- Email: [email protected]
Iron doped anatase for application in photocatalysis
Journal of the European Ceramic Society 36 (2016) 2991–2996
Contents lists available at www.sciencedirect.com
Journal of the European Ceramic Society journal homepage: www.elsevier.com/locate/jeurceramsoc
Iron doped anatase for application in photocatalysis Biljana Babic´ a,∗ , Aleksandra Zarubica b , Tamara Minovic´ Arsic´ a , Jelena Pantic´ a , Bojan Jokic´ c , Nadica Abazovic´ a , Branko Matovic´ a a b c
Vinˇca Institute of Nuclear Sciences, University of Belgrade, P.O. Box 522, 11000 Belgrade, Serbia Faculty of Science and Mathematics, Department of Chemistry, University of Niˇs, Viˇsegradska 33, 18000 Niˇs, Serbia Faculty of Technology and Metallurgy, University of Belgrade, Karnegijeva 4, 11000 Belgrade, Serbia
a r t i c l e
i n f o
Article history: Received 28 July 2015 Received in revised form 13 November 2015 Accepted 24 November 2015 Available online 8 December 2015 Keywords: TiO2 Sol–gel Fe-doping Optical properties Photocatalysis
a b s t r a c t Titanium dioxide nanopowders, doped with different amount of Fe3+ ions (0.3–5.0 mass%), were synthesized by acid-catalyzed sol–gel method in a non-aqueous medium. The obtained powders were characterized by field emission scanning electron microscopy (FESEM), energy dispersive X-ray spectrometry (EDX) and UV/vis spectroscopy. Degradation reaction of crystal violet (CV) azo dye is used to test the photocatalytic activity of iron doped titanium dioxide nanopowders. The results are correlated with previous investigations about the structure and physico-chemical characteristics of obtained nanopowders. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Nanometric titanium dioxide (TiO2 ) has a very important place among the various nanomaterials due to their unique properties such as high chemical and thermal stability, durability, nontoxicity, inexpensiveness etc. [1]. The wide applications of TiO2 [1–8] demands optimization of synthesis procedure in a sense of choice of the synthesis method, reproducibility and tailoring the properties of the materials depending on possible application. There are numerous methods for synthesis of TiO2 -based nanomaterials such as solid state reactions, hydrothermal and solvothermal method, precipitation, combustion method, biological synthesis, etc. [9]. From the scientific and practical point of view, sol–gel processing in organic solvents, under the exclusion of water is one of the most promising [6,10]. These processes involve the reaction of metal oxide precursor with organic solvent as a source of oxygen. These reactions take place at low temperatures and allow mixing of species at molecular level. Also, reactions in liquid phases allow doping of the final nanomaterial with different quantities of desired dopant. TiO2 nanopowders, obtained by sol–gel method, usually
∗ Corresponding author. Fax: +381 113408224. ´ E-mail address: [email protected] (B. Babic). http://dx.doi.org/10.1016/j.jeurceramsoc.2015.11.031 0955-2219/© 2015 Elsevier Ltd. All rights reserved.
have developed porosity, but shape and dimensions of pores are difficult to control [11,12]. The photocatalytic activity of TiO2 is well known. However, disadvantage of TiO2 is its low photocurrent efficiency due to its wide band gap which limits the absorption of the light from visible region [13,14]. This disadvantage could be overcome by synthesis of ordered structures of TiO2 and/or by doping of TiO2 nanoparticles with transition metal ions [9]. One of the common transition metal ion, which is used as dopant, is iron. In recent years, there is a significant growth of the literature data about doping TiO2 with this metal. But, the significant disagreements about the influences of the amount of doped iron, on photocatalytic activity, still exist [9]. We believe that there are a number of reasons for that. The certain concentration of iron ions in the structure of TiO2 will strongly influence on the band gap wideness but, at the same time, the structure and the morphology of the particles will changed. Consequently, as a function of morphology, active surface, particle size, isoelectric point of the material etc. will change. In our previous investigations [15] we synthesized Fe3+ doped titanium dioxide nanopowders by sol–gel method in non-aqueous medium. The amount of doped iron ions were between 0.3–5.0 mass%. Materials were characterized by X-ray diffraction (XRD), Raman spectroscopy, determination of isoelectric points and particle diameters. Specific surfaces and porosity properties were investigated by nitrogen adsorption–desorption method. The
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results showed that obtained nanopowders exhibited anatase crystal structure, independent of the amount of iron dopants. The crystallite size lies in nanometric range (10–11 nm). The presence of Fe3+ ions in anatase decreased the value of isoelectric point. But, unlike the crystal structure, porosity parameters are strongly affected by the amount of iron ions incorporated in the TiO2 lattice.
In this work, we continued the investigations about the structure, physical and chemical properties of the obtained nanopowders. Samples were characterized by field emission scanning electron microscopy (FESEM), energy dispersive X-ray spectrometry (EDX) and UV/vis spectroscopy. Photocatalytic activity of iron-doped TiO2 nanoparticles was investigated in reaction of
Fig. 1. FESEM images of iron doped TiO2 . (a) 0.3 mass% Fe; (b) 0.5 mass% Fe; (c) 1.0 mass% Fe; (d) 2 mass% Fe; (e) 5 mass% Fe (bar size-1 m).
B. Babi´c et al. / Journal of the European Ceramic Society 36 (2016) 2991–2996
degradation of crystal violet (CV) azo dye. The results are correlated with previous collected data. 2. Experimental 2.1. Synthesis of iron doped TiO2 samples TiO2 gels, doped with Fe3+ ions (0.3–5 mass%), were prepared by modified acid-catalyzed sol–gel method in a non-aqueous medium [6]. The detailed procedure was described in our previous paper [15]. Briefly, the sols were prepared by adding 2 M solution of nitric acid to a solution of titanium isopropoxide C12 H28 O4 Ti (97%, Aldrich) in anhydrous ethanol (99.5%, Superlab), at room temperature, under continuous stirring. A corresponding amounts of Fe(NO3 )3 (p.a., Superlab) were added. The amounts of Fe(NO3 )3 were adjusted so the calculated final mass% of the Fe3+ in TiO2 nanopowders were 0.3; 0.5; 1.0; 2.0 and 5.0. Sols were mechanically stirred for 10 min, sealed and placed at room temperature for 5 days. Gels were dried at 60 ◦ C for 24 h, on air. After drying, samples were thermal treated at 400 ◦ C for 2 h, in air. 2.2. Characterization of iron doped TiO2 samples The morphology of iron doped TiO2 nanoparticles was studied by FESEM TESCAN Mira XMU, at 20 kV. The chemical composition of the samples was analyzed using EDX Isis 3.2, with a SiLi X-ray detector (Oxford Instruments, UK) connected to the scanning electron microscope (SEM) JEOL JSM-5800, at 20 kV and a computer multi-channel analyzer. Diffuse spectral reflectance measurement were done in the spectral range 350–800 nm, on the Thermo Evolution 600 UV/vis spectrometer equipped with an integrating sphere, using BaSO4 as a blank. The color characteristics of specimens were calculated according to the CIE L*a*b* (1976) standard, using illuminant C spectral energy distribution [16]. In this system, L* (luminocity) measures the brightness (L* = 0 for black and L* = 100 for white), a* the amount of green (−) → red (+), and b* is the blue (−) → yellow (+) color. 2.3. Photocatalytic degradation of CV Photocatalytic activity of iron doped anatase nanopowders was studied using organic dye crystal violet (CV). The photochemical reactor consisted of a UV lamp (Roth Co., 16 W, 25 mW/cm2 , max = 366 nm) positioned annular to the 50 ml quartz flask. The rates of photocatalytical degradation of CV were followed at initial concentration 0.01 mmol/dm3 . The amount of catalyst was 30 ± 2 mg. The acidity of solutions was not adjusted and pH values were in the range of 6.7–7.0. 3. Results and discussion 3.1. Morphology of iron-doped TiO2 samples In our previous paper [15] we showed that all nanopowders have anatase structure and Fe3+ ions are completely incorporated in TiO2 lattice. The secondary crystalline phases containing iron was not detected, even in samples with 5 mass% of iron. FESEM images of samples (Fig. 1) confirmed our conclusion. The samples are homogeneous, particles have large distribution of shape and dimensions and grains consist of agglomerates of small particles. At the same time, nitrogen adsorption measurements showed that the amount of doped iron strongly influences on specific surface and porosity properties of obtained materials. Incorporation of small amounts of iron ions (below 2 mass%) in the lattice of TiO2
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Table 1 Fe3+ content in mass and atomic% of investigated iron doped TiO2 . Sample
Iron fraction in mass%
Iron fraction in atomic (%)
0.3% Fe 0.5% Fe 1.0% Fe 2.0% Fe 5.0% Fe
0.34 0.48 0.91 1.83 4.97
0.16 0.21 0.40 0.86 2.04
can slightly decrease or increase the specific surface while higher amounts of iron ions (above 2 mass%) significantly increase the overall specific surface, in comparison with pure TiO2 , synthesized by the same procedure. The microstructure of sample with 0.3 mass% of iron shows uniform size distribution of agglomerated grains (SBET = 47 m2 /g). The increasing of the amount of iron to 0.5 mass% leads to the further agglomeration and formation of clusters what is confirmed with the decreasing of specific surface (SBET = 26 m2 /g). The presence of 1.0 mass% of iron leads to the significant change in microstructure of the sample. The bimodal distribution of particles (larger particles of micrometer size and smaller particles in nanometric range) and, consequently, small increase of specific surface (SBET = 32 m2 /g) was detected. Sample with 2.0 mass% has even more homogeneous structure and particle size distribution in comparison with samples with lower content of iron (SBET = 62 m2 /g). And, finally, the FESEM image of the sample with 5 mass% of iron shows that powder consist of much larger particles whose agglomeration induced of intergranular porosity and significant increase of specific surface (SBET = 278 m2 /g). Considering the porosity properties of samples, nitrogen adsorption showed that samples are mesoporous and microporous and the ratio between microporous and mesoporous surface could be controlled by the amount of doped iron. According to these results, porosity properties can be predicted and controlled by the synthesis conditions. Despite the enormous growth of literature data which concern iron doped TiO2 , systematic investigations about influence of the amount of doped iron ions on porosity properties are still missing. Based on our experimental results we concluded that iron ions are incorporated into the anatase lattice but the crystallite size did not changed. Since Fe3+ ions replaced Ti4+ ions, the incorporation of iron ions caused structural disordering and excess of oxygen vacancies through valence effect, alternation in cation coordination or reduction/oxidation effects. Consequently, a certain number of unsaturated chemical bonds, on the surface of nanoparticles, are formed. This charge on the particle surface leads to differences in packaging and greater or lesser agglomeration which will strongly influence on specific surface and porosity. At higher iron concentrations the concentration of iron ions on the surface of the particle is higher, too. The influence of single iron ions is overlapped and, as a result, the shape of the particle is irregular and space between particles (mesoporosity) as well as pores on the particle surface (microporosity) is larger.
3.2. Energy dispersive X-ray spectroscopy (EDX)—elemental composition EDX analysis confirmed incorporation of iron in all samples of TiO2 . Representative EDX spectra of TiO2 sample with 5.0 mass% is presented on Fig. 2. Table 1 shows the iron content in mass and atomic percent. It is obvious that the amounts of doped iron systematically increase and are closed to the calculated values.
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Fig. 2. EDX spectrum of iron doped TiO2 (5.0 mass%).
3.3. UV/vis spectroscopy of iron doped TiO2 samples All the samples were analyzed by UV/vis spectroscopy in order to study the influence of the amount of doped iron on the optical band-gap. The diffuse reflectance spectra (DRS) are shown in Fig. 3—inset. UV/vis spectra clearly show that incorporation of iron ions leads to a shift of optical response toward visible part of spectra and reduction of the band-gap energy. Fig. 3 shows the band-gap calculation curves for iron doped anatase samples. The band-gap energies were calculated by applying Kubelka–Munk function on obtained DRS spectra, F(R)1/2 = f(h) functionality was observed. Extrapolation of linear part of acquired curves gave values of band-gap energies for doped samples [17]. Calculated values for the optical band-gap of iron doped anatase samples are shown in Table 2. For all doped samples optical band-gaps are lower in comparison with pure anatase (3.2 eV) and continually decrease with increasing the amount of doped iron. According to our results, Fe3+ doping enhance the adsorption in the visible light zone. Literature data show that there is significant disagreement about the influence of the amount of doped iron to the values of the bandgap energy [18]. It can be concluded that optical properties of iron doped anatase strongly depend on the synthesis conditions.
Table 2 The effects of Fe3+ content on the band-gap values and color properties of iron doped TiO2 samples. Sample
Bandgap (eV)
L
a
b
0.3% Fe 0.5% Fe 1.0% Fe 2.0% Fe 5.0% Fe
3.19 3.18 3.11 3.09 2.98
77.455 84.083 91.821 74.160 81.980
2.689 3.215 2.996 2.980 4.469
14.783 16.981 20.681 17.694 23.841
Ionic radius of Fe3+ ion has similar values to that of Ti4+ ion, hence, iron ion can easy occupy substitutional positions in TiO2 lattice. Modification of optical properties that occurs upon doping by iron ions is usually expressed by existence of two peaks/shoulders in UV–vis spectra: (i) one at about 400 nm, which originates from excitation of 3d electrons of Fe3+ to the TiO2 conduction band (charge transfer transition) [19,20], and (ii) the other cantered at about 500 nm, which is usually ascribed to the d–d transition of Fe3+ (2 T2g → 2 A2g , 2 T1g ) or the charge transfer transition between interacting iron ions (Fe3+ + Fe3+ → Fe4+ + Fe2+ ) [19,21,22] and which is common for higher dopant concentrations [22]. In DRS spectra of our samples there is no such obvious features, but spectra of all doped samples exhibit a red shift, comparing to undoped TiO2 . As
Fig. 3. Band-gap calculation curves for iron doped TiO2 samples. Inset: diffuse reflectance spectra of iron doped TiO2 samples.
B. Babi´c et al. / Journal of the European Ceramic Society 36 (2016) 2991–2996
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Fig. 4. Chromatic diagram of Ti1− y Fey O2 .
1.1
iron. The increasing of color intensity may support the thesis about the incorporation of iron ions in the lattice of TiO2 . At higher concentrations color saturation decreases (increases L) and color tone (hue) change. Judging by the changes in the absorption maxima, Fe3+ enters a lattice site in Ti1−y Fey O2 , changing the overall level of absorption. The change in relative intensities of the absorption produces a shift in color tone. For six-fold coordination, the iron ion (Fe3+ ) has an ionic radius (0.654 Å) and it is similar with the Ti4+ ion (0.605 Å), finds itself replacing Ti4+ in octahedral [25]. Therefore the ligands along the a-axis are more distant from the central ion in comparison with the others. Consequently, splitting of the d orbitals of the Fe3+ ion and the transition of energy from the higher energy level to lower energy level gives rise to the color. When the central ion, inside an octahedrally coordinated cage is iron, the field is strong enough to allow absorption in the visible spectrum, for wavelengths located in the violet and blue region.
0.3 % Fe 0.5 % Fe 1.0 % Fe 2.0 % Fe 5.0 % Fe
1.0 0.9 0.8
C/C0
0.7 0.6 0.5 0.4 0.3 0.2 0
5
10
15
20
25
Illumination time (h) Fig. 5. Photocatalytic degradation of crystal violet (CV) azo dye.
enters in TiO2 lattice, Fe3+ introduces new energy levels in bandgap of host [23] that are placed below original conduction band [24], consequently band-gap energy of doped samples decreases i.e., optical response of doped materials is shifted toward visible spectral region. UV/vis spectroscopy enables us to calculate the change in color of iron doped TiO2 samples, in comparison wit pure anatase. The colorimetry results of the iron doped TiO2 is presented in Table 2 and illustrated in Fig. 4. The values of L indicate that all of the colors are very pale. Also, all samples present red (a is positive) and yellow (b is positive) shades. Color changing is intensive with the increase of iron content from 0.3 to 5.0 mass% what is obvious from the corresponding L*a*b* coordinates (Table 1). For the prepared sample with 0.3 mass% of iron, a* (2.689), b* (14.783) and L* (77.455) were detected. On further increasing of iron concentration, L* increases until it reaches 1.0 mass% of iron. Above 2.0 mass% of iron, L* decrease and, again, increases with 5 mass% of
3.4. Photocatalytic degradation of CV Photocatalytic degradation of CV over iron doped TiO2 nanopowders was investigated at initial concentration of 0.01 mmol/dm3 . The photocatalytic degradation kinetics of CV is shown in Fig. 5. It is obvious that the degradation kinetics increases with increasing the amount of doped iron ions. Reasons for the increasing of reaction kinetics are multiple. The optical band-gaps decrease with the increasing the amount of doped iron ions. At the same time, the specific surfaces increase. The specific surface of sample with 5.0 mass% of iron is about ten time grater than surface of pure anatase, synthesized by the same method [15]. Consequently, the active surface available for chemical reaction increases. Additionally, the isoelectric point of pure anatase, synthesized by this method is at pH 7.25 ± 0.2 [15]. Presence of Fe3+ ions in titania crystal lattice can cause shifts in isoelectric point due to changes in cation coordination, structural charge, ion exchange capacity etc. In our case, the presence of iron ions in all TiO2 samples shift the isoelectric point at pH 6.5 ± 0.2. The pH of the initial solution was not adjusted and it was in the range 6.7–7.0. On that pH values, the surface of doped anatase is
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slightly negative while the surface of pure anatase is slightly positive. The excess of negative charge on surface of iron-doped anatase will accelerate the degradation of positively charged CV. 4. Conclusion Titanium dioxide nanopowders, doped with different amount of Fe3+ ions (0.3–5.0 mass%), were successfully synthesized by acid-catalyzed sol–gel method in a non-aqueous medium. Characterization by FESEM, EDX confirmed our previous results that Fe3+ ions are incorporated in TiO2 lattice. The morphology of doped samples strongly depends of the amount of doped iron. UV/vis spectroscopy showed that optical band-gap decreases with increasing the amount of doped iron ions. Degradation reaction of crystal violet (CV) dye showed that kinetics of the reaction increase with increasing the amount of doped iron due to decreasing the optical band-gap, increasing the overall specific surface and decreasing the isoelectric point of doped samples in comparison with pure TiO2 obtained by the same method. Acknowledgment This project was financially supported by the Serbian Education and Science Ministry (Project number: 45012). References [1] A. Fujishima, X. Zhang, D.A. Tryk, TiO2 photocatalysis and related surface phenomena, Surf. Sci. Rep. 63 (2008) 515–582. [2] R. Dastjerdi, M. Montazer, A review on the application of inorganic nano-structured materials in the modification of textiles: focus on anti-microbial properties, Colloid Surf. B 79 (2010) 5–18. [3] K. Han, M. Yu, Study of preparation and properties of UV-blocking fabrics of a PET/TiO2 nanocomposite prepared by in situ polycondensation, J. Appl. Polym. Sci. 100 (2006) 1588–1593. [4] D. Li, H. Haneda, S. Hishita, N. Onashi, Visible-light driven N–F–codoped TiO2 photocatalysts. 2. Optical characterization, photocatalysis and potential application to air purification, Chem. Mater. 17 (2005) 2596–2602. [5] L. Cermenati, P. Pichat, C. Guillard, A. Albini, Probing the TiO2 photocatalyst mechanisms in water purification by use of quinoline, photo-fenton generated OH radicals and superoxide dismutase, J. Phys. Chem. B 101 (1997) 2650–2658. [6] S. Boujday, F. Wunsch, P. Portes, J.F. Bocquet, C. Colbeau-Justin, Photocatalytic and electronic properties of TiO2 powders elaborated by sol–gel route and supercritical drying, Sol. Energy Mater. Sol. C 83 (2004) 421–433. [7] C. Lee, R.H. Lin, C.Y. Yang, M.H. Lin, W.Y. Wang, Preparation and characterization of novel photocatalysts with mesoporous titanium-dioxide (TiO2 ) via a sol–gel method, Mater. Chem. Phys. 109 (2008) 275–280.
[8] M. Ni, M.K.H. Leung, D.Y.C. Leung, K. Sumathy, A review and recent developments in photocatalytic water-splitting using TiO2 for hydrogen production, Renew. Sustain. Energy Rev. 11 (2007) 401–425. [9] M. Raileanu, M. Crisan, I. Nitoi, A. Ianculescu, P. Oancea, D. Crisan, A. Todan, TiO2 -based nanomaterials with photocatalytic properties for the advanced degradation of xenobiotic compounds from water. A literature survey, Water Air Soil Pollut. 224 (2013) 1548. [10] C. Sung, K.Z. Fung, I.M. Hung, M.H. Hon, Synthesis of highly ordered and worm-like mesoporous TiO2 assisted by tri-block copolymer, Solid State Ion. 179 (2008) 1300–1304. [11] Q. Dui, Z. Zhang, N. He, P. Li, C. Yuan, Preparation and characterization of mesostructured titanium dioxide and its application as a photocatalyst for the waste water treatment, Mater. Sci. Eng. C 8–9 (1999) 417–423. [12] B. Xin, Z. Ren, P. Wang, J. Liu, L. Jing, H. Fu, Study on the mechanisms of photoinduced carriers separation and recombination for Fe3+ -TiO2 photocatalysts, Appl. Surf. Sci. 253 (2007) 4390–4395. [13] H. Tang, K. Prasad, R. Sanjnes, P.E. Schmid, F. Levy, Electrical and optical properties of TiO2 anatase thin films, J. Appl. Phys. 75 (1994) 2042–2047. [14] T. Lindgren, J.M. Mwabora, E. Avendano, J. Jonsson, A. Hoel, C.G. Granqvist, et al., Photochemical and optical properties of nitrogen doped titanium dioxide films prepared by reactive dc magnetron sputtering, J. Phys. Chem. B 107 (2003) 5709–5716. [15] B. Babic, J. Gulicovski, Z. Dohcevic-Mitrovic, D. Bucevac, M. Prekajski, J. Zagorac, B. Matovic, Synthesis and characterization of Fe3+ doped titanium dioxide nanopowders, Ceram. Int. 38 (2012) 635–640. [16] CIE, Commision Internationale De L’eclairage Proceedings 1931, Cambridge University Press, Cambridge, 1932. [17] A.B. Murphy, Band-gap determination from diffuse reflectance measurements of semiconductor films, and application to photoelectrochemical water-splitting, Sol. Energy Mater. Sol. C 91 (2007) 1326–1337. [18] T.-h. Lee, H. Ryu, W. Lee, Photochemical properties of iron (III)-doped TiO2 , Ceram. Int. 41 (2015) 7582–7589 http://dx.doi.org/10.1016/j.ceramic.2015. 02.082. [19] J. Zhu, F. Chen, J. Zhang, H. Chen, M. Anpo, Fe3+ -TiO2 photocatalysts prepared by combining sol–gel method with hydrothermal treatment and their characterization, J. Photochem. Photobiol. A 180 (2006) 196–204. [20] T. Umebayashi, T. Yamaki, T. Itoh, K. Asai, Analysis of electronic structures of 3d transition metal-doped TiO2 based on band calculation, J. Phys. Chem. Solids 63 (2002) 1909–1920. [21] X. Li, P.-L. Yue, C. Kutal, Synthesis and photocatalytic oxidation properties of iron doped titanium dioxide nanosemiconductor particles, New J. Chem. 27 (2003) 1264–1269. [22] C. Adán, A. Bahamonde, M. Fernández-García, A. Martínez-Arias, Structure and activity of nanosized iron-doped anatase TiO2 catalysts for phenol photocatalytic degradation, Appl. Catal. B—Environ. 72 (2007) 11–17. [23] S. Manu, M. Abdul Khadar, Non-uniform distribution of dopant iron ions in TiO2 nanocrystals probed by X-ray diffraction, Raman scattering, photoluminescence and photocatalysis, J. Mater. Chem. C 3 (2015) 1846–1853. [24] W. Choi, A. Termin, M.R. Hoffmann, The role of metal ion dopants in quantum-sized TiO2 : correlation between photoreactivity and charge carrier recombination dynamics, J. Phys. Chem. 98 (1994) 13669–13679. [25] A. Shannon, Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides, Acta Crystallogr. A 32 (1976) 751–767.
Contents lists available at www.sciencedirect.com
Journal of the European Ceramic Society journal homepage: www.elsevier.com/locate/jeurceramsoc
Iron doped anatase for application in photocatalysis Biljana Babic´ a,∗ , Aleksandra Zarubica b , Tamara Minovic´ Arsic´ a , Jelena Pantic´ a , Bojan Jokic´ c , Nadica Abazovic´ a , Branko Matovic´ a a b c
Vinˇca Institute of Nuclear Sciences, University of Belgrade, P.O. Box 522, 11000 Belgrade, Serbia Faculty of Science and Mathematics, Department of Chemistry, University of Niˇs, Viˇsegradska 33, 18000 Niˇs, Serbia Faculty of Technology and Metallurgy, University of Belgrade, Karnegijeva 4, 11000 Belgrade, Serbia
a r t i c l e
i n f o
Article history: Received 28 July 2015 Received in revised form 13 November 2015 Accepted 24 November 2015 Available online 8 December 2015 Keywords: TiO2 Sol–gel Fe-doping Optical properties Photocatalysis
a b s t r a c t Titanium dioxide nanopowders, doped with different amount of Fe3+ ions (0.3–5.0 mass%), were synthesized by acid-catalyzed sol–gel method in a non-aqueous medium. The obtained powders were characterized by field emission scanning electron microscopy (FESEM), energy dispersive X-ray spectrometry (EDX) and UV/vis spectroscopy. Degradation reaction of crystal violet (CV) azo dye is used to test the photocatalytic activity of iron doped titanium dioxide nanopowders. The results are correlated with previous investigations about the structure and physico-chemical characteristics of obtained nanopowders. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Nanometric titanium dioxide (TiO2 ) has a very important place among the various nanomaterials due to their unique properties such as high chemical and thermal stability, durability, nontoxicity, inexpensiveness etc. [1]. The wide applications of TiO2 [1–8] demands optimization of synthesis procedure in a sense of choice of the synthesis method, reproducibility and tailoring the properties of the materials depending on possible application. There are numerous methods for synthesis of TiO2 -based nanomaterials such as solid state reactions, hydrothermal and solvothermal method, precipitation, combustion method, biological synthesis, etc. [9]. From the scientific and practical point of view, sol–gel processing in organic solvents, under the exclusion of water is one of the most promising [6,10]. These processes involve the reaction of metal oxide precursor with organic solvent as a source of oxygen. These reactions take place at low temperatures and allow mixing of species at molecular level. Also, reactions in liquid phases allow doping of the final nanomaterial with different quantities of desired dopant. TiO2 nanopowders, obtained by sol–gel method, usually
∗ Corresponding author. Fax: +381 113408224. ´ E-mail address: [email protected] (B. Babic). http://dx.doi.org/10.1016/j.jeurceramsoc.2015.11.031 0955-2219/© 2015 Elsevier Ltd. All rights reserved.
have developed porosity, but shape and dimensions of pores are difficult to control [11,12]. The photocatalytic activity of TiO2 is well known. However, disadvantage of TiO2 is its low photocurrent efficiency due to its wide band gap which limits the absorption of the light from visible region [13,14]. This disadvantage could be overcome by synthesis of ordered structures of TiO2 and/or by doping of TiO2 nanoparticles with transition metal ions [9]. One of the common transition metal ion, which is used as dopant, is iron. In recent years, there is a significant growth of the literature data about doping TiO2 with this metal. But, the significant disagreements about the influences of the amount of doped iron, on photocatalytic activity, still exist [9]. We believe that there are a number of reasons for that. The certain concentration of iron ions in the structure of TiO2 will strongly influence on the band gap wideness but, at the same time, the structure and the morphology of the particles will changed. Consequently, as a function of morphology, active surface, particle size, isoelectric point of the material etc. will change. In our previous investigations [15] we synthesized Fe3+ doped titanium dioxide nanopowders by sol–gel method in non-aqueous medium. The amount of doped iron ions were between 0.3–5.0 mass%. Materials were characterized by X-ray diffraction (XRD), Raman spectroscopy, determination of isoelectric points and particle diameters. Specific surfaces and porosity properties were investigated by nitrogen adsorption–desorption method. The
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B. Babi´c et al. / Journal of the European Ceramic Society 36 (2016) 2991–2996
results showed that obtained nanopowders exhibited anatase crystal structure, independent of the amount of iron dopants. The crystallite size lies in nanometric range (10–11 nm). The presence of Fe3+ ions in anatase decreased the value of isoelectric point. But, unlike the crystal structure, porosity parameters are strongly affected by the amount of iron ions incorporated in the TiO2 lattice.
In this work, we continued the investigations about the structure, physical and chemical properties of the obtained nanopowders. Samples were characterized by field emission scanning electron microscopy (FESEM), energy dispersive X-ray spectrometry (EDX) and UV/vis spectroscopy. Photocatalytic activity of iron-doped TiO2 nanoparticles was investigated in reaction of
Fig. 1. FESEM images of iron doped TiO2 . (a) 0.3 mass% Fe; (b) 0.5 mass% Fe; (c) 1.0 mass% Fe; (d) 2 mass% Fe; (e) 5 mass% Fe (bar size-1 m).
B. Babi´c et al. / Journal of the European Ceramic Society 36 (2016) 2991–2996
degradation of crystal violet (CV) azo dye. The results are correlated with previous collected data. 2. Experimental 2.1. Synthesis of iron doped TiO2 samples TiO2 gels, doped with Fe3+ ions (0.3–5 mass%), were prepared by modified acid-catalyzed sol–gel method in a non-aqueous medium [6]. The detailed procedure was described in our previous paper [15]. Briefly, the sols were prepared by adding 2 M solution of nitric acid to a solution of titanium isopropoxide C12 H28 O4 Ti (97%, Aldrich) in anhydrous ethanol (99.5%, Superlab), at room temperature, under continuous stirring. A corresponding amounts of Fe(NO3 )3 (p.a., Superlab) were added. The amounts of Fe(NO3 )3 were adjusted so the calculated final mass% of the Fe3+ in TiO2 nanopowders were 0.3; 0.5; 1.0; 2.0 and 5.0. Sols were mechanically stirred for 10 min, sealed and placed at room temperature for 5 days. Gels were dried at 60 ◦ C for 24 h, on air. After drying, samples were thermal treated at 400 ◦ C for 2 h, in air. 2.2. Characterization of iron doped TiO2 samples The morphology of iron doped TiO2 nanoparticles was studied by FESEM TESCAN Mira XMU, at 20 kV. The chemical composition of the samples was analyzed using EDX Isis 3.2, with a SiLi X-ray detector (Oxford Instruments, UK) connected to the scanning electron microscope (SEM) JEOL JSM-5800, at 20 kV and a computer multi-channel analyzer. Diffuse spectral reflectance measurement were done in the spectral range 350–800 nm, on the Thermo Evolution 600 UV/vis spectrometer equipped with an integrating sphere, using BaSO4 as a blank. The color characteristics of specimens were calculated according to the CIE L*a*b* (1976) standard, using illuminant C spectral energy distribution [16]. In this system, L* (luminocity) measures the brightness (L* = 0 for black and L* = 100 for white), a* the amount of green (−) → red (+), and b* is the blue (−) → yellow (+) color. 2.3. Photocatalytic degradation of CV Photocatalytic activity of iron doped anatase nanopowders was studied using organic dye crystal violet (CV). The photochemical reactor consisted of a UV lamp (Roth Co., 16 W, 25 mW/cm2 , max = 366 nm) positioned annular to the 50 ml quartz flask. The rates of photocatalytical degradation of CV were followed at initial concentration 0.01 mmol/dm3 . The amount of catalyst was 30 ± 2 mg. The acidity of solutions was not adjusted and pH values were in the range of 6.7–7.0. 3. Results and discussion 3.1. Morphology of iron-doped TiO2 samples In our previous paper [15] we showed that all nanopowders have anatase structure and Fe3+ ions are completely incorporated in TiO2 lattice. The secondary crystalline phases containing iron was not detected, even in samples with 5 mass% of iron. FESEM images of samples (Fig. 1) confirmed our conclusion. The samples are homogeneous, particles have large distribution of shape and dimensions and grains consist of agglomerates of small particles. At the same time, nitrogen adsorption measurements showed that the amount of doped iron strongly influences on specific surface and porosity properties of obtained materials. Incorporation of small amounts of iron ions (below 2 mass%) in the lattice of TiO2
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Table 1 Fe3+ content in mass and atomic% of investigated iron doped TiO2 . Sample
Iron fraction in mass%
Iron fraction in atomic (%)
0.3% Fe 0.5% Fe 1.0% Fe 2.0% Fe 5.0% Fe
0.34 0.48 0.91 1.83 4.97
0.16 0.21 0.40 0.86 2.04
can slightly decrease or increase the specific surface while higher amounts of iron ions (above 2 mass%) significantly increase the overall specific surface, in comparison with pure TiO2 , synthesized by the same procedure. The microstructure of sample with 0.3 mass% of iron shows uniform size distribution of agglomerated grains (SBET = 47 m2 /g). The increasing of the amount of iron to 0.5 mass% leads to the further agglomeration and formation of clusters what is confirmed with the decreasing of specific surface (SBET = 26 m2 /g). The presence of 1.0 mass% of iron leads to the significant change in microstructure of the sample. The bimodal distribution of particles (larger particles of micrometer size and smaller particles in nanometric range) and, consequently, small increase of specific surface (SBET = 32 m2 /g) was detected. Sample with 2.0 mass% has even more homogeneous structure and particle size distribution in comparison with samples with lower content of iron (SBET = 62 m2 /g). And, finally, the FESEM image of the sample with 5 mass% of iron shows that powder consist of much larger particles whose agglomeration induced of intergranular porosity and significant increase of specific surface (SBET = 278 m2 /g). Considering the porosity properties of samples, nitrogen adsorption showed that samples are mesoporous and microporous and the ratio between microporous and mesoporous surface could be controlled by the amount of doped iron. According to these results, porosity properties can be predicted and controlled by the synthesis conditions. Despite the enormous growth of literature data which concern iron doped TiO2 , systematic investigations about influence of the amount of doped iron ions on porosity properties are still missing. Based on our experimental results we concluded that iron ions are incorporated into the anatase lattice but the crystallite size did not changed. Since Fe3+ ions replaced Ti4+ ions, the incorporation of iron ions caused structural disordering and excess of oxygen vacancies through valence effect, alternation in cation coordination or reduction/oxidation effects. Consequently, a certain number of unsaturated chemical bonds, on the surface of nanoparticles, are formed. This charge on the particle surface leads to differences in packaging and greater or lesser agglomeration which will strongly influence on specific surface and porosity. At higher iron concentrations the concentration of iron ions on the surface of the particle is higher, too. The influence of single iron ions is overlapped and, as a result, the shape of the particle is irregular and space between particles (mesoporosity) as well as pores on the particle surface (microporosity) is larger.
3.2. Energy dispersive X-ray spectroscopy (EDX)—elemental composition EDX analysis confirmed incorporation of iron in all samples of TiO2 . Representative EDX spectra of TiO2 sample with 5.0 mass% is presented on Fig. 2. Table 1 shows the iron content in mass and atomic percent. It is obvious that the amounts of doped iron systematically increase and are closed to the calculated values.
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Fig. 2. EDX spectrum of iron doped TiO2 (5.0 mass%).
3.3. UV/vis spectroscopy of iron doped TiO2 samples All the samples were analyzed by UV/vis spectroscopy in order to study the influence of the amount of doped iron on the optical band-gap. The diffuse reflectance spectra (DRS) are shown in Fig. 3—inset. UV/vis spectra clearly show that incorporation of iron ions leads to a shift of optical response toward visible part of spectra and reduction of the band-gap energy. Fig. 3 shows the band-gap calculation curves for iron doped anatase samples. The band-gap energies were calculated by applying Kubelka–Munk function on obtained DRS spectra, F(R)1/2 = f(h) functionality was observed. Extrapolation of linear part of acquired curves gave values of band-gap energies for doped samples [17]. Calculated values for the optical band-gap of iron doped anatase samples are shown in Table 2. For all doped samples optical band-gaps are lower in comparison with pure anatase (3.2 eV) and continually decrease with increasing the amount of doped iron. According to our results, Fe3+ doping enhance the adsorption in the visible light zone. Literature data show that there is significant disagreement about the influence of the amount of doped iron to the values of the bandgap energy [18]. It can be concluded that optical properties of iron doped anatase strongly depend on the synthesis conditions.
Table 2 The effects of Fe3+ content on the band-gap values and color properties of iron doped TiO2 samples. Sample
Bandgap (eV)
L
a
b
0.3% Fe 0.5% Fe 1.0% Fe 2.0% Fe 5.0% Fe
3.19 3.18 3.11 3.09 2.98
77.455 84.083 91.821 74.160 81.980
2.689 3.215 2.996 2.980 4.469
14.783 16.981 20.681 17.694 23.841
Ionic radius of Fe3+ ion has similar values to that of Ti4+ ion, hence, iron ion can easy occupy substitutional positions in TiO2 lattice. Modification of optical properties that occurs upon doping by iron ions is usually expressed by existence of two peaks/shoulders in UV–vis spectra: (i) one at about 400 nm, which originates from excitation of 3d electrons of Fe3+ to the TiO2 conduction band (charge transfer transition) [19,20], and (ii) the other cantered at about 500 nm, which is usually ascribed to the d–d transition of Fe3+ (2 T2g → 2 A2g , 2 T1g ) or the charge transfer transition between interacting iron ions (Fe3+ + Fe3+ → Fe4+ + Fe2+ ) [19,21,22] and which is common for higher dopant concentrations [22]. In DRS spectra of our samples there is no such obvious features, but spectra of all doped samples exhibit a red shift, comparing to undoped TiO2 . As
Fig. 3. Band-gap calculation curves for iron doped TiO2 samples. Inset: diffuse reflectance spectra of iron doped TiO2 samples.
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Fig. 4. Chromatic diagram of Ti1− y Fey O2 .
1.1
iron. The increasing of color intensity may support the thesis about the incorporation of iron ions in the lattice of TiO2 . At higher concentrations color saturation decreases (increases L) and color tone (hue) change. Judging by the changes in the absorption maxima, Fe3+ enters a lattice site in Ti1−y Fey O2 , changing the overall level of absorption. The change in relative intensities of the absorption produces a shift in color tone. For six-fold coordination, the iron ion (Fe3+ ) has an ionic radius (0.654 Å) and it is similar with the Ti4+ ion (0.605 Å), finds itself replacing Ti4+ in octahedral [25]. Therefore the ligands along the a-axis are more distant from the central ion in comparison with the others. Consequently, splitting of the d orbitals of the Fe3+ ion and the transition of energy from the higher energy level to lower energy level gives rise to the color. When the central ion, inside an octahedrally coordinated cage is iron, the field is strong enough to allow absorption in the visible spectrum, for wavelengths located in the violet and blue region.
0.3 % Fe 0.5 % Fe 1.0 % Fe 2.0 % Fe 5.0 % Fe
1.0 0.9 0.8
C/C0
0.7 0.6 0.5 0.4 0.3 0.2 0
5
10
15
20
25
Illumination time (h) Fig. 5. Photocatalytic degradation of crystal violet (CV) azo dye.
enters in TiO2 lattice, Fe3+ introduces new energy levels in bandgap of host [23] that are placed below original conduction band [24], consequently band-gap energy of doped samples decreases i.e., optical response of doped materials is shifted toward visible spectral region. UV/vis spectroscopy enables us to calculate the change in color of iron doped TiO2 samples, in comparison wit pure anatase. The colorimetry results of the iron doped TiO2 is presented in Table 2 and illustrated in Fig. 4. The values of L indicate that all of the colors are very pale. Also, all samples present red (a is positive) and yellow (b is positive) shades. Color changing is intensive with the increase of iron content from 0.3 to 5.0 mass% what is obvious from the corresponding L*a*b* coordinates (Table 1). For the prepared sample with 0.3 mass% of iron, a* (2.689), b* (14.783) and L* (77.455) were detected. On further increasing of iron concentration, L* increases until it reaches 1.0 mass% of iron. Above 2.0 mass% of iron, L* decrease and, again, increases with 5 mass% of
3.4. Photocatalytic degradation of CV Photocatalytic degradation of CV over iron doped TiO2 nanopowders was investigated at initial concentration of 0.01 mmol/dm3 . The photocatalytic degradation kinetics of CV is shown in Fig. 5. It is obvious that the degradation kinetics increases with increasing the amount of doped iron ions. Reasons for the increasing of reaction kinetics are multiple. The optical band-gaps decrease with the increasing the amount of doped iron ions. At the same time, the specific surfaces increase. The specific surface of sample with 5.0 mass% of iron is about ten time grater than surface of pure anatase, synthesized by the same method [15]. Consequently, the active surface available for chemical reaction increases. Additionally, the isoelectric point of pure anatase, synthesized by this method is at pH 7.25 ± 0.2 [15]. Presence of Fe3+ ions in titania crystal lattice can cause shifts in isoelectric point due to changes in cation coordination, structural charge, ion exchange capacity etc. In our case, the presence of iron ions in all TiO2 samples shift the isoelectric point at pH 6.5 ± 0.2. The pH of the initial solution was not adjusted and it was in the range 6.7–7.0. On that pH values, the surface of doped anatase is
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slightly negative while the surface of pure anatase is slightly positive. The excess of negative charge on surface of iron-doped anatase will accelerate the degradation of positively charged CV. 4. Conclusion Titanium dioxide nanopowders, doped with different amount of Fe3+ ions (0.3–5.0 mass%), were successfully synthesized by acid-catalyzed sol–gel method in a non-aqueous medium. Characterization by FESEM, EDX confirmed our previous results that Fe3+ ions are incorporated in TiO2 lattice. The morphology of doped samples strongly depends of the amount of doped iron. UV/vis spectroscopy showed that optical band-gap decreases with increasing the amount of doped iron ions. Degradation reaction of crystal violet (CV) dye showed that kinetics of the reaction increase with increasing the amount of doped iron due to decreasing the optical band-gap, increasing the overall specific surface and decreasing the isoelectric point of doped samples in comparison with pure TiO2 obtained by the same method. Acknowledgment This project was financially supported by the Serbian Education and Science Ministry (Project number: 45012). References [1] A. Fujishima, X. Zhang, D.A. Tryk, TiO2 photocatalysis and related surface phenomena, Surf. Sci. Rep. 63 (2008) 515–582. [2] R. Dastjerdi, M. Montazer, A review on the application of inorganic nano-structured materials in the modification of textiles: focus on anti-microbial properties, Colloid Surf. B 79 (2010) 5–18. [3] K. Han, M. Yu, Study of preparation and properties of UV-blocking fabrics of a PET/TiO2 nanocomposite prepared by in situ polycondensation, J. Appl. Polym. Sci. 100 (2006) 1588–1593. [4] D. Li, H. Haneda, S. Hishita, N. Onashi, Visible-light driven N–F–codoped TiO2 photocatalysts. 2. Optical characterization, photocatalysis and potential application to air purification, Chem. Mater. 17 (2005) 2596–2602. [5] L. Cermenati, P. Pichat, C. Guillard, A. Albini, Probing the TiO2 photocatalyst mechanisms in water purification by use of quinoline, photo-fenton generated OH radicals and superoxide dismutase, J. Phys. Chem. B 101 (1997) 2650–2658. [6] S. Boujday, F. Wunsch, P. Portes, J.F. Bocquet, C. Colbeau-Justin, Photocatalytic and electronic properties of TiO2 powders elaborated by sol–gel route and supercritical drying, Sol. Energy Mater. Sol. C 83 (2004) 421–433. [7] C. Lee, R.H. Lin, C.Y. Yang, M.H. Lin, W.Y. Wang, Preparation and characterization of novel photocatalysts with mesoporous titanium-dioxide (TiO2 ) via a sol–gel method, Mater. Chem. Phys. 109 (2008) 275–280.
[8] M. Ni, M.K.H. Leung, D.Y.C. Leung, K. Sumathy, A review and recent developments in photocatalytic water-splitting using TiO2 for hydrogen production, Renew. Sustain. Energy Rev. 11 (2007) 401–425. [9] M. Raileanu, M. Crisan, I. Nitoi, A. Ianculescu, P. Oancea, D. Crisan, A. Todan, TiO2 -based nanomaterials with photocatalytic properties for the advanced degradation of xenobiotic compounds from water. A literature survey, Water Air Soil Pollut. 224 (2013) 1548. [10] C. Sung, K.Z. Fung, I.M. Hung, M.H. Hon, Synthesis of highly ordered and worm-like mesoporous TiO2 assisted by tri-block copolymer, Solid State Ion. 179 (2008) 1300–1304. [11] Q. Dui, Z. Zhang, N. He, P. Li, C. Yuan, Preparation and characterization of mesostructured titanium dioxide and its application as a photocatalyst for the waste water treatment, Mater. Sci. Eng. C 8–9 (1999) 417–423. [12] B. Xin, Z. Ren, P. Wang, J. Liu, L. Jing, H. Fu, Study on the mechanisms of photoinduced carriers separation and recombination for Fe3+ -TiO2 photocatalysts, Appl. Surf. Sci. 253 (2007) 4390–4395. [13] H. Tang, K. Prasad, R. Sanjnes, P.E. Schmid, F. Levy, Electrical and optical properties of TiO2 anatase thin films, J. Appl. Phys. 75 (1994) 2042–2047. [14] T. Lindgren, J.M. Mwabora, E. Avendano, J. Jonsson, A. Hoel, C.G. Granqvist, et al., Photochemical and optical properties of nitrogen doped titanium dioxide films prepared by reactive dc magnetron sputtering, J. Phys. Chem. B 107 (2003) 5709–5716. [15] B. Babic, J. Gulicovski, Z. Dohcevic-Mitrovic, D. Bucevac, M. Prekajski, J. Zagorac, B. Matovic, Synthesis and characterization of Fe3+ doped titanium dioxide nanopowders, Ceram. Int. 38 (2012) 635–640. [16] CIE, Commision Internationale De L’eclairage Proceedings 1931, Cambridge University Press, Cambridge, 1932. [17] A.B. Murphy, Band-gap determination from diffuse reflectance measurements of semiconductor films, and application to photoelectrochemical water-splitting, Sol. Energy Mater. Sol. C 91 (2007) 1326–1337. [18] T.-h. Lee, H. Ryu, W. Lee, Photochemical properties of iron (III)-doped TiO2 , Ceram. Int. 41 (2015) 7582–7589 http://dx.doi.org/10.1016/j.ceramic.2015. 02.082. [19] J. Zhu, F. Chen, J. Zhang, H. Chen, M. Anpo, Fe3+ -TiO2 photocatalysts prepared by combining sol–gel method with hydrothermal treatment and their characterization, J. Photochem. Photobiol. A 180 (2006) 196–204. [20] T. Umebayashi, T. Yamaki, T. Itoh, K. Asai, Analysis of electronic structures of 3d transition metal-doped TiO2 based on band calculation, J. Phys. Chem. Solids 63 (2002) 1909–1920. [21] X. Li, P.-L. Yue, C. Kutal, Synthesis and photocatalytic oxidation properties of iron doped titanium dioxide nanosemiconductor particles, New J. Chem. 27 (2003) 1264–1269. [22] C. Adán, A. Bahamonde, M. Fernández-García, A. Martínez-Arias, Structure and activity of nanosized iron-doped anatase TiO2 catalysts for phenol photocatalytic degradation, Appl. Catal. B—Environ. 72 (2007) 11–17. [23] S. Manu, M. Abdul Khadar, Non-uniform distribution of dopant iron ions in TiO2 nanocrystals probed by X-ray diffraction, Raman scattering, photoluminescence and photocatalysis, J. Mater. Chem. C 3 (2015) 1846–1853. [24] W. Choi, A. Termin, M.R. Hoffmann, The role of metal ion dopants in quantum-sized TiO2 : correlation between photoreactivity and charge carrier recombination dynamics, J. Phys. Chem. 98 (1994) 13669–13679. [25] A. Shannon, Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides, Acta Crystallogr. A 32 (1976) 751–767.