Photocatalytic integral water splitting (OWS) is a simple, economical, clean, and sustainable method for hydrogen production. Although some wide-bandgap semiconductors react very actively to OWS under UV irradiation, there is an urgent need to develop narrow-bandgap semiconductors in order to achieve efficient solar energy conversion. It is important to note that many narrow bandgap semiconductors are quite active to the hydrolysis half-reactive but remain inert to OWS, despite having a suitable band-edge position for the redox reaction of water.
Early reports have shown that LaTiO2N is a highly active photocatalyst that can undergo water splitting semi-reactions with the aid of sacrificial agents. Despite such high activity in the hydrolysis half-reaction, OWS on LaTiO2N has not yet been realized. In particular, LaTiO2N consists only of cheap elements with low toxicity and crystallizes in a simple perovskite structure, which is technical-economically competitive for large-scale production. Therefore, it is necessary to find strategies to enhance the OWS activity of TiO2N photocatalysts.
Recently, Xiaoxiang Xu's group at Tongji University has prepared a LaTiO2N monocrystalline nanosheet by topological conversion method, which has excellent photocatalytic OWS reaction ability and an initial wave of up to 600 nm. These LaTiO2N monocrystalline nanosheets have low concentrations of Ti3+ defects and a controlled defect distribution. According to the PL emission data, the energy level of Ti3+ is 0.25 eV lower than that of CB, and the Ti3+ defects on the surface form a shallow energy level state, which can easily capture electrons from CB for surface reduction reaction, and the photogenerated holes are driven to the surface due to the upward bending of the VB edge.
Considering the small thickness of the nanosheets, based on the above mechanism, a large number of photogenerated carriers can reach the surface, thereby increasing the OWS activity of LaTiO2N. The concomitant VO defects are detrimental to OWS activity due to the formation of deep energy level states by VO defects, which are essentially photogenerated carrier recombination centers.
The experimental results show that the prepared LaTiO2N exhibits significant wavelength-dependent activity against OWS and remains active at 600 nm, and the apparent quantum efficiency (AQE) of LaTiO2N for OWS is as high as 0.42% at 420±20n m. LaTiO2N also exhibited good durability and increased OWS activity under AM 1.5 G (100 mW cm-2) irradiation, with a solar-hydrogen (STH) conversion efficiency of 0.023%, which was quite competitive among the reported photocatalysts (still active at 600 nm).
It is worth noting that the activity of the photocatalytic OWS reaction is achieved without a water oxidation cocatalyst, and the photocatalytic activity can be further improved by loading an appropriate water oxidation cocatalyst. Overall, this work demonstrates the feasibility of constructing tunable defects on photocatalysts to improve activity, and provides guidance for the future design of OWS narrow bandgap semiconductors.
Single-crystalline LaTiO2N nanosheets with regulated defects for photocatalytic overall water splitting under visible light up to 600 nm. ACS Catalysis, 2023. DOI: 10.1021/acscatal.3c04743