Fabrication of Single-Crystalline BaTaO₂N from Chloride Fluxes for Photocatalytic H₂ Evolution under Visible Light

Abstract

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.

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

“…13b ), and XPS analysis (Supplementary Fig. 11 ), RbCl flux-assisted nitridation process enabled the formation of well-crystallized BaTaO 2 N with minimized defects (reduced Ta 5+ species and anion vacancies) 24 , 34 . Moreover, the amount of alkali metal ions incorporated in the BaTaO 2 N materials during the flux-assisted nitridation became less with the increase of their ionic radii (Supplementary Table 5 and Supplementary Fig.…”

“…Preparation of Pt-modified BaTaO 2 N. The preparation of BaTaO 2 N, loading of Pt nanoparticles on BaTaO 2 N, and the photocatalytic H 2 production and Z-scheme water splitting, are described in detail in the subsections of "Methods". Briefly, BaTaO 2 N was synthesized by one-pot nitridation of a BaCO 3 and Ta 2 O 5 mixture with the assistance of molten RbCl or other alkali chlorides (NaCl, KCl, or CsCl) fluxes at 1223 K for 8 h under a flow of gaseous NH 3 (200 mL min −1 ) 24,34 . BaTaO 2 N prepared by using RbCl, NaCl, KCl, and CsCl fluxes are denoted as BaTaO 2 N (RbCl), BaTaO 2 N (NaCl), BaTaO 2 N (KCl), and BaTaO 2 N (CsCl), respectively.…”

“…The materials properties of BaTaO 2 N (RbCl) were examined because BaTaO 2 N (RbCl) exhibited greater H 2 evolution activity than BaTaO 2 N synthesized by using the other alkali chloride fluxes when being decorated with Pt by photodeposition 24 , 34 . The X-ray diffraction pattern (Supplementary Fig.…”

“…BaTaO 2 N powder was synthesized by flux-assisted one-pot nitridation 24 , 34 . Ta 2 O 5 (99.9%; Kojundo Chemical Laboratory Co., Ltd.) and BaCO 3 (99.9%; Kanto Chemical Co., Inc.) were mixed at a Ta:Ba molar ratio of 1:1.1.…”

“…Synthesis of BaTaO 2 N particulate photocatalyst. BaTaO 2 N powder was synthesized by flux-assisted one-pot nitridation24,34 . Ta 2 O 5 (99.9%; Kojundo Chemical…”