Titania-based heterogeneous photocatalysis. Materials, mechanistic issues, and implications for environmental remediation*

Using hexavalent chromium [Cr(VI)] and methylene blue (MB) as model substrates, we discuss three aspects of TiO2-based heterogeneous photocatalysis. We show first that a given TiO2 sample may not be simultaneously optimal for photocatalytically driving the reduction of Cr(VI) and the oxidation of MB. We further show that a TiO2 sample that strongly adsorbs either of these substrates in the dark is not optimal as a photocatalyst. The other two aspects concern circumventing the rather poor surface catalytic properties and visible light photoresponse of TiO2, respectively. Strategies revolving around the visible light photoexcitation of the substrate itself and metal-modification of the TiO2 surface, are described as possible solutions.