Research ArticleFacile fabrication of ultrathin freestanding nanoporous Cu and Cu-Ag films with high SERS sensitivity by dealloying Mg-Cu(Ag)-Gd metallic glasses
Introduction
Nanoporous metals have attracted extensive attention due to their intriguing physical, chemical, and mechanical properties, which give rise to a wide range of potential applications in catalysts, sensors, supercapacitors, surface-enhanced Raman scattering (SERS)-active substrates, and so on [[1], [2], [3], [4], [5]]. During the past decades, numerous strategies have been proposed to synthesize a variety of nanoporous metals, including template methods [1,2,6], dealloying [7,8], galvanic replacement reactions [9], and electrochemical deposition [10,11]. Among these methods, the dealloying method has become one of the most efficient ways to fabricate nanoporous metals [[3], [4], [5], [6], [7], [8]]. During dealloying, as the less noble metal is selectively removed from the precursor alloy, the more noble metal atoms diffuse and reorganize into a well-defined three-dimensional bi-continuous porous structure with nanoscale ligaments/pores [7]. To date, many nanoporous metals (e.g., Au, Pt, Pd, Ag, Cu, CuAg, PtFe) have been prepared by dealloying various crystalline precursor alloys [3,4,6,7,12,13] or metallic glass precursor alloys [5,[14], [15], [16], [17], [18]]. In recent decades, as a special type of nanoporous metals, metallic ultrathin freestanding nanoporous films (UF-NPFs) have attracted increasing attention due to their unique structures, properties, and considerable potential for a broad range of applications, such as separation membranes, sensors, flexible SERS substrates, and catalysts [[19], [20], [21], [22], [23], [24]]. However, because nanoporous metals prepared by the dealloying approach usually maintain the thickness and shape as of the corresponding precursor alloys [12,25], the primary challenge in preparing metallic UF-NPFs by dealloying is the preparation of ultrathin freestanding precursor alloy films. Generally, the simplest and most direct way to prepare ultrathin freestanding precursor alloy films is continuous rolling. For example, an Au-Ag alloy sample with superplasticity can be hammer-rolled into ultrathin freestanding Au-Ag alloy film with a thickness of 100−200 nm. Thus, Au UF-NPFs (100−200 nm in thickness) can be easily prepared by dealloying ultrathin freestanding Au-Ag alloy films [7,19]. However, such a superplastic characteristic is only demonstrated in very few alloy systems, and most precursor alloys used for dealloying usually contain brittle intermetallic compounds, which cannot be rolled into ultrathin freestanding precursor alloy films. For example, to prepare nanoporous Cu or Ag by dealloying, the less noble elements in the precursor alloys mainly include Mg, Zn, Al, and Mn, resulting in the formation of brittle intermetallic compounds (such as Mg2Cu and Zn3Ag) in the precursor alloys [12,13,26,27]. Thus, it is unrealistic to prepare Cu or Ag UF-NPFs by dealloying rolled ultrathin freestanding precursor alloy films. An alternative approach to prepare precursor alloy films is melt spinning. However, the minimum thickness of the alloy films prepared by melt spinning is usually greater than 10 μm. Therefore, the preparation of metallic UF-NPFs by the ordinary dealloying approach remains a great challenge.
SERS, featured with superior surface sensitivity and specificity, has become a very powerful technique in several sensing fields, such as detection of pollutants in water, air and soil [28,29]; in vivo drug tracking [30]; and in situ monitoring of chemical reactions [31]. In addition to the high sensitivity of analyses, the recently practical implementation of SERS substrates for real in-situ analyses requires the introduction of flexible SERS substrates for express, outdoors measurements [32]. Compared with conventional rigid substrates (e.g., silicon, glass), flexible substrates can be wrapped onto curved surfaces for in situ detection and can easily be cut into arbitrary shapes and sizes [33]. Generally, flexible SERS substrates can be prepared by the assembly of metal nanoparticles on flexible substrates [22,34], deposition of metal nanoparticles on the surfaces of polymer thin films [32,35], embedding of metal nanostructures in fiber arrays, and using template-assisted techniques [[36], [37], [38]]. However, most of these methods are complex and require multiple steps.
In this work, we present a novel dealloying strategy on the facile preparation of flexible Cu and Cu-Ag (Cu3Ag and CuAg) UF-NPFs by dealloying thick Mg-Cu(Ag)-Gd metallic glass ribbons with a low glass transition temperature (Tg < 150 °C). The phase constitution, morphology, and element distribution of the as-prepared Cu, Cu3Ag, and CuAg UF-NPFs were investigated in detail. The formation mechanism of the as-prepared Cu, Cu3Ag, and CuAg UF-NPFs was revealed by investigating the morphology and composition of the sequential intermediates sampled at different reaction times. Moreover, the SERS performance of the as-prepared Cu, Cu3Ag, and CuAg UF-NPF substrates was investigated and discussed.
Section snippets
Experimental
Mg61Cu28Gd11, Mg61Cu21Ag7Gd11, and Mg61Cu14Ag14Gd11 (at.%) alloy ingots were prepared by induction melting mixtures of pure Mg (>99.95 wt.%), Cu (>99.95 wt.%), Ag and Gd (>99.9 wt.%) metals in an argon atmosphere. Mg61Cu28Gd11, Mg61Cu21Ag7Gd11, and Mg61Cu14Ag14Gd11 metallic glass ribbons with widths of 3–5 mm and thicknesses of 150−200 μm were prepared by a single copper roller melt-spinning process. The Cu, Cu3Ag, and CuAg UF-NPFs were prepared by simply dipping Mg61Cu28Gd11, Mg61Cu21Ag7Gd11,
Preparation and characterization of Cu, Cu3Ag, and CuAg UF-NPFs
Mg-Cu(Ag)-Gd metallic glass (Mg61Cu28Gd11, Mg61Cu21Ag7Gd11, and Mg61Cu14Ag14Gd11) ribbons with thicknesses of 150−200 μm were used as precursor alloys for the preparation of Cu, Cu3Ag, and CuAg UF-NPFs, respectively. Dilute HCl/ethanol solutions with volume ratios of 12 mol L−1 HCl aqueous solution to ethanol of 1:200, 1:500, and 1:500 were used as the erosion solution for the synthesis of Cu, Cu3Ag, and CuAg UF-NPFs, respectively. Because of the significant difference in the standard electrode
Conclusion
In conclusion, we present a novel strategy to prepare flexible Cu, Cu3Ag, and CuAg UF-NPFs with thicknesses of ∼200 nm by dealloying the outer surface of thick Mg-Cu(Ag)-Gd metallic glass ribbons. The formation mechanism of such metallic UF-NPFs was investigated by analyzing the different intermediate products in detail. It was revealed that the local reaction latent heat-induced glass transition of the metallic glass ribbons during dealloying plays a key role in the self-peeling of the Cu, Cu3
Acknowledgements
This work was financially supported by the National Natural Science Foundation of China (Nos. 51671206 and 51871056), the foundation from the Department of Education of Guangdong Province (No. 2018KZDXM069), and the Natural Science Foundation of Guangdong Province (No. 2019B030302010).
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