White phosphorus and metal nanoparticles: A versatile route to metal phosphide nanoparticles

Article abstract of DOI:10.1039/c0cc00684j

P₄ reaction with metal NPs (In, Pb, Zn) provides an easy access to the corresponding metal phosphide NPs in a soft and stoichiometric reaction. Size-influence on the reactivity is investigated in the case of indium. The Royal Society of Chemistry 2010.

Full text of DOI:10.1039/c0cc00684j

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White phosphorus and metal nanoparticles: a versatile route to metal
phosphide nanoparticlesw
a
a
b
b
Sophie Carenco,ab Matthieu Demange, Jing Shi, Cedric Boissiere, Clement Sanchez,
´
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a
a
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Pascal Le Flochz and Nicolas Mezailles*
Received 31st March 2010, Accepted 1st June 2010
First published as an Advance Article on the web 24th June 2010
DOI: 10.1039/c0cc00684j
P4 reaction with metal NPs (In, Pb, Zn) provides an easy access
to the corresponding metal phosphide NPs in a soft and stoichio-
metric reaction. Size-influence on the reactivity is investigated in
the case of indium.
most metals. The influence of the size of the particles on the
kinetics of the reaction is also presented herein.
Very few synthetic procedures are known for Zn NPs, because
they are readily oxidized by oxygen (E1(Zn) = ꢀ0.76 V).
This can only be prevented by surface passivation14 which
makes them unreactive. The synthesis of In NPs is also quite
challenging and usually yield large NPs (>10 nm),15 even
though smaller In NPs (5 nm) could be obtained by Chaudret
et al.16 The above mentioned syntheses of either small sized
NPs of Zn or In did not suit our purposes in terms of
stabilizing the ligand system and we devised a novel strategy
toward these NPs based on a strong reduction.
Molecular reactivity of P4 (the most reactive allotrope of
phosphorus) toward transition metal fragments has attracted
much attention in the previous four decades because
of its room-temperature reactivity.1 P4 may behave as a
‘‘classical’’ phosphine ligand via coordination of the lone
pair(s) at phosphorus in well-designed organometallic
complexes.2 Alternatively, the initial P4 tetrahedron may be
deeply transformed in the coordination sphere, providing
complex species incorporating various Pn moieties (from P1
to P12), opening routes to P4 functionalization in solution
under mild conditions.3 The key feature for these low-
temperature activations of P4 is the use of its molecular form
(in solution) and a carefully designed molecular reactant
(transition metal complexes or organic species). Surprisingly,
this enhanced reactivity has been very rarely exploited in the
field of nanomaterials.4 Indeed, while some studies report the
use of P4 in the synthesis of metal phosphide nanoparticles
(NPs),5 it is generally added as a solid in hydrothermal
conditions, at high temperature for days, as a source of PH3.5,6
Alternative ‘P’ donors have also been used in relatively mild
conditions, such as P(SiMe3)3,7 TOP (tri-n-octylphosphine)8
or PPh3.9
It is actually known in inorganic/organometallic chemistry
that strong one-electron reducing agents are able to transform
in a stoichiometric manner, oxidized transition metal com-
plexes into stable M(0) complexes provided that appropriate
ligands are also bound to the metal center.17 We extended this
strategy to the challenging synthesis of metal NPs (In, Zn) by
promoting a strong nucleation rate using Na/naphthalenide
and insuring surface stabilization with trialkyl-phosphine/amine
ligands. In the field of NPs, this reducing agent has been used
once on SiCl4,18 and very recently for Pt containing alloys.19
The fast reduction of InCl3 and ZnCl2 by stoichiometric
amounts of sodium naphthalenide in the presence of an excess
of ligand (either trioctylphosphine or trioctylamine) was
carried out at room temperature (Scheme 1). This reaction
yielded M(0) NPs species together with NaCl (which was
observed by XRD after centrifugation of the crude mixture)
as the expected byproduct of the reduction. In both cases a
brown colloidal suspension of highly reactive M(0) NPs was
obtained.
In this work, we demonstrate that soluble metal NPs
(from groups 12 to 14: Zn, In and Pb) successfully substitute
M(0) complexes which are not accessible in these cases, for the
activation of the whole P4 molecule, in a controlled and
stoichiometric reaction. The M(0) NPs provide a versatile
access to highly desirable metal phosphide NPs, which exhibit
relevant properties in the fields of catalysis,5d,10 lumines-
cence,11 magnetic applications,10b,12 and electrodes for lithium
batteries.13 Our approach could, in principle, be extended to
Because of their extremely fast reactivity toward either
O2 or H2O, their complete characterization was then not
attempted. However, TEM observations of the crude mixtures
showed the presence of small NPs (TOP as surface ligand). In
the case of indium, the NPs appeared to be in the 2–7 nm range
with some larger aggregates (up to 10 nm) (Fig. 1, left). The
NPs appeared smaller in the case of Zn (1–5 nm) (ESIw).
In a second step, without treatment, these NPs were reacted
with P4 at room temperature showing a reactivity comparable to
that of M(0) complexes, because of their high surface-to-volume
a
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Laboratoire Heteroelements et Coordination, Ecole Polytechnique,
CNRS, 91128 Palaiseau Cedex, France.
E-mail: nicolas.mezailles@polytechnique.edu;
Fax: (+33) 1 69 33 44 49; Tel: (+33) 1 69 33 44 02
b
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´
Laboratoire de Chimie de la Matiere Condensee de Paris,
UPMC Univ Paris 06 – Colle`ge de France, 11 place
Marcelin Berthelot, 75231 Paris Cedex 05, France.
Fax: (+33) 1 44 27 15 04; Tel: (+33) 1 44 27 15 02
w Electronic supplementary information (ESI) available: Synthetic
procedures, TEM/HRTEM observations and XRD measurements.
See DOI: 10.1039/c0cc00684j
z Deceased.
Scheme 1 Synthetic pathway for InP and Zn3P2 NPs.
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Fig. 1 As-synthesized NPs obtained by InCl3 reduction, and InP NPs
obtained after reaction of the In NPs with P4.
Fig. 2 XRD on InP NPs (top, circles from JCPDS 03-065-2889) obtained
ratio. Indeed, the reaction, followed by 31P NMR spectro-
scopy, showed the disappearance of P4 (singlet at ꢀ521 ppm)
within a few minutes, without the appearance of any signal.
This correlates with a stoichiometric reaction of P4 at room
temperature with the In NPs, yielding InP NPs. Similarly,
1/6 equiv. of P4 were added on the solution of Zn NPs at room
temperature to form Zn3P2. TEM observations on the NPs
synthesized with TOP show particles in the 2–10 nm range for
InP (Fig. 1, right) and 1–5 nm for Zn3P2 (ESIw). Note that
TEM analysis of TOA-capped NPs shows slightly larger
particles and aggregates (ESIw), indicating that the surface
stabilization by the amine is poorer than by the phosphine.
Elemental analysis (EDS on a MEB) was conducted on the
TOA-capped NPs in both cases, after centrifugation and
extensive washing of the NPs. It indicated ratios of 1.0 : 1.0
for In : P, in agreement with a stoichiometric reaction of white
phosphorus on the metal NPs, and 3.0 : 1.7 for Zn : P, slightly
off the theoretical ratio.
from isolated In NPs (bottom, crosses from JCPDS 03-065-9682).
Fig. 3 TEM observations of ‘large’ In and InP NPs.
reaction is not complete at room temperature. Toluene was
evaporated under vacuum and the solution was heated at 180 1C
for 2 h providing a P4-free solution as shown by the lack of
signal in the 31P NMR spectrum, suggesting complete reaction
of P4 with the NPs. The powder-XRD of the NPs after
isolation and washing showed only the InP structure
(Fig. 2). TEM observations showed highly crystallized NPs
in the same size range as the starting In NPs, suggesting direct
insertion of P in the metal template (Fig. 3 and ESIw).
We finally turned our attention to the least reactive species
of our series, both because of higher oxidation potential
(E1(Pb) = ꢀ0.13 V) and low surface-to-volume ratio: large
Pb NPs. The synthesis of Pb NPs was achieved according to a
procedure reported by Aubin et al.20 TEM observations
showed polydispersed NPs in the 5–20 nm range (ESIw). The
isolated Pb NPs were dispersed in 1-octadecene under an inert
atmosphere. We used a low-P stoichiometry (2 Pb for 1 P)
because of early reports indicating that only low-phosphorus
content alloys are stable.21 After the addition of a white
phosphorus solution in toluene (1/8 equiv. of P4) at room
temperature, the 31P NMR spectrum of the solution showed a
singlet at ꢀ521 ppm. In fact, no evolution of the peak intensity
on a period of 30 min was observed, indicating a slow kinetics
of the reaction. The solution was therefore heated at 150 1C
for 1 h after which time no more signal in the 31P NMR
spectrum was recorded. The NPs were recovered by addition
of isopropanol and centrifugation and were washed exten-
sively with isopropanol. Elemental analysis of the NPs by EDS
gave the expected Pb : P stoichiometry of 2.0 : 1.0. TEM images
revealed polydispersed NPs in the 5–10 nm range (ESIw).
The kinetics of the oxidation of M(0) NPs appears to
depend on the outmost importance on their size. Indeed, the
works of Chaudret et al. and Feldmann et al. had demon-
strated different kinetics of oxidation,15a,16 from instantaneous
oxidation in the first case (5 nm NPs), and a resistance for
a couple of hours in the other case (diameter > 10 nm),
while other works do not mention oxidation of the particles
(50 nm NPs).15b A similar kinetic behavior in their reaction
with P4 was expected, which prompted us to study larger
systems: In, to provide a direct comparison, and Pb.
We synthesized In NPs under nitrogen using a modified
single-phase procedure.15a (ESI) The isolated black powder
was analyzed by powder-XRD, confirming the formation of
crystalline In NPs (Fig. 2). TEM observation of the NPs
dispersed in hexane showed 5–25 nm spherical NPs. High-
resolution images confirmed the crystallinity and structure of
the NPs. Importantly, the same spectra were obtained when
the isolation of the particles was performed under nitrogen, or
in air (ca. 15 min for the whole process). Indeed, no shell of
In(OH)3 nor In2O3 could be observed, suggesting that both the
relatively large size of the particles and their coating with
oleylamine provided a good kinetic protection against oxida-
tion in air (Fig. 3, left and ESIw).
1/4 equiv. of P4 in toluene was added to a solution of
isolated In NPs in 10 mL of diethylene glycol. 31P NMR
spectrum of the solution showed the presence of significant
amounts of P4 (singlet at ꢀ521 ppm) indicating that the
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Scheme 2 Proposed metal phosphide formation mechanism.
We investigated the stoichiometric and low-temperature
activation of P4 using soluble metal NPs as reactants. We
propose an original synthesis for zinc (groups 12) and indium
(group 13) small metal phosphide NPs, based on the strong
reduction of a chloride precursor followed by low-temperature,
stoichiometric reaction of P4 on the as-synthesized NPs. This
one-pot strategy circumvents the lack of stability of these
species toward oxidation. We also showed that crystalline
InP particles can be obtained from crystalline In NPs (size
and morphology of the starting NPs are preserved), particles
in the relevant size range for luminescence and photovoltaic
applications. Finally, a kinetic dependence of the reaction of
P4 with M(0) NPs on the size of the particles was observed.
While small indium NPs were shown to exhibit a high reactivity
at room temperature, reactions involving larger NPs required
heating.
This suggests that the mechanism proceeds through reaction
on the surface of the NP followed by diffusion of phosphorus
in the NP lattice (Scheme 2), the kinetics of which is slowed
down in an extended lattice. Finally, we demonstrated here by
three examples that M(0) NPs offer a versatile route toward
metal phosphide NPs. Detailed studies on the reaction of P4
toward other metal NPs and on the detailed mechanism of this
reaction are underway in our laboratories.
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The LSI (Ecole Polytechnique) is acknowledged for the use
of their TEM. The LPEM (ESPCI, Paris) is acknowledged
for the use of their EDS device. S. C. thanks the DGA for
financial support. The CNRS, the Ecole Polytechnique and the
UPMC are also acknowledged for financial support.
Notes and references
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5580 | Chem. Commun., 2010, 46, 5578–5580