Active sites for ice nucleation differ depending on nucleation mode

The nucleation of ice crystals in clouds is poorly understood, despite being of critical importance for our planet's climate. Nucleation occurs largely at rare "active sites" present on airborne particles such as mineral dust, but the nucleation pathway is distinct under different meteorological conditions. These give rise to two key nucleation pathways where a particle is either immersed in a supercooled liquid water droplet (immersion freezing mode) or suspended in a supersaturated vapor (deposition mode). However, it is unclear if the same active sites are responsible for nucleation in these two modes. Here, we directly compare the sites that are active in these two modes by performing immersion freezing and deposition experiments on the same thin sections of two atmospherically important minerals (feldspar and quartz). For both substrates, we confirm that nucleation is dominated by a limited number of sites and show that there is little correlation between the two sets of sites operating in each experimental method: across both materials, only six out of 73 sites active for immersion freezing nucleation were also active for deposition nucleation. Clearly, different properties determine the activity of nucleation sites for each mode, and we use the pore condensation and freezing concept to argue that effective deposition sites have size and/or geometry requirements not of relevance to effective immersion freezing sites. Hence, the ability to nucleate is pathway dependent, and the mode of nucleation has to be explicitly considered when applying experimental data in cloud models. [Abstract copyright: Copyright © 2021 the Author(s). Published by PNAS.

One-step fabrication of hollow-channel gold nanoflowers with excellent catalytic performance and large single-particle SERS activity.

Hollow metallic nanostructures have shown potential in various applications including catalysis, drug delivery and phototherapy, owing to their large surface areas, reduced net density, and unique optical properties. In this study, novel hollow gold nanoflowers (HAuNFs) consisting of an open hollow channel in the center and multiple branches/tips on the outer surface are fabricated for the first time, via a facile one-step synthesis using an auto-degradable nanofiber as a bifunctional template. The one-dimensional (1D) nanofiber acts as both a threading template as well as a promoter of the anisotropic growth of the gold crystal, the combination of which leads to the formation of HAuNFs with a hollow channel and nanospikes. The synergy of favorable structural/surface features, including sharp edges, open cavity and high-index facets, provides our HAuNFs with excellent catalytic performance (activity and cycling stability) coupled with large single-particle SERS activity (including ∼30 times of activity in ethanol electro-oxidation and ∼40 times of single-particle SERS intensity, benchmarked against similar-sized solid gold nanospheres with smooth surfaces, as well as retaining 86.7% of the initial catalytic activity after 500 cycles in ethanol electro-oxidation). This innovative synthesis gives a nanostructure of the geometry distinct from the template and is extendable to fabricating other systems for example, hollow-channel silver nanoflowers (HAgNFs). It thus provides an insight into the design of hollow nanostructures via template methods, and offers a versatile synthetic strategy for diverse metal nanomaterials suited for a broad range of applications

The role of phase separation and related topography in the exceptional ice-nucleating ability of alkali feldspars

Our understanding of crystal nucleation is a limiting factor in many fields, not least in the atmospheric sciences. It was recently found that feldspar, a component of airborne desert dust, plays a dominant role in triggering ice formation in clouds, but the origin of this effect was unclear. By investigating the structure/property relationships of a wide range of feldspars, we demonstrate that alkali feldspars with certain microtextures, related to phase separation into Na and K-rich regions, show exceptional ice-nucleating abilities in supercooled water. We found no correlation between ice-nucleating efficiency and the crystal structures or the chemical compositions of these active feldspars, which suggests that specific topographical features associated with these microtextures are key in the activity of these feldspars. That topography likely acts to promote ice nucleation, improves our understanding of ice formation in clouds, and may also enable the design and manufacture of bespoke nucleating materials for uses such as cloud seeding and cryopreservation

The Intrinsic Resolution Limit in the Atomic Force Microscope: Implications for Heights of Nano-Scale Features

Background; Accurate mechanical characterization by the atomic force microscope at the highest spatial resolution requires that topography is deconvoluted from indentation. The measured height of nanoscale features in the atomic force microscope (AFM) is almost always smaller than the true value, which is often explained away as sample deformation, the formation of salt deposits and/or dehydration. We show that the real height of nano-objects cannot be obtained directly: a result arising as a consequence of the local probe-sample geometry. Methods and Findings; We have modeled the tip-surface-sample interaction as the sum of the interaction between the tip and the surface and the tip and the sample. We find that the dynamics of the AFM cannot differentiate between differences in force resulting from 1) the chemical and/or mechanical characteristics of the surface or 2) a step in topography due to the size of the sample; once the size of a feature becomes smaller than the effective area of interaction between the AFM tip and sample, the measured height is compromised. This general result is a major contributor to loss of height and can amount to up to ∼90% for nanoscale features. In particular, these very large values in height loss may occur even when there is no sample deformation, and, more generally, height loss does not correlate with sample deformation. DNA and IgG antibodies have been used as model samples where experimental height measurements are shown to closely match the predicted phenomena. Conclusions; Being able to measure the true height of single nanoscale features is paramount in many nanotechnology applications since phenomena and properties in the nanoscale critically depend on dimensions. Our approach allows accurate predictions for the true height of nanoscale objects and will lead to reliable mechanical characterization at the highest spatial resolution

Confinement generates single-crystal aragonite rods at room temperature

The topic of calcite and aragonite polymorphism attracts enormous interest from fields including biomineralization and paleogeochemistry. While aragonite is only slightly less thermodynamically stable than calcite under ambient conditions, it typically only forms as a minor product in additive-free solutions at room temperature. However, aragonite is an abundant biomineral, and certain organisms can selectively generate calcite and aragonite. This fascinating behavior has been the focus of decades of research, where this has been driven by a search for specific organic macromolecules that can generate these polymorphs. However, despite these efforts, we still have a poor understanding of how organisms achieve such selectivity. In this work, we consider an alternative possibility and explore whether the confined volumes in which all biomineralization occurs could also influence polymorph. Calcium carbonate was precipitated within the cylindrical pores of track-etched membranes, where these enabled us to systematically investigate the relationship between the membrane pore diameter and polymorph formation. Aragonite was obtained in increasing quantities as the pore size was reduced, such that oriented single crystals of aragonite were the sole product from additive-free solutions in 25-nm pores and significant quantities of aragonite formed in pores as large as 200 nm in the presence of low concentrations of magnesium and sulfate ions. This effect can be attributed to the effect of the pore size on the ion distribution, which becomes of increasing importance in small pores. These intriguing results suggest that organisms may exploit confinement effects to gain control over crystal polymorph

Confinement increases the lifetimes of hydroxyapatite precursors

The mineral component of bone is a carbonated, nonstoichiometric hydroxyapatite (calcium phosphate) that forms in nanometer confinement within collagen fibrils, the principal organic constituent of bone. We here employ a model system to study the effects of confinement on hydroxyapatite precipitation from solution under physiological conditions. In common with earlier studies of calcium carbonate and calcium sulfate precipitation, we find that confinement significantly prolongs the lifetime of metastable phases, here amorphous calcium phosphate (ACP) and octacalcium phosphate (OCP). The effect occurs at surprisingly large separations of up to 1 μm, and at 0.2 μm the lifetime of ACP is extended by at least an order of magnitude. The soluble additive poly(aspartic acid), which in bulk stabilizes ACP, appears to act synergistically with confinement to give a greatly enhanced stability of ACP. The reason for the extended lifetime appears to be different from that found with CaCO3 and CaSO4, and underscores both the variety of mechanisms whereby confinement affects the growth and transformation of solid phases, and the necessity to study a wide range of crystalline systems to build a full understanding of confinement effects. We suggest that in the case of ACP and OCP the extended lifetime of these metastable phases is chiefly due to a slower transport of ions between a dissolving metastable phase, and the more stable, growing phase. These results highlight the potential importance of confinement on biomineralization processes

Forces between mica surfaces in ethylene glycol

Measurements are presented of the force as a function of separation between two molecularly smooth mica surfaces immersed in ethylene glycol, and in solutions of lithium chloride and sulfuric acid in ethylene glycol. At surface separations greater than 3 nm the measured force is in very good agreement with double-layer theory, but at smaller separations there is an oscillatory solvation force which is superimposed on the double-layer repulsion. In contrast to the case in water, the adsorption of hydrogen ions at the mica surface does not markedly affect the short-range forces.<br /

Dataset for "Nucleation- and Emergence-Limited Growth of Ice from Pores"

Nucleation of ice from vapour on atmospheric aerosols has been attributed to the condensation and freezing of supercooled water in small pores. This dataset accompanies a paper in which we use wedge pores on mica to directly observe the growth of ice in confinement prior to the growth of bulk crystals. We report a transition in behaviour with decreasing temperature: at low temperatures the limiting step is not nucleation, but a free energy barrier associated with growth of ice through a narrow pore mouth to become a bulk phase