Nanoparticles of CuBi ₂ O ₄ Function as Efficient and Stable Photocathode for Photoelectrochemical Water‐Splitting Reaction

Among narrow‐bandgap p‐type semiconductors, CuBi ₂ O ₄ (CBO) has received potential interest as a photocathode for photoelectrochemical (PEC) water‐splitting due to its acceptable bandgap of ∼1.8 eV, positive onset potential, and reasonable stability in aqueous media. Here, a novel, solution‐based two‐step method is developed for the synthesis of CBO with different nanoparticle (NP) sizes. An electrodeposition process, followed by calcination, is explored for the synthesis of CBO NPs. To synthesize CBO NPs of various sizes (∼215 to 120 nm), different Cu(II) precursors are explored. PEC performance indicates that the CBO of the smallest particle size can generate the highest photocurrent density, −1.27 mA cm ⁻² under an applied potential of 0.2 V versus reversible hydrogen electrode, which is nearly three times higher compared to the CBO, having a NP size of ∼215 nm. The enhanced PEC activity is mainly attributed to the increase in the surface‐to‐volume ratio of CBO. It is observed that with decreasing NP size, the carrier density increases; as a result, charge–transfer efficiency also varies. Finally, the optimized photocathode shows considerable stability in Na ₂ SO ₄ medium, generating unaltered photocurrent density over a 2 h study. In addition to water‐splitting, CBOs also exhibit efficient activity toward CO ₂ photoreduction.