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Applied Surface Science 256 (2010) 6076–6082

Contents lists available at ScienceDirect

Applied Surface Science

journa l homepage: www.e lsev ier .com/ locate /apsusc

ynthesis and characterization of ZnO–Ag core–shell nanocomposites withniform thin silver layers

ei Li a,b,c,∗, Yuliang Yuana, Junyang Luoa, Qinghua Qina, Jianfang Wua, Zhen Lia,c, Xintang Huangb

Faculty of Materials Science and Chemical Engineering, China University of Geosciences, Wuhan 430074, PR ChinaDepartment of Physics, Central China Normal University, Wuhan 430079, PR ChinaEngineering Research Center of Nano-Geomaterials of Ministry of Education, China University of Geosciences, Wuhan 430074, PR China

r t i c l e i n f o

rticle history:eceived 15 May 2009eceived in revised form 27 January 2010ccepted 24 March 2010vailable online 31 March 2010

ACS:1.07.Bc1.16.Be1.46.Hk

a b s t r a c t

This paper presents an investigation on the synthesis and characterization of ZnO–Ag core–shellnanocomposites. ZnO nanorods were employed as core material for Ag seeds, and subsequent nucle-ation and growth of reduced Ag by formaldehyde formed the ZnO–Ag core–shell nanocomposites. TheZnO–Ag nanocomposites were annealed at different temperature to improve the crystallinity and bind-ing strength of Ag nanoparticles. The morphology, microstructure and optical properties of the ZnO–Agcore–shell nanocomposites were characterized by X-ray diffraction, field emission scanning electronmicroscopy, transmission electron microscopy, energy-dispersive X-ray spectroscopy, ultraviolet–visible(UV–vis) absorption and photoluminescence measurement. It was demonstrated that very small face-center-cubic Ag nanoparticles were coated on the surface of ZnO nanorods. The ultraviolet absorption and

eywords:nO–Agore–shellltraviolet–visible absorption

surface plasmon absorption band of ZnO–Ag core–shell nanocomposites exhibited some redshifts relativeto pure ZnO nanorods and monometallic Ag nanoparticles. The coating of Ag nanocrystals onto the ZnOnanorods completely quenched the photoluminescence. These observations reflected the strong interfa-cial interaction between ZnO nanorods and Ag nanoparticles. The effect of Ag coating thickness on the

ropeism o

hotoluminescencerowth mechanism

morphology and optical pover, the growth mechandetail.

. Introduction

In the past few years, a significant step after the remarkableuccess in growing single-component nanocrystals is the prepa-ation of nanostructures that are composed of different materials.he attraction of multicomponent nanostructures is that multipleunctions can be integrated into one system for specific applications1,2]. Moreover, the interactions between nanoscale-spaced com-onents can greatly improve the overall application performancef the nanostructured system and even generate new synergeticroperties. One extremely attractive example of a nanocompos-

te material is the core–shell structured particles because of theiriverse applications, for example, as building blocks for photonicrystals [3], heterogeneous catalysts [4], surface-enhanced Raman

cattering [5] and drug-delivery applications [6].

Semiconductors and metal nanoparticles also have attracteduch attention because of their novel physical and chemical prop-

rties, as well as the possibility for applications in the fields of

∗ Corresponding author at: Faculty of Materials Science and Chemical Engineering,hina University of Geosciences, No. 388, Lumo Road, Wuhan 430074, PR China.el.: +86 27 678 83737; fax: +86 27 678 83732.

E-mail address: alexfly2002@sina.com (F. Li).

169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.apsusc.2010.03.123

rties of ZnO–Ag core–shell nanocomposites was also investigated. More-f ZnO–Ag core–shell nanocomposites was also proposed and discussed in

© 2010 Elsevier B.V. All rights reserved.

catalysis, third-order nonlinearity, size quantum effects, lumines-cence, and so on [7–10]. Although the core–shell geometry canmaximize the interfacial area and thus provide a platform forstudying the plasmon–exciton interactions and charge distribu-tions, only a few examples of semiconductor–metal core–shellnanostructures have been reported to date [11–14]. Furthermore,control of the shapes and sizes of noble metal components remainsa challenge. This controllability is highly important for study-ing the plasmon–exciton interactions and even tailoring themfor improved application performances, because the properties ofnoble metals are strongly dependent on their shapes and sizes.

Among the various materials, the well-known ZnO and metalAg can still offer unexplored opportunities for the realization ofnovel nanocomposite systems. ZnO is one of the most investi-gated oxide materials owing to its important role in applicationsrelated to optoelectronic fields [15,16] and Ag nanoparticles aresome of the most well-developed materials because they possessgood chemical and physical properties [17,18]. It is expected thatthe surface coating of the ZnO nanomaterials with Ag nanopar-

ticles enables us to construct ZnO–Ag core–shell nanostructureswith novel optical and electrical properties. However, ZnO–Agcore–shell nanocomposites are rarely reported because it is verydifficult to form tight coupling between ZnO core and the shell of Agnanoparticles.

F. Li et al. / Applied Surface Science 256 (2010) 6076–6082 6077

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In this paper, we develop a facile solution strategy to syn-hesize ZnO–Ag core–shell nanocomposites via the pre-activationf ZnO nanorods, and primarily explore the structural and opti-al properties of the core–shell nanocomposites. To our bestnowledge, the synthesis of ZnO–Ag core–shell nanostructuresia the pre-activation of ZnO nanorods has not been reported soar. The structural and optical properties of ZnO–Ag core–shellanocomposites with different Ag thickness or after anneal were

nvestigated. Combination of the two nanomaterials of ZnO andg provides a model structure for further understanding theore–shell heterostructures and their properties. The compositeolloids produced are also expected to have potential applicationsn electronics, photonics, catalysis, and sensors. We believe that our

ethod can open a new avenue for deposition of metallic nanos-ructures based on solid substrates for desired properties.

. Materials and methods

.1. Materials

All chemicals were used as received without further purification.inc acetate dihydrate (Zn(CH3COO)2·2H2O, AR), sodium dodecylulfate (SDS, C12H25SO4Na, AR), ethanol (C2H5OH, AR), stannoushloride hydrate (SnCl2·H2O, AR), silver nitrate (AgNO3, AR), tri-thanolamine (TEA, C6H15O3N, AR), and formaldehyde (HCHO, AR)ere purchased from Shanghai Chemical Industrial Company.

.2. Synthesis of ZnO nanorods

ZnO nanorods were prepared by a SDS-assisted hydrothermalethod according to our previous work [19] with minor changes.

n a typical procedure, 0.22 g of Zn(CH3COO)2·2H2O and 1.5 g ofDS were dissolved in 45 ml double distilled water under vigor-usly stirring, then 5 ml NaOH solution (5 M) was introduced intohe above solution under continuous stirring and the pH value ofhe solution was adjusted to 12. Subsequently, the solution wasransferred into a teflon-lined autoclave 60 ml in volume. The auto-lave was sealed and maintained at 160 ◦C for 12 h, and then cooledaturally to room temperature. After that, the precipitates wereentrifuged and washed with distilled water and ethanol severalimes to remove the impurities. The products were dried in vacuumt 60 ◦C for 3 h, and ZnO nanorods were obtained.

.3. Preparation of ZnO–Ag core–shell nanocomposites

In a typical synthesis, 0.05 g of ZnO nanorods was dispersednto 40 ml of ethanol solution containing 0.025 g of SnCl2·H2O

ith vigorous stirring for about 30 min at room temperature,hen centrifuged to obtain the activated ZnO nanorods. After this,he activated ZnO nanorods were dispersed into 25 ml 0.0045 Mg(TEA)2

+ ethanol solution and stirred for about 1 h, then cen-rifuged to obtain ZnO nanorods containing Ag seeds. Subsequently,he Ag-seeded ZnO nanorods were redispersed into 25 ml 0.0045 M

g(TEA)2

+ ethanol solution and stirred. In another flask, 2 ml oformaldehyde solution and 48 ml of ethanol were mixed. One

illiliters of the mixture was added into the above Ag(TEA)2+

olution containing Ag-seeded ZnO nanorods drop by drop. Theolution was stirred for another 2 h. During the reaction, the reac-

nO–Ag core–shell nanocomposites.

tion mixture turned brown, indicating the generation of silver. Andthen, the products were centrifuged and washed with ethanol.Finally, it was dried in vacuum at 60 ◦C for 3 h to obtain ZnO–Agcore–shell nanocomposites. The detailed procedure was illustratedin Fig. 1. In order to improve the crystallinity and bonding strengthof Ag nanoparticles, the ZnO–Ag nanocomposites were annealedat 150 ◦C and 250 ◦C for 2 h. To investigate the effect of Ag coat-ing thickness on the morphology and optical properties of ZnO–Agcore–shell nanocomposites in our synthetic method, the concen-tration of Ag(TEA)2

+ and the volume of formaldehyde solutionwas used by 1.5 times and 2 times in last step with other stepsunchanged.

2.4. Characterization

The X-ray diffraction (XRD) patterns of the products weremeasured with a Dmax-3� diffractometer with nickel-filteredCu K� radiation (� = 1.54178 Å). The chemical composition analy-sis was achieved by energy-dispersive X-ray spectroscopy (EDS;Tecnai F20 transmission electron microscope). Morphology andmicrostructures of the products were studied by field emissionscanning electron microscopy (FESEM; JEOL-6300F) and transmis-sion electron microscopy (TEM; Tecnai F20). Specimens for TEMinvestigations were prepared by placing a small drop of samplesuspension in ethanol on a carbon-coated TEM grid, followed byair-drying to remove the solvent. The ultraviolet–visible (UV–vis)spectra were measured at room temperature with a Lambda 35UV–Vis spectrophotometer. The photoluminescence (PL) spectrawere carried out on a fluorescence spectrophotometer (F-4500)using Xe lamp with excitation wavelength of 254 nm.

3. Results and discussion

3.1. Structure and morphology of ZnO–Ag core–shellnanocomposites

The structure, crystalline phase, size, and morphology of ZnOnanorods and ZnO–Ag core–shell nanocomposites were deter-mined by XRD, EDS, FESEM and TEM.

Fig. 2a–d show the XRD patterns of pure ZnO nanorods,ZnO–Ag core–shell nanocomposites, the nanocomposites annealedat 150 ◦C and 250 ◦C, respectively. All the peaks in Fig. 2a canbe identified as hexagonal wurtzite ZnO with lattice constants ofa = 3.250 Å, and c = 5.207 Å, which is consistent with the literaturedata of JCPDS 36-1451. No impurity peaks are detected. In Fig. 2b,two additional diffraction peaks appear compared to pure ZnOnanorods (Fig. 2a). The two additional peaks at 2� = 38.18◦ and44.42◦ are assigned to the diffraction lines of (1 1 1) and (2 0 0)planes of the face-center-cubic (fcc) silver, respectively. Their posi-tion and relative intensities are in good agreement with the JCPDS4-0783 of bulk silver, which proves the formation of crystalline Ag.After anneal, the diffraction peaks of (2 2 0) and (3 1 1) planes of thefcc silver begin to appear and the diffraction intensities are higher

and higher with the increase of anneal temperature. This indicatesthat the crystallinity of Ag nanocrystals is better after anneal.

SEM and TEM images of the uncoated ZnO nanorods are pre-sented in Fig. 3. It can be seen that the needlelike ZnO nanorods arevery smooth on the surface, usually 40–100 nm in diameter, and

6078 F. Li et al. / Applied Surface Scien

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coated copper grids. It can be seen that zinc, oxygen and silver ele-

ig. 2. Typical XRD patterns of (a) pure ZnO nanorods, (b) ZnO–Ag core–shellanocomposites. (c) ZnO–Ag nanocomposites annealed at 150 ◦C, (d) ZnO–Aganocomposites annealed at 250 ◦C.

everal microns in length. The aspect ratio of the nanorods is in theange of 15–20. High-resolution TEM (HRTEM) image of an indi-idual ZnO nanorod shown in Fig. 3C reveals that the interplanarpacing is 0.52 nm, which corresponds to the (0 0 1) crystal plane

Fig. 3. SEM and TEM images of ZnO nanorods. (A) Low-magnification SEM image, (B)

ce 256 (2010) 6076–6082

of ZnO crystal. The high-resolution image indicates that the ZnOnanorod is single crystal and grow along the [0 0 1] direction.

Fig. 4 shows the typical SEM and TEM images of thesynthesized ZnO–Ag core–shell nanocomposites. Representativelow-magnification SEM image of the ZnO–Ag core–shell nanocom-posites is illustrated in Fig. 4A, where the needlelike ZnO nanorodsare not smooth on the surface in comparison with Fig. 3.High-magnification SEM image of the ZnO–Ag core–shell nanocom-posites is shown in Fig. 4B. It can be clearly observed that the ZnOnanorods are coated by uniform thin nanoshell. The nanoshell iscomposed of large number of spherical Ag nanoparticles, whosesize ranges from 10 nm to 40 nm. The measured average diameter ofthe Ag nanoparticles is about 20 nm. The TEM images in Fig. 4C andD confirm the results of the SEM images. By comparison with Fig. 3C,the nanorods coated with Ag nanoparticles appeared much darkerthan the uncoated nanorods, which is characteristic of the presenceof Ag nanoparticles partially or entirely covered on ZnO nanorods’surface. The Ag nanoparticle marked black in Fig. 4D is furtherobserved in Fig. 4E. The HRTEM image reveals the lattice spacing ofd = 0.236 nm, corresponding to the (1 1 1) planes of the metallic Agwith fcc structure. The result demonstrates that the Ag nanoparti-cles in the nanocomposites are metallic single crystalline Ag withfcc structure, which is in good agreement with the XRD result.Fig. 4F shows the energy-dispersive spectrum (EDS) of the ZnO–Agcore–shell nanocomposites dispersed on amorphous carbon-

ments are present in the samples. The detected carbon and copperelements come from the amorphous carbon-coated copper grids.

Fig. 5 shows the SEM and TEM images of the ZnO–Ag nanocom-posites annealed at 150 ◦C and 250 ◦C. On the whole, the coating

high-magnification SEM image, (C) TEM image, (D) high-resolution TEM image.

F. Li et al. / Applied Surface Science 256 (2010) 6076–6082 6079

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ig. 4. SEM, TEM images and EDS spectrum of ZnO–Ag core–shell nanocomposagnification TEM image, (D) TEM image of a single ZnO–Ag nanocomposite, (E) hi

f Ag nanoparticles on the ZnO nanorod is denser after anneal,hich implies stronger bond strength between Ag nanoparticles

nd ZnO nanorod. The size of the Ag nanoparticles after anneal isearly unchanged comparing to those without anneal. The HRTEM

mages in the insets show the lattice spacing of d = 0.236 nm (the1 1 1) planes of the metallic Ag with fcc structure), indicating theingle crystalline nature of the Ag nanoparticles.

Fig. 6 presents the SEM images of ZnO–Ag core–shell nanocom-osites prepared by higher concentration of Ag(TEA)2

+. Obviously,ith the increase of the concentration of Ag(TEA)2

+, Ag nanopar-icles attached on the ZnO nanorods gets more and more densely,hose sizes are almost unchanged. It indicates that the concen-

A) Low-magnification SEM image, (B) high-magnification SEM image, (C) low-solution TEM image marked in (D), (F) EDS spectrum.

tration of Ag(TEA)2+ has great effect on the content of Ag on ZnO

nanorods and little effect on the size of Ag nanoparticles.

3.2. Synthesis of ZnO–Ag core–shell nanocomposites

A solution-based synthetic method is utilized to produce het-erostructure ZnO–Ag core–shell nanocomposites under ambient

conditions. The formation of ZnO–Ag core–shell nanocompositesis based on nucleation and growth of Ag by the reduction reactionbetween Ag(TEA)2

+ and formaldehyde on the selective and suit-able surface sites of ZnO nanorods that could serve as Ag seedsfor the formation of heterogeneous nanostructured composites.

6080 F. Li et al. / Applied Surface Science 256 (2010) 6076–6082

Fig. 5. SEM image (A), TEM image (B) and HRTEM image (C) of ZnO–Ag core–shell nanocomposites annealed at 150 ◦C; SEM image (D), TEM image (E) and HRTEM image (F)of ZnO–Ag core–shell nanocomposites annealed at 250 ◦C.

Fig. 6. SEM images of ZnO–Ag core–shell nanocomposites prepared by higher concentration of Ag(TEA)2+ (A, B) 1.5 times, (C, D) 2 times.

F. Li et al. / Applied Surface Science 256 (2010) 6076–6082 6081

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ig. 7. UV-vis absorption spectra of (a) ZnO nanorods, (b) Ag nanoparticles,c) ZnO–Ag core–shell nanocomposites, (d) ZnO–Ag nanocomposites annealed at50 ◦C, (e) ZnO–Ag nanocomposites annealed at 250 ◦C.

he synthetic mechanism of ZnO–Ag core–shell nanocompositess proposed and illustrated in Fig. 1, which comprises three majorteps.

nO nanorod + Sn2+ → ZnO nanorod-Sn2+ (1)

nO nanorod-Sn2+ + Ag(TEA)2+ → ZnO nanorod-Ag seeds (2)

nO nanorod-Ag seeds + Ag(TEA)2+ + HCHO

→ ZnO nanorod-Ag nanoparticles (3)

In detail, the first step of the synthetic process consisted ofreparation of the ZnO nanorods with hydroxy groups as men-ioned above. In the second step, the ZnO nanorods were dispersedn ethanol solution containing SnCl2 ethanol solution during vig-rous stirring. Sn2+ was absorbed on ZnO nanorods due to thelectrostatic attraction between Sn2+ and the surface of ZnOanorods. Then, Sn2+ reacted with Ag(TEA)2

+ in ethanol to formg seeds, thus, Ag seeds located selectively at suitable sites on

he surface of ZnO nanorods. Sn2+ played an irreplaceable rolen the formation of Ag seeds. Subsequently, those sites on thenO surface acted as centers for the reduction of Ag(TEA)2

+ byormaldehyde and then form nucleation sites for reduced Ag atoms.inally, reduction of Ag(TEA)2

+ occurred at the preexisting nucle-tion sites followed by nucleation and growth of reduced Ag atomsr nanoclusters. Shan et al. [20] have also reported the synthe-is of ZnO–Ag core–shell nanocrystals. In their synthetic method,nO colloids were used as templates. The reaction was followedy reduction of AgNO3 with hydroquinone in ZnO colloids. How-ver, there existed large number of separated Ag nanoparticles inhe final products. Our method guarantees the controllable and uni-orm growth of Ag nanoparticles on the surface of the ZnO nanorodsnd all the Ag nanoparticles are firmly attached on the selectiveites of the surface of ZnO nanorods.

.3. UV–vis absorption

The optical absorption spectra of pure ZnO nanorods, pure Ag

anoparticles, ZnO–Ag core–shell nanocomposites and annealedanocomposites are shown in Fig. 7. The ZnO nanorods exhibit UVbsorption at 346 nm, as shown in line a. This peak is the exci-on absorption of ZnO nanorods. The pure Ag nanoparticle samplesere prepared through the reaction of Ag(TEA)2

+ and formalde-

Fig. 8. UV–vis absorption spectra of ZnO–Ag core–shell nanocomposites using dif-ferent concentration of Ag(TEA)2

+. (a) 1.5 times, (b) 2 times.

hyde solution without the presence of ZnO nanorods. The amountof Ag(TEA)2

+ and formaldehyde solution is the same as the syn-thesis of Ag nanoparticles in the presence of ZnO nanorods. Thepure Ag nanoparticles are in the range of 20–60 nm (SEM imagesnot shown here), with an average size of 40 nm, a little largerthan the Ag nanoparticles coated on the ZnO nanorods. The surfaceplasmon absorption of Ag nanoparticles prepared by the formalde-hyde reduction method in the absence of ZnO nanorods is verybroad and centered at 417 nm (line b). However, the absorptionof the ZnO–Ag core–shell nanocomposites is not a simple super-position of those of their individual single-component materials.As can be seen from line c, the surface plasmon band of ZnO–Agcore–shell nanocomposites shows a redshift of 18 nm compared topure Ag nanocrystals, which is similar to the previous report [21].The quantum size effect can be ruled out because the Ag nanopar-ticles coated on the ZnO nanorods are smaller than those pure Agnanoparticles.

The reason for the redshift of the surface plasmon absorp-tion is due to the strong interfacial coupling between neighboringnanoparticles. In detail, electron transfers from Ag to ZnO inZnO–Ag core–shell nanocomposites because the Fermi energy levelof Ag is higher than that of ZnO, resulting in the deficient elec-trons on the surface of the Ag nanoparticles. Therefore, the deficientelectrons on the surface of the Ag nanoparticles subsequently leadto the redshift of the surface plasmon absorption. Upon anneal-ing at higher temperature, the redshift gets a rise. Particularly,the surface plasmon absorption has a redshift of about 24 nm forthe nanocomposites annealed at 150 ◦C and a remarkable red-shift of 67 nm for the nanocomposites annealed at 250 ◦C. Thebig redshifts should originate from stronger interfacial electroniccoupling between Ag and ZnO after anneal. Anneal treatmentchanges the electronic coupling of the supported Ag nanoparti-cles on the ZnO nanorods and correspondingly influence its surfaceplasmon energy as well. Similar results are also reported forZnO–Au nanocomposites [22]. However, the absorption peaks ofthe pure Ag nanoparticles annealed at 150 ◦C and 250 ◦C are locatedat 453 nm and 466 nm (UV–vis spectra not shown here), respec-tively. Comparing to Fig. 7b, anneal treatment has the absorption

of the pure Ag nanoparticles redshifted. Therefore, the nanoclus-ter coupling or coalescence between Ag nanoparticles cannot beruled out. Fig. 8 shows the UV–vis absorption spectra of ZnO–Agcore–shell nanocomposites with different Ag coating thickness.

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ig. 9. PL spectra of (a) pure ZnO nanorods, (b) ZnO–Ag core–shell nanocompos-tes, (c) ZnO–Ag nanocomposites annealed at 150 ◦C, (d) ZnO–Ag nanocompositesnnealed at 250 ◦C.

hen the concentration of Ag(TEA)2+ is 1.5 times and 2 times as

riginal, the surface plasmon absorptions have redshifts of about2 nm and 49 nm, respectively. This indicates the stronger interfa-ial electronic coupling between Ag and ZnO when the Ag coatinghickness is increased. The UV absorption of ZnO–Ag core–shellanocomposites and annealed nanocomposites also redshift bybout 24 nm compared to pure ZnO nanorods, which implies thathe size of ZnO nanorods turn larger during the synthesis ofnO–Ag core–shell nanocomposites because the absorption shiftf pure ZnO nanocrystals is usually related to their sizes’ change23].

.4. Photoluminescence

The photoluminescence spectrum of an unmodified sample ofnO nanorods is shown in Fig. 9a and exhibits a shoulder peak cen-ered at about 410 nm and a sharp peak at 465 nm. The two emissioneaks can be assigned to the recombination of a photogeneratedole with the single ionized charged state of the defect [24,25].he growth of Ag nanocrystals onto the ZnO nanorods completelyuenches the PL, as shown in Fig. 9b–d. As we know, the emissioneaks at 410 nm and 465 nm is due to the defect-related emissions.n one hand, the defect-related emission peaks can be reducedy the surface modification on ZnO nanorods, on the other hand,hey may also be reduced by the decline of crystal defects afternneal. In contrast to the ZnO–Ag core–shell nanocomposites inur work, Wang et al. found that the ZnO band gap emission wasnhanced greatly for core–shell ZnO–Au nanocrystals [26]. Theyttributed the ZnO band gap emission enhancement to the transferf Au surface electrons to ZnO nanocrystals, which promotes elec-

ron density in the conduction band of ZnO. The difference betweenur work and Wang’s is that ZnO nanorods here have no bandap emission. The PL spectra of ZnO–Ag core–shell nanocompos-tes with different Ag coating thickness were all quenched (PL nothown here).

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ce 256 (2010) 6076–6082

4. Conclusion

In summary, we have developed a facile and general strat-egy for the synthesis of ZnO nanorod-Ag nanoparticles core–shellnanocomposites. Detailed structural characterization and elemen-tal analysis demonstrate the presence of a core made of ZnOnanorod, surrounded by a uniform thin shell of Ag nanoparticles.The synthesis employs ZnO nanorods as the cores, where silverseeds are grown on the surface, and silver growth occurs on theZnO nanorods to form the unique silver nanoshell. The formationof unique silver shells on ZnO nanorods is facilitated by silver seeds.The anneal treatment can improve the crystallinity of Ag nanocrys-tals and enhance the bond strength between Ag and ZnO. Theabsorption band of ZnO–Ag core–shell nanocomposites has someredshifts and the PL is quenched, revealing the strong interfacialinteraction between ZnO nanorods and Ag nanoparticles. With theincrease of anneal temperature, the redshifts of optical absorptionand the quenching effects of PL are more obvious. The effect of Agthickness on the optical properties of ZnO–Ag core–shell nanocom-posites was also studied. We believe that our synthetic strategycan be readily extended to the preparation of core–shell and het-erostructures of semiconducting metal oxide nanocrystals coatedwith Ag nanoparticles. Such core–shell and heterostructures willfind many applications in photocatalysis, photovoltaics, plasmon-ics, and optics.

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