Photoelectrochemical Water Splitting of Modified Tungsten Oxide Nanostructures via Vapour Deposition

Semiconductor photocatalysts have been used for water splitting for several decades. By utilizing solar energy, splitting water into H2 and O2 is considered to be a promising way for generating renewable energy. The aim of this project was to synthesize and study different nanostructured photoelectrode materials for photoelectrochemical (PEC) water splitting. Tungsten(VI) oxide (WO3) is a promising photoanode material which is active under visible light illumination. Flat WO3 films and nanostructured WO3 films were deposited via aerosol-assisted chemical vapour deposition (AACVD). The morphology, structure and PEC properties of the films were compared, with nanostructured WO3 films showing improved performance compared to flat WO3 films (~ 0.41 mA/cm2 ) with an average photocurrent density of ~ 0.68 mA/cm2 at 1.23 V vs. RHE (reversible hydrogen electrode). Nanostructured WO3 modified with Au, Ag and Pt nanoparticles (NPs) were deposited via AACVD on nanostructured WO3 films, and deposition parameters optimized. The band gap of metal NP-modified WO3 films appeared narrower than that for WO3 alone. The size, morphology and density of metal nanoparticles were controlled by deposition temperature and precursor amount with (0.1 mg) Au/WO3 and (100 μL) Ag/WO3 both having the highest photocurrent density amongst all samples at 0.96 mA/cm2 at 1.23 V (vs. RHE), which was 1.5 times that of undecorated WO3. Pt/WO3 showed 1.33 times PEC enhancement. Highly uniform, dense, amorphous TiO2 and Al2O3 films were synthesized via atomic layer deposition (ALD). Precursor vapour pressure, dose/purge times, gas flow rate and deposition temperature were optimised. The growth rate of TiO2 deposition was in the range of 0.5 – 0.7 Å/cycle. For Al2O3, a stable ALD process with a growth rate of 1 – 1.4 Å/cycle was observed. Heterojunction films composed of WO3, metal NPs, TiO2 and Al2O3 thin films were constructed together in specific sequence in order to optimise PEC water splitting performance. Ag/WO3 films with and without ALD layers had higher PEC performance and were more stable than undecorated WO3 films after 24 h, however the use of ALD layers wasn’t considered to be fully successful, and the parameters need further optimization