Applied Catalysis B: Environmental 164 (2015) 344351

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Applied Catalysis B: Environmental

j ourna l h omepa ge: www.elsev ier .com/ locate /apcatb

Efcien gen(VI) up

Hongyan YinDongxuea State Key Lab al TecChemistry, Chib State Key Lab hineseChinac University of

a r t i c l

Article history:Received 20 JuReceived in re16 SeptemberAccepted 22 September 2014Available online 28 September 2014

Keywords:PhotocatalysisPlasma chemistrySemiconductoHexavalent chSchottky junct

otocepresructu

area (42 times than normal AgCl material), which are the two critical factors to improve the pho-toreduction activity. Further, the Schottky junction of the plasmonic system facilitates separation ofphoto-induced electrons and holes, which absolutely benets the photocatalysis reactions. The pho-tocurrent intensity, kinetic responses with apparent kinetic rate constant (kapp) and apparent quantumefciency (AQE) (within 6 min) of AgCl:Ag-Hollow NCs were about 48, 5.3 and 2.7 times as that of nor-mal AgCl material, respectively. Most strikingly, we anticipate such AgCl:Ag-Hollow NCs might become

1. Introdu

Over thhas been a coping with[16]. Withindustrial rpollution inronment, eslive and deptaminant inleather tanntion, etc. [7,invade the levels, whicgenic primethe removatreatment,

CorresponE-mail add

http://dx.doi.o0926-3373/ rsromium (Cr (VI))ion

a promising candidate for practical application of water cleansing and environmental pollution purifyingunder visible light irradiation in the future.

2014 Elsevier B.V. All rights reserved.

ction

e past decades, semiconductor-based photocatalysishot research eld due to its potential applications in

the energy crisis as well as environment pollution unceasingly increasing industrial development afterevolution, carcinogenic hexavalent chromium (Cr (VI))cident continuously challenges our fragile living envi-pecially the precious water resource that human beingsend on. It is well known that Cr (VI) is a frequent con-

wastewater arising from industrial processes such asing, paint making, electroplating and chromate produc-8]. What troubled most is that the Cr (VI) ions can easilyhuman food chains and gradually accumulate to toxich becomes the main source of carcinogen and muta-r [9]. Hence, multifarious conventional techniques forl of Cr (VI) have been reported, including biochemistryion exchange, membrane separation, precipitation and

ding author. Tel.: +86 43185262425; fax: +86 43185262800.resses: dxhan@ciac.ac.cn, handongxue1206@gmail.com (D. Han).

adsorption, etc. [10]. Nevertheless, these approaches commonlysubject to large requirement of chemicals, extra organic additives,high cost, and secondary pollution, all of which greatly hindertheir practical applications. Consequently, pursuing environmentalfriendly and efcient technologies for cleaning Cr (VI)-containingwastewater has become an urgent task. Compared with thosemethods, photocatalysis has been proved to be a more attractive,efcient and clean strategy for reduction of toxic Cr (VI) species intothe less harmful benign form, which is of paramount importancefor human health [9,11].

Recently, there arose several research works concerning on pho-toreduction of Cr (VI) contaminants [12,13]. Unfortunately, thosephotocatalysts are far from commercial applications due to theirlong-term preparation process and low efciency. To date, fewworks have been reported on plasmonic photocatalyst with well-dened morphology and excellent performance for photoreductionof Cr (VI) into the benign form. In this case, in the previous investi-gations, we have contributed a lot of theoretical and experimentalefforts to study and improve the silver halide (AgX (X = Cl, Br, I))based plasmonic photocatalysis, such as manipulation of the energyband structure and amelioration of the active crystal plane or hier-archical structure [11,14]. However, there still remains an immense

rg/10.1016/j.apcatb.2014.09.0492014 Elsevier B.V. All rights reserved.tly photocatalytic reduction of carcinoon robust AgCl:Ag hollow nanocrystals

Lia,c, Tongshun Wua, Bin Caia,c, Weiguang Maa,c, Hana,, Li Niua

oratory of Electroanalytical Chemistry, c/o Engineering Laboratory for Modern Analyticnese Academy of Sciences, Changchun, 130022, Jilin, Chinaoratory of polymer physics and chemistry, Changchun Institute of Applied Chemistry, C

Chinese Academy of Sciences, Beijing, 100049, China

e i n f o

ly 2014vised form

2014

a b s t r a c t

Herein, we newly present a robust phexhibit fast photoreduction rate for rform(10 min). The provided hollow stic contaminant Cr

gjuan Sunb,c, Shiyu Gana,c,

hniques, Changchun Institute of Applied

Academy of Sciences, Changchun, 130022, Jilin,

atalyst in terms of AgCl:Ag-Hollow nanocrystals (NCs) whichentative carcinogenic contaminant (Cr (VI)) into the benignre signicantly enhanced light absorption and specic surface

H. Li et al. / Applied Catalysis B: Environmental 164 (2015) 344351 345

space for further development of the AgX-based plasmonic photo-catalysis [15,16]. To meet the high engineering requirements ofthe solar-powered sewage treatment, the catalytic efciency andchemical durability of photocatalyst are the two key factors to becrucially sosuperb phoconductor mcandidates

Consequof AgCl:Ag-monic photCr (III) via athe colorimtocatalytic investigated(VI)-containand in-deptfor the reduand discuss

2. Experim

2.1. Chemic

Sodium purchased ftion. Silver were obtaiChina) and with ultra-p(Millipore).cleaned in and rinsed

2.2. Prepara

The AgCuble salt disome modiwere dissoture with ulthe as-prep100 mL absodiately, the ethanol solAgNO3/PVPsuspensionfor 24 h to fobtained yebreaker andjing Trusttecoreshell sple suspensand the colltimes to remAgCl:Ag-HoAg/AgCl maand NaCl (0temeratureaforementioAg/AgCl maexperiment

2.3. Photoc

The carcAgCl:Ag-Ho

photoreduction was followed with similar that aforementionedabove and equipped with a UV cutoff lter ( 420 nm), andthe irradiation height was 15 cm. In a typical procedure, the as-prepared AgCl:Ag-Hollow NCs (20 mg) were well dispersed into

of Cr(VI) me-mled empDTAreactards,

(DPCxture

in thposehe phle liawnatalyon th

nm)rence

C0 is remelity oas alas wafterlutiotion

ults

ectroties

rphog-Hoand g-Hot dif Nal (0.0moue abs

quicl cryouredagn

ilizef th

of A of t

in Fed hostal rtinged uagelear

cubiuctud TEsolulved [17,18]. Thanks to its facile plasmonic properties,tocatalysis properties and photo-stabilities, AgCl semi-aterial is considered to be one of the most promising

to break through this bottleneck [19,20].ently, in the present work, we newly introduced a kindHollow NCs which has been proved to be a robust plas-ocatalytic reduction agent to reduce the Cr (VI) into the

facile and handy preparation approach. By means ofetrical diphenylcarbazide (DPC) method [21], the pho-activity for reduction of Cr (VI) contaminants has been

and showed that such NCs could facilely clean the Cring waste water within 10 min. Furthermore, a novelh perspective of photocatalytic reduction mechanismction of Cr (VI) on the AgCl:Ag-Hollow NCs is proposeded.

ental

als

chloride (NaCl) and Polyvinylpyrrolidone (PVP) wererom Sigma-Aldrich and used without further purica-nitrate (AgNO3) and Potassium bichromate (K2Cr2O7)ned from Sinopharm Chemical Reagent (Shanghai,used as received. All aqueous solutions were preparedure water (>18.2 M cm) from a Milli-Q Plus system

All glassware used in the following procedures werea bath of freshly prepared HCl:HNO3 (3:1, aqua regia)thoroughly by water prior to use.

tion of AgCl:Ag-Hollow NCs

l:Ag-Hollow NCs were prepared through a water sol-ssolution method according to previous reports withcations [19,20]. Briey, AgNO3 (0.17 g) and PVP (1.5 g)

lved in 20 mL absolute ethanol at ambient tempera-trasonication (the solution became deep brown). Then,ared NaCl saturated aqueous solution was injected intolute ethanol under vigorously magnetic stirring. Imme-white suspension granules were observed formed in theution (NaCl cubic crystal). Soon afterwards, the above

ethanol solution was poured into the NaCl ethanol, and then, the mixture was around-the-clock stirredorm NaCl@AgCl coreshell structure. Subsequently thellow colored NaCl@AgCl emulsion was placed into a

irradiated by Xenon arc lamp (CHF-XM35-500 W, Bei-ch Co. Ltd, China) light for 30 min to form NaCl@AgCl:Agtructure (the solution became purple). Finally, the pur-ion was treated by centrifugation (8500 rpm, 5 min)ections were washed with ultra-pure water for severalove PVP and residual ions (Na+, NO3 and Cl), and thellow purple NCs were achieved. For comparison, normalterials were prepared by mixing AgNO3 (0.01 mol L1).01 mol L1) aqueous solution under stirring in ambient. Some Ag NPs generated on the surface of AgCl duringned Xenon arc lamp light irradiation. And the obtainedterial was denoted as AgCl-Normal for further controls.

atalytic reduction hexavalent chromium

inogen of Cr (VI) was photoreduced with as-preparedllow NCs. In the whole process, the optical system for

20 mL on Cr in a hoassembstant tacid (Eof the afterwbazidethe mi30 minthen exAfter tof visibwithdrphotocbased = 540as refetime t,measureliabiNCs walyst wtimes ture soas men

3. Res

3.1. Elproper

MoAgCl:ASTEM AgCl:Aelemenbility oethanosmall ainto thwouldof NaCwas pously mto stabation osurfaceimagesshowndepictthe crySuppodisplayTEM imcould con thelow strSEM anhigh re (VI) solution under ultrasonication (10 mg L1 basedin a dilute K2Cr2O7 solution as the source of Cr (VI))ade reactor equipped with a cooling water circulator

to keep the whole reaction system maintaining at a con-erature condition. Then, ethylene diamine tetraacetic, 0.01 M) was injected into the mixture, and the pHion suspension was adjusted to 2.0 with HClO4. Soon

the 0.02 M of color developing reagent of diphenylcar-) in acetone was added into the mixture, and the color of

would turn into violet. The suspension was stirred fore dark to reach adsorptiondesorption equilibrium andd to visible-light irradiation to start the photocatalysis.otoreduction experiment was triggered by irradiationght, the solution of aliquots volume were periodically

from the reaction vessel and centrifuged to remove thest powders. And the Cr (VI) reduction was measurede spectrophotometric colorimetrical DPC method (at

by using a UV-Vis spectrophotometer (ultra-pure water). Normally, Ct is the concentration of Cr (VI) solution atthe initial concentration (10 mg L1). The photocatalyticnt mentioned above was repeated twice to ensure thef the results. The reproducibility of the AgCl:Ag-Hollowso tested with the following procedure: the photocat-ashed with ultra-pure water and ethanol for several

photoreduction and then retested in the fresh mix-n of Cr (VI) under the same experimental conditionsed above.

and discussion

nic images, crystal structure and photophysical

logy and element distribution of the as-preparedllow NCs were investigated by SEM, TEM, HAADF-elemental mapping (Fig. 1). The mappings of presentllow NCs are colored to distinctly mark parts of thestribution in this structure. Compared with the solu-Cl in water (26.5 wt%, 20 C), its solubility in absolute65 wt%, 20 C) is much lower. Base on the fact, when ant of saturated aqueous solution of NaCl was injectedolute ethanol with ration volume, the reaction systemkly turn into white color originated from precipitatestal. Subsequently, the AgNO3/PVP in absolute ethanol

into the above-mentioned dispersion under vigor-etic stirring. Herein, PVP served as the capping agent

the generated AgCl particles and prevent agglomer-e resultant [22]. The Ag NPs were generated on thegCl under visible light irradiation (Fig. 1a). The SEMhe AgCl:Ag-Hollow NCs with different resolutions areig. 1bd. It is observed that the AgCl:Ag-Hollow NCsllow cubic structure, which might be beneted frommorphology of NaCl in the synthesis phase (Fig. S1 in

information) [23]. Obviously, the AgCl:Ag-Hollow NCsniform morphology with some pores on the shells. Thes (Fig. 1e) and elemental mapping images (Fig. 1fh)ly reveal that both the Cl and Ag elements distributedc shell instead of center, which further proved the hol-re obtained in such a AgCl:Ag material. In addition, theM images of the as-prepared AgCl:Ag-Hollow NCs withtion were corresponding performed. The pore and Ag0

346 H. Li et al. / Applied Catalysis B: Environmental 164 (2015) 344351

Fig. 1. Electroimages of thetransmission e

species canbombardmTEM analys

The crysNCs and Agray diffractsamples dis27.7, 32.2nic images and element distribution images of the AgCl:Ag-Hollow NCs. (a) Schematic as-prepared AgCl:Ag-Hollow NCs with different resolutions (e) The TEM image of thelectron microscopy (HAADF-STEM) (f) and element mapping images for Ag (g) and Cl (h)

be attributed to the water dissolution process and theent of high energy electron beam during the SEM andis, respectively (Fig. S2 in Supporting information).tallographic structure of as-prepared AgCl:Ag-HollowCl-Normal materials were determined by means of X-ion (XRD) analysis (Fig. 2a). XRD pattern of the twoplayed distinct diffraction peaks at approximately (2), 46.2, 54.8, 57.5, 67.5, 74.4 and 76.7, which can

be assignedand (4 2 0) ctions of cryS3 (in Supption peaks can be assital phases diffraction illustration of formation of the AgCl:Ag-Hollow NCs. (bd) The SEM AgCl:Ag-Hollow NCs. (fh) High-angle annular dark-eld scanning

distribution in the AgCl:Ag-Hollow NCs. The scale bar is 200 nm.

to (1 1 1), (2 0 0), (2 2 0), (3 1 1), (2 2 2), (4 0 0), (3 3 1)rystal phases respectively, corresponding to the diffrac-stalline AgCl (JCPDS le: 31-1238) [24]. As shown in Fig.orting information), the NaCl@AgCl displayed diffrac-(2) at 31.9, 45.7, 56.7, 66.4 and 75.5. These peaksgned to (2 0 0), (2 2 0), (2 2 2), (4 0 0) and (4 2 0) crys-respectively, which was corresponding to the crystalof NaCl (JCPDS le: 5-0628) [20]. Comparing the XRD

H. Li et al. / Applied Catalysis B: Environmental 164 (2015) 344351 347

Fig. 2. XRD an31-1238). (bd

spectra of Ahave been cAdditionall38.1 and 4can be assig(JCPDS le:detected onsmall amoucess. More in these twcrystal phasonly compo

X-ray phther investstates of thetrum (Fig. 2the main inthe results oO 1s peaks athe XPS instconsists of tto Ag 3d5/2of Ag 3d5/2 aat 367.2 eV,the peaks aof Ag 3d5/2at 368.2 anof metallic d XPS characterizations of the AgCl:Ag-Hollow NCs. (a) XRD patterns of the AgCl:Ag-H) The survey XPS spectrum (b), Ag 3d (c) and Cl 2p (d) of the AgCl:Ag-Hollow NCs.

gCl and NaCl@AgCl (Fig. S3), it revealed that NaCl coreompletely removed after the water dissolution process.y, as shown in Fig. 2a, the weak diffraction peaks (2) at4.3 were founded in the AgCl:Ag-Hollow NCs, whichned to (1 1 1) and (2 0 0) crystal phases of metallic Ag

65-2871). It is noticeable that the Ag0 NPs were not the surface of AgCl-Normal material due to the verynt of Ag generated under visible light irradiation pro-specically, we did not nd other characteristic peakso crystal structures, such as impurities or other silveres, which demonstrated that the obtained product wassed of metallic Ag and AgCl.otoelectron spectroscopy (XPS) was performed to fur-igate the surface chemical composition and chemical

as-prepared samples (Fig. 2bd). The survey XPS spec-b) of the as-prepared AgCl:Ag-Hollow NCs conrmedgredient elements of Ag and Cl, which agreed well withf elemental mapping analysis (Fig. 1eg). The C 1s andre probably due to the adventitious hydrocarbon fromrument itself. Fig. 2c shows that the spectrum of Ag 3dwo peaks at 367.2 eV and 373.2 eV, which are ascribedand Ag 3d3/2, respectively [25]. Furthermore, the peaksnd Ag 3d3/2 could be further divided into different peaks

368.2 eV and 373.2 eV, 374.2 eV, respectively. Namely,t 367.2 eV and 374.2 eV could be attributed to the peaksof AgCl and Ag 3d3/2 of metallic Ag0, whereas the peaksd 373.2 eV could be ascribed to the peaks of Ag 3d5/2Ag0 and Ag 3d3/2 of AgCl. For Cl 2p (Fig. 2d), two peaks

were obtainsponding toenergies co

The phyAgCl:Ag-Hoshown in Fthe dispersthe megascthe commoSupportingcoloured NC

To autheof AgCl:Ag-catalyst samA strong anAgCl:Ag-Horegion is muP25 (Fig. 3bHollow NCabsorption the inner oshell followstructure, tthis is also(ii) In additeffect of thof the optiton path leollow NCs in comparison with AgCl-Normal materials (JCPDS le:

ed at binding energies of 197.6 eV and 199.2 eV, corre- Cl 2p3/2 and Cl 2p1/2, respectively. All the XPS bindingincide well with the reported literature [25,26].sical properties and light-absorption properties of thellow NCs, AgCl-Normal and commercial P25 wereig. 3. As shown in Fig. 3a, it can be clearly seen thation and desiccation of AgCl:Ag-Hollow NCs performedopic purple color, which is conspicuously different fromnly observed white AgCl-Normal materials (Fig. S4 in

information). Hence, it is supposed that this distinctives would have a doughty absorption in the visible scope.nticate the aforementioned light absorption propertiesHollow NCs, the absorption spectra of these photo-ples were recorded by a UV-Vis spectrophotometer.d broad absorption peak at about 550 nm upon thellow NCs was obtained, and its absorption in visiblech stronger than those of AgCl-Normal and commercial). The large absorption improvement of the AgCl:Ag-s can be ascribed to three factors: (i) Except directon the surface, the incident light could partially get intof the AgCl:Ag-Hollow NCs structure through the outering with several times reections inside the interiorhus greatly enhanced the optical absorption. Namely,

the typical property of the porous materials [27];ion, according to concerned literatures, Mie scatteringe AgCl:Ag:Hollow NCs also leads to an enhancementcal absorption through the increase of average pho-ngth in the AgCl:Ag:Hollow NCs [2830]; (iii) Most

348 H. Li et al. / Applied Catalysis B: Environmental 164 (2015) 344351

Fig. 3. The phHollow NCs anand desiccatio(DRS) of the A

importantlythe localizesurface of mination [2AgCl:Ag-HoFig. S5 (in Slarge band which madeHowever, sband gap vaKubelkaMthan AgCl-Nsignicantlcan further AgCl:Ag-Ho

3.2. BET sur

The BrBarrettJoinHollow NCnitrogen adisotherms oBrunauerDimplying thmore, the dNCs shows heterogenerials, the Agpressures (informationobserved ato over 10 nand AgCl-Nand 1.382 m

he BET surface area and photoelectric conversion properties of AgCl:Ag-NCs and AgCl-Normal. (a) Nitrogen adsorptiondesorption isotherms.toelectric conversion performances of AgCl:Ag-Hollow NCs and AgCl-materials in 0.1 M Na2SO4 aqueous solutions under visible light irradiation

nm).ysical properties and light absorption properties of the AgCl:Ag-d AgCl-Normal materials. (a) The digital photographs of dispersionn of AgCl:Ag-Hollow NCs. (b) UVvisible diffuse reectance spectragCl:Ag-Hollow NCs, AgCl-Normal and commercial P25.

, the enhancement could be furthermore ascribed tod surface plasmon characteristics of Ag0 NPs on theAgCl, which were generated under visible light illu-9,31]. The content of Ag0 species on the surface ofllow NCs and AgCl-Normal materials are shown inupporting information). Commonly, AgCl material has agap (direct band gap: 5.2 eV, indirect band gap: 3.2 eV),

it show negligible absorption in the visible region [32].

Fig. 4. THollow (b) PhoNormal ( > 420uch AgCl:Ag-Hollow NCs demonstrated a much smallerlue (2.0 eV, by the band-gap estimation according tounk theory, shown in Fig. S6 in Supporting information)ormal and commercial P25 materials, which implies a

y increased light absorption in the visible region [3]. Itexplain the enhanced optical absorption capacity of thisllow NCs material.

face area and photoelectrochemical measurements

unauerEmmetTeller (BET) surface area anderHalenda (BJH) pore structure of the AgCl:Ag-s and AgCl-Normal materials were investigated bysorptiondesorption isotherms (Figs. 4a and S7). Thef the two samples are identied as type IV from theemingDemingTeller (BDDT) classication [33],e presence of mesopores (250 nm) (Fig. 4a). Further-esorption branch of the isotherm for AgCl:Ag-Hollowa inclined ladder state, which indicted the existence ofous pore structure. Compared with AgCl-Normal mate-Cl:Ag-Hollow NCs show higher adsorption at relativeP/P0) around 1.0. As shown in Fig. S7 (in Supporting), the formation of large mesopores can be clearlynd the relative pores size distribution ranged from 2m. The BET surfaces area of the AgCl:Ag-Hollow NCsormal materials were calculated to be 58.737 m2 g12 g1, respectively. It is obviously found that AgCl:

Ag-Hollow that of AgCsites existinessentially

An excetive properexception. AgCl:Ag-Hoover severaunder visibin Supporticurves for tmittent irra48 times hthan that oAgCl:Ag-Hotion turnedturned on amaterial rarespectivelthe AgCl:Agphotodetecphotoelectrand AgCl-Nalytic reducNCs presented much higher specic surface area thanl-Normal, which indicated more photoreduction activeg in such AgCl:Ag-Hollow NCs material and they willbenet to photocatalytic reactions.llent material always accompany with many distinc-ties. Of course, the AgCl:Ag-Hollow NCs are of noHerein, photoelectric conversion experiments of thellow NCs and AgCl-Normal materials were carried outl onoff irradiation cycles in Na2SO4 aqueous solutionsle light irradiation (Experimental details were shownng information). Fig. 4b shows a comparison of the Ithese two samples over several onoff cycles of inter-diation. Or rather, the AgCl:Ag-Hollow NCs exhibit aigher photoelectric current and conversion efciencyf AgCl-Normal material. The photocurrent value of thellow NCs gradually decreased to zero when the irradia-

off, and would return to a constant value when the lightgain. By comparison, the photocurrent of AgCl-Normalpidly increased or decreased with the light on or off,y. Owing to its excellent photo-electric reproducibility,-Hollow NCs showed great potentials in the areas oftor, solar cell, ber-optic communication etc[25]. Theic conversion difference between AgCl:Ag-Hollow NCsormal material is also consistent with their photocat-tion results in the following discussion.

H. Li et al. / Applied Catalysis B: Environmental 164 (2015) 344351 349

Fig. 5. (a) Timcurves for reduAgCl:Ag-Hollophotocatalyst

3.3. Evalua

It is wortby-productand ne cheheavy metahealth and ecation. The would beneour knowle(III) by photviously repo(DPC) in aciwhich dispHerein, thethe concent

Fig. 5a plex compothe AgCl:Ation of Cr (had a suddhalf amoun10 min, Cr (Fig. S8 (in Sucolor decayCr (VI) formduction of Ce-dependent UVvis absorption spectra for photoreduction of Cr (VI) ion over AgCl:Ag-ction of Cr (VI) ion over commercial P25, AgCl-Normal and AgCl:Ag-hollow NCs. (c) Ct /C0w NCs. Normally, Ct stands for the concentration of Cr (VI) at time t, C0 represents to the and the number of successive reaction employing AgCl:Ag-Hollow NCs.

tion of photocatalytic reduction activity

h mentioning that carcinogenic Cr (VI) ion is a common in the industry processes of electroplating, tanning,mical manufacturing. The most important thing is thatl Cr (VI) ions have long-term adverse impacts to humancosystems due to bioaccumulation as well as biomagni-effective conversion of toxic Cr (VI) into the benign formt both environment protection and human healthy. Todge, the carcinogen of Cr (VI) could be reduced to Crocatalysis, and the mechanism of which has been pre-rted [11,21]. Briey, the Cr (VI) and diphenylcarbazided solution would generate a violet complex compound,lays a characteristic absorption peak at max = 540 nm.

variation in the content of Cr (VI) will be equivalent toration changing of the violet complex compound [9].presents the UVvis absorption spectra of the com-und during the photoreduction process of Cr (VI) overg-Hollow NCs. It is clearly seen that the concentra-VI) decreased with the increased irradiation time anden-drop from start to 2 min irradiation, while nearlyt of Cr (VI) was reduced within 4 min irradiation. AfterVI) ion in the reaction solution could not be detected.pporting information) shows the typical photograph of

from violet to pale (before and during the conversion of 0 to 10 min), which indicated the successful photore-r (VI). Fig. 5b displays photoreduction dynamic curves

over commvisible irradreduction amaterials. I10 min irracatalyst, whAgCl-Normiments revein the preseirradiation

Photocaation of Cr(correspondagainst thetionship anequation [8

ln(

C0Ct

)=

where Ct is initial concrate constacorrelationNCs and A0.06 min1,ity of AgCl:AapplicationHollow NCs based on the DPC method. (b) Photoreduction dynamicand ln(Ct /C0) against reaction time for photoreduction of Cr (VI) overinitial Cr (VI) concentration (10 mg L1). (d) kapp and AQE of different

ercial P25, AgCl-Normal and AgCl:Ag-hollow NCs. Underiation, the AgCl:Ag-hollow NCs exhibit higher Cr (VI)ctivity than both AgCl-Normal and commercial P25t is more convincing that Cr (VI) was fully reduced afterdiation by employing AgCl:Ag-hollow NCs as photo-ile only 44.7% and 16.5% of Cr(VI) was reduced over

al and commercial P25, respectively. The control exper-aled that the concentration of Cr (VI) remain unchangednce of AgCl:Ag-hollow NCs without irradiation or underbut without catalysts (data not shown).talytic reduction kinetics was monitored by the vari-

(VI) concentration (Fig. 5c). Furthermore, ln(Ct/C0)ing to ln(At/A0), where A represents to the absorbance)

reaction time for photoreduction exhibits a linear rela-d the kinetic study is described by pseudo-rst order,19], which can be expressed as following:

kappt

the concentration of Cr (VI) at irradiation time t, C0 is theentration of Cr (VI). Then, kapp is the apparent reactionnt, which can be obtained from the slope of the liner

[34]. As shown in Fig. 5d, the kapp of AgCl:Ag-HollowgCl-Normal were calculated to be 0.317 min1 and

respectively. Signicantly, the stability and recyclabil-g-Hollow NCs would also play the key role for practical

s. The plots of kapp against the number of successive

350 H. Li et al. / Applied Catalysis B: Environmental 164 (2015) 344351

Fig. 6. Schem on-eNCs.

reduction reshown in Fhas a little during the SEM, TEM aafter severawere depicillustrated such AgCl:AMoreover, ttoreductionof photons wmethod [11informationHollow NCsof the AgCl-

3.4. Hypoth

Accordinthat the carover the Agcrucial and which are pity as well acrystal can combine witon under vform on theof photons.be generatefaces becauof the AgCl,tion of phophotolysis high stabili

SubsequAg0 nanocluductor due AgCl = 6.3 etrons will twhom withregion untiequilibrated

riumce bous nar th

rang a up, thes sepelecten p

lightAg Nrodunsitardsctionf the ed pcienhilel couhereged be hig-Ho

arethe liatic diagram illustrating the production of Ag NPs on AgCl and the proposed plasm

actions for repeated using of AgCl:Ag-Hollow NCs wereig. 5d. It is noticed that the catalytic efciency onlyshrinkage which can be attributed to a small mass lossreclaim of photocatalyst in recycling reaction process.nd XRD analysis of this AgCl:Ag-Hollow NCs materiall photoreductions were also performed and the resultsted in Fig. S9 and S10 (in Supporting information). Itthat both the morphology and the crystal structure ofg-Hollow NCs material did not signicantly changed.he apparent quantum efciency (AQE) of Cr (VI) pho-

reaction within 6 min was also calculated. The numberas estimated by means of the ferrioxalate actinometer

,35] (the experiment details can be found in Supporting). The results showed that the AQE of the AgCl:Ag-

reached 7.5%, which presented much higher than thatNormal materials (2.8%).

esis of photocatalytic reduction mechanism

g to the above experimental results, it is conrmedcinogenic Cr (VI) ions can be essentially photoreducedCl:Ag-Hollow NCs within 10 min. Furthermore, it is alsonecessary to discuss the possible reaction mechanismsroposed to explain the enhanced photocatalytic activ-s the detailed reduction process. Commonly, the AgClgenerate an electronhole pair and the electron will

+ 0

equilibinterfanumerlish nea longrise toreasoncarrierof the

Whvisiblein the were ptron deAfterwcondulevel orepeatalso efMeanwof AgC(III), wscavention, thAgCl:Asurfacein all, th an Ag to form an Ag species after absorbing a pho-isible light irradiation [29]. Then, Ag0 nanoclusters will

surface of AgCl hollow NCs upon repeated absorption Simultaneously, under irradiation, the electrons wouldd in the Ag NPs and forced far away from the AgCl inter-se of the SPR effect of the Ag NPs and polarization eld

respectively [32] (Fig. 6, left). Whats more, the migra-to-excited electrons away from the AgCl also preventsof AgCl to Ag0 nanoclusters, which nally insured thety of the AgCl:Ag-Hollow NCs [36].ently, as shown in Fig. 6(right), the free electrons of thesters would prefer to ow to the n-type AgCl semicon-to their different work function [37] (Ag = 4.26 eV[36],V [38]). According to the previous report, the elec-ransfer from the material with low work function to

high work function and accumulate in the space chargel the two systems run up to equilibrium and form the

Fermi level [39,40] (EF,Ag, EF,AgCl and EF,eq). Under this

ture, the Schphoto-inducontribute system.

4. Conclus

In summphotoreduchollow NCsignicanceservation. reduction ation of remafacilitates cactivity anThe high pnhanced photoreduction mechanism of Cr (VI) over AgCl:Ag-Hollow

, a Schottky barrier would be formed at the junctionetween semiconductor AgCl and Ag NPs [37,41]. Theanoscale quasi-Helmholtz double layers would estab-e interface and tend to interfere coherently to achievee enhanced local electromagnetic eld, and then giveward bending in the band edges [39,42,43]. Stand to

Schottky junction could facilitate the photo-inducedaration, which can benet to suppress recombinationronhole [31,44].hotoreduction of Cr (VI) reaction is carried out under

irradiation, a number of increased electrons will storePs and thus lifted the Fermi energy level of Ag, whichced by photo-excitation [36]. And the incremental elec-y will disequilibrates the Fermi level of the system [45]., the superuous electrons would be injected into the

band (CB) of the neighbouring AgCl until the Fermisystems runs up to equilibrium again (Fig. 6, right). Thisrocess not only stabilizes the system equilibrium, buttly facilitate the separation of electronhole [15,39]., the enriched photo-excited electrons on the surfaceld be trapped by Cr (VI) ions in solution to form Cras the holes on the valence band (VB) of AgCl would bey EDTA (a hole scavenger) to form EDTA+ [21]. In addi-gher photoreduction activity that performed over thellow NCs photocatalyst can also be ascribed to its largera, which provides copious active sites for catalysis. Allght absorption, coefcient of utilization, hollow struc-

ottky junction, SPR-effect, generation and separation ofced charge carriers, interaction with molecules togetherto the photoactivity enhancement of AgCl:Ag-Hollow

ions

ary, the carcinogenic Cr (VI) has been highly efcientted very fast over eco-friendly as well as robust AgCl:Ags in aqueous solution. And thus it is of paramount

for both the human health and environmental con-Simultaneously, such AgCl:Ag-Hollow photocatalyticgent plays a crucial role in this process. The forma-rkable heterostructure between Ag and AgCl effectivelyharge carrier transfer, which resulted in extremely highd stability in photoreduction of carcinogenic Cr (VI).erformance of catalytic activity should arise from the

H. Li et al. / Applied Catalysis B: Environmental 164 (2015) 344351 351

following factors: (i) the SPR-effects not only drastically enhancedthe visible-light harvesting, but also created an intensive local elec-tric eld which signicantly facilitates the photocatalytic reactions;(ii) the Schottky junction between Ag NPs and AgCl could facili-tate the charge separation, which further benet to suppress therecombination of electronholes; (iii) The hollow structure couldsignicantly enhance the light absorption by multiple reectionsin the interior structure, and amplied specic surface area (42times than normal AgCl material). As expected, the apparent kineticrate constant (kapp) and apparent quantum efciency (AQE) of theAgCl:Ag-Hollow NCs are of 0.317 min1 and 7.5% (within 6 min),respectively. And it demonstrated 5.3 and 2.7 times higher than thenormal Ag/AgCl material. It is expected that this class of AgCl-basedphotocatalyst provides a novel perspective for understanding themechanism of photocatalytic reduction Cr (VI). Thanks greatly to itsenvironmental friendly solvent and handy preparation approachfor AgCl:Ag-Hollow NCs, this kind of photocatalyst might havea wide perspective in mass production and become a promisingcandidate for practical application of water cleansing and envi-ronmental pollution purifying under visible light irradiation in thefuture.

Acknowled

The authNo. 212255of Science 201105031of Sciences

Appendix A

Supplemfound, in th2014.09.049

References

[1] D.M. Schu[2] X. Chen,

6570.[3] Q. Xiang,

299303.[4] Y. Chen, J[5] J.M. Meic

ronmenta[6] S. Wang, D

144 (2014[7] S. Huang,

16 (2014)

[8] Y. Yang, G. Wang, Q. Deng, D.H.L. Ng, H. Zhao, ACS Applied Materials & Interfaces6 (2014) 30083015.

[9] J. Shang, W. Hao, X. Lv, T. Wang, X. Wang, Y. Du, S. Dou, T. Xie, D. Wang, J. Wang,ACS Catalysis (2014) 954961.

[10] Q. Wang, X. Chen, K. Yu, Y. Zhang, Y. Cong, Journal of Hazardous Materials246-247 (2013) 135144.

[11] B. Cai, J. Wang, S. Gan, D. Han, Z. Wu, L. Niu, Journal of Materials Chemistry A 2(2014) 5280.

[12] L. Wang, N. Wang, L. Zhu, H. Yu, H. Tang, Journal of Hazardous Materials 152(2008) 9399.

[13] P. Mohapatra, S.K. Samantaray, K. Parida, Journal of Photochemistry and Pho-tobiology A: Chemistry 170 (2005) 189194.

[14] B. Cai, J. Wang, D. Han, S. Gan, Q. Zhang, Z. Wu, L. Niu, Nanoscale 5 (2013)1098910995.

[15] H. Kisch, Angewandte Chemie 52 (2013) 812847.[16] H. Tong, S. Ouyang, Y. Bi, N. Umezawa, M. Oshikiri, J. Ye, Advanced Materials 24

(2012) 229251.[17] C. Yu, G. Li, S. Kumar, K. Yang, R. Jin, Advanced Materials 26 (6) (2013) 892898.[18] R. Marschall, Advanced Functional Materials 24 (2014) 24212440.[19] C. An, S. Peng, Y. Sun, Advanced Materials 22 (2010) 25702574.[20] Y. Tang, Z. Jiang, G. Xing, A. Li, P.D. Kanhere, Y. Zhang, T.C. Sum, S. Li, X. Chen, Z.

Dong, Z. Chen, Advanced Functional Materials 23 (2013) 29322940.[21] P. Wang, B. Huang, X. Zhang, X. Qin, Y. Dai, Z. Wang, Z. Lou, ChemCatChem 3

(2011) 360364.[22] D. Chen, S.H. Yoo, Q. Huang, G. Ali, S.O. Cho, Chemistry 18 (2012) 51925200.[23] M.R. Jones, K.D. Osberg, R.J. Macfarlane, M.R. Langille, C.A. Mirkin, Chemical

Reviews 111 (2011) 37363827.[24] B. Ma, J. Guo, W.-L. Dai, K. Fan, Applied Catalysis B: Environmental 130131

(2013) 257263.Han, 349ang, Li, Y. Dinic, Phang,iety 7u, C. RHou, Song, By C 11ang, 13) 49i, S. GamistrSchiavChemang, HJiang,doi.orGoldm

Non132hang,Tian, T276lavero. Bott. Schaong, Ku, Y.

. Ha, Tmicalgements

ors are most grateful to the NSFC, China (No. 21205112,24, No. 21475122 and No. 21127006), the Departmentand Techniques of Jilin Province (No. 20120308, No., No. 201215091 and SYHZ0006) and Chinese Academy(YZ201354, YZ201355) for their nancial support.

. Supplementary data

entary data associated with this article can bee online version, at http://dx.doi.org/10.1016/j.apcatb..

ltz, T.P. Yoon, Science 343 (2014) 1239176.S. Shen, L. Guo, S.S. Mao, Chemical Reviews 110 (2010) 6503

B. Cheng, J. Yu, Applied Catalysis B: Environmental 138139 (2013)

. Zhang, M. Zhang, X. Wang, Chemical Science 4 (2013) 3244.htry, C. Colbeau-Justin, G. Custo, M.I. Litter, Applied Catalysis B: Envi-l 144 (2014) 189195.. Li, C. Sun, S. Yang, Y. Guan, H. He, Applied Catalysis B: Environmental) 885892.

L. Gu, N. Zhu, K. Feng, H. Yuan, Z. Lou, Y. Li, A. Shan, Green Chemistry 2696.

[25] L. 495

[26] J. Ji[27] Y. L[28] S. L[29] X. Z

Soc[30] X. X[31] W. [32] R. D

istr[33] J. W

(20[34] H. L

Che[35] M.

on [36] J. Ji[37] R.

dx.[38] A.

and125

[39] Z. Z[40] Y.

763[41] C. C[42] A.W[43] G.C[44] R. L

X. W[45] J.W

CheZ. Xu, P. Wang, S. Dong, Chemical Communications 49 (2013)55.. Zhang, Chemistry 17 (2011) 37103717.ing, The Journal of Physical Chemistry C 114 (2010) 31753179.. Christopher, D.B. Ingram, Nature Materials 10 (2011) 911921.

Y.L. Chen, R.S. Liu, D.P. Tsai, Reports on progress in physics, Physical6 (2013) 046401.andorn, P. Efstathiou, J.T. Irvine, Nature Materials 11 (2012) 595598..B. Cronin, Advanced Functional Materials 23 (2013) 16121619.. Tian, C. Zeng, T. Li, T. Wang, J. Zhang, The Journal of Physical Chem-7 (2013) 213220.A.-H. Lu, M. Li, W. Zhang, Y.-S. Chen, D.-X. Tian, W.-C. Li, ACS Nano 7024910.n, D. Han, W. Ma, B. Cai, W. Zhang, Q. Zhang, L. Niu, Journal of Materialsy A 2 (2014) 3461.ello, V. Augugliaro, V. Loddo, M. Lopez-Munoz, L. Palmisano, Researchical Intermediates 25 (1999) 213227.. Li, L. Zhang, Chemistry 18 (2012) 63606369.

B. Li, C. Fang, J. Wang, Advanced Materials (2014), http://g/10.1002/adma.201400203.ann, in: A. Goldmann (Ed.), Noble Metals, Noble Metal Halides

magnetic Transition Metals, Springer, Berlin/Heidelberg, 2003, pp..

J.T. Yates Jr., Chemical Reviews 112 (2012) 55205551.. Tatsuma, Journal of the American Chemical Society 127 (2005)

37., Nature Photonics 8 (2014) 95103., Current Separations 17 (1998) 8792.tz, Accounts of Chemical Research 17 (1984) 370376.. Mao, M. Gong, S. Zhou, J. Hu, M. Zhi, Y. You, S. Bai, J. Jiang, Q. Zhang,

Xiong, Angewandte Chemie 126 (12) (2014) 32693273..P. Ruberu, R. Han, B. Dong, J. Vela, N. Fang, Journal of the American

Society 136 (2014) 13981408.

Efficiently photocatalytic reduction of carcinogenic contaminant Cr (VI) upon robust AgCl:Ag hollow nanocrystals1 Introduction2 Experimental2.1 Chemicals2.2 Preparation of AgCl:Ag-Hollow NCs2.3 Photocatalytic reduction hexavalent chromium

3 Results and discussion3.1 Electronic images, crystal structure and photophysical properties3.2 BET surface area and photoelectrochemical measurements3.3 Evaluation of photocatalytic reduction activity3.4 Hypothesis of photocatalytic reduction mechanism

4 ConclusionsAcknowledgementsAppendix A Supplementary dataReferences