Core-Sheath Paraffin-Wax-Loaded Nanofibers by Electrospinning for

Mar 28, 2018 - Paraffin wax (PW) is widely used for smart thermoregulation materials due to its good thermal performance. However, the leakage and low...
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Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 12759−12767

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Core-Sheath Paraffin-Wax-Loaded Nanofibers by Electrospinning for Heat Storage Yuan Lu,†,‡,§,∥ Xiudi Xiao,*,†,‡,§ Yongjun Zhan,†,‡,§ Changmeng Huan,†,‡,§,∥ Shuai Qi,†,‡,§,⊥ Haoliang Cheng,†,‡,§ and Gang Xu†,‡,§,#

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Guangzhou Institute of Energy Conversion and ‡Key Laboratory of Renewable Energy, Chinese Academy of Sciences, Guangzhou 510640, P. R. China § Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development, Guangzhou 510640, P. R. China ∥ University of Chinese Academy of Sciences, Beijing 100049, P. R. China ⊥ Nano Science and Technology Institute, University of Science and Technology of China, Suzhou 215123, P. R. China # Tibet New Energy Research and Demonstration Centre, Lhasa, Tibet 850000, P. R. China ABSTRACT: Paraffin wax (PW) is widely used for smart thermoregulation materials due to its good thermal performance. However, the leakage and low thermal conductivity of PW hinder its application in the heat storage field. Accordingly, developing effective methods to address these issues is of great importance. In this study, we explored a facile approach to obtain PW-loaded core-sheath structured flexible nanofibers films via coaxial electrospinning technique. The PW as the core layer was successfully encapsulated by the sheath-layer poly(methyl methacrylate). The diameter of the fiber core increased from 395 to 848 nm as the core solution speed rate increased from 0.1 to 0.5 mL/h. In addition, it can be seen that higher core solution speed rate could lead to higher PW encapsulation efficiency according to the transmission electron microscopy results. The core-sheath nanofiber films, moreover, possessed the highest latent heat of 58.25 J/g and solidifying enthalpy of −56.49 J/g. In addition, we found that after 200 thermal cycles, there was little change in latent heat, which demonstrated that it is beneficial for the PW-loaded core-sheath structure to overcome the leakage issue and enhance thermal stability properties for the thermoregulation film. KEYWORDS: phase change material, paraffin wax, electrospinning, core-sheath, heat storage

1. INTRODUCTION The discovery and improvement of materials for energy storage and conversion are important for providing clean and renewable power. The phase change material (PCM), owing to its great potential for heat storage, plays important roles in various application fields such as energy storage, thermal protection, thermoregulation, energy saving, and so on.1−3 Notably, PCMs can absorb and store thermal energy with latent heat and then release the stored heat at constant phase transition temperature.4,5 It has been reported that PCMs, including eutectics and inorganic and organic compounds, could retain the constant thermal property after multiple solid−liquid state transitions.6,7 Although PCMs, such as pentaerythritol and poly(ethylene glycol), have been widely investigated for energy storage,8−10 a significant barrier that these materials possess is that their high water solubility restricts their practicability. In addition, organic PCM alkanes with high latent heat enthalpy were also applied, but the high price limited their applicability. As is well known, paraffin wax (PW), composed of a mixture of alkanes (C18− C30), is a common heat storage organic PCM due to its obvious © 2018 American Chemical Society

advantages in terms of high storage density, high latent heat, low cost, negligible supercooling, and noncorrosive properties.11,12 Nevertheless, it is still a significant challenge that PW suffers from low thermal conductivity and leakage issues during the solid-to-liquid phase change process. In the past, considerable efforts have been also devoted to overcome the aforementioned limitations. On the one hand, higher thermal conductivity materials, such as carbon and metal materials, were introduced to enhance thermal conductivity; notably considerable number of studies have reported that the thermal conductivity of carbon nanotubes (CNTs) was about 3000 W/ (m K), which is higher than that of other materials.13−16 On the other hand, encapsulation was a good technique for solving the leakage of PW, which strongly suppressed the usable lifetime. More recently, stable wax-loaded materials have been prepared by different approaches, such as sol−gel, microencapsulation, and electrospinning.17−19 Among these methReceived: February 8, 2018 Accepted: March 28, 2018 Published: March 28, 2018 12759

DOI: 10.1021/acsami.8b02057 ACS Appl. Mater. Interfaces 2018, 10, 12759−12767

Research Article

ACS Applied Materials & Interfaces

Then, the PMMA and the PW solution were mounted in the double spinneret to produce the sheath and core layers, respectively. The double spinneret consisted of a stainless-steel needle with an inner needle (outer diameter of 0.9 mm and inner diameter of 0.5 mm) and an outer needle (outer diameter of 2.0 mm and inner diameter of 1.2 mm). The solutions were fed into two 10 mL plastic syringes equipped with a blunt metal needle (Beijing Technova Technology Co., Ltd) with an internal diameter of 0.8 mm, which was driven by two syringe pumps (TYD02-04, Baoding Lead Fluid Technology Co., Ltd; WZ50c6, Zhejiang Medical Instrument Co. Ltd., China). The positive electrode of the high-voltage direct current power supply (High Voltage Technology Institute, China) was clamped to the metal needle tip of the syringe with 16 kV, with a distance of 15 cm from the tip to the collector. The sheath feed speed rate was 1 mL/h, whereas the core feed speed rate varied from 0.1 to 0.5 mL/h with relative humidity (45%) and room temperature (25 °C). Finally, the prepared electrospun core-sheath nanofiber films were dried at room temperature under vacuum to remove the solvent. 2.3. Thermal Cycle Test. Furthermore, to explore the thermal stability of the as-obtained film, melting−cooling cycles were conducted in a thermostatic oven between 25 and 80 °C. The films were heated above melting temperature and then cooled to room temperature as a melting and solidifying thermal periodical cycle. The above-mentioned process was carried out consecutively until the times of thermal cycle were 50, 100, 150, and 200. Then, differential scanning calorimetry (DSC) was applied to characterize the thermal stability of the samples. 2.4. Characterizations. The morphologies of as-prepared nanofibers were observed by a field emission scanning electron microscope (FE-SEM, S-4800, FEI Ltd., Japan) at an acceleration voltage of 2 kV. All samples were coated with a thin layer of gold prior to FE-SEM observations. The core-sheath structure of the nanofibers was investigated by a transmission electron microscope (TEM, JEM2100, JEOL, Japan) under an acceleration voltage of 200 kV. To prepare the samples for TEM analysis, the nanostructures were deposited onto 400-mesh carbon-coated Cu grids. The thermal conductivity of the nanofiber film was characterized by the transient plane source method (TPS2500S, Hot Disk, Sweden).36 The samples were cut into 5 cm × 5 cm × 1 cm, and then, a disk sensor (Kapton) was imbedded into the sample. Thermal conductivity was measured by placing the sensor between two samples with parallel plane surfaces to ensure complete contact. Heat transfer was assumed to be purely conductive, and thermal transport properties were determined by the instrument considering the constant electric current supplied by the sensor to monitor the resistance. Differential scanning calorimetry (DSC214, Netzsch, Germany) was carried out in nitrogen flow (20 mL/min). The samples were heated from 30 to 120 °C at a heating rate of 5 K/min and then cooled from 120 to 30 °C.

ods, coaxial electrospinning, using concentrically aligned spinnerettes linking to separate channels for different solutions, can offer a straightforward manner to generate core-sheath nanofibers.20−26 Xia et al. encapsulated the alkanes via melting coaxial electrospinning with an additional complicated heating and temperature controller system,27 whereas there was no need for extra heating equipment and temperature controller in the electrospinning setup for solution coaxial electrospinning.28−32 To date, solution electrospinning is still the most popular electrospinning technique, where the solvent evaporates and the left-behind polymer fibers solidify and can be collected.33 It can provide a facile method to encapsulate PW to obtain core-sheath nanofiber films. For instance, Yu et al. chose soy wax as the encapsulated material, and latent heat of the material was obtained as 36.47 J/g.12 Jassal et al. fabricated nanoencapsulated octadecane nanofibers, and the latent heat obtained was 4.3 J/g.34 Unfortunately, the latent heat enthalpy of these obtained materials was low, which indicates that it is of great importance to further improve their encapsulation efficiency. Furthermore, there is scarce study investigating paraffin-wax-encapsulated materials via coaxial electrospinning. Therefore, the mixture of paraffin wax, with almost the same latent enthalpy as that of n-alkane, was chosen and employed in this work. Here, we present a study of paraffin-wax-loaded core-sheath nanofibers using the simple solution coaxial electrospinning method. After exploring the appropriate solvents for PW, toluene with competitive solubility was chosen as the solvent for the core layer. In addition, to improve the thermal conductivity, CNTs with higher thermal conductivities were added in the sheath layer. It is worthwhile to mention that the well-defined nanofiber film shows some attractive advantages, such as flexibility, high latent heat, mechanical strength, specific surface area, and so on. Additionally, the shape of the nanofiber can be maintained via the polymeric sheath, which stops leakage from occurring. In this work, consecutive 200 recycle phase change experiments have been conducted to estimate the reutilization of the PW-loaded electrospun nanofiber membrane. Thus, we demonstrated that the PW-loaded electrospun core-sheath nanofiber film displayed good thermal storage capacity by encapsulating PW into PMMA using solution coaxial electrospinning.

2. EXPERIMENTAL SECTION 2.1. Materials. The chemicals reagents used in this research were obtained commercially. Poly(methyl methacrylate) (PMMA, Mw = 120 000) was purchased from Shanghai Macklin Biochemical Technology Co., Ltd., China. Paraffin wax (PW, melting point 52− 54 °C) and multiwalled carbon nanotubes (CNTs, length 10−20 μm, internal diameter 5−15 nm, and outer diameter 50 nm) were purchased from Aladdin Chemistry Co., Ltd. Toluene and N,Ndimethylformamide (DMF) were obtained from Guangzhou Chemical Reagent Factory. Other reagents used were of analytical grade. 2.2. Fabrication of the Electrospun Core-Sheath Nanofiber Film. The core-sheath nanofiber film with PW was fabricated via coaxial electrospinning technique, as reported previously.35 Briefly, the PMMA solution as the sheath-layer precursor was obtained by dissolving the PMMA in N,N-dimethylformamide (DMF) with gentle magnetic stirring in ambient temperature with a concentration of 20% (w/w) for 12 h. The core layer precursor solution was prepared using PW dissolved in toluene with different concentrations, ranging from 10 to 50% (w/v). Furthermore, to improve the thermal conductivity, the CNTs and PMMA with different ratios (1, 5, and 10% (w/w)) were added into the DMF with ultrasonic treatment of 2 h and then the mixture solution was stirred at room temperature for 24 h.

3. RESULTS AND DISCUSSION The typical coaxial electrospinning technique was applied to fabricate the PW-loaded core-sheath nanofibers. As shown in Figure 1, coaxial electrospinning using concentrically aligned spinnerettes linking with two separate channels for different solutions. Two kinds of liquids were fed into the inner (PW solution) and outer (PMMA solution or PMMA−CNT blend solution) metallic capillaries, where the PMMA served as a support material to obtain the PW-encapsulated core-sheath nanofibers. The suitable high voltage was applied between the spinneret and the collector, and the nanofibers could be obtained in the surface of the collector after solvent evaporation resulting from the stretched solution jet by electrostatic forces. Figure 2 shows the morphology of the PW-loaded nanofiber films with different PW core concentrations and PMMA sheath feed speed rates. The corresponding detailed average fiber diameters are shown in Figure 3. When the PW concentrations were 10 and 20% (w/v), the films were straight and cylindrical 12760

DOI: 10.1021/acsami.8b02057 ACS Appl. Mater. Interfaces 2018, 10, 12759−12767

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feed speed rate play a great role in the morphology of the films. Taking the latent heat into consideration, the obtained film with a PW concentration of 40% (w/v) and a core feed speed rate of 0.5 mL/h was chosen for further study. To improve the thermal conductivity of materials, CNTs with high thermal conductivity were incorporated into the PMMA. The morphology of the PMMA nanofibers and CNTs could be observed in Figure 4a,b, respectively. The effect of CNT concentrations on the morphology of the nanofibers is shown in Figure 4c−e. Obviously, the as-prepared film with CNT-loaded core-sheath nanofibers displayed continuous and randomly oriented fibers. When the added content was 1% (w/ w), the fiber surface was still smooth without protuberances, maybe because a little amount of CNTs could not change the viscosity and conductivity of the solutions to maintain a delicate balance.37 However, when the CNT content was increased to 5 and 10% (w/w), the nanofibers became much rougher with some irregular protuberances, which was ascribed to the increase of the uniform charge density of the fiber surface.38 In addition, it might be explained that a high content of CNTs cannot stretch well and some can be exposed on the surface of nanofibers, which could induce irregular protuberances. Theoretically, the exposure of CNTs on the surface is beneficial for the improvement of thermal conductivity.39 As displayed in Figure 4f, the thermal conductivity increased after adding various contents of CNTs. Unexpectedly, the thermal conductivity was not obviously enhanced. It is known that phonon scattering, having a great effect on the thermal conductivity of polymers, could arise from the correlation in the spatial fluctuation of the sound velocity at the interfaces between the crystallites and the amorphous matrix in the acoustic mismatch polymer.16,40 In addition, some researches have reported that a disordered structure, especially micro- or nanometer materials, could also decrease thermal conductivity.40,41 Although the thermal conductivity of the film increased with CNT addition, the change of film conductivity by CNTs was not sufficient to mitigate the influence of phonon scattering. Hence, the amorphous polymer film with a disordered nanoscale fiber just possessed low thermal conductivity. Further investigations on this question would be under way in our future work. As displayed in the TEM images of the PW-loaded composite nanofiber films prepared by coaxial electrospinning with different core feed rates (Figure 5), the PW was completely covered by polymer, forming an ideal core-sheath structure. In addition, the encapsulated PW content could be adjusted by the feed speed rates. As the feed rate increased from 0.1 to 0.5 mL/h, the diameter of the inner core layer increased from 395 to 848 nm, illustrating that a higher speed rate could induce higher encapsulated content of PW; at the same time, the corresponding outer-layer diameters changed from 999 to 1095 nm. Hence, it can be further verified that the PW concentration of 40% (w/v) and the core feed speed rate of 0.5 mL/h were optimal conditions for the encapsulated coresheath nanofiber film. In addition, as shown in Figure 5d, the TEM image demonstrated that the CNT distributed in the nanofibers obviously and the CNT in the nanofibers induced some irregular protuberances, which is consistent with the results of SEM. To study the thermal property and latent heat storage capacity of films with core-sheath nanofiber, the films were measured in nitrogen flow via DSC. The melting and cooling processes of the films are shown in Figure 6, with the

Figure 1. Schematic illustration of the coaxial electrospinning technique used to fabricate the PW-loaded core-sheath nanofiber film.

fibers with a bead-free structure, with the average diameters of fibers being 544 ± 70 and 556 ± 55 nm, respectively. When the PW concentration increased to 30 and 40% (w/v), the fibers became obviously heterogeneous, especially at the concentration of 40% (w/v) (Figure 2j−l), which was attributed to the change of viscosity and conductivity with the augmentation of the PW-loaded content. Unfortunately, when the PW concentration was 50% (w/v), big spindlelike beads appeared and some PWs were exposed on the surface of the nanofibers because the redundant PWs cannot be encapsulated by the outer polymer layer. In addition, a porous structure can be obtained, which is due to the solvent volatilization of the PW solution. Hence, the PW concentration of 50% (w/v) was not appropriate for the fabrication of the PW-loaded nanofiber films due to leakage of the PW. To further explore the influence of the core feed speed rate on the film morphology, the different feed speed rates at different PW concentrations were discussed. The diameter of the core-sheath nanofibers was measured by software “Nano Measure”. The averaged diameter and deviation were calculated via the 100 randomly measured fiber diameter. As can be seen from Figure 3, at the PW concentration of 10% (w/v), there were no obvious changes in the films gained from different rates, indicating that the rate has no effect on morphology under this condition. However, according to the fiber diameter, the deviation value changed from 71 to 135 nm; the fibers showed relatively little heterogeneous nature when the speed rate increased from 0.1 to 0.5 mL/h at the PW concentration of 20% (w/v). Interestingly, the same change trend of the film morphology could be observed between the PW concentrations of 30 and 40% (w/v) at the rate of 0.1−0.5 mL/h for core feed (Figure 3g−l). Obvious nonuniformity can be observed especially for the sample with a PW concentration of 40% (w/v), exhibiting increased deviation of the fiber diameter ranging from 130 nm (0.1 mL/h) to 267 nm (0.5 mL/h). It can be explained that the higher speed rate could induce higher encapsulation efficiency, and the more easily evaporated process of PW solution can also produce increased diameter deviation. It follows that the PW concentration and the core 12761

DOI: 10.1021/acsami.8b02057 ACS Appl. Mater. Interfaces 2018, 10, 12759−12767

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Figure 2. FE-SEM images of the electrospun core-sheath PW-loaded nanofiber film with different core feed rates: (a), (d), (j), (g), and (m): 0.1 mL/ h. (b), (e), (h), (k), and (n): 0.3 mL/h. (c), (f), (i), (l), and (o): 0.5 mL/h. Samples with different PW concentrations (w/v): (a−c) 10%, (d−f) 20%, (g−i) 30%, (j−l) 40%, and (m−o) 50%. The sheath feed speed rate of all samples was 1 mL/h. Scale bars = 1 μm.

transition peak could not be observed for the film with the core-sheath nanofibers, which is attributed to the strong melting peak of PW. In addition, the melting temperature (Tm) and melting enthalpy (ΔHm) of PW was 47.26 °C and 135.18 J/g, respectively. The PW-loaded core-sheath nanofibers possessed ΔHm of 17.46 and 57.65 J/g at the core feed speed

corresponding summarized data listed in Table 1, where CNT5% or CNT10% represents the CNT concentration with 5 or 10% (w/w), respectively, and V0.1 or V0.5 represents the core speed rate of 0.1 or 0.5 mL/h, respectively. It can be found that a weak peak for the pristine PMMA film appears at 104.4 °C, which is the glass transition temperature. However, the glass 12762

DOI: 10.1021/acsami.8b02057 ACS Appl. Mater. Interfaces 2018, 10, 12759−12767

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rates of 0.1 and 0.5 mL/h, respectively, indicating that a higher core feed speed rate could induce higher latent heat capability as a result of more effective encapsulation of PW into the polymer. Furthermore, nanofiber films containing CNT contents of 5 and 10% (w/w) were investigated. In addition, there is no obvious influence on the melting temperature and latent heat of the films. The storage of thermal energy can be achieved by means of latent heat via PW changes from solid-to-liquid phase, as presented in Figure 7a. The PW-loaded thermoregulated films can regulate the temperature as the ambient temperature altered by absorbing or releasing heat when the PW changed the phases from solid to liquid or from liquid to solid. In Figure 7b, the temperature gradient (T1 > T2 > T3) is defined as the color change. When the PW-loaded films (melting point of 50 °C, defined as T2) were located in the environment of the temperature of T1, as shown in Figure 7b, the solid PW can change into liquid after absorbing heat from temperature T1 area with the heat collected in PW. Meanwhile, the temperature of the PW is constant during the phase change. Then, the stored heat can be released to T3 area due to solidifying. To test

Figure 3. Diameter distributions of the electrospun core-sheath nanofiber films prepared under different conditions.

Figure 4. FE-SEM images of (a) PMMA nanofibers with polymer concentration of 20% (w/w), (b) CNTs, and the coaxial electrospun nanofiber film with PW concentration of 40% (w/v) and CNT concentrations (w/w) of (c) 1%, (d) 5%, and (e) 10%. (f) Corresponding thermal conductivity of the nanofiber films containing CNTs. 12763

DOI: 10.1021/acsami.8b02057 ACS Appl. Mater. Interfaces 2018, 10, 12759−12767

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Figure 5. TEM images of core-sheath nanofibers with PW concentration of 40% (w/v) at core feed rates of (a) 0.1 mL/h, (b) 0.3 mL/h, and (c) 0.5 mL/h. (d) Core-sheath nanofibers with PW concentration of 40% (w/v) and CNT concentration of 10% at a core feed rate of 0.5 mL/h. Illustration of PW-loaded electrospun core-sheath nanofibers fabricated by different core speed rates.

Figure 6. DSC curves of the PW, PMMA, and electrospun core-sheath nanofiber films. (a) Melting process and (b) cooling process.

and verify the thermal stability of the film, thermal cycles have also been conducted many times. The 200 melting−cooling cycles of the optimal film were conducted, and the corresponding DSC curves were measured, as shown in Figure 8. The phase transition temperature range was calculated between the onset temperature and the peak temperature,7 and then, the melting/freezing point and latent

heat and solidifying could be obtained, as listed in Table 2. The results indicate that a little change can be found for the melting (from 45.56 to 45.14 °C) and freezing (from 51.38 to 52.11 °C) points, with negligible enthalpy changes after 50 and 200 thermal cycles. In addition, the latent heat, ΔHm, just reduced a little from 60.79 to 56.64 J/g with decrease of ΔHc from −59.69 to −55.71 J/g, indicating that the enthalpy value of the 12764

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ACS Applied Materials & Interfaces Table 1. Enthalpies of the Films Fabricated under Different Conditions sample

Tm (°C)

ΔHm (J/g)

Tc (°C)

ΔHc (J/g)

PW PW−PMMA−CNT5%−V0.5 PW−PMMA−CNT10%−V0.5 PW−PMMA−V0.5 PW−PMMA−V0.1

47.26 46.56 47.04 46.98 46.15

135.18 56.77 58.25 57.65 17.46

50.90 51.60 51.51 51.44 48.67

−134.19 −48.14 −56.49 −47.57 −7.82

nanofiber film did not decrease obviously after 200 cycles. Hence, it could be seen that the core-sheath nanofiber film showed good thermal stability and storage capacity and the PW could not leak from the sheath layer. The morphology of the core-sheath nanofiber films after different thermal cycles is shown in Figure 9. No obvious change was found for the core-sheath nanofiber film after 150 thermal cycles because the heated temperature (80 °C) was lower than the glass transition temperature of PMMA. However, for 200 (Figure 9d) thermal cycles, some fibers merged together with the adjacent fibers in the cross-part, indicating that the morphology of the core-sheath nanofibers could have been influenced due to the frequent thermal cycles. The fiber surface was smooth, and no PW could be seen. It illustrated that no PW leakage occurred in the surface of the nanofibers, which is consistent with the results of DSC. The results further exhibited the practicability of the core-sheath nanofiber film.

Figure 8. DSC curves of electrospun nanofiber films prepared at a core feed rate of 0.5 mL/h and with PW concentration of 40% (w/v) and CNT concentration of 10% (w/w) before and after different thermal cycles.

Table 2. Enthalpies of the Film Prepared at a Core Feed Rate of 0.5 mL/h and with PW Concentration of 40% (w/v) and CNT Concentration of 10% (w/w) before and after Heating−Cooling Thermal Cycles

4. CONCLUSIONS In summary, the PW was encapsulated by PMMA, successfully forming a well-defined core-sheath structure via coaxial electrospinning technique, thereby addressing the leakage issue of PW as latent heat materials. The SEM image showed that the film was continuous with randomly oriented fibers with higher PW core concentration and feed speed rates, which could lead to the higher encapsulation efficiency of PW and the inhomogeneity of the fibers. Furthermore, the clear core-sheath structures consisting of PW as core and PMMA as sheath can be demonstrated by the results of TEM images. Moreover, the core-sheath nanofiber films possessed the best latent heat of 58.25 J/g and solidifying enthalpy of −56.49 J/g, as seen from the DSC results. After 200 thermal cycles, the films still keep

times

Tonset (°C)

ΔHm (J/g)

Tc (°C)

ΔHc (J/g)

50 cycles 100 cycles 150 cycles 200 cycles

45.56 45.45 45.35 45.14

60.79 57.11 56.64 56.52

51.38 51.46 51.94 52.11

−59.69 −58.14 −55.71 −55.29

the constant latent heat, illustrating that the PW-loaded coresheath materials exhibit good performance in energy thermal storage capacity and stability. This work offers good support for the fabrication method of phase change materials in the heat storage field.

Figure 7. (a) Heat stored process of the core-sheath nanofibers fabricated at a core feed rate of 0.5 mL/h and with PW concentration of 40% and CNT concentration of 10%. (b) Simulation of the heat transfers at temperature gradient. 12765

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Figure 9. SEM images of electrospun core-sheath nanofiber films prepared at a core feed rate of 0.5 mL/h and with PW concentration of 40% (w/v) after different thermal cycles: (a) 50 cycles, (b) 100 cycles, (c) 150 cycles, and (d) 200 cycles. Scale bar = 1 μm.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: 86-20-37279344. Fax: 8620-37278821. ORCID

Changmeng Huan: 0000-0003-1351-0125 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by the Major Science and Technology Project of Guangdong Province (No. 2013A011401011), Youth Innovation Promotion Association CAS (No. 2017400), Special Plan of Guangdong Province (2015TQ01N714), Pearl River Star of Science and Technology (No. 2014J2200078), Science and Technology Project of Guangdong Province (No. 2014A010106018), the National Natural Science Foundation of China (No. 51506205), Project of Science and Technology Service Network initiative (No. KFJ-STS-QYZD-010), and Major Science and Technology Project of Tibet Autonomous Region (No. ZD20170017).

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DOI: 10.1021/acsami.8b02057 ACS Appl. Mater. Interfaces 2018, 10, 12759−12767