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.     

Twisted Surfaces in Porous Single Crystals to Deliver Enhanced Catalytic Activity and Stability

Porous single crystals which combine ordered lattice structures and disordered inter‐connected pores would provide an alternative to create twisted surface in porous microstructures. Now, transition‐metal nitride Nb₄N₅ and MoN single crystals are grown on a 2 cm scale to create well‐defined active structures at twisted surfaces. High catalytic activity and stability toward non‐oxidative dehydrogenation of ethane to ethylene is observed. Unsaturated metal–nitrogen coordination structures including Nb‐N_1/5, Nb‐N_2/5, Mo‐N_1/3, and Mo‐N_1/6 at the twisted surface mainly account for the C?H activation with chemisorption of H in molecular ethane at the twisted surface, which not only improves dehydrogenation performance but also avoids the deep cracking of ethane to enhance coking resistance. 11–25 % ethane conversion and 98–99 % ethylene selectivity is demonstrated without degradation being observed even after the operation of 50 hours.     

Charge Carrier Transport Mechanism in Ta2O5, TaON,and Ta3N5 Studied by Applying Polaron Hopping and Bandlike Models

TaON and Ta₃N₅ are considered promising materials for photocatalytic and photoelectrochemical water splitting. In contrast, their counterpart Ta₂O₅ does not exhibit good photocatalytic performance. This may be explained with the different charge carrier transport mechanisms in these materials, which are not well understood yet. Herein, we investigate the charge transport properties in Ta₂O₅, TaON, and Ta₃N₅ by polaron hopping and bandlike models. First, the polaron binding energies were calculated to evaluate whether the small polaron occurs in these materials. Then we performed calculations to localize the excess carriers as small polarons using a hybrid density functional. We find that the small polaron hopping is the charge transfer mechanism in Ta₂O_5, whereas our calculations indicate that this mechanism may not occur in TaON and Ta₃N₅. We also investigated the bandlike model mechanism by calculating the charge carrier mobility of these materials using the effective mass approximation, but the calculated mobility is not consistent with experimental results. This study is a first step towards understanding charge transport in oxynitrides and nitrides and furthermore establishes a simple rule to determine whether a small polaron occurs in a material.     

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.     

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.     

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.     

Tungsten doping of Ta3N5-nanotubes for band gap narrowing and enhanced photoelectrochemical water splitting efficiency

Ordered W-doped Ta₂O₅ nanotube arrays were grown by self-organizing electrochemical anodization of TaW alloys with different tungsten concentrations and by a suitable high temperature ammonia treatment, fully converted to W:Ta₃N₅ tubular structures. A main effect found is that W doping can decrease the band gap from 2 eV (bare Ta₃N₅) down to 1.75 eV. Ta₃N₅ nanotubes grown on 0.5 at.% W alloy and modified with Co(OH)_x as co-catalyst show ~ 33% higher photocurrents in photoelectrochemical (PEC) water splitting than pure Ta₃N₅.     

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.     

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.     

In situ Observation of Porosity Formation in Porous Single-crystalline TiO2 Monolith for Enhanced and Stable Catalytic CO Oxidation

Introducing pores in single crystals creates a new type of porous materials that incorporate porosity and structural coherence. Herein, we use in situ transmission electron microscopy to disclose the porosity formation by converting KTiOPO₄ (KTP) single crystals into porous single-crystalline (PSC) TiO₂ monoliths in a solid-solid transformation. The isolated crystalline nuclei of TiO₂ clusters with identical lattice orientation on KTP surface moves TiO₂/KTP interface toward mother phase for growing PSC TiO₂ monoliths. The relative density in PSC TiO₂ monoliths dominates porosity while the macroscopic dimensions remain unchanged in the transformation. The single-crystalline nature of porous architecture stabilizes oxygen vacancy to activate lattice oxygen while the three-dimensional percolation enhances species diffusion. PSC TiO₂ monoliths with deposited Pt clusters show enhanced and stable catalytic CO oxidation in air at ∼75 °C for 200 hours of operation.     

Coordinatively Unsaturated Metal–Nitrogen Active Sites at Twisted Surfaces in Metallic Porous Nitride Single Crystals Delivering Enhanced Electrocatalysis Activity

To create active sites on surfaces, the identification of structural features that could confine the local-defect structure in the lattice is required. Porous nitride single crystals, combining the advantages of porosity and structural coherence, provide the possibility to create coordinatively unsaturated metal–nitrogen active sites confined on surfaces. For the first time, ordered active sites and tailor the atomically resolved Fe−N and Co−N local structures are created through control of the unsaturated nitrogen coordination at twisted surfaces in porous single-crystalline Fe_nN (n=2–4) and Co_nN (n=1–3) nanocubes. The precise tailoring of the electronic structures of these coordinatively unsaturated active sites therefore engineer the catalytic activity. Optimum electrocatalysis performances are observed with the porous Fe₄N and Co₃N nanocubes with highly unsaturated nitrogen coordination for selective nitrate reduction to ammonia and nitrobenzene amination to aminobenzene, while the structural coherence of these porous nitride single crystals delivers excellent durability.     

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.     

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.     

Determination of the Ta and Nb ratio in LiNb1−xTaxO3 by total reflection X-ray fluorescence spectrometry

A procedure for evaluation of the mass ratio between Ta and Nb in LiNb_1−xTa_xO₃ single crystals by total reflection X-ray fluorescence spectrometry (TXRF) is described. The developed procedure does not require chemical preparation of the samples and it proves to be fast (30 min for sample preparation and 10 min for analysis), precise (down 1% RSD) and easy to perform. To check the methodology, the lattice parameter of each LiNb_1−xTa_xO₃ sample was determined by powder X-ray diffraction. While the a hexagonal lattice parameter remains constant, a strong and linear dependence of the c hexagonal lattice parameter with Ta content in molar percentage has been observed for the first time.     

Fluorine‐Doped Porous Single‐Crystal Rutile TiO2 Nanorods for Enhancing Photoelectrochemical Water Splitting

Fluorine‐doped hierarchical porous single‐crystal rutile TiO₂ nanorods have been synthesized through a silica template method, in which F^? ions acts as both n‐type dopants and capping agents to make the isotropic growth of the nanorods. The combination of high crystallinity, abundant surface reactive sites, large porosity, and improved electronic conductivity leads to an excellent photoelectrochemical activity. The photoanode made of F‐doped porous single crystals displays a remarkably enhanced solar‐to‐hydrogen conversion efficiency (≈0.35 % at ?0.33 V vs. Ag/AgCl) under 100 mW cm^?2 of AM=1.5 solar simulator illumination that is ten times of the pristine solid TiO₂ single crystals.     

Crystallization mechanism and microstructure evolution of Li2O–Al2O3–SiO2 glass-ceramics with Ta2O5 as nucleating agent

Li₂O–Al₂O₃–SiO₂ glass-ceramics were prepared with Ta₂O₅ as nucleating agent, the crystallization mechanism and microstructure evolution were investigated by DTA, XRD, and SEM technologies. With increasing amount of Ta₂O₅ from 2 to 6 mol%, the crystallization activation energy decreased from 297.73 to 218.66 kJ mol^?1, while the crystallization index increased from 1.76 to 3.39. In addition, the cluster of dendritic crystals and lamellar structure obtained in T-2 glass-ceramics indicated a typical two-dimensional crystallization mechanism, and the formation of spherical β-quartz solid solution in T-4 specimens, with average size of 50–70 nm, was mainly due to bulk crystallization mechanism. It was considered that Ta₂O₅ promoted the nucleation and crystallization of LAS glass by precipitating the crystalline precursor phase of Ta₂O₅, which acted as nuclei for the subsequent crystal growth. Eventually, the diffusion and crystallization process, microstructure morphology, as well as the secondary grain growth were also investigated.     

Synthese und Struktur von M4Ta18O53 (M = Th,U); die ersten Vertreter eines neuen Strukturtyps à la Jahnberg

Synthesis and Structure of M₄Ta₁₈O₅₃ (M = Th, U); the First Representatives with a New Structure Type à la Jahnberg Colorless {brownblack}, hexagonal crystals of Th₄Ta₁₈O₅₃ {U₄Ta₁₈O₅₃} were obtained by chemical transport. Several mixtures of powders were used as starting materials for chemical transport reactions. Cl₂ or TaCl₅/Cl₂ were applied as transport agents in several temperature gradients. The lattice constants are a = 6.2554(5) Å {6.253(1) Å}, c = 27.709(5) Å {27.609(6) Å} and Z = 1. Structure determination in the space group P6₃/mcm (no. 193) let to R1 = 0.0269 (wR2 = 0.0629) {R1 = 0.0295 (wR2 = 0.0674)}. Thorium as well as Uran is surrounded by oxygen like an trans-bis-capped octahedron (CN = 8) and Tantalum1 and Tantalum2 like pentagonal-bipyramids (CN = 7). The formula (Th₂O₃)₂(Ta₃O₈)₆ corresponds to the principle of building for the ideal construction. Because of necessarity of valence compensation the number of O-atoms in the real structure is diminished by one. The Th? O- and U? O-coordination polyhedra are for themselves arranged to o-layers and the Ta1? O- and Ta2? O-bipyramids for each other to p-layers. These layers are edge linked along the c-axes with a sequence of p-p-p-o-p-p-p-o. In fact of this these compounds are the first examples of the new structure type p-p-p-o as a member of a structure-family first described by Jahnberg. The various members differ in the sequence of layers with octahedral (o) and pentagonal-bipyramids (p).     

Crystal growth of a series of lithium garnets Ln3Li5Ta2O12 (Ln=La, Pr, Nd): Structural properties, Alexandrite effect and unusual ionic conductivity

We report the single crystal structures of a series of lanthanide containing tantalates, Ln₃Li₅Ta₂O₁₂ (Ln=La, Pr, Nd) that were obtained out of a reactive lithium hydroxide flux. The structures of Ln₃Li₅Ta₂O₁₂ were determined by single crystal X-ray diffraction, where the Li⁺ positions and Li⁺ site occupancies were fixed based on previously reported neutron diffraction data for isostructural compounds. All three oxides crystallize in the cubic space group (No. 230) with lattice parameters a=12.7735(1), 12.6527(1), and 12.5967(1) Å for La₃Li₅Ta₂O₁₂, Pr₃Li₅Ta₂O₁₂, and Nd₃Li₅Ta₂O₁₂, respectively. A UV-Vis diffuse reflectance spectrum of Nd₃Li₅Ta₂O₁₂ was collected to explain its unusual Alexandrite-like optical behavior. To evaluate the transport properties of Nd₃Li₅Ta₂O₁₂, the impedance data were collected in air in the temperature range 300?T(°C)?500.     

High-Density and Thermally Stable Palladium Single-Atom Catalysts for Chemoselective Hydrogenations

Single-atom catalysts (SACs) have shown superior activity and/or selectivity for many energy- and environment-related reactions, but their stability at high site density and under reducing atmosphere remains unresolved. Herein, we elucidate the intrinsic driving force of a Pd single atom with high site density (up to 5 wt %) under reducing atmosphere, and its unique catalytic performance for hydrogenation reactions. In situ experiments and calculations reveal that Pd atoms tend to migrate into the surface vacancy-enriched MoC surface during the carburization process by transferring oxide crystals to carbide crystals, leading to the surface enrichment of atomic Pd instead of formation of particles. The Pd₁/α-MoC catalyst exhibits high activity and excellent selectivity for liquid-phase hydrogenation of substituted nitroaromatics (>99 %) and gas-phase hydrogenation of CO₂ to CO (>98 %). The Pd₁/α-MoC catalyst could endure up to 400 °C without any observable aggregation of single atoms.     

Interface Engineering of a CoOx/Ta3N5 Photocatalyst for Unprecedented Water Oxidation Performance under Visible‐Light‐Irradiation

Cocatalysts have been extensively used to promote water oxidation efficiency in solar‐to‐chemical energy conversion, but the influence of interface compatibility between semiconductor and cocatalyst has been rarely addressed. Here we demonstrate a feasible strategy of interface wettability modification to enhance water oxidation efficiency of the state‐of‐the‐art CoO_x/Ta₃N₅ system. When the hydrophobic feature of a Ta₃N₅ semiconductor was modulated to a hydrophilic one by in situ or ex situ surface coating with a magnesia nanolayer (2–5 nm), the interfacial contact between the hydrophilic CoO_x cocatalyst and the modified hydrophilic Ta₃N₅ semiconductor was greatly improved. Consequently, the visible‐light‐driven photocatalytic oxygen evolution rate of the resulting CoO_x/MgO(in)–Ta₃N₅ photocatalyst is ca. 23 times that of the pristine Ta₃N₅ sample, with a new record (11.3 %) of apparent quantum efficiency (AQE) under 500–600 nm illumination.