Supporting Information. Probing the structure of water oxidizing anodic. Iridium oxide catalyst using Raman spectroscopy

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1 Supporting Information Probing the structure of water oxidizing anodic Iridium oxide catalyst using Raman spectroscopy Zoran Pavlovic, Chinmoy Ranjan, Qiang Gao, Maurice van Gastel, and Robert Schlögl Department of Heterogeneous Catalysis, Max Planck Institute for Chemical Energy Conversion, Muelheim an der Ruhr, Germany 1

2 1. Coordinates of molecules used in DFT Calculations IrO x naturally occurs in an octahedral environment. Various Ir complexes (chlorides, fluorides) and oxides (rutile IrO 2 ) have an octahedral environment of ligands around Ir. Even results from EXAFS studies show a coordination number of 6. Thus for all calculations, an octahedral environment around Ir was conserved. OH was used for charge balancing and H 2 O was added at the empty coordination sites. Cartesian coordinates of the optimized geometry of the molecules are provided below. Table S1: Ir(OH) 3 6, Formal charge on Ir: +3 Atom X Y Z Ir O H O H O H O O H O H H

3 Table S2: Ir 2 H 12 O 1 : IrO x - Dimer, Formal charges on Ir: +4 Atom X Y Z O O O O H H H H O O Ir Ir H H H H O O H H O H O H

4 Table S3: Ir 3 H 16 O 14 : IrO x Trimer, Formal charges on Ir: +4 Atom X Y Z O O O O H H H O O Ir Ir H H O O H H O H O Ir O H H O H H O H O H H H

5 Table S4: Ir 3 H 16 O 14 : IrO x - Trimer, Formal charges on Ir: +4, +4 and +5 Atom X Y Z O O O O H H H O O Ir Ir H H O O H H O H O Ir O H H O H H O H O H H H

6 2. Raman data collection The sample is inhomogeneous with various particles. Raman spectra collection amounts to collecting spectra from individual particles at a time. Since we sample many sites (not the same sites during each measurement), variations can arise from simply scattering intensity differences or differences in chemical composition of individual sites. Small variations also arise from the fact that all the measured particles do not transform exactly in the same way (to the same extent), at a given potential. As can be seen in Figure S1, although the entire IrO x foil was measured at 1.4 V, some particles were more, or less oxidized than others e.g. particles P1 and P2 at potentials 1.4 and 1.6 V. As clearly seen, the movement (redshift) of gamma and epsilon peaks are linked to each other, and the gap between β and γ peaks, closes as the material is increasingly oxidized. We can safely say that as the material gets oxidized, gamma and epsilon peaks redshift resulting in the closing of the gap in spectra between beta and gamma peaks. 6

7 1.1 V α β γ * * δ * ε * P1 P2 1.4 V * * P1 * P2 1.5 V 1.6 V Figure S1: Shows IrO x grown in H 2 O at various potentials. Raman spectra were collected at various sites in a given sample, at a given potential. Each color represents spectra collected from a certain particle P. As can be seen, not all the particles transform in exactly the same way. At 1.4 V, particle P1 (pink) is less oxidized that particle P2 (red). In particle P1, a hint of δ peak is also visible. At 1.5 V, particle P1 (red) is more oxidized than particle P2 (blue). For a single particle, the position of γ and ɛ are linked to each other and shift together. The arrow points at the dip in spectra around 4 cm 1. As the material gets oxidized (between 1.1 V and 1.6 V), the γ and ɛ peaks redshift resulting in the closure of this gap. 7

8 Intensity (a.u.) Raman shift (cm -1 ).4 V.5 V.6 V.7 V.8 Intensity (a.u.) Raman shift (cm -1 ).8 V.9 V 1. V 1.1 V 1.2 V Figure S2: The structural transformation of IrO x under a potential regime: V. IrO x was grown in.5 M H 2 SO 4 by cycling from. to 1.5 V for 2 cycles at 1 mv/s. The Raman collection was carried out using 2 s exposures averaged over five collections. A 78 nm laser source was used with 15 mw laser power was incident upon an area of.7 µm 2. 8

9 Intensity (a.u.) Raman shift (cm -1 ) 1.2 V 1.3 V V 1.6 V Intensity (a.u.) Raman shift (cm -1 ) 1.6 V 1.7 V 1.8 V 1.9 V 2. V Figure S3: The structural transformation of IrO x under a potential regime: V. IrO x was grown in.5 M H 2 SO 4 by cycling from. to 1.5 V for 2 cycles at 1 mv/s. The Raman collection was carried out using 2 s exposures averaged over five collections. A 78 nm laser source was used with 15 mw laser power was incident upon an area of.7 µm 2. 9

10 Intensity (a.u.) Raman Shift (cm -1 ) V.5 V.6 V.7 V.8 V V.9 V Intensity (a.u.) V 1.1 V V Raman Shift (cm -1 ) Figure S4: The structural transformation of IrO x under a potential regime: V. IrO x was grown in D 2 O (.5 M H 2 SO 4, 98% D 2 O) by cycling from. to 1.5 V for 2 cycles at 1 mv/s. The Raman collection was carried out using 2 s exposures averaged over five collections. A 78 nm laser source was used with 15 mw laser power was incident upon an area of.7 µm 2. 1

11 Intensity (a.u.) V 1.3 V 1.4 V 1.5 V Raman Shift (cm -1 ) 1.6 V Intensity (a.u.) Raman Shift (cm -1 ) V 1.7 V V 2. V Figure S5: The structural transformation of IrO x under a potential regime: V. IrO x was grown in D 2 O (.5 M H 2 SO 4, 98% D 2 O) by cycling from. to 1.5 V for 2 cycles at 1 mv/s. The Raman collection was carried out using 2 s exposures averaged over five collections. A 78 nm laser source was used with 15 mw laser power was incident upon an area of.7 µm 2. 11

12 In te n s ity (a.u.) α β γ δ ε R a m a n s h ift (c m -1 ).4 V.8 V 1. V 1.2 V 1.4 V 1.6 V 1.8 V 2. V Intensity (a.u.) Raman shift (cm -1 ).4 V.8 V 1. V 1.2 V 1.4 V 1.6 V 1.8 V 2. V Figure S6: The structural transformation of IrO x under a potential regime:.8-2. V. IrO x was grown in H 2 O 18 (.5 M H 2 SO 4, 98% H 2 O 18 ) by cycling from. to 1.5 V (1 cycles at 1 mv/s). The γ and ɛ peaks redshift with increasing potential (dashed red guidelines), also discernible through disappearance of the gap between β and γ peaks. The Raman collection was carried out using 2 s exposures averaged over five collections at 24 mw incident upon an area of.7 µm 2. The Raman data was smoothed using Savitsky Golay filtering over window size of 5 points. The peak around 3 cm 1 at 1.6 V is an artifact of spectral smoothing. The unprocessed (raw) data are provided in the figure at right. 12

13 3. UV spectra from calculations Ir(OH) 6 2- Ir(OH) Intensity (a.u).5 Intensity (a.u) Wavelength (nm) Wavelength (nm).15 Dimer Ir 4+ - Ir 4+.1 Intensity (a.u) Wavelength (nm) Figure S7: Shows the computed UV-VIS spectra of Iridium oxo-hydroxo monomer and dimer complexes. The UV-VIS spectra of monomers with Ir 4+ and Ir 3+ are essentially colorless. The dimer shows a strong band at 5 nm which arises from a Ir Ir charge transfer band. The difference density plot is provided in the main text. 13

14 4. Raman spectra from calculations 7 6 Dimer Ir(III)-Ir(IV) Intensity Raman Shift [cm 1 ] 615 cm cm -1 Figure S8: Shows the computed Raman spectrum of Iridium oxo-hydroxo dimer complex with one Ir 3+ and other Ir 4+. The calculation resulted in the movement of hydrogen atom from one of the terminal waters to bridging oxygens. The most intense vibrations are shown. The 642 cm 1 vibration involves the motion of µ oxo oxygen atom that is not bound to H atom. The arrows show the magnitude of atomic movement in a particular vibrational mode. 14

15 7 Ir(IV)-Dimer Intensity Raman Shift [cm 1 ] 676 cm cm -1 Figure S9: Shows the computed Raman spectrum of Iridium oxo-hydroxo dimer complex. The most intense vibrations are shown. These vibrations involve significant bending movement of the OH group from the water molecules. The arrows show the magnitude of atomic movement in a particular vibrational mode. The involvement of µ oxo moieties is minimal in these modes. 15

16 Intensity Timer Ir(IV)-Ir(III)-Ir(IV) Raman Shift [cm 1 ] 685 cm cm -1 Figure S1: Shows the computed Raman spectrum of Iridium oxo-hydroxo trimer complex that has Ir 4+ -Ir 3+ -Ir 4+ centers. The calculation resulted in the movement of hydrogen atom from one of the terminal waters to bridging oxygens. The most intense vibrations are shown. The spectrum has one major band composed of Ir-µ oxo stretches for the oxygen atom not bound to H atom. The arrows show the magnitude of atomic movement in a particular mode. 16

17 7 Ir(IV)-Trimer Intensity Raman Shift [cm 1 ] 486 cm cm cm cm -1 Figure S11: Shows the computed Raman spectrum of Iridium oxo-hydroxo trimer complex. The Raman spectrum computed for the trimer shows a bimodal distribution of bands as observed in the experiments. The vibrations that contribute the most to the Raman spectra are shown in the figure. The arrows show the magnitude of atomic movement in a particular mode. The strongest Raman intensities correspond to vibrations that involve the strong movement of µ oxo bonds, often coupled to the movement of hydroxides (O-H bending modes). 17

18 7 Ir(IV)-Ir(V)-Ir(IV) Trimer, Quintet state Intensity Raman Shift [cm 1 ] 57 cm cm cm cm -1 Figure S12: Shows the computed Raman spectrum of Iridium oxo-hydroxo trimer complex oxidized by one electron. So instead of three Ir 4+, the complex has two Ir 4+ and one Ir 5+. Quintet state was calculated to be the most stable electronic configuration. The Raman spectrum computed for the trimer still shows a bimodal distribution of bands although slightly blueshifted compared to the unoxidized trimer. The vibrations that contribute most to the Raman spectra are shown in the figure. The arrows show the magnitude of atomic movement in a particular mode. Similar to the unoxidized trimer, the strongest Raman intensities correspond to vibrations that involve the strong movement of µ oxo bonds. 18