Anatase and rutile crystallites were isolated from Degussa (Evonik) P25 by selective dissolution with a hydrogen peroxide-ammonia mixture and diluted hydrofluoric acid, respectively, and used as standard samples for calibration curves of X-ray diffraction analyses.The results showed that P25 contains more than 70% anatase with a minor amount of rutile and a small amount of amorphous phase. The composition anatase/rutile/amorphous could be determined by analysis of P25 mixed with an internal standard, nickel(II) oxide.However, it was also found that the composition of P25 used in this study was inhomogeneous and changed depending on the position of sampling from the same package.Comparison of activities of original P25 and reconstructed P25 with those of isolated anatase and rutile particles suggested a less probable synergetic effect of the co-presence of anatase and rutile.
Decahedral single-crystalline anatase particles prepared by a newly developed gas-phase process showed photocatalytic activities superior to those of P25 photocatalyst despite their relatively smaller specific surface area compared with that of P25, probably because of the low density of particle boundaries and crystalline defects.
Octahedral titanium(IV) oxide (TiO 2 ) crystallites exposing anatase {101} facets exhibited relatively high photocatalytic activity for oxidative decomposition of organic compounds and low activity for hydrogen liberation in the absence of molecular oxygen probably due to the characteristics of anatase {101} surface.Anatase has been the most studied polymorph of TiO 2 owing to its higher photocatalytic activity for oxidative decomposition of organic compounds in the presence of molecular oxygen, 1,2 hydrogen (H 2 ) liberation, and so on. Surface free energy of anatase {101} lattice plane is reported to be the smallest among the other planes of anatase.
3,4In agreement with natural minerals, a truncated octahedral bipyramid, exposing eight {101} facets as well as two {001} facets, has been shown as a thermodynamically most stable shape of anatase crystallites based on Wulff construction. 3,4 In spite of the important applications of anatase TiO 2 , experimental studies on the photoactive property of singlecrystalline surface are still limited due to the difficulty to obtain high quality anatase samples.
5A few studies suggested the significant role of facets of TiO 2 polyhedral particles for photocatalytic reactions. 6,7 Recent development of hydrothermal reaction method has enabled to prepare anatase crystalline particles with well-developed facets in a relatively high yield. [8][9][10] The aim of the present study is to investigate the roles of {101} facets, which is the most likely observed on anatase mesoparticles, 11 for practical photocatalytic reaction system such as powder suspensions. In order to compare the
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