Sol–gel iron-doped TiO2 nanopowders with photocatalytic activity

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Sol–gel iron-doped TiO2 nanopowders with photocatalytic activity Maria Cris¸an a , M˘alina R˘aileanu a,∗ , Nicolae Dr˘agan a , Dorel Cris¸an a , Adelina Ianculescu b,∗∗ , Ines Nit¸oi c , Petrut¸a Oancea d , Simona S¸om˘acescu a , Nicolae St˘anic˘a a , Bogdan Vasile b , Cristina Stan a a

“Ilie Murgulescu” Institute of Physical Chemistry, Romanian Academy, 202 Splaiul Independent¸ei, 060021 Bucharest, Romania “Politehnica” University of Bucharest, Department of Oxide Materials Science and Engineering, 1-7 Gh. Polizu, 011061 Bucharest, Romania National Research and Development Institute for Industrial Ecology, ECOIND, 71-73, Drumul Podu Dâmbovit¸ei Street, 060652 Bucharest, Romania d Department of Physical Chemistry, Faculty of Chemistry, University of Bucharest, 4-12 Bd. Regina Elisabeta, Bucharest 0300016, Romania b c

a r t i c l e

i n f o

Article history: Received 25 August 2014 Received in revised form 13 October 2014 Accepted 18 October 2014 Available online xxx Keywords: Sol–gel nanopowders Titanium dioxide Fe-doped TiO2 Structural study Nitrobenzene photodegradation

a b s t r a c t The aim of the present work was to establish the influence of the Fe-dopant on the structure and photocatalytic properties of the sol–gel TiO2 nanopowders. The relationship between the synthesis conditions and the properties of titania nanosized materials, such as thermal stability, phase composition, crystallinity, morphology and size of particles was investigated. Undoped, 0.5, 1, 2 and 5 wt% Fe-doped TiO2 samples have been prepared and structurally characterized by the XRD method. Lattice parameters, crystallite sizes, internal strains, as a measure of structural disorder, were determined. X-ray photoelectron spectroscopy (XPS) and magnetic measurements completed the structural data study. The acceptance of the dopant by the titania lattice was proved by the XRD measurements and the positive values of the magnetic susceptibilities. Its addition is responsible for supplementary defects in the crystalline lattice (paramagnetic behaviour). The dopant was present in the low spin state (LS) of Fe3+ in the sample with 0.5 wt% iron concentration and in the high spin state (HS) in the other samples. It influenced the photocatalytic properties. The photocatalytic activity of the prepared nanopowders has been tested in the degradation of nitrobenzene from water, as a first mention in literature. The sample with 0.5 wt% Fe dopant concentration thermally treated at 400 ◦ C presented the best photocatalytic activity. © 2014 Elsevier B.V. All rights reserved.

1. Introduction The photochemistry of nano semiconductor particles has been one of the fastest growing research areas in the physical chemistry field in the last time. The semiconductor assisted photocatalysis is considered an economic and environmental friendly water treatment technology in order to efficiently remove the organic pollutants from wastewaters [1]. The toxic and refractory pollutants like nitroaromatic compounds represent a special class of pollutants. Their variety (nitrobenzenes, nitrophenols, nitrotoluenes), toxicity and persistence directly affect the ecosystems health and the human beings by the contamination of surface and ground water supplies [2–4]. The effective removal of nitroaromatic pollutants from wastewaters has become a necessity and a duty, in order to assure a good management of water resources.

∗ Corresponding author. Tel.: +40 213 167 912; fax: +40 213 121 147. ∗∗ Corresponding author. Tel.: +40 214 023 884; fax: +40 213 181 010. E-mail addresses: malina [email protected], [email protected] (M. R˘aileanu), a [email protected] (A. Ianculescu).

It is well known that among the various photocatalysts, titania occupies a very important place, due to its high photocatalytic activity, excellent functionality, high chemical stability, thermal stability and non-toxicity. In search for a photocatalyst with optimal features, titania remains a benchmark against which any alternative photocatalyst must be compared. Enormous studies have been focused to the research of TiO2 material, which led to many promising applications in different fields, ranging from optics to gas sensors via solar energy [5–9]. These applications can be roughly divided into “energy” and “environmental” categories, many of which depending not only on the properties of the TiO2 material itself but also on the modifications of the TiO2 material host and of the interactions of TiO2 materials with the environment [10,11]. Recently, titanium dioxide has been extensively used for the decomposition and finally mineralization of environmental pollutants as a possible alternative to conventional water treatment technologies [12–15]. Generally, doping of TiO2 with transition metal cations was reported as a good tool to improve photocatalytic properties and for enhancement of visible light response [16–19]. The selection of synthesis technique constitutes an important factor for the

http://dx.doi.org/10.1016/j.apcata.2014.10.031 0926-860X/© 2014 Elsevier B.V. All rights reserved.

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efficiency of the nanopowders in the photocatalysis process. All the mentioned properties are improved in the case of nanostructured TiO2 , which can be easily obtained by the sol–gel method. Attention has been paid to Fe-doped TiO2 . Results have been controversial, depending on multiple factors such as iron precursor and content, preparation technique, temperature of calcination [20–23]. Doping with Fe3+ could introduce much more oxygen vacancies in the crystal lattice and on the surface of TiO2 , which favours the adsorption of water and formation of surface hydroxyl groups, as well as promoting the photocatalytic activity [24]. Iron adsorbed on the surface of TiO2 can be an electron or hole scavenger and results in the improvement of the separation of free carriers [25,26]. Previous studies regarding sol–gel Fe/TiO2 catalysts for oxidative degradation of recalcitrant water contaminants [27], degradation of Orange II azo dye solutions [28], decomposition of phenol, nitrophenol and 2,4,6-trichlorophenol [29–31] from aqueous solutions, oxidation of p-xylene in gas phase [32] and application in hydrogen evolution by water splitting under visible light irradiation [33,34] should be mentioned. In a recent review we have presented an exhaustive study of the last literature data about sol–gel Fe-doped TiO2 nanopowders from point of view of synthesis conditions and some of their properties [35]. The aim of the present work is to prepare and structurally characterize undoped and 0.5, 1, 2 and 5 wt% Fe-doped sol–gel TiO2 nanopowders as well as to establish the correlation between the structure, magnetic properties and photocatalytic activity (underlying the role of Fe dopant on morpho-structural changes of TiO2 nanopowders). The influence of iron on TiO2 crystallization is studied in detail by means of the XRD method. XPS and magnetic measurements are employed in order to complete the structural data study. The photocatalytic activity of the obtained nanopowders is tested in the advanced degradation of nitrobenzene from water, which is first mentioned in the literature.

2. Experimental 2.1. Samples preparation The alkoxide route of the sol–gel method has been used for both pure TiO2 and Fe-doped TiO2 nanopowders syntheses. The TiO2 precursor was titanium ethoxide for synthesis, Ti(OC2 H5 )4 Merck, and the iron source was Fe (III) nitrate nonahydrate p. a., Fe(NO3 )3 ·9H2 O Merck. Ethanol purris p. a. absolute, C2 H5 OH provided by Sigma–Aldrich (Riedel de Haën) was used as solvent. Four series of samples of 0.5, 1, 2 and 5 wt% Fe-doped TiO2 have been prepared, starting from the same amounts of reactants, except water and iron nitrate. The hydrolysis of the Ti alkoxide proceeded with a water excess and in noncatalysed conditions, for all dopant concentrations, except 5 wt% Fe, in which case a few drops of ammonia have been used. A volume of 15 mL tetraethyl orthotitanate has been solved in 150 mL ethanol, under stirring, at room temperature, followed by the hydrolysis step in which a mixture of water and alcohol has been used. The volume of water was 6.30 mL in all cases, except the sample with the maximum iron concentration when only 5.60 mL have been added. The supplementary volume of alcohol was of 100 mL. The last step of the syntheses consisted of the iron nitrate addition. According to the proposed iron concentration in the final samples 0.2076, 0.4173, 0.8431 and 2.1743 g Fe (III) nitrate nonahydrate, respectively have been solved in 104 mL ethanol and added, in drops and under stirring to the reaction mixtures. Then the solutions were maintained 2 h under stirring, at room temperature. The final pH value was 6. The resulting sols have been converted to xerogels by drying at 80 ◦ C. The thermal schedule of the prepared powders was established based on thermal analysis of the dry powders. Thus, thermal treatments at 300,

400 and 500 ◦ C have been applied with heating rate of 1 ◦ C/min and 1 h plateau in all cases. The un-doped samples were noticed “T” and the Fe-doped ones were noticed “TF”. The numbers corresponding to the concentration value of the dopant and to temperature of thermal treatment were added (e.g. TF0.5 300; TF1 400; TF2 400; and TF5 500). 2.2. Characterization methods The structural changes of the obtained nanopowders due to the thermal treatment and the dopant presence were followed by: • Thermal analysis, on dried samples performed using a SHIMADZU DTG-TA-51H thermal analyser in static air atmosphere up to 1000 ◦ C, with a heating rate of 5 ◦ C min−1 . • X-ray diffraction analyses (XRD) performed with a SHIMADZU XRD 6000 diffractometer using Ni-filtered CuK␣ radiation (40 kV/30 mA) with scan step increments of 0.02◦ and with a counting time of 1 s/step, for 2 ranged between 20◦ and 80◦ , for the establishment of the phase composition and structural parameters. Parameters to define the position, magnitude, shape and integral breadth or full width at half maximum of profile (FWHM) of the individual peaks were obtained using the pattern fitting and profile analysis of an own calculus program. The lattice constants calculation is based on the Least Squares Procedure (LSP) using the linear multiple regressions for all the lines of the identified phase. To deconvolute size-D and strain-S broadening from the XRD spectra the multiple line analysis and integral breadth methods applied to the Pearson-VII analytic profiles were used. • X-ray photoelectron spectroscopy (XPS) for surface analysis was carried out on PHI Quantera equipment with a base pressure in the analysis chamber of 10−9 Torr. The X-ray source was monochromatized Al K␣ radiation (1486.6 eV), and the overall energy resolution was estimated at 0.65 eV by the full width at half-maximum (FWHM) of the Au4f7/2 photoelectron line (84 eV). Although the charging effect was minimized by using a dual beam (electrons and Ar ions) as neutralizer, the spectra were calibrated using the C1s line (binding energy (BE) = 284.8 eV) of the adsorbed hydrocarbon on the sample surface (C–C or (CH)n bondings). As this spectrum was recorded at the start and the end of each experiment, the energy calibration during experiments was quite reliable. • TEM (transmission electron microscopy) and HRTEM (high resolution transmission electron microscopy) coupled with SAED (surface area electron diffraction) investigations carried out with a high resolution transmission electron TECNAI F30 S-Twin microscope for estimate the powders morphology and the crystallinity degree, together with EDX spectra. • The room temperature magnetic susceptibility was measured on Faraday Balance (FB) and on Lake Shore’s fully integrated Vibrating Sample Magnetometer (VSM) system 7404. The following calibrates were used: CoHg(SCN)4 for intercalibration FB-VSM and SRM 2853/NIST together with Ni sphere SRM 772a/NIST for VSM. • The photodegradation experiments were carried out in a laboratory scale UV reactor - Heraeus system using a medium-pressure mercury lamp which emits in the UV–vis range ( = 300–500 nm). The lamp equipped with a quartz water cooling jacket was immersed in the centre of the reactor containing the pollutant solution. The photon’s flow of the emitted radiations was determined by ferrioxalate actinometry [36] and a value of incident radiation, I0 = 6 × 10−6 einstein s−1 was found. The solution containing 2.45 × 10−4 M nitrobenzene (NB) content was photo-oxidized in the following working conditions: pH 7; concentration of samples T and TF = 100 mg L−1 ; air flow Qair = 100 L h−1 ; and irradiation

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Fig. 1. Thermal behaviour of the TiO2 (sample T) (a) and Fe-doped TiO2 (samples TF0.5; TF2 and TF5) (b) (c) and (d) respectively.

time  irr = 30–240 min. The NB concentration was analysed by gas chromatography coupled with mass spectroscopy (GC–MS) analysis using an Agilent 7890A gas chromatograph. 3. Results and discussion 3.1. Thermal analysis The thermal behaviour of the undoped and Fe-doped TiO2 sol–gel nanopowders, for dopant concentrations of 0.5; 2 and 5 wt%, respectively is presented in Fig. 1. In the absence of dopant, the differential thermal analysis (DTA) curve of the titania sample (T) (Fig. 1a) presents an endothermic effect at 56 ◦ C which corresponds to the removal of loosely adsorbed water molecules from the gel network. This effect is associated with 19.45% mass loss on the thermogravimetric (TG)

curve. The exothermic broad feature in the temperature range of 110–400 ◦ C is most likely the resultant of the thermo-oxidative process of Ti-OR (Ti-OH) groups belonging to TiO2 precursor [37] pointed out by the peak centred at 199 ◦ C, accompanied by a mass loss of ∼4.9% and the crystallization to anatase phase observed on the DTA curve as an asymmetric large profile centred at ∼350 ◦ C. Above 400 ◦ C, the DTA curve shows a large concave feature which can be assimilated with a broad endothermic peak centred between 600 ◦ C and 800 ◦ C, due to structural OH removal. This is associated with a mass loss step of 4.86%. These data have been reported previously [38,39]. The presence of Fe dopant in TF0.5 (Fig. 1b), TF2 (Fig. 1c) and TF5 (Fig. 1d) samples modifies the thermal behaviour of TiO2 which can be seen both in thermic effects and the weight losses. The endothermic effect from the DTA curve is shifted (from 56 ◦ C) towards higher temperatures (at 86 ◦ C for TF0.5; 109 ◦ C for TF2; and 115 ◦ C for

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TF5, respectively), being accompanied by mass losses of 20.4, 14.3 and 14.9%. The exothermic peak present in sample T at 199 ◦ C is also shifted towards higher temperatures for the doped samples: 208 ◦ C for TF0.5, a very sharp one at 211 ◦ C, for TF2 and 253 ◦ C, for TF5. The DTA curves corresponding to the TF0.5 and TF2 samples present well-defined exothermic effects, at 358 ◦ C (Fig. 1b) and 401 ◦ C, respectively (Fig. 1c), attributed to the phase transformation from amorphous TiO2 to anatase. Two small exothermic peaks centred around 242 and 315 ◦ C can also be observed in the case of TF2 powder (Fig. 1c) which can be attributed to the decomposition of nitrate species from the dopant source along with the dehydroxylation of Ti species [40]. For TF5 sample (Fig. 1d) a slight endothermic effect centred at 350 ◦ C accompanied by mass loss of 8.6% could be observed. It could be considered as the result of overlapping of two effects: the removal of both hydroxyl groups and nitrates and the crystallization of anatase phase over a large temperature range (320–500 ◦ C). The last peak could be explained by the influence of dopant which in higher concentration (5 wt%) strongly influences the thermal history of iron doped TiO2 . 3.2. XRD Room temperature XRD analyses were carried out on the undoped and Fe-doped samples thermally treated at different temperatures, in the temperature range of 300–500 ◦ C with a constant plateau of 1 h. The undoped TiO2 powders have been used as standards, in order to easily compare the contribution of the iron dopant to the structural modifications, for each annealing temperature. The XRD patterns of the powders under investigation show the main diffraction peaks specific to the tetragonal structure of the anatase phase of TiO2 (JCPDS file no. 04-0477), for both “T” and “TF” samples, regardless of the dopant concentration (0.5; 1; 2 and 5 wt%) and thermal treatment temperature (300–500 ◦ C) (see Fig. 2 and Table 1). In the case of the undoped TiO2 samples resulted after annealing at lower temperatures (300 and 400 ◦ C), small amounts of brookite phase were identified at the detection limit. The minor brookite phase was identified as a slight peak (the second one as intensity) corresponding to the (121) crystalline plane, located at the diffraction angle 2 ∼ 30.8◦ (JCPDS no. 15-0875) and marked by a star-symbol in their XRD patterns. It is worthy to mention that the most intense (1 2 0) peak of brookite located at 2 ∼ 25.3◦ cannot be identified because it is overlapped by the most intense (1 0 1) peak of the major anatase phase. The quantitative brookite–anatase transformation seems to occur in the temperature range of 400–500 ◦ C, so that the undoped TiO2 sample annealed at 500 ◦ C tends toward the unique anatase phase. ˇ ´ From this point of view, our results differ from those of Sjakovi cVujiˇcic´ et al., who reported a mixture of a dominant anatase phase and a minor rutile phase for the sol–gel undoped titania powder thermally treated in similar conditions as ours [19]. This suggests that the type of TiO2 polymorphs formed in the final oxide powders is strongly dependent not only by the annealing parameters, but also by the synthesis factors (type of precursors and solvent, pH, reflux time, synthesis environment etc.), which are responsible for modifying the size and surface properties of the precursor particles. For the doped powders thermally treated at low temperature (300 ◦ C), the incorporation of iron ions into the anatase lattice induces a gradual disappearance of the minor brookite phase. This effect is even more significant for the samples annealed at 400 ◦ C, when an addition of iron of only 0.5 wt% contributed to the complete conversion of the brookite phase, suggesting the concurrent effect of the increase of both iron content and annealing temperature in favouring the anatase over the brookite polymorph.

Fig. 2. XRD patterns of samples T; TF0.5; TF1; TF2 and TF5 thermally treated at 300 (a), 400 (b) and 500 ◦ C (c); the star-symbol (*) in (a) and (b) indicate the brookite phase.

Consequently, the higher is the annealing temperature (in the investigated temperature range of 300–500 ◦ C), the lower is the iron content required for the quantitative transformation of the residual brookite into anatase. Thus, when annealing is carried out at 500 ◦ C, even the undoped titania sample consists only of anatase form, as already mentioned. Unlike our case, López et al. found a mixture of brookite, anatase and rutile phases for the sol–gel TiO2 powder doped with 1 wt% of Fe and thermally treated at 550 ◦ C [41]. The XRD data also indicated that even in the case of the most heavily-doped sample (5 wt%) analysed here, the iron dopant is completely solubilized in the anatase lattice. These results are

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Table 1 The structural parameters from XRD data, magnetic susceptibilities and photocatalytic activities variation with temperature for undoped TiO2 (T) and Fe-doped TiO2 (TF) samples. Temperature (◦ C)

Structural parameters

T

TF TF0.5

TF1

TF2

TF5

300

a (Å) c (Å) UCV (Å3 ) D (Å) S × 103 NB %  × 10−6 (cm3 g−1 )

3.7812(13) 9.5148(46) 136.04(16) 138(4) 0.06(10) 50.07 −0.04(DM)

3.7858(7) 9.4721(25) 135.75(9) 165(5) 0.75(14) 56.95 1.07(PM)

3.7700(18) 9.4700(64) 134.60(22) 183(8) 1.22(13) 50.33 2.08(PM)

3.7808(15) 9.4815(53) 135.53(18) 114(4) 0.53(41) 68.47 4.44(PM)

3.7845(14) 9.0986(88) 130.31(22) < 50 <0.01 20.10 9.45(PM)

400

a (Å) c (Å) UCV (Å3 ) D (Å) S × 103 NB %  × 10−6 (cm3 g−1 )

3.7809(6) 9.4993(21) 135.80(7) 166(6) 1.27(14) 54.14 –

3.7811(3) 9.4786(10) 135.51(4) 236(6) 1.19(7) 84.91 0.59(PM)

3.7847(6) 9.4716(23) 135.96(8) 249(9) 0.97(13) 74.77 2.49(PM)

3.7792(15) 9.5178(55) 135.93(19) 176(7) 0.67(12) 71.77 4.34(PM)

3.7839(6) 9.4903(20) 135.45(7) 188(7) 1.06(16) 67.97 8.88(PM)

500

a (Å) c (Å) UCV (Å3 ) D (Å) S × 103 NB %  × 10−6 (cm3 g−1 )

3.7792(2) 9.4959(5) 135.62(2) 302(6) 0.54(6) 57.24 –

3.7809(1) 9.4950(4) 135.73(1) 368(14) 0.95(8) 82.34 0.80(PM)

3.7829(2) 9.5015(6) 135.97(2) 337(9) 0.97(6) 72.09 2.16(PM)

3.7833(6) 9.4973(20) 135.94(7) 245(7) 0.74(12) 63.66 3.39(PM)

3.7801(6) 9.4838(23) 135.51(8) 217(5) 0.27(15) 47.93 6.69(PM)

a, c – lattice parameters; UCV – unit cell volume; D – crystallite size; S – lattice strain;  – magnetic susceptibility; DM: diamagnetic; PM: paramagnetic; NB = photocatalytic activity after 2 h irradiation. ˚ c = 9.5139 A; ˚ UCV = 136.31 A˚ 3 . Lattice parameters of TiO2 -anatase from powder diffraction file nr. 21-1272: a = 3.7852 A;

somewhat different from those reported by author authors. Thus, Ganesh et al. [42] found small amounts of secondary ␣-Fe2 O3 and FeTiO3 phases in their Fe-doped TiO2 powders (with Fe content ≥4 wt%) prepared by coprecipitation and thermally treated at 550 ◦ C, while Zhu et al. [43] have identified a residual ␣-Fe2 O3 phase in their sol–gel TiO2 powders with iron content ≥5 wt%. Concerning the aspect of the XRD patterns, Fig. 2a shows larger diffraction peaks suggesting that the powders thermally treated at 300 ◦ C present a lower crystallinity degree (smaller crystallites). The increase of the annealing temperature at 400 and 500 ◦ C, respectively, promotes crystallization, reflected in the considerably sharper and well-defined diffraction peaks (Figs. 2b and c). Thus, the triplet consisting of the diffraction peaks (1 0 3), (0 0 4) and (1 1 2) of the anatase phase, placed in the 2 range of 36.9–38.6◦ , is clearly revealed only for the powders thermally treated at 500 ◦ C, irrespective of the iron content. For all the annealing temperatures, a lower dopant concentration (≤1 wt%) seems to be beneficial for the crystallization process (as in the case of the samples TF0.5 and TF1), relative to the undoped samples, while the further increase of the iron content to 5 wt%, is less effective from this point of view, mainly for the powder thermally treated at 300 ◦ C (TF5 300), which is rather amorphous.

3.2.1. Unit cell volume (UCV) variation with temperature and dopant concentration Excepting the FT5 sample, for the other dopant concentrations the increase of temperature does not significantly influence the unit cell volume (UCV). This fact could be considered as a proof of the acceptance of the dopant by the anatase lattice in TF samples, where Fex Ti1−x O2−ı type solid solutions are formed. In the case of undoped TiO2 a slight tendency of compaction of the UCV with the increase of temperature could be observed. This is not maintained in the presence of the dopant, excepting the lowest iron concentration (0.5 wt%). In this particular case, a very similar behaviour could be observed with only one difference, at 400 ◦ C, where the biggest UCV contraction was noticed. Along with the increase of the dopant concentration (1–5 wt%) the UCV evolution

is reversed, that is the Fex Ti1−x O2−ı unit cell tends to slightly dilate with the temperature (see Fig. 3 and Table 1). The variation of the UCV values could give some indications about the spin state of the Fe3+ ions. Shannon [44] has demonstrated that the ionic radius of the elements can vary depending on coordination, oxidation state, and spin state. Thus, he gives the following values of the ionic radii: Ti4+ = 0.605 A˚ (coordination = 6; electronic configuration = 3p6 ); Fe3+ (LS) = 0.550 A˚ (coordination = 6; electronic configuration = 3d5 ), and Fe3+ (HS) = 0.645 A˚ (coordination = 6; electronic configuration = 3d5 ), where “LS” indicates the low spin state, and “HS” refers to the high spin state, respectively. Based on these data, one can see that when the dopant is present in the low spin state (TF0.5), its ionic radius being lower than that of the Ti4+ (0.550 < 0.605), the Fex Ti1−x O2−ı cells will contract. On the contrary, in the case of the high spin state (TF1-TF5), the ionic radius value of the dopant becomes higher than that of the Ti4+ (0.645 > 0.605), thus the cells of the solid solution will expand.

Fig. 3. Unit cell volume (UCV) variation versus temperature and dopant concentration.

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Table 2 The binding energies and the atomic relative concentrations for T sample, thermally treated at 300, 400, and 500 ◦ C. Sample

T 300 T 400 T 500

Binding energy (eV)

Atomic relative concentration (%)

O1s

Ti2p

O

Ti

529.8 529.9 529.8

458.5 458.6 458.5

72.69 72.13 71.84

27.31 27.87 28.16

Fig. 4. Average crystallite size (D) evolution versus temperature and dopant concentration.

3.2.2. Crystallite size (D) variation with temperature and dopant concentration The mean crystallite size values D increase with temperature for both T and TF samples no matter the dopant concentration. This proves that a higher temperature is responsible for the increase of the crystallites, which suggests the increase of the order in the lattice. Besides, irrespective of the thermal treatment temperature, lower iron concentrations (0.5 and 1 wt%) induce higher values of the average crystallite size, relative to those ones corresponding to the undoped samples, while an obvious decrease of the crystallite size was obtained in the case of the heavily-doped samples (2 and 5 wt%) (Fig. 4 and Table 1). These calculations sustain the qualitative observations based on the analysis of the peaks profiles of the corresponding XRD patterns. 3.2.3. Lattice strain (S) variation with temperature and dopant concentration The evolution of the tensile strain factor S surprising does not follow a decreasing tendency, as the order extinction of the lattice (the crystallinity evolution) would suggest (Table 1). For the undoped TiO2 powder the value of S at 300 ◦ C is minimum

indicating only very low strains rather residual ones. Considering this structural factor as a measure of the level of the defects of the crystalline lattice, one can consider that in the mentioned case the number of the single point defects is very low. The increase of the temperature to 400 ◦ C significantly changes the behaviour of the T sample. It seems that it is a critical temperature for the stability of the crystalline edifice. The tensile strain grows twentyfold indicating a strong destabilization of the lattice. This could be attributed to a high number of single point and clusters defects [45]. Finally, at 500 ◦ C the number of defects decreases, leading to a lower S value, which is sustained by the sudden increase of the mean crystallite size value D from 166 A˚ ˚ to 302 A. The variation of S factor in the case of TF samples is even more complicated. For all iron concentrations (0.5–5 wt%) and for both 300 ◦ C and 500 ◦ C, one can see that the presence of the dopant leads (with only one exception–sample TF5 500) to the increase of tensile strain compared to the corresponding T samples. The temperature of 400 ◦ C is again the “different one”. In this case, S presents a slight decrease with the increase of the iron concentration (from 0.5 to 2 wt%) and then it grows for 5 wt% Fe. Thus, for 400 ◦ C the highest value of the average internal strain of the lattice S corresponded to the lowest dopant concentration (0.5 wt%). This maximum is apparently in contradiction with increasing crystallinity, which does not exclude the presence of a trace Fe2+ oxidation state. We can suppose that these ions facilitate the migration of oxygen vacancies to the surface, so improving the photocatalytic activity of the TF0.5 400 sample. As it can be seen in Table 1, the best photocatalytic test result corresponded to this nanopowder. 3.3. XPS 3.3.1. XPS analysis of undoped TiO2 nanopowders (T) XPS results for undoped TiO2 are presented in Fig. 5 and Table 2. Fig. 5 presents the Ti2p (a) and O1s (b) superimposed spectra of the undoped TiO2 nanopowder, thermally treated at 300, 400, and 500 ◦ C, respectively. The Ti 2p spectra with 2p3/2 peak (BE = 458.6 eV) and the spin-orbit parameter  = 6.2 eV are typically attributed to Ti 4+ oxidation state as TiO2 . O1s peaks at 529.9 eV are typically assigned to TiO2 , and the asymmetry on the high BE side

Fig. 5. Ti2p and O1s XPS superimposed spectra for TiO2 sample (T) thermally treated at 300, 400 and 500 ◦ C.

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Fig. 6. Ti2p and O1s XPS superimposed spectra for TF0.5 (a and e); TF1 (b and f); TF2 (c and g); and TF5 (d and h) samples thermally treated at 300, 400 and 500 ◦ C.

suggests the presence of a small amount of OH adsorbed on the top of the surface. The XPS data from Table 2 show that no change can be noticed with temperature neither in the lines profile nor in the binding energies (BEs). Therefore, it can be concluded that TiO2 on the surface of the samples is very stable over the studied temperature range. From Table 2, one can notice an O/Ti average ratio of ∼2.6 which, apparently, exceeds the stoichiometric ratio of TiO2 . This is due to the presence of OH and H2 O adsorbed on the outermost surface layer and to the oxygen bonded on the unavoidable carbon contaminant as O–C and O C chemical bonds. These data have been reported elsewhere [38,39].

3.3.2. XPS analysis of Fe-doped TiO2 nanopowders (TF) The XPS survey spectra recorded on a wide binding energy range (0–1200 eV) revealed that all the elements have been detected on the surface. The binding energies (BEs) of the most prominent XPS transitions (O1s, Ti2p, Fe2p) together with the atomic relative concentrations are presented for all the samples calcined at 300, 400 and 500 ◦ C in Table 3. In order to determine the oxidation states of Ti and Fe on the surface of the Fe doped TiO2 samples, the high resolution XPS spectra were recorded (Figs. 6 and 8). The binding energy for Ti2p line clearly exhibits Ti4+ oxidation state as TiO2 (Ti2p3/2 ∼ 458.6 eV) for all the samples calcined at 300, 400 and 500 ◦ C (Fig. 6a–d). Within our experimental errors we

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Table 3 The binding energies and the atomic relative concentrations for TF0.5, TF1, TF2 and TF5 samples, thermally treated at 300, 400 and 500 ◦ C. Sample

Binding energy (eV)

TF0.5 300 TF0.5 400 TF0.5 500 TF1 300 TF1 400 TF1 500 TF2 300 TF2 400 TF2 500 TF5 300 TF5 400 TF5 500

Atomic relative concentration (atom %)

O1s

Ti2p

Fe2p

O

Ti

Fe

530.0 529.9 529.9 530.0 529.8 529.9 529.8 529.9 530.0 529.8 529.9 530.0

458.6 458.6 458.6 458.6 458.5 458.6 458.6 458.4 458.6 458.5 458.6 458.6

710.6 710.5 710.5 710.9 710.8 711.0 711.3 711.1 711.2 711.2 711.0 711.1

71.9 71.6 72.6 72.2 71.5 71.5 70.9 70.7 70.4 72.0 69.8 69.7

27.7 27.7 26.8 25.6 26.2 26.0 28.1 27.9 28.0 25.1 25.5 23.8

0.4 0.7 0.6 2.2 2.3 2.5 1.0 1.4 1.6 2.9 4.7 6.5

Fig. 7. O1s deconvoluted spectrum for the TF2 500 sample.

cannot distinguish between different crystalline phases on the surface layer (<10 nm). Following the procedure showed in Fig. 7, the O1s spectra exhibit three features after spectral deconvolution, for all samples, Fig. 6: O2− which is bonded in the lattice as well as OH− and H2 O adsorbed on the top of the surface. One can notice that with the temperature increasing in the range (300–500 ◦ C) the

percentage of OH groups is continuously diminished on the surface, as expected. Ti features of the 2p doublet are characteristic to TiO2 . However, in O1s and Ti2p spectra some differences occur: for the lower content of Fe, TF0.5 sample (Fig. 6a and e) the spectra are narrower as compared to the higher Fe concentration, TF5 sample (Fig. 6d and h). As no chemical shift in the BEs takes place these different profiles suggests different sites of the Fe cations around Ti ions leading to rather weak interactions between Fe and Ti cations for higher iron content. In the sample with low Fe concentration (TF0.5 sample) this is just embedded into the TiO2 matrix without any interaction. The Fe 2p doublets (Fig. 8a and c) display slight shifts in the BEs for the TF0.5 sample compared with TF2 one. Thus, the BE of the Fe2p3/2 line (710.5 eV, Table 3) corresponding to the sample with lowest Fe content is shifted toward lower BEs by ∼0.6 eV, beyond our experimental errors of ±0.2 eV. This suggests that a mixture of FeOOH and Fe2 O3 [46,47] chemical state are found on the surface layer with FeOOH confined on the top of the surface layer and Fe2 O3 present in the subsurface layer. This statement is supported by the evidence that OH groups are adsorbed on the outermost surface layer as well as by the lack of the satellites characteristic to Fe2 O3 chemical state. Since a close inspection of these spectra seems to exhibit a shoulder on the lower

Fig. 8. Fe2p XPS superimosed spectra for TF0.5, TF1, TF2, and TF5 samples thermally treated at 300, 400 and 500 ◦ C.

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Fig. 9. TEM/HRTEM images for: (a–c) sample T 500; (d–f) sample T0.5 500 and (g–i) sample T5 500 (SAED patterns are presented as insets in (a), (d) and (g)).

BE side, the presence of a trace Fe2+ oxidation state cannot be completely ruled out. It is worth to notice that despite the presence of a large noise in the Fe2p XPS spectra (very low Fe percentages) the agreement between the surface experimental relative concentrations and the bulk (nominal) ones is rather good. 3.4. TEM The TEM and HRTEM images for samples T, TF0.5, and TF5, thermally treated at 500 ◦ C are presented in Fig. 9, as an example. TEM/HRTEM/SAED investigations performed on the undoped and Fe doped TiO2 -based powders, thermally treated at 500 ◦ C, indicate the formation of small, well-crystallized, almost uniform (as shape and size) polyhedral nanoparticles. These nanoparticles exhibit an obvious tendency of self-assembly into close-packed, dense, spherical submicron aggregates, especially in the case of

the undoped powder (Fig. 9a) [48,49]. Taking into account that the primary nanoparticles can be clearly distinguished just at the periphery of the aggregates, determining of the average particle size was a difficult task. Therefore, we were able only to roughly estimate particle sizes of 38–40 nm. The presence of a small content (0.5 wt%) of iron as dopant induces the reduction of the average size of the spherical mesocrystals (Fig. 9d), while the further increase of the dopant content determines the formation of increasingly thinner (and consequently more “transparent”), unevenly sized and irregular, plate-like agglomerates, as in the case of TF5 500 sample (Fig. 9g) [20,26,28,50]. Besides, with increasing iron doping, the polyhedral and relatively larger primary particles observed in the undoped titania sample gradually changed to almost spherical and smaller nanoparticles, with sizes of 20–22 nm in the powder doped with 5 wt% Fe. These observations are in good agreement with those ˇ ´ c-Vujiˇ cic´ et al. [19]. reported by Sjakovi

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Fig. 10. EDX spectra for the samples thermally treated at 500 ◦ C: (a) T 500 and TF0.5 500 and (b) TF2 500 and TF5 500.

It is worthy to mention that the sizes of the particles estimated from the TEM images of higher magnification are very close to the average size of the crystallites calculated from the X-ray diffraction data, proving the single crystal nature of these particles (Fig. 9b, e and h). The high crystallization degree of the primary particles is pointed out by the bright spots, forming well-defined diffraction rings in the SAED patterns (insets of Fig. 9a, d and g), as well as by the HRTEM images from Fig. 9c, f and i, which clearly show highlyordered fringes spaced at 0.351 nm and 0.232 nm, corresponding to the crystalline (1 0 1) and (1 1 2) planes of the anatase structure. EDX spectra from Fig. 10a and b point out the presence of iron in the doped TiO2 samples (TF0.5 500, TF2 500 and TF5 500).

quantity of impurities diffuses, thus forming “magnetic molecules” by reactions in the solid phase, paramagnetic defects being generated as tensile strain source. A low paramagnetism could be possible to coexist with some defects which can behave as active photocatalytic centres. For the smaller concentration of the dopant (0.5 wt%), the intracrystalline field probably screens the interaction between paramagnetic centres from the host lattice, thus favouring the appearance of active photocatalytic centres. Thus, for the prepared Fe-doped TiO2 nanopowders, after 2 h of irradiation, the best yield of nitrobenzene removal (NB = 84.91%) has been obtained for TF0.5 400 sample with internal strain S = 1.19 × 10−3 and magnetic susceptibility  = 0.59 × 10−6 cm3 /g.

3.5. Magnetic measurements

3.6. Kinetic and photodegradation studies of nitrobenzene in water

The magnetic susceptibilities measurements are presented in Table 1 together with structural parameters from XRD data and photocatalytic activities, both for T and TF samples. Undoped TiO2 sample (T) has a diamagnetic behaviour ( < 0) and TF samples are paramagnetic ( > 0), regardless of the dopant concentration and the temperature of thermal treatment. Thus, the presence of the dopant which partially replaces the Ti ions and/or occupies interstitial positions leads to paramagnetic behaviour of anatase phase [51]. The positive values of the magnetic susceptibilities increase almost in the same ratio with the increase of the dopant concentration, no matter the temperature. Combining this observation with the low fluctuations of the unit cell volumes, it can be concluded that the dopant has been accepted by the TiO2 lattice, where Fex Ti1−x O2−ı solid solutions are formed, conclusion which is supported by the XRD results. On the other hand, the growth of the magnetic susceptibility with the iron content suggests that the more dopant concentration is higher, the more

The photocatalytic activity has been evaluated by advanced degradation of nitrobenzene from water and has been expressed as yield of nitrobenzene removal (NB ) (see Table 1). The presence of metal ion dopant assures the enhancement of the photocatalytic activity due to the fact that the catalyst activity modification is the result of changes that occur in: interfacial changes transfer, light absorption capacity of TiO2 and catalyst adsorption capacity of pollutant molecules. The effect of dopant content on NB photocatalytic degradation was evaluated into solution with 31 mg L−1 pollutant in aqueous suspended 0.5–5 wt% Fe-doped TiO2 under UV–vis irradiation. The time profiles of NB concentrations were compared in Fig. 11. An increase of the photocatalytic activity has been mentioned for iron used as dopant in our experiments (samples TF thermally treated at 400 ◦ C). The value of NB obtained for the undoped powder (T) has increased for all TF samples (see Table 1). Our results showed that the kinetics of NB photodegradation catalysed

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[NB], mg/L

30 25 20 15

4.0

400C 

TF 0.5%= 100mg/L

3.5

ln([NB] 0 /[NB]t

TF0.5 TF1 TF2 TF5 T

35

11

 500C

3.0 2.5 2.0

300C 

1.5 1.0 0.5

10

0.0

5

0

50

100

150

200

250

t, min 0

20

40

60

80

100

120

t, min

Fig. 13. The pseudo first order kinetic of NB photocatalytic degradation for TF0.5 sample at different calcination temperatures.

Fig. 11. The kinetic curves of NB degradation for T and TF samples at 400 ◦ C.

by Fe-doped TiO2 could be well described as first-order kinetics, according to the expression: ln

Co = kobs t Ct

where Co is the initial concentration of NB, Ct is the concentration of NB at reaction time t, kobs represents the apparent kinetic rate constant of the first-order reaction model and t is the reaction time. Our experimental results are in accordance with data reported by other authors regarding first-order kinetic degradation of some nitroaromatic compounds like mono, di, tri nitrobenzenes, nitrotoluenes and nitrophenols in UV/TiO2 systems [52–56]. First-order rate constants kobs were calculated and the following values were obtained: kTF0.5 = 2.54 × 10−4 s−1 ; kTF1 = 2.09 × 10−4 s−1 , kTF2 = 1.76 × 10−4 s−1 and kTF5 = 1.69 × 10−4 s−1 . The optimum concentration level which assures the higher pollutant degradation for the Fe dopant is 0.5 wt%. The effect of calcination temperature on NB degradation was evaluated for 300, 400 and 500 ◦ C, into solution with 31 mg L−1 NB, at optimum concentration of Fe (0.5%) and 240 min irradiation time. The kinetic curves obtained in this condition are presented in Fig. 12. The pseudo first-order rate constants kobs were calculated from the slopes of the linear plots from Fig. 13. They are presented in Table 4.

Fig. 12. The kinetic curves of NB degradation for TF0.5 sample at different calcination temperatures.

It can be seen that, both rate constant and degradation efficiency increase by increasing the calcination temperature from 300 to 400 ◦ C. The catalytic efficiency of NB was diminished at calcination temperature of 500 ◦ C. This behaviour could be determined by the increase of the crystallite sizes of samples with temperature and can lead to decrease of pollutant adsorption at the solid/liquid interface. 3.7. Microstructure-properties correlation The correlations between the structure and the properties of the prepared undoped and Fe-doped TiO2 nanopowders are difficult to be established, due to the high complexity of the factors which to a large extent influences the final result. There is always a competition between parameters which have contrary effects and the resultant will change from case to case (see Table 1). The situation is a little easier to be explained for the undoped samples. As it was already presented in Section 3.2, at 300 ◦ C it could be considered that the number of defects in the crystalline lattice is minimum based on the lowest calculated value of the tensile strain (sample T 300 was considered as standard). The variation of the D and S values with the temperature for sample (T) shows that the highest disorder state was found at 400 ◦ C, where a high number of single point defects appear and S value increases 20 times. The temperature confers them a big mobility because of which some of these defects could recombine and thus, disappear. The mean free path is insufficient and less of defects (vacancies and interstitials of oxygen) could be arrived at the surface, as the defects-boundary interactions. These defects become active photocatalytic centres and lead to the increase of the photocatalytic activity (see Table 1). But the fact that part of defects migrates from the volume of the lattice to the surface could explain the increase of the medium crystalline size D in the same time with the tensile strain increasing. When the temperature increases at 500 ◦ C, two effects are predominant: (a) the interaction of point defects with a representative grain boundary and (b) the kinetics of those defects in the bulk material. Thus, the local intern strain S decreases, D has a twice value, and the photocatalytic activity increases by ∼3% from the case of 400 ◦ C. Introducing iron in the titania lattice, things became more complicated. In the presence of the dopant Fex Ti1−x O2−ı type solid solutions are formed and besides mobile single point defects due to the temperature, additional paramagnetic defects appear. The contribution of the iron to the crystallinity and to the tensile strain of the titania lattice, as well as to the catalytic activity of the samples, in addition to the contribution of the thermal treatment could be easily followed by direct comparing of the D, S and ␩NB values

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Table 4 Effect of calcination temperature on NB degradation efficiency (120 and 240 min) and rate constants for TF0.5 sample. Calcination temperature (◦ C)

kobs × 104 (s−1 )

NB (%) (120 min irrad.)

NB (%) (240 min irrad.)

300 400 500

0.95 2.54 2.29

56.95 84.91 82.34

90.16 97.35 96.55

to the surface but rather is prevented (which is sustained by the decrease of D values). 4. Conclusions

Fig. 14. The photocatalytic activity (NB – after 2 h of irradiation) and mean lattice strain (S) variation with temperature and dopant concentration.

corresponding to the pairs of samples T–TF at a certain temperature (see Table 1 and Fig. 14). Following Table 1 one can see that generally, the presence of iron in TF samples leads to the increase of the photocatalytic activity (except samples TF5 300 and TF5 500). Taking into account that sample TF0.5 400 has been shown to be the best one from point of view of its catalytic activity, a detailed discussion about the correlations between the structure and the properties of sample TF0.5 will be made. Starting with the temperature of 300 ◦ C, the minimum addition of iron in the anatase lattice (sample TF0.5 300) led to the increase of 12.5 times of the tensile strain, comparing to the same sample in the absence of dopant (T 300). The source of these tensions could be most likely attributed to the paramagnetic defects and it is due to the iron ions. Literature data [57] confirm that the paramagnetism could induce supplementary defects like colour centres. The thermal treatment at 300 ◦ C probably creates only very few mobile single point defects, deficit that paramagnetism is unable to recover. Thus, only few defects will be stimulated to have enough mobility to reach the boundaries of the nanocrystals. After their migration the disorder slightly decreases in the interior of the crystal such that the crystallinity (D) slightly increases. For 400 ◦ C, the experimental data presented in Table 1 show that the number of defects mobilized by temperature is high. They are furthermore stimulated by the paramagnetism, and probably by the presence of a trace Fe2+ ions. Thus, they reduce the disorder by massive leaving the volume. Therefore D increases. The high number of defects from the surface is responsible for the high catalytic activity (the best NB ). The reason for a higher value of S than expected could be the excess of Ti remained after the migration of the mentioned defects which could represent a source of local tensions. The discussion is the same for 500 ◦ C, the potential fluctuations of the parameters depending on the results of the competition between the mentioned factors. For higher concentrations than 0.5 wt% of the dopant it seems that the mechanisms are basically similar. Yet, as the iron concentration increases it seems that the stimulation of the migration of defects which present a higher mobility (O defects) could be in gradually decreasing. The decrease of the catalytic activities of TF1-TF5 samples both at 400 ◦ C and 500 ◦ C could be considered as an indication of the fact that starting with a certain value of the paramagnetism it is not possible to stimulate the defects migration

• Undoped (T) and Fe-doped TiO2 sol–gel nanopowders (TF) were prepared by controlled hydrolysis-condensation of titanium ethoxide. The use of tetraethyl orthotitanate as titania precursor for the preparation of iron-doped TiO2 nanopowders is a novelty. • The acceptance of the dopant by the anatase lattice in TF samples was proved by the UCV measurements and the positive values of the magnetic susceptibilities. • The variation of the UCV values with the dopant concentration were explained on the basis of the spin state of the Fe3+ ions (low in sample TF0.5 and high in samples TF1-TF5). • The role of the dopant on nanopowders structure was proved. Its addition is responsible for supplementary defects in the crystalline lattice. Besides the mobile single point defects due to the thermal treatment, the iron induces the paramagnetic ones. • The correlation between structural and magnetic properties of the obtained Fe-doped TiO2 nanopowders that explains their using properties was established and could be considered as a novelty point of the paper. • The application of sol–gel Fe-doped TiO2 nanopowders in the nitrobenzene degradation from water is for the first time mentioned in the literature. The photocatalytic properties of TiO2 have been significantly improved by iron doping. The sample TF0.5 400 has presented the best yield of nitrobenzene removal from water. Acknowledgements This work was supported by a grant from the Romanian National Authority for Scientific Research, CNDI-UEFISCDI, project number PN-II-PT-PCCA-2011-3.1-0031. It has also been performed in the frame of the 4.12 Project of the Romanian Academy Program: “Oxide systems obtained by the sol-gel method” (2014). References [1] U.I. Gaya, A.H. Abdullah, J. Photochem. Photobiol. C: Photochem. Rev. 9 (2008) 1–12. [2] R.J. Tayade, R.G. Kulkarni, R.V. Jasra, Ind. Eng. Chem. Res. 45 (2006) 922–927. [3] M. Tong, S. Yuan, H. Long, M. Zheng, L. Wang, J. Chen, J. Contam. Hydrol. 122 (2011) 16–25. [4] Y. Yuan, H. Li, M. Luo, S. Qin, W. Luo, L. Li, H. Yan, Water Air Soil Pollut. 225 (2014) 1881, http://dx.doi.org/10.1007/s11270-014-1881-5 (9 pp.). [5] M. Haro, R. Abargues, I. Herraiz-Cardona, J. Martínez-Pastor, S. Giménez, Electrochim. Acta 144 (2014) 64–70. [6] A. Djeddi, I. Fechete, F. Garin, Top. Catal. 55 (2012) 700–709. [7] N. Pourreza, T. Naghdi, Talanta 128 (2014) 164–169. [8] C. Dumitriu, A.B. Stoian, I. Titorencu, V. Pruna, V.V. Jinga, R.-M. Latonen, J. Bobacka, I. Demetrescu, Mater. Sci. Eng. C 45 (2014) 56–63. [9] J.O. Carneiro, G. Vasconcelos, S. Azevedo, C. Jesus, C. Palha, N. Gomes, V. Téixeira, Energy Buildings 81 (2014) 1–8. [10] X. Chen, S.S. Mao, Chem. Rev. 107 (2007) 2891–2959. [11] S. Malato, P. Fernandez-Ibanez, M.I. Maldonado, J. Blanco, W. Gernjak, Catal. Today 147 (2009) 1–59. [12] S.-Y. Lee, S.-J. Park, J. Ind. Eng. Chem. 19 (2013) 1761–1769. ˇ [13] C.J. Philippopoulos, M.D. Nikolaki, in: B. Sramová (Ed.), Photocatalytic Processes on the Oxidation of Organic Compounds in Water. New Trends in Technologies, In-Teh., Croatia, 2010, pp. 89–107.

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ARTICLE IN PRESS M. Cris¸an et al. / Applied Catalysis A: General xxx (2014) xxx–xxx

˜ [14] J.M. Poyatos, M.M. Munio, M.C. Almecija, J.C. Torres, E. Hontoria, F. Osorio, Water Air Soil Pollut. 205 (2010) 187–204, http://dx.doi.org/10.1007/s11270009-0065-1. [15] M. Takeuchi, M. Matsuoka, M. Anpo, Res. Chem. Intermed. 38 (2012) 1261–1277. [16] S. Ahmed, M.G. Rasul, W.N. Martens, R. Brown, M.A. Hashib, Water Air Soil Pollut. 215 (2011) 3–29. [17] A.N. Banerjee, Nanoteh. Sci. Appl. 4 (2011) 35–65. [18] M. Cris¸an, M. R˘aileanu, A. Ianculescu, D. Cris¸an, N. Dr˘agan, in: R.E. Morris (Ed.), Sol-gel TiO2 -based Oxide Systems. The Sol-Gel Process: Uniformity, Polymers and Applications, Nova Science Publishers, Inc., New York, 2011, pp. 1–135. ˇ ´ ´ M. Gotic, ´ S. Music, ´ M. Ivanda, S. Popovic, ´ J. Sol-Gel Sci. [19] N. Sijakovi c-Vujiˇ cic, Technol. 30 (2004) 5–19. ´ J. Gulikovski, Z. Dohˇcevic-Mitrovi ´ ´ D. Buˇcevac, M. Prekajski, J. Zagorac, [20] B. Babic, c, ´ Ceram. Int. 38 (2012) 635–640. B. Matovic, [21] J. Liu, Z. Zheng, K. Zuo, Y. Wu, J. Wuhan Univ. Technol. – Mater. Sci. Ed. 21 (3) (2006) 57–60. [22] J.A. Navío, G. Colón, M. Macías, C. Real, M.I. Litter, Appl. Catal. A: Gen. 177 (1999) 111–120. [23] D. Mardare, A. Yildiz, R. Apetrei, P. Rambu, D. Florea, N.G. Gheorghe, D. Macovei, C.M. Teodorescu, D. Luca, J. Mater. Res. 27 (2012) 2271–2277. [24] F. Han, V.S.R. Kambala, M. Srinivasan, D. Rajarathnam, R. Naidu, Appl. Catal. A: Gen. 359 (2009) 25–40. [25] B. Tryba, Int. J. Photoenergy (2008), doi:10.1155/2008/721824 (article ID 721824, 15 pp.). [26] I. Medina-Ramírez, J.L. Liu, A. Hernández-Ramírez, C. Romo-Bernal, G. PedrozaHerrera, J. Jáuregui-Rincón, M.A. Gracia-Pinilla, J. Mater. Sci. 49 (2014) 5309–5323. [27] C.A. Castro-López, A. Centeno, S.A. Giraldo, Catal. Today 157 (2010) 119–124. [28] M.P. Seabra, I.M. Miranda Salvado, J.A. Labrincha, Ceram. Int. 37 (2011) 3317–3322. [29] C. Adán, J. Carbajo, A. Bahamonde, A. Martínez-Arias, Catal. Today 143 (2009) 247–252. [30] N. Riaz, M.A.B. Bustam, Proc. 2013 Int.Conf. Biol. Biomed. 72-77, ISBN: 978-161804-199-9; Bio 2013: The 2013 Int. Conf. Biol. Biomed., July 16–19, 2013, Rhodes Islands, Greece. [31] P. Vijayan, C. Mahendiran, C. Suresh, K. Shanti, Catal. Today 141 (2009) 220–224. [32] C.L. Luu, Q.T. Nguyen, S.T. Ho, Adv. Nat. Sci.: Nanosci. Nanotechnol. 1 (2010) 015008 (5 pp.). [33] T. Sun, J. Fan, E. Liu, L. Liu, Y. Wang, H. Dai, Y. Yang, W. Hou, X. Hu, Z. Jiang, Powder Technol. 228 (2012) 210–218. [34] J.K. Reddy, K. Lalitha, P.V.L. Reddy, G. Sadanandam, M. Subrahmanyam, V.D. Kumari, Catal. Lett. 144 (2014) 340–346.

13

[35] M. R˘aileanu, M. Cris¸an, I. Nit¸oi, A. Ianculescu, P. Oancea, D. Cris¸an, L. Todan, Water Air Soil Pollut. 224 (2013) 1548, http://dx.doi.org/10.1007/s11270-013-1548-7 (45 pp.). [36] J.G. Calvert, J.N. Pitts, Photochemistry, John Wiley & Sons, New York, 1966. [37] M. Cris¸an, A. Br˘aileanu, M. R˘aileanu, D. Cris¸an, V.S. Teodorescu, R. Bîrjega, V.E. Marinescu, J. Madarász, G. Pokol, J. Therm. Anal. Calorim. 88 (2007) 171–176. [38] M. R˘aileanu, M. Cris¸an, A. Ianculescu, D. Cris¸an, N. Dr˘agan, P. Osiceanu, S. S¸om˘acescu, N. St˘anic˘a, L. Todan, I. Nit¸oi, Water Air Soil Pollut. 224 (2013) 1773, http://dx.doi.org/10.1007/s11270-013-1773-0 (10 pp.). [39] N. Dr˘agan, M. Cris¸an, M. R˘aileanu, D. Cris¸an, A. Ianculescu, P. Oancea, S. S¸om˘acescu, L. Todan, N. St˘anic˘a, B. Vasile, Ceram. Int. 40 (2014) 12273–12284. [40] K. Panpae, S. Angkaew, C. Sritara, C. Ngernsuttichaiporn, Kasetsart J. (Nat. Sci.) 41 (2007) 178–185. [41] J. López, J.A. Moreno, R. Gómez, X. Bokhimi, J.A. Wang, H. Yee-Madeira, G. Pecchi, P. Reye, J. Mater. Chem. 12 (2002) 714–718. [42] I. Ganesh, P.P. Kumar, A.K. Gupta, P.S.C. Sekhar, K. Radha, G. Padmanabham, G. Sundararajan, Process. Appl. Ceram. 6 (2012) 21–36. [43] S. Zhu, W. Liu, C. Fan, Y. Li, Hyperfine Interact. 165 (2005) 273–278. [44] R.D. Shannon, Acta Crystallogr. Sect. A: Found. Crystallogr. 32 (1976) 751–767. [45] B.P. Uberuaga, X.M. Bai, J. Phys.: Condens. Matter 23 (2011) 435004 (11 pp.). [46] A.V. Naumkin, A. Kraut-Vass, S.W. Gaarenstroom, C.J. Powell, NIST X-ray Photoelectron Spectroscopy Database. NIST Standard Reference Database 20, Version 4.1, U.S. Secretary of Commerce on behalf of the United States of America, 2012. [47] J.F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, Handbook of X-ray Photoelectron Spectroscopy, Physical Electronics, USA Inc., Minnesota 55317, 1995. [48] M. Zaharescu, M. Cris¸an, D. Cris¸an, N. Dr˘agan, A. Jitianu, M. Preda, J. Eur. Ceram. Soc. 18 (1998) 1257–1264. [49] M. Cris¸an, A. Jitianu, M. Zaharescu, F. Mizukami, Shu-ichi. Niwa, J. Dispersion Sci. Technol. 24 (2003) 129–144. [50] M. Popa, L. Diamandescu, F. Vasiliu, C.M. Teodorescu, V. Cosoveanu, M. Baia, M. Feder, L. Baia, V. Danciu, J. Mater. Sci 44 (2009) 358–364. [51] J.A. Wang, R. Limas-Ballesteros, T. López, A. Moreno, R. Gómez, O. Novaro, X. Bokhimi, J. Phys. Chem. B 105 (2001) 9692–9698. [52] D.C. Schmelling, K.A. Gray, P.V. Kamat, Water Res. 31 (1997) 1439–1447. [53] M. Nahen, D. Bahnemann, R. Dillert, G. Fels, J. Photochem. Photobiol. A: Chem. 110 (1997) 191–199. [54] K. Tanaka, W. Luesaiwong, T. Hisanaga, J. Mol. Catal. A: Chem. 122 (1997) 67–74. [55] M.S. Vohra, K. Tanaka, Water Res. 36 (2002) 59–64. [56] D.S. Bhatkhande, S.P. Kamble, S.B. Sawant, V.G. Pangarkar, Chem. Eng. J. 102 (2004) 283–290. [57] Ch. Kittel, Introduction to Solid State Physics, fourth ed., John Wiley & Sons, Inc., New York/London/Sydney/Toronto, 1971.

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