Preparation and characterization of Ag/ZnO composites via a simple hydrothermal route
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.
KeywordsSilver Zinc oxide Composite Hydrothermal process SEM Nanomaterial
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.
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
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.
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.