Halide Welding for Silver Nanowire Network Electrode

Solvent Welding-Based Methods Gently and Effectively Enhance the Conductivity of a Silver Nanowire Network

To enhance the conductivity of a silver nanowire (Ag NW) network, a facile solvent welding method was developed. Soaking a Ag NW network in ethylene glycol (EG) or alcohol for less than 15 min decreased the resistance about 70%. Further combined solvent processing via a plasmonic welding approach decreased the resistance about 85%. This was achieved by simply exposing the EG-soaked Ag NW network to a low-power blue light (60 mW/cm2). Research results suggest that poly(vinylpyrrolidone) (PVP) dissolution by solvent brings nanowires into closer contact, and this reduced gap distance between nanowires enhances the plasmonic welding effect, hence further decreasing resistance. Aside from this dual combination of methods, a triple combination with Joule heating welding induced by applying a current to the Ag NW network decreased the resistance about 96%. Although conductivity was significantly enhanced, our results showed that the melting at Ag NW junctions was relatively negligible, which indicates that the enhancement in conductivity could be attributed to the removal of PVP layers. Moreover, the approaches were quite gentle so any potential damage to Ag NWs or polymer substrates by overheating (e.g., excessive Joule heating) was avoided entirely, making the approaches suitable for application in devices using heat-sensitive materials.

Flexible Nanocomposite Conductors for Electromagnetic Interference Shielding

HIGHLIGHTS

Convincing candidates of flexible (stretchable/compressible) electromagnetic interference shielding nanocomposites are discussed in detail from the views of fabrication, mechanical elasticity and shielding performance. Detailed summary of the relationship between deformation of materials and electromagnetic shielding performance. The future directions and challenges in developing flexible (particularly elastic) shielding nanocomposites are highlighted. With the extensive use of electronic communication technology in integrated circuit systems and wearable devices, electromagnetic interference (EMI) has increased dramatically. The shortcomings of conventional rigid EMI shielding materials include high brittleness, poor comfort, and unsuitability for conforming and deformable applications. Hitherto, flexible (particularly elastic) nanocomposites have attracted enormous interest due to their excellent deformability. However, the current flexible shielding nanocomposites present low mechanical stability and resilience, relatively poor EMI shielding performance, and limited multifunctionality. Herein, the advances in low-dimensional EMI shielding nanomaterials-based elastomers are outlined and a selection of the most remarkable examples is discussed. And the corresponding modification strategies and deformability performance are summarized. Finally, expectations for this quickly increasing sector are discussed, as well as future challenges.

Direct Writing of Silver Nanowire Patterns with Line Width down to 50 μm and Ultrahigh Conductivity

Direct writing of one-dimensional nanomaterials with large aspect ratios into customized, highly conductive, and high-resolution patterns is a challenging task. In this work, thin silver nanowires (AgNWs) with a length-to-diameter ratio of 730 are employed as a representative example to demonstrate a potent direct ink writing (DIW) strategy, in which aqueous inks using a natural polymer, sodium alginate, as the thickening agent can be easily patterned with arbitrary geometries and controllable structural features on a variety of planar substrates. With the aid of a quick spray-and-dry postprinting treatment at room temperature, the electrical conductivity and substrate adhesion of the written AgNWs-patterns improve simultaneously. This simple, environment benign, and low-temperature DIW strategy is effective for depositing AgNWs into patterns that are high-resolution (with line width down to 50 μm), highly conductive (up to 1.26 × 105 S/cm), and mechanically robust and have a large alignment order of NWs, regardless of the substrate's hardness, smoothness, and hydrophilicity. Soft electroadhesion grippers utilizing as-manufactured interdigitated AgNWs-electrodes exhibit an increased shear adhesion force of up to 15.5 kPa at a driving voltage of 3 kV, indicating the strategy is very promising for the decentralized and customized manufacturing of soft electrodes for future soft electronics and robotics.

Origin of Capillary-Force-Induced Welding in Ag Nanowires and Ag Nanowire/Carbon Nanotube Conductive Networks

Capillary-force-induced welding can effectively reduce the contact resistance between two silver nanowires (AgNWs) by merging the NW-NW junctions. Herein, we report a model for quantifying the capillary force between two nano-objects. The model can be used to calculate the capillary force generated between AgNWs and carbon nanotubes (CNTs) during water evaporation. The results indicate that the radius of one-dimensional nano-objects is crucial for capillary-force-induced welding. AgNWs with larger radii can generate a greater capillary force (F_AgNW-AgNW) at NW-NW junctions. In addition, for AgNW/CNT hybrid films, the use of CNTs with a radius close to that of AgNWs can result in a larger capillary force (F_AgNW-CNT) at NW-CNT junctions. The reliability of the model is verified by measuring the change in sheet resistance before and after capillary-force-induced welding of a series of AgNW and AgNW/CNT conductive films with varying radii.

Performance Enhancement of Silver Nanowire-Based Transparent Electrodes by Ultraviolet Irradiation

Silver nanowires (AgNWs) are used as transparent electrodes (TE) in many devices. However, the contact mode between the nanowires is the biggest reason why the sheet resistance of silver nanowires is limited. Here, simple and effective ultraviolet (UV) irradiation welding is chosen to solve this problem. The influence of the power density of the UV irradiation on welding of the silver nanowires is studied and the fixed irradiation time is chosen as one minute. The range of the UV (380 nm) irradiation power is chosen from 30 mW/cm² to 150 mW/cm². First of all, the transmittance of the silver nanowire film is not found to be affected by the UV welding (400-11,000 nm). The sheet resistance of the silver nanowires decreases to 73.9% at 60 mW/cm² and increases to 127.6% at 120 mW/cm². The investigations on the UV irradiation time reveal that the sheet resistance of the AgNWs decreases continuously when the UV irradiation time is varied from 0 to 3 min, and drops to 57.3% of the initial value at 3 min. From 3-6 min of the continuous irradiation time, the change of the sheet resistance is not obvious, which reflects the self-limiting and self-termination of AgNWs welding. By changing the wavelength of the UV irradiation from 350-400 nm, it is found that the welding effect is best when the UV wavelength is 380 nm. The average transmittance, square resistance, and the figure of merit of the welded AgNWs at 400-780 nm are 95.98%, 56.5 Ω/sq, and 117.42 × 10^-4 Ω^-1, respectively. The UV-welded AgNWs are also used in silicon-based photodetectors, and the quantum efficiency of the device is improved obviously.

Advances in constructing silver nanowire-based conductive pathways for flexible and stretchable electronics

With their soaring technological demand, flexible and stretchable electronics have attracted many researchers' attention for a variety of applications. The challenge which was identified a decade ago and still remains, however, is that the conventional electrodes based on indium tin oxide (ITO) are not suitable for ultra-flexible electronic devices. The main reason is that ITO is brittle and expensive, limiting device performance and application. Thus, it is crucial to develop new materials and processes to construct flexible and stretchable electrodes with superior quality for next-generation soft devices. Herein, various types of conductive nanomaterials as candidates for flexible and stretchable electrodes are briefly reviewed. Among them, silver nanowire (AgNW) is selected as the focus of this review, on account of its excellent conductivity, superior flexibility, high technological maturity, and significant presence in the research community. To fabricate a reliable AgNW-based conductive network for electrodes, different processing technologies are introduced, and the corresponding characteristics are compared and discussed. Furthermore, this review summarizes strategies and the latest progress in enhancing the conductive pathway. Finally, we showcase some exemplary applications and provide some perspectives about the remaining technical challenges for future research.

Electrochemical Redox In-Situ Welding of Silver Nanowire Films with High Transparency and Conductivity

Silver nanowire (AgNW) networks with high transparency and conductivity are crucial to developing transparent conductive films (TCFs) for flexible optoelectronic devices. However, AgNW-based TCFs still suffer from the high contact resistance of AgNW junctions with both the in-plane and out-of-plane charge transport barrier. Herein, we report a rapid and green electrochemical redox strategy to in-situ weld AgNW networks for the enhanced conductivity and mechanical durability of TCFs with constant transparency. The welded TCFs show a marked decrease of the sheet resistance (reduced to 45.5% of initial values on average) with high transmittance of 97.02% at 550 nm (deducting the background of substrates). The electrochemical welding treatment enables the removal of the residual polyvinylpyrrolidone layer and the in-situ formation of Ag solder in the oxidation and reduction processes, respectively. Furthermore, local conductivity studies confirm the improvement of both the in-plane and the out-of-plane charge transport by conductive atomic force microscopy. This proposed electrochemical redox method provides new insights on the welding of AgNW-based TCFs with high transparency and low resistance for the development of next-generation flexible optoelectronic devices. Furthermore, such conductive films based on the interconnected AgNW networks can be acted as an ideal supporter to construct heterogeneous structures with other functional materials for wide applications in photocatalysis and electrocatalysis.

Tailoring Silver Nanowire Nanocomposite Interfaces to Achieve Superior Stretchability, Durability, and Stability in Transparent Conductors

Silver nanowires (AgNWs) have been considered as a promising candidate for transparent stretchable conductors (TSCs). However, the strong interface mismatch of stiff AgNWs and elastic substrates leads to the stress concentration at their interface and ultimately the low stretchability and poor durability of TSCs. Here, to address the interfacial mismatch of AgNWs-based TSCs we put forward a universal interface tailoring strategy that introduces the mercapto compound as the intermediate cross-linked layer. The mercapto compound strongly interacts with the AgNWs, forming a dense protective layer on their surface to improve their corrosion resistance, and reacts with the polymer substrate, forming a buffer layer to release the concentrated stress. As a result, the optimized TSCs showed superior stretchability (160%), exceptional durability (230 000 cycles), competent optoelectrical performance (18.0 ohm·sq^-1 with a transmittance of 86.5%), and prominent stability. This work provides clear guidance and a strong impetus for the development of transparent stretchable electronics.

Electroplated core-shell nanowire network electrodes for highly efficient organic light-emitting diodes

In this study, we performed metal (Ag, Ni, Cu, or Pd) electroplating of core-shell metallic Ag nanowire (AgNW) networks intended for use as the anode electrode in organic light-emitting diodes (OLEDs) to modify the work function (WF) and conductivity of the AgNW networks. This low-cost and facile electroplating method enabled the precise deposition of metal onto the AgNW surface and at the nanowire (NW) junctions. AgNWs coated onto a transparent glass substrate were immersed in four different metal electroplating baths: those containing AgNO₃ for Ag electroplating, NiSO₄ for Ni electroplating, Cu₂P₂O₇ for Cu electroplating, and PdCl₂ for Pd electroplating. The solvated metal ions (Ag⁺, Ni²⁺, Cu²⁺, and Pd²⁺) in the respective electroplating baths were reduced to the corresponding metals on the AgNW surface in the galvanostatic mode under a constant electric current achieved by linear sweep voltammetry via an external circuit between the AgNW networks (cathode) and a Pt mesh (anode). The amount of electroplated metal was systematically controlled by varying the electroplating time. Scanning electron microscopy images showed that the four different metals (shells) were successfully electroplated on the AgNWs (core), and the nanosize-controlled electroplating process produced metal NWs with varying diameters, conductivities, optical transmittances, and WFs. The metal-electroplated AgNWs were successfully employed as the anode electrodes of the OLEDs. This facile and low-cost method of metal electroplating of AgNWs to increase their WFs and conductivities is a promising development for the fabrication of next-generation OLEDs.

Flexible, transparent and tough silver nanowire/nanocellulose electrodes for flexible touch screen panels

Flexible touch screen panel (f-TSP) has been emerging recently and metallic nanowire transparent conductive electrodes (TCEs) are its key components. However, most metallic nanowire (MNW) TCEs suffer from weak bonding strength between metal nanowire electrode layers and polymer substrates, which causes delamination of TCEs and produces serious declines in durability of f-TSPs. Here, we introduce AgS bonding and develop tough and strong electrode-substrate bonded MNW TCEs, which can enhance durability of f-TSPs significantly. We used silver nanowires (AgNWs) as metal conductive electrode and thiol-modified nanofibrillated cellulose (NFC-HS) nanopaper as substrates. Because of the existence of Ag from AgNWs and S from NFC-HS, strong AgS bonding was generated and tough TCEs were obtained. The TCEs exhibit excellent electrical stability, outstanding optical and electrical properties. The f-TSP devices integrated with the TCEs illustrate striking durability. This technique may provide a promising strategy to produce flexible and tough TCEs for next-generation high-durability f-TSPs.

Rapid nanowelding of silver nanowires by focused-light-scanning for high-performance flexible transparent electrodes

Silver nanowires (AgNWs) have been considered as one of the most promising flexible transparent electrodes (FTEs) material for next-generation optoelectronic devices. However, the large contact resistance between AgNWs could deteriorate the conductivity of FTEs. In the present work, high-performance AgNWs FTEs were obtained by means of focused-light-scanning (FLS), which could lead to the large-area, rapid and high-quality welding between AgNWs within a short time, forming the reliable and stable AgNWs network. The results of the optoelectronic tests show that after FLS, the sheet resistance of the AgNWs FTEs sharply decreased from 5113 Ω/sq to 7.7 Ω/sq, with maintaining a high transmittance (∼94%). Finally, a high-performance flexible transparent heater was fabricated by using FLS, showing reach a relatively high temperature in a short response time and rapid response at low input voltage. The findings offer an effective pathway to greatly improve the conductivity of AgNWs FTEs.

Radio Frequency Induction Welding of Silver Nanowire Networks for Transparent Heat Films

Transparent heat films (THFs) are attracting increasing attention for their usefulness in various applications, such as vehicle windows, outdoor displays, and biosensors. In this study, the effects of induction power and radio frequency on the welding characteristics of silver nanowires (Ag NWs) and Ag NW-based THFs were investigated. The results showed that higher induction frequency and higher power increased the welding of the Ag NWs through the nano-welding at the junctions of the Ag NWs, which produced lower sheet resistance, and improved the adhesion of the Ag NWs. Using the inductive welding condition of 800 kHz and 6 kW for 60 s, 100 ohm/sq of Ag NW thin film with 95% transmittance at 550 nm after induction heating could be decreased to 56.13 ohm/sq, without decreasing the optical transmittance. In addition, induction welding of the Ag NW-based THFs improved haziness, increased bending resistance, enabled higher operating temperature at a given voltage, and improved stability.

Recent progress in post treatment of silver nanowire electrodes for optoelectronic device applications

Owing to the economical and practical solution synthesis and coating strategies, silver nanowires (AgNWs) have been considered as one of the most suitable alternative materials to replace commercial indium tin oxide (ITO) transparent electrodes. The primitive AgNW electrode cannot meet the requirements for preparing high performance optoelectronic devices due to its high contact resistance, large surface roughness and poor stability. Thus, various post-treatments for AgNW film optimization are needed before its actual applications, such as welding treatment to decrease contact resistance and passivation to increase film stability. This review investigates recent progress on the preparation and optimization of AgNWs. Moreover, some unique fabrication strategies to produce highly oriented AgNW films with unique anisotropic properties have also been carried out with detailed analysis. The representative devices based on the AgNW electrode have been summarized and discussed at the end of this review.

Improved thermal/chemical stability of flexible Ag NWs/Chitosan composite electrode integrated by chemical welding and sequential protection for better PV performance

An integration strategy of chemical welding and subsequent protection was demonstrated to address silver nanowires (Ag NWs)-based issues. Preferentially, a halogenated salt of NaCl solution was used to stimulate the junction welding thus to reduce the junction resistance, by virtue of the autocatalytic redox of Ag atoms with halogen ions and dissolved oxygen molecules. Subsequently, chitosan, possessing the biocompatible, degradable, environmentally friendly non-toxic features, was embedded to protect Ag NWs. With these two steps, the composite electrode consisting Ag NWs and chitosan reaches a lowest sheet resistance of ∼8 Ω, with a transmittance over 80% at 550 nm, along with high thermal and chemical stabilities, accompanying with excellent flexibility. Besides, it also prompts a synergistic improvement when pioneered in Cu(In, Ga)Se₂(CIGS) device as a transparent conductive electrode. It yields a power conversion efficiency of 6.6%, with 32% improvement relative to that bare Ag NWs, and 85% of the conventional one. Our findings present a new strategy for addressing instable/inefficient Ag NWs-based devices, driving their rapid development and its practical applications.

Microchemical Engineering in a 3D Ordered Channel Enhances Electrocatalysis

The kinetics of electrode reactions including mass transfer and surface reaction is essential in electrocatalysis, as it strongly determines the apparent reaction rates, especially on nanostructured electrocatalysts. However, important challenges still remain in optimizing the kinetics of given catalysts with suitable constituents, morphology, and crystalline design to maximize the electrocatalytic performances. We propose a comprehensive kinetic model coupling mass transfer and surface reaction on the nanocatalyst-modified electrode surface to explore and shed light on the kinetic optimization in electrocatalysis. Moreover, a theory-guided microchemical engineering (MCE) strategy has been demonstrated to rationally redesign the catalysts with optimized kinetics. Experimental measurements for methanol oxidation reaction in a 3D ordered channel with tunable channel sizes confirm the calculation prediction. Under the optimized channel size, mass transfer and surface reaction in the channeled microreactor are both well regulated. This MCE strategy will bring about a significant leap forward in structured catalyst design and kinetic modulation.

Solution-Processed MoS2 Film with Functional Interfaces via Precursor-Assisted Chemical Welding

Molybdenum disulfide (MoS₂) presents fascinating properties for next-generation applications in diverse fields. However, fully exploiting the best properties of MoS₂ in largescale practical applications still remains a challenge due to lack of proper processing methods. Solution-based processing can be a promising route for scalable production of MoS₂ nanosheets, but the resulting assembled film possesses an enormous number of interfaces that significantly compromise the intrinsic electrical properties. Herein, we demonstrate the solution processing of MoS₂ and subsequent precursor-assisted chemical welding to form defective MoS_2-x at the nanosheet interfaces. The formation of defective MoS_2-x significantly reduces the electrical contact resistances, and thus the chemically welded MoS₂ film exhibits more than 2 orders of magnitude improved electrical conductivity. Furthermore, the chemical welding provides MoS_2-x interface induced additional defect originated functionalities for diverse applications such as broadband photodetection over the near-infrared range and improved electrocatalytic activity for hydrogen evolution reactions. Overall, this precursor-assisted chemical welding strategy can be a facile route to produce high-quality MoS₂ films with low-quality defective MoS_2-x at the interfaces having multifunctionalities in electronics, optoelectronics, and electrocatalysis.

Failing Forward: Stability of Transparent Electrodes Based on Metal Nanowire Networks

Metal nanowire (MNW)-based transparent electrode technologies have significantly matured over the last decade to become a prominent low-cost alternative to indium tin oxide (ITO). Beyond reaching the same level of performance as ITO, MNW networks offer additional advantages including flexibility and low materials cost. To facilitate adoption of MNW networks as a replacement to ITO, they must overcome their inherent stability issues while maintaining their properties and cost-effectiveness. Herein, the fundamental failure mechanisms of MNW networks are discussed in detail. Recent strategies to computationally model MNWs from the nano- to macroscale and suggest future work to capture dynamic failure to unravel mechanisms that account for convolution of the failure modes are highlighted. Strategies to characterize MNW network failure in situ and postmortem are also discussed. In addition, recent work about improving the stability of MNW networks via encapsulation is discussed. Lastly, a perspective is given on how to frame the requirements of MNW-encapsulant hybrids with reference to their target applications, namely: solar cells, transparent film heaters, sensors, and displays. A cost analysis to comment on the feasibility of implementing MNW hybrids is provided, and critical areas to focus on for future work on MNW networks are suggested.

Printable Liquid Metal Microparticle Ink for Ultrastretchable Electronics

Liquid metal confined in the elastomer represents an ideal platform for stretchable electronics with ultimate deformability. To enable facile and scalable patterning of conductive features, bulk liquid metal is typically dispersed into fine particles to formulate printable inks. The presence of native oxide or organic ligands stabilizing these liquid metal particles unfortunately inhibits their direct coalescence to recover the metallic conductivity and liquid-state deformability. Here, we report a chemical sintering process that converts printed liquid metal microparticles into a highly deformable conductor. The process involves the removal of surface passivating oxide by a short exposure to acid fume and subsequent selective wetting of liquid metal microparticles onto copper nanoplates present in the ink formulation. The chemical reaction provides the basis for a facile and scalable procedure to print conductive features over a large area with exceptional conductivity (>10⁴ S cm^-1) and ultrahigh stretchability (∼1000% strain). Their practical suitability is demonstrated by the fabrication of an ultrastretchable ribbon cable and an epidermal heater.

High Aspect Ratio and Post-Processing Free Silver Nanowires as Top Electrodes for Inverted-Structured Photodiodes

Silver nanowires (Ag NWs) as transparent conducting electrodes are widely used in many applications such as organic light-emitting diodes (OLEDs), polymer light-emitting diodes, touch screens, solar cells, and transparent heaters. In this work, using a large-scale synthesis, the synthesized Ag NWs had a high aspect ratio of 2820. The Ag NWs could be applied as a top transparent electrode in a device by simple drop-casting without any post-processing steps. The fabricated device comprised 4,4'-bis(carbazol-9-yl)biphenyl/MoO3 organic/inorganic layers which are parts of the inverted structure OLEDs or solar cells. The photodiode characteristics at the UV range were observed in the device. The ability of Ag NWs to replace opaque metals as top electrodes in a device has been demonstrated.

Enhancement of the Conductivity and Uniformity of Silver Nanowire Flexible Transparent Conductive Films by Femtosecond Laser-Induced Nanowelding

In order to improve the performance of silver nanowire (AgNW) flexible transparent conductive films (FTCFs), including the conductivity, uniformity, and reliability, the welding of high repetition rate femtosecond (fs) laser is applied in this work. Fs laser irradiation can produce local enhancement of electric field, which induce melting at the gap of the AgNWs and enhance electrical conductivity of nanowire networks. The overall resistivity of the laser-welded AgNW FTCFs reduced significantly and the transparency changed slightly. Meanwhile, PET substrates were not damaged during the laser welding procedure in particular parameters. The AgNW FTCFs can achieve a nonuniformity factor of the sheet resistance as 4.6% at an average sheet resistance of 16.1 Ω/sq and transmittance of 91%. The laser-welded AgNW FTCFs also exhibited excellent reliability against mechanical bending over 10,000 cycles. The welding process may open up a new approach for improvement of FTCFs photoelectric property and can be applied in the fabrication of silver nanostructures for flexible optoelectronic and integration of functional devices.

Empowering Electroless Plating to Produce Silver Nanoparticle Films for DNA Biosensing Using Localized Surface Plasmon Resonance Spectroscopy

To facilitate the implementation of biosensors based on the localized surface plasmon resonance (LSPR) of metal nanostructures, there is a great need for cost-efficient, flexible, and tunable methods for producing plasmonic coatings. Due to its simplicity and excellent conformity, electroless plating (EP) is well suited for this task. However, it is traditionally optimized to produce continuous metal films, which cannot be employed in LSPR sensors. Here, we outline the development of an EP strategy for depositing island-like silver nanoparticle (NP) films on glass with distinct LSPR bands. The fully wet-chemical process only employs standard chemicals and proceeds within minutes at room temperature. The key step for producing spread-out NP films is an accelerated ripening of the silver seed layer in diluted hydrochloric acid, which reduces the nucleation density during plating. The reaction kinetics and mechanisms are investigated with scanning (transmission) electron microscopy (SEM/STEM), X-ray photoelectron spectroscopy (XPS), and UV-vis spectroscopy, with the latter enabling a convenient live monitoring of the deposition, allowing its termination at a stage of desired optical properties. The sensing capabilities of chemically deposited NP films as LSPR transducers are exemplified in DNA biosensing. To this end, a sensing interface is prepared using layer-by-layer (LbL) buildup of polyelectrolytes (PE), followed by adsorption and covalent immobilization of ssDNA. The obtained LSPR transducers demonstrate robustness and selectivity in sensing experiments with binding complementary and unrelated DNA strands.

Efficient metallic nanowire welding using the Eddy current method

In this study, metallic nanowires (M-NWs) such as silver nanowires (AgNWs) and copper nanowires (CuNWs) were welded only at junctions resistively by a novel method using an indirect Eddy current through an inductive power transfer. By applying an inductive power of 45 kHz alternating current power indirectly for 6 s to the M-NW network deposited on polymer substrates, a decrease of sheet resistance up to ∼67.9% for AgNWs and ∼49.9% for CuNWs could be obtained without changing the optical transmittance. For AgNWs, after the welding a decrease of surface roughness could also be observed from 44.5 nm to 26.3 nm, which is similar to the height of a single layer AgNW (22.2 nm) for a bilayer junction. For AgNWs coated on a transparent flexible substrate, after the cyclic bending of 10 000 times, no change of resistance (ΔR/R0) of the AgNWs after the welding was observed and the welded AgNWs were not easily peeled off from the substrate. It is believed that this novel welding method can be applied not only to all kinds of M-NWs on various flexible low-temperature polymer substrates, but also to large areas at a short time and at low cost.

Metal Nanotube/Nanowire-Based Unsupported Network Electrocatalysts

Combining 1D metal nanotubes and nanowires into cross-linked 2D and 3D architectures represents an attractive design strategy for creating tailored unsupported catalysts. Such materials complement the functionality and high surface area of the nanoscale building blocks with the stability, continuous conduction pathways, efficient mass transfer, and convenient handling of a free-standing, interconnected, open-porous superstructure. This review summarizes synthetic approaches toward metal nano-networks of varying dimensionality, including the assembly of colloidal 1D nanostructures, the buildup of nanofibrous networks by electrospinning, and direct, template-assisted deposition methods. It is outlined how the nanostructure, porosity, network architecture, and composition of such materials can be tuned by the fabrication conditions and additional processing steps. Finally, it is shown how these synthetic tools can be employed for designing and optimizing self-supported metal nano-networks for application in electrocatalysis and related fields.

Roll-to-roll redox-welding and embedding for silver nanowire network electrodes

We developed a continuous roll-to-roll redox-welding and embedding method for the fabrication of electrodes of silver nanowire (AgNWs) networks. The roll-to-roll welding method involved a sequence of oxidation and reduction reactions in an aqueous solution. The redox-welding significantly decreased the sheet resistance of the AgNW film owing to the strong fusion and interlocking at the nanowire junction, while the optical transmittance was maintained. The first oxidation step using HNO₃ generated ionized silver (Ag⁺) which got re-deposited onto the nanowire junctions via an autocatalytic reaction. The oxide layers, which formed on the nanowire surface by both air exposure and the first step of oxidation, were removed by the second reduction step using NaBH₄. The redox-welded AgNW electrodes exhibited a sheet resistance of 11.3 Ω sq^-1 at the optical transmittance of 90.5% at 550 nm. Furthermore, redox-welding of the AgNWs significantly enhanced their mechanical robustness compared to that of the as-coated AgNWs. The redox-welded AgNWs embedded in a UV curable resin, using a roll-to-roll embedding process, were successfully applied as anode electrodes for large-area and flexible organic light emitting diodes (OLEDs). The device performance is superior to that of a device based on the as-coated AgNW electrode, and is also comparable to that of a device using commercial ITO as the electrode. The redox-welding and embedding processes provide a facile and reliable method for fabricating large-area transparent flexible electrodes for next-generation flexible optoelectronic devices.

Controllable assembly of a hierarchical multiscale architecture based on silver nanoparticle grids/nanowires for flexible organic solar cells

In this work, an effective and facile strategy was developed to assemble a flexible hierarchical multiscale architecture by incorporating microscale silver nanoparticles (AgNPs) grids into random silver nanowires (AgNWs) networks combined with a room-temperature chemical sintering mechanism. The microscale AgNPs grids was fabricated by assemble AgNPs into a series of twin lines directly on a hydrophilic PET substrate based on coffee-ring effect with ink-jet printing technique. By regulating the assembly architecture, a flexible hierarchical multiscale conductor based on AgNPs grids/AgNWs was successfully fabricated and demonstrated a high transmittance of 87.5%, low sheet resistance of 16.5 Ω/sq and excellent mechanical flexibility. The hierarchical multiscale architecture was fairly favorable to efficiently collect free charges among the gaps in the AgNWs network, as well as to enhance the stability of conductivity by creating continuous conduction pathways. As an anode electrode in a flexible organic solar cell, the hierarchical multiscale AgNPs grids/AgNWs conductor demonstrated a more power photoelectric conversion efficiency, which was even superior to the corresponding properties of the ITO network at a similar transmittance. This simple, low-cost and nonlithographic solution-based approach would further enhance current fabrication approaches to create patterned microstructures, and have great potential to fabricate multifarious functional patterns in flexible electronic devices.

Epitaxial-Growth-Induced Junction Welding of Silver Nanowire Network Electrodes

In this study, we developed a roll-to-roll Ag electroplating process for metallic nanowire electrodes using a galvanostatic mode. Electroplating is a low-cost and facile method for deposition of metal onto a target surface with precise control of both the composition and the thickness. Metallic nanowire networks [silver nanowires (AgNWs) and copper nanowires (CuNWs)] coated onto a polyethylene terephthalate (PET) film were immersed directly in an electroplating bath containing AgNO₃. Solvated silver ions (Ag⁺ ions) were deposited onto the nanowire surface through application of a constant current via an external circuit between the nanowire networks (cathode) and a Ag plate (anode). The amount of electroplated Ag was systematically controlled by changing both the applied current density and the electroplating time, which enabled precise control of the sheet resistance and optical transmittance of the metallic nanowire networks. The optimized Ag-electroplated AgNW (Ag-AgNW) films exhibited a sheet resistance of ∼19 Ω/sq at an optical transmittance of 90% (550 nm). A transmission electron microscopy study confirmed that Ag grew epitaxially on the AgNW surface, but a polycrystalline Ag structure was formed on the CuNW surface. The Ag-electroplated metallic nanowire electrodes were successfully applied to various electronic devices such as organic light-emitting diodes, triboelectric nanogenerators, and a resistive touch panel. The proposed roll-to-roll Ag electroplating process provides a simple, low-cost, and scalable method for the fabrication of enhanced transparent conductive electrode materials for next-generation electronic devices.

Spontaneous and Selective Nanowelding of Silver Nanowires by Electrochemical Ostwald Ripening and High Electrostatic Potential at the Junctions for High-Performance Stretchable Transparent Electrodes

Metal nanowires have been gaining increasing attention as the most promising stretchable transparent electrodes for emerging field of stretchable optoelectronic devices. Nanowelding technology is a major challenge in the fabrication of metal nanowire networks because the optoelectronic performances of metal nanowire networks are mostly limited by the high junction resistance between nanowires. We demonstrate the spontaneous and selective welding of Ag nanowires (AgNWs) by Ag solders via an electrochemical Ostwald ripening process and high electrostatic potential at the junctions of AgNWs. The AgNWs were welded by depositing Ag nanoparticles (AgNPs) on the conducting substrate and then exposing them to water at room temperature. The AgNPs were spontaneously dissolved in water to form Ag⁺ ions, which were then reduced to single-crystal Ag solders selectively at the junctions of the AgNWs. Hence, the welded AgNWs showed higher optoelectronic and stretchable performance compared to that of as-formed AgNWs. These results indicate that electrochemical Ostwald ripening-based welding can be used as a promising method for high-performance metal nanowire electrodes in various next-generation devices such as stretchable solar cells, stretchable displays, organic light-emitting diodes, and skin sensors.

Size and surface effects on chemically-induced joining of Ag conductive inks

The underlying roles of particle size effects and ionic salts are uncovered for optimal chemically-induced sintering as a scalable approach to join metallic nanomaterials to create efficient sensors.