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Jun 29, 2010 - Titanium dioxide (TiO2) with a rutile phase and tetragonal structure ... nescence (TL) properties for dosimeters which could be used in a wide ...
J Nanopart Res (2011) 13:77–85 DOI 10.1007/s11051-010-0002-7

RESEARCH PAPER

Structural and thermoluminescence properties of undoped and Fe-doped-TiO2 nanopowders processed by sol–gel method Marin Cernea • Mihail Secu • Corina Elisabeta Secu • Mihaela Baibarac Bogdan S. Vasile



Received: 10 November 2009 / Accepted: 11 June 2010 / Published online: 29 June 2010 Ó Springer Science+Business Media B.V. 2010

Abstract In this study, we report on the nanocrystalline powders of TiO2 and Fe-doped TiO2 (anatase and rutile phases) prepared by sol–gel method. The X-ray diffraction and Raman spectroscopy measurements indicated the presence of anatase or rutile phase in nanopowders. TEM micrographs showed 10 and 112 nm average particle sizes for anatase and rutile, respectively. Furthermore, their thermoluminescence properties were analyzed. Keywords Sol–gel method  TiO2  Fe-doped TiO2  Nanopowders  Thermoluminescence

Introduction Titanium dioxide (TiO2) with a rutile phase and tetragonal structure has been extensively used as a whitening agent in paints and pigments due to its good scattering effect against ultraviolet light. Thus, M. Cernea (&)  M. Secu  C. E. Secu  M. Baibarac National Institute of Materials Physics, Str. Atomistilor 105 Bis, RO-77125, P.O. BOX: MG-7, Magurele-Bucharest, Romania e-mail: [email protected] B. S. Vasile University Politehnica of Bucharest, Bucharest, Romania

the rutile phase protects materials from ultraviolet light (Kim et al. 2004). The anatase phase has been used as a photocatalysts for photodecomposition of organics dyes and also solar energy conversion because of its high photocatalytic activity (Usami 2000; Rothenberger et al. 1985; Bahnemann et al. 1997; Wang et al. 1997). In recent years, the TiO2-based ceramics have increased considerable interest due to thermoluminescence (TL) properties for dosimeters which could be used in a wide variety of research activities and applications of ionizing radiation dosimetry (AzorinVega et al. 2007). The TL investigations have also shown that defect centers play a crucial role in TL. The formation and the stability of the defect centers also depend on the method of preparation and dopants (Furetta and Weng 1998). Several articles have focused on the relationship between chemical, crystallographic, and morphologic characteristics of the pure and doped-TiO2 and its TL and photoluminescence (PL) properties (Zhang et al. 2000; Wang et al. 2006; Abazovic et al. 2006; Jin et al. 2001; Krishna et al. 1997; Frindell et al. 2003; Stouwdam and van Veggel 2004; Prociow et al. 2007; Lange et al. 2004; Bettinelli et al. 2006). Some transition metal ion dopants, such as Fe3?, V5?, Mn2?, Co3?, and Ni2? were used to extend the absorption threshold of TiO2 to visible light. Although the solubility of rare earth elements ions in TiO2 is expected to be low (due to mismatch of ionic radius, etc.;

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e.g., ionic radii of the Ti4? and Fe3? cations are 0.605 ˚ , respectively), the molecular-level mixing and 0.645 A of liquid precursors in the sol–gel process and low operation temperature allows achieving uniform distributions of dopants. Thus, in this work, we have performed a study of the structural and TL properties of sol–gel prepared TiO2 and Fe-doped-TiO2 powders, crystallized on the anatase and rutile phase.

Experimental Samples preparation An alkoxide sol–gel route was employed for preparing TiO2 and Fe-doped-TiO2 powders. The alkoxide was molecular modified with chelating agents like acetic acid and acetylacetone. Titanium (IV) butoxide, 97%, (reagent grade, Aldrich) and iron (III) nitrate nonahydrate, C99.99%, (Aldrich) were used as starting precursors, n-butanol, C99.4% (Aldrich) as solvent and acetylacetone, C99% (Aldrich) as chelating agent, to prepare TiO2 doped with 0.5 at% Fe. Acetic acid, C99.7% (Aldrich) was used instead of water to decrease the kinetics of the hydrolysis and polycondensation reactions of Ti(O–Bun)4. Also, the acetylacetone is added with the aim of decreasing the reactivity of titanium butoxide and stabilizing the sol. Ti(O–Bun)4, n-butanol, acetylacetone, and ferric nitrate were mixed under reflux with strong magnetic agitation at 75 °C for 2 h. Acetic acid is added gradually to the sol. Acetic acid initiates hydrolysis via an esterification reaction (Buscema et al. 2002): R  OH þ R0  COOH $ H2 O þ RCOOR0 Moreover, it has been shown that acetic acid and acetylacetone induce modifications of the molecular structure of the precursor (Chaput et al. 1990). The molar ratios n acetic acid/(nTi ? nFe) and nacac/ (nTi ? nFe) used were 0.2 and 0.3, respectively. The proper amount of n-butanol to obtain a stable sol with pH = 2–2.5 and a concentration of 2.6 M, was used. The as-obtained sol was maintained at reflux with magnetic agitation, for 4 h, to achieve the gelification process. The solvents were evaporated from the gel by drying at 100 °C. The gel was heated at 450 and 800 °C, 3 h in air to obtain a Fe-doped-TiO2 powder

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crystallized as anatase and rutile, respectively. The same experimental procedure was used to prepare TiO2 powder, anatase, and rutile phases. Samples characterization The gels were investigated by thermal analyses, Raman spectroscopy, and X-ray Diffraction (XRD). The thermogravimetric analysis (TGA) and differential thermal analysis (DTA) were recorded at temperatures from 25 to 850 °C with a heating rate of 10 °C min-1, in air using a TGA Instrument (model Pyris Diamond), Perkin Elmer Instruments. The structure of the samples was characterized by XRD technique using a Bruker AXS tip D8 ADVANCE diffractometer, a TecnaiTM G2 F30 STWIN transmission electron microscope with a line ˚ , in High Resolution Transmission resolution of 1 A Electron Microscopy (HRTEM) mode, and by Raman spectroscopy. Raman spectra were recorded at room temperature in a backscattering geometry under excitation wavelength of 1064 nm using a spectrophotometer FT Raman Bruker RFS 100. For powder ˚ ), diffraction, CuKa1 radiation, (wavelength 1.5406 A LiF crystal monochromator, and Bragg–Brentano diffraction geometry were used. The data were acquired at 25 °C with a step-scan interval of 0.020° and a step time of 10 s. The Raman spectrum was recorded in the range 100–1000 cm-1 using for excitation, the 1064 nm line of a laser. Thermoluminescence (TL) measurements were carried out with a home-made setup at the heating rate of b = 7 °C s-1, and using as light detector an EMI 9656 photomultiplier and a Keithley current amplifier. In front of the photomultiplier, a filter was inserted to avoid the thermal radiation of the heater. Prior to the TL measurements, the samples were irradiated at room temperature for 40 min (copper anode, 40 kV, 40 mA).

Results and discussion Thermal analyses The thermal stabilities of the precursor gels of TiO2 and Fe-doped TiO2 was analyzed using thermogravimetry (TG) and DTA, in synthetic air (80% N2/20% O2), at a flow rate of 16 mL min-1 (Fig. 1).

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0

8%

DTA

487

TG

-30

118 0

200

400

600

800

24 22 20 1000

Temperature (°C) Fig. 1 Thermal analyses results for a TiO2 and b the Fe-doped-TiO2 precursor gels

According to the TG curves of undoped and Fe-doped-TiO2 gels in the temperature range of 25–850 °C (Fig. 1b), we can divide these thermograms into four domains: 25–80, 80–160, 160–280, and 280–360 °C. The volatile decomposition products correspond to a mixture of species. Owing to drying of the gels at 100 °C, in the first step, occurs the volatilization of the absorbed water on the grain surface. The first major step occurs between 80 and 160 °C. The main species evolved during this range are the n-butanol, acetic acid, acetylacetone, and nitric acid. In this temperature domain, the DTA curve shows an endothermic peak at 118 °C that is associated to this process (Fig. 1b). The third step, in the range of 160–280 °C, shows the weight loss due to the decomposition of the acetate groups, acetylacetonate groups, and nitrate groups bonded to Ti and Fe. The next step observed in the TG curve occurs between 280 and 360 °C and corresponds to the pyrolysis of the organic groups (exothermic peak at 345 °C). Two further exothermic peaks at 404 and

X-ray diffraction X-ray diffraction patterns of the undoped-TiO2 and Fe-doped-TiO2 precursor gels, powder heat treated at 450 °C, 3 h in air and 800 °C, 3 h in air, are shown in Fig. 2. The presence of diffraction peaks can be used to evaluate the structural order at long range or periodicity of the material (Cavalcante et al. 2008). The XRD analyses indicate that when the annealing temperature was 450 °C, the TiO2 and Fe-dopedTiO2 powders were in the TiO2 anatase phase forms; on annealing at temperature of 800 °C, a complete TiO2 rutile phase was formed. All diffraction peaks

(d)

(202)

26

(R)

(c)

(R)

(b)

(A)

(a)

20

30

40

50

60

70

(215)

30

Weight (mg)

28

(301) (112)

(b)

13%



Heat flow (mw) endo exo →

30

345 404

60

(116) (220)

800

(002) (310)

600

(204)

400

Temperature (°C)

(211)

200

(220)

0

(105) (211)

-25

2.1

0.8%

(200)

6% -20

(111)

TG

(210)

2.3 -15

(101)

13%

-10

(200)

2.5

(103) (004)

336 400

(110)

Heat flow (mw) exo →

-5

2.7

487 °C are probably due to the crystallization of the anatase and rutile phases, respectively. Thermal analyses results indicated that the mass loss of the gels occurs up to 360 °C, and then starts the crystallization of the anatase phase (TiO2). The rutile to anatase phase transformation of the heat-treated TiO2 powder gel begins around 360 °C Lottici et al. (1993), and the reverse anatase to rutile phase transformation takes place at temperatures higher than 600 °C. No distinct difference between the thermal analyses results for TiO2 (Fig. 1a) and for the Fe-dopedTiO2 precursor gels are observed. The thermal analyses results for TiO2 and Fedoped-TiO2 precursor gels revealed that the temperatures as 450 and 700–800 °C are enough to obtain anatase and rutile phases, respectively.

(101)

98

0

← endo

(a)

Intensity, (a.u)

DTA

Weight (mg)

5

79

(A) 80

2 Theta (degrees)

Fig. 2 X-ray diffraction patterns of the precursor gel heated at 450 °C (a undoped TiO2, b Fe-doped TiO2) and 800 °C (c undoped TiO2 and d Fe-doped TiO2); A anatase, R rutile

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can be assigned to the body-centered tetragonal structure (anatase, Fig. 2a, b) and tetragonal structure (rutil, Fig. 2c, d). Anatase-TiO2 exhibited characteristic diffraction peaks corresponding to an ordered structure at long range. As it can be seen on Fig. 2b and d, the intensities of the diffraction peaks for Fe-doped powders heated at 450 °C and 800 °C are less than the corresponding peaks of undoped powders heated at the same temperatures. Also, all the diffraction peaks of Fedoped powder heated at 450 °C are more diffuse than those of undoped gel powder calcined at 450 °C. This indicates that the Fe addition decreased the crystallization velocity, mainly for TiO2 anatase phase. The solubility of Fe in TiO2 was reported to be around 1 wt% (*1.4 at% Fe) (Kim et al. 2004), and the diffraction angle (2h) of 44° corresponds to the main peak position for elemental Fe. Diffraction patterns obtained suggested that anatase structure of the TiO2 powder analyzed revealed a preferential (101) orientation of titanium oxide and, rutile structure with a preferential (110) orientation of titanium oxide. The observed line broadening was used to estimate the average grain size. Assuming that the particles are stress free, the size was estimated from a single diffraction peak using the Scherrer’s equation (Patterson 1939; Cullity 1978), (Table 1). As can be seen in Table 1, the powders of pure and Fe-doped TiO2 with the average nanopolycrystalline sizes from 12.3 nm up to about 67.2 nm were obtained. The crystallites size of Fe-doped-TiO2 gels, heated at 450 and 800 °C being smaller than those of undoped TiO2 indicated the inhibitory role of Fe for the TiO2 crystallization. The lattice parameters of undoped and Fe-doped TiO2 were determined by full pattern fitting (Pawley method) using TOPAS 3 (Bruker AXS 2005) (Table 1). The results are in good agreement with the results of Howard et al. 1991. The incorporation of 0.5 at% Fe in the TiO2 crystalline

Table 1 The crystallite’s size of pure and Fe-dopedTiO2, gel heated at 450 and 800 °C

TEM–HRTEM analysis The TEM and HRTEM (inset) micrographs of Fedoped-TiO2 sample calcined at 450 and 800 °C are shown in Fig. 3a and b. The TEM bright field image obtained on Fe-doped-TiO2 powder heated at 450 °C reveals that the powder is composed of polyhedralshaped particles, with an average grain size of approximately 10 nm. The nanopowder also has the tendency to form soft agglomerates. The inset from the bright field image represents the HRTEM image of a selected area of the sample. The HRTEM image shows clear lattice fringes of poly˚ crystalline nanopowder of d = 3.52 and 1.89 A corresponding to the (101) and (200) crystallographic planes of the anatase phase. Also, the regular succession of the atomic planes indicates that the nanocrystallites are structurally uniform and crystalline with almost no amorphous phase present. The TEM bright field image obtained on Fe-doped-TiO2 powder heated at 800 °C reveals that the powder is composed of polyhedral-shaped particles, with an average grain size of approximately 112 nm. The HRTEM image shows clear lattice fringes of poly˚ crystalline nanopowder of d = 3.52 and 1.89 A corresponding to the (101) and (200) crystallographic planes of the anatase. The HRTEM inset image ˚ corresponding to shows lattice fringe of d = 1.68 A the (211) crystallographic plane of the rutile phase. Raman spectroscopy The Raman spectra of the undoped TiO2 powders calcined at 450 and 800 °C, and Fe-doped-TiO2

Sample

Undoped-TiO2 gel Fe doped-TiO2 gel Undoped-TiO2 gel Fe doped-TiO2 gel heated at 450 °C heated at 450 °C heated at 800 °C heated at 800 °C

Crystallite size (nm)

12.3

Lattice a = 3.78074 ˚) parameters (A c = 9.48846

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lattice (anatase and rutile) causes a very small change of the unit cell. Thus, the lattice parameters (a and c) of Fe-doped anatase were only smaller by 0.012 and 0.18%, respectively and, only higher by 0.004 and 0.015%, respectively, for Fe-doped rutile.

10.9

67.2

56.8

3.78028

4.5921

4.59230

9.4706

2.95928

2.95973

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Raman intensity (a.u.)

449

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(c)

611

237

146

(b) (a) 0

197 100

200

300

400

638

516

400

500

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Wavenumbers (cm-1)

Fig. 4 Raman spectra results for the gels heated at 450 °C (a undoped TiO2) and 800 °C (b undoped TiO2 and c Fe-doped TiO2)

range 350–700 cm-1 is of 10:1; and (ii) in the case of the TiO2 sample synthesized at 800 °C, one remarks that the dominant Raman lines are localized at 449 and 611 cm-1 and that the ratio between the relative intensities of the Raman lines situated in spectral ranges 50–350 and 350–700 cm-1 is of 1:10. Besides, in this last case a new Raman band with maximum at 237 cm-1 is observed, too. Taking into account the assignments of Raman lines shown in Table 2, all the above variations indicate that the syntheses carried out at temperatures of 450 and 800 °C result in a TiO2 of the type anatase and rutile, respectively. Table 2 Raman lines (cm-1) observed on the TiO2 samples synthesized at 450 and 800 °C and their assignment Fig. 3 TEM–HRTEM micrographs of a Fe-doped-TiO2 sample calcined at 450 and b 800 °C

sample calcined at 800 °C, are shown in Fig. 4. The Raman spectrum of Fe-doped-TiO2 powders calcined at 450 °C was not obtained with our equipment. This inconvenience is determined by the strong PL of Fe-doped TiO2. Raman spectra of the two samples of TiO2 in undoped state show significant differences: (i) for the TiO2 sample synthesized at 450 °C, one observes that the dominant Raman line is those peaked at 146 cm-1; the ratio between the relative intensities of the Raman line peaked at 146 and in the spectral

Sample

This work

Lj et al.

Assignment (Ohsake et al. 1978)

TiO2 synthesized at 450 °C

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Eg

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Eg

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B1g

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A1g; B1g

TiO2 synthesized at 800 °C

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B1g Multi-photon process

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Eg

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A1g

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Thermoluminescence, (arb. units.)

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TiO2, 800oC

600 450

TiO2, 450oC

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400

Temperature, (°C)

Thermoluminescence (arb. units)

Fig. 5 TL curves recorded, after X-ray irradiation, for TiO2 nano-crystalline powder, annealed at 450 and 800 °C for 3 h

750 o

TiO2, 800 C

600 450 300 150 0 50

100

150

200

250

300

Temperature (°C)

Fig. 6 TL curve recorded in TiO2 nano-crystalline powder annealed at 800 °C for 3 h, after X-ray irradiation (solid curve), and its deconvolution into single peaks (dashed curves)

Undoped and Fe-doped TiO2, calcined at 800 °C shown the same Raman lines, with a very small flattening of Fe-doped TiO2 Raman peaks. This indicates that iron slows down the crystallization process of TiO2, which is in good agreement with the XRD data. Thermoluminescence In the Figs. 5 and 6, are depicted the TL curves recorded immediately after the X-rays irradiation of undoped and Fe-doped-TiO2 powders, respectively, annealed at 450 and 800 °C. It is known that the TL in materials is related to the defects created by the irradiation at ‘‘low

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temperature,’’ and it results from the thermally activated recombination of the detrapped carriers; the thermic energy corresponding to each glow peak is equal to the trap depth (Kirsh 1992). According to this picture, ‘‘shallow’’ traps will empty at low temperatures (e.g., around 150 °C), and ‘‘deeper’’ traps will empty at higher temperatures (e.g., around 300 °C). The rutile and anatase phases of TiO2 have different crystal structures which affect the distribution and the nature of the traps and, therefore, different shapes of the TL curves are observed (Figs. 5, 6). The TL curves recorded in undoped and Fe-doped TiO2 (in the rutile phase) have shown a common TL peak at 173 and 178 °C, respectively, accompanied by a second peak at 135 °C (in undoped sample) which shifts to 117 °C in the doped one. This shows a clear influence of the Fe doping on the shallow traps responsible for this TL peak. It was shown that in transition metal-ion (V, Cr, Mn or Fe)-doped TiO2 single-crystal, the dopants may act as electron or hole traps and consequently alter electron–hole pair recombination rates. In the particular case of Fe(III)-doped TiO2, it has been accepted that Fe(III) ions act as shallow charge traps in the TiO2 lattice (Zhang et al. 1998; Mizushima et al. 1979). Therefore, we attribute the 117 °C TL peak to the recombination of the shallow charge traps related to the Fe(III) dopant ions; their role as electron and/or hole traps is still controversial (Zhang et al. 1998; and references therein). The TL curves recorded in undoped and Fe-doped TiO2 (in the atanase phase) are similar; they show only a broad peak at high temperatures at around 340 °C. This indicates the same nature of the deep traps responsible for the peak, i.e., they are not affected by the doping, but eventually only by their number since the TL signal is weaker in the doped samples. In this case, the nano-crystals are smaller (only about 10 nm) which leads to larger surface areas and increases the available surface active sites. The X-ray irradiation generates most of the electron– hole pairs sufficiently close to the surface. They quickly reach the surface, and undergo rapid surface recombination mainly due to abundant surface trapping sites and the lack of driving force for electron– hole pair separation (Mizushima et al. 1979). Accordingly, we expected a decrease of the TL signal as we have observed (Figs. 5, 6). Since both

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Thermoluminescence, (arb. units.)

350

Fe-TiO2, 800oC

300 250 200

350

Thermoluminescence (arb. units)

undoped and Fe-doped TiO2 samples have shown TL signal at high temperatures, this shows that some of the charge carriers are trapped in the ‘‘deep’’ traps, and their recombination gives rise to the TL peak at around 340 °C. By analogy with Ba[Zr0.25Ti0.75]O3 (BZT) thin films (Cavalcante et al. 2009) and SrZrO3 powders (Longo et al. 2008), we suppose that a certain degree of structural order–disorder in the TiO2 nano-ceramic is responsible for the appearance of intermediary energy levels within the band gap. These levels decrease the band gap which is linked to the presence of deep holes and large concentration of defects (Cavalcante et al. 2009). We tentatively attribute the high temperature TL peaks to the recombination of the deep trap holes related to defects in low optical band gap values. In order to compute the traps’ depths, we have performed the deconvolution of the TL curves recorded in undoped and Fe-doped TiO2 (in the rutile phase) into single-peaks (Figs. 7, 8) using the best-fit method, and the general order kinetics equation of TL (May and Patridge 1964). The TL peak temperature maximum Tm and the kinetic parameters, activation energy (or trap depth) E, frequency factor S, and kinetic order b, are listed in the Tables 3 and 4. In both cases, there is an uncertainty in the parameters of the last two TL peaks due to their overlapping. Similar analysis was performed in undoped and Fe-doped TiO2 (in the anatase phase). The broad TL peak at around 340 °C has been

o

Fe-TiO2, 800 C

300 250 200 150 100 50 0 -50 50

100

150

200

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300

Temperature (°C)

Fig. 8 Thermoluminescence curve recorded for Fe-dopedTiO2 nano-crystalline powder annealed at 800 °C for 3 h, after X-ray irradiation (solid curve), and its deconvolution into single peaks (dashed curves)

Table 3 The kinetic parameters obtained after deconvolution into single peaks of the TL curve recorded in TiO2 nanocrystalline powder annealed at 800 °C Tm (oC)

S (s-1)

E (eV)

b

130

7 9 1012

1.05 ± 0.04

2.1

173

2 9 1012

1.13 ± 0.05

2.0

217

9 9 1012

1.30 ± 0.06

2.1

296

5 9 1012

1.49 ± 0.07

2.1

Table 4 The kinetic parameters obtained after deconvolution into single peaks of the TL curve recorded in Fe-doped-TiO2 nano-crystalline powder annealed at 800 °C Tm (oC)

S (s-1)

E (eV)

b

118

4 9 1012

1.00 ± 0.04

1.5

178

2 9 1012

1.13 ± 0.05

1.6

237

7 9 1012

1.34 ± 0.06

2.1

300

5 9 1012

1.49 ± 0.07

2.1

150

Fe-TiO2, 450oC

100 50

decomposed with two peaks at around 300 and 355 °C (not shown) with the activation energies of 1.5 ± 0.07 and 1.6 ± 0.07 eV, respectively.

0 -50 50

100

150

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250

300

350

Conclusions

Temperature (°C)

Fig. 7 Thermoluminescence curves recorded on Fe-dopedTiO2 nano-crystalline powder annealed at 450 and 800 °C for 3 h, after X-ray irradiation

Ti0.995Fe0.005O2 and TiO2 powders with structures characteristic to anatase and rutile, respectively, have been prepared by a particulate sol–gel technique from

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precursor-type alkoxide modified molecular with acetic acid and acetylacetone. Both pure TiO2 and Fe-doped-TiO2 powders with rutile and anatase structure have crystalline sizes between 10 and 112 nm. The exclusive presence of anatase and rutile monophase of the calcined gels at 450 and 800 °C, respectively, were revealed by the XRD and Raman analyses. Thermoluminescence in TiO2 nano-powders (in the rutile phase) is related to the recombination of shallow traps. In the doped sample, the 117 °C TL peak has been assigned to the recombination of the shallow charge traps related to the Fe(III) dopant ions. The high temperature TL peaks in TiO2 nanopowders (in the atanase phase) have been tentatively attributed to the recombination of the deep trap holes related to defects in low optical band gap values. Acknowledgments This study was supported by the ‘‘Nucleu’’-project, PN09-450102, from the National plan for RDI, funded by the Romanian Ministry of Education and Research, and the National Authority for Scientific Research.

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