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International Journal of Photoenergy
Volume 2013, Article ID 616139, 7 pages
http://dx.doi.org/10.1155/2013/616139
Research Article

Nanocrystalline N-Doped Powders: Mild Hydrothermal Synthesis and Photocatalytic Degradation of Phenol under Visible Light Irradiation

Department of Inorganic Nonmetallic Materials, School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China

Received 10 January 2013; Revised 22 April 2013; Accepted 23 April 2013

Academic Editor: Jiaguo Yu

Copyright © 2013 Junna Xu et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Nitrogen-doped TiO2 powders have been prepared using technical guanidine hydrochloride, titanyl sulfate, and urea as precursors via a mild hydrothermal method under initial pressure of 3MPa, holding for 2h without any postheat treatment for crystallization. The nanocrystalline N-doped TiO2 powders were composed of anatase TiO2 by XRD. The grain size was estimated as about 10 nm, and the BET specific surface area of the powder was measured as 154.7 m2/g. The UV-visible absorption spectra indicated that the absorption edge of the N-doped TiO2 powders had been red shifted into the visible light region. The photocatalytic performance of the synthesized powders was evaluated by degradation of phenol under visible light irradiation. And the effects of the catalyst load and the initial pH value on the photodegradation were also investigated.

1. Introduction

Water pollution has become a major problem in recent years. The biological, chemical, and physical-chemical water treatment methods are often ineffectively or environmentally incompatible [1, 2]. Heterogeneous photocatalytic oxidation has been considered as one of the effective methods to treat the pollutants in water through the photo-generated hydroxyl radials and super oxygen species [3, 4]. photocatalyst has attracted much attention due to its low cost, high stability, nontoxicity, and environmental friendly properties [5, 6]. However, can only be excited by UV light ( nm, accounts only about 5% of sunlight) because of its large band gap of 3.2 eV for anatase which limits its application in practical use. Many methods have been adopted to extend its optical response to visible light (accounts about 45% of sunlight), such as doping with impurities [710], sensitizing by dyes [11], and coupling with semiconductors [12].

Doping with various elements is the most effective way to narrow its band gap. Among these dopants, nitrogen has attracted much attention in recent years due to its small ionization energy, comparable atomic size with oxygen, metastable center formation, and enhanced UV-Vis photocatalytic performance [7]. Many works on the preparation of N- by physical or chemical methods have been published [1322]. Among these methods, N- powders were mainly prepared either by calcination of powders under nitrogen containing atmosphere or by postheat treatment of the Sol-Gel/hydrothermally prepared amorphous with nitrogen agents at high temperature for both doping nitrogen atoms into the lattice and crystallization. However, calcination under high temperature may cause agglomeration and abnormal growth of crystals which results in small surface area and decreases the number of photoactive sites. So, it is necessary to seek an appropriate route to synthesize N- effectively under mild conditions. However, it is not so easy to dope nitrogen into under mild hydrothermal conditions because the bond energy between Ti–O is greater than that of Ti–N bond [23]. Some researchers have tried to synthesize N- powders by one-step hydrothermal method without postheat treatment in a relatively lower temperature [24]. It was found that N- could be synthesized through one-step hydrothermal method at 150°C for 8 h [25]. N- nanoparticles were prepared by using Degussa P25 and triethanol amine as precursors via hydrothermal method at 140°C holding for 24 h [26]. So, it always needs longer reaction time in the one-step hydrothermal method to prepare N-doped .

In this paper, well crystallized N-doped powders with enhanced visible light absorption have been prepared by a mild hydrothermal method at 150°C holding for 2 h without any postheat treatment for crystallization. The photocatalytic performance of the synthesized powders was evaluated by degradation of phenol in aqueous solution under visible light irradiation.

2. Experimental

N-doped TiO2 powders were prepared via a hydrothermal method at 150°C holding for 2 h. Technical grade , CO(NH2)2 and were used as received and without any further purification. In a typical synthesis process, titanyl sulfate (1 M), urea (1.5 M), and guanidine hydrochloride (3 M) were dissolved in 1 L distilled water and stirred for 30 min, and then the mixed solution was transferred into an autoclave with an internal volume of 2 L. The initial pressure was set at 3 MPa, and the stirring speed was fixed at 300 r/min. The autoclave was heated to 150°C holding for 2 h. The obtained suspension was centrifuged and washed with deionized water until no was detected in the washes. After drying at 90°C in air, the yellow colored N-doped powders were obtained. The white undoped TiO2 was also synthesized under the same processing parameters.

The synthesized N-doped powders were characterized by X-ray diffraction, Brunauer-Emmett-Teller (BET), Transmission electron microscopy, UV-Visible diffuse reflectance spectra, and X-ray photoelectron spectroscopy. The photocatalytic activity of N-doped powders was evaluated by photodegradation of phenol in aqueous solution under 400~650 nm visible light irradiation with illuminant intensity of 15.54 mW/cm2. Firstly, 0.25 g of N-doped powders were mixed into 100 mL phenol aqueous solution with a concentration of 25 mg/L, and then the suspension was stirred in the dark for 2 h to reach the balance of adsorption/desorption. The balanced concentration of phenol was treated as the initial phenol concentration. Then, the light was turned on to start photodegradation. The suspension was sampled at an interval of 2 h and centrifuged to remove the N-doped TiO2 particles. The concentration of phenol was measured on a UV-Visible spectrophotometer (TU-1901) at the wavelength of 270 nm to obtain the absorbance of the solution.

3. Results and Discussion

Figure 1 shows the XRD patterns of the synthesized pure and N-doped TiO2 powders. All the peaks are corresponding to those of anatase. The intensity of the peaks of N-TiO2 is slightly weakened compared with that of undoped TiO2. From the line broadening of the (101) diffraction peak by Scherrer's method, the grain sizes of undoped and N-doped TiO2 are about 9.7 and 10.0 nm, respectively. The specific surface areas for undoped and N-doped TiO2 are 150.6 and 154.7 m2/g, respectively.

616139.fig.001
Figure 1: XRD patterns of the undoped and N-doped TiO2 powders.

Figure 2 shows the TEM micrograph of N-doped TiO2 powders. The particles are approximately spherical in shape and 10 nm in size, which is consistent with the results calculated by Scherrer's method.

616139.fig.002
Figure 2: TEM micrograph of N-doped TiO2 powders.

The UV-Visible absorption spectra of undoped and N-doped powders are shown in Figure 3(a). It is clear that N-doped powders exhibit stronger visible light absorption while the undoped can absorb only UV light. The straight-line extrapolation method was used to determine the light absorption edge of the powders, as indicated in Figure 3(a). The absorption edge of the undoped is approximately 390 nm while that of N-doped is around 420 nm. According to the equation , the band gap energies of the undoped and N-doped are 3.18 and 2.95 eV, respectively. As has been considered as an indirect semiconductor [27, 28], the band gap energy of the samples can be estimated by Kubelka-Munk function [29, 30], as shown in Figure 3(b). Band gap energy calculated from the intercept of the tangent lines in the plots are 3.10 and 2.90 eV for undoped and N-doped , respectively, which is in accordance with the UV-Vis results.

fig3
Figure 3: (a) UV-Visible absorption spectra of undoped and N-doped TiO2 powders and (b) plots of (αhν)0.5 versus photoenergy of the synthesized samples.

The concentration and the electronic state of nitrogen at the surface of the N-doped powders were measured by XPS, as shown in Figure 4. The XPS survey spectra as indicated in Figure 4(a) show that the surface of N-doped is composed of Ti, O, N, and carbon. The concentration of nitrogen is about 1.6% (atomic percent). Yang et al. [31] suggested that when the doping concentration of nitrogen was less than 2.1% (atomic percent), an isolated N2p states would be created above the O2p valance band, which might contribute to the visible light sensitivity. Figure 4(b) shows the binding energy of N1s located at both 396.0 and 399.9 eV. The weaker peak located at 396.0 eV can be attributed to the Ti–N bond which indicates that nitrogen atoms have been incorporated into the lattice of [32, 33]. The stronger peak located at 399.9 eV can be assigned to Ti–O–N linkage which is considered as the interstitial nitrogen atoms in the lattice of and has been reported in many literatures [3436].

fig4
Figure 4: XPS survey spectra of (a) survey spectrum and (b) N1s.

The photodegradation of phenol aqueous solution under different conditions is shown in Figure 5. In order to confirm any possible self-degradation of phenol under visible light irradiation, the light irradiation of phenol solution with the same illumination intensity was processed. It can be seen in Figure 5 that the phenol concentration remains almost unchanged with the elapse of irradiation time. That is, the phenol will not be degraded by the visible light irradiation.

616139.fig.005
Figure 5: Photodegradation of phenol with 0.25 g catalysts at under different conditions.

The effect of the phenol adsorption by the catalysts on the photocatalytic degradation process has been investigated by stirring the phenol-catalysts suspensions in the dark for 10 h. The concentration of phenol was decreased only 2% during the 10 h black stirring as indicated in Figure 5. So, the decrease of the concentration of the phenol caused by the adsorption of the catalysts can be ignored. Forty-seven percent of phenol has been degraded by N-TiO2 powders while that by undoped TiO2 is 15%. N-doped TiO2 exhibits better photoactivity than that of undoped TiO2, which indicates that nitrogen doping has improved the photocatalytic activity of TiO2 in the visible light region.

Catalyst load is an important factor that affects the photocatalytic degradation of phenol. The influence of the load of photocatalyst was performed by varying the amount of N-doped from 0.1 to 2.0 g, as indicated in Figure 6. The maximum photocatalytic degradation is obtained when using 0.25 g N-doped powders. Further increase or decrease, the amount of catalysts will reduce the photodegradation. The results are in agreement with the data published by other researchers [37, 38]. When the amount of the charged catalyst is too high, turbidity will hinder the light penetration and enhance the light scattering [39]; thus, the photocatalytic activity of the catalyst will be reduced due to the decreased light intensity. Besides, the aggregation of particles increases with the increase of the amount of the photocatalyst which may also decrease the total active sites for the adsorption of photos and phenol [34], resulting in the decrease of photodegradation.

616139.fig.006
Figure 6: Effects of N-doped TiO2 powders load on the photodegradation of phenol at .

The pH value of the aqueous solution affects the surface charge of the catalyst as well as phenol. The effect of pH value on photodegradation of phenol was investigated in the range of 4 to 10 by adjusting the solution with 1 mol/L HCl and NaOH, as shown in Figure 7. The maximum photodegradation is observed at , and it can reach 81% after 10 h of visible light irradiation. The similar results were also reported in [40]. Figure 8 shows the zeta potential of N-doped in phenol aqueous solution at different pH values. The isoelectric point of N-doped particle is 6.4. According to the isoelectric point of N-doped , the following reactions may occur on the surface of N-doped at different pH values: That is, the surface of N-doped TiO2 will be positively charged at pH < 6.4 and negatively charged at pH > 6.4. As the dissociation constants of phenol are 9.96 () [41], the phenol will mainly exist as neutral species when pH value is less than the , while it will be negatively charged when the pH is larger than the . That is, phenol molecules can be scarcely adsorbed on the surface of N-doped particles due to the columbic repulsion between them as the surface of N-doped particles and phenol molecules are both negatively charged when pH = 10. Therefore, very little phenol could be decomposed. At the case when , phenol is primarily presented in neutral molecular form while the surface of particles are positively (or negatively when pH > 6.4) charged. The electrostatic attraction between particles and phenol molecule increases the amount of adsorbed phenol and then enhances the photodegradation. Besides, agglomeration has also been considered as one of the key factors that affect the photocatalytic activity. When the pH is relatively low, large repulsive forces between the particles may occur due to the positive charges caused by the reaction (1); thus, the dispersion of the particles in the suspension could be improved and the photocatalytic performance could be enhanced as well due to the decreased agglomeration of the particles. Therefore, the better photodegradation occurs at the case when pH value is 4.

616139.fig.007
Figure 7: Effect of pH value on the photodegradation of phenol with 0.25 g N-TiO2 powders.
616139.fig.008
Figure 8: Zeta potential of N-doped TiO2 powders in phenol aqueous solution at different pH values.

The photodegradation of phenol is complex and may produce complex intermediates such as toxic aromatic intermediates as reported in many researches [42, 43]. Generally, phenol is firstly oxidized to aromatic intermediates such as hydroquinone, catechol, benzoquinone, and 4, 4′-dihydroxybiphenyl, and then phenyl rings of theses aromatic intermediates break up to form aliphatic acid which is further oxidized to short-chain organic acid such as oxalic acid, acetic acid, and formic acid. Finally, they are decomposed to carbon dioxide and water completely [11, 37]. The aromatic intermediates always have colors. For example, the color of the mixture of catechol and hydroquinone aqueous solution is pink, while that of m-dihydroxybenzene and benzoquinone are yellow and brown, respectively [44]. The absorption peaks corresponding to aromatic intermediates and ring cleavage products have been detected in the UV-Vis absorption spectra of phenol degradation by researchers [45, 46]. However, in our experiments, the solution after photodegradation is colorless, and no clear absorption peaks corresponding to aromatic intermediates and ring cleavage products are detected in the spectra, as shown in Figure 9, which indicates that there is little photodegradation intermediates produced in the photodegradation of phenol process and the good photocatalytic performance of our samples.

616139.fig.009
Figure 9: UV-Vis absorption spectra of phenol degradation with 0.25 g N-doped TiO2 at pH 4.

4. Conclusion

N-doped was successfully synthesized by a mild hydrothermal method at 150°C and holding for 2 h without any postheat treatment for crystallization. The grain size of the powders was about 10 nm, and the specific surface area of N-doped TiO2 was 154.7 m2/g. The absorption edge of N-doped TiO2 had been shifted to 420 nm, and the band energy was about 2.90 eV estimated by Kubelka-Munk function after nitrogen doping. Nitrogen atoms have been successfully incorporated into the lattice of TiO2. The amount of catalyst and pH value was optimized as 0.25 g and 4, respectively, for better photocatalytic performance in our experiment conditions. The photodegradation reached 81% after 10 h of visible light irradiation.

Acknowledgments

This work has been financially supported by the National High Technology Research and Development Program of China (Grant no. 2012AA030302), the National Natural Science Foundation of China (Grant no. 51072019), and the Opening Project of State Key Laboratory of High Performance Ceramics and Superfine Microstructure under Grant SKL201112SIC.

References

  1. D. Gümüş and F. Akbal, “Photocatalytic degradation of textile dye and wastewater,” Water, Air, and Soil Pollution, vol. 216, no. 1–4, pp. 117–124, 2011. View at Publisher · View at Google Scholar · View at Scopus
  2. A. M. De Luis, J. I. Lombraña, A. Menéndez, and J. Sanz, “Analysis of the toxicity of phenol solutions treated with H2O2/UV and H2O2/Fe oxidative systems,” Industrial and Engineering Chemistry Research, vol. 50, no. 4, pp. 1928–1937, 2011. View at Publisher · View at Google Scholar · View at Scopus
  3. N. Kashif and F. Ouyang, “Parameters effect on heterogeneous photocatalysed degradation of phenol in aqueous dispersion of TiO2,” Journal of Environmental Sciences, vol. 21, no. 4, pp. 527–533, 2009. View at Publisher · View at Google Scholar · View at Scopus
  4. S. Ahmed, M. G. Rasul, W. N. Martens, R. Brown, and M. A. Hashib, “Advances in heterogeneous photocatalytic degradation of phenols and dyes in wastewater: a review,” Water, Air, and Soil Pollution, vol. 215, no. 1–4, pp. 3–29, 2011. View at Publisher · View at Google Scholar · View at Scopus
  5. A. Fujishima and K. Honda, “Electrochemical photolysis of water at a semiconductor electrode,” Nature, vol. 238, no. 5358, pp. 37–38, 1972. View at Publisher · View at Google Scholar · View at Scopus
  6. A. Fujishima, T. N. Rao, and D. A. Tryk, “Titanium dioxide photocatalysis,” Journal of Photochemistry and Photobiology C, vol. 1, no. 1, pp. 1–21, 2000. View at Google Scholar · View at Scopus
  7. X. Qiu and C. Burda, “Chemically synthesized nitrogen-doped metal oxide nanoparticles,” Chemical Physics, vol. 339, no. 1–3, pp. 1–10, 2007. View at Publisher · View at Google Scholar · View at Scopus
  8. Y. Li, W. Cao, F. Ran, and X. Zhang, “Photocatalytic degradation of methylene blue aqueous solution under visible light irradiation by using N-doped titanium dioxide,” Key Engineering Materials, vol. 336-338, pp. 1972–1975, 2007. View at Google Scholar · View at Scopus
  9. H. Q. Wang, Z. B. Wu, and Y. Liu, “A simple two-step template approach for preparing carbon-doped mesoporous TiO2 hollow microspheres,” Journal of Physical Chemistry C, vol. 113, no. 30, pp. 13317–13324, 2009. View at Publisher · View at Google Scholar · View at Scopus
  10. K. Huang, L. Chen, M. Liao, and J. Xiong, “The photocatalytic inactivation effect of Fe-doped TiO2 nanocomposites on Leukemic HL60 cells-based photodynamic therapy,” International Journal of Photoenergy, vol. 2012, Article ID 367072, 8 pages, 2012. View at Publisher · View at Google Scholar
  11. P. Chowdhury, J. Moreira, H. Gomaa, and A. K. Ray, “Visible-solar-light-driven photocatalytic degradation of phenol with dye-sensitized TiO2: parametric and kinetic study,” Industrial & Engineering Chemistry Research, vol. 51, no. 12, pp. 4523–4532, 2012. View at Google Scholar
  12. R. Khan and T. J. Kim, “Preparation and application of visible-light-responsive Ni-doped and SnO2 -coupled TiO2 nanocomposite photocatalysts,” Journal of Hazardous Materials, vol. 163, no. 2-3, pp. 1179–1184, 2009. View at Publisher · View at Google Scholar · View at Scopus
  13. M. H. Chan and F. H. Lu, “Characterization of N-doped TiO2 films prepared by reactive sputtering using air/Ar mixtures,” Thin Solid Films, vol. 518, no. 5, pp. 1369–1372, 2009. View at Publisher · View at Google Scholar · View at Scopus
  14. W. Guo, Y. Shen, G. Boschloo, A. Hagfeldt, and T. Ma, “Influence of nitrogen dopants on N-doped TiO2 electrodes and their applications in dye-sensitized solar cells,” Electrochimica Acta, vol. 56, no. 12, pp. 4611–4617, 2011. View at Publisher · View at Google Scholar · View at Scopus
  15. Y. Wang, C. Feng, M. Zhang, J. Yang, and Z. Zhang, “Visible light active N-doped TiO2 prepared from different precursors: origin of the visible light absorption and photoactivity,” Applied Catalysis B, vol. 104, no. 3-4, pp. 268–274, 2011. View at Publisher · View at Google Scholar · View at Scopus
  16. D. Huang, S. Liao, S. Quan et al., “Synthesis and characterization of visible light responsive N-TiO2 mixed crystal by a modified hydrothermal process,” Journal of Non-Crystalline Solids, vol. 354, no. 33, pp. 3965–3972, 2008. View at Publisher · View at Google Scholar · View at Scopus
  17. R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, and Y. Taga, “Visible-light photocatalysis in nitrogen-doped titanium oxides,” Science, vol. 293, no. 5528, pp. 269–271, 2001. View at Publisher · View at Google Scholar · View at Scopus
  18. S. Z. Hu, F. Y. Li, and Z. P. Fan, “The influence of preparation method, nitrogen source, and post-treatment on the photocatalytic activity and stability of N-doped TiO2 nanopowder,” Journal of Hazardous Materials, vol. 196, pp. 248–254, 2011. View at Google Scholar
  19. B. Kosowska, S. Mozia, A. W. Morawski, B. Grzmil, M. Janus, and K. Kałucki, “The preparation of TiO2-nitrogen doped by calcination of TiO2 · xH2O under ammonia atmosphere for visible light photocatalysis,” Solar Energy Materials and Solar Cells, vol. 88, no. 3, pp. 269–280, 2005. View at Publisher · View at Google Scholar · View at Scopus
  20. B. Baruwati and R. S. Varma, “Synthesis of N-doped nano TiO2 using guanidine nitrate: an excellent visible light photocatalyst,” Journal of Nanoscience and Nanotechnology, vol. 11, no. 3, pp. 2036–2041, 2011. View at Publisher · View at Google Scholar · View at Scopus
  21. S. Livraghi, A. M. Czoska, M. C. Paganini, and E. Giamello, “Preparation and spectroscopic characterization of visible light sensitized N doped TiO2 (rutile),” Journal of Solid State Chemistry, vol. 182, no. 1, pp. 160–164, 2009. View at Publisher · View at Google Scholar · View at Scopus
  22. M. D'Arienzo, R. Scotti, L. Wahba et al., “Hydrothermal N-doped TiO2: explaining photocatalytic properties by electronic and magnetic identification of N active sites,” Applied Catalysis B, vol. 93, no. 1-2, pp. 149–155, 2009. View at Publisher · View at Google Scholar · View at Scopus
  23. H. Diker, C. Varlikli, K. Mizrak, and A. Dana, “Characterizations and photocatalytic activity comparisons of N-doped nc-TiO2 depending on synthetic conditions and structural differences of amine sources,” Energy, vol. 36, no. 2, pp. 1243–1254, 2011. View at Publisher · View at Google Scholar · View at Scopus
  24. D. Wu, M. Long, W. Cai, C. Chen, and Y. Wu, “Low temperature hydrothermal synthesis of N-doped TiO2 photocatalyst with high visible-light activity,” Journal of Alloys and Compounds, vol. 502, no. 2, pp. 289–294, 2010. View at Publisher · View at Google Scholar · View at Scopus
  25. N. Bao, J. J. Niu, Y. Li, G. L. Wu, and X. H. Yu, “Low-temperature hydrothermal synthesis of N-doped TiO2 from small-molecule amine systems and their photocatalytic activity,” Environmental Technology, vol. iFirst, pp. 1–11, 2012. View at Google Scholar
  26. F. Peng, L. Cai, L. Huang, H. Yu, and H. Wang, “Preparation of nitrogen-doped titanium dioxide with visible-light photocatalytic activity using a facile hydrothermal method,” Journal of Physics and Chemistry of Solids, vol. 69, no. 7, pp. 1657–1664, 2008. View at Publisher · View at Google Scholar · View at Scopus
  27. G. Wang, L. Xu, J. Zhang, T. Yin, and D. Han, “Enhanced photocatalytic activity of TiO2 powders (P25) via calcination treatment,” International Journal of Photoenergy, vol. 2012, Article ID 265760, 9 pages, 2012. View at Publisher · View at Google Scholar
  28. G. Shang, H. Fu, S. Yang, and T. Xu, “Mechanistic study of visible-light-induced photodegradation of 4-chlorophenol by TiO2xNx with low nitrogen concentration,” International Journal of Photoenergy, vol. 2012, Article ID 759306, 9 pages, 2012. View at Publisher · View at Google Scholar
  29. X. W. Cheng, X. J. Yu, Z. P. Xing, and L. S. Yang, “Enhanced visible light photocatalytic activity of mesoporous anatase TiO2 co-doped with nitrogen and chlorine,” International Journal of Photoenergy, vol. 2012, Article ID 593245, 6 pages, 2012. View at Publisher · View at Google Scholar
  30. K. Villa, A. Black, X. Domnech, and J. Peral, “Nitrogen doped TiO2 for hydrogen production under visible light irradiation,” Solar Energy, vol. 86, no. 1, pp. 558–566, 2012. View at Google Scholar
  31. K. Yang, Y. Dai, and B. Huang, “Study of the nitrogen concentration influence on N-doped TiO2 anatase from first-principles calculations,” Journal of Physical Chemistry C, vol. 111, no. 32, pp. 12086–12090, 2007. View at Publisher · View at Google Scholar · View at Scopus
  32. H. Irie, Y. Watanabe, and K. Hashimoto, “Nitrogen-concentration dependence on photocatalytic activity of TiO2xNx powders,” Journal of Physical Chemistry B, vol. 107, no. 23, pp. 5483–5486, 2003. View at Google Scholar · View at Scopus
  33. M. Kurtoglu, T. Longenbach, K. Sohlberg, and Y. Gogotsi, “Strong coupling of Cr and N in Cr-N-doped TiO2 and its effect on photocatalytic activity,” Journal of Physical Chemistry C, vol. 115, no. 35, pp. 17392–17399, 2011. View at Google Scholar
  34. H. Sun, Y. Bai, H. Liu et al., “Mechanism of nitrogen-concentration dependence on pH value: experimental and theoretical studies on nitrogen-doped TiO2,” Journal of Physical Chemistry C, vol. 112, no. 34, pp. 13304–13309, 2008. View at Publisher · View at Google Scholar · View at Scopus
  35. X. Zhang, K. Udagawa, Z. Liu et al., “Photocatalytic and photoelectrochemical studies on N-doped TiO2 photocatalyst,” Journal of Photochemistry and Photobiology A, vol. 202, no. 1, pp. 39–47, 2009. View at Publisher · View at Google Scholar · View at Scopus
  36. J. Fang, F. Shi, J. Bu et al., “One-step synthesis of bifunctional TiO2 catalysts and their photocatalytic activity,” Journal of Physical Chemistry C, vol. 114, no. 17, pp. 7940–7948, 2010. View at Publisher · View at Google Scholar · View at Scopus
  37. H. J. Jung, J. S. Hong, and J. K. Suh, “A study on removal efficiency of phenol and humic acid using spherical activated carbon doped by TiO2,” Korean Journal of Chemical Engineering, vol. 28, no. 9, pp. 1882–1888, 2011. View at Google Scholar
  38. C. C. Chen, C. S. Lu, Y. C. Chung, and J. L. Jan, “UV light induced photodegradation of malachite green on TiO2 nanoparticles,” Journal of Hazardous Materials, vol. 141, no. 3, pp. 520–528, 2007. View at Publisher · View at Google Scholar · View at Scopus
  39. W. Bahnemann, M. Muneer, and M. M. Haque, “Titanium dioxide-mediated photocatalysed degradation of few selected organic pollutants in aqueous suspensions,” Catalysis Today, vol. 124, no. 3-4, pp. 133–148, 2007. View at Publisher · View at Google Scholar · View at Scopus
  40. D. Zhao, J. Cheng, and M. R. Hoffmann, “Kinetics of microwave-enhanced oxidation of phenol by hydrogen peroxide,” Frontiers of Environmental Science & Engineering in China, vol. 5, no. 1, pp. 57–64, 2011. View at Publisher · View at Google Scholar · View at Scopus
  41. S. H. Lin, C. H. Chiou, C. K. Chang, and R. S. Juang, “Photocatalytic degradation of phenol on different phases of TiO2 particles in aqueous suspensions under UV irradiation,” Journal of Environmental Management, vol. 92, no. 12, pp. 3098–3104, 2011. View at Google Scholar
  42. Z. Guo, R. Ma, and G. Li, “Degradation of phenol by nanomaterial TiO2 in wastewater,” Chemical Engineering Journal, vol. 119, no. 1, pp. 55–59, 2006. View at Publisher · View at Google Scholar · View at Scopus
  43. E. B. Azevedo, F. R. D. A. Neto, and M. Dezotti, “TiO2-photocatalyzed degradation of phenol in saline media: lumped kinetics, intermediates, and acute toxicity,” Applied Catalysis B, vol. 54, no. 3, pp. 165–173, 2004. View at Publisher · View at Google Scholar · View at Scopus
  44. B. Yang, “Study on intermediates varation in the course of phenol degradation,” Sci-Tech Information Development & Economy, vol. 7, no. 13, pp. 129–123, 2003. View at Google Scholar
  45. L. Xiong, L. Zheng, J. Xu et al., “Photocatalytic degradation of phenol with mesoporous TiO2xBx,” Environmental Chemistry Letters, vol. 9, no. 2, pp. 251–257, 2011. View at Publisher · View at Google Scholar · View at Scopus
  46. L. Liu, H. Liu, Y. P. Zhao et al., “Directed synthesis of hierarchical nanostructured TiO2 catalysts and their morphology-dependent photocatalysis for phenol degradation,” Environmental Science and Technology, vol. 42, no. 7, pp. 2342–2348, 2008. View at Publisher · View at Google Scholar · View at Scopus