Investigating and correlating photoelectrochemical, photocatalytic, and antimicrobial properties of TiO 2 nanolayers

Abstract

Semiconducting transition metal oxides such as TiO 2 are promising photo(electro)catalysts for solar water splitting and photoreduction of CO 2 as well as for antibacterial, self-, water and air-cleaning coatings and admixtures in paints, building materials, on window glass or medical devices. In photoelectrocatalytic applications TiO 2 is usually used as photoanode only catalyzing the oxidation reaction. In coatings and admixtures TiO 2 works as heterogeneous catalyst and has to catalyze a complete redox cycle. While photoelectrochemical charge transport parameters are usually quite well accessible by electrochemical measurements, the quantitative description of photocatalytic properties is more challenging. Here, we present a systematic structural, photoelectrocatalytic, photocatalytic and antimicrobial study to understand if and how photoelectrochemical parameters can be used to predict the photocatalytic activity of TiO 2. For this purpose TiO 2 thin films on flourine-doped tin oxide substrates were prepared and annealed at temperatures between 200 and 600 ∘C. The film morphologies and thicknesses 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 degradation and Escherichea coli inactivation and correlated with the deduced photoelectrocatalytic parameters. We found that the respective photoactivities of amorphous and crystalline TiO 2 nanolayers can be best correlated, if the extracted photoelectrochemical parameters such as charge transfer and recombination rates, charge transfer efficiencies and resistances are measured close to the open circuit potential (OCP). Hence, the interfacial charge transport parameters at the OCP can be indeed used as descriptors for predicting and understanding the photocatalytic activity of TiO 2 coatings.