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Synthesis of graphene oxide-BiPO4 composites with enhanced photocatalytic properties
Applied Surface Science 284 (2013) 308–314
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Synthesis of graphene oxide-BiPO4 composites with enhanced photocatalytic properties Hongwei Lv a , Xiaoping Shen a,∗ , Zhenyuan Ji b , Dezhou Qiu a , Guoxing Zhu a , Yongliang Bi a a b
School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, PR China School of Materials Science and Engineering, Jiangsu University, Zhenjiang 212013, PR China
a r t i c l e
i n f o
Article history: Received 27 May 2013 Received in revised form 18 July 2013 Accepted 18 July 2013 Available online 26 July 2013 Keywords: Graphene oxide Bismuth phosphate Nanocomposites Synthesis Photocatalysis
a b s t r a c t Graphene has attracted considerable attention for developing highly efficient photocatalysts due to its unique two dimensional structure and extraordinary physicochemical properties. In this paper, we demonstrate a facile two-phase self-assembly approach for the synthesis of graphene oxide (GO)-BiPO4 nanocomposites. The as-prepared samples were characterized by X-ray diffraction, transmission electron microscopy, Raman spectra, UV–vis diffuse reflectance spectra and Fourier transform infrared spectra. The significantly enhanced photocatalytic activity of the GO-BiPO4 nanocomposites in comparison with bare BiPO4 nanoparticles was revealed by the degradation of methylene blue under simulated sunlight irradiation, which can be attributed to the improved separation of electron–hole pairs. This facile method could be extended to design other graphene-based photocatalysts for environment and energy applications. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Photocatalysis, which provides a new promising way to meet the challenges of the environment, energy and sustainability with abundant solar light, has attracted considerable attention during the past decades [1–3]. Among the nanocrystal photocatalytic materials, bismuth-based photocatalysts have recently received much attention due to their superior photocatalytic activities in decomposing organic pollutants [4–6], oxidation of water [7,8], and reduction of NO [9–11]. Especially, bismuth phosphate (BiPO4 ), an oxoacid salt photocatalyst, has exhibited potential applications in catalysis [12], ion sensing [13], and separating radioactive elements [14]. However, due to the large band gap (3.85 eV) of BiPO4 [15], its excitation requires ultraviolet light below 322 nm. It is well known that 45% of the total energy in solar spectrum belongs to visible light, and the energy of UV light represents only 5% in that [16]. Therefore, the development of efficient visible-light responsive photocatalysts is highly expected and has become an active research area in photocatalysis research [17–24]. Another issue for BiPO4 photocatalyst is that the photoproduced electrons and holes in BiPO4 may experience a rapid recombination, which diminishes the efficiency of the photocatalytic reaction significantly [25]. Therefore, it is still a big challenge to explore an effective way
∗ Corresponding author. Tel.: +86 511 88791800; fax: +86 511 88791800. E-mail address: [email protected] (X. Shen). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.07.098
to improve the photocatalytic activity of bismuth phosphate for practical application. Graphene, a monolayer or few layers (<10) of hexagonally arrayed sp2 -bonded carbon atoms, has attracted intense scientific interest mainly due to its unique structure and excellent physicochemical properties [26–29]. Recently, the synthesis and application of graphene-based nanocomposites have been intensively pursued [30]. Among them, novel photocatalysts originating from the combination of graphene or graphene oxide (GO) nanosheets with photocatalytically active components such as TiO2 [31,32], ZnO [33], CoFe2 O4 [34], BiVO4 [35], and Ag/AgX (X = Br, Cl) [36] have been reported in degradation of organic pollutants for water purification. It was found that the hybridization of the above photocatalysts with graphene or GO can significantly improve the photocatalytic and photoconversion efficiency of the photocatalytic materials. For example, graphene/ZnO nanocomposites exhibit twice higher photocatalytic efficiency than bare ZnO in the degradation of RhB dye under UV irradiation [33]. However, to the best of our knowledge, the synthesis and photocatalytic performance of GO-BiPO4 nanocomposites have not been reported until now. In this study, GO-BiPO4 nanocomposites are prepared through a facile two-phase self-assembly approach. Compared with the synthesis of other graphene-based hybrids, our synthesis route has two features: (1) The synthesis of GO-BiPO4 nanocomposites is performed at room temperature. (2) The self-assembly method makes it easy to control the distribution and loading amount of BiPO4 on GO sheets. Furthermore, the synthesized GO-BiPO4 photocatalysts exhibit enhanced photocatalytic activities toward the degradation
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of methylene blue (MB) in comparison with bare BiPO4 nanocrystals (NCs) under simulated sunlight irradiation. 2. Experimental 2.1. Materials Natural flake graphite was purchased from Qingdao Guyu graphite Co. Ltd., with a particle size of 150 m (99.9% purity). All of the other chemical reagents are of analytical grade and used without further purification. 2.2. Preparation of graphite oxide Graphite oxide was synthesized through chemical exfoliation of graphite powder via a modified Hummers’ method [37]. Typically, 2.0 g of graphite powder was added into cold (0 ◦ C) concentrated H2 SO4 (80 mL) and NaNO3 (4.0 g) solution in a 500 mL flask. Under vigorous stirring, KMnO4 (10.0 g) was added gradually and the temperature of the mixture was kept to below 15 ◦ C. The reaction mixture was stirred at 35 ◦ C for 4–5 h until it became pasty brownish, and then diluted with de-ionized water (100 mL). The addition of water was performed in an ice bath to keep the temperature below 100 ◦ C. Then, the mixture was stirred for 30 min and 15 mL of 30 wt% H2 O2 was slowly added to the mixture to reduce the residual KMnO4 , after which the color of the mixture changed to reddish-brown. The mixture was filtered and washed with 5% HCl aqueous solution (800 mL) to remove metal ions followed by 1.0 L of de-ionized water to remove the acid. The resulted solid was centrifuged and dried at 45 ◦ C for 24 h. For further purification, the as-obtained graphite oxide was re-dispersed in de-ionized water and then was dialyzed for one week to remove residual salts and acids.
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2.5. Characterization The morphology and structure of the products were determined by transmission electron microscopy (TEM, JEOL JEM-2100) and Xray diffraction (XRD, Bruker D8 ADVANCE) with Cu K␣ radiation. Samples for TEM observation were prepared by dropping the products on a carbon-coated copper grid after ultrasonic dispersion in absolute ethanol and allowed them to dry in air before analysis. Fourier transform infrared (FT-IR) spectra were recorded on a Nicolet Nexus 470 spectrometer with KBr pellets in the 4000–400 cm−1 region. Raman spectra were measured at room temperature using a DXR Raman microscope with 514.5 nm excitation source from an Ar+ laser. Ultraviolet–visible (UV–vis) spectroscopy measurements were performed on a UV-2450 UV-vis spectrophotometer. 2.6. Photocatalytic measurements The photocatalytic activities of the as-prepared samples were evaluated by the photocatalytic decomposition of MB. Photoirradiation was conducted on a GHX-2 photochemical reactor with a 500 W tungsten lamp as simulated sunlight source. Experiments were conducted at room temperature as follows: 50 mg of photocatalyst was added to 100 mL of 20 mg/L dye aqueous solution. The mixed suspension was stirred for 1 h in the dark to reach the adsorption–desorption equilibrium between the catalyst and the dye. The dispersion was irradiated by the tungsten lamp after adding 1.0 mL of 30% H2 O2 solution. During the reaction process, 3 mL of the suspension was withdrawn at a regular time interval of 20 min and was centrifuged to remove the remnant photocatalyst. The supernatant was then recorded in the UV-vis spectrophotometer at ambient temperature for an optical absorbance measurement. The initial dye concentration (C0 ) and the concentrations (C) of the remnant dye were determined by measuring the absorbance of the solutions at 664 nm with UV–vis spectroscopy.
2.3. Preparation of OM-BiPO4 NCs Oleylamine-capped BiPO4 (OM-BiPO4 ) NCs were synthesized through a simple and reproducible route based on the reaction between bismuth ions, phosphoric acid and OM in toluene at room temperature [38]. Typically, bismuth nitrate (0.05 mol) and OM (0.1 mol) were dissolved in 150 mL of toluene, into which a H3 PO4 solution containing 0.05 mol of H3 PO4 and 50 mL of ethanol was gradually added. The resulting mixture was stirred for 30 min, and excess ethanol was added to precipitate OM-BiPO4 NCs. The obtained OM-BiPO4 product was dried at 45 ◦ C in a vacuum oven. 2.4. Preparation of GO-BiPO4 nanocomposites The GO-BiPO4 nanocomposites were produced through a twophase approach. In a typical synthesis, 50 mg of graphite oxide was added to 50 mL of water and then sonicated for 30 min to form a uniform GO aqueous dispersion. OM-BiPO4 NCs (950 mg) were dispersed in toluene (50 mL) and then mixed with the GO aqueous dispersion. The mixture was kept stirring for 12 h at room temperature. The OM-GO-BiPO4 (containing OM) nanocomposites were obtained by centrifugation, and washed with water and ethanol, respectively. The OM-GO-BiPO4 NCs were further treated with acetic acid to remove OM molecules. Typically, 1.0 g of the as-synthesized OM-GO-BiPO4 nanocomposites was dispersed in 100 mL of glacial acetic acid (≥99.5%), and stirred continually for 24 h at room temperature. The GO-BiPO4 products were separated by centrifugation, washed thoroughly with water and ethanol to remove any impurities, and then dried in a vacuum oven at 45 ◦ C for 24 h.
3. Results and discussion 3.1. Synthesis and characterization of GO-BiPO4 nanocomposites The GO-BiPO4 nanocomposites were fabricated through a facile two-phase self-assembly approach, the schematic of which is illustrated in Fig. 1. The pre-synthesized toluene solution of OM-BiPO4 NCs was mixed with aqueous solution of GO and stirred for 12 h. The self-assembly of OM-BiPO4 NCs on the GO nanosheets occurs at the water/toluene interface [39]. Then the OM molecules were removed from the surface of OM-GO-BiPO4 nanocomposites by acetic acid so as to make the GO-BiPO4 nanocomposite readily dispersed in water for practical photocatalytic application [40,41]. TEM images of the as-prepared products are shown in Fig. 2. It can be seen from Fig. 2a that the as-prepared OM-BiPO4 NCs are well-dispersed, and show sphere-like morphology with an average size of about 30 nm. After the treatment of OM-BiPO4 NCs with acetic acid, the resulting BiPO4 NCs are transformed into short rod-like morphology, and some agglomeration occurs due to the shortage of OM protection (Fig. 2b). The morphology of OM-GO-BiPO4 nanocomposite is shown in Fig. 2c. It can be observed that OM-BiPO4 nanoparticles are well dispersed on the GO nanosheets. Neither free BiPO4 nanoparticles nor bare GO sheets were observed during the TEM observation, indicating a strong combination between the GO support and the BiPO4 nanoparticles. Figs. 2d-e show the TEM images of GO-BiPO4 nanocomposites, the morphology and size of the BiPO4 NCs in the GO-BiPO4 system are almost the same as those in OM-GO-BiPO4 , which could be explained in terms of the capping effects afforded by the GO
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Fig. 1. Scheme of the formation of GO-BiPO4 nanocomposites by the self-assembly method.
Fig. 2. TEM images of OM-BiPO4 NCs (a), BiPO4 NCs (b), OM-GO-BiPO4 (c), GO-BiPO4 nanocomposites (d–e), and HRTEM image of the GO-BiPO4 nanocomposites (f).
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Fig. 3. XRD patterns of the as-prepared graphite oxide, OM-BiPO4 NCs and GO-BiPO4 nanocomposite.
sheets [42]. Fig. 2f shows the high-resolution transmission electron microscopy (HRTEM) image of GO-BiPO4 nanocomposite. The lattice spacing of 0.437 nm corresponds to the (101) crystallographic planes of BiPO4 . Fig. 3 shows XRD patterns of the as-synthesized graphite oxide, OM-BiPO4 and GO-BiPO4 nanocomposites. The diffraction pattern of graphite oxide shows a peak centered at 2 = 10.2◦ , corresponding to the (0 0 1) interlayer spacing of graphite oxide [43]. All of the diffraction peaks in the pattern of OM-BiPO4 are in accordance with the standard data of BiPO4 (JCPDS 15-0766). The XRD pattern of GO-BiPO4 nanocomposites shows the diffraction peaks of BiPO4 , and does not have any obvious difference in comparison with that of OM-BiPO4 NCs. This phenomenon suggests that the regular stacking of GO sheets was inhibited by the spacer of BiPO4 NCs due to the formation of well-combined GO-BiPO4 nanocomposites [43,44]. Fig. 4 shows the FT-IR spectra of graphite oxide, OM-BiPO4 NCs and GO-BiPO4 nanocomposites. The spectrum of graphite oxide shows the characteristic absorption peaks of C O and C O C at ca. 1729 and 1041 cm−1 , respectively, due to the existence of
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Fig. 5. Raman spectra of graphite oxide and GO- BiPO4 nanocomposites.
oxygen-containing functional groups on the surface [45]. In the FT-IR spectrum of OM-BiPO4 NCs, the two peaks at 2920 and 2845 cm−1 correspond to the asymmetric and symmetric C H stretching vibrations of CH2 groups in the alkyl chain of OM, while the peak at 2961 cm−1 is associated with CH3 stretching vibration. In the FT-IR spectrum of GO-BiPO4 , the lack of the OM characteristic peaks indicates that the OM molecules were removed after treated with glacial acetic acid at room temperature. The peak of graphite oxide at 1726 cm−1 is also observed, confirming the existence of GO in the nanocomposites. The good combination of GO nanosheets with BiPO4 NCs can also be confirmed by Raman spectroscopy. As shown in Fig. 5, Raman spectrum of graphite oxide displays two prominent peaks at ca. 1357 and 1605 cm−1 , corresponding to the well-documented D and G bands, respectively [46]; while in the Raman spectrum of GOBiPO4 nanocomposite, besides the two peaks from GO, the peaks at ca. 203, 402, 445, 543, 585, 971 and 1059 cm−1 can be observed, which can be ascribed to BiPO4 NCs, and is consistent with the previous report [47], further confirming the formation of GO-BiPO4 nanocomposite. The UV–vis diffuse reflectance spectra of BiPO4 NCs and GOBiPO4 nanocomposites are shown in Fig. 6. It can be clearly seen that the absorption edge of the BiPO4 NCs occurs at about 330 nm, which is in agreement with the result reported previously [48]. However, the absorbance of GO-BiPO4 nanocomposites is largely enhanced at the range of 400–800 nm in comparison with that of BiPO4 NCs due to the existence of GO, suggesting that GO-BiPO4 photocatalysts may absorb solar spectrum more efficiently, and would probably have good visible-light photocatalytic activity. 3.2. Photocatalytic properties
Fig. 4. FT-IR spectra of the as-prepared graphite oxide, OM-BiPO4 NCs and GO-BiPO4 nanocomposites.
The photocatalytic performance of bare BiPO4 NCs and GOBiPO4 nanocomposites was evaluated by the degradation of the organic dye MB, which is a typical pollutant in the textile industry. The photocatalytic degradation process was investigated by examining the characteristic absorption peak of MB at 664 nm. Fig. 7a shows UV–vis absorption spectra of the aqueous MB solution in the presence of H2 O2 and GO-BiPO4 nanocomposites with 7.0 wt% GO (denoted as GO(7%)-BiPO4 ). It can be seen that the strength of the absorption peaks at 664 nm decreases with the increasing irradiation time, and the MB can be completely decomposed within 140 min. However, for the bare BiPO4 photocatalyst (Fig. 7b), the
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Fig. 6. UV–vis diffuse reflectance spectra of BiPO4 NCs and GO-BiPO4 nanocomposites.
Fig. 8. (a) Effect of different catalysts on photocatalytic degradation of MB: H2 O2 ; BiPO4 ; GO(7%)-BiPO4 ; BiPO4 + H2 O2 ; GO(7%)-BiPO4 + H2 O2 ; GO(7%)BiPO4 + H2 O2 + tert-butanol; under simulated sunlight irradiation, and GO(7%)BiPO4 + H2 O2 without irradiation. (b) Photocatalytic activities of GO-BiPO4 nanocomposites with different GO contents for MB degradation in the presence of H2 O2 .
Fig. 7. Absorption spectra of the MB solution taken at different degradation times using (a) GO(7%)-BiPO4 and (b) BiPO4 as a photocatalyst in the presence of H2 O2 , respectively.
degradation rate of MB was much slower than that for GO-BiPO4 photocatalyst. The photocatalytic degradation efficiency of MB was only 38.9% after irradiation for 240 min. Therefore, it was concluded that the GO-BiPO4 nanocomposites have much higher photocatalytic activity than bare BiPO4 NCs. As shown in Fig. 8a, the photodegradation rates of MB using different catalysts under simulated sunlight irradiation decrease in the following order: GO-BiPO4 + H2 O2 > GO + H2 O2 > BiPO4 + H2 O2 = GO-BiPO4 + H2 O2 + tert-butanol > GOBiPO4 > BiPO4 > GO-BiPO4 + H2 O2 (without irradiation) > H2 O2 . It can be seen that in the presence of H2 O2 alone, the absorbance of MB is almost unchanged after irradiation for 240 min. It means that H2 O2 was unable to decompose MB under simulated sunlight irradiation. For BiPO4 or GO-BiPO4 alone, both the degradation efficiencies were also very low under simulated sunlight irradiation though GO-BiPO4 shows a little higher photocatalytic activity than BiPO4 . When H2 O2 was introduced into the catalytic systems, the photodegradation efficiency was improved remarkably for GOBiPO4 but slightly for BiPO4 under simulated sunlight irradiation. However, when without irradiation, the degradation efficiencies were no more than 10% for GO-BiPO4 in the presence of H2 O2 . This result clearly demonstrates that the introduction of GO into BiPO4
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photocatalysts can greatly enhance the photocatalytic activity of BiPO4 under simulated sunlight irradiation, especially in the presence of H2 O2 . The above results show that the GO sheets and H2 O2 play key roles for the enhanced photocatalytic activity of the composite photocatalysts. The possible mechanism of the photocatalytic degradation can be proposed as follows: On the one hand, GO contains a mixture of sp2 and sp3 hybridized carbon atoms with large amount of oxygen bonding on the sp3 hybridized carbon. Because oxygen atoms have a larger electronegativity than carbon atoms, GO becomes a p-type semiconductor material where its conduction band is the antibonding * orbital and its valence band mainly the O 2p orbital [49]. When GO nanosheets are combined with ntype semiconductor BiPO4 [50], a p/n heterojunction can be formed. This structure can achieve a more efficient charge separation and an increased lifetime of the charge carriers [51]. Also, the GO can act as a sensitizer and enhance the visible-light absorption of the nanocomposites [52]. Moreover, the high-surface-area GO sheets offer more active adsorption sites and photocatalytic reaction sites, which favor improved photocatalytic activity. On the other hand, it is worthwhile noting that the photocatalytic activity of GO-BiPO4 nanocomposites was significantly enhanced when H2 O2 was added to the reaction system. H2 O2 as an efficient electron scavenger could trap the photogenerated electrons and generate hydroxyl radicals (• OH), which can directly destroy the ring structure of MB molecules, and then convert them into CO2 by either direct electron transfer or insertion, hence offering enhanced photocatalytic performance. To justify the presence of • OH radicals in the reaction system, the chemical of tert-butanol (a ·OH radical scavenger) was employed. As shown in Fig. 8a, after the addition of 45 mM tert-butanol to the GO-BiPO4 photocatalytic system, the photocatalytic activity of the GO-BiPO4 nanocomposites was largely decreased, implies that • OH radicals contribute to photocatalytic performance. Furthermore, the content of GO in the GO-BiPO4 nanocomposites is also found to be an important factor to affect photocatalytic activity. With the increase of GO content, the photocatalytic activity of the GO-BiPO4 nanocomposites increased at first, and then decreased (Fig. 8b). The GO-BiPO4 nanocomposites with 7 wt% GO exhibit the best photocatalytic activity toward MB degradation. This result indicates that a suitable loading amount of GO is crucial for optimizing the photocatalytic activity of GO-BiPO4 nanocomposites. It is possible that excess GO in the GO-BiPO4 nanocomposites would block the light absorption of BiPO4 and weaken the charge transfer, and thus decrease the photocatalytic activity.
4. Conclusions In summary, we have developed a facile two-phase route for self-assembling BiPO4 on GO nanosheets. Compared with bare BiPO4 , the GO-BiPO4 nanocomposites exhibit much higher photocatalytic activity toward MB degradation under simulated sunlight irradiation. The existence of GO can greatly enhance the visiblelight absorption of the catalysts, facilitate charge transfer and suppress the recombination of electron–hole pairs. This study demonstrate that graphene-based BiPO4 nanocomposite is a very promising candidate for development of high performance photocatalysts for environmental and energy application.
Acknowledgments The authors are grateful for financial support from the National Natural Science Foundation of China (No. 51272094, 51072071 and
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51102117) and Specialized Research Fund for the Doctoral Program of Higher Education of China (No. 20123227110018).
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Synthesis of graphene oxide-BiPO4 composites with enhanced photocatalytic properties Hongwei Lv a , Xiaoping Shen a,∗ , Zhenyuan Ji b , Dezhou Qiu a , Guoxing Zhu a , Yongliang Bi a a b
School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, PR China School of Materials Science and Engineering, Jiangsu University, Zhenjiang 212013, PR China
a r t i c l e
i n f o
Article history: Received 27 May 2013 Received in revised form 18 July 2013 Accepted 18 July 2013 Available online 26 July 2013 Keywords: Graphene oxide Bismuth phosphate Nanocomposites Synthesis Photocatalysis
a b s t r a c t Graphene has attracted considerable attention for developing highly efficient photocatalysts due to its unique two dimensional structure and extraordinary physicochemical properties. In this paper, we demonstrate a facile two-phase self-assembly approach for the synthesis of graphene oxide (GO)-BiPO4 nanocomposites. The as-prepared samples were characterized by X-ray diffraction, transmission electron microscopy, Raman spectra, UV–vis diffuse reflectance spectra and Fourier transform infrared spectra. The significantly enhanced photocatalytic activity of the GO-BiPO4 nanocomposites in comparison with bare BiPO4 nanoparticles was revealed by the degradation of methylene blue under simulated sunlight irradiation, which can be attributed to the improved separation of electron–hole pairs. This facile method could be extended to design other graphene-based photocatalysts for environment and energy applications. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Photocatalysis, which provides a new promising way to meet the challenges of the environment, energy and sustainability with abundant solar light, has attracted considerable attention during the past decades [1–3]. Among the nanocrystal photocatalytic materials, bismuth-based photocatalysts have recently received much attention due to their superior photocatalytic activities in decomposing organic pollutants [4–6], oxidation of water [7,8], and reduction of NO [9–11]. Especially, bismuth phosphate (BiPO4 ), an oxoacid salt photocatalyst, has exhibited potential applications in catalysis [12], ion sensing [13], and separating radioactive elements [14]. However, due to the large band gap (3.85 eV) of BiPO4 [15], its excitation requires ultraviolet light below 322 nm. It is well known that 45% of the total energy in solar spectrum belongs to visible light, and the energy of UV light represents only 5% in that [16]. Therefore, the development of efficient visible-light responsive photocatalysts is highly expected and has become an active research area in photocatalysis research [17–24]. Another issue for BiPO4 photocatalyst is that the photoproduced electrons and holes in BiPO4 may experience a rapid recombination, which diminishes the efficiency of the photocatalytic reaction significantly [25]. Therefore, it is still a big challenge to explore an effective way
∗ Corresponding author. Tel.: +86 511 88791800; fax: +86 511 88791800. E-mail address: [email protected] (X. Shen). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.07.098
to improve the photocatalytic activity of bismuth phosphate for practical application. Graphene, a monolayer or few layers (<10) of hexagonally arrayed sp2 -bonded carbon atoms, has attracted intense scientific interest mainly due to its unique structure and excellent physicochemical properties [26–29]. Recently, the synthesis and application of graphene-based nanocomposites have been intensively pursued [30]. Among them, novel photocatalysts originating from the combination of graphene or graphene oxide (GO) nanosheets with photocatalytically active components such as TiO2 [31,32], ZnO [33], CoFe2 O4 [34], BiVO4 [35], and Ag/AgX (X = Br, Cl) [36] have been reported in degradation of organic pollutants for water purification. It was found that the hybridization of the above photocatalysts with graphene or GO can significantly improve the photocatalytic and photoconversion efficiency of the photocatalytic materials. For example, graphene/ZnO nanocomposites exhibit twice higher photocatalytic efficiency than bare ZnO in the degradation of RhB dye under UV irradiation [33]. However, to the best of our knowledge, the synthesis and photocatalytic performance of GO-BiPO4 nanocomposites have not been reported until now. In this study, GO-BiPO4 nanocomposites are prepared through a facile two-phase self-assembly approach. Compared with the synthesis of other graphene-based hybrids, our synthesis route has two features: (1) The synthesis of GO-BiPO4 nanocomposites is performed at room temperature. (2) The self-assembly method makes it easy to control the distribution and loading amount of BiPO4 on GO sheets. Furthermore, the synthesized GO-BiPO4 photocatalysts exhibit enhanced photocatalytic activities toward the degradation
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of methylene blue (MB) in comparison with bare BiPO4 nanocrystals (NCs) under simulated sunlight irradiation. 2. Experimental 2.1. Materials Natural flake graphite was purchased from Qingdao Guyu graphite Co. Ltd., with a particle size of 150 m (99.9% purity). All of the other chemical reagents are of analytical grade and used without further purification. 2.2. Preparation of graphite oxide Graphite oxide was synthesized through chemical exfoliation of graphite powder via a modified Hummers’ method [37]. Typically, 2.0 g of graphite powder was added into cold (0 ◦ C) concentrated H2 SO4 (80 mL) and NaNO3 (4.0 g) solution in a 500 mL flask. Under vigorous stirring, KMnO4 (10.0 g) was added gradually and the temperature of the mixture was kept to below 15 ◦ C. The reaction mixture was stirred at 35 ◦ C for 4–5 h until it became pasty brownish, and then diluted with de-ionized water (100 mL). The addition of water was performed in an ice bath to keep the temperature below 100 ◦ C. Then, the mixture was stirred for 30 min and 15 mL of 30 wt% H2 O2 was slowly added to the mixture to reduce the residual KMnO4 , after which the color of the mixture changed to reddish-brown. The mixture was filtered and washed with 5% HCl aqueous solution (800 mL) to remove metal ions followed by 1.0 L of de-ionized water to remove the acid. The resulted solid was centrifuged and dried at 45 ◦ C for 24 h. For further purification, the as-obtained graphite oxide was re-dispersed in de-ionized water and then was dialyzed for one week to remove residual salts and acids.
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2.5. Characterization The morphology and structure of the products were determined by transmission electron microscopy (TEM, JEOL JEM-2100) and Xray diffraction (XRD, Bruker D8 ADVANCE) with Cu K␣ radiation. Samples for TEM observation were prepared by dropping the products on a carbon-coated copper grid after ultrasonic dispersion in absolute ethanol and allowed them to dry in air before analysis. Fourier transform infrared (FT-IR) spectra were recorded on a Nicolet Nexus 470 spectrometer with KBr pellets in the 4000–400 cm−1 region. Raman spectra were measured at room temperature using a DXR Raman microscope with 514.5 nm excitation source from an Ar+ laser. Ultraviolet–visible (UV–vis) spectroscopy measurements were performed on a UV-2450 UV-vis spectrophotometer. 2.6. Photocatalytic measurements The photocatalytic activities of the as-prepared samples were evaluated by the photocatalytic decomposition of MB. Photoirradiation was conducted on a GHX-2 photochemical reactor with a 500 W tungsten lamp as simulated sunlight source. Experiments were conducted at room temperature as follows: 50 mg of photocatalyst was added to 100 mL of 20 mg/L dye aqueous solution. The mixed suspension was stirred for 1 h in the dark to reach the adsorption–desorption equilibrium between the catalyst and the dye. The dispersion was irradiated by the tungsten lamp after adding 1.0 mL of 30% H2 O2 solution. During the reaction process, 3 mL of the suspension was withdrawn at a regular time interval of 20 min and was centrifuged to remove the remnant photocatalyst. The supernatant was then recorded in the UV-vis spectrophotometer at ambient temperature for an optical absorbance measurement. The initial dye concentration (C0 ) and the concentrations (C) of the remnant dye were determined by measuring the absorbance of the solutions at 664 nm with UV–vis spectroscopy.
2.3. Preparation of OM-BiPO4 NCs Oleylamine-capped BiPO4 (OM-BiPO4 ) NCs were synthesized through a simple and reproducible route based on the reaction between bismuth ions, phosphoric acid and OM in toluene at room temperature [38]. Typically, bismuth nitrate (0.05 mol) and OM (0.1 mol) were dissolved in 150 mL of toluene, into which a H3 PO4 solution containing 0.05 mol of H3 PO4 and 50 mL of ethanol was gradually added. The resulting mixture was stirred for 30 min, and excess ethanol was added to precipitate OM-BiPO4 NCs. The obtained OM-BiPO4 product was dried at 45 ◦ C in a vacuum oven. 2.4. Preparation of GO-BiPO4 nanocomposites The GO-BiPO4 nanocomposites were produced through a twophase approach. In a typical synthesis, 50 mg of graphite oxide was added to 50 mL of water and then sonicated for 30 min to form a uniform GO aqueous dispersion. OM-BiPO4 NCs (950 mg) were dispersed in toluene (50 mL) and then mixed with the GO aqueous dispersion. The mixture was kept stirring for 12 h at room temperature. The OM-GO-BiPO4 (containing OM) nanocomposites were obtained by centrifugation, and washed with water and ethanol, respectively. The OM-GO-BiPO4 NCs were further treated with acetic acid to remove OM molecules. Typically, 1.0 g of the as-synthesized OM-GO-BiPO4 nanocomposites was dispersed in 100 mL of glacial acetic acid (≥99.5%), and stirred continually for 24 h at room temperature. The GO-BiPO4 products were separated by centrifugation, washed thoroughly with water and ethanol to remove any impurities, and then dried in a vacuum oven at 45 ◦ C for 24 h.
3. Results and discussion 3.1. Synthesis and characterization of GO-BiPO4 nanocomposites The GO-BiPO4 nanocomposites were fabricated through a facile two-phase self-assembly approach, the schematic of which is illustrated in Fig. 1. The pre-synthesized toluene solution of OM-BiPO4 NCs was mixed with aqueous solution of GO and stirred for 12 h. The self-assembly of OM-BiPO4 NCs on the GO nanosheets occurs at the water/toluene interface [39]. Then the OM molecules were removed from the surface of OM-GO-BiPO4 nanocomposites by acetic acid so as to make the GO-BiPO4 nanocomposite readily dispersed in water for practical photocatalytic application [40,41]. TEM images of the as-prepared products are shown in Fig. 2. It can be seen from Fig. 2a that the as-prepared OM-BiPO4 NCs are well-dispersed, and show sphere-like morphology with an average size of about 30 nm. After the treatment of OM-BiPO4 NCs with acetic acid, the resulting BiPO4 NCs are transformed into short rod-like morphology, and some agglomeration occurs due to the shortage of OM protection (Fig. 2b). The morphology of OM-GO-BiPO4 nanocomposite is shown in Fig. 2c. It can be observed that OM-BiPO4 nanoparticles are well dispersed on the GO nanosheets. Neither free BiPO4 nanoparticles nor bare GO sheets were observed during the TEM observation, indicating a strong combination between the GO support and the BiPO4 nanoparticles. Figs. 2d-e show the TEM images of GO-BiPO4 nanocomposites, the morphology and size of the BiPO4 NCs in the GO-BiPO4 system are almost the same as those in OM-GO-BiPO4 , which could be explained in terms of the capping effects afforded by the GO
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Fig. 1. Scheme of the formation of GO-BiPO4 nanocomposites by the self-assembly method.
Fig. 2. TEM images of OM-BiPO4 NCs (a), BiPO4 NCs (b), OM-GO-BiPO4 (c), GO-BiPO4 nanocomposites (d–e), and HRTEM image of the GO-BiPO4 nanocomposites (f).
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Fig. 3. XRD patterns of the as-prepared graphite oxide, OM-BiPO4 NCs and GO-BiPO4 nanocomposite.
sheets [42]. Fig. 2f shows the high-resolution transmission electron microscopy (HRTEM) image of GO-BiPO4 nanocomposite. The lattice spacing of 0.437 nm corresponds to the (101) crystallographic planes of BiPO4 . Fig. 3 shows XRD patterns of the as-synthesized graphite oxide, OM-BiPO4 and GO-BiPO4 nanocomposites. The diffraction pattern of graphite oxide shows a peak centered at 2 = 10.2◦ , corresponding to the (0 0 1) interlayer spacing of graphite oxide [43]. All of the diffraction peaks in the pattern of OM-BiPO4 are in accordance with the standard data of BiPO4 (JCPDS 15-0766). The XRD pattern of GO-BiPO4 nanocomposites shows the diffraction peaks of BiPO4 , and does not have any obvious difference in comparison with that of OM-BiPO4 NCs. This phenomenon suggests that the regular stacking of GO sheets was inhibited by the spacer of BiPO4 NCs due to the formation of well-combined GO-BiPO4 nanocomposites [43,44]. Fig. 4 shows the FT-IR spectra of graphite oxide, OM-BiPO4 NCs and GO-BiPO4 nanocomposites. The spectrum of graphite oxide shows the characteristic absorption peaks of C O and C O C at ca. 1729 and 1041 cm−1 , respectively, due to the existence of
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Fig. 5. Raman spectra of graphite oxide and GO- BiPO4 nanocomposites.
oxygen-containing functional groups on the surface [45]. In the FT-IR spectrum of OM-BiPO4 NCs, the two peaks at 2920 and 2845 cm−1 correspond to the asymmetric and symmetric C H stretching vibrations of CH2 groups in the alkyl chain of OM, while the peak at 2961 cm−1 is associated with CH3 stretching vibration. In the FT-IR spectrum of GO-BiPO4 , the lack of the OM characteristic peaks indicates that the OM molecules were removed after treated with glacial acetic acid at room temperature. The peak of graphite oxide at 1726 cm−1 is also observed, confirming the existence of GO in the nanocomposites. The good combination of GO nanosheets with BiPO4 NCs can also be confirmed by Raman spectroscopy. As shown in Fig. 5, Raman spectrum of graphite oxide displays two prominent peaks at ca. 1357 and 1605 cm−1 , corresponding to the well-documented D and G bands, respectively [46]; while in the Raman spectrum of GOBiPO4 nanocomposite, besides the two peaks from GO, the peaks at ca. 203, 402, 445, 543, 585, 971 and 1059 cm−1 can be observed, which can be ascribed to BiPO4 NCs, and is consistent with the previous report [47], further confirming the formation of GO-BiPO4 nanocomposite. The UV–vis diffuse reflectance spectra of BiPO4 NCs and GOBiPO4 nanocomposites are shown in Fig. 6. It can be clearly seen that the absorption edge of the BiPO4 NCs occurs at about 330 nm, which is in agreement with the result reported previously [48]. However, the absorbance of GO-BiPO4 nanocomposites is largely enhanced at the range of 400–800 nm in comparison with that of BiPO4 NCs due to the existence of GO, suggesting that GO-BiPO4 photocatalysts may absorb solar spectrum more efficiently, and would probably have good visible-light photocatalytic activity. 3.2. Photocatalytic properties
Fig. 4. FT-IR spectra of the as-prepared graphite oxide, OM-BiPO4 NCs and GO-BiPO4 nanocomposites.
The photocatalytic performance of bare BiPO4 NCs and GOBiPO4 nanocomposites was evaluated by the degradation of the organic dye MB, which is a typical pollutant in the textile industry. The photocatalytic degradation process was investigated by examining the characteristic absorption peak of MB at 664 nm. Fig. 7a shows UV–vis absorption spectra of the aqueous MB solution in the presence of H2 O2 and GO-BiPO4 nanocomposites with 7.0 wt% GO (denoted as GO(7%)-BiPO4 ). It can be seen that the strength of the absorption peaks at 664 nm decreases with the increasing irradiation time, and the MB can be completely decomposed within 140 min. However, for the bare BiPO4 photocatalyst (Fig. 7b), the
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Fig. 6. UV–vis diffuse reflectance spectra of BiPO4 NCs and GO-BiPO4 nanocomposites.
Fig. 8. (a) Effect of different catalysts on photocatalytic degradation of MB: H2 O2 ; BiPO4 ; GO(7%)-BiPO4 ; BiPO4 + H2 O2 ; GO(7%)-BiPO4 + H2 O2 ; GO(7%)BiPO4 + H2 O2 + tert-butanol; under simulated sunlight irradiation, and GO(7%)BiPO4 + H2 O2 without irradiation. (b) Photocatalytic activities of GO-BiPO4 nanocomposites with different GO contents for MB degradation in the presence of H2 O2 .
Fig. 7. Absorption spectra of the MB solution taken at different degradation times using (a) GO(7%)-BiPO4 and (b) BiPO4 as a photocatalyst in the presence of H2 O2 , respectively.
degradation rate of MB was much slower than that for GO-BiPO4 photocatalyst. The photocatalytic degradation efficiency of MB was only 38.9% after irradiation for 240 min. Therefore, it was concluded that the GO-BiPO4 nanocomposites have much higher photocatalytic activity than bare BiPO4 NCs. As shown in Fig. 8a, the photodegradation rates of MB using different catalysts under simulated sunlight irradiation decrease in the following order: GO-BiPO4 + H2 O2 > GO + H2 O2 > BiPO4 + H2 O2 = GO-BiPO4 + H2 O2 + tert-butanol > GOBiPO4 > BiPO4 > GO-BiPO4 + H2 O2 (without irradiation) > H2 O2 . It can be seen that in the presence of H2 O2 alone, the absorbance of MB is almost unchanged after irradiation for 240 min. It means that H2 O2 was unable to decompose MB under simulated sunlight irradiation. For BiPO4 or GO-BiPO4 alone, both the degradation efficiencies were also very low under simulated sunlight irradiation though GO-BiPO4 shows a little higher photocatalytic activity than BiPO4 . When H2 O2 was introduced into the catalytic systems, the photodegradation efficiency was improved remarkably for GOBiPO4 but slightly for BiPO4 under simulated sunlight irradiation. However, when without irradiation, the degradation efficiencies were no more than 10% for GO-BiPO4 in the presence of H2 O2 . This result clearly demonstrates that the introduction of GO into BiPO4
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photocatalysts can greatly enhance the photocatalytic activity of BiPO4 under simulated sunlight irradiation, especially in the presence of H2 O2 . The above results show that the GO sheets and H2 O2 play key roles for the enhanced photocatalytic activity of the composite photocatalysts. The possible mechanism of the photocatalytic degradation can be proposed as follows: On the one hand, GO contains a mixture of sp2 and sp3 hybridized carbon atoms with large amount of oxygen bonding on the sp3 hybridized carbon. Because oxygen atoms have a larger electronegativity than carbon atoms, GO becomes a p-type semiconductor material where its conduction band is the antibonding * orbital and its valence band mainly the O 2p orbital [49]. When GO nanosheets are combined with ntype semiconductor BiPO4 [50], a p/n heterojunction can be formed. This structure can achieve a more efficient charge separation and an increased lifetime of the charge carriers [51]. Also, the GO can act as a sensitizer and enhance the visible-light absorption of the nanocomposites [52]. Moreover, the high-surface-area GO sheets offer more active adsorption sites and photocatalytic reaction sites, which favor improved photocatalytic activity. On the other hand, it is worthwhile noting that the photocatalytic activity of GO-BiPO4 nanocomposites was significantly enhanced when H2 O2 was added to the reaction system. H2 O2 as an efficient electron scavenger could trap the photogenerated electrons and generate hydroxyl radicals (• OH), which can directly destroy the ring structure of MB molecules, and then convert them into CO2 by either direct electron transfer or insertion, hence offering enhanced photocatalytic performance. To justify the presence of • OH radicals in the reaction system, the chemical of tert-butanol (a ·OH radical scavenger) was employed. As shown in Fig. 8a, after the addition of 45 mM tert-butanol to the GO-BiPO4 photocatalytic system, the photocatalytic activity of the GO-BiPO4 nanocomposites was largely decreased, implies that • OH radicals contribute to photocatalytic performance. Furthermore, the content of GO in the GO-BiPO4 nanocomposites is also found to be an important factor to affect photocatalytic activity. With the increase of GO content, the photocatalytic activity of the GO-BiPO4 nanocomposites increased at first, and then decreased (Fig. 8b). The GO-BiPO4 nanocomposites with 7 wt% GO exhibit the best photocatalytic activity toward MB degradation. This result indicates that a suitable loading amount of GO is crucial for optimizing the photocatalytic activity of GO-BiPO4 nanocomposites. It is possible that excess GO in the GO-BiPO4 nanocomposites would block the light absorption of BiPO4 and weaken the charge transfer, and thus decrease the photocatalytic activity.
4. Conclusions In summary, we have developed a facile two-phase route for self-assembling BiPO4 on GO nanosheets. Compared with bare BiPO4 , the GO-BiPO4 nanocomposites exhibit much higher photocatalytic activity toward MB degradation under simulated sunlight irradiation. The existence of GO can greatly enhance the visiblelight absorption of the catalysts, facilitate charge transfer and suppress the recombination of electron–hole pairs. This study demonstrate that graphene-based BiPO4 nanocomposite is a very promising candidate for development of high performance photocatalysts for environmental and energy application.
Acknowledgments The authors are grateful for financial support from the National Natural Science Foundation of China (No. 51272094, 51072071 and
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51102117) and Specialized Research Fund for the Doctoral Program of Higher Education of China (No. 20123227110018).
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