Biotemplated Lightweight γ-Alumina Aerogels - Chemistry of Materials

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Cite This: Chem. Mater. XXXX, XXX, XXX−XXX

Biotemplated Lightweight γ‑Alumina Aerogels Thanh-Dinh Nguyen,† Dorothy Tang,† Francesco D’Acierno,†,‡ Carl A. Michal,*,†,‡ and Mark J. MacLachlan*,†,§ †

Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver, British Columbia V6T 1Z1, Canada Department of Physics, University of British Columbia, 6224 Agricultural Road, Vancouver, British Columbia V6T 1Z1, Canada § Quantum Matter Institute, University of British Columbia, 2355 East Mall, Vancouver, British Columbia V6T 1Z4, Canada ‡

S Supporting Information *

ABSTRACT: We present the biotemplating of γ-Al2O3 aerogels with chitosan nanofibrils. Aluminum−chitosan interactions cause the swelling of iridescent chitosan structures into helicoidal hydrogels and the subsequent aqueous dissolution of swollen fibrils to form Al− chitosan solutions. Viscous aqueous solutions of Al3+−chitosan hybrid nanofibers were freeze-dried to give lightweight cotton-like aerogels. Homogeneous incorporation of Al3+ ions into chitosan yields water-soluble nanofibrils that can serve as polymeric templates to support Al3+ ions in the aerogel composites. We investigated thermal removal of chitosan in the composites to obtain lightweight γ-Al2O3 nanocrystal aerogels that retain the weblike fiber networks of the chitosan template. These biotemplated alumina aerogel materials are promising candidates for catalyst supports and thermal insulation.

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to increase the accessibility of reactants into adsorption sites. For this reason, the lightweight, open structure of γ-Al2O3 aerogels may be beneficial over γ-Al2O3 nanocrystals for catalysis. Chitin is a highly organized biopolymer of N-acetylglucosamine that is abundant in shells of crustaceans and insects.17 The structural arrangement of native chitin fibrils in the exoskeletons often varies between different species of arthropods.18 In fact, in some arthropods, such as jewel beetles19 and mantis shrimps,20 chitin fibrils self-organize into helicoidal structures to enhance photonic and mechanical properties; this structure, known as the Bouligand structure, is analogous to cholesteric lyotropic liquid crystals.21 Hierarchical structure and nanometer scale make chitin fibrils an exciting template for fabricating new materials.22−25 Recently, our group exploited helicoidal chitin nanofibrils from cuticles of crustacean shells to template photonic mesoporous silicas and convert them into nitrogen-doped carbon electrodes.26 Chitosan is a deacetylated form of chitin. The surface deacetylation often leads to disruption of hydrogen bonding, resulting in chitosan fibrils with low crystallinity. The mechanical flexibility and reactive chemistry of hierarchical chitosan nanofibrils have inspired scientists to use them as nanoscale templates.27−29 Gelation and dissolution of chitosan are interesting ways to exploit the templated assembly of nanofibrils with functional components. The surface amino groups can interact with metal ions to enable control over swelling, solubility, and organization.30 These host−guest

abrication of lightweight porous aerogel materials has been an interesting subject of research for catalyst supports, thermal insulation, absorption, and filtration.1−3 The challenge to obtaining aerogels is being able to capture porous networks in substances on solidification. Templated synthesis provides a straightforward strategy for the construction of porous materials.4 The structural orientation of precursors on hierarchical templates can afford ultralight and highly porous materials with large pore volumes and high surface areas. Some beautiful examples of ultralight functional aerogels are photopolymer-templated ultrastiff alumina superlattices,5 Ni-templated superelastic BN foams,6 and poly(methyl methacrylate) (PMMA)-templated inverse NiFe2O4 opals for enhanced microwave absorption.7 Aluminum oxide (Al2O3) aerogels are promising porous materials, useful as catalyst supports, thermal insulators, and absorptive filters because of their amphoteric surface sites and heat corrosion resistance.8,9 The crystallinity, nanoscale, and stability of the alumina aerogels are keys to improving their performance. Among different alumina structures known, gamma (γ)-Al2O3 is widely used as an active catalyst and catalyst support due to uncoordinated Al3+ centers creating electronic defects at the surface.10−12 For example, Mrabet et al.13 synthesized γ-Al2O3 nanocrystals to design highly active Ag/γ-Al2O3 catalysts for removal of toxic NOx gases. Zhang et al.14 reported materials for catalytic CH4 oxidation with high activity and stability made by distributing Co nanoparticles on γ-Al2O3. Other efforts have also shown excellent thermal and mechanical properties of porous Al2O3 materials. Notably, super heat-resistant δ, θ, or α-Al2O3 aerogels were prepared by Zu et al.15 and hardened α-Al2O3 porous films were reported by our group.16 In view of catalysis, it is desirable to obtain γ-Al2O3 aerogel-like materials with large pore size and high surface area © XXXX American Chemical Society

Received: November 15, 2017 Revised: February 12, 2018

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DOI: 10.1021/acs.chemmater.7b04800 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials

Figure 1. Aluminum-induced swelling of iridescent chitosan membranes into helicoidal hydrogels. (a) Photograph of iridescent twisted mesoporous chitosan membranes; (b) photograph of chitosan hydrogels; (c) POM images of chitosan hydrogels viewed along cross sections (left) and edges (right); and (d) cross-sectional SEM image of chitosan aerogels.

glucosamine unit compared with chitin samples. This proves a substantial deacetylation of chitin by hot concentrated alkali solution to generate chitosan fibrils. To form composites with aluminum ions, the CNMC membranes were immersed in aqueous aluminum nitrate under ambient conditions. Visibly, the CNMC membranes were strongly swollen, forming soft, transparent hydrogels (Figure 1b). The maximum swelling of the chitosan membranes was reached within 4−6 h, with the hydrogels substantially increasing in thickness and mostly retaining their shape. Polarized optical microscopy (POM) images (Figure 1c) of the hydrogels show fingerprint textures characteristic of a chiral nematic structure. Intact aerogels were prepared from the chitosan hydrogels by supercritical CO2 drying. Scanning electron microscopy (SEM) images (Figure 1d) viewed along cross sections of the chitosan aerogels show a twisting, layered structure that looks identical to the left-handed chiral nematic organization of chitin nanocrystals in the Bouligand structure. The gap between adjacent layers of the helicoidal gels is about 3 μm, characteristic of a half-pitch that is much larger than that of the CNMC membranes (Figure S2). These observations prove the helicoidal structure was preserved in the Al-induced swollen chitosan hydrogels. This result is also consistent with a recent report that native chitin fibrils can interact with Ca2+ in hot methanol to swell into helicoidal gels.43 Remarkably, the swollen chitosan hydrogels dissolved in the Al3+ aqueous solution after prolonged swelling for 48 h (Figure 2a). The transparent chitosan liquid has a pH of ∼7 and maintains homogeneity at ∼0.5 to 2.0 wt % Al/chitosan. The water-soluble chitosan appears transparent under crossed polarizers, which is in contrast to lyotropic chitin liquid crystals that can show birefringence and a fingerprint texture.21 This indicates that the helicoidal chitosan structure was destroyed by dissolution of the swollen fibrils to form a viscous fluid rather than an anisotropic liquid crystal. Pure chitosan dissolved in

interactions may lead to the formation of hybrid complexes in which chitosan nanofibrils can function as templates to generate porous materials. Previous studies have shown that chitosan with different structures can be used as a precursor for adsorbent porous sponges,31 plastics,32 a matrix for mineralization of nacre-like photonic calcium carbonate layers,33 or a template for porous inorganic components.34 Recently, we prepared helicoidal mesoporous chitosan templates from crustacean shells and then functionalized these with PMMA to form photonic hydrogels.35 However, there is a limited description of the use of chitosan nanofibrils to template alumina materials as there are only two reports, of chitin-templated mesoporous alumina36 and chitosan-templated alumina microspheres.37 Many papers have reported the preparation of alumina catalyst supports,9,38−40 but there are very few investigations of making γ-Al2O3 aerogel-like materials.41,42 Here, we report a new synthetic approach to lightweight γ-Al2O3 aerogels by biotemplating with chitosan nanofibrils. Unlike previous acetic acid-based methods, our new approach is based on aqueous solutions of Al3+. We initially found that chitosan nanofibrils can form hydrogels and solutions by swelling the samples in an aqueous solution of Al3+ ions. In this work, we used iridescent chiral nematic mesoporous chitosan (CNMC) nanofibrils as a starting material to investigate the tunability of photonic chitosan hydrogels and aerogel templating of macro-mesoporous alumina materials. The CNMC membranes were prepared following a previously reported procedure by repeated hot alkali treatment of native chitin purified from king crab shells (Figure 1a).26,35 Solid-state 13C cross-polarization/magic-angle spinning (CP/MAS) nuclear magnetic resonance (NMR) spectra (Figure S1 of the Supporting Information) of CNMC show the dramatic reduction in the intensity of the resonances of C7 carbonyl and C8 methyl carbons in N-acetyl-DB

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Figure 2. Polymeric biotemplating of γ-Al2O3 aerogels with water-soluble chitosan nanofibrils. (a) Photographs of Al−chitosan aqueous solution, Al−chitosan composites, and γ-Al2O3 aerogels (from left to right); (b) photograph of Al−chitosan composite on a feather; (c) photograph of γAl2O3 aerogel on a flower; and (d) schematic of chitosan-templated synthesis of γ-Al2O3 aerogels.

magnification images show ∼200 nm diameter fibrils to be bundles of smaller nanofibril assemblies. Under lyophilization, the water-soluble Al-absorbed chitosan released water to leave behind macroscale interspaces and regenerated nanofibril bundles, producing the cotton-like aerogels. This method is a reliable route to native chitosan cotton-like fibers that may be attractive for tissue engineering,47 wound dressings,48 and antibacterial studies.49 These highly porous nanofibrillar materials are also promising as a novel precursor to prepare hydrophobic aerogels by surface functionalization for chromatographic applications.50−52 Energy-dispersive X-ray (EDX) spectroscopy and thermogravimetric analysis (TGA) show that the Al−chitosan aerogels contain ∼8 wt % alumina (Figures S5 and S6). Powder X-ray diffraction (PXRD) patterns (Figure S7) of the Al−chitosan aerogels show that the positions of the (110) and (020) diffraction peaks of chitosan slightly shift to higher angles and a new peak appears at around 8.5° 2θ. The diffraction intensity of the aerogels is much lower than that of the pristine chitosan samples, indicating the reduced crystallinity of the chitosan fibrils. As well, no diffraction of Al2O3 was observed in these composites. Infrared spectra (Figure S8) of the Al−chitosan composites show bands from chitosan (e.g., amide II stretch at ∼1560 cm−1 and a carbonyl stretch at ∼1025 cm−1) along with a strong NO stretching band at ∼1315 cm−1 of nitrate and a sharp peak at ∼830 cm−1 corresponding to Al−O vibration.53,54 These bands

acidic media often precipitates at basic pH due to the deprotonation of ammonium groups. Unlike pure chitosan solutions, the chitosan in Al3+ aqueous solutions remains in solution over a broad pH range, from below pH 4 to pH ∼8. Although synthetic methods such as reacetylation44 and Maillard reaction45 have been used to dissolve chitosan, neutral aqueous solutions of native chitosan nanofibrils are actually rare. These Al3+-induced chitosan solutions may be useful polymeric liquids for investigating biodegradable membranes.32,46 The homogeneous Al3+/chitosan aqueous solutions were freeze-dried to form white aerogels that retained the shape of the container (Figure 2b). The aerogels are made up of highly distributed fibrils, and no phase separation could be observed. About ∼150 mg of the solidified aerogels can be obtained from 20 mL of ∼10 wt % chitosan aqueous solution. The bulk density of this Al−chitosan aerogel determined by the mass-tovolume ratio is estimated to be approximately 0.012 g cm−3. The Al−chitosan aerogels are as soft and lightweight as cotton balls (Figure S3). They are stable in air and no significant change in color or shape was observed over several months. The aerogel solidification is reversible as the Al−chitosan composites redisperse in water to form a homogeneous solution. SEM images (Figure 3a,b, Figure S4) of the Al− chitosan aerogels show highly porous open networks of the interconnected fibrils without phase separation. Higher C

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Figure 3. Structural organization of γ-Al2O3 aerogels templated by chitosan nanofibrils. SEM images of (a,b) Al−chitosan composites and (c,d) γAl2O3 aerogels; inset of Figure 3c shows a model structure of a single fiber bundle of aggregated γ-Al2O3 nanoparticles in the aerogels; (e) TEM image and (f) HRTEM image of γ-Al2O3 aerogels.

solution. The recovery of the intact fibrillar aerogels from the solution may be facilitated by the cross-linking of chitosan with Al3+ ions. This guest−host complexation is homogeneous as no phase separation in the hybrid fibrils is evident by SEM. The homogeneous incorporation of Al into chitosan led the water-soluble nanofibrils to function as a soft template to generate alumina aerogels (Figure 2c). Figure 2d shows the synthetic route to alumina aerogels templated by water-soluble chitosan nanofibrils. We were surprised to find that the calcination of the Al−chitosan composites in air afforded white alumina replicas that retained the shape of the Al−chitosan composites. The resulting solid is a little denser than the composite but still lighter than cellulose cotton (Figure 2a, Figure S9). These Al2O3 aerogels have a bulk density of approximately 0.017 g cm−3. The templating of Al2O3 materials with chitosan has been reported by Fajardo et al.,37 but their structures are microspheres rather than lightweight aerogels and the authors used conventional acetic acid−chitosan solution as a medium for Al3+. Although alumina is a hard, brittle ceramic material, the chitosan-templated alumina aerogel is soft and spongelike.

verify the prepared composites are a mixture of chitosan and aluminum nitrate. The disappearance of an amide I stretch at ∼1640 cm−1 in the composites may be due to bonding interactions between primary amino groups and aluminum ions. Thermal analyses (Figure S6) of the Al−chitosan composites show a loss of ∼70 wt % of the material by 230 °C. This rapid loss is associated with a quick decomposition of chitosan into carbonaceous intermediates that then gradually decompose up to ∼550 °C. For comparison, pure chitosan is thermally stable up to ∼300 °C and then gradually degrades. Most likely, residual nitrate anions from the Al salts are responsible for the rapid degradation, but the higher surface area of the chitosan aerogels and the presence of Al3+ ions may contribute to the metastability of the Al−chitosan composites. We also note that chitosan did not dissolve in water and its aqueous dissolution exclusively occurred with the addition of Al3+ ions. This indicates that the incorporation of Al3+ ions into the chitosan fibrils is crucial to solubilizing chitosan. Binding interactions between the amino and hydroxyl groups of chitosan and Al3+ ions may disrupt hydrogen bonding between chains in fibrils with a consequent decrease of crystallinity. This aids chitosan to first swell and then dissolve to form an aqueous D

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Figure 4. (a) PXRD patterns of amorphous Al2O3 and crystalline γ-Al2O3; (b) 13C MAS NMR spectra of Al−chitosan composites, amorphous Al2O3, and crystalline γ-Al2O3; (c) 27Al MAS NMR spectra of Al−chitosan composites, amorphous Al2O3, and crystalline γ-Al2O3; (d) nitrogen adsorption−desorption isotherms of amorphous Al2O3 and crystalline γ-Al2O3, inset of corresponding BJH pore size distributions.

analysis from the (400) diffraction peak to be ∼30 nm in accordance with TEM results (Figure 3f). To assess the local structure of the Al3+ ions in the aluminabased materials, these aerogels were examined using solid-state 27 Al MAS NMR spectroscopy (Figure 4c). All spectra show broad, asymmetric peaks as expected for these nanostructured materials.55 The Al−chitosan composite shows a single peak with a chemical shift of ∼0 ppm, characteristic of octahedral coordination.56 A minor peak in this spectrum near 62 ppm is a background signal from the zirconia rotor. The peak near 0 ppm is believed to correspond to [Al(H2O)6]3+ arising from the dissolution of Al(NO3)3 in water.56 After calcination at 540 °C, the structural water is fully evaporated and the Al spectrum changes dramatically, with a large fraction (∼59%) of the Al intensity having migrated to shifts consistent with tetrahedral sites. Much of the spectral intensity is relatively far upfield for tetrahedral Al, and is close to where pentahedral Al would be expected, ∼20−35 ppm. To distinguish between these possible interpretations, we performed a triple-quantum MQ-MAS experiment (Figure S16). No well-resolved pentahedral environment was observed; rather, a broad distribution of shifts largely confined to the tetrahedral and octahedral regions is observed. A second distinct tetrahedral site having a smaller second-order quadrupole effect (SOQE) was also detected. The upfield shifting of the peaks in this spectrum compared to what is expected for γ-Al2O3 may be due to the more disordered structure of the amorphous Al2O3.36,57 Upon calcination at 900 °C, the peaks sharpen and shift downfield, consistent with the improved crystallinity observed in the PXRD. The second minor tetrahedral site is again observed in this sample, both as a visible shoulder of the tetrahedral peak in the 1D spectrum (Figure 4c) and as a distinct peak in the MQ-MAS spectrum (Figure S16c). This peak also shows a smaller SOQE than the

Elemental analysis shows carbon, nitrogen, and hydrogen in the alumina to be below the detection limit. EDX analyses show the presence of only aluminum and oxygen in the alumina (Figure S10). X-ray photoelectron spectroscopy (XPS) surface analyses reveal that the Al−chitosan composites before and after calcination both show an Al 2p peak at 75.5 eV assigned to Al3+ (Figure S11).16 The alumina samples are stable up to 900 °C in air with only a ∼3.5 wt % loss of mass compared with the large decomposition of ∼92 wt % chitosan in the composites (TGA, Figure S12). Solid-state 13C CP/MAS NMR spectra (Figure 4b) of the Al−chitosan composites show resonances at 46−96 ppm for C1−C6 on N-acetyl-D-glucosamine units and a weak sharp peak at 163.5 ppm for the C7 carbonyl carbon. These peaks, which are characteristic of chitosan, are absent in the aerogels formed after calcination at 540 °C and at 900 °C. These analyses verify the complete removal of the organic template from the Al−chitosan composites, yielding the Al2O3 aerogels. The structural transformation of the intact Al2O3 aerogels from amorphous to crystalline phase can be accomplished by increasing the calcination temperature of the composites. PXRD patterns (Figure 4a) reveal that the alumina prepared at 540 °C has a broad halo at ∼23° 2θ characteristic of an amorphous phase. Conversely, the alumina prepared at 900 °C has distinct diffraction peaks at 2θ = 20°, 33°, 38°, 46°, 61°, and 67°, corresponding to (111), (220), (311), (400), (511), and (440) planes, respectively, of γ-Al2O3.13 There is no evidence from the diffraction patterns of other phases of Al2O3. The diffraction peaks are intense and broad to indicate the high degree of crystallinity and the small size of the γ-Al2O3 nanoparticles in the aerogels. The average crystallite size of the γ-Al2O3 particles in the aerogels was estimated by Scherrer E

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means to explore the applications of the Al2O3 aerogels. The high porosity, crystallinity, and nanoscale size make the γ-Al2O3 aerogels useful as catalysts and catalyst supports. The lowdensity γ-Al2O3 nanostructured aerogels may be attractive materials to investigate their potential for thermal insulation.

main peak. The chemical shifts and SOQEs for all peaks observed are reported in Table S1. Most of the spectral intensity in the amorphous sample is found in the tetrahedral sites, while in the crystalline sample the octahedral site dominates. This migration of Al3+ ions from tetrahedral to higher-order octahedral sites with higher-temperature calcination is expected.58 The observed chemical shifts and quadrupole parameters found are also well within the ranges found previously for γ-Al2O3.36,57,59 Electron microscopy studies provide evidence for the structural retention of weblike fibrillar networks in the Al2O3 aerogels at the nanoscale (Figure 3, Figure S13). The hierarchically nanostructured features of both the amorphous and crystalline Al2O3 aerogels resemble those of the Al− chitosan samples, albeit with some shrinkage. SEM images (Figure 3c,d) show rough-textured γ-Al2O3 fibrils with a diameter of ∼100 nm that is a little smaller than that of Al− chitosan hybrids (∼200 nm). Large spaces between the fibrils are about 100−300 μm, consistent with macroporous structures for the Al2O3 aerogels. This supports a transfer of the weblike organization of the chitosan template to the Al2O3 aerogels. TEM images (Figure 3e, Figure S15) of γ-Al2O3 aerogels also show weblike networks of nanoparticle-aggregated fibrils. These ∼20 nm sized γ-Al2O3 nanoparticles with quasi-spherical and hexagonal shapes are primary units to be oriented along the fibrils interconnected in the aerogels. Macropores of ∼100−500 nm are clearly observed in the TEM images of these composites. High-resolution TEM images (Figure 3f) show lattice spacings of ∼0.30 nm, characteristic of γ-Al 2 O 3 nanocrystals. This electron microscopy observation suggests that guest−host interactions allows Al3+ ions to be uniformly attached on the chitosan template to form the homogeneous fibril hybrids. Subsequent calcination of these composites led to the fused agglomeration of Al3+ into particles that formed the Al2O3 fibrils in the aerogels. Nitrogen sorption measurements (Figure 4d) were used to examine the porosity in the Al2O3 aerogels. The average surface area is ∼200 m2 g−1 for amorphous Al2O3 and ∼180 m2 g−1 for the crystalline Al2O3. The amorphous Al2O3 aerogels have bimodal pore size distributions centered at around 6 and 40 nm, similar to the diameter of the pristine chitosan nanofibrils (∼10 nm) and of the range of macroscale interspaces in the aerogel networks, respectively. This indicates that chitosan nanofibrils were thermally removed from the composites to leave behind Al2O3 aerogels with macro-mesoporous structure. A decrease of the surface area and pore size of the γ-Al2O3 aerogels is due to shrinkage of the crystalline networks by calcining at higher temperature. These macroscale pores are consistent with the weblike aerogel networks observed by electron microscopy. This straightforward, scalable method presents the fabrication of lightweight, purified γ-Al2O3 nanocrystal aerogels. We have shown for the first time the biotemplating of ultralight γ-Al2O3 aerogels with water-soluble chitosan nanofibrils. Al-induced swelling of chiral nematic chitosan nanofibrils yielded chitosan hydrogels that dissolved in water to form Al− chitosan solutions. Solidification of these aqueous solutions afforded cotton-like Al−chitosan aerogel composites. Calcination of the composites yielded γ-Al2O3 aerogels that truly replicate the weblike structural organization of chitosan nanofibrils. Solid-state NMR spectroscopic analyses revealed that γ-Al2O3 nanocrystal aerogels contain mostly [AlO6]3− environments. This new biotemplating route provides a

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EXPERIMENTAL SECTION

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ASSOCIATED CONTENT

Preparation of Al−Chitosan Hydrogels and Aqueous Solution. Iridescent chiral nematic mesoporous chitosan membranes were prepared following our previous procedure as presented in the Supporting Information.26,35 The chitosan membranes (∼400 mg) were added to 40 mL of deionized water containing 250 mg of aluminum nitrate and sonicated for 2 h and then stirred at room temperature within 48 h. After aging for several hours, the iridescent chitosan membranes strongly swelled in water to form transparent hydrogels. To investigate the chiral nematic structure of the chitosan hydrogels, these hydrogel samples were collected from the aqueous solution and then immersed in ethanol to form alcogels. These alcogels were dried with supercritical CO2 to recover intact chitosan fibrillar aerogels. To obtain the Al−chitosan solution, the swollen chitosan was aged for 48 h to mostly dissolve into a polymer solution. The resulting mixture was centrifuged and then filtered to separate some insoluble chitosan and obtain a homogeneous Al−chitosan aqueous solution. Preparation of Al−Chitosan Aerogel Composites and Al2O3 Aerogels. An Al−chitosan aqueous solution (20 mL, 10 wt %) was freeze-dried to form Al−chitosan aerogel composites (∼150 mg). These aerogel composites (∼150 mg) were calcined in air at 100 °C for 2 h and then heated to desired temperatures for 6 h with a heating rate of 5 °C min−1 to remove the chitosan template, yielding Al2O3 aerogels (∼10−15 mg). The calcination of the Al−chitosan composites was carried out at 540 °C to prepare amorphous Al2O3 and at 900 °C to prepare crystalline γ-Al2O3. S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b04800. Experimental details of materials synthesis, structural analyses, and extended POM, SEM, TEM, and photographs (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Thanh-Dinh Nguyen: 0000-0003-2226-048X Mark J. MacLachlan: 0000-0002-3546-7132 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding

We are grateful to the NSERC of Canada for funding (Discovery Grant and CREATE NanoMat grant). Notes

The authors declare no competing financial interest.

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REFERENCES

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DOI: 10.1021/acs.chemmater.7b04800 Chem. Mater. XXXX, XXX, XXX−XXX