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

Abstract

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 ΦH₂ = 0.046; in the presence of a Pt cathode the hydrogen generation yield is increased to ΦH₂ = 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 ΦH₂ 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 ΦH₂ 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, i.e. for I = 0.01 M, at pH 2–6 ΦH₂ = 0.112 (±0.007), and at pH 8–12 ΦH₂ = 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 H₂evolution of ca. 30% can be obtained.