Cartesian Decomposition of Infrared Spectra Reveals the Structure of Solution-Deposited, Self-Assembled Benzoate and Alkanoate Monolayers on Rutile (110)

Abstract

Cartesian polarization analysis transforms a set of surface infrared spectra obtained in different geometries into their Cartesian components using a mathematical transform, providing direct insight into the bonding geometry of adsorbed molecules. This technique was extended to uniaxial substrates and used to analyze solution-deposited, self-assembled benzoate and alkanoate monolayers on rutile (110). This analysis resolved a long-standing controversy regarding the existence of paired molecules in benzoate monolayers, showing that two distinct isomers exist within the monolayer: a tilted tetramer, which is paired, and a twisted monomer, which is not. The two isomers are nearly isoenergetic, as shown by analysis of STM images and complementary DFT simulations. Infrared and XPS spectra as well as STM images of heptanoate and octanoate monolayers showed the formation of complete monolayers (as opposed to sparse layers or multilayers); however, the alkyl chains in the monolayer are disordered and loosely packed with a significant density of conformational defectsa stark contrast to the near-crystalline, all-trans alkyl monolayers typically formed on Au and Si surfaces. The high disorder in the alkanoate monolayers was attributed to geometry, as the density of alkanoate binding sites on rutile (110) is 30% less than the density of alkyl monolayers on Si. The high density of gauche defects in alkanoate monolayers was attributed to the small energy difference between the all-trans and single-gauche-defect conformers in isolated alkyl chains. In contrast, strong intermolecular interactions in tight-packed alkyl monolayers on Au and Si surfaces suppress gauche defect formation.

Atomic-Scale Understanding of Catalyst Activation: Carboxylic Acid Solutions, but Not the Acid Itself, Increase the Reactivity of Anatase (001) Faceted Nanocatalysts

“…For example, Figure 5 Infrared Spectroscopy Confirms Benzoate Formation. The infrared spectra of benzoate-terminated anatase (001) nanocrystals were strikingly similar to spectra of benzoateterminated rutile (110), 25,26,39 a system in which benzoate is known to bond in a vertical, bridged bidentate geometry. The observed modes are summarized in Table 1 and discussed in the following.…”

“…As a result, all in-planepolarized spectra are identical on anatase (001), whereas there are two distinct in-plane polarizations (i.e., x and y) on rutile The rutile data are from refs. 25,26,39 The (110). Because of this higher symmetry and the multilayer TiO 2 /SrTiO 3 /TiO 2 substrate, we were unable to use the Cartesian transformation techniques 39 that enabled the resolution of two additional C−H stretch modes at 3056 and 3070 cm −1 on benzoate-terminated rutile (110).…”

Breaking π–π Interactions in Carboxylic Acid Monolayers on Rutile TiO2(110) Leads to Unexpected Long-Range Ordering

“…The Ti, O, and H atoms are represented by blue, red, and white spheres, respectively. See ref for more information.…”

“…As an example, benzoic acid, the simplest analogue of the dyes used in TiO 2 -based dye-sensitized solar cells, binds to TiO 2 (110) as benzoate in two nearly isoenergetic isomers 8 as shown in Figure 1. The H atoms produced in this dissociative adsorption reaction bind to surface "bridging" O atoms, forming hydroxyls.…”

Self-Assembly of Di(pyrazol-1-yl)pyridine-benzoic Acid on Underpotentially Deposited Ag from Solution

“…Whereas alkanoic acids have been studied along with thiols and silanes since the emergence of the field of self-assembled monolayers (SAMs), − corresponding investigations of SAMs formed from aromatic carboxylic acids (ArCAs), either in an ultrahigh vacuum environment or at the liquid/solid interface, have been rather scarce. − Spectroscopic studies showed that layers with a well-defined orientation of the molecules can be formed through coordinative bonding of the carboxylate moiety to the substrate, and more recent experiments employing scanning tunneling microscopy (STM) further revealed that ArCA SAMs form highly crystalline layers both on metals, such as Ag and Cu, and on metal oxides, such as TiO 2 . − …”