Synergistic Enhancement of H₂ and CH₄ Evolution by CO₂ Photoreduction in Water with Reduced Graphene Oxide-Bismuth Monoxide Quantum Dot Catalyst

Photocatalytic water splitting or CO₂ reduction is one of the most promising strategies for solar energy conversion into hydrogen-containing fuels. However, these two processes typically compete with each other, which significantly decreases the solar energy conversion efficiency. Herein, we report for the first time this competition can be overcome by modulation of reactive sites and electron transfer pathway of heterogeneous photocatalysts. As a prototype, BiO composite reduced graphene oxide quantum dots (RGO-BiO QDs) were synthesized, which can provide large amounts of photogenerated electrons as well as individual reactive sites for H⁺ and CO₂ reduction. The productivity of H₂, CH₄, and CO by the RGO-BiO QDs catalyst were 102.5, 21.75, and 4.5 μmol/(g·h), respectively, in pure water without the assistance of any cocatalyst or sacrificial agent. The apparent quantum efficiency at 300 nm reached to 4.2%, which is more than 10 times higher than that of RGO-TiO₂ QDs (0.28%) under the same conditions. In situ DRIFT, ESR, and photoelectrochemical studies confirmed that the unique circled electron transfer pathway (E_vb(BiO) → E_cb(BiO) → E_f(RGO) → E_Vo•(BiO)) and the large amount of separated different reactive sites are responsible for the highly efficient simultaneous H₂ evolution and CO₂ reduction performance.