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Colloids and Surfaces A: Physicochem. Eng. Aspects 425 (2013) 99– 107

Contents lists available at SciVerse ScienceDirect

Colloids and Surfaces A: Physicochemical andEngineering Aspects

jo ur nal homep a ge: www.elsev ier .com/ locate /co lsur fa

ecorating hierarchical Bi2MoO6 microspheres with uniformlyispersed ultrafine Ag nanoparticles by an in situ reduction processor enhanced visible light-induced photocatalysis

o Yuan, Changhua Wang ∗, Ying Qi, Xiaolong Song, Kai Mu, Peng Guo, Lingtao Meng,ongmin Xi ∗

ollege of Chemistry and Biology, Beihua University, Jilin 132013, People’s Republic of China

i g h l i g h t s

Synthesis of Ag-loaded Bi2MoO6

nanosheet-based microspheres byan in situ reduction process.Ag/Bi2MoO6 showed enhance photo-catalytic activity.The influence of the content of silverhad been investigated.The synergetic effects between Agand Bi2MoO6 had been discussed.

g r a p h i c a l a b s t r a c t

Decorating hierarchical Bi2MoO6 microspheres with uniformly dispersed ultrafine Ag nanoparticles byan in situ reduction process exhibits enhanced photocatalytic activity under visible light irradiationdue to good visible light absorption capability and excellent charge separation characteristics of theAg nanoparticles.

r t i c l e i n f o

rticle history:eceived 12 November 2012eceived in revised form 21 February 2013ccepted 28 February 2013vailable online 7 March 2013

a b s t r a c t

In this work, we have demonstrated an efficient Sn (II) mediated in situ reduction strategy to assem-ble silver nanoparticles on Bi2MoO6 microspheres. The Ag loading can be readily controlled by varyingthe volume of Ag (NH3)2OH aqueous solution. Meanwhile, the ultrafine silver spherical nanoparticleswith uniform diameters are free of protection ligands and are well dispersed on the surface of Bi2MoO6

nanosheet without aggregation even at high loading. Photocatalytic test demonstrated that as-adopted

eywords:i2MoO6

gicrospheres

hotocatalytic

in situ reduction strategy was an effective means to enhance the photocatalytic activity of Bi2MoO6

nanosheet-based microspheres under visible light irradiation (� > 420 nm). The enhancement perfor-mance is believed to be induced by the intimate contact interface, where silver nanoparticles serve as goodelectron acceptor for facilitating quick photoexcited electron transfer and thus decreasing electron–holerecombination.

n situ reduction

∗ Corresponding authors. Tel.: +86 432 64602528; fax: +86 432 64602528.E-mail addresses: wangch002@yahoo.com.cn (C. Wang), xhm63@126.com

H. Xi).

927-7757/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.colsurfa.2013.02.058

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

The applications in solar energy conversion and degradation ofpollutants by semiconductor photocatalysts have received exten-

sive attention in modern society [1–4]. To utilize solar energy moreeffectively, the development of efficient visible light active photo-catalysts has attracted worldwide attention [5,6]. As an importantAurivillius oxide, Bi2MoO6 has attracted considerable attention

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ecause of its excellent intrinsic properties, such as, dielectricature, catalytic behavior and luminescence. Previously, severaluthors have reported the efficiency of Bi2MoO6 as a photocata-yst under visible light irradiation [7–10]. It has been demonstratedhat Bi2MoO6 acts as active photocatalyst for the degradation ofrganic pollutants. However, the activity of Bi2MoO6 alone is stillot sufficiently high restricted by its low efficiency in separat-

ng photogenerated electron–hole pairs. Considering Bi2MoO6 as possible visible light responding photocatalyst, its photocatalyticfficiencies should be further improved in order to be suitable forractical application.

Recently, semiconductor–metal composites were designed tomprove the catalytic ability inspired by the synergetic actionetween semiconductor and metal component. The synergeticction works via facilitating the charge separation and improv-ng the photocatalytic activity. The noble metal nanoparticleseposited on the semiconductor oxides could act as a sink forlectrons and promote interfacial electron transfer process due tots high Schottky barriers at the metal–semiconductor interface11,12]. There have been reports that noble metal–semiconductoreterojunction such as Pt, Au or Ag deposited on TiO2, ZnO oraTiO3 can be faciley designed in order to enhance the photocat-lytic activity [13–18]. Among these noble metals, Ag is one of theost suitable for industrial applications due to its low cost and low

oxicity. However, up to now few report has been made about theg effects on the photocatalytic performance of Bi2MoO6.

It is also known that the normal chemical or photoreductionpproach is employed for fabrication of semiconductor–metal com-osites. However, to stabilize metal nanoparticles during synthesis,olymers or surfactants are usually employed, which leads to theresence of capping layer at the surfaces of both the metal particlesnd the semiconductor. The unfavorable bad contact would causencertainties in regard to improving the efficiency of charge sepa-ation and transport. Alternatively, photoreduction reaction occursapidly, making it difficult to simultaneously control the nucleationnd growth of the metal nanoparticles. And usually, metal nanopar-icles with uneven size and sererious aggregation are deposited onhe surface of semiconductors. Therefore, direct growth of metalanoparticles with controlled size and dispersion on semiconduc-or substrate through an in situ redox reaction without any foreigntabilizing molecules is highly desired.

In the past, Zhong et al. reported an in situ method to reduced2+ by Sn2+ and subsequently deposit highly dispersed palladiumPd) NPs onto the surface of hydroxyl-group-rich TiO2 precursor19]. Later, Tang et al. made a successful attempt to deposit AgNPsn carbon spheres by using the similar method [20]. These worksmplied that it might be possible to in situ assemble highly dis-ersed AgNPs on the surface of Bi2MoO6 microspheres. Motivatedy these facts, we attempt to develop an in situ reduction approacho assemble ultrafine Ag nanoparticles on three dimensional urchinike Bi2MoO6 microspheres. Photodegradations of rhodamine BRB), methyl orange (MO) and methyl blue (MB) were employedo evaluate the photocatalytic activities of Ag/Bi2MoO6 photocata-ysts under visible-light irradiation (� > 420 nm). Furthermore, theelationship between photocatalytic activity and content of loadedg was also discussed. Last, the possible mechanism of photocat-lytic activity enhancement and the effect of silver deposition werenvestigated on the basis of characterization results.

. Experimental

.1. Materials and instruments

Bi(NO3)3·5H2O, Na2MoO4·2H2O, AgNO3 with analytical gradeere purchased from Tianjing Fuchen Chemical Reagent Co. Ltd.,

chem. Eng. Aspects 425 (2013) 99– 107

China. Ethylene glycol, were purchased from Beijing Chemical Co.Ltd., China. The chemicals with analytical grade were obtained asreagents and used without further purification. Deionized waterwas used throughout this study.

X-ray diffraction (XRD) patterns of the samples were recordedon a Rigaku, D/max-2500 X-ray diffractometer. Surface morphol-ogy of samples was observed with a Hitachi S-4800 field emissionscanning electron microscope (FESEM). Energy-dispersive X-ray(EDX) spectroscopy being attached to FESEM was used to ana-lyze the composition of samples. High-resolution transmissionelectron microscope (HRTEM) images were acquired using aJEOL JEM-2100 instrument working at an acceleration voltage of200 kV. UV–vis diffusion reflectance (DR) spectra of the sampleswere recorded on a Lambda 900 UV–vis–NIR spectrophotometer(PerkinElmer) and BaSO4 was used as reference. Specific surfacearea of samples was measured with a Micromeritics ASAP 2010instrument and analyzed by the Brunauer–Emmett–Teller (BET)equation. The pore size distribution plots were obtained by theBarret–Joyner–Halenda (BJH) model. X-ray photoelectron spec-troscopy (XPS) was performed on a VG-ESCALAB LKII instrumentwith Mg KR ADES (h� = 1253.6 eV) source at a residual gas pressureof below 1 × 10−8 Pa. Photoluminescence spectra were recordedwith a Jobin-Yvon HR800 instrument in macroscopic configurationwith 325 nm line of He–Cd laser.

2.2. Preparation of photocatalysts

The pure Bi2MoO6 samples were synthesized by a solvother-mal method. 0.05 mol of Bi(NO3)3·5H2O was added into a 20 mlof ethylene glycol solution containing dissolved Na2MoO4·2H2Owith the equivalent molar ratio. The mixture was stirred at roomtemperature for 30 min until the solution presented the homo-geneous phase and then transferred into the 25 ml Teflon-linedstainless steel autoclave and kept at 160 ◦C for 12 h. Subsequently,the autoclave was cooled to room temperature gradually. Finally,the precipitate was centrifuged, washed with ethanol and deion-ized water for three times until there were no other possibleimpurities in the product. The resulting product was dried in avacuum oven at 60 ◦C for 2 h.

Ag/Bi2MoO6 composites were prepared by an in situ reduc-tion method. Firstly, 0.01 g of SnCl2 was dissolved in 20.0 mL of5 mM HCl solution. Then, 0.01 g of as-prepared Bi2MoO6 was addedto the above solution and stirred for 6 h at room temperature.After this, the precipitate was recovered by centrifugation, followedby washing with distilled water three times. Thus, the activatedBi2MoO6 was obtained. Afterward, the activated Bi2MoO6 wereadded to 20.0 mL appropriate concentration of Ag (NH3)2OH aque-ous solution under vigorous stirring for about 2 h in dark at roomtemperature. The obtained Ag/Bi2MoO6 composites were washedwith deionized water and ethanol to remove any ionic residualand then dried in a vacuum oven at 40 ◦C for 24 h. A series ofAg/Bi2MoO6 with Ag content of 0.83%, 2.06% and 3.24% (wt. Ag/wt.Bi2MoO6), respectively, were prepared by changing the concentra-tion of Ag(NH3)2OH aqueous solution(0.05, 0.1, 0.2 mM). And, thecatalyst loaded with 0.83%, 2.06% and 3.24% (wt.) Ag was denotedas 0.8%Ag/Bi2MoO6, 2.1%Ag/Bi2MoO6 and 3.2%Ag/Bi2MoO6.

2.3. Photocatalytic test

A 100 mL of dye solution with an initial concentration of10 mg L−1 in the presence of solid catalyst (0.05 g) was filled in aphotoreactor designed with an internal light source (150 W xenon

lamp) with a cutoff glass filter transmitting � > 420 nm surroundedby a water-cooling quartz jacket to cool the lamp. Prior to lightillumination, the suspension was strongly magnetically stirred for30 min in the dark for adsorption/desorption equilibrium. At given

B. Yuan et al. / Colloids and Surfaces A: Physico

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ig. 1. XRD patterns of different samples: (a) Bi2MoO6 (b) 0.8%Ag/Bi2MoO6 (c).1%Ag/Bi2MoO6 (d) 3.2%Ag/Bi2MoO6.

ntervals of illumination, a sample of suspension was taken outnd centrifuged. The clear upper layer solution was analyzed by

Cary 500 UV–vis–NIR spectrophotometer. The dye concentrationas measured at the maximum absorption wavelength of the dyes.

. Results and discussion

.1. Crystal structure

The powder X-ray diffraction (XRD) patterns of the Bi2MoO6,.8%Ag/Bi2MoO6, 2,1%Ag/Bi2MoO6 and 3.2%Ag/Bi2MoO6 areecorded. Fig. 1 shows the XRD patterns of the as-prepared sam-les. All the reflections can be readily indexed to the characteristiceak of Bi2MoO6 [JCPDS file No. 76-2388]. No impurities cane detected from the pattern of Bi2MoO6, which indicates thatell-crystallized Bi2MoO6 can be prepared by the hydrothermalrocess. In addition, there is no any diffraction peak of silverpecies (38.1◦, 44.2◦, 64.4◦, and 77.4◦ for Ag and 32.2◦ for AgCl)an be observed for the silver deposited samples, suggesting thatll the prepared photocatalysts possess the same crystal structure.his may be attributed to the low concentration (0.8–3.2 wt.%) ormall crystal size of silver. Moreover, the changes of all diffractionsnd lattice parameters of Bi2MoO6 in all Ag-loaded Bi2MoO6amples are not detectable, which implies that Ag related speciesre deposited on the surface of Bi2MoO6 rather than doping intohe lattice of Bi2MoO6.

.2. Morphology and microstructure

Bi2MoO6, 0.8%Ag/Bi2MoO6, 2.1%Ag/Bi2MoO6 and.2%Ag/Bi2MoO6 were examined by SEM and their imagesre presented in Fig. 2. It can be seen clearly that both Bi2MoO6nd Ag/Bi2MoO6 samples are microsphere with a rough surface,he diameters of these spheres are in the range of 2–3 �m. Higher-

agnification SEM images reveal that these microspheres areomposed of nanosheets with several nanometers in thickness,nd the structure of all Ag/Bi2MoO6 samples (Fig. 2c–h) is similar tohat of the pure Bi2MoO6 (Fig. 2b). These nanosheets align radically

o form hierarchical microspheres. Meanwhile, panels a, b, c and

in Fig. 3 show the energy-dispersive X-ray (EDX) spectra frommages a, c, e and g in Fig. 2, respectively. It is indicated that Bi,, and Mo elements exist in pure Bi2MoO6 microsphere, whereas

chem. Eng. Aspects 425 (2013) 99– 107 101

Bi, O, Mo and Ag exist in Ag/Bi2MoO6 hierarchical microspheres,respectively. In addition, the EDX analysis indicates that theloading percentage of Ag was 0.83, 2.06 and 3.24%, respectively.However, these SEM results did not provide clear informationabout the size distribution of Ag deposits on Ag/Bi2MoO6 samples,TEM and HRTEM studies are necessary to further determine thesize distribution of Ag deposits. Fig. 4 shows TEM images ofBi2MoO6 and 2.1%Ag/Bi2MoO6. It can be seen that surface of pureBi2MoO6 microspheres is composed of nanosheets. In addition,some small nanoparticles are observed in the nanosheets ofAg/Bi2MoO6 (Fig. 4b), indicating that Ag nanoparticles are highlydispersed on the surface of the Bi2MoO6 microspheres, which iswell consistent with the EDX results. Under higher magnification,the very small Ag NPs (3–5 nm) with well-dispersed distributionare located on the outside surface of Bi2MoO6 microspheres(Fig. 4c). A representative high resolution TEM (HRTEM) image isshown in Fig. 4d. The two types of lattice fringes are clearly visiblewith a spacing of about 0.23 and 0.315 nm, which correspond tothe lattice spacing of the (1 1 1) planes of Ag and (1 3 1) latticespacing of the orthorhombic phase of Bi2MoO6. The TEM image ofthe Ag/Bi2MoO6 reveals that Ag nanoparticle is deposited on thesurface of Bi2MoO6, coinciding with the above characterizationresults.

3.3. XPS spectra

Further evidence for the chemical composition and oxidationstates of the as-prepared 2.1%Ag/Bi2MoO6 was obtained by theX-ray photoelectron spectroscopy (XPS) technique. The XPS of2.1%Ag/Bi2MoO6 in Fig. 5a shows the main peaks of Bi4f, Mo3d,O1s, Ag3d and C1s. This Ag3d peak confirms the existence of Agelement in the Ag/Bi2MoO6 and the C 1s peak resulted from theadsorption of carbon in atmosphere by the catalyst. Fig. 5b showsthe high-resolution XPS of Bi4f. Two specific peaks with the bindingenergy of 159.05 eV and 164.35 eV are found, representing Bi4f7/2and Bi4f5/2 electrons, respectively, which corresponded to Bi(III)according to the literatures. As for Fig. 5c, the binding energiesat around 232.2 eV and 235.3 eV can be ascribed to Mo 3d. As forFig. 5d, the fitted peak at 530.9 eV is ascribed to the Bi O bonds inBi2MoO6, while the peak at 532.1 eV toward higher binding energyis probably ascribed to the O H bonds, respectively. Fig. 5e showsthe high-resolution XPS of Ag3d. Two peaks from Ag 3d5/2 and 3d3/2were observed at 368.7 eV and 374.5 eV, and the splitting of the 3ddoublet is 6.1 eV, indicating that Ag ions in the in situ reductionreaction were completely reduced to Ag0, And Ag species may bemerely dispersed on the surface of Bi2MoO6 crystal without dopinginto the crystal lattice. [21,22] The calculated content of Ag0 is2.13 wt.%, coinciding with the above EDX results.

3.4. Specific surface areas and pore structure

N2 adsorption–desorption isotherms (Fig. 6a) and the corre-sponding BJH pore size distribution plots (Fig. 6b) of the as-obtainedsamples are performed to determine the surface area of thecomposites, which can further explore the possibility of intercon-nected pores in the Ag/Bi2MoO6 spheres. The BET surface areasof the prepared Bi2MoO6, 0.8%Ag/Bi2MoO6, 2.1%Ag/Bi2MoO6 and3.2%Ag/Bi2MoO6 are 37.6, 38.2, 39.5 and 37.9 m2 g−1, respectively.It is noteworthy that the BET surface areas of the Bi2MoO6 andAg/Bi2MoO6 spheres are much higher than that of the previouslyreported samples [23]. This is attributed to the fact that the lat-ter has a comparatively compact structure while the former has a

loose structure. In addition, the corresponding pore size distribu-tion curve for as-obtained samples are obtained by the BJH method,and indicates a relatively narrow pore size distribution centeredat about 20 nm. The larger BET surface area and porous structure

102 B. Yuan et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 425 (2013) 99– 107

d) 0.8%

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Fig. 2. SEM images of different samples: (a, b) Bi2MoO6 (c,

ould facilitate more efficient contact of Ag/Bi2MoO6 spheres withrganic contaminants and thus improve its photocatalytic activity.

.5. UV–vis diffuse reflectance spectra and band gap

To investigate the optical properties, the as-prepared samplesre analyzed by UV–vis diffuse reflectance spectra in the wave-ength range of 200–800 nm, and the UV–vis diffuse reflectancepectra of Bi2MoO6 and Ag/Bi2MoO6 samples are shown in Fig. 7.

Ag/Bi2MoO6 (e, f) 2.1%Ag/Bi2MoO6 (g, h) 3.2%Ag/Bi2MoO6.

In this work, the band gap of the pure Bi2MoO6 is calculated tobe about 2.64 eV, which is consistent with previous studies. Itis apparent that the absorption edges of all the samples loadedwith Ag show the slight red-shift with the increase of Ag content,the absorption becomes stronger in the visible light range. These

may be attributed to the surface plasma resonance (SPR) effectof spatially confined electrons in metallic Ag nanoparticles. Theseresults support that all these samples have suitable band gaps,which is able to be activated by visible-light for photocatalytic

B. Yuan et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 425 (2013) 99– 107 103

Fig. 3. EDX spectra of different samples: (a) Bi2MoO6 (b) 0.8%Ag/Bi2MoO6 (c) 2.1%Ag/Bi2MoO6 (d) 3.2%Ag/Bi2MoO6.

Fig. 4. (a) TEM image of Bi2MoO6; (b) TEM image of 2.1%Ag/Bi2MoO6; (c) high magnification TEM image of 2.1%Ag/Bi2MoO6; (d) HRTEM image of the 2.1%Ag/Bi2MoO6.

104 B. Yuan et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 425 (2013) 99– 107

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Fig. 5. XPS spectra of the 2.1%Ag/Bi2MoO6:(a) su

ecomposition of organic contaminants. It is also suggested thathe visible-light-responses of Ag/Bi2MoO6 composites are signif-cantly improved by the Ag doping and therefore it should haveetter photoactivity than the pure Bi2MoO6.

.6. Photocatalytic activity and postulated mechanism ofhotodegradation of dyes

The photocatalytic activities of the samples were evaluated byeasuring the decomposition of RB, MO and MB as the decom-

osed target molecules in an aqueous solution under visible light

rradiation. The blank test in Fig. 8a–c shows that the three dyes can-ot be degraded under visible light irradiation without catalysts,hich indicate that the three dyes have high structural stability.

ig. 8a exhibits the photocatalytic activities of different catalysts

pectrum, (b) Bi 4f, (c) Mo 3d, (d) O 1s, (e) Ag 3d.

in the degradation of RB under visible light irradiation. It can beseen that the RB degradation over pure Bi2MoO6 is 64.9% after3 h of visible-light irradiation. Compared to this, the Ag deposi-tion has a significant enhancement. That is, when the depositionis 2.1%, the catalytic activity is up to 97.1%. Moreover, the photo-catalytic efficiency increases with the increased of Ag content from0.8 to 2.1 wt.% and further decreases with the increase of Ag con-tent from 2.1 to 3.2 wt.% suggesting that the Ag content of 2.1 wt.%is the best value to enhance the photocatalytic activity of Bi2MoO6.Fig. 8b and c display the photodegradation of MO and MB as afunction of irradiation time. After irradiation for 3 h, about 64.9%

of MO and 69.3% of MB are decomposed for Bi2MoO6. In compar-ison, the Ag/Bi2MoO6 composites show much better activity thanpure Bi2MoO6 and the optimal deposited content of silver is foundto 2.1%. For 2.1%Ag/Bi2MoO6, the MO and MB removal efficiency

B. Yuan et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 425 (2013) 99– 107 105

ifferen

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Fig. 6. (a) N2 adsorption–desorption isotherm curves of d

eaches 86.6% and 98.3%, respectively. Meanwhile, the reaction rateonstants (kapp) of the samples were calculated, and the resultsere summarized in Fig. 8d. It is showed that the kapp for the

s fabricated Ag/Bi2MoO6 photocatalysts are higher than that ofingle component Bi2MoO6. More importantly, these results fur-her demonstrates that the 2.1%Ag/Bi2MoO6 possessed the highesthotocatalytic activity in our present work.

Additionally, it can be seen that the photocatalytic efficiency cane significantly affected by the content of Ag deposition. The effi-iency increases with the increase of Ag content initially and thenchieves a maximum value with 2.1%Ag/Bi2MoO6. Once the con-ent of Ag deposition is further increased, the efficiency decreases.his increase may be related to the capturing of electrons by theeposited Ag to reduce the recombination of hole–electron pairs,24,25] whereas the decrease is probably due to the blocking ofisible light by the over-deposited Ag crystalline [26].

The photocatalytic activity of Bi2MoO6 toward dyes degrada-ion is enhanced by the Ag loading arising from the following

echanisms: (i) Ag nanoparticles could act as electron traps toacilitate the separation of photogenerated electron–hole pairs andromotes interfacial electron transfer process; (ii) The surface plas-on resonance of Ag nanoparticles under visible light irradiationay enhance the photocatalytic activity of Bi2MoO6. A pathway

f enhanced photocatalytic activity over Ag-loaded Bi2MoO6 ishown in Scheme 1. The photogenerated electrons excited to theonduction band of Bi2MoO6 under visible light irradiation are

Fig. 7. UV–vis diffuse reflectance spectra of different samples.

t samples; (b) pore size distribution of different samples.

entrapped by Ag nanoparticles due to its high Schottky barri-ers at the metal–semiconductor interface, and thus reduces therecombination possibility with photogenerated holes. The betterseparation of electrons and holes in the Ag/Bi2MoO6 is confirmedby PL emission spectra of pure Bi2MoO6 and 2.1%Ag/Bi2MoO6(Fig. 9). It is found that 2.1%Ag/Bi2MoO6 exhibits much loweremission intensity than pure Bi2MoO6, indicating that the recom-bination of photogenerated charge carrier is inhibited greatly in theAg/Bi2MoO6. The electrons can be scavenged by present molecularoxygen absorbed on the surface and produce the reactive oxygenradicals, while the valence hole photogenerated on the surface ofBi2MoO6 could not react with OH− or H2O molecules to form •OHradicals. Because the standard redox potential of the valence bandis more negative than that of •OH/HO− [27]. Nevertheless, both •OHand •O2

− do have been detected on the surface of Bi2MoO6 undervisible-light irradiation in aqueous solution [21]. The redox poten-tials of the electrons of conduction band � (e−) is more negativethan � (O2/•O2

−), which allows the yield of •O2− radicals via the

reduction of dissolved O2 by CB electrons. Lately, the yield of •OHfrom •O2

− with the assistance of photoinduced electrons has beenrevealed [28]. Since the Bi2MoO6 (h+) has a redoxpotential of about2.3 V vs. NHE, it could oxidize a suitable substrate (R), together withrecovery of the original Bi2MoO6.

Besides, the surface plasmon resonance of Ag metals onBi2MoO6 is excited by visible light, enhancing the surface electron

excitation and interfacial electron transfer [29,30]. The excitation ofplasmon resonance can also contribute to the enhancement in thephotocatalytic activity under visible light irradiation, as reported

Scheme 1. Scheme for the separation and transport of charge carriers under vis-light between Bi2MoO6 and Ag.

106 B. Yuan et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 425 (2013) 99– 107

F er vist

fgaA

ig. 8. Photocatalytic activities of different samples on the degradation of dyes undhe RB, MO and MB decomposition over different samples.

or Ag–TiO2, [30] Au–TiO2 [31] and Pt–TiO2 [32]. Thus, we sug-

est that the cooperative effects of Ag deposits according to thebove two mechanisms lead to the significant enhancement in theg/Bi2MoO6 photocatalytic activity under visible light irradiation.

Fig. 9. PL spectra of different samples.

ible-light irradiation: (a) RB; (b) MO; (c) MB. (d) The apparent rate constants (K) of

4. Conclusions

In conclusion, we have developed a simple and fast methodto decorate Bi2MoO6 microspheres with silver nanoparticles. Themethod is based on in situ Ag (I) reduction by Sn (II) species thatare linked to the surface of the Bi2MoO6 microspheres, allowinguniform control of the size and density of silver nanoparticles overthe large surface area of the Bi2MoO6 nanosheet surface. Charac-terization results reveal that the as-synthesized heterostructurespossess following attractive features:

(1): High surface area caused by ultrafine Ag nanoparticles(several nanometers in diameter); (2) Active sites on Ag nanoparti-cles not blocked by the protection ligands, inferring high catalyticactivity; (3) Facile mass transfer due to the well-dispersed Agnanoparticles onto the Bi2MoO6 nanosheet surface without aggre-gation; (4) Facile separation and recycling of the catalyst due tothe hierarchical composite structure. We believe that this novelAg/Bi2MoO6 photocatalyst will be promising material for degrad-ing organic pollutants and other applications owing to its multiplebenefits.

Acknowledgement

This work was supported by National Natural Science Founda-tion of China (Grant No. 51102001).

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