Micron-scale Morphology Research Articles

Effect of a fluorinated surfactant on Langmuir monolayer properties of minimal-linker gemini surfactants

A new class of carboxylic acid-terminated gemini surfactants which contain the smallest possible headgroup linker (a single bond) has recently been reported in the literature. In this current work, we have explored how Langmuir monolayers of two different alkyl tail chain length variants (n = 12, n = 16) of these surfactants, dubbed Cn-0-Cn, are impacted by mixing with a benchmark perfluorinated surfactant, perfluorotetradecanoic acid (PF). Pure PF and C16-0-C16 monolayers share similar general characteristics, yielding compact, incompressible, solid-like films at the air-water interface. In contrast, the shorter tail chain variant, C12-0-C12 forms expanded, compressible liquid-like films. While both tail chain variants formed mixed films with PF that were generally expanded in comparison with their pure components, and were also phase-separated, the extent of interactions between film components and the resulting micron-scale morphology of the mixed films were different for the two alkyl chain lengths. Overall, PF induces different packing behavior in both the systems and the observations are attributed to the difference in the dispersion forces originating from the tail chain length differences.

Synthesis of a novel hierarchical pillared Sep@Fe3O4/ZnAl-LDH composite for effective anionic dyes removal

A novel magnetic hierarchically pillared Sep@Fe3O4/ZnAl-LDH composite was successfully synthesized via co-precipitation methods. The prepared Sep@Fe3O4/ZnAl-LDH composite were characterized via FTIR, XRD, XPS, SEM, TEM, BET and VSM. The results show that the Sep@Fe3O4/ZnAl-LDH composite have a hierarchical columnar structure with nanoscale structural unit (Fe3O4/ZnAl-LDH) and overall micron-scale morphology, and the obtained graded adsorbent has a large pore capacity and pore size distribution. The maximum adsorption capacities of the obtained samples for methyl orange ( MO) and Congo red ( CR) were 430 mg·g−1 and 832 mg·g−1, respectively. Kinetic data of MO and CR adsorbed on as-prepared samples can well fit with pseudo-second-order model. The isothermal data of MO and CR adsorbed on Sep@Fe3O4/ZnAl-LDH match well with the Langmuir model. The thermodynamic results indicate that the adsorption process is spontaneous and endothermic. Besides, the adsorbent shows good stability and reproducibility in a reusable cycle. This study demonstrated that Sep@Fe3O4/ZnAl-LDH provides a new idea for the development of a novel hierarchical magnetic pillared adsorbent for the efficient removal of anionic dyes.

Droplet rolling angle model of micro-nanostructure superhydrophobic coating surface.

The droplet rolling angle is one of the important indicators to measure the coating's hydrophobic performance, but the specific factors affecting the droplet rolling angle on the micro-nanostructured superhydrophobic coating surface are not yet known. Based on the rolling mechanism of droplets on rough surfaces, and from the perspective of coating microscopic energy conservation, this paper points out that the micron-scale morphology and the nanoscale morphology can comprehensively affect the droplet rolling angle. From the above perspective, a mathematical model of the droplet rolling angle on the micro-nanostructure superhydrophobic coating surface was established. The model shows that the droplet rolling angle is positively correlated with the ratio of nano-sized pillar width to spacing, the ratio of micron-sized papilla radius to spacing, and the liquid-gas interfacial tension, and is negatively correlated to the droplet intrinsic contact angle, droplet volume and droplet density. The droplet rolling angle calculated by the presented model is in good agreement with the experimentally tested results. This model can provide good accuracy in predicting the droplet rolling angle on the micro-nanostructured superhydrophobic coating surface.

The influence of surfactant head group on miscibility in mixed hydrocarbon-perfluorocarbon monolayers

Simple binary mixtures of hydrogenated and perfluorinated surfactants are typically immiscible in monolayers, with mixtures forming highly-structured films at the air-water and solid-air interfaces. While dispersion interactions between tail groups of the surfactants play a key role in dictating the micron-scale morphology of such mixed films, the influence of the head group on film morphology has been minimally explored. In this work, surfactant head group effects are investigated by comparing thermodynamic miscibility in combination with film morphology in binary mixed films comprised of nonadecanoic acid (ND; CH3(CH2)17COOH) with perfluorotetradecanoic acid (PF; CF3(CF2)12COOH), as well as the corresponding fatty alcohol nonadecanol (NDOH; CH3(CH2)18OH) with PF. Both the fatty acid and fatty alcohol were immiscible with PF in monolayers, with the systems exhibiting small, negative deviations from ideal mixing and forming phase-separated domains under a variety of preparation conditions. Micron and nanometer-scale film morphologies for the two sets of films were significantly different, however, with fatty acid mixtures tending to form polygonal, faceted domains enriched in hydrocarbon whereas the fatty alcohol mixtures formed significantly less structured domains, both at the solid- and liquid-air interfaces. Differences in film morphologies are discussed in terms of head and tail group interactions, and placed in context of existing literature on closely-related monolayer systems.

Random field finite element models with cohesive-frictional interactions of a hardened cement paste microstructure

Cement paste is a disordered material whose properties and structure differ spatially and cannot be precisely predicted. We propose a new approach for developing finite element models of a hardened microstructure that accounts for the stochastic nature of cement paste. The initial model configuration is a random field that is generated according to probability density functions measured through nanoindentation experiments on samples composed of Portland cement and a volcanic ash additive. We study the influence of cohesive-frictional interactions through a Mohr-Coulomb yield criterion applied to the finite element constitutive relations. An ensemble of microstructures are simulated to assess the influence of spatial fluctuations and plasticity on the mechanical response to compressive loading. Our results indicate that the micron-scale morphology has a limited influence on the macroscopic strength behavior. These findings suggest that the shear strength scaling and dilatant behavior of cement paste originates within nanometer-scale features of the composite.

Thermochemical properties of nanometer CL-20 and PETN fabricated using a mechanical milling method

2,4,6,8,10,12-Hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20) and pentaerythritol tetranitrate (PETN), with mean sizes of 73.8 nm and 267.7 nm, respectively, were fabricated on a high-energy ball-mill. Scanning electron microscope (SEM) analysis was used to image the micron-scale morphology of nano-explosives, and the particle size distribution was calculated using the statistics of individual particle sizes obtained from the SEM images. Analyses, such as X-ray diffractometer (XRD), infrared spectroscopy (IR), and X-ray photoelectron spectroscopy (XPS), were also used to confirm whether the crystal phase, molecular structure, and surface elements changed after a long-term milling process. The results were as expected. Thermal analysis was performed at different heating rates. Parameters, such as the activation energy (ES), activation enthalpy (ΔH≠), activation free energy (ΔG≠), activation entropy (ΔS≠), and critical temperature of thermal explosion (Tb), were calculated to determine the decomposition courses of the explosives. Moreover, the thermal decomposition mechanisms of nano CL-20 and nano PETN were investigated using thermal-infrared spectrometry online (DSC-IR) analysis, by which their gas products were also detected. The results indicated that nano CL-20 decomposed to CO2 and N2O and that nano PETN decayed to NO2, which implied a remarkable difference between the decomposition mechanisms of the two explosives. In addition, the mechanical sensitivities of CL-20 and PETN were tested, and the results revealed that nano-explosives were more insensitive than raw ones, and the possible mechanism for this was discussed. Thermal sensitivity was also investigated with a 5 s bursting point test, from which the 5 s bursting point (T5s) and the activation of the deflagration were obtained.

Morphology effect on photocatalytic activity in Bi3Fe0.5Nb1.5O9

In this work, the Aurivillius-phase ferroelectric Bi3Fe0.5Nb1.5O9 were synthesized by hydrothermal (BFNO-H) and solid state methods (BFNO-S), respectively. The BFNO-H shows a hierarchical morphology, which is stacked by intersecting single-crystal nanosheets with {001} and {110} exposed facets, while the BFNO-S shows disorganized micron-scale morphology. BFNO-H shows a much stronger photodegradation activity (10.4 times and 9.8 times) than BFNO-S in the visible-light photodegradation of rhodamine B (RhB) and salicylic acid. The higher photodegradation activity of BFNO-H was firstly ascribed to the hierarchical structure and the larger specific surface area (16.586 m2 g−1) because a large specific surface area can increase reactive sites and shorten photogenerated carrier migration distance. However, after being normalized by the specific surface area, BFNO-H still performs better than BFNO-S, implying that the specific surface area is not the only factor that determines the photocatalytic activity. Considering that the built-in electric field originating from spontaneous polarization in Bi3Fe0.5Nb1.5O9 has existed in both ab plane and c direction, it matches well with the {001} and {110} exposed facets of BFNO-H nanosheets. This appropriate matching in BFNO-H nanosheets may improve the separation and transmission of photogenerated electron–hole pairs and further enhance its photocatalytic activity. Moreover, the trapping experiments reveals that holes (h+) are the main active species and hole-derived oxidation is the main redox reaction during photodegradation of organic pollutions.

(Invited) Combined Experimental-Numerical Approaches for Transport Characterization in Nano and Micron-Sized Multi-Component and Multi-Functional Materials

Porous and heterogeneous materials are core components in energy conversion and storage devices such as batteries, fuel cells and electrolyzers, or photoelectrochemical fuel generators. The heterogeneity and structural complexity of these components are a result of three main drivers: i) The multi-functional nature of the applications requiring the presence of various functional materials in close vicinity, ii) nano- and micron-scale structuring of the material required to overcome the bulk material transport limitations, and iii) cheap and simple synthesis methods resulting in stochastic and complex morphologies. Consequently, understanding of the multi-physical transport phenomena and optimization of the component for enhanced performance, requires an accurate modelling and prediction of the transport properties, which heavily rely on the complex nano to micron-scale morphology. In this presentation, I will show how a previously developed combined experimental-numerical approach1 can be used for the accurate numerical characterization of the heterogeneous components’ transport properties. The methodology consists of (i) the experimental characterization of the material’s exact microstructure via non-intervening, non-destructive imaging techniques, and the subsequent use of this data in (ii) the discrete-scale numerical determination of the medium’s effective transport properties in conjunction with continuum-scale theory. This approach can be extended by using digital image processing on the structure data2,3 to allow for artificial changes in the morphological characteristics – corresponding to real variations in the fabrication process – and subsequent investigation of the changes in the material’s transport properties. I will show results from the application of the combined experimental-computational methodology to three morphologically complex components at very different length-scales and in three different applications: (i) nano-structured multi-component photoelectrodes for the production of solar fuels, (ii) nano-structured porous electrodes in high-temperature electrolysis, and (iii) porous ceramic receivers for the production of concentrated solar power. The approach allowed to quantify for the first time morphological and transport properties of nano-structured photoelectrodes (example i), to characterize novel transport properties such as permeability for porous electrodes in high-temperature electrolysis (example ii), and to propose pore-scale engineering design guidelines for optimized structures of solar receivers (example iii). 1 J. Petrasch, Ph.D. thesis, ETH Zurich, 2007. 2 S. Haussener, and A. Steinfeld, Materials, 2012, 5, 192–209. 3 S. Suter, A. Steinfeld, and S. Haussener, Int. J. of Heat and Mass Transfer, 2014, 78, 688-698. 4 S. Suter, and S. Haussener, JOM, 2013, 65, 1702-1709.

The effect of post-annealing on the interface between the aluminum electrode and the active layer in polymer/fullerene solar cells

The post-annealing effects on the performance of poly (3-hexylthiophene) (P3HT)/(6,6)-phenyl C61 butyric acid methyl ester (PCBM) solar cells with conventional bulk heterojunction (CBHJ) and layer-evolved bulk heterojunction (LBHJ) have been compared. It is found that contrary to the much better performance obtained from CBHJ cells, the post-annealing deteriorates the performance of LBHJ devices. Aqueous contact angle and X-ray photoelectron spectroscopy measurements show that P3HT is dominant at the top surface of CBHJ film, while PCBM is dominant at the top surface of LBHJ film. The micron-scale morphology evolution of the active layer/Al interface upon post-annealing reveals that the PCBM-rich surface is beneficial for the nucleation and growth of PCBM crystal, which does harm to the contact between the active layer and the electrode and results in the decrease of the fill factor. However, the original P3HT-rich surface prevents the formation of large surface-segregated PCBM clusters upon post-annealing, which is highly desirable for the efficient polymer/fullerene solar cells.

Probing the supra-nano-scale and micron-scale structure of liquid-crystal and polymer dispersions using laser light scattering

— Liquid-crystal and polymer dispersions are typically made utilizing one of three methods of phase separation. Laser light scattering is a convenient method to probe the evolving supra-nano scale and micron scale morphology of phase-separating mixtures. We used laser light scattering to probe the evolving supra-nano and micron scale structure of phase-separating liquid crystal and monomer mixtures induced with photon-initiated polymerization. We report on the evolutionary phase separation for mixtures of the E7 and the thiol-ene based optical adhesive Norland 65 from low to high weight percentages of E7.