Raman Scattering of 4-Aminobenzenethiol and Its Analog Molecules on Ag, Au, and Pt Nanostructures: Contribution of Photoinduced Charge-Transfer

Abstract

The surface-enhanced Raman scattering (SERS) of 4-aminobenzenehtiol (4-ABT) has seen a surge of interest recently, since its SERS spectral features are dependent not only on the kinds of SERS substrates but also on the measurement conditions. A problem was initially encountered in the interpretation of the SERS spectrum of 4-ABT due to difficulty in correlating several peaks therein with the normal Raman peaks, but the SERS spectral pattern of 4-ABT looked similar to that of 4,4′-dimercaptoazobenzene (4,4′-DMAB). To clarify the issue, the SERS characteristics of 4-ABT and its analog molecules adsorbed on metal surfaces are carefully reinvestigated to understand the charge-transfer chemical enhancement mechanism in this Ph.D. thesis. In chapter 1, the general overview of the Raman scattering theory and SERS mechanisms are described concisely. SERS is an abnormal surface optical phenomenon resulting in strongly increased Raman signals for molecules adsorbed onto nanostructured coinage metals. In recent years, it has been reported that even single-molecule detection is possible by surface-enhanced resonance Raman scattering (SERRS), suggesting that the enhancement factor (EF) can reach as much as 1014-1015. Two enhancement mechanisms, one called a long-range electromagnetic (EM) effect and the other called a short-range charge-transfer (CT) chemical effect, are simultaneously operative. Both mechanisms suggest the possibility of enhanced absorption and enhanced photochemistry for surface-adsorbed molecules. In chapter 2, the current status of SERS studies on 4-ABT are described. For a long time, 4-ABT, also known as p-aminothiophenol (PATP) or p-mercaptoaniline (pMA), is one of the important surface probe molecules in SERS community and the nanoscience field. The main reasons are that 4-ABT molecules are strongly and easily adsorbed onto the most metal substrates and generate a strong and unique SERS signal which is very sensitive to the type of substrates and measurement conditions. The normal Raman (NR) spectrum of 4-ABT is mostly featureless in the region of 1100~1500 cm-1, but three to four peaks appear newly in that region in its SERS spectra. Since these peaks can be assigned to the b2-type vibration, which are arising from charge transfer process through the Herzberg-Teller vibronic coupling term, 4-ABT has been regarded for two decades as a model adsorbate for probing the CM effect in SERS. Very recently, however, a number of researchers have come up with a different explanation for the appearance of these b2-type bands that the b2-type bands appearing in the SERS of 4-ABT must be the N=N stretching vibrations of 4,4-DMAB produced from 4-ABT via a catalytic coupling reaction on the metal substrates. The elucidation of the SERS characteristics of 4-ABT and its analog molecules including 4,4-DMAB is thus needed to see another or why we have made a wrong assignment during the past 15 years. In chapter 3, the SERS characteristics of 4-ABT and 4-ABT adsorbed on Pt nanoparticles are investigated. Pt is a well-known catalyst that has a high catalytic activity. It is found, however, that 4-nitrobenzenethiol (4-NBT) is barely subjected to photoreaction on a Pt surface. On the contrary, the SERS spectra of 4-ABT on Pt clearly show that the b2-type bands are increasing in relative intensity toward shorter wavelength. In addition, the SERS spectral pattern of 4-ABT on Pt is variable not only with changes in the electrode potential but also by altering the excitation wavelength. These spectral variations could be understood by presuming that the chemical enhancement mechanism is also operating in this system, along with the electromagnetic enhancement. Interestingly, similar spectral variation is also observed even under ambient conditions by exposure of 4-ABT on Ag to volatile organic chemicals (VOCs) such as acetone and ammonia. Based on the potential-dependent SERS data, the effect of acetone appeared to correspond to an application of +0.15 V to the Ag substrate vs a saturated Ag/AgCl electrode, while the effect of ammonia corresponded to the application of -0.45 V to Ag. In chapter 4, as one of several attempts to explore the origin of the b2-type bands observable in the SERS of 4-ABT, the pH dependence has been investigated. Consulting the claim that those b2-type bands might be associated with a surface-induced photoreaction product such as 4,4′-DMAB, the pH dependence of the SERS spectral feature of 4,4′-DMAB was also examined. Distinct differences were observed in the SERS spectra of 4-ABT and 4,4′-DMAB. The SERS spectral feature of 4,4′-DMAB had virtually no dependence either on the excitation wavelength or on the kind of metal substrate or even on the solution pH. On the other hand, the SERS spectral pattern of 4-ABT displayed substantial changes, depending not only on the excitation wavelength and the kind of SERS substrates but also on the solution pH. It is presumed that when the amine group of 4-ABT is protonated at acidic pHs, the electron population in the benzene ring moiety decreases, resulting in the up-shift of the LUMO level of 4-ABT, as revealed by UV-vis spectra and from an ab initio calculation, thereby prohibiting the charge transfer resonance chemical enhancement. In chapter 5, the similarity and dissimilarity in the SERS of 4-ABT and 4,4′-DMAB, along with the SERS spectrum of their analog molecule 4,4′-dimercaptohydrazobenzene (4,4′-DMHAB) were carefully examined. Under ambient conditions, the SERS spectra of 4-ABT, 4,4′-DMAB, and 4,4′-DMHAB on Ag looked in fact comparable to one another, but the spectral dissimilarity was evidenced not only from the SERS spectra taken after treating the probing substrates with a borohydride solution but also from the potential-dependent SERS spectra. It was found that 4,4′-DMAB on Ag could convert to 4-ABT not only by contact with a 100 mM borohydride but also by lowering the potential below -1.0 V. The reverse reaction from 4-ABT on Ag to 4,4′-DMAB appeared insignificant electrochemically as well as photochemically. Furthermore, it was found that the conversion of 4,4′-DMAB to 4-ABT on Ag is a more feasible process upon irradiation with a 514.5-nm (not 632.8-nm) laser under ambient conditions. The SERS spectral pattern of 4,4-DMAB on Ag varied as a function of laser irradiation time, finally becoming the same as that of 4-ABT on Ag. The photoconversion of 4,4-DMAB upon 514.5-nm radiation was further confirmed not only by the coupling reaction with 4-cyanobenzoic acid to form amide bonds but also by the selective growth of calcium carbonate. After considering all the experiments conducted in the work, it is concluded that the appearance of the so-called b2-type bands in the SERS of 4-ABT must be attributed wholly to the involvement of the chemical enhancement mechanism, not due to the formation of 4,4′-DMAB.