Recently it was found that amorphous microporous titanium dioxide doped in the bulk with [PtCl4] can catalyse photodegradation of 4-chlorophenol (4-CP) with visible light [1-3]. Even higher degradation rates and significant photocurrent generation were achieved when the platinum complex was chemisorbed onto the surface of the anatase [4, 5]. Upon visible light irradiation an oxidising weakly bound chlorine atom (E = 1.3-2.4 V) and a reducing Pt(III) species (ca. -0.7 V) are formed. The transient Pt(III) is capable of electron injection into the conduction band of TiO2. The Cl atoms oxidise the adsorbed 4-CP molecules. Another important oxidising agent, the hydroxyl radical (ca. 2.4 V), is formed in the reductive pathway as a product of superoxide transformations, as proven in experiments of salicylic acid formation from benzoic acid.
Electrochemical and photoelectrochemical studies enabled to estimate potential levels and to clarify several mechanistic details. The anatase modification of titania gave the best results both in 4-CP photocatalytic degradation and photocurrent generation. The behaviour of various [PtCl4]/TiO2 materials, based on different crystalline forms of TiO2 was parallel to the activity of unmodified TiO2 upon UV irradiation. Flatband potential determinations  have proven that the difference between rutile and anatase photosensitisation does not originate from the positions of the CB and VB edges, but is caused rather by adsorption properties and charge carrier mobilities.
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