DOI: 10.1002/cctc.201000380

Highly Efficient Visible Light Plasmonic PhotocatalystsAg@Ag(Cl,Br) and Ag@AgCl-AgIPeng Wang,[a] Baibiao Huang,*[a] Xiaoyang Zhang,[a] Xiaoyan Qin,[a] Ying Dai,[b] Zeyan Wang,[a]

and Zaizhu Lou[a]

Introduction

Since the first Ag@AgCl plasmonic photocatalyst was pro-posed,[1–2] plasmonic photocatalysts have attracted more andmore attention.[3–10] Unlike traditional photocatalysts,[11–14] plas-monic photocatalysts are based on the localized surface plas-mon resonance (LSPR) of the noble metal, metal-semiconduc-tor contact, and semiconductor photocatalysts. The LSPR is es-sentially light waves trapped on the metal surface as a resultof the interaction with the free electrons of the metal.[15–16] Ex-citation of the conduction electrons occurs when there is reso-nance between the electrons and the oscillating electric fieldof the incident light, and this LSPR can be monitored as an ex-tinction spectrum as light is passed through the sample. TheLSPR of metal nanoparticles can be modulated by their size,shape, and surrounding medium.[17–19] The plasmonic photoca-talyst Ag@AgCl has proven to be a highly efficient and stablephotocatalyst under visible-light illumination. Based on the re-search of Ag@AgCl, we have changed the surroundingmedium of silver nanoparticles and succeed in fabricating thenew plasmonic photocatalysts Ag@Ag(Cl,Br) and Ag@AgCl-AgI.By changing the surrounding medium, the photocatalytic per-formance and LSPR have been tuned.[20] The new plasmonicphotocatalysts Ag@Ag(Cl,Br) and Ag@AgCl-AgI are highly effi-cient for the photooxidization of methylic orange (MO) dyeand the photoreduction of CrVI.

The properties of Ag(Cl,Br) are similar to halide silver, butAg(Cl,Br) is seldom researched. Ag(Cl,Br) is also photosensitiveand can be decomposed by visible light. Just as AgCl, Ag(Cl,Br)can also be used as a photocatalyst if photogenerated elec-trons are prevented from combining with Ag+ ions.

Herein, we have synthesized the plasmonic photocatalystsAg@Ag(Cl,Br) and Ag@AgCl-AgI. The oxidative ability of theplasmonic photocatalysts (photogenerated holes, degradationof organic pollution) and reduction ability of the plasmonicphotocatalysts (photogenerated electrons, reducing CrVI) were

examined. The oxidative abilities of the plasmonic photocata-lysts Ag@Ag(Cl,Br) and Ag@AgCl-AgI are stronger than N-TiO2.The reducing ability of the plasmonic photocatalyst Ag@AgCl-AgI is stronger than N-TiO2, whereas the reducing ability of theplasmonic photocatalyst Ag@Ag(Cl,Br) is weaker than N-TiO2.The surrounding medium plays important role in the plasmon-ic performance of the photocatalysts. By changing the sur-rounding medium, the performance of the photocatalyst couldbe tuned.

Results and Discussion

X-Ray diffraction and X-ray photoelectron spectroscopyanalysis of Ag@Ag(Cl,Br) and Ag@AgCl-AgI.

The XRD patterns of the obtained Ag@Ag(Cl,Br) and Ag@AgCl-AgI products are shown in Figure 1. Figure 1 a can be indexedto the cubic phase of Ag (JCPDS file : 65-2871) coexisting withthe cubic phase of Ag(Cl,Br) (JCPDS file: 14-255). As seen fromFigure 1 b, all the diffractions can be indexed to the cubicphase of Ag (JCPDS file: 65-2871), the cubic phase of AgCl(JCPDS file: 31-1238), and the hexagonal phases of AgI (JCPDSfile: 9-374).

The elemental composition, chemical status, and silver con-tent of Ag@Ag(Cl,Br) and Ag@AgCl-AgI were also examined byX-ray photoelectron spectroscopy (XPS). Before the visible-light

New plasmonic photocatalysts Ag@Ag(Cl,Br) and Ag@AgCl-AgIwere synthesized by the ion-exchange process between AgCland a potassium halide (KBr, KI), then by reducing some Ag+

ions in the surface region of Ag(Cl,Br) and AgCl-AgI particles toAg0 species. The Ag nanoparticles were formed from Ag(Cl,Br)and AgCl-AgI by a light-induced chemical reduction. TheAg@Ag(Cl,Br) and Ag@AgCl-AgI particles have irregular shapesand their sizes vary between 100 nm and 1.3 mm. The as-

grown plasmonic photocatalysts show strong absorption inthe visible-light region, owing to the plasmon resonance of Agnanoparticles. The ability of this compound to oxidize methylicorange and reduce CrVI under visible light was compared withthose of other reference photocatalysts. These plasmonic pho-tocatalysts have been shown to be highly efficient under visi-ble-light irradiation.

[a] Dr. P. Wang, Prof. Dr. B. Huang, Prof. X. Zhang, X. Qin, Dr. Z. Wang, Z. LouState Key Lab of Crystal Materials, Shandong UniversityJinan 250100 (China)Fax: (+ 86) 531-8836-5969E-mail : bbhuang@sdu.edu.cn

[b] Prof. Dr. Y. DaiSchool of Physics, Shandong UniversityJinan 250100 (China)

360 � 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemCatChem 2011, 3, 360 – 364

irradiation, XPS results indicated that the Ag@Ag(Cl,Br) con-tained Ag, Cl, Br, and C elements. The C element was attribut-ed to adventitious hydrocarbon from the XPS instrument itself.The Ag, Cl and Br peaks were attributed to the as-obtainedAg@Ag(Cl,Br) samples. Shown in Figures 2 and 3 are the XPSspectra of Ag@Ag(Cl,Br). The binding energy values taken from

the XPS spectra were calibrated by using C 1s (284.8 eV). In Fig-ure 2 a, the Ag 3d spectra of Ag@Ag(Cl,Br) consists of two indi-vidual peaks at 373 and 367 eV, which were attributed toAg 3d3/2 and Ag 3d5/2 binding energies, respectively. TheAg 3d3/2 and Ag 3d5/2 peaks can be further divided in two,373.64, 374.11 eV, and 367.65 eV, 368.49 eV, respectively. Thepeaks at 374.11 and 368.49 eV were attributed to metal Ag0,whereas the peaks at 367.65 and 373.64 eV were attributed toAgI of Ag(Cl,Br). The surface Ag0 content (4.13 mol %) and thesurface Ag+ content (51.00 mol %) of the corresponding sam-

ples was also calculated. The content of Ag0 of the as-obtainedAg@AgCl-AgI (3.31 mol %) was also calculated from XPS data(not shown here). The spectra of Cl 2p and Br 3d are shown inFigure 3, the binding energies of Cl 2p1 and Cl 2p3 are 199.40and 197.75 eV, and the binding energies of Br 3d3 and 3d5 are69.26 and 68.20 eV. The calculated surface content values forCl� and Br� are 16.41 and 28.47 mol %, respectively.

Electron microscopy analysis and UV/Vis diffuse reflectancespectra of Ag@Ag(Cl,Br) and Ag@AgCl-AgI.

The scanning electron microscopy (SEM) images of Figure 4show the morphologies of the as-prepared Ag@Ag(Cl,Br) andAg@AgCl-AgI samples. The size of the Ag@Ag(Cl,Br) (100 nm–1 mm, Figure 4 a) and Ag@AgCl-AgI (165 nm–1.3 mm, Figure 4 b)

particles could also be calculated from the SEM images. Thesize and position of the Ag nanoparticles was difficult to con-firm. Higher resolution images could be achieved because Ag-(Cl,Br) and AgCl-AgI were decomposed under the high energyelectron beam.

The UV/Vis diffuse reflectance spectra of Ag@Ag(Cl,Br),Ag@AgCl-AgI, Ag@AgCl, and N-TiO2 are presented in Figure 5.The absorption in the UV and visible-light region was verygood for Ag@Ag(Cl,Br), Ag@AgCl-AgI, and Ag@AgCl, which allexhibit stronger absorption in the visible region than N-TiO2.Owing to the different surrounding medium of silver nanopar-ticles, the spectra of the plasmonic photocatalysts were differ-ent from each other; Ag@Ag(Cl,Br) shows the weakest absorp-

Figure 1. The XRD patterns of a) Ag@Ag(Cl,Br) and b) Ag@AgCl-AgI.

Figure 2. XPS spectra of Ag 3d in Ag@Ag(Cl,Br) samples.

Figure 3. XPS spectra of a) Cl 2p and b) Br 3d of the as-obtained Ag@Ag-(Cl,Br) samples.

Figure 4. SEM images of a) Ag@Ag(Cl,Br) and b) Ag@AgCl-AgI samples.

Figure 5. UV/Vis diffuse reflectance spectra of the photocatalysts:a) Ag@AgCl, b) Ag@AgCl-AgI, c) Ag@Ag(Cl,Br), and d) N-TiO2.

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tion in visible-light region, whereas Ag@AgCl shows the stron-gest absorption in visible-light region.

Photocatalytic activities of Ag@Ag(Cl,Br) and Ag@AgCl-AgI.

The photooxidation capabilities of the Ag@Ag(Cl,Br) andAg@AgCl-AgI samples were evaluated by measuring the de-composition of methylic orange (MO) dye in solution (concen-tration = 20 mg L�1) under visible-light irradiation. Shown inFigure 6 is the degradation rate of MO over different photoca-

talysts. Prior to visible-light irradiation, the MO solution overthe catalyst was kept in the dark for 30 min to reach an equi-librium adsorption state. The concentration of the MO solutionslightly decreased as it was kept in the dark; C0 is the equilibri-um concentration of MO after equilibrium adsorption state,and C is the concentration of MO after visible-light irradiation.A blank experiment in the absence of the photocatalyst, butunder visible-light irradiation, showed that no MO decom-posed. Another blank experiment without irradiation, usingAg@Ag(Cl,Br) and Ag@AgCl-AgI as the photocatalysts, demon-strated that the concentration of MO remained unchanged.The MO solution was decolorized completely by using Ag@Ag-(Cl,Br) after exposure to visible-light irradiation for 15 min(Figure 6). As shown in Figure 6, about 90 percent of the MOhad decomposed after exposure for 25 min over the Ag@AgCl-AgI photocatalyst. Provided that the reaction follows apseudo-first order reaction, the rate of MO decomposition overAg@Ag(Cl,Br) is estimated to be about 0.133 mg min�1 (equalto the rate over Ag@AgCl), faster than that over Ag@AgCl-AgI (�0.072 mg min�1). The rates over the Ag@Ag(Cl,Br) andAg@AgCl-AgI plasmonic photocatalysts are faster than thatover N-doped TiO2 (�0.017 mg min�1).[1]

CrVI can be reduced to CrIII by photocatalytic reduction,[21–23]

the mechanism of which has been reported many times.[24–25]

Shown in Figure 7 a is the photocatalytic reduction of CrVI overthe plasmonic photocatalyst Ag@AgCl-AgI, which exhibited ahigh photocatalytic activity for CrVI reduction under visiblelight. The concentration of CrVI decreased with increasing irra-

diation time and nearly half the amount of CrVI was photore-duced after only 5 min irradiation; almost all of the CrVI was re-duced to CrIII after irradiation for 10 min. Shown in Figure 7 b isthe photocatalytic reduction of CrVI over Ag@Ag(Cl,Br),Ag@AgCl-AgI, Ag@AgCl, and N-TiO2. The results indicate thatthe rate of CrVI reduction with the Ag@AgCl-AgI as a photoca-talyst was faster than that with Ag@Ag(Cl,Br), N-TiO2, orAg@AgCl. Blank experiments in the absence of the photocata-lyst, but under visible-light irradiation, and blank experimentsusing Ag@Ag(Cl,Br) and Ag@AgCl-AgI without irradiation, dem-onstrated that the concentration of CrVI remained unchanged.

The plasmonic photocatalysts Ag@Ag(Cl,Br) and Ag@AgCl-AgI can be used to decompose organic pollution, such as ourexample methylic orange (MO), and reduce CrVI. By varying thesurrounding medium, plasmonic photocatalysts can show dif-ferent efficiency in decomposing MO and reducing CrVI. Duringthe process of the destruction of MO, the amount of photo-generated holes that transfer to the surface of the system cor-responds to the oxidation of the MO dye. The photogeneratedelectrons are likely trapped by O2 in solution to form superox-ide ions and other reactive oxygen species.[26] During the pro-cess of CrVI decay, the photogenerated electrons correspond tothe reduction of CrVI. The photogenerated holes were expectedto be trapped by EDTA.[24] The activities of the photogenerated

Figure 6. Photodecomposition of MO dye in solution (20 mg L�1) overAg@Ag(Cl,Br) and Ag@AgCl-AgI under visible-light irradiation (l�400 nm). Cis the concentration of MO dye at time t, and C0 that in the MO solution im-mediately after it is kept in the dark to obtain equilibrium adsorption state.

Figure 7. a) Changing UV/Vis spectrum of the aqueous solution in the pres-ence of Ag@AgCl-AgI under visible-light irradiation. (using diphenycarbazideas a developer at l = 540 nm) b) Photocatalytic reduction of CrVI in a slurrysystem over plasmonic photocatalysts Ag@Ag(Cl,Br), Ag@AgCl-AgI,Ag@AgCl, and N-TiO2 under visible-light irradiation (l�400 nm). C is theconcentration of CrVI at time t, and C0 that in the slurry system immediatelyafter it is kept in the dark to obtain equilibrium adsorption state.

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B. Huang, Z. Lou et al.

electrons and photogenerated holes determined the reductionand oxidative abilities of the photocatalysts, respectively. Fromthe photocatalytic reaction, we found that the oxidative abilityof Ag@Ag(Cl,Br) was stronger than that of Ag@AgCl-AgI,whereas the reducing power of Ag@Ag(Cl,Br) was weaker thanthat of Ag@AgCl-AgI. Compared with the traditional photoca-talysts, the plasmonic photocatalysts show outstanding activi-ties, owing to the strong absorption of visible light (surfaceplasmon resonance of silver nanoparticles) and effective sepa-ration of the photogenerated electrons and holes (good sur-face contact of Ag metal particles to the semiconductors). Theperformance of the plasmonic photocatalysts can be tuned bychanging the surrounding medium of silver nanoparticles.

Conclusions

Plasmonic photocatalysts Ag@Ag(Cl,Br) and Ag@AgCl-AgI havebeen fabricated by a simple ion-exchange process and light-in-duced chemical reduction. The photocatalysts have strong ab-sorption in the visible-light region for the plasmon resonanceof Ag nanoparticles. The plasmonic photocatalysts show highlyefficiency in reduction of CrVI and degradation of MO undervisible-light irradiation. The study on plasmonic photocatalystsAg@Ag(Cl,Br) and Ag@AgCl-AgI strongly suggests that the per-formance of the plasmonic photocatalysts and the LSPR of thenoble metals can be tuned by changing the medium surround-ing the noble metal nanoparticles.

Experimental Section

Preparation of AgCl: Silver molybdate was synthesized as previous-ly described.[1] AgCl was synthesized by the ion-exchange reactionbetween the prepared silver molybdate and HCl. Ag2MoO4 was so-nicated in concentrated HCl until completion of the ion-exchangeprocess. The precipitated AgCl was collected, washed with deion-ized water, and dried in air.

Preparation of Ag@Ag(Cl,Br) and Ag@AgCl-AgI: AgCl (1.432 g) wasadded into the reaction vessel, which contained a solution of KBr(100 mL, 0.05 mol L�1, and KI). The vessel was stirred for about 3–5 days. The Ag(Cl,Br) and AgCl-AgI precipitate was then collected,washed with deionized water and dried in air.

The as-obtained Ag(Cl,Br) and AgCl-AgI powders were put into asolution of MO dye, which was then irradiated with a 300 W Xe arclamp equipped with an ultraviolet cutoff filter to provide visiblelight with l�400 nm. The resulting precipitates, which consist ofsilver NPs and Ag(Cl,Br), AgCl-AgI particles, were then washed anddried in air.

The crystal structures of the samples were examined by using X-ray diffraction (XRD, Bruker AXS D8), their morphology by usingscanning electron microscopy (SEM, Hitachi S-4800 microscopy),and their diffuse reflectance by using UV/Vis spectroscopy (UV-2550, Shimadzu). The content of Ag element in Ag(Cl,Br) and AgCl-AgI photocatalysts was confirmed by using X-ray photoelectronspectroscopy measurements (VG MicroTech ESCA 3000 X-ray pho-toelectron spectroscope using monochromatic A1Ka with a photonenergy of 1486.6 eV at a pressure of >1 � 10�9 Torr, a pass energyof 40 eV, an electron takeoff angle of 60 8C, and an overall resolu-tion of 0.05 eV). The XPS spectra were fitted by using a combined

polynomial and Shirley-type background function. A referencephotocatalyst, N-doped TiO2, was prepared by nitridation of com-mercially available TiO2 powder (with a surface area of 50 m2 g�1) at773 K for 10 h under NH3 flow (flow rate of 350 mL min�1).[27]

Evaluation of photocatalytic activities: The activities of the photo-catalysts were evaluated by studying the degradation of methylicorange (MO) dye and reduction of CrVI. The photocatalytic degrada-tion of MO dye was performed with Ag@Ag(Cl,Br) and Ag@AgCl-AgI (0.2 g) photocatalysts suspended in a solution of MO dye(100 mL, 20 mg L�1). The degradation of MO dye was monitored byusing UV/Vis spectroscopy (UV-7502PC, Xinmao, Shanghai). The re-duction of the CrVI was performed with Ag@Ag(Cl,Br) andAg@AgCl-AgI (0.1 g) photocatalysts suspended in a solution ofK2CrO4 (100 mL, 14.24 mg L�1). The pH of the reaction suspensionwas adjusted to 2 with dilute HClO4, and the ethylene diamine tet-raacetic acid (EDTA) has also been added as sacrificial reagent.After the photocatalytic experiment was started by the irradiationof visible light, 5 mL aliquots were periodically withdrawn from thereaction vessel, and the CrVI concentration was measured by usingthe diphenycarbazide (DPC) method at l= 540 nm[28–29] by using aUV/Vis spectrophotometer (UV-7502PC, Xinmao, Shanghai). The op-tical system for detecting the catalytic reaction included a 300 WXe arc lamp (PLS-SXE300, Beijing Trusttech Co. Ltd) with UV cutofffilter (providing visible light l�400 nm).

Acknowledgements

This work was financially supported by the National Basic Re-search Program of China (973 Program, Grant 2007CB613302),the National Natural Science Foundation of China under Grant50721002, 20973102 and 10774091 and China Postdoctoral Sci-ence Foundation under Grant 20090461200.

Keywords: catalysis · photocatalysis · photochemistry ·plasmon · silver

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Received: October 27, 2010Published online on January 3, 2011

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B. Huang, Z. Lou et al.