Synthesis of BaTaO₂N oxynitride from Ba-rich oxide precursor for construction of visible-light-driven Z-scheme overall water splitting

Abstract

Barium tantalum oxynitride (BaTaON) with an absorption edge of ca. 660 nm is one of the most promising photocatalysts for solar water splitting, and is usually synthesized by nitriding a mixture of Ba and Ta-containing compounds with a Ba/Ta molar ratio of unity under ammonia flow at high temperature, usually causing a high density of defect sites. Herein, we introduce a novel synthesis method for BaTaON (BTON) by employing Ba-rich LiBaTaO, prepared by a flux method, as a precursor of nitridation. As a comparison, BaTaO was prepared by conventional solid state reaction and used as the precursor. The as-nitrided samples were correspondingly denoted as BTON-Flux and BTON-SSR. It was found that well-crystallized BTON oxynitride can be similarly obtained by both methods, but the BTON-Flux sample exhibits significantly decreased defect density and enhanced surface area relative to the BTON-SSR sample. As a result of their structural differences, the photocatalytic water splitting performance of the BTON-Flux sample, regardless of the H-evolving half reaction in the presence of methanol or Z-scheme overall water splitting, is much better than that of BTON-SSR. This study may open up a novel strategy for preparing oxynitride photocatalyst with decreased defect density for the promotion of solar water splitting.

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

“…The UV-vis diffuse reflectance spectrum ( Supplementary Fig. 1b) demonstrates a light absorption edge at 650 nm, which is characteristic of BaTaO 2 N. The background absorption beyond 650 nm is negligible owing to the low concentration of reduced Ta 5+ species and anion vacancies 22,23 . The BaTaO 2 N was composed of cuboid particles smaller than 500 nm in size, as indicated in the scanning electron microscopy (SEM) image ( Supplementary Fig.…”

“…Perovskite-type BaTaO 2 N with a bandgap of around 1.8 eV is a photocatalyst material that has been intensively studied for Z-scheme water splitting 15,16,22,23 , as well as H 2 or O 2 evolution half-reactions using sacrificial reagents 15,[24][25][26][27] (see Supplementary Table 2). The BaTaO 2 N photocatalyst intrinsically exhibits a weak driving force for surface redox reactions 14 , and loading of cocatalysts such as nanoparticulate Pt is essential to promote the extraction of photogenerated charge carriers from BaTaO 2 N for efficient H 2 evolution 22,23,[28][29][30][31][32][33] . However, Pt loaded by conventional impregnation or photodeposition methods tends to form aggregates on BaTaO 2 N and to have weak contact with BaTaO 2 N particles, resulting in the inadequate formation of active catalytic sites and inefficient electron transfer.…”

“…However, Pt loaded by conventional impregnation or photodeposition methods tends to form aggregates on BaTaO 2 N and to have weak contact with BaTaO 2 N particles, resulting in the inadequate formation of active catalytic sites and inefficient electron transfer. In addition, BaTaO 2 N particles produced by thermal nitridation are generally polycrystalline and incorporate structural defects and midgap states that act as recombination and trapping centres for photogenerated electron-hole pairs 15,16,22,23,25 . Therefore, the efficiency of photocatalytic H 2 evolution on BaTaO 2 N needs to be boosted by utilizing a single-crystalline particulate photocatalyst and establishing a strategy to realize an active cocatalyst/photocatalyst reciprocal structure.…”

Defect Engineering for Photocatalysis: From Ternary to Perovskite Oxynitrides

“…reported the development of a layered Sr 2 Ta 2 O 7−x N x /SrTaO 2 N heterojunction by phase transformation of Sr 2 Ta 2 O 7 nanosheets via a thermal ammonolysis process . Whereas other reported oxynitride heterojunctions led to increased defects at the contact interface (despite an overall increased charge separation), Zeng et al. generated the Sr 2 Ta 2 O 7−x N x /SrTaO 2 N heterojunction by simultaneously intergrowing the SrTaO 2 N and Sr 2 Ta 2 O 7−x N x phases.…”

Development of Mixed‐Anion Photocatalysts with Wide Visible‐Light Absorption Bands for Solar Water Splitting

“…Several effective strategies have been successfully developed and adopted to tackle these problems. For example, newly synthesized BaTaO 2 N from Ba‐rich precursor oxides can effectively decrease the amount of recombination centers that originate from low‐valence metal cations, further improving the charge‐separation ability and Z‐scheme OWS performance . In addition, surface/interface modification, such as the choice of cocatalyst, surface protective layer, or the construction of a heterostructure, of the photocatalyst would be another effective approach to accelerate charge transfer.…”