Electrodeposition and properties of nanostructured platinum films studied by quartz crystal impedance measurements at 10 MHz

The electrodeposition of platinum films from aqueous solutions and lyotropic liquid crystalline mixtures of the non-ionic surfactant octaethyleneglycol monohexadecyl ether, water, and hexachloroplatinic acid (HCPA) has been studied by rapid in-situ measurements of the quartz crystal electroacoustic impedance with an electrochemical quartz crystal microbalance (EQCM). For solutions and liquid crystalline template mixtures of high HCPA content large changes of the damping resistance, R have been observed at the initial stages and the end of the plating process as a result of composition changes in the depletion layer. They were corrected forte yield accurate in-situ mass data for the electroplating process. For the measured film thicknesses (< 200 nm) the faradayic efficiency depends inversely on the concentration of HCPA in the aqueous solutions and the template mixtures. This was attributed mainly to migrational transport of the intermediate [PtCl4](2-) anion away from the electrode and to the concentration dependence of the disproportionation/comproportionation reaction in the depletion layer between the Pt(0), Pt(II), and Pt(IV) species involved in the electrode reaction. A 65% increase in specific surface area was measured for the template deposited material compared to Pt deposits from aqueous solutions. This difference can be regarded as a minimum value for the thin films studied and becomes much larger for thicker films. Transmission electron microscopy (TEM) analysis of the former show the films to be mesoporous with a hexagonal nanostructure identical to that produced by template depositions onto non-oscillating substrates. The high specific surface area of the Pt films from lyotropic liquid crystalline templates results in a sensitivity enhancement of the in-situ electroacoustic impedance EQCM technique. It could hence be applied to mono- and sub-monolayer adsorption studies for polycrystalline Pt in sulfuric acid. An increase in damping resistance was found when stepping the potential from the double layer region into the hydrogen adsorption region and attributed to a higher degree of acoustic shear wave coupling to the bulk liquid. It is believed that the adsorbed hydrogen forms hydronium species, which others have observed earlier by IR spectroscopy. (C) 2000 Elsevier Science Ltd. All rights reserved.