Nanocrystalline-TiO2-Pt photo-electrochemical cells--UV induced hydrogen evolution from aqueous solutions of alcohols

Addition of a Pt cathode in electrical contact with a thin nanocrystalline TiO2 film gives a photo-electrochemical cell (PEC) which will, under band-gap illumination, generate hydrogen from a range of aliphatic alcohols. When the TiO2 film is irradiated in contact with 0.1 M HCl in 50%, v/v MeOH-H2O in the absence of a Pt cathode phiH2 = 0.046; in the presence of a Pt cathode the hydrogen generation yield is increased to phiH2 = 0.193. In the absence of a Pt cathode hydrogen yields are independent of the thickness (and hence optical absorbance) of the TiO2 film and phiH2 falls with increasing film thickness, indicating a surface controlled reaction. In the presence of a Pt cathode hydrogen yields increase linearly with absorption efficiency and phiH2 remains constant. In the presence of 0.01 M HCl hydrogen evolution from the PEC occurs at both the TiO2 surface (30%) and at the Pt surface (70%), while in the presence of 0.01 M NaOH hydrogen evolution is exclusively from the TiO2 surface. Provided the total ion concentration, I, is kept constant, quantum yields as a function of pH across the range 2-12 take one of two values depending upon whether the solution is acidic or alkaline, ie. for I = 0.01 M, at pH 2-6 phiH2 = 0.112 (+/- 0.007), and at pH 8-12 phiH2 = 0.029 (+/- 0.003). Studies with a series of chloro and methyl substituted alcohols suggest that the stability of the alcohol anion is an important factor in determining the efficiency of the alcohol as a sacrificial electron donor. Quantum yields for H2 evolution of ca. 30%, can be obtained.