Phase separation of binary self-assembled thiol monolayers composed of 1-hexadecanethiol and 3-mercaptopropionic acid on Au(111) studied by scanning tunneling microscopy and cyclic voltammetry

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Abstract

The mixing characteristics of binary self-assembled monolayers composed of 1-hexadecanethiol (HDT) and 3-mercaptopropionic acid (MPA) on Au(111) have been studied by cyclic voltammetry and scanning tunneling microscopy. Two distinctive peaks, ∼0.45V apart, are observed over the entire range of surface composition on cyclic voltammograms for the reductive desorption of the adsorbed thiol molecules, which reflects the presence of two different types of phase-separated domains greater than several tens of nm2 which can be imaged by scanning tunneling microscopy. The peak potential of 1-hexadecanethiol is nearly independent of the surface composition whereas a slight shift of the peak potential is observed in the case of MPA, suggesting that the HDT is slightly soluble in the MPA domains, while MPA is insoluble in the HDT domains. The minimum number of the adsorbed thiol molecules required for exhibiting two distinctive peaks (i.e. two-dimensional bulk properties) is estimated to be ca. 50 by comparing the cyclic voltammograms with the distribution of the domain size observed by scanning tunneling microscopy.

Introduction

Self-assembled monolayers of thiol derivatives provide metal surfaces having a variety of chemical properties and functions by introducing different substituents as a terminal functional group in adsorbed thiols. Binary self-assembled monolayers composed of thiols having different chain lengths and/or terminal functional groups are particularly attractive in designing and controlling the surface properties [1]. It has been shown that wetting 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, electron transfer 19, 20, 21, 22, and other adsorption properties of monolayers 23, 24, 25, 26, 27, 28, 29, 30on metal surfaces can be controlled by changing the type and the composition of thiols in the monolayers. Two methodologies for preparing mixed monolayers on metal surfaces have been reported. One is to form a prescribed two-dimensional pattern on the millimeter to micrometer scale using several techniques such as UV photolithography 28, 31, 32, 33, UV irradiation 34, 35, 36, 37, microstamping 28, 38, 39, 40, 41, 42, microcontact pinning 28, 43, micromachining 14, 26, 28, micropens 26, 28, 39, 44, and electron beam bombardment [45]. Fabrication on the nanometer scale by shaving a thiol monolayer using an atomic force microscopy (AFM) tip has also been reported recently [46]. The other is to utilize spontaneous formation of structured mixed monolayers on the nanometer scale through adsorption from binary mixed solution 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 27, 29, 47, 48, 49, 50, 51, 52, 53, 54, replacement of adsorbed thiols [55], or selective electrochemical desorption of mixed monolayers followed by readsorption [56]. Nanometer-scale mixing behavior of binary monolayers on metal surfaces is extremely important in two respects, first, to elucidate the fundamental properties of phase transitions of two-component monolayers adsorbed on a well-defined two-dimensional lattice of a metal surface, and, second, to design desired surface properties at a molecular level. Contact angle and other macroscopic measurements 4, 5, 13, 49, 50have suggested that the binary monolayers do not phase-segregate into macroscopic islands. On the other hand, Stranick et al. reported the presence of nanometer scale domains in two-component self-assembled monolayers of HS(CH2)15CH3 and HS(CH2)15CO2CH3, formed on an Au surface, using scanning tunneling microscopy (STM) [52]. A recent AFM study has also shown nanometer-scale domains for binary thiol monolayers composed of HS(CH2)3CH3 and HS(CH2)17CH3 [57]. The absence of the segregation deduced from macroscopic approaches and the above scanning probe microscopic evidence for the presence of nanometer-scale phase separation have motivated us to study the relationship between the nanometer-scale phase separation and the macroscopic behavior of binary self-assembled monolayers, which is reflected in their electrochemical properties.

In the present study, we have investigated the phase behavior of binary monolayers composed of 1-hexadecanethiol (HDT) and 3-mercaptopropionic acid (MPA) using cyclic voltammetry and STM. Using cyclic voltammetry of the reductive desorption of adsorbed thiols, which can provide a sensitive indication of the state of self-assembled monolayers 58, 59, 60, 61, 62, and STM imaging, we will show the presence of two distinctive two-dimensional bulk phases, each of which is mainly composed of either HDT or MPA.

Section snippets

Experimental

HDT (Aldrich Chemical Co.) and MPA (Wako Pure Chemical Ind.) were used without further purification. Water was distilled and purified through a Milli-Q water system (Millipore). All other chemicals were reagent grade. Au films were vapor deposited on freshly cleaved mica sheets (Nilaco Co.) at less than 3×10−7 Torr. The mica was baked at 580°C prior to the vapor deposition and maintained at 580°C during the deposition of Au (99.99%). The Au substrates were then annealed at 550°C for 6 h in

Results and discussion

Fig. 1a shows a cyclic voltammogram for a single-component HDT monolayer which was formed from a 1 mM ethanol solution of HDT. A sharp peak was observed at −1.07 V in the negative scan. Fig. 1f shows a cyclic voltammogram for a single-component MPA monolayer formed from a 1 mM ethanol solution of MPA. A peak, whose shape was similar to that in Fig. 1a, was observed at −0.63 V in the negative scan. These peaks correspond to the reductive desorption of the adsorbed HDT and MPA, respectively 58, 62

Conclusions

The present results demonstrate that the adsorbed monolayer of HDT and MPA on Au(111) exhibits two distinctive domains, which are large enough to give two well-separated peaks in cyclic voltammograms. Although the exact molecular composition of the domains could not be confirmed by the STM measurements, the shift of Peak I observed in the voltammogram (Fig. 2) suggests that HDT is sparingly soluble in MPA domains, while MPA is insoluble in HDT domains. The observed phase separation contrasts

Acknowledgements

This work was partially supported by the New Energy and Industrial Technology Development Organization (NEDO), Research Fellowships of the Japan Society for the Promotion of Science for Young Scientists (D.H.), the Saneyoshi Scholarship Foundation (S.I.), and Yokohama Kogyo-Kai (T.K.).

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