Composite Photocatalysts Containing BiVO4 for Degradation of Cationic Dyes

The creation of composite structures is a commonly employed approach towards enhanced photocatalytic performance, with one of the key rationales for doing this being to separate photoexcited charges, affording them longer lifetimes in which to react with adsorbed species. Here we examine three composite photocatalysts using either WO3, TiO2 or CeO2 with BiVO4 for the degradation of model dyes Methylene Blue and Rhodamine B. Each of these materials (WO3, TiO2 or CeO2) has a different band edge energy offset with respect to BiVO4, allowing for a systematic comparison of these different arrangements. It is seen that while these offsets can afford beneficial charge transfer (CT) processes, they can also result in the deactivation of certain reactions. We also observed the importance of localized dye concentrations, resulting from a strong affinity between it and the surface, in attaining high overall photocatalytic performance, a factor not often acknowledged. It is hoped in the future that these observations will assist in the judicious selection of semiconductors for use as composite photocatalysts.

ammonium hydroxide (NH4OH) solution. The vanadium precursor solution was slowly added to the bismuth nitrate solution under stirring over 30 min. Then 3 M ammonia solution was added drop wise until pH 7 was attained. The resultant precipitate was washed with deionized water, centrifuged and dried at 60°C for 12 h.
Finally, the dried powder was then calcined at 450°C for 2 h to obtain BiVO4 powder.
After that, 25 mL of 3 M ammonium hydroxide (NH4OH) was slowly added into the above solution, and the transparent solution changed to a yellowish suspension. The suspension was kept under stirring at 50°C for a further 24 h and the precipitate was finally collected by centrifugation, washed 3 times with deionized water and then dried at 60°C for 24 h. The obtained powder was subsequently calcined at 450°C for 2 h. The as-synthesized composites were characterized and actual the ratio of BiVO4:CeO2 was confirmed by elemental analysis of Bi (from to BiVO4) and Ce (CeO2). For control experiments, pure CeO2 was also prepared by the procedure described above without the BiVO4 addition step, which this synthetic method have been previously reported by our group. 27

Preparation of TiO2 and TiO2/BiVO4 composite powder
BiVO4/TiO2 nanocomposite catalysts with different mole ratios (4:1, 3:2, 1:1, 2:3 and 1:4.) were synthesized by coupling a precipitation and sol-gel methods. Firstly, pure BiVO4 powder was synthesized as mention above. A sol-gel method of anatase TiO2 synthesis, reported by Wetchakun et al. , 28 was used here. Firstly, 20 mL titanium tetraisopropoxide (TTIP, Sigma-Aldrich) was dissolved in 250 mL 6 M nitric acid solution and mixed until a homogeneous solution was obtained. The mixture of TTIP and nitric acid solution was put into a cellophane membrane and then placed in solution containing a 1:1 v/v ratio (350 mL) of absolute ethanol and deionized water with 0.5-1.0 vol% concentrated (25%) ammonia. The BiVO4 powder was subsequently added to the above mixture in the cellophane pouch to synthesize BiVO4/TiO2 composite powders. The mixture inside the cellophane pouch was kept stirring with magnetic stirrer bar and heated to 80°C for 1 h.
After the completion of the dialysis process, the suspension was centrifuged at 5000 rpm for 10 min, washed with deionized water and then dried in an oven at 60°C for 24 h. The obtained powder was finally calcined in a furnace at the temperature of 450°C for 2 h. For control experiments, pure TiO2 photocatalyst was also synthesized by the procedure described above, without any BiVO4 added.

Preparation of WO3 and BiVO4/WO3 composite powder
BiVO4/WO3 nanocomposite photocatalysts with different mole ratios between were synthesized by a twostep precipitation processes, because the BiVO4 powder is not stable in acidic conditions, WO3 powder was first synthesized by dissolving 5 g of sodium tungstate dihydrate (Na2WO4·2H2O, Sigma-Aldrich) in a

Figure S13
Absorption spectra of RhB after 24h in the dark where an adsorption/desorption equilibrium is obtained. Figure S14 pH dependence of zeta potentials of BiVO4, CeO2, TiO2 and WO3 in aqueous solutions.