Nanocrystalline Anatase

Bose, F. Ayral, A. Albouy, P. A. Guizard, C. 2003. A simple route for low-temperature synthesis of mesoporous and nanocrystalline anatase thin films. Chem. Mater. 15 2463-2468. 

Choi, S. Y. Mamak, M. Coombs, N. Chopra, N. Ozin, G. A. 2004. Thermally stable two-dimensional hexagonal mesoporous nanocrystalline anatase, meso-nc-Ti02 Bulk and crack-free thin film morphologies. Adv. Fund. Mater. 14 335-344. 

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... 

An, H. J., Jang, S. R., Vittal, R., Lee, J. and Kim, K. J. (2005). Cationic surfactant promoted reductive electrodeposition of nanocrystalline anatase Ti02 for application to dye-sensitized solar cells. Electrochimica Acta 50(13), 2713-2718. 

Figure 33 shows a Gerischer-like diagram which summarizes the kinetic processes that can be initiated with light excitation of a dye-sensitized nanocrystalline anatase... 

Figure 7. Salient features of the electronic structure of trapping sites in nanocrystalline anatase particles.

Solvothermal decomposition of titanium tcrt-butoxide and titanium oxy-acetylacetonate (TiO(acac)2) in toluene at 300°C yields nanocrystalline anatase. It should be noted that the lowest temperature required for the formation of crystalline titania by CVD synthesis was reported to be 400 to 450°C. ° ° ... 

Trentler et al."" also proposed that nanocrystalline products are obtained at elevated temperatures because TLX4 serves as a crystallization agent as well as a reactant. They pointed out the importance of a chemical reversibility, that is, Ti-0 bond breaking and forming, that would erase defects incorporated into growing titania crystals. This statement seems to be made because they do not know that the solvothermal reaction of titanium tert-butoxide and titanium oxyacetylaceto-nate in toluene yields nanocrystalline anatase. However, this point is closely connected with one of the important features of solvothermal products, and therefore will be discussed here. 

Zhang and Banfield (1998) have made a detailed thermodynamic analysis of the nanocrystalline anatase and rutile system. Results suggested that at standard pressure, anatase is more stable than rutile when their particle sizes are below -14.5 nm (curve 1 in Fig. 16). In their calculation, the surface free energies for rutile and anatase were obtained through published data of surface energy calculated by molecule dynamics simulations and experimental data of heat capacity of ultrafme rutile samples ... 

The density data are pr = 4.249 g/cm and Pa = 3.893 g/cm. Both the density data and their structure analysis of the unsatisfied surface charges associated with several exposed faces of anatase and rutile support the fact that rutile has higher surface energy than anatase. Unfortunately, direct measurements of surface energies were not available. Based on the available data, the nanocrystalline anatase-rutile system is classified as Case 3 discussed above (a-phase = anatase and P-phase = rutile in Figs. 14 and 15). 

We have studied the phase transformation from nanometer-sized amorphous titania (Ti02) to nanocrystalline anatase at 300 - 400° C (unpublished). Amorphous titania samples were prepared by fast hydrolysis of titanium ethoxide in water at 0° C (Zhang et al. 2001). The extent of transformation was monitored using XRD determination of the phase mixture as a function of time. We also found that the transformation kinetics do not follow the widely employed JMAK equation. 

New kinetic models were developed to incorporate interface nucleation (Zhang and Banfield 1999) and surface nucleation (Zhang and Banfield 2000), thus to quantitatively interpret the kinetic behavior in the nanocrystalline anatase-rutile system. Surface nucleation and bulk nucleation come into play as temperature increases (Zhang and Banfield 2000). Particle size has been explicitly incorporated into the kinetic equations. The transformation rate scales with the square of the number of anatase nanoparticles in the case of interface nucleation (Zhang and Banfield 1999), or with the number of anatase nanoparticles in the case of surface nucleation (Zhang and Banfield 2000). If the transformation is governed only by interface nucleation, the kinetic equation is ... 

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... 

Agrawal A, Cizeron J and Colvin V L 1998 In situ high-temperature transmission electron microscopy observations of the formation of nanocrystalline TIC from nanocrystalline anatase (Ti02) Microsc. Microanal. 4 269... 

H. Kusama, M. Kurashige, K. Sayama, M. Yanagida, H. Sugihara, Improved performance of Black-dye-sensitized solar cells with nanocrystalline anatase Ti02 photoelectrodes prepared from TiCU and ammonium carbonate . Journal of Photochemistry and Photobiology A Chemistry, 189,100-104, (2007). 

Something commonly seen in the synthesis of nanomaterials is that many metastable structures appear stable in the nanometric range. A typical case is the synthesis of Ti02 polymorphs. Ti02 has three crystalline polymorphs, anatase, brookite, and rutile [84]. Although several papers report on the synthesis of nanocrystalline anatase [12, 85-89], few report on nanocrystalline rutile [90,91] as an example. However, several papers state that the rutile formation passes through the three metastable phases, and it has been established that rutile is the most stable Ti02 polymorph (observations of micrometric anatase are scarce) [92-96]. 

H, Z, Zhang and J, F, Banfield, Kinetics of crystallization and crystal growth of nanocrystalline anatase in nanometer-sized amorphous titania, Chem. Mater, 14 4145 154, 2002... 

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