Photocatalytic activity of sulfated Mo-doped TiO2@fumed SiO2 composite: A mesoporous structure for methyl orange degradation

Chemical Engineering Journal 225 (2013) 695–703

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Photocatalytic activity of sulfated Mo-doped TiO2@fumed SiO2 composite: A mesoporous structure for methyl orange degradation Changchao Zhan a,b, Feng Chen a,⇑, Honghu Dai a, Jintao Yang a, Mingqiang Zhong a,⇑ a b

College of Chemical Engineering and Materials, Zhejiang University of Technology, Hangzhou 310014, China College of Chemistry and Environmental Engineering, Jiujiang University, Jiujiang 332005, China

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 We developed a novel sulfated

Mo-doped TiO2@fumed SiO2 photocatalyst.  The catalyst exhibited a mesoporous structure and anatase crystallites.  The catalyst showed good photocatalytic activity for MO degradation.  The catalyst with 0.5% Mo dosage showed the highest photocatalytic activity.  The catalyst could be recycled for 6 times, remaining 90.0% MO degradation ratio.

a r t i c l e

i n f o

Article history: Received 9 January 2013 Received in revised form 18 March 2013 Accepted 24 March 2013 Available online 6 April 2013 Keywords: Mo-doped TiO2 Fumed silica Mesoporous structure Photocatalytic activity Methyl orange

a b s t r a c t In this paper, a novel ultraviolet and visible light responsive photocatalyst, sulfated Mo-doped TiO2@fumed SiO2 composite, was synthesized by a feasible sol–gel method. A mesoporous structure and anatase crystallites were comfirmed by TEM, XRD and BET. The prepared catalysts exhibited good photocatalytic activity under both ultraviolet and visible light irradiation for methyl orange (MO) degradation. The XPS, UV–vis and FT-IR spectra indicated the high active sites on the surface were caused by doping Mo and performing the sulfation. The composite catalyst with 0.5% Mo dosage showed the highest photocatalytic activity. And the composite catalyst can be recycled for 6 times, remaining 90.0% MO degradation ratio under UV irradiation. The high surface area and high active sites on the composite surface are considered as the key factors. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction The improvement and optimisation of TiO2 as a photocatalyst is an important task for technical applications of heterogeneous photocatalysis. Many investigations on the basic principles and enhancements of the photocatalytic activity either in the ultraviolet or visible light (VL) have been carried out [1,2]. It was found ⇑ Corresponding authors. Tel.: +86 571 88320856 (M. Zhong), tel.: +86 571 88320219 (F. Chen). E-mail addresses: [email protected] (F. Chen), [email protected] (M. Zhong). 1385-8947/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cej.2013.03.110

that the activity of TiO2 catalyst could be enhanced by doping some transition metals [3–6], such as W6+, Mo6+, Fe3+ in the past two decades. The energy gap of TiO2 could be efficiently narrowed, which results in the optical response shifting to VL region. Furthermore, Yang et al. [7], Do et al. [8], Papp et al. [9] and Fu et al. [10] have published the photocatalysts based on TiO2/MO3, TiO2/WO3, TiO2/SiO2 and TiO2/ZrO2 systems and their photocatalytic performance. They found a correlation between the enhanced photoreactivity and a higher surface acidity resulted from the addition of metal oxides. The surface acidity was thought to take the form of stronger surface hydroxyl groups, which could accept holes generated by illumination and oxidize adsorbed molecules. Yeung et al.

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[11–13] have also reported various TiO2/SiO2 aerogel displayed better catalytic activity for gas phase photocatalytic oxidation of volatile organic compounds than commercial Degussa P25 TiO2. Recently, many researchers reported that sulfation of TiO2 could efficiently enhance its photocatalytic activity due to the increase in the fraction of anatase, the surface area, and especially the surface acidity [14–19]. In general, the sulfated TiO2 is obtained by calcining TiO2 soaked in H2SO4. High calcination temperature is required to obtain high photocatalytic activity [18]. The strong acid sites on the sulfated TiO2 increase the adsorption strengths and coverages of different organics, which also benefit photocatalytic activity [19]. Fu et al. [20] reported that the photocatalytic activity of sulfated TiO2 for the photodegradation of CHBr3, C6H6 and C2H4 in air was six times higher than that of TiO2. Colón et al. [21,22] stated that anatase stabilization was enhanced by sulfate ions up to 700 °C and the creation of bulk oxygen vacancies through a dehydroxylation process of the excess of adsorbed protons seemed to be responsible for the generation of a highly defective material. The function of sulfate during calcination provides the stabilizations in both of surface and the structure, leading to the best situation for this reaction even though the amount of sulfate is almost negligible. Mesoporous TiO2 has high surface area, high surface-to-volume ratio, and more active sites, which are important to photocatalytic reaction [23–29]. However, TiO2 mesostructures have such poor thermal stability that the thermal treatment of mesoporous TiO2 often results in the collapse of mesostructures. In order to protect the mesostructures, the heat treatment temperature was usually below 400 °C [26–29]. But this temperature cannot offer high crystallinity. A probable solution is to regenerate TiO2 on the mesoporous frameworks, such as mesoporous SiO2, targeting the thermal stability and large surface area [30]. In order to enhance the photocatalysistic properties of TiO2, the catalyst system needs the anatase phase with thermal stability, high crystallinity, large surface area, and high active sites on the surface, these features can be realized by doping active substances or performing the acidification. Herein, we in situ synthesized TiO2 on the fumed SiO2 framework to fabricate a mesoporous structure. Non-ionic surfactant Platonic P123 was used as a template agent. To increasing the surface activity, the transition metal Mo was doped and sulfation of TiO2 was performed. In this paper, we investigated the effects of various synthesis conditions on the photocatalytic activity for the degradation of methyl orange (MO) in water.

2. Experimental 2.1. Preparation of photocatalyst composite The sulfated Mo-doped TiO2@fumed SiO2 mesoporous photocatalyst was in situ synthesized on the fumed SiO2 via sol–gel method. For instance, 5.0 mL acetic acid and 0.2 g P123 (Sigma Aldrich, Mn  5800) were first dissolved into 15 mL anhydrate ethanol. Then tetrabutyl titanate (98%), fumed silica (Cabot, A-300, 99.8%), deionized water, ammonium molybdate (98%) and ammonium persulfate (1.0 mol/L) were successively added at room temperature (molar ratio was 1:1:20:0.005:0.1). The mixture was stirred for two hours and a light yellow sol was obtained, aging for 24 h at room temperature and followed by drying at 70 °C for 24 h. The obtained powder was ground and calcined at various temperatures. A series of sulfated composite photocatalyst with different Mo components were prepared. For comparison, the non-sulfated Mo-doped TiO2@fumed SiO2 mesoporous photocatalyst was prepared without ammonium persulfate. All chemicals in this study were used as received without any further purification.

2.2. Characterization Transmission electron micrographs (TEM) were obtained using Philips Tecnai G2 F30 S-Twin. The powder X-ray diffraction (XRD) pattern was collected on a Thermo ARL SCLNTAG X’TRA X-ray diffractometer using Cu Ka radiation (k = 0.154178 nm) operated at 40 kV and 40 mA at a scan rate of 4.08/min. The average crystallite size was calculated from Scherrer equation (d = 0.89k/b1/2 cos h), where k is the characteristic X-ray wavelength applied, b1/2 is the half width of the peak at the 2h value. The surface area of the samples was measured by N2 adsorption at 77 K using the dynamic BET method using a Zeton Altamira (Model AMI-200) sorptometer. The samples were purged in a He atmosphere at 473 K for an hour prior to adsorption. X-ray photoelectron spectroscopy (XPS) data were obtained with an ESCALab220i-XL electron spectrometer from VG Scientific using 300 W Al K radiation. The base pressure was about 3  109 mbar. The binding energies were referenced to the C 1s line at 284.6 eV from adventitious carbon. The fitting results of XPS curves were analyzed with a curve fitting software. UV–vis absorption spectra were obtained using UV–vis spectrophotometer (UV 2550, shimadzu, Japan). Pure BaSO4 was used as a reflectance standard in the Uv–vis absorbance experiment. FT-IR spectra on pellets of the samples mixed with KBr were obtained utilizing a Nicolet-6700 Fourier transform infrared spectrophotometer at a resolution of 0.01 cm1. The photoluminescence (PL) spectra were collected on a Hitachi F-4500 Fluorescence spectrometer. The samples excitation was done at 300 nm at room temperature, and the emission spectra emission spectra were scanned between 320 and 650 nm wavelength ranges. The thermal stability of the samples was performed under the oxygen atmosphere by thermal gravimetric analysis (TGA) (DSC1+TGA/DSC1, Mettler Toledo, Switzerland). 2.3. Photocatalytic activity The photocatalytic activities of the samples were evaluated by the degradation of MO in an aqueous solution. 250 mL MO solution(20 mg/L) and 0.5 g photocatalyst were put into a 1 L beaker which was surrounded by circulated water kept at 25 °C. A 400 W halogen lamp and a 500 W mercury bulb (Institute of Electrical Light Source, Beijing, China) were used as the VL and UV irradiation source respectively and were positioned over MO solution at the height of 20 cm. A 400 nm filter was used to cut off UV light below 400 nm when detected VL photocatalytic activity. Before irradiation, the solution was continuously stirred for 45 min to ensure the establishment of an adsorption–desorption equilibrium between the photocatalyst and MO. After the suspension was irradiated for certain time, 5 mL suspension was taken out and centrifugalized, then the MO concentration of clean solution was measured by a 752 spectrometer (Shanghai third analysis instrument factory, China). The degradation of reactant could be calculated by g (%) = ((C0  C)/C0)  100%, where C0 was the initial concentration of MO and C was the concentration of MO after ‘‘t’’ hours or minutes irradiation. The photocatalytic activity lifetime of Mo-doped TiO2@fumed SiO2 mesoporous photocatalyst was carried out as follows. After the mixture of 250 mL MO solution (20 mg/L) and 0.5 g Mo-doped TiO2@fumed SiO2 mesoporous photocatalyst sample was irradiated for 2 h under the same conditions above, the Mo-doped TiO2@fumed SiO2 mesoporous photocatalyst sample was carefully collected by centrifugalization and then was dried in an oven at 60 °C for 6 h. The next cycle test was done using the collected Mo-doped TiO2@fumed SiO2 and 250 mL MO solution (20 mg/L).

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Fig. 1. TEM images of (a) fumed SiO2 (scale bar is 100 nm), (b) 0.1% Mo doped TiO2@SiO2 (scale bar is 50 nm), (c) 0.5% Mo doped TiO2@SiO2 (scale bar is 50 nm) and (d) higher magnification TEM images of 0.5% Mo doped TiO2@SiO2 (scale bar is 5 nm) with its electron diffraction pattern characteristic of anatase phase (inset), respectively.

In our test, the loss of Mo-doped TiO2@fumed SiO2 had been measured by gravimetry and was tiny. 3. Results and discussion 3.1. The morphology and the mesoporous structure Typical morphology of fumed SiO2 is shown in the TEM image of Fig. 1a. It is found that the size of primary particles is in the range 10–25 nm and the shape is irregular. The fumed SiO2 particle has high surface area (SBET = 297 m2/g) and shows mesoporous structure in it. Fig. 1b–d shows TEM images of the typical sulfated Mo-doped TiO2@fumed SiO2 particles after calcinations at 600 °C. These oxide composites also show the similar size and shape to those of fumed SiO2 particles. The mesoporous structure of higher Mo dosage composite (Fig. 1c) is similar to that of lower Mo content catalyst (Fig. 1b). Both the nanostructure of TiO2 and the framwork of fumed SiO2 are clearly observable in the higher magnification image of Fig. 1d. The nanostructured TiO2 was composed of nanocrystals at the edge of fumed SiO2. The electron diffraction of a selected area (see the inset of Fig. 1d) shows clear Debye–Scherrer ring corresponding to the reflection of a TiO2 anatase phase. This also indicates a highly crystalline nature of the pore wall, which is in accordance with the WXRD results. Fig. 2a shows WAXD patterns of synthesized composite powders calcined at 600 °C with different Mo dosage. The XRD peaks at 2h = 25.28° (1 0 1) and 2h = 27.4° (1 1 0) are often taken as the characteristic peaks of anatase and rutile crystal phase, respectively. The examination of the diffractograms of samples indicates that there is a decrease of the intensities of all anatase and rutile

peaks when the Mo dosage is increasing. Fig. 2b shows WAXD patterns of 0.5 mol% Mo doped samples with different calcination temperature. With increasing calcination temperature, the peak intensity of anatase increases, and the width of the (1 0 1) peak becomes narrower due to the growth of anatase crystallites. The rutile phase begins to appear when the sample is calcined at 500 °C. The TiO2 component consists of major anatase phase and minor rutile phase. The average crystal size, calculated with the Scherrer equation from the (1 0 1) plane of anatase, is in a range of 9.6– 27.7 nm (summarized in Table 1). It can also be observed that the crystalline size was increased slowly with the increasing of temperature. It can be concluded that the addition of Mo inhibits the growth of TiO2 crystalline size and suppress the transformation from anatase to rutile. It is noted that there is no detected MoO3 phase. This might be ascribed to the incorporation of Mo6+ ion into the TiO2 lattice. The ionic radius of Mo6+ is 0.062 nm, and that of Ti4+ is 0.0605 nm. Because of the similarity in their ionic sizes, Mo can easily be incorporated into the TiO2 lattice. Isostructural substitution can be confirmed by the examination of the lattice size of doped structures. The (10 1) and (20 0) peaks of the anatase crystal planes were selected to determine the lattice parameters of the doped photocatalysts (Table 2). The interlayer spacing, d, along the [10 1] direction of the anatase crystal and the unit cell parameters along c-axis increased with the amount of Mo, which indicates crystal lattice expansion due to the incorporation of Mo dopants. The mesoporous structure is confirmed by SAXD patterns (inset in Fig. 3a). The single SAXD diffraction peak in mesoporous samples indicates that the pore structure is worm-like. The isotherms of fumed SiO2 and sulfated Mo-doped TiO2@SiO2 (shown in

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to the capillary condensation associated with a larger pore channel. Based on the observation of adsorption isotherm, it can be concluded that P123 is effective in increasing the pore volume of the composite, which is in good agreement with the investigation that the block polymer P123 as an effective template could be used to direct the formation of mesoporous TiO2 with larger surface and pore volume [32,33]. The pore size distribution, calculated from the desorption branch of the nitrogen isotherm by BJH (Barrett–Joyner–Halenda) method, shows a wide range of 1.7–55.0 nm. These mesopores are from the aggregation of primary particles and the small size pore (<10 nm) are derived from the templating of P123. The results of BET surface area, pore volume and average pore size are summarized in Table 1. It is shown that TiO2@SiO2 samples have mesoporous structures with an average pore size of 18–21 nm and high pore volume (1.0 cm3/g). These mesoporous structures are expected to enhance the rate of photocatalytic reaction, due to the rapid diffusion of various pollutant molecules within the mesopores during the photocatalytic reaction. It can be seen from Table 1 that the 0.1% Mo-doped TiO2@SiO2 composite shows a larger SBET value of 237.0 m2/g. However, higher Mo doped TiO2@SiO2 composite shows smaller surface areas and larger average pore sizes. With a combination of these results and the invariable pore volumes, it is suggested that the number of mesopores decreased. The formation mechanisms of the mesoporous TiO2@SiO2 composite can be explained according to the process as schematic Fig. 4. A small amount of P123 was added to TiO2 colloid particles suspension solution. The TiO2 colloid particles deposited on the surface of the fumed SiO2 framework, resulting in the formation of worm-like pores by the templating of P123. This phenomenon is consistent with the illustration of TEM image (Fig. 1d). Fig. 2. WAXD patterns of (a) samples with different Mo dosage (calcined at 600 °C) and (b) samples with different calcination temperature (Mo/Ti = 0.5 mol%), respectively.

3.2. Surface element analysis

Fig. 3a) are type IV N2 isotherms with hysteresis loops, which clearly indicate the mesoporous nature [31]. The considerable hysteresis loop especially at a high relative pressure region is related

To identify the alteration of surface properties after the Mo doping and sulfation, XPS measurements were carried out to calibrate the binding energy (BE) of Ti 2p, O 1s, Mo 3d and S 2p. It is noted the surface element contents of Mo and S are very low (shown in

Table 1 XRD, BET surface areas and XPS data of samples’ powders. Samples

Fumed SiO2 0.1% Mo–TiO2@SiO2 0.5% Mo–TiO2@SiO2 1.0% Mo–TiO2@SiO2 a b c d e

TiO2 average crystalline size

a

SBETb (m2/g)

(nm)

– 27.7 22.7 19.2

297.0 237.0 194.0 188.7

Vpc (cm3/g)

0.61 1.0 0.99 0.97

Dd (nm)

9.5 18.3 19.5 21.0

XPS surface element content

e

Mo

S

– 0.02 0.06 0.10

– 0.57 0.55 0.48

(atom%)

Average crystal size, calculated from the (1 0 1) plane of anatase with the Scherrer equation(K = 0.89, k = 0.154178 nm). BET surface area calculated from the linear part of the BET plot (P/P0 = 0.1  0.25). Total pore volume, taken from the volume of N2 adsorbed at P/P0 = 0.990. Average pore diameter estimated using the adsorption branch of the isotherm by the BJH method. The content data of Mo and S element originated from XPS surface element analysis.

Table 2 Structural parameters for the mesoporous sulfated Mo–TiO2 doped materials. Samples

Crystal plane

2h (°)

dhkl

Unit cell volume (Å)3

Lattice parameters (Å)

Neat SiO2

101 200 101 200 101 200 101 200

25.436 48.016 25.277 48.049 25.269 48.064 25.270 48.035

3.512 1.887 3.522 1.891 3.526 1.896 3.519 1.893

132.929

a = b = 3.788, c = 9.264

136.65

a = b = 3.791, c = 9.508

136.79

a = b = 3.792, c = 9.513

136.46

a = b = 3.789,c = 9.505

0.1% Mo–TiO2@SiO2 0.5% Mo–TiO2@SiO2 1.0% Mo–TiO2@SiO2

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Fig. 5. XPS spectra of (a) Ti 2p and (b) O 1s of non-sulfated or sulfated 0.5% Modoped TiO2@SiO2.

Fig. 3. (a) Nitrogen adsorption–desorption isotherms of the SiO2 and various Modoped TiO2@SiO2 samples (Mo/Ti = 0.1%, 0.5% and 1.0%) and (b) the corresponding pore size distribution curves calculated from the desorption branch of the nitrogen isotherm by the BJH method, respectively.

Table 1). The actual Mo content is below its dosage. And the sulfate residual is also rarely detected. As shown in Fig. 5a, the Ti 2p3/2 peak of sulfated Mo-doped TiO2@SiO2 can be broken into two parts, one at 458.4 eV corresponding to Ti4+ species and another one at 459.4 eV resulting from the TiO2 combining with the surface sulfate species [20,34]. Additionally, it is worth mentioned that sulfate species can be detected in the FT-IR spectra (Fig. 9) of sulfated Mo-doped TiO2@SiO2 sample. XPS spectra of the O 1s region are given in Fig. 5b. It is observed that the O 1s region of neat TiO2 can be resolved into lattice oxygen (Ti–O–Ti, Ti–O–Si) and surface hydroxyl groups (Ti–OH), while that

of sulfated sample contains more sulfates (SO2 4 ). As the symmetrical peak centered at 531.6 eV is the characteristic XPS peak of the O 1s in surface sulfate species. And it is suggests the existence of lattice oxygen (Ti–O–Ti, Ti–O–Si), oxygen in surface sulfate species (SO2 4 ), and surface hydroxyl groups (Ti–OH) [35]. Mo 3d spectrum (Fig. 6a) has four peaks at 232.2 eV, 235.3 eV, 231.2 eV and 234.3 eV, which can be attributed to Mo in Mo6+ 3d5/2, Mo6+ 3d3/2, Mo5+ 3d5/2 and Mo5+ 3d3/2 peak respectively. The presence of Mo5+ in the photocatalysts may be due to the reduction of Mo6+ by orangic compound generated during the calcination of the catalysts. And S 2p spectrum (Fig. 6b) of sulfated sample has two peaks at 168.5 and 169.7 eV, which can be attributed to S 2p3/2 and 2p1/2 peak, respectively [36,37]. 3.3. TG and DTG analysis Fig. 7 shows the TG and DTG curves of sulfated 0.5% Mo-doped TiO2@SiO2 sample obtained before the calcination. The TG curve in Fig. 7 can be divided into three stages. A weight loss (10%) is in the temperature range from room temperature to 100 °C. And the DTG

Fig. 4. Schematic model of the formation mechanisms of mesoporous structure with P123 as a template.

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Fig. 8. Uv–vis spectra of sulfated Mo-doped TiO2@SiO2 with different Mo dosage: (a) neat TiO2; (b) 0.1%; (c) 0.25%; (d) 0.5% and (e) 1.0%.

3.4. UV–vis and FTIR spectral analysis

Fig. 6. XPS spectra of (a) Mo 3d and (b) S 2p of sulfated 0.5% Mo-doped TiO2@SiO2.

Fig. 7. TG and DTG curve of sulfated 0.5% Mo-doped TiO2@SiO2 crude product.

curve shows a peak at 67.9 °C, which can be attributed to the evaporation of the physically absorbed water and ethanol solvent. The second stage is from 100.0 to 450.0 °C and a 15% weight loss is observed. The DTG peak at 233.8 °C comes from the decomposition of organic substances contained in sulfated Mo-doped TiO2@SiO2 xerogel, which includes thermal decomposition of small-molecule organic chemicals in the gels and P123 template [35,38]. In the third stage from 450 to 800 °C, the mass loss is around 15%. This is assigned to the evaporation of chemisorbed water and bulk sulfate. At 520.7 °C, a sharp peak of DTG curve is observed due to the loss of sulfate.

UV–vis absorption spectra of sulfated Mo-doped TiO2@SiO2 with different Mo dosage are presented in Fig. 8. It can be seen that the composites show strong light absorption property in the UV region and further photocatalysis of semiconductors is the direct absorption of a photon by band gap of the material and generates electron–hole pairs in the semiconductor particles. In all the sulfated Mo-doped TiO2@SiO2 samples, a broad band approximately centered at 600 nm can be ascribed to a Mo5+–Mo6+charge transfer or a Mo5+ d–d transition [39]. The extended absorbance of the composites in the VL region provides a possibility for enhancing the photocatalytic performance of TiO2. Fig. 9 shows the FT-IR spectra of neat TiO2 (P25 sample) and sulfated Mo-doped TiO2@SiO2 with different Mo dosage. The peaks at 3426.9 and 1646.9 cm1 are the bending peaks of hydroxyl group and carbondioxide. Meanwhile, the absorption peak at 468 cm1 can be assigned to the Ti–O–Ti vibration, the absorption peak at 925 and 2364.3 cm1 are associated with the formation of Si–O– Ti bridges [40] and Mo–O vibration [41], and the absorption peak at 1122 cm1 can be assigned to the SO2 4 vibration, which reveals that the structure of sulfate species in this sample is of inorganic chelating bidentate containing strongly covalent S@O bonds. Such a structure has a strong electron-withdrawing effect on the neighboring Ti species, which is believed to be a strong driving force to modify the surface properties of TiO2 [34,42,43]. No other adsorption peaks from organic components are detected. Therefore, the results above indicate that the template P123 has been completely removed from samples and the anatase framework has been formed after calcination at 600 °C.

3.5. Photoluminescence spectra and electron/hole recombination The PL technique is also useful to reveal the migration, transfer and recombination processes of photogenerated electron hole pairs in semiconductors. Therefore, the sulfated TiO2@SiO2 and sulfated xMo–TiO2@SiO2 (x = 0.0, 0.1, 0.25, 0.5 and 1.0 mol%) composites are characterized by PL, and the result is shown in Fig. 10. As can be seen from this figure, sulfated TiO2@SiO2 shows a strong PL emission at 380 and 460 nm, indicating that the electron holes recombine rapidly. However, over sulfated xMo–TiO2@SiO2 (x = 0.0, 0.1, 0.25, 0.5 and 1.0 mol%) samples, the peak intensity is greatly decreased. The sulfated 0.5%Mo–TiO2@SiO2 shows the lowest peak intensity. It suggests that the doping of Mo suppresses the

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Fig. 9. FT-IR spectra of (a) neat TiO2 P25 and Mo-doped TiO2 /SiO2 with different Mo dosage: (b) 0.1% and (c) 0.5%.

Fig. 10. Photoluminescence spectra of sulfated xMo–TiO2@SiO2 (x = 0.0, 0.1, 0.25, 0.5 and 1.0 mol%) photocatalysts with excitation wavelength of 300 nm.

electron hole pair recombination, which is in good agreement with the photocatalytic activity. 3.6. Photocatalytic activity and endurance The photoactivity of the catalysts was evaluated by heterogeneous degradation of methyl orange (MO) wastewater under UV irradiation. As shown in Fig. 11, the degradation rate of MO increases with the increment in Mo dosage. It is because the fact that doped Mo6+ ion can shift the band gap of titania by changing its electronic properties through the formation of shallow trap within the TiO2 matrix. In contrast, for any photocatalyst reaction, the lifetimes of electrons and holes can be long enough to allow them to reach the surface of photocatalyst. Many investigations showed that the photocatalytic activity of Mo-doped TiO2 was strongly dependent on the dopant content [6]. The small amount of cation doping could act as electron–hole separation centers, and inhibit the hole–electron recombination, leading to the increase of photo-

Fig. 11. Curves of MO degradation under UV irradiation.

catalytic activity. The large amount of cation doping resulted in an increase in the recombination rate of photo-generated electrons and holes, leading to the decrease of photocatalytic activity. In our case, the small amount of doping Mo6+ ion can alter the intrinsic properties of titania and act as electron or hole traps that increase the photoinduced electron/hole charge recombination lifetimes. For comparison, the non-sulfated sample exhibits poor photocatalytic activity, which is similar to other reports [19,20]. The sulfation of TiO2 does increase the bulk oxygen vacancies on the TiO2 surface [21,22], and as a result the photocatalytic activity is greatly improved. Fig. 12 shows the MO degradation results photocatalyzed by sulfated various Mo-doped TiO2@SiO2 mesoporous composites, Degussa P25 and blank test under VL irradiation. It is obviously observed that the sample with 0.5% Mo dosage shows better effective VL photocatalytic activity. When the suspension system was irradiated for 40 h, the degradation percentage of MO reached 80.2%, compared to 68.3% and 9.2% by the sample with 0.1% Mo dosage and Degussa P25.

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C. Zhan et al. / Chemical Engineering Journal 225 (2013) 695–703 Table 3 Photodegradation of MO over different cycles under UV irradiation.

a

Cycle numbersa

1

2

3

4

5

6

MO degradation ratio (%)

100

98.85

94.39

92.05

90.56

90.0

The test conditions are described in experimental part.

Moreover, excessive Mo doping result in decrease of BET surface area and total pore volume, which are detrimental to photocatalytic activity enhancement. Meanwhile, SO2 anchored on the surface of TiO2 and oxygen 4 vacancy could also capture photogenerated electrons [46]. Thus, the recombination of photogenerated electrons and holes can be  suppressed effectively, resulting in formation of O 2 and OH. As a consequence, the quantum efficiency of photocatalytic reaction catalyzed by co-modified TiO2@SiO2 is improved. In the experiment, the optimum Mo dosage is about 0.50 mol%. And sulfated Mo-doped TiO2@SiO2 mesoporous composites show a higher degradation rate compared with P25 samples under both UV and VL light irradiation. The reason might be that the mesoporous structure of sulfated Mo-doped TiO2@SiO2 composite has better distribution, larger surface area and stronger electronwithdrawing effect. In addition, the sulfated composite with 0.5% Mo dosage showes good endurance of six cycles for 20 mg/L MO solution degradation under UV irradiation. which degradation ratio also keep up 90.0% without any deal with. The detailed results were listed in Table 3.

Fig. 12. Curves of MO degradation under VL irradiation.

4. Conclusions

Fig. 13. Photocatalytic mechanism of sulfated xMo–TiO2@SiO2.

Pure TiO2 is n-type semiconductor because of the presence of oxide vacancies (VÖ). A defect was created in the crystal when Mo6+ substituted Ti4+ in the lattice. In the Kroger–Vink notation, this is written as follows [6,44,45]:

MoO3 ! MoTi þ 2e þ 2Ox0 þ 1=2O2 " 

ð1Þ

After Ti4+ was substituted, the electron concentration increased because of charge equilibrium. Mo6+ became the electron donor, the Fermi level in band gap moved upwards and the width of band gap narrowed compared with pure TiO2. The reason the sulfated xMo–TiO2@SiO2 exhibited high visible light photocatalytic activity can be explained using the scheme shown in Fig. 13 for sulfated 0.5%Mo–TiO2@SiO2 photocatalyst, electrons can be excited from valence band to the Mo doping energy level (process (1)) and from S3p energy level to the conduction band (process (2)). Besides, it is possible that the electrons can be migrate from S3p level to the Mo 4d energy level (process(3)).As a result, more photoinduced charge carriers could be effectively separated to participate in the photocatalytic process, leading to a higher photocatalytic activity than that of single doping and neat TiO2 samples. But the photoactivity of the samples decreases with the adding of Mo dosage when the dopant content is higher (1.0 mol %) under UV irradiation. This can be ascribed to the fact that when the dopants are excessive, Mo6+ cannot enter the TiO2 lattice but cover on the surface of TiO2 in MO3 form, and form heterogeneity junction. The valence bands and conduction bands of two crystals may be link paratactically and charge capture centers maybe become recombination center, so photocatalytic activity reduce.

A novel UV and VL responsive sulfated Mo-doped TiO2@SiO2 mesoporous photocatalyst was successfully in situ synthesized by a sol–gel method. Moreover, both UV and VL photocatalytic activity of this composite photocatalyst was greatly enhanced compared with Degussa P25. It was founded that the photocatalytic activity of MO degradation depended on the concentration of the Mo dopant. A 0.5% Mo dosage was optimal for both UV and VL photoactivity. The suitable amount dopants can capture photogenerated electrons and decrease the rate of recombination of electron–hole and accelerate photocatalytic reaction. Furthermore, the composite showed good recycling endurance after six times under UV irradiation. Acknowledgements This material is based upon work funded by both Natural Science Foundation of China under Grant Nos. 21274131, 5127378 and 5123139. This work is also supported by Science and Technology Innovative Research Team of Zhejiang Province (No. 2009R50010) and Qianjiang talent Project of Zhejiang Province (2010R10018). References [1] Y. Wang, M. Zhong, F. Chen, J. Yang, Visible light photocatalytic activity of TiO2/ D-PVA for MO degradation, Appl. Catal. B 90 (2009) 249–254. [2] D. Hufschmidt, D. Bahnemann, J.J. Testa, C.A. Emilio, M.I. Litter, Enhancement of the photocatalytic activity of various TiO2 materials by platinisation, J. Photochem. Photobiol. A 148 (2002) 223–231. [3] Y. Yang, H. Wang, X. Li, C. Wang, Electrospun mesoporous W6+-doped TiO2 thin films for efficient visible-light photocatalysis, Mater. Lett. 63 (2009) 331–333. [4] G. Colón, M. Maicu, M.C. Hidalgo, J.A. Navío, Cu-doped TiO2 systems with improved photocatalytic activity, Appl. Catal. B 67 (2006) 41–51. [5] Y. Yang, H. Zhong, C. Tian, Z. Jiang, Single-step preparation, characterization and photocatalytic mechanism of mesoporous Fe-doped sulfated titania, Surf. Sci. 605 (2011) 1281–1286. [6] Y. Yang, X. Li, J. Chen, L. Wang, Effect of doping mode on the photocatalytic activities of Mo/TiO2, J. Photochem. Photobiol. A 163 (2004) 517–522.

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