Synthesis of the nickel selenophosphinates [Ni(Se2PR 2)2] (R = iPr, tBu and Ph) and their use as single source precursors for the deposition of nickel phosphide or nickel selenide nanoparticles

Article abstract of DOI:10.1039/b816903a

Nickel phosphide (Ni₂P and Ni₁₂P₅) or nickel selenide (NiSe) nanoparticles were prepared from the single molecule precursor, dialkyldiselenophosphinato nickel(II), [Ni(Se₂PR ₂)₂] (R =

Full text of DOI:10.1039/b816903a

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PAPER
www.rsc.org/dalton | Dalton Transactions
i
Synthesis of the nickel selenophosphinates [Ni(Se2PR2)2] (R = Pr, tBu and
Ph) and their use as single source precursors for the deposition of nickel
phosphide or nickel selenide nanoparticles†
Weerakanya Maneeprakorn, Chinh Q. Nguyen, Mohammad A. Malik, Paul O’Brien* and James Raftery
Received 29th September 2008, Accepted 9th January 2009
First published as an Advance Article on the web 5th February 2009
DOI: 10.1039/b816903a
Nickel phosphide (Ni2P and Ni12P5) or nickel selenide (NiSe) nanoparticles were prepared from the
i
single molecule precursor, dialkyldiselenophosphinato nickel(II), [Ni(Se2PR2)2] (R = Pr, tBu and Ph) by
thermolysis in trioctylphosphine oxide (TOPO) or hexadecylamine (HDA). The chemical composition
of these nanoparticles depends on the precursors, capping agents, and reaction temperature.
NiSe2, Ni0.85Se and Ni3Se2 have been prepared by solvothermal
Introduction
process,9,10 hydrothermal synthesis,6,11 and mechanical alloying.12,13
Nickel phosphides are excellent corrosion-resistant, oxidation-
NiSe2 and Ni3Se2 materials have been produced using mechanical
resistant and waterproofing materials.1 Nanocrystalline phos-
phides have distinctive mechanical and thermal properties. Pre-
vious studies, and the calculated Ni-P binary phase diagram,
indicate that the possible nickel phosphide phases are: Ni3P, Ni12P5,
Ni5P2, and Ni2P. 2 Nickel phosphide is an n-type semiconductor
with a band gap of 1.0 eV.3 There are very few reports on the
preparation of nickel phosphide nanoparticles; most of these, use
a phosphine. The phosphide was prepared by the direct reaction
between phosphine (PH3) and the metal or metal salts.4 It was also
reported that nickel phosphide particles can be easily prepared
from metal or metal oxide on a silica-support by reaction with
PH3.5 Recently, spherical metal phosphide nanoparticles were
synthesised by the reaction of metal carbonyl complexes with
phosphine. In this case, the phosphine surfactant can serve as
both stabilising ligand and phosphorous source.6 Moreover, some
organic reagents containing phosphorus, such as P(SiMe3)3 or
trioctylphosphine (TOP), have been used as phosphorus sources
for the preparation of transition metal phosphide nanocrys-
tals by solution phase or sol–gel methods.7 The phosphorous
sources NaH2PO2 has been used in a hydrothermal-microemulsion
method.1
Nickel selenides have also attracted attention due to their
interesting electrical and magnetic properties and have found
applications in the field of material science. NiSe2 is a good
electrical conductor and Pauli paramagnetic metal compound.
It is weakly paramagnetic with paramagnetism increasing weakly
with temperature.8 According to the phase diagram, nickel and
selenium can form a variety of nickel selenides. There are three
stable phases at room temperature, Ni1-xSe (the nickel content
can vary from 1.00 to 0.85 relative to Se), Ni3Se2 and NiSe2,
and other phases, such as Ni2Se3, Ni2Se, and Ni3Se4.6 Nickel
selenide is p-type with a band gap of 2.0 eV. Many methods
have been employed for synthesis of nickel selenides, in particular
alloying12 starting from blended Ni and Se element powders
with nominal composition Ni25Se75 and Ni75Se25, respectively. A
series of nickel selenides (NiSe2, Ni1-xSe and Ni3Se2) including
NiSe nanowires have been synthesised through hydrothermal
methods.6
In order to develop a route for the preparation of metal
phosphide or metal selenide nanoparticles, the use of precursors
in the presence of a coordinating solvent, has been investigated
especially for the TOPO/TOP system.7,14 There are some reports
on this method, for Ni–P nanoparticles, Hyeon and co-workers
have used a continuous-injection method to synthesise Ni2P
nanorods by reacting the acetylacetonate [Ni(acac)2] and TOP
◦
in TOPO at 330 C.15 More recently, Senevirathne and co-
workers have synthesised Ni2P nanoparticles by using a solution-
phase method with bis(1,5-cycloocdtadiene) nickel(0) as the
nick◦el source and TOP as the phosphorous source in TOPO at
345 C.16
The use of single-source molecular precursors, which have
both metal source and phosphorous/selenium source in the
same structure, has been extensively studied for the synthesis
of semiconductor nanoparticles. However, to the best of our
knowledge, there is only one report of a single-source precursor
(SSP) for nickel phosphide and/or nickel selenide nanoparticles.17
The metal complexes containing bifunctional phosphine ligands
that possess alkoxysilyl functional groups were used as the
precursor incorporated into silica xerogel matrixes using sol–gel
chemistry. Recently, we have reported the deposition of nickel
phosphide or selenide films from the single source precursor
i
[Ni{ Pr2P(S)NP(Se)iPr2}2] by adjusting the growth temperature
in chemical vapor deposition.3
The current work employs chalcogenophosphinates as SSP for
nickel selenides or nickel phosphides. This method has previ-
ously proven useful for the preparation of metal chalcogenides,18
based on the injection of dialkyldiseleno-phosphinato nickel(II)
complex, [Ni(Se2PiPr2)2] (1), [Ni(Se2PtBu2)2] (2), [Ni(Se2PPh2)2]
(3), and into hot coordinated solvents at different temperatures.
Trioctylphosphine oxide (TOPO) or hexadecylamine (HDA) were
used as the coordinating solvent.
The School of Chemistry and the School of Materials, The University
of Manchester, Oxford Road, Manchester, UK M13 9PL. E-mail: paul.
obrien@manchester.ac.uk; Fax: 44 161 275 4598; Tel: 44 161 275 4653
† CCDC reference numbers 692535 and 692536. For crystallographic data
in CIF or other electronic format see DOI: 10.1039/b816903a
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Table 1 Crystal data and structure refinement of [Ni(Se2PiPr2)2] (1) and
[Ni(Se2PtBu2)2] (2)
Experimental
Preparation of [Ni(Se2PR2)2], R = Pr, tBu, Ph
i
Compound
[Ni(Se2PiPr2)2]
[Ni(Se2PtBu2)2]
A solution of NiCl2·6H2O (1.19 g, 5 mmol) in 5 ml MeOH
was added drop wise to solution of (HNEt3)(iPr2PSe2) (3.77 g,
10 mmol) in 100 ml of MeOH. The mixture was stirred for
1 h at room temperature under atmospheric pressure forming
green precipitate, which was filtered, washed with MeOH and
re-crystallized in dichloromethane to obtain green crystals of
[Ni(Se2PiPr2)2] (2.58 g, 84.75%); mp 212 ◦C. Elemental analysis
(Found: C, 23.8; H, 4.5; P, 10.6; Ni, 9.4. Calc. for C12H28P2Se4Ni:
C, 23.7; H, 4.6; P, 10.2; Ni, 9.6%; dH(300 MHz; CDCl3; Me4Si)
1.43 (dd, 36H, J 6.88 and 20.06, 6 ¥ CH(CH3)2), 2.07 (dd, 6H, J
5.49 and 12.66, 6 ¥ CH(CH3)2). TGA: 220–300 ◦C (74.01 wt%
loss).
In case of [Ni(Se2PtBu2)2] and [Ni(Se2PPh2)2], follow the process
of making [Ni(Se2PiPr2)2] but used (HNEt3)(tBu2PSe2) (4.05 g,
10 mmol) and (HNEt3)(Ph2PSe2) (4.45 g, 10 mmol), obtained green
crystals of [Ni(Se2PtBu2)2] (2.4 g, 72.2%) and dark yellow crystals
of [Ni(Se2PPh2)] (3.0 g, 80.5%), respectively. For [Ni(Se2PtBu2)2],
elemental analysis (Found: C, 28.7; H, 5.4; P, 9.3; Ni, 8.7. Calc.
for C16H36P2NiSe4: C, 28.9; H, 5.5; P, 9.3; Ni, 8.8%); 1H-NMR d =
1.54 ppm (br, s, CH3, t-Bu). TGA: 220–290 ◦C (61.14 wt% loss),
290–350 ◦C (5.90 wt% loss). For [Ni(Se2PPh2)], elemental analysis
(found: C, 38.9; H, 2.6; P, 8.4; Ni, 7.6. Calc. for C24H20P2Se4Ni: C,
38.7; H, 2.7; P, 8.3; Ni, 7.9%). TGA: 220–350 ◦C (61.25 wt% loss).
Thermogravimetric analysis (TGA) measurements were per-
formed on Seiko SSC/S200 thermal analyzer◦ under nitrogen
atmosphere from 25 ◦C to 500 ◦C at ramp of 10 C min-1.
Empirical formula
Fw
C12H28NiP2Se4
608.83
100(2)
0.71073
Monoclinic
P2(1)/n
9.2865(9)
10.9349(10)
10.1364(10)
90
97.904(2)
90
1019.54(17)
2
1.983
C16H36NiP2Se4
664.94
100(2)
0.71073
Monoclinic
P2(1)/c
17.8391(8)
16.2638(7)
33.5402(15)
90
94.1650(10)
90
9705.4(7)
16
1.820
Temperature/K
˚
Wavelength/A
Crystal system
Space group
˚
a/A
˚
b/A
˚
c/A
a/◦
b/◦
g /◦
3
˚
Volume/A
Z
d
calc/Mg m-3
Abs. coeff./mm-1
F (000)
8.236
588
6.930
5216
Crystal size
Index ranges
0.55 ¥ 0.55 ¥ 0.50
-11 ≤ h ≤ 9,
-13 ≤ k ≤ 13,
-10 ≤ l ≤ 12
5643/2067
0.0216
2067/0/92
1.074
0.0198, 0.0470
0.0224, 0.0479
0.543, -0.446
0.44 ¥ 0.25 ¥ 0.15
-22 ≤ h ≤ 22,
-20 ≤ k ≤ 20,
-42 ≤ l ≤ 43
83345/23010
0.0730
23010/0/877
1.009
0.0515, 0.0975
0.0888, 0.1107
1.562, -1.414
Collected/unique
Rint
Data/restraints/pars.
Goodness-of-fit on F2
R1, wR2 [I > 2s(I)]
R1, wR2 (all data)
˚
Diff. peak, hole/e A
carried out at 200 ◦C, 280 ◦C and 330 ◦C and used both HDA and
TOPO as the coordinating solvent.
X-Ray single crystallography
Characterization of nanoparticles
Single-crystal X-ray crystallography measurements for the com-
The X-ray powder diffraction experiments were performed using
a Bruker D8 AXE diffractometer (Cu Ka). TEM samples were
prepared by evaporating a drop of the material suspended in
toluene onto a carbon coated copper grids. Transmission elec-
tron microscopy (TEM) was performed using a Pillips CM200
microscope at an accelerating voltage of 200 kV. High resolution
transmission electron microscopy (HRTEM) was performed using
Tecnai F30 FEG TEM instrument, operating at 300 kV, all samples
deposited over carbon coated copper grids.
pounds were collected using graphite monochromated Mo Ka
˚
radiation (l = 0.71073 A) on a Bruker APEX diffractometer.
The structure was solved by direct methods and refined by
full-matrix least-squares on F2.19 All non-H atoms were refined
anisotropically. H atoms were placed in calculated positions,
assigned isotropic thermal parameters and allowed to ride on their
parent carbon atoms. All calculations were carried out using the
SHELXTL package.20 Crystal data and structure refinement are
given in Table 1. CCDC reference numbers 692535 and 692536.†
Results and discussion
Preparation of nickel selenide and nickel phosphide nanoparticles
Nickel phosphide and nickel selenide nanoparticles were synthe-
sised by the thermal decomposition of [Ni(Se2PR2)2] precursors,
The method used was essentially as described by Trindade and
O’Brien.18 In a typical experiment hexadecyl◦amine, HDA (10 g)
was degassed under reduced pressure at 140 C and then heated
i
t
R = Pr, Bu, and Ph, in a hot surfactant solutions of TOPO
or HDA. The experiments were carried out by dissolving the
precursor in TOP and quickly injecting the solution into the hot
TOPO or HDA at different temperatures including 200 ◦C, 280 ◦C
and 330 ◦C. The precursors were synthesised by the deprotonation
◦
to 330 C under nitrogen. [Ni(Se2PiPr2)2] precursor (1.0 g) was
dissolved in TOP (10 ml) and quickly injected into the hexade-
cylamine. The reaction temperature dropped to approximately
270 ◦C. The reaction mixture was heated again to 330 ◦C and
then maintained at 330 ◦C for 1 h. The dark solution formed
was cooled to approximately 70 ◦C. After cooling the reaction
mixture, an excess of hot methanol was added and the solid
isolated by centrifugation. The solid was washed several times with
hot methanol and left to dry at room temperature. Other materials
were also prepared by similar procedures. All experiments were
of the ligand (HNEt3)(R2PSe2), R = Pr, tBu, and Ph to form the
i
anion which is then reacted with nickel(II) chloride hexahydrate
in methanol to produce a dialkyldiselenophosphinato nickel(II)
complex. The nanoparticles prepared from these precursors show
both nickel selenide and nickel phosphide depending upon the
alkyl group of the precursor, reaction temperature and capping
agent.
2104 | Dalton Trans., 2009, 2103–2108
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◦
Table 2 Nickel phosphide and nickel selenide nanoparticles accessible by
˚
Table 3 Selected bond distances (A) and the angles ( ) for [Ni(Se2PR2)2]
i
t
i
(R = Pr (1); and tBu (2))
reacting single source precursors for [Ni(Se2PR2)2] (R = Pr (1); Bu (2);
and Ph (3)) with TOPO or HDA
˚
Bond distances/A
Bond angles/◦
Capping agent
Reaction
Compound 1
Se(1)–P(1)
Se(1)–Ni(1)
Se(2)–P(1)
Se(2)–Ni(1)
Ni(1)–Se(2A)
Ni(1)–Se(1A)
temperature/◦C
TOPO
HDA
2.1703(6)
2.3591(3)
2.1728(6)
2.3566(3)
2.3566(3)
2.3591(3)
P(1)–Se(1)–Ni(1)
P(1)–Se(2)–Ni(1)
84.823(2)
84.830(2)
180.0
90.263(9)
89.736(9)
89.737(9)
90.264(9)
180.0
1
2
330
280
200
Ni2P(hex) + Ni5P4(hex)
—
—
Ni12P5(tet)
Ni12P5(tet)
—
Se(2)–Ni(1)–Se(2A)
Se(2)–Ni(1)–Se(1A)
Se(2A)–Ni(1)–Se(1A)
Se(2)–Ni(1)–Se(1)
Se(2A)–Ni(1)–Se(1)
Se(1A)–Ni(1)–Se(1)
330
280
Ni2P(hex)
NiSe(hex)
Ni12P5(tet)
NiSe(hex) +
NiSe(rhom)
—
200
330
280
200
Compound 2
Ni(1)–Se(3)
Ni(1)–Se(4)
Ni(1)–Se(1)
Ni(1)–Se(2)
Ni(2)–Se(6)
Ni(2)–Se(8)
Ni(2)–Se(7)
Ni(2)–Se(5)
Ni(3)–Se(10)
Ni(3)–Se(12)
Ni(3)–Se(9)
Ni(3)–Se(11)
Ni(4)–Se(14)
Ni(4)–Se(15)
Ni(4)–Se(16)
Ni(4)–Se(13)
P(1)–Se(2)
P(1)–Se(1)
P(2)–Se(3)
P(2)–Se(4)
P(3)–Se(6)
P(3)–Se(5)
P(4)–Se(7)
P(4)–Se(8)
P(5)–Se(10)
P(5)–Se(9)
2.3408(9)
2.3467(9)
2.3482(9)
2.3520(9)
2.3442(9)
2.3473(9)
2.3621(9)
2.3671(9)
2.3408(9)
2.3499(9)
2.3563(9)
2.3626(9)
2.3305(9)
2.3396(10)
2.3409(10)
2.3568(9)
2.1777(2)
2.1809(2)
2.1782(2)
2.1785(2)
2.1725(15)
2.1792(16)
2.1792(16)
2.1807(16)
2.1770(16)
2.1806(16)
2.1749(16)
2.1789(15)
2.1703(17)
2.1800(17)
2.1663(18)
2.1690(18)
Se(3)–Ni(1)–Se(4)
Se(3)–Ni(1)–Se(1)
Se(4)–Ni(1)–Se(1)
Se(3)–Ni(1)–Se(2)
Se(4)–Ni(1)–Se(2)
Se(1)–Ni(1)–Se(2)
Se(6)–Ni(2)–Se(8)
Se(6)–Ni(2)–Se(7)
Se(8)–Ni(2)–Se(7)
Se(6)–Ni(2)–Se(5)
Se(8)–Ni(2)–Se(5)
Se(7)–Ni(2)–Se(5)
Se(10)–Ni(3)–Se(12)
Se(10)–Ni(3)–Se(9)
Se(12)–Ni(3)–Se(9)
Se(10)–Ni(3)–Se(11)
Se(12)–Ni(3)–Se(11)
Se(9)–Ni(3)–Se(11)
Se(14)–Ni(4)–Se(15)
Se(14)–Ni(4)–Se(16)
Se(15)–Ni(4)–Se(16)
Se(14)–Ni(4)–Se(13)
Se(15)–Ni(4)–Se(13)
Se(16)–Ni(4)–Se(13)
P(1)–Se(1)–Ni(1)
P(1)–Se(2)–Ni(1)
P(2)–Se(3)–Ni(1)
P(2)–Se(4)–Ni(1)
P(3)–Se(5)–Ni(2)
P(3)–Se(6)–Ni(2)
P(4)–Se(7)–Ni(2)
P(4)–Se(8)–Ni(2)
P(5)–Se(9)–Ni(3)
P(5)–Se(10)–Ni(3)
P(6)–Se(11)–Ni(3)
P(6)–Se(12)–Ni(3)
P(7)–Se(13)–Ni(4)
P(7)–Se(14)–Ni(4)
P(8)–Se(15)–Ni(4)
P(8)–Se(16)–Ni(4)
Se(2)–P(1)–Se(1)
Se(3)–P(2)–Se(4)
Se(6)–P(3)–Se(5)
Se(7)–P(4)–Se(8)
Se(10)–P(5)–Se(9)
Se(11)–P(6)–Se(12)
Se(14)–P(7)–Se(13)
Se(15)–P(8)–Se(16)
89.66(3)
90.39(3)
170.57(4)
170.77(4)
91.54(3)
89.91(3)
173.10(4)
90.53(3)
89.42(3)
89.45(3)
90.89(3)
177.60(4)
174.92(4)
89.45(3)
90.70(3)
90.44(3)
89.58(3)
178.03(4)
89.43(3)
179.13(4)
89.71(3)
89.72(3)
179.14(4)
91.14(3)
85.24(5)
85.22(5)
85.90(4)
85.75(5)
83.38(5)
84.07(5)
83.25(5)
83.56(5)
83.51(5)
83.96(5)
83.70(5)
83.91(5)
84.88(5)
85.74(5)
85.46(5)
85.37(5)
99.28(6)
98.67(6)
99.26(6)
98.91(6)
98.67(6)
99.38(6)
98.94(6)
99.19(7)
3
Ni2P(hex)
NiSe(ortho) +
Ni3Se2(rhom)
NiSe(ortho) +
Ni3Se2(rhom)
—
—
—
The summary of the nanoparticles formed from this work is
shown in Table 2. The decomposition reaction of the precursors
was carried out at temperature 200 ◦C, 280 ◦C and 330 ◦C. In
TOPO, Ni2P nanoparticles were observed at 330 ◦C for all
precursors except [Ni(Se2PiPr2)2] (1 which show the trace amounts
of hexagonal Ni5P4. NiSe nanoparticle was only formed by using
[Ni(Se2PtBu2)2] (2) at 280 ◦C. No deposition of nanoparticles
occurred at 200 ◦C which was shown by the absence of any
precipitate on addition of methanol after 1 h reaction at this
temperature. This observation corresponds to the TGA results
which show no decomposition at this temperature (Fig. 1). TGA
showed that all complexes start decomposing at ca. 220 ◦C.
[Ni(Se2PiPr2)2] (1) and [Ni(Se2PPh2)2] (3) decompose in one step
whereas [Ni(Se2PtBu2)2] (2) decompose in two steps. The decom-
position of complex (1) completes at 300 ◦C whereas the for com-
plex (2) and (3) the decomposition completes at 350 ◦C (Fig. 1).
P(6)–Se(11)
P(6)–Se(12)
P(7)–Se(14)
P(7)–Se(13)
P(8)–Se(15)
P(8)–Se(16)
Fig. 1 TGA diagram of [Ni(Se2PR2)2] with R = Pr, tBu, Ph.
i
four different molecules in the asymmetric unit. The molecular
structures of [Ni(Se2PiPr2)2] (1) and [Ni(Se2PtBu2)2] (2) are shown
in Fig. 2.
The nickel atom is coordinated by four selenium atoms
in square-planar coordination. However, the NiSe4 plane in
[Ni(Se2PtBu2)2] (2) (Fig. 2(b)) is in a distorted square planar
geometry. The Se(4)–Ni(1)–Se(1) and Se(3)–Ni(1)–Se(2) bond
X-Ray structures
Crystallographic details and selected interatomic distances and an-
gles of [Ni(Se2PiPr2)2] (1) and [Ni(Se2PtBu2)2] (2) are summarised
in Tables 1 and 3 respectively. For [Ni(Se2PtBu2)2] (2), there are
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Fig. 3 XRD patterns of the particles obtained from [Ni(Se2PtBu2)2]
precursor in TOP/TOPO (a) 330 ◦C (b) 280 ◦C.
Fig. 2 ORTEP diagram showing the molecular structure of (a) the
asymmetric unit of [Ni(Se2PiPr2)2] (1) and (b) one of the four similar
molecules in the asymmetric unit of [Ni(Se2PtBu2)2] (2);thermal ellipsoids
are at the 50% probability level.
angles (170.57(4)◦ and 170.77(4)◦)◦are considerably smaller than
that of [Ni(Se2PiPr2)2] (1) (180.0 ) (Fig. 2(a)). The geometry
around phosphorous shows tetrahedral and the Se–P–Se bite angle
is reduced in [Ni(Se2PtBu2)2] due to more bulky alkyl group as in
[Ni(Se2PiPr2)2] (1). In this work, the structure of [Ni(Se2PPh2)2]
(3) could not be determined because of not getting good quality
crystals for X-ray study. However, the crystal structure for
[Ni(Se2PPh2)2] (3) at room temperature is already reported.21
Fig. 4 HRTEM images and size distributions of (a) hexagonal Ni2P
nanoparticles obtained from [Ni(Se2PtBu2)2] precursor in TOP/TOPO at
Characterization of nanoparticles
◦
330 C; (b) hexagonal NiSe nanoparticles obtained from [Ni(Se2PtBu2)2]
precursor in TOP/TOPO at 280 ◦C.
The crystalline nanoparticles have been characterized by using
X-ray diffraction (XRD) and transmission electron microscope
(TEM). Fig. 3 shows XRD patterns of the nanoparticles prepared
of hexagonal Ni2P, respectively. The NiSe nanoparticles are also
generally spherical with an average size of 6.2 nm and some area of
the sample also formed agglomerate of nanomeric particles. The
fringe spacing observed in this image is 0.270 nm corresponding
to (101) plane of hexagonal NiSe (Fig. 4(b)).
In order to prove a role for TOPO in the reaction, HDA were
used as an alternative capping agent under the same reaction
conditions. Deposition from HDA and TOPO both gave different
phases of phosphide nanoparticles for complex (1) whereas
complex (2) gave phosphide as well as selenide nanoparticles.
Complex (3) deposited only nickel selenide nanoparticles using
HDA whereas the deposition from TOPO gave only phosphide
(Table 2). Fig. 5 shows the XRD patterns of the stable phases
◦
◦
by using TOPO as the capping agent at 330 C and 280 C. The
samples show hexagonal Ni2P (JCPDS 03-0953) and hexagonal
NiSe (JCPDS 02-0892), respectively. However, the minor inter-
mediate phase of hexagonal Ni5P4 was also observed from the
Ni2P nanoparticles prepared from [Ni(Se2PiPr2)2] (1) precursor at
330 ◦C.
Typical TEM images are given in Fig. 4. Nanoparticles with
close to spherical shape are commonly observed for both Ni2P and
NiSe. The results from TEM images showed that monodispersed
Ni2P nanoparticles were obtained with an average size of 4 nm
(Fig. 4(a)). The lattice fringes observed for this particle are
0.228 nm and 0.205 nm corresponding to (111) and (201) plane
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Fig. 5 XRD patterns of the particles prepared from (a) [Ni(Se2PtBu2)2]
precursor; (b) [Ni(Se2PiPr2)2] precursor; (c) [Ni(Se2PPh2)2] precursor; all
particles prepared at 280 ◦C by using HDA as the capping agent.
in HDA; tetragonal Ni12P5 (JCPDS 74-1381) and hexagonal NiSe
(JCPDS 02-0892), as well as orthorhombic NiSe (JCPDS 29-0935).
In Fig. 5(a), there are small peaks of the rhombohedral NiSe as
the impurity phase. The formation of rhombohedral NiSe can
be explained in terms of a competitive process between kinetic
and thermodynamic control. At the beginning of the reaction,
hexagonal NiSe is produced by kinetic control. When all the Se
and Ni sources are consumed, the precipitation solubility process
for NiSe in solution becomes the main reaction. At this time,
thermodynamic control is the main factor. Longer reaction time
leads to a preference for the formation of the thermodynamically
stable phase. Thus, hexagonal NiSe will gradually transform into
rhombohedral NiSe.6 The mixed phases between orthorhombic
NiSe and rhombohedral Ni3Se2 were also observed by using
[Ni(Se2PiPr2)2] (1) precursor (Fig. 4(b)). The XRD patterns show
orthorhombic NiSe as the major phase with small amount of
rhombohedral Ni3Se2.
Typical TEM images for these samples are shown in Fig. 6. All
samples are nearly spherical in shape. Fig. 6(a) shows monodis-
persed Ni12P5 nanoparticles with an average size of 5.6 nm. The
lattice spacing observed in the image is 0.216 nm corresponding to
400 planes of tetragonal Ni12P5. Spherical nanoparticles of hexag-
onal NiSe show in Fig. 6(b). The average particle size is 6.6 nm. The
lattice fringes show a d-spacing of 0.203 nm which corresponds
to the 102 plane. In case of orthorhombic NiSe, the nanoparticles
are also generally spherical with an average size of 5.1 nm; in some
areas of the image agglomerates of nanomeric particles are also
formed. The lattice spacing observed in this image is 0.155 nm
corresponding to 103 plane of hexagonal NiSe (Fig. 6(c)).
Fig. 6 HRTEM images and size distribution of (a) tetragonal Ni12P5
obtained from [Ni(Se2PPh2)2] precursor; (b) hexagonal NiSe obtained
from [Ni(Se2PtBu2)2] precursor; (c) the orthorhombic NiSe obtained from
[Ni(Se2PiPr2)2] precursor; all samples prepared at 280 ◦C by using HDA
as the capping agent.
In this work, we generally observed that higher temperatures
tend to favour the deposition of nickel phosphide nanoparticles
probably because of either higher phosphorous availability or
via TOP decomposition.22–24 However, the product also depends
on the nature of the single source precursor, for instance,
[Ni(Se2PPh2)2] (3) only formed NiSe. Moreover, the coordinating
solvents including TOPO and HDA play an important role in the
chemical composition of nickel phosphide nanoparticles. Whilst,
the richer phosphorous phase Ni2P (P : Ni = 0.5) was always
formed when using TOPO as the capping agent, the phase Ni12P5
(P : Ni = 0.42) was formed when using HDA as the capping agent.
Conclusions
In this paper, we have shown that nickel phosphide and nickel
selenide nanoparticles can be synthesised in a one pot reaction
using single source molecular precursors of the type [Ni(Se2PR2)2],
i
(R = Pr, tBu, or Ph) in coordinating solvent such as TOPO and
HDA at moderate temperatures. The chemical compositions of
the nanoparticles can easily be controlled by varying tempera-
ture, capping agent, and type of precursor. The monodispersed
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nanoparticles prepared from this method are spherical in shape.
The nanoparticles can be formed at low temperatures (<370 ◦C),
which represents an alternative, convenient and simple route to
synthesise nickel selenide or phosphide nanoparticles.
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Acknowledgements
WM would like to acknowledge the Royal Thai Government
for funding. The authors also thank EPSRC, UK for financial
assistance.
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