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J. Phys. Chem. C 2008, 112, 16022–16027
Effect of TiO2 Nanoparticles on the Improved Performances on Electrochemically Prepared Surface-Enhanced Raman Scattering-Active Silver Substrates Yu-Chuan Liu,*,†,§ Chung-Chin Yu,‡,§ and Ting-Chu Hsu‡,§ Department of Chemical and Materials Engineering, Department of EnVironmental Engineering, and Nano Materials Applications R&D Center, Vanung UniVersity, 1, Van Nung Road, Shuei-Wei Li, Chung-Li City, Taiwan ReceiVed: July 7, 2008; ReVised Manuscript ReceiVed: August 14, 2008
In this work, we use electrochemical oxidation-reduction cycles (ORC) methods to prepare surface-enhanced Raman scattering (SERS)-active silver substrates modified with TiO2 nanoparticles to improve the corresponding SERS performances. On the basis of the modified substrates, the SERS of Rhodamine 6G (R6G) exhibits a higher intensity by 4-fold of magnitude, as compared with that of R6G adsorbed on a SERS-active Ag substrate without the modification of TiO2 nanoparticles. Moreover, the SERS enhancement capabilities of the modified and the unmodified Ag substrates are seriously destroyed at temperatures higher than 200 and 125 °C, respectively. These results indicate that the modification of TiO2 nanoparticles can improve the thermal stability of SERS-active substrates. More interestingly, the SERS intensity of R6G was significantly increased by 9-fold of magnitude, as the temperature of the modified Ag substrate was raised from 25 to 125 °C. The aging in SERS intensity is also depressed on this modified Ag substrate due to the contribution of TiO2 nanoparticles to SERS effects. Introduction For structural studies, Raman spectroscopies have been recently employed to investigate the nanostructures of titania1,2 and heterogeneous individual and single- and double-wall carbon nanotubes.3,4 However, only poor information can be provided due to weak signal or interference from noise.5,6 This issue was successfully resolved by using surface-enhanced Raman scattering (SERS), of which nearly 106-fold enhancement of Raman spectra of surface species provides a dramatic example for the modification of the optical properties of molecules near solidstate surfaces in view of its unique sensitivity and excellent frequency resolution.7,8 As shown in the literature, roughened silver and gold substrates9,10 or metals colloids11,12 were generally used for SERS studies, depending critically on the presence of structural features of dimensions 10-100 nm.13,14 Among other techniques used to obtain roughened metal substrates,13,15 a controllable and reproducible surface roughness can be generated through the electrochemical oxidation-reduction cycles (ORC).16,17 To further improve the SERS performance, some systems including metal/metal alloy colloids,18 metalcoated metal colloids,19 and semiconductor/adsorbate/metal sandwiches20 have been developed. In other nanoscale fields, titania is one of the most investigated oxide materials recently owning to its important applications in environmental cleanup,21 photocatalysts,22 solar cells,23 and polarimetric interference sensor.24 Dai et al.25 presented a surface sol-gel process to be an effective method for surface modification of silver island films as unique SERS substrates for monitoring molecular adsorption on a dielectric titania surface. This layer-by-layer approach allows control of the thickness of the dielectric surface with a monolayer precision on silver surfaces. Leon et al.26 reported work regarding the * Corresponding author. Tel: 886-3-4515811 ext 540. Fax: 886-34514814. E-mail:
[email protected]. † Department of Chemical and Materials Engineering. § Nano Materials Applications R&D Center. ‡ Department of Environmental Engineering.
adsorption of probe molecules on mesoporous anatase titania films to clarify the role of the carboxylate groups in the anchoring process of the dyes on the semiconductor surface. The results of the Raman experiments at different excitation wavelengths demonstrate that photoinduced charge-transfer processes take place efficiently between the adsorbate and the substrate. Moreover, this is the first time that the Raman spectrum of the dye N719 adsorbed on TiO2 has been obtained without the resonance condition only by means of SERS enhancement. To our knowledge, the effects of TiO2 nanoparticles on the preparation of roughened metal substrates, and on the corresponding SERS effects have not been well investigated. In this work, silver substrates were roughened by an electrochemical oxidation-reduction cycles (ORC) procedure in 0.1 M HCl aqueous solution containing 0.1 mM TiO2 nanoparticles. The improved SERS performances were investigated in detail. Experimental Section Chemical Reagents. The electrolytes and Rhodamine 6G (R6G) reagents (p.a. grade) purchased from Acros Organics were used as received without further purification. Commercial TiO2 nanoparticles (model: P25, mixed types of 70 mol % anatase and 30 mol % rutile) with particle sizes from 20 to 30 nm were purchased from Degussa Co., Japan. Generally, anatase TiO2 nanoparticles are responsible for the photocatalytical activities. Rutile TiO2 nanoparticles are responsible for the structural stabilities of TiO2 nanoparticles. All of the solutions were prepared with deionized 18.2 MΩ cm water provided from a MilliQ system. Preparation of SERS-Active Ag Substrates. All of the electrochemical experiments were performed in a threecompartment cell at room temperature, 24 °C, and were controlled by a potentiostat (model PGSTAT30, Eco Chemie). A sheet of silver foil with a bare surface area of 0.238 cm2, a 2 × 2 cm2 platinum sheet, and a silver-silver chloride (Ag/ AgCl) electrode were employed as the working, counter, and reference electrodes, respectively. Before the oxidation-reduction
10.1021/jp805997n CCC: $40.75 2008 American Chemical Society Published on Web 09/23/2008
SERS-Active Ag Substrates cycles (ORC) treatment, the silver electrode was mechanically polished (model Minimet 1000, Buehler) successively with 1 and 0.05 µm of alumina slurry to a mirror finish. Then the electrode was cycled in a deoxygenated 0.1 M HCl aqueous solution containing 0.1 mM TiO2 nanoparticles from -0.3 to +0.3 V vs Ag/AgCl at 5 mV/s for three scans without any duration at the cathodic and anodic vertexes (called modified roughened Ag substrate prepared with this procedure). Finally, the potential was held at the cathodic vertex before the roughened Ag electrode was taken from the solution and rinsed throughout with deionized water. For comparison, roughened Ag substrates without the additive of TiO2 were also prepared by using the same roughening condition (called unmodified roughened Ag substrate prepared with this procedure). SERSactive substrates at different temperatures were prepared by mounting the samples on a thermal heater (THMS 600, Linkam Scientific Instruments, UK) at a heating rate of 1 deg/min in air. The aging test was performed via placing the samples in an atmosphere of 50% relative humidity (RH) and 20% volume concentration (v/v) of O2 in the mixture of O2 and N2 at 30 °C for 60 days. Adsorption of R6G on SERS-Active Ag Substrates. For SERS measurements, the roughened Ag substrates were incubated in 2 × 10-5 M R6G aqueous solutions for 30 min. Then the substrates were rinsed throughout with deionized water, and finally dried in a vacuum-dryer with dark atmosphere for 1 h at room temperature for subsequent tests. Characteristics of SERS-Active Ag Substrates. The surface morphologies of Ag substrates were examined by scanning electron microscopy (SEM, model S-4700, Hitachi, Japan). Raman spectra were obtained by using a Renishaw InVia Raman spectrometer employing an Ar+ ion laser of 1 mW radiating on the sample operating at 514 nm. A 50×, 0.50 NA Leica objective with long working distance was used to focus the laser light on the samples. The laser spot size is ca. 1-2 µm. A thermoelectrically cooled charge-coupled device (CCD) with 1024 × 256 pixels operating at -60 °C was used as the detector with 1 cm-1 resolution. All spectra were calibrated with respect to silicon wafer at 520 cm-1. The acquisition time for each measurement was 10 s. Replicate measurements of three times on different areas were made to verify the spectra were a true representation of each experiment. The relative standard deviation is controlled under 5% based on the strongest peak intensity of R6G on the Raman spectrum. For high-resolution X-ray photoelectron spectroscopy (HRXPS) measurements, a ULVAC PHI Quantera SXM spectrometer with monochromatized Al KR radiation, 15 kV and 25 W, and an energy resolution of 0.1 eV was used. To compensate for surface-charging effects, all HRXPS spectra are referred to the C 1s neutral carbon peak at 284.8 eV. Surface chemical compositions were determined from peak-area ratios corrected with the approximate instrument sensitivity factors. Results and Discussion The aim of the addition of TiO2 nanoparticles in the ORC treatment to prepare SERS-active Ag substrates is to let a few quantities of TiO2 nanoparticles be incorporated into the redeposited Ag nanoparticles on Ag substrates. Then the incorporated TiO2 nanoparticles can work to improve the corresponding SERS performances. Higher quantities of TiO2 nanoparticles may destroy the microstructure of the roughened Ag substrate, and thus reduce the SERS effect. Figure 1a shows the SEM image of the roughened Ag substrates without the modification of TiO2 nanoparticles. The surface morphology of
J. Phys. Chem. C, Vol. 112, No. 41, 2008 16023
Figure 1. SEM images of silver substrates roughened in different solutions: (a) 0.1 M HCl; (b) 0.1 mM TiO2 and 0.1 M HCl.
the Ag substrate roughened in 0.1 M HCl exhibits thin metal islands with good Raman activity, which results from a microstructure of
