Alexander V. Karabulin

Structure and Properties of Platinum, Gold and Mercury Nanowires Grown in Superfluid Helium

Webs consisting of nanowires made of gold, platinum and mercury were produced by the technique based on laser ablation of metals inside superfluid helium. Their morphology and structure as well as their electrical conductivity have been studied. Diameters of gold and platinum nanowires are 4.5 and 3 nm, respectively. Fortunately, they are close to diameters of nanospheres made of these metals, which, as known from the literature, possess anomalous catalytic activity. Web resistivities for all metals up to room temperature are controlled by conductive electron scattering on a wire surface, thus they are almost independent of T. Nanowires in the webs are electrically interconnected, and therefore the web can be used as a catalyst without any support. Possible advantages of this type of nanocatalyst are outlined.

Production of ultrathin nanowires from refractory metals (Nb, Re, W, Mo) by laser ablation in superfluid helium

The ablation of targets in superfluid helium with a short-pulse laser allows introducing into liquid the atoms and small clusters of any metal. The metal is then concentrated in the core of 1D quantized vortices nucleating in the laser focus and expanding into the liquid. Subsequent metal coagulation within the vortex results in the formation of thin nanowires with perfect shape and structure. For refractory metals these wires are expected to be especially thin. The diameters of nanowires grown from niobium, molybdenum and tungsten are indeed 4.0, 2.0 and 2.5 nm, respectively. Unfortunately, under ablation of unannealed rhenium the main product is flat flakes having irregular shape and 20–50 nm size. Short nanowires (with a 1.5 nm diameter) were present in small amounts. The wires produced by this method contain no additional impurities and have a free lateral surface. By using a diode-pumped Nd:LSB microlaser (1.06 μm wavelength, 0.4 ns pulse duration, 100 μJ pulse energy and 4 kHz repetition rate) the amounts of nanoweb sufficient for many physical and chemical applications could be produced in a single low-temperature experiment. Ultrathin wires of molybdenum and tungsten show promise for creating cold cathodes whereas a niobium nanoweb should be an excellent catalyst.

Application of Au–Cu nanowires fabricated by laser ablation in superfluid helium as catalysts for CO oxidation

The copper-doped gold nanowires (4xa0nm in diameter) were produced by the novel technique based of laser ablation of Au–Cu alloy inside superfluid helium. The principle of the method is using the quantized vortices as the 1D template for the condensation of the ablation products into thin threads. The nanowires were applied as the catalyst in СО oxidation with oxygen. The activity of Au–Cu nanowires deposited on glass filters was compared with that for monometallic and bimetallic Au and Cu particles (3–8xa0nm in diameter) deposited on alumina by traditional deposition–precipitation and impregnation techniques. The apparent activation energies of the reaction (Еа) were 95 and 98, 150, and 147xa0kJ/mol for Au–Cu nanowires and Au–Cu, Au, and Cu particles, respectively. During running-in of the Au–Cu nanowire–based catalyst, Еа decreased to 20xa0kJ/mol and retained at this level in the subsequent cycles of lowering and raising the reactor temperature.

Experimental study of thermal stability of thin nanowires.

Thin (D < 10 nm) nanowires are in principle promising for their application as catalysts and as elements of nanocomputers and quantum devices. To perform these tasks, their structure and properties must be stable at least at standard conditions. Using our technique based on the capture of small particles to the core of quantized vortices in superfluid helium, we synthesized nanowires made of various metals and alloys and investigated their thermal stability. The indium nanowires (D = 8 nm) were shown to be stable when heated to 100 °C, i.e., almost to the melting point, whereas the silver nanowires (D = 5 nm) disintegrated into traces of individual nanoclusters at 300 K. The gold and platinum nanowires also decomposed at temperatures more than twice as low as the melting point. A model is proposed to explain the premature decay of thin nanowires by unfreezing of the surface-atom mobility in combination with the anomalous dependence of the surface tension on the nanowire radius. Methods for improving the stability limits of thin nanowires by saturation of their surface with immobilized atoms as well as by surface oxidation have been proposed and experimentally tested.

Stability of micron-sized spheres formed by pulsed laser ablation of metals in superfluid helium and water

It has been shown that micrometer-sized balls, resulting from the condensation of products of laser ablation of fusible metals in both superfluid helium and water, are in the state of strong internal tension counterbalanced by external compression. By radiation-induced or chemical damage to the integrity of their surface, they break up, ejecting a plurality of nanoparticles. The empty shells of the microspheres, which nonetheless remain intact, are identical to the “hollow spheres” of unclear origin that have been observed previously under laser ablation in usual liquids. The metastability of the microparticles produced by ablation in a liquid should be taken into consideration in their use in engineering and medicine.

Catalysis of carbon monoxide oxidation with oxygen in the presence of palladium nanowires and nanoparticles

A new synthesis method based on the coagulation of metal nanoparticles, introduced by laser ablation into superfluid helium, inside of quantized vortices has been used for the fabrication of nanoweb consisting of interconnected palladium wires of a 4 nm diameter. It has been found that at temperatures above 523 K, the Pd nanoweb effectively catalyzes the oxidation of CO with molecular oxygen. Temperature cycling leads to a shift of Pd nanoweb activity to lower temperatures. The catalytic action of the Pd nanoweb has been compared to that of Pd nanoparticles with a diameter of about 2 nm prepared by laser electrodispersion.

Coagulation of Metals in Superfluid and Normal Liquid Helium

The thermal emission study in this work has shown that coagulation of metals in liquid helium is accompanied by enormous local overheating of several thousand degrees. Direct experiments demonstrated, for the first time, that condensation of metals in superfluid helium occurs via the specific mechanism which is substantially faster than that in normal liquid helium. It has been stated that coagulation of metals in superfluid helium indeed occurs in two stages, a hot one of nanoparticles coalescence with the formation of molten nanospheres and the subsequent stage of their sticking together into nanowires. It turned out that if a laser ablation of metal targets immersed in superfluid helium was used for introducing a metal into liquid, the formation of nanowires occurs at distances of only about 1 mm from the laser focus. This leads to the presence of a considerable number of spherical inclusions in nanowires grown in such a way.

Formation of nanostructures during coagulation of semiconductors in superfluid helium

As in the case of metals, laser ablation of germanium and silicon in superfluid helium leads to the formation of thin nanowires; spherical nanoclusters are also present in ablation products. A decrease in the resistance of bundles of silicon nanowires with increasing temperature is typical of semiconductors. Laser ablation of amorphous graphite also leads to the formation of quasi-one-dimensional structures, but there are no spherical clusters among the coagulation products. The structure of carbon filaments is amorphous; they contain onion-like entities, and nanodiamonds appear as coagulation products when the concentration of carbon introduced into superfluid helium increases. The peculiar behavior of carbon is associated with the impossibility of its melting at low pressures.