3. Optical properties and radiation stability of micro and nanopowders of titanium dioxide before modification

The study objects were the TiO2 submicron sized powders of industry production with an average particle size of 240 nm (m�240), as well as nanopowders with an average particle size of 60 (n�60), 80 (n�80), and 160 (n�160) nm. The specific surface of m�240, n�160, n�80, and <sup>n</sup>�60 powders was 8.0, 13, 16, and 28 m<sup>2</sup> /g, respectively [17].

The m�240 powder possessed rutile lattice, and the nanopowders were the mixture of anatase and rutile. The ratio of these phases changed with the change in particle size. With a decrease in particle size from 160 down to 80 and 60 nm, the concentration of rutile changed from 50 down to 33.9 and 98.7 mass%, the anatase concentration was 49.8, 66.1, and 1.3 mass%.

The largest value of the reflection coefficient (r0) was registered in rλ<sup>0</sup> spectra of micropowder m�240. The nanopowders n�160, n�80, and n�60 follow in order of its decrease (Figure 1).

The absolute values of the reflection coefficient on the various regions of the spectrum reduce with decreasing powder particle sizes. Its largest values are registered in the range from 500 to 1200 nm. The reflection coefficient reduces in more short-wavelength (λ < 500 nm) and more long-wavelength (λ >1200 nm) ranges. The more is the decrease, the less is the size particles of powders. In the short-wavelength region, the decrease in the reflection coefficient is sharp with pronounced outlines of the absorption bands, and in the long-wavelength region, the decay is

Figure 1. The diffuse reflection spectra of TiO2 powders measured in vacuum (in situ).

smooth. The more is the reflection coefficient, the less is the specific surface of the powders in the range of values 8–13 m2 /g.

Such a character of the difference in the reflection coefficient on the various regions of the spectrum allows to believe that its reduction is determined by a variety of factors. For their identification, the difference diffuse reflection spectra were obtained by means of subtraction from 100% value of r in rE0 spectra, which depend on energy:

$$
\Delta \rho\_{\rm E0} = (100 - \rho\_{\rm E0}) \tag{3}
$$

The third region in the vicinity above 1.5 eV is determined by increasing values of absorption coefficient with an increase in energy. The slope of these absorption contours is different for various powders: with reducing particle size, the slope increases. In the near-IR range, the absorption of semiconductors is defined by free electrons, which is described by a power law

Figure 2. The absorption spectra of the TiO2 powders before modification: 1—Tii˙, 2—Oi´, 3—VO˙, 4—Tii˙˙, 5—Tii˙˙˙,

X

The power function is defined by free electron transitions between levels in the conductance band. That is why, the exponent can be a measure of free electron concentration [18]. The calculations showed that with the decrease in particle sizes of powders the exponent n increases (Table 1). It means that the concentration of free electrons increases with decreasing

Thus, the reflection coefficient of initial powders is determined by native point defects and free electrons. The less is the particle sizes of powders, the larger is the concentration of native point defects on the surface and the concentration of free electrons. The particle size defines a value of the reflection coefficient on the first and third regions of the reflection spectra. The

Average particle size of the powders, nm 60 80 160 240 n 1.67 1.53 1.48 0.85

Table 1. The dependence of exponent n on the particle sizes of TiO2 powders.

<sup>Δ</sup><sup>r</sup> <sup>¼</sup> ηλ<sup>n</sup> (4)

, 12–0.69 eV, 13—free carriers of charge.

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489

of absorption coefficient from wavelength:

6—VO˙˙, 7—Oi´´, 8—Tii˙˙˙˙, 9—VTi´´´´, 10—VTi´´´, 11—VO

grain sizes or increasing specific surface.

Such spectra (Figure 2) are the absorption spectra of powders before modification. They indicate which absorption bands define a decrease in reflection coefficient. Qualitatively, all spectra are similar. They consist of three regions. First region from absorption edge up to 2.5 eV is characterized by several absorption bands. These bands are encased in integral contour, which is defined by native point defects of titanium dioxide. The intensity of this band in the maximum is 15, 33, 38, and 60%. It is inversely proportional to particle sizes of powders m�240, n�160, n�80, and n�60.

The second region is characterized by an absence of relationship between Δr and energy. Its length depends on grain size of powders: the less is the grain size, the shorter is the region. The second region of m�240 powder is in the range of 3–1 eV, n�60 powder—in the range of 2.25–1.5 eV, which has an absorption by Tii˙, Oi´, VO˙, and Tii˙˙ defects. There is a dependence of Δr values on particle size of powders: the less is the particle size, the larger is the absorption coefficient. Since nanoparticles possess the large specific surface, the concentration of native lattice defects on their surface is higher than in micropowders, and consequently, the larger concentration of interstitial titanium ions (Tii X) will be in the nanoparticles.

Investigation of Optical Properties and Radiation Stability of TiO2 Powders before and after… http://dx.doi.org/10.5772/intechopen.74073 489

Figure 2. The absorption spectra of the TiO2 powders before modification: 1—Tii˙, 2—Oi´, 3—VO˙, 4—Tii˙˙, 5—Tii˙˙˙, 6—VO˙˙, 7—Oi´´, 8—Tii˙˙˙˙, 9—VTi´´´´, 10—VTi´´´, 11—VO X , 12–0.69 eV, 13—free carriers of charge.

The third region in the vicinity above 1.5 eV is determined by increasing values of absorption coefficient with an increase in energy. The slope of these absorption contours is different for various powders: with reducing particle size, the slope increases. In the near-IR range, the absorption of semiconductors is defined by free electrons, which is described by a power law of absorption coefficient from wavelength:

$$
\Delta \rho = \eta \lambda^n \tag{4}
$$

The power function is defined by free electron transitions between levels in the conductance band. That is why, the exponent can be a measure of free electron concentration [18]. The calculations showed that with the decrease in particle sizes of powders the exponent n increases (Table 1). It means that the concentration of free electrons increases with decreasing grain sizes or increasing specific surface.

Thus, the reflection coefficient of initial powders is determined by native point defects and free electrons. The less is the particle sizes of powders, the larger is the concentration of native point defects on the surface and the concentration of free electrons. The particle size defines a value of the reflection coefficient on the first and third regions of the reflection spectra. The


Table 1. The dependence of exponent n on the particle sizes of TiO2 powders.

smooth. The more is the reflection coefficient, the less is the specific surface of the powders in

Such a character of the difference in the reflection coefficient on the various regions of the spectrum allows to believe that its reduction is determined by a variety of factors. For their identification, the difference diffuse reflection spectra were obtained by means of subtraction

Such spectra (Figure 2) are the absorption spectra of powders before modification. They indicate which absorption bands define a decrease in reflection coefficient. Qualitatively, all spectra are similar. They consist of three regions. First region from absorption edge up to 2.5 eV is characterized by several absorption bands. These bands are encased in integral contour, which is defined by native point defects of titanium dioxide. The intensity of this band in the maximum is 15, 33, 38, and 60%. It is inversely proportional to particle sizes of

The second region is characterized by an absence of relationship between Δr and energy. Its length depends on grain size of powders: the less is the grain size, the shorter is the region. The second region of m�240 powder is in the range of 3–1 eV, n�60 powder—in the range of 2.25–1.5 eV, which has an absorption by Tii˙, Oi´, VO˙, and Tii˙˙ defects. There is a dependence of Δr values on particle size of powders: the less is the particle size, the larger is the absorption coefficient. Since nanoparticles possess the large specific surface, the concentration of native lattice defects on their surface is higher than in micropowders, and consequently, the larger

ΔrЕ<sup>0</sup> ¼ 100 � rЕ<sup>0</sup> ð Þ (3)

X) will be in the nanoparticles.

the range of values 8–13 m2

488 Titanium Dioxide - Material for a Sustainable Environment

/g.

Figure 1. The diffuse reflection spectra of TiO2 powders measured in vacuum (in situ).

from 100% value of r in rE0 spectra, which depend on energy:

powders m�240, n�160, n�80, and n�60.

concentration of interstitial titanium ions (Tii

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

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

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

powders, the negatively charged defects VTi´´´´, VTi´´´, and VO

VO˙, and Tii˙˙ in the UV ranges are formed.

X

TiTiX þ OO

TiTi

<sup>X</sup> <sup>þ</sup> OO

4.1. Reflection spectra of modified powders

<sup>X</sup> <sup>þ</sup> <sup>e</sup>

<sup>X</sup> <sup>þ</sup> <sup>e</sup>

' <sup>∗</sup> ! TiTi

after heating and modification with nanoparticles

VTi´´´´, VTi´´´, and VO

(OO

IR range, and then positively charged defects Tii˙˙˙, VO˙˙, Oi´´, Tii˙˙˙˙ in the visible and Tii˙, Oi´,

Table 2. The dependence of integral area of absorption bands in the spectra of TiO2 powders on electron fluence.

Fluence, 1016 cm�<sup>2</sup> <sup>m</sup>�<sup>240</sup> <sup>n</sup>�<sup>160</sup> <sup>n</sup>�<sup>80</sup> <sup>n</sup>�<sup>60</sup> F0.5 46 30 32.22 51.5 F1 51.77 39.28 42.76 56.43 F2 56.40 43.56 46.99 62

The analysis of the integral area of the bands during a variety of fluences of electron exposure with energy of 30 keV shows (Table 2) that the highest radiation stability belongs to powders with 80–160 nm particle size and the powders with 60 nm particle size possess the lowest radiation stability. Advantageously, the difference is due to a greater concentration of defects

band generation close to 2.17 eV in difference diffuse reflection spectra. The similar band is registered in the spectrum of n�80 nanopowder, but its intensity is significantly less than n�60. Elementary processes leading to formation and accumulation of such defects are described by

X)\*—titanium and oxygen atoms and ions in points of lattice; (H˙)\*, H˙, (e´)\*, e´—accelerated and thermolyzed proton and electron, correspondingly; Tii˙˙˙˙, Tii˙˙˙, Tii˙˙, Tii˙, VTi´´´´, VTi´´´, VTi´´, VTi´, Oi´´, Oi´, VO˙˙, VO˙—interstitial ions and vacancies of titanium and oxygen in various charge states; and h˙—hole. The impact of accelerated electrons leads to interstitial titanium and oxygen generation and corresponding vacancies due to ionizing displacement mechanisms, electrical repulsion from the same charged ions placed close to each other, or the

reactions given below, where the following designations were accepted: TiTi

displacement of neighboring simultaneously ionized atoms by the following reactions:

<sup>X</sup> <sup>þ</sup> VO˙˙ <sup>þ</sup> Oi

'' \$ VO

4. Optical properties and radiation stability of titanium dioxide powders

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

' <sup>∗</sup> ! <sup>V</sup>Ti'''' <sup>þ</sup> Tii˙˙˙˙ <sup>þ</sup> OO

VO˙ þ Oi

, as well as due to Tii˙˙˙ and VO˙˙ formed defects, which lead to absorption

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

<sup>X</sup> <sup>þ</sup> <sup>e</sup>

'' <sup>þ</sup> <sup>e</sup>

<sup>X</sup> <sup>þ</sup> Oi

<sup>V</sup>Ti'''' <sup>þ</sup> Tii˙˙˙ \$ <sup>V</sup>Ti''' <sup>þ</sup> Tii˙˙ \$ <sup>V</sup>Ti'' <sup>þ</sup> Tii˙ (6)

' \$ TiTi

' \$ <sup>V</sup>Ti'''' <sup>þ</sup> Tii˙˙˙ <sup>þ</sup> OO

<sup>X</sup> <sup>þ</sup> VO˙ <sup>þ</sup> Oi

' (8)

<sup>X</sup> are formed, mostly, in the near-

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X, OO

X, (TiTi

<sup>X</sup> (5)

'' (7)

X)\*,

491

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). 1—Tii˙, 2—Oi´, 3—VO˙, 4—Tii˙˙, 5—Tii˙˙˙, 6—VO˙˙, 7—Oi´´, 8—Tii˙˙˙˙, 9—VTi´´´´, 10—VTi´´´, 11—VO X , 12—0.69 eV.


Table 2. The dependence of integral area of absorption bands in the spectra of TiO2 powders on electron fluence.

powders, the negatively charged defects VTi´´´´, VTi´´´, and VO <sup>X</sup> are formed, mostly, in the near-IR range, and then positively charged defects Tii˙˙˙, VO˙˙, Oi´´, Tii˙˙˙˙ in the visible and Tii˙, Oi´, VO˙, and Tii˙˙ in the UV ranges are formed.

The analysis of the integral area of the bands during a variety of fluences of electron exposure with energy of 30 keV shows (Table 2) that the highest radiation stability belongs to powders with 80–160 nm particle size and the powders with 60 nm particle size possess the lowest radiation stability. Advantageously, the difference is due to a greater concentration of defects VTi´´´´, VTi´´´, and VO X , as well as due to Tii˙˙˙ and VO˙˙ formed defects, which lead to absorption band generation close to 2.17 eV in difference diffuse reflection spectra. The similar band is registered in the spectrum of n�80 nanopowder, but its intensity is significantly less than n�60.

Elementary processes leading to formation and accumulation of such defects are described by reactions given below, where the following designations were accepted: TiTi X, OO X, (TiTi X)\*, (OO X)\*—titanium and oxygen atoms and ions in points of lattice; (H˙)\*, H˙, (e´)\*, e´—accelerated and thermolyzed proton and electron, correspondingly; Tii˙˙˙˙, Tii˙˙˙, Tii˙˙, Tii˙, VTi´´´´, VTi´´´, VTi´´, VTi´, Oi´´, Oi´, VO˙˙, VO˙—interstitial ions and vacancies of titanium and oxygen in various charge states; and h˙—hole. The impact of accelerated electrons leads to interstitial titanium and oxygen generation and corresponding vacancies due to ionizing displacement mechanisms, electrical repulsion from the same charged ions placed close to each other, or the displacement of neighboring simultaneously ionized atoms by the following reactions:

$$\text{Ti}\_{\text{Ti}}X + \text{O}\_{\text{O}}^{X} + \text{(e}^{\text{\textquotedblleft}}\text{\textquotedblright} \rightarrow \text{V}\_{\text{Ti}}\overset{\text{\textquotedblleft}}{\text{}}^{\text{\textquotedblleft}} + \text{Ti}\_{\text{i}}\overset{\text{\textquotedblleft}}{\text{}}^{\text{\textquotedblleft}} + \text{O}\_{\text{O}}^{X} + \text{e}^{\text{\textquotedblleft}} \leftrightarrow \text{V}\_{\text{Ti}}\overset{\text{\textquotedblright}}{\text{}}^{\text{\textquotedblleft}} + \text{Ti}\_{\text{i}}\overset{\text{\textquotedblleft}}{}^{\text{\textquotedblleft}} + \text{O}\_{\text{O}}^{X} \tag{5}$$

$$V\_{\text{Ti}}\stackrel{\text{??}}{}+\text{Ti}\_{\text{i}}\stackrel{\text{??}}{}\leftrightarrow V\_{\text{Ti}}\stackrel{\text{??}}{}+\text{Ti}\_{\text{i}}\stackrel{\text{??}}{}\leftrightarrow V\_{\text{Ti}}\stackrel{\text{??}}{}+\text{Ti}\_{\text{i}}\text{'}\tag{6}$$

$$\text{Ti}\_{\text{Ti}}^X + \text{O}\_{\text{O}}^X + \text{(e}^\circ\text{)}^\* \rightarrow \text{Ti}\_{\text{Ti}}^X + \text{V}\_{\text{O}}^\circ\text{--}^\circ + \text{O}\_{\text{i}}^\circ + \text{e}^\circ \leftrightarrow \text{Ti}\_{\text{Ti}}^X + \text{V}\_{\text{O}}^\circ\text{--}^\circ \tag{7}$$

$$\boldsymbol{V\_O}^{\cdot} + \mathbf{O}\_i^{\cdot \cdot} \leftrightarrow \boldsymbol{V\_O}^{\times} + \mathbf{O}\_i^{\cdot \cdot} \tag{8}$$
