Crystal anatase

K. Lagarec and S. Desgreniers, Raman study of single crystal anatase Ti02 up to 70 GPa, Sol. Si. Comm. 

The samples prepared have a good surface area after calcination at 500°C, as can be seen in table 1. Alumina-titania mixed oxide supported samples have surface areas larger than those of the alumina and titania single oxides. As expected x-ray diffraction results show that the mixed oxide catalysts are amorphous, but alumina shows a y phase structure, and Ti02 is a well crystallized anatase phase. No nickel metal or nickel oxide was detected in any of the samples, including Ti02 sample, suggesting the metal was well dispersed, and present as small crystallites (< 50A). 

Fig. 15 Conversion of light to electric current by dye-sensitized solar cells. The incident photon to current conversion efficiency is plotted as a function of the excitation wavelength. Left single crystal anatase cut in the (001) plane. Right nanocrystaUine anatase film. Pictures of the two electrodes used as current collectors are also presented. The electrolyte consisted of a solution of 0.3 M Lil and 0.03 M fr in acetonitrile...

L. Kavan, M. Gratzel, S. E. Gilbert etal.. Electrochemical and photoelectrochemical investigation of single-crystal anatase, j. Amer. Chem. Soc. 1996,118(28),6716-6723. 

Single-crystal anatase Ti02 was synthesized by hydrothermal treatment of aqueous titanium trichloride (TiCls 0.15 mol dm ) solutions with sodium dodecyl sulfate (SDS 0-0.133 mol dm. The solution of TiCls in dilute aqueous HCl was supplied by Wako Pure Chemicals. The precursor solution (40 mL) was placed in a Teflon-lined autoclave. Glass slides were used as substrates and were immersed in the solutions. The solutions were then heated at 200 °C for 3 h in a dry oven. The Ti02 deposited on the substrates was rinsed with deionized water and dried at room temperature. ... 

Figure C2.17.1. Transmission electron micrograph of a Ti02 (anatase) nanocrystal. The mottled and unstmctured background is an amorjihous carbon support film. The nanocrystal is centred in die middle of die image. This microscopy allows for die direct imaging of die crystal stmcture, as well as the overall nanocrystal shape. This titania nanocrystal was syndiesized using die nonhydrolytic niediod outlined in [79].

Figure C2.17.8. Powder x-ray diffraction (PXRD) from amoriDhous and nanocry stalline Ti02 nanocrystals. Powder x-ray diffraction is an important test for nanocrystal quality. In the top panel, nanoparticles of titania provide no crystalline reflections. These samples, while showing some evidence of crystallinity in TEM, have a major amoriDhous component. A similar reaction, perfonned with a crystallizing agent at high temperature, provides well defined reflections which allow the anatase phase to be clearly identified.

Unlike melting and the solid-solid phase transitions discussed in the next section, these phase changes are not reversible processes they occur because the crystal stmcture of the nanocrystal is metastable. For example, titania made in the nanophase always adopts the anatase stmcture. At higher temperatures the material spontaneously transfonns to the mtile bulk stable phase [211, 212 and 213]. The role of grain size in these metastable-stable transitions is not well established the issue is complicated by the fact that the transition is accompanied by grain growth which clouds the inteiyDretation of size-dependent data [214, 215 and 216]. In situ TEM studies, however, indicate that the surface chemistry of the nanocrystals play a cmcial role in the transition temperatures [217, 218]. 

A significant advantage of the PLM is in the differentiation and recognition of various forms of the same chemical substance polymorphic forms, eg, brookite, mtile, and anatase, three forms of titanium dioxide calcite, aragonite and vaterite, all forms of calcium carbonate Eorms I, II, III, and IV of HMX (a high explosive), etc. This is an important appHcation because most elements and compounds possess different crystal forms with very different physical properties. PLM is the only instmment mandated by the U.S. Environmental Protection Agency (EPA) for the detection and identification of the six forms of asbestos (qv) and other fibers in bulk samples. 

The concentrated mother Hquor contains a large amount of sulfuric acid in a free form, as titanium oxy-sulfate, and as some metal impurity sulfates. To yield the purest form of hydrated TiOg, the hydrolysis is carried out by a dding crystallizing seeds to the filtrate and heating the mixture close to its boiling temperature, - 109° C. The crystal stmcture of the seeds (anatase or mtile) and their physical properties affect the pigmentary characteristics of the final product. 

During calcination, water is removed at temperatures between 200 and 300°C sulfur trioxide is removed at temperatures between 480 and 800°C. At about 480°C the crystals of Ti02 are being formed and continue to grow with increasing temperature. To prepare the anatase pigment, the final calcination temperature of the hydrolysate prepared in the presence of anatase seeds should reach about 800—850°C. 

To ensure the mtile crystal form, seed crystals are added, otherwise anatase is obtained. The precipitate is thoroughly washed using water and sulfuric acid to remove all traces of discoloring elements, eg, iron, chromium, vanadium, and manganese. The TiO(OH)2 is finally calcined at 1000°C to Ti02 (8). 

Physical and Chemical Properties. Titanium dioxide [13463-67-7] occurs in nature in three crystalline forms anatase [1317-70-0] brookite [12188-41 -9] and mtile [1317-80-2]. These crystals are essentially pure titanium dioxide but contain small amounts of impurities, such as iron, chromium, or vanadium, which darken them. Rutile is the thermodynamically stable form at all temperatures and is one of the two most important ores of titanium. 

Anatase and mtile are produced commercially, whereas brookite has been produced by heating amorphous titanium dioxide, which is prepared from an alkyl titanate or sodium titanate [12034-34-3] with sodium or potassium hydroxide in. an autoclave at 200—600°C for several days. Only mtile has been synthesized from melts in the form of large single crystals. More recentiy (57), a new polymorph of titanium dioxide, Ti02(B), has been demonstrated, which is formed by hydrolysis of K Ti O to form 20, followed by subsequent calcination/dehydration at 500°C. The relatively open stmcture... 

The commercially important anatase and mtile both have tetragonal stmctures consequentiy, the values of physical properties such as refractive index and electrical conductivity depend on whether these are being measured parallel or perpendicular to the principal, ie, axis. However, in most appHcations, this distinction is lost because of random orientation of a large number of small crystals. It is thus the mean value that is significant. Representative physical properties are coUected in. Table 6. 

Both anatase and mtile are broad band gap semiconductors iu which a fiUed valence band, derived from the O 2p orbitals, is separated from an empty conduction band, derived from the Ti >d orbitals, by a band gap of ca 3 eV. Consequendy the electrical conductivity depends critically on the presence of impurities and defects such as oxygen vacancies (7). For very pure thin films, prepared by vacuum evaporation of titanium metal and then oxidation, conductivities of 10 S/cm have been reported. For both siugle-crystal and ceramic samples, the electrical conductivity depends on both the state of reduction of the and on dopant levels. At 300 K, a maximum conductivity of 1 S/cm has been reported at an oxygen deficiency of... 

A high purity titanium dioxide of poorly defined crystal form (ca 80% anatase, 20% mtile) is made commercially by flame hydrolysis of titanium tetrachloride. This product is used extensively for academic photocatalytic studies (70). The gas-phase oxidation of titanium tetrachloride, the basis of the chloride process for the production of titanium dioxide pigments, can be used for the production of high purity titanium dioxide, but, as with flame hydrolysis, the product is of poorly defined crystalline form unless special dopants are added to the principal reactants (71). 

Fig. 5. A 90° polished cross section of a production white titania enamel, with the microstructure showing the interface between steel and direct-on enamel as observed by reflected light micrography at 3500 x magnification using Nomarski Interface Contrast (oil immersion). A is a steel substrate B, complex interface phases including an iron—nickel alloy C, iron titanate crystals D, glassy matrix E, anatase, Ti02, crystals and F, quart2 particle.

---