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, Bi_xO_yBr_z 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.