Immobilization of Cobalt-Based Polyoxometalate on Titanium Dioxide Nanorods for Photoelectrochemical Water Oxidation

The production of hydrogen from water as a source of renewable energy is a crucial technology to overcome the energy crisis and world pollution issues. However, there are challenges due to its non-thermodynamically favorable reaction and the complex electron transfer process. Titanium dioxide (TiO₂) has been widely used as a photocatalyst due to its high stability and activity towards light radiation. It is considered one of the promising types of catalysts that can catalyze the water oxidation reaction under light radiation, but its performance is often hampered by the fast recombination of photogenerated electron-holes pairs. Polyoxometalates (POMs), a class of transition metal oxides, with good redox properties, tunable structure and high durability could act as an effective hole scavenger layer to enhance the activity of TiO₂towards water oxidation. In this study, a nanocomposite film, consisting of TiO₂and cobalt-based POMs, was synthesized by the growth of TiO₂ on a conductive fluorine-doped tin oxide (FTO) substrate <i>via</i> the hydrothermal technique. This step was followed by layer-by-layer deposition of POMs. The photoelectrochemical water oxidation test showed that the onset potential of POMs coated TiO₂photoanode was lower. The induced photocurrent at the applied potential of 1.23 V <i>vs.</i> reversible hydrogen electrode (RHE) was approximately two times that of pristine TiO₂. POMs layers would reduce the recombination rate of electron-hole pairs by extracting and transporting the generated holes, thus improving the overall PEC performance. The photoelectrochemical response was further enhanced with increasing POMs layers, but there was an optimal number beyond which the produced current density began to decline owing to light penetration limitation. In term of durability, the generated photocurrent density remained relatively stable throughout the test period with minor fluctuations that could be caused by the formation and evolution of oxygen bubbles. This work provides a facile layer-by-layer deposition method to immobilize layers of POMs on TiO₂ to promote PEC water oxidation under neutral conditions, which is attractive for long-term applications.