of 5
Supporting Information
A. VLS
–Catalyzed Wire Growth
Arrays
of
undoped
Si
microwires
were
grown
on
both
on
planar
n
+
-
and
p
+
-Si(111)
substrates
using
the
vapor–liquid–solid
(VLS)
method,
with
thermally
evaporated
Cu
(ESPI,
99.9999%)
as the
VLS
growth
catalyst.
The
degenerately
doped
(111)–oriented
n
+
-
and
p
+
-Si
wafers
were
patterned
with
3 μm
diameter
circular
holes,
with
a 7 μm
center-to-center
spacing,
in
a square
array
using
a positive
photoresist
(Microchem
S1813).
The
wafers
were
etched
in
buffered
HF(aq)
(BHF,
Transene
Inc.)
for
5 min
to
remove
the
exposed
thermal
oxide,
and
450–
600
nm
Cu
was
thermally
evaporated
onto
the
patterned
growth
substrates.
The
wafers
were
thoroughly
rinsed
in
acetone
and
isopropyl
alcohol,
and
then
cleaved
into
1.3
x 2.0
cm
pieces.
The
lithographically
patterned
planar
substrates
with
the
Cu
catalyst
were
annealed
in
a tube
furnace
at
1000º
C
for
20
min
with
500
sccm
of
H
2
(Research
grade,
ALPHAGAZ™
2,
Air
Liquide)
at atmospheric
pressure.
VLS
wire
growth
occurred
at 1000
ºC
using
SiCl
4
(99.9999%-
Si PURATREM,
Strem)
in
50 sccm of H
2
, without the introduction of dopants, for
20–45 min.
B. Corrections of
J
-
E
Data
for
Concentration Overpotential and Series Resistance
The
J
-
E
data
from
the
Me
2
Fc
+/0
–CH
3
OH
cell
were
corrected
for
concentration
overpotential
(
η
conc
)
and
series resistance (
R
s
) losses using
eq. 1 and 2:
(1)
c
o
n
c
k
B
T
n
q
l
n
J
l
,
a
J
l
,
c
l
n
J
l
,
a
J
J
J
l
,
c
(2)
E
c
o
r
r
E
m
e
a
s
i
R
s
c
o
n
c
1
Electronic
Supplementary
Material
(ESI)
for
Energy
&
Environmental
Science.
This
journal
is
©
The
Royal
Society
of
Chemistry
2014
where
k
B
is Boltzmann’s
constant;
T
is the
absolute
temperature;
q
is the
(unsigned)
charge
on
an
electron;
n
is stoichiometric
number
of
electrons
transferred
in
the
electrode
reaction
(
n
= 1 for
Me
2
Fc
+/0
);
and
J
l,a
and
J
l,c
are
the
anodic
and
cathodic
mass-transport-limited
current
densities,
respectively.
A
Pt
foil
working
electrode
of
comparable
area
to
the
Si
working
electrodes
was
used
to
measure
J
l,a
and
J
l,c
,
and
R
s
of
the
cell.
The
measured
limiting
anodic
current
density
was
72
mA
cm
-2
and
the
limiting
cathodic
current
densities
were
0.15
and
15
mA
cm
-2
,
for
0.4
mM
and
40
mM
Me
2
FcBF
4
,
respectively.
The
value
of
R
s
varied
from
40–300
Ω,
due
to
variations
in
the
electrode
placement
relative
to
the
Luggin
capillary
reference;
a conservative
value
of
R
s
=
50
Ω
was used
in
the
calculations to avoid overcorrection of the data.
In
J
-
E
measurements,
the
electrode
areas
were
less
than
0.03
cm
-2
,
to limit
resistance
losses
within
the
electrochemical
cell,
but
greater
than
0.02
cm
-2
,
such
that
the
electrode
areas
could
be accurately
measured.
Electrodes
with
areas
less
than
0.02
cm
-2
exhibited
measurable
effects
from
epoxy
creeping
into
the
wire
array,
and
also
exhibited
artificially
enhanced
photocurrent
from
light
scattering
into
the
array
from
the
surrounding
epoxy.
For
all
J
-
E
measurements,
the
light
calibration
was
confirmed
by
measurements
using
a planar,
single
crystal
n-Si
photoelectrode
with
an area
of
~0.03
cm
-2
in the
electrochemical
cell.
The
behavior
of these electrodes has been
well established under
1 Sun
illumination.
C. Chemical–mechanical polishing of Si microwire arrays
After
removal
of
the
Cu
VLS
catalyst,
arrays
of
Si
microwires
of
dimensions
of
~ 3 cm
x
2
cm
were
cleaved
in
half
longitudinally.
Half
of
the
array
was
reserved
for
the
fabrication
of
unpolished,
control
electrodes.
The
other
half
of the
array
was
again
cut
in half,
to
create
to
smaller
chips
for
more
uniform
polishing.
Each
chip
was
mounted
on
a flat,
1 inch
diameter
stainless
steel
mounting
block
using
a small
amount
of mounting
wax
(Quickstick
135,
South
2
Bay
Technology)
on
a hot
plate
at
~ 150
ºC.
The
array
was
subsequently
infilled
with
mounting
wax,
and
the
wax
was
allowed
to
equilibrate
within
the
array
for
30
min;
the
resulting
array
was
completely
filled
to
the
tops
of
the
wire
arrays
with
wax.
Additional
mounting
wax
was
placed
around
the
perimeter
of
the
array,
to
prevent
the
removal
of
wires
at the
edge
of
the
array
during
polishing.
The
array
infilled
with
wax
was
polished
using
a succession
of
aluminum
oxide
suspensions
of
3
μm,
1
μm
and
0.3
μm
with
polishing
cloth
(MultiTex™,
South
Bay
Technology).
The
array
was
thoroughly
rinsed
in
> 18
M
Ω-cm
resistivity
H
2
O
periodically
and
between
different
suspensions.
To
gauge
the
polishing
rate,
the
focal
planes
of
the
top
most
wires
and
the
shortest
wires
were
determined
using
an
optical
microscope.
Polishing
was
terminated
shortly
after
all
of
the
wires
were
the
same
height,
and
the
array
was
then
subjected
to
a final polish using a
colloidal silica suspension (SBT, 0.02–0.06 μm).
SI Figure Captions.
Figure
S1.
J
-
E
behavior
of
lightly
doped
Si
microwire
arrays
grown
on
an
n
+
-Si
substrate,
in
contact
with
the
Me
2
Fc
+/0
–CH
3
OH
redox
system,
with
an
increased
concentration
of
Me
2
FcBF
4
and the corrected
J
-
E
response.
Figure
S2.
A)
Top
view
SEM
image
of
a mechanically
polished
Si
microwire
array,
scale
bar
= 4
μm B) Side view
SEM
image of the same array. Scale
bar = 20 μm.
Table
S1.
Figures of merit of undoped Si microwire array photoelectrodes.
3
Figure S1.
Figure S2.
4
Table
S1.
5
V
oc
(mV)
J
sc
(mA cm
-2
)
ff
Efficiency
(%)
n
+
/i-Si/ Me
2
Fc
+/0
–CH
3
OH
i-Si on
n
+
substrate (ELH)
445 ± 13
12.8 ± 2.1
0.41 ± 0.03
2.3
± 0.3
i-Si on
n
+
substrate (808 nm)
436 ± 14
12.8 ± 2.1
0.58 ± 0.02
5.9
± 1.0
Corrected i-Si
on n
+
substrate
445 ± 13
12.9 ± 2.1
0.62 ± 0.04
3.5
± 0.6
Wires Removed,
n
+
substrate
7.5 ± 0.7
0.9 ± 0.01
0.34 ± 0.07
0.002 ± 0.003
p
+
/i-Si/ CoCp
2
+/0
–MeCN
i-Si on
p
+
substrate (ELH)
421 ± 14
10.9 ± 0.3
0.32 ± 0.02
1.5
± 0.1
Wires Removed,
p
+
substrate
253
± 1
1.75 ± 0.11
0.27 ± 0.05
0.11 ± 0.01
p
+
/i-Si/ Me
2
Fc
+/0
–CH
3
OH
i-Si on
p
+
substrate (ELH)
0.14 ± .07
0.10 ± 0.03
i-Si on
p
+
substrate (dark)
0.42 ± .09
0.17 ± 0.04
n
+
/i-Si/ CoCp
2
+/0
–MeCN
i-Si on
n
+
substrate (ELH)
0
0.02 ± 0.01
i-Si on
n
+
substrate (dark)
0
0.04 ± 0.02