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Fabrication of the CN co-doped rod-like TiO2 photocatalyst with visible-light responsive photocatalytic activity
Materials Research Bulletin 47 (2012) 1508–1512
Contents lists available at SciVerse ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Fabrication of the C–N co-doped rod-like TiO2 photocatalyst with visible-light responsive photocatalytic activity Liang-Hai Li a, Juan Lu a, Zuo-Shan Wang a,b,*, Lu Yang a, Xiu-Feng Zhou a, Lu Han a a b
College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215021, China State Key Laboratory of Explosion Science and Technology, Beijing Institute of Technology, Beijing 100081, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 10 August 2011 Received in revised form 12 January 2012 Accepted 19 February 2012 Available online 25 February 2012
The C–N co-doped TiO2 nanorods were synthesized by the vapor transport method of water molecules, and urea was used as the carbon and nitrogen source. The samples were characterized by X-ray diffraction and photoelectron spectroscopy analysis. The scanning electron microscope images showed that as-prepared TiO2 powders were nanorods, which were formed by the stacking of nanoparticles with a uniform size around 40 nm. The degradation of methylene blue with the prepared nanorods demonstrated the photocatalytic activities of TiO2 under visible light are improved by doping with C and N elements. The main reasons were discussed: doping with C and N elements could enhance the corresponding visible-light absorption of TiO2. On the other hand, doping C and N could create more oxygen vacancies in the TiO2 crystals, which could capture the photogenerated electrons more effectively. Thus, more photogenerated holes could be left to improve the photocatalytic activity of TiO2. ß 2012 Elsevier Ltd. All rights reserved.
Keywords: A. Nanostructures A. Semiconductors B. Crystal growth D. Catalytic properties
1. Introduction The titanium dioxide (TiO2) has attracted many attentions since it was served as a functional material for the decomposition of organic compounds. Most of the researches focused on two issues: on one hand, the decrease of energy gap of TiO2 was explored to modify the optical response in visible light range not only ultraviolet [1]; on the other hand, how to prevent photogenerated electrons and holes from recombination and improve the photocatalytic activity [2]? Asahi firstly reported the TiO2, which was doped with N element, exhibited a high photocatalytic activity under visible light [3]. The result revealed that doping was an effective way to solve the above problems. Further study has shown that TiO2 powders doped with two different elements, such as F–N [1], B–N [4], P–N [5] and C–N [6–8] have better photocatalytic properties than single nonmetal doping. Recently, the C–N co-doped TiO2 has attracted many attentions for its excellent photocatalytic properties under visible light. However, the morphology of samples mostly has focused on nanoparticles [6–8], and the pH value should be adjusted by adding acid or alkali in almost all of the preparations. Here we report a new synthesis of C–N co-doped TiO2 nanorods by the vapor transport method of water molecules. No acid or any surfactant was used throughout
* Corresponding author at: No. 199, Ren’ai Road, Suzhou Industry Park, Suzhou 215123, China. Tel.: +86 0512 85187680; fax: +86 0512 85187680. E-mail address: [email protected] (Z.-S. Wang). 0025-5408/$ – see front matter ß 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2012.02.032
the process, and as-prepared powders possessed an excellent photocatalytic activity under visible light. 2. Experimental 2.1. Preparation of photocatalyst Typically, ethanol and tetra-n-butyl titanate were mixed with a volume ratio of 1:40, and certain urea was dissolved in the mixture by ultrasonic oscillation for 3 min. Then, the solution was transferred to the reaction flask of the rotary evaporator, in which several agate granules with a diameter of about 1 cm had been put in advance. The rotary evaporator was turned on and the system maintained at 25 8C. In addition, a vacuum pump connected with a round bottom flask containing some deionized water was turned on to provide air with a rate of 40 L min 1. At the same time, vapor molecules were brought into the reaction flask with air to react with the mixture. It could be observed that the solution changed from colorless and transparent to turbid, and finally became a cream color emulsion. Reaction stopped after 3 h, and then the emulsion was distillated at 55 8C for 10 min in a vacuum to turn to well dispersed white powders. Finally, C–N co-doped TiO2 nanorods were obtained by calcining the white powders for 2 h at 300, 400 and 500 8C. The pure and C–N co-doped TiO2 were designated as P-x and Dx, respectively, where ‘‘x’’ is the annealing temperature. S-x represented the samples annealed at the corresponding temperature (x), whether doping or not.
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2.2. Characterization of photocatalyst The crystal structure of the sample was identified by X-ray diffraction (XRD, PANalytical, X-Pert-Pro MPD) instrument with Cu-Ka radiation (voltage: 40 kV, electric current: 100 mA), performed over angular ranges of 2u = 20–808. A field-emission scanning electron microscopy (FE-SEM, Hitachi, S-4700) was used for the morphology characterization. The elemental composition of samples was determined by X-ray photoelectron spectroscopy (XPS, USA Thermo ESCALAB 250) with Al/Ka radiation (hn = 1486.6 eV). The X-ray anode runs at 150 W, and the X-ray beams spot size was 500 mm. The UV–vis reflection spectra of TiO2 samples was recorded in the range of 200–800 nm using a Japan Shimadzu UV240 UV–vis spectrophotometer equipped with an integrating sphere, and BaSO4 was used as the reference. ESR tests were carried out on Germany Bruker A300-10/12 to investigate defects of powders. 2.3. Photocatalytic degradation of methylene blue (MB) solution The photocatalytic experiment was performed in a 0.5 L cylindrical glass photocatalytic reactor (Xujiang electromechanical plant, Nanjing), where a 500 W Xe lamp was selected as the visible light source. A cut filter (ZJB 420) was inserted between the xenon lamp and reactor to eliminate ultraviolet light. The flow rate of air was kept at a constant value of 500 mL min 1. Typically, 300 mL of methylene blue (10 mg L 1) aqueous solution containing 0.05 g of photocatalyst powder was mixed in a beaker. Prior to the photocatalytic reaction, the suspension was allowed to reach adsorption/desorption equilibrium by maintaining the solutions in dark for 1 h. At a defined time interval, 5 mL of methylene blue was taken out from the reactor, and was analyzed using the UV–vis spectrophotometer. The percentage of degradation was calculated by the formula [1 (A/A0)] 100%, where A0 is the absorbance of original methylene blue solution before irradiation and Ai is the
Fig. 1. XRD patterns of pure and C–N co-doped TiO2 calcined at different temperatures. (a) D-300 8C; (b) P-300 8C; (c) D-400 8C; (d) P-400 8C; (e) D-500 8C; and (f) P-500 8C.
absorbance of methylene blue solution measured every 15 min in the process of photodegradation. 3. Results and discussion 3.1. XRD analysis XRD was used to investigate the phase structure of the pure TiO2 and C–N co-doped TiO2 powders calcined at different temperatures, and the results are shown in Fig. 1. Diffraction peaks appeared at 2u = 25.38 and 37.88 can be indexed to (1 0 1) and (0 0 4) planes of anatase as the samples were calcined at 300 8C
Fig. 2. SEM images of C–N co-doped TiO2 calcined at different temperatures: (a) 300 8C; (b) 400 8C; and (c) 500 8C.
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(Fig. 1a, b) and 400 8C (Fig. 1c, d). The rutile phase (1 1 0) appeared when the calcination temperature increased to 500 8C (Fig. 1c, d). Neither carbon nor nitrogen-derived peaks could be detected in all of the samples, mainly due to the low dosage of the dopant. The average sizes of the samples (Fig. 1a–f) were 9.7 nm, 10.2 nm, 15.9 nm, 15.1 nm, 25.9 nm and 24.9 nm, calculated with the Scherrer formula. 3.2. SEM analysis Fig. 2 shows the typical SEM images of TiO2 samples doped with C and N, calcined at 300 8C (Fig. 2a), 400 8C (Fig. 2b) and 500 8C (Fig. 2c) for 2 h. It could be clearly observed that as-prepared samples were nanorods (40–50 nm 400–500 nm), piling up together in disorder. The results of XRD indicated that the sizes of nanoparticles are in the range of 10–25 nm, so it can be deemed that the rod-like structure was formed by several TiO2 nanoparticles self-organizing along with the same direction. Fig. 2 shows that some nanoparticles had not yet grown into a nanorod and attached on the surface of the grown nanorods, which could also demonstrate our conjecture. According to the existing mechanism of crystal growth [9], a schematic illustration for the forming process of rod-like TiO2 is shown in Fig. 3. In our experiment, vapor molecules were transported with the air into the flask containing tetra-n-butyl titanate, resulting in the hydrolysis reaction in the gas-liquid interface. Colloidal nucleus was formed in this process (Fig. 3a). The rate of hydrolysis could be controlled by adjusting the flow velocity of the air. In addition, rotating and ball milling could improve the dispersion of colloidal nucleus in three-dimensional space, which is also conducive to the high-energy surface combination. The colloidal nucleus would
choose the high-energy surface to pile up, which was called epitaxy. The final products were nanorods piling up by colloidal nucleus (Fig. 3b). However, colloidal nucleuses are so easy to reunion that there are still some nanoparticles in the products (Fig. 3c), because of their small sizes and huge surface energy. As Fig. 2 presents, among all of the samples, nanorods gained by calcination at 400 8C are the most regular. In addition, the size of the nanorods could increase with the rise of the calcining temperature. 3.3. XPS analysis Fig. 4 illustrates the XPS survey spectra of C–N co-doped TiO2 calcined at 400 8C (Fig. 4a) and 500 8C (Fig. 4b), respectively. It showed that both of the two samples contained C, N, O, and Ti. The peaks appeared at 284.5 eV, 458.5 eV and 529.8 eV could be attributed to C1s, Ti2p and O1s, respectively, and the mini peak around 399.6 eV could be assigned to N1s. The accessory figures of Fig. 4a and b presented the XPS spectra of Ti2p. The micro peak at 454.9 eV was attributed to Ti–C bond [10], reflecting the small amount of dopant C since it is hard for C to enter the TiO2 lattice [11]. The binding energy of 458.5 eV was attributed to Ti–O bond. Two peaks at 455.1 eV and 461.0 eV assigned to Ti–C and Ti–N bonds illustrated that calcination generates a small amount of TiC and TiN. To further investigate the states of the dopant C and N, curves fitting of C1s and N1s XPS spectra of D-400 and D-500 with high resolution was recorded and shown in Fig. 5. The C1s core levels of C–N co-doped TiO2 revealed three peaks structures at binding energies of 284.5, 286.2 and 288.2 eV (Fig. 5a, b). The carbon peak at 284.5 eV assigned to the C–C bond may be due to the organic
Fig. 3. The schematic illustration for TiO2 nanorods.
Fig. 4. XPS spectra of C–N co-doped TiO2 at different temperatures: (a) 400 8C and (b) 500 8C.
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Fig. 5. The C1s and N1s XPS spectra of C–N co-doped TiO2 at different temperatures: (a) C1s-400 8C; (b) C1s-500 8C; (C) N1s-400 8C; and (d) N1s-500 8C.
impurities [12]. The two features of 286.2 and 288.2 eV are attributed to C–O and C–N bonds, respectively. Ti–C bond has been proved above. All these results indicate that a small quantity of C has been doped into the TiO2 lattice. The N1s core levels of C–N co-doped TiO2 revealed three peaks structures at binding energies of 395.7, 400.1, 402.0 eV (Fig. 5c, d) and 406.5 eV (Fig. 5d), corresponding to C–N bond, N2O22 , g-N2 chemisorbed on TiO2 [13] and NO3 [14], proving that some N has also been doped into the TiO2 lattice. So, it could be concluded that C–N co-doped TiO2 has been obtained by this vapor transport method. XPS results showed that the molar ratios of Ti/C and Ti/N are approximately 0.8 and 40, respectively. It suggested that a lot of C and N existed in the samples. In fact, the amount of C and N doped into TiO2 lattices is small. So, it can be ascribed to formation of carbonated species [12,15,16] and nitrides [14] which was also proved by above analysis.
Firstly, dopants C and N could expand the optical response range of TiO2 from ultraviolet to visible light, which was proved by UV–vis diffuse reflex spectra (Fig. 7) of the powders. It can be observed that the pure TiO2 has a weak reflection under the ultraviolet light (400 nm) and a strong reflection under visible
3.4. Photocatalytic activity Methylene blue (MB) was chosen as model dye to evaluate the photocatalytic activity of the photocatalysts under visible light irradiation, and the results were shown in Fig. 6. It could be observed that the C–N co-doped TiO2 has a higher catalytic efficiency, compared with the pure TiO2 prepared under the same calcination temperature. As mentioned above, the reasons mainly focused on two aspects as follows.
Fig. 6. Photocatalytic degradation of methylene blue under visible light irradiation using C–N co-doped TiO2 as the catalyst.
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intensity of C–N co-doped TiO2 is about 25–30 times stronger than that of the pure TiO2, which suggested that co-doping of C and N can create more oxygen vacancies. The g value (2.00306) can be obtained by equation: g = hv/mBHr, which is very close to the g value (2.003 0.001) of single electron associated with the oxygen vacancy [17–19]. It is well known that oxygen vacancies can capture the photogenerated electrons effectively, preventing photogenerated electrons recombining with photogenerated holes, which is finally responsible for the degradation of pollutants into H2O and CO2. So, it could be concluded that the photocatalytic activity of TiO2 under visible light can be enhanced by co-doping C and N. 4. Conclusions
Fig. 7. UV–vis diffuse reflectance spectra of pure and C–N co-doped TiO2 nanorods.
In summary, C–N co-doped TiO2 nanorods were synthesized by the vapor transport method, without acid or any surfactants. Results of XPS analysis revealed that C and N were successfully doped into the lattice of TiO2. The photocatalytic activity of samples was evaluated by the degradation of methylene blue under visible light irradiation. Compared with the pure TiO2, C–N co-doping TiO2 exhibited a significantly enhanced photocatalytic activity. The reason reflected in two aspects: On one hand, C–N codoping could narrow the band gap of TiO2 and enhance the absorption of visible light. On the other hand, compared with the pure TiO2, doping C and N could create more oxygen vacancies, which could capture the photogenerated electrons effectively, and left more photogenerated holes, which are finally responsible for the degradation of pollutants into H2O and CO2. Acknowledgment The authors are grateful for the financial support of Explosion Science and Technology Key Laboratory Foundation of China (KFJJ09-7). References
Fig. 8. ESR spectra of pure and C–N co-doped TiO2 nanorods.
light (400–800 nm), implying the strong absorption of ultraviolet and the little absorption of visible light. On the contrary, C–N codoped TiO2 exhibited a significant absorption of both ultraviolet and visible light, which resulted in more photogenerated electrons and holes and further improvement in the photocatalytic efficiency of TiO2. In addition, samples calcined at 400 8C had a stronger absorption of visible light than samples calcined at 500 8C, whether doping or not, suggesting that compared with S-500, S-400 possessed the better photocatalytic efficiency. Secondly, the recombination of the photogenerated electrons and holes can occur very easily, resulting in low light quantum efficiency. Fig. 8 displays the ESR spectra of the samples. Both pure and C–N co-doped TiO2 have ESR signals, resulting from the electron trapped on the oxygen vacancy. However, the signal
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Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Fabrication of the C–N co-doped rod-like TiO2 photocatalyst with visible-light responsive photocatalytic activity Liang-Hai Li a, Juan Lu a, Zuo-Shan Wang a,b,*, Lu Yang a, Xiu-Feng Zhou a, Lu Han a a b
College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215021, China State Key Laboratory of Explosion Science and Technology, Beijing Institute of Technology, Beijing 100081, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 10 August 2011 Received in revised form 12 January 2012 Accepted 19 February 2012 Available online 25 February 2012
The C–N co-doped TiO2 nanorods were synthesized by the vapor transport method of water molecules, and urea was used as the carbon and nitrogen source. The samples were characterized by X-ray diffraction and photoelectron spectroscopy analysis. The scanning electron microscope images showed that as-prepared TiO2 powders were nanorods, which were formed by the stacking of nanoparticles with a uniform size around 40 nm. The degradation of methylene blue with the prepared nanorods demonstrated the photocatalytic activities of TiO2 under visible light are improved by doping with C and N elements. The main reasons were discussed: doping with C and N elements could enhance the corresponding visible-light absorption of TiO2. On the other hand, doping C and N could create more oxygen vacancies in the TiO2 crystals, which could capture the photogenerated electrons more effectively. Thus, more photogenerated holes could be left to improve the photocatalytic activity of TiO2. ß 2012 Elsevier Ltd. All rights reserved.
Keywords: A. Nanostructures A. Semiconductors B. Crystal growth D. Catalytic properties
1. Introduction The titanium dioxide (TiO2) has attracted many attentions since it was served as a functional material for the decomposition of organic compounds. Most of the researches focused on two issues: on one hand, the decrease of energy gap of TiO2 was explored to modify the optical response in visible light range not only ultraviolet [1]; on the other hand, how to prevent photogenerated electrons and holes from recombination and improve the photocatalytic activity [2]? Asahi firstly reported the TiO2, which was doped with N element, exhibited a high photocatalytic activity under visible light [3]. The result revealed that doping was an effective way to solve the above problems. Further study has shown that TiO2 powders doped with two different elements, such as F–N [1], B–N [4], P–N [5] and C–N [6–8] have better photocatalytic properties than single nonmetal doping. Recently, the C–N co-doped TiO2 has attracted many attentions for its excellent photocatalytic properties under visible light. However, the morphology of samples mostly has focused on nanoparticles [6–8], and the pH value should be adjusted by adding acid or alkali in almost all of the preparations. Here we report a new synthesis of C–N co-doped TiO2 nanorods by the vapor transport method of water molecules. No acid or any surfactant was used throughout
* Corresponding author at: No. 199, Ren’ai Road, Suzhou Industry Park, Suzhou 215123, China. Tel.: +86 0512 85187680; fax: +86 0512 85187680. E-mail address: [email protected] (Z.-S. Wang). 0025-5408/$ – see front matter ß 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2012.02.032
the process, and as-prepared powders possessed an excellent photocatalytic activity under visible light. 2. Experimental 2.1. Preparation of photocatalyst Typically, ethanol and tetra-n-butyl titanate were mixed with a volume ratio of 1:40, and certain urea was dissolved in the mixture by ultrasonic oscillation for 3 min. Then, the solution was transferred to the reaction flask of the rotary evaporator, in which several agate granules with a diameter of about 1 cm had been put in advance. The rotary evaporator was turned on and the system maintained at 25 8C. In addition, a vacuum pump connected with a round bottom flask containing some deionized water was turned on to provide air with a rate of 40 L min 1. At the same time, vapor molecules were brought into the reaction flask with air to react with the mixture. It could be observed that the solution changed from colorless and transparent to turbid, and finally became a cream color emulsion. Reaction stopped after 3 h, and then the emulsion was distillated at 55 8C for 10 min in a vacuum to turn to well dispersed white powders. Finally, C–N co-doped TiO2 nanorods were obtained by calcining the white powders for 2 h at 300, 400 and 500 8C. The pure and C–N co-doped TiO2 were designated as P-x and Dx, respectively, where ‘‘x’’ is the annealing temperature. S-x represented the samples annealed at the corresponding temperature (x), whether doping or not.
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2.2. Characterization of photocatalyst The crystal structure of the sample was identified by X-ray diffraction (XRD, PANalytical, X-Pert-Pro MPD) instrument with Cu-Ka radiation (voltage: 40 kV, electric current: 100 mA), performed over angular ranges of 2u = 20–808. A field-emission scanning electron microscopy (FE-SEM, Hitachi, S-4700) was used for the morphology characterization. The elemental composition of samples was determined by X-ray photoelectron spectroscopy (XPS, USA Thermo ESCALAB 250) with Al/Ka radiation (hn = 1486.6 eV). The X-ray anode runs at 150 W, and the X-ray beams spot size was 500 mm. The UV–vis reflection spectra of TiO2 samples was recorded in the range of 200–800 nm using a Japan Shimadzu UV240 UV–vis spectrophotometer equipped with an integrating sphere, and BaSO4 was used as the reference. ESR tests were carried out on Germany Bruker A300-10/12 to investigate defects of powders. 2.3. Photocatalytic degradation of methylene blue (MB) solution The photocatalytic experiment was performed in a 0.5 L cylindrical glass photocatalytic reactor (Xujiang electromechanical plant, Nanjing), where a 500 W Xe lamp was selected as the visible light source. A cut filter (ZJB 420) was inserted between the xenon lamp and reactor to eliminate ultraviolet light. The flow rate of air was kept at a constant value of 500 mL min 1. Typically, 300 mL of methylene blue (10 mg L 1) aqueous solution containing 0.05 g of photocatalyst powder was mixed in a beaker. Prior to the photocatalytic reaction, the suspension was allowed to reach adsorption/desorption equilibrium by maintaining the solutions in dark for 1 h. At a defined time interval, 5 mL of methylene blue was taken out from the reactor, and was analyzed using the UV–vis spectrophotometer. The percentage of degradation was calculated by the formula [1 (A/A0)] 100%, where A0 is the absorbance of original methylene blue solution before irradiation and Ai is the
Fig. 1. XRD patterns of pure and C–N co-doped TiO2 calcined at different temperatures. (a) D-300 8C; (b) P-300 8C; (c) D-400 8C; (d) P-400 8C; (e) D-500 8C; and (f) P-500 8C.
absorbance of methylene blue solution measured every 15 min in the process of photodegradation. 3. Results and discussion 3.1. XRD analysis XRD was used to investigate the phase structure of the pure TiO2 and C–N co-doped TiO2 powders calcined at different temperatures, and the results are shown in Fig. 1. Diffraction peaks appeared at 2u = 25.38 and 37.88 can be indexed to (1 0 1) and (0 0 4) planes of anatase as the samples were calcined at 300 8C
Fig. 2. SEM images of C–N co-doped TiO2 calcined at different temperatures: (a) 300 8C; (b) 400 8C; and (c) 500 8C.
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(Fig. 1a, b) and 400 8C (Fig. 1c, d). The rutile phase (1 1 0) appeared when the calcination temperature increased to 500 8C (Fig. 1c, d). Neither carbon nor nitrogen-derived peaks could be detected in all of the samples, mainly due to the low dosage of the dopant. The average sizes of the samples (Fig. 1a–f) were 9.7 nm, 10.2 nm, 15.9 nm, 15.1 nm, 25.9 nm and 24.9 nm, calculated with the Scherrer formula. 3.2. SEM analysis Fig. 2 shows the typical SEM images of TiO2 samples doped with C and N, calcined at 300 8C (Fig. 2a), 400 8C (Fig. 2b) and 500 8C (Fig. 2c) for 2 h. It could be clearly observed that as-prepared samples were nanorods (40–50 nm 400–500 nm), piling up together in disorder. The results of XRD indicated that the sizes of nanoparticles are in the range of 10–25 nm, so it can be deemed that the rod-like structure was formed by several TiO2 nanoparticles self-organizing along with the same direction. Fig. 2 shows that some nanoparticles had not yet grown into a nanorod and attached on the surface of the grown nanorods, which could also demonstrate our conjecture. According to the existing mechanism of crystal growth [9], a schematic illustration for the forming process of rod-like TiO2 is shown in Fig. 3. In our experiment, vapor molecules were transported with the air into the flask containing tetra-n-butyl titanate, resulting in the hydrolysis reaction in the gas-liquid interface. Colloidal nucleus was formed in this process (Fig. 3a). The rate of hydrolysis could be controlled by adjusting the flow velocity of the air. In addition, rotating and ball milling could improve the dispersion of colloidal nucleus in three-dimensional space, which is also conducive to the high-energy surface combination. The colloidal nucleus would
choose the high-energy surface to pile up, which was called epitaxy. The final products were nanorods piling up by colloidal nucleus (Fig. 3b). However, colloidal nucleuses are so easy to reunion that there are still some nanoparticles in the products (Fig. 3c), because of their small sizes and huge surface energy. As Fig. 2 presents, among all of the samples, nanorods gained by calcination at 400 8C are the most regular. In addition, the size of the nanorods could increase with the rise of the calcining temperature. 3.3. XPS analysis Fig. 4 illustrates the XPS survey spectra of C–N co-doped TiO2 calcined at 400 8C (Fig. 4a) and 500 8C (Fig. 4b), respectively. It showed that both of the two samples contained C, N, O, and Ti. The peaks appeared at 284.5 eV, 458.5 eV and 529.8 eV could be attributed to C1s, Ti2p and O1s, respectively, and the mini peak around 399.6 eV could be assigned to N1s. The accessory figures of Fig. 4a and b presented the XPS spectra of Ti2p. The micro peak at 454.9 eV was attributed to Ti–C bond [10], reflecting the small amount of dopant C since it is hard for C to enter the TiO2 lattice [11]. The binding energy of 458.5 eV was attributed to Ti–O bond. Two peaks at 455.1 eV and 461.0 eV assigned to Ti–C and Ti–N bonds illustrated that calcination generates a small amount of TiC and TiN. To further investigate the states of the dopant C and N, curves fitting of C1s and N1s XPS spectra of D-400 and D-500 with high resolution was recorded and shown in Fig. 5. The C1s core levels of C–N co-doped TiO2 revealed three peaks structures at binding energies of 284.5, 286.2 and 288.2 eV (Fig. 5a, b). The carbon peak at 284.5 eV assigned to the C–C bond may be due to the organic
Fig. 3. The schematic illustration for TiO2 nanorods.
Fig. 4. XPS spectra of C–N co-doped TiO2 at different temperatures: (a) 400 8C and (b) 500 8C.
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Fig. 5. The C1s and N1s XPS spectra of C–N co-doped TiO2 at different temperatures: (a) C1s-400 8C; (b) C1s-500 8C; (C) N1s-400 8C; and (d) N1s-500 8C.
impurities [12]. The two features of 286.2 and 288.2 eV are attributed to C–O and C–N bonds, respectively. Ti–C bond has been proved above. All these results indicate that a small quantity of C has been doped into the TiO2 lattice. The N1s core levels of C–N co-doped TiO2 revealed three peaks structures at binding energies of 395.7, 400.1, 402.0 eV (Fig. 5c, d) and 406.5 eV (Fig. 5d), corresponding to C–N bond, N2O22 , g-N2 chemisorbed on TiO2 [13] and NO3 [14], proving that some N has also been doped into the TiO2 lattice. So, it could be concluded that C–N co-doped TiO2 has been obtained by this vapor transport method. XPS results showed that the molar ratios of Ti/C and Ti/N are approximately 0.8 and 40, respectively. It suggested that a lot of C and N existed in the samples. In fact, the amount of C and N doped into TiO2 lattices is small. So, it can be ascribed to formation of carbonated species [12,15,16] and nitrides [14] which was also proved by above analysis.
Firstly, dopants C and N could expand the optical response range of TiO2 from ultraviolet to visible light, which was proved by UV–vis diffuse reflex spectra (Fig. 7) of the powders. It can be observed that the pure TiO2 has a weak reflection under the ultraviolet light (400 nm) and a strong reflection under visible
3.4. Photocatalytic activity Methylene blue (MB) was chosen as model dye to evaluate the photocatalytic activity of the photocatalysts under visible light irradiation, and the results were shown in Fig. 6. It could be observed that the C–N co-doped TiO2 has a higher catalytic efficiency, compared with the pure TiO2 prepared under the same calcination temperature. As mentioned above, the reasons mainly focused on two aspects as follows.
Fig. 6. Photocatalytic degradation of methylene blue under visible light irradiation using C–N co-doped TiO2 as the catalyst.
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intensity of C–N co-doped TiO2 is about 25–30 times stronger than that of the pure TiO2, which suggested that co-doping of C and N can create more oxygen vacancies. The g value (2.00306) can be obtained by equation: g = hv/mBHr, which is very close to the g value (2.003 0.001) of single electron associated with the oxygen vacancy [17–19]. It is well known that oxygen vacancies can capture the photogenerated electrons effectively, preventing photogenerated electrons recombining with photogenerated holes, which is finally responsible for the degradation of pollutants into H2O and CO2. So, it could be concluded that the photocatalytic activity of TiO2 under visible light can be enhanced by co-doping C and N. 4. Conclusions
Fig. 7. UV–vis diffuse reflectance spectra of pure and C–N co-doped TiO2 nanorods.
In summary, C–N co-doped TiO2 nanorods were synthesized by the vapor transport method, without acid or any surfactants. Results of XPS analysis revealed that C and N were successfully doped into the lattice of TiO2. The photocatalytic activity of samples was evaluated by the degradation of methylene blue under visible light irradiation. Compared with the pure TiO2, C–N co-doping TiO2 exhibited a significantly enhanced photocatalytic activity. The reason reflected in two aspects: On one hand, C–N codoping could narrow the band gap of TiO2 and enhance the absorption of visible light. On the other hand, compared with the pure TiO2, doping C and N could create more oxygen vacancies, which could capture the photogenerated electrons effectively, and left more photogenerated holes, which are finally responsible for the degradation of pollutants into H2O and CO2. Acknowledgment The authors are grateful for the financial support of Explosion Science and Technology Key Laboratory Foundation of China (KFJJ09-7). References
Fig. 8. ESR spectra of pure and C–N co-doped TiO2 nanorods.
light (400–800 nm), implying the strong absorption of ultraviolet and the little absorption of visible light. On the contrary, C–N codoped TiO2 exhibited a significant absorption of both ultraviolet and visible light, which resulted in more photogenerated electrons and holes and further improvement in the photocatalytic efficiency of TiO2. In addition, samples calcined at 400 8C had a stronger absorption of visible light than samples calcined at 500 8C, whether doping or not, suggesting that compared with S-500, S-400 possessed the better photocatalytic efficiency. Secondly, the recombination of the photogenerated electrons and holes can occur very easily, resulting in low light quantum efficiency. Fig. 8 displays the ESR spectra of the samples. Both pure and C–N co-doped TiO2 have ESR signals, resulting from the electron trapped on the oxygen vacancy. However, the signal
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