Electronic Supplementary Information

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1 Electronic Supplementary Material (ESI) for Sustainable Energy & Fuels. This journal is The Royal Society of Chemistry 2018 Electronic Supplementary Information Proton coupled electron transfer from Co 3 O 4 nanoparticles to photogenerated Ru(bpy) 3 3+ : base catalysis and buffer effect. Giulia Alice Volpato, a Alessandro Bonetto, b Antonio Marcomini, b Pierre Mialane, c Marcella Bonchio, a Mirco Natali* d and Andrea Sartorel* a a. Department of Chemical Sciences University of Padova and Institute on Membrane Technology, Unit of Padova, via F. Marzolo 1, Padova, 35131, Italy. b. Dept. Environmental Sciences, Informatics and Statistics, University Ca' Foscari Venice, Vegapark, Via delle Industrie 21/8, Marghera, Venice, Italy. c. Institut Lavoisier de Versailles, UMR 8180, Université Paris-Saclay, Université de Versailles Saint-Quentin en Yvelines, 45 Avenue des Etats-Unis Versailles cedex, 78035, France. d. Department of Chemical and Pharmaceutical Sciences, University of Ferrara, Via L. Borsari 46, Ferrara, 44121, Italy.

2 Figure S1. Transmission Electron Microscopy image of cubic Co 3 O 4 NPs. Average NP size: 5±1 nm. Figure S2. Powder X-ray diffraction pattern of Co 3 O 4 NPs: cubic spinel structure, F d -3 m space group. Source: Co Kα 1 (λ = Å).

3 450 nm Figure S3. Cyclic voltammetry for Co 3 O 4 NPs deposited onto glassy carbon, performed at ph 8 in different buffers. Working electrode: Glassy carbon (0.07 cm 2 ); counter electrode: Pt (0.02 cm 2 ); reference electrode: Ag/AgCl (3M NaCl); scan rate: 100 mv/s. Figure S4. ATR-IR of Co 3 O 4 NPs Figure S5. Laser flash photolysis ( exc = 355 nm) of 50 M Ru(bpy) 3 2+ in the presence of 5 mm Na 2 S 2 O 8 and 100 M CoFe 2 O 4 NPs in 10 mm borate buffer at ph 8.

4 k 0 (s -1 ) k obs (s -1 ) nm (a) mm 10 mm 20 mm 30 mm 40 mm (b) pd [Base] (M) Figure S6. (a) Laser flash photolysis ( exc = 355 nm) of 50 M Ru(bpy) 3 2+ in the presence of 5 mm Na 2 S 2 O 8 and 100 M Co 3 O 4 NPs in 4-30 mm borate buffer at pd 8.6. (b) plot of k obs vs [Base] [OH - ] ( M) Figure S7. Plot of k 0 vs [OH - ]. See table 1 in the main text for k 0 values, determined from flash photolysis experiments in borate buffer, ph

5 k obs (s -1 ) nm nm (a) (b) mm 2.5 mm 5 mm 7.5 mm 10 mm mm 2.5 mm 5 mm 7.5 mm 10 mm (c) 30 ph 8.0 ph [Base] (M) Figure S8. (a) laser flash photolysis ( exc = 355 nm) of 50 M Ru(bpy) 3 2+ in the presence of 5 mm Na 2 S 2 O 8 and 100 M Co 3 O 4 NPs in 1-10 mm phosphate buffer at ph 8.0. (b) Laser flash photolysis ( exc = 355 nm) of 50 M Ru(bpy) 3 2+ in the presence of 5 mm Na 2 S 2 O 8 and 100 M Co 3 O 4 NPs in 1-10 mm phosphate buffer at ph 8.9. (c) Plot of k obs vs [Base].

6 450 nm mm 25 mm 50 mm 100 mm Figure S9. Laser flash photolysis ( exc = 355 nm) of 50 M Ru(bpy) 3 2+ in the presence of 5 mm Na 2 S 2 O 8 and 100 M Co 3 O 4 NPs at ph 8.0, in mm carbonate buffer.

7 nm nm nm (a) mm 100 mm 200 mm 400 mm (b) mm 100 mm 200 mm 400 mm (c) mm 100 mm 200 mm 400 mm Figure S10. Laser flash photolysis ( exc = 355 nm) of 50 M Ru(bpy) 3 2+ in the presence of 5 mm Na 2 S 2 O 8 and 100 M Co 3 O 4 NPs at: (a) ph 8.0, in mm borate buffer; (b) ph 8.5, in mm borate buffer; (c) ph 8.9, in mm borate buffer.

8 Table S1. Ru(III) lifetime ( and associated kinetic rate constants (k) for the flash photolysis experiments conducted at different ph and borate buffer concentration. For the sake of comparison, (and derived k values) are reported from: (i) experimentally determination at half recovery of the OD 450 in the experimental traces (values used in the discussion in the main text) and (ii) from mono or bi-exponential fittings. Bi-exponential fittings converged into a monoexponential fitting, unless in the cases indicated by footnote (b). Uncertainty in the values determined at the half recovery of the OD traces is estimated as 10 or 15% of the value; uncertainty in the values determined by exponential fitting is provided by the fitting (see traces below). Determined at the half recovery of the Determined from exponential fittings OD traces buffer (s) k obs (s -1 ) (s) k obs (s -1 ) Borate, ph 8 4 mm 0.254±0.038 (a) 2.73± ± ± mm 0.186±0.028 (a) 3.72± ± ± mm 0.147±0.022 (a) 4.71± ± ± mm 0.160± ± ± ± mm 0.151± ± ± ± mm 0.218± ± ± ± mm 1.280± ± mm Borate, ph mm 0.191±0.029 (a) 3.63± ± ± mm 0.121±0.018 (a) 5.73± ± ± mm 0.083± ± ± ± mm 0.066± ± ± ± mm 0.059± ± ± ± mm 0.065± ± ± ± mm 0.184± ± ± ± mm 0.326± ± ± ±0.03 Borate, ph mm 0.058± ± ± ± mm 0.053± ± ± (b) 12.42± mm 0.034± ± ± (b) 17.77± mm 0.026± ± ± (b) 16.78± mm 0.020± ± ± (b) 20.32± mm 0.016± ± ± (b) 24.41± mm 0.020± ± ±0.001 (b) 12.83± mm 0.023± ± ±0.001 (b) 9.37±0.13 Borate, ph mm 0.037± ± ± ± mm 0.033± ± ± ± mm 0.022± ± ± ± mm 0.020± ± ± ± mm 0.017± ± ± ± mm 0.012± ± ± ± mm 0.029± ± ± ± mm 0.040± ± ± ±0.58 (a) determined from a linear fitting of the traces; (b) determined from a weighted average of the two values from a bi-exponential fitting.

9 k obs (s -1 ) Table S2. Kinetic rate constants for PCET from Co 3 O 4 NPs to Ru(III)(bpy) 3 3+ in flash photolysis experiments in borate buffer, determined from: (i) experimentally determination at half recovery of the OD 450 in the experimental traces (values used in the discussion in the main text) and (ii) from mono or bi-exponential fittings, see Table S1. Determined plotting k obs (determined at the half recovery of the OD traces) Determined plotting k obs (from exponential fittings) ph k 0 (s -1 ) k B /100 (M -1 s -1 ) k 0 (s -1 ) k B /100 (M -1 s -1 ) ± ± ± ± ± ± ± ± [Base] (M) Figure S11. Plot of k obs (from exponential fittings) vs. [Base], where Base is B(OH) 4 -, and the concentrations are given by [Base]=f B [buffer], where f B is the fraction of the basic form of the buffer in solution, and [buffer] is the total buffer concentration; pk a for the H 3 BO 3 /B(OH) 4 - couple = 8.6.

10 4 mm borate, ph 8 Fit of Sheet1 B"" Reduced Chi-S 3,9969E-5 Adj. R-Square 0,69357 A ,86762E-4 t1 0, k 2, tau 0, mm borate, ph 8 Fit of Sheet1 C"" Reduced Chi-S 4,37123E-5 Adj. R-Square 0,72317 A ,08372E-4 t1 0, k 3, tau 0, mm borate, ph 8 Fit of Sheet1 D"" Reduced Chi-S 4,56054E-5 Adj. R-Square 0,78874 A ,1972E-4 t1 0, k 4, tau 0,

11 Reduced Chi-S 30 mm borate ph 8 Fit of Sheet1 E"" 5,23645E-5 Adj. R-Square 0,71957 Value Standard Error A ,38459E-4 t1 0, k 3, tau 0, mm borate ph 8 Fit of Sheet1 F"" Reduced Chi-S 4,82491E-5 Adj. R-Square 0,75096 A ,27574E-4 t1 0, k 4, tau 0, mm borate ph 8 Fit of Sheet1 G"" Reduced Chi-S 5,16069E-5 Adj. R-Square 0,57608 A ,32593E-4 t1 0, k 2, tau 0,

12 mm borate ph mm borate ph mm borate ph 8.5 Fit of Sheet1 B"" Reduced Chi-S 4,62426E-5 Adj. R-Square 0,72847 A ,14215E-4 t1 0, k 3, tau 0,

13 10 mm borate ph 8.5 Fit of Sheet1 C"" Reduced Chi-S 4,74166E-5 Adj. R-Square 0,8311 A ,35063E-4 t1 0, k 5, tau 0, mm borate ph 8.5 Fit of Sheet1 D"" Reduced Chi-S 5,07253E-5 Adj. R-Square 0,87981 A ,70707E-4 t1 0, ,52144E-4 k 8, tau ,59976E mm borate ph 8.5 Fit of Sheet1 E"" Reduced Chi-S 6,10146E-5 Adj. R-Square 0,8782 A ,1147E-4 t1 0, ,60556E-4 k 9, tau ,96492E-4

14 40 mm borate ph 8.5 Fit of Sheet1 F"" Reduced Chi-S 5,87041E-5 Adj. R-Square 0,88211 A ,05982E-4 t1 0, ,1276E-4 k 9, tau ,63362E mm borate ph 8.5 Fit of Sheet1 G"" Reduced Chi-S 5,7603E-5 Adj. R-Square 0,86432 A ,00387E-4 t1 0, ,03847E-4 k 9, tau ,26499E mm borate ph 8.5 Fit of Sheet1 H"" Reduced Chi-S 5,36572E-5 Adj. R-Square 0,51498 A ,41395E-4 t1 0, k 3, tau 0,

15 Reduced Chi-S 400 mm borate ph 8.5 Fit of Sheet1 I"" 6,09919E-5 Adj. R-Square 0,26749 Value Standard Error A ,52682E-4 t1 0, k 1, tau 0, mm borate ph 8.9 ExpGro1 Fit of Sheet1 B"" ExpGro1 y = A1*exp(x/t1) + y0 Reduced Chi-S 5,23739E-5 Adj. R-Square 0,94858 A ,93145E-4 t ,69435E-4 k -11, tau ,25387E-4 10 mm borate ph 8.9 ExpDec2 Fit of Sheet1 C"" ExpDec2 y = A1*exp(-x/t1) + A2*exp(-x/t2) + y0 Reduced Chi-S 4,89018E-5 Adj. R-Square 0,91415 A t1 03 4,15718E-4 A ,75532E-4 t ,36789E-4 k1 333, ,23753 k2 10, tau ,88154E-4 tau ,49332E-4

16 20 mm borate ph 8.9 ExpDec2 Fit of Sheet1 D"" ExpDec2 y = A1*exp(-x/t1) + A2*exp(-x/t2) + y0 Reduced Chi-S 5,19071E-5 Adj. R-Square 0,91834 A t1 02 2,72105E-4 A ,62273E-4 t ,29749E-4 k1 499, ,96124 k2 13, ,11952 tau ,88609E-4 tau ,36509E-4 30 mm borate ph 8.9 ExpDec2 Fit of Sheet1 E"" ExpDec2 y = A1*exp(-x/t1) + A2*exp(-x/t2) + y0 Reduced Chi-S 5,62441E-5 Adj. R-Square 0,90167 A t ,13479E-4 A t k1 171, ,93948 k2 13, ,20869 tau ,25232E-4 tau ,44485E-4 40 mm borate ph 8.9 ExpDec2 Fit of Sheet1 F"" ExpDec2 y = A1*exp(-x/t1) + A2*exp(-x/t2) + y0 Reduced Chi-S 5,95848E-5 Adj. R-Square 0,90466 A t ,83135E-4 A t ,17857E-4 k1 16, ,23065 k2 204, ,94364 tau ,12143E-4 tau ,28266E-4

17 mm borate ph 8.9 ExpDec2 Fit of Sheet1 G"" ExpDec2 y = A1*exp(-x/t1) + A2*exp(-x/t2) + y Reduced Chi-S 6,2711E-5 Adj. R-Square 0,88875 A t A t k1 17, ,66285 k2 100, ,68383 tau tau ,33278E mm borate ph 8.9 ExpDec2 Fit of Sheet1 H"" -4-6 ExpDec2 y = A1*exp(-x/t1) + A2*exp(-x/t2) + y Reduced Chi-S 6,11535E-5 Adj. R-Square 0,78676 A t ,89855E-4 A ,3447E-4 t2 0, k1 179, ,50864 k2 8,0774 0,23065 tau ,70227E-4 tau mm borate ph 8.9 ExpDec2 Fit of Sheet1 I"" ExpDec2 y = A1*exp(-x/t1) + A2*exp(-x/t2) + y0 Reduced Chi-S 5,87994E-5 Adj. R-Square 0,74435 A t ,71857E-4 A ,77146E-4 t2 0, k1 147, ,19946 k2 5, ,24115 tau ,27066E-4 tau2 0,

18 5 0 4 mm borate ph 9.5 Fit of Sheet1 B"" Reduced Chi-S 1,73642E-4 Adj. R-Square 0,95258 A ,84248E-4 t ,49911E-4 k 23, ,13769 tau ,73225E-4 10 mm borate ph 9.5 Fit of Sheet1 C"" Reduced Chi-S 1,3262E-4 Adj. R-Square 0,9596 A ,07509E-4 t ,12613E-4 k 26, ,1444 tau 266 1,47372E mm borate ph 9.5 Fit of Sheet1 D"" Reduced Chi-S 1,32554E-4 Adj. R-Square 0,95578 A ,35192E-4 t ,80451E-4 k 36,0445 0,23444 tau ,25079E-4

19 5 30 mm borate ph 9.5 Fit of Sheet1 E"" Reduced Chi-S 1,44667E-4 Adj. R-Square 0,95403 A t ,67115E-4 k 41, ,29276 tau ,15835E mm borate ph 9.5 Fit of Sheet1 F"" Reduced Chi-S 1,97042E-4 Adj. R-Square 0,93819 A t ,73911E-4 k 51, ,46823 tau ,20546E mm borate ph 9.5 Fit of Sheet1 G"" Reduced Chi-S 1,38376E-4 Adj. R-Square 0,93582 A t ,41735E-4 k 69, ,68118 tau 1 9,82432E-5 5

20 200 mm borate ph 9.5 Fit of Sheet1 H"" Reduced Chi-S 6,87994E-5 Adj. R-Square 0,94701 A ,43888E-4 t ,83892E-4 k 20, ,12254 tau ,96779E mm borate ph 9.5 Fit of Sheet1 I"" Reduced Chi-S 9,44334E-5 Adj. R-Square 0,68027 Value Standard Error A ,96901E-4 t1 0, k 7, ,10916 tau