This paper gives results demonstrating the production of nanoporous platinum through the de-alloying of Cu0.75Pt0.25alloy 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.

We present the results of dealloying studies of Ag 0.7 Au 0.3 and Ag 0.65 Au 0.35 alloys in 0.1 M HClO 4 with the addition of either 0.1 M KCl, 0.1 M KBr, or 0.1 M KI. The critical overpotential decreases with the addition of halides with KI having the largest potential reduction of almost 50%. This decrease is discussed with respect to a competition between the rates of increase of Au surface diffusivity and Ag exchange current density with halide additions. The size scale of porosity produced during the dealloying of Ag 0.65 Au 0.35 in the above electrolytes was found to increase with the addition of halides. Without the addition of halides, a pore size of approximately 8 nm is produced while 17, 16, and 67 nm is measured in the KCl-, KBr-, and KI-containing electrolytes. The value of surface diffusivity required to coarsen the dealloyed structure to these size scales has been calculated to be 2 ϫ 10 Ϫ16 cm 2 s Ϫ1 ͑0.1 M HClO 4 ), 3 ϫ 10 Ϫ15 cm 2 s Ϫ1 ͑KBr or KCl͒, and 8 ϫ 10 Ϫ13 cm 2 s Ϫ1 ͑KI͒.Dealloying is a corrosion process in which one or more elements are selectively removed leaving behind a bicontinuous porous residue of the remaining element͑s͒. This bicontinuous metal-void structure is highly brittle in nature and has been linked to stress corrosion cracking in many alloy systems ͑see, for example Ref. 1-6͒. Dealloying has been observed in numerous systems including the systems Cu-and even during the reduction of titanium dioxide in molten calcium chloride. 24,25 This is an illustrative rather than an exhaustive list as it appears that dealloying can be electrochemically driven in any system where a large electrochemical potential difference exists between the alloying elements. Beyond its direct relevance to stress corrosion cracking, dealloying has been linked to the accelerated corrosion of Al2024-T3, 26,27 the corrosion of austenitic stainless steel in acidified chloride-containing electrolyte, 2,28 and the production of Raney metal catalysts. 29,30 New Pt-based alloy systems have been investigated by the authors with the motivation of producing nanoporous metals for various highsurface area electrode applications particularly in the biomedical area. 31 Not until 1980 was the first detailed micromorphological study of alloy dissolution reported. 18 This study was conducted by Forty et al. for the dealloying of Ag from Ag-Au alloys in nitric acid. It is now well recognized that the morphology of dealloyed structures consist of bicontinuous metal-void phases in what is loosely referred to as a sponge-like structure ͑see, for example, Fig. 5͒. The smallangle neutron scattering from structures obtained through leaching of one phase of a spinodally decomposed system ͑e.g., porous Vycor͒ is quantitatively similar to that of dealloyed Au. 32-34 Interestingly, one recent theory views the formation of porosity as a spinodal decomposition process occurring at the dealloyed surface. 35 The as-dealloyed structure typically consists of pore diameters on the nanometer length scale; the smallest size that we are ...

The results of morphological and electrochemical characterizations of Cu 0.8 Pt 0.2 , Cu 0.75 Pt 0.25 , and Cu 0.71 Pt 0.29 dealloying in 1 M H 2 SO 4 are presented. The critical potential values for these systems were measured and compared to previously reported results for the Ag-Au system. The differences between the systems are discussed in terms of their relative surface diffusivity values. Based on potential hold data, the dealloying critical potentials for Cu 0.8 Pt 0.2 , Cu 0.75 Pt 0.25 , and Cu 0.71 Pt 0.29 are 0.565, 0.730, and 0.800 V, respectively, vs a normal hydrogen reference electrode. The critical overpotentials for dealloying in the Cu-Pt system were found to be approximately 100 mV greater than that for similar compositions in the Ag-Au system.Dealloying is a corrosion process that results in the selective dissolution of one or more elements from an alloy. This process typically results in the production of a nanoporous material of the more noble element͑s͒ 1 with a completely interconnected metalvoid structure containing pores as small as 3.5 nm diam. 2 The structure can be coarsened to larger length scales; at room temperature pore sizes up to 200 nm can be easily produced 3 and at elevated temperatures structures as large as several micrometers can be achieved. 4 Various applications have been found for these structures. [5][6][7][8][9][10][11][12] One of the most important characteristics of a dealloying system is the value of the critical potential. 13 The critical potential marks the transition from alloy passivity ͑planar stability͒ to rapid dealloying ͑porosity formation͒. The dealloying process has most recently been treated as a kinetically controlled morphological transition, 14 a phase separation process, 15,16 and as a nucleation and clustering process. 17 The measurement of the critical potential is typically determined through analysis of electrochemical polarization data, although recently we have shown that this technique may overestimate the critical potential by as much as 115 mV. 18 In the same publication, an alternative method was introduced which utilizes a series of potential hold measurements. This method was later refined in a subsequent manuscript. 19 In this paper we compare the potential hold method for the measurement of critical potentials in the Cu-Pt system to that of polarization data. Not only is this system interesting due to potential applications in the area of biomedical sensors, but this system represents a contrast to the previous investigated Ag-Au system. Specifically, Pt has a value of surface diffusivity that is at least 3-4 orders of magnitude lower than that of Au under their respective electrochemical environments. c This difference in surface diffusivity allows us to make a comparison of the two systems and specifically adds to the body of knowledge aimed at understanding the role of surface diffusivity on the dealloying process. ExperimentalThe alloy samples were prepared at the Materials Preparation Center, Ames Laboratory ͑Ames, Iowa͒, by arc ...

One of the important characteristics of dealloying systems is the location of the critical potential which marks the transition from a “passivated” alloy surface to the sustained formation of a bicontinuous porous structure. A steady-state method for accurately determining the dealloying critical potential is compared with the more traditional approach of extrapolation from anodic polarization data. Misinterpretation in the literature of short-term potential hold data is discussed in light of these results. The dealloying critical potentials for Ag0.80Au0.20, Ag0.75Au0.25, and Ag0.70Au0.30 were 0.80, 0.94, and 1.01 V (normal hydrogen electrode), respectively. The polarization data is shown to overestimate these values by 100 mV. Morphological investigations confirm the presence of porosity for a potential hold only 10 mV above the critical potential. For alloys with compositions between 70-80 atom % Ag, the critical potential increased by 20 mV per atom % decrease in the Ag composition. Long-term current fluctuations were observed during the potential hold experiments that suggest the presence of a surface corrosion instability occurring over 1-5% of the total surface area. © 2005 The Electrochemical Society. All rights reserved.

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