Vapor phase deposited cobalt oxide: A highly versatile material system for photoelectrochemical water splitting

Dennis Zywitzki, Ruhr-University Bochum, Bochum, Germany
Dereje Taffa, Carl von Ossietzky University Oldenburg, Oldenburg, Germany
Jyeon Kim, Ruhr-University Bochum, Bochum, Germany
Michael Wark, Carl von Ossietzky University Oldenburg, Oldenburg, Germany
Radim Beranek, Ulm University, Ulm, Germany
Anjana Devi, Ruhr-University Bochum, Bochum, Germany

Spinel type cobalt oxide is a well-known oxygen evolution reaction (OER) catalyst and a p-type semiconductor with a direct bandgap of 2.07 eV and band positions suitable for the hydrogen evolution reaction (HER). Furthermore, the general chemical stability of the material, as well as the comparably low costs and toxicity render Co3O4 a highly promising candidate for energy related applications. While studies investigating the electrocatalytic properties are abundant,[1] the utilization of Co3O4 as a photoabsorber material is rarely reported.[2]

In two separate studies we successfully applied Co3O4 thin films in conjunction with TiO2 underlayers as absorber material for the HER and as electrocatalyst for the OER, respectively.[3] Co3O4/TiO2 composite thin film electrodes were deposited on fluorine doped tin oxide (FTO) substrates by newly developed atomic layer deposition (ALD) and chemical vapor deposition (CVD) processes and investigated with regard to their composition structure and morphology by means of XRD, RBS/NRA, SEM and XPS analyses. The deposits were found to be polycrystalline and phase pure Co3O4. Photoelectrochemical studies, such as chopped light voltammetry or electrochemical impedance spectroscopy, were conducted on samples deposited on FTO in standard three-electrode setups.

When applying a thin ALD layer of Co3O4 on 10 nm of plasma enhanced (PE-)ALD deposited TiO2 an increased photo current in the OER regime could be observed upon UV-light irradiation, proving its suitability as an OER catalyst when applied in conjunction with an absorber material. On the other hand however, a thicker (100 nm) CVD layer of Co3O4 showed minute photocurrents under visible light in the HER regime. With the introduction of PE-ALD TiO2 underlayer of 100 nm thickness a considerable increase in photocurrent was observed and attributed to enhanced charge carrier separation as a result of the generation of a p-n heterojunction, which was confirmed by photoluminescence measurements. These findings are one of the rare reports on Co3O4 as an absorber material and demonstrate the versatility of this material system for photoelectrochemical water splitting applications.

[1] a) A. T. Marshall et al, Electrocatalysis 2014, 5, 445; b) F. E. Osterloh et al, J. Mater. Chem. A 2014, 2, 9405.
[2] K. Sopian et al., RSC Adv. 2015, 5, 36820.
[3] a) A. Devi et al., Chem. Mater. 2017, 29, 5796; b) A. Devi et al., Inorganic chemistry 2018, 57, 5133.

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