Polycrystalline rutile

Fig. 8.10. Experimental ESR spectra of the polycrystalline rutile doped with 0.1 at. % of V4, ...

Fig. 14. AFM images of polycrystalline rutile surfaces before (a,c,e) and after (b,d,f) the photochemical deposition of silver (white contrast).

In addition to electron diffraction, the measurement of the film refractive index also enables distinction between anatase and rutile. According to Hass [27], polycrystalline rutile films have a refractive index value of 54 = 2.70 and that of the anatase films is 546 = 2.39. 

Intensity Analysis of Raman Scattering Characterizes Columnar Grain Orientation in Polycrystalline Rutile Films... 

Polycrystalline oxide materials, both undoped and doped, have been extensively examined for use as photoanodes. Ti02 electrodes have been prepared by thermal oxidation of a Ti plate in an electric furnace in air at 300-800°C (15-60 min) and in a flame at 1300°C (20 min) [27-30]. XRD analysis of thermally oxidized samples indicates the formation of metallic sub-oxide interstitial compounds, i.e. TiOo+x (x < 0.33) or Ti20i y (0 < y < 0.33) and Ti30 together with rutile Ti02 [27]. The characteristic reflection of metallic titanium decreases in intensity after prolonged oxidation (60 min) at 800° C indicating the presence of a fairly thick oxide layer (10-15 pm). Oxidation at 900°C leads to poor adhesion of the oxide film... 

Figure 2.2. (a) Coiled whisker of rutile in cabochon-cut quartz (courtesy of E. A. Jobbins). (b) Crysotile crystals resembling stacked ice cream cones. (Transmission electron microphotograph taken by K. Yada.) (c) Pyrolusite polycrystalline dendritic pattern formed along a bedding plane of sedimentary rock. 

It should be noted that all above-mentioned results have been obtained using polycrystalline titanium dioxide (anatase, rutile) [49, 51] on the whole, the same regularities are observed during the control experiments with the monocrystalline rutile. When going from poly- to nanocrystalline Ti02 obtained by zol-gel method, the EER spectrum of the oxide substantially changes [53]. 

Fig. 8.7 presents our experimental results plotted by the data published in [128, 129] obtained for polycrystalline Ti02 (rutile) lattice doped with vanadium (IV) ions at different content. The linear dependence of V4+ amount (in spin/g) on total vanadium content (in at.%) shows that all vanadium ions in these samples are in (+4) state while the non-linear graph of Cioc allows to assume that some part of V4+ centers is distributed in the lattice not randomly. Such systems will be discussed in detail in section 8.5. One can see from Fig. 8.7 that Cloc and (r) values can be easily estimated by such a simple approximation as Eqs. (8.7) and (8.8). 

Computer simulations of V4+ signals were also very useful for the determination of the EPR parameters, and they confirmed the interpretation of the experimental spectra as it had been done in [145-149] for vanadium doped Ti02 colloid powders and bulk samples. The EPR parameters of V4+ ions in Ti02 lattices (rutile, anatase and brookite) are summarized in Table 8.6 for nano-, polycrystalline and single crystal samples (NC, PC, SC). 

Samples of the ceramic polycrystalline Ti02 (rutile) doped electrodes of the VxTi . x02 composition were studied at different vanadium content (0.001 < x < 0,05) in [128, 129]. It was shown that at x < 0,003 the EPR spectra perform a well resolved hyperfine structure (hfs) typical of V4+-doped rutile (Fig. 8.10), in which V4+ ions substituted Ti4+ ions in the crystal lattice. At 0.003 < x < 0.01, the dipolar broadening of the individual lines 8H occurred. At x > 0.01, in parallel with continuing broadening of the hfs lines, a broad single line appears (Fig. 8.11). Its part in the spectrum increased with the increase of vanadium content. 

Rutile is anisotropic, with the values of er at room temperature being approximately 170 and 90 in the c and a directions respectively. In the polycrystalline ceramic form er averages to intermediate values with a... 

The external surface of a rutile crystal is almost entirely composed of the three crystal planes (1 10), (1 0 0) and (101). The relative area of each crystal face in a sample of finely divided rutile probably varies from one polycrystalline sample to another but it is generally assumed that 60-80% of the overall surface area of the powder is provided by the (1 10) plane, with the remainder divided equally between the two other planes (Jaycock and Waldsax, 1974 Boddenberg and Eltzner 1991). 

A fairly low-area sample of polycrystalline Ti02 (98.5% rutile) was used by Grillet... 

The materials mentioned in the previous sections had permittivities approximately equal to 6. Values exceeding 10 are rare among standard substances. The first exception discovered was the permittivity of 86 to 170 exhibited by Ti02 (rutile) as a function of crystallographic orientation. This discovery was exploited in the manufacture of rutile ceramics in the thirties. The theoretically attainable value for polycrystalline materials is about 105. In practice, the permittivity of rutile ceramics is always lower owing to the presence of pores and foreign phases. 

Figure 9. Electron micrograph of polycrystalline (Al,Sb,V,W)204 (left), and the corresponding electron diffraction pattern with basic rutile rings (right).

The optical properti es of thin polycrystalline films are influenced by the extent of grain orientation, which is also manifested in the Raman band intensities for vibrational modes of different symmetry. Figure 9a illustrates the two strongest Raman active modes for the rutile phase of Ti02. The Eg assigned mode exhibits a vibrational frequency of 444 cm"l, while the frequency for the Aig mode is 608 cm l. A series of 0.6 micrometer thick rutile films with variable ordering in the grain structure was prepared by sputter deposition techniques. Refracti ve indices... 

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