W. Freyland

Electrochemical in situ STM study of phase formation during Ag and Al electrodeposition on Au(111) from a room temperature molten salt

The electrodeposition of Ag and Al on flame annealed Au(111) films from an acidic aluminium chloride–1-methyl-3-butylimidazolium chloride room temperature molten salt has been investigated by electrochemical scanning tunneling microscopy, cyclic voltammetry and potential step experiments. The cyclic voltammogram of Ag on Au(111) is characterized by adsorption controlled Ag underpotential deposition (upd) and diffusion controlled Ag overpotential deposition (opd). Starting from the anodic limit, bulk oxidation of Au is observed to start near +1.25V vs. Ag/Ag+ reference electrode (RE). In the upd range, two dimensional Ag islands form which merge in a coherent Ag monolayer near 0.05 V vs. RE. With further reduction of the potential, a second monolayer grows. The corresponding chronoamperometric measurements show exponential behaviour with time constants of the order 10 s-1 consistent with a Langmuir adsorption model. In the opd range, a diffusion controlled layer by layer growth of Ag clusters occurs, the bulk Ag+ diffusion coefficient being (1.4±0.2)×10-6 cm2 s-1. Alloying of Ag with codeposited Al from the electrolyte has to be considered. For the Al electrodeposition on Au(111), strong indications for alloying have been observed starting at a potential of +0.95 V vs. Al/Al3+ RE. Below +0.55V, the formation of two dimensional Al islands is seen followed by a three dimensional growth whereby a strong tendency for alloying has to be considered.

Peculiarity of the liquid/vapour interface of an ionic liquid: study of surface tension and viscoelasticity of liquid BMImPF6 at various temperatures

We have measured the surface tension and the capillary wave spectra at the liquid/vapour interface of the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate at various temperatures up to 400 K. From the weak temperature dependence of the surface tension a low value of the surface excess entropy of approximately 3.5 x 10(5) J K(-1) m(-2) results which is consistent with a strongly aligned surface layer of imidazolium cations previously predicted by MD-calculations. The capillary wave spectra recorded at different wave numbers in the range 170 cm(-1) < or = q < or =500 cm(-1) exhibit strong deviations from the behaviour expected for the free surface of simple liquids. With an extended dispersion relation including the contributions of surface dipole moment density gamma and shear surface excess viscosity mu the spectra have been analyzed. It is found that mu is negligibly small, whereas gamma substantially influences the capillary wave spectra. The electrostatic potential across the interface, which corresponds to the measured dipole moment densities, qualitatively agrees with simulation calculations. The distinct temperature dependence of gamma suggests that with increasing temperature an order-disorder transformation occurs in the surface layer.

WETTING AND PREWETTING TRANSITION IN METALLIC FLUID K-KCL SOLUTIONS STUDIED BY SECOND HARMONIC GENERATION

We report a detailed optical investigation of the wetting phenomena in fluid metalrich KxKCl1−x solutions for temperatures up to 820 °C and x≥0.86. To this end the fluid sample–sapphire interface has been probed by second harmonic generation (SHG) with continuous variation of temperature T at various x. At coexistence a wetting transition near Tw≊500 °C is signaled by a clear change in the temperature dependence of the SH intensity. This observation is in agreement with previous ellipsometric measurements. The precise location of the transition in these systems is strongly influenced by the presence of oxygen impurities segregating at the interface. This is manifested in a strong enhancement of the SH signal in the vicinity of 450 °C. In the homogeneous metal‐rich phase the SH intensity shows a pronounced maximum as a function of temperature for T≳Tw. The occurrence of these maxima is explained consistently by assuming a salt‐rich microscopic wetting film in between the metallic bulk fluid and the sapphir...

Interfacial wetting in a liquid binary alloy

We report the first study of interfacial wetting at the liquid-vapour surface of metallic gallium-bismuth alloys approaching the miscibility gap. Ellipsometry has been used to probe the interface continuously with increasing temperature along the coexistence curve. Below the monotectic (eutectic) temperature Tmono a liquid Ga-rich phase is in equilibrium with solid bismuth and the real part epsilon 1 of the complex dielectric function measured at the liquid surface clearly reflects Ga-like behaviour. Above Tmono the second liquid Bi-rich phase becomes stable and epsilon 1 changes from Ga-like to Bi-like behaviour. We conclude that the Bi-rich phase, which has the higher mass density, now covers the low-density phase. This can be consistently interpreted as complete wetting of the interface between the vapour and the Ga-rich phase by a coexisting Bi-rich layer.

Layer-by-layer growth of zinc during electrodeposition on Au(111) from a room temperature molten salt

The initial stages of electrochemical phase formation of Zn on Au(111) in a solution of AlCl3–MBIC (58∶42) containing 1 mmol l−1 of Zn(II) have been investigated by in situ electrochemical scanning tunneling microscopy (STM) and spectroscopy (STS). In the underpotential range (UPD) three successive Zn monolayers are clearly resolved by STM which are correlated with reduction peaks in the cyclic voltammogram at 300, 100 and 10 mV vs. Zn(II)/Zn, respectively. This layer-by-layer growth mechanism seems to continue into the bulk deposition regime. The thickness of the Zn layers in the UPD and OPD range as determined from STM measurements has a value of 2.4 ± 0.2 A, with the exception of the first UPD Zn monolayer which is 2.2 ± 0.2 A thick. Measurements of the effective tunneling barrier ϕ by STS indicate no significant variation of the interfacial electronic structure at anodic potentials, with the exception of a clearly reduced value of ϕ ∼0.7 eV for the first monolayer. These observations are indicative of Zn–Au surface alloying. This is strongly supported by STM images of the Au(111) substrate taken after dissolution of bulk Zn deposits. Surface alloying is manifested by holes with a depth of 1 or 2 monolayers in the Au(111) terraces.

Electron localization in metal–molten salt solutions: An optical study with in situ variation of composition

Optical absorption spectra are reported for liquid KxKCl1−x (10−5≤x≤5×10−2) and NaxNaBr1−x (10−3≤x≤3×10−2) solutions at 800 °C in the energy range 0.5≤hν≤5 eV. The composition has been varied in situ by Coulometric titration of the metal. With this method of simultaneous measurement of optical spectra and metal activity relative changes in the spectra are detected with high precision. In this way it has been possible, for the first time, to separate distinct components in the optical spectra of metal solutions. A contribution which is quantitatively described by the Drude model is attributed to weakly localized electrons in conduction band tail states with low mobility. The determined electronic mobilities are of the order of 0.1 cm2 V−1 s−1. The concentration dependence of the corresponding electronic conductivity quantitatively agrees with independent measurements of the dc‐electronic conductivity. The main component in the absorption spectra is due to excitations of localized electrons which are differ...

Electrochemical in situ STM study of Al and Ti–Al alloy electrodeposition on Au(111) from a room temperature molten salt electrolyte

The electrodeposition of Al and Ti–Al alloys on Au(111) from an acidic aluminium chloride-1-methyl-3-butylimidazolium chloride (AlCl3–MBImC) room-temperature molten salt electrolyte containing TiCl4 was studied by cyclic voltammetry and chronoamperometry and in particular, nucleation and growth processes were monitored in situ by means of electrochemical scanning tunneling microscopy (EC-STM) for the first time. Two underpotential deposition (UPD) states for Al were observed in the cyclic voltammogram at 0.51 and +0.22 V vs. Al/Al3+. At corresponding UPD conditions STM images show different nucleation behavior for Al: first a growth of clusters all over the Au surface, and later preferentially along the step edges leading to a step edge decoration. Potential step experiments imply progressive nucleation and growth and are evidenced by STM images obtained at different time intervals showing the growth of Al clusters of up to 8 nm diameter in size. After anodic dissolution of the deposited Al film, the appearance of circular holes of a size comparable with the Al clusters notably along Au step edges indicates Al–Au surface alloy formation. Tunneling current vs. bias voltage (I–U) curves of the Al clusters exhibit a reduced tunneling barrier height (Φ) of 0.23 eV. A distinct two-dimensional nucleation of Ti–Al clusters begins to occur at 0.5 V vs. Al/Al3+ exclusively on the Au terraces. They grow in the form of chains reaching a size of about 2–3 nm diameter in size, smaller in comparison with Al clusters. X-Ray photoelectron spectra of the potentiostatically deposited film under overpotential deposition (OPD) conditions confirm the formation of a Ti–Al alloy phase.

Electron localization and metal–nonmetal transition in fluid KxKCl1−x: An electron spin resonance study of the magnetic properties with insitu variation of x (10−4⩽x⩽10−1)

We have measured the electron spin resonance (ESR) spectra of saltrich KxKCl1−x melts at temperatures near 800 °C and over a broad composition range, 10−4≤x≤10−1, approaching the metal–nonmetal (M–NM) transition in these solutions. Emphasis has been given to a precise in situ variation of composition which has been achieved for the first time in a high temperature ESR experiment by Coulometric titration. The spectra are characterized by a motional linewidth narrowing with a Lorentzian line shape. The ESR characteristics as determined from the Lorentz fits of the spectra exhibit the following features as a function of x: In the nonmetallic regime the g factor (g=1.9938±0.0002) and the peak‐to‐peak halfwidth (ΔBPP=0.2 mT) stay constant up to x∼0.05. Above this composition a clear increase of ΔBPP is observed indicating the NM–M transition. The spin susceptibility ϰs has been determined from the imaginary part of the Lorentz fits and has been calibrated against a sapphire signal measured simultaneously with ...

Surface freezing in liquid Ga–Bi alloys: optical second harmonic and plasma generation study

The phenomenon of surface freezing has been studied by second harmonic and plasma generation measurements in the entire composition range of liquid Ga–Bi alloys. Surface freezing was observed for the first time for all alloys with compositions between the eutectic (xBi = 0.0022, Teut = 29.48 °C) and monotectic point (xBi = 0.085, Tmono = 222 °C). On cooling of such alloys a Bi-rich solid-like film forms on top of the bulk liquid phase at temperatures well above the liquidus line. On melting of these films a clear hysteresis behaviour is found, characterizing this type of transition as a first order phase transition. The maximum difference of 20 K between the liquidus temperature and the melting temperature of the surface freezing films was observed for the eutectic alloy. The line of surface freezing temperatures merges with the liquidus approaching the monotectic point. An estimate of the thickness of the Bi-rich surface freezing films from the interfacial free energies yields a value of the order of 10 nm. The correlation of the surface freezing and wetting transition occurring in the Ga–Bi system as well as the thermodynamics of surface freezing are qualitatively discussed.

Two-dimensional connective nanostructures of electrodeposited Zn on Au (111) induced by spinodal decomposition

Phase formation of surface alloying by spinodal decomposition has been studied at an electrified interface. For this aim Zn was electrodeposited on Au(111) from the ionic liquid AlCl(3)-MBIC (58:42) containing 1 mM Zn(II) at different potentials in the underpotential range corresponding to submonolayer up to monolayer coverage. Structure evolution was observed by in situ electrochemical scanning tunneling microscopy (STM) at different times after starting the deposition via potential jumps and at temperatures of 298 and 323 K. Spinodal or labyrinth two-dimensional structures predominate at middle coverage, both in deposition and in dissolution experiments. They are characterized by a length scale of typically 5 nm which has been determined from the power spectral density of STM images. Structure formation and surface alloying are governed by slow kinetics with a rate constant k with activation energy of 120 meV and preexponential factor of 0.17 s(-1). The evolution of the structural features is described by a continuum model and is found to be in good agreement with the STM observations. From the experimental and model calculation results we conclude that the two-dimensional phase formation in the Zn on Au(111) system is dominated by surface alloying. The phase separation of a Zn-rich and a Zn-Au alloy phase is governed by two-dimensional spinodal decomposition.