Total Internal Reflection Sum-Frequency Spectroscopy:  A Strategy for Studying Molecular Adsorption on Metal Surfaces

Abstract

Total internal reflection sum-frequency spectroscopy (TIR-SFS) is shown to be capable of detecting molecules adsorbed on ultrathin gold films (e10 nm) deposited on a sapphire prism. Octadecanethiol (ODT) and thiocyanate (SCN -) were used as probe molecules in order to assess the usefulness of the approach. For ODT adsorbed on 5-nm Au films, SF signal enhancements of over an order of magnitude were observed with TIR-SFS compared to the standard external reflection geometry. While TIR-SF spectra were obtained for ODT on 5-and 10-nm Au films, no molecular signals were detected for 20-nm Au films. The CtN stretch of SCNadsorbed on a 5-nm Au film was detected by TIR-SFS in the presence of either water or air. A theoretical model is presented to rationalize the different SF signal levels observed under various conditions. Future prospects of TIR-SFS for studying other oxide-supported metals are discussed, along with possible applications in the fields of heterogeneous catalysis and electrochemistry.

Total-internal-reflection Broad-bandwidth Sum Frequency Generation Spectroscopy of Hexadecanethiol Adsorbed on Thin Gold Film Deposited on CaF, 2

“…31 It is noted that χ NR (2) and χ R,v (2) contain the Fresnel coeffcients, 24,28 and that Av is normalized by χ NR (2) . For the orientation analysis of the methyl groups, the vibrational intensity of v, Vv, may be defined as follows: Figure 1 shows an SF vibrational spectrum for the HDTadsorbed 10 nm thickness thin gold film deposited on the CaF2 prism in the TIR condition.…”

“…29 This difference in φ results from both the change in the Fresnel coeffcients 35,36 and the contribution of the gold/CaF2 interface to χ NR (2) . 24 We normalized this spectrum by that for a GaAs surface; the susceptibility of the GaAs surface is independent of the wavelength in the region studied and we hence can measure the IR envelope. 37 The plot in Fig.…”

“…2a, a small band can be seen at ∼2910 cm -1 , corresponding to the methylene asymmetric stretching, which is SFG-active when the methylene groups are in gauche conformation. 24,40 This indicates that a detectable amount of gauche conformations is present, unlike alkanethiols adsorbed on Au(111)-prevalent gold surface.…”

“…[22][23][24][25] There are two advantages when one uses the TIR condition. First, both incident light to the interface and reflected light or generated light from the interface travel in only one of the two phases adjacent to the interface, which circumvents bulk absorption by the other phase.…”

“…[10][11][12] Recently, Williams et al showed that the TIR arrangement can also be employed to detect SFG signals from alkanethiols adsorbed on a thin gold film. 24 However, detailed analysis of the sum frequency vibrational spectra has not been reported. SFG in the TIR arrangement has some potential as a powerful tool to study molecular orientations at metal/liquid interfaces, such as electrochemical interfaces, because it is free from bulk absorption and has interfacial sensitivity in the order of nanometers.…”