1,6-Hexanedithiol Self-Assembled Monolayers on Au(111) Investigated by Electrochemical, Spectroscopic, and Molecular Mechanics Methods

Abstract

1,6-Hexanedithiol (HSC 6 SH) self-assembled monolayers (SAMs) on Au(111) were formed in a 1 mM ethanolic solution and were investigated by voltammetry in 0.1 M aqueous KOH solution. HSC 6 SH SAMs on Au(111) were further probed by X-ray photoelectron spectroscopy (XPS), scanning tunneling microscopy (STM), sum frequency generation spectroscopy (SFG), and molecular mechanics calculation (MMC). An identical investigation was performed with the self-assembly of hexanethiol (C 6 SH) on Au(111) for comparison. Linear sweep voltammograms showed that a full monolayer of thiolate (7.6 ((0.2) × 10 -10 mol -2) chemisorbed on Au(111) with both HSC 6 SH and C 6 SH, while the peak potential of the electroreduction of the former assembly was more negative than that of the latter. XPS revealed that two different forms of sulfur existed at HSC 6 SH SAMs on Au(111), i.e. a thiolate and a thiol. The STM image could not be resolved with HSC 6 SH SAMs, while it was atomically well-defined with the C 6 SH case. SFG results showed that alkyl chains within the HSC 6 SH SAMs were in an all-trans conformation. Thus the experimental results concluded that densely packed and higly oriented HSC 6 SH SAMs could be successfully formed on Au(111) with the incubation conditions in the practical solution. MMC presented that HSC 6 SH SAMs were more stable than C 6 SH ones because of the additional end-end interaction among the thiol groups faced up to the air, albeit both SAMs were tilted by 28°from the surface normal.

Electrochemical Properties of Alkanethiol Monolayers Adsorbed on Nanoporous Au Surfaces

“…13 Shown in Figure 2A is the cyclic voltammogram for the reductive desorption of dodecanethiol (DT) adsorbed on flat Au surfaces, where a main desorption wave is observed at -1.06 V. This desorption peak potential is close to that previously reported for DT monolayers on Au surfaces. 8,13,14 The origin of a satellite desorption wave observed at -0.92 V can be ascribed to the difference in the microscopic roughness on the Au substrates used in this study. 15 The total charge associated with the reductive desorption of DT on flat Au surfaces is estimated to be ca.…”

“…This corresponds to the charge density of 110 µC cm -2 , which is slightly larger than that observed for alkanethiol SAMs with a monolayer coverage of (√3 × √3) R30 o structures on Au(111) surfaces. 14 On NPG surfaces, the reductive peak potential of DT monolayers nega-…”

Influence of Thiol Molecular Backbone Structure on the Formation and Reductive Desorption of Self-Assembled Aromatic and Alicyclic Thiol Monolayers on Au(111) Surface

“…[38][39][40][41][42] In addition, it was found that reductive desorption peaks for COOH-terminated alkanethiol SAMs on Au electrodes appeared at less negative potential compared to the corresponding CH 3 -terminated alkanethiol SAMs, 42 and fluorinated aromatic thiol SAMs on Ag electrodes showed similar reductive desorption behavior to the corresponding hydrogenated aromatic thiol SAMs. 43 These results imply that the reductive desorption for organic thiol SAMs on metal electrodes occurs more easily when the organic thiols contain electron withdrawing groups, since such groups act as good electron acceptors in the reduction process.…”

“…To understand these fundamental issues, STM has been used as one of the very powerful tools for visualizing the atomic-and molecular-scale surface structures of SAMs on metal surfaces, as demonstrated in many previous reports. [1][2][3][4][6][7][8][9][10][11][12][13][14][15][16][20][21][22][23][24][25][26][27][28][29][30][31][32][33] Additionally, thermal desorption spectroscopy (TDS) 10,11,32,[34][35][36][37] and electrochemical measurements [38][39][40][41][42][43] are very useful techniques to investigate the adsorption conditions, thermal stability, and the electrochemical behavior and stability of SAMs. TDS measurements revealed that alkanethiol SAMs exist as a form of monomer (alkanethiolate), not dimers (dialkyl disulfides) derived from associa-tive reactions of the sulfur headgroups, 10,11,34,…”

“…TDS measurements revealed that alkanethiol SAMs exist as a form of monomer (alkanethiolate), not dimers (dialkyl disulfides) derived from associa-tive reactions of the sulfur headgroups, 10,11,34,36 while electrochemical measurements have shown that alkanethiolates with longer alkyl chains have more negative peak potential for reductive desorption compared to those with shorter chains, implying that van der Waals interactions among alkyl chains are a crucial factor for enhancing electrochemical stability. [38][39][40][41][42] To date, however, there have been no reports directly comparing the formation and reductive desorption behavior of organic thiol SAMs with aromatic and alicyclic molecular backbones. Therefore, to better understand these issues, we investigated three types of molecules: BT with an aromatic ring, CHT with a six-membered aliphatic ring, and cyclopentanethiol (CPT) with a five-membered aliphatic ring ( Figure 1).…”