On the Origin of the Decay of the Photocatalytic Activity of TiO₂ Powders Ground at High Energy

Abstract

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.

Effect of ball-milling and Fe-/Al-doping on the structural aspect and visible light photocatalytic activity of TiO2 towards Escherichia coli bacteria abatement

“…1). The TiO 2 II phase was only identified in the P25 powder ground for 30 min as already reported and disappeared after heat treatment at 500°C [5]. The proportions of each phase and the mean grain sizes in the different samples have been calculated using the Rietveld method (Table 2) [31].…”

“…In addition, an intermediate phase called TiO 2 II and isomorphous to α-PbO 2 was already encountered when anatase transforms into rutile under high energy ball milling [4,5]. Anatase is well-known to be more active in photocatalysis under UV light than the rutile allotropic form.…”

Hallmarks of mechanochemistry: from nanoparticles to technology

“…The influence of milling on the phase transformation of titania with and without dopants has been widely investigated [141][142][143][144][145][146][147][148][149][150][151][152][153][154][155][156] because of the industrial interest in this material as a photocatalyst. It has been generally reported that the anatase -rutile transformation takes place through the intermediate formation of a non-equilibrium high-pressure phase.…”

Continuous Synthesis of Ag/TiO2 Nanoparticles with Enhanced Photocatalytic Activity by Pulsed Laser Ablation

“…Because of the difference in Fermi levels of anatase and rutile, the holes and electrons can be efficiently separated through charge carrier transfer. Hence, the mixed phase in TiO 2 may benefit the photocatalytic activity due to the reduced recombination between the excited electrons and holes, as found in the widely studied commercial Aeroxide P-25 TiO 2 [41].…”