Letter pubs.acs.org/NanoLett
Ultralight Conductive Silver Nanowire Aerogels Fang Qian,*,† Pui Ching Lan,† Megan C. Freyman,† Wen Chen,‡ Tianyi Kou,§ Tammy Y. Olson,† Cheng Zhu,‡ Marcus A. Worsley,† Eric B. Duoss,‡ Christopher M. Spadaccini,‡ Ted Baumann,† and T. Yong-Jin Han*,† †
Physical and Life Science Directorate, ‡Engineering Directorate, Lawrence Livermore National Laboratory, Livermore, California 94550, United States § Department of Chemistry and Biochemistry, University of California, Santa Cruz, California 95064, United States S Supporting Information *
ABSTRACT: Low-density metal foams have many potential applications in electronics, energy storage, catalytic supports, fuel cells, sensors, and medical devices. Here, we report a new method for fabricating ultralight, conductive silver aerogel monoliths with predictable densities using silver nanowires. Silver nanowire building blocks were prepared by polyol synthesis and purified by selective precipitation. Silver aerogels were produced by freeze-casting nanowire aqueous suspensions followed by thermal sintering to weld the nanowire junctions. As-prepared silver aerogels have unique anisotropic microporous structures, with density precisely controlled by the nanowire concentration, down to 4.8 mg/cm3 and an electrical conductivity up to 51 000 S/m. Mechanical studies show that silver nanowire aerogels exhibit “elastic stiffening” behavior with a Young’s modulus up to 16 800 Pa. KEYWORDS: Metal foams, silver nanowires, aerogel, ultralight, conductive
M
176 S/m, and Young’s moduli from 1.74 to 12.96 Pa. A year later, the same group improved the CuNW aerogel properties by applying a trace amount of poly(vinyl alcohol) to protect the aerogel surface; such CuNW−polymer aerogels exhibited enhanced mechanical robustness and chemical stability while maintaining good electrical conductivity.12 Jung et al. reported an alternative method to produce CuNW aerosponge, through the gelation of a concentrated CuNW suspension followed by critical point drying.13 Because the aerosponge was made from a direct assembly of interconnected 3D network, rather than from a dilute nanowire suspension, the authors obtained a very good electrical conductivity from 1300 to 11 600 S/m, with densities of 3.79−7.5 mg/cm3, and a Young’s modulus up to 1200 Pa. Besides CuNWs, there are also attempts to use silver nanowire (AgNWs) to produce conductive aerogels, because Ag is more conductive than Cu and more chemically stable against oxidation. The most widely used method is to dip-coat a macroporous foreign template into AgNW suspension, so that the template becomes conductive due to the surface coating of AgNWs.15−20 Numerous template materials, including cotton,15 polymer,16,17 carbon,18 and graphene19,20 have been demonstrated to generate lightweight and conductive foams. However, the binary composition of these foams may limit their application when the organic template is deleterious. The first
etal foams (or porous metals) represent a new class of materials with a unique combination of properties including lightweight, high surface area, high electrical conductivity, and low thermal conductivity, that can advance technologies in electronics, thermal insulation, sensing, catalysis, and energy storage.1−3 Conventional methods for producing metal foams include powder metallurgical process,4 combustion methods,5 dealloying,6,7 or plating of metal films on existing porous templates.8−10 Often these methods require demanding manufacturing conditions (e.g., high temperature, high pressure, strict oxygen exclusion) or are not scalable for practical device applications. In addition, ultralight monolithic metal foams with various densities are technically challenging to fabricate because (1) the densities of metals are much higher than those of nonmetal aerogel materials. For example, the density of silver, 10.49 g/cm3, is ca. 5-times larger than silica (2.65 g/cm3) or carbon (2.27 g/cm3); (2) conventional methods usually cannot offer tunable foam densities below 10 mg/cm3 due to respective limitations. Recent advances in metal nanowire (NW) synthesis enable new methodologies of ultralight metal foam production under mild conditions, with predictable densities and flexible material choices, and can potentially be scaled up.11−14 Starting with an aqueous suspension of metal NWs, ultralight porous metal aerogels can be formed by freeze-drying or critical point drying. Tang et al. reported the fabrication of copper nanowire (CuNW) aerogel monoliths by freeze-drying CuNW suspensions.11 Asmade Cu aerogels have various densities from 15.55 mg/cm3 down to 4.6 mg/cm3, with electrical conductivities from 90 to © 2017 American Chemical Society
Received: July 1, 2017 Revised: August 21, 2017 Published: September 5, 2017 7171
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Nano Letters
growth as well as NP growth. For the purpose of creating ultralight conductive Ag foams, NPs contribute negatively to metal aerogel performance, as they do not percolate yet add significant weight. We removed the AgNPs by selective precipitation of NWs in acetone.23 In short, AgNW solution in EG was first washed repeatedly with water to transfer all of the AgNWs/NPs into an aqueous stock solution. Then acetone was added to the stock solution, causing AgNW to precipitate selectively, while the AgNPs remained in the supernatant. After discarding the supernatant and redispersing the pellet in water, the nanowires were significantly enriched and purified. The process can be repeated as needed. The application of the selective precipitation procedure proves to effectively remove NPs and generate almost 100% pure NW morphologies (Figure 1d, Figure S1). More details of synthesis and purification procedure can be found in Supporting Information. The crystal structure of Ag NWs was further characterized by X-ray powder diffraction (XRD) and transmission electron microscopy (TEM). According to the XRD results, purified Ag NWs exhibited two distinct peaks at 2θ = 38.1, 44.3, and a small peak at 64.4, corresponding to the {111}, {200}, and {220} planes, respectively (Figure S1). The calculated lattice constant from this XRD pattern is 4.086 Å, matching close to the reported value of 4.09 Å of bulk Ag (JCPDS card no. 04-0783), indicating the fcc structure of silver.24 In addition, TEM analysis together with selective area electron diffraction pattern shows that Ag NWs had a penta-twinned crystal structure and grow along [011] direction (Figure 1e,f). These results suggest Ag NWs are enclosed by five {100} side facets and ten {111} end facets, consistent with previous reports.21,22 High-resolution TEM images reveal the NW surface was capped by a few nm thin layer of PVP (Figure 1g). Although the PVP coating helps protect the Ag NW surface from oxidation and enables good NW dispersion in water, it also prevents NWs from direct contact and as a result yields high electrical resistivity. The effect of PVP coating will be discussed in following text. Purified AgNWs were used as the building blocks to fabricate conductive aerogels. As shown in Figure 2a, the AgNW suspension with a known NW concentration was vortexed in a glass vial and then immediately placed on a metal block precooled in liquid N2. Due to the vertical temperature gradient, ice crystals nucleated at the bottom and grew upward. As ice crystal grew, Ag NWs were redistributed accordingly into well-aligned ice lamella. The frozen NW suspension was then lyophilized, leaving behind a highly porous percolated network of nanowires, with NW junctions being weakly connected by the van der Waals force. The AgNW aerogels were then sintered in hydrogen gas (H2) at 250 °C for 1 h to burn out the surface layer of PVP and to weld the NW junctions. Figure 2c shows a typical aerogel cylinder with a 4 mm diameter and 25 mm length in a glass vial. A variety of other geometries, such as discs, spheres, and hemispheres, were also made using specially designed molds. Importantly, the use of NW suspension allows us to prepare aerogels of predictable densities. Figure 2b shows a plot of measured aerogel density vs NW concentration. The linear curve fitting yields a correlation coefficient of 0.9845, indicating a minimal loss of NW materials during the aerogel fabrication. By using diluted NW suspensions, the lowest density we achieved is 4.8 mg/cm3. Below this density, aerogels collapse due to the lack of building block materials to form an interconnected network. A representative SEM image of the aerogel made by freezecasting reveals highly anisotropic pore structures, with an
(and also the only) AgNW aerogel monoliths was reported by Jung et al., via the hydrogel formation from dilute NW suspensions to the isotropic concentrated regime.14 Using a commercially available suspension of AgNWs, the authors fabricated a Ag aerogel sample having a density of 88 mg/cm3 and a very high conductivity of 3 × 106 S/m. Here we report a new method to fabricate ultralight AgNW aerogels with tunable densities, controlled pore structures, and improved electrical conductivity and mechanical properties compared to previously reported materials. Using this method, we for the first time produced high-performance Ag foams with an ultralow density down to 4.8 mg/cm3, a high electrical conductivity up to 51 000 S/m, and a high Young’s modulus of 16 800 Pa. AgNWs were prepared through a modified polyol process.21,22 Figure 1a shows a schematic of our synthetic
Figure 1. Synthesis and purification of AgNWs. (a) Schematic illustration of the synthesis and purification approach. (b) Digital photograph of AgNW suspension in ethylene glycol. (c, d) SEM images of unpurified and purified AgNWs. Scale bars are 20 μm. (e) A representative low-resolution TEM image of Ag NWs. Scale bar is 1 μm. (f) A zoom-in TEM image of the NW end. The scale bar is 100 nm. Inset: electron diffraction pattern collected from the same NW. (g) High-resolution TEM image of the NW surface. The scale bar is 10 nm.
and purification approach. For a typical synthesis, ∼100 mL of ethylene glycol (EG) containing 0.05 mM NaCl, 0.189 M polyvinylpyrrolidone (PVP, MW = 55k), 0.0014 mM AgNO3, and 0.017 mM CuCl2 were added to a round-bottom flask and preheated at 185 °C in an oil bath for 1 h. Subsequently, 30 mL of freshly prepared AgNO3 in EG (0.12M) was added dropwise with vigorous stirring. After the reaction was completed, the flask was removed from the oil bath and cooled down to room temperature. As-prepared Ag NWs in EG suspension appeared to be shiny, silky white (Figure 1b). Scanning electron microscope (SEM) images show these NWs have a uniform diameter of 50−100 nm, with a typical length of 40−80 μm, and accompanied by a large amount of Ag nanoparticles (AgNPs) as the main byproduct (Figure 1c, Figure S1). NPs are commonly observed in the polyol synthesis of NWs due to the generation of different seed types which results in both NW 7172
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from a welded junction shows double diffraction spots along two perpendicular directions, which is ascribed to epitaxial recrystallization, consistent with previous report (Figure S4).25 Furthermore, high-resolution TEM analysis show that the PVP coating on the NW surface was removed, and there is no evidence of silver oxide or silver hydroxide on the surface (Figure 2h). Fourier transform infrared spectroscopy (FT-IR) and thermogravimetric analysis (TGA) studies confirmed this observation (Figure S5). In the FT-IR spectra, pure PVP powder has five characteristic IR peaks at 1290, 1424, 1663, 2953, and 3449 cm−1, while these peaks can be barely seen in as-made aerogels and are absent in sintered aerogels.26 TGA analysis reveals that PVP started to decompose slowly above 100 °C and rapidly above 300 °C; therefore, our thermal treatment at a relatively low temperature (250 °C) is still effective to remove the few-nanometers-thick PVP coating given enough time. The electrical conductivity of aerogels was characterized as a function of density by four-probe measurements. First, compared to nonsintered aerogels at similar densities, sintered samples exhibited 2−3 orders of magnitude higher conductivity (Figure S6a). This observation again confirms sintering effectively lowers the junction resistances that dominate the apparent aerogel conductivity. Second, for the sintered AgNW aerogels, when the foam density increases from ca. 10 to 50 mg/cm3, the sheet resistance drops from 2673 to 0.079 Ω/□, yielding increasing electrical conductivity accordingly from 1.5 to 5.1 × 104 S/m (Figure 3a, Figure S6b). At densities larger than 50 mg/cm3, we obtained a large deviation of electrical
Figure 2. Fabrication of AgNW aerogels. (a) Schematic illustration of the aerogel fabrication procedure. (b) Plot of measured aerogel density vs estimated NW concentration. (c) Digital photograph of an aerogel cylinder contained in a vial. (d,e) SEM images of aerogel microstructures. Scale bars are 100 and 10 μm, respectively. (f,g) Lowresolution TEM images of welded NW junctions. Red arrows indicated the position of two such junctions. Scale bars are 500 and 100 nm, respectively. (h) High-resolution TEM image of NW surface. The scale bar is 5 nm.
averaged pore size of 25 μm (Figure 2d, Figure S2). The pores are defined by a higher density aligned NWs in the perpendicular direction and filled with low-density interconnected NWs (Figure 2e), which we believe helps reinforce the mechanical stability and enhance electrical conductivity of the aerogel, yet contributes minimal weight. In contrast, aerogels prepared by an “isotropic-freezing”, that is, submersion of the NW suspension into liquid N2 which causes solidification to converging inward, showed similar pore sizes yet spherical pore structures (Figure.S3). Our preliminary data showed qualitatively that freezing Ag NW suspension in different cooling agents (e.g., liquid N2, dry ice/acetone, or −50 °C freezer) yielded distinct pore sizes. Further studies are ongoing to enable quantitative control/analysis of pore geometries. It also worth mentioning that different micropore anisotropies of aerogels may exert impact on their mechanical, electrical and thermal properties, and this would be an interesting research topic to explore in the future. The thermal annealing process did not change the aerogel shape or cause any observable shrinkage. Aerogels with and without sintering appeared the same in color and morphology, although they behaved differently in contact with water. While sintered aerogels kept integrity and floated on water, nonsintered aerogel immediately fell apart and “dissolved” because PVP is soluble in water (Figure S4). TEM analysis of annealed samples showed that thermal sintering caused significant local change in morphology of NW junctions. As shown in Figure 2f,g and Figure S4, prior to sintering, individual NWs were distinct throughout the junctions, whereas after sintering, the junctions were clearly welded, while away from the junctions the NWs were not affected. Selected area electron diffraction pattern collected
Figure 3. Electrical properties of AgNW aerogels. (a) Representative data collected from sintered aerogels; (b) Relative conductivity (σ/σs) vs relative density (ρ/ρs) plot of AgNW aerogel samples were compared with previously reported CuNW aerogels, CuNW aerosponges and AgNW aerogel. The dashed line is a guide for the eyes. 7173
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Nano Letters conductivity, depending on where the probes were placed, possibly due to NW segregations which causes local inhomogeneity. We also note that the AgNW aerogels reported in this work have much enhanced conductivities compared to the CuNW aerogels prepared by the same freeze-casting method (176 S/m),11 which we attributed to junction welding and high aspect-ratio of AgNWs. The AgNW aerogel made by the hydrogel method previously reported has the highest electrical conductivities (3 × 106 S/m) to date.14 Here we would like to point out that these conductivity measurements were performed using various sample geometries, equipment, and calculation methods. Results may not be directly comparable, and calculation errors might be present. For example, two-terminal measurement does not exclude the influence of contact resistance and could generate underestimated conductivity results. Third, based on percolation theory, a power-law scaling of relative conductivity (σ/σs) vs relative density (ρ/ρs) yielded an exponent of 2.9 (for Ag, its bulk conductivity σs ≈ 6.3 × 107 S/m, and bulk density ρs = 10.49 g/cm3) (Figure 3b). This value is larger than that of Cu foams fabricated by the lost carbonate sintering method (n = 0.91)27 and the CuNW aerogels fabricated by freeze-drying (n = 0.81).11 It has been predicted previously that for a perfectly random percolated network where junction resistance dominates, the scaling exponent would be 1.3 for 2D systems and 2.0 for 3D systems.13,28,29 Our results of n = 2.9 are consistent with the previously reported CuNW aerosponge (n = 3.2) and could be an indication of good conductivity from both individual nanowires and the contacts between them.13 However, considering the 3D geometry, structural anisotropy at multiple length scales, and the substantially reduced junction resistance of our AgNW aerogels, current models for 2D/3D network may not be sufficient to describe the behavior of our Ag aerogels. To better understand such complicated NW-based percolated 3D systems, more simulation and experimental work are needed in the future. In future work, to further reduce the density and enhance the electrical conductivity of AgNW aerogels, we believe several factors should be considered: (1) Increase the length of NW building blocks. Because the global aerogel resistance is largely determined by the junction resistance, using long nanowire to construct the aerogel is expected to reduce the total number of junctions while still maintaining the mechanical stability. (2) Reduce the diameter of NWs. The Ag NWs used in this study have an average diameter of 80 nm, larger than the electron mean free path in bulk silver (∼52 nm).30 Reducing the NW diameter to ∼50 nm will drastically reduce the weight of aerogels yet not affect much the single nanowire conductivity. (3) Reduce the pore size. Previous research has shown that the slow freezing rate leads to a small mean pore size and high aerogel strength.31 Our current freeze-casting method involving using liquid N2 yielded a large mean pore size of ∼25 μm, suggesting nonuniform force distribution in the network that may cause aerogel collapse at low densities. Tuning the freezing rate can enable the control of pore size and potentially improved aerogel performance. We also carried out mechanical tests on the AgNW aerogels. From a practical viewpoint, the mechanical properties are critical to ensure the structural robustness of the synthesized highly porous AgNW aerogels. To examine the mechanical properties of these AgNW aerogels, we performed uniaxial compression tests of five different AgNW aerogels covering an order of magnitude in relative densities (ρ/ρs) from 4.6 × 10−4 to 4.6 × 10−3 under a quasi-static strain rate of 5 × 10−4 s−1
(Figure 4). We found that all considered, AgNW aerogels exhibit a typical mechanical response of highly porous aerogels,
Figure 4. Mechanical properties of AgNW aerogels. (a) Compressive stress−strain curves of AgNW aerogels with various densities up to ∼50% compressive strain. (b) Relative Young’s modulus (E/Es) as a function of relative density (ρ/ρs) of AgNW aerogels. The data of previously reported CuNW aerogels and aerosponges were included in the same graph for comparison.
that is, elastic deformation followed by densification at large strains where stress increases dramatically.32 Furthermore, we found that AgNW aerogels can fully recover under an applied nonlinear strain, suggesting a significant nonlinear elasticity by elastic buckling of the Ag nanowires within the aerogel networks. Based on the classical beam theory, a porous foam architecture can deform by elastic buckling of the constituent beams when the relative density of the foam decreases below a critical value,
() ρ ρs
, which is determined by the yield strength,
crit
σy, and the Young’s modulus, Es, of the parent material.33 For example,
() ρ ρs
≈2 3
crit
uniaxial compression. σy ≈ 125 MPa,
33
() ρ ρs
σy E
for a honeycomb-like foam under
In particular, for Ag, Es ≈ 70 GPa and ≈ 0.62% can be approximated. The
crit
considered AgNW aerogels have significantly lower relative densities than such critical value, and hence nonlinear elastic buckling takes place during the densification stage, which results in the observed elastic recovery of ∼50% total strain. In this regard, the densification of the AgNW aerogels can be treated as an “elastic stiffening” behavior, which is fundamentally different than the plastic densification in conventional metal foams34 yet practically encouraging for maintaining the structural robustness upon loading. 7174
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The relative Young’s modulus (E/Es) of each considered AgNW aerogel is extracted from the slope of the stress−strain curve within the initial linear elastic regime and is summarized in Figure 4b. We found that E/Es rapidly increases with increasing the relative density, ρ/ρs, of the AgNW aerogels. For example, E/Es dramatically increases from 1.1 × 10−9 to 2.4 × 10−7 when increasing ρ/ρs from 4.6 × 10−4 to 4.6 × 10−3 (Figure 4b). A quantitative scaling behavior of E/Es ∼ (ρ/ρs)n is revealed, where n ≈ 2.35 is the scaling exponent. Such a scaling behavior has been extensively reported for a variety of other porous foams.33 The exponent n of the scaling relationship depends on the specific microarchitecture of the foam. In general, for open foams which deform predominantly through stretching of the constituent beams, n = 1, for open foams which deform predominantly through bending of the constituent beams, n = 235 for periodic foams as represented by some 3D printed Ni−P microlattices8 or n = 336 for stochastic foams as represented by some CuNW aerogels or aerosponges.11,13 Interestingly, for our stochastic AgNW aerogels, we found n ≈ 2.35, which suggests a bending dominant deformation mechanism. However, this scaling exponent is lower than that for conventional stochastic open foams (n = 3) and locates between n = 2 for CuNW aerosponges13 and n = 3 for CuNW aerogels.11 This is likely to be attributed to the different aspect ratios (AR) of the metal nanowires although the diameters (D) are comparable (D ≈ 50−100 nm, AR ≈ 1000 for our AgNW aerogels, D ≈ 80 nm, AR ≈ 3000 for CuNW aerosponges, and D ≈ 60 nm, AR ≈ 300 for CuNW aerogels). A higher AR suggests a higher degree of entanglement/connectivity and even some hidden periodicity of the nanowire network, which can trigger the nanowire architecture to behave stiffer, and vice versa. Besides the AR, it should be mentioned that the loading conditions, particularly the applied loading rate, can be another origin responsible for the differences among the three nanowire networks considering the viscoelastic nature of the lightweight architectures.33 Overall, the low degrading exponent revealed for the introduced AgNW aerogels than that for conventional stochastic foams or aerogels sets another structural benefit, although the mechanistic origin may require further modeling efforts to reveal in the future. In summary, we report for the first time the fabrication of ultralight and highly conductive silver aerogels with predicable and tunable densities. The aerogels were produced by freezecasting AgNW suspensions, followed by lyophilizing and thermal sintering. By using NW suspensions of different concentrations, the density of aerogels can be rationally tuned down to 4.8 mg/cm3. The AgNW aerogels feature a unique hierarchical porous structure with well-aligned, longitudinal pores. Thermal sintering effectively removed the polymer coating and welded the NW junctions and, as a result, enhanced the electrical conductivity of aerogel significantly by several orders of magnitude up to 51 000 S/m at a density of 50 mg/cm3. Mechanical tests show as-made AgNW aerogels exhibit “elastic stiffening” behavior, and a Young’s modulus up to 16 800 Pa. The high porosity and excellent mechanical/ electrical properties of these AgNW aerogels may lead to new device applications in fuel cells, energy storage, medical devices, catalysis, and sensors.
Letter
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.7b02790. Experimental methods and instrumentation; optical and SEM images of nonpurified AgNWs, purified AgNWs and AgNPs in supernatant; XRD data recorded from purified AgNWs; SEM images of AgNW aerogels prepared by freeze-casting method and “isotropicfreezing” method; TEM images of AgNW junctions before and after thermal sintering; FT-IR spectra and TGA data of PVP, as-made aerogels and sintered aerogels; relative conductivity vs relative density plot of aerogels before and after sintering; raw data of electrical conductivity of samples of increasing densities (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Fang Qian: 0000-0002-8782-4285 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Dr. Zurong Dai for performing TEM characterization, Drs. Benjamin Yancey and Joshua Kuntz for TGA experiments, Dr. Anna Hiszpanski for discussing electrical data. This work was supported by Lawrence Livermore National Laboratory under the auspices of the U.S. Department of Energy under Contract DE-AC52-07NA27344, through LDRD award 13-ERD-022, 14-SI-004, and 16-ERD-019. (LLNLJRNL-730883)
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