Investigating and Correlating Photoelectrochemical, Photocatalytic, and Antimicrobial Properties of TiO2 Nanolayers

Semiconducting transition metal oxides such as TiO2 are promising photo(electro)catalysts for solar water splitting and photoreduction of CO2. Titania admixtures are also used in paints and building materials or as coating on window glass and medical devices, giving the modified materials antimicrobial, self-or even air-cleaning properties. Although TiO2 is an effective catalyst for all these applications, it is mechanistically important to distinguish between photoelectrocatalytic, photocatalytic and antimicrobial processes. In the former, TiO2 is usually electrically contacted as photoanode, i.e. only the oxidation reaction takes place at the titania surface. In the two latter applications, TiO2 works as heterogeneous catalyst and has to catalyze a complete redox cycle. The underlying common and diverging rate-determining photochemical and photoelectrochemical mechanisms are still not well understood. Here, we thus present a systematic structural, photoelectrocatalytic, photocatalytic and antimicrobial study to directly compare and correlate these properties. We prepared TiO2 thin films on flourine-doped tin oxide (FTO) substrates by a sol-gel spin-coating technique. The materials were annealed at temperatures between 200 and 600°C and their morphologies were studied by GIXRD, FESEM and EDX. Photoelectrochemical properties were measured by linear sweep voltammetry, photoelectrochemical impedance spectroscopy, chopped light chronoamperometry, and intensity modulated photocurrent/ photovoltage spectroscopy. For comparison, photocatalytic rate constants were determined by methylene blue and Escherichea coli degradation and correlated with the deduced photoelectrocatalytic parameters.