Sol–gel titanium oxides sensitized by nanometric carbon blacks: Comparison with the optoelectronic and photocatalytic properties of physical mixtures

The preparation, optical properties and photocatalytic activity of TiO2/carbon blacks composites as a function of TiO2 crystallite size are reported. Carbon-doped TiO2, evidenced by a well defined visible absorption at 1.3 eV, is grown in situ by sol–gel techniques, using TiCl4 as the oxide precursor and ammonia as the basic catalyst. For the physical mixtures of carbon blacks with titanium oxides, at similar composition and thermal treatment, sensitization is not observed regardless of the oxide crystallite size. The core-shell microstructure of the sol–gel materials (i.e. defects at the interface) and/or the presence of a nitrogen base seem relevant for carbon-doping. In general, sol–gel composites outperformed the physical mixtures in the photocatalytic degradation of methyl violet dyes, and the data suggest that this enhancement is mainly due to the role of carbon during carrier photogeneration and transport and not just to the superior surface area of the sol–gel catalysts.