Cobalt-doped CdS quantum dots enhanced photoelectroreduction of CO₂ to formic acid with high selectivity

Excessive carbon dioxide (CO₂) emission has caused problems associated with environmental pollution and climate deterioration. As a consequence, the selective conversion of CO₂ into liquid fuels by artificial photosynthesis has gained increasing attention. However, the rational design of photocathode to achieve selective CO₂ photoelectroreduction is challenging. Here, we sensitized cuprous oxide (p-nCu₂O) loaded on hydroxyl iron oxide (FeOOH) with cobalt-doped cadmium sulfide (Co:CdS) quantum dots to prepare a novel photocathode FeOOH/p-nCu₂O/Co:CdS by sequential electrodeposition and chemical bath deposition. The composite photocathode exhibited a larger photovoltage, which is 1.9 times higher than the pristine counterpart, and was efficient for CO₂ reduction to produce formic acid with high selectivity of up to 82.9% (Faradaic efficiency). Theoretical calculations revealed that the photocathode out-layer Co:CdS quantum dots had increased binding energy toward the key intermediate *OOCH through additional hybridization orbitals to exclusively favor the formation of formic acid. An impurity energy level was revealed to form by doping Co to the CdS-containing composite, which could reduce the photocathode band gap with improved absorption toward visible light, thus remarkably increasing the photoelectrochemical properties. This is the first work undertaking the energy band structure optimization of the photocathode enabled by elemental doping to improve its photoelectrocatalytic performance.