Raman Nanospectroscopy of a Photo-Catalytic Reaction

Abstract

A chemical reaction between molecules requires a certain amount of energy to occur. A catalyst can lower this activation energy, resulting in a faster, greener and more sustainable chemical process. Ideally, one would like to follow a reaction over a single catalytic nanoparticle, to fully understand the dynamics involved in a reaction to guide the development of even more efficient catalysts. The nanophotonic effect in silver and gold nanoparticles, surface-enhanced Raman scattering (SERS), can be used to obtain nano-scale spatial resolution and overcome the inherent low scattering intensity of normal Raman spectroscopy. Here, SERS is optimized for the investigation of heterogeneous catalytic reactions on the nano-scale. The photo-catalytic reduction of p-nitrothiophenol (pNTP) was chosen as a model conversion. This chemical reaction is easily triggered via the combination of green laser excitation and a silver SERS substrate, but is influenced by many parameters. The reduction is typically performed within a self-assembled monolayer and is thus confined in a two-dimensional space. The reaction kinetics show that the reaction rate is most likely limited by an uneven catalytic activity over the full SERS-enhancing area. The reaction was found to resemble second-order reaction kinetics, and this observation was confirmed via dilution experiments. This two-dimensional catalytic approach is shown to yield new interesting possibilities in the characterization of heterogeneous catalysts. It also proves that p,p’-dimercaptoazobisbenzene is the reaction product in the photo-catalytic reduction of pNTP, settling a long-standing discussion in literature. Single-hotspot SERS studies can show strong spectral and intensity fluctuations due to the small ensemble of molecules in the measurement volume. These spectral fluctuations are very interesting in the study of heterogeneous catalytic reactions as reaction intermediates are expected to have short lifespans and might only be observed in the most intense hotspots. A novel chemometric method was developed to separate short-term fluctuations from bulk reactivity in time-resolved spectra on the basis of a time-dependent one-component principal component analysis. The fit of reaction kinetics is thereby improved, while enabling the investigation of components with limited lifetime. Tip-enhanced Raman scattering (TERS) allows nano-scale imaging, by using a gold- or silver-coated needle of an atomic force microscope to create the SERS-enhancement effect. Only when and where the needle is in contact with the sample, will an enhanced Raman signal be obtained. The incorporation in an atomic force microscope allows localized analysis with nanometer precision. This technique was used to study the reduction of pNTP over a single catalytic particle: the TERS-tip itself. A dual wavelength approach was found to be crucial to follow reaction dynamics, with 633 nm laser excitation for monitoring purposes, and 532 nm laser excitation to induce the reduction. Chemometric analysis allows separation of the data into reaction kinetics and short-term fluctuations that could belong to reaction intermediates. This is the first description of a catalytic reaction over a single catalytic particle, as observed by TERS. Herewith, TERS has extended the possibilities to have both the time-resolution and sufficient enhancement effect to follow a chemical reaction over a single catalytic particle.