Preparation and Photocatalytic Properties of g-c 3 N 4 /TiO 2 Hybrid Composite

Transcription

1 J. Mater. Sci. Technol., 2010, 26(10), Preparation and Photocatalytic Properties of g-c 3 N 4 /TiO 2 Hybrid Composite Xifeng Lu 1,2,3), Qilong Wang 1) and Deliang Cui 1) 1) State Key Lab of Crystal Materials, Shandong University, Jinan , China 2) Lunan Institute of Coal Chemical Engineering, Jining , China 3) State Quality Supervisor and Inspection Center of Coal and Coal Chemical Product, Jining , China [Manuscript received April 16, 2009, in revised form July 17, 2009] g-c 3 N 4 /TiO 2 composite were prepared by hydrolysis of Ti(OC 4 H n 9 ) 4 and the precursors of g-c 3 N 4 at room temperature and annealing in nitrogen atmosphere. X-ray diffraction results revealed that all the products were anatase structure. The chemical nature of O, N of the g-c 3 N 4 /TiO 2 were identified by X-ray photoelectron spectroscopy, presenting N-Ti-O and N-Ti-N doping status of the composite. The g-c 3 N 4 /TiO 2 composite showed better photocatalytic activity for the UV and visible-light degradation of Rhodamine B. KEY WORDS: Composite materials; Chemical synthesis; X-ray photoelectron spectroscopy (XPS) 1. Introduction Over the last few years, the design and synthesis of photocatalysts have attracted considerable interest due to their potential applications in environmental cleaning and water purification by the degradation of organic compounds [1 5]. Among the various photocatalytic materials, TiO 2 is recognized as the most promising candidate for its low cost, nontoxicity, high stability, and high efficiency for eliminating persistent organic pollutants [6 8]. However, the high efficient photocatalysis of TiO 2 has failed to be realized, owing to its wide band gap (3.2 ev) which requires ultraviolet irradiation (λ 387 nm), i.e., less than 5% of the solar energy at the earth surface, to generate photocatalytic activation [8]. Currently, one promising way to overcome this problem was doping TiO 2 with some metals (FeIII, MoV, RuIII, ReV, VIV, RhIII and GdIII) [3,9] as well as nonmetals such as B, C, N, F and S [6, 7, 10 14], so as to change the electronic structure of TiO 2 and enhance its photocatalytic reactivity. A lot of efforts have been devoted to optimizing the choice of the dopant precursors in the syn- Corresponding author. Prof., Ph.D.; Tel./Fax: ; address: cuidl@sdu.edu.cn (D.L. Cui). thesis process. However, the low quantum efficiency still limits its practical application [8] for the recombination of photogenerated electrons and holes. It has been reported that hybrid composite based on TiO 2 can promote the separation of photogenerated charge carriers [15 17] to improve the photocatalytic efficiency. As a novel function materials, carbon nitride has attracted attention because of its particular properties [18 20]. It is expected that the g-c 3 N 4 can alter the surface charge of titania and enhance its photocatalytic activity. To our knowledge, the synthesis of g-c 3 N 4 /TiO 2 hybrid materials was firstly reported. In the present work, we prepared g-c 3 N 4 /TiO 2 composite by hydrolyzing a mixture of Ti(OC 4 H 9 n) 4 and g-c 3 N 4 precursors (sol-gel), and annealing them in nitrogen flow. Furthermore, the photocatalytic degradation of Rhodamine B was investigated. 2. Experimental All the chemicals were analytical grade and were used as received without further purification. The principal processing steps are illustrated by the flowchart given in Fig. 1.

2 926 X.F. Lu et al.: J. Mater. Sci. Technol., 2010, 26(10), Fig. 2 X-ray diffraction patterns of (a) as-synthesized pure TiO 2, (b) precursors of g-c 3N 4/TiO 2, (c) hybrid composite of g-c 3N 4/TiO 2, (d) hybrid composite of g-c 3N 4/TiO 2 after hydrothermal reaction were obtained (XRD results, Fig. 2(c) and (d)). 2.3 Preparation of TiO 2 TiO 2 was synthesized through a similar procedure used for preparing g-c 3 N 4 /TiO 2 composite except that the mixture of sol-gel and Ti(OC 4 H n 9 ) 4 was replaced by Ti(OC 4 H n 9 ) Characterization Fig. 1 Flowchart of the preparation procedure 2.1 Preparation of sol-gel precursors The sol-gel (carbon nitride precursors) was prepared according to literature [21]. 2.2 Preparation of g-c 3 N 4 /TiO 2 The above sol-gel (1 ml) and Ti(OC 4 H 9 n) 4 (6 ml) were mixed at room temperature. A homogeneous colloidal solution was obtained after vigorously stirring for 5 h, followed by dropwise addition of 5 ml de-ionized water into the mixture under continuously stirring. Then a brown precipitate of precursors of g-c 3 N 4 /TiO 2 was obtained. The precipitate was filtered and washed with deionized water and dried in the air at room temperature, which was amorphous powder (according to XRD results, Fig. 2(b)). A quantity of the precursor was put into a 20 ml Teflon-lined stainless clave, which was filled with 12 ml deionized water and sealed. The autoclave was maintained at 180 C for 5 h, and then allowed to cool to room temperature. The resulting product was filtered and washed with deionzed water, and then dried at room temperature. The above precursors were further calcined at 600 C for 5 h under N 2 flow, respectively, and the g-c 3 N 4 /TiO 2 composites X-ray powder diffraction (XRD) patterns were obtained with a Rigaku D/Max-γA X-ray powder diffractometer, using CuKα radiation (λ= nm). Raman spectra were recorded on a Thermal NXR Raman Module spectrometer with an excitated wavelength of 1064 nm. Surface property of the samples was characterized by X-ray photoelectron spectroscopy (XPS) on a PHI 5300 X-ray photoelectron spectrometer with a monochromatic AlKα source. Scanning electron microscopy (SEM) images, energy dispersive X-ray spectroscopy (EDS) pattern and EDS map were performed on a scanning electron microscope (S-4800) at an accelerating voltage of 5 kv. UV-vis diffuse reflectance spectra were recorded on a Shimadzu UV-2550 spectrometer, using BaSO 4 powder as an internal reference. 2.5 Photocatalytic activity The liquid-phase photocatalytic degradation of rhodamine B (RhB) was carried out at room temperature in a 100 ml glassy reactor containing catalyst (0.15 g) and RhB aqueous solution (30 ml, 0.01 g/l). The reaction system was exposed in the air, and the solution was irradiated by a 500 W xenon lamp (λ>365 nm) and a 250 W UV light source (λ= nm), respectively, which were located at 5 cm above the solution. The RhB solution containing catalyst was magnetically stirred for 2 h to reach adsorption equilibrium in darkness. After a defined

3 X.F. Lu et al.: J. Mater. Sci. Technol., 2010, 26(10), Fig. 3 Raman spectra of (I) hybrid composite of g- C 3N 4/TiO 2 after hydrothermal reaction, (II) hybrid composite of g-c 3N 4/TiO 2, (III) assynthesized pure TiO 2. (a) full length spectra from cm 1, (b) expanded spectra in the range cm 1 interval, the change of solution absorption at 549 nm was applied to identify the concentration of the RhB by using a U-4100 spectrophotometer. The reproducibility of the results was checked by repeating the results three times. 3. Results and Discussion 3.1 XRD patterns and Raman spectra Figure 2 shows a typical set of XRD patterns for as-synthesized TiO 2, g-c 3 N 4 /TiO 2 precursors and the corresponding hybrid composites. In comparison with Fig. 2(c) and (d), it is found that hydrothermal treatment dramatically improves the crystalline perfection of g-c 3 N 4 /TiO 2 composite. The well-established peaks in Fig. 2(d) can be ascribed to (101), (103), (004), (112), (200), (105), (211), (206) and (116) planes of anatase TiO 2 (JCPDS card No ), and no other rutile or brookite secondary phases can be observed. In addition, the shift of peak position of the samples caused by hybrid is not identified. However, in comparison with Fig. 2(a), the broadness of the peaks shown in Fig. 2(c) exhibits the hybrid effect. It is difficult or impossible for the C and N atoms to dope in the TiO 2 crystal lattice, because g-c 3 N 4 is a huge polymer molecule (supplementary data, Fig. S-I), It may be reasonably concluded that the N atoms with lone electron pairs of carbon nitride coordinate with the Ti centers on the exterior surface of TiO 2 matrix (supplementary data, Fig. S-I), i.e., the presence of interface of g-c 3 N 4 /TiO 2, leading to the formation of N-Ti-N (O) and surface reconstruction of Ti which confines the growth of TiO 2 nanoparticles in the annealing process, resulting in the broadness of the peaks. Figure 3 shows the Raman spectra of assynthesized TiO 2 and g-c 3 N 4 /TiO 2 hybrid composites. Generally, the Ti-O-Ti network structure can be identified by the Raman peaks over cm 1 range [22 25], wherein six characteristic Raman active fundamental modes at 142 (E g ), 197 (E g ), 397 (B 1g ), 518 (A 1g +B 1g ), and 640 cm 1 (E g ) correspond to anatase TiO 2. As is shown in Fig. 3(a), the positions of these peaks can be attributed to the typical anatase TiO 2. However, in comparison with Fig. 3(bIII) for pure TiO 2 sample, the Raman peaks in Fig. 3(bI) and (bii) (a) and (b) for g-c 3 N 4 /TiO 2 samples redshift slightly, i.e., the Eg mode at cm 1 for pure TiO 2 shifts to 144 cm 1 and 145 cm 1 for hybrid g- C 3 N 4 /TiO 2 samples, which can be ascribed to the formation of g-c 3 N 4 /TiO 2 hybrid composites. 3.2 X-ray photoelectron spectroscopy and UV-vis diffuse reflectance spectra Figure 4 shows the XPS spectra of the Ti2p, O1s and N1s region of pure TiO 2 and g-c 3 N 4 /TiO 2. In comparison with those of pure TiO 2, the binding energies of Ti2p in g-c 3 N 4 /TiO 2 shift to lower energies (Fig. 4a, Table 1), indicating that the electron interaction between Ti and N in g-c 3 N 4 /TiO 2 occurs [26], which results from the formation of the coordination bond between N and Ti. In addition, in comparison with that of pure TiO 2, the O1s core level peaks for g-c 3 N 4 /TiO 2 become broadening (Fig. 4b), which can be fitted into two peaks at and ev by Gaussian technique (supplementary data, Fig. S-II), corresponding to the Ti O and O H bonds, respectively [27,28]. The broadness of O1s peaks of g-c 3 N 4 /TiO 2 gives further evidence of the presence of N element in TiO 2 lattice [26]. As for N1s core level spectra, the binding energy of N element is different with changing its chemical environment, such as chemisorbed nitrogen or TiN at ev and NO or NO 2 type species at above 400 ev [27,29,30,31]. Nitrogen 1s core level of g-c 3 N 4 /TiO 2 presents a main peak at and ev (Fig. 4(C)). As shown in Fig. S-III (supplementary data), three different binding energies are observed, i.e., , , and ev, which can be attributed to the formation of N-Ti-N [32], O-Ti-N [11] and C=N [33], respectively. The above results and analysis lead to a conclusion that nitrogen element is successfully doped into the

4 928 X.F. Lu et al.: J. Mater. Sci. Technol., 2010, 26(10), Table 1 XPS data of the samples, the core level binding energies (ev) of Ti2p, N1s, and O1s Sample Ti2p O1s N1s Ti2p3/2 Ti2p1/2 As-synthesized TiO 2(a) g-c 3N 4/TiO 2(b) g-c 3N 4/TiO 2(c) Fig. 5 Diffuse reflectance UV-vis spectra of assynthesized pure TiO 2 (I), hybrid composite of g-c 3N 4/TiO 2 (II) and hybrid composite of g-c 3N 4/TiO 2 after hydrothermal reaction (III) ites. The absorbance edge of g-c 3 N 4 /TiO 2 (Fig. 5(II), (III)) obviously shifts into visible-light region g- C 3 N 4 /TiO 2 compared with that of pure TiO 2 (Fig. 5(I)) due to a change of intrinsic band gap, indicating the sufficient hybride of g-c 3 N 4 and TiO SEM image, EDS pattern and EDS maps Fig. 4 X-ray photoelectron spectra (XPS) for g- C 3N 4/TiO 2 hybrid composite and TiO 2 (a) Ti2p, (b) O1s, and (c) N1s. (I) as-synthesized pure TiO 2, (II) hybrid composite of g-c 3N 4/TiO 2, (III) hybrid composite of g-c 3N 4/TiO 2 after hydrothermal reaction anatase TiO 2 lattice, resulting in the formation of N- Ti-N and O-Ti-N bonds, which is consistent with the analyses of Fig. 4(a) and (b). The surface spectral properties of the obtained photocatalysts were also investigated. Figure 5 shows the diffuse reflectance UV-vis spectra of the assynthesized TiO 2 and g-c 3 N 4 /TiO 2 hybrid compos- Figure 6 presents the SEM image, EDS pattern and EDS maps of Ti, C, and N element for g- C 3 N 4 /TiO 2 composite after hydrothermal treatment. It is found that the composite is composed of irregular nanoparticles (Fig. 6(a)), and the diameters of the particles are in the range of nm. The EDS pattern (Fig. 6(b)) indicates the composite contains Ti, O, C and N element, and the Al element is attributable to the aluminum foil. In addition, the uniformly distributed bright dots, which is assigned to Ti (Fig. 6(c)), C (Fig. 6(d)), and N element (Fig. 6(e)), respectively, displays an uniform composite. 3.4 Photocatalytic properties Figure 7 shows the change of RhB concentration with reaction time under UV-Vis and visible light irradiation in the presence of different TiO 2 -based materials. It is found that photocatalytic reaction proceed hardly in the absence of catalysts (Fig. 7, curve I). In addition, similar photocatalytic activity is observed in the presence of different TiO 2 -based materials under UV-Vis light irradiation (Fig. 7(a), curve I, II, and IV), and the degradation rate within 5 h is 60%, 50%, and 55%, respectively. However, a dramatic enhance-

5 X.F. Lu et al.: J. Mater. Sci. Technol., 2010, 26(10), Fig. 6 SEM image (a), EDS pattern (b) corresponding to the framed area in Fig. 5(I), EDS map of Ti element (c), EDS map of C element (d), and EDS map of N element (e) for g-c 3N 4/TiO 2 composite after hydrothermal treatment. Al can be attributed to the aluminum foil five hours is 35% (Fig. 7(b), curve II), 38% (Fig. 7(b), curve III) and 80% (Fig. 7(b), curve IV), respectively, for pure TiO 2 and g-c 3 N 4 /TiO 2 composites. 4. Conclusion In summary, g-c 3 N 4 /TiO 2 hybrid composites have been synthesized by using a simple hydrolysis and hydrothermal method and annealing under nitrogen atmosphere. The structures and the photocatalytic properties were characterized. It is worthy of noting that hybrid g-c 3 N 4 /TiO 2 after hydrothermal treatment displays a good photocatalytic activity for the degradation of rhodamine B under visible light. Acknowledgements This word was supported by the National Natural Science Foundation of China (NSFC, No , ), Ministry of Science and Technology, China (No. 2005CCA00900). Supplementary data Supplementary data associated with this article can be found in the online version at Fig. 7 Photocatalytic degradation curves of rhodamine B in the presence of (I) no catalyst, (II) assynthesized pure TiO 2, (III) hybrid composite of g-c 3N 4/TiO 2, (IV) hybrid composite of g- C 3N 4/TiO 2 after hydrothermal reaction under the irradiation of UV-Vis (a) and visible light (b) ment in degradation rate under visible irradiation is achieved in the presence of g-c 3 N 4 /TiO 2 composite after hydrothermal treatment (Fig. 7(b), curve IV). The degradation rate under visible irradiation within REFERENCES [1 ] A.L. Linsebigler, G.Q. Lu and J.T. Yates: Chem. Rev., 1995, 95, 735. [2 ] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki and Y. Taga: Science, 2001, 293, 269. [3 ] M.R. Hoffmann, S.T. Martin, W. Choi and D.W. Bahnemann: Chem. Rev., 1995, 95, 69. [4 ] B. Ohtani, Y. Ogawa and S. Nishimoto: J. Phys. Chem. B, 1997, 101, [5 ] H. Yamashita, Y. Ichihashi and M. Harada: J. Catal., 1996, 158, 97.

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