Photocatalytic activity of pulsed laser deposited TiO2 thin films

Nanostructured TiO2 thin films were prepared by pulsed laser deposition (PLD) on indium-doped tin oxide (ITO) substrates. Results from X-ray photoelectron spectroscopy (XPS) show that Ti 2p core level peaks shift toward the lower binding energy with decrease in the buffer gas pressure (O2:Ar = 1:1). This suggests that oxygen vacancies are created under insufficient oxygen conditions. Anatase-to-rutile ratio is also found to be system pressure dependent. Under deposition pressure of 100 Pa, only anatase phase was observed even at 1073 K substrate temperature which is much higher that the bulk anatase-to-rutile phase transformation temperature. The deposited TiO2 thin films were fabricated as photoanodes for photoelectrochemical (PEC) studies. PEC measurements on TiO2 photoanodes show that the flatband potential (Vfb) increases by 0.088 eV on absolute vacuum energy scale (AVS) with decrease in the deposition pressure from 100 to 33 Pa at 873 K. The highest incident photon to current conversion efficiency [IPCE(λ)] of 2.5 to 6% at λ = 320 nm was obtained from the thin films prepared at substrate temperature of 873 K. Combining the results from XPS and PEC studies, we conclude that the deposition pressure affects the concentration of the oxygen vacancies which changes the electronic structure of the TiO2. With reference to photoelectrochemical catalytic performance, our results suggest that it is possible to adjust the Fermi energy level and structure of TiO2 thin films by controlling the buffer gas pressure and temperature to align the energy of the flatband potential (Vfb) with respect to specific redox species in the electrolyte.