Cotton modified with silver-nanowires/polydopamine for a wearable thermal management device

Abstract

Currently, indoor heating technology supports people's comfortable working and living conditions, but a large portion of energy is wasted on heating empty space. Target-selective heating can focus energy on specific objects, thus achieving high efficiency at a low cost. Here, we report a personal thermal management cloth that uses a coating of sliver nanowire (AgNW)/polydopamine nanocomposite; the cotton was modified with polydopamine to enhance the adhesive ability of AgNWs through intermolecular cross-linking. The AgNW/polydopamine cloth (ADNC) not only highly reflects middle-to-far infrared radiation from the human body (the average reflectance is up to 86%, approximately 66 times higher than normal cloth), but it also allows Joule heating with a quick thermal response (1 min, from 22 °C to 40 °C). Additionally, the AgNW/polydopamine cloth is durable and washable, making it suitable for a personal thermal management cloth.

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1. Introduction

The heating of residential buildings accounts for 19% of the total final energy consumption, which is greater than the energy share from electricity (17%).¹ New technologies for reducing the loss of indoor heat usually focus on developing insulating building skins, such as low heat-conductivity materials and structures for walls, floors, roofs and window glass.² Currently, indoor temperatures are sustained with air-conditioning and heating, which is wasteful because it heats empty space and inanimate objects rather than focusing on humans. Cui et al. used a silver nanowire coated textile for thermal management,³ which can save 8.5 kW h of heating energy daily per person.

A personal heat management cloth should be wearable.^4,5 A flexible conductor was coated onto existing textiles.^6,7 Conducting polymers have been widely used as flexible conductors in various types of flexible devices;^8–10 they are compatible with mass production through cost-effective solution process. However, conducting polymers are not yet suitable for commercial products because of their poor conductivity. Emerging candidates for improving conductivity include carbon nanotubes (CNT),^11–14 graphite nanoplatelets,¹⁵ graphene^16–18 and metallic nanowires.¹⁹ Personal heat management cloth fabrication based on the graphene sheets is carried out in our group now by taking advantage of the high electrical conductivity. But as far as we know, carbon-based materials require an expensive vacuum environment or a toxic chemical process to exhibit enhanced conductivity.²⁰ The low aspect ratio of most metallic nanowires and complex fabrication methods required for their preparation limits their use.²¹ Among metallic nanowires, silver nanowires (AgNWs) are the most promising choice for wearable conductive material because of their high electrical conductivity and superior yield strength.²²

To use AgNWs in industrial applications, there are issues that must be addressed, especially issues related to poor substrate adhesion.²³ Several strategies have been proposed to solve this problem, such as high-temperature thermal treatment,^24,25 high-intensity pulsed light sintering^26,27 and conformal pressure.²⁸ However, the use of mechanical pressure is constrained by cotton's endurance and compatible temperatures. Huang²⁹ et al. prepared a smart cloth using a AgNWs/poly(dimethylsiloxane) nanocomposite, which showed robust adhesion and a machine-washable character. Lee³⁰ et al. introduced a conducting polymer poly(3,4-ethyl-enedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) to the top layer of an AgNW film. Actually, the preparation of AgNW transparent conducting film by adhesive binding has been widely applied in various fields of science research and actual production. However, cotton fibers have more complex surface than the flat surface films, adhesives should theoretically be selected based upon the physical chemistry of the surface.³¹ Besides, this adhesive is expected to be well-dispersed in polar solvents of AgNW dispersions. Furthermore, this adhesive is expected to be resistant to water and should be able to be washed quickly. Polydopamine has attracted much attention for its excellent chemical stability and good adhesion to virtually all types of surfaces, even wet surfaces.³²

In this study, we fabricated an AgNW/polydopamine nanocomposite cloth (ADNC) via the dip-coating technique. Polydopamine improved the adhesion of AgNWs to cotton and demonstrated excellent adhesion of crosslinked Ag wires.³³ The ADNC has excellent radiation insulation and can generate Joule heating with a quick thermal response (1 min, from 22 °C to 40 °C). Importantly, the ADNC is bendable and water-washable without causing a degradation in performance. Based on the above properties, ADNC holds great potential as a personal thermal management cloth.

2. Experimental section

2.1 Materials

The following materials were used in this study: anhydrous ethylene glycol (99.8%, Aladdin), sodium chloride (NaCl) (99.5%, Aladdin), silver nitrate (99%, AgNO₃) (Aladdin) and polyvinylpyrro-lidone (PVP) (M_w ≈ 1 300 000, Aladdin), dopamine hydrochloride (98%, Aladdin), alcohol (95%, Aladdin), tris(hydroxymethyl)aminomethane (99.9%, Aladdin), hydrochloric acid (HCl) (37%, Sinopharm), and 1-octadecanethiol (97%, Aladdin).

2.2 Synthesizing AgNWs

Silver nanowires were synthesized according to the literature using a polymer-mediated polyol process,^34,35 which produces the nanostructures in large quantities with controlled morphologies. In a typical synthetic process, a mixture of 0.4 g of PVP, 0.2 g of AgNO₃ and 20 mL of ethylene glycol was stirred in a flask for 30 min. Due to the shading treatment, the color of the solution was substantially unchanged. Then, 200 μL of 0.1 mol L⁻¹ NaCl was added, and the flask was heated at 170 °C for ∼2 h for the complete formation of AgNWs, which was indicated by a final opaque gray color. The resulting precipitate was filtered and then washed with distilled water three times to remove ethylene glycol, PVP, and other impurities. Finally, the AgNWs' precipitates were redispersed in 50 mL of alcohol. The concentration of the AgNW dispersion in the follow-up experiments is about 2 mg mL⁻¹.

2.3 Preparation of ADNC

A 10 mM Tris buffer solution was prepared using water, and the pH was adjusted to ∼8.5 with the addition of HCl. Then, 20 mg of dopamine hydrochloride was dissolved into 10 mL of Tris–HCl solution to prepare the dopamine solution. The cotton cloth (2 × 2 cm²) was modified by soaking it into the dopamine solution for 24 h. The polydopamine-coated cotton cloth was then air-dried. Subsequently, 0.01 g dopamine hydrochloride was added to the prepared AgNW dispersion (10 mL). Finally, the dopamine treated cotton cloth was dip-coated with the AgNW dispersion for 3 times and dried in vacuum.

2.4 Preparation of AgNW cloth

The original cotton cloth was ultrasonically cleaned in ethanol and deionized water successively for 30 min to remove surface stains and oils. Then, pre-cured cotton cloth (2 × 2 cm²) was dip-coated with the AgNW dispersion for 3 times and dried in vacuum.

2.5 Characterization

The surface morphology and the distribution of the AgNWs were observed using a Scanning Electron Microscope (SEM, JEOL Manufacturing, Japan, JSM-6700F). The sheet resistance of ADNC was measured by a four-point probe surface resistivity meter (JG, ST2263). The composition and chemical structure of ODT-modified ADNC surfaces were investigated by X-ray photoelectron spectroscopy (XPS, THERMO SCIENTIFIC K-ALPHA). Fourier transform infrared (FTIR) spectroscopy was measured using a BRUKER EQUINOX55 infrared spectrophotometer. All thermal images were taken by Mikron thermal imager, and the working distance was approximately 30 cm.

3. Results and discussion

To date, the physicochemical properties of the surface interaction and the self-polymerization mechanism of dopamine are still unknown.^36–40 However, this lack of understanding has not prevented research groups from developing a multifunctional and nanometer-scale ad-layer that is comprised of a polydopamine coating. The polydopamine-coated surface typically reacts with a variety of molecules via Schiff-base and Michael addition chemistries.³² Fig. 1a presents a schematic illustration of the interactions between polydopamine and cotton/AgNWs. Cloth was first soaked in polydopamine hydrochloride to produce a modified cloth (polydopamine is drawn in purple). Then, the modified cloth was coated with AgNWs via dip-coating in a ploydopamine–AgNW dispersion. This method is versatile because of its simple ingredients, mild reaction conditions, and applicability to a cotton-cloth with a complex and changeable surface. Though there were some previous reports have shown that AgNWs have excellent adhesion with micro-scale fibers because of entanglement and electrostatic attraction.^3,41–44 To investigate the function of polydopamine, we designed a comparative experiment. In the comparative experiment, there was no variation of the concentration of AgNW dispersion and dipping times. The SEM images in Fig. 1b and c reveal that AgNWs were coated more uniformly on the cotton surfaces by using dopamine. The AgNWs were even filled into the gaps and spaces between neighboured microfibers and stuck together to form networks. Comparing the SEM images of Fig. 1d and inset suggests that the AgNWs are intercalated and coated evenly on a cotton fiber surface with a homogeneous distribution, which is very difficult to accomplish through entanglement and electrostatic attraction. We note that the amount of the dopamine hydrochloride in AgNW dispersion play a crucial role in this experiment. The excess dopamine hydrochloride accelerates the aggregation process of AgNW through the self-polymerization of monomer dopamine (as shown in Fig. S1†), which lead to a large number of defects and obvious uncovered areas. Fig. S1† also show that the maximum spacing of the AgNW network is approximately 300 nm, which is much larger than a water molecule (0.2 nm). Therefore, water vapor generated by perspiration can easily escape. In addition, the spacing is much smaller than the main wavelength of human body radiation (9 μm); this finding means that ADNC can reflect human body radiation. The reflectance spectrum of ADNC and the normal cloth is measured from 2 to 16 μm (Fig. 1e). Remarkably, the average reflectance of human body radiation (86%) for ADNC is significantly higher than that of the normal cloth (1.8%). This finding indicates that ADNC is an effective IR reflector, which was expected. The dependence of the Ag nanowires loading on the dip-coating times was also studied. The sheet resistance of ADNC shows a very high value of 312.15 Ω □⁻¹ after the first dip-coating process and then drops significantly to 19.65 Ω □⁻¹ after the second dip-coating process. The sheet resistance has little change after the third dip-coating process. The results indicate that dip-coating with the AgNW dispersion for 3 times leads to the optimum loading amount of Ag nanowires.

Fig. 1 (a) Schematic description of interactions between polydopamine and AgNWs/cotton surface, (b) SEM image of ADNC, (c) SEM image of AgNW cloth, (d) SEM image of pristine cotton fiber and ADNC fiber image (inset), and (e) reflectance measurement of normal cloth and ADNC performed by an FTIR microscope using gold film as the reference.

The conducting network of AgNWs can be heated electrically, which is compatible with the purpose of personal thermal management. Time-dependent temperatures for a 2 × 2 cm² sample at various applied voltages were measured (Fig. 2a, details in ESI†). Only a low voltage of 0.7 V can induce a temperature change up to 40 °C; this temperature is higher than the average body temperature of 37 °C. Fig. 2b shows the cycling heating performance of ADNC. The heating performance of ADNC is extremely accurate during 8 cycles, which is critical to its use as a personal thermal management cloth. To date, there have been several reports on wearable and portable heating devices,^4,45–48 but equipment safety is still a major concern. Here, we report a thermo-color dye that colors quickly when overheated. The temperature distribution of ADNC when it is coated with a thermo-color dye was captured with an IR camera and a corresponding digital camera, as shown in Fig. 2c and insets. The thermo-color dye demonstrated the temperature changes; it was blue at a low temperature and white at a high temperature. Therefore, a thermo-color dye is an indicator of temperature suitable for general use.

Fig. 2 (a) Dependence of temperature on time by applying different voltage to a 2 cm × 2 cm sample of ADNC, (b) the temperature response curve during the cycling heating test of ADNC, and (c) infrared (IR) camera images of ADNC at an applied voltage of 0.7 V; the inset is the corresponding image of ADNC dyed with thermo-color dyes and captured by digital camera.

To investigate the durability of ADNC, the electrical resistance of a 2 × 2 cm² ADNC sample was measured under a bending curvature radius of ∼0.5 cm for 2000 bending cycles (Fig. 3a). The resistance was maintained at a constant value during the cyclic bending test (Fig. 3b). Tape testing was also performed, and the results are shown in Fig. S2.† To investigate the washability of ADNC, sheet resistance was used to characterize the change of ADNC before and after washing in swirling clean water (details in ESI†). As shown in Fig. 3c, each measurement was conducted after the samples were dried. The resistance remained stable for 10 cycles, showing the high washability of ADNC. Compared to previous reports,³ the electrical resistance did not severely decrease after the first washing cycle, which might be due to the chemical combination of AgNW with cotton by DOPA.

Fig. 3 (a) ADNC sample before (left) and after (right) bending with a curvature radius of ∼0.5 cm, (b) change of resistance after 2000 bending tests on ADNC, and (c) change of resistance after several washing tests on ADNC.

One concern about metal nanofibers is their chemical stability against oxidation in the air. For example, an increase in sheet resistance from 10 to 18 Ω was observed for a transparent Cu nanofiber electrode under ambient conditions for 3 months due to slow oxidation.²¹ For ADNC in which AgNWs are exposed to ambient atmosphere, a common practice of encapsulation is needed to increase long-term stability. We studied the addition of a self-cleaning function to ADNC to keep it clean without washing or to decrease cleaning times and prolong the life expectancy of ADNC. In our previous work,⁴⁹ we demonstrated that the 1-octadecanethiol (ODT)-modified noble metal materials exhibit good superhydrophobic stability by applying various external forces. And according to the literature, catechols can react with thiols via a Michael addition or Schiff base reactions.⁵⁰ Thus, we soaked ADNC in an ODT ethanol solution to prepare the hydrophobic surface through thiol-adduct formation (Fig. 4a). The XPS spectra revealed the presence of ODT in the modified surface (Fig. 4b and c). The arrow in Fig. 4b represents the sulfur 2p (163 eV) signal derived from the surface-immobilized ODT molecules.³¹ Interestingly, the surface hydrophobicity of ODT-modified ADNC increased while maintaining good conductive properties (Fig. 4d). In addition, ADNC kept under ambient conditions for 3 months showed a sheet resistance that was almost unchanged (7.13 Ω □⁻¹ vs. 7.53 Ω □⁻¹). By comparing Fig. 4e with 1c, it appears that the surface topography of ADNC barely changed after modification.

Fig. 4 (a) Schematic illustration of alkanethiol monolayer grafting on a polydopamine-coated surface, (b) the XPS survey spectrum after reaction between octadecanethiol and ADNC, (c) the high-resolution spectrum of the sulfur 2p region marked by the arrow, (d) the change in the average sheet resistance of ADNC sample before (left) and after (right) modified with ODT; the inset is the corresponding water contact angle images, and (e) the SEM image of the surface topography of ODT-modified ADNC; the inset is the enlarged view.

4. Conclusions

In summary, we fabricated a personal thermal management cloth based on the sliver nanowires (AgNWs)/polydopamine nanocomposite that had high safety, durability and washability. ADNC has a significantly high IR reflectance, suggesting that it has a higher thermal insulation capability than normal cloth. In addition to an excellent electrical property of 7.13 Ω □⁻¹ due to the dense electrical connection of Ag nanowires, ADNC exhibited outstanding stability against repeated external deformations (2000 bending tests). We demonstrated that ADNC is capable of heating from 22 °C to 42 °C within 1 min under 0.8 V. Further, the anti-oxidative capability and self-cleaning property of ADNC can be improved by ODT modification while still maintaining the good conductive properties. Moreover, there are numerous technical advantages because ADNC is fabricated using an inexpensive, environment-friendly, efficient and easily scalable manufacturing process. These promising results exemplify the practicability of using ADNC as a personal thermal management cloth.

Acknowledgements

This work is supported by the National Natural Science Foundation of China (No. 11274328 and 51325203), the Ministry of Science and Technology of the People's Republic of China (No. 2014AA032802), the Science and Technology Commission of Shanghai Municipal (No. 13521102100 and 15XD1501700).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra13104b

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