Raman spectroscopic and computational studies on 4-mercaptopyridine adsorption on zinc oxide/graphene oxide- Supporting information
Description
Raman spectroscopy is a technique with great potential for molecular identification and detection. However, its limited sensitivity renders it incompatible with trace analysis. Some materials, when employed as substrates, are able to amplify the Raman signal of an adsorbate molecule through an effect called Surface-Enhanced Raman Scattering (SERS), making the technique viable for this purpose. The SERS effect is typically applied to noble metal nanoparticles, namely gold and silver, but can also be brought about by other non-plasmonic materials, such as semiconductors and other 2D nanostructures of different compositions and morphologies. A deeper understanding of the mechanisms responsible for non-plasmonic SERS would allow for its exploration in cheaper, more customizable materials than can be tailored to a multitude of purposes. In the present work, experimental and computational studies were conducted on ZnO, graphene oxide (GO) and a ZnO-GO hybrid. Powders of the materials were synthesized through various strategies and characterized. SERS studies were then conducted by treating the materials with 4-mercaptopyridine (4-MPy). In parallel, computational studies were conducted to analyse the charge-transfer mechanism responsible for the experimentally observed SERS effect. From the combination of the two approaches, we were able to find that ZnO is able to amplify the signal of 4-MPy, both in the presence and absence of GO. The results also suggest that charge-transfer happens between the valence band of ZnO and the lowest unoccupied molecular orbital of 4-MPy. This work succeeds that of Lopes et al. (JPCC, 2020, 23, 12742), in which ZnS and GO are investigated as SERS substrates for 4-MPy. Whilst isolated ZnS and GO are not SERS-active, the interaction between the two leads to the formation of intragap levels, as confirmed through computational studies, that assist the charge-transfer mechanism and bring about the SERS effect.