Synthesis of free-standing Ta3N5nanotube membranes and flow-through visible light photocatalytic applications

We report on free-standing Ta₃N₅nanotubular membranes with open top and bottom, used as visible-light-active, flow-through photocatalytic micro-reactors.

Transition-Metal (Co, Ni, and Fe)-Based Electrocatalysts for the Water Oxidation Reaction

Increasing energy demands and environment awareness have promoted extensive research on the development of alternative energy conversion and storage technologies with high efficiency and environmental friendliness. Among them, water splitting is very appealing, and is receiving more and more attention. The critical challenge of this renewable-energy technology is to expedite the oxygen evolution reaction (OER) because of its slow kinetics and large overpotential. Therefore, developing efficient electrocatalysts with high catalytic activities is of great importance for high-performance water splitting. In the past few years, much effort has been devoted to the development of alternative OER electrocatalysts based on transition-metal elements that are low-cost, highly efficient, and have excellent stability. Here, recent progress on the design, synthesis, and application of OER electrocatalysts based on transition-metal elements, including Co, Ni, and Fe, is summarized, and some invigorating perspectives on the future developments are provided.

Gradient doping of phosphorus in Fe2O3 nanoarray photoanodes for enhanced charge separation

Highly-oriented Fe₂O₃ nanoarrays with a gradient phosphorus concentration result in enhanced charge separation in the bulk for photoelectrochemical water oxidation.

Hematite (α-Fe₂O₃) is a promising candidate for solar-to-hydrogen energy conversion. However, the low carrier mobility and extremely high charge recombination rate limit the practical application of hematite in solar water splitting. This paper describes the fabrication of a Fe₂O₃ photoanode with gradient incorporation of phosphorus (P) employing a facile dipping and annealing method to improve the charge separation for enhanced photoelectrochemical water oxidation. This gradient P incorporation increases the width of band bending over a large region in Fe₂O₃, which is crucial for promoting the charge separation efficiency in the bulk. Although both gradient and homogeneous P-incorporated Fe₂O₃ samples exhibit similar electrical conductivity, the Fe₂O₃ electrode with a gradient P concentration presents an additional charge separation effect. A photocurrent of ∼1.48 mA cm^–2 is obtained at 1.23 V vs. reversible hydrogen electrode (vs. RHE) under air mass 1.5G illumination. Additionally, the H₂O oxidation kinetics of Fe₂O₃ with gradient P incorporation was further improved upon loading cobalt phosphate as cocatalyst, reaching a photocurrent of ∼2.0 mA cm^–2 at 1.23 V vs. RHE.

Photocatalytic Water Splitting-The Untamed Dream: A Review of Recent Advances

Photocatalytic water splitting using sunlight is a promising technology capable of providing high energy yield without pollutant byproducts. Herein, we review various aspects of this technology including chemical reactions, physiochemical conditions and photocatalyst types such as metal oxides, sulfides, nitrides, nanocomposites, and doped materials followed by recent advances in computational modeling of photoactive materials. As the best-known catalyst for photocatalytic hydrogen and oxygen evolution, TiO₂ is discussed in a separate section, along with its challenges such as the wide band gap, large overpotential for hydrogen evolution, and rapid recombination of produced electron-hole pairs. Various approaches are addressed to overcome these shortcomings, such as doping with different elements, heterojunction catalysts, noble metal deposition, and surface modification. Development of a photocatalytic corrosion resistant, visible light absorbing, defect-tuned material with small particle size is the key to complete the sunlight to hydrogen cycle efficiently. Computational studies have opened new avenues to understand and predict the electronic density of states and band structure of advanced materials and could pave the way for the rational design of efficient photocatalysts for water splitting. Future directions are focused on developing innovative junction architectures, novel synthesis methods and optimizing the existing active materials to enhance charge transfer, visible light absorption, reducing the gas evolution overpotential and maintaining chemical and physical stability.

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.

Fabrication of porous nanoflake BiMO x (M = W, V, and Mo) photoanodes via hydrothermal anion exchange

Most Bi-based photoelectrodes have suitable band gaps and can effectively promote the water oxidation reaction. However, simple preparation methods for Bi-based binary metal oxides as photoanodes are scarce. This paper describes a simple hydrothermal anion exchange method to synthesize Bi-based binary metal oxides with controlled morphologies. This synthesis process uses BiOI as the template and Bi source, which is eventually converted to Bi-based porous nanoflake photoanodes upon reaction with MO _x (M = W, V, and Mo)-containing precursors. The photoanodes show well-shaped porous nanoflake morphologies and exhibit impressive photoelectrochemical properties compared to Bi-based photoanodes synthesized by conventional methods. These three samples possess long-term stability under solar irradiation and show considerable photocurrent for sulfite oxidation.

Effective Charge Carrier Utilization in Photocatalytic Conversions

Continuous efforts have been devoted to searching for sustainable energy resources to alleviate the upcoming energy crises. Among various types of new energy resources, solar energy has been considered as one of the most promising choices, since it is clean, sustainable, and safe. Moreover, solar energy is the most abundant renewable energy, with a total power of 173 000 terawatts striking Earth continuously. Conversion of solar energy into chemical energy, which could potentially provide continuous and flexible energy supplies, has been investigated extensively. However, the conversion efficiency is still relatively low since complicated physical, electrical, and chemical processes are involved. Therefore, carefully designed photocatalysts with a wide absorption range of solar illumination, a high conductivity for charge carriers, a small number of recombination centers, and fast surface reaction kinetics are required to achieve a high activity. This Account describes our recent efforts to enhance the utilization of charge carriers for semiconductor photocatalysts toward efficient solar-to-chemical energy conversion. During photocatalytic reactions, photogenerated electrons and holes are involved in complex processes to convert solar energy into chemical energy. The initial step is the generation of charge carriers in semiconductor photocatalysts, which could be enhanced by extending the light absorption range. Integration of plasmonic materials and introduction of self-dopants have been proved to be effective methods to improve the light absorption ability of photocatalysts to produce larger amounts of photogenerated charge carriers. Subsequently, the photogenerated electrons and holes migrate to the surface. Therefore, acceleration of the transport process can result in enhanced solar energy conversion efficiency. Different strategies such as morphology control and conductivity improvement have been demonstrated to achieve this goal. Fine-tuning of the morphology of nanostructured photocatalysts can reduce the migration distance of charge carriers. Improving the conductivity of photocatalysts by using graphitic materials can also improve the transport of charge carriers. Upon charge carrier migration, electrons and holes also tend to recombine. The suppression of recombination can be achieved by constructing heterojunctions that enhance charge separation in the photocatalysts. Surface states acting as recombination centers should also be removed to improve the photocatalytic efficiency. Moreover, surface reactions, which are the core chemical processes during the solar energy conversion, can be enhanced by applying cocatalysts as well as suppressing side reactions. All of these strategies have been proved to be essential for enhancing the activities of semiconductor photocatalysts. It is hoped that delicate manipulation of photogenerated charge carriers in semiconductor photocatalysts will hold the key to effective solar-to-chemical energy conversion.

Complex-Mediated Synthesis of Tantalum Oxyfluoride Hierarchical Nanostructures for Highly Efficient Photocatalytic Hydrogen Evolution

In this work, we have, for the first time, developed a facile wet-chemical route to obtain a novel photocatalytic material of tantalum oxyfluoride hierarchical nanostructures composed of amorphous cores and single crystalline TaO2F nanorod shells (ACHNs) by regulating the one-step hydrothermal process of TaF5 in a mixed solution of isopropanol (i-PrOH) and H2O. In this approach, elaborately controlling the reaction temperature and volume ratio of i-PrOH and H2O enabled TaF5 to transform into intermediate coordination complex ions of [TaOF3·2F](2-) and [TaF7](2-), which subsequently produced tantalum oxyfluoride ACHNs via a secondary nucleation and growth due to a stepwise change in hydrolysis rates of the two complex ions. Because of the unique chemical composition, crystal structure and micromorphology, the as-prepared tantalum oxyfluoride ACHNs show a more negative flat band potential, an accelerated charge transfer, and a remarkable surface area of 152.4 m(2) g(-1) contributing to increased surface reaction sites. As a result, they exhibit a photocatalytic activity for hydrogen production up to 1.95 mmol h(-1) g(-1) under the illumination of a simulated solar light without any assistance of co-catalysts, indicating that the as-prepared tantalum oxyfluoride ACHNs are a novel promising photocatalytic material for hydrogen production.

Strongly Enhanced Water Splitting Performance of Ta3 N5 Nanotube Photoanodes with Subnitrides

Subnitrides strongly enhance the efficiency of Ta3 N5 -nanotube photoanodes in photochemical water splitting. The fabrication of Ta3 N5 nanotube layers with a controlled subnitride layer at the interface to the back contact is demonstrated. The insertion of this subnitride layer has a strong influence on the electron transfer to the back contact, and as a result leads to a drastic shift in photocurrent onset potential and a considerable enhancement of photocurrent conversion efficiency.

Gray Ta2O5 Nanowires with Greatly Enhanced Photocatalytic Performance

Black TiO2, with enhanced solar absorption and photocatalytic activity, has gained extensive attention, inspiring us to investigate the reduction of other wide-bandgap semiconductors for improved performance. Herein, we report the preparation of gray Ta2O5 nanowires with disordered shells and abundant defects via aluminum reduction. Its water decontamination is 2.5 times faster and hydrogen production is 2.3-fold higher over pristine Ta2O5. The reduced Ta2O5 also delivers significantly enhanced photoelectrochemical performance compared with the pristine Ta2O5 nanowires, including much higher carrier concentration, easier electron-hole separation and 11 times larger photocurrent. Our results demonstrate that Ta2O5 will have great potentials in photocatalysis and solar energy utilization after proper modification.

Theoretical study on the surface stabilities, electronic structures and water adsorption behavior of the Ta3N5(110) surface

DFT calculations were performed to study the surface stabilities, electronic structures and water adsorption behavior of the Ta₃N₅(100) surface.

S-doped mesoporous nanocomposite of HTiNbO5 nanosheets and TiO2 nanoparticles with enhanced visible light photocatalytic activity

S-doped mesoporous TiO₂/HTiNbO₅ nanocomposite showed dramatically enhanced visible-light photocatalytic activity and stability owing to the combined effects of nano-heterojunction, S doping and morphology engineering.

Studies on structural defects in bare, PVP capped and TPPO capped copper oxide nanoparticles by positron annihilation lifetime spectroscopy and their impact on photocatalytic degradation of rhodamine B

Enhanced photocatalytic dye degradation by reducing sizes of surface defects.

Effective photo-reduction to deposit Pt nanoparticles on MIL-100(Fe) for visible-light-induced hydrogen evolution

An effective photo-reduction approach was developed to deposit highly dispersed Pt nanoparticles on MIL-100(Fe) (Pt/MIL-100(Fe)) for superior visible-light-induced H₂ evolution.

Facile hydrolysis synthesis of Bi4O5Br2 photocatalyst with excellent visible light photocatalytic performance for the degradation of resorcinol

A Bi₄O₅Br₂ photocatalyst had been synthesized successfully through a facile, one-step, and energy-saving hydrolysis route.

Improving KNbO3 photocatalytic activity under visible light

An efficient strategy for improving KNbO₃ photocatalytic activity under visible light is presented using N and W as dopant pair, which reduces the band gap mainly by elevation of the valence band maxima.

Single-layer cadmium chalcogenides: promising visible-light driven photocatalysts for water splitting

Single-layer CdSe and CdTe sheets cut along the (001) lattice plane of the wurtzite phase are promising photocatalysts for water splitting.

Self-assembly and photocatalytic H2 evolution activity of two unprecedented polytantalotungstates based on the largest {Ta18} and {Ta18Yb2} clusters

Two unprecedented polytantalotungstates, 1 and 2 based on the largest {Ta₁₈} and {Ta₁₈Yb₂} clusters, respectively, were synthesized. 1 and 2 exhibit significant UV photocatalytic water splitting activity.

Amorphous and Crystalline Sodium Tantalate Composites for Photocatalytic Water Splitting

A facile hydrothermal synthesis protocol for the fabrication of sodium tantalates for photocatalytic water splitting is presented. Mixtures of tantalum and sodium ethoxide precursors were dispersed in ethanol, and ammonium hydroxide solution was used as mineralizer. By adjusting the amount of mineralizer, a variety of sodium tantalates with various morphologies, textural parameters, band gaps, crystal phases, and degrees of crystallinity were fabricated. The reaction was carefully monitored with a pressure sensor inside the autoclave reactor, and the obtained samples were characterized using X-ray diffraction, transmission electron microscopy, N2-physisorption, and ultraviolet-visible light spectroscopy. Among the series, the amorphous sample and the composite sample that consists of amorphous and crystalline phases showed superior activity toward photocatalytic hydrogen production than highly crystalline samples. Particularly, an amorphous sodium tantalate with a small fraction of crystalline nanoparticles with perovskite structure was found to be the most active sample, reaching a hydrogen rate of 3.6 mmol h(-1) from water/methanol without the use of any cocatalyst. Despite its amorphous nature, this photocatalyst gave an apparent photocatalyst activity of 1200 μmol g(-1) L(-1) h(-1) W(1-), which is 4.5-fold higher than highly crystalline NaTaO3. In addition, the most active sample gave promising activity for overall water splitting with a hydrogen production rate of 94 μmol h(-1), which is superior to highly crystalline NaTaO3 prepared by conventional solid-solid state route.

Tungsten Oxides for Photocatalysis, Electrochemistry, and Phototherapy

The conversion, storage, and utilization of renewable energy have all become more important than ever before as a response to ever-growing energy and environment concerns. The performance of energy-related technologies strongly relies on the structure and property of the material used. The earth-abundant family of tungsten oxides (WOx ≤3 ) receives considerable attention in photocatalysis, electrochemistry, and phototherapy due to their highly tunable structures and unique physicochemical properties. Great breakthroughs have been made in enhancing the optical absorption, charge separation, redox capability, and electrical conductivity of WOx ≤3 through control of the composition, crystal structure, morphology, and construction of composite structures with other materials, which significantly promotes the efficiency of processes and devices based on this material. Herein, the properties and synthesis of WOx ≤3 family are reviewed, and then their energy-related applications are highlighted, including solar-light-driven water splitting, CO2 reduction, and pollutant removal, electrochromism, supercapacitors, lithium batteries, solar and fuel cells, non-volatile memory devices, gas sensors, and cancer therapy, from the aspect of function-oriented structure design and control.

Effects of oxygen impurities and nitrogen vacancies on the surface properties of the Ta3N5 photocatalyst: a DFT study

Surface defects and impurities play important roles in the photocatalytic performance of semiconductors. In this study, DFT calculations are performed to investigate the effects of oxygen impurities and nitrogen vacancies on the surface stability and electronic structures of Ta3N5(100), (010) and (001) low-index surfaces. The results show that, for each surface, the oxygen impurities and nitrogen vacancies are beneficial and harmful, respectively, to the surface stability of Ta3N5. The oxygen impurities and nitrogen vacancies have mainly two effects on the surface electronic structures of Ta3N5. One is saturating surface states on the clean surface, and the other is inducing the downshift of conduction band minimum. In addition, the Ta3N5(100) surface with oxygen impurities is expected to have the strongest reduction ability in practice, providing useful guidance for further investigations of Ta3N5 in the photocatalytic hydrogen evolution.

Designing Photocatalysts for Hydrogen Evolution: Are Complex Preparation Strategies Necessary to Produce Active Catalysts?

A facile synthetic route for the preparation of highly active photocatalysts was developed. The protocol involves the preparation of a photocatalyst through the direct injection of metal alkoxide precursors into solutions in a photoreactor. As a proof of concept, a tantalum oxide based photocatalyst was chosen as a model system. Tantalum ethoxide [Ta(OEt)5 ] was injected rapidly into a photoreactor filled with a water/methanol mixture, and a TaOx (OH)y composite formed and was able to produce hydrogen under light illumination. Compared to commercial and mesostructured Ta2 O5 and NaTaO3 materials, TaOx (OH)y produced by direct injection shows superior hydrogen production activity. Notably, the samples prepared by direct injection are amorphous; however, their photocatalytic performance is much higher than those of their crystalline equivalents. If Ta(OEt)5 was dispersed in methanol before injection, an amorphous framework with higher surface area and larger pore volume was formed, and the hydrogen production rate increased further. The addition of a sodium precursor during the injection further boosted the photocatalytic activity. Furthermore, this concept has also been applied to a titanium-based photocatalyst, and a much better hydrogen production rate has been obtained in comparison with that of commercial TiO2 (P25-Degussa); therefore, the direct-injection synthesis is a flexible method that opens the door to the facile preparation of highly active nanostructured photocatalysts for hydrogen production.

Bridging the transport pathway of charge carriers in a Ta3N5 nanotube array photoanode for solar water splitting

This paper describes an approach to synthesize a tightly adhered Ta3N5 nanotube array (NTA) photoanode with enhanced electron conductivity between the Ta3N5 layer and the substrate via a two-step anodization method. The obtained tightly adhered Ta3N5 NTA photoanode exhibits excellent photoelectrochemical properties with an optimal photocurrent up to 5.3 mA cm(-2) at 1.6 V vs. the reversible hydrogen electrode. This approach provides an effective strategy to address the adhesion issue of one dimensional semiconductor photoanodes.

Hierarchical nanostructures of γ-TaON flowers for enhanced visible light driven photocatalytic activities

Hierarchical nanostructures of single phase γ-TaON flowers as novel photocatalysts were synthesized for the first time via a facile hydrothermal method exhibiting a high photocatalytic hydrogen production rate under visible light irradiation.

Research progress of perovskite materials in photocatalysis- and photovoltaics-related energy conversion and environmental treatment

Perovskite materials are shown to be active in the applications of photocatalysis- and photovoltaics-related energy conversion and environmental treatment.

Hierarchical CdS nanostructure by Lawesson's reagent and its enhanced photocatalytic hydrogen production

Lawesson's reagent (LR) has been effectively exploited for the synthesis of hierarchical architectures of cadmium sulphide (CdS) nanostructures for the first time.

Three-dimensional Porous Networks of Ultra-long Electrospun SnO2 Nanotubes with High Photocatalytic Performance

Recent progress in nanoscience and nanotechnology creates new opportunities in the design of novel SnO₂ nanomaterials for photocatalysis and photoelectrochemical. Herein, we firstly highlight a facile method to prepare three-dimensional porous networks of ultra-long SnO₂ nanotubes through the single capillary electrospinning technique. Compared with the traditional SnO₂ nanofibers, the as-obtained three-dimensional porous networks show enhancement of photocurrent and photocatalytic activity, which could be ascribed to its improved light-harvesting efficiency and high separation efficiency of photogenerated electron-hole pairs. Besides, the synthesis route delivered three-dimensional sheets on the basis of interwoven nanofibrous networks, which can be readily recycled for the desirable circular application of a potent photocatalyst system.

Photogeneration of two reduction-active charge-separated states in a hybrid crystal of polyoxometalates and naphthalene diimides

Upon irradiation, two kinds of long-lived charge-separated states for the reduction reactions can be generated in a hybrid crystal composed of two-dimensional naphthalene diimide coordination networks and polyoxometalates.

Effects of intrinsic defects and extrinsic doping on the electronic and photocatalytic properties of Ta3N5

The improvement of n-type conductivity is an effective strategy to enhance the photocatalytic activity of Ta₃N₅.

Solar light driven dye degradation using novel organo–inorganic (6,13-pentacenequinone/TiO2) nanocomposite

The 6,13-pentacenequinone loaded TiO₂ catalyst was successfully synthesized via simple wet-impregnation. The highest apparent rate constant was observed among the prepared catalysts is 5.2 × 10⁻² min⁻¹ using a PQ/TiO₂ catalyst having 0.2 wt% PQ.

Durability, inactivation and regeneration of silver tetratantalate in photocatalytic H2 evolution

The prepared Ag₂Ta₄O₁₁ photocatalyst exhibits durable activity for H₂ production from water, which can achieve the dual recovery of the photocatalyst and activity.

Graphene-based photocatalysts for oxygen evolution from water

Recent achievements of GR-based photocatalysts for oxygen evolution from water are summarized with perspectives on major challenges and opportunities.

BiAg alloy nanospheres: a new photocatalyst for H2 evolution from water splitting

We demonstrate for the first time that Bi and BiAg alloy nanospheres, fabricated with a facile hydrothermal method, display evident photocatalytic H2 production activities. Element Bi can serve as an active photocatalyst for both water splitting and photoelectrochemical applications. More interestingly, these activities of Bi can be greatly enhanced by introducing Ag to form BiAg alloy nanoparticles, which may be ascribed to the improved charge separation and enlarged carrier concentration. The constituent of the BiAg alloy can be rationally tuned by varying the amount of Ag nanowires, and it is found that Bi0.7Ag0.3 exhibits the highest photoelectrochemical property.