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BiOBr composites under visible light
Applied Surface Science 258 (2012) 7826–7832
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Synthesis and photocatalytic activity of graphene/BiOBr composites under visible light Xingmiao Zhang a , Xiaofeng Chang a , M.A. Gondal b , Bin Zhang a , Yousong Liu a , Guangbin Ji a,∗ a b
College of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 211100, China Laser Research Group, Physics Department, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia
a r t i c l e
i n f o
Article history: Received 7 November 2011 Received in revised form 5 April 2012 Accepted 8 April 2012 Available online 30 April 2012 Keywords: Graphene BiOBr Adsorption Photocatalysis Sulforhodamine 640
a b s t r a c t The study presented in this work deals with the synthesis of graphene/BiOBr composite following hydrothermal reaction between graphene oxide and BiOBr. The results achieved demonstrated that the presence of graphene on the surface of BiOBr significantly improved the photocatalytic activity, under visible light irradiation, owing to the low isoelectric characteristics of graphene and better interfacial electron transfer between BiOBr and graphene. The effect of different amounts of graphene such as 1, 3, 6 and 10 wt% on the photocatalytic and adsorption efficiency was investigated. Our results showed that there exists an optimum concentration of graphene (∼6 wt%) for the best photocatalytic response of BiOBr which could be due to crucial energy dissipation. The photocatalytic and adsorption efficiency of the composites were investigated by studying the removal of Sulforhodamine 640 dye as a probe reaction.
1. Introduction Recent scientific reports present that the photocatalysts prepared in conjunction with carbon materials such as that carbon nanotubes, fullerenes, graphene etc. have fascinating photocatalytic activity [1,2]. Among the applications of carbon materials, integrating graphene with other photocatalysts to fabricate composites has been the focus of many researchers [3,1]. Owing to its unique two-dimensional structure, high mechanical strength, high surface area, special electronic, optical and other wonderful properties, graphene shows potential applications in different fields such as energy storage, catalysis, transducer, photoelectric etc. It has been reported that graphene oxide modification could improve the performance of TiO2 in its photocatalytic activity [4]. After first experimental discovery of chemically modified graphene sheets produced by the solvothermal reduction of graphite oxide in 2008 [5], the hydrothermal reaction was applied to synthesize graphene/photocatalysts composites using the graphite oxide and photocatalysts [6] to enhance the photocatalytic activity. The creative work of Zhang et al. is a good example [7], who prepared a chemically bonded TiO2 (P25)-graphene composite photocatalyst by using a hydrothermal reduction method. In addition, Liang et al. also prepared the graphene/TiO2 nanocrystals by directly
∗ Corresponding author. Tel.: +86 25 52112902; fax: +86 25 52112900. E-mail address: [email protected] (G. Ji).
Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved.
growing TiO2 nanocrystals on GO sheets followed with a subsequent hydrothermal reduction in water and ethanol [8]. Brominated oxygen bismuth (BiOBr) is a semiconductor with highly anisotropic layered structure, and its crystal structure (PbFCl type, symmetry D4h, space group P4/nmm) belongs to the tetragonal structure of photocatalyst [9]. BiOBr has been applied in many applications recently like photocatalytic water splitting into hydrogen [10], decomposition of pollutants [11–13] etc. In the present study, an attempt has been made to synthesize graphene/BiOBr (Gra/BiOBr) composites using hydrothermal reaction between graphene oxide and BiOBr. The photocatalytic response of the composites was investigated by studying the removal of Sulforhodamine 640 (Srh640) dye as a probe reaction under visible light irradiation. The effect of different amounts of graphene such as 1, 3, 6 and 10 wt% on the photocatalytic and adsorption efficiency was investigated. The physical changes in the surface features of composites were also explored by interface adsorption mechanism. 2. Experimental Preparation of graphite oxide: graphite oxide (GO) was prepared according to the modified Hummers method [14,15]. For this purpose, 10 g of graphite was weighed and washed with 500 mL aqueous HCl (5 wt%) solution followed by filtering, drying, and mixing with 30 g KMnO4 . The resulting sample was added to 230 mL concentrated H2 SO4 . Then it was kept on an ice bath (T < 293 K) and stirred for 30 min. Then the temperature was increased to 35 ◦ C and
0169-4332/$ – see front matter. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.04.049
(C − C ) n 0 C0
× 100%
(1)
where C0 and Cn are the concentration of Srh640 before and after photocatalytic reaction. The crystal structures of the graphite oxide, 6 wt% Gra/BiOBr and graphite powder samples were determined by X-ray diffractometer (XRD, Bruker D8 ADVANCE) with 2 scope of 5–90◦ at 40 kV and 40 mA using Cu K␣ as the irradiation source. Microscopic characterizations were carried out using a field emission scanning electron microscopy (FE-SEM, JEOL S4800) and high-resolution transmission electron microscope (JEOL JSM-2010). The UV–vis diffuse reflectance spectra (UV–vis DRS) of samples were recorded on a UV–vis spectrometer (Shimadzu UV-2550) with an integrating sphere. BaSO4 was used as a reference sample. 3. Results and discussion The XRD patterns of the graphite, graphite oxide (GO), and Gra/BiOBr composites are presented Fig. 1. It could be seen that the diffraction peak at 26◦ disappeared and emergence of new peak at 10◦ , a characteristic peak of graphene oxide, indicating that the flake shaped graphite was oxidized to layered graphene oxide. The calculating results conformed that the spacing of graphite oxide layer (0.88 nm). The diffraction peaks of 6 wt% Gra/BiOBr such at 25◦ , 31.5◦ , and 32◦ could attribute to the characteristic peaks of BiOBr. Moreover, it could be noticed that the characteristic diffraction peak of GO at 10◦ disappeared in the synthesized Gra/BiOBr
212
104
004
112
212
104
004
112
110 003
102 002
BiOBr
001
101
001
6%Gra/BiOBr
110
002 101
001
Relative intensity/a.u
102
004
Graphite
GO
5
10
15
20
25
30
35
40
45
50
55
60
65
70
2Theta/deg Fig. 1. XRD diffraction patterns of Graphite, Graphite oxide (GO), Gra/BiOBr.
samples, which could be attributed to the low amount and relatively low diffraction intensity of the graphene [7,18]. The Raman spectra were measured to examine the changes in the structure of the graphite oxide as compared to the as-prepared Gra/BiOBr and graphene sheets of the Gra/BiOBr composition which may occurred in the photocatalytic process, as shown in Fig. 2. 2D bands of the graphene (oxide) sheets of Gra/BiOBr were observed at around 2694 cm−1 , indicating formation of multilayer (2–4 layers) graphene sheets after the hydrothermal reduction [19]. Intense peaks at 1348 and 1595 cm−1 corresponding to graphite oxide and graphene D band and G band are visible in the spectrum. The smaller ID /IG peak intensity ratio of in Raman spectrum can be assigned to lower defects and disorders of the graphitized structures containing the disorders caused at the edges of the carbon platelets. It was found that the ID /IG ratio increased from 0.85 for GO to 0.93 for Gra/BiOBr after hydrothermal process, which could be attributed to formation of the defects and disorders in the graphene sheets [20]. The phenomenon indicated that the layers
G
2500
GO ID/IG:0.85 0h ID/IG:0.93 6h ID/IG:0.89 10h ID/IG:0.89
D 2000
Intensity/a.u
the solution was again stirred for another 1 h. After this, the solution was slowly added to 460 mL distilled water and heated at 50 ◦ C for 1 h. The solution was added to 1.4 L distilled water and 100 mL H2 O2 (30 wt%) which was kept under static conditions for 24 h. Then the obtained precipitate was filtered, washed, and dried under vacuum. BiOBr was prepared according to the reported method in scientific literature [16]. According to the reported method in literatures [5,7], Gra/BiOBr composites were prepared by a simple hydrothermal method. Briefly, required mass ratio of graphite oxide and BiOBr were mixed with water and ethanol solution (volume ratio of 2:1) and treated ultrasonically for 60 min to dissolve. The resulting solution was then transferred to a Teflon autoclave and heated at 120 ◦ C for 4 h. After this treatment, the obtained precipitate was filtered and dried at less than 80 ◦ C for 10 h. To study the sorption kinetics, batch experiments were performed. In this process, 0.1 g of the BiOBr and 6 wt% Gra/BiOBr were added into 200 mL aqueous solution of Srh640 with desired concentration and the mixture was shaken for 10 h in an orbital shaker (150 rpm) at a constant temperature. The resulting mixture was centrifuged at 2500 rpm and the absorbance of Srh640 was recorded at 586 nm by means of UV–vis spectrophotometer. The experiments were performed on a thermostatic shaker at 150 rpm at 30 ◦ C for 24 h, using a batch equilibrium technique in which 0.01 g of photocatalysts was added into 10 mL of Srh640 solution. The change in concentration of Srh640 was monitored by UV–vis spectrophotometer at 586 nm wavelength. The photocatalytic activity of the different samples was evaluated using a glass beaker equipped with filter and a xenon lamp. For irradiation experiments, 200 mL of Srh640 solution was taken into the beaker and 0.05 g catalyst was added into the solution. Irradiation was carried out using a filter and 420 xenon lamp and 4 mL samples were drawn from the reaction vessel. The decomposition (decrease in absorption intensity vs. irradiation time) of the Srh640 was monitored by measuring the change in absorbance at 586 nm using a UV–vis spectrophotometer. The photocatalytic efficiency was determined by calculating the degradation rate. The degradation rate was calculated as given below [17]:
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1500
2D
6h 1000
10h
500
0 1000
1500
2000
2500
Wavenumber/cm-1 Fig. 2. Raman absorption spectra of Graphite oxide (GO) and Gra/BiOBr 0 h, 6 h, and 10 h visible light irradiation time.
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Fig. 3. SEM (a) and TEM (a) images of BiOBr, TEM (c) images of Gra/BiOBr composites, (b and d) HR-TEM image of the selected area. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
of graphite oxide were partially reduced to single and multi-layer graphene during hydrothermal process. Furthermore, as depicted in Fig. 2 showed that we also found that the ID /IG ratio of Gra/BiOBr decreased from 0.93 to 0.89 after 6 h irradiation under visible light, which be assigned to formation of further sp2 bonds after photocatalytic reduction of the graphene sheets [21]. However, we further found that the ID /IG ratio of Gra/BiOBr was not changed by further increasing the time of irradiation to 10 h, which indicated that Gra/BiOBr composite was stable after a long time irradiation under visible light. It was reported that typical semiconductors such as ZnO [20] and TiO2 [21] can photodegrade the graphene oxide sheets resulted in decrease of the carbon content of the reduced graphene oxides and increase of the carbon defects in the photocatalytic process under the longer irradiations. However, this phenomenon was not observed into our experiment, which could be explained that the oxidation ability should be dependent on the valence band edge of semiconductor. It has been well know that the oxidation ability of photo-generated holes is strongly dependent on the potential of valence band (VB) edge in the corresponding semiconductor. It can be initially deduced that the quite good stability should be attributed to the much more negative potential of VB edge in BiOBr which has been reported by theoretical calculation [22]. Gra/BiOBr composites were prepared which could be explained as follows. Initially GO can be delaminated in water to form colloidal dispersion due to the nature of interactions between the GO layers and the solvent using a suitable ultrasonic treatment [23]. Thermal decomposition of the labile oxygen-containing functional groups could contribute to solvo-thermal reactions with strong conversion. Moreover, the autogenous pressures developed inside the sealed autoclave also play an important role in the conversion of GO sheets to graphene sheets [5]. Microstructure and morphology of the samples were studied using field emission scanning electron microscope and high resolution transmission electron microscope. Typical images obtained
from these techniques are illustrated in Fig. 3. The SEM image (Fig. 3a of embedded in the left) indicates that the BiOBr was in the form of sheet, which might facilitate the insertion of BiOBr into the graphene layers to form intercalation compounds. A close observation of the samples by the TEM (Fig. 3a) demonstrated further details of the BiOBr lamellar structure. One can see from selected area electron diffraction pattern (SAED, inset in Fig. 3a) that the BiOBr samples were tetragonal and had good crystallinity. Fig. 3b depicts the high-resolution transmission electron microscopy (HRTEM) image of the red circle part in the Fig. 3a. It can be seen that intergranular spacing of BiOBr is about 0.25 nm. Gra/BiOBr composite structure also showed flaky distribution simmilar as the BiOBr, as shown in Fig. 3c. SAED pattern taken from the interface of the BiOBr with graphene composite are also illustrated. Part of the left red circle is a square crystalline structure of BiOBr and part of the right red circle is the amorphous graphene. A typical TEM image is depicted in Fig. 3d showing a clear interface between BiOBr and graphene, which could be attributed to the insertion of BiOBr into the graphene layers to form intercalation compounds. Thomas et al. had established that the affinity of surface hydroxyl groups of TiO2 surface to undergo charge transfer interaction with carboxylic acid functional groups [24]. It was reported that the direct interaction between surface hydroxyl groups of TiO2 and carboxylic acid functional groups of graphene provides the basis to obtain TiO2 graphene composites [1]. Weng et al. provided quantitative evidence that the dye adsorbed onto a TiO2 surface through a carboxylic group via ester-like linkage [25]. Moreover, the creative work of Zhang et al., who prepared a chemically bonded TiO2 (P25)-graphene composite photocatalyst using simile hydrothermal reduction method, indicated that graphene oxide with the residual carboxylic acid functional groups firmly interacted with the surface hydroxyl groups of P25 nanoparticles and finally formed the chemically bonded P25-GR composites [7]. Therefore, the phenomenon could be credited to the residual
X. Zhang et al. / Applied Surface Science 258 (2012) 7826–7832
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1.6 BiOBr
1.4
1% Gra/BiOBr 3% Gra/BiOBr
Absorbance/a.u.
1.2
6% Gra/BiOBr
1.0
10% Gra/BiOBr
0.8 0.6 0.4 0.2 0.0 200
300
400
500
600
700
800
Wavelength/nm Fig. 4. UV–visible absorption spectra of diffuse reflection of the obtained BiOBr and different proportions of Gra/BiOBr.
carboxylic acid functional groups of graphene oxide firmly interacted with the surface hydroxyl groups of BiOBr. Then, it maybe finally form new chemical bonds of Gra/BiOBr composites during the hydrothermal reduction. Moreover, It is important that carboxyl acid groups of GO were likely situated at the edge [26], which was conducive to the formation of intercalation compounds. The optical properties of prepared samples were measured using a UV–vis spectroscopy. Fig. 4 depicts the diffuse reflectance absorption spectra of pure BiOBr and Gra/BiOBr composites. It is obvious that the absorption edges of BiOBr and 1 wt% Gra/BiOBr were at 437 and 432 nm, respectively, which are near to the UV absorption region. Whereas, the absorption edges of 3, 6 and 10 wt% Gra/BiOBr were at 450, 445, and 442 nm, respectively, which are extending to the visible region. The band gaps, based on g = 1239.8/Eg [27], of the BiOBr, 1, 3, 6, and 10 wt% Gra/BiOBr samples were estimated to be 2.83, 2.86, 2.75, 2.78 and 2.80 eV, respectively. The color of powders was gradually changed from light yellow to dark grey. Semiconductor intrinsic absorption coefficient ␣ is functions of the wavelength of incident light and band transitions between solid types. The optical absorption near the band edge follows the formula [28]: ˛h = A(h − Eg)
n/2
Fig. 5. UV–visible diffuse reflectance spectroscopy of the obtained BiOBr and different proportions of Gra/BiOBr (Gra/BiOBr).
The effects of graphene on the adsorption kinetics and isotherm of Srh 640 on BiOBr were also investigated and the results are presented in Fig. 6. In this study, the pseudo-first-order adsorption kinetic model and the pseudo-second-order adsorption kinetic model were adopted to evaluate and calculate the solute uptake
(2)
where ˛, h, v, Eg and A are absorption coefficient, Planck constant, light frequency, band gap and a constant, respectively. Among them, n depends on the characteristics of the transition of the semiconductor. For BiOBr, the value of n is 4 for the indirect transition [29]. As shown in Fig. 5, the band gap energies (Eg values) of BiOBr, 1, 3, 6, 10 wt% Gra/BiOBr were estimated to be 2.72, 2.68, 2.40, 2.44 and 2.50 eV, respectively. The presence of different amounts of graphene obviously affected the optical property of BiOBr. The addition of graphene increased the light absorption in the UV–vis region, as observed in all of the Gra/BiOBr samples with different amounts of graphene. The corresponding band gap energy was also reduced extending to the visible region. This phenomenon could be attributed to the chemical bonding between BiOBr and the specific sites of carbon [17]. Furthermore, the presence of graphene leads to a continuous absorption band in the 400–800 nm region, which is in agreement with the black color of the samples. The stronger absorption of light by the Gra/BiOBr composite than bare BiOBr suggested that they could have higher photocatalytic activity for a given reaction. This hypothesis was further confirmed by the following photocatalytic experiments.
Fig. 6. (a) Pseudo-first-order kinetic plots and (b) pseudo-second-order kinetic plots for the adsorption of BiOBr and 6 wt% Gra/BiOBr onto Srh640 at 30 ◦ C.
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Table 1 Kinetics constants for the adsorption of Srh640 sorbed on BiOBr and 6 wt% Gra/BiOBr at 30 ◦ C. Pseudo-first-order kinetic model
Pseudo-second-order kinetic model
Sample
qe (mg/g)
k1
r2
qe (mg/g)
k2
r2
BiOBr Gra/BiOBr
6.32942 4.52314
16.67418 13.66961
0.9359 0.88641
6.32899 4.52241
0.00948 0.0162
0.9359 0.88641
Fig. 8. Photodegradation efficiencies of Srh640 as a function of irradiation time.
Fig. 7. Langmuir isotherms and Freundlich isotherms of BiOBr and 6 wt% Gra/BiOBr sorbed on Srh640.
rate and fitted values are listed in Table 1. The pseudo-first-order adsorption kinetic equation can be written as follows [30]: qt =
qe t k1 + t
does not have any significant impact on the adsorption behavior of Srh640. Fig. 8 depicts the adsorption (dark) and photocatalytic degradation of dye Srh 640 in presence of BiOBr and 6 wt% Gra/BiOBr composite under Xe arc lamp irradiation with a UV cutoff filter ( > 420 nm). A rapid decrease in the intensity of absorption peak of Srh640 at the wavelength of 586 nm was observed indicating that the 6 wt% Gra/BiOBr sample has excellent photocatalytic activity for the degradation of selected dye derivative. The adsorption of Srh640 dye with 8.93 ppm concentration onto the surface of BiOBr and 6 wt% Gra/BiOBr was found to be 3.60 and 3.69 mg/g, respectively. This suggested that the adsorption performance of composite was slightly improved. However, overall BiOBr, 6 wt% Gra/BiOBr adsorption properties were poor because of their macroporous nature. The photocatalytic response of 6 wt% Gra/BiOBr for the degradation of Srh 640 was around 2 fold to that of BiOBr, (38.2% for 6 wt% Gra/BiOBr composite and 21.6% for BiOBr) under visible light irradiation and identical experimental conditions. The absorption spectra changes of Srh 640 in presence of BiOBr and Gra/BiOBr composite as a function of irradiation time have been presented in Fig. 9(a) and (b), respectively. Fig. 9 further describes that the Gra/BiOBr composites have improved photocatalytic efficiency as compared to bare BiOBr. The continuous decrease in the absorption peak intensity at 586 nm, without formation of any other peak, in the presence of all photocatalysts indicated that the degradation of dye occurs without formation of any stable intermediate products.
(3)
The pseudo-second-order adsorption kinetic equation can be written as follows [31]: qt =
q2e k2 t 1 + qe k2 t
(4)
It was noticed that the presence of graphene in BiOBr did not change the adsorption kinetics constant and time of adsorption to adsorption desorption equilibrium. Fig. 7 shows the adsorption isotherms of 6 wt% Gra/BiOBr and BiOBr. The Langmuir and Freundlich adsorption models were used to express the sorption phenomenon of the sorbent for which fitting results are presented in Table 2. The Langmuir equation is given in the following equation [32]: qe =
qm kL Ce 1 + KL Ce
(5)
The Freundlich equation can be written [33]: 1/n
qe = KF Ce
(6)
The adsorption capacities of 6 wt% Gra/BiOBr and pure phase of BiOBr for in Srh640 solution were found to be 6.06181 and 7.07885 mg/g, respectively. This showed that a small amount of graphene present in BiOBr sample or the interface of Gra/BiOBr
Table 2 Isotherm constants for the adsorption of Srh640 sorbed on BiOBr and 6 wt% Gra/BiOBr at 30 ◦ C. Langmuir isotherm model
Freundlich isotherm model
Sample
qm (mg/g)
KL (L/g)
r2
1/n
KF
r2
BiOBr Gra/BiOBr
7.07885 6.06181
3.15267 1.04398
0.86227 0.89893
0.16771 0.26799
5.17663 3.29189
0.75904 0.79978
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Table 3 The photocatalysis kinetics constants of the Srh640 degradation.
Fig. 9. UV–visible absorption spectral changes observed for the BiOBr/Srh640 and 6 wt% Gra/BiOBr/Srh 640 mixtures as a function of irradiation time under Xe arc lamp ( > 420 nm): (a) BiOBr (b) 6 wt% Gra/BiOBr.
The pseudo-first order kinetic model, as expressed by Eq. (7) [34], was applied to study the photocatalytic kinetics of the Srh640 degradation; ln
C 0
C
= Kt
(7)
where C0 and C are the concentrations of organic dye at time 0 and t, respectively, and K is the pseudo-first-order rate constant. Fig. 10 shows the fit obtained by pseudo-first-order kinetics model for the
Sample
m
K
r2
BiOBr 6 wt% Gra/BiOBr
0.08729 0.09617
0.004 0.00798
0.9911 0.9977
photocatalytic degradation of Srh640 in aqueous suspensions of BiOBr and 6 wt% Gra/BiOBr composite. The values obtained after fitting are listed in Table 3. As can be seen from the table, a good correlation to the pseudo-first-order reaction kinetics (R2 > 0.99) was found, which Indicated that the current photocatalytic process is controlled by pseudo-first order kinetics. The rate constants for the removal of dye under investigation in presence of pure BiOBr and 6 wt% Gra/BiOBr composites were found to be KBiOBr = 0.004 min−1 and K6wt% Gra/BiOBr = 0.00798 min−1 respectively. The enhanced photocatalytic response of Gra/BiOBr composites could be explained in terms of the conductance and valence band (CB and VB) edge potentials. The CB and VB edge electrochemical potentials of BiOBr semiconductor are important factors to understand the photo-degradation mechanism of contaminants over BiOBr. For pure BiOBr, the valence band was mainly consisting of O 2p and Br 4p occupied while conduction band is mainly constituted by the Bi 6p track. When electrons of Br 4p are photoexcited to the track of Bi 6p under irradiation, it produces a photo-hole-electron pairs. Owing to the two-dimensional structure of graphene, it can accept the photoexcited electrons from BiOBr and thus inhibits the electron–hole recombination and improves the efficiency of photocatalytic degradation [7]. Furthermore, the effect of graphene amount in composite was studied and found that the resulting activity of composite significantly depends on the amount of graphene. Fig. 11 shows the effect of graphene amount on the photocatalytic degradation of dye. It can be seen that the photocatalytic activity of composite catalyst increases with the increase in graphene amount and the highest (34.8% higher than other composites) activity was achieved with 6 wt% graphene. However, a further increase in graphene amount (10 wt%) led to decrease in photocatalytic efficiency of the composite. The control experiments, as depicted in Fig. 11 showed that when the amount of graphene in composite is higher, the degradation activity of the composite remained close to the pure BiOBr. This may be attributed to the fact that when the amount of graphene
Fig. 11. Degradation performance towards Srh640 after 1 h photocatalytic process over different proportions of Gra/BiOBr. Fig. 10. The photocatalysis kinetics of the Srh640 degradation.
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Acknowledgments Financial supports from the National Natural Science Foundation of China (no: 51172109) and Natural Science Foundation of Jiangsu Province of China (no: BK2010497) are gratefully acknowledged. References [1] [2] [3] [4] [5] [6]
Fig. 12. Degradation performance towards Srh640 after 40 min photocatalytic process over Gra/BiOBr composite, Gra/BiOBr mixture and BiOBr.
[7] [8] [9] [10]
in composite is in excess, the graphene which cover the surface of the system blocked the absorption of the visible light of the photocatalyst. Consequently, increasing the energy dissipation of the whole photocatalytic system, the absorption of visible light is greatly enhanced which is also reported by Zhang et al. [17] is consistent with the experimental trend. In order to see the significance of the composite structure in enhancement of photocatalytic activity, comparative experiments of Gra/BiOBr composite and Gra/BiOBr mixture were conducted. For this purpose, the hydrothermal reaction of BiOBr and GO were performed respectively. Then the obtained two precipitates were mixed (mixture of 6 wt% + 94 wt% BiOBr) and photocatalytic irradiation experiments were performed. The comparative experiments, as depicted in Fig. 12 demonstrated that the highest activity was obtained with 6 wt% Gra/BiOBr composite. However, the photocatalytic activity of 6 wt% Gra/BiOBr mixture was much lower than that of Gra/BiOBr composite, further demonstrating that the Gra/BiOBr composites could improve photocatalytic efficiency as compared to simple mixture. The combination of graphene on BiOBr could effectively facilitate the photo-generated electrons transportation from semiconductor to the graphene sheet thus improve the carries lifetime generated from BiOBr semiconductor. 4. Conclusions In summary, the synthesis of novel composite consisting graphene and BiOBr was demonstrated using a hydrothermal method. The results indicated that the photocatalytic response of BiOBr could be significantly improved under visible light irradiation. The photocatalytic activity of the composite strongly depends on the amount of Graphene and the composite prepared with 6 wt% graphene showed the highest photocatalytic activity.
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Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Synthesis and photocatalytic activity of graphene/BiOBr composites under visible light Xingmiao Zhang a , Xiaofeng Chang a , M.A. Gondal b , Bin Zhang a , Yousong Liu a , Guangbin Ji a,∗ a b
College of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 211100, China Laser Research Group, Physics Department, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia
a r t i c l e
i n f o
Article history: Received 7 November 2011 Received in revised form 5 April 2012 Accepted 8 April 2012 Available online 30 April 2012 Keywords: Graphene BiOBr Adsorption Photocatalysis Sulforhodamine 640
a b s t r a c t The study presented in this work deals with the synthesis of graphene/BiOBr composite following hydrothermal reaction between graphene oxide and BiOBr. The results achieved demonstrated that the presence of graphene on the surface of BiOBr significantly improved the photocatalytic activity, under visible light irradiation, owing to the low isoelectric characteristics of graphene and better interfacial electron transfer between BiOBr and graphene. The effect of different amounts of graphene such as 1, 3, 6 and 10 wt% on the photocatalytic and adsorption efficiency was investigated. Our results showed that there exists an optimum concentration of graphene (∼6 wt%) for the best photocatalytic response of BiOBr which could be due to crucial energy dissipation. The photocatalytic and adsorption efficiency of the composites were investigated by studying the removal of Sulforhodamine 640 dye as a probe reaction.
1. Introduction Recent scientific reports present that the photocatalysts prepared in conjunction with carbon materials such as that carbon nanotubes, fullerenes, graphene etc. have fascinating photocatalytic activity [1,2]. Among the applications of carbon materials, integrating graphene with other photocatalysts to fabricate composites has been the focus of many researchers [3,1]. Owing to its unique two-dimensional structure, high mechanical strength, high surface area, special electronic, optical and other wonderful properties, graphene shows potential applications in different fields such as energy storage, catalysis, transducer, photoelectric etc. It has been reported that graphene oxide modification could improve the performance of TiO2 in its photocatalytic activity [4]. After first experimental discovery of chemically modified graphene sheets produced by the solvothermal reduction of graphite oxide in 2008 [5], the hydrothermal reaction was applied to synthesize graphene/photocatalysts composites using the graphite oxide and photocatalysts [6] to enhance the photocatalytic activity. The creative work of Zhang et al. is a good example [7], who prepared a chemically bonded TiO2 (P25)-graphene composite photocatalyst by using a hydrothermal reduction method. In addition, Liang et al. also prepared the graphene/TiO2 nanocrystals by directly
∗ Corresponding author. Tel.: +86 25 52112902; fax: +86 25 52112900. E-mail address: [email protected] (G. Ji).
Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved.
growing TiO2 nanocrystals on GO sheets followed with a subsequent hydrothermal reduction in water and ethanol [8]. Brominated oxygen bismuth (BiOBr) is a semiconductor with highly anisotropic layered structure, and its crystal structure (PbFCl type, symmetry D4h, space group P4/nmm) belongs to the tetragonal structure of photocatalyst [9]. BiOBr has been applied in many applications recently like photocatalytic water splitting into hydrogen [10], decomposition of pollutants [11–13] etc. In the present study, an attempt has been made to synthesize graphene/BiOBr (Gra/BiOBr) composites using hydrothermal reaction between graphene oxide and BiOBr. The photocatalytic response of the composites was investigated by studying the removal of Sulforhodamine 640 (Srh640) dye as a probe reaction under visible light irradiation. The effect of different amounts of graphene such as 1, 3, 6 and 10 wt% on the photocatalytic and adsorption efficiency was investigated. The physical changes in the surface features of composites were also explored by interface adsorption mechanism. 2. Experimental Preparation of graphite oxide: graphite oxide (GO) was prepared according to the modified Hummers method [14,15]. For this purpose, 10 g of graphite was weighed and washed with 500 mL aqueous HCl (5 wt%) solution followed by filtering, drying, and mixing with 30 g KMnO4 . The resulting sample was added to 230 mL concentrated H2 SO4 . Then it was kept on an ice bath (T < 293 K) and stirred for 30 min. Then the temperature was increased to 35 ◦ C and
0169-4332/$ – see front matter. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.04.049
(C − C ) n 0 C0
× 100%
(1)
where C0 and Cn are the concentration of Srh640 before and after photocatalytic reaction. The crystal structures of the graphite oxide, 6 wt% Gra/BiOBr and graphite powder samples were determined by X-ray diffractometer (XRD, Bruker D8 ADVANCE) with 2 scope of 5–90◦ at 40 kV and 40 mA using Cu K␣ as the irradiation source. Microscopic characterizations were carried out using a field emission scanning electron microscopy (FE-SEM, JEOL S4800) and high-resolution transmission electron microscope (JEOL JSM-2010). The UV–vis diffuse reflectance spectra (UV–vis DRS) of samples were recorded on a UV–vis spectrometer (Shimadzu UV-2550) with an integrating sphere. BaSO4 was used as a reference sample. 3. Results and discussion The XRD patterns of the graphite, graphite oxide (GO), and Gra/BiOBr composites are presented Fig. 1. It could be seen that the diffraction peak at 26◦ disappeared and emergence of new peak at 10◦ , a characteristic peak of graphene oxide, indicating that the flake shaped graphite was oxidized to layered graphene oxide. The calculating results conformed that the spacing of graphite oxide layer (0.88 nm). The diffraction peaks of 6 wt% Gra/BiOBr such at 25◦ , 31.5◦ , and 32◦ could attribute to the characteristic peaks of BiOBr. Moreover, it could be noticed that the characteristic diffraction peak of GO at 10◦ disappeared in the synthesized Gra/BiOBr
212
104
004
112
212
104
004
112
110 003
102 002
BiOBr
001
101
001
6%Gra/BiOBr
110
002 101
001
Relative intensity/a.u
102
004
Graphite
GO
5
10
15
20
25
30
35
40
45
50
55
60
65
70
2Theta/deg Fig. 1. XRD diffraction patterns of Graphite, Graphite oxide (GO), Gra/BiOBr.
samples, which could be attributed to the low amount and relatively low diffraction intensity of the graphene [7,18]. The Raman spectra were measured to examine the changes in the structure of the graphite oxide as compared to the as-prepared Gra/BiOBr and graphene sheets of the Gra/BiOBr composition which may occurred in the photocatalytic process, as shown in Fig. 2. 2D bands of the graphene (oxide) sheets of Gra/BiOBr were observed at around 2694 cm−1 , indicating formation of multilayer (2–4 layers) graphene sheets after the hydrothermal reduction [19]. Intense peaks at 1348 and 1595 cm−1 corresponding to graphite oxide and graphene D band and G band are visible in the spectrum. The smaller ID /IG peak intensity ratio of in Raman spectrum can be assigned to lower defects and disorders of the graphitized structures containing the disorders caused at the edges of the carbon platelets. It was found that the ID /IG ratio increased from 0.85 for GO to 0.93 for Gra/BiOBr after hydrothermal process, which could be attributed to formation of the defects and disorders in the graphene sheets [20]. The phenomenon indicated that the layers
G
2500
GO ID/IG:0.85 0h ID/IG:0.93 6h ID/IG:0.89 10h ID/IG:0.89
D 2000
Intensity/a.u
the solution was again stirred for another 1 h. After this, the solution was slowly added to 460 mL distilled water and heated at 50 ◦ C for 1 h. The solution was added to 1.4 L distilled water and 100 mL H2 O2 (30 wt%) which was kept under static conditions for 24 h. Then the obtained precipitate was filtered, washed, and dried under vacuum. BiOBr was prepared according to the reported method in scientific literature [16]. According to the reported method in literatures [5,7], Gra/BiOBr composites were prepared by a simple hydrothermal method. Briefly, required mass ratio of graphite oxide and BiOBr were mixed with water and ethanol solution (volume ratio of 2:1) and treated ultrasonically for 60 min to dissolve. The resulting solution was then transferred to a Teflon autoclave and heated at 120 ◦ C for 4 h. After this treatment, the obtained precipitate was filtered and dried at less than 80 ◦ C for 10 h. To study the sorption kinetics, batch experiments were performed. In this process, 0.1 g of the BiOBr and 6 wt% Gra/BiOBr were added into 200 mL aqueous solution of Srh640 with desired concentration and the mixture was shaken for 10 h in an orbital shaker (150 rpm) at a constant temperature. The resulting mixture was centrifuged at 2500 rpm and the absorbance of Srh640 was recorded at 586 nm by means of UV–vis spectrophotometer. The experiments were performed on a thermostatic shaker at 150 rpm at 30 ◦ C for 24 h, using a batch equilibrium technique in which 0.01 g of photocatalysts was added into 10 mL of Srh640 solution. The change in concentration of Srh640 was monitored by UV–vis spectrophotometer at 586 nm wavelength. The photocatalytic activity of the different samples was evaluated using a glass beaker equipped with filter and a xenon lamp. For irradiation experiments, 200 mL of Srh640 solution was taken into the beaker and 0.05 g catalyst was added into the solution. Irradiation was carried out using a filter and 420 xenon lamp and 4 mL samples were drawn from the reaction vessel. The decomposition (decrease in absorption intensity vs. irradiation time) of the Srh640 was monitored by measuring the change in absorbance at 586 nm using a UV–vis spectrophotometer. The photocatalytic efficiency was determined by calculating the degradation rate. The degradation rate was calculated as given below [17]:
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1500
2D
6h 1000
10h
500
0 1000
1500
2000
2500
Wavenumber/cm-1 Fig. 2. Raman absorption spectra of Graphite oxide (GO) and Gra/BiOBr 0 h, 6 h, and 10 h visible light irradiation time.
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Fig. 3. SEM (a) and TEM (a) images of BiOBr, TEM (c) images of Gra/BiOBr composites, (b and d) HR-TEM image of the selected area. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
of graphite oxide were partially reduced to single and multi-layer graphene during hydrothermal process. Furthermore, as depicted in Fig. 2 showed that we also found that the ID /IG ratio of Gra/BiOBr decreased from 0.93 to 0.89 after 6 h irradiation under visible light, which be assigned to formation of further sp2 bonds after photocatalytic reduction of the graphene sheets [21]. However, we further found that the ID /IG ratio of Gra/BiOBr was not changed by further increasing the time of irradiation to 10 h, which indicated that Gra/BiOBr composite was stable after a long time irradiation under visible light. It was reported that typical semiconductors such as ZnO [20] and TiO2 [21] can photodegrade the graphene oxide sheets resulted in decrease of the carbon content of the reduced graphene oxides and increase of the carbon defects in the photocatalytic process under the longer irradiations. However, this phenomenon was not observed into our experiment, which could be explained that the oxidation ability should be dependent on the valence band edge of semiconductor. It has been well know that the oxidation ability of photo-generated holes is strongly dependent on the potential of valence band (VB) edge in the corresponding semiconductor. It can be initially deduced that the quite good stability should be attributed to the much more negative potential of VB edge in BiOBr which has been reported by theoretical calculation [22]. Gra/BiOBr composites were prepared which could be explained as follows. Initially GO can be delaminated in water to form colloidal dispersion due to the nature of interactions between the GO layers and the solvent using a suitable ultrasonic treatment [23]. Thermal decomposition of the labile oxygen-containing functional groups could contribute to solvo-thermal reactions with strong conversion. Moreover, the autogenous pressures developed inside the sealed autoclave also play an important role in the conversion of GO sheets to graphene sheets [5]. Microstructure and morphology of the samples were studied using field emission scanning electron microscope and high resolution transmission electron microscope. Typical images obtained
from these techniques are illustrated in Fig. 3. The SEM image (Fig. 3a of embedded in the left) indicates that the BiOBr was in the form of sheet, which might facilitate the insertion of BiOBr into the graphene layers to form intercalation compounds. A close observation of the samples by the TEM (Fig. 3a) demonstrated further details of the BiOBr lamellar structure. One can see from selected area electron diffraction pattern (SAED, inset in Fig. 3a) that the BiOBr samples were tetragonal and had good crystallinity. Fig. 3b depicts the high-resolution transmission electron microscopy (HRTEM) image of the red circle part in the Fig. 3a. It can be seen that intergranular spacing of BiOBr is about 0.25 nm. Gra/BiOBr composite structure also showed flaky distribution simmilar as the BiOBr, as shown in Fig. 3c. SAED pattern taken from the interface of the BiOBr with graphene composite are also illustrated. Part of the left red circle is a square crystalline structure of BiOBr and part of the right red circle is the amorphous graphene. A typical TEM image is depicted in Fig. 3d showing a clear interface between BiOBr and graphene, which could be attributed to the insertion of BiOBr into the graphene layers to form intercalation compounds. Thomas et al. had established that the affinity of surface hydroxyl groups of TiO2 surface to undergo charge transfer interaction with carboxylic acid functional groups [24]. It was reported that the direct interaction between surface hydroxyl groups of TiO2 and carboxylic acid functional groups of graphene provides the basis to obtain TiO2 graphene composites [1]. Weng et al. provided quantitative evidence that the dye adsorbed onto a TiO2 surface through a carboxylic group via ester-like linkage [25]. Moreover, the creative work of Zhang et al., who prepared a chemically bonded TiO2 (P25)-graphene composite photocatalyst using simile hydrothermal reduction method, indicated that graphene oxide with the residual carboxylic acid functional groups firmly interacted with the surface hydroxyl groups of P25 nanoparticles and finally formed the chemically bonded P25-GR composites [7]. Therefore, the phenomenon could be credited to the residual
X. Zhang et al. / Applied Surface Science 258 (2012) 7826–7832
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1.6 BiOBr
1.4
1% Gra/BiOBr 3% Gra/BiOBr
Absorbance/a.u.
1.2
6% Gra/BiOBr
1.0
10% Gra/BiOBr
0.8 0.6 0.4 0.2 0.0 200
300
400
500
600
700
800
Wavelength/nm Fig. 4. UV–visible absorption spectra of diffuse reflection of the obtained BiOBr and different proportions of Gra/BiOBr.
carboxylic acid functional groups of graphene oxide firmly interacted with the surface hydroxyl groups of BiOBr. Then, it maybe finally form new chemical bonds of Gra/BiOBr composites during the hydrothermal reduction. Moreover, It is important that carboxyl acid groups of GO were likely situated at the edge [26], which was conducive to the formation of intercalation compounds. The optical properties of prepared samples were measured using a UV–vis spectroscopy. Fig. 4 depicts the diffuse reflectance absorption spectra of pure BiOBr and Gra/BiOBr composites. It is obvious that the absorption edges of BiOBr and 1 wt% Gra/BiOBr were at 437 and 432 nm, respectively, which are near to the UV absorption region. Whereas, the absorption edges of 3, 6 and 10 wt% Gra/BiOBr were at 450, 445, and 442 nm, respectively, which are extending to the visible region. The band gaps, based on g = 1239.8/Eg [27], of the BiOBr, 1, 3, 6, and 10 wt% Gra/BiOBr samples were estimated to be 2.83, 2.86, 2.75, 2.78 and 2.80 eV, respectively. The color of powders was gradually changed from light yellow to dark grey. Semiconductor intrinsic absorption coefficient ␣ is functions of the wavelength of incident light and band transitions between solid types. The optical absorption near the band edge follows the formula [28]: ˛h = A(h − Eg)
n/2
Fig. 5. UV–visible diffuse reflectance spectroscopy of the obtained BiOBr and different proportions of Gra/BiOBr (Gra/BiOBr).
The effects of graphene on the adsorption kinetics and isotherm of Srh 640 on BiOBr were also investigated and the results are presented in Fig. 6. In this study, the pseudo-first-order adsorption kinetic model and the pseudo-second-order adsorption kinetic model were adopted to evaluate and calculate the solute uptake
(2)
where ˛, h, v, Eg and A are absorption coefficient, Planck constant, light frequency, band gap and a constant, respectively. Among them, n depends on the characteristics of the transition of the semiconductor. For BiOBr, the value of n is 4 for the indirect transition [29]. As shown in Fig. 5, the band gap energies (Eg values) of BiOBr, 1, 3, 6, 10 wt% Gra/BiOBr were estimated to be 2.72, 2.68, 2.40, 2.44 and 2.50 eV, respectively. The presence of different amounts of graphene obviously affected the optical property of BiOBr. The addition of graphene increased the light absorption in the UV–vis region, as observed in all of the Gra/BiOBr samples with different amounts of graphene. The corresponding band gap energy was also reduced extending to the visible region. This phenomenon could be attributed to the chemical bonding between BiOBr and the specific sites of carbon [17]. Furthermore, the presence of graphene leads to a continuous absorption band in the 400–800 nm region, which is in agreement with the black color of the samples. The stronger absorption of light by the Gra/BiOBr composite than bare BiOBr suggested that they could have higher photocatalytic activity for a given reaction. This hypothesis was further confirmed by the following photocatalytic experiments.
Fig. 6. (a) Pseudo-first-order kinetic plots and (b) pseudo-second-order kinetic plots for the adsorption of BiOBr and 6 wt% Gra/BiOBr onto Srh640 at 30 ◦ C.
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Table 1 Kinetics constants for the adsorption of Srh640 sorbed on BiOBr and 6 wt% Gra/BiOBr at 30 ◦ C. Pseudo-first-order kinetic model
Pseudo-second-order kinetic model
Sample
qe (mg/g)
k1
r2
qe (mg/g)
k2
r2
BiOBr Gra/BiOBr
6.32942 4.52314
16.67418 13.66961
0.9359 0.88641
6.32899 4.52241
0.00948 0.0162
0.9359 0.88641
Fig. 8. Photodegradation efficiencies of Srh640 as a function of irradiation time.
Fig. 7. Langmuir isotherms and Freundlich isotherms of BiOBr and 6 wt% Gra/BiOBr sorbed on Srh640.
rate and fitted values are listed in Table 1. The pseudo-first-order adsorption kinetic equation can be written as follows [30]: qt =
qe t k1 + t
does not have any significant impact on the adsorption behavior of Srh640. Fig. 8 depicts the adsorption (dark) and photocatalytic degradation of dye Srh 640 in presence of BiOBr and 6 wt% Gra/BiOBr composite under Xe arc lamp irradiation with a UV cutoff filter ( > 420 nm). A rapid decrease in the intensity of absorption peak of Srh640 at the wavelength of 586 nm was observed indicating that the 6 wt% Gra/BiOBr sample has excellent photocatalytic activity for the degradation of selected dye derivative. The adsorption of Srh640 dye with 8.93 ppm concentration onto the surface of BiOBr and 6 wt% Gra/BiOBr was found to be 3.60 and 3.69 mg/g, respectively. This suggested that the adsorption performance of composite was slightly improved. However, overall BiOBr, 6 wt% Gra/BiOBr adsorption properties were poor because of their macroporous nature. The photocatalytic response of 6 wt% Gra/BiOBr for the degradation of Srh 640 was around 2 fold to that of BiOBr, (38.2% for 6 wt% Gra/BiOBr composite and 21.6% for BiOBr) under visible light irradiation and identical experimental conditions. The absorption spectra changes of Srh 640 in presence of BiOBr and Gra/BiOBr composite as a function of irradiation time have been presented in Fig. 9(a) and (b), respectively. Fig. 9 further describes that the Gra/BiOBr composites have improved photocatalytic efficiency as compared to bare BiOBr. The continuous decrease in the absorption peak intensity at 586 nm, without formation of any other peak, in the presence of all photocatalysts indicated that the degradation of dye occurs without formation of any stable intermediate products.
(3)
The pseudo-second-order adsorption kinetic equation can be written as follows [31]: qt =
q2e k2 t 1 + qe k2 t
(4)
It was noticed that the presence of graphene in BiOBr did not change the adsorption kinetics constant and time of adsorption to adsorption desorption equilibrium. Fig. 7 shows the adsorption isotherms of 6 wt% Gra/BiOBr and BiOBr. The Langmuir and Freundlich adsorption models were used to express the sorption phenomenon of the sorbent for which fitting results are presented in Table 2. The Langmuir equation is given in the following equation [32]: qe =
qm kL Ce 1 + KL Ce
(5)
The Freundlich equation can be written [33]: 1/n
qe = KF Ce
(6)
The adsorption capacities of 6 wt% Gra/BiOBr and pure phase of BiOBr for in Srh640 solution were found to be 6.06181 and 7.07885 mg/g, respectively. This showed that a small amount of graphene present in BiOBr sample or the interface of Gra/BiOBr
Table 2 Isotherm constants for the adsorption of Srh640 sorbed on BiOBr and 6 wt% Gra/BiOBr at 30 ◦ C. Langmuir isotherm model
Freundlich isotherm model
Sample
qm (mg/g)
KL (L/g)
r2
1/n
KF
r2
BiOBr Gra/BiOBr
7.07885 6.06181
3.15267 1.04398
0.86227 0.89893
0.16771 0.26799
5.17663 3.29189
0.75904 0.79978
X. Zhang et al. / Applied Surface Science 258 (2012) 7826–7832
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Table 3 The photocatalysis kinetics constants of the Srh640 degradation.
Fig. 9. UV–visible absorption spectral changes observed for the BiOBr/Srh640 and 6 wt% Gra/BiOBr/Srh 640 mixtures as a function of irradiation time under Xe arc lamp ( > 420 nm): (a) BiOBr (b) 6 wt% Gra/BiOBr.
The pseudo-first order kinetic model, as expressed by Eq. (7) [34], was applied to study the photocatalytic kinetics of the Srh640 degradation; ln
C 0
C
= Kt
(7)
where C0 and C are the concentrations of organic dye at time 0 and t, respectively, and K is the pseudo-first-order rate constant. Fig. 10 shows the fit obtained by pseudo-first-order kinetics model for the
Sample
m
K
r2
BiOBr 6 wt% Gra/BiOBr
0.08729 0.09617
0.004 0.00798
0.9911 0.9977
photocatalytic degradation of Srh640 in aqueous suspensions of BiOBr and 6 wt% Gra/BiOBr composite. The values obtained after fitting are listed in Table 3. As can be seen from the table, a good correlation to the pseudo-first-order reaction kinetics (R2 > 0.99) was found, which Indicated that the current photocatalytic process is controlled by pseudo-first order kinetics. The rate constants for the removal of dye under investigation in presence of pure BiOBr and 6 wt% Gra/BiOBr composites were found to be KBiOBr = 0.004 min−1 and K6wt% Gra/BiOBr = 0.00798 min−1 respectively. The enhanced photocatalytic response of Gra/BiOBr composites could be explained in terms of the conductance and valence band (CB and VB) edge potentials. The CB and VB edge electrochemical potentials of BiOBr semiconductor are important factors to understand the photo-degradation mechanism of contaminants over BiOBr. For pure BiOBr, the valence band was mainly consisting of O 2p and Br 4p occupied while conduction band is mainly constituted by the Bi 6p track. When electrons of Br 4p are photoexcited to the track of Bi 6p under irradiation, it produces a photo-hole-electron pairs. Owing to the two-dimensional structure of graphene, it can accept the photoexcited electrons from BiOBr and thus inhibits the electron–hole recombination and improves the efficiency of photocatalytic degradation [7]. Furthermore, the effect of graphene amount in composite was studied and found that the resulting activity of composite significantly depends on the amount of graphene. Fig. 11 shows the effect of graphene amount on the photocatalytic degradation of dye. It can be seen that the photocatalytic activity of composite catalyst increases with the increase in graphene amount and the highest (34.8% higher than other composites) activity was achieved with 6 wt% graphene. However, a further increase in graphene amount (10 wt%) led to decrease in photocatalytic efficiency of the composite. The control experiments, as depicted in Fig. 11 showed that when the amount of graphene in composite is higher, the degradation activity of the composite remained close to the pure BiOBr. This may be attributed to the fact that when the amount of graphene
Fig. 11. Degradation performance towards Srh640 after 1 h photocatalytic process over different proportions of Gra/BiOBr. Fig. 10. The photocatalysis kinetics of the Srh640 degradation.
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X. Zhang et al. / Applied Surface Science 258 (2012) 7826–7832
Acknowledgments Financial supports from the National Natural Science Foundation of China (no: 51172109) and Natural Science Foundation of Jiangsu Province of China (no: BK2010497) are gratefully acknowledged. References [1] [2] [3] [4] [5] [6]
Fig. 12. Degradation performance towards Srh640 after 40 min photocatalytic process over Gra/BiOBr composite, Gra/BiOBr mixture and BiOBr.
[7] [8] [9] [10]
in composite is in excess, the graphene which cover the surface of the system blocked the absorption of the visible light of the photocatalyst. Consequently, increasing the energy dissipation of the whole photocatalytic system, the absorption of visible light is greatly enhanced which is also reported by Zhang et al. [17] is consistent with the experimental trend. In order to see the significance of the composite structure in enhancement of photocatalytic activity, comparative experiments of Gra/BiOBr composite and Gra/BiOBr mixture were conducted. For this purpose, the hydrothermal reaction of BiOBr and GO were performed respectively. Then the obtained two precipitates were mixed (mixture of 6 wt% + 94 wt% BiOBr) and photocatalytic irradiation experiments were performed. The comparative experiments, as depicted in Fig. 12 demonstrated that the highest activity was obtained with 6 wt% Gra/BiOBr composite. However, the photocatalytic activity of 6 wt% Gra/BiOBr mixture was much lower than that of Gra/BiOBr composite, further demonstrating that the Gra/BiOBr composites could improve photocatalytic efficiency as compared to simple mixture. The combination of graphene on BiOBr could effectively facilitate the photo-generated electrons transportation from semiconductor to the graphene sheet thus improve the carries lifetime generated from BiOBr semiconductor. 4. Conclusions In summary, the synthesis of novel composite consisting graphene and BiOBr was demonstrated using a hydrothermal method. The results indicated that the photocatalytic response of BiOBr could be significantly improved under visible light irradiation. The photocatalytic activity of the composite strongly depends on the amount of Graphene and the composite prepared with 6 wt% graphene showed the highest photocatalytic activity.
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