Influence of the properties of TiO2 particles on a photocatalytic acrylate polymerisation

Samples of TiO2 having different BET surface areas and different numbers of primary crystallites per secondary particle were prepared by annealing amorphous TiO2 at different temperatures. The photocatalytic activity of these TiO2 samples was checked using the polymerisation of a trisacrylate as test reaction. Amorphous TiO2 is not able to initiate the polymerisation of the trisacrylate used. The maximum polymerisation rate as well as the monomer conversion after an illumination time of 120 s increases with increasing number of primary crystallites per particle. That means it increases with increasing grain boundary area inside the secondary particles. To get a better understanding of the photocatalytic properties the photoelectric primary processes in the TiO2 samples were investigated using measurements of the transient photoelectromotive force (Photo-EMF). Amorphous TiO2 does not show any Photo-EMF signal. This means illumination does not generate mobile charge carriers. For this reason, in the case of amorphous TiO2 no photocatalytic activity may be expected. The maximum Photo-EMF of TiO2 as a measure of the efficiency of charge separation increases with increasing number of primary crystallites per particle. A direct correlation between the maximum Photo-EMF U-max and the maximum rate of the acrylate photopolymerisation r(p)(max) shows that the photoelectric primary processes in the catalyst are of great importance for its photocatalytic activity. (c) 2005 Elsevier B.V. All rights reserved.