Synthesis of low-melting-point metallic nanoparticles with an ultrasonic nanoemulsion method

Sonochemistry of molten metals

Ultrasonic irradiation of molten metals in liquid media causes dispersion of the metals into suspensions of micro- and nanoparticles that can be separated. This is applicable mainly to low-m_p elemental metals or alloys, but higher m_p elemental metals or alloys were also reported. Among metals, mercury and gallium exhibit especially-low melting points and are thus considered as liquid metals (LMs). Sonication of mercury in aqueous solutions of certain metal ions can cause simultaneous reduction of the ions and reactions between the metals. Gallium can be melted and sonicated in warm water, as well as in aqueous solutions of various solutes such as metal ions and organic compounds, which opened a wide window of interactions between the gallium particles and the solutes. Sonication of molten metals in organic liquids, such as polyethylene glycol (PEG) 400, forms carbon dots (C-dots) doped with nanoparticles of these metals. This review article summarizes the various interactions and reactions that occur upon sonication of metals in liquid media.

Ultrasound-assisted production and optimization of mini-emulsions in a microfluidic chip in continuous-flow

The use of ultrasound to generate mini-emulsions (50 nm to 1 μm in diameter) and nanoemulsions (mean droplet diameter < 200 nm) is of great relevance in drug delivery, particle synthesis and cosmetic and food industries. Therefore, it is desirable to develop new strategies to obtain new formulations faster and with less reagent consumption. Here, we present a polydimethylsiloxane (PDMS)-based microfluidic device that generates oil-in-water or water-in-oil mini-emulsions in continuous flow employing ultrasound as the driving force. A Langevin piezoelectric attached to the same glass slide as the microdevice provides enough power to create mini-emulsions in a single cycle and without reagents pre-homogenization. By introducing independently four different fluids into the microfluidic platform, it is possible to gradually modify the composition of oil, water and two different surfactants, to determine the most favorable formulation for minimizing droplet diameter and polydispersity, employing less than 500 µL of reagents. It was found that cavitation bubbles are the most important mechanism underlying emulsions formation in the microchannels and that degassing of the aqueous phase before its introduction to the device can be an important factor for reduction of droplet polydispersity. This idea is demonstrated by synthetizing solid polymeric particles with a narrow size distribution starting from a mini-emulsion produced by the device.

Comparison between multi-walled carbon nanotubes and titanium dioxide nanoparticles as additives on performance of turbine meter oil nano lubricant

This research aims of compare the impact of the mass fraction of multi-walled carbon nanotubes (MWCNTs) and titanium dioxide (TiO₂) nano additive on the tribological and thermophysical attributes of turbine meter oil. These attributes include the average friction coefficient, pressure drop, wear, flash point, pour point, relative viscosity, kinematics viscosity, and viscosity index. The pressure drops and the average friction coefficient inside the copper tube were simulated and compared with experimental results. In this study, for the synthesis of nano lubricants from turbine meter oil as a pure fluid and from MWCNTs and TiO₂ as nano additives in the mass fraction of 0.05, 0.1, 0.2, 0.3, and 0.4 wt.% and from oleic acid and Triton x100 as surfactants were utilized. The results illustrated that the wear depth of copper pins in the presence of nano lubricant with 0.4 wt.% of MWCNTs and 0.1 wt.% TiO₂ was improved by 88.26% and 71.43%, respectively. Increasing 0.3 wt.% of TiO₂ and MWCNTs into the oil caused to improvement in viscosity index. The simulation data and experimental data for the pressure drop were closer together and indicated a minor error that the maximum error is less than 10%.

Size of liquid metal particles influences actuation properties of a liquid crystal elastomer composite

Composites of liquid crystal elastomer (LCE) that are electrically conductive have the potential to function as soft "artificial muscle" actuators that can be reversibly stimulated with electrical Joule-heating. Conductivity can be achieved by embedding the LCE with droplets of an alloy of gallium and indium that is liquid at room temperature. These soft artificial muscles are capable of >50% reversible actuation with an applied load. The key to actuation at high loadings of liquid metal (LM) is that the droplets deform with the surrounding matrix. By controlling the size of LM droplets through simple processing techniques, we show that the actuator properties of the LM-LCE muscle can be tuned. For example, composites with smaller liquid metal particles (ca. 10 μm or less) are stiffer than those with larger liquid metal particles (ca. >100 μm) and are capable of greater force output. However, smaller particles reduce actuation strain and composites with large particles exhibit significantly greater stroke length. Such tunability in actuation properties permits the fabrication of specialized soft artificial muscles, where processing of the composite controls actuation strain and actuation force.

Melting and Recrystallization of Copper Nanoparticles Prepared by Microwave-Assisted Reduction in the Presence of Triethylenetetramine

The small sized copper nanoparticles (Cu-NPs), prepared in the presence of triethylene tetramine (TETA) and assisted with microwave irradiation, have an extremely low melting temperature. Melting of the small sizezd Cu-NPs can be triggered by the heat generated from the e-beam irradiation during SEM and TEM image construction. The dispersed Cu atoms around the agglomerated big Cu particles can undergo recrystallization immediately due to the strong driving force of the huge temperature difference to normal melting temperature (T_m = 1085 °C). Some of the Cu-NPs with bigger sizes also recrystallize and agglomerate into dense, big particles. According to X-ray diffraction patterns, these particles can agglomerate into compact, ordered Cu crystals in less than five minutes at 60 °C. The melting and recrystallization related endothermic and exothermic phase transitions of Cu-NPs can be found from differential scanning calorimeter (DSC) thermograms and optical microscopic pictures.

Synthesis and multifunctionality of (CeO2-NiO) nanocomposites synthesized via sonochemical technique

CeO₂, NiO and their nanocomposite were synthesized using facile sonochemical technique. XRD assure single phase CeO₂ and NiO while the nanocomposite consists of the two phases only. CeO₂ nanoparticles possess cubic shape, NiO was formed in nanorods, and CeO₂ decorated the NiO nanorods in the nanocomposite. The magnetic behavior of the nanocomposite lies between those of the two parents with a ferromagnetic tendency. Metal oxide nanoparticles acted as catalyst in the formation of carbon nanofibers (CNFs), while the nanocomposite leads to the production of carbon nanotubes. The photocatalyst (CeO₂-NiO) achieved complete dye degradation (100%) in light for the tested dye at 50 min. The decay products were analyzed using GC mass confirming mineralization of Bb red dye.

Formation of Metal Nanoparticles Directly from Bulk Sources Using Ultrasound and Application to E-Waste Upcycling

A method for creating nanoparticles directly from bulk metal by applying ultrasound to the surface in the presence of a two-part surfactant system is presented. Implosive collapse of cavitation bubbles near the bulk metal surface generates powerful microjets, leading to material ejection. This liberated material is captured and stabilized by a surfactant bilayer in the form of nanoparticles. The method is characterized in detail using gold, but is also demonstrated on other metals and alloys, and is generally applicable. It is shown that nanoparticles can be produced regardless of the bulk metal form factor, and the method is extended to an environmentally important problem, the reclamation of gold from an electronic waste stream.

An Experimental Study on Flow and Heat Transfer Characteristics of Ethanol/Polyalphaolefin Nanoemulsion Flowing Through Circular Minichannels

This work experimentally studied the convective flow and heat transfer characteristics of a novel nanostructured heat transfer fluid: "ethanol/polyalphaolefin nanoemulsion" flowing through 12 circular minichannels of 1-mm diameter each. Ethanol/polyalphaolefin nanoemulsion is a thermodynamically stable system formed by dispersing ethanol into a mixture of "polyalphaolefin (PAO)" and surfactants. In this study, ethanol/PAO nanoemulsion is used as the working fluid to study the effect of ethanol nanodroplets on its convective flow and heat transfer characteristics. In addition, the effect of flow regime on its heat transfer is examined. It is found that using ethanol/PAO nanoemulsion fluids can improve convective heat transfer compared to that of pure PAO under both single- and two-phase flow regimes. For single-phase flow, there is no significant difference in Nusselt number between ethanol/PAO nanoemulsion and pure PAO in laminar flow regime. However, when entering transition flow regime, the ethanol/PAO nanoemulsion fluid showed a substantial increase in Nusselt number. Meanwhile, there is an increase in pressure drop and early onset of the laminar-turbulent transitional region for the ethanol/PAO nanoemulsion compared to pure PAO. The heat transfer coefficient of ethanol/PAO nanoemulsion can be further enhanced when the ethanol nanodroplets undergo phase change, which is hypothesized that such an effect is likely related to the enhanced interfacial thermal transport between the nanodroplets and base fluid under elevated temperature and the latent heat of phase changeable nanodroplets inside nanoemulsion. Further studies are needed to fully explore the convective heat transfer properties of nanoemulsion fluids.

Ultrasonic assisted synthesis of palladium-nickel/iron oxide core-shell nanoalloys as effective catalyst for Suzuki-Miyaura and p-nitrophenol reduction reactions

In this study, ultrasonic assisted synthesis of Pd-Ni/Fe₃O₄ core-shell nanoalloys is reported. Unique reaction condition was prepared by ultrasonic irradiation, releasing the stored energy in the collapsed bubbles and heats the bubble contents that leads to Pd(II) and Ni(II) reduction. Co-precipitation method was applied for the synthesis of Fe₃O₄ nanoparticles (NPs). Immobilized solution was produced by sonicating the aqueous mixture of Fe₃O₄ and mercaptosuccinic acid to obtain Pd-Ni alloys on Fe₃O₄ magnetic NP cores. The catalytic activity of the synthesized Pd-Ni/Fe₃O₄ core-shells was investigated in the Suzuki-Miyaura CC coupling reaction and 4-nitrophenol reduction, which exhibited a high catalytic activity in both reactions. These magnetic NPs can be separated from the reaction mixture by external magnetic field. This strategy is simple, economical and promising for industrial applications.

Analysis of the Efficiency of Surfactant-Mediated Stabilization Reactions of EGaIn Nanodroplets

A methodology based on light scattering and spectrophotometry was developed to evaluate the effect of organic surfactants on the size and yield of eutectic gallium/indium (EGaIn) nanodroplets formed in organic solvents by ultrasonication. The process was subsequently applied to systematically evaluate the role of headgroup chemistry as well as polar/apolar interactions of aliphatic surfactant systems on the efficiency of nanodroplet formation. Ethanol was found to be the most effective solvent medium in promoting the formation and stabilization of EGaIn nanodroplets. For the case of thiol-based surfactants in ethanol, the yield of nanodroplet formation increased with the number of carbon atoms in the aliphatic part. In the case of the most effective surfactant system-octadecanethiol-the nanodroplet yield increased by about 370% as compared to pristine ethanol. The rather low overall efficiency of the reaction process along with the incompatibility of surfactant-stabilized EGaIn nanodroplets in nonpolar organic solvents suggests that the stabilization mechanism differs from the established self-assembled monolayer formation process that has been widely observed in nanoparticle formation.

Biocompatible nano-gallium/hydroxyapatite nanocomposite with antimicrobial activity

Intensive research in the area of medical nanotechnology, especially to cope with the bacterial resistance against conventional antibiotics, has shown strong antimicrobial action of metallic and metal-oxide nanomaterials towards a wide variety of bacteria. However, the important remaining problem is that nanomaterials with highest antibacterial activity generally express also a high level of cytotoxicity for mammalian cells. Here we present gallium nanoparticles as a new solution to this problem. We developed a nanocomposite from bioactive hydroxyapatite nanorods (84 wt %) and antibacterial nanospheres of elemental gallium (16 wt %) with mode diameter of 22 ± 11 nm. In direct comparison, such nanocomposite with gallium nanoparticles exhibited better antibacterial properties against Pseudomonas aeruginosa and lower in-vitro cytotoxicity for human lung fibroblasts IMR-90 and mouse fibroblasts L929 (efficient antibacterial action and low toxicity from 0.1 to 1 g/L) than the nanocomposite of hydroxyapatite and silver nanoparticles (efficient antibacterial action and low toxicity from 0.2 to 0.25 g/L). This is the first report of a biomaterial composite with gallium nanoparticles. The observed strong antibacterial properties and low cytotoxicity make the investigated material promising for the prevention of implantation-induced infections that are frequently caused by P. aeruginosa.

Controlling and predicting droplet size of nanoemulsions: scaling relations with experimental validation

Nanoemulsions possess powerful nano-scale properties that make them attractive for diverse applications such as drug delivery, food supplements, nanoparticle synthesis and pharmaceutical formulation. However, there is little knowledge in nanoemulsion literature about controlling and predicting droplet size. In this article, we propose a scaling relation to predict the dependence of nanoemulsion droplet size with physical properties such as viscosity of the droplet phase and continuous phase, and process parameters such as input power density. We validate our proposed scaling with a wide range of droplet size data from nanoemulsions prepared with high pressure homogenization and ultrasonication. Our proposed scaling also compares favorably with experimental data from literature. The scaling relation can serve as a guiding principle for rational design of nanoemulsions.

Formation of particles of bismuth-based binary alloys and intermetallic compounds by ultrasonic cavitation

This work reports an important understanding of the bismuth reactivity with other low m_p metals (Sn, In, Ga, and Zn).

Synthesis of liquid core-shell particles and solid patchy multicomponent particles by shearing liquids into complex particles (SLICE)

We report a simple method that uses (i) emulsion shearing with oxidation to make core-shell particles, and (ii) emulsion shearing with surface-tension driven phase segregation to synthesize particles with complex surface compositions and morphologies. Subjecting eutectic gallium-indium, a liquid metal, to shear in an acidic carrier fluid we synthesized smooth liquid core-shell particles 6.4 nm to over 10 μm in diameter. Aggregates of these liquid particles can be reconfigured into larger structures using a focused ion beam. Using Field's metal melts we synthesized homogeneous nanoparticles and solid microparticles with different surface roughness and/or composition through shearing and phase separation. This extension of droplet emulsion technique, SLICE, applies fluidic shear to create micro- and nanoparticles in a tunable, green, and low-cost approach.

Synthesis and Characterization of Solid-State Phase Change Material Microcapsules for Thermal Management Applications

Polyalcohols such as neopentyl glycol (NPG) undergo solid-state crystal transformations that absorb/release significant latent heat. These solid–solid phase change materials (PCM) can be used in practical thermal management applications without concerns about liquid leakage and thermal expansion during phase transitions. In this paper, microcapsules of NPG encapsulated in silica shells were successfully synthesized with the use of emulsion techniques. The size of the microcapsules range from 0.2 to 4 μm, and the thickness of the silica shell is about 30 nm. It was found that the endothermic phase transition of these NPG-silica microcapsules was initiated at around 39 °C and the latent heat was about 96.0 J/g. A large supercooling of about 43.3 °C was observed in the pure NPG particles without shells, while the supercooling of the NPG microcapsules was reduced to about 14 °C due to the heterogeneous nucleation sites provided by the silica shell. These NPG microcapsules were added to the heat transfer fluid PAO to enhance its heat capacity and the effective heat capacity of the fluid was increased by 56% with the addition of 20 wt. % NPG-silica microcapsules.

Template-free preparation of crystalline Ge nanowire film electrodes via an electrochemical liquid-liquid-solid process in water at ambient pressure and temperature for energy storage

The direct electrodeposition of crystalline germanium (Ge) nanowire film electrodes from an aqueous solution of dissolved GeO(2) using discrete 'flux' nanoparticles capable of dissolving Ge(s) has been demonstrated. Electrodeposition of Ge at inert electrode substrates decorated with small (<100 nm), discrete indium (In) nanoparticles resulted in crystalline Ge nanowire films with definable nanowire diameters and densities without the need for a physical or chemical template. The Ge nanowires exhibited strong polycrystalline character as-deposited, with approximate crystallite dimensions of 20 nm and a mixed orientation of the crystallites along the length of the nanowire. Energy dispersive spectroscopic elemental mapping of individual Ge nanowires showed that the In nanoparticles remained at the base of each nanowire, indicating good electrical communication between the Ge nanowire and the underlying conductive support. As-deposited Ge nanowire films prepared on Cu supports were used without further processing as Li(+) battery anodes. Cycling studies performed at 1 C (1624 mA g(-1)) indicated the native Ge nanowire films supported stable discharge capacities at the level of 973 mA h g(-1), higher than analogous Ge nanowire film electrodes prepared through an energy-intensive vapor-liquid-solid nanowire growth process. The cumulative data show that ec-LLS is a viable method for directly preparing a functional, high-activity nanomaterials-based device component. The work presented here is a step toward the realization of simple processes that make fully functional energy conversion/storage technologies based on crystalline inorganic semiconductors entirely through benchtop, aqueous chemistry and electrochemistry without time- or energy-intensive process steps.