One‐Pot Synthesis of Co(OH)2‐ and/or Co3O4‐Decorated Cobalt‐Doped ZnO Nanorod Arrays and Their Potential as (Photo‐)Anode Materials

Metal oxides like zinc oxide (ZnO) are particularly promising materials to be used in core technologies for the generation and storage of clean energy such as batteries, photovoltaics or solar fuel production and solar water splitting. The deposition of the electrode materials used in these applications should be as cost effective as possible, while maintaining good device characteristics. Here, a simple low‐temperature solution‐based deposition method is reported that allows for the growth of high surface area, cobalt‐doped ZnO nanorod‐arrays decorated with cobaltic over‐coatings. Control over the visible light absorption and the nature of the cobaltic over‐coating (e. g. Co(OH)2 and/or Co3O4) can be achieved by changing the growth parameters during the one‐pot synthesis. Focusing on the evaluation of the underlying growth principles and resulting material properties, the study discusses the crucial role of the organic growth modifier used (monoethanolamine) as a complexing‐, growth‐directing‐ and reducing agent. Furthermore, the (solar driven) oxidation of water is taken as an example reaction in order to gain further insight into the functionality of the structures. Accounting for the temperature dependent breakdown of metal‐amine complexes, a two‐stage growth mechanism is proposed that will allow the optimization of the resulting structures for individual applications, but which could also prove valuable for other metal‐oxide/hydroxide material combinations. The visible light absorption of ZnO:Co nanorod arrays as well as their decoration with Co(OH)2 or Co3O4 is controlled by the solution chemistry applied during the herein presented low‐temperature one‐pot two‐step hydrothermal growth method. These low‐cost absorber/catalyst structures are investigated electrochemically, taking the (photo‐)oxidation of water as an example reaction.