The photocatalytic activity of g-C3N4/Ta2O5 composite under visible light irradiation - Nguyen Thi Viet Nga

Vietnam Journal of Chemistry, International Edition, 55(2): 172-177, 2017 DOI: 10.15625/2525-2321.2017-00439 172 The photocatalytic activity of g-C3N4/Ta2O5 composite under visible light irradiation Nguyen Thi Viet Nga 1* , Vo Vien 1,2 1 Department of Chemistry, Quy Nhon University 2 Applied Research Institute for Science and Technology, Quy Nhon University Received 28 February 2017; Accepted for publication 11 April 2017 Abstract The g-C3N4/Ta2O5 composites were synthesized by heating mixtures of urea and Ta2O5 at 500 o C. The as-prepared samples were denoted as CN-500/TaO-W, where W is weight ratio of urea/Ta2O5 and equals to 3, 4 and 5. The materials were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, and Ultraviolet-visible diffuse reflectance spectroscopy. The photocatalytic activity of CN-500/TaO-W samples was assessed by degradation of methylene blue in aqueous solution under visible light. Among them, CN-500/TaO-4 exhibited the best performance. An enhancement in photocatalytic activity of the composites is believed to be the presence of g-C3N4. Keywords. g-C3N4, Ta2O5, photocatalytic activity, methylene blue, g-C3N4/Ta2O5 composite. 1. INTRODUCTION The study of materials that could absorb light from the sun to decompose organic pollutants in water and air is not only an opportunity but also a challenge for many researchers and attracts a lot of interest from society. The semiconductors that highly exhibit the catalytic performance under light have been known as potential photocatalysts. They possess many advantages including high efficiency in conversion of solar energy, strongly catalytic activity, highly capable decomposition of organic pollutants, reusability and especially environmental friend. Among the semiconductors, in recent years, Ta2O5 has attracted much attention in photocatalysis. However, with the large band gap energy (about 3.8 to 5.3 eV), its photocatalytic activity only exhibits in ultraviolet region [1], leading to limit its application in practice. To solve this problem, many works have been done to modify Ta2O5 by doping with some other elements to improve optical absorbance in visible light [2, 3]. At the same time, g-C3N4 material, a polymer organic semiconductor with graphitic-like structure, can absorb visible light (the band gap energy of about 2.7 eV), taking a lot interest of researchers due to its wide range of applications. However, the fast recombination rate of photoinduced electron-hole pair for this material leads to its low photoefficiency. This is a drawback when using it in invidual status. Therefore, many attempts to enhance the photocatalytic performance of g-C3N4 by modifying it with other elements have been done [4, 5]. To improve the photocatalytic capability of each material when using them in separate systems, we synthesized g-C3N4/Ta2O5 composite as described in details in the previous works [6]. In this article, we focused on effect of the ratio (urea/Ta2O5) on properties of the products and evaluated their photocatalytic activity by photodegrading methylene blue in aqueous solution. 2. EXPERIMENTAL 2.1. Chemicals Tantalum pentoxide (Ta2O5), urea (CH4N2O), and methylene blue (C16H18N3S) were purchased from Merck. All chemicals were analytical grade and used without further treatment. 2.2. Synthesis Urea and Ta2O5 were mixed with different weight ratios (urea/Ta2O5 in weight = 3, 4 and 5). The mixtures were put into a porcelain cup sealed with aluminum foil and then calcined at 500 o C with a heating rate 10 o C min -1 for 2 hours. The samples were denoted as CN-500/TaO-W, where W is VJC, 55(2), 2017 Nguyen Thi Viet Nga et al. 173 weight ratio between urea and Ta2O5. For comparison, pure g-C3N4 was also prepared by heating urea at 500 o C, and denoted as CN-500. 2.3. Characterization Powder X-ray diffraction (XRD) patterns were conducted using a Bruker D8 Advance with Ni- filtered CuKα radiation (λ = 1.5418 Å). X-ray photoelectron spectroscopy (XPS) was performed using an ESCALab spectrometer (Thermo VG, UK) with monochromated AlKα radiation. Ultraviolet- Visible Diffuse reflectance spectroscopy (UV-Vis DRS) was investigated on a GBC Instrument - 2885 spectrophotometer. 2.4. Photocatalytic Activity To evaluate photocatalytic activity of the prepared materials, methylene blue (MB) was selected as an organic pollutant. To 200 mL of 50 mg/L MB solution, 0.1 g of the prepared sample was dispersed under stirring and the solution kept in dark condition for 3 h to reach equilibrium of adsorption- desorption process. Then, the solution was irradiated by visible light using a 75W-220V lamp with a filter cutting UV rays. MB degradation was monitored by taking suspension at irradiation time intervals of 1 h. Each suspension was centrifuged to separate the catalyst from the MB solution. The degradation rate was calculated as a function of irradiation time from the change in absorbance at wavelength of 663 nm as measured using a UV-Vis spectrophotometer (Jenway 6800). 3. RESULTS AND DISCUSSION 3.1. Characterization of catalysts 3.1.1. XRD The XRD patterns of the samples in figure 1 show that the main diffraction peaks of all samples are from orthorhombic Ta2O5 [7], indicating that orthorhombic Ta2O5 phase still remains in the as- prepared samples under thermal treatment of the mixture of Ta2O5 and urea at 500 o C. In addition, it is worth to note that an extra peak with 2θ value at about 13.2 and 27.3o belonging to g- C3N4 [8] (figure 1) can be seen clearly for CN- 500/TaO-4. This diffraction is indexed to (100) and (002) plane, respectively, for graphite-like layer structure of g-C3N4. This peak is very strong for the CN-500/TaO-5, proving a larger amount of g-C3N4 in this material. Figure 1: XRD patterns of Ta2O5, CN-500 and CN-500/TaO-3, CN-500/TaO-4 and CN-500/TaO-5 3.1.2. XPS XPS was used to examine the surface composition and chemical state of C, N, O and Ta in the samples. Figure 2 and table 1 show XPS of C, N, O and Ta of a representative sample, CN-500/ TaO-4. Figure 2: XPS spectrum of CN-500/TaO-4 Table 1: Binding energy of Ta4f, O1s, C1s, N1s Orbital Ta4f O1s C1s N1s Ebinding (eV) 25.5 27.3 530.6 287.7 284.5 398.6 For C1s XPS, in addition to the peak at 287.7 eV corresponding to the referenced C, a strong peak at 284.5 eV can be attributed to carbon in the C-N-C configurations of g-C3N4 [9, 10]. The presence of g- C3N4 in the sample can be further confirmed by N1s XPS in table 1. The intense pic at 398.6 eV may be assigned to sp 2 hybridized aromatic nitrogen atoms bonded to carbon atoms (N=C) [11]. The peak at 530.6 eV for O1s is usually reported as O in Ta2O5 VJC, 55(2), 2017 The photocatalytic activity of g-C3N4/Ta2O5 174 [12]. Also shown in table 1, Ta 4f7/2, and Ta 4f5/2 peaks appeared at 25.5 eV and 27.3 eV, respectively, which correspond to Ta 5+ in Ta2O5 [12]. However, compared to pure Ta2O5 (26.6 eV and 28.5 eV, respectively), a slight shift to lower energy has been obtained. Such a shift may be caused by presence of an interaction between Ta and g-C3N4 in the composite. These results rather demonstrated that CN-500/TaO-4 is a composite consisting of Ta2O5 and g-C3N4. 3.1.3. UV–Vis DRS The optical absorption spectra and band gap energy of Ta2O5, CN-500/TaO-3, CN-500/TaO-4, and CN-500/TaO-5 are shown in figure 3. Figure 3: (A) UV–Vis DRS and (B) band gap energy of Ta2O5, CN-500/TaO-3, CN-500/TaO-4, CN-500/TaO-5 In figure 3A, Ta2O5 exhibits an absorption peak centered at approximately 275 nm which is assigned to the O2p to Ta5d charge transfer band of the Ta2O5 [13]. This peak can be observed in the composites, however their intensity strongly reduced. Compared to the pristine Ta2O5, a broad absorption band covering the range of 300-500 nm with a peak centered at about 380 nm for the composites has been obtained. This broad absorption in the visible light region may come from g-C3N4 in the composites. This indicates that the g-C3N4 may serve as a sensitizer to extend the absorption in visible light region for the composites and thus improve their photocatalytic activities under visible light irradiation. Based on the document [14] and Kubelka-Munk equation [15], the band gap energy of the materials was determined and shown in Figure 3B. With a reduced band gap energy, CN-500/TaO-3, CN- 500/TaO-4, CN-500/TaO-5 materials might possess good catalyst activity in visible light region. The formation of the composites can be explained by that when heating the mixture of urea and Ta2O5, first, polymerization of urea can occur via some following steps [16, 17]: N N N NH2 NH2 + 6CO2 + 3NH36CO(NH2)2 H2N melamine to urea N N N NH2 NH2H2N 2 to + 2NH3 Melem N N N N N NN NH2 NH2H2N to + 3NH3 Melon N N N N N NN NH2 NH2H2N N N N N N N NHN N N NH N N N N N N N N N N N N N NH2 NH2H2N 3 Melon N N N N N N NHN N N NH N N N N N N N N N N N N N NH2 NH2H2N to n - NH3 N N N N NN N N N N NN N N NN N N NN N N NN N N N N NN N N N NN N N N N NN NN N N NN N NN N N N g-C3N4 The g-C3N4 was then formed on surface of the Ta2O5 particles to yield the composites. 3.2. Photocatalytic Activity The photocatalytic activity of the samples was determined by degradation of MB in water under visible light. For comparison, photocatalytic activity of the pristine Ta2O5 and g-C3N4 (CN-500) was also presented. Figure 4 shows the variation of MB concentration (C/C0) versus irradiation time on the five samples. Ta2O5 exhibited a low photocatalytic activity in visible light which may be due to its large band gap energy. For g-C3N4 (CN-500), although relatively VJC, 55(2), 2017 Nguyen Thi Viet Nga et al. 175 small band gap energy, about 2.70 eV [1], its photocatalytic activity is still low. This is explained by that the high recombination rate of electron-hole pair in pure g-C3N4 [18] leads to reduced photocatalytic activity. Figure 4: Photocatalytic activity of Ta2O5, CN-500, CN-500/TaO-3 and CN-500/TaO-4, CN-500/TaO-5 toward the degradation of MB For g-C3N4/Ta2O5 composits, a difference in catalytic performance of the materials can be observed. On the CN-500/TaO-4 sample, the decrease in C/C0 is much faster than for the others. For the CN-500/TaO-3 and CN-500/TaO-5, the decrease in C/C0 is slower compared to CN- 500/TaO-4. This may be attributed to a reduced recombination rate of electron-hole in CN-500/TaO- 4. The photocatalytic activity of CN-500/TaO-4 is better than that of Ta2O5 and g-C3N4, indicating that a synergistic effect occurs in the composite during the photocatalytic reaction. Graphitic g-C3N4 playing a key role in enhancement of photocatalytic activity for composites was discussed in several reports [19]. On the basis of a number of works published [20, 21], decomposition mechanism of MB on the g- C3N4/Ta2O5 composites is proposed in figure 5A. In these composites, the photogenerated electrons on the conduction band of the g-C3N4 can directly inject into the conduction band of Ta2O5. This phenomenon can lead to a significant decrease in the electron–hole recombination for the pure g-C3N4. Hence, this may contribute to the enhancement of photocatalytic reactivity. However, for CN-500/TaO-3 and CN-500/ TaO-5 in this work, the photocatalytic activity is lower than that of the pure g-C3N4. This may be explained by that a small or too large amount of g- C3N4 in the two materials as shown in the characterizations does not form the synergistic effect mentioned above. In CN-500/TaO-5, when covered with a quite thick layer of g-C3N4, this material will show photocatalyst closing to that of pure g-C3N4. In contrast, with a smaller amount of g-C3N4, the activity of CN-500/TaO-3 is close to Ta2O5. This shows that a proper amount of g-C3N4 in composite is very important. Figure 5: (A) Proposed mechanism for the photodegradation of MB on the g-C3N4/Ta2O5 composites; (B) linear plots of ln(C0/Ct) vs. time for degradation of MB in aqueous solution under visible light In order to investigate kinetics of the photocatalytic reactions, the Langmuir-Hinshelwood model has been usually employed. For solid-liquid reactions, the Langmuir–Hinshelwood equation can be expressed: r = k·θ = k·KC/(1 + KC) (1) where r and k are the reaction rate and rate constant, respectively, θ is the surface coverage, K is the adsorption coefficient of the reactant, and C is the equilibrium concentration of the reactant. When C is at low concentration, K·C << 1, hence r = k·KC/(1 + KC) ≈ k·K·C = kapp·C. Therefore, the pseudo first- order kinetics equation (Eq. (2)) is applied: ln(C0/Ct) = kapp·t (2) where C0 and Ct are the reactant concentrations at reaction times, t = 0 and t = t, respectively, kapp is the apparent reaction rate constant. A relationship between ln(C0/Ct) and t was plotted in figure 5B. It can been clearly seen that the plots of ln(C0/Ct) versus reaction time (t) are well VJC, 55(2), 2017 The photocatalytic activity of g-C3N4/Ta2O5 176 fitted with the pseudo first-order rate model with high correlation coefficients (R ≥ 0.995). From the plots, the kapp values were calculated for CN- 500/TaO-4, CN-500 và Ta2O5 to be 0.129 và 0.047 và 0.006 h -1 , respectively. These results show that the reaction rate on the catalyst CN-500/TaO-4 is much stronger than CN-500 and Ta2O5. The above obtained results show that weight ratio of urea and Ta2O5 in reaction mixture for the synthesis of g-C3N4/Ta2O5 composites plays an important role to determine their band gap energy and photocatalytic properties. 4. CONCLUSIONS 1. Composites of Ta2O5 and g-C3N4 with different weight ratios of the precursors (urea/Ta2O5 = 3, 4 và 5) have been successfully synthesized with a new method by calcining ure with Ta2O5 at temperature of 500 °C. 2. The composites demonstrate photocatalytic activity for degradation of methylene blue in aqueous solution under visible light irradiation. Among the composites, the sample derived from thermal treatment at 500 o C, weight ratio of urea/Ta2O5 = 4 exhibits the best photocatalytic activity. 3. Decomposition rate of methylene blue on CN-500/TaO-4 composite is more than 2.7 times the g-C3N4 and 21.5 times higher than Ta2O5. Acknowledgement. This research was partly funded by TEAM project (code ZEIN2016PR431). REFERENCES 1. Ramy Nashed, Walid M. I. Hassan, Yehea Ismail and Nageh K. Allam. Unravelling the interplay of crystal structure and electronic band structure of tantalum oxide (Ta2O5), Phys. Chem. Chem. Phys., 15, 1352- 1357 (2013). 2. Tomiko M. Suzuki, Tadashi Nakamura, Shu Saeki, Yoriko Matsuoka, Hiromitsu Tanaka, Kazuhisa Yano, Tsutomu Kajino and Takeshi Morikawa. Visible light-sensitive mesoporous N-doped Ta2O5 spheres: synthesis and photocatalytic activity for hydrogen evolution and CO2 reduction, J. Mater. Chem., 22, 24584-24590 (2012). 3. Chao Zhou, Lu Shang, Huijun Yu, Tong Bian, Li- Zhu Wu, Chen-Ho Tung and Tierui Zhang. Mesoporous plasmonic Au-loaded Ta2O5 nanocomposites for efficient visible light photocatalysis, Catal. Today, 225(15), 158-163 (2014). 4. Sergey Stolbov and Sebastian Zuluaga. Sulfur doping effects on the electronic and geometric structures of graphitic carbon nitride photocatalyst: insights from first principles, J. Phys.: Condens. Matter, 25, 085507 (2013). 5. Muhammad Tahir, Chuanbao Cao, Faheem K. Butt, Sajid Butt, Faryal Idrees, Zulfiqar Ali, Imran Aslam, M. Tanveer, Asif Mahmoodc and Nasir Mahmood. Large scale production of novel g-C3N4 micro strings with high surface area and versatile photodegradation ability, CrystEngComm, 16, 1825- 1830 (2014). 6. Nguyen Van Kim, Nguyen Thi Viet Nga, Sai Cong Doanh, Le Truong Giang, Vo Vien. Synthesis of g- C3N4/Ta2O5 composite materials with various contents of g-C3N4. Vietnam Journal of Chemistry, 53(6e1,2), 116-119 (2015). 7. T. J. Bright, D. B. Tanner, J. I. Watjen, Z. M. Zhang, and D. J. Arenas, C. Muratore, A. A. Voevodin, D. I. Koukis, T. J. Bright, J. I. Watjen, Z. M. Zhang, C. Muratore, A. A. Voevodin, D. I. Koukis, D. B. Tanner, and D. J. Arenas. Infrared optical properties of amorphous and nanocrystalline Ta2O5 thin films, Journal of Applied Physic, 114, 083515-10 (2013). 8. Gang Xin and Yali Meng. Pyrolysis Synthesized g-C3N4 for Photocatalytic Degradation of Methylene Blue, Journal of Chemistry, 2013, 5 pages (2013). 9. Yongchao Bao, Kezheng Chen. AgCl/Ag/g-C3N4 Hybrid Composites: Preparation, Visible Light- Driven Photocatalytic Activity and Mechanism, Nano-Micro Letters, 8(2), 182-192 (2016). 10. H. Katsumata, T. Sakai, T. Suzuki, S. Kaneco. Highly efficient photocatalytic activity of g-C3N4/Ag3PO4 hybrid photocatalysts through z-scheme photocatalytic mechanism under visible light, Ind. Eng. Chem. Res., 53(19), 8018-8025 (2014). 11. Y. W. Zhang, J. H. Liu, G. Wu, W. Chen. Porous graphitic carbon nitride synthesized via direct polymerization of urea for efficient sunlight-driven photocatalytic hydrogen production, Nanoscale, 4(17), 5300-5303 (2012). 12. Rupesh S. Devan, Ching-Ling Lin, Shun-Yu Gao, Chia-Liang Cheng, Yung Liou and Yuan-Ron Ma. Enhancement of green-light photoluminescence of Ta2O5 nanoblock stacks, Phys. Chem. Chem. Phys., 13, 13441-13446 (2011). 13. Sreethawong T., Ngamsinlapasathian S., Suzuki Y., Yoshikawa S. Nanocrystalline mesoporous Ta2O5- based photocatalysts prepared by surfactant-assisted templating sol-gel process for photocatalytic H2 evolution, Journal of Molecular Catalysis A: Chemical, 235, 1-11 (2005). 14. Sebastian Zuluaga, Li-Hong Liu, Natis Shafiq, Sara Rupich, Jean-Franc¸ois Veyan, Yves J. Chabal and Timo Thonhauser, Structural band-gap tuning in g- C3N4, Phys. Chem. Chem. Phys., 17, 957-962 (2015). 15. Kubelka P., Munk F. The Kubelka-Munk Theory of Reflectance, Zeits. f. Techn. Physik, 12, 593-601 (1931). VJC, 55(2), 2017 Nguyen Thi Viet Nga et al. 177 16. Hideo Kinoshita. Synthesis of melamine from urea, The Review of Physical of Japan, 23(1), 1-9 (1953). 17. Yuexiang Li, Ying Xianga, Shaoqin Peng, Xuewen Wang, Lang Zhou. Modification of Zr-doped titania nanotube arrays by urea pyrolysis for enhanced visible-light photoelectrochemical H2 generation, Electrochimica Acta, 87, 794-800 (2013). 18. Muhammad Tahir, Chuanbao Cao, Faheem K. Butt, Sajid Butt, Faryal Idrees, Zulfiqar Ali, Imran Aslam, M. Tanveer, Asif Mahmoodc and Nasir Mahmood. Large scale production of novel g-C3N4 micro strings with high surface area and versatile photodegradation ability, CrystEngComm, 16, 1825- 1830 (2014). 19. Huang L., Xu H., Li Y., Li H., Cheng X., Xia J., Xu Y., Cai G. Visible-light-induced WO3/g-C3N4 composites with enhanced photocatalytic activity, Dalton Trans., 42 (24), 8606-16 (2013). 20. C. Miranda, H. Mansilla, J. Yanez, S. Obregón, G. Colón. Improved photocatalytic activity of g- C3N4/TiO2 composites prepared by a simple impregnation method, Journal of Photochemistry and Photobiology A: Chemistry, 253, 16-21 (2013). 21. Haiyan Ji, Xiaocui Jing, Yuanguo Xu, Jia Yan, Hongping Li, Yeping Li, Liying Huang, Qi Zhang, Hui Xu and Huaming Li. Magnetic g-C3N4/NiFe2O4 hybrids with enhanced photocatalytic activity, RSC Adv., 5, 57960-57967 (2015). Corresponding author: Nguyen Thi Viet Nga Department of Chemistry, Quy Nhon University No. 170, An Duong Vuong Street Quy Nhon City, Binh Dinh Province E-mail: nguyenthivietnga@qnu.edu.vn; Telephone number: 0914481795.