Effect of Ti Doping on the Photoelectrochemical Water Splitting Efficiency of WO3 Photoanode
Authors: Kalanur Shankara S , Seo Hyungtak 
Published: February 2020

Photocatalytic/photo-electrochemical (PEC) water splitting is a process whereby H2O is split into H2 and O2 using sunlight. This solar to chemical conversion (specifically, solar-energy-driven H2 production) is an ambitious yet crucial objective for futuristic production of eco-friendly energy that is storable and transportable and can be efficiently converted into electricity using fuel cells whenever necessary. Most importantly, this process is believed to lay the foundation for a sustainable hydrogen-based energy economy, which represents a carbon-neutral approach for producing hydrogen gas using the most abundant renewable resources: water and sunlight. WO3, an n-type semiconductor, has been extensively studied as a photoanode for PEC water splitting because of its advantages such as non-toxicity, low cost, and chemical stability in acidic aqueous media. Furthermore, WO3 has a moderate hole diffusion length (~150 nm) compared to other oxides such as Fe2O3 (2–4 nm) and TiO2 (~100 nm), with inherently good electron transport properties. After absorbing a portion of the visible light during illumination, WO3 generates electron–hole pairs and its valence band edge can provide enough potential for the oxidation of water since it is located at approximately 3.0 eV versus NHE. Several strategies have been implemented to enhance the photocatalytic properties of WO3, such as morphological control, transition metal doping, noble metal loading, surface sensitization, and formation of composite materials. In particular, doping and co-doping are possible ways of tailoring the electronic band structure as well as the PEC properties of WO3. Therefore, photo-electrochemical properties of tungsten oxide (WO3) can be tailored by doping foreign atoms from which conduction band (CB)/valence band (VB) edge positions can be altered. Band edge engineering for WO3 is essential since VB maximum of WO3 is more positive more the photo-oxidation potential of H2O but CB minimum of WO3 is lower than the reduction potential of H2O to limit spontaneous solar water reduction. In view of this, here we propose a simple, single step and efficient strategy for band edge tailoring of WO3 by Ti doping for efficient solar water splitting. The physical properties of synthesized Ti doped WO3 thin films were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM), energy-dispersive X-ray analysis (EDS), UV-Vis absorption spectra and X-ray photoelectron spectroscopy (XPS). The incorporation of Ti into WO3 reduces the crystallinity and increases the band gap of WO3. From XPS valence band analysis, we confirm that, substitution Ti into WO3 leads to slight shift up the CB maximum upwards more closed to the H+/H2 redox potential without any change in VB maximum. Compared to un-doped WO3, Ti doped WO3 exhibited an unprecedented photocurrent of 1.6 mA cm-2 (at 0.8 V vs Ag/AgCl) under simulated 1.5 AM sunlight without an added water oxidation catalyst due to the improved electrical conductivity and reaction kinetics after Ti doping. The method presented here, demonstrates a simple but systematic and efficient approach for the design and fabrication of band edge tailored WO3 photoanodes via Ti doping efficient for photo-electrochemical water splitting.

Figure 1