Charge-influenced structural properties of electrically connected platinum nanoparticles
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Charge-influenced structural properties of electrically connected platinum nanoparticles R.N. Viswanath1), J. Weissmüller1), R. Würschum1,2) and H. Gleiter1 ) 1) Institut für Nanotechnologie, Forschungszentrum Karlsruhe, Karlsruhe, Germany 2) Technische Universität Graz, Institut für Technische Physik, Graz, Austria Abstract We present results of a study motivated by the recent suggestion that the properties of nanocrystalline materials with a large surface-to-volume ratio can be tuned by inducing spacecharge regions at interfaces by means of an applied voltage. As an example, we investigate the reversible variation of the lattice constant of platinum nanoparticles immersed in an aqueous 1M KOH electrolyte as a function of applied potential. It is found that a reversible volumetric strain of up to 1.2 % can be induced, corresponding to pressures of up to 3.2 GPa. We present the experimental set-ups for in-situ X-ray diffraction with an electrochemical cell. The variation of the space charge at the metal-electrolyte interface results in a variation of the surface stress f as a function of the applied potential, which is not an electrocapillary effect. Introduction Nanocrystalline materials with their high fraction of surfaces or interfaces may open prospects of achieving novel properties since the modified properties of the matter at the surface or at internal interfaces may contribute measurably to the overall materials properties [1,2]. Recently, there has been a growing body of evidence suggesting that space-charge layers at interfaces can have a significant influence on a variety of physical properties. The notion that a suitable experimental set-up may allow space charge layers to be included and manipulated by an externally applied voltage suggests that nanocrystalline material with tunable structure and electronic properties may be prepared [3]. The purpose of this paper is to present experimental results in support of this concept. As the tunable property in question we study the lattice constant of nanoporous Pt. A schematic view of our charging X-ray experiment is shown in Fig.1: inter-connected electrically conducting arrays of metallic (a) nanoparticles with a large surface-to-volume (b) ratio immersed in an electrolyte are used as a (c) symmetrical set of electrode and counter (d) electrode. Upon applying a voltage typical (+) (-) charge densities of 0.3 C/m2 can be achieved in (a) PVC container, (b) Porous n-Pt electrode, this way at the metal-electrolyte interfaces. (c) Ni support, (d) 1M KOH electrolyte The thickness of the layer over which the excess charge in the metal is distributed Fig.1: Schematic illustration of the electrochemical cell for in-situ diffraction experiment. corresponds to the screening length, roughly one or few interatomic spacings. It is readily estimated (compare, for instance, Ref.[3]) that the extra charge which can be reversibly added or withdrawn corresponds to about 0.2 electron per atom in the first atomic layer at the surface. Some measurements in the literature indicate
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