Synthesis of CoPt nanorods in ionic liquids

Article abstract of DOI:10.1021/ja043625w

Cobalt platinum nanorods, hyperbranched nanorods, and nanoparticles were synthesized in 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [BMIM][Tf₂N], ionic liquid though the thermal reduction of platinum acetylacetonate, and coba

Full text of DOI:10.1021/ja043625w

Published on Web 03/24/2005
Synthesis of CoPt Nanorods in Ionic Liquids
Yong Wang† and Hong Yang*,†,‡
Department of Chemical Engineering and Laboratory for Laser Energetics, 206 GaVett Hall,
UniVersity of Rochester, Rochester, New York 14627-0166
Received October 20, 2004; E-mail: hongyang@che.rochester.edu
Although various synthetic methods have been developed for
the morphological controls of nanoparticles, rods, and wires of
semiconductors, metals, and metal oxides, only limited success has
been made in making alloy or intermetallic nanoparticles, such as
FePt, CoPt, and AuCu.1 Not all the compositions or broad particle
sizes have been achieved at the nanometer scale. The formation of
one-dimensional nanorods and nanowires has rarely been reported.1
For technologically important Co-Pt and Fe-Pt systems, shape,
crystal phase, and composition are essential for the outstanding
catalytic, magnetic, and other properties.2,3 For example, the ordered
CoPt3 and FePt3 have been predicted to have higher electrocatalytic
activities than pure Pt in hydrogen oxidation reactions.3 The face-
centered tetragonal FePt nanoparticles are the most designable
material among the Fe-Pt system for magnetic data storage media
applications.2 The reaction temperatures, however, are usually high
in order to obtain crystal phase- and composition-specific alloy
nanomaterials,1h,4,5 except for a biotemplate synthetic method.6
These temperatures are typically above the boiling points of most
of the conventional solvents. Postsynthesis high-temperature treat-
ments often affect the particle size, size distribution, and structural
property of the nanoparticles.
Figure 1. TEM images at low (left) and high (right) magnifications of the
CoPt nanorods obtained at the Pt(acac)2:Co(acac)3:CTAB molar ratio of
0.5:1.5:10.
Ionic liquids (ILs),7,8 known for their nonvolatile, nonflammable,
and thermally stable properties, have recently been used in making
nanoparticles.9-15 The thermal stability and nonvolatility of ILs,
however, have not been utilized as a high-temperature environment
for the synthesis of nanomaterials, especially crystal phase- and
composition-specific alloy nanomaterials. In this work, we dem-
onstrate that nanorods, hyperbranched nanorods, and nanoparticles
with different CoPt compositions can be synthesized in 1-butyl-3-
methylimidazolium bis(trifluoromethylsulfonyl) imide, [BMIM]-
[Tf2N].
[BMIM][Tf2N] was chosen as the solvent because of its thermal
stability.8,16 This IL was synthesized using a method reported
previously (Supporting Information).8 Cetyltrimethylammonium
bromide (CTAB) was used as the capping reagent because it has
good solubility in 1-ethyl-3-methylimidazolium bis(trifluorometh-
ylsulfonyl)imide, [EMIM][Tf2N],17 which has a chemical structure
similar to that for [BMIM][Tf2N].
Figure 2. TEM images of the nanomaterials obtained at the different
reactant molar ratios of Pt(acac)2:Co(acac)3:CTAB: (a) 10:3.3:10, (b)
3.3:3.3:10, (c) 3.3:10:10, and (d) 0.37:1:10. Insets show micro-EDs from
an individual nanoparticle (panel b) and the tip of a nanorod (panel c).
bundles of CoPt nanorods. These nanorods had an average diameter
of ∼8 nm. Energy-dispersive X-ray (EDX) analysis indicated the
average Co/Pt atomic ratio of these nanorods was Co61Pt39, Figure
S1.
We examined the reaction at different Pt(acac)2:Co(acac)3:CTAB
molar ratios to understand the formation mechanism and control
the compositions and crystal phases. In all the tests, [BMIM][Tf2N]
used was kept at 4 mL and CTAB was at 0.375 mmol. Nanoparticles
with an average diameter of ∼5 nm formed when the Pt(acac)2:
Co(acac)3 molar ratio was g∼1, Figure 2a and b. Most of these
particles formed aggregates similar to some other nanoparticles
formed in ionic liquids.13 EDX analysis of nanoparticles shown in
Figure 2a indicated the composition was Co22Pt78, which was close
to the atomic ratio of CoPt3 alloy. The Co/Pt atomic ratio of the
particles shown in Figure 2b was Co35Pt65, which was very close
to those for individual nanoparticles obtained using an ultrahigh
vacuum scanning transmission electron microscope (UHV-STEM,
Cornell VG HB501 STEM), Co37Pt63, Table S2 and Figure S2.
Micro-electron diffraction (ED) analysis indicated that these nano-
Scheme 1. Molecular Structure of [BMIM][Tf2N]
Cobalt platinum nanorods were made in freshly dried [BMIM]-
[Tf2N] at 350 °C. Platinum acetylacetonate (Pt(acac)2) and cobalt
acetylacetonate (Co(acac)3) were used as Pt and Co precursors,
respectively. The Pt(acac)2:Co(acac)3:CTAB molar ratio was
0.5:1.5:10 (See Supporting Information). Figure 1 shows the
representative transmission electron microscope (TEM) images of
† Department of Chemical Engineering.
‡ Laboratory for Laser Energetics.
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10.1021/ja043625w CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
formation of CoPt alloy nanoparticles and nanorods with different
compositions and crystal phases. The decrease in melting point of
nanoparticles21 could be relevant for the formation of stable CoPt
alloys.
Acknowledgment. This work is supported partially by NSF
(CTS-0417722) and DOE through LLE (DE-FC03-92SF19460).
This work made use of Shared Experimental Facilities at the Cornell
Center for Materials research (CCMR) supported by NSF. We thank
Dr. Mick Thomas for help.
Supporting Information Available: Synthetic procedures for
[BMIM][Tf2N], CoPt nanoparticles, nanorods, and hyperbranched
nanorods. TEM, EDX, and UHV-STEM analysis of nanoparticles,
Figures S1-5 and Tables S1-3. This material is available free of charge
via the Internet at http://pubs.acs.org.
Figure 3. PXRD patterns of the Co-Pt nanomaterials obtained at the
different reactant mole ratios of Pt(acac)2:Co(acac)3:CTAB: (a) 10:3.3:10,
(b) 3.3:3.3:10, and (c) 3.3:10:10, respectively.
particles were highly crystalline, inset of Figure 2b. When the
Pt(acac)2:Co(acac)3 molar ratio was below one, bundles of nanorods
were the major product, Figure 2c. The average composition of
the bundles of Co-Pt nanorods shown in Figure 2c was Co68Pt32,
Table S3. This value was close but smaller than the number of
Co75Pt25 (or Co3Pt) obtained on selected individual rods. The micro-
electron diffraction on the selected single rod revealed the single-
crystalline nature of the individual rods, inset of Figure 2c. The
estimated yield of the reactions based on Co elemental analysis
was about 60%. The solvent properties18 of [BMIM][Tf2N] IL
appeared to play an important role in the formation of CoPt
nanorods. Polydispersed nanoparticles, instead of nanorods, were
obtained when we replaced [BMIM][Tf2N] with trioctylamine as
solvent and reacted at ∼340 °C, Figure S4. Hyperbranched nanorods
were obtained at relatively low concentrations of Pt(acac)2 and
Co(acac)3, Figure 2d.
The anisotropic interaction between the capping agents and the
different facets of Co-Pt crystals should also be essential for the
formation of nanorods and hyperbranched nanorods. To examine
the effect of surfactants on the growth mechanism, we replaced
CTAB with N,N-dimethylhexadecylamine (DMHA), which had
similar alkane chain length and an amine function group. Further-
more, DMHA could form from CTAB at high temperatures.19
Similar morphological control over Co-Pt systems was achieved
using DMHA as surfactant, Figure S5. This result suggested that
the amine should be the crucial functional group in the control of
the anisotropic growth of Co-Pt nanorods.
Our EDX data indicated that different Co-Pt alloy nanoparticles
and nanorods could be made. This result was confirmed by the
powder X-ray diffraction (PXRD) study, Figure 3. The main
diffraction peaks from nanoparticles made at Pt(acac)2:Co(acac)3
molar ratio of ∼3 matched those for CoPt3 alloy (space group:
Pm3m). The peak position of (111) diffraction between 40° and
43° 2θ shifted toward high angle with the decrease of Pt(acac)2:
Co(acac)3 molar ratio, suggesting the Co-Pt nanoparticles and
nanorods form over a wide range of Co-Pt atomic ratios. The (111)
diffraction of Co-Pt alloy for the nanorods shown in Figure 2c
was at 41.6° 2θ, which belongs to CoPt alloy, Figure 3c.20 It is
noted that Weller et al. have studied the formation of Co-Pt
nanoparticles in conventional organic solvents below 300 °C.1j In
those cases, only CoPt3 nanoparticles could form over a wide range
of Pt/Co precursor ratios. The combined high-temperature environ-
ment and the unique property of IL18 could attribute to the observed
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