Electrostatically assembled CdS-Co₃O₄ nanostructures for photo-assisted water oxidation and photocatalytic reduction of dye molecules

Water oxidation catalysts are promising materials for running artificial photosynthesis. In Nature, the oxygenevolving center (OEC) of photosystem II uses water as an electron source, which is the major source of carbon fixation on Earth. Nature's unique mechanism of oxidizing water to oxygen while using the generated electrons to reduce other molecules such as quinones has been extensively explored. While some approaches to mimic photosynthesis have used natural photosystems extracted from plants, other methods have used the entire thylakoid membrane to shuttle electrons to electrodes and produce electrical energy or fuels. However, due to some of the inherent difficulties of using biological systems in non-natural environments, a wide variety of photoresponsive organic and inorganic materials coupled with specific catalysts have also been tested. With the advent of nanotechnology and new materials and architectures, a wide range of inorganic nanostructures have emerged as catalysts for water oxidation, including semiconductors such as TiO₂, CdS, and BiVO₄. However, while semiconductors should theoretically be able to oxidize water under photoillumination, kinetic barriers have often hindered their ability to run complete oxidation-reduction reactions processes to produce higher energy products. Although there has been one recent breakthrough that reported a single cobalt catalyst (CoO) that could simultaneously oxidize water and reduce protons to hydrogen in solution, which has been difficult to achieve with most other materials. Furthermore, many semiconductor nanostructures have often shown rapid degradation upon continuous exposure to light due to oxidation of surface ions by hole migration.