Cobalt and sulfur co-doped nano-size TiO2 for photodegradation of various dyes and phenol

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photodegradation of various dyes and phenol

Item Type Article

Authors Siddiqa, Asima;Masih, Dilshad;Anjum, Dalaver H.;Siddiq, Muhammad

Citation Cobalt and sulfur co-doped nano-size TiO2 for photodegradation of various dyes and phenol 2015, 37:100 Journal of Environmental Sciences

Eprint version Post-print

DOI 10.1016/j.jes.2015.04.024

Publisher Elsevier BV

Journal Journal of Environmental Sciences

Rights NOTICE: this is the author’s version of a work that was accepted for publication in Journal of Environmental Sciences. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document.

Changes may have been made to this work since it was submitted for publication. A definitive version was subsequently published in Journal of Environmental Sciences, 1 November 2015. DOI:

10.1016/j.jes.2015.04.024 Download date 2023-12-01 08:09:30

Link to Item http://hdl.handle.net/10754/582655

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Cobalt and sulfur co-doped nano-size TiO 2 for

2

photodegradation of various dyes and phenol

3Q1

Asima Siddiqa

^1,2,3

, Dilshad Masih

^1,

⁎, Dalaver Anjum

⁴

, Muhammad Siddiq

^3,

⁎

4 1. KAUST Catalysis Center, King Abdullah University of Science and Technology (KAUST), Thuwal 23955, Saudi Arabia 5 2. National Centre for Physics, Quaid-i-Azam University Complex, Islamabad 44000, Pakistan

6 3. Chemistry Department, Quaid-i-Azam University, Islamabad 44000, Pakistan 7 4. Imaging and Characterization Lab, KAUST, Thuwal 23955, Saudi Arabia 8

10 A R T I C L E I N F O

11 A B S T R A C T

12 Article history:

13 Received 14 January 2015 14 Revised 31 March 2015 15 Accepted 8 April 2015 16 Available online xxxx

17 Various compositions of cobalt and sulfur co-doped titania nano-photocatalyst are synthesized 18 via sol–gel method. A number of techniques including X-ray diffraction (XRD), ultraviolet–visible 19 (UV–Vis), Rutherford backscattering spectrometry (RBS), thermal gravimetric analysis (TGA), 20 Raman, N2sorption, electron microscopy are used to examine composition, crystalline

21 phase, morphology, distribution of dopants, surface area and optical properties of

22 synthesized materials. The synthesized materials consisted of quasispherical nanopar- 23 ticles of anatase phase exhibiting a high surface area and homogeneous distribution of

24 dopants. Cobalt and sulfur co-doped titania demonstrated remarkable structural and

25 optical properties leading to an efficient photocatalytic activity for degradation of dyes 26 and phenol under visible light irradiations. Moreover, the effect of dye concentration,

27 catalyst dose and pH on photodegradation behavior of environmental pollutants and

28 recyclability of the catalyst is also examined to optimize the activity of nano-photocatalyst

29 and gain a better understanding of the process.

31 Published by Elsevier B.V.

32 Keywords:

33 Photocatalysis 34 Nanoparticles 35 Co-doped TiO2

36 Dye degradation 37 Phenol

38

3940 4142

4344

Introduction

45 Among all the various fields of nanotechnology, photocatalysis 46 has attracted a huge scientific interest with significant develop- 47 ments during the last decade. Modern day photocatalysis began 48 with Fujishima and Honda reporting photocatalytic splitting of 49 water in 1972 (Fujishima and Honda, 1972). And since then, it has 50 been making prompt progress as illustrated with a huge number 51 of publications toward theoretical investigations and practi- 52 cal applications (Oaxaca and Becerril, 2010). Finding out an 53 appropriate semiconductor, a practicable photocatalyst, is 54 one of the greatest challenges of material science. A practicable 55 photocatalyst with suitable band gap should have the ability to

56 absorb light in visible range, and remain stable in aqueous phase

57 (Hwang et al., 2006; Ma et al., 2008). Above and beyond, better it is

58 nontoxic, and easily available.

59 In the past 30 years, numerous semiconductors and molecu-

60 lar assemblies have been investigated as photocatalyst candi-

61 dates (Carp et al., 2004). Among all the various materials, titania

62 (TiO2) has long been a promising material for photocatalytic

63 applications because of its stability, natural abundance,

64 non-toxicity and high oxidative power of its photogenerated

65 holes (Huang et al., 2001; Carlier et al., 2006). And, in the last

66 few decades, titania has been investigated extensively for

67 environmental remediation, solar energy utilization and hydro-

68 gen production (Deepa et al., 2006; Bahadur et al., 2007). As

J O U R N A L O F E N V I R O N M E N T A L S C I E N C E S X X ( 2 0 1 5 ) X X X–X X X

⁎ Corresponding author.E-mail:[email protected](D. Masih),[email protected](M. Siddiq).

http://dx.doi.org/10.1016/j.jes.2015.04.024

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69 compared to other photocatalyst materials, a better performance 70 of titania-based materials is mainly attributed to its oxidizing 71 holes and hydroxyl radicals that are formed after photoex- 72 citation of electrons. These hydroxyl radicals in terms of 73 oxidation potential possess high oxidation power than those 74 of the other available oxidants (Huang et al., 2001; Bahadur et 75 al., 2007).

76 Titania is a well-known wide band gap semiconductor, 77 and since long has remained first choice for photocatalysis.

78 However, there are some limitations regarding its relatively 79 large band gap (3 to 3.2 eV), and a high number of electron- 80 hole recombination (Deepa et al., 2006; Mor et al., 2006;

81 Bahadur et al., 2007). Therefore, substantial efforts have been 82 devoted toward modifications of titania-based photocatalysts 83 that would consent utilization of visible spectrum of solar 84 radiation (Rigby et al., 2006; Khan et al., 2006). Development 85 of an efficient, economical and sustainable photocatalyst 86 material capable of working under visible-light irradiation 87 represents a central challenge of this field. Several strategies, 88 involving noble metal deposition, transition-metal ion dop- 89 ing, and coupled semiconductor systems have been explored 90 previously (Yu et al., 2003; Samarghandi et al., 2007). Among 91 the reported studies, modifications of titania by doping have 92 attracted much attention for the enhancement of photo- 93 catalytic activity. Main attractions of doped materials are 94 attributed to modified electronic and structural properties, 95 along with controlled growth of crystallite size leading to 96 variations in surface area and porosity (Masakazu, 2000;

97 Chatterjee and Mahata, 2009).

98 Simultaneous doping with anions and transition metal 99 ions (co-doping) may modify conductivity and optical proper- 100 ties of titania. Furthermore, such a co-doping may introduce 101 new electronic states that may lie closely to conduction or 102 valence band of titania (Bouras et al., 2007). Recently, an 103 innovative technique of doping titania with anions such as 104 carbon, nitrogen, phosphorous, sulfur and fluorine was reported 105 for desired control of band gap along with an enhanced photo- 106 degradation efficiency (Masakazu, 2000; Bouras et al., 2007; Chen, 107 2009). Doping with transition metals such as chromium, vanadi- 108 um and iron has also been explored in extending spectral 109 response of titania and improving its photoactivity (Chatterjee 110 and Mahata, 2009). It is believed that transition metals could act 111 as shallow traps in lattice of titania, thereby suppressing 112 recombination of photoinduced electron-hole pairs on the 113 surface of a photocatalyst (Belessi et al., 2000; Yamashita et al., 114 2001).

115 Uniform distribution of dopants along with a large surface 116 area and suitably lower band gap of a semiconductor material 117 are indispensable properties that strongly favor its photocat- 118 alytic properties (Zhu et al., 2010). Recently we have developed 119 a new generation titania photocatalyst that was co-doped 120 with cobalt and sulfur and synthesized through sol–gel 121 method. Sol–gel method is useful in controlling particle 122 size, morphology, homogeneity and surface area of produced 123 material (Zhou et al., 2010). Here in this study, photocatalytic 124 activity of materials prepared via sol–gel method is investi- 125 gated for decomposition of common environmental pollut- 126 ants, dyes and phenol. Furthermore, important strategies for 127 making improvements in photocatalytic activity of co-doped 128 titania are also examined.

129

1. Materials and methods

130

131 1.1. Synthesis of titania nanoparticles

132 Syntheses of pure, doped (1 wt.% sulfur), and co-doped

133 (1–5 wt.% cobalt and 1 wt.% sulfur) titania nanoparticles was

134 carried out by a conventional sol–gel method (Asahi et al.,

135 2001). The chemicals used for synthesis include titanium

136 isopropoxide (TTIP) (97%, Aldrich, USA), thiourea (99.9%, _Q2 137 Merck, USA), acetic acid (99.9%, Aldrich, USA), cobaltous Q3Q4

138 nitrate hexahydrate (99.9%, Aldrich, USA) and absolute Q5 139 ethanol (99.9%, Merck, USA). Milli-Q water was used in all Q6 140 the experiments. At first, a desired liquid medium was

141 prepared from 7:3:1 molar ratio of ethanol, acetic acid and

142 water, respectively. Then, measured amounts of thiourea

143 and cobaltous nitrate were added into the above solution,

144 followed by stirring for 15 min to ensure a homogeneous

145 solution. Next, a sol of titania was slowly formed with a drop

146 wise addition of TTIP into the above mixture, and stirred

147 magnetically for 6 hr at ambient temperature. The resultant

148 sol was gelled overnight followed by drying at 100°C to

149 eliminate water and ethanol, and ground to a fine powder.

150 Thus obtained product was finally treated at 500°C in static

151 air for 4 hr to achieve a desired anatase phase of titania. The

152 resultant materials are designated as pure TiO2(PT), sulfur

153 doped TiO₂(1% ST), 1 wt.% cobalt and 1 wt.% sulfur co-doped

154 TiO2 (1% Co–ST), 2 wt.% cobalt and 1 wt.% sulfur co-doped

157 TiO2 (4% Co–ST) and 5 wt.% cobalt and 1 wt.% sulfur

158 co-doped TiO2(5% Co–ST).

159 1.2. Characterization

160 Crystal structure, phase and size of the synthesized materials

161 were analyzed using X-ray diffractometer (XRD) (X'Pert

162 PRO 3040/60, Philips, USA) employing Cu-Kα radiation. N2 Q7 163 adsorption–desorption isotherms were recorded on a NovaWin2

164 apparatus (NovaWin2, Quantachrome, USA) to estimate Q8 165 Brunauer–Emmett–Teller (BET) surface areas and porosity.

166 Fourier transform infrared spectrometer (FT-IR) spectra were recorded with Spectrum-1000 of Perkin–Elmer (Spectrum-1000, 167

168 Perkin-Elmer, USA) for determination of the phase purity and Q9 169 monitoring the functional groups. For stability and phase purity

170 confirmation, thermal gravimetric analysis (TGA) profiles were

171 recorded with Diamond Series TGA/DT (Diamond Series TGA/DT,

172

Perkin–Elmer, USA). _Q10

173 The microstructure and morphology of synthesized mate-

174 rials were observed using scanning electron microscopy (SEM)

175 equipped with energy dispersive X-ray (EDX) spectrometry

176 (XL30, EDAX DX4, Philips, USA). Transmission electron mi- Q11

177 croscopy (TEM) investigations of the samples were completed

178 with an electron microscope (Titan G²80-300 ST, FEI, USA). Q12

179 The point-to-point resolution of this microscope was mea-

180 sured to be 0.196 nm with an amorphous carbon specimen.

181 The TEM investigations were carried out by operating it at

182 primary beam energy of 300 keV. The selected area electron

183 diffraction (SAED) analysis was performed to determine the

184 crystal structure of the samples. Circular Hough transform

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185 (CHT) was applied to determine thed-spacing present in the 186 acquired SAED micrographs (Mitchell, 2008). High resolution 187 TEM (HR-TEM) analysis of samples was also accomplished to 188 determine the size and crystal structure of nanoparticles directly.

189 Moreover, the spatial frequencies (or latticed-spacing) emanating 190 from the crystal structure of the samples were measured 191 from the calculated fast-Fourier transforms (FFTs) of HR-TEM 192 micrographs.

193 Optical properties were measured using Perkin-Elmer 194 Lambda 950 UV/VIS/NIR spectrophotometer (Lambda 950 UV/

195Q13 VIS/NIR, Perkin-Elmer, USA). Raman spectra were obtained by 196 Jobin-Yvon T64000 Raman spectrometer (T64000, Jobin-Yvon, 197Q14 Japan) using excitation laser of 647.1 nm in a backscattering 198 geometry with liquid nitrogen cooled charge-coupled device 199 (CCD) detector. The elemental analysis and dopant distribution of 200 the nanoparticles was determined by Rutherford backscattering 201 spectrometry (RBS) using collimated 2.0-MeV He²⁺beam of 20 nA 202 produced by tandem pelletron accelerator (5UDH-2, National 203Q15 Electrostatic Corporation, USA). Computer code software SIMNRA 204_Q16 version 6.02 (SIMNRA, Germany) was used for analyzing the RBS 205 spectra, recorded after calibration.

206 1.3. Photocatalytic degradation of dyes and phenol

207 Photodegradation of various dyes, Crystal Violet (dye 208_Q17 content≥35%) (Aldrich, USA), Malachite Green (MG, dye 209Q18 content≥90%) (Merck, USA), Procion Blue MXR (PB-MXR, 210Q19 dye content 95%) (Aldrich, USA), Alizarin Red S (ARS, dye 211Q20 content≥95%) (Merck, USA), and phenol (PH, 99%) (Aldrich, 212Q21 USA) was explored over titania-based photocatalysts. Photo- 213 catalytic degradation experiments were performed in 214 500 mL Pyrex beaker placed on magnetic stirrer and irradi- 215Q22 ated with 500 W Xenon lamp (UI-502Q, Ushio, Japan). Fig. S1 216 (in supplementary data) shows experimental setup for 217 photocatalytic degradation experiments. Stock solution of 218 each dye was prepared with Milli-Q water. In order to 219 determine adsorption–desorption equilibrium; required 220 amount of a catalyst was added into dilute solution of a dye 221 and stirred for 30 min in dark. Then, for photocatalytic test, 222 the suspension was irradiated with Xenon lamp. The lamp 223 was equipped with cutoff filters to selectively eliminate UV 224 and/or visible light in order to obtain a required range of 225 wavelength. Photodegradation reaction was carried out 226 under both UV (with cutoff filter λ< 380 nm) and visible 227 light irradiations (with cutoff filterλ> 420 nm) at 25°C. The 228 intensity of light in the UV and visible range was 225 and 229 200μW/cm², respectively. An aliquot was drawn at regular 230 intervals and residual concentration of dye was determined 231 by UV–visible spectrophotometer.

232 For optimizing reaction parameters, photodegradation was 233 carried out by varying initial concentration of a pollutant from 234 20 to 100 ppm at pH ~7, under visible-light irradiation. In 235 second step, visible-light irradiation experiments were per- 236 formed for 10 to 70 mg of catalyst added into 100 mL of 237 20 ppm dye solution at pH of ~ 7. Next, the role of acidic and 238 alkaline solution was determined at pH of ~4.5, ~7 and ~ 9.5, 239 while all other parameters were kept constant. Under optimized 240 conditions of dye concentration, catalyst loading and pH, the 241 experiments were performed with catalyst materials having 242 different wt.% of dopant. Finally, comparative studies were

243 carried out under UV and visible light irradiations, and

244 recyclability of optimized catalyst was also determined.

245 Extent of dye degradation with reaction time is estimated

246 by the following equation (Di Valentin et al., 2007):

Percent degradationð Þ ¼% ðCo−CtÞ=Co100% ð1Þ 247 whereCois the initial concentration of a dye, andCtis the 248 residual concentration at timet. 249

250

2. Results and discussion

251

252 2.1. Crystal structure and morphology

253 XRD patterns for all the samples are shown inFig. 1a, and the

254 obtained data are summarized in Table 1. X-ray diffraction

255 peaks of TiO2, 1% S-doped TiO2, Co–S co-doped TiO2 (with

256 1 wt.% S, and Co contents varying from 1–5 wt.%) matched to

257 typical peaks for anatase phase of titania (JCPDS standard files

258

#21-1272). No extra peaks of crystalline impurity are seen in 259 any of the samples, indicating that dopants are well dispersed

260 in titania. Doped species may occupy either interstitial

261 position or substitution sites in TiO2crystal structure (Chen,

262 2009; Belessi et al., 2000). In contrast to XRD patter of un-

263 doped TiO2, a slight shift in peak position along with decrease

264 in intensity is observed upon doping. A distinct change in

265 lattice parameters andd-spacing is also observed after doping

266 titania with sulfur and cobalt (Table 1). As reported previously,

267 this may imply that the dopants have introduced some dis-

268 tortions in structure (Zhou et al., 2010; Luu et al., 2010). Growth

269 of titania nanoparticles is presumably suppressed with

270 doping, leading to a good interaction and consequent incorpora-

271 tion of dopant ions into TiO2framework. Average crystallite size

272 calculated by Scherrer's formula, and lattice parameters are

273 summarized inTable 1.

274 2.2. Band gap and BET surface area

275 Optical properties of TiO2, 1% sulfur doped TiO2, 1%–5% cobalt

276 and 1% sulfur co-doped TiO2nanoparticles are evaluated from

277 their UV–vis diffuse reflectance spectra (DRS). Results of UV–

278 vis spectra are reported in Fig. 1b as Tauc plot, (F(R) ×hν)^0.5

279 versus energy (hν) (eV) (Zhu et al., 2010; Asahi et al., 2001).

280 Band gap energies are derived from Tauc plot and reported in

281 Table 1. It is noted that band gap for pure TiO₂ (3.17 eV) is

282 decreased with the addition of sulfur (2.89 eV). Upon sulfur

283 doping of titania there may be some mixing of S3porbital with

284 O2porbitals resulting in the formation of mid gap levels and

285 hence shifting absorption toward visible-light region (Di

286 Valentin et al., 2007). A distinctive band gap narrowing trend

287 is also observed with incorporation of cobalt and further

288 increasing its contents. A decrease in band gap from 2.45 to

289 1.75 eV is achieved with increase in cobalt contents from 1 to

290 5 wt.%. Among all the materials, 5% Co–S co-doped TiO2

291 exhibited the highest visible-light absorption capacity. This trend

292 suggests that addition of cobalt may promote visible-light

293 absorption by introducing its energy levels near conduction

294 band (Yu et al., 2010). Textural characterization results in

295 Table 1show that BET specific surface area and average pore

296 volume increased after doping with sulfur and cobalt. It is

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297 however noted that the highest BET specific surface area 298 observed for 5% Co–S co-doped TiO2(111 m²/g) as compared 299 to pure TiO2(68 m²/g) is caused by change in particle size to 300 lower nano-size range after doping.

301 2.3. Morphological studies

302 Morphologies of pure, sulfur-doped, and cobalt and sulfur 303 co-doped TiO2are revealed by SEM micrographs and present- 304 ed here inFig. 2. All the samples appeared as agglomerates of 305 smaller particles with a homogenous size distribution. SEM 306 micrographs of doped and un-doped powder exhibit particles 307 in the range of 12–22 nm, that is similar to the one obtained 308 from XRD analyses (Table 1). A marked difference in particles 309 with a gradual reduction of size is seen upon increase in 310 cobalt contents (Fig. 2c–g), describing that growth of particles 311 is strongly dependent on its concentration (Yu et al., 2010).

312 This trend illustrates that cobalt addition may hinder the 313 growth of TiO2 nanoparticles without much affecting their 314 crystallinity.

315 TEM analyses of pure TiO2, and 5 wt.% cobalt and sulfur 316 co-doped TiO2are presented inFig. 3, respectively. Bright-field 317 TEM (BF-TEM) micrographs for 1 wt.% sulfur doped TiO2are 318 shown in Fig. S2. It is obvious that particles are agglomerated 319 to some extent and exhibit narrow size distribution with 320 qausispherical morphology and distinguishable grain bound- 321 aries. TEM micrographs further suggest that nanoparticles are 322 single crystals of anatase phase matching well with XRD 323 results. Crystal lattice fringes present in HR-TEM micrographs 324 depict irregular, highly crystallinity and sharp edges of 325 nanoparticles. The corresponding FFT patterns, obtained 326 from an area of interest in HR-TEM micrographs exhibit an 327 ordered array of spots that confirm single crystalline nature of 328 the material (Luu et al., 2010; Yu et al., 2010). FFT patterns 329 consist of well resolved concentric rings arising from several 330 of these single crystal nanoparticles giving specific values of 331 inter-planard-spacing whose Miller indices (hkl) were deter- 332 mined as well (Khan et al., 2008). These inter-planar values

333 obtained from diffraction patterns closely resemble with

334 d-spacing values calculated from XRD patterns. The space

335 between lattice planes is precise, which are 3.51, 3.52, and 3.56 Å

336 for TiO2, 1% S-doped TiO2and 5% Co–S co-doped TiO2, respec-

337 tively. Essentially, all these values correspond tod-spacing values

338 of (101) plane for anatase phase of titania, and also are in good

339 agreement with theoretical data from XRD analyses (Table 1) (Yu

340 et al., 2010).

341 2.4. Composition and dopant distribution

342 RBS spectra demonstrate (Fig. 4A) that all dopants are

343 homogeneously distributed in titania structure and this

344 corroborates EDX results obtained from electron microscopy.

345 To find out stoichiometry of nano-photocatalysts, RBS data

346 was simulated and the resultant curves showed a good agree-

347 ment with that of experimental observations. Chemical compo-

348 sitions established from EDX and RBS analyses confirm that

349 targeted amount of sulfur and cobalt is obtained in all of these

350 modified titania samples (Table 2).

351 2.5. Raman studies

352 Raman spectroscopy provides useful information related to

353 phase composition, oxygen vacancies, defect concentration

354 and crystallinity of the material (Chen et al., 2005).Fig. 4B

355 exhibits Raman spectra of TiO2, sulfur doped TiO2, and catalyst

356 co-doped with cobalt and sulfur with various amounts of cobalt.

357 Raman spectra with well resolved peaks appearing at 144.3, 196,

358 397, 514.33 and 640 cm⁻¹depict a well-defined repetitive struc-

359 ture of tetragonal anatase phase of TiO2(Khan et al., 2008). The

360 strongest Egmode appearing at 144.3 cm⁻¹basically originated

361 from extension vibrations of anatase crystal structure (Chen,

362 2009). It was observed that upon doping of titania with sulfur and

363 cobalt, Raman bands shifted toward higher wavenumber along

364 with a peak broadening and a decrease in intensity (Luu et al.,

365 2010; Yu et al., 2010). This phenomena indicates the inclusion of

366 cobalt and sulfur ions into crystal structure of TiO2distorting its Fig. 1–(A) X-ray diffraction patterns and (B) Tauc plots derived from UV–vis spectra. (a) pure TiO2, (b) 1% S-doped TiO2, (c) 1%

Co–S co-doped TiO2, (d) 2% Co–S co-doped TiO2, (e) 3% Co–S co-doped TiO2, (f) 4% Co–S co-doped TiO2, and (g) 5% Co–S co-doped TiO2calcined at 500°C.F(R): the adsorption coefficient;hv: the energy of the light.

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367 perfect symmetry. Consequently, these distortions induced 368 defects in framework structure, creating oxygen vacancies and 369 limiting growth of TiO2crystals. All these factors led to distortions 370 in lattice, resulting in discrepancy of vibration frequency, and 371 thereby causing changes in peak intensity, bandwidth and 372 position of Raman bands (Hussain and Siddiqa, 2011). Besides, 373 no new Raman band appeared in the spectra, suggesting that 374 TiO2retained its anatase phase and no extra phase appeared with 375 doping of sulfur and cobalt (Zhu et al., 2010).

376 2.6. TGA analysis

377 Thermal analysis of as-synthesized samples was carried out 378 to evaluate phase transformation from amorphous to crystal- 379 line TiO2, effect of dopant level on thermal stability and mass 380 loss behavior of TiO2. Two distinct weight loss steps appeared 381 in TGA curves of as-prepared samples, and are presented here 382 inFig. 4C. The first weight loss that occurred from 25 to 110°C 383 in all the samples can be attributed to desorption of water and 384 elimination of residual organic solvents like ethanol and 385 acetic acid (Chatterjee and Mahata, 2009; Masakazu, 2000). At 386 second stage, a significant weight loss begins at 130°C and 387 ends at 400°C in all the samples. This second peak represents 388 the removal of water of crystallization or decomposition of 389 un-hydrolyzed TTIP. This peak can also come from elimina- 390 tion of nitrates or organic moieties in doped samples 391 (Fig. 4Cb–g). Comparatively, a reduction in total weight loss 392 is however noted with increasing dopant level, indicating that 393 a higher level of dopant induces thermal stability (Hussain et 394 al., 2011). A constant line of TGA profile beyond 400°C renders 395 that the final sample is thermally stable and essentially free of 396 any residual impurities.

397 2.7. Photodegradation of dyes and phenol

398 2.7.1. Effect of initial concentration of dyes and phenol

399 Effect of initial concentration of dyes and phenol on photo- 400 degradation is studied by varying their concentrations from 20 401 to 100 ppm. Fig. S3 in Supplementary data represents ob- 402 served percent degradation over TiO2, 1% S-doped TiO2, and 403 5% Co–S co-doped TiO2. These results demonstrate that 404 degradation is decreasing with the increase in initial concen- 405 tration of a pollutant, and a maximum is achieved at around 406 20 ppm. A possible explanation for this behavior may be the 407 adsorption–desorption equilibrium. In principle, adsorption of

408 enhanced number of pollutant molecules onto surface of

409 photocatalyst may cause a shielding effect (Khan et al., 2006).

410 Consequently, for a high concentration of pollutant, degra-

411 dation decreases due to this screening effect when a limited

412 number of photon can come out to the surface of the photo-

413 catalyst (Chatterjee and Mahata, 2009; Hussain and Siddiqa,

414 2011).

415 2.7.2. Effect of catalyst dose

416 Amount of catalyst is also one of the crucial factors which

417 may affect photodegradation efficiency. Effect of catalyst

418 amount for degradation of dyes and phenol was studied in

419 order to acquire an optimized amount of photocatalyst for

420 efficient degradation of noted pollutants. Fig. S4 (Supplemen-

421 tary data) demonstrates this dependence of degradation of

422 dyes on the amount of photocatalyst at various loadings. An

423 increase in the amount of catalyst drastically enhanced the

424 activity for photocatalytic decomposition of dyes and phenol.

425 For TiO2, 1% S-doped TiO2 and 5% Co–S co-doped TiO2,

426 maximum degradation percentage was observed at around

427 50 mg of catalytic dose, upon visible-light irradiation for

428 50 min. This enhancement in catalytic activity with the

429 increase in catalyst amount up to 50 mg is thus rationalized

430 on the basis of an increased availability of active sites onto the

431 photocatalyst surface (Hussain et al., 2011; Seery et al., 2007).

432 Any further increase in the catalyst amount above 50 mg

433 caused a decrease in photocatalytic activity, mainly because the

434 suspended photocatalyst has a high probability of agglomeration.

435 As reported previously, this aggregation of nano-size titania

436 may block the photogenerated active species for subsequent

437 dye degradation reactions on the catalyst surface (Hussain

438 and Siddiqa, 2011; Hussain et al., 2011). Therefore, an

439 optimum dose of photocatalyst is needed to ensure maxi-

440 mum absorption of solar light for an efficient photocatalytic

441 activity (Rigby et al., 2006).

442 2.7.3. Effect of pH

443 Study of pH effect on dye degradation is also investigated

444 because it may vary for textile effluents and influence

445 surface charge properties of a catalyst. Photodegradation

446 over un-doped, S-doped and Co–S co-doped TiO2was carried

447 out at solution pH values ranging from 4.5 to 9.5 (Fig. S5 in

448 Supplementary data). For TiO2and 1% S-doped TiO2, only a

449 small change is noticed in photodegradation activity with

450 changing solution pH. While photodegradation over cobalt Table 1

t1:1 –Structural parameters, band gap and physical properties of pure, S-doped, and Co–S co-doped TiO2. t1:2

t1:3t1:4 Catalyst Particle size d-Spacing Cell parameters Band gap BET surface area Pore volume

t1:5 (nm) (Å) a (Å) c (Å) (eV) (m²/g) (cm³/g)

t1:6 TiO2 20.0 3.51 3.781 9.541 3.17 68 0.33

t1:7 1% ST 19.2 3.52 3.781 9.551 2.89 78 0.53

t1:8 1% Co–ST 18.3 3.52 3.781 9.556 2.45 81 0.98

t1:9 2% Co–ST 16.4 3.54 3.781 9.559 2.26 91 1.03

t1:10 3% Co–ST 15.6 3.54 3.781 9.563 2.15 97 1.10

t1:11 4% Co–ST 14.2 3.56 3.781 9.569 2.02 105 1.18

t1:12 5% Co–ST 13.3 3.56 3.781 9.570 1.75 111 1.20

t1:13 Particle size, d-spacing and cell parameters were calculated from X-ray diffraction patterns. Band gap was calculated from Tauc plot derived of t1:14 UV-Vis spectra. BET (Brunauer–Emmett–Teller) surface area and pore volume were estimated from N2adsorption-desorption isotherms.

t1:15

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451 and sulfur co-doped TiO2 demonstrated a pH dependent 452 behavior (Fig. S5c). Over co-doped titania, a complete photo- 453 degradation of cationic (Malachite Green and Crystal Violet) 454 and anionic (Alizarin Red S, Procion Blue MXR and phenol) 455 species was observed under basic and acidic media, respectively.

456 The effect of solution pH was not so profound for pure titania and

457 upon replacing its O^2–with S^2–. However, pertaining to surface

458 charge, a drastic change with pH was observed when Ti⁴⁺species

459 are replaced with Co^{2 +}. Actually, surface charge properties of

460 a catalyst are modified with change in solution pH, thereby

461 controlling adsorption of dyes and consequently their photo-

462 degradation efficiencies. Under acidic and basic media, surface Fig. 2–Scanning electron microscopy images of (a) pure TiO2, (b) 1% S-doped TiO2, (c) 1% Co–S co-doped TiO2, (d) 2% Co–S co-doped TiO2, (e) 3% Co–S co-doped TiO2, (f) 4% Co–S co-doped TiO2, and (g) 5% Co–S co-doped TiO2calcined at 500°C.

a1 b1 c1 d1

a2 b2 c2 d2

Fig. 3–TEM analysis of pure TiO2(1) and 5% Co–S co-doped TiO2(2). (a) BF-TEM micrograph, (b) the corresponding SAED along with Circular Hough transform (CHT) analysis of (a), (c) HR-TEM electron micrograph, and (d) the corresponding calculated fast-Fourier transform (FFT) of micrograph given in (c). BF-TEM: bright-field transmission electron microscopy; SAED: selected area electron diffraction; HR-TEM: high-resolution TEM.

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463 of titania may be protonated and deprotonated, respectively, 464 according to following equations (Seery et al., 2007; Hussain 465 et al., 2012):

TiOHþH^þ→TiOH^þ₂ ð2Þ

466 467

TiOHþOH⁻→TiO⁻þH2O ð3Þ

468 469470 It has been reported that titania may act as positively

471 charged surface in acidic medium and negatively charged

472 surface in alkaline medium (Hussain et al., 2011; Seery et al.,

473 2007). At high pH value, negatively charged surface of TiO2may

474 repel anionic molecules showing a decrease in photodegradation

475 efficiency to anionic dyes in alkaline media. On the other hand, a

476 high photodegradation of cationic species is observed in

477 alkaline pH due to an increased electrostatic attraction of

478 opposite charges on catalyst surface and organic pollutant.

479 Similarly, it was observed that removal efficiencies for anionic

480 species increased against decrease in solution pH and decreased

481 for cationic species (Hussain et al., 2012).

482 2.7.4. Effect of dopant contents

483 Effect of cobalt contents of sulfur doped TiO2on photocata-

484 lytic degradation of dyes and phenol is demonstrated in

485 Figs. 5a and S6. Only cobalt doped TiO2prepared by sol–gel

486 method showed around 20% photodegradation of Malachite

487 Green dye after 50 min irradiation (Narayana et al., 2011).

488 Photodegradation after 50 min irradiation over pure TiO2and

489 sulfur-doped TiO2is also included for comparison. As the amount

490 of cobalt is increased from 1 to 5 wt.%, photodegradation is

491 enhanced in giving a complete mineralization at 5 wt.% cobalt

492 and 1 wt.% sulfur co-doped titania. Since the time of irradiation

493 was the same, this increase may come from a high extent of

494 visible-light absorption and specific surface area of 5 wt.% cobalt

495 and sulfur co-doped TiO2(Hussain et al., 2012). Anatase phase

496 purity, structural stability, and recycling of the doped titania

497 photocatalyst restricted the extent of the dopants beyond 5 wt.%

498 cobalt and 1 wt.% sulfur.

499 2.8. Comparison of photocatalytic activity under UV and

500 visible light irradiations

501 Fig. 5b presents a comparison of photocatalytic degradation

502 of dyes and phenol over 5 wt.% cobalt and 1 wt.% sulfur

503 co-doped TiO2under UV and visible light irradiations. Consider-

504 ably low photodegradation of dyes and phenol was observed

505 even for 6 hr of UV-light irradiation. On the other hand, upon

400 600 800 1000 1200 1400

Normalized yield

Channel

(a) (b) (c)(d) (e) (f) (g) O

S

Ti Co

A

200 300 400 500 600 700 800

Raman shifts (cm^-1)

Intensity (a.u)

Eg

Eg B1g A1g

Eg

(a) (b) (c) (d) (e) (f) (g)

B

100 200 300 400 500 600 700 800

70 75 80 85 90 95 100

Weight (%)

Temperature (°C) (a) (b) (c) (d)

(e) (f) (g)

C

Fig. 4–(A) RBS spectra, (B) Raman spectra, and (C) TGA profiles of (a) pure TiO2, (b) 1% S-doped TiO2, (c) 1% Co–S co-doped TiO2, (d) 2% Co–S co-doped TiO2, (e) 3% Co–S co-doped TiO2, (f) 4% Co–S co-doped TiO2, and (g) 5% Co–S co-doped TiO2calcined at 500°C. RBS: Rutherford

backscattering spectrometry; TGA: thermal gravimetric analysis.

Table 2–Elemental analyses of pure, S-doped, and Co–S t2:1 t2:2 co-doped TiO2.

t2:3t2:4 Catalyst EDX elemental

composition (wt.%)

RBS elemental composition (wt.%)

t2:5

Ti O S Co Ti O S Co

t2:6

PT 59.9 40.1 – – 58.7 41.3 – –

t2:7

1% ST 59.1 39.9 1.0 – 59.1 39.9 1.0 –

t2:8 1% Co–ST 59.1 39.9 1.0 1.0 58.2 39.9 1.0 1.0

t2:9 2% Co–ST 57.7 39.4 0.9 1.9 57.7 39.4 0.9 1.9

t2:10 3% Co–ST 57.2 38.9 0.9 2.9 57.1 38.9 0.9 2.9

t2:11 4% Co–ST 57.0 38.2 0.9 3.9 57.0 38.1 0.9 3.9

t2:12 5% Co–ST 56.5 37.7 0.9 4.9 56.5 37.7 0.9 4.9

t2:13 EDX: energy dispersive X-ray spectrometry; RBS: Rutherford

t2:14

backscattering spectrometry. t2:15

7

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506 visible-light irradiation a complete photodegradation was 507 achieved within 50 min. This difference in rate upon UV 508 and visible light irradiation may be attributed to narrow

509 band gap of cobalt and sulfur co-doped TiO2catalyst which

510 lies essentially in visible region, and thereby enabling a

511 visible-light induced photodegradation (Khan et al., 2010). Q23

512 2.9. Reuse of the photocatalyst

513 One major focus of industrial waste water treatment strate-

514 gies is to develop greener technologies with fairly repeated

515 usage, and that shall be beneficial for environmental and

516 economic implications. Therefore, recycling of spent catalyst

517 after photocatalytic reaction is a good criterion for sustainable

518 waste water treatment process, as it stands for any other

519 catalytic process. Optimized nano-photocatalyst, 5 wt.% co-

520 balt and 1 wt.% sulfur co-doped TiO2was selected for repeated

521 testing of material for photodegradation reaction. The results

522 for re-use of nano-photocatalyst are shown in Fig. 5c.

523 For recyclability of the photocatalyst, after degradation of

524 pollutants, the solid phase was separated from the reaction

525 mixture, washed thoroughly with distilled water, dried and

526 reused for photodegradation of a fresh dye solution. As shown

527 inFig. 5c, a comparable efficiency was observed on second

528 and third cycles. Next, for the fourth and fifth repeated usage,

529 relatively lesser photodegradation efficiency was noticed.

530 This trend may be explained by the deposition of organic

531 species on active sites of catalyst, inhibiting its catalytic

532 activity (Yu et al., 2010; Yang et al., 2004). For the sixth cycle,

533 photocatalyst material is separated, dried and calcined at

534 500°C to eliminate surface adsorbed impurities. As expected

535 after heat treatment and with decomposition of surface

536 adsorbed organics, almost 100% efficiency is regained for the

537 sixth cycle.

538 Fig. 6schematically illustrates photodegradation process

539 of environmental pollutants over nanosize titania catalyst.

540 Optimized photocatalyst semiconductor, 5% Co–S co-doped

541 titania depicted moderate activity under neutral conditions.

542 While under acidic and alkaline media, surface of material is

543 respectively charged, and attracted opposite species demon-

544 strating their enhanced photodegradation.

UV (6 hr) Visible (50min) 0

20 40 60 80 100

Irradiation source ARS

MG CV PB-MXR PH

a

PT ST 1Co-ST 2Co-ST 3Co-ST 4Co-ST 5Co-ST 0

20 40 60 80 100

Percent degradation (%)Percent degradation (%)Percent degradation (%)

Dopant concentration (%) MG

PH CV ARS PB-MXR

b

1 2 3 4 5 6

0 20 40 60 80 100

Cycles

ARS MG CV PB-MXR PH

c

Fig. 5–Degradation of dyes and phenol over (a) 5% Co–S co-doped TiO2(5Co–ST) under UV and visible light

irradiations, (b) titania photocatalyst as a function of dopant contents under visible-light irradiation, and (d) recycling of 5% Co–S co-doped TiO2catalyst under visible-light

irradiation. Reaction conditions: time = 50 min, catalyst = 50 mg, and pollutant concentration = 20 ppm. ARS: Alizarin Red S; MG: Malachite Green; CV: Crystal Violet; PB-MXR:

Procion Blue MXR; PH: phenol; PT: pure TiO2.

Degradation Light

Adsorbed O2

Adsorbed H2O, OH^- Adsorbed pollutant Adsorbed pollutant O2-. H⁺

h⁺h⁺h⁺ h⁺ h⁺h⁺h⁺

H₂O₂ e^-

e^-e^-e^-e^-e^-e^-e^-

OH

OH Activation

Recombination

Conduction band

Valence band

Fig. 6–Schematic illustration of photodegradation process of environmental pollutants over optimized nano-size titania semiconductor under acidic and alkaline media upon light irradiation.

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545

546

3. Conclusions

547 For photocatalytic degradation of dyes and reduction of carbon 548 dioxide, 5 wt.% cobalt and 1 wt.% sulfur co-doped titania nano- 549 photocatalyst was found superior to un-doped and only sulfur 550 and only cobalt doped materials. Nano-size anatase phase of 551 titania materials was successfully synthesized via sol–gel method 552 and characterized by XRD. HR-TEM analyses confirmed phase 553 purity of nano-size titania. Cobalt concentration was varied from 554 1 to 5 wt.%, and found to have a significant effect on structure, 555 optical, and photocatalytic properties of the obtained titania 556 nanoparticles. A synergistic effect of co-doping cobalt with sulfur 557 was found for an enhanced absorption of visible-light by nano- 558 size titania and consequently demonstrating its better photocat- 559 alytic efficiency. Among all the catalysts investigated in this 560 study, nano-size titania with 5 wt.% cobalt and 1 wt.% sulfur 561 co-doping demonstrated the highest photocatalytic efficiency.

562 The highest photocatalytic efficiency of optimized catalyst may 563 be accounted to its reduced band gap and high specific surface 564 area for its smallest particle size. Recycling experiments depicted 565 that catalyst was regenerated after each photcatalysis process, 566 suggesting that it can be reused for several cycles thus 567 making it economically feasible material. On the basis of the 568 obtained results, it can be concluded that optimized co-doped 569 titania nano-photocatalyst may be recommended for practical 570 application.

571572

Acknowledgments

573 Sincere thanks to Prof. Kazuhiro Takanabe at KAUST Catalysis 574 Center, King Abdullah University of Science and Technology for 575 sharing laboratory facilities for completion of this research 576 project. A. Siddiqa and M. Siddiq are grateful to Higher Education 577 Commission, Pakistan for providing financial assistance.

578579

Appendix A. Supplementary data

580 Supplementary data to this article can be found online at 581 http://dx.doi.org/10.1016/j.jes.2015.04.024.

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