Simultaneous Control of Composition, Size, and Morphology in Discrete Ni2-xCox P Nanoparticles

Article abstract of DOI:10.1021/acs.chemmater.5b00958

A synthetic protocol developed to produce phase-pure, nearly monodisperse Ni_2-xCo_xP nanoparticles (x ≤ 1.7) is described. The Ni_2-xCo_xP particles vary in size, ranging from 9-14 nm with standard deviations of <20% (based on transmission electron microscopy analysis), and the actual metal ratios obtained from energy-dispersive spectroscopy closely follow the targeted ratios. With increasing Co, samples with larger size distributions are obtained and include particles with voids, attributed to the Kirkendall effect. To probe the mechanism of ternary phosphide particle formation, detailed studies were conducted for Ni:Co = 1:1 as a representative composition. It was revealed that the P:M ratio, heating temperature, and heating time have a large impact on the nature of both intermediate and final crystalline particles formed. By tuning these conditions, nanoparticles can be produced with different sizes (from ca. 7-25 nm) and morphologies (hollow versus dense).

Full text of DOI:10.1021/acs.chemmater.5b00958

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Article
Simultaneous control of composition, size and
morphology in discrete Ni2-xCoxP nanoparticles
D. Ruchira Liyanage, Samuel J Danforth, Yi Liu, Mark E Bussell, and Stephanie L. Brock
Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b00958 • Publication Date (Web): 20 May 2015
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Chemistry of Materials
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Simultaneous control of composition, size and morphology in dis-
crete Ni2-xCoxP nanoparticles
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D. Ruchira Liyanage,1 Samuel J. Danforth,2 Yi Liu,3,† Mark E. Bussell2 and Stephanie L Brock1,*
1. Department of Chemistry, Wayne State University, Detroit Michigan 48202, (USA)
2. Department of Chemistry, Western Washington University, Bellingham, WA 98225 (USA)
3. Electron Microscopy Facility, Oregon State University, Corvallis Oregon 97331 (USA)
Center, Chrysler LLC, 2301 Feather Stone Road, Auburn Hills, MI 48326 (USA)
.
†Present address: Technical
ABSTRACT: A synthetic protocol developed to produce phase-pure, nearly monodisperse Ni2-xCoxP nanoparticles (x≤1.7)
is described. The Ni2-xCoxP particles vary in size, ranging from 9-14 nm with standard deviations of <20% (based on TEM
analysis) and the actual metal ratios obtained from EDS closely follow the targeted ratios. With increasing Co, samples
with larger size distributions are obtained and include particles with voids, attributed to the Kirkendall effect. In order to
probe the mechanism of ternary phosphide particle formation, detailed studies were conducted for Ni:Co = 1:1 as a repre-
sentative composition. It was revealed that the P:M ratio, heating temperature and heating time have a large impact on
the nature of both intermediate and final crystalline particles formed. By tuning these conditions, nanoparticles can be
produced with different sizes (from ca 7-25 nm) and morphol0gy (hollow vs. dense).
Introduction
grammed reduction, etc.), solution-phase arrested-
precipitation reactions enable excellent control of size,
shape and composition, producing samples comprising
narrowly polydisperse, discrete nanoparticles.18-21 This
approach has been extensively employed for the synthesis
of binary phosphides, and these have recently been
demonstrated to be effective as model catalysts for
HDS22,23 as well as electrocatalysts for HER.6-10 In contrast,
few examples of arrested precipitation reactions deliver-
ing ternary phosphide nanoparticles are reported.24,25
Such reports often target a limited range of composition
space and do not perform the systematic evaluation
needed to control the reaction characteristics to yield
targeted size, composition, and shape. To our knowledge,
reports of discrete Ni2-xCoxP nanoparticles are limited to a
2014 report by Peng and co-workers on the synthesis of
Co1.33Ni0.67P nanorods. This material proved to be active
for HER,16 underscoring the need for a rational synthetic
approach that would enable tuning of catalytic function
in ternary phosphide nanoparticles by independently
addressing size, shape and composition.
Transition metal phosphides are an important class of
materials for applications in energy storage,1 optics,2
magnetism,3 and catalysis.4,5 With respect to catalysis,
transition metal phosphides are being evaluated for both
hydrodesulfurization (HDS) and hydrogen evolution re-
action (HER) processes, among others.6-11 While binary
phases have been extensively studied,6,10,12 only recently
have researchers started to explore opportunities for syn-
ergism afforded by ternary phases.13-16 Ternary phos-
phides of formula Ni2-xCoxP are of particular interest as
they have shown improved HDS activity relative to the
best of the binary phases (Ni2P) for low concentrations of
Co (x ≤ 0.1).14,17 Likewise Zhang and co-workers observed
Ni2-xCoxP to be an active catalyst for hydrazine decompo-
sition, with the highest activity observed for the
Ni1.0Co1.0P1.5 composition.15 While these studies point to
the promise of these phases as active catalysts with a
range of potential applications, the lack of synthetic
methodologies enabling nanoparticle formation with
independent tuning of composition and size impedes the
development of a comprehensive model for their func-
tion.
In this work, a protocol to synthesize narrow polydisper-
sity phase-pure Ni2-xCoxP nanoparticles for different
compositions (x≤1.7) is established. The role of reaction
temperature, initial precursor ratio and heating times on
the resultant size, composition and shape of the products
In contrast to traditional methods of preparing heteroge-
neous catalysts (incipient wetness, temperature pro-
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will be presented. Thus, in addition to providing a route from the ICDD database for phase identification. The
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to new materials for catalytic testing, this paper also con-
tributes to our fundamental knowledge of the operations
of synthetic levers in control of phase and composition
for transition metal phosphide nanoparticles.
collected patterns were analyzed using Jade 5.0 software
for crystallite size (using the Scherrer equation), while
analyses of the patterns using the Rietveld method were
carried out using PANalytical HighScore Plus software.
Co fluorescence incited by the Cu radiation results in
significant noise, and hence a diminished spectrum quali-
ty, in data acquired on Co-rich samples.
Experimental Section
Materials
Nickel acetylacetonate (95%) was purchased from Alfa
Aesar, cobalt(II) acetylacetonate (99%) was purchased
from Sigma-Aldrich, n-octyl ether (95%) and oleylamine
(>50%) were purchased from TCI America, tri-n-
octylphosphine (TOP, 97%) was purchased from STREM
Chemicals, chloroform was purchased from Fisher Scien-
tific, and methanol and ethanol (200 proof) were pur-
chased from Decon laboratories. All chemicals were used
as received. The purity of oleylamine was assessed by
NMR (Figure S1) and it was determined that impurities
consist largely of other saturated and unsaturated prima-
ry amines.
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Transmission electron microscopy (TEM) and energy
dispersive spectroscopy (EDS) were carried out with a
JEOL 2010 electron microscope operating at a voltage of
200 kV and coupled to an EDS detector (EDAX Inc).
Bright field images were captured by Amtv 600 software.
TEM specimens were prepared by dispersion of nanopar-
ticles in chloroform and depositing a drop of solution on
a formvar carbon coated 200 mesh Cu grid, followed by
drying in air. The peak positions in the EDS were cali-
brated using Al (1.486 keV) and Cu (8.040 keV) metals.
The particle sizes were analyzed by NIS-Elements D3.10
software. The spherical boundary of a particle was as-
signed by manually selecting 3 points on the edge of each
particle. To obtain the histograms, the sizes of particles
obtained were categorized into 0.2 nm bin sizes. EDS
elemental mappings were collected using an FEI Titan
80-200 scanning transmission electron microscope
(STEM) with ChemiSTEM technology operated at 200 kV.
The STEM images were taken using a high angle annular
dark field detector (HAADF). The so-called "Super X"
EDS system, which includes 4 windowless silicon-drift
detectors (SDD) around the specimen made by Bruker
Corporation, provides a solid angle of more than 0.9 srad,
and the X-ray signal is usually more than 1000 cps at a
magnification of about 640,000x.
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Synthesis of Ni2-xCoxP nanoparticles (0≤x≤2)
All reactions were carried out under argon atmosphere
using standard Schlenk line techniques. In a typical syn-
thesis, the corresponding amounts (overall 2.0 mmol) of
the two metal precursors (Ni(acac)2, Co(acac)2) were
combined with 5.0 mL (8.0 mmol) of oleylamine (coordi-
nating ligand), 10.0 mL of octyl ether (solvent) and 4.0
mL (10.0 mmol) of trioctylphosphine (TOP, P source) in a
200 mL Schlenk flask with a condenser. The flask was
placed on a heating mantle operated by a temperature
controller and the thermocouple was inserted between
the flask and the heating mantle. The flask was thermally
insulated by wrapping with glass wool. The mixture was
degassed at 110 °C for 20 min to remove any moisture or
oxygen, followed by purging with argon for 20 min at this
temperature (the reaction is conducted under slight posi-
tive pressure of Ar). The temperature was then increased
to 230 °C and maintained for 90 min. Next, the tempera-
ture was set to heat rapidly (ca 40 °C/min) to 350 °C; as
soon as the temperature started increasing, a 6 mL (15
mmol) aliquot of TOP was injected, followed by heating
for 4.5 h at 350 °C. The black product was isolated by pre-
cipitation with ethanol. The product was dispersed again
in chloroform, sonicated for 5-10 min and reprecipitated
with ethanol. This sonication and precipitation process
was carried out at least three times.
X-ray photoelectron spectroscopy (XPS) analyses were
carried out using a Surface Science Instruments S-probe
spectrometer. This instrument has a monochromatized
Al Kα X-ray source and a low energy electron flood gun
for charge neutralization of non-conducting samples. The
samples were dusted onto double-sided tape and run as
insulators. X-ray spot size for these acquisitions was ap-
proximately 800 µm. Pressure in the analytical chamber
during spectral acquisition was less than 5 x 10-9 Torr. The
pass energy for survey spectra (to calculate composition)
was 150 eV and the pass energy for high resolution scans
was 50 eV. The binding energy scales of the high-
resolution spectra were calibrated by assigning the most
intense C1s high-resolution peak a binding energy of
285.0 eV.
Characterization
Powder X-ray diffraction (PXRD) patterns were collected
on a Rigaku RU 200B diffractometer with Cu Kα radiation
(0.154 nm) operated at 40kV, 150mA. Samples were de-
posited on a zero background quartz holder using mini-
mal grease and data were collected over the 2Ɵ range 30-
80°. Silicon was used as an internal 2Ɵ standard. The pat-
terns were compared to powder diffraction files (PDFs)
Results and Discussion
The protocol developed to synthesize Ni2-xCoxP nanopar-
ticles is based on the method refined by our group to
synthesize discrete Ni2P phase-pure nanoparticles.20 To
make the binary nanoparticles, Ni and Co acetylacetonate
(acac) complexes are combined at the outset of the reac-
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Chemistry of Materials
The products of the different targeted compositions ob-
tion with trioctylphosphine (TOP) in a solution of octyl
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ether and oleylamine (OA) as shown in Scheme 1. This
was heated to 230°C to prepare the Ni-Co-P alloy precur-
sor particles, which are then converted to the crystalline
ternary phosphide by adding an additional aliquot of
TOP and heating to 350°C. In the original Ni2P study, we
observed that the initial TOP:M:OA=10.0:2.0:8.0 molar
ratio resulted in nearly monodisperse (±20% S.D., in the
terminology coined by Finke26) and discrete amorphous
intermediate NixPy precursor particles. Accordingly, in all
the Ni, Co compositions targeted, the initial molar ratio
was set to TOP:M:OA = 10.0:2.0:8.0 while the final
TOP:M ratio was increased to 11.2 by injecting an addi-
tional 6.0 mL of TOP at 350°.
tained were characterized for phase by PXRD, morpholo-
gy by TEM, and composition by EDS. Figure 1 illustrates
the PXRD patterns for different targeted compositions.
Ni2P and Co2P adopt the hexagonal Fe2P and
orthorhombic Co2P structure-types, respectively. Both
have two metal sites, M(1) and M(2) with tetrahedral and
squre pyramidal geometries, respectively, defined by P.
However, the two structures differ in the packing of
rhombohedral subcells containing M(1) and M(2) sites
within the crystal (Figure 2). Each P atom is in a tri-
capped trigonal prismatic geometry defined by Ni atoms.
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Structural, morphological and surface chemical
changes of ternary phosphide nanoparticles with
composition
C
o
2 P
- M1
- M2
▪
*
Ni0.10Co1.90
▪
*
*
*
▪
*
*
Ni0.25Co1.75
Ni0.40Co1.60
*
*
*
*
*
*
*
*
*
Ni0.50Co1.50
*
*
Ni0.67Co1.33
Ni1.00Co1.00
*
*
*
*
Figure 2. Orthorhombic Co2P structure-type (top) hexago-
nal Fe2P structure-type (bottom).
*
Ni1.50Co0.50
Ni1.92Co0.08
*
*
*
*
In Figure 1 the patterns match the Ni2P (hexagonal)
structure in the nickel rich region and this phase is
clearly maintained up to a targeted composition of
Ni0.67Co1.33P, at which point we observed a change in the
pattern. For the nickel rich compositions, there is a
broad peak near 55° 2Ɵ corresponding to the overlapping
reflections of (300) and (211) planes. Due to the small size
of the crystallites these two peaks overlap, resulting in
one broad peak. However, for the composition
Ni0.67Co1.33P, we observed the appearance of two peaks
that shift away from each other as the Co is increased.
This splitting can be attributed to a separation of the
(300) and (211) reflections upon Co-incorporation, or a
shift to the Co2P structure type. Importantly, all observed
peaks for compositions up to x=1.75, could be assigned to
the desired ternary phase, suggesting that homogeneous
materials can be prepared in this composition space (i.e.,
no detectable crystalline impurities are present).
However, at the very Co-rich end (x=1.9), CoP is observed
*
*
*
*
*
Ni2.00Co0.00
*
N i2P
4 0
5 0
6 0
7 0
8 0
2 -T h e ta (d e g re e s)
Figure 1. PXRD patterns for different targeted compositions
of Ni2-xCoxP. Reference patterns for Co2P and Ni2P are
shown for comparison with drop lines indicating the major
distinguishing peaks for the two phases. The sharp peaks
denoted with * arise from an internal Si standard. Peaks
denoted with ▪ arise from a CoP impurity.
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as an impurity. Likewise, this methodology is
unsuccessful at preparing phase pure Co2P; attempts to
do so produce a mixture of Co2P, CoP and unidentified
impurities (Figure S2).
Page 4 of 12
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Table 1. Ni:Co target and actual (as assessed by EDS) metal ratios, crystallite sizes (by Scherrer application to
PXRD data), particle size (by TEM) and refined lattice parameters for different compositions of Ni2-xCoxP
Lattice parameters (Å)
TEM
size
Crystallite
size
Molecular
Actual
Target ratio
(Ni:Co)
9
Ni2P type refined
Co2P type refined
Volume
(Å3)a
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ratio (Ni:Co)
(nm)
(nm)
A
c
a
-
-
-
-
-
b
-
-
-
-
-
c
2.00 : 0.00
1.92 : 0.08
1.50 : 0.50
1 : 1
2.00 : 0.00
1.91 : 0.09
1.52 : 0.48
1.06 : 0.94
0.71 : 1.29
0.55 : 1.45
0.44 : 1.56
0.29 : 1.71
9.1
10.3
9.6
8.1
11.26±0.34
12.36±0.65
11.09±0.97
9.77±0.94
13.28±2.04
12.73±1.98
11.57±2.07
12.5±1.88
5.889(5)
5.877(9)
5.877(5)
5.853(6)
5.824(9)
3.364(2)
3.372(3)
3.354(3)
3.381(3)
3.398(4)
-
33.69
33.62
33.43
33.44
33.27
32.62
31.98
32.30
-
-
-
0.67 : 1.33
0.5 : 1.5
10.9
10.0
9.1
-
5.851(9)
5.83(3)
5.801(7)
6.530(7)
6.44(3)
3.414(6)
3.42(2)
0.4 : 1.6
-
-
-
-
0.25 : 1.75
9.5
6.468(5) 3.443(3)
constants for the phase-pure Ni2-xCoxP materials (Figure
S3). The refined lattice constants and molecular volumes
(Ni2P structure: (unit cell volume)/3; Co2P structure: (unit
cell volume)/4) are listed in Table 1. For nominal
compositions x<1.5, the PXRD patterns were refined
starting from the Ni2P structure (PDF#74-1385) and good
fits were obtained. However, attempts to refine the x=1.5
composition on the hexagonal structure resulted in a
poorer quality fit of the pattern. Thus for the three most
Co rich compositions, refinement was carried out based
on the Co2P structure (PDF#32-0306), resulting in a
notable improvement in the fit and more reasonable
lattice parameters. Intriguingly, the phase transformation
appears at a somewhat more Ni-rich composition in our
nanoparticles than is reported for the bulk material (x =
1.7), 27-29
5.90
5.89
5.88
5.87
5.86
5.85
5.84
5.83
5.82
5.81
3.41
3.40
3.39
3.38
3.37
3.36
3.35
3.34
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
Cobalt Fraction (Co/(Co+Ni))
Figure 3. Hexagonal unit cell parameters (Ni2P structure)
plotted as a function of the Co content ( polynomial fits
are guides for the eye).
The unit cell parameters for the Ni2-xCoxP materials
adopting the Ni2P structure (x<1.5) are consistent with
those reported for bulk phases of similar composition,27-29
and exhibit clear trends as a function of Co content as
shown in Figure 3. The “a” lattice parameter decreases
with increasing Co content while the “c” lattice parameter
shows a minimum for the Ni1.50Co0.50P composition. The
orthorhombic unit cell parameters for the three most Co-
rich compositions, which were fitted based on the Co2P
structure, are also consistent with those for bulk phase
Ni2-xCoxP materials.27-29 Except for the Ni0.4Co1.6P composi-
tion, which yielded the poorest fit, the molecular volumes
calculated for the nanoparticle samples (Table 1) using
the results of the Rietveld refinement analyses are in ex-
cellent agreement with those noted previously for bulk
All the phase pure PXRD patterns were calibrated using
silicon as an internal standard. Crystallite sizes were
calculated by application of the Scherrer equation to the
most intense peak, around 41°, corresponding to the (111)
plane of Ni2P or to the overlapping(121), (201) planes of
Co2P (Table 1). The crystallite sizes range from 8-11 nm
with no discernible size trends based on composition.
The PXRD patterns were analyzed using the Rietveld
refinement method in order to determine the lattice
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For Co1.0Ni1.0P, Artigas et al. observed that 80.5% of the
phase Ni2-xCoxP materials over the entire composition
range investigated (0≤x≤1.75).27 The unusual trend ob-
served for the hexagonal unit cell parameters, with “c”
M(2) sites were occupied by Ni atoms, while only 19.5% of
the more spatially confined M(1) sites were occupied by
Ni atoms.30 The extent of ordering of the metal atoms in
the M(1) and M(2) sites increased with increasing Ni con-
tent. Preferential ordering of the metal atoms has been
observed for Ni and Fe in bulk and nanoscale Ni2-xFexP
materials as measured by Mössbauer spectroscopy.32,33
1
2
3
exhibiting
a minimum value near the composition
4
5
Ni1.50Co0.50P, has been attributed previously to ordering of
the metal atoms in the M(1) and M(2) sites.
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Neutron diffraction measurements carried out on bulk
phase Ni2-xCoxP materials (x = 0.8, 1.0, 1.2) showed a
strong preference for Ni atoms to occupy the square py-
ramidal (M(2)) sites,30 which is unexpected based on
atomic size (covalent radii: Ni - 121 pm, Co - 126 pm).31
Ni
Co
P
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X=0.08
X=0.25
12.36±0.65 nm
12.08±0.80 nm
Ni
100
80
Co
P
60
5 nm
20 nm
20 nm
5 nm
40
X=0.5
9.77±0.94 nm
X=1.0
11.09±0.97 nm
20
0
20
40
60
80
100
Point number
Figure 5. STEM images and elemental mapping data for a
Ni0.67Co1.33P sample. In the plot of intensity vs. point
number, Ni is shown in red, Co in green and P in blue.
5 nm
20 nm
20 nm
5 nm
X=1.33
X=1.5
12.73±1.98 nm
13.28±2.04 nm
The morphology, particle size and composition of the Ni2-
xCoxP nanoparticle materials were analysed by TEM and
EDS, respectively, and the results are shown in Figure 4
and Table 1. The particle sizes are in the range 9-14 nm,
slightly larger, but comparable to values obtained from
crystallite size analysis of the PXRD data (Table 1). The
standard deviations range from ± 5-18% with greater
deviations manifesting for x>1.0.
20 nm
20 nm
5 nm
5 nm
X=1.6
12.50±1.88 nm
11.57±2.07 nm
X = 1.75
The particle size distribution histograms with Gaussian
profiles are shown in Figure S4 for all compositions. The
histograms reveal a larger size distribution of particles for
cobalt rich compositions. However, for all compositions
the standard deviation is < ±20. High resolution TEM
(HRTEM) images were taken for all compositions and are
shown as insets in each TEM image. Lattice planes are
evident throughout the crystallites, but at the Co-rich
compositions, hollow particles are also observed. The
formation of hollow particles may be attributed to the
diffusion rate differences between metals and P,
according to the Kirkendall effect.34 In previous studies
the formation of hollow particles for cobalt phosphides
has also been noted,35 suggesting that Co is a highly
mobile metal within the phosphide lattice.
20 nm
5 nm
20 nm
5 nm
Figure 4. TEM images for Ni2-xCoxP nanoparticles (tar-
geted compositions indicated). The insets illustrate
HRTEM images for each composition showing lattice
fringes and, for x=1.75, hollow particle formation.
In comparing the sizes obtained from PXRD (by
application of the Scherrer equation) to TEM (average
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from histrogram), it is evident that the former method is observed for reduced P species (129.1-129.7 eV) is lower
than that for the elemental P (129.9-130.2 eV).37,38 This is
attributed to a higher electron density present in these
phosphides caused by transfer of electron density from
the metal species to the more electronegative
phosphorus. The lower binding energies for P are
consistent with previous studies done for Ni2P/SiO2 and
Co2P/SiO2 materials prepared by temperature-programed
reduction (TPR).39-41 The peaks observed for reduced
species of Ni (852.5-852.7 eV) and Co (777.8-777.9 eV) are
consistent with the binding energies reported for zero-
valent Ni (852.5-852.9 eV) and Co (778.1-778.2 eV).39-41 The
Ni(2P3/2) peak is slightly shifted to lower binding energies
(852.7-852.5 eV) with increased Co content suggesting the
presence of slightly higher electron density.
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slightly underestimating the size. This may be due to an
amorphous layer on the particle surface, polydispersity of
particles and/or the presence of hollow particles for some
compositions (x=1.75 in Figure 4). Additionally, the TEM
size measurement method could be contributing as the
software models assume a perfect sphere, but TEM
images indicate that some particles are elongated along
the a or c axes (see Figure 4 inset, x = 1.5).
9
From the EDS analysis the compositions of the materials
were calculated and the metal ratio is found to be close
(within 5-20%) to the ratio employed in the synthesis, as
shown in Table 1. STEM images and elemental mapping
data for the Ni0.67Co1.33P composition show that the two
metals are homogeneously distributed within a particle,
consistent with solid-solution formation (Figure 5).
Overall the developed protocol is ideal to synthesize Ni2-
xCoxP nanoparticles as nearly monodisperse samples
(≤±20% S.D.)26 with good control of composition in the
range x<1.7.
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The binding energies for the present ternary phosphide
components are in the range of values observed by Bussell
and coworkers in their study of TPR-prepared Ni2-
xCoxP/SiO2.14 XPS studies were performed for three
compositions (x=0.25, 1.00, 1.75), but there was no clear
pattern in binding energy shifts with increasing cobalt.
Mar and co-workers studied the effect of metal
substitution for bulk (Ni1-xMx)2P (M=Cr, Fe, Co) using XPS
and X-ray absorption near-edge spectroscopy (XANES).42
By comparing the satellite intensity of Ni 2p3/2 XPS
spectra and the Ni and metal (Cr, Fe, Co) edge XANES
spectra, Mar and coworkers concluded that charge
transfer from M to Ni does occur in the (Ni1-xMx)2P
phases, although this is small in magnitude for bulk Ni2P
doped with Co.
The surfaces of some of the Ni2-xCoxP compositions (x =0,
0.08, 0.25, 0.50) were probed with XPS and the resulting
spectra in the Ni(2P3/2), Co(2P3/2) and P(2P) regions are
shown in Figure 6.
The surface compositions of the Ni2-xCoxP nanoparticles
(x =0, 0.08, 0.25, 0.50) were evaluated by XPS and the
normalized metal ratios are shown in Table S1. The
surface metal compositions closely track those measured
by EDS, while the P:M ratios determined by XPS are in
the range 2-2.9, indicating that the surfaces are P rich.
Surface enrichment in P might be due to the stripping of
metal ions from the surface during the nanoparticle
washing procedure. The loss of surface metal ions on
metal chalcogenide nanoparticles during ligand exchange
has been noted by Owen and co-workers.43 It is likely that
a similar mechanism is operable here. Intriguingly, in
TPR-prepared Ni rich materials the surface compositions
were found to be quite P enriched for bulk phase
materials (P:M = 1.2-4.8),17 but less so for nanoscale Ni2-
xCoxP particles prepared on a silica support (P:M = 0.51-
0.76).14
Figure 6. XPS spectra of Ni2-xCoxP nanoparticle composi-
tions: (a) Ni2P, (b) Co0.08Ni1.92P, (c) Co0.25Ni1.175P and (d)
Co0.50Ni1.50P in the Ni(2P3/2), Co(2P3/2) and P(2P) regions.
Mechanistic studies on particle formation: NiCoP
Effect of intermediate heating temperature and heat-
ing time on precursor particle formation and tem-
plating
The binding energies observed for Ni (856.0-856.1 eV), Co
(781.4-781.5 eV) and P (133.1-133.8 eV) correspond to the
oxidized phase of each component resulting from the
oxide layer formation on the nanoparticle surface due to
air exposure. Those binding energies noted for each
component correspond to Ni2+, Co2+ and P5+,
respectively.36 The oxide layer is not an issue for catalytic
studies as these materials are preheated under reducing
conditions prior to catalytic testing. The binding energy
Based on our experience with Ni2P, the initial hypothesis
regarding the mechanism of NixCo2-xP particle formation
was that an intermediate mixed-metal phase was formed
at 230 °C and these particles acted as templates for the
final ternary phosphide. To test this hypothesis, interme-
diate particles were isolated after the synthesis step at
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Scheme 1. Reaction protocols for the synthesis of Ni2-xCoxP nanoparticles
230 °C with TOP:M=4.48 (8.96 mmol:2.00 mmol) for a
representative composition, Ni:Co=1:1 (Scheme 1). As
shown in Figure 7a, precursor particles formed at 230 °C
were found to have CoO, NiO or a mixed metal oxide as
the only crystalline phase. NiO and CoO are virtually in-
distinguishable by PXRD but CoO is the more likely im-
purity phase as we do not see NiO in the binary Ni2P syn-
thesis.20 Our presumption of CoO formation is also con-
sistent with a study done by Seo et al.44 In their work, a
reaction protocol of Co(acac)3 and oleylamine was devel-
oped at 200°C to synthesize two different phases of CoO,
presuming the oxygen was contributed by the acety-
lacetonate ligand in the metal precursor complex. From
the TEM image in Figure 7(a), it is evident that while
some spherical particles form, much of the product con-
sists of features < 1 nm, most likely nuclei. We do observe
an amorphous shell on these particles that may be due to
an amorphous layer of reactant species or uncrystallized
oxide on the particle surface. Thus, it appears that while
nucleation starts at 230°C,particle growth is incomplete at
the 1.5 h mark, unlike the case for pure Ni2P.20 Neverthe-
less the final ternary phosphide particles obtained are
nearly monodisperse and spherical, as shown in Figure 4.
synthesis, where a second aliquot of TOP is added before
increasing the temperature to form the crystalline phos-
phide. When less TOP is present, the concentration of P
diffusing into the particles is decreased and the effect of
Co-diffusion outward is magnified according to the
Kirkendall effect.
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Prior studies have shown a strong relationship between
the composition and size of the precursor particles and
that of the final crystalline phosphide. Robinson and co-
workers did a thorough investigation on incorporation of
P into Ni nanocrystals, observing that phosphorus incor-
poration increases with time and temperature.45 Accord-
ingly, we sought to investigate the effect of time on parti-
cle shape and composition at 230 °C and 260 °C.
c
b
a
NiCoP
Co
In order to better understand the chemical and structural
changes that occur upon heating the precursor particles
to the crystallization temperature (350°C), the initial reac-
tion temperature was changed from 230°C to 260°C and
290°C and particles were isolated. For the particles heated
at 260 °C no CoO was observed in the PXRD pattern (Fig-
ure 7b). Instead a broad peak is observed near 44° 2θ that
can be indexed to the (111) reflection of fcc Ni (or Co).
Based on the breadth of the peak and the absence of any
higher order reflections, we presume this to be a metal-P
amorphous alloy.35,45,46 The absence of CoO is attributed
to reduction to fcc Co metal by oleylamine at this tem-
perature, as observed by Nam et al.47
Ni
CoO
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70
2-Theta (degrees)
11.88±1.13 nm
12.24±1.72 nm
(a)
(b)
(c)
20 nm
20 nm
20 nm
5 nm
5 nm
5 nm
When the intermediate temperature was increased to 290
°C, the formation of crystalline NiCoP was observed (Fig-
ure 7c), indicating this temperature is sufficient to crys-
tallize the material. The particle size is slightly larger (8.8
nm from the PXRD data and 12.24 ± 1.72 nm from TEM)
than that obtained at 350 °C and some hollow particles
are observed (Figure 7c). Hollow particle formation for
this composition can be attributed to the lower amount of
TOP used in the reaction medium relative to the two step
Figure 7. PXRD patterns and TEM images of NiCoP nanopar-
ticles obtained from heating at (a) 230 °C (b) 260 °C (c) 290
°C for 1.5 h with TOP:M=4.48 (reference PXRD patterns for
CoO, fcc Ni, Co and for NiCoP are shown). The insets in TEM
micrographs show HRTEM images of representative particles
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As shown in Figure S5, heating for 1.5 or 3.5 h at 230 °C the large precursor particle size and the absence of P-
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resulted in CoO as the only detectable phase by PXRD,
whereas by 7.5 h the fcc (111) peak manifests as a very
broad reflection superposed over peaks for CoO. At the
same time, the TEM data show a shift from poorly-
defined particles at shorter times to ca 12 nm spherical
particles at 7.5 h (Figure S6). EDS analysis was carried out
for regions with spherical particles and it was revealed
that the P amount increased with time (@1.5 h M:P = 24,
@3.5 h M:P=5.5, @7.5 h M:P=3.4).
incorporation inducing a large diffusion gradient for
phosphidation.
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A similar series of reactions was done for precursor parti-
cle formation at 260 °C at different time intervals, yield-
ing only the (111) reflection by PXRD and a particle size of
(again) ca. 12 nm (Figure S7 and S8). Relative to 230 °C,
more P is being incorporated at 260 °C and the extent of
incorporation also increases with time (@1.5 h M:P=3.5,
@3.5 h M:P=3.2, @7.5 h M:P=2.9). This pattern of P incor-
poration is similar to that reported by Robinson and co-
workers.45
a
NiCoP
Co
Ni
To investigate whether spherical precursor particles
formed at low temperatures act as templates for the final
crystalline ternary phosphide particles, reactions were
done by heating at 230 °C and 260 °C for 7.5 h and 1.5 h,
respectively, with TOP:M=4.48, and then heated to 350 °C
for 4.5 h after injecting an additional 6 mL of TOP, thus
modifying only the intermediate step in our standard
preparation. In both cases, spherical particles adopting
the NiCoP phase were obtained. The size (ca. 13 nm, Fig-
ure S9) was slightly larger than the precursor particles
(ca. 12 nm, Figure S6, S8), attributed to crystallization and
lattice expansion upon inclusion of more P.
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50
60
70
2-Theta (degrees)
20.63±2.14 nm
23.62±2.85 nm
(a)
(b)
(b)
100 nm
100 nm
100 nm
Modifying the precursor particles to increase the size
Figure 8. PXRD patterns (reference patterns for Ni, Co, and
NiCoP are shown) and TEM images of nanoparticles ob-
tained from reactions (a) initially heated at 230 °C with
TOP:M=1.12 for 7.5 h (b) followed by heating at 350 °C for
4.5 h after injection of an additional 20.2 mmol of TOP.
The inset shows the enlarged image to highlight the hollow
particles.
We sought to modify the size of the precursor particles as
a means to control the size of the NiCoP product. This
was achieved by using a minimum of TOP in the initial
synthesis (1 mL of initial TOP to produce TOP:M=1.12) to
force formation of crystalline NiCo nanoparticles over Ni-
Co-P amorphous alloys (Scheme 1). As shown in Figure 8,
when the reaction mixture is heated for 7.5 h at 230 °C,
crystalline peaks appeared that could be assigned to the
(111) and (200) reflections of fcc Ni-Co alloy, which has a
very similar pattern to each of the pure metals as ob-
served by Li et al.48 The TEM image indicates the particles
are larger (20.63±2.14 nm) compared to reactions using 4
mL of TOP, attributed to the lesser degree of stabilization
of the particles’ surface. In the isolation process the parti-
cles adhered to the stirring bar, suggesting ferromagnetic
or superparamagnetic behavior that is indicative of Ni-Co
alloys, and not of amorphous Ni-Co-P particles, similar to
our previous observations for amorphous Ni-P particles.20
Further size-control by direct heating to the crystal-
lization temperature
Based on the fact that the 230 °C step at short times does
not produce well-formed nanoparticles, precursor particle
formation likely occurs en route to heating to 350 °C. This
led us to question whether the low temperature step is
necessary or whether direct heating would be suitable to
produce ternary phosphide nanoparticles. To test this
hypothesis, the reaction mixture was heated directly to
350 °C with a high initial TOP:M=11.2 (10 mL TOP,
Scheme 1). To investigate whether any alloy precursor
particles are formed at the initial stage of reaction or
whether there is any growth in the particle size with the
heating time, particles were isolated at different time in-
tervals at 350 °C. As observed by PXRD, during the time
interval 1.5-4.5 h at 350 °C, the phase of materials was
consistent with crystalline NiCoP (Figure S10) and the
average particle sizes were the same regardless of time
intervals investigated, with a low degree of polydispersi-
The precursor particles were subsequently converted to
the final ternary phosphide phase by introducing a fur-
ther 9 mL (20.2 mmol) of TOP at 350 °C to make overall
TOP:M=11.2. The final ternary phosphide particles ob-
tained have the correct phase and the size of these parti-
cles (23.62±2.85) is again similar to the large precursor
particles from which they formed, consistent with tem-
plated formation (Figure 8). Some hollow particles were
also present in the reaction product, attributed to both
8
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Chemistry of Materials
size of these particles was smaller (7-8 nm) compared to
Funding
1
the particles observed in the approach with the interme-
diate heating step (ca 12 nm). We expect this is the case
because the particle growth takes place in the presence of
an excess of TOP which facilitates the stabilization of the
surface of the particles, inhibiting the growth. If this is
the case, it should be possible to increase the size of the
product particles by decreasing the quantity of TOP em-
ployed. Accordingly, a direct heating reaction was con-
ducted at 350 °C with 4 mL of TOP (TOP:M=4.48)
(Scheme 1). As shown in Figure S11 the particles obtained
index to NiCoP in the PXRD and are larger in size (ca 14-
15 nm) compared to those obtained from TOP:M=11.2. A
consequence of the reduced TOP is, once again, for-
mation of hollows within the spherical NiCoP particles,
which accounts for a portion of the increased particle
diameter, in addition to the reduced surface stabilization.
This material is based upon work supported by the Na-
tional Science Foundation under Grant Nos. 1361702,
1361741 (CHE) and 1064159 (DMR).
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Acknowledgment
HAADF/STEM data were acquired on a FEI Titan 80-200
field emission transmission electron microscope with
ChemiSTEM technology purchased with funds provided by
NSF MRI award 1040588, the Murdock Charitable Trust and
Oregon Nanoscience and Microtechnologies Institute
(ONAMI). TEM and HRTEM data were acquired on a JEOL
2010 TEM with funds provided by NSF MRI award 0216084.
TEM, NMR and powder X-ray diffraction were acquired at
the Lumigen Instrument Center, Wayne State University.
The XPS analyses were performed at the Surface Analysis
Recharge Center (SARC) at the University of Washington.
We thank Akhila Kuda Wedagedara and Jessica Davis for
their assistance with NMR.
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Conclusions
A solution-phase synthetic protocol is established for
production of Ni2-xCoxP nanoparticles in the range x≤1.7 as
narrow polydispersity samples with control of size, shape,
and morphology (dense vs. hollow particles). Composi-
tion modulation (x) in the product reflects the starting
composition until the very Co-rich end is reached, at
which point impurity phases become apparent. Increasing
Co-concentration results in an increase in polydispersity
and is accompanied by formation of voids within the solid
particles due to the Kirkendall effect. A comprehensive
study of the role of key parameters on particle formation
and growth of the NiCoP phase reveals that precursor
particles formed below the crystallization temperature act
as templates for the final crystalline product. At low M:P
ratios the precursor particles are amorphous metal phos-
phides of small size (< 10 nm) that transform to dense
particles, whereas at high M:P ratios, the precursor parti-
cles are large (> 20 nm) crystalline NiCo alloy particles
that transform to large hollow NiCoP particles. Formation
of precursor particles by a moderate temperature soak is
not a priori a requirement for NiCoP particle formation.
Direct heating to the crystallization temperature also re-
sults in narrow polydispersity samples with size control-
lable by the M:P ratio. The ability to tune the particle
size, composition and morphology in Ni2-xCoxP opens the
door to an understanding of how these factors impact the
catalytic function.
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Supporting information
NMR spectrum for oleylamine, particle size histograms,
PXRD patterns and TEM images of intermediate particles
heated at 230 °C and 260 °C for different time intervals, crys-
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Ni2-xCoxP
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Composition Control (x≤1.7)
1
2
3
Ni
Co
P
Ni(acac)2
+
350 °C
Co(acac)2
+
4
5
6
20 nm
20 nm
20 nm
P(C18H17)3
(a)
(b)
Size and Morphology Control
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9
(a)
Small, Dense
Low
M:P ratio
20 nm
20 nm
5 nm
5 nm
High
Large, Hollow (b)
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