The growth, structure formation and thermal disordering of (sub-)monolayer films of 1-butyl-1-methylpyrrolidinium-bis(trifluoromethylsulfonyl)imide BMPTFSA 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.
The growth, structure formation and thermal disordering of (sub-)monolayer films of 1-butyl-1-methylpyrrolidiniumbis(trifluoromethylsulfonyl)imide on Au(111) have been investigated using STM and ARXPS under UHV conditions and between 100 and 298 K.
Ten C8C1Im+ (1‐methyl‐3‐octylimidazolium)‐based ionic liquids with anions Cl−, Br−, I−, NO3−, BF4−, TfO−, PF6−, Tf2N−, Pf2N−, and FAP− (TfO=trifluoromethylsulfonate, ...Tf2N=bis(trifluoromethylsulfonyl)imide, Pf2N=bis(pentafluoroethylsulfonyl)imide, FAP=tris(pentafluoroethyl)trifluorophosphate) and two C8C1C1Im+ (1,2‐dimethyl‐3‐octylimidazolium)‐based ionic liquids with anions Br− and Tf2N− were investigated by using X‐ray photoelectron spectroscopy (XPS), NMR spectroscopy and theoretical calculations. While 1H NMR spectroscopy is found to probe very specifically the strongest hydrogen‐bond interaction between the hydrogen attached to the C2 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 C8C1Im+ and C8C1C1Im+ 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.
Charges in charged systems: Anion and cation interactions in ionic liquids strongly modify the charges on the ions as proven by XPS, NMR spectroscopy and DFT calculations. The smaller, more basic and, thus, more strongly coordinating the anion, the more pronounced the charge transfer to the cation (see graphic). Hydrogen‐bonding‐type interactions between anion and cation do not significantly influence the amount of charge transferred.
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.
Ultrathin films of two imidazolium-based ionic liquids (ILs), C1C1ImTf2N (=1-methyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide) and C4C1ImTf2N (=1-butyl-3-methylimidazolium ...bis(trifluoromethylsulfonyl)imide) were deposited on mica surfaces by physical vapor deposition in ultrahigh vacuum. Using angle-resolved X-ray photoelectron spectroscopy (ARXPS), the initial wetting behavior, the growth characteristics, and the molecular arrangement of the ions at the interface were investigated. The measurements were performed on freshly air-cleaved mica surfaces with different carbon precoverages. ARXPS clearly reveals that the initial IL adsorption behavior strongly depends on the amount of preadsorbed carbon: On clean mica, 3D growth (complete dewetting) occurs, whereas on a fully carbon covered surface, initially a complete 2D wetting layer forms, followed by 3D growth.
The concept of employing photonic crystal fibers for chemical and (photo)‐catalytical transformations is presented. These optofluidic microdevices represent a versatile platform where light and ...fluids can interact for spectroscopic or photoactivation purposes. The use of photonic crystal fibers in chemistry and sensing is reviewed and recent applications as catalytic microreactors are presented. Results on homogeneous catalysis and the immobilization of homogeneous and heterogeneous catalysts in the fiber channels are discussed. The examples demonstrate that combining catalysis and the excellent light guidance of photonic crystal fibers provides unique features for example, for photocatalytic activation and quantitative photospectroscopic reaction analysis.
Set photons to stun! Photonic crystal fibers are versatile microreactors for photochemical and catalytic transformations. Their excellent light guidance properties offer unique opportunities for both photocatalytic activation and quantitative photospectroscopic reaction analysis, while offering all the advantages that are commonly linked to the use of microreactors, that is, strict control of reaction conditions.
Photo-oxidation of formaldehyde is important because it is an atmospheric pollutant and a possible intermediate along the pathway for CO2 reduction to methanol. By using temperature-programmed ...reaction spectroscopy as the main tool, we demonstrate that the efficiency for photo-oxidation of formaldehyde to adsorbed formate on rutile TiO2(110) is strongly dependent on the degree of reduction of the surface and near-surface region. The most efficient photoreaction occurs when O adatoms are present on titania that is highly bulk reduced. These data suggest that electron–hole pairs created deep in the bulk do not play a significant role in the photo-oxidation of formaldehyde. Exposure of the material to O2 heals most bridging oxygen vacancies, creates O adatoms in the Ti five-coordinate rows, and quenches charge at the surface due to the presence of interstitial defects in the near-surface region. Under these conditions, the surface is oxidized but the bulk remains reduced. The major product of formaldehyde photo-oxidation is adsorbed formate; the highest yield is for a highly reduced sample containing O adatoms. The efficiency of formate production on a highly reduced surface is about four times smaller than on one with a similar degree of bulk reduction but containing O adatoms on the surface. On reduced surfaces (not containing O adatoms), the source of the additional O to make formate is derived from a less efficient and more complex photodecomposition pathway of formaldehyde. With increasing bulk reduction of the crystal, the photoefficiency on the reduced surface is severely diminished, suggesting that bridging O vacancies quench photo-oxidation. All photoproducts remain on the surface; only formaldehyde desorption is detected during exposure to UV light. A combination of scanning tunneling microscopy and density functional theory calculations were used to establish that formaldehyde forms a surface dioxyalkylene complex (H2CO–Oad) with oxygen adatoms. This intermediate loses hydrogen to a hole at a nearby bridging O to yield the formate photoproduct. These results are discussed in the broader context of photo-oxidation.
Ultrathin films of the ionic liquid (IL) 1‐ethyl‐3‐methylimidazolium bis(trifluoromethylsulfonyl)imide, EMIMTf2N, 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 Tf2N− anions at the vacuum side. The ultrathin IL films are found to be stable under ambient conditions.
Preferential ionic distribution: Nanometer‐thick ionic liquid films produced on planar glass substrates by means of thermal evaporation and recondensation under ultrahigh‐vacuum conditions exhibit a bilayered structure within the first molecular layer (see picture).
Carboxylates are important intermediates in oxidative reactions on gold, as they are precursors to carboxylic acids and CO2; they may also act as site-blockers in oxidative coupling of alcohols, ...thereby decreasing both catalyst activity and selectivity. We demonstrate that the reaction selectivity and pathways for a prototype carboxylate, acetate, adsorbed on Au(111), are dramatically altered by the presence of coadsorbed atomic O. Finely tuning the initial oxygen coverage affords control of the product selectivity and the reaction pathway. Oxygen-assisted γ-C–H activation occurs with coadsorbed oxygen near 425 K, yielding mainly CO2 and formaldehyde, and a kinetic isotope effect is observed for these products. In the absence of coadsorbed oxygen, acetate reacts at 530 K by C–C bond cleavage to form CO2, methyl, and methyl acetate as well as minor products. These studies have led to the identification of a new synthetic pathway for ester formation, in which methyl (either produced in the reaction or introduced externally using methyl iodide) reacts with surface acetate to form methyl acetate. Detailed isotopic labeling studies using d 3-acetate, 13C-acetate, and 18O show that the methyl carbon forms mainly formaldehyde in the oxygen assisted reaction and methyl in the clean-surface reaction and that surface oxygen is incorporated into products in the low temperature, oxygen-assisted pathway. A complete mechanism is proposed and compared to the reaction of acetate on silver. These studies provide a detailed fundamental understanding of acetate chemistry on gold and demonstrate how the oxygen concentration can be used to tune selectivity.
The chemical surface composition of Rh‐containing ionic liquid solutions has been studied by angle‐resolved X‐ray photoelectron spectroscopy (ARXPS). Rh–tppts complexes of high relevance for ...multiphase hydroformylation catalysis showed remarkable surface activity, whereas non‐tppts containing systems showed Rh depleting from the surface (tppts=tris(3‐sodium sulfonatophenyl)phosphine).
