Probing the role of surface energetics of electrons and their accumulation in photoreduction processes on TiO2

We address the role of the energetics of photogenerated electrons in the reduction of 4-nitrobenzaldehyde on TiO2. This model molecule bears two functional groups featuring different reducibility. Electrochemistry shows that reduction to 4-aminobenzyl alcohol occurs in entirely distinct potential ranges. Partial reduction of the –NO2 group, affording 4-aminobenzaldehyde, takes place via surface states at potentials positive of the flatband potential (Efb). Dark currents due to reduction of the aldehyde group are observed only at potentials more negative than Efb and the process requires an electron accumulation regime. Photocatalysis with TiO2 suspensions agrees with the electrochemical data. In particular, reduction of the nitro group is a relatively fast process (k = 0.059 s-1), while that of the aldehyde group is slower (k = 0.001 s-1) and requires electrons photoaccumulation. Control of the photogenerated charge is a prospective means to achieve chemoselective reductions.