An in situ STM and DTS study of the extremely pure [EMIM]FAP/Au(111) interface

Herein the structure of the interfacial layer between the air- and water-stable ionic liquid 1-ethyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate ([EMIM]FAP) and Au(111) is investigated using in situ scanning tunneling microscopy (STM), distance tunneling spectroscopy (DTS) and cyclic voltammetry (CV) measurements. The in situ STM measurements reveal that structured interfacial layers can be probed in both cathodic and anodic regimes at the IL/Au(111) interface. The structure of these layers is dependent on the applied electrode potential, the number of subsequent STM scans and the scan rate. Furthermore, first DTS results show that the tunneling barrier during the 1st STM scan does not seem to change significantly in the cathodic potential regime between the ocp (-0.2 V) and -2.0 V.     

In situ STM investigation of gold reconstruction and of silicon electrodeposition on Au(111) in the room temperature ionic liquid 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide

The electrodeposition of silicon on Au(111) was investigated by cyclic voltammety (CV) and by in situ scanning tunneling microscopy (STM) in the room temperature ionic liquid 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide with a SiCl(4) concentration of 0.1 mol/L. A main reduction process begins in the cyclic voltammogram at about -1800 mV versus ferrocene/ferrocinium, which is correlated to the electrodeposition of elemental semiconducting silicon. It has been found that at an electrode potential more negative than the open circuit potential (OCP), the Au(111) surface is subject to a restructuring/reconstruction both in the case of the pure ionic liquid and in the presence of SiCl(4). The first STM-probed silicon islands with 150-450 pm in height appear at about -1700 mV versus ferrocene/ferrocinium. Their lateral and vertical growth leads to the formation of a rough layer with silicon islands of up to 1 nm in height. At about -1800 mV the islands merge and form silicon agglomerates. In situ I/U tunneling spectroscopy reveals a band gap of 1.1 +/- 0.2 eV for layers of about 5 nm in height, a value that has to be expected for semiconducting silicon.     

Determination of the solubilising character of 2-methoxyethyl-(dimethyl)ethylammonium tris(pentafluoroethyl)trifluorophosphate based on the Abraham solvation parameter model

Gas–liquid chromatographic retention factors have been measured for 42 different organic probes on a 2-methoxyethyl(dimethyl)ethylammonium tris(pentafluoroethyl)trifluorophosphate, ([MeoeM₂EAm]⁺[FAP]^–), stationary phase capillary column at 323 and 353 K. The measured retention factors were combined with published gas-to-liquid partition coefficient data for solutes dissolved in ([MeoeM₂EAm]⁺[FAP]^–) and with published gas-to-water partition coefficient data to yield 107 gas-to-anhydrous ionic liquid and 105 water-to-anhydrous ionic liquid partition coefficients. Abraham model correlations for describing solute transfer into ([MeoeM₂EAm]⁺[FAP]^–) were derived from the three sets of experimental partition coefficient data. The derived correlations back-calculated the experimental gas-to-([MeoeM₂EAm]⁺[FAP]^–) and water-to-([MeoeM₂EAm]⁺[FAP]^–) partition coefficient data to within 0.13 and 0.15 log units, respectively.     

Decorated Ru/Au(111) and Os/Au(111) surfaces: An in situ STM and electrochemistry study

The structure and reactivity of bimetallic electrodes obtained by spontaneous deposition of Ru and Os on Au(111) single-crystal surfaces are studied. In situ electrochemical STM and cyclic voltammetry are used to characterize a wide range of surface morphologies thus produced. The STM results on Ru/Au(111) demonstrate a pronounced step decoration, while a random distribution of Ru nuclei, quite uniform in size, occurs on terraces. Osmium deposits show a slight preference for deposition on steps, but it also occurs readily on terraces. However, many of the Os islands grow into multilayer heights. The coverage of the Au(111) by the deposited Ru or Os islands for a particular solution concentration depends on the deposition time. Nanostructures of Ru and Os are tested for catalytic behavior and correlated to CO oxidation activity as measured by CO stripping voltammetry. Published in Russian in Elektrokhimiya, 2006, Vol. 42, No. 11, pp. 1385–1392. Based on the report delivered at the 8th International Frumkin Symposium “Kinetics of the Electrode Processes,” October 18–22, 2005, Moscow. The text was submitted by the authors in English.     

Electrodeposition of Al in 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ionic liquids: in situ STM and EQCM studies

In the present paper, the electrodeposition of Al on flame-annealed Au(111) and polycrystalline Au substrates in two air- and water-stable ionic liquids namely, 1-butyl-1-methyl-pyrrolidinium bis(trifluoromethylsulfonyl)amide, [Py(1,4)]Tf(2)N, and 1-ethyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl)amide, [EMIm]Tf(2)N, has been investigated by in situ scanning tunneling microscopy (STM), electrochemical quartz crystal microbalance (EQCM), and cyclic voltammetry. The cyclic voltammogram of aluminum deposition and stripping on Au(111) in the upper phase of the biphasic mixture of AlCl(3)/[EMIm]Tf(2)N at room temperature (25 degrees C) shows that the electrodeposition process is completely reversible as also evidenced by in situ STM and EQCM studies. Additionally, a cathodic peak at an electrode potential of about 0.55 V vs Al/Al(III) is correlated to the aluminum UPD process that was evidenced by in situ STM. A surface alloying of Al with Au at the early stage of deposition occurs. It has been found that the Au(111) surface is subject to a restructuring/reconstruction in the upper phase of the biphasic mixture of AlCl(3)/[Py(1,4)]Tf(2)N at room temperature (25 degrees C) and that the deposition is not fully reversible. Furthermore, the underpotential deposition of Al in [Py(1,4)]Tf(2)N is not as clear as in [EMIm]Tf(2)N. The frequency shift in the EQCM experiments in [Py(1,4)]Tf(2)N shows a surprising result as an increase in frequency and a decrease in damping with bulk aluminum deposition at potentials more negative than -1.8 V was observed at room temperature. However, at 100 degrees C there is a frequency decrease with ongoing Al deposition. At -2.0 V vs Al/Al(III), a bulk aluminum deposition sets in.     

Long-range self-assembly of a polyunsaturated linear organosilane at the n-tetradecane/Au(111) interface studied by STM

We report on the formation of self-assembled monolayers of 13-(trimethylsilyl)-1-tridecene-6,12-diyne [C13H17-Si(CH3)3], an organosilane derivative with a linear polyunsaturated chain, on Au(111) substrates. Molecular resolution STM images recorded at the liquid-solid interface between gold and tetradecane reveal a long-range and densely packed hexagonal lattice with a ( radical3 x radical3)R30 degrees -like structure commensurate against gold adlattice.     

In situ STM evidence for the adsorption geometry of three N-heteroaromatic thiols on Au(111)

The electrochemical behavior of three heteroaromatic thiols (MBs) (2-mercaptobenzimidazole (MBI), 2-mercaptobenzothiazole (MBT), and 2-mercaptobenzoxazole (MBO)) on a Au(111) surface has been investigated by electrochemical scanning tunneling microscopy (ECSTM) and cyclic voltammetry (CV) in 0.1 M HClO(4) solution. All three thiols form oriented molecular cluster lines along the reconstruction line direction at 0.55 V. With the electrode potential shifting negatively, the molecules undergo a disordered-ordered structural transition. Molecularly resolved STM images show that all three molecules form striped adlayers in the desorption region on the Au(111) surface. The different heteroatoms in the heteroaromatic rings result in different electrochemical behavior of the MB self-assembled monolayers (SAMs). MBI, MBT, and MBO are proposed to interact with the substrate via the S-Au bonds from thiol group and the coordination interaction of N, S, and O with the substrate from the heteroaromatic ring, respectively. These results provide direct evidence of the electrochemical behavior and the adlayer structures of MB SAMs on the Au electrode.     

In situ STM study of zinc electrodeposition on Au(111) from the ionic liquid 1-ethyl-3-methylimidazolium trifluoromethylsulfonate

The electrodeposition of Zn on Au(111) was investigated with cyclic voltammetry (CV) and in situ scanning tunneling microscopy (STM) in the air and water stable ionic liquid 1-ethyl-3-methylimidazolium trifluoromethylsulfonate ([EMIm]TfO) with a Zn(TfO)₂ concentration of 0.2 M. It has been found that the structure [EMIm]TfO/Au(111) is very complex. Furthermore, the addition of Zn(TfO)₂ changes the interfacial structure significantly. The first STM-probed Zn islands appear at +0.3 V, and their growth leads to the formation of a thin zinc layer. A bulk deposition of Zn is obtained with in situ STM at ?0.1 V. Furthermore, in situ STM reveals that the deposition of Zn is accompanied by the formation of Au-Zn surface alloys.     

Probing the buried C60/Au(111) interface with atoms

To characterize the C(60)/Au(111) interface, we send Au atoms "diving" through the C(60) layer and observe their behavior at the interface. Our observations show that the interfacial diffusion of gold atoms and the nucleation of small Au islands at the interface are strongly dependent on the local C(60)-Au(111) bonding which varies from one domain to another. The contrast-disordered domain consisting of a large fraction of molecules bonded to Au vacancies has a special structure at the interface allowing Au atoms to be inserted beneath the bright-looking molecules while the dim molecules present a much stronger resistance to the diffusing Au atoms. This leads to the formation of isolated Au islands with discrete sizes, with the smallest island just about 1 nm across.     

Assembly dynamics and detailed structure of 1-propanethiol monolayers on Au(111) surfaces observed real time by in situ STM

1-Propanethiol is chosen as a model alkanethiol to probe detailed mechanisms of the self-assembled monolayer (SAM) formation at aqueous/Au(111) interfaces. The assembly processes, including initial physi- and chemisorption, pit formation, and domain growth, were recorded into movies in real-time with high resolution by in situ scanning tunneling microscopy (STM) under potential control. Two major adsorption steps were disclosed in the propanethiol SAM formation. The first step involves weak interactions accompanied by the lift of the Au(111) surface reconstruction, which depends reversibly on the electrochemical potentials. The second step is chemisorption to form a dense monolayer, accompanied by formation of pits as well as structural changes in the terrace edges. Pits emerged at the stage of the reconstruction lift and increased to a maximum surface coverage of 4.0 +/- 0.4% at the completion of the SAM formation. Well-defined triangular pits in the SAM were found on the large terraces (more than 300 nm wide), whereas few and small pinholes appeared at the terrace edge areas. Smooth edges were converted into saw-like structural features during the SAM formation, primarily along the Au(111) atomic rows. These observations suggest that shrinking and rearrangement of gold atoms are responsible for both formation of the pits and the shape changes of the terrace edges. STM images disclose a (2 square root 3 x 3)R30 degrees periodic lattice within the ordered domains. Along with electrochemical measurements, each lattice unit is assigned to contain four propanethiol molecules exhibiting different electronic contrasts, which might originate in different surface orientations of the adsorbed molecules.     

Methanethiolate adsorption site on Au(111): a combined STM/DFT study at the single-molecule level

The chemisorptive bonding of methanethiolate (CH(3)S) on the Au(111) surface has been investigated at a single-molecule level using low-temperature scanning tunneling microscopy (LT-STM) and density functional theory (DFT). The CH(3)S species were produced by STM-tip-induced dissociation of methanethiol (CH(3)SH) or dimethyl disulfide (CH(3)SSCH(3)) at 5 K. The adsorption site of an isolated CH(3)S species was assigned by comparing the experimental and calculated STM images. We conclude that the S-headgroup of chemisorbed CH(3)S adsorbs on the 2-fold coordinated bridge site between two Au atoms, consistent with theoretical predictions for CH(3)S on the nondefective Au(111) surface. Our assignment is also supported by the freezing of the tip-induced rotational dynamics of a single CH(3)SH molecule upon conversion to CH(3)S via deprotonation.     

In situ STM study of potential-driven transitions in the film of a cationic surfactant adsorbed on a Au(111) electrode surface

Electrochemical scanning tunneling microscopy (EC-STM) has been employed to study the structure of a film formed by cationic surfactant N-decyl-N,N,N-trimethylammonium triflate (DeTATf) adsorbed on the Au(111) electrode surface. The film is disordered at potentials corresponding to either large negative charge densities or to positive charge densities. At small negative charge densities, an ordered adlayer of flat-lying DeTATf molecules is formed. High-resolution images of this adlayer reveal that the triflate anion is coadsorbed with the N-decyl-N,N,N-trimethylammonium cation, effectively forming an ion pair at the electrode surface. This is a significant result because it explains why this surfactant behaves like a zwitterionic surfactant at the metal/solution interface.     

Layer by layer characterization of 1-octadecanol films on a Au(111) electrode surface. "In situ" polarization modulation infrared reflection absorption spectroscopy and electrochemical studies

Bilayers of 1-octadecanol were transferred from the gas-solution interface onto a Au(111) electrode surface using either the horizontal touch or the Langmuir-Blodgett (LB) technique. The horizontal touch method gives an X-type film in which the molecule in the bottom layer is oriented with the tail toward the metal and the head toward the tail of the molecule in the top layer (head to tail orientation). The LB technique allows one to build not only X but also a Y bilayer, in which the molecules in the two leaflets are oriented tail to tail with the polar heads facing the metal and the solution. The properties of these films were characterized by electrochemistry and polarization modulation infrared reflection absorption spectroscopy (PM IRRAS). The freshly prepared X bilayer is defect free and is characterized by a low capacity of 0.84 microF cm(-2). The bilayer may be desorbed from the electrode surface at E < -0.6 V and readsorbed again at more positive potentials. The potential-driven readsorption gives a Y film with a higher capacity of 2.2 microF cm(-2). The IR studies of a mixed bilayer composed of one hydrogenated leaflet and a second deuterated leaflet showed that 1-octadecanol molecules assume a more vertical orientation in the leaflet facing the solution than in the leaflet in contact with the metal. Upon desorption of the X film, the molecules in the bottom layer flip-flop and the readsorbed film assumes the Y structure. Smaller tilt angles are observed in the X film as opposed to the Y film.     

Kinetics of the oxygen transfer reaction at the (U0.5Sc0.5)O2±x/YSZ interface by impedance spectroscopy

The complex impedance dispersion analysis technique was used to study the electrode kinetics of (U_0.5Sc_0.5)O_2±x, a fluorite type solid solution material potentially suitable as electrode for low temperature oxygen sensors. Variables included the temperature and oxygen partial pressure. The effect of heat treatment on the interfacial contact resistance and the electrode morphology was also investigated. A single are for the electrode reaction was observed over most of the experimental ranges of temperature and oxygen partial pressure. The angle of depression of the electrode are was small (8–18°) compared with platinum electrodes (20–45°). The activation energy for the overall electrode reaction was between 170 and 180 kJ mol^?1. The average value for the pressure exponent, determined from the oxygen partial pressure dependence of the electrode resistance, was 0.16. A mechanism for the oxygen transfer reaction is proposed. Materials of this type show promise for future use in low temperature oxygen sensors.     

Liquid/solid interface of ultrathin ionic liquid films: [C1C1Im][Tf2N] and [C8C1Im][Tf2N] on Au(111)

Ultrathin films of two imidazolium-based ionic liquids (IL), [C(1)C(1)Im][Tf(2)N] (= 1,3-dimethylimidazolium bis(trifluoromethyl)imide) and [C(8)C(1)Im][Tf(2)N] (= 1-methyl-3-octylimidazolium bis(trifluoromethyl)imide) were prepared on a Au(111) single-crystal surface by physical vapor deposition in ultrahigh vacuum. The adsorption behavior, orientation, and growth were monitored via angle-resolved X-ray photoelectron spectroscopy (ARXPS). Coverage-dependent chemical shifts of the IL-derived core levels indicate that for both ILs the first layer is formed from anions and cations directly in contact with the Au surface in a checkerboard arrangement and that for [C(8)C(1)Im][Tf(2)N] a reorientation of the alkyl chain with increasing coverage is found. For both ILs, geometry models of the first adsorption layer are proposed. For higher coverages, both ILs grow in a layer-by-layer fashion up to thicknesses of at least 9 nm (>10 ML). Moreover, beam damage effects are discussed, which are mainly related to the decomposition of [Tf(2)N](-) anions directly adsorbed at the gold surface.     

Ionic mobility, ionic transport, and charge transfer mechanism in solid solutions (1 ? x )PbF2- x MF n by the NMR and impedance spectroscopy data

The internal mobility and ionic conduction of solid solutions in systems PbF₂-MF₂ and PbF₂-MF₃ are studied by the NMR (¹⁹F, ²⁷Al, ²⁰⁷Pb) and impedance spectroscopy methods. Factors that define the form of ionic movements and their energy characteristics in the temperature region 150 to 550 K are considered and analyzed. Temperature shifts of the frequency range that defines the ionic conduction of solid solutions are discovered on the basis of impedance spectroscopy data and explained. It is established that the high ionic conductance in lead difluoride doped with fluorides of metals of Groups II and III is caused by the diffusion of fluoride ions. The large values of specific conductance (10⁻⁴ to 10⁻³ S cm⁻¹) at a relatively low activation energy (less than 0.6 eV) allow one to consider the solid solutions studied in the role of a basis for the obtaining of new fluoride materials with high ionic conductance. Original Russian Text ? V.Ya. Kavun, A.B. Slobodyuk, S.L. Sinebryukhov, E.V. Tararako, V.K. Goncharuk, S.V. Gnedenkov, V.I. Sergienko, 2007, published in Elektrokhimiya, 2007, Vol. 43, No. 6, pp. 643–656. Based on the report delivered at the 8th International Meeting on Fundamental Problems of Solid-State Ionics, Chernogolovka (Russia), 2006.