Fabrication and Synthesis of Highly Ordered Nickel Cobalt Sulfide

Sep 27, 2017 - Well-aligned NiCo2S4 nanowires, synthesized hydrothermally on the surface of woven Kevlar fiber (WKF), were used to fabricate composite...
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Cite This: ACS Appl. Mater. Interfaces 2017, 9, 36311-36319

Fabrication and Synthesis of Highly Ordered Nickel Cobalt Sulfide Nanowire-Grown Woven Kevlar Fiber/Reduced Graphene Oxide/ Polyester Composites Ankita Hazarika, Biplab K. Deka, DoYoung Kim, Hyung Doh Roh, Young-Bin Park, and Hyung Wook Park* Department of Mechanical Engineering, Ulsan National Institute of Science and Technology, 50 UNIST-gil, Ulsan, Republic of Korea 44919 S Supporting Information *

ABSTRACT: Well-aligned NiCo2S4 nanowires, synthesized hydrothermally on the surface of woven Kevlar fiber (WKF), were used to fabricate composites with reduced graphene oxide (rGO) dispersed in polyester resin (PES) by means of vacuum-assisted resin transfer molding. The NiCo2S4 nanowires were synthesized with three precursor concentrations. Nanowire growth was characterized using scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. Hierarchical and high growth density of the nanowires led to exceptional mechanical properties of the composites. Compared with bare WKF/PES, the tensile strength and absorbed impact energy were enhanced by 96.2% and 92.3%, respectively, for WKF/NiCo2S4/rGO (1.5%)/PES. The synergistic effect of NiCo2S4 nanowires and rGO in the fabricated composites improved the electrical conductivity of insulating WKF/ PES composites, reducing the resistance to ∼103 Ω. Joule heating performance depended strongly on the precursor concentration of the nanowires and the presence of rGO in the composite. A maximum surface temperature of 163 °C was obtained under low-voltage (5 V) application. The Joule heating performance of the composites was demonstrated in a surface deicing experiment; we observed that 17 g of ice melted from the surface of the composite in 14 min under an applied voltage of 5 V at −28 °C. The excellent performance of WKF/NiCo2S4/rGO/PES composites shows great potential for aerospace structural applications requiring outstanding mechanical properties and Joule heating capability for deicing of surfaces. KEYWORDS: aramid fiber, NiCo2S4 nanowires, reduced graphene oxide, composite, mechanical properties, Joule heating

1. INTRODUCTION Frosting of wing edges, wind turbines, and tails of aerospace structures in extreme cold environments can be very dangerous; ice accumulation damages aircraft structures and the performance of the aircraft, creating a serious safety threat.1 Aircraft blades are carefully designed to provide the airflow necessary to assist in buoyancy; the existence of ice on the blade and wings of aircraft promotes shape change and surface roughness that can affect aerodynamic performance by disrupting airflow around the blades.2,3 Among the various deicing strategies, deicing chemicals are primarily used; despite their costeffectiveness, these chemicals are harmful as they pollute groundwater and cause corrosion.4,5 Joule heating is an efficient deicing electrothermal practice occurring in an electrically conductive material in which voltage-applied heating ensues from the dissipation of electrical power. Electric heating devices are also used in industrial processes, plane heating, water heating, among others. For electric heating purposes, electrically conductive polymer composites offer many advantages owing to their easy processing, low cost, corrosion resistance, © 2017 American Chemical Society

lightweight, and good thermomechanical stability, compared with metallic materials.6,7 Thermomechanical stability is an especially important property of electric heating devices throughout their service term for practical applications. Conductive polymer composites must have high thermal and mechanical stability to be deployed in an electric heating device.7 Aramid fiber is applied in a wide range of applications due to its exceptional ballistic resistance, high strength properties, and oxidative and thermal stabilities. It has commercial significance for its utility in various components of spacecraft and body armor.8 The high thermal stability of aramid fibers also assists in fabricating heat-resistant materials suitable for fire safety. Exceptional mechanical strength and thermal stability make aramid fiber an ideal candidate for use in electric heating devices through the incorporation of a conductive filler.9 The Received: August 6, 2017 Accepted: September 27, 2017 Published: September 27, 2017 36311

DOI: 10.1021/acsami.7b11712 ACS Appl. Mater. Interfaces 2017, 9, 36311−36319

Research Article

ACS Applied Materials & Interfaces

Figure 1. Scanning electron microscopy images of (a) woven Kevlar fiber (WKF), (b) NiCo2S4 nanowires grown with conc. A, (c) NiCo2S4 nanowires grown with conc. B, (d) NiCo2S4 nanowires grown with conc. C with high-resolution images of the nanowires in the inset, and (e) energy-dispersive X-ray spectrometry (EDX) analysis of NiCo2S4 nanowire-grown WKF with conc. B.

and rGO into the WKF/PES composites enhanced electrical conductivity, and the resulting mechanical properties of the fabricated composites promoted electric heating behavior. Specifically, NiCo 2 S 4 nanowires and rGO significantly influenced the Joule heating performance of the composites, as demonstrated in the deicing experiment. During deicing, the interface between the heated composite and the ice melted first, producing a thin water layer via Joule heating. This enabled easy removal of the ice as its adhesion to the composite surface deteriorated rapidly. To our knowledge, the effect of NiCo2S4 nanowires on electric heating performance has yet to be investigated. The fabricated lightweight WKF/NiCo2S4/rGO/ PES composites can be suitably applied in various aircraft components with improved mechanical properties and deicing technology.

inclusion of any conductive nanomaterial into insulating Kevlar fiber generates an electrical network throughout the fiberreinforced polymer composite. Carbon nanofiller, such as reduced graphene oxide (rGO) nanosheets, holds exceptional thermal conductivity in the range of 3000−5000 W m−1 K−1, and its inclusion enhances heat conduction with uniform temperature throughout the composite.10 Transition metal oxide nanostructures, such as CuO or SnO2, play a significant role in enhancing interlaminar resistive heating in woven carbon fiber-reinforced polymer composites, as reported in the literature.11,12 Recently, transition metal sulfides, particularly the ternary transition metal sulfides, have received much attention, due to their outstanding properties and potential applications in electronic, optical, and optoelectronic devices, and they offer better performance than monometal sulfides.13 The low electrical conductivity of transition metal oxides/hydroxide materials due to their smaller band gap over the ternary transition metal sulfides makes them a fascinating class of materials. Ternary nickel cobalt sulfide (NiCo2S4) is commonly utilized as an electrode material for supercapacitors and other energy storage devices; its electrical conductivity is ∼100-fold higher than that of NiCo2O4, even though NiCo2O4 exhibits much-improved electric conductivity compared with NiO and Co3O4,14,15 being very close to metallic conductivity.16 In this work, different concentrations of hydrothermally synthesized, hierarchical NiCo2S4 nanowire-grown woven Kevlar fiber (WKF) and polyester resin (PES) with dispersed rGO were utilized to fabricate unique hybrid WKF/NiCo2S4/ rGO/PES composites via vacuum-assisted resin transfer molding (VARTM). The introduction of NiCo2S4 nanowires

2. EXPERIMENTAL SECTION 2.1. Materials. Woven Kevlar fibers sheets (Kevlar 49) were received from JMC Co., Ltd. (Gyeongsangbuk-do, South Korea). Nickel chloride hexahydrate (NiCl2·6H2O), cobalt chloride hexahydrate (CoCl2·6H2O), urea, and sodium sulfide were obtained from Sigma-Aldrich (South Korea). Hexadecyl trimethylammonium bromide and rGO were supplied by Sigma-Aldrich (South Korea) and IDT International (South Korea), respectively. PES (LSP-8020B) and methyl ethyl ketone peroxide were purchased from CCP Composites and Arkema (South Korea). 2.2. Synthesis of NiCo2S4 Nanowires and Fabrication of Composites. Prior to synthesizing NiCo2S4 nanowires on the surface of WKF, the samples were cut to a size of 75 × 75 mm2, washed thoroughly with acetone and ethanol solution, and dried in the oven at 80 °C. Typically, three precursor concentrations were used during nanowire synthesis labeled as concentrations A, B, and C, and details 36312

DOI: 10.1021/acsami.7b11712 ACS Appl. Mater. Interfaces 2017, 9, 36311−36319

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Figure 2. (a) Transmission electron microscopy (TEM) image of NiCo2S4 nanowires, (b) a high-resolution image of NiCo2S4 nanowire, and (c) scanning TEM image and elemental mapping of Ni, Co, and S for NiCo2S4 nanowires. of the concentration were provided in Table S1. CoCl2·6H2O, NiCl2· 6H2O, hexadecyl trimethylammonium bromide, and urea were dissolved in 600 mL of deionized water to form a pink transparent solution. This solution was transferred to a Teflon-lined stainless-steel autoclave. Under a tightly sealed autoclave, the WKF sheet was immersed in the solution and kept at 120 °C for 6 h. The samples covered with NiCo2(CO3)1.5(OH)3 nanowire arrays were then taken out and washed carefully several times with deionized water. In the second step, the sample was immersed in Na2S solution, and hydrothermal treatment was conducted for 12 h at 150 °C. Sulfur anions (S2−), discharged from sodium sulfide, substituted for CO32− and OH− anions in the NiCo2(CO3)1.5(OH)3 nanowire arrays via an anion-exchange reaction to form ordered NiCo2S4 nanowire arrays. Finally, the samples coated with NiCo2S4 nanowires were taken out, rinsed several times with deionized water, and dried in oven at 60 °C overnight. Composites were fabricated using five sheets of NiCo2S4 nanowire-grown WKF samples with rGO diffused PES, using a VARTM technique, as described in our previous work.17 2.3. Characterization. The morphologies, growth densities, and microstructural features of as-synthesized NiCo2S4 nanowires on the surface of WKF were observed by scanning electron microscopy (SEM; Nova Nano SEM 230 FEI, Hillsboro, OR, USA) operating at 15 kV; the SEM system was equipped with an energy-dispersive X-ray spectrometer (EDS/EDX), which was used to detect the elemental percentage composition of the nanowire. High-resolution transmission electron microscopy (HR-TEM, JEM-2100F, JEOL, Tokyo, Japan) was also used to characterize the nanowire growth. X-ray diffraction (XRD) measurements were conducted with a wide-angle X-ray diffractometer (Bruker, Billerica, MA) working at 40 kV/20 mA with monochromatic Cu Kα radiation with 2θ from 10° to 90°. The chemical states of the elements present in the nanowires were investigated by X-ray photoelectron spectroscopy (XPS; Thermo Fisher, UK).

Tensile tests were performed using an Instron 5982 universal testing machine at a displacement rate of 2 mm/min with a maximum load of 100 kN, in accordance with the ASTM D3039 standard. Tensile test results correspond to the average of three test specimens. Impact resistance tests of the composites were carried out using a drop-weight impact tester (model 5982; Instron, Norwood, MA) following the ASTM D5628−10 standard. Energy of 200 J was applied to each sample to perforate the samples completely. The round clamp was laden with a 5 kg impactor having a 40 mm diameter. Data were analyzed between the initial impact contact and until penetration occurred. A 6517 multimeter (Keithley Instruments, Beachwood, OH) was used to perform electrical resistance tests. The Joule heating behavior of the composites was evaluated at three voltages: 1, 3, and 5 V. Infrared thermographic investigations were conducted with an infrared camera (SE/A325; FLIR Systems Inc.), and the images were collected for each sample during heating when the maximum temperature is attained. The surface temperature of the sample was measured using thermocouples attached to its surface. Current was applied through electrodes positioned on the sample, and the distance between them was kept at 50.0 mm. The electrodes were connected to an electrical power supply, and the surface temperature of the samples was recorded. The deicing experiment was conducted by applying a constant direct current (DC) voltage across the electrodes attached to the samples in a styrofoam box. The temperature inside the box was maintained at −28 °C by placing dry ice inside it. Ice was formed on the samples by spraying water on the sample surface.

3. RESULTS AND DISCUSSION Figure 1 shows SEM images of the nanowires grown with different concentrations of the precursors. The growth density of the nanowires was very sparse when concentration A of the 36313

DOI: 10.1021/acsami.7b11712 ACS Appl. Mater. Interfaces 2017, 9, 36311−36319

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Figure 3. (a) X-ray diffraction spectra of WKF and NiCo2S4 nanowire-grown WKF with different concentrations. X-ray photoelectron spectroscopy measurements of (b) NiCo2S4 nanowires, (c) Ni 2p, (d) Co 2p, and (e) S 2p.

precursors was used (Figure 1b). The low precursor concentration suppressed homogeneous nucleation for uniform growth of the nanowires. When concentration B of the precursors was used in the growth solutions, vertically aligned, highly dense nanowire growth was obtained, with uniform distribution over the WKF surface (Figure 1c). Uniform growth density and orientation of the nanowires is very critical for effective performance of the device. The density of the nanowires on the filament as well as the fiber interlacement region was determined from SEM images. The nanowire density grown with conc. B on the single fiber filament was around 54/μm2 and in the fiber interlacement region was around 82/μm2. The growth orientation of the nanowires throughout the fabric is mostly perpendicular to the substrate and is the energetically favorable growth direction. The nanowires are aligned at different angles to the substrate. The diameter of the nanowires ranged from 50 to 90 nm, and the length of the nanowires varied from 2 μm to several microns (Figure S1). Concentration C, corresponding to a high precursor concentration, produced agglomerated nanowire structures (Figure 1d). The nanowires bent gradually toward each other near their tip until agglomeration occurred, resulting in the formation of extensive nanowire bundles enveloping the WKF surface. The coalescing structures of the nanowires were attributed to the liquid solution concentration and capillary and van der Waals forces.18 The EDX spectrum (Figure 1e) of NiCo2S4 nanowires confirmed the presence of Ni, Co, and S as the main constituent elements. An atomic ratio of 1:1.9:3.8 was obtained for Ni:Co:S with the expected stoichiometry of 1:2:4; this result was consistent within experimental error. Figure 2 shows transmission electron microscopy (TEM) images of the nanowires. Each nanowire consisted of NiCo2S4

nanoparticles (Figure 2a). Higher-resolution images (Figure 2b) indicated that the lattice planes of the nanowires were oriented randomly, suggesting polycrystalline behavior. The presence of lattice fringes having interplane gaps of 0.287 and 0.239 nm was also observed in the micrographs that corresponded to the (311) and (400) planes of NiCo2S4 (Figure 2b). Similar findings were also reported by Bai et al. while performing HR-TEM analysis of NiCo2S4.19 Scanning TEM (STEM) images (Figure 2c) and EDX elemental mapping images of NiCo2S4 nanowires revealed that all constituent elements (Ni, Co, and S) of the NiCo2S4 nanowires had a spatially homogeneous distribution over the investigated detection range of the nanowires. XRD patterns of woven Kevlar fiber and NiCo2S4 nanowires grown on WKF with different precursor concentrations are shown in Figure 3a. Untreated WKF exhibited diffraction peaks at 2θ of 20.88° (110) and 23.03° (200), and these peaks were present in all NiCo2S4-nanowire-grown WKF samples.20 The growth of NiCo2S4 nanowires was confirmed from the presence of characteristic peaks of 2θ at 26.95°, 32.4°, 38.6°, 50.6°, and 55.4° corresponding to the (220), (311), (400), (511), and (440) planes of the cubic structured phase of NiCo2S4, respectively.13 The synthesized NiCo2S4 nanowires retained very high purity as no other peaks were observed in the spectra. The peak intensity of the nanowires depended on various factors, including alignment, crystallization quality, growth density, and the aspect ratio of the nanowires. The nanowires grown with concentration B showed intense peaks that were attributed to the aligned, high growth density, and enhanced aspect ratio of the nanowires with concentration B. The growth of nanowires obtained with concentration A was very sparse and not properly aligned; hence, a lower peak intensity was observed. The apparent grain boundaries reduced nanowire 36314

DOI: 10.1021/acsami.7b11712 ACS Appl. Mater. Interfaces 2017, 9, 36311−36319

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Figure 4. (a) Energy−time response curves and (b) tensile stress−strain curves of WKF/polyester resin (WKF/PES) composites.

enhanced the interfacial interaction area, thereby increasing the absorbed impact energy by accruing damage within the composite. The high specific surface area and high stiffness of rGO nanosheets induced favorable impact resistance properties to the composites. The toughening and reinforcing effect of rGO in polycarbonate in enhancing its impact resistance are well-addressed by Wang et al.24 Nanowires grown with concentration C of the precursor solution exhibited the least impact resistance, whereas concentration B-grown nanowires showed the highest impact resistance, followed by samples grown with nanowires having concentration A. To examine the influence of nanowires, the impact resistance of concentration B composites was checked without adding rGO. Uniform well-aligned nanowire growth was observed, which may explain the highly enhanced impact resistance of WKF/NiCo2S4 (conc. B)/PES samples, compared with WKF/ NiCo2S4 (conc. A)/rGO (1.5%)/PES and WKF/NiCo2S4 (conc. A)/rGO (1.5%)/PES samples. Thus, the impact resistance of the composites was highly influenced by the growth density and homogeneity of NiCo2S4 nanowires. The tensile stress−strain curves (Figure 4b) were utilized to investigate the synergistic reinforcing effect of NiCo2S4 nanowires and rGO nanosheets in the composites. WKF/PES composites incorporated with NiCo2S4 (conc. B) and rGO displayed the highest tensile properties, specifically, a 96.2% improvement compared with bare WKF/PES samples. The high surface area of the nanowires and rGO nanosheets facilitated effective load bearing ability due to the improved interfacial interaction with the polymer and fiber. WKF/PES composites embodied with nanowires (conc. A) exhibited lower tensile strength and modulus, followed by samples incorporated with nanowires (conc. C). The longer dimensions and large aspect ratio of the nanowires with conc. B may assist in limiting the relaxation of polymer chains, particularly those in the amorphous region, thus providing higher load-bearing ability and enhanced tensile properties.25 WKF/NiCo2S4(conc. C)/rGO (1.5%)/PES showed the lowest tensile properties due to the lack of interfacial adhesion of the polymer matrix with the fiber attributed to nonaligned, agglomerated, and uneven nanowire growth forming networklike structures within themselves without interacting with the polymer matrix. Figure 5 depicts the electrical resistance (R) of WKF/PES composites plotted as a function of NiCo2S4 nanowire concentration. The electrical resistivity (ρ) of the composite samples was also evaluated using the formula R = ρ (L/A), where L represents the length between electrodes and A is the

growth with concentration C, thereby disrupting its crystal structure leading to a decrease in peak intensity. X-ray photoelectron spectroscopy (XPS) is an efficient technique for investigating surface elemental composition and its chemical states. A full scan survey spectrum of the sample ranging from 0 to 1400 eV can be used to detect Ni, Co, and S (Figure 3b). The presence of C (as a reference) and O elements in the spectrum may be due to air exposure of the samples or the elemental composition of WKF. The spectrum of Ni 2p (Figure 3c) was deconvoluted using a Gaussian fitting method into two spin−orbit doublets distinctive of Ni2+ and Ni3+ and two shakeup satellites (designated as “Sat.”). The existence of both Ni2+ and Ni3+ was confirmed from the binding energy peaks for Ni 2p3/2 and Ni 2p1/2, appearing at 854.7 and 871.8 eV, respectively. The spectrum of Co 2p (Figure 3d) was fitted with two spin−orbit doublets, appearing at 796.2 and 779.2 eV indexed to Co 2p1/2 and Co 2p3/2, respectively, which confirms the presence of Co2+ and Co3+ and two shakeup satellites. The S 2p spectrum (Figure 3e) shows that the S 2p peak can be split into two main peaks with binding energies of 162.8 and 163.4 eV, ascribed to S 2p3/2 and S 2p1/2, respectively, and one shakeup satellite peak. The peak at 163.8 eV is characteristic of a metal−sulfur bond, while 162.6 eV is indexed to the sulfur ion in low coordination on the surface. Thus, the elemental composition of NiCo2S4 consists of Co2+, Co3+, Ni2+, Ni3+, and S2−, which is in good accordance with the results obtained in the literature.21,22 The low velocity impact energy is converted into absorbed and rebounded energy, in which the absorbed energy is the total energy distributed throughout the composite laminate at the end of an impact. In the beginning, the energy is absorbed by elastic deformation on application of impact to the composite samples. A certain amount of the energy is engrossed by friction owing to the intrinsic brittleness of fiber-reinforced composites, whereas the remaining energy is absorbed by the composites by means of various damage mechanisms once the level of energy is outside the limits needed for full elastic deformation.23 The impact resistance of the WKF composites is illustrated from absorbed energy versus time curves, as shown in Figure 4a. Untreated WKF/PES composites displayed the lowest impact resistance. The growth of NiCo2S4 nanowires on the surface of WKF and incorporation of rGO improved the absorbed impact energy of the WKF/PES composites. The NiCo2S4 nanowires facilitated fiber-matrix interfacial bonding. Homogeneous dispersion of rGO in the polymer matrix further 36315

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electrically conductive material. NiCo2S4 nanowires reportedly exhibit metallic conductivity;16 the growth concentration of the nanowires must be above the electrical percolation threshold, in which a conductive nanowire network forms. WKF/NiCo2S4 (conc. B)/rGO (1.5%)/PES showed the highest electrical conductivity; the high aspect ratio of the nanowires and increased surface area provided an effective conductive network with well-dispersed rGO nanosheets present in the polymer matrix. WKF/PES composites incorporated with a fixed amount of rGO (1.5%) and NiCo2S4 nanowires with concentrations A and C displayed lower electrical conductivity than that of composites with concentration B; this was attributed to the low growth density of the nanowires with concentration A and highly agglomerated structures of nanowires with concentration C. Interestingly, a drastic decrease in electrical resistance was observed from 105 Ω for WKF/NiCo2S4(conc. B)/PES to ∼103 Ω for WKF/NiCo2S4 (conc. B)/rGO(1.5)/PES composites, indicating that the electrical resistance of the composites is also governed by the existence of rGO in the composites. The electron charge transport properties in graphene layers are the main mechanism determining electrical conductivity that sequentially relies on the functionality, defects, and layer disorder of the graphene sheets. Ruoff and co-workers suggested that, despite possessing oxygen functional groups, rGO exhibited high electrical conductivity due to (1) an enhanced conductive network

Figure 5. Electrical resistance and resistivity of WKF/PES composites.

cross-sectional area of the sample; the results are plotted as a function of NiCo2S4 nanowire concentration. The electrical resistance properties of the composites were dependent on both nanowire growth and the presence of rGO in the composites. The different growth concentrations of the nanowires had a remarkable impact on the electrical resistivity of the composites as it turned insulating WKF/PES into an

Figure 6. Time-dependent temperature plot during resistive heating [A-C] and average temperature [D] at applied voltages of 5, 3, and 1 V for (a) WKF/PES, (b) WKF/NiCo2S4(conc. B)/PES, (c)WKF/NiCo2S4(conc. C)/rGO(1.5%)/PES, (d)WKF/NiCo2S4(conc. A)/rGO(1.5%)/PES, and (e) WKF/NiCo2S4(conc. B)/rGO(1.5%)/PES. 36316

DOI: 10.1021/acsami.7b11712 ACS Appl. Mater. Interfaces 2017, 9, 36311−36319

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Figure 7. (a) Deicing experimental setup with the sample in a dry ice box (temperature: −28 °C), (b) ice formation in the sample WKF/ NiCo2S4(conc. B)/rGO(1.5%)/PES before applying a voltage, (c) ice removal after 14 min for the same sample, (d) ice formation in the WKF/PES sample before applying the voltage, and (e) 14 min after applying the voltage to the WKF/PES sample.

having concentrations A and C. Furthermore, the presence of rGO promoted even heat distribution throughout the composite sample. Zhou et al. reported outstanding heat conduction and temperature uniformity when rGO was incorporated into a poly(3,4-ethylenedioxythiophene):poly(styrene sulfonic acid) (PEDOT:PSS) elastomeric waterborne polyurethane composite film.10 WKF/PES composites incorporated with NiCo2S4(conc. B) without rGO exhibited the lowest surface temperature compared with all of the composite samples loaded with rGO; thus, the existence of rGO highly influenced the Joule heating performance of the composite samples. The performance of m-aramid/multiwalled carbon nanotube composite film was studied as electric heating device by Jeong et al.7 The film achieved a maximum temperature of 65 °C at 5 V on incorporation of 10 wt % MWCNT to maramid film, and on application of 10 V the surface temperature raised to 176 °C. Raji et al. prepared graphene nanoribbon (GNR) stacks-epoxy composites and reported a maximum surface temperature of 60 °C at 20 V for the composite samples.27 In our study, the combined effect of NiCo2S4 nanowires and rGO nanosheets has remarkable influence in electric heating performance of WKF/PES composites as the maximum surface temperature achieved for the composites was 163 °C on application of 5 V. A high surface temperature in a short duration with faster heating rate on application of low voltage (5 V) is indicative of the suitability of WKF/NiCo2S4/ rGO/PES composites in deicing applications. A deicing experiment was performed in which WKF/ NiCo2S4 (conc. B)/rGO (1.5%)/PES and WKF/PES samples were placed inside of a styrofoam box cooled to a temperature of −28 °C by placing chunks of dry ice. WKF/NiCo2S4 (conc. B)/rGO (1.5%)/PES and WKF/PES were used in the deicing tests to compare the deicing capabilities of the two composites. Figure 7 shows digital images of the experimental setup with the samples with the ice forming on their surfaces before applying the voltage and 14 min after voltage application. The WKF/PES sample incorporated with NiCo2S4 (conc. B) and rGO could melt 17 g of ice in 14 min on application of 5 V, whereas the bare WKF/PES showed no deicing capability (see video 1 and video 2). It is clearly evident from the digital images of the samples that no delamination was visible, and the sample remained unimpaired following cooling and heating cycles. Technically, the ice would be removed from the surface of the aircraft in a shorter time. As soon as a layer of water forms underneath, the ice over that segment detaches and

with cross-linked connections, (2) the percolation threshold determined by conductive particle-to-particle interfaces, and (3) charge transport in the sample through ionic channels. The high conductivity of the rGO is mainly linked with interparticle associates, and it is also much less susceptible to the surroundings.26 Joule heating behavior of the composites was investigated by applying voltages of 1, 3, and 5 V to the composites via the electrodes attached to the composite sample. Time-dependent temperature profiles and the average temperature attained by the sample on applied varied voltages are shown in Figure 6. Infrared images of individual samples after applying different voltages are shown at the side of each profile. The electric heating behavior of the composite samples was evident from infrared images, which changed from blue to dark yellow, red, and then white over time with voltage application; an even temperature distribution over the composite was observed. In the temperature versus time plots, all samples displayed three distinct regions: the heating region, the maximum temperature region, and the cooling region. The heating performance of the composite relied strongly on the nanowire growth concentration and rGO content. The temperature rose concomitantly with the upsurge of the applied voltage, signifying the temperature-regulating ability of the electric heaters and excellent electric heating performance. The high conductivity of the composite allowed the temperature to reach the equilibrium state (maximum point referred to as the steadystate temperature) from room temperature quickly, irrespective of the applied voltage. The surface temperature of the composite is constant in the equilibrium state because the supplied electrical power is stabilized by the dissipated power. For the same samples, a higher voltage resulted in improved maximum surface temperature as the power distributed throughout the composite enhanced, according to the formula P = V2/R, triggering increased surface temperature. The enhanced surface temperature promoted higher heating rates for the same samples on application of higher voltages. The maximum temperature was highest for the WKF/NiCo2S4(conc. B)/rGO(1.5%)/PES sample, followed by WKF/ PES composites loaded with concentrations A and C, whereas WKF/NiCo2S4(conc. B)/PES showed the lowest maximum temperature. The high growth density of the nanowires with conc. B and its large surface area to volume ratio initiated a faster thermal response and increased the surface temperature of the sample, compared with samples loaded with nanowires 36317

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leaves with the airflow; moreover, the centrifugal force of the spinning turbines and blades should be able to easily discard the ice. The fabricated composite has potential effectiveness in structural applications such as the wings and tail of aircraft, offering exceptionally high mechanical strength and satisfactory anti-icing performance upon application of a low voltage.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Hyung Wook Park: 0000-0002-7751-1402 Notes

The authors declare no competing financial interest.

4. CONCLUSION NiCo2S4 nanowires were grown on the surface of WKF by a hydrothermal technique with three concentrations (A, B, and C) of precursor solution. Polymer composite was fabricated with NiCo2S4 nanowire-grown WKF and RGO dispersed in PES. SEM confirmed well-aligned nanowire growth when concentration B of the precursor solution was used. The growth of nanowires was also confirmed by TEM, EDX, XRD, and XPS. Both tensile and impact resistance properties were enhanced considerably after NiCo2S4 nanowire and rGO incorporation. WKF/NiCo2S4 (conc. B)/rGO(1.5%)/PES exhibited the highest mechanical performance, followed by the composites incorporated with nanowires having concentrations A and C. Thus, the high growth density and wellaligned growth of nanowires highly influenced the mechanical properties of the composites. The existence of NiCo2S4 nanowires and rGO in WKF/NiCo 2 S 4 (conc. B)/ rGO(1.5%)/PES subsidizes insulating WKF/PES composites to display electrically conducting properties. The electrical resistance increased sharply from 103 Ω for WKF/NiCo2S4 (conc. B)/rGO (1.5%)/PES to ∼105 Ω for WKF/NiCo2S4 (conc. B)/PES composites, confirming that the presence of rGO had a significant effect on the electrical resistance properties of the composites. Moreover, the electrical conductivity can be tuned by varying the nanowire concentration in the composites. Accordingly, faster thermal response, as well as efficient Joule heating performance, was exhibited by WKF/NiCo2S4 (conc. B)/rGO (1.5%)/PES; this composite attained a maximum temperature of 163 °C under a low applied voltage of 5 V. The Joule heating and deicing efficiencies of the composites were established by demonstrating a deicing experiment at −28 °C in which 17 g of ice was removed from the surface of WKF/NiCo2S4 (conc. B)/ rGO(1.5%)/PES in 14 min at a very low applied voltage of 5 V. The fabricated WKF/NiCo2S4/rGO/PES composite, with promising mechanical performance as well as deicing efficiency, is very attractive for aerospace structural applications and can also provide satisfactory electromagnetic interference shielding to aircraft. The WKF composite can also be used as an electric heating device in various applications such as window or mirror deicing, floor heating, water heating, road deicing, and functional textiles.

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ACKNOWLEDGMENTS This work was supported by the Midcareer Researcher Program through the National Research Foundation of Korea (NRF) Grant funded by the Ministry of Science and ICT (NRF2015R1A2A2A01005499) and the National Research Foundation of Korea (NRF) Grant funded by the Ministry of Science and ICT (NRF-2017R1A5A1015311).

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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.7b11712. Concentration details and different concentrations of precursors during synthesis of NiCo2S4 nanowires (PDF) Deicing experiment of WKF/PES composite (AVI) Deicing experiment of WKF/NiCo2S4(conc. B)/rGo (1.5%)/PES composite (AVI) 36318

DOI: 10.1021/acsami.7b11712 ACS Appl. Mater. Interfaces 2017, 9, 36311−36319

Research Article

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DOI: 10.1021/acsami.7b11712 ACS Appl. Mater. Interfaces 2017, 9, 36311−36319