Synthesis and photocatalytic activity of sulfate modified Nd-doped TiO2 under visible light irradiation

JOURNAL OF RARE EARTHS, Vol. 33, No. 5, May 2015, P. 491

Synthesis and photocatalytic activity of sulfate modified Nd-doped TiO2 under visible light irradiation SUN Dongfeng (孙东峰)1, WANG Kai (王 凯)2, XU Zhijian (徐志坚)2, LI Ruixing (李锐星)1,* (1. Key Laboratory of Aerospace Materials and Performance (Ministry of Education), School of Materials Science and Engineering, Beihang University, Beijing 100191, China; 2. College of Physical Science and Technology, Dalian University, Dalian 116622, China) Received 1 August 2014; revised 7 April 2015

Abstract: Nd-doped TiO2 (NT) photocatalysts with different contents of Nd were synthesized by sol-gel method. Then sulfated Nd-doped TiO2 (SNT) solid superacid photocatalysts were prepared by an incipient wetness impregnation technique. The photocatalytic activity of catalysts was evaluated by the photodegradation of methylene blue under visible light irradiation. Analytical results demonstrated that Nd doping inhibited the growth of TiO2 crystallite and enhanced the thermal stability of anatase TiO2. Meanwhile, sulfate ions modification increased the specific surface area of samples. In addition, the optical absorption edges of SNT photocatalysts shifted to longer wavelength compared with the undoped TiO2. Such SNT with Nd dosage of 0.25 at.% exhibited the highest photocatalytic activity in the degradation of methylene blue upon irradiation with visible light. Keywords: TiO2; solid superacid; photocatalyst; visible light; methylene blue; rare earths

Photocatalytic oxidation techniques have attracted much attention because of their potential application in water treatment and air purification[1,2]. Among various photocatalysts, TiO2 has been regarded as one of the most promising photocatalysts owing to its strong oxidizing power, nontoxicity, low cost and stablility to light irradiation[3−5]. However, TiO2 has wide band gap energy (3.2 eV), which limits its application in visible light range of solar spectrum. Moreover, the recombination of photogenerated electron-hole pairs leads to low photoquantum efficiency of TiO2[6−8]. It is, therefore, necessary to modify the optical and electronic properties of TiO2 to make it work under visible light. Rare earth ions are known for their f-orbitals to form complexes with various Lewis bases and their oxides having characteristics of adsorption selectivity and thermal stability[9]. In recent years, many researchers have examined the effects of rare earth ions doping on photocatalytic properties of TiO2[10−14]. Results indicate that the visible light photocatalytic activity of TiO2 can be enhanced by doping rare earth ions[15−18]. However, the recombination of photogenerated electron-hole pairs of such photocatalysts is still higher thus leading to lower photoquantum efficiency. Recently, modified solid acid catalysts have gained much attention in isomerization reactions owing to their non-toxicity, high strength of acidity, and high activity at low temperatures[19−22]. This

is mainly because of the thermal stability and acidity gained after sulfation process[23−25]. A few reports on sulfated TiO2 solid superacid photocatalysts demonstrated that the sulfation could affect the crystalline structure and crystallinity, the surface chemistry and the pore structure, etc.[26], thus enhancing the photocatalytic activity of photocatalysts. In the present work, we tried to combine the effects of both rare earth and sulfate ions to improve the visible light photocatalytic activity of TiO2. A series of Nddoped TiO2 nanoparticles with different contents of Nd were prepared by sol-gel method. And then the solid superacid photocatalysts of sulfated Nd-doped TiO2 were prepared by an incipient wetness impregnation technique. Finally, their photocatalytic activities were evaluated by using methylene blue aqueous solution as a model contaminant under visible light irradiation.

1 Experimental 1.1 Starting materials All major chemicals were of AR grade and used without further purification. All solutions were prepared with deionized water. Tetrabutyl titanate and methylene blue were supplied by Tianjin Kemiou Chemical Reagent Co., Ltd., China. Neodymium nitrate was purchased from

Foundation item: Project supported by the National Natural Science Foundation of China (51372006), the Scientific Research Starting Foundation for Returned Overseas Chinese Scholars, Ministry of Education; the Start-up Fund for High-end Returned Overseas Talents (Renshetinghan 2010, 411), Ministry of Human Resources and Social Security, China * Corresponding author: LI Ruixing (E-mail: [email protected]; Tel.: +86-10-82316500) DOI: 10.1016/S1002-0721(14)60446-4

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Guoyao Group Chemical Reagent Co., Ltd., China. Acetic acid and ammonium sulfate were obtained from Tianjin Damao Reagent Factory, China. 1.2 Synthesis of sulfated Nd-doped TiO2 solid superacid photocatalyst Undoped and Nd-doped TiO2 photocatalysts with different contents of neodymium (Nd) were prepared by sol-gel method with the following procedure: Solution A, 10 mL tetrabutyl titanate was dissolved into 30 mL absolute ethanol with stirring for 30 min; solution B, a given amount of neodymium nitrate in the required stoichiometry (molar ratio of Nd/Ti=0, 0.125, 0.25, 0.5 and 1) was fully dissolved in 10 mL acetic acid. Then 10 ml distilled water and 30 mL absolute ethanol were added. Subsequently, solution B was added into solution A dropwise using a separatory funnel under vigorous stirring for 30 min. Afterward, the resulting colloidal suspension was stirred for 1 h and then aged for 2 h at room temperature. The gel was dried at 100 ºC under vacuum and then ground. Then Nd-doped TiO2 with different Nd contents of 0.125 at.%, 0.25 at.%, 0.5 at.% and 1 at.% (simply “NT x at.%” for short) photocatalysts were obtained. Sulfated Nd-doped TiO2 solid superacid photocatalysts were prepared by incipient wetness impregnation technique using ammonium sulfate as a source of sulfate ions. The as-prepared Nd-doped TiO2 photocatalysts were added into ammonium sulfate solution (0.5 mol/L) and stirred for 8 h. Then the sulfated samples were filtrated and dried in an air oven at 100 ºC and subsequently calcined at 300, 400, 500, 600 and 700 ºC for 2 h. Sulfated Nd-doped TiO2 (simply “SNT” for short) solid superacid photocatalysts were then obtained. For comparison, sulfated TiO2 (simply “ST” for short) sample was also prepared using undoped TiO2. 1.3 Catalyst characterization X-ray diffraction (XRD) spectra was recorded with an AD/MAX 2500 diffractometer using Cu Kα radiation. The measurements were carried out at 40 kV tube voltages and 200 mA current. The crystallite size of the samples was calculated using the Debye-Scherrer equation. The specific surface areas (BET) of the powder samples were determined by the amount of nitrogen adsorption at liquid nitrogen temperature (Quantachrome, USA). The morphology of samples was evaluated by field emission scanning electron microscopy (FESEM, JEOL-6340F) with an accelerating voltage of 15 kV. The chemical composition of the sample was determined using an energy dispersive X-ray spectrometer (EDS) attached to the SEM. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images were taken over a JEOL JEM-2100F transmission electron microscope with an accelerating

JOURNAL OF RARE EARTHS, Vol. 33, No. 5, May 2015

voltage of 200 kV. FT-IR spectra of samples were recorded with a Perkin-Elmer (Spectrum One-B) FT-IR spectrometer in the range of 4000−400 cm−1. All IR measurements were carried out at room temperature using KBr technique. UV-vis absorption spectra of the samples were obtained using an UV-vis spectrophotometer (Cary 100) with BaSO4 as reference. 1.4 Evaluation of photocatalytic activity The photocatalytic activity of the catalysts was evaluated by degradation of methylene blue (MB) under visible light irradiation. A 300 W Xe lamp (PLS-SXE300) was used as the light source and the visible wavelength was controlled through a 420 nm cut filter, which was hanged in a dark box and kept at about 15 cm on the top of photoreactor. A general photocatalytic procedure was carried out as follows: 0.1 g of catalyst was suspended in a fresh aqueous MB dye solution (C0=0.01 g/L, 100 mL). The suspension was stirred in the dark for 30 min to ensure establishment of adsorption-desorption equilibrium of MB dye. The samples were collected at regular irradiation intervals, and the concentration changes of MB solution were measured using an UV-vis spectrometer at 665 nm (λ max). The photocatalytic activity of catalyst was evaluated by the degradation rate (D) of the samples. The equation of the degradation ratio is as follows: A − At (1) D= 0 × 100% A0 where D is the degradation ratio, A0 is the initial absorbance of MB, and At is the absorbance of MB after ‘‘t’’ minutes.

2 Results and discussion Fig. 1 shows the XRD patterns of the SNT photocatalysts calcined at 600 ºC with different Nd contents. All the identified peaks can be assigned to the tetragonal

Fig. 1 XRD patterns of SNT photocatalysts with different Nd contents (1) 0; (2) 0.125 at.%; (3) 0.25 at.%; (4) 0.5 at.%; (5) 1 at.%

SUN Dongfeng et al., Synthesis and photocatalytic activity of sulfate modified Nd-doped TiO2 under visible light …

structure of anatase TiO2 (JCPDS No. 21-1272). The major peaks at 2θ values of 25.3°, 37.9°, 48.0°, 53.8°, 54.9° and 62.5° correspond to the (101), (004), (200), (105), (211) and (204) planes, respectively. No other crystalline impurities were detected, demonstrating that the products are composed of a single phase anatase TiO2. Based on the XRD data, the average crystallite sizes of SNT photocatalysts were calculated using the Debye-Scherrer equation and the results are shown in Table 1. It reveals that the crystallite size decreases from 18.17 to 13.78 nm with increasing Nd content, indicating that the Nd doping can inhibit the growth of crystallite size. The XRD patterns of SNT photocatalysts with 0.25 at.% Nd content calcined at different temperatures are shown in Fig. 2. It is found that the diffraction intensity of the samples increases with the rise of calcining temperatures from 300 to 600 ºC, and the diffraction planes become narrow, which indicates that the crystallization of the Table 1 Crystallite size of samples with different Nd contents synthesized using the sol-gel method followed by calcination at 600 °C for 2 h Nd content/at.%

0

0.125

0.25

0.5

1

Crystallite size/nm

18.17

16.12

15.69

15.05

13.78

Fig. 2 XRD patterns of SNT photocatalysts with 0.25 at.% Nd content synthesized using the sol-gel method followed by calcination at different temperatures (1) 300 °C; (2) 400 °C; (3) 500 °C; (4) 600 °C; (5) 700 °C for 2 h

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samples increased with the increasing calcination temperature. In addition, rutile phase TiO2 appears when the calcination temperature is 700 ºC. Nd doping of TiO2 seems to stabilize anatase phase structure on TiO2, allowing a slow phase transition during thermal treatments even at 600 ºC, where typically the presence of rutile TiO2, instead of anatase TiO2, is not found[27]. Several studies have demonstrated that rare earth ions could inhibit the anatase-to-rutile phase transformation, which is ascribed to the formation of Ti−O−rare earth element bonds by the surrounding rare earth ions, resulting in the stabilization of the anatase phase[28,29]. The morphology and microstructure of as-prepared samples were measured by the electron microscopy. Fig. 3 shows the micrographs of undoped TiO2, NT and SNT (0.25 at.%) calcined at 600 ºC for 2 h. It can be seen from Fig. 3(a) that the undoped TiO2 sample has a slight agglomeration between particles and the average size of particle is ca. 50 nm. Whereas the morphology of NT and SNT (Fig. 3(b) and (c)) both present relatively uniform spherical particles with average size of ca. 25 nm. It indicates that the NT and SNT samples contained smaller grains and a correspondingly higher exposed surface area than that undoped TiO2. The specific surface areas of the samples calcined at 600 ºC are shown in Table 2. It is found that the specific surface area increased owing to Nd doping. For SNT photocatalysts with 0.25 at.% Nd content, the specific surface area increased remarkably, possibly owing to the coordination between SO42− and TiO2 in the network, which preferably retarded the aggregation and growth of well dispersed particles. In addition, the increase in the surface area of sulfated samples may be due to the formation of porous surface sulfate compounds between the sulfate species and the supporting oxides[30]. This was confirmed by the following FT-IR spectrum. To understand the structure of the sample further, the undoped TiO2, NT and SNT (0.25 at.%) samples calcined at 600 ºC for 2 h were also characterized by TEM. As shown in Fig. 4(a), the undoped TiO2 sample exhibits irregular shape with an average particle size of ca. 50 nm and the aggregation phenomenon is also observed. The

Fig. 3 SEM images of TiO2 (a), NT (0.25 at.%) (b) and SNT (0.25 at.%) (c) synthesized using the sol-gel method followed by calcination at 600 °C for 2 h

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Fig. 4 TEM and HRTEM (inset) images of TiO2 (a), NT (0.25 at.%) (b) and SNT (0.25 at.%) (c) synthesized using the sol-gel method followed by calcination at 600 °C for 2 h Table 2 Specific surface area of the samples synthesized using the sol-gel method followed by calcination at 600 °C for 2 h Samples

TiO2

NT (0.25 at.%)

SNT (0.25 at.%)

Specific surface area (m2/g)

42.73

53.74

93.38

NT and SNT (0.25 at.%) samples (Fig. 4(b) and (c)) present a small particle size and narrow distribution, which is consistent with the conclusion drawn from the SEM images. The HRTEM images (inset Fig. 4(a) and (c)) show the surface micrographs of prepared photocatalysts, where the set of fringes correspond to the (101) lattice planes of anatase phase TiO2. Thus, it is proved that anatase phase TiO2 has well formed in the undoped and SNT samples, which is in good agreement with the XRD analysis. EDS analysis was carried out to identify the elemental composition of SNT (0.25 at.%), as shown in Fig. 5, which reveals the existence of Ti, O, Nd and S elements. According to the EDS result, the Nd/Ti atomic ratio was estimated to be 0.00232, which is slightly lower than the defined value. In addition, S element might be in the form of SO42– adsorbed on the surface of SNT photocatalysts. FT-IR spectra of NT and SNT (0.25 at.%) photocatalysts calcined at 600 ºC are shown in Fig. 6. The peak corresponding to 1623 cm−1 can be attributed to H−O−H bending vibration of adsorbed water and the broad absorption band in the region of 3300−3500 cm−1, which

Fig. 5 EDS spectrum of the as-synthesized SNT (0.25 at.%) sample

Fig. 6 FT-IR spectra of samples NT (0.25 at.%) (1) and SNT (0.25 at.%) (2) photocatalysts synthesized using the solgel method followed by calcination at 600 °C for 2 h

indicates the presence of hydroxyl groups on the surface of samples[31]. And the absorption band intensity of NT photocatalysts is less than that of SNT photocatalysts, indicating that the SO42− is favorable for the absorption of water and the formation of hydroxyl bonds on the surface of photocatalysts which are essential to enhance its photocatalytic activity. The peaks at 1046 and 1136 cm−1 are corresponding to the S−O stretching vibrations. Meanwhile, the peak corresponding to 1219 cm–1 appears due to symmetric stretching of S=O vibration[32]. It suggests that most sulfates are incorporated into the TiO2 bulk, i.e. S−O−Ti network and covalently bound to TiO2 in chelating or bridged bidentate mode[33]. In this case, the oxygen atoms are in electron deficient state and the surface acidity, especially brönsted site acid, increases significantly. Finally, it can effectively prevent the recombination of photogenerated electron-hole pairs, thus enhancing the photocatalytic efficiencies[34,35]. In addition, surface acidity was determined by the spectrophotometric method on the basis of irreversible adsorption of organic base pyridine[21]. The amounts of pyridine adsorbed by the 0.1 g of as-prepared undoped TiO2, NT and SNT (0.25 at.%) are 142, 148 and 190 μg, respectively. This reveals that SNT has more acidic sites when compared to undoped TiO2 and NT. To further investigate the optical absorption properties of the samples, UV-vis spectra of prepared undoped TiO2 and SNT photocatalysts with different Nd contents were

SUN Dongfeng et al., Synthesis and photocatalytic activity of sulfate modified Nd-doped TiO2 under visible light …

measured in the range of 200–800 nm, as shown in Fig. 7. It is found that the optical absorption edges of SNT photocatalysts shift signally to longer wavelength (red shift) and the absorbance in visible light region strengthened compared with that of undoped TiO2 (Fig. 7(2)– (4)). It is indicated that SNT photocatalysts might have photocatalytic activity under visible light irradiation. In this series of SNT photocatalysts, the 4f states seem to take part in the chemical bonding, hybridizing with the 2p oxygen states and pushing the valence band to the direction of lower binding energy. And finally, it leads to the decrease of band gap of SNT photocatalysts. With increasing of Nd content (Fig. 4(d)), the SNT photocatalysts exhibit absorption peak in the visible region located at 586 nm, which corresponded to transitions of 4g7/2 to 2g7/2[36]. Fig. 8 shows the effect of Nd doping content on the photocatalytic activity of SNT photocatalysts under visible light. All the photocatalysts were calcined at 600 ºC. In order to make sure the important role of catalyst, a blank experiment (self-photosensitized process) was performed under identical conditions. As can be seen from Fig. 8, the blank test confirmed that MB was only

Fig. 7 UV-vis spectra of undoped TiO2 (1) and SNT samples with different Nd content: 0.125 at.% (2); 0.25 at.% (3); 0.5 at.% (4) synthesized using the sol-gel method followed by calcination at 600 °C for 2 h

Fig. 8 Photocatalytic activity for degradation of MB under visible light irradiation without catalysts and in the present of different samples

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slightly degraded in the absence of photocatalyst, indicating that the photolysis can be ignored. The results of adsorption capability in the dark revealed that SNT photocatalysts exhibit larger adsorption affinity for MB compared to the undoped TiO2. In the presence of SNT photocatalysts and in the absence of irradiation, the MB adsorption equilibrium is 5%. Meanwhile, it is found that Nd doping significantly improved the photocatalytic activity of TiO2 under visible light. The photocatalytic activity of SNT photocatalysts is improved along with the increase of doping amount until 0.25 at.%, but an inverse relationship appears over 0.5 at.%. These results demonstrated that Nd doping content is an important factor affecting photocatalytic performance and shows an optimal dosage. The reason might be that the Nd doping improves the absorbance of visible light, which helps to enhance the visible light photocatalytic activity of SNT. Furthermore, Nd doping could increase specific surface area of TiO2, thus the photocatalytic activity of SNT photocatalysts is remarkably enhanced. However, it becomes a recombination center of electrons with holes as the Nd doping content increases further[37]. The photocatalytic activities of undoped TiO2, NT and SNT photocatalysts with 0.25 at.% Nd content calcined at different calcining temperatures are shown in Fig. 9. The degradation of MB was measured after 2 h irradiation under visible light. It can be seen from Fig. 9 that the photocatalytic activity of SNT photocatalysts is much higher than that of NT photocatalysts under the same conditions. This may be ascribed to the following two aspects. On the one hand, SNT photocatalysts have larger specific surface area that provide more available surface active sites for adsorption of the photoenergy and MB molecules. On the other hand, the Brönsted site acid on the surface of SNT photocatalysts can effectively prevent the recombination of photoinduced electron-hole pairs and then enhance the photocatalytic efficiencies. The photocatalytic activity of SNT photocatalysts reaches the maximum 70% at 600 ºC of calcining temperature and

Fig. 9 Photocatalytic activities of undoped TiO2, NT and SNT with 0.25 at.% Nd content calcined at different temperatures

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then decreases. The specific surface area of SNT calcined at 700 ºC is 91.76 m2/g which is very close to the specific surface area of SNT calcined at 600 ºC (93.38 m2/g). In this way, the specific surface area has little effect on the photocatalytic activity. As a result, the possible reason is the phase transition of TiO2 from anatase to rutile at 700 ºC, as shown in Fig. 2. Compared with rutile phase, anatase phase can contain more adsorbed water and hydroxyl groups on the surface of titania TiO2, which is beneficial to improving the photocatalytic activity of SNT photocatalysts. To confirm the stability of the SNT photocatalysts, the circulating runs in the photocatalytic degradation of MB in the presence of SNT photocatalysts under visible light irradiation were checked. As shown in Fig. 10, the photocatalytic activity of SNT photocatalysts does not exhibit any conspicuous loss after four recycles for the photo-degradation of MB. This fact implies that the obtained SNT photocatalysts have high stability and are not photo-corroded during the photocatalytic oxidation of the model pollutant molecules, which is especially important for its practical applications.

Fig. 10 Cycling runs in the photocatalytic degradation of MB under under visible light irradiation of SNT with 0.25 at.% Nd

3 Conclusions SNT solid superacid photocatalysts were prepared via sol-gel method combined with incipient wetness impregnation technique. Results indicated that Nd doping inhibited the growth of TiO2 crystallite and enhanced the thermal stability of anatase TiO2. Sulfated modification increased the specific surface area of SNT solid superacid photocatalysts. In comparison with undoped TiO2, SNT photocatalysts exhibited red shifts of optical absorption edge and strong absorbance in the visible light region. The optimal Nd content and calcining temperature of SNT were 0.25 at.% and 600 ºC, respectively, with the best degradation ratio of 70% under visible light irradiation for 2 h. Acknowledgements: Authors would like to thank Innovation

JOURNAL OF RARE EARTHS, Vol. 33, No. 5, May 2015 Foundation of BUAA for PhD Graduates (YWF-14-YJSY-005) for financial support.

References: [1] Gole J L, Stout J D, Burda C, Lou Y B, Chen X B. Highly efficient formation of visible light tunable TiO2–xNx photocatalysts and their transformation at the nanoscale. J. Phys. Chem. B, 2004, 108: 1230. [2] Baia L, Diamandescu L, Barbu-Tudoran L, Peterd A, Melintea G, Danciud V, Baia M. Efficient dual functionality of highly porous nanocomposites based on TiO2 and noble metal particles. J. Alloys Compd., 2011, 509: 2672. [3] Higarashi M M, Jardim W F. Remediation of pesticide contaminated soil using TiO2 mediated by solar light. Catal. Today, 2002, 76: 201. [4] Ramacharyulu P V R K, Prasad G K, Ganesan K, Singh B. Photocatalytic decontamination of sulfur mustard using titania nanomaterials. J. Mol. Catal. A: Chem., 2012, 353-354: 132. [5] An T C, Zhang W B, Xiao X M, Sheng G Y, Fu J M, Zhu X H. Photoelectrocatalytic degradation of quinoline with a novel three-dimensional electrode-packed bed photocatalytic reactor. J. Photochem. Photobiol. A: Chem., 2004, 161: 233. [6] Wang C, Ao Y H, Wang P F, Hou J, Qian J. Preparation of cerium and nitrogen co-doped titania hollow spheres with enhanced visible light photocatalytic performance. Powder Technol., 2011, 210: 203. [7] Liu J J, Qin W, Zuo S L, Yu Y C, Hao Z P. Solvothermalinduced phase transition and visible photocatalytic activity of nitrogen-doped titania. J. Hazard. Mater., 2009, 163: 273. [8] Rengaraj S, Venkataraj S, Yeon J W, Kim Y H, Li X Z, Pan G K H. Preparation, characterization and application of Nd-TiO2 photocatalyst for the reduction of Cr(VI) under UV light illumination. Appl. Catal., B: Environ., 2007, 77: 157. [9] Liang T, Ding S M, Song W C, Chong Z Y, Zhang C S, Li H T. A review of fractionations of rare earth elements in plan. J. Rare Earths, 2008, 26: 7. [10] Bolare A, Pai M, Athawale A A. Surface modified Nd doped TiO2 nanoparticles as photocatalysts in UV and solar light irradiation. Sol. Energy, 2013, 91: 111. [11] Khataee A R, Hosseini M, Hanifehpour Y, Safarpour M. Hydrothermal synthesis and characterization of Nd-doped ZnSe nanoparticles with enhanced visible light photocatalytic activity. Res. Chem. Intermed., 2014, 40: 495. [12] Zhang P L, Yin S, Sekino T, Lee S W, Sato T. Nb and N co-doped TiO2 for a high-performance deNOx photocatalyst under visible LED light irradiation. Res. Chem. Intermed, 2013, 39: 1509. [13] Nasir M, Bagwasi S, Jiao Y C, Chen F, Tian B Z, Zhang J L. Characterization and activity of the Ce and N co-doped TiO2 prepared through hydrothermal method. Chem. Eng. J., 2014, 236: 388. [14] Lan X, Wang L Z, Zhang B Y, Tian B Z, Zhang J L. Preparation of lanthanum and boron co-doped TiO2 by modified sol-gel method and study their photocatalytic ac-

SUN Dongfeng et al., Synthesis and photocatalytic activity of sulfate modified Nd-doped TiO2 under visible light … tivity. Catal. Today, 2013, 224: 163. [15] Václav S, Snejana B J, Nataliya M. Preparation and photocatalytic activity of rare earth doped TiO2 nanoparticles. Mater. Chem. Phys., 2009, 114: 217. [16] Meng Z, Zhu L, Choi J G, Park C Y, Oh W C. Rare earth oxide-treated fullerene and titania composites with enhanced photocatalytic activity for the degradation of methylene blue. Chin. J. Catal., 2011, 32: 1457. [17] Bagwasi S, Tian B Z, Zhang J L, Nasir M. Synthesis, characterization and application of bismuth and boron co-doped TiO2: a visible light active photocatalyst. Chin. J. Catal., 2013, 217: 108. [18] Nasir M, Xi Z H, Xing M Y, Zhang J L, Chen F, Tian B Z, Bagwasi S. Study of synergistic effect of Ce- and Scodoping on the enhancement of visible-light photocatalytic activity of TiO2. J. Phys. Chem. C, 2013, 117: 9520. [19] Krishnakumar B, Swaminathan M. A recyclable and highly effective sulfated TiO2-P25 for the synthesis of quinoxaline and dipyridophenazine derivatives at room temperature. J. Organomet. Chem., 2010, 695: 2572. [20] Krishnakumar B, Velmurugan R, Jothivel S, Swaminathan M. An efficient protocol for the green synthesis of quinoxaline and dipyridophenazine derivatives at room temperature using sulfated titania. Catal. Commun., 2010, 11: 997. [21] Jothivel S, Velmurugan R, Selvam K, Krishnakumar B, Swaminathan M. Preparation, characterization and photocatalytic activity of acidic sulfated nano titania for the degradation of reactive orange 4 under UV light. Sep. Purif. Technol., 2011, 77: 245. [22] Krishnakumar B, Swaminathan M. A convenient method for the N-formylation of amines at room temperature using TiO2-P25 or sulfated titania. J. Mol. Catal. A, 2010, 334: 98. [23] Hassan M S, Amna T, Yang O, Kimd H C, Khil M S. TiO2 nanofibers doped with rare earth elements and their photocatalytic activity. Ceram. Int., 2012, 38: 5925. [24] Krishnakumar B, Swaminathan M. An expeditious and solvent free synthesis of azine derivatives using sulfated anatase-titania as a novel solid acid catalyst. Catal. Commun., 2011, 16: 50. [25] Krishnakumar B, Velmurugan R, Swaminathan M. TiO2SO42− as a novel solid acid catalyst for highly efficient, solvent free and easy synthesis of chalcones under microwave irradiation. Catal. Commun., 2011, 12: 375. [26] Man K L, Keat T L, Abdul R M. Sulfated tin oxide as solid superacid catalyst for transesterification of waste cooking

497

oil: An optimization study. Appl. Catal., B: Environ., 2009, 93: 134. [27] Parida K M, Sahu N, Biswal N R, Naik B, Pradhan A C. Preparation, characterization, and photocatalytic activity of sulfate-modified titania for degradation of methyl orange under visible light. J. Colloid Interface Sci., 2008, 31: 231. [28] Hu Y, Tsai H L, Huang C L. Effect of brookite phase on the anatase-rutile transition in titania nanoparticles. J. Eur. Ceram. Soc., 2003, 23: 691. [29] Shi H X, Zhang T Y, An T C, Li B, Wang X. Enhancement of photocatalytic activity of nano-scale TiO2 particles codoped by rare earth elements and heteropolyacids. J. Colloid Interface Sci., 2012, 38: 121. [30] Yang X L, Li Z, Yang L M, Zhou W Y, Xu Y H. Preparation and photocatalytic activity of neodymium doping titania loaded to silicon dioxide. Trans. Nonferrous Met. Soc. China, 2011, 21: 335. [31] Park Y M, Lee D W, Kim D K, Lee J S, Lee K Y. The heterogeneous catalyst system for the continuous conversion of free fatty acids in used vegetable oils for the production of biodiesel. Catal. Today, 2008, 131: 238. [32] Li H X, Li G S, Zhu J, Wan Y. Preparation of an active SO42−/TiO2 photocatalyst for phenol degradation under supercritical conditions. J. Mol. Catal. A: Chem., 2005, 226: 93. [33] Yang Y, Zhong H, Tian C X, Jiang Z Q. Single-step preparation, characterization and photocatalytic mechanism of mesoporous Fe-doped sulfated titania. Surf. Sci., 2011, 605: 1281. [34] Yang Y, Tian C X. Effects of calcining temperature on photocatalytic activity of Fe-doped sulfated titania. Photochem. Photobiol., 2012, 88: 816. [35] Mohamed M M, Mater A M. Characterization, adsorption and photocatalytic activity of vanadium-doped TiO2 and sulfated TiO2 (rutile) catalysts: Degradation of methylene blue dye. J. Mol. Catal. A: Chem., 2006, 255: 53. [36] Yuan M Q, Zhang J, Yan S, Luo G X, Xu Q, Wang X, Li C. Effect of Nd2O3 addition on the surface phase of TiO2 and photocatalytic activity studied by UV Raman spectroscopy. J. Alloys Compd., 2011, 509: 6227. [37] de la Cruz Romero D, Torres Torres G, Arévalo J C, Gomez R, Aguilar-Elguezabal A. Synthesis and characterization of TiO2 doping with rare earths by sol-gel method: photocatalytic activity for phenol degradation. J. Sol-Gel Sci. Technol., 2010, 56: 219.