Facile fabrication of ultrathin freestanding nanoporous Cu and Cu-Ag films with high SERS sensitivity by dealloying Mg-Cu(Ag)-Gd metallic glasses

doi:10.1016/j.jmst.2020.08.049

Abstract

Abstract:

Nanoporous metals prepared by dealloying have attracted increasing attention due to their interesting size-dependent physical, chemical, and biological properties. However, facile fabrication of metallic ultrathin freestanding nanoporous films (UF-NPFs) by dealloying is still challenging. Herein, we report a novel strategy of facile preparation of flexible Cu, Cu₃Ag, and CuAg UF-NPFs by dealloying thick Mg-Cu(Ag)-Gd metallic glass ribbons. During dealloying, the local reaction latent heat-induced glass transition of the precursor ribbons leads to the formation of a solid/liquid interface between the initially dealloyed nanoporous layer and the underlying supercooled liquid layer. Due to the bulging effect of in situ generated H₂ on the solid/liquid interface, Cu, Cu₃Ag, and CuAg UF-NPFs with thicknesses of ~200 nm can self-peel off from the outer surface of the dealloying ribbons. Moreover, it was found that the surface-enhanced Raman scattering (SERS) detection limit of Rhodamine 6G (R6G) on the Cu and CuAg UF-NPF substrates are 10 ^-6 M and 10 ^-11 M, respectively, which are lower than most of the Cu and Cu-Ag substrates prepared by other methods. This work presents a reliable simple strategy to synthesize a variety of cost effective and flexible metallic UF-NPFs for functional applications.

Key words: Metallic glasses, Dealloying, Nanoporous metals, Ultrathin freestanding films, Surface-enhanced Raman scattering (SERS)

Yuan-Yun Zhao, Feng Qian, Chengliang Zhao, Chunxiao Xie, Jianguo Wang, Chuntao Chang, Yanjun Li, Lai-Chang Zhang. Facile fabrication of ultrathin freestanding nanoporous Cu and Cu-Ag films with high SERS sensitivity by dealloying Mg-Cu(Ag)-Gd metallic glasses[J]. J. Mater. Sci. Technol., 2021, 70: 205-213.

Figures/Tables 8

Fig. 1. Synthesis of Cu and Cu-Ag UF-NPFs via dealloying Mg-Cu(Ag)-Gd metallic glass ribbons. (a) Photographs of a typical synthetic procedure for Cu UF-NPFs by dealloying thick Mg61Cu28Gd11 metallic glass ribbon. (b) Photograph of a capsule shell-like Cu3Ag UF-NPF by dealloying Mg61Cu21Ag7Gd11 metallic glass ribbon. (c) Photograph showing that the remaining Mg61Cu21Ag7Gd11 metallic glass ribbon can be removed by tearing off the capsule shell-like Cu3Ag UF-NPF. (d-f) SEM images of the as-prepared Cu, Cu3Ag, and CuAg UF-NPFs, respectively. (g-i) High-magnification SEM images of the as-prepared Cu, Cu3Ag, and CuAg UF-NPFs, respectively.

Fig. 2. XRD patterns of the Mg-Cu(Ag)-Gd metallic glass precursors and the as-prepared Cu, Cu3Ag, and CuAg UF-NPFs. The green and red vertical lines represent the reference diffraction peaks of pure Ag and Cu, respectively.

Fig. 3. TEM images of the ligaments in Cu (a), Cu3Ag (b), and CuAg UF-NPFs (c). High-magnification TEM images of the ligaments in Cu (d), Cu3Ag (e), and CuAg UF-NPFs (f). SAED patterns of the Cu (g), Cu3Ag (h), and CuAg UF-NPFs (i).

Fig. 4. STEM images, in a mapping mode, of typical ligaments in a Cu3Ag UF-NPF: (a) TEM image, (b) high-angle annular dark field image (HAADF), (c) Cu distribution; and (d) Ag distribution. (e) High-magnification TEM image of the marked region in (a). (f, g) High-magnification TEM images in region A and B, respectively. STEM images of typical ligaments in a CuAg UF-NPF: (h) TEM image, (i) HAADF image, (j) Cu distribution, and (k) Ag distribution.

Fig. 5. Morphology of the sequential intermediates sampled at different reaction times. (a) SEM image of the outer surface of the Mg61Cu28Gd11 metallic glass precursor ribbon. (b) DSC curves of the Mg-Cu(Ag)-Gd metallic glass precursor ribbons. (c) SEM image of the rudimentary nanoporous Cu layer (layer I) and the local “melted” layer (layer II) at 3 min. (d) High-magnification SEM image of the local “melted” layer at 3 min. (e) SEM image of the Cu-rich UF-NPF at 6 min. (f) High-magnification SEM image of the Cu-rich UF-NPF at 6 min.

Fig. 6. Schematic illustration of the formation mechanism of the metallic UF-NPFs. (a) Formation of rudimentary nanoporous Cu(Ag) on the outer surface of the metallic glass precursor ribbon. (b) Glass transition (T>Tg) occurs within a layer between the rudimentary nanoporous Cu(Ag) and the metallic glass ribbon core. (c) Separation of the Cu(Ag)-rich UF-NPF from the supercooled liquid layer due to the bulging effect of in situ-generated H2 on the solid/liquid interface. (d) Self-peeling of the Cu(Ag) UF-NPF from the outer surface of the ribbon.

Fig. 7. Raman spectra of Cu and Cu-Ag UF-NPFs at different concentrations of R6G. (a) Raman spectra of the Cu UF-NPF at different concentrations of R6G, together with the Raman spectra of a 10-4 M R6G ethanol solution on a glass substrate; (b) Raman spectra of the Cu3Ag and CuAg UF-NPFs at different concentrations of R6G.

Table 1 Comparison of the detection limit of R6G between the proposed Cu, Cu3Ag, CuAg UF-NPFs, and previously reported Cu-based SERS substrates.

SERS substrates	Prepared method	Detection limit of R6G	Refs.
Cu films	Magnetron sputtering	10^-6 M	[52]
Cu nanodot films	Electrodeposition	10^-4 M	[53]
Nanoporous Cu	Dealloying	10^-5 M	[4]
Nanoporous Cu foils	Hydrothermal method	10^-6 M	[22]
Nanoporous Cu	Dealloying	10^-5 M	[54]
Ag@Nanoporous Cu	Dealloying + displacement reaction	10^-8 M	[6]
Cu-Ag nanowires	Displacement reaction	10^-8 M	[55]
Cu-Ag films	Vacuum thermal evaporation	10^-9 M	[56]
Nanoporous Cu-Ag	Dealloying	10^-11 M	[5]
Nanoporous Au	Dealloying	10^-10 M	[57]
Cu UF-NPFs	Dealloying	10^-6 M	This work
Cu₃Ag UF-NPFs	Dealloying	10^-8 M	This work
CuAg UF-NPFs	Dealloying	10^-11 M	This work

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