Photocatalytic nitrate reduction to ammonia

Abstract

A system for photocatalytic synthesis of ammonia includes an aqueous solution including nitrate ions and a photocatalytic device immersed in the aqueous solution. The photocatalytic device includes a substrate having a surface, a conductive structure supported by the substrate at the surface, the conductive structure including a semiconductor interface in contact with the aqueous solution, the semiconductor interface being configured to reduce the nitrate ions to nitrite ions, and a plurality of catalyst nanostructures supported by the conductive structure. Each catalyst nanostructure of the plurality of catalyst nanostructures includes a metal configured to reduce the nitrite ions to ammonia.

Description

Atty. Docket No. 10110-25003APHOTOCATALYTIC NITRATE REDUCTION TO AMMONIACROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of U.S. provisional application entitled “Photocatalytic Nitrate Reduction to Ammonia,” filed February 7, 2025, and assigned Serial No. 63 / 755,694, 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 Army Research Office. The government has certain rights in the invention.BACKGROUND OF THE DISCLOSUREField of the Disclosure

[0003] The disclosure relates generally to ammonia production.Brief Description of Related Technology

[0004] Ammonia (NH3) is a critical feedstock for fertilizers and a promising carbon-free hydrogen carrier, easily stored in liquid form. Currently, industrial NH3production relies on the energy-intensive and carbon-emissive Harber-Bosch process, which operates under high temperatures and pressure. As a clear alternative, the electrochemical nitrogen (N2) reduction reaction in aqueous electrolytes allows for NH3synthesis under milder conditions. However, the high dissociation energy of the N N bond (941 kJ / mol) limits both the selectivity and efficiency. In contrast, the electrochemical nitrate reduction reaction (NO3RR) is more feasible due to the significantly lower dissociation energy of the N=O bond (204 kJ / mol) and the much higher solubility of NO3_in aqueous electrolytes - approximately 40,000 times greater than that of N2. Furthermore, the abundance of NO3_in wastewater makes its reduction a sustainable approach to NH3synthesis, while also providing an effective solution for mitigating environmental pollution through runoff purification.Atty. Docket No. 10110-25003A

[0005] The NO3RR to NH3is a complex eight-electron process (NO3_+ 9H++ 8e_— > NH3+ 3H2O) involving 9 protons (H+) for deoxygenation and hydrogenation steps. Previous studies have explored various metal-based catalysts, including Cu-PTCDA, strained Ru nanoclusters, Fe single-atom catalysts, and strained Bi, to enhance the faradaic efficiency for NH3(FENH3) and reduce the overpotential. While several catalysts have demonstrated success for NO3RR, detailed mechanistic studies remain limited.

[0006] Recently, binary metal compound catalysts such as CuNi, CuCoSP, Rh@Cu, and NiCo2O4have shown improved NH3production compared to their single-metal counterparts, due to metal-support interactions and the transfer of reaction intermediates between multiple active phases. These heterogeneous catalysts enhance reaction kinetics and lower thermodynamic barriers by modulating the binding energy of adsorbates on the catalytic surface. For example, Rh clusters on Cu surfaces provide sufficient hydrogen, activating the hydrogenation process and promoting selective NH3production. Similarly, Co(OH)2aids in the hydrogenation of adsorbed NO3_in the CoP / Co(OH)2catalytic system.

[0007] In microorganisms, NH3synthesis from NO3RR occurs through two tandem reactions: (1) NO3_reduction to NO2_by nitrate reductase, and (2) NO2_conversion to NH3by nitrite reductase or nitrogenase. This tandem process has been artificially replicated using CuCoSP, where Cu / CuOxphases reduce NO3_to NO2_, and the NO2_intermediate is transferred and converted to NH3on Co / CoO phases.SUMMARY OF THE DISCLOSURE

[0008] In accordance with one aspect of the disclosure, a system for photocatalytic synthesis of ammonia includes an aqueous solution comprising nitrate ions and a photocatalytic device immersed in the aqueous solution. The photocatalytic device includes a substrate having a surface, a conductive structure supported by the substrate at the surface, the conductive structure including a semiconductor interface in contact with the aqueous solution, the semiconductor interface being configured to reduce the nitrate ions to nitrite ions, and a plurality of catalyst nanostructures supported by the conductive structure, each catalyst nanostructure of the plurality of catalyst nanostructures including a metal configured to reduce the nitrite ions to ammonia.

[0009] In accordance with another aspect of the disclosure, a system for photocatalytic synthesis of ammonia includes an aqueous solution including nitrate ions and a photoelectrode immersed in the aqueous solution. The photoelectrode includes a substrate having a surface, the substrate being configured to generate charge carriers uponAtty. Docket No. 10110-25003A absorption of solar radiation, 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 having a nitride semiconductor interface in contact with the aqueous solution, the nitride semiconductor interface being configured to reduce the nitrate ions to nitrite ions, and a plurality of catalyst nanostructures disposed over the array of conductive projections, the charge carriers generated in the substrate migrating to the plurality of catalyst nanostructures via the array of conductive projections, each catalyst nanostructure of the plurality of catalyst nanostructures including a metal configured to reduce the nitrite ions to ammonia.

[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 conductive structure includes a conductive projection extending outward from the surface of the substrate. The semiconductor interface includes a nitride semiconductor. The nitride semiconductor is gallium nitride. Each nanostructure of the plurality of catalyst nanostructures includes metallic gold. The conductive structure is partially covered by the plurality of catalyst nanostructures. The substrate includes a semiconductor material. The semiconductor material is doped to define a junction to generate charge carriers upon absorption of solar radiation. The conductive structure includes a nanowire configured to extract the charge carriers generated in the substrate. The aqueous solution includes KNO3. The aqueous solution is free of a sacrificial agent for the synthesis of ammonia from the nitrate ions. The photocatalytic device is configured as a working electrode. The system further includes a counter electrode immersed in the aqueous solution and a voltage source configured to apply a bias voltage between the working electrode and the counter electrode. The bias voltage is at a level for the synthesis of ammonia at the working electrode. The level of the bias voltage falls in a range from about 0 to about -2 Volts versus reversible hydrogen electrode (VRHE). A method of using one of the systems disclosed herein includes illuminating the photocatalytic device and capturing the ammonia synthesized by the photocatalytic device. The method further includes applying a bias voltage to the photocatalytic device. The photoelectrode is configured as a working electrode, and the system further includes a counter electrode immersed in the aqueous solution and a voltage source configured to apply a bias voltage between the working electrode and the counter electrode. The bias voltage is at a level for the synthesis of ammonia at the working electrode. The level of the bias voltage falls in a range from about 0 to about -2 Volts versus reversible hydrogen electrode (VRHE). A method of using one of the systems disclosedAtty. Docket No. 10110-25003A herein includes illuminating the photoelectrode and capturing the ammonia synthesized by the photoelectrode. The method further includes applying a bias voltage to the photoelectrode.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 45Q-tilt-view scanning electron microscopy (SEM) image of an example Au10 / GaN / Si photoelectrode, (b) a scanning transmission electron microscopy (STEM) image and energy-dispersive X-ray spectroscopy (EDS) elemental maps of Au / GaN, in which Ga and N from GaN nanowires were uniformly distributed while Au from Au nanoclusters was aggregated on an upper region of the GaN nanowires, X-ray photoelectron spectroscopy (XPS) spectra of (a) Ga 2p2 / 3, (b) N 1s, (c) Au 4f for GaN / Si, Au1 / GaN / Si, Au10 / GaN / Si, and Au100 / GaN / Si photoelectrodes, (d) Au I Ga+N ratio calculated from XPS analysis, and (e) X-ray diffraction (XRD) patterns of the example photoelectrodes.

[0013] Figure 2 depicts graphical plots of (a, b) linear sweep voltammetry (LSV) curves of (a) GaN / Si and (b) Au100 / GaN / Si photoelectrodes measured in 0.1 M K2SO4with KNO3 concentrations of 0, 0.02, 0.1 , 0.5, and 1 M, in which measurements were conducted under 1-sun light illumination (under dark conditions, the photoelectrodes were tested in 0.1 M K2SO4electrolyte), (c) Vonset at -1 .0 mA / cm2and (d) J saturation at -2 VRHE with KNO3 concentration, and (e, f) Faradaic efficiency of (e) NH3and (f) NO2_for GaN / Si, Au 1 / GaN / Si, Au10 / GaN / Si, and Au100 / GaN / Si photoelectrodes measured in 0.1 M K2SO4with different KNO3 concentration at -0.4 VRHE. The Au10 / GaN / Si example photoelectrode exhibited the highest FENHS (91.8%) among the tested examples.

[0014] Figure 3 depicts graphical plots of (a) LSV curves of Si, Au10 / Si, GaN / Si, Au10 / GaN / Si, and Au100 / GaN / Si photoelectrodes measured in 0.1 M K2SO4with 0.5 M KNO3, (b) Faradaic efficiency and (c) production rate of NH3(YNHS) plotted as a function of cathodic potentials from -0.2 to -0.8 VRHE (in which NH3production was measured three times, and the mean values along with their standard deviations are represented as error bars, and in which the Au10 / GaN / Si example had the widest potential window (-0.4 to -0.8 VRHE) for high FENHS > 90% and achieved the best YNHS = 131.1 pmol / cm2 / h at -0.8 VRHE), and (d) FENHS and YNHS for 8 cycles of reaction (each cycle was performed for 1 hour at -0.4 VRHE), as well as (e) schematic illustrations of adsorption of NOs-and NO3-reduction reactionAtty. Docket No. 10110-25003A on the (i) GaN / Si, (ii) Au10 / GaN / Si, and (iii) Au100 / GaN / Si photoelectrodes (in which the dashed line represents adsorption of molecules and the arrow line indicates a reduction reaction).

[0015] Figure 4 depicts graphical representations of (a) relative free energies of NO3-reduction to NO2-on GaN surfaces, in which, after reduction to NO2-, the reduction to NO (and then further to NH3) shown in light grey involves high reductive potentials, while expulsion of the NO2-by incoming NOs-is favorable (black), making NO2-the main product in PEC NO3-reduction on GaN, and (b) relative free energies of NO2-reduction on Au (211) surface, in which expulsion of NO2-by NO3-(light grey) is unfavorable, meaning NO2-can be fully reduced towards NH3.

[0016] Figure 5 is a flow diagram of a method of fabricating a device (e.g., a photoelectrode) for synthesis of ammonia via nitrate reduction in accordance with one example.

[0017] Figure 6 is a schematic view and block diagram of an electrochemical system having a working electrode with a nanowire-nanostructure architecture for synthesis of ammonia via nitrate reduction in accordance with one example.

[0018] The embodiments of the disclosed systems and methods may assume various forms. Specific embodiments are illustrated in the drawing and hereafter described with the understanding that the disclosure is intended to be illustrative. The disclosure is not intended to limit the invention to the specific embodiments described and illustrated herein.DETAILED DESCRIPTION OF THE DISCLOSURE

[0019] Systems and methods for photoelectrochemical and other photocatalytic synthesis of ammonia are described. Methods for fabricating photocatalytic devices of the disclosed systems are also described. The photocatalytic devices are immersed in an aqueous solution having nitrate ions for the synthesis of ammonia at semiconductor interfaces of the photocatalytic device in contact with the aqueous solution. The semiconductor interfaces may be provided by an array of conductive projections, such as nanowires, projecting outward from a substrate configured for photogeneration of charge carriers. The conductive projections may be composed of, or otherwise include, a nitride semiconductor, such as GaN.

[0020] The nitrate reduction to ammonia is supported by a metal-semiconductor combination of the photocatalytic devices of the disclosed systems. The conductiveAtty. Docket No. 10110-25003A projections or other support structures are decorated with metal nanoclusters or other nanostructures. As described herein, synergistic interactions with the metal nanoclusters and semiconductor interfaces implement a reduction strategy that provides dual active sites for tandem reactions. The metal-semiconductor combination leverages photon-to-electron conversion in semiconductors to further reduce the overpotential involved in NO3RR to provide energy-efficient photoelectrochemical reactions.

[0021] Described herein are examples of solar-driven photoelectrochemical systems and methods for sustainable NH3production via NO3_reduction. In the example systems, Au nanoclusters on GaN nanowires enable a tandem catalysis mechanism: NO3" reduces to NO2- on GaN, followed by NH3synthesis on Au. The examples demonstrate the synergistic metal-support interactions between Au nanoclusters (NCs) and GaN nanowires (NWs) grown on an n+-p Si wafer for photoelectrochemical (PEC) NO3RR to NH3under simulated solar light. By optimizing the surface coverage and size of the Au NCs on the GaN NWs, rapid adsorption and reduction of NO3_to NO2- on the GaN NWs was achieved, followed by efficient NH3generation on the Au NCs. This optimization led to a significant increase in a Faradaic efficiency for NH3(FENH3), reaching 91.8% at -0.4 V vs. reversible hydrogen electrode (VRHE). Density functional theory (DFT) calculations corroborated the test results, showing that NO3_is reduced to NO2_on the GaN surface and displaced by remaining NO3_, while NO2‘ is further reduced to NH3on the Au NCs. The sequential reduction reactions occurring on the adjacent Au and GaN surfaces resulted in a high production rate of NH3(YNH3) of 131 .1 pmol / cm2 / h at -0.8 VRHE with selective NH3generation (FENH3> 90%) sustained for 8 hours at -0.4 VRHE. The examples establish that metal / semiconductor systems can achieve high-efficiency PEC NO3RR to NH3under solar illumination for green synthesis of ammonia.

[0022] Although described in connection with photoelectrochemical systems, the disclosed systems and methods are not limited to applications in which an applied bias voltage is relied upon. The disclosed systems and methods may thus be used in photocatalytic contexts and applications in which a bias voltage is not applied to a photocatalytic device.

[0023] Although described in connection with nanowires having a GaN semiconductor composition, alternative or additional nitride semiconductors may be used. 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. The disclosed devices are also not limited to nanowires having a uniform semiconductor composition. For instance, the conductive projections of the photocatalytic devices mayAtty. Docket No. 10110-25003A 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.

[0024] Although described in connection with nanowires, the nitride interfaces of the disclosed photocatalytic devices may be provided by a variety of different nanostructures or other conductive projections. The nature, construction, configuration, characteristics, shape, and other aspects of the conductive projections may vary accordingly.

[0025] Although described in connection with gold nanostructures, the catalytic arrangements of the disclosed photocatalytic devices may alternatively or additionally include other catalysts. For instance, nanostructures composed of, or otherwise including, additional and / or alternative metal catalysts, such as Ag, Co, Ni, and Ir, may be used for ammonia synthesis.

[0026] 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.

[0027] Although described in connection with solar radiation, the disclosed photocatalytic 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.

[0028] 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-type metal-nitride nanowire arrays," Nat. Common. 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 Application No. PCT / US23 / 24569 ("Photocatalytic CO2 Reduction with Co-Catalyst-Decorated Nanostructures"), the entire disclosures of which are hereby incorporated by reference.Atty. Docket No. 10110-25003A

[0029] Described below in connection with Figures 1-4 are examples of photoelectrochemical systems and methods in which metal-support interactions are used in the electrochemical synthesis of ammonia (NH3) from the nitrate (NO3) reduction reaction, enabling efficient NH3 production under mild conditions. The example photoelectrochemical systems include gold (Au) nanoclusters supported on gallium nitride (GaN) nanowires, grown on a n+-p Si wafer, for selective reduction of NO3-to NH3 under solar illumination. NO3-ions are preferentially adsorbed and reduced to nitrite (NO2) on the GaN nanowires, which then transfer to adjacent Au nanoclusters to complete the NH3synthesis. This mechanism is confirmed by both test data and theoretical calculations. Optimizing the surface coverage and size of the Au nanoclusters on the GaN nanowires significantly enhanced catalytic activity compared to that on planar n+-p Si photoelectrodes. A faradaic efficiency of 91.8% at -0.4 VRHE and a high NH3production rate of 131.1 pmol / cm2 / h at -0.8 VRHE were achieved. These results establish the synergetic effect between the metal cocatalysts and the semiconductor supports in providing photoelectrodes for multi-step NO3-reduction.

[0030] Examples of Au / GaN / Si photoelectrodes were fabricated using plasma-assisted molecular beam epitaxy to grow GaN NWs on an n+-p Si wafer, followed by photodeposition of Au NCs. Scanning electron microscopy (SEM) confirmed the vertical growth of the GaN NWs on the planar n+-p Si wafer (Figure 1 , part a). The GaN NWs measured approximately 450 nm in length and 50 nm in diameter. Three variations of the Au / GaN / Si photoelectrodes were prepared by adjusting the volume of 0.4 M HAuCL precursor solution used during the photodeposition, denoted as Au# / GaN / Si, where # indicates the precursor solution volume. As the precursor volume increased from 1 to 100 pL, the size of Au NCs increased, particularly on the upper region of the GaN NWs.

[0031] The microstructure of the GaN NWs and Au NCs was examined using high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) (Figure 1, part b). Due to the significant atomic number difference between Au (79) and Ga (31), Z-contrast imaging revealed locally aggregated Au NCs on both the top and sidewalls of the GaN NWs. Elemental mapping by energy-dispersive X-ray spectroscopy (EDS) showed a uniform distribution of Ga, and N across the GaN NWs, while Au NCs, ranging from 2 to 20 nm in size, were sparsely distributed on the GaN surface, indicating a non-uniform decoration of Au NCs on the GaN NWs. The non-uniform distribution of Au NCs can be attributed to preferential photodeposition at the tips of GaN NWs, where enhanced light absorption and higher photo-excited electron density promote localized growth. In other cases, the Au NCs may be more uniformly distributed across each nanowire via adjustmentAtty. Docket No. 10110-25003A of one or more of the photodeposition parameters, such as light wavelength and deposition time.

[0032] X-ray photoelectron spectroscopy (XPS) was used to analyze the surface chemical composition of the photoelectrodes. The Ga 2p3 / 2 XPS spectrum was deconvoluted into a dominant Ga-N bond peak at 1116.8 eV and a minor Ga-0 bond peak at 1118.0 eV (Figure 1 , part c). As the amount of Au NCs increased, the intensity of the Ga 2p3 / 2 signal diminished due to the screening effect of the Au NCs, which blocked photoelectrons emitted from the GaN surface. Similarly, the N 1s XPS spectrum (Figure 1 , part d) was split into a major N-Ga bond at 396.7 eV and a minor N-0 bond at 398.1 eV, both of which showed decreasing intensity with increasing Au NC loading. The Au NCs exhibited the metallic Au° phase, with binding energies at 83.4 and 87.1 eV, alongside a trace amount of Au+at 84 and 87.7 eV (Figure 1 , part e). As the Au loading increased, the intensity of the Au 4f peaks increased and the Au 4f peak shifts toward a higher binding energy, approaching the reference value of 84 eV for bulk Au 4f7 / 2°- This result indicates that smaller Au NCs have weaker electron binding compared to bulk Au, primarily due to electron transfer from GaN NWs to Au NCs at the interface. This interfacial charge transfer is particularly pronounced for smaller Au NCs, as shown by the shifts of Ga and N peaks to higher binding energies and Au peaks to lower binding energies in the XPS spectra. As the size of the Au NCs increases, thicker Au layers gradually screen the interfacial charge transfer effect, leading to bulk-like electronic properties.

[0033] The surface atomic ratios of Au with respect to Ga and N, calculated from the XPS data, were 0.09, 1.06, and 8.91 for the Au1 / GaN, Au10 / GaN, and Au100 / GaN examples, respectively, as shown in Figure 1 , part f.

[0034] X-ray diffraction (XRD) pattern of GaN / Si revealed a distinct GaN (002) peak [JCPDS #02-1078], confirming GaN NWs were single crystals (Figure 1 , part g). The Au (111) peak was only detected in the Au100 / GaN / Si example, indicating that the crystallite size of the Au NCs was sufficiently large for XRD detection [JCPDS #04-0784], In contrast, smaller Au NCs, such as those in the Au1 and Au10 examples, likely had a higher proportion of surface-exposed Au atoms, particularly at terrace sites, compared to bulk crystal facets. This increased surface exposure, especially on high-index crystal planes, can enhance catalytic properties by providing a greater number of active sites, potentially enhancing reactivity.

[0035] PEC NO3RR was performed in an H-type cell using simulated AM1.5G 1-sun solar light, with the light sources positioned perpendicular to the planar n+-p Si photoelectrodes.Atty. Docket No. 10110-25003A Linear sweep voltammetry (LSV) curves were recorded in 0.1 M K2SO4 with varying KNO3 concentrations (0, 0.02, 0.1, 0.5, and 1 M). As the NO3-concentration increased from 0 to 1 M, the onset potential (Vonset) at -1.0 mA / cm2for GaN / Si shifted significantly from -0.63 to -0.21 VRHE (Figure 2, part a), indicating that GaN / Si exhibits higher catalytically activity for NO3 RR compared to the hydrogen evolution reaction (HER). However, as the loading of Au NCs increased, the difference in Vonset between electrolytes with different NO3-concentrations diminished. Eventually, the Au100 / GaN / Si example exhibited minimal variation in LSV curves regardless of NOs-concentration, with Vonset ranging between -0.17 to -0.21 VRHE (Figure 2, parts b and c). These results are indicative of competitive behavior between NO3 RR and HER on the Au 100 co-catalyst. Additionally, the saturated photocurrent density (Jsaturation) at -2 VRHE decreased from -41.0 to -16.8 mA / cm2as the Au NCs loading increased, likely due to the Au NCs blocking incident light and reducing light absorption by the underlying n+-p Si photoelectrode (Figure 2, part d). Under dark conditions, all of the photoelectrode examples exhibited a current density close to zero, attributed to the absence of photon-induced charge carrier generation. These results indicate that the reduction reactions on the photoelectrodes are entirely reliant on photo-generated charge carriers.

[0036] To further investigate photovoltage generation by the example photoelectrodes, an electrochemical reaction electrode was fabricated by growing GaN NWs on an electrically conductive n-type silicon substrate (n-Si) and loading it with Au NCs. LSV curves measured in 0.1 M K2SO4 with 0.1 M KNO3 electrolyte revealed that the onset potential for the Au / GaN / n-Si electrode under dark conditions was approximately 0.5 V more negative compared to the Au / GaN / n+-p Si photoelectrode under illuminated conditions. This difference in onset potential highlights the role of the underlying n+-p Si substrate in reducing the voltage requirement by effectively harnessing solar energy.

[0037] Faradaic efficiencies (FEs) for GaN / Si, example Au 1 / GaN / Si, Au10 / GaN / Si, and Au100 / GaN / Si photoelectrodes were assessed in electrolytes with varying NO3-concentrations at -0.4 VRHE (Figure 2, parts e and f). Concentrations of NH3, NO2-, and N2H4products were determined via spectrocolorimetric analysis. In 0.1 M K2SO4(without NO3 ), all of the example photoelectrodes exhibited selective HER with a FEH2greater than 95%. Upon introducing 0.02 M KNO3, the production of NH3, NO2-, and N2H4 was observed, with the Au10 / GaN / Si photoelectrode achieving the highest FENHS of 59.5% (Figure 2, part e).Notably, the GaN / Si photoelectrode effectively suppressed HER (FEH2less than 3%) and selectively converted NO3-into NO2-(FENO2- greater than 80%), even with just 0.02 M NO3-(Figure 2, part f). However, H2production remained significant on the Au / GaN / SiAtty. Docket No. 10110-25003A photoelectrode, with the highest FEH2 of 41.6% observed for the Au100 / GaN / Si example. As NOs-concentration increased, the FENHS for Au / GaN / Si gradually rose. The Au10 / GaN / Si photoelectrode demonstrated the optimal balance of Au coverage, achieving a maximum FENHS of 91.8% at 0.5 M NOs-. Accordingly, one or more characteristics of the Au / GaN metalsupport interactions may be further adjusted to improve catalytic performance of the photoelectrodes.

[0038] To further investigate the metal-support interactions of the example photoelectrodes, planar n+-p Si and Au-coated n+-p Si (Au 10 / Si) photoelectrodes were also fabricated and tested. The LSV curve of an Au10 / Si photoelectrode exhibited a more positive Vonset = -0.41 VRHE compared to Si (-0.81 VRHE) in 0.1 M K2SO4with 0.5 M KNOs (Figure 3, part a). However, both remained more negative than those of the GaN / Si or Au / GaN / Si photoelectrodes, highlighting that the GaN NWs are useful platforms for loading the Au cocatalysts for the NO3 RR. Additionally, the Jsaturation of the GaN / Si photoelectrode was about 1.5 times higher than planar Si, which may be attributed to the antireflective properties of the GaN NWs.

[0039] The influence of the applied potential on FE and production rates was evaluated for Si, Au / Si, GaN / Si, Au10 / GaN / Si, and Au 100 / GaN / Si photoelectrodes, with a summary of FENHS shown in Figure 3, part b. Planar Si achieved an FENHS of 36.3% at -0.8 VRHE, while Au deposition on Si (Au / Si) marginally increased the NH3selectivity (FENHS = 47.2% at -0.8 VRHE). In contrast, GaN / Si predominantly produced NO2-(FENOS- above 90% at potential less than -0.3 VRHE), with low NH3(FENHS less than 16%) and N2H4(FEN2H4less than 3%) selectivities. GaN NWs effectively reduce NO3-to NO2-, which was desorbed from the photoelectrode surface before further reduction. The Au10 / GaN / Si example photoelectrode demonstrated a broad potential window (from about -0.4 to about -0.8 VRHE) for selective NH3production (FENHS above 90%). As a result, the Au10 / GaN / Si example photoelectrode exhibited the highest FENHS among the tested photoelectrodes, achieving NH3production rates (YNHS) of 110 pmol / cm2 / h at -0.4 VRHE and 131.1 pmol / cm2 / h at -0.8 VRHE (Figure 3, part c). The FENHS and YNHS values for the Au10 / GaN / Si example photoelectrode outperformed recent photoelectrodes and were comparable to state-of-the-art electrocatalysts.

[0040] The stability of the Au10 / GaN / Si example was assessed over 8 cycles in 0.1 M K2SO4with 0.5 M KNO3 (Figure 3, part d). Each cycle was run for 1 hour at a constant potential of -0.4 VRHE, with fresh electrolyte introduced for every cycle. Chronoamperometric curve analysis revealed an initial high photocurrent density, which gradually decreased over 1 hour of reaction. This behavior was likely due to the depletion of NOs-concentration near the surface as the reaction progresses, and the accumulation of by-products that poisonedAtty. Docket No. 10110-25003A the surface. Upon replacing the electrolyte with a fresh solution, the photocurrent density was restored to its initial high level, confirming that the observed decrease was primarily due to changes in the reaction environment rather than irreversible degradation of the photoelectrode. Throughout the 8 cycles, a high NH3faradaic efficiency (FENHS above 90%) was consistently maintained. The NH3production rate (YNHS) increased to 100.1 pmol / cm2 / h during the first 3 cycles but gradually declined to 69.3 pmol / cm2 / h over the last 5 cycles. This reduction in YNHS was attributed to the detachment of Au NCs from the GaN NWs. Although Au NCs remained on the surface of GaN NWs after 8 cycles, the Au / Ga+N ratio dropped from 1.06 to 0.47, indicating loss of Au NCs. To address this issue, incorporating protective layers, or applying pre- or post-treatments, may be used to enhance the interfacial binding between the Au NCs and GaN NWs, thereby improving the long-term stability of the photoelectrode. Despite this challenge, the Au10 / GaN / Si example remained a useful photoelectrode for selective and sustained NH3production with extended stability.Additionally, isotope labeling experiments confirmed that the NH3originated from the NOs-rather than from the nitrogen of GaN, demonstrating that the GaN NWs remained chemically stable during the PEC NOs-RR.

[0041] The findings highlight that initial NO3-adsorption is involved for selective NH3production. On the GaN surface, NO3-primarily adsorbs and is converted to NO2-, which desorbs without further reduction, leading to a high FENOS- above 80% (Figure 3, part e). However, when a moderate amount of small Au NCs was deposited on the GaN surface (the Au10 / GaN / Si example), the NO2-produced on the GaN NWs migrates to nearby Au sites where further reduction to NH3occurs via deoxygenation and hydrogenation with H+, resulting in high FENHS above 90%. In contrast, larger Au crystallites with higher surface coverage on GaN (Au100 / GaN / Si) impaired NO3-adsorption and reduced the number of Au-GaN active sites. This led to competition between NO3 RR and HER, causing a drop in FENHS to below 70%.

[0042] The mechanistic insights from DFT calculations support these results. In the photoelectrochemical NOs-reduction on GaN nanowires, NO2-is the primary product. This is because the reduction of NOs-to NO2-is thermodynamically favorable, involving only one endergonic electron transfer step with an energy of +0.29 eV, which is easily provided by the photo-excited electrons. In contrast, the reduction of NO2-to NO to form the first intermediate towards reduction to NH3requires significantly more energy (+0.65 eV) (Figure 4, part a). Additionally, GaN shows a stronger affinity for NOs-over NO2-, with the binding free energy of NOs-being more favorable by about -0.15 eV. This preferential binding of NOs-leads to the displacement of NO2-, which explains why NO2-is main product of the NOs-reduction. Thus,Atty. Docket No. 10110-25003A the combination of the lower energy requirements for NO3-reduction and the stronger binding affinity of NOs-over NO2-results in NO2-being expelled as the main product during the NOs-reduction.

[0043] On an Au surface, the binding preference between NO3-and NO2-is reversed: NO2-exhibits a more negative binding free energy than NO3-by approximately -0.30 eV on Au (211) surface. This indicates that once NO2-has been formed, it preferentially binds to Au making the initial adsorption of NO3-on Au (211) more difficult. Additionally, the reduction of NO3-to NO2-on Au involves an endergonic step of +0.30 eV during the conversion of *NOs to *NO2-, indicating that NO3-reduction to NO2-on Au is relatively slow and easily outcompeted by GaN. It is also worth noting that, while both NO2-and NO3-chemisorption on the inert Au surface are entropically disfavored in vacuum, the chemisorption of NO2-is more favorable than that of NO3-.

[0044] In contrast, the further reduction of NO2-is strongly favored on Au (Figure 4, part b). This process involves a transition in the binding mode from *ONO * (where NO2-binds via its two oxygen atoms to the Au step) to the *NO2-(where nitrogen binds to the Au step, with one of oxygen atom pointing down and adsorbing onto the lower terrace). This switch from oxygen to nitrogen binding is useful for the dissociation of the second N=O bond. A minimal free energy of 0.01 eV - easily surmountable thermally - enables irreversible bond dissociation to *NO and *OH intermediates on the surface. The subsequent removal of *OH through protonation, coupled to the reduction of the oxidized Au surface, is highly exergonic.

[0045] Further reduction steps to *NHO, *NHOH, *NH, and *NH2, culminating in the release of NH3are also exergonic, leading to complete reduction to NH3once NO2-dissociates. These computational findings align well with the test results, explaining why Au NCs without GaN show low NH3production yield - because little NO2-is formed and made available for further reduction - and why GaN alone exhibits high faradaic efficiency towards NO2-rather than to NH3. High and selective production of NH3is observed when GaN and Au are present and used together, with GaN reducing NO3-to NO2-, which desorbs and undergoes further reduction on Au, ultimately forming NH3.

[0046] The examples described above demonstrate the utility of the metal-support interactions between Au NCs and GaN NWs, which act as synergetic catalytic sites for PEC NO3 RR to NH3. Via this combination, NO3-is preferentially reduced to NO2-on GaN NWs, while the NO2-is subsequently converted to NH3on adjacent Au NCs. The sequential reduction of NO3-and NH3synthesis occur across two adjacent Au / GaN phases, making the optimization of Au NCs coverage on GaN NWs useful for achieving selective NH3Atty. Docket No. 10110-25003A production. As a result, example Au / GaN / Si photoelectrodes exhibited excellent performance, with faraday efficiency (FENHS) of 91.8% at -0.4 VRHE and a high production rate (YNHS) of 131.1 pmol / cm2 / h at -0.8 VRHE, with no significant degradation over 8 hours.

[0047] A number of examples of the disclosed devices, systems, and methods are now described in connection with the flow diagram of Figure 5 and the schematic diagram of Figure 6.

[0048] Figure 5 depicts a method 500 of fabricating a photocatalytic device for ammonia synthesis in accordance with one example. The method 500 may be used to manufacture any of the devices described herein or another device. The method 500 may include additional, fewer, or alternative acts. For instance, the method 500 may or may not include one or more acts directed to annealing the device (act 530).

[0049] The method 500 may begin with an act 502 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.

[0050] In some cases, the act 502 includes an act 504 in which 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 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.

[0051] The act 502 may include fewer, additional, or alternative acts. For instance, in the example of Figure 1 , the act 502 includes an act 506 in which the substrate is cleaned (e.g., with acetone, IPA and hydrofluoric acid), and an act 508 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.

[0052] The method 500 includes an act 510 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 512 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-corrosionAtty. Docket No. 10110-25003A 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.

[0053] 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 514. Beam equivalent pressures may be also adjusted in the act 514.

[0054] The act 510 includes doping the nanowires n-type in an act 516. For example, Ga and silicon and / or other fluxes may be controlled by using thermal effusion cells. In some cases, a dopant cell temperature is adjusted in an act 518 to control the doping (e.g., n-type doping) of the nanowires.

[0055] During the act 510, 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.

[0056] The act 510 may include additional, fewer, or alternative acts. For instance, the act 510 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 documents.

[0057] As shown in Figure 5, the method 500 further includes an act 520 in which the array is decorated with a catalyst arrangement. Catalysts nanostructures are deposited across the array of nanowire. In the example of Figure 5, the act 520 include an act 522 in which the nanowires are decorated with a catalyst arrangement. The act 522 may include depositing metal nanoclusters or other nanostructures on the nanowires. The nanostructures may be composed of, or otherwise include, gold and / or other metals, as described herein. In some cases, the deposition of the nanostructures includes implementation of a photo-deposition procedure in an act 524. Alternative or additional deposition procedures may be used to deposit the nanostructures, 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.Atty. Docket No. 10110-25003A

[0058] In photo-deposition cases, the array of nanowires or other conductive projections are immersed in a precursor solution, as described herein. The precursor solution has a precursor molar concentration at a level to form nanostructures of a desired size. For instance, the precursor molar concentration may be set to a level such that each nanostructure of the plurality of nanostructures has a size that falls in a range from about 1 nm to about 10 nm. The molar concentration may be sufficiently low to avoid formation of plate-shaped structures on the array of conductive projections. The molar concentration level may also be selected to limit the plurality of nanostructures to partial coverage of each conductive projection of the array of conductive projections.

[0059] The method 500 may include one or more additional acts directed to forming the photocatalytic structures of the device. For instance, in some cases, the method 500 includes an act 528 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).

[0060] The order of the above-described acts of the method 500 may differ from the example shown. For instance, the annealing of the act 528 may be implemented before or after the deposition of the nanostructures in the act 520.

[0061] Figure 6 depicts a system 600 for synthesis of ammonia in accordance with one example. The system 600 may also be configured for additional reactions, including, for instance, the evolution of H2. The system 600 may be configured as an electrochemical system. In this example, the electrochemical system 600 is a photoelectrochemical (PEC) system in which solar or other radiation is used to facilitate the ammonia synthesis. The manner in which the PEC system 600 is illuminated may vary.

[0062] The electrochemical system 600 includes one or more electrochemical cells 602. A single electrochemical cell 602 is shown for ease in illustration and description. The electrochemical cell 602 and other components of the electrochemical system 600 are depicted schematically in Figure 6 also for ease in illustration. The cell 602 contains an aqueous electrolyte solution 604 to which a source 606 of nitrate ions is applied. In the example of Figure 6, the nitrate source 607 provides KNO3, but alternative or additional sources may be used. Potassium bicarbonate KHCO3 may be included in the electrolyte. Additional or alternative aqueous solutions may be used. As described herein, the aqueous solution may be free of a sacrificial agent for the ammonia production. Further details regarding an example of the electrochemical system 600 are provided hereinabove.Atty. Docket No. 10110-25003A

[0063] The electrochemical cell 602 includes a working electrode 608, a counter electrode 610, and a reference electrode 612, each of which is immersed in the aqueous solution 604. The counter electrode 610 may be or include a metal wire or mesh, such as a platinum wire or mesh. The reference electrode 612 may be configured as a reversible hydrogen electrode (RHE). The configuration of the counter and reference electrodes 610, 612 may vary. For example, the counter electrode 610 may be configured as, or otherwise include, a photoanode at which water oxidation (2H2O <=> 02 + 4e + 4H+) occurs.

[0064] The synthesis of ammonia may occur at the working electrode 612 as described herein. To that end, electrons flow from the counter electrode 610 through a circuit path external to the electrochemical cell 602 to reach the working electrode 608. The working and counter electrodes 608, 610 may thus be considered a cathode and an anode, respectively. The competition between synthesis of NH3and evolution of H2may be managed or controlled (e.g., to favor NH3reduction) via the composition of the components of the nanoarchitecture and / or the applied voltage, as described herein.

[0065] In the example of Figure 6, the working and counter electrodes are separated from one another by a membrane 614, e.g., a proton-exchange membrane. In some cases, the membrane 614 is configured as, or otherwise includes, a Nation membrane. The construction, composition, configuration and other characteristics of the membrane 614 may vary.

[0066] In this example, the circuit path includes a voltage source 616 of the electrochemical system 600. The voltage source 616 is configured to apply a bias voltage between the working and counter electrodes 608, 610. The bias voltage may be used to establish a desired product ratio at the working electrode. The circuit path may include additional or alternative components. For example, the circuit path may include a potentiometer in some cases.

[0067] In some cases, the working electrode 608 is configured as a photocathode. Light 618, such as solar radiation, may be incident upon the working electrode 608 as shown. The electrochemical cell 602 may thus be considered and configured as a photoelectrochemical cell. In such cases, illumination of the working electrode 608 may cause charge carriers to be generated in the working electrode 608. Electrons that reach the surface of the working electrode 608 may then be used in the ammonia synthesis. The photogenerated electrons may augment the electrons provided via the current path. A number of examples of, and further details regarding, photocathodes are provided hereinabove in connection with, for instance, Figures 1-5.Atty. Docket No. 10110-25003A

[0068] The working electrode 608 includes a substrate 620. The substrate 620 of the working electrode 608 may constitute a part of an architecture, or a support structure, of the working electrode 608. The substrate 620 may be uniform or composite. For example, the substrate 620 may include any number of layers or other components. The substrate 620 thus may or may not be monolithic. The shape of the substrate 620 may also vary. For instance, the substrate 620 may or may not be planar or flat.

[0069] The substrate 620 of the working electrode 608 may be active (functional) and / or passive (e.g., structural). In the latter case, the substrate 620 may be configured and act solely as a support structure for a catalyst arrangement formed along an exterior surface of the working electrode 608, as described below. Alternatively or additionally, the substrate 620 may be composed of, or otherwise include, a material suitable for the growth or other deposition of the catalyst arrangement of the working electrode 608.

[0070] The substrate 620 may include a light absorbing material. The light absorbing material is configured to generate charge carriers upon solar or other illumination. The light absorbing material has a bandgap such that incident light generates charge carriers (electron-hole pairs) within the substrate. Some or all of the substrate 620 may be configured for photogeneration of electron-hole pairs. To that end, the substrate 620 may be composed of, or otherwise include, a semiconductor material. In some cases, the substrate 620 is composed of, or otherwise includes, silicon. For instance, the substrate 620 may be provided as a silicon wafer. The silicon may be doped. In some cases, the substrate 620 is heavily n-type doped, and moderately or lightly p-type doped, to form a junction as described herein. The doping arrangement may vary. For example, one or more components of the substrate 620 may be non-doped (intrinsic), or effectively non-doped. The substrate 620 may include alternative or additional layers, including, for instance, support or other structural layers. In other cases, the substrate 620 is not light absorbing. In these and other cases, one or more other components of the photocathode (e.g., nanowires) may be composed of, or otherwise include, a semiconductor material configured to act as a light absorber. Thus, in photoelectrochemical cases, the semiconductor material of the substrate and / or other components supported by the substrate may be configured to generate charge carriers upon absorption of solar (or other) radiation, such that the chemical cell is configured as a photoelectrochemical system.

[0071] The substrate 620 of the working electrode 608 establishes a surface at which a catalyst arrangement of the electrode 608 is provided. The catalyst arrangement may include a conductive projection (e.g., nanowire)-nanostructure architecture as described below.Atty. Docket No. 10110-25003A

[0072] The electrode 608 includes an array of nanowires 622 and / or other conductive projections supported by the substrate 620. Each nanowire 622 extends outward from the surface of the substrate 620. The nanowires 622 may thus be oriented in parallel with one another. Each nanowire 622 may have a semiconductor composition for synthesis of ammonia. In some cases, the semiconductor composition includes gallium nitride (GaN). Additional or alternative nitride materials (e.g., semiconductor nitrides) may be used, including, for instance, indium nitride, indium gallium nitride, aluminum nitride, boron nitride, and / or their alloys.

[0073] The nanowires 622 may facilitate the conversion in one or more ways. For instance, each nanowire 622 may be configured to extract the charge carriers (e.g., electrons) generated in the substrate 620. The extraction brings the electrons to external sites along the nanowires 622 for use in the ammonia synthesis. The composition of the nanowires 622 may also form an interface well-suited for the ammonia synthesis, as explained herein.

[0074] Each nanowire 622 may be or include a columnar, post-shaped, or other elongated structure that extends outward (e.g., upward) from the plane of the substrate 620. The nanowires 622 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 dimensions, size, shape, composition, and other characteristics of the nanowires 622 (and / or other conductive projections) may vary. For instance, each nanowire 622 may or may not be elongated like a nanowire. Thus, other types and shapes of nanostructures or other conductive projections from the substrate 620, such as various shaped nanocrystals, may be used.

[0075] In some cases, one or more of the nanowires 622 is configured to generate electron-hole pairs upon illumination. For instance, the nanowires 622 may be configured to absorb light at frequencies different than other light absorbing components of the electrode 608. For example, one light absorbing component, such as the substrate 620, 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 nanowires 622 are the only light absorbing component of the electrode 608.

[0076] The electrode 608 further includes nanoclusters or other nanostructures 624 disposed over the array of nanowires 622. Each nanostructure 624 is configured for the catalytic synthesis of ammonia (NH3) in the chemical cell 602. A plurality of the nanostructures 624 are disposed on each nanowire 622, as schematically shown in Figure 6. The nanostructures 624 are distributed across the outer surface of each nanowire 622. For example, each nanowire 622 has a plurality of the nanostructures 624 distributed across orAtty. Docket No. 10110-25003A along sidewalls of the nanowire 622. The nanostructures 624 may also be disposed on a top or upper surface of each nanowire 622. The distribution may or may not be uniform or symmetric as shown. As described herein, each nanostructure 624 may include or be composed of a metal, such as gold, for the reduction of nitrates in the chemical cell 602 for the synthesis of ammonia. In some cases, additional or alternative nanostructures may be used, including, for instance, nanostructures composed of, or otherwise including, metals other than the noble metals.

[0077] The nanostructures 624 may be sized in a manner to facilitate the ammonia synthesis. The size of the nanostructures 624 may be useful in catalyzing the reaction, as described herein. The size of the nanostructures 624 may be promote the ammonia synthesis in additional or alternative ways. For instance, the nanostructures 624 may also be sized to avoid inhibiting the illumination of the light absorber (e.g. the substrate 620).

[0078] The manner in, or extent to, which the array of nanowires 622 is ordered may vary. In some cases, the nanowires 622 may be arranged laterally in a regular or semi-regular pattern. In other cases, the lateral arrangement of the nanowires 622 is irregular. In such cases, the ordered nature of the nanowires 622 is instead limited to the parallel orientation of the nanowires 622.

[0079] In some cases, each nanowire 622 is coated with the nanostructures 624. The extent of the coating may vary. For instance, a top surface of each nanowire 622 may be entirely coated with the nanostructures 624, while one or more portions of the sidewalls of the nanowires 622 may be partially coated. The distribution of the nanostructures 624 may accordingly be uniform or non-uniform. The nanostructures 624 may thus be distributed randomly across each nanowire 622. The schematic arrangement of Figure 6 is shown for ease in illustration.

[0080] The nanowires 622 may be configured to generate electron-hole pairs upon illumination. The nanowires 622 may be configured to generate the electron-hole pairs upon absorption of light at certain wavelengths (e.g., LIV solar wavelengths). In some cases, each nanowire 622 has a uniform composition. For instance, each nanowire may be composed entirely of a single semiconductor material (e.g., a Ill-nitride semiconductor material, such as GaN). The semiconductor composition establishes one or more bandgaps that establish the wavelength(s) at which charge carrier generation occurs. In some cases, a single bandgap is established for absorption of light of ultraviolet wavelengths in solar radiation.

[0081] In other cases, the nanowires 622 have multiple segments, with each segment being configured to absorb light over a respective range of wavelengths. For instance, eachAtty. Docket No. 10110-25003A nanowire 622 may include a stacked or layered arrangement of semiconductor materials. Each layer in the arrangement may be configured for absorption of light at other wavelengths of the solar spectrum (e.g., infrared, visible, and / or ultraviolet wavelengths).

[0082] 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. 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.

[0083] The nanowires 622 and the nanostructures 624 are not shown to scale in the schematic depiction of Figure 6. The shape of the nanowires 622 and the nanostructures 624 may also vary from the example shown.

[0084] 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.

[0085] 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.

[0086] The foregoing description is given for clearness of understanding only, and no unnecessary limitations should be understood therefrom.

Claims

Atty. Docket No. 10110-25003A What is Claimed is:

1. A system for photocatalytic synthesis of ammonia, the device comprising:an aqueous solution comprising nitrate ions; anda photocatalytic device immersed in the aqueous solution, the photocatalytic device comprising:a substrate having a surface;a conductive structure supported by the substrate at the surface, the conductive structure comprising a semiconductor interface in contact with the aqueous solution, the semiconductor interface being configured to reduce the nitrate ions to nitrite ions; anda plurality of catalyst nanostructures supported by the conductive structure, each catalyst nanostructure of the plurality of catalyst nanostructures comprising a metal configured to reduce the nitrite ions to ammonia.

2. The system of claim 1 , wherein the conductive structure comprises a conductive projection extending outward from the surface of the substrate.

3. The system of claim 1 , wherein the semiconductor interface comprises a nitride semiconductor.

4. The system of claim 3, wherein the nitride semiconductor is gallium nitride.

5. The system of claim 1 , wherein each nanostructure of the plurality of catalyst nanostructures comprises metallic gold.

6. The system of claim 1 , wherein the conductive structure is partially covered by the plurality of catalyst nanostructures.

7. The system of claim 1 , wherein:the substrate comprises a semiconductor material; andthe semiconductor material is doped to define a junction to generate charge carriers upon absorption of solar radiation.

8. The system of claim 7, wherein the conductive structure comprises a nanowire configured to extract the charge carriers generated in the substrate.

9. The system of claim 1 , wherein the aqueous solution comprises KNO3.Atty. Docket No. 10110-25003A 10. The system of claim 1 , wherein the aqueous solution is free of a sacrificial agent for the synthesis of ammonia from the nitrate ions.

11. The system of claim 1 , wherein the photocatalytic device is configured as a working electrode, and the system further comprises:a counter electrode immersed in the aqueous solution; anda voltage source configured to apply a bias voltage between the working electrode and the counter electrode;wherein the bias voltage is at a level for the synthesis of ammonia at the working electrode.

12. The system of claim 11 , wherein the level of the bias voltage falls in a range from about 0 to about -2 Volts versus reversible hydrogen electrode (VRHE).

13. A method of using the system of claim 1 , the method comprising:illuminating the photocatalytic device; andcapturing the ammonia synthesized by the photocatalytic device.

14. The method of claim 13, further comprising applying a bias voltage to the photocatalytic device.

15. A system for photocatalytic synthesis of ammonia, the system comprising:an aqueous solution comprising nitrate ions; anda photoelectrode immersed in the aqueous solution, the photoelectrode comprising:a substrate having a surface, the substrate being configured to generate charge carriers upon absorption of solar radiation;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 having a nitride semiconductor interface in contact with the aqueous solution, the nitride semiconductor interface being configured to reduce the nitrate ions to nitrite ions; anda plurality of catalyst nanostructures disposed over the array of conductive projections, the charge carriers generated in the substrate migrating to the plurality of catalyst nanostructures via the array of conductive projections, each catalyst nanostructure of the plurality of catalyst nanostructures comprising a metal configured to reduce the nitrite ions to ammonia.Atty. Docket No. 10110-25003A 16. The system of claim 15, wherein the photoelectrode is configured as a working electrode, and the system further comprises:a counter electrode immersed in the aqueous solution; anda voltage source configured to apply a bias voltage between the working electrode and the counter electrode;wherein the bias voltage is at a level for the synthesis of ammonia at the working electrode.

17. The system of claim 16, wherein the level of the bias voltage falls in a range from about 0 to about -2 Volts versus reversible hydrogen electrode (VRHE).

18. A method of using the system of claim 15, the method comprising:illuminating the photoelectrode; andcapturing the ammonia synthesized by the photoelectrode.

19. The method of claim 18, further comprising applying a bias voltage to thephotoelectrode.

Images