Formation of nanoporous platinum by selective dissolution of Cu from Cu 0.75 Pt 0.25
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This paper gives results demonstrating the production of nanoporous platinum through the de-alloying of Cu0.75Pt0.25 alloy in 1 M H2SO4. Both field emission scanning electron microscopy and small angle neutron scattering confirm the presence of porosity with a diameter of approximately 3.4 nm. This is the smallest porosity quantitatively reported from a de-alloying process to date. The small size is attributed to the extremely small values of surface diffusivity expected for Pt at room temperature, effectively eliminating room-temperature coarsening processes. The results also show that larger length scales can be achieved through coarsening at elevated temperatures. The ease of production of porous platinum makes it attractive for possible applications, such as high surface area electrodes for biomedical devices or as catalyst materials.
I. INTRODUCTION
Selective dissolution of one or more elements from an alloy is achieved through a corrosion process known as de-alloying. Consider a binary alloy, ApB1−p, where the reactivities of A and B are significantly different relative to a specific corrosive environment, and element A is more reactive. Under the appropriate driving force (applied voltage or presence of oxidizing species), we can selectively remove A from the alloy. The alloy will now evolve in one of two directions. At moderate driving force, the alloy surface will enrich in component B, resulting in a protective B-rich layer and thus hindering further dissolution. However, at a slightly higher driving force, a structural instability occurs, resulting in the formation of porosity. This porosity allows for the ingress of the corrosive environment and a continuation of the process. We are unaware of any investigations that define the limit of the depth of porosity that can be created, but it has been demonstrated that depths of 2 mm are easily achievable.1 The mechanism of the structural instability that leads to porosity formation has been the focus of many recent investigations.2,3 De-alloying has been observed in numerous systems including Cu–Au,4–8 Zn–Cu,4,9–11 Mg–Cd,12,13 Al–Cu,9 Ag–Au,1,14–21 Mn–Cu,9,22 Pd–Cu,23 Ni–Cu,9 and even during the reduction of titanium dioxide in molten calcium chloride.24,25 Beyond its potential for the creation of new porous materials, a better understanding of the de-alloying processes is relevant to stress corrosion cracking of some alloy, alloy-environment systems (see for example Refs. 26–31), the accelerated corrosion in 216
J. Mater. Res., Vol. 18, No. 1, Jan 2003
AA2024-T3,32,33 corrosion of austenitic stainless steel in acidified chloride containing electrolyte,27,34 and the production of Raney metal catalysts.35,36 The morphology of typical de-alloyed structures consists of a highly tortuous, completely interconnected porosity with a pore diameter as small as 3 nm. The structure can be coarsened to larger length scales (up to micrometers) at elevated temperatures1 and has been shown to coarsen at room temperature (to size scales as large as a few 100 nm in the ca
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