Spectroelectrochemical characterization of euhedral anatase TiO2 crystals – Implications for photoelectrochemical and photocatalytic properties of {001} {100} and {101} facets

Abstract

Tailored crystals of anatase TiO2 have been synthesized using a hydrothermal method with hydrofluoric acid and sodium fluoride as capping agents. Titanium dioxide in the form of sheets, tetragonal truncated bipyramids, cuboids and belts offer different exposed facets: {001}, {001}/{101}, {001}/{100} and {100}, respectively. The strongest oxygen adsorption was observed for crystals with exposed {001} facets. Adsorbed fluorides also improve oxygen adsorption. Our spectroelectrochemical (SE-DRS) measurements of density of electronic states (DOS) revealed that the energy of conduction band edge increases in the order: {101}<{100}<{001}. Fluoride ions significantly influence the electronic states distribution, which in the case of sheets introduce new low energy electronic states. In the case of tetragonal truncated bipyramids the effect is opposite: low energy states present in fluoride-free materials disappear upon F− adsorption. Moreover, grain boundaries formed between crystals introduce new low energy electronic states, which can act as deep electron traps. Photocurrent measurements have shown that the oxidation of methanol occurs most efficiently at TiO2 sheets offering mainly 001 facets. The photocatalytic oxidation of terephthalic acid (test for HO generation) also depends on the exposed facets and increases in the order: cuboids, sheets, tetragonal truncated bipyramids, belts. Furthermore, fluoride ions drastically decrease the photocatalytic production of TAOH.