Structural Investigation of Cobalt Oxide Clusters Derived from

Article pubs.acs.org/JPCC

Structural Investigation of Cobalt Oxide Clusters Derived from Molecular Cobalt Cubane, Trimer, and Dimer Oligomers in a Phosphate Electrolyte Xiaobo Li,† Edwin B. Clatworthy,† Stuart Bartlett,† Anthony F. Masters,† and Thomas Maschmeyer*,†,‡ †

Laboratory of Advanced Catalysis for Sustainability, School of Chemistry, The University of Sydney, Sydney, NSW 2006, Australia Australian Institute of Nanoscale Science and Technology, The University of Sydney, Sydney, NSW 2006, Australia

‡

S Supporting Information *

ABSTRACT: Cobalt oxide clusters were formed from three molecular cobalt oligomers, the Co-cubane, [Co4(μ3-O)4(μ-OAc)4(py)4], Co-trimer, [Co3(μ3O)(μ-OAc)6(py)3][PF6], and Co-dimer, [Co2(μ-OH)2(μ-OAc)(OAc)2(py)4][PF6], in phosphate buffer electrolyte after aging. Phosphate is essential for the formation of these cobalt oxide clusters. XAS characterization shows the cobalt oxide clusters are CoII/IIIOx clusters with CoII(O)4 and CoIII(O)6 subunits. The cobalt oxide clusters formed have a structure similar to that of aged “CoPi” oxygen evolution catalysts prepared by electrodeposition.

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INTRODUCTION Of all the proposed renewable energy sources, hydrogen obtained from photocatalytic water splitting is potentially the most sustainable. Water is the cheapest and most abundant hydrogen feedstock acting as both the fuel precursor and combustion product.1,2 Water oxidation is arguably the most difficult half-reaction in water splitting as it requires the transfer of four oxidative equivalents to generate four protons and electrons and the formation of the molecular oxygen bond. A water oxidation catalyst (WOC) with a low overpotential is required to enable this reaction to proceed as efficiently as possible. Additionally, to be applicable at a large scale the WOC must avoid noble metal elements, such as iridium and ruthenium. Among the limited set of candidates, cobalt-based WOCs have attracted considerable attention. Nocera et al. demonstrated a low-overpotential heterogeneous cobalt oxide “CoPi” WOC under neutral conditions.3,4 Molecular cobalt compounds with cubane structures analogous to the Mn3Ca(μO)4 cluster of photosystem II have also been examined widely.5−9 However, there is evidence showing that, in some cases, the real active WOC species are probably amorphous species generated by the transformation of the cobalt compounds.10 In our previous study three cobalt molecular clusters, the Co-cubane, [Co4(μ3-O)4(μ-OAc)4(py)4], Cotrimer, [Co3(μ3-O)(μ-OAc) 6(py)3][PF 6], and Co-dimer, [Co2(μ-OH)2(μ-OAc)(OAc)2(py)4][PF6], were investigated as water oxidation reaction (WOR) catalysts using electrochemical, photochemical, and photoelectrochemical methodologies in a phosphate buffer electrolyte. It was found that the species responsible for the water oxidation activity observed are © XXXX American Chemical Society

derived from the transformation of these cobalt clusters into amorphous active clusters.11 It is of fundamental interest to investigate the structural influence of the initial cobalt cluster size/nuclearity on the nature of the active amorphous species obtained, so as to better understand their structure and functionality. Although the structures of the cobalt WOCs have previously been studied, those studies concentrated on the cobalt WOCs from Co(II) precursors.12,13 Herein we report that the stability of the initial molecular cobalt oligomers in phosphate buffer is highly dependent on their structure. The structure of the resulting amorphous cobalt oxide clusters has been established as cobalt oxide CoII/IIIOx clusters comprising CoII(O)4 and CoIII(O)6 subunits. They have a structure similar to that of the CoPi with a reduced cobalt valence state after storage. The larger the initial molecular cobalt oligomer size, the larger the amorphous cobalt clusters.

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RESULTS AND DISCUSSION The Co-cubane is soluble in aqueous solution, whereas the Cotrimer and Co-dimer are not. To investigate their electrochemical behavior in water and the role of phosphate buffer, four different electrolytes were prepared: electrolyte I, CH3CN/H2O with 0.167 M Na2SO4 (CH3CN/H2O = 1:2 (v/v)), pH 7.0; electrolyte II-Pi, CH3CN/H2O with 0.167 M Na2SO4/0.033 M KPi (K2HPO4 + KH2PO4) (CH3CN/H2O = Received: November 17, 2016 Revised: February 1, 2017 Published: May 3, 2017 A

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Figure 1. (a) Background-corrected CVs of Co-cubane (0.27 mM) in electrolytes I, II-Pi, III, and IV-Pi. Cyclic voltammograms of fresh Co-cubane (0.27 mM) and after 48 h storage in (b) II-Pi and (c) IV-Pi. All scan rates were 20 mV/s. (d) Time course UV−vis spectra of Co-cubane in II-Pi and IV-Pi.

disappearance of absorption features (Figure 1d). The change of the Co-cubane in electrolytes I and III is also evident in the associated CV experiments (Figure S1). The reversible Co(III)4/Co(III)3Co(IV) process could still be observed in electrolyte III and is shifted to a more positive potential in electrolyte I. A catalytic process at ∼1.3−1.4 V is also observed after 48 h of aging. In the corresponding UV−vis spectra no obvious change was noticeable for the Co-cubane in electrolyte III (Figure S2). Changes in the UV−vis spectra of the Cocubane in electrolyte I are visible, but the initial absorption features are still present after 48 h. This indicates that when only minor transformation occurs, cyclic voltammetry is a more sensitive and reliable technique to determine Co-cubane stability as compared to UV−vis spectroscopy. Time course CV and UV−vis spectroscopy studies of the Co-cubane in four electrolytes show that phosphate facilitates the transformation of the Co-cubane. The transformation of the Co-cubane in electrolyte without phosphate, such as sulfate, is also observed but to a much lesser degree during the same time period. This observation justifies our focus on the behavior of molecular cobalt oligomers in phosphate buffer electrolyte. This choice is supported further by the fact that phosphate buffer is used widely to evaluate WOCs. Addition of Na2EDTA to fresh or aged solutions does not change the CV of the Co-cubane in either electrolyte II-Pi or

1:2 (v/v)), pH 7.0 (the CH3CN is to help solubilize the Codimer and Co-trimer in aqueous solution); electrolyte III, 0.5 M Na2SO4, pH 7.0; and electrolyte IV-Pi, 0.5 M Na2SO4/0.1 M KPi, pH 7.0, solution. Potentials are reported against the Ag/ AgCl reference electrode. Figure 1a shows the cyclic voltammograms (CVs) of Co-cubane in the four electrolyte solutions. A reversible Co(III)4/Co(III)3Co(IV) electron transfer is observed, but no catalytic process was observed scanning to 1.4 V irrespective of the electrolyte solution used. It has been reported that the Co-cubane is not a WOC at neutral pH.11,14 The phosphate and Na2EDTA have no effect on the electron transfer process of Co(III)4/Co(III)3Co(IV). A shift of the oxidation potential in solution with acetonitrile is observed, which is consistent with literature reports.5,6 The time course CVs and UV−vis spectra show that the Cocubane is unstable in aqueous solution and to an extent highly dependent on the electrolyte used (Figure 1b,c and Figures S1 and S2). After 48 h of storage in either electrolyte II-Pi or IV-Pi, the reversible Co(III)4/Co(III)3Co(IV) process disappears. In addition, a catalytic process at ∼1.3 V and an oxidation step with an onset at ∼0.6 V become noticeable. These observations indicate the formation of active WOR species from the molecular Co-cubane after aging in either of the two electrolytes. UV−vis spectra also clearly show the change of Co-cubane over time in both electrolytes, as indicated by the B

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Figure 2. (a) Cyclic voltammograms of Co-trimer (0.27 mM) in electrolyte II-Pi and subsequent scans of the blank II-Pi solution after the electrodes had been rinsed with water (blue line). (b) Time course cyclic voltammograms of Co-dimer (0.27 mM) in II-Pi.

Figure 3. (a) Normalized XAS spectra of the precipitates from molecular cobalt oligomers in the electrolyte II-Pi after 4 weeks of aging. (b) Comparison to cobalt oxide references. (Only precipitate from Co-dimer was plotted for clarification.)

IV-Pi. This indicates that the oxidation process at ∼1.3 V is unlikely to derive from free Co(II) species. The free Co(II) signifies the cobalt species, which could be quenched by the addition of EDTA. The lack of quenching on addition of EDTA indicates that the Co-cubane transforms into heterogeneous cobalt oxide clusters. Interestingly, green precipitates start to appear in Co-cubane II-Pi solutions after ∼14 h of aging. In contrast, no precipitate was observed from the Co-cubane in electrolyte I, III, and IV-Pi solutions within 1 week of aging. This indicates that both phosphate and CH3CN have significant roles in the appearance of precipitate. Time course CV studies were performed with small time intervals in IV-Pi and II-Pi solutions as shown in Figure S3. In electrolyte IV after 0.5 h the CV indicates negligible change of the Co-cubane (blue line). Over time the current gradually decreases, due to the reversible Co(III)4/ Co(III)3Co(IV) process, and new electron transfer processes become apparent in the range of 0.8−0.9 V. After 7 h of aging, an oxidation step occurs at a potential of ∼0.6 V along with the gradual disappearance of the currents of the electron transfer steps in the range of 0.8−0.9 V. A gradual increase of the catalytic current at ∼1.3 V was observed with aging. After 48 h, no obvious changes were apparent in the CVs. Free Co(II) species were not detected by the Na2EDTA quenching test at

any time (Figure S4). A similar transformation behavior of the Co-cubane in II-Pi electrolyte was observed (Figures S5 and S6). The intermediate electron transfer processes are in the range of 0.75−0.85 V. After 48 h, oxidations with onset potential at ∼0.6 V were observed. In fact, in both electrolytes similar oxidations with onset potential at ∼0.6 V were visible, suggesting the formation of similar cobalt oxide clusters after 72 h (Figure S7). Furthermore, after 72 h of aging of the Cocubane in II-Pi, a lower current is observed for the catalytic process at ∼1.3 V. This is due to the gradual formation of green precipitates from solution. In contrast, similar oxidations were observed for the Co-cubane in IV-Pi between 72 and 144 h. This is consistent with the lack of precipitates in the Co-cubane IV-Pi solution with aging. These results demonstrate that the green species in II-Pi solution is precipitated by CH3CN. During the transformation, CVs with multiple oxidations were observed in both electrolytes. This is typical of CVs of multimetal clusters. The potentials of these oxidations are different in two electrolytes, 0.75−0.85 V in electrolyte II-Pi versus 0.80−0.90 V in electrolyte IV-Pi, indicating a quasimolecular state of multicobalt oxide clusters as intermediates. In fresh solutions, the reversible electron transfer of the Cocubane is cathodically shifted by ∼0.1 V with CH3CN present in the electrolyte. Finally, similar oxidations with an onset at C

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The Journal of Physical Chemistry C ∼0.6 V were observed after 72 h (Figure S7). This indicates a heterogeneous state of the cobalt oxide clusters formed, in other words, a larger cluster showing bulk properties not responding to the electrolytes. The CV of the Co-trimer in II-Pi shows a catalytic process centered at ∼1.3 V (Figure 2a). This catalytic process has been observed from the transformed Co species from the Co-trimer under electrochemical conditions.11 After 4 h, the Co-trimer IIPi solution becomes cloudy, and a green precipitate is observed after 6 h. No precipitate is observed in electrolyte I, indicating that phosphate is essential for the transformation of the oligomer. Time course CVs of the Co-trimer in II-Pi show that the current of the catalytic process at ∼1.3 V decreases over time. This is different from the Co-cubane system and, likely, due to the formation of green precipitates continually consuming the Co-trimer from the solution. The instability of the Co-trimer is also observed from UV−vis spectra. A concomitant decrease of the absorbance intensity in the time course UV−vis spectra was also seen (Figure S8). We observed no cyclic voltammetric oxidative processes for the Co-dimer in aqueous/MeCN solutions. The Co-dimer has higher stability compared to the Co-cubane and Co-trimer in II-Pi from time course CVs and UV−vis results (Figure 2b and Figure S9). However, green precipitate was noticed after approximately 4 weeks of aging in II-Pi solution. The green precipitate was not noticeable for solutions of the Co-dimer in electrolyte I. The EDX results (Figure S10) show that the precipitates formed from the three molecular cobalt oligomers (collected after 4 weeks of aging in II-Pi) contain Co, P, Na, and K with Co/P atom ratios in the range of 1.46−1.58. This indicates the phosphate participates in the transformation of molecular cobalt oligomers into precipitates. The ratios of Na/K are close to 5, similar to the ratio of Na/K in the electrolyte solution. XRD results show that the precipitates are amorphous. EDX, FTIR (Figure S11a), Raman (Figure S12a), and XANES (Figure 3a and Figure S13) results indicate that the green precipitates obtained from the three cobalt oligomers have similar structures and average Co oxidation states. The formation of other phosphate salts was not detected by XRD, FTIR, and Raman. To investigate the structure of the precipitates, cobalt oxides, CoO, CoOOH, Co3O4, and Co(OH)2, were used as references. A sample of CoPi WOC on FTO glass was prepared from Co(NO3)2 by electrodeposition at 1.1 V with a total charge of 0.2 C/cm2. The average Co oxidation state of the precipitates from the molecular cobalt oligomers determined from XANES measurements is 2.6 (Figure 3b and Figure S13), indicating some reduction of Co3+ to Co2+ in the electrolyte. This establishes that the precipitate comprises a mixture of 60% Co(III) and 40% Co(II) species. Under aqueous conditions the Co3+ valence state is not stable and would be reduced to the Co2+ valence state when it reacts with water. CoPi also contains a mixture of Co(III) and Co(II) species with an average Co oxidation state of 2.5. As the XAS of CoPi was performed ex situ, some of the Co3+ is reduced to Co2+ by adventitious water, consistent with that reported for CoPi during storage.15 The precipitate has XANES spectra similar to those of CoPi, indicating the two species have similar structures. The Raman spectra of the precipitates show a broad absorption from 450 to 700 cm−1, which has been previously assigned to amorphous CoOx species.16−18 The assignment of the absorption to oxidic cobalt species such as Co(OH)2, CoOOH, and Co3O4 can be ruled out because the spectra of

these oxides do not contain absorptions in the region around 600 cm−1 (Figure S12b). This is also supported by the FTIR results (Figure S11b). An exception is CoPi, although differences in the absorption wavelengths are observed. This precipitate shows a broad absorption between 450 and 700 cm1, which is similar to those observed from precipitates derived from the cobalt oligomers. FTIR results also show that these species have similar absorptions. The structures of the precipitates derived from the molecular cobalt oligomers were characterized with EXAFS (Figure 4; see

Figure 4. EXAFS spectra and Fourier transforms of CoPi (a) and precipitates from Co-cubane (b), Co-trimer (c), and Co-dimer (d). Black line is from experiment; red line is from fitting. Fitting in k-space (Å−1), 3−12; R-space (Å), 1.0−2.8.

the Supporting Information for details of EXAFS and fitting of samples, ESI). The EXAFS data for CoO, CoOOH, Co3O4, and Co-cubane are consistent with their known structures. For the precipitates, a coordination number N (the number of scatters per absorber at distance R (Å)) of cobalt of ∼5 oxygen atoms at a distance of 1.88−1.89 Å was obtained for each of the three samples (Table 1). The sample of CoPi WOC on FTO glass was prepared from Co(NO3)2 in electrolyte IV-Pi by electrodeposition at 1.1 V with total charge to 0.2 C/cm2. The EXAFS is fitted with a Co−O distance of 1.89 Å (Figure 4; Table 1). For the CoPi sample the N number of oxygen atoms is 4.8, notably less than 6.12,13 Again, due to the EXAFS of CoPi being performed ex situ some of the Co3+ is reduced to Co2+ by adventitious water.13,15 This conclusion is supported by the XANES spectra. It has been reported that CoPi WOC has a decreasing Co−O coordination number (from 6 to 5) with an increasing Co(II) percentage.19 For the most reduced of the CoPi samples reported, which has an average Co oxidation state of ∼2.6, an N value of ∼5 was obtained. This is consistent with our fitting results. XANES results show the precipitates consist of Co(III) and Co(II) mixtures with an average Co oxidation state of 2.6. We infer the presence of both tetrahedral CoII(O)4 and octahedral CoIII(O)6 in the precipitates isolated, resulting in an observed average N of ∼5. A value of N of ∼5.5 for Co−O was obtained from Co3O4, which has tetrahedral CoII(O)4 and octahedral CoIII(O)6 cobalt atoms (Figure S14; Table S1), supporting the above inference. In our green precipitates, the number of adjacent cobalt atoms is >3. This indicates the core of the precipitates is composed of CoOx clusters. By comparison, the largest number of adjacent cobalt atoms in the cobalt oligomers is 3 (Co-cubane). The number of adjacent cobalt atoms in the CoPi is 3.2. The Co−Co distances of the green precipitates are ∼2.79−2.78 Å, and the Co···Co distance of CoPi is 2.80 Å, consistent with the cobalt atoms D

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The Journal of Physical Chemistry C Table 1. EXAFS Fitting Parameters of Green Precipitates from Cobalt Oligomers and CoPia sample

shell

R (Å)

N

S

S02

E0

Rf

from Co-cubane

Co−O

1.89 (0.011) 2.79 (0.015)

5.1 (1.1) 4.4 (1.6)

0.003 (0.002) 0.007 (0.003)

0.70

−9.3 (2.4)

11.7

1.89 (0.011) 2.79 (0.014)

5.1 (0.6) 3.9 (1.5)

0.003 (0.001) 0.006 (0.003)

0.70

−9.7 (2.4)

10.0

1.88 (0.010) 2.78 (0.012)

5.2 (0.6) 3.6 (0.2)

0.003 (0.001) 0.005 (0.002)

0.70

−10.2 (2.3)

14.2

1.89 (0.012) 2.80 (0.015)

4.8 (1.1) 3.2 (1.3)

0.003 (0.002) 0.006 (0.003)

0.70

−8.4 (2.7)

10.0

Co−Co

from Co-trimer

Co−O Co−Co

from Co-dimer

Co−O Co−Co

CoPi

Co−O Co−Co

a

R, scatterer/absorber distance (Å); N, number of scatterers per absorber at distance R (Å); S02, amplitude reduction factor; S, Debye−Waller factor; E0, energy shift; Rf, R factor (defined as R = [X2EXAFS/X2O]1/2) for the refinement reported to serve as a criterion of the goodness of fit between the calculated and observed EXAFS. Values in parentheses are uncertainties.

being connected by oxo or hydroxo bridges.20,21 There is a correlation between the size of the CoOx cluster formed and the size of the cobalt precursors. This is indicated by the coordination number N of the cobalt shell (Co···Co interactions) of the CoOx clusters: 3.2 when derived from Co(NO3)2, 3.6 from Co-dimer, 3.9 from Co-trimer, and 4.4 from Co-cubane, respectively. The slight increase in the apparent number of adjacent cobalt atoms could be due to a difference in cluster size, although this remains inconclusive due to the standard deviation for the coordination number. A higher N value in bulk CoPi compared to surface CoPi was reported.13 Thus, the CoOx cluster size appears to be tunable by the selection of different-sized cobalt compounds. The EXAFS of Co-cubane was fitted with two Co−Co distances of 2.71 and 2.84 Å20,21 (the average distance is 2.73 Å if only one Co···Co shell was considered) (Figures S15 and S16 and Table S2 and S3). This value is smaller than that of the precipitates and CoPi. The fittings of the EXAFS of the green precipitates and CoPi give consistent Debye−Waller parameters of 0.003 and 0.005− 0.007 for the Co−O and Co···Co distances, respectively. The amplitude reduction factor is 0.70. For the green precipitates generated from the cobalt oligomers, a third peak appears in the Fourier transform at R > 3 Å. This is most evident in the Fourier transformed data from the precipitates from the Codimer. However, a single additional shell with either Co−O or Co−Co or Co−Na bonds does not fit the data. The best fit was obtained by fitting two shells, Co−O and Co−Co. However, the fitting is not reliable due to large uncertainties, possibly indicating another shell is contributing to this peak. This last possibility was not tested due to the low intensity of the peak and consequent uncertainties in the fitting. On the basis of the structural data from the XAS, we infer that the green precipitates formed from the cobalt oligomers are CoII/IIIOx clusters with CoII(O)4 and CoIII(O)6 subunits. The cobalt atoms are connected by oxo or hydroxo bridges. It seems the CoII/IIIOx species obtained from the cobalt oligomers have a structure similar to that of aged CoPi from electrodeposition

with a Co(II) precursor. It is possible that the peak at R > 3 Å does indicate some difference in long-range order, but could not be determined by EXAFS. The CV of CoPi (1.5 mC/cm2 at 1.0 V) electrodeposited on a glassy carbon electrode from a solution of Co(NO3)2 in electrolyte IV-Pi shows an oxidation step with an onset potential of ∼0.65 V (Figure S17). This oxidation step is attributed to the oxidative charging (oxidation of Co2+ to Co3+ at the onset potential) in the CoPi WOC.3,4,19 On the basis of the XAS results, we infer that the oxidation step with onset potential at ∼ +0.6 V in aged Co-cubane cluster solutions arises from the oxidation of Co2+ to Co3+ of CoII/IIIOx clusters. This is similar to the oxidative charging of the CoPi with applied potential. However, only a negligible effect was observed following addition of Na2EDTA to the aged Co-cubane solutions. This indicates that the Co(II) of CoII/IIIOx exists within a cluster unit of sufficient size/bulk to hinder access of EDTA to the Co(II) sites. In other words, free Co(II) species are not released during the transformation of the Co-cubane into CoII/IIIOx clusters. In addition, the failure of EDTA to quench the solution indicates this quenching methodology is not a reliable way to exclude the presence of Co(II) in a WOR system if such a species exists in a cluster of sufficient size to block the access of EDTA ligands.

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CONCLUSION The molecular cobalt oligomers, Co-cubane, Co-trimer, and Co-dimer, are not stable in phosphate electrolyte and undergo transformation into amorphous CoII/IIIOx clusters over time with the participation of phosphate. The Co-dimer has the greatest stability (with respect to oxide formation) among them, followed by Co-cubane and then Co-trimer. The addition of acetonitrile precipitates the CoII/IIIOx clusters from the aqueous solution. Comparison of the thus obtained CoII/IIIOx clusters with aged CoPi, derived from the electrodeposition of cobalt nitrate in phosphate buffer, shows that they can be described by very similar structures, based on our XAS results. E

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The Journal of Physical Chemistry C The size of the CoII/IIIOx cluster is found to be related to the size of the cobalt oligomer precursor. This provides an alternative way to finely tune the CoPi WOC catalysts by choosing different-sized cobalt precursors.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b11607. Analysis, instrumentation, and chemicals; X-ray absorption spectroscopy; preparation of molecular cobalt(III) and cobalt (III) compounds; procedures for cyclic voltammetry scan; Figures S1−S17; Tables S1−S4 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*(T.M.) E-mail: [email protected]. ORCID

Xiaobo Li: 0000-0002-2752-749X Thomas Maschmeyer: 0000-0001-8494-9907 Author Contributions

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

This work was funded by the Australian Research Council (DP150102515). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Support from the Australian Research Council is greatly acknowledged. E.B.C. acknowledges the receipt of an Australian Postgraduate Award. This research was undertaken on the wiggler XAS beamline at the Australian Synchrotron, Victoria, Australia.

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REFERENCES

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DOI: 10.1021/acs.jpcc.6b11607 J. Phys. Chem. C XXXX, XXX, XXX−XXX