Linda Stappers

Growth of sputter-deposited gold nanoparticles in ionic liquids.

The growth of gold nanoparticles (NPs) synthesized by sputter deposition on an ionic liquid surface is studied in situ in the bulk phase of the ionic liquids (ILs) 1-butyl-3-methylimidazolium dicyanamide [C(1)C(4)Im][N(CN)(2)], 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide [C(1)C(4)Im][Tf(2)N], 1-butyl-3-methylimidazolium tetrafluoroborate [C(1)C(4)Im][BF(4)], 1-butyl-3-methylimidazolium hexafluorophosphate [C(1)C(4)Im][PF(6)] and 1-butyl-3-methylimidazolium triflate [C(1)C(4)Im][TfO]. It is found that primary nanoparticles with a diameter smaller than 2.5 nm are present in the sample immediately after sputtering. Growth of these primary particles proceeds after the end of the sputtering process and stops when the nanoparticles reach a certain size. Depending on the viscosity of the ionic liquid this growth process can proceed several hours to several days. The growth speed is fastest for the least viscous ionic liquid and follows the trend [C(1)C(4)Im][N(CN)(2)] > [C(1)C(4)Im][Tf(2)N] > [C(1)C(4)Im][TfO] > [C(1)C(4)Im][BF(4)] > [C(1)C(4)Im][PF(6)]. It is also found that a higher concentration of sputtered gold results in faster growth of the gold nanoparticles. A discussion on the growth mechanism of sputtered gold NPs is included.

Stability of sputter-deposited gold nanoparticles in imidazolium ionic liquids

The stability of gold nanoparticles synthesised by sputter deposition has been studied in situ in 1-butyl-3-methylimidazolium ionic liquids with bis(trifluoromethylsulfonyl)imide, tetrafluoroborate, hexafluorophosphate and dicyanamide anions with UV-VIS absorption spectroscopy and transmission electron microscopy. Besides the growth of the gold nanoparticles, two other processes were observed after sputtering, namely aggregation and sedimentation of these nanoparticles. To model the absorption spectra of the sputtered gold nanoparticles, generalized multiparticle Mie calculations were performed. These theoretical calculations confirm the increase in absorbance at longer wavelength for larger aggregates and are in agreement with the experimental observations. It was found that the kinetics of aggregation and sedimentation scale with the viscosity of the ionic liquid. Small amounts of water were found to have a large detrimental influence on the stability of the colloidal suspensions of the gold nanoparticles in ionic liquids. From the large discrepancy between the theoretical and the experimentally observed stability of the NPs, it was concluded that structural forces stabilize the gold nanoparticles. This was also borne out by AFM measurements.

The effect of electrolyte conductivity on electrophoretic deposition

In this work, the influence of electrolyte conductivity on the electrophoretic deposition of alumina particles from ethanol suspensions was studied. Deposition experiments in a Hull cell showed that high-conductivity ethanol-based suspensions yield uniform deposits, while low-conductivity suspensions result in nonuniform deposits. The difference in the deposition, behavior is due to the resistance increase over the deposit during polarization. Impedance measurements during electrophoretic deposition showed that during EPD the relative deposit resistance increases much faster for the high- than for the low-conductivity suspension. The impedance measurements also showed that the resistance increase dropped almost to the suspension resistance after the electric field was turned off. This means that the resistance over the deposit is caused by the interaction of ions with the deposit and by the depletion of ions at the deposition electrode. Negatively charged ions are depleted in the deposit by migration toward the positively charged counterelectrode, while positively charged ions undergo electrochemical reactions at the deposition electrode. This change in ion concentrations near the deposition electrode changes the acid/base properties of the particles in the deposit, as proven by adsorbed pH indicators on the particles. The change in acid/base behavior is quasi-irreversible and results in a memory effect of the deposit resistance when the voltage is reapplied.

Growth of Metal around Particles during Electrodeposition

The deposition profile of metal around particles during electrodeposition was studied by atomic force microscopy measurements.In addition to these measurements, the local metal deposition rate around particles was determined from electrodeposited nickel–iron multilayers using “process archeology.” A strong correlation was found between the metal deposition profile and the surfaceproperties of the particles. Hydrophilic particles are pushed ahead by metal that deposits underneath the particles before theybecome incorporated. Metal deposition does not take place underneath hydrophobic particles, hence incorporating from the startof the metal deposition. For electrically conductive particles, metal deposits on the particles which causes such particles toincorporate almost immediately.© 2006 The Electrochemical Society. DOI: 10.1149/1.2198090 All rights reserved.Manuscript submitted January 10, 2006; revised manuscript received March 10, 2006. Available electronically May 4, 2006.

Novel Composite Coatings for Heat Sink Applications

Department of Metallurgy and Materials Engineering, Katholieke Universiteit Leuven, B-3001 Heverlee,BelgiumNovel composite coatings with interesting thermal properties were prepared by incorporation of phase change materials inelectrodeposited metals. Such composite coatings combine a high thermal conductivity with a high heat absorption capacity andare potentially useful for heat sink applications where heat should be dissipated as fast as possible. Microencapsulation was usedto make polymer particles with a core of phase change material which formed stable suspensions in the plating electrolyte andwhich incorporated well in copper deposits. It was found that under optimal processing conditions, copper coatings with 35 vol %of incorporated phase change material could be obtained. Such coatings have a heat absorption capacity of 10.9 J/g, as determinedby differential scanning calorimetry.© 2005 The Electrochemical Society. @DOI: 10.1149/1.1926207# All rights reserved.Manuscript submitted December 3, 2004; revised manuscript received January 12, 2005. Available electronically May 24, 2005.

On the electrochemical deposition of metal–organic frameworks

The electrochemical deposition of Metal–Organic Frameworks (MOFs) is an interesting technique to synthesise adherent, microporous layers on top of conductive substrates. The technique can be subdivided in two approaches: anodic and cathodic deposition. While the mechanism of the cathodic approach has already been well investigated, at least for MOF-5, up to now not much is known about the anodic approach. In this paper, a four-step mechanism is proposed to better understand the anodic deposition, and the same MOF used for the investigation, HKUST-1, is also deposited cathodically to compare the two approaches. This study focuses on how nucleation starts and proceeds, on the influence of the potential applied, the stresses in the growing layers, and the origin of defects like delamination and MOF detachment. The study is followed by critical considerations on the methods and on the technique, together with suggestions and guidelines to synthesise new MOF layers.

Synthesis and Characterization of Composite Coatings for Thermal Actuation

The synthesis and thermal expansion of metal coatings containing phase change material (PCM) prepared by electrolytic deposition were investigated. Such a composite combines the thermomechanical properties of the PCM with the high thermal conductivity of the metal, and can be used as a thermal actuator material. This study used paraffin as the PCM and copper as the metal matrix. The paraffin was first microencapsulated by emulsion polymerization to obtain microcapsules with a diameter of 1-5 μm containing 90 vol% paraffin to facilitate the incorporation of paraffin in copper by electrocodeposition. The microcapsules were added to a copper sulfate bath up to a concentration of 500 g/L. The electrocodeposition was performed at room temperature with a current density between 2 and 5 A dm -2 . These composites were examined by scanning electron microscopy, differential scanning calorimetry, and vertical dilatometry. Coatings with 40 vol % of microcapsules and a heat capacity of 12 kJ kg -1 during phase transformation were obtained. The thermal expansion of the composite showed a sharp increase in a small temperature range above the melting point. Although this behavior is ideal for thermal actuators, the effect decreased by thermal cycling. This remarkable thermomechanical behavior is explained by a thermoelastoplastic model for two-phase composites.

The Effect of Turbulence on the Electrodeposition of Composite Coatings

The effect of turbulence on the incorporation of particles in metal deposits during electrodeposition was studied experimentally by comparing particle incorporation in deposits made under laminar and turbulent flow conditions. Flow conditions were chosen such that the average hydrodynamic shear force acting on particles on the electrode is equal for both flows. Under these conditions, possible differences in the amount of particle incorporation could be related to the fluctuations of the hydrodynamic shear force due to turbulence. The mean shear force on particles in laminar and turbulent flow was calculated from the velocity gradients. The velocity gradient on a rotating cylinder electrode in turbulent flow was experimentally determined using microelectrodes. It was shown that the incorporation of neutrally buoyant particles on a rotating cylinder and a rotating disk electrode is equal, which proves that the incorporation of particles on a rotating cylinder electrode is not influenced by flow variations due to turbulence.

Metallic and bi-metallic Janus nanofibers: electrical and self-propulsion properties

Aligned polymeric nanofibers obtained via electrospinning were sputter-coated with metals on both their front and back sides. When both sides were coated with the same metal (gold, silver or platinum/palladium alloy), the nanofibers behaved as pure conductors. However, coating front and back of the fibers with different metals causes a potential difference when they are immersed in an aqueous solution. This potential difference propelled the Janus fibers in the direction perpendicular to their long axis when hydrogen peroxide was present in the solution.

Study of the deposit resistance during electrophoretic deposition

Deposition experiments in a Hull cell showed that high conductivity suspensions yield uniform deposits while low conductivity suspensions result in non-uniform deposits. This difference in deposition behavior is related to the resistance increase of the deposit during EPD. Impedance measurements during EPD showed that the ratio of the deposit resistance to the suspension resistance increases much more for high than for the low conductivity suspensions. They also showed that the total resistance of the EPD cell dropped almost to the suspension resistance after the electric field was turned off. This means that the deposit has no inherent resistance, but that its resistance during polarization is caused by the interaction of ions with the deposit and by the depletion of ions at the deposition electrode. The change in ion concentrations near the deposition electrode changes the acid/base properties of the particles in the deposit, as proven by adsorbed pH indicators on the particles. The change in acid/base behavior is quasi irreversible and results in a memory effect of the deposit resistance when the voltage is reapplied.