DOI: 10.1002/cplu.201200159

Facile Synthesis of an Ag2O–ZnO Nanohybrid and Its HighPhotocatalytic ActivityMing Wu, Jun-Min Yan,* Ming Zhao, and Qing Jiang*[a]

Introduction

Semiconductor photocatalysts have attracted much attentionsfor their applications in solving environmental problems, suchas the photocatalytic degradation of polluted air and wastewa-ter.[1–5] Over the past few decades, TiO2 has been regarded asan efficient photocatalyst for its high activity, great stability,nontoxicity, and low cost.[6–8] However, further practical applica-tion of TiO2 is restricted because of its wide bandgap of 3.2 eV,which absorbs mainly ultraviolet (UV) light (l<400 nm) thataccounts for about only 4 % of solar energy.[9–11] Therefore, thechallenge is to find other photocatalysts with high catalytic ac-tivity in visible light.

Recently, Ag2O was found to be a promising visible-lightphotocatalyst owing to its narrow bandgap of 1.2 eV compar-ing with that of TiO2 (3.2 eV).[12, 13] However, Ag2O is too expen-sive for practical application as a photocatalyst. Nowadays,nanohybrids consisting of two or more semiconductors areconsidered a great choice to improve the efficiencies of photo-catalysts owing to the synergistic effects on reducing the re-combination rate of the photogenerated electron holepairs.[14–18] ZnO, an important semiconductor with a bandgapof 3.37 eV and a large excitation binding energy of60 meV,[19–22] is believed to be an alternative photocatalyst ma-terial to TiO2 because of their similar bandgaps and similarphotocatalytic mechanisms.[23, 24] Moreover, it has been report-ed that ZnO even exhibits better photocatalytic activity thanTiO2 in degradation of some dyes.[25, 26] Thereafter, partial sub-stitution of Ag2O by ZnO to form a nanohybrid may lead toa low cost and highly active photocatalyst owing to the elec-tronic synergistic effect of Ag2O and ZnO.

Herein, the first Ag2O–ZnO nanohybrid is synthesized witha basic coprecipitation-assisted hydrothermal (BCAH) method.The as-synthesized Ag2O–ZnO nanohybrid has an average par-ticle size of 30 nm with uniformly distributed Ag2O and ZnOphases. The small particle size and uniform distribution of con-stituents effectively enhance the photocatalytic activity of thehybrid for degradation of rhodamine B (RhB) aqueous solutionwhether under sunlight, visible light, or UV light irradiation.

Results and Discussion

Figure 1 shows the field-emission scanning electron microsco-py (FESEM) images of the as-synthesized Ag2�2xZnxO–ZnO mid-product, Ag2O–ZnO (0.3:0.7) nanohybrid, ZnO, and Ag2O parti-cles. It can be seen that after addition of NaOH into the aque-ous solution containing Ag+ and Zn2+ cations, the formedgray precipitate has an irregular shape (Figure 1 a), and itsX-ray diffraction (XRD) pattern (Figure 2 a) reveals that it con-sists of the hexagonal wurtzite ZnO (JCPDS card no. 36-1451)and an unknown new phase. This new phase has similar crystalstructure as cubic Ag2O (JCPDS card no. 41-1104) and sphaler-ite ZnO (JCPDS card no. 65-2880), and the d (111) of this newphase is calculated to be 0.2698 nm, which is between thed (111) values of Ag2O (0.2729 nm) and ZnO (0.2673 nm).Therefore, it is reasonable that the new phase is a solid solu-tion of Ag2O and ZnO, which can be expressed as Ag2�2xZnxO.After hydrothermal treatment of the mid-product, the irregu-lar-shape material transforms into spherical nanoparticleswhich have an average diameter of around 30 nm (Figure 1 b).Based on its XRD result (Figure 2 b), this specimen is made ofthe crystallized cubic Ag2O and hexagonal ZnO; which meansthat an Ag2O–ZnO nanohybrid has been successfully synthe-sized through a facile BCAH method. Figure 1 c,d indicate themorphologies of pure ZnO and Ag2O prepared by the samemethod. It is clear that ZnO particles are about 100 nm in di-ameter, meanwhile Ag2O particles are much bigger rangingfrom 100 to 800 nm. Therefore, it is interesting that during thesynthesis of the Ag2O–ZnO nanohybrid, the counterparts ofAg2O and ZnO exert size-depression effects on each other andlead to a nanohybrid of smaller particle size, which may bebeneficial to its catalytic activity owing to the larger surface-to-volume ratio.

[a] M. Wu, Dr. J.-M. Yan, Dr. M. Zhao, Dr. Q. JiangKey Laboratory of Automobile Materials, Ministry of EducationDepartment of Materials Science and EngineeringJilin University, Changchun 130022 (P. R. China)Fax: (+ 86) 431-85095876E-mail : junminyan@jlu.edu.cn

jiangq@jlu.edu.cn

An Ag2O–ZnO nanohybrid is synthesized for the first timethrough a facile, basic coprecipitation-assisted hydrothermalstrategy. This novel method endows the Ag2O–ZnO nanohy-brid (Ag2O/ZnO = 0.3:0.7) with small particle size and strongelectronic synergistic effects owing to the uniform distribution

of the constituents. The as-synthesized nanohybrid shows ex-cellent photocatalytic properties for degradation of rhodami-ne B, even under visible light irradiation. This effect is muchbetter than pure or physical mixtures of Ag2O and ZnO, as wellas the traditional homogenous Fenton catalyst (Fe3 +/H2O2).

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Figure 3 shows the transmission electron microscopy (TEM),energy-dispersive X-ray spectroscopy (EDS), and elementalmapping images of the Ag2O–ZnO nanohybrid. The micro-structure (Figure 3 a) and composition (Figure 3 b) analysesagree well with the FESEM and XRD results. Moreover, the ele-mental mapping data of a typical particle (Figure 3 c–f) indi-cates the uniform distribution of Ag, Zn, and O elements inthe Ag2O–ZnO nanohybrid particle. This uniform distributionmay further lead to enhanced catalytic activity owing to stron-ger synergistic effect between the two phases, Ag2O and ZnO.

Figure 4 a shows the photocatalytic activity of the Ag2O–ZnOnanohybrid together with pure Ag2O, ZnO, and a physical mix-ture of Ag2O and ZnO under the irradiation of simulated sun-light. Pure Ag2O and ZnO particles show similar poor activities,where the photodegradations are both completed in 20 mi-nutes. Although the physical mixture of Ag2O and ZnO (Ag2O/ZnO = 0.3:0.7) exhibits better activity (8 min for completeddegradation) than pure Ag2O and ZnO, its activity is still lowerthan that of the Ag2O–ZnO nanohybrid. The as-synthesized

Ag2O–ZnO nanohybrid has the best photocatalytic activity, andthe photodegradation was complete in only 5 minutes. Thisgood catalytic performance may be due to its small particlesize as well as the strong synergistic effect between the uni-formly distributed Ag2O and ZnO phases. Moreover, Ag2O–ZnOhas a good recycle stability, which can be seen clearly fromFigure 4 b where, after the 4th recycling test, there is no obvi-ous decrease in degradation ratio of RhB during 5 minutes. Itshould be noted that, whether increasing or decreasing themolar ratio of Ag2O/ZnO from 0.3:0.7, the catalytic activities ofAg2O–ZnO nanohybrids are obviously decreased, as shown inFigure 4 c. In addition, without any catalyst, RhB is stable andshows no degradation even after irradiation with simulatedsunlight for 1 hour (Figure 4 d).

Finding highly active photocatalysts that are effective in visi-ble light is an attractive research goal.[12] Therefore, the pre-pared catalysts are further evaluated under visible light (l>420 nm). As shown in Figure 5 a. ZnO shows the lowest visible-light photocatalytic activity and the degradation can onlyreach about 40 % even after 40 minutes. Meanwhile Ag2Oshows better activity (32 min for completed degradation) thanthat of ZnO, which agrees well with reported results.[12, 15, 27, 28]

Moreover the activity of Ag2O in visible light is even betterthan that of a Ag2O/ZnO mixture (40 min for completed degra-dation), thus indicating the prominence of Ag2O (even withthe large particle size) as a photocatalyst under these condi-tions. However, although most of the content of Ag2O hasbeen substituted by the less active phase, ZnO, the synthe-sized Ag2O–ZnO nanohybrid shows the best visible-light pho-tocatalytic activity. With the nanohybrid, RhB can be complete-ly degraded in only 20 minutes.

To find out why the Ag2O–ZnO nanohybrid has such excel-lent photoactivity, UV/Vis diffuse reflectance spectra of the pre-pared catalysts were measured. As shown in Figure 6, theAg2O–ZnO hybrid has stronger absorption in the visible andUV regions when compared with the other catalysts, includinga Ag2O/ZnO mixture, Ag2O, and ZnO. Consequently, the photo-catalytic activity of the Ag2O–ZnO nanohybrid, whether undersimulated sunlight or visible-light irradiation, is better than itspure or physically mixed components (Figure 4 a and Fig-

Figure 1. FESEM images of the synthesized (a) Ag2�2xZnxO–ZnO mid-product,(b) Ag2O–ZnO (0.3:0.7) nanohybrid, (c) ZnO particles, and (d) Ag2O particles.

Figure 2. XRD patterns of the synthesized (a) Ag2�2xZnxO–ZnO mid-product,(b) Ag2O–ZnO (0.3:0.7) nanohybrid, (c) ZnO particles, and (d) Ag2O particles.

Figure 3. a) TEM image of Ag2O–ZnO (0.3:0.7) nanohybrid and (b) the corre-sponding EDS pattern, (c) a selected particle for elemental mapping analysisof (d) Ag, (e) O, and (f) Zn elements.

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J.-M. Yan, Q. Jiang et al.

ure 5 a). The excellent activity of the Ag2O–ZnO nanohybridproves again that the present BCAH synthetic method endowsthe hybrid with strong and positive synergistic effect betweenthe uniformly distributed Ag2O and ZnO phases, which can beunderstood from Figure 7. Under irradiation with UV light (Fig-ure 7 a), ZnO and Ag2O are both excited, and the photogener-ated electrons from the lower conduction band (CB) of ZnO(�0.5 eV vs. SHE)[29] are transferred to the higher CB of Ag2O(0.2 eV vs. SHE).[12] Therefore, the recombination of electron-hole pairs can be inhibited. Under irradiation with visible light

(Figure 7 b), only Ag2O can be excited, and the photogeneratedelectrons on Ag2O can partially reduce it to form Ag nanoparti-cles.[12] Where after, electrons on Ag nanoparticles then migrateto the CB of ZnO because of the larger work function ofZnO,[30] and the recombination of electron-hole pairs are thenreduced. Meanwhile, the interface between ZnO and Ag2Ophases can further inhibit the electron-hole recombination,[14]

and thus the catalytic activity is enhanced.

Figure 4. a) Photodegradation of RhB in the presence of different catalysts.b) Recycle tests of Ag2O–ZnO (0.3:0.7) for photodegradation of RhB. c) Pho-todegradation rate of RhB in the presence of Ag2O/ZnO with different Ag2Oto ZnO molar ratios. d) Photodegradation of RhB without catalyst. All reac-tions are applied under the simulated sunlight irradiation.

Figure 5. a) Photodegradation of RhB in the presence of different catalysts.b) UV/Vis spectrum of RhB solution after different photodegradation time inthe presence of Ag2O–ZnO (0.3:0.7) nanohybrid. All reactions are appliedunder visible light irradiation (l>420 nm).

Figure 6. UV/Vis diffuse reflectance spectra of different catalysts.

Figure 7. Schematic view for electron-hole separations and energy bandmatching of Ag2O-ZnO nanohybrid under (a) UV and (b) visible light irradia-tion.

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Photocatalysis

It is well-known that, homogenous Fe3+/H2O2, a Fenton cata-lytic system, is highly efficient for degradation of organic com-pounds under UV light irradiation.[31–33] In this sense, the activi-ties/stabilities of the Ag2O–ZnO nanohybrid and Fe3 +/H2O2 fordegradation of RhB are also investigated under irradiation ofUV light for comparison. As results in Figure 8 a show, thenanohybrid has the same high activity as Fe3 +/H2O2, where thedegradations can be completed within 6 minutes. However,after the 3rd recycle test (Figure 8 b), the degradation ratiousing Fe3 +/H2O2 is decreased to only 27.5 % during 6 minutes,meanwhile that with Ag2O–ZnO can still reach 88 %. Therefore,the present nanohybrid shows superiority in photocatalytic sta-bility even compared with a traditional homogenous catalyst.

Conclusion

In summary, the novel Ag2O–ZnO (0.3:0.7) nanohybrid was suc-cessfully synthesized though a basic coprecipitation-assistedhydrothermal method. The partial substitution of Ag2O by ZnOin the hybrid of Ag2O–ZnO not only effectively lowers the costof the catalyst but also greatly enhances its photocatalytic ac-tivity. Moreover, compared with pure Ag2O, ZnO, a physicalmixture of Ag2O/ZnO, and homogenous Fe3+/H2O2 system, theas-prepared nanohybrid shows the best activity and stabilityfor degradation of rhodamine B aqueous solution, even underirradiation with visible light. Furthermore, this study also pro-poses a new idea for designing and synthesizing heterogene-ous nanohybrid photocatalysts.

Experimental Section

Chemicals

Zinc nitrate hexahydrate (Zn(NO3)·6H2O, Tianjin Windship ChemistryTechnological Co., Ltd,>99.0 %), Silver nitrate (AgNO3, BeijingChemicals Works, �99.8 %), sodium hydroxide (NaOH, BeijingChemicals Works,>96 %), ferric chloride hexahydrate (FeCl3·6H2O,Beijing Chemicals Works,�99 %), hydrogen peroxide (H2O2, BeijingChemicals Works, 30 %), and poly(vinylpyrrolidone) K30 (PVP,(C6H9NO)n, average Mw = 58000 g mol<-M>1, Sinopharm ChemicalReagent Co., Ltd) were used as received without any purification.Ultrapure water with the specific resistance of 18.2 MW cm was ob-tained by reversed osmosis and subsequent ion-exchange and fil-tration (RFU 555DA, ToyoRoshi Kaisha, Ltd. , Japan).

Preparation of Ag2O–ZnO nanohybrids

Ag2O–ZnO nanohybrids with different molar ratios (0.1:0.9, 0.3:0.7,0.5:0.5, 0.7:0.3, and 0.9:0.1) were prepared by using a facile BCAHmethod. Typically, Ag2O–ZnO nanohybrid with the molar ratio of0.3:0.7 was synthesized as follows: AgNO3 (0.6 mmol), Zn(NO3)2

(0.7 mmol), and PVP (100 mg) were dissolved in water (27.5 mL),and then NaOH aqueous solution (12.5 mL, 0.24 mol L�1) wasslowly added at room temperature with magnetic stirring for30 min. This gray alkaline suspension was placed in a Teflon-sealedautoclave (50 mL) and maintained at 130 8C for 100 min. The as-prepared specimen was then washed with deionized water untilthe solution reached a neutral pH value. Finally, the washed speci-men was dried at 80 8C for 10 h. For comparison, the pure Ag2Oand ZnO specimens and their physical mixture (Ag2O/ZnO =0.3:0.7) were also synthesized by the same method.

Photocatalytic activities and stabilities

The prepared catalysts (10 mg) and RhB aqueous solution (5 mL,40 mg L�1) were added to water (35 mL). The photocatalytic reac-tions were begun with magnetic stirring under the irradiation witha Xe lamp (300 W, 320<l<2500 nm; CEL-HXF 300), which wasused as the simulated sunlight source. This source was placed25 cm away from the reaction solution. The degraded solution wasanalyzed by UV/Vis measurements at various time intervals aftercentrifugation and filtration. A cutoff filter (AULIGHT) was used forthe photocatalytic reactions under visible light (l>420 nm).

The activity of the Ag2O–ZnO (0.3:0.7) nanohybrid for degradationof RhB was also tested under irradiation with UV light (GY 250,with the maximum emission of 365 nm). For comparison, the samephotocatalytic reaction was also applied with the homogenous cat-alyst Fe3 +/H2O2 as follows: FeCl3 (0.18 mmol, including 10 mg ofFe3 +), H2O2 (13.5 mL, 0.98 m), and RhB (5 mL, 40 mg L�1) were addedto water (35 mL), and the reaction was begun with magnetic stir-ring under UV light.

After the above degradations of RhB with different catalysts werecompleted under irradiations with the nominated light sources,the new RhB solution (5 mL, 40 mg L�1) was re-added into the reac-tion solution containing catalysts. And then, the recycle tests werebegun with magnetic stirring under the same irradiation of lightsource as the 1st run.

Figure 8. a) Photodegradation of RhB with Fe3 +/H2O2 and Ag2O–ZnO(0.3:0.7) nanohybrid under UV light irradiation. b) Recycle tests of Fe3 +/H2O2

and Ag2O–ZnO (0.3:0.7) for photodegradation of RhB under UV light irradia-tion.

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J.-M. Yan, Q. Jiang et al.

Catalyst characterization

Powder X-ray diffraction (XRD) patterns were measured ona Rigaku D/max-2500pc X-ray diffractometer with CuKa irradiation(l= 1.5406 �) by a scan rate of 2 8min�1. JEOL JSM-6700F field-emission scanning electron microscope (FESEM) with an accelera-tion voltage of 8 kV and transmission electron microscope (TEM,Hitachi H-9000NA) together with an energy-dispersive X-ray spec-troscopy (EDS) were applied to analyze the microstructure andcomposition of the specimens. UV/Vis absorption spectra of thedegraded RhB solution and UV/Vis diffuse reflectance spectra ofthe as-prepared solid catalysts were recorded on spectrophotome-ters of Agilent Cary 50 and Shimadzu UV-3600, respectively.

Acknowledgements

This study was supported by the National Key Basic Research, De-velopment Program (2010CB631001), National Natural ScienceFoundation of China (51101070), Program for Changjiang Schol-ars and Innovative Research Team in University, Program for NewCentury Excellent Talents in University of the Ministry of Educa-tion of China (NCET-09–0431), Jilin Province Science and Technol-ogy Development Program (201101061), and Jilin University Fun-damental Research Funds.

Keywords: heterogeneous catalysis · nanohybrids · synergisticeffects · synthetic methods · visible light

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Received: June 21, 2012

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Photocatalysis

FULL PAPERS

M. Wu, J.-M. Yan,* M. Zhao, Q. Jiang*

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Facile Synthesis of an Ag2O–ZnONanohybrid and Its HighPhotocatalytic Activity

Be active! A facile, basic coprecipita-tion-assisted hydrothermal strategy hasbeen employed to prepare an Ag2O–ZnO (0.3:0.7) nanohybrid. The nanohy-brid shows excellent photocatalyticproperties for degradation of rhoda-mine B, even under visible light irradia-tion. This effect is much better thanpure or a physical mixture of Ag2O andZnO (see graph).

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