Nitrogen-Doped Carbon Fiber Paper by Active Screen Plasma

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Nitrogen-Doped Carbon Fiber Paper by Active Screen Plasma Nitriding and Its Microwave Heating Properties Naishu Zhu,*,† Shining Ma,‡ and Xiaofeng Sun‡ †

National Key Laboratory for Disaster Prevention & Mitigation of Explosion & Impact, PLA University of Science and Technology, Nanjing 210007, P.R. China ‡ National Key Laboratory for Remanufacturing, Academy of Armored Force Engineering, Beijing 100072, P.R. China ABSTRACT: In this paper, active screen plasma nitriding (ASPN) treatment was performed on polyacrylonitrile carbon fiber papers. Electric resistivity and microwave loss factor of carbon fiber were described to establish the relationship between processing parameters and fiber’s ability to absorb microwaves. The surface processing effect of carbon fiber could be characterized by dynamic thermal mechanical analyzer testing on composites made of carbon fiber. When the process temperature was at 175 °C, it was conducive to obtaining good performance of dynamical mechanical properties. The treatment provided a way to change microwave heating properties of carbon fiber paper by performing different treatment conditions, such as temperature and time parameters. Atomic force microscope, scanning electron microscope, and X-ray photoelectron spectroscopy analysis showed that, during the course of ASPN treatment on carbon fiber paper, nitrogen group was introduced and silicon group was removed. The treatment of nitrogen-doped carbon fiber paper represented an alternative promising candidate for microwave curing materials used in repairing and heating technology, furthermore, an efficient dielectric layer material for radar-absorbing structure composite in metamaterial technology. KEYWORDS: carbon fiber paper, active screen plasma nitriding, surface treatment, microwave heating, interface, dynamic thermal mechanical properties cloth) are widely used for carbon fiber reinforced polymer structure composites.14 The use of carbon fiber papers as structural phase while ignoring its microwave heating absorption capacity greatly deduced its value. To realize the microwave heating absorption capacity of carbon fiber papers, it was necessary to conduct surface treatment on them. Untreated fibers whose high electrical conductivity resulted in poor electromagnetic properties limited the application scope of microwave heating materials. Electrodeposition15,16 and nitrogen-doped17−19 carbon fibers have been used to overcome the obvious reflection of carbon materials, such as short-cut carbon fiber, while filament carbon fiber has been rarely reported, leading to limited demand for the low cost and convenience of surface treatment of carbon fiber paper. Compared to hydrothermal20 and plasma21 methods to realize those depositions, dielectric barrier discharge (DBD) treatment22 offers many advantages over traditional gas nitriding and bath nitriding, particularly, in terms of reduced gas consumption, reduced energy consumption, and the complete removal of any environmental hazard. Usually, the DBD treatment is used for alloy steel, polymer or oxide film

1. INTRODUCTION In recent years, microwave technology gained more and more attention for material processing.1−6 As one important subfield of the technology,7 microwave heating gained very fast popularity for processing of ceramics, polymers, metallicbased materials, and advanced materials such as metal matrix composite, ceramic matrix composite, and polymer matrix composite. First, the uniform heating characteristic of microwave yields was found to be better polymeric products than the conventionally cured. Then additional materials, called microwave absorbers, which interact with microwaves to produce heat, were widely used as selected heating, whose mode of heating was found more effective in high conductive fibers.8 The addition of particles and fibers,9−11 such as metal, metal oxide, silicon carbide, and carbon materials, in hybrid composite had enhanced bonding strength and better ability to absorb microwave energy. Among those microwave absorbers, carbon materials, graphene, and carbon nanotube are,12,13 in general, very good absorbents of microwaves; they are easily heated by microwave radiation. However, very little literature is available on microwave heating of carbon fiber paper that is made of continuous carbon fiber filament and difficult for treating as microwave absorbing phase. Because of excellent mechanical strength, modulus, and low cost, carbon fiber papers (or called © XXXX American Chemical Society

Received: August 16, 2016 Accepted: November 30, 2016 Published: November 30, 2016 A

DOI: 10.1021/acsami.6b10262 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces deposition,23,24 carbon materials syntheses,25 or modification.26,27 However, in Direct Current plasma nitriding, the components to be treated are subject to a high cathodic potential so that plasma forms directly on the component surface to provide the active nitriding species and to heat the components. This brings some inherent shortcomings for DC plasma nitriding, such as damage caused to parts by arcing, the “edging effect”, and “hollow cathode effect”. Furthermore, the most important drawbacks of the DC process was that carbon fibers show evidence of being partially burned with the process. Active screen plasma nitriding (ASPN) treatment that developed from DBD treatment has been proved to be an effective way to alleviate the edge effect of alloy.28−30 In such a novel nitriding process, the entire workload was surrounded by a large metal screen, on which a high voltage cathodic potential was applied. The nitridation principle within traditional ASPN treatment was quite different from that of DBD treatment and the composition of the equipment was different. Compared with the DBD treatment, a bell-jar-shaped screen was fixated within the ASPN equipment. Because of this screen design, uneven discharge on the surface of steel alloy by the ASPN could be handled. However, tiny filaments could still be produced when carbon fiber was placed in the traditional belljar-type screen, leading to the damage of carbon fiber, which could not be used for the surface treatment of the carbon fiber paper. We have developed an effective screen to provide the upgraded ASPN in the treatment of carbon fiber paper so that carbon fiber paper shall avoid being partially burned,31,32 although its contribution to the nitriding effect was still uncertain. In this paper, details of ASPN treatment on fiber’s microwave absorption properties, correspondingly affecting the microwave heating of it, were considered for application and development of ASPN technology.

Papers B, C, and D were treated in ammonia plasma apparatus by adjusting the temperature and time of treatment parameters, listed in Table 1. The powers in the apparatus were controlled at 700−800 W and the pressure was controlled at 85 Pa. The solder wires containing 1.8−2.1% Si were used for making active screens which could justify functional groups and contents of fiber. B, C, and D were put into a sealed vacuum bag under room temperature after surface treatment. 2.3. Characterization and Measurement. The surface observations of carbon fibers were presented by using CP-II scanning probe microscope atomic force microscope (AFM) and scanning electron microscope (SEM). The roughness data were computed by Image Process software after correcting quadric surface of topography. The surface analysis studies were performed by X-ray photoelectron spectroscopy (XPS) employing an ESCALAB 250 (ThermoFisher Scientific USA, Waltham, MA) spectrometer. The XPS was equipped with a Monochromated Al Kα (150 W), spot size 500 μm, pass energy 150 eV for survey, and 30 eV for narrow scans. As the ASPN equipment had been used for ion-sulfurizing, sulfur and iron impurities could not be cleaned out completely from the ion-nitriding cavity. And the content of Fe and S is less than 1.5%. So the atomic number fraction of Fe and S elements could be ignored in discussion. Dynamic mechanical properties of carbon fiber composites were carried out on the TA2980 DMA equipment. By manual scraping method, the self-made epoxy adhesive was coated on double layers on carbon fiber paper (A, B, C, and D), and then under the condition of microwave curing it was molded into composite specimens AC, BC, CC, and DC. The single cantilever mode was adopted. The test frequency was 1 Hz, and the temperature rise rate at 5 °C/min was scanned from room temperature. SEM and energy-dispersive spectrometer (EDS) were used to observe the morphology of the interface zone of carbon fiber composites. To discuss fiber’s microwave heating properties, carbon fiber samples were compounded with CYD-128 epoxy resin and made to form composite flake. The flake AF (50 × 50 × 2 mm), which consisted of one-layer fiber paper (T300), and epoxy resin were cured, so did the other flakes, B, C, and D. The loss factor of flakes was detected by HP4342A, which was tested as follows by China GB/T 1409-2006, recommended methods for the determination of the permittivity and dielectric dissipation factor of electrical insulation materials at power, audio, and radio frequencies, including meter wavelengths. The electrical resistivity of carbon fiber samples, cut at about 1 cm length of a branch of carbon fiber, was tested by a Keithley 6517A.

2. MATERIAL AND METHODS 2.1. Materials. Polyacrylonitrile carbon fiber plain weave papers were used (3k), whose radial and weft direction tensile rigidity were 80 N·mm−1. The low-temperature ammonia plasma nitriding apparatus was shown in Figure 1, made by National Key Laboratory for Remanufacturing (Bejing, China). 2.2. Methods. Carbon fiber paper A was taken from carbon fiber production line in Weihai Guangwei Composite Company (Weihai City, China) and used as reference samples without any pretreatment.

3. RESULTS AND DISCUSSION 3.1. AFM and SEM Observation of Carbon Fiber. Compared with the smooth surface A in Figure 2a, relatively weak linear projections could be found on surface B in Figure 2b, which were along the longitudinal fibers. The granular projections connecting into lines could be found on surface C in Figure 2c.The dense granular projections could be found on surface D in Figure 2d. With the improvement of the processing temperature, the grooves and granular projections on the carbon fiber surface were more intensive. It could be observed from AFM that the surface of fiber A untreated as reference sample was superficially striated in Figure 3a.The widths of strips of B, C, and D positively correlated with processing temperature in (b), (c), and (d), respectively, of Figure 3. It was found that vertical stripes became apparent on the surface of the carbon fiber and interlinked with horizontal thin stripes. It could be explained that the surface change, which was produced by plasma treatment process, was an energy exchange process, for which N atom was constantly introduced and Si atom was constantly substituted. The increased content of N and reduced content of Si has been proved by XPS analysis in Table 1. Because of the point effect, particles on the top of the thin stripe on fiber

Figure 1. Low-temperature ammonia plasma nitriding apparatus. B

DOI: 10.1021/acsami.6b10262 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Table 1. Treatment Conditions, Surface Roughness, Electrical Resistivity, and the Atomic Number Fraction of Elements of O 1s, Si 2p, N 1s, Fe 2p3, and S 2p Peaks of Carbon Fibera sample

condition

roughness (nm)

electrical resistivity (Ω· cm)

C 1s (%)

O 1s (%)

N 1s (%)

Si 2p (%)

Fe 2p3 (%)

S 2p (%)

A B C D

untreated surface-treated at 125 °C for 80 min surface-treated at 150 °C for 60 min surface-treated at 175 °C for 40 min

47.07 111.1 68.68 106.3

1.93 3.22 3.66 3.74

70.31 70.26 70.82 74.37

18.57 16.01 16.71 12.00

1.05 7.28 6.24 9.74

10.07 5.28 3.63 2.34

none 0.76 1.09 0.67

none 0.41 1.49 0.88

“None” means the element could not be detected in X-ray photoelectron spectroscopy analysis, which meant the samples that were taken from raw material did not contain the element, such as Fe 2p3 and S 2p of sample A. a

Figure 2. Surface SEM photos of untreated and treated carbon fibers: (a) A, (b) B, (c) C, and (d) D.

3.2. Electrical Resistivity of Carbon Fibers and Loss Factor of Flakes. The electrical resistivity value of fibers was listed in Table 1. All the electrical resistivities of fibers became much higher as the process temperature was enhanced. It could be attributed to the enhancement of structure disorder degree of carbon fiber, which resulted in increased interstitial and discontinuity of the fiber’s surface that finally restricted free movement of currents to flow through the fiber. As a result, the reflection of microwave reduced which made the fiber’s microwave heating effective. The enhancement of electrical resistivity of fibers demonstrated that ASPN was an effective way to improve microwave heating performance of carbon fiber. Furthermore, the dominant microwave heating mechanism of fiber with different ASPN process parameters was discussed. There are two sorts of mechanisms for microwave heating.33 Microwave heating results from an electric field that is able to polarize the charges in the material, which is subject to the microwaves. The delay of this polarization follows the extremely rapid reversal of the electric field. This mechanism is known as polarization loss. Another mechanism for microwave heating is resistance loss which is due to electrical resistance within the material that dissipates the energy as heat. The dielectric constant and loss tangent are important as it can characterize the microwave properties of materials.

collided with each other. Si atom was substituted by N atom and this process caused the surface area and the disorder extent of surface structure of carbon fiber to increase. With the decrease of Si content and enhancement of nitrogenization, the amino functional groups gradually increased and the thick area could not take in N atom anymore. The thin area, which was also the weak area, could be nitrogenized. And this process deepened into the inner area of carbon fiber. So the horizontal stripes could be observed. When the nitrogenization process deepened into the inner area of carbon fiber, the graphite sheets on the etching part of fiber’s surface were stripped; thus, it led to the appearance of a new surface. In this way, the exchange of active particles went on and morphology of fibers B, C, and D could be obtained at different temperatures correspondingly. From Table 1, the surface roughness of carbon fibers was improved after surface treatment. This was because that different temperatures might lead to different collision speeds and exchange process of surface particles. Besides, the degree of atom exchange on the surface of carbon fiber behaved differently during the varying stages of process. Thus, carbon fibers were dominated by different microwave absorption mechanisms because different levels of atom exchange appeared on ASPN-treated carbon fibers as shown in section 3.2 and deconvolution of the C 1s and N 1s peak area into surface functional groups is presented in section 3.3. C

DOI: 10.1021/acsami.6b10262 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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on the surface of carbon fiber, such as amino and carboxyl groups, were reacted with epoxy resin. Thus, the effect of polarization loss could be neglected and judged whether conductive loss was the main microwave heating mechanism. In other words, if microwave heating of the treated fibers was dominated by conductivity loss mechanism, ε″ should became higher as resistivity increased. In contrast, when ε″ became lower, polarization loss was mainly guided. In view of all samples’ resistivity becoming higher, comparing ε″ of AF with those of BF, CF, and DF was all right. As could be seen in Table 2, ε″ of BF was higher than that of AF and it could be suggested that the conductive loss mechanism was the dominant mechanism for the B sample. The ε″ of CF and DF were lower than that of AF. So it could be indicated that the polarization loss mechanism was the dominant mechanism of C and D. The meaning of the distinguishment of microwave heating mechanism was to perform carbon fiber processing purposefully to obtain the carbon fiber with different microwave heating properties. 3.3. XPS Analysis. The chief function of XPS in this article served to prove that the introduction of N atom and extraction Si atom on the surface of carbon fiber paper were part of objective substitution process that changed the microwave absorption of carbon fiber paper, correspondingly improving the microwave heating of it. This has been mentioned in AFM for description of ASPN processing. The secondary function of XPS was that C 1s and N 1s decomposition analysis could reflect the quality of interface adhesion of ASPN-treated carbon fiber paper; thus, detailed C 1s and N 1s decomposition analysis would be provided for experiment proof in the following. Survey spectra in the binding energy ranging 0−1400 eV were obtained to identify the surface elements present and a quantitative analysis has been provided in Figure 4. The carbon C 1s peaks could be fitted to four line shapes with binding energies at about 286.5, 287.5−288.2, and 288.5−290 eV, which were assigned to C−O−R or C−NR2, CO, and O− CO in Figure 5.34 The N 1s region could be deconvoluted

Figure 3. Surface AFM photos of untreated and treated carbon fibers: (a) A, (b) B, (c) C, and (d) D.

The loss factors of flakes were shown in Table 2, in which ε′ and ε″ were represented as dielectric constant and loss tangent. Table 2. Loss Factor of Flakes flake

ε′

AF BF CF DF

3.62 3.86 4.38 4.33

ε″ 1.58 1.81 3.74 9.25

× × × ×

10−2 10−2 10−3 10−3

The polarization loss was more induced by the functional groups and the conductive loss was more affected by resistivity. As ε″ was a composite index to reflect both polarization loss and conductive loss, it was necessary to distinguish the dominant loss impact on ε″. Considering this point and demand of China GB/T 1409-2006, the following method had been adopted. During preparation of flakes, functional groups

Figure 4. XPS surface survey spectra of A, B, C, and D. D

DOI: 10.1021/acsami.6b10262 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 5. Deconvolution of the C 1s peak area into surface functional groups: (a)A, (b) B, (c) C, and (d) D.

Figure 6. Deconvolution of the N 1s peak area into surface functional groups (a) A, (b) B, (c) C, and (d) D.

into three functional components −N(such as 2,2′-biquinoline), −NH2(−NH) (such as triphenylamine), oxidized N(such as Reichardt’s Dye) with binding energies of about 398.4, 399.9, and 401.7 eV in Figure 6.35−37 On the basis of this deconvolution method, the contents of O−CO and −NH2(−NH) could be recognized easily in Table 3. The content of −CO, O−CO, −NH2(−NH) on the carbon fiber’s surface showed apparent tendency of increase after the carbon fiber surface was treated by ASPN. The groups were conducive to the reaction between carbon fiber paper and epoxy resin under microwave heating to enhance the interface bonding strength. The treatment condition of 175 °C/85 Pa,

that was when -NH2 (−NH) content in Sample D reached its highest level, was conducive to its chemical bonding with epoxy resin. Besides, the introduction of amino functional groups on the surface of carbon fiber could react directly with epoxy resin. In contrast, C−OH and O−CO oxygen containing functional groups must react with the curing agent first to be bonded with the epoxy resin. From the above SEM, AFM, XPS analysis and the discussion of the microwave heating mechanism of fiber, the microscopic physical and chemical characteristics of carbon fiber could be obtained. In the following, the influence of microproperties of carbon fiber on the macro-properties of fiber composites E

DOI: 10.1021/acsami.6b10262 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces Table 3. Surface Functional Components Obtained from the Deconvolution of C 1s and N 1s Peaksa samples A B C D

C−C and C−C/C 1s

C−O−R and C− O−R/C 1s

−CO and −CO /C 1s

O−CO and O−CO /C 1s

−N and − N/N 1s

−NH2(−NH) and −NH2(−NH)/N 1s

oxidized N and oxidized N/N 1s

50.25 0.73 48.04 0.68 50.76 0.72 54.67 0.74

16.04 0.23 14.52 0.21 14.84 0.21 13.69 0.18

0.53 0.008 5.24 0.07 3.28 0.05 4.56 0.06

1.50 0.02 2.41 0.04 2.0 0.02 1.55 0.02

none

none

none

1.67 0.23 1.47 0.24 1.55 0.17

4.73 0.65 4.56 0.73 7.56 0.82

0.89 0.12 0.21 0.03 0.06 0.01

“None” means the element could not be detected in XPS analysis which meant the samples that were taken from raw material did not contain the element, such as −N, −NH2(−NH), and oxidized N, the deconvolution of N 1s peaks of sample A. a

prepared by microwave heating was discussed as the data comparison of macro performance was intuitive and could be used as the adjusting criterion of parameter processing of ASPN. 3.4. Dynamic Mechanical Properties and Interfacial Bond Morphology of Carbon Fiber Composites. DMA was an important method to study the stiffness and damping of carbon fiber composites at a certain temperature, frequency, stress, or strain level. With just a small sample, in a relatively short time (0.5−1 H), material’s elastic modulus and damping in a wide temperature range of continuous change, the dynamic mechanical temperature spectrum, could be obtained.38 The damping factor Tan δ, reflecting the condition of interfacial adhesion, was the ratio of loss modulus to storage modulus. As the damping factor was lower and Tg was higher, it was indicated that the interface of composite materials was more firm. As the curve center axis of damping factor shifted to the right, it was indicated that Tg was improved due to the fact that the main chain of the epoxy resin movement was more constrained. As could be seen in Figure 7a, the Tan δ of DC obviously decreased compared with that of AC. The storage modulus E′ was a measure of stiffness of composite materials. E′ decreased as the temperature increased, as shown in Figure 7b because matrix resin was in the glassy state whose molecular chain segment movement was frozen. With the increase of temperature, the macromolecular chains were prone to moving and E′ would be reduced. Furthermore, as the temperature continued to rise at high elastic zone, E′ sharply declined and arrived at a relatively stable value. The E′ of DC was 10.9 GPa, higher than that of AC. As mentioned above, to compare these macro-mechanical properties for evaluating the microwave heating mechanism of fibers, it could be realized that processing parameters of sample D, dominated by polarization loss mechanism, should be taken as the optimum treatment method. The microwave absorption mechanism of the carbon fibers C and D was mechanism loss polarization; however, the dynamic mechanical properties of the composite CC were not as good as the composite DC, which was related to the penetration depth of the microwave. When the carbon fiber composite was heated by microwave, the energy of the carbon fiber composite was attenuated in the form of the microwave radiation, and the energy was released to the composite material by the microwave field. When the composite material’s depth exceeded the ultimate penetration limit of microwave, uneven temperature distribution would be produced within the composite materials. Therefore, it was necessary to adjust composite material’s ultimate penetration depth to meet the

Figure 7. Dynamic mechanical properties of composites.

requirements of the material’s thickness. The penetration depth of microwave in the material could be calculated by the following formula: Dp =

λ0 π ε′ × ε″

(1)

In the formula λ0 is vacuum electromagnetic wavelength, 0.1224 m, ε′ is dielectric constant, and ε″ is loss tangent. Calculated from data in Table 2, the DpAC is 1.2968 m, DpBC is 1.0962 m, DpCC is 4.9797 m, and DpDC is 2.0250 m. According to the DMA data, it could be inferred that the penetration depth of the DpDC was conducive to even heating and complete curing of the interface of carbon fiber and resin. Besides, it should be noted that when designing the penetration F

DOI: 10.1021/acsami.6b10262 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 8. Energy-dispersive spectrometer line scanning spectrum of interface of the composite: (a) AC; (b) DC.

depth of the composite material, the dielectric constant of other parts within the composite material should be adjusted to make sure that the thickness of the composite material could meet the needs of microwave conversion. This paper focused on the method of adjusting the microwave loss factor by modulating the ASPN process parameters to make sure the penetration depth limit could reach the designed index. Besides, on the basis of this discovery of the phenomenon that DpDC of the material could be changed by the way of nitrogen doping, it might be used for an efficient dielectric layer material for radarabsorbing structure composite in metamaterial technology, not just used for microwave heating. In addition, element analysis of the interface was presented to illustrate the enhancement of mechanical dynamic properties of DC compared with those of AC. In Figure 8a, the carbon fiber of AC was surrounded with the creased resin, reflecting that it was a weak interface. In line scanning, the C element was represented by red, the Si element was represented by blue, the O element was represented by green, and the Fe element was represented by purple. The element distribution showed that a small Si peak, which was in the oval frame region, could be found next to a large Si peak in the interface region near the fiber, showing that the interface reaction was not sufficient. As shown in Figure 8b, DC’s carbon fiber and resin combined into an intact interface. Si element was concentrated in the interfacial region and only a large Si peak was formed near the fiber, which showed that the interface reaction was complete. Besides, through quantitative analysis of Si/C, the interface reaction could be further judged to be sufficient or not. This is because Si element comes from the silane coupling agents in resin, the content of Si/C in the interface can be used to judge whether the interface chemical reaction is sufficient or not. When the same dosage of the silane coupling agents was applied, the carbon fiber surface treated by ASPN was more conducive to the reaction between the interface phase. As shown in Table 4, the surface analysis of resin matrix of A showed that the atomic percentage of elements Si and C was Si/C = 0.0016. Similarly, through surface analysis of resin

Table 4. Surface Element Analysis of A and D element

A (wt %)

A (at. %)

D (wt %)

D (at. %)

CK OK Si K Fe K

81.10 18.43 0.31 0.16

85.28 14.55 0.14 0.04

58.63 17.60 23.08 0.69

71.62 16.14 12.06 0.18

matrix of D, the atomic percentage of elements Si and C was Si/C = 0.134.

4. CONCLUSIONS The surface treatment on carbon fiber was carried out by an upgraded low-temperature ASPN. It has been confirmed that ASPN was an effective method to adjust microwave heating properties of carbon fiber. The dominant microwave heating mechanisms of the treated fiber were judged by analyzing indication of resistivity, ε′ and ε″. The surface processing effect of carbon fiber could be indirectly characterized by DMA testing on composites made of carbon fiber. Besides, a carbon fiber function formation course was put forward according to AFM and XPS. It has been found that the atom exchange course during ASPN treatment was responsible for structure disorder, which led to the change of microwave attenuation constant, electric resistivity, and surface roughness. For practical applications, temperature and time parameters of ASPN could be used to change microwave heating properties of fiber as their resistivity, ε′ and ε″, of fiber were influenced by treatment conditions.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (N.Z.). ORCID

Naishu Zhu: 0000-0002-7651-8960 Notes

The authors declare no competing financial interest. G

DOI: 10.1021/acsami.6b10262 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACKNOWLEDGMENTS This work was supported by the National Nature Science Foundation of China (51505497). We thank Prof. Fan Hualin for his assistance in writing this paper. And we also thank Mr. Li Youzi for his help with the language.

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DOI: 10.1021/acsami.6b10262 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX