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Applied Catalysis A: General 488 (2014) 11–18

Contents lists available at ScienceDirect

Applied Catalysis A: General

jou rn al hom ep age: www.elsev ier .com/ locate /apcata

itrogen-doped graphene modified AgX@Ag (X = Br, Cl) compositesith improved visible light photocatalytic activity and stability

hao Donga, Kong-Lin Wua, Xian-Wen Weia,∗, Jing Wanga, Li Liua, Bin-Bin Jiangb

College of Chemistry and Materials Science, Key Laboratory of Functional Molecular Solids, The Ministry of Education, Anhui Laboratory ofolecular-Based Materials, Anhui Key Laboratory of Functional Molecular Solids, Anhui Normal University, Wuhu 241000, PR ChinaSchool of Chemical and Engineering, Anhui University of Technology, Maanshan 243002, PR China

r t i c l e i n f o

rticle history:eceived 8 March 2014eceived in revised form6 September 2014ccepted 20 September 2014vailable online 26 September 2014

a b s t r a c t

Nitrogen-doped graphene modified AgX@Ag (NG–AgX@Ag, X = Br, Cl) composites were prepared by asimple co-precipitation method under ambient condition. The composites possessed higher photocat-alytic activity than corresponding bare AgX@Ag and TiO2–AgX@Ag towards degradation of rhodamine B(RhB) aqueous solution under visible light irradiation, which can be ascribed to the corporative effectsof more light harvest, enhanced adsorption capacity and more efficient separation of photogeneratedelectron–hole pairs after integrated with nitrogen-doped graphene. The NG–AgX@Ag (X = Br, Cl) com-

eywords:itrogen-doped graphenegX@Agompositesisible-light plasmonic photocatalyst

posites exhibited efficiencies of 86% and 89% for photodegradation of RhB, which were about 2 times and1.8 times of corresponding bare AgX@Ag (X = Br, Cl), respectively. The composites also had a relativelyhigh stability.

© 2014 Elsevier B.V. All rights reserved.

atalytic properties

. Introduction

Photocatalysts are expected to play an increasingly impor-ant role in solving the challenging problems of modern society,amely, energy shortage and environment pollution [1]. However,he narrow excitation wavelength, the fast recombination rate ofhotogenerated electron–hole pairs and poor adsorption capacityreatly inhibit the practical applications of photocatalysts [2]. Overhe past decades, the visible-light plasmonic photocatalysts haveroused much attention [3–5] since they can strongly absorb visi-le light and efficiently separate the photogenerated electrons andoles due to surface plasmon resonance (SPR). In general, the plas-onic photocatalyst is a composite composed of noble-metal (i.e.g, Au, Pt) nanoparticles (NPs) and a polar semiconductor, suchs AgX@Ag (X = Br [3], Cl [4,5]), etc. But recently, other materialsuch as BiOX (X = Br, Cl, I) [6–8], graphene oxide (GO) [9–13] and

raphene (or reduced graphene oxide (rGO)) [14–17] have alsoeen used as supports and/or charge carriers to form plasmonichotocatalysts.

∗ Corresponding author at: No. 1 East Beijing Road, College of Chemistry and Mate-ials Science, Anhui Normal University, Wuhu 241000, PR China.el.: +86 553 3869303; fax: +86 553 3869303.

E-mail addresses: xwwei@mail.ahnu.edu.cn, xwwei@ahut.edu.cn,eixianwen@yahoo.com (X.-W. Wei).

ttp://dx.doi.org/10.1016/j.apcata.2014.09.025926-860X/© 2014 Elsevier B.V. All rights reserved.

Graphene is a major focus of recent research due to its largetheoretical specific surface area, high mobility of charge carri-ers and outstanding mechanical, electrical, thermal, and opticalproperties because of a flat monolayer of hexagonally arrayedsp2-bonded carbon atoms tightly packed into a two-dimensionalhoney-comb lattice [18]. Moreover, the properties of graphenecan be adjusted by controlling its morphology and tailoring itselectronic structure [19]. Many studies have revealed that nitro-gen doping could significantly increase the electron conductivity,improve the electron–donor properties of graphene and enhancethe binding ability of graphene [20–22]. Recently, nitrogen-dopedgraphene (NG) has been applied in many fields including ultra-capacitors [23], solar cell [24], lithium-ion batteries [25], andelectrochemical biosensors [21,26], etc. It has been confirmed thatNG had better properties compared to pristine graphene. Surpris-ingly, several recent publications reported that NG–semiconductorcomposites possessed enhanced visible light photocatalytic per-formance [27–29]. For example, NG/CdS nanocomposites had ahigher photocatalytic activity than pure CdS towards hydrogenevolution from water under visible light irradiation [27], NG/ZnSenanocomposites exhibited remarkably enhanced photocatalyticactivity for bleaching of methyl orange [28], and NG/CdS hollow

spheres nanocomposite had an improved photocatalytic perfor-mance for degradations of methylene blue and salicylic acid[29]. Thus, the combination of NG nanosheets and photocatalystis a promising way to enhance the photocatalytic performance.

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2 C. Dong et al. / Applied Cata

onsidering the remarkable properties of NG nanosheets andisible-light plasmonic photocatalysts, their combinations coulde regarded as an ideal strategy to construct efficient and sta-le composite photocatalyst. To the best of our knowledge, there

s rare report on the visible light driven NG–AgX@Ag compos-te photocatalyst. Herein, we prepared nitrogen-doped graphene

odified AgX@Ag (X = Br, Cl) (NG–AgX@Ag) composites by a sim-le co-precipitation method under ambient condition for the firstime. The as-prepared NG–AgX@Ag composites showed higherhotocatalytic activity than corresponding bare AgX@Ag towardsegradation of rhodamine B (RhB) aqueous solution under visible

ight irradiation, and had a relatively high stability.

. Experimental

.1. Preparation

NG powders (Fig. S1 in Supplementary data) were purchasedrom XFNANO Materials Tech Co., Nanjing, China. All the chemi-als were of analytical grade and used as received without furtherurification. Typically, 8 mL N,N-dimethylformamide (DMF) dis-ersion of NG nanosheet (0.5 mg/mL) was treated with ultrasonicor more than 2 h, then 10 mL aqueous solution of AgNO3 (0.1 M)as added in under magnetic stirring, and kept stirring for another

h in dark. At last, 10 mL aqueous solution of cetyltrimethylammo-ium bromide (CTAB) or cetyltrimethylammonium chloride (CTAC)0.1 M) was added dropwise into the above mixture under magnetictirring, and then kept stirring for another 1 h under ambient con-ition. The final products were collected by centrifugation, washedith de-ionized water and absolute ethanol for several times andried in vacuum at room temperature for 24 h. The correspondinggX@Ag (X = Br, Cl) species were synthesized via a parallel pro-ess but without the NG. The TiO2–AgX/Ag (X = Br, Cl) compositesere prepared through a similar process by using commercial TiO2

nstead of the NG. Graphene oxide (GO)–AgX@Ag (X = Br, Cl) com-osites were synthesized via a similar method to NG–AgX@Ag butsing GO instead of NG.

.2. Characterization

Structure and morphologies of the products were characterizedy X-ray powder diffraction (XRD, Philips X‘Pert PRO), scan-ing electron microscopy (SEM, Hitachi S-4800), and transmissionlectron microscopy (TEM, FEI Tecnai G20), respectively. X-rayhotoelectron spectroscopy (XPS) measurements were performedn a Thermo ESCALAB 250Xi XPS system with Al K� source and

charge neutralizer. The binding energies were referenced to thedventitious C 1s line at 284.8 eV. UV–vis diffuse reflectance spectraDRS) were recorded by a Shimadzu UV-2450 spectrometer using

gO (Light) as a standard at room temperature. Infrared spectraere measured by Fourier transform infrared spectrophotometer

FT-IR, FTIR-8400S, Shimadzu) using KBr disk as a standard.

.3. EIS measurements

Electrochemical impedance spectroscopy (EIS) was performedn an electrochemical workstation (CHI660C, Chen Hua Instru-ents Company, Shanghai, China) by using an electrochemical

nalyzer with a standard three-electrode configuration, whichmployed a Pt wire as a counter electrode, a saturated Hg/Hg2Cl2lectrode as a reference electrode and a glass carbon electrode

s working electrode. 20 mg of samples was dispersed ultrasoni-ally in 10 mL of DMF, and 5 �L of the resulting colloidal dispersion2 mg/mL) was dropped onto the surface of a glass carbon electrodend dried in air at room temperature to form catalysts modified

: General 488 (2014) 11–18

glass carbon electrode. The EIS were performed in a 0.1 M KCl solu-tion containing 5 mM Fe(CN)6

3−/Fe(CN)64−.

2.4. Photocatalytic evaluation

The optical system for the photocatalytic reaction was com-posed of a 500 W Xe lamp (Shanghai Jiguang Special Light, China)and a cutoff filter (� > 400 nm). RhB aqueous solution (50 mL,10 mg/L) containing 20 mg of catalyst was put in a sealed glassbeaker and first ultrasonicated for several minutes, and then stirredin the dark for 12 h to ensure absorption–desorption equilib-rium. After visible light illumination at regular time intervals, theabsorbance of RhB solution was monitored by a Hitachi U-4100UV–Vis-NIR spectrophotometer.

The cycling experiments were conducted as follows: at the endof each cycle, the suspension was centrifuged at 10,000 rpm for10 min and the supernatant was discarded. The recovered catalystwas washed with de-ionized water and absolute ethanol for 3 timesand dried at ambient temperature under vacuum for the next cycle.The initial content of catalyst is 0.4 mg/mL at the beginning of everycycle.

3. Results and discussion

3.1. Phase structure and morphologies

The XRD patterns of the products AgX/Ag obtained before andafter integrated with NG are shown in Fig. 1. All diffractions of theproducts can be indexed to face-centered cubic AgBr crystal (JCPDSNo. 79-0149) and AgCl crystal (JCPDS No. 85-1355), respectively.No obvious peaks assigned to Ag or NG was detected, probably dueto their relative low contents.

The morphologies of AgX/Ag products obtained before and afterintegrated with NG were observed by SEM (Fig. 2). It could be seenthat AgBr and AgCl particles were agglomerated and had a largedifference in size, their sizes were in the ranges of 200–1000 nmand 100–400 nm, respectively (Fig. 2a and c), they became smaller(about 100–700 nm and 100–350 nm, respectively) while inte-grated with NG (Fig. 2b and d). This phenomenon is similar to thatof GO enwrapped Ag/AgX nanocomposite [9].

Fig. 3 shows typical transmission electron microscopy images ofthe as-obtained NG–AgX@Ag composites. It was obvious that thediameters of AgX nanoparticles were in the range of 40–150 nm.The AgX nanoparticles were deposited on the surface of NG sheets(Fig. 3a), even wrapped by NG sheets (Fig. 3c), and a hetero-junction structure was formed. The interaction between the AgXnanoparticles and the NG layered materials was so strong thatultrasonication during the sample preparation procedure for TEManalysis could not peel off these nanoparticles. Furthermore, theinterface between AgX and NG could be clearly seen (pointed toby red arrows in Fig. 3b and d), which could offer more photocat-alytic reaction centers, was helpful for the efficient photogeneratedcharge carriers transfer process across the interface, and mightlead to the improved photocatalytic activity and inhibited photo-corrosion. The interplanar spacings of the adjacent lattice fringewere estimated to be about 0.33 nm (Fig. 3b) and 0.24 nm (Fig. 3d),attributed to the (1 1 1) plane of the AgBr lattice and the (2 1 0)plane of the AgCl lattice, respectively. The interplanar spacings ofthe (1 1 1) plane of the Ag lattice were also observed (Fig. 3b).

3.2. Optical property

Generally, bare AgX species could only display distinctabsorption in the ultraviolet (UV) region but negligible absorp-tion in the visible region [9,30,31]. The optical absorbanceof NG–AgX@Ag composites was measured by UV–vis diffuse

C. Dong et al. / Applied Catalysis A: General 488 (2014) 11–18 13

10 20 30 40 50 60 70 80

(a)

JCPDS No.79-0149

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JCPDS No.85-135 5

AgCl@A g NG-AgCl@Ag

Fig. 1. XRD patterns of (a) AgBr@Ag (black line), NG–AgBr@Ag (red line); and (b) AgCl@Ag (black line), NG–AgCl@Ag (red line).(For interpretation of the references to colorin this figure legend, the reader is referred to the web version of the article.)

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Fig. 2. SEM images of the as-obtained (a) AgBr@A

eflectance spectroscopy. As shown in Fig. 4, all of the as-obtainedgX@Ag-involved species exhibited strong absorption both in theV and visible-light regions. This suggested the existence of AgPs in all of samples, which could arouse plasmon resonancebsorptions in the visible region. For bare AgBr@Ag (black line inig. 4a), the absorption range at 200–470 nm can be ascribed tohe characteristic absorption of the AgBr semiconductor, and thetrong absorption in the visible-light region of above 470 nm cane attributed to the SPR of Ag NPs. The existence of Ag NPs had beenonfirmed by the HRTEM image (Fig. 3b). Similarly, the character-stic absorption range of the AgCl semiconductor was 200–400 nm,nd the strong absorption in the visible-light region can be alsottributed to the SPR of Ag NPs (black line in Fig. 4b). Furthermore,G–AgX@Ag composites displayed stronger visible light absorp-

ion than bare AgX@Ag, which indicated their potential higherhotocatalytic activities.

.3. FT-IR spectra analysis

To further confirm the fact that NG had been integrated with

gX@Ag, FT-IR spectra of NG and NG-AgX@Ag (Fig. S2) were also

nvestigated. A broad peak at about 3438 cm−1 is ascribed to the H stretching mode of interacted water, the characteristic band

or the stretching vibration of sp2 hybridized C C (1642 cm−1)

NG–AgBr@Ag, (c) AgCl@Ag and (d) NG–AgCl@Ag.

and C OH groups (1346 cm−1), and the deformation mode of O Hgroups (1399 cm−1) can be clearly identified. In addition, a peak atabout 1594 cm−1 is certainly identified, which can be assigned tosp2 bonded C N in NG [26]. In the case of the NG–AgX@Ag compos-ites, the characteristic peaks of NG did not move (red shift or blueshift) after the introduction of AgX@Ag nanoparticles. This resultindicated that in the NG–AgX@Ag system, there was no covalentbond between NG and AgX@Ag nanoparticles.

3.4. XPS analysis

Moreover, XPS analysis was carried out to analyze the sta-tus of NG. The existences of peaks of C 1s, O 1s and N 1s (Fig.S3a) indicated that NG contained C, O and N elements. In its C 1sdeconvolution spectrum (Fig. S3b), three different peaks centeredat 284.8, 285.7 and 289.4 eV are observed, corresponding to C C,C N and C N groups, respectively. The peaks centered at 530.5,532.3 and 532.7 eV in Fig. S3c are consistent with the feature peaksof oxygen in O C, O C and O C O, respectively. Three peaks atabout 398.3, 400.0 and 402.9 eV in the high-resolution XPS spec-

trum of N 1s (Fig. S3d) can be assigned to pyridinic N, pyrrolic Nand quaternary N, respectively.

For comparison, XPS spectra of AgX@Ag and NG–AgX@Ag com-posites were also given in Figs. 5 and 6. It can be seen that new

14 C. Dong et al. / Applied Catalysis A: General 488 (2014) 11–18

s of (

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Fig. 3. TEM and high resolution TEM image

PS peaks of Ag 3d and Br 3d appeared besides the three peaks ofG (Fig. 5c and d). It was reported that the peaks at 368.32 and73.99 eV could be attributed to metallic Ag, whereas the peaks at67.4 and 373.36 eV can be attributed to Ag+ ions of AgBr [32]. Inur XPS results, one peak that assigned to Ag 3d5/2 binding energys at 367.48 eV (red line in Fig. 5c), which is right in between the

inding energy counterparts of metallic Ag (368.32 eV) and Ag+ ions367.4 eV). Similarly, the other peak that assigned to Ag 3d3/2 bind-ng energy is at 373.47 eV (red line in Fig. 5c), which is right inetween the binding energy counterparts of metallic Ag (373.99 eV)

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ig. 4. UV–vis DRS of (a) AgBr@Ag (black line), NG–AgBr@Ag (red line); and (b) AgCl@Ag

his figure legend, the reader is referred to the web version of the article.)

a, b) NG–AgBr@Ag and (c, d) NG–AgCl@Ag.

and Ag+ ions (373.36 eV). These results verified the coexistence ofmetallic Ag and Ag+ ions in as-obtained NG–AgBr@Ag composites.In the same way, we could prove the coexistence of metallic Ag andAg+ ions in as-synthesized AgBr@Ag species.

The XPS peaks of Ag 3d and Cl 2p were shown in Fig. 6c andd. It was found that the peaks of Ag 3d5/2 and Ag 3d3/2 were cen-

tered at 367.22 and 373.27 eV (red line in Fig. 6c), respectively. Ithas been reported that the peaks of metallic Ag observed at 368.4and 374.4 eV could be ascribed to Ag 3d5/2 and Ag 3d3/2, respec-tively [33]. In the case of the NG–AgCl@Ag composites, the peaks

200 300 400 500 60 0 70 0 80 0

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Wavelength(nm)

AgCl@AgNG-AgCl@Ag

(black line), NG–AgCl@Ag (red line).(For interpretation of the references to color in

C. Dong et al. / Applied Catalysis A: General 488 (2014) 11–18 15

F t is th( er is

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ig. 5. XPS spectra of (a) survey in AgBr@Ag, (b) survey in NG–AgBr@Ag (insert plored line).(For interpretation of the references to color in this figure legend, the read

scribed to Ag 3d exhibited the negative shift, which was differ-nt from the pure metallic Ag. It might be due to the interactionetween Ag NPs and AgCl particles [33]. The above XPS analyses hadonfirmed that the metallic Ag and AgCl coexisted in as-obtainedG–AgCl@Ag composites. In the same way, we could prove that

he metallic Ag and AgCl coexisted in as-prepared AgCl@Ag species.he above XPS results verified the existence of metallic Ag in as-btained NG–AgX@Ag and AgX@Ag, as suggested by the UV–visiffuse reflectance spectra.

ig. 6. XPS spectra of (a) survey in AgCl@Ag, (b) survey in NG–AgCl@Ag (insert plot is the

ine).(For interpretation of the references to color in this figure legend, the reader is refer

e peak of N 1s); (c) Ag 3d and (d) Br 3d in AgBr@Ag (black line) and NG–AgBr@Agreferred to the web version of the article.)

3.5. Photocatalytic activity

The photocatalytic activity of the prepared samples was eval-uated by photocatalytic decolorization of RhB aqueous solutionunder visible light. The RhB solution containing the catalyst was

kept in the dark for 12 h to reach the adsorption–desorption equi-librium. In the dark, the adsorption percentages of RhB in thepresences of AgBr@Ag and AgCl@Ag were 5.8% and 2.5% (Fig.S4 and Fig. 7), respectively. While the adsorption percentages of

peak of N 1s); (c) Ag 3d and (d) Cl 2p in AgCl@Ag (black line) and NG–AgCl@Ag (redred to the web version of the article.)

16 C. Dong et al. / Applied Catalysis A: General 488 (2014) 11–18

0 10 20 30 40 500.0

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NG-AgBr@AgTiO2-AgBr@Ag

C t/C0

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NG-AgCl@AgTiO2-AgCl@Ag

C t/C

0

Time(min)

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ig. 7. Photocatalytic degradation of RhB solution over (a) AgBr@Ag (black line), NG–red line), TiO2–AgCl@Ag (green line) under visible light irradiation.(For interpretatif the article.)

hB in the presences of NG–AgBr@Ag and NG–AgCl@Ag compos-tes were 28.2% and 21.5%, respectively. These results indicatedhat the NG–AgX@Ag composites possessed enhanced adsorptionapacity, which could be ascribed to the addition of NG that canbsorbed RhB via �–� conjugation between RhB and aromaticegions of nitrogen-doped graphene. Such an increased adsorp-ivity of NG–AgX@Ag composites is beneficial for photocatalyticegradation of dye on the surface of the photocatalysts under vis-

ble light irradiation. As is known, the reactive oxidative speciesROSs) produced on the surface of the photocatalyst have a shortifetime and need to diffuse to participate in the oxidation reac-ion during heterogeneous photocatalysis. Thus if the photocatalysttself has a considerable adsorption capacity, it is favorable toesolve this contradiction by enrichment of organic dyes from theulk solution and shorten the diffusion distance [2].

When the bare AgBr@Ag and AgCl@Ag species were employed ashotocatalysts, ca. 44% and 49% of RhB molecules were decomposedfter being irradiated for 50 min under visible light irradiation.owever, in the case of NG–AgBr@Ag and NG–AgCl@Ag com-osites, the degradation efficiencies were about 86% and 89%,espectively (Fig. 7). These results indicated that the photocatalyticctivities of NG–AgX@Ag (X = Br, Cl) composites are essentiallyigher than those of the corresponding bare AgX@Ag species by

factor of ca. 2 and 1.8, respectively. The degradation efficienciesf RhB over TiO2–AgX/Ag (X = Br, Cl) are 64% and 67%, respectivelyFig. 7), which are higher than those of AgX/Ag but lower thanhose of NG-AgX/Ag. The UV–vis spectral changes for photocatalyticegradation of RhB over NG–AgX@Ag composites under visible

ight irradiation (Fig. S5) showed the sharp decrease in the maxi-um absorption band, which implied that RhB molecules suffered

rather cleavage of the whole conjugated chromophore. Further-ore, the major absorption band shifted from 552 nm to 500 nm

or NG–AgBr@Ag composites and to 506 nm for NG–AgCl@Ag com-osites step by step with the increasing of irradiation time (Fig.6), which was presumed to result from the formation of a seriesf N-de-ethylated intermediates in a stepwise manner [34,35].

In order to understand the causes for enhancing the photo-atalytic performance of NG–AgX@Ag composites, electrochemicalmpedance spectra (EIS) Nyquist plot, a very useful tool to char-cterize the charge-carrier migration [36], has been performedFig. S7). It can be observed that AgX@Ag electrodes showedonsiderably small semicircles in radius over the high frequencyange, followed by a linear part in the low frequency range, whichttributed to the high electrical conductivity of materials. After NG

odified, the electric transfer resistance of AgX@Ag decreased sig-

ificantly, which indicated that the photo-induced electron–holeairs in the NG–AgX@Ag electrodes could be easily separatedhrough an interfacial interaction between NG and AgX@Ag [37],

Ag (red line), TiO2–AgBr@Ag (green line) and (b) AgCl@Ag (black line), NG–AgCl@Aghe references to color in this figure legend, the reader is referred to the web version

and transferred to the sample surface. The effective separation ofphotogenerated electron–hole pairs provides more opportunitiesto transfer electron and holes to the surface of photocatalyst and toproduce various ROSs [2].

It was noted that AgCl@Ag had a higher electric transfer resis-tance than AgBr@Ag because AgCl@Ag had a bigger semicircle,ranging from 120 to 240 �, in EIS Nyquist plot. To the best ofour knowledge, there are three key factors to determine theperformance of the photocatalyst, such as the band gap of thephotocatalyst, the behavior of photogenerated charge carriers andthe adsorption capacity of the photocatalyst [2]. As shown inFigs. 4 and 7 and Fig. S7, AgCl@Ag herein exhibited more lightabsorption than AgBr@Ag, while broader bandgap width, biggercharge transfer resistance and poorer adsorption capacity for dye,but the AgCl@Ag had a higher decomposition rate for RhB. The samephenomenon was found for NG-AgCl@Ag and NG-AgBr@Ag. There-fore, for the AgX@Ag species, beside the above three factors, someother factors such as particle size, the dispersion of the catalystmay be under consideration [38] for the observed photocatalyticactivities.

The degradation efficiencies of RhB over GO-AgX@Ag (X = Br, Cl)was 73% and 65% (Fig. S8), respectively, which were higher thanthose of AgX@Ag but lower than those of NG-AgX@Ag. NG-AgX@Agphotocatalyst exhibited the highest activity among NG-AgX@Ag,GO-AgX@Ag and AgX@Ag, the possibility was that NG had the high-est electrical conductivity, and the hybrid effect also resulted inremarkably enhanced photocatalytic performance [27].

3.6. Stability of the catalyst

The recyclability and stability of a photocatalyst was impor-tant for practical use. Therefore, the recycled experiments ofNG–AgX@Ag composites for the photodegradation of RhB wereinvestigated, and the results were shown in Fig. 8. The photo-catalytic activity of NG-AgBr@Ag gradually decreased during thefive consecutive recycle experiments, which could be ascribedto the photocorrosion [39]. While for NG-AgCl@Ag, its photocat-alytic activity dramatically changed, namely, first an increase andthen a decrease, which is the same as that of Ag@AgCl/GO [31].In the XRD patterns of NG–AgX@Ag composites after five cycles(Fig. S9), the obvious Ag peaks appeared in both NG–AgBr@Agand NG–AgCl@Ag composites after five cycles. The formation ofmetallic Ag could decrease their photocatalytic efficiencies to someextent. With the cycle experiments increased to 4 times, the pho-

tocatalytic activity of NG–AgCl@Ag was enhanced due to graduallyraising the valance band, resulting in a better match to the Fermienergy of Ag0. With cycling up to 5 times, the high valance bandwas not enough to maintain the photocatalytic system due to the

C. Dong et al. / Applied Catalysis A: General 488 (2014) 11–18 17

0 50 100 150 200 2500.0

0.2

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C t/C

0

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Fig. 8. Cycling runs in the photodegradation of RhB by (a) NG–AgBr@Ag and (b) NG–AgCl@Ag under visible light irradiation.

0 10 20 30 40 50 60 70 80 900.0

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IPA BQ EDT A AgN O3

C t/C

0

0 10 20 30 40 50 60 70 80 900.0

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0

No sca venger s IP A BQ EDTA AgN O3

F eous

diopSsTw

3

ccpaessssiR(fseRpmfii

t

Time(min)

ig. 9. Effects of a series of scavengers on the photocatalytic degradation of RhB aqu

ifficulty of the transferring of holes or accepting electrons, lead-ng to a decline in the photocatalytic activity [31]. The morphologyf the NG-AgBr@Ag catalysts changed so much due to suffer fromhotocorrosion after five cycling runs (as marked by arrows in Fig.10a). Meanwhile, NG-AgCl@Ag catalysts were separated into NGheets and agglomerated particles after five cycling runs (Fig. S10b).hese results suggested that as-prepared NG–AgX@Ag compositesere relatively stable visible-light plasmonic photocatalysts.

.7. Proposed photocatalytic mechanism

It is generally accepted that the dyes and organic pollutantsan be photo-decomposed via photocatalytic oxidation (PCO) pro-ess [36,40–42]. In order to further understand the underlyinghotocatalytic mechanism for the degradation of RhB over thes-prepared NG-AgX@Ag photocatalysts, a series of controlledxperiments with addition of different scavengers for the activepecies produced during the PCO process were performed. Ashown in Fig. 9, when the trapping agent of isopropanol (IPA),cavenger for hydroxyl radicals (•OH), was added to the reactionystem, compared with the original experiments without the rad-cal scavengers, there was only a little change on the conversion ofhB. When benzoquinone (BQ), scavenger for superoxide radicals

•O2−), and ethylene diamine tetraacetic acid (EDTA), scavenger

or holes, were added, the degradation efficiencies of RhB wereignificantly decreased. Surprisingly, the addition of AgNO3, scav-nger for electrons, could promote the degradation efficiency ofhB. It is noted that AgX was unstable under visible light due tohotocorrosion resulting from the transformation of Ag+ ions toetallic Ag. AgNO3, a good electron acceptor, also could act as sacri-

cial reagent, thus inhibited the photocorrosion of AgX and furthermproved the photocatalytic activity of photocatalysts.

On the basis of the above experimental results, a possible pho-ocatalytic mechanism was proposed (Fig. S11). Under visible light

Time (min)

solution over (a) NG–AgBr@Ag and (b) NG–AgCl@Ag under visible light irradiation.

irradiation, the electrons in the valence band of AgX particlescould be excited to the conduction band, leaving holes in thevalence band. Then, the photogenerated electrons could be trans-ferred to NG nanosheets due to the interfacial contact between AgXand NG, which greatly reduced the recombination possibility ofelectron–hole pairs. Meanwhile, the SPR produced by the collectiveoscillations of surface electrons on Ag NPs could induce enhance-ment of the local inner electromagnetic field [43], which could helpto separate efficiently the electrons and holes generated by the AgX.Due to the local electromagnetic field and excellent conductivityof Ag NPs, the electrons could be transferred quickly and inducedaway from AgX as far as possible, instead of remaining in the Ag+

ions of the AgX lattice. Electrons generated by AgX could also betransferred to Ag NPs and then reduced the present molecular oxy-gen to form the •O2

− radical species. The RhB molecules could beoxidized by the holes directly and •O2

− radicals generated in thephotocatalytic reaction to produce CO2, H2O and other mineralizedintermediates.

4. Conclusion

In summary, a facile co-precipitation method was developedto synthesize the nitrogen-doped graphene modified AgX@Ag(X = Br, Cl) visible-light plasmonic photocatalysts. Compared to thebare AgX@Ag, the NG-AgX@Ag photocatalysts possessed greatlyimproved photocatalytic activity towards degradation of RhB aque-ous solution under visible light irradiation. The hybridization ofAgX@Ag with NG nanosheets caused more light harvest, enhancedadsorption capacity and more efficient separation of photogenera-ted electron–hole pairs, which resulted from the hybridization of

NG, contributed to the improved photocatalytic performance. Inaddition, the NG-AgX@Ag photocatalysts still exhibited high pho-toactivity even after five successive cycles, namely, had a relativelyhigh stability. This investigation should open up new possibilities

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or the development of new highly efficient and stable NG-basedlasmonic photocatalysts that utilize visible light as an energyource.

cknowledgments

This work was supported by the National Natural Science Foun-ation of China (Nos. 21071005, 21271006), and the Researchulture Funds of Anhui Normal University (No. 2011rcpy038).

ppendix A. Supplementary data

Supplementary data associated with this article can beound, in the online version, at http://dx.doi.org/10.1016/j.apcata.014.09.025.

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