Resolution of Intermediate Adsorbate Structures in the Potential-Dependent Self-Assembly of n-Hexanethiolate on Silver by in situ Surface-Enhanced Raman Spectroscopy

Journal Article · Sun Aug 01 00:00:00 EDT 2004 · Applied Spectroscopy

DOI:https://doi.org/10.1366/0003702041655395· OSTI ID:2480878

Uibel, Rory H. ^[1]; Harris, Joel M. ^[1]

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1. University of Utah, Salt Lake City, UT (United States)

Resolution of the reaction steps and the associated component Raman spectra during the formation or desorption of self-assembled monolayers is challenging because intermediate adsorbate populations are present at low concentrations and their spectral bands overlap. By collecting Raman spectra versus applied potential into a two-dimensional data set, one can utilize multivariate statistical techniques to resolve the component concentration profiles along with their corresponding Raman spectra. In situ surface-enhanced Raman spectroscopy (SERS) spectra were collected during the potential-dependent formation and desorption (–1.50 to –0.70V versus Ag/AgCl) of n-hexanethiolate monolayer at a polycrystalline Ag electrode. Resolution of the pure component spectra from these components was accomplished by using self-modeling curve resolution (SMCR), which does not require a physical model. For monolayer adsorption, the potential-dependent Raman spectra could be described by three significant eigenvectors; the eigenvectors could be rotated into a set of pure component spectra and concentration profiles using a linear least-squares step to find a common plane in the space of the eigenvectors representing the linear combination of the real-component responses. The convex hull surrounding the data in the plane and positive amplitude criteria were utilized to identify the coordinates of the pure component responses. The C– S stretching vibrations of the resolved spectra show that the initial adsorbate is a gauche conformer, which allows the hydrocarbon chain to lie on the metal surface; a second phase arises at higher coverage with trans C–S conformation, where the hydrocarbon chains are oriented off the surface plane, and a final complete monolayer is formed with a well-ordered, all-trans C–S configuration. In contrast, desorption studies showed only two surface phases, the initial well-ordered monolayer and the low-density phase dominated by gauche conformations. Furthermore, the results illustrate the utility of self-modeling curve resolution to unravel interfacial reaction mechanisms and intermediate structures from two-dimensional SERS data, without requiring prior knowledge of a physical model for the process.

• Research Organization: University of Utah, Salt Lake City, UT (United States)
• Sponsoring Organization: USDOE Office of Science (SC), Basic Energy Sciences (BES). Chemical Sciences, Geosciences & Biosciences Division (CSGB)
• Grant/Contract Number: FG03-93ER14333
• OSTI ID: 2480878
• Journal Information: Applied Spectroscopy, Journal Name: Applied Spectroscopy Journal Issue: 8 Vol. 58; ISSN 0003-7028
• Publisher: Society for Applied Spectroscopy
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
factor analysis
self-assembled monolayers
self-modeling curve resolution
spectroelectrochemistry
surface-enhanced Raman spectroscopy (SERS)