Diffusion-limited Conditions Research Articles

New insights into thallium(I) behaviors at birnessite surfaces: Effects of an organic buffer and goethite

Understanding the environmental behavior of thallium (Tl) is crucial due to its high toxicity and increasing anthropogenic presence. This study investigated the adsorption and redox behaviors of Tl(I) with acid birnessite (AcBi) in the presence of 1,4-piperazine-diethanesulfonic acid (PIPES) and goethite under diffusion-limited conditions using Donnan reactors in aerobic and anaerobic environments. Our findings indicate that Tl(I) preferentially adsorbs onto AcBi, with capacities 20 to 100 times higher than onto goethite, even when AcBi is partial reduced by PIPES. No net Tl(I) oxidation occurred in the Donnan reactors, likely due to complex electron transfer processes between Tl(I), birnessite, and PIPES. Any Tl(III) generated from Tl(I) oxidation by birnessite was rapidly reduced back to Tl(I) by PIPES. This was confirmed in batch experiments where reduced Tl(III) on birnessite surfaces and in Tl(III) salts. These findings highlight the need to assess the impact of Good's buffers on redox reactions involving manganese oxides and Tl, while also providing insights into the competitive retention of Tl on manganese and iron (hydr)oxides, with implications for Tl mobility and bioavailability in natural environments.

Uncovering the predictive pathways of lithium and sodium interchange in layered oxides.

Ion exchange is a powerful method to access metastable materials with advanced functionalities for energy storage applications. However, high concentrations and unfavourably large excesses of lithium are always used for synthesizing lithium cathodes from parent sodium material, and the reaction pathways remain elusive. Here, using layered oxides as model materials, we demonstrate that vacancy level and its corresponding lithium preference are critical in determining the accessible and inaccessible ion exchange pathways. Taking advantage of the strong lithium preference at the right vacancy level, we establish predictive compositional and structural evolution at extremely dilute and low excess lithium based on the phase equilibrium between Li0.94CoO2 and Na0.48CoO2. Such phase separation behaviour is general in both surface reaction-limited and diffusion-limited exchange conditions and is accomplished with the charge redistribution on transition metals. Guided by this understanding, we demonstrate the synthesis of NayCoO2 from the parent LixCoO2 and the synthesis of Li0.94CoO2 from NayCoO2 at 1-1,000 Li/Na (molar ratio) with an electrochemical assisted ion exchange method by mitigating the kinetic barriers. Our study opens new opportunities for ion exchange in predictive synthesis and separation applications.

Cation Effects on Hydrogen Oxidation Reaction on Pt Single-Crystal Electrodes in Alkaline Media.

The exact mechanism behind the cation-assisted hydrogen oxidation reaction (HOR) on platinum electrodes in alkaline media remains disputed. We show that the cation effects at platinum display a remarkable structure sensitivity: not only the H adsorption but also the HOR activity on (111) terrace sites are independent of the nature of cation and cation concentration. On (110) step sites, at low cation concentration and mildly alkaline media, cations promote the HOR, whereas at more alkaline pH and consequently higher near-surface cation concentrations, the HOR is inhibition by the cations. Moreover, the role of the cation on terrace-OHad is different from that on step-OHad, as can also be observed from the inhibition of the HOR current by terrace-OHad at higher potentials. These results suggest that near the onset potential, HOR mainly takes place on steps, but under diffusion-limited conditions at higher overpotential, HOR mainly takes place on terraces.

Electrooxidation of aminomethyl phosphonic acid (AMPA) using a boron doped diamond anode

Glyphosate, a commonly used herbicide, can transform into aminomethyl phosphonic acid (AMPA, CH6NO3P) in the aquatic environment. This AMPA can bioaccumulate in the food chain, damage living organisms at cellular and embryonic levels, and trigger eutrophication by releasing bioavailable phosphorus. Electrooxidation (EO) offers a promising strategy to neutralize organic pollutants, yet limited research has extended EO to manage AMPA in diluted water matrices. In this study, we determine EO’s kinetics in degrading low-concentration AMPA and evaluate operational strategies to boost EO’s performance under diffusion-limited conditions. Results of the batch systems indicate that AMPA can be degraded via direct and indirection oxidation under 2.5 V. The former takes place on an anode surface through direct electron transfer, while indirect oxidation is achieved via electrochemically generated reactive species such as •OH and H2O2. We find all degradation products to be inorganic ions, with no accumulation of organic intermediates. Further decrease of AMPA levels from 0.1 to 0.01 mM poses a diffusion-limited condition to our EO system, suppressing AMPA degradation efficiency from 97.2% to 84.2%. We subsequently increased the electric driving force from 2.5 to 3.0 V and obtained a 98.8% 0.01 mM AMPA removal. The long-term performance of a continuous-flow reactor reveals EO is governed by electrode surface area and hydraulic retention time (HRT), with the highest removal rate of 6.6 mg m−2 h−1 at 3.5-h HRT. Our results indicate that EO can efficiently remove diluted AMPA in water matrices and warrants future efforts in electrode and reactor design to effectively counter mass transfer constraints.

Radial flow enhances QCM biosensor sensitivity

QCM biosensors are widely used for the detection of a variety of pathogenic microorganisms, and there remains a need for further increases in sensitivity. The majority of QCM biosensors are operated under diffusion-limited conditions that, when used with a standard flow cell, lead to the majority of analyte being captured onto regions of the crystal surface that have limited to no mass sensitivity. Here we show that simple changes to a flow cell architecture (reduction of flow cell height) can provide increases in sensing signal. Furthermore, we show that the use of radial flow will lead to analyte capture directly in the sensitive region of a crystal, thus providing dramatic increases in the biosensor’s sensitivity. We examine this problem from both a theoretical and numerical perspective and present a new method by which to consider how changes to a sensing assay will affect sensor sensitivity. We demonstrate this experimentally through the detection of E. coli using one commercial and two custom-built flow cells having large differences in height, one of which has radial flow. We show that simple reduction in flow cell height using a standard flow cell leads to a 2.4 × in sensitivity, where the further inclusion of radial flow leads to a 5.1 × increase. We further show that these performance increases are only slightly offset by changes in the sensing noise, which were seen to vary slightly when using different flow cells. These results are directly applicable to nearly every QCM biosensor operated using affinity-based detection.

Vapor distribution changes evaporative flux profiles of a sessile droplet

HypothesisWe propose that during the evaporation of sessile droplets, the evaporative flux profile is primarily influenced by droplet geometry and composition under diffusion-limited conditions. Most studies have focused solely on the evaporation feature from the liquid to the gas phase, neglecting the extent to which the evaporated vapors affect the evaporation process. We hypothesize that if the molecular weight of the evaporated vapors is significantly high or low compared to the ambient gas, it could alter the evaporative flux. ExperimentsWe employed a direct optical measurement technique, specifically Mach-Zehnder interferometry. This demonstrated that the distribution of evaporated vapor molecules can substantially modify the evaporative flux profile. FindingsOur study discovered that substantial density gradients between vapor and air could either suppress or enhance the evaporative flux, depending on the droplet's orientation. This research offers fresh insights into evaporative fluxes by taking into account the relative vapor concentration and gravitational effects.

Smooth Thin Film of a CoNiCu Medium-Entropy Alloy Consisting of Single Nanometer-Sized Grains Formed by Electrodeposition in a Water-in-Oil Emulsion

High-entropy and medium-entropy alloys are new classes of alloys that exhibit excellent strength and toughness. These alloys are expected to be used as multifunctional catalysts given that methods for producing nanoparticles of such alloys have been established. By contrast, the production methods to form thin films of such alloys are still very limited. Here, we show that electrodeposition in a chemically stabilized water-in-oil emulsion functions as a general method to form thin films of such alloys. The key to forming thin films of these alloys by electrodeposition is to conduct reduction under diffusion-limited conditions and to confine the distribution of concentration in a droplet. This situation is not achieved in conventional bulk aqueous or nonaqueous single-phase solution but is achieved in a water-in-oil emulsion. Here, we found aerosol OT as a suitable surfactant for the present water-in-oil emulsion to stabilize the emulsion during electrodeposition. It is also clarified that the increase in viscosity of the aqueous solution within water droplets by the addition of glycerol decreases the roughness greatly. The strategy demonstrated here is expected to spur further applied research into thin films of high- and medium-entropy alloys.

Branched growth of ZSM-12 zeolite on seeds

In this work, two ZSM-12 zeolites with stellar morphology and dendritic morphology were synthesized from the synthesis system containing nanorod-aggregated hexagonal prism seed and microrod seed, respectively, in the presence of CTAB. The stellar ZSM-12 zeolite was composed of core and branches, while the dendritic ZSM-12 zeolite consisted of stem and branches. For the two zeolites, the branches grew on {310} planes of seed and were in twin relation with the seed. The c-axes of branch and seed were parallel to each other, while the b-axes of them exhibited the angular relation of 65°. The crystallization process of the two zeolites were investigated by using XRD, SEM, and TEM characterizations. It was demonstrated that the branched growth on seeds was controlled by two factors: diffusion of reactive species and adsorption of CTAB. The Mullins-Sekerka instability occurred under the diffusion-limited condition, resulting in the formation of branches on seed. The adsorption of CTAB on side facets of branches inhibited the occurrence of secondary branching.

(Invited) Effect of Additive Species on the Nucleation and Growth Process on Zn Negative Electrode Surface

Zn is one of the promising candidates as negative electrode materials for next-generation secondary batteries, especially those for large capacity applications such as grid-scale energy storage. While the Zn has numbers of attracting features such as crustal abundance, applicability of aqueous system, etc., which enable to realize the secondary batteries with high performance and safety with lower cost, the major drawback is evolution of irregular morphology called mossy structure with highly filamentous features at the electrode surface during charge-discharge cycles. Unlike the conventional dendritic growth which proceeds under diffusion-limited conditions, formation of the mossy structure takes place without such a condition. At the same time, it has been reported that the growth of these irregular structures could be suppressed by applying metallic species as additives to form uniform electrodeposition [1]. We have investigated formation process of the mossy structure on the Zn anode surface, focusing on its initial stage in combination with the additive effects such as Pb and Sn [2]. In this presentation, we will introduce the results of our research on the nucleation and growth process of the mossy structure through experimental analysis and computational modeling. For the theoretical calculation, we attempted to develop multi-scale simulation model by employing density functional theory (DFT) and kinetic Monte Carlo (KMC) approaches [3], and the results will be described focusing on the effects of the metallic additives.This work is financially supported in part by MEXT/JSPS Grant-in-Aid for Scientific Research No. 21H01642.[1] For example, F. Mansfeld, S. Gilman, J. Electrochem. Soc. 117, 1328 (1970); Y. Ito, M. Nyce, R. Plivelich, M. Klein, D. Steingart, S. Banerjee, J. Power Sources, 196, 2340 (2011).[2] For example, T. Otani, M. Nagata, Y. Fukunaka, T. Homma, Electrochim. Acta, 206, 366 (2016); T. Otani, Y. Fukunaka, T. Homma, Electrochim. Acta, 242, 364 (2017).[3] Y. Onabuta, M. Kunimoto, S. Wang, Y. Fukunaka, H. Nakai, T. Homma, J. Phys. Chem.C, submitted.

Measuring DNA target search at the single molecule level

We study DNA target search by type II restriction endonucleases. In our single molecule technique, micron sized beads tethered with single DNAs are imaged using video microscopy. Under diffusion limited conditions, release of individual beads due to cleavage identifies the instant a single protein acquires its DNA target. In our experiments, we study NdeI, an endonuclease that is found in Neisseria denitrificans. In vivo, many other proteins bound on the DNA act as ‘roadblocks’ and prevent DNA binding proteins from sliding on the DNA during target search.

Mathematical Analysis of Cyclic and Sampled Current Voltammetries for the Description of Nucleation and Growth Processes of Metallic Nanoparticles

The electrodeposition of metallic nanoparticles was studied using numerical methods under progressive and instantaneous nucleation regimes to calculate cyclic and sampled current voltammetries. The cyclic voltammograms (CV) for both types of nucleation were calculated using diffusion and kinetics limited growth conditions. The sampled current voltammograms (SCV) were built from numerically generated chronoamperometric curves. Comparison of the SCV and CV curves shows that the current response corresponding to the progressive nucleation was similar under both growth regimes, although the nuclei density was considerably different. Under instantaneous nucleation, the CV and SCV showed similar current responses only under kinetic control due to similar cluster growth, and the diffusion-controlled growth showed a similar cluster growth, but a different current response. The comparison made in this work provides a guideline to understand experimental electrodeposition results and makes a good argument for using SCV to complement the CV curves.

Rectangular electrodes: Simulation of accurate steady state currents and the behaviour of square electrode arrays

In this paper, rectangular electrodes embedded flush in an insulating plane have been simulated under diffusion limited chronoamperometric conditions, with the goal of estimating steady diffusion limited currents to an accuracy of about 1%, for a range of relative lengths vs. width, as an update of our previous work. Following this, the behaviour of square electrode arrays was studied, for various separations on an infinite array, and as small arrays, and diffusion limited time-varying and steady state currents were obtained. For increasing separations between the electrodes, the steady state currents at small arrays fall below the expected theoretical values.

Interactive patches over amyloid-β oligomers mediate fractal self-assembly.

The monomeric units of intrinsically disordered proteins self-assemble into oligomers, protofilaments, and eventually fibrils which may turn into amyloid. The aggregation of these proteins is primarily studied in bulk with no restriction on their degrees of freedom. Herein we experimentally demonstrate that amyloid-β (Aβ) aggregation under diffusion-limited conditions leads to its fractal self-assembly. Confocal microscopy and scanning electron microscopy with energy dispersion x-ray analysis were used to confirm that the fractal self-assemblies were formed from Aβ rather than the salt present in the two supporting media: deionized water and phosphate buffered saline. The results from the molecular docking experiments implicated that electrostatic and hydrophobic patches on the solvent-accessible surface area of the Aβ oligomers mediate the fractal self-assembly. These implications were tested with laser light scattering experiments on the oligomers formed by breaking mature fibrils of Aβ through sonication, which were observed to self-assemble into fractals when sonicated solutions were drop casted. The electrostatic interactions modulate the fractal morphologies with pH of the solution, which leads to a morphological phase transition observed through the variation in their fractal dimension. These transitions provide experimental evidence for the existing theoretical framework in terms of different kinetic models. The higher surface-to-volume ratio of these fractal self-assemblies may have applications in drug delivery, biosensing, and other biomedical applications.

Synthesis, characterization, structural, redox and electrocatalytic proton reduction properties of cobalt polypyridyl complexes

A monoanionic amido pentadentate ligand bpaqH (2-(bis(pyridin-2-ylmethyl)amino)-N-(quinolin-8-yl)acetamide) and its corresponding cobalt(III) chloro complex [Co(bpaq)Cl]Cl: 1 and aqua derivative [Co(bpaq)(OH2)](ClO4)2: 2 were successfully synthesized and fully characterized by different analytical and spectroscopic techniques such as FT-IR, 1H NMR, UV–vis spectroscopy, ESI mass spectra. The structures of 1 and 2 have been determined by the single-crystal X-ray diffraction. Spectral and redox properties were investigated along with free ligand under electrochemical conditions. Both complexes performed proton reduction activity under soluble, diffusion-limited conditions in acetonitrile with acetic acid as an external proton source with overpotentials of 0.412 V for 1 and 0.394 V for 2. The stability of the catalysts was inspected by the time-dependent UV–vis spectroscopy; 1 and 2 were found to be highly stable in the absence and presence of acetic acid. There was no significant spectral change before and after the controlled potential electrolysis suggesting no change in molecular integrity during electrocatalysis.

Synthesis of Nanostructured Metallic Foams By Plasma Electrolysis Deposition

Within the framework of fundamental physics studies at CEA, laser target experiments are conducted to investigate laser-matter interactions. Some of these experiments require metallic foams samples in a milimetric scale with an apparent volumetric mass within few hundreds milligrams per cube centimeters. Metallic foams materials are developed and studied since few years for their unique properties offered by their aerated structure. For example, due to a high specific surface, metallic foams could find applications in batteries, supercapacitors, as well as chemical sensors. However, all the metallic foams obtained by the processes referenced in the literature do not meet the specifications needed for laser experiments. This is why, the CEA has developed a new and innovative technique to synthetize metallic foams by plasma electrolysis deposition [1].This process consists in applying a voltage within the 20-100V range at a cathode immerged in a solution containing metallic ions. Under these specific conditions, a uniform gaseous sheath is created around the cathode which is then isolated from the liquid. Due to the high electrical field applied at the electrode surface, several electrical plasma discharges are formed between the cathode and the gas/liquid interface. When the electrical discharges meet the solution, metallic cations are reduced to form metallic strands about 100 nm in diameter. The formed strands are all interconnected together to form metallic foam with micrometric porosities [2]. A few millimeters wide foam with a volumetric mass up to 100mg/cc is obtained in about 10 seconds which is very fast compared to classical electrochemical deposition techniques. The foams synthetized can be made up of pure metals (Cu, Au, Pt ...) or alloys (AuCu ...) in accordance to the metallic ions diluted in the solution.In order to control the shape and homogeneity of the synthetized foams, a better understanding of the mechanisms involved in the foams’ growth is required. By direct camera observation, Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM), we can show that the growth behavior and the structure of the foams vary with the applied voltage. We observe three main growth patterns which seem to correspond to the different propagation modes of streamer type electrical discharges reported in the literature on plasma discharges in dielectric liquids [3,4]. The first one obtained at “low” voltage is composed of very thin strands (Ø < 100 nm) growing linearly with few ramifications. The second one is formed by highly branched strands about 100 to 600 nm in diameter. TEM observations of first and second mode strands show that their structure is monocrystalline and field oriented. A third mode obtained at the highest voltages is made up of strands above 1 µm thick with a shape very similar to the aggregates obtained in diffusion limited conditions [5]. Voltammetry studies conducted on the electrolytes show that the reduction of metallic species are indeed diffusion limited. Moreover a lower limit current corresponds to a more predominant germination phenomenon on strands. The mass transportation plays a major role alongside the applied voltage in defining the growth mode of the metallic foams and the microstructure of the strands. The structure of the metallic strands and it’s evolution with the synthesis parameters seems in accordance to the electrodeposition theory [6]. However if our results indicate that the reduction and crystallization of the strands may result from an electrochemical process, the field oriented crystalline structure obtained is unlikely to be possible inside an aqueous electrolyte with the speed of growth observed. We conclude this work by proposing an original mechanism for the growth of the foam involving electrochemistry within a plasma medium.

Removal of toluene in a biotrickling filter in the presence of methanol vapors: Experimental study, mathematical modeling, and kinetic parameters optimization

The performance of biotrickling filters (BTFs) has been significantly affected during the co-treatment of volatile organic compounds (VOCs) with different hydrophilicities. Removal of toluene vapor as a hydrophobic compound in the absence and presence of methanol as a hydrophilic pollutant was studied in a BTF. The inlet loading ranges were 18−36 g.m−3.h−1 for toluene and 0−225 g.m−3.h−1 for methanol at constant empty bed residence time (EBRT) of 60 s. The removal efficiency of toluene (RET) varied from 30 to 80 %, while RE of methanol (REM) remained almost constant at > 90 % within different phases of experiments due to the unlimited solubility in water. A comprehensive dynamic mathematical model consisting of mass transfer through gas, liquid, and biofilm phases with the kinetic of biodegradation in the BTF and accumulation of methanol in liquid was developed. The interaction parameters were optimized by genetic algorithm and validated by experimental data. Sensitivity analysis on the optimized parameters showed that biofilm thickness and methanol interaction parameter had the most effects on RET as 45 % and 38 %, respectively. Model simulation showed that 43–90 % of biofilm depth was active for toluene degradation at different inlet gas-phase concentrations, indicating a diffusion-limited condition in the process. In contrast, all depth of biofilm was fully active for methanol degradation, showing a kinetic-limited regime dominant for methanol removal in the BTF under all operating conditions. Results showed that the developed model could effectively predict the dynamics of VOCs removals along with the determination of kinetic constants.

A novel self-sustained single step process for synthesizing activated char from ligno-cellulosic biomass

A gasification-based single step process is reported for synthesizing moderately activated char (surface area in the range of 100–600 m2/g) from ligno-cellulosic biomass. The novelty of the process lies in the use of mixtures of O2-CO2 and O2-steam as gasification agents to obtain significantly higher activation in a single-step as compared to gasification with air (<100 m2/g) in a self-sustained process. Experiments were conducted in a counter-current packed bed reactor with three biomass (agro-residue pellets, wood based pellets and coconut shells). Yield and Brunauer-Emmett-Teller (BET) surface area are reported for char obtained from experiments covering an overall equivalence ratio range of 3.9–1.2. The volatiles equivalence ratio (ϕv), identified as the unifying parameter for analysis from the earlier studies of the authors, emerges as the relevant parameter to characterize the relationship between yield and activation as well. With mixtures of O2-CO2, as ϕv decreases from 2.7 to 1.1 (fuel-rich to stoichiometric) the yield decreases from 100 to 10% (of fixed carbon content) with a corresponding increase in BET surface area from 10 to 570 m2/g. With mixtures of O2-steam, as ϕv decreases from 2.1 to 1.04 the yield decreases from 92 to 5% with a corresponding increase in BET surface area from 106 to 561 m2/g. The zone of transition of activation from reaction rate limited to diffusion limited conditions is identified from the experimental results. It occurs at around ϕv ~ 1.7 (T ~ 1450 K) for O2-CO2 mixtures and around ϕv ~ 2.1 (T ~ 1200 K) for O2-steam mixtures. The activation obtained with the current single step process is an order of magnitude higher compared to that from other single step processes like pyrolysis and air-gasification. Char obtained from the current process can be used as activated biochar and for other applications requiring only moderate activation (100<SBET<600 m2/g).

Dunite carbonation in batch-tubular reactor.

For geological carbon sequestration, the reaction of aqueous CO2 with silicate rock permits carbonate formation, achieving permanent carbon sequestration. The fractures available in silicate rock provide significant surface area for the precipitation of carbonates. The experiments were performed in a batch tubular reactor under diffusion-limited condition, with a special arrangement of a narrow tube filled with a 2800 g/L dunite slurry. The tube was kept open from the top, standing vertically filled with a CO2-rich bulk solution under 1 barg CO2 and temperatures ranging from 25 to 75 oC for 7-30 days. After 7 days of the experiment, magnesite precipitation was seen inside the tube and the precipitation was continued for up to 30 days. The magnesite precipitation was identified by micro-Raman spectroscopy, X-ray diffraction, and scanning electron microscopy. Additionally, SiO2 formation was seen in relative close vicinity to the magnesite precipitation. The precipitation on the surface of silicate rock might cover the fractures and pore spaces available, which may over time reduce the dissolution rate of dunite. Graphical Abstract.

Growth Mechanisms of Low Density Metallic Foams Synthetized By Plasma Electrolysis Deposition

Within the framework of fundamental physics studies at CEA, laser target experiments are conducted to investigate laser-matter interactions. Some of these experiments require metallic foams samples in a milimetric scale with an apparent volumetric mass within few hundreds milligrams per cube centimeters. Metallic foams materials are developed and studied since few years for their unique properties offered by their aerated structure. For example, due to a high specific surface, metallic foams could find applications in batteries, supercapacitors, as well as chemical sensors. However, all the metallic foams obtained by the processes referenced in the literature do not meet the specifications needed for laser experiments. This is why, the CEA has developed a new and innovative technique to synthetize metallic foams by plasma electrolysis deposition [1].This process consists in applying a voltage within the 20-100V range at a cathode immerged in a solution containing metallic ions. Under these specific conditions, a uniform gaseous envelop is created around the cathode which is then isolated from the liquid. Due to the high electrical field applied at the electrode surface, several electrical plasma discharges are formed between the cathode and the gas/liquid interface. When the electrical discharges meet the solution, metallic cations are reduced to form metallic strands about 100 nm in diameter. The formed strands are all interconnected together to form metallic foam with micrometric porosities [2]. A few millimeters wide foam with a volumetric mass up to 100mg/cc is obtained in about 10 seconds which is very fast compared to classical electrochemical deposition techniques. The foams synthetized can be made up of pure metals (Cu, Au, Pt ...) or alloys (AuCu ...) in accordance to the metallic ions diluted in the solution.In order to control the shape and homogeneity of the synthetized foams, a better understanding of the mechanisms involved in the foams’ growth is required. By direct camera observation and Scanning Electron Microscopy (SEM), we can show that the growth behavior and the structure of the foams vary with the applied voltage. We observe three main growth patterns which seem to correspond to the different propagation modes of electrical discharges reported in the literature on plasma discharges in dielectric liquids [3,4]. The first one observed at “low” voltage is composed of very thin strands (Ø < 100 nm) growing linearly with few ramifications. The second one is formed by highly branched strands about 100 to 600 nm in diameter. And the third one obtained at the highest voltages is made up of strands above 1 µm thick with a shape very similar to the aggregates obtained in diffusion limited conditions [5]. Furthermore, we observe that the potential predominance range of each growing mode can be shifted by modifying the concentration or the nature of the metallic ions. Our investigations show that the mass transportation plays a major role with the applied voltage in defining the growth mode of the metallic foams. Electron BackScatterred Diffraction (EBSD) and X-ray Diffraction (XRD) analyses have been conducted to study the crystalline structure of each mode and thus providing information on the conditions of electro-crystallization of the strands. This work concludes by proposing an electrochemical model of the foam growth.

Formation of hierarchical structures from functionalized multi-walled carbon nanotubes in aerosil containing solutions

Specific features and regularities of self-assembly and self-organization of multi-walled carbon nanotubes (MWCNT) have been studied for diffusion-limited conditions (method of drops) in water-based (deionized water) colloidalal solutions with aerosil exposed to DC electric fields with magnitudes of 15 to 25 V. Studies of hierarchical structure formation during drop evaporation in uniform electric fields have revealed the formation of 40–120 nm sized linear piecewise formations, 25–45 nm sized fractal structures and 250 nm sized diffuse structures from MWCNT – COOH + aerosil + H2Odw. The structures have been studied using confocal microscopy, X-ray diffraction, Raman scattering, atomic force microscopy, IR spectroscopy and scanning electron microscopy. The sizes of the observed micro- and nanostructures decrease following the hyperbolic law d = 1/U in the approximation d → 2R, their growth rate increasing as U2. We show that intense ultrasonication of functionalized MWCNT – COOH + aerosil + H2Odw in colloidal solutions causes the formation of the so-called “breathing" modes in axis centered single-walled carbon nanotubes. This is confirmed by short-wave Raman scattering excitation and enables the existence of both combined sp2-hybridization types with π- and σ- carbon bonds and the metallic and semiconductor conductivity types in the material thus showing good promise of these structures for nanoelectronics.