  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 750 mTorr 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 750 to 250 mTorr at 873K. The highest incident
photon to current conversion efficiency [IPCE(lambda)] of 2.5 to 6 % was
obtained from the thin films prepared at substrate temperature of 873K.
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.
