Stable and efficient multi-crystalline n+p silicon photocathode for H2
production with pyramid-like surface nanostructure and thin Al2O3
protective layer
Supplementary Material
Additional experimental details
Scanning Electron Microscopy analysis
The surface morphology of sample surface was analyzed by Field-Emission Scanning
Electron Microscopy using a SU8010 from Hitachi. In order to avoid surface charging,
Pt was coated by the sputtering method for 120s using the BAL-TEC/SCD 005 model
prior to the SEM analysis.
Transmission Electron Microscopy analysis
Specimens for cross-sectional transmission electron microscopy analysis were
prepared by mechanical polishing, followed by argon ion milling using a Gatan PIPS
system at an angle of 3° and a voltage of 3.6 kV , cooled with 79K LN2. The
microscope was conducted by FEI Tecnai G2 F20 S-TWIN transmission electron
microscope operating at 200 kV.
X-Ray Photoelectron Spectroscopy analysis
X-ray photoelectron spectroscopy (XPS) measurements were performed at room
temperature using a PHI-550 ESCA spectrometer (Perkin Elmer) hemispherical
analyzer and an Al K (h=1486.6 eV). X-ray source is in a vacuum below 1.0×10−9
Torr.
UV-vis Diffuse Reflectance Spectroscopy analysis
The reflection spectra of the Si specimens were measured using a Perkin Elmer
Lambda 750 spectrophotometer in a wavelength range of 300~1300 nm, which uses
BaSO4 as a reference. The minority carrier lifetimes were measured using a silicon
wafer lifetime tester (WT-1000, Semilab Semiconductor Physics Laboratory, Inc.).
Pt Deposition
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The Si samples were chemically etched in HF solution (5%) to remove oxide layer.
Afterward, they were immediately transferred to a solution containing H2PtCl6 (5mM)
(Sigma-Aldrich, reagent grade) for 3 min for optimum energy conversion efficiency.
The electrode was photoirradiated at a negative potential (0.1V vs. Ag/AgCl reference
electrode) allowing for reduction of the Pt salt to Pt metal.
Contact Angle Measurement
The contact angle analysis was carried out with a standard optical contact angle meter
(SL200A, KINO Industry Ltd., USA.).
PEC Measurement
The samples for PEC measurements were laser-cut into 1.51.5 cm2. In order to
establish an ohmic contact between the copper wire and the unpolished side (back
side) of Si, tinned Cu wire was connected to the Al bottom electrode by
gallium-indium eutectic. The exposed backside, edges, and some part of the front of
the electrodes were sealed with an industrial epoxy (PKM12C-1, Pattex).
Prior to the PEC measurement for the samples without Al2O3 protection, native
oxide on the Si surface was removed with 5% HF. A 300 W Xe lamp (Oriel, Newport
Co.) with an IR cutoff filter was used as a light source. The emission spectrum of the
Xe lamp is shown as Fig. S1. During the PEC measurement, the light intensity was
carefully maintained at 100 mW/cm2, measured using an optical power meter
(Newport Co.) just before the light enters into the PEC cell. PEC experiments were
performed in a one-compartment quartz cell (150mm100mm70mm). To minimize
the sticking of hydrogen bubbles on the Si surface, violent stirring of the electrolyte
was performed during the measurement. No additional surfactant was introduced,
because the harsh stirring can remove the bubbles effectively. The measurements
were conducted in a solution containing 0.5mol K2SO4 (Sinopharm Chemical Reagent
Co., Ltd., Analytical reagent) and H2SO4 (pH=1) (Enox, Analytical reagent), using the
Si photocathode as working electrode, a Ag/AgCl (3M KCl) as reference electrode,
and a Pt as auxiliary electrode. The potentials were rescaled to the ones versus the
reversible hydrogen electrode (RHE) according to the following equation:
E(RHE)=E(Ag/AgCl)+0.197V at pH 0. The potential of the Si electrode was
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controlled using a potentiostat (CHI600D, CH Instrument).
Figure S1. The emission spectrum of the Xe lamp.
Figure S2. SEM image of the mc-Si surface after the Ag assisted etching.
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Reflectance / %
50
40
30
Normal-Si
20
Pyramid-Si
10
400
600
800
1000 1200
Wavelength / nm
Figure S3. The total hemispherical optical reflectance of normal-Si and pyramid-Si
measured in air. The nano-pyramid structure lowers the reflection over the entire solar
spectrum obviously.
Figure
S4.
Potentiostatic
photocurrent
density
traces
for
the
Pt-loaded
Al2O3-protected pyramid electrode during the reduction reaction to produce H2 biased
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0.1V vs. RHE in H2SO4 and 0.5 M K2SO4 solution under 100 mW/cm2 Xe lamp. Inset:
PEC J-V curves before and after a 42 h long time test. The photocurrent is stable at
about 30 mA/cm2 throughout the 42 h run under 0.1V vs. RHE.
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