Ecofriendly Synthesis and Photocatalytic Activity of Uniform Cubic Ag

Dec 13, 2012 - Here, we report a simple green synthetic route for the preparation of ... For each measurement, 0.1 g of photocatalyst sample was added...
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Ecofriendly Synthesis and Photocatalytic Activity of Uniform Cubic Ag@AgCl Plasmonic Photocatalyst Rongfang Dong, Baozhu Tian,* Cuiyun Zeng, Taoyun Li, Tingting Wang, and Jinlong Zhang* Key Lab for Advanced Materials and Institute of Fine Chemicals, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, P. R. China S Supporting Information *

ABSTRACT: Uniform cubic Ag@AgCl plasmonic photocatalyst was synthesized by a facile green route in the absence of organic solvent, in which a controllable double-jet precipitation technique was employed to fabricate homogeneous cubic AgCl grains and a photoreduction process was used to produce Ag nanoparticles (NPs) on the surface of AgCl. During the double-jet precipitation process, the presence of gelatin and Cl− ions at low concentration was necessary for the formation of cubic AgCl grains. Atomic force microscopy (AFM) was used to probe the morphological structure of Ag@AgCl grains for the first time, which showed that Ag NPs are anchored on the surface of AgCl grains like up-and-down mounds. Further characterization of the photocatalyst was also done by scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and UV−visible diffuse reflectance spectroscopy (DRS). The as-prepared Ag@AgCl plasmonic photocatalyst exhibited excellent photocatalytic efficiency for the degradation of the azo dye acid orange 7 (AO7), phenol, and 2,4-dichlorophenol (2,4-DCP). The photocatalytic mechanism was studied by radical-trapping experiments and the electron spin resonance (ESR) technique with 5,5-dimethyl-1-pyrroline N-oxide (DMPO), and the results indicated that •O2− and Cl0 are responsible for the rapid degradation of organic pollutants under visible-light irradiation. plasmon resonances (SPRs).22 As a result, plasmonic NPs can serve as an alternative type of sensitizers to enhance the visiblelight absorption of photocatalysts without the problem of degradation encountered with organic sensitizers.23,24 In 2008, Huang et al.25 fabricated Ag@AgCl plasmonic photocatalyst by an ion-exchange method, which triggered an upsurge of research in Ag@AgCl plasmonic photocatalst. Most such studies were focused on fabricating Ag@AgCl with different morphological structures and exploring the relationship between morphology and photocatalytic activity.26−30 Recently, cubic Ag@AgCl plasmonic photocatalyst has attracted much interest because of its various merits compared with spherical counterparts of similar size such as higher specific surface area, more active sites, and more active facets,31−33 all of which are favorable to the enhancement of photocatalytic activity.33 Sun et al.34 synthesized cube-like Ag@AgCl plasmonic photocatalyst by a precipitation reaction between Ag+ and Cl− ions in ethylene glycol solution with the assistance of polyvinylpyrrolidone. Dong et al.35 prepared cube-like Ag@AgCl by a hydrothermal method using methylene dichloride as the chlorine source. By a facile reverse micelle method in the presence of polyvinylpyrrolidone and p-tolyl sulfonic acid, An

1. INTRODUCTION Semiconductor photocatalysis has received increasing attention as a promising solution to solve the worldwide energy crisis, environmental pollution, and global warming through hydrogen generation by water splitting,1−3 degradation of environmental pollutants,4−6 and conversion of carbon dioxide into gaseous hydrocarbons.7−9 Among various semiconductor materials, TiO2 has been widely studied because of its excellent optical and electronic properties, low cost, chemical stability, and nontoxicity.10,11 However, pristine TiO2 can only be excited by UV light because of its wide band gap, leading to low utilization efficiency of the solar spectrum. To solve this problem, considerable efforts have been devoted to the exploitation of new and more efficient visible-light-active photocatalysts to meet the requirements of future environmental and energy technologies driven by solar energy. The typical strategies used to date can be classified into two categories: The first involves doping with metals12,13 or nonmentals14,15 and anchoring organic sensitizers16,17 for wide-band-gap semiconductors such as TiO2, ZnO, and InTaO4; the second is to exploit new semiconductor materials with narrow band gaps such as CdS,18 Bi2WO6,19 and In1−xNixTaO4 (x = 0−0.2).20 However, these photocatalysts are still not usable for practical applications because of a limited visible-light response and low stability.17−21 Noble-metal nanoparticles (NPs), such as Au, Ag, Pt, and Cu, exhibit strong UV−vis absorption because of their surface © 2012 American Chemical Society

Received: August 11, 2012 Revised: December 13, 2012 Published: December 13, 2012 213

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et al.22 prepared AgCl nanocubes with an average edge length of 85 nm. Very recently, Chen et al.33 fabricated quasicubic Ag@AgCl plasmonic photocatalyst by a surfactant-assisted method in an oil-in-water system. It should be noted that the currently available approaches require plenty of organic reagents as well as high temperature, both of which are contrary to the principle of “green chemistry”. Therefore, a simpler, more environmentally friendly, and less energyintensive synthetic method is highly desirable. The plasmonic oscillation of noble-metal NPs depends on their size, shape, and distribution,26,28,36 which determines the visible-light absorption of plasmonic photocatalysts. Unfortunately, the morphological structures of Ag NPs formed in situ on a AgCl surface are hard to probe accurately by transmission electron microscopy because both AgCl and Ag nanoparticles can be destroyed by high-energy electron beams during measurements.26,33 Therefore, it is also necessary to employ an appropriate technique to probe the morphological structures of Ag NPs formed in situ on AgCl surfaces. Here, we report a simple green synthetic route for the preparation of uniform cubic Ag@AgCl plasmonic photocatalyst in the absence of organic solvent, in which a controllable double-jet precipitation technique is employed to fabricate cubic AgCl grains and photoreduction is used to form Ag NPs in situ on the AgCl surface. In particular, the key factors (presence/absence of gelatin, Cl− ion concentration) that determine the morphology of AgCl grains were investigated. The morphological structure of Ag NPs formed on the AgCl surface was probed for the first time by atomic force microscopy (AFM), which can preserve the pristine morphology of Ag NPs on AgCl grains. The photocatalytic activity of Ag@AgCl plasmonic photocatalyst was investigated in terms of the degradation of the azo dye acid orange 7 (AO7), phenol, and 2,4-dichlorophenol (2,4,-DCP). Moreover, the photocatalysis mechanism was studied in detail by radicaltrapping experiments and the electron spin resonance (ESR) technique with 5,5-dimethyl-1-pyrroline N-oxide (DMPO).

2.2. Characterization. The morphology of the samples was characterized by scanning electron microscopy (SEM) on a a JEOL JSM-6360 LV microscope at 15 kV. A NanoScope IIIa MultiMode atomic force microscope was used to probe the morphological structures of Ag particles formed on the surface of AgCl grains. X-ray diffraction (XRD) measurements were carried out with a Rigaku D/max 2550 VB/PC X-ray diffractometer using Cu Kα radiation (λ = 0.154056 nm). Xray photoelectron spectroscopy (XPS) was carried out on a Perkin-Elmer PHI 5000 Versaprobe system with Al K1 radiation at 300 W. UV−vis diffuse reflectance spectra were measured with a Shimadzu UV-2450 spectrometer equipped with an integrating sphere assembly, using BaSO4 as the reference material. The electron spin resonance (ESR) technique (with DMPO) was used to detect the radical species over the catalyst on a Bruker EMX-8/2.7 spectrometer by accumulating three scans at a microwave frequency of 9.85 GHz and a power of 6.35 mW. Before testing, DMPO was added to the suspension system, and then the system was irradiated by visible light using a halogen tungsten lamp with a UV cutoff filter (λ ≥ 420 nm). 2.3. Measurement of Photocatalytic Activity. The photocatalytic activities of photocatalysts were evaluated in terms of the degradation of the azo dye acid orange 7 (AO7), phenol, and 2,4-dichlorophenol (2,4-DCP). The photocatalytic reaction was conducted under visible light by using a 500-W halogen tungsten lamp with a UV cutoff filter (λ ≥ 420 nm). For each measurement, 0.1 g of photocatalyst sample was added into a quartz tube containing 100 mL of an aqueous solution of AO7 (20 mg L−1), phenol (10 mg L−1), or 2,4-DCP (40 mg L−1). Prior to light irradiation, the suspension was stirred for 30 min in the dark to attain the adsorption− desorption equilibrium for AO7, phenol, or 2,4-DCP. At a given time interval, about 4 mL of suspension was withdrawn, centrifuged, and filtered to remove the remaining particles. The residual concentration of AO7 was determined with a UV−vis spectrophotometer, whereas the residual concentrations of phenol and 2,4-DCP were detected with a CTO-10 ASVP highperformance liquid chromatograph.

2. EXPERIMENTAL SECTION 2.1. Preparation of Photocatalyst. Cubic AgCl grains were prepared by a double-jet precipitation method as follows: AgNO3 aqueous solution (150 mL, 0.6 M), NaCl aqueous solution (160 mL, 0.6 M), and gelatin aqueous solution (200 mL, 2 wt %) were prepared beforehand. When the temperature of the gelatin solution approached 80 °C, the AgNO3 and NaCl solutions were simultaneously injected into the gelatin solution by peristaltic pumps under vigorous stirring. The charging rate of AgNO3 solution was kept at 2 mL/min, whereas that of NaCl solution was adjusted by a feedback control system to keep the concentration of Cl− ions (cCl−) in the reaction solution constant at 0.063 M throughout the whole reaction process. After AgNO3 solution had been completely injected, the mixture solution was stirred for 15 min to eliminate the unstable nuclei or small particles. Subsequently, the obtained AgCl emulsion was placed in a quartz tube and irradiated by UV light for 2 h. Finally, the product was collected by centrifugation and then washed with deionized water several times to remove the gelatin and residual ions (Na+, NO3−, and Cl−). To investigate the influence of cCl− and gelatin on the morphology of the AgCl grains, the samples were also prepared at cCl− = 0.40 M and in the absence of gelatin. For comparison, N-doped TiO2, denoted as N−TiO2, was perepared according to the previous method.37

3. RESULTS AND DISCUSSION 3.1. Morphology and Crystal Structure. It has been considered that the morphology of AgCl is hard to control because of the high reaction rate between silver and chloride ions.35 Here, we used peristaltic pumps to precisely control the concentrations of silver and chloride ions in the reaction system instead of using an organic chlorine source.35 The influences of gelatin and Cl− ion concentration (cCl−) in the double-jet precipitation process on the morphology of the AgCl samples were analyzed by SEM. As shown in Figure 1A, when cCl− was maintained at 0.063 M, the as-prepared AgCl grains showed a homogeneous cubic morphology with a length of 0.7 μm. In contrast, when cCl− was increased to 0.40 M, the shape of the AgCl grains became irregular, and the particle size increased (Figure 1B). By careful observation, it was found that most of the AgCl grains exhibited a polyhedral morphology. In the absence of gelatin, irregular and interconnected AgCl grains were obtained (Figure 1C). These results indicate that the morphology of AgCl grains is closely related to the concentration of Cl− ions, as well as the presence or absence of gelatin. Based on the above experimental results, a probable mechanism was proposed to illustrate the formation of cubic 214

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The X-ray diffraction (XRD) patterns of cubic AgCl and Ag@AgCl samples are shown in panels A and B, respectively, of Figure 2. The two samples exhibited diffraction peaks at

Figure 1. (A−C) SEM images of AgCl grains prepared under different conditions: (A) cCl− = 0.063 M, gelatin; (B) cCl− = 0.40 M, gelatin; (C) cCl− = 0.063 M, without gelatin. (D) SEM image of Ag@AgCl plasmonic photocatalyst.

Figure 2. XRD patterns of (A) cubic AgCl and (B) Ag@AgCl.

approximately 2θ = 27.8°, 32.2°, 46.2°, 54.8°, 57.5°, and 76.7°, which can be indexed to the AgCl (111), (200), (220), (311), (222), and (420) plane reflections, respectively (JCPDS file 311238). Unlike AgCl (Figure 2A, inset), Ag@AgCl shows a weak peak around the scattering angle of 2θ = 38.2° (Figure 2B, inset), corresponding to the cubic phase of Ag (111) (JCPDS file 65-2871). Other characteristic peaks of Ag can also be clearly seen, as shown in Figure S1 (Supporting Information). This result indicates that photoreduction is an effcient method for producing metallic Ag on the surface of AgCl grains. 3.3. Surface Composition and Microstructures. The elemental composition and surface chemical status of cubic AgCl and Ag@AgCl were analyzed by X-ray photoelectron spectroscopy (XPS) (Figure 3 and Figure S2, Supporting

AgCl grains. During the nucleation stage, the spontaneous nucleation of AgCl begins to take place rapidly when the concentrations of Ag+ and Cl− ions are higher than the critical supersaturation point of AgCl, resulting in the formation of AgCl nuclei. In the subsequent grain growth process, gelatin and the concentration of Cl− ions play vital roles in determining the morphology of the AgCl grains. Gelatin will adsorb on the surface of AgCl nuclei, forming an efficient protective layer.38 This protective layer can hinder the Ag+ ions from reaching the particle surface to form AgCl,29 which is favorable to the orientation growth of AgCl nuclei. Another role of gelatin is to restrain the aggregation of AgCl grains. Therefore, irregular and interconnected AgCl grains will be produced in the absence of gelatin. Cl− ions can tune the morphology of AgCl grains by changing the surface energies of different crystal surfaces. Based on theoretical calculations and experimental results, Dai et al.39 demonstrated that the surface energies of AgCl crystals can be reversed by adsorption of enough Cl− ions, enabling the possibility of tuning the crystal morphology and surface exposure. In this study, when the concentration of Cl− ions was controlled at 0.063 M, the surface energy of the (100) surface was lower than those of the (110) and (111) surfaces. Therefore, the growth of AgCl nuclei along the ⟨100⟩ direction will be restricted, leading to the preservation of {100} facets and the formation of {100}-facetexposed cubic AgCl. In contrast, when the concentration of Cl− ions was controlled at 0.4 M, the surface energies of the (110) and (111) surfaces were lower than that of the (100) surface, leading to the emergence of {110} and {111} facets and the formation of AgCl polyhedra (Figure 1B). When the concentration of Cl− ions surpassed the critical concentration for grain growth, new AgCl crystal nuclei were able to survive during the crystal growth process, thereby widening the AgCl size distribution.40 Figure 1D shows an SEM image of Ag@AgCl plasmonic photocatalyst that was obtained by photoreducing 0.7-μm cubic AgCl. The Ag@AgCl grains retained the cubic profile of its precursor AgCl, but the corners of the cubes became slightly rounded. This can be attributed to the formation of Ag NPs on the AgCl surface.

Figure 3. XPS spectra of the (A) Cl 2p and (B) Ag 3d regions: (a) cubic AgCl and (b) cubic Ag@AgCl.

Information). As shown in Figure 3A, both AgCl and Ag@AgCl display double peaks located at 197.8 and 199.4 eV, which can be assigned to the characteristic doublets of Cl 2p3/2 and Cl 2p1/2, respectively.26,35 Figure 3B presents the Ag 3d XPS spectra of the two samples. AgCl exhibits two peaks centered at 367.55 and 373.55 eV. As for Ag@AgCl, the fitted Ag 3d peaks 215

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indicate that two components coexist in Ag 3d5/2 and Ag 3d3/2 signals, giving peaks at 367.55 and 368.25 eV for 3d5/2 as well as 373.55 and 374.25 eV for Ag 3d3/2. According to previous reports,26,35 the peaks at 367.55 and 373.55 eV can be attributed to Ag+ (AgCl), whereas the peaks at 368.25 and 374.25 eV can be ascribed to Ag0. The XPS results confirm the existence of Ag0 in Ag@AgCl, consistent with the XRD results. For Ag@AgCl, the calculated surface content of chlorine was 41.77 at. %, which was higher than the total silver content at 40.48 at. % (4.82 at. % for Ag0 and 35.66 at. % for Ag+). Throughout the double-jet precipitation process, the concentration of Cl− ions was slightly higher than that of Ag+ ions, which was favorable for the formation of a Cl−-terminated surface. As a result, the surface chlorine content became higher than that of silver. To further explore the morphological structures of Ag NPs formed in situ on the surface of Ag@AgCl grains, AFM was employed. Figure 4 shows AFM images of cubic AgCl and Ag@

Figure 5. (A) UV−vis diffuse reflectance spectra and (B) photographs of cubic AgCl and Ag@AgCl samples.

3.4. Photocatalytic Activity and Mechanism. The photocatalytic activity of the obtained Ag@AgCl plasmonic photocatalyst was evaluated in terms of the degradation of AO7 dye, phenol, and 2,4-DCP under visible-light irradiation (λ ≥ 420 nm). For comparison, photodegradation of AO7, phenol, and 2,4-DCP over the visible-light photocatalyst N-doped TiO2 (denoted as N−TiO2) was also conducted under the same conditions. The degradation curves of AO7, phenol, and 2,4DCP over Ag@AgCl and N−TiO2 photocatalysts as a function of visible-light irradiation time are plotted in panels A−C, respectively, of Figure 6, in which C0 is the concentration of AO7 (or phenol, or 2,4-DCP) after adsorption−desorption equilibrium, whereas C represents the corresponding concentration at a certain time interval. It can be seen from Figure 6A that, after visible-light irradiation for 20 min, the degradation of AO7 over Ag@AgCl reached 97%, whereas only 12% of AO7 was decomposed over N−TiO2. Figure 6D shows the evolution of the absorption spectra of AO7 as a function of visible-light irradiation time over Ag@AgCl. It is shown that AO7 can be rapidly decomposed in the presence of Ag@AgCl plasmonic photocatalyst under visible-light irradiation. It has been confirmed that some dyes can be degraded under visible-light irradiation by a self-photosensitization mechanism.41 To ascertain that the degradation of organic pollutants over Ag@ AgCl photocatalyst was realized by an SPR-induced photocatalytic mechanism and not by a self-photosensitization process, we further selected phenol and 2,4-DCP, which have no absorption in the visible-light region, as model pollutants to evaluate the photocatalytic activity of Ag@AgCl under visiblelight irradiation. As shown in Figure 6B, after visible-light irradiation for 50 min, the degradation of phenol over Ag@ AgCl reached 93%, whereas it reached only 14% over N−TiO2. Similarly, 97% of 2,4-DCP was degraded in 60 min over Ag@ AgCl, whereas the extent of degradation of 2,4-DCP over N− TiO2 was only 12% (Figure 6C). Because AgCl does not absorb visible light, it is reasonable to ascribe the remarkable visible

Figure 4. AFM images of (A) cubic AgCl grains, (B) local surface of a cubic AgCl grain, and (C) local surface of a cubic Ag@AgCl grain.

AgCl. As shown in Figure 4A,B, the surface of the AgCl grains was flat and smooth, consistent with the SEM results (Figure 1A). After UV-light reduction, as shown in Figure 4C, the surface of the Ag@AgCl grains became up-and-down like a sequence of small mounds, which were silver clusters linked together. This phenomenon probably corresponds to the SEM results that the corners of Ag@AgCl grains became rounded (Figure 1D). 3.3. Light Absorption. The light absorption properties of cubic AgCl and Ag@AgCl samples were analyzed by UV−vis diffuse reflectance spectroscopy. As shown in Figure 5A, both of the samples exhibited strong characteristic absorption of AgCl in the range of 200−400 nm, in which the two peaks around 230 and 301 nm should be ascribed to the direct and indirect exciton transitions of AgCl, respectively.34 In contrast to AgCl, Ag@AgCl displayed a marked absorption enhancement in the visible-light region, which can be attributed to the surface plasmon resonance of Ag NPs formed on the surface of AgCl. Consistent with the DRS results, the sample’s color changed from white for AgCl to gray-purple for Ag@AgCl (Figure 5B). Because of the undulation in size of the silver clusters, as shown in Figure 4C, the Ag@AgCl sample showed strong light absorption in the whole visible-light region, making the sample use sunlight more efficiently. 216

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Figure 6. Photocatalytic degradation curves of (A) AO7, (B) phenol, and (C) 2,4-DCP over N−TiO2 and Ag@AgCl. (D) UV−vis spectral evolution of AO7 as a function of visible-light irradiation time over Ag@AgCl sample.

used to quench •OH in solution, NaHCO3 for h+ and •OH adsorbed on the photocatalyst surface, and p-benzoquinone for • O2−.42 As shown in Figure 8, methanol quencher had almost

photocatalytic activity of Ag@AgCl to the SPR of Ag NPs. As photostability is very important to a photocatalyst for its practical application, the photostability of Ag@AgCl was investigated by cycle degradation experiments. As shown in Figure 7, the photocatalytic efficiency of Ag@AgCl photo-

Figure 8. Photocatalytic degradation curves of AO7 over Ag@AgCl catalyst under different conditions: (a) no scavenger, (b) 10 mM methanol, (c) 0.1 M methanol, (d) 6 mM NaHCO3 (pH = 10), (e) NaOH solution (pH = 10), (f) 0.05 mM p-benzoquinone, and (g) 0.1 mM p-benzoquinone.

Figure 7. Cycling runs in the photocatalytic degradation of AO7 over Ag@AgCl.

catalyst underwent no obvious loss even after five cycles of degrading AO7. The SEM and XRD results also demonstrate that Ag@AgCl is a stable photocatalyst under visible-light irradiation (Figures S3 and S4, Supporting Information). It is generally accepted that organic pollutants can be degraded by photocatalytic oxidation processes, in which a series of photoinduced reactive species, such as h+, •OH, and • O2−, are suspected to be involved in the photocatalytic degradation reaction. To elucidate the main reactive species responsible for the degradation of organic contaminants over Ag@AgCl photocatalyst, a series of quenchers were employed to scavenge the relevant reactive species. Here, methanol was

no influence on the degradation rate of AO7, indicating that no • OH existed in the solution. The degradation rate exhibited a slight depression in the presence of NaHCO3, but a contrast test indicated that this result should be ascribed to an increase of the pH value. This result indicates that h+ is not the directly reactive species responsible for AO7 degradation. In contrast, the introduction of p-benzoquinone significantly restrains the degradation of AO7, suggesting that •O2− plays a key role in degrading AO7. 217

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confirm that •O2− radicals exist in the Ag@AgCl system under visible-light irradiation. As for •OH, the appearance of DMPO−•OH adducts seems to be in conflict with the result of reactive species trapping experiments. However, when AO7 dye was added to the Ag@AgCl system, with other conditions remaining unchanged, no peak of DMPO−•OH adducts emerged under visible-light irradiation (Figure 9C). This result indicates that no •OH exists in the AO7 degradation system. To further investigate the origin of •OH radicals, an ESRDMPO test in the Ag@AgBr system without AO7 dye was also carried out under the same conditions. No signal of DMPO−•OH adducts was detected (Figure S4, Supporting Information), indicating that no •OH radicals were produced. On the basis of these results, we speculate that the •OH detected in the Ag@AgCl system is the oxidation product of OH− by Cl0. Because AO7 is more easily oxidized by Cl0 than OH−, no DMPO−•OH signal was detected when AO7 was added to the Ag@AgCl system. Similarly, no •OH radicals formed in the Ag@AgBr system because the oxidizability of Br0 is lower than that of Cl0.46 The formation process of Ag nanoclusters and the degradation mechanism of organic pollutants over Ag@AgCl plasmonic photocatalyst are schematically illustrated in Figure 10. Under UV-light irradiation, AgCl will produce an electron

To further ascertain the active oxygen species in the Ag@ AgCl degradation system under visible-light irradiation, the ESR spin-trap technique with 5,5-dimethyl-1-pyrroline N-oxide (DMPO) was employed. Under visible-light irradiation, Ag@ AgCl gives the characteristic four peaks of DMPO−•OH with an intensity ratio 1:2:2:1 (Figure 9A), similar to the spectra

Figure 10. Schematic diagram illustrating the production of Ag NPs on AgCl and the proposed degradation mechanism of organic pollutants over Ag@AgCl plasmonic photocatalyst.

and a hole after absorbing a photon, and subsequently, the electron combines with an interstitial silver ion to form a Ag0 atom. Upon repeated absorption of photons, a cluster of silver atoms will ultimately be formed. When Ag@AgCl is irradiated by visible light, Ag NPs produce photogenerated electrons and holes, which can be separated by the SPR-induced local electromagnetic field (eq 1).25 As mentioned previously, the surface of AgCl grains is terminated by Cl− ions and, therefore, is negatively charged. Because of the polarization effect of the negatively charged AgCl surface, the electrons are transferred to the surface of the Ag NP farthest from the Ag@AgCl interface, whereas holes are transferred to the surface of AgCl. Subsequently, the electrons will be trapped by adsorbed O2 to form •O2− (eq 2), whereas the holes will combine with Cl− ions to form Cl0 atoms (radicals) (eq 3). Both Cl0 and •O2− are reactive active species responsible for the degradation of organic pollutants (eqs 4 and 5). After the organic pollutants have been oxidized, Cl0 can return to chloride ion form again (eq 4).25,26 In the absence of organic pollutants, Cl0 will react with OH− to form •OH (eq 6).47,48

Figure 9. DMPO spin-trapping ESR spectra recorded at ambient temperature with Ag@AgCl photocatalyst in (A) aqueous dispersion (for DMPO−•OH), (B) methanol dispersion (for DMPO−•O2−), and (C) aqueous dispersion with AO7 under visible-light irradiation (λ ≥ 420 nm).

reported in previous studies of •OH adduct.43,44 In contrast, no such signals were detected in the dark. Given that •O2− radicals in water are very unstable and undergo facile disproportionation rather than slow reaction with DMPO,44,45 DMPO−•O2− adducts in the Ag@AgCl system were detected in methanol solution. As shown in Figure 9B, six characteristic peaks with intensity similar to that of DMPO−•O2− adducts were observed only under visible-light irradiation, which is consistent with the former studies for •O2− adduct.43,44 These ESR results 218

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Ag NPs + hv → Ag NPs* → Ag NPs⊖ + Ag NPs⊕

(1)

Ag NPs⊖ + O2 → •O2−

(2)

Ag NPs⊕ + Cl− → Cl0

(3)

Cl0 + organics → degradation products + Cl−

(4)

•

−

O2 + organics → degradation products 0

−

−

•

Cl + OH → Cl + OH

Shanghai Natural Science Foundation (10ZR1407400), the National Basic Research Program of China (973 Program, 2010CB732306), the Project of International Cooperation of the Ministry of Science and Technology of China (2011DFA50530), and the Science and Technology Commission of Shanghai Municipality (10JC1403900).

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4. CONCLUSIONS Uniform cubic Ag@AgCl plasmonic photocatalyst was fabricated by a facile green synthetic route in the absence of organic solvent, in which a mild controllable double-jet precipitation technique was employed to fabricate homogeneous cubic AgCl grains and a photoreduction process was used to produce Ag nanoparticles on the surface of AgCl. It was found that gelatin and Cl− ion concentration play vital roles in determining the morphology of the AgCl grains. Gelatin can adsorb on the surface of AgCl to form a protective layer, which not only facilitates the orientation growth of AgCl nuclei but also restrains the aggregation of AgCl particles. At low Cl− ion concentration, the surface energy of the (100) surface is lower than those of the (110) and (111) surfaces, which is favorable for forming {100}-facet-exposed cubic AgCl grains. Excess Cl− ions can reverse the energy order between the (100) surface and the (110) or (111) surface, leading to the formation of AgCl polyhedral grains. The morphological structure of Ag NPs formed on the AgCl surface was probed for the first time by atomic force microscopy. It was revealed that the Ag NPs exist as up-and-down mounds anchored on the surface of AgCl grains. The prepared cubic Ag@AgCl exhibits excellent photocatalytic activity for the degradation of AO7, phenol, and 2,4-DCP. Radical-trapping experiments and ESR tests with DMPO confirmed that •O2− and Cl0 are likely to be the reactive species responsible for the degradation of organics. The synthetic route explored in this study could potentially be used to synthesize other morphology-controllable silver salt plasmonic photocatalysts.

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ASSOCIATED CONTENT

* Supporting Information S

Preparation of N-doped TiO2. Method for cycle degradation experiments. XRD patterns of sample Ag@AgCl: (A) with enlarging Y coordinate and (B) standard XRD pattern of Ag. XPS survey spectra of AgCl and Ag@AgCl. SEM image and XRD pattern of Ag@AgCl grains after cycle degradation experiments. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (B.T.), [email protected]. cn (J.Z.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21277046, 21047002, 21173077), the 219

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