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.

Ta N 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 Ta N film photoanodes owing to successful synthesis of the vital TaO precursors. An effective GaN coating strategy is developed to remarkably stabilize Ta N by forming a crystalline nitride-on-nitride structure with an improved nitride/electrolyte interface. A stable, high photocurrent density of 8 mA cm was obtained with a CoPi/GaN/Ta N photoanode at 1.2 V under simulated sunlight, with O and H 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.

Low charge separation efficiencies are regarded as obstacles that limit the improvement in the photocatalytic performance of BaTaO2N. In this study, we demonstrated that the anisotropic facets ({100} and {110} facets) of BaTaO2N for efficient spatial charge separation were successfully constructed using the one-pot flux-assisted nitridation approach. As a result, the photocatalytic activity for H2 production on BaTaO2N with coexposed {100} and {110} facets was nearly 10-fold over that of BaTaO2N with only {100} facets and that of the conventional irregularly shaped sample. This finding provides an innovative approach to the development of efficient (oxy)­nitride photocatalysts for solar energy conversion.

BaTaO 2 N is an important oxynitride photocatalyst for the splitting of water under visible light. However, the preparation of BaTaO 2 N crystals with low defect densities and high crystallinities is challenging via ammonolysis at high temperatures. Thus, we herein report the preparation of BaTaO 2 N crystals using an NH 3 -assisted flux method. We initially studied the effects of the flux type (RbCl, CsCl, and BaCl 2 •2H 2 O) on the morphology, crystallinity, and photocatalytic performance of the BaTaO 2 N crystals. It was found that the BaTaO 2 N crystals grown using a RbCl flux exhibited a significantly higher photocatalytic H 2 evolution rate than those grown using the other two fluxes. In particular, the observed activity was almost twice as high as that reported over BaTaO 2 N crystals grown using a KCl flux. Such an excellent activity could be attributed to the lower defect density and higher crystallinity, which were confirmed by UV−vis diffuse reflectance spectroscopy and composition analysis. Subsequently, the effects of the source ratio, solute concentration, reaction temperature, and time on the crystallinity and morphology of the BaTaO 2 N crystals grown using a RbCl flux were studied systematically to gain insight into the BaTaO 2 N crystal growth mechanism.

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