Using self-assembly to prepare a graphene-silver nanowire hybrid film that is transparent and electrically conductive

Abstract

Silver nanowires (AgNWs), modified by cysteamine, with a high electrical conductivity can be combined with high surface area graphene nanosheets (GNs) to form AgNW–GN hybrid nanomaterials. These materials with $''–''{\text{NH}}_3^{+}$ functional groups in an alkaline environment can be deposited on waterborne polyurethane surfaces with the attraction of sulfonate functional groups to prepare transparent conductive films with high transmittance and low surface electrical resistance. This self-assembly method provides highly controllable transmittance and surface electrical resistance. The AgNWs can inhibit GNs from restacking and aggregation after reduction from graphene oxide, increasing the electrical conductivity between the GN interlayers. The AgNW–GN hybrid nanomaterial films show a sheet resistance of 86 Ω/sq with 80% light transmittance, and the value of DC conductivity to optical conductivity ratio reaches 19.81.

Introduction

With the development of flexible optoelectronic devices, advanced transparent conductive films (TCFs) must be light, cheap, high mechanical properties and flexibility. New transparent conductor materials are made to replace indium tin oxide (ITO) because of the lack of indium resources and its flexible property [1]. These materials include carbon nanotubes (CNTs) [2], graphene nanosheets (GNs) [3], conductive polymers [4], metal grids [5], and metallic nanowires [1], [6]. Among these materials, GN thin films and silver nanowire (AgNW) networks possess outstanding optoelectronic performances that close to that of ITO [1].

A GN, which is a 2D honeycomb carbon lattice with a highly specific surface area, possesses emerged as one of the thinnest and strongest promising next-generation nanomaterials [6]. Researchers have developed various methods of synthesizing GNs, including bottom-up and top-down approaches [7]. Bottom-up methods generally involve the direct synthesis of GNs from carbon molecular sources (chemical vapor deposition (CVD)) and the graphitization of carbon structural from small aromatic molecules to grow large-area, single or few-layer GN thin films [8]. The structure of GNs that was prepared by bottom-up approach closes to perfect aromatic bonding. However, an efficient approach to produce GNs in large quantities at a low cost is needed [9]. The top-down methods are the alternative approaches to obtain GNS in terms of high yield which contains the oxidation reaction [10] and a non-destructive exfoliation process with intercalation in an ultrasonic water bath [11]. The modified Hummers oxidation process recently becomes the most popular approach for preparing exfoliated the carbon layer material, which is graphene oxide (GO) [10]. Various types of reduction processes can be used to obtain GNs from GO, including chemical reduction [12] and thermal expansion [10]. Although the high degree reduction of GNs can be achieved by the annealing process, it is difficult to disperse in aqueous or organic solvents. Chemical reduction is the better method to obtain GNs that are dispersed in aqueous or solvent media. However, up to now, the surface electrical resistance of GNs reduced by the chemical reduction agent remains at ∼10³ Ω/sq [12]. Previous research showed that GNs incorporated with silver nanoparticles (AgNPs) can improve the performance of GN TCFs [13], [14]. Furthermore, AgNPs act as intercalation agents in preparing exfoliated GNs.

According to recent research [1], 1D pioneering AgNW materials to construct networks can yield a high optoelectronic performance that is better than that of GN films and close to that of ITO. Moreover, AgNW thin films are suitable for advanced optoelectronic devices because of their flexibility. So far, researchers have presented many methods of depositing AgNWs on a substrate to prepare thin films. Similar to 1D CNTs, vacuum filtration [15], drop-cast [16], Meyer rod coating [17], and the transferring [18] methods can be used to prepare AgNW thin films. Although many methods are capable of preparing thin films, they encounter problems with film quality, processing complexity, and selection of substrate. Unlike 1D nanomaterials, 2D GN nanomaterials provide a better approach to preparing TCFs, which is dip coating (self-assembly). This self-assembly process is a convenient, powerful, and versatile method of obtaining thin films with homogeneous surfaces, and it can create highly controllable conformal thin films with consistent thickness, transmittance, and surface resistance [19].

This study reports the preparation of exfoliated GO using a modified Hummers method. The GNs were prepared using a two-step chemical reduction (sodium borohydride (NaBH₄) and ethylene glycol (EG) as reducing agents). To maintain the GNs as an exfoliated structure, 1D AgNWs were grafted on 2D GN surfaces to form 2D AgNW–GN hybrid nanomaterials. The AgNWs increased the performance of GN-based TCFs. Fig. 1 shows the reaction process. Cysteamine was grafted on AgNWs to form modified AgNWs, which are existence with cysteamine on the surfaces which possess a number of –NH₂ functional groups (NH₂–AgNWs). The NH₂–AgNWs were mixed with GO, and then the NH₂–AgNWs reacted with the epoxy functional groups of GO to form AgNW–GO. The AgNW–GNs were obtained from AgNW–GO using a two-step reduction process. The AgNWs on the GN surfaces played a vital role of inhibiting the restacking of GNs and increasing electrical conductivity between the interlayers. For easier processing, a self-assembly method was used to prepare TCFs. Based on previous work [3], waterborne polyurethane (WPU) was selected as the substrate for preparing the TCF because WPU possesses outstanding transmittance and mechanical properties. Moreover, negative charges exist on the WPU film surface because of intrinsic sulfonate salts functional groups. When the pH value of the AgNW–GN solution is approximately 10, the amine functional groups (–NH₂) on the GNs become positively charged ($''–''{\text{NH}}_3^{+}$), which can be attracted by the sulfonate salt functional groups of WPU and adsorbed on the WPU film surfaces. This simple self-assembly process produced AgNW–GN hybrid nanomaterial TCFs with low-surface electrical resistance and high optical transmittance.