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Kartik Venkatraman1 Joshua Vincent1 Peter Rez2 Katia March3 Peter Crozier1

1, Arizona State University, Tempe, Arizona, United States
2, Arizona State University, Tempe, Arizona, United States
3, Arizona State University, Tempe, Arizona, United States

The ability to perform high-resolution mapping of adsorbate molecules or layers and correlate this with atomic structure would provide a transformative new tool for investigating the surface chemistry taking place on nanoparticles. Such a tool can be used to detect dissociative and non-dissociative sites on adsorbent surfaces and study different bonding arrangements between the adsorbate and the adsorbent. Recent work using high-resolution electron energy-loss spectroscopy (EELS) in the scanning transmission electron microscope (STEM) has shown how surface and interfaces modify the spatial extent of vibrational excitations [1]. For a comprehensive understanding of the detection of such vibrational signals, experiments need to be performed on simple model systems. The spectra need to be compared with results from conventional vibrational spectroscopies like Raman and Fourier-transform infrared (FTIR) spectroscopy to understand how electron excitation differs from photon excitation. Radiation damage may be substantial when an electron probe is placed directly on the adsorbate layer of interest. We adopt the aloof beam EELS technique that makes use of the long-range Coulomb interaction between electrons and adsorbate molecules to minimize damage when the probe is placed just outside the adsorbate layer [3]. We explore three classes of adsorbate/substrate systems because of their overall scientific importance and their suitability for developing the aloof beam vibrational EELS technique, viz. PVP ligand shell on Au nanoparticles, CO on Pt nanoparticles supported on CeO2 nanocubes, and CO2 on MgO nanocubes.
All our results were obtained using a NION UltraSTEM 100 aberration-corrected microscope operated at 60 kV and equipped with a monochromator. The aloof beam energy-loss spectrum from the PVP ligand shell/Au nanoparticle system suffers from a low signal-to-noise ratio and consists of two weak and broad vibrational signals centered at 160 meV and 205 meV corresponding to peaks associated with the C-N stretch, CH2 scissor and wag, and C=O stretch modes, convolved with the experimental energy-resolution. Comparison shows that the FTIR spectrum convolved with the EELS energy-resolution is a better match to the vibrational energy-loss spectrum than the similarly convolved Raman spectrum [4]. For CO chemisorbed on a Pt nanoparticle/CeO2 nanocube system, diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS) was performed to provide a reference data set for the aloof beam EELS experiment. Further vibrational EELS results from the CO on Pt/CeO2 and CO2 on MgO systems will be presented, along with comparisons with theory, FTIR and Raman spectra.
References:
[1] K. Venkatraman et al., Microscopy (under review).
[2] Y. Borodko et al., J. Phys. Chem. B, 110 (2006), p. 23052.
[3] The support from National Science Foundation CHE-1508667 and the use of (S)TEM at John M. Cowley Center at Arizona State University is gratefully acknowledged.

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