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BiFeO3 nanocomposites with enhanced visible light photocatalytic activities
Journal of Colloid and Interface Science 448 (2015) 17–23
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Facile fabrication of highly efficient g-C3N4/BiFeO3 nanocomposites with enhanced visible light photocatalytic activities Xingfu Wang a,b, Weiwei Mao a,b, Jian Zhang a, Yumin Han a, Chuye Quan a, Qiaoxia Zhang a, Tao Yang a, Jianping Yang b, Xing’ao Li a,⇑, Wei Huang a,c,⇑ a Key Laboratory for Organic Electronics & Information Displays (KLOEID), Synergetic Innovation Center for Organic Electronics and Information Displays (SICOEID), Institute of Advanced Materials (IAM), School of Materials Science and Engineering (SMSE), Nanjing University of Posts and Telecommunications (NUPT), Nanjing 210023, PR China b School of Science, Nanjing University of Posts and Telecommunications (NUPT), Nanjing 210023, PR China c Key Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials (IAM), National Synergistic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University (NanjingTech), 30 South Puzhu Road, Nanjing 211816, PR China
g r a p h i c a l a b s t r a c t
a r t i c l e
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Article history: Received 18 November 2014 Accepted 31 January 2015 Available online 9 February 2015 Keywords: g-C3N4 BiFeO3 Nanocomposites Photocatalysis
a b s t r a c t Graphitic carbon nitride/bismuth ferrite (g-C3N4/BiFeO3) nanocomposites with various g-C3N4 contents have been synthesized by a simple method. After the deposition–precipitation process, the novel BiFeO3 spindle-like nanoparticles with the size of 100 nm were homogeneously decorated on the surfaces of the C3N4 nanosheets. A possible deposition growth mechanism is proposed on the basis of experimental results. The as-prepared g-C3N4/BiFeO3 composites exhibit high efficiency for the degradation of methyl orange (MO) under visible light irradiation, which can be mainly attributed to the synergic effect between g-C3N4 and BiFeO3. The ability to tune surface and interfacial characteristics for the optimization of photophysical properties suggests that the deposition growth process may enable formation of hybrids suitable for a range of photocatalytic applications based on g-C3N4. Ó 2015 Elsevier Inc. All rights reserved.
1. Introduction In recent years, the scientific community has focused intently on new photocatalysts based on semiconductor that enhance the photocatalytic activities as well as being earth abundant in water splitting and degradation of organic pollutants [1–3]. Various ⇑ Corresponding authors at: Key Laboratory for Organic Electronics & Information Displays (KLOEID), Synergetic Innovation Center for Organic Electronics and Information Displays (SICOEID), Institute of Advanced Materials (IAM), School of Materials Science and Engineering (SMSE), Nanjing University of Posts and Telecommunications (NUPT), Nanjing 210023, PR China (W. Huang). E-mail addresses: [email protected] (X. Li), [email protected] (W. Huang). http://dx.doi.org/10.1016/j.jcis.2015.01.090 0021-9797/Ó 2015 Elsevier Inc. All rights reserved.
semiconductor photocatalysts, such as metal oxides (ZnO [4], TiO2 [5]) and metal sulfides (ZnS [6]), have been explored in the degradation of organic pollutants under UV light irradiation only because of the large band gap (>3 eV). However, the large band gap restricts the absorption in the UV region, which accounts for only 5% of the entire solar spectrum. As a result of the development of band-gap engineering, the fabrication of composites by coupling of two semiconductors with narrow band gap has attracted considerable attention [7,8]. Graphitic carbon nitride (g-C3N4), with a narrow band gap of 2.7 eV, is a promising metal-free photocatalyst due to its nontoxicity, abundance and stability [9]. To enhance the photocatalytic activity, a number of methods have been developed. In particular,
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combining g-C3N4 with other semiconductors to form composites or heterostructures, provides a feasible route to inhibit the recombination of photogenerated electron–hole pairs [10–12]. In recent years, a C3N4/Bi2WO6 photocatalyst hybridized with monolayer C3N4 exhibited the highest photocatalytic activity which was 68.9% higher than that of pure Bi2WO6 [13]. g-C3N4–Ag2O composites with the mass ratio of 1:4 degraded nearly 50% of MO within 4 min, and 90% after 30 min [14]. On the other hand, bismuth ferrite (BiFeO3), with simultaneous ferroelectric and ferromagnetic ordering, was studied as one of the most important multiferroic materials in photocatalytic applications recently [15–17]. With a relatively narrow band gap of 2.2 eV [18,19], BiFeO3-based photocatalysts show strong absorption of visible light and hence exhibit excellent photocatalytic abilities under visible light irradiation. In the previous reports, Zhu et al. modified TiO2-nanotubes surface by BiFeO3 nanoparticles, which remarkably improves the photoresponses because of significantly enhanced visible-light utilization [20]. Recently, hybrids of BiFeO3 with nanoscale carbon materials have attracted significant attention. Li et al. reported a hybrid approach by decorating BiFeO3 nanoparticles on graphene nanosheets, and the rate for the photo-degradation of Congo Red under visible light is six times higher than that of bare BiFeO3 particles [21]. In order to enhance the photoinduced charges separation efficiency for improved photocatalytic degradation activity, the novel g-C3N4/BiFeO3 photocatalysts were firstly synthesized by a green approach at ambient temperature. To the best of our knowledge, there are no reports on the synthesis and application of g-C3N4/ BiFeO3 nanocomposites for pollutants degradation under visible light irradiation. The photoactivity of the catalysts was evaluated by degrading methyl orange (MO) dye solution, and the results showed that the optimal amount of g-C3N4 in the composites was 50 wt%. A possible mechanism for enhancing photocatalytic activity of g-C3N4/BiFeO3 was critically discussed.
2. Experimental section 2.1. Fabrication of g-C3N4/BiFeO3 nanocomposite photocatalysts Melamine (C3H6N6) and sodium hydroxide (NaOH) were obtained from Sinopharm Chemical Reagent Corp, P.R. China. Bismuth nitrate (Bi(NO3)35H2O) and ferric chloride hexahydrate (FeCl36H2O) were purchased from Guangdong Xilong Chemical Co., Ltd., P.R. China. All chemicals were used as received without further purification. The g-C3N4 power was synthesized according to the literature [12]. Typically, 3 g of melamine was put into an alumina crucible with a cover and heated to 550 °C in a muffle furnace and then kept at this temperature for 4 h. The resulting yellow product was collected and ground into powder for further use. BiFeO3 micro-cubes were synthesized by the hydrothermal method. In a typical synthesis, Bi(NO3)35H2O (4.850 g) and FeCl36H2O (2.704 g) in a stoichiometric ratio (1:1 in molar ratios) were mixed in 100 mL of acetone (99.8%) and sonicated for 30 min. Then 300 mL of deionized (D.I.) water and concentrated ammonia were added under vigorous stirring until the pH value of the solution reached 10–11. The sediment was centrifuged out and washed with D.I. water several times until the pH value was neutral. Next, the red co-precipitate was redispersed in D.I. water. Under vigorous stirring, 15 g NaOH was added into the suspension. Then, the solution was placed inside a stainless steel autoclave with a Teflon liner and heated at 180 °C for 72 h. After cooling down to room temperature, the precipitate was harvested by filtration, washed with D.I. water and pure ethanol for three times, respectively, and dried at 80 °C for 2 h.
The typical preparation of g-C3N4/BiFeO3 photocatalysts was as follows: an appropriate amount of g-C3N4 and BiFeO3 powder were completely dispersed in methanol assisted by ultrasonication for 3 h, respectively. The as-prepared g-C3N4 solution and BiFeO3 solution were mixed together and stirred in a fume hood for 24 h. After volatilization of the methanol, an opaque powder was obtained after drying at 80 °C in air. According to this method, various mass ratios of g-C3N4 from 10% to 80% were synthesized. 2.2. Characterization The microstructural morphologies of the g-C3N4/BiFeO3 composites were observed via scanning electron microscopy (SEM, JEOL-6380LV Japan; accelerating voltage = 200 kV) and energy dispersive X-ray spectroscopy (EDS, HORIBA EMAX Energy EX-250). Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) imaging was performed on a JEOL JEM-2100 microscope operating at 200 kV, by depositing a drop of sample dispersion onto 300 mesh Cu grids coated with a carbon layer. The obtained samples were characterized by XRD on a Bruker D8 Advance X-ray powder diffractometer with Cu Ka radiation (k = 1.5418 Å) in the 2h range of 10–70° with a step size of 0.002° and a scan speed of 0.5 s per step. The accelerating voltage and the applied current were 40 kV and 40 mA, respectively. UV–vis absorption spectra were obtained using a Ruili UV-1201 spectrophotometer. The visible light (k > 420 nm) was obtained using a 500 W Xe lamp with a 420 nm cutoff filter to completely remove any radiation below 420 nm. The photoluminescence (PL) spectra were performed using a FL3-TCSPC spectrofluorometer (FL, Horiba Jobin Yvon, France) equipped with a xenon arc lamp as the light source and a quartz cell (1 1 cm). 2.3. Photocatalytic tests The photocatalytic activities of g-C3N4/BiFeO3 nanocomposites were evaluated by degradation of MO aqueous solution under UV light irradiation and visible light irradiation. A 300 W high-pressure Hg lamp was used as the UV light source. A 500 W xenon lamp was used as the other light source, and visible-light irradiation was realized by attaching a 420 nm cutoff filter. In a typical photocatalytic experiment, 50 mg catalyst was dispersed in 50 mL MO aqueous solution with a concentration of 5 mg L 1. Prior to irradiation, the suspensions were magnetically stirred in the dark for 30 min to ensure the establishment of an adsorption–desorption equilibrium. Then, at selected time intervals, samples were collected and filtered to remove the photocatalyst particles by centrifugation. After that, the solution was analyzed using a ultraviolet– visible light spectrophotometer. A blank test was also carried out on an aqueous MO solution without photocatalyst under the same condition. In addition, the recyclability of the catalyst was also studied by washing and drying the catalyst before the next cycle.
3. Results and discussion 3.1. Characterization of samples In order to investigate the formation process of the composites, the morphology of the as-synthesized pure BiFeO3 was observed by SEM and TEM, as shown in Fig. 1. It can be detected that the BiFeO3 samples present excellent cubes under SEM observation and an edge length ranging from 0.5 to 1 lm. The same result can be obtained from the TEM micrograph (Fig. 1b). The inset of Fig. 1b is the corresponding HRTEM imaging, which match well with the (0 1 2) of the BiFeO3 lattice (d = 3.95 Å). According to the
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Fig. 1. SEM and TEM images of pure BiFeO3 cubes. Inset: HRTEM image.
previous report [22], the relatively high alkaline concentration would better result in the cubic BiFeO3. The introduction of g-C3N4 results in interesting change of the morphology of BiFeO3 micro cubes. The dispersion state and the structure of all the as synthesized g-C3N4/BiFeO3 nanocomposite were obtained by TEM. Fig. 2e shows that the pure g-C3N4 has a layered structure with few stacking layers. More attractively, the spindle-like BiFeO3 nanoparticles were attached to the surface of g-C3N4 sheets and were randomly distributed, as shown in Fig. 2a–d. From the corresponding size distribution column graph (inset in Fig. 2c), it showed that the particle sizes are mainly distributed around the sizes of 50–250 nm. With the increase of the g-C3N4 concentration, the spindle-like BiFeO3 nanoparticles reveal a better dispersion. However, the high g-C3N4 concentration also reduces dispersion of itself, which suppress the formation of the spindle-like morphology, as can be found in Fig 2d. As discussed above, it can be concluded that the mass ratios of g-C3N4/BiFeO3 photocatalysts could be crucial for this hybrid, which might promote the separation of electron–hole pairs and, subsequently accelerate photocatalytic reaction [23]. In addition, EDS analysis showed that the selected areas of the 50% g-C3N4/BiFeO3 were found with C, N, O, Fe and Bi elements (Fig. 2f and g). Based on the SEM–EDS and TEM analyses, it can be concluded that the fascinating structure is formed in the composite. The crystal phase of the materials is determined by XRD, as shown in Fig. 3. All peaks of pure BiFeO3 samples (Fig. 3a) can be indexed to a rhombohedral lattice with the space group R3c (JCPDS card No. 86-1518) [24]. The pure g-C3N4 has two distinct diffraction peaks at 13.0° and 27.2° (Fig. 3f), which correspond to the graphitic materials as the (1 0 0) and (0 0 2) diffraction planes [25]. For g-C3N4/BiFeO3 composites (Fig. 3b–e), no impurity diffraction planes were observed. With an increasing amount of g-C3N4 from 10% to 80%, the diffraction peaks of g-C3N4 are intensified gradually, in accordance with the TEM results. The growth mechanism has been widely employed to synthesize a variety of g-C3N4-based composites [13,26,27]. As shown in Fig. 4, a similar strategy was used for the preparation of the gC3N4/BiFeO3 hybrid nanocomposites. Firstly, the ultrasonication of g-C3N4 and BiFeO3 powder completely dispersed in methanol for 3 h, resulted in the formation of g-C3N4 nanosheets and broken BiFeO3 nanoparticles, respectively. As a result of its unique surface and electronic distribution, the g-C3N4 nanosheets, create numerous active sites. Secondly, the small BiFeO3 nanoparticles attached to the surface by the effection of the surface charge. In order to minimize the overall energy of the reaction system, BiFeO3 nanoparticles had a tendency to aggregate along a certain direction. The preferential growth of the BiFeO3 nanoparticles
can be attributed to the different nanoparticle-aggregation potentials [28], leading to the anisotropic growth rates in different directions. The self-assembly and anisotropic growth process could be explained by the ‘‘oriented attachment’’ mechanism [29,30]. Thirdly, the primary self-aggregate nanoparticles recrystallized according to the well-known Gibbs–Thomson law [31]. With the assistance of Ostwald ripening, the surface energy will be further decreased, thus the as-prepared spindle-like BiFeO3 nanoparticles become much more stable [32]. In some cases, small BiFeO3 nanoparticles were aligned, but have not yet fused. With the decrease of the BiFeO3 content, the concentration of spindle-like nanoparticles also decreased. In order to further understand the effect of g-C3N4 on the growth of the g-C3N4/BiFeO3 spindle-like composites, we also stirred BiFeO3 solution for 24 h at the same condition without g-C3N4. The result indicated that there was no remarkable change of the broken BiFeO3 nanoparticle. Therefore, the introduction of g-C3N4 results in, not only novel formation and dispersion of spindle-like BiFeO3 structure, but also high reinforced the visible light absorption, which provides largely improved photogenerated electron–hole separation. The optical properties of the as-prepared g-C3N4/BiFeO3 samples, as well as pure g-C3N4 and BiFeO3 were investigated by UV– vis diffuse reflectance spectroscopy (DRS). As can be seen from Fig. 5, the band gap absorption edge of pure g-C3N4 is around 460 nm, which can be assigned to the intrinsic band gap of gC3N4. On the basis of the equation ahm = A(hm Eg)n/2 [33],the band gap values of the 50% g-C3N4/BiFeO3 sample were estimated to be 2.7 and 2.2 eV from the inset of the absorption edges, corresponding to the pure g-C3N4 and BiFeO3 respectively, which is in good agreement with the values reported in the literature [20,34,35]. Obviously, the g-C3N4/BiFeO3 composite samples exhibit a red shift in visible light compared with the pure g-C3N4, indicating that the absorption of the g-C3N4/BiFeO3 photocatalysts is shifted to the lower energy region. By introducing BiFeO3 to the g-C3N4, the photocatalyst could absorb more visible light to produce electron–hole pairs, which will be favorable for a photocatalytic reaction, as will be demonstrated below. 3.2. Photocatalytic activity of catalysts The photocatalytic performance of g-C3N4/BiFeO3 hybrids was evaluated by the photodegradation of methyl orange (MO) aqueous solution under visible light irradiation after adsorption/desorption equilibration. Fig. 6a shows the photocatalytic activity of gC3N4, BiFeO3 and the g-C3N4/BiFeO3 photocatalysts with different loading amounts of g-C3N4. As can be clearly seen, the decrease in the concentration of MO is faster and more prominent with gC3N4/BiFeO3 nanocomposites than the pure g-C3N4 or BiFeO3 under
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Fig. 2. TEM images of 10%, 30%, 50% and 80% CN/BiFe (a–d), pure g-C3N4 (e), EDS of the 50% CN/BiFe sample (f–g). Inset: size distribution of spindle-like BiFeO3 nanoparticles.
the same experimental conditions. By comparison, the optimal content of g-C3N4 in g-C3N4/BiFeO3 hybrids was found to be 50 wt%. In order to evaluate the photocatalytic performance, we compared the 50% g-C3N4/BiFeO3 photocatalysts with that of
g-C3N4/WO3 [36], which is another nontoxic visible light-responsed composite photocatalysts prepared by Zang et al. Nearly 50% of MO is degraded within a period of 2 h under visible light, which is has lower activity than our photocatalysts. Further
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Fig. 3. XRD patterns of the nanocomposites samples: pure BiFeO3 (a), 10%, 30%, 50% and 80% CN/BiFe (b–e), and pure g-C3N4 (f).
comparative experiments were carried out to investigate the photocatalytic activity of the as-prepared g-C3N4/BiFeO3 photocatalysts and Degussa P25 titania powders under UV light irradiation, as seen in Fig. 6b. All g-C3N4/BiFeO3 photocatalysts had high photocatalytic efficiency under UV light irradiation, but lower than the Degussa P25. However, under UV light irradiation, every sample had a photocatalytic efficiency higher than that of under visible light irradiation. In addition to the photocatalytic efficiency, the stability of a photocatalyst is also very important to practical application. The result shown in Fig. 7 revealed that the photocatalytic activity of 50% g-C3N4/BiFeO3 did not exhibit significant loss after four recycles for the photodegradation of MO. Only a slight decrease was observed, which can be attributed to the loss of catalyst during the recycle process. Once BiFeO3 nanoparticles were added, the emission intensity of the PL spectra for the g-C3N4/BiFeO3 hybrids significantly decreased, as shown in Fig. 8. In the g-C3N4/BiFeO3 hybrid nanocomposites, the BiFeO3 and g-C3N4 have closely contacted interfaces, and the photo-generated electron–hole pairs can migrate easily between g-C3N4 and BiFeO3 through their interface
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Fig. 5. UV–vis spectra of BiFeO3, g-C3N4 and g-C3N4/BiFeO3 hybrids with varied gC3N4 content. The inset is the calculation diagram of band gaps of the 50% g-C3N4/ BiFeO3 sample.
due to the matching band potentials and therefore, the recombination of electrons and holes is suppressed. The result suggests that the g-C3N4/BiFeO3 hybrid nanocomposite is crucial to reduce the recombination rate of the photo-generated charge carriers, and improve the corresponding photocatalytic activity. Thus, the g-C3N4/BiFeO3 hybrid composites with matched energy band positions could be promising photocatalysts for environmental applications. On the basis of the above structure characterizations and the visible light photocatalytic tests, a proposed mechanism is discussed to explain the enhanced photocatalytic activity and stability of the g-C3N4/BiFeO3 photocatalysts, illustrated in Fig. 9. Under visible light irradiation, a high energy photon excites an electron from the valence band (VB) to the conduction band (CB) of BiFeO3 and g-C3N4. The photoinduced electrons in g-C3N4 can easily transfer to BiFeO3, while the holes can transfer to the VB of g-C3N4 conveniently [35]. As a result, the photogenerated electrons and holes are efficiently separated between BiFeO3 and g-C3N4. The redistribution of electrons and holes reduces the electron–hole recombination in the hybrid composite photocatalysts and, thereby
Fig. 4. Schematic representation of the deposition of BiFeO3 nanoparticles on g-C3N4 sheet.
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Fig. 8. Comparison of PL spectra of pure g-C3N4 and 50 wt% g-C3N4/BiFeO3 samples.
Fig. 6. Photocatalytic activity of different catalysts for the degradation of MO solution at room temperature: (a) visible light irradiation, and (b) UV light irradiation.
Fig. 9. Scheme for electron–hole transport at the interface of the g-C3N4/BiFeO3 nanocomposites.
polluted dyes. Meanwhile, based on Fig. 2, it can be deduced that the BiFeO3 content was pivotal for achieving the high photocatalytic activity of the g-C3N4/BiFeO3 nanocomposites. The suitable BiFeO3 content causes good dispersion in the catalyst, which benefits the formation of heterojunctions between the g-C3N4 and BiFeO3 particles. As a result, a high separation of the charge carriers and photocatalytic activity were obtained on the 50 wt% g-C3N4/ BiFeO3 sample.
4. Conclusions
Fig. 7. Stability study of photocatalytic degradation of MO by 50 wt% g-C3N4/BiFeO3 composite under visible light irradiation.
improves the photo-oxidation efficiency [37]. The electrons in g-C3N4 could also adsorb surface O2 form various reactive oxygen species, thus could assist the degradation of organic MO effectively. Meanwhile, the photogenerated holes on BiFeO3 could also oxidize
A novel g-C3N4/BiFeO3 nanocomposite photocatalyst have been successfully synthesized by a simple method. More interestingly, the introduction of g-C3N4 results in the change of BiFeO3 morphology, from micro cubes to spindle-like nanoparticles. Compared with the pure g-C3N4 photocatalyst, the photocatalytic activity of the as-prepared g-C3N4/BiFeO3 nanocomposite was increased over 2 times for methyl orange (MO) photodegradation under visible light irradiation, and the results showed that the optimal amount of g-C3N4 in the composites was 50 wt%. The enhanced photocatalytic activity of the g-C3N4/BiFeO3 photocatalysts could be ascribed to the formation of a heterojunction between the g-C3N4 and BiFeO3, which improved the charge separation efficiency. Therefore,
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the g-C3N4/BiFeO3 nanocomposite is a promising photocatalytic material for environmental applications as well as water splitting. Acknowledgments We acknowledge the financial support from the National Basic Research Program of China (2012CB933301, 2014CB648300), the Key Project of National High Technology Research of China (2011AA050526), the Ministry of Education of China (No. IRT1148), National Synergistic Innovation Center for Advanced Materials (SICAM), Natural Science Foundation of Jiangsu Province, China (BM2012010), the Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD, YX03001), the National Natural Science Foundation of China (51172110, 51372119, 61377019, 61136003, 51173081). References [1] M.R. Hoffmann, S.T. Martin, W.Y. Choi, D.W. Bahnemann, Chem. Rev. 95 (1995) 69–96. [2] Y.H. Li, Y.J. Sun, F. Dong, W.K. Ho, J. Colloid Interface Sci. 436 (2014) 29–36. [3] J. Zhang, R. He, X.H. Liu, Nanotechnology 24 (2013) 505401. [4] B.P. Nenavathu, A.V.R.K. Rao, A. Goyal, A. Kapoor, R.K. Dutta, Appl. Catal., AGen. 459 (2013) 106–113. [5] H.G. Wang, X.L. Fei, L. Wang, Y.P. Li, S.F. Xu, M.D. Sun, L. Sun, C.Q. Zhang, Y.X. Li, Q.B. Yang, Y. Wei, New J. Chem. 35 (2011) 1795–1802. [6] W. Chen, H. Ruan, Y. Hu, D.Z. Li, Z.X. Chen, J.J. Xian, J. Chen, X.Z. Fu, Y. Shao, Y. Zheng, CrystEngComm 14 (2012) 6295–6305. [7] J.Q. Zhang, K. Yu, Y.F. Yu, L.L. Lou, Z.Q. Yang, J.W. Yang, S.X. Liu, J. Mol. Catal. A: Chem. 391 (2014) 12–18. [8] J. Fu, B.B. Chang, Y.L. Tian, F.N. Xi, X.P. Dong, J. Mater. Chem. A 1 (2013) 3083– 3090. [9] X.C. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, J.M. Carlsson, K. Domenet, M. Antonietti, Nat. Mater. 8 (2009) 76–80. [10] Y.L. Tian, B.B. Chang, J.L. Lu, J. Fu, F.N. Xi, X.P. Dong, ACS Appl. Mater. Interfaces 5 (2013) 7079–7085. [11] C.S. Xing, Z.D. Wu, D.L. Jiang, M. Chen, J. Colloid Interface Sci. 433 (2014) 9–15. [12] Y.L. Min, X.F. Qi, Q.J. Xu, Y.C. Chen, CrystEngComm 16 (2014) 1287–1295. [13] Y.J. Wang, X.J. Bai, C.S. Pan, J. He, Y.F. Zhu, J. Mater. Chem. 22 (2012) 11568– 11573.
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Facile fabrication of highly efficient g-C3N4/BiFeO3 nanocomposites with enhanced visible light photocatalytic activities Xingfu Wang a,b, Weiwei Mao a,b, Jian Zhang a, Yumin Han a, Chuye Quan a, Qiaoxia Zhang a, Tao Yang a, Jianping Yang b, Xing’ao Li a,⇑, Wei Huang a,c,⇑ a Key Laboratory for Organic Electronics & Information Displays (KLOEID), Synergetic Innovation Center for Organic Electronics and Information Displays (SICOEID), Institute of Advanced Materials (IAM), School of Materials Science and Engineering (SMSE), Nanjing University of Posts and Telecommunications (NUPT), Nanjing 210023, PR China b School of Science, Nanjing University of Posts and Telecommunications (NUPT), Nanjing 210023, PR China c Key Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials (IAM), National Synergistic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University (NanjingTech), 30 South Puzhu Road, Nanjing 211816, PR China
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Article history: Received 18 November 2014 Accepted 31 January 2015 Available online 9 February 2015 Keywords: g-C3N4 BiFeO3 Nanocomposites Photocatalysis
a b s t r a c t Graphitic carbon nitride/bismuth ferrite (g-C3N4/BiFeO3) nanocomposites with various g-C3N4 contents have been synthesized by a simple method. After the deposition–precipitation process, the novel BiFeO3 spindle-like nanoparticles with the size of 100 nm were homogeneously decorated on the surfaces of the C3N4 nanosheets. A possible deposition growth mechanism is proposed on the basis of experimental results. The as-prepared g-C3N4/BiFeO3 composites exhibit high efficiency for the degradation of methyl orange (MO) under visible light irradiation, which can be mainly attributed to the synergic effect between g-C3N4 and BiFeO3. The ability to tune surface and interfacial characteristics for the optimization of photophysical properties suggests that the deposition growth process may enable formation of hybrids suitable for a range of photocatalytic applications based on g-C3N4. Ó 2015 Elsevier Inc. All rights reserved.
1. Introduction In recent years, the scientific community has focused intently on new photocatalysts based on semiconductor that enhance the photocatalytic activities as well as being earth abundant in water splitting and degradation of organic pollutants [1–3]. Various ⇑ Corresponding authors at: Key Laboratory for Organic Electronics & Information Displays (KLOEID), Synergetic Innovation Center for Organic Electronics and Information Displays (SICOEID), Institute of Advanced Materials (IAM), School of Materials Science and Engineering (SMSE), Nanjing University of Posts and Telecommunications (NUPT), Nanjing 210023, PR China (W. Huang). E-mail addresses: [email protected] (X. Li), [email protected] (W. Huang). http://dx.doi.org/10.1016/j.jcis.2015.01.090 0021-9797/Ó 2015 Elsevier Inc. All rights reserved.
semiconductor photocatalysts, such as metal oxides (ZnO [4], TiO2 [5]) and metal sulfides (ZnS [6]), have been explored in the degradation of organic pollutants under UV light irradiation only because of the large band gap (>3 eV). However, the large band gap restricts the absorption in the UV region, which accounts for only 5% of the entire solar spectrum. As a result of the development of band-gap engineering, the fabrication of composites by coupling of two semiconductors with narrow band gap has attracted considerable attention [7,8]. Graphitic carbon nitride (g-C3N4), with a narrow band gap of 2.7 eV, is a promising metal-free photocatalyst due to its nontoxicity, abundance and stability [9]. To enhance the photocatalytic activity, a number of methods have been developed. In particular,
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combining g-C3N4 with other semiconductors to form composites or heterostructures, provides a feasible route to inhibit the recombination of photogenerated electron–hole pairs [10–12]. In recent years, a C3N4/Bi2WO6 photocatalyst hybridized with monolayer C3N4 exhibited the highest photocatalytic activity which was 68.9% higher than that of pure Bi2WO6 [13]. g-C3N4–Ag2O composites with the mass ratio of 1:4 degraded nearly 50% of MO within 4 min, and 90% after 30 min [14]. On the other hand, bismuth ferrite (BiFeO3), with simultaneous ferroelectric and ferromagnetic ordering, was studied as one of the most important multiferroic materials in photocatalytic applications recently [15–17]. With a relatively narrow band gap of 2.2 eV [18,19], BiFeO3-based photocatalysts show strong absorption of visible light and hence exhibit excellent photocatalytic abilities under visible light irradiation. In the previous reports, Zhu et al. modified TiO2-nanotubes surface by BiFeO3 nanoparticles, which remarkably improves the photoresponses because of significantly enhanced visible-light utilization [20]. Recently, hybrids of BiFeO3 with nanoscale carbon materials have attracted significant attention. Li et al. reported a hybrid approach by decorating BiFeO3 nanoparticles on graphene nanosheets, and the rate for the photo-degradation of Congo Red under visible light is six times higher than that of bare BiFeO3 particles [21]. In order to enhance the photoinduced charges separation efficiency for improved photocatalytic degradation activity, the novel g-C3N4/BiFeO3 photocatalysts were firstly synthesized by a green approach at ambient temperature. To the best of our knowledge, there are no reports on the synthesis and application of g-C3N4/ BiFeO3 nanocomposites for pollutants degradation under visible light irradiation. The photoactivity of the catalysts was evaluated by degrading methyl orange (MO) dye solution, and the results showed that the optimal amount of g-C3N4 in the composites was 50 wt%. A possible mechanism for enhancing photocatalytic activity of g-C3N4/BiFeO3 was critically discussed.
2. Experimental section 2.1. Fabrication of g-C3N4/BiFeO3 nanocomposite photocatalysts Melamine (C3H6N6) and sodium hydroxide (NaOH) were obtained from Sinopharm Chemical Reagent Corp, P.R. China. Bismuth nitrate (Bi(NO3)35H2O) and ferric chloride hexahydrate (FeCl36H2O) were purchased from Guangdong Xilong Chemical Co., Ltd., P.R. China. All chemicals were used as received without further purification. The g-C3N4 power was synthesized according to the literature [12]. Typically, 3 g of melamine was put into an alumina crucible with a cover and heated to 550 °C in a muffle furnace and then kept at this temperature for 4 h. The resulting yellow product was collected and ground into powder for further use. BiFeO3 micro-cubes were synthesized by the hydrothermal method. In a typical synthesis, Bi(NO3)35H2O (4.850 g) and FeCl36H2O (2.704 g) in a stoichiometric ratio (1:1 in molar ratios) were mixed in 100 mL of acetone (99.8%) and sonicated for 30 min. Then 300 mL of deionized (D.I.) water and concentrated ammonia were added under vigorous stirring until the pH value of the solution reached 10–11. The sediment was centrifuged out and washed with D.I. water several times until the pH value was neutral. Next, the red co-precipitate was redispersed in D.I. water. Under vigorous stirring, 15 g NaOH was added into the suspension. Then, the solution was placed inside a stainless steel autoclave with a Teflon liner and heated at 180 °C for 72 h. After cooling down to room temperature, the precipitate was harvested by filtration, washed with D.I. water and pure ethanol for three times, respectively, and dried at 80 °C for 2 h.
The typical preparation of g-C3N4/BiFeO3 photocatalysts was as follows: an appropriate amount of g-C3N4 and BiFeO3 powder were completely dispersed in methanol assisted by ultrasonication for 3 h, respectively. The as-prepared g-C3N4 solution and BiFeO3 solution were mixed together and stirred in a fume hood for 24 h. After volatilization of the methanol, an opaque powder was obtained after drying at 80 °C in air. According to this method, various mass ratios of g-C3N4 from 10% to 80% were synthesized. 2.2. Characterization The microstructural morphologies of the g-C3N4/BiFeO3 composites were observed via scanning electron microscopy (SEM, JEOL-6380LV Japan; accelerating voltage = 200 kV) and energy dispersive X-ray spectroscopy (EDS, HORIBA EMAX Energy EX-250). Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) imaging was performed on a JEOL JEM-2100 microscope operating at 200 kV, by depositing a drop of sample dispersion onto 300 mesh Cu grids coated with a carbon layer. The obtained samples were characterized by XRD on a Bruker D8 Advance X-ray powder diffractometer with Cu Ka radiation (k = 1.5418 Å) in the 2h range of 10–70° with a step size of 0.002° and a scan speed of 0.5 s per step. The accelerating voltage and the applied current were 40 kV and 40 mA, respectively. UV–vis absorption spectra were obtained using a Ruili UV-1201 spectrophotometer. The visible light (k > 420 nm) was obtained using a 500 W Xe lamp with a 420 nm cutoff filter to completely remove any radiation below 420 nm. The photoluminescence (PL) spectra were performed using a FL3-TCSPC spectrofluorometer (FL, Horiba Jobin Yvon, France) equipped with a xenon arc lamp as the light source and a quartz cell (1 1 cm). 2.3. Photocatalytic tests The photocatalytic activities of g-C3N4/BiFeO3 nanocomposites were evaluated by degradation of MO aqueous solution under UV light irradiation and visible light irradiation. A 300 W high-pressure Hg lamp was used as the UV light source. A 500 W xenon lamp was used as the other light source, and visible-light irradiation was realized by attaching a 420 nm cutoff filter. In a typical photocatalytic experiment, 50 mg catalyst was dispersed in 50 mL MO aqueous solution with a concentration of 5 mg L 1. Prior to irradiation, the suspensions were magnetically stirred in the dark for 30 min to ensure the establishment of an adsorption–desorption equilibrium. Then, at selected time intervals, samples were collected and filtered to remove the photocatalyst particles by centrifugation. After that, the solution was analyzed using a ultraviolet– visible light spectrophotometer. A blank test was also carried out on an aqueous MO solution without photocatalyst under the same condition. In addition, the recyclability of the catalyst was also studied by washing and drying the catalyst before the next cycle.
3. Results and discussion 3.1. Characterization of samples In order to investigate the formation process of the composites, the morphology of the as-synthesized pure BiFeO3 was observed by SEM and TEM, as shown in Fig. 1. It can be detected that the BiFeO3 samples present excellent cubes under SEM observation and an edge length ranging from 0.5 to 1 lm. The same result can be obtained from the TEM micrograph (Fig. 1b). The inset of Fig. 1b is the corresponding HRTEM imaging, which match well with the (0 1 2) of the BiFeO3 lattice (d = 3.95 Å). According to the
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Fig. 1. SEM and TEM images of pure BiFeO3 cubes. Inset: HRTEM image.
previous report [22], the relatively high alkaline concentration would better result in the cubic BiFeO3. The introduction of g-C3N4 results in interesting change of the morphology of BiFeO3 micro cubes. The dispersion state and the structure of all the as synthesized g-C3N4/BiFeO3 nanocomposite were obtained by TEM. Fig. 2e shows that the pure g-C3N4 has a layered structure with few stacking layers. More attractively, the spindle-like BiFeO3 nanoparticles were attached to the surface of g-C3N4 sheets and were randomly distributed, as shown in Fig. 2a–d. From the corresponding size distribution column graph (inset in Fig. 2c), it showed that the particle sizes are mainly distributed around the sizes of 50–250 nm. With the increase of the g-C3N4 concentration, the spindle-like BiFeO3 nanoparticles reveal a better dispersion. However, the high g-C3N4 concentration also reduces dispersion of itself, which suppress the formation of the spindle-like morphology, as can be found in Fig 2d. As discussed above, it can be concluded that the mass ratios of g-C3N4/BiFeO3 photocatalysts could be crucial for this hybrid, which might promote the separation of electron–hole pairs and, subsequently accelerate photocatalytic reaction [23]. In addition, EDS analysis showed that the selected areas of the 50% g-C3N4/BiFeO3 were found with C, N, O, Fe and Bi elements (Fig. 2f and g). Based on the SEM–EDS and TEM analyses, it can be concluded that the fascinating structure is formed in the composite. The crystal phase of the materials is determined by XRD, as shown in Fig. 3. All peaks of pure BiFeO3 samples (Fig. 3a) can be indexed to a rhombohedral lattice with the space group R3c (JCPDS card No. 86-1518) [24]. The pure g-C3N4 has two distinct diffraction peaks at 13.0° and 27.2° (Fig. 3f), which correspond to the graphitic materials as the (1 0 0) and (0 0 2) diffraction planes [25]. For g-C3N4/BiFeO3 composites (Fig. 3b–e), no impurity diffraction planes were observed. With an increasing amount of g-C3N4 from 10% to 80%, the diffraction peaks of g-C3N4 are intensified gradually, in accordance with the TEM results. The growth mechanism has been widely employed to synthesize a variety of g-C3N4-based composites [13,26,27]. As shown in Fig. 4, a similar strategy was used for the preparation of the gC3N4/BiFeO3 hybrid nanocomposites. Firstly, the ultrasonication of g-C3N4 and BiFeO3 powder completely dispersed in methanol for 3 h, resulted in the formation of g-C3N4 nanosheets and broken BiFeO3 nanoparticles, respectively. As a result of its unique surface and electronic distribution, the g-C3N4 nanosheets, create numerous active sites. Secondly, the small BiFeO3 nanoparticles attached to the surface by the effection of the surface charge. In order to minimize the overall energy of the reaction system, BiFeO3 nanoparticles had a tendency to aggregate along a certain direction. The preferential growth of the BiFeO3 nanoparticles
can be attributed to the different nanoparticle-aggregation potentials [28], leading to the anisotropic growth rates in different directions. The self-assembly and anisotropic growth process could be explained by the ‘‘oriented attachment’’ mechanism [29,30]. Thirdly, the primary self-aggregate nanoparticles recrystallized according to the well-known Gibbs–Thomson law [31]. With the assistance of Ostwald ripening, the surface energy will be further decreased, thus the as-prepared spindle-like BiFeO3 nanoparticles become much more stable [32]. In some cases, small BiFeO3 nanoparticles were aligned, but have not yet fused. With the decrease of the BiFeO3 content, the concentration of spindle-like nanoparticles also decreased. In order to further understand the effect of g-C3N4 on the growth of the g-C3N4/BiFeO3 spindle-like composites, we also stirred BiFeO3 solution for 24 h at the same condition without g-C3N4. The result indicated that there was no remarkable change of the broken BiFeO3 nanoparticle. Therefore, the introduction of g-C3N4 results in, not only novel formation and dispersion of spindle-like BiFeO3 structure, but also high reinforced the visible light absorption, which provides largely improved photogenerated electron–hole separation. The optical properties of the as-prepared g-C3N4/BiFeO3 samples, as well as pure g-C3N4 and BiFeO3 were investigated by UV– vis diffuse reflectance spectroscopy (DRS). As can be seen from Fig. 5, the band gap absorption edge of pure g-C3N4 is around 460 nm, which can be assigned to the intrinsic band gap of gC3N4. On the basis of the equation ahm = A(hm Eg)n/2 [33],the band gap values of the 50% g-C3N4/BiFeO3 sample were estimated to be 2.7 and 2.2 eV from the inset of the absorption edges, corresponding to the pure g-C3N4 and BiFeO3 respectively, which is in good agreement with the values reported in the literature [20,34,35]. Obviously, the g-C3N4/BiFeO3 composite samples exhibit a red shift in visible light compared with the pure g-C3N4, indicating that the absorption of the g-C3N4/BiFeO3 photocatalysts is shifted to the lower energy region. By introducing BiFeO3 to the g-C3N4, the photocatalyst could absorb more visible light to produce electron–hole pairs, which will be favorable for a photocatalytic reaction, as will be demonstrated below. 3.2. Photocatalytic activity of catalysts The photocatalytic performance of g-C3N4/BiFeO3 hybrids was evaluated by the photodegradation of methyl orange (MO) aqueous solution under visible light irradiation after adsorption/desorption equilibration. Fig. 6a shows the photocatalytic activity of gC3N4, BiFeO3 and the g-C3N4/BiFeO3 photocatalysts with different loading amounts of g-C3N4. As can be clearly seen, the decrease in the concentration of MO is faster and more prominent with gC3N4/BiFeO3 nanocomposites than the pure g-C3N4 or BiFeO3 under
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Fig. 2. TEM images of 10%, 30%, 50% and 80% CN/BiFe (a–d), pure g-C3N4 (e), EDS of the 50% CN/BiFe sample (f–g). Inset: size distribution of spindle-like BiFeO3 nanoparticles.
the same experimental conditions. By comparison, the optimal content of g-C3N4 in g-C3N4/BiFeO3 hybrids was found to be 50 wt%. In order to evaluate the photocatalytic performance, we compared the 50% g-C3N4/BiFeO3 photocatalysts with that of
g-C3N4/WO3 [36], which is another nontoxic visible light-responsed composite photocatalysts prepared by Zang et al. Nearly 50% of MO is degraded within a period of 2 h under visible light, which is has lower activity than our photocatalysts. Further
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Fig. 3. XRD patterns of the nanocomposites samples: pure BiFeO3 (a), 10%, 30%, 50% and 80% CN/BiFe (b–e), and pure g-C3N4 (f).
comparative experiments were carried out to investigate the photocatalytic activity of the as-prepared g-C3N4/BiFeO3 photocatalysts and Degussa P25 titania powders under UV light irradiation, as seen in Fig. 6b. All g-C3N4/BiFeO3 photocatalysts had high photocatalytic efficiency under UV light irradiation, but lower than the Degussa P25. However, under UV light irradiation, every sample had a photocatalytic efficiency higher than that of under visible light irradiation. In addition to the photocatalytic efficiency, the stability of a photocatalyst is also very important to practical application. The result shown in Fig. 7 revealed that the photocatalytic activity of 50% g-C3N4/BiFeO3 did not exhibit significant loss after four recycles for the photodegradation of MO. Only a slight decrease was observed, which can be attributed to the loss of catalyst during the recycle process. Once BiFeO3 nanoparticles were added, the emission intensity of the PL spectra for the g-C3N4/BiFeO3 hybrids significantly decreased, as shown in Fig. 8. In the g-C3N4/BiFeO3 hybrid nanocomposites, the BiFeO3 and g-C3N4 have closely contacted interfaces, and the photo-generated electron–hole pairs can migrate easily between g-C3N4 and BiFeO3 through their interface
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Fig. 5. UV–vis spectra of BiFeO3, g-C3N4 and g-C3N4/BiFeO3 hybrids with varied gC3N4 content. The inset is the calculation diagram of band gaps of the 50% g-C3N4/ BiFeO3 sample.
due to the matching band potentials and therefore, the recombination of electrons and holes is suppressed. The result suggests that the g-C3N4/BiFeO3 hybrid nanocomposite is crucial to reduce the recombination rate of the photo-generated charge carriers, and improve the corresponding photocatalytic activity. Thus, the g-C3N4/BiFeO3 hybrid composites with matched energy band positions could be promising photocatalysts for environmental applications. On the basis of the above structure characterizations and the visible light photocatalytic tests, a proposed mechanism is discussed to explain the enhanced photocatalytic activity and stability of the g-C3N4/BiFeO3 photocatalysts, illustrated in Fig. 9. Under visible light irradiation, a high energy photon excites an electron from the valence band (VB) to the conduction band (CB) of BiFeO3 and g-C3N4. The photoinduced electrons in g-C3N4 can easily transfer to BiFeO3, while the holes can transfer to the VB of g-C3N4 conveniently [35]. As a result, the photogenerated electrons and holes are efficiently separated between BiFeO3 and g-C3N4. The redistribution of electrons and holes reduces the electron–hole recombination in the hybrid composite photocatalysts and, thereby
Fig. 4. Schematic representation of the deposition of BiFeO3 nanoparticles on g-C3N4 sheet.
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Fig. 8. Comparison of PL spectra of pure g-C3N4 and 50 wt% g-C3N4/BiFeO3 samples.
Fig. 6. Photocatalytic activity of different catalysts for the degradation of MO solution at room temperature: (a) visible light irradiation, and (b) UV light irradiation.
Fig. 9. Scheme for electron–hole transport at the interface of the g-C3N4/BiFeO3 nanocomposites.
polluted dyes. Meanwhile, based on Fig. 2, it can be deduced that the BiFeO3 content was pivotal for achieving the high photocatalytic activity of the g-C3N4/BiFeO3 nanocomposites. The suitable BiFeO3 content causes good dispersion in the catalyst, which benefits the formation of heterojunctions between the g-C3N4 and BiFeO3 particles. As a result, a high separation of the charge carriers and photocatalytic activity were obtained on the 50 wt% g-C3N4/ BiFeO3 sample.
4. Conclusions
Fig. 7. Stability study of photocatalytic degradation of MO by 50 wt% g-C3N4/BiFeO3 composite under visible light irradiation.
improves the photo-oxidation efficiency [37]. The electrons in g-C3N4 could also adsorb surface O2 form various reactive oxygen species, thus could assist the degradation of organic MO effectively. Meanwhile, the photogenerated holes on BiFeO3 could also oxidize
A novel g-C3N4/BiFeO3 nanocomposite photocatalyst have been successfully synthesized by a simple method. More interestingly, the introduction of g-C3N4 results in the change of BiFeO3 morphology, from micro cubes to spindle-like nanoparticles. Compared with the pure g-C3N4 photocatalyst, the photocatalytic activity of the as-prepared g-C3N4/BiFeO3 nanocomposite was increased over 2 times for methyl orange (MO) photodegradation under visible light irradiation, and the results showed that the optimal amount of g-C3N4 in the composites was 50 wt%. The enhanced photocatalytic activity of the g-C3N4/BiFeO3 photocatalysts could be ascribed to the formation of a heterojunction between the g-C3N4 and BiFeO3, which improved the charge separation efficiency. Therefore,
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the g-C3N4/BiFeO3 nanocomposite is a promising photocatalytic material for environmental applications as well as water splitting. Acknowledgments We acknowledge the financial support from the National Basic Research Program of China (2012CB933301, 2014CB648300), the Key Project of National High Technology Research of China (2011AA050526), the Ministry of Education of China (No. IRT1148), National Synergistic Innovation Center for Advanced Materials (SICAM), Natural Science Foundation of Jiangsu Province, China (BM2012010), the Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD, YX03001), the National Natural Science Foundation of China (51172110, 51372119, 61377019, 61136003, 51173081). References [1] M.R. Hoffmann, S.T. Martin, W.Y. Choi, D.W. Bahnemann, Chem. Rev. 95 (1995) 69–96. [2] Y.H. Li, Y.J. Sun, F. Dong, W.K. Ho, J. Colloid Interface Sci. 436 (2014) 29–36. [3] J. Zhang, R. He, X.H. Liu, Nanotechnology 24 (2013) 505401. [4] B.P. Nenavathu, A.V.R.K. Rao, A. Goyal, A. Kapoor, R.K. Dutta, Appl. Catal., AGen. 459 (2013) 106–113. [5] H.G. Wang, X.L. Fei, L. Wang, Y.P. Li, S.F. Xu, M.D. Sun, L. Sun, C.Q. Zhang, Y.X. Li, Q.B. Yang, Y. Wei, New J. Chem. 35 (2011) 1795–1802. [6] W. Chen, H. Ruan, Y. Hu, D.Z. Li, Z.X. Chen, J.J. Xian, J. Chen, X.Z. Fu, Y. Shao, Y. Zheng, CrystEngComm 14 (2012) 6295–6305. [7] J.Q. Zhang, K. Yu, Y.F. Yu, L.L. Lou, Z.Q. Yang, J.W. Yang, S.X. Liu, J. Mol. Catal. A: Chem. 391 (2014) 12–18. [8] J. Fu, B.B. Chang, Y.L. Tian, F.N. Xi, X.P. Dong, J. Mater. Chem. A 1 (2013) 3083– 3090. [9] X.C. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, J.M. Carlsson, K. Domenet, M. Antonietti, Nat. Mater. 8 (2009) 76–80. [10] Y.L. Tian, B.B. Chang, J.L. Lu, J. Fu, F.N. Xi, X.P. Dong, ACS Appl. Mater. Interfaces 5 (2013) 7079–7085. [11] C.S. Xing, Z.D. Wu, D.L. Jiang, M. Chen, J. Colloid Interface Sci. 433 (2014) 9–15. [12] Y.L. Min, X.F. Qi, Q.J. Xu, Y.C. Chen, CrystEngComm 16 (2014) 1287–1295. [13] Y.J. Wang, X.J. Bai, C.S. Pan, J. He, Y.F. Zhu, J. Mater. Chem. 22 (2012) 11568– 11573.
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