Controllable Preparation and Optimisation of Bi4O5Br2 for Photocatalytic Reduction of CO2 to CO

The use of photocatalytic CO2 reduction as a green technology has attracted the attention of scholars. Nevertheless, the lower visible-light utilisation and photocatalytic efficiency of catalysts remain a challenge. In this work, BixOyBrz photocatalysts were synthesised using a hydrothermal method by adjusting the molar ratio of Bi(NO3)3·5H2O and C19H42BrN (Bi:Br ratio) and the pH value of the precursor solution. The obtained samples were characterised, and the CO2 reduction performance was tested. The results showed that the phase composition for most of the samples was Bi4O5Br2, and BiOBr or Bi5O7Br was also confirmed in a small number of samples. Owing to the effects of pH and the Bi:Br ratio on the reaction process, BiOBr→Bi4O5Br2→Bi5O7Br transformation occurred. Acidic conditions are conducive to the formation of BiOBr. In alkaline environments, bismuth-rich Bi4O5Br2 or even Bi5O7Br easily forms. Bi4O5Br2 has self-assembled microsphere and irregular polyhedron morphologies. The polyhedron Bi4O5Br2 results in CO and CH4 yields of 10.34 μmol·g−1·h−1 and 1.86 μmol·g−1·h−1 in CO2 reduction, respectively. Although the microsphere Bi4O5Br2 has a maximum light absorption wavelength of 438 nm, the polyhedron Bi4O5Br2 has the best photocatalytic CO2 reduction performance and CO selectivity. This work describes the controllable preparation of Bi4O5Br2 at various pH values and Bi:Br ratios and the optimisation of its photocatalytic performance.