Preparation and optical properties of iron-modified titanium dioxide obtained by sol–gel method

Optical Materials xxx (2015) xxx–xxx

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Preparation and optical properties of iron-modified titanium dioxide obtained by sol–gel method Agnieszka Hreniak, Katarzyna Gryzło, Bartosz Boharewicz, Andrzej Sikora, Jacek Chmielowiec, Agnieszka Iwan ⇑ Electrotechnical Institute, Division of Electrotechnology and Materials Science, M. Sklodowskiej-Curie 55/61 Street, 50-369 Wroclaw, Poland

a r t i c l e

i n f o

Article history: Received 7 January 2015 Received in revised form 26 March 2015 Accepted 31 March 2015 Available online xxxx Keywords: Fe Nanoparticles Doped TiO2 TiO2 Optical and photocatalytic properties Zeta potential

a b s t r a c t In this paper twelve TiO2:Fe powders prepared by sol–gel method were analyzed being into consideration the kind of iron compound applied. As a precursor titanium (IV) isopropoxide (TIPO) was used, while as source of iron Fe(NO3)3 or FeCl3 were tested. Fe doped TiO2 was obtained using two methods of synthesis, where different amount of iron was added (1, 5 or 10% w/w). The size of obtained TiO2:Fe particles depends on the iron compound applied and was found in the range 80–300 nm as it was confirmed by SEM technique. TiO2:Fe particles were additionally investigated by dynamic light scattering (DLS) method. Additionally, for the TiO2:Fe particles UV–vis absorption and the zeta potential were analyzed. Selected powders were additionally investigated by magnetic force microscopy (MFM) and X-ray diffraction techniques. Photocatalytic ability of Fe doped TiO2 powders was evaluated by means of cholesteryl hemisuccinate (CHOL) degradation experiment conducted under the 30 min irradiation of simulated solar light. Ó 2015 Elsevier B.V. All rights reserved.

1. Introduction Titanium dioxide is presently widely investigated in various areas of sciences being into consideration its application as a photocatalytic material for self-cleaning coatings, environmental purifiers, antifogging mirrors and many others [1–7]. Properties of TiO2 could be improved by doping with metal ions, such as silver, nickel, chromium, iron, vanadium, and zinc [3–14]. The electronic structure of TiO2 nanoparticles doped with various metals creates the new chemical compositions and modifies their optical properties [e.g. 3–38]. For example, Liu et al. prepared the Fe-doped TiO2 nanorod clusters and monodispersed nanoparticles by a modified hydrothermal and solvothermal method [23]. Shi et al. [24] obtained Fe–La–TiO2 photocatalysts by a sol–gel method. Ranjit et al. [17] showed that Fe (III)-doped TiO2 improves photocatalytic activity up to a certain doping level (1.8 wt. %) of Fe (III). Parida et al. [25] applied iron (II) sulphate and Ti-isopropoxide as the precursors and sodium dodecyl sulphate as the surfactant to obtain mixed phase of mesoporous iron–titanium mixed oxide by used sol–gel technique. Fe/Ti ratio of 1:1 shows highest catalytic activity i.e. 42% conversion of clyclohexane oxidation and 62% selectivity of cyclohexanol [25]. Naik et al. [26] presented catalytic activity ⇑ Corresponding author. E-mail address: [email protected] (A. Iwan).

evaluation of an efficient mesoporous FexTi1xO2yNy nanophotocatalyst (FeNT) for the degradation of phenol under direct solar light illumination. For fabrication of the FeNT, Fe doped TiO2 was prepared by a nonhydrolytic sol–gel route and N doping was subsequently performed by heating the resultant materials under a flow of NH3 [26]. Barakat et al. [27] investigated ZnOFe2O3-incoported TiO2 nanofibers as super effective photocatalyst for water splitting under visible light radiation. Electrospinning of a mixture composed of titanium isopropoxide, zinc acetate, iron acetate and polyvinylpyrrolidone leads to produce good morphology electrospun nanofibers. Photocatalytic activities of Fe–Cu/ TiO2 on the mineralization of oxalic acid and formic acid under visible light irradiation were studied by Apiwong-Ngarm et al. [29]. Authors showed that 0.5 mol% (Fe,Cu)-codoped TiO2 sample exhibited remarkably higher activity than pure TiO2. Devi et al. [30] prepared TiO2 by sol–gel method through the hydrolysis of TiCl4 and its surface derivatization was carried out with molecular catalyst like Hemin (chloro(protoporhyinato) iron (III)). Authors showed that the enhancement in the photocatalytic activity is attributed to the presence of iron (III) porphyrin ring on the TiO2 surface, which reduces the electron-hole recombination rate and also by acting as a mediator for continuous production of enriched concentration of hydroxyl radicals along with various other reactive free radicals [30]. Photocatalytic activity of titanium dioxide modified by Fe2O3 nanoparticles were studied by Wodka et al. [31]. Nanocrystalline

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Fig. 1. SEM micrographs of TiO2:Fe obtained from Fe(NO3)3 (I) and FeCl3 (II).

rutile TiO2 powders were modified with small amounts of CuOx and FeOx clusters by impregnation and drying. The modified rutile samples exhibited drastically enhanced photocatalytic degradation of 4-chlorophenol under UV–vis irradiation [32]. Nakamura et al. [36] showed improvement of visible light responsivity of rutile TiO2 nanorods by site-selective modification of iron (III) ion on newly exposed faces formed by chemical etching treatment. In [36] authors investigated shape-controlled brookiteTiO2 nanorods with various aspect ratios by a hydrothermal process in the presence of a shape-control agent (polyvinyl alcohol and/or polyvinyl pyrrolidone). An improvement in the photocatalytic activity of a morphology-controlled brookite TiO2 nanorod for acetaldehyde

oxidation was achieved by site-selective modification of the exposed crystal surface of a brookite TiO2 nanorod with Fe3+ ions [36]. The aim of this study was to develop a method of synthesis for obtaining TiO2 modified with iron and to characterize the obtained nanopowders by dynamic light scattering (DLS), SEM, MFM, X-ray and UV–Vis spectroscopy. Amount of iron along with source of iron in titanium dioxide were investigated in details. Finally, photocatalytic activity, band gap and energy value measurement are presented and discussed taking into consideration the effects of Fe doping on TiO2 behavior under the 30 min irradiation of simulated solar light.

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A. Hreniak et al. / Optical Materials xxx (2015) xxx–xxx Table 1 Fe weight content analysis performed with EDS and average size of TiO2:Fe detected by SEM along with TiO2:Fe size distribution at 25 °C detected by DLS. Code

Fe content (% mass)

Average size (nm)

Average size by DLS (nm)

200/180 350/150 340/160 350/170 240/120 340/150 160

829/614 580/540 485/504 860/671 697/296 787/519 440

Fe(NO3)3/FeCl3 TiO2:Fe TiO2:Fe TiO2:Fe TiO2:Fe TiO2:Fe TiO2:Fe TiO2

1% 5% 10% 1% (a) 5% (a) 10% (a)

0.94/0.8 2.54/3.2 4.72/7.5 0.51/1.2 2.35/5.6 5.80/13.4 

3

0.4 g of powder (TiO2, TiO2:Fe 10%, TiO2:Fe 10a% from FeCl3 and Fe(NO3)3) was added to 4 ml of water and stirred 30 minutes in ultrasonic scrubber. CHOL (0.2 g) was soluble in 20 ml of ethyl alcohol and added to TiO2:Fe solution. Mixture was stirred 4 h at 60 °C and next was collected by means of centrifugation. Finally, powders with CHOL were dried at 60 °C during 20 h. 2.2. Materials and synthesis 2.2.1. Materials Titanium (IV) isopropoxide (TIPO) (99 + %) was purchased from Alfa Aesar. Ethanol (96%), isopropyl alcohol, Fe(NO3)3  9 H2O and FeCl3 were purchased from POCh Gliwice. Cholesteryl hemisuccinate was purchased from Aldrich.

2. Experimental 2.1. Methods Size of TiO2:Fe was determined by dynamic light scattering (DLS) method. The measurements were performed by Zetasizer Analyzer Nano ZS - Malvern Instruments, at temperature of 25 °C. In 20 ml H2O 0.01 g TiO2 was added. The pH of suspensions was adjusted using 0.1 M HCl or 0.1 M NaOH. For each pH five samples were tested. Scanning electron microscopy (SEM) studies were performed with a tungsten cathode Vega II SBH (TESCAN) to examine the morphology of the TiO2:Fe. UV–vis spectra were recorded by using Jasco V670 spectrophotometer. Magnetic force microscopy (MFM) measurements were performed with Innova system from Bruker (formerly Veeco) in air, at temperature 23 °C and humidity 35% RH. MESP-LC probes from Bruker were used to avoid the demagnetization of the particles by the scanning tip [39–41]. The liftmode height was set to 40 nm, in order to obtain selective detection of the long range magnetic forces. The structure of prepared powders was determined on the basis of the X-ray diffraction results performed at room temperature with DRON-2 powder (Fe-filtered Co Ka radiation). The photocatalytic activity of TiO2:Fe powders was evaluated by performing cholesteryl hemisuccinate (CHOL) degradation experiment conducted under the 30 min irradiation of simulated solar light using a Solar Simulator Model SS100AAA with AM 1.5G with an irradiation intensity of 100 mW/cm2. In a typical experiment,

2.2.2. Synthesis of TiO2:Fe Titania doped with Fe (TiO2:Fe x%) were obtained by using two methods of synthesis. In both cases such components as TIPO, ethanol, and distilled water were applied and stirred during 4 h. Differences in both methods of synthesis were in the time and order to added Fe(NO3)3 (or FeCl3) to the mixture. In the first method Fe(NO3)3 (or FeCl3) was stirred 4 h along with other components, while in the second case Fe(NO3)3 (or FeCl3) was added to sol mixture and stirred only 2 h. Details of the TiO2:Fe synthesis are presented below. First method (TiO2:Fe 1%, TiO2:Fe 5%, TiO2:Fe 10%): TiO2:Fe x% powder was prepared by sol–gel method using as titanium precursor titanium (IV) isopropoxide (TIPO). Titanium precursor was dissolved in ethanol. Briefly, 4.5 ml of TIPO and 21 ml of ethanol were mixed with 3.5 ml of distilled water and addition of Fe(NO3)3 (or FeCl3) to the obtaining sol with the molar ratio Fe/TIPO equal 1%, 5%, 10%. The solution was stirred in a plastic flask at room temperature for 4 h. During the stirring the titanium dioxide powder was formed and after filtering was dried at room temperature. The TiO2:Fe x% powder was heated at 500 °C for 1 h. Second method (TiO2:Fe 1% (a), TiO2:Fe 5% (a), TiO2:Fe 10% (a)): TiO2:Fe x% (a) powder was prepared by sol–gel method using as titanium precursor titanium (IV) isopropoxide (TIPO). Titanium precursor was dissolved in ethanol. Briefly, 4.5 ml of TIPO and 21 ml of ethanol were mixed with 3.5 ml of distilled water yielding a titania sol. The solution was stirred in a plastic flask at room temperature for 2 h. Next addition of Fe(NO3)3 (or FeCl3) to the obtaining sol with the molar ratio Fe/TIPO equal 1%, 5%, 10% and mixed for 2 h. During the stirring the titanium dioxide powder was formed and after filtering was dried at room temperature. The TiO2:Fe x% (a) powder was heated at 500 °C for 1 h. 3. Results and discussion 3.1. Characterization of TiO2:Fe

Fig. 2. TiO2:Fe 5% from Fe(NO3)3 (top) and FeCl3 (bottom) particles size distribution at 25 °C detected by DLS.

The TiO2:Fe powders were investigated by SEM technique. Morphologies of TiO2:Fe obtained from Fe(NO3)3 or FeCl3 detected by SEM micrographs are shown in Fig. 1. TiO2:Fe samples appeared as agglomerations of smaller particles. The grains size shape depends on the synthetic conditions and kind of iron used. The smallest grain sizes were observed for the reactions with FeCl3. The size of obtained TiO2:Fe particles from FeCl3 was found about 120–180 nm, while for TiO2:Fe particles from Fe(NO3)3 was about 80–300 nm, as it was confirmed by SEM (see Table 1). Moreover, the amount of Fe in titanium dioxide was detected with EDS, and differences depend on the kind of iron compound used. The obtained results are schematically presented in Table 1. In the case of TiO2:Fe obtained from FeCl3 the amount of Fe in titanium dioxide was in good agreement with our

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3500

JCPDS 19-635 (Fe2Ti3O9) 3000

Intensity [a.u.]

2500 TiO2:Fe10% (a)

2000

from FeCl3

TiO2:Fe10%

1500 TiO2:Fe10% (a) from Fe(NO3)3

TiO2:Fe10%

1000

500

TiO2 - Anatase

0 10

20

30

40

50

60

70

2Θ [deg.] Fig. 3. X-ray patterns of TiO2:Fe 10% and TiO2:Fe 10% (a) powders from Fe(NO3)3 and FeCl3 and pure TiO2.

Table 2 Zeta potentials of TiO2:Fe powders. Code

TiO2:Fe TiO2:Fe TiO2:Fe TiO2:Fe TiO2:Fe TiO2:Fe TiO2

Zeta potentials (mV) Fe(NO3)3/FeCl3

1% 5% 10% 1% (a) 5% (a) 10% (a)

pH 4

pH 6

pH 8

pH 10

12/9 13/11 10/14 14/13 10/15 12/11 11

20/17 18/16 21/21 21/19 19/7 -23/-15 24

25/21 20/26 24/19 23/25 25/30 24/23 30

29/14 29/31 26/30 29/27 26/33 20/31 34

experimental methods. This observation confirmed that better for practical application of TiO2 taking into consideration magnetic properties should be FeCl3 than Fe(NO3)3 and the second method of synthesis propose (see Experimental). Additionally, we investigated size of TiO2:Fe particles by DLS method at 25 °C. Average size of TiO2:Fe particles was found about 504-860 nm and strongly depend on the kind of iron compound used (see Table 1). Similar as was observed by SEM the DLS method showed also that the size of obtained TiO2:Fe particles from FeCl3 was smallest than from Fe(NO3)3. As an example TiO2:Fe 5% (from Fe(NO3)3 and FeCl3) particles size distribution at 25 °C detected by DLS are presented in Fig. 2.

Fig. 4. Topography, Sobel transform of the topography and MFM maps (from left to right) acquired using magnetic force microscopy for TiO2:Fe 1% and TiO2:Fe 1% (a) (from top to bottom) respectively.

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90 80 80

from Fe(NO3)3

from FeCl3

70 70 60

50

TiO2:Fe1%

40

TiO2:Fe5%

30 20

TiO2:Fe1%

40

TiO2:Fe5%

TiO2:Fe10%

30

TiO2:Fe10%

TiO2:Fe1%a

20

TiO2:Fe1% a

TiO2:Fe5%a

10 0

50

R [%]

R [%]

60

TiO2:Fe5% a

10

TiO2:Fe10%a

TiO2:Fe10% a

0 300

400

500

600

700

800

900

1000

λ [nm]

300

400

500

600

700

800

900

1000

λ [nm]

(a) 24

16

20

14

18

((1-R) /2R) (a.u.)

12

TiO2:Fe1%

10 8

TiO2:Fe10%

6

TiO2:Fe1%a

4

TiO2:Fe5%a

2

TiO2:Fe5%

2

((1-R) /2R) (a.u.)

from FeCl3

22

from Fe(NO3)3

TiO2:Fe10%a

2

16

TiO2:Fe1%

14

TiO2:Fe5%

12 10

TiO2:Fe10%

8

TiO2:Fe1% a

6

TiO2:Fe5% a

4

TiO2:Fe10% a

2 0 300

400

500

600

0

700

λ [nm]

300

400

500

600

700

λ [nm]

(b)

Fig. 5. Diffuse reflective UV–vis spectra (a) and Kubelka–Munk function at different wavelengths (b) of TiO2:Fe from Fe(NO3)3 (left side) and FeCl3 (right side).

Table 3 Absorption UV–vis properties of TiO2:Fe powders detected from the Kubelka–Munk function. Code

TiO2:Fe TiO2:Fe TiO2:Fe TiO2:Fe TiO2:Fe TiO2:Fe *

UV–vis Fe(NO3)3/FeCl3

1% 5% 10% 1% (a) 5% (a) 10% (a)

kmax (nm)

Eg* (eV)

314/315 322/324 332/339 318/317 318/316 325/330

3.21/3.26 3.07/2.95 2.69/2.63 3.21/3.25 2.92/3.06 2.77/2.63

X-ray diffraction patterns of TiO2:Fe 10% and TiO2:Fe 10% (a) powders obtained from Fe(NO3)3 and FeCl3 and pure TiO2 are presented in Fig. 3. It can be seen that investigated powders are crystalline materials, and the pure TiO2 is in the form of anatase. The Fe-doped TiO2 powders are mainly in anatase phase together with some minor phases including Fe2Ti3O9 (JCPDS 19-635). The shape of X-ray peaks at 2H  44°, 53° and 56° observed as sharp peaks in pure TiO2 are broader for the Fe-doped TiO2 powders (see Fig. 3) confirms presence of Fe in TiO2.

Eg = 1240nk.

3.2. Zeta potentials Table 4 Absorption UV–vis properties of selected TiO2:Fe powders with CHOL detected from the Kubelka–Munk function. Code

TiO2:Fe 10%-CHOL TiO2:Fe 10%-CHOL (after 30 min) TiO2:Fe 10% (a)-CHOL TiO2:Fe 10% (a)-CHOL (after 30 min) TiO2-CHOL TiO2-CHOL (after 30 min) *

Eg = 1240nk.

UV–vis Fe(NO3)3/FeCl3 kmax (nm)

Eg* (eV)

312/348 339/348 334/334 339/334 309 318

2.64/2.19 2.67/2.19 2.63/2.66 2.77/2.66 3.30 3.34

Presence of iron changed the zeta potentials of titanium dioxide. For the doped with Fe titanium dioxide negative zeta potentials were found as is presented in Table 2. Along with the increase of the Fe amount and used kind of iron compound, some differences were found. In the most cases for the TiO2:Fe obtained from FeCl3 the lowest values of zeta potentials were found (see Table 1). For example, in the presence of 1% of Fe from Fe(NO3)3 (or FeCl3) TiO2 had negative zeta potentials about 12 (9) mV at pH 4, and at 29 (14) mV at pH 10. Some differences were also observed along with change of the TiO2 with Fe synthesis method (see Table 2). The change of zeta potential values of TiO2:Fe with increase of Fe-content suggests that the isoelectric point of TiO2 powder is changing with the increase of Fe amount. It

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100

100

90

80

80

70

70

60

60

R (%)

R (%)

90

from Fe(NO 3) 3

50 TiO2:Fe10%CHOL

40

TiO2:Fe10%CHOL (30 min)

30

from FeCl 3

50 TiO2:Fe10%CHOL

40

TiO2:Fe10%CHOL (30 min)

30

TiO2:Fe10% aCHOL

TiO2:Fe10% aCHOL

20

TiO2:Fe10% aCHOL (30 min)

20

TiO2:Fe10% aCHOL (30 min)

10

TiO2 CHOL

10

TiO2 CHOL

0

TiO2 CHOL (30 min) 300

400

500

600

700

800

900

0

1000

TiO2 CHOL (30 min) 300

400

(a)

Wavelength (nm) 16

500

600

700

800

900

1000

Wavelenght (nm)

13 from Fe(NO 3) 3

14

from FeCl 3

12 11 10

(1-R) /2R (a.u.)

10 8

2

TiO2:Fe10%CHOL TiO2:Fe10%CHOL (30 min)

2

(1-R) /2R (a.u.)

12

6

TiO2:Fe10% aCHOL TiO2:Fe10% aCHOL (30 min)

4

TiO2 CHOL 2

TiO2 CHOL (30 min) 400

500

600

700

800

900

1000

Wavelength (nm)

8 7 6

TiO2:Fe10%CHOL

5

TiO2:Fe10%CHOL (30 min)

4

TiO2:Fe10% aCHOL

3

TiO2:Fe10% aCHOL (30 min)

2

TiO2 CHOL

1

TiO2 CHOL (30 min)

0

0 300

9

(b)

300

400

500

600

700

800

900

1000

Wavelenght (nm)

Fig. 6. Diffuse reflective UV–vis spectra (a) and Kubelka–Munk function at different wavelengths (b) of TiO2:Fe 10% from Fe(NO3)3 (left side) and FeCl3 (right side) before and after 30 min under a xenon lamp with an irradiation intensity of 100 mW/cm2.

indicates that the amount and kind of Fe compound along with method of TiO2:Fe synthesis influences on the properties of titanium dioxide. Probably Fe ions imparted negative charge to nanoparticles TiO2 surfaces and increased their absolute surface potentials. Moreover, the solution pH also has an influence on the surface catalytic reactions of TiO2 in aqueous medium. In our experiments, TiO2:Fe powders have negative zeta potentials, and probably the reactions of organic molecules with positive charge would be higher on the surfaces of these TiO2:Fe powders [20,42,43]. Additional work is necessary to confirm our suggestions.

3.3. Magnetic force microscopy study Fig. 4 shows the examples of the MFM data. The results revealed ferromagnetic nature of observed particles, as well as the specific regularity of shapes typical for Ferrite submicron particles [39]. One can notice also some morphological variety as well as differences of magnetic interaction. It is very likely caused by the preparation procedure influencing the dispersion and organization level of the particles. Advanced investigation using wide temperature range magnetization measurements confirmed the various magnetic properties of investigated samples [44].

3.4. UV–vis study Fig. 5 shows the diffuse reflective UV–vis spectra and Kubelka– Munk function of TiO2:Fe samples. All investigated samples exhibited one absorption maximum band in the range 314–332 nm, dependly on the method of synthesis applied to obtained TiO2:Fe and amount of iron. Moreover, weak absorption was observed in the range 400–600 nm and is caused by the presence of Fe in titanium dioxide. Moreover, along with increase the amount of Fe in TiO2, increase of second absorption band intensity was found (see Fig. 5). Similar behavior was observed for TiO2 with Ag [45]. The values of maximum of UV– vis absorption band detected from the Kubelka–Munk function are presented in Table 3. Along with the increase of the amount of iron in TiO2, bathochromic shift in maximum of absorption band was found. Samples TiO2:Fe 10% and TiO2:Fe 10% (a) were red shifted in comparison with other investigated compounds and exhibited kmax in the range 325–339 nm (see Table 3). Sample TiO2:Fe 1% has kmax at about 314 nm and was blue shifted in comparison with other compounds. Moreover, in the most cases, maximum of absorption band of TiO2:Fe obtained from FeCl3 was red shifted in comparison with TiO2:Fe obtained from Fe(NO3)3 (see Table 3). Photocatalytic ability of Fe doped TiO2 powders was evaluated by means of cholesteryl hemisuccinate (CHOL) degradation

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experiment conducted under the 30 min irradiation of simulated solar light. The values of maximum of UV–vis absorption band detected from the Kubelka–Munk function are presented in Table 4, while in Fig. 6 the diffuse reflective UV–vis spectra and Kubelka–Munk function of TiO2:Fe samples with CHOL are showed. It can be seen that among the eight powders with Fe tested, only powders obtained from FeCl3 showed the photocatalytic activity, and the photocatalytic efficiency was not decreased with changing stirring time from 2 h to 4 h (samples TiO2:Fe 10%CHOL and TiO2:Fe 10% (a)-CHOL) as is presented in Fig. 6. Maximum of absorption band of TiO2:Fe 10%-CHOL powder obtained from Fe(NO3)3 under the 30 min irradiation of simulated solar light was 27 nm bathochromically shifted compare with TiO2:Fe 10%-CHOL before irradiation. In the case of TiO2:Fe 10% (a)-CHOL powder obtained from Fe(NO3)3 small red shift (5 nm) was found under irradiation (see Table 4). On the other hand TiO2:Fe 10%-CHOL and TiO2:Fe 10% (a)-CHOL powders obtained from FeCl3 did not exhibit shift in maximum of absorption band under irradiation of samples. For the reference TiO2-CHOL sample maximum of absorption band was under the 30 min irradiation of simulated solar light 9 nm red shifted compared with not irradiated sample. Moreover, changes were found also in the estimated energy band gap (Eg) of TiO2:Fe-CHOL powders and in the most cases irradiation caused decrease the Eg value of powders TiO2 doped with Fe (see Table 4). Additionally, Fe in TiO2-CHOL influences on the decrease the Eg compared with TiO2-CHOL without Fe. TiO2:Fe powders obtained from FeCl3 shows the highest photo-catalytic activity for CHOL degradation under irradiation of simulated solar light. 4. Conclusions Titanium dioxide nanoparticles with different amount of Fe were synthesized by a sol–gel method. Six TiO2:Fe powders were investigated taking into consideration kind and amount of iron source and method synthesis. In this contribution it was shown that:  Method of TiO2:Fe synthesis influence on their structural and optical properties.  SEM images revealed that the grains size shape depends on the synthetic conditions and kind of iron used. The smallest grain sizes (120–180 nm) were observed for the reactions with FeCl3.  The lowest values of zeta potentials in the most cases for the TiO2:Fe obtained from FeCl3 were found. For pH = 6 zeta potentials was found from 7 to 21 mV.  UV–vis absorption spectra showed that along with increase the amount of iron in TiO2 bathochromic shift in maximum of absorption band was found. Moreover, in the most cases, maximum of absorption band of TiO2:Fe obtained from FeCl3 was red shifted in comparison with TiO2:Fe obtained from Fe(NO3)3.  The highest photo-catalytic activity for CHOL degradation under irradiation of simulated solar light was found for TiO2:Fe powders obtained from FeCl3.

Acknowledgements The research was supported by Wroclaw Research Centre EIT+ under the project ‘‘The Application of Nanotechnology in

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