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Resolution of Electronic and Structural Factors Underlying OxygenEvolving Performance in Amorphous Cobalt Oxide Catalysts Gihan Kwon, Hoyoung Jang, Jun-Sik Lee, Anil Mane, Sarah Rose Soltau, Lisa M. Utschig, Alex B. F. Martinson, David M. Tiede, Hacksung Kim, and Jungho Kim J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b02719 • Publication Date (Web): 20 Jul 2018 Downloaded from http://pubs.acs.org on July 20, 2018

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Journal of the American Chemical Society

Resolution of Electronic and Structural Factors Underlying OxygenEvolving Performance in Amorphous Cobalt Oxide Catalysts Gihan Kwon,1,2,4,5 Hoyoung Jang,8,+ Jun-Sik Lee,8 Anil Mane,6 Sarah Soltau,5,‡ Lisa M. Utschig,5 Alex B.F. Martinson,1,4 David M. Tiede,1,5* Hacksung Kim,3,5* Jungho Kim7* 1

Argonne-Northwestern Solar Energy Research (ANSER) Center, 2Northwestern Argonne Institute of Science and Engineering (NAISE), 3Center for Catalysis and Surface Science, Northwestern University, Evanston, IL 60208, USA 4

Materials Science Division, 5Chemical Sciences and Engineering Division, 6Energy Systems Division, 7X-ray Science Division, Argonne National Laboratory, Lemont, IL 60439, USA

8

Stanford Synchrotron Radiation Light Source, SLAC National Accelerator Laboratory, Menlo Park, CA 94025, USA

+ ‡

Current address: PAL-XFEL Beamline Division, Pohang Accelerator Laboratory, Gyeongbuk 37673, Republic of Korea

Current address: Department of Chemical Sciences, Bridgewater State University, MA 02325, USA

Cobalt oxide, Cobalt borate, Cobalt phosphate, Water oxidation, Water splitting, Resonant X-ray emission spectroscopy (RXES), Resonant inelastic X-ray scattering (RIXS), Resonant Raman spectroscopy, Cobalt L3 edge X-ray absorption spectroscopy Supporting Information ABSTRACT: Non-noble-metal, thin-film oxides are widely investigated as promising catalysts for oxygen evolution reactions (OER). Amorphous cobalt oxide films electrochemically formed in the presence of borate (CoBi) and phosphate (CoPi) share a common cobaltate domain building block, but differ significantly in OER performance that derives from different electron-proton charge transport properties. Here, we use a combination of Ledge synchrotron X-ray absorption (XAS), resonant X-ray emission (RXES), resonant inelastic X-ray scattering (RIXS), resonant Raman (RR) scattering, and high-energy X-ray pair distribution function (PDF) analyses that identify electronic and structural factors correlated to the charge transport differences for CoPi and CoBi. The analyses show that CoBi is composed primarily of cobalt in octahedral coordination, while CoPi contains approximately 17% tetrahedral Co(II), with the remainder in octahedral coordination. Oxygen-mediated 4p-3d hybridization through Co-O-Co bonding was detected by RXES and the inter-site dd excitation was observed by RIXS in CoBi, but not in CoPi. RR shows that CoBi resembles a disordered layered LiCoO2-like structure while CoPi is amorphous. Distinct domain models in the nanometer range for CoBi and CoPi have been proposed on the basis of the PDF analysis coupled to XAS data. The observed differences provide information on electronic and structural factors that enhance OEC performance.

INTRODUCTION Achieving fuels production from sunlight through artificial photosynthesis is a fundamental research challenge. Earthabundant catalysts that operate stably for long periods and that can be implemented on an industrial scale are intensively

sought.1 Thin film transition metal oxides are widely investigated as catalysts for artificial photosynthesis,1, 2 because of the possibilities for utilization in artificial leaf devices,3-5 and by the relevance of the fundamental chemistry to both artificial and photosynthetic oxygen evolving catalysis, OEC.4, 6 Nocera and co-workers have shown that amorphous cobalt oxide OEC can readily be produced by anodic electrochemical deposition from cobalt solutions with phosphate, methyl phosphate, and borate oxyanions, which serve to limit domain growth and yield CoPi, CoMePi, and CoBi OEC, respectively.3-5, 7, 8 A variety of spectroscopic and analytical techniques have been employed to obtain information relevant to the oxygen evolving reaction, OER, performance of Co-OECs: atomic pair distribution function (PDF) analysis,9-11 electron paramagnetic resonance (EPR) spectroscopy,12, 13 solid state NMR,14 X-ray emission spectroscopy,15, 16 electrochemical methods,2, 17-23 X-ray absorption spectroscopy (XAS),22, 24 time-resolved infrared spectroscopy25 and differential electrochemical mass spectrometry.18, 26 This work has shown that catalysis is likely to occur from di-µ-oxo-bridged cobalt sites at the domain edges.13, 17, 20-22, 27 A key challenge for gaining deeper insight into mechanisms for thin-film photo- and electro-catalytic OER function lies in resolving the interplay between intrinsic catalytic activity of catalytic sites and the electronic structure and charge transport properties of the thin-film OEC.28-33 Transition metal (oxy)hydroxides show linked electron-proton conductivity which confers a volume rather than a surface area dependent activity to the films.31, 32 This is understood to arise from homogeneously distributed and accessible catalytic sites though out the film, and with the catalytic current densities determined by the product (kcatkH,e)1/2 of the catalytic rate, kcat, and electron-proton conductivity, kH,e.31, 32 As a result, understanding OER function for transition metal (oxy)hydroxide requires the structural basis for both catalysis and electron-proton conductivity to be addressed.

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A comparative study of CoPi and CoBi offers a good chance to provide insights into the structural and electronic factors in cobalt (oxy)hydroxides that underlie OER performance. Both OEC share the same cobaltate building block, but differ significantly in OER performance. Farrow, Nocera and coworkers showed that catalytic currents for CoPi and CoBi have different film thickness dependencies, and proposed that enhanced catalytic water-splitting performance in CoBi compared to CoPi is derived from increased charge delocalization and hole mobility in the former compared to the latter.9 This hypothesis was founded on PDF measurements for the Co-OEC powders that resolve larger domain size and mesoscale ordering for CoBi compared to CoPi,9 but did not directly interrogate electronic properties of the two OEC materials.

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standing enhanced conductivities for CoBi compared to CoPi. In particular, the results show that the cobaltate domains for these OEC differ in the content of tetrahedral Co(II) “defect” sites, extent of oxygen-mediated metal-metal delocalization, and mesoscale ordering that can be understood to combine and support enhanced electron-proton conductivity in CoBi compared to CoPi. More generally, this work demonstrates opportunities to use the combination of soft XAS, RXES, RIXS for investigating the interplay between intrinsic catalytic activity of catalytic sites and the electronic structure and charge transport properties of thin-film catalysis. RESULTS Oxygen evolution performance of cobalt oxide catalysts

In this report, we provide a comparison of CoBi and CoPi OER performance and bulk electronic properties, and correlate these to the electronic and structural properties measured at the atomic scale using a combination of soft X-ray absorption (XAS), resonant X-ray emission (RXES), resonant inelastic Xray scattering (RIXS), and resonant vibrational Raman scattering (RR) techniques. In particular, electric dipole-allowed p-d transitions observed by synchrotron soft (L-edge) XAS data provide high-quality, high-resolution electronic signatures on 3d state and thus allow to distinguish Co(II) from Co(III) and low-spin from high-spin states. RXES also provides information on p-d electronic transitions, but more detailed 2-D electronic signature by the photon-in resonant absorption and photon-out emission processes. RIXS shows spin-allowed, element-specific d-d electronic transitions. RR spectrum excited at the d-d absorption band energy appearing in RIXS provides information on the molecular vibration that is resonantenhanced. While XAS, RXES, and RIXS provide atomspecific electronic information, RR and PDF provide molecular bonding and the distances of all-atom to all-atom in a few nanometer ranges, respectively. Accordingly, this combination of advanced X-ray and vibrational analyses provide complementary detailed information on the atomic and electronic structures of OEC materials. This combination of analytical approaches show CoBi and CoPi have characteristic differences in electronic structure measured at the atomic scale that provide a basis for under-

A comparison of CoPi and CoBi OER function is shown in Figure 1. Part a shows plots of catalytic current densities measured as a function of film thickness and electrochemically poised with overpotentials of 400 mV, 500 mV, 600 mV. At each overpotential, CoPi and CoBi have approximately comparable current densities that increase with film thicknesses below 0.1 µm Co/cm2. With further increases in film thicknesses, the current densities for CoBi continue to increase and significantly exceed those for CoPi. Analogous results were reported previously, by assaying CoPi and CoBi OER in using different electrolytes, 1 M KPi, pH 7.0 and 1 M KBi, pH 9.2, respectively.9 The film thickness dependent activity for CoBi measured in KBi, pH 9.2 shows a straight, linear dependence on film thickness with no leveling off due to charge transport limitations in the KBi electrolyte.9, 34 The results presented here provide a complementary measurement of a direct comparison of OER function for both catalysts in the same KPi electrolyte. The electrochemical OER experiments in Figure 1 indicate that the CoPi and CoBi films are structurally stable throughout the electrochemical assays. Each data point in Figure 1 represents a measuremnt from a individual sample, with catalytic currents measured as described in the Supporting Information. Repeated scans across the 5 min to 15 min time range were reproducible and showed no indication of drift or evolution. PDF measurements of CoPi and CoBi following electrolyte exchange and overnight soaking showed no change in domain structures at the atomic scale.11 We note that analysis of CoPi OER function has also been investigated following electrolyte exchange.32 Prior SEM and EXAFS mearsurements for CoPi films that varied in thickness from 15 nm to 1.5 µm showed the film surface topology and internal structures to be invariate.23 Finally, in order to verify domain structures of the Co-OECs formed initially at electrochemical potentials below OER with those following OER, PDF paterns, Figure S5 and S6 confirm that the CoPi and CoBi structures at the atomic scale are equivalent before and following OER. Hence, we consider the domain structures for CoPi and CoBi to remain constant during the timecourse of the electrochemical assays in Figure 1. As a result, we identify the film thickness dependencies in electrocatalytic performance to refect enhanced electron-proton charge transport properties for CoBi compared to CoPi.9

Figure 1. Current density, Part A, and turnover frequency (TOF), Part B, for CoPi and CoBi as a function of film thickness, measured by cobalt loading, and recorded with various overpotentials for OEC in 0.1 M KPi, pH 8. Each current data point was taken This conclusion is corroborated by plots of turnover frequency, at the end of 5 min electrolysis at a given overpotential. The corTOF, normalized by the extent of ICP-determined cobalt loadrelation between deposition charge and cobalt amount is shown in ing (See A.2 and Figure S3, Supporting Information) versus Figure S4. In Part B, the slopes of the straight lines, TOF/film film thickness, Figure 1b. At each overpotential, CoBi and thickness, for CoPi are -0.66 at 400mV, -0.81 at 500mV, -0.88 at 600mV. The corresponding slopes of the straight lines for CoBi 2 Further Plus Environment are -0.41 at 400mV, -0.61 at 500mV, -0.74 at 600mV. ACS Paragon experimental information is provided in the Supporting Information.

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CoPi show linear dependencies of TOF on film thickness in the log-log plots with slopes that increase with overpotential. This linear dependence of TOF on film thickness and slope dependence on overpotential has been previously analyzed for CoPi, and demonstrated to result from the volume activity of the CoPi film, limited by the product (kcatkH,e)1/2.23 Here we show comparable behavior for CoBi. CoBi is seen to have a shallower dependence of TOF with increasing film thickness compared to CoPi. Interestingly, the slope differences cause CoPi to have higher TOF for films with thickness < 0.05 µmol/cm2 loading, while CoBi activity exceeds that for CoPi in thicker films. This is consistent with the interpretation that the intrinsic catalytic site turnover frequency maybe higher for CoPi, but that charge transport is higher for CoBi. Electronic conductivity of Co-OECs as ex-situ films Charge transport properties of transition metal (oxy)hydroxides have been extensively investigated. CoPi and CoBi, and related oxides can be understood as p-type semiconductors which undergo an electrochemical potential-driven insulator to conductor transition.22, 35, 36 Under electrochemical conditions, the conductivity of the CoBi is about 0.6 mS/cm, while that of the CoPi is about 0.2 mS/cm.37 We have corroborated these measurements by conductivity measurements of CoPi and CoBi as solid state materials in the absence of aqueous electrolyte as described in Figures S7-S11. Conductivity of air-dried catalyst films is of interest as a measure of a bulk electronic property that correlates directly to the conditions used for electronic structure measurements at the atomic scale by the ex-situ X-ray analyses described below. We note that in on-going work we are developing methods to extend the X-ray analyses to in-situ, operando electrochemical conditions. Most transition metal oxide thin film catalysts are fragmented by volume contraction during the drying process. Obtaining continuous film that is large enough for conductivity measurement poses challenges, so electrical conductivity is usually measured in a dry pressed pellet or in a powder compression cell.38, 39 Here a new method, the GHK method shown in Figure S7, was used to obtain free standing CoPi and CoBi tubes with 1.1 mm lengths that permitted direct electrode connections. The method used porous working electrodes, fabricated by atom layer deposition (ALD) of conductive ITO layers on porous glass microchannel plate supports, to template CoPi and CoBi tubular growth, as described in the Supporting Information. The setup for ex-situ conductivity measurements as a function of applied voltage is shown in Figure S9, and measured I-V and conductivity plots are shown in Figures S10 and S11, respectively. Across the scan of applied voltage, 0.1 V to 50 V (1x102 to 5x104 V/m), the current density of CoBi is seen to be about 100-fold greater than CoPi, Figure S11. A linear, approximately ohmic region is seen region with applied voltage 1V (103 V/m) to 5V (5x103 V/m) showing the conductivity of CoBi to be about 60-fold greater than CoPi, Figure S11a. Notable too, is that the I-V plots and conductivity plots for CoBi are almost entirely reversible, while the forward and reverse scans for CoPi show considerable hysteresis. Although CoBi is about 60-fold more conductive than CoPi, both OECs are insulating materials. There are two general mechanisms of Frenkel-Poole (FP) and Space-Charge Limited (SCL) emission40 which distinctly describe the electrical conduction properties of insulators such as Co-OECs. As shown in Figure 12b,

the I-E plots for both are almost straight lines, suggestive of the FP mechanism rather than the SCL emission in the applied voltage region < 4.2×104 V/m. Above this the curves are constant and thus no longer follow the FP emission mechanism.40 These results demonstrate characteristic differences in conductivity for the CoPi and CoBi materials. The experimental conditions for the ex-situ conductivity measurements are comparable to those used for the X-ray and Raman spectroscopy analyses described below. The absence of electrolyte eliminates electrochemical redox processes, and both CoPi and CoBi are found to behave as insulators in which applied potential-driven charge transport is dominated by Frenkel-Poole emission, although with CoBi having 60 times higher conductivity than CoPi. The finding of conductivity differences for CoPi and CoBi in the absence of electrolyte is in accord with electrochemical conductivity measurements37and the catalytic activity differences described above. In the following sections we utilize a combination of X-ray and vibrational Raman spectroscopy measurements to interrogate electronic and structural differences between CoPi and CoBi at the atomic scale. X-ray absorption at the Co L3 edge Figure 2 compares Co L3 edge X-ray absorption (XAS) spectra of Co3O4, CoBi, and CoPi measured for ex-situ powders in a total electron yield (TEY) mode. Co3O4 is known to be comprised of 2/3 octahedral (Oh) Co(III) and 1/3 tetrahedral (Td) Co(II). Co(II) ions are in the high-spin (HS) state and Co (III) low-spin (LS) state. 41 42 Hereafter the spin-state label will be omitted unless otherwise noted. The higher photon energy peaks of C and D are mainly due to Oh Co(III) ions, while B peak reflects the contribution from both the Td Co(II) and Oh Co(II) ions (See Fig S1). The contribution percentages of the Td Co(II) and Oh Co(II) ions can be determined from an additional lower photon energy peak A appearing at 777 eV (Figure 2c), which is assigned to Oh Co(II) ions.42 The B peak is much more intense in CoPi spectrum than in CoBi, indicating the presence of a significant amount of Td Co(II)43 in CoPi, unlike CoBi on the basis of the curve fit (Figure 2, Table 1, and Figs. S1, S2). Hence, XAS results show that the electronic and structural properties of the CoPi is qualitatively different from that of the CoBi. Relative quantitative fractions of the Oh Co(III), Oh Co(II), and Td Co(II) can be obtained by fitting a linear combination of reference spectra to the measured spectra. The relative absorption intensity is given by the number of 3d holes multiplied by the number of ions present. In the case of the Co3O4, this yields a relative value of 8/3 (2/3 ions times 4 holes) for Oh Co(III) and 1 (1/3 ion times 3 holes) for Td Co(II). The published spectra41 42 of LiCoO2 and K5H[CoW12O40]·xH2O were used as reference spectra for the Oh Co(III) and Td Co(II), respectively. (See A.4 in supporting information for details) The fit curves are shown in Figure 2a. In the case of the CoBi, the same Oh Co(III) spectrum from the Co3O4 fit is assumed and the literature spectrum of the Cobased polyoxometalates was used for the Oh Co(II).42 The measured spectrum is reasonably fit by a linear combination of the reference spectra as shown in Figure 2c, Figures S1 and S2. Based on the relative spectral weight, we find that the CoBi is comprised of 65.3 % Oh Co(III) and 34.7 % Oh Co(II). The same spectra of the Oh Co(III), Td Co(II) and Oh Co(II) used


3


for the Co3O4 and CoBi were used to fit the measure spectrum of the CoPi as shown in Figure 2d. The CoPi spectrum is described as a sum of 43.5 % Oh Co(III), 39.2 % Oh Co(II), and 17.3 % Td Co(II). In the CoPi, the Td Co(II) chemical composition exists at the expense of the Oh Co(III). Table 1 summarizes the fit results of the relative fractions of the HS Td Co(II), LS Oh Co(III) and HS Oh Co(II) in Co3O4, CoBi, and CoPi.

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to Oh Co(III), while the B peak is primarily due to Td Co(II) contribution. The A peak of Co-OECs (CoPi and CoBi) reflects the Oh Co(II) contribution. Linear combination fittings of the Oh Co(III), Td Co(II), Oh Co(II) reference spectra to the measured spectra allowed to determine the relative fractions of Oh Co(III), Oh Co(II), and Td Co(II) contributions. Table 1 Summary of the relative fractions of the HS Td Co(II), LS Oh Co(III) and HS Oh Co(II) in Co3O4, CoBi, and CoPi. Co ions

Co3O4

CoBi

CoPi

HS Td Co(II)

33.3

0

17.3

LS Oh Co(III)

66.7

65.3

43.5

HS Oh Co(II)

0

34.7

39.2

Resonant X-ray emission at the Co K pre-edge

Figure 2. Co L3 absorption spectra of a) Co3O4, b) LiCoO2, c) CoBi, and d) CoPi. The C and D peaks are mainly attributed

The schematic of the physical process of resonant x-ray emission spectroscopy (RXES)44-47 at the Co K pre-edge are presented in Figure 3 a). The incident X-ray energy tuned to the pre-edge of the 3d transition metal (TM) excites the 1s core electron to the unoccupied 3d state. Then, the created 1s core hole is filled by the shallower 3p core electron through the 1s3p electric dipole transition, which is called the Kβ transition. Relative intensities of Kβ emission is much weaker by 3-6 times than those of Kα emission (1s-2p dipole transition). However, Kβ RXES measurements provide higher spectral resolution than Kα RXES measurements. The observed highresolution Co Kβ RXES spectra are shown in Figure 3. In RXES, both the incident and emitted X-rays were analyzed, yielding two-dimensional (2D) spectral surface. Figures 3b–3e show contour plots of experimental Kβ RXES intensities at Co K pre-edge for Co3O4, LiCoO2, CoBi and CoPi, respectively. The incident X-ray energy (Ei) is on the x-axis of the plots. The energy transfer (Etr) is on the y-axis of the plots and is equal to the incident X-ray energy minus the emitted X-ray energy.


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Figure 3 Schematics of the RXES and RIXS processes a) and contour plots of Co Kβ RXES intensities at Co K pre-edge for b) Co3O4, c) LiCoO2, d) CoPi, e) CoBi.. There are three distinct regions, indicated by dashed circles (α α, β , and γ). ‘α α’ peak reflects LS Oh Co(III) contribution. The Td and Oh Co(II) comprise the ‘α α’ and ‘β β’ peaks. RXES spectra of CoBi and CoPi are found to be different, indicating different chemical compositions between CoBi and CoPi. The ‘γγ’ region intensity reflects the non-local excitation of Oh Co(III),48-50 which was observed for Co3O4 and CoBi, but unseen for CoPi. A comparison of constant energy-transfer cut plots f) at Etr ~61.5 eV for the three samples clearly show the presence and absence of the ‘γγ’ region peak at Ei ~7711.5 eV.

As shown in Figure 3b, at Ei = 7709 eV two emission peaks were observed at Etr~ 61 eV (denoted ‘α’) and 58 eV (denoted ‘β’). At Ei = 7711.5 eV, one emission peak was observed at Etr~ 61 eV (denoted ‘γ’). The former two peaks originate from the 1s-3d quadrupole transitions. RXES spectrum of CoO (not shown here, see ref. 50), a reference for Oh Co(II) is characterized by the medium-intensity ‘α’ peak and weak ‘β’ peak. RXES spectrum of LiCoO2 in Figure 3c, a reference for Oh Co(III) is characterized by the intense ‘α’ peak and mediumintensity ‘γ’ peak, and does not show a ‘β’ peak. RXES features of Oh Co(III) is seen also in Co3O4, Figure 3b, which has 66.7% Oh Co(III). While ‘β’ peak is weakly seen in Oh Co(II), it appears very strong in Co3O4 where Td Co(II) is included by 33.3%. In Td symmetry, 3d-4p orbital mixing becomes significant and leads to intensify the 1s-3d quadrupole transition which partially gains dipole transition in character. Dipole transition (1s-4p) is much more intense than the quadrupole (1s-3d) transition. In Td symmetry, the 3d-4p orbital mixing has been reported to be so large that the intensity of Td Co(II) increases by approximately four times.50 Theoretically, the Td Co(II) is responsible for all the intensity of the ‘β’ region and the majority intensity (60%) of the ‘α’ peak.50 In contrast, the emission from the Oh Co(III) mainly appears in the ‘α’ region (Etr~ 61 eV). RXES spectra of the CoPi and CoBi were found to be significantly different (Figure 3d and 3e). The ‘α’ peak is very

strong but ‘β’ peak is very weak in the CoBi spectrum. In contrast, ‘β’ peak is the strongest among the three peaks in the CoPi spectrum. Thus, RXES data suggest that a significant amount of Td Co(II) is present in CoPi, but not in CoBi. CoBi includes mainly Oh Co(III) instead. The results are in good agreement with the above XAS results. On the basis of the relative abundance values of 2.61 (= 65.3% ions times 4 holes) for Oh Co(III) and 1.04 (= 34.7% ions times 3 holes) for Oh Co(II) by above XAS result (Table 1), CoBi’s RXES spectrum would be dominated by the Oh Co(III) emission (72%). Note that the Oh Co(II) does not have an enhancement factor due to the 3d-4p mixing. On the other hand, the ‘α’ and ‘β’ peaks in CoPi spectrum show similar in intensity to those of Co3O4 spectrum. Let us calculate the spectral intensity ratios between Co(III) and Co(II) in the CoPi. On the basis of the above relative abundance values (Table 1) of 1.7 (0.435 ions times 4 holes) for Oh Co(III), 1.2 (0.392 ions times 3 holes) for Oh Co(II), and 2.1 (0.173 ions times 3 holes multiplied by 4 of 3d-4p mixing factor) for Td Co(II), Co(II) ions are responsible for the majority intensity (66%) of the spectral intensities of the CoPi RXES. This ratio of the CoPi is close to that of the Co3O4, 66%. Hence, the similar α and β peak intensities between the CoPi and Co3O4 are understood. Both Co3O4, LiCoO2, CoBi RXES image plots (Figure 3b and 3e, respectively) clearly show the ‘γ’ peak at Etr~ 61 eV for Ei = 7711.5 eV. The γ peak originates from non-local dipolar


5


excitation arising from the Co-O-Co intersite hybridization between Co 4p states and neighboring Co 3d orbitals mediated by oxygen 2p in the Oh Co(III) oxide systems.48-50 Because these transitions only exist by virtue of coupling to neighboring cobalt sites, the spectral weight is sensitive to changes in the electronic and structural coupling to neighboring cobalt ions. Figure 3f shows plots of the constant energy transfer cut at Etr ~ 61.5 eV. As seen in the image plots, both Co3O4 and CoBi line curves show the non-local excitation peak at Ei ~ 7711.5 eV. In the case of CoPi, however, there is no visible intensity of a possible peak at Ei ~ 7711.5 eV. Because the fraction of the Oh Co(III) is not negligible (43.5%), this is not simply due to a weak intensity but imply a weak 3d-4p coupling between neighboring cobalt sites in the CoPi, contrary to the strong intersite coupling in the CoBi.

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process also involves electronic excitation of core electrons to a 3d-orbital and then the transition to the core-hole from the d electrons of another d-orbital and thus reflects the information on the electronic states of d orbitals. The positions of RIXS peaks due to CF dd excitation are identical to dd transition energies found in optical (UV-vis) spectroscopy and thus can be compared with the optical values. A big difference is that RIXS is element-specific unlike UV-vis spectroscopy.

Resonant inelastic X-ray scattering at the Co K pre-edge As described in Figure 3a, there is another decay channel of the 1s core hole directly by the 3d electron. This process is called resonant inelastic X-ray scattering (RIXS).46, 51 The spin-allowed crystal-field (CF) excitations (so-called dd excitation) are created when a 3d orbital above EF is occupied by the 1s core electron and subsequently a different 3d orbital below EF is emptied to fill the 1s core hole. When neighboring ligands or 3d orbitals were involved, inter-site excitations could be created. The RIXS spectra of Co3O4, CoBi, and CoPi are shown in Figure 4. A very intense peak at zero energy transfer is due to elastic scattering. The two well-resolved RIXS peaks below 1.2 eV of Co3O4 are assigned to two CF excitations of 4T2 and 4 T1 from the ground state 4A2 for Td Co(II). The peaks at 1.8 eV and at ~ 2.1 eV are assigned to the first CF excitation of the Oh Co(III) (1A1 → 1T1), and the second CF excitation of 4 T1 (4A2 → 4T1) for Td Co(II), respectively. The RIXS spectrum of CoBi in the middle layer of Figure 4 shows two peaks at ~ 0.8 eV and at ~ 2 eV. The 0.8 eV peak is assigned to the CF excitations of Oh Co(II): 4T1 → 4T2. Thus, three distinct peaks appear below 1 eV, two from Td Co(II) and one from Oh Co(II). The peak at 2 eV is attributed mainly to the first CF excitation of Oh Co(III) (1A1 → 1T1) and a little due to the second CF excitation of Oh Co(II) (4T1 → 4A2). The RIXS spectrum of CoPi is noticeably different from that of CoBi. First, the spectral weight around 0.8 eV is larger than that of CoBi. This can be explained by the presence of Td Co(II) ions in CoPi in addition to the Oh Co(II). Second, all observed peaks are much broader than those of the CoBi. This is due to relatively poorly-defined low-energy dd excitations and implies a significant disorder in electronic structure for CoPi, consistent with resonant vibrational Raman and PDF results. All CF excitation peaks below 2 eV were fit with Gaussian functions, which are included in Figure 4. In cases of the Co3O4 and CoBi, broad Gaussian peaks are added at higher energy to fit the broad high energy peaks, which will be discussed later. For all cases, a gradually increasing background with increasing energy was used to fit intensity from a higher energy valence (Kβ2,5) emission. The same energy value was assumed for the Oh Co(II) peak of CoBi and CoPi. The result of the curve fit is shown in Figure 5 and is summarized in Table 2. Unlike X-ray absorption spectroscopy, RIXS process involves core-hole occupancy in the final state and thus includes the information on the ground electronic state. RIXS

Figure 4. Resonant inelastic X-ray scattering (RIXS) of Co3O4, CoBi, and CoPi. RIXS data were collected at the maximum of the pre-edge absorption (Ei = 7.7105 keV). Excitation peaks below 2 eV are assigned to the CF excitations of the Oh Co(III), Oh Co(II), and Td Co(II). The broad peak at around 2.9 eV is attributable to the inter-site excitations between neighboring Co(III) ions. The negligible amplitude of this peak in the CoPi suggests that an effective hopping 3d electron between neighboring Co ions is much weaker in CoPi than in CoBi.

The values of crystal-field splitting energy (10Dq) for Co3O4, CoBi, and CoPi were obtained from Tanabe-Sugano (TS) diagram and are included in Table 2. Table 2 also includes optical CF energies for CoO as a reference material for Oh Co(II) ions. The 10Dq values for Co-OEC’s Oh Co(II) has been found smaller by 0.2 eV than that of CoO (0.9 eV vs 1.1 eV). The -10Dq values of CoPi’s Td Co(II) is also slightly smaller by 0.07 eV than that of Co3O4. However, the 10Dq values of Co-OEC’s Oh Co(III) were greater by 0.2 eV than that of Co3O4 (~2.25 eV vs 2.05 eV). The distinctively broad peaks centered around 2.7-2.9 eV in the Co3O4 and CoBi RIXS spectra are attributed to the inter-site dd excitation, whose energy corresponds to the energy cost of the hopping of 3d electron to neighboring Co: 3d6 + 3d6 → 3d5 + 3d7. In the multi-electron Hamiltonian (see A.7 in supporting information), the inter-site dd excitation energy between neighboring Co ions is calculated to be 10Dq + U - 5J, where U is the effective Coulomb repulsion and J is Hund’s coupling energy. In the spinel structure of Co3O4, the nearest neighbor pair is between the edgesharing Oh Co(III) ions. Using the literature values (U = 5 and


6

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J = 0.9) and our fit values of the 10Dq, the inter-site dd excitation energy can be calculated to be 10Dq + U - 5J = 2.05 + 5 4.5 = 2.55 eV for the Co3O4, which is reasonably close to the measured excitation energy of 2.7 eV. In the CoBi, the Co ions are dominantly in the Oh Co(III) state and the majority of the nearest neighbor is between the LS Oh Co(III) ions. The inter-site dd excitation energy is calculated to be 10Dq + U 5J = 2.2 + 5 - 4.5 = 2.7 eV for the CoBi which is close to the measured excitation energy of 2.8 eV. There is another nearest neighbor pair between the Oh Co (II) and Oh Co (III). In the multi-electron Hamiltonian, the inter-site dd excitation of this pair is in much higher energy (> 4 eV) and not relevant to the observed 2.8 eV peak. In the case of CoPi, its feature is clearly unseen. The negligible amplitude of this peak in the CoPi suggests that an effective hopping 3d electron between neighboring Co ions is not effective in CoPi unlike in CoBi. Note that the observed fraction of the Oh Co(III) in CoPi is 43% by the above soft XAS and non-negligible. RIXS and RXES data show that the inter-site 3d-3d excitation and 4p-3d hybridization, respectively are present in CoBi and absent in CoPi. Table 2 Spin-allowed CF and inter-site dd excitations from the RIXS spectra of Co3O4, CoBi, and CoPi. Gaussian functions were used to fit the measured spectra. All units are eV. The CF excitation of the CoO is from the optical absorption spectrum.52 Co3O4 Exp 10Dq LS Oh Co (III)

1

A1→ 1T1

HS Oh Co (II)

HS Td Co (II)

1.8

TS

1.93

1.95

TS

2

1.99

2.25

3.1

10Dq

3.16

0.9

T1→ 4T2

0.79

0.79

0.79

0.9

1.1

0.79

0.99

→ A2

4

1.69

1.69

→ 4T1

2.18

2.18

10Dq

Ref.5 2

The Raman spectrum for CoBi shows two bands centered at ~607 and ~503 cm-1, which we assign to the symmetric Co-O stretching (A1g) and the bridging Co-O-Co (Eg) vibrations, respectively. The two vibrational bands are similar in position to the two characteristic LiCoO2 bands at 595 and 487 cm-1, but are significantly broader. This can be explained by CoBi’s LiCoO2-like, layered structure with much broader distribution of force constants and angles of Co-O and Co-O-Co bonds in CoBi structure compared to crystalline LiCoO2. In contrast, only a broad Co-O stretching vibration, notably shifted to around 597 cm-1, is seen as a much weaker component in the Raman spectrum of CoPi. Raman data shows that CoPi is “amorphous” solid lacking the long range order, in contrast to CoBi’s pseudo-layered structure.

2.9

4

4

Exp

2.2

2.95 2.7

CoO

CoPi

Exp

2.05 1.8

→ 1T2 inter-site dd

CoBi TS

Co3O4, Co(II)Co(III)2O4 has the normal spinel structure (Fd3m, Oh symmetry) with Co(II) ions in tetrahedral sites and Co(III) ions in the octahedral sites of the cubic close-packed lattice of oxygen anions. The Raman band appearing at 482 cm-1 has been assigned to Eg vibrations,56 which involves the motion of bridging oxygen in Co(II)-O-Co(III) and Co(III)-O-Co(III) bonds.57 Note that the bridging Co-O-Co (Eg) frequencies for LiCoO2 and Co3O4 appear very closely at 487 and 482 cm-1, respectively. The vibrational frequency in the bridging Co-OCo structure is sensitive to the bridging angle.58 The bridging O-Co-O angles for LiCoO2 and Co3O4 are quite similar (differ by 4o): 98.6o and 81.4o in the Co3O4 structure,59 94.2 and 85.8o in the LiCoO2 structure.53

0.49

0.42

A2→ T2

4

0.53

0.49

0.45

0.42

→ 4T1

0.88

0.92

0.78

0.79

→ 4T1

2.1

2.05

2

1.97

Resonant vibrational Raman Spectroscopy Resonant vibrational Raman spectra of Co3O4, LiCoO2, CoBi, and CoPi upon excitation at 2.71 eV are compared in Figure 5. The excitation energy of 2.71 eV (458 nm) is close to the absorption maximum of 2.7-2.9 eV for the intersite d-d electronic hopping (transition) between neighboring Co ions in Co-O-Co molecular unit (See Figure 4 and Table 2). It is thus expected that vibrational bands of Co-O-Co unit in cobalt oxide structures are vibronically enhanced with 2.71 eV excitation. LiCoO2 is commonly used in the positive electrodes for Li-ion batteries and has a layered trigonal (rhombohedral) structure with R3-m (D3d) space group symmetry.53 The Li and Co ions occupy alternate layers in octahedral sites between the cubic close-packed oxygen planes. Or, both Li and Co are octahedrally coordinated by oxygen with edge-sharing. The analysis of factor group D3d yields two Raman active modes of A1g and Eg. Vibrational Raman bands of LiCoO2 appearing at 595 and 487 cm-1 (Figure 5) are assigned to the symmetric CoO stretching (A1g) of the CoO6 octahedra and the bridging CoO-Co (Eg) vibrations, respectively.54, 55 Both the modes involve the motion of, approximately, only the oxygen atoms.

Figure 5. Resonant vibrational Raman spectra excited at 2.71 eV (458 nm) of Co3O4, LiCoO2, CoBi, and CoPi in the 300800 cm-1 region.

Energy dispersive X-ray (EDX) and X-ray photoelectron spectroscopy (XPS) The presence of added and adventitious Fe atoms have been found to change the catalytic activities of Ni (oxy)hydroxides, and at higher concentration levels those of Co-OEC as well.35, 60, 61 We have examined the possibility of whether the presence of variable adventitious Fe could account for the differences seen here in the electronic and catalytic properties of the CoPi and CoBi OEC by carrying out comparative elemental analyses. Energy dispersive X-ray (EDX) spectroscopy of the CoPi and CoBi ex-situ tubes were measured using 12 kV and 30 kV acceleration voltages, Figure S13. The Co:P:K ratio measured for CoPi from the 12 kV EDX was 2.8:1.2:1 and the Co:K ratio from CoBi was 10.9:1, in agreement with previous measurements.2, 11, 19, 34 These spectra show no discernable


7


contribution was from elemental Fe. Further details are discussed in Figure S13. In addition, Al Kα X-ray photoelectron spectroscopy (XPS) spectra were measured for CoPi and CoBi, Figure S14. With this excitation energy, although the Co 2p binding energy lines are well-separated from other elemental lines and permit quantitative elemental analysis, each of the Fe binding energy lines are overlapped with transitions from Co. As described in Figure S14, from the XPS we estimate that the total amount of Fe for each sample is less than 8% of the Co content, consistent with prior measurements of adventitious Fe in electrochemically deposited CoOOH.35 However, we note that there was no detectable difference in Fe content between the CoPi and CoBi samples. The absence of detectable Fe by EDX and XPS suggests that Fe contamination cannot account for the electronic and catalytic differences observed between CoPi and CoBi, and accordingly, we can attribute the electronic and structural factors resolved by the X-ray and Raman analyses. DISCUSSION Td Co(II) Content This work reveals the absence and presence of tetrahedral (Td) Co(II) ions in CoBi and CoPi, respectively, as indicated by the large B peak in XAS, the enhanced quadruple transition peak at the ‘β’ region in RXES, and the CF excitation peaks mainly below 1 eV in RIXS. The identification of Td Co(II) content in CoPi has not been previously reported, to the best of our knowledge. Recognition of Td Co(II) sites as a component in CoPi could be significant for understanding structural, electronic and chemical differences between CoPi and CoBi OER function. It has been found by PDF analysis that CoPi’s nano-domains are composed of about 13 cobalt atoms and the terminal oxygen atoms at the domain edge sites are distorted in coordination geometry.10, 11 However, within a general model of edgesharing Oh CoO6 cobaltate domains,9-11, 24 it has been unexplained why such coordinative distortions are present. We found that the coordinative displacements of the terminal oxygen atoms that are required to optimally model the CoPi PDF,10, 11, 24 nearly exact fit of oxygen-oxygen distances in Td Co(II) extracted from Co3O4 crystal structures. In the Co3O4 structure, Td Co(II) makes an equivalent coordination to oxygen atoms that are axial to di-µ-oxo-coupled Oh Co(III) atoms, and are accompanied by analogous distortions to the Oh Co(III) coordination geometry. This suggests the possibility that Td Co(II) coordination at edge atom sites can contribute to coordination geometry distortions seen in CoPi,10 which are absent or of reduced content in CoBi.11 Figure 6 illustrates such possible Td Co(II) sites superimposed onto edge sites of the CoPi PDF model.10, 11 The 17.3 % atom fraction of Td Co(II) content analyzed here corresponds to approximately 2 Td Co(II) atom sites in a 13 cobalt atom domain.

Page 8 of 13

Figure 6. Td Co(II) association with the CoPi OEC domain model. The structure used to model CoPi PDF data is shown by the Oh Co(III) atoms (blue) in blue and oxygen, red. Fitting to CoPi PDF data required distortions to the coordination geometries for the terminal oxygen atoms (red).10, 11 Td Co(II) (green) from the Co3O4 structure are shown to superimpose onto sites created by these distortions, two of which are illustrated in the graphic, and suggest a possible mechanism for the distortion of the edge atom coordination geometry. A rocking disorder of about 0.25 Å would be sufficient to broaden Td Co(II) contributions to atom pair correlations.

Bridging atom coordination across terminal Co-oxo atoms has been proposed for Pi and Bi oxy anions and potentially functions as capping groups to limit cobaltate domain growth and serve as proton acceptors during water-splitting catalysis.4, 18, 20, 21 From an analysis of PDF data as noted previously,10 edge capping by Pi, or the Td Co(II) proposed here, would have disorder sufficient to broaden pair correlations to these atoms. Evidence for domain disorder is discussed below. Non-local Excitation Non-local transition via intersite orbital hybridization provides an important mechanism for rapid charge redistribution (delocalization) across metal oxide domains. For example, enlarging the Co-O-Co dihedral angle by substituting strontium in LaCoO3 perovskite leads to enhanced overlap between Co-3d and O-2p orbitals and a higher charge conductivity.62, 63 In this study, two hard X-ray spectroscopy data (RXES in Figure 3 and RIXS in Figure 4) revealed that the Oh Co(III) in the CoPi has a vanishing inter-site charge transfer coupling. Non-local dipolar excitation in the RXES (the ‘γγ’ region in Figure 3) arises from the inter-site hybridization by oxygen 2p between 4p states and 3d orbitals of neighboring LS Oh Co(III) ions. Co Kβ RXES spectrum of CoBi in Figure 3 e) clearly shows this non-local dipolar excitation, analogous to that seen in the Co3O4 spinel,50 LiCoO2,48 and a molecular Co4O4 cubane,16 while that of CoPi does not. The inter-site dd excitation in the RIXS reflects the inter-site hybridization by oxygen 2p between neighboring 3d orbitals in the Oh Co(III). This inter-site excitation is observed as clear peaks in the CoBi but is not visible in the CoPi. Non-local inter-site excitation intensity in TM oxide systems depends on the bond length and angle. In edge-sharing octahedra, a larger bond angle gives rise to a larger intensity of the inter-site coupling through oxygen.48 Similar cases have been shown in MnO264 and CoS2.50 The bond distance of 1st (Co-O) and 2nd shell (Co-Co) in CoPi and CoBi are estimated from the full range of PDF spectra shown in Figure S6 b) and c). in the supporting information. The bond angles are estimated from


8

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CoOECs

Co-O(Å)

Co-Co (Å)

Co-O-Co angle (o)

CoPi-1

1.915

2.819

94.7889

CoPi-2

1.915

2.819

94.7889

CoBi-1

1.901

2.836

96.4769

CoBi-2

1.908

2.841

96.2318

Average bond angle difference 1.57o

these bond distances shown in Figure S12 a) and b). The resulting bond distance and angles are summarized in the Table 3. Table 3. Average Co-O-Co bond angle difference obtained from 1st (Co-O) and 2nd (Co-Co) shell bond distance of experimental PDF spectrum (Figure S12 a) and b) in the supplementary section). Two different samples in both OECs are measured.

Co(4p)-O(2p)-Co(3d) hybridization is sensitive to the Co-O bond length and the Co-O-Co bridging angle.48 The degree of hybridization becomes greater at shorter Co-O length and at larger bridging angle, i.e., greatest at 180 degree. The Co-OCo bridging angle for CoBi is larger than CoPi, but only by 1.57 degree (Table 3), and similarly, the Co-O length for CoBi compared to CoPi is shorter by 0.01 Å. In addition, Co-O-Co hybridization is also most effective for Oh Co(III) pairs. The mole fraction of Oh Co(III) is higher in CoBi than CoPi, (65.3 % vs 43.5 %, Table 1). While these structure and mole fraction differences would predict a stronger Co-O-Co hybridization in CoBi compared to CoPi, the relatively small differences in length and angle and mole fraction do not fully explain the large difference in the degree of Co-O-Co hybridization between CoBi and CoPi, as shown by the ‘γ’ region RXES peak (Figure 3) and the RIXS peak at 2.9 eV (Figure 4). The large difference in hybridization can be fully explained if most of Oh Co(III) would be isolated in CoPi, while most of Oh Co(III) in CoBi would be µ-oxo-bridged and is thus effective in hybridization. This is supported by resonant Raman evidence (Figure 5) which show a higher fraction of µ-oxobridged Oh Co(III) pairs in the pseudo-layered CoBi structure, and nearly zero fraction in fully-amorphous CoPi. Co(III)-O-Co(III) structural order Analogous to high energy X-ray scattering lineshapes in Figure S6 a), resonant Raman spectra in Figure 5, clearly show differences in vibrational lineshapes between CoPi and CoBi. A very weak and broad Co-O stretching vibration was observed for CoPi, suggesting a significantly disordered bonding pattern. In contrast, two characteristic strong bands due to CoO stretching and Co-O-Co bridging vibrations were detected for CoBi that can be correlated to a disordered LiCoO2-like layered structure. Note that the resonant Raman spectra were obtained with the excitation energy at which the intersite electronic absorption is favorable. Thus resonant Raman spectral intensity is expected to be vibronically enhanced for CoBi which has an intersite excitation. For CoPi, the vanishing intersite excitations identified by both the RXES and RIXS is consistent with the lack of contributions of Co-O-Co bridging structures in the resonant Raman spectrum.

The higher degree of coordination disorder for CoPi compared to CoBi is also indicated in the RIXS data. For example, RIXS transitions for Oh Co(II) and Oh Co(III) show crystal field excitation widths which are approximately 2-fold broader in CoPi compared to CoBi as shown in Figure 4. Since edge atom sites make a proportionally higher contribution in CoPi,10, 11 this result is in keeping with the interpretation that distortions in the coordination geometry at domain edges contribute to the broadening of spectroscopic crystal field parameters reported here. The Oh Co(III) RIXS peak is seen to have a comparable excitation width in both nanocrystalline Co3O4 and CoBi, suggesting a similar 3d electronic structure of Oh Co(III). Resonant vibrational Raman suggests that the molecular structure of CoBi is a layered LiCoO2-like structure, far from that of a spinel Co3O4 crystalline structure. Observed vibrational Raman bands of CoBi is much broader in width than those of LiCoO2, indicating CoBi’s higher degree of disorder in coordination, bond length and angles than that of LiCoO2. Correlations of structure and disorder to charge transport In terms of OER activity, we found that CoPi is a slightly better catalyst in very thin films (cobalt loading < 0.05 µmol/cm2), when scaled on a per-metal-atom basis, while CoBi shows better catalytic activity in thicker films. Enhanced OER for CoBi compared to CoPi with increasing film thickness would be consistent with the differing charge transport properties. Charge transport across amorphous Co-OEC thin films can be considered to arise from a combination of both intra- and inter-domain transport characteristics. At the domain level, the increased geometry distortions in CoPi compared to those in CoBi can be expected to dampen collective properties of the domain, such as normal vibrational modes, non-local electronic excitations, and charge redistribution or delocalization. The absence of an inter-site dd excitation in CoPi unlike in CoBi as discussed above, suggest a possible restriction of charge delocalization across the CoPi domain. In addition to the difference in electronic properties of the CoPi and CoBi domains shown here, PDF analyses have shown that the two Co-OEC are distinguished by the degree of cobaltate layer stacking, with CoPi consisting of disordered single layers, and CoBi having turbostratic disordered stacking, with an order parameter of approximately 3 layers.9, 11 Differences in cobaltate layer associations can be anticipated to influence inter-domain electron-proton charge transport, potentially by disrupting either or both electronic overlap integrals and proton transfer networks. Our findings of conductivity differences for CoPi and CoBi in the absence of electrolyte is in accord with electrochemical conductivity measurements37 and the catalytic activity differences described in this manuscript. These electronic and activity properties measured for “bulk” CoPi and CoBi are correlated to characteristic differences in electronic properties and structure measured at the atomic scale. In particular, the results show that the cobaltate domains for CoPi and CoBi differ in the content of tetrahedral Co(II) “defect” sites, the extent of oxygen-mediated metal-metal delocalization, and mesoscale ordering. The proposed location of Td Co(II) “defect atoms” at random edge-coordinated sites in Figure 6 suggests a model by which Td Co(II) could form dis-aligning connections between individual cobaltate domains, Figure 7 left, which can be understood to impede inter-domain charge transport, com-


9


bined with the absence of delocalized charge transfer bonding within the CoPi domain. In contrast, CoBi is organized in stacked motifs, providing facilitated charge transport between

Page 10 of 13

domains possessing extended charge delocalization Figure 7, right.

Figure 7. Models for cobaltate domain organization in CoPi, left, and CoBi viewed edge-on, is shown on the right.

CONCLUSION We have compared the OEC performance of CoPi and CoBi under equivalent electrolyte conditions and show that the OEC performance are distinguished by their differing electronproton charge transport properties and bulk conductivity properties, and that these functional properties can be correlated to characteristic differences in electronic properties and structure at the atomic scale. In particular, the results show that the cobaltate domains for CoPi and CoBi differ in the content of tetrahedral Co(II) “defect” sites, the extent of oxygenmediated metal-metal delocalization, and mesoscale ordering that can be understood to combine and support enhanced electron-proton conductivity in CoBi compared to CoPi. More generally, this work demonstrates opportunities to use the combination of soft XAS and resonant X-ray emission spectroscopy for investigating the interplay between intrinsic catalytic activity of catalytic sites and the electronic structure and charge transport properties of thin-film catalysts. In particular, the intra and inter-domain structures and electronic structures of CoBi compared to CoPi are found to support facile charge transport, which compensates for a presumed lower density of domain edge located catalytic sites. In-situ measurements are on-going which will allow a more direct interrogation of electronic and atomic structure linked to OEC function. ASSOCIATED CONTENT Supporting information The supporting information is available free of charge on the ACS Publication website.

AUTHOR INFORMATION Corresponding Author [email protected] [email protected] [email protected]

Notes The authors declare no competing financial interest

ACKNOWLEDGMENT

This research was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences (DOE BES). Argonne National Laboratory (ANL) is a U.S. Department of Energy laboratory. ANL and the Advanced Photon Source (APS) and Center for Nanoscale Materials (CNM) user facilities are managed by UChicago Argonne, LLC, under contract DE-AC0206CH11357. G. K., A. B. F. M., and RIXS and RXES data collection and analysis were supported by the Argonne Northwestern Solar Energy Research (ANSER) Center, an Energy Frontier Research Center funded by DOE BES under Award Number DESC0001059. PDF analyses were supported by the DOE BES Chemical Sciences, Geosciences and Biosciences Division under Contract DE-AC02-06CH11357. G. K. acknowledges use of the Center for Nanoscale Materials, including resources in the Electron Microscopy Center (Rachel Koritala). X-ray spectroscopy experiments were carried out at the beamline 8-2 of the Stanford Synchrotron Radiation Lightsource, a Directorate of SLAC and an Office of Science User Facility operated for DOE BES by Stanford University. J.-S. L. acknowledges support by the DOE BES, Materials Sciences and Engineering Division, under Contract No. DE-AC02-76SF00515. The Raman work at ANL was supported by the U.S. DOE BES under contract DE-AC02-06CH11357 and DE-FG02-03-ER15457. H. K. acknowledges the financial support from the R&D Convergence Program of MSIP (Ministry of Science, ICT and Future Planning) and NST (National Research Council of Science & Technology) of Republic of Korea (CRC14-1-KRICT). G. K. also gratefully acknowledges support of 11ID-B and 6ID-D and Eunjae Lee for the cobalt standard powders, and the APS 3D printer shop (Michelle Givens, Brenda Davis, Neil Bartkowiak, and Brian Rusthoven) for making several tools for the experiments. REFERNCES

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Resolution of Electronic and Structural Factors ... - ACS Publications

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