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Review of the Anatase to Rutile Phase Transformation Dorian Hanaor, Charles Sorrell Review of the anatase to rutile phase transformation Dorian Hanaor, Charles Sorrell To cite this version: Dorian Hanaor, Charles Sorrell. Review of the anatase to rutile phase transformation. Journal of Ma- terials Science, Springer Verlag, 2011, 46 (4), pp.855-874. 10.1007/s10853-010-5113-0. hal-02308408 HAL Id: hal-02308408 https://hal.archives-ouvertes.fr/hal-02308408 Submitted on 8 Oct 2019 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Distributed under a Creative Commons Attribution - NonCommercial - NoDerivatives| 4.0 International License J Mater Sci (2011) 46:855–874 DOI 10.1007/s10853-010-5113-0 REVIEW Review of the anatase to rutile phase transformation Dorian A. H. Hanaor • Charles C. Sorrell Received: 21 May 2010 / Accepted: 23 November 2010 / Published online: 8 December 2010 Ó Springer Science+Business Media, LLC 2010 Abstract Titanium dioxide, TiO2, is an important pho- Titanium dioxide occurs as two important polymorphs, tocatalytic material that exists as two main polymorphs, the stable rutile and metastable anatase. These polymorphs anatase and rutile. The presence of either or both of these exhibit different properties and consequently different phases impacts on the photocatalytic performance of the photocatalytic performances. Anatase transforms irrevers- material. The present work reviews the anatase to rutile ibly to rutile at elevated temperatures. This transformation phase transformation. The synthesis and properties of does not have a unique temperature and the processes that anatase and rutile are examined, followed by a discussion are involved in the transformation as well as the methods to of the thermodynamics of the phase transformation and the inhibit or promote this transformation have not been factors affecting its observation. A comprehensive analysis reviewed comprehensively to date. of the reported effects of dopants on the anatase to rutile The present work aims to clarify the differences between phase transformation and the mechanisms by which these the two main polymorphs of titanium dioxide, the nature of effects are brought about is presented in this review, the anatase to rutile transformation, and the principles of yielding a plot of the cationic radius versus the valence controlling phase composition through the inhibition or characterised by a distinct boundary between inhibitors and promotion of the transformation of anatase to rutile. promoters of the phase transformation. Further, the likely effects of dopant elements, including those for which experimental data are unavailable, on the phase transfor- Titania polymorphs mation are deduced and presented on the basis of this analysis. Titanium dioxide, the only naturally occurring oxide of titanium at atmospheric pressure, exhibits three poly- morphs: rutile, anatase, and brookite [1–7]. While rutile is Background the stable phase, both anatase and brookite are metastable; the latter is difficult to synthesise and so is seldom studied Titanium dioxide, also known as titania, is of growing [8]. Another five high-pressure phases of TiO2 have been interest due to its proven ability to function as a photo- reported: catalyst and facilitate important environmentally beneficial • TiO II or srilankite, an orthorhombic polymorph of the reactions, such as water splitting to generate hydrogen and 2 lead oxide structure treatment of polluted air and water. • Cubic fluorite-type polymorph • Pyrite-type polymorph D. A. H. Hanaor (&) Á C. C. Sorrell • Monoclinic baddeleyite-type polymorph School of Materials Science and Engineering, University of New • Cotunnite-type polymorph South Wales, Sydney, NSW 2052, Australia e-mail: [email protected] The stability of these phases has been discussed in sev- C. C. Sorrell eral publications [4, 7, 9–12]. However, these are of minor e-mail: [email protected] significance for research and development applications. 123 856 J Mater Sci (2011) 46:855–874 Titania properties • Energy Electrolysis of water to generate hydrogen [26–30]. Table 1 outlines the basic properties of rutile and anatase. Dye-sensitised solar cells (DSSCs) [31–33]. Titania applications • Environment Air purification [34, 35]. The primary application of titanium dioxide is as a white Water treatment [36–40]. pigment in paints, food colouring, cosmetics, toothpastes, polymers, and other instances in which white colouration is • Built Environment desired [13]. The reason for this is the high refractive Self-cleaning coatings [34, 38, 39, 41–50]. indices of rutile and anatase, which result in high reflec- Non-spotting glass [47, 51]. tivity from the surfaces. Consequently, titanias of small particle size and correspondingly high surface areas are • Biomedicine used owing to the resultant opacifying power and bright- Self-sterilising coatings [52, 53]. ness. However, paints utilise polymeric binders to fix the pigment and, when in contact with titania, the polymer may Photocatalytic effect oxidise when exposed to sunlight. This effect is known as chalking and, in addition to the direct degrading effect of Photocatalysed reactions for applications such as those ultraviolet (UV) radiation, is accelerated by the photocat- mentioned above are facilitated through the presence of alytic activity of TiO , which also is enhanced by the high 2 adsorbed radicals (from air or water) on the TiO surface [28, surface area of this material [25]. 2 44–47, 49, 54–56]. These radicals, which are atomic species The potential for the application of the photocatalytic with a free unpaired electron, are formed upon reaction of an effect in TiO has attracted considerable interest over the 2 adsorbed molecule (such as O or H O) with a photo-gen- last three decades. Titania photocatalysts are known to be 2 2 erated charge carrier (from an electron–hole pair or exciton) applicable in a range of important technological areas: when TiO2 is exposed to radiation exceeding its band gap; this radiation normally is in the UV wavelength region (290–380 nm). These electron–hole pairs are formed when an electron is elevated from the valence to the conducting Table 1 Properties of anatase and rutile band, leaving behind an electron hole, as shown in Fig. 1. The electrons in the conduction band facilitate reduction Property Anatase Rutile Reference of electron acceptors and the holes facilitate oxidation of Crystal structure Tetragonal Tetragonal [13] electron donors [57]. Examples of the photo-generation of Atoms per unit cell (Z) 4 2 [14, 15] radicals in atmospheric and aqueous environments are Space group 4 42 [14, 16] given in the following reactions [28, 57–59]: lamd Pmnm þ À Lattice parameters (nm) a = 0.3785 a = 0.4594 [14, 15] TiO2 þ hv $ h þ e ð1Þ c = 0.9514 c = 0.29589 hþ þ H O $ Hþ þ OH ð2Þ Unit cell volume (nm3)a 0.1363 0.0624 2 absorbed -3 þ À Density (kg m ) 3894 4250 [14, 15] h þ OHabsorbed $ OH ð3Þ Calculated indirect band gap [8, 17–20] (eV) 3.23–3.59 3.02–3.24 (nm) 345.4–383.9 382.7–410.1 (e-) Experimental band gap [8, 19, 21] hν Conduction band (eV) *3.2 *3.0 (nm) *387 *413 Band Refractive index 2.54, 2.49 2.79, 2.903 [13, 22] gap Solubility in HF Soluble Insoluble [23] Solubility in H2O Insoluble Insoluble [13] Hardness (Mohs) 5.5–6 6–6.5 [24] Valence band Bulk modulus (GPa) 183 206 [20] (h+) a Since the numbers of atoms per unit cell is halved upon going from rutile to anatase, the lattice parameters and unit cell volumes must be Fig. 1 Schematic illustration of photo-generation of charge carriers viewed accordingly in a photocatalyst 123 J Mater Sci (2011) 46:855–874 857 À À e þ O2 absorbed $ O2 ð4Þ larger grain size [55, 73] and its resultant lower capacity to In this notation, an unpaired electron is represented by a adsorb species [60, 74, 75]. point, a valence band electron hole is represented by h?, It may be noted that, owing to the different crystal struc- and a conduction band electron is represented by e-. tures and associated exposed planes of the two polymorphs, These mechanisms have been described elsewhere in anatase has been reported to have a lower surface enthalpy greater detail for the decomposition of organic pollutants, and lower surface free energy than rutile [76]. Hence, it would as shown in Fig. 2 [38, 46, 56, 57, 60] and the splitting of be expected that the wetting of anatase by water would be less water [26, 27, 29]. than that of rutile since higher surface free energies generally The generation of positive and negative charge carriers contribute to hydrophilicity [77]. Since a high density of by UV radiation and their tendency to recombine are adsorbed species would be expected from a hydrophilic competing phenomena, the rates of which govern whether material, rutile could be anticipated to exhibit superior pho- or not a semiconductor can function as a photocatalyst tocatalytic performance. It may be noted that there are no [61]. A key factor in titania’s photocatalytic ability is its reports of rutile’s exhibiting higher levels of adsorbed species. high surface area, the same property that contributes to its The photoactivity of anatase and rutile have been optical properties. A high surface area leads to a higher examined and interpreted by Sclafani and Herrmann [58] density of localised states, which involve electrons with with reference to the densities of surface-adsorbed species.
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