Fabrication of Meso-Porous Sintered Metal Thin Films by Selective Etching of Silica Based Sacrificial Template

Abstract

Meso-porous metal materials have enhanced surface energies offering unique surface properties with potential applications in chemical catalysis, molecular sensing and selective separation. In this paper, commercial 20 nm diameter metal nano-particles, including silver and copper were blended with 7 nm silica nano-particles by shear mixing. The resulted powders were cold-sintered to form dense, hybrid thin films. The sacrificial silica template was then removed by selective etching in 12 wt% hydrofluoric acid solutions for 15 min to reveal a purely metallic meso-porous thin film material. The impact of the initial silica nano-particle diameter (7-20 nm) as well as the sintering pressure (5-20 ton·m^-2) and etching conditions on the morphology and properties of the final nano-porous thin films were investigated by porometry, pyknometery, gas and liquid permeation and electron microscopy. Furthermore, the morphology of the pores and particle aggregation during shear mixing were assessed through cross-sectioning by focus ion beam milling. It is demonstrated that meso-pores ranging between 50 and 320 nm in average diameter and porosities up to 47% can be successfully formed for the range of materials tested.

Keywords: meso-porous metal materials; metal nano-particle sintering; metal particle coalescence; silica template etching; surface texturing.

Figure 1

Schematic of the powder mixing, cold-compression and silica etching process leading to a meso-porous pure metal material.

Figure 2

Representative SEMs of the silica - silver cold-pressed composites at 20 ton·m⁻²; (A–C) are surface and (D–F) cross section images of a 10 wt% silica content composite. The scale bars of (A,D), (B,E) and (C,F) respectively represent 100 μm, 1 μm and 100 nm.

Figure 3

Representative EDS distributions of Si, Ag and O elements across a 10 wt% silica content cold-pressed composite before silica etching presented in Figure 1—carbon is present due to the sputtering step for sample preparation for SEM imaging.

Figure 4

Representative SEMs of the surface of an etched 10 wt% SiO₂-Ag composite similar to those presented in Figure 1; scale bars of (A), (B), (C) and (D), respectively correspond to 100 μm, 10 μm, 1 μm and 100 nm.

Figure 5

Representative SEMs of the cross-section of an etched 10 wt% SiO₂-Ag composite; scale bars of (A) and (B) are 1 μm and 200 nm respectively. The images were taken approximately 10 μm from the surface.

Figure 6

Representative EDS distributions of Si, Ag and O elements across a 10 wt% silica content cold-pressed composite after silica etching–a small amount of carbon can be detected residual from sample preparation for SEM imaging; no Si and minutes amount of O could be detected; the scale bar corresponds to 100 μm.

Figure 7

(A) Average pore size distribution obtained for the series of Ag-Si nano-composites after Si etching; (B) Average porosity for the same series; (C) BET surface area for the series of samples before and after etching and theoretical BET surface area calculated for an estimated individual surface area of the Si and Ag NP powders of 23.81 and 30 m²·g⁻¹. The surface area of the cold press sample at 0 wt% of SiO₂ was extremely low and thus 0 due to strong particle coalescence; and (D) FTD calculated surface energy for the main elements of the periodic table and visual representation of the position of Si, Cu and Ag. The 0 wt% silica data points for pore size and porosity were extrapolated since no data could be obtained neither by perm-porometry nor pyknometry.

Figure 8

N₂ permeance across the etched Si-Ag NP composites.

Figure 9

Representative SEMs of the surface of a 10 wt% Si-Cu composite before (A,B) and after (C,D) HF etching; scale bars of (A,C) and (B,D) respectively correspond to 2 μm and 200 nm.