At the ionic liquid|metal interface: structure formation and temperature dependent behavior of an ionic liquid adlayer on Au(111).

The growth, structure formation and thermal disordering of (sub-)monolayer films of 1-butyl-1-methylpyrrolidinium-bis(trifluoromethylsulfonyl)imide [BMP][TFSA] grown under ultrahigh vacuum (UHV) conditions on Au(111) have been investigated using scanning tunneling microscopy (STM) and angle resolved X-ray photoelectron spectroscopy (ARXPS) under UHV conditions at temperatures between 100 and 298 K. At room temperature, two-dimensional film growth occurs up to one monolayer coverage, with both cations and anions in direct contact with the gold substrate, as shown by ARXPS, and STM images reveal a 2D liquid state of the adlayer. At lower temperatures, motion is frozen and a disordered 2D glass state as well as a 2D crystalline phase with long-range order are formed. The structure of the 2D crystalline phase is influenced by the underlying Au(111) reconstruction pattern. Annealing experiments reveal that the 2D crystalline phase is thermally more stable against melting than the 2D glass state, and that the stability is strongly affected by the adsorbate coverage.

Visible-light photocurrent response of TiO2-polyheptazine hybrids: evidence for interfacial charge-transfer absorption.

We investigated photoelectrodes based on TiO(2)-polyheptazine hybrid materials. Since both TiO(2) and polyheptazine are extremely chemically stable, these materials are highly promising candidates for fabrication of photoanodes for water photooxidation. The properties of the hybrids were experimentally determined by a careful analysis of optical absorption spectra, luminescence properties and photoelectrochemical measurements, and corroborated by quantum chemical calculations. We provide for the first time clear experimental evidence for the formation of an interfacial charge-transfer complex between polyheptazine (donor) and TiO(2) (acceptor), which is responsible for a significant red shift of absorption and photocurrent response of the hybrid as compared to both of the single components. The direct optical charge transfer from the HOMO of polyheptazine to the conduction band edge of TiO(2) gives rise to an absorption band centered at 2.3 eV (540 nm). The estimated potential of photogenerated holes (+1.7 V vs. NHE, pH 7) allows for photooxidation of water (+0.82 V vs. NHE, pH 7) as evidenced by visible light-driven (λ > 420 nm) evolution of dioxygen on hybrid electrodes modified with IrO(2) nanoparticles as a co-catalyst. The quantum-chemical simulations demonstrate that the TiO(2)-polyheptazine interface is a complex and flexible system energetically favorable for proton-transfer processes required for water oxidation. Apart from water splitting, this type of hybrid materials may also find further applications in a broader research area of solar energy conversion and photo-responsive devices.

Towards a molecular understanding of cation-anion interactions--probing the electronic structure of imidazolium ionic liquids by NMR spectroscopy, X-ray photoelectron spectroscopy and theoretical calculations.

Ten [C(8)C(1)Im](+) (1-methyl-3-octylimidazolium)-based ionic liquids with anions Cl(-), Br(-), I(-), [NO(3)](-), [BF(4)](-), [TfO](-), [PF(6)](-), [Tf(2)N](-), [Pf(2)N](-), and [FAP](-) (TfO=trifluoromethylsulfonate, Tf(2)N=bis(trifluoromethylsulfonyl)imide, Pf(2)N=bis(pentafluoroethylsulfonyl)imide, FAP=tris(pentafluoroethyl)trifluorophosphate) and two [C(8)C(1)C(1)Im](+) (1,2-dimethyl-3-octylimidazolium)-based ionic liquids with anions Br(-) and [Tf(2)N](-) were investigated by using X-ray photoelectron spectroscopy (XPS), NMR spectroscopy and theoretical calculations. While (1)H NMR spectroscopy is found to probe very specifically the strongest hydrogen-bond interaction between the hydrogen attached to the C(2) position and the anion, a comparative XPS study provides first direct experimental evidence for cation-anion charge-transfer phenomena in ionic liquids as a function of the ionic liquid's anion. These charge-transfer effects are found to be surprisingly similar for [C(8)C(1)Im](+) and [C(8)C(1)C(1)Im](+) salts of the same anion, which in combination with theoretical calculations leads to the conclusion that hydrogen bonding and charge transfer occur independently from each other, but are both more pronounced for small and more strongly coordinating anions, and are greatly reduced in the case of large and weakly coordinating anions.

Physical vapor deposition of [EMIM][Tf2N]: a new approach to the modification of surface properties with ultrathin ionic liquid films.

Ultrathin films of the ionic liquid (IL) 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [EMIM][Tf(2)N], are prepared on a glass substrate by means of an in situ thermal-evaporation/condensation process under ultrahigh-vacuum conditions. By using X-ray photoelectron spectroscopy (XPS), it is demonstrated that the first layer of the IL film grows two dimensionally, followed by the three-dimensional growth of successive layers. The first molecular layer consists of a bilayer, with the [EMIM](+) cations in contact to the surface and the [Tf(2)N](-) anions at the vacuum side. The ultrathin IL films are found to be stable under ambient conditions.

Tuning the Stability of Surface Intermediates Using Adsorbed Oxygen: Acetate on Au(111).

Selective oxidative reactions promoted by gold depend critically on controlling the coverage and stability of adsorbed intermediates, as well as promoting specific bond activations of those intermediates. We demonstrate that acetate, a common intermediate in the oxidation of olefins, aldehydes, and alcohols, is destabilized by 7-10 kcal/mol by coadsorbed oxygen relative to its stability on the clean gold surface. The amount of destabilization depends on the oxygen coverage. Peak temperatures of products indicative of oxygen-assisted and clean-surface bond activation differ by up to 130 K. Experiments with d3-acetate show a kinetic isotope effect of 6.9 at 400 K, indicating that the rate-limiting step of the low temperature oxygen-assisted reaction is γ-CH bond breaking. This clearly demonstrates that coadsorbed oxygen activates γ-CH bonds on gold and suggests that an oxygen-assisted activation may also occur for ß-CH bonds crucial in oxygen-assisted alcohol coupling on metallic gold catalysts, as predicted by theory.