Heterogeneous catalysts play a pivotal role in the chemical industry, but acquiring molecular insights into functioning catalysts remains a significant challenge. Recent advances in micro-spectroscopic approaches have allowed spatiotemporal information to be obtained on the dynamics of single active sites and the diffusion of single molecules. However, these methods lack nanometre-scale spatial resolution and/or require the use of fluorescent labels. Here, we show that time-resolved tip-enhanced Raman spectroscopy can monitor photocatalytic reactions at the nanoscale. We use a silver-coated atomic force microscope tip to both enhance the Raman signal and to act as the catalyst. The tip is placed in contact with a self-assembled monolayer of p-nitrothiophenol molecules adsorbed on gold nanoplates. A photocatalytic reduction process is induced at the apex of the tip with green laser light, while red laser light is used to monitor the transformation process during the reaction. This dual-wavelength approach can also be used to observe other molecular effects such as monolayer diffusion.

Straightforward analysis of chemical processes on the nanoscale is difficult, as the measurement volume is linked to a discrete number of molecules, ruling out any ensemble averaging over rotation and diffusion processes. Raman spectroscopy is sufficiently selective for monitoring chemical changes, but is not sufficiently sensitive to be applied directly. Surface‐enhanced Raman spectroscopy (SERS) can be applied for studying reaction kinetics, but adds additional variability in the signal as the enhancement factor is not the same for every location. A novel chemometric method described here separates reaction kinetics from short‐term variability, based on the lack of fit in a principal‐component analysis. We show that it is possible to study effects that occur on different time scales independently without data reduction using the photocatalytic reduction of p‐nitrothiophenol as a showcase system. Using this approach a better description of the nanoscale reaction kinetics becomes available, while the short‐term variations can be examined separately to examine reorientation and/or diffusion effects. It may even be possible to identify reaction intermediates through this approach. With only a limited number of reactive molecules in the studied volume, an intermediate on a SERS hot spot may temporarily dominate the spectrum. Now such events can be easily separated from the bulk conversion process by making use of this chemometric method.

Heterogeneous catalysis is a surface phenomenon. Yet, though the catalysis itself takes place on surfaces, the reactants and products rapidly take the form of another physical state, as either a liquid or a gas. Catalytic reactions within a self‐assembled monolayer are confined within two dimensions, as the molecules involved do not leave the surface. Surface‐enhanced Raman spectroscopy is an ideal technique to probe these self‐assembled monolayers as it gives molecular information in a measured volume limited to the surface. We show how surface‐enhanced Raman spectroscopy can be used to determine the reaction kinetics of a two‐dimensional reaction. As a proof of principle, we study the photocatalytic reduction of p‐nitrothiophenol. A study of the reaction rate and dilution effects leads to the conclusion that a dimerization must take place as one of the reaction steps.

The integration of Atomic Force Microscopy and Raman spectroscopy is tested for use in heterogeneous catalysis research by a preliminary investigation, the photo-oxidation of rhodamine-6G. Temperature and atmosphere were varied in an in situ cell to show compatibility with realistic reaction conditions. Supported metal nanoparticles (NPs) are important heterogeneous catalysts in many industrial processes.1-3 A thorough understanding of which specific surfaces have the highest catalytic activity is required in order to tune the shape and size of NPs to achieve maximum catalytic activity. [4][5][6][7][8] A variety of spectroscopic techniques are employed for the study of heterogeneous catalysts, both ex situ and in situ. 9,10Vibrational spectroscopy is one of the most valuable methods for obtaining chemical information about a catalytic system. However, IR spectroscopy is severely limited spatially, while Raman spectroscopy has a better spatial resolution, but lacks sensitivity under normal measurement circumstances. The use of surface enhancement makes Raman spectroscopy a more sensitive and versatile tool for studying chemical reactions. 11-14Surface Enhanced Raman Scattering (SERS) occurs principally on roughened noble metal surfaces or noble metal NPs, and allows chemical imaging of adsorbate-surface interactions with high sensitivity. [15][16][17] This makes it uniquely suited for investigations of reactions at a catalytic surface. 18,19 The integration of SERS and Atomic Force Microscopy (AFM) forms an even more powerful tool for in situ chemical imaging of catalytic solids, allowing nanoscale morphological features to be correlated directly with chemical information. 20-22Here we demonstrate the potential and limitations of this integrated approach for identifying exactly which NP morphologies and sizes are the most active through a study of the photo-oxidation of rhodamine-6G (Rh6G) over Al 2 O 3 -supported Ag NPs. We show that it is possible to follow the reactants or products of a reaction under in situ conditions (i.e. controlled temperature and atmosphere), and pinpoint the active section of the substrate on the nanoscale. Using the integrated setup, Raman maps can be measured in combination with AFM. The resolution of Raman measurements is, however, still dependent on the excitation wavelength that is used, and typically not better than 300 nm. With AFM it is possible to obtain topography images of the surface that is being measured with Raman spectroscopy with a resolution of less than a nanometre. A top-view set-up is required as in general the support oxide material for dispersing the catalytic particles is not transparent. In the integrated setup an AFM cantilever is used with a tip that protrudes at the end of the cantilever, so that it is possible to focus the laser beam onto the tip itself. The optical focus on the AFM tip, as shown in Fig. 1, means that the focus stays with the surface during sample scanning. At every measurement point, an AFM height point is recorded along with a Raman spectr...

Vibrational spectroscopy usually provides structural information averaged over many molecules. We report a larger peak position variation and reproducibly smaller FWHM of TERS spectra compared to SERS spectra indicating that the number of molecules excited in a TERS experiment is extremely low. Thus, orientational averaging effects are suppressed and micro ensembles are investigated. This is shown for a thiophenol molecule adsorbed on Au nanoplates and nanoparticles.