Ta3N5 Photoanodes Research Articles

Semi-transparent quaternary oxynitride photoanodes on GaN underlayers.

Conformal atomic layer deposition (ALD) technique is employed to make semi-transparent TaOxNy, providing the possibility to build semi-transparent oxy(nitride) heterojunction photoanodes on conductive substrates. A generalized approach was developed to manufacture semi-transparent quaternary metal oxynitrides on conductive substrates beyond semi-transparent binary Ta3N5 photoanodes aiming for wireless tandem photoelectrochemical (PEC) cells.

Preparation of Nanostructured Ta3N5 Electrodes by Alkaline Hydrothermal Treatment Followed by NH3 Annealing and Their Improved Water Oxidation Performance.

Solar water splitting is a clean and sustainable process for green hydrogen production. It can reduce the fossil fuel consumption. Tantalum nitride (Ta3N5) is one of the limited candidates of semiconductors, which absorb a broad range of visible light and are thermodynamically able to split water without external bias potential. In the present work, we introduce a facile method to prepare a nanostructured Ta3N5 photoanode in a two-step process: hydrothermal deposition of perovskite-type NaTaO3 in a hydrofluoric acid-free NaOH aqueous solution followed by heat treatment in NH3 atmosphere. The resulted bare Ta3N5 electrode was subsequently modified with a Ni-doped CoFeOx (Ni:CoFeOx) as a water oxidation catalyst. After the cocatalyst loading, the electrode shows a photocurrent of about 5.3 mA cm–2 at 1.23 V vs reversible hydrogen electrode. The electrode maintained about 82% of its initial photocurrent after 7 h irradiation. In addition, a continuous oxygen evolution occurred for 3 h at Faraday efficiency of 96%. This performance is superior to that of the single-layer-modified Ta3N5 photoanodes reported so far. This remarkable improvement on the photochemical performance could be due to the uniform nanostructured surface morphology of the present Ta3N5 photoanode. Other alkaline salt treatments, such as LiOH and KOH, do not give such nanostructured morphology and accordingly exhibit lower performance than the one treated in NaOH.

A Semitransparent Nitride Photoanode Responsive up to λ=600 nm Based on a Carbon Nanotube Thin Film Electrode

AbstractThe development of semitransparent photoanodes is required for the construction of tandem photoelectrochemical (PEC) water splitting cells incorporating photocathodes. However, the poor stability of transparent conductive oxides at high temperatures hampers the growth of non‐oxide photoanodes with intense visible light absorption. In this work, semitransparent Ta3N5 thin film photoanodes were prepared on quartz glass substrates coated with carbon nanotubes (CNTs) by sputtering and thermal nitridation. This process makes use of the high thermal and chemical stability as well as the tunable conductivity and transmittance of CNT substrates. The photoanodic current produced by these Ta3N5 photoanodes at negative potentials is also enhanced by surface modification with Mg species. Conductive semitransparent CNT substrates such as these will assist in the development of new tandem PEC cells for water splitting.

Boosting photoelectrochemical water splitting performance of Ta3N5 nanorod array photoanodes by forming a dual co-catalyst shell

Concerning both the activity and stability of the promising solar-driven Ta3N5-based photoanodes for photoelectrochemical water splitting, the strategy for simultaneously promoting charge separation, enhancing catalytic activity and also improving the resistance to self-oxidation is highly desirable and actively pursued. In this study, a novel dual co-catalyst shell consisting of a continuous CoPi layer at the bottom and many non-continuous Co(OH)2 islands at the top of the CoPi layer is designed to meet the strict requirements for efficient Ta3N5 photoanodes. As a result of the synergistic effects of such a shell in collectively addressing the concerns, the constructed photoanode of CoPi/Co(OH)2-Ta3N5 nanorod arrays show the remarkably enhanced photoelectrochemical water splitting performance compared with the photoanodes with single co-catalyst. The results demonstrated in this study are expected to shed some light on constructing efficient photoelectrodes of the light absorbers that have wide absorption range but low resistance to self-oxidation.

Interfacial Effects on the Band Edges of Ta3N5 Photoanodes in an Aqueous Environment: A Theoretical View

SummaryTa3N5, as a fascinating photoanode for solar hydrogen production, is expected to split water without any bias, because its band edge potentials straddle H2O redox potentials. Unfortunately, Ta3N5 photoanodes can split water only when a bias of at least 0.6–0.9 V is applied. It means that they exhibit an onset potential as high as 0.6–0.9 VRHE (reversible hydrogen electrode). In this study, density functional theory calculations show that the band edge potentials of Ta3N5 have a shift of approximately −0.42 eV relative to vacuum level when exposed to water. The increased ratio of dissociated water at Ta3N5-water interface will further make the band edge potentials shift −0.85 eV relative to vacuum level, implying the anodic shifts of the onset potential for water oxidation. The findings may reveal the mystery of the unexpectedly high onset potential of Ta3N5, as high as 0.6–0.9 VRHE.

Transparent Ta3 N5 Photoanodes for Efficient Oxygen Evolution toward the Development of Tandem Cells.

Photoelectrochemical water splitting is regarded as a promising approach to the production of hydrogen, and the development of efficient photoelectrodes is one aspect of realizing practical systems. In this work, transparent Ta3 N5 photoanodes were fabricated on n-type GaN/sapphire substrates to promote O2 evolution in tandem with a photocathode, to realize overall water splitting. Following the incorporation of an underlying GaN layer, a photocurrent of 6.3 mA cm-2 was achieved at 1.23 V vs. a reversible hydrogen electrode. The transparency of Ta3 N5 to wavelengths longer than 600 nm allowed incoming solar light to be transmitted to a CuInSe2 (CIS), which absorbs up to 1100 nm. A stand-alone tandem cell with a serially-connected dual-CIS unit terminated with a Pt/Ni electrode was thus constructed for H2 evolution. This tandem cell exhibited a solar-to-hydrogen energy conversion efficiency greater than 7 % at the initial stage of the reaction.

Transparent Ta3N5 Photoanodes for Efficient Oxygen Evolution toward the Development of Tandem Cells

AbstractPhotoelectrochemical water splitting is regarded as a promising approach to the production of hydrogen, and the development of efficient photoelectrodes is one aspect of realizing practical systems. In this work, transparent Ta3N5 photoanodes were fabricated on n‐type GaN/sapphire substrates to promote O2 evolution in tandem with a photocathode, to realize overall water splitting. Following the incorporation of an underlying GaN layer, a photocurrent of 6.3 mA cm−2 was achieved at 1.23 V vs. a reversible hydrogen electrode. The transparency of Ta3N5 to wavelengths longer than 600 nm allowed incoming solar light to be transmitted to a CuInSe2 (CIS), which absorbs up to 1100 nm. A stand‐alone tandem cell with a serially‐connected dual‐CIS unit terminated with a Pt/Ni electrode was thus constructed for H2 evolution. This tandem cell exhibited a solar‐to‐hydrogen energy conversion efficiency greater than 7 % at the initial stage of the reaction.

Ta3N5 Photoanodes Fabricated by Providing NaCl–Na2CO3 Evaporants to Tantalum Substrate Surface under NH3 Atmosphere

Photoelectrochemical (PEC) water splitting is an attractive technique to convert solar energy into chemical fuels of hydrogen and oxygen. Here, we report novel fabrication of Ta3N5 photoanodes usin...

Novel Cobalt Germanium Hydroxide for Electrochemical Water Oxidation.

Developing efficient and stable oxygen evolution catalyst (OEC) is a critical step to overcome the sluggish kinetics of water oxidation. Here, we hydrothermally synthesized a novel OEC, cobalt germanium hydroxide, CoGeO2(OH)2. The inherent Co-bonded hydroxyl groups facilitate the formation of active oxygen evolution reaction intermediates. Meanwhile, the facile leaching of Ge at the OEC-electrolyte interface contributes to surface reconstruction, generating Co-based (oxy)hydroxides, which would weaken its lattice constraint and suppress the excessive corrosion in the OEC bulk. As a result, CoGeO2(OH)2 reveals good catalytic activity and stability. This CoGe-based OEC achieves the overpotential at 10 mA cm-2 (η@10mA) of ∼340 mV, and the turnover frequency of ∼0.08 s-1. And the electrolysis kept at ∼10 mA cm-2 could be sustained for over 350 h. In addition, this p-type CoGeO2(OH)2 is demonstrated to be an effective electrocatalytic overlayer on n-type Ta3N5 photoanode, remarkably decreasing the onset for nearly 400 mV and increasing the photocurrent density at 1.23 VRHE about 3.8 times.

Nanostructured TaON/Ta3N5 as a highly efficient type-II heterojunction photoanode for photoelectrochemical water splitting.

Enhancing the charge separation by a semiconductor heterojunction is greatly promising and challenging for photoelectrochemical (PEC) water splitting. Here, we report for the first time the design and fabrication of a TaON/Ta3N5 heterojunction photoanode, in which the electrode Ta3N5 is the primary light absorber and TaON acts as an electron conductor. By combining the merits of the substantial light harvesting of Ta3N5 with the excellent charge transport capability of TaON, the TaON/Ta3N5 heterojunction photoanode, without any co-catalysts, shows a 350 mV negative shift of photocurrent onset potential to 0.65 V versus the reversible hydrogen electrode (RHE) compared to that of the Ta3N5 photoanode. The design and fabrication scheme can be readily extended to other (oxy)nitride semiconductors for heterojunction construction.

Charge separation properties of Ta3N5 photoanodes synthesized via a simple metal-organic-precursor decomposition process.

Here, we successfully synthesized a Ta3N5 thin film using a simple metal-organic-precursor decomposition process followed by its conversion to nitride and studied its photoelectrochemical (PEC) properties to understand charge separation on the surface. Newly synthesized Ta3N5 photoanodes showed a significant difference in the PEC activity in relation to the annealing temperature under ammonia flow, although similar light absorption properties or electronic states were obtained. Charge separation related PEC properties were analyzed using intensity modulated photocurrent density spectroscopy (IMPS) and photocurrent measurements in the absence/presence of scavengers. The charge transfer and recombination rate constants which are related to the photogenerated charge-separation dynamics on the Ta3N5 surface were found to be more sensitively influenced by the ammonia annealing temperatures, and low temperature (700 °C) treated Ta3N5 showed a fast recombination rate constant (kr). In addition, high-efficiency charge injection into the electrolyte on the surface was critically associated with the greatly enhanced photocurrent density of Ta3N5 synthesized at a higher temperature (900 °C) of ammonia annealing.

Fabrication of a Robust Tantalum Nitride Photoanode from a Flame‐Heating‐Derived Compact Oxide Film

AbstractPhotoelectrochemical (PEC) water splitting provides an attractive way of converting solar energy into hydrogen energy. To make this approach practical, it is crucial to develop strategies that can be used to fabricate efficient photoelectrodes in a scalable manner. Herein, we report a rapid flame heating method to prepare a Ta2O5 precursor film for a Ta3N5 photoanode. By varying the parameters of flame heating, rational control over the physicochemical nature of the Ta2O5 precursor film can be realized. With the following nitridation treatment, the compact precursor film can be transformed into a tight‐knit Ta3N5‐based nitride film which strongly stuck to the underlying conductive Ta substrate. As a result of the close interfacial connections and reasonable conductivity achieved through the manipulation of the duration of flame heating, an enhanced charge‐separation efficiency was obtained for the as‐prepared Ta3N5 photoanode. Furthermore, with the help of a ferrihydrite layer, the photocurrent density for PEC water oxidation on the optimum Ta3N5 photoanode reached circa 3.53 mA cm−2 at 1.23 V vs. RHE, which is among the best values reported for planar Ta3N5‐based photoanodes. This report highlights the advantages of flame heating over traditional heating methods for preparing precursor films for further processing or functionalization.

Enhanced Performance of Pristine Ta3N5 Photoanodes for Solar Water Splitting by Modification with Fe–Ni–Co Mixed-Metal Oxide Cocatalysts

Solar water splitting is an alternative way of clean and sustainable hydrogen production. Tantalum nitride (Ta3N5) is one of the promising candidates that recently attracted a great amount of attention as photoelectrodes for solar water splitting. Nevertheless, it suffers severely from photocorrosion in an aqueous solution. Therefore, the precise selection of a cocatalyst, in terms of the material, the amount, and the way of its deposition, is indispensable to highly improve its water splitting performance. In the present work, we introduce a Fe–Ni–Co mixed-metal oxide as a water oxidation cocatalyst that remarkably improved the photocurrent and photostability of the pristine Ta3N5 photoanode. The cocatalyst-modified electrode showed a photocurrent of about 4.0 mA cm–2 at 1.23 V vs RHE in 1 M NaOH. The electrode maintained 100% and 96% of the initial photocurrent after irradiation times of 1 and 2 h, respectively. In addition, a continuous evolution of hydrogen and oxygen occurred for 2 h at quantitative ...

Tandem Core-Shell Si-Ta3N5 Photoanodes for Photoelectrochemical Water Splitting.

Nanostructured core-shell Si-Ta3N5 photoanodes were designed and synthesized to overcome charge transport limitations of Ta3N5 for photoelectrochemical water splitting. The core-shell devices were fabricated by atomic layer deposition of amorphous Ta2O5 onto nanostructured Si and subsequent nitridation to crystalline Ta3N5. Nanostructuring with a thin shell of Ta3N5 results in a 10-fold improvement in photocurrent compared to a planar device of the same thickness. In examining thickness dependence of the Ta3N5 shell from 10 to 70 nm, superior photocurrent and absorbed-photon-to-current efficiencies are obtained from the thinner Ta3N5 shells, indicating minority carrier diffusion lengths on the order of tens of nanometers. The fabrication of a heterostructure based on a semiconducting, n-type Si core produced a tandem photoanode with a photocurrent onset shifted to lower potentials by 200 mV. CoTiOx and NiOx water oxidation cocatalysts were deposited onto the Si-Ta3N5 to yield active photoanodes that with NiOx retained 50-60% of their maximum photocurrent after 24 h chronoamperometry experiments and are thus among the most stable Ta3N5 photoanodes reported to date.

Tantalum nitride films integrated with transparent conductive oxide substrates via atomic layer deposition for photoelectrochemical water splitting.

Tantalum nitride, Ta3N5, is one of the most promising materials for solar energy driven water oxidation. One significant challenge of this material is the high temperature and long duration of ammonolysis previously required to synthesize it, which has so far prevented the use of transparent conductive oxide (TCO) substrates to be used which would allow sub-bandgap light to be transmitted to a photocathode. Here, we overcome this challenge by utilizing atomic layer deposition (ALD) to directly deposit tantalum oxynitride thin films, which can be fully converted to Ta3N5via ammonolysis at 750 °C for 30 minutes. This synthesis employs far more moderate conditions than previous reports of efficient Ta3N5 photoanodes. Further, we report the first ALD of Ta-doped TiO2 which we show is a viable TCO material that is stable under the relatively mild ammonolysis conditions employed. As a result, we report the first example of a Ta3N5 electrode deposited on a TCO substrate, and the photoelectrochemical behavior. These results open the door to achieve efficient overall water splitting using a Ta3N5 photoanode.

Insight into the charge transfer in particulate Ta3N5 photoanode with high photoelectrochemical performance.

Charge separation is one of the most critical factors for generating solar fuels via photoelectrochemical water splitting, but it is still not well understood. This work reveals the fundamental role of charge transfer in photoanodes for achieving high charge separation efficiency. Specifically, we fabricated a particulate Ta3N5 photoanode by a bottom-up method. By improving the charge separation with refined necking treatment, the photocurrent is increased by two orders of magnitude. The charge separation efficiency (ηsep) is analyzed by dividing it into charge generation efficiency (Φgene) and transportation efficiency (Φtrans). Necking treatment is found to substantially improve the electron transfer. Transient photovoltage (TPV) measurements based on the Dember effect is used to confirm the benefit of necking treatment in improving the charge transportation. The superior electron transfer in the necked-Ta3N5 electrode is further evidenced by the facile electron exchange reaction with the ferri/ferrocyanide redox couple. Moreover, cobalt phosphate is found to promote both charge separation and surface reaction, resulting in a photocurrent of 6.1 mA cm-2 at 1.23 V vs. RHE, which is the highest response for a particulate photoanode.

Charge Carrier Transfer in Ta3N5 Photoanodes Prepared by Different Methods for Solar Water Splitting

The preparation method of a photoanode can affect its water splitting property. Here, as examples, we prepared Ta3N5 photoanodes by an electrophoresis deposition (EPD) method and an oxidation and nitridation of Ta foil (ONTF) method. The light harvest, interfacial charge transfer, and charge separation of the two Ta3N5 photoanodes were analysed to gain insight into the role of the preparation method on the water splitting property. The results suggested that the ONTF-prepared Ta3N5 showed a higher solar energy conversion efficiency, arising from its better interfacial charge transfer efficiency and higher charge separation efficiency. The higher charge separation efficiency was mainly attributed to good electron transfer, and the inter-particle connectivity was key for the electron transfer in the photoanodes. Especially, the dense, small particle structure of ONTF-prepared Ta3N5 was beneficial for increasing the connectivity between inter-particles. This comparison of preparation methods can be used as a reference for future photoanode preparation to improve the water splitting property of photoelectrochemical cells.

Recent progress in photoelectrochemical water splitting for solar hydrogen production

A water splitting photoelectrochemical (PEC) cell can convert solar energy to hydrogen fuels directly. The challenges for practical application are to fabricate photoelectrodes with high efficiency, good durability and low cost. In this review, we focus on recent progress of some promising photoelectrode materials, including BiVO4, α-Fe2O3, Ta3N5 photoanodes and Cu2ZnSnS4 photocathodes. Several new strategies to enhance the performance of a PEC cell, such as surface exfoliation, suppressing back reaction and loading dual-layer catalysts, are discussed.

High-Temperature Ammonolysis of Thin Film Ta2O5 Photoanodes: Evolution of Structural, Optical, and Photoelectrochemical Properties

Tantalum nitride (Ta3N5) and oxynitride (TaON) are promising materials for photoelectrochemical (PEC) water splitting due to near-ideal band gaps and band edge positions. However, the high-temperature ammonolysis process that is usually used to make these materials depends sensitively on the process parameters and specific design of the annealing system, and reproducing highly efficient (oxy)nitride photoanodes from recipes reported by other laboratories has proven to be challenging. To understand and monitor the nitridation process in more detail, we employ an optical absorption spectroscopy technique that allows us to follow the transformation of thin Ta2O5 films in situ at temperatures up to 800 °C. Our results show that the incorporation of nitrogen in a dry ammonia atmosphere starts at 575 °C and is accompanied by a gradual red-shift of the Ta2O5 absorption edge and an expansion of the lattice due to the larger ionic radius of N3– relative to O2–. Although coloration of the material due to an N-2p → Ta-3d transition occurs readily, the films do not show any visible-light PEC activity until the nitrogen concentration is high enough to form a continuous N-2p impurity band. Ta3N5 is found to be the only thermodynamically stable phase between 575 and 800 °C, with no traces of TaON. Longer nitridation times result in lower defect concentrations, larger grain sizes, and improved PEC performance. The photocurrent of well-crystallized films is limited by slow water oxidation kinetics. This can be effectively remedied by depositing IrO2 nanoparticles as a water oxidation cocatalyst, which results in external quantum efficiencies of up to 45%. The smaller enhancement of the PEC performance at longer wavelengths reveals that hole transport in Ta3N5 limits the water splitting performance of IrO2-catalyzed Ta3N5 photoanodes.

Ge-mediated modification in Ta3N5 photoelectrodes with enhanced charge transport for solar water splitting.

Ta3 N5 is a promising photoanode candidate for photoelectrochemical water splitting, with a band gap of about 2.1 eV and a theoretical solar-to-hydrogen efficiency as high as 15.9 % under AM 1.5 G 100 mW cm(-2) irradiation. However, the presently achieved highest photocurrent (ca. 7.5 mA cm(-2) ) on Ta3 N5 photoelectrodes under AM 1.5 G 100 mW cm(-2) is far from the theoretical maximum (ca. 12.9 mA cm(-2) ), which is possibly due to serious bulk recombination (poor bulk charge transport and charge separation) in Ta3 N5 photoelectrodes. In this study, we show that volatilization of intentionally added Ge (5 %) during the synthesis of Ta3 N5 promotes the electron transport and thereby improves the charge-separation efficiency in bulk Ta3 N5 photoanode, which affords a 320 % increase of the highest photocurrent comparing with that of pure Ta3 N5 photoanode under AM 1.5 G 100 mW cm(-2) simulated sunlight.