3.1 Graphene coating onto CGYs
GO possesses good dispersion stability in solution, which is due to the strong electrostatic repulsion between the charged GO [23]. Although GO can be easily attached onto CGYs by physical bonging, it is difficult to obtain a completely covered structure due to the intrinsic repulsion between GO and the coated yarns. We, therefore, employ a commercialized pad dyeing instrument to accomplish the modification. Figure 1a shows the detailed preparation procedure of rGO-CGYs. The pristine CGYs were firstly immersed into GO solution to make surface fully in contact with the GO solution. Then the excess GO solution on the pristine CGYs surface was removed by a rolling mill to obtain continuous structure and full coverage for high-efficiency electron transport. After the GO modification, the GO-coated yarn experienced reduction through being transferred into ascorbic acid solvent (with a weight ratio of 10:1 to GO) at 80ºC to acquire rGO-CGYs.
Figure 1b presents the schematic diagrams of pristine CGY, GO-CGY, and rGO-CGY, and the photos of corresponding yarns. The size distribution of GO in Fig. 1c exhibits that the size of the GO flakes is mainly between 2 µm and 6 µm and we can see that the GO flakes possess a uniform size, which is beneficial to pad dyeing of CGYs. In order to optimize the experimental parameters, we analyze the GO concentration, pad dyeing cycles, and reduction time in determining the properties of graphene-coated CGYs through an orthogonal design with three factors and three levels. Table 1 shows the sample specifications. We choose the samples with good volume conductivity (Sample 2, Sample 5, and Sample 8) to present and evaluate their performance, which are denoted as rGO-CGY-4, rGO-CGY-7 and rGO-CGY-10 respectively. The factors affecting the yarn conductivity are analyzed using range analysis with all sample data included. Figure 1d illustrates that the viscosity of GO solutions of 3, 5, and 7 mg/ml is 26.6 ± 4.8, 41.4 ± 7.4, and 61.6 ± 7.0 respectively, indicating a higher GO concentration obviously increases its viscosity.
Table 1
The specifications of rGO-CGYs through pad dyeing.
Sample | Additive concentration (mg/mL) | Reduction time (h) | Pad dyeing cycles | Contact angles (°) | Volume conductivities (S/m) |
1 | 3 | 12 | 4 | 133.83 ± 5.12 | 0.29 ± 0.08 |
2 | 5 | 36 | 4 | 147.47 ± 5.21 | 1.99 ± 0.35 |
3 | 7 | 24 | 4 | 135.23 ± 4.08 | 0.56 ± 0.12 |
4 | 3 | 24 | 7 | 139.03 ± 8.49 | 0.33 ± 0.05 |
5 | 5 | 12 | 7 | 150.27 ± 4.94 | 3.50 ± 0.44 |
6 | 7 | 36 | 7 | 145.03 ± 2.97 | 0.98 ± 0.09 |
7 | 3 | 36 | 10 | 143.83 ± 5.00 | 0.74 ± 0.05 |
8 | 5 | 24 | 10 | 150.1 ± 3.52 | 6.90 ± 0.19 |
9 | 7 | 12 | 10 | 142.33 ± 5.87 | 1.40 ± 0.22 |
3.2 Surface morphology and elements
The pristine CGY and rGO-CGY-10 in Fig. 2a, b show that the nature-color yarn becomes black after the functionalization, which can be attributed to the attachment of rGO onto the fiber surface. After the modification, the surface elements of the yarns are significantly changed. Figure 2c shows the atomic percentage of C, N and O on the surface of pristine CGY, GO-CGY-10, and rGO-CGY-10. It presents that the atomic percentage of C, N, and O of the pristine yarn is 57.780%, 4.678%, and 37.542% respectively, while that becomes 59.323%, 3.133%, and 37.544% after GO modification. The increase of C, and O, decrease of N can be attributed to the attachment of GO on the surface of the CGY. After reduction, the C increases by 61.855%, N and O decrease by 2.536% and 35.609% respectively. The surface elements change indicates that the reduction process successfully facilitates the transformation of rGO from GO, which is consistent with our previous observation [24].
Figure 2d-d2 show the surface morphology of the pristine CGY, GO-CGY-10, and rGO-CGY-10 respectively. Comparing with the pristine CGYs with smooth surfaces, the GO-CGY-10 shows rough and uneven surfaces due to the existence of GO layers, and rGO-CGY-10 has the wrinkled and crumpled surface which is similar to graphene fibers [25–27]. This volume shrinkage is mainly due to intermittent pad dyeing and dehydration shrinkage [28]. The volume shrinkage is beneficial to compacting the rGO coatings into a dense structure and forming a strong interaction between the rGO and the pristine CGYs [29]. Ascorbic acid, as a green reducing agent, promotes the reduction of GO on the surface of the pristine CGY without observable damage to the yarn[30]. Figure 2e-e2, Fig. 2f-f2, and Fig. 2g-g2 show the surface element distribution of the pristine CGY, GO-CGY-10, and rGO-CGY-10 respectively. The surface morphology and element distribution indicate that graphene is uniformly covered onto the CGFs, which is of great significance to both conductivity and stability of the yarn.
Figure 3a shows the Raman spectra of the GO and rGO-modified CGYs. The D-band at 1370 cm− 1 (related to defects or heteroatom-doping) and G-band at 1590 cm− 1 (attributed to the vibrations of the sp2-bonded crystalline carbon), indicate that graphene wrapped onto the pristine CGYs successfully. The GO-CGY shows a higher D band and rGO-CGY shows a higher G band, which can be seen in the high-resolution fitting spectrum of Raman spectra of the GO-CGY (Fig. 3b) and the high-resolution fitting spectrum of Raman spectra of the rGO-CGY (Fig. 3c) respectively. The intensity ratio of D-band and G-band increases from 0.9 to 1.2 and the full width at half maximum (FWHM) of the D-band decreases from 71 to 53 (Fig. 3d,). The enhancement of D-band is mainly attributed to the formation of small-size sp2 domains (disordered carbon) during reduction process. The decrease of FWHM suggests an increase in the structural order. These changes show that GO-CGYs are chemically converted into rGO-CGYs, which is consistent with graphene-coated elastic wearable electronics reported in literature [31].
Figure 3e shows changes in surface wettability of the pristine CGYs and the CGYs with graphene coating. The contact angles increase greatly from around 20° (the pristine CGYs) to about 150° (the rGO-CGY) after functionalization. With rGO modification for 4 pad dyeing cycles, the contact angle has reached up to 145°. While with the increase of pad dyeing, the contact angle raises slowly up to 150° after 10 cycles. The increase of the surface contact angle can be attributed that the pristine CGY is hydrophilic material[17], while graphene is the hydrophobic material[32]. The coating element (i.e., graphene) on the CGYs surface directly contacting water drops leads to higher contact angles. In addition, the surface wettability depends on not only the hydrophilicity and hydrophobicity but also the surface roughness of CGY. The graphene coating can increases the surface roughness of CGY, resulting in high hydrophilicity [33]. Such change of the surface roughness of CGY can be confirmed through surface morphology (Fig. 2d-d2).
Figure 3f shows the thermal stability of the CGYs. With the increase in temperature, the mass of CGYs becomes lighter. When the temperature rises from 20ºC to 700ºC, the corresponding weight loss reaches about 85%. The masses of CGYs only have a little loss at 100 ºC, which is mainly due to the loss of moisture. At around 400 ºC, rGO begins to be decomposed, the rGO-CGY-10 shows a more obvious weight loss. The loss can be attributed to the thermal-induced destroy in CGYs, as graphene has remarkable temperature stability and tolerance. It also proves that increasing dyeing cycles can increase the graphene amount on the CGYs, thus enhance their conductivity.
3.3 Hydrophilicity and mechanical properties
The typical tensile strain curves of pristine and rGO-modified CGYs in Fig. 4a exhibit that graphene coating can result in a higher breaking strength and a shorter shifting distance as well as a higher modulus. The breaking strength of the pristine CGYs is about 168 cN, and it will gradually increase with the dyeing times (Fig. 4b). After 10-cycle dyeing, the breaking strength finally becomes 195 cN. The growth in breaking strength is due to the attachment of graphene on the fiber surface, promoting a stronger combination among fibers[34]. Figure 4c shows that the increase in dyeing cycles slightly decreases the tensile strain of the CGYs, of which tensile strain is 4.1% and 7.2% for pristine CGYs and the rGO-CGY-10 respectively.
Figure 4d shows the electrical resistance of CGYs after graphene coating. The electrical resistance of the rGO-CGY drastically decreases with pad dyeing cycles, from about 800 kΩ for rGO-CGY-4 to 210 kΩ for rGO-CGY-10. We further calculate the electrical conductivity of the rGO-CGYs (Fig. 4e). Compared with the electrical conductivity of rGO-CGY-4, the electrical conductivity of rGO-CGY-10 is obviously improved. The rGO-CGY-10 exhibits high electrical conductivity of 6.9 S/m, while the rGO-CGY-4 only presents a volume conductivity of 2 S/m. With the combination of graphene, the strong π-π interactions between adjacent graphene form interconnected electron-transport pathways. With more pad dyeing cycles, rGO-CGY-10 holds more electron-transport pathways and obtains high electrical conductivity.
The range analysis of the coating parameters in determining the electrical conductivity of rGO-CGYs in Fig. 4f shows that range value of the additive concentration of GO solution, the reduction time, and the pad dyeing cycles is 3.665, 1.344, and 2.048 respectively. It exhibits that the electrical conductivity of rGO-CGYs largely depends on the additive concentration of GO solution and pad dyeing cycles, and the reduction time has little effect on it. When the concentration of GO solution is 3 mg/mL, the content of graphene is low, while the concentration of GO solution is 7 mg/mL, the viscosity of graphene solution is too high to adhere to the surface of CGY. Both of them lead to fewer electron-transport pathways. The degree of graphene coating on yarns’ surface can be augmented with the increase of pad dyeing cycles, which also can raise the amounts of electron-transport pathways on the surface of CGYs.
Figure 5a presents the FTIR spectrum of the pristine CGY, GO-CGY-10, and rGO-CGY-10. The FTIR spectrum of the pristine CGY displays peaks around 3340, 2900, 1645, 1430 and 1060 cm− 1, which are mainly attributed to the OH stretching, asymmetrical C-H stretching, C = O stretching, CH2 symmetrical bending and C-O stretching respectively [35]. After graphene coating, no evident change is found in the FTIR spectrum of GO-CGY-10, which is consistent with the previous research [35]. Whereas, the FTIR spectrum of rGO-CGY-10 changed obviously after the treatment of ascorbic acid. The peak at 1645 cm− 1 disappears and the peak at 2900 and 3340 cm− 1 become weaker, which implies that GO on the surface of CGY is successfully reduced to rGO after the treatment of ascorbic acid [35].
Figure 5b shows the characteristic absorption wavenumbers of intramolecular hydrogen bond O(2)H···O(6), O(3)···O(5), and O(6)···O(3') in CGYs are 3310 − 3233 cm− 1, 3375 − 3340 cm− 1, and 3455 − 3410 cm− 1, which are named Ⅰ, Ⅱ, and Ⅲ respectively [36]. The percentage of Ⅰ changes slightly from 61.32 to 64.74, the percentage of Ⅱ increases moderately from 25.14 to 31.97, and the percentage of Ⅲ decreases obviously from 13.54 to 3.29. It reveals that the content of Ⅲ in CGY decreases during the treatment. CGYs consisting of celluloses are rich in positively charged hydrogen, while GO has plenty of negative groups such as carboxyl and hydroxyl groups [36, 37]. We postulate a bonding mechanism between CGYs and rGO, which is shown in Fig. 5c. The pristine CGY is coated with GO and using ascorbic acid to convert GO into rGO. The negatively charged GO species and positively charged hydrogen in CGY are combined by electrostatic attraction, which is consistent with reported work [38]. Such postulation can also be supported by the FTIR and EDS results. The atomic percentage of O not only does not increase obviously after GO coating but also decreases significantly after reduction. Meanwhile, the content of the Ⅲ in CGYs greatly drops during the whole process.
To evaluate the washability of the graphene-functionalized CGY, we wash the rGO-CGY-10 with deionized water and laundry liquid. With the washing in deionized water, the volume conductivity of the rGO-CGY-10 slightly decreases with the increase of washing time (Fig. 6a). Similarly, the volume conductivity shows the same trend decrease when the yarn is washed in laundry liquid (Fig. 6b). The electrostatic attraction between negatively charged GO species and positively charged hydrogen in CGY enhances the bonding. Notably, the washed yarns still maintain very good conductivity after washing in both deionized water and laundry liquid, showing the good stability of the graphene coating.
Figure 6c-c2 show the photos of the rGO-CGY-10 with bending (0–75%). Figure 6d and Fig. 6e present the relative resistance change of the rGO-CGY-10 with the bending of 50% and 75% respectively. The bending was calculated by distance change/initial length of CGYs during the bending test. We show the resistance change with (R-R0)/R0, where the R0 and R refer to the initial resistance and the resistance under bending respectively. The resistance of the yarn slightly increases from 255 to 262 kΩ with the bending ranging from 0 to 50%, and this trend is very stable throughout the cyclic bending process. However, the resistance change is as low as 3.2%. Even under high-degree bending up to 75%, the resistance change is still very minor (4.3%). The CGY has a very limited response to bending, which means that the graphene-coated CGYs have a certain anti-interference performance to bending and show great potential in flexible wearable electronic devices.