Enzymatic hydrogen production by light-sensitized anodized tubular TiO2 photoanode

Preliminary experiments with a slurry system of enzyme and powdery photocatalyst mixed in one compartment suggested that the electron transfer from light-sensitized photocatalyst to enzyme is the rate-determining step. Hence, in this study an anodized tubular TiO2 electrode (ATTE) on a titanium substrate was examined as a photoanode in an anodic cell for enzymatic hydrogen production in a cathodic cell. Anodization of Ti foil in a two-electrode electrochemical cell followed by annealing in an O2 atmosphere led to the formation of a tube-shaped TiO2 arrays, destroyed tube arrays, or spongelike TiO2 dense film. Samples were proven based on methylene blue (MB) discoloration to be photocatalytically active. The rate of photocatalytic hydrogen production in one of the samples (20 V–25 °C in a mixed electrolyte/350 °C–5 h) was 40 μmol/(h cm2) with a 0.1 M Na2S electrolyte in one compartment reactor system, while the enzymatic hydrogen production rate with light-sensitized photoanode was 30 μmol/(h cm2) in the cathodic compartment with an oxygen production rate of 15 μmol/(h cm2) in the anodic compartment. These results confirmed the successful evolution of stoichiometric H2 and O2 separately. For the system with a sample (20 V–5 °C in 0.5% HF/650 °C–5 h), a hydrogen production rate was ca. 43 μmol/(h cm2) in the cathodic compartment and an oxygen production rate was ca. 20 μmol/(h cm2) in the anodic compartment. X-ray diffraction (XRD) results clearly indicated that the samples showing the highest evolution rate were composed of both anatase and rutile, while those made of either anatase or rutile showed a lower evolution rate. Higher annealing temperatures increased the thickness of the oxide barrier layer and obstructed the charge transfer to the back contact.