of 8
S
1
Supporting Information for
Enhanced Stability and Efficiency
f
or
Photoelectrochemical Iodide Oxidation by
Methyl Termination and Electrochemical Pt
Deposition of
n
-
Si Microwire Arrays
Shane Ardo,
a
Elizabeth A. Santori,
a
Hal S. Emmer,
b
Ronald L. Grimm,
a
Ma
tthew J.
Bierman,
a
Bruce S. Brunschwig,
c
Harry A. Atwater,
bd
and Nathan S. Lewis
acd*
a
Division of Chemistry and Chemical Engineering, California Institute of Technology,
1200 E. California Blvd., Pasadena, California 91125, USA
b
Thomas J. Watson Laborator
ies of Applied Physics, California Institute of Technology,
1200 E. California Blvd., Pasadena, California 91125, USA
c
Beckman Institute, California Institute of Technology, 1200 E. California Blvd.,
Pasadena, California 91125, USA.
E
-
mail: nslewis@calte
ch.edu; Fax: +1 626 395
-
8867; Tel: +1 626 395
-
6335
d
Kavli Nanoscience Institute, California Institute of Technology, 1200 E. California
Blvd., Pasadena, California 91125, USA
*corresponding author
Additional Experimental Details
Chemicals
and Material
s
All chemicals
and materials
were used as received unless noted otherwise. The
hydriodic acid was unstabilized (Sigma Aldrich, 7.6 M, aq, 99.99%, or Alfa Aesar, 7.3
7.7 M, aq, ACS grade), except for the evaluation of planar Si samples for which
stabil
ized hydriodic acid was used (Sigma Aldrich, 7.6 M, aq, 99.95%, < 1.5%
hypophosphorous acid as stabilizer). All water was obtained from a Barnsted Nanopure
system and had a resistivity
>18 MΩ
-
cm.
Nafion™ membranes were purchased from Ion
Power (NR
-
212,
50.8 μm thick,
equivalent weight of 1100 g
of dry Nafion per
mol
e
sulfonic acid groups
).
S
2
Materials Synthesis and Processing
Crystalline Si microwire arrays were grown on planar n
-
type S
i(111) wafer substrates
(University Wafer, Si(111) wafers, As
-
doped, 0.001
-
0.004 Ω
-
cm resistivity, 406.6 nm
thermal oxide). Using photolithography, an array of holes (3 μm x 7 μm pitch) was
patterned into the thermal oxide as a hexagonal pattern, and the
holes were then filled
with thermally evaporated Cu (EPSI, 6N). Si microwire arrays were grown on a ~2 cm x
3 cm chip at 1000 °C using flow rates of 500 sccm for H
2
, 50 sccm for SiCl
4
, and 0.6
1.0 sccm for PH
3
for 8
20 min. These growth conditions ro
utinely produced
microwires ~60
μm in height. After microwire growth, the samples were cooled to ~800
°C under H
2
(g), cooled to ~200 °C under ambient pressure with a He headspace, and then
removed from the reactor.
The samples were then cleaned via a standard procedure, involving a ri
nse with H
2
O,
an etch for 10 s with buffered HF(aq) (Transene, Inc.),
followed immediately by rinsing
with H
2
O,
two consecutive RCA2 cleans consisting of H
2
O with equal parts HCl(12 M,
aq) and H
2
O
2
(9.8 M, aq) (6:1:1 v/v/v) at 70 °C for 20 min. The sample
was then
immersed for 10 s in BHF(aq) followed
immediately by rinsing with H
2
O,
60 s in
KOH(30 wt%, aq), and then 10 s in HF(~6 M, aq), followed immediately by rinsing with
H
2
O, drying under a stream of N
2
(g), and subsequent thermal oxidation for 100 min
at
1100 °C under a 4 L min
-
1
flow of O
2
(g). Oxide “boots” were defined using
polydimethylsiloxane (PDMS). PDMS was cast from a freshly prepared and degassed
solution that contained toluene, PDMS, and an initiator (5 mL: 1 g: 0.1 g). This solution
was sp
in cast on the microwire arrays for 30 s at 150 RPM, then for 30 s at 750 RPM, and
then for 30 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. The oxide was
removed from the exposed regions of the microwires using a 5 min soak in BHF(aq),
followed
immediately by rinsing with H
2
O,
a PDMS etch for ~5 sec that consisted of
N
-
methyl
-
2
-
pyrrolidone with tetra
-
n
-
butylammonium fluoride (2.7 M, aq) (3:1, v/v)
,
and
the
n a PDMS etch that consisted of
N
,
N
-
dimethylformamide with tetra
-
n
-
butylammonium fluoride (1.0 M, tetrahydrofuran) (1:1, v/v) until the PDMS was
completely removed.
S
3
Surface Methylation
Immediately after etching samples with HF(~6 M, aq), samples were ri
nsed with H
2
O,
dried under a stream of N
2
(g), and transferred to a
n
N
2
(g)
-
purged glovebox (<10 ppm
O
2
(g)).
Samples were then immersed in a test tube containing chlorobenzene with
saturated
PCl
5
and
freshly
added benzoyl peroxide radical initiator, and
hea
ted at
90 °C
for ~50
min
to chlorinate the Si surface.
T
he samples were rinsed ten times with
neat
chlorobenzene
followed by ten times with neat
tetrahydrofuran
, dried briefly on a hot
plate,
rinsed three more times with neat tetrahydrofuran,
and then tra
nsferred to another
test tube containing a Grignard solution consisting of methy
l
magnesium chloride (
~
1
M)
in tetrahydrofuran
, which was used to
m
ethylate the Si surface via a
reaction
at
~
5
5
°C
for
~4
hr
.
The samples were then rinsed ten times
with
neat
tetrahydrofuran followed by
ten times with
neat
methanol
, removed from the glovebox,
rinsed
three more times with
neat methanol followed by three times with H
2
O,
dried under a stream of N
2
(g)
,
and
stored under N
2
(g)
until use
.
S
uccess of the two
-
step chlo
rination/alkylation reaction
sequence in methyl
ating atop s
ites on the silicon microwires i
s supported by large
increases in water contact angle, X
-
ray photoelectron spectroscopy data, and electrode
stability measurements (Figure S
5
).
Photoelectrode Fabri
cation and Photoelectrochemical Evaluation
The wafer was diced into several ~2.0 mm
2
pieces, and each piece was fashioned into
an electrode
of approximately the same area
by lightly scratching the back side with In
Ga eutectic and using Ag paint to affix
the eutectic
-
coated electrode onto a coiled, tinned
Cu wire. 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, if
desired, Pt was then deposited on
the electrode either electrochemically or by electron
-
beam evaporation (vide infra). Prior to electrochemical evaluation, each electrode was
briefly etched in HF
(~6 M,
aq
), and the current
versus
potential data were measured
using a three
-
electrode conf
iguration with a Si MW array working electrode, a Pt wire or
small carbon
-
cloth quasi
-
reference electrode, and a Pt mesh or large carbon
-
cloth counter
electrode. The electrolyte consisted of ~7.6 M HI(aq) that contained adventitious I
3
, and
air was conti
nuously blown onto the cell for temperature control. The electrolyte was
S
4
rapidly stirred and was continuously purged with Ar(g). 1 Sun of simulated solar
illumination at an intensity of 100 mW cm
-
2
was obtained from an ELH
-
type W
halogen
lamp. The rever
sible
formal
potential for the aqueous I
3
/I
redox couple (
E
(I
3
/I
)) was
determined using a Pt disk/button working electrode and a standard calomel electrode
(KCl saturated) (SCE) as the reference electrode. The light intensity was determined
using a calibrate
d 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. All reported
curr
ent densities were referenced to the projected geometric area of the electrolyte
contact to the electrode. Ultraviolet
visible electronic absorption spectra of the dissolved
I
3
were obtained using an HP 8452A diode
-
array spectrophotometer.
Electrochemic
al Deposition of Platinum
Electrodes were wetted with H
2
O, etched for 15 s in HF(~6 M,
aq
),
followed
immediately by
rins
ing
with H
2
O
,
and then immersed in an aqueous solution of 5 mM
K
2
PtCl
4
(99.9%, Alfa Aesar) and 200 mM LiCl(aq). Using a three
-
electrod
e
configuration with an SCE reference electrode and a Pt mesh counter electrode, the
working electrode was held potentiostatically at
1.0 V
vs
SCE. The deposition was
performed until at least 100 mC cm
-
2
of cathodic charge
density
had passed. The sample
s
were then rinsed with H
2
O and dried under a stream of N
2
(g).
Electron
-
beam Evaporation of Platinum
Immediately after etching samples with HF(~6 M, aq),
samples were rinsed with H
2
O,
dried under a stream of N
2
(g),
mounted onto a sample holder using Kapt
on tape affixed at
the corners, placed into an evaporator (Denton Explorer) and positioned at an angle, set
on planetary rotation, and pumped down to < 1 x 10
-
5
Torr. Pt was then deposited until
the desired planar equivalent value was obtained on a calibr
ated quartz crystal
microbalance.
Scanning Electron Microscopy with Energy Dispersive Spectroscopy
Imaging of Si microwire arrays during and after fabrication and processing, and when
S
5
affixed as
electrodes, was performed on field
-
emission scanning
-
electron microscopes
(Zeiss 1550VP). Validation that Pt was present on samples was determined using an
energy
-
dispersive spectrometer with a silicon drift detector (Oxford X
-
Max
N
).
Stability
Analysis
Based on the data in Figure
5
, th
e turnover number for net h
+
passed per Si atom was
calculated to be ~900 as follows
.
A net integrated charge density of 837 C cm
-
2
was
passed through the microwire array and b
ecause each microwire array
contain
s
one
microwire
per 7 x 7 μm unit cell
, the
average net charge passed per microwire was 410
μC = 4.25 nmol e
and h
+
. A ~50 μm tall microwire that is ~1.2 μm in diameter,
representative of the microwires used in this study, has a volume of ~57 μm
3
, which
b
ased on
the density of crystalline Si
(
2.33
g mL
-
1
= 83 mmol Si
cm
-
3
)
,
yields ~4.7 pmol
Si per microwire.
Division of 4.25 nmol h
+
by ~4.7 pmol Si, yields the turnover number
of h
+
per microwire. Since complete
oxid
ation of
each Si atom requires four holes
,
the
observed photoanodic current cannot
predominantly be ascribed to the oxidation of Si.
S
6
Additional Figures
0
1
2
3
4
0
1x10
17
2x10
17
3x10
17
4x10
17
5x10
17
Dopant Density (cm
-3
)
Phosphine Flow Rate (sccm)
1.8x10
18
9.45x10
17
7.3x10
17
Figure S1
.
Mean (± standard deviation) d
opant density
data
as a function of PH
3
flow
rate for
single
n
-
type Si microwires
taken from arrays
grown at 1000 °C.
Each point was
obtain
ed from a different microwire growth and error bars
are
shown
when
measurements
were performed on
multiple single microwires
for a
sample.
Figure S2
. Three
-
electrode current density
versus
potential data recorded in the dar
k
(dashed) or under 100 mW cm
-
2
of simulated AM1.5 G solar illumination
(solid lines)
provided by an ELH
-
type W
-
halogen bulb with a dichroic rear reflector for an n
-
type Si
microwire array electrode immersed in Ar
-
purged 10 mM 1,1’
-
Me
2
Fc
+
,
10
0
m
M Me
2
Fc
(wh
ere Me
2
Fc is 1,1’
-
dimethylferrocene)
in
1.0 M LiClO
4
CH
3
OH. Also shown is the
response after correcting for concentration overpotential (solid, purple
; CORR
)
,
45
and the
calculated ideal regenerative
-
cell energy
-
conversio
n efficiency, η
,
for each.
-0.4
-0.2
0.0
0
5
10
15
= 2.9%
CORR
= 3.4%
1sun of ELH
Current Density (mA/cm
2
)
Potential (V vs.
E
(Fc
+/o
))
S
7
400
600
800
1000
0.00
0.05
0.10
0.15
0.20
0.25
External Quantum Yield
Wavelength (nm)
Figure S3
.
Mean
(± standard deviat
ion) normal
-
incidence spectral response data at 0 V
vs.
E
(Me
2
Fc
+/0
) for two n
-
type Si
microwire
array electrode
measurement
s in Ar
-
purged,
1 mM Me
2
Fc
+
, 10
0
m
M Me
2
Fc
in
1.0 M LiClO
4
CH
3
OH.
The dip in the data at
wavelengths less than 600 nm is due to solution absorption by Me
2
Fc
+
.
52
Figure S4
. Three
-
electrode current density
versus
potential data recorded in the dark
(dashed) or under 100 mW cm
-
2
of simulate
d AM1.5 G solar illumination
(solid lines) for
methylated n
-
type Si
microwire
array electrodes immersed in Ar
-
purged ~7.6 M HI(aq)
containing adventitious I
3
, with (purple) or without (orange) electrochemically deposited
Pt. The purple data depict the be
havior of the best
-
performing sample containing Pt,
whereas the orange data represent a typical response in the absence of Pt
, along with the
calculated ideal regenerative
-
cell energy
-
conver
sion efficiency, η
, for each
. The
reference electrode was a Pt wire poised at the Nernstian potential of the I
3
/I
(aq) redox
couple.
-0.4
-0.2
0.0
0
5
10
15
20
= 0.38%
= 3.6%
Current Density (mA/cm
2
)
Potential (V vs.
E
(I
-
3
/I
-
))
1sun of ELH
HI(7.6M)/I
3
-
Ar
(1
atm
)
n
-
Si
CH
3
n
-
Si
CH
3
Pt
1sun ELH illumination
HI(7.6M)/I
3
-
Ar
(1atm)
Pt
pseudo
-
RE
S
8
-0.4
-0.2
0.0
0
5
10
Initial
After 50 hours
Current Density (mA/cm
2
)
Potential (V vs.
E
(I
3
-
/I
-
))
Figure S5
. Three
-
electrode current density
versus
potential data recorded under 100 mW
cm
-
2
of simulated AM1.5 G
solar illumination
for a methylated n
-
type Si
microwire
array
electrode immersed in Ar
-
purged ~7.6 M HI(aq) containing adventitious I
3
, initially
(black) and after 50 h of continuous cyclic voltammetric operation (black) that pass
ed
a
net
total
anodic
ch
arge
density o
f 529 C cm
-
2
.
Figure S6
.
Representative s
canning
-
electron micrograph images at 70,000 magnification
(in lens detector) of
nominally identical
Si microwires
obtained from the sample used in
Figure
5
showing P
t nanoparticles (white dots)
(a, b)
before and
(c)
after
photoelectrochemical analysis for 21 h, per the operation shown in Figure
5
.
Within
variation
between microwires
no substantial change
was observed
in
the
microwire
dimensions or in the shape or cov
erage
of Pt
, indicating that etching of Si and Pt was
slow compared to oxidation of I
(aq). All three scale bars are 500 nm.
(a)
(b)
(c)