Fluorinated, Sulfur-Rich, Covalent Triazine Frameworks for Enhanced

Oct 9, 2017 - Lithium–sulfur battery represents a promising class of energy storage technology owing to its high theoretical energy density and low ...
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Research Article Cite This: ACS Appl. Mater. Interfaces 2017, 9, 37731-37738

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Fluorinated, Sulfur-Rich, Covalent Triazine Frameworks for Enhanced Confinement of Polysulfides in Lithium−Sulfur Batteries Fei Xu,*,† Shuhao Yang,† Guangshen Jiang,† Qian Ye,† Bingqing Wei,†,‡ and Hongqiang Wang*,† †

State Key Laboratory of Solidification Processing, Center for Nano Energy Materials, School of Materials Science and Engineering, Northwestern Polytechnical University and Shaanxi Joint Laboratory of Graphene (NPU), Xi’an 710072, P. R. China ‡ Department of Mechanical Engineering, University of Delaware, Newark, Delaware 19716, United States S Supporting Information *

ABSTRACT: Lithium−sulfur battery represents a promising class of energy storage technology owing to its high theoretical energy density and low cost. However, the insulating nature, shuttling of soluble polysulfides and volumetric expansion of sulfur electrodes seriously give rise to the rapid capacity fading and low utilization. In this work, these issues are significantly alleviated by both physically and chemically restricting sulfur species in fluorinated porous triazine-based frameworks (FCTF-S). One-step trimerization of perfluorinated aromatic nitrile monomers with elemental sulfur allows the simultaneous formation of fluorinated triazine-based frameworks, covalent attachment of sulfur and its homogeneous distribution within the pores. The incorporation of electronegative fluorine in frameworks provides a strong anchoring effect to suppress the dissolution and accelerate the conversion of polysulfides. Together with covalent chemical binding and physical nanopore-confinement effects, the FCTF-S demonstrates superior electrochemical performances, as compared to those of the sulfur-rich covalent triazine-based framework without fluorine (CTF-S) and porous carbon delivering only physical confinement. Our approach demonstrates the potential of regulating lithium−sulfur battery performances at a molecular scale promoted by the porous organic polymers with a flexible design. KEYWORDS: fluorinated, trimerization, porous organic polymers, covalent triazine frameworks, polysulfides confinement, lithium−sulfur batteries

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INTRODUCTION Advanced energy storage technologies are essential to the future society and sustainable economy. Lithium−sulfur (Li−S) batteries deliver fivefold higher energy density than conventional lithium-ion batteries by taking advantage of the high theoretical specific capacity of sulfur (1675 mAh g−1).1−4 Besides, the low cost, natural abundance, and environmental benignity of sulfur together make Li−S batteries competitive as a kind of next-generation energy storage devices. In spite of the significant potential, several major problems have to be overcome before Li−S batteries can find their widespread practical realization.5−7 The sulfur electrode works on the redox transformation between cyclic S8 and lithium sulfide accompanied by large volume changes, during which highly soluble linear polysulfides intermediates are produced. The intrinsic insulation of sulfur and lithium sulfide leads to poor utilization of the active material; the large volume expansion (∼80%) by lithiation of sulfur during discharge gives rise to mechanical damage of cathode materials; particularly important is that the loss of sulfur from the electrode into the electrolyte by dissolution of polysulfides with the so-called shuttling effect causes serious degradation of cycling stability and worse Coulombic efficiency. Much effort has been devoted to solving these problems in the past several years. One of efficient strategies is to develop © 2017 American Chemical Society

composite cathode materials by physical encapsulation of sulfur into conductive porous hosts that are capable of immobilizing polysulfides and improving conductivity.8−13 Exciting progress has been made recently by using various carbon hosts such as micro/mesoporous carbons, carbon nanotubes/nanofibers, hollow carbon spheres, carbon aerogels, and graphene.10,14−25 While enhanced sulfur utilization and better electrochemical performance have been achieved using carbon hosts, a gradual decrease in capacity is still observed with prolonged cycling. This is likely due to the fact that the nonpolar property of carbons is insufficient and kinetically unfavorable in immobilizing the polar polysulfides by physical adsorption/confinement alone.26 Accordingly, efforts have been made to modify the carbon hosts with heteroatom doping or surface functionalities,27,28 so as to intensify specific interactions between sulfur species and carbon hosts. However, the restraining efficiency is still not satisfying due to the inherently low doping or modification ratio in carbons, giving rise to limited active sites available for chemisorption. In this context, constructing strong chemical bonding with sulfur species seems essential for chemical trapping of Received: July 25, 2017 Accepted: October 9, 2017 Published: October 9, 2017 37731

DOI: 10.1021/acsami.7b10991 ACS Appl. Mater. Interfaces 2017, 9, 37731−37738

Research Article

ACS Applied Materials & Interfaces polysulfides. Organic materials bearing rich functional groups show their advantages in this respect. For example, the covalent chemical bonding with elemental sulfur was used for producing sulfur-containing polymers via inverse vulcanization strategy,29 ring-opening polymerization of sulfur along the thiol surfaces of trithiocyanuric acid,30 and also in cross-linked sulfur−polyaniline composite with an interconnected disulfide bond.31 Benefiting from the strong covalent chemical interaction, the dissolution of polysulfides is effectively suppressed.29,32 Nevertheless, these sulfur-rich polymers generally exhibit poor conductivity and lack micro-/mesopores for physical encapsulation, precluding the accomplishment of high utilization of active materials, good cycling, and improved rate performance. Therefore, it would be highly desirable to develop novel, alternative cathode host materials that enable strong chemical trapping of polysulfides, while maintaining good physical confinement in a similar manner to that in carbons. Porous organic polymers (POPs), including conjugated microporous polymers and covalent organic frameworks, are a class of porous materials that combine the predesignable polymeric frameworks and permanent nanopores.33−40 They offer a high flexibility for the molecular design of both skeletons and nanopores. In principle, there would be great opportunity to rationally design POPs as hosts for sulfur, in consideration of complementary utilization of predesignable skeletons for strong chemical trapping of sulfur species and tailored nanopores for physical adsorption of polysulfides. Some POPs were employed to serve as sulfur hosts, whereas most of them showed moderate restriction efficiency owing to the lack of strong chemical interactions,41−46 and very few works witnessed enhanced interactions with sulfur species by covalent bonding or dual chemical adsorption of Li+ and Sx2−.47,48 However, the rational design of POP hosts with a suitable pore environment for efficiently sequestering the soluble polysulfides still remains to be well explored. Meanwhile, it is also highly demanding to unravel the relationship of structure−property between the POP hosts and cell performances at a molecular level. Herein, we present the fluorinated sulfur-rich covalent triazine frameworks (FCTF-S) as efficient sulfur immobilizers for Li−S batteries by combining its physical and chemical confinement effects. The FCTF-S was synthesized by one-step sulfur-assisted trimerization of perfluorinated aromatic nitriles, resulting in the simultaneous formation of triazine-based frameworks, covalent attachment of sulfur, and its homogeneous distribution within the pores (Figure 1). The welldesigned structure of FCTF-S has several features: (1) the abundant nanopores and covalent chemical binding of sulfur could help to physically and chemically trap sulfur and thus suppress the diffusive loss of soluble polysulfides; (2) the incorporation of fluorine species with a polar characteristic in FCTF-S would further facilitate its strong interactions with polar polysulfides; (3) the semiconducting property of triazinebased frameworks would be beneficial for the conductivity, compared to that of the most organic materials for Li−S batteries. Owing to these structural features, FCTF-S demonstrated a high capacity of 1296 mAh g−1 at 0.1 C, a stable cycling performance of 833 mAh g−1 after 150 cycles at 0.5 C, and improved high-rate capability, superior to CTF-S without fluorine.

Figure 1. (a) Schematic synthesis of the FCTF-S composite by onestep sulfur-assisted cyclotrimerization of aromatic nitriles. (b) Chemical structures of FCTF-S and CTF-S from cyclotrimerization with sulfur.

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EXPERIMENTAL SECTION

A description of chemicals, synthetic procedures, material characterizations, and cell fabrication and electrochemical measurements of Li− S batteries is provided in the Supporting Information.

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RESULTS AND DISCUSSION Sulfur-rich covalent triazine-based frameworks were synthesized by the sulfur-assisted cyclotrimerization of aromatic nitriles (Figure 1). To introduce the fluorine in the frameworks, perfluorinated aromatic nitrile was used as the monomer. A stepwise heating at 160 °C and then at 400 °C was applied to first dissolve the sulfur and then initiate the ring-opening polymerization of sulfur into polymeric sulfur, accompanied by the trimerization of the nitrile.47 In such a process, the formation of fluorinated triazine-based frameworks, the covalent attachment by sulfur polymerization/insertion, and the nanoconfinement of sulfur within the micropores can be simultaneously accomplished. A notable color change from khaki to black was observed (Supporting Information, Figure S1), indicating the trimerization and partial graphitization of the framework. The occurrence of the trimerization reaction was evidenced by the significant attenuation of the otherwise intense carbonitrile band at 2230 cm−1 found in monomers, whereas the presence of the characteristic absorption bands at about 1500 and 1320 cm−1 indicates the formation of triazine rings in Fourier transform infrared (FTIR) (Figure 2a).49 Such triazine-based frameworks formation can be also validated by C1s X-ray photoelectron spectra in both FCTF-S and CTF-S (Figure 2c). The lowest peak of 284.6 eV was associated with the CC of the aryl rings in frameworks, whereas the peak at 286.9 eV corresponded to C in triazine NC−N rings, in accordance with the result from FTIR. For sulfur, both covalent attachment in frameworks owing to the sulfur polymerization/insertion and nanoconfinement 37732

DOI: 10.1021/acsami.7b10991 ACS Appl. Mater. Interfaces 2017, 9, 37731−37738

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Figure 2. (a) Fourier transform infrared (FTIR) spectroscopy of FCTF-S, CTF-S, and terephthalonitrile monomer. (b) F1s X-ray photoelectron spectrum of FCTF-S. (c) C1s and (d) S2p spectra of FCTF-S and CTF-S.

Figure 3. (a) Scanning electron microscopy (SEM) and (b) transmission electron microscopy (TEM) images of FCTF-S. (c) Annular dark-field TEM image and the corresponding elemental mappings of (d) sulfur and (e) carbon of the region indicated by the white square in (c).

within the micropores were accomplished in the preparation process. The covalent attachment in the frameworks can be revealed by the X-ray photoelectron spectra of S2p and C1s

(Figure 2c,d). The high-resolution S2p X-ray photoelectron spectra can be deconvoluted into three peaks at 164.8, 163.7, 163.1 eV for FCTF-S and 164.8, 163.6, 161.7 eV for CTF-S, 37733

DOI: 10.1021/acsami.7b10991 ACS Appl. Mater. Interfaces 2017, 9, 37731−37738

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Figure 4. (a) CV profiles of the FCTF-S and CTF-S cathodes at a scan rate of 0.1 mV s−1 in the potential range from 1.7 to 2.8 V. (b) Galvanostatic discharge−charge profiles of the FCTF-S and CTF-S cathodes recorded at 0.5 C. (c) Cycling performance of the FCTF-S, CTF-S, and porous carbon/S composite cathodes at 0.5 C for 150 cycles. (d) Nyquist plots of FCTF-S and CTF-S cathodes from 100 kHz to 10 mHz.

Nitrogen adsorption/desorption shows that the adsorbed amount is very low (Supporting Information, Figure S3), suggesting that the inherent pores of triazine-based frameworks were fully occupied by elemental sulfur. Such physically confined sulfur can be removed by dynamic vacuum heating, judging from the color change from black to brown and the appearance of sulfur on the upper wall of the dynamic vacuum tube (Supporting Information, Figure S1c). The powder X-ray diffraction (PXRD) analysis shows that some crystalline peaks remained in the FCTF-S and CTF-S, consistent with that of crystalline elemental sulfur (Supporting Information, Figure S4). This result reveals that the physically trapped sulfur crystallizes inside the nanopores after the high-temperature treatment. The morphology of as-synthesized materials in FCTF-S appears as well-defined nanoparticles (Figure 3a), in agreement with the transmission electron microscopy (TEM) observation in Figure 3b, whereas CTF-S consists of seriously aggregated large particles with average sizes of 1 μm (Supporting Information, Figure S5). TEM with elemental mapping was used to track the elemental distribution of sulfur in the FCTF-S (Figure 3c). The elemental mapping shows the spatial distributions of sulfur and carbon (Figure 3d,e), implying that the sulfur is homogeneously encapsulated in the nanopores of frameworks. It is speculated that such fluorinated, sulfur-rich-based triazine frameworks will perform well for energy storage in Li−S batteries. The physical nanoconfinement from nanopores, chemical covalent binding with frameworks, and polar and ionic characteristic of incorporated fluorine species will work co-

respectively (Figure 2d). The former two dominated peaks can be attributed to the S2p1/2 and S2p3/2 of S−S bonds in sulfur. The latter peak at a low binding energy represents the C−S bond in both FCTF-S and CTF-S, indicating the existence of bonding interactions between the C atom and S atom.30,32 Such covalent bonding interactions can also be confirmed by the presence of a peak at 286.1 eV in C1s spectra, which can be assigned to the C−S bonding in both FCTF-S and CTF-S (Figure 2c). The incorporation of fluorine in FCTF-S was verified by the presence of the C−F bond at 993 cm−1 in FTIR and the F1s signal in survey X-ray photoelectron spectra of FCTF-S, which otherwise cannot be found in CTF-S (Figures 2a and S2, Supporting Information). To investigate the chemical bonding nature, the F1s of FCTF-S could be deconvoluted into three peaks at 688.6, 687.4, and 685.6 eV, respectively (Figure 2b). The value at a high binding energy of 688.6 eV attributes to the covalent C−F bond, whereas the peaks at 687.4 and 685.6 eV correspond to F atoms semi-ionically bound to the sp2 carbon.50,51 This result indicates that the ionic contribution to the C−F bonding is also noticeable owing to the hightemperature preparation process. Furthermore, an additional peak of 288.3 eV at a higher binding energy was found in the C1s spectrum of FCTF-S (Figure 2c), corresponding to the C atom in C−F bonding in FCTF-S.50 However, this peak was absent in CTF-S (Figure 2c). It is expected that the polar C−F bonding will increase the interactions with polar polysulfides, and their semi-ionic properties with positively charged frameworks also help to adsorb negatively charged polysulfides. 37734

DOI: 10.1021/acsami.7b10991 ACS Appl. Mater. Interfaces 2017, 9, 37731−37738

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Figure 5. (a) Cycle stability of the FCTF-S and CTF-S cathodes and Coulombic efficiency of FCTF-S at a current rate of 1 C. (b) Digital picture of the fast adsorption of lithium polysulfides by FCTF and CTF frameworks.

the charge process, respectively, in agreement with the redox peaks (Figure 4a) observed in the CV measurements. The polarization, reflected by the gap between the oxidation and reduction potential plateaus, provides information on the redox reaction kinetics. Notably, FCTF-S shows a lower polarization than CTF-S (Figure 4b), further manifesting the promoted kinetics caused by the introduction of fluorine, similar to the result from CV. FCTF-S delivers an initial discharge capacity of 1131 mAh g−1 at 0.5 C, whereas CTF-S without a polar C−F bond and a porous carbon/S composite with only physical confinement show only a low capacity of 862 and 488 mAh g−1, respectively (Figure 4c). More significantly, the plateaus retain their shape with cycling, implying the stable structure of FCTFS for immobilizing sulfur species (Supporting Information, Figure S8). FCTF-S is able to maintain a stable cycling performance after 150 cycles with a capacity retention ratio of 73.7% and a fading rate of 0.17% per cycle (Figure 4c). In contrast, CTF-S fades at a higher rate of 0.30% with a capacity retention of 55.5% after 150 cycles. Thus, the FCTF-S cathode could retain a much higher reversible capacity of 833 mAh g−1 than that of the CTF-S cathode (478 mAh g−1) after 150 cycles. To show the potential of FCTF-S for practical applications, we increased the sulfur loading to 1.3 mg cm−2 and found that FCFT-S still demonstrated stable cycling performances at 0.5 C, in spite of a slight decrease in capacity (Supporting Information, Figure S9). At low rate of 0.1 C, capacities of up to 1296 and 1243 mAh g−1 are achieved for FCTF-S and CTFS at the initial cycle, respectively. With further chargedischarging to 50 cycles, the difference between capacities becomes significant (Supporting Information, Figure S10) for FCTF-S and CTF-S. These results vividly show that the chemical and physical restraining effect, together with the polar and ionic C−F bond, works effectively to improve sulfur utilization and mitigate the dissolution of polysulfides into the organic electrolyte. Owing to the low conductivity of the organic host materials, a large amount of the conductive agent was used in the conventional cathode electrodes using POPs as hosts (around 30 wt %, Supporting Information, Table S1). However, the capacities of sulfur-rich triazine-based frameworks, for example, FCTF-S, are still superior to most of the previously reported POP hosts even with a reduced amount of the conductive agent (10 wt %, Supporting Information, Table S1) in the electrode. This is thought to be attributed to the semiconductivity properties of the triazine-based frameworks.55 The conductivity of FCTF-S and CTF-S was measured to be

operatively to restrain the polysulfides shuttling effect and increase the active sulfur utilization. Therefore, we carried out electrochemical performances by using FCTF-S as the cathode material for Li−S batteries. The sulfur content is around 51 wt %, as determined by repeated energy dispersive spectrometer line scans in SEM (51 wt %), TEM with elemental mapping (48.5 wt %, Figures 3d and S6, Supporting Information), and thermogravimetric analysis (53 wt %, Supporting Information, Figure S7). The sulfur mass loading is around 0.7 mg cm−2, unless otherwise stated. For comparison, CTF-S and porous carbon/S were used as references. The cathode electrode was prepared by integrating active materials (e.g., FCTF-S), conductive carbon Super P, and binder polyvinylidene fluoride using the doctor-blade method. Cyclic voltammetry (CV) profiles were measured within a potential window of 1.7−2.8 V at a scan rate of 0.1 mV s−1 (Figure 4a), both showing two main cathodic peaks and two anodic peaks. The cathodic peak located at 2.31 V is assigned to the reduction of cyclic S8 to long-chain lithium polysulfide (Li2Sx, 4 ≤ x ≤ 8), whereas another peak appears at 2.03 V that reflects the further reduction via long-chain lithium polysulfide to short-chain polysulfides (Li2Sx, x ≤ 3) and Li2S. In the subsequent anodic scan, two broad oxidation peaks at 2.23−2.43 V are observed, assignable to the conversion of Li2S2/Li2S to sulfur via the formation of the intermediate lithium polysulfides. Interestingly, the reduction peaks of the FCTF-S cathode (2.31 and 2.03 V) appear at higher potentials than those of CTF-S without a fluorine cathode (2.28 and 2.02 V), whereas the oxidation peaks appear at lower potentials (2.43 vs 2.46 V for FCTF-S and CTF-S, respectively). Such distinct positive shift in the reduction peaks (i.e., 34 and 8 mV) and negative shift in the oxidation peaks (i.e., 30 mV) of FCTF-S indicate a decrease in polarization and an accelerated kinetics for the polysulfide redox. Similar improvement of redox reaction kinetics was observed by several works using the precious metals or metal nitride/oxide (Pt,52 VN,53 Nb2O554) as electrocatalysts to facilitate the conversion of polysulfide back to soluble longchain polysulfide. These results indicate that the incorporation of polar F atoms in the framework helps to boost the redox reaction kinetics of polysulfide owing to their strong interactions. Galvanostatic discharge−charge tests were further carried out at a current rate of 0.5 C, as shown in Figure 4b. Two plateaus at 2.30 and 2.06 V were observed in the discharge curves, and two charge plateaus between 2.24 and 2.40 V were present in 37735

DOI: 10.1021/acsami.7b10991 ACS Appl. Mater. Interfaces 2017, 9, 37731−37738

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2.26 × 10−4 and 2.02 × 10−4 S cm−1, respectively, which is ∼21 orders of magnitude higher than sulfur (∼5 × 10−26 S cm−1) and is also greater than that of many insulating organic porous hosts. Electrochemical impedance spectra in Nyquist plots of the FCTF-S and CTF-S were also compared (Figure 4d). The charge transfer resistance of FCTF-S, determined by the semicircle at the high-frequency region, is much smaller than that of the CTF-S. This can be explained by increased interfacial affinity between fluorinated triazine-based frameworks with polar and ionic characteristics and polysulfides. Moreover, FCTF-S also delivered an excellent high-rate performance, as shown in Figure 5a. The electrode was cycled at a high rate of 1 C for 100 cycles. The cell of FCTF-S was able to deliver a stable discharge capacity. In contrast, the CTFS electrode exhibited a lower discharge capacity under the same conditions. In addition, the Coulombic efficiency is stable and close to 98%. These results demonstrate the higher rate capabilities of FCTF-S due to the introduction of the polar and ionic C−F bond, as compared to that of CTF-S. To verify the strong anchoring ability of FCTF-S for polysulfides, we compared the polysulfide adsorption ability of the pure frameworks of FCTF and CTF, which were prepared by conventional ZnCl2-assisted ionothermal synthesis (Experimental Section in Supporting Information). Photos were taken after adding 5 mg of CTF or FCTF powders to glass vials containing Li2S6 solution (5 mmol L−1, 1.5 mL) for 1 h. As shown in Figure 5b, the digital image shows a visible decoloration of the Li2S6 solution after the adsorption, whereas the solution containing CTF showed less decoloration. Furthermore, the cell was disassembled in an Ar-filled glovebox after discharging the cell to 2.08 V at the 5th cycle and keeping at this potential for 12 h, followed by washing several times with 3 mL of 1,3dioxolane to collect the polysulfide-containing solution. The polysulfides dissolved in the electrolyte solution were examined by UV−vis adsorption spectra (Supporting Information, Figure S11). The results show that the amount of polysulfides is less for the FCTF-S battery, as compared to that of CTF-S, implying that the incorporation of F is helpful to minimize the loss of polysulfides into the electrolyte. Such a difference suggests strong adsorption of Li2S6 molecules to frameworks with fluorine functionalities, owing to polar and ionic bonding of C−F in FCTF-S.

Research Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b10991. Description of the material; experimental section, digital pictures, X-ray photoelectron spectra, nitrogen adsorption/desorption isotherm, PXRD patterns, SEM images, energy dispersive spectra, thermogravimetric analysis curve, galvanostatic discharge−charge curves, cycling performance and Coulombic efficiency, UV−vis absorption spectra, and summary of the electrode composition and capacity of representative porous organic polymers host cathodes (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (F.X.). *E-mail: [email protected] (H.W.). ORCID

Fei Xu: 0000-0003-2446-8903 Bingqing Wei: 0000-0002-9416-1731 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Projects of NSFC (51702262, 51472204, and 51672225), the Natural Science Foundation of Shaanxi Province (2017JQ5003 and 2017JM5028), the Fundamental Research Funds for the Central Universities (3102017OQD057 and G2017KY0002), the Project of Young Talent Fund of University Association for Science and Technology in Shaanxi, China (20160103), Creative Research Foundation of Science and Technology on Thermostructural Composite Materials Laboratory (6142911030512), the Key laboratory of Polymeric Composite & Functional Materials of Ministry of Education (PCFM201602), and the Program of Introducing Talents of Discipline to Universities (B08040). H.W. also acknowledges the financial support of the 1000 Youth Talent Program of China.

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CONCLUSIONS We have designed and prepared fluorinated, sulfur-rich covalent triazine-based frameworks for Li−S batteries, by one-step trimerization of fluorinated aromatic nitrile monomers with elemental sulfur. This structure benefits from the advantages of (i) the physical confinement of polysulfides by nanopores of frameworks, (ii) chemical confinement of sulfur by covalent binding with frameworks, and (iii) a strong anchoring effect for polysulfides by highly electronegative fluorine atoms in frameworks to accelerate the polysulfide conversion. With this strategy, the FCTF-S electrode exhibited a high specific capacity of 1296 mAh g−1 at 0.1 C, stable cycling performance of 833 mAh g−1 after 150 cycles at 0.5 C, and improved highrate capability, superior to CTF-S without fluorine and porous carbon/S composite. Our finding will provide an impetus for the utilization of these porous organic materials in a broad spectrum of applications for energy storage, especially by modulating the performances via a molecular design.

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DOI: 10.1021/acsami.7b10991 ACS Appl. Mater. Interfaces 2017, 9, 37731−37738

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ACS Applied Materials & Interfaces

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DOI: 10.1021/acsami.7b10991 ACS Appl. Mater. Interfaces 2017, 9, 37731−37738

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DOI: 10.1021/acsami.7b10991 ACS Appl. Mater. Interfaces 2017, 9, 37731−37738