In situ synthesis of silver/chemically reduced graphene nanocomposite and its use for low temperature conductive paste

Figure 1a shows the XRD patterns of the Ag/G nanocomposite and bare Ag. The dominant diffraction peaks can be indexed to Ag (space group Fm3m, JCPDS No. 04-0783) for both Ag/G nanocomposite and bare Ag. The diffraction peak of GO was not observed in Ag/G which indicated that GO was reduced into graphene during the reduction process [16]. And the concentration of graphene is estimated to be 1 wt% by carbon content analysis.

Figure 1b gives the Raman spectra of Ag/G, graphene and GO. For all the three samples, two bands at 1350 and 1580 cm⁻¹ appear, corresponding to the disordered (D) and graphitic (G) bands of carbon-based materials [17]. The D-to-G peak intensity ratios of GO, bare graphene and graphene in Ag/G are calculated to be 0.79, 1.11 and 1.06, respectively. Note that, compared with GO, both Ag/G and graphene exhibit an increased D/G intensity ratio, which is caused by a reduction of the average size of the sp² domains and an increased number of these domains, indicating the reduction of GO to graphene [18]. It should be noted that the G peak shows an asymmetric feature. In fact, it consists of two overlapped peaks, namely, G and D′ bands, located at 1580 and 1620 cm⁻¹, respectively. The D′ peak is a defect peak due to intra-valley scattering [17]. The asymmetric feature of G peak of graphene was also observed in other work [19, 20]. The peaks at around 2700 and 2900 cm⁻¹ are related to 2D mode [21] and D + G mode [22], respectively, of carbon-based materials. Note that Ag/G exhibits a broad 2D peak, indicating the few-layer feature of graphene [21]. Broad 2D peak was also observed in few-layer graphene prepared by either solution chemical route [23] or chemical vapor deposition (CVD) [24]. Raman spectra only give the statistical average of the powder sample, where a large number of graphene sheets in single-, double- or few-layer form coexist in the powder. It should be stressed that when defined in a general sense, “graphene” can be distinguished as three different types: single-, double-, and few layer (3 to <10 layers) according to Geim et al. [25]. In this sense, the graphene in Ag/G can be regarded as “graphene” in few-layer form.

Figure 1c presents the C1s XPS of Ag/G,graphene and GO. The XPS is fitted into five peaks, corresponding to carbon atoms in four functional groups: non-oxygenated carbon (C–C 285.6 eV or C=C 284.8 eV), carbon in C–O group (epoxide or hydroxyl, 286.3 eV), carbonyl carbon (C=O 287.6 eV) and carboxylate carbon (O–C=O, 289.0 eV) [18, 26]. Note that the peak intensity of carbons in C–O, C=O and O–C=O groups exhibits a significant decrease in Ag/G and graphene compared with that in GO, indicative of a sufficient reduction of GO into graphene. Note that the solvothermal products still contain residual epoxide and/or hydroxyl groups, in consistent with the theoretical calculation that these groups are difficult to be removed when located at the edges of the GO [27].

The samples were further checked by FTIR to check the reduction status of GO as indicated in Fig. 1d. For GO, the strong absorption band at 1724 cm⁻¹ is due to the C=O stretching, the band around 1622 cm⁻¹ due to aromatic C=C, while the bands at 1412, 1233, and 1060 cm⁻¹ correspond to carboxy C–O, epoxy C–O and alkoxy C–O, respectively [28]. Of note is the obvious decrease of the peak intensity related to C=O and C–O, indicative of remarkable reduction of GO after the solvothermal reactions, agreeing well with the Raman and XPS results.

The microstructure of Ag/G nanocomposite and bare Ag was characterized by SEM as shown in Fig. 2. Figure 2a show typical SEM of Ag/G nanocomposite, indicating nano-sized Ag particles are warpped by graphene sheet. The graphene sheet exhibits a transparent feature as denoted by the arrows, indicating that it is rather thin. The microstructure of bare Ag was also characterized to check the effect of graphene on the morphology. Note that bare Ag crystals tend to aggregate without the dispersing and confining effects of graphene as shown in Fig. 2b.

The schematic of the preparation and curing process of the Ag/G conductive paste is shown in Fig. 3a. The Ag/G nanocomposite was blended with organic mixture to form the conductive paste. After dispersed by a three-roller mill, the paste can be easily printed on substrates for further curing process. Thermal analysis was conducted to determine the proper curing process. Figure 3b gives the TGA and DSC curves of the Ag/G conductive paste. An obvious mass loss observed between 150 and 245 °C corresponded to evaporation of the solvents. And an apparent exothermic peak appeared at the heating temperature about 225 °C, which indicated the state of the conductive paste changed and the curing process was completed at 225 °C. Isothermal TGA measurement at 150 °C indicate that all the solvent will evaporate (asymptotic at 71%) after about 18 min. Therefore, the conductive paste was cured at 150 °C for 30 min to ensure the solvent volatilize completely and the resin cure fully.

The conductivity of the synthesized Ag/G nanocomposites is the most important thing we care about, which decides whether the Ag/G nanocomposite is suitable for conductive paste or not. Figure 4 exhibit the photos of printed tracks on paper substrate. The printed tracks were used as a part of an electrical circuit as shown in Fig. 4a. When the traces were electrified from the end point, the LED was lighted, indicating a good electrical conductivity of the traces, as demonstrated in Fig. 4b. In addition, the Ag/G conductive paste is also fit for printing fine line pattern and irregular pattern. Figure 4c shows the good electrical conductivity of the trace of a line pattern with width of 300 µm and Fig. 4d shows the case for an irregular pattern. The sheet resistance of Ag/G conductive pastes that contain 0.3 and 0.6 wt% graphene are 32 mΩ/sq/25 µm and 22 mΩ/sq/25 µm. And the sheet resistance of conductive paste without graphene is 37 mΩ/sq/25 µm. Note that the Ag/G pastes show a better electrical property than pure Ag paste prepared by the same method. The sheet resistance of Ag/G conductive pastes is even comparable with some commercial products, such as Heraeus Ag paste LTC 3601 (15 mΩ/sq/25 µm) and Engineered Conductive Materials CI-1035c (35 mΩ/sq/25 µm). In addition, the Ag/G paste also exhibit good thermal stability and oxidation resistance. The printed tracks were heated in an electric oven at 80 °C for 10 days and the sheet resistance remain the same.

Figure 5 show cross-section SEM images of the printed track of the Ag/G paste. As shown in Fig. 5a, most of the Ag particles had fused together and connected with graphene to form a network throughout the entire film. Figure 5b gives a magnified image. The silver particles can be clearly identified. Between neighboring Ag particles or clusters, there were gaps. These gaps were much smaller than the size of graphene and thus the Ag particles were easily connected by the graphene sheets.