Seeded Nanowire and Microwire Growth from Lithium Alloys

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Seeded Nanowire and Microwire Growth from Lithium Alloys Sang Yun Han, Matthew G. Boebinger, Neha P Kondekar, Trevor J. Worthy, and Matthew T. McDowell Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b01334 • Publication Date (Web): 03 Jun 2018 Downloaded from http://pubs.acs.org on June 3, 2018

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Seeded Nanowire and Microwire Growth from Lithium Alloys Sang Yun Han1, Matthew G. Boebinger2, Neha P. Kondekar2, Trevor J. Worthy2, Matthew T. McDowell*1, 2

1

George W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology,

Atlanta, GA, 30332 2

School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA,

30332

*Corresponding author; [email protected]

KEYWORDS: Nanowires, phase transformations, lithium, nanomaterials

ACS Paragon Plus Environment

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ABSTRACT Although vapor-liquid-solid (VLS) growth of nanowires from alloy seed particles is common in various semiconductor systems, related wire growth in all-metal systems is rare. Here, we report the spontaneous growth of nano- and microwires from metal seed particles during the cooling of Li-rich bulk alloys containing Au, Ag, or In. The as-grown wires feature Au-, Ag-, or In-rich metal tips and LiOH shafts; the results indicate that the wires grow as Li metal and are converted to polycrystalline LiOH during and/or after growth due to exposure to H2O and O2. This new process is a simple way to create nanostructures, and the findings suggest that metal nanowire growth from alloy seeds is possible in a variety of systems.


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INTRODUCTION Nanowires have been extensively investigated because of the remarkable optical, electrical, mechanical, and thermal properties1–3 which arise from their one-dimensional geometry, high surface area, tunable length, controllable chemistry, and mechanical flexibility. Because of these properties, nanowires have been used for energy storage and energy-harvesting devices,3–5 lightweight structural composites,6 flexible electronic devices,7 and nanoelectronics such as field-effect transistors and logic gates.8–10 Many types of nanowires are synthesized via colloidal fabrication routes.2,11 However, seeded growth of nanowires from metal catalysts via the vapor-liquid-solid (VLS) mechanism and its derivatives is potentially more versatile, as these procedures are inherently scalable and allow for the encoding of different compositions, structures, and morphologies along the wire axis.12 Wagner and Ellis first introduced the growth of silicon “whiskers” via the VLS mechanism in 1964.13 In the VLS mechanism, droplets of catalyst material form a liquid alloy with another atomic species supplied via the vapor phase. Upon continued alloying and eventual saturation of the added species, unidirectional singlecrystalline growth of the nanowire material occurs from the catalyst seed droplet.2,13 While the family of VLS-type growth mechanisms has proved to be useful for the growth of semiconductor nanowires (e.g., InAs, InSb, and GaAs),2,14,15 this type of growth for nano/microwires in allmetal systems has not been widely investigated. Growth of metallic nanowires via seed-based processes could open the door to a variety of new one-dimensional nanomaterials. Lithium forms alloys with many other metals at relatively low temperatures (< 300 °C) due to its high reactivity and diffusivity. Thus, Li/metal alloy systems may be interesting for investigating the occurrence of nano/microwire growth. Li/metal alloys are used in a variety of applications; for instance, Li-containing Al alloys have been developed and deployed in


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aerospace systems because of their lower density and higher stiffness than conventional Al alloys.16 Li-rich alloys have also been investigated as high-capacity electrode materials and interfacial protection layers for Li-ion batteries.17–19 In addition to these applications, lithium alloying reactions have recently been shown to be useful for fabricating nanostructured materials. For instance, Chen et al. created porous nanostructures and hollow core-shell particles by removing Li from Li-Sn alloys.20 Lei et al. extracted Li from Li-Al and Li-Mg alloys to create alkoxide nanowires; this method is suitable for large-scale synthesis of oxide nanostructures.21 These examples show that Li alloying/dealloying reactions could enable the fabrication of novel materials, but it is also critical to develop an improved understanding of nanostructure growth mechanisms so these materials can be tailored for applications. Here, we report the growth of LiOH nanowires and microwires directly from Li-rich alloys at relatively low temperature. Multiple metals (Au, Ag, and In) were observed to induce nanowire/microwire growth from a macroscale liquid melt during cooling, and the results indicate these wires convert from Li metal to LiOH during and/or after growth. The observation of Au-, Ag-, or In-rich alloy tips and the high surface diffusion rate of Li suggest a mechanism similar to VLS growth. This work introduces a new and simple method to fabricate nanowires, and the insights gained herein could be important for seeded growth of various metallic nanostructures.


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RESULTS AND DISCUSSION Simple heating and cooling procedures were found to induce wire growth. Au, Ag, or In foils were pressed onto Li foils and heated to temperatures above the melting point of Li on a hot plate in an argon-filled glove box, followed by natural cooling to room temperature. Li was controlled to be in excess (molar ratios of between 15 and 20 moles Li to 1 mole of the other metal were used in most experiments). After this procedure, the surfaces of the metal samples were found to be covered in nano- and microwires. Figure 1 shows scanning electron microscopy (SEM) images of wires grown from a Li-Au sample. The wires have uniform diameters, and the wire tips exhibit brighter contrast. Figure 1b shows that wires are observed to cover the entire surface. Figure S1 in the Supporting Information (SI) contains additional SEM images of wires grown from Li-Ag and Li-In samples heated to the same temperature and cooled in the same manner. These wires appear similar to the wires from the Li-Au sample, except that the wires from the Li-Ag sample are slightly tapered with thinner sections near the tips.

Figure 1. (a) SEM image of wires grown from a Li-Au alloy (a Li:Au molar ratio of 20:1 was used). (b) Top-down SEM images of a Li-Au sample covered with wires.


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Experiments were carried out to understand the conditions that produced wire growth. For Au, Ag, and In, temperatures of at least ~220 °C were required for at least two hours for wires to form. Heating for longer durations and at higher temperatures (up to 320 °C) resulted in more uniform coverage of wires on the surface of samples, as shown in Fig. S2. Other metal foils (Sn and Cu) were also tested under similar conditions, but heating these metals with Li did not result in the growth of wires despite repeated tests at different temperatures. For the materials that did grow wires, it was found that wires grew upon cooling by imaging the sample during the cooling process. A camera in a glove box was used to record images of a Li-Au sample during heating and cooling. Upon heating, the sample (with Au foil initially on top of the Li foil) changed from a metallic gold color to a dark gray color, indicating the formation of the Li-Au alloy. The sample was then removed from the hot plate to induce rapid cooling over a few minutes; during this process, the majority of the surface of the sample turned bright red, as shown by the optical images in Fig. S3 in the SI. SEM investigation of the same sample after this process revealed that the red portions contained a high concentration of wires, while dull gray portions did not (Fig. S4 in the SI). The red color likely arises because of optical effects due to the small diameters of the wires.22 Although the samples shown in Fig. 1 were cooled more slowly over a period of ~1.5 h since they remained on the hot plate after it was turned off, they also showed a similar red color and the wires exhibited similar shapes. Further experiments showed that slower cooling rates resulted in the growth of longer wires, as shown in Fig. S5. Finally, wires were observed to grow for Li:Au molar ratios of 5:1 and greater (Fig. S6). Experiments were carried out to examine the composition and structure of the wires. Xray energy dispersive spectroscopy (EDS) in the SEM was used to investigate the elemental content of the tips of wires that were transferred to the SEM through atmosphere. Figure 2 shows


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an SEM image and EDS maps of two wires grown from a Li-Au sample. As shown in Fig. 2b, only the tips of the wires contain Au, while both the tips and the lengths of the wires contain oxygen (Fig. 2c). Similarly, Li-Ag wire samples also featured Ag primarily in the tips of the wires (Fig. S7). We note that Li cannot be detected with EDS. The structure of these wires is similar to semiconductor nanowires grown via VLS (such as Si), which share the common characteristic of tips that are rich in Au or other metallic catalyst material. X-ray diffraction (XRD) was also used to investigate Li-Au wire samples. The XRD spectrum in Fig. S8a shows that the heated and cooled Li-Au samples are primarily made up of the cubic Li15Au4 alloy phase, with weaker Li3Au peaks also present. Notably, diffraction peaks from Li or Au were not detected, even though these were the primary peaks detected in pristine Li-Au bilayers before heating (Fig. S8b). It is unclear from the XRD alone whether the wires are made of Li15Au4 alloy or some other material since the entire underlying sample likely contributes to the diffraction spectrum, and the wires only covered the surface.

Figure 2. (a) SEM image of the tips of two wires grown from a Li-Au sample (20:1 molar ratio). (b) EDS map of the image in (a) showing the presence of Au in the wire tips but not in the shafts of the wires. (c) EDS map showing the element O within the wires.


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To examine the structure of the wires in more detail, transmission electron microscopy (TEM) and selected area electron diffraction (SAED) were used. Cryo-TEM using liquid nitrogen was found to be necessary to stabilize the wires when imaging in a 300 kV TEM (see experimental sections for details).23,24 A cryo TEM image of a wire exposed to atmosphere for ~30 s during transfer into the instrument is shown in Fig. 3a. As shown by the magnified view in Fig. 3b, this wire has a Au-rich tip (the darker region) which is sheathed by lighter material. The lower section of the wire has a hollow or porous center with lighter contrast (Fig. 3c). Selectedarea electron diffraction (SAED) patterns of the tip and of the shaft (Fig. 3d and e) show that the wire is made of polycrystalline LiOH (ICDD 00-032-0564, space group P4/nmm), and the contrast in the image indicates small grain sizes on the order of tens of nm. Although the SAED patterns clearly show that LiOH is the majority phase, it is possible that small amounts of Li-Au phases or Li itself may also be present; the overlap of diffraction peaks among these phases and the few expected peaks from individual Li or Li-Au crystals precludes identification. Finally, the seemingly hollow structure of the shaft of the wire is notable; such structures were repeatedly observed with SEM and TEM (although wires were not always hollow). Taken together, these TEM results show that the wires are made up of LiOH with Au-rich tips, and the results furthermore suggest that the wires convert to polycrystalline LiOH after initially growing as Li metal (as will be described in detail subsequently). X-ray photoelectron spectroscopy (XPS) analysis of the wire samples confirmed the presence of LiOH and the absence of Au at the surface of the wires (Fig. 3f-h). In these experiments, a vacuum transfer holder was used to transfer the samples from the glove box directly to the XPS without exposure to atmosphere (unlike in the SEM and TEM experiments). The Li 1s peak at 54.9 eV and O 1s peak at 531.3 eV in Fig. 3g and h show the presence of Li


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and O-H species. A small amount of Li2O species may also be present based on the O 1s peak. The Li 1s peaks for Li metal and LiOH can be separated by small binding energy values,25 which makes it difficult to identify the exact chemical nature of the Li species.

Figure 3. (a) Cryo-TEM image of a wire grown from a Li-Au sample (Li:Au molar ratio of 20:1) after transfer through atmosphere. The colored boxes correspond to the locations of the magnified images in (b,


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c). (b) A magnified image of the tip of the wire. (c) A magnified image of the shaft of the wire. (d, e) SAED patterns of the tip and the shaft show the presence of polycrystalline LiOH. (f-h) XPS spectra from a separate wire sample grown from Li-Au (Li:Au molar ratio of 20:1). (f) Au 4f, (g) Li 1s, (h) O 1s. For XPS, the sample was transferred in a vacuum transfer holder without exposure to atmosphere.

Separate experiments in which pure Li metal was analyzed within the XPS before and after argon sputtering to remove the oxidized surface layer showed that the LiOH species produces a peak at ~54.5 eV, while Li metal produces a peak at 53.3 eV peak (Fig. S9 in the SI). Since the wires exhibited a Li 1s peak at ~54.9 eV, these results indicate that the outermost surfaces of the wires, including the Au tips, are entirely covered with LiOH, which agrees with the TEM analysis. Finally, Ar sputter depth-profiling of the wires did not reveal the presence of Li0 (only LiOH contributed to the Li 1s spectrum), which further supports the conclusion that LiOH was formed during the growth process (before atmospheric exposure) because these samples were transferred without exposure to atmosphere. Given these observations, we postulate that the wire formation process involves the growth of Li metal wires from Li-Au alloy seed particles. Figure 4 shows the Li-rich portion of the Li-Au binary phase diagram26 and schematics that illustrate the proposed growth mechanism. Heating the Li and Au foils between ~220 °C and 320 °C causes a Li-rich liquid alloy to form. Since Li was used in significant excess in most experiments, the composition of this liquid is likely near point (a) in the phase diagram in Fig. 4e. Upon cooling, the temperature of the bulk liquid alloy begins to decrease along the red line in Fig. 4e until the two-phase region separating the solid Li phase and the liquid alloy phase is reached at point (b) in the phase diagram. At this point, solid Li metal (with a small amount of dissolved Au) nucleates from the liquid alloy phase, and based on our observations, it is likely that this growth occurs from individual liquid alloy


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catalyst droplets, as shown in Fig. 4b. These Li metal crystals continue to grow as the sample is further cooled through the two-phase region (Fig. 4c), and these wires push up the liquid Li-Au droplets as they grow. A critical aspect of this growth mode is that additional Li atoms must continually enter and alloy with the droplet at the tip instead of directly attaching to the sidewalls of the Li wires. We postulate that the formation of passivating LiOH plays a key role in this step. If LiOH forms on the surface of the Li wires by reacting with trace H2O in the glove box as growth occurs, as shown in Fig. 4c, it could cause Li atoms migrating from the underlying liquid

Figure 4. (a-d) Schematics of the postulated growth mechanism. (e) The Li-rich portion of the Li-Au phase diagram.


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alloy reservoir to diffuse along the surface of the wires to dissolve within the tips of the wires instead of directly attaching to the solid Li. The LiOH could also play a role in stabilizing the Au-rich droplets at the tips of the wires. Recent work has shown the importance of surface passivation in catalyzed nanowire growth,27,28 and we predict that it plays a key role here. In addition, Li is known to exhibit fast surface diffusion on a variety of materials,29 which could enable the facile surface transport of Li atoms up the wires to enter into the alloy tips during the continuous precipitation of the Li metal from the alloy droplets (Fig. 4c). As long as a sufficient flux of Li enters the particulate tip from the wire surface, the wires should experience growth. Li surface migration to the tip is driven by the chemical potential difference between Li in the liquid metal at the tip and Li in the bulk liquid alloy below (which has excess Li during cooling). Thermodynamically-driven removal of Li atoms from the tip and inclusion into the wire during cooling therefore results in steady-state surface migration of Li atoms up the wire from the bulk liquid. The solid Li metal formed in the wire is thermodynamically stable in contact with the liquid alloy at the tip because the system is traversing the two-phase region in the phase diagram. Growth likely stops upon further cooling past the eutectic temperature of 155 °C, although subeutectic growth is also possible. The final product of this growth mechanism would be LiOH wires, as shown in Fig. 4d. We note that the formation of LiOH due to reaction with H2O is kinetically preferred to the formation of Li2O in the presence of H2O and O2.30 The formation of LiOH during growth is likely due to the low concentrations (~0.1-2 ppm) of O2 and H2O present in the glove box. The observed hollow shafts of the wires strongly suggest such a conversion process, since Li would diffuse radially outward to react with moisture or oxygen at the surface in a manner akin to the Kirkendall effect.31 We note that LiOH was not detected in significant quantities in the XRD studies, which is probably because the LiOH grains


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were too small and there was not enough material to produce significant diffraction intensity. Furthermore, the Au-containing Li15Au4 alloy should diffract more strongly than Li or LiOH because of the higher mass absorption coefficient of Au.32 The XRD results likely detect the bulk Li15Au4 alloy that is beneath the wires, since most of the Au used during the synthesis remained below the wires and did not participate in wire growth. Although most experiments involved Li:M ratios of ~20:1, wire growth was also observed in the Li-Au and the Li-In systems when lower molar ratios (~5:1) were used (Fig. S6b). Growth of Li wires from liquid metal seeds when the overall composition is less Li-rich than that shown in Fig. 4e could occur due to kinetics effects and/or local enrichment of the liquid Li-Au alloy with Li. Experiments with lower molar ratios (1:1) did not yield wire growth, however (Fig. S6a). In addition, growth of wires was observed in multiple metal systems (Li-Au, Li-Ag, and Li-In). Both the Li-Ag and Li-Au systems have similar Li-rich portions of their phase diagrams despite different intermediate phases and compounds;26,33 thus, the same growth mechanism is likely at play. The phase diagram of the Li-In system features a much smaller twophase region between the Li and liquid phases (the eutectic point occurs at 99.8 atomic % Li),34 but non-equilibrium effects may also play a significant role in the growth of wires from Li-In alloys. A better understanding of the growth mechanisms in these three cases could open up a wider variety of materials for which wire growth is possible using these simple methods. In the growth mechanism postulated here, the Li atoms are added to the catalyst droplet by diffusion from the underlying bulk liquid phase instead of from a surrounding vapor phase as in VLS, or a surrounding liquid phase as in solution-liquid-solid (SLS) growth. The mechanism observed here would thus only be active for elements that exhibit high surface diffusion rates (such as Li). Additionally, a surface passivation layer is likely critical, as it prevents direct


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growth that would cause widening of the wires. However, these other growth mechanisms (VLS, SLS, and related mechanisms) could be activated for the growth of nanowires within other metal systems beyond the Li-based systems studied here. In particular, for the growth of nanowires from other metals, the requirement of high surface mobility could be bypassed by introducing metal atoms from the vapor phase to induce VLS growth. This opens up possibilities for the growth of a variety of metals via the use of vapor-phase organometallic precursors. The variety of different observations herein indicates that cooling through the two-phase region was the underlying cause of wire growth. This includes the finding that slower cooling rates resulted in longer wires, since slower cooling would cause the materials to remain in the two-phase growth region for longer durations. Furthermore, the temperature and Li molar ratio had to be above certain values to cause wire growth, which suggests that being in the liquid Lirich region of the phase diagram is necessary. However, a number of aspects of this proposed growth mechanism are still unclear, and various experimental difficulties related to the reactivity of Li and Li alloys make these aspects challenging to study. For instance, the sensitivity of Li alloys to LiOH formation upon air exposure makes it difficult to determine exactly when this phase forms during the growth process, and whether Li grows directly from the Li/metal catalyst droplets. In addition, the reason for formation of Au-rich droplets as growth catalysts, as opposed to the liquid Li-Au remaining within the bulk of the melt, is unclear; it is possible that surface oxidation plays a role. Future work, including experiments to uncover the effects of controlled O2/H2O exposure during growth, is needed to fully understand this novel growth mechanism.


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CONCLUSIONS In summary, we have demonstrated a new, simple, low-temperature method for the fabrication of nanowires and microwires via the cooling of liquid Li-metal alloys that are rich in Li. These wires have Au-, Ag-, or In-rich tips and LiOH shafts. Although the wires consist of polycrystalline LiOH, analysis of the final material strongly suggests that they grow as Li metal wires and are converted to LiOH during and/or after growth. If this is the case, this would indicate that it is possible to grow metallic nanowires with VLS-like mechanisms. If expanded to other, less reactive metals beyond Li, such an advance could more effectively enable the bottomup integration of metal nanowires into electronic device architectures. Thus, these initial experiments revealing wire growth from Li-metal alloys hint at interesting and important applications for related metal systems that require dedicated further study.

METHODS Synthesis of wires. Au, Ag, or In metal foils (Sigma-Aldrich, 99.99%) were physically roll-pressed onto the top of Li metal foil (Sigma-Aldrich, 99.9%) inside an Ar-filled glove box (MBraun) with H2O and O2 levels < 2.0 ppm. The surface contamination layer was physically removed from the Li foil in the glove box before pressing. For most experiments, a molar ratio of between 15-20:1 was used (Li:M, where M = Au, Ag, or In). Additional experiments were undertaken in which the molar ratio was varied from 20:1 to 1:1 (Li:M). The contact area of the metal foils was usually less than 25 mm2. After pressing the foils, samples were heated in a machined stainless steel boat on a hot plate at 290 °C and held at this temperature for a duration of 4 h. Other experiments were carried out with varying temperatures between 190 °C and


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320 °C and varying durations between 1 h and 8 h. A Signstek 6802 II Dual Channel Digital Thermometer with a K-Type thermocouple was used to measure the temperature of the samples. After heating, the hot plate was turned off and the sample was cooled on the hot plate to ~50 °C over a period of 1.5 h. Characterization. SEM images were collected on a Hitachi SU8230 SEM with an accelerating voltage between 3 kV and 10 kV and a working distance between 2.3 mm and 8 mm. During loading into the SEM, the samples were exposed to atmosphere for < ~10 s, and evacuation of the loading chamber took ~30 sec. EDS analysis was carried out using an accelerating voltage of 30 kV and a working distance of 15 mm. An X-MaxN X-ray detector (Oxford Instruments) was used for EDS analysis. AZtec 2.3 software was used for EDS mapping. Before mapping, energy calibration was performed to measure the shift in the position of the spectral peaks of each element for normalized quantitative analysis. XRD of Li-Au samples was carried out using a PANalytical Empyrean instrument with a Cu Kα radiation source (λ = 1.54 Å). The sample was placed on a glass slide and sealed underneath a Mylar film within a glove box to minimize oxidation during X-ray investigation. Cryo-TEM characterization was performed using an FEI Tecnai F30 TEM operating at 300 kV. The TEM sample was prepared by scraping wires from the growth sample surface onto a lacey carbon grid inside a glove box. The grid was then loaded into a Gatan cryo holder inside the glove box. The sample was exposed to atmosphere for < ~30 seconds as the holder was transferred from the argon environment into the TEM antechamber. Liquid nitrogen was then added to the holder’s external dewar, and the temperature of the sample was brought to and


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maintained at -170 °C. After temperature stabilization, imaging was conducted and images were recorded using a Gatan OneView CMOS camera. XPS analysis was performed using a Thermo K-Alpha instrument with an Al Kα source, a 400 µm spot size, and 15 W X-ray gun power. The base pressure was 2.1 Χ 10-8 Torr, and the analyzer pass energy was 50 eV with a resolution of 0.05 eV and a dwell time of 100 ms. A flood gun with slow electrons and Ar+ ions was used to compensate for surface charging. Charge referencing for the peaks was performed using the C 1s peak at 284.8 eV. Depth profiling was carried out with an Ar ion gun (3 keV, 10 µA) for a series of five etches of 60 s each. CasaXPS software was used for fitting with a Lorentzian Gaussian (GL30) line shape and a Shirley background.

ACKNOWLEDGMENTS This work was supported by the Air Force Office of Scientific Research (AFOSR) under Grant FA9550-17-1-0130. This research was performed in part at the Georgia Tech Institute for Electronics and Nanotechnology, a member of the National Nanotechnology Coordinated Infrastructure, which is supported by the National Science Foundation (Grant ECCS-1542174). The authors acknowledge helpful discussions and comments from Prof. Michael Filler.

AUTHOR INFORMATION ⃰ Corresponding Author: [email protected] The authors declare no conflicts of interest.


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SUPPORTING INFORMATION The supporting information contains additional SEM images of Li-Ag and Li-In alloys, optical images of a Li-Au sample during cooling, EDS analysis of Li-Ag samples, and X-ray diffraction and XPS data.

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Figure 1 165x57mm (300 x 300 DPI)


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Figure 2 82x39mm (300 x 300 DPI)


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Figure 3 165x183mm (300 x 300 DPI)


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Figure 4 82x138mm (300 x 300 DPI)


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Table of Contents Graphic 80x39mm (300 x 300 DPI)


Seeded Nanowire and Microwire Growth from Lithium Alloys

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