Surface-Enhanced Raman Spectra of Metallotetraphenyl Porphyrin Adsorbed on AgI Colloids

Surface-enhanced Raman Spectra(SERS)of four metallotetraphenylporphyrins(MTPP,M=Ag,Mg,Cu,Pd)adsorbed on AgI colloid were studied using 632.8nm excitation.It was shown that MgTPP on AgI colloid undergoes a metal exchange reaction with the substrate to form AgTPP,evidenced by appearing characteristic Raman bands of AgTPP in the SERS of MgTPP.In contrary,metal exchange reaction was not observed for CuTPP and PdTPP molecules adsorbed on AgI colloids.In addition to the porphyrin-ring related bands,several monosubstituted benzene bands are also enhanced in the SERS spectra of MTPP.The SERS enhancement patterns of PdTPP and CuTPP in the region of 1400～1600cm-1 are very different from that of AgTPP,which can be attributed to the different stereochemistry of the adsorbents around Cm atoms.The present study demonstrates that semiconductors such as AgI can be suitable SERS substrates to study the structure and chemical reactions of adsorbed molecules.