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1 SUPPLEMETARY IFORMATIO DOI: /CHEM.1874 Time-Resolved Observations of Water Oxidation Intermediates on a Cobalt Oxide anoparticle Catalyst Miao Zhang, Moreno de Respinis, and Heinz Frei * Physical Biosciences Division, Lawrence Berkeley ational Laboratory, University of California, Berkeley, CA HMFrei@lbl.gov ATURE CHEMISTRY 1

2 DOI: /CHEM.1874 SUPPLEMETARY IFORMATIO Table of contents Supplementary Scheme S1. Method of visible light sensitization of Co3O4 nanoparticle catalyst for water oxidation using Ru(bpy)32+ as the sensitizer and S2O82- as electron acceptor. 3 Supplementary Figure S1. FT-IR spectra of Ru(bpy)32+, Ru(bpy)33+ and the change of the Ru(bpy)33+ spectra when Co3O4 nanoparticles were added 4 Supplementary Figure S2. Rapid-scan FT-IR spectra of Co(III)-(OO)--Co(III) intermediate in isotopic water solution with labeled [Ru(bpy-d8)3]2+ sensitizer 5 Supplementary Figure S3. Survey difference FT-IR spectra of [Ru(bpyh8)3]2+/S2O82- sensitized water oxidation by Co3O4 in H2O 6 Supplementary Figure S4. Mass spectroscopic monitoring of oxygen gas evolution in H218O 7 Supplementary Figure S5. Electrochemical monitoring of O2 evolution in D2O with 300 ms and 1 s laser pulse 7 Supplementary Figure S6. Electrochemical monitoring of O2 evolution in H2O with 300 ms, 10 s and 20 s laser pulse 8 Supplementary Figure S7. Kinetics of Co(III)-OO(-)-Co(III) intermediate (995 cm-1 and 966 cm-1) upon water oxidation in H218O 8 Supplementary Figure S8. Rapid-scan FT-IR spectra of 840 cm-1 Co(IV)=O intermediate in isotopic water solution with labeled [Ru(bpy-d8)3]2+ as sensitizer 9 Supplementary Figure S9. Kinetics of 840 cm-1 Co(IV)=O intermediate upon water oxidation in H216O and H218O 10 Supplementary Figure S10. FT-IR spectra of Co-O modes of Co3O4 catalyst nanoparticles before and after treatment with H218O 10 Supplementary Table S1. FT-IR spectra of sensitizer and acceptor species 11 Deconvolution method of infrared bands 11 References 12 2 ATURE CHEMISTRY

3 SUPPLEMETARY IFORMATIO DOI: /CHEM.1874 * Ru2+ S2O82- Ru(bpy)32+ SO42- + SO4 Ru2+ Ru(bpy)33+ SO42- Ru3+ Co3O4 H + + O2 H 2O Supplementary Scheme S1. Process of visible light sensitization of Co3O4 catalyst nanoparticles by 476 nm excitation of [Ru(bpy)3]2+ in the presence of S2O82- electron acceptor1. Excitation of [Ru(bpy)3]2+ and transfer of an electron to S2O82- generates an oxidized [Ru(bpy)3]3+ species with a potential of V(SHE)2. Sequential hole transfer to Co3O4 upon collision with the nanoparticles results in the oxidation of water. While [Ru(bpy)3]3+ is known to be unstable in basic (ph 8) solution due to OHnucleophilic attack on the bpy carbon3, the rate constant of 10 L mol-1 s-1 is too small for resulting in noticeable impact of this reaction on the hundreds of millisecond time scale of relevance for the time resolved experiments reported here. Only [Ru(bpy)3]3+ species is expected to inject electrons into Co3O4 particles, not SO4- radical anions, as shown experimentally in previous work4. This is because the concentration of the [Ru(bpy)3]2+ sensitizer, with which the radical reacts very efficiently (SO4- has a potential of V (SHE)), exceeds the concentration of Co3O4 particles by 3 to 4 orders of magnitude. Furthermore, the Co3O4 particle surface is negatively charged at the ph used, thus favoring reaction with the positively charged sensitizer. Using the characteristic bpy ligand infrared bands of [Ru(bpy)3]2+ and [Ru(bpy)3]3+, Supplementary Figure S1 shows the conversion of oxidized to reduced sensitizer in the presence of Co3O4 catalyst, but not if no catalyst is present. While we do not have direct experimental evidence for the absence of any hole injection from SO4- into Co3O4 (for precedents see Refs. 5-7), the fact that the B1013 cm-1 intermediate continues to grow in the time interval 450 ms 1350 ms after termination of the laser pulse under concurrent consumption of [Ru(bpy)3]3+ (decrease of 1496 cm-1 band in Figure 3 and Figure S7) confirms that hole injection from [Ru(bpy)3]3+ is responsible for the growth of the B1013 cm-1 intermediate. In the same time interval, the SO42- band at 980 cm-1 (reduction product of SO4-) does not increase, confirming that no concurrent hole injection from SO4- radical anion takes place. 3 ATURE CHEMISTRY

4 DOI: /CHEM.1874 SUPPLEMETARY IFORMATIO c) Ru(bpy) Co 3 O 4 Absorbance b) Ru(bpy) 3 3+ a) Ru(bpy) Wavenumber (cm -1 ) Supplementary Figure S1. a) FT-IR spectrum of [Ru(bpy) 3 ] 2+ /as 2 O 8 in H 2 O. The mixture of [Ru(bpy) 3 ] 2+ and a 2 S 2 O 8 was irradiated with a 5 s pulse of 476 nm laser to generate [Ru(bpy) 3 ] 3+. b) 0.1 ml pure H 2 O added to [Ru(bpy) 3] 3+ solution. c) 0.1 ml Co 3 O 4 suspension in H 2 O added to [Ru(bpy) 3 ] 3+ solution. While no change of the [Ru(bpy) 3 ] 3+ spectrum is observed upon addition of pure H 2 O, the [Ru(bpy) 3] 3+ bands decrease and [Ru(bpy) 3] 2+ bands grow in when adding Co 3 O 4 solution, confirming hole transfer from the oxidized sensitizer to the catalyst particle under reduction of the sensitizer. ATURE CHEMISTRY 4

5 DOI: /CHEM.1874 SUPPLEMETARY IFORMATIO a Absorbance th pulse 1 st pulse b c d Wavenumber/cm -1 Supplementary Figure S2. Rapid-scan FT-IR spectra of bands assigned to superoxide intermediate Co(III)-(OO) - -Co(III) in parent and isotopic water solution at 5850 ms after onset of the 300 ms photolysis laser pulse (476 nm, 160 mw). The sensitizer was [Ru(bpy-d 8 ) 3 ] 2+. a) H 2 16 O, b) H 2 18 O, c) D 2 O, d) control experiment in H 2 16 O, no Co 3 O 4 present. The dotted signal in this trace is due to unreacted [Ru(bpyd 8 ) 3 ] 3+. The red components are superoxide bands, the blue component is due to SO 4 2- growth. The deconvolution procedure is described below. The inset of b) shows the spectral trace after irradiation with 5 laser pulses, each of 300 ms duration. Samples were discarded after each pulse (except inset of b). ATURE CHEMISTRY 5

6 DOI: /CHEM.1874 SUPPLEMETARY IFORMATIO a) Wavenumber/cm ms 5850 ms b) Wavenumber/cm -1 Supplementary Figure S3. Survey difference FT-IR spectra of [Ru(bpyh 8 ) 3 ] 2+ /S 2 O 8 2- sensitized water oxidation by Co 3 O 4 in H 2 O after 300 ms photolysis pulse at 476 nm. (a) Spectral range cm -1 (no IR transmission above 3200 cm -1 in our ATR setup). (b) Expanded spectra for the fingerprint region. Black trace: 450 ms after start of pulse. Red trace: 5850 ms after start of pulse. All spectral assignments are presented in Supplementary Table S1, or are due to reaction intermediates B 1013 cm -1 or A 840 cm -1. ATURE CHEMISTRY 6

7 DOI: /CHEM.1874 SUPPLEMETARY IFORMATIO O 2 amount (µmol) O 2 16 O 18 O 16 O time (min) Supplementary Figure S4. Oxygen evolution in H 2 18 O suspension of Co 3 O 4 nanoparticle catalysts monitored by mass spectrometry. O 2 conc. (nmol/ml) photolysis pulse 1s 300 ms time (sec) c 1s 0.3c 1s Supplementary Figure S5. Oxygen evolution upon water oxidation in D 2 O catalyzed by visible light sensitized Co 3 O 4 nanoparticles monitored electrochemically with a Clark electrode. Black trace: The sample was irradiated with a laser pulse of 1 s duration pulse at 476 nm (160 mw). Red trace: The sample was irradiated with a 300 ms pulse at 476 nm laser (160 mw). The dashed section shows the missing O 2 in the 300 ms experiment, which is still be present as intermediates on the Co 3 O 4 surface. ATURE CHEMISTRY 7

8 DOI: /CHEM.1874 SUPPLEMETARY IFORMATIO O 2 conc. (nmol/ml) time of laser illumination 20 s 10 s 300 ms *0.015 = *0.03 = Time (sec) Supplementary Figure S6. Electrochemical monitoring of O 2 evolution in H 2 O solution by photolysis 476 nm pulse (160 mw) using Clark electrode. Black trace: Sample irradiated with a laser pulse of 20 s duration. Red trace: Sample irradiated with a 10 s pulse. Green trace: Sample irradiated with a 300 ms pulse. The yield of the 10 s pulse is within 1% of the yield expected from the 20 s result. By contrast, the yield of the 300 ms pulse falls short of the yield expected based on the 10 s pulse by 36% cm cm -1 Ru(bpy-h8) 3 3+, 1496 cm -1 Absorbance time (ms) Supplementary Figure S7. Temporal behavior of the infrared bands of 18 O 16 O (995 cm -1 ) and 18 O 18 O (966 cm -1 ) superoxide intermediate and oxidized sensitizer [Ru(bpyh 8 ) 3 ] 3+ (1496 cm -1 ) infrared bands upon visible light sensitized water oxidation at Co 3 O 4 catalyst in H 2 18 O. ATURE CHEMISTRY 8

9 DOI: /CHEM.1874 SUPPLEMETARY IFORMATIO in H 2 16 O 840 Absorbance in H 2 18 O in D 2 O control Wavenumber/cm -1 Supplementary Figure S8. Rapid-scan FT-IR spectra of A 840 cm -1 species assigned to Co(IV)=O in parent and isotopic water solution at 450 ms after onset of the 300 ms photolysis pulse (476 nm, 160 mw). The sensitizer was [Ru(bpy-d 8 ) 3 ] 2+. a) H 2 16 O, b) H 2 18 O, c) D 2 O, d) control experiment in H 2 16 O, no Co 3 O 4 present. The dotted peaks at 870, 856 and 839 cm -1 are due to unreacted [Ru(bpy-d 8 ) 3 ] 3+. Samples were discarded after each pulse. ATURE CHEMISTRY 9

10 DOI: /CHEM.1874 SUPPLEMETARY IFORMATIO Absorbance in H 2 16 O in H 2 18 O time (ms) Supplementary Figure S9. Temporal behavior of A 840 cm -1 intermediate assigned to Co(IV)= 16 O upon visible light sensitized water oxidation at Co 3 O 4 catalyst in H 2 18 O (red trace) and H 2 16 O (black trace). The sensitizer was [Ru(bpy-d 8 ) 3 ] 2+. The duration of the 300 ms laser pulse is indicated by the grey area. Co 3 O 4 without treatment Co 3 O 4 treated with H 18 2 O Absorbance Wavenumber/cm -1 Supplementary Figure S10. FT-IR spectra of Co-O modes of Co 3 O 4 catalyst nanoparticles before and after treated by stirring in H 2 18 O for 8 h under irradiation with 476 nm laser light (500 mw). The bands agree with literature reports on Co 3 O 4 infrared spectra 8. o frequency shifts are noted. Expected 18 O shifts are 28 cm -1 and 26 cm -1 for the 663 cm -1 and the 576 cm -1 bands, respectively, if isotopic exchange were to occur. ATURE CHEMISTRY 10

11 SUPPLEMETARY IFORMATIO DOI: /CHEM.1874 Supplementary Table S1. FT-IR spectra of [Ru(bpy-h8)3] and [Ru(bpy-d8)3] complexes in 2+ and 3+ state, and of S2O82- and SO42- ions in aqueous solution Ru(bpy-h8) (s) Ru(bpy-h8) (s) Ru(bpy-d8)3 2+ Ru(bpy-d8) (m) 1581 (s) 1548 (s) 1572 (s) 1465 (s) 1496 (m) 1521 (s) 1538 (m) 1445 (s) 1472 (m) 1462 (m) 1527 (m) 1424 (s) 1452 (s) 1313 (m) 1322 (m) 1332 (s) 1360 (s) 1270 (m) 1273 (m) 1286 (s) 1340 (m) 1245 (m) 1165 (m) 1432 (s) 1246 (m) 3+ S 2O (s) 1273 (s) 1167 (w) 1178 (w) 1068 (w) 1047 (m) 1025 (w) 1032 (w) 1025 (w) 1036 (m) 1002 (w) 1012 (m) 1010 (w) 1022 (m) 982 (m) 990 (s) 1124 (w) 1104 (s) 969 (w) 902 (m) SO (m) 980 (w) 962 (m) 862 (w) 868 (w) 870 (w) 773 (vs) 820 (m) 856 (w) 733 (s) 791 (w) 839(w) 659 (w) 726 (m) Deconvolution of infrared bands: Spectral deconvolution of the infrared bands in Figure 1 and Supplementary Figure S2 was conducted with software OPUS 6.5 (Bruker Optics). The procedure was as follows: With the sulfate band at 980 cm-1 clearly identified as a component contributing to the absorption profile (see Table S1), one other component was used for the superoxide band for experiments in H2O and D2O. For the experiment in H218O, it is to be expected that the red shift of the profile between the 1st and 5th pulse is caused by a change from partially labeled 18O16O to 18O18O fully labeled superoxide growth (because no unlabeled 16O16O superoxide and no unlabeled 16O2 product are observed, the assignment of the deconvoluted bands with maxima at 955 and 966 cm-1 to partially and fully labeled superoxide is the only possible explanation we can conceive of). With this minimal number of spectral components, the analysis affords the same explanation for the two isotopic sensitizer experiments (Figure 1 and Supplementary Figure S2), which serves as validation of the deconvolution result. 11 ATURE CHEMISTRY

12 DOI: /CHEM.1874 SUPPLEMETARY IFORMATIO References [1] Hara, M., Waraksa, C. C., Lean, J. T., Lewis, B. A., Mallouk, T. E. J. Phys. Chem. A 104, 5275 (2000). [2] CRC Handbook of Chemistry and Physics, 85 th ed.; Lide, D. R., Ed.; CRC: Boca Raton, FL, 2004; p [3] Ghosh, P.K., Brunschwig, B.S., Chou, M., Creutz, C., Sutin,. J. Am. Chem. Soc. 106, 4772 (1984). [4] Jiao, F., Frei, H. Energy Environ. Sci. 3, 1018 (2010). [5] La Ganga, G., Puntoriero, F., Campagna, S., Bazzan, I., Berardi, S., Bonchio, M., Sartorel, A., atali, M., Scandola, F. Faraday Disc. 155, 177 (2012). [6] atali, M., Orlandi, M., Berardi, S., Campagna, S., Bonchio, M., Sartorel, A., Scandola, F. Inorg. Chem. 51, 7324 (2012). [7] Sartorel, A., Bonchio, M., Campagna, S., Scandola, F. Chem. Soc. Rev. 42, 2262 (2013). [8] Tang, C. W.; Wang, C. B.; Chien, S. H. Thermochimica Acta 473, 68 (2008). ATURE CHEMISTRY 12