Photocatalytic Reduction of Bicarbonate to Formic Acid Using Hierarchical ZnO Nanostructures

Zinc oxide (ZnO) is an earth abundant, nontoxic, and low-cost material that has been used widely for photocatalytic water splitting, gas sensing, and dye degradation. In this study, several ZnO structures were tested for the photochemical reduction of bicarbonate to formic acid, an intermediate to methanol, a high-octane-number fuel with higher energy density than compressed hydrogen. The different ZnO morphologies studied included micron- and nanoparticulate ZnO, rods, wires, belts, and flowers. ZnO was also synthesized from the direct calcination of zinc acetate, which provided a cheap and large-scale synthesis method to produce ZnO. The photocatalytic efficiency of the synthesized ZnO was compared to commercial micron- and nanoparticulate ZnO, and was proven to be just as efficient. ZnO flowers, possessing the largest surface area of 12.9 m2/g, were found to be the most efficient reaching an apparent quantum efficiency (AQE) of 10.04 ± 0.09%, with a superior performance over commercial TiO2 (P25), a benchmark photocatalyst. This is the first study to compare different shapes and sizes of ZnO for bicarbonate reduction in an aqueous system with excellent photocatalytic performance.