www.sciencemag.org/content/342/6160/836/suppl/DC1

Supplementary Materials for

High-Performance Silicon Photoanodes Passivated with Ultrathin Nickel

Films for Water Oxidation

Michael J. Kenney, Ming Gong, Yanguang Li, Justin Z. Wu, Ju Feng, Mario Lanza,

Hongjie Dai*

*Corresponding author. E-mail: hdai@stanford.edu

Published 15 November 2013, Science 342, 836 (2013)

DOI: 10.1126/science.1241327

This PDF file includes:

Materials and Methods

Figs. S1 to S13

1

Experiment Details Fabrication of electrodes: Nickel films (2 – 20 nm) were deposited on as-received phosphorous-doped [100] n-type silicon wafers (0.3 – 0.5 ohm·cm) from University Wafer by electron beam evaporation at a deposition rate of ~0.2 Å/s. Ohmic contact was made to the backside of the wafer by e-beam deposition of titanium (20 nm). Copper tape was used to contact the Ti on the backside for electrochemical experiments. Before experiments, both the Ti side and Ni side were cleaned with isopropanol and dried. The same procedure was followed to deposit nickel films on metallic, heavily arsenic-doped [100] n++ silicon wafers (0.001 – 0.005 ohm·cm) from University Wafer. Electrochemical characterization: Electrodes were analyzed in a square cell with a circular 0.38 cm2 aperture that was sealed by the electrode. Light was provided by a 150 W Xenon lamp from Newport Corporation. The power density of the light irradiating the sample was measured with a Thorlabs PMT50 power meter to be 225 +/- 10 mW/cm2 (~ 2 suns) through the cell filled with electrolyte. This illumination condition was used throughout this work. No OER current was observed for the photoanodes in the absence of illumination. An SCE reference electrode was used together with a stainless steel counter electrode. SCE was converted to RHE using the following relationship. E(RHE) = E(SCE) + 0.244 V + 0.059*pH Before every measurement, bubbles were removed from the aperture. Electrochemical experiments were carried out in a three electrode system controlled by a CHI 760D potentiostat. The K-borate electrolyte was made by mixing 2 M boric acid and 1 M KOH. The K-borate + Li-borate electrolyte was made by dissolving 6 g of boric acid, 1.9 g KOH and 0.65 g LiOH in 50 mL of water. Before data was recorded, the electrodes were pre-scanned under light until CV curves became stable. For K-borate the curve became stabilized after ~50 scans and for KOH ~20 scans. All CVs were taken at 100 mV/s. During experiments longer than 12 hours, the electrolyte solution was replaced with fresh solution every 12 hours to prevent changes in concentration due to electrolyte evaporation. Materials Characterization: Prior to Scanning electron microscopy (SEM) characterization, the samples were washed with water and toluene to remove any organic contamination. SEM analysis was measured by an FEI XL30 Sirion scanning electron microscope. Auger electron spectra were taken by PHI 700 Scanning Auger Nanoprobe operating at 10 kV and 10 nA. XPS spectra and depth profiles were collected on a PHI VersaProbe Scanning XPS Microprobe. UV-Vis thickness study: Ni films of 2, 5, 10, 20 and 30 nm were deposited on Precleaned 25x75x1 mm Superfrost Plus VWR Micro Slides and light transmission was measured using a Varian Cary 300 Scan UV-Visible Spectrophotometer. Chemical stability study: To evaluate the chemical stability of Ni in 1 M KOH and 1 M K-borate, 2 nm Ni-coated n-Si wafers were exposed to 1 M KOH and 1 M K-borate for 24

2

hours. To evaluate the chemical stability of NiOx the sample (2 nm Ni-coated n-Si) was oxidized under PEC conditions for 30 minutes and then exposed to 1 M KOH and 1 M K-borate for 24 hours. The chemical stability of Si was evaluated by exposing bare Si wafers to 1 M KOH and 1 M K-borate. Flatband potential measurement: The flatband voltage was measured in impedance-potential mode on a CHI 760D potentiostat at 1 kHz for 2 nm Ni/n-Si and bare n-Si in both 1 M KOH and 1 M K-borate in the dark. The Si wafer was used as the working electrode with SCE as the counter electrode and a stainless steel foil as the counter electrode.

Work Function Effect We investigated n-Si coated by thin Ni layers combined with other metals with various working functions as the surface passivation layer. As expected, for n-Si coated with Ni combined with a high work function metal (such as Pt, Figure S12), high photocurrents were measured, whereas much smaller photoactivity was detected for n-Si coated with Ni combined with a low work function metal (such as Ti, Figure S13) due to the formation of nearly ohmic contact instead of a Schottky junction at the interface.

Supplementary Figures

Figure S1. Mott-Schottky plots for 2 nm Ni/n-Si electrodes in 1 m KOH and 1 M K-borate. The sharp increase in 1/Cp2 at -0.6 V vs SCE and -0.45 V vs SCE in 1 M KOH

3

and 1 M K-borate respectively imply the formation of a depletion region and are indicative of the flat band potential.

Figure S2. Mott-Schottky plots for bare n-Si electrodes in 1 m KOH and 1 M K-borate. The sharp increase in 1/Cp2 at -0.6 V vs SCE and -0.45 V vs SCE in 1 M KOH and 1 M K-borate respectively imply the formation of a depletion region and are indicative of the flat band potential.

4

Figure S3. A zoom-in of Figure 2A to show onset potentials for Ni/n-Si photoanodes with different thicknesses of Ni in 1 M KOH.

-0.2 0.0 0.2 0.4 0.6

-2

0

2

4

6

8

10

Cu

rre

nt

De

ns

ity

(mA

/cm

2)

Potential (V vs SCE)

2 nm Ni

5 nm Ni

10 nm Ni

20 nm Ni

2 nm Ni

(n++ doped Si)

KOH

(pH=14)

5

Figure S4. Comparing the OER onset of a two-electrode cell containing a 2 nm Ni/n++Si anode and a Ni foam cathode (black) with a two-electrode cell containing a Ni foam cathode and a 2 nm Ni/n-Si photoanode under illumination (red). The voltage difference was determined by applying iR compensation to both curves and measuring the voltage at which 20 mA/cm2 of OER current was achieved. The difference (1.47 V and 2.02 V) was found to be ~500 mV.

6

Figure S5. UV-Vis data of various Ni film thicknesses on glass: light transmittance of 2, 5, 10, 20 and 30 nm Ni films on glass slides.

400 500 600 700 800

0

20

40

60

80

100

30 nm Ni

20 nm Ni

10 nm Ni

5 nm Ni

Tra

ns

mit

tan

ce

(%

)

Wavelength (nm)

2 nm Ni

7

Figure S6. 48hr stability data – Stability data for 2 nm Ni/n-Si anode operating for 48 hr operation at 10 mA/cm2 under illumination in both 1M KOH and 1M K-borate. Both samples begin to steadily lose activity after the 24hr point.

8

Figure S7. Microscopy and Auger electron spectroscopy mapping of Ni/nSi photoanode surfaces. (A) An SEM image showing an etch pit due to corrosion after a 2 nm Ni/n-Si photoanode was operated at 10 mA/cm2 under illumination for 5 h in 1 M KOH (pH = 14). (B) An AES map showing exposed silicon (red: Auger intensity of Si) under the Ni film (blue: Auger intensity of Ni) after the test in (A) in 1 M KOH. (C) An SEM image showing increased etching/corrosion of Ni/n-Si in 1 M KOH after 12 h of PEC operation as in (A). (D) An AES map showing increased Si exposed to KOH after the 12 h experiment in (C). (E) A representative featureless SEM image showing no obvious etching/corrosion after a 2 nm Ni/n-Si anode was operated at 10 mA/cm2 under illumination for 12 h in 1 M K-borate (pH = 9.5). (F) An AES map showing no Si etching/exposure after the experiment (E). (G) An SEM showing no obvious etching/corrosion of a 2 nm Ni/n-Si anode after operation at 10 mA/cm2 under illumination for 80 h in 0.65 M K-borate + 0.35 M Li-borate (pH = 9.5). (H) An AES map showing no Si etching/exposure after 80 h operation of the photoanode in (G) in 0.65 M K-borate + 0.35 M Li-borate. (I) An AES spectrum recorded over area 1 in (D) (an intact area on Ni/n-Si after 12 h PEC operation in 1M KOH) showing predominantly Ni signals

9

in the area without Si exposure. (J) An AES spectrum recorded over area 2 in (D) (in an etch hole) after the Ni/n-Si was operated in 1M KOH for 12 h.

Figure S8. Cyclic voltammograms taken before and after 80 hour stability tests in K-borate+L-borate pH = 9.5 electrolyte. All CVs were taken at 100 mV/s with no iR compensation applied.

10

Figure S9. Stability data showing that the stabilization effect of lithium addition persists after the lithium is removed from the solution, implying that the lithium is incorporated into the nickel oxide film.

11

Figure S10. Addition of Li+ to 1 M KOH – Constant current data (10 mA/cm2) of 2 nm Ni-coated n-Si electrode in 1 M KOH + 0.15 M LiOH under illumination (red) and a 2 nm Ni-coated n-Si electrode in 1 M KOH.

12

Figure S11. Chemical stability of Si, Ni and NiOx in 1 M K-borate (A, B and C) and 1 M KOH (D, E and F) for 24 hours. SEM shows a morphology change in the samples exposed to 1 M KOH.

13

Figure S12. High work function metal interface – Cyclic voltammogram of 2 nm Ni on 2 nm Pt on an n-Si anode under illumination at a scan rate of 100 mV/s. The high work function of Pt allows for a relatively low onset voltage, however it is still more positive than the ultrathin 2 nm Ni/n-Si photoanode.

14

Figure S13. Low work function metal interface – Cyclic voltammogram taken under illumination in 1 M KOH for an n-Si anode coated with 2 nm Ti followed by 2 nm Ni. The onset potential is shifted 350 mV positive of the 2 nm Ni case and the current achieved at 1 V vs. SCE is significantly smaller than the 2 nm Ni case.

-0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0-2

0

2

4

6

8

10

12

14

16

Cu

rre

nt

Den

sit

y (

mA

/cm

2)

Potential (V vs SCE)