The Influence of Physico-Chemical Properties of Bare Titania Powders Obtained from Various Synthesis Routes on Their Photo-Electrochemical Performance

Titania nanopowders prepared by different chemical routes as well as commercially available were investigated in terms of structure, chemistry, morphology, surface and optical properties by using various physical-chemical characterization techniques; these properties were related to their photoelectrochemical behaviour in a photo-electrolysis half-cell. Bare powders were used as photo-anodes without any doping treatment or addition of surface promoters and the photocurrent associated to the oxygen evolution rate was determined. This approach appears to provide suitable information about the powder quality for photo-electrochemical applications in the absence of any other modification or treatment. The investigated preparation procedures included sol-gel and combustion synthesis. Most of the powders showed an organic residue and the presence of contaminants both in the bulk and surface even if at a small level; moreover, only a few formulations showed a pure anatase phase. The highest photocurrent was obtained for the P25 commercial powder as a compromise of good chemical purity i.e. low occurrence of impurities acting as recombination or trapping centers for the photogenerated carriers and appropriate surface area/crystallite size characteristics associated with a suitable number of surface sites promoting the oxygen evolution under illumination. Good bulk and surface chemical purity, suitable surface area, proper particle size and appropriate morphology are among the most relevant properties that influence the photo-electrochemical behaviour.