Nanophotocatalyst for Photocatalytic Degradation under Visible Light

Apr 13, 2014 - Department of Chemical and Petroleum Engineering, Sharif University of Technology, Tehran, Iran. ‡. Department of Engineering, Univer...
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Enhancement of Efficient Ag−S/TiO2 Nanophotocatalyst for Photocatalytic Degradation under Visible Light Mehrzad Feilizadeh,† Manouchehr Vossoughi,*,† S. Mohammad Esmaeil Zakeri,‡ and Mohammad Rahimi† †

Department of Chemical and Petroleum Engineering, Sharif University of Technology, Tehran, Iran Department of Engineering, University of Kashan, Kashan, Iran

‡

ABSTRACT: A new photocatalyst (Ag−S/PEG/TiO2) was synthesized by adding polyethylene glycol (PEG) to an efficient Ag−S/TiO2 photocatalyst, to obtain a photocatalyst that is highly active under visible light. In addition to Ag−S/PEG/TiO2, Ag−S/TiO2 and pure TiO2 were prepared to compare their properties and activities. Specifically, the morphologies and microstructures of the nanophotocatalysts were characterized by means of powder X-ray diffraction (XRD), N2 adsorption−desorption measurements, scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) microanalysis, transmission electron microscopy (TEM), UV−visible diffuse reflectance spectroscopy (DRS), photoluminescence (PL) spectroscopy, Fourier transform infrared (FTIR) spectroscopy, and X-ray photoelectron spectroscopy (XPS). Moreover, to evaluate their activities, the synthesized powders were used for the degradation of acid orange 7 (AO7) and methylene blue (MB) azo dyes in aqueous solution under low-voltage lightemitting diodes (LEDs) as the visible-light source. It was found that addition of PEG to Ag−S/TiO2 increases the photodegradation of AO7 and MB by about 28.82% and 24.24%, respectively, in comparison with Ag−S/TiO2 without PEG addition.

1. INTRODUCTION Heterogeneous photocatalysis using semiconductors is an effective and significant method for the removal of many environmental contaminants.1−3 Titanium dioxide (TiO2) is one of the most efficient semiconductor photocatalysts for environmental applications because of its high oxidizing power, nontoxicity, high stability, low cost, resistance against photocorrosion and chemical corrosion, and ability to mineralize refractory organic pollutants under ambient pressure and temperature.4−6 However, because of the large band gap of TiO2, it can be activated only upon irradiation with photons of light in the UV domain (λ ≤ 387 nm for the anatase phase). This fact limits the practical efficiency of TiO2 for visible-light applications.7,8 Therefore, to utilize visible light for TiO2 excitation, it is necessary to modify the nanomaterial to facilitate visible-light absorption. Several attempts have been made to shift the onset of TiO2 absorption from the ultraviolet to the visible region and make the photocatalyst sensitive to visible-light irradiation.9 One of the most effective strategies is the doping of TiO2 with metal ions10,11 and nonmetal elements.12,13 Although only metal doping can create electron traps in the metal centers, it can reduce the thermal stability of the resulting material.14 Therefore, to overcome this drawback and also to increase the photocatalytic activity, doping of a metal and nonmetal at the same time has been researched.15−17 Among metals and nonmetals, the doping of silver and sulfur has been widely investigated.18−20 Although Sobana et al.21 observed that Ag doping increases the visible-light photodegradation performance, Gunawan et al.22 did not obtain the desired result by modifying TiO2 through Ag doping. In addition, it appears that doping with S makes an enormous improvement in the performance of TiO2 photocatalyst.23,24 In this line of investigation, Ag, S codoped TiO2 has been synthesized by a few researchers. Hamal and Klabunde25 showed that a photocatalyst based on silver and sulfur-doped TiO2 degrades acetaldehyde 10 © 2014 American Chemical Society

times faster under visible-light illumination and 3 times faster under UV-light illumination than the certified photocatalyst P25 TiO2. Consequently, it can be concluded that the presence of sulfur and silver enhanced the visible-light photocatalytic activity of the synthesized catalyst. Another strategy to achieve a more efficient photocatalyst is the addition of polyethylene glycol (PEG) as a template, which could increase the specific surface area and the visible-light absorption of the synthesized photocatalysts.26,27 Addition of PEG can introduce some extra trap states (impurity levels) within the valence and conduction bands of TiO2.28 Chang et al.29 reported that PEG suppressed the crystal growth and enlarged the specific surface area of a photocatalyst. Moreover, this morphology modification by PEG addition was found to lead to an active photocatalyst. Likewise, Shamaila et al.30 showed that the photocatalytic degradation of phenol was noticeably enhanced upon addition of PEG as a template to TiO2. Furthermore, Sun et al.31 reported that PEG-modified TiO2 evolved 1.5 times more hydrogen than the photocatalyst without PEG at the optimum conditions. Hence, given the apparent advantage of PEG addition, it is valuable and rational to investigate the enhancement of the photocatalytic activity under visible-light irradiation of the efficient Ag−S/TiO2 catalyst using PEG. Most researchers who have performed photodegradation under visible-light irradiation have utilized high-voltage lamps as the visible-light source. For example, 1000- and 500-W mercury lamps, 500- and 300-W halogen lamps, a 690-W Xe lamp, and a 450-W OSRAM bulb have been used in some studies.32−35 In this work, low-voltage (3-W) light-emitting diodes (LEDs), Received: Revised: Accepted: Published: 9578

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efficiency was measured on a UV−vis spectrophotometer (PerkinElmer Lambda2S). X-ray photoelectron spectroscopy (XPS) measurements were made using a Multilab 2000 XPS system with a monochromatic Mg Kα source. 2.3. Evaluation of Photocatalytic Activity. Photocatalytic experiments were performed in a Pyrex glass for the degradation of azo dyes under visible-light irradiation at room temperature. Ten 3-W low-voltage LED lamps were used as the visible-light source. Figure 1 shows the spectrum distribution of

which consume a much lower amount of electrical energy than most lamps, were used as the visible-light source. Furthermore, another important merit of the LED lamps is their narrow irradiation wavelength distribution in the visible region. In this study, a new photocatalyst (Ag−S/PEG/TiO2) was synthesized by adding PEG to the efficient Ag−S/TiO2 photocatalyst, to obtain a photocatalyst that is highly active under visible light. In addition, Ag−S/TiO2 and pure TiO2 were synthesized with the aim of comparing the properties and photocatalytic activities of the photocatalysts. Moreover, the photodegradation of two azo dyes using these photocatalyst under low-voltage LED lamps was investigated.

2. MATERIALS AND METHODS 2.1. Sample Preparation. TiO2 photocatalyst was prepared by a sol−gel method using titanium tetraisopropoxide (TTIP, ≥98%, Sigma-Aldrich) as the titanium source. 5 mL of TTIP was added to 9.62 mL glacial acetic acid (Merck KgaA, Darmstadt, Germany), and the mixture was stirred vigorously for 30 min until it formed sol. Afterwards, appropriate amount of deionized water was added dropwise to the sol. Again, the mixture was stirred vigorously for 2 h. Then, the prepared solution was kept for a day at room temperature in a dark area, after which it was gelated at 76 °C for 12 h and dried at 120 °C for 3 h. Finally, the dried gel was calcined at 550 °C for 3 h. Ag−S/TiO2 (0.8% Ag, 1% S molar ratio) was synthesized similarly to the above method, but in the sol-forming step, the required volumes of AgNO3 and thiourea (as silver and sulfur precursors, respectively) were dissolved in deionized water, and this solution was added dropwise to the TTIP solution prepared in the first step. To add PEG to Ag−S/TiO2, 2 g of polyethylene glycol (MW = 4000, Merck KgaA, Darmstadt, Germany) was added to the TTIP solution prepared in the first step. The other steps were similar to those in the above process. It is worth noting that the Ag- and S-doped sample is labeled Ag−S/TiO2 whereas the Ag−S/TiO2 sample with PEG addition is labeled Ag−S/PEG/TiO2. Moreover, pure TiO2 denotes the photocatalyst without any modification, neither PEG addition nor element doping. 2.2. Photocatalyst Characterization. The X-ray diffraction (XRD) analysis of the synthesized photocatalysts was carried out on a Philips (X’pert Pro MPD) X-ray diffractometer, employing Cu Kα radiation (wavelength 1.5406 Å) equipped with a Ni filter over the range of 10−90° (2θ). The crystallite sizes of the samples were calculated using the Scherrer equation with the full width at half-maximum (fwhm).32 The specific surface area and pore volume of the samples were determined from the nitrogen adsorption−desorption isotherms at 77 K using a Micromeritics ASAP 2101 instrument. The morphologies of the modified photocatalysts and their particle sizes were observed by field-emission scanning electron microscopy (SEM, Hitachi S4160) and transmission electron microscopy (TEM, Phillips CM10). Energy-dispersive X-ray (EDX) spectra were also obtained using the Hitachi S4160 scanning electron microscope. Ultraviolet−visible (UV−vis) diffuse reflectance spectra (DRS) were recorded with an Ava Spec-2048TEC Scan UV− vis−near-infrared (NIR) spectrophotometer using BaSO4 as the reference in the range of 380−750 nm. Photoluminescence (PL) spectra of the samples were measured with a fluorospectrophotometer (Cary Eclipse fluorescence spectrometer) using the 300-nm line of a Xe lamp as the excitation source at room temperature. The bond vibrations of powder samples were analyzed by Fourier transform infrared (FTIR) spectrometry using an ABB Bomem MB 100 spectrometer with KBr pellets over the range of frequencies from 400 to 4000 cm−1. The degradation

Figure 1. Spectrum distribution of the visible-light source provided by 3-W LED lamps.

the visible-light source provided by the 3-W LED lamps. It clearly demonstrates that the distribution of the lampirradiation wavelength was located in the visible region (430 nm ≤ λ ≤ 500 nm). The distance between the lights and the Pyrex glass was fixed at 10 cm. During the reaction process, oxygen was continuously bubbled through the mixture so that the concentration of dissolved oxygen did not change. To keep the lamps at a constant temperature during the photocatalytic reaction, the lamps were cooled with an external fan at the light box. Prior to light irradiation, the suspension was sonicated for 10 min. Subsequently, the mixture was magnetically stirred in the dark for 30 min to reach the steady state of adsorption equilibrium on the photocatalyst surface. Then, the lamps were turned on, and degradation was performed for 2 h. Every 30 min, a sample was taken. Each sample was centrifuged to extract solid particles. The collected samples were used for analysis by UV−vis spectrometer, which monitored the concentration changes of the pollutants using their maximum absorption wavelengths. In this study, acid orange 7 (AO7, Merck KgaA, Darmstadt, Germany) and methylene blue (MB, Sigma-Aldrich) azo dyes were utilized to measure the photodegradation activities of the synthesized photocatalysts. For each run, 0.1 g of photocatalyst sample was added to the reactor containing 100 mL of 10 ppm AO7 or MB aqueous solution (pH 3 for AO7 and pH 10 for MB). Figure 2 presents a schematic diagram of the photocatalytic reactor.

3. RESULTS AND DISCUSSION 3.1. Characterization of the Synthesized Photocatalysts. 3.1.1. XRD Analysis and Brunauer−Emmett−Teller (BET) Measurements. XRD patterns of pure TiO2, Ag−S/TiO2, and Ag−S/PEG/TiO2 samples are presented in Figure 3. All Bragg peaks of the three samples can be well indexed to the phase of 9579

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listed in Table 1. As can be seen, pure TiO2 had the lowest surface area and pore volume (i.e., 33 m2·g−1 and 0.10 cm3·g−1, respectively). After the TiO2 had been doped with silver and sulfur, the surface and pore volume enhanced to 53 m2·g−1 and 0.13 cm3·g−1, respectively, similar to values that were reported in previous studies.38,39 Moreover, as a more important result, addition of PEG dramatically increased the BET surface and pore volume of the photocatalyst (i.e., 68 m2·g−1 and 0.19 m3·g−1, respectively) compared to the values for the sample without PEG addition. Indeed, the porous structures of Ag−S/PEG/ TiO2 were formed by PEG combustion during calcination. PEG started to decompose at around 250 °C in air, and a calcination temperature of 550 °C caused removal of PEG from the photocatalyst.40,41 This burning of PEG during calcination led to a photocatalyst with a porous and rougher surface. The more porous structure resulted in the higher specific surface area of the photocatalyst. 3.1.2. SEM, TEM, and EDX Analyses. Figure 4 shows SEM images of the modified TiO2 samples synthesized with and without PEG addition. According to this analysis, the average particle sizes of both samples were almost equal (∼20 nm), with some nanoagglomerates. Figure 5 shows TEM images of the doped photocatalysts. The photocatalysts have a cube-like shape. According to this figure, Ag−S/TiO2 and Ag−S/PEG/TiO2 have average sizes of 19 ± 3 and 16 ± 3 nm, respectively. After PEG addition to Ag−S/TiO2, it can be seen that the nanoparticles maintained their original crystalline morphology, but their size decreased slightly. Figure 6 presents the EDX analyses of Ag−S/PEG/TiO2 and Ag−S/TiO2. According to these analyses, the presence of Ag and S in the structure of the photocatalysts is confirmed. Despite the low doping amount of silver, a peak was observed for both samples. Therefore, the silver was distributed uniformly in the photocatalyst structure. In addition, Ag−S/PEG/TiO2 showed higher peaks for C and O in comparison with Ag−S/TiO2. This extra amount of carbon and oxygen came from unburned PEG during calcination, which remained in the synthesized Ag−S/PEG/TiO2 photocatalyst. 3.1.3. DRS and PL Spectroscopy. Figure 7 presents the DR spectra of pure TiO2, Ag−S/TiO2, and Ag−S/PEG/TiO2 photocatalysts. The pure TiO2 absorption is often exhibited in the UV region; however, for the modified TiO2 samples, a noticeable shift of the optical absorption edges toward the visible-light region occurred. The visible-light absorption for

Figure 2. Schematic diagram of the photocatalytic reactor.

Figure 3. XRD patterns of different samples calcined at 550 °C (A = anatase).

anatase. As can be seen, the low doping degree with Ag and S (i.e., Ag, 0.8%; S, 1% molar ratio) led to a light shifting of the anatase (101) peak to higher angles, which is in accordance with previous investigations.31,36 Compared to pure TiO2, the modified samples exhibited a new peak at 2θ = 45°, indicating the good dispersion of Ag within the crystal matrix of TiO2.37 Moreover, according to the Scherrer equation, the average crystal sizes of pure TiO2, Ag−S/TiO2, and Ag−S/PEG/TiO2 were 34, 21, and 19 nm, respectively. The Brunauer−Emmett−Teller surface areas and pore volumes of the pure TiO2, Ag−S/TiO2, and Ag−S/PEG/TiO2 samples are

Figure 4. SEM images of the photocatalysts synthesized using the sol−gel method and calcined at 550 °C: (a) Ag−S/TiO2, (b) Ag−S/PEG/TiO2. Scale bars: 200 nm. 9580

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Figure 5. TEM images of (a) Ag−S/TiO2 and (b) Ag−S/PEG/TiO2. Scale bars: 20 nm.

Figure 6. EDX analysis of the modified TiO2 nanophotocatalysts obtained from two different regions for each sample: (a,b) Ag−S/TiO2, (c,d) Ag− S/PEG/TiO2. Panels b and d show the S and Ag peaks at higher magnification.

Ag−S/TiO2, and Ag−S/PEG/TiO2 were estimated using the equation43

Ag−S/TiO2 and Ag−S/PEG/TiO2 was associated with the doped elements. Dopants might create a level between the valence and conduction bands and narrow the band gap. This means that the small amount of the dopants might be incorporated into the lattice of TiO2 and the dopant’s level appeared between the valence band and the conduction band of titanium dioxide.42 The band-gap energies (Eg) of pure TiO2,

Eg =

hc λ

(2)

where Eg is the band-gap energy (eV), h is Planck’s constant, and c is the velocity of light (m/s). Moreover, λ is the 9581

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Table 1. Summary of Properties of the Synthesized Photocatalysts sample

crystallite sizea (nm)

surface areab (m2·g−1)

pure TiO2 Ag−S/TiO2 Ag−S/PEG/TiO2

34 21 19

33 53 68

pore volumec band-gap (cm3·g−1) energyd (eV) 0.10 0.13 0.19

3.05 2.39 2.23

a

Calculated by the Scherrer equation. bObtained by BET analysis. Obtained from the volume of N2 adsorbed at a relative pressure (p/p0) of 0.984. dCalculated using the DRS figure by eq 2.

c

Figure 8. Photoluminescence emission spectra of pure TiO2 and Ag− S/PEG/TiO 2 photocatalysts measured at room temperature (excitation wavelength = 300 nm).

Figure 7. DR spectra of pure and modified TiO2 samples prepared by the sol−gel method and calcined at 500 °C.

wavelength (nm) corresponding to the intersection point of the horizontal axis and the tangent line to the absorbance curve of the spectra (at the point where the slope of the absorbance curve suddenly varies). Utilizing the equation, the band-gap energies of pure TiO2, Ag−S/TiO2 and Ag−S/PEG/TiO2 were calculated as 3.05, 2.39, and 2.23 eV, respectively. These results clearly illustrate that the modified TiO2 samples can be used in visible-light irradiation for photocatalytic degradation. Photoluminescence (PL) emission spectroscopy was used to investigate the efficiency of charge-carrier trapping, migration, and transfer, which can provide information about the separation and recombination of photoinduced carriers, surface structure, and surface defects.44,45 Figure 8 shows the PL emission spectra of the pure TiO2 and Ag−S/PEG/TiO2 samples recorded in the wavelength range of 300−600 nm with an excitation wavelength of 300 nm. This figure illustrates the similar curve shapes for all samples in the range of 330−430 nm with different intensities. The higher PL intensity of the doped sample indicates a higher oxygen vacancy content associated with the dopant elements. This oxygen vacancy could create local levels within the valence and conduction bands and decrease the band gap of the modified TiO2 samples (see Figure 9).44 A comparison between the PL spectra of the modified TiO2 and pure TiO2 in the range of 430−550 nm indicates completely different patterns for the prepared photocatalysts. In contrast to pure TiO2, the doped TiO2 samples exhibit strong and broad PL signals. This signal confirms the presence of local levels within the valence and conduction bands. As a result, Ag−S/TiO2 and Ag−S/PEG/TiO2 can be excited under

Figure 9. Schematic diagram of the energy levels within the conduction and valence bands of the modified photocatalysts.

visible-light irradiation. These results clearly confirm the DRS analysis and band-gap calculations for the modified samples. 3.1.4. FTIR Spectra. The FTIR spectra of the pure TiO2, Ag−S/TiO2, and Ag−S/PEG/TiO2 samples are presented in Figure 10. For all three samples, three absorption peaks located around 3430, 1630, and 475 cm−1 appeared. The peaks at 3430 and 1630 cm−1 can be assigned to the stretching and bending vibrations, respectively, of surface −OH groups.46−48 The peak at 475 cm−1 shows Ti−O stretching vibrations, which can be attributed to the formation of anatase TiO2 nanoparticles.46,47 Also, there is no peak around 608 cm−1 corresponding to rutile TiO2.47 This result confirms the XRD pattern, which indicated only the anatase crystal phase for all samples. Compared with Ag−S/TiO 2 , Ag−S/PEG/TiO 2 exhibited peaks around 1465 cm−1 indicating −CH2 bending vibrations, due to the low amount of unburned PEG during calcination as confirmed in the EDX analysis.49 In contrast, the spectra of the modified photocatalysts (Ag−S/TiO2 and Ag- S/PEG/TiO2) do not show any bands corresponding to silver and sulfur oxide. This might be because the contents of incorporated silver and sulfur were very low (Ag, 0.8%; S, 1% molar ratio); therefore, the 9582

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intensities of the bands related to Ag−O−Ti and S−O−Ti were also very low. A shift of Ti−O peaks to higher wavenumbers occurred for Ag−S/TiO2 and Ag−S/PEG/TiO2 photocatalysts. This shift could be due to the size decrease of the nanoparticles, which occurred because of the addition of PEG.50 3.1.5. XPS Results. The XP spectra of Ag−S/PEG/TiO2 are presented in Figure 11. The C 1s peak at 289.5 eV was used for calibration. The XPS peaks show that Ag−S/PEG/TiO2 powder contained Ti, O, Ag, S, and C, in accordance with the EDX analysis. Several previous studies reported that, if thiourea were used as a sulfur precursor, the substitution of Ti4+ by S6+ and, consequently, the doping of TiO2 with S would be highly likely.51−53 From Figure 11b, the binding energy of S 2p was observed to be at 168 eV, which should be assigned to S6+. Therefore, it can be concluded that the lattice titanium sites of TiO2 were substituted by S6+ and formed a new energy band structure. Figure 11c shows the XP spectrum of Ag 3d, which consists of two major peaks at 373.2 and 367.1 eV, corresponding to the binding energies of Ag 3d3/2 and Ag 3d5/2, respectively,

Figure 10. FTIR spectra of (a) pure TiO2, (b) Ag−S/TiO2, and (c) Ag−S/PEG/TiO2 photocatalysts prepared by the sol−gel method and calcined at 550 °C.

Figure 11. XP spectra of Ag−S/PEG/TiO2 photocatalyst: (a) overall, (b) S 2p, (c) Ag 2p, and (d) Ti 2p. 9583

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TiO2, the modified photocatalysts showed significant increases in the AO7 and MB photodegradation rates, as illustrated in Figure 12. According to this figure, the degradation efficiencies of AO7 and MB using Ag−S/TiO2 were about 4 and 4.8 times greater, respectively, than the degradation efficiencies of these two dyes using pure TiO2. This behavior could be explained by the effect of Ag and S as dopants. Ag in the photocatalyst can operate as electron traps, increasing the electron−hole separation and the subsequent transfer of the trapped electron to the adsorbed O2 acting as an electron acceptor.21,57 Moreover, doping of sulfur could help to narrow the band gap, decreasing the crystallite size and controlling the crystallization.58,59 In addition, according to the DRS analysis, the absorption edge of TiO2 doped with silver and sulfur moved strongly to longer wavelengths in comparison with that of pure TiO2 and made excitation by LED visible-light irradiation sensible. Furthermore, as a key result, addition of PEG to Ag−S/TiO2 caused a 28.82% increase in AO7 degradation and a 24.24% increase in MB degradation in comparison with the efficient Ag−S/TiO2 catalyst. Although the two modified samples had almost equal particle sizes, this difference in photocatalytic activity can be explained in terms of the larger surface area of Ag−S/PEG/TiO2 photocatalyst, as determined by the BET analysis.60,61 Furthermore, addition of PEG might cause a decrease of the band-gap energy, which led to an enhancement of the photocatalytic activity.62

and may indicate silver incorporation into the TiO2 crystal matrix.54,55 The effects of Ag and S doping could be studied in more depth by investigating the Ti 2p XP spectrum (Figure 11d). The binding energies of Ti 2p1/2 and Ti 2p3/2 were found to be 464.7 and 458.5 eV, respectively, or shifted to higher binding energies in comparison with those of pure TiO2.56 These shifts are due to the substitution of Ti4+ by S6+, as well as the decrease of the electron density around the Ti4+ ions when TiO2 is doped with Ag+ ions. This result confirms the doping of silver and sulfur in the TiO2 nanoparticles. 3.2. Photocatalytic Activity for Dye Degradation. To investigate the photocatalytic activities of the synthesized materials, the photodegradation of acid orange 7 (AO7) and methylene blue (MB) using low-voltage LEDs as a visible-light source was performed at room temperature. The degradation efficiencies are summarized in Table 2. Compared to pure Table 2. Summary of Photodegradation Efficiencies of AO7 and MB under Visible-Light Irradiation sample

AO7 degradation (%)

MB degradation (%)

pure TiO2 Ag−S/TiO2 Ag−S/PEG/TiO2

15.98 62.37 91.19

11.16 53.27 77.51

4. CONCLUSIONS In this article, with the aim of approaching a highly active photocatalyst under visible light, PEG was added to the efficient Ag−S/TiO2 photocatalyst for the first time. In this way, a new photocatalyst (Ag−S/PEG/TiO2) was synthesized, and the effects of PEG addition to Ag−S/TiO2 on the photocatalyst properties and activity were investigated. In addition to Ag− S/PEG/TiO2, Ag−S/TiO2 and pure TiO2 were prepared to compare their properties and activities. XRD analysis indicated that all samples were made up of the homogeneous anatase crystalline phase and also that doping of Ag and S caused a decrease of the crystallite size. BET measurements showed a higher surface area and pore volume for Ag−S/PEG/TiO2 than for Ag−S/TiO2, due to the burning of PEG during calcination. Moreover, SEM analysis confirmed the porous and rougher surface of Ag−S/PEG/TiO2. DR and PL emission spectra confirmed the presence of local levels within the valence and conduction bands that could decrease the band-gap energy of the doped photocatalyst. The photodegradation results illustrated that the degradation efficiencies of AO7 and MB using Ag−S/TiO2 were about 4 and 4.8 times greater, respectively, than the degradation efficiencies of these two dyes using pure TiO2. Furthermore, in comparison to the Ag−S/TiO2 photocatalyst, the Ag−S/PEG/TiO2 photocatalytic degradation efficiencies of AO7 and MB increased by 28.82% and 24.24%, respectively, under LED visible-light irradiation, because of the effect of PEG on the modification of the Ag−S/TiO2 structure. All in all, the characterization analysis and degradation tests lead to the conclusion that Ag−S/PEG/ TiO2 is a new highly active photocatalyst for photodegradation under visible light. Moreover, the utilization of PEG was confirmed to be an efficient technique for improving photocatalytic performance and could be an important method for the future modification of photocatalysts.

Figure 12. Photocatalytic degradations of (a) AO7 and (b) MB under visible-light irradiation for 2 h. 9584

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efficient solar driven photocatalysis. J. Solid State Chem. 2012, 189, 117. (19) Lee, M. S.; Hong, S.-S.; Mohseni, M. Synthesis of Nanosized TiO2−Ag Particles by One-Step Process Using the Sol−Gel Method and Their Photocatalytic Activity. J. Mol. Catal. A: Chem. 2005, 242, 135. (20) Han, C.; Pelaez, M.; Likodimos, V.; Kontos, A. G.; Falaras, P.; O’Shea, K.; Dionysiou, D. D. Innovative visible light-activated sulfur doped TiO2 films for water treatment. Appl. Catal. B: Environ. 2011, 107, 77. (21) Sobana, N.; Muruganadham, M.; Swaminathan, M. Nano-Ag particles doped TiO2 for efficient photodegradation of Direct azo dyes. J. Mol. Catal. A: Chem. 2006, 258, 124. (22) Gunawan, C.; Teoh, W. Y.; Marquis, C. P.; Lifia, J.; Amal, R. Reversible Antimicrobial Photoswitching in Nanosilver. Small 2009, 5, 341. (23) Bayati, M. R.; Moshfegh, A. Z.; Golestani-Fard, F. On the photocatalytic activity of the sulfur doped titania nano-porous films derived via micro-arc oxidation. Appl. Catal. A: Gen. 2010, 389, 60. (24) Niu, Y.; Xing, M.; Zhang, J.; Tian, B. Visible light activated sulfur and iron co-doped TiO2 photocatalyst for the photocatalytic degradation of phenol. Catal. Today 2013, 201, 159. (25) Hamal, D. B.; Klabunde, K. J. Synthesis, characterization, and visible light activity of new nanoparticle photocatalysts based on silver, carbon, and sulfur-doped TiO2. J. Colloid Interface Sci. 2007, 311, 514. (26) Matsuda, A.; Kotani, Y.; Kogure, T.; Tatsumisago, M.; Minami, T. Transparent Anatase Nanocomposite Films by the Sol−Gel Process at Low Temperatures. J. Am. Ceram. Soc. 2000, 83, 229. (27) Liau, L.C.-K.; Chang, H.; Yang, T.C.-K.; Huang, C.-L. Effect of poly(ethylene glycol) additives on the photocatalytic activity of TiO2 films prepared by sol−gel processing and low temperature treatments. J. Chin. Inst. Chem. Eng. 2008, 39, 237. (28) Luo, S. X.; Wang, F. M.; Shi, Z. S.; Xin, F. Preparation and photocatalytic activity of Zr doped TiO2. Mater. Res. Innov. 2009, 13, 64. (29) Chang, H.; Jo, E.-H.; Jang, H. D.; Kim, T.-O. Synthesis of PEGmodified TiO2−InVO4 nanoparticles via combustion method and photocatalytic degradation of methylene blue. Mater. Lett. 2013, 92, 202. (30) Shamaila, S.; Sajjad, A. K. L.; Chen, F.; Zhang, J. Mesoporous titania with high crystallinity during synthesis by dual template system as an efficient photocatalyst. Catal. Today 2011, 175, 568. (31) Sun, W.; Zhang, S.; Liu, Z.; Wang, C.; Mao, Z. Studies on the enhanced photocatalytic hydrogen evolution over Pt/PEG-modified TiO2 photocatalysts. Int. J. Hydrogen Energy 2008, 33, 1112. (32) Jaimy, K. B.; Ghosh, S.; Sankar, S.; Warrier, K. G. K. An aqueous sol−gel synthesis of chromium(III) doped mesoporous titanium dioxide for visible light photocatalysis. Mater. Res. Bull. 2011, 46, 914. (33) Lv, K.; Zuo, H.; Sun, J.; Deng, K.; Liu, S.; Li, X.; Wang, D. (Bi, C and N) codoped TiO2 nanoparticles. J. Hazard. Mater. 2009, 161, 396. (34) Guo, H.; Lin, K.; Zheng, Z.; Xiao, F.; Li, S. Sulfanilic acidmodified P25 TiO2 nanoparticles with improved photocatalytic degradation on Congo red under visible light. Dyes Pigm. 2012, 92, 1278. (35) Stylidi, M.; Kondarides, D. I.; Verykios, X. E. Visible lightinduced photocatalytic degradation of Acid Orange 7 in aqueous TiO2 suspensions. Appl. Catal. B: Environ. 2004, 47, 189. (36) Wu, Q.; Yang, C.-C.; Krol, R. A dopant-mediated recombination mechanism in Fe-doped TiO2 nanoparticles for the photocatalytic decomposition of nitric oxide. Catal. Today 2013, 225, 96. (37) Inturi, S. N. R.; Boningari, T.; Suidan, M.; Smirniotis, P. G. Visible-light-induced photodegradation of gas phase acetonitrile using aerosol-made transition metal (V, Cr, Fe, Co, Mn, Mo, Ni, Cu, Y, Ce, and Zr) doped TiO2. Appl. Catal. B 2014, 144, 333. (38) Xiao, J.; Peng, T.; Li, R.; Peng, Z.; Yan, C. Preparation, phase transformation and photocatalytic activities of cerium-doped mesoporous titania nanoparticles. J. Solid State Chem. 2006, 179, 1161.

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*Tel.: +98-021-66164104. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the Institute for Biotechnology and Environment (IBE) at Sharif University of Technology for financial support.

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REFERENCES

(1) Hoffmann, M. R.; Martin, M. T.; Choi, W. Y.; Bahnemann, D. W. Environmental Applications of Semiconductor Photocatalysis. Chem. Rev. 1995, 95, 69. (2) Fujishima, A.; Rao, T. N.; Tryk, D. A. Titanium dioxide photocatalysis. J. Photochem. Photobiol. C 2000, 1, 1. (3) Carp, O.; Huisman, C. L.; Reller, A. Photoinduced reactivity of titanium dioxide. Prog. Solid State Chem. 2004, 32, 33. (4) Fujishima, A.; Zhang, X.; Tryk, D. A. TiO2 photocatalysis and related surface phenomena. Surf. Sci. Rep. 2008, 63, 515. (5) Gaya, U. I.; Abdullah, A. H. Heterogeneous photocatalytic degradation of organic contaminants over titanium dioxide: A review of fundamentals progress and problems. J. Photochem. Photobiol. C 2008, 9, 1. (6) Diebold, U. The surface science of titanium dioxide. Surf. Sci. Rep. 2003, 48, 53. (7) Irie, H.; Watanabe, Y.; Hashimoto, K. Carbon-Doped Anatase TiO2 Powders as a Visible-Light Sensitive Photocatalyst. Chem. Lett. 2003, 32, 772. (8) Morikawa, T.; Asahi, R.; Ohwaki, T.; Aoki, K.; Taga, Y. Band-Gap Narrowing of Titanium Dioxide by Nitrogen Doping. Jpn. J. Appl. Phys. 2001, 40, 561. (9) Huang, D.-G.; Liao, S.-J.; Liu, J.-M.; Dang, Z.; Petrik, L. Preparation of visible-light responsive N−F-codoped TiO2 photocatalyst by a sol−gel-solvothermal method. J. Photochem. Photobiol. A 2006, 184, 282. (10) Shu, X.; He, J.; Chen, D. Visible-Light-Induced Photocatalyst Based on Nickel Titanate Nanoparticles. Ind. Eng. Chem. Res. 2008, 47, 4750. (11) Moradi, S.; Vossoughi, M.; Feilizadeh, M.; Zakeri, S. M. E.; Mohammadi, M. M.; Rashtchian, D.; Yoosefi Booshehri, A. Photocatalytic degradation of dibenzothiophene using La/PEG-modified TiO2 under visible light irradiation. Res. Chem. Intermed., published online Jan 14, 2014, 10.1007/s11164-013-1519-z. (12) Kabra, K.; Chaudhary, R.; Sawhney, R. L. Treatment of Hazardous Organic and Inorganic Compounds through AqueousPhase Photocatalysis: A Review. Ind. Eng. Chem. Res. 2004, 43, 7683. (13) Daghrir, R.; Drogui, P.; Robert, D. Modified TiO2 for Environmental Photocatalytic Applications: A Review. Ind. Eng. Chem. Res. 2013, 52, 3581. (14) Burda, C.; Lou, Y. B.; Chen, X. B.; Samia, C. S.; Stout, J.; Gole, J. L. Enhanced Nitrogen Doping in TiO2 Nanoparticles. Nano Lett. 2003, 3, 1049. (15) Li, L.; Zhuang, H.; Bu, D. Characterization and activity of visible-light-driven TiO2 photocatalyst codoped with lanthanum and iodine. Appl. Surf. Sci. 2011, 257, 9221. (16) Cao, Y.; Yu, Y.; Zhang, P.; Zhang, L.; He, T.; Cao, Y. An enhanced visible-light photocatalytic activity of TiO2 by nitrogen and nickel−chlorine modification. Sep. Purif. Technol. 2013, 104, 256. (17) Wang, W.; Zhang, L.; An, T.; Li, G.; Yip, H.-Y.; Wong, P.-K. Comparative study of visible-light-driven photocatalytic mechanisms of dye decolorization and bacterial disinfection by B−Ni-codoped TiO2 microspheres: The role of different reactive species. Appl. Catal. B: Environ. 2011, 108−109, 108. (18) Gong, D.; Ho, W. C. J.; Tang, Y.; Tay, Q.; Lai, Y.; Highfield, J. G.; Chen, Z. Silver decorated titanate/titania nanostructures for 9585

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(60) Fresno, F.; Coronado, J. M.; Tudela, D.; Soria, J. Influence of the structural characteristics of Ti1−xSnxO2 nanoparticles on their photocatalytic activity for the elimination of methylcyclohexane vapors. Appl. Catal. B: Environ. 2005, 55, 159. (61) Anpo, R.; Shima, T.; Kodama, S.; Kubokawa, Y. Photocatalytic hydrogenation of propyne with water on small-particle titania: Size quantization effects and reaction intermediates. J. Phys. Chem. 1987, 91, 4305. (62) Yin, S.; Sato, T. Photocatalytic activity of platinum loaded fibrous titania prepared by solvothermal process. J. Photochem. Photobiol. A 2005, 169, 89.

(39) Khan, R.; Kim, S. W.; Kim, T.-J.; Nam, C.-M. Comparative study of the photocatalytic performance of boron−iron co-doped and boron-doped TiO2 nanoparticles. Mater. Chem. Phys. 2008, 112, 167. (40) Atsunori, M.; Yoshihiro, M.; Shinya, K.; Toshio, T.; Noboru, T.; Tsutomu, M. Physical and Chemical Properties of Titania−Silica Films Derived from Poly(ethylene glycol)-Containing Gels. J. Am. Ceram. 1990, 73, 2217. (41) Yu, J.; Zhao, X.; Zhao, Q. Photocatalytic activity of nanometer TiO2 thin films prepared by the sol−gel method. Mater. Chem. Phys. 2001, 69, 25. (42) Dong, Y. L.; Won, J. L.; Jae Sung, S.; Jung, H. K.; Yang, S. K. Electronic surface state of TiO2 electrode doped with transition metals, studied with cluster model and DV-X method. Comput. Mater. Sci. 2004, 30, 383. (43) Deng, F.; Li, Y.; Luo, X.; Yang, L.; Tu, X. Preparation of conductive polypyrrole/TiO2 nanocomposite via surface molecular imprinting technique and its photocatalytic activity under simulated solar light irradiation. Colloids Surf. A: Physicochem. Eng. Aspects 2012, 395, 183. (44) Liqiang, J.; Xiaojun, S.; Baifu, X.; Baiqi, W.; Weimin, C.; Honggang, F. The preparation and characterization of La doped TiO2 nanoparticles and their photocatalytic activity. J. Solid State Chem. 2004, 177, 3375. (45) Cong, Y.; Zhang, J.; Chen, F.; Anpo, M. Synthesis and Characterization of Nitrogen-Doped TiO2 Nanophotocatalyst with High Visible Light Activity. J. Phys. Chem. C 2007, 111, 6976. (46) Bezrodna, T.; Puchkovska, G.; Shymanovska, V.; Baran, J.; Ratajczak, H. IR analysis of H- bonded H2O on the pure TiO2 surface. J. Mol. Struct. 2004, 700, 175. (47) Erkov, V. G.; Devyatova, S. F.; Molodstova, E. L.; Malsteva, T. V.; Yanovskii, U. A. Si− TiO2 interface evolution at prolonged annealing in low vacuum or N2O ambient. Appl. Surf. Sci. 2000, 166, 51. (48) Yates, D. J. C. Infrared studies of the surface hydroxyl groups on titanium dioxide, and of the chemisorption of carbon monoxide and carbon dioxide. J. Phys. Chem. 1961, 65, 746. (49) Zhang, J.-Y.; Boyd, I. W.; O’Sullivan, B. J.; Hurley, P. K.; Kelly, P. V.; Senateur, J.-P. Nanocrystalline TiO2 films studied by optical, XRD and FTIR spectroscopy. J. Non-Cryst. Solids 2002, 303, 134. (50) Ghasemi, S.; Rahimnejad, S.; Rahman Setayesh, S.; Rohani, S.; Gholami, M. R. Transition metal ions effect on the properties and photocatalytic activity of nanocrystalline TiO2 prepared in an ionic liquid. J. Hazard. Mater. 2009, 172, 1573. (51) Ohno, T.; Akiyoshi, M.; Umebayashi, T.; Asai, K.; Mitsui, T.; Matsumura, M. Preparation of S-doped TiO2 photocatalysts and their photocatalytic activities. Appl. Catal. A 2004, 265, 115. (52) Ohno, T.; Mitsui, T.; Matsumura, M. Photocatalytic activity of S-doped TiO2 photocatalyst under visible light. Chem. Lett. 2003, 32, 364. (53) Ohno, T. Preparation of visible light active S-doped TiO2 photocatalysts and their photocatalytic activities. Water Sci. Technol. 2004, 49, 159. (54) Zhang, L.; Yu, J. C.; Yip, H.; Li, Q.; Kwong, K.; Xu, A.; Wong, P. Langmuir 2003, 19, 10372. (55) Wagner, C. D.; Riggs, W. M.; Davis, L. E.; Moulder, J. F.; Muilenberg, G. E. Handbook of X-ray Photoelectron Spectroscopy; Perkin-Elmer Corporation: Eden Prairie, MN, 1979; p 112. (56) Kim, K. S.; Winograd, N. X-ray photoelectron spectroscopic studies of nickel−oxygen surfaces using oxygen and argon ionbombardment. Surf. Sci. 1974, 43, 625. (57) Xin, B.; Ren, Z.; Hu, H.; Zhang, X.; Dong, C.; Shi, K.; Jing, L.; Fu, H. Photocatalytic activity and interfacial carrier transfer of Ag− TiO2 nanoparticle films. Appl. Surf. Sci. 2005, 252, 2050. (58) Liu, S.; Chen, X. A visible light response TiO2 photocatalyst realized by cationic S-doping and its application for phenol degradation. J. Hazard. Mater. 2008, 152, 48. (59) Tian, H.; Ma, J.; Li, K.; Li, J. Hydrothermal synthesis of S-doped TiO2 nanoparticles and their photocatalytic ability for degradation of methyl orange. Ceram. Int. 2009, 35, 1289. 9586

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