Effect of Carbon Doping on the Mesoporous Structure of

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Langmuir 2008, 24, 3510-3516

Effect of Carbon Doping on the Mesoporous Structure of Nanocrystalline Titanium Dioxide and Its Solar-Light-Driven Photocatalytic Degradation of NOx Yu Huang,†,‡ Wingkei Ho,† Shuncheng Lee,*,† Lizhi Zhang,*,‡ Guisheng Li,§ and Jimmy C. Yu§ Department of CiVil and Structural Engineering, Research Center for EnVironmental Technology and Management, The Hong Kong Polytechnic UniVersity, Hong Kong, China and Key Laboratory of Pesticide & Chemical Biology of Ministry of Education, College of Chemistry, Central China Normal UniVersity, Wuhan 430079, People’s Republic of China, and Department of Chemistry and EnVironmental Science Programme, The Chinese UniVersity of Hong Kong, Shatin, New Territories, Hong Kong, China ReceiVed October 25, 2007. In Final Form: NoVember 14, 2007 Effective mesoporous nanocrystalline C-doped TiO2 photocatalysts were synthesized through a direct solutionphase carbonization using titanium tetrachloride and diethanolamine as precursors. X-ray photoelectron spectroscopy (XPS) results revealed that oxygen sites in the TiO2 lattice were substituted by carbon atoms and formed a C-Ti-O-C structure. The absorption region of the as-prepared TiO2 was extended to the visible light region in view of the substitution for oxygen sites by carbon atoms. The photocatalytic activities of the as-prepared samples were tested in a flow system on the degradation of NO at typical indoor air levels under simulated solar-light irradiation. The samples showed a more effective removal efficiency than commercial photocatalyst (P25) on the degradation of the common indoor pollutant NO. The parameters significantly affecting the mesoporous structure and removal efficiency on indoor air were also investigated.

1. Introduction Researchers have paid more and more attention to indoor air quality (IAQ) with increasing awareness of the public environment and health, especially in urban cities.1 Gaseous pollutants such as NOx, SO2, and VOCS are common hazardous species in the indoor environment. The overly high concentrations of these species in indoor environments can be attributed to wall finishing, use of liquefied petroleum gas cooking stoves, and infiltration from nearby vehicular emissions.2-5 Lengthened exposure to such gaseous pollutants may cause various health problems. Conventionally, these gaseous pollutants were removed mainly by remediation techniques including adsorption and filtration methods. However, besides their low efficiency to eliminate gases at the parts per billion level in the indoor environments,6 these methods cannot solve the postdisposal and regeneration problems. In this study, nitrogen oxide, which is the most common gaseous pollutant found in the indoor environment,7 is chosen as the target pollutant. Much research has been performed in the past 10 years on removal of NO using the photocatalysis technique.8-11 Anpo * To whom correspondence should be addressed. E-mail: ceslee@ polyu.edu.hk (S.C.L.), [email protected] (L.Z.Z.). † Hong Kong Polytechnic University. ‡Central China Normal University. § The Chinese University of Hong Kong. (1) Lee, S. C.; Wang, B. Atmos. EnViron. 2004, 38, 941. (2) Lee, S. C.; Kwok, N. H.; Guo, H.; Hung, W. T. Sci. Total EnViron. 2003, 302, 75. (3) Baek, S. O.; Kim, Y. S.; Perry, R. Atmos. EnViron. 1997, 31, 529. (4) Lee, S. C.; Chan, L. Y. EnViron. Int. 1998, 24, 729. (5) Weschler, C. J.; Shields, H. C. EnViron. Sci. Technol. 1994, 28, 2120. (6) Ao, C. H.; Lee, S. C.; Yu, J. C. J. Photochem. Photobiol. A: Chem. 2003, 156, 171. (7) Ao, C. H.; Lee, S. C.; Mak, C. L.; Chan, L. Y. Appl. Catal. B: EnViron. 2003, 42,119. (8) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann D. W. Chem. ReV. 1995, 95, 69.

and co-workers investigated the decomposition of NO into N2 and O2 in a large-scale flow system using TiO2. Moreover, the pretreatment and reaction conditions were also investigated to achieve the highest photodecomposition efficiency.12 Recently, they reported that they directly decomposed NO using a series of Ti-MCM-41 photocatalysts involving different loadings of Ti oxides in a closed system. The coordination and dispersibility of Ti oxides in the mesoporous photocatalysts were found to be strongly affecting the photocatalytic degradation of NO.13,14 Additionally, NO can also be removed by applying photoassisted selective catalytic reduction with NH3 over TiO2 photocatalyst at low temperature.15,16 Madras et al. developed a new photocatalyst, Pd-ion-substituted TiO2, for NO reduction and CO oxidation. Reduction of NO was carried out in both the presence and the absence of CO. They found that NO was photodissociated on the oxide-ion vacancy site on the surface of the photocatalysts.17 Photocatalytic decomposition and reduction of NO, using carbon monoxide (CO) as a reducing gas, have been investigated over Degussa P25 titanium dioxide photocatalysts by Walker et al. using a continuous flow reactor. It was found that the photocatalytic activity for both the decomposition and the reduction (9) Ollis, D. F.; Pelizzetti, E.; Serpone, N. EnViron. Sci. Technol. 1991, 25, 1523. (10) Ibusuki, T.; Takeuchi, K. J. Mol. Catal. 1994, 88, 93-102. (11) Sano, T.; Negishi, N.; Koike, K.; Takeuchi, K.; Matsuzawa, S. J. Mater. Chem. 2004, 14, 380-384. (12) Zhang, J. L.; Azusa, T.; Minagawa, M.; Kinugawa, K.; Yamashita, H.; Matsuoka, M.; Anpo, M. J. Catal. 2001, 198, 1. (13) Hu, Y.; Martra, G.; Zhang, J. L.; Higashimoto, S.; Coluccia, S.; Anpo, M. J. Phys. Chem. B 2006, 110, 1680. (14) Hu, Y.; Higashimoto, S.; Martra, G.; Zhang, J. L.; Matsuoka, M.; Coluccia, S.; Anpo, M. Catal. Lett. 2003, 90, 161. (15) Tanaka, T.; Teramura, K.; Arakaki, K.; Funabiki, T. Chem. Commun. 2002, 22, 2742. (16) Yamazoe, S.; Okumura, T.; Teramura, K.; Tanaka, T. Catal. Today. 2006, 111, 266. (17) Roy, S.; Hegde, M. S.; Ravishankar, N.; Madras, G. J. Phys. Chem. C 2007, 111, 8153.

10.1021/la703333z CCC: $40.75 © 2008 American Chemical Society Published on Web 02/22/2008

Effect of Carbon Doping

Figure 1. XRD patterns of the resulting samples: (a) pure TiO2; (b) C-doped TiO2 calcined at 500 °C; (c) C-doped TiO2 calcined at 600 °C. (A, anatase; R, rutile).

reactions decreased with increasing pretreatment temperature. They attributed this to removal of surface hydroxyl species that act as active sites for reaction.18 The effects of initial NO concentration, gas-residence time, reaction temperature, and ultraviolet (UV) light intensity on the photocatalytic decomposition of NO have also been determined in an annular flow type photoreactor and a modified two-dimensional fluidized bed photoreactor.19 However, it should be noted that the pollutant concentrations adopted by these studies during the photocatalytic degradation process were usually at the parts per million (ppm) level. In the indoor environment the NO concentration was found normally at parts per billion (ppb) levels only. It is reported that gaseous pollutants at parts per billion levels were more difficult to be removed compared to those at parts per million levels. Moreover, removal of NO in indoor environments mainly proceeded through an oxidation rather than a reduction process in air because there is abundant O2 in the mixed gas system. Anpo et al. also confirmed that in the presence of abundant O2, NO can be removed by adsorption and oxidation rather than by a decomposition reaction.20 Previously, we successfully demonstrated that TiO2 immobilized on different substrates, such as activate carbon and glass fibers, can photocatalytically degrade indoor air pollutants at parts per billion levels in a flow system under UV light irradiation.21-25 Different parameters such as humidity level, residence time, and coexistence of binary pollutants, which are the vital factors affecting the photodegradation efficiency, were also investigated.7,26,27 However, since the traditional TiO2 (18) Bowering, N.; Walker, G. S.; Harrison, P. G. Appl. Catal. B: EnViron. 2006, 62, 208. (19) Lim, T. H.; Jeong, S. M.; Kim, S. D.; Gyenis, J. J. Photochem. Photobiol. A: Chem. 2000, 134, 209. (20) Zhang, J. L.; Azusa, T.; Minagawa, M.; Kinugawa, K.; Yamashita, H.; Matsuoka, M.; Anpo, M. J. Catal. 2001, 198, 1. (21) Ao, C. H.; Lee, S. C. J. Photochem. Photobiol. A: Chem. 2004, 161, 131. (22) Ao, C. H.; Lee, S. C Appl. Catal. B: EnViron. 2003, 44,191. (23) Ao, C. H.; Lee, S. C. Chem. Eng. Sci. 2005, 60, 103. (24) Ao, C. H.; Lee, S. C.; Zou, S. C.; Mark, C. L. Appl. Catal. B: EnViron. 2004, 49,187. (25) Ao, C. H.; Lee, S. C.; Yu, J. C. J. Photochem. Photobiol. A: Chem. 2003, 156, 171. (26) Yu, H. G.; Lee, S. C.; Yu, J. G.; Ao, C. H. J. Mol. Catal. A: Chem. 2006, 246, 206. (27) Ao, C. H.; Lee, S. C.; Yu, J. Z.; Xu, J. H. Appl. Catal. B: EnViron. 2004, 54, 41.

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photocatalyst can only be activated under UV light, but more than 95% of indoor light is in the visible region,28 it is significant to develop visible-light-sensitive photocatalysts for indoor environments where UV light intensity is insufficient. Compared with the doping of selective metal ions into TiO2, modification of TiO2 with nonmetal doping is a promising way to extend the absorption of light from the UV to the visible region because it can avoid both the deteriorating thermal stability of the TiO2 lattice and the possible increase in carrier-recombination centers.29-31 It has been proven that doping TiO2 with carbon can extend the optical response of TiO2 into the visible region to enhance its photocatalytic activity.32-34 For instance, Irie and co-workers reported that carbon-doped TiO2 powder was 5 times more active than nitrogen-doped TiO2 on the degradation of 4-chlorophenol under visible light irradiation (λ g 455 nm).35 Novel carbon-doped TiO2 nanotube arrays with high aspect ratios were successfully obtained by Bard and co-workers. They found that the synthesized TiO2-xCx nanotube arrays showed much higher photocurrent densities and more efficient water splitting under visible-light illumination (λ > 420 nm) than pure TiO2 nanotube arrays.36 Carbon-doped TiO2 spheres formed on Ti substrates and carbon-doped TiO2 nanotubes within nanochannels of alumina template using the CVD method were obtained by Chen et al. In particular, they found that the photocurrent of the carbon-doped TiO2 spheres was much higher than that of commercial P-25, which is currently considered one of the best TiO2 photocatalysts.37 TiO2-2xCx[V··O2]x photocatalysts, where [V··O2]x is the oxide ion vacancy created for charge balance, were obtained by the solution combustion method in a single step, showing a significant red shift in their UV absorption spectra.38 Most recently, carbon and nitrogen co-doped TiO2 with different nitrogen and carbon contents were prepared by a sol-gel method, and the resulting photocatalysts showed enhanced visible-light photocatalytic activity.39 In this study, effective C-doped TiO2 nanocrystalline photocatalysts have been obtained through a direct solution-phase carbonization using titanium tetrachloride and diethanolamine as precursors. To the best of our knowledge, there have been no reports on mesoporous carbon-doped TiO2 photocatalysts used for indoor air control. The photocatalytic activities of the asprepared samples were tested in a flow system on the degradation of NO at typical indoor air levels under simulated solar-light irradiation. The results showed enhanced removal efficiency compared to that of commercial P25 photocatalyst in the common indoor air control of nitrogen oxide under simulated solar light irradiation. In addition, the effects of carbon doping on the mesoporous structure and photocatalytic efficiency of nanocrystalline TiO2 were also discussed. (28) Joung, S. K.; Amemiya, T.; Murabayashi, M.; Itoh, K. Chem. Eur. J. 2006, 12, 5526. (29) Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. Science 2001, 293, 269. (30) Khan, S. U. M.; Al-Shahry, M.; Ingler, W. B., Jr. Science 2002, 297, 2243. (31) Li, J. G.; Yang, X. J.; Ishigaki, T. J. Phys. Chem. B 2006, 110, 14611. (32) Sakthivel, S.; Kisch, H. Angew. Chem., Int. Ed. 2003, 42, 4908. (33) Di Valentin, C.; Pacchioni, G.; Selloni, A. Chem. Mater. 2005, 17, 6656. (34) Yin, S.; Komatsu, M.; Zhang, Q.; Saito, F.; Sato, T. J. Mater. Sci. 2007, 42, 2399. (35) Irie, H.; Watanabe, Y.; Hashimoto, K. Chem. Lett. 2003, 32, 772. (36) Park, J. H.; Kim, S. W.; Bard, A. J. Nano Lett. 2006, 6, 24. (37) Wu, G. S.; Nishikawa, T.; Ohtani, B.; Chen, A. C. Chem. Mater. 2007, 19, 4530. (38) Nagaveni, K.; Hegde, M. S.; Ravishankar, N.; Subbanna, G. N.; Madras, G. Langmuir 2004, 20, 2900. (39) Chen, D.; Jiang, Z.; Geng, J.; Wang, Q.; Yang, D. Ind. Eng. Chem. Res. 2007, 46, 2741.

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Figure 2. TEM images of the C-doped TiO2 sample calcined at 500 °C.

Figure 3. N2 adsorption-desorption isotherms and Barret-JoynerHalenda (BJH) pore size distribution plot (inset) of the resulting photocatalyst samples.

2. Experimental Section All chemicals used in the present work were of reagent grade and purchased from Aldrich. 2.1. Synthesis of the Nanocrystalline TiO2-xCx Photocatalysts. Nanocrystalline TiO2-xCx photocatalysts were directly obtained based on a solution-phase carbonization method without adding any surfactants as template. In a typical synthesis, 10 mmol of titanium tetrachloride was added dropwise into 30 mL of ethanol under stirring. After stirring for 1 h, a transparent yellowish sol was formed, and then 4 mL of diethanolamine was added into the sol. The solution turned colorless after the addition finished. Under stirring for 24 h

at ambient temperature, the solution was maintained at 60 °C for several hours, resulting in a vivid yellow gel. The resulting gels were then calcined at different temperatures for 5 h in a muffle furnace. Calcination temperatures were attained at a heating rate of 1 °C/min. After calcination, the powder photocatalysts were obtained. As a controlled experiment, undoped TiO2 was also obtained via the same method after being calcined at 500 °C without addition of diethanolamine. 2.2. Characterization. The X-ray powder diffraction (XRD) patterns were recorded on a Bruker D8 Advance X-ray diffractometer with Cu KR radiation (λ ) 1.54178 Å) at a scan rate of 0.05° 2θ/s. The accelerating voltage and applied current were 40 kV and 40 mA, respectively. A transmission electron microscopy (TEM) study was carried out on a Philips CM-120 electron microscopy instrument. The samples for TEM were prepared by dispersing the final powders in ethanol; the dispersion was then dropped on carbon copper grids. The nitrogen adsorption and desorption isotherm at 77 K was measured using a Micrometritics ASAP2010 system after the sample was vacuum dried at 473 K overnight. FT-IR spectra were recorded on a Nicolet Nexus spectrometer on samples embedded in KBr pellets. A Varian Cary 100 Scan UV-Visible system equipped with a Labsphere diffuse reflectance accessory was used to obtain the reflectance spectra of the catalysts over a range of 200-800 nm. Labsphere USRS-99-010 was employed as a reflectance standard. The spectra were converted from reflection to absorbance by the Kubelka-Munk method. X-ray photoelectron spectroscopy (XPS) measurements were performed on a PHI Quantum 2000 XPS System with a monochromatic Al KR source and a charge neutralizer. All binding energies were calibrated to the C1s peak at 284.8 eV of the surface adventitious carbon. 2.3. Photocatalytic Activity Measurement. The photocatalytic activity experiments on the resulting samples for oxidation of NO in air were performed at ambient temperature in a continuous flow reactor. The volume of the rectangular reactor which was made of stainless steel and covered with Saint-Glass was 27.3 L (13 cm ×

Effect of Carbon Doping

Figure 4. Diffuse reflectance absorption spectra of the as-prepared samples. Data are plotted as transformed Kubelka-Munk function versus the energy of light. Insert shows the absorption spectra between 200 and 800 nm. 70 cm × 30 cm (H × L × W)). Three sample dishes containing the photocatalyst powders were placed on a single path in the reactor. A 300 W commercial tungsten halogen lamp (General Electric) was used as the simulated solar-light source. The lamp was vertically placed outside the reactor above the three sample dishes. Four minifans were fixed around the lamp to avoid the temperature rise of the flow system. The integrated UV intensity in the range 310400 nm was 720 ( 10 µW/cm2. The photocatalyst samples were prepared by coating an aqueous suspension of our sample onto three dishes with a diameter of 5.0 cm. The weight of the photocatalysts used for each experiment was kept at 0.3 g. The dishes containing the photocatalyst were pretreated at 70 °C until complete removal of water in the suspension and then cooled to room temperature. NO gas was selected as the target pollutant for the photocatalytic degradation at ambient temperature. The NO gas was acquired from a compressed gas cylinder at a concentration of 48 ppm NO (N2 balance, BOC gas) with traceable National Institute of Stands and Technology (NIST) standard. The initial concentration of NO was diluted to about 400 ppb by the air stream supplied by a zero air generator (Thermo Environmental Inc. model 111). The desired humidity level of the NO flow was controlled at 70% (2100 ppmv) by passing the zero air streams through a humidification chamber. The gas streams were premixed completely by a gas blender, and the flow rate was controlled at 4 L min-1 by a mass flow controller. After the adsorption-desorption equilibrium among water vapor, gases, and photocatalysts was achieved, the lamp was turned on. The concentration of NO was continuously measured by a chemiluminescence NO analyzer (Thermo Environmental Instruments Inc. model 42c), which monitors NO, NO2, and NOx (NOx represents NO + NO2) with a sampling rate of 0.7 L/min. The removal rate (%) of NO was defined according to the following equation NO removal rate (%) ) [NO]inlet - [NO]outlet/[NO]inlet × 100% where [NO]inlet represents the concentration of NO in the feeding stream and [NO]outlet is the concentration of NO in the outlet stream. The reaction of NO with air was ignorable when performing a control experiment with or without light in the absence of photocatalyst.

3. Results and Discussions 3.1. XRD Patterns. X-ray diffraction was used to characterize the phase composition of the products. Figure 1 shows the XRD patterns of the as-prepared samples. It was found that only anatase (JCPDS, file No. 84-1285) existed in the samples calcined at 500 °C. The anatase in the resulting samples underwent anatase-

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Figure 5. Infrared spectra of the samples: (a) pure TiO2; (b) C-doped TiO2 calcined at 500 °C; (c) C-doped TiO2 calcined at 600 °C.

to-rutile phase transformation and subsequent crystal growth from 500 to 600 °C. When the calcination temperature was increased to 600 °C, anatase coexisting with rutile (JCPDS, file No. 77-442) phases were found in the sample. The crystal sizes estimated from the (101) peaks show that the anatase crystallites are 15, 16, and 18 nm for undoped TiO2, C-doped TiO2 calcined at 500 °C, and C-doped TiO2 calcined at 600 °C, respectively. With the increase of calcination temperature, the crystallites’ sizes increase slightly. In Figure 1 it is clear that the diffraction peaks of carbon-doped samples shifted to higher diffraction angles compared with that of undoped TiO2. The results imply that an oxygen atom in the TiO2 lattice is substituted by a carbon atom. It is known that phase transformation among brookite, anatase, and rutile occurs irreversibly when titanium dioxide is treated under high temperature. Besides, there is no evidence for any degree of conversion from the anatase to the rutile structure on carbon-doped TiO2 photocatalysts.34 Thus, the phase transformation of the resulting photocatalysts from anatase to rutile is exclusively induced by the thermal treatment rather than by the existence of carbon dopant in the TiO2 lattice. 3.2. TEM Images. Figure 2 shows the TEM images of the C-doped TiO2 sample calcined at 500 °C. The image shows that sample consists of large amounts of small particulate with a size of around 20 nm, which agrees with the XRD results calculated by the Scherrer equation. 3.3. Nitrogen Adsorption-Desorption. Figure 3 shows the nitrogen adsorption-desorption isotherms and pore size distribution curves (inset) of the resulting photocatalyst samples. The pore size distribution was calculated from the desorption branch of a nitrogen isotherm by the Barret-Joyner-Halenda (BJH) method using the Halsey equation. The adsorption isotherm can be classified as type IV with hysteresis loops in the IUPAC classification,40 which was characteristic of mesoporous materials. Obviously, in the nitrogen adsorption-desorption isotherms there is a hysteresis loop at 0.40 < P/P0 < 0.96 in the isotherms of the both undoped and C-doped TiO2 samples, corresponding to filling of mesopores produced by the agglomeration of primary particles in the resulting samples. It is known that the mesoporous structure can be obtained through the agglomeration of small particulates using titanium tetrachloride as the precursor in the (40) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pierotti, R. A.; Rouquerol, J.; Sieminewska, T. Pure Appl. Chem. 1985, 57, 603.

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Figure 6. High-resolution XPS spectra of the C1s for the C-doped TiO2 calcined at 500 °C.

formation of TiO2. The pore size distributions of these samples are shown in the inset of Figure 3. It reveals that the pore size distribution of both undoped TiO2 and C-doped TiO2 calcined at 500 °C are similar, with mostly about 8.5 nm in pore diameter. However, the pore diameter of the C-doped TiO2 sample calcined at 600 °C decreases intensively to about 2.5 nm, indicating the destruction of mesoporous structure by high-temperature calcination. The Brunauer-Emmett-Teller (BET) specific surface areas of undoped TiO2, C-doped TiO2 calcined at 500 °C, and C-doped TiO2 calcined at 600 °C were 43.4, 22.4, and 6.6 m2/g, respectively. It illustrates that both carbon doping and phase transformation from anatase to rutile decrease the surface area of the samples. Therefore, it can be concluded that carbon doping only affects the surface area of the TiO2. 3.4. Diffuse Reflectance UV-Vis Spectroscopy. The UVvis diffuse reflectance spectra of the samples are shown in Figure 4. A strong absorption in the ultraviolet region ascribed to the band-band transition can be observed clearly for all samples. However, comparing to the unmodified TiO2, the C-doped TiO2 crystallites showed typical absorption in the visible light region from 400 to 550 nm, especially for the sample calcined at 500 °C. As reported by Kish and co-workers, the carbon content in the resulting carbon-modified TiO2 has a great effect on light absorption,32 which was consistent with our experimental results. Assuming the material to be an indirect semiconductor, the band gap energies of undoped TiO2, C-doped TiO2 calcined at 500 °C, and C-doped TiO2 calcined at 600 °C calculated by the transformed Kubelka-Munk function were 2.85, 2.64, and 2.78 eV, respectively. A series of localized occupied states would appear in the band gap in view of carbon doping, which can explain the experimentally observed absorption edge shift to the visible region for the carbon-doped TiO2 samples.33 In addition, it has been proven that when the titanium ions in the TiO2 lattice were substituted by carbon, no visible-light response was observed.41 Thus, we believe that the band gap of our carbondoped TiO2 samples decrease significantly compared with that of pure TiO2 sample because of the substitution of O ions in the TiO2 lattice by C atoms, leading to visible light absorption. 3.5. FT-IR Spectra. The bonding characteristics of functional groups in the as-prepared samples were identified by the FT-IR spectroscopy. The absorption peaks at about 3400 and 1630 cm-1 (41) Kamisaka, H.; Adachi, T.; Yamashita, K. J. Chem. Phys. 2005, 123, 084704.

Huang et al.

Figure 7. Photocatalytic activity of the as-prepared samples for oxidation of NO. Residence time 3.72 min, humidity levels 2200 ppmv, and 400 ppb NO.

were associated with the stretching vibrations of surface water molecules.42,43 Additionally, Li et al. found that peaks around 3000 cm-1 were often assigned to molecularly chemisorbed water, which was in relation to the surface defect sites.44 These observations confirmed the coexistence of dissociated water (hydroxyl groups) and molecular water on the samples. Meanwhile, the peaks corresponding to the stretching vibrations of water and hydroxyl groups were broader and stronger in the C-doped TiO2 calcined at 500 °C than that in the C-doped TiO2 calcined at 600 °C, indicating the different hydrophilic ability of the samples. It is well known that the surface hydroxyl groups are important to the photocatalytic activity because these groups can be trapped by the holes generated under UV light irradiation to form hydroxyl radicals which can suppress electron-hole recombination, hence increasing the photocatalytic efficiency. 3.6. XPS Spectra. X-ray photoelectron spectroscopy (XPS) was used to investigate the chemical compositions of C-doped TiO2 calcined at 500 °C. Figure 6 shows the high-resolution spectra of C1s in which the C1s peaks can be fitted as two peaks at binding energies of 284.8 and 283 eV, implying two different chemical environments of carbon existing in the sample. The peak at around 284.8 eV was assigned to carbon adsorbed on the surface of the photocatalyst as a contaminant,45 while the peak at around 283 eV was ascribed to the existence of Ti-C bonds.34,35,46 No peaks can be detected at around 289 eV, suggesting that carbonate species were not found in the C-doped sample.47 This reveals that the oxygen sites in the TiO2 lattice were substituted by carbon atoms and formed a C-Ti-O structure, which was consistent with the UV-vis diffuse reflectance results. The atomic concentration of carbon in the carbon-doped TiO2 sample was 17.43%, in which 4.85% and 12.58% contributed to Ti-C species and surface-adsorbed carbon species, respectively. 3.7. Photocatalytic Degradation of Nitrogen Oxide. Figure 7 shows the relative variations of the NO removal rate against irradiation time in the presence of the pure TiO2, C-doped TiO2 (42) Burgos, M.; Langlet, M. Thin Solid Films 1999, 349, 19. (43) Brownson, J. R. S.; Tejedor-Tejiedor, M. I.; Anderson, M. A. Chem. Mater. 2005, 17, 6304. (44) Li, G.; Li, L.; Boerio, J.; Woodfield, B. F. J. Am. Chem. Soc. 2005, 127, 8569. (45) Gopinath, C. S.; Hegde, S. G.; Ramaswamy, A. V.; Mahapatra, S. Mater. Res. Bull. 2002, 37, 1323. (46) Zhang, L.; Koka, R. V. Mater. Chem. Phys. 1998, 57, 23. (47) Ohno, T.; Tsubota, T.; Nishijima, K.; Miyamoto, Z. Chem. Lett. 2004, 33, 750.

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calcined at 500 °C, C-doped TiO2 calcined at 600 °C, and P25 under simulated solar-light irradiation at a single pass flow through the reactor under a humidity level of 2100 ppmv. Prior to the simulated solar-light irradiation, the adsorption/desorption equilibrium between the indoor air with NO and photocatalysts had been reached. When the lamp was switched on, the photodegradation reaction of NO was initiated. It was found that both the photolysis of NO without photocatalysts and its degradation in the presence of P25 were negligible under simulated solar light irradiation. After 40 min simulated solar-light irradiation, 25% and 12% NO was photodegraded by the C-doped TiO2 calcined at 500 and 600 °C, respectively, in a single pass flow. However, for pure TiO2 sample, only 8% NO was photodegraded, indicating its lower photoresponse to simulated solar light irradiation with carbon doping. This NO degradation for the pure TiO2 is mainly due to the photocatalysis activated by the UV part from simulated solar light. Moreover, the NO removal rate increased rapidly in the first 10 min and reached the highest value of about 8% in a single pass for the pure TiO2. Then the NO removal rate decreased slowly with the irradiation time, which was ascribed to accumulation of HNO3 on the catalyst surface resulting in deactivation of TiO2 photocatalysts.22,48 However, for the C-doped TiO2 calcined at 500 °C, the removal rate reached the highest value after being irradiated for 30 min. Comparing to the C-doped TiO2 calcined at 600 °C, the carbon-doped TiO2 calcined at 500 °C showed superior photocatalytic activity on the degradation of NO at parts per billion levels, which can be explained by the band gap energy, the surface properties, as well as the mesoporous architecture. It is known that the photocatalytic process mainly takes place on the surface of catalysts and involves comprehensive competing reactions. Therefore, the surface properties of TiO2, such as surface acidity, defects, and hydroxyl groups, can greatly affect the reaction efficiency.49 Among them, hydroxyl groups can give rise to a great influence on the chemical properties of TiO2.50-52 The importance of surface hydroxyl groups is related to two aspects: scavenging of holes and adsorption centers for reactants and intermediates.53 Moreover, the oxidation reaction of NO in our experiment was believed to be initiated by ‚OH radicals. In the presence of O2, the OH radicals are formed in the following reactions54

C-TiO2 + solar light T h+VB + e-CB

(1)

e-CB + O2 f O2-

(2)

-

+

O2 + 2H + e

CB

f H 2O 2

produce hydroxyl radical group ‚OH, which is possibly beneficial for oxidation of NO. A recent study revealed that the holes formed for carbon-doped TiO2 photocatalysts under visible light irradiation were less reactive than those formed under UV light irradiation for pure TiO2.33 For the carbon-doped TiO2 samples, the holes were trapped at midgap levels and showed less mobility, which was beneficial for the capture of surface hydroxyl to produce ‚OH. However, the density and nature of the localized states in the band gap was significantly influenced by the carbon dopant concentration,33 which may be used to explain the difference in photocatalytic activity of carbon-doped TiO2 calcined at 500 and 600 °C on the degradation of NO at typical parts per billion levels. For our C-doped TiO2 samples, the C atoms substituted some O sites in the TiO2 lattice, which was confirmed by the XRD, UV-vis, and XPS results. The loss of lattice-C in the carbon-doped TiO2 calcined at 600 °C was inevitable compared to that calcined at 500 °C. It was further confirmed by the UV-vis results that the carbon-doped TiO2 calcined at 500 °C showed more intensive absorption in the visible light region than the sample calcined at 600 °C. In addition, oxidation of lattice-C at high preparation temperature may be another reason for the loss of C in the resulting sample. Thus, the difference in carbon concentration in the carbon-doped TiO2 samples leads to the localized states in the band gaps with different density and nature, resulting in different surface properties of the carbon-doped TiO2 photocatalysts. Compared to the carbon-doped TiO2 calcined at 600 °C, having a higher surface area than that calcined at 500 °C may be another reason for its superior photocatalytic activity on degradation of NO under simulated solar-light irradiation. From the nitrogen adsorption results we can see that the pore structure of the sample at higher calcination temperature was nearly destroyed entirely because of the crystal growth during calcination under high temperatures up to 600 °C. The BET specific surface area of sample calcined at 500 °C was 22.42 m2/g, but it dropped to 6.58 m2/g for sample calcined at 500 °C. It is known that a large surface area provides more surface sites for the adsorption of reactants molecules, making the photocatalytic process more efficient.55 Additionally, the transport of NO through the interior space can be feasible because of the mesoporous structure of the resulting photocatalysts. The porous structure facilitates the harvesting of solar light due to the enlarged surface area. These can also contribute to the higher photocatalytic activity of the carbon-doped TiO2 calcined at 500 °C.

4. Conclusions (3)

H2O2 + O2- f ‚OH + OH- + O2

(4)

h+ + H2O f ‚OH + H+

(5)

According to the above mechanism, inhibiting the undesirable electron-hole pair recombination is important to enhance the photocatalytic activity because it can improve the ability to (48) Ibusuki, I.; Yakeuchi, K. J. Mol. Catal. 1994, 88, 93. (49) Tang, J. W.; Quan, H. D.; Ye, J. H. Chem. Mater. 2007, 19, 116. (50) Li, D.; Haneda, H.; Hishita, S.; Ohashi, N.; Labhsetwar, N. K. J. Fluorine Chem. 2005, 126, 69. (51) Kwon, Y. T.; Song, K. Y.; Lee, W. I.; Choi, G. J.; Do, Y. R. J. Catal. 2000, 191, 192. (52) Keller, V.; Bernhardt, P.; Grain, F. J. Catal. 2003, 215, 129. (53) Cao, L.; Spiess, F.-J.; Huang, A.; Suib, S. L.; Obee, T. N.; Hay, S. O.; Freihaut, J. D. J. Phys. Chem. B 1999, 103, 2912.

In this study, effective mesoporous nanocrystalline C-doped TiO2 photocatalysts have been obtained through a direct solutionphase carbonization using TiCl4 and diethanolamine as precursors. The resulting materials were characterized by XRD, TEM, nitrogen adsorption, FT-IR spectroscopy, UV-vis diffraction spectroscopy, and XPS analysis. The characterization results revealed that oxygen sites in the TiO2 lattice were substituted by carbon atoms. The absorption region of TiO2 was extended to the visible region in view of carbon doping. The photocatalytic activities of the resulting samples were tested in a flow system on the degradation of NO at typical indoor air levels under simulated solar-light irradiation. The resulting samples showed more effective removal efficiency than commercial P25 photocatalyst of the common indoor pollutant NO control in a single (54) Yamashita, H.; Honda, M.; Harada, M.; Ichihashi, Y.; Anpo, M.; Hirao, T.; Itoh, N.; Iwamoto, N. J. Phys. Chem. B 1998, 102, 10707. (55) Wang, X. C.; Yu, J. C.; Chen, Y. L.; Wu, L.; Fu, X. Z. EnViron. Sci. Technol. 2006, 40, 2369.

3516 Langmuir, Vol. 24, No. 7, 2008

pass flow due to the doped carbon, which significantly affects the mesoporous structure and removal efficiency on indoor air. Acknowledgment. This project was funded by the Research Grants Council of Hong Kong (PolyU 5204/07E) and the Hong Kong Polytechnic University (GYF08 and GYX75). The work

Huang et al.

described in this paper was also partially supported by the National Science Foundation of China (20503009 and 20673041) and Open Fund of Key Laboratory of Catalysis and Materials Science of the State Ethnic Affairs Commission & Ministry of Education, Hubei Province (CHCL0508 and CHCL06012). LA703333Z