Supercritical Preparation of a Highly Active S-Doped TiO2

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Environ. Sci. Technol. 2007, 41, 4410-4414

Supercritical Preparation of a Highly Active S-Doped TiO2 Photocatalyst for Methylene Blue Mineralization HEXING LI,* XINYU ZHANG, YUNING HUO, AND JIAN ZHU Department of Chemistry, Shanghai Normal University, Shanghai 200234, P. R. China

Sulfur-modification of the TiO2 was achieved by treating the TiO2 precursor (xerogel) under supercritical conditions in CS2/ethanol fluid. Nuclear magnetic resonance, X-ray photoelectron spectroscopy and Fourier transform infrared spectra demonstrated that the TiO2 was modified by the S-species incorporated into the TiO2 network by forming S-Ti-O bonds rather than by the adsorbed CS2. During liquidphase photocatalytic degradation of methylene blue (MB) under visible light irradiation (>420 nm), the asprepared S-doped TiO2 exhibited much higher activity than the undoped TiO2 obtained via either supercritical treatment or direct calcinations and even the N-doped TiO2 obtained via supercritical treatment. The promoting effects of both the supercritical treatment and S-modification on the photocatalytic activity were discussed by considering the high surface, large porous channels, well crystallized anatase phase, excellent thermal stability, and strong absorbance for visible lights, corresponding to the high quantum efficiency. The maximum activity was obtained at 1.8% S/Ti molar ratio, nearly 8 times higher than that of the commercially available P25 TiO2.

1. Introduction Photocatalysis has caused much attention owing to its potential applications in purifying water and air by thoroughly decomposing organic compounds. The TiO2 has been most frequently employed as the photocatalyst owing to its cheapness, nontoxicity, and structural stability (1-4). Many attempts have been made to improve the photocatalytic efficiency and/or to enhance the absorbance for visible lights by either designing the TiO2 with unusual porous structures or by modifying the TiO2 with various dopants (5-11). It is well-known that both the N- and the S-modifications may result in the strong absorbance for visible lights (10, 12-14). Up to now, nearly all the N- and S-doped TiO2 are prepared by treating the precursors in NH3 or H2S atmosphere at high temperature to ensure the entrance of the N- or S-dopants into the TiO2 network. Besides the energy waste, the hightemperature treatment usually causes the low surface area due to agglomeration and the collapse of pore structure. As reported previously, the TiO2 prepared under supercritical conditions exhibited high surface area since the original porous structure in the gel could be preserved owing to the lack of surface tension (13, 15). Meanwhile, the high pressure and excellent solubility under supercritical conditions also ensure the strong incorporation of various dopants into the TiO2 network (16, 17), even at low temperature. In this paper, * Corresponding author e-mail: [email protected]. 4410

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we report a novel S-doped TiO2 prepared by treating the TiO2 precursor under supercritical conditions with CS2/ ethanol as a fluid. Its activity was evaluated and compared with other TiO2-based catalysts. The correlation of the catalytic performance to the structural properties and the promoting effect of the S-modifications are discussed briefly based on systematic characterizations.

2. Experimental Section 2.1. Catalyst Preparation. The S-doped TiO2 samples were prepared according to the following procedure. At 313 K, a solution containing 10 mL ethanol (EtOH) and 2.5 mL dilute HNO3 aqueous solution (1:5, volume ratio between HNO3 and H2O) was added dropwise into a solution containing 40 mL EtOH and 10 mL Ti(O-C4H9)4 within 20 min under vigorous stirring. After being aged for 48 h at 313 K, the asprepared TiO2 xerogel was transferred into a 500 mL autoclave containing 250 mL CS2/EtOH solution and was heated slowly to 553 K at the speed of 4 K/min. After being treated under supercritical conditions (553 K and 15 MPa) for 2 h, the vapor inside was released and the system was allowed to cool slowly to room temperature in the N2 flow. The solid was then further calcined at high temperature for 8 h to remove the residual organic compounds resulting from the hydrolysis of the Ti(O-C4H9)4. The as-prepared samples were denoted as X% TiO2-S(SC), where SC refers to the sample obtained via supercritical treatment and X% refers to the molar ratio of S/Ti. For comparison, the TiO2(DC) was also prepared by direct calcination instead of supercritical treatment to remove organic species. P25 TiO2 was commercially available and used without further treatment. 2.2 Characterization. X-ray diffraction (XRD) patterns were collected on Rigacu Dmax-3C (Cu KR radiation). Selected area electronic diffraction (SAED) and transmission electronic micrograph (TEM) were recorded on a JEM-2010. Raman spectra, UV-visible diffuse reflectance spectra (DRS), photoluminescence spectra (PLS), and solid-state 13C MAS NMR spectra were conducted on a Dilor Super LabRam II, a MC-2530, a Varian Cary-Eclipse 500, and a Bruker DRX400 NMR, respectively. The N2 sorption isotherms were obtained on a NOVA 4000 at 77 K, from which the surface area (SBET), pore volume (VP), and average pore diameter (dP) were calculated by using BJH method (see the Supporting Information). The X-ray photoelectron spectroscopy (XPS) analysis was performed on a Perkin-Elmer PHI 5000C, and the S/Ti molar ratio was determined by using 0.54 and 1.2 as the PHI sensitivity factors (see the Supporting Information) corresponding to XPS peaks in the S2P3/2 and Ti2P3/2 levels (18). All the binding energies were calibrated by using the contaminant carbon (C1S ) 284.6 eV) as a reference. 2.3. Activity test. The liquid-phase photocatalytic degradation of methylene blue (MB) was carried out at 303 K in an 80 mL self-designed quartz photochemical reactor containing 0.050 g catalyst and 50 mL 0.01 g/L MB aqueous solution. After reaching adsorption equilibrium, the photocatalytic reaction was initiated by irradiating the system with three 150 W xenon lamps (characteristic wavelength ) 420 nm) located at 30 cm away from the reaction solution. All the UV lights with the wavelength less than 420 nm were removed by a glass filter. Each run of reactions was lasted for 3 h, and the leftover MB was analyzed by a UV spectrophotometer (UV 7504/PC) at its characteristic wavelength (λ ) 665 nm) to determine the degradation yield (19). Preliminary tests demonstrated a good linear relationship between the light absorbance and the MB concentration. Only less than 7% MB decomposed after reaction for 3 h 10.1021/es062680x CCC: $37.00

 2007 American Chemical Society Published on Web 05/12/2007

FIGURE 1. XPS spectra of (a) TiO2(SC) and (b) 1.8% TiO2-S(SC) calcined at 673 K

TABLE 1. Structural Parameters and Photocatalytic Activity of Different Samplesa sample TiO2(DC) TiO2(SC) P25 TiO2 0.50 %TiO2-S(SC) 1.0% TiO2-S(SC) 1.4% TiO2-S(SC) 1.8% TiO2-S(SC) 2.6% TiO2-S(SC) 1.7% TiO2-N(SC)

[CS2] SBET Vp dp band gap degradation (eV) (%) (M) (m2/g) (cm3/g) (nm) 0 0 0 0.68 1.4 2.0 2.7 3.4 /

21 68 45 88 91 97 104 97 90

0.031 0.37 0.25 0.38 0.38 0.40 0.42 0.43 0.59

5.7 20 20 23 23 24 25 27 17

3.2 3.2 3.1 2.8 2.7 2.7 2.6 2.7 2.90

7.0 11.2 10.0 74.5 78.0 83.3 88.6 82.7 64.5

a Reaction conditions: 0.050 g catalyst after being calcined at 673 K for 8 h, 50 mL 0.01 g/L MB aqueous solution, three 150 W xenon lamps (characteristic wavelength >420 nm) located at 30 cm away from the reaction solution, T ) 303 K, stirring rate is 1000 rpm, reaction time is 3 h.

under the same conditions in the absence of either the photocatalyst or the light irradiation and, thus, could be neglected in comparison with that in the presence of both the catalyst and the light irradiation. The reproducibility of the results was checked by repeating the results at least three times and was found to be within acceptable limits ((5%).

3. Results and Discussion The XPS spectra (Figure 1) demonstrated the presence of S species in the TiO2-S(SC) samples, corresponding to the binding energy (BE) around 163.2 eV in S2P3/2 level. The 13C CPMAS NMR spectra (Figure S1, Supporting Information) demonstrated that the CS2-adsorbed TiO2(SC) displayed a strong signal around 193.2 ppm indicative of the carbon species. However, no significant signal indicative of carbon species was observed in the TiO2-S(SC) sample. Thus, it was reasonable to conclude that the TiO2 was modified by only the sulfur resulted from the decomposition of CS2 rather than by the adsorbed CS2. According to the calculation, by using 0.54 and 1.2 as the PHI sensitivity factors corresponding to XPS peaks in the S2P3/2 and Ti2P3/2 levels, the S-content in the TiO2-S(SC) increased with the CS2 concentration in CS2/ EtOH fluid (see Table 1). The FTIR spectra (Figure S2, Supporting Information) revealed that, besides the absorbance peaks from the TiO2, the TiO2-S(SC) displayed two additional absorbance peaks at 1138 and 1048 cm-1 indicative of the S-Ti bond (20). Meanwhile, the BE of the S in the TiO2-S(SC) was 4.8 eV lower than that in the sulfur oxides (168.0 eV) but 3.2 eV higher than that in the TiS2 (160.0 eV) (21-23). Furthermore, the BE values of the Ti and the O in

FIGURE 2. XRD patterns of (a) TiO2(DC), (b) TiO2(SC), and (c) 1.8% TiO2-S(SC) samples calcined at 673 K. The inset is the XRD patterns of the 1.8% TiO2-S(SC) sample calcined at elevated temperatures from 623 to 1173 K. the TiO2-S(SC) shifted negatively by 0.2 and 0.5 eV in comparison with those in the undoped TiO2(SC). These results demonstrated that the S-dopants in the TiO2-S(SC) were present in the S-Ti-O bond, in which partial electrons transferred from the S to the Ti and further to the O atom due to the higher electronegativity of oxygen than that of sulfur, making the S electron-deficient and making both the Ti and the O electron-enriched (Figure S3, Supporting Information). As shown in Figure 2, the XRD patterns demonstrated that all the TiO2(DC), TiO2(SC) and TiO2-S(SC) samples calcined at 673 K were present in unique anatase phase. The TiO2(SC) exhibited higher crystallization degree than the TiO2(DC), obviously owing to the high pressure and supersaturation which allowed the nucleation and the growth of the anatase crystal similar to those observed in solvothermal synthesis (13). The S-modification could further enhance the crystallization degree of the anatase. The attached XRD patterns revealed that the increase of the calcination temperature from 623 to 1123 K caused only a slight enhancement of the crystallization degree of the TiO2-S(SC), suggesting that the TiO2-S(SC) after being calcined at 623 K was already present in well-crystallized anatase. Further increase of the calcination temperature to 1173 K resulted in the transformation from anatase to rutile phase. Taking into account that the TiO2(DC) and the TiO2(SC) samples displayed rutile phase at 673 and 973 K (15, 16), it was clear that both the supercritical treatment and the S-modification could enhance VOL. 41, NO. 12, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. TEM morphologies of (a) TiO2(DC), (b) TiO2(SC), and (c) 1.8% TiO2-S(SC) samples calcined at 673 K. The attached are the indexed SAED images. the thermal stability of the TiO2 against the structural transformation. The DSC curve (Figure S4, Supporting Information) further confirmed the high thermal stability since only one exothermic peak appeared around 1252 K. Increase of the S-content from 0.5% to 2.6% caused an increase in the temperature of the anatase-rutile phase transformation, which could be attributed to the enhancement of the straining force in the TiO2-S(SC) resulting from structural distortion owing to the formation of more S-Ti-O bonds and the porous structure. Besides, the increase of both the particle size and the crystallization degree of anatase caused by S-modification could also enhance the phase transformation temperature. Figure 3 shows the TEM morphologies of the TiO2(DC), TiO2(SC), and TiO2-S(SC) samples after being calcined at 673 K for 8 h. From TiO2(DC) to TiO2(SC) to TiO2-S(SC), uniform cubic particles with increasing size were observed which could be understood by considering the model of the anatase crystal cell (Figure S5, Supporting Information). The high crystallization degree of the anatase could be confirmed by both the attached SAED images and the aforementioned XRD patterns (Figure 2). Table 1 summarizes the surface area (SBET), pore volume (Vp), and average pore diameter (dp) calculated by using the BJH method from the N2 adsorption-desorption isotherms at 77 K. Although the particle size increased, the TiO2(SC) still exhibited much higher SBET than the TiO2(DC) and P25 TiO2, obviously owing to the reservation of the porous structure inside the particle under supercritical conditions (see the Vp and the dp values). The S-modification further increased the Vp and the dp since the O atom in the O-Ti-O network was replaced by the S atom with relatively larger atomic radius, corresponding to the further increase of SBET though the particle size increased. At very high S-content (S/Ti molar ratio ) 2.6%), although both the Vp and the dp still increased, the SBET decreased slightly, possibly due to extremely large particle size. The Raman spectra (Figure S6, Supporting Information) demonstrated that all the TiO2(DC), TiO2(SC), and TiO2S(SC) samples were present in the unique anatase phase, corresponding to four characteristic peaks with the principal peak locating around 143 cm-1 (24), which was in good accordance with the XRD patterns. The absence of the weak peak around 140 cm-1 demonstrated that no rutile phase was present in the as-prepared TiO2-S(SC) sample. The position of the principal peak shifted toward higher wavenumber in the order of TiO2(DC), TiO2(SC), and TiO2-S(SC), indicating that both the supercritical treatment and the S-modification might increase the number of surface oxygen vacancies and/or defects in the TiO2-S(SC) sample (24). From Figure 4, one could see that all the TiO2(DC), TiO2(SC), and TiO2-S(SC) samples displayed two kinds of PLS peaks. The peak around 382 nm could be attributed to an emission peak from band edge free excitation, mainly corresponding to the 4412

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FIGURE 4. PLS spectra of (a) TiO2(DC), (b) TiO2(SC), and (c) 1.8% TiO2-S(SC) samples calcined at 673 K excited by 280 nm.

FIGURE 5. UV-vis DRS spectra of (a) TiO2(DC), (b) TiO2(SC), (c) 1.4% TiO2-S(SC), (d) 1.8% TiO2-S(SC), and (e) 2.6% TiO2-S(SC) samples calcined at 673 K. number of surface oxygen vacancies and/or surface defects (25). While, the PLS peak around 560 nm could be attributed to the light absorption coefficient, known as dual-frequency peak (26). From TiO2(DC) to TiO2(SC) to TiO2-S(SC), the intensity of the peak around 382 nm increased while the intensity of the peak around 560 nm decreased, indicating that the supercritical treatment and the S-modification resulted in the increase of both the number of the oxygen vacancies and/or defects in the TiO2 crystal and the ability for light absorbance. The UV-vis DRS spectra (Figure 5) revealed that the TiO2(SC) exhibited stronger light absorbance in the UV area (200∼300 nm) than the TiO2(DC) and the TiO2-S(SC) showed the strongest light absorbance. On one hand, the formation of well crystallized anatase might facilitate the transfer of photoelectrons from the bulk to the surface (9), which could reduce the probability of the recombination between the

FIGURE 6. Effect of the calcination temperature on the activity of 1.8% TiO2-S(SC). Reaction conditions: 0.050 g catalyst, 50 mL 0.01 g/L MB aqueous solution, three 150 W xenon lamps (characteristic wavelength >420 nm) located at 30 cm away from the reaction solution, T ) 303 K, stirring rate is 1000 rpm, reaction time is 3 h. photoinduced electrons and holes. On the other hand, the more oxygen vacancies and/or defects in the TiO2 crystal might capture photoelectrons and thus inhibit the recombination between the photoinduced electrons and holes (26, 27). Both the TiO2(DC) and the TiO2(SC) displayed no significant absorbance for the visible light due to the large band gap (3.2 eV). However, the TiO2-S(SC) exhibited remarkable light absorbance in visible region owing to the appearance of intermediate energy levels resulting from the formation of the S-Ti-O bonds, making the energy band gap narrower. The energy band gaps of different samples were calculated based on the optical absorption edge obtained from UV-vis DRS spectra by using the following formula (28, 29):

(Rhυ)n ) k(hυ - Eg) Where R is the absorption coefficient, k is the parameter that related to the effective masses associated with the valence and conduction bands, n is 1/2 for a direct transition, hυ is the absorption energy, and Eg is the band gap energy. The variation of (Rhυ)1/2 with hυ gives the extrapolated intercept corresponding to the Eg value. As shown in Table 1, the absorbance for visible lights increased with the S-content in the TiO2-S(SC) owing to a decrease in the energy band gaps since more S-Ti-O bonds were formed. Figure 6 shows the dependence of the activity of the TiO2S(SC) on the calcination temperature during liquid-phase photocatalytic degradation of MB. Only a slight increase in activity was observed when the calcination temperature increased from 623 to 673 K, since the TiO2-S(SC) was already present in well-crystallized anatase and the increase of calcination temperature exhibited very little improvement on the crystallization degree. Further increase in the calcination temperature caused a smooth decrease in the activity due to the decrease in SBET caused by the agglomeration of nanoparticles and the collapse of porous structure at high temperature. An abrupt activity decrease was observed when the calcination temperature reached 1173 K, obviously due to the transformation from the active anatase to the relative inactive rutile (9). Table 1 summarized the activities of different catalysts, from which the following results could be obtained. (1) The TiO2(SC) exhibited higher activity than the TiO2(DC), showing the promoting effect of the supercritical treatment. On one hand, it could be attributed to the high

surface area (SBET) and the large porous channels (dP) in the TiO2(SC) which was favorable for the diffusion and the adsorption of MB molecules. On the other hand, it could be attributed to the high crystallization degree of the anatase in the TiO2(SC) which might facilitate the transfer of photoelectrons from the bulk to the surface and thus, could decrease their recombination with the photogenerated holes, resulting in the high quantum efficiency of photocatalysis (9). (2) The TiO2-S(SC) exhibited higher activity than the TiO2(SC), showing the promoting effect of the S-modification which could be understood by considering the following factors: (i) The S-modification further increased the SBET and the dP. (ii) The S-modification further enhanced the crystallization degree of the anatase. (iii) The S-modification generated more oxygen vacancies and /or defects which could capture the photoinduced electrons and thus could effectively inhibit the recombination of the photoinduced electrons and holes (26, 27). (iv) More importantly, the S-modification resulted in a slight absorbance for visible lights and thus, the catalyst could be easily activated by visible lights. (v) According to the XPS spectra, partial electrons transferred from the S to the O in the TiO2-S(SC), making the S electrondeficient. The electron-deficient S could capture the photoelectrons and thus inhibited their recombination with the photoinduced holes, resulting in high quantum efficiency (26, 27). (vi) In addition, the S-modification also improved the thermal stability against the transformation from anatase to rutile, which was also favorable for the photocatalytic activity since rutile was less active than the anatase (9). (vii) The activity of the TiO2-S(SC) increased with the increase of the S-content owing to the enhancement of above promoting effects. The maximum activity was obtained on the 1.8% TiO2-S(SC) which was nearly 8 times higher than that obtained on the P25 TiO2, showing a good potential in practical application. Very high S-content (S/Ti molar ratio >2.6%) was harmful for the activity due to the decrease of the SBET since very large particles were formed. (3) The 1.8% TiO2-S(SC) exhibited much higher activity than the 1.7% TiO2N(SC) obtained by treating the TiO2 precursor under supercritical conditions with NH3/EtOH as a fluid (17). Besides the higher SBET and the larger dP, the stronger light absorbance in the visible area owing to the narrow band gaps in the TiO2-S(SC) than that in the TiO2N(SC) might be a principal cause accounting for the higher activity of the TiO2-S(SC) than that of the TiO2-N(SC).

Acknowledgments This work was supported by Science Foundation of Chinese Science and Technology (2005CCA01100) and Shanghai Government (T0402, 065412070, 06DZ013).

Supporting Information Available Description of the BJH method and the PHI factor, and six additional figures. This material is available free of charge via the Internet at http://pubs.acs.org.

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Received for review November 8, 2006. Revised manuscript received February 25, 2007. Accepted February 26, 2007. ES062680X