Room-Temperature Chemical Welding and Sintering of Metallic

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Room-temperature Chemical Welding and Sintering of Metallic Nanostructures by Capillary Condensation Sung-Soo Yoon, and Dahl-Young Khang Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.6b00621 • Publication Date (Web): 09 May 2016 Downloaded from http://pubs.acs.org on May 10, 2016

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Room-temperature Chemical Welding and Sintering of Metallic Nanostructures by Capillary Condensation Sung-Soo Yoon and Dahl-Young Khang* Department of Materials Science and Engineering, Yonsei University 50 Yonsei-ro, Seodaemun-gu, Seoul 03722, KOREA KEYWORDS: capillary condensation, metal nanostructures, welding, sintering

ABSTRACT

Room-temperature welding and sintering of metal nanostructures, nanoparticles and nanowires, by capillary condensation of chemical vapors have successfully been demonstrated. Nanoscale gaps or capillaries that are abundant in layers of metal nanostructures have been found to be the preferred sites for the condensation of chemically oxidizing vapor, H2O2 in this work. The partial dissolution and re-solidification at such nano-gaps completes the welding/sintering of metal nanostructures within ~10min. at room-temperature, while other parts of nanostructures remain almost intact due to negligible amount of condensation on there. The welded networks of Ag

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nanowires have shown much improved performances, such as high electrical conductivity, mechanical flexibility, optical transparency, and chemical stability. Chemically sintered layers of metal nanoparticles, such as Ag, Cu, Fe, Ni, and Co, have also shown orders of magnitude increase in electrical conductivity and improved environmental stability, compared to nontreated ones. Pertinent mechanisms involved in the chemical welding/sintering process have been discussed. Room-temperature welding and sintering of metal nanostructures demonstrated here may find widespread application in diverse fields, such as displays, deformable electronics, and wearable heaters, etc.


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Capillary condensation,1-4 a pre-mature condensation of vapor into liquid even at vapor pressures lower than its thermodynamic saturation vapor pressure, is a ubiquitous phenomenon in our daily life. The phenomenon can easily occur especially under geometrical confinement, such as divided media, cracks, contacts or nanoscale gaps between surfaces, when the mutual interaction (van der Waals interaction, in general) among vapor molecules exceeds certain limit. It has been found to play an important role in a wide variety of science and engineering fields that involve small gaps or capillaries, such as scanning probe microscopy (SPM),5,6 micro- or nanoelectromechanical system (MEMS/NEMS),7,8 porous media and oil recovery,9 and colloidal and sol-gel based film formation.10,11 We demonstrate in this work that the capillary condensation can be used as a valuable tool in welding and sintering of metal nanostructures chemically at room temperature. The nanoscale gaps abundant in the network or film of metallic nanostructures have been found to be the preferred locations of capillary condensation. When chemical vapors of highly oxidizing agents, hydrogen peroxide (H2O2) in this work, condense at such nanogaps, welding and sintering can be accomplished at room temperature, due to dissolution and redeposition of materials at such gaps only. Welding or joining of material pieces together has long been practiced in history. Contrary to the welding of bulk materials, which is now a mature technique, joining of nanoscale counterparts12-15 imposes significant barriers not seen in the bulk. Limited success of nanowelding using electron beam16,17 and laser irradiation,18-21 or diffusion bonding22 has been demonstrated, but those are largely limited to laboratory scale. Recent demonstration of plasmonic welding23 exploits the novel and unique optical phenomenon occurring in metal nanostructures, but it suffers from the shadow effect. Traditional thermal welding is not compatible with low thermal budget substrates such as flexible polymers. The room-temperature


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chemical welding and sintering of metal nanostructures by capillary condensation is a simple and readily scalable approach, and thus may find wide range of applications, especially for transparent electrical conductors in highly bendable or stretchable forms.24

Figure 1. Room-temperature chemical welding of Ag nanowires (AgNWs) network by capillary condensation of H2O2 vapor. (a) Schematic illustration of chemical welding by capillary condensation. (b) Pristine AgNWs network on plastic (PET) substrate by spin coating from 0.08 wt% solution in de-ionized water. Dotted red circle shows the junction between AgNWs. (c) Chemically welded AgNWs junctions are highlighted in dotted red circles. (d) HRTEM image of the welded junction. (e) AFM image of welded AgNWs network on a Si substrate. (f) Plot for the change in junction height as a function of welding time, measured from multiple AFM images like shown in (e).


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Figure 1a schematically shows the welding of metal nanowires using capillary condensation. When deposited on a substrate, metal nanowires form percolating thus electrically conducting network. Here, the physical contacts between nanowires form nanogap or capillary. Upon exposure to oxidizing vapor of H2O2, condensation occurs preferentially at such nanogaps, leading to dissolution and joining of nanowires altogether. The capillary condensation phenomenon is explained by the following Kelvin equation:2

P  2γ V  v = exp− l   P  r RT  sat

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Here, Pv and Psat denote equilibrium and saturation vapor pressures, respectively, and γ the liquid surface tension, Vl the liquid molar volume, r the mean radius of curvature of meniscus, R the gas constant, and T the temperature, respectively. All things being constant, the mean radius of curvature of meniscus governs the condensation. It can easily be verified that the smaller the radius of curvature (or, nanoscale gap), the earlier the vapor condensation occurs at such spots compared to areas having large radius of curvature. Indeed, the exposed nanowires’ surface remains almost intact while the junctions between them are welded together. Before the welding by capillary condensation, the overlying nanowire bends conformally along the contour of bottom-lying one at the physical junction, as shown in Figure 1b (Supporting Information, Figure S1 for low-magnification large area view of the sample surface, before/after the chemical welding). After the chemical welding by exposing the sample on the vapor of H2O2, however, the junction seems to be flattened by partial sinking of the overlying nanowire, as can be seen in Figure 1c (Experimental details can be found in Supporting Information). Note here that there is no noticeable change in nanowire morphology, under the SEM observation, after the welding


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process, except the junctions (Upon prolonged exposure to H2O2 vapor, however, the nanowires were broken into small dots and short segments. Figure S2). The nanowire morphology, especially in the neighborhood of the welded junctions, showed slight change upon high-resolution transmission electron microscopy (HR-TEM) observation, as shown in Figure 1d. The outer surface of nanowires became wobble and there are very small (100%, by adding more and more Ag nanowires,25-27 if the optical transparency is not an essential requirement.


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Also, the high value of optical haze in 3-D networked samples may be useful in certain applications that require photon management or the effective trapping of incident photons, such as in solar cell devices.28,29 It is worth to mention that non-welded AgNWs networks having optical transparency of >80 % (i.e., spin-coated samples) did not show any electrical conductance when embedded into PDMS, due to the intervention of percolating contacts among nanowires by PDMS. Even for spray-coated 3-D network of AgNWs, the resistance for nonwelded network is >3X larger than that for chemically welded counterpart, as shown in Figure 3e. More importantly, the stretchability is almost halved down to ~20 % in the non-welded sample. All the physics and chemistry discussed above for AgNWs have been found to be equally applicable in chemical sintering of other metal nanoparticles at room temperature. In fact, the nanoparticles layer has numerous contact gaps among particles, which greatly facilitate the capillary condensation of oxidizing H2O2 vapor. Shown in Figure 4a is the ~200 nm-thick layer of Ag nanoparticles, before and after the room-temperature chemical sintering by capillary condensation. The individual nanoparticles are hard to be discerned after the chemical sintering. At the same time, the sheet resistance of the layer decreased ~7 orders of magnitude upon chemical sintering. Similar results were obtained for ~700 nm-thick Cu nanoparticles film, as shown in Figure 4b. The chemical sintering or fusion among metal nanoparticles has led to ~5 orders of magnitude reduction in electrical resistance, too. Ni and Co nanoparticles showed similar sintered morphology and orders of magnitude reduction in electrical resistance upon ~10 min of exposure to H2O2 vapor, as shown in Figure 4c and Figure 4d, respectively. Also, the sintering time can be reduced down to ~5 min by using higher vapor pressure, i.e., heating the H2O2 container to 70 oC. Prolonged exposure to vapor, however, has led to increase in resistance


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again, due to segregation of nanoparticle film into rather big chunks of metal aggregates (Figure S11). It should also be noted that the chemical sintering by capillary condensation can be applicable to rather thick, up to ~1 µm in this work, layers without any noticeable shadow effect.

Figure 4. Room-temperature chemical sintering of various metallic nanoparticles. Left panels in each row are the SEM images of nanoparticles layer before chemical sintering, while the middle


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panels show the sample images after the chemical sintering. Right panel plots are the change in resistance or sheet resistance as a function of sintering time. (a) ~200 nm-thick Ag nanoparticles layer, (b) ~700 nm-thick Cu nanoparticles layer, (c) and (d) are ~1 µm-thick Ni and Co nanoparticles layers, respectively.

The chemical welding and sintering of metallic nanostructures by the capillary condensation of strongly oxidizing chemical species is believed to be universal one, i.e., the process can be applicable to other pure metals (Figure S12 for the chemical welding of Cu nanowires and sintering of Fe nanoparticles). The key enabling mechanisms of the process are the redox potential difference of species involved and the easy availability of free electrons. The hydrogen peroxide has very high redox potential value of 1.78 V. This means its high affinity to electrons and thus to be reduced to H2O. Considering the fact that almost all pure metals have lower redox potential values than that of H2O2 (except Au, which has redox potential of 1.8 V, and thus could not be welded/sintered chemically),30 vaporized molecules of H2O2 that condense at the contact gaps among metallic nanostructures can act as an oxidizing agent. Being full of free electrons, metals can easily be reduced to metallic ions (Mz+) upon contact with the strong oxidizing agents such as H2O2. This dissolution of metals occurs at the contact gaps only, due to preferential capillary condensation on there. After drying (i.e., evaporation of H2O), the junctions in the layer of metallic nanostructures become tightly welded/sintered, instead of simple physical contact. It is worthy of noting that chemicals other than H2O2 may be used for the welding/sintering of metallic nanostructures by the capillary condensation of their vapors. When strong acids, such as HCl and HNO3, were applied for the process, the preferential dissolution of metallic nanostructures at nanoscale junctions has been observed, leading to orders of magnitude


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reduction in electrical resistance. This highly conducting state could not last long enough, however (Figure S13). The dissolution by-products, metal salts of halide (MX) or nitrates (MNO3), preferentially deposited at the junction upon drying, leading back to very high resistance state, even higher than the non-treated initial state. Therefore, the sintered layers are highly conductive only when the reaction by-products are in liquid state, then the layers become highly resistive upon complete drying, typically within ~5 min. On the contrary, reaction byproduct between metals and H2O2 is a simply reduced form of H2O2, i.e., water (H2O), and the water does not leave any non-conductive residues upon complete drying. This is another key enabler for the present approach to be successful.

Figure 5. Stretchable, transparent heaters using chemically welded AgNWs network. (a) Photo images of samples before (left) and after (right) stretching to 20%. (b) Thermal images by IR


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camera for welded (top) and non-welded AgNWs heater samples under 4 V bias. The inlaid scale bars denotes 120 oC (red) and 20 oC (blue), respectively. (c) Temperature variation as a function of stretching strain for chemically welded AgNWs network heater under different voltage bias. (d) Temperature variation as a function of stretching strain for non-welded AgNWs network heater at 4 V bias.

Figure 5 shows the performance of stretchable heater17,20,31,32 made of chemically welded AgNWs network on latex glove. A piece of latex glove was cut, coated with AgNWs and welded chemically by exposure to H2O2 vapor for 15 min. Then, the processed piece was bonded onto another latex glove with PDMS as an adhesive, as shown in Figure 5a. The temperature of the sample by Joule heating was monitored by IR camera, thermal images as shown in Figure 5b, for welded and non-wleded AgNWs network, respectively. The temperature of heaters made of welded AgNWs network, shown in Figure 5c, has shown slight hysteresis upon the 1st stretching, then showed stable and constant temperature upon following mechanical stretching cycles. The temperature rises to >100 oC under 4 V bias, due to high electrical conductivity of the chemically welded AgNWs network. On the other hand, the non-welded AgNWs network shows much inferior performance to welded one, as shown in Figure 5d. The temperature reached to ~50 oC at 4 V, and showed rather large hysteresis, due to poor electrical conductance of the non-welded network. The high-performing stretchable, transparent heater may find various practical applications, such as in wearable thermotherapy, de-icing, heating gloves for outdoor activities, etc. In summary, we have demonstrated a simple chemical method to weld or sinter metallic nanostructures altogether. Upon exposure to the chemical vapors of hydrogen peroxide, the


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capillary condensation occurs preferentially at physical contact gaps among metallic nanostructures. The high electron affinity of the oxidant and easy availability of free electrons from metals have enabled the nanoscale welding and sintering at room temperature in ~10 min The approach neither involves high thermal budget steps nor suffers from shadowing effect. The welded or sintered metal nanostructures have enabled highly bendable or stretchable electrodes formation, with very high optical transparency and low haze, which may be very helpful in diverse applications such as in deformable or wearable electronics, photovoltaics, etc.

ASSOCIATED CONTENT Supporting Information. Experimental details, SEM, TEM, EDX, and AFM analysis of welded AgNWs, thermal welding by Joule heating, bending test, stretching test, areal coverage of AgNWs, optical haze and transmittance, chemically sintered AgNPs, welded CuNWs, chemically sintered FeNPs, HNO3-based sintering of CoNPs, 2-point I-V vs. 4-points probe measurements. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ACKNOWLEDGMENT This work was supported in part by the National Research Foundation of Korea Grant funded by the Korean Government (MEST) (NRF-2010-C1AAA001-0029061). The authors thank LG Display Co. Ltd. for optical measurement and Prof. J.-W. Park (Yonsei University) for thermal imaging with IR camera.


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CONFLICT OF INTEREST The authors declare no competing financial interest.


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(24) Hecht, D.S.; Hu, L.; Irvin, G. Adv. Mater. 2011, 23, 1482-1513. (25) Yun, S.; Niu, X.; Yu, Z.; Hu, W.; Brochu, P.; Pei, Q. Adv. Mater. 2012, 24, 1321-1327. (26) Xu, F.; Zhu, Y. Adv. Mater. 2012, 24, 5117-5122. (27) Park, M.; Im, J.; Shin, M.; Min, Y.; Park, J.; Cho, H.; Park, S.; Shim, M.-B.; Jeon, S.; Chung, D.-Y.; Bae, J.; Park, J.; Jeong, U.; Kim, K. Nat. Nanotech. 2012, 7, 803-809. (28) Strudley, T.; Zehender, T.; Blejean, C.; Bakkers, E.P.A.M.; Muskens, O.L. Nat. Photon. 2013, 7, 413-418. (29) Wallentin, J.; Anttu, N.; Asoli, D.; Huffman, M.; Aberg, I.; Magnusson, M.H.; Siefer, G.; Fuss-Kailuweit, P.; Dimroth, F.; Witzigmann, B.; Xu, H.Q.; Samuelson, I.; Deppert, K.; Borgstrom, M.T. Science 2013, 339, 1057-1060. (30) Lide, D.R. CRC Handbook of Chemistry and Physics; 87th ed.; CRC Press; Boca Raton; 2006. (31) Choi, S.; Park, J.; Kim, J.; Kim, J.; Lee, Y.B.; Song, C.; Hwang, H.J.; Kim, J.H.; Hyeon, T.; Kim, D.-H. ACS Nano 2015, 9, 6626-6633. (32) An, B.W.; Gwak, E.-J.; Kim, K.; Kim, Y.-C.; Jang, J.; Kim, J.-Y.; Park, J.-U. Nano Lett. 2016, 16, 471-478.


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Room-temperature Chemical Welding and Sintering of Metal Nanostructures by Capillary Condensation

Sung-Soo Yoon and Dahl-Young Khang


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Nano Letters

ToC Figure

Chemical Welding


Room-Temperature Chemical Welding and Sintering of Metallic

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