关键词: 表面增强拉曼光谱, 硫脲, 水, 电荷转移效应

Abstract: As one of the important mechanisms of SERS, the charge transfer (CT) enhancement requires the strong interaction of the adsorbed species with the substrate in order to permit the transition of charge between the metal Fermi level (energy state) and the molecular orbital [1] . The high enhancement needs the match of the energy gap between the Fermi level (or surface state) and the orbital energy level of the adsorbed molecules with the energy of the incident light. The electrode Fermi level is usually adjusted by the applied potential to satisfy the CT resonance. For the electrochemical reaction process, the frontier orbital energy level of the reacting surface species should be greatly different from that of the adsorbed molecules. Thus, it is interesting to test the additional SERS enhancement induced by the electrochemical reaction. In the present study, the influence of the electrochemical reaction on SERS intensity of thiourea (TU) and water adsorbed on silver electrode surfaces were investigated, respectively. The Raman experiments were performed on a confocal microprobe Raman system (LabRam I). The details of the Raman system and pretreatment of the Ag electrode can be found elsewhere [2] . The SERS spectra of TU in pH 1 and 7 are showed in the Fig. 1 (a) and (b), respectively. The major bands of TU locate at～710 cm -1 and～1 091 cm -1 . The strong 933 cm -1 band is assigned to the symmetric stretching vibration of ClO 4 - as electrolyte anion, which coadsorbed on the surface. The electrochemical measurements indicate that TU can adsorb strongly at Ag electrode in a wide potential region from -0.2 V to -1.5 V (vs SCE). It is of interest that the SERS intensity reaches the maximum at different potential in acidic and neutral solutions. In the low pH solution, one can find that when the electrode potential shifted to -0.8 V, all the band intensities increase remarkably. In the high pH solution (Fig. 1 (b)), at potentials positive of -1.0 V only solution signal can be discerned. The intensity of the surface signals underwent a sharp increase at -1.2 V. Interestingly, we found the maximum intensities in the two pH solutions have a certain relation with the occurrence of the electrochemical hydrogen evolution reaction (HER). A systematic SERS experiments were performed in solutions with pH of 2.0, 2.5, 3..0, 3.5, 4.0 and 7.0 respectively. The profiles of the integrated band intensities (～710 cm -1 ) and the potentials are shown in Fig. 2. It can be found that the maxima of the intensities are located at -0.8 V, -1.1 V for pH 1.0 and 2.0 respectively and -1..2 V for pH 2.5 ～7.0. Correspondingly, the current densities for each potential in different pH solution were presented in Fig. 3. From the two figures, it can be found that the potentials of the maximum intensity of SERS are right at the initial potentials of HER. It implies that there must have some relation between the HER and the additional enhancement of the SERS. A study on the SERS of water will be helpful for further understanding of this relation.