Crystal face dependent intrinsic wettability of metal oxide surfaces.

Knowledge of intrinsic wettability at solid/liquid interfaces at the molecular level perspective is significant in understanding crucial progress in some fields, such as electrochemistry, molecular biology and earth science. It is generally believed that surface wettability is determined by the surface chemical component and surface topography. However, when taking molecular structures and interactions into consideration, many intriguing phenomena would enrich or even redress our understanding of surface wettability. From the perspective of interfacial water molecule structures, here, we discovered that the intrinsic wettability of crystal metal oxide is not only dependent on the chemical components but also critically dependent on the crystal faces. For example, the [Formula: see text] crystal face of α-Al2O3 is intrinsically hydrophobic with a water contact angle near 90°, while another three crystal faces are intrinsically hydrophilic with water contact angles <65°. Based on surface energy analysis, it is found that the total surface energy, polar component and Lewis base portion of the hydrophobic crystal face are all smaller than the other three hydrophilic crystal faces indicating that they have different surface states. DFT simulation further revealed that the adsorbed interfacial water molecules on each crystal face hold various orientations. Herein, the third crucial factor for surface wettability from the perspective of the molecular level is presented, that is the orientations of adsorbed interfacial water molecules apart from the macro-level chemical component and surface topography. This study may serve as a source of inspiration for improving wetting theoretical models and designing controllable wettability at the molecular/atomic level.

Interactions in stanene centred van der Waals trilayers structures of boron-nitride and graphene: effect of mirror symmetry on electronic interactions.

Dispersion-corrected density functional theory was used to investigate structures consisting of a stanene layer sandwiched between atomically-thin boron nitride and graphene. The parameters controlling the mirror symmetry, lattice rotation and stacking sequences were varied systematically to generate fifteen candidate trilayers. Two types of structural buckling occur in the heterostructures depending on whether the lattice vectors are co-aligned or non-collinear. The configurations with the honeycomb lattices rotated by π/6 with respect to the stanene generally have lower binding energy. In the majority of the trilayers, the electronic structures deviate strongly from the band structures of the isolated components. The boron nitride/stanene/boron nitride structure is identified as a special case where stanene has an electronic structure that is not perturbed by interlayer interactions and resembles the ideal monolayer form. For the other candidate structures, however, interlayer interactions drive significant modifications in the electronic structure thus indicating emergent features that go beyond the pure van der Waals description.

Voltage-induced penetration effect in liquid metals at room temperature.

Room-temperature liquid metal is discovered to be capable of penetrating through macro- and microporous materials by applying a voltage. The liquid metal penetration effects are demonstrated in various porous materials such as tissue paper, thick and fine sponges, fabrics, and meshes. The underlying mechanism is that the high surface tension of liquid metal can be significantly reduced to near-zero due to the voltage-induced oxidation of the liquid metal surface in a solution. It is the extremely low surface tension and gravity that cause the liquid metal to superwet the solid surface, leading to the penetration phenomena. These findings offer new opportunities for novel microfluidic applications and could promote further discovery of more exotic fluid states of liquid metals.

Discovery of a Voltage-Stimulated Heartbeat Effect in Droplets of Liquid Gallium.

Chemomechanical effects are known to initiate fluid oscillations in certain liquid metals; however, they typically produce an irregular motion that is difficult to deactivate or control. Here we show that stimulating liquid gallium with electrochemistry can cause a metal drop to exhibit a heart beating effect by shape shifting at a telltale frequency. Unlike the effects reported in the past for mercury, the symmetry-breaking forces generated by using gallium propel the drop several millimeters with velocities of the order of 1 cm per second. We demonstrate pulsating dynamics between 0 and 610 beats per minute for 50-150 µL droplets in a NaOH electrolyte at 34 °C. The underlying mechanism is a self-regulating cycle initiated by fast electrochemical oxidation that adjusts the drop's surface tension and causes a transformation from spherical to pancake form, followed by detachment from the circular electrode. As the beat frequency can be activated and controlled using a dc voltage, the electrochemical mechanism opens the way for fluid-based timers and actuators.

Tuning the electronic structure in stanene/graphene bilayers using strain and gas adsorption.

Epitaxial growth of stanene monolayers on graphene substrates is an attractive synthesis route for atomically-thin electronic components, however, it remains unclear how such composites will tolerate lattice strain and exposure to ambient atmosphere. Using density functional theory, we identified several epitaxial configurations for the stanene-graphene bilayer system and determined the effect of strain and water adsorption. In addition to previously reported co-aligned bilayers, we identify a second family of low energy structures involving rotation of one layer by thirty degrees. The band structures of the rotated configurations exhibit a fully metallic interface, whereas the co-aligned structures are poised at the transition between semimetallic and semiconductor characteristics. In general, the electronic states are directly correlated with differences in the buckling parameter of the tin layer assigned to the competition between sp2 and sp3 hybridization schemes. This can be controlled by strain to yield a metal-insulator transition in special circumstances. For the equilibrium structure, H2O preferentially adsorbs on the stanene layer, and the system remains metallic with a mixture of Dirac and parabolic bands at the Fermi surface.

Defect introduced paramagnetism and weak localization in two-dimensional metal VSe2.

We have carried out a detailed investigation of the magnetism, valence state, and magnetotransport in VSe2 bulk single crystals, as well as in laminates obtained by mechanical exfoliation. In sharp contrast to the ferromagnetic behavior reported previously, here, no ferromagnetism could be detected for VSe2 single crystal and laminate from room temperature down to 2 K. Neither did we find the Curie paramagnetism expected due to the 3d 1 odd-electronic configuration of covalent V4+ ions. Rather, intrinsic VSe2 is a non-magnetic alloy without local moment. Only a weak paramagnetic contribution introduced by defects is noticeable below 50 K. A weak localization effect due to defects was also observed in VSe2 single crystals for the first time.

Desert Beetle-Inspired Superwettable Patterned Surfaces for Water Harvesting.

With the impacts of climate change and impending crisis of clean drinking water, designing functional materials for water harvesting from fog with large water capacity has received much attention in recent years. Nature has evolved different strategies for surviving dry, arid, and xeric conditions. Nature is a school for human beings. In this contribution, inspired by the Stenocara beetle, superhydrophilic/superhydrophobic patterned surfaces are fabricated on the silica poly(dimethylsiloxane) (PDMS)-coated superhydrophobic surfaces using a pulsed laser deposition approach with masks. The resultant samples with patterned wettability demonstrate water-harvesting efficiency in comparison with the silica PDMS-coated superhydrophobic surface and the Pt nanoparticles-coated superhydrophilic surface. The maximum water-harvesting efficiency can reach about 5.3 g cm-2 h-1 . Both the size and the percentage of the Pt-coated superhydrophilic square regions on the patterned surface affect the condensation and coalescence of the water droplet, as well as the final water-harvesting efficiency. The present water-harvesting strategy should provide an avenue to alleviate the water crisis facing mankind in certain arid regions of the world.