On the origin of the decay of the photocatalytic activity of TiO2 powders ground at high-energy
Résumé
The influence of high-energy grinding on the photocatalytic activity of nanocrystalline titania (TiO2) powders was investigated. Either in gas or in liquid reactions, the photocatalytic activity is observed to decrease with milling time. The structural and surface characterizations showed that the catalytic decrease is not correlated to the milling induced transformation of anatase TiO2 into TiO2 rutile via the high-temperature and high-pressure phase TiO2-II but to the introduction of defects and to the amorphization of the surface of the TiO2 nanocrystals (nc). This has been correlated to the observed change of the photoluminescence properties of the powders. Spectrally resolved photoluminescence (PL) measurements as a function of temperature are sensitive to the well-established transformations of ground anatase TiO2 nc into TiO2 rutile via TiO2-II. For all TiO2 nc in their different modifications, increasing the temperature produces a quenching of the PL intensity that is shown to follow a double activation energy law. These PL measurements first show that the electronic properties are strongly affected by the grinding process. Second, new states are generated in ground TiO2, which have a higher PL quantum efficiency than those of the original TiO2 nc. Thus, new channels of radiative recombination are opened due to defects produced by grinding, especially surface defects due to surface amorphization, and the number of electron−hole pairs, which are responsible for the photocatalytic activity, diminishes.