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Carbon-doped mesoporous TiO2 film and its photocatalytic activity
Microporous and Mesoporous Materials 142 (2011) 276–281
Contents lists available at ScienceDirect
Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Carbon-doped mesoporous TiO2 film and its photocatalytic activity Xiaoxia Lin a,b, Fei Rong b, Xiang Ji a,b, Degang Fu a,b,⇑ a b
State Key Laboratory of Bioelectronics, Southeast University, Nanjing 210096, China Suzhou Key Laboratory of Environment and Biosafety, Suzhou, 215123, China
a r t i c l e
i n f o
Article history: Received 30 July 2010 Received in revised form 8 December 2010 Accepted 9 December 2010 Available online 14 December 2010 Keywords: Titania Photocatalysis Carbon-doping Mesoporous film
a b s t r a c t A visible-light-active C-doped mesoporous TiO2 film was prepared by sol–gel process combined with hydrothermal treatment. Glucose was used as carbon source and structure-directing agent. The structure and physicochemical properties of the film were characterized by X-ray diffraction (XRD), N2 adsorption– desorption isotherms, scanning electron microscopy (SEM), Fourier transform-infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), UV–vis diffuse reflectance spectrum (DRS) and photoluminescence (PL). The results indicated that oxygen sites in the TiO2 lattice were substituted by carbon atoms and an O–Ti–C bond was formed. The C-doped mesoporous TiO2 film had a high surface area of 283 m2 g 1. The film was about 10 lm thick and was composed of anatase TiO2 particles. The prepared C-doped TiO2 film exhibited excellent photocatalytic activity in the degradation of dye Reactive Brilliant Red X-3B (C.I. reactive red 2) under UV and visible light irradiation compared with that of the smooth TiO2 film and P25 film. The recycle ability of the C-doped mesoporous TiO2 film was also investigated. The degradation ratio which was still higher than 87% after 5 cycles, decreased by 3% compared to the first cycle. Ó 2010 Elsevier Inc. All rights reserved.
1. Introduction The application of titanium dioxide as heterogeneous photocatalyst is attracting considerable attention for water and air purification and remediation [1]. Unfortunately, owing to its large band gap of 3.2 eV, TiO2 can only be activated under ultraviolet irradiation, which occupies <5% of the total solar irradiance at the Earth’s surface. For the sake of efficient use of sunlight, scientists are exploring methods to modify titanium dioxide to absorb visible light. Nonmetal doping, such as C and N doping, has displayed promising results in shifting the light absorption of TiO2 into visible light region [2–5]. As to the visible light photocatalytic activity, C-doped TiO2 was proved to be more active than N-doped TiO2 [3]. Wang et al. [6] reported the shift of photo response of TiO2 from UV to the infrared region by a carbon dopant. Besides, Janus et al. [7] also reported that photocatalytic ability of C-doped TiO2 could be improved by decreasing the recombination rate in photogenerated electron–hole pair with doped carbon as electron scavengers. Meanwhile, the problem of separation and recovery of photocatalyst from the reaction medium exists, which enhances the overall capital and running cost of the treatment. Recently, TiO2 films have been widely studied in photocatalytic degradation of organic contaminants because it overcomes the need for post-treatment separation in a slurry system [8]. However, film-type ⇑ Corresponding author at: State Key Laboratory of Bioelectronics, Southeast University, Nanjing 210096, China. Tel.: +86 025 83793091. E-mail address: [email protected] (D. Fu). 1387-1811/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2010.12.010
photocatalysts normally have lower photocatalytic activity than powdered ones due to their lower surface area. Therefore, porous TiO2 film with high surface areas, such as 299 m2 g 1 [9] has been prepared [9–12]. Li and coworkers synthesized carbon nanostructures under mild aqueous condition using glucose as the carbon source [13]. Subsequently, Ren and coworkers prepared carbon-doped TiO2 by using glucose as the carbon source [14]. Puangpetch also synthesized mesoporous SrTiO3 using glucose as structure-directing agent, which could enhance photocatalytic activity of splitting water [15]. Green chemical glucose which was used as carbon doping and structure-directing agent could avoid using expensive or unstable precursors and production of undesirable byproducts in the synthesis process. Their discovery inspires us to prepare Carbon-doped mesoporous TiO2 film use glucose as carbon source and structure-directing agent. Porous TiO2 film is deposited by various methods including electron beam [16], magnetron sputtering [17], mist plasma evaporation [18], pulsed laser deposition [19], sol–gel method [20], etc. Among these methods, sol–gel combing dip-coating needs several deposition cycles [21], other methods involve sophisticated equipment and complex condition [22,23]. In this study, we prepared C-doped mesoporous TiO2 film supported on glass substrates from Ti(OBu)4 precursors via sol–gel process combined with hydrothermal treatment, which avoided expensive equipment and complex disposal condition. To our best knowledge, the preparation of C-doped mesoporous TiO2 film by sol–gel process combined with hydrothermal treatment has never been reported. The photocatalytic activity of as-prepared
X. Lin et al. / Microporous and Mesoporous Materials 142 (2011) 276–281
composite film was determined by degradation of X-3B in aqueous solution. The recycle ability of the film was also investigated. 2. Experiment 2.1. Photocatalysts preparation The preparation procedure of C-doped mesoporous TiO2 film consisted of two steps. Firstly, the TiO2 particles were prepared by sol–gel method. The pH of the solution was adjusted to 2.0. The mixture of Ti(OBu)4 and i-PrOH was added gradually into the as-prepared acid solution under vigorous stirring until Ti(OBu)4 was hydrolyzed completely. The solution was kept refluxing at 348 K for 24 h. The sol was then dried in an oven at 333 K and the fine TiO2 powders were obtained eventually. Then C-doped mesoporous TiO2 film was synthesized as follows: 1.0 g of TiO2 and 0.5 g glucose were added into 70 ml of deionized water in a 100 ml Teflon-lined stainless autoclave. Soda-lime glass substrate (30 20 1 mm) was rinsed by acetone and distilled water in turn before dipped into the mixture solution. The autoclave was maintained at 423 K for 12 h and then air-cooled to room temperature. The resulting C-doped mesoporous TiO2 film was collected and washed through with water and ethanol, and then calcined in the furnace. The furnace temperature was increased at a rate of 3.0 K min 1 until 373 K; this temperature was held for 1 h. The temperature of the oven was subsequently increased at a rate of 3.0 K min 1 to 723 K and was held at this value for 1 h. We also prepared other two C-doped TiO2 mesoporous film by changing the added amount of glucose from 0.25 to 0.5 g or 1.0 g. The corresponding samples were marked as C-doped TiO2-L (low concentration) film, C-doped TiO2-M (middle concentration) film, and C-doped TiO2-H (high concentration) film, respectively. For comparison, undoped TiO2 and P25 films were prepared by the similar method without adding glucose. 2.2. Characterization The crystalline structure of C-doped mesoporous film was measured by X-ray diffraction (XD-3A, Shimadzu Corporation, Japan) using graphite monochromatic copper radiation (CuKa) at 40 kV, 30 mA over the 2h range of 20–80°. Nitrogen adsorption–desorption isotherms were collected at 77 K using an ASAP2020 instrument (BET and BJH models for specific surface area and porosity evaluation) for samples scratched off the substrate. Prior to measurement, all samples were degassed at 403 K for 5 h. The morphology and size were observed by scanning electron micrographs (SEM, Sirion, FEI). Infrared spectrum was recorded on Shimadzu Fourier transform infrared (FT-IR) spectrometer. All the samples were dried in the oven to get rid of water. The binding energy was identified by X-ray photoelectron spectroscopy (XPS) with Mg Ka radiation (ESCALB MK-II) with charge correction. X-ray photoelectron spectra are referenced to the C 1s peak (Eb = 284.8 eV) resulting from the adventitious hydrocarbon (i.e., from the XPS instrument itself) present on the sample surface. A UV–Vis spectrophotometer (Shimadzu UV-8500) was used to record the diffuse reflectance spectra (DRS) of samples. The photoluminescence emission spectra of the samples were measured at room temperature by a LS-55 (Perkin Elmer) devise illuminated with a 325 nm He–Cd laser.
277
tively, undoped TiO2 and P25 films were used for comparison. A photoreactor system consisted of a cylindrical silica reactor with glass plate at the bottom, and an external light source with vertical irradiation. A 250 W ultraviolet lamp with a wavelength peak at 365 nm was used as light source with an average irradiation intensity of 9 mW cm 2 at upper surface of the reactor. The visible light (>420 nm) was achieved with a light filter cutting the short wavelength components. The distance between the light source and the reactor was 20 cm. A set of photocatalytic degradation experiments were performed following this procedure: the substrate with the film was merged into the X-3B solution with a volume of 50 ml and an initial concentration of 10 mg l 1. Prior to photoreaction, air was pumped into the reactor in the dark for 30 min to reach adsorption–desorption equilibrium. During the photoreaction, samples were collected at a time interval of every 20 min for analysis. The recycle experiments were designed to examine photoactivity of C-doped mesoporous TiO2 film. After finishing a cycle, the film was rinsed and dried in the atmosphere, without any other treatments, and then for next cycle.
3. Results and discussion 3.1. Characterization of C-doped mesoporous TiO2 film The X-ray diffraction (XRD) spectra of undoped TiO2 film and Cdoped TiO2 film are shown in Fig. 1. The characteristic diffraction peaks at 25.4°, 37.9°, 48.2°, 54.7° and 62.8°, show that both of the samples have formed anatase-phase. Compared with undoped TiO2 film, the C-doped sample has an increased intensity due to its higher crystallinity. Meanwhile, C-doped sample shows diffraction peak broadening a little compared to undoped TiO2 film, which indicates the formation of smaller C-doped TiO2 nanoparticles. The crystal sizes of undoped TiO2 and C-doped TiO2 are determined to be about 5.5 and 5.0 nm respectively by Scherrer’s equation. Two peaks at 464.2 and 458.4 eV observed in XPS spectrum (Fig. 2(a)) are assigned to Ti 2p1/2 and Ti 2p3/2, respectively. These values agree well with XPS data in the literature and are known to be due to Ti4+ in pure anatase TiO2 form [24]. Fig. 2(b) shows the C 1s spectrum of C-doped mesoporous TiO2-M film sample with range from 281.0 to 292.0 eV. The peaks at 284.8, 286.2 and 288.6 eV can be thought to signal the presence of adventitious elemental C or the residual carbon from precursor. While the peak at 282.4 eV is close to the C1s peak (281.8 eV) of TiC [25]. Therefore,
2.3. Photocatalytic reactions The photocatalytic activity of as-prepared C-doped mesoporous film was studied by the degradation of X-3B (C.I. reactive red 2) in aqueous solution under UV and visible light irradiation respec-
Fig. 1. XRD patterns of as-prepared C-doped TiO2-M film and undoped TiO2 film.
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Fig. 3 shows UV–Vis spectra for C-doped mesoporous TiO2-M film and undoped TiO2 film. It can be seen that C-doped TiO2 film absorbs more light in the range of 400–500 nm compared to undoped TiO2 film. This result implies that C-doped TiO2 film may have higher visible light photocatalytic activity. In semiconductor particles, the photoluminescence emission spectra (PL) have been widely used to investigate the efficiency of charge trapping, immigration, transfer and to understand the fate of electron–hole pairs [27]. Fig. 4 shows the PL spectra of Cdoped TiO2 film and undoped TiO2 film. It can be seen that the PL intensity of C-doped sample is much lower than that of undoped TiO2. This indicates that carbon doping can effectively inhibit the recombination of photogenerated electrons and holes. The surface hydroxyl groups on TiO2 have been recognized to play an important role in the photocatalytic process, as these groups can inhibit the recombination of photogeneration charges and interact with photogenerated holes to product active oxygen species. The FT-IR transmittance spectra of different samples are shown in Fig. 5. The peak at about 1640 cm 1 is associated with the bending vibration absorption of free water, while the peaks at 3200–3600 cm 1 are attributed to the stretching vibration
Fig. 2. XPS spectra of Ti 2P (a) and C 1s (b) for C-doped TiO2-M film.
Fig. 4. PL spectra of C-doped and undoped TiO2 samples.
Fig. 3. UV–Vis spectra of undoped TiO2 film and C-doped TiO2-M film.
the C1s peak at 282.4 eV originates from O–Ti–C bonding, which can be ascribed to carbon substituting for an oxygen atom in the lattice of TiO2 [26].
Fig. 5. FT-IR for C-doped TiO2-M film and undoped-TiO2 film.
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Fig. 6. SEM images of C-doped TiO2-M film (a) and undoped TiO2 film (b).
absorption of hydroxyl function groups. It is often believed that the surface TiO2–OH bonds arise from the hydrolysis reaction in the preparing process [28]. No peaks corresponding to –CH3, CH2, or –CH bonds are found, confirming that the C-doped TiO2 are free of organic species on the surface. Further observation shows that the peaks corresponding to the stretching vibrations of water and hydroxyl groups are broader and stronger in the C-doped TiO2 film than those of undoped TiO2 film. The larger surface hydroxyl group density in the C-doped TiO2 film can lead to enhancement of the photocatalytic activity. The SEM images in Fig. 6 show the film morphology of C-doped TiO2-M film (a) and undoped TiO2 film (b). The images show that both samples consist of large amounts of monodispersed particulates with a size of around 5 nm, which is consistent with the XRD result. Thickness of the film is measured by the method reported by Rebrov et al. [11]. The average thickness of the C-doped film is about 10 lm (seen in Supplementary material). Besides, we can see that the C-doped TiO2 film is porous, while undoped TiO2 film shows more agglomeration. The porous structure has been shown to be very important for the photocatalytic activity of TiO2 films due to its enhanced surface area [29]. It is worth pointing out that the presence of glucose in the course of reaction is critical for the formation of the mesoporous structure in C-doped TiO2 film. By contrast, no such relatively uniform pores can be obtained in undoped TiO2 film. It is suggested that the glucose template could be present in forms of aggregations or assembly of the aggregations, whose interactions with the TiO2 species through
Fig. 7. Nitrogen adsorption–desorption isotherm (a) and pore size distribution (b) of undoped TiO2 film and C-doped TiO2-M film.
hydrogen bonding might play a significant role in directing the mesophase formation during the course of reaction [30,31]. Fig. 7 shows the nitrogen adsorption–desorption isotherms and BJH pore size distribution curve of C-doped TiO2-M film and undoped TiO2 film. The pore size distribution curves are calculated from desorption branch of a nitrogen isotherm with an H2 hysteresis loop, which are typically characteristic of mesoporous structure. The mesopores of undoped TiO2 and C-doped TiO2 are 3.0 and 5.6 nm, respectively. Table 1 shows the results from surface area measurements of the different samples. As it can be seen, the C-doped TiO2 film has larger surface area.
3.2. Photocatalytic activity of different films The blank experiment without catalysts indicates that the photolysis can be ignored as it is about 1.1% after illumination for 120 min. The photocatalytic activity of the samples for degradation of X-3B under UV and visible light irradiation are shown in Fig. 8. It
Table 1 Surface area measurement of the different samples. Films
BET surface area (m2/g)
Pore volume (cm3/g)
Undoped TiO2 film C-doped TiO2-M film
190 283
0.19 0.25
280
X. Lin et al. / Microporous and Mesoporous Materials 142 (2011) 276–281
Fig. 9. Recycle of the composite film for the degradation of X-3B (initial concentration of X-3B 10 mg l 1, 50 ml).
Table 3 Data for recycle photocatalytic experiments for different samples under UV irradiation. Samples
Decomposed amount (%)
P25 film Undoped TiO2 film C-doped TiO2-M film
Cycle 1
Cycle 2
Cycle 3
Cycle 4
Cycle 5
72.0 62.8 91.0
68.4 58.6 90.4
64.6 53.9 89.7
60.1 48.5 88.8
56.1 43.1 87.8
above the valence band, which could narrow the band gap of TiO2 and make the catalyst to absorb visible light efficiently. Finally, it is also proposed that these carbonaceous species lead to new active sites which are responsible for the high photocatalytic activity. Fig. 8. UV irradiation photocatalytic activity of P25, TiO2, C-doped TiO2 films and self-degradation of X-3B (initial concentration 10 mg l 1, 50 ml) (a) and visible light photocatalytic activity of P25, TiO2, C-doped TiO2 films and self-degradation of X-3B (initial concentration 10 mg l 1, 50 ml) (b).
is found that photocatalytic activity of C-doped porous TiO2 film is higher than that of undoped TiO2 film under both UV (Fig. 8a) and visible light (Fig. 8b). The obtained apparent rate constants kapp are listed in Table 2. The kapp of porous film is more than 2.4 times under UV light irradiation and 3.6 times under visible light irradiation as that of undoped film. Three reasons may account for the high photocatalytic activity of C-doped mesoporous TiO2 film. First, C-doped mesoporous TiO2 film can provide more active sites and adsorb more reactive species due to large surface area and pore volume, which causes the enhanced photocatalytic activity of the film. Second, the reduction of glucose employed in the hydrothermal process lead to carbonaceous species embedded in the TiO2 matrix. It is generally accepted that substitutional carbon can form a new state lies just
3.3. Recycle of the composite film The regeneration of TiO2 photocatalyst is one of key steps to make practical applications for this heterogeneous photocatalysis technology. An examination of the photocatalytic activity of the recycled film is carried out under UV light irradiation. The results are shown in Fig. 9 and Table 3. The degradation rate is still higher than 87%, decreased by 3% after 5 cycles. While for P25 and undoped TiO2 films, the photocatalytic activity decreased by 16% and 18% respectively for the same cycles. 4. Conclusions A novel photocatalyst C-doped mesoporous TiO2 film was prepared by sol–gel process combined with hydrothermal treatment. Glucose was used as carbon source and structure-directing agent. For comparison, undoped TiO2 film was also prepared by the same method. It was found that the absorbance spectra of C-doped porous TiO2 film exhibited a significant red shift to the visible region.
Table 2 Degradation parameter of X-3B by different samples. Samples
P 25 film Undoped-TiO2 film C-doped TiO2-M film
UV
Visible light
X-3B degradation (%)
Apparent rate constant kapp (min
72.0 62.8 91.0
0.0112 0.00855 0.0210
1
)
X-3B degradation (%)
Apparent rate constant kapp (min
9.30 22.1 55.6
0.000659 0.00194 0.00708
1
)
X. Lin et al. / Microporous and Mesoporous Materials 142 (2011) 276–281
The photocatalytic activity of C-doped mesoporous TiO2 film was highly improved compared undoped TiO2 film on the degradation of X-3B under UV and visible light irradiation (k > 420 nm). The recycle of C-doped mesoporous TiO2 film was also investigated under UV light irradiation. The degradation rate decreased only 3% after 5 cycles. Acknowledgements This work is financially supported from the National Science Foundation of China (No. 30670522), the Hi-Tech Research and Development Program (863 Program) of China (No. 2006AA10Z436) and Social Developing Program of Jiangsu Province (BE2008643). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.micromeso.2010.12.010. References [1] A. Fujishima, T.N. Rao, D.A. Tryk, J. Photochem, Photobiol. C: Photo-chem. Rev. 1 (2000) 1–21. [2] Z.B. Wu, F. Dong, W.R. Zhao, H.Q. Wang, Y. Liu, B.H. Guan, Nanotechnology 20 (2009) 235701. [3] S. Sakthivel, H. Kisch, Angew. Chem. Int. Ed. 42 (2003) 4908–4911. [4] Y.H. Ao, J.J. Xu, D.G. Fu, C.W. Yuan, Microporous Mesoporous Mater. 118 (2009) 382–386. [5] A.R. Gandhe, S.P. Naik, J.B. Fernandes, Microporous Mesoporous Mater. 87 (2005) 103–109. [6] X. Wang, S. Meng, X. Zhang, H. Wang, W. Zhang, Q. Du, Chem. Phys. Lett. 444 (2007) 292–296.
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Contents lists available at ScienceDirect
Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Carbon-doped mesoporous TiO2 film and its photocatalytic activity Xiaoxia Lin a,b, Fei Rong b, Xiang Ji a,b, Degang Fu a,b,⇑ a b
State Key Laboratory of Bioelectronics, Southeast University, Nanjing 210096, China Suzhou Key Laboratory of Environment and Biosafety, Suzhou, 215123, China
a r t i c l e
i n f o
Article history: Received 30 July 2010 Received in revised form 8 December 2010 Accepted 9 December 2010 Available online 14 December 2010 Keywords: Titania Photocatalysis Carbon-doping Mesoporous film
a b s t r a c t A visible-light-active C-doped mesoporous TiO2 film was prepared by sol–gel process combined with hydrothermal treatment. Glucose was used as carbon source and structure-directing agent. The structure and physicochemical properties of the film were characterized by X-ray diffraction (XRD), N2 adsorption– desorption isotherms, scanning electron microscopy (SEM), Fourier transform-infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), UV–vis diffuse reflectance spectrum (DRS) and photoluminescence (PL). The results indicated that oxygen sites in the TiO2 lattice were substituted by carbon atoms and an O–Ti–C bond was formed. The C-doped mesoporous TiO2 film had a high surface area of 283 m2 g 1. The film was about 10 lm thick and was composed of anatase TiO2 particles. The prepared C-doped TiO2 film exhibited excellent photocatalytic activity in the degradation of dye Reactive Brilliant Red X-3B (C.I. reactive red 2) under UV and visible light irradiation compared with that of the smooth TiO2 film and P25 film. The recycle ability of the C-doped mesoporous TiO2 film was also investigated. The degradation ratio which was still higher than 87% after 5 cycles, decreased by 3% compared to the first cycle. Ó 2010 Elsevier Inc. All rights reserved.
1. Introduction The application of titanium dioxide as heterogeneous photocatalyst is attracting considerable attention for water and air purification and remediation [1]. Unfortunately, owing to its large band gap of 3.2 eV, TiO2 can only be activated under ultraviolet irradiation, which occupies <5% of the total solar irradiance at the Earth’s surface. For the sake of efficient use of sunlight, scientists are exploring methods to modify titanium dioxide to absorb visible light. Nonmetal doping, such as C and N doping, has displayed promising results in shifting the light absorption of TiO2 into visible light region [2–5]. As to the visible light photocatalytic activity, C-doped TiO2 was proved to be more active than N-doped TiO2 [3]. Wang et al. [6] reported the shift of photo response of TiO2 from UV to the infrared region by a carbon dopant. Besides, Janus et al. [7] also reported that photocatalytic ability of C-doped TiO2 could be improved by decreasing the recombination rate in photogenerated electron–hole pair with doped carbon as electron scavengers. Meanwhile, the problem of separation and recovery of photocatalyst from the reaction medium exists, which enhances the overall capital and running cost of the treatment. Recently, TiO2 films have been widely studied in photocatalytic degradation of organic contaminants because it overcomes the need for post-treatment separation in a slurry system [8]. However, film-type ⇑ Corresponding author at: State Key Laboratory of Bioelectronics, Southeast University, Nanjing 210096, China. Tel.: +86 025 83793091. E-mail address: [email protected] (D. Fu). 1387-1811/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2010.12.010
photocatalysts normally have lower photocatalytic activity than powdered ones due to their lower surface area. Therefore, porous TiO2 film with high surface areas, such as 299 m2 g 1 [9] has been prepared [9–12]. Li and coworkers synthesized carbon nanostructures under mild aqueous condition using glucose as the carbon source [13]. Subsequently, Ren and coworkers prepared carbon-doped TiO2 by using glucose as the carbon source [14]. Puangpetch also synthesized mesoporous SrTiO3 using glucose as structure-directing agent, which could enhance photocatalytic activity of splitting water [15]. Green chemical glucose which was used as carbon doping and structure-directing agent could avoid using expensive or unstable precursors and production of undesirable byproducts in the synthesis process. Their discovery inspires us to prepare Carbon-doped mesoporous TiO2 film use glucose as carbon source and structure-directing agent. Porous TiO2 film is deposited by various methods including electron beam [16], magnetron sputtering [17], mist plasma evaporation [18], pulsed laser deposition [19], sol–gel method [20], etc. Among these methods, sol–gel combing dip-coating needs several deposition cycles [21], other methods involve sophisticated equipment and complex condition [22,23]. In this study, we prepared C-doped mesoporous TiO2 film supported on glass substrates from Ti(OBu)4 precursors via sol–gel process combined with hydrothermal treatment, which avoided expensive equipment and complex disposal condition. To our best knowledge, the preparation of C-doped mesoporous TiO2 film by sol–gel process combined with hydrothermal treatment has never been reported. The photocatalytic activity of as-prepared
X. Lin et al. / Microporous and Mesoporous Materials 142 (2011) 276–281
composite film was determined by degradation of X-3B in aqueous solution. The recycle ability of the film was also investigated. 2. Experiment 2.1. Photocatalysts preparation The preparation procedure of C-doped mesoporous TiO2 film consisted of two steps. Firstly, the TiO2 particles were prepared by sol–gel method. The pH of the solution was adjusted to 2.0. The mixture of Ti(OBu)4 and i-PrOH was added gradually into the as-prepared acid solution under vigorous stirring until Ti(OBu)4 was hydrolyzed completely. The solution was kept refluxing at 348 K for 24 h. The sol was then dried in an oven at 333 K and the fine TiO2 powders were obtained eventually. Then C-doped mesoporous TiO2 film was synthesized as follows: 1.0 g of TiO2 and 0.5 g glucose were added into 70 ml of deionized water in a 100 ml Teflon-lined stainless autoclave. Soda-lime glass substrate (30 20 1 mm) was rinsed by acetone and distilled water in turn before dipped into the mixture solution. The autoclave was maintained at 423 K for 12 h and then air-cooled to room temperature. The resulting C-doped mesoporous TiO2 film was collected and washed through with water and ethanol, and then calcined in the furnace. The furnace temperature was increased at a rate of 3.0 K min 1 until 373 K; this temperature was held for 1 h. The temperature of the oven was subsequently increased at a rate of 3.0 K min 1 to 723 K and was held at this value for 1 h. We also prepared other two C-doped TiO2 mesoporous film by changing the added amount of glucose from 0.25 to 0.5 g or 1.0 g. The corresponding samples were marked as C-doped TiO2-L (low concentration) film, C-doped TiO2-M (middle concentration) film, and C-doped TiO2-H (high concentration) film, respectively. For comparison, undoped TiO2 and P25 films were prepared by the similar method without adding glucose. 2.2. Characterization The crystalline structure of C-doped mesoporous film was measured by X-ray diffraction (XD-3A, Shimadzu Corporation, Japan) using graphite monochromatic copper radiation (CuKa) at 40 kV, 30 mA over the 2h range of 20–80°. Nitrogen adsorption–desorption isotherms were collected at 77 K using an ASAP2020 instrument (BET and BJH models for specific surface area and porosity evaluation) for samples scratched off the substrate. Prior to measurement, all samples were degassed at 403 K for 5 h. The morphology and size were observed by scanning electron micrographs (SEM, Sirion, FEI). Infrared spectrum was recorded on Shimadzu Fourier transform infrared (FT-IR) spectrometer. All the samples were dried in the oven to get rid of water. The binding energy was identified by X-ray photoelectron spectroscopy (XPS) with Mg Ka radiation (ESCALB MK-II) with charge correction. X-ray photoelectron spectra are referenced to the C 1s peak (Eb = 284.8 eV) resulting from the adventitious hydrocarbon (i.e., from the XPS instrument itself) present on the sample surface. A UV–Vis spectrophotometer (Shimadzu UV-8500) was used to record the diffuse reflectance spectra (DRS) of samples. The photoluminescence emission spectra of the samples were measured at room temperature by a LS-55 (Perkin Elmer) devise illuminated with a 325 nm He–Cd laser.
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tively, undoped TiO2 and P25 films were used for comparison. A photoreactor system consisted of a cylindrical silica reactor with glass plate at the bottom, and an external light source with vertical irradiation. A 250 W ultraviolet lamp with a wavelength peak at 365 nm was used as light source with an average irradiation intensity of 9 mW cm 2 at upper surface of the reactor. The visible light (>420 nm) was achieved with a light filter cutting the short wavelength components. The distance between the light source and the reactor was 20 cm. A set of photocatalytic degradation experiments were performed following this procedure: the substrate with the film was merged into the X-3B solution with a volume of 50 ml and an initial concentration of 10 mg l 1. Prior to photoreaction, air was pumped into the reactor in the dark for 30 min to reach adsorption–desorption equilibrium. During the photoreaction, samples were collected at a time interval of every 20 min for analysis. The recycle experiments were designed to examine photoactivity of C-doped mesoporous TiO2 film. After finishing a cycle, the film was rinsed and dried in the atmosphere, without any other treatments, and then for next cycle.
3. Results and discussion 3.1. Characterization of C-doped mesoporous TiO2 film The X-ray diffraction (XRD) spectra of undoped TiO2 film and Cdoped TiO2 film are shown in Fig. 1. The characteristic diffraction peaks at 25.4°, 37.9°, 48.2°, 54.7° and 62.8°, show that both of the samples have formed anatase-phase. Compared with undoped TiO2 film, the C-doped sample has an increased intensity due to its higher crystallinity. Meanwhile, C-doped sample shows diffraction peak broadening a little compared to undoped TiO2 film, which indicates the formation of smaller C-doped TiO2 nanoparticles. The crystal sizes of undoped TiO2 and C-doped TiO2 are determined to be about 5.5 and 5.0 nm respectively by Scherrer’s equation. Two peaks at 464.2 and 458.4 eV observed in XPS spectrum (Fig. 2(a)) are assigned to Ti 2p1/2 and Ti 2p3/2, respectively. These values agree well with XPS data in the literature and are known to be due to Ti4+ in pure anatase TiO2 form [24]. Fig. 2(b) shows the C 1s spectrum of C-doped mesoporous TiO2-M film sample with range from 281.0 to 292.0 eV. The peaks at 284.8, 286.2 and 288.6 eV can be thought to signal the presence of adventitious elemental C or the residual carbon from precursor. While the peak at 282.4 eV is close to the C1s peak (281.8 eV) of TiC [25]. Therefore,
2.3. Photocatalytic reactions The photocatalytic activity of as-prepared C-doped mesoporous film was studied by the degradation of X-3B (C.I. reactive red 2) in aqueous solution under UV and visible light irradiation respec-
Fig. 1. XRD patterns of as-prepared C-doped TiO2-M film and undoped TiO2 film.
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Fig. 3 shows UV–Vis spectra for C-doped mesoporous TiO2-M film and undoped TiO2 film. It can be seen that C-doped TiO2 film absorbs more light in the range of 400–500 nm compared to undoped TiO2 film. This result implies that C-doped TiO2 film may have higher visible light photocatalytic activity. In semiconductor particles, the photoluminescence emission spectra (PL) have been widely used to investigate the efficiency of charge trapping, immigration, transfer and to understand the fate of electron–hole pairs [27]. Fig. 4 shows the PL spectra of Cdoped TiO2 film and undoped TiO2 film. It can be seen that the PL intensity of C-doped sample is much lower than that of undoped TiO2. This indicates that carbon doping can effectively inhibit the recombination of photogenerated electrons and holes. The surface hydroxyl groups on TiO2 have been recognized to play an important role in the photocatalytic process, as these groups can inhibit the recombination of photogeneration charges and interact with photogenerated holes to product active oxygen species. The FT-IR transmittance spectra of different samples are shown in Fig. 5. The peak at about 1640 cm 1 is associated with the bending vibration absorption of free water, while the peaks at 3200–3600 cm 1 are attributed to the stretching vibration
Fig. 2. XPS spectra of Ti 2P (a) and C 1s (b) for C-doped TiO2-M film.
Fig. 4. PL spectra of C-doped and undoped TiO2 samples.
Fig. 3. UV–Vis spectra of undoped TiO2 film and C-doped TiO2-M film.
the C1s peak at 282.4 eV originates from O–Ti–C bonding, which can be ascribed to carbon substituting for an oxygen atom in the lattice of TiO2 [26].
Fig. 5. FT-IR for C-doped TiO2-M film and undoped-TiO2 film.
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Fig. 6. SEM images of C-doped TiO2-M film (a) and undoped TiO2 film (b).
absorption of hydroxyl function groups. It is often believed that the surface TiO2–OH bonds arise from the hydrolysis reaction in the preparing process [28]. No peaks corresponding to –CH3, CH2, or –CH bonds are found, confirming that the C-doped TiO2 are free of organic species on the surface. Further observation shows that the peaks corresponding to the stretching vibrations of water and hydroxyl groups are broader and stronger in the C-doped TiO2 film than those of undoped TiO2 film. The larger surface hydroxyl group density in the C-doped TiO2 film can lead to enhancement of the photocatalytic activity. The SEM images in Fig. 6 show the film morphology of C-doped TiO2-M film (a) and undoped TiO2 film (b). The images show that both samples consist of large amounts of monodispersed particulates with a size of around 5 nm, which is consistent with the XRD result. Thickness of the film is measured by the method reported by Rebrov et al. [11]. The average thickness of the C-doped film is about 10 lm (seen in Supplementary material). Besides, we can see that the C-doped TiO2 film is porous, while undoped TiO2 film shows more agglomeration. The porous structure has been shown to be very important for the photocatalytic activity of TiO2 films due to its enhanced surface area [29]. It is worth pointing out that the presence of glucose in the course of reaction is critical for the formation of the mesoporous structure in C-doped TiO2 film. By contrast, no such relatively uniform pores can be obtained in undoped TiO2 film. It is suggested that the glucose template could be present in forms of aggregations or assembly of the aggregations, whose interactions with the TiO2 species through
Fig. 7. Nitrogen adsorption–desorption isotherm (a) and pore size distribution (b) of undoped TiO2 film and C-doped TiO2-M film.
hydrogen bonding might play a significant role in directing the mesophase formation during the course of reaction [30,31]. Fig. 7 shows the nitrogen adsorption–desorption isotherms and BJH pore size distribution curve of C-doped TiO2-M film and undoped TiO2 film. The pore size distribution curves are calculated from desorption branch of a nitrogen isotherm with an H2 hysteresis loop, which are typically characteristic of mesoporous structure. The mesopores of undoped TiO2 and C-doped TiO2 are 3.0 and 5.6 nm, respectively. Table 1 shows the results from surface area measurements of the different samples. As it can be seen, the C-doped TiO2 film has larger surface area.
3.2. Photocatalytic activity of different films The blank experiment without catalysts indicates that the photolysis can be ignored as it is about 1.1% after illumination for 120 min. The photocatalytic activity of the samples for degradation of X-3B under UV and visible light irradiation are shown in Fig. 8. It
Table 1 Surface area measurement of the different samples. Films
BET surface area (m2/g)
Pore volume (cm3/g)
Undoped TiO2 film C-doped TiO2-M film
190 283
0.19 0.25
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Fig. 9. Recycle of the composite film for the degradation of X-3B (initial concentration of X-3B 10 mg l 1, 50 ml).
Table 3 Data for recycle photocatalytic experiments for different samples under UV irradiation. Samples
Decomposed amount (%)
P25 film Undoped TiO2 film C-doped TiO2-M film
Cycle 1
Cycle 2
Cycle 3
Cycle 4
Cycle 5
72.0 62.8 91.0
68.4 58.6 90.4
64.6 53.9 89.7
60.1 48.5 88.8
56.1 43.1 87.8
above the valence band, which could narrow the band gap of TiO2 and make the catalyst to absorb visible light efficiently. Finally, it is also proposed that these carbonaceous species lead to new active sites which are responsible for the high photocatalytic activity. Fig. 8. UV irradiation photocatalytic activity of P25, TiO2, C-doped TiO2 films and self-degradation of X-3B (initial concentration 10 mg l 1, 50 ml) (a) and visible light photocatalytic activity of P25, TiO2, C-doped TiO2 films and self-degradation of X-3B (initial concentration 10 mg l 1, 50 ml) (b).
is found that photocatalytic activity of C-doped porous TiO2 film is higher than that of undoped TiO2 film under both UV (Fig. 8a) and visible light (Fig. 8b). The obtained apparent rate constants kapp are listed in Table 2. The kapp of porous film is more than 2.4 times under UV light irradiation and 3.6 times under visible light irradiation as that of undoped film. Three reasons may account for the high photocatalytic activity of C-doped mesoporous TiO2 film. First, C-doped mesoporous TiO2 film can provide more active sites and adsorb more reactive species due to large surface area and pore volume, which causes the enhanced photocatalytic activity of the film. Second, the reduction of glucose employed in the hydrothermal process lead to carbonaceous species embedded in the TiO2 matrix. It is generally accepted that substitutional carbon can form a new state lies just
3.3. Recycle of the composite film The regeneration of TiO2 photocatalyst is one of key steps to make practical applications for this heterogeneous photocatalysis technology. An examination of the photocatalytic activity of the recycled film is carried out under UV light irradiation. The results are shown in Fig. 9 and Table 3. The degradation rate is still higher than 87%, decreased by 3% after 5 cycles. While for P25 and undoped TiO2 films, the photocatalytic activity decreased by 16% and 18% respectively for the same cycles. 4. Conclusions A novel photocatalyst C-doped mesoporous TiO2 film was prepared by sol–gel process combined with hydrothermal treatment. Glucose was used as carbon source and structure-directing agent. For comparison, undoped TiO2 film was also prepared by the same method. It was found that the absorbance spectra of C-doped porous TiO2 film exhibited a significant red shift to the visible region.
Table 2 Degradation parameter of X-3B by different samples. Samples
P 25 film Undoped-TiO2 film C-doped TiO2-M film
UV
Visible light
X-3B degradation (%)
Apparent rate constant kapp (min
72.0 62.8 91.0
0.0112 0.00855 0.0210
1
)
X-3B degradation (%)
Apparent rate constant kapp (min
9.30 22.1 55.6
0.000659 0.00194 0.00708
1
)
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