Surface Reorientation Research Articles

Selective furfural conversion via parallel hydrogenation–oxidation on MOF-derived CuO/RuO2/C electrocatalysts via pulsed laser

Herein, Cu-metal–organic framework-derived CuO/C and CuO/RuO2/C were fabricated via pulsed laser ablation and calcination, demonstrating multifunctional electrocatalytic performances toward hydrogen and oxygen evolution, furfural hydrogenation, and oxidation reactions (HER, OER, FHR, and FOR) in alkaline solution. CuO/RuO2/C showed a high HER activity with 127-mV overpotential and 98.4-mV/dec Tafel slope. In contrast, during FHR, CuO/C converted 14.93-mM furfural to furfuryl alcohol (FFA) in 120-min with a Faradaic efficiency (FE) of 80.12 %, demonstrating a high yield. Similarly, CuO/RuO2/C showed a high OER activity with lowest overpotential ∼346-mV; subsequently, adding 50-mM furfural reduced that to 1.536 V vs. RHE. Furthermore, during FOR, furfural conversion was highly-favorable to achieve furoic acid (FA) upto 13.28-mM with a FE ∼77.56 % and C-balance of ∼77.56 %. Moreover, CuO/C‖CuO/RuO2/C electrolyzer was fabricated to simultaneously convert furfural into FA and FFA, saving-energy with enhanced-efficiency. Finally, in situ/operando electrochemical-Raman spectroscopy revealed the surface reorientation and MIII–O intermediate formation during HER and OER.

Polar in-plane surface orientation of a ferroelectric nematic liquid crystal: Polar monodomains and twisted state electro-optics

We show that surface interactions can vectorially structure the three-dimensional polarization field of a ferroelectric fluid. The contact between a ferroelectric nematic liquid crystal and a surface with in-plane polarity generates a preferred in-plane orientation of the polarization field at that interface. This is a route to the formation of fluid or glassy monodomains of high polarization without the need for electric field poling. For example, unidirectional buffing of polyimide films on planar surfaces to give quadrupolar in-plane anisotropy also induces macroscopic in-plane polar order at the surfaces, enabling the formation of a variety of azimuthal polar director structures in the cell interior, including uniform and twisted states. In a π-twist cell, obtained with antiparallel, unidirectional buffing on opposing surfaces, we demonstrate three distinct modes of ferroelectric nematic electro-optic response: intrinsic, viscosity-limited, field-induced molecular reorientation; field-induced motion of domain walls separating twisted states of opposite chirality; and propagation of polarization reorientation solitons from the cell plates to the cell center upon field reversal. Chirally doped ferroelectric nematics in antiparallel-rubbed cells produce Grandjean textures of helical twist that can be unwound via field-induced polar surface reorientation transitions. Fields required are in the 3-V/mm range, indicating an in-plane polar anchoring energy of w P ∼3 × 10-3 J/m2.

Detection of surface structural changes during adsorption events using two-trace two-dimensional (2T2D) correlation spectroscopy

We demonstrate a method by which a pair of spectra acquired using surface-sensitive second-order nonlinear vibrational spectroscopy, visible-infrared sum-frequency generation, can be analyzed in order to readily determine whether changes that occur as a result of varying environmental conditions results in a change in the population of surface molecules, or a structural rearrangement of the surface. This is accomplished via two-trace two-dimensional correlation analysis, a recently developed technique that uses spectral phase information to provide a level of comparison beyond what can be achieved by simple spectral subtraction or division. While surface reorientation and/or a change in the degree of ordering are often difficult to detect from spectra obtained in a single beam polarization, without detailed analysis of the lineshape, we show that such changes provide strong asynchronous cross peaks in the frequency correlation map.

Mechanism of oxygen and argon low pressure plasma etching on polyethylene (UHMWPE)

Different preferential sites for etching are observed for low pressure argon and oxygen plasma on ultra-high molecular weight polyethylene. Atomic force microscopy analysis showed an organized pattern after 1 min of oxygen plasma treatment. Roughness decreases after 3 min and then after 5 min increases again, when surface is completely damaged. A different etching pattern is observed after argon plasma treatment, where nanometric pits are formed with size increasing with treatment time. Infrared spectroscopy shows that, by analyzing the relative intensity of the different peaks, the different etching mechanisms are explained where oxygen plasma reduces coupling between parallel chains in the crystalline short axis direction and argon plasma suppresses hydrogen and induces surface reorientation of functional groups.

The Stability of a Nanoparticle Diamond Lattice Linked by DNA.

The functionalization of nanoparticles (NPs) with DNA has proven to be an effective strategy for self-assembly of NPs into superlattices with a broad range of lattice symmetries. By combining this strategy with the DNA origami approach, the possible lattice structures have been expanded to include the cubic diamond lattice. This symmetry is of particular interest, both due to the inherent synthesis challenges, as well as the potential valuable optical properties, including a complete band-gap. Using these lattices in functional devices requires a robust and stable lattice. Here, we use molecular simulations to investigate how NP size and DNA stiffness affect the structure, stability, and crystallite shape of NP superlattices with diamond symmetry. We use the Wulff construction method to predict the equilibrium crystallite shape of the cubic diamond lattice. We find that, due to reorientation of surface particles, it is possible to create bonds at the surface with dangling DNA links on the interior, thereby reducing surface energy. Consequently, the crystallite shape depends on the degree to which such surface reorientation is possible, which is sensitive to DNA stiffness. Further, we determine dependence of the lattice stability on NP size and DNA stiffness by evaluating relative Gibbs free energy. We find that the free energy is dominated by the entropic component. Increasing NP size or DNA stiffness increases free energy, and thus decreases the relative stability of lattices. On the other hand, increasing DNA stiffness results in a more precisely defined lattice structure. Thus, there is a trade off between structure and stability of the lattice. Our findings should assist experimental design for controlling lattice stability and crystallite shape.

Kinetic studies of attachment and re-orientation of octyltriethoxysilane for formation of self-assembled monolayer on a silica substrate

The present study deals with kinetic studies of the chemical modification for synthesizing a hydrophobic silica surface. Surface silanization (modification) via formation of Self-Assembled Monolayer (SAM) using a short chain triethoxyoctylsilane (TEOS) was carried out under inert atmosphere at room temperature. Fourier transmission infrared (FTIR) spectroscopy, water contact angle (WCA) and atomic force microscopy (AFM) were employed to investigate surface modification. FTIR analysis in the range from 900–1200cm−1 and 2850–3000cm−1 confirmed surface modification and re-orientation of the attached molecules. Kinetic studies of TEOS SAM formation were fitted by Exponential Association function. Kinetic fitting of FTIR data in the range from 900–1200cm−1 revealed a very fast attachment of TEOS molecules resulting in total surface coverage within 16min whereas re-orientation rate was slow and continued till 512min. Further, change in orientation from lying-down to standing-up state was supported by contact angle analysis. AFM images initially showed small islands of ~20nm, which in-fill with time indicating formation of a smooth monolayer. Our findings indicate that formation of octyl SAM is fast process and completes within 8.5h in contrary to reported 24h in conventional SAM formation protocols. The kinetic fitting data can be explored to design a nanopatterned surface for a specific application.

Theoretical methods for understanding advanced magnetic materials: The case of frustrated thin films

Materials science has been intensively developed during the last 30 years. This is due, on the one hand, to an increasing demand of new materials for new applications and, on the other hand, to technological progress which allows for the synthesis of materials of desired characteristics and to investigate their properties with sophisticated experimental apparatus. Among these advanced materials, magnetic materials at nanometric scale such as ultra thin films or ultra fine aggregates are no doubt among the most important for electronic devices.In this review, we show advanced theoretical methods and solved examples that help understand microscopic mechanisms leading to experimental observations in magnetic thin films. Attention is paid to the case of magnetically frustrated systems in which two or more magnetic interactions are present and competing. The interplay between spin frustration and surface effects is the origin of spectacular phenomena which often occur at boundaries of phases with different symmetries: reentrance, disorder lines, coexistence of order and disorder at equilibrium. These phenomena are shown and explained using of some exact methods, the Green's function and Monte Carlo simulation. We show in particular how to calculate surface spin-wave modes, surface magnetization, surface reorientation transition and spin transport.

Simple and improved approaches to long-lasting, hydrophilic silicones derived from commercially available precursors.

Three types of commercially derived methylsilicone materials, Sylgard-184, Q(V)Q(H) (an MQ-based silicone containing no dimethylsiloxane, D units), and D(V)D(H) (a D-based silicone with no additives), were judiciously chosen to study the conditions under which long-lasting hydrophilicity after oxygen plasma treatment can be obtained. A 30 s plasma treatment time under controlled conditions was found to be optimal in terms of achieving the lowest initial advancing and receding contact angles of θ(A)/θ(R) = 10°/5° with undetectable surface damage. Vacuum treatment, a necessary step prior to plasma ignition that has been overlooked in previous studies, as well as room temperature curing were explored as means to remove low molecular weight species. For thin films (a few micrometers), 40 min vacuum treatment was sufficient to achieve low dynamic contact angles of θ(A)/θ(R) = 51-56°/38-43° on all three types of silicones measured more than 30 days after the plasma treatments. These values indicate superior hydrophilicity relative to what has been reported. The small and slow rise in contact angle over time is likely caused by the intrinsic nature of the silicone materials, i.e., surface reorientation of hydrophilic functional groups to the bulk and condensation of surface silanol groups, and is thus unavoidable. For thick films (∼1 mm), room temperature curing in addition to vacuum treatment was required to reduce hydrophobic recovery and to achieve long-lasting hydrophilicity. The final contact angles for thick samples were slightly higher than the corresponding thin film samples due to the greater "reservoir" depth and migration length for mobile species. In particular, Sylgard exhibited inferior performance among the thick samples, and we attribute this to the additives in its commercial formulation. Furthermore, unlike polydimethylsiloxane-based silicones, Q(V)Q(H) does not contain equilibration products of the Dn-type; its thin films perform as well as those of Sylgard and D(V)D(H). Silicones without D units are promising materials with intrinsically low hydrophobic recovery characteristics and long-lasting hydrophilicity after oxygen plasma treatment.

Implications of solution chemistry effects: Direction-specific restraints on the step kinetics of calcite growth

Classical crystallization models successfully depict the dependence of growth kinetics on thermodynamic driving force but cannot predict the roles of solution chemistry. Yet, it has become increasingly clear that crystal growth rate at fixed supersaturations depends on pH, ionic strength, and the relative abundance of cations and anions in the parent solutions. We conducted experiments to isolate the effect of individual solution-chemistry parameters on calcite step growth kinetics. Our results revealed a distinct correlation between step velocity and pH in acute and obtuse directions and a simultaneous trend change at pH ∼9.5. Step speeds varied with solution stoichiometry, and each direction reached its maximal rate at a different [Ca2+]/[CO32-] value. In addition, the solution cation/anion at which maximal step speed occurred appeared to be pH dependent as well. Limited effect of ionic strength on step growth was observed in the obtuse direction at low background electrolyte concentrations, but no obvious change was found for growth at the acute side. We proposed a growth model based upon independent incorporation of cations and anions and interpreted these observations as: (1) solute constituents enter kinks through direct solution diffusion at obtuse steps but surface diffusion at acute steps; (2) growth is limited by Ca2+ dehydration at the obtuse side, but by surface adsorption and reorientation and rearrangement of CO32− in the acute direction; and (3) background electrolyte (NaCl in our study) competes with solutes for surface sites at more easily accessible kinks.

Detection of surface mobility of poly (2, 3, 4, 5, 6-pentafluorostyrene) films by in situ variable-temperature ToF-SIMS and contact angle measurements

Poly (2, 3, 4, 5, 6-pentafluorostyrene) (5FPS) was prepared by bulk radical polymerization. The spin-cast films of this polymer were analyzed using time-of-flight secondary ion mass spectrometry (ToF-SIMS) at various temperatures ranging from room temperature to 120°C. Principal component analysis (PCA) of the ToF-SIMS data revealed a transition temperature (TT) at which the surface structure of 5FPS was rearranged. A comparison between the results of the PCA of ToF-SIMS spectra obtained on 5FPS and polystyrene (PS) indicate that the pendant groups of 5FPS and PS moved in exactly opposite directions as the temperature increased. More pendant groups of 5FPS and PS migrated from the bulk to the surface and verse versa, respectively, as the temperature increased. These results clearly support the view that the abrupt changes in the normalized principal component 1 value was caused by the surface reorientation of the polymers and not by a change in the ion fragmentation mechanism at temperatures above the TT. Contact angle measurement, which is another extremely surface sensitive technique, was used to monitor the change in the surface tension as a function of temperature. A clear TT was determined by the contact angle measurements. The TT values determined by contact angle measurements and ToF-SIMS were very similar.

Origin of the Enhanced Electrocatalysis for Thermally Controlled Nanostructure of Bimetallic Nanoparticles

The thermal annealing process is a common treatment used after the preparation step to enhance the electrocatalytic properties of the oxygen reduction reaction (ORR). The structure of a Pt-based bimetallic nanoparticle, which is significantly affected by the catalytic properties, is reconstructed by thermal energy. We investigated the effect of structural reconstruction induced by thermal annealing on the improvement of the ORR using various physical and electrochemical methods. We found that the structural evolution of PtNi nanoparticles, i.e., the Pt–Ni ordering with the Pt shell and the surface reorientation into the (111) facet, is the source of the enhanced ORR activity as well as electrochemical stability through the thermal annealing. This result confirms the crucial factors for the ORR properties by the thermal annealing process and proposes a way to design advanced electrocatalysts.

The Genesis and Development of the Commercial BP Doubly Promoted Catalyst for Ammonia Synthesis

In the 1970s BP started developing a new catalyst for ammonia synthesis which in final form comprises ruthenium, two promoters and a graphitised carbon support. This became the first new catalyst for ammonia synthesis to be commercialised since Fritz Haber’s promoted iron catalyst. The catalyst is commercialised in the KBR Advanced Ammonia Process (KAAP). We describe the development of the catalyst, starting in the 1960s with studies to elucidate the mechanism of selective hydrogenation catalysts using nickel single crystals and LEED. These identified a phenomenon dubbed surface reorientation, in which an adsorbate such as sulphur causes the structure of the surface monolayer of a metal to change e.g. from fcc [111] to [100]. These ideas led to discovery of an active platinum/alkali metal/carbon catalyst for the dehydrocyclisation of hexane. That catalyst was not commercial, but it led in turn to the identification of highly active ruthenium/alkali metal/carbon catalysts for ammonia synthesis. The best new catalyst is about 20 times more active than a commercial Haber catalyst. The special carbon support is mesoporous and graphitic. Scale up of catalyst production and testing are described briefly, as are some process considerations.

Magnetic reorientation in three layer antiferromagnetic films

Abstract Magnetic reorientation in three layer antiferromagnetic films with simple cubic lattice and with (001) surfaces is investigated theoretically. We applied the Green function method to the Heisenberg model. Self-consistent calculations for layer magnetizations and the surface reorientation temperatures for different exchange anisotropy and single-ion anisotropy parameters are carried out. Particularly, we compare the effect of both mentioned anisotropies on the surface reorientation temperature.

Origin of Size Dependency in Coherent-Twin-Propagation-Mediated Tensile Deformation of Noble Metal Nanowires

Researchers have recently discovered ultrastrong and ductile behavior of Au nanowires (NWs) through long-ranged coherent-twin-propagation. An elusive but fundamentally important question arises whether the size and surface effects impact the twin propagation behavior with a decreasing diameter. In this work, we demonstrate size-dependent strength behavior of ultrastrong and ductile metallic NWs. For Au, Pd, and AuPd NWs, high ductility of about 50% is observed through coherent twin propagation, which occurs by a concurrent reorientation of the bounding surfaces from {111} to {100}. Importantly, the ductility is not reduced with an increase in strength, while the twin propagation stress dramatically increases with decreasing NW diameter from 250 to 40 nm. Furthermore, we find that the power-law exponent describing the twin propagation stress is fundamentally different from the exponent describing the size-dependence of the yield strength. Specifically, the inverse diameter-dependence of the twin propagation stress is directly attributed to surface reorientation, which can be captured by a surface energy differential model. Our work further highlights the fundamental role that surface reorientations play in enhancing the size-dependent mechanical behavior and properties of metal NWs that imply the feasibility of high efficiency mechanical energy storage devices suggested before.

Singularities in hydrophobic recovery of plasma treated polydimethylsiloxane surfaces under non-contaminant atmosphere

Herein, the hydrophobic recovery of argon plasma treated polydimethylsiloxane surfaces is explored. In contrast to previous works, environmental contamination is here taken into account. We find that diffusion and reorientation strongly dominate the hydrophobic recovery under high thermal activation (60°C), no matter the surrounding environment during storage. However, at lower temperature (24°C and below), we find that contamination plays a major role in lab atmosphere environment. By working at low temperature and under inert nitrogen atmosphere to slow down the diffusion and reorientation dynamics and to avoid contamination, we identify two different temperature-dependent regimes in the kinetics of the hydrophobic recovery. One is fast and involves exclusively relaxation processes occurring in the surface region. The second, much slower, concerns diffusion phenomena in the sub-surface region. Thereby, the specific impact of bulk diffusion and surface reorientation processes can be distinguished during aging by slowing down the surface dynamics and by eliminating all possible sources of contamination.

All-optical control of localized plasmonic resonance realized by photoalignment of liquid crystals

Large shifts in the plasmonic resonances of a thin film of gold nanorods (GNRs) are induced through the modulation of the local refractive index of the neighboring dielectric medium. This change is enabled through light-induced surface reorientation of a nematic liquid crystal in contact with a nanometer-thin photoalignment layer coating the GNR film. The presence of isolated and well distributed GNRs, both before and after the photoalignment layer deposition, is shown through atomic force and scanning electron microscopy. Exposure of the photoalignment layer to polarized light is shown to reorient aligned nematic liquid crystal molecules to an orthogonal direction, thereby changing the local refractive index of the medium in close proximity to the GNR film. This large change in the local dielectric strength is shown to cause a broad red shift of the localized plasmonic longitudinal resonance and almost no shift in the local transverse resonance. The ability to remotely and quickly change the plasmonic properties of this GNR system marks a breakthrough towards the realization of all-optical plasmonic and photonic devices such as plasmonic colour filters.

Optical Control of Surface Anchoring and Reorientation of Liquid Crystals via a Plasmon-Enhanced Local Field

Gold nanoparticles deposited on the windows of a liquid crystal (LC) cell were found to be able to reduce the surface anchoring energy of the LC, and hence the threshold for its reorientation phase transition, by 1 to 2 orders of magnitude when a cw pump light was used to excite the local plasmon resonance of the nanoparticles. The effect was due to the disorientation of LC molecules between nanoparticles by the plasmon-enhanced local field that softens the effective surface anchoring. A light-controlled variation of surface anchoring energy could provide new opportunities for optoelectronic applications of a LC.

Factors determining electrostatic fields in molecular dynamics simulations of the ras/effector interface

Using molecular dynamics simulations, we explore geometric and physical factors contributing to calculated electrostatic fields at the binding surface of the GTPase Ras with a spectroscopically labeled variant of a downstream effector, the Ras-binding domain of Ral guanine nucleotide dissociation stimulator (RalGDS). A related system (differing by mutation of one amino acid) has been studied in our group using vibrational Stark effect spectroscopy, a technique sensitive to electrostatic fields. Electrostatic fields were computed using the AMBER 2003 force field and averaged over snapshots from molecular dynamics simulation. We investigate geometric factors by exploring how the orientation of the spectroscopic probe changes on Ras-effector binding. In addition, we explore the physical origin of electrostatic fields at our spectroscopic probe by comparing contributions to the field from discrete components of the system, such as explicit solvent, residues on the Ras surface, and residues on the RalGDS surface. These models support our experimental hypothesis that vibrational Stark shifts are caused by Ras binding to its effector and not the structural rearrangements of the effector surface or probe reorientation on Ras-effector binding, for at least some of our experimental probes. These calculations provide physical insight into the origin, magnitude, and importance of electrostatic fields in protein-protein interactions and suggest new experiments to probe the field's role in protein docking.

Coverage‐Dependent Variation of Adsorption Configurations of Methionine on Ge(100)

The adsorption configurations of methionine molecules on the Ge(100) surface have been studied by using DFT calculations, core-level photoemission spectroscopy (CLPES), and low-energy electron diffraction (LEED) to scrutinize the adsorption structure as a function of coverage. At first, we obtained two important and stable structures. One is the most stable structure between these structures described as an "O-H dissociated-N dative-S dative-bonded structure" and the other is a less stable adsorption structure of these indicating an "O-H dissociated-S dative-bonded structure" by using DFT calculations. We also performed CLPES to clarify our DFT calculation results. Through the spectral analysis of the S 2p, C 1s, N 1s, and O 1s core-level spectra, we acquired the reasonable results that also revealed quite different bonding configurations depending on the methionine coverage. At low coverage (ca. 0.30 ML), a single type of sulfur and charged nitrogen peaks, which indicate an "O-H dissociated-N dative-S dative-bonded structure", were observed. On the other hand, two types of sulfur peaks with thiol formation and two nitrogen peaks with neutralized and charged characteristics were monitored at a higher coverage (0.60 ML and above), which can be described as an "O-H dissociated-S dative-bonded structure". Hence, we can clearly demonstrate that our results obtained from CLPES spectra and DFT calculations are matched well with each other. Moreover, we additionally confirmed that the relative population of the two types of thiols and amines being included in methionine in between half monolayer induces a surface reorientation in the ordering from 2×1 to 1×1 employing LEED. This interesting variation of the methionine adsorbed on the Ge(100) surface by coverage dependence will be precisely discussed by using DFT calculations, CLPES, and LEED.

Surface viscosity and reorientation process in an asymmetric nematic cell

The influence of surface viscosity and anchoring energy on the reorientation process of a nematic liquid crystal cell is theoretically investigated. The cell is a slab of thickness, d, whose limiting surfaces are characterised by different anchoring strengths and present easy directions parallel to the bounding surfaces, changing with time due to some external action. The exact space-time profile of the director angle is obtained by means of integral transform techniques and a Green function approach. From this formalism, the time dependence of the optical path difference is exactly determined and its behaviour is analysed in connection with the presence of surface viscosity and different anchoring energies. The problem is also exactly solved in the presence of a constant electric field. It is shown that the compatibility problem between the time derivative of the director field on the surface and in the bulk can be avoided.