Cu@Ag core–shell NWs for printed electronics: synthesis, stability, and device integration

Abstract

Printed electronics is an emerging technology that has already transformed the world around us by enabling the fabrication of flexible, lightweight, and large-area electronic devices. However, achieving simultaneous optimization of electrical conductivity, environmental stability, mechanical flexibility, and cost efficiency remains a critical challenge. Nanowires (NWs) have gained prominence in various applications due to their high surface area to volume ratio. Silver NWs (Ag NWs) in particular have established themselves as leading platforms for transparent conductive electrodes (TCE), however their high cost and material consumption limit widespread adoption. While copper NWs (Cu NWs) offer cost advantages and comparable conductivity to silver; rapid oxidation and corresponding increase in resistivity in ambient conditions restrict their widespread adoption. Cu@Ag core–shell NWs (Cu@Ag NWs) are a promising solution, combining the cost efficiency of copper with the oxida0074ion resistance of silver through a strategic thin-shell design. This mini-review focuses on recent advances in Cu@Ag NWs synthesis, focusing particularly on kinetically controlled approaches that achieve conformal silver shells lowering the consumption of silver. We examine synthesis methodologies, including galvanic displacement and chemical reduction, discuss mechanisms underlying long-term environmental stability, and evaluate integration pathways for printed electronics applications including, transparent conductive films, flexible displays, wearable devices, and electromagnetic interference (EMI) shielding. Challenges and future considerations, including shell uniformity control, ink formulation optimization, and multi-functional coating systems incorporating protective oxide layers, are also addressed. This review provides a comprehensive framework for advancing Cu@Ag NWs technology toward practical commercial implementation in next-generation flexible and printed electronics.

Cu@Ag NWs combine low cost with superior oxidation resistance. This review highlights key synthesis strategies and their integration into durable transparent networks for printed, flexible, and wearable devices.

Introduction

Printed and flexible electronics are an important platform for lightweight and large-area electronic systems, spanning applications from flexible displays and touch panels to wearable sensors, radio-frequency (RF) devices, and energy harvesters.^1–4

These technologies leverage solution-processable materials and scalable patterning methods such as inkjet, aerosol-jet, screen, and roll-to-roll printing to reduce manufacturing cost and enable form factors that are difficult or impossible to achieve with conventional methods.^1,3,5 However, the stringent materials requirements of printed electronics; combining high electrical conductivity, optical transparency where relevant, mechanical robustness under repeated deformation, environmental stability, and low cost continue to impose significant constraints on the choice of industrial applications.^3,6,7 Transparent conductive electrodes (TCEs) represent a particularly critical component in displays, touch sensors, solar cells, transparent heaters amongst other applications.^7–9

Indium tin oxide (ITO), and transparent conductive oxides (TCO) have long been the industrial standard due to its excellent combination of low sheet resistance and high optical transmittance in the visible range, but its brittleness, scarcity of indium, and incompatibility with low-temperature plastic substrates motivate the search for alternative transparent conductors compatible with high-throughput printing.^2,7,10 Solution-processable transparent conductors based on metallic nanostructures, conductive polymers, carbon nanomaterials, and hybrid metal–polymer or metal–oxide architectures have therefore been intensively explored as ITO replacements.^{3,4,6,11–14}

Among these candidates, silver NWs (Ag NWs) networks have emerged as a leading platform for high-performance TCEs due to the combination of silver's high bulk conductivity and advantages of high-aspect-ratio NWs percolation networks.^8,15 Optimized Ag NWs films can achieve sheet resistances below 10 Ω sq⁻¹ at visible transmittance above 90%, surpassing commercial ITO benchmarks.⁸ Furthermore, Ag NWs networks exhibit excellent mechanical flexibility, retaining low resistance under repeated bending or stretching, which makes them highly attractive for flexible and wearable electronics.^4,8,16 Nevertheless, the high costs of silver impose economic and sustainability constraints, especially for large-area applications such as building-integrated photovoltaics, automotive glazing, or disposable wearable devices.^1,5

Copper NWs (Cu NWs) networks have therefore been investigated as a low-cost alternative to Ag NWs because of their comparable bulk conductivity (σ_Cu ≈ 5.96 × 10⁷ S m⁻¹vs. σ_Ag ≈ 6.30 × 10⁷ S m⁻¹) and much lower raw material cost.^17–20 Cu NWs-based flexible transparent electrodes have been reported with sheet resistances and transmittance close to those of Ag NWs and ITO films.^18–20 However, rapid oxidation of copper under ambient conditions remains a fundamental barrier: the formation of Cu₂O and CuO leads to orders-of-magnitude increases in sheet resistance over short timescales, severely compromising the reliability of the device.^17,21 These stability issues are exacerbated at elevated temperatures or humid environments, which are highly relevant for practical device operation.^17,18,20

To prevent copper oxidation, a variety of protective strategies have been explored, such as coating with metal oxides or carbon-based layers, encapsulating NWs with barrier polymers, or integrating them with graphene and carbon nanotubes.^4,19,22 However, many of these methods still face challenges, including incomplete surface coverage, limited long-term stability in the air, and increased junction resistance over time.^4,23 These limitations highlight the continued need for copper-based systems that maintain the cost advantages of copper while reaching stability and performance levels comparable to, or even surpassing, those of Ag NWs networks. For Ag nanowire-based transparent electrodes, hybrid electrical-photonic sintering provides a practical route to reduce junction resistance while preserving high transmittance and substrate compatibility.²⁴

Copper–silver core–shell NWs (Cu@Ag NWs) are a promising solution to this challenge by combining a copper core, with a thin silver shell that serves as a protective and conductive outer layer.^25–27 The higher standard reduction potential of silver (E_Ag⁺/_Ag⁰ = +0.80 V) relative to copper (E_Cu²⁺/_Cu⁰ = +0.34 V) enables selective deposition of Ag onto Cu NWs via galvanic displacement or related wet-chemical routes, forming a core–shell geometry that minimizes silver consumption while preserving high electrical conductivity and suppressing copper oxidation.^{23,25,26,28,28–30}

Despite progress, the performance and stability of Cu@Ag NWs networks depend on the morphology of the shell. Non-uniform or porous silver shells, electrostatically bound Ag clusters on the surface, and mechanically induced defects such as cracks or delamination lead to accelerated copper oxidation.^17,28,31,32 At the same time, optimization of ink formulation and printing conditions is required. In practical printed electronics, film morphology, junction resistance, and interface adhesion are important in determining overall device performance.^3,33 Several reviews have surveyed metal NWs for transparent conductors, flexible energy storage, and printed electronics.^4,6,17 However, they typically treat Cu@Ag systems as an example within a class of bimetallic NWs, without comparing synthesis routes, shell structures, stability metrics, and showing device demonstrations.^23,34

In this mini-review, Cu@Ag NWs are considered a platform at the intersection of low-cost copper and high-performance silver-based transparent electrodes. The available literature on Cu@Ag NWs is surveyed with emphasis on synthesis strategies and control of core–shell formation, structural and chemical factors governing oxidation resistance and long-term stability, ink formulation and printed film processing, and demonstrations in transparent conductors, wearables, and related applications. Particular attention is paid to identifying design principles that minimize silver usage while achieving robust shells and to outlining the challenges that must be addressed for the commercial deployment of Cu@Ag NWs technologies in next-generation printed and flexible electronics.

Synthesis strategies for Cu@Ag NWs

The performance and stability of Cu@Ag core–shell NWs (NWs) are determined to a large extent by how the silver shell is formed around the copper core. Reported strategies can be broadly grouped into: galvanic displacement of preformed Cu NWs by Ag species, galvanic-replacement-free routes in which noble metal ions are reduced onto Cu NWs, and adsorption and subsequent decomposition of Ag–amine complexes.^{26,28,35–37} The typical fabrication workflow begins with the solvothermal synthesis of pure Cu NWs, followed by their purification to remove impurities. The critical coating step, where the shell strategy is applied can be seen in Fig. 1 and show the transformation from pure copper core to a core–shell structure where silver shell covers and protects the copper core interior.

Fig. 1. Schematic overview of the bottom-up synthesis process for Cu NWs and subsequent coating with Ag for Cu@Ag NWs preparation for conductive applications.

Fig. 2 visually summarizes this multi-step process, showing the progression from raw precursor reduction to the final functional material. The upper sequence details the initial Cu NWs growth, typically involving the reduction of copper salts in the presence of capping agent like PVP. The final coating phase is the most delicate; depositing a uniform Ag shell without pitting the underlying Cu core is essential for high electrical conductivity. The final stage is the preparation of coated NWs into conductive pastes and dispersion on substrates, which are then ready for sintering applications.

Fig. 2. Schematic overview of the synthesis and processing workflow for Cu@Ag NWs preparation for conductive applications.

Table 1 shows the specific chemical routes available for the silver coating step. Standard galvanic displacement is chemically simple, but it often compromises the copper core. In contrast, modified approaches such as ammonia-assisted complexation or the use of external reducing agents (e.g., glucose, ascorbic acid) offer superior control over the shell morphology. The table categorizes these methods by their reaction mechanism, and their key conditions: pH, temperature, and specific additives. For instance, thermal decomposition routes offer an alternative that completely avoids galvanic corrosion, whereas electrochemical deposition enables thickness control suitable for pre-deposited films. Selecting the appropriate route from Table 1 depends heavily on the desired balance between process scalability, cost, and the ambient stability of the NWs dictated by their desired applications.

Table 1. Synthesis routes for coating pre-synthesized copper NWs with silver.

Methods	Ag precursor	Recent agent	Additives	Conditions	Remarks	Ref.
Ammonia-assisted galvanic	AgNO3	None	NH4OH, PVP	RT, pH 10–11	Complexes Ag+, slows pitting	37
Adsorption & decomposition	[Ag(NH2R2)]+	Thermal decomposition	NH3	Ads. RT + Ann. 200 °C	Avoids wet-galvanic pitting	25
Green solvent-mediated	AgNO3	DMSO2	DMSO2	25 °C, aq	Surfactanat DMSO2	38
Electrochemical	AgNO3	Applied pot	Electrolyte	Applied volt	Precise thickness	39

Kinetic regimes for conformal shell growth

The cross-over depends on three coupled parameters. First, the activity of free Ag⁺ by complexing with NH₃ to form [Ag(NH₃)₂]⁺, the standard reduction potential drops from E°(Ag⁺/Ag⁰) = +0.80 V to E°([Ag(NH₃)₂]⁺/Ag⁰) = −0.37 V vs. SHE,³⁸ lowering the galvanic driving force across the Cu/Ag couple from ΔE ≈ 0.46 V to ΔE ≈ −0.71 V and suppressing pinhole formation by Cu oxidation-undercutting.³⁹

To date literature has no cohesive mechanistic framework although many Cu@Ag nanowire coating methods exist.⁴⁰ Very little systematic study of their interaction is routinely conducted, and the morphological effect is observed empirically rather than predictively.⁴¹ However in most of previously reported approaches it is the reduction potential of the Ag precursor species that constitutes the most relevant governing parameter whether free Ag⁺ or a stabilised complex [Ag(NH₃)₂]⁺ are adopted. The form of the Ag ion during coating sets the galvanic driving force at the Cu surface and whether deposition is controlled and conformal or results in discontinuous, decorated surfaces of poor stability.

Galvanic displacement on copper NWs

The most frequently used route to Cu@Ag structures is classical galvanic displacement, exploiting the higher standard reduction potential of silver relative to copper. In this approach, pre-synthesized Cu NWs are immersed in an aqueous Ag(i) solution; reduction of Ag⁺ on the Cu surface is coupled with oxidation and partial dissolution of the underlying copper.^28,42 This process is driven by the transmetallation reaction, where the redox potential difference (ΔE° ≈ 0.46 V) between the Ag⁺/Ag (0.80 V vs. SHE) and Cu²⁺/Cu (0.34 V vs. SHE) couples provides the thermodynamic driving force for the spontaneous replacement of surface copper atoms by silver:

Cu_(s) + 2Ag_(aq)⁺ → Cu_(aq)²⁺ + 2Ag_(s) 1

He et al.⁴³ demonstrated a representative example by introducing well-dispersed Cu NWs into a silver–ammonia solution, where [Ag(NH₃)₂]⁺ species drive galvanic etching of Cu and nucleation of Ag nanocrystals on the NWs surfaces.⁴³ XRD, SEM, and TEM analyses showed Ag domains decorating the Cu NWs to form Cu@Ag heterostructures, with the size and density of Ag controlled by the [Ag(NH₃)₂]⁺ concentration and reaction time.^28,44 Similar “silver-coated Cu NWs” have since been obtained with modified galvanic conditions (complexing agents, pH, agitation), often improving shell continuity and oxidation stability.^20,45

However, galvanic deposition of Ag, Au, and Pt onto copper NWs can oxidize and etch the Cu core, leading to broken or hollow NWs if the reaction is not carefully controlled. Additives like ascorbic acid or applied potentials are often required to obtain continuous, smooth shell coatings.³⁵ Consequently, more recent work has focused on kinetic and chemical control of the displacement reaction to favor conformal Ag growth while minimizing core damage.^26,37,45

Galvanic-replacement-free noble-metal deposition

To circumvent core loss associated with galvanic displacement, Stewart et al. reported a room-temperature, solution-phase process in which ascorbic acid simultaneously removes the passivating copper oxide layer and reduces noble-metal ions (Ag⁺, AuCl₄⁻, PtCl₆²⁻) onto Cu NWs, while preventing net galvanic replacement.^34,37 In this chemistry, ascorbic acid acts as a sacrificial reductant; instead of Cu oxidizing to drive Ag deposition, Ag⁺ is reduced directly at the Cu surface and the Cu core remains largely intact, as confirmed by the SEM images in Fig. 3A–C which show that the NWs diameter increases slightly (from 79 ± 22 nm to 92 ± 23 nm for Cu@Ag) without the pitting characteristic of galvanic corrosion.

Fig. 3. SEM images of (A) Cu NWs, (B) Cu@Ag core–shell NWs, and (C) Cu–Au core–shell NWs. Average diameters were calculated from 50 distinct NWs. (D and E) TEM-EDS images of Cu@Ag and Cu@Au NWs. This figure has been adapted from Stewart et al.³⁴ with permission from ACS Publications, copyright 2015.

The resulting Cu@Ag, Cu@Au, and Cu@Pt core–shell NWs exhibit good noble metal shells, as evidenced by the TEM-EDS elemental maps in Fig. 3D and E. These maps reveal a relatively continuous sheath of silver or gold (red/blue signals) encasing the copper core (green signal).^25,34 For Cu@Ag specifically, films of these core–shell NWs are conductive as-printed and remain resistant to oxidation at 160 °C in dry air and at 85 °C, 85% RH, matching the performance of pure Ag NWs with significantly reduced Ag usage.^25,34 This work established a guideline for galvanic-replacement-free core–shell formation, which has since been extended to other systems like Cu@Ni and Cu@Pt.^28,28,37

Adsorption–decomposition of Ag–amine complexes

An alternative strategy is using Ag–amine complexes as a shell precursor, decoupling Ag adsorption from metallic Ag formation. Zhang et al. developed a galvanic-replacement-free, large-scale synthesis in which Cu NWs were exposed to a reactive Ag–amine complex, [Ag(NH₂R)₂]⁺, that adsorbed onto the Cu surface to form a Cu@Ag–amine core–shell structure.²⁶ Subsequent mild thermal annealing in air decomposed the complex into a continuous metallic Ag shell without net dissolution of the copper core.²⁶

By tuning the concentration of the Ag–amine complex in the Cu NWs dispersion, the authors could control shell thickness, obtaining Cu@Ag NWs that retained high conductivity and transparency comparable to pure Cu NWs networks while exhibiting improved thermal and humidity stability (up to 140 °C and 85% RH) due to the protective Ag shell.²⁶ This adsorption–decomposition concept has also been generalized to other Cu@metal systems and integrated with additional processing steps (alloying and embedding in elastomer matrices) to realize ultrastable stretchable transparent electrodes.³⁶

A summary of the synthesis methods discussed above and their is provided in Table 2. The synthesis routes are grouped into four: galvanic-displacement growth, galvanic-replacement-free reduction with a sacrificial reductant, Ag–amine adsorption–decomposition, and hybrid physical-chemical routes that pair a Cu core with a wet-chemical Ag shell. For each entry the table compiles the synthesis route and reagents, the resulting Cu core dimensions and Ag shell thickness, the ink-formulation and printing technique used, the sintering or curing protocol, the film electrical performance, the mechanical and oxidative stability achieved, and the demonstrated device. This layout makes it clear how the choice of synthesis route propagates through each step: galvanic-replacement-free routes consistently yield smooth thin shells suitable for high-transmittance transparent electrodes; Ag–amine routes trade an additional thermal step for outstanding long-term stability (≥500 h at 85 °C/85% RH) and electrospun hybrid systems deliver the lowest reported sheet resistances but at the cost of multi-step fabrication. Two benchmark rows on non-NW Cu@Ag morphologies are included to substantiate NW data against the wider Cu@Ag conductive-ink landscape.

Table 2. Summary of Cu@Ag nanowire synthesis routes, properties, and device performance.

Synthesis route	Ag shell & characterisation	Ink formulation	Printing	Sintering/curing	Electrical performance	Mechanical/stability	Device	Reference
Ascorbic acid removes CuOx and simultaneously reduces Ag+ (AgNO3, 0.025 M) onto Cu NW surface at RT; prevents galvanic replacement	PVP/DEHA stabiliser; Ag : Cu mol 0.04–0.15. Cu NW : D = 79 ± 22 nm, L = 28 ± 10 µm. 5 nm (L/D 320) and 15 nm (L/D 260) shells; TEM–EDS mapping; no HAADF line-scan confirmed	Nitrocellulose binder in acetone/EtOH/ethyl acetate/pentyl acetate/IPA/toluene; PVP + DEHA	Meyer rod	Conductive as-printed (no thermal step); brief acetone/H2O rinse removes residual binder	R sh = 29 Ω sq−1 @ T550 = 84%	Stable 24 h, 160 °C/dry air (L/D 320 & 260)	Transparent conducting film (TCF); envisioned OPV/OLED/touch	Stewart et al. (2015)34
					R sh = 31 Ω sq−1 @ T550 = 79% (as-printed, no rinse)	15 nm shell stable 24 h at 85 °C/85% RH
						5 nm shell: Rsh rises ∼100× in 24 h at 85% RH
Scale-up of family ii; multigram (4.4 g Cu@Ag NW h−1); Ag : Cu mol 0.04; D ≈ 240 nm (SEM)	≈3 nm shell; shell-continuity characterisation n/r	Polycaprolactone (PCL) composite filament; 5 vol% NW loading	3D printing	None (composite matrix consolidation only)	Bulk resistivity in composite: 0.002 Ω cm (=2000 µΩ cm)	T n/r (opaque filament)	Wireless power transfer coil (3D printed)	Cruz et al. (2018)47
					Current density 2.5 × 105–4.5 × 105 A m−2	Oxidation protection stated vs. bare Cu NW; no quantitative cycling data
[Ag(NH2R)2]+ Ag-amine complex adsorbs on Cu NW surface forming Cu@Ag-amine core–shell; thermal annealing in air decomposes complex to pure Ag shell; shell thickness tunable by Ag-amine concentration; galvanic replacement prevented	Tunable thickness; characterisation method n/a	n/a	Drop-cast on glass or PDMS; no formal rheological ink formulation	Thermal annealing in air; T and t n/r from abstract	Conductivity and T stated “similar to pure CuNWs” (qualitative); Rsh n/r	≥500 h stable at 140 °C and at 85 °C/85% RH	Flexible conductor (generic); flexible electronics	Zhang et al. (2019a)26
						Bending data n/r
Family iv + iii hybrid: Electrospun CuAc2/PVP NFs (12.9 kV, 1 mL h−1, 40 min) → H2-reduction at 300 °C/1 h in Ar/H2 → Cu NF (D = 91 ± 20 nm); then Ag(NH3)2+ electroless plating → Cu/Ag core/shell NF (D = 202 ± 52 nm)	Shell formed by Ag-amine complex (NH4OH); EDS mapping + HR-TEM (Ag lattice d = 0.2359 nm) + XPS; no HAADF line-scan explicitly stated	n/a	ES web deposition	No explicit sintering of final film; ES webs self-contact at junctions	R sh = 7.85 Ω/sq (random web)	Stable after 100 bending cycles on PDMS	Flexible PLED (62.53 cd m−2 at 10 V)	Jiang et al. (2019)49
					T > 80% (aligned); best random web T n/r at fixed λ	Resists 85 °C/80% RH oxidation vs. bare Cu NF	Solar cell potential noted
						20% strain without cracks
Electroless plating using amino-alcohol precursor; prevents galvanic replacement; Cu NW D and L n/a	Ag shell thickness n/a; characterisation method n/a	n/a	n/a	Chemical sintering: 5% aqueous H2O2 (welds NW junctions, improves substrate adhesion)	R sh ≈ 13 Ω/sq @ T = 89%	Oxidation stability improved vs. bare Cu NW	Transparent flexible electrode (TFE)	Navik et al. (2020)20
						20% strain repeated stretching cycles (saturation T n/r)	Transparent stretchable heater (TSH)
Pre-formed Cu@Ag NW (synthesis by family iii, Zhang 2019a); HIPL induces rapid Cu–Ag inter-diffusion → alloy NW; simultaneously embeds alloy NW into stretchable substrate	Shell alloyed during HIPL; thickness n/r; no HAADF post-HIPL	n/a	n/a	HIPL (high-intensity pulsed light); fluence 0.8–3.2 J cm−2 (single step: alloy + embed)	T = 89%	500 h at 140 °C and 85 °C/85% RH	Stretchable transparent heater (TSH); Tmax = 130 °C	Zhang et al. (2019b)51
					R sh n/r from abstract	1000 stretching–relaxation cycles at 30% strain
						Heater up to 60% strain
Note: Ag core/Cu shell (inverted). Reactive Ag ink + PEO/PVP electrospun at 15 kV; calcination 300 °C/30 s → Ag NF; UV-ozone 10 s; electroless Cu deposition (35 °C, 15–120 min; CuSO4/Rochelle salt/HCHO)	Cu shell confirmed by SEM (junction fusing); no EDS line-scan	Reactive Ag ink + PEO + PVP in EtOH/H2O	ES	Calcination 300 °C/30 s + UV-ozone 10 s	R sh = 0.33 Ω/sq @ VLT = 86%	+4% Rsh after 6 months (>4000 h) ambient	Transparent conducting electrode (TCE); all-atmospheric fabrication	Koga et al. (2023)128
					Haacke FoM = 652 × 10−3 Ω−1	+2.5% Rsh after 1000 bending cycles at 2.5 mm radius
Cu@Ag NW prepared by prior synthesis (route n/r from abstract; likely family ii)	Ag shell characterisation n/r from abstract	NW ink in EtOH; PVP K120 binder on substrate	Vacuum filtration + transfer (VFT) onto polymer-coated glass	Capillary immersion pressing (ambient, no thermal sinter)	R sh = 61.3 Ω/sq @ T = 89.4%	Film survives spin-coating post-deposition without delamination	TCF for optoelectronic devices fabricated by spin coating	Digregorio et al. (2022)126
Pseudo-one-pot: Cu(OH)2 + l-ascorbic acid + PVP-K30 in EtOH, 80 °C/15 min → Cu NPs; AgNO3 added dropwise → Cu@Ag NPs (≈100 nm DLS)	HAADF–STEM + EDS confirmed core–shell structure	60 wt% NPs in ethylene glycol (87.5 wt%) + diethylene glycol (12.5 wt%) + cellulose/glycerol; viscosity = 1.56 mPa s at 100 s−1	Screen printing on flexible PI	290 °C/15 min/N2	Resistivity = 25.5 µΩ cm	Oxidation onset delayed ≈70 °C vs. pure Cu NPs (ca. 240 °C)	LED circuit illumination	Li et al. (2023)127
						Stable 60 days ambient
						200 inner/100 outer bending cycles (gradual R increase)
Cu flakes ball-milled; Ag(NH3)2+ reduced by sodium potassium tartrate onto flake surface (family iii analogue)	EDS/XRD confirm Ag layer on Cu flake surface	Silicone-acrylic emulsion (SAE), water-based; 70 wt% filler	Spray coating on PET	Ambient cure (waterborne emulsion)	Conductivity: 1073.61 S cm−1 at 70 wt%	93.4% conductivity retained after 2000 bending at 3 mm radius	EMI shielding film	Huang et al. (2023)57

Table 2 not only summarizes the performance characteristics of NWs systems across diverse applications. Oxidation resistance, which can be characterized via oxidation induction time or indirectly as aging by measuring conductivity, is one of the most important factors for long-term electrical performance.⁴⁶ Systems that employ Ag shells or protective coatings have improved stability under ambient conditions, establishing that the presence and effective thickness of the Ag shell influence the initiation of oxidation. Oxidation induction time is not always explicitly reported in the literature, but extended operational lifetimes reinforce the core@shell importance in maintaining conductivity. Also, mechanical durability measurements, including critical strain and bending stability, are important indicators of the use cases of NW networks in flexible and stretchable electronics. Ag shell thickness and critical strain under bending give a good reference for performance comparison across studies in a consistent way and a basis for the design of next-generation conductive networks.

The minimum shell thickness required to suppress oxidation of the copper core is determined by the interplay between shell morphology, deposition mechanism, and the conditions of exposure. There are regimes distinguishable in the literature. First, twin-mediated epitaxial Ag shells grown via vapor-phase deposition on 〈110〉-twinned Cu NW cores of ∼100 nm diameter can provide effective passivation at thicknesses as low as ≈3 nm.⁴⁷ The epitaxial registry between the Cu{111} facets and the Ag shell minimizes structural defects that would otherwise serve as diffusion paths for oxygen. Second, polycrystalline shells produced by solution-phase chemical reduction require greater minimum thicknesses (≈5–15 nm) for full wire coverage, because grain boundaries in the deposit provide fast inward transport routes for oxidants.⁴⁸ The mechanisms governing all of these deposition methods is scarcely found in literature but understanding of these will be beneficial and lead to generation of newer more refined applications that might redefine how applications will be introduced in industry over the coming years (Table 2).

Applications of Cu@Ag core–shell NWs

Transparent conductive electrodes for optoelectronic devices

Cu@Ag NWs have been used as TCEs in displays, LEDs, and solar cells. Ye et al. first demonstrated Cu@Ag, Cu@Au, and Cu@Pt core–shell NWs films that match Ag NWs performance.³⁵ Cu@Ag NWs films were conductive as printed and remained resistant to oxidation at 160 °C and 85 °C/85% RH, making them viable anodes for organic optoelectronics.^17,35 Jiang et al. employed electrospinning followed by electroless Ag plating to form Cu@Ag NWs webs as TCEs.⁴⁹ Random webs achieved sheet resistances as low as 7.85 Ω sq⁻¹ at ∼82% transmittance (550 nm), while aligned webs maintained more than 80% transmittance.⁴⁹ The authors integrated these TCEs into flexible PLEDs on PDMS, demonstrating stability under repeated bending up to 20% strain.⁴⁹

More recently, Cu@Ag NWs have been used to fabricate TCEs on flexible substrates for display and sensor applications.²⁷ By minimizing grain-boundary diffusion pathways, Kuo et al. demonstrated TCEs that combine low sheet resistance, high transmittance, and significantly improved long-term stability under ambient and elevated temperature conditions.²⁷ These studies position Cu@Ag core–shell architectures as serious contenders for next-generation transparent electrodes where cost, flexibility, and stability must be balanced.^27,35,49

Wearable, stretchable, and electronic-skin devices

The mechanically compliant, high-aspect-ratio nature of metal NWs networks makes them attractive for wearable and electronic-skin (e-skin) platforms, where conformability and cyclic strain tolerance are important, which can be seen in Fig. 4 showing the results of compression tests. Wei et al. exploited Cu@Ag core–shell NWs produced by a simple room-temperature galvanic replacement reaction as conductive fillers in a bio-inspired piezoresistive e-skin.⁵⁰ A thin (≈20 nm) Ag shell imparted excellent oxidation resistance to Cu NWs, and embedding these Cu@Ag NWs into a rose-petal-mimetic PDMS microstructure yielded e-skin devices with high sensitivity (1.35 kPa⁻¹), very low detection limit (<2 Pa), fast response and relaxation times (36/30 ms), and stability over more than 5000 loading cycles.⁵⁰ The authors demonstrated applications in voice recognition, wrist pulse monitoring, and spatial pressure mapping.⁵⁰

Fig. 4. (a and b) Digital photographs to show the compression test of the e-skin. This figure has been reproduced from Wei et al.⁵⁰ with permission from Royal Society of Chemistry, copyright 2015.

Electrospun Cu@Ag core/shell NWs TCEs from Jiang et al. were also integrated into flexible PLEDs on elastomeric substrates.⁴⁹ The Cu@Ag NWs electrodes maintained conductivity and light emission under repeated bending and stretching, indicating suitability for wearable displays and conformable optoelectronics.⁴⁹ More broadly, Kuo et al. used Cu@Ag NWs in wearable and flexible electronic platforms, correlating shell microstructure with mechanical robustness and stable electrical performance under cyclic strain.²⁷ These results collectively show that when shell continuity and adhesion are properly engineered, Cu@Ag-based networks can serve as mechanically resilient electrodes for wearable sensors, e-skin, and soft optoelectronic systems.^27,49,50

The mechanical robustness and optoelectronic versatility of Cu@Ag core–shell nanostructures enable their direct integration into high-performance wearable devices. As demonstrated in Fig. 5, Cu@Ag NWs (Cu@Ag NWs) can serve as transparent, stretchable electrodes for polymer light-emitting diodes (PLEDs). After transferring the nanofiber network onto a flexible PDMS substrate (Fig. 5a), the resulting device maintains uniform blue light emission even under significant mechanical deformation, such as 20% stretching or twisting.⁴⁹ Furthermore, the transferred films exhibit excellent flexibility on various substrates (Fig. 5b), maintaining a low sheet resistance of ∼100 Ω sq⁻¹ even after being peeled and bent.

Fig. 5. (a) Schematic and photographs of a stretchable PLED device using Cu@Ag NWs on PDMS, showing stable light emission under 20% strain and twisting. (b) Transfer process of the transparent conductive film onto a flexible substrate, demonstrating preserved conductivity (∼100 Ω sq⁻¹) after handling. This figure has been reproduced from Jiang et al.⁴⁹ with permission from ACS Publications, copyright 2019.

To quantify this durability, mechanical bending tests were used. As shown in Fig. 6, Cu@Ag NWs electrodes exhibit superior fatigue resistance electrodes maintaining a stable electrical connection (ΔR/R₀ < 1) over 1000 cycles, regardless of whether the bending is tensile or compressive.²⁶ This enhanced electromechanical stability is attributed to the silver shell, which not only prevents oxidation-induced embrittlement but also mechanically reinforces the NWs junctions, preventing fracture propagation during repeated deformation.

Fig. 6. Electromechanical stability tests comparing pure Cu NWs and Cu@Ag core–shell NWs. (a and b) Photographs of the flexible LED circuit under bending. (c and d) Relative resistance change over 1000 bending cycles, showing the rapid failure of pure Cu NWs versus the high stability of Cu@Ag NWs. (e) Resistance change as a function of bending radius, confirming stability down to 2 mm. This figure has been adapted from Navik et al.⁴⁵ and corrected with Author's approval with permission from ACS Publications, copyright 2020.

Transparent heaters and anti-fog elements

The high electrical conductivity and excellent optical transmittance of Cu@Ag core–shell NWs make them ideal candidates for high-performance transparent heaters. Unlike traditional indium tin oxide (ITO) films, which are brittle and exhibit slow thermal response, Cu@Ag NWs networks can rapidly reach elevated temperatures via Joule heating at low driving voltages. For instance, Jiang et al. demonstrated that Cu@Ag nanofiber heaters could achieve a steady-state temperature of ∼130 °C at only 3.5 V with a thermal response time of less than 4 seconds, significantly outperforming pure copper NWs counterparts which often fail due to oxidation-induced resistance drift at elevated temperatures.⁴⁹ The silver shell is critical in this application, as it prevents the rapid degradation of the heating element under the thermal stress of repeated on–off cycles, enabling stable operation for hundreds of cycles without resistance hysteresis.^49,51,52

Beyond basic heating, these nanostructured films have been successfully deployed as efficient anti-fog and de-icing elements for smart windows and automotive applications. The percolating network of core–shell NWs provides uniform heat distribution over large areas, which is essential for rapid defrosting. Experimental validations have shown that Cu@Ag-based heaters can completely clear frost and fog from glass surfaces within seconds of voltage application, maintaining high visibility (>85% transmittance) throughout the process.^27,51,52 Furthermore, the intrinsic mechanical flexibility of the NWs network allows these heating elements to be integrated into curved surfaces, such as goggles or helmet visors, where rigid oxide coatings would crack.⁴⁵

EMI shielding and RF components

High-conductivity, low-thickness metallic conductors are essential for lightweight EMI shielding and RF structures such as antennas and RFID tags. While much recent progress in printed EMI and RF devices has employed copper pastes with protective coatings, Cu@Ag architectures offer a NW-based route to similar functionality with potentially lower weight and higher optical transparency.^53,54

The effectiveness of such NWs-based composites is vividly illustrated in Fig. 7, which summarizes the EMI shielding performance of silver NWs (Ag NWs)/cellulose papers fabricated via a dip-coating process. Fig. 6 serves as a comparative guide and shows Ag NW/cellulose paper data. While representative of the general scaling of specific shielding efficacy with electrical conductivity for high AR metallic NW networks being described for its analogical reference. However, actual measurements on Cu@Ag NW networks under identical conditions can be different and only rarely cited in the literature, while the elevated surface conductivity of Cu@Ag NWs should provide similar performance at lower noble-metal loadings. As shown in Fig. 7A, the specific shielding effectiveness (SE_T) at 1.0 GHz scales linearly with electrical conductivity, significantly outperforming other carbon-based and polymer-based composites reported in literature.⁵⁵ The mechanism, depicted in Fig. 7C, is reflection-dominated shielding where the dense, interconnected NWs network on the paper surface efficiently reflects incident electromagnetic waves (SE_R), while internal scattering within the porous cellulose structure contributes to absorption (SE_A).⁵⁵ This trend is supported by recent bimetallic-system reports. Huang et al. presented waterborne Cu@Ag composites with EMI for X-band SE_T >76 dB of only 60 µm, on the basis of dc conductivity of 1074 S cm⁻¹ and a much lower noble metal fraction achieved from AgNW/cellulose paper with similar noble-metal content.⁵⁶ These data are for flakes, not NWs and cover various frequency ranges, but show the desired behavior of confinement by isolating Ag to the conduction skin while replacing the bulk noble metal with Cu, allowing an experimental limit, assuming enough percolation, for Cu@Ag NW networks.

Fig. 7. SE_T values (at 1.0 GHz) of Ag NWs/cellulose papers as functions of (A) electrical conductivity and (B) density in comparison with those of other materials in the literature. (C) Schematic EMI shielding mechanism for Ag NWs/cellulose papers with thin or thick Ag NWs coating layers. EMI shielding characteristics of Ag NWs/cellulose papers: (D) coefficients of reflection, absorption, and transmittance at 1.0 GHz and their dependence on electrical conductivity; (E) SE_R, SE_A, and SE_T in decibels at 1.0 GHz; (F) SE_T in the frequency range of 0.5–1.0 GHz. This figure has been adapted from Lee et al.⁵⁵ with permission from ACS Publications, copyright 2016.

For RF applications, this high surface conductivity is crucial. The skin depth of copper at UHF frequencies (e.g., for RFID tags) is only a few microns; thus, a Cu@Ag core–shell structure that concentrates the highly conductive silver on the surface (SE ∝ σ^1/2) where σ is conductivity is advantageous.^53,57 By achieving high conductivity (>60 S cm⁻¹) at very low metal volume fractions (<1 vol%), these core–shell NWs composites enable the printing of lightweight, flexible antennas that maintain performance even under mechanical deformation, offering a distinct advantage over rigid, heavy copper foils.^45,55

Structural characterization of core–shell morphology

Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) provide indication of successful shell formation, revealing diameter growth and contrast differences between core and shell.^35,43,43 In bright-field TEM, Cu@Ag NWs exhibit a lighter-contrast core surrounded by a darker-contrast annulus corresponding to the higher-Z Ag shell, while HAADF-STEM enhances this Z-contrast.^58,59

Thermal analysis can be used to establish caping agent decomposition or the energy onset for oxidation. Khalil et al. synthesized copper NWs via electrospinning of a copper acetate-polyvinyl alcohol precursor followed by calcination and hydrogen reduction, with TGA analysis showing the onset of weight loss due to PVA decomposition around 85 °C and completion by 450 °C.⁶⁰ In contrast, Xu et al. reported bare copper NWs oxidize rapidly in water or nonpolar solvents to form mace-like CuO structures at room temperature, while TGA in air revealed two-stage oxidation to CuO with activation energies of 101 kJ mol⁻¹ (Cu to Cu₂O) and 128 kJ mol⁻¹ (Cu₂O to CuO).⁶¹ Comparable analyses in other non-galvanic systems consistently show Cu-rich interiors surrounded by continuous Ag layers, effectively passivating the core.³⁵

It has been demonstrated that insufficient kinetic control produces “decorated” rather than core–shell NWs. Galvanic displacement under poorly complexed conditions often yields isolated Ag nanoparticles attached to Cu NWs surfaces, appearing as discontinuous bright spots in TEM rather than uniform shells.^28,31,32 These discontinuous structures correlate strongly with inferior oxidation resistance and accelerated degradation of performance, as the exposed Cu surface remains vulnerable to oxidation.^28,31,32

X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) provide structural evidence. As shown in Fig. 8, the decomposition of the Ag–amine precursor is temperature-dependent; the XRD patterns (Fig. 8a) reveal that while the complex remains stable at room temperature, distinct metallic Ag reflections (FCC structure) emerge and sharpen as the annealing temperature reaches 140 °C.²⁶ The corresponding Cu@Ag NWs patterns (Fig. 8b) display dual sets of FCC peaks for both Cu and Ag, with the relative intensity of the Ag reflections increasing proportionally with the silver content (from Cu–5Ag to Cu–30Ag).²⁶ The nomenclature Cu–xAg denotes the silver weight percentage in the Ag–amine complex precursor, where Cu–5Ag, Cu–20Ag, and Cu–30Ag correspond to 5 wt%, 20 wt%, and 30 wt% silver content, respectively.²⁶ This systematic variation in composition enables tunable control over the resulting silver shell thickness. This confirms the successful formation of a crystalline Ag shell. XPS surveys of such heterostructures are typically dominated by Ag 3d signals, whereas Cu 2p signals emerge only after Ar⁺ etching, proving that metallic copper resides beneath a silver outer layer.^27,62

Fig. 8. (a) XRD patterns of Ag–amine complex at different decomposition temperatures. (b) XRD patterns of the Cu@Ag core–shell NWs with different amounts of silver. This figure has been reproduced from Zhang et al.²⁶ with permission from ACS Publications, copyright 2019.

Cu@Ag interface, shell-defect mitigation and characterization

The stability of Cu@Ag NWs is achieved through the control of the lattice mismatch, interfacial energetics, and thermally activated interdiffusion. Recent work demonstrates that the significant Cu/Ag lattice mismatch (≈12.6%) is accommodated via twin-mediated epitaxial growth and supersaturated interfacial solid-solution regions instead of a simple coherent interface.⁴⁷ Upon heating, the process of structural evolution is characterised by Ag shell ripening, partial dewetting, and grain-boundary-mediated Cu diffusion which result in the formation of Ag nanonodules and Cu exposure at higher temperatures, whereas a thin, dense Ag layer can additionally prevent oxidation compared to bare Cu.

Empirical processing windows, when the temperature is sufficiently low and at short dwell times are able to reduce Ag dewetting and shell discontinuities on the surface of Cu@Ag nanowire networks without affecting their sheet resistance.⁶³ Here, pinholes and grain-boundary diffusion-induced oxidation are alleviated by means of plating conditions that generate continuous Ag shells and restrict sintering temperatures to regimes below the onset of pronounced Ag dewetting observed in Cu@Ag particles; however, in the future, these conditions can possibly be reduced with interfacial solid-solution techniques that suppress outward Cu diffusion along grain boundaries.⁶⁴

In literature the term “continuous shell” is applied inconsistently, with some reports inferring continuity from low bulk resistance or from XPS surface composition alone. To impose a more rigorous standard, we adopt the following set of evidentiary criteria, which is recommended as a minimum reporting checklist for future Cu@Ag NWs studies.

Firstly, a HAADF-STEM cross-section with EDS line-scan showing a continuous Ag plateau coincident with the Cu plateau, with an Ag/Cu interfacial width <2nm and no Ag-intensity drop greater than 30% along ≥200 nm of NW length.^47,65

Secondly, a TEM-EDS or STEM-EDS elemental mapping of at least 5 wires, showing a closed Ag perimeter on more than 90% of the imaged length, with no exposed Cu segments longer than 5 nm. Thirdly, an XPS depth profile showing the Ag 3d/Cu 2p ratio decreasing monotonically from the surface and only reaching the bulk Cu signature beyond the nominal shell thickness; absence of Cu²⁺/Cu⁺ satellite features at the surface confirms that no oxide nucleates through pinholes.⁴⁹

Using the above characteristic and imaging methods, authors should aim to link the morphological criteria to a stability metric: an oxidation-induction time R/R₀ < 1.5 after 500 h at 85 °C/85% RH is achieved only by samples that simultaneously satisfy criteria 1–4.⁴⁹ Reports that satisfy these can reasonably be described as having demonstrated a continuous shell; reports that rely solely on bulk resistance or unresolved XPS are considered partially shell covered or decorated in morphology rather than a true core@shell structure.

Oxidation mechanisms and thermal/environmental stability

Copper oxidizes in air via formation of Cu₂O and CuO, electrical resistance of these are orders of magnitude higher than metallic Cu.^17,20 In Cu@Ag NWs, the silver shell acts as a kinetic barrier, but oxidation can proceed through diffusion along grain boundaries, penetration through pinholes, or interdiffusion at elevated temperature.^{21,27,31,32,45}

Quantitative stability studies illustrate the improvement in maintaining conductivity by the silver shell. As shown in Fig. 9, Jiang et al. demonstrated that while pure Cu NWs underwent rapid oxidation evidenced by a sharp increase in electrical resistance (R/R₀)—the Cu@Ag core–shell nanofibers remained highly stable. Specifically, under ambient conditions (Fig. 9a), the Cu NWs showed a continuous resistance drift over 30 days, whereas the core–shell structures maintained a near-constant R/R₀. Under harsh accelerated aging conditions (85 °C, 80% relative humidity in Fig. 9b), the pure Cu control failed catastrophically with great increase in resistance over 15 hours. In contrast, the Ag-protected network exhibited negligible degradation.⁴⁹

Fig. 9. Oxidation test of Cu NWs and Cu@Ag core@shell NWs at (a) room temperature and (b) humidity 80 ± 10% and 85 °C. This figure has been reproduced from Jiang et al.⁴⁹ with permission from ACS Publications, copyright 2019.

Other reports corroborate these findings with varying degrees of thermal tolerance. Zhang et al. reported that Cu@Ag alloyed NWs embedded in a polymer matrix maintained high conductivity unchanged at 140 °C and 85 °C/85% RH for over 500 hours, significantly outperforming standard Cu NWs which typically fail within hours.⁵¹ Similarly, thermal gravimetric analysis (TGA) in related systems has shown that a complete Ag coating can delay the onset of rapid oxidation from ∼150 °C to over 250 °C, expanding the materials processing window.⁴⁹

Kuo et al. correlated optimized shell microstructure with suppressed oxygen transport and stable electrical performance under cyclic strain.²⁷ These findings align with earlier galvanic-replacement-free Cu@Ag systems reporting modest resistance changes after hundreds of hours at elevated temperature and humidity when shells were formed.^26,35

By contrast, incomplete or discontinuous silver shells on Cu NWs leave exposed copper sites vulnerable to oxidation. While uniform shells ≥5–15 nm provide excellent stability under ambient, elevated temperature, or humidity conditions, partial coatings result in rapid resistance degradation as Cu oxidizes directly at defect sites.^66,67 These results support a picture where long-term stability is controlled by shell continuity, defect density, and grain-boundary structure, not merely nominal thickness.^27,31,32,45 Molecular dynamics simulations further suggest that Cu@Ag NWs exhibit distinctive thermal behavior, including size-dependent thermal conductivity, thermal expansion, and melting characteristics, which are relevant to their stability under processing and operation.⁶⁸

The structural integrity and long-term stability of Cu@Ag nanowires are directly linked to the morphological quality of the shell. While standard characterization via TEM and XRD confirms the successful formation of dual-phase heterostructures, the transition from a decorated to a truly continuous core–shell surface is the threshold for functional passivity. The literature consistently shows the silver in the shell acts as a barrier, effectively shielding the copper core from oxidation typically observed in many environments. However, current evidence indicates that the field should shift its focus from simple shell thickening toward the fine-tuned engineering of the Ag microstructure since degradation ultimately proceeds via grain-boundary diffusion and pinhole defects, the mere presence of silver is insufficient; rather, attaining a low-defect, epitaxial interface which could be layered is the true prerequisite for realizing the long term stability required for commercial flexible electronics. By optimizing these interfacial kinetics, Cu@Ag NWs will bridge the gap between the cost-effectiveness of copper and the reliability of silver, ensuring their role as a primary candidate for next-generation applications.

Substrate integration and post-processing: coating, drying, and sintering

Even with a high-quality Cu@Ag core–shell, the electrical and mechanical performance is determined by: efficient and uniform deposition of the NWs network onto substrate and post-deposition processing. Drying, junction welding, ligand removal, and densification are key for reducing junction resistance and enhanced stability. This section addresses both aspects, emphasizing the coupling between the coating method, ink formulation, substrate choice, and sintering strategy.

Deposition of NWs networks onto substrates

Types of substrate and temperature constraints

The choice of substrate determines what sintering method will be used. Si/SiO₂ substrates allow high-temperature annealing (>250 °C) and are used for material characterization and fundamental studies using optical and structural techniques. However, the widespread use of polymeric substrates in printed and flexible electronics necessitates thermal processing considerations. Polyethylene terephthalate (PET) begins to soften around 100–130 °C and shows significant dimensional changes above 140 °C; polyimide (PI) and polyetherimide (PEI) extend this window to ca. 200 °C; while the newer generations of transparent polyurethanes (TPU), polydimethylsiloxane (PDMS), and cellulose based substrates offer varied thermal budgets.⁶⁹ For stretchable and wearable devices, soft silicone elastomers and thermoplastic polyurethanes withstand only 60–100 °C without permanent deformation.⁷⁰ Consequently, the development of low-temperature sintering routes below 150 °C for PET and below 100 °C for ultra-flexible materials has become of high interest.⁷¹

Coating and printing methods

The deposition method used for metal nanowires (NWs) significantly influences film uniformity, thickness, and susceptibility to defects, which in turn dictates the long-term stability of Cu@Ag systems. In laboratory environments, simple approaches such as drop-casting and spin coating are frequently utilized due to their convenience.^72,73 However, for Cu@Ag NWs, spin coating often results in radial non-uniformity and significant material waste, which is economically disadvantageous given the cost of the silver precursor. Furthermore, vacuum filtration followed by mechanical transfer can yield high-quality reference films, but the mechanical stresses involved during the transfer process can induce micro-cracks or “delamination” of the silver shell from the copper core, creating localized oxidation sites.⁴⁹

For industrial scale-up, printed electronics methods such as slot-die coating and inkjet printing are preferred. These methods must be carefully optimized for Cu@Ag NWs to prevent mechanical breakage. High-aspect-ratio NWs are susceptible to fragmentation under the high shear rates encountered in slot-die heads or during ultrasonic ink dispersion.^72,73 Literature suggests that the silver shell actually enhances the mechanical robustness of the copper core, with Ag-coated NWs showing superior resistance to bending fatigue compared to bare Cu NWs.^49,51

The choice of deposition also impacts environmental stability. While spin-coated films may harbor solvent residues that accelerate corrosion, roll-to-roll (R2R) compatible methods like slot-die coating allow for the integration of in-line intensive pulsed light (IPL) or thermal sintering. These processes not only improve wire-to-wire contact but also help “seal” the silver shell at junction points, significantly enhancing the oxidation resistance of the network compared to air-dried laboratory samples.^51,73 Consequently, the transition to automated printing is not merely a matter of scale, but a requisite for achieving the structural density needed for high-performance Cu@Ag transparent conductors.

Screen printing

In screen printing, ink is deposited through a patterned mesh, enabling good control over precision onto the substrate which can be seen in Fig. 10a. It is well-established for conductive traces and buses but has coarse resolution (≳ 50 µm) and high ink viscosity requirements. This can lead to reduced transmittance due to film thickness and packing density.^74,75

Fig. 10. Schematic diagrams of three ink deposition methods for conductive inks: (a) screen printing, where a squeegee forces ink through a patterned mesh onto the substrate; (b) Mayer rod bar coating, using a wire-wound rod dragged over a substrate holder to meter and spread ink into a uniform film with controlled gap; (c) ultrasonic spray coating, where piezoelectric transducers in the nozzle vibrate to form capillary waves, atomizing liquid into micrometer-sized droplets under a nitrogen shroud for patterned deposition. This figure has been adapted from ^75,84 with permission from Nature Publishing Group UK and Elsevier, copyright 2019 and 2020.

Slot-die and mayer-rod (bar) coating

This is a classic roll-to-roll compatible technique that deposits a thin amount of liquid onto a moving substrate. Mayer-rod coating, enables precise thickness control and has been demonstrated for Cu@Ag NWs systems with subsequent drying at moderate temperatures (130 °C, 5 minutes) on PET without substrate damage. These result in good coating uniformity and minimal waste compared to spray, but require rheological optimization of the ink. A schematic overview of the bar coating using the mayer rod can be seen in Fig. 10b.

Spray coating

Ultrasonic and electrostatic spray coating is used for large area printing. In spray coating, atomized droplets of NWs suspension are projected onto a substrate, and the solvent is left to evaporate leaving a percolated network.^76,77 This method is sensitive to conditions of atomization (nozzle size, gas flow rate), wettability of the substrate and drying rate. When solvent evaporates too quickly the remaining NWs concentrate near the droplet edges, which creates uneven distributions across the film.⁷⁸ Controlling substrate temperature, humidity, and spray pitch can reduce sheet resistance to as low as ≈20 Ω sq⁻¹ with more than 85% optical transmittance.^79,80 Although the method can be scaled up, problems like overspray and nozzle clogging are sometimes encountered.

Inkjet printing

Inkjet printing offers sub-micrometer resolution which can create complex patterns and integration with other materials in a single pass. However, Ag NWs inks present challenges: NWs can sediment if not properly stabilized and nozzle clogging is a persistent problem due to their aspect ratio this is exacerbated when inks contain particulate matter or polymer stabilizers.⁸¹ Recent work has demonstrated inkjet printing of ethylene glycol/isopropanol formulations with 10–50 mg mL⁻¹ NWs concentration, followed by room-temperature nano-welding.⁸² Rheology of NWs inks must be considered and optimised to avoid nozzle blockage.^74,75 Though a small amount of NWs may not have a profound effect directly on the ink, the wettability of the substrate after deposition can be strongly influenced.⁸³

Ink formulation and solvent-driven stability

The rheology of Cu@Ag NW dispersions is controlled by the particle aspect ratio L/D and the chemistry of organic capping layers that remain after synthesis. For slender rigid rods, the geometric percolation threshold in a quasi-two-dimensional NW film scales as ϕ_c ∝ D/L, with the prefactor depending on dimensionality and rod-orientation distribution. Monte-Carlo simulations for isotropic 2D stick networks (L/D = 25–800) yield a critical number density N_cL² ≈ 5.64,^85,86 which for high-aspect-ratio Cu@Ag NWs (L/D = 200–500) corresponds to volume fractions well below 1%. In the dilute regime below percolation, the relative viscosity of a rigid-rod dispersion follows a Huggins-type expansion,

2

where the intrinsic viscosity scales as [η] ≈ 2(L/D)²/[15 ln(L/D)] for slender rods) and rises rapidly with L/D; the Doi–Edwards tube model further predicts pronounced shear thinning once the Péclet number for rod rotation exceeds unity.^87,88 This means that the Newtonian rheological window for Cu@Ag NW dispersions of L/D = 200–500 is lower than for spherical Ag particles of comparable volume fraction:⁸⁹ appreciable shear thinning is observed at shear rates above ∼10² s⁻¹, several orders of magnitude below the 10⁴–10⁶ s⁻¹ shear experienced in an inkjet nozzle.⁹⁰

This kinetic mismatch is the primary reason that NW-ink jetting is unusually sensitive to capping-agent type and concentration.⁹¹ Polyvinylpyrrolidone (PVP, M_w = 40–360 kDa), hexadecylamine, ethylenediamine, and oleylamine each leave a distinct organic corona on the NW surface that sets the interparticle pair potential and therefore the colloidal stability in alcohol/glycol vehicles as well as the elastic storage modulus G′ of the resulting suspension: thicker PVP shells suppress sedimentation but raise post-drying junction resistance and require a more aggressive ligand-removal or sintering step to reach the metallic limit.^92,93 From a process-design standpoint, aspect ratios L/D > 500 minimise the percolation threshold and the sheet resistance per unit volume fraction, but such long wires also fragment in the shear field of slot-die coaters and ultrasonic dispersers, typically losing 20–30% of their length during ink processing.^94,95 Solvent choice, surfactant selection, and homogenouse mixing mode must be co-optimised with the synthesis route in order to preserve both the shell integrity and the final aspect ratio in the printable ink.⁹⁶

The composition of NWs ink such as solvent, surfactants and wetting agents are highly dependent on desired uniformity of the coating. Water or alcohols are commonly used due to environmental reasons. However, their low boiling point can lead to uneven coatings.⁹⁷ Ethylene glycol can be mixed with water or IPA and evaporates slower which promotes lateral spreading and uniform drying.⁹⁷ The choice of surfactant, polymeric stabiliser or wetting agent can help in maintaining ink stability during storage. It can also help to deposit more uniform coatings but often remains after drying reducing contact points between individual NWs increasing sheet resistance of the coating and further treatments are required to remove them.⁹⁸

For Cu@Ag NWs synthesized via Ag–amine or Ag–carboxylic-acid routes, the film may retain metal–organic complexes on the surface. Exposure to low heating (130–150 °C for 5–15 minutes) can decompose these residuals and increase conductivity even before junction welding which is a distinct process in itself and useful particularly for flexible substrates.²⁶

Adhesion to the substrate is often poor for deposited NWs networks, because of weak van der Waals interactions and the hydrophobic or hydrophilic mismatch between the NWs and polymer surface.⁹⁹ Surface pretreatment using O₂ plasma or UV-ozone can increase substrate wettability and promote electrostatic or hydrogen-bonded adhesion to the NWs film.⁹⁹ As previously mentioned small amounts of polymers are often added to ink or deposited as a thin overcoat, improving adhesion at the cost of small increase in sheet resistance.¹⁰⁰

Sintering, annealing, and post-treatment routes

Origins of junction resistance and why sintering is required

The NWs network deposited exhibits sheet resistance (R_s) that is often several orders of magnitude higher than the target for suitable applications. This high resistance arises from multiple sources. Organic surface layers that electrically isolate NWs from each other, poor physical contact between NWs due to random orientation and oxide layers, especially on Cu cores if the Ag shell is discontinuous or damaged during solution deposition and film formation, all contribute to increased junction resistance. During processes such as spin coating, spray coating, or doctor-blading, NWs experience shear, impact, and capillary forces from flowing and evaporating solvent; these stresses can reorganize the particle orientation,¹⁵ thin or crack the relatively thin Ag shell at nanowire – nanowire contacts and at wire ends, locally exposing the underlying Cu core to oxygen and moisture. The exposed Cu oxidises readily under ambient conditions, and the formation of poorly conducting Cu oxides at these partially protected junctions significantly increases the resistance of percolation pathways through the network, particularly in films formed by aggressive, multi-step coating and drying compared with gentler, single-pass depositions. Drying at room temperature or mild temperature (∼60–80 °C) removes solvent but does not address these junction issues. Sintering, whether thermal, photonic, electrical, or chemical, aims to reduce junction resistance by removing organics, promoting metal-to-metal contact, and establishing conductive pathways through the network.^101,102

Conventional thermal annealing

Heating the film (T_anneal ≈ 120–250 °C) for 5–30 minutes enables solvent and volatile organics evaporation, decomposition of polymeric stabilizers and NWs at contact points undergo solid-state diffusion, forming metallic bridges.^103,104 For Cu@Ag systems synthesized via Ag–amine routes, thermal treatment at 140 °C for 5 minutes converts Ag–amine complexes to metallic Ag, yielding sheet resistances <50 Ω sq⁻¹ and environmental stability exceeding 500 hours at 140 °C and 85% relative humidity.²⁶

The nature of Ag shell deposition usually leads to incomplete coverage accelerating Cu oxidation when annealing in atmospheric conditions negating sintering benefits. Inert atmospheres or mild reducing gases suppress oxidation. Formic-acid vapor offers a mild, room-temperature-compatible alternative but has only been observed in literature for metallic nanoparticles.¹⁰⁵

Rapid and low-thermal-budget sintering

Photonic (intense pulsed light, IPL) sintering delivers high irradiance (6–16 kW cm⁻²) for very short durations (100–500 µs per pulse), with NWs absorbing photons via surface-plasmon resonance and reaching ≈ 400–600 °C at junctions while the substrate remains near room temperature due to low absorptivity and brief pulse duration.^106,107 This selective heating promotes junction welding without substrate degradation, reducing sheet resistance by 25–30% in a single 300 µs pulse and improving mechanical durability.¹⁰⁷ The method is inherently scalable but has a narrow optimization window.

Laser sintering with CO₂ or diode lasers selectively heats the NWs network via direct absorption, enabling patterning or large-area coverage, though disadvantages include slow throughput for large areas and risk of localized overheating and vaporization. Low-temperature oxidation-free selective laser sintering has been demonstrated for Cu nanoparticles.¹⁰⁸ Joule heating applies DC voltage across the NW network, causing resistive heating (Q = I²Rt), with high current density at junctions generating local temperatures exceeding 300 °C while the substrate remains cooler, similar to photonic sintering. This suits large-area or thick films and integrates into roll-to-roll equipment. Current crowding and localized fusion risks require careful voltage/time control. Mechanical pressing and rolling reduce junction gaps through perpendicular pressure, improving contact area and reducing contact resistance without heat. Light mechanical rolling after deposition improves R_s by 15–20% and substantially enhances bending durability.¹⁰⁹

Balancing conductivity, stability, and substrate constraints

Higher sintering temperatures and more aggressive processing improve junction conductivity and reduce sheet resistance, but introduce trade-offs. Thermal annealing above 150–200 °C improves electrical properties but incurs the risk of polymer substrate damage. It can also lead to accelerated Cu core migration and oxidation if the Ag shell is incomplete. This can also increase haze and optical scattering from NWs coarsening and surface roughening. Rapid sintering methods, particularly photonic sintering and mechanical approaches successfully preserve substrate integrity and maintain optical clarity of the film, but leave isolated high-resistance defects and compromise performance by leaving incompletely welded junctions across NWs networks.

Notably, UV photochemical sintering has demonstrated promise for nanoparticle ink systems, where UV-light-induced photolysis of PVP and other organic stabilizers can achieve room-temperature sintering with minimal substrate damage however, this approach remains less explored in NWs literature, representing a potential gap for future investigation.^110–112 The choice of deposition and sintering strategy must treat these processes as an integrated design system, balancing coating method, ink rheology and organic loading. Substrate thermal tolerance, target electrical performance, throughput requirements, and encapsulation compatibility must also be considered. For rigid substrates or applications permitting >200 °C, conventional thermal annealing remains optimal despite their slower processing. For PET and flexible electronics with modest sheet-resistance targets (<50 Ω sq⁻¹), spray or bar-coat deposition combined with IPL sintering (100–300 µs) or mechanical calendering offers a balanced approach that preserves optical properties. Stretchable and ultra-low-temperature substrates (<100 °C) demand inks engineered for minimal organic content.

Efficient substrate coating and post-deposition sintering are core elements of the Cu@Ag NWs device platform. The field is increasingly recognizing that the entire workflow: from ink formulation through deposition, drying, sintering, and encapsulation must be co-designed to meet application-specific constraints. Cu@Ag core–shell NWs, combining Cu's cost and thermal/electrical properties with Ag's oxidation resistance, are well-positioned to meet the demands of printed and flexible electronics, provided that substrate integration and sintering strategies are optimized in parallel with synthesis advances.

Challenges, outlook, and future directions

Despite significant progress in the synthesis and applications of Cu@Ag core–shell, several key challenges remain before widespread commercial application.

Scalability and process integration

While laboratory-scale syntheses have demonstrated excellent performance, translating these to industrial volumes remains difficult. Batch-to-batch reproducibility in shell thickness, uniformity, and NWs aspect ratio must be maintained across large reaction volumes. Continuous-flow synthesis strategies, have recently been adapted to core–shell systems and require further optimization to achieve the kinetic control needed for uniform Ag shells.^113,114 Roll-to-roll printing of Cu@Ag NWs inks also demands long-term colloidal stability, precise viscosity control, and compatibility with high-speed coating heads.^115,116

Stability at elevated temperatures and harsh environments

Although Cu@Ag NWs outperform bare Cu NWs, their stability above ∼100 °C remains limited, especially in humid or corrosive atmospheres. Grain-boundary diffusion of oxygen and water through the Ag shell accelerates degradation, and mechanical stress from thermal cycling can induce microcracks.^{21,27,31,32,45} For automotive, aerospace, or industrial applications requiring operation at 150–200 °C, additional protective layers (e.g., TiO₂, SiO₂, Al₂O₃, or polymer encapsulants) will likely be necessary, adding process complexity and cost.^117,118 Developing self-healing or passivating shells that can repair minor defects in situ is an emerging research direction but remains at an early stage.¹¹⁹

Standardization and benchmarking

The literature on Cu@Ag NWs suffers from a lack of standardized testing protocols. Stability is variously reported as “room-temperature aging”, “85 °C/85% RH”, or “thermal cycling”, making cross-study comparisons difficult.^3,6 Similarly, figures of merit (sheet resistance vs. transmittance, bending radius, strain limit) are measured under different conditions. Establishing community-agreed benchmarks for Cu@Ag NWs similar to those for Ag NWs or ITO would accelerate technology transfer and enable fair comparison with alternative materials (Cu@graphene, Cu@Ni, Cu@Sn, etc.).^3,6,20

Recyclability and reusability

Whilst Cu@Ag NWs can lower noble-metal consumption as compared to pure Ag NWs and the Ag : Cu mol ratio can be as low as 0.02–0.04 and still protect against oxidation, the environmental effect of end-of-life printed electronics should not be discounted. Increasing pressure on the global front with the regulation of electronic waste has pushed recyclability and reusability as key design criteria for conductive inks. The latest research by van Impelen et al. for Cu@Ag microparticles is an example of the possibility of in situ recycling. Conductive films sintered beneath ∼200 °C in air maintain the continuous Ag shell and can be mechanically detached from the substrate to return the metal powder for the second generation paste.¹²⁰ However, sintering greater than about 250 °C causes Ag dewetting, which re-exposes the Cu core and native-oxide growth takes place on the recovered particles, causing an 8-fold increase in second-generation sheet resistance. First-generation print dries in vacuum at room temperature to preserve the shell, restricting the second-generation increase in resistance to a factor of ∼2. The same trade-off is anticipated for Cu@Ag NWs, but under a narrower processing window due to the thinner shells (5–15 nm compared to 30–60 nm microspheres) and their higher dewet capacity. This poses a clear gap in research and a systematic reporting of recycling routes is yet to be done for Cu@Ag NWs.

End of life promising approaches would be mechanical separation following low temperature sintering such as the microparticle process and electrochemical Ag stripping in thiosulphate or aminopolycarboxylate based electrolytes which is then re-deposited on fresh Cu cores. LCA (life-cycle-assessment) studies that combine the embodied energy of NW synthesis and the recyclability of completed prints would strengthen the sustainability case for Cu@Ag NWs.

In addition to device recyclability, several reports show that the Cu@Ag NWs synthesis process in the upstream phase can be radically greener. Through reagent and solvent recovery and replacing hazardous reductants and complexing compound with milder, biodegradable or aqueous alternatives. Maji et al. proposed the first gram-scale, zero-discharge synthesis of Cu NWs powder via ethylenediamine mediated anisotropic growth in which unreacted reactants are quantified and reused throughout successive batch cycles to lower the reported ∼2.4 kg of NaOH waste per gram of product, compared to traditional protocols to a level of zero, in a manner that does not affect wire morphology and leads to substantial production cost reduction.¹²¹

Future directions

For Cu@Ag NWs to transition from lab to commercial scale, future research must address many aforementioned challenges. One critical point is the development of multifunctional shells that go beyond coverage providing only oxidation protection. Alloy or composite coatings, such as Ag@Au or Ag@metal oxide, enable environmental stability to be combined with catalytic, biosensing, or tunable optical functionalities,as highlighted by recent breakthroughs in solderable silver NWs-based flexible composites.^122–125 Simultaneously, the environmental footprint of NW production must be reduced by adopting green synthesis protocols; shifting away from toxic reducing agents like hydrazine toward water-based, room-temperature, or photochemical routes will be essential for sustainable, large-scale manufacturing.

Performance gains can also be realized through hybrid integration, where Cu@Ag NWs are co-assembled with 2D materials (e.g., graphene, MXenes) or conductive polymers like PEDOT: PSS. Such multilayer architectures offer a pathway to decouple trade-offs between conductivity, transparency, and mechanical compliance, creating robust composites superior to any single component.^12,22 Finally, the reliability of these materials in industrial settings will require advanced in situ monitoring techniques. Addressing the processing, and integration challenges through approaches spanning chemistry, engineering, and device physics will unlock the full commercial potential of Cu@Ag NWs technologies.

Conclusions

Cu@Ag core–shell NWs can be considered as the core for next-generation printed and flexible electronics, merging the cost advantages of copper with the oxidation resistance of silver. Recent advances in synthesis have formulated complete Ag shells with tunable thickness. Aging tests confirm that continuous shells are essential for long-term stability, with properly engineered Cu@Ag NWs retaining >90% conductivity after weeks to months under ambient conditions.

In summary, Cu@Ag NWs have shown to be a tangible, low-cost alternative to incumbent transparent conductors; provided that the remaining materials and processing hurdles are addressed.

Conflicts of interest

There are no conflicts to declare.

Data availability

All data accompanying this publication are directly available within the article and its associated references. No additional primary data were created or analyzed in the preparation of this review