AgCl photocatalyst with enhanced photocatalytic performance

Journal of Molecular Catalysis A: Chemical 381 (2014) 114–119

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Journal of Molecular Catalysis A: Chemical journal homepage: www.elsevier.com/locate/molcata

Cocatalyst modification and nanonization of Ag/AgCl photocatalyst with enhanced photocatalytic performance Ping Wang a , Tingsen Ming a , Guohong Wang b , Xuefei Wang a , Huogen Yu a,∗ , Jiaguo Yu c a

Department of Chemistry, School of Science, Wuhan University of Technology, Wuhan 430070, People’s Republic of China Hubei Key Laboratory of Pollutant Analysis & Reuse Technology, Hubei Normal University, Huangshi 435002, People’s Republic of China State Key Laboratory of Advanced Technology for Material Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, People’s Republic of China b c

a r t i c l e

i n f o

Article history: Received 26 August 2013 Received in revised form 12 October 2013 Accepted 16 October 2013 Available online 25 October 2013 Keywords: Graphene Ag/AgCl-rGO Nanonization In situ oxidation Photocatalysts

a b s t r a c t Usually, cocatalyst modification and nanonization of photocatalytic materials have been demonstrated to be two kinds of effective strategies to improve the photocatalytic performance. For the well-known Ag/AgCl photocatalyst, it is difficult to obtain AgCl nanoparticles by a conventional precipitation reaction in aqueous solutions. It is highly required to develop a facile and effective strategy to simultaneously realize the cocatalyst modification and nanonization of Ag/AgCl photocatalysts. In this study, cocatalyst modification and nanonization of Ag/AgCl photocatalyst were simultaneously realized via a facile reduction–reoxidization route by using graphene oxide (GO) as the cocatalyst modifier. It was found that the chemical reduction of both Ag+ and GO by NaBH4 leaded to the formation of nanoscale Ag grafted on the reduced GO (rGO), whereas the following in situ reoxidization of metallic Ag in FeCl3 solution resulted in the final formation of well-dispersed Ag/AgCl nanoparticles on the rGO surface. Owing to a good encapsulation of Ag nanoparticles by rGO nanosheets, the resultant AgCl nanoparticles could be easily controlled to be 20–200 nm and were tightly grafted on the rGO cocatalyst surface. The photocatalytic experimental results indicated that all the Ag/AgCl-rGO (1–5 wt% rGO) nanocomposites exhibited a much higher photocatalytic decomposition of phenol than the Ag/AgCl under visible light irradiation, and the Ag/AgCl-rGO (3 wt% rGO) showed the highest performance. The enhanced photocatalytic activity of Ag/AgCl-rGO can be attributed to the cooperation effect of rGO nanosheet cocatalyst promoting the effective transfer of photogenerated electrons, and the nanonization of AgCl particles that provide more surface active sites for the decomposition of organic substances. This work may provide new insights into the fabrication of high-performance visible-light photocatalytic materials. © 2013 Elsevier B.V. All rights reserved.

1. Introduction The utilization of photocatalysts for the degradation of organic pollutions has been considered to be an effective solution to environmental problems. Titanium dioxide (TiO2 ), a traditional photocatalyst, is regarded as a suitable material for various photocatalytic applications owing to its strong oxidizing power, high chemical inertness, low cost, and long-term stability [1,2]. However, a large bandgap (3.2 eV) of anatase TiO2 restricts its use only to the narrow light-response range of ultraviolet in the solar spectrum (only about 3–5% of total sunlight) [3,4]. Therefore, the development of efficient visible-light-responded photocatalysts in a wide range of solar spectrum plays an important role in this field. Various strategies have been explored to improve the visible-light absorption and enhance the photocatalytic performance of TiO2

∗ Corresponding author. Tel.: +86 27 87871029; fax: +86 27 87879468. E-mail addresses: [email protected], [email protected] (H. Yu). 1381-1169/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molcata.2013.10.013

photocatalyst, such as surface modification [5,6], dye/quantum-dot sensitization [7,8], and doping of metallic or nonmetallic elements [9,10]. In addition, many other materials with desirable band structures have been extensively explored to develop new visiblelight-sensitive photocatalysts. Recently, Ag/AgX (X = Cl, Br, I) composites have been demonstrated to be a new and promising family of visible-light photocatalytic materials [11–18]. Huang et al. [19] prepared highly efficient visible-light plasmonic photocatalyst Ag@AgCl via treating Ag2 MoO4 with HCl to form AgCl powder which was irradiated by visible light to produce metallic Ag0 , and Hu et al. [20] synthesized a highly efficient and stable visible-light plasmonic Ag–AgI photocatalyst supported on mesoporous alumina (Ag–AgI/Al2 O3 ) by a deposition-precipitation and photoreduction method. For the Ag/AgX photocatalyst, however, it is difficult to obtain small and homogeneous nanoparticles by a traditional precipitation reaction in aqueous solutions. To further enhance photocatalytic performance, nanonization of photocatalytic materials has been demonstrated to be one of the effective strategies. Ye and

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coworkers [21] prepared highly efficient Ag/AgCl core-shell nanowires in large quantities via an in situ oxidation reaction between pentagonal Ag nanowires and FeCl3 solution, and Sun [22] also synthesized high-performance AgCl nanowires decorated with Au nanoparticles by using Ag nanowires as chemical templates. It is clear that the reported results showed that the nanonization of Ag/AgX photocatalytic materials was a desirable route to enhance photocatalytic performance. On the other hand, cocatalyst modification has also been demonstrated to be an effective method to improve photocatalytic performance. Especially, graphene is a promising modified material to improve the photocatalytic performance owing to its unique structure and property such as high electron mobility (250,000 cm2 V−1 s−1 ) and high surface area (2630 m2 g−1 ) [23–28]. In the photocatalytic reactions, the photocatalytic performance of silver halide photocatalysts can be further improved by the modification of graphene, which can accelerate the separation of photogenerated electrons and holes [29–32]. For examples, Zhang et al. [30] reported the enhanced photocatalytic performance of Ag@AgCl by the rGO cocatalyst via a deposition–precipitation method, and Zhu et al. [31] prepared the high efficient Ag/AgBr photocatalysts grafted on the graphene oxide by a surfactant-assisted assembly protocol. Obviously, graphene modification is one of the effective strategies to improve the photocatalytic activity of AgX photocatalysts. However, the previous studies about AgX photocatalyst are mainly restricted to the nanonization or surface modification, and seldom investigations have been focused on their simultaneous realization. Therefore, it is expected that the photocatalytic performance of Ag/AgX photocatalysts can be further improved by simultaneous nanonization and graphene modification. Herein, nanonization and rGO modification of Ag/AgCl photocatalyst were simultaneously realized via a facile reduction–reoxidization route. It was found that the chemical reduction of both Ag+ and GO by NaBH4 leaded to the formation of nanoscale Ag grafted on the rGO, whereas the following in situ reoxidization of metallic Ag in FeCl3 solution resulted in the final formation of well-dispersed Ag/AgCl nanoparticles on the rGO surface. The nanonization and rGO modification of Ag/AgCl photocatalyst accompanied with the effective reduction of GO to rGO were carefully characterized and their photocatalytic activities were evaluated by the phenol solution under visible-light irradiation. To the best of our knowledge, this is the first report about the simultaneous realization of cocatalyst modification and nanonization for Ag/AgCl photocatalyst with enhanced photocatalytic performance. This work may provide new insights into the fabrication of high-performance visible-light photocatalytic materials. 2. Experimental The graphene oxide was synthesized from natural graphite powder (99.95%) [33,34]. All the other reagents (analytical grade) were supplied by Shanghai Chemical Reagent Ltd. (P.R. China) and used as received without further purification. 2.1. Synthesis of Ag-rGO nanocomposite Ag-rGO composite was synthesized by a chemical method in the mixture solution of GO and AgNO3 . Typically, a 32 mL of AgNO3 (0.1 M) was added to GO aqueous solution to form a homogeneous mixing solution. After stirring for 30 min, 58 mL of NaBH4 (0.1 M) was then added to the mixing solution and was intensively stirred for another 30 min. The resulting precipitate was centrifuged, washed and dried at 60 ◦ C for 5 h to obtain the Ag-rGO nanocomposite. To prepare the Ag-rGO nanocomposites with a controllable

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component, the weight ratio of GO to Ag was controlled to be 1, 3, 5, 7 and 10 wt%, respectively, via adjusting the Ag+ amount and GO solution. To simplify the sample name, the resulting product will be referred to as Ag-rGO (X wt%) (X = 1, 3, 5, 7, and 10), with X representing weight ratio of GO to Ag. For comparison, the rGO was also prepared by a similar method without the addition of AgNO3 solution. 2.2. Synthesis of Ag/AgCl-rGO nanocomposite The Ag/AgCl-rGO nanocomposite was prepared by an in situ oxidation reaction between Ag-rGO and FeCl3 aqueous solution in a dark condition. To prevent the rapid hydrolysis of Fe3+ ions, the pH of the FeCl3 solution (0.05 mol L−1 ) was adjusted to ca. 2.5 by using HCl solution (1 mol L−1 ). Typically, 0.5 g of the Ag-rGO (X wt%) nanocomposite was added into 150 mL of FeCl3 solution under stirring at room temperature (25 ◦ C). After stirring for 30 min, the product was filtered, washed with DI water, and dried at 60 ◦ C for 5 h to obtain Ag/AgCl-rGO (X wt%) nanocomposites. For comparison, AgCl was prepared by a similar method without GO and the resulted sample was denoted as Ag/AgCl. 2.3. Characterization X-ray diffraction (XRD) patterns were obtained on a D/MAXRBX-ray diffractometer (Rigaku, Japan). Morphological analysis was performed with an S-4800 field emission scanning electron microscope (FESEM) (Hitachi, Japan) with an acceleration voltage of 10 kV. Fourier Transform Infrared (FTIR) spectra were acquired using a Nexus FT-IR spectrophotometer (Thermo Nicolet, America). Raman spectra were collected using an INVIA spectrophotometer (Renishaw, UK). X-ray photoelectron spectroscopy (XPS) measurements were done on a KRATOA XSAM800 XPS system with Mg K␣ source. UV–vis absorption spectra were obtained using a UV-visible spectrophotometer (UV-2550, SHIMADZU, Japan). Photoluminescence (PL) spectra were measured at room temperature on an F-7000 fluorescence spectrophotometer (Hitachi, Japan). The excitation wavelength was 365 nm. 2.4. Photocatalytic activity The evaluation of photocatalytic activity of the samples for the photocatalytic decomposition of phenol was performed [16,35,36]. Experimental details were shown as follows: 50 mg of the sample was dispersed into 10 mL of phenol solution (10 mg L−1 ) in a disk with a diameter of ca. 5 cm. To reach the adsorption–desorption equilibrium between the photocatalyst and phenol before irradiation, the above mixed suspension was placed in dark for 1 h. Under room conditions, the disk was exposed to 350 W Xe lamp equipped with a UV cutoff filter ( > 400 nm) and the illumination intensity was ca. 40 mW cm−2 . At certain time intervals, the solution was centrifuged to measure the concentration of phenol. For the phenol solution with a low concentration, its photocatalytic decomposition is a pseudo-first-order reaction: ln(c/c0 ) = −kt, where k is the apparent rate constant, and c0 and c are the phenol concentrations at initial state and after irradiation for t min, respectively [35]. 3. Results and discussion 3.1. Strategy for the synthesis of Ag/AgCl-rGO Fig. 1 displays the graphical illustration for simultaneously realization of nanonization and rGO-cocatalyst modification of Ag/AgCl photocatalyst. It is well known that the GO can be well dispersed into water to form a homogeneous and stable light-brown solution

P. Wang et al. / Journal of Molecular Catalysis A: Chemical 381 (2014) 114–119

Ag

c

37.5

38.0

d

38.5

39.0

39.5

rGO(002)

c

A A

d 10

20

30

B

B

40

B

B

B A

AA

50

60

A(331) A(420) B(311)

B

B(220) A(400)

B(111)

b

B(200) A(220)

A:AgCl B:Ag

A(200)

a A(111)

Relative intensity (a.u.)

rGO(002)

A(311) A(222)

116

BA

70

AB

80

o

2 Theta ( ) Fig. 2. XRD patterns of the (a) GO, (b) Ag/AgCl, (c) Ag-rGO (3 wt%), and (d) Ag/AgClrGO (3 wt%). Fig. 1. Graphical illustration for the synthesis of Ag/AgCl-rGO composite.

owing to the existing of a lot of oxygenous groups such as OH, C O, C O C, and COOH (Fig. 1a). In the GO suspension, these negative functional groups can easily combine with Ag+ ion to form Ag+ GO by an electrostatic attraction interaction after the addition of AgNO3 solution (Eq. (1) in Fig. 1), resulting in the formation of light brown color for the mixing solution (Fig. 1b). The well dispersion of Ag+ ions is beneficial to the following formation of nanonized metallic Ag via in situ reduction of Ag+ ions on the GO nanosheet surface. After the addition of excess NaBH4 solution into the Ag+ GO solution, immobilized Ag+ ions are in situ reduced to form Ag0 species on the graphene surface [37], which is accompanied with the simultaneous reduction of GO to rGO nanosheets, resulting in the one-step formation of Ag-rGO nanocomposite (Eq. (2) in Fig. 1) [38]. The residual solution is very clear after the participation of AgrGO composite (Fig. 1c), suggesting that the Ag nanoparticles can be well encapsulated by the rGO nanosheets. When FeCl3 solution is added to the Ag-rGO composite, Cl− ions can reduce the redox potential of Ag species markedly from +0.80 V (Ag+ /Ag vs. SHE) to +0.223 V (AgCl/Ag), which is much lower than that of the Fe3+ /Fe2+ couple (E0 = +0.771 V) [21]. Thereby, the metallic Ag nanoparticles can be in situ reoxidized to form AgCl nanocrystals, leading to the final formation of Ag/AgCl-rGO composites (Eq. (3) in Fig. 1). In addition, it is found that the resident solution is very clear after the black precipitation of Ag/AgCl-rGO composite (Fig. 1d), suggesting that the well interaction between metallic Ag and rGO can be well preserved after the transformation of metallic Ag to AgCl via the in situ reoxidation reaction. The light green color of the resident solution, which is caused by the formation of Fe2+ ions, further demonstrates the effective transformation of metallic Ag to AgCl via Eq. (3) in Fig. 1. Therefore, the chemical reduction of both Ag+ and GO by NaBH4 leaded to the formation of nanoscale Ag grafted on the rGO, whereas the following in situ reoxidization of metallic Ag in FeCl3 solution resulted in the final formation of well-dispersed Ag/AgCl nanoparticles on the rGO surface. 3.2. Microstructure of the Ag/AgCl-rGO composites The above formation strategy of the Ag/AgCl-rGO composites could be first demonstrated by XRD and FESEM technologies, and the corresponding results were shown in Figs. 2 and 3, respectively. The diffraction peak at 2 = 11.0 (0 0 2) could be attributed to the characteristic peak of GO (Fig. 2a), in good agreement with the previous results [39]. The FESEM image of the GO (Fig. 3a) showed

a smooth surface with a wrinkled structure, which is a typical character of nanosheet-like structure. After the addition of NaBH4 solution into the Ag+ -GO mixing solution, the characteristic peaks of metallic Ag (JCPDS file: 65-2871), could be clearly seen (Fig. 2c). Simultaneously, a new and broad peak appeared at ca. 25o (inset in Fig. 2), which can be ascribed to the characteristic diffraction peak of rGO [39]. The corresponding FESEM image suggested that the Ag nanoparticles with a size of 20–200 nm were well encapsulated by the rGO nanosheets (Fig. 3c), suggesting the successful formation of a homogeneous composite of Ag-rGO nanoparticles. After FeCl3 solution was added into the Ag-rGO composite, the new diffraction peaks corresponding to AgCl phase (JCPDS file: 31-1238) could be easily observed in addition to the metallic Ag (Fig. 2d), indicating the effective in situ reoxidation reaction of metallic Ag to AgCl by FeCl3 . Compared with the Ag/AgCl sample in the absent of GO (Fig. 2b), the resultant Ag/AgCl-rGO showed a significantly weaker diffraction peak of metallic Ag at ca. 38.4◦ (inset in Fig. 2). In view of a smaller size (20–200 nm) of Ag/AgCl nanoparticles in Ag/AgCl-rGO composite (Fig. 3d) than the Ag/AgCl particles with a size of 1–2 ␮m (Fig. 3b), the weaker diffraction peak of metallic Ag in Ag/AgCl-rGO could be attributed to a more complete in situ reoxidation reaction owing to the nanonization of metallic Ag. Further observation revealed that the Ag/AgCl nanoparticles were well dispersed and encapsulated by the rGO nanosheets (Fig. 3d), indicating the well coupling between the Ag/AgCl nanoparticles and the rGO nanosheets, in good agreement with the observation in Fig. 1. According to the EDX result (Fig. S1), the ratio of Ag to AgCl is 1.1:1 for the Ag/AgCl-rGO (3 wt%) sample. In fact, the real ratio of Ag to AgCl should be lower owing to the partial decomposition of AgCl under the irradiation of electron beam. Therefore, it is clear that rGO-cocatalyst modification and nanonization of Ag/AgCl photocatalyst can be simultaneously realized via a facile reduction–reoxidization route. FTIR, Raman and XPS technologies can further demonstrate the formation process of Ag/AgCl-rGO and the effective reduction of GO to rGO. Fig. 4 showed the FTIR spectra of GO, rGO, Ag/AgCl, Ag-rGO and Ag/AgCl-rGO samples. It could be seen clearly that the as-prepared GO (Fig. 4a) displayed many strong characteristics bands at 3412, 1727, 1626, 1420, 1223, 1056 cm−1 , which are ascribed to water OH stretching, carboxylates or ketones C O stretching, water OH bending and C C stretching-alcoholic C OH bending, epoxide C O C or phenolic C O H stretching and C O stretching, respectively [40,41]. After the addition of NaBH4 solution into the GO and Ag+ -GO, the intensity of characteristics bands

P. Wang et al. / Journal of Molecular Catalysis A: Chemical 381 (2014) 114–119

117

3000

2000

1000 -1

Wavenumber (cm ) Fig. 4. FTIR spectra of the (a) GO, (b) rGO, (c) Ag/AgCl, (d) Ag-rGO (3 wt%), and (e) Ag/AgCl-rGO (3 wt%).

corresponding to oxygen functional groups had a significant decrease (Fig. 4b and d), suggesting the effective reduction of GO to rGO by NaBH4 , in good agreement with the XRD results (Fig. 2). Consequently, it is clear that the resulting Ag/AgCl-rGO composites (Fig. 4e) also showed low characteristics bands at 900–1800 cm−1 due to the removal of oxygen functional groups. Therefore, the above results further confirmed the effective reduction of GO to rGO in the Ag/AgCl-rGO composites. The structural change of graphene was further proved by Raman spectra of GO, rGO, Ag/rGO and Ag/AgCl-rGO samples (Fig. 5). It is well reported that the intensity ratio of the D band to the G band usually reflects the disorder/defects in graphene, and a smaller intensity ratio of ID /IG can be assigned to less sp3 defects/disorders and larger average size (or less amount) of the in-plane graphitic crystallite sp2 domains [42,43]. Compared with the GO with a ID /IG value of 0.809, the rGO shows an obvious higher ID /IG (1.519) after reduction by NaBH4 , in good agreement with the our previous studies [33,34]. This result suggests that the rGO prepared from

Relative intensity (a.u.)

2850, -CH22925, -CH2-

4000

1055, C-O 1227, C-O-C, C-O-H 1410, C-OH 1629, -OH,C=C 1734, C=O

a b c d e

3412, -OH

Relative intensity (a.u)

Fig. 3. FESEM images of the (a) GO, (b) Ag/AgCl, (c) Ag-rGO (3 wt%), and (d) Ag/AgCl-rGO (3 wt%).

D,1348 cm-1

G,1592 cm-1

d c b a

ID/IG=1.301 ID/IG=1.191 ID/IG=1.519 ID/IG=0.809

500

1000

1500

2000

-1

Raman shift (cm ) Fig. 5. Raman spectra of the (a) GO, (b) rGO, (c) Ag-rGO (3 wt%), and (d) Ag/AgCl-rGO (3 wt%).

NaBH4 has more sp3 defects after the removal of oxygen functional groups. For the Ag-rGO and Ag/AgCl-rGO composites, the ID /IG ratio slightly decreased to 1.191 and 1.301, respectively, indicating less sp3 defects in carbon than rGO. Therefore, it is clear that the loading of metallic Ag or AgCl nanoparticles on the rGO nanosheet surface can partially reduce the formation of sp3 defects, which is beneficial to the effective transfer of photogenerated charges during photocatalytic reactions. High resolution C1s peaks in the XPS of rGO could further prove that oxygen-containing groups were removed after the reduction by NaBH4 . The C 1s XPS spectrum of the original GO nanosheets (Fig. 6a) clearly revealed that it consisted of non-oxygenated ring C (284.9 eV, including C C, C C, and C H), hydroxyl and epoxy groups (286.6 eV, C O in C O C or C OH), carbonyl C (287.6 eV, C O), and carboxylate carbon (288.6 eV, O C OH), suggesting a considerable degree of oxidation for the GO nanosheets [44]. After reduction by NaBH4 , the intensity of the hydroxyl and epoxy groups in the resulted rGO (Fig. 6b and c), that are the majority of oxygen-containing groups in GO, indicated a dramatic decrease, and the C C bonds became dominant, suggesting an effective deoxygenation of GO nanosheets. To further

P. Wang et al. / Journal of Molecular Catalysis A: Chemical 381 (2014) 114–119

Relative intensity (a.u.)

118

C-C C=C C-H

C-O-C C-OH C=O

284

288

HO-C=O

a

b c

280

292

Fig. 8. Schematic diagram illuminating the possible photocatalytic mechanism of Ag/AgCl/rGO composite photocatalysts.

Binding energy (eV) Fig. 6. XPS spectra of C 1s of the (a) GO, (b) Ag-rGO (3 wt%), and (c) Ag/AgCl-rGO (3 wt%). Table 1 Peak area (A) ratios oxygen-containing bonds to total area (obtained by XPS). Sample

AC

GO Ag-rGO (3 wt%) Ag/AgCl-rGO (3 wt%)

0.42 0.84 0.90

C /A

AC

O /A

0.32 0.09 0.04

AC

O /A

ACOOH /A

0.07 0.05 0.03

0.19 0.02 0.03

demonstrate the reduction degree of the GO in the Ag/AgCl-rGO composite, the peak area ratios of oxygen-containing bonds to total area were calculated on the basis of XPS results and the corresponding results were shown in Table 1. Obviously, the amount of oxygen-containing groups such as C OH, C O C, and C O had a significant decrease, which further demonstrates the effective reduction of GO and the formation of rGO in the Ag/AgCl-rGO composites. In addition to the reduction of GO, the transformation from metallic Ag0 to Ag+ can be clearly demonstrated by the XPS spectra of Ag3d (Fig. S2) after the addition of FeCl3 solution, in good agreement with the XRD results. 3.3. Photocatalytic performance and mechanism The photocatalytic activities of the Ag/AgCl-rGO composites with a different amount of rGO were evaluated by the photocatalytic decomposition of phenol solution under visible light irradiation, as shown in Fig. 7. It is clear that the Ag/AgCl photocatalyst (Fig. 7a) showed an obvious photocatalytic activity and the corresponding apparent rate constant (k) was 1.85 × 10−2 min−1 owing to its well-known visible-light photocatalytic performance caused by its strong absorption ability in the visible-light region (Fig. S3). After the Ag/AgCl was coupled with rGO nanosheets, the resultant Ag/AgCl-rGO composites (Fig. 7b–d) showed a remarkable enhanced photocatalytic activity. Especially, when the amount

0.04

c

k (min-1)

0.03 b 0.02

d

a e

0.01

of the rGO was controlled to be 3 wt%, the Ag/AgCl-rGO composite (Fig. 7c) exhibited the highest photocatalytic activity with a rate constant of 3.66 × 10−2 min−1 , a value larger than that of the Ag/AgCl by a factor of ca. 2. Further repeated experimental results suggest that the Ag/AgCl-rGO (3 wt%) can exhibit a good photocatalytic performance even after five cycles of photocatalytic reactions (Fig. S4). With further increase of the rGO amount to 5 wt%, although the photocatalytic activity of the Ag/AgCl-rGO composite (Fig. 7d) had a light decrease (k = 2.50 × 10−2 min−1 ), it still showed a higher photocatalytic performance than the Ag/AgCl photocatalyst. Apparently, graphene modification is an efficient route to improve the photocatalytic performance of Ag/AgCl photocatalysts. Further experimental results suggested that the resultant Ag/AgCl-rGO nanocomposite showed an obviously decreased photocatalytic activity when the rGO amount was larger than 7 wt% (Fig. 7e and f). This is probably due to the fact that the rGO with a high amount in the Ag/AgCl-rGO nanocomposites can shield the visible-light absorption of Ag/AgCl nanoparticles, which results in a rapid decrease of irradiation passing through the reaction suspension solution. The photocatalytic mechanism of the Ag/AgCl-rGO photocatalyst was shown in Fig. 8. Usually, the visible light absorption in AgCl with wide band gap of 3.25 eV can be attributed to the plasmonic absorption of metallic Ag nanoparticles (Ag/AgCl). Under visible-light irradiation, the plasmon-induced electrons of Ag nanoparticles transfer to the rGO nanosheets via the conduction band (CB) of AgCl to reduce oxygen, while the plasmon-induced holes are retained on the surface of Ag nanoparticles to oxidize organic substances [19]. In the case of Ag/AgCl-rGO composites, the rGO nanosheets work as a highly efficient cocatalyst for the rapid transfer of photogenerated electrons to promote the charge separation and enhance photocatalytic activity [33]. Moreover, the well interface interaction between Ag/AgCl nanoparticles and rGO nanosheets (Fig. 1d and 3d) is beneficial to the rapid transfer of photogenerated electrons from the CB of AgCl to the rGO surface (Fig. 8) [29]. The rapid transfer of photogenerated charges in Ag/AgCl-rGO can be further demonstrated by the PL spectra. From the PL spectra (Fig. S5), it was found that the PL intensity of the Ag/AgCl-rGO had an obvious decrease after the addition of rGO, suggesting a more effective separation of photogenerated electrons and holes. On the other hand, compared with the bulk Ag/AgCl photocatalyst (Fig. 8), the nanonization of Ag/AgCl phase also contributes to the enhanced photocatalytic performance of the Ag/AgCl-rGO photocatalyst. As a consequence, graphene as a cocatalyst and nanonization of Ag/AgCl photocatalysts have been realized simultaneously to further improve photocatalytic activity.

f 0.00 Fig. 7. The photocatalytic degradation rate constant (k) of phenol for various photocatalysts: (a) Ag/AgCl, (b) Ag/AgCl-rGO (1 wt%), (c) Ag/AgCl-rGO (3 wt%), (d) Ag/AgCl-rGO (5 wt%), (e) Ag/AgCl-rGO (7 wt%) and (f) Ag/AgCl-rGO (10 wt%).

4. Conclusions In summary, graphene modification and nanonization of Ag/AgCl photocatalysts have been realized simultaneously via a

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facile reduction–reoxidization route by using the GO as the cocatalyst modifier. The chemical reduction of both Ag+ and GO by NaBH4 leaded to the formation of nanoscale Ag grafted on the rGO nanosheets, whereas the following in situ reoxidization of metallic Ag in FeCl3 solution resulted in the final formation of well-dispersed Ag/AgCl nanoparticles on the rGO surface. Due to a good encapsulation of Ag nanoparticles by rGO nanosheets, the resultant AgCl nanoparticles can be controlled to be 20–200 nm and well grafted on the rGO surface. The photocatalytic experimental results indicated that all the Ag/AgCl-rGO (1–5 wt%) nanocomposites had a much higher rate constant than the Ag/AgCl, and the Ag/AgCl-rGO (3 wt%) showed the highest rate constant, which was larger than that of the Ag/AgCl by a factor of ca. 2. The enhanced photocatalytic activity of Ag/AgCl-rGO can be attributed to the cooperation effect of cocatalyst modification by rGO, and the nanonization of Ag/AgCl particles that provide more surface active sites for the decomposition of organic substances. This work may provide new insights into the fabrication of high-performance visible-light photocatalytic materials. Acknowledgments This work was partially supported by the National Natural Science Foundation of China (21277107, 61274129, and 51208396). This work was also financially supported by the 973 Program (2013CB632402), 863 Program (2012AA062701), the Natural Science Foundation of Hubei Province (2012FFB05002 and 2012FFB05105), the Project-sponsored by SRF for ROCS, SEM and Fundamental Research Funds for the Central Universities (Grant 2013-1a-039 and 2013-1a-036). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.molcata.2013.10.013. References

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