Journal of Nanoparticle Research

, Volume 11, Issue 5, pp 1159–1166

Preparation and characterization of Ag/ZnO composites via a simple hydrothermal route

Authors

  • Xiao-Yun Ye
    • School of Chemistry and Chemical EngineeringSoutheast University
    • School of Chemistry and Chemical EngineeringSoutheast University
  • Yan-Qing Sun
    • School of Chemistry and Chemical EngineeringSoutheast University
  • Jing Chen
    • School of Chemistry and Chemical EngineeringSoutheast University
  • Zhi-Qiang Wang
    • School of Chemistry and Chemical EngineeringSoutheast University
Research Paper

DOI: 10.1007/s11051-008-9511-z

Cite this article as:
Ye, X., Zhou, Y., Sun, Y. et al. J Nanopart Res (2009) 11: 1159. doi:10.1007/s11051-008-9511-z

Abstract

Silver/zinc oxide (Ag/ZnO) composites were fabricated by a facile one-pot synthesis method under hydrothermal conditions. By choosing glucose as the reductant, metal Ag was fabricated from Ag2O with the growth of the crystalline ZnO. Structure measurements revealed that obtained Ag/ZnO composites comprised wurtzite ZnO and face-centered cubic structure of nanosized Ag, which uniformly distributed in the composites. Moreover, the morphology of the ZnO was varied regularly with the formation of Ag nanoparticles from flower-like to rod-like and finally returned to flower-like. The optical properties of UV–Vis and surface-enhanced Raman scattering spectra of the composites as well as effects of the dimension of metal Ag fabricated during the prior period of reaction on the morphology of ZnO were discussed.

Keywords

SilverZinc oxideCompositeHydrothermal processSEMNanomaterial

Introduction

Advanced materials derived from metallodielectric composites are of extensive scientific and technological interest, due to their unique electrical, physical, and mechanical properties for various application areas in materials science and chemical science (Caruso et al. 1998, 2001; Correa-Duarte et al. 1998). Especially, metal-modified oxide semiconductor materials have their potential uses as catalysts, sensors, substrates for surface-enhanced Raman scattering (SERS), and colloidal entities with unique optical properties (Zhong and Maye 2001; Gittins et al. 2002; Pham et al. 2002; Liz-Marzan 2006; Tian et al. 2007). Many examples can be found, such as colloid crystals (Shan et al. 2007), thin films (Zhang et al. 2005), and other one-dimensional nanostructures of rods (Wu et al. 2004), wires (Lee et al. 2005), etc. Recently, a number of reports have been published on the oxide semiconductor modification by noble metal of silver, which was used as building blocks toward functional nanostructures (Pastoriza-Santos and Liz-Marzan 2002; Okada et al. 2004). Up to now, several routes have been reported for the fabrication of Ag-modified semiconductor composites such as mechanical mixing of powders (Joshi et al. 1995), electroless coating process (Kobayashi et al. 2001; Ye et al. 2007a), spray-coprecipitation (Kang and Park 1999), sono-chemical synthesis (Pol et al. 2002, 2003; Ye et al. 2007b), and hydrothermal reaction (Zhang and Mu 2007). Various reductants have been described, including Sn2+ ion, NaBH4, H2, and HCHO. In the present work, a one-pot synthesis method of Ag/ZnO composites via a simple hydrothermal process is described. During the synthesis process, zinc salt and silver ion were added together for the composite fabrication. The semiconductor materials of ZnO used in our experiment, with wide band gap, have exceptionally important applications in both fundamental research and practical studies (Shiosaki and Kawabata 1974; Pan et al. 2001; Vayssieres et al. 2001a, b; Vayssieres 2003). Modification of ZnO properties by impurity incorporation is currently an important issue for their potential applications. For example, Ando and co-workers have demonstrated that the magnetic coupling of ZnO is enhanced by Mn and Ni doping (Ando et al. 2001). Similar research has been done by Wei et al. with Co doping (Wei et al. 2006).

Herein, we present a novel route to synthesize Ag/ZnO composites, using glucose as reductant for the formation of nanosized Ag nanoparticles (Nersisyan et al. 2003). The deposition of silver on pillar-like ZnO is discussed previously (Zhang and Mu 2007). However, the monodispersity and particle size of Ag still need to be ameliorated for the improvement of electrical, physical, and mechanical properties in the composites. By varying the amount of silver salt (Zhu et al. 2006), we have found the variation of the ZnO morphology. In particular, well-dispersed Ag nanoparticles in the Ag/ZnO composites were observed without aggregation. The structural and optical properties are studied in detail by XRD, SEM, TEM, UV–Vis, and SERS.

Experimental section

Zinc nitrate [Zn (NO3)2 · 6H2O, 99%], sodium hydroxide (NaOH, 98%), silver nitrate (AgNO3, 99.5%), cetyl trimethylamine bromide (CTAB, 99%), glucose (CH2OH(CHOH)4CHO, 99%) were all obtained from Shanghai Chemical Reagent Co. and used as received without further purification. Deionized water was used in all preparations. The preparation of Ag/ZnO composites by hydrothermal reaction is described as followed. Five milliliter of NaOH (5 M), 2.5 mL of Zn (NO3)2 (1 M), and the required amount of CTAB were mixed and then transferred into a Teflon-lined stainless steel autoclave of capacity 100 mL. A certain amount of water was added to make the total volume up to 60 mL. Subsequently, 1 mL of AgNO3 with known concentration was poured into the reactor under vigorous stirring, followed by the addition of excessive glucose with fixed mass of 1 g. The autoclave was sealed and heated in an oven up to 180 °C at a heating rate of 1 °C/min. Such temperature was maintained for 24 h. Finally, the resultant was centrifugally separated from the solution after natural cooling and then thoroughly washed by acetone and water by turns. The content of Ag in Ag/ZnO composites was varied from 0% to 22%.

TEM was performed with a Hitachi H-600 microscope operating at 120-kV accelerating voltage. HRTEM was measured on a JEOL JEM-2010 microscope operating at 200-kV accelerating voltage. Samples were prepared by placing drops of the colloid dispersion on a Cu grid (200 mesh; placed onto filter paper to remove excess solvent) and letting the solvent evaporate at room temperature. SEM was performed with a microscope of JSM-5610LV. XRD was performed with an X-ray diffractometer (XD-3A) with CuKα radiation operating at 40 kV and 30 mA. UV–Vis absorption spectra were measured with a Shimadzu UV-2201 spectrophotometer. Raman spectra were done with a JY-HR800 laser light scattering spectrograph.

Results and discussion

The XRD patterns of the Ag/ZnO composites with various Ag content of 0%, 0.8%, 3%, 8%, 15%, 22% are shown in Fig. 1a–f. All samples illustrate the typical patterns of hexagonal zincite. From Bragg’s formula, the calculated interplanar spacing matches well with those of the wurtzite ZnO structure with the diffraction peaks of the (100), (002), (101), (102), (110), and (103) planes (JCPDS file no. 36-1451). With the addition of Ag, the peaks of ZnO remain almost the same. When the content of Ag reaches 3%, a pattern resembling a mixture of wurtzite ZnO and silver appears (note the peak at 38° in Fig. 1c). Furthermore, the diffraction peaks at 38°, 44°, 64°, which correspond to the (111), (200), (220) planes of metallic Ag with face-centered cubic (fcc) structure, are observed (JCPDS file no. 4-783) as the content of Ag up to 8%. In addition, the average lattice constants a and c of ZnO after the addition of Ag are about 3.241 and 5.19 Å, respectively, which are close to those of the wurtzite ZnO crystal structure in the standard card. It is indicative that the crystallization of ZnO is not affected by the Ag nanoparticles affixion.
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Fig. 1

XRD patterns of Ag/ZnO composites with various Ag content: (a) without Ag, (b) 0.8%, (c) 3%, (d) 8%, (e) 15%, (f) 22%. The characteristic peaks of metallic Ag are marked by asterisk

Figures 2 and 3 show the SEM and TEM images of the Ag/ZnO composite samples. All the samples are treated at the same CTAB-assisted hydrothermal condition at 180 °C for 24 h. It can be seen that flower-like pure ZnO with an average size of ~5 μm consisted of the ZnO sword-like nanorods with about 200–300 nm in width and average 2.5 μm in length (Fig. 2a and b). A similar morphology of ZnO was previously observed by Zhang et al. (2004). However, the morphology of the ZnO was varied from flower-like to rod-like, finally to flower-like pattern after the increased addition of Ag. When the content of Ag reaches 0.8% (Fig. 2c), the flower-like ZnO is partially disappeared, going with some ZnO nanorods. Figure 3b exhibits the magnified end of the flower-like Ag/ZnO composites marked by white oval in Fig. 3a, which shows an enlarged figure of the Ag scatter and obvious Ag nanoparticles of sphere-like dot. The trend of the growth of rod-like ZnO becomes more obvious with increased amount of Ag (Fig. 2d). Only ZnO nanorod is observed with homogeneous distribution of Ag nanoparticles until the Ag content increases to 8% (Figs. 2e and 3d). Whereas when the addition amount of Ag is up to 15%, flower-like ZnO appears again with less petals (Figs. 2f and 3e). Simultaneously, the density of Ag is obviously increased. In the case of the Ag content to 22%, flower-like Ag/ZnO composites are completely obtained (Figs. 2h and 3f). It is clear that Ag nanoparticles are distributed along each petal of flower-like ZnO or ZnO nanorod. At the same time, the size of the Ag nanoparticles decreased with the Ag content addition to 22%, which is in accord with the report by Tojo et al. (1997). Moreover, the electron diffraction pattern of a twin Ag particle is presented as shown in the inset in Fig. 3g, which corresponds to the fcc structure of the metallic silver (Sarkar et al. 2005). The Ag nanoparticle marked white in Fig. 3g is further observed (Fig. 3h). The image reveals the lattice spacing of d = 0.234 nm, which is attributed to the (111) planes of the metallic silver with fcc structure. The result demonstrates that the Ag nanoparticles in the composites are metallic silver with fcc structure, which is in good agreement with the XRD results. Little free Ag nanoparticles could be observed, as illustrated in TEM images.
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Fig. 2

SEM images of Ag/ZnO composites: (a, c, d, e, f, h) with Ag content of 0%, 0.8%, 3%, 8%, 15%, and 22%; (b, g, i) the magnified images correspond to (a), (f), and (h), respectively

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Fig. 3

TEM micrographs of Ag/ZnO composites: (a, b) low-resolution and magnified images with Ag content of 0.8%; (cf) with Ag content of 3%, 8%, 15%, and 22%; (g) low-resolution image with Ag content of 8% (the inset: corresponding selected-area electron diffraction (SAED) pattern from a twin particle; (h) high-resolution image of the particle white marked in g

EDX analysis of the Ag/ZnO composites with Ag content of 0%, 3%, 15% shows that the concentrations of Ag in the nanofeatures are the same (to within the limits of error) as the ratios of Ag in the original precursor solution. The typical EDX spectra are illustrated in Fig. 4. All of these elements (C, O, Zn, and Ag) are observed. Moreover, with the increased amount of Ag content, the peaks signed to Ag become obvious, further indicating that silver is successfully introduced to the composites. Besides, Pt signals in these spectra came from the platinum coating for SEM specimens.
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Fig. 4

EDX spectra of Ag/ZnO composites with various Ag content: (a) without Ag; (b) 3%; (c) 15%

The formation of composites was monitored by UV–Vis absorption spectroscopy. The optical plasmon resonance of single metal nanoparticles is confined to relatively narrow wavelength ranges. The optical properties toward metal particles in the composites would be different from that of individual metal particles. Moreover, the position of absorption peak depends on the dimension of the metal as well as the surroundings. Figure 5 shows the characteristic spectra of the ZnO before and after silver addition. The absorption spectrum of ZnO shows a peak at about 370 nm (Fig. 5a), due to surface plasmon resonance of ZnO. After the addition of silver nanoparticles, new broad peaks at around 440 nm appear in all samples containing silver (Fig. 5b–f), which are attributed to the surface plasmon resonance of Ag nanoparticles (Michaelis et al. 1994; Li et al. 2003). In comparison to the narrow absorption peak of pure Ag nanoparticles, the observed broad bands are evidence of strong interactions between ZnO and silver nanoparticles.
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Fig. 5

UV–Vis absorption spectra of Ag/ZnO composites with different Ag content: (a) without Ag, (b) 0.8%, (c) 3%, (d) 8%, (e) 15%, (f) 22%

The SERS spectra of the as-grown ZnO and Ag/ZnO composites (8% of Ag content) are shown in Fig. 6. As for ZnO is concerned, it has a wurtzite structure and belongs to C6V space group. According to the symmetry selection rules, eight modes of 2E2, 2E1, 2A1, and 2B2 are for the wurtzite phase, in which four modes of A1 + 2E2 + E1 are Raman-active (Xu et al. 2004). As illustrated in Fig. 6a, peaks are observed at 332 and 438 cm−1, corresponding to IInd-E2 and E2high of the ZnO lattice vibrations, respectively (Pachauri et al. 2006), which are in good agreement with the theoretical calculations of Balandin et al. for bulk ZnO (Alim et al. 2005). Besides, the full width at half maximum (fwhm) value of the E2high is ~8 cm−1, indicating high crystalline of the ZnO as revealed by XRD. By comparison, the peak of E2high of ZnO fades in Ag/ZnO composites with additional vibration modes observed at 580 and 670 cm−1 (Fig. 6b). The vibration mode at 580 cm−1 is LO modes with E1 symmetry of ZnO. It is believed that the appearance of the E1(LO) mode generally results from impurities and structural defects. No impurities in the composites were found by EDX analysis. Therefore, structural defects caused by Ag are the main reason for the appearance of the E1(LO) mode, showing the formation of Ag/ZnO composites. Toward the mode at 670 cm−1, it is considered to associate with Ag addition as it is not found in pure ZnO. The similar kind of phenomenon was also observed in other system of Fe-doped ZnO films by Bundesmann et al. (2003).
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Fig. 6

Raman scattering spectra of (a) the as-grown ZnO flower-like structure, (b) Ag/ZnO composites with Ag content of 8%

Ag/ZnO composites were prepared by one-pot hydrothermal synthesis, which involved the growth of ZnO and the formation of Ag nanoparticles using glucose as reductant in the presence of CTAB. Glucose, which acts as a moderate and convenient reducing agent, represents a practical interest in preparing colloidal dispersion of silver at the boiling temperature of water. The hydrothermal surroundings offer suitable temperature and pressure for the formation of ZnO and Ag. In general, bulk ZnO is a polar crystal, whose positive polar plane is rich in Zn and the negative polar plane is rich in O. According to Laudise’ argument on the growth of polar crystals, the growth rate along the <0001> direction is the fastest, which results in the disappearance of the (0001) faces in the end (Laudise and Ballman 1960). ZnO22−, the growth unit of ZnO, leads to the fastest growth rate of plane of (0001) in the hydrothermal process. So, the sword-like ends of the ZnO petal and rod are observed in all samples. Before the addition of Ag, flower-like ZnO was fabricated using CTAB as morphology control agent. From the following chemical equation, it can be seen that OH is firstly introduced into Zn2+ aqueous solution for the formation of Zn(OH)2, which can partly transform into ZnO nuclei (Eqs. 1 and 2). Simultaneously, the Zn(OH)2 could be gradually converted into soluble ZnO22− (Eq. 3), which interacts with capsules of CTAB generated in the saturation solution by the coulomb force to form complexing agents. Such complexing agents capped on the surface of ZnO nuclei result in the formation of flower-like ZnO (Eq. 4) (Zhang et al. 2004).
$$ {\text{Zn}}^{{ 2 + }} {\text{ + 2OH}}^{ - } \rightleftharpoons {\text{Zn}}\left( {\text{OH}} \right)_{2} $$
(1)
$$ {\text{Zn}}\left( {\text{OH}} \right)_{ 2} \to {\text{ZnO} + \text{H}}_{ 2} {\text{O}} $$
(2)
$$ {\text{Zn}}\left( {\text{OH}} \right)_{ 2} + {\text{ 2OH}}^{ - } \rightleftharpoons {\text{ZnO}}_{ 2}{}^{2 - } + 2 {\text{H}}_{ 2} {\text{O}} $$
(3)
$$ {\text{ZnO}}_{ 2}{}^{2 - } + {\text{H}}_{ 2} {\text{O}} \rightleftharpoons {\text{ZnO}} + {\text{2OH}}^{ - } $$
(4)
$$ 2{\text{Ag}}^{ + } + 2{\text{OH}}^{ - } \to 2{\text{AgOH}} \to {\text{Ag}}_{ 2} {\text{O}} + {\text{H}}_{ 2} {\text{O}} $$
(5)
$$ {\text{Ag}}_{ 2} {\text{O}} + {\text{CH}}_{ 2} {\text{OH}}\left( {\text{CHOH}} \right)_{ 4} {\text{CHO}} \to {\text{Ag}} + {\text{CH}}_{ 2} {\text{OH}}\left( {\text{CHOH}} \right)_{ 4} {\text{COOH}} $$
(6)
Why the addition of Ag has a great influence on the morphology of ZnO? Mu et al. observed that the morphology of ZnO was varied from pillar-like to rod-like with the increased concentration of Ag+. It was considered that the changes of the equilibrium between zinc ions and ammonia led to varied morphology of ZnO (Zhang and Mu 2007). Whereas in our experiment, the addition amount of Ag+ is negligible in contrast to high concentrated alkali. Therefore, the fabricated Ag nanoparticles are effective during the growth of ZnO, while other experimental parameters remain unchanged. Herein, the fabricated Ag nanoparticles assemble with ZnO nuclei to provide heterogeneous nucleation sites for the growth of the ZnO (Pachauri et al. 2006). Moreover, various species and dimension of metal (Govender et al. 2002; Pachauri et al. 2006; Wei et al. 2006) used as structure directing agent could lead to dissimilar morphology of ZnO. During the reduction of Ag by glucose (Eq. 6), the dimension of the silver nanoparticles is decreased with the increase of Ag+ concentration, resulting in the variation of ZnO from rod-like to flower-like. The illustration of the growth mechanism of Ag/ZnO composites is shown in Fig. 7.
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Fig. 7

Illustration of the growth mechanism of Ag/ZnO composites

Conclusion

In summary, Ag/ZnO composites with various structure and well-distributed sliver nanoparticles were fabricated through a simple one-step hydrothermal synthesis route. Ag nanoparticles in composites are generated from Ag2O using glucose as reductant. The morphology of the composites was varied with the increased amount of Ag. The silver nanoparticles of different size assembled with ZnO nuclei result in the various oriented growth of ZnO. The obtained Ag/ZnO composites exhibit a mixed structure which comprises wurtzite of ZnO and fcc of nanosized Ag. Analysis shows that the addition of silver has great effects on the UV–Vis and Raman spectra. Moreover, this work not only obtained Ag/ZnO composites, which are synthesized by a simple approach and possess great potential applications in various fields, but also afforded a simple and effective way to synthesis other similar composites with special morphology.

Acknowledgments

The authors are grateful to the National Nature Science Foundation of China (50873026), the Six Talents Pinnacle Program” of Jiangsu Province of China (06-A-033), and Science and Technology Support Program of Jiangsu Province of China for financial supports of this research. We are also grateful to Mr. Aiqun Xu from the Analysis and Testing Centre of Southeast University for his kind help with the measurements.

Copyright information

© Springer Science+Business Media B.V. 2008