Surface Diffusion Processes Research Articles

Formation and modifications of nanostructures of tin oxide by SHI irradiation

We deposited thin films of tin oxide (SnO2) by the thermal evaporation method on quartz and Si substrates. The as-deposited films were irradiated using 100 Mev Ag ions at different fluences ranging from 1×1012 to 1×1013 ions/cm2. Pristine and irradiated films were characterized for surface topographical study using atomic force microscopy. Pristine films on both types of substrates (Si and Quartz) are featureless. Irradiation at the lowest fluence (1×1012 ions/cm2) induces the nanostructures at the surface and higher fluence irradiations change the shape and size of these nanostructures. The value of roughness and growth exponent, as evaluated from power spectral density analysis, indicates that the formation of nanostructures is mainly due to the swift heavy ion-induced surface diffusion process. We have tried to explain the results in the framework of the thermal spike model.

Capillary-driven interdiffusion along interphase boundaries in solids

We consider chemical interdiffusion along interphase boundary (IB) between two immiscible solid phases. We derive explicit expressions for capillary-related excess chemical potentials of the atoms diffusing along the IB. The obtained expressions contain both a local term which depends on local curvature of the IB, and a non-local one which depends on overall system geometry and on energy of all surfaces and interfaces in the system. The obtained expressions are employed for describing the sintering of two immiscible two-dimensional solid particles controlled by surface and IB diffusion processes. We demonstrate that the sintering of particles is accompanied by relative rigid-body rotation of the particles even for fully isotropic surfaces and interphase boundaries.

A cluster expansion model for rate constants of surface diffusion processes on Ag, Al, Cu, Ni, Pd and Pt(100) surfaces

Rate constants of surface diffusion processes in metals often depend on the local atomic environment. Accurate models that describe this local environment dependence for specific material systems are not easily available in the literature despite the need for such information in kinetic models. In this work, we find the prefactor and activation barrier associated with adatom hop processes for a large number of local environments and use this information to systematically train cluster expansion (CE) models for Ag, Al, Cu, Ni, Pd and Pt. We demonstrate that although the local environments used for training is a small subset of all possible local environments, the CE model can predict the rate constants for many local environments that have not been used while training the model with reasonable accuracy. It is found that up to fourth nearest neighbor positions can be relevant to the activation barrier. Furthermore, many-body effects play an important role. We also find that the CE model depends on the direction of the hop.

Self-organized graphene crystal patterns

The phenomenon of ordered pattern formation is universal in nature but involves complex non-equilibrium processes that are highly important for both fundamental research and applied materials systems. Among countless pattern systems, a snowflake is possibly the most fascinating example offered by nature. Here, we report that single-layered and single-crystalline graphene flakes (GFs) with highly regular and hexagonal symmetric patterns can be grown on a liquid copper surface using a CH4 chemical vapor deposition (CVD) method. The different morphologies of these GFs can be precisely tailored by varying the composition of the inert gas/H2 carrier gas mixture used to produce the GFs, and the GF edges can be continuously tuned over the full spectrum from negative to zero to positive curvature in a controllable way. The family of GF crystal patterns is remarkably analogous to that of snowflakes, representing an ideal two-dimensional (2D) growth system. Pattern formations from compact to dendritic GFs can be explained by the continuous modulation of the competition between adatom diffusion along island edges or corners and surface diffusion processes.

Sustained release of naproxen in a new kind delivery system of carbon nanotubes hydrogel.

In this paper, carbon nanotubes (CNTs) were added into chitosan (CS) hydrogels in the form of chitosan modified CNTs (CS-CNTs) composites to prepare carbon nanotubes hydrogels (CNTs-GEL). The products, named CS-MWCNTs, were characterized by scanning electron microscope (SEM) and Fourier transform infrared (FTIR) spectroscopy. Swelling properties and effect of pH on controlled release performance of the two kinds of hydrogels, CNTs- GEL and pure chitosan hydrogels without CNTs (GEL), were investigated respectively. The results showed that CNTs-GEL possess better controlled release performance than GEL. The releasing equilibrium time of CNTs-GEL was longer than that of GEL in both pH = w7.4 and pH=1.2 conditions, although the release ratios of the model drug are similar in the same pH buffer solutions. It is found that release kinetics is better fitted Ritger-Peppas empirical model indicating a fick-diffusion process in pH = 1.2, while in pH = 7.4 it was non-fick diffusion involving surface diffusion and corrosion diffusion processes.

Effect of Boron Deposition and Poisoning on the Surface Exchange Properties of LSCF Electrode Materials of Solid Oxide Fuel Cells

The relationship between the surface segregation, boron poisoning and surface exchange coefficients of La0.6Sr0.4Co0.2Fe0.8O3−δ, (LSCF) cathodes of solid oxide fuel cells (SOFCs) is studied on dense bar samples using SEM, SIMS and conductivity relaxation method. The SEM results clearly indicate that the segregation on the LSCF surface occurs after heat-treatment at 700–800°C for 48 h, forming isolated particles on the LSCF surface. The presence of volatile boron species accelerates grain growth of the segregated particles and reacts with LSCF. The depth of boron reaction layer after heat-treatment in the presence of E-glass at 700°C, 750°C and 800°C for 48 h was 2, 5 and 40 nm, respectively. The depth profiles analysis of SIMS indicates that there is segregation and enrichment of constituent elements of LSCF on the electrode surface, in line with the depth profile of boron species on the LSCF sample surface layer. Boron deposition and poisoning deteriorates the surface exchange and diffusion processes for the oxygen reduction reaction on LSCF. After exposed to boron at 800°C for 48 h, the surface exchange coefficient, Kchem is 6.0 × 10−5 cm s−1, more than one magnitude lower than 1.1 × 10−3 cm s−1 of as-prepared LSCF samples.

Dynamic Surface Adsorption Properties of Sodium Dodecyl Sulfate Aqueous Solution

The dynamic surface adsorption properties of aqueous sodium dodecyl sulfate(SDS) solutions were investigated at different concentrations of NaCI using bubble pressure tensiometry MPTC.In the case of ionic surfactants,the existence of a diffuse electric double layer on the surface adsorption layer and around the micelle produces a surface charge.Here,we discuss the influence of the surface charge on the dynamic surface diffusion processes and the micelle properties.It was found that the SDS adsorption process occurred in the presence of a 5.5 kJ ? mol~(-1) adsorption barrier(E_a) that was generated by the surface charge;this barrier significantly decreased the effective diffusion coefficient(D_(eff)) of the dodecyl sulfate ions(DS~-).The ratio of the effective diffusion coefficient to the monomer self-diffusion coefficient(D) (D_(eff)/D) was only 0.013.This indicated that at the beginning,the adsorption of SDS followed the mixed kinetic-diffusion controlled model;this is different from the behavior observed for nonionic surfactants.The adsorption barrier was reduced when NaCI was added.E_a was less than 0.3 kJ ? mol~(-1) after the addition of 80 mmol ? L~(-1) of NaCI.This resulted in values of between 0.8 and 1.2 for D_(eff)/D,which was consistent with the diffusion-controlled model that describes the behavior of nonionic surfactants.The characteristic constants for the micelle dissociation rate(k_2) were determined from the dynamic surface tension of the SDS micelle solutions.The calculated k values decreased as the NaCI concentration was increased,which demonstrated the existence of surface charge on the SDS micelles;this surface charge increased the repulsive forces between the dodecyl sulfate ions,and promoted the dispersion of the micelles.

Enhanced shape stability of Pd–Rh core–frame nanocubes at elevated temperature: in situ heating transmission electron microscopy

Shape stability of Pd-Rh core-frame nanocubes was studied by in situ heating transmission electron microscopy. Pd-Rh nanocubes could maintain cubic shape at elevated temperature compared with pure Pd. The surface diffusion process of Rh onto {100} side surfaces is believed to postpone the degradation to higher temperature.

Polarization mechanism of high temperature electrolysis in a Ni–YSZ/YSZ/LSM solid oxide cell by parametric impedance analysis

Comprehensive modeling of the spectra of the state-of-the-art Ni–YSZ/YSZ/LSM solid oxide cells including the instrumental stray impedance allowed the systematic deconvolution of the four major polarization losses ranging over one order of impedance magnitude. The stray impedance can be successfully modeled as an inductor connected in parallel with a parasitic resistor whose resistance was shown proportional to the inductance. From the high frequency the ohmic losses, the ‘charge-transfer’ impedance of the Ni–YSZ electrode, the surface diffusion and reaction co-limited impedance of LSM electrode, and the gas phase transport impedance of Ni–YSZ electrode were successfully distinguished. The latter two were satisfactorily described by the ideal Gerischer impedance with two independent parameters, respectively. The gas-concentration impedance increases with electrolysis due to the gas density decrease with hydrogen production, while the LSM polarization decreases due to the increased oxygen activity. Compensation of the opposite polarization behavior of Ni–YSZ and LSM electrodes explains the apparently ohmic polarization over a wide electrolysis range until the upturn where exponentially increasing gas-concentration impedance of Ni–YSZ electrode prevails. Apparently being quite distinct from the fuel cell polarization behavior, the polarization of the high temperature electrolysis can be consistently explained by the chemical potential variations of the reactants and products, which is suggested to be general characteristic of the gas electrodes of solid oxide cells, co-limited by surface diffusion and reaction process. The finite-length Gerischer model constituted of series resistors, shunt resistors, and shunt capacitors, allows the evaluation of the surface diffusivity (ca. 2⋅10−4cm2s−1), reaction constant (ca. 103s−1), and the utilization length (ca. 5μm) among the LSM–YSZ composite functional layer of thickness of ca. 10μm. The strong decrease in LSM polarization with electrolysis at the humidity of 30% can be contributed by the increase in surface diffusivity, chemical capacitance, and the surface reaction constant in the decreasing order, while the adsorption capacitance increases is mainly responsible for the polarization decreases at higher humidity condition of 50%.

Atomic layer deposition of zinc indium sulfide films: Mechanistic studies and evidence of surface exchange reactions and diffusion processes

The authors present the elaboration of zinc indium sulfide (ZnInxSy) thin films in the context of a cadmium-free buffer layer development for copper indium gallium diselenide photovoltaic solar cells. The films were deposited by atomic layer deposition (ALD) from ZnEt2 (DEZ), In(acac)3 (acac = acetylacetonate), and H2S at 200 °C. In situ growth kinetics studies were performed with the quartz crystal microbalance technique to determine the respective mass gain per cycle of ZnS and In2S3 layers, allowing determination of the atomic compositions of the ZnInxSy thin films to be expected if the deposition strictly follows the rule of mixtures. As the experimental atomic compositions of the ZnInxSy films differ significantly from this rule, a comprehensive study of the growth mechanism was performed to determine the nature of the side reactions. First, an exchange reaction between In2S3 and the Zn precursor was identified, though this process is not sufficient to account for the experimental data, and therefore, a second process which corresponds to the diffusion of species within the film was also found to take place. Ultimately, the atomic compositions of the ZnInxSy films can be explained by a rate-limited exchange reaction at the surface between DEZ and the In2S3 layer, combined with diffusion of the species in the whole film. More generally, such side reactions should be considered in ALD of multinary compounds, even at low temperature.

Specific features of the formation of terrace-step nanostructures on the (0001) surface of sapphire crystals

The terrace-step structures on atomically smooth vicinal surfaces of sapphire crystals subjected to thermal annealing after mechanical and chemical treatments have been investigated by atomic-force microscopy. The influence of the vicinal angles and conditions of thermal treatment (including annealings in air and vacuum) of sapphire plates on the metric parameters of terrace-step structures has been analyzed. It is shown that steps with a height from monoatomic (0.22 nm) to 3 nm or more can be formed on the sapphire surface by changing the vicinal angle and thermal-treatment conditions. The results are interpreted in terms of the minimum surface energy and surface diffusion processes.

Role of surface composition in morphological evolution of GaAs nano-dots with low-energy ion irradiation

The surface chemistry of GaAs (100) with 50-keV Ar+ ion beam irradiation at off-normal incidence has been investigated in order to elucidate the surface nano-structuring mechanism(s). Core level and valence band studies of the surface composition were carried out as a function of fluences, which varied from 1 × 1017 to 7 × 1017 ions/cm2. Core-level spectra of samples analyzed by X-ray photoelectron spectroscopy confirmed the Ga enrichment of the surface resulting in bigger sized nano-dots. Formation of such nano-dots is attributed to be due to the interplay between preferential sputtering and surface diffusion processes. Valence band measurement shows that the shift in the Fermi edge is higher for Ga- rich, bigger sized nano-dots due to the partial oxide formation of Ga. ‘One-dimensional power spectral density’ extracted from atomic force micrographs also confirms the significant role of surface diffusion in observed nano-structuring.

Adsorption mechanism of copper and lead ions onto graphene nanosheet/δ-MnO2

The removal behaviors of Cu(II) and Pb(II) ions onto graphene nanosheet/δ-MnO2 (GNS/MnO2) adsorbent were investigated under various conditions including contact time and initial metal concentrations. The surface interactions involved in the adsorption process and the regeneration were also explored. The equilibrium adsorption data could be well described by the pseudo-second-order kinetic model and the Langmuir equation. Weber–Morris model suggested that the sorption was a multi-step with the pore and surface diffusion process. The maximum adsorption capacity of GNS/MnO2 for Cu(II) and Pb(II) ions were 1620 and 781 μmol g−1, respectively. FTIR and XPS studies illuminated that the formation of complexes on GNS/MnO2 surface mainly consisted of the oxygen in hydroxyl (C–OH, Mn–OH) and carboxyl groups. Moreover, XRD analysis further confirmed that Cu(II) or Pb(II) could not only adsorb on the surface of GNS/MnO2, but also intercalate the layered of MnO2. GNS/MnO2 can be used four times by HCl solution.

Transient Mobility Mechanisms of Deposited Metal Atoms on Insulating Surfaces: Pd on MgO (100)

The importance and mechanisms of transient mobility of atoms and molecules adsorbing at surfaces have been a subject of controversy for many years. We used classical molecular dynamics simulations to examine transient mobility of Pd atoms adsorbing on the MgO (100) surface with incident kinetic energies not exceeding 0.4 eV. The calculations show that deposited Pd atoms exhibit high mobility at temperatures below 80 K where the contribution from thermal diffusion processes should be negligible. For our selected deposition conditions, aimed at simulation of Pd cluster growth experiments, an estimated 76 of the impinging Pd atoms are expected to travel up to 20 A away from the collision site before capture on a 5 K surface. We find that mobility of metal atoms on oxide surfaces is expected to decrease with decreasing incident energy and increase with decreasing incident angle. Comparison with prior studies highlights similarities and differences with other surface diffusion processes, such as long jumps. At higher surface temperatures, the observed mobility will mainly be due to thermally activated processes rather than transient mobility mechanisms. Atoms that exhibit transient mobility upon deposition may quickly migrate to surface features and defects affecting kinetics of growth and structures of nanoclusters and surface layers. © 2012 American Chemical Society.

Detachment Induced by Rayleigh-Instability in Metal Oxide Nanorods: Insights from TiO2

The comprehension of the physical-chemistry processes related to the nanocrystals growth mechanism is fundamental in controlling the shape and assembly of inorganic nanoparticles. In this paper, we describe TiO2 nanocrystal synthesis by a colloidal process where the final shape and size of the nanocrystal is influenced by a phenomenon similar to the Rayleigh-instability of freestanding TiO2 nanorods in solution. In this process, a larger structure (the nanorod) is divided into smaller units (nanocrystals), by a kind of detachment process. A detailed high-resolution transmission electron microscopy (HRTEM) characterization facilitates the identification of a morphological feature, which characterizes a surface diffusion process. Through a classical thermodynamic analysis, energetic and geometrics parameters, which lead to fragmentations of TiO2 nanorods, were established.

Impedance Studies on LSCM/GDC Composite Cathode for High Temperature CO2 Electrolysis

The cathode performance of LSCM/GDC composite for high temperature CO2 electrolysis has been studied by electrochemical impedance spectroscopy (EIS) measurements in a three-electrode electrolysis cell. The main feature in the impedance of LSCM/GDC is that it showed a Gerischer like impedance dispersion at 900oC in all CO2-CO mixtures investigated here. The Rs of LSCM/GDC changes little with the variations in CO2/CO ratio and operating voltage, however, the Rp of LSCM/GDC composite reduces with increasing CO concentration and operation potential. The activation energy for area specific resistance and for cathode polarization resistance is ~58 and 72 kJ mol-1 in CO2/CO 70/30 fuel, respectively. The surface adsorption/desorption equilibration and/or surface diffusion processes play a dominant role in the electrochemical reduction of CO2 by SOEC.

One-Step Synthesis of Stable Gold Nanoparticles on Eco-friendly Ionic Liquid

ABSTRACTMetallic nanoparticles are often obtained by chemical decomposition or reactive techniques involving the extensive usage of harmful reducing or stabilizing agents. A facile green synthesis technique resulting in readily exploitable nanoparticle dispersion in ionic liquid without the use of any additional agents is reported here. 1-Propyl- 3- Methyl Imidazolium Iodide (PMIM(I)) is a non-volatile, thermally stable and non-toxic ionic liquid. This eco-friendly liquid is used as the substrate for thermal evaporation of gold to obtain stable gold nanoparticles. On being examined by Transmission Electron Microscopy the high monodispersity in their sizes was revealed. The byproduct free, ‘clean’ processing technique helps in obtaining un-contaminated particles. The thermal evaporation method used (for the generation of metallic vapor) plays a significant role in the difference in kinetics of the formation and growth of nanoparticles, unlike the widely reported sputtering technique for vapor generation. The formed particles are deposited only on the top surface of the liquid. Thus the nucleation and growth of the particles can be considered to have occurred by surface diffusion process only. A deeper investigation into the formation kinetics has the potential application for synthesizing other nanomaterials via this environmental friendly approach.

Impedance Studies on LSCM∕GDC Cathode for High Temperature CO2 Electrolysis

The cathode performance of LSCM/GDC composite for high temperature CO2 electrolysis has been studied by electrochemical impedance spectroscopy (EIS) measurements in a three-electrode electrolysis cell using Pt paste as current collector. The main feature of LSCM/GDC cathode is that it showed a Gerischer like impedance dispersion at 900°C in all CO2-CO mixtures investigated here. With increasing CO concentration and operating potential, the Rs changes little whilst, the Rp declines markedly for LSCM/GDC composite cathode. The surface adsorption/desorption equilibration and/or surface diffusion processes play a dominant role in the electrochemical reduction of CO2 by SOEC.

Diffusion in quasi-one-dimensional nano- and microobjects

A new approach to investigations of the volume and surface diffusion in quasi-one-dimensional nano- and microobjects is described in application to the diffusion of tungsten from the substrate into a carbon whisker crystal. The proposed experimental technique employs special positioning marks formed on the whisker surface. The interrelated processes of volume and surface diffusion have been simulated using a combination of the integrated field sources and finite difference methods. The coefficients of volume and surface diffusion of tungsten atoms in carbon whiskers have been estimated.

Ripples and dots generated by lattice gases

We show that the emergence of different surface patterns (ripples, dots) can be well understood by a suitable mapping onto the simplest nonequilibrium lattice gases and cellular automata. Using this efficient approach difficult, unanswered questions of surface growth and its scaling can be studied. The mapping onto binary variables facilitates effective simulations and enables one to consider very large system sizes. We have confirmed that the fundamental Kardar–Parisi–Zhang (KPZ) universality class is stable against a competing roughening diffusion, while a strong smoothing diffusion leads to logarithmic growth, a mean-field type behavior in two dimensions. The model can also describe anisotropic surface diffusion processes effectively. By analyzing the time-dependent structure factor we give numerical estimates for the wavelength coarsening behavior.