Morphological evolution of Pt-modified nanoporous gold after thermal coarsening in reductive and oxidative environments

Abstract

Nanoporous gold made by dealloying AgAuPt (NPG-Pt) has been shown to exhibit several interesting catalytic properties, tied to its exceptionally high surface area; however, structural degradation may occur owing to thermal coarsening. To understand the effect of atmosphere chemistry on thermal coarsening and degradation, and means of limiting it, this study focuses on the high-resolution characterization of NPG-Pt layers coarsened in reductive Ar-H2 atmosphere, and in oxidative air. Atom probe tomography (APT) analysis is performed on NPG-Pt, coarsened separately in either Ar-H2 or air, to characterize the atomic-scale chemical changes in the nanoligaments and to develop a mechanistic view of the inherent processes. A tendency of Ag to segregate to the surface during coarsening is found to lead to complete elimination of the nanoligament core-shell structures in both cases. Large Pt segregates form during coarsening in Ar-H2, but under the surface of the ligaments, having relatively little effect on the coarsening rate. The oxygen-induced segregation of Pt was observed to cause the inhibition of thermal coarsening after minor loss in surface area-to-volume ratio. Findings in this paper help in understanding further the thermal coarsening of heterogeneous nanomaterials made by dealloying, and the pertinent factors that come into play in different chemical environments.

Evolution of micro-pores in Ni–Cr alloys via molten salt dealloying

“…The slower morphological coarsening may be attributed to the presence of the oxides, primarily due to the use of a less pure NaCl starting material where a higher water and oxygen contents may be present. It has been shown in prior studies that surface oxides or doped elements can create defects, thereby acting as diffusion barriers to slow down or even prohibit the coarsening of bicontinuous structures [52][53][54][55] . The process may benefit certain functional applications where a smaller size and thus higher surface areas are desirable to maintain a higher surface reactivity.…”

Advanced Characterization and Modification of Nanoporous Metals

“…One drawback of ordinary NPG for such purposes is the continued mobility of Au at the surface, which leads to coarsening of the structure, especially at elevated temperatures. The addition of small amounts of Pt [1] hinders such coarsening, even at low Au contents [2] and also has obvious relevance to catalysis.Atom Probe Tomography (APT) has proved very successful for the nanoscale analysis of NPG [2][3][4]. The essential step was to learn how to electroplate Cu into the pores, achieving complete filling, at least locally.…”

“…The essential step was to learn how to electroplate Cu into the pores, achieving complete filling, at least locally. This strengthened the material against the otherwise inevitable fractures that would occur during the APT analysis.Heating of NPG, in air or inert environments, causes surface segregation that can be beneficial, especially for catalysis [4,5]. Much remains to be learned about the operative mass transport processes.Adsorption on to the ligament surfaces in NPG can alter the electrical resistance of the metal itself, leading to potential applications in gas sensing [6].…”

“…Atom Probe Tomography (APT) has proved very successful for the nanoscale analysis of NPG [2][3][4]. The essential step was to learn how to electroplate Cu into the pores, achieving complete filling, at least locally.…”

“…Heating of NPG, in air or inert environments, causes surface segregation that can be beneficial, especially for catalysis [4,5]. Much remains to be learned about the operative mass transport processes.…”