Recent advances of metal nanowire based transparent conductive electrodes for �exible chromatic devices

Transparent conductive electrodes (TCEs) that can maintain structure integrity and remain highly electrically conductive under large mechanical deformations are strongly desired and urgently needed in �exible optoelectronics. Nevertheless, the TCE technology in �exible electronics remains at a standstill, in which brittle indium tin oxide (ITO) is main material being used. Metal nanowires (NWs) networks with high optoelectronic performance as next-generation TCEs have widespread applications in �exible electronics, such as smart windows, touch panels, solar cells and �at displays. Here we review the latest advances that have been made for metal NWs TCEs, including the strategies for the synthesis of metal NWs, metal NWs �lms assembly via different solution-based processes and their practical applications as electrodes in various �exible chromic devices, as well as discuss their challenges and perspectives for the future practice.


Introduction
Over the last decade, our lives have been fundamentally changed through smart electronics and optoelectronics, such as smart windows, touch panels and solar cells, etc (Chen et al. 2018a; Ke et al. 2018).Transparent conductive electrodes (TCEs), with an outstanding combination of high conductivity and good transparency, are an essential component of these optoelectronics (Liu et al. 2018;Ren et al. 2020; Yang et al. 2020).Typically, TCEs consist of transparent conductive materials deposited on transparent glass or polyethylene terephthalate (PET).The global TCE market is prospective to increase with a growth rate exceeding 17%, reaching about $8.04 billion by 2022 (Swallow et al. 2020).Currently, liquid crystal devices (LCDs) is the largest user of TCEs, however, many other modern electronics are catching up fast (Hecht et al. 2011).
The main materials used today for TCEs are transparent conductive oxides.These materials were rst reported in 1907 by Badeker (Badeker 1907).He found that Cd and Pb lms absorbed in glow discharge chambers could be oxidized and became transparent while maintaining its conductive property.After that, researchers recognized the commercial and academic value of these transparent conductive oxides and expanded the list of potential TCE materials, including F-doped In 2 O 3 , Al-doped ZnO, GdInOx and etc (Chen et al. 2000;Crockett et al. 2019).Among which, indium tin oxide (ITO) is the most successful transparent conductive metal oxide both in commercial market and academic sector.ITO electrodes have been the industrial standard of TCEs in most of electronics and optoelectronics for several decades (Kim et al. 2020; Lahav and van der Boom 2018; Li et al. 2019;Xu et al. 2021).However, the utilization of ITO as TCEs has certain limitations, such as complicated fabrication process, sensitivity to basic and acidic environments as well as increasing cost (Jin et al. 2013;Sannicolo et al. 2016;Zhang et al. 2017).In addition, ITO is quite brittle and easy to crack, which is incompatible with exible substrates (Chen et  So far, more and more efforts have been paid to exploit cost-effective, solution-processable and exible TCEs by introducing some new materials for the next-generation optoelectronics.One of the most mature alternative materials are conducting polymers, such as polyaniline (Kulkarni et al. 1989), poly (3,4-ethylenedioxythiophene:poly(styrenesulfonate) (PEDOT:PSS) (Vosgueritchian et al. 2012), polypyrrole (Sakkopoulos et al. 1998) and polythiophene (Li et al. 2007), etc.After a simple chemical doping, these polymers were found to exhibit high electrical conductivities.For example, Ouyang and co-workers demonstrated a simple approach to signi cantly improve the electrical conductivity (from ~0.3 to ~3000 S/cm) of PEDOT:PSS lms via being doped with sulfuric acid (Xia et al. 2012).Though the great achievements in conducting polymers, one of the major challenges is their poor stability when exposure to UV light, high humidity and high temperature.
Carbon nanotubes (CNTs) thin network has attracted great interest on account of their high electrical conductivity, desirable optical properties, outstanding structural stability and excellent exibility (Ebbesen et al. 1996;Jiang et al. 2018a).The theoretical calculation shows that the electrical conductivity of CNTs network can be as high as 9×10 4 S/cm, which is 10 times better than that of ITO TCE (Pereira et al. 2009).Recently, a signi cant advance for single-walled CNTs (SWCNTs) lms was achieved by Cheng's group (Chen et al. 2018b;Jiang et al. 2018a; Wang et al. 2018a).In one study, they reported highperformance SWCNTs-based networks with an average sheet resistance of 41 Ω/sq at transmittance of 90% at 550 nm (Jiang et al. 2018a).The transmittance is hereafter referred to the transmittance at 550 nm in this review, unless otherwise speci ed.In another study, meter-scale dimension SWCNT thin networks were developed with outstanding photoelectric performance (transmittance: 90%; sheet resistance: 65 Ω/sq) (Wang et al. 2018a).However, several issues remain: i) high production costs and energy consumption; ii) large junction resistance between CNTs; iii) severe aggregation of CNTs into bundles, which dramatically reduces transmittance; and iv) lacking of scalable and simple deposition techniques for robust, uniform and large-area CNT-based TCEs.
Meantime, graphene, a single layer of sp 2 -bonded carbon atoms, has emerged as a promising candidate for ITO replacement because of its speci c physical properties, such as high transparency, good conductivity, excellent stability as well as outstanding exibility (Bae et al. 2010; Kim et al. 2009; Park et al. 2018;Xu and Liu 2016).Theoretical calculations estimate that the sheet resistance of graphene lm is equal to 62.4/N Ω/sq, where N is the number of monolayer of graphene (Wu et al. 2010).Graphene based TCEs can be obtained using various methods, such as mechanical exfoliation (Novoselov et al. 2004), chemical vapor deposition (CVD) (Kim et al. 2009) and chemical exfoliation (Stankovich et al. 2006).
Nevertheless, the high processing temperature (~1000°C) of CVD method limits the utilization of graphene, especially, makes it incompatible with mass production required for real application.
As is known to all, most metals show relatively high electrical conductivity because of their high freeelectron density.Whereas the metals display low transparency due to large re ection.Therefore, metal mesh or ultrathin lm is required to meet both optical and electrical requirements while with improved exibility of TCE.Unfortunately, the transmittance for ultrathin metal lm is normally below 56% with comparable electrical conductivity to ITO lm (Formica et al. 2012; Guo et al. 2016).As for conventional metal mesh, the width of metal line is generally greater than 100 µm, which results in big shadowing loss and high surface roughness (Yu et al. 2013).What's more, the electrohydrodynamic printing for metal mesh is expensive and unsuitable for high-throughput production.
As an alternative candidate, metal NWs networks hold greater promise for cost-effective TCE application due to their high photoelectric performance and good solution-processability.As shown in Fig. 1, the bibliometric data depict the publication number and citation number of NWs based TCE over the past decade.The continuous increase in the number of citations testi es the prosperity of the metal NW based TCE.The key point for metal NW TCE is to produce highly smooth and pure metal NWs with large aspect ratio (long lengths and small diameters).Up to now, in this research direction, efforts have been mainly focused on Ag NWs and Cu NWs because of their excellent electrical conductivity, simple wet-chemical synthesis as well as relatively low materials costs.In this review, we rst describe the different methods to synthesize metal NWs with the desired electrical and optical properties, followed by a summary on the assembly process of metal NWs lms via solution-based process.Then we will discuss the practical applications of these metal NW lms as electrodes in various solution-based exible chromic devices.Finally, the challenges and perspectives of metal NW TCEs are discussed.It is clear that the TCEs will continue to grow and play a crucial part in future exible smart electronics.
2. Metallic Nws Networks: Synthesis, Fabrication And Properties

Cu NW TCEs
Cu has the second highest intrinsic conductivity among all the metals with an electrical conductivity of 5.96×10 5 S/cm, which is very close to that of Ag (6.30×10Generally, Cu NWs could be obtained via a solution-processable approach by reducing copper ions in the presence of capping agents.The slow reduction of Cu ions, together with the use of an appropriate capping ligand, such as hexadecylamine (HDA), can lead to the formation of decahedral seeds, which further grew into penta-twinned NWs (Jin et al. 2011).In 2005, Zeng and co-workers demonstrated that high-quality ultralong Cu NWs with aspect ratios >350-450 (length: 40-50 µm, diameter: 90-120nm) could be synthesized on a large scale via a simple aqueous reduction method for the rst time (Fig. 2A-C) (Chang et al. 2005).In a typical protocol, the Cu(NO 3 ) 2 precursor was reduced by hydrazine using ethylenediamine (EDA) as capping ligand in NaOH solution at low temperatures.Wiley and co-workers further scaled up Zeng's reaction by 200 times (from 0.006 to 1.2 g) via optimizing the experimental parameters to develop the high prospect of this synthetic strategy for mass production (Fig. 2D) (Rathmell et al. 2010).Afterwards, they used vacuum ltration method to fabricate Cu NWs network lms (Fig. 2E).However, the lms exhibited a relatively low transmittance of 65% at a sheet resistance of 15 Ω/sq (Fig. 2F) because of the aggregation and low aspect ratio (lengths: 10 ±3 µm; diameters: 90±10 nm) of NWs.
To address this issue, Wiley and co-workers modi ed this solution-phase synthesis method to produce longer and thinner (length from 10 to > 20 µm, diameter from 90 nm to < 60 nm) Cu NWs that were well dispersed in water (Rathmell and Wiley 2011).In order to grow high-aspect-ratio NWs, after only 3 min at 80°C, polyvinylpyrrolidone (PVP) was added into the heated reaction mixture (composed of NaOH, Cu(NO 3 ) 2 , EDA and N 2 H 4 ) to suppress the Cu NWs aggregation and the reaction solution was then quickly cooled to room temperature (RT).Such prepared Cu NWs were then formulated into an ink to make exible Cu NWs TCEs on PET substrates with a transmittance of 85% at a sheet resistance of 30 Ω/sq.Recently, they further increased the average aspect ratio of Cu NWs to as high as 2280 (diameter: ~35 nm, length: ~80 µm) by modifying the EDA-based synthesis (Fig. 3A-B) (Ye et al. 2014c).These Cu NWs were then used to develop TCEs via rod-coating method with high transmittance (around 95%) and low sheet resistance (< around 100 Ω/sq) (Fig. 3C).Besides, other groups also made a big progress in the long and small-diameter Cu NWs synthesis.For example, Xia and co-workers synthesized ultra-long and thin Cu NWs by reducing CuCl 2 with glucose in water using HDA as a capping ligand (Jin et al. 2011).The Cu NWs showed long lengths (up to hundreds of µm) with small diameters of 24±4 nm (Fig. 3D-F).Lu and co-workers demonstrated a novel approach to produce ultra-long single-crystalline Cu NWs with superior dispersibility in toluene (Zhang et al. 2012).The Cu NWs were synthesized by self-catalytic growth within a liquid-crystalline medium of cetyltriamoninum bromide (CTAB) and HDA and showed long lengths of hundreds of micrometers and an average diameter of ~78 nm.The transparent electrodes prepared from these Cu NWs exhibited high transmittance (90%) and relatively low sheet resistance (90 Ω/sq) among solution-based transparent conductors.Recently, Yang and co-workers demonstrated a new approach to synthesize high-quality and monodispersed Cu NWs by using tris(trimethylsilyl)silane as a mild reducing agent for the rst time, with a mean length of 17 µm and an average diameter of 17.5 nm (Fig. 3G) (Cui et al. 2015).The uniform Cu NW TCEs were then fabricated by vacuum ltration, which exhibited high transmittance (90%) and low sheet resistance (34.8 Ω/sq) as well as low optical haze factors (Fig. 3H).
During the last decade, signi cant efforts have been achieved to develop Cu NW TCEs with high optoelectronic performance by numerous groups (Han et (Liu et al. 2020), have been explored as the protective shell.In general, these shells not only protected the Cu NWs from oxidation but also maintained their high conductivity.For example, Lee and co-workers developed Cu-graphene core-shell NWs by a plasma-enhanced CVD (PE-CVD) method at temperature of 400°C for the rst time (Fig. 4A) (Ahn et al. 2015).The Cu-graphene NW TCE exhibited comparable optoelectronic performance to a traditional ITO TCE.In addition, it revealed outstanding chemical and thermal stability because of the tight graphene encapsulation (Fig. 4B).Furthermore, the practical perspective of Cu-graphene NW TCEs was studied by fabricating polymer photovoltaic devices, which exhibited higher power conversion e ciency (PCE) than pure Cu NW TCEs based devices.Nonetheless, it should be noted that CVD method used during the graphene preparation is incompatible with low-cost and mass production required for industrial applications.Reduced GO (rGO) has been accepted as a cost-effective strategy for highthroughput production of graphene.Yang and co-workers reported a solution-based method to develop ultrathin and high-quality Cu-rGO cores-shell NWs (Fig. 4C) (Dou et al. 2016).High optoelectronic performance TCEs (transmittance: ~90%, sheet resistance: ~28 Ω/sq, haze: ~2%) were then obtained with these NWs, which showed outstanding stability in air over 200 days.In addition to graphene-based materials, Wiley and co-workers utilized different metals, such as Au, Pt, Ag and Ni, to protect Cu NWs from oxidation (Rathmell et al. 2012;Stewart et al. 2014;Stewart et al. 2015).However, the encapsulation is not complete because of the galvanic replacement during reduction process and the surface of formed core-shell NWs is quite rough.They also developed an electrodeposition of In, Sn or Zn shells onto Cu NWs core, followed by the shells oxidation, enabled the protection of Cu NW networks against oxidation without obvious photoelectric performance degradation (Chen et al. 2014).Recently, Yang and co-workers reported ultrathin and smooth Cu-Au core-shell NWs by using strong binding ligand (trioctylphosphine) to suppress galvanic replacement reactions (Niu et al. 2017).The epitaxial overgrowth of a thin Au shell can signi cantly improve their stability under 80% humidity at 80°C for more than 700 h (see Fig. 4D-E), and thus maintain their excellent optoelectronic performance (transmittance of ~89% with sheet resistance of 35 Ω/sq and haze < 3%) under environment attacks.More recently, Li and co-workers demonstrated a galvanic-replacement-free approach to synthesize highly stable Cu-Ag core-shell NWs in by a simple adsorption and decomposition process (Zhang et al. 2019a).The Cu-Ag NW lms exhibited outstanding stability and could maintain their optoelectronic properties at high-temperature environment (140°C) or under standard harsh condition (humidity of 85% RH and temperature of 85°C) for more than 500 hours.

Ag NW TCEs
Ag NWs have been the most widely explored form of metal NWs in TCEs, with recent efforts on overcoming the poor stability of Cu NWs (Huo et Wang et al. 2018b).Since Ag NWs networks have excellent electrical conductivity (highest electrical conductivity of 6.3×10 5 S/cm among all the elements) and high optical transparency when NWs are well synthesized and assembled into lms (Azani et al. 2020;Papanastasiou et al. 2020).In addition, as a noble metal, Ag NWs show relatively good oxidation/corrosion resistance under ambient conditions as well as outstanding mechanical strength (Ahn et al. 2017;Zhao et al. 2021).As a result, Ag NWs TCEs are most promising to be incorporated into commercial products so far.
To our knowledge, Yang and co-workers used mesoporous silica template for the rst time to synthesize Ag NWs in 2000 (Huang et al. 2000).However, these NWs (0.5-5 µm in length) were not long enough to be applied in TCE.In 2002, Xia and co-workers developed uniform and high-aspect-ratio Ag NWs with long length up to ~50 µm and small diameters of 30-60 nm by two-step polyol synthesis for the rst time (Sun et al. 2002).Later, they condensed this method into one step.In a typical synthesis (Sun and Xia 2002), the ethylene glycol (EG) solutions containing both PVP and Ag NO 3 , respectively, were simultaneously injected into pre-heated 160°C EG by a two-channel syringe pump.The Ag atoms formed at beginning of the reduction reaction served as seeds for the subsequent NWs growth through controlling the injection rate.They proposed that each NW was evolved from a decahedral seed with the assistance of PVP ligand (Fig. 5A) (Sun et al. 2003).The anisotropic growth was enabled by selective binding of PVP agent toward the Ag (100) facts while leaving the Ag (111) facets mostly uncoated, which are thus greatly reactive.This results in the passivation of the (100) facets that nally become the side faces of the penta-twined Ag NWs featuring a pentagonal cross section (Fig. 5B).Since then, polyol reduction has become the most favorite strategy to produce high-quality and uniform Ag NWs (Da Silva  Various inorganic additives were introduced to optimize the synthesis of Ag NWs.For instance, Buhro and co-workers found that the addition of sodium chloride at the beginning of polyol synthesis induced the heterogeneous nucleation of metallic Ag in the form of AgCl nano-cubes (Schuette and Buhro 2013).
Subsequently, the Ag NWs grew from these nucleation sites.In another study, Benjamin and co-workers produced ultrathin Ag NWs with small diameters (20 nm) and high aspect ratios (>2000) via introducing 2.2 mM sodium bromide to the synthesis solution, where AgBr nanoparticles act as heterogeneous nucleants (Li et al. 2015b).However, the NWs were associated with nanoparticles and needed further selective precipitation for the purity.Xia and co-workers developed a simple method to produce Ag NWs with small diameters of <20 nm and high aspect ratios of >1000 under atmospheric pressure (Da Silva et al. 2016).The main point to the successful synthesis of Ag NWs is to retard the reduction kinetics through injecting the Ag precursor via a syringe pump and, meanwhile, adding PVP ligand with a high molecular weight and bromide ion.Recently, Yang and co-workers demonstrated a modi ed polyol synthesis of ultrathin Ag NWs with an average diameter of 13 nm and aspect ratios >3000 (Fig. 5C-D) (Niu et al. 2018).They added the benzoin into EG solution containing Ag NO 3 , PVP, NaCl and NaBr to decrease the reduction temperatures of Ag precursors, leading to enhanced surface passivation of the Ag (100) facets by Br − ions.The benzoin generated reactive radicals under exposure to heating, where the strong reducing power of radicals could reduce silver precursors at relatively low temperatures.However, the use of PVP introduced an electronically insulating layer around each NW and thus sharply increased the contact resistance between NWs.To address this issue, Lim and co-workers demonstrated a modi ed polyol reduction of Ag NWs without using any capping agents (Sim et al. 2016).In a typical synthesis, EG solutions containing tiny amount of Fe(NO 3 ) 3 and NaCl were introduced into EG heated at 110°C.Subsequently, an EG solution containing Ag NO 3 was injected to the mixture and kept at 110°C without stirring for 15 h.At the beginning of the synthesis, the formation of AgCl particles offered preferential sites for the heterogeneous nucleation and growth of Ag NWs (Fig. 5E).At the later stages of the synthesis, the oxidative etching by O 2 /Cl − pairs dramatically reduced the number of Ag nanoparticles, and led to the selective formation of NWs with a high aspect ratio of ~1000 without using any capping agents.
In 2008, Peumans and co-workers rst demonstrated solution-processable Ag NW network lms, deposited by drop coating technique, as TCEs, which showed comparable optoelectronic performance to ITO electrodes (Lee et al. 2008).The organic photovoltaic devices built upon these Ag NW TCEs exhibited similar PCEs to those of ITO TCEs.Since then, high optoelectronic performance Ag NW networks lms have been reported by many research groups (Bellew et et al. 2012b).Typically, this method involves two steps: (i) deposit Ag NW percolation network on a lter membrane by vacuum ltration and (ii) pressure-aided transfer of the NW networks onto target substrates (Fig. 6A).However, the area of such NWs lm is usually quite small because of the limited size of the lter membrane, which is unsuitable for large-area device application.Rod coating is well-known technique to make lms in a controlled and continuous manner.It can be applied in a scalable way for roll-to-roll (R2R) production in industry.In one study, Cho and co-workers reported a continuous R2R production of exible Ag NW network lm with good optoelectronic properties via the rod coating method (Lee et al. 2014).This coating process includes three crucial steps: (i) Meyer-rod coating of Ag NWs, (ii) roll compression of NWs lms and (iii) salt-treatment and washing to improve electrical property (Fig. 6B).The resulting Ag NWs showed high transmittance (~ 92%) with a low sheet resistance (~5 Ω/sq), which surpassed those previously reported solution-processed metal NW electrodes.Moreover, the Ag NWs electrodes exhibited enhanced mechanical stability and were successfully used in various organic devices.Besides, spray coating is another attractive method to prepare uniform and large-area Ag NW networks due to its simple and straightforward setup.For example, Lee and co-workers reported a facile strategy to make large-area transparent Ag NWs lms with high optoelectronic performance via spray coating (Lee et al. 2013a).
Interestingly, Lim and co-workers demonstrated a simple method to prepare network lms with Ag NW rings through a traditional spray-coating method (Lim et al. 2017).They found that Ag NWs were elastically bent into curved shapes within micrometer-sized liquid droplets generated via the elastocapillary interaction during the spraying process (Fig. 6C).The curving phenomenon enabled the Ag NW electrodes with good electromechanical stability as well as excellent stretchability.Recently, Jiang and co-workers developed aligned Ag NW networks with anisotropic conductivity via a simple conical bers array-guided liquid transfer strategy, which yielded drastically different conductivity in different directions with respect to the brushing direction (Fig. 6D) (Meng et al. 2018).The conductivity of the as-prepared Ag NW networks along the brushing direction was 270 times higher than that of the spin-coated Ag NW networks under the same condition.Similarly, Ko and co-workers modi ed a traditional bar-coating technique to produce highly cross-aligned and large-area (> 20 cm × 20 cm) exible Ag NW networks TCEs with excellent optoelectronic properties (transmittance of 95.0% and sheet resistance of 21.0 Ω/sq) (Cho et al. 2017).Despite the signi cant process, the development of cost-effective and scalable deposition techniques for making robust, exible and large-area Ag NW networks TCEs remains a big challenge.
For the widespread practical application of Ag NW network lms, there are certain issues that need to be solved.Firstly, effective interconnections between NWs are the critical parameters to obtain high optical and electrical properties.However, the use of caping agents during the polyol synthesis of Ag NWs brings an insulating layer around each NW and can thus signi cantly increase their contact resistance, increasing the overall sheet resistance of resultant TCEs.To solve this issue, researchers used various kinds of treatments, including thermal annealing (De et  ).In one study, it was demonstrated that thermal annealing at the temperature of 200°C for 20 min could decrease the sheet resistance of Ag NW TCEs from >1000 Ω/sq to ~100 Ω/sq due to the partial decomposition of PVP as well as NWs welding (Lee et al. 2008).Whereas, continuous heating (about 40 min) gradually increased the sheet resistance of the Ag NW networks as the Ag NWs broke into disconnected nanorods or nanoparticles (Fig. 7A).Such a thermal annealing process is, however, an issue for the preparation of Ag NW TCE on heat-sensitive substrates.To address this issue, Suganuma and co-workers improved the electrical conductivity of Ag NW TCEs by mechanical pressing the NWs at 25 MPa for 5 s at RT (Tokuno et al. 2011).This simple process resulted in a steep drop of sheet resistance by more than four orders of magnitude from 18 kΩ/sq to 8.6 Ω/sq, while maintaining their transmittance of 80% (Fig. 7B).In another study, Brongersma and co-workers reported a light-induced-plasmonic welding of NW junctions to reduce the sheet resistance of Ag NW lms (Fig. 7C) (Garnett et al. 2012).The welding process was self-limited at junction points because the plasmonic light concentration was strongly dependent on the distance between the NWs.The small gaps formed naturally at NW junctions enabled effective light concentration and the heating only at junction points, preventing damage to heat-sensitive substrates.Similarly, Lee and co-workers demonstrated a ash-induced plasmonic welding of Ag NWs networks with high transmittance (90%), extremely low sheet resistance (~5 Ω/sq) and strong adhesion to plastic substrates (Park et al. 2017b).Recently, Choy and co-workers reported a novel approach to chemically grow and selectively integrate Ag nanoparticles at junctions of Ag NWs networks.(Fig. 7D) (Lu et al. 2014).This plasmon-induced chemical reaction was environmentally-friendly and could be carried at RT.The results indicated that the sheet resistance of the Ag NW networks can be dramatically reduced from 2787.3 Ω/sq to 13.4 Ω/sq at transmittance of 88.7% after plasmon-induced chemical treatment.Besides, they developed a facile alcohol-based solution strategy to fuse Ag NWs together via chemically growing crystalline Ag nanoparticles at the junctions of NWs, making Ag NWs networks with high optoelectronics performance and good operational stability (Fig. 7E) (Lu et al. 2015).More recently, an interesting phenomenon emerged, in which capillary force was used to drive self-limited cold sintering of the junctions for Ag NWs networks at RT (Liu et al. 2017).The capillary-force-induced nano-welding could be simply carried out by applying moisture, such as breathing-out water vapor, to the Ag NWs networks without any additional materials or facilities (Fig. 7F).The moisture-processed Ag NW TCEs exhibited a remarkable reduction in sheet resistance as well as improved exibility, while maintaining their transparency.
Besides, efforts have also been made to address their weak adhesion to plastic substrate, poor stability in a harsh environment as well as high surface roughness (Chen et (Xiong et al. 2016).In one study, Peumans and co-workers created inorganic-organic hybrid composite transparent electrodes by burying Ag NWs networks into PEDOT:PSS lms (Fig. 8B) (Gaynor et al. 2011).The resulting Ag NW TCEs showed strong adhesion to the substrate as well as low root mean square roughness (11.9 nm).Finally, the Ag NW TCEs based organic photovoltaic cells exhibited PCE of 3.8%, which was much higher than those of ITO TCEs based cells.In another study, Lee and co-workers developed a sandwich-structured graphene/Ag NW networks/graphene TCE, which exhibited excellent optoelectronics properties (sheet resistance of 19.9 ±1.2 Ω/sq at transmittance of 88.6%) and excellent exibility (Lee et al. 2015).Moreover, the resulting hybrid electrodes showed signi cantly improved long-term stability due to the gasbarrier property of graphene.Recently, Blom and co-workers reported an e cient TCE via coupling highquality solution-processable exfoliated graphene layers and Ag NW networks (Fig. 8C) (Ricciardulli et al. 2018).These TCEs showed high opto-electrical properties (transmittance of 89% with a sheet resistance of 13.7 Ω/sq), low-surface roughness of 4.6 nm and superior stabilities (Fig. 8D).

Au NW TCEs
Compared with Cu NWs and Ag NWs, Au NWs show better chemical and thermal stability and are highly resistant against oxidation under common environment, which makes them a promising candidate for TCEs (Chandni et Pong et al. 2007), have been developed for the production of Au NWs.In 2004, Lee and co-workers reported a very simple method for the preparation of Au NWs via UV irradiation photoreduction of HAuCl 4 in the bulk phase of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) block copolymer (Kim et al. 2004).However, the Au NWs showed a low aspect ratio of ~100 with lengths of 3 µm to 6 µm and diameters of 50 nm on average (Fig. 9A-B).The early synthesis of ultrathin Au NWs was published in 2007-2008 (Halder and Ravishankar 2007;Huo et al. 2008;Lu et al. 2008;Wang et al. 2008).In one report, ultrathin Au NWs were prepared by aging a mixture of Au(I)-oleylamine (OLA) complexes and ascorbic acid (Halder and Ravishankar 2007).The as-synthesized Au NWs showed an average diameter of 2 nm (aspect ratio: 500) with main byproduct of 10-20 nm sized Au nanoparticles (Fig. 9C).The high-resolution TEM image indicated that the NWs grew through the oriented attachment of small Au nanoparticles along the <111> direction (Fig. 9D).In 2008, Yang and co-worker reported a simple solution method to produce high-quality ultrathin Au NWs by mixing HAuCl 4 with OLA (Huo et al. 2008).
The OLA here served as solvent, recuing agent and surface capping agent.The Au NWs showed a uniform diameter of 1.6 nm and lengths up to ~ 4 µm.High resolution TEM images revealed that all theses Au NWs were single crystalline and grew along the <111> direction.At about the same time, Xia and co-worker produced Au NWs with a mean diameter of 1.8 nm (aspect ratio>1000) by reducing the 1dimensional chain of (OLA-Au(I)Cl) complexes via Ag nanoparticles.The polymeric complexes were rst formed due to the formation of the aurophilic bond between OLA and AuCl, which then served as the backbone of the Au NWs.Afterwards, the Ag nanospheres with size of 10 nm were added into the above growth solution as a reducing agent and signi cantly enhanced the yield of Au NWs.Different diameters of Au NWs have also been synthesized by slightly changing the reaction condition.For example, Sun and co-workers mixed HAuCl 4 with OLA and OA, and heated the reaction to 80°C for 5 h (Wang et al. 2008).
The diameters of the formed Au NWs were 3 nm and 9 nm in absence and presence of OA, respectively.Both types of Au NWs grew along the direction of <111>.
Following these pioneering studies, further efforts have been made to develop the controlled synthesis of Au NWs with high aspect ratios and explore their application in TCEs (Azulai et (Azulai et al. 2009).In another study, Coleman and co-worked developed a thin lm of Au NWs on PET substrate (Fig. 10A-B) (Lyons et al. 2011).These TCEs showed low sheet resistance of 49 Ω/sq with high transmittance of 83%, which are very close to industry requirements.They rst synthesized Au NWs with a mean diameter of 47 nm by template-assisted electrodeposition method (Fig. 10C).The resultant NWs were then dispersed in dichloromethane and deposited by vacuum ltration on cellulose lter membranes, followed by pressure and heat-aided NW networks transfer to the PET substrates.Finally, the Au NW TCEs were obtained via removing the lter membrane in acetone and methanol baths.Although the values of sheet resistance are promising in the above reports, the transparency is still too low.In order to address this issue, Correa-Duarte and co-workers reported a facile and one-step approach to produce Au NWs lm by self-assembly with extremely high transmittance of 96.5% (sheet resistance: 400 Ω/sq) without any further treatment (Fig. 10D) (Sánchez-Iglesias et al. 2012).The ultrathin and high aspect ratio Au NWs were prepared by an OLA reduction approach in solution phase, showing a typical dark red color (inset in Fig. 10E).The as-prepared Au NWs showed an average diameter of 1.6 nm and lengths of several micrometers (Fig. 10E and F).Afterwards, the aligned monolayers of Au NWs lms were developed by the self-assembly of NWs at a liquid (diethyleneglycol, DEG)-air interface and transferred to a TEM grid (Fig. 10G).Likewise, mechanically-robust and free-standing giant Au NW nanomembranes were developed from ultrathin single-crystalline Au NWs via the Langmuir-Blodgett technique (Chen et al. 2013).The lms showed high transmittance from 90-97% over a wide spectral window (300-1100 nm) with super mechanical exibility.
High-performance Au NW TCEs have also been prepared using alternative methods (Gong et  successfully demonstrated a solution-based approach to prepare self-assembled mesh lm using ultrathin Au NWs at the water/air interface (Gong et al. 2016b).The formation of mesh lm was based on a ligand-removal aggregation process of ultrathin Au NWs and could be well controlled by altering aging temperature and time.With the optimal aging time of 12 h, the Au NW mesh lm exhibited good optoelectronic performance, where the sheet resistance was 130.1 Ω/sq at transmittance of 92%.In addition, these Au NW TCEs were washable and patternable with excellent exibility, which were further used in touch screen and exible circuit of LEDs.In another study, Kraus and co-workers fabricated exible and transparent metal grids as TCEs with tunable performance by direct nanoimprinting of selfassembled Au NWs (Fig. 11A) (Maurer et al. 2016).The Au NWs were rst synthesized via surfactantassisted method (Feng et al. 2009), which showed ultrathin diameters of ~1.6 nm and high aspect ratios of >1000 (Inset in Fig. 11B).Then, a drop of Au NWs in cyclohexane was dispensed on a PET substrate.Afterwards, a polydimethylsiloxane (PDMS) stamp was placed onto the above Au NW dispersion and made conformal contact with the substrate.After the complete evaporation of solvent, the stamp can be removed.Finally, the highly exible Au NW lm TCEs with a sheet resistance of 29 Ω/sq were obtained via sintering by a hydrogen plasma at RT.These TCEs showed a hexagonal network of Au NWs bundles (Fig. 11B).It was found that the Au NW TCEs had better exibility than ITO-PET TCEs and could maintain 94.4% of conductivity under a bending radius of 5 mm after 450 bending cycles (Fig. 11C).Au NW TCEs featuring high stretchability have also been prepared.As described by Cheng and co-workers (Gong et al. 2016a), a simple yet e cient method was developed to fabricate highly transparent and stretchable Au NWs conductive thin lms.These self-assembled, well aligned Au NWs lms were then successfully transferred to a soft PDMS substrate (Fig. 11D) using a water-assisted method, which showed high stretchability of up 50% with stable and reversible electrical responses.The optoelectronic performance of the lms can be tuned by changing the times of the transfer process (Fig. 11E-F).The resultant Au NW TCEs were further applied in stretchable and transparent supercapacitors, which showed an areal-speci c capacitance of 226.8 µFcm −2 and a speci c capacitance of 8.5 Fg −1 without degradation even over 80 stretching cycles.
Table 1 shows some representative results of the photovoltaic devices by using optimal Cu, Ag and Au NWs lms as TCEs.From these results, Ag NWs lms should be the most promising candidate for the replacement of traditional ITO electrode in the near future.conductive lms composed of rGO and Cu NWs (Fig. 12A) (Kholmanov et al. 2013).These hybrid lms showed better electrical conductivity, stronger adhesion to the substrate as well as higher oxidation resistance than pure Cu NW lms.The RG-O/Cu NW lms were then used as TCEs in Prussian blue-base EC devices (Fig. 12B), which performed much better than those of Cu NW lms.The optical transmittance of the devices corresponding to the colored and bleached states were 33.4% and 81.3%, respectively (Fig. 12C).Likewise, Samori and co-workers demonstrated a highly conducting and transparent hybrid Cu NW/rGO lms as TCEs used in high-performance EC device, which showed remarkably high coloration e ciency of 948 cm 2 C −1 (Aliprandi et al. 2017).More recently, Wang and co-workers developed Cu/Ag-Au alloy core/shell NW network via co-electrodepositing method (Fig. 12D) (Zhang et al. 2020a).These core/shell NW networks showed good chemical stability, outstanding optoelectronic performance (transmittance of 90.1% at a sheet resistance of 14.2 Ω/sq) and excellent mechanical exibility.The bifunctional devices with EC and supercapacitor performance were then fabricated on these core-shell NW lms, which could change color and store energy simultaneously under different bias voltages (Fig. 12E).The devices showed excellent transmittance variation of 61.43% at wavelength of 633 nm (Fig. 12F) and high areal capacitance of 12.12 mF/cm 2 .In another study, Dong and co-workers reported a facile electrochemical method to produce robust Cu-Au alloy NWs network lms (Zhang et al. 2021).These alloy NWs network lms presented high optical-electrical performance (transmittance of 87% with a sheet resistance of 23.2 Ω/sq), excellent exibility and outstanding electrochemical stability.The asymmetric-EC devices based on the alloyed NWs TCEs exhibited high coloration e ciency of 153.77 cm 2 /C at wavelength of 633 nm and areal capacitance of 2.29 mF/cm 2 , which demonstrated their potential for multifunctional, exible and smart optoelectronics.Unfortunately, the application of the devices built on Cu NWs lms are seriously limited due to the concern of long-term stability.
Therefore, more and more researchers have been focused on the practical application of Ag NW TCE.
Indeed, Ag NW TCE has been so far the most widely studied and successfully applied material among all metal NWs.For example, Peng an co-workers reported a continuous R2R method to develop exible TCEs based on a Ag NWs lm fully encapsulated between a monolayer of graphene and a plastic substrate (Fig. 13A) (Deng et al. 2015).A roll of continuous graphene lm was rst grown on an industrial copper foil via a R2R CVD process.Then, the as-prepared graphene on copper foil was transferred onto Ag NWsprecoated ethylene vinyl acetate (EVA)/PET by hot-lamination.Finally, the graphene/Ag NWs network/EVA/PET lm was delaminated by electrochemical bubbling, preserving the copper foil for reutilization.The encapsulated structure, minimized the electrical resistance of both graphene grain boundaries and NW-to-NW junctions, and strengthened adhesion of graphene and NWs to exible substrates.Moreover, the graphene/Ag NWs network lms exhibited outstanding optoelectronic performance with a transmittance of 90% and a sheet resistance of ~22 Ω/sq (Fig. 13B), excellent corrosion resistance and remarkable exibility.To evaluate the potential for practical application, these lms were used as TCEs in exible EC devices, which showed excellent cycle stability up to 10000 cycles (Fig. 13C).Similarly in another study, Wu and co-workers developed a nonheated R2R process to produce continuous, extra-large and exible Ag NW network lms, which exhibited superior optoelectronic properties with a transmittance of 95% and a sheet resistance of 12 Ω/sq (Lin et al. 2017).The entire fabrication process was environmental-friendly and could perform in RT at atmospheric pressure.Moreover, an A4-sized EC device was successfully assembled by using PEDOT:PSS and transparent Ag NWs network lms as active layer and electrode, respectively (Fig. 13D).The devices showed switching time (4.3 s), good coloration e ciency and outstanding exibility (Fig. 13E).
Recently, Yu and co-workers reported exible Ag/W 18 O 49 NWs lms on PET substrate via the co-assembly of functional NWs for both transparent electrodes and EC devices application (Wang et al. 2017b).In their work, the NWs lms were prepared by using a modi ed Langmuir-Blodgett technique with tunable transmittance (58-86%) and sheet resistance (7-40 Ω/sq).NWs networks in a polymer lm, which showed high transparency (>85%), low haze (<0.2%) and smooth surface (Li et al. 2020a).In addition, the Ag NW network electrodes showed high electromechanical stability and good structural integrity due to the embedment of the NWs.Furthermore, the EC device based on this Ag NW network electrode showed excellent electromechanical stability and high coloring e ciency of 87.0 cm 2 C −1 (Fig. 14E-F).
Apart elastomer as an active layer and stretchable conductors, respectively (Fig. 15A).The devices showed fast bleaching (4 s) and coloration (1 s) time and good cyclic stability at the relaxed state.Furthermore, the stretchable devices were mechanically robust and could be twisted, folded, crumpled and stretched without obvious performance degradations (Fig. 15B-C), making them as the most promising candidates for next-generation wearable smart optoelectronics applications.Fig. 15C showed the patterned EC device in bleached and colored states under 0 and 50% tensile strain, respectively.Recently, Zhang and co-workers demonstrated a new approach to fabricate a stretchable EC device by integrating a stretchable EC hydrogel as well as asymmetric stretchable electrodes where Au nanosheets and Ag NWs network used as the anode and the cathode, respectively (Fig. 15D) (Yang et al. 2019).The EC hydrogel containing a transparent, conductive and elastic polyacrylamide (PAAm) and water-soluble EC molecules was used as both electrolyte and EC active layer.The stretchable device based on hydrogel-R could reversibly stretched and easily changed its color from yellow to red (Fig. 15D).It showed promising performance with high coloration e ciency of 92.10 cm 2 C −1 , good reversibility (> 100 cycles) and outstanding stretchability (> 100 cycles without degradation after under 20% tension).
In real-world applications, stretchable EC device needs a delicate design of each part to satisfy the stringent requirements for stretchability and deformation stability (Hao et 16A) (Yun et al. 2019).The as-prepared PAAm hydrogel electrolyte showed high stretchability of up to 80%, as well as good ionic conductivity.Besides, the WO 3 nanotube/PEDOT:PSS hybrid active layer could sustain high EC property even under large mechanical deformations.The integrated wearable EC device showed good reversibility during the coloration (-1.5 V) and bleached (-0.1 V) states and could be xed onto the wrist (Inset in the Fig. 16B).Furthermore, the device demonstrated outstanding stability in the ambient condition for 2 weeks and could maintain 98.6% of capacitance under 20% tensile strain (Fig. 16B).In another study, Zhao and co-workers developed a facile hydrothermal method to prepare the core-shell structure of WO 3 @Ag NWs (Fig. 16C) (Hao et al. 2021).
The stretchable electrode was then prepared by embedding the WO 3 @Ag NWs network in a PDMS lm, which could be simultaneously used as stretchable TCE (12 Ω/sq) and EC active layer.The embedded electrode exhibited large transmittance modulation of 72% at 550 nm and presented excellent stretchability (> 1000 cycles at 70% strain).The stretchable EC device was fabricated by using WO 3 @Ag NWs embedded PDMS, LiClO 4 /propylene carbonate and Ag NW embedded PDMs as working electrode, electrolyte and counter electrode, respectively, which exhibited excellent stretchability and good EC behavior even after 200 stretching cycles (Fig. 16D).However, the Ag NW networks enhanced the electro-optical characteristics of the device.Goldthorpe and co-workers got the similar results (Khaligh et al. 2015).They integrated the Ag NWs network electrodes into the LCD for smart window applications (Fig. 17A).The Ag NWs based devices showed larger transmittance modulation (∆T on−off =57.3%) than the ITO electrodes-based devices (∆T on−off =46.4%) (Fig. 17B).Additionally, they found that the costs of the materials and fabrication process of Ag NWs electrodes were much lower than those of ITO electrodes.Kim and co-workers demonstrated a continuous R2R slot die coating approach to develop Ag NWs networks electrodes for large-area and exible LCDs application (Kim et al. 2018a).The Ag NWs electrodes-based LCDs exhibited large-area optical modulation of 70% at an external voltage of 80 V (Fig. 17C).Moreover, they demonstrated a large LCD smart window with ultra-long length of 2 m and width of 400 mm, which showed outstanding exibility (Fig. 17D).

Other Chromatic Devices
There are only a few reports on the integration of metal NWs electrodes into SPDs.Recently, Ma and coworkers introduced the metal NWs electrode into SPD for the rst time.In 2018, they rst developed a cost-effective approach to produce Cu-rGO core-shell NWs lms with high electrical conductivity and optical transparency, as well as excellent chemical and thermal stabilities (Huang et al. 2018).A novel sandwich-structured SPD, built upon the core-shell NW lm, showed wide transmittance modulation of 42% at 550 nm between bleaching and coloring states, fast switching time and superior reversibility (Fig. 18A and B).These high performances are comparable to, and even better than, the SPDs based on commercial ITO electrodes (Fig. 18B).However, the area of such device was quite small due to the limited size of electrodes obtained through vacuum ltration method.Later, they reported a facile and scalable automatic blade-coating method to prepare large-area Ag NW networks TCEs with high optoelectronic performance and superior exibility (Huang et al. 2020a).For instance, these TCEs could be bent up to 10,000 cycles without any performance degradation, obviously better than ITO electrodes.In addition, the Ag NW networks TCEs exhibited good long-term stability as well as strong adhesion of the Ag NW networks to the substrate.To evaluate the practical potential, they incorporated these Ag NW TCEs into exible SPDs.The Ag NW TCEs based SPDs exhibited large transmittance modulation (60.2%), superior reversibility and fast switching time (21 s).The devices showed signi cantly better exibility than those fabricated on ITO TCEs and could be applied in smart beakers (Fig. 18C) with any performance loss.Moreover, the Ag NW TCEs based SPDs could maintain 85% of the initial performance after folding by 180° for 200 cycles (Fig. 18D).
Yu and co-workers demonstrated a versatile strategy to produce exible and stretchable intelligent ETC smart windows (Huang et al. 2019).First, they designed a large-area Ag NWs-nylon electrode (7.5 m 2 ) by coating Ag NWs on nylon mesh with high transmittance of 86.05% and extremely low sheet resistance of 8.87 Ω/sq (Fig. 19A).The Ag NWs-nylon hybrid electrodes with high heating e ciency was very responsive to the bias voltage and could be used as reversibly exible and stretchable electrothermochromic (ETC) smart windows by simply coating thermochromic dye layer (Fig. 19B-C).The Ag NWs-nylon electrodes based ETC devices showed fast switching time, outstanding mechanical exibility, superior reversible stability and excellent PM2.5 removal performance (Fig. 19D-E).Similarly, Gong and co-workers combined Cu NWs network electrodes with thermochromic lms to develop an ETC devices (Yu et al. 2021b).Fig. 20 summarizes applications of the metal NW based TCEs in exible chromatic devices in this review.In each case, the NW TCEs based devices showed better or at least similar performance to those of ITO TCEs, whereas addressing any mechanical exibility issues.

Conclusion And Outlook
Over the past several decades, we have witnessed a big progress in various modern optoelectronics, such as touch panels, solar cells, at displays and smart windows.TCEs are one of the key components in these smart electronic and optoelectronic devices.ITO has dominated the TCEs market for long time, and is likely to continue playing a crucial role in the near future.However, the utilization of ITO as TCEs has certain limitations, such as complicated fabrication procedures, sensitivity to acid and basic environments as well as increasing cost due to indium scarcity.In addition, ITO is quite brittle and easy to crack under continuous bending, which is absolutely incompatible with the strong demands for exible and wearable devices.Therefore, it is extremely urgent to develop cost-effective, solution-processable and exible TCEs by introducing some new materials for the next-generation optoelectronics.Many novel materials, such as, conductive polymers, graphene, CNTs, have been regarded as potential candidates, however, the poor environmental stability and/or high production costs limit their practical applications.
One-dimensional metal NWs lms have great potential for cost-effective and scalable TCEs application due to their high optoelectronic performance and good solution-processability.In general, metal NWs can be obtained via a facile and low-cost method and are compatible with universal solution-based techniques.Subsequently, large-area robust NW lms can be prepared through a continuous R2R process.Therefore, metal NWs based TCEs have attracted widespread attention as next-generation TCEs for modern and future optoelectronic devices.Especially, Ag NW networks based TCEs were the most widely studied and successful material among all metal NWs.It is worth mentioning that metal NW lms make lots of exible and wearable chromic devices come true, such as EC devices, LCDs and SPDs.
Up to now, there are still certain issues that need to be addressed for industrial applications, such as the environmental stability, surface atness as well as performance reproducibility in the resulting devices.
For instance, long-term stability remains a major challenge for commercially available Ag NWs TCEs during their real-world applications.In addition, developing NWs TCEs with base metals, such as Cu and Ni, should be more interesting and attractive.Recent developments of Cu hybrid NWs TCEs are prospective and could reduce the material cost.However, the stability studies for the TCEs in the related devices under real environments are still lacking, and more attentions should be paid to the integration of these materials into industrial products.Even though we made great progress in TCEs eld, it is still at very early stage and more efforts are required for further exploitation of these novel TCEs and their promising applications in smart optoelectronics.
Fig.14Apresented the uniform distribution of two kinds of NWs in the networks in which Ag NWs served as conductive materials and W 18 O 49 NWs played a role as chromic components.The Ag/W 18 O 49 NWs networks displayed good reversibility and stability during bending tests, implying the excellent exibility of the lms (Fig.14B).Furthermore, the asprepared exible solid EC device applied in EC eyeglass and smart windows showed clear transmittance change compared to the original state (Fig.14C-D).Other noteworthy examples used newly developed metal NW electrodes.Very recently, Huang and co-workers reported a robust electrode by embedding Ag Other metal NWs based chromatic devices, such as LCDs (Choi et al. 2018b; Khaligh et al. 2015; Kim et al. 2018a; Kim et al. 2018b; Park et al. 2015), suspended particle devices (SPDs) (Huang et al. 2018; Huang et al. 2020a) and electro-thermochromic (ETC) devices (Huang et al. 2019; Wang et al. 2017a; Yu et al. 2021b), were also demonstrated.For example, Seo and co-workers found that Ag NW TCEs based LCDs showed similar liquid crystal alignment characteristics to those using ITO electrodes (Park et al. 2015).

Figure 4 Three
Figure 4

Table 1
Kim et al. 2018c;Kortz et al. 2019;Yang et al. 2020ice by using optimal metal NW lms as TCEs Chen et al. 2021;Kim et al. 2018c;Kortz et al. 2019;Yang et al. 2020).In the last decade, a large amount of work has been done to develop various ITO-free EC devices, by using CNTs, graphene, conductive polymer, metal NWs and so on.Among these materials, metal NWs networks hold bright prospect due to their high optoelectronic properties, excellent mechanical exibility and outstanding solution-processability.
electronic displays (Cheng et al. 2018; Gu et al. 2018b; Lahav and van der Boom 2018; Lang et al. 2019; Stec et al. 2017; Zhang et al. 2020c).Many efforts have been made to these devices due to their unique advantages, such as fast switching time, low working voltage and low cost (Azam et al. 2018; Cao et al. 2019; Gu et al. 2018a; Macher et al. 2020; Savagian et al. 2018; Zhang et al. 2018a).Traditionally, the EC devices are based on brittle ITO TCEs, which however substantially limits the scope of their applications, especially in exible EC devices ( from exible EC devices, stretchable EC devices have great potential application in future e-skins, implantable electronics and smart clothes.Stretchable EC devices based on Ag NWs were developed by a number of groups (Cai et al. 2016; Lee et al. 2016; Yan et al. 2014).For example, Lee and co-workers reported stretchable and wearable EC devices on Ag NWs/PDMS electrodes (Yan et al. 2014).The devices were fabricated by using electrochemically deposited WO 3 and Ag NWs lms embedded in PDMS al. 2021; Koo et al. 2018; Lee et al. 2018; Yun et al. 2019).For example, in one study, Kim and co-workers demonstrated a stretchable and wearable EC supercapacitor device, which consisted of Ag-Au core-shell NWs embedded PDMS, PAAm hydrogel electrolyte, bis-tacked WO 3 nanotube/PEDOT:PSS wrapping layer and Ag NWs embedded PDMS electrode (Fig.