Environment-Sensitive Thermal Coarsening of Nanoporous Gold

Abstract

In order to investigate environmental effects on the ligament/pore coarsening of nanoporous gold (NPG), we studied the thermal coarsening of NPG both in air and vacuum by ex situ observation, and found that it has high structural stability against heat treatment in vacuum. To clarify the nature of this phenomenon, we investigated the thermal coarsening of NPG by in situ environmental transmission electron microscopy. At an elevated temperature (400°C), the coarsening of ligaments/pores was triggered by introducing either pure N 2 or O 2 gas into the transmission electron microscopy (TEM) chamber (but not by Ar gas). We thus conclude with a discussion on the mechanism for thermal coarsening of NPG.

Influence of gas atmospheres and ceria on the stability of nanoporous gold studied by environmental electron microscopy and in situ ptychography

“…Nevertheless, systematic investigations of the inuence of the X-ray dose should be performed in future. In contrast to previous studies, 18,24 it could be shown that np-Au samples did not start to change in vacuum at temperatures below 700 C. At those high temperatures, a simultaneous material loss and sintering were observed. Interestingly, the loss observed during in situ ptychography in 20% O 2 /He at ambient pressure started at much lower temperature than under vacuum conditions, but at a comparable temperature to the onset of coarsening during ETEM analysis in oxygen atmosphere.…”

“…Applying IR thermography with a spatial resolution better than 10 mm offers a distinct advantage over classic temperature sensors, which are generally only capable of determining the average temperature of the heated area. 11,24 Using IR thermography, imaging of the heated area can also account for temperature inhomogeneities on the microscale, which can occur when catalysts are studied during reactions. 47,48 In contrast to the use of pre-calibrated heating chips in vacuum, 25 the direct determination of the temperature allows its use in a gas atmosphere as well, where the calibration performed in vacuum would no longer be valid.…”

“…Lattice diffusion of gold was suggested as an alternative mechanism in this case. Furthermore, both Kuwano-Nakatani et al 24 and Sun et al 18 did not observe any strong coarsening in vacuum. Kuwano-Nakatani et al 24 did not detect any coarsening behavior at 400 C in argon either, whereas Chen et al 10 reported a pronounced coarsening at 600 C. Moreover, a stabilization of the structures could be reached by the latter in CO atmosphere and Biener et al 16 reported a stabilization by treatment with ozone compared to annealing in helium.…”

“…In addition, although gold is oen considered as inert metal, a stabilization has been further attributed to surface adsorbates. 16 Recently, Kuwano-Nakatani et al 24 found different coarsening behavior in vacuum or argon, compared to nitrogen or oxygen atmosphere. From different activation energies, it was concluded that different coarsening mechanisms dominate depending on the atmosphere.…”

Plasmonic nanosponges

“…During the coarsening, the small pores collapse initially very fast, and then this is followed by a stable stage of the coarsened structure at a constant temperature. The coarsening kinetics was recently well studied [56][57][58]. The details about the statistic analysis of the particle size, pore size, and ligament size can be found in the Ref.…”