The Vibrational Sum Frequency Generation Spectrum of Toluene On HOPG

The high versatility, low cost, and high electrochemical conductivity of carbon make carbon materials ideal candidates to fulfill societal needs for high-performance energy and conversion systems. While these materials are intensely studied, surprisingly little focus has been placed on understanding the surface region of carbon substrates, where many energy-relevant processes occur. Here, vibrational sum frequency generation (SFG) is used to gain a molecular level understanding of the interfacial environment on carbon substrates. We focus on understanding the nonlinear optical properties of carbon substrates, in particular highly ordered pyrolytic graphite (HOPG), and obtaining molecular level vibrational signatures of a prototypical aromatic hydrocarbon namely toluene, adsorbed to their surfaces. The SFG signal of SPI-2 grade HOPG is dominated a large non-resonant (NR) response which is attributed to the mobile π-electrons. The NR response was found to be invariant with azimuthal rotation of the substrate due to the substrate’s crystal domains, having 3m symmetry, being smaller than the focal spots of the incident beams. While we find that bare HOPG does not exhibit vibrational CH stretching modes, vibrational signatures of toluene adsorbed onto the surface are successfully resolved by employing IR-upconverter time-delay methods. The NR SFG response is almost quantitatively suppressed with a 500 fs time delay to resolve vibrational resonances of toluene on the surface. Spectroscopic signatures are clearly identified at 2855 cm^-1, 2912 cm^-1 and 2967 cm^-1 which account for the symmetric, Fermi resonance, and overtone with the aromatic ring of the CH₃ group of toluene. The implications of our results for fluid-solid interface chemistry are discussed.