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Palygorskite–TiO2 nanocomposites: Part 1. Synthesis and characterization
Applied Clay Science 83–84 (2013) 191–197
Contents lists available at ScienceDirect
Applied Clay Science journal homepage: www.elsevier.com/locate/clay
Research paper
Palygorskite–TiO2 nanocomposites: Part 1. Synthesis and characterization D. Papoulis a,⁎, S. Komarneni b, D. Panagiotaras c, A. Nikolopoulou a, Huihui Li d, Shu Yin d, Sato Tsugio d, H. Katsuki e a
Department of Geology, University of Patras, 26504 Patras, Greece Department of Crop and Soil Science and Materials Research Laboratory, The Pennsylvania State University, University Park, PA 16802, USA Department of Mechanical Engineering, Technological Educational Institute of Patras, 26334 Patras, Greece d Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan e Saga Ceramics Research Laboratory, 3037-7, Arita-machi, Saga 844-0022, Japan b c
a r t i c l e
i n f o
Article history: Received 1 October 2011 Received in revised form 3 September 2013 Accepted 4 September 2013 Available online 21 September 2013 Keywords: Palygorskite Anatase Titania Nanocomposite Photocatalytic activity
a b s t r a c t In this paper we describe the synthesis and characterization of small-sized TiO2 particles supported on palygorskite (Pal) with Pal to TiO2 mass ratios of 10:90, 20:80, 30:70, 40:60 and 50:50. The above Pal–TiO2 nanocomposites were prepared by deposition of anatase form of TiO2 on the Pal surfaces using a sol-gel method with titanium isopropoxide as a precursor under hydrothermal treatment at 180 °C. Phase composition, particle morphology and physical properties of these samples were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), attenuated total reflection using Fourier transform infrared spectroscopy (ATR-FTIR) and N2 surface area analysis by BET. In order to investigate the absorption properties of the catalysts, UV–vis reflection spectra were measured. Preparation of Pal–TiO2 nanocomposites led to good dispersion of TiO2 on Pal surfaces. By increasing the amount of TiO2, the deposited 3– 10 nm TiO2 particles were found to be aggregated on the surfaces of the Pal particles. However, by decreasing the amount of TiO2, the Pal particles were found to be aggregated. After treating with TiO2, Pal samples largely showed interparticle mesopores of about 5.8 nm. It was observed that the commercial titania P25 showed no absorption in visible light region. In contrast, the prepared Pal–TiO2 samples showed gray color and absorption in visible light region. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Ultra fine TiO2 powders have good catalytic activities because of their large specific surface areas where reactions take place (Zhao et al., 2006). Titanium dioxide (TiO2) has proven to be the most suitable semiconductor for widespread environmental applications such as the degradation of oil spills and the decomposition of many organic pollutants in water and air (Fujishima et al., 2000; Hoffmann et al., 1995; Martínez-Ortiz et al., 2003; Michael et al., 1995). However, TiO2 powders easily agglomerate into larger particles, resulting in poor catalyst performance (Zhao et al., 2006). TiO2 in pillared clay minerals improved the photocatalytic activity (e.g. Chmielarz et al., 2009, 2011; Nikolopoulou et al., 2009; Ooka et al., 2004; Sun et al., 2006). TiO2 pillared clay minerals have many potential uses (e.g. adsorbents, catalysts and catalyst supports) due to their mesoporous structure and
⁎ Corresponding author at: Department of Geology, Section of Earth Materials, University of Patras, GR-265 04 Patras, Greece. Tel.: +30 2610 997842. E-mail address: [email protected] (D. Papoulis). 0169-1317/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.clay.2013.09.003
large specific surface area (Kun et al., 2006; Ménesi et al., 2008; Occelli, 1986; Sterte, 1986; Yamanaka et al., 1987). The use of acidic sol precursors (pH b 1.8) to prepare pillared clay minerals generally yield amorphous or poorly crystalline TiO2 (Liu et al., 2009) and such phases may lead to a reduction in photocatalytic activity when compared to anatase (Kun et al., 2006). Dispersing the TiO2 particles onto clay minerals under mild conditions is a promising method to resolve the agglomeration problem of TiO2. In aqueous dispersions, clay minerals have been used in combination with TiO2 to enhance the removal of organic pollutants by photocatalytic degradation (Kibanova et al., 2009; Mogyorósi et al., 2002). Previous experiments using clay minerals with microfibrous morphology showed an increase in TiO2 photocatalytic activity (Aranda et al., 2008; Karamanis et al., 2011; Papoulis et al., 2010). In this paper we describe the synthesis and characterization of small-sized TiO2 particles supported on palygorskite (Pal) in five different proportions (mass ratios of Pal to TiO2 were 10:90, 20:80, 30:70, 40:60 and 50:50) using a novel method under mild conditions, which neither requires stabilizing agents, nor clay minerals calcinations. The main goal of this study was to prepare Pal–TiO2 nanocomposites that
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Fig. 1. XRD patterns of a) Pal and b) TiO2 treated Pal samples (P: Palygorskite (Pal), A: Anatase).
could increased the photocatalytic efficiency in decomposing NOx and organic pollutants. 2. Materials and methods 2.1. Sample preparation Pal-rich samples from the Ventzia continental basin, in western Macedonia, Greece, were collected. The samples were size fractionated to obtain b2 μm by gravity sedimentation. Separation of the clay minerals fraction was carried out by using centrifugation method. The clay minerals fraction of the most Pal-rich sample was used for the preparation of clay mineral-TiO2 nanocomposites. 2.2. TiO2 sol stock dispersion A large batch of well-dispersed TiO2 sol was prepared by mixing titanium tetraisopropoxide, Ti(OC3H7)4, with hydrochloric acid, three distilled (3D) water and absolute ethanol (Langlet et al., 2003). The above TiO2 stock dispersion was diluted with absolute ethanol to give a 0.05 M concentration of Ti(OC3H7)4.
Fig. 2. ATR-FTIR spectra showing the only characteristic band of Pal affected (at about 974 cm−1) by TiO2 treatment. The shifting of the band in the nanocomposites is increasing with increasing amount of TiO2 (980, 982, 984, 985, 987 cm−1 for 50:50, 60:40,70:30, 80:20, 90:10 respectively).
D. Papoulis et al. / Applied Clay Science 83–84 (2013) 191–197
2.3. Pal-supported TiO2 The Pal-water dispersion (1% w/w) was stirred for 2 h and an aliquot of TiO2 sol was added to the dispersion in order to obtain Pal to TiO2
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mass ratios of 10:90, 20:80, 30:70, 40:60 and 50:50. The slurry was stirred for 24 h and the resulting dispersion was centrifuged for 10 min (3800 rpm) followed by three times centrifuge washing with 3D water. The clay mineral-TiO2 composite was then dispersed in a
Fig. 3. (a) SEM micrographs showing Pal fibers; SEM micrographs showing uniform TiO2 grains of about 10–30 nm deposited on Pal particles in samples: 50:50 (b), 60:40 (c), 70:30 (d, e), 80:20 (f) and 90:10 (g, h).
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1:1 water:ethanol solution, prior to hydrothermal treatment in an autoclave for 5 h (180 °C). The solid product was separated by centrifugation for 15 min (at 3800 rpm), and oven-dried for 3 h at 60 °C.
2.4. Characterization The phase composition of untreated and TiO2 treated clay mineral samples were determined by X-ray diffraction (using a Bruker D8 advance diffractometer, with Ni-filtered Cu Kα radiation). XRD patterns were obtained from oriented or random powder samples in a 2θ range of 2 to 60° at a scanning rate of 2°/min. Random powder mounts for selected samples were prepared by gently pressing the powder into the cavity holder. Oriented clay mineral powder samples were prepared by the dropper method. Morphology and chemical composition were examined using a scanning electron microscope (SEM JEOL 6300) equipped with an energy dispersive spectometer (EDS). The chemical composition of the phases by EDS was determined using natural and synthetic standards and 20 kV accelerating voltage with 10 mA beam current. Microanalyses were performed on epoxy resin-impregnated polished and gold or carbon coated thin sections and sample powders were mounted directly on the sample holder. Clay minerals morphology and chemical composition were also examined with a SEM LEO SUPRA 35VP. Morphology of a few samples was also determined by transmission electron microscopy (TEM; Model 2010, JEOL, Tokyo, Japan). Attenuated total reflection infrared (ATR/FTIR) measurements were made with the ATR Miracle accessory of PIKE technologies (diamond crystal) attached to the EQUINOX 55 FT-IR spectrometer (BRUKER). It is interesting to note that ATR-FTIR spectroscopy is suitable for characterization of materials which are either too thick
or too strongly absorbing to be analyzed by TEM and no sample preparation is required. The nitrogen adsorption–desorption isotherms for each sample degassed at 100 °C for 3 h were obtained at 77 K using Autosorb (Quantachrome corporation). Brunauner–Emmet–Teller (BET) surface areas and pore size distribution were determined from the isotherms. Pore size distribution of each sample was obtained using density functional theory (DFT) method in which a N2 adsorption branch model was selected. The ultraviolet visible diffuse reflectance spectrum was taken by a UV–vis spectrophotometer (Shimadzu, UV2450). Band gap analysis was carried out following standard procedures by plotting (hna)1/2 (hn = excitation energy, a = absorption coefficient) vs. energy (Christoforidis et al., 2013).
3. Results and discussion 3.1. X-Ray diffraction The XRD pattern of untreated Pal sample (Fig. 1a) confirming its purity and the absence of other minerals except from traces of quartz. The basal reflection of Pal is intense and appears at 10.54 Å. The XRD patterns of Pal–TiO2 samples are presented in Fig. 1b. The basal reflections of Pal are relatively smaller while anatase reflections are slightly stronger with increasing amount of TiO2 (Fig. 1b) verifying the accurate synthesis of the Pal–TiO2 nanocomposites in the correct ratios. XRD patterns for TiO2 treated Pal samples exhibited the characteristic reflections of anatase (γ-TiO2) at 25.3°, 37.9°, 47.6°, 54.8° 2θ. From the above observations it is evident that the temperature of 180 °C involved in the synthesis was too low to modify the original structure of Pal as has been reported by previous investigators (Kuang et al., 2004).
Fig. 4. TEM images showing TiO2 grains of about 3–10 nm on Pal in samples: 50:50 (a), 60:40 (b), 70:30 (c), 80:20 (d) and 90:10 (e, f).
D. Papoulis et al. / Applied Clay Science 83–84 (2013) 191–197
3.2. ATR-FTIR spectrum The reflections of the ATR-FTIR spectra indicate that the structure of Pal did not change significantly after the TiO2 treatment (Fig. 2). The intensity of all bands of Pal was weakened after the TiO2 treatment and the weakening was found to be more significant with decreasing amount of Pal (because of the significant decrease in the amount of Pal after the treatment), as it was expected by Beer's law (Petit, 2006). The reflections of the ATR-FTIR spectra indicated that the structure of Pal remained largely unaffected by the TiO2 treatment (Fig. 2). The only characteristic band of Pal that was affected was the band at about 974 cm−1 corresponding to stretching of the Si\O bond (Blanco et al., 1989). The shift of the band at about 974 cm−1 in the Pal–TiO2 nanocomposites increased with increasing amount of TiO2 (980, 982, 984, 985, 987 cm−1 for 50:50, 60:40,70:30, 80:20, 90:10 respectively), indicating a small distortion of the symmetry of the tetrahedral sheets (Shuali et al., 1989) which gradually increased with increasing amount of TiO2. The other characteristic bands including the asymmetric band centered at 1650 cm−1 that corresponds to the presence of two partially resolved peaks at 1655 and 1630 cm−1 (Mendelovici, 1973; Mendelovici and Portillo, 1976) appeared in all studied samples, which could be attributed to water. Thus, ATR-FTIR confirmed, XRD observations, that Pal was not significantly affected by the treatment temperature of 180 °C involved in the synthesis of the Pal–TiO2 nanocomposites.
3.3. Scanning electron microscopy (SEM) SEM micrographs of the Pal sample are presented in Fig. 3. Pal crystals formed matted fibers of planar structures (Fig. 3a), which is a typical morphology of this mineral. Even though the XRD patterns of the sample indicated the presence of traces of quartz, not even one quartz crystal was observed using SEM. That clearly verifies the very low amount of quartz in the samples and definitely does not mean that quartz is absent. Many uniform TiO2 grains most of them about 10–30 nm were deposited on the Pal particles (Fig. 3) after treatment with Ti isopropoxide, as revealed by the SEM. The homogeneous distribution of TiO2 grains is anticipated to improve the properties and potential uses of these Pal–TiO2 nanocomposites. Generally the distribution of TiO2 grains appears to be very good but not in all areas (e.g. Fig. 3d, h). By increasing the amount of TiO2 content, the deposited TiO2 particles were found to be aggregated on the surfaces of the Pal particles (Fig. 3f, g, h) while decreasing the amount of TiO2 led to aggregation of Pal particles (Fig. 3b, c and d).
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Table 1 Average pore diameter, pore volume, specific surface area (SBET), band gap energy and TiO2 content. Sample
Average pore diameter (nm)
Pore volume (mL/g)
SBET (m2/g)
Palygorskite 50:50 60:40 70:30 80:20 90:10
11.0 8.27 8.45 6.95 6.78 6.30
0.810 0.568 0.552 0.422 0.406 0.351
297 274 261 243 240 223
Band gap energy (eV)
TiO2 content (wt.%)
3.05 3.05 3.04 3.04 3.04
0 50 60 70 80 90
3.4. Transmission electron microscopy (TEM) TEM study confirmed SEM observations which showed increased aggregation of TiO2 particles by increasing the amount of TiO2 that was deposited on the surfaces of the Pal particles (especially in the Pal–TiO2 10–90 sample) (Fig. 4e, f). On the contrary, decreasing the amount of TiO2 led to aggregation of Pal particles (Fig. 4a, b) which is definitely not an anticipated observation. TEM study also revealed that the grain size of the dispersed TiO2 particles varied from 3–10 nm and most commonly was in the 3–5 nm range. The TiO2 grain size observed by TEM was significantly lower than that observed by SEM, because the lower resolution of SEM did not allow us to discriminate the nanoparticles that form TiO2 aggregates of about 10–30 nm (Fig. 3). It should be noted that even though TEM observations are in higher magnification than SEM it is not easy to discriminate every anatase crystal, it is therefore possible that some of the larger anatase crystals are observed (about 10 nm) to be aggregates rather than single crystals. 3.5. Determination of specific surface area and porosity The adsorption–desorption isotherms of nitrogen and pore size distributions for prepared Pal–TiO2 samples were obtained and presented in (Fig. 5). Although the isotherm shapes of the samples after treatment were somewhat different from each other, every isotherm seemed to be of the type IV indicating both meso and micropores (IUPAC classification) (Sing and Williams, 2004; Sing et al., 1985). The pore volume and pore size of all the Pal–TiO2 samples were found to be significantly smaller than those of the starting material (Table 1, Fig. 6). After treating with TiO2, Pal samples largely showed mesopores of about 5.8 nm, which could be attributed to interparticle pores. The total pore volume as well as the SSA decreased with increasing amount of TiO2 while generally the same trend was observed with the average pore diameter
Fig. 5. Nitrogen adsorption (ad)–desorption (de) isotherms.
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Fig. 6. Pore size distribution curves of Pal and Pal–TiO2 samples.
(Table 1). That trend is consistent with previous investigations (Papoulis et al., 2010) and therefore expected. 3.6. Ultraviolet visible diffuse reflectance spectrum To investigate the absorption properties of the catalysts, UV–vis reflection spectra were measured (Fig. 7). The increase of the absorption at wavelength close to 380 nm can be assigned to the band gap absorption of anatase TiO2. The corresponding band gap energies of the prepared materials are presented in Table 1. Based on the data in Table 1, it is observed that by changing the Pal content, the band gap energy is not affected. However, all Pal–TiO2 samples showed a gray color and absorption in the visible region of light (Fig. 7). This was not detected for the commercial titania P25. The absorption in the visible light of the Pal–TiO2 samples (extending between 400–550 nm) slightly increased with increasing Pal content in the nanocomposite. However, the increased absorption in the visible light may not justify a better photoactivity due to other influencing factors. Although no conclusive
assignment regarding this last contribution can be made, the UV– visible spectra provide information that the visible light absorption is linked with the presence of Pal. Alternatively, it might be also caused by carbon doping during the preparation. 4. Conclusions Preparation of Pal–TiO2 nanocomposites led to good dispersion of TiO2 on Pal surfaces. The temperature of 180 °C involved in the synthesis is too low to modify the original structure of Pal. When the amount of TiO2 deposited increased, the TiO2 particles (3–10 nm) were found to be aggregated on the surfaces of the Pal particles. On the other hand, when the amount of TiO2 deposited decreased, Pal particles were found to be aggregated. After treating with TiO2, Pal samples largely showed interparticle mesopores of about 5.8 nm. The total pore volume as well as the SSA decreased with increasing amount of TiO2 deposited on Pal while generally the same trend was observed with average pore diameter. Commercial titania, P25 showed no absorption in visible light region. In contrast, the prepared Pal–TiO2 samples showed a gray color and absorption in visible light region. Acknowledgments The authors wish to thank Dr. Drakopoulos of the Foundation for Research and Technology-Hellas (FORTH) and the Institute of Chemical Engineering and High Temperature Chemical Processes (ICE/HT) Rio-Patras, Greece. References
Fig. 7. Ultraviolet visible defuse reflectance spectrum of commercial titania P25 and prepared Pal–TiO2 samples.
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D. Papoulis et al. / Applied Clay Science 83–84 (2013) 191–197 clays and porous clay heterostructures modified with copper and iron as catalysts of the DeNOx process. Appl. Clay Sci. 53, 164–173. Christoforidis, K.C., Iglesias-Juez, A., Figueroa, S.J.A., Di Michiel, M., Newton, M.A., Fernández-García, M., 2013. Structure and activity of iron-doped TiO2-anatase nanomaterials for gas-phase toluene photo-oxidation. Catal. Sci. Technol. 3, 626–634. Fujishima, A., Tata, N.R., Donald, A.T., 2000. Titanium dioxide photocatalysis. J. Photochem. Photobiol. C 1, 1–21. Hoffmann, M.R., Martin, S.T., Choi, W., Bahnemann, D.W., 1995. Environmental application of semiconductor photocatalysis. Chem. Rev. 95, 69–96. Karamanis, D., Ökte, A.N., Vardoulakis, E., Vaimakis, T., 2011. Water vapor adsorption and photocatalytic pollutant degradation with TiO2–Sepiolite nanocomposites. Appl. Clay Sci. 53, 181–187. Kibanova, D., Trejo, M., Destaillats, H., Cervini-Silva, J., 2009. Synthesis of hectorite–TiO2 and kaolinite–TiO2 nanocomposites with photocatalytic activity for the degradation of model air pollutants. Appl. Clay Sci. 42, 563–568. Kuang, W., Facey, A.G., Detellier, C., 2004. Dehydratation and rehydratation of palygorskite and the influence of water on the nanopores. Clays Clay Miner. 52, 635–642. Kun, R., Mogyorósi, K., Dékány, I., 2006. Synthesis and structural and photocatalytic properties of TiO2/montmorillonite nanocomposites. Appl. Clay Sci. 32, 99–110. Langlet, M., Jenouvrier, P., Kim, A., Manso, M., Trejo-Valdez, M., 2003. Functionality of aerosol-gel deposited TiO2 thin films processed at low temperature. J. Sol-Gel Sci. Technol. 26, 759–763. Liu, J., Dong, M., Zuo, S., Yu, Y., 2009. Solvothermal preparation of TiO2/montmorillonite and photocatalytic activity. Appl. Clay Sci. 43, 156–159. Martínez-Ortiz, M.J., Fetter, G., Domínguez, J.M., 2003. Catalytic hydrotreating of heavy vacuum gas oil on Al- and Ti-pillared clays prepared by conventional and microwave irradiation methods. Micropor. Mesopor. Mater. 58, 73–80. Mendelovici, E., 1973. Infrared study of attapulgite and HCl treated attapulgite. Clays Clay Miner. 21, 115–119. Mendelovici, E., Portillo, D., 1976. Organic derivatives of attapulgite: I. infrared spectroscopy and X-ray diffraction studies. Clays Clay Miner. 24, 177–182. Ménesi, J., Korösi, L., Bazsó, É., Zöllmer, V., Richardt, A., Dékány, I., 2008. Photocatalytic oxidation of organic pollutants on titania-clay composites. Chemosphere 70 (3), 538–542. Michael, R.H., Scot, T.M., Wonyong, C., Detlef, W.B., 1995. Environmental applications of semiconductor photocatalysis. Chem. Rev. 95, 69–96.
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Contents lists available at ScienceDirect
Applied Clay Science journal homepage: www.elsevier.com/locate/clay
Research paper
Palygorskite–TiO2 nanocomposites: Part 1. Synthesis and characterization D. Papoulis a,⁎, S. Komarneni b, D. Panagiotaras c, A. Nikolopoulou a, Huihui Li d, Shu Yin d, Sato Tsugio d, H. Katsuki e a
Department of Geology, University of Patras, 26504 Patras, Greece Department of Crop and Soil Science and Materials Research Laboratory, The Pennsylvania State University, University Park, PA 16802, USA Department of Mechanical Engineering, Technological Educational Institute of Patras, 26334 Patras, Greece d Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan e Saga Ceramics Research Laboratory, 3037-7, Arita-machi, Saga 844-0022, Japan b c
a r t i c l e
i n f o
Article history: Received 1 October 2011 Received in revised form 3 September 2013 Accepted 4 September 2013 Available online 21 September 2013 Keywords: Palygorskite Anatase Titania Nanocomposite Photocatalytic activity
a b s t r a c t In this paper we describe the synthesis and characterization of small-sized TiO2 particles supported on palygorskite (Pal) with Pal to TiO2 mass ratios of 10:90, 20:80, 30:70, 40:60 and 50:50. The above Pal–TiO2 nanocomposites were prepared by deposition of anatase form of TiO2 on the Pal surfaces using a sol-gel method with titanium isopropoxide as a precursor under hydrothermal treatment at 180 °C. Phase composition, particle morphology and physical properties of these samples were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), attenuated total reflection using Fourier transform infrared spectroscopy (ATR-FTIR) and N2 surface area analysis by BET. In order to investigate the absorption properties of the catalysts, UV–vis reflection spectra were measured. Preparation of Pal–TiO2 nanocomposites led to good dispersion of TiO2 on Pal surfaces. By increasing the amount of TiO2, the deposited 3– 10 nm TiO2 particles were found to be aggregated on the surfaces of the Pal particles. However, by decreasing the amount of TiO2, the Pal particles were found to be aggregated. After treating with TiO2, Pal samples largely showed interparticle mesopores of about 5.8 nm. It was observed that the commercial titania P25 showed no absorption in visible light region. In contrast, the prepared Pal–TiO2 samples showed gray color and absorption in visible light region. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Ultra fine TiO2 powders have good catalytic activities because of their large specific surface areas where reactions take place (Zhao et al., 2006). Titanium dioxide (TiO2) has proven to be the most suitable semiconductor for widespread environmental applications such as the degradation of oil spills and the decomposition of many organic pollutants in water and air (Fujishima et al., 2000; Hoffmann et al., 1995; Martínez-Ortiz et al., 2003; Michael et al., 1995). However, TiO2 powders easily agglomerate into larger particles, resulting in poor catalyst performance (Zhao et al., 2006). TiO2 in pillared clay minerals improved the photocatalytic activity (e.g. Chmielarz et al., 2009, 2011; Nikolopoulou et al., 2009; Ooka et al., 2004; Sun et al., 2006). TiO2 pillared clay minerals have many potential uses (e.g. adsorbents, catalysts and catalyst supports) due to their mesoporous structure and
⁎ Corresponding author at: Department of Geology, Section of Earth Materials, University of Patras, GR-265 04 Patras, Greece. Tel.: +30 2610 997842. E-mail address: [email protected] (D. Papoulis). 0169-1317/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.clay.2013.09.003
large specific surface area (Kun et al., 2006; Ménesi et al., 2008; Occelli, 1986; Sterte, 1986; Yamanaka et al., 1987). The use of acidic sol precursors (pH b 1.8) to prepare pillared clay minerals generally yield amorphous or poorly crystalline TiO2 (Liu et al., 2009) and such phases may lead to a reduction in photocatalytic activity when compared to anatase (Kun et al., 2006). Dispersing the TiO2 particles onto clay minerals under mild conditions is a promising method to resolve the agglomeration problem of TiO2. In aqueous dispersions, clay minerals have been used in combination with TiO2 to enhance the removal of organic pollutants by photocatalytic degradation (Kibanova et al., 2009; Mogyorósi et al., 2002). Previous experiments using clay minerals with microfibrous morphology showed an increase in TiO2 photocatalytic activity (Aranda et al., 2008; Karamanis et al., 2011; Papoulis et al., 2010). In this paper we describe the synthesis and characterization of small-sized TiO2 particles supported on palygorskite (Pal) in five different proportions (mass ratios of Pal to TiO2 were 10:90, 20:80, 30:70, 40:60 and 50:50) using a novel method under mild conditions, which neither requires stabilizing agents, nor clay minerals calcinations. The main goal of this study was to prepare Pal–TiO2 nanocomposites that
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Fig. 1. XRD patterns of a) Pal and b) TiO2 treated Pal samples (P: Palygorskite (Pal), A: Anatase).
could increased the photocatalytic efficiency in decomposing NOx and organic pollutants. 2. Materials and methods 2.1. Sample preparation Pal-rich samples from the Ventzia continental basin, in western Macedonia, Greece, were collected. The samples were size fractionated to obtain b2 μm by gravity sedimentation. Separation of the clay minerals fraction was carried out by using centrifugation method. The clay minerals fraction of the most Pal-rich sample was used for the preparation of clay mineral-TiO2 nanocomposites. 2.2. TiO2 sol stock dispersion A large batch of well-dispersed TiO2 sol was prepared by mixing titanium tetraisopropoxide, Ti(OC3H7)4, with hydrochloric acid, three distilled (3D) water and absolute ethanol (Langlet et al., 2003). The above TiO2 stock dispersion was diluted with absolute ethanol to give a 0.05 M concentration of Ti(OC3H7)4.
Fig. 2. ATR-FTIR spectra showing the only characteristic band of Pal affected (at about 974 cm−1) by TiO2 treatment. The shifting of the band in the nanocomposites is increasing with increasing amount of TiO2 (980, 982, 984, 985, 987 cm−1 for 50:50, 60:40,70:30, 80:20, 90:10 respectively).
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2.3. Pal-supported TiO2 The Pal-water dispersion (1% w/w) was stirred for 2 h and an aliquot of TiO2 sol was added to the dispersion in order to obtain Pal to TiO2
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mass ratios of 10:90, 20:80, 30:70, 40:60 and 50:50. The slurry was stirred for 24 h and the resulting dispersion was centrifuged for 10 min (3800 rpm) followed by three times centrifuge washing with 3D water. The clay mineral-TiO2 composite was then dispersed in a
Fig. 3. (a) SEM micrographs showing Pal fibers; SEM micrographs showing uniform TiO2 grains of about 10–30 nm deposited on Pal particles in samples: 50:50 (b), 60:40 (c), 70:30 (d, e), 80:20 (f) and 90:10 (g, h).
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1:1 water:ethanol solution, prior to hydrothermal treatment in an autoclave for 5 h (180 °C). The solid product was separated by centrifugation for 15 min (at 3800 rpm), and oven-dried for 3 h at 60 °C.
2.4. Characterization The phase composition of untreated and TiO2 treated clay mineral samples were determined by X-ray diffraction (using a Bruker D8 advance diffractometer, with Ni-filtered Cu Kα radiation). XRD patterns were obtained from oriented or random powder samples in a 2θ range of 2 to 60° at a scanning rate of 2°/min. Random powder mounts for selected samples were prepared by gently pressing the powder into the cavity holder. Oriented clay mineral powder samples were prepared by the dropper method. Morphology and chemical composition were examined using a scanning electron microscope (SEM JEOL 6300) equipped with an energy dispersive spectometer (EDS). The chemical composition of the phases by EDS was determined using natural and synthetic standards and 20 kV accelerating voltage with 10 mA beam current. Microanalyses were performed on epoxy resin-impregnated polished and gold or carbon coated thin sections and sample powders were mounted directly on the sample holder. Clay minerals morphology and chemical composition were also examined with a SEM LEO SUPRA 35VP. Morphology of a few samples was also determined by transmission electron microscopy (TEM; Model 2010, JEOL, Tokyo, Japan). Attenuated total reflection infrared (ATR/FTIR) measurements were made with the ATR Miracle accessory of PIKE technologies (diamond crystal) attached to the EQUINOX 55 FT-IR spectrometer (BRUKER). It is interesting to note that ATR-FTIR spectroscopy is suitable for characterization of materials which are either too thick
or too strongly absorbing to be analyzed by TEM and no sample preparation is required. The nitrogen adsorption–desorption isotherms for each sample degassed at 100 °C for 3 h were obtained at 77 K using Autosorb (Quantachrome corporation). Brunauner–Emmet–Teller (BET) surface areas and pore size distribution were determined from the isotherms. Pore size distribution of each sample was obtained using density functional theory (DFT) method in which a N2 adsorption branch model was selected. The ultraviolet visible diffuse reflectance spectrum was taken by a UV–vis spectrophotometer (Shimadzu, UV2450). Band gap analysis was carried out following standard procedures by plotting (hna)1/2 (hn = excitation energy, a = absorption coefficient) vs. energy (Christoforidis et al., 2013).
3. Results and discussion 3.1. X-Ray diffraction The XRD pattern of untreated Pal sample (Fig. 1a) confirming its purity and the absence of other minerals except from traces of quartz. The basal reflection of Pal is intense and appears at 10.54 Å. The XRD patterns of Pal–TiO2 samples are presented in Fig. 1b. The basal reflections of Pal are relatively smaller while anatase reflections are slightly stronger with increasing amount of TiO2 (Fig. 1b) verifying the accurate synthesis of the Pal–TiO2 nanocomposites in the correct ratios. XRD patterns for TiO2 treated Pal samples exhibited the characteristic reflections of anatase (γ-TiO2) at 25.3°, 37.9°, 47.6°, 54.8° 2θ. From the above observations it is evident that the temperature of 180 °C involved in the synthesis was too low to modify the original structure of Pal as has been reported by previous investigators (Kuang et al., 2004).
Fig. 4. TEM images showing TiO2 grains of about 3–10 nm on Pal in samples: 50:50 (a), 60:40 (b), 70:30 (c), 80:20 (d) and 90:10 (e, f).
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3.2. ATR-FTIR spectrum The reflections of the ATR-FTIR spectra indicate that the structure of Pal did not change significantly after the TiO2 treatment (Fig. 2). The intensity of all bands of Pal was weakened after the TiO2 treatment and the weakening was found to be more significant with decreasing amount of Pal (because of the significant decrease in the amount of Pal after the treatment), as it was expected by Beer's law (Petit, 2006). The reflections of the ATR-FTIR spectra indicated that the structure of Pal remained largely unaffected by the TiO2 treatment (Fig. 2). The only characteristic band of Pal that was affected was the band at about 974 cm−1 corresponding to stretching of the Si\O bond (Blanco et al., 1989). The shift of the band at about 974 cm−1 in the Pal–TiO2 nanocomposites increased with increasing amount of TiO2 (980, 982, 984, 985, 987 cm−1 for 50:50, 60:40,70:30, 80:20, 90:10 respectively), indicating a small distortion of the symmetry of the tetrahedral sheets (Shuali et al., 1989) which gradually increased with increasing amount of TiO2. The other characteristic bands including the asymmetric band centered at 1650 cm−1 that corresponds to the presence of two partially resolved peaks at 1655 and 1630 cm−1 (Mendelovici, 1973; Mendelovici and Portillo, 1976) appeared in all studied samples, which could be attributed to water. Thus, ATR-FTIR confirmed, XRD observations, that Pal was not significantly affected by the treatment temperature of 180 °C involved in the synthesis of the Pal–TiO2 nanocomposites.
3.3. Scanning electron microscopy (SEM) SEM micrographs of the Pal sample are presented in Fig. 3. Pal crystals formed matted fibers of planar structures (Fig. 3a), which is a typical morphology of this mineral. Even though the XRD patterns of the sample indicated the presence of traces of quartz, not even one quartz crystal was observed using SEM. That clearly verifies the very low amount of quartz in the samples and definitely does not mean that quartz is absent. Many uniform TiO2 grains most of them about 10–30 nm were deposited on the Pal particles (Fig. 3) after treatment with Ti isopropoxide, as revealed by the SEM. The homogeneous distribution of TiO2 grains is anticipated to improve the properties and potential uses of these Pal–TiO2 nanocomposites. Generally the distribution of TiO2 grains appears to be very good but not in all areas (e.g. Fig. 3d, h). By increasing the amount of TiO2 content, the deposited TiO2 particles were found to be aggregated on the surfaces of the Pal particles (Fig. 3f, g, h) while decreasing the amount of TiO2 led to aggregation of Pal particles (Fig. 3b, c and d).
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Table 1 Average pore diameter, pore volume, specific surface area (SBET), band gap energy and TiO2 content. Sample
Average pore diameter (nm)
Pore volume (mL/g)
SBET (m2/g)
Palygorskite 50:50 60:40 70:30 80:20 90:10
11.0 8.27 8.45 6.95 6.78 6.30
0.810 0.568 0.552 0.422 0.406 0.351
297 274 261 243 240 223
Band gap energy (eV)
TiO2 content (wt.%)
3.05 3.05 3.04 3.04 3.04
0 50 60 70 80 90
3.4. Transmission electron microscopy (TEM) TEM study confirmed SEM observations which showed increased aggregation of TiO2 particles by increasing the amount of TiO2 that was deposited on the surfaces of the Pal particles (especially in the Pal–TiO2 10–90 sample) (Fig. 4e, f). On the contrary, decreasing the amount of TiO2 led to aggregation of Pal particles (Fig. 4a, b) which is definitely not an anticipated observation. TEM study also revealed that the grain size of the dispersed TiO2 particles varied from 3–10 nm and most commonly was in the 3–5 nm range. The TiO2 grain size observed by TEM was significantly lower than that observed by SEM, because the lower resolution of SEM did not allow us to discriminate the nanoparticles that form TiO2 aggregates of about 10–30 nm (Fig. 3). It should be noted that even though TEM observations are in higher magnification than SEM it is not easy to discriminate every anatase crystal, it is therefore possible that some of the larger anatase crystals are observed (about 10 nm) to be aggregates rather than single crystals. 3.5. Determination of specific surface area and porosity The adsorption–desorption isotherms of nitrogen and pore size distributions for prepared Pal–TiO2 samples were obtained and presented in (Fig. 5). Although the isotherm shapes of the samples after treatment were somewhat different from each other, every isotherm seemed to be of the type IV indicating both meso and micropores (IUPAC classification) (Sing and Williams, 2004; Sing et al., 1985). The pore volume and pore size of all the Pal–TiO2 samples were found to be significantly smaller than those of the starting material (Table 1, Fig. 6). After treating with TiO2, Pal samples largely showed mesopores of about 5.8 nm, which could be attributed to interparticle pores. The total pore volume as well as the SSA decreased with increasing amount of TiO2 while generally the same trend was observed with the average pore diameter
Fig. 5. Nitrogen adsorption (ad)–desorption (de) isotherms.
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Fig. 6. Pore size distribution curves of Pal and Pal–TiO2 samples.
(Table 1). That trend is consistent with previous investigations (Papoulis et al., 2010) and therefore expected. 3.6. Ultraviolet visible diffuse reflectance spectrum To investigate the absorption properties of the catalysts, UV–vis reflection spectra were measured (Fig. 7). The increase of the absorption at wavelength close to 380 nm can be assigned to the band gap absorption of anatase TiO2. The corresponding band gap energies of the prepared materials are presented in Table 1. Based on the data in Table 1, it is observed that by changing the Pal content, the band gap energy is not affected. However, all Pal–TiO2 samples showed a gray color and absorption in the visible region of light (Fig. 7). This was not detected for the commercial titania P25. The absorption in the visible light of the Pal–TiO2 samples (extending between 400–550 nm) slightly increased with increasing Pal content in the nanocomposite. However, the increased absorption in the visible light may not justify a better photoactivity due to other influencing factors. Although no conclusive
assignment regarding this last contribution can be made, the UV– visible spectra provide information that the visible light absorption is linked with the presence of Pal. Alternatively, it might be also caused by carbon doping during the preparation. 4. Conclusions Preparation of Pal–TiO2 nanocomposites led to good dispersion of TiO2 on Pal surfaces. The temperature of 180 °C involved in the synthesis is too low to modify the original structure of Pal. When the amount of TiO2 deposited increased, the TiO2 particles (3–10 nm) were found to be aggregated on the surfaces of the Pal particles. On the other hand, when the amount of TiO2 deposited decreased, Pal particles were found to be aggregated. After treating with TiO2, Pal samples largely showed interparticle mesopores of about 5.8 nm. The total pore volume as well as the SSA decreased with increasing amount of TiO2 deposited on Pal while generally the same trend was observed with average pore diameter. Commercial titania, P25 showed no absorption in visible light region. In contrast, the prepared Pal–TiO2 samples showed a gray color and absorption in visible light region. Acknowledgments The authors wish to thank Dr. Drakopoulos of the Foundation for Research and Technology-Hellas (FORTH) and the Institute of Chemical Engineering and High Temperature Chemical Processes (ICE/HT) Rio-Patras, Greece. References
Fig. 7. Ultraviolet visible defuse reflectance spectrum of commercial titania P25 and prepared Pal–TiO2 samples.
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