Enhanced Water Oxidation on Ta3N5 Photocatalysts by Modification with Alkaline Metal Salts

Photogenerated charge carrier dynamics on Pt-loaded SrTiO3 nanoparticles studied via transient-absorption spectroscopy

Loading cocatalysts on semiconductor-based photocatalysts to create active reaction sites is a preferable method to enhance photocatalytic activity and a widely adopted strategy to achieve effective photocatalytic applications. Although theoretical calculations suggest that the broad density of states of noble metal cocatalysts, such as Pt, act as a recombination center, this has never been experimentally demonstrated. Herein, we employed pico-nano and nano-micro second transient absorption spectroscopy to investigate the often overlooked photogenerated holes, instead of the widely studied electrons on Pt- and Ni-loaded SrTiO3 to evaluate the effects of cocatalysts as a recombination center. It is demonstrated that Pt serves as the recombination center with no sacrificial agent; recombination can be suppressed by a hole scavenger, while recombination is not significant on Ni with localized density of states. It is also found that photo-generated holes in SrTiO3 tend to migrate to Pt within 400 ps, and photo-generated holes generated in the bulk gradually migrate to Pt cocatalysts in a micro-second regime.

Engineering band structuring via dual atom modification for an efficient photoanode

Efficient carrier separation is important for improving photoelectrochemical water splitting. Here, the morphology modification and band structure engineering of Ta3N5 are accomplished by doping it with Cu and Zr using a two-step method for the first time. The initially interstitially-doped Cu atoms act as anchors to interact with subsequently doped Zr atoms under the influence of differences in electronegativity. This interaction results in Cu,Zrg-Ta3N5 having a dense morphology and higher crystallinity, which helps to reduce carrier recombination at grain boundaries. Furthermore, the gradient doping of Zr generates a band edge energy gradient, which significantly enhances bulk charge separation efficiency. Therefore, a photoanode based on Cu,Zrg-Ta3N5 delivers an onset potential of 0.38 VRHE and a photocurrent density of 8.9 mA cm-2 at 1.23 VRHE. Among all the Ta3N5-based photoanodes deposited on FTO, a Cu,Zrg-Ta3N5-based photoanode has the lowest onset potential and highest photocurrent. The novel material morphology regulation and band edge position engineering strategies described herein provide new ideas for the preparation of other semiconductor nanoparticles to improve the photoelectrochemical water splitting performance.

Narrow-Band-Gap Particulate Photocatalysts for One-Step-Excitation Overall Water Splitting

ConspectusSunlight-driven one-step-excitation overall water splitting (OWS) using a single particulate photocatalyst is a simple and cost-effective approach to producing sustainable hydrogen on a large scale, providing an important impetus to achieving a carbon-neutral society. Technoeconomic studies have determined that a minimum solar-to-hydrogen (STH) energy conversion efficiency of 5% must be achieved to allow this process to be economically competitive. Meeting this goal will require the fabrication of particulate photocatalysts comprising composites of semiconductors and cocatalysts that are sufficiently active under sunlight. A one-step-excitation OWS system based on a metal oxide semiconductor having a wide bandgap was first reported in 1980, and the performance of such systems has been improved significantly over the past decade. In particular, work by the authors' group increased the apparent quantum yield (AQY) obtainable for ultraviolet (UV)-active SrTiO₃ to more than 90% in 2020. However, the STH conversion efficiency of a photocatalyst that absorbs only UV light (that is, λ < 400 nm) is limited to 1.7% even at an AQY of unity. It is therefore highly important to develop one-step-excitation OWS processes utilizing narrow bandgap photocatalysts having absorption edge wavelengths equal to or longer than 500 nm. Such systems would be expected to meet the desired 5% STH energy conversion efficiency once a constant AQY of approximately 63% is obtained.This Account summarizes the development and application of narrow-band-gap (oxy)nitride and oxysulfide photocatalysts in the authors' laboratory that are able to split water in response to wavelengths as high as 500 to 650 nm via single-step photoexcitation. At first, the authors briefly recount the key steps required to progress from the initial utilization of a UV-active SrTiO₃ photocatalyst as an OWS-active material to the realization of an AQY of almost unity. Multiple design and refinement strategies applied to both the semiconductor and cocatalysts associated with this benchmark photocatalyst are summarized, providing insights into the rational design of narrow-band-gap OWS-active photocatalysts. Furthermore, the necessity, target, and current status of developing narrow-band-gap OWS-active photocatalysts are discussed, followed by a comprehensive discussion of progress in the fabrication of OWS-active (oxy)nitride and oxysulfide photocatalysts with absorption edge wavelengths at up to the range of 500-650 nm in our laboratory. Specific examples are used to show the importance of several factors. First, adjusting the properties of the semiconducting material based on designing appropriate precursors, optimizing the synthetic conditions and aliovalent doping is described. Second, loading of efficient dual cocatalysts is examined. Lastly, the effectiveness of coating the particulate photocatalysts with surface nanolayers is addressed. Deficits related to the performance of present-day photocatalysts are also evaluated. Expectations with regard to future improvements of (oxy)nitride- and oxysulfide-based photocatalysts as a means of increasing the AQY are considered. The strategies summarized in this Account are expected to promote the development of nonsacrificial long-wavelength-responsive photosynthesis systems using water as a hydrogen/oxygen source.

Water electrolysis: from textbook knowledge to the latest scientific strategies and industrial developments

Replacing fossil fuels with energy sources and carriers that are sustainable, environmentally benign, and affordable is amongst the most pressing challenges for future socio-economic development. To that goal, hydrogen is presumed to be the most promising energy carrier. Electrocatalytic water splitting, if driven by green electricity, would provide hydrogen with minimal CO2 footprint. The viability of water electrolysis still hinges on the availability of durable earth-abundant electrocatalyst materials and the overall process efficiency. This review spans from the fundamentals of electrocatalytically initiated water splitting to the very latest scientific findings from university and institutional research, also covering specifications and special features of the current industrial processes and those processes currently being tested in large-scale applications. Recently developed strategies are described for the optimisation and discovery of active and durable materials for electrodes that ever-increasingly harness first-principles calculations and machine learning. In addition, a technoeconomic analysis of water electrolysis is included that allows an assessment of the extent to which a large-scale implementation of water splitting can help to combat climate change. This review article is intended to cross-pollinate and strengthen efforts from fundamental understanding to technical implementation and to improve the 'junctions' between the field's physical chemists, materials scientists and engineers, as well as stimulate much-needed exchange among these groups on challenges encountered in the different domains.

Interface engineering of Ta3N5 thin film photoanode for highly efficient photoelectrochemical water splitting

Interface engineering is a proven strategy to improve the efficiency of thin film semiconductor based solar energy conversion devices. Ta₃N₅ thin film photoanode is a promising candidate for photoelectrochemical (PEC) water splitting. Yet, a concerted effort to engineer both the bottom and top interfaces of Ta₃N₅ thin film photoanode is still lacking. Here, we employ n-type In:GaN and p-type Mg:GaN to modify the bottom and top interfaces of Ta₃N₅ thin film photoanode, respectively. The obtained In:GaN/Ta₃N₅/Mg:GaN heterojunction photoanode shows enhanced bulk carrier separation capability and better injection efficiency at photoanode/electrolyte interface, which lead to a record-high applied bias photon-to-current efficiency of 3.46% for Ta₃N₅-based photoanode. Furthermore, the roles of the In:GaN and Mg:GaN layers are distinguished through mechanistic studies. While the In:GaN layer contributes mainly to the enhanced bulk charge separation efficiency, the Mg:GaN layer improves the surface charge inject efficiency. This work demonstrates the crucial role of proper interface engineering for thin film-based photoanode in achieving efficient PEC water splitting.

Solar-to-fuel energy conversion requires well-designed materials properties to ensure favorable charge carrier movement. Here, authors employ interface engineering of Ta₃N₅ thin film to enhance bulk carrier separation and interface carrier injection to improve the water-splitting efficiency.

Photochromic immunoassay for tumor marker detection based on ZnO/AgI nanophotocatalyst

A photochromic immunoassay was built for tumor marker detection based on ZnO/AgI nanophotocatalyst. Frist, ZnO/AgI nanoparticles were synthesized and characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray powder diffraction (XRD), and Fourier transform infrared spectrometry (FTIR). The color development is caused by tetramethyl benzidine (TMB) solution oxidated by ZnO/AgI nanomaterials. The electron transitions in ZnO/AgI nanomaterials are driven by visible light irradiation, generating photogenerated hole and oxidizing TMB to blue solution. Appropriate band width between ZnO and AgI promotes separation of photogenerated electrons and holes and enhances oxidation efficiency. A sandwich-type immunoassay was constructed based on ZnO/AgI nanomaterial as labels. The absorbance at 650 nm of reaction solution is positively correlated with antigen concentration. The developed immunoassay showed good performance for carcinoma embryonic antigen (CEA) detection in the range 0.1-7.0 ng/mL with a detection limit of 65 pg/mL. The photochromic immunoassay also exhibited preferable selectivity, repeatability, and stability. A novel colorimetric immunoassay was constructed based on ZnO/AgI photocatalyst. ZnO/AgI nanomaterials occur electron transitions under visible light irradiation and generate photogenerated hole, which can oxidize TMB to blue solution. Carcinoembryonic antigen in sample was detected sensitively due to the high catalytic efficiency of ZnO/AgI nanomaterials.

Heteroepitaxial Growth of a Ta3N5 Thin Film with Clear Anisotropic Optical Properties

Ta₃N₅ is a promising semiconductor photocatalyst which can generate H₂ gas from water under visible light illumination. It is expected that Ta₃N₅ exhibits a strong anisotropy in its physical properties stemming from its highly anisotropic crystal structure. However, such anisotropic properties have not been verified experimentally due to the difficulty in synthesizing a large single crystal. Here, we report the synthesis of (010)-oriented Ta₃N₅ single-crystalline thin films by solid phase epitaxy on the (110) plane of perovskite LaAlO₃ substrates. The obtained epitaxial thin films of Ta₃N₅ exhibited clear optical anisotropy (pleochroism) as predicted by previous first-principles calculations. The optical gap for E||[100] polarization (∼2.12 eV) was smaller than that for E||[100] polarization (∼2.27 eV).

Efficient solar photocatalytic hydrogen production using direct Z-scheme heterojunctions

We report the preparation of a series of heterojunctions made of Ta₃N₅ and TiO₂ nanoparticles that show good properties for photocatalytic hydrogen production. The composite photocatalyst with a light-response range up to 620 nm shows a hydrogen evolution rate of 250 μmol h^-1. The apparent quantum efficiency at 330 nm can be as high as 46%. Particularly, normalized spectral studies indicate that the heterojunction is more active upon full-spectrum (without using optical filters) irradiation, and its activity is even superior  to the total activity exhibited upon UV-light irradiation (λ ≤ 420 nm) and visible-infrared light irradiation (λ ≥ 420 nm). Moreover, in situ photodeposition of platinum nanoparticles on the surface of the photocatalyst as well as the band alignment analysis demonstrate the Z-scheme mechanism associated with the photocatalytic process. Specifically, photogenerated electrons from TiO₂ will rapidly combine with the photogenerated holes from Ta₃N₅ through interfacial charge transfer, leaving the more active electrons and holes in Ta₃N₅ and TiO₂, respectively, to facilitate redox reactions. Basically, TiO₂ is only UV-light active, while Ta₃N₅ can be activated under visible-light irradiation. In this case, a synergy effect, upon simultaneous UV-light excitation and visible-light excitation, can be achieved by full-spectrum irradiation, leading to a much higher photocatalytic activity. This work thus provides a favorable and upward direction for the establishment of heterojunctions for high-efficiency hydrogen production and solar energy applications.

Co(OH)2 water oxidation cocatalyst-decorated CdS nanowires for enhanced photocatalytic CO2 reduction performance

Photocatalytic CO2 reduction is a promising technology to resolve the greenhouse effect and energy crisis. In this work, a Co(OH)2 nanoparticle decorated CdS nanowire (Co(OH)2/CdS) based heterostructured photocatalyst was prepared via a solvothermal and subsequent co-precipitation method, and it was used for photocatalytic CO2 reduction. The optimal Co(OH)2/CdS photocatalyst achieves a CO production rate of 8.11 μmol g-1 h-1 under visible light irradiation (λ > 420 nm), which is about 2 times higher than that of bare CdS. The experimental results show that a Co(OH)2 cocatalyst possesses a great capability of consuming holes, which promotes the oxygen-producing half-reaction and accelerates charge separation, thus enhancing the CO2 photoreduction performance of CdS. Notably, without using complex synthesis processes, hazardous substances or expensive ingredients, Co(OH)2/CdS shows high light absorption, efficient charge separation and complete CO product selectivity. This work offers a new pathway for the construction of cost-effective photocatalytic materials to achieve highly efficient CO2 reduction activity by the integration of a Co(OH)2 cocatalyst.

Nanostructure Engineering and Modulation of (Oxy)Nitrides for Application in Visible-Light-Driven Water Splitting

(Oxy)nitride-based nanophotocatalysts have been extensively investigated for solar-to-chemical conversion, and not only allow wide spectral utilization to achieve high theoretical energy conversion efficiency but also exhibit suitable conduction and valence band positions for robust reduction and oxidation of water. During the past decades, a few reviews on the research progress in designing and synthesizing new visible-light-responsive semiconductors for various applications in solar-to-chemical conversion have been published. However, those on the effects of their bulk and composite (surface/interface) nanostructures on basic processes as well as solar water splitting performances to produce hydrogen are still limited. In this review, a brief introduction on the relationship between the nanostructure photocatalytic properties is included. Three main processes of solar water splitting are involved, allowing the elucidation of the correlation with the nanostructural properties of the photocatalyst such as surface/interface, size, morphology, and bulk structure. Subsequently, the development of methodologies and strategies for modulating the bulk and composite structures to improve the efficiencies of the basic processes, particularly charge separation, is summarized in detail. Finally, the prospects of (oxy)nitride-based photocatalysts such as controlled synthesis, modulation of 1D/2D morphology, exposed facet regulation, heterostructure formation, theoretical simulation, and time- and space-resolved spectroscopy are discussed.

Simultaneously Tuning the Defects and Surface Properties of Ta3N5 Nanoparticles by Mg-Zr Codoping for Significantly Accelerated Photocatalytic H2 Evolution

The simultaneous control of the defect species and surface properties of semiconducting materials is a crucial aspect of improving photocatalytic performance, yet it remains challenging. Here, we synthesized Mg-Zr-codoped single-crystalline Ta₃N₅ (Ta₃N₅:Mg+Zr) nanoparticles by a brief NH₃ nitridation process, exhibiting photocatalytic water reduction activity 45 times greater than that of pristine Ta₃N₅ under visible light. A coherent picture of the relations between the defect species (comprising reduced Ta, nitrogen vacancies and oxygen impurities), surface properties (associated with dispersion of the Pt cocatalyst), charge carrier dynamics, and photocatalytic activities was drawn. The tuning of defects and simultaneous optimization of surface properties resulting from the codoping evidently resulted in the generation of high concentrations of long-lived electrons in this material as well as the efficient migration of these electrons to evenly distributed surface Pt sites. These effects greatly enhanced the photocatalytic activity. This work highlights the importance and feasibility of improving multiple properties of a catalytic material via a one-step strategy.

Boosting Photocatalytic Water Oxidation Over Bifunctional Rh0 -Rh3+ Sites

Photocatalytic water splitting provides an economically feasible way for converting solar energy into hydrogen. Great efforts have been devoted to developing efficient photocatalysts; however, the surface catalytic reactions, especially for the sluggish oxygen evolution reaction (OER), still remain a challenge, which limits the overall photocatalytic energy efficiency. Herein, we design a Rh_n cluster cocatalyst, with Rh⁰ -Rh³⁺ sites anchoring the Mo-doped BiVO₄ model photocatalytic system. The resultant photocatalyst enables a high visible-light photocatalytic oxygen production activity of 7.11 mmol g^-1  h^-1 and an apparent quantum efficiency of 29.37 % at 420 nm. The turnover frequency (TOF) achieves 416.73 h^-1 , which is 378 times higher than that of the photocatalyst only with Rh³⁺ species. Operando X-ray absorption characterization shows the OER process on the Rh⁰ -Rh³⁺ sites. The DFT calculations further illustrate a bifunctional OER mechanism over the Rh⁰ -Rh³⁺ sites, in which the oxygen intermediate attacks the Rh³⁺ sites with assistance of a hydrogen atom transfer to the Rh⁰ sites, thus breaking the scaling relationship of various oxygen intermediates.

Synthesis and Characterization of Amorphous Molybdenum Sulfide (MoSx)/CdIn2S4 Composite Photocatalyst: Co-Catalyst Using in the Hydrogen Evolution Reaction

Co-catalyst deposition is used to improve the surface and electrical properties of photocatalysts. In this work, MoSx/CdIn2S4 nanocomposites were prepared by a facile hydrothermal and photodeposition route. The basic crystalline phases and morphology of the as-prepared samples were determined, and these results showed that MoSx was tightly anchored onto CdIn2S4 by sharing the same S atom. In the hydrogen production experiments, MoSx/CdIn2S4-40 displayed the optimal photocatalytic hydrogen production yield in 4 h. The H2 evolution rate reached 2846.73 μmol/g/h, which was 13.6-times higher than that of pure CdIn2S4. Analyzing the photocatalytic enhancement mechanisms revealed that this unique structure had a remarkable photogenerated electron-hole pair separation efficiency, rapid charge carrier transfer channels, and more abundant surface reaction sites. The use of co-catalyst (MoSx) greatly improved the photocatalytic activity of CdIn2S4.

CO2 and H2O Coadsorption and Reaction on the Low-Index Surfaces of Tantalum Nitride: A First-Principles DFT-D3 Investigation

A comprehensive mechanistic insight into the photocatalytic reduction of CO2 by H2O is indispensable for the development of highly efficient and robust photocatalysts for artificial photosynthesis. This work presents first-principles mechanistic insights into the adsorption and activation of CO2 in the absence and presence of H2O on the (001), (010), and (110) surfaces of tantalum nitride (Ta3N5), a photocatalysts of significant technological interest. The stability of the different Ta3N surfaces is shown to dictate the strength of adsorption and the extent of activation of CO2 and H2O species, which bind strongest to the least stable Ta3N5(001) surface and weakest to the most stable Ta3N5(110) surface. The adsorption of the CO2 on the Ta3N5(001), (010), and (110) surfaces is demonstrated to be characterized by charge transfer from surface species to the CO2 molecule, resulting in its activation (i.e., forming negatively charged bent CO2−δ species, with elongated C–O bonds confirmed via vibrational frequency analyses). Compared to direct CO2 dissociation, H2O dissociates spontaneously on the Ta3N5 surfaces, providing the necessary hydrogen source for CO2 reduction reactions. The coadsorption reactions of CO2 and H2O are demonstrated to exhibit the strongest attractive interactions on the (010) surface, giving rise to proton transfer to the CO2 molecule, which causes its spontaneous dissociation to form CO and 2OH− species. These results demonstrate that Ta3N5, a narrow bandgap photocatalyst able to absorb visible light, can efficiently activate the CO2 molecule and photocatalytically reduce it with water to produce value-added fuels.

A Selective Synthesis of TaON Nanoparticles and Their Comparative Study of Photoelectrochemical Properties

A simplified ammonolysis method for synthesizing single phase TaON nanoparticles is presented and the resulting photoelectrochemical properties are compared and contrasted with as-synthesized Ta2O5 and Ta3N5. The protocol for partial nitridation of Ta2O5 (synthesis of TaON) offers a straightforward simplification over existing methods. Moreover, the present protocol offers extreme reproducibility and enhanced chemical safety. The morphological characterization of the as-synthesized photocatalysts indicate spherical nanoparticles with sizes 30, 40, and 30 nm Ta2O5, TaON, and Ta3N5 with the absorbance onset at ~320 nm, 580 nm, and 630 nm respectively. The photoactivity of the catalysts has been examined for the degradation of a representative cationic dye methylene blue (MB) using xenon light. Subsequent nitridation of Ta2O5 yields significant increment in the conversion (ζ: Ta2O5 < TaON < Ta3N5) mainly attributable to the defect-facilitated adsorption of MB on the catalyst surface and bandgap lowering of catalysts with Ta3N5 showing > 95% ζ for a lower (0.1 g) loading and with a lamp with lower Ultraviolet (UV) content. Improved Photoelectrochemical performance is noted after a series of chronoamperometry (J/t), linear sweep voltammetry (LSV), and electrochemical impedance spectroscopy (EIS) measurements. Finally, stability experiments performed using recovered and treated photocatalyst show no loss of photoactivity, suggesting the photocatalysts can be successfully recycled.

Prismatic Ta3N5-composed spheres produced by self-sacrificial template-like conversion of Ta particles via Na2CO3 flux

Ta₃N₅ crystals were grown from Na₂CO₃ flux using spherical Ta powders.

Water Oxidation Catalysts for Artificial Photosynthesis

Water oxidation is the primary reaction of both natural and artificial photosynthesis. Developing active and robust water oxidation catalysts (WOCs) is the key to constructing efficient artificial photosynthesis systems, but it is still facing enormous challenges in both fundamental and applied aspects. Here, the recent developments in molecular catalysts and heterogeneous nanoparticle catalysts are reviewed with special emphasis on biomimetic catalysts and the integration of WOCs into artificial photosystems. The highly efficient artificial photosynthesis depends largely on active WOCs integrated into light harvesting materials via rational interface engineering based on in-depth understanding of charge dynamics and the reaction mechanism.

Merging Single-Atom-Dispersed Iron and Graphitic Carbon Nitride to a Joint Electronic System for High-Efficiency Photocatalytic Hydrogen Evolution

Scalable and sustainable solar hydrogen production via photocatalytic water splitting requires extremely active and stable light-harvesting semiconductors to fulfill the stringent requirements of suitable energy band position and rapid interfacial charge transfer process. Motivated by this point, increasing attention has been given to the development of photocatalysts comprising intimately interfaced photoabsorbers and cocatalysts. Herein, a simple one-step approach is reported to fabricate a high-efficiency photocatalytic system, in which single-site dispersed iron atoms are rationally integrated on the intrinsic structure of the porous crimped graphitic carbon nitride (g-C₃ N₄ ) polymer. A detailed analysis of the formation process shows that a stable complex is generated by spontaneously coordinating dicyandiamidine nitrate with iron ions in isopropanol, thus leading to a relatively complicated polycondensation reaction upon thermal treatment. The correlation of experimental and computational results confirms that optimized electronic structures of Fe@g-C₃ N₄ with an appropriate d-band position and negatively shifting Fermi level can be achieved, which effectively gains the reducibility of electrons and creates more active sites for the photocatalytic reactions. As a result, the Fe@g-C₃ N₄ exhibits a highlighted intramolecular synergistic effect, performing greatly enhanced solar-photon-driven activities, including excellent photocatalytic hydrogen evolution rate (3390 µmol h^-1 g^-1 , λ > 420 nm) and a reliable apparent quantum efficiency value of 6.89% at 420 nm.

Solar Hydrogen Production from Cost Effective Stannic Oxide Under Visible Light Irradiation

Visible-light-driven stannic oxide was synthesized by facile one-pot solvothermal method from SnCl2·2H2O and methanol. The as-prepared powder was identified by XRD as the low crystalline phase of SnO2, and its absorption edge reached about 530 nm, presenting good potential to respond to visible light. Under visible light irradiation (λ > 420 nm), the as-prepared tin oxide showed good anodic photocurrent effects on FTO photoelectrode, and showed hydrogen and oxygen evolution activities under electron donor (methanol) and acceptor (AgNO3), respectively, even without any co-catalyst loading. The visible-light-driven mechanism for this SnO2-x maybe ascribed to Sn2+ self-doped into Sn4+ and formed an energy gap between the band gap of SnO2.

Particulate Photocatalysts for Light-Driven Water Splitting: Mechanisms, Challenges, and Design Strategies

Solar-driven water splitting provides a leading approach to store the abundant yet intermittent solar energy and produce hydrogen as a clean and sustainable energy carrier. A straightforward route to light-driven water splitting is to apply self-supported particulate photocatalysts, which is expected to allow solar hydrogen to be competitive with fossil-fuel-derived hydrogen on a levelized cost basis. More importantly, the powder-based systems can lend themselves to making functional panels on a large scale while retaining the intrinsic activity of the photocatalyst. However, all attempts to generate hydrogen via powder-based solar water-splitting systems to date have unfortunately fallen short of the efficiency values required for practical applications. Photocatalysis on photocatalyst particles involves three sequential steps: (i) absorption of photons with higher energies than the bandgap of the photocatalysts, leading to the excitation of electron-hole pairs in the particles, (ii) charge separation and migration of these photoexcited carriers, and (iii) surface chemical reactions based on these carriers. In this review, we focus on the challenges of each step and summarize material design strategies to overcome the obstacles and limitations. This review illustrates that it is possible to employ the fundamental principles underlying photosynthesis and the tools of chemical and materials science to design and prepare photocatalysts for overall water splitting.

Heterostructure of 1D Ta3 N5 Nanorod/BaTaO2 N Nanoparticle Fabricated by a One-Step Ammonia Thermal Route for Remarkably Promoted Solar Hydrogen Production

Heterostructures are widely fabricated for promotion of photogenerated charge separation and solar cell/fuel production. (Oxy)nitrides are extremely promising for solar energy conversion, but the fabrication of heterostructures based on nitrogen-containing semiconductors is still challenging. Here, a simple ammonia thermal synthesis of a heterostructure (denoted as Ta₃ N₅ /BTON) composed of 1D Ta₃ N₅ nanorods and BaTaO₂ N (BTON) nanoparticles (0D), which is demonstrated to result in a remarkable increase in photogenerated charge separation and solar hydrogen production from water, is introduced. As analyzed and discussed, the Ta₃ N₅ /BTON heterostructure is type II and tends to create intimate interfaces between the 1D nanorods and 0D nanoparticles. The 1D Ta₃ N₅ nanorods are demonstrated to transfer electrons along the rod orientation direction. Furthermore, the intimate interfaces of the heterostructure are believed to originate from the similar Ta-based octahedron units of Ta₃ N₅ and BTON. All of the above features are expected to integrally endow increased photoinduced charge separation and one order of magnitude higher solar overall water splitting activity with respect to counterpart systems. These results may open a new avenue to fabricate heterostructures on the basis of nitrogen-containing semiconductors that is extremely promising for solar energy conversion.

A mononuclear tantalum catalyst with a peroxocarbonate ligand for olefin epoxidation in compressed CO2

A mononuclear tantalum complex bonded to a peroxocarbonate ligand has been proved to be particularly important in the epoxidation reactions.

Nanostructured materials for photocatalysis

Photocatalysis is a green technology which converts abundantly available photonic energy into useful chemical energy.

Reductive Transformation of Layered-Double-Hydroxide Nanosheets to Fe-Based Heterostructures for Efficient Visible-Light Photocatalytic Hydrogenation of CO

Conversion of syngas (CO, H2 ) to hydrocarbons, commonly known as the Fischer-Tropsch (FT) synthesis, represents a fundamental pillar in today's chemical industry and is typically carried out under technically demanding conditions (1-3 MPa, 300-400 °C). Photocatalysis using sunlight offers an alternative and potentially more sustainable approach for the transformation of small molecules (H2 O, CO, CO2 , N2 , etc.) to high-valuable products, including hydrocarbons. Herein, a novel series of Fe-based heterostructured photocatalysts (Fe-x) is successfully fabricated via H2 reduction of ZnFeAl-layered double hydroxide (LDH) nanosheets at temperatures (x) in the range 300-650 °C. At a reduction temperature of 500 °C, the heterostructured photocatalyst formed (Fe-500) consists of Fe0 and FeOx nanoparticles supported by ZnO and amorphous Al2 O3 . Fe-500 demonstrates remarkable CO hydrogenation performance with very high initial selectivities toward hydrocarbons (89%) and especially light olefins (42%), and a very low selectivity towards CO2 (11%). The intimate and abundant interfacial contacts between metallic Fe0 and FeOx in the Fe-500 photocatalyst underpins its outstanding photocatalytic performance. The photocatalytic production of high-value light olefins with suppressed CO2 selectivity from CO hydrogenation is demonstrated here.

Visible-Light-Responsive Photoanodes for Highly Active, Stable Water Oxidation

Solar energy is a natural and effectively permanent resource and so the conversion of solar radiation into chemical or electrical energy is an attractive, although challenging, prospect. Photo-electrochemical (PEC) water splitting is a key aspect of producing hydrogen from solar power. However, practical water oxidation over photoanodes (in combination with water reduction at a photocathode) in PEC cells is currently difficult to achieve because of the large overpotentials in the reaction kinetics and the inefficient photoactivity of the semiconductors. The development of semiconductors that allow high solar-to-hydrogen conversion efficiencies and the utilization of these materials in photoanodes will be a necessary aspect of achieving efficient, stable water oxidation. This Review discusses advances in water oxidation activity over photoanodes of n-type visible-light-responsive (oxy)nitrides and oxides.

Highly efficient near-infrared light photocatalytic hydrogen evolution over MoS2 supported Ta2O5 combined with an up-conversion luminescence agent (β-Tm3+,Yb3+:NaYF4/MoS2–Ta2O5 nanocomposite)

The photocatalytic hydrogen evolution activity of Ta₂O₅ can be greatly enhanced by adding up-conversion luminescence agent (β-Tm³⁺,Yb³⁺:NaYF₄) and co-catalyst (MoS₂).

Molten salt-assisted a-axis-oriented growth of Ta3N5 nanorod arrays with enhanced charge transport for efficient photoelectrochemical water oxidation

Molten salt-assisted a-axis-oriented growth of Ta₃N₅ nanorod arrays with enhanced charge transport for efficient photoelectrochemical water oxidation.

Surface-Modified Ta3N5 Nanocrystals with Boron for Enhanced Visible-Light-Driven Photoelectrochemical Water Splitting

Photocatalysts for water splitting are the core of renewable energy technologies, such as hydrogen fuel cells. The development of photoelectrode materials with high efficiency and low corrosivity has great challenges. In this study, we report new strategy to improve performance of tantalum nitride (Ta₃N₅) nanocrystals as promising photoanode materials for visible-light-driven photoelectrochemical (PEC) water splitting cells. The surface of Ta₃N₅ nanocrystals was modified with boron whose content was controlled, with up to 30% substitution of Ta. X-ray photoelectron spectroscopy revealed that boron was mainly incorporated into the surface oxide layers of the Ta₃N₅ nanocrystals. The surface modification with boron increases significantly the solar energy conversion efficiency of the water-splitting PEC cells by shifting the onset potential cathodically and increasing the photocurrents. It reduces the interfacial charge-transfer resistance and increases the electrical conductivity, which could cause the higher photocurrents at lower potential. The onset potential shift of the PEC cell with the boron incorporation can be attributed to the negative shift of the flat band potential. We suggest that the boron-modified surface acts as a protection layer for the Ta₃N₅ nanocrystals, by catalyzing effectively the water splitting reaction.

First-Principles Study of the Band Diagrams and Schottky-Type Barrier Heights of Aqueous Ta3N5 Interfaces

The photo(electro)chemical production of hydrogen by water splitting is an efficient and sustainable method for the utilization of solar energy. To improve photo(electro)catalytic activity, a Schottky-type barrier is typically useful to separate excited charge carriers in semiconductor electrodes. Here, we focused on studying the band diagrams and the Schottky-type barrier heights of Ta₃N₅, which is one of the most promising materials as a photoanode for water splitting. The band alignments of the undoped and n-type Ta₃N₅ with adsorbents in a vacuum were examined to determine how impurities and adsorbents affect the band positions and Fermi energies. The band edge positions as well as the density of surface states clearly depended on the density of O_N impurities in the bulk and surface regions. Finally, the band diagrams of the n-type Ta₃N₅/water interfaces were calculated with an improved interfacial model to include the effect of electrode potential with explicit water molecules. We observed partial Fermi level pinning in our calculations at the Ta₃N₅/water interface, which affects the driving force for charge separation.