Fabrication of superhydrophobic, conductive and microwave-absorbing CF
The synthesis of SNFs and the process of stabilizing MWCNTs in water medium to fabricate a multifunctional CF were briefly introduced in Fig. 1. Aqueous SNFs were obtained after the silk was hydrolyzed with the help of urea under alkaline condition. With the incorporation of SNFs, aqueous dispersion of SNFs stabilized MWCNTs was prepared via ultrasonication and homogenization assisted liquid-phase method. The unwound MWCNTs are stabilized by electrostatic repulsive interactions with SNFs dispersant (Paredes and Villar-Rodil, 2016). Notably, SNFs include aromatic amino acid residues, such as tyrosine, tryptophan, and phenylalanine, which have strong π-π interactions with the surface of MWCNTs (Liang et al. 2020). The β-sheet structure, as well as the high negative charge density, endowed SNFs with hydrophobicity and good aqueous dispersibility, allowing the SNFs to act as a surfactant to prevent the restacking of MWCNTs in water (Bai et al. 2014) (Fig. 1a). As shown in Fig. 1b, the placed water droplet wetted the cotton fibers and was absorbed fast representing the obvious hydrophilicity of pristine CF. Subsequently, MWCNTs deposited CF was fabricated by dip-coating and thermal treatment induced chemical immobilization cycles, which were labeled as MWCNTs-CF-n. After further treatment with stearoyl chloride, hydrophobic CF was obtained with the tethered octadecanoyl chains (C18-MWCNTs-CF). Compared with the water droplet on the surface of pristine CF, the C18-MWCNTs-CF demonstrated an obviously varied wettability and the water droplet maintained a nearly spherical shape exhibiting prominent water repellency.
Structure and morphology of MWCNTs and C18-MWCNTs-CF
The successful preparation of SNFs dispersant was verified by FT-IR characterization (Fig. 2). Compared with the spectrum of pristine silk (Liu et al. 2017), the characteristic peaks of SNFs were associated to the amide III stretching vibration at about 1290 cm− 1, and amide I (C═O or asymmetric carboxylic group vibration) at around 1689 and 1630 cm− 1. Besides, the newly appearing absorption peaks at 3448 and 3351 cm− 1 corresponding to N-H, and the peak of the 1160 cm− 1 originating from the existence of C − N.
To confirm the successful tethering of the octadecanoyl chain, characterization of CF samples was carried out by using ATR FT-IR, which were shown in Fig. 3a. Compared with the spectrum of pristine CF, the newly appearing absorption peaks at 2919 and 2850 cm− 1 attributing to CH2 groups from the hydrophobic octadecanoyl groups, and the peaks of the C═O stretching vibration at 1816 cm− 1 and C − H deformation vibration at 1707 cm− 1 were also assigned to the existence of octadecanoyl groups. These results confirmed that the octadecanoyl groups were chemically bonded on the CF surface by esterification reactions between active hydroxyl groups of cotton fiber and stearoyl chloride. To further verify the above reaction mechanism, the local chemical composition of the surface was detected by XPS (Fig. 3b). Compared with pristine MWCNTs, the XPS spectrum of the modified CF showed new peaks of N element, indicating that SNFs were successfully introduced on the surface of the modified MWCNTs. After dip-coating of aqueous SNFs stabilized MWCNTs and octadecanoyl chain bonding processes, the appearance of N 1s and Cl 2p signals testified the successful immobilization of SNFs-MWCNTs and octadecanoyl groups on the surface of CF, respectively. The addition of SNFs on the pristine MWCNTs led to deconvolution of the spectrum into three peaks, namely the original two peaks and a new C═N peak at 286.7 eV (Fig. 3c, d). Moreover, different from the C 1s peak pattern of pristine CF, the new peaks located at 285.8, 287.2, and 288.1 eV appeared in the C 1s peak spectra of the C18-MWCNTs-CF, corresponding to the binding energies of C − N, O − C−O, and O − C═O, respectively (Fig. 3e, f). These XPS analyses are consistent with that of ATR-FTIR results, verifying the successful introduction of SNFs-MWCNTs and octadecanoyl groups.
The X-ray diffraction data collected on the pristine CF, pristine MWCNTs, and C18-MWCNTs-CF samples were shown in Fig. 4. Compared with pristine MWCNTs (Fig. 4a), the characteristic diffraction peak of C18-MWCNTs-CF appeared at 2θ = 26.06° corresponding to the (002) crystal plane. The untreated CF and treated CF had the same peak located at 35.2°, which attributed to the crystalline structure of CF, suggesting that crystalline structure of treated CF was not destroyed. The noncovalent interaction between SNFs and MWCNTs preserved the inherent characteristic of MWCNTs backbone and is superior to chemical modification, which weakened the intact structure and electronic properties. It can be speculated that the loading of MWCNTs and subsequent octadecanoyl group bonding had not changed the major structure of CF during the preparation of C18-MWCNTs-CF.
The surface morphology of pristine MWCNTs and SNFs-stabilized MWCNTs were investigated via SEM analyses. Without the addition of SNFs dispersant, pristine MWCNTs agglomerated into microsized bundles even after ultrasonication and homogenization processes due to the fact that the commercialized MWCNTs are supplied in the form of heavily entangled bundles (Fig. 5a, b). With the incorporation of SNFs dispersant, microsized MWCNTs bundles disappeared and were scattered into well-dispersed MWCNTs with the wrapping of SNFs (Fig. 5c). The β-sheet structure and the high negative charge density endowed SNFs with hydrophobicity and good aqueous dispersibility, which provided electrostatic repulsive interactions and strong π-π interactions towards MWCNTs (Bai et al. 2014). The disentangled MWCNTs are stabilized and prevented from the restacking (Liang et al. 2020). With the further enlargement of magnification, the aggregates of MWCNTs were hardly examined (Fig. 5d). As shown in the inset of Fig. 5d, no precipitation was observed in 5 mg/mL MWCNTs dispersion whether the sample bottle was placed upside down, which was in sharp contrast to the observation of Fig. 5b. These results suggested that SNFs was a suitable biomolecule to effectively disperse MWCNTs and efficiently stabilize them in water.
It can be seen that pristine CF presented quite a smooth surface without visible impurities (Fig. 6a, b) (Cheng et al. 2018). Compared with that of pristine CF, the deposition of MWCNTs on the CF surface was presented in the form of gray tubes with a size of approximately 20–30 nm (Fig. 6c, d). It was observed that the connected MWCNTs formed a conductive network owing to the extended percolative structure. With the enlargement of magnification, MWCNTs appeared to be adhered tightly on CF because amino groups of SNFs and hydroxyl groups of CFs reacted with each other during drying process to achieve curing effect (Fig. 6d). After octadecanoyl chain tethering, a mass of grooves emerged on the surface of C18-MWCNTs-CF owing to the detachment of tiny cotton fibers during reaction and stirring processes (Fig. 6e, f). To achieve superhydrophobicity, the key is to construct micro/nano roughed hierarchical surface (Zhang et al. 2017). Compared with the pristine CF, the combination of the enhanced surface roughness induced by MWCNTs disposition and the hydrophobic octadecanoyl group bonding dramatically strengthened the hydrophobicity of C18-MWCNTs-CF. The post-treatment did not destroy the conductive network of MWCNTs in the coating and weakened the adhesion of MWCNTs towards CF surface, which is expected to achieve high conductivity and hydrophobicity.
Properties of multifunctional CF
Superhydrophobic durability of CF
The wetting behavior of pristine CF and C18-MWCNTs-CF were explored using WCA measurements. As displayed in Fig. 7a, the WCA of pristine CF was approximately 26° and upon approaching, the water droplet was adsorbed completely within 2 s due to the abundant hydrophilic hydroxyl groups of pristine CF. The hydrophilicity of CF was enhanced by the disposition of SNFs stabilized MWCNTs, and the water droplets were quickly absorbed so that the contact angle can’t be captured (Movie S1). The octadecanoyl chain-tethered C18-MWCNTs-CF was expected to enable the hydrophilic CF with superhydrophobic characteristics after the active sites of CF and SNFs were reacted with stearoyl chloride. Therefore, C18-MWCNTs-CF displayed superhydrophobicity with a WCA of about 154° due to the increased surface roughness induced by hydrophobic octadecanoyl chain (Fig. 7b and Movie S2) (Fan et al. 2019). The pristine CF was wetted as soon as it was immersed in common contaminated liquids (dyed with methylene blue), and the CF was dyed after being taken out (Fig. 7c1, c2). However, C18-MWCNTs-CF possessed obviously antiwetting performance remarkable towards the examined water (dyed with methylene blue), milk, and coffee due to the superhydrophobicity of C18-MWCNTs-CF (Fig. 7d). Fine sand as pollutant was placed on the surfaces of the C18-MWCNTs-CF and pristine CF. As presented in Fig. 8, in contrast to the pristine CF, water drops falling on the C18-MWCNTs-CF could automatically roll and take away fine sand, demonstrating the remarkable self-cleaning property.
The robustness of medical clothing is required to fulfil the practical application of superhydrophobic interfacial materials. Therefore, the mechanical stability of C18-MWCNTs-CF was explored to evaluate its durability by means of utilizing extreme mechanical force. As presented in Fig. 9a, the C18-MWCNTs-CF was subjected to rubbing and scratching process under external force. After 20 cycles, C18-MWCNTs-CF still possessed superhydrophobicity with a WCA approaching 150° verifying its mechanical durability (Fig. 9b). Additionally, the stability of C18-MWCNTs-CF was confirmed by repeatedly employing certain stress under the action of axial tensile force. The C18-MWCNTs-CF remained the hydrophobicity with the WCA approximately 148° after being stretched in case of severe axial tensile force (Fig. 9c). The chemically bonded octadecanoyl groups endowed C18-MWCNTs-CF with excessive abrasion resistance (Wang et al. 2019). Further, the amino and carboxyl groups of SNFs can interact with hydroxyl groups on the surface of CF, which facilitated the strong adhesion of MWCNTs towards CF. The robust superhydrophobicity of C18-MWCNTs-CF are capable of strong repellency to water and effectively prevent water penetration, which efficiently avoid performance deterioration. These results reflected that the microstructure and surface composition of C18-MWCNTs-CF fiber were not damaged during mechanical drawing and thus the mechanical wear stability and long-term durability of superhydrophobic fiber was realized consequently.
Electrical conductivity
The variation in the loading capacity of MWCNTs on pure CF (mg/g) under different dipping-drying cycles directly affect the conductivity of the C18-MWCNTs-CF. The loading capacity of MWCNTs as a function of dipping-drying cycle was shown in Fig. 10a. The loading capacity of MWCNTs gradually increased after each dipping-drying cycle and reached the maximum content of 259.2 mg/g after 4 dipping-drying cycles. As could be observed in Fig. 10b, the sheet resistivity of MWCNTs-CF-n and C18-MWCNTs-CF exhibited similar downward trends at incremental dipping-drying cycles. The sheet resistance of MWCNTs-CF-1 decreased dramatically with the LC of 115.3 mg/g (580 Ω) in comparison with that of pristine CF (2\(\times\)1010 Ω). With the dipping-drying cycles increasing to 2 and 3, the surface resistance of MWCNTs-CF-2 was 187 Ω containing MWCNTs of 203.8 mg/g and MWCNTs-CF-3 was 40 Ω containing MWCNTs of 247.5 mg/g. But the loading of MWCNTs seemed to be saturated at 3 dipping-drying cycles, as further increase cycle of 4, the sheet resistivity increased slightly with fluctuation around 40 Ω. Therefore, the dipping-drying cycles of CF samples in subsequent studies were 1, 2, 3. The surface resistivity of MWCNTs-CF-n is slight lower than that of C18-MWCNTs-CF, which may be due to the fact that the adsorbed MWCNTs fell off a little after being stirred in the solvent for 1 d and washed several times during MWCNTs-CF was modified with stearoyl chloride. The mild decrease in WCA of MWCNTs deposited with CF was ascribed to the certain hydrophilicity of the functionalized MWCNTs, but the superhydrophobicity of the CF was not affected. Furthermore, a test of a bulb lighting experiment was carried out, as shown in Fig. 11. Compared with the pure CF (Fig. 11a), MWCNTs-CF exhibited excellent electrical conductivity and was conducive to lighting the LED at 1.5V power supply (Fig. 11b). The strengthened conductivity was originated from the three-dimension electrical paths that were formed by direct contact of MWCNTs.
Thermal stability
The variation of thermal decomposition and pyrolysis behavior of pristine CF, MWCNTs-CF-n, and C18-MWCNTs-CF samples with temperature under nitrogen atmosphere by TGA analysis were presented in Fig. 12. The T5wt% and Tmax of MWCNTs-CF-n and C18-MWCNTs-CF moved towards lower temperature compared with that of pristine CF due to the decomposition of SNFs, which confirmed that SNFs stabilized MWCNTs were deposited on CF. Particularly, carbon residue of CF was significantly increased from 5.328% (pristine CF) to 10.119%, 13.370%, 13.765% and 17.581% of MWCNTs-CF-1, MWCNTs-CF-2, MWCNTs-CF-3, and C18-MWCNTs-CF at 600°C, respectively. The results of carbon residue were consistent with the increasing LC of MWCNTs on CF, which was conductive to the formation of residue and did not affect the thermal stability. After that, weight loss rate became slower as the remainder was mainly the relatively stable MWCNTs carbon skeleton (Su et al. 2020).
Mechanical property
The impact of chemical treatments on the mechanical property of CF samples were investigated by monitoring the variation of tensile strength and elongation at break (Fig. 13). Compared with pristine CF, the loading of the MWCNTs layer slightly strengthened tensile strength and elongation at break of MWCNTs-CF-n and C18-MWCNTs-CF. This is because the chemical treatment mainly enhanced the combination of MWCNTs and stearoyl chloride on the CF surface, while the core structure of cellulose cotton (which is mainly responsible for showing mechanical strength) has changed modestly ( Bhattacharjee et al. 2020).
Microwave absorption performance
Because of its light weight, nanocarbon-based absorbing materials can absorb electromagnetic energy maximally in a wide frequency range, which has gradually become the focus of attention in the field of absorbing waves in the new era (Qi et al. 2020; Zhang et al. 2021). The microwave absorption performance in case of various thickness were characterized by calculating RL values (Fig. 14). Compared with pristine MWCNTs, SNFs-MWCNTs has a relatively greater absorption performance, the RLmin of SNFs-MWCNTs is -22.33 dB as thick as 2.5 mm, while its average RL is only − 6.57 dB (Fig. 14a). As shown in Fig. 14b, the RLmin of SNFs-MWCNTs were larger than − 20 dB covering the testing thicknesses from 2.5 to 3.0 mm. Generally speaking, RL values under − 10 dB indicates > 90% microwave absorption (Mu et al. 2018) and > 99% microwave were absorbed when RL is below − 20 dB (Qi et al. 2016). Obviously, as expected, SNFs-MWCNTs showed the optimum enhanced microwave absorption with the RLmin for − 36.08 dB at 9.28 GHz with 2.7 mm thickness, when the filling amount of MWCNTs was merely 0.78%. The applicable bandwidth of presents the absorption frequency range, which is the critical property for an absorber. The corresponding effective bandwidths over − 10 dB of SNFs-MWCNTs from 2.5 to 3.0 mm thickness were calculated to be 3.72 (7.48–11.20), 3.90 (7.16–11.06), 4.00 (6.96–10.86), 3.88 (6.81–10.69), 3.88 (6.67–10.55), and 4.16 GHz (6.47–10.63 GHz), respectively. Therefore, SNFs-MWCNTs provided multiple microwave reflection access to obtain advanced microwave absorption performance (Liu et al. 2015), which could be adjusted by facile controlling LC of MWCNTs in the targeted CF composites.
Performance evaluation
In order to highlight the superiority of as-prepared C18-MWCNTs-CF, a comparison of the C18-MWCNTs-CF in current study with the state-of-the-art of modified cotton fabrics reported in the literature. Compared with the following five similar materials, the greatly enhanced conductivity and superhydrophobicity of C18-MWCNTs-CF were contributed to the efficient SNFs-stabilized MWCNTs and tethered octadecanoyl groups. SNFs can not only disperse MWCNTs effectively, but also have the superiority of chemical bonding with CF at high temperature (the amino and carboxyl groups of SNFs can interact with hydroxyl groups on the surface of CF, which facilitated the strong adhesion of MWCNTs towards CF), further providing active sites for subsequent hydrophobic treatment. As shown in Table 1, C18-MWCNTs-CF displayed the competitive performance that promises an ideal application prospect in the field of smart textile and wearable electronic devices.
Table 1
Performance comparison of the C18-MWCNTs-CF in current study with the state-of-the-art of multifunctional cotton fabrics reported in the literature.
Materials
|
Conductivity
(Ω)
|
Superhydrophobicity
(°)
|
Microwave absorption
(dB)
|
References
|
C18-MWCNTs-CF
|
40.0
|
154
|
-36.08
|
This work
|
CO + MWCNTs
|
710.0
|
155
|
—
|
(Makowski et al. 2014)
|
MTMS-MWCNT
|
4.0×104
|
146
|
—
|
(Nasirizadeh et al. 2015)
|
C-PDA-CNT
|
51.8
|
—
|
—
|
(Shak et al. 2019)
|
(bPEI/CNTs) n /APP/PDMS
|
113.3
|
151
|
—
|
(Xue et al. 2020)
|
CCNT/cotton composites
|
2.1×103
|
—
|
—
|
(Li L et al. 2017)
|