Construction of Spatial Charge Separation Facets on BaTaO2N Crystals by Flux Growth Approach for Visible-Light-Driven H2 Production

Enhanced Photocatalytic O2 Evolution over Layered Perovskite Oxyiodide Ba2Bi3Nb2O11I through Flux Synthesis and Surface Modifications

Sillén-Aurivillius oxyiodides, particularly Ba2Bi3Nb2O11I with double-perovskite layers, are promising photocatalysts for visible-light-driven water splitting due to their excellent light absorption and carrier transport properties. However, efforts to enhance their photocatalytic performance through advancements in synthesis methods or surface modifications remain limited. Here, we report the flux synthesis of Ba2Bi3Nb2O11I and the optimization of cocatalyst loading. Single-phase Ba2Bi3Nb2O11I was successfully synthesized using molten alkali iodide salts under appropriate reaction conditions. The heating rate during the synthesis significantly influenced crystallinity and carrier lifetime, as shown by time-resolved microwave conductivity measurements. By optimizing the reaction conditions to enhance crystallinity (prolong carrier lifetime), the flux-synthesized sample exhibited a higher sacrificial O2 evolution rate than that prepared via the conventional solid-state reaction. Furthermore, precise control over the loading conditions of the iron-ruthenium oxide cocatalyst ((Fe,Ru)Ox) significantly enhanced nonsacrificial O2 evolution from an aqueous Fe3+ solution. Electrochemical analysis revealed that the tuned loading conditions enhanced the catalytic activity of the (Fe,Ru)Ox cocatalyst for both water oxidation and Fe3+ reduction. Finally, Z-scheme water splitting using the optimized (Fe,Ru)Ox-loaded Ba2Bi3Nb2O11I photocatalyst showed superior efficiency than that using the previously reported unoptimized sample. This study provides valuable insights into enhancing the O2 evolution activity of oxyiodide photocatalysts for water-splitting applications.

Elucidation of the Ta(O,N)6 Octahedral Arrangement in Flux-Grown BaTaO2N Photocatalysts by Experimental and Computational Structural Modeling

BaTaO2N (BTON) is a visible-light-responsive photocatalyst used for water splitting. The flux method, which involves the use of a molten salt, is an effective synthetic strategy for achieving a high photocatalytic activity. Fluxes with alkali metal cations strongly affect the photocatalytic activity of the BTON crystals. In particular, RbCl flux-grown BTON exhibits hydrogen evolution activity over several times higher than that grown in other chloride fluxes, such as NaCl, under visible-light irradiation. One factor of this difference is presumably owing to the change in the Ta(O,N)6 octahedral arrangement in the BTON triggered by the doping of alkali metal ions into the crystal lattice. However, the precise Ta(O,N)6 octahedral arrangement remains unclear. Herein, X-ray absorption spectroscopy, structural analysis by neutron diffraction, and computational structural modeling based on comprehensive structural energy predictions were performed for two types of BTONs. The results suggested that the configuration manners of Ta(O,N)6 octahedral units strongly depend on the flux composition. Specifically, in NaCl-flux-grown BTON crystals, the number of N20 cis planes parallel to the (100), (010), and (001) crystal planes in a TaO4N2 unit is anisotropic, resulting in differences in their electron-hole conduction characteristics. The findings indicate that in addition to lattice defects, the interconnections of mixed-anion units such as Ta(O,N)6 should be taken into account to improve the photocatalytic activity of BTONs and develop other mixed-anion compounds.

Untangling the Effect of Carbonaceous Materials on the Photoelectrochemical Performance of BaTaO2N

The water oxidation reaction is a rate-determining step in solar water splitting. The number of surviving photoexcited holes is one of the most influencing factors affecting the photoelectrochemical water oxidation efficiency of photocatalysts. The solar-to-hydrogen energy conversion efficiency of BaTaO2N is still far below the benchmark efficiency set for practical applications, notwithstanding its potential as a 600 nm-class photocatalyst in solar water splitting. To improve its efficiency in photoelectrochemical water splitting, this study offers a straightforward route to develop photocatalytic materials based on the combination of BaTaO2N and carbonaceous materials with different dimensions. The impact of diverse carbonaceous materials, such as fullerene, g-C3N4, graphene, carbon nanohorns, and carbon nanotubes, on the photoelectrochemical behavior of BaTaO2N has been examined. Notably, the use of graphene and g-C3N4 remarkably improves the photoelectrochemical performance of the composite photocatalysts through a higher photocurrent and acting as electron reservoirs. Consequently, a marked reduction in recombination rates, even at low overpotentials, leads to a higher accumulation of photoexcited holes, resulting in 2.6- and 1.7-fold increased BaTaO2N photocurrent densities using graphene and g-C3N4, respectively. The observed trends in the dark for the oxygen reduction reaction (ORR) potential align with the increase in the photocurrent density, revealing a good correlation between opposite phenomena. Importantly, the enhancement observed implies an underlying accumulation phenomenon. The verification of this concept lies in the evidence provided by oxygen reduction and is in line with photoredox flux matching during photocatalysis. This research underscores the intricate interplay between carbonaceous materials and oxynitride photocatalysts, offering a strategic approach to enhancing various photocatalytic capabilities.

Perovskite BaTaO2 N: From Materials Synthesis to Solar Water Splitting

Barium tantalum oxynitride (BaTaO2 N), as a member of an emerging class of perovskite oxynitrides, is regarded as a promising inorganic material for solar water splitting because of its small band gap, visible light absorption, and suitable band edge potentials for overall water splitting in the absence of an external bias. However, BaTaO2 N still exhibits poor water-splitting performance that is susceptible to its synthetic history, surface states, recombination process, and instability. This review provides a comprehensive summary of previous progress, current advances, existing challenges, and future perspectives of BaTaO2 N for solar water splitting. A particular emphasis is given to highlighting the principles of photoelectrochemical (PEC) water splitting, classic and emerging photocatalysts for oxygen evolution reactions, and the crystal and electronic structures, dielectric, ferroelectric, and piezoelectric properties, synthesis routes, and thin-film fabrication of BaTaO2 N. Various strategies to achieve enhanced water-splitting performance of BaTaO2 N, such as reducing the surface and bulk defect density, engineering the crystal facets, tailoring the particle morphology, size, and porosity, cation doping, creating the solid solutions, forming the heterostructures and heterojunctions, designing the photoelectrochemical cells, and loading suitable cocatalysts are discussed. Also, the avenues for further investigation and the prospects of using BaTaO2 N in solar water splitting are presented.

Transition-metal (oxy)nitride photocatalysts for water splitting

Solar-driven water splitting based on particulate semiconductor materials is studied as a technology for green hydrogen production. Transition-metal (oxy)nitride photocatalysts are promising materials for overall water splitting (OWS) via a one- or two-step excitation process because their band structure is suitable for water splitting under visible light. Yet, these materials suffer from low solar-to-hydrogen energy conversion efficiency (STH), mainly because of their high defect density, low charge separation and migration efficiency, sluggish surface redox reactions, and/or side reactions. Their poor thermal stability in air and under the harsh nitridation conditions required to synthesize these materials makes further material improvements difficult. Here, we review key challenges in the two different OWS systems and highlight some strategies recently identified as promising for improving photocatalytic activity. Finally, we discuss opportunities and challenges facing the future development of transition-metal (oxy)nitride-based OWS systems.

Metallic Powder Promotes Nitridation Kinetics for Facile Synthesis of (Oxy)Nitride Photocatalysts

Nitrogen-containing semiconductors (including metal nitrides, metal oxynitrides, and nitrogen-doped metal oxides) have been widely researched for their application in energy conversion and environmental purification because of their unique characteristics; however, their synthesis generally encounters significant challenges owing to sluggish nitridation kinetics. Herein, a metallic-powder-assisted nitridation method is developed that effectively promotes the kinetics of nitrogen insertion into oxide precursors and exhibits good generality. By employing metallic powders with low work functions as electronic modulators, a series of oxynitrides (i.e., LnTaON2 (Ln = La, Pr, Nd, Sm, and Gd), Zr2 ON2 , and LaTiO2 N) can be prepared at lower nitridation temperatures and shorter nitridation periods to obtain comparable or even lower defect concentrations compared to those of the conventional thermal nitridation method, leading to superior photocatalytic performance. Moreover, some novel nitrogen-doped oxides (i.e., SrTiO3- x Ny and Y2 Zr2 O7- x Ny ) with visible-light responses can be exploited. As revealed by density functional theory (DFT) calculations, the nitridation kinetics are enhanced via the effective electron transfer from the metallic powder to the oxide precursors, reducing the activation energy of nitrogen insertion. The modified nitridation route developed in this work is an alternative method for preparing (oxy)nitride-based materials for energy/environment-related heterogeneous catalysis.

Flux-assisted synthesis of tungsten-doped layered perovskite oxychloride with promoted visible-light-responsive O2 evolution performance

Here, we successfully prepared Ba₂Bi₃Ta₂O₁₁Cl via a simple one-step molten salt method and adjusted its crystal morphology and structure, based on which the O₂-evolving activity was significantly improved. In addition, W doping promotes the charge separation efficiency, lowers the energy barrier for water oxidation reaction, and thus improves the activity. Finally, the optimized W-doped sample after molten salt treatment shows the best O₂ production activity (55 μmol h^-1) without loading any cocatalyst, which is 6 times higher than that of pristine Ba₂Bi₃Ta₂O₁₁Cl and 2 times higher than that of the undoped Ba₂Bi₃Ta₂O₁₁Cl treated with molten salt, respectively.

Manipulation of charge carrier flow in Bi4NbO8Cl nanoplate photocatalyst with metal loading

Separation of photoexcited charge carriers in semiconductors is important for efficient solar energy conversion and yet the control strategies and underlying mechanisms are not fully established. Although layered compounds have been widely studied as photocatalysts, spatial separation between oxidation and reduction reaction sites is a challenging issue due to the parallel flow of photoexcited carriers along the layers. Here we demonstrate orthogonal carrier flow in layered Bi4NbO8Cl by depositing a Rh cocatalyst at the edges of nanoplates, resulting in spatial charge separation and significant enhancement of the photocatalytic activity. Combined experimental and theoretical studies revealed that lighter photogenerated electrons, due to a greater in-plane dispersion of the conduction band (vs. valence band), can travel along the plane and are readily trapped by the cocatalyst, whereas the remaining holes hop perpendicular to the plane because of the anisotropic crystal geometry. Our results propose manipulating carrier flow via cocatalyst deposition to achieve desirable carrier dynamics for photocatalytic reactions in layered compounds.

First-principles calculations of the structural, energetic, electronic, optical, and photocatalytic properties of BaTaO2N low-index surfaces

We present here the influence of different surface terminations on the electronic, optical, and photocatalytic properties of trans and cis BaTaO2N using density functional theory calculations.

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.

The origins of charge separation in anisotropic facet photocatalysts investigated through first-principles calculations

It was recently discovered that the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) can be completed on the {110} and {001} facets, respectively, of a 18-facet SrTiO₃ mono-crystal. The effective charge separation is attributed to the facet junction at the interface between two arbitrary anisotropic crystal planes. Theoretical estimation of the built-in potential at the facet junction can greatly improve understanding of the mechanism. This work employs density functional theory (DFT) calculations to investigate such potential at the (110)/(100) facet junction in SrTiO₃ crystals. The formation of the facet junction is verified by a calculated work function difference between the (110) and (100) planes, which form p-type and n-type segments of the junction, respectively. The built-in potential is estimated at about 2.9 V. As a result, with the ultra high built-in potential, electrons and holes can effectively transfer to different anisotropic planes to complete both photo-oxidative and photo-reductive reactions.

Photocatalytic and Photoelectrochemical Hydrogen Evolution from Water over Cu2SnxGe1-xS3 Particles

Cu₂Sn_xGe_1-xS₃ (CTGS) particles were synthesized via a solid-state reaction and assessed, for the first time, as both photocatalysts and photocathode materials for hydrogen evolution from water. Variations in the crystal and electronic structure with the Sn/Ge ratio were examined experimentally and theoretically. The incorporation of Ge was found to negatively shift the conduction band minimum, such that the bandgap energy could be tuned over the range 0.77-1.49 eV, and also increased the driving force for the photoexcited electrons involved in hydrogen evolution. The effects of the Sn/Ge ratio and of Cu deficiency on the photoelectrochemical performance of Cu₂Sn_xGe_1-xS₃ and Cu_ySn_0.38Ge_0.62S₃ (1.86 < y < 2.1) based photocathodes were evaluated under simulated sunlight. Both variations in the band-edge position and the presence of a secondary impurity phase affected the performance, such that a particulate Cu_1.9Sn_0.38Ge_0.62S₃ photocathode was the highest performing specimen. This cathode gave a half-cell solar-to-hydrogen energy conversion efficiency of 0.56% at 0.18 V vs a reversible hydrogen electrode (RHE) and an incident-photon-to-current conversion efficiency of 18% in response to 550 nm monochromatic light at 0 V_RHE. More importantly, these CTGS particles also demonstrated significant photocatalytic activity during hydrogen evolution and were responsive to radiation up to 1500 nm, representing infrared light. The chemical stability, lack of toxicity, and high activity during hydrogen evolution of the present CTGS particles suggest that they may be potential alternatives to visible/infrared light responsive Cu-chalcogenide photocatalysts and photocathode materials such as Cu(In,Ga)(S,Se)₂ and Cu₂ZnSnS₄.

Sequential cocatalyst decoration on BaTaO2N towards highly-active Z-scheme water splitting

Oxynitride photocatalysts hold promise for renewable solar hydrogen production via water splitting owing to their intense visible light absorption. Cocatalyst loading is essential for activation of such oxynitride photocatalysts. However, cocatalyst nanoparticles form aggregates and exhibit weak interaction with photocatalysts, which prevents eliciting their intrinsic photocatalytic performance. Here, we demonstrate efficient utilization of photoexcited electrons in a single-crystalline particulate BaTaO₂N photocatalyst prepared with the assistance of RbCl flux for H₂ evolution reactions via sequential decoration of Pt cocatalyst by impregnation-reduction followed by site-selective photodeposition. The Pt-loaded BaTaO₂N photocatalyst evolves H₂ over 100 times more efficiently than before, with an apparent quantum yield of 6.8% at the wavelength of 420 nm, from a methanol aqueous solution, and a solar-to-hydrogen energy conversion efficiency of 0.24% in Z-scheme water splitting. Enabling uniform dispersion and intimate contact of cocatalyst nanoparticles on single-crystalline narrow-bandgap particulate photocatalysts is a key to efficient solar-to-chemical energy conversion.

Visible-Light-Responsive Oxyhalide PbBiO2Cl Photoelectrode: On-Site Flux Synthesis on a Fluorine-Doped Tin Oxide Electrode

The performance of photoelectrodes is hugely affected by the preparation method. Although a flux synthesis is useful to endow semiconductor particles with the desired properties such as high crystallinity, there are only a few reports on its application to photoelectrode fabrication, probably because relatively high temperatures are necessary. In the present study, we introduce a new concept for on-site flux synthesis of semiconductor crystals on a commonly used fluorine-doped tin oxide (FTO) substrate; a seed layer is predeposited and then treated with an appropriate flux containing other required elements at a right temperature lower than the limit temperature of FTO but sufficiently high to transform the seed layer to the target material with the aid of flux. Here, an oxyhalide PbBiO₂Cl, one of the promising semiconductors for achieving visible-light water splitting, is selected as a target material. Combination of a BiOCl seed layer and the NaCl-PbCl₂ flux containing other precursors enables the seed layer to transform into PbBiO₂Cl crystals even at 450 °C. The thickness of the PbBiO₂Cl layer can be controlled by changing the thickness of the BiOCl seed layer for efficient photon-to-current conversion. Owing to a good contact at the semiconductor-substrate interfaces as well as the high quality of PbBiO₂Cl crystals, the flux-synthesized PbBiO₂Cl photoelectrode shows a significantly improved PEC performance compared with those prepared from the particulate PbBiO₂Cl samples via the conventional squeegee method. In addition, the present PbBiO₂Cl photoelectrodes exhibit both anodic and cathodic photoresponses with substantially high current values depending on the applied potentials; the unusual phenomenon is affected by the conditions in flux-assisted synthesis. The present study provides a new and effective way for fabricating efficient photoelectrodes of various semiconductors on various substrates and a possible option to control their morphologies and p/n types for further improvement in performance.

Flower-like MoS2 microspheres compounded with irregular CdS pyramid heterojunctions: highly efficient and stable photocatalysts for hydrogen production from water

An irregular CdS pyramid/flower-like MoS₂ microsphere composite photocatalyst was successfully synthesized using a simple one-step hydrothermal method. The as-prepared samples were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, ultraviolet visible absorption spectroscopy, fluorescence spectroscopy and photoelectrochemical tests. The composite photocatalysts showed superior photocatalytic activities for hydrogen evolution from water under visible light irradiation (λ ≥ 420 nm) with an extremely high apparent quantum yield (AQY = 64.8%) at 420 nm. To our knowledge, this value is the highest reported efficiency value for CdS/MoS₂ photocatalysts. Further detailed characterization revealed that the special structure for some CdS pyramid structures dispersed in the MoS₂ microsphere structures and surrounded by MoS₂ nanosheets led to the photogenerated electrons migrating from the conduction band of different faces of the CdS pyramid to the conduction band of different MoS₂ nanosheets while photogenerated holes remained in the CdS pyramid structures, which greatly promoted the separation of photogenerated electrons and holes, improving the photoactivity of the CdS/MoS₂ catalyst. The catalyst also exhibited perfect stability, and the photoactivity displayed no significant degradation during continuous hydrogen production over nearly 70 h.

Schematic diagram of the photogenerated carrier migration between CdS and MoS₂.

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.

Ni(OH)2-modified SrTiO3 for enhanced photocatalytic hydrogen evolution reactions

Strontium titanate (SrTiO₃) is a promising photocatalyst because of its high chemical stability and excellent photocatalytic activity.

Shallow Trap State-Induced Efficient Electron Transfer at the Interface of Heterojunction Photocatalysts: The Crucial Role of Vacancy Defects

Constructing vacancies has been demonstrated to be an effective way to modulate charge flow in semiconductor photocatalysts. However, the role of vacancies in the interfacial electron transfer (IET) of heterojunction photocatalysts remains poorly understood, which hinders the general design of heterojunction photocatalysts. Herein, by taking g-C₃N₄/MoS₂ as a heterojunction photocatalyst prototype, we unravel that vacancies play a critical role in the IET of heterojunction photocatalysts. Theoretical simulations, combined with femtosecond time-resolved diffuse reflectance spectroscopy, give a clear physical picture that N vacancy states act as shallow trap states (STSs) for photogenerated electrons and thereby facilitate the IET process due to a large energy difference between STSs and charge separation states. Moreover, the excess electrons left by the loss of N atoms (producing N vacancies) could partially transfer to MoS₂ to generate STSs in the forbidden band of MoS₂, where the transferred photogenerated electrons could be further trapped to efficiently drive H₂ evolution. This work suggests a promising strategy to tune IET of heterojunction photocatalysts for achieving highly efficient photocatalytic reactions.