Exploring the surface properties of the liquid metals

Gallium based liquid metal alloys feature unique properties that set promising pathways for developing future technologies. In contrast to mercury, gallium is considered a non-hazardous option as a liquid metal. A variety of metals can be alloyed into gallium in order to create mixtures with exciting physical and chemical properties. Galinstan is a eutectic liquid metal alloy, consisting 68.5 wt% gallium, 21.5 wt% indium, and 10 wt% tin which possesses a lower (below 0 ºC) freezing point and higher surface tension than that of gallium liquid metal. Surface tension manipulations of liquid metals in electrolyte environments play an important role in the development of their emerging applications in electrochemical systems and microfluidics. However, the effects of electrolytes on the surface tension of liquid metals were not fully explored and an opportunity existed for a comprehensive study of such liquids. Due to significant potential outcomes, exploring the effects of different electrolyte types and concentrations on gallium based liquid metal dynamics was chosen as one of the main foci of this research. In this PhD thesis, the author establishes a new framework to study the effects of different acidic and basic electrolytes on dynamics of galinstan droplets. The droplet is placed in a recess between two microfluidic channels. Each of the channels carries different acidic and basic electrolytes to induce pH gradient across the droplet. Adjusting pH gradient by changing the concentrations of electrolytes on either side of droplets is shown to change the potential of zero charge and the chemical potentials. The author of this thesis hypothesises that according to the Lippmann’s equation this condition should induce surface tension gradient to the droplet. The PhD candidate observes two distinct dynamics of deformation and surface Marangoni flow. To measure the deformation, the author mathematically modelled the change in shape as an aspect ratio between major and minor axes. The surface Marangoni flow is measured by adding micro particles to the electrolytes and tracking their velocities on the surface of the droplet. When low concentrations of electrolytes are used, the hydroxides are less dominant on the surface to enable the deformation of the liquid metal. Instead, when concentrations of both acidic and basic electrolytes are increased, the surface hydroxides are thickened. The formation of thicker surface hydroxides leads to less droplet deformability. Under this condition, the surface tension gradient appears mostly as surface Marangoni flows. The deformation and surface Marangoni flows are replicated and characterised by applying external potentials. The PhD candidate derives theoretical equations for both dynamics which are in good agreement with the experimental results. The author also became involved in the development of applications in low pH gradients when the deformation is the more dominant dynamic. The author discovers that under the deformation dominant conditions, self-propulsion of the droplets is possible. Applications including pumping and switching are demonstrated by only changing the electrolytes surrounding the droplet without the need for an external power supply. After investigating the surface manipulation of liquid metals using electrolyte environments, this PhD research is extended to explore the fundamental understanding of the oxide compounds that form on the surface of gallium liquid metals. In ambient conditions, self-limiting layers of metal oxides spontaneously form on the surface of liquid metal alloys. These oxide layers are naturally occurring two-dimensional (2D) materials. The author of this PhD thesis demonstrates two novel methods for delaminating the surface oxides. In the first method, surface oxides are directly transferred onto a substrate via a van der Waals exfoliation technique. In the second method, a reactor is devised for the large scale production of the 2D sheets. During this process, compressed air is injected into the liquid metal and the 2D exfoliated sheets are collected in a solvent that is situated above the liquid metal. PhD candidate then investigates the possibility of modifying the composition of the surface oxides. To achieve this, the author of the thesis uses galinstan liquid metal as a reaction solvent and incorporates other metals, such as transition metals, post transition metals and lanthanides. The author hypothesised that the surface oxide composition is dominated by the metal oxide with more favourable Gibbs energy of oxide reaction in comparison to the base alloy. Author uses this method to synthesise the thinnest ever reported 2D hafnium oxide sheets and develops insulating layers for applications in electronics. In the final stage of this PhD project, the author of this PhD thesis furthers this concept to grow low dimensional materials at the interface of galinstan liquid metal with water molecules. 2D sheet and One-dimensional (1D) fibre morphologies of aluminium oxide compounds are successfully produced by exposing galinstan-aluminium alloy to liquid and vapour phases of water molecules, respectively. Annealing is shown to retain both 1D and 2D morphologies when aluminium oxide hydroxides are converted into aluminium oxides. Both 1D and 2D oxide hydroxides of aluminium feature very large surface areas. The author demonstrates that each of the morphologies have exciting and unique characteristics. 1D morphologies that are grown from the surface of liquid metal are presented to have high transparency. Instead, the 2D grown morphologies are of strong stiffness. Freestanding membranes are made from the self-assembled 2D structures for the filtration of lead contaminated water and oil-water separation. Membranes demonstrate extraordinary high flux rates which author believes to be due to the highly wrinkled structures of the 2D sheets. A green and sustainable synthesis method is presented to reuse liquid metal for many synthesis cycles. The synthesis cycle is shown to repeat several times with negligible loss of liquid metal and a yield of 100%. The synthesis cycles are shown to only require aluminium and water as pre-cursers. Altogether the author successfully demonstrates several significant discoveries in the course of this PhD research, developed new concepts based on liquid metals and also created a few unique devices with extraordinary properties. It is expected that the outcomes of this PhD research to impact future of many industries including electronics, optics and also filtration

Wafer-scale two-dimensional semiconductors from printed oxide skin of liquid metals

A variety of deposition methods for two-dimensional crystals have been demonstrated; however, their wafer-scale deposition remains a challenge. Here we introduce a technique for depositing and patterning of wafer-scale two-dimensional metal chalcogenide compounds by transforming the native interfacial metal oxide layer of low melting point metal precursors (group III and IV) in liquid form. In an oxygen-containing atmosphere, these metals establish an atomically thin oxide layer in a self-limiting reaction. The layer increases the wettability of the liquid metal placed on oxygen-terminated substrates, leaving the thin oxide layer behind. In the case of liquid gallium, the oxide skin attaches exclusively to a substrate and is then sulfurized via a relatively low temperature process. By controlling the surface chemistry of the substrate, we produce large area two-dimensional semiconducting GaS of unit cell thickness (∼1.5 nm). The presented deposition and patterning method offers great commercial potential for wafer-scale processes

Surface water dependent properties of sulphur-rich molybdenum sulfides: electrolyteless gas phase water splitting

Sulfur-rich molybdenum sulfides are an emerging class of inorganic coordination polymers that are predominantly utilized for their superior catalytic properties. Here we investigate surface water dependent properties of sulfur-rich MoSx (x = 32/3) and its interaction with water vapor. We report that MoSx is a highly hygroscopic semiconductor, which can reversibly bind up to 0.9 H2O molecule per Mo. The presence of surface water is found to have a profound influence on the semiconductor's properties, modulating the material's photoluminescence by over 1 order of magnitude, in transition from dry to moist ambient. Furthermore, the conductivity of a MoSx-based moisture sensor is modulated in excess of 2 orders of magnitude for 30% increase in humidity. As the core application, we utilize the discovered properties of MoSx to develop an electrolyteless water splitting photocatalyst that relies entirely on the hygroscopic nature of MoSx as the water source. The catalyst is formulated as an ink that can be coated onto insulating substrates, such as glass, leading to efficient hydrogen and oxygen evolution from water vapor. The concept has the potential to be widely adopted for future solar fuel production

Corrigendum: Wafer-scale two-dimensional semiconductors from printed oxide skin of liquid metals.

Nature Communications 8: Article number: 14482; published: 17 February 2017; Updated: 22 March 2017 The original version of this Article contained a typographical error in the spelling of the author Omid Kavehei, which was incorrectly given as Omid Kevehei. This has now been corrected in both the PDF and HTML versions of the Article.</jats:p

Wafer-scale two-dimensional semiconductors from printed oxide skin of liquid metals

© The Author(s) 2017. A variety of deposition methods for two-dimensional crystals have been demonstrated; however, their wafer-scale deposition remains a challenge. Here we introduce a technique for depositing and patterning of wafer-scale two-dimensional metal chalcogenide compounds by transforming the native interfacial metal oxide layer of low melting point metal precursors (group III and IV) in liquid form. In an oxygen-containing atmosphere, these metals establish an atomically thin oxide layer in a self-limiting reaction. The layer increases the wettability of the liquid metal placed on oxygen-terminated substrates, leaving the thin oxide layer behind. In the case of liquid gallium, the oxide skin attaches exclusively to a substrate and is then sulfurized via a relatively low temperature process. By controlling the surface chemistry of the substrate, we produce large area two-dimensional semiconducting GaS of unit cell thickness (∼1.5 nm). The presented deposition and patterning method offers great commercial potential for wafer-scale processes

Chemical analysis of acoustically levitated drops by Raman spectroscopy

An experimental apparatus combining Raman spectroscopy with acoustic levitation, Raman acoustic levitation spectroscopy (RALS), is investigated in the field of physical and chemical analytics. Whereas acoustic levitation enables the contactless handling of microsized samples, Raman spectroscopy offers the advantage of a noninvasive method without complex sample preparation. After carrying out some systematic tests to probe the sensitivity of the technique to drop size, shape, and position, RALS has been successfully applied in monitoring sample dilution and preconcentration, evaporation, crystallization, an acid–base reaction, and analytes in a surface-enhanced Raman spectroscopy colloidal suspension

A gallium-based magnetocaloric liquid metal ferrofluid

We demonstrate a magnetocaloric ferrofluid based on a gadolinium saturated liquid metal matrix, using a gallium-based liquid metal alloy as the solvent and suspension medium. The material is liquid at room temperature, while exhibiting spontaneous magnetization and a large magnetocaloric effect. The magnetic properties were attributed to the formation of gadolinium nanoparticles suspended within the liquid gallium alloy, which acts as a reaction solvent during the nanoparticle synthesis. High nanoparticle weight fractions exceeding 2% could be suspended within the liquid metal matrix. The liquid metal ferrofluid shows promise for magnetocaloric cooling due to its high thermal conductivity and its liquid nature. Magnetic and thermoanalytic characterizations reveal that the developed material remains liquid within the temperature window required for domestic refrigeration purposes, which enables future fluidic magnetocaloric devices. Additionally, the observed formation of nanometer-sized metallic particles within the supersaturated liquid metal solution has general implications for chemical synthesis and provides a new synthetic pathway toward metallic nanoparticles based on highly reactive rare earth metals

Spring Graduation 2016 - 8:30am Commencement

Published on Jun 8, 2017 Commencement for candidates from the School of Health Professions and Seventh-day Adventist Theological Seminary.https://digitalcommons.andrews.edu/auvideo/1119/thumbnail.jp

Nickel Phosphides Electrodeposited on TiO2 Nanotube Arrays as Electrocatalysts for Hydrogen Evolution

Nickel phosphide (NiPx)-based nanocomposites emerged as a new class of highly promising non-noble-metal-based electrocatalysts for hydrogen evolution reactions (HER). However, conventional synthesis for this type of nanocomposite involves using harsh organic solvents and multiple complicated procedures as well as the release of concomitant toxic gases, thus significantly restricting their practical applications. To explore a much greener and more sustainable synthesis approach, a facile one-step electrodeposition technique for nanoelectrode toward HER is developed in this research, in which amorphous NiPx is anchored onto TiO2 nanotube arrays to form a nanocomposite material. The synthesized nanocomposites have a synergistic coupling of material properties in two components of the nanocomposites, with an enhanced charge transfer and electrochemically active surface areas. Because of such unique characteristics, the system exhibits a remarkable catalytic reduction activity and superior stability in alkaline electrolytes, requiring overpotentials of 104 and 129 mV to achieve current densities of 10 and 20 mA cm-2, respectively. This method provides an effective and green approach to fabricate NiPx-based nanocomposites with enhanced HER performances