J. Phys. Chem. B 2006, 110, 24585-24592
24585
Electrochemical Fabrication of Two-Dimensional Palladium Nanostructures as Substrates for Surface Enhanced Raman Scattering Yin Li, Gewu Lu, Xufeng Wu, and Gaoquan Shi* Department of Chemistry and Key Lab of Bio-organic Phosphorus and Biochemistry of Education Commission of China, Tsinghua UniVersity, Beijing 100084, People’s Republic of China ReceiVed: June 21, 2006; In Final Form: September 28, 2006
Two-dimensional palladium (Pd) nanostructures have been fabricated by electrochemical deposition of Pd onto an indium tin oxide glass substrate modified with a thin flat film of polypyrrole or a nanofibril film of polyaniline. The experimental results demonstrated that the morphology of Pd nanoparticles strongly depended on the properties of conducting polymers and the conditions of electrochemical deposition. Two-dimensional nanostructures composed of flower-like (consisting of staggered nanosheets) or pinecone-like Pd nanoparticles were successfully synthesized. They can be used as substrates for surface-enhanced Raman scattering after partly decomposing the polymer components by heating in air, and the enhancement factor of the substrate composed of flower-like Pd nanoparticles was measured to be as high as 105 for 4-mercaptopyridine.
Introduction Nanoparticles of noble metals have attracted steadily growing attention due to their interesting optical,1 electrochemical,2 electronic,3 and photoelectrochemical4 properties. Among these materials, extensive efforts have been devoted to palladium (Pd) nanoparticles, mainly due to their wide applications in catalysis, sensors, hydrogen storage and separation, and electronic and optoelectronic devices.5-9 Recently, ordered nanostructures of transition metals including Pd also have been used as substrates for surface enhanced Raman scattering (SERS), and their surface enhancement factor was calculated to be 1-4 orders of magnitude.10-12 The nanostructured metallic SERS substrates are usually prepared by electrochemical oxidation-reduction cycle (ORC),13,14 chemical etching,15 film deposition,16 and template guided growth of metal nanowire arrays.17 On the other hand, electrochemical deposition of metals on the electrodes modified with thin conducting polymer films provides a convenient and cheap route for growing metal nanostructures.18,19 Here, we desire to report the electrodeposition of Pd on the indium tin oxide (ITO) electrodes modified with thin flat polypyrrole (PPY) films or films of polyaniline (PAN) nanowires. Two-dimensional nanostructures composed of flower-like (consisting of staggered nanosheets) or pinecone-like Pd nanoparticles were successfully synthesized. Furthermore, these nanostructured Pd films were tested to be novel substrates for Raman scattering with high enhancement factors. Experimental Section Chemicals and Materials. PdCl2, HCl, sodium dodecylbenzene sulfonate (DBS), HClO4, LiClO4, and 4-mercaptopyridine (4MP) were products of Beijing Chem. Co. (Beijing, China) and used as received. The ITO glass plates were purchased from Asahi Beer Optical, Ltd. (Japan), and they were used after being washed in acetone, ethanol, and deionized water by sonication * Corresponding author. E-mail:
[email protected]. Fax: +86-1062771149.
and drying with a nitrogen stream. In all the procedures, we used deionized water prepared using a Millipore Autopure WR600A apparatus. Pyrrole and aniline were purchased from Chin. Army Med. Inst. (Beijing, China) and distilled under reduced pressure before use. Instruments. Scanning electron micrographs (SEM) were taken out by the use of a FEI Sirion 200 or a S530 (Hitachi) scanning electron microscope. Electrochemical deposition and examinations were performed in a one-compartment cell by the use of a Model 273A potentiostat (EG&G Princeton Applied Research) under computer control. Raman spectra were recorded with a RM 2000 microscopic confocal Raman spectrometer (Renishaw PLC, England) employing a 514 nm or a 785 nm laser beam and a CCD detector with 4 cm-1 resolution. The spectra were obtained by focusing a 1-2 µm laser spot on the sample using a 20× long distance objective lens and accumulated three times for 10 s each. The power was always kept very low (
