Fourier-transform surface-enhanced Raman scattering study on thiourea and some substituted thioureas adsorbed on chemically deposited silver films

Adsorption of thiourea, N,N′-diphenylthiourea (DIPTU), N-ethyl-N′-phenylthiourea (ETPTU) and N-(p-nitrophenyl)thiourea (NPTU), from acidic and neutral solutions, on chemically deposited silver film substrates are investigated using Fourier transform surface-enhanced Raman scattering (FT-SERS) technique. A strong band in the low frequency region, around 230 cm⁻¹ in the FT-SERS spectra of all these systems, assigned to Ag–S stretch, indicates a preferential adsorption through the sulfur atom. The frequencies and bandwidths of benzene ring vibrational modes in ETPTU as well as DIPTU are almost unaffected by adsorption indicating that benzene rings do not interact directly with the silver surface. A cis-cis conformation (both imino H-atoms cis with respect to thione sulfur atom) is proposed for DIPTU and ETPTU on the silver surface. In the case of NPTU, an additional interaction through the benzene ring is also observed as shown by the frequency shifts and band broadening of some benzene ring modes and weak intensity of the C–H stretching band.