Munekazu Motoyama

Electrodeposition of Cu nanowire arrays with a template

Cu nanowire arrays were electrodeposited with a template of a polycarbonate (PC) filter. Pt–Pd alloy film was sputtered on one side of the PC filter and then used as cathode in 0.6 M CuSO4 aqueous solution (pH 2). Potentiostatic electrodeposition at −400 mV vs. Cu reference electrode was conducted in two kinds of electrolytic cell configurations, cathode over anode (C/A) and anode over cathode (A/C). The transient variation of the cathodic current clearly showed four stages in both cell configurations; (I) nucleation and crystal growth mode of Cu on the Pt–Pd film, (II) filling up with electrodeposited Cu in the nanosized pores, (III) covering of the surface of the template with Cu, and (IV) growth of a Cu thick film. In Stage II, the cathodic current increased in the A/C configuration, while it decreased in the C/A. The duration time of Stage II was shorter in the A/C configuration than that in the C/A. The difference of the cathodic current variation between the two configurations was smaller with smaller sized pores. These phenomena suggest that the ionic mass transfer rate of Cu2+ ion accompanied by electrodeposition is enhanced by a kind of natural convection even in and around such a nanosized pore and that the pores are filled up faster with electrodeposited Cu in the A/C than in the C/A configuration.

Producing Shape-Controlled Metal Nanowires and Nanotubes by an Electrochemical Method

Nanowire and nanotube arrays of transition metal were produced by potentiostatic electrochemical deposition using a polycarbonate membrane filter template (15 to 200 nm in nanopore diameter). Nanotubes are produced by appropriately controlled partial filling of the pore cross section; nanowires result when the filter pore is completely filled. In this manner, nanotubes and nanowires of transition metals (Ni, Co, and Fe) were produced as well as Cu nanowire. The nanowires have an aspect ratio ranging from 50 to 400 dependent on the transient variation of cathodic current. The metal deposition rate must be balanced with that of hydrogen generation (dissolved or evolved as bubbles). The wall thickness of nanotubes, varying from 10 to 70 nm, may be controlled conveniently by adjusting the pH of the solution and the applied electrode potential. Suppression of hydrogen bubble evolution in the nanocapillary structure clearly plays an important role.

Effect of surface pH on electrodeposited Ni films

Ni metal was potentiostatically electrodeposited on a vertical plane cathode in a Watts bath in order to determine the effect of the cathode surface pH on the Ni film microstructure. Polarization curves and current efficiencies were measured at pH values of 1.5, 3.4, and 5.5. The surface pH value (pH s ) was estimated from the measured partial current density for hydrogen gas evolution based on a steady-state one-dimensional mass transfer model. Any species relating to the buffering function through the dissociation reactions (HSO - 4 , H 3 BO 3 , Ni 4 (OH) 4+ 4 , H 3 O + , OH - ) were taken into account. The pH s abruptly rose to above 6 as the partial current density for H 2 gas evolution increased. The preferred orientation of electrodeposited Ni thin film was plotted in electrode potential-pH s diagram. It was found that the transition boundary between {110} and {100} preferred orientations was located along a ridge 500 mV below the H + /H 2 equilibrium potential line. This relationship suggests that the dissolved hydrogen atoms in Ni metal are partly responsible for the evolution of structural texture of the Ni films.

Nanotubular array solid oxide fuel cell.

This report presents a demonstration and characterization of a nanotubular array of solid oxide fuel cells (SOFCs) made of one-end-closed hollow tube Ni/yttria-stabilized zirconia/Pt membrane electrode assemblies (MEAs). The tubular MEAs are nominally ∼5 μm long and have <500 nm outside diameter with total MEA thickness of nearly 50 nm. Open circuit voltages up to 660 mV (vs air) and power densities up to 1.3 μW cm(-2) were measured at 550 °C using H2 as fuel. The paper also introduces a fabrication methodology primarily based on a template process involving atomic layer deposition and electrodeposition for building the nanotubular MEA architecture as an important step toward achieving high surface area ultrathin SOFCs operating in the intermediate to low-temperature regime. A fabricated nanotubular SOFC theoretically attains a 20-fold increase in the effective surface, while projections indicate the possibility of achieving up to 40-fold.

Impact of Accompanying Hydrogen Generation on Metal Nanotube Electrodeposition

Ni is electrodeposited into polycarbonate porous membrane templates at various cathodic potentials and bulk solution pH values. The membrane pore diameters are 200 nm. Transmission electron microscope observations reveal that the electrodeposited Ni nanostructures are nanowires but occasionally nanotubes. The nanotube wall thicknesses vary from 10 to 70 nm. Nanotubes with thinner walls are deposited under more acidic and less noble conditions. The numerical model for pH values at the cathode surface in the template is developed. Calculated pH values at the cathode surface are lower in the template than those at a vertical planar cathode. The current efficiency of Ni is measured by inductively coupled plasma emission spectrometry. As the predicted by the pH calculations, the current efficiency of Ni is lower for a porous template than for a vertical planar cathode. The observed nanotube walls become thinner as the current efficiency decreases. Thus, it is deduced that accompanying H 2 evolutions promote nanotube growth in the pores. Finally, the pH values in the template are more than 1 to 2 times smaller than at the vertical planar cathode surface.

Electrodeposition and Behavior of Single Metal Nanowire Probes

This paper describes the fabrication of scanning probes with single metal nanowires (NWs) at the probe tip. The porous-template technique can produce NWs of various kinds of metals, with diameters down to 10-20 nm, which compete with multiwall carbon nanotube diameters. Metal NWs are grown by electrodeposition on the scanning probe tip. One NW can be selected to remain by focused ion beam technique. A variety of metals can be chosen as the tip material. Electric potentials of NWs at the probe tip can be measured. Single NW probes can measure surface topographies, electrode potentials, and their mechanical bending properties.

Thermal conduction in nanoporous copper inverse opal films

Copper inverse opal films offer an attractive combination of conduction and convection transport properties that yield a low total thermal resistance for microfluidic heat exchanger applications. In this work, we present an integrated synthesis and characterization strategy to fabricate nanoporous copper inverse opal films and to measure the effective thermal conductivity. We synthesize inverse opal films with sub-micron pore diameters using a sacrificial packed multilayer nanosphere bed to mold the geometry of an electrodeposited copper film. We characterize the effective thermal conductivity using the 3ω method, where the nanoporous copper film is deposited directly above a microfabricated and electrically-passivated 3ω device. The effective thermal conductivity is measured to be as large as 170 W m-1 K-1. This experimental data is compared to finite element simulations as well as common conduction models for heterogeneous media, including Maxwells model and differential effective medium theory. This provides insight into the design of nanoengineered surfaces and two-phase vapor-venting microfluidic heat exchangers for ultrahigh heat flux cooling.

Nanoscale conformable coatings for enhanced thermal conduction of carbon nanotube films

Vertically aligned carbon nanotube (CNT) arrays can provide the required combination of high thermal conductivity and mechanical compliance for thermal interface applications. Much work in the last 15 years has focused on improving the quality and intrinsic thermal conductivity of the nanotube arrays. Currently the thermal interface resistance between nanotube arrays and surrounding materials limits the overall thermal performance. To reduce this interface resistance, we propose coating the nanotube film with a continuous layer of metal. In this work, we electroplate 1 to 20 μm-thick continuous copper films directly on the carbon nanotube array. We measure the thermal conductivity of CNT arrays after electroplating using cross-sectional infrared microscopy. For low volume fraction, vertically-aligned carbon nanotubes arrays with copper electroplating (0.5 vol. %), the film thermal conductivity is nearly 3 W/m/K. These results demonstrate the feasibility of the electroplating method to coat CNT films.

Nickel Electrodeposition under Magnetic Field

Nickel was electrodeposited on a vertical silver cathode immersed in aqueous electrolyte solutions with various pH values under an external magnetic field of 0.5 T. The current density decreased, while the current efficiency slightly increased in a 0.5 T field, especially in the negative electrode potential region at pH 3.4 and 5.5. Surface morphology of Ni films in a 0.5 T field is hardly different from that without magnetic field. Crystal preferred orientation of Ni films varied with the pH as well as electrode potential. The electrode potential at which the preferred orientation transition from {100} to {110} appeared was shifted in the negative direction at pH 3.4 and 5.5 under a 0.5 T field. Ni films showed the dominant {110} preferred orientation in the high overpotential region regardless of the pH.