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Multifunctional Janus fibrous hybrid membranes with sandwich structure for on-demand personal thermal management
Nano Energy 63 (2019) 103808
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Multifunctional Janus fibrous hybrid membranes with sandwich structure for on-demand personal thermal management
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Xuejie Yuea, Tao Zhanga,b,*, Dongya Yanga, Fengxian Qiua,**, Gengyao Weia, Hao Zhoua a b
School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang, 212013, Jiangsu Province, China Institute of Green Chemistry and Chemical Technology, Jiangsu University, Zhenjiang, 212013, Jiangsu Province, China
ARTICLE INFO
ABSTRACT
Keywords: Personal thermal management Janus membrane MnO2 Infrared emissivity Cu nanowires
Developing functional Janus membrane with asymmetric infrared emissivity for personal thermal management is of significance but challenging, due to the difficulty in controlling the infrared insulation and infrared dissipation to satisfy the personal thermal comfort. Herein, a multifunctional Janus Cu/MnO2/cellulose@Layered Double hydroxide fiber (CMCFL) membrane with sandwich structure has been fabricated by vacuum filtrating ultralong MnO2 nanowires and Cu nanowires sequentially on cellulose fiber@Layered Double Hydroxide (LDH) basement membrane. The obtained CMCFL membrane allows for an integration of low infrared emission, promising electrical conductivity, antibacterial property from Cu nanowire layer, and high infrared emission from cellulose@LDH layer. Moreover, the Janus CMCFL membrane shows the asymmetrical characteristics of infrared radiation for on-demand personal thermal management: the low-emissivity layer (Cu nanowires layer) is facing outside to reduce the human thermo radiation, and high-emissivity layer (cellulose@LDH layer) is facing outward to enhance the human thermo radiation. In addition, the promising electrical conductivity of Cu nanowires layer endows the superior Joule heating for extra warmth of 19 °C using a low supply voltage around 8.4 V to enhance the thermal comfort in the cold environment. Besides, the obtained Janus CMCFL membrane not only shows good breathability and flexibility, but also possesses other desired properties including excellent interfacial compatibility and antibacterial activity for wearability. The outstanding integrated properties and corresponding design strategy of Janus CMCFL membrane are expected to be applicable in the fields of personal thermal management, providing a promising direction for the development of wearable textile to enhance the adaptability of human skin to the environment.
1. Introduction To maintain the body at a metabolically favorable temperature is vital for the proper function of humans. Although the human body has a precise and reliable body temperature regulation mechanism including the perspiration, piloerection, shivering, and blood circulation, fluctuating weather can destroy the thermal comfort and even hurt people's overall health [1]. For example, under the sudden and dramatic changes in temperature, people are prone to suffer from respiratory infections and even heart-related diseases due to the imbalance of immune system [2]. In the indoor environment, there are usually two ways to maintain the human thermal comfort: keeping the suitable temperature for the use of air conditioning equipment and directing adjustment the human body temperature using clothing. For air conditioning, a large portion of energy waste to the vast empty space and
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inanimate objects of the building, resulting in enormous energy consumption and contributing to the global warming [3,4]. In the US, 37% of the primary energy consumption is spent simply on indoor temperature control [5]. Clothing is another strategy to maintain the human body temperature. However, traditional clothes usually fail to adapt to the fluctuating weather due to their unchangeable thermal insulation performance [6]. Recently, personal thermal management has been successfully demonstrated to maintain thermal human comfort through heating or cooling textiles [7]. Personal thermal management fabric material with controllable thermal insulation performance could have great potential since it not only provides localized thermal control near the human body instead of the entire building, but also can on-demand maintain the human body temperature according to changes in the surrounding environment [8]. Based on the radiation heat transfer law, control of human body
Corresponding author. School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang, 212013, Jiangsu Province, China. Corresponding author. E-mail addresses: [email protected], [email protected] (T. Zhang), [email protected] (F. Qiu).
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https://doi.org/10.1016/j.nanoen.2019.06.004 Received 7 February 2019; Received in revised form 29 May 2019; Accepted 3 June 2019 Available online 20 June 2019 2211-2855/ © 2019 Elsevier Ltd. All rights reserved.
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infrared radiation flow has a great impact on the personal thermal management. Currently, development of personal thermal management materials with human body infrared radiation control has garnered considerable attention. For example, Mylar blankets with a dense metallic coating are commercially available, and the metallic coating can work as the infrared reflection layer to block the radiation heat loss [9]. However, the lack of breathability causes the user uncomfortable, dramatically limiting their widespread applications for daily use. To obtain good breathability, Cai et al. [10] fabricated a nanoporous metallized polyethylene by constructing a metallic infrared reflective layer in the polyethylene with embedded nanopores, simultaneously having minimal infrared emissivity and good breathability. Similarly, Liu et al. [11] have presented a multifunctional cloth using the cotton fabric as the starting material, one side of which was modified for hydrophobic layer, while the other side can work as an infrared reflector by depositing a nanoporous silver layer. Moreover, the porosity of cotton cloth is retained, allowing the fabric to be breathable. Despite tremendous advancements of human body infrared radiation control for thermal management, most of the personal thermal management materials focus on the radiative heating textiles, ignoring the demand for cooling. Although several technologies provide the radiative human body cooling through use infrared-transparent nanoporous polyethylene textile [12,13], to date, it is desirable but remains a great challenge to develop an efficient thermal management material with both infrared insulation and infrared dissipation features in one material for on-demand personal thermal management. In order to achieve the integration of human body heating and cooling functions in one material, Janus membrane materials, an emerging concept which attracts widespread interest, can provide a promising alternative to the traditional membrane materials with single property in a wide range of applications [14–16]. Janus membrane shows highly asymmetric properties on each side, endowing such membranes with various characteristics and widely applications in nature. For example, lotus leaf with superhydrophobicity on one side and hydrophilicity/superoleophobicity on the other side displays dual functional property, which allows self-cleaning in the air and anti-oilfouling in water, and simultaneously protect the lotus leaf from pollution [17]. Moreover, the inter-relationship between the two sides endows the Janus membrane with promising applications in many emerging fields, such as fog collection, biomimetic ion channel, solar distillation, and switchable permeation [18–20]. Although extensive research has been performed on Janus membrane materials for various applications, little information is available on infrared radiation Janus membrane materials. Janus membranes with asymmetric infrared radiation properties could be a good candidate, because such asymmetric infrared radiation enabled the controllable infrared emissivity to help the users adapt to the changing environment. By taking advantage of Janus membrane, a Janus-type membrane with sandwich structure and asymmetric infrared emissivity has been fabricated based on vacuum filtration technology for on-demand personal thermal management. The high infrared emissivity surface was firstly prepared by in situ constructing LDH layer on the surface of the cellulose membrane with high roughness, and subsequently coating with ultralong MnO2 nanowires and Cu nanowires, respectively. The ultralong MnO2 layer was used as the transition layer to achieve the strong bonding and high interfacial compatibility of Janus membrane. The Cu nanowires layer plays three important roles as an infrared reflector for heating, a flexible heating providing additional Joule heat for enhanced heating, and an antibacterial layer. Due to the joined presence of high infrared emissivity and low infrared emissivity sides, the Janus membrane shows two different infrared radiation properties. The Janus membrane can achieve the cooling mode when the high-emissivity layer is facing outside, and heating mode can be achieved by wearing the Janus membrane inside out, resulting that the Cu-NWs layer is facing outside. Moreover, the Joule heat of the Cu-NWs provides additional Joule heat using a low supply voltage. In addition, the
multifunctional Janus membrane can maintain excellent breathability like cotton cloth and antibacterial property. 2. Experimental section 2.1. Materials Filter paper (moderate speed) was purchased from special paper Co., Ltd (Hangzhou, China). Aluminum nitrate nonahydrate (Al (NO3)3·9H2O), ammonium hydroxide (NH3·H2O), magnesium nitrate hexahydrate (Mg(NO3)2·6H2O), hexamethylenetetramine (C6H12N4), magnesium sulfate heptahydrate (MgSO4·7H2O), potassium sulfate (K2SO4), potassium persulphate (K2S2O8), copper(II) sulfate pentahydrate (CuSO₄·5H2O), ethylenediamine (C2H8N2), polyvinylpyrrolidone (PVP), 1-Hexadecylamine (C16N35N), ethylenediamine (C2H8N2) and hydrazine hydrate (N2H4·H2O) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China), and they were analytical grade and used as received without further purification. Distilled water was used throughout the experiment. 2.2. Preparation of the Janus membrane The Janus CMCFL membrane with sandwich structures was obtained by layer-by-layer assembly using the cellulose@LDH fiber, ultralong MnO2 nanowires and Cu nanowires as building blocks. Methods of preparation of the ultralong MnO2 nanowires, Cu nanowires, and cellulose@LDH fiber basement membrane were provided in the supporting information. In a typical procedure, 20 mL of the obtained MnO2 nanowire suspension was dispersed in 30 mL of ethanol under sonication. Subsequently, the above MnO2 nanowire uniform suspension was filtered on the cellulose fiber@LDH fiber basement membrane (the diameter is 4 cm) by a vacuum-filtration setup with the vacuum degree at 0.05 MPa. After that, 30 mL of Cu nanowires suspension with the concentration of 0.15 g/L was filtered on the surface of MnO2 nanowire by the same vacuum filtrating process. Finally, the obtained trilayer Janus CMCFL membrane was incubated in the oven at 50 °C for 18 h under 2 kPa pressure, ensuring that the MnO2 layer and Cu nanowires layer are packed closely. The schematic illustration of the preparation method of the trilayer Janus membrane was shown in Fig. 1. 2.3. Thermal management performance Thermal management properties were investigated by Joule heating and infrared radiation processes. For Joule heating test, a trilayer Janus CMCFL membrane with 4 cm diameter was fixed on each side by two wire clamps. The voltage was provided by a Direct Current (DC) regulated power supply (YB1730A) and the temperature was recorded by using a thermal couple. The heating properties of the Janus CMCFL membrane are investigated by measuring the change of temperature. Human body thermal radiation was obtained according to the Planck's law at the skin temperature of 34 °C [21]. The thermal insulation properties of the Janus CMCFL membrane are evaluated by the infrared emissivity, which were measured with a fourier transformation infrared spectrometer with Gold integrating sphere. All thermal images were taken by an infrared camera (FLIR ONE Gen 3) with a working distance approximately 30 cm. 2.4. Antibacterial experiment The antibacterial test of the Janus CMCFL membranes was done against Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) with the plate count method. Typically, trilayer Janus CMCFL membranes and culture media were first sterilized at 121 °C for 10 h. Then E. coli and S. aureus solution (300 μL, 107 CFU/mL) was spread on culture media respectively. The sample was cut into a 6 mm circle and placed 2
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Fig. 1. Schematic illustration of the preparation method and cooling mode and heating mode of trilayer Janus membrane with sandwich structure.
Fig. 2. Schematic illustration of the preparation process of trilayer Janus CMCFL membrane (A). SEM images of LDH/cellulose fiber layer (B and C), MnO2-NWs layer (D and E), and Cu-NWs layer (F and G) of the trilayer Janus CMCFL membrane, respectively.
into the agar plate. After culturing at 37 °C for 24 h, the forming inhibitory zone was used to evaluate the antimicrobial properties of samples.
2.6. Sample characterization Surface and cross-sectional morphology of the trilayer Janus CMCFL membranes were observed using a field emission scanning electron microscopy (SEM, HITACHI, S-4800, Japan). Static contact angle (CA) in air for both sides of the Janus membrane was measured using an optical contact angle meter (DSA100, KRUSS, Germany). The crystalline structures of the trilayer Janus CMCFL membranes were analyzed using a Shimadzu XRD-6100 instrument. X-ray photoelectron spectroscopy (XPS) was performed using ESCALAB 250Xi-AER to analyze the chemical composition of the Cu nanowires.
2.5. Breathability experiment Breathability was evaluated by comparing the water vapor transmission rate of samples. In a typical procedure, a total of 40 g of desiccant (95 wt% CaSO4 and 5 wt% silica gel) was thoroughly dried in an oven at 120 °C for 48 h. Then the desiccant was placed into a glass vial with open-top cap. The trilayer Janus membrane and ordinary cotton cloth were used to seal the glass vial, respectively. All the glass vials were tested simultaneously in the same environment and the total mass of the glass vial was recorded periodically. 3
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3. Results and discussion
MnO2 nanowires show excellent self-entanglement and entanglement with other fibers to enhance the stability and mechanical strength of the Janus membrane, which is demonstrated by our previous work [22]. The SEM images in Fig. 2D and E show the general morphology of the MnO2 nanowires layer. The MnO2 nanowires layer consisted of a large amount of randomly oriented ultralong nanowires with lengths of more than 120 μm. These nanowires intertwined with each other to form a porous structure, indicating the extraordinary flexibility. The highmagnification SEM image in Fig. 2E reveals that the network skeleton originates from entangled nanowires bundles and nanowires. The nanowires bundles compose of smooth nanowires, which have a uniform diameter of ~50 nm, indicating exceptionally large aspect ratios. It is noted that the surface of the MnO2 nanowires layer is relatively flat, and no splits are detected, due to the high flexibility and large aspect ratios. Compared to the rough surface of cellulose@LDH layer, the MnO2 nanowires layer could provide a much flatter substrate for the depositing of Cu nanowires layer. The forming mechanism of MnO2 nanowires layer was elucidated by depositing different volumes of MnO2 nanowires suspension (Figure S4). Although there is a huge difference in the size between MnO2 nanowires and cellulose@LDH fibers, the MnO2 nanowires can closely attached and wrapped around the surface of cellulose@LDH fibers. With an increase in the nanowires loading, MnO2 nanowires tend to self-entangle and entangle with cellulose@LDH fibers, forming a porous layer. This growth process would facilitate a good interfacial compatibility between cellulose fiber@LDH membrane and MnO2 nanowires layer, which could potentially enhance the mechanical stability of laminated membrane. Usually, a network made of low emissivity nanomaterials can emit less infrared radiation generated from the human body to reduce the heat loss. The general and detailed morphologies of the Cu nanowires layer are shown in Fig. 2F and G. Fig. 2F illustrates that plenty of Cu nanowires randomly arrange together, forming a connected porous network, which is also the key factor of electronic transportation for Joule heat. Cu nanowires with the diameter ranging from 50 to 200 nm show a wide length distribution ranging from 50 to 100 μm, with an aspect ratio of above 200. The high magnification SEM image in Fig. 2G indicates that a small number of Cu nanoparticles attach to nanowires surface. They may be extra copper seed particles during the Cu nanowires growth process. The low-emissary layer needs high infrared reflectively. The average pore of the Cu nanowires layer is about 4 μm, which is smaller than the infrared emission of the human body which focuses in the mid-infrared range between 7 and 14 μm, indicating that the potential high infrared reflectivity for human thermal insulation. Moreover, the interconnected porous structure can provide access for air and water vapor transmission.
3.1. Morphology and structural characterization The fabrication process of trilayer Janus CMCFL membrane is illustrated schematically in Fig. 1. The cellulose fiber@LDH membrane was used as the base membrane, and the MnO2 nanowires layer were coated onto cellulose fiber@LDH membrane via a vacuum filtration technique. At last, another Cu nanowires layer was prepared to cover the MnO2NWs layer. Actually, the MnO2 nanowires can be entirely retained by the frameworks that constructed by cellulose fiber@LDH and Cu nanowires can also be retained by the dense MnO2 nanowire layer, which was confirmed by UV measurements for the filtrate from MnO2 nanowire suspension and Cu nanowires suspension (Figure S1). Namely, a sandwiched Janus CMCFL membrane composed of cellulose fiber@LDH membrane, MnO2-NWs layer and Cu nanowires layer was obtained. The trilayer Janus membrane was obtained by layer-by-layer assembly, allowing for an integration of different materials, as shown in Fig. 2A. The morphology of the obtained Janus membrane was evaluated by SEM analysis. The cellulose fiber@LDH membrane is derived from pristine cellulose fiber membrane with a smooth surface consisted of cellulose fibers with lengthens of ~300 μm, which crosslink with each other, forming porous structures (Figure S2). To meet the high infrared emission for person thermal management applications, the hierarchical cellulose@LDH fibers layer was obtained by in situ growth of oriented LDH films on the cellulose fiber membrane. These fibers displayed a wide diameter of ~25 μm, providing enough space for the fabrication of micro- and nano-texture, providing the potential to fabricate rough surface. As compared with the pristine cellulose fiber membrane, the cellulose fiber membrane after the facile hydrothermal process shows high rough surface, due to the forming of hierarchical Mg-Al LDH microcrystals on the surface of cellulose fiber (Fig. 2B). The surface roughness of CMCFL membrane has a significant effect on thermal management properties. More details of Mg-Al LDH coated cellulose fiber membrane are shown in Fig. 2C. The LDH microcrystals, with unified plate-like morphology, stand vertically on the surface of the cellulose fibers randomly, indicating the high quality of the LDH microsheets. Obviously, densely LDH microsheets formed a nest-like hierarchical porous structure. The nest-like hierarchical porous structure can increase the area of infrared absorption and decrease the reflection, and thus enhances the capture of infrared energy from the surrounding environment, resulting in higher infrared emission. Moreover, the elemental mapping images and EDS analysis of cellulose@LDH fibers affirm the successful coating and the uniform distribution of Mg-Al LDH nanosheets on the cellulose surface (Figure S3). Due to the extraordinary flexibility and cross-link effect, ultralong
Fig. 3. (A) The XRD patterns of cellulose fiber@Mg-Al LDH base membrane, MnO2 nanowires, and Cu nanowires, respectively. (B) The XRD patterns of both sides of the trilayer Janus membrane.
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Usually, the crystal phase and chemical composition of materials have an important influence on their infrared radiation properties, the crystal structures of materials were characterized by the XRD, and results are presented in Fig. 3. As shown in Fig. 3A, the XRD pattern of cellulose@LDH fiber layer shows two intense peaks at 10.5° and 21.9°, which are indexed to (003) and (006) reflections, indicating the presence of the typical hexagonal system LDH compound and good crystallinity of Mg-Al LDH. The weak and broad diffraction peaks marked with asterisks at 14.68° and 16.56° can be assigned to the cellulose I crystalline structure of the cellulose fiber. These results can provide evidence for the incorporation of Mg-Al LDH on the cellulose fiber. Apart from these two peaks, other peaks are in good qualitative agreement with the pattern of Mg-Al LDH crystal (JCPDS: 14–0191). For ultralong MnO2 nanowires, all the diffraction peaks match well with the standard card of typical tetragonal α-MnO2 (JCPDS card 72–1982), indicating the high quality of ultralong MnO2 nanowires. The Cu nanowires showed an XRD pattern with three clearly distinguishable diffraction peaks at 2θ = 43.2°, 50.3°, and 74.4°, corresponding to (111), (200), and (220) crystals planes of face-centeredcubic Cu, which were indexed as per JCPDS card no. 03–1005. The strong and sharp peaks prove the high quality of the Cu nanowires. It is worth noting that the XRD pattern shows no evidence of other impurities like CuO or Cu2O, indicating the high purity of the Cu nanowires for low infrared emissivity. For trilayer Janus CMCFL membrane, the XRD patterns of both sides of it exhibit the typical diffraction peaks of Cellulose@LDH fiber layer, MnO2 nanowires layer, and Cu nanowires layer, indicating the integration of them in one Janus membrane (Fig. 3B). To minimize the spontaneous oxidation process, the Cu nanowires were protected by a PVP layer on their surface. As shown in Figure S5, a PVP layer with a thickness of about 1.6 nm appears on the surface of the Cu nanowire, and the surface information of the Cu nanowires can be properly obtained using XPS techniques. In Fig. 4A, the XPS survey spectrum of Cu nanowires shows a pronounced C 1s peak at around 284.6 eV, various Cu binding energy peaks, and a weak O 1s peak at around N 1s, confirming the presence of PVP molecules. For clarity, the
fitted XPS spectra of C 1s indicates four peaks of individual components at 284.8, 285.6, 286.1 and 287.8 eV, which can be attributed to carbon atoms C1 to C4 of PVP molecule (the inset image in Fig. 4B), respectively, based on the different chemical environments. Furthermore, in the O 1s spectrum in Fig. 4C, Cu nanowires have two clear peaks at 531.6 and 532.7 eV, which are assigned to carboxyl (C=O) and hydroxyl (C-OH) oxygen atoms, respectively. Interestingly, compared to the O 1s peak from carboxyl (C=O) carbon in the pure PVP, the peak of Cu nanowires shifts to higher binding energy, suggesting the enhanced electron density, because of the interaction between PVP molecule and the Cu nanowire. The presence of the N 1s peak in Fig. 4D can be a relatively reliable indication that PVP is absorbed on the surface of the Cu nanowires. Moreover, the N 1s peak at 399.6 eV does not seem to be influenced by the Cu nanowires. In Fig. 4E, there are no peaks at 940 eV around, suggesting that there are negligible Cu2O or CuO in the Cu nanowires. The high quality of Cu nanowires can result from the PVP coating of Cu nanowires, which is a good barrier to prevent surface oxidation of Cu nanowires. 3.2. Interface stability and wettability For a practical application, another interfacial stability between three layers should be carefully considered for the stability of the obtained Janus membrane. The interfacial compatibility of the Janus membrane was characterized by a soak test. As shown in Fig. 4A, the Janus membranes (1 cm × 4 cm) were fully immersed in water. After 48 h of soaking, the water in the bottles was transparent, and no detachment was observed. Moreover, the SEM images in Figure S6A and B showed no change of the two sides of the Janus membrane. The interfacial compatibility was further evaluated in the hot water soak test. As excepted, the similar results appeared, as shown in Fig. 5B and Figure S6C and D. Apart from the visual observation (Fig. 5A and B) and SEM (Figure S6), UV–Vis measurements (Figure S7) also indicate that the Janus membrane exhibited a satisfactory stability, which can be attributed to the strong compatible entanglement between three separated layer.
Fig. 4. (A to E) XPS spectra of the Cu nanowires and O1s, C1s, N1s, and Cu 3p3/2 regions.
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radiative heat dissipation. Usually, the control of infrared emissivity on the outer surface of textiles can have a substantial impact on regulating radiative heat of clothed human for on-demand personal thermal management. To achieve warmth in the cold environment, the infrared radiation loss should be suppressed, because the continuous infrared emission from the human body into the ambient air is the main part of the heat loss. Usually, normal cotton textiles with a high emissivity of 0.895 lack the human radiation control in the cold environment [10]. By contrast, the Cu nanowires layer of trilayer Janus CMCFL membrane show the desired infrared radiation properties, and the results are shown in Fig. 6. In Fig. 6A, the emissivity of Cu nanowires layer is approximately 0.3–0.5 between the wavelengths of 2–18 μm. The weighted average emissivity based on human body radiation was 0.436 for the Cu nanowires layer, indicating the low emissivity, compared to the normal cotton cloth. Fig. 6B illustrates the thermal image of a hand with both the normal cotton and Janus membrane with heating mode (Cu nanowires layer facing outsides). They are in thermal equilibrium on the hand at atmosphere temperature of 18 °C. As shown in Fig. 6B, due to the low emissivity, the Cu nanowire layer emits less infrared radiation than normal cotton, making the Janus membrane appear “cold”. The infrared radiation from normal cotton was detected by an infrared camera and the obtained temperature is 26.4–28.6 °C. While the temperature of the Janus membranes is lower, in the range of 24.6–26.1 °C, indicating a decreasing of 1.8–2.5 °C. The Janus membrane and normal cotton were further compared under a hot plate with a high temperature of 75 °C. Under thermal equilibrium, the center temperatures of Cu nanowire layer and normal cotton were 51.5 and 64.0 °C, respectively, indicating Cu nanowires layer is low infrared emitter to decrease the heat loss (Fig. 6C). These results demonstrate the effectiveness of the strategy of using the trilayer Janus CMCFL membrane with Cu nanowire layer facing outside on warming human bodies. An illustration, as shown in Fig. 6D, is provided to understand the heating mode of the Janus membrane. The Cu nanowires layer with low infrared emission can work as a low infrared emitter and suppress human body infrared
Figure 5. (A and B) Optical photos of the trilayer Janus CMCFL membrane before and after soaking in water with different temperature. (C and D) The wettability of the two sides of the trilayer Janus CMCFL membrane.
In addition, the wettability of the trilayer Janus membrane was evaluated by the static water contact angle test, as illustrated in Fig. 4C and D. When the water droplets are dropped on the two sides of the trilayer Janus CMCFL membrane, the two sides can be wetted by water droplet with water contact angle of ~ 0°, indicating the superhydrophilicity of the Janus membrane, endowing the hygroscopicity and breathability of the Janus membrane. 3.3. Thermal control and electrical property The objective of on-demand thermal management is to maintain the thermal comfort of the human in different environments by controlling
Fig. 6. The infrared emissivity of the Cu nanowires layer and the human body radiation at 33 °C (A). The thermal image of the cotton cloth and Janus membrane with Cu nanowires layer facing outside on a hand (B) and a hot plate (C). Schematics depicting the heating mode of the trilayer Janus CMCFL membrane (D). 6
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Fig. 7. Joule heat property of the Janus membrane. (A) Temperature change vs time after applying different voltages. (B) Temperature difference of the heater compared to room temperature based on supplied voltage. (C) Infrared images of Janus CMCFL membrane applied a voltage of 12.1 V.
same palm temperature, but the Janus membrane can emit more infrared radiation for the human cooling due to the higher infrared emissivity. The cooling property of Janus membrane was further demonstrated by evaluating the skin temperature under different conditions (Fig. 8C). Under the bare skin condition, the skin temperature is about 31.3 °C. Placing a piece of normal cotton cloth onto the skin increases its temperature to 36.2 °C (an increase of 3.9 °C). Janus membrane with cooling mode (cellulose@LDH layer facing outside) is placed onto the skin, and the temperature of the skin is 33.8 °C, indicating a small skin temperature rise (2.5 °C). Thus, the cooling mode can enhance the adaptability of human skin to the environment. Moreover, an illustration (Fig. 8D) is provided to understand the cooling mode of the Janus membrane. When the cellulose@LDH fiber layer is facing out, the high infrared emissivity can allow more radiative heat dissipation from a clothed human body to the ambient environment, and the Janus membrane works in cooling mode.
radiation loss; thus, the Janus membrane works in heating mode. Active warming up is one of the most important features of the Janus membrane under cold environment. Fig. 7A shows the generated temperature profiles with variations of the supplied voltages. The temperature was measured by a thermal couple in close contact with the samples. Specially, for Janus membrane, a low voltage supply nearly 8 V can already meet the normal requirement of the human body. This is due to the high electrical conductivity of Cu nanowires layer. To ensure safety of the human body, 8–12V is more than enough, however, higher temperature can also be achieved. When the supply voltage reaches 24V, the temperature difference becomes up to 80 °C, which might be useful in some limited conditions (Fig. 7B). All the heating procedures were accomplished within the initial 100 s and remained stable before power off. Moreover, the electrothermal response was confirmed by an infrared thermal camera under a typical supply voltage of 8.4 V (Fig. 7C). As can be seen, it only takes 15 s for the Janus membrane to rise from room temperature to 36 °C. In addition to the satisfied temperature response, a homogeneously distributed heat flow was observed due to the high thermal conductivity of Cu nanowires layer. In heating process, the hot Janus membrane is the primary heat source which can transfer heat to surrounding of human body by radiation and conduction, in which both thermal radiation and conduction are beneficial. Moreover, it can act as a great bodyguard against human body radiative heat loss after Joule heating under cold temperature conditions (such as < 15 °C). Human skin is a good infrared emitter with an excellent emissivity of 0.98 [21]. The normal cotton cloth usually has a lower infrared emissivity of 0.895, resulting in inhibition of human body heat dissipation under a hot environment. An excellently high infrared emissivity is necessary for the human body heat dissipation. As shown in Fig. 8A, the cellulose@LDH fiber layer shows an extremely high infrared emissivity with a weighted average value of 0.973, which is very close to human skin radiation cooling performance. Fig. 8B shows the thermal image of the normal cotton and Janus membrane with cooling mode (cellulose@LDH layer facing outside). For the normal cotton cloth, the temperature obtained by infrared camera is in a range of 32.1–33.4 °C, which is smaller than the temperature of Janus membrane (in a range of 32.8–34.1 °C). The two samples are tested under the
3.4. Breathability and antibacterial property When the human skin perspires, the Janus membrane must not prevent the moisture efflux. The breathability of the Janus membrane was tested by weighing the mass increase of desiccant sealed with a normal cotton cloth and Janus membrane. To avoid the basic by environment temperature and humidity, all the samples are tested at the same time and the same environment. During the test, water vapor in the air permeates the samples and finally is captured by the desiccant, leading to the increase in weight. In Fig. 9A, the results show that the water vapor permeabilities of Janus membrane and normal cotton cloth are 11 mg/cm2∙hr and 12 mg/cm2∙hr, respectively, with a slight weight gain rate reduction of only ~8.3% compared with the cotton cloth. This result displayed the extreme breathability of Janus membrane to ensure the comfortable wearing feeling. The antibacterial properties of the Janus membrane were tested on Gram-negative E. coli and Gram-positive S. aureus using the modified Kirby-Bauer method, in which the radius of inhibition zone was used to evaluate the antibacterial activity. As shown in Fig. 9B, inhibition zones of 12 mm and 13 mm in diameter were clearly observed around Janus membrane on the agar plates of E. coli and S. aureus, respectively, revealing the antibacterial property. 7
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Fig. 8. The infrared emission of the cellulose@ LDH layer and the human body radiation at 33 °C (A). The thermal image of the cotton cloth and Janus membrane with cellulose@LDH layer facing outside on a hand (B). The steady-state skin temperature of various conditions: bare skin, normal cotton, Janus membrane with cooling mode (C). Schematics depicting the cooling mode of the trilayer Janus CMCFL membrane (D).
Nanoscale copper can be responsible for the antibacterial property, which can restrain the reproduction of bacteria to lengthen the lifespan of the trilayer Janus CMCFL membrane. In addition, the Janus membrane can be washed by dry cleaning, and the electrical resistance of it shows a slight increase after six cycles of dry cleaning tests (Figure S8). The pores in the obtained Janus membrane plays several important roles. First, the superhydrophilic and porosity maintain excellent hygroscopicity (Fig. 5C and D) and breathability (Fig. 8A) of the Janus membrane, allowing water vapor to diffuse from one side to the other. The resistance to diffusion in perforated membranes has been studies with the equation [23]:
Rw =
t
+ 0.71d (
1
1
)
dense MnO2 nanowires layer and Cu nanowires layer contribute positively to the breathability of the Janus membrane. It might be the reason that although the Janus membrane was filled up with micro-/ nanopores, it showed good breathability like cotton cloth. The Janus membrane that is both superhydrophilic and breathable can provide the wearer with a high level of comfort. Second, the pore diameter of the Cu nanowires layer is smaller than the wavelength of the infrared radiation of the human body. Therefore, the Cu nanowires layer can work as a perfect infrared reflector without sacrificing the breathability of the Janus membrane. 4. Conclusions
(1)
In conclusion, a multifunctional trilayer Janus CMCFL membrane with sandwich structure was fabricated by vacuum filtrating ultralong MnO2 nanowires and Cu nanowires sequentially on cellulose@LDH fiber basement membrane. Compared to the normal cotton cloth, the asprepared Janus membrane can switch its radiative property to control human body infrared radiation for on-demand thermal management
Where Rw, is the resistance of the membrane, t is the thickness of the membrane, d is the average pores of the membrane, and β is the percentage area of the pores. According to the equation, the decreasing of pore size on a constant porosity and thickness can be beneficial to the water vapor transmission. Thus, the small pores that caused by the
Fig. 9. (A) The breathability test by weight mass increase of desiccants sealed with normal cotton cloth and Janus membrane, respectively. (B) The antibacterial property of trilayer Janus CMCFL membrane. 8
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based on the asymmetrical infrared emissivity. When Cu nanowires layer with a low weighted average emissivity of 0.476 is facing outside, reducing the human thermo radiation in the cold environment. The cooling mode can be achieved by wearing the membrane inside out when the cellulose@LDH fiber layer with an extremely high weighted average emissivity of 0.973 is facing outward. The great conductivity of Cu nanowires layer will further improve the heating mode by providing superior Joule heating for extra warmth of 19 °C using a low supply voltage around 8 V. Moreover, the obtained Janus membrane not only shows excellent superhydrophilicity, porous feature, and high breathability for wearing comfort, but also possesses desired antibacterial activity. These advantages can endow the Janus membrane applications in complex and varied environments without compromising breathability.
[16] Z. Wang, Y. Wang, G. Liu, Rapid and efficient separation of oil from oil‐ in‐ water emulsions using a Janus cotton fabric, Angew. Chem. Int. Ed. 55 (2016) 1291–1294. [17] H.-C. Yang, J. Hou, V. Chen, Z.-K. Xu, Janus membranes: exploring duality for advanced separation, Angew. Chem. Int. Ed. 55 (2016) 13398–13407. [18] M. Cao, J. Xiao, C. Yu, K. Li, L. Jiang, Hydrophobic/hydrophilic cooperative Janus system for enhancement of fog collection, Small 11 (2015) 4379–4384. [19] H. Dan, C. Chen, T. Ma, Y. Shang, B. Gao, B. Jin, Q. Li, Q. Yue, Y. Li, Y. Wang, X. Xu, In-situ pyrolysis of Enteromorpha as carbocatalyst for catalytic removal of organic contaminants: Considering the intrinsic N/Fe in Enteromorpha and non-radical reaction, Appl. Catal., B 250 (2019) 382–395. [20] Y. Ito, Y. Tanabe, J. Han, T. Fujita, K. Tanigaki, M. Chen, Multifunctional porous graphene for high-efficiency steam generation by heat localization, Adv. Mater. 27 (2015) 4302–4307. [21] J. Steketee, Spectral emissivity of skin and pericardium, Phys. Med. Biol. 18 (1973) 686. [22] X. Yue, T. Zhang, D. Yang, F. Qiu, Z. Li, Janus ZnO-cellulose/MnO2 hybrid membranes with asymmetric wettability for highly-efficient emulsion separations, Cellulose 25 (2018) 5951–5965. [23] M.E. Whelan, L.E. MacHattie, A.C. Goodings, L.H. Turl, The diffusion of water vapor through laminae with particular reference to textile fabrics:introduction, Text. Res. J. 25 (1955) 197–198.
Acknowledgements This work was financially supported by the Natural Science Foundation of Jiangsu Province (BK20160500, BK20161362 and BK20161264), State Key Laboratory of Pollution Control and Resource Reuse Foundation (NO. PCRRF18003), the National Natural Science Foundation of China (21706100 and 21878132), Postgraduate Research & Practice Innovation Program of Jiangsu Province (SJKY19_2577, SJKY19_2525, and SJCX19_1158), and Natural Science Foundation of Hebei Province (B2019108017).
Xuejie Yue started his Ph.D. degree in Jiangsu University. His research interest is mainly focused on the thermal management material for saving energy.
Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.nanoen.2019.06.004.
Tao Zhang received his Ph.D. degree in Material Physics and Chemistry from Southeast University in 2015. He is currently working toward the functional materials field in materials science and engineering in the Jiangsu University, China. His research interests include design, synthesis, and fabrication of biomass materials for advanced applications.
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Dongya Yang received her Ph.D. degree in materials science from East China University of Science and Technology in 2006. She is now working as an associate professor at School of Chemistry and Chemical Engineering, Jiangsu University. She mainly works on materials for hyperbranched polymers and nonlinear optical materials.
Fengxian Qiu obtained her Ph.D. degree in 2005 from Southeast University. And now she is a professor of materials physics and chemistry in Jiangsu University. She has published over 150 peer-review research papers and patents. Her research interests include energy-saving technology and chemical separation technology
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Hao Zhou is currently a postgraduate student in Jiangsu University of Science and Technology, China. Since 2018, he worked in Prof. Fengxian Qiu's research group at the Jiangsu University as an exchange student, working on biomass materials, especially as those relate to thermal management applications.
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Full paper
Multifunctional Janus fibrous hybrid membranes with sandwich structure for on-demand personal thermal management
T
Xuejie Yuea, Tao Zhanga,b,*, Dongya Yanga, Fengxian Qiua,**, Gengyao Weia, Hao Zhoua a b
School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang, 212013, Jiangsu Province, China Institute of Green Chemistry and Chemical Technology, Jiangsu University, Zhenjiang, 212013, Jiangsu Province, China
ARTICLE INFO
ABSTRACT
Keywords: Personal thermal management Janus membrane MnO2 Infrared emissivity Cu nanowires
Developing functional Janus membrane with asymmetric infrared emissivity for personal thermal management is of significance but challenging, due to the difficulty in controlling the infrared insulation and infrared dissipation to satisfy the personal thermal comfort. Herein, a multifunctional Janus Cu/MnO2/cellulose@Layered Double hydroxide fiber (CMCFL) membrane with sandwich structure has been fabricated by vacuum filtrating ultralong MnO2 nanowires and Cu nanowires sequentially on cellulose fiber@Layered Double Hydroxide (LDH) basement membrane. The obtained CMCFL membrane allows for an integration of low infrared emission, promising electrical conductivity, antibacterial property from Cu nanowire layer, and high infrared emission from cellulose@LDH layer. Moreover, the Janus CMCFL membrane shows the asymmetrical characteristics of infrared radiation for on-demand personal thermal management: the low-emissivity layer (Cu nanowires layer) is facing outside to reduce the human thermo radiation, and high-emissivity layer (cellulose@LDH layer) is facing outward to enhance the human thermo radiation. In addition, the promising electrical conductivity of Cu nanowires layer endows the superior Joule heating for extra warmth of 19 °C using a low supply voltage around 8.4 V to enhance the thermal comfort in the cold environment. Besides, the obtained Janus CMCFL membrane not only shows good breathability and flexibility, but also possesses other desired properties including excellent interfacial compatibility and antibacterial activity for wearability. The outstanding integrated properties and corresponding design strategy of Janus CMCFL membrane are expected to be applicable in the fields of personal thermal management, providing a promising direction for the development of wearable textile to enhance the adaptability of human skin to the environment.
1. Introduction To maintain the body at a metabolically favorable temperature is vital for the proper function of humans. Although the human body has a precise and reliable body temperature regulation mechanism including the perspiration, piloerection, shivering, and blood circulation, fluctuating weather can destroy the thermal comfort and even hurt people's overall health [1]. For example, under the sudden and dramatic changes in temperature, people are prone to suffer from respiratory infections and even heart-related diseases due to the imbalance of immune system [2]. In the indoor environment, there are usually two ways to maintain the human thermal comfort: keeping the suitable temperature for the use of air conditioning equipment and directing adjustment the human body temperature using clothing. For air conditioning, a large portion of energy waste to the vast empty space and
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inanimate objects of the building, resulting in enormous energy consumption and contributing to the global warming [3,4]. In the US, 37% of the primary energy consumption is spent simply on indoor temperature control [5]. Clothing is another strategy to maintain the human body temperature. However, traditional clothes usually fail to adapt to the fluctuating weather due to their unchangeable thermal insulation performance [6]. Recently, personal thermal management has been successfully demonstrated to maintain thermal human comfort through heating or cooling textiles [7]. Personal thermal management fabric material with controllable thermal insulation performance could have great potential since it not only provides localized thermal control near the human body instead of the entire building, but also can on-demand maintain the human body temperature according to changes in the surrounding environment [8]. Based on the radiation heat transfer law, control of human body
Corresponding author. School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang, 212013, Jiangsu Province, China. Corresponding author. E-mail addresses: [email protected], [email protected] (T. Zhang), [email protected] (F. Qiu).
**
https://doi.org/10.1016/j.nanoen.2019.06.004 Received 7 February 2019; Received in revised form 29 May 2019; Accepted 3 June 2019 Available online 20 June 2019 2211-2855/ © 2019 Elsevier Ltd. All rights reserved.
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infrared radiation flow has a great impact on the personal thermal management. Currently, development of personal thermal management materials with human body infrared radiation control has garnered considerable attention. For example, Mylar blankets with a dense metallic coating are commercially available, and the metallic coating can work as the infrared reflection layer to block the radiation heat loss [9]. However, the lack of breathability causes the user uncomfortable, dramatically limiting their widespread applications for daily use. To obtain good breathability, Cai et al. [10] fabricated a nanoporous metallized polyethylene by constructing a metallic infrared reflective layer in the polyethylene with embedded nanopores, simultaneously having minimal infrared emissivity and good breathability. Similarly, Liu et al. [11] have presented a multifunctional cloth using the cotton fabric as the starting material, one side of which was modified for hydrophobic layer, while the other side can work as an infrared reflector by depositing a nanoporous silver layer. Moreover, the porosity of cotton cloth is retained, allowing the fabric to be breathable. Despite tremendous advancements of human body infrared radiation control for thermal management, most of the personal thermal management materials focus on the radiative heating textiles, ignoring the demand for cooling. Although several technologies provide the radiative human body cooling through use infrared-transparent nanoporous polyethylene textile [12,13], to date, it is desirable but remains a great challenge to develop an efficient thermal management material with both infrared insulation and infrared dissipation features in one material for on-demand personal thermal management. In order to achieve the integration of human body heating and cooling functions in one material, Janus membrane materials, an emerging concept which attracts widespread interest, can provide a promising alternative to the traditional membrane materials with single property in a wide range of applications [14–16]. Janus membrane shows highly asymmetric properties on each side, endowing such membranes with various characteristics and widely applications in nature. For example, lotus leaf with superhydrophobicity on one side and hydrophilicity/superoleophobicity on the other side displays dual functional property, which allows self-cleaning in the air and anti-oilfouling in water, and simultaneously protect the lotus leaf from pollution [17]. Moreover, the inter-relationship between the two sides endows the Janus membrane with promising applications in many emerging fields, such as fog collection, biomimetic ion channel, solar distillation, and switchable permeation [18–20]. Although extensive research has been performed on Janus membrane materials for various applications, little information is available on infrared radiation Janus membrane materials. Janus membranes with asymmetric infrared radiation properties could be a good candidate, because such asymmetric infrared radiation enabled the controllable infrared emissivity to help the users adapt to the changing environment. By taking advantage of Janus membrane, a Janus-type membrane with sandwich structure and asymmetric infrared emissivity has been fabricated based on vacuum filtration technology for on-demand personal thermal management. The high infrared emissivity surface was firstly prepared by in situ constructing LDH layer on the surface of the cellulose membrane with high roughness, and subsequently coating with ultralong MnO2 nanowires and Cu nanowires, respectively. The ultralong MnO2 layer was used as the transition layer to achieve the strong bonding and high interfacial compatibility of Janus membrane. The Cu nanowires layer plays three important roles as an infrared reflector for heating, a flexible heating providing additional Joule heat for enhanced heating, and an antibacterial layer. Due to the joined presence of high infrared emissivity and low infrared emissivity sides, the Janus membrane shows two different infrared radiation properties. The Janus membrane can achieve the cooling mode when the high-emissivity layer is facing outside, and heating mode can be achieved by wearing the Janus membrane inside out, resulting that the Cu-NWs layer is facing outside. Moreover, the Joule heat of the Cu-NWs provides additional Joule heat using a low supply voltage. In addition, the
multifunctional Janus membrane can maintain excellent breathability like cotton cloth and antibacterial property. 2. Experimental section 2.1. Materials Filter paper (moderate speed) was purchased from special paper Co., Ltd (Hangzhou, China). Aluminum nitrate nonahydrate (Al (NO3)3·9H2O), ammonium hydroxide (NH3·H2O), magnesium nitrate hexahydrate (Mg(NO3)2·6H2O), hexamethylenetetramine (C6H12N4), magnesium sulfate heptahydrate (MgSO4·7H2O), potassium sulfate (K2SO4), potassium persulphate (K2S2O8), copper(II) sulfate pentahydrate (CuSO₄·5H2O), ethylenediamine (C2H8N2), polyvinylpyrrolidone (PVP), 1-Hexadecylamine (C16N35N), ethylenediamine (C2H8N2) and hydrazine hydrate (N2H4·H2O) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China), and they were analytical grade and used as received without further purification. Distilled water was used throughout the experiment. 2.2. Preparation of the Janus membrane The Janus CMCFL membrane with sandwich structures was obtained by layer-by-layer assembly using the cellulose@LDH fiber, ultralong MnO2 nanowires and Cu nanowires as building blocks. Methods of preparation of the ultralong MnO2 nanowires, Cu nanowires, and cellulose@LDH fiber basement membrane were provided in the supporting information. In a typical procedure, 20 mL of the obtained MnO2 nanowire suspension was dispersed in 30 mL of ethanol under sonication. Subsequently, the above MnO2 nanowire uniform suspension was filtered on the cellulose fiber@LDH fiber basement membrane (the diameter is 4 cm) by a vacuum-filtration setup with the vacuum degree at 0.05 MPa. After that, 30 mL of Cu nanowires suspension with the concentration of 0.15 g/L was filtered on the surface of MnO2 nanowire by the same vacuum filtrating process. Finally, the obtained trilayer Janus CMCFL membrane was incubated in the oven at 50 °C for 18 h under 2 kPa pressure, ensuring that the MnO2 layer and Cu nanowires layer are packed closely. The schematic illustration of the preparation method of the trilayer Janus membrane was shown in Fig. 1. 2.3. Thermal management performance Thermal management properties were investigated by Joule heating and infrared radiation processes. For Joule heating test, a trilayer Janus CMCFL membrane with 4 cm diameter was fixed on each side by two wire clamps. The voltage was provided by a Direct Current (DC) regulated power supply (YB1730A) and the temperature was recorded by using a thermal couple. The heating properties of the Janus CMCFL membrane are investigated by measuring the change of temperature. Human body thermal radiation was obtained according to the Planck's law at the skin temperature of 34 °C [21]. The thermal insulation properties of the Janus CMCFL membrane are evaluated by the infrared emissivity, which were measured with a fourier transformation infrared spectrometer with Gold integrating sphere. All thermal images were taken by an infrared camera (FLIR ONE Gen 3) with a working distance approximately 30 cm. 2.4. Antibacterial experiment The antibacterial test of the Janus CMCFL membranes was done against Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) with the plate count method. Typically, trilayer Janus CMCFL membranes and culture media were first sterilized at 121 °C for 10 h. Then E. coli and S. aureus solution (300 μL, 107 CFU/mL) was spread on culture media respectively. The sample was cut into a 6 mm circle and placed 2
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Fig. 1. Schematic illustration of the preparation method and cooling mode and heating mode of trilayer Janus membrane with sandwich structure.
Fig. 2. Schematic illustration of the preparation process of trilayer Janus CMCFL membrane (A). SEM images of LDH/cellulose fiber layer (B and C), MnO2-NWs layer (D and E), and Cu-NWs layer (F and G) of the trilayer Janus CMCFL membrane, respectively.
into the agar plate. After culturing at 37 °C for 24 h, the forming inhibitory zone was used to evaluate the antimicrobial properties of samples.
2.6. Sample characterization Surface and cross-sectional morphology of the trilayer Janus CMCFL membranes were observed using a field emission scanning electron microscopy (SEM, HITACHI, S-4800, Japan). Static contact angle (CA) in air for both sides of the Janus membrane was measured using an optical contact angle meter (DSA100, KRUSS, Germany). The crystalline structures of the trilayer Janus CMCFL membranes were analyzed using a Shimadzu XRD-6100 instrument. X-ray photoelectron spectroscopy (XPS) was performed using ESCALAB 250Xi-AER to analyze the chemical composition of the Cu nanowires.
2.5. Breathability experiment Breathability was evaluated by comparing the water vapor transmission rate of samples. In a typical procedure, a total of 40 g of desiccant (95 wt% CaSO4 and 5 wt% silica gel) was thoroughly dried in an oven at 120 °C for 48 h. Then the desiccant was placed into a glass vial with open-top cap. The trilayer Janus membrane and ordinary cotton cloth were used to seal the glass vial, respectively. All the glass vials were tested simultaneously in the same environment and the total mass of the glass vial was recorded periodically. 3
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3. Results and discussion
MnO2 nanowires show excellent self-entanglement and entanglement with other fibers to enhance the stability and mechanical strength of the Janus membrane, which is demonstrated by our previous work [22]. The SEM images in Fig. 2D and E show the general morphology of the MnO2 nanowires layer. The MnO2 nanowires layer consisted of a large amount of randomly oriented ultralong nanowires with lengths of more than 120 μm. These nanowires intertwined with each other to form a porous structure, indicating the extraordinary flexibility. The highmagnification SEM image in Fig. 2E reveals that the network skeleton originates from entangled nanowires bundles and nanowires. The nanowires bundles compose of smooth nanowires, which have a uniform diameter of ~50 nm, indicating exceptionally large aspect ratios. It is noted that the surface of the MnO2 nanowires layer is relatively flat, and no splits are detected, due to the high flexibility and large aspect ratios. Compared to the rough surface of cellulose@LDH layer, the MnO2 nanowires layer could provide a much flatter substrate for the depositing of Cu nanowires layer. The forming mechanism of MnO2 nanowires layer was elucidated by depositing different volumes of MnO2 nanowires suspension (Figure S4). Although there is a huge difference in the size between MnO2 nanowires and cellulose@LDH fibers, the MnO2 nanowires can closely attached and wrapped around the surface of cellulose@LDH fibers. With an increase in the nanowires loading, MnO2 nanowires tend to self-entangle and entangle with cellulose@LDH fibers, forming a porous layer. This growth process would facilitate a good interfacial compatibility between cellulose fiber@LDH membrane and MnO2 nanowires layer, which could potentially enhance the mechanical stability of laminated membrane. Usually, a network made of low emissivity nanomaterials can emit less infrared radiation generated from the human body to reduce the heat loss. The general and detailed morphologies of the Cu nanowires layer are shown in Fig. 2F and G. Fig. 2F illustrates that plenty of Cu nanowires randomly arrange together, forming a connected porous network, which is also the key factor of electronic transportation for Joule heat. Cu nanowires with the diameter ranging from 50 to 200 nm show a wide length distribution ranging from 50 to 100 μm, with an aspect ratio of above 200. The high magnification SEM image in Fig. 2G indicates that a small number of Cu nanoparticles attach to nanowires surface. They may be extra copper seed particles during the Cu nanowires growth process. The low-emissary layer needs high infrared reflectively. The average pore of the Cu nanowires layer is about 4 μm, which is smaller than the infrared emission of the human body which focuses in the mid-infrared range between 7 and 14 μm, indicating that the potential high infrared reflectivity for human thermal insulation. Moreover, the interconnected porous structure can provide access for air and water vapor transmission.
3.1. Morphology and structural characterization The fabrication process of trilayer Janus CMCFL membrane is illustrated schematically in Fig. 1. The cellulose fiber@LDH membrane was used as the base membrane, and the MnO2 nanowires layer were coated onto cellulose fiber@LDH membrane via a vacuum filtration technique. At last, another Cu nanowires layer was prepared to cover the MnO2NWs layer. Actually, the MnO2 nanowires can be entirely retained by the frameworks that constructed by cellulose fiber@LDH and Cu nanowires can also be retained by the dense MnO2 nanowire layer, which was confirmed by UV measurements for the filtrate from MnO2 nanowire suspension and Cu nanowires suspension (Figure S1). Namely, a sandwiched Janus CMCFL membrane composed of cellulose fiber@LDH membrane, MnO2-NWs layer and Cu nanowires layer was obtained. The trilayer Janus membrane was obtained by layer-by-layer assembly, allowing for an integration of different materials, as shown in Fig. 2A. The morphology of the obtained Janus membrane was evaluated by SEM analysis. The cellulose fiber@LDH membrane is derived from pristine cellulose fiber membrane with a smooth surface consisted of cellulose fibers with lengthens of ~300 μm, which crosslink with each other, forming porous structures (Figure S2). To meet the high infrared emission for person thermal management applications, the hierarchical cellulose@LDH fibers layer was obtained by in situ growth of oriented LDH films on the cellulose fiber membrane. These fibers displayed a wide diameter of ~25 μm, providing enough space for the fabrication of micro- and nano-texture, providing the potential to fabricate rough surface. As compared with the pristine cellulose fiber membrane, the cellulose fiber membrane after the facile hydrothermal process shows high rough surface, due to the forming of hierarchical Mg-Al LDH microcrystals on the surface of cellulose fiber (Fig. 2B). The surface roughness of CMCFL membrane has a significant effect on thermal management properties. More details of Mg-Al LDH coated cellulose fiber membrane are shown in Fig. 2C. The LDH microcrystals, with unified plate-like morphology, stand vertically on the surface of the cellulose fibers randomly, indicating the high quality of the LDH microsheets. Obviously, densely LDH microsheets formed a nest-like hierarchical porous structure. The nest-like hierarchical porous structure can increase the area of infrared absorption and decrease the reflection, and thus enhances the capture of infrared energy from the surrounding environment, resulting in higher infrared emission. Moreover, the elemental mapping images and EDS analysis of cellulose@LDH fibers affirm the successful coating and the uniform distribution of Mg-Al LDH nanosheets on the cellulose surface (Figure S3). Due to the extraordinary flexibility and cross-link effect, ultralong
Fig. 3. (A) The XRD patterns of cellulose fiber@Mg-Al LDH base membrane, MnO2 nanowires, and Cu nanowires, respectively. (B) The XRD patterns of both sides of the trilayer Janus membrane.
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Usually, the crystal phase and chemical composition of materials have an important influence on their infrared radiation properties, the crystal structures of materials were characterized by the XRD, and results are presented in Fig. 3. As shown in Fig. 3A, the XRD pattern of cellulose@LDH fiber layer shows two intense peaks at 10.5° and 21.9°, which are indexed to (003) and (006) reflections, indicating the presence of the typical hexagonal system LDH compound and good crystallinity of Mg-Al LDH. The weak and broad diffraction peaks marked with asterisks at 14.68° and 16.56° can be assigned to the cellulose I crystalline structure of the cellulose fiber. These results can provide evidence for the incorporation of Mg-Al LDH on the cellulose fiber. Apart from these two peaks, other peaks are in good qualitative agreement with the pattern of Mg-Al LDH crystal (JCPDS: 14–0191). For ultralong MnO2 nanowires, all the diffraction peaks match well with the standard card of typical tetragonal α-MnO2 (JCPDS card 72–1982), indicating the high quality of ultralong MnO2 nanowires. The Cu nanowires showed an XRD pattern with three clearly distinguishable diffraction peaks at 2θ = 43.2°, 50.3°, and 74.4°, corresponding to (111), (200), and (220) crystals planes of face-centeredcubic Cu, which were indexed as per JCPDS card no. 03–1005. The strong and sharp peaks prove the high quality of the Cu nanowires. It is worth noting that the XRD pattern shows no evidence of other impurities like CuO or Cu2O, indicating the high purity of the Cu nanowires for low infrared emissivity. For trilayer Janus CMCFL membrane, the XRD patterns of both sides of it exhibit the typical diffraction peaks of Cellulose@LDH fiber layer, MnO2 nanowires layer, and Cu nanowires layer, indicating the integration of them in one Janus membrane (Fig. 3B). To minimize the spontaneous oxidation process, the Cu nanowires were protected by a PVP layer on their surface. As shown in Figure S5, a PVP layer with a thickness of about 1.6 nm appears on the surface of the Cu nanowire, and the surface information of the Cu nanowires can be properly obtained using XPS techniques. In Fig. 4A, the XPS survey spectrum of Cu nanowires shows a pronounced C 1s peak at around 284.6 eV, various Cu binding energy peaks, and a weak O 1s peak at around N 1s, confirming the presence of PVP molecules. For clarity, the
fitted XPS spectra of C 1s indicates four peaks of individual components at 284.8, 285.6, 286.1 and 287.8 eV, which can be attributed to carbon atoms C1 to C4 of PVP molecule (the inset image in Fig. 4B), respectively, based on the different chemical environments. Furthermore, in the O 1s spectrum in Fig. 4C, Cu nanowires have two clear peaks at 531.6 and 532.7 eV, which are assigned to carboxyl (C=O) and hydroxyl (C-OH) oxygen atoms, respectively. Interestingly, compared to the O 1s peak from carboxyl (C=O) carbon in the pure PVP, the peak of Cu nanowires shifts to higher binding energy, suggesting the enhanced electron density, because of the interaction between PVP molecule and the Cu nanowire. The presence of the N 1s peak in Fig. 4D can be a relatively reliable indication that PVP is absorbed on the surface of the Cu nanowires. Moreover, the N 1s peak at 399.6 eV does not seem to be influenced by the Cu nanowires. In Fig. 4E, there are no peaks at 940 eV around, suggesting that there are negligible Cu2O or CuO in the Cu nanowires. The high quality of Cu nanowires can result from the PVP coating of Cu nanowires, which is a good barrier to prevent surface oxidation of Cu nanowires. 3.2. Interface stability and wettability For a practical application, another interfacial stability between three layers should be carefully considered for the stability of the obtained Janus membrane. The interfacial compatibility of the Janus membrane was characterized by a soak test. As shown in Fig. 4A, the Janus membranes (1 cm × 4 cm) were fully immersed in water. After 48 h of soaking, the water in the bottles was transparent, and no detachment was observed. Moreover, the SEM images in Figure S6A and B showed no change of the two sides of the Janus membrane. The interfacial compatibility was further evaluated in the hot water soak test. As excepted, the similar results appeared, as shown in Fig. 5B and Figure S6C and D. Apart from the visual observation (Fig. 5A and B) and SEM (Figure S6), UV–Vis measurements (Figure S7) also indicate that the Janus membrane exhibited a satisfactory stability, which can be attributed to the strong compatible entanglement between three separated layer.
Fig. 4. (A to E) XPS spectra of the Cu nanowires and O1s, C1s, N1s, and Cu 3p3/2 regions.
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radiative heat dissipation. Usually, the control of infrared emissivity on the outer surface of textiles can have a substantial impact on regulating radiative heat of clothed human for on-demand personal thermal management. To achieve warmth in the cold environment, the infrared radiation loss should be suppressed, because the continuous infrared emission from the human body into the ambient air is the main part of the heat loss. Usually, normal cotton textiles with a high emissivity of 0.895 lack the human radiation control in the cold environment [10]. By contrast, the Cu nanowires layer of trilayer Janus CMCFL membrane show the desired infrared radiation properties, and the results are shown in Fig. 6. In Fig. 6A, the emissivity of Cu nanowires layer is approximately 0.3–0.5 between the wavelengths of 2–18 μm. The weighted average emissivity based on human body radiation was 0.436 for the Cu nanowires layer, indicating the low emissivity, compared to the normal cotton cloth. Fig. 6B illustrates the thermal image of a hand with both the normal cotton and Janus membrane with heating mode (Cu nanowires layer facing outsides). They are in thermal equilibrium on the hand at atmosphere temperature of 18 °C. As shown in Fig. 6B, due to the low emissivity, the Cu nanowire layer emits less infrared radiation than normal cotton, making the Janus membrane appear “cold”. The infrared radiation from normal cotton was detected by an infrared camera and the obtained temperature is 26.4–28.6 °C. While the temperature of the Janus membranes is lower, in the range of 24.6–26.1 °C, indicating a decreasing of 1.8–2.5 °C. The Janus membrane and normal cotton were further compared under a hot plate with a high temperature of 75 °C. Under thermal equilibrium, the center temperatures of Cu nanowire layer and normal cotton were 51.5 and 64.0 °C, respectively, indicating Cu nanowires layer is low infrared emitter to decrease the heat loss (Fig. 6C). These results demonstrate the effectiveness of the strategy of using the trilayer Janus CMCFL membrane with Cu nanowire layer facing outside on warming human bodies. An illustration, as shown in Fig. 6D, is provided to understand the heating mode of the Janus membrane. The Cu nanowires layer with low infrared emission can work as a low infrared emitter and suppress human body infrared
Figure 5. (A and B) Optical photos of the trilayer Janus CMCFL membrane before and after soaking in water with different temperature. (C and D) The wettability of the two sides of the trilayer Janus CMCFL membrane.
In addition, the wettability of the trilayer Janus membrane was evaluated by the static water contact angle test, as illustrated in Fig. 4C and D. When the water droplets are dropped on the two sides of the trilayer Janus CMCFL membrane, the two sides can be wetted by water droplet with water contact angle of ~ 0°, indicating the superhydrophilicity of the Janus membrane, endowing the hygroscopicity and breathability of the Janus membrane. 3.3. Thermal control and electrical property The objective of on-demand thermal management is to maintain the thermal comfort of the human in different environments by controlling
Fig. 6. The infrared emissivity of the Cu nanowires layer and the human body radiation at 33 °C (A). The thermal image of the cotton cloth and Janus membrane with Cu nanowires layer facing outside on a hand (B) and a hot plate (C). Schematics depicting the heating mode of the trilayer Janus CMCFL membrane (D). 6
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Fig. 7. Joule heat property of the Janus membrane. (A) Temperature change vs time after applying different voltages. (B) Temperature difference of the heater compared to room temperature based on supplied voltage. (C) Infrared images of Janus CMCFL membrane applied a voltage of 12.1 V.
same palm temperature, but the Janus membrane can emit more infrared radiation for the human cooling due to the higher infrared emissivity. The cooling property of Janus membrane was further demonstrated by evaluating the skin temperature under different conditions (Fig. 8C). Under the bare skin condition, the skin temperature is about 31.3 °C. Placing a piece of normal cotton cloth onto the skin increases its temperature to 36.2 °C (an increase of 3.9 °C). Janus membrane with cooling mode (cellulose@LDH layer facing outside) is placed onto the skin, and the temperature of the skin is 33.8 °C, indicating a small skin temperature rise (2.5 °C). Thus, the cooling mode can enhance the adaptability of human skin to the environment. Moreover, an illustration (Fig. 8D) is provided to understand the cooling mode of the Janus membrane. When the cellulose@LDH fiber layer is facing out, the high infrared emissivity can allow more radiative heat dissipation from a clothed human body to the ambient environment, and the Janus membrane works in cooling mode.
radiation loss; thus, the Janus membrane works in heating mode. Active warming up is one of the most important features of the Janus membrane under cold environment. Fig. 7A shows the generated temperature profiles with variations of the supplied voltages. The temperature was measured by a thermal couple in close contact with the samples. Specially, for Janus membrane, a low voltage supply nearly 8 V can already meet the normal requirement of the human body. This is due to the high electrical conductivity of Cu nanowires layer. To ensure safety of the human body, 8–12V is more than enough, however, higher temperature can also be achieved. When the supply voltage reaches 24V, the temperature difference becomes up to 80 °C, which might be useful in some limited conditions (Fig. 7B). All the heating procedures were accomplished within the initial 100 s and remained stable before power off. Moreover, the electrothermal response was confirmed by an infrared thermal camera under a typical supply voltage of 8.4 V (Fig. 7C). As can be seen, it only takes 15 s for the Janus membrane to rise from room temperature to 36 °C. In addition to the satisfied temperature response, a homogeneously distributed heat flow was observed due to the high thermal conductivity of Cu nanowires layer. In heating process, the hot Janus membrane is the primary heat source which can transfer heat to surrounding of human body by radiation and conduction, in which both thermal radiation and conduction are beneficial. Moreover, it can act as a great bodyguard against human body radiative heat loss after Joule heating under cold temperature conditions (such as < 15 °C). Human skin is a good infrared emitter with an excellent emissivity of 0.98 [21]. The normal cotton cloth usually has a lower infrared emissivity of 0.895, resulting in inhibition of human body heat dissipation under a hot environment. An excellently high infrared emissivity is necessary for the human body heat dissipation. As shown in Fig. 8A, the cellulose@LDH fiber layer shows an extremely high infrared emissivity with a weighted average value of 0.973, which is very close to human skin radiation cooling performance. Fig. 8B shows the thermal image of the normal cotton and Janus membrane with cooling mode (cellulose@LDH layer facing outside). For the normal cotton cloth, the temperature obtained by infrared camera is in a range of 32.1–33.4 °C, which is smaller than the temperature of Janus membrane (in a range of 32.8–34.1 °C). The two samples are tested under the
3.4. Breathability and antibacterial property When the human skin perspires, the Janus membrane must not prevent the moisture efflux. The breathability of the Janus membrane was tested by weighing the mass increase of desiccant sealed with a normal cotton cloth and Janus membrane. To avoid the basic by environment temperature and humidity, all the samples are tested at the same time and the same environment. During the test, water vapor in the air permeates the samples and finally is captured by the desiccant, leading to the increase in weight. In Fig. 9A, the results show that the water vapor permeabilities of Janus membrane and normal cotton cloth are 11 mg/cm2∙hr and 12 mg/cm2∙hr, respectively, with a slight weight gain rate reduction of only ~8.3% compared with the cotton cloth. This result displayed the extreme breathability of Janus membrane to ensure the comfortable wearing feeling. The antibacterial properties of the Janus membrane were tested on Gram-negative E. coli and Gram-positive S. aureus using the modified Kirby-Bauer method, in which the radius of inhibition zone was used to evaluate the antibacterial activity. As shown in Fig. 9B, inhibition zones of 12 mm and 13 mm in diameter were clearly observed around Janus membrane on the agar plates of E. coli and S. aureus, respectively, revealing the antibacterial property. 7
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Fig. 8. The infrared emission of the cellulose@ LDH layer and the human body radiation at 33 °C (A). The thermal image of the cotton cloth and Janus membrane with cellulose@LDH layer facing outside on a hand (B). The steady-state skin temperature of various conditions: bare skin, normal cotton, Janus membrane with cooling mode (C). Schematics depicting the cooling mode of the trilayer Janus CMCFL membrane (D).
Nanoscale copper can be responsible for the antibacterial property, which can restrain the reproduction of bacteria to lengthen the lifespan of the trilayer Janus CMCFL membrane. In addition, the Janus membrane can be washed by dry cleaning, and the electrical resistance of it shows a slight increase after six cycles of dry cleaning tests (Figure S8). The pores in the obtained Janus membrane plays several important roles. First, the superhydrophilic and porosity maintain excellent hygroscopicity (Fig. 5C and D) and breathability (Fig. 8A) of the Janus membrane, allowing water vapor to diffuse from one side to the other. The resistance to diffusion in perforated membranes has been studies with the equation [23]:
Rw =
t
+ 0.71d (
1
1
)
dense MnO2 nanowires layer and Cu nanowires layer contribute positively to the breathability of the Janus membrane. It might be the reason that although the Janus membrane was filled up with micro-/ nanopores, it showed good breathability like cotton cloth. The Janus membrane that is both superhydrophilic and breathable can provide the wearer with a high level of comfort. Second, the pore diameter of the Cu nanowires layer is smaller than the wavelength of the infrared radiation of the human body. Therefore, the Cu nanowires layer can work as a perfect infrared reflector without sacrificing the breathability of the Janus membrane. 4. Conclusions
(1)
In conclusion, a multifunctional trilayer Janus CMCFL membrane with sandwich structure was fabricated by vacuum filtrating ultralong MnO2 nanowires and Cu nanowires sequentially on cellulose@LDH fiber basement membrane. Compared to the normal cotton cloth, the asprepared Janus membrane can switch its radiative property to control human body infrared radiation for on-demand thermal management
Where Rw, is the resistance of the membrane, t is the thickness of the membrane, d is the average pores of the membrane, and β is the percentage area of the pores. According to the equation, the decreasing of pore size on a constant porosity and thickness can be beneficial to the water vapor transmission. Thus, the small pores that caused by the
Fig. 9. (A) The breathability test by weight mass increase of desiccants sealed with normal cotton cloth and Janus membrane, respectively. (B) The antibacterial property of trilayer Janus CMCFL membrane. 8
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based on the asymmetrical infrared emissivity. When Cu nanowires layer with a low weighted average emissivity of 0.476 is facing outside, reducing the human thermo radiation in the cold environment. The cooling mode can be achieved by wearing the membrane inside out when the cellulose@LDH fiber layer with an extremely high weighted average emissivity of 0.973 is facing outward. The great conductivity of Cu nanowires layer will further improve the heating mode by providing superior Joule heating for extra warmth of 19 °C using a low supply voltage around 8 V. Moreover, the obtained Janus membrane not only shows excellent superhydrophilicity, porous feature, and high breathability for wearing comfort, but also possesses desired antibacterial activity. These advantages can endow the Janus membrane applications in complex and varied environments without compromising breathability.
[16] Z. Wang, Y. Wang, G. Liu, Rapid and efficient separation of oil from oil‐ in‐ water emulsions using a Janus cotton fabric, Angew. Chem. Int. Ed. 55 (2016) 1291–1294. [17] H.-C. Yang, J. Hou, V. Chen, Z.-K. Xu, Janus membranes: exploring duality for advanced separation, Angew. Chem. Int. Ed. 55 (2016) 13398–13407. [18] M. Cao, J. Xiao, C. Yu, K. Li, L. Jiang, Hydrophobic/hydrophilic cooperative Janus system for enhancement of fog collection, Small 11 (2015) 4379–4384. [19] H. Dan, C. Chen, T. Ma, Y. Shang, B. Gao, B. Jin, Q. Li, Q. Yue, Y. Li, Y. Wang, X. Xu, In-situ pyrolysis of Enteromorpha as carbocatalyst for catalytic removal of organic contaminants: Considering the intrinsic N/Fe in Enteromorpha and non-radical reaction, Appl. Catal., B 250 (2019) 382–395. [20] Y. Ito, Y. Tanabe, J. Han, T. Fujita, K. Tanigaki, M. Chen, Multifunctional porous graphene for high-efficiency steam generation by heat localization, Adv. Mater. 27 (2015) 4302–4307. [21] J. Steketee, Spectral emissivity of skin and pericardium, Phys. Med. Biol. 18 (1973) 686. [22] X. Yue, T. Zhang, D. Yang, F. Qiu, Z. Li, Janus ZnO-cellulose/MnO2 hybrid membranes with asymmetric wettability for highly-efficient emulsion separations, Cellulose 25 (2018) 5951–5965. [23] M.E. Whelan, L.E. MacHattie, A.C. Goodings, L.H. Turl, The diffusion of water vapor through laminae with particular reference to textile fabrics:introduction, Text. Res. J. 25 (1955) 197–198.
Acknowledgements This work was financially supported by the Natural Science Foundation of Jiangsu Province (BK20160500, BK20161362 and BK20161264), State Key Laboratory of Pollution Control and Resource Reuse Foundation (NO. PCRRF18003), the National Natural Science Foundation of China (21706100 and 21878132), Postgraduate Research & Practice Innovation Program of Jiangsu Province (SJKY19_2577, SJKY19_2525, and SJCX19_1158), and Natural Science Foundation of Hebei Province (B2019108017).
Xuejie Yue started his Ph.D. degree in Jiangsu University. His research interest is mainly focused on the thermal management material for saving energy.
Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.nanoen.2019.06.004.
Tao Zhang received his Ph.D. degree in Material Physics and Chemistry from Southeast University in 2015. He is currently working toward the functional materials field in materials science and engineering in the Jiangsu University, China. His research interests include design, synthesis, and fabrication of biomass materials for advanced applications.
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Dongya Yang received her Ph.D. degree in materials science from East China University of Science and Technology in 2006. She is now working as an associate professor at School of Chemistry and Chemical Engineering, Jiangsu University. She mainly works on materials for hyperbranched polymers and nonlinear optical materials.
Fengxian Qiu obtained her Ph.D. degree in 2005 from Southeast University. And now she is a professor of materials physics and chemistry in Jiangsu University. She has published over 150 peer-review research papers and patents. Her research interests include energy-saving technology and chemical separation technology
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X. Yue, et al. Gengyao Wei is currently a postgraduate student in Jiangsu University, China. His research focused on thermochromic materials, dielectric materials, hydrogels, and their applications in solar thermal energy conservation.
Hao Zhou is currently a postgraduate student in Jiangsu University of Science and Technology, China. Since 2018, he worked in Prof. Fengxian Qiu's research group at the Jiangsu University as an exchange student, working on biomass materials, especially as those relate to thermal management applications.
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