Article Cite This: Chem. Mater. XXXX, XXX, XXX−XXX
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Multivalent Electrochemistry of Spinel MgxMn3−xO4 Nanocrystals
Chunjoong Kim,†,§,∥ Abdullah A. Adil,†,∥ Ryan D. Bayliss,†,∥ Tiffany L. Kinnibrugh,⊥ Saul H. Lapidus,⊥ Gene M. Nolis,†,∥ John W. Freeland,⊥ Patrick J. Phillips,∥,‡ Tanghong Yi,†,∥ Hyun Deog Yoo,†,∥ Bob Jin Kwon,†,∥ Young-Sang Yu,# Robert Klie,∥,‡ Peter J. Chupas,⊥ Karena W. Chapman,⊥ and Jordi Cabana*,†,∥ †
Department of Chemistry and ‡Department of Physics, University of Illinois at Chicago, Chicago, Illinois 60607, United States § Department of Materials Science and Engineering, Chungnam National University, Daejeon, South Korea ∥ Joint Center for Energy Storage Research (JCESR) and ⊥X-ray Science Division, Advanced Photon Source, Argonne National Laboratory, Lemont, Illinois 60439, United States # Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States S Supporting Information *
ABSTRACT: Oxides undergoing reversible electrochemical cycling of Mg2+ ions would enable novel battery concepts beyond Li+, capable of storing large amounts of energy. However, materials showing this chemical reactivity are scarce. Suitable candidates require small particles to shorten transport lengths, together with chemically complex structures that promote cation mobility, such as spinel. These goals pose a challenge for materials chemists. Here, nanocrystals of spineltype Mg0.5Mn2.5O4 were prepared using colloidal synthesis, and their electrochemical activity is presented. Cycling in an aqueous Mg2+ electrolyte led to a reversible transformation between a reduced spinel and an oxidized layered framework. This reaction involves large amounts of capacity because of the full oxidation to Mn4+, through the extraction of both Mg2+ and, in the first cycle, Mn2+ ions. Re-formation of the spinel upon reduction resulted in enrichment with Mg2+, indicating that its insertion is more favorable than that of Mn2+. Incorporation of water into the structure was not indispensable for the transformation, as revealed by experiments in non-aqueous electrolytes and infrared spectroscopy. The findings open the door for the use of similar nanocrystals in Mg batteries provided that electrolytes with suitable anodic stability are discovered, thereby identifying novel routes toward electrode materials for batteries with high energy.
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INTRODUCTION Electrochemical energy storage devices in the form of secondary Li-ion batteries are currently the principal technology for portable electronic devices and electrical automotive applications. The development of efficient energy storage is now one of the main obstacles in reducing our reliance on fossil fuel energy sources. Recent research efforts in the development of novel Li-ion battery materials have highlighted that the most substantial limiting factor in energy density at the cell level remains the capacity of the cathode materials. In an effort to discover higher-capacity intercalation cathodes, the search has turned to multivalent ions to replace Li+, such as Mg2+, Ca2+, and Zn2+, to double the amount of charge stored per intercalated ion. Intercalation of multivalent species into highvoltage oxides has received little attention until recently because of the presumed large barriers for cation migration. However, several computational studies suggest there is potential for some divalent cations to migrate sufficiently well in certain lattices to sustain reversible electrochemical reactions.1,2 The combination of full Mg2+ cycling at high redox potentials in the © XXXX American Chemical Society
cathode with the ability to strip and plate Mg metal in the anode could result in truly substantial energy gains in energy storage compared to existing Li-ion technology, as a function of both mass and, especially, volume. Oxides with the spinel crystal structure, first proposed by Thackeray and co-workers,3 are well-known Li-ion battery cathode materials. The spinel crystal structure is recognized for its relatively rapid Li+-ion transport through three-dimensional migration pathways. According to computational predictions, the spinel crystal structure of Mn2O4 allows Mg2+ mobility with reasonable kinetics, making it promising as a positive electrode material for a rechargeable magnesium battery,1 especially because its components would be relatively cheap, safe, and environmentally benign. Most importantly, the predicted intercalation voltage is sufficiently high to lead to high energy densities. Recently, reversible intercalation of Mg2+ into Mn2O4 Received: August 28, 2017 Revised: February 19, 2018 Published: February 20, 2018 A
DOI: 10.1021/acs.chemmater.7b03640 Chem. Mater. XXXX, XXX, XXX−XXX
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Chemistry of Materials was verified experimentally,4 providing comprehensive evidence of electrochemical intercalation of Mg2+ into a transition metal oxide. To further advance toward a functional Mg battery cathode, two major obstacles need to be overcome. First, the empty Mn2O4 (or, equivalently, λ-MnO2) spinel-type lattice is metastable with respect to other MnO2 polymorphs. Indeed, the only known way to prepare it is through chemical/ electrochemical delithiation of LiMn2O4,4 a process for which a scalable method does not presently exist. Therefore, preparation of a viable electrode material for a Mg battery will, at this point, require targeting spinel-type MgMn2O4. Conventional synthetic methods in the solid state have been used to prepare this oxide.5 The other major obstacle relates to the sluggish kinetics of Mg intercalation in submicrometer crystals. Reducing the particle size to the nanoscale would partially alleviate this problem by shortening ion transport pathways. Methods based on colloidal synthesis are known to lead to 550 mAh/g was recorded. This value was twice the theoretical capacity corresponding to full demagnesiation or removal of the cations from tetrahedral sites (∼270 mAh/g), which implies that secondary reactions existed in addition to D
DOI: 10.1021/acs.chemmater.7b03640 Chem. Mater. XXXX, XXX, XXX−XXX
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Chemistry of Materials the content of Mn2+ was 32%, close to the value of 33.3% for Mn3O4. Upon oxidation, the spectrum closely resembled the Mn4+ signatures of LiNi0.5Mn1.5O4, with a large peak centered at 642.5 eV and a secondary feature at 640 eV. Subsequent reduction produced a complex spectrum with multiple peaks, the largest being centered at 641 eV. The shift in the center of gravity led to a position comparable to the position of the signals in ordered (Mg)[Mn]2O4, a spinel oxide containing only Mn3+, providing direct evidence of reduction. A low intensity was observed at 639 eV, especially compared to that of the pristine oxide, indicating that the reduced state was significantly enriched with Mn3+ over Mn2+. Nonetheless, the intensity at this energy, as well as 642.5 eV, was notably higher than that in MgMn2O4, suggesting that Mn2+ and Mn4+ persisted in the discharged state. Similar trends in oxidation− (partial) reduction were observed in Mn K-edge spectra (Figure S5). These spectra were collected in transmission, providing a complementary evaluation of the bulk of the electrode. The O K-edge spectra of Mg0.5Mn2.5O4 in different electrochemical states are compared to those of Mn3O4 in Figure 3b. All spectra show complex signals in the pre-edge region, below 540 eV, due to the mixing of Mn 3d and O 2p states in covalent Mn−O interactions. The presence of multiple peaks in the pre-edge of MgMn2O4 and in the Mn3O4 and Mg0.5Mn2.5O4 nanocrystals is consistent with the splitting of 3d states due to the presence of a Jahn−Teller distortion. However, the specific distortions appear to be subtly different in all cases, likely because of the presence of varying amounts of Mg2+, Mn2+, and Mn3+. In turn, the clear doublet in the case of Mg0.5Mn2.5O4 harvested after oxidation is reminiscent of Mn4+ in an undistorted octahedral environment, as in LiNi0.5Mn1.5O4, consistent with Mn L-edge data. The changes were accompanied by a shift to a lower energy of the onset of the pre-edge, which could be ascribed to changes in the position of the Fermi level and unoccupied states upon oxidation. Upon reduction, the spectrum recorded in total electron yield mode (Figure 3b) became dominated by a broad feature above the edge (i.e., above 535 eV), likely due to the growth of secondary species on the electrode surface that masked the signals from the particles. Nonetheless, a return of some of the fine pre-edge structure due to Jahn−Teller distortions was observed, especially when fluorescence detection was used to enhance signals from a larger volume into the electrode (Figure S6a). In both cases, the changes were not completely reversible, suggesting that Mn4+ was left in the structure. Lastly, the Mg K-edge spectra of the same Mg0.5Mn2.5O4 samples were compared to those of MgMn2O4 (Figure 3c). The spectrum of MgMn2O4 showed sharp features at 1310, 1315, and 1318 eV, together with a subtle bump centered at 1313 eV. The signals are consistent with data on related Mg-containing spinels in the literature.27 In contrast, the spectrum of pristine Mg0.5Mn2.5O4 was broad and featureless, centered at 1315 eV. The differences between samples could reflect the extensive chemical and atomic disorder of Mg0.5Mn2.5O4, which, as described above, contained extensive amounts of Mn2+ and Mg2+ in both octahedral and tetrahedral sites. Upon oxidation, some Mg was found to remain in the structure, but its coordination environment underwent significant changes. The changes were considerably reversed upon reduction. The spectrum of the pristine state suffered from sample charging, which led to greater noise, compared to the cycled samples because it was collected from pure powder, as opposed to
composite electrodes containing carbon. This problem was alleviated through collection of fluorescence yield signals (Figure S6b), which confirmed the trends mentioned above. Synchrotron diffraction and PDF analysis on electrodes recovered from different states of charge were used to explore the structural reversibility of the electrochemical reaction. Highenergy diffraction data were collected in parallel with total scattering data for PDF analysis. Mg0.5Mn2.5O4 in different states was recovered from cells stopped at regular intervals with respect to charge, and subsequent discharge. Clear structural changes in the Mg0.5Mn2.5O4 nanocrystals upon oxidation were observed by diffraction (Figure 4a and Figure S7). The peaks
Figure 4. (a) Powder X-ray diffraction and (b) pair distribution functions of Mg0.5Mn2.5O4 at various states of charge (oxidation) and discharge (reduction) in aqueous Mg(NO3)2 solutions. Diffraction was originally collected by high-energy X-rays (λ = 0.2114 Å) and then converted to the energy of averaged Cu Kα (λ = 1.5418 Å) to enable comparison with other figures.
associated with the tetragonal cubic spinel of the pristine state were eliminated for the oxidized phase. This effect was most evident between 27° and 35° (2θ) (the original synchrotron data were again converted to Cu Kα scale to enable comparisons with the literature and HRXRD mentioned above, collected at different wavelengths). The diffraction data for the oxidized phase had very broad and anisotropic peaks, indicative of defects and microstrain in a poorly ordered structure. These features rendered an accurate Rietveld refinement untenable, even when HRXRD was used (Figure S8). Qualitative comparison with the parent spinel pattern indicated many features reminiscent of spinel and layered structure, with metal cations arranged within a face-centered cubic array of oxygen anions.24,28 New broad features were evident at 20°, 25.5°, 43°, and 66°, which can be more clearly E
DOI: 10.1021/acs.chemmater.7b03640 Chem. Mater. XXXX, XXX, XXX−XXX
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displaying an electrochemical profile consistent with similar layered manganese oxides (Figure S10).30,31 To assess the role of H2O in the reaction and compare the results with those of previous studies reporting incorporation of water during charging of Mn3O4,29 FTIR spectra were collected from the sample in the fully oxidized state (Figure 3d). Upon comparison to the pristine nanocrystals, there was a rather subtle increase in intensity in the regions containing signals typically associated with the H−O−H bending (1600 cm−1 region) or stretching (3400 cm−1 region) modes of H2O. Further comparison of the intensity in these regions with that in the fingerprint region, and with reference spectra for hydrated birnessites,30,32 clearly indicates that the concentration of H2O in the oxidized material was too small to play a determining role in the bulk spinel−layered transformation. Because the spectra were collected in diffuse reflectance mode, it is even possible that significant portions of H2O in the sample were adsorbed at surfaces. The passive role of H2O was also suggested by the outcome of attempting the same oxidation reaction in a non-aqueous cell. Because of the lack of Mg electrolytes with suitable anodic stability,33 the oxidation was performed in a two-electrode cell with Mg0.5Mn2.5O4 as the working electrode and Li metal as the counter electrode, in the presence of standard Li electrolytes used in Li-ion technologies. When the cell was charged to 5.0 V versus Li+/Li0 (4.3 V vs Mg2+/Mg0), a very large capacity of 350 mAh/g was measured (Figure 2b). The profile of the cell showed a quasi-asymptotic increase in potential of ≤5 V. Redox processes at ∼4.0 and ∼4.3 V (vs Li+/Li0) were resolved in differential capacity plots as shown in the inset, followed by a large oxidation at close to 5.0 V. The operation at these potentials surely involved, again, decomposition of the electrolyte. But XRD and PDF data of the electrode harvested from the cell at 5.0 V, denoted by “Li Ch 100” in Figure 4 and Figure S9a, also showed the same loss of superstructure peaks, line broadening (in XRD), and PDFs as the most oxidized samples collected from aqueous cells. Therefore, the nanocrystals could still undergo electrochemical reactions in the presence of electrolyte decomposition. Nonetheless, some residual intensity of the superstructure peaks was still observed in the XRD patterns, indicating that the reaction was less efficient in non-aqueous than aqueous electrolytes. Unfortunately, the reversibility could not be evaluated under these conditions because of the overwhelming presence of Li+ ions in the electrolyte, which would be preferentially involved in the electrochemical reaction over Mg2+ (or Mn2+). The anodic response of the aqueous cell containing Mn3O4 nanocrystals as a working electrode was similar when compared to that of a cell with Mg0.5Mn2.5O4 (see the LSV data in Figure S4). However, the anodic currents were significantly lower in the case of Mn3O4, and the anodic signal at 0.7 V versus SCE was largely absent for the Mn3O4 cell. Accordingly, comparison of the S-XRD patterns of pristine and fully oxidized Mn3O4 revealed similar but much less prominent changes than in the case of Mg0.5Mn2.5O4 (Figure S9b). Most notably, the peaks between 27° and 35°, resulting from the tetragonal distortion of the spinel structure, were clearly visible in the spectrum of the oxidized Mn3O4 sample. Again, no visible peaks were found below 15°. The diffraction data collected upon discharge of the cell showed a progressive reappearance of the peaks corresponding to a tetragonal structure (Figure 4). However, comparison of the patterns collected in the pristine state and after a full
seen in Figures S8 and S9a. No peaks indicative of hydrated layered manganese oxides such as birnessite were observed between 10° and 15° (2θ) (Cu Kα).29 Overall, it was clear that the transformation involves the elimination of the original crystalline tetragonal spinel framework, with the topotactic preservation of the basic oxygen stacking. The loss of intensity from the tetragonal spinel was observed throughout but it was most obvious between samples at 80% (Ch 80) and full oxidation (Ch 100). The difficulties with the quantitative structural analysis of diffraction peaks upon charge motivated a study of the local structure by PDF. The corresponding data for the same samples harvested upon oxidation are presented in Figure 4b. The intensity of the signals above 5 Å, and especially above 10 Å, significantly decreased as the reaction progressed, indicating an increase in the level of structural disorder in the long range. Locally, notable shifts, accompanied by changes in shape, were seen in the peaks around r values of 4.7 and 8.0 Å, corresponding to the length scale-defined neighboring octahedra. In the most oxidized state, the signals were located at 4.5 and 7.7 Å, respectively, consistent with the radius of Mn4+ being smaller than those of Mn2+ and Mn3+. Concurrently, there was a change in the ratio of the intensity for the peaks in the low-r region (2.9 and 3.1 Å), associated with the local atomic structure. Fits of the PDF data for the fully oxidized sample (Ch 100) using either a cubic spinel or a mixture of cubic and tetragonal spinels did not produce satisfactory results (Figure 5a). In contrast, fits significantly improved when a layered
Figure 5. Representative fits of the PDF data for Mg0.5Mn2.5O4 nanocrystals after a full oxidation in aqueous Mg(NO3)2 solutions, using (a) a spinel and (b) a layered structured model.
structure with edge-sharing MnO6 octahedra, similar to birnessite, was used (Figure 5b). The lack of coherence above 10 Å is consistent with the lack of diffraction peaks at small angles, which would otherwise be expected from birnessite with significant levels of hydration. Indeed, the atomic displacement parameters refined perpendicular to the layer were more than 4 times larger than those in the layer plane, indicating weak bonding interactions and, thus, registry between neighboring layers. The resulting framework presented a high capacity for the electrochemical intercalation of lithium, F
DOI: 10.1021/acs.chemmater.7b03640 Chem. Mater. XXXX, XXX, XXX−XXX
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electrolyte (1 M), chemical potential effects favored the preferential introduction of the alkaline earth ion over the transition metal. HRXRD also revealed a composition consistent with the presence of Mn in the +2, +3, and +4 oxidation states, for an average state of +3, after a full spinel− layered cycle. The mixtures of redox states likely contribute to the Mn LIII-edge spectrum as an elevated background at energies typical of Mn2+ and Mn4+ compared to that of a pure Mn3+ standard (Figure 3a).35 Therefore, during the reactions, the Mn remaining in the structure acts as the redox center to compensate for the charge involved in the movement of ions, leading to a reversible yet complex change in structure. A similar reaction could be conducted on Mn3O4 nanocrystals of similar sizes, but it was found to be much less extensive by analysis of the resulting XRD patterns. Because this difference cannot be triggered by particle size, the result indicates that, although Mn2+ is extracted during the electrochemical process, the presence of Mg2+ in Mg0.5Mn2.5O4 enables increased completeness, possibly because of a mobility that is higher than that of Mn2+ in the spinel framework. The layered framework obtained upon oxidation was reminiscent of layered manganese oxide structures such as birnessite. Therefore, the electrochemical transformation in this work is reminiscent of what has been reported for Mn3O4.29 However, some significant differences were observed in this work. First, previous reports indicated the need for the oxide to undergo multiple oxidation−reduction cycles before the transformation from spinel to layered upon oxidation was extensive and could be detected by XRD. In this case, such a point was reached after the first oxidation. This effect could be a trivial consequence of the very small particle sizes achieved here. Small sizes will shorten the diffusion lengths that multivalent ions must cross before leaving the oxide framework. Second, and more importantly, evidence collected in this work supported the conclusion that water was not incorporated to a great degree into the layered framework. Such incorporation would lead to XRD reflections at very small angles, which were notably absent in the patterns in Figure 4, and very intense signals associated with water molecules in an FTIR spectrum. If water incorporation was a critical step in the transformation, as suggested by Nam et al.,29 the reaction would not be possible in non-aqueous electrolytes. However, the same layered framework, albeit in a less extensive transformation, was observed after oxidation of the Mg0.5Mn2.5O4 nanocrystals in nonaqueous electrolytes. Therefore, we can conclude that the incorporation of water reported by Nam et al.29 is likely the result of the susceptibility of the layered framework made electrochemically to take up water when exposed to aqueous environments for a long period of time, as observed by many layered manganese oxides.36,37 To achieve sufficiently anodic potentials, non-aqueous electrolytes with Li salts had to be employed, precluding evaluation of the possible reversibility of the transformation, as in aqueous electrolytes. Nonetheless, this observation suggests that the reversibility found more than a handful of cycles in aqueous electrolytes would also be possible in non-aqueous environments should systems containing Mg instead of Li salts with adequate anodic stability be discovered.
reduction revealed a significant increase in peak widths (Figure 1d), suggesting loss of crystallinity. The PDF features associated with the tetragonal structure were also largely recovered upon discharge (see especially boxed regions in Figure 4b). However, the ratios of intensity of these features and, overall, at long radial distances (i.e., >5 Å) did not return to the pristine state, reinforcing the notion that the recovered spinel framework was defective. Nonetheless, the return of the tetragonally distorted crystal structure qualitatively suggests at least 50% of the total B site occupancy was Mn3+, which is Jahn−Teller active. Rietveld refinements of HRXRD data were undertaken for the cycled state to obtain quantitative insight. The final lattice cell parameters were as follows: a = b = 5.7650(2) Å, and c = 9.3696(6) Å with goodness-of-fit values of 8.82 (rwp) and 6.90 (rp). This is a minor contraction compared to the pristine material, though it remains larger than the reported structures for various inverted MgMn2O4 spinels.5 Fixing atomic fractions to three cations per unit formula and allowing free refinement of those relative ratios led to a Mg/Mn ratio of 1/2, with a final chemical composition of (Mg2+0.6Mn2+0.4)[Mg2+0.4Mn3+1.2Mn4+0.4]O4. Therefore, some degree of inversion was preserved, but the material was significantly enriched with Mg2+. The electrochemical response of the material was found to be sustained, but for only a few cycles (Figure S11a). Nonetheless, laboratory (Cu Kα) XRD patterns collected during the first two cycles indicated that the spinel−layered transition still occurred (Figure S11b), indicative of the reversibility of the reaction. The capacity loss likely stems from a combination of chemical inefficiencies in the redox reaction of the nanocrystals and the passivation introduced by the continuous decomposition of the electrolytes due to operation outside their window of stability, aggravated by the large surface area of the nanopowders.
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DISCUSSION The oxidation of Mg0.5Mn2.5O4 nanocrystals in an electrochemical cell led to the extensive transformation of the tetragonal spinel structure into a poorly ordered layered framework. However, the basic oxygen stacking was preserved, as proven by the fact that both structures share many diffraction peaks. Therefore, the process had a topotactic character typical of intercalation reactions. However, this observation contrasts with the cubic spinel−tetragonal spinel observed upon intercalation of Mg2+ into λ-MnO2.4 The difference is probably triggered by the presence of Mn2+ in tetrahedral sites, as well as Mg2+ in both octahedral and sites of Mg0.5Mn2.5O4. In λ-MnO2, only tetrahedral sites are available, because all octahedral sites are occupied by Mn4+. The preservation of structural features runs contrary to the existence of conversion reactions, which lead to the destruction of the original structure. According to spectroscopy and diffraction data, the transformation was accompanied by the oxidation of Mn from an average of +2.8 to the +4 state. As a result, the reaction involved large specific capacities. The spinel−layered transformation was reversible; subsequent reduction in electrolytes containing Mg2+ induced the reformation of a tetragonal spinel, which, according to Rietveld fits of HRXRD data, was enriched with Mg with respect to the pristine state. The increase in Mg/Mn ratio can be explained only by a mechanism whereby both Mg and Mn, most likely as Mn2+, and not just due to corrosion,34 were extracted during oxidation, leaving sites available for insertion of ions from the electrolyte. Given the very high concentration of Mg2+ in the
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CONCLUSIONS The redox properties of Mg0.5Mn2.5O4 nanocrystals were evaluated using electrochemical cells with aqueous and nonaqueous environments. In aqueous cells, a reversible transformation between the tetragonal spinel structure and a layered G
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framework was observed, leading to large accumulated specific capacities due to a change in redox state from Mn2.8+ to Mn4+. The complex structural transformation preserved the basic oxygen framework and involved the extraction of divalent ions, Mg2+ and Mn2+. The absence of a critical role of water and the reversibility of the transformation open the door to using this reaction as the basis for an electrode material in a high-energy multivalent battery, such as with Mg. For this purpose, two follow-up steps will be critical. First, to simplify the reaction, it will be desirable to design synthetic protocols that lead to nanocrystals of stoichiometric MgMn2O4 at sizes comparable to what was achieved here. At this stoichiometry, only Mn3+ is present, meaning that only Mg2+ will be expected to be mobile. The study of such nanocrystals would precisely further probe the role of Mg2+ versus Mn2+ in this complex transformation. Second, it is vital that electrolytes that contain Mg2+ salts are discovered but offer anodic windows that compare with the state of the art in Li-ion technologies.
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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/acs.chemmater.7b03640.
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Article
Material characterization and electrochemical properties (XRD, TEM, PDF, LSV, and GCPL) (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Hyun Deog Yoo: 0000-0001-5188-481X Karena W. Chapman: 0000-0002-8725-5633 Jordi Cabana: 0000-0002-2353-5986 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported as part of the Joint Center for Energy Storage Research (JCESR), an Energy Innovation Hub funded by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES). ThemUIC JEOL JEM-ARM 200CF instrument is supported by an MRI-R2 grant from the National Science Foundation (Grant DMR-0959470). C.K. acknowledges additional support by the National Research Foundation of Korea (NRF-2015R1D1A1A01056874). A.A.A. acknowledges support through the UIC Chancellor Undergraduate Research Award (CURA) and the UIC Liberal Arts and Sciences Undergraduate Research Initiative (LASURI). This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract DE-AC02-06CH11357. Use of the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract DE-AC02-76SF00515. The authors thank Dr. Tham Hoang, from Loyola UniversityChicago for assistance with the ICP-MS measurements. H
DOI: 10.1021/acs.chemmater.7b03640 Chem. Mater. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.chemmater.7b03640 Chem. Mater. XXXX, XXX, XXX−XXX
