Materials Research Bulletin 47 (2012) 3383–3389

Room temperature synthesis and photocatalytic property of AgO/Ag2Mo2O7

heterojunction nanowires

Muhammad Hashim a, Chenguo Hu a,*, Xue Wang a, Buyong Wan b, Jing Xu a

a Department of Applied Physics, Chongqing University, Chongqing 400044, PR Chinab Key Laboratory of Optical Engineering, College of Physics and Electronic Engineering, Chongqing Normal University, Chongqing 400047, PR China

A R T I C L E I N F O

Article history:

Received 28 December 2011

Received in revised form 25 June 2012

Accepted 28 July 2012

Available online 4 August 2012

Keywords:

A. Nanostructures

B. Chemical synthesis

D. Catalytic properties

A B S T R A C T

AgO/Ag2Mo2O7 heterojunction nanowires were synthesized at temperatures of 25 8C, 50 8C, 80 8C, and

110 8C, under magnetic stirring in solution reaction. The catalytic activity of AgO/Ag2Mo2O7 nanowires

was evaluated by the degradation of Rhodmine B dye under the irradiation of the simulated sunlight. The

synthesized samples were characterized by X-ray diffractometer, energy dispersive spectrometry, X-ray

photoelectron spectrometer, scanning electron microscopy, and transmission electron microscopy. The

results show that the AgO nanoparticles are attached on the surface of the Ag2Mo2O7 nanowires to form a

heterojunction structure. The length of the nanowires is up to 10 mm and the size of the AgO

nanoparticles is 10–20 nm. The length of nanowires increases with increasing reaction time and

temperature while the AgO particles are gradually embedded into the nanowires. The photocatalytic

activity is greatly improved for the AgO/Ag2Mo2O7 heterojunction nanowires compared with that of the

pure Ag2Mo2O7 nanowires, indicating a remarkable role of AgO particles on the Ag2Mo2O7 nanowires in

the photodegradation.

� 2012 Elsevier Ltd. All rights reserved.

Contents lists available at SciVerse ScienceDirect

Materials Research Bulletin

jo u rn al h om ep age: ww w.els evier .c o m/lo c ate /mat res b u

1. Introduction

Among different nanostructures, recently considerable atten-tion has been paid to the synthesis of heterojunction nanos-tructures due to their wide applications in modern electronicdevices such as single electron transistors [1], resonant tunnelingdiodes [2], field emitters [3], light emitting diodes [4], energystorage [5], photoelectric conversion [6], efficient switching [7],photocatalyst [8,9]. It has been demonstrated that the coupling ofdifferent nanostructured semiconductors or loading of a metal ormetal oxide on the surface of semiconductors much affects theirchemical and physical properties. For example, Kolmakov et al.found that the deposition of Pd particles in/on the surface of SnO2

nanobelts can dramatically enhance its oxygen and hydrogensensitivity performance [10]. It has been proved that the loading ofnoble metal particles, such as platinum, gold, and palladium, onTiO2 photocatalysts can improve its photocatalytic activity [11–13]. Li and Wang reported that hierarchical-structured ZnO–CuOnanocomposites synthesized through a simple one-step homoge-neous coprecipitation method exhibited improved photocatalyticperformance as compared with those of the monocomponentoxides [14]. Zhou et al. reported that Ag2O/TiO2 heterostructures

* Corresponding author. Tel.: +86 23 65678362; fax: +86 23 65678362.

E-mail addresses: hucg@cqu.edu.cn, hu_chenguo@yahoo.com (C. Hu).

0025-5408/$ – see front matter � 2012 Elsevier Ltd. All rights reserved.

http://dx.doi.org/10.1016/j.materresbull.2012.07.019

exhibited enhanced photocatalytic activity in comparison withAg2O nanoparticles or TiO2 nanobelts. They concluded that theTiO2 photocatalysts modified by some metal oxides, such as Ag2O,W2O3, NiO, and PdO, would increase photoelectron capture andvisible-light sensitization capability, which further improvephotocatalytic activity[15]. To the best of our knowledge we havenot yet found any report of the AgO/Ag2Mo2O7 heterostructuredphotocatalyst, therefore, it is necessary to evaluate whether theAgO nanoparticles on the surface of Ag2Mo2O7 nanowires wouldpresent enhanced photocatalytic property.

In this paper, we report room temperature synthesis of AgO/Ag2Mo2O7 heterojunction nanowires under magnetic stirringwithout any surfactant or template for the first time. We observedthe length change of the nanowires and evolution of the AgO particlesunder the different reaction temperature and time. The characteri-zation and optical properties of as-synthesized product were alsostudied. We observed remarkable effect on the photodegradationafter loading AgO particles on the Ag2Mo2O7 nanowires. Thephotodegradation experiment in presence of the AgO/Ag2Mo2O7

nanowires was also compared with that in presence of a commercialphotocatalyst TiO2 (50–200 nm). We believe that the roomtemperature synthesis route of AgO/Ag2Mo2O7 heterojunctionnanowires under magnetic stirring significantly reduces thesynthesis difficulties of molybdates nanostructures and the investi-gation of the catalytic property of the AgO/Ag2Mo2O7 heterojunctionnanowires will play crucial role for further important applications.

Fig. 1. (a) XRD patterns of pure Ag2Mo2O7 nanowires synthesized by hydrothermal

method at 150 8C for 24 h (S1) and AgO/Ag2Mo2O7 nanowires synthesized under

magnetic stirring at 25 8C for 15 h (S2), 24 h (S3), 72 h (S4); and for 15 h, at 50 8C(S5), 80 8C (S6), 110 8C (S7), and (b) EDS spectrum (S2) of AgO/Ag2Mo2O7 nanowires

synthesized at 25 8C for 15 h.

M. Hashim et al. / Materials Research Bulletin 47 (2012) 3383–33893384

2. Experimental

2.1. Synthesis

All the chemical reagents were of the analytical grade and wereused without further purification. In a typical procedure, 0.5 mmolof AgNO3 and 0.25 mmol of H2MoO4�2H2O were dissolved in 12 mLdeionized water. Then the AgNO3 solution was added into theH2MoO4 solution drop by drop under magnetic stirring to form ahomogeneous mixture at room temperature and then pH value ofthe mixture was adjusted to 2 using HNO3 solution. The beakercontaining the solution was put on the hot plate at differenttemperature and for different time. For room temperaturesynthesis, the temperature of the hot plate was kept constant at25 8C for 15 h, 24 h, and 72 h, while for other experiments, thetemperature was adjusted at 50 8C, 80 8C, and 110 8C for 15 h,respectively, as are listed in Table 1. After reacting for a specifictime, the mixture was allowed to cool down to room temperaturenaturally. The resulting greenish-yellow products were collectedfrom the solution by centrifugation, and washed several times bydistilled water and ethanol, then saved for further investigations.

2.2. Characterization

X-ray diffractometer (XRD: BDX320) and energy dispersivespectrometry (EDS) were used to characterize the crystalline phaseand the chemical composition of as-synthesized samples respec-tively. X-ray photoelectron spectrometry (XPS) analysis wasperformed on an ESCALab MKII using Mg Ka as the excitingsource. Morphology and size of the products were investigated byfield emission scanning electron microscope (FE-SEM: Nova 400Nano SEM) at 15 kV, resolution transmission electron microscopeat 400 kV (TEM JEOL 4000EX). Hitachi U-4100 UV-VIS-NIRSpectrophotometer was used to record reflection spectrum.

2.3. Degradation of RhB

The photocatalytic activity of AgO/Ag2Mo2O7 nanowiressynthesized at 25 8C for 15 h was evaluated by degradation ofRhodamine B (RhB). 100 mL RhB aqueous solution with an initialconcentration of 0.02 mmol/L and 20 mg of each photocatalyst wasmagnetically stirred for 30 min in the dark to ensure adsorptionequilibrium. The photocatalytic reactivity tests were then carriedout using a simulated sunlight instrument (CHF-XM-500W). Atgiven irradiation time intervals, 3 mL of the suspension wasextracted and subsequently centrifuged to remove the catalyst.The concentration change of RhB was then determined bymeasuring the maximum absorbance using an UV–visible Spec-trophotometer (Hitachi U-4100).

3. Results and discussion

3.1. Characterizations of the AgO/Ag2Mo2O7 nanowires

Fig. 1a shows the XRD patterns of the pure Ag2Mo2O7 nanowiressynthesized at 150 8C for 24 h by hydrothermal method and AgO/

Table 1Experimental condition for the preparation of samples.

Sample no. Temperature (8C) Time (h) Method Composition

S1 150 24 Hydrothermal Ag2Mo2O7

S2 25 15 Stirring AgO/Ag2Mo2O7

S3 25 24 Stirring AgO/Ag2Mo2O7

S4 25 72 Stirring AgO/Ag2Mo2O7

S5 50 15 Stirring AgO/Ag2Mo2O7

S6 80 15 Stirring AgO/Ag2Mo2O7

S7 110 15 Stirring AgO/Ag2Mo2O7

Ag2Mo2O7 nanowires synthesized under magnetic stirring at 25 8Cfor 15 h, 24 h, 72 h and at 50 8C, 80 8C, 110 8C for 15 h, respectively.All the peaks of the samples can be indexed with pure anorthicphase Ag2Mo2O7, which matches well with the standard JCPDS fileno. 75-1505, except the marked diffraction peaks. The markeddiffraction peaks 32.35 (2 0 0), 34.10 (0 0 2), and 52.12 (0 2 0)agree with monoclinic AgO (JCPDS, 74-1750) and no other peaksfrom possible impurities were detected. The EDS pattern in Fig. 1bindicates that the elements in the sample are Ag, Mo, and O only (Siis from the substrate). Further evidence of the components of thesample is obtained by the X-ray photon spectroscopy (XPS)measurements, which is an excellent technique for understandingoxidation state and relative composition of a material. The surveyspectrum of the synthesized sample at 25 8C for 15 h in Fig. 2aindicates the presence of Mo, Ag, O, and C (from reference). The Ag3d peaks obtained from the nanowires are shown in Fig. 2b. Thebinding energy of the Ag 3d5/2 peak for AgO is in 367.22 eV withHFWD 1.81 eV in accord with the previous report [16], confirmingthe presence of AgO particles on the surface of the Ag2Mo2O7

nanowires. In Fig. 2c and d, Mo 3d peaks centered at 232.61 and235.53 eV are attributed to Mo 3d3/2 and Mo 3d5/2 respectively, andO 1s peak centered at 531.87 eV is attributed to O 1s (2), where O 1s(2) represents binding energy of bridging oxygen atom of Mo–O–Mo units [17]. The binding energy of core level Ag 3d and Mo 3d inthe AgO/Ag2Mo2O7 heterostructure nanowires are listed in Table 2.The XPS results agree well with XRD results, demonstrating thatthe synthesized sample is AgO/Ag2Mo2O7.

360 365 370 375 380 385

800

1000

374.01

367.91

367.22

Inte

nsity (

arb

.units)

Binding energy (eV)

0 200 400 600 800 10 00 12 00

0

1000

2000

3000

4000

5000

Ag4d Mo4p

Mo3d

C1s

Ag3dMo3p

O1s

Ag3p

O(KLL)

Inte

nsity (

arb

. units)

Binding Energy ( eV)

(a) (b) Ag 3d5/2

Ag 3d3/2

220 225 230 235 240 245

400

450

500

550

600

650

700

750

235.8

232.8

Binding Energy (eV)

520 525 530 535 540 545800

900

1000

1100

1200

1300

1400

1500

532.12

Inte

nsity (

arb

. units)

Inte

nsity (

arb

. units)

Binding Energy (eV)

(c) (d)

Mo 3d O 1s

Fig. 2. XPS spectra of AgO/Ag2Mo2O7 nanowires synthesized under magnetic stirring at 25 8C for 15 h: a typical survey spectrum (a); Ag 3d core level spectrum (b); Mo 3d core

level spectrum (c); O 1s core-level spectrum (d).

M. Hashim et al. / Materials Research Bulletin 47 (2012) 3383–3389 3385

FESEM images of the samples prepared at room temperature(25 8C) for 15 h, 24 h, and 72 h are shown in Fig. 3a–c, from which wecan see that the average length of nanowires is 3 mm, 15 mm, and40 mm, respectively, indicating the increase of the nanowire lengthwith the reaction time. Similarly, the average length of nanowires is7 mm, 15 mm, 30 mm for the samples prepared at 50 8C, 80 8C, and110 8C for 15 h respectively, as shown in Fig. 3d–f, indicates that thelength of nanowires also increases with temperature. TEM images indark and bright field for the sample synthesized at roomtemperature for 15 h are shown in Fig. 3g and h, which clearlyillustrate the wire-like morphology with nanoparticle on the surfaceof the nanowires. Fig. 3h (inset) presents the selective area electronicdiffraction pattern of AgO decorated Ag2Mo2O7 nanowires, indicat-ing the nanowires are polycrystals.

TEM and HRTEM images in Fig. 4 show an important phenome-non that as the temperature increases from room temperature tohigher temperature the particle starts embedding into the surface ofthe nanowires. Low and high magnified TEM images are shown forthe sample synthesized for 15 h at room temperature (Fig. 4a and b),80 8C (Fig. 4c and d), and 110 8C (Fig. 4e–h). We can see the AgOparticles on the surface of the nanowire at low temperature.However, the particles gradually embed into the nanowires as thetemperature increases due to the fact that nanowire grows layer by

Table 2Binding energy of core level Ag 3d and Mo 3d in the AgO/Ag2Mo2O7 heterostructure

nanowires.

Compound XPS core-level electrons

Ag 3d5/2 Ag 3d3/2 Mo 3d5/2 Mo 3d3/2 O 1s

Ag2Mo2O7 367.91 373.95 235.53 232.61 531.87

AgO 367.22 528.5

Ag2O 367.91 373.95 528.8

Note: uncertainty is �0.05 eV.

layer. Similar phenomenon has been found when there is a rise ingrowth reaction time at a constant temperature. To compare thesamples synthesized by the stirring method at low temperature andambient pressure with the sample synthesized at higher tempera-ture and higher pressure, we synthesized the sample by thehydrothermal method at 150 8C for 24 h [18]. The SEM and TEM(inset) images in Fig. 4i exhibit clean surface (no particles were foundattached on the surface of nanowires) of the Ag2Mo2O7 nanowires.Fig. 4j and k exhibit the corresponding TEM and HRTEM images for asingle clean surface nanowire with SAED (inset) demonstrating apure anorthic phase structure of the Ag2Mo2O7 nanowires inagreement with XRD result (JCPDS No. 75-1505) in Fig. 1. Therefore,we conclude that AgO/Ag2Mo2O7 heterojunction nanowires can beobtained at room temperature and ambient pressure for growthtime (�15 h), and the AgO particles will partially embed into thenanowires by increasing the temperature or growth time. Thermo-gravimetric analysis showed that AgO powder is stable at thetemperature less than 150 8C [19]. However, with an increase intemperature and pressure, the smooth pure Ag2Mo2O7 nanowirescan be obtained by the hydrothermal method at 150 8C, where thewater is in supercritical fluid.

3.2. Optical properties of the AgO/Ag2Mo2O7 nanowires

Optical properties of the AgO/Ag2Mo2O7 heterojunctionnanowires synthesized at 25 8C for 15 h were investigatedthrough UV–visible diffuse reflection spectrum, as is shown inFig. 5. From reflection spectrum, the Kubelka–Munk function canbe known, which is defined as the ratio between the absorptioncoefficient and scattering factor from the optical diffusereflectance spectrum [20]. By extrapolating the linear part ofKubelka–Munk function, a clear optical gap of about 2.74 eVcorresponding to the absorption edge of 454 nm can be obtained,which is 0.2 eV, less than 2.94 eV for the pure Ag2Mo2Onanowires reported before [18].

Fig. 3. SEM images of AgO/Ag2Mo2O7 nanowires synthesized under magnetic stirring at 25 8C for 15 h (a), 24 h (b), 72 h (c); and for 15 h at 50 8C (d), 80 8C (e), 110 8C (f). TEM

images (g and h) and SAED pattern (inset h) of AgO/Ag2Mo2O7 nanowires synthesized under magnetic stirring at 25 8C for 15 h.

M. Hashim et al. / Materials Research Bulletin 47 (2012) 3383–33893386

3.3. Degradation of RhB

RhB, a widely used dye was selected to examine thephotocatalytic activity of catalysts. In order to investigate thephotocatalytic behavior of the AgO particles toward the degrada-tion of RhB dye, comparative experiments were carried out withthe catalysts AgO/Ag2Mo2O7 heterojunction nanowires synthe-sized for 15 h at 25 8C, 110 8C and the Ag2Mo2O7 nanowires(synthesized by hydrothermal method). The absorption spectra inFig. 6a clearly shows an enhanced catalytic activity of AgO/Ag2Mo2O7 nanowires synthesized at 25 8C for 15 h compared with

those of the AgO/Ag2Mo2O7 nanowires synthesized at 110 8C for15 h and pure Ag2Mo2O7 nanowires as shown in Fig. 6b and c,indicating the key role of AgO particles on the Ag2Mo2O7

nanowires in the RhB photodegradation. To better evaluate theAgO/Ag2Mo2O7 nanowires catalytic activity, we carried outdegradation experiment with a commercial catalyst of TiO2

nanoparticles (50–200 nm) at the same conditions, and the resultis shown in Fig. 6d. The plots of the catalytic degradationpercentage versus illumination time in Fig. 6d shows that themore AgO exposure on Ag2Mo2O7 nanowires, the better degrada-tion effect is achieved. The degradation rate of the AgO/Ag2Mo2O7

Fig. 4. TEM/HRTEM images of AgO/Ag2Mo2O7 nanowires synthesized under magnetic stirring for 15 h at 25 8C (a and b), 80 8C (c and d), and 110 8C (e–h), and TEM/HRTEM

images (i–k) and SAED (inset k) of Ag2Mo2O7 nanowires synthesized by hydrothermal method at 150 8C for 24 h.

M. Hashim et al. / Materials Research Bulletin 47 (2012) 3383–3389 3387

heterojunction exceeds that of the commercial catalyst of TiO2

nanoparticles (50–200 nm) in the initial 4.5 h, however, the rate ofthe AgO/Ag2Mo2O7 heterojunction decreases a little after 4.5 h. Noremarkable change in the concentration of the RhB was observedby catalysis of the Ag2Mo2O7 nanowires. Based on the above result,

it can be concluded that AgO/Ag2Mo2O7 nanowires display a muchbetter photocatalytic activity as compared with Ag2Mo2O7

nanowires due to the important role of AgO particles inphotodegradation of RhB under the simulated sunlight irradiation.Both the AgO/Ag2Mo2O7 heterojunction nanowires and TiO2

0 21 -10

0

10

20

30

40

50

60

70

80

(e)

De

gra

da

tio

n (

%) AgO/Ag

2Mo

2O

7 (RT

TiO2 nan

Ti

300 400 500 600 700 800

0.0

0.2

0.4

0.6

0.8

1.0

Ag2Mo

2O

7

(c)0 h

1 h

2 h

3 h

4 h

5 h

6 h

Inte

nsity (

a.u

)

2 T het a ( deg.)

300 400 500 600 700 800-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

AgO/A g2Mo

2O

7

(RT)

(a)0 h

1 h

2 h

3 h

4 h

5 h

6 h

Inte

nsity (

a.u

)

2 Theta (deg.)

Fig. 6. Absorption spectrum of 0.02 mmol/L RhB solution with 20 mg in 100 mL solution

degradation percentage versus illumination time (e).

1.5 2.0 2.5 3.0 3. 5 4.0 4.5 5.0 5.50

1

2

3

4

5

6

7

8

9

0

1

2

3

4

5

Eg=2.74 eV

Photon Energy (eV)

K-M

fu

nctio

n

Re

fle

ctio

n %

Fig. 5. Reflection spectrum and its corresponding Kubelka–Munk function of the

AgO/Ag2Mo2O7 heterojunction nanowires synthesized at 25 8C for 15 h under

magnetic stirring.

M. Hashim et al. / Materials Research Bulletin 47 (2012) 3383–33893388

nanoparticles are good photocatalysts. Due to easy and simplesynthesis route at room temperature and ambient pressureconditions, the AgO/Ag2Mo2O7 heterojunction nanowires will befavorable to the potential applications for photodegradation ofvarious organic pollutants.

To illustrate a clearer understanding of the photodegradationwith AgO/Ag2Mo2O7 nanowires, we have drawn a diagramshowing the conduction band, valence band of Ag2Mo2O7 andAgO. It has been demonstrated that AgO (best described as acombination of Ag(I) and Ag(III)) is thermal stability below thetemperature of 200 8C [21,23]. The band gap is 2.94 eV for theAg2Mo2O7 nanowires [18] and 1.1 eV for AgO [19]. For the very lowphotocatalytic activity of the Ag2Mo2O7 nanowires presented inFig. 6b, we propose that the transition of photogenerated electronsshould transfer from the conduction band of AgO to the conductionband of Ag2Mo2O7, as is shown in Fig. 7, while holes transfer fromvalence band of Ag2Mo2O7 to the valence band of AgO causes theseparation of electrons and holes. When light falls on the AgO/

300 400 500 600 700 800-0.2

0.0

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0.4

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1.4

TiO2 (50-200nm )

(d)0 h

1 h

2 h

3 h

4 h

5 h

6 h

Inte

nsity (

a.u

)

2 Theta (deg.)

543 6

)

AgO/Ag2Mo

2O

7 (110 C)

opa rticles (50-20 0 nm)

Ag2Mo

2O

7

me (h)

300 40 0 500 600 700 80 0

0.0

0.2

0.4

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AgO/A g2Mo

2O

7

(110oC)

(b)0 h

1 h

2 h

3 h

4 h

5 h

6 h

Inte

nsity (

a.u

)

2Theta (deg.)

of the S2 (a), S7 (b), S1 (c), TiO2 nanoparticles (d), and their corresponding plots of

Fig. 7. Schematic diagram of the band structures of AgO/Ag2Mo2O7 heterojunction

and transition of the photogenerated electrons and holes in the photocatalytic

degradation of RhB with the AgO/Ag2Mo2O7 heterojunction nanowires under the

sunlight irradiation.

M. Hashim et al. / Materials Research Bulletin 47 (2012) 3383–3389 3389

Ag2Mo2O7 nanowires both the AgO particles and Ag2Mo2O7

nanowires can produce electron–hole pairs and these photogen-erated electrons will accumulate on the Ag2Mo2O7 nanowireswhile holes will accumulate on the AgO particles. The photo-generated electrons and holes react with the adsorbed surfacesubstances, like O2, OH�, etc., to form reactive species O2

�, OH*,which is the major oxidative species for the decomposition oforganic pollutants. Then the oxidative species degrade the organicpollutant (RhB) into the small molecules like CO2, H2O, etc. [22]. Onthe other hand, AgO is not stable under irradiation of visible light.The photochemical behavior of AgO is similar to that of Ag2O,which yields Ag clusters under irradiation of visible light [23]. Itwas found that the structure stability and high photocatalyticactivity of Ag2O under visible-light irradiation can be wellmaintained by partial formation of metallic Ag on its surfaceduring the photodecomposition of organic substances [24].Therefore, under the irritation of sunlight, once a certain amountof metallic Ag is formed on the surface of AgO, the followingphotogenerated electrons tend to transfer to the metallic-Ag sitesdue to the Schottky barrier and then are captured by adsorbedoxygen (Fig. 7). The metallic Ag works as an electron pool andtransfers the photogenerated electrons to oxygen through multi-electron-transfer routes [24].

4. Conclusion

AgO/Ag2Mo2O7 heterojunction nanowires have been synthe-sized at room temperature and above the room temperature fordifferent time under magnetic stirring without any surfactant ortemplate. XRD, XPS, TEM, and SAED results confirm that thesynthesized nanowires are polycrystalline anorthic phaseAg2Mo2O7 and particles attached on the surface of the nanowiresare single crystalline monoclinic AgO nanoparticles. The length ofAgO/Ag2Mo2O7 heterojunction nanowires increases with reaction

time and temperature while AgO nanoparticles are partiallyembedded into the nanowires. The optical band gap of AgO/Ag2Mo2O7 heterojunction nanowires is 2.74 eV from the UV–visreflection spectrum, which is 0.2 eV narrower compared with thepure Ag2Mo2O7 nanowires. The enhanced photocatalytic activity ofthe AgO/Ag2Mo2O7 heterojunction nanowires demonstrates asignificant role of AgO particles in photodegradation, as thephotogenerated electrons and holes can be efficiently separated bya certain amount of metallic Ag formed on the surface of AgO underthe irritation of sunlight.

Acknowledgments

This work has been funded by the NSFC (60976055), SRFDP(20110191110034), and sharing fund of large-scale equipment ofChongqing University.

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