4.3. The effect of temperature at heating and modification by SiO2 nanoparticles on the radiation stability of TiO2 powders

A comparison of the r<sup>λ</sup> spectra of heated TiO2 powders shows (Figure 7) that with an increase in the heating temperature from 150 to 400�C, the reflection coefficient varies in different regions of the spectrum according to various regularities [26]. In the region from the absorption edge up to 600 nm, it increases so that an absorption band with a maximum at 400–405 nm is formed in the difference spectrum determined by the relation:

$$
\Delta \rho\_{\lambda} = \rho\_{\lambda 150} - \rho\_{\lambda 400} \tag{9}
$$

where rλ<sup>150</sup> and rλ<sup>400</sup> are the reflection coefficients of the powder heated at a temperature of 150 and 400�C, respectively.

In the 600–900 nm region, the reflection coefficient slightly decreases with a minimum value of 1.7% at 700 nm. In the longer wavelength region, it increases in accordance with power law of the wavelength [30, 31].

An increase in the heating temperature up to 800�C leads to the appearance of an absorption band at 380–390 nm in the difference spectrum (Δr<sup>λ</sup> = rλ<sup>150</sup> � rλ800). At 450–680 nm, the changes are close to zero, and in the region of 680–2100 nm, a power function of the wavelength with a maximum value Δr = 8.7% is recorded.

Modification of the TiO2 powder with SiO2 nanoparticles and heating at 400�C, both lead to a decrease in the reflection coefficient over the entire spectrum (Figure 8). At the same time, an absorption band is recorded in the region from the absorption edge up to 600 nm with a maximum at 500 nm, and in the longer wavelength region, the reflection coefficient changes without certain regularities, the Δr values are 2–3%.

The radiation stability of TiO2 powders modified by all types of nanoparticles is higher compared to unmodified powder but heated at 150С. A reduction in Δr values of modified powders is registered in entire range of Δr<sup>λ</sup> spectra (Figure 6). The effectiveness of modification (Δr150/Δrmod) reaches almost two times in the visible range. In the near-IR range, it is even larger and reaches more than six times. The best result in the visible range corresponds to n-

Figure 6. The Δr values of heated and modified TiO2 powders by nanoparticles of different oxide compounds for various

Radiation stability in the visible region of the spectrum of TiO2 powder heated at 800С is the same or even higher in comparison to the modified powders. Only the modification with n-ZrO2 gives, although not significant, an increase in radiation stability compared to the heated powder. Modification with n-SiO2 nanopowder results in a slight decrease and the modification with n-MgO, n-ZnO, n-Al2O3, and n-TiO2 to a noticeable decrease in radiation stability in

In the near-IR region, the modification with some nanopowders has a significant effect on the radiation stability. The largest effect was obtained using n-SiO2 and n-ZrO2. Then, mixtures with n-Al2O3 and n-TiO2 follow. The least effect from the modification was obtained using n-

With respect to the aggregate values of Δr in the visible and near-IR regions of the spectrum, the series of the largest effect at modifying with nanoparticles is as follows: 1—SiO2, 2—ZrO2, 3—Al2O3, 4—TiO2, 5—MgO, and 6—ZnO. The largest effect in increasing the radiation stability of micropowders of titanium dioxide is obtained by modifying with n-SiO2 and n-ZrO2 and

ZrO2 modification, in the near-IR range—n-SiO2.

494 Titanium Dioxide - Material for a Sustainable Environment

the smallest by the modification with n-MgO and n-ZnO.

comparison to the heated powder.

MgO and n-ZnO.

wavelengths.

An increase in the heating temperature up to 800�C at modifying TiO2 powder leads to a decrease in the reflection coefficient in the region from the absorption edge up to 600 nm and

Figure 7. The diffuse reflection spectra of unmodified titanium dioxide powders heated at various temperatures.

η ¼ Δas

where <sup>Δ</sup>a<sup>s</sup> values after irradiation with 0.5, 1, and 2 � 1016 cm�<sup>2</sup> electron fluence: <sup>Δ</sup>a<sup>s</sup>

(Figure 10) in comparison to its changes in the range of F = (0.5–1) � 1016 cm�<sup>2</sup>

With an increase in the electron fluence from F = 1 � 1016 cm�<sup>2</sup> to F = 2 � 1016 cm�<sup>2</sup>

coefficient η varies insignificantly for all values of the heating and modifying temperature

us to assume that the values obtained at F = 2 � 1016 cm�<sup>2</sup> are close to the steady-state values. The decrease in the efficiency of silica nanopowder modification with increasing exposure time or electron fluence is, probably, determined by the contribution of the surface preradiation defects in the total concentration of radiation defects formed by irradiation both in the TiO2

the diffuse reflection spectra and the integral absorption coefficient is made by preradiation

Upon irradiation by accelerated electrons with prethreshold energies (e\*), the processes of formation and separation of charge carriers in titanium dioxide proceed according to the scheme:

<sup>∗</sup> ! <sup>e</sup>

b. Hole drift toward negatively charged surface, then an interaction with sorbed radicals and

Holes move to the surface, where they first interact with surface oxygen, then with the oxygen of the lattice that leads to its radiolysis. If there are defects on the surface, then holes relax on these defects and SiO2 nanoparticles do not play a special role in increasing the radiation stability. And only at an optimal value of the nanoparticle concentration which is sufficient to create the necessary number of relaxation centers along with the available native surface

Based on the results of studies of the temperature effect on optical properties (Figures 9 and 10) at the TiO2 powders heating and their modification by SiO2 nanoparticles, the following can be

defects—native relaxation centers, the modification becomes effective.

TiO2 þ e

powders heated at 150�C, Δa<sup>s</sup>

micropowder and in SiO2 nanoparticles.

c. Neutralization of surface oxygen by holes

d. Oxygen formation and its escape in vacuum

a. Hole-electron formation

their oxidation

concluded:

SiO2 nanoparticles at a temperature of 400 or 800�C.

When the electron fluence is less than 0.5 � 1016 cm�<sup>2</sup>

defects on the grain surface of the crystal lattice of the TiO2 powder.

<sup>150</sup>=Δas

<sup>T</sup> (10)

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

, the main contribution to the change in

� þ p<sup>þ</sup> (11)

p<sup>þ</sup> þ R ! RO� (12)

p<sup>þ</sup> þ Osð Þ! Ol O (13)

O þ O ! O2↑ (14)

<sup>T</sup> are the Δa<sup>s</sup> values of TiO2 powder modified with 7 mass% of

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

<sup>150</sup>—TiO2

. This allows

, the

497

Figure 8. Diffuse reflection spectra of TiO2 powders heated at different temperatures and modified with SiO2 nanopowders.

its increase in the longer wavelength region. The Δr values are no more than 1% except for two regions: the bands with a maximum at 400–405 nm and the "tail" of absorption in the range of 900–2100 nm reaching 3.5% and 3.7%, respectively.

Analysis of the r<sup>λ</sup> spectra of heated and modified by nanoparticles at different temperatures of titanium dioxide powders shows that the change in the reflection coefficient in the entire spectral region is determined by several processes, which lead to the appearance of qualitatively different dependences. A band with a maximum at 400–500 nm caused by defects of cationic sublattice (interstitial ions or absorption vacancies) is on the first region in the range from the absorption edge to 600 nm of TiO2. The range from 680 or 900 nm to 2100 nm is the second region and can be described by a power-law dependence of the absorption coefficient from the wavelength because of electron transitions between levels in the conduction band of TiO2. In the interval between these two regions, the difference values of the reflection coefficient change insignificantly or form an absorption band with a maximum at 700 nm.

Therefore, heating the TiO2 powder at 400 and 800C without nanoparticles and in mixtures with SiO2 nanoparticles leads to a change in the concentration of native point defects and free electrons on the surface. Such changes are determined by the desorption of physically and chemically sorbed gases, the release of bonds, their filling with other molecules and atoms, i.e., redistribution of defects and electronic state on the surface.

Exposure with electrons leads to a decrease in the reflection coefficient over the entire spectrum as a result of the formation of radiation defects and the appearance of the absorption bands induced by them. With an increase in the heating temperature from 150 to 400C, very slight changes in the absorption coefficient occur (Figure 9).

Since the heating at 150C does not give noticeable changes in the absorption coefficient as, then the Δa<sup>s</sup> values of the heated powder at 150C are taken as the values of the initial (unheated) powder, and the efficiency of the TiO2 micropowder modification by the SiO2 nanopowder in an amount of 7 mass% at a temperature of 400 or 800 C was computed as follows:

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$$
\eta = \Delta \mathbf{a}\_s^{150} / \Delta \mathbf{a}\_s^{T} \tag{10}
$$

where <sup>Δ</sup>a<sup>s</sup> values after irradiation with 0.5, 1, and 2 � 1016 cm�<sup>2</sup> electron fluence: <sup>Δ</sup>a<sup>s</sup> <sup>150</sup>—TiO2 powders heated at 150�C, Δa<sup>s</sup> <sup>T</sup> are the Δa<sup>s</sup> values of TiO2 powder modified with 7 mass% of SiO2 nanoparticles at a temperature of 400 or 800�C.

With an increase in the electron fluence from F = 1 � 1016 cm�<sup>2</sup> to F = 2 � 1016 cm�<sup>2</sup> , the coefficient η varies insignificantly for all values of the heating and modifying temperature (Figure 10) in comparison to its changes in the range of F = (0.5–1) � 1016 cm�<sup>2</sup> . This allows us to assume that the values obtained at F = 2 � 1016 cm�<sup>2</sup> are close to the steady-state values.

The decrease in the efficiency of silica nanopowder modification with increasing exposure time or electron fluence is, probably, determined by the contribution of the surface preradiation defects in the total concentration of radiation defects formed by irradiation both in the TiO2 micropowder and in SiO2 nanoparticles.

When the electron fluence is less than 0.5 � 1016 cm�<sup>2</sup> , the main contribution to the change in the diffuse reflection spectra and the integral absorption coefficient is made by preradiation defects on the grain surface of the crystal lattice of the TiO2 powder.

Upon irradiation by accelerated electrons with prethreshold energies (e\*), the processes of formation and separation of charge carriers in titanium dioxide proceed according to the scheme:

a. Hole-electron formation

its increase in the longer wavelength region. The Δr values are no more than 1% except for two regions: the bands with a maximum at 400–405 nm and the "tail" of absorption in the range of

Figure 8. Diffuse reflection spectra of TiO2 powders heated at different temperatures and modified with SiO2

Analysis of the r<sup>λ</sup> spectra of heated and modified by nanoparticles at different temperatures of titanium dioxide powders shows that the change in the reflection coefficient in the entire spectral region is determined by several processes, which lead to the appearance of qualitatively different dependences. A band with a maximum at 400–500 nm caused by defects of cationic sublattice (interstitial ions or absorption vacancies) is on the first region in the range from the absorption edge to 600 nm of TiO2. The range from 680 or 900 nm to 2100 nm is the second region and can be described by a power-law dependence of the absorption coefficient from the wavelength because of electron transitions between levels in the conduction band of TiO2. In the interval between these two regions, the difference values of the reflection coeffi-

cient change insignificantly or form an absorption band with a maximum at 700 nm.

Therefore, heating the TiO2 powder at 400 and 800C without nanoparticles and in mixtures with SiO2 nanoparticles leads to a change in the concentration of native point defects and free electrons on the surface. Such changes are determined by the desorption of physically and chemically sorbed gases, the release of bonds, their filling with other molecules and atoms, i.e.,

Exposure with electrons leads to a decrease in the reflection coefficient over the entire spectrum as a result of the formation of radiation defects and the appearance of the absorption bands induced by them. With an increase in the heating temperature from 150 to 400C, very

Since the heating at 150C does not give noticeable changes in the absorption coefficient as, then the Δa<sup>s</sup> values of the heated powder at 150C are taken as the values of the initial (unheated) powder, and the efficiency of the TiO2 micropowder modification by the SiO2 nanopowder in an

amount of 7 mass% at a temperature of 400 or 800 C was computed as follows:

900–2100 nm reaching 3.5% and 3.7%, respectively.

496 Titanium Dioxide - Material for a Sustainable Environment

nanopowders.

redistribution of defects and electronic state on the surface.

slight changes in the absorption coefficient occur (Figure 9).

$$\text{TiO}\_2 + e^\* \to e^- + p^+ \tag{11}$$

b. Hole drift toward negatively charged surface, then an interaction with sorbed radicals and their oxidation

$$p^{+} + \text{R} \rightarrow \text{RO}^{-} \tag{12}$$

c. Neutralization of surface oxygen by holes

$$p^{+} + O\_{s}(O\_{l}) \rightarrow O \tag{13}$$

d. Oxygen formation and its escape in vacuum

$$O + O \to O\_2\uparrow\tag{14}$$

Holes move to the surface, where they first interact with surface oxygen, then with the oxygen of the lattice that leads to its radiolysis. If there are defects on the surface, then holes relax on these defects and SiO2 nanoparticles do not play a special role in increasing the radiation stability. And only at an optimal value of the nanoparticle concentration which is sufficient to create the necessary number of relaxation centers along with the available native surface defects—native relaxation centers, the modification becomes effective.

Based on the results of studies of the temperature effect on optical properties (Figures 9 and 10) at the TiO2 powders heating and their modification by SiO2 nanoparticles, the following can be concluded:


The fact that the radiation stability at the same heating temperature is higher in mixtures of TiO2 + SiO2 powders than TiO2 powders may indicate about a change in the catalytic activity, specific surface area, and concentration of chemisorbed gases upon the addition of SiO2 nanoparticles. The reduction in the catalytic activity which depends on the anionic vacancy concentration and excess titanium on the surface [32] will occur at heating in an atmosphere with a large oxygen concentration. Such a heating, in turn, will lead to an increase in radiation stability.

Figure 10. Effectiveness of the heating and TiO2 powder modification (η) with SiO2 nanoparticles.

Figure 9. Dependence of Δa<sup>s</sup> values on the electron fluence with 30 keV energy of TiO2 powder heated at the temperature of 150, 400, and 800C and modified with SiO2 nanoparticles in an amount of 7 mass% by heating at the same temperature

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499

values.

Investigation of Optical Properties and Radiation Stability of TiO2 Powders before and after… http://dx.doi.org/10.5772/intechopen.74073 499

1. Heating at 400C does not give a noticeable increase in optical properties stability during electron exposure in comparison with the heating at 150C. The increase is significant for modified powders, and the coefficient η is 2.38, 1.8, and 1.73 for electron fluence (0.5, 1,

2. Heating at 800C leads to an increase in the radiation stability of both types of powders. In this case, the coefficient η is 1.82, 1.48, and 1.43 for unmodified and 2.65, 2.31, and 2.26 for

3. A comparison of these values shows that, with respect to the coefficient η and its dependence on the electron fluence, the modification at a temperature of 400C leads to approximately the same effect (η = 2.38, 1.8 and 1.73) as the heating at 800C (η = 1.82, 1.48, and 1.43). The effectiveness of such a method of processing powders is manifested: modification at a reduced temperature gives the same effect as heating at a higher temperature. 4. The lack of improvement in radiation stability from heating at 400C can be a consequence of the fact that at such a temperature, the surface of titanium dioxide particles is released only from physically bounded water on the surface and OH groups. Surface bonds are not released, and there is no chemisorption of oxygen, which does not give noticeable

5. A noticeable improvement in the radiation stability of the powder modified at 400C testifies to the effect of SiO2 nanoparticles, which are on the surface of grains and granules of titanium dioxide, as centers of relaxation of electronic excitations arise upon irradiation. This effect is more pronounced at low electron fluences (η = 2.38 at

determine a degradation. At such a temperature and electron fluence of 0.5 <sup>10</sup><sup>16</sup> and <sup>1</sup> <sup>10</sup>16, these defects, basically, transform to color centers. This is evidenced by the almost complete equality of the absorption coefficient at an increase in the heating temperature from 400–800C: Δa<sup>s</sup> = 0.029 for T = 400C and Δa<sup>s</sup> = 0.026 for T = 800C. From this point, the following can be concluded that in the case of small values of the electron fluence (or absorbed dose), a rather low heating temperature (400C) is suffi-

specimens heated at 400 and 800C increases from 0.03 up to 0.1 and 0.13, respectively. This means that for large values of fluence, the SiO2 nanoparticles on the surface of TiO2 grains and granules are not sufficient to perform the function of relaxation centers of electronic excitations. Therefore, in order to clean the surface from chemosorbed gases,

The fact that the radiation stability at the same heating temperature is higher in mixtures of TiO2 + SiO2 powders than TiO2 powders may indicate about a change in the catalytic activity, specific surface area, and concentration of chemisorbed gases upon the addition of SiO2 nanoparticles. The reduction in the catalytic activity which depends on the anionic vacancy concentration and excess titanium on the surface [32] will occur at heating in an atmosphere with a large oxygen concentration. Such a heating, in turn, will lead to an increase in radiation stability.

the appropriate conditions, such as heating at 800C, should be provided.

), i.e., when the surface preradiation defects in titanium dioxide

, respectively.

, then the change in Δa<sup>s</sup> values of

and 2) 1016 cm<sup>2</sup>

498 Titanium Dioxide - Material for a Sustainable Environment

F = 0.5 <sup>10</sup><sup>16</sup> cm<sup>2</sup>

cient at modification with nanoparticles.

6. If the electron fluence increases to (1–2) <sup>10</sup><sup>16</sup> cm<sup>2</sup>

, respectively.

modified powders for electron fluence (0.5, 1, and 2) 1016 cm<sup>2</sup>

changes in the concentration of anionic sublattice defects.

Figure 9. Dependence of Δa<sup>s</sup> values on the electron fluence with 30 keV energy of TiO2 powder heated at the temperature of 150, 400, and 800C and modified with SiO2 nanoparticles in an amount of 7 mass% by heating at the same temperature values.

Figure 10. Effectiveness of the heating and TiO2 powder modification (η) with SiO2 nanoparticles.

The reflectivity of modified powders can both increase, in comparison to the initial powders, and decrease. The reflection coefficient is determined by the grain sizes and, with their decrease, it increases, which occurs when nanopowders are added. The decrease in the reflection coefficient may be due to a large absorption by native point defects in the UV and visible ranges and by chemisorbed gases in the near-IR range of the spectrum, determined by the

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

Modification with nanopowders leads to an increase in the radiation stability of reflective powders, which is determined by the relaxation of electronic excitations on the surface of nanoparticles and by a smaller concentration of absorption centers formed upon irradiation. The maximum effect of increasing radiation stability is achieved by modification by nanopowders with a larger specific surface area and a smaller particle size. An additional factor affecting the increase in radiation stability is the heating during the modification, and the largest effect was obtained at

This work was supported by Ministry of Education and Science of Russian Federation (Agree-

, Vitaly V. Neshchimenko1,2, Semyon A. Yuryev1

1 Radiation and Space Materials Laboratory, Tomsk State University of Control Systems and

[1] Ma Y, Wang X, Jia Y, Chen X, Han H, Li C. Titanium dioxide-based Nanomaterials for Photocatalytic fuel generations. Chemical Reviews. 2014;114(19):9987-10043. DOI: 10.10

[2] Akpan UG, Hameed BH. Parameters affecting the photocatalytic degradation of dyes using TiO2-based photocatalysts: A review. Journal of Hazardous Materials. 2009;170(2–3):

2 Space Materials Laboratory, Amur State University, Blagoveshchensk, Russia

\* and

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501

larger specific surface area of nanopowders.

T = 800C.

Acknowledgements

ment No 1.8575.2017/8.9).

Author details

References

21/cr500008u

Mikhail M. Mikhailov<sup>1</sup>

Alexey N. Sokolovskiy<sup>3</sup>

Radio-electronics, Tomsk, Russia

3 University of Washington, Seattle, USA

\*Address all correspondence to: yusalek@gmail.com

520-529. DOI: 10.1016/j.jhazmat.2009.05.039

Figure 11. X-ray diffraction patterns of TiO2 powders modified with SiO2 nanoparticles at a variety of heating temperatures.

The heating of TiO2 powder mixtures with SiO2 nanoparticles at a temperature of 150, 400, and 800С cannot lead to the formation of new phases, because even at a higher heating temperature (900, 1000, and 1200С) of TiO2 + SiO2 powder mixtures, the Ti(1x)SixO2 solid solution is not formed [33].

A certain contribution to the increase in radiation stability at the highest heating temperature in the present studies, equaled to 800C, can be made by changing the phase state of TO2 powder the conversion of anatase to rutile, which is carried out at a temperature of 450–900C. The transition temperature is determined by the degree of crystallinity of the compound (films, polycrystals, and single crystals), the concentration of defects, and other factors [34, 35].

When SiO2 particles are added to the TiO2 powder, the phase transition is facilitated: transition temperature reduces and the relative rutile concentration increases [33]. Therefore, the TiO2 powder mixtures with SiO2 nanoparticles heated at 800C can give an increase in the relative rutile concentration, a change in the particle size and specific surface area, the concentration of absorbed gases, and stability to an impact of electron exposure. A confirmation of the increase in rutile concentration with increasing temperature is the results of X-ray phase analysis (Figure 11), which show that an increase in the heating temperature from 150–400C does not change the phase ratio, and further increase to 800C leads to the formation of rutile in an amount 10 mass%.
