Supporting Information for:
Spontaneous
Mesostructure
Formation
Produces
Optically
Transmissive
NiP
Films
That
are
Catalytically Active for the Photoelectrochemical Hydrogen
Evolution Reaction
Zachary
P.
Ifkovits,
1
Jillian
T.
Reed,
1
Paul
A.
Kempler,
1
Madeline
C.
Meier,
1
Sean
T. Byrne,
1
Shaoyang Lin,
1
Alexandre Z. Ye,
1
Azhar I. Carim,
1,2
and Nathan S. Lewis
1,2*
1
Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena,
California 91125, USA.
2
Beckman Institute Molecular Materials Resource Center, California Institute of Technology,
Pasadena, California
91125, USA.
*
Corresponding author:
nslewis@caltech.edu
Electronic
Supplementary
Material
(ESI)
for
Sustainable
Energy
&
Fuels.
This
journal
is
©
The
Royal
Society
of
Chemistry
2023
Experimental Details
Materials:
Copper
sheets
(Cu,
99.999%,
Alfa
Aesar),
indium-gallium
eutectic
(In-Ga,
99.99%,
Alfa
Aesar),
sodium
hypophosphite
monohydrate
(NaPO
2
H
2
·H
2
O,
Sigma),
nickel
chloride
(NiCl
2
,
98%,
Sigma),
boric
acid
(H
3
BO
3
,
>99.5%,
Sigma),
and
sodium
chloride
(NaCl,
99%,
Macron
Chemicals)
were
commercially
available
and
used
as
received.
Sulfuric
acid
(H
2
SO
4
,
TraceMetal
grade,
Thermo
Fisher
Scientific),
and
hypophosphorous
acid
(H
3
PO
2
,
50%
weight
in
water,
VWR
International)
were
diluted
with
deionized
water
with
a resistivity
of
>18.2
MΩ·cm
produced
by
a
Barnstead
Nanopure
system.
100
mm
diameter,
525
μm
thick
n
+
-type
(100)-oriented
Si
wafers
with
a nominal
resistivity
of
< 0.005
Ω·cm,
and
p-type
(100)-oriented
Si
wafers
of
the
same
dimensions with a
nominal resistivity of
1-10
Ω·cm,
were
purchased from Addison
Engineering.
Working Electrode Fabrication:
15-30
mm
2
chips
of n
+
-Si
or
doped
homojunction
n
+
-p-Si
were
used
as
the
substrates
for
working
electrodes.
To
promote
adhesion
between
the
substrates
and
the
electrochemically
deposited
layer,
metallic
Ti and
then
Ni
were
each
sputtered
(AJA
Orion)
onto
the
samples
for
90
s
at
deposition
powers
of 130
W
and
100
W,
respectively,
Ohmic
contacts
were
formed
by
scratching
In-Ga
eutectic
into
the
backside
of
the
chips
with
a scribe.
Insulated
Sn-coated
Cu
wires
with
stripped
ends
were
fed
through
a 6 mm
borosilicate
glass
tube.
The
wire
was
sealed
into
the
glass
tube using
epoxy
(Hysol
9460).
Samples
were
attached
to the
wire
with
Ag
paint
that
had
a
grain
size
< 1.0
μm
(PELCO,
Ted
Pella),
and
the
back
of
the
electrode
was
sealed
using
clear
nail
polish
(Sally
Hansen).
After
the
film
had
been
deposited,
the
sealant
was
readily
removed
with
a
razor
blade
and
the
sample
was
separated
from
the
wire.
To
prepare
electrodes
with
Cu
substrates,
a Cu
sheet
was
hand
polished
using
a circular
motion
for
5 min
for
each
different
grit
size:
2400
(European
FEPA
standard),
followed
by
3000,
and
then
by
4000.
This
sheet
was
cut
with
scissors
into
smaller
10-20
mm
2
pieces.
The
Cu
substrates
were
attached
to
an electrode
assembly
by
the
same process
as described above
for Si substrates, but without the In-Ga contact.
Film Deposition:
Shortly
before
film
deposition,
metalized
n
+
-Si
samples
were
sprayed
with
deionized
H
2
O
and
dried
under
a stream
of N
2
(g).
The
Cu
substrates
were
etched
for
60
s in 6.6
M HCl(aq)
to
remove
surface
oxides.
Electrodeposition
of
the
NiP
film
was
performed
in a single-compartment
borosilicate
glass
cell
that
contained
0.15
M
H
3
BO
3
,
0.10
M NaCl,
0.30
M
H
2
NaPO
2
· H
2
O,
and
0.20
M
NiCl
2
.
The
pH
of
the
solution
was
adjusted
from
6.5
to 3.5
by
dropwise
addition
of
H
3
PO
2
(aq)
(10%
wt).
Control
over
either
the
current
or
potential
of
the
working
electrode
was
obtained
using
a Biologic
SP-200
potentiostat.
The
electrolyte
was
purged
with
N
2
(g)
for
at least
30
min
prior
to the
deposition
and
the
flow
of N
2
(g)
was
maintained
during
the
deposition
process.
A
saturated
calomel
electrode
(SCE)
was
used
as
the
reference
electrode,
and
a Ni
foil
electrode
was used as the
counter
electrode.
A
thermocouple
was
immersed
directly
into
the
electrolyte
to
monitor
the
deposition
temperature.
The
cell
was
secured
to
a ring
stand
with
a clamp
and
held
in
a water
bath
so
that
the
water
completely
surrounded
the
solution
without
contaminating
the
cell.
A hot
plate
was
used
to
raise
the
temperature
of the
water
bath
above
room
temperature
(20
ºC).
A
magnetic
stir
bar
was
placed
inside
the
cell
and
stirring
at a rate
of
280
RPM
was
performed
for
the
duration
of
the
deposition.
Depositions
onto
n
+
-Si
and
Cu
were
performed
at
a current
density
of
-20
mA
cm
-2
.
Electrodepositions
onto
the
photoactive
n
+
-p-Si/Ti/Ni
electrodes
were
performed
using
a light-
emitting
diode
(LED)
source
that
had
an
intensity-weighted
average
wavelength
of
630
nm
and
approximate
intensity
of
600
mW
cm
-2
.
In
these
photoelectrochemical
depositions,
the
working
electrode was
held
potentiostatically
at -0.6 V vs. SCE.
Electrochemical Measurements and Physical Characterization
The
catalytic
behavior
of the
Ni-P
films
for
the
hydrogen-evolution
reaction
was
measured
at
ambient
temperature
in
0.50
M
H
2
SO
4
(aq)
that
was
purged
with
H
2
(g).
A
saturated
calomel
electrode
(SCE)
was
used
as
the
reference
electrode,
and
a graphite
rod
was
used
as
the
counter
electrode. Measurements
were
obtained
using
a
Biologic
SP-200
potentiostat.
Photoelectrochemical
hydrogen
evolution
experiments
were
performed
in
H
2
-purged
0.50
M
H
2
SO
4
(aq)
under
the
equivalent
of
100
mW
cm
-2
of
solar
Air
Mass
1.5
illumination
produced
by
a calibrated Xe lamp.
Scanning-electron
micrographs
(SEMs)
were
recorded
with
a FEI
Nova
NanoSEM
450
at
an
accelerating
voltage
of
X kV
with
a working
distance
of
5 mm
and
in-lens
secondary
electron
detector. Energy-dispersive
X-ray
(EDX)
spectroscopy
was
performed
in the
SEM
to
determine
the
bulk
composition
of
the
NiP
films.
An
accelerating
voltage
of
10
kV
and
an Oxford
Instruments
X-Max
Si drift
detector
was
utilized.
Spectra
were
collected
in the
range
of
0 to
10
keV
and
quantitative
film
compositions
were
derived
from
these
spectra
using
the
“Inca”
software
package
(Oxford
Instruments).
All
other
observed
elements
were
either
attributable
to
the
electrode
base
(Ti, Si, or Cu) or were negligible in intensity.
X-ray
photoelectron
spectroscopy
(XPS)
was
performed
using
a Surface
Science
M-Probe
ESCA/XPS.
The
chamber
base
pressure
was
< 2 × 10
-8
.
The
X-ray
source
was
a monochromatic
Al
Kα
source.
The
data
were
analyzed
using
CasaXPS
computer
software.
A Shirley
background
was
used
for
all
spectra,
which
were
referenced
to
adventitious
C at a binding
energy
of 284.8
eV.
Atomic
force
microscopy
was
performed
using
a Bruker
Dimension
Icon
to image
the
surface
morphology
of
the
deposited
films.
The
instrument
was
operated
in ScanAsyst
mode
using
a silicon
nitride
probe
(ScanAsyst-Air).
The
data
were
collected
using
scan
areas
of
500
nm
x 500
nm
with
a probe
velocity
500
nm
s
-1
and
a pixel
size
of
1.95
nm
x 1.95
nm.
Raw
data
were
corrected
for sample
tilt
using the
Bruker NanoScope
Analysis
software.
Samples
of NiP
films
electrodeposited
onto
FTO/Ti/Ni
substrates
were
analyzed
using
a
Cary
5000
UV-vis-NIR
with
an
integrating
sphere.
Optical
transmission
measurements
were
performed
after
subsequent
etching
cycles
of
the
same
samples
at
open
circuit
in 0.50
M
H
2
SO
4
(aq).
Samples
of NiP
films
electrodeposited
onto
n
+
Si/Ti/Ni
substrates
were
removed
from
the
electrode
body
by
use
of
a razor
blade.
The
nail
polish
used
to seal
the
back-contact
of
the
electrode
was
rigorously
removed
with
the
razor
blade
and
isopropyl
alcohol,
when
necessary.
The
sample
chips
were
then
placed
in
screw
cap
septum
vials
(2
mL)
and
submerged
in
a mineral
oil
bath.
The
bath
was
stepped
from
ambient
temperature
to
200
°C
in 25
°C
increments.
The
data
shown
in
Figure
3 were
collected
after
samples
had
been
held
for
10
min
at 200
°C.
For
each
measurement,
0.2
mL
of the
headspace
was
injected
into
an
Agilent
7890A
gas
chromatography
system
equipped
with
a thermal
conductivity
detector
(TCD),
Ar
as
the
carrier
gas,
a backflush
system
with
a
PoraPLOT
Q column
as
the
pre-column,
and
a HP-PLOT
Molesieve
column
as
the
main
column.
The quantity
of the
hydrogen
was
calculated
based
on
a calibration curve.
The
optical
properties
of
the
NiP
samples
were
characterized
using
a spectroscopic
ellipsometer
with
a rotating
analyzer
(J.A.
Woolam
Co.,
Inc.).
Data
were
recorded
from
300
to
1100
nm
in
10
nm
increments
at
an
angle
of
incidence
of
70º.
The
refractive
index
(
n
)
and
extinction
coefficient
(
k
) of
the
film
were
determined
using
a Maxwell-Garnett
effective
medium
approximation.
Supporting Figures
Cross-Sectional SEMs
Figure S1.
Cross-sectional SEMs
of as-deposited NiP films with a cathodic charge density
passed of 400 mC cm
-2
in electrodeposition baths at (a) 20 °C,
(b) 40 °C, (c) 60 °C, and (d) 80
°C.
Figure S2.
Cross-sectional SEMs
of as-deposited Ni-P films at 20 °C, for a range of
cathodic
charge density passed between 100-400 mC cm
-2
.
Figure S3.
Thickness of deposited films as determined from
cross-sectional SEMs in (a) Figure
S1 for films deposited
at 400
mC
cm
-2
(b) from Figure S2
of films deposited
at 20 °C
over
a
range of cathodic charge density, and (c) of films deposited at 80 °C over a
range
of cathodic
charge density.
Cross-sectional
SEMs
used
to
determine
the
thickness
of
electrodeposited
films
were
based
on
n
>
7 measurements
from
multiple
spots
along
duplicate
samples.
The
Ni-P
film
thickness
was
determined
to be ~90-100
nm
at all
temperatures,
when
400
mC
cm
-2
of
charge
density
was
passed
during
electrodeposition
(Figure
S3a).
The
measured
film
thicknesses
deposited
at
20
°C
and
80
°C
increased
linearly
as
a function
of
the
charge
density
passed
(Figure
S3b-c).
A linear
fit
showed
the
growth
rates
to
be 0.23
nm
cm
2
mC
-1
and
0.26
nm
cm
2
mC
-1
for
the
NiP
20°C
and
NiP
80°C
films,
respectively.
Forcing
the
line
in
S3c
through
the
origin
reduces
the
rate
to
0.24
nm
cm
2
mC
-1
,
and
a
straight
line
fit through
the
origin
to
the
point
at 400
mC
cm
-2
changes
the
rate
to 0.255
nm
cm
2
mC
-1
.
Measurements
for
the
NiP
20°C
films
were
taken
across
the
top
of
the
film,
so the
reported
deposition
rate
is likely
an overestimation
as
those
films
were
not
as
conformal
in
cross-section
as
films deposited
at higher temperatures.
Photos of Working
Electrodes during
Electrodeposition
B)
80°C
A)
20°C
Figure
S4.
Still
frames
taken
from
videos
of
electrodeposition
of
NiP
at
-20
mA
cm
-2
onto
n
+
Si/Ti/Ni electrodes in NiP deposition baths at A) 20 °C and
B) 80 °C.
Figure
S4a
shows
the
presence
of
H
2
(g)
bubbles
across
the
entire
surface
of
the
n
+
Si/Ti/Ni
working
electrode
while
the
electrode
was
passing
-20
mA
cm
-2
in a deposition
bath
at
20
°C.
Figure
S4b
shows
the
surface
of
an
identical
electrode
at -20
mA
cm
-2
in
a deposition
bath
at 80
°C.
The
slight
yellow
hue
in
Figure
S4b
is due
to
the
yellow/beige-colored
top
surface
of
the
hot
plate
that
was
used to heat the deposition solution.
Faradaic Efficiency Measurements
Figure S5.
(a) Summary of anodic
stripping measurements demonstrating the faradaic efficiency
of Ni-P electrodepositions
at 20°C
(blue)
and 80°C (red), as well as dashed lines showing
expected values at 65%
and 95% faradaic
efficiency,
(b) net anodic charge density passed under
a linear sweep of potential for films deposited at
20°C, (c)
net
anodic
charge
density passed
under a
linear sweep
of
potential for
films deposited at 80°C.
EDX compositional
data
Figure S6.
Nickel composition of deposited Ni-P films before (light
blue circle)
and
after (dark
blue circle) etching for 300 s
in 0.50 M H
2
SO
4
(aq)
as
determined by EDX from
≥
12 regions of
replicate samples.
The
long-term
stability
of
these
photocathodes
was
not
assessed
as
the
Ni-P
is
electrodeposited
directly
onto
an
unprotected
silicon
surface,
which
is
susceptible
to
chemical
oxidation
and
degradation
in
acidic
electrolytes,
but
the
photoelectrodes
showed
stability
for
as
long
as they
were
analyzed
(~30
minutes
to
1 hour).
The
catalytic
films
deposited
onto
metallic
substrates
showed
stable
operation
for
hours
(Figure
4b).
We
did
not
determine
the
behavior
of
films
deposited
at temperatures
below
room
temperature,
but
we
do
not
believe
this
experiment
is
necessary to support our conclusions.
Table S1.
Summary
of top-down and cross-sectional EDX measurements of NiP
20°C
films
Sample
Collection Angle
Depth from Surface (nm)
Ni Composition
(Ni/Ni+P)
As
deposited
Top-Down
N/A
92.1% ±
0.8%
As
deposited
Cross section
~10 nm
90.6% ±
0.6%
As
deposited
Cross section
~50 nm
91.3% ±
0.6%
As
deposited
Cross section
~90 nm
90.6% ±
2.3%