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Solar photocatalytic degradation of methylene blue in carbon-doped TiO2 nanoparticles suspension
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Solar Energy 82 (2008) 706–713 www.elsevier.com/locate/solener
Solar photocatalytic degradation of methylene blue in carbon-doped TiO2 nanoparticles suspension Qi Xiao *, Jiang Zhang, Chong Xiao, Zhichun Si, Xiaoke Tan School of Resources Processing and Bioengineering, Central South University, Changsha 410083, PR China Received 8 March 2007; received in revised form 21 August 2007; accepted 9 February 2008 Available online 6 March 2008 Communicated by: Associate Editor G. Calzaferri
Abstract Carbon-doped TiO2 nanoparticles were prepared by sol–gel auto-combustion method and characterized by X-ray diffraction (XRD), X-ray photoelectron spectra (XPS), Brunauer–Emmett–Teller method (BET), UV–vis diffuses reflectance spectroscopy (DRS). UV–vis diffuse reflectance spectra showed that carbon-doped TiO2 exhibited obvious absorption in the visible light range. The visible light photocatalytic activity of carbon-doped TiO2 was ascribed to the presence of oxygen vacancy state between the valence and the conduction bands because of the formation of Ti3+ species in the as-synthesized carbon-doped TiO2. The sample calcined at 873 K showed the highest photocatalytic activity under solar irradiation. The effects of photocatalyst concentration, initial concentration of methylene blue, and pH value in aqueous solution were also presented. Ó 2008 Elsevier Ltd. All rights reserved. Keywords: Carbon-doped TiO2 nanoparticles; Solar photocatalysis; Methylene blue
1. Introduction Heterogeneous photocatalysis was a promising technology for the removal of toxic organic and inorganic contaminates from water. However, the development of a practical photocatalytic system focused on the cost effectiveness of the process by the use of renewable solar energy source. Photocatalytic degradation of organic contaminants using solar irradiation could be highly economical compared with the processes using artificial UV irradiation, which required substantial electrical power input. Abundant solar energy could be utilized efficiently in the photocatalytic processes for the degradation of organic pollutants. Photocatalytic degradation of various organic and inorganic pollutants using solar energy has been reported (Neppolian et al., 2002; Sakthivel et al., 2003; Kuo and Ho, 2001).
*
Corresponding author. Tel.: +86 731 8830543; fax: +86 731 8879815. E-mail address: [email protected] (Q. Xiao).
0038-092X/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.solener.2008.02.006
TiO2 was the most widely used photocatalyst because of its good activity, chemical stability, commercial availability, and inexpensiveness. However, only a small UV fraction of solar light (3–5%) could be utilized because of the wide band gap of TiO2. Therefore, it was urgent to develop a particular photocatalyst sensitive to sunlight (Tang et al., 2004). Recently, doping TiO2 with nonmetal elements such as nitrogen (Asashi et al., 2001), carbon (Khan et al., 2002), sulfur (Umebayashi et al., 2002), iodine (Hong et al., 2005) noticeably improved the photocatalytic activity of TiO2 under visible light. Various synthetic routes have been studied for carbondoped TiO2 nanoparticles. Khan et al. (2002) synthesized chemically modified TiO2 by controlled combustion of Ti metal in a natural gas flame. Irie and coworkers (Irie et al., 2003) prepared carbon-doped anatase TiO2 nanoparticles by oxidative annealing of TiC under O2 flow at 873 K. Sakthivel and Kwasch (2003) reported the wet process synthesis of carbon-doped n-TiO2 by hydrolysis of TiCl4 with tetrabutylammonium hydroxide (C16H36NOH)
Q. Xiao et al. / Solar Energy 82 (2008) 706–713
followed by calcination of the precipitates. Especially, Nagaveni et al. (2004a,b) reported that the solar photocatalytic degradation rates with combustion synthesized nanoTiO2 were 20 times higher for remazol brill blue R (RBBR), 4 times higher for methylene blue (MB) and 1.6 times higher for orange G (OG), compared to Degussa P25 TiO2. Their discovery inspired us to further develop combustion synthesis method to prepare carbon-doped TiO2. In the present study, we focused on the synthesis of carbon-doped TiO2 nanoparticles using sol–gel auto-combustion technique with a unique combination of the chemical sol–gel process and the combustion process (Selvan et al., 2003). In addition, the excess fuel could be used as carbon source for fuel/oxidizer >1 in the reaction system. Therefore, crystallization of TiO2 as well as carbon doping could take place at the same time during the calcination. The prepared carbon-doped TiO2 nanoparticles were characterized by X-ray diffraction (XRD), X-ray photoelectron spectra (XPS), Brunauer–Emmett–Teller method (BET), UV–vis diffuses reflectance spectroscopy (DRS). In addition, the photocatalytic activity of carbon-doped TiO2 nanoparticles was evaluated by measuring degradation rates of methylene blue (MB) under solar irradiation.
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angle. b and h of anatase and rutile were taken from anatase (1 0 1) and rutile (1 1 0) diffraction line, respectively. The amount of rutile in the samples was calculated using the following equation (Spurr and Myers, 1957) XR = (1 + 0.8IA/IR)1, where XR was the mass fraction of rutile in the samples, IA and IR were the X-ray integrated intensities of (1 0 1) reflection of the anatase and (1 1 0) reflection of rutile, respectively. The specific surface area of the nanoparticles was measured by the dynamic Brunauer–Emmett–Teller (BET) method, in which a N2 gas was adsorbed at 77 K using a Micromeritics ASAP 2000 system. The diffuse reflectance spectra (DRS) of the photocatalyst sample in the wavelength range of 200–800 nm were obtained using a UV–vis scanning spectrophotometer (Shimadzu UV-3101), while BaSO4 was used as a reference. X-ray photoelectron spectra (XPS) measurements were performed in a VG Scientific ESCALAB Mark II spectrometer equipped with two ultrahigh vacuum (UHV) chambers using Al Ka radiation (1486.6 eV). The XPS binding energies were calibrated with respect to the C 1s peak from the carbon tape at 284.6 eV. The potentials of carbon-doped TiO2 nanoparticles were measured using a Zetaplus zeta potential analyzer (Brookhadn, USA).
2. Experimental 2.3. MB adsorption experiment 2.1. Preparation of carbon-doped TiO2 nanoparticles Carbon-doped TiO2 nanoparticles were synthesized by sol–gel auto-combustion technique. The analytical grade titanium isopropoxide (Ti (OPri)4), C2H6O2 (ethylene glycol, abbreviated as EG), C6H8O7 (citric acid, abbreviated as CA), ammonia (25%) and nitride acid (65–68%) were used as raw materials. The detailed process could be described as follows. Appropriate amount of Ti (OPri)4 was added to CA and EG mixture under constant stirring condition. The molar ratios of CA/Ti, NO 3 /CA, and CA/ EG were kept constant at 2:1, 1:3, and 1:1, respectively. After adjusting the pH value with ammonia to 6–7, the mixture solution was evaporated at 363 K to gradually form a clear precursor gel. The precursor gel was baked at 423 K in muffle furnace and expanded, then was autoignited at about 523 K. The puffy, porous gray powders were calcined at the temperature of 673–1073 K for 2 h in air. 2.2. Characterization of carbon-doped TiO2 nanoparticles The crystalline structure of the carbon-doped TiO2 nanoparticles was determined by a D/max-cA diffractometer (Cu Ka radiation, k = 0.154056 nm) studies. The crystallite sizes D were determined from the XRD pattern according to the Scherrer equation D = Kk/bcos h, where k was a constant (shape factor, about 0.9), k was the Xray wavelength (0.15418 nm), b the full width at half maximum (FWHM) of the diffraction line, and h the diffraction
All batch equilibrium experiments were conducted in the dark. The study of methylene blue adsorption has been performed at 298 K. A pH value of about 7 was used. In each test, 0.02 g of carbon-doped TiO2 nanoparticles were added to 20 mL of 10 mg/L methylene blue (MB) aqueous solution. The equilibrium concentration was determined using a UV–vis spectrophotometer (Shimadzu UV-3101) after centrifugation and filtration, through Millipore filters (0.45 lm diameter) of the suspension. The amounts of methylene blue adsorbed were calculated as follows: nðadsÞ ¼ V DC
ð1Þ
where n(ads) was the number of moles adsorbed; DC the difference between the initial concentration, C0 and equilibrium concentration, Ce; V was the volume (20 ml). 2.4. Photocatalytic activity All the solar photocatalytic experiments were carried out at the same conditions on August, 2006 from 11.00 a.m. to 15.00 p.m. Solar light was used as the irradiation source, and the average insolation of the solar irradiation was 21.28 W/m2 measured by an UV irradiance meter at the range of 375–475 nm, (model UV-A, made in photoelectric instrument factory of Beijing Normal University). Photocatalytic experiments were carried out by adding 0.20 g of carbon-doped TiO2 nanoparticles to 200 mL of 10 mg/L methylene blue (MB) aqueous solution, and then the mixture was sonicated in the dark for 30 min to obtain
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the saturated absorption of methylene blue before illumination. A series of certain volume of samples were withdrawn at selected times for analysis. To avoid a volume change due to the volatility of the solvent, a certain amount of distilled water was added into the system at different intervals. After recovering the catalyst by centrifugation, the light absorption of the clear solution was measured at 660 nm (kmax for MB) at a set time. The decolorization of methylene blue was calculated by formula: decolorization = (C0 C)/C0, where C0 and C was the concentration of the primal and remaining MB, respectively, and (C0 C) was the concentration of the decomposed MB. All the data were corrected for adsorbed MB. The absorbance of the methylene blue (MB) solution was measured with a UV–vis spectrophotometer (Shimadzu UV-3101). 3. Results and discussion 3.1. XRD analysis Fig. 1 showed the XRD patterns of carbon-doped TiO2 nanoparticles calcined at various temperatures between 673 and 1073 K. It was shown that the sample calcined at
673 K had amorphous structure. The X-ray diffraction peak at 25.5° corresponded to characteristic peak of crystal plane (1 0 1) of anatase, and the peak at 27.6° corresponded to characteristic peak of crystal plane (1 1 0) of rutile between 773 and 1073 K. The intensities of the anatase peaks decreased, while the intensities of the rutile peaks greatly increased and contents of rutile phase increased with the increase of the calcination temperature (shown in Table 1). When calcined at 1073 K, the pattern exhibited a complete rutile TiO2 structure. 3.2. XPS analysis According to the X-ray photoelectron spectroscopy (XPS) survey spectrum (Fig. 2A) of carbon-doped TiO2 nanoparticles calcined at 873 K for 2 h, the sample contained only Ti, O, and C. And binding energies for Ti 2p3/2, O 1s, and C 1s were 458.1, 529.9 and 288.2 eV, respectively. To investigate the carbon states in the photocatalyst, we measured C 1s core levels, as shown in Fig. 2B. Deconvolution of the C 1s spectrum (Fig. 2B) revealed three components at 284.5 eV, 288.0 eV and 290.3 eV. The smaller component at a binding energy of 284.5 eV could be attrib-
Fig. 1. XRD patterns of carbon-doped TiO2 nanoparticles at different calcination temperatures.
Table 1 The characteristics of carbon-doped TiO2 prepared at various calcinations temperatures Calcinations temperature (°C)
500 600 700 800
Anatase
Rutile
Crystallite size D(101) (nm)
XA (%)
Crystallite size D(110) (nm)
XR (%)
13.5 – – –
54.76 10.76 0.41 0
16.00 17.73 23.64 24.22
45.24 89.24 97.59 100
Specific surface area (m2/g)
Decolourization of MB at 80 min (%)
20.00 18.8 16.65 15.50
95.84 100 90.32 82.6
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Fig. 2. XPS spectra of (A) survey spectrum, (B) C 1s, (C) Ti 2p, and (D) O 1s for carbon-doped TiO2 nanoparticles calcined at 873 K for 2 h.
uted to C 1s electrons from the carbon tape. Sakthivel and Kwasch (2003) observed the two kinds of carbonate species with binding energies of 287.5 and 288.5 eV. Ohno et al. (2004) observed only one kind of carbonate species with binding energies of 288.0 eV, and they thought that C4+ ions were incorporated into the bulk phase of TiO2. Li et al. (2005) also observed only one kind of carbonate species with binding energies of 288.2 eV. Recently, Ren et al. (2007) observed only one kind of carbonate species with binding energies of 288.6 eV and reveal that carbon may substitute for some of the lattice titanium atoms and form a Ti–O–C structure. These results indicated that the C 1s XPS peak (288.0 eV) could be assigned to Ti–O–C structure in carbon-doped titania by substituting some of the lattice titanium atoms by carbon. In addition, the smaller component at a binding energy of 290.3 eV could be attributed to O@C–O components, which was similar to that of Tseng et al. (2006). XPS signals of Ti 2p were observed at binding energies at around 458.1 eV (Ti 2p3/2) and 463.9 eV (Ti 2p1/2), as
shown in Fig. 2C. The typical binding energy of Ti 2p3/2 peak in TiO2 crystals was 458.5–459.7 eV (Yoshitake et al., 2002; Leprince-Wang, 2002). Compared to the binding energy of Ti4+ in pure titania (458.5–459.7 eV), there was a red shift of binding energy of Ti 2p3/2 peak for the carbon-doped titania, which suggested that Ti3+ species was formed in the carbon-doped titania (Ohno et al., 2004). Deconvolution of the O 1s spectrum was shown in Fig. 2D. The binding energies values of the individual components were 529.6 (Ti4+–O) and 533 eV (OH), which was in good agreement with previous work (Pouilleau et al., 1997). The binding energy components at 531.5 eV were unambiguously assigned to oxygen bonded to Ti3+ (Madhu Kumar et al., 2000). 3.3. UV–vis diffuse reflectance spectra Fig. 3 showed the UV–vis diffuse reflectance spectra of carbon-doped TiO2 nanoparticles calcined at different calcination temperatures. The absorption edge of the
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Fig. 4. The MB nanoparticles.
Fig. 3. UV–vis absorption spectra of carbon-doped TiO2 nanoparticles calcined at different calcination temperatures.
carbon-doped TiO2 samples calcined at 673 K, 773 K, 873 K, 973 K, and 1073 K occurred at 418, 422, 433, 428, and 425 nm, respectively, and accordingly the band gap energy was estimated to be about 2.97, 2.94, 2.86, 2.90, and 2.92 eV, respectively. This showed that the band gap of the carbon-doped TiO2 samples monotonically became narrower with the increase of calcination temperatures, and the band gap of carbon-doped TiO2 samples calcined at 873 K became the narrowest among all the carbondoped TiO2 samples. According to Fig. 2, the presence of Ti3+ species led to the formation of oxygen vacancy state between the valence and the conduction bands in the carbon-doped TiO2. It was reported that reducing TiO2 introduced localized oxygen vacancy states located at 0.75–1.18 eV below the conduction band edge of TiO2 (Nakamura et al., 2000). So, for TiO2 containing localized oxygen vacancy, the band gap between valence band and localized oxygen vacancy state was 2.02–2.45 eV, which corresponded to a wavelength of 506–614 nm. Therefore, the visible light photocatalytic activity was ascribed to the presence of oxygen vacancy state between the valence and the conduction bands because of the formation of Ti3+ species in the as-synthesized carbon-doped TiO2.
adsorption
isotherms
on
carbon-doped
TiO2
the increase of the calcination temperatures, but the difference of specific surface areas for all the samples was much small. 3.5. Photocatalytic activity of carbon-doped TiO2 Fig. 5 showed the results of photocatalytic decomposition of methylene blue (MB) over carbon-doped TiO2 prepared at various calcination temperatures. It was found that the photocatalytic activity increased up to 873 K and then decreased with increasing calcination temperatures. The carbon-doped TiO2 calcined at 873 K showed the highest photocatalytic activity. It could be related to the absorption spectra presented in Fig. 3, because carbondoped TiO2 calcined at 873 K showed stronger absorption not only in the 400–500 nm but also in the 300–400 nm range compared to the other carbon-doped TiO2 samples.
3.4. Adsorption behavior of methylene blue The kinetics of adsorption were given in Fig. 4 for C0 = 10 mg/L (initial concentration of methylene blue). It was found that most of adsorption occurred within 30 min, and the saturated adsorption amount of MB onto the carbon-doped TiO2 decreased with the increase of calcination temperatures. To further study the surface characteristics of the prepared catalysts, we carried out the BET analysis (shown in Table 1). The BET results showed that the specific surface areas of the catalysts decreased with
Fig. 5. Photocatalytic decomposition profiles of methylene blue over carbon-doped TiO2 nanoparticles at different calcination temperatures in neutral (pH 7) suspension (initial concentration of methylene blue:10 mg/ L; catalyst loading: 1 g/L).
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3.6. Factors influencing the photocatalytic degradation 3.6.1. Effect of catalyst loading The effect of catalyst loading on the photocatalytic degradation of methylene blue (MB) was studied by varying the amount of carbon-doped TiO2 nanoparticles calcined at 873 K. Fig. 6 showed the degradation profile of methylene blue (MB) with an initial concentration of 10 mg/L under various catalyst loadings from 0.5 to 4 g/L. It could be seen from Fig. 5 that photocatalytic degradation efficiency has increased up to 1 g/L and then declined with increasing catalyst loading. This could be attributed to shadowing effect, wherein the high turbidity due to high carbon-doped TiO2 nanoparticles concentration decreased the penetration depth of solar radiation. Hence, the optimal catalyst loading of 1 g/L was employed throughout the present study. 3.6.2. Effect of initial concentration of methylene blue Fig. 7 showed the results of the photocatalytic decomposition of methylene blue (MB) as a function of the initial concentration of methylene blue (MB) under solar light irradiation. The increased concentration of methylene blue (MB) always decreased the photocatalytic efficiency under solar light irradiation. This could be due to the fact that the carbon-doped TiO2 nanoparticles played an important role in depreciating the effect of the apparent reduction of light penetration into solution with the increase of the concentration of methylene blue (MB). 3.6.3. Effect of solution pH value The solution pH was a very important operation parameter of photocatalytic reaction. Hoffmann et al. (1995) reviewed a lot of studies and concluded that the interaction of TiO2 with cationic electron donors and electron acceptors would be favored for heterogeneous photocatalytic
Fig. 6. Effect of catalyst loading on the degradation of methylene blue with initial concentration of 10 mg/L over carbon-doped TiO2 nanoparticles calcined at 873 K in neutral (pH 7) suspension.
Fig. 7. Effect of initial concentration of methylene blue on the degradation of methylene blue over carbon-doped TiO2 nanoparticles calcined at 873 K with catalyst loading of 1 g/L in neutral (pH 7) suspension.
activity at high pH greater than the zero point charge of TiO2, while anionic electron donors and electron acceptors would be favored at low pH less than the zero point charge of TiO2. Therefore, a suitable solution pH was needed for photocatalytic reactions. Fig. 8 showed the zeta potentials of carbon-doped TiO2 nanoparticles calcined at 873 K as a function of pH. As expected, the zeta potential was positive at low pH and decreases as the pH increases, and the catalyst suspensions were stable in both basic and acidic pH because of the electrostatic repulsion due to either positive or negative surface charges. In this study, the pHpzc of carbon-doped TiO2 nanoparticles calcined at 873 K was about 5. Fig. 9 presented the effect of pH value in the suspension on photocatalytic efficiency. The variation of pH value showed its strong influence on the methylene blue (MB)
Fig. 8. Zeta potential of carbon-doped TiO2 nanoparticles calcined at 873 K.
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pH-dependent adsorption and photodecomposition were in good agreement. 4. Conclusion
Fig. 9. Effect of pH value on the degradation of methylene blue with initial concentration of 10 mg/L over carbon-doped TiO2 nanoparticles calcined at 873 K.
photodegradation. The photodegradation efficiency as a function of pH value decreased in the order of 10 > 7 > 3. It was generally accepted that the pH-dependent photodecomposition was mainly ascribed to the variations of surface charge properties of a photocatalyst. Consequently, this changed the adsorption behavior of a dye on catalyst surface. Since methylene blue (MB) had a cationic configuration, its adsorption was favored in alkaline solution as demonstrated in Fig. 10. The increase of pH value resulted in a higher adsorption amount of methylene blue (MB) on the carbon-doped TiO2 nanoparticles surface. As MB decolorization took place mainly on powder surface under solar light irradiation, positive holes or hydroxyl radicals might effectively oxidize a suitable amount of methylene blue (MB) in close contact with the catalyst. As seen comparatively in Fig. 9 and Fig. 10, the
Fig. 10. pH-dependent adsorption of MB on carbon-doped TiO2 nanoparticles calcined at 873 K.
Carbon-doped TiO2 nanoparticles was prepared by sol– gel auto-combustion technique. UV–vis diffuse reflectance spectra showed that carbon-doped TiO2 exhibited obvious absorption in the visible light range, which was ascribed to the presence of oxygen vacancy state between the valence and the conduction bands because of the formation of Ti3+ species in the as-synthesized carbon-doped titania. The sample calcined at 873 K showed the highest photocatalytic activity under solar light irradiation. At low catalyst loadings, such as 0.5 g/L, the catalyst surface and absorption of light were the limiting factors; thus, an increase in catalyst loading greatly enhances the process efficiency. At high loadings, penetration of the light inside the reaction medium was reduced because of the light scattering and shielding effect by catalyst particles. In addition, a basic pH level and a lower initial concentration of methylene blue (MB) solution were found to be beneficial for photocatalytic degradation. Acknowledgements This work was supported by the Provincial Excellent PhD Thesis Research Program of Hunan (No. 2004-141) and the Postgraduate Educational Innovation Fund of Central South University (No. 2006-48). References Asashi, R., Morikawa, T., Ohwaki, T., Aoki, K., Taga, Y., 2001. Science 293, 269. Hoffmann, M.R., Martin, S.T., Choi, W., Bahnemann, D.W., 1995. Chem. Rev. 95, 69. Hong, X.T., Wang, Z.P., Cai, W.M., Lu, F., Zhang, J., Yang, Y.Z., Ma, N., Liu, Y.J., 2005. Chem. Mater. 17, 1548. Irie, H., Watanabe, Y., Hashimoto, K., 2003. Chem. Lett. 32, 772. Khan, S.U.M., Al-shahry, M., Ingler Jr., W.B., 2002. Science 297, 2243. Kuo, W.S., Ho, P.H., 2001. Chemosphere 45, 77. Leprince-Wang, Y., 2002. Surf. Coat. Technol. 150, 257. Li, Yuanzhi, Hwang, Doo-Sun, Lee, Nam Hee, Kim, Sun-Jae, 2005. Chem. Phys. Lett. 404, 25. Madhu Kumar, P., Badrinarayanan, S., Sastry, Murali, 2000. Thin Solid Films 358, 122. Nagaveni, K., Sivalingam, G., Hegde, M.S., Madras, G., 2004a. Appl. Catal. B: Environ. 48, 83. Nagaveni, K., Hegde, M.S., Ravishankar, N., Subbanna, G.N., Madras, G., 2004b. Langmuir 20, 2900. Nakamura, Isao, Negishi, Nobuaki, Kutsuna, Shuzo, Ihara, Tatsuhiko, Sugihara, Shinichi, Takeuchi, Koji, 2000. J. Mol. Catal. A 161, 205. Neppolian, B., Choi, H.C., Sakthivel, S., Arabindoo, B., Murugesan, V., 2002. Chemosphere 46, 1173. Ohno, Teruhisa, Tsubota, Toshiki, Toyofukum, Maki, Inaba, Ryoji, 2004. Catal. Lett. 98, 255. Pouilleau, J., Devilliers, D., Groult, H., Marcus, P., 1997. J. Mater. Sci. 32, 5645. Ren, Wenjie, Ai, Zhihui, Jia, Falong, Zhang, Lizhi, Fan, Xiaoxing, Zou, Zhigang, 2007. Appl. Catal. B: Environ. 69, 138.
Q. Xiao et al. / Solar Energy 82 (2008) 706–713 Sakthivel, S., Kwasch, H., 2003. Angew. Chem. Int. Ed. 42, 4908. Sakthivel, S., Neppolian, B., Shankar, M.V., Arabindoo, B., Palanichamy, M., Murugesan, V., 2003. Sol. Energy Mater. Sol. Cells 77, 65. Selvan, R.K., Augustin, C.O., Berchmans, L.J., Saraswathi, R., 2003. Mater. Res. Bull. 38, 41. Spurr, R.A., Myers, H., 1957. Anal. Chem. 29, 760. Tang, J., Zou, Z., Ye, J., 2004. Chem. Mater. 16, 1644.
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Solar Energy 82 (2008) 706–713 www.elsevier.com/locate/solener
Solar photocatalytic degradation of methylene blue in carbon-doped TiO2 nanoparticles suspension Qi Xiao *, Jiang Zhang, Chong Xiao, Zhichun Si, Xiaoke Tan School of Resources Processing and Bioengineering, Central South University, Changsha 410083, PR China Received 8 March 2007; received in revised form 21 August 2007; accepted 9 February 2008 Available online 6 March 2008 Communicated by: Associate Editor G. Calzaferri
Abstract Carbon-doped TiO2 nanoparticles were prepared by sol–gel auto-combustion method and characterized by X-ray diffraction (XRD), X-ray photoelectron spectra (XPS), Brunauer–Emmett–Teller method (BET), UV–vis diffuses reflectance spectroscopy (DRS). UV–vis diffuse reflectance spectra showed that carbon-doped TiO2 exhibited obvious absorption in the visible light range. The visible light photocatalytic activity of carbon-doped TiO2 was ascribed to the presence of oxygen vacancy state between the valence and the conduction bands because of the formation of Ti3+ species in the as-synthesized carbon-doped TiO2. The sample calcined at 873 K showed the highest photocatalytic activity under solar irradiation. The effects of photocatalyst concentration, initial concentration of methylene blue, and pH value in aqueous solution were also presented. Ó 2008 Elsevier Ltd. All rights reserved. Keywords: Carbon-doped TiO2 nanoparticles; Solar photocatalysis; Methylene blue
1. Introduction Heterogeneous photocatalysis was a promising technology for the removal of toxic organic and inorganic contaminates from water. However, the development of a practical photocatalytic system focused on the cost effectiveness of the process by the use of renewable solar energy source. Photocatalytic degradation of organic contaminants using solar irradiation could be highly economical compared with the processes using artificial UV irradiation, which required substantial electrical power input. Abundant solar energy could be utilized efficiently in the photocatalytic processes for the degradation of organic pollutants. Photocatalytic degradation of various organic and inorganic pollutants using solar energy has been reported (Neppolian et al., 2002; Sakthivel et al., 2003; Kuo and Ho, 2001).
*
Corresponding author. Tel.: +86 731 8830543; fax: +86 731 8879815. E-mail address: [email protected] (Q. Xiao).
0038-092X/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.solener.2008.02.006
TiO2 was the most widely used photocatalyst because of its good activity, chemical stability, commercial availability, and inexpensiveness. However, only a small UV fraction of solar light (3–5%) could be utilized because of the wide band gap of TiO2. Therefore, it was urgent to develop a particular photocatalyst sensitive to sunlight (Tang et al., 2004). Recently, doping TiO2 with nonmetal elements such as nitrogen (Asashi et al., 2001), carbon (Khan et al., 2002), sulfur (Umebayashi et al., 2002), iodine (Hong et al., 2005) noticeably improved the photocatalytic activity of TiO2 under visible light. Various synthetic routes have been studied for carbondoped TiO2 nanoparticles. Khan et al. (2002) synthesized chemically modified TiO2 by controlled combustion of Ti metal in a natural gas flame. Irie and coworkers (Irie et al., 2003) prepared carbon-doped anatase TiO2 nanoparticles by oxidative annealing of TiC under O2 flow at 873 K. Sakthivel and Kwasch (2003) reported the wet process synthesis of carbon-doped n-TiO2 by hydrolysis of TiCl4 with tetrabutylammonium hydroxide (C16H36NOH)
Q. Xiao et al. / Solar Energy 82 (2008) 706–713
followed by calcination of the precipitates. Especially, Nagaveni et al. (2004a,b) reported that the solar photocatalytic degradation rates with combustion synthesized nanoTiO2 were 20 times higher for remazol brill blue R (RBBR), 4 times higher for methylene blue (MB) and 1.6 times higher for orange G (OG), compared to Degussa P25 TiO2. Their discovery inspired us to further develop combustion synthesis method to prepare carbon-doped TiO2. In the present study, we focused on the synthesis of carbon-doped TiO2 nanoparticles using sol–gel auto-combustion technique with a unique combination of the chemical sol–gel process and the combustion process (Selvan et al., 2003). In addition, the excess fuel could be used as carbon source for fuel/oxidizer >1 in the reaction system. Therefore, crystallization of TiO2 as well as carbon doping could take place at the same time during the calcination. The prepared carbon-doped TiO2 nanoparticles were characterized by X-ray diffraction (XRD), X-ray photoelectron spectra (XPS), Brunauer–Emmett–Teller method (BET), UV–vis diffuses reflectance spectroscopy (DRS). In addition, the photocatalytic activity of carbon-doped TiO2 nanoparticles was evaluated by measuring degradation rates of methylene blue (MB) under solar irradiation.
707
angle. b and h of anatase and rutile were taken from anatase (1 0 1) and rutile (1 1 0) diffraction line, respectively. The amount of rutile in the samples was calculated using the following equation (Spurr and Myers, 1957) XR = (1 + 0.8IA/IR)1, where XR was the mass fraction of rutile in the samples, IA and IR were the X-ray integrated intensities of (1 0 1) reflection of the anatase and (1 1 0) reflection of rutile, respectively. The specific surface area of the nanoparticles was measured by the dynamic Brunauer–Emmett–Teller (BET) method, in which a N2 gas was adsorbed at 77 K using a Micromeritics ASAP 2000 system. The diffuse reflectance spectra (DRS) of the photocatalyst sample in the wavelength range of 200–800 nm were obtained using a UV–vis scanning spectrophotometer (Shimadzu UV-3101), while BaSO4 was used as a reference. X-ray photoelectron spectra (XPS) measurements were performed in a VG Scientific ESCALAB Mark II spectrometer equipped with two ultrahigh vacuum (UHV) chambers using Al Ka radiation (1486.6 eV). The XPS binding energies were calibrated with respect to the C 1s peak from the carbon tape at 284.6 eV. The potentials of carbon-doped TiO2 nanoparticles were measured using a Zetaplus zeta potential analyzer (Brookhadn, USA).
2. Experimental 2.3. MB adsorption experiment 2.1. Preparation of carbon-doped TiO2 nanoparticles Carbon-doped TiO2 nanoparticles were synthesized by sol–gel auto-combustion technique. The analytical grade titanium isopropoxide (Ti (OPri)4), C2H6O2 (ethylene glycol, abbreviated as EG), C6H8O7 (citric acid, abbreviated as CA), ammonia (25%) and nitride acid (65–68%) were used as raw materials. The detailed process could be described as follows. Appropriate amount of Ti (OPri)4 was added to CA and EG mixture under constant stirring condition. The molar ratios of CA/Ti, NO 3 /CA, and CA/ EG were kept constant at 2:1, 1:3, and 1:1, respectively. After adjusting the pH value with ammonia to 6–7, the mixture solution was evaporated at 363 K to gradually form a clear precursor gel. The precursor gel was baked at 423 K in muffle furnace and expanded, then was autoignited at about 523 K. The puffy, porous gray powders were calcined at the temperature of 673–1073 K for 2 h in air. 2.2. Characterization of carbon-doped TiO2 nanoparticles The crystalline structure of the carbon-doped TiO2 nanoparticles was determined by a D/max-cA diffractometer (Cu Ka radiation, k = 0.154056 nm) studies. The crystallite sizes D were determined from the XRD pattern according to the Scherrer equation D = Kk/bcos h, where k was a constant (shape factor, about 0.9), k was the Xray wavelength (0.15418 nm), b the full width at half maximum (FWHM) of the diffraction line, and h the diffraction
All batch equilibrium experiments were conducted in the dark. The study of methylene blue adsorption has been performed at 298 K. A pH value of about 7 was used. In each test, 0.02 g of carbon-doped TiO2 nanoparticles were added to 20 mL of 10 mg/L methylene blue (MB) aqueous solution. The equilibrium concentration was determined using a UV–vis spectrophotometer (Shimadzu UV-3101) after centrifugation and filtration, through Millipore filters (0.45 lm diameter) of the suspension. The amounts of methylene blue adsorbed were calculated as follows: nðadsÞ ¼ V DC
ð1Þ
where n(ads) was the number of moles adsorbed; DC the difference between the initial concentration, C0 and equilibrium concentration, Ce; V was the volume (20 ml). 2.4. Photocatalytic activity All the solar photocatalytic experiments were carried out at the same conditions on August, 2006 from 11.00 a.m. to 15.00 p.m. Solar light was used as the irradiation source, and the average insolation of the solar irradiation was 21.28 W/m2 measured by an UV irradiance meter at the range of 375–475 nm, (model UV-A, made in photoelectric instrument factory of Beijing Normal University). Photocatalytic experiments were carried out by adding 0.20 g of carbon-doped TiO2 nanoparticles to 200 mL of 10 mg/L methylene blue (MB) aqueous solution, and then the mixture was sonicated in the dark for 30 min to obtain
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the saturated absorption of methylene blue before illumination. A series of certain volume of samples were withdrawn at selected times for analysis. To avoid a volume change due to the volatility of the solvent, a certain amount of distilled water was added into the system at different intervals. After recovering the catalyst by centrifugation, the light absorption of the clear solution was measured at 660 nm (kmax for MB) at a set time. The decolorization of methylene blue was calculated by formula: decolorization = (C0 C)/C0, where C0 and C was the concentration of the primal and remaining MB, respectively, and (C0 C) was the concentration of the decomposed MB. All the data were corrected for adsorbed MB. The absorbance of the methylene blue (MB) solution was measured with a UV–vis spectrophotometer (Shimadzu UV-3101). 3. Results and discussion 3.1. XRD analysis Fig. 1 showed the XRD patterns of carbon-doped TiO2 nanoparticles calcined at various temperatures between 673 and 1073 K. It was shown that the sample calcined at
673 K had amorphous structure. The X-ray diffraction peak at 25.5° corresponded to characteristic peak of crystal plane (1 0 1) of anatase, and the peak at 27.6° corresponded to characteristic peak of crystal plane (1 1 0) of rutile between 773 and 1073 K. The intensities of the anatase peaks decreased, while the intensities of the rutile peaks greatly increased and contents of rutile phase increased with the increase of the calcination temperature (shown in Table 1). When calcined at 1073 K, the pattern exhibited a complete rutile TiO2 structure. 3.2. XPS analysis According to the X-ray photoelectron spectroscopy (XPS) survey spectrum (Fig. 2A) of carbon-doped TiO2 nanoparticles calcined at 873 K for 2 h, the sample contained only Ti, O, and C. And binding energies for Ti 2p3/2, O 1s, and C 1s were 458.1, 529.9 and 288.2 eV, respectively. To investigate the carbon states in the photocatalyst, we measured C 1s core levels, as shown in Fig. 2B. Deconvolution of the C 1s spectrum (Fig. 2B) revealed three components at 284.5 eV, 288.0 eV and 290.3 eV. The smaller component at a binding energy of 284.5 eV could be attrib-
Fig. 1. XRD patterns of carbon-doped TiO2 nanoparticles at different calcination temperatures.
Table 1 The characteristics of carbon-doped TiO2 prepared at various calcinations temperatures Calcinations temperature (°C)
500 600 700 800
Anatase
Rutile
Crystallite size D(101) (nm)
XA (%)
Crystallite size D(110) (nm)
XR (%)
13.5 – – –
54.76 10.76 0.41 0
16.00 17.73 23.64 24.22
45.24 89.24 97.59 100
Specific surface area (m2/g)
Decolourization of MB at 80 min (%)
20.00 18.8 16.65 15.50
95.84 100 90.32 82.6
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Fig. 2. XPS spectra of (A) survey spectrum, (B) C 1s, (C) Ti 2p, and (D) O 1s for carbon-doped TiO2 nanoparticles calcined at 873 K for 2 h.
uted to C 1s electrons from the carbon tape. Sakthivel and Kwasch (2003) observed the two kinds of carbonate species with binding energies of 287.5 and 288.5 eV. Ohno et al. (2004) observed only one kind of carbonate species with binding energies of 288.0 eV, and they thought that C4+ ions were incorporated into the bulk phase of TiO2. Li et al. (2005) also observed only one kind of carbonate species with binding energies of 288.2 eV. Recently, Ren et al. (2007) observed only one kind of carbonate species with binding energies of 288.6 eV and reveal that carbon may substitute for some of the lattice titanium atoms and form a Ti–O–C structure. These results indicated that the C 1s XPS peak (288.0 eV) could be assigned to Ti–O–C structure in carbon-doped titania by substituting some of the lattice titanium atoms by carbon. In addition, the smaller component at a binding energy of 290.3 eV could be attributed to O@C–O components, which was similar to that of Tseng et al. (2006). XPS signals of Ti 2p were observed at binding energies at around 458.1 eV (Ti 2p3/2) and 463.9 eV (Ti 2p1/2), as
shown in Fig. 2C. The typical binding energy of Ti 2p3/2 peak in TiO2 crystals was 458.5–459.7 eV (Yoshitake et al., 2002; Leprince-Wang, 2002). Compared to the binding energy of Ti4+ in pure titania (458.5–459.7 eV), there was a red shift of binding energy of Ti 2p3/2 peak for the carbon-doped titania, which suggested that Ti3+ species was formed in the carbon-doped titania (Ohno et al., 2004). Deconvolution of the O 1s spectrum was shown in Fig. 2D. The binding energies values of the individual components were 529.6 (Ti4+–O) and 533 eV (OH), which was in good agreement with previous work (Pouilleau et al., 1997). The binding energy components at 531.5 eV were unambiguously assigned to oxygen bonded to Ti3+ (Madhu Kumar et al., 2000). 3.3. UV–vis diffuse reflectance spectra Fig. 3 showed the UV–vis diffuse reflectance spectra of carbon-doped TiO2 nanoparticles calcined at different calcination temperatures. The absorption edge of the
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Fig. 4. The MB nanoparticles.
Fig. 3. UV–vis absorption spectra of carbon-doped TiO2 nanoparticles calcined at different calcination temperatures.
carbon-doped TiO2 samples calcined at 673 K, 773 K, 873 K, 973 K, and 1073 K occurred at 418, 422, 433, 428, and 425 nm, respectively, and accordingly the band gap energy was estimated to be about 2.97, 2.94, 2.86, 2.90, and 2.92 eV, respectively. This showed that the band gap of the carbon-doped TiO2 samples monotonically became narrower with the increase of calcination temperatures, and the band gap of carbon-doped TiO2 samples calcined at 873 K became the narrowest among all the carbondoped TiO2 samples. According to Fig. 2, the presence of Ti3+ species led to the formation of oxygen vacancy state between the valence and the conduction bands in the carbon-doped TiO2. It was reported that reducing TiO2 introduced localized oxygen vacancy states located at 0.75–1.18 eV below the conduction band edge of TiO2 (Nakamura et al., 2000). So, for TiO2 containing localized oxygen vacancy, the band gap between valence band and localized oxygen vacancy state was 2.02–2.45 eV, which corresponded to a wavelength of 506–614 nm. Therefore, the visible light photocatalytic activity was ascribed to the presence of oxygen vacancy state between the valence and the conduction bands because of the formation of Ti3+ species in the as-synthesized carbon-doped TiO2.
adsorption
isotherms
on
carbon-doped
TiO2
the increase of the calcination temperatures, but the difference of specific surface areas for all the samples was much small. 3.5. Photocatalytic activity of carbon-doped TiO2 Fig. 5 showed the results of photocatalytic decomposition of methylene blue (MB) over carbon-doped TiO2 prepared at various calcination temperatures. It was found that the photocatalytic activity increased up to 873 K and then decreased with increasing calcination temperatures. The carbon-doped TiO2 calcined at 873 K showed the highest photocatalytic activity. It could be related to the absorption spectra presented in Fig. 3, because carbondoped TiO2 calcined at 873 K showed stronger absorption not only in the 400–500 nm but also in the 300–400 nm range compared to the other carbon-doped TiO2 samples.
3.4. Adsorption behavior of methylene blue The kinetics of adsorption were given in Fig. 4 for C0 = 10 mg/L (initial concentration of methylene blue). It was found that most of adsorption occurred within 30 min, and the saturated adsorption amount of MB onto the carbon-doped TiO2 decreased with the increase of calcination temperatures. To further study the surface characteristics of the prepared catalysts, we carried out the BET analysis (shown in Table 1). The BET results showed that the specific surface areas of the catalysts decreased with
Fig. 5. Photocatalytic decomposition profiles of methylene blue over carbon-doped TiO2 nanoparticles at different calcination temperatures in neutral (pH 7) suspension (initial concentration of methylene blue:10 mg/ L; catalyst loading: 1 g/L).
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3.6. Factors influencing the photocatalytic degradation 3.6.1. Effect of catalyst loading The effect of catalyst loading on the photocatalytic degradation of methylene blue (MB) was studied by varying the amount of carbon-doped TiO2 nanoparticles calcined at 873 K. Fig. 6 showed the degradation profile of methylene blue (MB) with an initial concentration of 10 mg/L under various catalyst loadings from 0.5 to 4 g/L. It could be seen from Fig. 5 that photocatalytic degradation efficiency has increased up to 1 g/L and then declined with increasing catalyst loading. This could be attributed to shadowing effect, wherein the high turbidity due to high carbon-doped TiO2 nanoparticles concentration decreased the penetration depth of solar radiation. Hence, the optimal catalyst loading of 1 g/L was employed throughout the present study. 3.6.2. Effect of initial concentration of methylene blue Fig. 7 showed the results of the photocatalytic decomposition of methylene blue (MB) as a function of the initial concentration of methylene blue (MB) under solar light irradiation. The increased concentration of methylene blue (MB) always decreased the photocatalytic efficiency under solar light irradiation. This could be due to the fact that the carbon-doped TiO2 nanoparticles played an important role in depreciating the effect of the apparent reduction of light penetration into solution with the increase of the concentration of methylene blue (MB). 3.6.3. Effect of solution pH value The solution pH was a very important operation parameter of photocatalytic reaction. Hoffmann et al. (1995) reviewed a lot of studies and concluded that the interaction of TiO2 with cationic electron donors and electron acceptors would be favored for heterogeneous photocatalytic
Fig. 6. Effect of catalyst loading on the degradation of methylene blue with initial concentration of 10 mg/L over carbon-doped TiO2 nanoparticles calcined at 873 K in neutral (pH 7) suspension.
Fig. 7. Effect of initial concentration of methylene blue on the degradation of methylene blue over carbon-doped TiO2 nanoparticles calcined at 873 K with catalyst loading of 1 g/L in neutral (pH 7) suspension.
activity at high pH greater than the zero point charge of TiO2, while anionic electron donors and electron acceptors would be favored at low pH less than the zero point charge of TiO2. Therefore, a suitable solution pH was needed for photocatalytic reactions. Fig. 8 showed the zeta potentials of carbon-doped TiO2 nanoparticles calcined at 873 K as a function of pH. As expected, the zeta potential was positive at low pH and decreases as the pH increases, and the catalyst suspensions were stable in both basic and acidic pH because of the electrostatic repulsion due to either positive or negative surface charges. In this study, the pHpzc of carbon-doped TiO2 nanoparticles calcined at 873 K was about 5. Fig. 9 presented the effect of pH value in the suspension on photocatalytic efficiency. The variation of pH value showed its strong influence on the methylene blue (MB)
Fig. 8. Zeta potential of carbon-doped TiO2 nanoparticles calcined at 873 K.
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pH-dependent adsorption and photodecomposition were in good agreement. 4. Conclusion
Fig. 9. Effect of pH value on the degradation of methylene blue with initial concentration of 10 mg/L over carbon-doped TiO2 nanoparticles calcined at 873 K.
photodegradation. The photodegradation efficiency as a function of pH value decreased in the order of 10 > 7 > 3. It was generally accepted that the pH-dependent photodecomposition was mainly ascribed to the variations of surface charge properties of a photocatalyst. Consequently, this changed the adsorption behavior of a dye on catalyst surface. Since methylene blue (MB) had a cationic configuration, its adsorption was favored in alkaline solution as demonstrated in Fig. 10. The increase of pH value resulted in a higher adsorption amount of methylene blue (MB) on the carbon-doped TiO2 nanoparticles surface. As MB decolorization took place mainly on powder surface under solar light irradiation, positive holes or hydroxyl radicals might effectively oxidize a suitable amount of methylene blue (MB) in close contact with the catalyst. As seen comparatively in Fig. 9 and Fig. 10, the
Fig. 10. pH-dependent adsorption of MB on carbon-doped TiO2 nanoparticles calcined at 873 K.
Carbon-doped TiO2 nanoparticles was prepared by sol– gel auto-combustion technique. UV–vis diffuse reflectance spectra showed that carbon-doped TiO2 exhibited obvious absorption in the visible light range, which was ascribed to the presence of oxygen vacancy state between the valence and the conduction bands because of the formation of Ti3+ species in the as-synthesized carbon-doped titania. The sample calcined at 873 K showed the highest photocatalytic activity under solar light irradiation. At low catalyst loadings, such as 0.5 g/L, the catalyst surface and absorption of light were the limiting factors; thus, an increase in catalyst loading greatly enhances the process efficiency. At high loadings, penetration of the light inside the reaction medium was reduced because of the light scattering and shielding effect by catalyst particles. In addition, a basic pH level and a lower initial concentration of methylene blue (MB) solution were found to be beneficial for photocatalytic degradation. Acknowledgements This work was supported by the Provincial Excellent PhD Thesis Research Program of Hunan (No. 2004-141) and the Postgraduate Educational Innovation Fund of Central South University (No. 2006-48). References Asashi, R., Morikawa, T., Ohwaki, T., Aoki, K., Taga, Y., 2001. Science 293, 269. Hoffmann, M.R., Martin, S.T., Choi, W., Bahnemann, D.W., 1995. Chem. Rev. 95, 69. Hong, X.T., Wang, Z.P., Cai, W.M., Lu, F., Zhang, J., Yang, Y.Z., Ma, N., Liu, Y.J., 2005. Chem. Mater. 17, 1548. Irie, H., Watanabe, Y., Hashimoto, K., 2003. Chem. Lett. 32, 772. Khan, S.U.M., Al-shahry, M., Ingler Jr., W.B., 2002. Science 297, 2243. Kuo, W.S., Ho, P.H., 2001. Chemosphere 45, 77. Leprince-Wang, Y., 2002. Surf. Coat. Technol. 150, 257. Li, Yuanzhi, Hwang, Doo-Sun, Lee, Nam Hee, Kim, Sun-Jae, 2005. Chem. Phys. Lett. 404, 25. Madhu Kumar, P., Badrinarayanan, S., Sastry, Murali, 2000. Thin Solid Films 358, 122. Nagaveni, K., Sivalingam, G., Hegde, M.S., Madras, G., 2004a. Appl. Catal. B: Environ. 48, 83. Nagaveni, K., Hegde, M.S., Ravishankar, N., Subbanna, G.N., Madras, G., 2004b. Langmuir 20, 2900. Nakamura, Isao, Negishi, Nobuaki, Kutsuna, Shuzo, Ihara, Tatsuhiko, Sugihara, Shinichi, Takeuchi, Koji, 2000. J. Mol. Catal. A 161, 205. Neppolian, B., Choi, H.C., Sakthivel, S., Arabindoo, B., Murugesan, V., 2002. Chemosphere 46, 1173. Ohno, Teruhisa, Tsubota, Toshiki, Toyofukum, Maki, Inaba, Ryoji, 2004. Catal. Lett. 98, 255. Pouilleau, J., Devilliers, D., Groult, H., Marcus, P., 1997. J. Mater. Sci. 32, 5645. Ren, Wenjie, Ai, Zhihui, Jia, Falong, Zhang, Lizhi, Fan, Xiaoxing, Zou, Zhigang, 2007. Appl. Catal. B: Environ. 69, 138.
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