J. Phys. Chem. C 2009, 113, 8065–8069
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SERS Spectroscopy Used To Study an Adsorbate on a Nanoscale Thin Film of CuO Coated with Ag Yunxin Wang,†,‡ Wei Song,† Weidong Ruan,† Jingxiu Yang,† Bing Zhao,*,† and John R. Lombardi*,§ State Key Laboratory of Supramolecular Structure and Materials, Jilin UniVersity, Changchun 130012, People’s Republic of China, Jilin Center for Disease Control and PreVention, Changchun, 130031, People’s Republic of China, and Department of Chemistry, The City College of New York, New York, New York 10031 ReceiVed: January 4, 2009; ReVised Manuscript ReceiVed: April 1, 2009
In this work, 5, 15, and 30 nm Ag island films over CuO thin films (AgFOCuF) were prepared as SERS substrates. The spectra of 4-mercaptopyridine (4-Mpy) adsorbed on these AgFOCuF and bare Ag island films are compared. The Raman spectrum of a bare 5 nm Ag island film is weak. However, the Raman signal of 4-Mpy from 5 nm AgFOCuF substrate is enhanced and can be readily ascribed to surface plasma resonance of silver and charge transfer between the silver and CuO nanoparticles. Moreover, the difference between the spectra of 4-MPY adsorbed on CuO film partially covered with Ag island film and CuO film entirely covered with Ag film confirms that the observed Raman scattering arises from the interaction between CuO film and Ag island film. 1. Introduction Raman spectroscopy has always been attractive to surface scientists as it can be, in principle, used for the investigation of adsorbed molecules on chemical, physical, and biological surfaces and interfaces at the molecule scale. However, the major limitation of conventional Raman spectroscopy is its low sensitivity due to the inherently inefficient scattering process. One possible approach, which overcomes this drawback, is surface enhanced Raman scattering (SERS) spectroscopy. It has evolved as a powerful and sensitive tool for detecting and identifying a wide range of adsorbed molecules down to the limit of single molecule detection. SERS has become one of the most important spectroscopic methods for surface science,1 analytical chemistry,2,3 and biology.4,5 This is due to the highly enhanced vibrational signals, low detection requirements, and good selectivity for adsorbates. For many years SERS was restricted primarily to analytes adsorbed on coinage (Au, Ag, and Cu) and Pt, Ni, and Pd6 rough metallic surfaces. Recently, it has been found that various semiconductor nanoparticles such as ZnO, 7,8 TiO2,9 and Cu2O10 can also directly generate weak SERS activity with typical prominent enhancement factors ranging from 101 to 103. Our group has succeeded in observing SERS effects on shuttle-like CuO nanocrystals with lengths of about 80 nm, although the signal was not very strong (the enhancement factor was calculated at about 100).11 CuO is a narrow band gap p-type semiconductor and has been recognized as an essential material for diverse practical applications, including catalysis, batteries, magnetic storage media, solar energy conversion, gas sensing, and field emission.12–17 More recently, nanoscale CuO systems are of great current interest, mainly because of their unique properties and diverse industrial applications.18,19 * Author to whom correspondence should be addressed. † Jilin University. ‡ Jilin Center for Disease Control and Prevention. § The City College of New York.
Recently, there have been many reports on the fabrication of semiconductor/metal composites because for a semiconductor one can tune the localized surface plasmonic resonance (LSPR) of metallic nanostructures.20,21 In this paper, we describe the observation of a SERS signal from 4-mercaptopyridine (4MPy) on a nanoscale CuO thin film coated with a discontinuous layer of Ag (AgFOCuF). We were not able to detect the SERS signal of 4-MPy from the CuO surface without the deposited Ag film. By virtue of the SERS of 4-MPy on the CuO thin film coated with Ag nanoparticles, the enhancement mechanism suggests that Ag nanoparticles would excite localized surface plasmon resonance (LSPR) under the irradiation of a suitable laser, further enhancing the SERS signals of 4-MPy adsorbed on the CuO thin film through long-range electromagnetic enhancement. In addition, the obvious enhancement of 5 nm AgFOCuF may be ascribed to the charge transfer between the nanoscale metal and the semiconductor. Furthermore, a stronger enhanced electromagnetic field is produced between the interface of the metal and the semiconductor. These results permit us to further understand the influence on semiconductors of localized surface plasmon resonance of metal and extract more information about the surface of CuO as functional components for future electronic, optoelectronic, and biocompatible material. 2. Experimental Section 2.1. Chemicals and Materials. All the reagents were used as received without further purification. 4-Mpy was obtained from Aldrich. Silver (99.95%) was from Alfa. Copper acetate and poly(ethylene glycol) (PEG) were obtained from the Guangfu Chemical Co. (China). Isopropanol and ethanolamine were purchased from the Beijing Chemical Co. (China). 2.2. Experimental Details. CuO thin films were prepared by the sol-gel-like processing method.22 In a typical procedure, the solutions were prepared by dissolving copper acetate in isopropanol and ethanolamine. Afterward, PEG was added to the resulting solution. Then the mixture was stirred for 10 min. The resulting solutions were mixed for 10 min with use of a
10.1021/jp900052q CCC: $40.75 2009 American Chemical Society Published on Web 04/21/2009
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J. Phys. Chem. C, Vol. 113, No. 19, 2009
Wang et al.
Figure 1. XRD pattern of a five-layer CuO thin film.
magnetic bar and filtered (
