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Applied Surface Science 319 (2014) 189–196 Contents lists available at ScienceDirect Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc…
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Applied Surface Science 319 (2014) 189–196 Contents lists available at ScienceDirect Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc Effect of calcination temperature on physical parameters and photocatalytic activity of mesoporous titania spheres using chitosan/poly(vinyl alcohol) hydrogel beads as a template R. Jiang a,b,c , H.-Y. Zhu a,b,∗ , H.-H. Chen c , J. Yao a,b , Y.-Q. Fu b , Z.-Y. Zhang a , Y.-M. Xu c,∗∗ a Department of Environmental Engineering, Taizhou University, Taizhou 318000, PR China Zhejiang Provincial Key Laboratory of Plant Evolutionary Ecology and Conservation, College of Life Science, Taizhou University, Taizhou 318000, PR China c State Laboratory of Silica Materials and Department of Chemistry, Zhejiang University, Hangzhou 310027, PR China b a r t i c l e i n f o Article history: Received 12 May 2014 Received in revised form 30 June 2014 Accepted 30 June 2014 Available online 7 July 2014 Keywords: Mesoporous titania Phenol Photocatalytic activity Chitosan Poly(vinyl alcohol) Calcination temperature a b s t r a c t Mesoporous titania spheres were prepared by modified sol–gel method using chitosan/poly(vinyl alcohol) hydrogel beads as a template. Effects of calcination temperature on physical parameters were investigated by X-ray diffraction (XRD), N2 adsorption–desorption, Fourier transform infrared (FT-IR) spectra, thermogravimetry and differential thermal analyses (TG-DTA), high-resolution transmission electron microscope (HRTEM) and scanning electron microscopy (SEM). The photocatalytic activity of mesoporous titania spheres prepared was also evaluated by photocatalytic degradation of phenol as a model molecule under UV irradiation. With increasing calcination temperature, average crystallite size and pore size increased. In contrast, Brunauer–Emmett–Teller (BET) specific surface areas, porosity and pore volumes steadily decreased. Results of characterization proved that prepared titania spheres with highly organized pores were mesoporous structure. The photocatalytic activity of mesoporous titania spheres calcined at 500 ◦ C was more effective than those calcined at other temperatures, which were attributed to the porous structure, large BET surface area, crystalline, and smaller crystallite size. This work may provide new insights into the preparation of novel mesoporous titania spheres and further practical applications in the treatment of wastewater. © 2014 Elsevier B.V. All rights reserved. 1. Introduction To solve increasingly serious problems of water pollution, various novel materials are being applied in the field of environmental pollution control [1–4]. Among all kinds of materials, titanium oxide (TiO2 ) is an extremely interesting semi-conductor material due to its excellent properties such as high photocatalytic activity, low cost, environmental friendliness and chemical stability [2,5]. It is well known that effectiveness of titania as a photocatalyst is very sensitive to its crystal phase, particle size, surface area and crystallinity [6–8]. Recently, many research attentions have been focused on the fabrication of mesoporous TiO2 with high BET surface area and photocatalytic activity [7–11]. Mesoporous titania materials have been reported for applications in photocatalysis, ∗ Corresponding author at: No. 1139 Avenue, Jiaojiang District, Taizhou, Zhejiang Province 318000, PR China. Tel.: +86 139 8967 2070; fax: +86 576 8866 0338. ∗∗ Corresponding author. Tel.: +86 571 87952410; fax: +86 571 87951895. E-mail addresses: a class= __cf_email__ href= /cdn-cgi/l/email-protection data-cfemail= 0c76647964796d7579694c3d3e3a226f6361 [email protected] /a script data-cfhash='f9e31' type= text/javascript /* ![CDATA[ */!function(t,e,r,n,c,a,p){try{t=document.currentScript||function(){for(t=document.getElementsByTagName('script'),e=t.length;e--;)if(t[e].getAttribute('data-cfhash'))return t[e]}();if(t&&(c=t.previousSibling)){p=t.parentNode;if(a=c.getAttribute('data-cfemail')){for(e='',r='0x'+a.substr(0,2)|0,n=2;a.length-n;n+=2)e+='%'+('0'+('0x'+a.substr(n,2)^r).toString(16)).slice(-2);p.replaceChild(document.createTextNode(decodeURIComponent(e)),c)}p.removeChild(t)}}catch(u){}}()/* ]] */ /script (H.-Y. Zhu), a class= __cf_email__ href= /cdn-cgi/l/email-protection data-cfemail= 99e1ece0f4d9faeaeab7e3f3ecb7fcfdecb7faf7 [email protected] /a script data-cfhash='f9e31' type= text/javascript /* ![CDATA[ */!function(t,e,r,n,c,a,p){try{t=document.currentScript||function(){for(t=document.getElementsByTagName('script'),e=t.length;e--;)if(t[e].getAttribute('data-cfhash'))return t[e]}();if(t&&(c=t.previousSibling)){p=t.parentNode;if(a=c.getAttribute('data-cfemail')){for(e='',r='0x'+a.substr(0,2)|0,n=2;a.length-n;n+=2)e+='%'+('0'+('0x'+a.substr(n,2)^r).toString(16)).slice(-2);p.replaceChild(document.createTextNode(decodeURIComponent(e)),c)}p.removeChild(t)}}catch(u){}}()/* ]] */ /script (Y.-M. Xu). http://dx.doi.org/10.1016/j.apsusc.2014.06.185 0169-4332/© 2014 Elsevier B.V. All rights reserved. adsorption, separation, etc. [9,11–14]. However, practical use of such mesoporous titania materials in nano grade presents some drawbacks, such as agglomeration during utility and difficulty in separating and reclaiming them from treated effluent [15]. To overcome above drawbacks, the development of mesoporous TiO2 microspheres supported in porous frameworks such as structured silica and alumina (Al2 O3 ) became a good alternative to extra beneficial properties due to synergistic effects with the porous matrix [16–19]. Additionally, those mesoporous TiO2 materials obtained by such synthesis method have been defined particle size and suitable mean pore size [19]. However, it is difficult to remove those porous frameworks used in the preparation of mesoporous TiO2 materials. Chitosan (CS), (1,4)2-amino-2-deoxy-d-glucosamine, is a natural basic, hydrophilic, nontoxic and biocompatible biopolymer obtained by the alkaline deacetylation of chitin [20]. Moreover, chitosan has good chelating ability with transition metal ions, which makes it possible for its metal ion complexes to be used as precursors to synthesize nanoparticles [3,21–23]. All the above-mentioned properties also make chitosan a very good candidate to design all kinds of functional materials [21,23]. Previous study has revealed that chitosan 190 R. Jiang et al. / Applied Surface Science 319 (2014) 189–196 exhibited a multifunctional performance with TiO2 in heterogeneous photocatalysis technology by enhancing adsorptionphotocatalysis [23]. As distinctive intermolecular interactions and formation of hydrogen bonds between poly(vinyl alcohol) (PVA) and chitosan, chitosan/PVA composite has good mechanical property, unique three-dimensional structure, favorable film- and particle-forming property and adjustable pore size, which allows considering chitosan/PVA hydrogel beads as ideal adsorbents for removal of metal ions and organic dye from aqueous solutions [24–29]. Therefore, it also becomes possible that chitosan/PVA hydrogel beads will be used as an alternative template to prepare mesoporous materials. However, to the best of our knowledge, study on the effect of calcination temperature on physical parameters and photocatalytic activity of mesoporous titania microspheres using chitosan/PVA hydrogel beads as a template has not yet been studied. In this study, mesoporous titania spheres were prepared using chitosan/PVA hydrogel beads as a template. Effects of calcination temperature on physical parameters of mesoporous titania microspheres were studied by using X-ray diffraction (XRD), N2 adsorption–desorption, Fourier transform infrared (FT-IR) spectra, high-resolution transmission electron microscope (HRTEM) and scanning electron microscopy (SEM). The photocatalytic activity of mesoporous titania spheres was also evaluated by photocatalytic degradation of phenol as a probe pollutant under UV irradiation. This work may provide a new insight into preparation of novel mesoporous titania spheres. 2. Experimental 2.1. Chemicals and materials Titanium dioxide (P25) with purity of at least 99.5% was obtained from Degussa. A commercial anatase TiO2 (a-TiO2 , purity >99.7%) was purchased from Xiamen Micaren Technology Co., Ltd. (Xiamen, China). Chitosan with 91% of deacetylation degree prepared from shrimp shell was purchased from Yuhuan Ocean Biochemical Co., Ltd. (Zhejiang, China). Titanium isporpoxide (reagent grade, 98%) was purchased from Aladdin Industrial Co., Ltd (Shanghai, China). Poly(vinyl alcohol) (98% hydrolyzed, average molecular weight 105,000) purchased from Shanghai Chemical Reagent Co. Ltd. (Shanghai, China) was of analytical reagent grade. Other chemical agents used were all analytical grade and all solutions were prepared with double distilled water. 2.2. Preparation of mesoporous titania spheres A formation scheme of mesoporous titania spheres using chitosan/PVA hydrogel beads as a template is presented in Fig. 1. Firstly, chitosan/PVA hydrogel beads were prepared by an instantaneous gelation method [29]. Two grams of chitosan was dissolved into 50 mL of 5% (v/v) aqueous acetic acid to obtain a chitosan solution. Two grams of PVA was dissolved in 50 mL of double distilled water to form PVA aqueous solution under mechanical stirring at 70 ± 1 ◦ C. Then, PVA solution was mixed homogenously with chitosan solution to form composite gel-forming mixture with vigorous stirring for 3.0 h at 30 ◦ C. The resulting mixture was dropped into sodium hydroxide bath (500 mL, 0.5 M) through a 0.8 mm internal diameter syringe needle, forming chitosan/PVA hydrogel beads. The hydrogel beads were kept overnight in sodium hydroxide solution for complete solidifying, and then washed with double distilled water, ethanol and isopropyl alcohol to obtain swollen chitosan/PVA hydrogel beads, respectively. Swollen chitosan/PVA hydrogel beads were soaked in the 98% titanium isopropoxide solution for 48 h at room temperature under closed conditions. Titanium isporpoxide molecules diffused slowly into swollen chitosan/PVA hydrogel matrix. Then, hydrogel beads with titanium isopropoxide were immersed into excess water for 24 h to cause the hydrolysis and condensation of titanium isopropoxide to take place in the chitosan/PVA polymer matrix. The resultant spheres were washed with double distilled water to remove titanium hydroxide deposited on the surface of these spheres. Finally, the spheres were dried in the oven at 60 ◦ C till constant weight. Uncalinated titania spheres prepared in the above-described method were labeled as UTS. Following the drying process, the samples were calcined under 400, 500, 600, or 700 ◦ C for 4 h in air at a heating rate of 1 ◦ C/min, respectively. During calcination, the polymeric template was burnt off, and the inorganic precursor crystallized, forming inorganic spheres of mesoporous titania spheres [30]. Calcined titania spheres were labeled as CTS-400, CTS-500, CTS-600 and CTS-700 according to different calcination temperature, respectively. 2.3. Characterization X-ray diffraction (XRD) patterns of the samples were recorded on a Rigaku diffractometer using Cu K␣ irradiation (0.1540 nm) in an 2 range of 5–75◦ . Thermogravimetric (TG) curves were performed on Model STA 409 PC Luxx at a heating rate of 10 ◦ C/min. Nitrogen adsorption–desorption isotherms were carried out on a Micromeritics ASAP2020 apparatus at 77 K. Scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM) measurements were performed on a Hitachi S-4800 and JEOL JEM-2010, respectively. FT-IR spectra were measured at room temperature on a FT-IR-8400 spectrometer (Shimadzu, Japan). 2.4. Photocatalytic activity measurements Photocatalytic experiments were carried out in a Pyrex-glass reactor (inner diameter: 29 mm and height: 9.1 mm), thermostated at 25 ◦ C with a water jacket. An aqueous suspension (50 mL) containing 0.43 mM phenol and 1.0 g/L photocatalyst was first stirred in dark for 1 h to attain adsorption–desorption equilibrium and then irradiated with a high pressure mercury lamp (300 W, Shanghai Mengya) with a 320 nm cutoff filter. The distance between the reactor and the lamp was fixed at 10 cm. The light intensity reaching external surface of the reactor was 1.41 mW/cm2 , as measured by an UV-irradiance meter (UV-A, Instruments of Beijing Normal University, China). At given intervals, 2.5 mL of suspension was withdrawn by a microsyringe and filtered through a mephenolrane (0.22 ␮m in pore size). Organic substrates were analyzed by high-performance liquid chromatography (HPLC) on a Dionex P680 (Apollo C18 reverse column, and 50% CH3 OH aqueous solution as an eluent). 3. Results and discussion 3.1. Optical photographs Fig. 2 displays typical optical photographs of chitosan/PVA hydrogel beads, UTS and CTS-500. Transparent chitosan/PVA hydrogel beads and white UTS are spherical with a diameter of ca. 3–4 mm as shown in Fig. 2a and b. Fig. 2c indicates that calcined titania spheres (UTS-500) have a little bit poor sphericity. At the same time, average out diameter of UTS-500 was decreased from 3–4 mm to 1.5–2 mm, which resulted from a shrinking from original size during calcination [19]. The result is in a good agreement with the fact that surface area and pore volume decreased significantly after calcination. R. Jiang et al. / Applied Surface Science 319 (2014) 189–196 191 mesoporous pore chitosan Calcination PVA Self-assembly Instantaneous gelation chitosan/PVA aqueous solution chitosan/PVA hydrogel bead De-templation uncalcined titania sphere calcined titania sphere Fig. 1. A formation scheme of mesoporous titania spheres using chitosan/PVA hydrogel beads as template. Fig. 2. Optical photographs of CS-PVA beads (a), UTS (b) and CTS-500 (c). 3.2. XRD analysis Fig. 3 shows XRD patterns of as-synthesized UTS, CTS-400, CTS500, CTS-600 and CTS-700. Uncalcined titania spheres (UTS) are nearly amorphous (Fig. 3a), which indicated that the as-dried precursor showed noncrystalline nature. When titanium isopropoxide is hydrolyzed at room temperature, the rate of hydrolysis reaction is relatively low and hydrolysis may be not complete. There was a great amount of un-hydrolyzed alkyls in UTS, which resulted in preventing phase transformation of amorphous to anatase [32]. Generally, XRD pattern of chitosan exhibited two broad diffraction peaks with different intensities at 2 = 10◦ and 20◦ [33]. However, two characteristic peaks of chitosan disappeared in XRD pattern of as-synthesized UTS, which revealed that chitosan has some unordered existence in uncalcined titania spheres. In addition, due to its high content of amino and hydroxyl groups of chitosan in UTS, CS has also been combined with titanium in UST. Diffraction intensity (CPS) R R A R R R R R RR RA e d c b a 10 20 30 40 50 60 70 o Two theta ( ) Fig. 3. XRD patterns of as-synthesized and calcined titania microspheres. (a) UTS, (b) CTS-400, (c) CTS-500, (d) CTS-600 and (e) CTS-700. The peaks marked A and R represents the anatase and rutile phase, respectively. The CTS-400 and CTS-500 samples display characteristic peaks of anatase TiO2 (JCPDS card No. 21-1272). With increasing calcination temperature (from 400 to 500 ◦ C), the peak intensity of anatase increases and the width of (1 0 1) plane diffraction peak of anatase (2 = 25.7◦ ) become narrower, which indicated that the crystallization of mesoporous titania was enhanced [31]. Anatase crystallite size of calcined titania spheres was estimated by employing Debye–Scherrer equation. Average crystallize size of UTS-400 is about 5.4 nm, which is much smaller than that of CTS-500 (9.8 nm). Therefore, relative low-temperature is advantageous to maintain small crystallize size of TiO2 . When calcination temperature increases up further to 600 ◦ C, CTS-600 contains both anatase and rutile phase. An additional phase of rutile was observed in CTS-600, indicating that anatase (A) begins to transform into rutile (R) at 600 ◦ C. Almost complete transformation from anatase to rutile took place at 700 ◦ C (CTS-700). According to some research, anatase phase is irreversibly converted to rutile phase in the range of 600–700 ◦ C due to its lower thermal stability [34]. In addition, among three different forms of titania (anatase, rutile and brookite), anatase is the most photoactive form [34]. As a result, the photocatalytic activity of mesoporous titania spheres calcined at 500 ◦ C (CTS-500) could be more effective than those calcined at other temperatures. 3.3. N2 adsorption–desorption isotherms and porous nature Fig. 4 shows nitrogen adsorption–desorption isotherms and Barrett–Joyner–Halenda (BJH) pore-size distribution plots of UTS and calcined titania spheres at different temperatures. Isotherms of calcined titania spheres exhibited the typical type IV adsorption isotherms, indicating the characteristics of mesoporous materials according to the IUPAC classification (Fig. 4a) [7]. With increase of calcined temperature, hysteresis loop shifted to higher relative pressure, indicating the loss of mesoporous structure at higher temperature. Except for as-synthesized samples (UTS), with increasing the calcined temperature from 400 to 700 ◦ C, the surface area and pore volume decline significantly. The as-prepared sample showed a larger surface area of 233.56 m2 /g. While the sample was calcined at 400 and 500 ◦ C for 4 h, it still presents a high specific surface area of 136 and 95 m2 /g (Table 1), respectively. CTS-700 exhibits 192 R. Jiang et al. / Applied Surface Science 319 (2014) 189–196 250 0.010 a UTS CTS-400 CTS-500 CTS-600 CTS-700 b UTS CTS-400 CTS-500 CTS-600 CTS-700 0.008 150 0.006 3 dV/dD (cm /g nm) 3 Quantity Adsorbed (cm /g STP) 200 100 50 0.004 0.002 0.000 0 0.0 0.2 0.4 0.6 0.8 1.0 0 P/P0 20 40 60 80 100 Pore size diameter (nm) Fig. 4. N2 adsorption–desorption isotherms (a) and pore size distribution curves (b) of (a) UTS, CTS-400, CTS-500, CTS-600 and (e) CTS-700. a much lower surface area (13.39 m2 /g). As reported in literature, this is caused by a collapse of pore structure and an increase of particle size [18]. It is clear that corresponding pore volumes of CTS-400 and CTS-500 are greater than UTS, indicating that chitosan/PVA template was removed gradually from those materials. However, when calcination temperature increase further from 500 to 700 ◦ C, pore volume of CTS decreased obviously from 0.354 to 0.119 cm3 /g. On the other hand, the crystalline size and the mean pore size of the samples increased gradually with an increase of calcination temperature. Pore size distribution curves indicate that the sample exhibit maxima at 4.74, 10.12 and 13.58 nm for UTS, CTS300 ◦ C and CTS-500, respectively (Fig. 4b). Pore-size distribution of TS-500 is narrow and centered at 13.58 nm, implying its welldefined mesoporous structure. Therefore, calcination temperature should be well at 500 ◦ C unless the crystal size is controlled by other methods. 3.4. TEM and SEM TEM images of as-synthesized CTS-400, CTS-500, CTS-600 and CTS-700 were illustrated in Fig. 5. Average particle sizes of CTS400, CTS-500, CTS-600 and CTS-700 are approximately 9, 16, 19, 28 nm in size (Fig. 5a, b, d and e), respectively. Corresponding measured sizes from TEM image are in good agreement with the values determined by XRD analysis. Obviously, with increasing calcination temperatures, average crystallite size and average pore size increased, which indicated an enhancement of crystallite growth of mesoporous titania [18]. Further observation suggests that a large number of mesopores come from the aggregation of primary particles. Fig. 5c shows corresponding HRTEM image of CTS-500 sample. It shows clear lattice rings, indicating that CTS-500 was well crystalline. The fringes of 0.35 nm match that of (1 0 1) crystallographic plane of anatase TiO2 . According to Yu’s model [32], nucleation of primary TiO2 particles res
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