of 8
S
-
1
Supporting
Information
Phase
D
irecting
A
bility of an
I
onic
L
iquid
S
olvent for the
S
ynthesis of
HER
-
Active
Ni
2
P
N
anocrystals
Emily J. Roberts,
Carlos G. Read,
Nathan S. Lewis,
,
*
and Richard L. Brutchey
,
*
Department of Chemistry, University of
Southern Cal
ifornia, Los Angeles, California
90089, USA
Division of Chemistry and
Chemical Engineering, California Institute of T
echnology, Pasadena, California
91125, USA
*Email: brutchey@usc.edu; nslewis@caltech.edu
Table of contents
Materials
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Experimental Procedure.......
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1
Characterization Methods....................
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-
2
Fig. S1
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Fig. S2
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Fig. S3
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Fig. S4
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S
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4
Fig. S5.
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5
Fig. S6.
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S
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5
Fig. S7.
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5
Fig. S8.
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Fig. S9.
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Fig. S10
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S
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6
Table S1.
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Fig. S11
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Fig. S12
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.S
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8
Materials
Nickel(II)
acetylacetonate (Ni(acac)
2
, 95%, Lot #W05A005) and
triphenylphosphine (PPh
3
, 99+%, Lot #X04CO11) were
purchased from Alfa Aesar. Co(II) acetylacetonate (Co(acac)
2
,
97
%, Lot#MKBW8226V),
o
leylamine (OAm, 90%, Lot
#STBG3454V), 1
-
octadecene (ODE, 90%, Lot #MKBW6045V)
, and
t
itanium foil
(
99.7%, 0.25 mm
thickness
)
were
purchased from Sigma
-
Aldrich. 1
-
butyl
-
3
-
methylimidazolium bis(trifluoromethylsulfonyl)imide (BMIM
-
Tf
2
N,
99%) was
purchased from IoLiTec (Lot # P00164.1.Inc
-
IL
-
0029).
OAm and ODE were degased at 120 ̊C for 5 h and stored under
nitrogen befor
e use; all other reagents were used as received.
High
-
quality colloidal Ag paint was purchased from SPI
Supplies. Two
-
part epoxy
(
HYSOL 9460
)
was purchased from McMaster
-
Carr.
Experimental
Synthesis of Nickel Phosphide
Nanocrystals
The Ni
2
P
nanocrystals
were prepared by combining Ni(acac)
2
(200 mg, 0.780 mmol), PPh
3
(2.5 g, 9.5 mmol), OAm (2.0
mL, 6.1 mmol) and BMIM
-
Tf
2
N
(5 mL) in a 25
-
mL round bottom flask with a PTFE
-
coated stir bar. The flask was fitted
with a condenser and vacuum inlet adapter. The r
eaction mixture was stirred at 1000 rpm and heated to 100
̊C under
vacuum for 1 h to remove residual water and other low
-
boiling
species
. The flask was subsequently placed under
flowing
N
2
(g)
and heated to 310 ̊C
over a period of
~18 min
,
and held at that
temperature for 15, 30, and 60
min,
respectively
. The reaction was
quenched
by removing the flask from the sand bath and allowing it to cool naturally to
S
-
2
room temperature. The resulting reaction mixture was transferred to two 50
-
mL centrifuge tubes with th
e aid of 10
mL of ethanol
,
and 30 mL of hexanes was added to each
tube
. The mixture was vortex mixed (30 s), sonicated (5 min),
and centrifuged for 5 min (6000 rpm). The resulting supernatant was biphasic
,
and
the top layer (hexanes)
was
decanted
off to le
ave a bottom layer (emulsion of
ethanol
and BMIM
-
Tf
2
N). The Ni
2
P
nanocrystals
were washed twice
with, 1:1.5:1.5 (v/v/v) of CHCl
3
/acetone/hexanes and
were then
centrifuged for 10 min (6000 rpm). The final two
washes were performed with 1:3 (v/v) of
ethanol
/
hexanes to give a
nanocrystal
suspension that was colloidally stable
in
ethanol
(~40 mL).
For mixed
-
metal Ni
2
x
Co
x
P nanocrystals, the stoichiometry was altered by changing the nominal
precursor ratio while maintaining a total of 0.780 mmol of metal precurs
or. The reaction time was also increased to
1
h, but the reaction and workup were otherwise analogous to that used to synthesize the Ni
2
P nanocrystals.
Determination of Yield
To estimate the
yield of
Ni
2
P
nanocrystals
, a known volume of the final suspension was dried
under N
2
(g) in a pre
-
weighed vial,
to remove the solvent. The resulting
nanocrystal
powder was analyzed
thermo
gravimetrically (
to
750 ̊C
under flowing
N
2
(g)
) to estimate the organic ligand content. The ove
rall yield was calculated according to Equation S1:
푚푎푠푠
표푓
푝푟표푑푢푐푡
푖푛
푑푟푖푒푑
푣푖푎푙
푘푛표푤푛
푣표푙푢푚푒
표푓
푐표푙푙표푖푑푎푙
푠푢푠푝푒푛푠푖표푛
푖푛
푣푖푎푙
×
푡표푡푎푙
푣표푙푢푚푒
표푓
푠푢푠푝푒푛푠푖표푛
×
푁푖
!
푤푡
%
푓푟표푚
푇퐺퐴
=
푡표푡푎푙
푚푎푠푠
표푓
푁푖
!
푐표푟푟푒푐푡푒푑
푓표푟
푙푖푔푎푛푑
푐표푛푡푒푛푡
!"#$!%
!"##
!"
!"
!
!
!"##$!%$&
!"#
!"#$%&
!"#$%#$
!"
!
!
!"!#$%&'(")
!
!
!"#!$%&'(
!"#
!"
×
100%
=
푁푖
!
푛푎푛표푐푟푦푠푡푎푙
푦푖푒푙푑
(S1)
For example, the yield from the 7 mL precursor solution
(
containing 200 mg Ni(acac)
2
) that led
to 40 mL of a Ni
2
P
nanocrystal
product suspension in
ethanol
would be determined as follows:
A 2
0 mL aliquot of the Ni
2
P
nanocrystal
product suspension
was
dried in a pre
-
weighed vial to yield 21.8 mg of
product.
A
~13 wt% ligand content of the resulting powder was determined using TGA. The yield was determined
according to equation S1:
26
.
98
푚푔
표푓
푝푟표푑푢푐푡
20
푚퐿
표푓
푐표푙푙표푖푑푎푙
푠푢푠푝푒푛푠푖표푛
×
40
푚퐿
×
0
.
87
푁푖
!
푤푡
%
=
46
.
95
푚푔
푎푐푡푢푎푙
푁푖
!
푛푎푛표푐푟푦푠푡푎푙
푦푖푒푙푑
46
.
82
푚푔
표푓
푝푟표푑푢푐푡
57
.
80
푚푔
푒표푟푒푡푖푐푎푙
푁푖
!
푛푎
푛표푐푟푦푠푡푎푙
푦푖푒푙푑
×
100%
=
81%
푁푖
!
푛푎푛표푐푟푦푠푡푎푙
푦푖푒푙푑
Preparation of Working Electrodes
Ni
2
P
nanocrystal
suspensions were drop
cast onto
~
0.25 cm
2
pieces of Ti foil
,
to
produce
a loading of ~1 mg cm
2
.
After drying, the
nanocrystal
-
modified Ti foils were annealed for 1 h at 400 °C under a flow of 5%
H
2
/N
2
(g)
. The foils
were
affixed
to
a polyvinyl chloride
-
coated copper wire using Ag paint. Two
-
part epoxy was then used to cover all
surfaces except for the
nanocrystal
films
.
Electrochemical Measurements
Electrochemical measurements were performed using a Bio
-
Logic SP
-
200 potentiostat. The measurements in 0.5 M
H
2
SO
4
were performed using a two
-
compartment, three
-
electrode cell. The two compartments were separated by a
Nafion
membrane to minimize contamination of the working electrode by the contents of the counter electrode
solution. Mercury/mercurous sulfate (Hg/Hg
2
SO
4
, saturated K
2
SO
4
) was used as the reference electrode whereas the
counter electrode was a Pt mesh. In
alkaline conditions
,
a single
-
compartment, three
-
electrode cell was utilized,
along with a Hg/HgO reference and a carbon mesh counter electrode. Polarization data were collected at a sweep
rate of 10 mV/s while bubbling the solution with ~1 atm of H
2
(g). T
he solutions were rapidly stirred using a magnetic
stir bar. The current
-
interrupt method was used to compensate for uncompensated resistance.
Characterization
Thermogravimetric
a
nalysis (TGA):
TGA m
easurements were made on a TA Instruments TGA Q50 instru
ment, using
sample sizes of
5 mg in an alumina crucible under flowing
N
2
(g)
.
Prior to analysis,
TGA samples were prepared by
fully drying the colloid
suspension
overnight
under flowing
N
2
(g) at ambient temperature
.
S
-
3
Powder X
-
ray diffraction (XRD):
XRD
patterns were collected on a Rigaku Ultima IV diffractometer functioning at
40
mA and 40
kV with a Cu
K
α
X
-
ray source (
λ
=
1.5406
Å). The step size and collection time were 0.02 ̊ and 19 s per
step,
respectively.
Transmission electron microscopy (
TEM) and
e
nergy
d
ispersive X
-
r
ay
s
pectroscopy (EDS):
TEM images were obtained
using a JEOL JEM2100F (JEOL Ltd.) microscope operating at 200
kV. Samples were prepared on 400 mesh Cu grids
coated with a lacey carbon film (Ted Pella, Inc.) by drop
-
casting dilute
suspensions of colloidal Ni
2
P nanocrystals in
ethanol or
hexanes.
The average particle diameter and size distribution
of the
Ni
2
P
nanocrystals
synthesized in BMIM
-
NTf
2
were
determined by counting 500 individual particles.
For the particles synthesized in O
DE
,
150 individual
particles were counted by measuring the
anisotropic particles
longest length.
Elemental maps of Ni
1.0
Co
1.0
P
nanocrystals
were acquired using a JEOL JEM2100F (JEOL Ltd) transmission electron microscope operating at 200 kV,
equipped with an EDAX Octane T Plus silicon drift detector. Elemental maps were acquired by signal averaging over a
period of 5 min. Sam
ples for STEM
-
EDS
studies pre
-
pared by drop
-
casting a stable suspension of nanocrystals in
ethanol on a 400 mesh Cu grid coated with a lacey carbon film (Ted Pella, Inc.) and dr
ying
under vacuum overnight
prior to imaging.
X
-
ray photoelectron spectroscopy (XPS):
XPS spectra
were acquired on a Kratos Axis Ultra X
-
ray photoelectron
spectrometer with the analyzer lens in hybrid mode. High resolution scans were performed using a monochromatic
Al
anode with an operating current of 5 mA and voltage of 10 kV
,
a step size of 0.1 eV,
a pass energy of 20 eV, and a
pressure range between 1 and 3
×
10
8
Torr. The binding energies for all spectra were referenced to the adventitious
C
1
s
core level at 284.8 eV.
Casa XPS analysis software was used for peak deconvolution
.
T
he stoichiometry of
samples was determined from corrected peak areas and employing sensitivity factors
provided by Kratos
for each
element of interest.
Fourier
-
Transform Infrared Spectroscopy (FT
-
IR):
FT
-
IR spectra were acquired on a Bruker Vertex 80
using drop
-
casted
suspensions of the Ni
2
P
nanocrystals
after
purification on ZnSe plates.
Fig
ure
S1
.
Thermogravimetric analysis of Ni
2
P nanocrystals synthesized using BMIM
-
Tf
2
N.
Fig
ure
S2
.
TEM micrograph of Ni
2
P nanocrystals synthe
sized using ODE as the solvent and corresponding size
distribution.
S
-
4
Fig
ure
S3
.
High
-
resolution XPS spectra of (a) F 1
s
, (b) S 2
p
, and (c) the survey scan of Ni
2
P nanocrystals
synthesized
using BMIM
-
Tf
2
N.
Fig
ure
S4
.
FT
-
IR spectra of pure BMIM
-
Tf
2
N
(blue), PPh
3
(green), OAm (pink), and
Ni
2
P nanocrystals synthesized using
BMIM
-
Tf
2
N
post
-
purification (tan).
S
-
5
Fig
ure
S5
.
Galvanostatic stability measurement, held at a
potential of
-
20 mA/cm
2
for 15 h
in 0.5 M H
2
SO
4
.
Figure S6.
Polarization data for
Ni
2
P
nanocrystals
in 0.5 M H
2
SO
4
initially and after 100, 200, 300, 400, and 500 CV
sweeps between 0 and
-
0.20 V vs
.
RHE at a rate of 20 mV/s.
Figure S
7
.
Chronopotentiometr
ic stability measurement
at
-
10 mA
/
cm
2
for 1
5
h in 1 M NaOH.
0
5
10
15
-0.4
-0.3
-0.2
-0.1
0.0
Time (hours)
Overpotential (V vs. RHE)
-0.25
-0.20
-0.15
-0.10
-0.05
0.00
-20
-15
-10
-5
0
Potential (V vs. RHE)
Current Density (mA/cm
2
)
500
400
300
200
100
Initial
0
5
10
15
-0.4
-0.3
-0.2
-0.1
0.0
Time (hours)
Overpotential ( V vs. RHE)