Harvesting energy directly from the sun offers an ideal approach towards fulfilling the need for clean and renewable energy with minimal environmental impact. Photoelectrochemical (PEC) water splitting using semiconductors as both light absorber and energy converter, to store solar energy in the simplest chemical bond, H2, is one of the most efficient approaches, since this approach requires no other energy input except sunlight. Even though extensive research has been carried out since Titanium dioxide (TiO2) was first used as a photocatalyst in the early 1970s, a high efficiency, stable and low-cost PEC water splitting cell has not yet been fully realised.
To address the challenges impeding the efficiency of TiO2, we investigated the creation of surface defects in TiO2 inverse opal (IO) structures to enhance PEC performance. Surface states such as Ti3+ and oxygen vacancies have been reported to reduce the bandgap of TiO2 and prevent recombination of photogenerated carriers. The underlying interconnected TiO2 porous network of IO, facilitates diffusion of the electrolyte as well as improves charge transport. As measured by PEC performance, a four-fold increase in the photocurrent density was observed. Electrochemical impedance studies and Mott-Schottky analyses were used to evaluate the charge transfer between the porous electrode and electrolyte interface. Future studies will be devoted to coupling TiO2 IO with narrow bandgap semiconductors such as hematite (bandgap 2.1 eV) and tantalum oxynitride (tunable bandgap from 1.9 eV to 2.5 eV) to form heterojunctions, which together with the slow photon effect of IO to enhance photon absorption, may provide an alternative strategy to improve overall water splitting performance.