Ag Plasmonic Photocatalyst with Enhanced - American Chemical

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Facile Synthesis of the Novel Ag3VO4/AgBr/Ag Plasmonic Photocatalyst with Enhanced Photocatalytic Activity and Stability Qing Zhu,† Wan-Sheng Wang,† Ling Lin, Gui-Qi Gao, Hong-Li Guo, Hong Du, and An-Wu Xu* Division of Nanomaterials and Chemistry, Hefei National Laboratory for Physical Sciences at Microscale, University of Science and Technology of China, Hefei 230026, China S Supporting Information *

ABSTRACT: A novel ternary plasmonic Ag3VO4/AgBr/Ag hybrid photocatalyst was successfully fabricated via an in situ anion-exchange reaction between Ag3VO4 and KBr, followed by light reduction. The obtained samples were characterized by X-ray diffraction, scanning electron microscopy, energydispersive X-ray spectroscopy, UV−visible diffuse-reflectance spectroscopy (UV−vis DRS), and X-ray photoelectron spectroscopy. The photocatalytic activities of obtained photocatalysts were measured by the degradation of Rhodamine B and methylene blue under visible-light irradiation (λ ≥ 400 nm). As-prepared Ag3VO4/AgBr/Ag plasmonic photocatalysts exhibit wide absorption in the visible-light region and display superior visible-light-driven photocatalytic activities in degradation of organic contamination compared with pristine Ag3VO4, Ag3VO4/AgBr, and AgBr/Ag. This enhanced photocatalytic activity is attributed to the synergistic effects between Ag3VO4/AgBr-based heterostructured semiconductor photocatalysis and the surface plasmon resonance (SPR) of Ag nanoparticles (NPs). On the basis of UV−vis DRS and valence band X-ray photoelectron spectroscopy, a possible mechanism of enhanced photocatalytic activity of Ag3VO4/AgBr/Ag is proposed; the vectorial electron transfer driven by the matching band potentials of AgBr and Ag3VO4 and the SPR of Ag NPs contribute to its high photocatalytic activity and the improved stability. Therefore, the present study provides helpful insight into the design of novel and highly efficient visible-light photocatalysts in the future. has been studied by several groups15−17 as potential photocatalysts for splitting water and decomposing organic pollutants under visible-light irradiation. However, its practical application is still limited because of the high electron−hole recombination rate in the photocatalytic reaction. Recently, a series of new hybrid photocatalysts on the basis of surface plasmon resonance (SPR) of noble metal nanoparticles (NPs),18−20 namely, plasmonic photocatalysts,21 have attracted considerable attention due to application in the degradation of various organic pollutants under visible-light irradiation. Xiang et al.22 have reported plasmonic photocatalyst Ag/TiO2 with high activity for degradation of RhB. Chen et al.23 have developed Au/ZrO2 and Au/SiO2 plasmonic photocatalysts with efficient photocatalytic activity under visible-light illumination. Silver halides are well known for their wide applications in photographic films as the photosensitive materials. On absorbing photons, silver halide particles generate electrons in the CB and holes in the VB; subsequently, the photogenerated electrons are trapped by Ag+ ions to form clusters of Ag0 atoms within the silver halide particles. So, pure silver halides can be photodecomposed under light irradiation

1. INTRODUCTION During the past decades, advances in heterogeneous photocatalysis have received considerable attention focused on the viewpoint of environmental accountability and energy conversion. Titanium dioxide (TiO2) is the oldest and still the most widely used photocatalyst1 because of its large availability, low cost, nontoxicity, and relatively high chemical stability.2,3 However, its relatively wide band gap (3.2 eV) significantly limits its photocatalytic applications because only ∼4% of the available sunlight energy can be utilized. There are usually two ways to exploit visible-light-responsive photocatalysts. One is to generate intermediate energy levels between conduction band (CB) and valence band (VB) by doping metal or nonmetal elements to shift the absorption edge into the visible-light range.4−7 However, this way is not so effective because dopants will also serve as sites for electron−hole recombination to decrease photocatalytic activity. Another alternative approach is to develop new materials to effectively utilize the visible light that constitutes 43% of the total sunlight. Because Zou et al.8 reported water splitting into H2 and O2, numerous new visiblelight-driven catalysts have also been reported. Among them, Agbased compounds such as AgMO2 (M = Al, Ga, In),9−11 Ag2CO3,12 and Ag3PO413,14 have been widely studied for their unique crystal structures and several interesting properties including photocatalytic and photochromic activity. Ag3VO4 © 2013 American Chemical Society

Received: January 24, 2013 Revised: February 24, 2013 Published: March 12, 2013 5894

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Figure 1. Typical SEM images of the obtained Ag3VO4 (a), /Ag3VO4/AgBr (b), and Ag3VO4/AgBr/Ag (c). White arrows point to Ag nanoparticles and the scale bar is 200 nm.

the reaction was continued for 30 min under vigorously stirring. Then, the dispersion was irradiated under visible light for 20 min to convert some Ag+ ions on the surfaces of AgBr/Ag3VO4 to Ag0 species. The solution color changed to dark brown, indicating the formation of silver nanoparticles (Ag NPs) on the surfaces of AgBr/Ag3VO4, and the ternary Ag3VO4/AgBr/ Ag hybrid photocatalysts were prepared, after which the photocatalysts were collected by centrifugation and dried at 60 °C for 12 h. 2.3. Photocatalyst Characterization. The X-ray powder diffraction (XRD) patterns of the samples were performed on a Rigaku/Max-3A X-ray diffractometer with Cu Kα radiation (λ = 1.54178 Å); the operation voltage and current were maintained at 40 kV and 200 mA, respectively. The SEM images were taken using a field-emission scanning electron microscope (JSM-6701F, JEOL) operated at an accelerating voltage of 5 kV, and energy-dispersive X-ray spectroscopy (EDS) analyses were performed on a JEOL-2010 microscope with an accelerating voltage of 200 kV. A Shimadzu spectrophotometer (Model 2501 PC) was used to record the UV−vis absorbance spectra of the samples in the region of 300 to 800 nm. BaSO4 was used as the reflectance standard material. The X-ray photoelectron spectroscopy (XPS) was performed at a PerkinElmer RBD upgraded PHI-5000C ESCA system. 2.4. Test of Photocatalytic Activity. The photocatalytic activities were conducted at room temperature with exterior irradiation. The light source was a 500 W Xe lamp (GX-500). In brief, 50 mg of photocatalyst was dispersed in 50 mL of a 10 mg L−1 aqueous solution of RhB (or MB) in a reactor with a double layer cooled by running water to keep the temperature unchanged. Prior to irradiation, the suspensions were magnetically stirred in the dark for 1 h to ensure the establishment of an adsorption/desorption equilibrium between the photocatalyst and dye molecules. Then, the suspension was illuminated by the Xe lamp combined with a UV cutoff filter (λ ≥ 400 nm) with magnetic stirring. At given time intervals, about 3 mL aliquots were sampled, centrifuged, and filtered through a 0.22 μm membrane filter to remove the remaining particles. The degradation of RhB (or MB) was monitored with a UV 1800PC spectrophotometer (Shanghai Mapada Instruments). For comparison, blank experiments without catalyst were also carried out. Additionally, the recycle experiments were performed for five consecutive cycles to test the durability. After each cycle, the catalyst was filtrated and washed thoroughly with distilled water several times to remove residual dye impurities and then dried at 60 °C for the next test.

and are seldom used as photocatalysts. However, recent studies24 have shown that AgBr can exhibit high stability with Ag NPs formed on the surface under successive UV illumination. Inspired by this idea, Ag/AgX (X = Cl, Br) as plasmonic photocatalysts were prepared by Huang’s group25−28 through an ion-exchange reaction between aqueous solutions of Ag2MoO4 and HX, followed by converting some Ag+ ions to Ag0 species via light irradiation. An et al.29 reported a one-pot approach for the synthesis of AgCl/Ag as a highly efficient photocatalyst that can drive degradation of organics under sunlight. Kuai et al.30 also reported that a stable and highly efficient direct sunlight plasmonic photocatalyst Ag/AgBr was successfully fabricated through a facile hydrothermal and subsequently sunlight-induced route. Herein we report a novel Ag3VO4/AgBr/Ag hybrid photocatalyst synthesized via an in situ anion-exchange reaction between Ag3VO4 and Br− ions with subsequent visible-light irradiation. The as-prepared novel nanostructured materials show intrinsic high adsorption for dye molecules, which is crucial for improved photocatalytic performance. Evaluated by degradation of Rhodamine B (RhB) and methylene blue (MB), Ag3VO4/AgBr/Ag plasmonic photocatalyst shows superior photocatalytic activity under visible irradiation (λ ≥ 400 nm), which could be ascribed to the strong SPR effect of metallic Ag component in the hybrid NPs and effective transfer of the photogenerated carriers during the photocatalytic reaction. Notably, no significant loss is observed in the photocatalytic efficiency after five successive cycles, indicating that these novel Ag3VO4/AgBr/Ag heterostructures are very stable.

2. EXPERIMENT SECTION 2.1. Preparation of Ag3VO4. Silver nitrate (AgNO3), sodium orthovanadate (Na3VO4), sodium hydroxide (NaOH), and potassium bromide (KBr) were purchased from Sinopharm Chemical Reagent; RhB and other chemicals were of analytical reagent grade and used without further purification. In a typical synthetic route, AgNO3 (60 mL, 0.1 M) solution was added dropwise into Na3VO4 (20 mL, 0.1 M) aqueous solution under vigorously stirring. The orange precipitates formed and were stirred for another 30 min. Then NaOH solution (5 M) was slowly added to the mixture under stirring until pH 10. After stirring for 2 h at room temperature, the suspension was transferred into a 100 mL Teflon-lined stainless autoclave and heated to 140 °C for 8 h. After cooling to room temperature naturally, the brownish yellow products were collected by centrifugation and then washed with deionized water several times. Finally, the products were dried in air at 60 °C overnight. 2.2. Synthesis of Heterostructured Ag3VO4/AgBr/Ag Photocatalyst. The obtained Ag3VO4 (0.1 g) powders were dispersed in 20 mL of ethylene glycol solution. After an ethylene glycol solution of KBr (10 mL, 25 mM) was added,

3. RESULTS AND DISCUSSION 3.1. Characterization of Catalysts. The microstructures of the obtained samples were examined with scanning electron microscopy (SEM) measurements. As shown in Figure 1a, the 5895

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high-resolution Ag 3d XPS spectra of Ag3VO4/AgBr/Ag. The two peaks at approximately 368.6 and 374.5 eV can be ascribed to the binding energies of Ag 3d5/2 and Ag 3d3/2, respectively. The 3d5/2 and Ag 3d3/2 peaks can be further divided into four different bands, that is, 368.1, 368.6, 374.1, and 374.7 eV, respectively. The bands at 368.1 and 374.1 eV are attributed to Ag+, and those at 368.6 and 374.7 eV are attributed to Ag0 species.32 On the basis of XPS peak areas, the mole ratio between Ag0 and Ag+ is calculated to be 1:10.7. The Br 3d XPS peaks can also be resolved into two typical peaks, 68.8 and 69.8 eV (Figure 3c), which are ascribed to AgBr.33 In addition, as shown in Figure 3d, two peaks at 516.55 and 524.4 eV are separately attributed to V 2p5/2 and V 2p3/2 of V5+ in Ag3VO4.17 To study the optical response of the obtained samples, we measured the UV−vis absorption spectra. As shown in Figure 4, the Ag3VO4 sample prepared by hydrothermal method displays absorbance in both the ultraviolet and visible regions. After loading with AgBr, the absorbance curve of Ag3VO4/AgBr can be thought to be the superimposition of the spectrum contributed from AgBr and Ag3VO4. In contrast with Ag3VO4 and Ag3VO4/AgBr, the Ag3VO4/AgBr/Ag sample exhibits a much stronger response in the whole visible region, which should be attributed to the SPR of the Ag NPs on the surfaces of Ag3VO4/AgBr/Ag. Remarkable absorption enhancement in visible-light region is beneficial for improving photocatalyic activity. 3.2. Test of Photocatalytic Activity. The photocatalytic activity and stability of the as-prepared Ag3VO4/AgBr/Ag photocatalysts were evaluated by photodegradation RhB (or MB) dye under visible-light irradiation. For comparison, the performances of pure Ag3VO4, Ag3VO4/AgBr, and Ag/AgBr photocatalysts were also investigated. Total concentrations of dye aqueous solution were simply determined from the maximum absorption (λ = 554 nm for RhB and λ = 664 nm for MB) measurements by UV−vis spectra. C/C0 was used to describe the degradation, which stands for the concentration ratio after and before a certain period of reaction time. As shown in the Figure 5a, Ag3VO4/AgBr/Ag hybrid displays superior photocatalytic activity, and ∼96.5% of RhB dye molecules are degraded only in 15 min. The characteristic absorption band of RhB at 554 nm diminishes quickly, accompanied by a slight concomitant blue shift from 554 to 496 nm of the maximum absorption. The inset photographs show the corresponding color changes of the dye solution with the reaction time increasing (Figure 6). This hypsochromic shift of the major absorption peak has been attributed to a stepby-step de-ethylation derivative generated from the RhB reduction process.33,34 Whereas, in the case of Ag3VO4/AgBr, ∼86.8% of RhB dye molecules is degraded under visible-light irradiation in 15 min, and it takes ∼25 min to completely decompose the dye molecules. In contrast, only 36.6 and 75% of RhB dye molecules could be decomposed within 30 min for Ag3VO4 and Ag/AgBr, respectively. Additionally, as shown in Figure 5b, the experimental results for the MB degradation under the same conditions clearly indicate that their photoreactivity order is highly consistent with the above results for the RhB degradation; UV−vis absorption peak intensity decreases in the MB solution (Figure S3 of the Supporting Information) when exposed to visible light in the presence of Ag3VO4/AgBr/Ag photocatalyst. These results clearly demonstrate that coupling Ag/AgBr with

as-prepared Ag3VO4 sample displays polyhedral shapes with their sizes in the range of 1−1.5 μm. After reacting with KBr, it can be clearly seen that the diameter of the obtained Ag3VO4/ AgBr/Ag particles is decreased to about 800 nm, as shown in Figure 1b. Figure 1a and b show that the obtained Ag3VO4 and Ag3VO4/AgBr particles have a relatively smooth surface. Typical SEM image of the obtained Ag3VO4/AgBr/Ag sample is shown in Figure 1c, and it can be clearly seen that small Ag NPs with diameters in the range of 10 − 20 nm are well scattered over the surfaces of Ag3VO4/AgBr binary heterocrystals, as indicated by white arrows. To investigate the crystallographic structure of the asobtained products, we show the XRD patterns of as-prepared Ag3VO4, AgBr/Ag3VO4, and Ag3VO4/AgBr/Ag products in Figure 2, respectively. Figure 2a shows the typical XRD pattern

Figure 2. XRD patterns of the obtained Ag3VO4 sample (a), Ag3VO4/AgBr (b), and Ag3VO4/AgBr/Ag (c).

of the Ag3VO4 sample, and all of the diffraction peaks could be indexed as the monoclinic phase of α-Ag3VO4 (JCPDS no. 43− 0542) with the lattice constants a = 8.735 Å, b = 6.708 Å, and c = 6.516 Å. The XRD pattern of Ag3VO4/AgBr sample is shown in Figure 2b, with peaks with 2θ values of 26.7, 44.3, and 55.0 corresponding to (111) (220) and (222) crystal planes of cubic AgBr (JCPDF no. 79-0149), respectively, indicating that AgBr crystals are generated after reacting with Br−. The diffraction peak at 31° assigned to the (200) crystal plane of AgBr is overlaid by the strong diffraction peak at 30.9° of α-Ag3VO4. Figure 2c displays XRD pattern of the as-prepared Ag3VO4/ AgBr/Ag sample; no peaks assigned to Ag0 were detected in the XRD pattern (Figure 2c), probably due to its relative low content and high dispersity.32 The chemical compositions of Ag3VO4/AgBr/Ag were further analyzed using energy-dispersive spectroscopy (EDS) (Figure S1 in the Supporting Information). The peaks of the elemental Ag and V are detected in the EDS spectrum. Except for V, Ag peaks coming from the Ag3VO4, the Br diffraction peaks coming from AgBr are also observed, further confirming that the ion-exchange reaction could readily take place and AgBr is produced. (The signals of C and Cu elements come from the carbon-coated copper grid.) To investigate the elemental compositions and chemical states of Ag3VO4/AgBr/Ag, we carried out XPS measurements, and the results are shown in Figure 3. The obvious peaks of C, O, Br, V, and Ag in the survey spectrum of Ag3VO4/AgBr/Ag sample can be clearly detected (Figure 3a). Figure 3b shows the 5896

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Figure 3. Survey XPS spectrun (a), Ag 3d XPS spectra (b), Br 3d XPS spectra (c), and V 2p and O 1s spectra (d) of the as-prepared Ag3VO4/AgBr/ Ag sample.

Figure 4. UV−vis diffuse-reflectance spectra of Ag3VO4, Ag3VO4/ AgBr, and Ag3VO4/AgBr/Ag.

Figure 5. Photocatalytic degradation of (a) RhB and (b) MB in the presence of pure Ag3VO4 (●), Ag/AgBr (▲), AgBr/Ag3VO4 (▼), and Ag3VO4/AgBr/Ag (◆) under visible-light irradiation.

Ag3VO4 photocatalytic system can greatly enhance their photocatalytic activity. In addition to photocatalytic activity, the stability of photocatalysts is another important issue for their practical applications. Therefore, we carried out cycling experiments by evaluating the decreased concentration of RhB under visiblelight irradiation, and all processes and parameters were kept unchanged during the cycling tests. As shown in Figure 7, Ag3VO4/AgBr/Ag sample for the degradation of RhB shows a slight decline after five cycling experiments tests; ∼92% of the original RhB is degraded, whereas it is 99.3% for the first run, confirming that our heterostructured Ag 3VO4/AgBr/Ag

plasmonic photocatalysts are stable enough during the photocatalytic process. It is noted that silver-based photocatalyst is usually unstable.35 Our heterostructured Ag3VO4/ AgBr/Ag plasmonic photocatalysts exhibit the improved stability because the AgBr shell grows on the surface of Ag3VO4, thus protecting Ag3VO4 from decomposition, which is due to the fact that AgBr is more stable than Ag3VO4. Therefore, it is not surprising that our Ag3VO4/AgBr/Ag sample for the degradation of RhB shows a slight decline after five cycles. 3.3. Proposed Mechanism for Enhanced Photocatalytic Activity. To explain the photocatalytic processes 5897

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Figure 6. UV−vis spectra of RhB in aqueous Ag3VO4/AgBr/Ag dispersions as a function of irradiation time with visible light (λ ≥ 400 nm).

Figure 8. VB-XPS spectra of the (a) AgBr and (b) Ag3VO4. The Fermi level EF is located at E = 0 eV, as marked by the vertical dot line.

see reactions 1−3. Charge carriers can emigrate to the surface of the semiconductor under the action of electric field or recombine with energy dissipation (reaction 4). Under the visible-light irradiation, photogenerated electron−hole pairs are formed on the surface of Ag NPs, and the electrons transfer from the photoexcited Ag NPs to the CB of AgBr (reaction 5),31,42 then further transfer from AgBr to the CB of Ag3VO4. Holes migrate from Ag3VO4 to VB of AgBr simultaneously (reactions 6 and 7), which promotes the effective separation of photoexcited electron−hole pairs and decreases the recombination of electron−hole pairs effectively. As a consequence, a relative high concentration of electrons on the surface of Ag3VO4 is formed, which can be trapped by O2 and H2O at the surface of photocatalysts to form •O2−, •OH radicals and other reactive oxygen species. These reactive oxygen species also help the degradation of dye molecules (reactions 8−10). Some holes located on the VB top of AgBr could combine with Br− to form Br0 atoms. As Br0 atoms are reactive radical species and are able to oxidize dye molecules directly (reaction 11) then become reduced to Br− again, according to the previous report,26,43 the detailed roles of Br0 radical species need further clarification in future work. Whereas other holes accumulated in AgBr could react with surface-bound H2O or OH− in the solvent, generating •OH radicals adsorbed at the surface (reactions 12 and 13), which is an extremely strong oxidant for degradation of organic pollutants due to their high oxidizing potential (2.80 V).44,45 The reactive species, such as Br0, •OH, •O2−, and holes, could attack the organic pollutants (P) until dye molecules are completely degraded (reaction 14).

Figure 7. Cycling runs in the photodegradation of RhB in the presence of Ag3VO4/AgBr/Ag under visible-light irradiation.

of Ag3VO4/AgBr/Ag as a photocatalyst toward decomposing organic contaminants, we carried out band position calculations were. The band gap energy estimated from the intercept of the tangents to the plots is 2.16 eV for pristine Ag3VO4 (Figure S4 of the Supporting Information), which is consistent with the previous report.15 The AgBr has a relatively larger band gap (Eg = 2.6 eV) compared with Ag3VO4.36,37 To further clarify the migration direction of the photogenerated charge carriers, we provided valence band X-ray photoelectron spectroscopy (VB XPS) to investigate the electronic structure of the as-prepared samples.38−40 Figure 8 shows the VB XPS spectra near the Fermi level. By extrapolating the leading edge of the VB to its intersection with background counts near the Fermi level, the position of the Fermi level with respect to the VB maximum (VBM) could be determined. The Fermi level is located at 0.97 eV above the VBM for the AgBr, whereas it is 1.11 eV for the Ag3VO4, which is in agreement with the previous study.41 On the basis of the above results, the relevant reactions during photodegradation process are given by eqs 1−14, where P represents the organic pollutants. First, the photocatalytic reaction is initiated by the absorption of visible-light photons with energy equal or higher than the band gap, leading to the creation of electron−hole pairs derived from both plasmonexcited Ag NPs and photoexcited semiconductor components; 5898

AgBr + hν → AgBr(e + h)

(1)

Ag 3VO4 + hν → Ag 3VO4 (e + h)

(2)

Ag + hν → Ag*

(3)

e + h → heat

(4)

Ag* + AgBr → AgBr(e) + Ag +*

(5)

AgBr(e) + Ag 3VO4 → Ag 3VO4 (e) + AgBr

(6)

Ag 3VO4 (h) + AgBr → AgBr(h) + Ag 3VO4

(7)

O2 + e → •O2−

(8)

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•O2− + H+ → HO2 •

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hanced photocatalytic activity for degradation of organic contaminants compared with prestine Ag3VO4, Ag3VO4/AgBr, and Ag/AgBr under visible-light irradiation. Even after five cyclings, the photocatalytic activity does not show any significant decrease. On the basis of the experimental and theoretical calculating results, the CB and VB energy levels of AgBr are more negative than those of Ag3VO4, which is beneficial for charge transfer and suppressing recombination of photogenerated electron−hole pairs effectively. Thus, we attribute this enhanced photocatalytic performance and good structural stability to synergistic effects between Ag3VO4/AgBrbased heterostructured semiconductor photocatalysis and SPR of Ag NPs. It can be expected that this kind of Ag3VO4/AgBr/ Ag plasmonic photocatalyst may provide a new route for designing and fabricating other metal NPs-based heterostructured semiconductor photocatalysts.

(9)

2e + HO2 • + H+ → •OH + OH−

(10)

AgBr(h) + Br − → AgBr + Br 0

(11)

AgBr(h) + H 2O → H+ + •OH

(12)

AgBr(h) + OH− → •OH

(13)

Br 0, •OH, •O2− , h, + P → degraded products (P = organic pollutants)

(14)

Taken together, a plasmon-induced charge-transfer process is proposed for enhanced photocatalytic activity. As shown in Scheme 1, under visible-light irradiation, photogenerated

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ASSOCIATED CONTENT

S Supporting Information *

Scheme 1. Schematic Photocatalytic Reaction Processes and Charge Transfer of the Ag3VO4/AgBr/Ag Hybrid Photocatalyst under Visible-Light Illumination

EDS spectrum, SEM image, and XRD pattern; UV−vis spectra and photographs; and band gap calculation. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: 86-551-63602346. Fax: 86-551-63600246. E-mail: [email protected]. Author Contributions †

Qing Zhu and Wan-Sheng Wang contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Support from the National Basic Research Program of China (2011CB933700, 2010CB934700) and the National Natural Science Foundation of China (21271165) is gratefully acknowledged.

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REFERENCES

(1) Fujishima, A.; Honda, K. Electrochemical Photolysis of Water at a Semiconductor Electrode. Nature 1972, 238, 37−38. (2) Fujishima, A.; Zhang, X.; Tryk, D. A. TiO2 Photocatalysis and Related Surface Phenomena. Surf. Sci. Rep. 2008, 63, 515−582. (3) Chen, X. B.; Shen, S. H.; Guo, L. J.; Mao, S. S. SemiconductorBased Photocatalytic Hydrogen Generation. Chem. Rev. 2010, 110, 6503−6570. (4) Yang, K. S.; Dai, Y.; Huang, B. B. Understanding Photocatalytic Activity of S- and P-Doped TiO2 under Visible Light from FirstPrinciples. J. Phys. Chem. C 2007, 111, 18985−18994. (5) Chen, X. B.; Clemens, B. The Electronic Origin of the VisibleLight Absorption Properties of C-, N- and S-Doped TiO2 Nanomaterials. J. Am. Chem. Soc. 2008, 130, 5018−5019. (6) Zhou, J. K.; Zhang, Y. X.; Zhao, X. S.; Ray, A. K. Photodegradation of Benzoic Acid over Metal-Doped TiO2. Ind. Eng. Chem. Res. 2006, 45, 3503−3511. (7) Peng, B.; Meng, X. W.; Tang, F. Q.; Ren, X. L.; Chen, D.; Ren, J. General Synthesis and Optical Properties of Monodisperse Multifunctional Metal-Ion-Doped TiO2 Hollow Particles. J. Phys. Chem. C 2009, 113, 20240−20245. (8) Zou, Z. G.; Ye, J. H.; Sayama, K.; Arakawa, H. Direct Splitting of Water Under Visible Light Irradiation With An Oxide Semiconductor Photocatalyst. Nature 2001, 414, 625−627.

electron−hole pairs are formed on the surface of Ag NPs owing to the SPRs. The Ag NPs convert the plasmonic energy of incident photons into localized SPR oscillations and electrons transfer from the photoexcited Ag NPs to the CB of AgBr. Because the CB of AgBr is more negative than that of Ag3VO4, the electrons would sequentially migrate to the surface of Ag3VO4, which can be trapped by molecular oxygen in solution to form O2− and other oxidative species. Both AgBr (2.6 eV) and Ag3VO4 (2.16 eV) can be excited by visible light. The photoinduced electrons could migrate to the surface of Ag3VO4 from AgBr and are further trapped by O2 to form O2−, preventing their recombination with the interstitial Ag+. Meanwhile, the photoinduced holes in the VB of Ag3VO4 and AgBr could directly oxidize the organic pollutants.

4. CONCLUSIONS A novel and efficient photocatalyst with double visible-light active components has been successfully fabricated via an in situ anion-exchange reaction, followed by light reduction. The Ag3VO4/AgBr/Ag nanocomposite photocatalysts display en5899

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The Journal of Physical Chemistry C

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dx.doi.org/10.1021/jp400842r | J. Phys. Chem. C 2013, 117, 5894−5900