Research Article www.acsami.org
Control Synthesis of Tubular Hyper-Cross-Linked Polymers for Highly Porous Carbon Nanotubes Xiaoyan Wang,† Pan Mu,† Chong Zhang,† Yu Chen,† Jinghui Zeng,† Feng Wang,‡ and Jia-Xing Jiang*,† †
Shaanxi Key Laboratory for Advanced Energy Devices, Key Laboratory for Macromolecular Science of Shaanxi Province, School of Materials Science and Engineering, Shaanxi Normal University, Xi’an, Shaanxi 710062, P. R. China ‡ Key Laboratory for Green Chemical Process of Ministry of Education, Wuhan Institute of Technology, Wuhan 430073, P. R. China S Supporting Information *
ABSTRACT: Porous carbon nanotubes (PCNTs) have attracted considerable attention due to their large specific surface areas and unique one-dimensional (1D) structures. However, most of the reported synthetic strategies for PCNTs are complex and expensive. Herein, we present a selftemplated, surfactant-free strategy for the synthesis of highquality PCNTs with high surface area by direct carbonization of 1D hyper-cross-linked polymer nanotubes. The precursors of the 1D hyper-cross-linked polymer nanotubes were synthesized by FeCl3 catalyzed Friedel−Crafts alkylation of aromatic hydrocarbons with formaldehyde dimethyl acetal. It was found that the monomer concentration and mechanical agitation play crucial roles in the formation of the 1D tubular hyper-cross-linked polymer precursor. The tube size of the resulting PCNTs could be finely controlled by the aromatic monomers with different molecular sizes. The excellent electrochemical performances of the supercapacitors fabricated from the PCNTs demonstrate that these PCNTs are promising for the electrode materials of high-performance supercapacitors. This work highlights that the facile synthetic strategy for PCNTs would open up new avenues of porous carbon nanotube materials with promising applications. KEYWORDS: porous carbon nanotube, aromatic hydrocarbon, controllable tube size, surface area, energy storage
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INTRODUCTION Since the discovery of carbon nanotubes (CNTs) in 1991,1 there have been increasing research interests on this subject in the past decades because of their unique one-dimensional (1D) structure and promising applications,1−3 such as for conductive films,4 fuel cells,5 solar cells,6 supercapacitors,7 and sensors.8 Recently, a new class of carbon materials, porous carbon nanotubes (PCNTs), with 1D tubular morphology and high surface area has been developed. Many efforts have focused on the preparation methodologies and potential applications of PCNTs.9−11 These 1D PCNTs exhibit very high surface area (up to 1000 m2 g−1), which exceeds that of the traditional CNTs.11 Due to the high surface area and 1D tubular instinct, PCNTs hold great potential for electrode materials of lithium ion batteries and supercapacitors, catalyst supports, and gas storage media. Several strategies have been developed to prepare PCNTs, mainly including carbonization of 1D polymer precursors (e.g., cross-linked conjugated polymers or polypyrrole),11−13 multiwalled carbon nanotubes (MWCNTs) model14 or skeleton method,15 and solvothermal synthesis from different carbon sources (e.g., hexachlorobenzene or ferrocene).9,10 Among these strategies, direct carbonization of tubular polymers, such as conjugated microporous polymers (CMPs),11,16 is significantly efficient and convenient to produce PCNTs with large surface areas. In fact, the tubular morphology was observed in the first example of CMPs.17 CMPs could be tailored into 1D tubular polymers by rational © 2017 American Chemical Society
design of the molecular structure and optimizing reaction conditions.11,18−20 However, 1D CMP networks are mostly synthesized by the well-established transition metal catalyzed polycondensation reactions (e.g., Sonogashira reaction).11,19 In these polymerizations, deliberately made multifunctional monomers and expensive noble metal-based catalysts (e.g., Pd) are normally used, which involve in the complicated preparation procedures and harsh preparation conditions.11,20 Hyper-cross-linked polymers (HCPs) represent a subclass of advanced porous polymer materials with high porosity and physiochemical stability.21 HCPs could be facilely synthesized by one-step Friedel−Crafts polymerization using cheap catalyst (e.g., FeCl3) and inexpensive chemicals such as benzene,22 naphthalene,23 thiophene,24 and triphenylamine,25 which avoids the use of noble metal catalysts and the monomers with specific polymerizable groups,11 leading to the low cost and scalable synthesis of HCPs.26 Most studies on HCPs to date have focused on the development of synthetic approaches for improving surface area and gas storage performance using various building blocks, and the reported HCPs commonly have nanobead or nanosphere morphologies,27,28 although hollow tube-shaped porous polymer and carbons were obtained using 4-tritylaniline as the build block, where the ratio of the Received: April 17, 2017 Accepted: June 1, 2017 Published: June 1, 2017 20779
DOI: 10.1021/acsami.7b05345 ACS Appl. Mater. Interfaces 2017, 9, 20779−20786
Research Article
ACS Applied Materials & Interfaces
Figure 1. Schematic illustration of the preparation of HCPTs and PCNTs.
morphology of the precursor of HCPT-B (Table S1). The HCPT-B produced from the monomer concentration of 0.05 M for benzene shows uniform tubular morphology, as evidenced by field-emission scanning electron microscopy (SEM; Figure 2a) and transmission electron microscopy
cross-linker of formaldehyde dimethyl acetal to the monomer of 4-tritylaniline has a large influence on the polymer morphology.29 Beyond this, there are few reports on the purposeful synthesis of tubular HCPs. The hyper-cross-linked knitting structure in HCPs would prevent the pore from collapsing during carbonization. Therefore, one could expect to obtain PCNTs with high surface area by pyrolysis of tubular HCPs precursors. With this in mind, we synthesized a series of 1D hyper-crosslinked polymer nanotubes (HCPTs) via a FeCl3 catalyzed Friedel−Crafts reaction of aromatic hydrocarbons (e.g., benzene, anthracene, phenanthrene, and pyrene) with formaldehyde dimethyl acetal without any template and surfactant. High-quality PCNTs were then obtained by direct carbonization of these 1D tubular HCPTs. The resulting PCNTs show high surface area up to 921 m2 g−1, which is much higher than that of all of the commercial CNTs. The morphology of the HCPTs was strongly dependent on the initial monomer concentration, and the tube size could be easily tuned by the aromatic monomers with different molecular sizes. The obtained PCNTs show excellent electrochemical performances for supercapacitors in a symmetric two-electrode system because of the uniform 1D tubular morphology and high specific surface areas. Compared to the PCNTs produced from the template method and solvothermal method, the facile developed strategy for PCNTs avoids the use of noble metal catalysts, the monomers with specific polymerizable groups and templates, which make it possible to prepare PCNTs on large scale at low cost. Additionally, the combination of controllable tube size and large surface area endow these PCNTs with great potentials in gas adsorption, energy storage, and shape-selective catalysis.
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Figure 2. Scanning electron microscope (SEM) images: (a) HCPT-B, (b) PCNT-B; transmission electron microscope (TEM) images: (c) HCPT-B, (d) PCNT-B; (e) outer and (f) inner diameter distribution histograms of PCNT-B from analysis of SEM and TEM images.
RESULTS AND DISCUSSION Figure 1 shows the synthetic route for the porous carbon nanotubes. First, the benzene-based tubular hyper-cross-linked porous polymer (HCPT-B) was synthesized by a FeCl3 catalyzed Friedel−Crafts polymerization of benzene with formaldehyde dimethyl acetal without any template and surfactant. Then, the benzene-based porous carbon nanotube (PCNT-B) was obtained by direct carbonization of HCPT-B at 700 °C for 3 h in nitrogen atmosphere. It was found that the monomer concentration has a large influence on the
(TEM) images (Figure 2c). After carbonization, PCNT-B retains an intact tubular morphology without any obvious collapse of the tubes (Figure 2b,d). The resulting PCNT-B shows a typical 1D tubular structure with an external diameter of around 57 nm and an inner diameter of around 34 nm (Figure 2e,f) and a length of more than 10 μm (Figure S1). To the best of our knowledge, such uniform and long 1D tubular 20780
DOI: 10.1021/acsami.7b05345 ACS Appl. Mater. Interfaces 2017, 9, 20779−20786
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pyrolysis of these tubular polymers. SEM and TEM images of the tubular polymers produced from ultrasonic and solvothermal methods also showed exclusively polymer nanotubes (Figure S10 and S11). Although the polymer nanotubes prepared by ultrasonic method show slightly inhomogeneous distribution, they still show exclusively 1D morphology in a long-range. After carbonization, all of the porous carbon nanotubes remain well 1D tubular morphology (Figure S10 and S11). The elemental mapping images of PCNT-B show the presence of C and O elements in the porous carbon nanotubes (Figure 4a−c). The line scanning profiles of C and O elements demonstrated that O element is relatively rare and the C signal on the wall is stronger than that in the core (Figure 4d), which again proves that PCNT-B is a nanotube with a hollow structure instead of a nanofiber. The atomic force microscope (AFM) images and topological height profiles crossing through the tube demonstrated that PCNT-B shows the rough tube surface and 1D tubular structure with the height of 60 nm (Figure 4e−g), which agree with the dimensions from TEM images. The high resolution transmission electron microscopy (HR-TEM) images illustrated that the nanotubular structures of PCNT-B are composed of a disordered carbon matrix (Figure 4h−j) and the presence of abundant micropores on the tube wall.9,37 The other three aromatic monomers of anthracene, phenanthrene, and pyrene were also used to produce the tubular porous polymers and corresponding porous carbon nanotubes. As shown in Figure 5, all of the monomers can generate polymer nanotubes with specific monomer concentration (“critical concentration”), indicating that the facile method is universal for a range of aromatic hydrocarbon monomers. Different monomers have different critical concentrations (Table S1 and Figure S12−15). It was found that the critical concentrations are 0.1, 0.05, 0.02, and 0.05 M for benzene, anthracene, phenanthrene, and pyrene, respectively, which are much lower than the normal monomer concentration of 1.0 M,22 where polymer nanobeads are commonly obtained. No nanotube could be obtained for this series of polymers in the monomer concentration higher than 0.1 M, although polyaniline nanofibers could be synthesized in a high concentration of 0.4 M.38 The formation of tubular polymers in low monomer concentration ( 0.9), indicating the presence of mesopores and/or macropores in the materials, which is attributed to the voids and interparticle porosity.35 The apparent Brunauer−Emmet− Teller (BET) surface areas were found to be 1034 and 921 m2 g−1 for HCPT-B and PCNT-B, respectively, demonstrating a little loss in surface area after carbonization. The surface area of 921 m2 g−1 for PCNT-B is higher than that of all of the commercial MWCNTs36 and comparable to that of the reported PCNTs.10,11 Figure 3b shows the pore size distributions of HCPT-B and PCNT-B as calculated using nonlocal density functional theory (NL-DFT). After carbonization, PCNT-B shows narrower micropore diameter centered at around 0.8 nm than the precursor of HCPT-B (1.0 nm). In addition, the tubular polymers could be also synthesized by solvothermal and ultrasonic preparation strategies as shown in Figure 1, and high quality PCNTs were also obtained by 20781
DOI: 10.1021/acsami.7b05345 ACS Appl. Mater. Interfaces 2017, 9, 20779−20786
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Figure 4. Elemental mappings for PCNT-B: (a) combined map, (b) carbon, (c) oxygen, (d) line scanning profiles of carbon and oxygen, (e) representative AFM image of PCNT-B, (f) corresponding height profile along a line scan (red line), (g) height profile along a line scan (black line), and (h−j) HR-TEM images of PCNT-B with different scale bars.
morphology (Figure 5b1−b3) and the similar tube size to the corresponding precursors (Figure 5d1−d3). The BET surface area of these PCNTs varied between 396 and 509 m2/g (Table S2). Time-dependent experiments were carried out to investigate the formation process of the tublar polymer by using benzene as the monomer. As shown in Figure 6, at the initial stage of polymerization, the monomer molecules accumulate to form nanoparticles with uniform size after heating for 20 min with stirring (Figure 6a). Then, nanotube clusters were formed after 40 min under the condition of stirring (Figure 6b), and the
Figure 5. SEM images of HCPTs (a1−a3) and PCNTs (b1−b3) with different monomers: (a1, b1) anthracene, (a2, b2) phenanthrene, and (a3, b3) pyrene. TEM images of HCPTs (c1−c3) and PCNTs (d1− d3) with different monomers: (c1, d1) anthracene, (c2, d2) phenanthrene, and (c3, d3) pyrene. Figure 6. SEM images of HCPT-B with different reaction duration and conditions: (a) 20 min with stirring, (b) 40 min with stirring, (c) 24 h with stirring, (d) 20 min without stirring, (e) 40 min without stirring, and (f) 24 h without stirring. (g) Time-dependent structural evolution of HCPT-B.
because of the thicker tube wall in HCPT-Ph compared with HCPT-An (Figure 5c1,c2), leading to more porous structures in the resulting polymer of HCPT-Ph. After carbonization, all of the porous carbon nanotubes remain well the 1D tubular 20782
DOI: 10.1021/acsami.7b05345 ACS Appl. Mater. Interfaces 2017, 9, 20779−20786
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Figure 7. Electrochemical performances of PCNT-B and PCNB-B: (a) CV curves of PCNT-B at various scan rates, (b) charge−discharge profiles of PCNT-B, (c) specific capacitances of PCNT-B and PCNB-B at various current densities, (d) Nyquist plots and the inset shows the close-up view of the high-frequency region, (e) Bode plots of phase angle vs frequency for PCNT-B and PCNB-B, and (f) cycling stability of PCNT-B at a current density of 1.5 A g−1.
layer capacitive behavior and efficient electrolyte ion transport in the PCNT-B electrode.44−46 Figure 7b shows the galvanostatic charge−discharge curves of the PCNT-B based electrode. The symmetric charge and discharge profiles with negligible voltage drop (IR drop) were obtained at different current densities, confirming a low internal resistance and an ideal capacitive behavior. The specific capacitance calculated from the discharge curve was found to be 172 F g−1 at a current density of 0.5 A g−1, which could remain 76.2% (131.4 F g−1) at a high current density of 30 A g−1 (Figure 7c). However, the initial specific capacitance of PCNB-B was only 113.4 F g−1 at a current density of 0.5 A g−1, and decreased to 81 F g−1 at the current density of 30 A g−1, which are much lower than those of PCNT-B at the same conditions. In addition, PCNT-B shows smaller IR drop of 0.07 V than PCNB-B (0.12 V) at the high current density of 20 A g−1 (Figure S18). The capacitance of 172 F g−1 for PCNT-B is comparable to that of the hollow carbon nanospheres (201 F g−1 at 0.1 A g−1)44 and carbon nonomesh (182 F g−1 at 0.25 A g−1),47 and much higher than that of the commercial MWCNT (50−60 F g−1)11 in a symmetric two-electrode system. The kinetic ion diffusion within the electrode was investigated by electrochemical impedance spectroscopy (EIS) with the frequency ranging from 10 mHz to 100 kHz. Figure 7d shows the Nyquist plots of the samples, both plots feature a vertical curve in low-frequency region, confirming primary contribution of electrostatic ion adsorption. PCNT-B shows a smaller semicircle diameter and a shorter Warburg region (the slope of 45° portion of the curve) in the high-frequency region compared to PCNB-B, indicating the better electrical conductivity in PCNT-B electrode because of the long-range 1D carbon nanotube nature (Figure 7d, inset). Figure 7e shows the dependence of impedance phase angle on frequency of PCNT-B and PCNB-B electrodes. The relaxation time constant (τo=1/fo) of 0.34 s for PCNT-B is much shorter than that of PCNB-B (0.98 s), which is also much smaller than conventional activated carbon-based ECs (10 s)48 and graphene aerogel based EC (0.73 s)49 and comparable to
tube is curved with homogeneous size. After 24 h, the polymer nanotubes become uniform and straight (Figures 6c and S16c). As a sharp contrast, the polymer nanoparticles aggregate to form the bigger spheres when reaction time was prolonged from 20 to 40 min without stirring (Figure 6d,e). Even at the final polymerization stage (24 h), no nanotube could be obtained, and only large spheres with different sizes were observed in the sample (Figures 6f and S16f). Figure 6g shows the time-dependent structural evolution of the tublar polymer under the conditions of stirring or no stirring, inidicating the mechanical agitation helps to form the nanotube structures. Based on the conventional coagulation theory of colloids, the mechanical agitation plays a key role to disturb aggregation of particles and help particles disperse well in solution.41,42 Unlike the formation of polyaniline nanofibers, the mechanical agitation helps to form homogeneous nucleation of hypercross-linked polymer nanotubes.42 More importantly, a suitable concentration is also essential for the generation of the tubular polymer as discussed above. Overall, these results demonstrated that both monomer concentration and mechanical agitation play crucial roles in the morphology control of the hyper-crosslinked polymers. The porous 1D structure and high surface area make the porous carbon nanotubes ideal electrode materials for supercapacitors. As a proof of concept, we have evaluated the electrochemical performances of PCNT-B in a symmetric twoelectrode system using 6.0 M KOH as an aqueous electrolyte. For comparison, we also prepared another hyper-cross-linked porous polymer (HCPB-B) with spherical morphology (Figure S17) by using 1.0 M monomer concentration of benzene according to the reported method,22,43 and porous carbon nanobeads (PCNB-B) were then obtained by carbonization of HCPB-B. Figure 7a shows the typical cyclic voltammetry (CV) curves of the PCNT-B based electrode over the scanning rates of 100−500 mV s−1 in 6.0 M KOH aqueous electrolyte. All of the CV curves dispalyed a quasirectangular shape without any obvious redox peak, indicating a nearly ideal electrical-double20783
DOI: 10.1021/acsami.7b05345 ACS Appl. Mater. Interfaces 2017, 9, 20779−20786
Research Article
ACS Applied Materials & Interfaces holey graphene frameworks (0.17 s).50 The very short time constant of PCNT-B highlights that the crucial role of nanopores promote the ion kinetic diffusion in the interior of the electrodes. The cycling stability of the PCNT-B based supercapacitor was carried out by a consecutive charge− discharge experiment at a constant current of 1.5 A g−1. As shown in Figure 7f, a capacitance retention of 93.6% was achieved after 15 000 cycles, indicating the excellent cycling stability of the PCNT-B based supercapacitor. The capability of PCNT-B integrated high rate performance with excellent cycling stability is of great importance for high-performance supercapacitors.
analysis (TGA) was carried out using a thermal analysis instrument (Q1000DSC + LNCS + FACS Q600SDT) over the temperature range from 30 to 800 °C. The samples were heated at a rate of 10 °C min−1 under a nitrogen atmosphere. Elemental analysis was performed on a EUROEA 3000 Elemental Analyzer. Powder X-ray diffraction patterns were carried out on X-ray diffractometer (D/Max-3c). The iron content was characterized by inductively coupled plasma atomic emission spectrometry (ICP-AES, Leeman). The morphology was collected using a field emission scanning electron microscope (SEM; JSM-6700F), and the chemical composition was characterized by energy dispersive X-ray spectroscopy (EDX, JSM-2010). Transmission electron microscopy (TEM) images and elemental mappings were obtained on a JEM-2100F. Atomic force microscopy (AFM) images were obtained on as Bruker Dimension ICON. X-ray photoelectron spectroscopy (XPS) was measured with a VG ESCALAB MKII spectrometer. Raman spectra were recorded on a Renishaw Laser Micro-Raman spectrometer. Surface areas and pore size distributions were measured by nitrogen adsorption and desorption at 77.3 K using a Micromeritics ASAP 2420-4 volumetric adsorption analyzer. The surface areas were calculated in the relative pressure range (P/P0) from 0.05 to 0.20 using Brunauer−Emmet−Teller (BET) method. Pore size distributions and pore volumes were derived from the nitrogen adsorption branch of the isotherms using the nonlocal density functional theory. The sample was degassed at 120 °C for 15 h under vacuum (10−5 bar) before analysis. Supercapacitors Fabrication and Measurements. The supercapacertor performances were carried out in 6 M KOH electrolyte using a symmetric two-electrode cell. The working electrodes were prepared by pressing a mixture of PCNT-B or PCNB-B samples, polytetrafluoroethylene, and commercial carbon black in the mass ratio of 8:1:1 and then dispersing in a small amount of ethanol for the formation of paste, which was pressed into tablets and dried at 100 °C for 8 h in vacuum. Then, the tablets were coated onto the nickel foam under the pressure of 10 MPa to obtain the electrode. A sandwich-type supercapacitor consisting of two of the same sample electrodes was assembled. The electrodes and separators were soaked in the electrolyte over 8 h before each assembling. All electrochemical measurements were performed with the assembled two-electrode supercapacitors at ambient temperature. Cyclic voltammetry (CV), galvanostatic charge−discharge tests, and electrochemical impedance spectroscopy (EIS) were carried out using an CHI650 electrochemical workstation. CV tests were carried out at sweep rates from 50 to 500 mV s−1. Electrochemical impedance spectroscopy measurements were conducted over the frequency range from 100 kHz to 10 mHz. The specific capacitance of the electrode was calculated using the following equation: Ctotal = IΔt/(ΔVm), where C is the total capacitance (F g−1), I is the response current density (A g−1), Δt is the discharge time, ΔV is the potential (V), and m is the total mass of the electroactive material in the electrodes (g). The specific capacitance of the single electrode is calculated as Cs = 4Ctotal.
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CONCLUSION In summary, we have developed a novel facile method to prepare PCNTs by direct carbonization of 1D hyper-crosslinked porous polymer nanotubes. This method involves a controllable synthesis mechanism for a tubular porous polymer, in which both monomer concentration and mechanical agitation are important for the formation of the 1D tubular hyper-cross-linked polymers. The tube size of the resulting PCNTs could be finely controlled by the aromatic monomers with different molecular sizes. These obtained PCNTs show uniform 1D tube morphology with outstanding porous structure, high specific surface areas, and excellent electrochemical performances for supercapacitors. The facile strategy makes it possible to prepare high-quality PCNTs with regular 1D tube morphology, controllable tube size, and high porous structure on a large scale at low cost.
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EXPERIMENTAL SECTION
Materials. Benzene (99.5%), anthracene (99%), phenanthrene (97%), and pyrene (98%) were purchased from Adamas. Formaldehyde dimethyl acetal (99%), anhydrous FeCl3, 1,2-dichloroethane, and other chemicals were obtained from J & K Scientific Ltd. All chemicals were used as received. Synthesis of HCPTs and PCNTs. All hyper-cross-linked polymers nanotubes (HCPTs) were synthesized by Friedel−Crafts alkylation of a series of aromatic hydrocarbons including benzene, anthracene, phenanthrene, and pyrene with a formaldehyde dimethyl acetal external cross-linker promoted by anhydrous FeCl3. The resulting tubular polymers from monomers of benzene, anthracene, phenanthrene, and pyrene were denoted as HCPT-B, HCPT-An, HCPT-Ph, and HCPT-Py, respectively. The obtained HCPTs were heated at 700 °C for 3 h with a heating rate of 2 °C min−1 under a N2 flow to produce various targeted PCNTs. The yield of the PCNTs is above 70% (Table S2). The PCNB-B was obtained by carbonization of benzene based HCPB-B, which was synthesized according to a previous reported method.22 A typical experimental procedure for HCPT-B is given as an example. Synthesis of HCPT-B Using Solution Method. Benzene (1 mmol, 78 mg) was diluted in 20 mL of 1,2-dichloroethane, and after stirring for several minutes, anhydrous FeCl3 (3 mmol, 486 mg) and formaldehyde dimethyl acetal (3 mmol, 228 mg) were added in the mixture. The mixture was heated to 80 °C and stirred for 24 h under a nitrogen atmosphere. The resulting mixture was cooled to room temperature, and the precipitate was washed by 2 M diluted hydrochloric acid, methanol, distilled water, dichloromethane, and acetone successively. Further purification of the polymer was carried out by Soxhlet extraction from methanol for 48 h. The product was dried in vacuum for 24 h at 70 °C to give a dark brown powder and denoted as HCPT-B (yield: 98.5%). The synthesis of HCPT-B using solvothermal and ultrasonic methods is given in the Supporting Information. Characterizations. FT-IR spectra were collected as KBr disks using a Tensor 27 FT-IR spectrometer (Bruker). Thermogravimetric
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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.7b05345. Details of the synthesis HCPTs use solvothermal method and ultrasonic method. FT-IR spectra, powder XRD patterns, XPS spectra, TGA curves in nitrogen, N2 adsorption−desorption isotherms, and TEM and SEM images for HCPTs and PCNTs. Tables showing the morphology evolution with monomer concentration and surface areas for HCPTs and PCNTs. (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. 20784
DOI: 10.1021/acsami.7b05345 ACS Appl. Mater. Interfaces 2017, 9, 20779−20786
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Jia-Xing Jiang: 0000-0002-2833-4753 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21574077 and 21304055), Shaanxi Innovative Team of Key Science and Technology (2013KCT17), the Fundamental Research Funds for the Central Universities (GK201501002 and GK201701007), and the Opening Project of State Key Laboratory of Polymer Materials Engineering from Sichuan University (Grant No. sklpme20164-22).
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DOI: 10.1021/acsami.7b05345 ACS Appl. Mater. Interfaces 2017, 9, 20779−20786
