Thermal coarsening of nanoporous gold: Melting or recrystallization
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Mamoru Mabuchi Department of Energy Science and Technology, Graduate School of Energy Science, Kyoto University, Yoshidahonmachi, Sakyo, Kyoto 606-8501, Japan (Received 13 May 2008; accepted 4 November 2008)
The thermal coarsening of nanoporous Au was examined and compared with the thermal instability of Au nanoparticles. The nanoporous Au was coarsened at temperatures far below the melting temperature of Au nanoparticles, which possess sizes similar to the nanoligaments. Differential scanning calorimetry characterization of nanoporous Au exhibited an exothermal peak around 470 K. These results suggest that solid-state process like recrystallization, rather than melting, is responsible for the thermal coarsening of nanoporous Au.
Nanoporous Au with an open-cell structure and nanosized ligaments can be fabricated via the dealloying (or electrochemical etching) of Au–Ag alloys.1,2 The pore ligament size can be tailored to sizes as small as 5 nm,3,4 resulting in a large specific surface area. Many applications of nanoporous Au, such as piezoelectric,5 sensor,6 and catalytic7 ones, are being expected using its extraordinarily large surface area effect. One feature of nanoporous Au, which is not observed in porous metals with a pore size greater than several micrometers, is the thermal coarsening of their nanoporous structure.8–10 Namely, the pore and ligament sizes of nanoporous Au increase, maintaining an open-cell porous structure itself, when subjected to thermal treatment at high temperatures. This thermal coarsening is useful for pore size control, but undesirable for hightemperature application. Although the thermal coarsening is known to occur and is used for pore (and ligament) size control,8–10 the discussion on the coarsening mechanism itself is not sufficient. Therefore, the thermal coarsening behavior must be understood for the practical use of nanoporous Au. On the other hand, metallic nanoparticles and nanowires are thermally instable. The melting temperature of metallic nanoparticles is lower than that (Tm,bulk) of the parent bulky metal, owing to the large surface effect.11–14 The thermal instability of metallic nanowires is also elucidated; i.e., when a metallic nanowire is heated at a temperature below Tm,bulk, it fragments to short nanorods or spherical nanodots.15–17
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0037 J. Mater. Res., Vol. 24, No. 2, Feb 2009
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When the nanoporous structure of the dealloyed nanoporous Au is regarded as an architecture composed of thermally instable nanoparticles, the following mechanism via melting is possible. First, ligaments in nanoporous Au have a lower melting temperature Tm,np (
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