Electrochemical synthesis of nanostructured tellurium films

Direct templating of materials via lyotropic liquid crystalline mesophases of non-ionic surfactants provides an elegant and highly versatile route to the production of a wide range of nanostructured materials with well-defined mesoporous architectures of extended spatial periodicities. This technique has now been applied in the electrochemical synthesis of adherent nanostructured tellurium films. This represents an important step towards the synthesis of II–IV semiconductor compounds such as cadmium telluride. Low angle X-ray scattering and transmission electron microscopy (TEM) studies of the resulting tellurium films indicate the presence of a system of uniform cylindrical pores organized in an hexagonal array.     

Electrochemical tailoring of lamellar-structured ZnO films by interfacial surfactant templating

Zinc oxide films with ordered lamellar structures can be electrochemically produced by interfacial surfactant templating. This method utilizes amphiphile assemblies at the solid-liquid interface (i.e., the surface of a working electrode) as a template to electrodeposit inorganic nanostructures. To gain the ability to precisely tailor inorganic lamellar structures, the effect of various chemical and electrochemical parameters on the repeat distances, homogeneity, orientation, and quality of the interfacial amphiphilic bilayers were investigated. Surfactants with anionic headgroups (e.g., 1-hexadecanesulfonate sodium salt, dodecylbenzenesulfonate sodium salt, dioctyl sulfosuccinate sodium salt, mono-dodecyl phosphate, and sodium dodecyl sulfate) are critical because they incorporate Zn(2+) ions into their bilayer assemblies as counterions and guide the lamellar growth of ZnO films. Unlike surfactant structures in solution, the interfacial surfactant assemblies are insensitive to the surfactant concentration in solution. The use of organic cosolvents (e.g., ethylene glycol, dimethyl sulfoxide) can increase the homogeneity of bilayer assemblies when multiple repeat distances are possible in a pure aqueous medium. In addition, organic cosolvents can make the interfacial structure responsive to the change in bulk surfactant concentrations. The presence of quaternary alkylammonium salts (e.g., cetyltrimethylammonium bromide) as cationic cosurfactants improves the ordering of anionic bilayers significantly. Consequently, it also affects the orientation of lamellar structures relative to the substrate as well as the surface texture of the films. The quality of lamellar structures incorporated in ZnO films is also dependent on the deposition potentials that determine deposition rates. A higher degree of ordering is achieved when a slower deposition rate (I < 0.15 mA/cm(2)) is used. The results described here will provide a useful foundation to design and optimize synthetic conditions for the electrochemical construction of broader types of inorganic nanostructures.     

Supramolecular staircase via self-assembly of disklike molecules at the solid-liquid interface.

A series of soluble hexabenzocoronene (HBC) derivatives with pendant optically active (S)-3,7-dimethyloctanyl and (R,S)-3,7-dimethyloctanyl (mixture of stereoisomers) hydrocarbon side chains with and without a phenylene spacer were assembled into differently ordered arrays at the interface between a solution and the basal plane of highly oriented pyrolytic graphite (HOPG). Molecularly resolved scanning tunneling microscopy (STM) images revealed that all derivatives self-assemble into oriented crystals in quasi-two dimensions. However, while for the alkyl-substituted HBCs (1,4) all of the single aromatic cores within a monolayer exhibit the same contrast in the STM, the single aromatic cores with a phenylene group between the alkyl side chains and the aromatic core (2a,2b,3) exhibit different contrasts within a monolayer. For the disks carrying racemic branched or n-alkyl side chains (2b,3) a random distribution of the two different contrasts within the 2D-crystal is observed, while the optically active phenylene-alkyl-substituted HBC (2a) exhibits a periodical distribution of three contrasts within the monolayer. We attribute the different contrasts of the aromatic cores in the presence of the phenylene groups to a loss of the planarity of the whole molecule and different conformations, which allow the conjugated disks to attain different equilibrium positions above the surface of HOPG. In the case of the optically active side chains a regular superstructure with three distinctly different positions such as in a staircase is attained. The self-assembly processes are governed by the interplay of intramolecular as well as intermolecular and interfacial interactions. In the present case, the interactions may induce both the molecules to acquire well distinct positions along the z axis and to adopt different conformations. The reported results open new avenues of exploration. For instance, the different couplings of conjugated molecules with the substrate at different separations can be investigated by means of scanning tunneling spectroscopy (STS). Furthermore, experiments on the STM tip-induced switching of single molecules embedded in a monolayer appear feasible.     

固-液界面上的吸附膜

利用Gibbs吸附公式处理了硅胶自四氯化碳和环己烷中吸附脂肪醇、环己醇、苯甲醛、苯甲醚、乙酸丙酯的实验结果,得到吸附膜的表面压(π)和分子面积(A)的关系曲线,这些曲线均可用描述不溶物液态扩张膜的Smith方程描述。文中对所得结果给出了初步的解释。     

Structural transitions at solid-liquid interfaces

In this paper we present the findings of our investigations using molecular dynamics, on molecularly thin films of n-octane confined between topographically smooth solid surfaces. We focus on the effect of increasing solid surface-methylene unit energetic affinity and the effect of increasing pressure (normal load) of the film in inducing liquid-solid phase transitions. We observed an abrupt transition in the structural features of the film at a critical value of the characteristic energy that quantified the affinity between solid surfaces and methylene units. This energetically driven transition was evident from the discontinuous increase of intermolecular order, a precipitous extension of the octane molecules and freezing of molecular migration and rotation. Increasing pressure had a similar effect in inducing a liquid-solid phase transition. The characteristics of the transition showed that it is a mild first order transition from a highly ordered liquid to a poorly organized solid. These findings demonstrate that the solidification of nanoscopically thin films of linear alkanes is a general phenomenon (driven either energetically or by increasing pressure), and does not require the aid of commensurate surface topography.     

Adsorption and desorption of polymer/surfactant mixtures at solid-liquid interfaces: substitution experiments

The adsorption of mixtures of aqueous solutions of cationic hydroxyethylcellulose polymer JR400 and anionic surfactant, sodium dodecyl sulfate, using atomic force microscopy (AFM) has been studied. Samples with various compositions from different regions of the ternary phase diagram presented in our previous work were imaged by atomic force microscopy on freshly cleaved mica, and hydrophobically modified mica and silica in soft-contact mode. A series of "washing" (subsequent injection of compositions with gradually decreasing polymer/surfactant ratio) and "scratching" (mechanical agitation of the surface material with an AFM tip) experiments were performed. It was revealed that the morphology of the adsorbed layer altered in a manner following the changes in morphology in the bulk solution. These changes were evidenced in cluster formation in the layer. The results suggest that the influence of the surface was limited to the formation of the adsorbed layer where the local concentrations of polymer and surfactant were higher than those in the bulk. All further modifications were driven by changes in the mixture composition in bulk. Force measurements upon retraction reveal the formation of network structures within the surface aggregates that will greatly slow structural reequilibration.     

Surfactant aggregates at rough solid-liquid interfaces

We demonstrate improved atomic force microscopic imaging of surfactant surface aggregates, featuring an increase in the topography contrast by several hundred percent with respect to all previous studies. Surfactant aggregates on rough gold surfaces, which could not be imaged previously because of low resolution, display substantially different morphologies when compared with atomically smooth materials.     

Electrochemical self-assembly of melanin films on gold

An electrochemical method for self-assembling melanin films on the Au(111) surface from melanin aggregates in alkaline media is reported. Electrochemical data combined with scanning tunneling microscopy (STM), atomic force microscopy, and Auger electron spectoscopy show that the amount and structure of the deposited melanin film depend on the potential (E) applied to the electrochemical interface and deposition time. Film formation takes place at a noticeable rate at E = -1.0 V (vs SCE). High-resolution STM images at the early stages of growth show small particles, 5-8 nm in size and 0.3-0.4 nm in height, forming ordered arrays that follow closely the Au(111) topography. The size of the melanin particles increases as the film thickness increases, reaching 150 nm for deposits grown for 16 h. The deposited films are electrochemically active, showing well-defined redox couples preceding the hydrogen evolution reaction.     

Particle self-assembly at ionic liquid-based interfaces

This review presents an overview of the nature of ionic liquid (IL)-based interfaces and self-assembled particle morphologies of IL-in-water, oil- and water-in-IL, and novel IL-in-IL Pickering emulsions with emphasis on their unique phenomena, by means of experimental and computational studies. In IL-in-water Pickering emulsions, particles formed monolayers at ionic liquid–water interfaces and were close-packed on fully covered emulsion droplets or aggregated on partially covered droplets. Interestingly, other than equilibrating at the ionic liquid–water interfaces, microparticles with certain surface chemistries were extracted into the ionic liquid phase with a high efficiency. These experimental findings were supported by potential of mean force calculations, which showed large energy drops as hydrophobic particles crossed the interface into the IL phase. In the oil- and water-in-IL Pickering emulsions, microparticles with acidic surface chemistries formed monolayer bridges between the internal phase droplets rather than residing at the oil/water–ionic liquid interfaces, a significant deviation from traditional Pickering emulsion morphology. Molecular dynamics simulations revealed aspects of the mechanism behind this bridging phenomenon, including the role of the droplet phase, surface chemistry, and inter-particle film. Novel IL-in-IL Pickering emulsions exhibited an array of self-assembled morphologies including the previously observed particle absorption and bridging phenomena. The appearance of these morphologies depended on the particle surface chemistry as well as the ILs used. The incorporation of particle self-assembly with ionic liquid science allows for new applications at the intersection of these two fields, and have the potential to be numerous due to the tunability of the ionic liquids and particles incorporated, as well as the particle morphology by combining certain groups of particle surface chemistry, IL type (protic or aprotic), and whether oil or water is incorporated.     

Multilayer adsorption with variable thickness at solid-liquid interfaces

 The multilayer adsorption on the solid/liquid interface in binary mixtures was studied by adsorption space filling with constant and variable layer thickness. Adsorption from benzene/n-heptane mixtures was examined on hydrophilic and hydro-phobic surfaces. The free enthalpy of adsorption, Δ₂₁ G=f (x ₁), was calculated from the adsorption excess isotherm by integration of the Gibbs equation. Supposing that the free enthalpy is mainly due to adsorption in the first layer, the composition of this layer can be calculated from the Δ₂₁ G=f (x ₁) function. It was established that the adsorption layer thickness in benzene/heptane mixtures increases significantly with increasing benzene content. This statement was supported by X-ray diffraction on hydrophobic clay minerals. Received: 2 April 1997 Accepted: 10 June 1997     

Self-aggregation of the SDS surfactant at a solid-liquid interface

Molecular dynamics simulations of sodium dodecyl sulfate (SDS) molecules on a graphite surface are presented. The simulations were conducted at low and high surface coverage to study aggregation at the water/graphite interface. Results showed that at low surface coverage, the SDS molecules form hemicylindrical aggregates, in agreement with AFM experiments, whereas at high surface coverage, the surfactants form full cylinders. The latter aggregates have not been reported in systems of SDS on hydrophobic substrates, such as graphite. The unexpected results are explained in terms of a water layer adsorbed at the solid surface which was the responsible for the formation of these aggregates. Moreover, the SDS tails in the full cylindrical configuration became straighter than those of the hemicylindrical aggregate. Hydrogen bond formation between water and surfactant head groups was also studied, and it was found that they did not depend on the surfactant concentration.     

Absolute orientation of molecules at interfaces

A method to determine the absolute orientation of molecules at liquid interfaces by sum frequency generation (SFG) is reported. It is based on measurements of the orientations of two nonparallel vibrationally active chromophores in the molecule of interest combined with a rotation matrix formulation to obtain the absolute molecular orientation. We chose m-tolunitrile, a planar molecule adsorbed to the air/water interface, as a proof-of-method experiment. Quantitative analysis of different polarization sum frequency intensities facilitate unique peak assignments of the methyl and nitrile groups of m-tolunitrile. The SFG analysis of the measurement yields a nitrile group tilting at 53 degrees to the surface normal, and the C3 axis of the methyl group is almost upright at 23 degrees with respect to the surface normal. Using a rotation matrix formulation, we found that the angle between the surface plane and the m-tolunitrile molecular plane is 70 degrees.     

Density depletion at solid-liquid interfaces: a neutron reflectivity study

Neutron reflectivity experiments conducted on self-assembled monolayers (SAMs) against polar (water) and nonpolar (organic) liquid phases reveal further evidence for a density reduction at hydrophobic-hydrophilic interfaces. The density depletion is found at the interface between hydrophobic dodecanethiol (C12) and hexadecanethiol (C16) SAMs and water and also between hydrophilic SAMs (C12/C11OH) and nonpolar fluids. The results show that the density deficit of a fluid in the boundary layer is not unique to aqueous solid-liquid interfaces but is more general and correlated with the affinity of the liquid to the solid surface. In water the variation of pH has only minor influence, while different electrolytes taken from the Hofmeister series seem to increase the depletion. On hydrophobic SAMs an increase in density depletion with temperature was observed, in agreement with Monte Carlo simulations performed on corresponding model systems. The increase in the water density depletion layer is governed by two effects: the surface energy difference between water and the substrate and the chemical potential of the aqueous phase.     

Molecular dynamics simulations of surfactant self-organization at a solid-liquid interface

Self-organization of aqueous surfactants at a planar graphite-like surface is studied by means of coarse-grain molecular dynamics simulations. The nonionic surfactant, n-alkyl poly(ethylene oxide), and water are both represented by coarse-grain models while an implicit representation is used for the graphite surface. The observed morphology of the aggregated surfactants depends on the alkyl chain length. Surfactants with a short chain form a monolayer on the graphite surface with a thickness roughly equal to that of the alkane tail. On the other hand, longer-tail surfactants form continuous hemicylinders on the surface with diameter approximately 5.0 +/- 0.5 nm, in good agreement with experimental AFM data.     

Oligo(p-phenylenevinylene)-peptide conjugates: synthesis and self-assembly in solution and at the solid-liquid interface

Two oligo(p-phenylenevinylene)-peptide hybrid amphiphiles have been synthesized using solid- and liquid-phase strategies. The amphiliphiles are composed of a pi-conjugated oligo(p-phenylenevinylene) trimer (OPV) which is coupled at either a glycinyl-alanyl-glycinyl-alanyl-glycine (GAGAG) silk-inspired beta-sheet or a glycinyl-alanyl-asparagyl-prolyl-asparagy-alanyl-alanyl-glycine (GANPNAAG) beta-turn forming oligopeptide sequence. The solid-phase strategy enables one to use longer peptides if strong acidic conditions are avoided, whereas the solution-phase coupling gives better yields. The study of the two-dimensional (2D) self-assembly of OPV-GAGAG by scanning tunneling microscopy (STM) at the submolecular level demonstrated the formation of bilayers in which the molecules are lying antiparallel in a beta-sheet conformation. In the case of OPV-GANPNAAG self-assembled monolayers could not be observed. Absorption, fluorescence, and circular dichroism studies showed that OPV-GAGAG and OPV-GANPNAAG are aggregated in a variety of organic solvents. In water cryogenic temperature transmission electron microscopy (cryo-TEM), atomic force microscopy (AFM), light scattering, and optical studies reveal that self-assembled nanofibers are formed in which the helical organization of the OPV segments is dictated by the peptide sequence.     

Electrochemical synthesis of SnO2 films containing three-dimensionally organized uniform mesopores via interfacial surfactant templating

SnO2 films containing organized mesopores and nanocrystalline frameworks with easily removable surfactant templates were produced electrochemically using interfacial amphiphilic assemblies formed on the working electrode as a template.