Photocatalytic urea synthesis

Abstract

A system for photocatalytic synthesis of urea includes an aqueous solution and a photocatalytic device immersed in the aqueous solution. The photocatalytic device includes a substrate having a surface, and a conductive structure supported by the substrate at the surface, the conductive structure including a nitride interface in contact with the aqueous solution. The aqueous solution includes nitrate ions and dissolved carbon dioxide for synthesis of urea at the nitride interface of the conductive structure.

Description

PHOTOCATALYTIC UREA SYNTHESISCROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of U.S. provisional application entitled “Photocatalytic Urea Synthesis,” filed January 29, 2024, and assigned Serial No. 63 / 626,377, the entire disclosure of which is hereby expressly incorporated by reference.STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

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

[0003] The disclosure relates generally to photoelectrochemical and other photocatalytic synthesis of urea.Brief Description of Related Technology

[0004] Sustainable carbon and nitrogen recycling has emerged as an energy and environmental issue with movement toward a future with zero carbon emissions. The electrochemical carbon dioxide (CO2) reduction reaction is a promising route for reducing CO2 emission and producing value-added chemicals such as CO, HCOOH, CH4, and multicarbon products. The development of electrocatalysts for CO2 reduction reactions has enabled the selective production of target chemicals in aqueous electrolytes. There are two main pathways for nitrogen recycling, including nitrogen (N2) or nitrate (NO3reduction reactions to ammonia (NH3), a useful raw material for modern chemistry. Both reactions offer a clean method for NH3synthesis compared to the conventional, energy-intensive, and carbon-emitting Haber-Bosch process. NO3_reduction is thermodynamically more feasible than N2reduction because the N=O bond has much lower dissociation energy (204 kJ / mol) than the N N bond (941 kJ / mol), and NO3‘ is about 40,000 times more soluble in water thanN2. Therefore, high selectivity and productivity of NH3have been achieved by electrochemical NO3_reduction using various electrocatalysts.

[0005] Urea (CO(NH2)2) is a useful fertilizer manufactured through a two-step process under high-temperature and high-pressure conditions. The Haber-Bosch process first yields NH3, as depicted in the chemical reaction: N2+ 3H2— > 2NH3. Subsequently, NH3reacts with CO2to produce urea, following the chemical equation: 2NH3+ CO2— > CO(NH2)2+ H2O. Recently, electrochemical synthesis of urea has been demonstrated by simultaneous CO2reduction and N2 / NO / NO27NO3_reduction at room temperature and under ambient pressure. For instance, PdCu-TiO2, Bi-BiVC heterostructure, CuPc nanotubes, InOOH nanoparticles, and polypyrrole-coated Pt electrode have produced urea from direct coupling of N2with CO2. However, the faradaic efficiency (FEurea) has remained lower than 20% due to the difficulty of activating N2.

[0006] As alternative nitrogen sources, NO and NO2_have been electrochemically converted to urea with higher FEurea (11 - 43%) using Zn nanobelts, Te-Pd nanocrystals, ZnO nanosheets, and Cu-TiO2catalysts. More recently, oxygen vacancy-enriched CeO2, ln(OH)3, and InOOH catalysts as well as Ti02-nafion, Cu-Zn nanowires, Fe-Ni diatomic catalysts, and Cu-N-C single atom catalysts have enabled the concurrent reduction of NO3_and CO2to urea in an aqueous electrolyte. In particular, experiments with ln(OH)3and InOOH have achieved high selectivity of urea generation (FEurea > 50%) at cathodic potentials of -0.6 and -0.5 V versus the reversible hydrogen electrode (VRHE), respectively. Theoretical calculations have suggested that the facilitated C-N coupling between *NO2_and *CO2intermediates on ln(OH)3at an early stage of the reaction can promote urea synthesis more effectively than the conventional C-N coupling between *NH2and *CO. Furthermore, rational design of the Cu-W bimetallic C-N coupling sites improved the performance of electrocatalysts, resulting in high FEurea greater than 70%. However, electrochemical urea synthesis still undesirably relies upon an applied potential, and the product selectivity is still much lower than the individual C02-to-CO and N03-to-NH3conversion reactions, which typically show faradaic efficiencies above 80%.SUMMARY OF THE DISCLOSURE

[0007] In accordance with one aspect of the disclosure, a system for photocatalytic synthesis of urea includes an aqueous solution and a photocatalytic device immersed in the aqueous solution. The photocatalytic device includes a substrate having a surface and a conductive structure supported by the substrate at the surface. The conductive structureincludes a nitride interface in contact with the aqueous solution. The aqueous solution includes nitrate ions and dissolved carbon dioxide for synthesis of urea at the nitride interface of the conductive structure.

[0008] In accordance with another aspect of the disclosure, a system for photocatalytic synthesis of urea includes an aqueous solution and a photoelectrode immersed in the aqueous solution. The photoelectrode includes 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 having a composition configured to catalyze a nitrate reduction reaction, and a plurality of catalyst nanoparticles disposed over the array of conductive projections, the charge carriers generated in the substrate migrating to the plurality of catalyst nanoparticles via the array of conductive projections, each catalyst nanoparticle of the plurality of catalyst nanoparticles including a metal configured to catalyze a carbon dioxide reduction reaction. The aqueous solution includes nitrate ions and dissolved carbon dioxide for synthesis of urea via the nitrate reduction reaction and the carbon dioxide reduction reaction, respectively.

[0009] In accordance with yet another aspect of the disclosure, a method of fabricating a device for photocatalytic synthesis of urea includes growing an array of conductive projections on a semiconductor substrate, each conductive projection of the array of conductive projections having a nitride semiconductor interface, the nitride semiconductor interface having a composition configured to catalyze a nitrate reduction reaction, and implementing a photo-deposition procedure to deposit a plurality of nanoparticles across each conductive projection of the array of conductive projections, each nanoparticle of the plurality of nanoparticles having a metallic composition configured to catalyze a carbon dioxide (CO2) reduction reaction. Implementing the photo-deposition procedure includes immersing the array of conductive projections in a precursor solution, the precursor solution having a precursor molar concentration at a level to avoid formation of plate-shaped structures on the array of conductive projections, the level of the precursor molar concentration limiting the plurality of nanoparticles to partial coverage of each conductive projection of the array of conductive projections.

[0010] In connection with any one of the aforementioned aspects, the systems, 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 structureincludes a conductive projection extending outward from the surface of the substrate. The nitride interface has a composition configured for catalyzing a nitrate reduction reaction. The nitride interface includes a nitride semiconductor. The nitride semiconductor is gallium nitride. The photocatalytic device further includes a plurality of catalyst nanoparticles disposed over the conductive structure. Each nanoparticle of the plurality of catalyst nanoparticles includes a metal configured for catalyzing a carbon dioxide reduction reaction. Each nanoparticle of the plurality of catalyst nanoparticles includes metallic silver. The conductive structure is partially covered by the plurality of catalyst nanoparticles. The substrate includes a semiconductor material, and 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 is a CO2-saturated electrolyte, and the CO2- saturated electrolyte includes KNO3. The aqueous solution is free of a sacrificial agent for the synthesis of urea from the nitrate ions and the dissolved carbon dioxide. The photocatalytic device 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 urea at the working electrode. The level of the bias voltage falls in a range from about -0.1 to about -0.6 Volts versus reversible hydrogen electrode (VRHE). A method of using a system as described herein includes illuminating the photocatalytic device, and capturing the urea 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 urea at the working electrode. The level of the bias voltage falls in a range from about -0.1 to about -0.6 Volts versus reversible hydrogen electrode (VRHE). A method of using a system as described herein includes illuminating the photoelectrode, and capturing the urea synthesized by the photoelectrode. The method further includes applying a bias voltage to the photoelectrode. The precursor molar concentration is such that each nanoparticle of the plurality of nanoparticles has a size that falls in a range from about 10 nm to about 80 nm.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 view of an example photoelectrode device for photoelectrochemical urea synthesis conducted under solar light with cathodic bias, showing Ag nanoparticle loading of GaN nanowires vertically grown on a n+-p Si substrate, (b) an energy band diagram of the Ag / GaN / Si photoelectrode under light illumination, in which photoexcited electrons migrate from Si to Ag nanoparticles through GaN nanowires and participate in urea synthesis in an aqueous solution containing NOs- and CO2, (c-e) tilt-view SEM images of example Ag / GaN / Si photoelectrodes with different loading amounts of Ag cocatalysts, in which the loading amount of Ag was controlled by adjusting the molar concentration of an AgNOs precursor solution during the photo-deposition step, (f) a HADDF-STEM image and (g) EDS elemental map of GaN nanowires loaded via a precursor solution having an Ag amount of 30 M ("Ag30"), and (h) a graphical plot of surface Ag / GaN ratio obtained from XPS measurements.

[0013] Figure 2 depicts graphical plots of example device performance, including (a) LSV curves, (b) Faradaic efficiency, and (c) production rate for GaN / Si, Ag3 / GaN / Si, Ag30 / GaN / Si, and Ag300 / GaN / Si photoelectrodes, in which the product analysis was conducted at -0.3 and -0.6 VRHE in a CO2-saturated 0.5 M KNO3 aqueous electrolyte under AM 1 .5 G-filtered 1 sun light, as well as graphical plots of Faradaic efficiency at different potentials for (d) GaN / Si and (e) Ag / GaN / Si devices, (f) urea production rate of GaN / Si and Ag / GaN / Si devices at different potentials, in which measurements were performed 4 times, (g) comparison of FEurea between an example Ag / GaN / Si photoelectrode and previously reported electrocatalysts, and (h) a stability test of an Ag30 / GaN / Si example at -0.3 VRHE. Each cycle was conducted for 1 hour.

[0014] Figure 3 depicts graphical plots of LSV curves of (a) GaN / Si and (d) Ag / GaN / Si devices measured in CO2-saturated 0.5 KNO3, Ar-saturated 0.5 M KNO3 (Ar-KNOs), and CO2-saturated 0.5 M KHCO3 (CO2-KHCO3), as well as graphical plots of Faradaic efficiency of H2, CO, NH3, NO2 , and urea for a GaN / Si device in (b) Ar-KNOs and (c) CO2-KHCO3, and graphical plots of Faradaic efficiency for an example Ag / GaN / Si photoelectrode in (e) Ar- KNO3 and (f) CO2-KHCO3. Urea was not produced in either Ar-KNOs, or CO2-KHCO3.

[0015] Figure 4 depicts schematic and graphical views of (a) a free energy profile of urea production on GaN without reduction of CO2 to CO, including a first step involving C-N bondformation, and subsequent reduction on a carbon site with further C-N bond formation and reduction, as well as (b) corresponding DFT optimized structures and reaction equations for each step.

[0016] Figure 5 depicts schematic and graphical views of (a) a free energy profile of urea production on GaN with Ag particles, in which Ag particles reduce CO2 to CO, so the C-N bond formation is facilitated by reaction of CO with either *NO2 (blue) or *NO2_(red) intermediates (this reaction is followed by further reduction and the second C-N bond formation), as well as (b) corresponding DFT optimized structures and reaction equations for each step.

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

[0018] Figure 7 is a flow diagram of a method of fabricating a device (e.g., a photocathode) for synthesis of urea in accordance with one example.

[0019] The embodiments of the disclosed systems, 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] Systems and devices for photoelectrochemical and other photocatalytic synthesis of urea from nitrate and carbon dioxide are described. Methods for fabricating photocatalytic devices are also described. The photocatalytic devices are immersed in an aqueous solution having nitrate ions and dissolved carbon dioxide for the synthesis of urea at a nitride interface of the photocatalytic device in contact with the aqueous solution. The nitride 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 including a nitride semiconductor, such as GaN.

[0021] The disclosed systems and devices address the bottleneck presented by previous electrochemical methods of urea synthesis by providing a photoelectrode. Semiconductor photoelectrodes generate a built-in potential under solar light, which can be used to shift the operating potential to a more positive value. As described herein, the semiconductormaterials of the disclosed devices are capable of efficiently and stably producing urea in aqueous electrolytes as photocatalysts for solar-driven urea production. For instance, example photoelectrodes with GaN nanowires grown on n+-p Si photoelectrodes exhibited catalytic activity for urea synthesis. Unprecedented high FEurea of 74.5 ± 4.1% was achieved at -0.3 VRHE by concurrent NO3- and CO2 reduction reactions. When Ag nanoparticle cocatalysts were loaded onto the GaN nanowires, the onset potential was positively shifted by about 0.1 V and the potential range for selective urea production (FEurea greater than 45%) was significantly broadened. This resulted in a four-fold increase in the urea production rate (4.4 ± 0.9 pmol / cm2 / h) at -0.6 VRHE. Measurements and theoretical calculations indicate that the GaN surface effectively produced NO2 intermediates, which then interacted with COO- radicals and resulted in C-N coupling at the early stage of urea synthesis. These examples show that GaN nanowires and other nitride interfaces are capable of selectively producing urea by promoting C-N bonds from NO3- and CO2.

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

[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 uniform semiconductor compositions. For instance, the conductive projections of the photocatalytic devices may have a multi-band configuration. For example, the arrays may include monolithically integrated multiple-band InGaN nanostructures or segments configured to act as photocatalysts. Each conductive projection may thus be capable of photoexcitation via a wider range of wavelengths, including, for instance, both ultraviolet and visible portions of the solar spectra. Any number or type of segments may be included.

[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 silver nanoparticles, the catalytic arrangements of the disclosed photocatalytic devices may alternatively or additionallyinclude other catalysts. For instance, nanoparticles composed of, or otherwise including, additional and / or alternative metal catalysts, such as Au, Co, Ni, and Ir, may be used for urea synthesis. In still other cases, the catalytic arrangements of the disclosed photocatalytic devices may not include nanoparticles.

[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 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. Commun. 6, 1-8 (2015); Wang, D. et al., "Wafer-level photocatalytic water splitting on GaN nanowire arrays grown by molecular beam epitaxy," Nano Lett. 11 , 2353-2357 (2011); Guan, X. et al., "Making of an industry-friendly artificial photosynthesis device," ACS Energy Lett. 3, 2230-2231 (2018); U.S. Patent Publication No. 2023 / 0017032 ("CO2 Conversion with Metal Sulfide Nanoparticles"); and, International Application No. PCT / US23 / 24569 ("Photocatalytic CO2 Reduction with Co-Catalyst- Decorated Nanostructures"), the entire disclosures of which are hereby incorporated by reference.

[0029] Disclosed herein are examples semiconductor photoelectrodes capable of synthesizing urea from carbon dioxide and nitrate under solar light. Example photoelectrodes include GaN nanowires, which have catalytic activity for nitrate conversion to nitrite, as well as Ag cocatalysts loaded onto the GaN nanowires that also promote the urea synthesis. In examples employed for photoelectrochemical urea synthesis, a high faradaic efficiency of 75.6 ± 2.6% was achieved at a potential of -0.3 vs. reversible hydrogen electrode. Measurements and theoretical calculations indicate that the high selectivity of urea originates from the facilitated C-N coupling between the intermediates of NO2 and COO- at an early stage of the reduction reaction. The disclosed examples demonstrate thatGaN nanowires and other nitride interfaces (e.g., loaded with Ag cocatalysts) achieve solar- powered urea synthesis with higher efficiency than previously reported techniques.

[0030] As described herein, the urea production was achieved by the example photoelectrodes without reliance on a sacrificial agent. Thus, the aqueous solutions in which the photoelectrodes were immersed were free of a sacrificial agent.

[0031] Example photoelectrodes 100 included n-type GaN nanowires 102 that were vertically grown along the c-axis of wurtzite crystal on an n+-p Si wafer 104 by plasma- assisted molecular beam epitaxy under nitrogen-rich conditions (Figure 1 , part a). In these examples, the n-type doping of the nanowires 102 may be or include silicon, but alternative or additional dopants may be used. Ag nanoparticle cocatalysts 106 were then loaded onto the GaN nanowires 102 by photo-deposition as described herein. When the photoelectrode 100 is irradiated with light 108, the photoexcited electrons in the Si wafer 104 move toward the GaN nanowires 102 and the Ag cocatalysts 106 due to the built-in potential of the n+-p Si junction and the applied cathodic bias. The photogenerated electrons migrate to the GaN and Ag surfaces without significant electrical resistance (see, e.g., Figure 1 , part b), because there is negligible energy offset between the conduction bands of n+-Si and n-GaN while the work function of Ag is below the conduction band edge of n-GaN. As described herein, the electrons on the surfaces of GaN and Ag participate in the reduction steps that synthesize urea from NOs- and CO2 under aqueous conditions.

[0032] Three different Ag / GaN / Si photoelectrodes 100 were prepared by changing the molar concentration of a precursor solution (AgNOs) used for photodeposition. The photoelectrodes 100 are denoted Ag# / GaN / Si herein, where # is the molar concentration of AgNOs in the unit of pM. As the amount of Ag precursor solution increased from 3 to 300 pM, the size of the Ag nanoparticle cocatalysts 106 became larger on the GaN nanowires 102 (Figure 1 , parts c-e). In the case of Ag3 and Ag30, small nanoparticles 106 were formed on the GaN nanowires 102, whereas in Ag300, large microscale plate-shaped Ag structures 106 were grown on the surface of the GaN nanowires 102. These results indicate that the morphology and surface coverage of the Ag cocatalysts 106, which are closely related to the photocurrent generation of underlying GaN / Si photoelectrode 100 and catalytic performance for urea synthesis, can be well controlled.

[0033] The microstructure of the Ag30 / GaN example photoelectrode 100 was characterized using a high angular annular dark-field scanning transmission electron microscopy (HAADF- STEM) after the Ag / GaN nanowire array was scraped from the Si wafer 104 (Figure 1 , part f). Ag nanoparticles 106 ranging in size from 10-80 nm were observed on the GaNnanowires 102 with a length of about 400 nm and a diameter of about 50 nm. The elemental distribution of N (red) from the GaN nanowires 102 and Ag (cyan) from the Ag nanoparticles 106 was measured by energy-dispersive X-ray spectroscopy (EDS) (Figure 1 , part g). Nitrogen was uniformly distributed over the GaN nanowires 102, whereas the Ag nanoparticles 106 were partially covered on the GaN surface. The area ratio of the Ag nanoparticles 106 to the GaN nanowires 102 was measured to be approximately 0.19, providing an estimate for the surface coverage of the Ag nanoparticles 106 on the GaN nanowires 102. In the high-resolution STEM images, Ag (11 1 ) planes with d-spacing of 0.235 nm were predominantly observed. X-ray diffraction (XRD) patterns showed a prominent GaN (002) peak, indicating the presence of single-crystalline GaN nanowires across all samples. Notably, the XRD pattern corresponding to Ag nanoparticles was absent in the Ag3 / GaN / Si example. However, a peak of Ag (1 11 ) emerged in the Ag30 / GaN / Si example, becoming more pronounced in the Ag300 / GaN / Si example due to increased crystallite size and density of Ag nanoparticles.

[0034] The surface bonding states of the cocatalyst arrangement were characterized by X- ray photoelectron spectroscopy (XPS) analysis. The Ag3 / GaN / Si example exhibited a minor Ag+state alongside a major Ag° state. Notably, the Ag+bonding state diminished in the Ag30 and Ag300 examples, indicating that the Ag nanoparticles 106 were primarily composed of metallic phases. The surface atomic ratio (Ag / Ga+N) was calculated by integration of Ga 2p3 / 2, N 1 s, and Ag 3d peaks, followed by multiplying the atomic sensitivity factors of each element. The Ag / GaN ratio was 0.03, 0.22, and 1 .05 for the Ag3 / GaN / Si, Ag30 / GaN / Si, and Ag300 / GaN / Si photoelectrodes, respectively (Figure 1 , part h). These measurements indicate an increasing surface coverage with higher AgNO3concentrations in the solution for the photo-deposition.

[0035] Turning to Figure 2, example photoelectrodes were then tested for photoelectrochemical (PEC) urea synthesis from NO3_and CO2. Linear sweep voltammetry (LSV) measurements were carried out to investigate PEC urea synthesis in CO2-saturated 0.5 M KNO3under AM 1 .5 G 1 sun light illumination. A GaN / Si photoelectrode showed an onset potential (VonSet) = -0.3 VRHE, where photocurrent density reached -1 mA / cm2(Figure 2, part a). The VonSet was improved to -0.2 VRHE after loading Ag nanoparticles on the GaN / Si photoelectrode, indicating that metallic Ag promotes cathodic reactions such as urea synthesis, NO3‘ reduction, CO2 reduction, or H2 evolution. However, when too many Ag microplates were loaded (the Ag300 / GaN / Si example), the photocurrent density rapidly decreased because the opaque Ag cocatalyst blocked the incident light and limited the number of photoexcited charge carriers in the GaN / Si photoelectrode.

[0036] Under dark conditions, no measurable photocurrent density was observed in any of the examples, indicating that the solar light was involved in the photoelectrochemical reaction.

[0037] The photocurrent utilization and product selectivity were assessed by measuring the faradaic efficiency (Figure 2, part b) and production rate of urea (Figure 2, part c) at applied voltages of -0.3 VRHE and -0.6 V HE. H2and CO gaseous products were measured by gas chromatograph, whereas urea, NH3, N2H4, and NO2_liquid products were spectrocolorimetrically measured. All photoelectrode examples produced very high FEurea greater than 70% (Figure 2, part b) with a trace amount of NH3, H2, and CO products at -0.3 VRHE. However, when operating at -0.6 V HE, there was a notable decline in the faradaic efficiency and production rate of urea on the GaN / Si and Ag3 / GaN / Si photoelectrodes likely due to the insufficient surface coverage of Ag nanoparticles. In contrast, the Ag30 / GaN / Si example retained high selectivity greater than 50% at an applied voltage of -0.6 V HE. AS a result, the Ag30 / GaN / Si photoelectrode exhibited the highest production rate of urea at -0.6 V HE among the tested examples due to the optimized balance between catalytic activity and photocurrent generation (Figure 2, part c).

[0038] The potential dependent faradaic efficiency of the examples was measured and compared on GaN / Si and Ag / GaN / Si photoelectrodes. The GaN / Si photoelectrode exhibited high FEUrea greater than 70% at -0.3 and -0.4 V HE (Figure 2, part d). Such high urea selectivity has never been reported among any semiconductor materials and photoelectrodes known to the inventors. At more negative potentials (< -0.5 V HE), NO2_was produced as the major product. The missing total FE (< 20%) may be attributed to the presence of undetected byproducts, such as nitrogen, acetamide, and formamide. Noticeably, the Ag / GaN / Si example showed a broader potential window (voltages ranging from -0.1 to -0.6 V HE) for high FEurea (Figure 2, part e). The Ag / GaN / Si example achieved about 4 times higher production rate of urea (4.4 ± 0.9 pmol / cm2 / h) than GaN / Si (1 .0 ± 0.1 pmol / cm2 / h) at -0.6 V HE (Figure 2, part f) because the higher photocurrent density flows at more negative potentials. The Ag nanoparticles supported on GaN nanowires thus improve the efficiency and productivity of urea synthesis. Owing to the built-in potential generated by solar light and the excellent catalytic selectivity, the Ag / GaN / Si photoelectrode showed a high FEurea of 75.6 ± 2.6% at a low potential of -0.3 V HE compared to previously reported electrocatalysts measured in dark conditions (Figure 2, part g). While the potential used to achieve a high FEurea of 75.6% is slightly more negative (-0.3 V HE) for the Ag / GaN / Si photoelectrode compared to the recent CuWO4electrocatalyst (FEurea = 70.1% at -0.2 V HE), this result indicates that the integration of high-performing cocatalysts with GaNnanowires / n+-p Si photoelectrodes has the potential to further shift the operating potential in a positive direction. Moreover, the stability of the Ag / GaN / Si photoelectrode was tested at - 0.3 V RHE for 6 cycles (Figure 2, part h). Each cycle was performed for 1 hour. The electrolyte was replaced after the reaction. The photoelectrode consistently produced urea with high FEUrea of 70.5 ± 5.0% and production rate of 0.8 ± 0.1 pmol / cm2 / h, all while preserving its morphology and chemical composition without any noticeable changes.

[0039] Control measurements were performed in three aqueous electrolytes of CO2- saturated 0.5 M KNO3, Ar-saturated 0.5 M KNO3, and CO2-saturated 0.5 M KHCO3 to confirm that the C and N elements in urea originated from CO2 and NOs- sources, respectively. First, the GaN / Si photoelectrode (without Ag cocatalysts) showed more positive onset potential in the Ar-KNOs solution than the CO2-KNO3 solution, and a low current density less than 1 mA / cm2was measured in the CO2-KHCO3 solution within the potential up to -0.6 VRHE (Figure 3, part a). This means that GaN is more selective for NOs- reduction reaction than CO2 reduction, and that CO2 dissolved in the solution slows down the reaction kinetics likely due to lowering the local concentration of NOs- near the photoelectrode.

[0040] Potential-dependent FEs were also measured in the Ar-KNOs (Figure 3, part b) and CO2-KHCO3 (Figure 3c). In the Ar-KNOs solution, NO2- was the major product with FENo2- greater than 70%, and minor products of H2and NH3were produced with FEs less than 20% (Figure 3, part b). In the CO2-KHCO3 solution, H2was the only product (Figure 3, part c). There was no CO evolution despite the presence of the CO2 reactant in the solution. These results reveal that GaN nanowires are capable of effectively reducing NOs- to NO2- but have no significant activity for the CO2 reduction reaction. That the GaN / Si photoelectrode selectively produced urea in the CO2-KNO3 electrolyte may be due to surface adsorbed intermediates such as *NO2 that interact with CO2 in the electrolyte and induce C-N coupling. This result is discussed further below.

[0041] The Ag / GaN / Si photoelectrode example exhibited improved LSV performance in the CO2-KHCO3 solution, with the Ag catalyst contributing via C02-to-CO conversion (Figure 3, part d). Therefore, in the potential range of -0.1 to -0.6 VRHE, where urea synthesis dominates in the CO2-KNO3 solution, there may be simultaneous reactions of NOs- reduction and CO2 reduction. The selectivity of NO3- reduction was evaluated in the Ar-KNOs solution and it was found that most electrons were utilized for NO2- production and some were consumed for NH3synthesis (Figure 3, part e). There was no urea production, indicating that carbon from CO2 molecules is provided to form the C-N bond. In the CO2-KHCO3 solution, H2was the dominant product but CO was also produced with a FECo of about 12% and a production rate of about 3.9 pmol / cm2 / h at -0.6 VRHE (Figure 3, part f). The CO produced viaC02reduction reaction on the Ag / GaN / Si photoelectrode can react with N-containing intermediates ( / .e., NO2_and NH2) produced via NO3_reduction reaction, resulting in more efficient C-N coupling and urea synthesis. There was no urea production without NO3_reactant in the electrolyte, confirming that urea is produced from both CO2and NO3_reactants. Furthermore, an isotope labeling test supported that NH3, one of the products generated from NO3_reduction, indeed originated from NO3_in the electrolyte.

[0042] Other cocatalysts may be used to improve urea selectivity and productivity via enhanced CO generation. For instance, Au cocatalysts are also efficient CO evolution catalysts. Au nanoparticles were loaded onto GaN / Si photoelectrodes and tested for PEC urea synthesis. The size of the Au nanoparticles became larger as the precursor solution concentration increased from 6 to 600 pM. Similarly, the intensities of Ga 2p3 / 2and N 1s XPS spectra gradually decreased and those of Au 4f XPS spectra increased with Au loading. Among the three photoelectrodes, an example of Au60 / GaN / Si showed the best LSV performance in CO2-saturated 0.5 M KNO3due to the optimal surface coverage of Au nanoparticles. An example of Au60 / GaN / Si showed a high FEurea of 76% at -0.2 VRHE and a high urea production rate of 2.1 pmol / cm2 / h at -0.6 VRHE, which outperformed the GaN / Si photoelectrode and was slightly inferior to the Ag / GaN / Si photoelectrode. However, as the applied potential was increased to -0.4 VRHE, the Au / GaN / Si photoelectrode experienced a significant decrease in FEurea to less than 1% and an increase in faradaic efficiency for NH3to 55%. This result indicates that Au nanoparticles have higher activity in converting NO3_to NH3compared to Ag nanoparticles, thereby resulting in a large deviation in the urea production. The stability of Au nanoparticles was confirmed by negligible degradation of FEUrea or the production rate during the 8 cycles of reaction. Furthermore, Ag and Au cocatalysts were directly coated on the n+-p Si photoelectrodes by photo-deposition, and the PEC measurements were carried out in CO2-saturated 0.5 M KNO3. With only Ag and Au cocatalysts, Si photoelectrode examples exhibited very low photocurrent densities less than 0.7 mA / cm2and low selectivity of urea (FEurea less than 10% at -0.4 VRHE), indicating that the nitride interface of the GaN nanowires is useful for reducing NO3_and stabilizing reaction intermediates for C-N coupling and consequent urea production.

[0043] Density Functional Theory (DFT) was applied to assess the catalytic mechanism of urea production from CO2and NO3_, with emphasis on the step associated with C-N bond formation and competing pathways on GaN. As shown in Figure 4, formation of urea by reduction of CO2and NO3_involves a 16-electron reaction with multiple steps along a complex reaction pathway. One step involves formation of the C-N bond, followed by subsequent reduction on the carbon site with further C-N bond formation and reduction. Therelative energies of the reaction steps shown in Figure 4, part a, correspond to the minimum energy structures shown in Figure 4, part b. The initial step of the mechanism involves adsorption of NO3- in a bidentate mode, with two oxygens bound to adjacent gallium sites. NO2 is formed on the surface upon further reduction and release of a water molecule. At this stage, the NO2 can either be reduced to NO2-, or react with an adsorbed COO- radical on the GaN surface (Figure 4, part b). Because both reactions are exergonic, the C-N bond formation step proceeds in competition with formation of NO2- which can desorb from the surface, or remain on the surface to be subsequently reduced to NH3. After formation of the C-N bond, further reduction of the O2N-CO2- species is thermodynamically downhill. The second C-N bond formation involves the OC-NH intermediate and the co-adsorbed *NH2. The resulting intermediate forms urea upon reduction. Therefore, after the first C-N coupling has taken place, formation of urea is energetically very favorable. Further reduction of NO2- to NO (and subsequent further reduction to NH3) is energetically uphill, making NO2- an important side product. The DFT calculations clearly show the negative potential that is involved in reducing C02to CO on GaN surface, in agreement with experimental results, while temporary production of the COO- radical on the surface is thermodynamically favorable. Overall, the calculations indicate that favoring the initial C-N bond formation over NO2- formation is useful, the two competing with one another because both reactions are exergonic. Unexpectedly, CO formation is disfavored so the C-N coupling takes place between the co-adsorbed NO2 and COO- through a radical coupling mechanism. Thus, stabilization of the COO- radical on the GaN surface allows for C-N bond formation without forming CO as in electrochemical processes.

[0044] When a cocatalyst that can electrochemically reduce CO2 to CO, such as Ag, is present, the initial C-N bond formation is significantly facilitated, as shown in Figure 5. While the reduction to NO2 and the formation of the second C-N bond remains the same, the initial coupling between carbon and nitrogen proceeds differently when CO is available. Since CO has a weak adsorption on silver, CO can approach the GaN surface and react with the adsorbed NO2 intermediate to form the O2N-CO intermediate. In contrast to the coupling with COO-, this aspect facilitates the urea production by making exergonic all subsequent catalytic steps after the NO2. Additionally, C-N bond formation by reaction with the adsorbed NO2- intermediate is found to be thermodynamically favorable, although it did not react with COO-. Therefore, the nitrite formation that leads to the main side product on GaN can still react by coupling with CO and form urea.

[0045] While the C-N bond formation on GaN between adsorbed NO2 and CO is favorable, it is unlikely on the Ag nanoparticles. This is shown by the formation free energy of the O2N-CO intermediate from co-adsorbed NO2 and CO on an Ag (111) surface, which is the most common facet of the Ag nanoparticles. Therefore, the Ag cocatalyst facilitates urea production by forming CO, as species that can diffuse towards the GaN and react with either NO2 or NO2’ intermediates adsorbed on the surface to form the C-N bond.

[0046] A number of examples of the disclosed devices, systems, and methods are now described in connection with the schematic diagram of Figure 6 and the flow diagram of Figure 7.

[0047] Figure 6 depicts a system 600 for synthesis of urea 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 urea synthesis. The manner in which the PEC system 600 is illuminated may vary.

[0048] 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 CO2 is applied. In some cases, the aqueous electrolyte solution is saturated with CO2. The aqueous solution also includes nitrate ions as described herein. Accordingly, a source 607 of nitrate ions may also be in communication with the cell 602. 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 urea production. Further details regarding an example of the electrochemical system 600 are provided hereinabove.

[0049] 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.

[0050] The synthesis of urea 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 reduction of CO2 and evolution of H2may be managed or controlled (e.g., to favor CO2reduction) via the composition of the components of the nanoarchitecture and / or the applied voltage, as described herein.

[0051] 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.

[0052] 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.

[0053] 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 urea 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.

[0054] 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.

[0055] 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.

[0056] 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.

[0057] 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)-nanoparticle architecture as described below.

[0058] 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 urea. In some cases, the semiconductor composition includes gallium nitride (GaN). Additional or alternative nitride materials (e.g., semiconductor nitrides) may be used, including, forinstance, indium nitride, indium gallium nitride, aluminum nitride, boron nitride, and / or their alloys.

[0059] 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 urea synthesis. The composition of the nanowires 622 may also form an interface well-suited for the urea synthesis, as explained herein.

[0060] 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.

[0061] 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.

[0062] The electrode 608 further includes nanoparticles 624 disposed over the array of nanowires 622. Each nanoparticle 624 is configured for the catalytic conversion of carbon dioxide (CO2) in the chemical cell 602. A plurality of the nanoparticles 624 are disposed on each nanowire 622, as schematically shown in Figure 6. The nanoparticles 624 are distributed across the outer surface of each nanowire 622. For example, each nanowire 622 has a plurality of the nanoparticles 624 distributed across or along sidewalls of the nanowire 622. The nanoparticles 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 nanoparticle 624 may include or be composed of a metal, such as silver or gold, for the reduction of carbon dioxide (CO2) in the chemical cell 602 and the synthesis of urea. In some cases, additional or alternative nanoparticles may be used,including, for instance, nanoparticles composed of, or otherwise including, metals other than the noble metals.

[0063] The nanoparticles 624 may be sized in a manner to facilitate the CO2 reduction and urea synthesis. The size of the nanoparticles 624 may be useful in catalyzing the reaction, as described herein. The size of the nanoparticles 624 may be promote the CO2 reduction and urea synthesis in additional or alternative ways. For instance, the nanoparticles 624 may also be sized to avoid inhibiting the illumination of the light absorber (e.g. the substrate 620).

[0064] 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.

[0065] In some cases, each nanowire 622 is coated with the nanoparticles 624. The extent of the coating may vary. For instance, a top surface of each nanowire 622 may be entirely coated with the nanoparticles 624, while one or more portions of the sidewalls of the nanowires 622 may be partially coated. The distribution of the nanoparticles 624 may accordingly be uniform or non-uniform. The nanoparticles 624 may thus be distributed randomly across each nanowire 622. The schematic arrangement of Figure 6 is shown for ease in illustration.

[0066] 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., UV 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.

[0067] 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, each 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).

[0068] 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.

[0069] The nanowires 622 and the nanoparticles 624 are not shown to scale in the schematic depiction of Figure 6. The shape of the nanowires 622 and the nanoparticles 624 may also vary from the example shown.

[0070] Figure 7 depicts a method 700 of fabricating a photocatalytic device for urea synthesis in accordance with one example. The method 700 may be used to manufacture any of the devices described herein or another device. The method 700 may include additional, fewer, or alternative acts. For instance, the method 700 may or may not include one or more acts directed to annealing the device (act 730).

[0071] The method 700 may begin with an act 702 in which a substrate is prepared or otherwise provided. The substrate may be or be formed from a silicon wafer. In one example, a 2-inch or 3-inch Si wafer was used, but other (e.g., larger) size wafers may be used. Other semiconductors and substrates may be used.

[0072] In some cases, the act 702 includes an act 704 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.

[0073] The act 702 may include fewer, additional, or alternative acts. For instance, in the example of Figure 1 , the act 702 includes an act 706 in which the substrate is cleaned (e.g., with acetone, IPA and hydrofluoric acid), and an act 708 in which oxide is removed (e.g., via annealing at a sufficiently high temperature). The oxide removal may be implemented in the MBE reaction chamber immediately before growth.

[0074] The method 700 includes an act 710 in which a nanowire or other nanostructure array is grown or otherwise formed on the substrate. Each nanowire is formed on the surface of the substrate such that each nanowire extends outward from the surface of the substrate. Each nanowire has a semiconductor composition, as described herein. The nanowire growth may be achieved in an act 712 in which molecular beam epitaxy (MBE) is implemented. The MBE procedure may be implemented under nitrogen-rich conditions to promote the formation of N-rich surfaces (which are useful for prevention of photo-corrosion and oxidation). Alternatively or additionally, the substrate may be rotated during the MBE procedure such that each nanostructure is shaped as a cylindrically shaped nanostructure. Each nanowire may thus have a circular cross-sectional shape, as opposed to a plateshaped or sheet-shaped nanostructure.

[0075] In some cases, the MBE procedure may be modified to fabricate the arrangement of layers or segments of each nanowire directed to providing a multi-band structure. Various parameters may be adjusted to achieve the different composition levels of the layers. For instance, the substrate temperature may be adjusted in an act 714. Beam equivalent pressures may be also adjusted in the act 714.

[0076] The act 710 includes doping the nanowires n-type in an act 716. For example, Ga and silicon and / or other fluxes may be controlled by using thermal effusion cells. In some cases, a dopant cell temperature is adjusted in an act 718 to control the doping (e.g., n-type doping) of the nanowires.

[0077] During the act 710, nitrogen radicals may be produced from a radio-frequency nitrogen plasma source. In one example, a nitrogen flow rate of 1 .0 seem and a forward plasma power of about 350 W were used in the growth process.

[0078] The act 710 may include additional, fewer, or alternative acts. For instance, the act 710 may include one or more acts directed to forming a seed other initial layer in preparation for growth of the nanowires. The seed layer may be configured to promote the nucleation of the nanowires. In some cases, the seed layer is composed of, or otherwise includes, Ga. Further details regarding the use of seed layers are set forth below in connection with a number of examples as well as in the above-referenced patent documents.

[0079] As shown in Figure 7, the method 700 further includes an act 720 in which the array is decorated with a catalyst arrangement. Catalysts nanoparticles are deposited across the array of nanowire. In the example of Figure 7, the act 720 include an act 722 in which the nanowires are decorated with a catalyst arrangement. The act 722 may include depositing metal nanoparticles on the nanowires. The nanoparticles may be composed of, or otherwiseinclude, silver and / or other metals, as described herein. In some cases, the deposition of the nanoparticles includes implementation of a photo-deposition procedure in an act 724. Alternative or additional deposition procedures may be used to deposit the nanoparticles, including, for instance, an e-beam evaporation procedure. Still further or alternative procedures may be used, including, for instance, other physical vapor deposition procedures, such as sputtering, as well as atomic layer deposition procedures.

[0080] 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 nanoparticles of a desired size. For instance, the precursor molar concentration may be set to a level such that each nanoparticle of the plurality of nanoparticles has a size that falls in a range from about 10 nm to about 80 nm. The molar concentration may be sufficiently low to avoid formation of plateshaped structures on the array of conductive projections. The molar concentration level may also be selected to limit the plurality of nanoparticles to partial coverage of each conductive projection of the array of conductive projections.

[0081] The method 700 may include one or more additional acts directed to forming the photocatalytic structures of the device. For instance, in some cases, the method 700 includes an act 728 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).

[0082] The order of the above-described acts of the method 700 may differ from the example shown. For instance, the annealing of the act 728 may be implemented before or after the deposition of the nanoparticles in the act 720.

[0083] Described above are examples of efficient GaN / Si photoelectrodes for PEC and other photocatalytic urea synthesis from simultaneous NO3- and CO2 reduction reactions under solar light. The built-in potential in a n+-p Si substrate and the catalytic activity of GaN nanowires led to the selective synthesis of urea at a low overpotential. Moreover, Ag and Au cocatalysts loaded onto GaN nanowires improved the performance of PEC urea synthesis, resulting in high FEurea of 75.6 ± 2.6% at a low potential of -0.3 VRHE. These examples outperformed previous electrocatalysts measured under dark conditions. Theoretical calculation suggests that GaN nanowires are capable of converting NO3- to the NO2 intermediate, which then interacts with a COO- radical on the surface, coupling carbon and nitrogen as the initial step for urea production. This circumvents the need for initial COformation. These findings reveal that GaN nanowires decorated with cocatalysts have high catalytic activity for C-N coupling, which is a useful step in the production of fertilizers. These findings establish that GaN and other nitride interfaces may be used to establish more efficient and sustainable methods for producing fertilizers.

[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 systems, devices, and methods.

[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

What is Claimed is:

1. A system for photocatalytic synthesis of urea, the device comprising: an aqueous solution; and a photocatalytic device immersed in the aqueous solution, the photocatalytic device comprising: a substrate having a surface; and a conductive structure supported by the substrate at the surface, the conductive structure comprising a nitride interface in contact with the aqueous solution; wherein the aqueous solution comprises nitrate ions and dissolved carbon dioxide for synthesis of urea at the nitride interface of the conductive structure.

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 nitride interface has a composition configured for catalyzing a nitrate reduction reaction.

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

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

6. The system of claim 1 , wherein the photocatalytic device further comprises a plurality of catalyst nanoparticles disposed over the conductive structure.

7. The system of claim 6, wherein each nanoparticle of the plurality of catalyst nanoparticles comprises a metal configured for catalyzing a carbon dioxide reduction reaction.

8. The system of claim 6, wherein each nanoparticle of the plurality of catalyst nanoparticles comprises metallic silver.

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

10. 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.

11. The system of claim 10, wherein the conductive structure comprises a nanowire configured to extract the charge carriers generated in the substrate.

12. The system of claim 1 , wherein: the aqueous solution is a CO2-saturated electrolyte; and the CO2-saturated electrolyte comprises KNO3.

13. The system of claim 1 , wherein the aqueous solution is free of a sacrificial agent for the synthesis of urea from the nitrate ions and the dissolved carbon dioxide.

14. 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; and a 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 urea at the working electrode.

15. The system of claim 14, wherein the level of the bias voltage falls in a range from about -0.1 to about -0.6 Volts versus reversible hydrogen electrode (VRHE).

16. A method of using the system of claim 1 , the method comprising: illuminating the photocatalytic device; and capturing the urea synthesized by the photocatalytic device.

17. The method of claim 16, further comprising applying a bias voltage to the photocatalytic device.

18. A system for photocatalytic synthesis of urea, the system comprising: an aqueous solution; and a 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 ofconductive projections having a nitride semiconductor interface in contact with the aqueous solution, the nitride semiconductor interface having a composition configured to catalyze a nitrate reduction reaction; and a plurality of catalyst nanoparticles disposed over the array of conductive projections, the charge carriers generated in the substrate migrating to the plurality of catalyst nanoparticles via the array of conductive projections, each catalyst nanoparticle of the plurality of catalyst nanoparticles comprising a metal configured to catalyze a carbon dioxide reduction reaction; wherein the aqueous solution comprises nitrate ions and dissolved carbon dioxide for synthesis of urea via the nitrate reduction reaction and the carbon dioxide reduction reaction, respectively.

19. The system of claim 18, wherein the photoelectrode is configured as a working electrode, and the system further comprises: 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; wherein the bias voltage is at a level for the synthesis of urea at the working electrode.

20. The system of claim 19, wherein the level of the bias voltage falls in a range from about -0.1 to about -0.6 Volts versus reversible hydrogen electrode (VRHE).

21. A method of using the system of claim 18, the method comprising: illuminating the photoelectrode; and capturing the urea synthesized by the photoelectrode.

22. The method of claim 21 , further comprising applying a bias voltage to the photoelectrode.

23. A method of fabricating a device for photocatalytic synthesis of urea, the method comprising: growing an array of conductive projections on a semiconductor substrate, each conductive projection of the array of conductive projections having a nitride semiconductor interface, the nitride semiconductor interface having a composition configured to catalyze a nitrate reduction reaction; and implementing a photo-deposition procedure to deposit a plurality of nanoparticlesacross each conductive projection of the array of conductive projections, each nanoparticle of the plurality of nanoparticles having a metallic composition configured to catalyze a carbon dioxide (CO2) reduction reaction; wherein implementing the photo-deposition procedure comprises immersing the array of conductive projections in a precursor solution, the precursor solution having a precursor molar concentration at a level to avoid formation of plate-shaped structures on the array of conductive projections, the level of the precursor molar concentration limiting the plurality of nanoparticles to partial coverage of each conductive projection of the array of conductive projections.

24. The method of claim 23, wherein the precursor molar concentration is such that each nanoparticle of the plurality of nanoparticles has a size that falls in a range from about 10 nm to about 80 nm.

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