Revealing Adsorption Mechanism of p-Mercaptobenzoic Acid with TiO2 Surfaces Using Electric Field-Enhanced Semiconductor SERS

Abstract

p-Mercaptobenzoic acid (4-MBA) is a typical molecular probe for a surface-enhanced Raman scattering (SERS) study of the enhancement performance of semiconductor nanoparticles. Understanding the molecular adsorption mechanism of 4-MBA on a semiconductor surface is crucial to reveal the enhancement mechanism of semiconductor SERS. Herein, two types of submicrometer-sized TiO2 particles with amorphous (denoted as a-TiO2) and anatase structures (denoted as c-TiO2) were fabricated, and their potential as SERS-active substrates with high electric-field enhancement was explored based on the near-field scattering theory and finite-element method simulation. The electric field-enhanced semiconductor SERS provide a better vision for us to study the adsorption modes of molecules on the TiO2 surface. On this basis, adsorption behaviors of 4-MBA on a-TiO2 and c-TiO2 particles were systematically studied by the semiconductor SERS and density functional theory. The results demonstrated that the adsorption mechanism of 4-MBA with TiO2 surfaces is highly dependent on the exposure of acid sites of TiO2 surfaces. 4-MBA adsorbs preferentially on Brønsted acid sites of a-TiO2 through a carboxyl group, in contrast on Lewis acid sites of c-TiO2 through a sulfhydryl group. Furthermore, 4-MBA molecules may form multilayer adsorption on TiO2 surfaces through the hydrogen bond and/or π–π stacking interaction. Research results not only provide a new insight to re-evaluate the chemical enhancement mechanism for TiO2–4-MBA systems but also provide a theoretical guidance for the modification of TiO2 surface with organic molecules containing carboxyl and sulfhydryl groups.