of 8
Supplemental
Information
for
Stablization
of
Si Microwire
Arrays
for
Solar-Driven
H
2
O
Oxidation
to
O
2
(g)
in
1.0
M
KOH(aq)
using
Conformal
Coatings
of
Amorphous
TiO
2
Methods
Chemicals
All
chemicals
were
used
as
received
unless
noted
otherwise. Water
was
filtered
using
a MilliPore
system
and
had
a resistivity
> 18
MΩ-cm.
Si
Microwire
Fabrication
Phosphorous-doped
(N
D
=3
x 10
17
cm
-3
)
Si microwire
arrays
were
grown
via
a Cu-
catalyzed
vapor-liquid-solid
(VLS)
process
on
As-doped
n
+
-Si
<111>
wafers
(<0.005
Ω-cm,
Addison)
6,25,2
. The
n
+
-Si
<111>
growth
wafers
were
received
as
3”
wafers
with
a 500
nm
thick
thermal
oxide
(SiO
2
)
and
were
photolithographically
patterned
to
form
an
array
of
3
μm
diameter
holes
filled
with
Cu
in
a 7
μm
x 7
μm
square
lattice. The
photolithography
process
began
with
the
application
of
a positive
photoresist
layer
(Shipley
S1813)
to
the
SiO
2
surface. Next,
the
photoresist
was
exposed
to
UV
illumination
through
a mask
that
contained
the
growth
pattern
(3
μm
diameter
holes
in
a 7
μm
x 7
μm
square
lattice),
and
the
exposed
and
cured
photoresist
was
removed
with
a developer
(Microposit
MF-319). This
process
left
3
μm
diameter
holes
in
the
photoresist
through
which
the
underlying
SiO
2
was
etched
with
buffered
HF(aq)
(BHF)
(Transene
Inc.),
to reveal
the
underlying
Si. 500nm
of
high-purity
Cu
(EPSI
6N)
was
then
thermally
evaporated
onto
the
sample. Through
a
lift-off
process,
all
of
the
Cu,
except
for
that
in
the
3
μm
diameter
holes,
was
removed. This
entire
process
produced
3”
growth
wafers
that
were
diced
to
fit
into
the
tube
furnace.
Electronic
Supplementary
Material
(ESI)
for
Energy
&
Environmental
Science.
This
journal
is
©
The
Royal
Society
of
Chemistry
2014
Growth
of Si microwire
arrays
was
performed
in
a chemical-vapor
deposition
(CVD)
tube
furnace
at
atmospheric
pressure
using
SiCl
4
(Strem,
99.9999+%)
at 25
sccm
flow
rate,
H
2
(Matheson,
research
grade)
at
500
sccm
flow
rate,
and
PH
3
(Matheson,
100ppm
in H
2
)
at 0.3
sccm
flow
rate
for
9 min. Following
growth,
the
samples
were
cooled
to
~
200
°C
under
a 500
sccm
flow
of
He.
Microwire
Array
Processing
Microwire
arrays
were
cleaned
using
a 6:1:1
(by
volume)
H
2
O:HCl(fuming,
aqueous):H
2
O
2
(30%
in
H
2
O)
metal
etch
(RCA
2)
for
20
min
at
60
°C.
The
samples
were
then
subjected
sequentially
to a 15
s BHF
etch,
a H
2
O
rinse,
a 10
min
organic
(piranha)
etch
in 3:1
H
2
SO
4
(99.6%,
aqueous):H
2
O
2
(30%
in
H
2
O)
at
room
temperature,
and
a H
2
O
rinse.
Following
a 30
s etch
in 10%
BHF
and
H
2
O
rinse,
a
150
nm
thick
SiO
2
layer
was
grown
via
dry
thermal
oxidation
in
a tube
furnace
for
2.5
h at
1050
°C
under
an
O
2
atmosphere. A
15
μm
thick
PDMS
layer
was
deposited
at
the
base
of
the
wires
by
spin
coating
a solution,
consisting
of 1.1
g of
polydimethylsiloxane
(PDMS,
Sylgard
185,
Dow
Corning)
and
0.1
g of PDMS
curing
agent
dissolved
in 5 mL
of
toluene,
onto
the
sample
at
3000
rpm
for
30
s, followed
by
a 30
min
cure
in a vacuum
oven
at
150
°C. These
PDMS-infilled
arrays
were
submerged
in
BHF
for
3 min,
to
remove
the
SiO
2
on
the
exposed
microwire
surfaces.
The
PDMS
was
removed
by
a 30
min
soak
in
3:1
N-methyl-2-
pyrrolidone(NMP):tetrabutylammonium
fluoride(TBAF,
aq
75
wt%),
followed
by
a
30
s H
2
O
rinse. The
samples
were
then
dried
under
a stream
of N
2
(g)
and
residual
organics
were
removed
by
a 10
min
piranha
etch.
p
+
Emitter
Formation
A boron-doped
p
+
-Si
radial
emitter
was
formed
on
the
processed
n-Si
microwire
arrays
by
exposure
of
the
samples
in
a CVD
furnace
to
a 20:400
sccm
flow
of
BCl
3
(Matheson,
0.25%
in
H
2
):H
2
(Matheson,
research
grade)
at
950
°C
for
30
min,
immediately
following
a 30
s etch
in 10%
BHF.
The
samples
were
then
rinsed
with
H
2
O
and
dried
under
a stream
of
N
2
(g).
TiO
2
Deposition
TiO
2
was
deposited
by
atomic-layer
deposition
(ALD)
on
np
+
-Si
microwires
at
150
°C
using
tetrakis(dimethylamino)titanium
(TDMAT)
as
the
Ti source
heated
to
75
°C
and
H
2
O
as
the
O source.
2000
cycles
were
performed
with
pulse
and
purge
times
of
0.1
s and
15
s for
the
TDMAT,
respectively
and
0.015
s and
15
s for
the
H
2
O
respectively,
with
the
H
2
O
pulse
beginning
each
new
cycle.
Prior
to ALD
deposition,
the
samples
were
etched
for
15
s in 10%
HF(aq),
cleaned
using
a RCA
SC-2
procedure,
and
dried
with
N
2
(g).
In
the
RCA
SC-2
procedure,
the
samples
were
immersed
in
a 5:1:1
(by
volume)
solution
of
H
2
O,
concentrated
hydrochloric
acid
(11.1
M),
and
hydrogen
peroxide
(conc.~
1 M)
for
10
min
at
75
°C.
NiCrO
x
Deposition
NiCrO
x
was
deposited
on
np
+
-Si/TiO
2
microwire
arrays
by
reactive
RF
magnetron
sputtering
(AJA)
with
separate
Ni
and
Cr
targets
at 150
W
and
90
W,
respectively.
The
deposition
chamber
was
at
a pressure
of 5 mTorr,
fed
by
20
sccm
N
2
(g)
and
1
sccm
of
O
2
(g).
The
deposition
time
was
20
min,
which
resulted
in
a thickness
of
~
40
nm
planar
equivalent.
Electrode
Fabrication
Electrodes
were
fabricated
from
diced
pieces
of
each
processed
microwire
growth
array. To
make
ohmic
contact
to the
samples,
In-Ga
(99.99%,
Alfa-Aesar)
eutectic
was
scratched
into
the
back-side
of
the
growth
wafer
with
a diamond-tipped
scribe.
Exposed
In-Ga
(Si
electrodes)
was
affixed
to
a coiled
Cu-Sn
wire
with
Ag
paint
(SPI
05001-AB). The
active
area
was
defined
with
epoxy
(Loctite
Hysol
9460)
and
the
entire
electrode
was
sealed
with
epoxy
to
the
bottom
of a glass
tube
(6mm
O.D.).
The
electrode
orientation,
down-
or
side-facing,
was
determined
by
the
orientation
of
the
coiled
wire
that
protruded
from
the
glass
tube. Geometric
areas
were
measured
by
scanning
the
active
area,
and
using
software
(ImageJ)
to
calculate
the
area.
Photoelectrochemical
Measurements
in
Aqueous
Solutions
Bottom-facing
electrodes
were
used
in both
1 M
KOH
and
aqueous
ferri-/ferro-
cyanide
solutions.
Four
np
+
-Si/TiO
2
/NiCrO
x
and
two
n-Si/TiO
2
/NiCrO
x
microwire
array
electrodes
were
made
and
tested,
with
the
reported
results
being
representative
of
all
electrodes
in each
specified
class. Three-electrode
cyclic
voltammetry
(CV)
measurements
were
conducted
using
a Biologic
(SP-200)
potentiostat
in
conjunction
with
a borosilicate
single-compartment
cell
that
had
a
flat-bottom
glass
window. A
mercurous/mercury
oxide
(Hg/HgO,
1.0
M
KOH
filling
solution)
(CH
Instruments)
reference
electrode
and
a fritted
carbon
counterelectrode
were
used
for
measurements
in 1 M
KOH,
whereas
a Luggin-
capillary
Pt
reference
electrode
placed
as
close
to
the
working
electrode
surface
as
possible
and
a Pt
mesh
counterelectrode
were
used
for
measurements
in
ferri-
/ferrocyanide
solutions. The
ferri-/ferrocyanide
data
were
plotted
versus
the
solution
potential
at
the
reference
electrode,
while
the
potential
axis
for
the
1 M
KOH
data
was
shifted
to
the
reversible
hydrogen
electrode
(RHE)
scale
based
on
the
value
provided
by
CH
Instruments,
0.14
V vs
RHE.
For
these
experiments,
illumination
was
provided
by
a Xe
lamp
(Oriel
67005,
Newport
Corporation)
with
an
AM1.5G
filter
(Newport
Instruments
81094)
that
was
calibrated
to
1 Sun
by
a
calibrated
Si
photodiode. A
mirror
(10Q20BB.HR
broadband
dielectric
mirror,
Newport
Corporation)
was
used
to
direct
the
horizontally
projected
illumination
from
the
lamp
in
the
vertical
direction,
to produce
normal
incidence
light
for
the
bottom-facing
electrodes.
Calibration
was
performed
such
that
the
highest
light
intensity
anywhere
in the
cell
was
at
the
cited
light
intensity,
and
the
position
of
the
photodiode
at
this
light
intensity
was
marked
to assure
sample
positioning
was
at
the
same
point.
Varying
illumination
intensities
were
achieved
through
the
use
of
neutral
density
filters
(ND
0.3)
(Newport
FSQ-OD30). This
approach
allowed
determination
of
the
diode
quality
factor
by
a linear
line
fit
to a plot
of
the
open-
circuit
potential
vs.
ln(J
ph
).
Photoelectrochemical
Characterization
in
Non-Aqueous
Solvents
Bottom-facing
electrodes
that
contained
n-p
+
-Si
microwire
arrays
were
etched
for
10
s in
BHF
immediately
prior
to
introducing
the
samples
into
a glove
box.
Solutions
for
photoelectrochemical
measurements
consisted
of
CH
3
CN
(anhydrous,
99.8%,
Sigma
Aldrich)
dried
through
Al
2
O
3
sieves
in
a solvent
column
system
and
stored
over
dry
molecular
sieves,
1M
LiClO
4
(battery
grade,
99.99%,
Sigma
Aldrich),
and
25
mM
bis(cyclopentadienyl)
iron(II)
(ferrocene,
FeCp
2
0
,
Sigma
Aldrich)
and
3 mM
bis(cyclopentadienyl)
iron(III)
tetrafluoroborate
(ferrocenium,
FeCp
2
+
∙BF
4
-
,
Sigma
Aldrich). Ferrocene
was
purified
by
vacuum
sublimation
at room
temperature,
and
ferrocenium
was
recrystallized
prior
to use. An
ELH-type
W–
halogen
lamp
with
a dichroic
rear
reflector
was
used
for
illumination,
and
was
set
to
produce
the
same
current
density
on
a calibrated
Si
photodiode
as
was
obtained
from
100
mW
cm
-2
of
1 Sun
AM1.5
G illumination.
Three-electrode
photoelectrochemical
data
were
obtained
in
a single-compartment
cell
by
use
of a
Biologic
potentiostat
(SP-200),
with
a Pt
counter
electrode
and
a Pt
quasi-reference
electrode
at
scan
rates
of 20
mV-s
-1
.
Spectral
Response
Spectral
response
measurements
were
obtained
to
determine
the
external
quantum
efficiency
(
ext
)
as
a function
of
wavelength.
Monochromatic
light
with
a bandwidth
of
10
nm
was
produced
using
an
Oriel
monochromator
and
an
Oriel
Xe
lamp. The
beam
was
chopped
at 20
Hz
and
was
focused
to a spot
size
that
under-filled
the
sample. A
mirror
(10Q20BB.HR
broadband
dielectric
mirror,
Newport
Corporation)
was
used
to
direct
the
horizontal
beam
vertically
for
the
bottom-facing
electrodes.
The
electrodes
were
tested
in the
same
photoelectrochemcial
cell
as
described
above
for
the
1 M KOH
measurements
. Each
electrode
was
held
potentiostatically
at
1.63
V vs
RHE
with
a Biologic
potentiostat
(SP200)
that
was
connected
to
a lock-
in
amplifier
(SRS
830).
A
second
lock-in
amplifier
was
connected
to
a second
photodiode
that
continuously
monitored
the
light
intensity
provided
by
a quartz
beam-split
portion
of
the
incident
beam.
Calibration
of the
light
incident
on
the
electrodes
was
performed
using
a bottom-facing
calibrated
photodiode
(Newport
FDS100-CAL)
that
was
placed
in
the
same
location
as
the
electrodes.
Oxygen-Evolution
Faradaic
Efficiency
The
Faradaic
efficiency
for
oxygen
evolution
of
a single
np
+
-Si/TiO
2
/NiCrO
x
microwire
array
electrode
was
measured
in
the
same
cell
as
for
the
1 M
KOH
measurements
described
above.
Both
the
oxygen
concentration
and
the
charge
passed
were
simultaneously
monitored
over
a 40
min
photoelectrochemical
measurement,
After
a 10-min
waiting
period
at open
circuit,
the
microwire
electrode
was
held
potentiostatically
at
1.63
V vs
RHE
with
a Biologic
potentiostat
(SP200)
for
30
min,
and
the
oxygen
concentration
in
solution
was
measured
over
the
entire
40
min
period
with
an
Ocean
Optics
fluorescent
probe
(NeoFox
HIOXY).
Illumination
was
provided
by
a Xe
lamp
(Oriel
67005,
Newport
Corporation)
with
a
AM
1.5G
filter
and
was
calibrated
to
1-Sun
intensity
with
a Si photodiode. Prior
to
measurement,
the
solution
(approximately
48
mL
in
volume)
was
purged
with
N
2
(g)
for
~
20
min
to
obtain
a near
oxygen-free
environment
The10-min
measurement
at
open
circuit
without
oxygen
production
provided
a measurement
of
the
oxygen
leak
rate
of
the
cell.
These
leak
rates
were
interpolated
during
the
40
min
photoelectrochemical
measurement
and
were
subtracted
from
the
overall
oxygen
detection
to yield
an
oxygen
evolution
rate
for
the
electrode
itself.
The
corrected
oxygen
concentration
data
were
then
converted
to
micrograms
of
O
2
,
using
the
amount
of
O
2
dissolved
in water
at
room
temperature
under
1 atm
(7700
μg∙L
-1
),
the
solution
volumne
(~48
mL),
and
the
concentration
of
O
2
in
air
under
1 atm
at
25
C
(20.9%).
100%
Faradaic
efficiency
was
assumed
as
4
electrons
are
used
to
generate
1 O
2
molecule,
i.e.
0.33
mA
h of charge
passed
to
generate
100
μg
of
O
2
. All
the
data
were
processed
using
a Matlab
script.
Imaging
All
scanning-electron
micrograph
(SEMs)
were
taken
on
an
FEI
Nova
NanoSEM
450.
Data
np
+
-Si
Microwire
Array
Non-aqueous
PEC
Prior
to
coating
the
np
+
-Si
and
n-Si
microwires,
small
areas
of each
growth
wafer
were
diced
and
made
into
bottom-facing
electrodes
for
use
in
non-aqueous
photoelectrochemical
measurements
with
a ferrocenium/ferrocene
redox
couple.
Figure
S1
shows
representative
photoelectrochemical
(PEC)
data
for
these
two
different
electrode
types.
The
np
+
-Si
microwire
arrays
exhibited
an
open-circuit
voltage,
V
OC
= 461
± 31
mV
and
a light-limited
current
density,
J
ph
= 11.8
± 2.2
mA
cm
-2
over
eight
electrodes,
while
the
n-Si
microwire
arrays
demonstrated
V
OC
= 311
mV
and
J
ph
= 8 mA
cm
-2
with
only
two
electrodes
tested.
Figure
S 1:
Non-aqueous
ferrocenium/ferrocene
photoelectrochemical
performance
of
a)
np
+
-Si
and
b)
n-Si
microwire
arrays
under
100
mW
cm
-2
of
ELH-type
W-halogen
illumination.