Surface-Enhanced Raman Scattering of 4-Nitrobenzenethiol and 4-Aminobenzenethiol on Silver in Icy Environments at Liquid Nitrogen Temperature

Abstract

The origin of b2-type bands in the surface-enhanced Raman scattering (SERS) of 4-nitrobenzenethiol (4-NBT) and 4-aminobenzenethiol (4-ABT) has recently been debated because these bands are very similar to those attributed to a photoreaction product such as 4,4′-dimercaptoazobenzene (4,4′-DMAB). To resolve the debate, we obtained the SERS spectra of both 4-NBT and 4-ABT on Ag under ambient conditions and icy environments at 77 K. Under ambient conditions, the b2-type bands distinctly appeared in the SERS spectra of both 4-NBT and 4-ABT. In contrast, no b2-type peaks appeared in the SERS of 4-NBT in icy environments, suggesting that 4-NBT did not undergo a photoreaction. However, the SERS spectral pattern of 4-ABT was the same both at room temperature and in icy conditions. Because hot electrons are plasmonically generated from Ag even in icy environments, the lack of photoreaction of 4-NBT is likely a result of the small spaces between the ice crystals, rendering the N–O bond difficult to break. The situation of 4-ABT on Ag is identical to that of 4-NBT on Ag in the same conditions; therefore, the b2-type bands observed in icy conditions must be because of the 4-ABT, and not because of the production of 4,4′-DMAB or other photoreaction products. Regardless of temperature, hot electrons were more easily generated at lower excitation wavelengths, and the b2-type bands appeared more distinctly with a decrease in the excitation wavelength. From these observations, it can be surmised that the hot electrons, as well as the b2-type bands of 4-ABT, are associated with the charge-transfer chemical enhancement mechanism in SERS.