Controllable Preparation of Silver Nanowires and Its Application in Flexible Stretchable Electrode

Abstract

Silver nanowires (AgNWs), as conductive materials for flexible stretchable electrodes, do not only have high conductivity but also have a high specific surface area, excellent stretchability, and mechanical stability, showing great potential applications in flexible electronics such as foldable, stretchable electrodes and wearable devices, etc. This work successfully synthesized AgNWs with controllable morphology by an improved dual-alcohol process. The diameter, length, and size uniformity of AgNWs were effectively regulated by studying the reaction temperature, different control agents, and the dropping rate of the AgNO₃ solution. The flexible stretchable electrodes were prepared using PDMS, paper, and nonwoven fabrics as substrate materials, and the impregnation method prepared the flexible stretchable electrodes of conductive fabrics. The properties of flexible stretchable electrodes of AgNWs based on different substrates were compared.

1. Introduction

Flexible electronic technology has a broad application prospect in information, energy, medicine, national defense, and other fields, such as flexible electronic displays, organic light-emitting diode (OLEDs), printed, thin film solar panels, and skin patches for electronics, due to its unique flexibility/extensibility and efficient and low-cost manufacturing process [1,2,3,4,5]. The manufacturing process and equipment are the main driving force for developing flexible electronic technology. Compared with the traditional rigid electronic equipment limited to planar structure, flexible electronic equipment can maintain a stable performance under tensile, bending, compression, and other deformations. Therefore, the development of flexible electronic equipment has aroused great concern. The wide application of flexible electronic devices depends on flexible electrode technology. With the emergence of portable, wearable, and implantable electronic products and the increasing demand for humanized intelligent soft robots, people have made great efforts on highly flexible functional materials, especially flexible stretchable electrodes [6,7]. For flexible stretchable electrodes, the stretching deformation is the most demanding and challenging deformation among different deformations due to the higher applied strain during its stretching deformation. The fracture of the flexible stretchable electrode during large deformation will directly lead to the failure of flexible electronic equipment. This problem restricts the development of flexible electronic technology. Therefore, the current research focuses on ensuring the stability and high reliability of the flexible stretchable electrode under immense strain. There are usually two methods to prepare flexible stretchable electrodes. The first is to make the electrode bear the strain force during the stretching process by changing the electrode structure; The second method is to add conductive materials to the elastic substrate. Now, there are many conductive materials used to prepare flexible stretchable electrodes. For example, Qi et al. [8] transferred graphene microstrip to PDMS substrate supported by tripod structure to prepare wavy flexible stretchable electrode; Shen et al. [9] coated AgNWs@ PEDOT: PSS composites on patterned PDMS substrates to prepare flexible stretch electrodes; Chu et al. [10] synthesized layered carbon nanotubes (hCTNs)/polyaniline (PANI) composite flexible stretchable electrode on the surface of stainless steel spring with spiral structure.

Conductive materials are the key to flexible stretchable electrodes and must be designed and prepared to maintain good conductivity under mechanical deformation. At present, the common conductive materials are nanometallic materials (gold, silver, copper, etc.) [11,12], carbon materials (carbon nanotubes, carbon black, graphene, etc.) [13,14,15], metal oxides (manganese dioxide, tin oxide, etc.) [16,17] and conductive polymers (polypyrrole, polyaniline, etc.) [18]. Among them, nanometallic materials are often used as conductive fillers for fabricating flexible stretchable electrodes due to their properties such as good electrical conductivity, high stability, and unique quantum effects. Among the various nano-precious metals, nanosilver has the most extraordinary properties. It has high electrical and thermal conductivity and can exhibit good electrical and optical properties in polymer composites [19]. In addition, nanosilver is safer for humans than other conductive materials, such as carbon nanotubes.

Nanosilver has the morphology of nanoparticles, nanowires, nanosheets, nanocubes, etc. Among them, silver nanowires (AgNWs), as one-dimensional nanomaterials, have high conductivity and a high specific surface area, excellent stretchability, and mechanical stability, showing great potential for flexible electronic applications such as foldable and stretchable electrodes and wearable devices. The properties of AgNWs are determined by their morphology and size. In general, the more uniform the size of AgNWs, the better the stability. However, the preparation of AgNWs generally has the problem of uncontrollable morphology and size. Therefore, many researchers are committed to synthesising AgNWs with uniform size, promoting the rapid development of the AgNWs preparation process. AgNWs are mainly synthesised by the template method, wet chemical method, electrochemical method, and ultrasonic reduction method [20,21,22]. The template method is divided into hard and soft templates, which are used to prepare AgNWs by a template. The process of this method is complex and the rough shape of the prepared AgNWs is not controllable and only suitable for small-scale production; electrochemical preparation of AgNWs can accurately control the morphology of AgNWs, but the steps are cumbersome and difficult to quantify the production of AgNWs. The wet chemical method is to control the diameter and length of AgNWs through control agents. The operation steps are relatively simple, but the surface of the prepared AgNWs is rough and its morphology cannot be accurately controlled. The ultrasonic reduction method makes silver molecules collide and gather strongly by using the shock wave and microfluidic phenomenon generated by ultrasonic pressure. Then silver molecules gradually generate AgNWs with the increase in time. This method is difficult to control the morphology of AgNWs accurately.

The liquid polyol method is the most effective method to achieve large-scale and high-quality production of AgNWs. Compared with other methods, this method has the advantages of simple operational steps, mild reaction conditions, fast reaction rate, and easy availability of raw materials. Moreover, the prepared AgNWs have better homogeneity and more controllable length and diameter, so it is widely used [23,24]. The conventional method of preparing AgNWs from alcohol solution is to prepare silver nanocrystalline seeds and then prepare AgNWs by heterogeneous nucleation and nucleation growth. The controllability and reaction efficiency of this preparation method is usually very low. In order to improve the controllability and reaction efficiency, the alcohol system was modified by introducing different control agents. Anionic control agents and neutral molecular control agents can provide nucleation seeds for the rapid growth of AgNWs, which in turn can improve the growth efficiency of AgNWs. Cationic control agents can reduce the influence of by-products on the morphology of AgNWs, thereby improving the controllability of the morphology of AgNWs. In this paper, AgNWs were prepared by an improved dual-alcohol process. The AgNWs were oriented by using AgNO₃ solution as a silver source and coated with polyvinyl pyrrolidone (PVP). By studying the effects of different controlling agents, reaction temperature and drop acceleration of AgNO₃ solution on the morphology of AgNWs, the process parameters for the preparation of AgNWs with controllable morphology and uniform size were determined. This leads to further optimization of the preparation process of AgNWs. In this work, self-made AgNWs were used as conductive fillers, and PDMS, paper, and nonwoven fabrics were selected as the substrate materials of flexible stretchable electrodes to study the performance of flexible stretchable electrodes based on different substrates. The conductive fabric flexible stretchable electrode was prepared by impregnation method to obtain the flexible stretchable electrode with stronger stretchability and better conductivity.

2. Experimental

2.1. Preparation of AgNWs

The preparation process of nanosilver wire is shown in Figure 1. The diol solution is obtained by mixing propylene glycol (Macklin, Shanghai, China) and ethylene glycol (Macklin, Shanghai, China) in a ratio of 2:1. A solution: Weigh 6.7 g PVP into the diol solution, stir magnetically until all PVP in the solution is dissolved and the solution turns pale yellow. Then, add 0.075 g KBr (variable 1, 0.1 g, 0.125 g, (Macklin, Shanghai, China)) and 0.255 g AgCl (Macklin, Shanghai, China), and stir magnetically for 90 min at 160 °C (variable 2, 110 °C, 130 °C, 150 °C). B solution: Put 3.4 g of ground AgNO₃ (Aladdin, Shanghai, China) powder into the diol solution and stir at room temperature until all AgNO₃ is dissolved. B Solution was added dropwise to A solution with a constant flow pump at a flow rate of 5.56 mL/min (variable 3, direct pour, 10 mL/min, 7.14 mL/min), and the reaction was continued with magnetic stirring for a period to obtain the AgNWs solution. The AgNWs solution was cooled to room temperature, and then 15 mL of acetone was added and left to stand for 4 h. The AgNWs solution was then centrifuged with the centrifuge speed set to 2000 r/min and a centrifugation time of 10 min. Finally, the silver nanowires that precipitated at the bottom of the centrifuge tube were dispersed in a solution (deionized water or ethanol) and stored. When studying the effect of one of the variables on the product, the remaining variables are kept constant and at their initial values.

2.2. Preparation of AgNWs Flexible Stretchable Electrode

First, cut the substrate film (PDMS (Macklin, Shanghai, China), cellulose paper (Macklin, Shanghai, China), nonwoven fabric (Macklin, Shanghai, China)) into small pieces of the same size, cleaned with methanol, acetone, ethanol, and deionized water ultrasonically four times, and then dried at 70 °C for use (Figure 2a). Prepare sodium alginate (NaAlg) solution: Weigh 1 g sodium alginate (Macklin, Shanghai, China) and 0.1 g calcium chloride (CaCl₂, (Macklin, Shanghai, China)), respectively, dissolve calcium chloride in 30 mL deionized water, and then add sodium alginate. Quickly stir at room temperature until the solution is viscous, and then leave it under seal for 12 h for deaeration. Preparation of flexible stretchable electrodes: The NaAlg solution is applied to the substrate (PDMS, paper, nonwoven fabric) by a bar coating method. Then, the AgNWs dispersion is evenly coated on the substrate material and finally dry at 60 °C (Figure 2b). The preparation method of AgNWs dispersion is the same as that in Section 2.1 (experimental variables are all initial values).

3. Results and Discussions

3.1. Growth Mechanism of AgNWs

Figure 3a shows the growth mechanism of AgNWs. The silver ions provided by the AgNO₃ solution are reduced to silver atoms, and the nucleation and growth of silver nanoparticles start after the silver atom concentration reaches saturation. The capping agent, PVP macromolecule, adsorbs on the surface of silver nanoparticles, and the reducing property of alcohol prevents the aggregation of silver heteronuclear. The effect of the two makes silver particles grow into AgNWs. Specifically, the growth of AgNWs can be divided into three stages: the first stage is that the Ag⁺ in the solution is reduced to silver atoms, and the silver nanowires grow into heteronuclear; the second stage is that these heteronuclear evolve into multiple twin crystals; in the third stage, the twin crystals grow into nanowires under the action of the coating agent PVP and the control agent. Figure 3b shows the color change of the solution during the whole reaction process. PVP solution was initially colorless and transparent, and turned yellow after adding a control agent. After dropping AgNO₃, the reaction solution gradually turned reddish brown. It turns grey–green after 12 min, and a large amount of white flocs are produced after 1 h; the reaction continues until the solution turns gray and AgNWs are formed. Nanosilver crystal nuclei grow in a wired shape, in which PVP plays an important role. PVP is an important polymer that can be used as a coating agent. The effectiveness of PVP is mainly due to its nitrogen and oxygen atoms that can make it adsorbed on the surface of silver crystal seeds or silver particles [25]. The silver nanoparticles reduced from the solution converge into a bicrystal decahedron. Since the (100) crystal plane of silver is higher than the (111) crystal plane, PVP acts more strongly on the (100) plane than on the (111) plane. The (100) crystal plane of silver is coated to prevent silver atoms from nucleating from this plane. Therefore, the (111) crystal plane of silver is the only nucleation plane.

3.2. Effect of Different Control Agents

For the method of preparing silver nanowires by polyols, the addition of control agents (AgCl, CuCl, NaCl, KBr, etc.) provides cations, such as Ag⁺, Cu⁺, and anions, such as Cl⁻, Br⁻, and I⁻, for the reaction system. Anions can provide crystal nuclei to promote the rapid growth of AgNWs. Cations can usually reduce the influence of nitric acid and oxygen by-products in the atmosphere on the AgNWs morphology [26,27,28]. Cl⁻ can coordinate with the silver core in the polyol reaction system to stabilize it and prevent its aggregation. It also promotes the anisotropic growth of the silver core, thus promoting the formation of AgNWs. Schuette et al. demonstrated that the role of the Cl⁻ is to generate AgCl, upon which the Ag⁺ reduction reaction is initiated and metallic Ag is nucleated [29]. In this work, we directly selected AgCl as the control agent. AgCl will gradually release Ag⁺ and Cl⁻ during the reaction process, providing Ag⁺ that can be reduced to serve as the seeds for the nucleation of silver nanoparticles. The silver seeds gather to form twin crystals and grow into AgNWs.

When AgCl is not added, the reaction solution will gradually become reddish brown after the AgNO₃ solution is dropped. After the reaction lasts for 5 h, the color of the solution will become darker (Figure 4a). Figure 4b shows that the products generated when AgCl is not added are irregular silver nanoparticles. It can be seen that AgCl plays an important role in the formation of AgNWs in the polyol reaction system. Figure 4c is the UV–vis absorption spectra (Trousse S-2600, Tianjin, China) of the product without adding AgCl. It can be seen that the broad absorption peak is at 420 nm, which is caused by the plasma resonance of silver nanoparticles. The absence of narrow peaks proves that there are no AgNWs in the product, which is consistent with the results shown in Figure 4b. Figure 5a shows the X-ray diffraction (XRD, D8 Advance, Bruker, Karlsruhe, Germany) pattern of AgNWs prepared after adding an AgCl control agent. The diffraction peaks at 27.82°, 32.24°, 46.23°, 54.83°, 57.48°, and 67.47° correspond to AgCl (PDF# 04-0783). The diffraction peaks at 38.12°, 44.32°, and 65.54° are very strong, corresponding to the (111), (200), and (220) crystal planes of Ag (PDF# 04-0783). The silver crystal is a face-centered cubic structure, and the diffraction peak intensity of the (111) crystal plane is 2.5 times higher than that of the (200) crystal plane, indicating that the sample has a high aspect ratio and is wire-shaped [30]. In order to further control the morphology of AgNWs, we added a KBr control agent after adding the AgCl control agent to provide Br⁻ for the reaction solution. The effect of KBr on the morphology of AgNWs was studied by changing the amount of KBr added.

Figure 5d–i are the SEM images of AgNWs prepared by adding different amounts of KBr in the reaction solution. The AgNWs show a trend of thinning with the increase in the amount of KBr added, its diameter has dropped from an average of 60 nm to an average of 40 nm. In addition, a small amount of silver nanoparticles were produced when 0.125 g of KBr was added. Figure 5b shows the UV–vis absorption spectra of products with different amounts of KBr. The amount of KBr added is 0.075 g, 0.1 g, and 0.125 g, respectively. The absorption peaks appear at 350 nm and 380 nm, which are characteristic absorption peaks of AgNWs. The half-peak width of the absorption peak decreased with the increase in the amount of KBr, indicating that the diameter of the AgNWs became thinner. Figure 5c shows the mechanism of Br⁻ in the preparation of AgNWs. The effect of Br⁻ on the morphology of AgNWs is mainly due to the fact that it can react with Ag⁺ to form AgBr. AgBr is a solid and difficult to dissolve, so the Ag⁺ content in the reaction solution is reduced to a certain extent. The reduction rate of silver atoms in the reaction system is reduced, the nucleation rate is increased, and the growth of crystal seeds along the (111) plane is accelerated under the action of the coating agent PVP [31]. On the other hand, Br⁻ adsorbs the surface of the AgNWs, thus reducing the deposition of silver particles on the AgNWs and making them thinner.

3.3. Effect of Reaction Temperature

Ethylene glycol and propylene glycol are oxidatively reduced to ethanol aldehyde and propylene aldehyde at high temperatures, with propylene glycol having a stronger reducing ability than ethylene glycol [32]. Generally, the reduction ability increases with the increase in temperature, so the reaction temperature is a key factor affecting the size of silver nanowires. Therefore, we conducted experiments at different temperatures (110 °C, 130 °C, 150 °C, 160 °C) to examine the temperature suitable for the growth of AgNWs. Figure 6 shows that the reaction temperature has a great influence on the morphology of AgNWs. When the reaction temperature is T = 110 °C, although AgNWs are produced, the particle size is uneven. There are also a large number of rod-like and block-like silver nanoparticles (Figure 6a,b). When the reaction temperature is T = 130 °C, the AgNWs produced are relatively uniform in size, large in diameter, and short in length, and many massive silver nanoparticles are included in them. When the reaction temperature T = 150 °C, the size of AgNWs produced is uniform and the diameter decreases. A small amount of silver nanocubes were also found. When the reaction temperature is T = 160 °C, a large number of AgNWs with the same diameter (d = 60 nm) and length are generated, and there are no silver nanoparticles of other shapes. When the temperature is lower, the reducing ability of alcohols decreases, which cannot provide enough energy for the anisotropic growth of specific surfaces of AgNWs. When Ag⁺ is reduced to Ag atoms, twin crystal growth on the (111) plane requires high energy, so the growth of AgNWs requires a relatively high temperature. In general, the yield of AgNWs increases with the reaction temperature, which is the same as the relatively high proportion of double decahedrons at high temperatures.

3.4. Effect of the Drop Acceleration of AgNO₃ Solution

The concentration of Ag⁺ in the reaction solution of preparing AgNWs by polyol is an important factor in determining the morphology of AgNWs. AgNO₃ is a silver source that provides Ag⁺ to the reaction solution, so the dropping speed of the AgNO₃ solution affects the concentration of Ag⁺ in the reaction solution. Figure 7a–d are the SEM images of AgNWs when the dropping accelerations of AgNO₃ solution are 5.56 mL/min, 7.14 mL/min, 10 mL/min, and poured directly, respectively. The diameter of AgNWs gradually increases with the increase in the AgNO₃ solution drop acceleration (the diameters are about 40 nm, 50 nm, and 60 nm, respectively). When the AgNO₃ solution is poured directly, only a small amount of silver nanowires are produced (Figure 7d). Figure 7e shows the UV–vis absorption spectra of the products at different dropping accelerations of the AgNO₃ solution. All products have absorption peaks at 350 nm and 378 nm, which are caused by the plasma resonance of AgNWs. The half-peak width of the absorption peak widened with the increase in the concentration of AgNO₃ solution, which was caused by the increase in the diameter of the AgNWs. It shows that the diameter of silver nanowires increases with the increase in the AgNO₃ solution drop acceleration.

3.5. Performance of AgNWs Flexible Stretchable Electrodes

PDMS (Polydimethylsiloxane) is a film cross-linked by a polydimethylsiloxane curing agent, which has stable chemical properties, strong insulation, low price, corrosion resistance, high- and low-temperature resistance, good dielectric property, and high resilience, and can be applied to flexible stretchable electrodes [33]. The paper is hydrophilic, and the interior is a 3D porous structure connected by cellulose, which can accommodate various functional materials and can be deformed. This allows the paper substrate to be used as a flexible substrate for wearable and portable flexible electronic products [34]. The raw material of the nonwoven fabric is polypropylene resin, which is very light, antibacterial, and anti-corrosive. The nonwoven fabric is formed by hot melting and bonding of fine fibers. It is soft, comfortable, porous, and has good air permeability [35]. Wearable electronic products need functionality and comfort, so it is of practical significance to study the conductivity of nonwoven fabrics.

The flexibility and excellent mechanical properties of the base material play a key role in the stability of the flexible stretchable electrode. Therefore, we choose PDMS, paper, and nonwoven fabric as the flexible stretchable electrode substrates. Figure 8a–c shows that PDMS has excellent flexibility and extensibility, which can be stretched to 300% of the original length. When the PDMS-based flexible stretchable electrode was stretched, it was found that the conductive functional layer composed of silver nanowires would peel off from the PDMS substrate. Therefore, the surface adhesion of the PDMS substrate is not good enough. Figure 8d–f shows the pictures of the paper-based flexible stretchable electrode under different deformation. The electrode can achieve 90° and nearly 360° bending deformation. Figure 8g–i shows the physical image of a nonwoven flexible stretchable electrode. The nonwoven surface still retains its original softness and lightness after depositing AgNWs. Moreover, the AgNWs are firmly attached to the nonwoven fabric surface. Under repeated bending, the functional layer formed by the conductive material of the AgNWs does not break and peel off the substrate.

Figure 9a,b shows that the AgNWs are evenly distributed on the PDMS-based surface. The substrate surface is not exposed and is completely covered by dense AgNWs. In general, the more evenly distributed and densely covered the AgNWs on the substrate surface, the better the conductivity of the conductive film. This is mainly due to the AgNWs being covered and deposited on the surface of the substrate to form a conductive path, making the initially insulated substrate conductive. Figure 9c,d shows that the AgNWs are interlaced on the paper surface, forming a uniform and dense AgNWs coating. It indicates that the AgNWs have good deposition on the paper-based. Due to the hydrophilicity and capillarity of the paper-based, the liquid of the AgNWs dispersion is rapidly absorbed, and the AgNWs are quickly deposited on its surface and attached. It avoids the uneven distribution of the conductive layer of AgNWs due to the accumulation or dissociation of silver nanowires when flowing with liquid. The internal 3D porous structure of the paper-based also increases the deposition rate of AgNWs dispersion, further enhancing the flatness of the AgNWs layer. Different from PDMS and paper base, it can be seen that the nonwoven fabric is composed of fibers with the naked eye. Therefore, we observed the SEM images of the nonwoven fabric not impregnated with AgNWs. Figure 9e shows that nonwoven fabric is interwoven by multiple fibers. Figure 9f shows that the AgNWs with uniform diameter form a dense conductive layer on the surface of the nonwoven fabric. The AgNWs only deposit on the nonwoven fabric surface to form a functional layer, instead of penetrating the interior and wrapping the interior fibers. Polypropylene, the raw material of the nonwoven fabric, is nonpolar in structure and has almost no hydrophilic group, so nonwoven fabric does not absorb water or even directly repel water. The AgNWs on the surface of the nonwoven fabric cannot penetrate the internal structure of the nonwoven fabric with the liquid.

The four-probe method is used to directly measure the resistance of the flexible stretch electrode surface at unequal intervals many times. The average value is the square resistance of the conductive film. The mechanical flexibility of the flexible stretchable electrode is determined by observing the change of its resistance at the bending angle and bending times. The square resistance of the conductive film under the bending angle of 0° is expressed as the initial square resistance R_0□, and the square resistance of the conductive film under the bending angle of 0~180° and bending for 0~30 times is expressed as R_□. By dividing the square resistance value measured after the conductive film is subjected to mechanical deformation from the initial square resistance value (R_□/R_0□), observe the change of the resistance of the conductive film under different degrees of mechanical deformation.

The initial square resistance of a PDMS-based flexible stretchable electrode is about 2 Ω·sq⁻¹. Figure 9g,h show that the square resistance of PDMS-based flexible stretchable electrode increases slightly with the bending angle when it is bent at 0~180°, and only increases somewhat after 30 times. It shows that its overall resistance is stable and flexible. The initial square resistance of the paper-based flexible stretchable electrode is about 1.7 Ω·sq⁻¹. The square resistance of the paper-based flexible stretchable electrode increases with the increase in the bending angle, and increases relatively fast from 80° to 180°, but does not exceed 1.5 times the initial square resistance (Figure 9g). The square resistance also increases slightly with the increase in bending times (Figure 9h), indicating that it has good flexibility. Although paper can have excellent conductivity under bending and folding conditions, it is almost not stretchable. It can be cut into special shapes to achieve extensibility. The initial square resistance of a nonwoven-based flexible stretchable electrode is 8 Ω·sq⁻¹. Figure 9g shows a small increase in the square resistance as the bending angle changes. However, when the bending angle is 180°, the conductive layer of AgNWs on the nonwoven fabric surface falls off, resulting in a significant increase in square resistance. Figure 9h shows that the square resistance of a nonwoven flexible stretchable electrode is almost unchanged after 30 times of bending, indicating that the electrode has good mechanical flexibility. Due to the poor extensibility of the nonwoven fabric itself, the flexible stretchable electrode on the nonwoven fabric substrate has almost no extensibility. However, it is flexible, thin, and breathable, and can be used to manufacture portable wearable electronic devices and small flexible electronic products in the medical and health industry.

Fabrics are the most common and widely used items in life. Compared with PDMS, paper-based and nonwoven fabrics, the fabric has better stretchable properties, can be deformed freely, is comfortable, and is wear-resistant. Therefore, the commonly used fabrics in daily life are made electrically conductive by physical or chemical methods to prepare flexible stretchable electrodes. The conductive fabric flexible stretchable electrode as a basic element to prepare wearable electronic devices is the future development direction of wearable electronic technology [36]. It is a challenge to develop a conductive fabric that how to keep the original properties of the fabric unchanged while maintaining the natural insulation of the fabric. As we all know, metal has high conductivity, and the combination of metal and fabric can make the fabric have conductivity. Therefore, the combination of metal and fabric is one of the key research directions of the flexible stretchable electrode. There are two main methods for fabric composite metal. The first is to embed metal wires (copper, aluminium, etc.) into fabric fibers through the weaving process. The second is depositing conductive fillers (gold, silver, copper, etc.) on ordinary fabrics through electrochemical deposition and immersion technologies [37]. Although the fabric embedded with metal wire has good conductivity, the metal wire will increase the hardness and weight of the fabric, seriously affecting the wearing comfort. The method of depositing nano conductive layer on the fabric will not affect the softness and comfort of the fabric in the macro view. Therefore, in this work, the self-made silver nanowires are used as conductive materials, and the conductive fabric is prepared by the impregnation process with simple operation and low cost.

As the substrate of conductive fabric, the performance of the fabric itself determines the performance of the conductive fabric. The performance of the fabric is mainly determined by its fiber and structure. Fibers are divided into natural fibers and chemical fibers. At present, chemical fibers are the most used [38]. Among many chemical fibers, polyester fiber has good elasticity, wrinkle resistance, and abrasion resistance, and can enhance water absorption through chemical modification. Therefore, polyester fabric is selected as the base of conductive fabric. As shown in Figure 10a–c, we have set three polyester fabrics with different structures: porous, coarse jacquard, and rib. The porous structure has good extensibility, but poor surface roughness resilience and easy looseness. The coarse jacquard structure is thick, and its pattern leads to an uneven cloth surface and uneven fiber thickness. Rib structure has good extensibility, great resilience, flat cloth, and dense connection of uniform fibers. Figure 10d–f shows the pictures of porous structure, coarse jacquard structure, and rib structure fabrics after conducting treatment. As shown in Figure 10g–l, the ultimate stretch rate of the three fabrics are 170%, 210%, and 200%, respectively, which proves that the stretch capacity is: coarse jacquard fabric > rib fabric > pore fabric.

A scanning electron microscope (SEM) was used to observe the coverage of silver nanowires on the fabric surface. Figure 11a,b show a single fiber of the porous fabric. The fiber surface is covered with fewer silver nanowires and more exposed surfaces. Figure 11c shows that several fine fibers form the coarse fibers in the coarse jacquard fabric. Figure 11d shows that AgNWs mainly cover the surface of coarse fibers, and there are many AgNWs in the gullies where fine fibers overlap. Figure 11e shows the uniform and compact arrangement of the fiber thickness inside the rib fabric, and the AgNWs are wrapped on the fiber surface. Figure 11f shows that the surface of the rib fabric fiber is covered unevenly as a whole, and some fibers are completely exposed. In general, conductive filler silver nanowires can form a coating on the fiber surface of conductive fabrics prepared by the impregnation process, but the coating is not uniform enough.

Figure 11g shows the influence of immersion time on the square resistance of conductive fabric. When the immersion time is between 6 h and 24 h, the fabric square resistance decreases faster. After the immersion time reaches 24 h, the decline speed of fabric square resistance tends to be gentle. Polyester fabric has low water absorption and resistance to organic solvents, bleaches, and oxidants. Therefore, the penetration rate of water molecules into fibers is relatively slow, requiring a long immersion time. The conductive material is gradually deposited on the fiber surface to increase the conductivity. After the immersion time reached 24 h, the water absorption of the fabric gradually approached saturation, and the deposition of AgNWs decreased. Finally, the square resistance of the porous fabric is 37 Ω·sq⁻¹, that of coarse jacquard fabric is 16 Ω·sq⁻¹, and that of rib fabric is 32 Ω·sq⁻¹. Figure 11g clearly shows that the square resistance of the coarse jacquard fabric is smaller than that of porous fabric and rib fabric. The coarse jacquard structure is more complex, the fiber surface has more gullies than the other two structures, and silver nanowires are easier to deposit in the gullies to increase conductivity.

Figure 11h shows the resistance change of conductive fabric under different stretching degrees. The square resistance of the three kinds of fabrics increases with the drawing degree. When the drawing exceeds 60%, the square resistance increases sharply. When the stretch rate is 0%~20%, the change of square resistance is relatively small, indicating that the tensile strain range of conductive fabric is 20%. In the macro aspect, when the fabric is stretched, the gap between the fibers becomes larger with the increase in the stretching degree, and the contact area of the fibers decreases gradually, leading to the interruption of the formed initially conductive channel. In the microscopic aspect, the originally interconnected AgNWs were separated, resulting in the disconnection of the conductive path. Stretch the conductive fabric to 20% of its original length and release it. Repeat this action 0~100 times, and observe the change of its square resistance. Figure 11i shows the change of square resistance of conductive fabric after repeated stretching. The overall growth rate of square resistance of porous fabrics is fast during repeated stretching. The resistance of coarse jacquard fabrics increases sharply after 30 times of repeated stretching. The resistance of rib fabrics increases slightly within 20 times of stretching and increases significantly after 50 times of stretching. It shows that the repeatability of coarse jacquard knitted conductive fabric is good at 20 times stretching. In contrast, the repeatability of rib conductive fabric is good with less than 20 times stretching, and the repeatability of porous fabric is poor. The difference between the three conductive fabrics in different strain ranges is mainly due to the difference in the surface structure of the two fabrics. Porous fabrics have poor fiber density and easily cause irrecoverable damage after stretching. The coarse jacquard fiber is relatively thick and its surface pattern is large, which makes it difficult to restore its original shape after repeated drawing. The surface fibers of rib fabric are uniform and flat, arranged regularly and densely, and have good resilience. With the distance between fibers increasing repeatedly, the resistance increases gradually. Therefore, the repeated resistance of rib fabric changes regularly with the stretching. After 50 times of stretching, the structure is damaged and cannot be restored to the initial state, and the resistance rises sharply. The conductivity of conductive fabrics prepared by impregnation can be further improved. The poor conductivity of conductive fabric under stretch is caused by the peeling of the conductive layer from the fabric. How to make the conductive layer and the fabric firmly combine or even blend without changing the characteristics of the fabric itself is one of the directions to further improve the conductivity of the fabric. We will continue to explore the in-situ growth process of conductive filler nanosilver on the fabric.

4. Conclusions

In this work, an improved dual-alcohol process successfully prepared AgNWs with controllable morphology. The growth mechanism of Ag⁺ reduction, heterogeneous nucleation, and crystal nucleus-oriented growth into AgNWs was analyzed. AgCl and KBr control agents provide chloride ions and Ag⁺ for the reaction system and promote the growth of the silver crystal nucleus. The silver nanoparticles grow into AgNWs with uniform size and a diameter range of 40–60 nm. The influence of reaction temperature on the morphology of AgNWs was studied in the range of 110~160 °C. When the reaction temperature was 110 °C, no AgNWs were formed; When the reaction temperature was 160 °C, AgNWs with uniform size and diameter of about 60 nm were formed. The influence of the dropping acceleration of AgNO₃ solution on the particle size of silver nanowires was studied. It was found that the diameter of AgNWs gradually increased with the increasing of the dropping acceleration of the AgNO₃ solution. Still, when the AgNO₃ solution was added too quickly, many silver nanoparticles would be produced. A flexible stretch electrode with silver nanowires was prepared by using PDMS/paper/nonwoven fabric as the base material. The initial square resistance values of the three base materials were 2 Ω·sq⁻¹, 1.7 Ω·sq⁻¹, and 8 Ω·sq⁻¹. After the measurement of bending angle and bending times, the square resistance of PDMS/nonwoven conductive film almost remains unchanged. The conductive fabric flexible stretchable electrode was prepared by immersion method, and the square resistance of the porous fabric flexible stretchable electrode was 37 Ω·sq⁻¹, that of the coarse jacquard fabric flexible stretchable electrode was 16 Ω·sq⁻¹, and that of the rib fabric flexible stretchable electrode was 32 Ω·sq⁻¹. AgNWs have high conductivity, high specific surface area, excellent extensibility and mechanical stability, and show great application potential in flexible electronics such as foldable, stretchable electrodes and wearable devices.