A general methodology for the synthesis of transition metal pnictide nanoparticles from pnictate precursors and its application to iron-phosphorus phases

Article abstract of DOI:10.1021/ja028180v

Nanoparticles of iron phosphide have been prepared through a new strategy involving the reductive annealing of nanoparticulate iron phosphate precursors cast onto atomically flat mica surfaces. This route appears to be general for a range of transition metals and pnicogens and avoids the use of highly toxic and pyrophoric agents such as Pn(SiMe₃)₃ (Pn = P, As), which are commonly employed in the synthesis of pnictide nanoparticles. Copyright

Full text of DOI:10.1021/ja028180v

Published on Web 03/14/2003
A General Methodology for the Synthesis of Transition Metal Pnictide
Nanoparticles from Pnictate Precursors and Its Application to
Iron-Phosphorus Phases
†
†
‡
,†
Kimber L. Stamm, Jayne C. Garno, Gang-yu Liu, and Stephanie L. Brock*
Department of Chemistry, Wayne State UniVersity, 5101 Cass AVenue, Detroit, Michigan 48202, and
Department of Chemistry, UniVersity of California, DaVis, One Shields AVenue, DaVis, California 95616
Received August 16, 2002; E-mail: sbrock@chem.wayne.edu
Three-dimensional limiting of extended solids to produce nano-
particles has a profound influence on the magnetic, electronic, and
optical properties of a material.1 The study of such size-dependent
phenomena is predicated on access to well-defined materials and
impacts both advancements in existing technologies (e.g. the effect
of decreasing bit size on device performance) as well as new,
unanticipated technologies. Although general routes to nanoparticles
of main group pnictides (pnicogen ) group 15 element) such as
InP and GaAs have been developed,3 nanoparticles of transition
metal analogues remain largely unexplored. This is surprising
because bulk transition metal pnictides exhibit properties of
,2
,4
significant interest including ferromagnetism, magnetoelastic and
Figure 1. AFM study of iron phosphate nanoparticles. (A) An 800 × 800
magnetooptical properties, semiconductivity, and superconductiv-
2
nm topographic image. (B) Cursor profile of two nanoparticles from (A),
5
ity. Understanding size-dependent phenomena in nanocrystalline
measuring 2.7 and 2.9 nm, respectively.
transition metal pnictides may contribute to the advancement of
technologies, such as magnetoresistive sensing, magnetic refrigera-
tion, and data storage and transmission. However, the lack of
synthetic methodologies has severely limited the opportunities for
systematic exploration of the size-dependent physical properties in
these materials.6
The reduction of transition metal pnictates (other than nitrate)
to pnictides is a proven route to bulk, microcrystalline pnictide
7
phases of Fe, Co, W, and Mo, among others. We are interested in
exploiting this method for the facile production of transition metal
pnictide nanoparticles by using preformed transition metal pnictate
nanoparticles as precursors. This route should be general for a
number of transition metals and pnicogens and has the benefit of
avoiding the use of highly toxic and pyrophoric agents such as
3 3
P(SiMe ) , commonly employed in main group nanoparticle
3,4
synthesis. Additionally, the method is expected to permit control
of stoichiometry, an important criterion as the phase diagrams of
transition metal pnictides are considerably richer than their main
group congeners. As proof of concept, we report herein the
application of pnictate reduction to the synthesis of transition metal
Figure 2. X-ray diffraction patterns of precipitated iron phosphate
nanoparticles annealed under H2/Ar at 700 and 1100 °C (inset), and
corresponding line diagrams for FeP (78-1443) and Fe2P (85-1725).
pnictide nanoparticles, specifically, the magnetic iron phosphides
8
2
FeP and Fe P.
1
3
The precursor iron phosphate nanoparticles were produced by a
route analogous to that reported for the synthesis of LaPO
4
indicative of the formation of large crystallite sizes (>500 nm),
indicating that sintering has occurred. The presence of crystalline
9
nanoparticles, and the morphology and size were determined using
atomic force microscopy (AFM).10 Figure 1A reveals the typical
morphology of these nanoparticles to be nearly spherical. Particle
diameter was measured from the height in the cursor profiles as
illustrated in Figure 1B11 and was found to range in size from 1.4
to 4.6 nm, with an average value of 2.8 ( 0.6 nm for this sample.
2 4
phosphates such as Fe (P O12) (powder diffraction file (PDF): 76-
0
223) at temperatures of ca. 400 °C, in turn, suggests that the
13
sintering occurs well before reduction to the pnictide. Between
00 and 1000 °C, both FeP and Fe P are observed, suggesting
transformation from FeP to Fe P with loss of phosphorus as a
8
2
2
Reduction of precipitated particles in H
crystalline patterns of phase-pure FeP (700 °C) and Fe
2
/Ar atmosphere produced
P (1100
C) (Figure 2).12 However, the sharp diffraction patterns are
12
volatile byproduct (likely as phosphine).
2
To prevent sintering, we have begun to explore techniques for
preventing aggregation during reduction by increasing the inter-
particle separation and/or restricting the particle mobility. These
can be achieved by decreasing particle concentration during
°
†
‡
Wayne State University.
University of California, Davis.
4038
9
J. AM. CHEM. SOC. 2003, 125, 4038-4039
10.1021/ja028180v CCC: $25.00 © 2003 American Chemical Society

C O MMU N I C A T I O N S
Acknowledgment. We thank Hyungrok Kim and Susan Kau-
zlarich for assistance with experiments performed at UCD, Ron
Baird and Eric McCullen for help with XPS, and Charles H. Winter
for critical reading of this manuscript. Acknowledgments are made
to the donors of the Petroleum Research Fund, administered by
the ACS, and the National Science Foundation (CAREER award
DMR-0094273 and IGERT-970952) for financial support of this
research.
Supporting Information Available: XPS spectra of Fe-P samples
reduced on mica (PDF). This material is available free of charge via
the Internet at http://pubs.acs.org.
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(
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(
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Figure 3. AFM topographic study of the particles after annealing. Surface-
confined precursors were calcined at 700 °C for 1 h, in 7% hydrogen in
nitrogen. (A) Precursor concentration: 2.3 mg/mL. (B) Cursor profile from
(
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(
A). (C) Precursor concentration: 0.19 mg/mL. (D) Cursor profile from
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reduction and confining the particles on surfaces, respectively.14
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(
6) (a) Previous reports of the synthesis of transition metal pnictide nano-
6b
particles have been limited to organometallic pyrolysis routes and
6
c,d
solvothermal syntheses. (b) Lukehart, C. M.; Milne, S. B.; Stock, S.
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The results of directly annealing surface-confined phosphate
nanoparticle precursors at 700 °C are illustrated in Figure 3.15 At
high initial coverage, the resulting iron phosphide particles are 3D
aggregates ranging from 0.8 to 37 nm (average 13.4 ( 8.7 nm) as
shown in Figure 3A. Quantitative measurements of two relatively
large particles are shown in Figure 3B. The scale of these 3D
aggregates represents a significant growth in particle size relative
to the phosphate precursor employed for this experiment (2.18 (
(
7) Gopalakrishnan, J.; Pandey, S.; Rangan, K. K. Chem. Mater. 1997, 9,
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113-2116.
(8) Fe P is ferromagnetic, T
25 K; other phases in the Fe-P phase diagram: FeP
2
c
) 266 K, and FeP is antiferromagnetic, T
N
)
1
Fe
2
(semicond.) and
5
3 c
P (ferromagnetic, T ) 716 K).
(9) (a) FeCl was treated with crystalline H PO and tri-n-octylamine in tris-
3
3
4
2
-ethylhexylphosphonate (TEHP) at 160 °C overnight, followed by
precipitation with methanol. The resultant nanoparticles have an Fe:P ratio
of 0.92 ( 5% (ICP-MS) and are soluble in pyridine, toluene, or hexane.
(
b) Riwotzki, K.; Meyssamy, H.; Kornowski, A.; Haase, M. J. Phys. Chem.
1.3 nm). However, at low coverage, phosphide nanoparticles exhibit
B 2000, 104, 2824-2828.
a near-spherical morphology with an average size of 1.41 ( 0.5
nm, as shown in Figure 3C and D. Assuming spherical geometry,
we found that annealing results in 35% reduction in particle
diameter (∼75% reduction in volume), consistent with oxygen loss
during the transformation of phosphate to phosphide (predicted
volume reduction from FePO : 72% for FeP, 79% for Fe P). On
4 2
the basis of data obtained for sintered samples (Figure 2), the
expected phase at 700 °C is FeP.16
The reduction of phosphorus is further confirmed in the XPS
spectra (Supporting Information). The P 2p XPS spectrum reveals
two peaks at 132.7 and 128.4 eV, respectively, with an intensity
ratio of 70:30. The lower energy peak is consistent with the binding
energy for FeP (129.5 eV),17 demonstrating that some reduction
has occurred. The presence of oxidized phosphorus (132.7 eV) is
not unexpected as the samples are exposed to air during introduction
to the XPS chamber. These data, including the observation of
oxidation, are consistent with those observed for other phosphide
nanoparticles (e.g., InP).3
In conclusion, the preparation of iron phosphide nanoparticles
presented here represents the first report of the use of preformed
nanoparticle precursors for the synthesis of nanoparticulate pnic-
tides. However, this general methodology is expected to be
applicable to a wide range of transition metals and pnicogens. To
date, we have successfully extended this approach to Ni-As
nanoparticulate phases and are also investigating other strategies
for limiting aggregation, including solution reduction routes, to
further expand the versatility of the method.
(
10) (a) Samples for AFM were prepared by depositing a 10 µL drop of
phosphate nanoparticle solution onto freshly cleaved mica (0001) surfaces,
followed by air-drying. The AFM scanner is home-constructed and
10b,c
employs the optical beam-deflection configuration.
controllers and software are from RHK Technology, Inc. Commercially
available Si cantilevers with force constants of 0.1 N/m (microsharp-
The electronic
3
N
4
ened cantilever, Thermo Microscopes) were used for imaging. The AFM
scanner was calibrated using Au(111) periodicity and step height. Contact-
resonance imaging (CRI) was used to minimize adhesion between the tip
10d
and surface, thus circumventing stick-slip motion during scanning. (b)
Kolbe, W. F.; Ogletree, D. F.; Salmeron, M. B. Ultramicroscopy 1992,
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S.; Liu, G.-Y. Appl. Surf. Sci. 2001, 175-176, 391-398.
11) Lateral AFM sizes are a convolution of tip geometry. Without aggressive
deconvolution, it is more reliable to use measurements of nanoparticle
heights. Over 150 cursor profiles were analyzed for average particle size
determinations.
(
(12) Safety Note: Because of the use of hydrogen mixtures (7-15% in Ar or
2 3
N ), as well as the possibility of producing toxic byproducts (e.g., PH ),
all reduction reactions were either conducted in, or vented to, a fume
hood.
(
(
(
13) Powder diffraction file (PDF) data were obtained from the ICDD-PDF
database, release 2000.
14) Chen, Z. X.; Smith, G. C.; Putman, C. A. J.; ter Voert, E. J. M. Catal.
Lett. 1998, 50, 49-57.
15) AFM imaging of plain mica substrates indicates that the surface remains
atomically flat under the annealing conditions employed here.
(16) Although both iron and phosphorus are detected, the stoichiometry of the
2
reduced nanoparticle phase on mica (FeP, Fe P) has not been determined
due to sensitivity issues with conventional methods (e.g., XPS) on these
8
highly dilute samples. Planned magnetic studies will probe whether the
reduction temperature dependence of the phase transformation is identical
to that observed in the bulk, or if there are differences due to size effects,
and this will be presented in a full paper.
(
17) Binding energy data are available online at http://srdata.nist.gov/xps/.
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