4. Optical properties and radiation stability of titanium dioxide powders after heating and modification with nanoparticles

#### 4.1. Reflection spectra of modified powders

joint influence of absorption coefficient of point defects and free electrons determines the reflection coefficient in the second region: the larger is the Δr on the first and third regions,

Qualitatively, the change in diffuse reflection spectra obtained after irradiation (ΔrF) is the same for all powders (Figure 3). They are absorption spectra induced by accelerated electron exposure. The spectra include the bands in the visible range with the maximum at 2.9 eV and

The ΔrE<sup>а</sup> spectra of powders n60 significantly differ from m240, n160, and n80, where three regions can be distinguished with qualitatively difference of absorption coefficient. The first region is characterized by the presence of absorption bands in UV and visible ranges. The second one has absorption in the range of 2–1.5 eV. The third one is in the range above 1.5 eV

In order to understand the origin of bands and to conduct an analysis of nanoparticle size effect on absorption center accumulation, the Δr<sup>E</sup> spectra were decomposed on elementary bands [19–23]. The function of decomposition consists of 80% of Gaussian and 20% of Lorentzian functions [24]. From decomposition of the induced irradiation spectra on absorption spectra, it follows that during electron exposure in m240, n160, n80, and n60 titanium dioxide

Figure 3. The change in diffuse reflection spectra of titanium dioxide powders m240 (A), n160 (B), n80 (C), and n60 (D) after electron irradiation with fluence of 0.5 (1), 1 (2), and 2<sup>10</sup><sup>16</sup> (3) cm<sup>2</sup> and after residual vacuum exposure (4).

X

, 12—0.69 eV.

1—Tii˙, 2—Oi´, 3—VO˙, 4—Tii˙˙, 5—Tii˙˙˙, 6—VO˙˙, 7—Oi´´, 8—Tii˙˙˙˙, 9—VTi´´´´, 10—VTi´´´, 11—VO

the larger is its value in the second region and the less the size of this region.

wide unstructured band in the near-IR range with maximums at 1 eV.

with absorption peak in the range of 1–0.7 eV.

490 Titanium Dioxide - Material for a Sustainable Environment

The rutile titanium dioxide pigment (m�240) was used for investigation of an influence of nanoparticle type of various oxide compounds on diffuse reflection spectra of modified TiO2 powder and their changes after accelerated electron irradiation [25]. The average grain size of nanoparticles was, nm: Al2O3—30, ZrO2—30, ZnO—50, MgO—60, SiO2—55, and TiO2—60. The specific surface area of the powders was equal to m2 /g: Al2O3—49, ZrO2—25, ZnO—20, MgO—26, SiO2—60, and TiO2—26. All nanopowders possess a crystalline structure except the SiO2 powder, which was amorphous.

Modification effects on r<sup>λ</sup> spectra are shown in Figure 4. In the range of 440 nm, the reflectivity of heated and modified powders increases by 2–3% (Figure 5). An exception is a powder modified with n-ZnO, the r value of which reduces by 2–4%. In the range of 580 nm, the reflectivity increases for powders doped by n-MgO and n-TiO2 only. For other modified powders, the reflectivity does not change noticeably, for the powder doped with ZnO nanoparticles—it decreases as well.

The r value of powders modified with n-TiO2 and n-SiO2 increases noticeably in the near-IR range at 850 nm. For other modified and heated powders, the r changes insignificantly. The increase in r is detected in the spectra of powders modified with n-MgO, n-ZrO2, n-ZnO, and n-SiO2 at 1200 nm. The largest increase is 4%. The increase in r in the range of 1800 nm is observed at both heating and modification by all nanopowders. The largest value, equaled to 4.7%, belongs to powder modified with n-ZnO.

The change in r<sup>λ</sup> spectra of powders at heating and high-temperature modification can be because of the distinction between the reflection coefficient of nanoparticles and the reflection coefficient of micropowder. This distinction is determined by large value of nanoparticle scattering coefficient in comparison to micro-sized particles and will appear, mainly, in the visible range of spectra.

Another reason of the reflection coefficient distinction is desorption of gases molecules from the surface at heating, the main component of which is OH-groups and molecules of water [24, 26]. The desorption leads to a reduction of absorption band intensity in the near-IR range

Figure 5. The value of reflection coefficient of unmodified TiO2 powders and modified with nanoparticle of various oxide

Investigation of Optical Properties and Radiation Stability of TiO2 Powders before and after…

http://dx.doi.org/10.5772/intechopen.74073

493

An oxygen chemisorption, simultaneously with desorption, on the freed bonds on the surface and oxygen diffusion into the bulk of powder grains occur at heating in atmosphere. This leads to a decrease in the concentration of native point defects, to a decrease in the intensity of the absorption bands, and to an increase in the reflection coefficient in the regions where the bands

Radiation stability was estimated from the difference diffuse reflection spectra (Δrλ), in which two parts can be distinguished, determined by the absorption bands. In the visible range the bands at 420–580 nm are registered, in the near-IR range at 850–1200 nm (Figure 6). The position of maximums changes depending on type of nanopowder. Two bands appear in some spectra and one band in others. Based on types and number of absorption bands in these ranges of TiO2 absorption spectra, it can be assumed that these integral contours include a large number of elementary bands. The Δr values in these regions are different: for some powders, they are approximately the same; for others, the intensity of the band in the near-IR region is substantially

Judging by the Δr<sup>λ</sup> values, it can be concluded that the largest changes occur during irradiation in the visible range in the powder heated at 150С and in the powders modified with n-

MgO and n-ZnO. The changes in r<sup>λ</sup> spectra reach 27% after exposure.

at 1240, 1420, 1950, and 2250 nm [27–29].

4.2. Radiation stability of modified powders

are located.

compounds.

larger.

Figure 4. The diffuse reflection spectra of heated at 150 and 800C unmodified TiO2 powder and modified with nanoparticles of various oxide compounds in quantity of 7 mass% at 800C.

Investigation of Optical Properties and Radiation Stability of TiO2 Powders before and after… http://dx.doi.org/10.5772/intechopen.74073 493

Figure 5. The value of reflection coefficient of unmodified TiO2 powders and modified with nanoparticle of various oxide compounds.

Another reason of the reflection coefficient distinction is desorption of gases molecules from the surface at heating, the main component of which is OH-groups and molecules of water [24, 26]. The desorption leads to a reduction of absorption band intensity in the near-IR range at 1240, 1420, 1950, and 2250 nm [27–29].

An oxygen chemisorption, simultaneously with desorption, on the freed bonds on the surface and oxygen diffusion into the bulk of powder grains occur at heating in atmosphere. This leads to a decrease in the concentration of native point defects, to a decrease in the intensity of the absorption bands, and to an increase in the reflection coefficient in the regions where the bands are located.
