Free-standing, membrane-embedded, Si microwire arrays have been used to affect the solar-driven, unassisted splitting of HI into H_2 and I_3−. The Si microwire arrays were grown by a chemical-vapor-deposition vapor–liquid–solid growth process using Cu growth catalysts, with a radial n+p junction then formed on each microwire. A Nafion proton-exchange membrane was introduced between the microwires and Pt electrocatalysts were then photoelectrochemically deposited on the microwires. The composite Si/Pt–Nafion membrane was mechanically removed from the growth substrate, and Pt electrocatalysts were then also deposited on the back side of the structure. The resulting membrane-bound Si microwire arrays spontaneously split concentrated HI into H_2(g) and I_3− under 1 Sun of simulated solar illumination. The reaction products (i.e. H_2 and I_3−) were confirmed by mass spectrometry and ultraviolet–visible electronic absorption spectroscopy

Ardo, Shane

Lewis, Nathan S.

Park, Sang Hee

Warren, Emily L.

Caltech Authors - Main

 S1 
 
Supporting Information for 
“Unassisted solar-driven photoelectrosynthetic HI splitting using membrane-embedded Si 
microwire arrays” 
 
 
  
 
 
 
Figure S1.  Digital photograph images of (a) a Nafion-embedded microwire-array electrode used 
to photoelectrochemically deposit Pt on the microwires, and (b) a fully functional, peeled 
composite membrane containing a partially embedded Si microwire arrays that demonstrated 
unassisted solar-driven HI splitting.  Both images are referenced in size to the penny shown in 
panel a.  (c) Scanning electron micrograph of microwires containing photoelectrochemically 
deposited Pt. 
 
 
 
a
b
c
10 μm
Electronic Supplementary Material (ESI) for Energy & Environmental Science.
This journal is © The Royal Society of Chemistry 2015
 S2 
      
 
  
 
Figure S2.  (a) Schematic showing the arrangement of a sample between two Viton sheets that 
had holes and that were capped by two borosilicate glass cuvettes having one hole each to wet the 
sample.  The catholyte cell was clear while anolyte cell was red due to visible-light absorption by 
I3
–. Digital photograph images of cell holders illustrating (b) an early design showing the excitation 
arrangement using the fiber optic, the location of the sample, and the probe beam path through the 
anolyte.  Also shown are (c) an intermediate design with the features from panel b, plus a port for 
the fiber optic and an inductive stirrer to circulate the anolyte, and (d) a final design including the 
features from panel c, plus a port for a borescope to monitor bubble formation on the sample 
surface, a gas-tight cathode compartment to sample the headspace for evolved H2, and two 
electrodes in order to measure the potential difference between the cells. 
 
 
 
sample
a “sunlight”
catholyte anolyte
UV–Vis probe
sample
b
c d
 S3 
 
 
Figure S3.  Real-time digital images using a borescope positioned on the photocathode side of the 
composite-material membrane (Figure S2d), demonstrating bubble formation due to light-driven 
H2 evolution (a) over the first 19.5 min, and subsequently (b) 5.7 h later over a time-span of 6.8 
min. 
  
0 1.7 3.6 5.1 8.0 11.4 15.5 19.5
Time (minutes)
3 mma
0 4.7 6.8
Time (minutes)
b
 S4 
 
Figure S4.  Schematic depiction of the general fabrication process for the free-standing composite 
membrane. 
 
p pClean Samples
and Grow
Barrier Oxide
PDMS
Polymer 
Infill
p pRemove 
Oxide
pDiffusion
Dope n+
pRemove
Polymer
p
pNafion
Infill
Photoelectro-
chemically 
Deposit Pt
n+
Peel
Array
Electron-Beam 
Evaporate Pt
p p
 S5 
Additional Experimental Details 
 
Chemicals 
 All chemicals were used as received unless noted otherwise.  All water was filtered using a 
Barnsted Nanopure system which resulted in a resistivity >18 MΩ-cm. 
 
Polydimethylsiloxane Deposition Protocol 
 Polydimethylsiloxane (PDMS) was cast from a freshly prepared and degassed solution that, 
unless noted otherwise, contained toluene, PDMS, and an initiator (5 mL: 1 g: 0.1 g).  This solution 
was spin cast on the microwire arrays for 30 s at 150 RPM, then for 30 s at 750 RPM, and then for 
60 s at 1500 RPM.  The PDMS was subsequently cured at 60 °C overnight in a vacuum oven 
followed by a final cure at 150 °C for 30 min on a hot plate. 
 
Materials Synthesis and Processing 
 Crystalline Si microwire arrays were grown on planar Si(111) wafer substrates pre-patterned 
with Cu in a hexagonal pattern having a 3 μm x 7 μm array pitch (Montco Silicon Tech., Inc., 
Si(111) wafers, 610 – 640 μm thick, B-doped, 0.005 – 0.010 Ω-cm resistivity, 300 nm thermal 
oxide).  The microwire arrays were grown on a ~2 cm x 3 cm chip using flow rates of 450 sccm 
for H2, 50 sccm for SiCl4, and 1.5 sccm for BCl3 for 18 min.  These growth conditions produced 
microwires ~65 μm in height. 
 The samples were then cleaned via a standard procedure of HF(~6 M, aq) for 10 s, followed by 
an RCA2 clean consisting of H2O with equal parts HCl(12 M, aq) and H2O2(9.8 M, aq) (6:1:1 
v/v/v) at 70 °C for 20 min, then 10 s in HF(~6 M, aq), 50 s in KOH(30 wt%, aq), 10 s in HF(~6 
M, aq), a second RCA2 etch at 70 °C for 20 min, immediately followed by rinsing with H2O, 
drying under a stream of N2(g), and subsequent thermal oxidation for 100 min at 1100 °C under 
an O2 flow at 4 L min
-1, Figure S4.  Oxide “boots” were defined using PDMS, per the procedure 
described above.  The oxide was removed from the exposed regions of the microwires using a 5.5 
min soak in buffered HF(aq) (Transene, Inc.), followed by a PDMS etch that consisted of N-
methyl-2-pyrrolidone with tetra-n-butylammonium fluoride (2.7 M, aq) (3:1, v/v), until the PDMS 
had been completely removed.  Prior to diffusion doping, the samples were then cleaned via a 
Piranha etch that consisted of H2SO4 with H2O2(9.8 M, aq) (3:1, v/v) at 80 °C for 30 min, 10 s in 
HF(~6 M, aq), an RCA1 clean consisting of H2O with equal parts of NH4OH(15 M, aq) and 
 S6 
H2O2(9.8 M, aq) (5:1:1, v/v/v) at 80 °C for 20 min, 10 s in HF(~6 M, aq), followed by another 
RCA2 clean.  Samples were etched for 10 s in HF(~6 M, aq), followed by drying under a stream 
of N2(g), and then were immediately placed into the first slot between solid-source ceramic 
CeP5O14 diffusion doping wafers (Saint-Gobain Ceramic Materials, PDS PH-900) on a quartz boat.  
The samples were placed into a furnace set at 900 °C with a 1 min push in, an 8 min soak, and a 1 
min pull out.  A p-type Si planar control that was placed in the second slot exhibited a sheet 
resistance of ~140 Ω/□. 
 
Photoelectrode Fabrication and Photoelectrochemical Evaluation 
 All four edges of the growth substrate wafer were mechanically removed leaving a wafer (~1.5 
cm x 2 cm) that was then diced into several ~3 mm x 3 mm pieces.  Each piece was fashioned into 
an electrode by lightly scratching the back side with In–Ga eutectic and affixing the eutectic-coated 
electrode onto a coiled tinned Cu wire using Ag paint.  The other end of the wire was then inserted 
into a glass tube and sealed using epoxy (Hysol 9460).  The epoxy was left to cure overnight in a 
60 °C oven and afterward the electrode was wetted with H2O, etched for 15 s in HF(~6 M, aq), 
rinsed with H2O, and then Pt was deposited potentiostatically (–100 mC cm-2), as described above.  
Before use, each electrode was briefly etched in HF(~6 M, aq), and the current versus potential 
data were measured using a three-electrode setup with a Si microwire-array working electrode, a 
Pt wire or small carbon-cloth pseudo-reference electrode, and a Pt mesh or large carbon-cloth 
counter electrode.  The electrolyte consisted of 12 M HCl under rapid stirring, a continuous H2(g) 
purge, and 1 Sun of simulated solar illumination at an intensity of 100 mW cm-2 from an ELH-
type W–halogen lamp with air blown onto the cell.  The reversible potential for H2(g) evolution 
(RHE) was determined using a Pt disk/button working electrode.  The light intensity was 
determined using a calibrated Si photodiode (ThorLabs, Inc., FDS100) positioned at the location 
of the electrode, with the incident light passing through the borosilicate glass window of the 
electrochemical cell as well as through a thin path length of electrolyte. 
 
Back-side Electrode Fabrication and Electrochemical Evaluation 
 A PDMS solution (2.5 mL toluene: 1 g PDMS: 0.1 g initiator) was spin cast onto oxide-booted 
p-type Si microwire arrays using the procedure described in the Supporting Information.  Each 
partially PDMS-embedded Si microwire array was etched for 5 s with the PDMS etch described 
 S7 
in the Supporting Information, followed by a 10 min ozone etch, a 10 s etch in HF(~6 M, aq) and 
a rinse with H2O, immediately followed by drying under a stream of N2(g).  Au was then thermally 
evaporated onto the microwires (300 nm planar equivalent; see Supporting Information), followed 
by covering the microwires in Ag paint, for structural stability. Once dried, the microwire–PDMS 
composite was mechanically removed from its growth substrate using a Teflon-coated steel razor 
blade.  Pt was then evaporated onto the back side of the free-standing PDMS-embedded microwire 
array (5 nm planar equivalent) and the assembly was diced into smaller samples (~3 mm x 3 mm) 
using a razor blade.  Each sample was fashioned into an electrode by mounting the Ag paint side 
of the sample onto a piece of Cu foil (99.9999%, Alfa Aesar) using Ag paint.  The samples were 
then affixed to coiled tinned Cu wire using Ag paint, followed by inserting the other end of the 
wire into a glass tube and sealing the electrode using Hysol 9460 epoxy.  After allowing the epoxy 
to cure overnight in a 60 °C oven, the electrode was wetted with H2O, etched for 15 s in HF(~6 M, 
aq), and then immediately immersed in ~7.6 M HI that contained adventitious I3
–, under rapid 
stirring and a continuous Ar(g) purge.  The current versus potential data were measured using a 
three-electrode setup with a back-side microwire–PDMS working electrode, a Pt wire pseudo-
reference electrode, and a Pt mesh counter electrode.  The reversible potential for I3
– generation 
was determined using a Pt button working electrode and was found to be the same potential 
recorded for the back-side electrode at zero current. 
 
Photoelectrochemical Deposition of Platinum 
 Electrodes were wetted with H2O, etched for 15 s in HF(~6 M, aq), and then immediately 
immersed in an aqueous solution of 5 mM K2PtCl4 (99.9%, Alfa Aesar) and 200 mM LiCl.  Using 
a three-electrode setup with a (KCl saturated) standard calomel reference electrode (SCE) and a Pt 
mesh counter electrode, the working electrode was held potentiostatically at –300 mV vs SCE 
while being illuminated by an ELH-type W–halogen lamp, with the light passing through a long-
pass RG830 filter prior to striking the sample.  The deposition was performed using a stirred 
solution and was terminated when 100 mC cm-2 of cathodic charge had passed.  The samples were 
then rinsed with H2O and dried under a stream of N2(g). 
 
Electron-beam Evaporation 
 Immediately after etching samples with HF(~6 M, aq) or mechanically removing membrane-
 S8 
embedded Si microwires from their growth substrate, samples were mounted onto a sample holder 
using Kapton tape affixed at the corners, placed into an evaporator (Temescal FC-1800 for Au 
(KNI) or Denton Explorer for all other metals) and positioned at an angle, set on planetary rotation, 
pumped down to < 1x10-5 Torr, and deposited metal until a desired planar equivalent registered on 
a calibrated quartz crystal. 
 
Scanning Electron Microscopy with Energy Dispersive Spectroscopy 
 Imaging of Si microwire arrays during and after fabrication and processing, and when affixed as 
front-side and back-side electrodes, was performed on field-emission scanning-electron 
microscopes (Zeiss 1550VP and FEI Quanta 200F (Kavli Nanoscience Institute at Caltech (KNI)).  
The elemental composition of the front-side and back-side electrodes and samples was determined 
using an energy-dispersive spectrometer with a silicon drift detector (Oxford X-MaxN). 
 
Electrochemistry 
 All reported current densities were referenced to the projected geometric area of the electrolyte 
contact to the electrode and/or membrane. 
 
Free-standing Composite Membrane Fabrication 
 Nafion (~11 wt%, DMF) was exchanged from a Nafion solution (5 wt%, aq/alcohol) (Alfa 
Aesar) by heating at 55 °C for several days, and the resulting polymer was spin cast on (n+)p-type 
Si microwire arrays three times at 1000 RPM, each followed by drying in a vacuum oven at 60 °C 
for 40 min and then a 140 °C cure on a hotplate for 20 min.  For n+p-type Si microwires, all four 
edges of the growth substrate wafer were mechanically removed and In–Ga eutectic was lightly 
scratched as ~10 spots, ~2 mm in diameter, on the back side of the wafer (~1.5 cm x 2 cm), 
followed by coating with Ag paint.  Each sample was fashioned into a large electrode using Ag 
paint to affix the wafer onto a coiled tinned Cu wire that had been partially sealed into a glass tube 
using Hysol 1C epoxy and hardener.  The exposed wire and silicon edges were coated in two coats 
of black nail polish (Sally Hansen, Black Out) (Figure S1a), each allowed to dry at room 
temperature for 60 min before Pt was deposited potentiostatically on the electrode (–100 mC cm-
2), as described above.  The samples were rinsed with H2O and then carefully removed from the 
tinned Cu wire using acetone and affixed to a glass slide using mounting wax (South Bay 
 S9 
Technology, Inc., Quickstick 135).  Each partially Nafion-embedded Si–Pt microwire array was 
removed from its growth substrate by mechanical peeling using a Teflon-coated steel razor blade 
(for p-Si) or an 8” heavy-duty scraper blade (Better Tools, LLC, Part #20107) and a home-built 
microwire array peel-off tool, to ensure the microwire–membrane integrity, with no tears, of the 
composite membranes (for n+p-Si).S1  The peel-off tool contained a spring-loaded scraper blade 
that traveled along the surface of the wafer to sever the microwire–Nafion composite from the Si 
growth substrate.  Pt was then evaporated onto the back side of the free-standing Nafion-embedded 
microwire array (5 nm planar equivalent) (Figure 1) followed by storage in a glove box under an 
inert Ar atmosphere. 
 
Free-standing Composite Membrane Photoelectrochemical Evaluation (additional information) 
 Before assembling the composite membrane sample between the two glass cuvettes, the anode 
back side of the sample was wetted with HI to ensure that bubbles were not trapped at the surface.  
Approximately 20 composite membranes were evaluated.  Before introducing the membranes into 
the H-cell, samples were qualitatively evaluated for their ability to split HI by immersing the 
membrane into the electrolyte, and visually monitoring, while under illumination, bubble 
formation and release from the microwire side of the composite membrane, as well as a darkening 
of the back side of the membrane due to I3
– formation.  In the H-cell, the anolyte was continuously 
stirred in the 1 mm path length cuvette which was vertically offset by ~0.5 cm from the bottom of 
the 10 mm path length cuvette, so that it could be probed using transmission-mode ultraviolet–
visible electronic absorption spectroscopy.  The 10 mm path length cuvette was equipped with a 
gas-tight sealable screw top with a silicone septum that contained 3 ports, each containing 1/16” 
Teflon tubing.  One port served as an outlet to the mass spectrometer and was under slightly 
negative pressure, due to pull from the differential pump at the inlet of the mass spectrometer.   
Another port served as an inlet for inert carrier gas, and was slightly over-pressurized using ultra-
high-purity Ar(g).  The final port was open to allow the system pressure to equilibrate with ambient 
pressure and thereby prevent over-pressurization or under-pressurization, which ultimately ripped 
the membrane.  Two traps were placed between the cuvette and the mass spectrometer: a dry-
ice/isopropanol trap, followed by a second trap filled with Drierite pellets to scrub any additional 
acid or water prior to entering the mass spectrometer (Hiden Analytical, HPR20 with a 100 amu 
column).  The molar decadic absorption coefficient of triiodide at its maximum (350 nm) is 
 S10 
~26,000 M-1 cm-1, and is ~100 M-1 cm-1 at the probing wavelength (550 nm).  These values were 
used to convert the sample absorbance to a concentration of I3
–, and subsequently to the current 
that generated I3
–, using the volume of the cell (~0.4 mL) and differentiation of the data over time.  
The catholyte consisted of ~2.8 mL of ~7.6 M HI, while the anolyte consisted of ~0.4 mL of HI/I3
–
.  The hydriodic acid was unstabilized (Sigma Aldrich, 7.6 M, aq, 99.99%, or Alfa Aesar, 7.3 – 7.7 
M, aq, ACS grade); thus, prior to transferring hydriodic acid into each cuvette, adventitious I3
– was 
electrochemically reduced in an H-cell that contained ~7.6 M HI in both compartments and a 
Nafion membrane separator.  (The catholyte compartment was continuously bubbled with Ar(g), 
and reduction occurred at a large carbon-cloth working electrode.  Oxidation occurred at a Pt mesh 
or large carbon cloth counter electrode in the anolyte compartment.)  The 1 mm path length cuvette 
had been exposed to air and was not purged further during the experiment, so the electrolyte inside 
this compartment contained adventitious I3
–.  The HI electrolyte in the 10 mm path length cuvette 
initially contained small amounts of adventitious H2 from the electrochemical reduction of I3
–.  It 
was continually bubbled with Ar(g) during the experiments, and no decrease in the volume of the 
catholyte was observed during the course of the experiments.  The custom acrylic sample holder 
had a port for a borescope (PCE Instruments UK Ltd., PCE-VE 310) to image and record the 
photoelectrochemical reactions (Figures S2d and S3). 
 
Reference 
(S1)  Tamboli, A. C.; Chen, C. T.; Warren, E. L.; Turner-Evans, D. B.; Kelzenberg, M. D.; Lewis, 
N. S.; Atwater, H. A. Wafer-Scale Growth of Silicon Microwire Arrays for Photovoltaics 
and Solar Fuel Generation. IEEE Journal of Photovoltaics 2012, 2, 294–297.


Unassisted solar-driven photoelectrosynthetic HI splitting using membrane-embedded Si microwire arrays

https://authors.library.caltech.edu/56993/2/c5ee00227c1_si.pdf

Unassisted solar-driven photoelectrosynthetic HI splitting using membrane-embedded Si microwire arrays

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