Low-temperature stability and high-temperature reactivity of iron-based core-shell nanoparticles

Article abstract of DOI:10.1021/ja0473891

Iron-based core-shell nanoparticles have been prepared that exhibit low-temperature stability and high-temperature reactivity toward oxygen in solution. The concentration of oxygen was determined from the fluorescence decay of pyrene. At low temperatures (<110 °C), the decays are short, indicating oxygen in solution. At higher temperatures (>110 °C), the decays become long and are consistent with no oxygen in solution. The change is abrupt, occurring over a narrow temperature range, and reproducible. Copyright

Full text of DOI:10.1021/ja0473891

Published on Web 08/17/2004
Low-Temperature Stability and High-Temperature Reactivity of Iron-Based
Core-Shell Nanoparticles
Christopher E. Bunker* and John J. Karnes
Air Force Research Laboratory, Propulsion Directorate, Wright-Patterson AFB, Ohio 45433
Received May 4, 2004; E-mail: christopher.bunker@wpafb.af.mil
Engineering the surface of nanoparticles to tailor physical or
chemical properties for desired effects is the focus of numerous
research efforts. Interesting examples of core-shell nanoparticles
with unique or enhanced optical, magnetic, and catalytic properties
are well documented.1-4 As the particle size decreases, the surface
area increases, offering the promise of enhanced reactivity, greater
efficiency, and potentially new and unusual active sites. A problem
arises when the active sites of the nanomaterial are unstable under
ambient conditions. Methods to protect the active material from
early or undesired reactions are required if easy, cost-effective
applications of these materials are to be realized. We have been
investigating the development of such methods using Fe0 nano-
particles as model compounds. These nanoparticles are easy to
produce by the sonochemical method5,6 and can be combined with
a number of organic materials both in situ and after-the-fact to form
core-shell nanoparticles.7,8 In some cases, the core-shell nano-
particles are resistant to oxidation, which prevents the conversion
of the Fe0 core to an oxide form (FexOy). These observations agree
well with results from other core-shell nanoparticle systems,
Figure 1. FTIR and TEM analyses of iron-based core-shell nanopar-
ticles: (a) top left, FTIR neat oleic acid; (b) bottom left, FTIR iron-oleic
acid nanoparticles; (c) top right, TEM of iron-AOT nanoparticles; (d)
bottom right, size-distribution analysis of iron-AOT nanoparticles.
specifically Co9,10 and FePt11 nanoparticles with various organic
stabilizers, prepared using high-temperature reduction or decom-
position methods. Here, we demonstrate the oxidative stability of
two Fe-based core-shell materials and, more importantly, the ability
to regain reactivity using temperature as a trigger.
quenching reaction that occurs between pyrene and oxygen in
solution. The reaction is known to follow Stern-Volmer kinetics
and can therefore be used to determine the oxygen concentration
quantitatively.
In the preparation of the iron nanoparticles, a deoxygenated
solution of Fe(CO)5 in dodecane (0.11 M) was sonicated for 15
min active time using a 1 s on, 1 s off procedure. In the absence of
the organic-shell component, the solution quickly turned dark, and
solid material was observed to settle out of solution. Once recovered,
the dark solid material (Fe0 nanoparticles) easily converted to a
reddish solid (FexOy) upon exposure to air or water. The same
reaction performed with dioctyl sulfosuccinate sodium salt (AOT)
or oleic acid present in solution at a concentration of 0.1 M
produced a dark solution that yielded no precipitate. The solutions
were stable for a time period on the order of months and showed
no visible changes upon exposure to air or water. FTIR analysis of
the core-shell nanoparticles indicated chemical bonding of the
organic component to the iron-particle surface (Figures 1a and 1b).7
TEM analysis revealed nanoparticles with an average size of ∼7
nm (Figure 1c and 1d) and an interesting ring pattern. Such patterns
have been observed in TEM images of sonochemically produced
bimetallic nanoparticles;12 however, in those cases, the darker
regions represent the cores. The particles shown in Figure 1c reveal
a darker region on the outside of the particle. The reactivity of the
Fe0 nanoparticles was investigated using the reaction of oxygen
with Fe0 to form FexOy. The reaction was monitored using the
fluorescence lifetime of pyrene as an indicator of oxygen concentra-
tion. In the absence of oxygen, pyrene has a fluorescence lifetime
of ∼450 ns; however, in the presence of oxygen, the lifetime is
much shorter (∼20 ns for an air-saturated solution of hexane).13
The shorter lifetime is due to the diffusion-controlled bimolecular-
To obtain the baseline no-oxygen analysis, a hexane solution of
pyrene was exposed to dried Fe0 nanoparticles in a sealed reaction
flask for ∼5 min with agitation. An aliquot (2.5 mL) of this solution
was then transferred to a high-temperature, high-pressure stainless
steel optical cell (7 mL volume) and sealed under N2. The
fluorescence decay of pyrene was then recorded using a home-
assembled time-resolved fluorescence spectrometer consisting of
a N2 laser (LSI VSL-337ND), narrow band-pass filters for
wavelength selection (Andover Corp., Salem, NH), and a fast-wired
R928 photomultiplier tube as the detector. The instrument response
function was ∼2 ns. All decays were single exponential and
sufficiently long to be characterized by a linear fit of the data. The
fluorescence lifetimes of pyrene in the absence of oxygen are plotted
in Figure 2 as a function of temperature. At room temperature, the
pyrene lifetime was measured to be 430 ns, which is in excellent
agreement with previously determined values.13 As the temperature
is increased, the lifetime is observed to decrease in a systematic
fashion.
To study the behavior of the core-shell nanoparticles, 2.5 mL
of a 0.1 mg/mL solution of the nanoparticles in hexane (air saturated
and containing pyrene) was transferred to the optical cell. The
headspace was purged with N2 to limit the initial oxygen concentra-
tion to that present in solution (∼2.7 × 10-3 M). At room
temperature, the lifetime of pyrene is short (∼50 ns), indicating
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C O M M U N I C A T I O N S
(Figure 2, inset). This value is somewhat smaller than previously
determined values for pyrene13 but is in good agreement with the
values obtained from studies of the temperature dependence of kisc
for other polyaromatic hydrocarbons.13-15
The results of these experiments indicate that the long lifetimes
obtained for pyrene at temperatures above ∼110 °C are indeed due
to the removal of oxygen from the optical cell. The fact that the
lifetime of pyrene remains short in the absence of the Fe
nanoparticles indicates the importance of Fe in this process. The
intriguing aspect of these results is the ability of the core-shell
nanoparticles to inhibit the oxidation of the iron core at low
temperatures and then to apparently open or release over a very
narrow temperature range, giving oxygen access to the iron core.
Such capability suggests applications with a view toward the
development of temperature-activated catalysts and reactive media.
Future efforts will be focused on understanding the activation
processsspecifically, how molecules gain access to the core
particle, whether size restrictions apply, and whether the temperature
of activation can be tunedsand the nature of the reaction between
oxygen and the Fe nanoparticles.
Figure 2. Plot of pyrene lifetime vs temperature for Fe0 nanoparticles (O),
iron-oleic acid nanoparticles (measured as a function of increasing
temperature 0 and decreasing temperature [), iron-AOT nanoparticles
(4), and for no nanoparticles (solutions with and without oleic acid, +).
Inset: Arrhenius plot of ln(kisc) vs temperature.
Acknowledgment. We thank Drs. J. R. Gord, D. K. Phelps, E.
A. Guliants, and S. W. Buckner for helpful discussions, P. Pathak
and Prof. Y.-P. Sun for TEM assistance, N. L. Sanders for
experimental assistance, and Ms. M. M. Whitaker for editorial
support. We acknowledge the continuing support of Dr. Julian
Tishkoff and the Air Force Office of Scientific Research (AFOSR)
for high-temperature fluids research.
the presence of oxygen in solution. The oxygen concentration
determined from this lifetime using the Stern-Volmer equation
and a diffusion rate constant of 1.7 × 1010 M-1s-1 for hexane was
∼1 × 10-3 M, which was lower than the initial concentration
because of partitioning between the solution and the headspace
within the cell. The lifetime of pyrene remained short for temper-
atures up to ∼110 °C, above which it increased to match the
baseline-study values exactly (Figure 2). The change in lifetime is
abrupt, occurring over a 10 °C temperature range, and reproducible.
Pyrene decays were also recorded from high to low temperature
(Figure 2). The lifetimes determined while reducing the temperature
remained long, but slightly shorter than those obtained in the
absence of oxygen, indicating for the most part an irreversible
reaction.
For comparison, pyrene decays were also obtained in air-saturated
hexane (both with and without oleic acid) without the Fe nano-
particles. Under these conditions, the lifetime of pyrene is short
and remains short throughout the temperature range investigated
(Figure 2).
For pyrene it is known that the decay of the first excited singlet
state S1* occurs through two routes, fluorescence (kF) and inter-
system crossing (kisc), and that the sum of the quantum efficiencies
of these two processes (ΦF and Φisc) is unity.13,14 Under these
conditions, the temperature dependence of the fluorescence lifetime
is attributed to intersystem crossing, and the dependence of kisc on
temperature follows Arrhenius kinetics.15-18 A plot of ln(kisc) vs
1/T (K) using all of the oxygen-free data from Figure 2 is linear,
which suggests that no other processes are competing with
intersystem crossing, and yields an energy barrier of 2.7 kcal/mol
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