Taoran Han, a Yajie Chen, a Guohui Tian,*a,b Jian-Qiang Wang, c Zhiyu Ren, a Wei Zhoua and Honggang Fu*

Oxygen generation is the key step for the photocatalytic overall water splitting and considered to be kinetically more challenging than hydrogen generation. Here, effective water oxidation catalyst of hierarchical FeTiO3-TiO2 hollow spheres are prepared via two-step sequential solvothermal processes and followed by thermal treatment. The existence of effective heterointerface and built-in electric field in the surface space charge region in FeTiO3-TiO2 hollow spheres plays a positive role in promoting the separation of photoinduced electron-hole pairs. Surface photovoltage, transient-state photovoltage, fluorescence and electrochemical characterization are used to investigate the transfer process of photoinduced charge carriers. The photogenerated charge carriers in the hierarchical FeTiO3-TiO2 hollow spheres with a proper molar ratio display much higher separation efficiency and longer lifetime than those in the FeTiO3 alone. Moreover, it is suggested that the hierarchical porous hollow structure can contribute to the enhancement of light utilization, surface active sites and material transportation through the framework walls. This specific synergy significantly contributes to the remarkable improvement of the photocatalytic water oxidation activity of the hierarchical FeTiO3-TiO2 hollow spheres under simulated sunlight (AM1.5).

Experimental section

Photocatalytic water oxidation test The photocatalytic O2 evolution from water was conducted in an online photocatalytic oxygen production system (AuLight, Beijing, CEL-SPH2N). For each reaction, 50 mg of catalyst powder was well dispersed in an aqueous solution (100 mL) containing AgNO3 (0.01 M) as an electron acceptor. The reaction was carried out by irradiating the suspension with light from a 300 W Xe lamp (AuLight, CEL-HXF-300，Beijing) lamp with a working current of 15 A. The wavelength of the incident light was controlled by applying some appropriate long-pass cutoff filters. Prior to the reaction, the mixture was deaerated by evacuation to remove O2 and CO2 dissolved in water. Gas evolution was observed only under photoirradiation, being analyzed by an on-line gas chromatograph (SP7800, TCD, molecular sieve 5 Å, N2 carrier, Beijing Keruida Limited). Apparent quantum yield (A.Q.Y.) was measured using a 420 nm, 450 nm and 520 nm band-pass filter and an irradiate-meter, and calculated according to the following equation:

Conclusions 

In summary, we have successfully synthesized hierarchical FeTiO3 - TiO2 hollow spheres via two-step sequential solvothermal processes and followed calcination. The as-prepared hierarchical FeTiO3 -TiO2 hollow spheres have shown enhanced photocatalytic water oxydation activity as well as excellent stability. This work confirms that the heterojunctions formed between these two kinds of semiconductors (FeTiO3 and TiO2 ) effectively promote the charge transfer. The photoinduced holes on the FeTiO3 rapidly localizes to the TiO2 , and therefore promoting the oxygen evolution ability compared to pure FeTiO3 sample. Our results simultaneously suggest that the particular hierarchical porous hollow structure plays a positive role in enhancement of light utilization, effective increase in catalyst surface active sites and material transportation. It is believed that these research results can provide useful information for designing an optimal semiconductor that would combine the abilities of dissociating water molecules and possessing a proper band gap that absorbs solar light while remaining high stability.