A Redox‐Mediator‐Free Solar‐Driven Z‐Scheme Water‐Splitting System Consisting of Modified Ta3N5 as an Oxygen‐Evolution Photocatalyst

Tantalum nitride (Ta₃N₅) modified with various O₂‐evolution cocatalysts was employed as a photocatalyst for water oxidation under visible light (λ>420 nm) in an attempt to construct a redox‐mediator‐free Z‐scheme water‐splitting system. Ta₃N₅ was prepared by nitriding Ta₂O₅ powder under a flow of NH₃ at 1023–1223 K. The activity of Ta₃N₅ for water oxidation from an aqueous AgNO₃ solution as an electron acceptor without cocatalyst was dependent on the generation of a well‐crystallized Ta₃N₅ phase with a low density of anionic defects. Modification of Ta₃N₅ with nanoparticulate metal oxides as cocatalysts for O₂ evolution improved water‐oxidation activity. Of the cocatalysts examined, cobalt oxide (CoO_x) was found to be the most effective, improving the water‐oxidation efficiency of Ta₃N₅ by six to seven times. Further modification of CoO_x/Ta₃N₅ with metallic Ir as an electron sink allowed one to achieve Z‐scheme water splitting under simulated sunlight through interparticle electron transfer without the need for a shuttle redox mediator in combination with Ru‐loaded SrTiO₃ doped with Rh as a H₂‐evolution photocatalyst.     

Overall Water Splitting on the Transition‐Metal Oxynitride Photocatalyst LaMg1/3Ta2/3O2N over a Large Portion of the Visible‐Light Spectrum

One of the main targets of studies on water splitting photocatalysts is to develop semiconductor materials with narrower bandgaps capable of overall water splitting for efficient harvesting of solar energy. A series of transition‐metal oxynitrides, LaMg_xTa_1?xO_1+3xN_2?3x (x≥1/3), with a complex perovskite structure was reported as the first example of overall water splitting operable at up to 600 nm. The photocatalytic behavior of LaMg_1/3Ta_2/3O₂N was investigated in detail in order to optimize photocatalyst preparation and water‐splitting activity. Various attempts exploring photocatalyst preparation steps, that is, cocatalyst selection, coating material and method, and synthesis method for the oxide precursor, revealed photocatalyst structures necessary for achieving overall water splitting. Careful examination of photocatalyst preparation procedures likely enhanced the quality of the produced photocatalyst, leading to a more homogeneous coating quality and semiconductor particles with fewer defects. Thus, the photocatalytic activity for water splitting on LaMg_1/3Ta_2/3O₂N was largely enhanced.     

Efficient Hole Extraction from a Hole‐Storage‐Layer‐Stabilized Tantalum Nitride Photoanode for Solar Water Splitting

One of the major hurdles that impedes the practical application of photoelectrochemical (PEC) water splitting is the lack of stable photoanodes with low onset potentials. Here, we report that the Ni(OH)_x/MoO₃ bilayer, acting as a hole‐storage layer (HSL), efficiently harvests and stores holes from Ta₃N₅, resulting in at least 24 h of sustained water oxidation at the otherwise unstable Ta₃N₅ electrode and inducing a large cathodic shift of ≈600 mV in the onset potential of the Ta₃N₅ electrode.     

Highly Active GaN‐Stabilized Ta3N5 Thin‐Film Photoanode for Solar Water Oxidation

Ta₃N₅ is a very promising photocatalyst for solar water splitting because of its wide spectrum solar energy utilization up to 600 nm and suitable energy band position straddling the water splitting redox reactions. However, its development has long been impeded by poor compatibility with electrolytes. Herein, we demonstrate a simple sputtering‐nitridation process to fabricate high‐performance Ta₃N₅ film photoanodes owing to successful synthesis of the vital TaO_δ precursors. An effective GaN coating strategy is developed to remarkably stabilize Ta₃N₅ by forming a crystalline nitride‐on‐nitride structure with an improved nitride/electrolyte interface. A stable, high photocurrent density of 8 mA cm⁻² was obtained with a CoPi/GaN/Ta₃N₅ photoanode at 1.2 V_RHE under simulated sunlight, with O₂ and H₂ generated at a Faraday efficiency of unity over 12 h. Our vapor‐phase deposition method can be used to fabricate high‐performance (oxy)nitrides for practical photoelectrochemical applications.     

Synthesis of Ordered Porous Graphitic‐C3N4 and Regularly Arranged Ta3N5 Nanoparticles by Using Self‐Assembled Silica Nanospheres as a Primary Template

Uniform‐sized silica nanospheres (SNSs) assembled into close‐packed structures were used as a primary template for ordered porous graphitic carbon nitride (g‐C₃N₄), which was subsequently used as a hard template to generate regularly arranged Ta₃N₅ nanoparticles of well‐controlled size. Inverse opal g‐C₃N₄ structures with the uniform pore size of 20–80 nm were synthesized by polymerization of cyanamide and subsequent dissolution of the SNSs with an aqueous HF solution. Back‐filling of the C₃N₄ pores with tantalum precursors, followed by nitridation in an NH₃ flow gave regularly arranged, crystalline Ta₃N₅ nanoparticles that are connected with each other. The surface areas of the Ta₃N₅ samples were as high as 60 m² g⁻¹, and their particle size was tunable from 20 to 80 nm, which reflects the pore size of g‐C₃N₄. Polycrystalline hollow nanoparticles of Ta₃N₅ were also obtained by infiltration of a reduced amount of the tantalum source into the g‐C₃N₄ template. An improved photocatalytic activity for H₂ evolution on the assembly of the Ta₃N₅ nanoparticles under visible‐light irradiation was attained as compared with that on a conventional Ta₃N₅ bulk material with low surface area.     

Enhanced Spatial Charge Separation in a Niobium and Tantalum Nitride Core-Shell Photoanode: In Situ Interface Bonding for Efficient Solar Water Splitting

Tantalum nitride (Ta₃N₅) has emerged as a promising photoanode material for photoelectrochemical (PEC) water splitting. However, the inefficient electron-hole separation remains a bottleneck that impedes its solar-to-hydrogen conversion efficiency. Herein, we demonstrate that a core–shell nanoarray photoanode of NbN_x-nanorod@Ta₃N₅ ultrathin layer enhances light harvesting and forms a spatial charge-transfer channel, which leads to the efficient generation and extraction of charge carriers. Consequently, an impressive photocurrent density of 7 mA cm⁻² at 1.23 V_RHE is obtained with an ultrathin Ta₃N₅ shell thickness of less than 30 nm, accompanied by excellent stability and a low onset potential (0.46 V_RHE). Mechanistic studies reveal the enhanced performance is attributed to the high-conductivity NbN_x core, high-crystalline Ta₃N₅ mono-grain shell, and the intimate Ta−N−Nb interface bonds, which accelerate the charge-separation capability of the core–shell photoanode. This study demonstrates the key roles of nanostructure design in improving the efficiency of PEC devices.     

A Complex Perovskite‐Type Oxynitride: The First Photocatalyst for Water Splitting Operable at up to 600 nm

One of the simplest methods for splitting water into H₂ and O₂ with solar energy entails the use of a particulate‐type semiconductor photocatalyst. To harness solar energy efficiently, a new water‐splitting photocatalyst that is active over a wider range of the visible spectrum has been developed. In particular, a complex perovskite‐type oxynitride, LaMg_xTa_1?xO_1+3xN_2?3x (x≥1/3), can be employed for overall water splitting at wavelengths of up to 600 nm. Two effective strategies for overall water splitting were developed. The first entails the compositional fine‐tuning of a photocatalyst to adjust the bandgap energy and position by forming a series of LaMg_xTa_1?xO_1+3xN_2?3x solid solutions. The second method is based on the surface coating of the photocatalyst with a layer of amorphous oxyhydroxide to control the surface redox reactions. By combining these two strategies, the degradation of the photocatalyst and the reverse reaction could be prevented, resulting in successful overall water splitting.     

Ge‐Mediated Modification in Ta3N5 Photoelectrodes with Enhanced Charge Transport for Solar Water Splitting

Ta₃N₅ 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 Ta₃N₅ 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 Ta₃N₅ photoelectrodes. In this study, we show that volatilization of intentionally added Ge (5 %) during the synthesis of Ta₃N₅ promotes the electron transport and thereby improves the charge‐separation efficiency in bulk Ta₃N₅ photoanode, which affords a 320 % increase of the highest photocurrent comparing with that of pure Ta₃N₅ photoanode under AM 1.5 G 100 mW cm^?2 simulated sunlight.     

Ultrahigh‐Content Nitrogen‐doped Carbon Encapsulating Cobalt NPs as Catalyst for Oxidative Esterification of Furfural

It is an attractive and challenging topic to endow non‐noble metal catalysts with high efficiency via a nitrogen‐doping approach. In this study, a nitrogen‐doped carbon catalyst with high nitrogen content encapsulating cobalt NPs (CoO_x@N‐C(g)) was synthesized, and characterized in detail by XRD, HRTEM, N₂‐physisorption, ICP, CO₂‐TPD, and XPS techniques. g‐C₃N₄ nanosheets act as nitrogen source and self‐sacrificing templates, giving rise to an ultrahigh nitrogen content of 14.0 %, much higher than those using bulk g‐C₃N₄ (4.4 %) via the same synthesis procedures. As a result, CoO_x@N‐C(g) exhibited the highest performance in the oxidative esterification of biomass‐derived platform furfural to methylfuroate under base‐free conditions, achieving 95.0 % conversion and 97.1 % selectivity toward methylfuroate under 0.5 MPa O₂ at 100 °C for 6 h, far exceeding those of other cobalt‐based catalysts. The high efficiency of CoO_x@N‐C(g) was closely related to its high ratio of pyridinic nitrogen species that may act as Lewis basic sites as well as its capacity for the activation of dioxygen to superoxide radical O₂^.?.     

Rh/Cr2O3 and CoOx Cocatalysts for Efficient Photocatalytic Water Splitting by Poly (Triazine Imide) Crystals

In situ photo-deposition of both Pt and CoO_x cocatalysts on the facets of poly (triazine imide) (PTI) crystals has been developed for photocatalytic overall water splitting. However, the undesired backward reaction (i.e., water formation) on the noble Pt surface is a spontaneously down-hill process, which restricts their efficiency to run the overall water splitting reaction. Herein, we demonstrate that the efficiency for photocatalytic overall water splitting could be largely promoted by the decoration of Rh/Cr₂O₃ and CoO_x as H₂ and O₂ evolution cocatalysts, respectively. Results reveal that the dual cocatalysts greatly extract charges from bulk to surface, while the Rh/Cr₂O₃ cocatalyst dramatically restrains the backward reaction, achieving an apparent quantum efficiency (AQE) of 20.2 % for the photocatalytic overall water splitting reaction.     

Pristine Ta3N5 Nanotubes: Trap‐Driven High External Biasing Perspective in Semiconductor/Electrolyte Interfaces

Ta₃N₅ is a promising photoelectrode for solar hydrogen production; however, to date pristine Ta₃N₅ electrodes without loading co‐catalysts have presented limited photoelectrochemical (PEC) performance. In particular, large external biasing has been required to run water oxidation, the origin of which is investigated herein. Ta₃N₅ nanotubes (NTs) prepared by nitridation were characterized by a wide range of techniques. The bandgap was confirmed by a novel PEC technique. Nondestructive synchrotron‐excited XPS has shown the presence of reduced Ta species deeper in the Ta₃N₅ surface. Lower photocurrent and transient spikes that were intense at lower applied biasing were observed under water oxidation; however, spikes were inhibited in the presence of a sacrificial agent and photocurrent was improved even at low biasing. It was observed for the first time that the lower PEC performance under water oxidation can be attributed to the presence of interband trapping states associated with pristine Ta₃N₅ NTs/electrolyte junction. These states correspond to the structural defects in Ta₃N₅, devastate PEC performance, and present the necessity to apply higher biasing. The key to circumvent them is to use a sacrificial agent in the electrolyte or to load a suitable co‐catalyst to avoid hole accumulation under water oxidation, thereby improving the phootocurrent. The findings on the interband states could also provide guidance for the investigation of PEC properties of new types of semiconducting devices.     

Enhancement of the Photocatalytic Oxygen Production by Tantalum Nitride Supported Fe Atom Embedded N-Doped Graphene Oxide†

Energy crises and environmental pollution have become urgent problems with human civilization development. Photocatalysis technology is a green method to deal with these challenges. The key to improve photocatalytic efficiency lies in the effective separation of photogenerated electron–hole pairs. In this work, we designed the Fe atom embedded N-doped graphene oxide (Fe-NGO) supporting on tantalum nitride (Ta₃N₅) catalyst, which was employed to improve the photocatalytic oxygen production activity. The oxygen production of 5 wt% Fe atom embedded N-doped graphene oxide supporting on tantalum nitride (Fe-NGO/Ta₃N₅) was 184.7 μmol·g⁻¹, about 3.5 times higher than that of the pure Ta₃N₅. The introduction of the cocatalyst Fe-NGO acting as an electron conductor in the Fe-NGO/Ta₃N₅ accelerates the carrier migration of Ta₃N₅ and further enhances the photocatalytic oxygen production activity. N-doping increases the conductivity of graphene oxide (GO), and Fe atoms are used as the reactive sites to promote the combination of electron and sacrificial agent in the system. This work may provide insights into the research of new carbon cocatalyst materials.      

Porous TiO2 Nanotubes with Spatially Separated Platinum and CoOx Cocatalysts Produced by Atomic Layer Deposition for Photocatalytic Hydrogen Production

Efficient separation of photogenerated electrons and holes, and associated surface reactions, is a crucial aspect of efficient semiconductor photocatalytic systems employed for photocatalytic hydrogen production. A new CoO_x/TiO₂/Pt photocatalyst produced by template‐assisted atomic layer deposition is reported for photocatalytic hydrogen production on Pt and CoO_x dual cocatalysts. Pt nanoclusters acting as electron collectors and active sites for the reduction reaction are deposited on the inner surface of porous TiO₂ nanotubes, while CoO_x nanoclusters acting as hole collectors and active sites for oxidation reaction are deposited on the outer surface of porous TiO₂ nanotubes. A CoO_x/TiO₂/Pt photocatalyst, comprising ultra‐low concentrations of noble Pt (0.046 wt %) and CoO_x (0.019 wt %) deposited simultaneously with one atomic layer deposition cycle, achieves remarkably high photocatalytic efficiency (275.9 μmol h⁻¹), which is nearly five times as high as that of pristine TiO₂ nanotubes (56.5 μmol h⁻¹). The highly dispersed Pt and CoO_x nanoclusters, porous structure of TiO₂ nanotubes with large specific surface area, and the synergetic effect of the spatially separated Pt and CoO_x dual cocatalysts contribute to the excellent photocatalytic activity.     

Porous Tantalum Nitride Single Crystal at Two‐Centimeter Scale with Enhanced Photoelectrochemical Performance

Porous tantalum nitride (Ta₃N₅) single crystals, combining structural coherence and porous microstructure, would substantially improve the photoelectrochemical performance. The structural coherence would reduce the recombination of charge carriers and maintain excellent transport properties while the porous microstructure would not only reduce photon scattering but also facilitate surface reactions. Here, we grow bulk‐porous Ta₃N₅ single crystals on a two‐centimeter scale with (002), (023), and (041) facets, respectively, and show significantly enhanced photoelectrochemical performance. We show the preferential facet growth of porous crystals in a lattice reconstruction strategy in relation to lattice match and lattice channel. We present the facet engineering to enhance light absorption, exciton lifetime and transport properties. The porous Ta₃N₅ single crystal boosts photoelectrochemical oxidation of alcohols with the (002) facet showing the highest performance of >99 % alcohol conversion and >99 % aldehyde/ketone selectivity.     

Tuning the Electron Localization of Gold Enables the Control of Nitrogen‐to‐Ammonia Fixation

The (photo)electrochemical N₂ reduction reaction (NRR) provides a favorable avenue for the production of NH₃ using renewable energy in mild operating conditions. Understanding and building an efficient catalyst with high NH₃ selectivity represents an area of intense interest for the early stages of development for NRR. Herein, we introduce a CoO_x layer to tune the local electronic structure of Au nanoparticles with positive valence sites for boosting conversion of N₂ to NH₃. The catalysts, possessing high average oxidation states (ca. 40 %), achieve a high NH₃ yield rate of 15.1 μg cm^?2 h^?1 and a good faradic efficiency of 19 % at ?0.5 V versus reversible hydrogen electrode. Experimental results and simulations reveal that the ability to tune the oxidation state of Au enables the control of N₂ adsorption and the concomitant energy barrier of NRR. Altering the Au oxidation state provides a unique strategy for control of NRR in the production of valuable NH₃.     

A Highly Efficient All‐Solid‐State Lithium/Electrolyte Interface Induced by an Energetic Reaction

The energetic chemical reaction between Zn(NO₃)₂ and Li is used to create a solid‐state interface between Li metal and Li_6.4La₃Zr_1.4Ta_0.6O₁₂ (LLZTO) electrolyte. This interlayer, composed of Zn, ZnLi_x alloy, Li₃N, Li₂O, and other species, possesses strong affinities with both Li metal and LLZTO and affords highly efficient conductive pathways for Li⁺ transport through the interface. The unique structure and properties of the interlayer lead to Li metal anodes with longer cycle life, higher efficiency, and better safety compared to the current best Li metal electrodes operating in liquid electrolytes while retaining comparable capacity, rate, and overpotential. All‐solid‐state Li||Li cells can operate at very demanding current–capacity conditions of 4 mA cm^?2–8 mAh cm^?2. Thousands of hours of continuous cycling are achieved at Coulombic efficiency >99.5 % without dendrite formation or side reactions with the electrolyte.     

Synthesis of well‐defined star‐shaped poly(ε‐caprolactone)/poly(ethylbene glycol) amphiphilic conetworks by combination of ring opening polymerization and “click” chemistry

Well‐defined star‐shaped hydrophobic poly(ε‐caprolactone) (PCL) and hydrophilic poly(ethylene glycol) (PEG) amphiphilic conetworks (APCNs) have been synthesized via the combination of ring opening polymerization (ROP) and click chemistry. Alkyne‐terminated six arm star‐shaped PCL (6‐s‐PCLx‐C?CH) and azido‐terminated PEG (N₃‐PEG‐N₃) are characterized by ¹H NMR and FT‐IR. The swelling degree of the APCNs is determined both in water and organic solvent. This unique property of the conetworks is dependent on the nanophase separation of hydrophilic and hydrophobic phases. The morphology and thermal behaviors of the APCNs are investigated by SEM and DSC respectively. The biocompatibility is determined by water soluble tetrazolium salt reagents (WST‐1) assay, which shows the new polymer networks had good biocompatibility. Through in vitro release of paclitaxel (PTX) and doxorubicin (DOX), the APCNs is confirmed to be promising drug depot materials for sustained hydrophobic and hydrophilic drugs. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 407–417     

Na0.74Ta3O6, ein niedervalentes Oxotantalat mit einer Ta–Ta‐Mehrfachbindung

Na_0.74Ta₃O₆, a Low‐Valent Oxotantalate with Multiple Ta–Ta Bonds The title compound was prepared in a sealed tantalum tube through the reaction of Ta₂O₅, tantalum and Na₂CO₃ in a NaCl flux at 1570 K within 5 d. The crystal structure of Na_0.74Ta₃O₆ (a = 713.5(1), b = 1027.4(2), c = 639.9(1) pm, Immm, Z = 4) was determined by single crystal X‐ray means. The structure is isomorphous with NaNb₃O₅F [1]. The characteristic structural units are triply bonded Ta1₂ dumb‐bells with eight square‐prismatically co‐ordinated O ligands. Four Ta2, each octahedrally surrounded by O atoms, are side‐on bonded weakly to the binuclear Ta₂O₈ complex, thus forming a Ta₆ propellane‐like cluster. The lattice parameters of three additional M_xTa₃O₆ phases, M = Li, Mn, and Yb, are reported.     

The chemistry and thermal stability of HfTaO/Si interface by x‐ray photoelectron spectroscopy

The chemistry and thermal stability of HfTaO/Si interface as a function of annealing temperature have been investigated by x‐ray photoelectron spectroscopy. For the as‐deposited sample, the formation of Hf‐silicate bond is observed on Hf 4f core‐level spectra, which contributes to bulk HfO₂ and SiO₂. Besides, the suboxide of tantalum (Ta⁺¹) is formed at the interface at room temperature because of oxygen‐deficient conditions. HfSi₂, Hf_xSi_yO_4, and HfO₂ coexists in interfacial region at 850 °C, meanwhile, an evidence for transforming from the Ta¹⁺ to tantalum oxide (Ta⁵⁺) is verified. The peaks of Hf–O–Si and Hf–O have disappeared, only one peak of Hf silicide remained after the annealing at 950 °C. A stable SiO₂ phase in HfTaO/Si is formed under different annealing conditions. Copyright © 2011 John Wiley & Sons, Ltd.     

Defect‐Rich Co−CoOx‐Graphene Nanocatalysts for Efficient Hydrogen Production from Ammonia Borane

Chemical hydrogen storage ammonia borane has attracted extensive attention as a method of efficient utilization of hydrogen energy. The high‐efficiency catalysts are the main factor restricting the hydrogen production of ammonia borane. In this paper, the synergistic effect of Co and CoO_x supported on graphene (named Co?CoO_x@GO‐II) promotes the efficient hydrogen production of ammonia borane, and its catalytic hydrogen production rate can reach 5813 mL min^?1 g_Co^?1 at 298 K, the corresponding TOF is 15.33 min^?1. After five stability tests, Co?CoO_x @GO‐II maintained 65% of its original catalytic performance. The synergy of metal and metal oxide and the defects in the atomic arrangement ensure the catalytic activity, the large specific surface area of graphene ensures the dispersion and fixation. This strategy may provide a possibility to design high‐performance transition metal catalysts.