Rutile photoactive surface

It is well known that pure rutile titanium dioxide has a photoactive surface [5, 6]. When irradiated with UV light, highly reactive 0x0 radicals are formed that can photocatalytically degrade organic materials in contact with the pigment s surface. Commercial grades of rutile TiOj are passivated with coatings of other metal oxides such as those of aluminum, silicon, or zirconium to suppress this effect. 

Photocatalytically active rutile samples can be prepared by hydrolysis of TiCU. The presence of chloride ions and a refluxing treatment promote the formation of the rutile polymorph and increase the activity of the samples. Azide anions favour the formation of anatase and, in moderate amounts, enhances the photoreactivity of rutile. The fair photoactivity of the samples is scarcely related to the specific surface area and crystallite size of the powders and depends probably on the particular physico-chemical features of the surface. 

Doping the crystal lattice and coating the surface with metal oxides or hydroxides has reduced the photocatalytic polymer decomposition caused by titanium dioxide to such an extent that all stability requirements can be met. Since anatase is generally more photoactive than rutile, aftertreated and/or lattice-stabilized rutile pigments are preferred for exterior applications (exposure to sunlight). 

The importance of anatase crystallinity was emphasized in relation to the preparation and annealing procedures for achieving high photocatalytic performance [19,24,25,119,120]. The highest photoactivity always appeared at the onset of phase transformation of anatase to rutile. A linear relationship between photoactivity (normalized by the BET surface area) and crystallite size determined from FWHM of 101 diffraction line was reported for powders prepared from TTIP by either spray or gas pyrolysis, although the variations in BET surface area and crystallite size were limited to a narrow range, from 2.5 to 50 mVg and 10 to 33 nm [121],... 

Some studies reported enhancements in photoactivity in the presence of a small amount of rutile phase [122-124]. Even a mechanical mixture of anatase and rutile showed much higher photoactivity for naphthalene oxidation than either pnre anatase or rutile powders [123,124]. The P25 powder is produced from TiCl4 in a flow reactor [122]. Based on a detailed investigation by x-ray diffraction (XRD) and micro-Raman spectroscopy, the rutile (formed directly in the flame) was fonnd to be covered by anatase [122,124]. However, another study based on transmission electron microscopy (TEM) with selected-area electron diffraction reported the presence of separate particles of anatase and rutile in P25 [125,126]. Diffuse reflectance spectra of P25 could be reproduced by a mechanical mixture of anatase and rutile powders, and particles of pure rutile phase were isolated from P25 upon HF treatment. Photoactivity for the decomposition of 4-chlorophenol in water was compared on four commercial photocatalysts, applying criteria of (a) initial rate of pollutant disappearance, (b) amount of intermediate products formed, and (c) time necessary to achieve total mineralization [127]. Based on criterion (c), P25 was concluded to be the most efficient photocatalyst even though it contains 20% rutile and has a moderate BET surface area (ca. 50 m /g). It was also reported to have a higher photoactivity than catalyst All in the degradation of reactive black 5 (an azo-dye) [128]. 

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