Stable and sustainable photoanodes using zinc oxide and cobalt oxide chemically gradient nanostructures for water-splitting applications.

Amorphous cobalt oxide (CoO) encapsulated zinc oxide (ZnO) nanostructures were developed by adopting three low-temperature methods respectively atomic layer deposition, chemical bath deposition, and electrochemical deposition. The impact of CoO growth on the physical and chemical properties of ZnO nanostructures was investigated. Then, the ZnO/CoO core/shell nanostructures grown under optimized conditions were adopted for the fabrication of photoelectrochemical (PEC) water-splitting devices. The catalytic performance of ZnO nanostructures is substantially improved after their encapsulation with CoO layers. In addition, the chemical stability and durability of the structures are significantly enhanced. Under typical measurement conditions, these surface-modified ZnO nanostructures exhibited incident photon to charge carrier conversion efficiency (IPCE) higher than 16%, and a stable photocurrent density of 1.25 mA cm-2. Further, these ZnO/CoO nanostructured photoanodes showed a high illumination to dark current density ratio, ~2910.