Equilibrium Morphology Research Articles

Growth and Equilibrium Morphology of Hydrohalite (NaCl·2H2O) and Its Epitaxy with Hexagonal Ice Crystals

The experimental growth morphology of NaCl·2H2O, mineral hydrohalite (HH), is compared with that calculated at equilibrium, at 0 K. The HH surface structure is theoretically investigated through the Hartman–Perdok method: the growth characters of the different {hkl} forms are determined along with their possible surface profiles in order to calculate, at the DFT level, their specific surface energies in the presence of vacuum. The striking difference between the calculated equilibrium shape (ES) and the observed growth shape (GS) is interpreted on the ground of a privileged epi-adsorption of {00.1}, the most important form of hexagonal ice (Ih) on the observed dominant {010} form of HH. The sharp pseudo-hexagonality of the {010} HH-form (substrate) seems to be the keystone to understand the very good epitaxy with the {00.1} pinacoid (deposit) of Ice (Ih).

Theoretical Insights into Morphologies of Alkali-Promoted Cobalt Carbide Catalysts for Fischer–Tropsch Synthesis

Co2C catalysts were found to be structure-sensitive during Fischer–Tropsch synthesis (FTS), and reaction-induced Co2C nanoprisms exposing the (101) and (020) facets exhibited superior olefin selectivity, low methane selectivity, and high activity under mild conditions. Alkali metal promoters benefit the formation and stabilization of Co2C nanoprisms, although mechanistic understanding and theoretical support are lacking due to the great complexity of this catalytic reaction and the difficulty in characterizing the detailed structural changes. Here, density functional theory (DFT) calculations and ab initio atomistic thermodynamics simulations are combined to elucidate the “promoter–structure” relationship of Co2C catalysts decorated with different alkali metal promoters. Co- and C-terminated and stoichiometric low-index surfaces including (020), (101), (111), (011), and (110) are considered. Evolution of the equilibrium morphology versus K2O coverage (θK2O) and carbon chemical potential (μC) as determined by the temperature, pressure, and the H2/CO ratio are predicted. We find that increasing θK2O facilitates the preferential exposure of the (020) and (101) facets with different terminations at diverse μC, which are the active surfaces for Fischer–Tropsch to olefins (FTO). For θK2O = 1/6 ML, these two surfaces are predicted to cover 100, 53.4, and 48.4% of the exposed surface area at μC of −7.5, −8.5, and −9.5 eV, respectively, compared with 8.6, 18.3, 26.2% without this promoter. The dominant exposure of these two facets explains the experimentally observed structure of Co2C nanoprisms with a parallelepiped shape. Additionally, the promotional effect extends the preference of most Co- and C-terminated facets over stoichiometric facets to a larger range of μC. Furthermore, Na, K, and Rb promoters are predicted to have stronger effects than Li in stabilizing these two facets, which is also consistent with available experimental observations. Insights from this theoretical work provide a rational understanding of the promotional effect of alkali metals on the morphology of the Co2C catalyst, which may facilitate the design of more efficient FTO catalysts with controlled surface structures.

Molecular Basis for the Morphological Transitions of Surfactant Wormlike Micelles Triggered by Encapsulated Nonpolar Molecules.

Surfactant wormlike micelles are prone to experience morphological changes, including the transition to spherical micelles, upon the addition of nonpolar additives. These morphological transitions have profound implications in diverse technological areas, such as the oil and personal-care industries. In this work, additive-induced morphological transitions in wormlike micelles were studied using a molecular theory that predicts the equilibrium morphology and internal molecular organization of the micelles as a function of their composition and the molecular properties of their components. The model successfully captures the transition from wormlike to spherical micelles upon the addition of a nonpolar molecule. Moreover, the predicted effects of the concentration, molecular structure, and degree of hydrophobicity of the nonpolar additive on the wormlike-to-sphere transition are shown to be in good agreement with experimental trends in the literature. The theory predicts that the location of the additive in the micelle (core or hydrophobic-hydrophilic interface) depends on the additive hydrophobicity and content, and the morphology of the micelles. Based on the results of our model, simple molecular mechanisms were proposed to explain the morphological transitions of wormlike micelles upon the addition of nonpolar molecules of different polarities.

DNA Knot Malleability in Single-Digit Nanopores

Knots in long DNA molecules are prevalent in biological systems and serve as a model system for investigating static and dynamic properties of biopolymers. We explore the dynamics of knots in double-stranded DNA in a new regime of nanometer-scale confinement, large forces, and short time scales, using solid-state nanopores. We show that DNA knots undergo isomorphic translocation through a nanopore, retaining their equilibrium morphology by swiftly compressing in a lateral direction to fit the constriction. We observe no evidence of knot tightening or jamming, even for single-digit nanopores. We explain the observations as the malleability of DNA, characterized by sharp buckling of the DNA in nanopores, driven by the transient disruption of base pairing. Our molecular dynamics simulations support the model. These results are relevant not only for the understanding of DNA packing and manipulation in living cells but also for the polymer physics of DNA and the development of nanopore-based sequencing technologies.

Synthesis and characterization of pseudoboehmite by neutralization method

Pseudoboehmite was obtained through neutralization of ammonium aluminum sulfate solution [NH4Al(SO4)2·12H2O] with ammonia water [NH3 (aq)]. The neutralization process resulted in flake-like overlapping flower-like structures at 80 °C and 85 °C. When the temperature of the NH4Al(SO4)2·12H2O solution was raised to 90 °C, nanoparticles, microspheres with a diameter of less than 50 nm, were obtained. The (131) plane was studied by HRTEM imaging (high resolution transmission electron microscopy). Density functional theory was used to study the interface properties of the pseudoboehmite, and the surface energy and attachment energy were calculated for the optimized crystal planes of interest. The results show that the relaxed (131) plane has the lowest surface energy (0.0474 J/m2), and thus is the most stable surface. Additionally, the (200) plane of the pseudoboehmite has the lowest absolute value of attachment energy (0.2106 eV/unit), consistent with having the strongest X-ray diffraction peak and indicating that it is the preferred orientation. The formation mechanism of flower-like and microspherical particles is discussed, with consideration for solution temperature effects on morphology and the predicted equilibrium morphology and growth morphology of pseudoboehmite.

Effect of debris accumulation on scour evolution at bridge pier in bank proximity

Abstract Large debris transported by flood affects scour features at bridge piers and increases the risks of structural failure. Geometric characteristics of the debris and the relative position of the pier with respect to the river bank are important parameters for the scour process. The interaction between the water flow and debris accumulation increases the shear stress, turbulence and consequently enhances the scour depth at the pier. This paper aims at analyzing such effects on scour evolution at bridge piers. To this end, two series of tests were carried out under clear water condition with different debris geometries and percentage blockage ratios. Experimental evidences showed that the pier position only influences scour evolution and equilibrium morphology for low water depths. Conversely, its effect becomes negligible for scour at bridge piers with debris accumulation and higher water depths. Useful practical relationships have been derived, with satisfactory prediction capability of the scour evolution for all the tested configurations.

Storm impacts on a coupled human-natural coastal system: Resilience of developed coasts

Human occupation of and alteration of the world's coast has transformed large stretches of it into Coupled Human-Natural Systems (CHANS) in which humans both influence and are influenced by coastal evolution. In such systems, human activity is as critical on natural resilience as processes and sediment supply derived from the natural setting. Pre- and post-storm observations of these interactions on the intensively developed Atlantic coast of the Gulf of Cádiz, (Spain and Portugal) are examined to determine natural and engineering resilience. Three case studies are used in three CHANS, showing that human interventions interact in complex ways with the natural system influencing post-storm recovery. In natural coasts, storm impact is assessed in terms of geomorphological response; on developed coasts, it is quantified as damage to infrastructure or loss of amenity. Preparedness, availability of resources, choice of response and the speed at which human agencies respond affect resilience for post-storm beach behaviour. Results show in some sites natural resilience adjusting by post-storm sediment transfers and an equilibrium morphology that may differ from pre-storm morphology; engineering resilience ensured that CHANS regained their pre-storm human infrastructure and amenity. Their management requires a fundamentally different approach to that of natural coastlines. The current immature stage of understanding of CHANS (especially the human preparedness and response components) is illustrated by the case studies presented where short-term political decisions and reactions to storms play a strong role in post-storm response. The nature and extent of many developed coasts as CHANS is slowly becoming more widely acknowledged, but to increase natural resilience and decrease vulnerability in CHANS better planning is required so that future storms are anticipated and when they happen, pre-planned human response actions are activated. Storms are an integral and inevitable element in the behaviour of coastal CHANS, not a disaster or emergency.

Fault‐controlled base‐of‐scarp deposits

AbstractThe term base‐of‐scarp is proposed for those submarine deposits controlled by a fault and physically disconnected from their more proximal counterpart located on the footwall, although genetically linked to it. These systems differ from conventional fault‐controlled deltas, such as shoal‐ and Gilbert‐type, because they are entirely subaqueous and lack equilibrium morphology—a steady state in which the system grows in size without altering its shape. We present field examples of fault‐controlled deposits from the Crati Basin and the Messina Strait (southern Italy) consisting of stratigraphic clastic wedges that thin towards and onlap onto the active margin with primary inclined bedding. Beds are composed of immature coarse‐grained gravels and sand, lack structures representative of wave‐action and reflect gravity‐driven processes such as debris flow, debris fall and high‐density turbidity currents. These deposits represent the unsteady‐state phase in which the system grows reducing its slope angle leading to conditions under which the unsteady state may eventually turn into a Gilbert‐type or shoal‐water system. A diagram for fault‐controlled base‐of‐scarp (B), Gilbert (G) and shoal‐water (S) deposits is presented, including their steady‐ and unsteady states, and the conceptual conditions under which a base‐of‐scarp system might evolve into Gilbert‐type or shoal‐water systems and vice versa.

SPH simulations of binary droplet deformation considering the Fowkes theory

To analyze the complex droplet transient deformation involving two immiscible phases, a practical multi-interfacial tension model has been introduced in the SPH framework. The attraction, which operates on the interface between the different phases is explicitly defined as London dispersion force using the Fowkes theoretical model, and the interfacial tension can be calculated using the sum of the attraction magnitudes corresponding to the surrounding particles. The analytical solution demonstrated good agreement with the simulated results corresponding to the macroscopic multicontact angles and nature of interphase interfacial forces. This simple model is widely applicable to the dynamic analysis of capillary forces involving two or more liquids, and we demonstrated its application to the simulation of the deformation behavior of a binary droplet that contacts a solid. The solid-liquid-liquid contact angle is not sufficient to predict the equilibrium morphology, and the momentum exchange between the two liquids should be considered.

First-principles study of the surface properties of uranium carbides

Uranium carbides have attracted renewed interest as advanced nuclear fuels for Generation IV reactors. As an important property required for gas bubble modeling in nuclear fuels, the surface energy of uranium carbides is scarce in literature. In this work, we study the surface properties of uranium carbides by first-principles density functional theory calculations. Surface orientations with maximum Miller index up to 3, 2 and 2 are investigated for UC, U2C3 and α-UC2, respectively. By studying the effects of surface termination and chemical potential on surface energy, we identify the factors that determines the surface stability. From the calculated surface energies, the surface properties of uranium carbide single crystals are obtained from Wulff construction, including equilibrium morphology, dominant surface orientation and area weighted surface energy.

Morphology evolution and dendrite growth in Li- and Mg-metal batteries: A potential dependent thermodynamic and kinetic multiscale ab initio study

It is well accepted that dendrites grow on anode surface of lithium-metal based batteries, while magnesium-metal batteries do not exhibit such a behavior. However, it has been recently shown experimentally and theoretically that magnesium can form uneven deposits that lead to similar safety concerns as lithium dendrites. To investigate a complex phenomenon such as dendritic growth from a theoretical point of view, it is necessary to study it on a multiscale level, i.e. from atomistic to meso scale. Herein we investigate thermodynamics, kinetics and morphology evolution of magnesium and lithium surfaces using density functional theory (DFT) and kinetic Monte Carlo (kMC) simulations. To be realistic for a device such as a battery, the electrochemical dimension has also to be included: this study was done within the framework of Grand canonical DFT approach that enables potential dependent DFT calculations, simulating relevant battery operating conditions. The atomistic potential dependent DFT results were upscaled using kMC simulations allowing to extract mesoscopic behavior at various operating potentials. The results of these calculations demonstrate the effect of potential on both the energetics and the dynamics of diffusion at the electrode-electrolyte interface. In particular, the equilibrium morphology of the metal electrode is strongly modified by the potential, leading to the stabilization of different surface orientations with potential. We show that the applied potential impacts atomic diffusion barriers leading to different surface mesoscopic relaxation times in the overall kinetics. The obtained results provide not only potential dependent information about systems morphology evolution, but also demonstrate the importance of using a suitable potential range (i.e. coherent with the experimental conditions): an unrealistic potential could lead to a very different behavior of the system on atomic and on meso scale.

DFT-D study of single water adsorption on low-index surfaces of calcium silicate phases in cement

The hydration of calcium silicate phases dominates the application of cement clinkers and has important practical implications. The reaction mechanisms between water and calcium silicate phases are complex and remain poorly understood at the atomic level. Herein, the single water adsorption on all low-index surfaces of β-C2S and M3-C3S was investigated using DFT-D calculations. The surface energy was calculated and the influence of surface cleavage discussed. Both dissociative and molecular adsorptions were investigated. Molecular adsorption was energetically favoured on β-C2S surfaces while dissociative adsorption was energetically favoured on M3-C3S surfaces. A Wulff construction was used to describe the equilibrium morphology with and without an adsorbed water molecule for the β-C2S phase. Water adsorption promoted the solid dissolution by weakening the bond among surface atoms. Electron transfer was observed mainly from the surface atoms to water atoms during the adsorption. These findings provide a novel insight into the adsorption of a water molecule on different calcium silicate surfaces using the same level of theory, which includes a representation of van der Waals forces, therefore, laying the foundation for better understanding the hydration mechanism.

Morphology Evolution of Hcp Cobalt Nanoparticles Induced by Ru Promoter

AbstractThe modification of the equilibrium morphology to expose active facets of cobalt nanoparticles is essential for tailoring and controlling its catalytic performance. Periodic density functional theory (DFT) was applied to systematically investigate the structure and stability of exposed facets of hcp Co nanoparticles induced by Ru promoter in order to clarify the facet‐dependent equilibrium morphology of hcp Co nanoparticles stabilized by Ru promoter. The adsorption and migration of Ru atom can give rise to the change of the surface energy of exposed facets, thus affecting the equilibrium morphology of the hcp Co nanoparticles. Selectively exposed the high Miller index facets, such as Co(10‐12), Co(11‐20) and Co(11‐21) surfaces, can be tuned by adding Ru promoter. Our theoretical calculation results show that Co(11‐21) becomes the predominantly exposed facet accompanied with the decrease of exposed Co(10‐10) and Co(10‐11) with the increase of Ru loading. Further experimental studies suggest that the area of selectively exposed the Co(10‐11) surface for uniform Co nanoplates loaded with 3.0 wt % Ru will be reduced, which is in agreement with the theoretical predicted result. This work is helpful to design the desired Co‐based FTS catalysts rationally with well‐controlled surface structure.

Slow-Motion Self-Assembly: Access to Intermediates with Heterochiral Peptides to Gain Control over Alignment Media Development.

Understanding the intermediates or transition states in organic reactions has made it possible to develop theories and to synthesize important compounds. In contrast to organic reaction intermediates and even protein folding intermediates, the intermediates of peptide/protein self-assembly are not very well understood. Here we report that the self-assembly kinetics of linear heterochiral peptides are significantly slower than those of the corresponding homochiral peptides, which enables direct microscopic observation of assembly intermediates. By designing racemic or asymmetric heterochiral peptides, we were able to discover unusual mixed helical (MP-helix) and overtwisted intermediates. The convergence of equilibrium morphology between the homochiral and heterochiral peptides enables us to reasonably deduce the unobservable intermediates of rapidly assembling homochiral peptides. By utilizing the discovered information about the assembly intermediates, we were able to develop a functional NMR alignment medium that enables the measurement of residual dipolar couplings (RDCs) in a time-dependent manner. Although much less studied than their cyclic counterparts, the linear form of heterochiral peptides provides a means of obtaining a more in-depth understanding of the self-assembly pathway and of developing sophisticated bottom-up materials.

Equilibrium Shape of 2D Nuclei Obtained from Spiral Hillocks on {010} Form of Gypsum

Growth patterns on the {010} form of gypsum crystals were investigated, under controlled temperature and supersaturation, by atomic force microscope. With the morphology of hillocks formed by growth spirals generated by screw dislocations, the two-dimensional (2D) equilibrium morphology on the {010} form of gypsum was calculated. Edge energies of the [001], [102], [101], [201], and [100] steps were obtained by measuring the spacing between hillocks steps running in the same direction. Their values range between 8 × 10–7 and 2.3 × 10–6 erg/cm. The calculated critical size of 2D nucleus on {010} form is quite large (∼3400 nm2) and witnesses the low supersaturation value at which experiments were done.

First-Principles Insights into the Interface Chemistry between 4-Aminothiophenol and Zinc Phosphide (Zn3P2) Nanoparticles.

Accurate prediction of the structures, stabilities, and electronic structures of hybrid inorganic/organic systems is an essential prerequisite for tuning their electronic properties and functions. Herein, the interface chemistry between the 4-aminothiophenol (4ATP) molecule and the (001), (101), and (110) surfaces of zinc phosphide (Zn3P2) has been investigated by means of first-principles density functional theory calculation with a correction for van der Waals interactions. In particular, the atomic-level insights into the fundamental aspects of the 4ATP adsorption, including the lowest-energy adsorption configurations, binding energetics, structural parameters, and electronic properties are presented and discussed. The 4ATP molecule is demonstrated to bind most strongly onto the least stable Zn3P2(001) surface (Eads = −1.91 eV) and least strongly onto the most stable Zn3P2(101) surface (Eads = −1.21 eV). Partial density of states analysis shows that the adsorption of 4ATP on the Zn3P2 surfaces is characterized by strong hybridization between the molecule’s sulfur and nitrogen p-orbitals and the d-orbitals of the interacting surface Zn ions, which gave rise to electron density accumulation around the centers of the newly formed Zn–S and Zn–N chemical bonds. The thermodynamic crystal morphology of the nonfunctionalized and 4ATP-functionalized Zn3P2 nanoparticles was obtained using Wulff construction based on the calculated surface energies. The stronger binding of the 4ATP molecule onto the less stable (001) and (110) surfaces in preference to the most stable (101) facet resulted in the modulation of the Zn3P2 nanocrystal shape, with the reactive (001) and (110) surfaces becoming more pronounced in the equilibrium morphology.

Morphology control of metallic nanoparticles supported on carbon substrates in catalytic conditions

Metallic nanoparticles are highly reactive objects, often used for their catalytic properties which strongly depend on their shape and morphology. Here we show that controlling the wetting properties on a substrate enables one to control the nanoparticle’s shape, prevent their coalescence and maximize their surface reactivity. The highly ordered mesoporous carbon structures (CMK) are ideal to achieve such a control. CMK can be tuned during their synthesis and display convex and concave surfaces capable of modifying the wetting properties and the morphology of the nanoparticles incorporated. On a concave substrate, the nanoparticle tends to spread on the surface of the substrate resulting in a platelet particle shape, while on flat or convex ones, the nanoparticle shows a limited wetting behavior corresponding to a spherical shape. In addition, the carbon enrichment of the metallic nanoparticles in contact with CMK plays a key role in controlling their equilibrium morphology. This atomic scale study allows us to better understand the interaction between metal nanoparticles and CMK in order to master their morphology and improve their reactivity.

Aerosol Optical Tweezers Constrain the Morphology Evolution of Liquid-Liquid Phase-Separated Atmospheric Particles

Summary Chemical models of atmospheric particles are vital in understanding the role of aerosol particles in atmospheric chemistry, air pollution, human health, and climate change. Advancing these models requires new frameworks that can realistically predict how critical particle properties evolve. We present such a framework for predicting particle phase separation and which morphology will prevail; this controls how each particle interacts with and affects the atmosphere. We studied the mixing behavior of α-pinene secondary organic aerosol (SOA) with different organic phases, as relative humidity was varied to determine the interplay between polarity, miscibility, interfacial tension, and the resulting morphology. Using measurements from aerosol optical tweezers experiments and literature data, a general trend in morphology with increasing atmospheric oxidation was observed, from biphasic partially engulfed (where both phases are immediately accessible to the gas phase) to biphasic core shell (where the organic shell conceals the core) and finally to a single-phase, homogeneous morphology.

Equilibrium morphology evolution of FCC cobalt nanoparticle under CO and hydrogen environments

Metal nanoparticles could change their shape and exposed certain facet under specific reaction conditions, resulting in greatly influence the performance of catalysts in heterogeneous catalysis. Combining spin-polarized density functional theory and ab initio atomistic thermodynamics, the equilibrium morphology evolution of FCC cobalt nanoparticle had been investigated under CO and hydrogen environments. The phase diagram clearly revealed the stable coverage could be affected by temperature and CO and hydrogen partial pressure. According to the surface energies of four surfaces, Wulff construction was introduced to characterize the morphologies of the Co catalyst at diverse temperatures and pressures. At the typical temperature (675 K) of hydrogen reduction, the estimated surface proportion ratio of the Co(3 1 1) exposed the B5 site could exist a marked increase, which was also consistent with the available Co catalyst reduced under experimental conditions. Our results would give direct insights into the understanding and quantitative description of structure evolution as well as active facets of the Co nanoparticles under the realistic FTS reaction condition.

Thermodynamics of low-index surfaces and particle shapes of Y(OH)3 under typical hydrothermal conditions

Yttrium hydroxide Y(OH)3 is an important precursor for yttrium oxides, and its morphologies are inherited by the yttrium oxides, thus affecting the products’ properties and performance. Hydrothermally synthesized Y(OH)3 particles are typically elongated hexagonal bipyramid, but thermodynamic parameters are inadequate for quantitative modeling of the morphology. We calculate surface energies and surface stresses of low-index orientations using density functional theory, and present a thermodynamic morphology model based on these parameters. We find that the thermodynamically stable surface terminations are always stoichiometric, and the surface energies are in the order of γ(101¯0)<γ(101¯1)<γ(112¯0)<γ(0001). The equilibrium morphology is of hexagonal prism capped by hexagonal pyramid enclosed by (101¯1) and (101¯0) surfaces. Because these terminations are stoichiometric, varying temperature or pressure is ineffective for modifying Y(OH)3 particles’ morphology, whereas surfactants should be used for this purpose in hydrothermal synthesis.