Anatase to rutile, transformation

XRD analysis of the xerogels obtained by drying pure titanium dioxide sol at 70°C showed the presence of the nanocrystalline anatase phase [109]. Thermal treatment of this xerogel resulted in the growth of anatase crystallites up to 400°C. The anatase-to-rutile transformation began to occur at 450-500°C. This process was practically completed at 700°C, and only rutile phase existed at rcaic > 700°C. This feature of Ti02 xerogels is typical and well known (see, for example, [109]). Thus, it can be concluded that anatase-rutile transition temperature of nanosized particles is considerably lower than that of the... 

Zhang H, Banfield JF (1999) New kinetic model for the nanocrystalline anatase-to-rutile transformation revealing rate dependence on number of particles. Am Mineral 84 528-353 Zhang H, Perm RL, Hamers RJ, Banfield JF (1999) Enhanced adsorption of molecules on surfaces of nanocrystalline particles. J Phys Chem B 103 4656-4662 Zhang H, Banfield JF (2000a) Phase transformation of nanocrystalline anatase-to-mtile via combined interface and surface nucleation. J Mater Res 15 437-448... 

Zhang H and Banfield J F 1999 New kinetic model for the nanocrystalline anatase-to-rutile transformation revealing rate dependence on number of particles Am. Miner. 84 528... 

Vemury and Pratsinis (18) investigated the effect of dopants on the characteristics of titania particles made by oxidation of TiCU in diffusion flames. They found that introduction of Si + inhibited the anatase to rutile transformation and decreased the primary particle size of titania. However, the addition of Al + had the reverse results, presumably because Si + entered the lattice interstitially while Al + entered substitutionally into the titania lattice (3). 

It is accepted that, at normal pressures, rutile is the thermodynamically stable form of titanium dioxide at all temperatures. Calorimetric studies have demonstrated that rutile is more stable than anatase and that brookite and Ti02 have intermediate stabilities, although the relative stabilities of brookite and Ti02(ii) have not yet been defined. The transformation of anatase to rutile is exothermic, eg, 12.6 KJ/mol (9), although lower figures have also been reported (63). The rate of transformation is critically dependent on the detailed environment and may be either promoted or retarded by the presence of other substances. For example, phosphoms inhibits the transformation of anatase to rutile (64). 

Anatase, brookite and rutile are three polymorphs of titanium dioxide. Anatase is a kind of thermodynamically metastable form while rutile is a kind of stable one. Anatase can transform irreversibly to rutile at elevated temperatures ranged from 400 to 1200 °C according to particle size, morphology and additives. The solid-state phase transformation behavior has been widely investigated while the phase evolution between anatase and rutile under hydrothermal condition has been little paid attention to so far [5]. In this work, the structural evolution from anatase to rutile under milder hydrothermal conditions is proposed as well [7, 10]. 

Hydrothermal Phase Transformation from Anatase to Rutile the phase transformation from anatase to rutile was investigated under various hydrothermal conditions by using as-hydrothermally-synthesized nanocrystals of pure anatase or mixture of anatase and rutile as starting materials. 

Figure 6 implies that anatase could be transformed into rutile especially at higher temperatures. If a precursor of anatase particles prepared from hydrothermal reaction was loaded into a vessel again for aging in HCI medium, it would be transformed into a product of a mixture of anatase and rutile, even of a pure rutile phase. In the presence of rutile as a trace component, the precursor would be transformed into rutile more easily. Thus, the phase transformation from anatase to rutile could be achieved not only in solid-state reaction, but also in aqueous chemistry, especially in the medium of HCI aqueous solution, and the phase transformation in aqueous solution will be promoted with the existence of trace rutile. Some minelizers such as NaCl also affect the phase transformation from anatase to rutile. 

The results of XRD show that the anatase phase is formed at a temperature of 330°C for the pure titanium dioxide, while rutile phase is formed when the calcination temperature approached 630°C, indicating the beginning of the transformation from anatase to rutile. The anatase peaks in the XRD pattern disappears during the heat treatment of bare titanium dioxide at 700°C for Ih, suggesting the end of the transformation of anatase to rutile. The evolution of both anatase and rutile starts at higher temperature for silicon doped samples. However, the transition temperature of anatase to rutile progressively elevates when the silicon content is increased. Figure 1 shows the 29 diffraction scans for some of the samples annealed at 1000°C for Ih in air. It can be seen that the phase transition temperature... 

First we consider the transformation mechanism of anatase to rutile in order to determine the reason for the dependence of the transformation rate on particle size. Penn and Banfield (1998 1999) showed that the oriented assembly of nanoparticles to form larger crystals (see below for details) is accompanied by formation of twins that introduce new atomic arrangements at particle-particle interfaces. In the case of anatase, a 112 twin represents a slab of brookite and thus, a structural state intermediate between anatase and rutile. Penn and Banfield (1999) proposed that the activation barrier for rutile nucleation is lowered by the presence of these twins. Simultaneously, it was noted that the transformation of anatase to rutile in air (Gribb and Banfield 1997) and under hydrothermal conditions (Penn and Banfield 1999) rarely generates partially reacted crystals, suggesting a high activation barrier for rutile nucleation but rapid rutile growth. 

Once initiated, a cascade of atomic displacements and distortions occurs, converting anatase to rutile. Thus, it appears that the transformation rate is limited by the nucleation rate, which is increased by an increase in the number particle-particle interfaces, thus a decrease in particle size. Zhang and Banfield (1999) extended this more generally to particle-particle contacts. 

Rao CNR (1961a) Kinetics and thermodynamics of the crystal structure transformation of spectroscopically pure anatase to rutile. Can J Chem 39 498-500... 

In the case of titania, the stabilising effect of yttria was not observed below a heat treatment of 600 C as shown in Table 6. Pure titania shifted from anatase to rutile at 900 °C and the phase transformation was accompanied by a very strong decrease of the BET surface area. The doped sample, although it kept the anatase structure until reaching a treatment temperature of 1200 °C, was also characterised by a very strong diminution of its surface area at 900 °C which, however, could not be explained by a sole crystalline change as shown in Table 6. 

The XRD patterns as shown in Fig 1 obtained for all the samples after their reduction in hydrogen indicate the presence of unreduced platinum oxides even after HTR in the supported titania gel and mixed oxide sample. However, complete reduction to metallic platinum was observed on supported commercial titania system. It can be observed that the phase transformation of titania from anatase to rutile does not occur under the experimental... 

Rutile is the thermodynamically most stable Ti02 crystal structure, and the transformation from anatase to rutile takes place at about 1300 K. This temperature can be much lower in the presence of foreign atoms, which can catalyze the transformation. In a series of studies of the properties of titania, Shanon and Pask (45) postulated that the transformation to rutile... 

---