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Phosphonate-Derived Nanoporous Metal Phosphates and Their Superior Energy Storage Application Malay Pramanik,† Rahul R. Salunkhe,† Masataka Imura,† and Yusuke Yamauchi*,†,‡ †
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International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan ‡ Department of Nanoscience and Nanoengineering, Faculty of Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku, Tokyo 169-8555, Japan S Supporting Information *
ABSTRACT: Nanoporous nickel, aluminum, and zirconium phosphates (hereafter, abbreviated as NiP, AlP, and ZrP, respectively) with high surface areas and controlled morphology and crystallinity have been synthesized through simple calcination of the corresponding phosphonates. For the preparation of phosphonate materials, nitrilotris(methylene)triphosphonic acid (NMPA) is used as phosphorus source. The organic component in the phosphonate materials is thermally removed to form nanoporous structures in the final phosphate materials. The formation mechanism of nanoporous structures, as well as the effect of applied calcination temperatures on the morphology and crystallinity of the final phosphate materials, is carefully discussed. Especially, nanoporous NiP materials have a spherical morphology with a high surface area and can have great applicability as an electrode material for supercapacitors. It has been found that there is a critical effect of particle sizes, surface areas, and the crystallinities of NiP materials toward electrochemical behavior. Our nanoporous NiP material has superior specific capacitance, as compared to various phosphate nanomaterials reported previously. Excellent retention capacity of 97% is realized even after 1000 cycles, which can be ascribed to its high structural stability. KEYWORDS: nanoporous materials, phosphate, phosphonate, calcination, supercapacitors
1. INTRODUCTION The synthesis of nanoporous metal phosphates with different morphologies always remains very attractive due to their unique advantages, such as abundant active sites for reactions with high surface areas and fast interfacial transport of protons/electrons by decreasing the diffusion path length through porous structure.1−4 Various phosphate materials with nanoporous structures can be synthesized by cooperative assemblies of inorganic phosphate precursors and organic structure-directing agents (SDAs).5,6 Generally, ionic (cetyltrimethylammonium bromide or sodium dodecyl sulfate) and nonionic (Pluronic P123 or F127) SDAs have been employed to prepare nanoporous phosphate materials; however, removing these SDAs while maintaining the original nanostructures is really difficult.7−9 In general, the nanoporous phosphate materials prepared by this method often suffer from less-crystallinity or amorphous pore walls.10−12 Furthermore, the synthesis of nanoporous phosphates through a cooperative self-assembly process is very difficult to control, due to the mismatch of interaction energy between phosphate precursors and SDAs and the high reactivity of inorganic phosphate precursors toward hydrolysis and condensation.13−15 Although there have been some reports on the synthesis of nanoporous crystalline phosphates in the presence of smaller © 2016 American Chemical Society
organic molecules (such as 1,2-diaminocyclohexane and diethylenetriamine) as a template or in the absence of templates, the obtained materials are micrometer-sized crystals, 16,17 which are a thousand times larger than architectural pores (typically dAlP‑A > dZrP‑A). This is wellknown for phosphonate materials.13,26,27 Thus, as the valence of the central metal ion increases, the particle size of the final phosphonate material decreases and, consequently, the pore center-to-pore center distance (d) also decreases. The architectural porosity of the materials was characterized by N2 adsorption−desorption isotherms at the liquid N2 temperature (Figure S2a), and the corresponding pore-size distributions of the materials are determined by the density functional theory method (Figure S2b). All of the as-prepared materials (NiP-A, AlP-A, and ZrP-A) have isotherms with no
Figure 2. FE-SEM images of (a, b) NiP-300 and NiP-700, (c, d) AlP300 and AlP-700, and (e, f) ZrP-300 and ZrP-700, respectively. 9792
DOI: 10.1021/acsami.6b01012 ACS Appl. Mater. Interfaces 2016, 8, 9790−9797
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phosphate materials are displayed in Figure 3. The average crystallite sizes of the NiP-700, AlP-700, and ZrP-700 samples are calculated by using the Scherrer equation (D = 0.9λ/β·cos θ), where D is the average crystallite size (Å), λ is the X-ray wavelength (Cu Kα = 1.54 Å), β is the full width at halfmaximum (FWHM) in radians, and θ is Bragg’s diffraction angle. The average crystalline sizes for NiP-700, AlP-700, and ZrP-700 are estimated to be 85, 60, and 28 nm, respectively. Interestingly, materials prepared at a higher calcination temperature (700 °C) still have a signature peak at 2θ = 6.15° (d = 1.44 nm) (Figure 4), suggesting the periodicity of
compared to NiP-A (ca. 45 nm), the particle sizes of NiP-300 and NiP-700 are increased to ca. 305 and 450 nm, respectively; however, their spherical shapes remain intact (Figure 2a,b). A similar situation is observed in the case of ZrP-A. The average particle sizes of ZrP-300 and ZrP-700 are ca. 180 and 355 nm, respectively (Figure 2e,f). From the SEM image of ZrP-700, it can be clearly observed that the big particle is made up of a selfassembly of tiny nanoparticles. In the case of AlP-A, the individual size of particles is increased, and they are selfaggregated to form a dendritic structure with clear edges (Figure 2c,d).32 Figure 3 shows the corresponding wide-angle XRD patterns of the calcined materials. When the as-prepared materials are
Figure 4. Powder XRD patterns of NiP-700, AlP-700, and ZrP-700, showing signature peaks at 2θ = 6.15°.
micropores in highly crystallized phosphate materials (NiP-700, AlP-700, and ZrP-700).38−40 We have compared our results with the corresponding bulk phosphate materials as well. Although the bulk materials (bulk-NiP, bulk-AlP, and bulk-ZrP) have the same wide-angle XRD patterns as NiP-700, AlP-700, and ZrP-700, the materials do not have any signature peak in the region of 2θ = 5°−10° (Figure S4a). This signifies that the bulk materials have no microporosity which has been generated in NiP-700, AlP-700, and ZrP-700 due to the burning of organics from the parent phosphonate materials. The corresponding FE-SEM images of the bulk phosphate materials are shown in Figure S4. The SEM images (Figure S4b−d) also clearly demonstrate that the bulk phosphate materials do not have uniform morphologies. Although there have been a few reports of the synthesis of nanoporous crystalline metal phosphates through a variety of synthetic approaches, a general synthetic approach has not yet been demonstrated. Our simple, inexpensive, tunable, and scalable process of transforming nanoporous solid metal phosphonates to nanoporous metal phosphates is very useful for the synthesis of a range of phosphate materials. NiP-300, AlP-300, and ZrP-300 materials have surface areas of 55, 105, and 212 m2 g−1 with pore diameters of 3.76, 3.68, and 3.64 nm, respectively. When the materials are calcined at 300 °C, the structure of the phosphonate materials partially/ completely collapses due to the removal of organics. As the calcination temperature increases, the crystallization of phosphate materials increases and, consequently, the surface areas decrease. The corresponding N2 adsorption−desorption isotherms and pore-size distribution curves of the materials are shown in Figure 5. For comparison, the BET isotherms for bulk-NiP, bulk-AlP, and bulk-ZrP are also shown in Figure S5. The corresponding surface areas and the pore diameters of all materials are listed in Table S1.
Figure 3. (a, c, e) Wide-angle XRD patterns of NiP-300, -500, and -700; AlP-300, -500, and -700; ZrP-300, -500, and -700; respectively. (b, d, f) Crystal structures of NiP-700, AlP-700, and ZrP-700, respectively.
calcined at 300 °C to burn the organic units from the framework, the materials (NiP-300, AlP-300, and ZrP-300) show amorphous phases. Obviously, as the calcination temperature increases, the crystallinity of the materials increases.33−35 Unlike metal oxides, metal phosphates have higher crystallization temperatures. Thus, when the as-prepared materials are calcined at a relatively higher temperature (700 °C), perfectly crystalline phosphate frameworks are obtained.36,37 The XRD patterns for NiP-700, AlP-700, and ZrP-700 can be assigned to Ni2P2O7 (JCPDS card no. 01-0741604), AlPO4 (JCPDS card no. 01-070-4689), and Zr2.25(PO4)3 (JCPDS card no. 00-038-0017) structures, respectively, without any impurity phase. The corresponding crystal structures of the 9793
DOI: 10.1021/acsami.6b01012 ACS Appl. Mater. Interfaces 2016, 8, 9790−9797
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sizes and the crystallite sizes significantly increase. From the wide-angle XRD pattern (Figure 3), it is also clear that, as the calcination temperature increases, the peak width decreases and the peak intensity increases. This means that the primary crystallite sizes in the calcined materials become larger. From the transmission electron microscopy (TEM) images (Figure 6a−c) of NiP-300 and NiP-700, it is clear that both samples have spherical morphologies with nanoporous structures, as confirmed by the high-resolution TEM (HRTEM) image (Figure 6c). At a lower calcination temperature (300 °C), the spherical nickel phosphate possesses an amorphous phase. The selected area electron diffraction (ED) patterns for NiP-300 (taken from 100 nm2, 10 nm × 10 nm) do not show any patterns, indicating the amorphous nature of the material, which is in good agreement with XRD data (Figure 3a). Elemental analysis of NiP-300 proves the presence of Ni, P, and O over the entire area (Figure S6). When the calcination temperature increases, the crystallinity of the materials obviously increases, which can be confirmed from the HRTEM of NiP-700 material (Figure 6c). On the high-resolution TEM image of the edge part of the particle (Figure 6c), the crystal fringes and nanopores are observable on the particle surface of the sphere, and the distance between the two fringes is in good agreement with the (012) crystal plane of the monoclinic crystal structure. The selected area ED pattern with several intense spots also indicates the crystalline nature of the material (NiP-700). Like NiP-300, AlP-300 and ZrP-300 are also amorphous with no spots/rings in the ED patterns. Elemental analysis of ZrP300 proves the presence of Zr, P, and O over the entire area (Figure S7). Both AlP-700 and ZrP-700 show crystalline structures with nanopores. In the HR-TEM image of AlP-700 and ZrP-700, the distance between the two crystal fringes are in good agreement with the (206) and (012) crystal planes of triclinic and trigonal crystal structures, respectively (Figure 6f,i). Interestingly, for all of the materials (NiP-700, AlP-700, and
Figure 5. (a, c, e) N2 adsorption−desorption isotherms of NiP-300, -500, and -700; AlP-300, -500, and -700; ZrP-300, -500, and -700, respectively. (b, d, f) The corresponding pore-size distribution curves for NiP-300, -500, and -700; AlP-300, -500, and -700; ZrP-300, -500, and -700, respectively.
On the basis of the above results, when the calcination temperature of the NiP-A, AlP-A, and ZrP-A precursors are raised to 500 and 700 °C, the original spherical morphology of the as-prepared materials was retained; however, the particle
Figure 6. (a, b) TEM images of NiP-300 and NiP-700, respectively. (c) High-resolution (HR) TEM image of NiP-700 showing the lattice fringes and pores (indicated by dots); the inset shows the corresponding FFT pattern. (d, e) TEM images of AlP-300 and AlP-700, respectively. (f) HRTEM of AlP-700 showing the lattice fringes and pores (marked by dots); the inset shows the corresponding FFT pattern. (g, h) TEM images of ZrP300 and ZrP-700, respectively. (i) HR-TEM of ZrP-700 showing the lattice fringes and pores (marked by dots). The inset shows the corresponding FFT patterns. 9794
DOI: 10.1021/acsami.6b01012 ACS Appl. Mater. Interfaces 2016, 8, 9790−9797
Research Article
ACS Applied Materials & Interfaces ZrP-700), micropores are confirmed inside the crystals (as indicated by dots), although the arrangement of pores is not ordered. 3.3. Investigation of Electrochemical Capacitances. Supercapacitors are an important type of energy device whose applications dramatically increase in areas such as hybrid electrical vehicles, telecommunications, and back-up power supplies due to their fast charging−discharging and long cycle life.41,42 Various types of materials, including nanoporous carbon, metal-oxides, metal sulfides, and polymers, have been investigated as fundamental electrode materials for supercapacitors.43−46 There have been several previous studies on nanoporous transition metal oxides and chalcogenides representing the supercapacitor properties.47,48 However, the stability of these materials as electrode is insufficient.49,50 The electrochemical property of nanoporous metal-P for supercapacitors remains almost unexplored due to their lower electrical conductivity as compared to analogous oxide/ hydroxide materials.51 Still, the phosphate-based materials have been reported for electrode materials in supercapacitors due to their long-term cycling stability, low cost, and environmental friendliness.52 Herein, our nanoporous amorphous NiP-300 and nanoporous crystalline NiP-700 are used as electrode materials in supercapacitors. Electrochemical performances of NiP-300 and NiP-700 were investigated by cyclic voltammetry (CV) in a three-electrode system in a 3 M KOH electrolyte at room temperature. Figure 7a,b shows the CV curves of NiP-300 and NiP-700 in the 0.0−
From Figure 7b, it is clear that the intensity of the anodic and cathodic current is not the same; this implies that the redox reaction is not completely reversible, which has been identified as a common electrochemical phenomenon for Ni-based electrode materials.52,55,56 In the case of NiP-300, due to the amorphous nature of the material, the oxidation/reduction peaks are not so prominent. This indicates that the electrochemical capacitance of NiP-300 and NiP-700 electrodes mainly results from the pseudocapacitance behavior of the materials.57 Under a high scan rate, the potential difference between the anodic and cathodic peaks increases due to the polarization of electrodes at a high scan rate. Additionally, with an increase in the scan rates, the peak current increases gradually, which suggests good reversibility during the fast charge−discharge process.54,58 The prepared electrode from NiP-300 shows capacitance values of 265, 126, 95, and 73 F·g−1 at 20, 40, 60, and 80 mV s−1, respectively. The corresponding capacitance values for NiP-700 are 124, 108, 100, and 93 F·g−1 at the same scan rates (Figure 7c). The specific capacitances of NiP-300 and NiP-700 with various scan rates are summarized in Table S2. In the case of NiP-300, due to its amorphous nature with a higher surface area and relatively smaller particle size, the electrode−electrolyte contact area increases, and the transport path for electrons becomes shorter, which effectively increases the capacitance value of the material as compared to that of NiP-700 at lower scan rates.59−61 The specific capacitance value of NiP-300 is higher than or comparable to that of the previously reported various metal phosphate supercapacitors (Table S3). Jiang et al. prepared Ni(OH)2 nanoflowers, nanoplates, and nanosheets for supercapacitor applications with retention of capacitances of 41.4%, 53.2%, and 55.7%, respectively.62 Patil et al. prepared β-Ni(OH)2 for supercapacitor application with retention of capacitances of up to 23.2%.63 In our case, the corresponding capacitance retention values of NiP-700 and NiP-300 were 75.0% and 27.5%, respectively. Although the specific capacitances of our NiP materials were not comparable to those of Ni(OH)2 materials, our NiP materials’ cycling stability and retention of capacitance values at higher scan rates are superior. When the scan rate is increased, the capacitance of the electrode material starts to gradually decrease due to the lower redox time at higher scan rates, which inhibits the electrolytes from properly interacting with the catalytically active “inner” sites of electrodes.64 NiP700 with high crystallinity provides a more stable capacitance value as compared to amorphous NiP-300.60,61 To evaluate the electrochemical stability of the electrode material, the long-term cycling performance was investigated at a constant current density of ∼5A·g−1 for 1000 cycles. In the case of NiP-300, the retention of a specific capacitance is up to 97% after 1000 cycles, while the corresponding retention capacity for NiP-700 is up to 96%. To ensure the stability of the catalyst, the NiP-700 sample was characterized by SEM and wide-angle XRD (Figure S8) after long-term cycling tests. From the SEM image, the spherical morphology of the material is clearly observable, indicating that the morphology of the material is retained, even after electrochemical measurement. As seen in Figure S8b, no significant change is observed in the wide-angle XRD pattern before and after electrochemical measurement. From these data, it is clear that neither the morphology nor the crystallinity of the material was destroyed during the electrochemical test.
Figure 7. (a, b) Cyclic voltammograms (CVs) of the porous NiP-300 and NiP-700 electrode, respectively, in 3 M KOH electrolytes at scan rates of 20, 40, 60, and 80 mV s−1. (c) Scan-rate dependency of specific capacitances for porous NiP-300 and NiP-700. (d) Relationship of capacity retention vs cycling number.
0.5 V range at various scan rates from 20 to 80 mV S−1. For comparison, the electrochemical performance of as-prepared NiP-A was tested with the same experimental conditions; however, it did not produce any reasonable CV curves at different scan rates. The very poor supercapacitor properties of NiP-A are due to the presence of insulating organic groups in the framework, which drastically decreases the conductivity of the material.53 As seen in Figure 7b, NiP-700 shows two welldefined oxidation/reduction peaks at around 0.31 and 0.19 V, which correspond to the redox reaction of Ni(II)/Ni(III).54 9795
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(6) Schuth, F. Non-Siliceous Mesostructured and Mesoporous Materials. Chem. Mater. 2001, 13, 3184−3195. (7) Pramanik, M.; Imura, M.; Lin, J.; Kim, J.; Kim, J. H.; Yamauchi, Y. Shape-Controlled Synthesis of Mesoporous Iron Phosphate Materials with Crystallized Frameworks. Chem. Commun. 2015, 51, 13806− 13809. (8) Doi, T.; Miyake, T. Synthesis of a Novel Mesoporous VPO Compound. Chem. Commun. 1996, 1635−1636. (9) Bhaumik, A.; Inagaki, S. Mesoporous Titanium Phosphate Molecular Sieves with Ion-Exchange Capacity. J. Am. Chem. Soc. 2001, 123, 691−696. (10) Guo, X.; Ding, W.; Wang, X.; Yan, Q. Synthesis of a Novel Mesoporous Iron Phosphate. Chem. Commun. 2001, 709−710. (11) Mal, N. K.; Ichikawa, S.; Fujiwara, M. Synthesis of a Novel Mesoporous Tin Phosphate, SnPO4. Chem. Commun. 2002, 112−113. (12) Ciesla, U.; Froba, M.; Stucky, G.; Schuth, F. Highly Ordered Porous Zirconias from Surfactant-Controlled Syntheses: Zirconium Oxide-Sulfate and Zirconium Oxo Phosphate. Chem. Mater. 1999, 11, 227−234. (13) Vasylyev, M.; Neumann, R. Preparation, Characterizaton, and Catalytic Aerobic Oxidation by a Vanadium Phosphonate Mesoporous Material Constructed from a Dendritic Tetraphosphonate. Chem. Mater. 2006, 18, 2781−2783. (14) Mah, R. K.; Lui, M. W.; Shimizu, G. K. H. Enhancing Order and Porosity in a Highly Robust Tin(IV) Triphosphonate Framework. Inorg. Chem. 2013, 52, 7311−7313. (15) Mao, J. G. Structures and Luminescent Properties of Lanthanide Phosphonates. Coord. Chem. Rev. 2007, 251, 1493−1520. (16) Yu, J.; Xu, R. Insight into the Construction of Open-Framework Aluminophosphates. Chem. Soc. Rev. 2006, 35, 593−604. (17) Yang, G. Y.; Sevov, S. C. Zinc Phosphate with Gigantic Pores of 24 Tetrahedra. J. Am. Chem. Soc. 1999, 121, 8389−8390. (18) Pérez-Ramı ́rez, J.; Christensen, C. H.; Egeblad, K.; Christensen, C. H.; Groen, J. C. Hierarchical Zeolites: Enhanced Utilisation of Microporous Crystals in Catalysis by Advances in Materials Design. Chem. Soc. Rev. 2008, 37, 2530−2542. (19) Seo, Y.; Lee, S.; Jo, C.; Ryoo, R. Microporous Aluminophosphate Nanosheets and Their Nanomorphic Zeolite Analogues Tailored by Hierarchical Structure Directing Amines. J. Am. Chem. Soc. 2013, 135, 8806−8809. (20) Smeets, S.; Liu, L.; Dong, J.; Mccusker, L. B. Ionothermal Synthesis and Structure of a New Layered Zirconium Phosphate. Inorg. Chem. 2015, 54, 7953−7958. (21) Gu, D.; Schuth, F. Synthesis of Non-Siliceous Mesoporous Oxides. Chem. Soc. Rev. 2014, 43, 313−344. (22) Tian, B.; Liu, X.; Tu, B.; Yu, C.; Fan, J.; Wang, L.; Xie, S.; Stucky, G. D.; Zhao, D. Self-Adjusted Synthesis of Ordered Stable Mesoporous Minerals by Acid−base Pairs. Nat. Mater. 2003, 2, 159− 163. (23) Lu, Y.; Zhan, W.; He, Y.; Wang, Y.; Kong, X.; Kuang, Q.; Xie, Z.; Zheng, L. MOF-Templated Synthesis of Porous Co3O4 Concave Nanocubes with High Specific Surface Area and Their Gas Sensing Properties. ACS Appl. Mater. Interfaces 2014, 6, 4186−4195. (24) Ma, T. Y.; Yuan, Z. Y. Metal Phosphonate Hybrid Mesostructures: Environmentally Friendly Multifunctional Materials for Clean Energy and Other Applications. ChemSusChem 2011, 4, 1407−1419. (25) Salunkhe, R. R.; Lin, J.; Malgras, V.; Dou, S. X.; Kim, J. H.; Yamauchi, Y. Large-Scale Synthesis of Coaxial Carbon Nanotube/ Ni(OH)2 Composites for Asymmetric Supercapacitor Application. Nano Energy 2015, 11, 211−218. (26) Vasylyev, M. V.; Wachtel, E. J.; Popovitz-Biro, R.; Neumann, R. Titanium Phosphonate Porous Materials Constructed from Dendritic Tetraphosphonates. Chem. - Eur. J. 2006, 12, 3507−3514. (27) Alberti, G.; Costantino, U.; Marmottini, F.; Vivani, R.; Zappelli, P. Zirconiumphosphit-(3,3′,5,5′-Tetramethyl-Bipheny1) Diphosphonat: ein Mikroporoses Anorganisch-organisches Polymer Mit SaulenSchichtstruktur. Angew. Chem. 1993, 105, 1396−1398.
4. CONCLUSION A series of nanoporous phosphate materials with high surface areas were successfully prepared through the simple heat treatment of the parent phosphonate materials. The NMPA molecules play a vital role as a self-sacrificial template to produce the nanoporous structures and phosphorus sources in the final phosphate materials. The calcination temperature has a crucial role in controlling the crystallinities, nanoporous structures, and morphologies of the final phosphate materials. Due to the high surface areas and favorable structural stability of NiP materials, NiP materials show superior electrochemical performances as electrode materials in supercapacitors with excellent cycling stability. Our measurements indicate that both NiP-300 and NiP-700 have superior supercapacitor properties as compared to the parent phosphonate analogue (NiP-A) and various previously reported cobalt- and manganese-based phosphate materials. Furthermore, we also found that the electrochemical activity of amorphous mesoporous NiP-300 is more pronounced at lower scan rates, and the specific capacitance decreases frequently with increasing scan rates. In contrast, in the case of nanoporous NiP-700 with high crystallinity, the specific capacitance is not as high as that of amorphous NiP-300; however, the specific capacitance of the material is much more stable, even at high scan rates. The present thermal conversion strategy can be easily extended to prepare other nanoporous metal phosphates with high surface areas, which will be important for the further development of various energy storage materials.
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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.6b01012. Tables for comparison of textural properties, surface areas, and pore diameters of the materials and for comparison of electrochemical results, and figures of lowand high-angle XRD patterns, N2 adsorption−desorption isotherms, pore-size distribution curves, and FT-IR spectra, XRD and SEM images, and elemental mapping of the materials.. (PDF)
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AUTHOR INFORMATION
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[email protected]. Notes
The authors declare no competing financial interest.
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REFERENCES
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DOI: 10.1021/acsami.6b01012 ACS Appl. Mater. Interfaces 2016, 8, 9790−9797
