Effect of the electrolyte cations on photoinduced charge transfer at TiO2

Abstract

This work focuses on the effect of electrolyte cations on the behaviour of a probe photocatalytic system comprising 2-propanol and 4-nitrobenzaldehyde (O2NC6H4CHO) as the hole and electron scavenger, respectively. Photo-reduction of the latter occurs via a stepwise pathway involving 4-aminobenzaldehyde (H2NC6H4CHO) as the stable intermediate and 4-aminobenzyl alcohol (H2NC6H4CH2OH) as the final product. 2-propanol photo-oxidation produces protons at the surface.The complete reduction of O2NC6H4CHO to H2NC6H4CH2OH needs photo-charging. Excess negative charge is compensated by cations adsorption and by lattice insertion. In the case of bulky K+ and TEA+ (tetraethylammonium ions) that apparently are not inserted, protons are inserted instead. In contrast, Li+ (and to a lesser degree Na+) ions favour selective reduction of O2NC6H4CHO to H2NC6H4CHO.Electrochemical measurements in the dark provide evidence of a strong interaction of 2-propanol with the surface and of the influence exerted by cations thereon. Additionally, electron paramagnetic resonance (EPR) spin trapping spectroscopy gives information about the formation of radicals intermediates from the alcohol photo-oxidation. Repeated illumination/dark runs showed that alkoxy (R-O) radicals always formed when the electrolyte contains K+ or TEA+. Conversely, in prolonged experiments, Li+ favours the formation of hydroxyalkyl radicals (R-CHOH), which indicates a non-dissociative interaction of 2-propanol with the surface. This weaker interaction can be reasonably ascribed to the know ability of TiO2 inserted Li+ to cause the formation of a new phase of the type LixTiO2.