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RGO with high visible light photocatalytic activity
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CERAMICS INTERNATIONAL
Ceramics International 42 (2016) 4406–4412 www.elsevier.com/locate/ceramint
A novel photocatalyst AgBr/ZnO/RGO with high visible light photocatalytic activity Huihu Wanga,b,n, Daluo Penga, Tao Chena, Ying Changb,c, Shijie Donga,b b
a School of Mechanical Engineering, Hubei University of Technology, Wuhan, PR China Hubei Provincial Key Laboratory of Green Materials for Light Industry, Hubei University of Technology, Wuhan, PR China c School of Materials Science and Engineering, Hubei University of Technology, Wuhan, PR China
Received 4 September 2015; received in revised form 14 November 2015; accepted 22 November 2015 Available online 27 November 2015
Abstract A novel ternary photocatalyst AgBr/ZnO/RGO, where AgBr/ZnO is supported on reduced graphene oxide, is synthesized via a facile hydrothermal–impregnation method. The resultant composite presents a lamellar structure with AgBr nanoparticles homogeneously dispersing on the surface. The photocatalytic experiment for methyl orange dye degradation under visible light irradiation shows that ternary composite AgBr/ ZnO/RGO has an activity 12.8 times and 2.3 times higher than binary photocatalysts ZnO/RGO and AgBr/ZnO respectively. More importantly, the ternary composite also demonstrates a good photostability. Metallic Ag is produced during the photocatalytic process, which may serve as the electron transfer mediator in the vectorial Z-scheme transfer of photogenerated charge carriers at the interface of AgBr/ZnO/RGO. The effective separation of photogenerated electrons and holes was proposed to be responsible for the enhancement of visible light photoactivity. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: D. ZnO; Reduced graphene oxide; AgBr; Photocatalytic activity
1. Introduction With the depletion of fossil fuel reserves and a severe environmental crisis by burning of fossil fuel, worldwide efforts have been devoted to making use of sunlight for energy production, environmental protection and water purification. Semiconductor photocatalysis represents a potentially costeffective pathway for converting solar energy to chemical energy and environment remediation. Therefore, this method has received more and more attention in the past few decades. ZnO is one of the most commonly employed photocatalysts for its unique electronic structure. However, its wide band gap (3.37 eV) makes it can only absorb the ultraviolet part in solar spectrum. As sunlight consists of about 5–7% UV light, 46% visible light and 47% infrared irradiation [1], it is of n Corresponding author at: School of Mechanical Engineering, Hubei University of Technology, Wuhan 430068, Hubei, PR China. Tel./fax: þ86 027 59750418. E-mail address: [email protected] (H. Wang).
http://dx.doi.org/10.1016/j.ceramint.2015.11.124 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
significance to develop visible light active ZnO photocatalyst in order to efficiently utilize solar energy. Recently, AgBr as a visible light sensitive material has been used to improve the visible light activity of ZnO [2–5]. Fu et al. [2] prepared Ag–AgBr/ZnO photocatalyst through in situ oxidation of Ag/ZnO by bromine water. The Ag–AgBr/ZnO composite showed much higher performance for RhB degradation than ZnO and Ag/ZnO under simulated sun light illumination. Habibi-Yangjeh et al. [3] synthesized AgBr/ ZnO composites in water by refluxing method. The composites exhibited the higher photocatalytic activity than ZnO and AgBr for methylene blue degradation under visible light irradiation. Sun et al. [4] prepared Ag/AgBr/ZnO composites by deposition–precipitation method. The higher visible light photocatalytic activity of composites has been ascribed to its enhanced visible light absorption, narrowed band gap and the effective separation of photoinduced electron–hole pairs. Our group also prepared AgBr nanoparticles surface modified ZnO nanorod arrays by impregnation method. The high surface area of ZnO nanorod arrays and the visible light sensitivity of AgBr
H. Wang et al. / Ceramics International 42 (2016) 4406–4412
promoted ZnO nanorod arrays exhibiting the high visible light photocatalytic activity [5]. Although AgBr modification can effectively improve the utilization of solar energy for ZnO, the visible light activity of AgBr/ZnO composites may be restricted to some extent for the micron-meter size of AgBr particles and the fast recombination of electron–hole pairs. Therefore, it is essential to enhance the visible light activity of AgBr/ZnO composites by accelerating the separation of electron–hole pairs. Graphene sheet modified with oxygen functional groups, including GO and reduced GO (RGO), is an excellent electrontransport carbon material, which has been widely used to enhance the photodegradation efficiency of wide band gap semiconductor, such as TiO2 [6,7] and ZnO [8–15], by retarding the recombination of electron–hole pairs. Even the visible light photocatalytic activity of ZnO was obtained through the hybridization of graphene sheet [9,11]. Except for wide band gap semiconductor, it is interesting that GO and RGO were also employed to modify AgBr in order to enhance the separation of electron–hole pairs and the photostability of AgBr [16–18]. The high visible light activity and stability of AgBr/GO and AgBr/RGO were obtained. Therefore, it would be of great interest to investigate the synergistic effect of AgBr and graphene sheet modification on the visible light photocatalytic activity of ZnO. It is expected that the ternary composite AgBr/ZnO/RGO may have a highly efficient visible light activity due to the synergistic effect coming from the visible light sensitive of AgBr and the fast electron transfer ability of RGO. 2. Experimental 2.1. Preparation of ZnO/RGO composite For preparation of ternary composite AgBr/ZnO/RGO, graphene oxide (GO) was synthesized at first through Hummer's method [19]. Then, ZnO/RGO composite was prepared through hydrothermal method. Briefly, 0.58 g of Zn (CH3COO)2, 0.53 g of NaOH, and a certain amount of GO powders were mixed in 80 mL of ethanol solution and stirred for 30 min. Then, the mixtures were transferred to a Teflon sealed stainless steel autoclave, and solvothermally treated at 160 1C for 24 h in order to simultaneously achieve ZnO nanorods and reduced graphene oxide (RGO). Afterwards, the autoclave was cooled down to room temperature. The precipitates in autoclave was filtered, washed and dried. The amount of graphene oxide in ZnO/RGO composite was controlled to be 3 wt%. Pure RGO was also prepared using the similar method above without introducing Zn(CH3COO)2, while pure ZnO sample was obtained without adding GO. 2.2. Preparation of AgBr/ZnO/RGO composite Based on preparation of ZnO/RGO binary composite, ternary photocatalyst AgBr/ZnO/RGO was synthesized via a facile impregnation method [5]. First, ZnO/RGO composite was added into 0.04 mol/L KBr solution and dispersed by
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ultrasonic agitation for 10 min. The pH value of solution was adjusted to 6. Secondly, a certain amount of AgNO3 was added to the solution. The obtained mixtures were put in water bath and kept at 55 1C for 2 h. The molar ratio of Ag:Br was adjusted to 1:1. The amount of AgBr in ternary composite was controlled to be 2 wt%. For comparison, AgBr/ZnO binary composite was also prepared using pure ZnO as substrates. 2.3. Characterization X-ray diffraction patterns (XRD) of samples were carried out on a XD-2 type diffractometer with Cu Kα1 radiation. Surface morphology analysis was performed using a Quanta FEG 450 scanning electron microscope (FESEM). Further morphological and structural characterization was conducted by a FEI Tecnai G20 transmission microscope (TEM) using a 200 kV accelerating voltage. The surface chemical species of samples were analyzed on a VG Multilab 2000 X-ray photoelectron spectrometer (XPS). Diffuse reflectance spectra of different samples were measured by a JASCO V-560 UV–vis spectrophotometer. 2.4. Photocatalytic test The photocatalytic activity of different samples was examined by methyl orange (MO) degradation. The light source used was a 300 W Xe lamp with a 400 nm filter. The catalyst loading amount and MO concentration was set as 0.5 g/L and 10 mg/L, respectively. The MO concentration was analyzed on a UV2102 PC spectrophotometer. For photocatalytic stability test, the photodegradation experiments were repeated five times using the same testing conditions above. After each photoreaction, the samples were washed with deionized water for several times and dried at 60 1C. 3. Results and discussion Fig. 1 shows the XRD patterns of pure RGO, ZnO/RGO, AgBr/ZnO, and AgBr/ZnO/RGO samples. A typical diffraction peak is observed at 24.11 corresponding to graphite (002) in pure RGO sample, indicating hydrothermal method can effectively reduce GO to RGO. Therefore, when GO powders were mixed with Zn(CH3COO)2 and NaOH, ZnO/RGO composite can be obtained after hydrothermal treatment. However, no RGO diffraction peak is found in ZnO/RGO composite, which may be ascribed to the limited amount of RGO in composite. Similar phenomenon was also reported in the previous literature even the mass ratio of RGO increases to 29.6% in ZnO/RGO composite [20]. All the diffraction peaks in ZnO/RGO composite can be indexed to ZnO wurtzite phase. In AgBr/ZnO and AgBr/ZnO/RGO samples, the diffraction peaks of AgBr are also observed. XRD results demonstrate that ZnO/RGO, AgBr/ZnO, and AgBr/ZnO/RGO composites have been successfully synthesized in this experiment. Fig. 2 shows the FESEM images of ZnO/RGO (Fig. 2a and b), AgBr/ZnO (Fig. 2c and d), and AgBr/ZnO/RGO (Fig. 2e and f) samples. It can be found that ZnO in all samples presents the
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nanorod shape. Compared with AgBr/ZnO sample, lamellar aggregates appear in RGO/ZnO and AgBr/RGO/ZnO samples, which may be attributed to the presence of graphene sheet in composites. For the polar oxygen-containing functional groups on the surface of RGO sheet [21], zinc salt is inclined to hydrolyzing on its surface through hydrogen bonds or adsorp-
Fig. 1. XRD patterns of pure RGO, ZnO/RGO, AgBr/ZnO and AgBr/ZnO/ RGO samples.
tion during hydrothermal treatment. Therefore, ZnO nanorods are formed on the surface of RGO sheet. For AgBr/ZnO/RGO ternary composite, there are many tiny nanoparticles homogeneously dispersing on the surface of ZnO/RGO composite (Fig. 2f), which are suggested to be AgBr phase. However, this phenomenon is not observed in AgBr/ZnO sample, as shown in Fig. 2d. It is suggested that the surface abundant functional groups of RGO sheet, such as hydroxyl, carbonyl, carboxyl, and so on, can provide a lot of active sites for the heterogeneous nucleation of AgBr nanoparticles. On the other hand, the interaction between AgBr and surface functional groups can prevent the growth rate of AgBr nanoparticles. For AgBr/ZnO composite, there are no active sites for heterogeneous nucleation of AgBr. Therefore, the formation of AgBr phase may come from the homogeneous nucleation and growth in solution. The interaction role between ZnO and AgBr becomes very weak, which may affect the charge carriers transfer during photocatalytic process. TEM images of AgBr/ZnO/RGO sample are shown in Fig. 3. The presence of RGO in ternary composite is provided by Fig. 3a–c. It is clearly observed that ternary composite consist of one-dimensional ZnO nanorods, AgBr nanoparticles and RGO sheets. The high resolution TEM image of ZnO nanorods in Fig. 3d shows that it grows along the [0001] direction.
Fig. 2. FESEM images of ZnO/RGO (a and b), AgBr/ZnO (c and d) and AgBr/ZnO/RGO (e and f) samples.
H. Wang et al. / Ceramics International 42 (2016) 4406–4412
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Fig. 3. TEM images of AgBr/ZnO/RGO sample.
Diffuse reflectance spectra of pure RGO, ZnO and the other composites are displayed in Fig. 4. For pure RGO, a maximum absorption peak at 260 nm corresponding to the excitation of π-plasmon of graphite structure is observed. For pure ZnO and the other composites, there is no significant change on the absorption in UV region. However, the composites show the enhanced visible light absorption which is accordance with the color change from white to gray. Moreover, the absorption edge of AgBr/ZnO and AgBr/ZnO/RGO composites exhibit a little red shift to the higher wavelength relative to ZnO/RGO photocatalyst. The photocatalytic performance of different photocatalysts for MO degradation is shown in Fig. 5. It is clear that pure ZnO doesn't show any activity under visible light irradiation. Both RGO and AgBr addition can improve the visible light photocatalytic activity of ZnO. However, AgBr/ZnO composite exhibits much higher visible light photocatalytic activity (kapp. ¼ 0.01013 min 1) than ZnO/RGO composite (kapp. ¼ 0.00183 min 1), indicating AgBr addition plays a more important role on the improvement of visible light photocatalytic activity for ZnO than RGO introduction. When AgBr and RGO are simultaneously used to modify ZnO, the ternary composite AgBr/ZnO/RGO exhibits a highest photocatalytic activity (kapp. ¼ 0.02336 min 1), indicating a synergistic role of AgBr and RGO on the improvement of visible light activity of ZnO. Ternary composite AgBr/ZnO/RGO shows a visible light activity 12.8 times and 2.3 times higher than binary photocatalysts ZnO/RGO and AgBr/ZnO respectively.
Fig. 4. Diffuse reflectance spectra of pure RGO, ZnO and the composites.
Reusability is very important for the application of photocatalyst, especially the catalyst composites with photosensitive and unstable components AgBr. The photostability test of AgBr/ZnO/RGO composites for MO degradation was confirmed by five repeated experiments using a recycled ternary photocatalysts under the same conditions. As shown in Fig. 6, the ternary composites reveal a high visible light photostability for five repeated experiments, although a slight decrease of photocatalytic activity is observed compared to the first-run result.
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e þ Agi þ -Ag1 Agi þ : interstitial Ag þ AgBr-Ag þ Br
Fig. 5. Photocatalytic performance of different photocatalysts for MO degradation.
Fig. 6. The repeated experiments of AgBr/ZnO/RGO composite for MO degradation.
To further investigate the effect of recycle experiments on the structure of ternary composite AgBr/ZnO/RGO under visible light irradiation, the surface compositions of ternary composite were analyzed by XPS before photocatalytic test and after five repeated experiments, as shown in Fig. 7. Compared with the ternary composite AgBr/ZnO/RGO before photocatalytic test, the XPS peak of Ag3d in composite after five repeated experiments shows a slight higher binding energy. The peak position of Ag3d5/2 increases from 373.0 eV to 373.1 eV, while the peak position of Ag3d3/2 shifts from 367.0 eV to 367.1 eV (Fig. 7b). This slight increase on binding energy of Ag3d can be ascribed to the formation of metallic Ag due to its larger binding energy. Actually, this phenomenon can be expected because AgBr can absorb a light photon and simultaneously generate an electron and a positive hole under visible light irradiation, the photogenerated electrons can react with interstitial Ag þ ions to form metallic Ag1. This process is expressed as follows: AgBr þ hν-h þ þ e
ð1Þ
ð2Þ ð3Þ
Furthermore, it is also observed that the XPS peak of Br3d moves from 68.3 eV to 67.9 eV (Fig. 7c), which may be attributed to the photoreduction of AgBr under visible light irradiation. Fig. 7d shows the C 1s peak of ternary composites before photocatalytic test and after five repeated experiments. No obvious change is observed. The XPS peak of C 1s can be deconvoluted to three different peaks assigned to C–C (284.6 eV), C–O(284.6 eV), and C ¼ O(288.5 eV) bonds in RGO sheets, respectively [22,23]. Through the discussion above, the possible photocatalytic mechanism for the enhanced visible light photoactivity was proposed. During photocatalytic process, Ag nanoparticles were produced on the surface of AgBr particles for the photoreduction of AgBr [5], which has been verified from XPS results. Under visible light irradiation, AgBr can be photoexcited to generate electron–hole pairs. Simultaneously, the electron–hole pairs are also generated in metallic Ag nanoparticles owing to the surface plasmon resonance of Ag nanoparticles in visible light region. The photogenerated electrons in the CB of AgBr combine with the photogenerated holes in the highest occupied orbital of Ag. Meanwhile, the photogenerated electrons in the lowest unoccupied orbital of Ag can effectively inject to the CB of ZnO [2], and then to the RGO sheet. In this case, Ag nanoparticles can serve as the electron transfer mediator in the vectorial electron transfer of AgBr-Ag-ZnO-RGO, leading to an efficient charge separation and enhancing the photocatalytic activity. This process is consistent with the Z-scheme electron transfer path in Z-scheme structures, such as AgI–Ag–AgBr [24], AgBr–Ag– Bi2WO6 [25], BiOBr–Ag–AgBr [26], ZnO–Au-CdS [27], H2WO4.H2O–Ag–AgCl [28], and Bi20TiO32–Ag–AgCl [29]. In contrast to the heterojunction-type AgBr/ZnO photocatalyst, the ternary Z-scheme composite AgBr/ZnO/RGO may simultaneously possess the higher charge-separation efficiency and stronger redox ability. On the other hand, the homogeneous dispersion and small size of AgBr, as well as the good interface between AgBr and ZnO in ternary composite are favorable for the separation of electrons and holes, making it a perfect visiblelight-driven photocatalyst (Fig. 8). 4. Conclusions In this paper, a ternary photocatalyst AgBr/ZnO/RGO has been successfully synthesized. The composite presents a lamellar structure with AgBr nanoparticles homogeneously dispersing on the surface of composite. Owing to the addition of RGO and AgBr, the composite exhibits a higher absorption in visible light region in contrast to pure ZnO. Furthermore, an enhancement on visible light photocatalytic activity of ternary composite AgBr/ZnO/RGO for methyl orange dye degradation is observed relative to ZnO/RGO and AgBr/ZnO binary photocatalysts. Ternary composite AgBr/ZnO/RGO also shows a good photostability. The origin of this improvement lies in the formation of effective vectorial Z-scheme charge-carrier
H. Wang et al. / Ceramics International 42 (2016) 4406–4412
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Fig. 7. The XPS spectra of AgBr/ZnO/RGO composite before photocatalytic test (a) and after repeated experiments (b). (A) XPS survey spectrum; (B) highresolution XPS spectra of Ag3d; (C) high-resolution XPS spectra of Br3d; (D) high-resolution XPS spectra of C1s.
51202064) and Natural Science Foundation of Hubei Province of China (No. 2013CFA085).
References
Fig. 8. Proposed reaction mechanism over AgBr/ZnO/RGO composite.
transfer in ternary composite AgBr/ZnO/RGO, where metallic Ag formed in photocatalytic process serves as electron transfer mediator. Acknowledgment The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (No.
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CERAMICS INTERNATIONAL
Ceramics International 42 (2016) 4406–4412 www.elsevier.com/locate/ceramint
A novel photocatalyst AgBr/ZnO/RGO with high visible light photocatalytic activity Huihu Wanga,b,n, Daluo Penga, Tao Chena, Ying Changb,c, Shijie Donga,b b
a School of Mechanical Engineering, Hubei University of Technology, Wuhan, PR China Hubei Provincial Key Laboratory of Green Materials for Light Industry, Hubei University of Technology, Wuhan, PR China c School of Materials Science and Engineering, Hubei University of Technology, Wuhan, PR China
Received 4 September 2015; received in revised form 14 November 2015; accepted 22 November 2015 Available online 27 November 2015
Abstract A novel ternary photocatalyst AgBr/ZnO/RGO, where AgBr/ZnO is supported on reduced graphene oxide, is synthesized via a facile hydrothermal–impregnation method. The resultant composite presents a lamellar structure with AgBr nanoparticles homogeneously dispersing on the surface. The photocatalytic experiment for methyl orange dye degradation under visible light irradiation shows that ternary composite AgBr/ ZnO/RGO has an activity 12.8 times and 2.3 times higher than binary photocatalysts ZnO/RGO and AgBr/ZnO respectively. More importantly, the ternary composite also demonstrates a good photostability. Metallic Ag is produced during the photocatalytic process, which may serve as the electron transfer mediator in the vectorial Z-scheme transfer of photogenerated charge carriers at the interface of AgBr/ZnO/RGO. The effective separation of photogenerated electrons and holes was proposed to be responsible for the enhancement of visible light photoactivity. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: D. ZnO; Reduced graphene oxide; AgBr; Photocatalytic activity
1. Introduction With the depletion of fossil fuel reserves and a severe environmental crisis by burning of fossil fuel, worldwide efforts have been devoted to making use of sunlight for energy production, environmental protection and water purification. Semiconductor photocatalysis represents a potentially costeffective pathway for converting solar energy to chemical energy and environment remediation. Therefore, this method has received more and more attention in the past few decades. ZnO is one of the most commonly employed photocatalysts for its unique electronic structure. However, its wide band gap (3.37 eV) makes it can only absorb the ultraviolet part in solar spectrum. As sunlight consists of about 5–7% UV light, 46% visible light and 47% infrared irradiation [1], it is of n Corresponding author at: School of Mechanical Engineering, Hubei University of Technology, Wuhan 430068, Hubei, PR China. Tel./fax: þ86 027 59750418. E-mail address: [email protected] (H. Wang).
http://dx.doi.org/10.1016/j.ceramint.2015.11.124 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
significance to develop visible light active ZnO photocatalyst in order to efficiently utilize solar energy. Recently, AgBr as a visible light sensitive material has been used to improve the visible light activity of ZnO [2–5]. Fu et al. [2] prepared Ag–AgBr/ZnO photocatalyst through in situ oxidation of Ag/ZnO by bromine water. The Ag–AgBr/ZnO composite showed much higher performance for RhB degradation than ZnO and Ag/ZnO under simulated sun light illumination. Habibi-Yangjeh et al. [3] synthesized AgBr/ ZnO composites in water by refluxing method. The composites exhibited the higher photocatalytic activity than ZnO and AgBr for methylene blue degradation under visible light irradiation. Sun et al. [4] prepared Ag/AgBr/ZnO composites by deposition–precipitation method. The higher visible light photocatalytic activity of composites has been ascribed to its enhanced visible light absorption, narrowed band gap and the effective separation of photoinduced electron–hole pairs. Our group also prepared AgBr nanoparticles surface modified ZnO nanorod arrays by impregnation method. The high surface area of ZnO nanorod arrays and the visible light sensitivity of AgBr
H. Wang et al. / Ceramics International 42 (2016) 4406–4412
promoted ZnO nanorod arrays exhibiting the high visible light photocatalytic activity [5]. Although AgBr modification can effectively improve the utilization of solar energy for ZnO, the visible light activity of AgBr/ZnO composites may be restricted to some extent for the micron-meter size of AgBr particles and the fast recombination of electron–hole pairs. Therefore, it is essential to enhance the visible light activity of AgBr/ZnO composites by accelerating the separation of electron–hole pairs. Graphene sheet modified with oxygen functional groups, including GO and reduced GO (RGO), is an excellent electrontransport carbon material, which has been widely used to enhance the photodegradation efficiency of wide band gap semiconductor, such as TiO2 [6,7] and ZnO [8–15], by retarding the recombination of electron–hole pairs. Even the visible light photocatalytic activity of ZnO was obtained through the hybridization of graphene sheet [9,11]. Except for wide band gap semiconductor, it is interesting that GO and RGO were also employed to modify AgBr in order to enhance the separation of electron–hole pairs and the photostability of AgBr [16–18]. The high visible light activity and stability of AgBr/GO and AgBr/RGO were obtained. Therefore, it would be of great interest to investigate the synergistic effect of AgBr and graphene sheet modification on the visible light photocatalytic activity of ZnO. It is expected that the ternary composite AgBr/ZnO/RGO may have a highly efficient visible light activity due to the synergistic effect coming from the visible light sensitive of AgBr and the fast electron transfer ability of RGO. 2. Experimental 2.1. Preparation of ZnO/RGO composite For preparation of ternary composite AgBr/ZnO/RGO, graphene oxide (GO) was synthesized at first through Hummer's method [19]. Then, ZnO/RGO composite was prepared through hydrothermal method. Briefly, 0.58 g of Zn (CH3COO)2, 0.53 g of NaOH, and a certain amount of GO powders were mixed in 80 mL of ethanol solution and stirred for 30 min. Then, the mixtures were transferred to a Teflon sealed stainless steel autoclave, and solvothermally treated at 160 1C for 24 h in order to simultaneously achieve ZnO nanorods and reduced graphene oxide (RGO). Afterwards, the autoclave was cooled down to room temperature. The precipitates in autoclave was filtered, washed and dried. The amount of graphene oxide in ZnO/RGO composite was controlled to be 3 wt%. Pure RGO was also prepared using the similar method above without introducing Zn(CH3COO)2, while pure ZnO sample was obtained without adding GO. 2.2. Preparation of AgBr/ZnO/RGO composite Based on preparation of ZnO/RGO binary composite, ternary photocatalyst AgBr/ZnO/RGO was synthesized via a facile impregnation method [5]. First, ZnO/RGO composite was added into 0.04 mol/L KBr solution and dispersed by
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ultrasonic agitation for 10 min. The pH value of solution was adjusted to 6. Secondly, a certain amount of AgNO3 was added to the solution. The obtained mixtures were put in water bath and kept at 55 1C for 2 h. The molar ratio of Ag:Br was adjusted to 1:1. The amount of AgBr in ternary composite was controlled to be 2 wt%. For comparison, AgBr/ZnO binary composite was also prepared using pure ZnO as substrates. 2.3. Characterization X-ray diffraction patterns (XRD) of samples were carried out on a XD-2 type diffractometer with Cu Kα1 radiation. Surface morphology analysis was performed using a Quanta FEG 450 scanning electron microscope (FESEM). Further morphological and structural characterization was conducted by a FEI Tecnai G20 transmission microscope (TEM) using a 200 kV accelerating voltage. The surface chemical species of samples were analyzed on a VG Multilab 2000 X-ray photoelectron spectrometer (XPS). Diffuse reflectance spectra of different samples were measured by a JASCO V-560 UV–vis spectrophotometer. 2.4. Photocatalytic test The photocatalytic activity of different samples was examined by methyl orange (MO) degradation. The light source used was a 300 W Xe lamp with a 400 nm filter. The catalyst loading amount and MO concentration was set as 0.5 g/L and 10 mg/L, respectively. The MO concentration was analyzed on a UV2102 PC spectrophotometer. For photocatalytic stability test, the photodegradation experiments were repeated five times using the same testing conditions above. After each photoreaction, the samples were washed with deionized water for several times and dried at 60 1C. 3. Results and discussion Fig. 1 shows the XRD patterns of pure RGO, ZnO/RGO, AgBr/ZnO, and AgBr/ZnO/RGO samples. A typical diffraction peak is observed at 24.11 corresponding to graphite (002) in pure RGO sample, indicating hydrothermal method can effectively reduce GO to RGO. Therefore, when GO powders were mixed with Zn(CH3COO)2 and NaOH, ZnO/RGO composite can be obtained after hydrothermal treatment. However, no RGO diffraction peak is found in ZnO/RGO composite, which may be ascribed to the limited amount of RGO in composite. Similar phenomenon was also reported in the previous literature even the mass ratio of RGO increases to 29.6% in ZnO/RGO composite [20]. All the diffraction peaks in ZnO/RGO composite can be indexed to ZnO wurtzite phase. In AgBr/ZnO and AgBr/ZnO/RGO samples, the diffraction peaks of AgBr are also observed. XRD results demonstrate that ZnO/RGO, AgBr/ZnO, and AgBr/ZnO/RGO composites have been successfully synthesized in this experiment. Fig. 2 shows the FESEM images of ZnO/RGO (Fig. 2a and b), AgBr/ZnO (Fig. 2c and d), and AgBr/ZnO/RGO (Fig. 2e and f) samples. It can be found that ZnO in all samples presents the
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nanorod shape. Compared with AgBr/ZnO sample, lamellar aggregates appear in RGO/ZnO and AgBr/RGO/ZnO samples, which may be attributed to the presence of graphene sheet in composites. For the polar oxygen-containing functional groups on the surface of RGO sheet [21], zinc salt is inclined to hydrolyzing on its surface through hydrogen bonds or adsorp-
Fig. 1. XRD patterns of pure RGO, ZnO/RGO, AgBr/ZnO and AgBr/ZnO/ RGO samples.
tion during hydrothermal treatment. Therefore, ZnO nanorods are formed on the surface of RGO sheet. For AgBr/ZnO/RGO ternary composite, there are many tiny nanoparticles homogeneously dispersing on the surface of ZnO/RGO composite (Fig. 2f), which are suggested to be AgBr phase. However, this phenomenon is not observed in AgBr/ZnO sample, as shown in Fig. 2d. It is suggested that the surface abundant functional groups of RGO sheet, such as hydroxyl, carbonyl, carboxyl, and so on, can provide a lot of active sites for the heterogeneous nucleation of AgBr nanoparticles. On the other hand, the interaction between AgBr and surface functional groups can prevent the growth rate of AgBr nanoparticles. For AgBr/ZnO composite, there are no active sites for heterogeneous nucleation of AgBr. Therefore, the formation of AgBr phase may come from the homogeneous nucleation and growth in solution. The interaction role between ZnO and AgBr becomes very weak, which may affect the charge carriers transfer during photocatalytic process. TEM images of AgBr/ZnO/RGO sample are shown in Fig. 3. The presence of RGO in ternary composite is provided by Fig. 3a–c. It is clearly observed that ternary composite consist of one-dimensional ZnO nanorods, AgBr nanoparticles and RGO sheets. The high resolution TEM image of ZnO nanorods in Fig. 3d shows that it grows along the [0001] direction.
Fig. 2. FESEM images of ZnO/RGO (a and b), AgBr/ZnO (c and d) and AgBr/ZnO/RGO (e and f) samples.
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Fig. 3. TEM images of AgBr/ZnO/RGO sample.
Diffuse reflectance spectra of pure RGO, ZnO and the other composites are displayed in Fig. 4. For pure RGO, a maximum absorption peak at 260 nm corresponding to the excitation of π-plasmon of graphite structure is observed. For pure ZnO and the other composites, there is no significant change on the absorption in UV region. However, the composites show the enhanced visible light absorption which is accordance with the color change from white to gray. Moreover, the absorption edge of AgBr/ZnO and AgBr/ZnO/RGO composites exhibit a little red shift to the higher wavelength relative to ZnO/RGO photocatalyst. The photocatalytic performance of different photocatalysts for MO degradation is shown in Fig. 5. It is clear that pure ZnO doesn't show any activity under visible light irradiation. Both RGO and AgBr addition can improve the visible light photocatalytic activity of ZnO. However, AgBr/ZnO composite exhibits much higher visible light photocatalytic activity (kapp. ¼ 0.01013 min 1) than ZnO/RGO composite (kapp. ¼ 0.00183 min 1), indicating AgBr addition plays a more important role on the improvement of visible light photocatalytic activity for ZnO than RGO introduction. When AgBr and RGO are simultaneously used to modify ZnO, the ternary composite AgBr/ZnO/RGO exhibits a highest photocatalytic activity (kapp. ¼ 0.02336 min 1), indicating a synergistic role of AgBr and RGO on the improvement of visible light activity of ZnO. Ternary composite AgBr/ZnO/RGO shows a visible light activity 12.8 times and 2.3 times higher than binary photocatalysts ZnO/RGO and AgBr/ZnO respectively.
Fig. 4. Diffuse reflectance spectra of pure RGO, ZnO and the composites.
Reusability is very important for the application of photocatalyst, especially the catalyst composites with photosensitive and unstable components AgBr. The photostability test of AgBr/ZnO/RGO composites for MO degradation was confirmed by five repeated experiments using a recycled ternary photocatalysts under the same conditions. As shown in Fig. 6, the ternary composites reveal a high visible light photostability for five repeated experiments, although a slight decrease of photocatalytic activity is observed compared to the first-run result.
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e þ Agi þ -Ag1 Agi þ : interstitial Ag þ AgBr-Ag þ Br
Fig. 5. Photocatalytic performance of different photocatalysts for MO degradation.
Fig. 6. The repeated experiments of AgBr/ZnO/RGO composite for MO degradation.
To further investigate the effect of recycle experiments on the structure of ternary composite AgBr/ZnO/RGO under visible light irradiation, the surface compositions of ternary composite were analyzed by XPS before photocatalytic test and after five repeated experiments, as shown in Fig. 7. Compared with the ternary composite AgBr/ZnO/RGO before photocatalytic test, the XPS peak of Ag3d in composite after five repeated experiments shows a slight higher binding energy. The peak position of Ag3d5/2 increases from 373.0 eV to 373.1 eV, while the peak position of Ag3d3/2 shifts from 367.0 eV to 367.1 eV (Fig. 7b). This slight increase on binding energy of Ag3d can be ascribed to the formation of metallic Ag due to its larger binding energy. Actually, this phenomenon can be expected because AgBr can absorb a light photon and simultaneously generate an electron and a positive hole under visible light irradiation, the photogenerated electrons can react with interstitial Ag þ ions to form metallic Ag1. This process is expressed as follows: AgBr þ hν-h þ þ e
ð1Þ
ð2Þ ð3Þ
Furthermore, it is also observed that the XPS peak of Br3d moves from 68.3 eV to 67.9 eV (Fig. 7c), which may be attributed to the photoreduction of AgBr under visible light irradiation. Fig. 7d shows the C 1s peak of ternary composites before photocatalytic test and after five repeated experiments. No obvious change is observed. The XPS peak of C 1s can be deconvoluted to three different peaks assigned to C–C (284.6 eV), C–O(284.6 eV), and C ¼ O(288.5 eV) bonds in RGO sheets, respectively [22,23]. Through the discussion above, the possible photocatalytic mechanism for the enhanced visible light photoactivity was proposed. During photocatalytic process, Ag nanoparticles were produced on the surface of AgBr particles for the photoreduction of AgBr [5], which has been verified from XPS results. Under visible light irradiation, AgBr can be photoexcited to generate electron–hole pairs. Simultaneously, the electron–hole pairs are also generated in metallic Ag nanoparticles owing to the surface plasmon resonance of Ag nanoparticles in visible light region. The photogenerated electrons in the CB of AgBr combine with the photogenerated holes in the highest occupied orbital of Ag. Meanwhile, the photogenerated electrons in the lowest unoccupied orbital of Ag can effectively inject to the CB of ZnO [2], and then to the RGO sheet. In this case, Ag nanoparticles can serve as the electron transfer mediator in the vectorial electron transfer of AgBr-Ag-ZnO-RGO, leading to an efficient charge separation and enhancing the photocatalytic activity. This process is consistent with the Z-scheme electron transfer path in Z-scheme structures, such as AgI–Ag–AgBr [24], AgBr–Ag– Bi2WO6 [25], BiOBr–Ag–AgBr [26], ZnO–Au-CdS [27], H2WO4.H2O–Ag–AgCl [28], and Bi20TiO32–Ag–AgCl [29]. In contrast to the heterojunction-type AgBr/ZnO photocatalyst, the ternary Z-scheme composite AgBr/ZnO/RGO may simultaneously possess the higher charge-separation efficiency and stronger redox ability. On the other hand, the homogeneous dispersion and small size of AgBr, as well as the good interface between AgBr and ZnO in ternary composite are favorable for the separation of electrons and holes, making it a perfect visiblelight-driven photocatalyst (Fig. 8). 4. Conclusions In this paper, a ternary photocatalyst AgBr/ZnO/RGO has been successfully synthesized. The composite presents a lamellar structure with AgBr nanoparticles homogeneously dispersing on the surface of composite. Owing to the addition of RGO and AgBr, the composite exhibits a higher absorption in visible light region in contrast to pure ZnO. Furthermore, an enhancement on visible light photocatalytic activity of ternary composite AgBr/ZnO/RGO for methyl orange dye degradation is observed relative to ZnO/RGO and AgBr/ZnO binary photocatalysts. Ternary composite AgBr/ZnO/RGO also shows a good photostability. The origin of this improvement lies in the formation of effective vectorial Z-scheme charge-carrier
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Fig. 7. The XPS spectra of AgBr/ZnO/RGO composite before photocatalytic test (a) and after repeated experiments (b). (A) XPS survey spectrum; (B) highresolution XPS spectra of Ag3d; (C) high-resolution XPS spectra of Br3d; (D) high-resolution XPS spectra of C1s.
51202064) and Natural Science Foundation of Hubei Province of China (No. 2013CFA085).
References
Fig. 8. Proposed reaction mechanism over AgBr/ZnO/RGO composite.
transfer in ternary composite AgBr/ZnO/RGO, where metallic Ag formed in photocatalytic process serves as electron transfer mediator. Acknowledgment The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (No.
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