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A Scalable Synthesis Pathway to Nanoporous Metal Structures Christopher Coaty, Hongyao Zhou, Haodong Liu, and Ping Liu ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.7b06667 • Publication Date (Web): 08 Jan 2018 Downloaded from http://pubs.acs.org on January 9, 2018
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ACS Nano
A Scalable Synthesis Pathway to Nanoporous Metal Structures
Christopher Coaty, Hongyao Zhou, Haodong Liu, Ping Liu*
Department of NanoEngineering, University of California – San Diego, La Jolla, Ca, 92093, USA
*Corresponding Author:
[email protected] 1 ACS Paragon Plus Environment
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Abstract A variety of nanoporous transition metals, Fe, Co, Au, Cu, and others, have been readily formed by a scalable, room temperature synthesis process. Metal halide compounds are reacted with organolithium reductants in a nonpolar solvent to form metal/lithium halide nanocomposites. The lithium halide is then dissolved out of the nanocomposite with a common organic solvent, leaving behind a continuous, three-dimensional network of metal filaments that form a nanoporous structure. This approach is applicable to both noble metals (Cu, Au, Ag) and less-noble transition metals (Co, Fe, Ni). The microstructures of these nanoporous transition metals are tunable, as controlling the formation of the metal structure in the nanocomposite dictates the final metal structure. Microscopy studies and nitrogen adsorption analysis show these materials form pores ranging from 2 nm to 50 nm with specific surface areas from 1.0 m2/g to 160 m2/g. Our analysis also shows that pore size, pore volume and filament size of the nanoporous metal networks depend on the mobility of target metal and the amount of lithium halide produced by the conversion reaction. Further, it has been demonstrated that hybrid nanoporous structures of two or more metals could be synthesized by performing the same process on mixtures of precursor compounds. Metals (e.g., Co and Cu) have been found to stabilize each other in nanoporous forms, resulting in smaller pore sizes and higher surface areas than each element in their pure forms. This scalable and versatile synthesis pathway greatly expands our access to additional compositions and microstructures of nanoporous metals.
Keywords: nanoporous metals, nanopores, nanocomposites, lithium conversion reactions, transition metals, 3D nanostructures 2 ACS Paragon Plus Environment
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Nanoporous Metals (NPMs) are nanostructured pure metals with pores ranging from approximately 1-100 nm in diameter. They have been sought-after for their combination of metallic characteristics and nanostructured size-effect properties.
They retain the desirable
features of bulk metal, such as thermal/electrical conductivity, ductility, and malleability, while gaining beneficial nanostructured properties such as low density and high surface area.1 It has also been reported that these nanostructured metals exhibit size-enhanced effects, such as higher catalytic activity and plasmonic resonance.2-4
Their high surface area also marks them as
promising materials for battery and capacitor electrodes.5, 6 Further, NPMs are being explored for actuation applications because the topology and connectivity of these ligamented networks can be modulated with surface charge, causing dimensional change.7 Many synthesis routes for nanoporous metals have been developed in the past 30 years,8, 9 the oldest and most widely researched being the dealloying method. This method involves chemically or electrochemically removing the less-noble element from an alloy, leaving behind a nanoporous metal.10, 11 The prototypical example of this technique is the synthesis of nanoporous gold by etching the silver from a silver-gold alloy. Dealloying can produce structures with tunable porosities by varying reaction conditions and alloy compositions, and is most effective when the target is a more noble metal (Au, Ag, Cu, Pt, Pd).1, 11, 12 The formation of NPMs via dealloying has been explained by a continuum model by Erlebacher et al. which states that pores form when the less-noble alloy component dissolves and the noble element atoms left behind are continuously driven to aggregate into 2-D clusters by phase separation at the solid-electrolyte interface.12 It is also possible to use dealloying to fabricate nanoporous metals of less-noble transition metals (e.g. Ni, Fe, Co, etc.) from their respective alloys with even more reactive metals such as Al.13 In addition, there are more specialized synthesis methods for less-noble nanoporous metals, such as 3 ACS Paragon Plus Environment
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combustion synthesis,9 block-copolymer templating,14 or directed assembly of metal nanoparticles.15, 16 This study details a simple, versatile, scalable synthesis route to nanoporous metals via solidstate conversion reactions. Unlike the dealloying process, where the porous structure forms simultaneously while removing the less-noble element of an atomically mixed alloy, this method first fashions a nanocomposite of the desired pure metal with an ionic compound that can be removed by dissolution with a common organic solvent. This method is qualitatively similar to dealloying and other selective etching processes employed in nanomaterials synthesis, such as the techniques used to make MXenes.17, 18 The advantage of this process is that it utilizes an organic solvent rather than an acid as an etchant, so it is applicable to both noble and less-noble pure metals, as well as metal mixtures and alloys. Access to these nanoporous transition metal systems is particularly relevant to application in catalysis and magnetic materials, where alloys and nanometer-scale mixtures are highly desired.9, 19, 20 Conversion reactions between lithium and transition metal halides have been previously studied to yield high capacity battery electrodes. These reactions follow the general formula, 𝑀𝐴𝑥 + 𝑥𝐿𝑖 → 𝑀 + 𝑥𝐿𝑖𝐴, where the transition metal halide (𝑀𝐴x) is reduced by Li to form a pure metal (M) and a corresponding lithium halide (LiA).21 For example, the reaction between Li and FeF3 has been studied extensively due to its theoretical energy density of 1750 Wh/kg, 2-3 times the capacity of state-of-the-art oxide materials.22,
23
A TEM study observed that after
discharge and complete conversion of an FeF3 battery cathode following the equation above, the morphology of the Fe/LiF composite consisted of an ‘interconnected’ and ‘bicontinuous’ network of Fe nanoparticles surrounded by a LiF matrix.24 The study observed that the Fe nanoparticles were approximately 2-3 nm in width. We reasoned that if the LiF were removed from the 4 ACS Paragon Plus Environment
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nanocomposite without damaging the Fe nanoparticle network, the resulting material would be a nanoporous metal.
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Results and Discussion Nanoporous Pure Metals (NPMs)
To employ the conversion reaction for the synthesis of nanoporous metals, a chemical method is more desirable than an electrochemical one in terms of scalability and material purity. We chose to conduct the reactions at ambient temperatures using a reducing agent in a solution. The selection rule is that both the transition metal halide and the lithium halide should be insoluble in the solution containing the reducing agent. Then, the lithium halide must be soluble in a separate solution which is stable towards the metal. We chose an organolithium reducing agent, specifically nbutyllithium in hexane, which met all requirements.25 Since LiF is difficult to dissolve with common solvents, we focused on chloride and bromide compounds because the LiCl or LiBr produced by the reaction is easily dissolved with a variety of common organic solvents such as ethanol, acetone, and methanol. We chose methanol in this study because most lithium halides and transition metal halides have higher solubility in methanol compared to other organic solvents. This concept for conversion synthesis of nanoporous metals is illustrated in Figure 1. We utilized this conversion synthesis method to prepare nanoporous Co, Fe, Ni, Cu, Ag and Au from their respective chloride precursors. Scanning Electron Microscope (SEM) images of the resulting nanoporous metals are displayed in Figure 2a-h. Qualitatively, each porous structure is a network of thin metal filaments with each precursor producing a different morphology with a characteristic filament thickness and network density. Nanoporous Fe from the FeCl3 precursor showed exceptionally small pore size and its features are difficult to discern from SEM alone, so the sample was also characterized with Transmission Electron Microscopy (TEM), shown in Figure 3. The TEM images show that the nanoporous Fe is made of similar randomly packed 6 ACS Paragon Plus Environment
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filaments to the other samples, but at a smaller length scale of a few nanometers. The electron diffraction pattern indicates that the nanoporous Fe is apparently amorphous. Both the nanocomposites and the purified nanoporous metals were characterized via X-Ray Diffraction (XRD) to confirm the complete conversion of each precursor to a metal/Li halide nanocomposite and complete removal of the Li halide after the methanol purification. FourierTransform Infrared spectroscopy (FTIR) and Energy-Dispersive X-ray spectroscopy (EDX) also confirm that no residual organic compounds remain in the nanocomposites after the conversion reaction, as shown in supporting information (Figure S7, Figure S8). XRD characterization of the composites shows distinct peaks for both the Li halide and the target metal, indicating that they exist in the nanocomposite as two distinct phases (Figure 4 and Figure S1). All samples show a degree of crystallographic peak broadening; a sign of nanocrystal formation. For Co and Fe samples, the peak broadening is so extreme that the peaks are barely perceptible, suggesting that the metal crystal structure for these samples is largely amorphous.26, 27 We employed Brunauer-Emmett-Teller (BET) analysis on the purified nanoporous metals to measure specific surface area and to estimate pore size distribution and cumulative pore volume. N2 adsorption-desorption isotherms and pore size distributions are displayed in Figure 5 and in the supporting information (Figure S2) while structural and surface data for all samples are tabulated in Table 1 for comparative analysis. When comparing the porous structures, average pore width and ligament thickness correlate with one another and are both inversely proportional to specific surface area. All samples produce type IV N2 adsorption-desorption isotherms,28 and for samples with exceptionally small pore width, their isotherms exhibit type H3 hysteresis between the adsorption and desorption curves.29
This phenomenon is indicative of capillary condensation of
the nitrogen, and has been previously documented in similar porous metal and metal oxide 7 ACS Paragon Plus Environment
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structures.30-32 The adsorption-desorption isotherm hysteresis only appears for samples with pore width
