An integrated AFM-Raman instrument for studying heterogeneous catalytic systems: a first showcase

Abstract

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...

Surface- and Tip-Enhanced Raman Spectroscopy in Catalysis

“…To showcase the possibilities of the AFM–Raman methodology, the catalytic activity of silver nanocubes in rhodamine 6G degradation was linked to their distribution. 105 However, the diffraction-limited spatial resolution of Raman spectroscopy (typically 200–300 nm) is not overcome by combining SERS and AFM.…”

Tip in–light on: Advantages, challenges, and applications of combining AFM and Raman microscopy on biological samples

“…Likewise the formation of a pore network during in vitro drug release from a carrier polymer matrix has as well been observed by co‐located AFM‐Raman (Biggs et al, ). Moreover catalytic reactions of metal nanoparticles have also been proofed by this technology (Harvey et al, ). In this case, the metal nanoparticles enhanced the Raman signal.…”

Atomic Force Microscopy Meets Biophysics, Bioengineering, Chemistry, and Materials Science

“…[76] The experimentsw ere carried in ac ell in which gas and temperature could be varied. The study of catalytic phenomenaa tt he micro-and nanoscale requires ac ombination of AFM scanning and spectroscopy.H arvey and co-workers integrated Raman spectroscopy with AFM to study the photo-oxidation of rhodamine 6G over Al 2 O 3 -supported AgNP.…”