Hydroxyapatite Nanowire-Based All-Weather Flexible Electrically

Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX-XXX

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Hydroxyapatite Nanowire-Based All-Weather Flexible Electrically Conductive Paper with Superhydrophobic and Flame-Retardant Properties Fei-Fei Chen,†,‡ Ying-Jie Zhu,*,†,‡ Zhi-Chao Xiong,*,† Li-Ying Dong,† Feng Chen,† Bing-Qiang Lu,† and Ri-Long Yang†,‡ †

State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, P. R. China ‡ University of Chinese Academy of Sciences, Beijing 100049, P. R. China S Supporting Information *

ABSTRACT: How to survive under various harsh working conditions is a key challenge for flexible electronic devices because their performances are always susceptible to environments. Herein, we demonstrate the novel design and fabrication of a new kind of the all-weather flexible electrically conductive paper based on ultralong hydroxyapatite nanowires (HNs) with unique combination of the superhydrophobic surface, electrothermal effect, and flame retardancy. The superhydrophobic surface with water repellency stabilizes the electrically conductive performance of the paper in water. For example, the electrical current through the superhydrophobic paper onto which water droplets are deposited shows a little change (0.38%), and the electrical performance is steady as well even when the paper is immersed in water for 120 s (just 3.65% change). In addition, the intrinsic electrothermal effect of the electrically conductive paper can efficiently heat the paper to reach a high temperature, for example, 224.25 °C, within 10 s. The synergistic effect between the electrothermal effect and superhydrophobic surface accelerates the melting and removal of ice on the heated electrically conductive paper. Deicing efficiency of the heated superhydrophobic electrically conductive paper is ∼4.5 times that of the unheated superhydrophobic electrically conductive paper and ∼10.4 times that of the heated superhydrophilic paper. More importantly, benefiting from fire-resistant ultralong HNs, thermally stable Ketjen black, and Si−O backbone of poly(dimethylsiloxane), we demonstrate the stable and continuous service of the as-prepared electrically conductive paper in the flame for as long as 7 min. The electrical performance of the electrically conductive paper after flame treatment can maintain as high as 90.60% of the original value. The rational design of the electrically conductive paper with suitable building materials and structure demonstrated here will give an inspiration for the development of new kinds of all-weather flexible electronic devices that can work under harsh conditions. KEYWORDS: hydroxyapatite, nanowires, fire-resistant paper, superhydrophobic, flexible device

1. INTRODUCTION The traditional hard and brittle electronic components based on silicon and indium tin oxide cannot meet the requirements for flexible devices that can work under harsh environments.1 Because demands for flexible, lightweight, and portable electronic devices are ever-increasing, flexible electronic devices have been intensively investigated for applications in various fields, such as transistors,2 integrated circuits,3 touch screen panels,4 sensors,5 solar cells,6 electric heaters,7 supercapacitors,8 and wearable devices.9 However, flexible electronic devices still suffer from some drawbacks, and the susceptibility to the environments is one of the main disadvantages. For example, metals, as the first choice of conductive materials, are sensitive to oxidation and corrosion, which degrades their performance inevitably and even leads to a failure in function over time.10 In addition, contaminants, moisture, icing, and snowing are also © XXXX American Chemical Society

the critical factors that can deteriorate the performance of flexible electronic devices. Especially, the capability of flexible electronic devices to exhibit stable performance in the flame arouses much interest,11 which is particularly important for applications in fire disasters. In this work, we demonstrate a new kind of hydroxyapatite nanowire (HN)-based all-weather flexible electrically conductive paper with superhydrophobic, electrothermal, and flame-retardant properties. Superhydrophobic surfaces, inspired by the lotus leaf or other natural creations,12 are regarded as one of the most effective ways to stabilize the performance of devices by providing waterproof property and resistance to corrosion or moisture.9,13−15 Received: July 2, 2017 Accepted: October 23, 2017

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DOI: 10.1021/acsami.7b09484 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Scheme 1. Schematic Illustration of the Novel Design of a New Kind of the All-Weather Flexible Electrically Conductive Paper Based on Ultralong Hydroxyapatite Nanowires with Unique Combination of the Superhydrophobic Surface, Electrothermal Effect, and Flame Retardancy

For example, Samuel et al.16 reported that the superhydrophobic surface could reduce the adsorption of water and improve the specificity and sensitivity of gas sensors. The water-proof feature of the superhydrophobic surface also enables the flexible electronic devices to work steadily underwater.9 Additionally, the most attractive feature of the superhydrophobic surface is its self-cleaning ability.17 The contaminants on the superhydrophobic surface can be picked up and removed by water, such as raining.18,19 According to the previous studies,20,21 the shading of the collector area of photovoltaic modules by contaminants decreased the power output during the long-term service. The self-cleaning ability of the superhydrophobic surface offers an ideal strategy to avoid the accumulation of contaminants on the photovoltaic modules, thus improving their performance and extending their working lifetime. Icing and snowing are common but extremely harsh environments. Taking a serious snow storm occurred in South China in 2008 as a typical example,22 the excessive ice accretion on the transmission lines and power network towers caused severe damage and enormous economic loss. Fortunately, the superhydrophobic surface is capable of inhibiting the accumulation of water and ice formation.23 However, some tiny condensed water droplets can adhere to the surface and freeze into ice, further leading to ice adhesion and accretion. Although trapping liquid as a lubricating layer to reduce ice adhesion is an alternative solution,24 liquid infusion into electronic devices is not advisable. Among the traditional antiicing/deicing techniques, the electrothermal method is one of the most direct and efficient ways.25 Fortunately, the intrinsic Joule effect of the flexible conductors is capable of transforming the electric energy into thermal energy.26−28 The major problem of the electrothermal method is its high energy consumption and low anti-icing/deicing efficiency. The combination of superhydrophobic surface and electrothermal effect may improve the efficiency.29,30 Another issue is that the extremely tiny water droplets cannot automatically roll away

the superhydrophobic surface. In this case, the electrothermal effect can also accelerate the evaporation of water droplets on the superhydrophobic surface.31 One severe problem is that the use of electric power may cause fire accidents due to short circuit, overload, poor contact, overheating, electric leakage, lighting stroke, misuse, and so forth.32,33 The fire disasters inevitably damage or even destroy the electronic devices containing flammable materials. Therefore, flame-retardant or rapid self-extinguishing properties are highly desirable for flexile electronic devices that can withstand high temperatures and work in the fire. Flame-retardant and conductive networks were prepared using the building materials with good conductivity and thermal stability.34−40 On the basis of the above discussion, the all-weather flexible electronic devices can be designed by combining the superhydrophobic surface, electrothermal effect, and flame retardancy. In our previous reports,41−43 we have fabricated the fireresistant paper or fabric based on ultralong hydroxyapatite nanowires (HNs). Although the inorganic ultralong hydroxyapatite nanowires are highly insulating, their high flexibility and nonflammability are attractive for the flame-retardant flexible electronic devices. On the other hand, the superhydrophobic and fire-resistant paper was prepared using HNs modified with sodium oleate.44 However, the decomposition of sodium oleate at high temperatures (>200 °C) is not suitable for the electrothermal applications. Herein, we report a new kind of all-weather flexible electrically conductive paper with superhydrophobic, electrothermal and flame-retardant properties based on hydroxyapatite nanowires (HNs), Ketjen black (KB), and poly(dimethylsiloxane) (PDMS), as illustrated in Scheme 1. The superhydrophobic surface makes the as-prepared KB + HNs + PDMS an electrically conductive paper resistant to corrosion (pH 1−14) and moisture (up to 90%), and the paper also exhibits high-temperature stability (up to 300 °C in the oxidative atmosphere). The water-proof feature enables the KB + HNs + PDMS electrically conductive paper work steadily B


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Figure 1. (a) Schematic illustration of the preparation process. (b−d) TEM images of HNs (b), KB (c), and 10 wt % KB + HNs nanocomposite (d). autoclave (1 L), sealed, and then thermally treated at 180 °C for 24 h. HNs were washed with ethanol three times and deionized water three times at 60 °C under magnetic stirring. The obtained HNs were dispersed in ethanol to form a colloidal suspension for further use. 2.3. Preparation of All-Weather Flexible Electrically Conductive Paper. KB was first dispersed in ethanol under ultrasonic treatment for 30 min to obtain a KB colloidal suspension. To prepare the electrically conductive paper, a colloidal suspension containing KB was mixed with a colloidal suspension of HNs under magnetic stirring for 10 min; then, vacuum-assisted filtration was adopted to fabricate the KB + HNs paper. The KB + HNs paper could be easily peeled off after drying on a hot plate at 90 °C for 5 min. The dried KB + HNs paper was coated with PDMS according to a previously reported method.45 Briefly, by mixing the PDMS base agent and curing agent with ethyl acetate at a weight ratio of 10:1:100, a diluted PDMS solution was prepared. The KB + HNs paper was immersed in the diluted PDMS solution at room temperature for 30 min and then cured at 100 °C for 1 h. 2.4. Characterization and Instruments. The samples were characterized by transmission electron microscopy (TEM, Hitachi H800, Japan), scanning electron microscopy (SEM, Magellen 400, FEI; JSM 6510, JEOL, Japan), X-ray diffraction (XRD, Cu Kα radiation, λ = 1.54178 Å, Rigaku D/max 2550 V, Japan), and Fourier transform infrared (FTIR, FTIR-7600, Lambda Scientific, Australia) spectroscopy. The elemental analysis of the KB + HNs paper and KB + HNs + PDMS paper was conducted with an energy-dispersive X-ray (EDX) spectrometer (X-MAXN, Oxford Instruments, U.K.). The KB contents of the as-prepared paper sheets were determined by thermogravimetric (TG) analysis using a simultaneous thermal analyzer (STA 409PC, Netzsch, Germany) at a heating rate of 10 °C min−1 in flowing air. Electrical conductivities of the samples were measured using a four-pin probe on a semiconductor characterization system (4200-SCS, Keithley). Water contact angles were measured by an optical contact

when water droplets are deposited on it or even it is immersed in water. In addition, the synergistic effect between the superhydrophobic and electrothermal properties promotes the removal of water and deicing on the KB + HNs + PDMS electrically conductive paper. For example, the heated superhydrophobic KB + HNs + PDMS electrically conductive paper can completely deice within 23 s, whereas it takes 103 s for the unheated superhydrophobic KB + HNs + PDMS electrically conductive paper and 240 s for the heated superhydrophilic paper to deice. More importantly, the as-prepared KB + HNs + PDMS electrically conductive paper exhibits excellent flame retardancy. The stable and continuous service of the KB + HNs + PDMS electrically conductive paper in the flame for as long as 7 min is demonstrated in this work.

2. EXPERIMENTAL SECTION 2.1. Materials. CaCl2, NaOH, NaH2PO4·2H2O, and ethyl acetate were purchased from Sinopharm Chemical Reagent Co., Ltd. Oleic acid was purchased from Aladdin Industrial Corporation. Kejten black (KB, EC-600JD) and poly(dimethylsiloxane) (PDMS, Sylgard 184, Dow Corning) were commercially available. Ice was obtained from a refrigerator with a temperature of −20 °C. Contaminants (soils) were obtained from native garden. 2.2. Synthesis of Ultralong Hydroxyapatite Nanowires (HNs). HNs with high aspect ratios were synthesized via a modified calcium oleate precursor solvothermal method reported by this group.41 In brief, CaCl2 (2.200 g) aqueous solution (200 mL), NaOH (10.000 g) aqueous solution (200 mL), and NaH2PO4·2H2O (2.800 g) aqueous solution (100 mL) were added into a mixture of oleic acid (100.0 g) and ethanol (140.0 g) alternately under stirring. The resulting mixture was transferred into a Teflon-lined stainless steel C


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Figure 2. (a) Digital image, (b) SEM image, and (c) EDX pattern of 20 wt % KB + HNs paper. (d) Digital image, (e) SEM image, and (f) EDX pattern of KB + HNs + PDMS (KHP-2) paper. (g) The KHP-2 paper could be twisted without breaking (the inset shows the paper after twisting). (h) The electrically conductive (KHP-2) paper was connected with a direct current power supply at a voltage of 3 V and an light-emitting diode (LED) lamp, which was continuously lighting. angle system (model SL200B) at three different sites for each paper sheet using a water droplet of 3 μL. The surface temperature of the paper under an applied voltage was real-time monitored and recorded by an infrared thermal imaging camera (FLIR A300). The voltage was applied by a direct current power supply (KXN-305D, Zhaoxin Electronic, China). The thickness of the paper was measured on a high-precision paper thickness gauge (GDH-3, Qingtongboke Automation Technology Co., Ltd., China). The mechanical properties of the KB + HNs + PDMS paper were investigated using a universal

testing machine (DRK-101B, Drick, China) at a loading rate of 0.5 mm min−1 with a gauge length of 10 mm. The as-prepared paper was cut into multiple sheets with sizes of 25 mm × 10 mm.

3. RESULTS AND DISCUSSION 3.1. Preparation and Characterization of All-Weather Flexible Electrically Conductive Paper. The preparation process of the all-weather flexible electrically conductive paper D


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KHP-x paper sheets are shown in Table S1 (Supporting Information). The coating of PDMS on the KB + HNs paper had no obvious effect on the appearance of the electrically conductive paper due to the transparent feature of PDMS but could significantly change the surface wettability (Figures 2d and S7, Supporting Information). Water contact angles of the KHP-x (x = 1, 2, 3, 4) paper sheets were larger than 150°, but the KHP-0 (i.e., HNs + PDMS) paper without the addition of KB had a lower contact angle (137.3 ± 0.6°, Figure S8, Supporting Information). Because the surface wettability is determined by both the surface energy and microstructure, KB particles play a key role in regulating the surface wettability of the KHP-x paper. The textured surface of the HNs paper (Figure S9a, Supporting Information) was flattened after coating with PDMS (Figure S9b, Supporting Information), resulting in inadequate surface roughness and a lower contact angle of the KHP-0 paper. The addition of KB particles into HNs created a beads-on-string structure that introduced a second level of roughness (Figure 2b).57 The additional roughness was enough to offset the negative impact of PDMS (Figure 2e), making the KHP-x (x = 1, 2, 3, 4) paper superhydrophobic. In this regard, HNs and KB not only served as a stable substrate and electrically conductive filler, respectively, but also provided two levels of roughness. The successful coating of PDMS on the electrically conductive paper was characterized by FTIR analysis (Figure S4, Supporting Information). The absorption peaks at 2920 and 2850 cm−1 in the FTIR spectrum of the KHP-2 paper are assigned to asymmetric and symmetric C−H stretching vibration of the methyl groups, and these two absorption peaks show higher intensities than those of the HNs and 20 wt % KB + HNs paper. In addition, two new absorption peaks at 1259 and 800 cm−1 in the FTIR spectrum of the KHP-2 paper are ascribed to the Si−CH3 vibration of PDMS.58 Furthermore, compared to the 20 wt % KB + HNs paper, the obvious Si peak was detected in the KHP-2 paper by energy-dispersive X-ray (EDX) spectroscopy (Figure 2c,f), confirming the successful PDMS coating on the electrically conductive paper. After coating with PDMS, the intensities of the diffraction peaks in the XRD pattern became weaker, but no obvious shift of the diffraction peaks was observed (Figure S10, Supporting Information). As shown in Figures 2g and S11, Supporting Information, the as-prepared KB + HNs + PDMS paper could be twisted and bent without breaking, showing high flexibility. To test the electrical performance, the electrically conductive paper sheets with different KB contents (KHP-x (x = 1, 2, 3, 4)) were connected with an LED lamp under an applied voltage of 3 V (Figures 2h and S7, Supporting Information). The electrically insulated KHP-0 paper without addition of KB could not light the LED lamp, whereas the electrically conductive paper sheets with different KB contents (KHP-x (x = 1, 2, 3, 4)) could make the LED lamp continuously lighting. The KB content in the electrically conductive paper had a significant effect on the electrical conductivity (Figure S12, Supporting Information) and thus on the brightness of the LED lamp. The LED lamp was brighter when the KB content in the electrically conductive paper was larger. In addition, the thickness of the KB + HNs + PDMS electrically conductive paper could be easily controlled by adjusting the amounts of HNs and KB. The properties of the KB + HNs + PDMS electrically conductive paper are summarized in Table S2 (Supporting Information).

made of HNs, KB, and PDMS is illustrated in Figure 1a. Briefly, the free-standing KB + HNs paper is prepared by the vacuumassisted filtration of homogeneous colloidal suspension containing KB and HNs. After drying, the KB + HNs paper is coated with PDMS to obtain the all-weather flexible KB + HNs + PDMS electrically conductive paper. We select these three constituents based on the following considerations: (1) HNs possess high biocompatiblility46 and are considered as the ideal building material to construct the flexible fire-resistant paper.43 Moreover, inorganic hydroxyapatite nanowires with high thermal stability are desirable for flame-retardant applications;41,43 (2) KB is a kind of conductive carbon black with higher performance than ordinary carbon black due to its branched structure;47 (3) PDMS is a nontoxic, chemically inert, stretchy, and transparent polymer, making it promising for the application in next-generation stretchable and flexible electronic devices;48−50 (4) with the aid of the side alkyl groups and inorganic Si−O backbone, PDMS is desirable for constructing the superhydrophobic surface51,52 and shows higher stability at elevated temperatures in the oxidative atmosphere than ordinary organic polymers, which has been proven as an effective material to retard fire.53 In addition, the resulting products after the thermal decomposition of PDMS have no negative environmental impact and toxicity.54,55 It is worth emphasizing the important value of PDMS in the areas where the superhydrophobic and flame-retardant properties are required simultaneously. Ultralong hydroxyapatite nanowires with high aspect ratios were synthesized via a modified calcium oleate precursor solvothermal method,41 as shown in Figure 1b. Because KB particles (Figure 1c) could not form the stable and continuous paperlike network by filtration (Figure S1, Supporting Information), HNs with high aspect ratios were used as an ideal host material to load KB particles. X-ray diffraction (XRD) patterns of KB particles and HNs are shown in Figure S2 (Supporting Information). Transmission electron microscopy (TEM) images show that the KB + HNs nanocomposites with different KB contents exhibited a similar microstructure (Figure S3, Supporting Information). KB particles were prone to aggregate around HNs, attach to the surface of HNs, and insert into the interspace between HNs, forming a beads-onstring structure (Figure 1d). This microstructure of the KB + HNs nanocomposite contributed to the formation of the electrically conductive paper. Fourier transform infrared (FTIR) spectrum of KB shows a broad peak at 3300−3600 cm−1 (Figure S4, Supporting Information), which may be assigned to the adsorbed water and O−H group.56 Therefore, the high affinity of KB particles to HNs may result from the hydrogen-bonding interaction. After the filtration of KB + HNs colloidal suspensions with different KB contents, electrically conductive paper sheets with different KB contents were prepared (Figure 2a). KB contents of the electrically conductive KB + HNs paper sheets were determined by thermogravimetric (TG) analysis (Figure S5, Supporting Information). The experimental results are consistent with the theoretical values, as shown in Table S1 (Supporting Information). KB exhibited a hydrophilicity similar to that of HNs (Figures S1 and S6, Supporting Information). The as-prepared KB + HNs paper sheets with different KB contents were superhydrophilic (Figures 2a and S6, Supporting Information). For the convenience of discussion, the KB + HNs + PDMS paper sheets with different KB contents are labeled as KHP-x (x = 0, 1, 2, 3, 4), and the KB contents of the E


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Figure 3. (a, b) Water flow impacting the surface of the HNs + PDMS (KHP-0) paper and the KB + HNs + PDMS (KHP-2) paper with different surface wettabilities. The insets show the paper surfaces after water flow impacting. (c) Self-cleaning process of the KB + HNs + PDMS (KHP-2) paper. (d−f) Evaluation of corrosion resistance (d), moisture resistance (e), and thermal stability (f) of the KB + HNs + PDMS (KHP-2) paper by recording the changes of water contact angle (top panels) and electrical current (bottom panels).

experimental results, the KHP-2 paper was selected as a typical example for the following studies unless otherwise specified. To further demonstrate the properties of the KHP paper, we also monitored the change of the electrical performance after bending the KHP paper for different cycles. The bending angle of the KHP paper was ∼90°, and the electrical current through the KHP paper was recorded. The experimental results showed that electrical performance was relatively stable during the process of 500 bending cycles (Figure S13e, Supporting Information). In addition, the KHP paper was not broken after 500 bending cycles, indicating its high flexibility and good mechanical properties. 3.2. Superhydrophobic Property of the KB + HNs + PDMS Electrically Conductive Paper. The malfunction of the electronic devices due to aging, interference, and pollution under highly corrosive, oxidative, and extremely humid conditions is a frequent occurrence. The superhydrophobic surface provides an effective way to protect the electronic devices because of its water-repellent and self-cleaning properties. Both high water contact angle and low sliding angle are necessary to reduce the adhesion between water and the surface. As shown in Figure 3a, the water droplets could

Good mechanical properties are necessary for the electrically conductive paper to adapt to the mechanical deformation in the practical usage. The tensile tests were carried out to investigate the mechanical properties of the KB + HNs + PDMS electrically conductive paper (Figure S13a, Supporting Information). The architecture of the KB + HNs + PDMS electrically conductive paper is similar to the structure of the steel-reinforced concrete (Figure 2e). The addition of KB particles (concrete) would improve the tensile strength of HNs (steel bars). For example, the KHP-1 paper had a higher tensile strength (2.82 ± 0.50 MPa) than the KHP-0 paper without the addition of KB (1.71 ± 0.99 MPa). However, further increase in the KB content decreased the tensile strength of the KB + HNs + PDMS electrically conductive paper (Figure S13b, Supporting Information). The addition of KB particles in the paper led to decreased strain at failure (Figure S13c, Supporting Information) and higher Young’s modulus (Figure S13d, Supporting Information). The KHP-3 paper had the highest Young’s modulus (141.33 ± 11.68 MPa) among the KHP-x (x = 0, 1, 2, 3, 4) paper sheets. The values of the ultimate tensile strength, strain at failure, and Young’s modulus are summarized in Table S3 (Supporting Information). On the basis of the F


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Figure 4. (a) Schematic illustration of real-time monitoring of electrical current through the electrically conductive paper (20 mm × 30 mm) onto which water droplets were deposited; the paper was connected with a direct current power supply at a voltage of 3 V and an ammeter. (b, c) Schematic illustration of water droplets on the superhydrophilic 20 wt % KB + HNs paper and the superhydrophobic KB + HNs + PDMS (KHP-2) paper. (d) Electrical current through (1) the superhydrophobic KB + HNs + PDMS (KHP-2) paper and (2) the superhydrophilic 20 wt % KB + HNs paper, and the inset shows the relative electrical current change (φ). (e, f) Snapshots of real-time monitoring of the electrical current through the electrically conductive paper.

corrosion resistance of the KB + HNs + PDMS electrically conductive paper was investigated by measuring the water contact angle at different pH values, as shown in Figure 3d (top panel). In a broad range of pH values (2.06−13.42), water contact angles of the KB + HNs + PDMS electrically conductive paper were higher than 150°, indicating the stable water repellency under highly corrosive conditions. The chemically inert PDMS and water repellency were responsible for the high corrosion resistance of the KB + HNs + PDMS electrically conductive paper. However, under extremely acidic conditions (e.g., pH 1.24), the PDMS protective layer was not strong enough to survive completely (water contact angle = 136.2 ± 3.3°). The evaluation of the electrical performance was performed by recording the electrical current through the KB + HNs + PDMS electrically conductive paper before and after immersing the paper in corrosive water at different pH values for 30 s, as shown in the inset of the bottom panel in Figure 3d. It should be mentioned that an external force was needed to dip the paper in the corrosive water. As shown in the bottom panel of Figure 3d, the relative electrical current change (φ), a representative index, was below 3.5% even under extremely acidic conditions (e.g., pH 1.01), exhibiting stable electrical performance under corrosive conditions. The values of φ are shown in Table S4 (Supporting Information). To evaluate the resistance to moisture at high relative humidities, the KB + HNs + PDMS electrically conductive paper was stored in a chamber at a constant temperature (23 °C) and different humidities (50, 60, 70, 80, 90%) for 24 h and

attach to the KHP-0 paper without addition of KB due to the low water contact angle, below 150°. In contrast, the superhydrophobic KHP-2 paper with a high water contact angle (above 150°) and a low sliding angle (below 10°) could bounce the impacting water flow, as shown in Figure 3b, and water droplets automatically rolled away at a small slanting angle, enabling the KB + HNs + PDMS electrically conductive paper to keep dry. This water-proof feature is also beneficial to deicing and cleaning the contaminants on the surface of the KB + HNs + PDMS electrically conductive paper. The self-cleaning process of the KHP-2 paper is shown in Figure 3c. The KHP-2 paper was placed on the tilted glass slide at an angle of ∼5°; then, the contaminants (soil particles from a native garden) were put on the paper surface homogeneously. When water was injected by a syringe at a distance of 5−10 mm from the paper surface, water droplets rolled away, picked up the contaminants, and removed them from the paper surface. After the selfcleaning process, the surface of the KHP-2 paper became clean without any pollution. Such kind of the KB + HNs + PDMS electrically conductive paper with superhydrophobic and selfcleaning properties is suitable for the outdoor applications, such as flexible electronic devices that are applied in wearable fabrics,9 photovoltaic equipment,59 or the skin of robots. Other essential properties of the all-weather electrically conductive paper, including resistance to both corrosion and moisture, and thermal stability were quantitatively investigated by monitoring the change of both water contact angle and electrical performance before and after the treatment. The G


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Figure 5. (a) Schematic illustration of the testing setup for the underwater electrical performance of the superhydrophobic KB + HNs + PDMS (KHP-2) paper (10 mm × 20 mm) connected with an ammeter and a direct current power supply at a voltage of 3 V (or an LED lamp). (b) Electrical current through the superhydrophobic KB + HNs + PDMS (KHP-2) paper when it was immersed in water, and the inset shows the relative electrical current change (φ). (c) Snapshots of real-time monitoring of the electrical current through the superhydrophobic KB + HNs + PDMS (KHP-2) paper that was immersed in water. (d) Snapshots of real-time monitoring of the brightness of an LED lamp that was connected with the underwater superhydrophobic KB + HNs + PDMS (KHP-2) paper and a direct current power supply at a voltage of 3 V.

treatment could be recovered by recoating the paper with PDMS, as shown by the improved water contact angle (153.1 ± 0.5°, Figure S15, Supporting Information). In addition, the relative electrical current change was below 3.5% after the thermal treatment, indicating that the electrical performance of the KB + HNs + PDMS paper could be well preserved after the thermal treatment (bottom panel of Figure 3f). The high thermal stability under oxidative conditions and easy recovering of the superhydrophobicity of the KB + HNs + PDMS electrically conductive paper enable this new kind of paper promising for the high-temperature-related applications. To further understand the effect of surface wettability on the stability of the electrical performance, real-time monitoring of electrical current through the superhydrophobic or superhydrophilic electrically conductive paper was carried out using the superhydrophobic KB + HNs + PDMS paper and the superhydrophilic KB + HNs paper, respectively. First, water droplets injected by a syringe were deposited onto the electrically conductive paper that was connected with a direct current power supply at a voltage of 3 V and an ammeter, as illustrated in Figure 4a. The ammeter can real-time record the electrical current passing through the electrically conductive paper. For the KB + HNs electrically conductive paper with the superhydrophilic surface, water droplets rapidly wetted the paper surface and penetrated into the interior of the paper (Figure 4b), whereas water droplets stood on the superhydrophobic KB + HNs + PDMS paper without wetting

then the water contact angle and weight gain were measured. The water contact angles of the KB + HNs + PDMS electrically conductive paper were higher than 150° (top panel of Figure 3e), indicating that the KB + HNs + PDMS electrically conductive paper could maintain its water-proof feature at high relative humidities. The steady water-proof performance of the KB + HNs + PDMS electrically conductive paper could resist the moisture and reduce the adsorption of water molecules, which was confirmed by small weight gains of the paper (150°) up to 300 °C. The higher temperature would decompose PDMS (Figure S14, Supporting Information), which was responsible for a lower water contact angle after thermal treatment at 350 °C for 12 h (131.7 ± 0.6°, Figure S15, Supporting Information). The superhydrophobic surface of the KB + HNs + PDMS paper after thermal H


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Figure 6. (a) Schematic illustration of the surface temperature measurement of the KB + HNs + PDMS electrically conductive paper (paper size: 20 mm × 20 mm). (b) Surface temperature of the KHP-x (x = 0, 1, 2, 3, 4) paper sheets under different applied voltages at 9.6 s. (c) Surface temperature change of the KHP-x (x = 0, 1, 2, 3, 4) paper sheets over time under an applied voltage of 10 V. (d) Surface temperature change of the KHP-2 paper over time under applied voltages of 10, 15, and 20 V. (e) Repeated heating/cooling cycles. (f) Infrared thermal images of the KHP-2 paper when an applied voltage of 20 V was turned on and off.

(Figure 4c). The wetting behaviors of the two electrically conductive paper sheets were distinct from each other, leading to the different electrical performances (Figure 4d). The electrical current through the superhydrophilic KB + HNs paper dropped from the initial value of 14.18 mA to 12.95 mA at 28 s and then leveled off during the time period between 28 and 60 s (12.96 mA). The strong adsorption of water had an impact on the electrical performance of the superhydrophilic KB + HNs paper, and the relative electrical current change reached as high as 8.6% (inset of Figure 4d). In contrast, the electrical current through the superhydrophobic KB + HNs + PDMS paper was steady during the real-time monitoring process for 60 s, and the relative electrical current change was very low with a value of 0.38% (Figure 4d). The snapshots of real-time monitoring are also shown in Figure 4e,f and Movies S1 and S2 (Supporting Information). We also evaluated the electrical performance underwater of the superhydrophobic KB + HNs + PDMS paper (Figure 5a). It should be emphasized that it was hard for the super-

hydrophobic KB + HNs + PDMS paper to be immersed in water, and an external force was necessary. The electrical current was real-time recorded and is shown in Figure 5b. The testing process in the first 30 s is also shown in Figure 5c and Movie S3 (Supporting Information). Although the electrical current increased gradually, little change of electrical performance was observed during the testing process. The relative electrical current change was only 3.65% for the whole process. Different from the previous work,60 the external force and water had no obvious effects on the structure and mechanical properties of the superhydrophobic KB + HNs + PDMS paper during immersion in water (Figure S16, Supporting Information). The high stability of the electrical performance and structural integrity enabled the superhydrophobic KB + HNs + PDMS paper to work steadily underwater (Figure 5d). No obvious change in the brightness of the LED lamp was observed during the testing time period, indicating the stable and continuous underwater service of the superhydrophobic KB + HNs + PDMS paper. I


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Figure 7. (a, b) Evolution of a water droplet with a volume of 3 μL over time on the KB + HNs + PDMS electrically conductive paper (KHP-2) without and with an applied voltage of 20 V, respectively. (c, d) Evolution of ice over time on the KB + HNs + PDMS electrically conductive paper (KHP-2) without and with an applied voltage of 20 V, respectively. (e) The 20 wt % KB + HNs paper with an applied voltage of 20 V. All of the tests were carried out at room temperature (27 °C), and the paper sizes were 20 mm × 30 mm.

3.3. Electrothermal Property of the KB + HNs + PDMS Electrically Conductive Paper. The intrinsic Joule effect of the conductor can efficiently convert the electric energy into thermal energy and thus increase the surface temperature. To evaluate the electrothermal effect of the KB + HNs + PDMS electrically conductive paper, a direct voltage was applied to the paper covered by copper foils at the paper edges, and the surface temperature was recorded by an infrared thermal imaging camera, as illustrated in Figure 6a. Both the KB content and applied voltage had effects on the electrothermal efficiency. The electrically insulated KHP-0 paper without addition of KB produced no thermal energy, and there was no change in the temperature of the paper surface over time (Figure 6b,c). In contrast, the KB + HNs + PDMS electrically conductive paper exhibited the electrothermal behavior that could be divided into two stages. In the first stage (yellow region in Figure 6c), the paper surface temperature rose rapidly and reached the quasisteady-state temperature quickly, within 10 s, indicating the rapid temperature response. After that, the paper surface temperature leveled off in the second stage (red region in Figure 6c). A similar behavior was also observed for the KHP-2 paper when different voltages were applied (Figure 6d). In brief, the higher KB content or higher applied voltage, the higher the paper surface temperature. The surface temperature

of the KB + HNs + PDMS electrically conductive paper at 9.6 s after the voltage was applied is recorded in Figure 6b. For example, the surface temperature of the KHP-2 paper under an applied voltage of 20 V reached a temperature as high as 224.25 °C at 9.6 s. Such high temperature produced within a short time is suitable for efficient deicing and water evaporation because energy consumption can be significantly reduced by shortening the operation time. The steady electrothermal effect during the testing process was demonstrated by repeating heating/cooling cycles, as shown in Figure 6e. During five cycles, rapid thermoresponsive behavior and maximal temperature were maintained, indicating the good recyclability. The rapid temperature response was also shown by the infrared thermal images (Figure 6f). The inhomogeneous temperature distribution on the paper surface resulted from the contact between copper foils and the electrically conductive paper. Two problems related with the superhydrophobic surface are extremely tiny water droplets that adhere to the surface and ice accretion that is already formed on the surface. Fortunately, the electrothermal effect of the KB + HNs + PDMS electrically conductive paper can solve these two problems. We investigated the performance of the electrothermal effect of the KB + HNs + PDMS electrically conductive paper in removing the extremely tiny water droplets. A water droplet J


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Figure 8. (a, b) Flame-retardant tests of the cardboard and the KB + HNs + PDMS electrically conductive paper (KHP-2) based on the vertical combustibility method. (c, d) Digital images of the bottom surface and top surface of the KHP-2 paper after exposing to the flame for 7 min. SEM image (e), elemental analysis (f), and elemental mapping (g) of the bottom surface of the KHP-2 paper after exposing to the flame for 7 min.

with a volume of 3 μL was separately deposited on the unheated KB + HNs + PDMS electrically conductive paper without an applied voltage and on the heated one with an applied voltage of 20 V, as shown in Figure 7a,b. As expected, the higher surface temperature produced by the heated KB + HNs + PDMS electrically conductive paper significantly reduced the evaporation time of a water droplet. It took only 128 s for a water droplet to evaporate completely on the heated paper, whereas there was almost no change in the water volume on the unheated paper even after 10 min. We also investigated the impact of the electrothermal effect of the KB + HNs + PDMS electrically conductive paper on the melting of ice, as shown in Figure 7c−e. It took about 103 s for the melting and complete removal of ice on the unheated KB + HNs + PDMS paper (Figure 7c). It should be mentioned that the relatively short time for deicing was a result of high room temperature (27 °C). For the heated KB + HNs + PDMS

paper, the deicing time was much shorter (23 s) and the deicing efficiency is ∼4.5 times that of the unheated KB + HNs + PDMS paper (Figure 7d and Movie S4, Supporting Information). The key strategy for deicing by the electrothermal effect is to keep the surface temperature higher than the freezing point of water, which is efficient to prevent ice accretion and accelerate the melting of ice. In addition, the surface wettability also plays an important role in the deicing efficiency. As shown in Figure 7e, although the thermal energy of the heated KB + HNs paper accelerated the melting of ice, the melted water was absorbed by the paper because of its superhydrophilic property. It took 240 s (∼10.4 times that of the heated superhydrophobic KB + HNs + PDMS paper) for the superhydrophilic KB + HNs paper to become entirely dry. The experimental results indicate that the electrothermal effect can accelerate the melting of ice, and the superhydrophobic surface enables the rapid removal of the melted water. The high K


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Figure 9. (a) Electrical current through the KB + HNs + PDMS electrically conductive paper connected with a direct current power supply at a voltage of 3 V and an ammeter during the exposure process of the paper to the flame for 7 min; the inset shows the schematic illustration of the realtime monitoring of electrical current through the paper in the flame. (b) Relative electrical current change (φ): (1) maximal relative electrical current change and (2) total relative electrical current change. (c) Snapshots of the real-time monitoring of electrical current through the paper in the flame. (d) Snapshots of the real-time monitoring of brightness of an LED lamp connected with a direct current power supply at a voltage of 3 V and the KB + HNs + PDMS electrically conductive paper (KHP-2) in the flame; the KHP-2 paper had a diameter of 40 mm.

temperature reached in a short time period (e.g., 224.25 °C) did not degrade the superhydrophobicity of the KB + HNs + PDMS electrically conductive paper owing to the thermal stability of PDMS coating up to 300 °C (Figures 3f and S14, Supporting Information). This feature of PDMS is differentiated from other low-surface-energy compounds that decompose at low temperatures.44 Furthermore, we also investigated the deicing performance of the KB + HNs + PDMS electrically conductive paper at a low temperature (−20 °C). During storage in a refrigerator at a temperature of −20 °C for 24 h, water on the superhydrophobic KB + HNs + PDMS paper froze into ice (Figure S17a, Supporting Information). After applying a voltage, the ice melted into water droplets. However, these water droplets were too tiny to roll away automatically. As discussed above, the high temperature produced by the electrothermal effect of the KB + HNs + PDMS electrically conductive paper could promote the evaporation of the extremely small water droplets (Figure S17b−f, Supporting Information). It should be noted that big ice blocks instead of tiny ice particles are more common in the practical environments. Under these conditions, the superhydrophobic surface will have a superior advantage. Because the heated superhydrophobic KB + HNs + PDMS electrically conductive paper for deicing and water evaporation is more

efficient compared to the unheated superhydrophobic paper or heated superhydrophilic paper, it is necessary to emphasize the synergistic effect between the superhydrophobicity and electrothermal effect for the protection of the flexible electronic devices from water, ice, snow, and pollutants. 3.4. Flame-Retardant Property of the KB + HNs + PDMS Electrically Conductive Paper. As discussed in Section 1, the erroneous use of electronic devices may cause fire accident. It is desirable for the electronic devices to be flameretardant. In this work, we investigated the flame retardancy of the KB + HNs + PDMS electrically conductive paper. The experimental setup for the flame-retardant tests was designed on the basis of the vertical combustibility method (Figure S18, Supporting Information).61,62 As shown in Figure 8a, the cardboard made of plant cellulose fibers was used as the control sample. As expected, the cardboard was easily burnt within 10 s. In contrast, when the KB + HNs + PDMS electrically conductive paper was exposed to fire (Figure 8b), there was no obvious flame on the paper, and the paper could well preserve its structure integrity for at least 7 min. The flameretardant tests of the KB + HNs + PDMS electrically conductive paper (0−30 s) and cardboard (0−10 s) are shown in Movies S5 and S6 (Supporting Information), respectively. L


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ACS Applied Materials & Interfaces To understand the flame retardancy of the KB + HNs + PDMS electrically conductive paper, we carried out the flameretardant tests for the samples of KB, HNs paper, KB + HNs paper, HNs + PDMS paper, and KB + HNs + PDMS paper (Figure S19, Supporting Information). The experiments indicate that KB had a good thermal stability below 600 °C, as demonstrated by its nonflammable feature in fire (Figure S19a, Supporting Information). Because TG curve shows that KB lost its weight significantly when the temperature was higher than 550 °C (Figure S5, Supporting Information), we suppose that the temperature of the flame and the short heating time were not sufficient to burn KB. Moreover, the pure HNs paper exhibited excellent resistance to fire (Figure S19b, Supporting Information). As a result, the KB + HNs paper comprising KB and nonflammable HNs showed good resistance to fire as well (Figure S19c, Supporting Information). However, the HNs + PDMS paper could catch fire due to the side alkyl groups of PDMS (Figure S19d, Supporting Information). After exposing the KB + HNs + PDMS paper to the flame, a small amount of gas was observed, but there was no obvious flame on the paper (Figure S19e and Movie S7, Supporting Information). From these comparative studies, we deduce that KB could improve the flame retardancy of PDMS. Furthermore, we exposed the surface of the KB + HNs + PDMS paper to the flame for 7 min to investigate the products of the combustion (Figure S20, Supporting Information). Although the appearance of the bottom surface of the paper changed, the top surface of the paper survived with unbroken structure (Figure 8c,d). A thick layer was formed on the bottom surface after the combustion (Figure 8e), which was considered as mainly the inorganic residue of silica,53 as supported by the elemental analysis (Figure 8f). This feature of PDMS is distinct from the ordinary organic polymers and consequently the silica residue layer can serve as an insulating barrier to protect the interior materials from catching fire. The uniform physical barrier was regarded as an effective flame-retardant method.35,36 However, there were some cracks between residual islands, which may become the entrance for the fire. Fortunately, the underlying HNs could serve as a second barrier to resist the flame. The elemental mapping of the bottom surface showed the distribution of Ca element from HNs in these cracks (Figure 8g), indicating the presence of the underlying HNs. After the combustion, the superhydrophobic property of the KB + HNs + PDMS electrically conductive paper was destroyed due to thermal degradation of the PDMS coating. Fortunately, recoating the electrically conductive paper with PDMS could recover the superhydrophobicity of both the top and bottom surfaces (Figure S21, Supporting Information). We investigated the electrical performance of the KB + HNs + PDMS electrically conductive paper when the paper was exposed to the flame, as shown in Figure 9a. During the combustion process, the electrical current through the KB + HNs + PDMS electrically conductive paper increased from the initial value of 11.92 mA to 13.39 mA at 60 s and then became stable in the following combustion process (13.34 A at 7 min). The maximal current change during the combustion process was determined to be 15.5% (Figure 9b). After the combustion, the electrical current through the KB + HNs + PDMS electrically conductive paper was measured to be 10.80 mA (Figure S22, Supporting Information). Therefore, the total current change was relatively low (9.40%) and the electrical current after the flame treatment was 90.60% of the initial value (Figure 9b). Real-time monitoring of the electrical current

during the combustion process is shown in Figure 9c. The realtime monitoring of the brightness of an LED lamp connected with the KB + HNs + PDMS electrically conductive paper during the process of exposing the paper to the flame for 7 min is shown in Figure 9d. The experiment showed that the KB + HNs + PDMS electrically conductive paper could provide the stable and continuous service for the LED lamp under the extreme condition of combustion, and the brightness of the LED lamp was as high as the original state during the whole combustion process.

4. CONCLUSIONS In conclusion, we report a novel strategy for designing a new kind of all-weather flexible superhydrophobic electrically conductive paper based on HNs, KB, and PDMS. HNs and KB not only serve as a stable fire-resistant host material and a conductive filler, respectively, but also provide two levels of roughness, whereas the PDMS coating provides the superhydrophobicity and enhances the mechanical properties of the paper. It should also be mentioned that these three constituents show high thermal stability and good flame retardancy. Therefore, this new kind of all-weather flexible electrically conductive paper combines the superhydrophobic surface, electrothermal effect, and flame retardancy; thus, the paper exhibits excellent electrical performance under the corrosive, humid, and high-temperature conditions. The KB + HNs + PDMS electrically conductive paper can work well under extreme conditions, including underwater and in the flame. We expect that the as-prepared KB + HNs + PDMS electrically conductive paper is promising for applications in various flexible electronic devices with the ability of working under extreme conditions.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b09484. Supplementary characterization of KB, HNs, and their composites, including water contact angle measurements, XRD patterns, SEM and TEM images, FTIR spectra, TG curves; properties of the KHP-x (x = 0, 1, 2, 3, 4) paper, such as surface wettability, morphology, flexibility, electrical conductivity, mechanical properties, and flame retardancy (Figures S1−S21, Tables S1−S4) (PDF) Water droplets were deposited onto the 20 wt % KB + HNs paper that was connected with a direct current power supply at a voltage of 3 V and an ammeter (Movie S1) (AVI) Water droplets were deposited onto the KB + HNs + PDMS electrically conductive paper (KHP-2) that was connected with a direct current power supply at a voltage of 3 V and an ammeter (Movie S2) (AVI) KB + HNs + PDMS electrically conductive paper (KHP2) that was connected with a direct current power supply at a voltage of 3 V and an ammeter was immersed into water (Movie S3) (AVI) Deicing process of the KB + HNs + PDMS electrically conductive paper (KHP-2) without and with an applied voltage of 20 V (Movie S4) (AVI) Vertical combustibility test of the cardboard (Movie S5) (AVI) M


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Vertical combustibility test of the KB + HNs + PDMS electrically conductive paper (KHP-2) (Movie S6) (AVI) Flame-retardant test of the KB + HNs + PDMS electrically conductive paper (KHP-2) (Movie S7) (AVI)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel: 0086-21-52412616. Fax: 0086-21-52413122 (Y.-J.Z.). *E-mail: [email protected] (Z.-C.X.). ORCID

Ying-Jie Zhu: 0000-0002-5044-5046 Zhi-Chao Xiong: 0000-0002-0216-5699 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Science and Technology Commission of Shanghai (15JC1491001), the National Natural Science Foundation of China (21601199, 21501188, 51702342), and the Shanghai Sailing Program (16YF1413000) is gratefully acknowledged.

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ABBREVIATIONS KB, Ketjen black; HNs, ultralong hydroxyapatite nanowires; PDMS, poly(dimethylsiloxane); KHP, KB + HNs + PDMS REFERENCES

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