Electrochemical Dealloying Of Icosahedron Al-Pd-M (M=Mn, Fe, Cr,Mg) Quasicrystals

Dealloying is an important corrosion process involved in stress corrosion cracking of a wide range of industrial alloys. In such a process, one or several less noble constituent constituents are selected dissolved out from an alloy, leaving behind a porous structure made almost entirely of the more noble constituent. The corrosion process has recently been developed into a simple method for producing useful nanoporous metals. Nanoporous metals made by dealloying possess a network of3-dimensional bi-continuous pores and ligaments, The materials combine properties characteristic of metals, such as good thermal electrical conductivity and the extreme properties of nanostructures such as high surface area per volume, which gives rise to potential application in heterogeneous catalysis, electrochemical catalysis, and biosensing and so forth. Most of the recent studies have been focused on dealloying crystalline solid solution and amorphous alloys. The present study is devoted to the fabrication of nanoporous metals by dealloying quasicrystal alloys. The electrocatalytic activities of nanoporous structures thus obtained by dealloying quasicrystals towards electrooxidation of ethanol in alkaline environment have also been investigated.FCC icosahedral Al-Pd-M (M=Mn, Fe, Cr, Mg, etc) quasicrystals assume the common features of a dealloyable metallic alloy. Four typical icosahedral quasicrystals, namely, Al72Pd2oMn8, Al70Pd17Fe13, Al72Pd20Cr8and Al42.5Pd15Mg42.5were taken as typical subject materials in our study. Their electrochemical behaviors in dilute NaCl aqueous solution were measured using a standard three-electrode setup. The favorable electrochemical conditions for dealloying were then determined, under which dealloying of quasicrystals was carried out at room temperature. The structural evolution of quasicrystals during dealloying were investigated by a combined used of X-ray diffraction (XRD) and Transmission Electron Microscopy (TEM). The morphology and chemical composition of samples were analyzed with Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectrometer (EDX), respectively. The results of quasicrystal preparation, fabrication of nanoporous Pd metals and electrochemical measurement are presented as follows:(1) Melt-spun samples of Al72Pd20Mn8, Al70Pd17Fe13, Al72Pd2oCr8and Al42.5Pd15Mg42.5quasicrystals were made by means of rapid quenching with appropriate amounts of low purity metals Al, Pd, Mn, Fe, Cr and Mg. The rapidly quenched Al72Pd2oMn8is a single phase quasicrystal alloy; the Al70Pd17Fe13alloys is composed of the quasi crystalline phase as a majority phase and a small amount of Al3Pd phase. Besides of the quasicrystalline phase, the Al72Pd20Cr8and Al42.5Pd15Mg42.5samples also contain a large volume fraction of crystalline phases. The feature grain size in these as-cast samples are ranged from several hundred nanometers to a few micrometers.(2) Dealloying of Al72Pd20Mn8and Al70Pd17Fe13quasicrystal alloys was conducted in0.1M NaCl aqueous solution under an applied potential of0.0V (vs SCE). These two quasicrystals exhibited similar dealloying behaviors. After dealloying, surface cracking was observed in all of the samples, and the surface grains were found to preserve their initial shapes. The sample surface morphology evolution characteristic is similar to those found in crystalline solid solution alloys. In each case, a maze-like nanoporous pattern was formed throughout the entire sample. The nanoporous structures have a pore size of5-20nm and a homogeneous ligament thickness of-5nm. TEM results indicate that the collection of nanoligaments within a quasicrystal grain is polycrystalline, which is fundamentally different from that observed for solid solution alloys. It was also found that at the initial stage of dealloying, the quasicrystal grains first decomposed into nanometer scaled quasicrystal particles, and with continued dealloying these nanocrystals finally evolved into nanoporous structures. The Al3Pd phase in the Al70Pd17Fe13sample was also completely dealloyed, and the finally dealloyed structure is rather homogeneous. The Al72Pd20Cr8and Al42.5Pd15Mg42.5quasicrystals were partially dealloyed and the crystalline phases were left. The former nanoporous composites exhibit a pore size and a ligament thickness are both-10nm while the later exhibit-5nm.(3) A dealloying mechanism for the structural evolution is proposed in the present work: The surface atomic structure of these icosahedral quasicrystal is believed to play a crucial role for understanding the initiation of dealloying. Structurally, this quasicrystal is associated with a F-type6D supercubic lattice. There exist three atomic surfaces at site n=(0,0,0,0,0,0), n’=(1,0,0,0,0,0) and be=1/2(-1,1,1,1,1,-1) in the6D supercubic lattice with a superstructure induced by the chemical decoration of Al, Pd and M atoms. These atomic objects consist of successive atomic shells, in which Al atoms form the external shells at the n and n’atomic surfaces. The surface atomic structures of these quasicrystals are bulk-terminated, i.e. the surface layer planes are identical to planes from the bulk model of the quasicrystal. Therewith, local electrochemical attack can preferentially be initiated at these shell surface Al atom sites while the quasicrystal surface is being exposed in a NaCl aqueous solution. As Al atoms are continually removed from the atomic shells, the atomic objects located at sites n and n’get collapsed. The nanodecomposition of the quasicrystal grain is likely to be triggered by this kind of local event. Subsequently, massive dissolution of Al and M atoms from the nanocrystals can occur due to their strongly negative electrode potentials with respect to Pd. The Pd atoms are driven to agglomerate into clusters and gradually evolve into a3D network structure. The microstructural size reached in the dealloyed structure is limited by the nanodecomposition of quasicrystal grains.(4) The electrochemical catalytic activity of the nanoporous Pd metals made from Al72Pd20Mn8and Al70Pd17Fe13quasicrystals for the electrooxidation of ethanol in alkaline environment have been measured with cyclic voltammetry (CV) and chronoamperometry (CA). Our experimental results show that the nanoporous Pd electrodes exhibit good electrocatalytic activity and stability towards the oxidation of ethanol in alkaline circumstance superior to bulk Pd.Dealloying of quasicrystal has been successfully realized in FCC icosahedral Al72Pd20Mn8, Al70Pd17Fe13, Al72Pd20Cr8and Al42.5Pd15Mg42.5quasicrystals, and nanoporous Pd metals with good electrocatalytic activities towards electrooxidation of ethanol in alkaline environment have been fabricated. It is worth mentioning that the wide variety of Al-based quasicrystals would open a lot of room for the discovery of new type of nanoporous structures, and many useful properties in these nanoporous metals are awaiting exploration.