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1 An electrodeposited inhomogeneous metal insulator semiconductor junction for efficient photoelectrochemical water oxidation James C. Hill, Alan T. Landers, Jay A. Switzer * Missouri University of Science & Technology, Department of Chemistry and Graduate Center for Materials Research, Rolla, MO , USA. *Correspondence to: jswitzer@mst.edu NATURE MATERIALS 1

2 Photoelectrochemical efficiency calculations The fill factor is FF, the current density at the maximum power point is J mpp, the potential at the maximum power point is V mpp, the open circuit voltage is V oc, the short circuit current density is J sc, and the power input is P in. The fill factor is determined by finding the potential and current at which the maximum power output is generated from the device. The power input is 100 mw cm -2. The photovoltage is calculated by taking the difference between the voltage of n- Si/SiO x /Co/CoOOH in the light and the p ++ -Si/Co/CoOOH in the dark. The limiting current density is the short circuit current density. The maximum power point occurs at 31 ma cm -2 and 0.33 V. The short circuit current density is 35 ma cm -2 and the open circuit voltage is 0.47 V. Thus, the fill factor is 0.62 (Eq. S1) and the efficiency is 10.2 % (Eq. S2). If the photovoltage is calculated against the thermodynamic potential for water oxidation in 1 M KOH ( V vs. Ag/AgCl), the maximum power point occurs at 6.2 ma cm -2 and 0.06 V, the short circuit current density is 13 ma cm -2 and the open circuit potential is 0.20 V. Thus, the fill factor is 0.14 and the efficiency is 0.4 %. FF = Jmpp x V mpp (Eq. S1) Jsc x V oc FF x V oc sc % efficiency = (Eq. S2) P in x J Solid-state measurement calculations The Mott-Schottky equation is shown below as Eq. S3. Capacitance is C (F cm -2 ), the charge of an electron is q (1.60 x C), the vacuum permittivity is! o (8.85 x F cm -1 ), the permittivity of silicon is! s (1.05 x F cm -1 ), area is A, donor density is N D, the applied bias is V, the flat band voltage is V fb, Boltzmann s constant is k (1.38 x J K -1 ), and temperature 2 2 NATURE MATERIALS

3 SUPPLEMENTARY INFORMATION is T (294 K). The x-intercept of the Mott-Schottky plot is reached at the bias that needs to be applied to cause the bands to become flat. Also, the slope of the plot can be used to calculate the donor density of the electrode. The x-intercept plus kt/q (0.025 V) equals the flat band voltage, which is 0.66 V for the solid state cell with 10 mc cm -2 of cobalt electrodeposited on n-silicon. 1 C 2 = 2 q" s" o A 2 N D ( V! V fb! kt / q) (Eq. S3) The donor density (N D ) of the silicon wafer is calculated from Eq. S4 using the charge of an electron as q (1.60 x C), the electron mobility as µ (1500 cm 2 V -1 s -1 ), and the resistivity as ". A 4-point probe was used to measure the resistivity of the silicon wafers prior to scoring and breaking the wafers. The (100) n-silicon wafer used in this study had a resistivity of 2.3 # cm and a donor density of 1.8 x cm -3 (Eq. S4). N D 1 = (Eq. S4) qµ! Interface surface state density calculation Eq. S5 was used to estimate the density of surface states in the n-si/sio x /Co/CoOOH heterojunction 1. The expression was developed by Card and Rhoderick for metal-insulatorsemiconductor (MIS) solar cells. The following are constants used in this equation; the permittivity of silicon dioxide is! i (3.45 x F cm -1 ), the permittivity of silicon is! s (1.05 x F cm -1 ), and the charge of an electron is q (1.60 x C). The following variables were calculated: the diode quality factor is n (1.12), the thickness of silicon dioxide is $ (5 Å), the flat band voltage is V fb (0.66 V), and the donor density is N D (1.8 x cm -3 ). This shows that the density of surface states is dependent on the diode quality factor and the thickness of the insulating layer, in this case silicon dioxide. The density of surface states for the photoanode with 10 mc cm -2 of cobalt electrodeposited onto (100) n-silicon is 5.07 x states cm -2 ev NATURE MATERIALS 3

4 This low surface state density is consistent with minimal electron-hole recombination at midgap states. D ( n# 1)! i # " 2! sv q! s q N fb D = (Eq. S5) ss References: 1. Card, H. C., & Rhoderick, E. H. Studies of tunnel MOS diodes I. Interface effects in silicon Schottky diodes. J. Phys. D: Appl. Phys. 4, (1971). 2. Prabhakaran, K., Sumitomo, K., & Ogino T. Diffusion mediated chemical reaction in Co/Ge/Si(100) forming Ge/CoSi 2 /Si(100). Appl. Phys. Lett. 68, (1996). 3. Hwang, I. Y., Kim, J. H., Oh, S. K., Kang, H. J., & Lee Y. S. Ultrathin cobalt silicide film formation on Si(100). Surf. Interface Anal. 35, (2003). 4 4 NATURE MATERIALS

5 SUPPLEMENTARY INFORMATION Figure S1 Light attenuation and reflectance by the catalyst. UV-vis-NIR a, specular reflectance measurements of n-si (black), n-si/co (blue), and n-si/co/coooh (red); and b, transmittance measurements of FTO (black), FTO/Co (blue), and FTO/Co/CoOOH (red). The dashed vertical line represents the bandgap of silicon. All films had 10 mc cm -2 of cobalt electrodeposited. The Co is thin enough to be transparent, and the n-si/sio x /Co/CoOOH electrode shows lower reflectivity in the visible region than bare Si. 5 NATURE MATERIALS 5

6 Figure S2. Surface characterization of activated photoanode by Raman spectroscopy. Raman spectrum of the cobalt surface after photo-oxidation in 1 M KOH at 100 mw cm -2 AM 1.5 irradiation. The spectrum shows a surface layer of CoOOH. 6 6 NATURE MATERIALS

7 SUPPLEMENTARY INFORMATION Figure S3. Characterization of the n-si/co interface by XPS. a, X-ray photoelectron spectroscopy measurements of Si 2p comparing as-deposited n-si/co (10 mc cm -2 ) before (black), and after (red) dissolution of the cobalt layer in 5 M sulfuric acid. b, X-ray photoelectron spectroscopy measurements of Co 2p comparing as-deposited n-si/co before (black), and after (red) dissolution of the cobalt layer in 5 M sulfuric acid. These results are consistent with a layer of SiO x atop silicon, and show no evidence of a silicide layer NATURE MATERIALS 7

8 Figure S4. TEM cross-sections of the n-si/co interface. Cross-section TEM images of as-deposited n- Si/SiO x /Co thin films with a 200 mc cm -2 and b 10 mc cm -2 of cobalt. Cobalt islands are evident in the image (b) of the sample with 10 mc cm -2 of cobalt. 8 NATURE MATERIALS

9 SUPPLEMENTARY INFORMATION Figure S5 Measurements of the catalytic activity of p + -Si/SiO x /Co/CoOOH for the oxygen evolution reaction. a, Enhancement of the catalytic activity of degenerate Si (p + -Si) for water oxidation by the deposition of a thin Co/CoOOH layer. Linear sweep voltammetry of bare p + -Si (black) and p + -Si/SiO x /Co/CoOOH (red) at a 10 mv s -1 scan rate in 1 M KOH. b, Table of overpotential values necessary for p + -Si/SiO x /Co/CoOOH to generate the given current densities for water oxidation in 1 M KOH. c, Tafel plot of a p + -Si/SiO x /Co/CoOOH thin film at a 1 mv s -1 scan rate in 1 M KOH. NATURE MATERIALS 9

10 Figure S6 Photoelectrochemical characterization of n-si/sio x /Co/CoOOH in 1 M KOH and ph 9 borate buffer. a, Chopped light linear sweep voltammetry under 100 mw cm -2 AM 1.5 irradiation in 1 M KOH at a 10 mv s -1 scan rate. b, Linear sweep voltammetry under 100 mw cm -2 AM 1.5 irradiation in borate buffer at ph 9 at a 10 mv s -1 scan rate. The vertical line in b corresponds to the thermodynamic potential for water oxidation at ph NATURE MATERIALS

11 SUPPLEMENTARY INFORMATION Figure S7 Stability measurements of n-si/sio x /Co/CoOOH. a, Normalized photocurrent stability measurements at (red) 1.3 V vs. Ag/AgCl in borate buffer at ph 9 and (black) 1.0 V vs. Ag/AgCl in 1 M KOH to under 100 mw cm -2 AM 1.5 irradiation. b, Electrochemical stability measurements of Au/Co/CoOOH at 1.3 V vs. Ag/AgCl in borate buffer at ph 9 (red) and at 0.7 V vs. Ag/AgCl in 1 M KOH (black). Note that even the Au/Co/CoOOH electrode shows a steady decrease in activity in 1 M KOH. The improved stability at ph 9 for n-si/sio x /Co/CoOOH and Au/Co/CoOOH suggests cobalt dissolution in 1 M KOH. c, 5-day photocurrent stability measurement at 1.2 V vs. Ag/AgCl in borate buffer at ph 9 under 100 mw cm -2 AM 1.5 irradiation. 11 NATURE MATERIALS 11

12 Figure S8 Characterization of an n-si/sio x /Co/CoOOH photoanode after device failure. a, SEM image of n-si/sio x /Co/CoOOH photoanode after photocurrent significantly decreased in 1 M KOH at 100 mw cm -2 AM 1.5 irradiation. The quantity and density of the cobalt islands has decreased significantly due to dissolution or conversion to the CoOOH platelets. b, Cross-section TEM of the same film showing a region where all of the Co has been converted to CoOOH forming an n- Si/SiO x /CoOOH junction with a significantly thicker SiO x layer. 12 NATURE MATERIALS

13 SUPPLEMENTARY INFORMATION Figure S9 Solar water oxidation efficiency determination using the thermodynamic standard reduction potential. a, Linear sweep voltammetry of n-si/sio x /Co/CoOOH at 100 mw cm -2 AM 1.5 irradiation in 1.0 M KOH. The dashed vertical line represents the standard reduction potential for oxygen evolution. b, J-V plot of n-si/sio x /Co/CoOOH at 100 mw cm -2 AM 1.5 irradiation with 0 V bias equivalent to the standard reduction potential of water oxidation. NATURE MATERIALS 13

14 List of Symbols and Units A Area cm 2 A* Richardson s constant 120 A cm -2 K -2 C Capacitance F cm -2 D ss Density of surface states cm -3 FF Fill factor J Current density A cm -2 J L Limiting current density A cm -2 J mpp Current density at maximum power point A cm -2 J s Saturation current density A cm -2 J sc Short circuit current density A cm -2 k Boltzmann s constant 1.38 x J K -1 n Diode quality factor N C Density of states in the conduction band 2.8 x cm -3 N D Donor density cm -3 P in Power input W cm -2 q Charge of an electron 1.60 x C R s Series resistance # T Temperature 294 K V Potential or voltage V V fb Flat band voltage V V mpp Voltage at maximum power point V V n Conduction band position Fermi level ev 14 NATURE MATERIALS

15 SUPPLEMENTARY INFORMATION V oc Open circuit voltage V $ Oxide thickness Å! i Silicon dioxide permittivity 3.45 x F cm -1! s Silicon permittivity 1.05 x F cm -1! o Permittivity in vacuum 8.85 x F cm -1 µ Electron mobility in silicon 1500 cm 2 V -1 s -1 % bh Barrier height ev " Resistivity # cm NATURE MATERIALS 15