a method of fabricating a device includes providing a substrate of the device, forming an array of nanostructuresAtty. Docket No. 10110-24016A supported by the substrate, each nanostructure of the array of nanostructures including a sidewall surface that extends outward from the substrate and an end surface at an outer end of the nanostructure,

[0009] depositing a metal layer on the end surface of each nanostructure of the array of nanostructures, and after depositing the metal layer, implementing a reaction that redistributes and oxidizes metal atoms from the metal layer to form a protection architecture on each nanostructure of the array of nanostructures, the protection architecture including a capping layer that covers the end surface of each nanostructure and a distribution of metal oxide disposed on the sidewall surface of each nanostructure.

[0010] 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 continuous capping layer is thicker than the discontinuous distribution. The device further includes a plurality of catalyst nanoparticles disposed over the array of nanostructures. The discontinuous distribution is configured to stabilize respective catalyst nanoparticles of the plurality of catalyst nanoparticles along the sidewall surface. The metal oxide of the discontinuous distribution is located at interfaces between the respective catalyst nanoparticles and the nanostructure without blocking a plurality of active sites for the reaction along the sidewall surface. Each catalyst nanoparticle of the plurality of catalyst nanoparticles includes metal atoms. The metal atoms are anchored to the metal oxide of the discontinuous distribution via chemical bonds formed through an atom-trapping mechanism. The plurality of catalyst nanoparticles include Rh / Cr2O3 core-shell nanoparticles and CoOxnanoparticles. The protection architecture is composed of aluminum oxide. Each nanostructure of the array nanostructures has a semiconductor composition. The sidewall surface is an m-plane of a lattice of the semiconductor composition. The end surface is a c-plane of a lattice of the semiconductor composition. Implementing the reaction includes illuminating the array of nanostructures. The reaction is implemented for a time period sufficient to create a thermal gradient along each nanostructure between the end surface and the substrate. The time period is about 60 minutes or more. Implementing the reaction includes implementing a photocatalytic water splitting reaction. Depositing the metal capping layer includes implementing a directional deposition procedure. Depositing the metal capping layer includes implementing an electron beam evaporative deposition procedure. Depositing the metal capping layer includes implementing an anneal procedure in an oxygen environment after deposition of metal atoms of the metal capping layer. The capping layer is continuous. The distribution of metal oxide is discontinuous. The method further includes disposing aAtty. Docket No. 10110-24016A plurality of catalyst nanoparticles over the array of nanostructures. The distribution of metal oxide is configured to stabilize respective catalyst nanoparticles of the plurality of catalyst nanoparticles along the sidewall surface.BRIEF DESCRIPTION OF THE DRAWING FIGURES

[0011] 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.

[0012] Figure 1 depicts (a) a schematic illustration of photocatalytic corrosion and cocatalyst nanoparticle displacement during a water splitting reaction, (b) a 45° tilt SEM image of an example as-grown p-type InGaN nanowire (NW) array, with a schematic view of different layers of the p- InGaN nanowire photocatalyst shown in an inset, (c) a flow diagram of a method of fabricating the Al203-protected p-lnGaN nanowire sample with cocatalyst nanoparticles (NPs) in accordance with one example, (d) EDX elemental mapping of the AI2O3-capped cocatalyst-decorated p-lnGaN nanowires after 0 min, 30 min, and 60 min of photocatalytic reactions, (e) HAADF-STEM-EDX mapping of the nanowire showing the AI2Os thickness change after 0 min, 20 min, 40 min, and 60 min of photocatalytic reactions.

[0013] Figure 2 depicts graphical plots of (a) photocatalytic stability of AI2O3-capped cocatalyst-loaded p-lnGaN nanowire examples with different AI2Os capping thicknesses, and (b) stability testing of cocatalyst-decorated p-lnGaN nanowires with and without AI2Os- capping under concentrated light of 1000 mW cm-2at 70 °C temperature.

[0014] Figure 3 depicts (a, b) EDX elemental mappings of the cocatalyst-loaded samples for different reaction durations (a) without and (b) with AI2Os protection, showing that the cocatalyst nanoparticles are agglomerated in the case of no AI2Os protection, and (c,d) schematic depictions of (c) cocatalyst NP displacement in an unprotected nanostructure and (d) cocatalyst stabilization effect by an AI2Os protection layer in accordance with one example.

[0015] Figure 4 depicts (a) 45° tilt SEM images showing the time evolution of an undecorated, unprotected p-lnGaN nanowire structure during a 24-hour photocatalytic reaction, (b) a magnified HAADF-STEM image of the etched p-lnGaN nanowires from the yellow-boxed region in the nanowire structure of part (a), (c) a graphical plot of nominal nanowire height of the cocatalyst-decorated sample with and without AI2Os protection for different reaction durations, and (d, e) schematic illustrations of the (d) photocatalyticAtty. Docket No. 10110-24016A corrosion in an unprotected nanostructure and (e) reduced corrosion due to AI2O3 protection in accordance with one example.

[0016] Figure 5 depicts graphical plots of TRPL signals (from about 440 nm to about 460nm) of three sets of InGaN / GaN nanowire devices for 0, 2, 4, and 10 reaction days, including (a) AI2O3-capped cocatalyst-decorated nanowire examples, (b) bare InGaN / GaN nanowire samples with no protection and decoration, and (c) unprotected cocatalystdecorated nanowire samples.

[0017] Figure 6 is a schematic view of a photocatalytic device having capping layer-based protection in accordance with one example.

[0018] Figure 7 is a flow diagram of a method of fabricating a photocatalytic device with capping layer-based, dynamic protection in accordance with one example.

[0019] 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

[0020] Photocatalytic and other catalytic devices with capping layer-based protection are described. The capping layer-based protection scheme of the disclosed devices involves a dynamic process in which a metal oxide (e.g., AI2O3) protection layer evolves during the reaction. Methods for fabricating such devices via the dynamic process are also described.

[0021] The capping layer-based protection of the disclosed methods and devices addresses the challenges in achieving long-term photocatalytic stability of, for instance, InGaN nanowire-based photocatalysts and, in so doing, achieves enhanced longevity in those and other catalytic devices. For instance, the capping layer-based protection addresses the photocatalytic etching at the top c-plane of the nanowires that would otherwise occur. Cocatalyst nanoparticle aggregation along the lateral m-plane surfaces and the loss of co-catalyst nanoparticles are also addressed by the protection scheme of the disclosed methods and devices.

[0022] As described herein, the protection layer is formed via redistribution and subsequent oxidation of metal adatoms from a capping layer on a top surface (e.g., c-plane) of the nanowire to sidewalls (e.g., non-polar, m-plane sidewalls) of the nanowires. The presence of the capping layer thus prevents nanowire etching and nanoparticle aggregation on the topAtty. Docket No. 10110-24016A surface, while the redistribution and oxidation of the metal adatoms inhibits the detachment of nanoparticles by stabilizing the nanoparticles on the sidewalls. As a result, the metal oxide protection layer achieves a significant enhancement in photocatalytic longevity. The resulting protection may be used to support the implementation of long-term stable photocatalyst systems for practical large-scale sunlight-driven overall water splitting.

[0023] Described herein are examples of semiconductor photocatalysts with long-term stable operation under concentrated solar irradiation. The examples utilize a photocatalyst protection architecture that includes an AI2O3 capping layer on GaN-based photocatalyst nanostructures to mitigate two of the most dominant bottlenecks in long-term stability: photocatalytic corrosion and cocatalyst nanoparticle displacement. The examples exhibit stable operation for about 1500 hours without significant performance degradation under concentrated sunlight. Detailed investigation reveals the dynamic characteristic of the capping layer, in which Al atoms diffuse from the nanowire c-plane toward the m-plane during the reaction and subsequently become oxidized to form an AI2O3 layer, thereby stabilizing the cocatalyst nanoparticles on the sidewall m-plane and preventing the photocatalyst top c-plane corrosion at the same time. The examples thus demonstrate the manner in which the disclosed devices and methods overcome the challenge of simultaneously achieving high efficiency and stability in clean fuel generation, thereby supporting a variety of useful applications of photocatalytic devices and systems, e.g., for sustainable green energy.

[0024] Although described in connection with photocatalytic water-splitting examples, the disclosed photocatalytic devices and systems may be used in other chemical reaction contexts and applications. For instance, the disclosed photocatalytic devices and systems may be useful in connection with nitrogen reduction to ammonia, CO2 reduction to various fuels and other chemicals, and activation of C-H bonds for the production of various chemicals.

[0025] Although described in connection with photocatalytic devices and systems, the disclosed methods and devices may be applied to other catalytic systems. For instance, the disclosed devices and methods may be used in connection with photoelectrochemical (PEC) systems and other systems in which a bias voltage or other assistance is provided. The disclosed devices and methods may also be useful in other types of catalytic contexts and systems, including, for instance, electrocatalytic and thermocatalytic systems. In these and other cases, the protection arrangement of the disclosed devices and methods may be combined with one or more other protection schemes, including, for instance, the oxynitridebased stabilization scheme described in WO 2022 / 187133 ("Crystallographic- andAtty. Docket No. 10110-24016AOxynitride-Based Surface Stabilization"), the entire disclosure of which is hereby incorporated by reference.

[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. Thus, the nature, construction, configuration, composition, characteristics, shape, and other aspects of the conductive projections through which the water splitting is implemented may vary.

[0027] Although described in connection with AI2O3 capping and protection layers, the disclosed methods and devices may use additional or alternative metals and metal oxides. For instance, titanium (Ti) and TiO2 may be used. Still other oxides may be used, including, for instance, Ga2Os and HfOx.

[0028] Figure 1 , part b, depicts a scanning electron microscopy (SEM) image of an as- grown p-type InGaN / GaN nanowire array on a Si (111) substrate synthesized using molecular beam epitaxy. A schematic illustration showing the different layers of each nanowire structure is depicted in the inset. The details of the epitaxial growth can be found in, for example, P. Zhou, et al., Nature 613 (2023), and M. G. Kibria, et al., Nat. Comm. 6 (2015), the entire disclosures of which are hereby incorporated by reference. Further details regarding the growth of the nanowires are set forth in U.S. Patent No. 8,563,395 ("Method of growing uniform semiconductor nanowires without foreign metal catalyst and devices thereof"), the entire disclosure of which is hereby incorporated by reference.

[0029] In the example of Figure 1 , the length of the nanowires is about 850 nm with a diameter of about 150 nm. The dimensions may vary in other cases. Rh / Cr2O3 core / shell and CoOx nanoparticles were photo-deposited on the p-type InGaN / GaN nanowires as cocatalysts to promote H2and O2 generation, respectively.

[0030] Figure 1 , part c, depicts the steps involved in synthesizing the AhOs-protected InGaN nanowire photocatalyst. In this example, a thin (about 20 nm) protective AI / AI2O3 capping layer is deposited or otherwise formed on top of the cocatalyst decorated p-lnGaN nanowires. The capping layer is configured to suppress nanowire etching and cocatalyst nanoparticle displacement, as described herein. In this example, Al was deposited using electron-beam (e-beam) evaporation followed by rapid thermal annealing (RTA) in an O2 environment at 500 °C for 5 mins. In this preparation process, some portion of the deposited Al is oxidized to form an AhOs-coated Al layer. Apart from primarily protecting the top c- plane of the nanowire photocatalyst, the protection layer does not block the catalyticallyAtty. Docket No. 10110-24016A active sites on the m-plane, due to the directional nature of the e-beam deposition. The O2 treatment catalytically further stabilizes the material. As such, this protection scheme helps increase the long-term stability of the photocatalyst without sacrificing its activity. Upon conducting photocatalytic reactions on this structure, the dynamic nature of the oxide layer can be observed as the Al atoms of the capping layer deposited on the nanowire tip migrate to the sidewall m-plane and subsequently get oxidized, as depicted in the final step of Figure 1 , part c.

[0031] Scanning transmission electron microscopy (STEM) images along with the elemental mapping shown in Figure 1 , part d, confirm the successful e-beam deposition of Al followed by the O2 environment RTA, resulting in the formation of a thin AhOs-coated Al layer on top of each nanowire structure before the reaction. As can be observed in the STEM images with chemical mappings of the structure with different reaction durations in Figure 1 , part d, the AI / AI2O3 capping layer stays on the nanowire tip at the initial stage of the reaction without undergoing much change. However, the AI / AI2O3 protection layer starts changing, and is redistributed (e.g., partially redistributed) after 60 mins of the photocatalytic reaction. During the ongoing reaction, the AI / AI2O3 capping layer continues to absorb optical excitation from the light source, which results in the localized heating of Al metal species. Consequently, a thermal gradient from the photocatalyst nanowire tip towards the bottom is formed, with the heated side on top. As such, a thermomigration force is applied to the Al adatoms in the direction opposite to the thermal gradient and, therefore, the Al adatoms diffuse from the heated nanowire tip towards the relatively cold nanowire sidewalls and root. These thermally migrated Al atoms eventually get oxidized during the photocatalytic reaction to form AI2O3 on the nanowire sidewall. In this example, it took about 60 minutes for the capping layer Al atoms to acquire sufficient thermal energy from the optical excitation to overcome the diffusion energy barrier for the thermomigration, which is reflected by the significant capping layer redistribution after 60 minutes. The reaction time may vary in other cases (e.g., involving alternative or additional metals).

[0032] Figure 1 , part e, depicts the high-angle annular dark field (HAADF)-STEM elemental mappings of one of the nanostructures, showing the thickness change of the AI / AI2O3 capping layer over time. It can be observed that the decrease in the AI / AI2O3 top layer thickness is considerably small for the first 40 minutes of the reaction, even though the AI2O3 formation on the sidewall starts becoming noticeable after 40 minutes. However, after 60 minutes of the reaction, the AI2O3 top layer thickness is significantly reduced with much more enhanced oxide layer attachment on the sidewall.Atty. Docket No. 10110-24016A

[0033] Further analyses were performed in order to identify the dynamic nature of the AI2O3 protection layer on the InGaN photocatalyst. X-ray photoelectron spectrum (XPS) measurements were performed on the examples, in which the c-plane nanowire surfaces were predominantly measured. The XPS signal for the Al 2p orbital is initially strong on the top c-plane of the nanowire. Based on the STEM elemental mapping before reaction shown in Figure 1 , part d, the estimated Al amount is 51 atomic percentage (at%), whereas the O amount is 48 at%. The substoichiometric O amount indicates the presence of Al and AI2O3 in this capping layer at the initial stage. After a longer reaction time, the Al 2p signal strength considerably decreases due to the Al redistribution from the c-plane. The Al (e.g., metallic Al) in the c-plane capping layer migrates to the nanowire sidewall during the initial stage of the reaction (60 mins). As the reaction continues, some portion of this Al is oxidized and stays on the sidewall, while the rest starts to dissolve in the water solution. For the longer duration of the photocatalytic reaction (1 -64 days), the Al signal intensity in the STEM elemental mapping becomes much weaker which indicates that more Al from the nanowire sidewall continues to dissolve in water with the ongoing long-term reaction. This is further corroborated by the inductively coupled plasma-atomic emission spectrometer (ICP-AES) test shown in the Supporting Information Figure S3. It can be observed that the total Al content dissolved in the water continues to increase as the long-term reaction goes on.Moreover, high resolution (HR)-STEM images indicated that there is no formation of AI / AI2O3 lattices within the metal co-catalyst nanoparticles during the reaction. The nanoparticles are thus mainly composed of the lattices of Rh / Cr / Co oxides.

[0034] Photocatalytic stability analysis for overall water splitting (OWS) was performed in pure water at 70 °C under concentrated simulated solar illumination (about 10 suns or 1000 mW cm-2). The stability results for the samples with different AI / AI2O3 capping layer thicknesses (5 nm, 10nm, and 20 nm) are displayed in Figure 2, part a. It is noticeable that the stability of the structure increases with thicker oxide protection layers. If the capping layer is thin, there will not be enough Al atoms that can migrate and subsequently become oxidized to cover the nanowire sidewall entirely. As such, there will be exposed portions of the nanowire sidewall with no oxide protection resulting in worse photocatalytic stability for samples with sub-optimal capping layer thickness. Moreover, the solar-to-hydrogen (STH) efficiency of the photocatalyst nanowires is not adversely affected in this protection scheme, and STH values over 9% are achieved with different AI / AI2O3 top layer thicknesses. These results are achieved because the oxide protection architecture does not create any physical barrier between the catalytically active sites on the nanowire sidewall and the aqueous solution.Atty. Docket No. 10110-24016A

[0035] This aspect of the protection scheme was further corroborated by depositing (via ALD) a conformal atomic layer of AI2O3with a thickness of about 0.5 nm on a photocatalyst nanowire sample and testing its water splitting activity. Even with such a thin AI2O3 layer deposited by conformal ALD, the STH efficiency of the nanowire photocatalyst was significantly reduced because the conformal layer essentially blocks the active sites on the nanowire m-plane.

[0036] The photocatalytic test results over time with normalized STH efficiency for cocatalyst-loaded samples with 20 nm AI / AI2O3 capping layer and without protection are shown in Figure 2, part b. The cocatalyst-decorated p-type InGaN nanowire photocatalyst without any protection layer can retain more than 50% of its maximum activity for approximately 250 hours before the photocatalyst is deactivated (less than 50% of the maximum activity value). On the other hand, remarkably enhanced photocatalytic stability has been achieved in the ALOs-protected architecture, which was able to sustain more than 50% of its maximum activity for about 1500 hours (as compared to 250 hours for the sample without any protection layer). SEM images also indicated that, without AI2O3 protection, the etching and the cocatalyst nanoparticle agglomeration take place on the top c-plane of the nanowires after the reaction.

[0037] To analyze the cocatalyst displacement phenomenon in detail, STEM elemental mapping was performed on the unprotected and ALOs-protected samples after several hours of reaction as shown in Figure 3, parts a and b. There is a clear agglomeration of the cocatalyst nanoparticles observed for the sample with no AI2O3 protection after several hours of photocatalytic reaction (Figure 3, part a). Furthermore, EDX spectra also demonstrated the agglomeration of the cocatalyst nanoparticles (Cr, Co, and Rh) in the unprotected nanowires after several hours of photocatalytic reaction, mostly towards the top c-plane. Previous studies have reported that hydrogen bubble growth induces mechanical stress on the cocatalyst nanoparticle surface. This stress drives the cocatalyst nanoparticle migration and agglomeration towards nanowire facets with different formation energies, resulting in severe degradation of the photocatalytic performance.

[0038] Conversely, in an ALOs-protected example, the cocatalyst nanoparticles did not undergo much agglomeration even after about 200 hours of reaction, as shown in Figure 4, part b. As previously discussed, the AI / AI2O3 protection layer initially covers the top c-plane and Al subsequently redistributes itself to the sidewall m-plane of the nanowire followed by oxidation during the photocatalytic reaction. Thus, the AI2O3 layer occupies different photocatalyst facets, primarily the nanowire top c-plane and sidewall m-plane. As previously described, these different nanowire facets have different formation energies and thisAtty. Docket No. 10110-24016A variation in the formation energy causes the nanoparticles to migrate and aggregate from one photocatalyst surface to another in response to optical or thermal excitation. As such, the AI2O3 coverage on different surfaces of the nanowire minimizes the variation in the formation energies of different photocatalyst facets and this, in turn, results in the suppressed cocatalyst migration and agglomeration.

[0039] An ICP-AES test was conducted to compare the cocatalyst (Rh, Cr, Co) amount in water after the long-term reactions for the photocatalytic devices with and without AI2O3 protection. It can be observed that the amount of dissolved cocatalyst nanoparticles in water is reduced for the AhOs-protected sample compared to the one without protection, indicating a reduced amount of cocatalyst detachment for the oxide-coated device. Usually the metal nanoparticles on the Ga(ln)N nanowires are either physically adsorbed on the nanowire surface or there is a lattice alignment between metal nanoparticles and Ga(ln)N nanowires. The epitaxially deposited metal nanoparticles have a strong binding at the nanoparticle- nanowire interface and this helps anchor the nanoparticle cocatalysts on the photocatalyst nanowire surface even under harsh reaction conditions. However, the surface-adsorbed metal nanoparticles have much weaker bonding strength at the nanoparticle-nanowire interface and as such are more susceptible to mechanical detachment. In the case of the Al203-protected sample, Al atoms migrate to different photocatalyst sites at the interface between the metal nanoparticles and the Ga(ln)N nanowire as the reaction happens. Some portion of this redistributed Al that is oxidized ends up residing at the interface between the surface-adsorbed metal nanoparticles and Ga(ln)N nanowire with relatively weak bonding strength. With the presence of AI2O3 in these weakly bonded interfacial regions, the cocatalyst metal atoms are trapped on the surface O2-sites, which results in an increased activation energy for metal nanoparticle detachment from the O2-modified photocatalyst surface.

[0040] Furthermore, AI2O3 as a support is reported to possess relatively robust metalsupport interaction strength for a given metal due to its high Huttig and Tamman temperatures. Consequently, there is reduced dissociation of the metal species from the oxide surface and the metal species are anchored on AI2O3 by chemical bonds formed through atom-trapping mechanism. This leads to the further stabilization of the nanoparticle- nanowire binding strength in these relatively susceptible regions, thereby reducing cocatalyst detachment from the oxide-protected photocatalyst surface.

[0041] There was initial Cr dissolution in the long-term photocatalytic reaction for both the Al203-protected and unprotected samples. This could be due to the accelerated dissolutionAtty. Docket No. 10110-24016A of Cr through the formation of CrOH2+and Cr(OH)3in the solution at the early stage of the photocatalytic reaction. However, the amount of Cr dissolution in the long-term reaction (5 days or longer) is much more suppressed in the AhOs-protected devices than that of the unprotected devices due to the stabilization effect of the oxide-protected surface as described earlier, resulting in overall better long-term photocatalytic stability.

[0042] The change in Co amount over time was below the detection limit of the measurement. The overall cocatalyst displacement phenomenon including agglomeration and detachment in the unprotected photocatalyst is schematically illustrated in Figure 3, part c. The nanoparticle stabilization on the photocatalyst surface resulting from the oxide protection architecture is depicted in the schematic diagram of Figure 3, part d.

[0043] As schematically shown in Figure 3, part d, the resulting AI2O3 protection layer along the sidewalls of the nanowire is discontinuous or non-continuous. These sidewall portions of the resulting AI2O3 protection layer are also thinner than the capping layer on the top surface (e.g., c-plane) of the nanowire. In contrast to the sidewall portions, the AI2O3 protection layer along the top surface may be continuous and / or uniform and / or conformal.

[0044] When portions (e.g., c-plane surfaces) of the Ga(ln)N photocatalysts are directly exposed to water, the surface charge transfer toward the overall water splitting reaction is slow. As such, accumulated holes can instead oxidize the Ga(ln)N surface resulting in its dissolution into the surrounding water in the form of Ga2O3(Ga3+ions). In order to analyze this aspect, a control photocatalytic measurement was implemented using the same reaction condition as before on an undecorated as-grown nanowire sample. The SEM and HAADF- STEM images in Figure 4, parts a and b, show that some nanowires of the undecorated sample are severely corroded on the c-plane within just 24 hours. These results substantiate the fact that a nanowire photocatalyst without any nanoparticle coverage or protection layer is highly susceptible to catalytic corrosion leading to much faster deactivation. The nominal nanowire heights of the cocatalyst-loaded samples with and without AI2O3 protection for different reaction durations are shown in Figure 4, part c. After about 100 hours of reaction of the device without any protection, a significant nominal nanowire height reduction of about 200 nm is observed due to nanowire photo-corrosion, relative to devices measured before the reaction. The nanowire height further decreased after about 300 hours of reaction for the unprotected device.

[0045] Conversely, there was very little or no Ga(ln)N dissolution even after about 200 hours of reaction for the example device with AI2O3 protection. There was about 100 nm of nanowire height reduction after about 300 hours of reaction for this AhOs-protected exampleAtty. Docket No. 10110-24016A device. But SEM data established that there was no noticeable nanowire height reduction due to corrosion between that 300-hour reaction point and about the 1500-hour reaction point. This further confirms that the AI2O3 protection layer substantially mitigates the oxidative corrosion during the long-term photocatalytic reaction compared to the case of unprotected and exposed nanowires.

[0046] Up until the 300 hour point of the reaction, the oxidation of the capping (or tip) layer of Al may be incomplete, resulting in a relatively less robust AI2O3 protection. Therefore, nanowire height reduction to some extent is observed. Afterwards, the completely oxidized and stable AI2O3 is formed on the nanowire top plane in the long-term reaction, which prevents any further corrosion by providing a robust physical oxide barrier. The fast nanowire etching for the sample with no protection can be attributed to cocatalyst displacement and the subsequent dissolution of the exposed portions of the photocatalyst as illustrated in the schematic of Figure 4, part d. Moreover, HAADF-STEM imaging along with energy-dispersive X-ray (EDX) elemental mapping of a deactivated nanowire after reaction showed that the photocatalytic corrosion took place mostly on the c-plane of the nanowire during the reaction. In contrast, the cocatalyst stabilization effect along with a physical barrier provided by the AI2O3 layer minimized the direct exposure of the Ga(ln)N photocatalyst to water. This, in turn, substantially reduced the photocatalytic corrosion of the nanowires during the reaction as schematically shown in Figure 4, part e.

[0047] Time-resolved photoluminescence (TRPL) measurements were performed to investigate the influence of catalytic corrosion and nanoparticle agglomeration on the carrier dynamics of the nanowires. The abundance of surface trap states induces Fermi level pinning on the sidewall of the nanowire and therefore downward band bending in the p-type InGaN / GaN nanowire. Under the electric field caused by downward band bending, the photo-excited carriers experience separation, leading to electron accumulation (hole depletion) in the near-surface region. Such a separation process occurs in tens of picoseconds, which is much faster than radiative recombination. Therefore, it can be recorded by the TRPL signal, which is affected by the spatial overlap of carrier distribution.

[0048] Three sets of devices after 0, 2, 4, and 10 days of reaction were under measurement for comparison as shown in Figure 5. AI / ALOs-capped cocatalyst-decorated devices, which have minor catalytic corrosion and nanoparticle agglomeration, show similar PL decay curves for different reaction days (Figure 5, part a). Devices with bare InGaN nanowires, which experience severe catalytic corrosion primarily on the top c-plane but are absent of nanoparticle agglomeration, don’t show an apparent variation on PL decay curves with reaction days either (Figure 5, part b). The rapid PL decay in these two cases showsAtty. Docket No. 10110-24016A efficient carrier separation inside the nanowire. Meanwhile, it also indicates that catalytic corrosion on the nanowire c-plane is not an important factor affecting the carrier dynamics inside the nanowire.

[0049] In contrast, the unprotected, cocatalyst-decorated samples, which suffer from catalytic corrosion and nanoparticle agglomeration simultaneously, exhibit a prolonged PL decay time with increasing reaction days (Figure 5, part c). The nanoparticle migration and agglomeration change the coating thickness unevenly on the sidewall of the nanowire and form a thick coating on the upper side of the nanowire, which significantly affects the band bending of the nanowire. A thicker coating on the nanowire generally causes the surface band bending of the nanowire to align with the intrinsic band bending of the coating material. The metal cocatalyst nanoparticles, which contain numerous free electrons, maintain a constant Fermi level throughout. The metallic coating reduces Fermi-level pinning on the surface of nanowires, thereby diminishing the extent of band bending. Therefore, the degree of carrier separation is reduced, leading to more carriers undergoing recombination, which in turn extends the decay time.

[0050] The above-described TRPL data provides insight into the carrier dynamics during the catalyst reaction and also offers a potential nondestructive detection method for monitoring nanoparticle agglomeration and catalyst reaction efficiency after proper calibration.

[0051] The conventional surface protection via physical coating, such as ALD films, involves overcoating the metal nanoparticle-decorated photocatalyst nanostructure. Such coating layers preferentially decorate the low coordinated metal sites and spatially confine or stabilize the cocatalyst nanoparticles. Such nonporous ALD protection conformally coats all of the photocatalyst nanostructure facets and physically blocks the catalytically active sites of the nanostructure (e.g., the m-plane of a nanowire structure), causing catalytic efficiency degradation as shown in the analyses described herein. While the formation of porous ALD configuration upon high temperature calcination may be effective in enhancing both the catalytic activity and stability, the dynamic AI2O3 protection scheme of the disclosed methods and devices also provides atom trapping that stabilizes the cocatalyst metal nanoparticles on the photocatalyst surface, as described herein. Moreover, the dynamic architecture of the disclosed methods and devices is capable of selectively protecting the more susceptible top surface (e.g., c-plane) of the nanowire photocatalyst from corrosion (unlike the nonporous ALD films), while still not blocking the more robust catalytically active sites on the sidewalls (e.g., the m-plane surfaces).Atty. Docket No. 10110-24016A

[0052] A stability metric comparison of the conventional ALD AI2O3 protection and the dynamic AI2O3 protection provided via the disclosed methods and devices was implemented. The corrosion rate of the photocatalyst and the cocatalyst detachment / leaching were compared. There are a few studies reporting the corrosion current density (lCOrr) of ALD ALOs-protected catalysts. Daubert et al. studied the corrosion properties of copper metal protected with different ALD oxide films (ACS Appl. Mater. Interfaces 9, 4192-4201 (2017)). Gong et al. reported an ALD-AI2O3 protection scheme on wrought and additively manufactured (AM) stainless steel (SS) 316L (Mater. Lett. 331 , 133434:133431 -133435 (2023)). The lCOrr values reported in those studies were converted to corrosion rates using the following equationCorrosion Rate = IcorrA / nFd where A= Atomic Weight, n=Number of valence electrons, F=Faradaic constant, and d=Density. The dynamic ALOs-protected RhCrCo / lnGaN of the disclosed methods and devices was found to have a corrosion rate with a similar order of magnitude, and an identical percentage reduction in corrosion compared to the best ALD ALOs-protected catalyst samples.

[0053] Cocatalyst detachment or leaching is another useful photocatalytic stability metric. While this can be suppressed by applying a protective layer on the photocatalyst, it also depends on the nature of the cocatalyst nanoparticle. The dynamic AI2O3 protection of the disclosed methods and devices achieved about 93% and about 40% reduction of Rh and Cr cocatalyst detachment, respectively. In comparison, Lee et al. reported about 90% reduction in Co cocatalyst leaching with an ALD TiC>2 protection on Co / Ti02 catalyst (Energy Environ. Sci. 7, 1657-1660 (2014)), and Settle et al., up to about 96% decrease in Pd cocatalyst leaching for an ALD ALOs-coated Pd / TiO2 catalyst (Energy Environ. Sci. 7, 1657-1660 (2014)). Therefore, the dynamic AI2O3 protection scheme of the disclosed methods and devices exhibits mostly identical stability metrics compared to the conventional ALD AI2O3 protection, while not having the drawback of efficiency degradation. This is because the dynamic AI2O3 layer can selectively protect the more susceptible c-plane of the nanowire photocatalyst from corrosion while not blocking the catalytically active sites on the more robust m-plane. As such, this protection scheme does not compromise the photocatalytic efficiency in exchange for long-term stability, unlike the nonporous ALD films.

[0054] Dynamic formation of an AI2O3 protection layer may help stabilize other reaction systems, such as other artificial photosynthesis systems involving photoelectrochemical (PEC) devices. The dynamic protection architecture of the disclosed methods and devices may thus be used in PEC and other reactions in the interest of stability enhancement.Atty. Docket No. 10110-24016A

[0055] The dynamic protection scheme and architecture of the disclosed methods and devices provides a useful solution to challenges presented by electrochemical (EC) processes. EC processes suffer from different degradation mechanisms such as structural reconfiguration of electrode materials, dendrite growths on electrodes due to nonuniform plating / stripping of metal ions, and attack of chemical species on reactive electrodes etc. All of these factors result in poor EC stability. ALD has been utilized to apply precise nanoscale coatings of simple insulators (i.e., AI2O3) onto common EC electrode materials, creating thin and effective protective layers to achieve enhanced EC stability. This approach supported the idea of using thin-film synthesis methods, such as ALD, to form thin protective layers on EC electrodes. However, by introducing this additional protection layer between the electrode and electrolyte, both the chemical and electrochemical interactions between them are minimized, causing EC performance degradation. Generally, an intentional protection layer in an EC system should possess low electronic conductivity and high ionic conductivity, enabling ion transport between the electrolyte and electrode while preventing electron transfer. Additionally, it is useful to design the protection layer to suit a specific EC process, to prevent undesirable chemical species from reaching the electrode to mitigate electrode degradation. These combined properties present a formidable interfacial engineering challenge in EC systems that is met by the architecture and dynamic scheme of the capping layer-based protection of the disclosed methods and devices.

[0056] Described above are examples in which (1) an initially deposited oxide capping layer helps protect the nanowire c-plane from catalytic etching during the reaction, and (2) a dynamically redistributed thin oxide layer on the m-plane helps stabilize the cocatalyst nanoparticles on the photocatalyst without any sacrifice in the photocatalytic activity and while the remaining oxide layer on the top c-plane still protects against the corrosion. Therefore, this kind of oxide protection architecture mitigates the major limiting factors of long-term stability such as photocatalyst corrosion, nanoparticle agglomeration, and nanoparticle detachment, and consequently leads to a remarkably enhanced longevity of the photocatalyst material.

[0057] Further details regarding examples of the disclosed devices and methods are now provided in connection with Figures 6 and 7, respectively.

[0058] Figure 6 depicts a photocatalytic system 100 for photocatalytic water splitting and other chemical reactions. In this example, the photocatalytic system 100 includes a container 102 in which water 104 is disposed. The water 104 may or may not be pure water. The pH of the water 104 may vary accordingly. The container 102 may be configured to allow illumination of the water 104, such as solar illumination 105. The size, construction,Atty. Docket No. 10110-24016A composition, configuration, and other characteristics of the container 102 may vary. The system 100 may not include a container in other cases.

[0059] The photocatalytic system 100 includes a photocatalytic semiconductor device 106 immersed in the water 104. In the example of Figure 1 , the photocatalytic semiconductor device 106 is disposed in the container 102 in a manner to allow the incident light to illuminate the semiconductor device 106. In some cases, the photocatalytic semiconductor device 106 may be configured for photocatalytic water splitting in response to the illumination.

[0060] The semiconductor device 106 includes a substrate 108 and an array 110 of conductive projections 112 supported by the substrate 108. In some cases, each conductive projection 112 is or includes a nanowire or other nanostructure. In this example, each conductive structure 112 is or includes a cylindrically shaped nanostructure. The cylindrical shape has a circular (or roughly circular) cross-sectional shape (e.g., a circular cylinder), as opposed to, for instance, a plate-shaped or sheet-shaped nanostructure. The conductive projections 112 may thus be configured, and / or referred to herein, as nanowires. The nanowires 112 extend outward from a surface 114 of the substrate 108.

[0061] The substrate 108 may be active (e.g., functional) and / or passive (e.g., structural). In one example of the former case, the substrate 108 may be or include a reflective material or layer to direct light back toward the nanowires 112. In one example of the latter case, the substrate 108 may be configured and act solely as a support structure for the nanowires 112. Alternatively or additionally, the substrate 108 may be composed of, or otherwise include, a material suitable for the growth or other deposition of the nanowires 112.

[0062] The substrate 108 may include a light absorbing material. In such cases, the light absorbing material is configured to generate charge carriers upon solar or other illumination. The light absorbing material has a bandgap such that incident light generates charge carriers (electron-hole pairs) within the substrate. Some or all of the substrate 108 may be configured for photogeneration of electron-hole pairs.

[0063] The substrate 108 may include a semiconductor material. In some cases, the substrate 108 is composed of, or otherwise includes, silicon. For instance, the substrate 108 may be provided as a silicon wafer. The silicon may or may not be doped. The doping arrangement may vary. For example, one or more components of the substrate 108 may be non-doped (intrinsic), or effectively non-doped. The substrate 108 may include alternative or additional layers, including, for instance, support or other structural layers. The compositionAtty. Docket No. 10110-24016A of the substrate 108 may thus vary. For example, the substrate may be composed of, or otherwise include, metal films, GaAs, GaN, or SiOxin other cases.

[0064] The substrate 108 may establish a surface, e.g., the surface 114, at which a catalyst arrangement (e.g., a photocatalyst arrangement) of the semiconductor device 106 is provided. The photocatalyst arrangement may be provided by the nanowires 112 of the array 110. In some cases, the catalyst arrangement may be a co-catalyst arrangement including a nanowire-nanoparticle architecture, as described below.

[0065] In the example of Figure 6, each nanowire 112 has a semiconductor composition for photocatalytic water splitting. The semiconductor composition establishes a photochemical diode. In some cases, the semiconductor composition includes Ill-nitride semiconductor materials, such as gallium nitride (GaN) and / or one or more alloys of indium gallium nitride (InGaN). Additional or alternative semiconductor materials may be used, including, for instance, indium nitride, indium gallium nitride, aluminum nitride, boron nitride, aluminum oxide, and silicon, gallium phosphide, gallium arsenide, indium phosphide, tantalum nitride, and alloys thereof, as well as silicon and / or other semiconductor materials.

[0066] Each nanowire 112 may be or include a columnar, rod-shaped, post-shaped, or other elongated structure. The nanowires 112 may be grown or formed as described in U.S. Patent No. 8,563,395 ("Method of growing uniform semiconductor nanowires without foreign metal catalyst and devices thereof"), the entire disclosure of which is hereby incorporated by reference. The dimensions (e.g., length, diameter), size, shape, composition, and other characteristics of the nanowires 112 may vary.

[0067] The semiconductor composition of each nanowire 112 establishes a photochemical diode. In some cases, each nanowire 112 may be configured to have an anode side or surface 116 and a cathode side or surface 118. The anode and cathode sides 116, 118 may be parallel, opposing sides of the nanostructure, as shown. A photochemical diode may be established between the anode and cathode sides 116, 118 of a single one of the nanowires 112. As described herein, the water oxidation reaction (2H2O -> O2 + 4H++ 4e ) of the water splitting occurs along the anode side 116. The proton reduction reaction (4H++ 4e -> 2H2) of the water splitting occurs at the cathode side 118. Proton diffusion from the water oxidation reaction to the proton reduction reaction may occur across a single one of the nanowires 112. Alternatively or additionally, the proton diffusion may occur between two adjacent nanowires 112 in the array 110. As described herein, the configuration of the array 110 may be useful for promoting water splitting involving a pair of the nanowires 112 due to the proximity of the anode and cathode sides 116, 118 of the pair.Atty. Docket No. 10110-24016A

[0068] Each nanowire 112 extends outward from the surface 114 of the substrate 108. In this example, the surface 114 of the substrate 108 is planar. In other cases, the surface 114 may be nonplanar such that subsets of the array 110 are oriented at different angles. For instance, the nonplanar surface may include a multi-faceted surface, such as one having a number of pyramid-shaped sections. The nanowires 112 projecting from each pyramid face or other facet are oriented in parallel with one another. Further details regarding nonplanar substrates and the dopant gradients supported thereby are set forth in WO 2021 / 195484 ("Doping Gradient-Based Photocatalysis"), the entire disclosure of which is hereby incorporated by reference.

[0069] The nanowires 112 may be configured to generate electron-hole pairs upon illumination. The nanowires 112 may be configured to generate the electron-hole pairs upon absorption of light at certain wavelengths. In some cases, each nanowire 112 may be configured to absorb light over a wide range of wavelengths and, thus, improve the efficiency of the photocatalytic water splitting. For instance, each nanowire 112 may include a layered arrangement of semiconductor materials. Each layer in the arrangement may be configured for absorption of light of different wavelengths.

[0070] The layered arrangement of semiconductor materials is used to establish a multiband 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. For instance, in Ill-nitride examples, the layers or segments of the arrangement may have different indium and gallium compositions. In some cases, the layered arrangement is configured to provide a quadruple band structure.

[0071] The layered arrangement of the nanowires 112 may vary. 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.

[0072] The semiconductor composition of each nanowire 112 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 112 may include doping to promote charge carrier separation and extraction, as well as to facilitate the establishment of a photochemical diode (e.g., to promote charge carrier separation and extraction). For example, a dopant concentration of the semiconductor composition may vary laterally and / orAtty. Docket No. 10110-24016A from layer to layer. In the example of Figure 6, the dopant concentration decreases from the anode side 116 to the cathode side 118 to establish a lateral dopant gradient.

[0073] The dopant gradient may be formed during fabrication as a result of the orientation of the nanowires 112 relative to a dopant source, and / or as a result of the positioning and / or orientation of the dopant source. In some cases, the orientation of the nanowires 112 may be established via a non-planar substrate as described in the above-referenced patent publication. These and / or other techniques may be used to establish that the anode sides 116 of the nanowires 112 are more heavily doped.

[0074] 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, zinc, and beryllium, depending on the semiconductor light absorber of choice.

[0075] The semiconductor device 106 may further include one or more types of catalyst nanoparticles 122, 124 disposed over the array 110 of nanowires 112. Pluralities of each type of the nanoparticles 122, 124 are disposed on each nanowire 112, as schematically shown in Figure 1. The nanoparticles 122, 124 are distributed across or along the outer surface (e.g., sidewalls) of each nanowire 112. In the example of Figure 6, one type of nanoparticle 122 is disposed on the anode side 116 of each nanowire 112, and another type of nanoparticle 124 is disposed on the cathode side 118 of each nanowire 112. In other cases, the nanoparticles 122, 124 are positioned on the same side or sides of the nanowires 112.

[0076] The nanoparticles 122 are configured to facilitate or promote the water-oxidation reaction. The nanoparticles 124 are configured to facilitate or promote the proton reduction reaction. Further details regarding the formation, configuration, functionality, and other characteristics of nanoparticles in conjunction with a nanowire array are set forth in one or more of the above-referenced U.S. patents.

[0077] In some cases, the nanoparticles 122 on the water-oxidizing anode side 116 are composed of, or otherwise include, cobalt oxide. The nanoparticles 124 on the protonreducing cathode side may be composed of, or otherwise include, rhodium (Rh). For example, the Rh-based nanoparticles may have a core-shell configuration in which a Rh core is surrounded by a shell, such as a shell composed of, or otherwise including,Atty. Docket No. 10110-24016A chromium oxide (Cr2O3). However, additional or alternative materials may be used, including, for instance, iridium oxide, copper oxide, and nickel oxide for water oxidation, and platinum, gold, nickel, palladium, iron, and copper for proton reduction.

[0078] The nanoparticles 122, 124 may be sized in a manner to facilitate the water splitting. The size of the nanoparticles 122, 124 may be useful in catalyzing the reaction, as described herein. The size of the nanoparticles 122, 124 may promote the water splitting in additional or alternative ways. For instance, the nanoparticles 122, 124 may also be sized to avoid inhibiting the illumination of the nanowires 112.

[0079] The distribution of the nanoparticles 122, 124 may be uniform or non-uniform. The nanoparticles 122, 124 may thus be distributed randomly across each nanowire 112. The schematic arrangement of Figure 6 is shown for ease in illustration.

[0080] The nanowires 112 and the nanoparticles 122, 124 are not shown to scale in the schematic depiction of Figure 6. The shape of the nanowires 112 and the nanoparticles 122, 124 may also vary from the example shown. Further details regarding the nanowire- nanoparticle co-catalyst arrangement, including the fabrication thereof, are provided below.

[0081] The nanoparticle-nanowire co-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 the nanoparticles on the respective sides of the nanowire. Further details regarding example fabrication procedures are provided below, e.g., in connection with Figure 2.

[0082] The nanowires 112 may facilitate the water splitting in alternative or additional ways. For instance, each nanowire 112 may be configured to extract charge carriers (e.g., electrons) generated in the substrate 108 (e.g., as a result of light absorbed by the substrate 108). In such cases, the opposite side of the substrate 108 may be configured for hole extraction. The extraction brings the charge carriers to external sites along the nanowires 112 for use in the water splitting or other reactions. For instance, the nanowires 112 may thus form an interface well-suited for reduction of CO2, and / or other reactions.

[0083] The nanowires 112 are protected from corrosion, nanoparticle detachment, and / or other adverse effects via the capping layer-based scheme described herein. As shown in Figure 6 and described herein, each nanowire 112 is protected by a protection architecture that includes a capping layer 126 that covers an end surface, or crown, of each nanowire 112, and a distribution 128 of metal oxide disposed along one or more sidewall surfaces ofAtty. Docket No. 10110-24016A each nanowire 112. As described herein, the capping layer 126 may be continuous, e.g., thereby completely covering the crown, while the distribution 128 along the sidewalls is discontinuous, e.g., thereby not completely covering the sidewalls. For instance, the distribution 128 may be non-conformal or discontinuous as schematically shown in Figure 6 and described herein. The capping layer 126 may be thicker than the distribution 128 along the sidewalls. As schematically shown in Figure 6 and described herein, the metal oxide of the distribution 128 may be located at interfaces between the nanoparticles 122, 124 and the nanowire 112 without blocking the active sites for the reaction along the sidewall surfaces. The metal atoms of the nanoparticles 122, 124 may be anchored to the metal oxide of the distribution 128 via chemical bonds formed through an atom-trapping mechanism, as described herein.

[0084] Figure 7 depicts a method 200 of fabricating a semiconductor device for photocatalytic water splitting in accordance with one example. The method 200 may be used to manufacture any of the devices described herein or another device. The method 200 may include additional, fewer, or alternative acts. For instance, the method 200 may or may not include one or more acts directed to annealing the device before the implementation of the chemical reaction that forms the protection layer scheme described herein.

[0085] The method 200 may begin with an act 202 in which a substrate is prepared or otherwise provided. The substrate may be or be formed from a silicon wafer. In one example, a 2-inch Si wafer was used, but other (e.g., larger) size wafers may be used. Other semiconductors and substrates may be used.

[0086] The substrate may have a nonplanar surface as described above. In some cases, the act 202 includes an act 204 in which a wet or other etch procedure is implemented to define the surface. For example, the etch procedure may be or include a crystallographic etch procedure. In silicon substrate examples, the crystallographic etch procedure may be or otherwise include a KOH etch 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 202 may include fewer, additional, or alternative acts. For instance, in the example of Figure 1 , the act 202 includes an act 206 in which the substrate is cleaned, and an act 208 in which oxide is removed.

[0088] The method 200 includes an act 210 in which a nanowire or other nanostructure array is grown or otherwise formed on the substrate. Each nanowire is formed on theAtty. Docket No. 10110-24016A surface of the substrate such that each nanostructure extends outward from the surface of the substrate. Each nanostructure has a semiconductor composition, as described herein. The nanostructure growth may be achieved in an act 212 in which molecular beam epitaxy (MBE) is implemented. The MBE procedure may be implemented under nitrogen-rich conditions. Alternatively or additionally, the substrate may be rotated during the MBE procedure such that each nanostructure is shaped as a cylindrically shaped nanostructure. Each nanostructure may thus have a circular cross-sectional shape, as opposed to a plateshaped or sheet-shaped nanostructure.

[0089] 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 and beam equivalent pressures may be adjusted in an act 214. In some cases, a dopant cell temperature is adjusted to control the doping (e.g., Mg doping) of the nanowires.

[0090] In one example, Mg-doped InGaN nanowires were grown by plasma-assisted molecular beam epitaxy (MBE) under N-rich conditions. The growth parameters included a gallium (Ga) beam equivalent pressure of about 7E-8 Torr, a nitrogen flow rate of 1 seem, and a plasma power of 350 W. The substrate temperature, indium (In) beam equivalent pressure (BEP), and magnesium (Mg) cell temperature were tuned to synthesize different single-band or multi-band InGaN nanowires with various p-doping and alloy concentrations. For instance, for single-band p-GaN nanowires or a GaN layer of a multi-band structure, the substrate temperature was 685 °C, and Ga BEP was about 7E-8 Torr. The p-type doping level was tuned by using different Mg cell temperatures. For a p-lno.2oGao.8oN nanowire layer, the substrate temperature was 675 °C, the Ga BEP was about 7E-8 Torr, and the In BEP was about 7.3E-8. For p-lno.27Gao.73N nanowire layers, the substrate temperature was 662 °C, the Ga BEP was about 7E-8 Torr, and the In BEP was about 7.3E-8. For p- lno.35Gao.65N nanowire layers, the substrate temperature was 640 °C, the Ga BEP was about 7E-8 Torr, and the In BEP was about 3.5E-8. For quadruple-band InGaN nanowires, the growth conditions are similar to those of the constituting single-band nanowires but with varying thicknesses for each segment. The substrate temperature may refer to a thermocouple reading of a substrate heater, which may be different from the actual substrate surface temperature, which may depend on the sample size, substrate holder, and mounting configuration.

[0091] In the example of Figure 2, the method 200 further includes an act 216 in which one or more types of catalyst nanoparticles are deposited across the array of nanowires. AsAtty. Docket No. 10110-24016A described above, in some cases, two types of catalyst nanoparticles are deposited. One type of catalyst nanoparticle may be deposited on a water-oxidizing anode side of each nanowire. Another type of nanoparticle may be deposited on a proton-reducing cathode side of each nanowire.

[0092] The selective deposition of the nanoparticles may be achieved via implementation of two photo-deposition procedures. In the example of Figure 2, the act 216 includes an act 218 in which hydrogen evolution reaction (HER) co-catalyst nanoparticles (e.g., Rh / C^Ch) are deposited (e.g., on the proton reducing cathode sides), and an act 220 in which oxygen evolution reaction (OER) co-catalyst nanoparticles (e.g., CoOx) are deposited (e.g., on the water oxidizing anode sides). The photo-deposition procedures may be configured to direct the catalyst nanoparticles to the respective sides, as described herein. Further details regarding the photo-deposition procedures are set forth in one or more of the abovereferenced U.S. patents.

[0093] In one example, the co-catalyst nanoparticles were deposited using a photodeposition procedure in which an InGaN nanowire device was put in a glass chamber with a quartz lid. The chamber was pumped down and then illuminated using a 300 W Xenon lamp for 20 minutes to deposit cocatalyst nanoparticles on the InGaN nanowires. The deposition of Rh / CrOx core / shell structures included two steps. The first step involved the use of 55 ml deionized water, 11 mL methanol, and 2 pmol sodium hexachlororhodate (NasRhCle) for the formation of the Rh core. The second step involved the use of 55 mL deionized water, 11 mL methanol, and 4 pmol potassium chromate (K2CrO4) to form the CrOxshell. The deposition of CoOxnanoparticles included the use of 60 mL deionized water, 6 mL potassium iodate (KIO3, 0.01 M), and 4 pmol cobalt nitrate (Co(N03)2). The deposition time, volume of water, volume of methanol, precursors, and / or other characteristics may vary depending on the sample area and nanowire growth duration or size.

[0094] In an act 222, a metal layer is deposited on the end surfaces, or crowns, of the nanostructures. The act 222 may include implementation of a directional deposition procedure so that the deposition is limited to the end surfaces. For instance, an electronbeam evaporative procedure may be implemented in an act 224. After deposition of the metal atoms, an anneal procedure may be implemented in an oxygen environment in an act 226. For instance, the anneal procedure may be or include a rapid thermal anneal (RTA) procedure. The anneal procedure may oxidize a portion of the metal layer as described herein, leaving another, non-oxidized portion for redistribution to the sidewalls of the nanostructures.Atty. Docket No. 10110-24016A

[0095] After the deposition of the metal layer, a reaction is implemented to form a protection arrangement in an act 228. The reaction causes the non-oxidized metal atoms in the metal layer to be redistributed as described herein. In some cases, the reaction may be the same reaction that the device is configured to catalyze. For instance, the act 228 may include an act 230 in which a water splitting reaction (e.g., a photocatalytic water splitting reaction) is implemented. Alternatively or additionally, the reaction may include illuminating the nanostructures in an act 232 to establish a thermal gradient to promote the redistribution, or migration, of the metal atoms as described herein. The reaction may be implemented for a time period (e.g., about 60 minutes or more) sufficient to establish the thermal gradient.

[0096] The order of the above-described acts of the method 200 may differ from the example shown. For instance, an annealing step may be implemented before or after the deposition of the nanoparticles in the act 216.

[0097] Described above are examples of capping layer-based protection schemes that address the primary factors limiting the long-term stability of InGaN nanowire photocatalysts, those being photocatalytic corrosion and cocatalyst nanoparticle displacement. Detailed analysis indicates that these phenomena severely affect the top c-plane of the nanowire structure, rendering the achievable photocatalytic stability to about 250 hours under concentrated solar illumination. As such, protecting the nanowire tip with an oxide capping layer significantly suppresses the photocatalyst etching and cocatalyst nanoparticle displacement occurring at the nanowire surface. Consequently, AI / AhOs-capped Rh / Cr203 / CoOx-decorated p-lnGaN nanowire example devices exhibited a remarkably enhanced photocatalytic stability of about 1500 hours under the same reaction conditions. Further analysis of the architecture of the disclosed devices revealed the dynamic nature of the AI / AI2O3 protection layer, in which Al is redistributed from the c-plane toward the m-plane followed by oxidation during the reaction, simultaneously resulting in nanoparticle stabilization on the m-plane and prevention of the c-plane corrosion. Therefore, the disclosed devices and methods provide an effective protection architecture that substantially improves the stability of photocatalytic water splitting and also greatly supports the long-term stability of other catalytic reactions, such as nitrogen fixation, carbon dioxide reduction, and various electrochemical reactions.

[0098] 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.Atty. Docket No. 10110-24016A

[0099] 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.

[0100] The foregoing description is given for clearness of understanding only, and no unnecessary limitations should be understood therefrom.

Claims

Atty. Docket No. 10110-24016AWhat is Claimed is:

1. A device for catalyzing a reaction, the device comprising: a substrate; an array of nanostructures supported by the substrate, each nanostructure of the array of nanostructures comprising: a sidewall surface that extends outward from the substrate; and an end surface at an outer end of the nanostructure; and a protection architecture composed of a metal oxide and disposed on each nanostructure of the array of nanostructures, the protection architecture comprising: a continuous capping layer that covers the end surface of each nanostructure; and a discontinuous distribution of the metal oxide disposed on the sidewall surface of each nanostructure.

2. The device of claim 1 , wherein the continuous capping layer is thicker than the discontinuous distribution.

3. The device of claim 1 , further comprising a plurality of catalyst nanoparticles disposed over the array of nanostructures, wherein the discontinuous distribution is configured to stabilize respective catalyst nanoparticles of the plurality of catalyst nanoparticles along the sidewall surface.

4. The device of claim 3, wherein the metal oxide of the discontinuous distribution is located at interfaces between the respective catalyst nanoparticles and the nanostructure without blocking a plurality of active sites for the reaction along the sidewall surface.

5. The device of claim 3, wherein: each catalyst nanoparticle of the plurality of catalyst nanoparticles comprises metal atoms; and the metal atoms are anchored to the metal oxide of the discontinuous distribution via chemical bonds formed through an atom-trapping mechanism.

6. The device of claim 3, wherein the plurality of catalyst nanoparticles comprise Rh / Cr2O3 core-shell nanoparticles and CoOxnanoparticles.

7. The device of claim 1 , wherein the protection architecture is composed of aluminum oxide.Atty. Docket No. 10110-24016A8. The device of claim 1 , wherein each nanostructure of the array nanostructures has a semiconductor composition.

9. The device of claim 8, wherein the sidewall surface is an m-plane of a lattice of the semiconductor composition.

10. The device of claim 8, wherein the end surface is a c-plane of a lattice of the semiconductor composition.

11. A method of fabricating a device, the method comprising: providing a substrate of the device; forming an array of nanostructures supported by the substrate, each nanostructure of the array of nanostructures comprising: a sidewall surface that extends outward from the substrate; and an end surface at an outer end of the nanostructure; depositing a metal layer on the end surface of each nanostructure of the array of nanostructures; and after depositing the metal layer, implementing a reaction that redistributes and oxidizes metal atoms from the metal layer to form a protection architecture on each nanostructure of the array of nanostructures, the protection architecture comprising: a capping layer that covers the end surface of each nanostructure; and a distribution of metal oxide disposed on the sidewall surface of each nanostructure.

12. The method of claim 11 , wherein implementing the reaction comprises illuminating the array of nanostructures.

13. The method of claim 11 , wherein the reaction is implemented for a time period sufficient to create a thermal gradient along each nanostructure between the end surface and the substrate.

14. The method of claim 13, wherein the time period is about 60 minutes or more.

15. The method of claim 11 , wherein implementing the reaction comprises implementing a photocatalytic water splitting reaction.

16. The method of claim 11 , wherein depositing the metal capping layer comprises implementing a directional deposition procedure.Atty. Docket No. 10110-24016A17. The method of claim 11 , wherein depositing the metal capping layer comprises implementing an electron beam evaporative deposition procedure.

18. The method of claim 11 , wherein depositing the metal capping layer comprises implementing an anneal procedure in an oxygen environment after deposition of metal atoms of the metal capping layer.

19. The method of claim 11 , wherein the capping layer is continuous.

20. The method of claim 11 , wherein the distribution of metal oxide is discontinuous.

21. The method of claim 11 , further comprising disposing a plurality of catalyst nanoparticles over the array of nanostructures, wherein the distribution of metal oxide is configured to stabilize respective catalyst nanoparticles of the plurality of catalyst nanoparticles along the sidewall surface.

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