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

Abstract

IntroductionFor the study of surface phenomena at various interfaces, the total internal reflection (TIR) condition has been used in a number of spectroscopic measurements, 1 such as fluorometry, 2-9 surface enhanced infrared spectroscopy (SEIRA), [10][11][12] second harmonic generation, [13][14][15][16][17][18][19][20][21] and sum frequency generation (SFG). [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. One corollary is the freedom of changing the composition of the adjacent phase that has a lower refractive index. Second, high sensitivity to the interfaces can be achieved owing to the enhancement of the electric field at the interfaces. 1 The TIR arrangement can be employed at metal interfaces if a very thin metal film is used to avoid the absorption of light. It has been successfully employed to detect adsorbed species on metal in the SEIRA, where the thickness of the metal film is about 10 nm on a transparent substrate. [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. Unlike surface plasmon resonance and capacitance measurements that have been used as sensitive means of detecting adsorption and molecular interaction at the interfaces, TIR-SFG will enable us to obtain molecular information on the solution side of the interfaces. One drawback of the TIR arrangement is the roughness of the metal surfaces. The roughness is prerequisite for high sensitivity and can complicate the interpretation of the experimental data because of the orientational distribution of molecules in comparison with those adsorbed on a single crystal surface. It is therefore of crucial importance to assess the degree of the variation in orientation of molecules.By using broad-bandwidth sum frequency generation (BBSFG) [26][27][28] we recently analyzed the orientation of alkanethiols adsorbed on Au(111). 29It is worthwhile to examine the applicability of BBSFG in the TIR condition at metal interfaces to obtain SFG spectra with higher resolution. In the present paper, we report the results of TIR-BBSFG measurements for hexadecanethiol (HDT) adsorbed on a thin gold film on CaF2. We will show that BBSFG can be successfully employed in the orientation analysis of the HDT molecules and that the methyl groups of HDT are much more randomly orientated than those on Au(111) on mica. Experimental SubstrateA hemicylindrical CaF2 prism (Pier Optic...

Determination of the potential of zero charge of Au(1 1 1) electrodes modified with thiol self-assembled monolayers using a potential-controlled sessile drop method

“…This shift is consistent with the double layer capacitance measurements of x-hydroxyltetradecanethiolate SAM on gold in contact with submillimolar aqueous sodium trifluoroacetate at 0°C [2]. As the packing of alkanethiolate molecules on polycrystalline gold is likely to be less ordered [8], the contact angle and other electrocapillary properties of SAMs formed on a single crystal surface or a metal surface of similar quality are desirable in further studies of the double layer properties of SAM-covered electrodes. crystal surface, the sessile drop method is preferable to the Wilhelmy method.…”

Direct Observation of Adsorption Morphologies of Cationic Surfactants at the Gold Metal–Liquid Interface

“…In Figure 2, the C4 Quat ssp spectrum (Figure 2(A)) contained a prominent methylene asymmetric stretch peak (CH 2 AS) at ∼2907 cm −1 . 28,29 The other CH stretches are less prominent (peak assignments and positions are reported in the SI). 22,26,30,31 The methyl asymmetric stretch was not evident in the ssp spectrum compared to other vibrational modes, especially to the methylene groups that dominated the surface.…”

Momentum Relaxed, Plasmon-Resonant Vibrational Sum Frequency Generation of Electrochemical Interfaces: Direct Observation of Carbon Dioxide Electroreduction on Gold

“…The second method is for the visible and infrared beams to approach the interface through the electrode as shown in Figure 1b. 10,[16][17][18] While this technique avoids the need for ultrathin electrolyte layers, it requires the use of a transparent electrode. Since conductive materials are by nature strong absorbers (or reflectors) of infrared light, this geometry presents major challenges by severely limiting selection of the electrode material.…”

“…1,19 However, the signal-to-noise in these geometries is typically poor owing to the unfavorable Fresnel factor. 10,17,19 Additionally, films with average thickness less than 10 nm are often unstable and cannot sustain the current densities required for electrocatalysis due to resistive heating of the ultrathin film.…”