initial oxidation stage (t < 1 ns), the rate of oxygen
uptake quickly increases because oxygen can easily
access the surface Fe atoms to react and form a
continuous oxide shell. At longer times, the oxygen uptake rate decreases because of void formation at the Fe/FexOy interface, which leads to a
drop in its area. These results are consistent with
the experimental measurements (Fig. 4B).
Experimentally, when the reaction temperature
increases to 180°C, at which amorphous FexOy
can crystallize, the diffusion-limited behavior observed at 100° and 140°C no longer exists (Fig. 4A
and fig. S17). The amount of oxygen uptake increases abruptly at 269 min, whereas the maximum lateral dimension of the NPs does not
undergo an apparent change, indicating that the
oxygen uptake mechanism in this short period is
different from the oxidation at 100° and 140°C.
The real-time wide-angle x-ray scattering (WAXS)
patterns show that the amorphous FexOy
nanoshells start to crystallize at 265 min, coincident
with a jump in the WAXS signal (fig. S18). The
coincident increases in oxygen uptake and WAXS
signals at 265 to 270 min indicate that converting
amorphous FexOy nanoshells to crystalline ones
enhances the oxygen uptake rate. The oxygen
uptake quickly reaches a maximum at 273 min,
whereas the size of the nanoshells shows negligible change (Fig. 4A). Such dependence on reaction time implies that increasing crystallinity
of the nanoshells prompts a fast inward oxygen
diffusion in the crystalline iron oxide. Therefore,
the diffusion direction of materials involved in
the oxidation of colloidal metal NPs can be tuned
by controlling the crystallinity of the NPs.
Once the oxygen uptake reaches the maximum,
further crystallizing the nanoshells does not increase the amount of oxygen uptake, indicating
that the oxidation process stalls during the deep
crystallization process. The maximum lateral dimension of the nanoshells slightly fluctuates as a
result of the material reorganization associated
with the crystallization process. When the reaction atmosphere switches from air to N2, annealing the colloidal FexOy nanoshells releases oxygen
to reach a stable state, at which the crystallinity of
the nanoshells ceases to change, culminating in
well-defined Fe3O4 nanocrystals (fig. S18, C and
D). The loss of oxygen might be attributed to desorption of the oxygen adsorbed on the NP surface
and crystalline grain boundaries in the environment absent of molecular oxygen. The results indicate that the ratio of oxygen to iron in the iron
oxide nanoshells can be tuned by controlling the
We have used in situ SAXS and WAXS to track,
with high fidelity (fig. S19) and a spatial resolu-
tion of ~5 Å, the full 3D geometrical evolution of
hollow interiors in the transformation of amor-
phous Fe core–FexOy shell solid NPs into crystal-
line Fe3O4 hollow nanoshells. Large-scale reactive
MD simulations corroborate the experimental
observations and elucidate the underlying at-
omistic mechanism associated with the geomet-
rical transformation. The compositional evolution
of the NPs over the course of oxidation highlights
that material diffusion and the stoichiometric
ratio of oxygen to iron can be tuned to control
doping concentrations of anions and crystalline
defects in oxide NPs. More generally, we have
shown that in situ synchrotron x-ray scattering
techniques combined with ab initio structural
modeling enable quantitative reconstruction of
the 3D geometry of colloidal NPs in reactive
solutions and offer great potential for addressing
many fundamental questions in materials science
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Fig. 4. Quantitative analysis of
oxygen uptake kinetics.
(A) Dependence of the (relative)
amount of oxygen taken up by the
Fe-FexOy NPs (black squares) and
the maximum lateral dimension of
the NPs (red circles) on the reaction
time. The programmed reaction condition is plotted to help correlate the
jumps in reaction kinetics and the
changes in reaction conditions.
(B) Plot of the rate of oxygen uptake
(black squares) and the Fe/FexOy
interfacial area (red circles) in a single
NP as a function of reaction time in
the early oxidation stage. (C) Oxygen
uptake curve obtained from reactive
MD simulations of oxidizing a 10-nm
Fe NP. The red line is the simulated
curve, whereas the dashed black line
is the logarithmic fit to the simulated
data. The inset is a cross-sectional
snapshot of the oxidized Fe-FexOy NP,
highlighting the distribution of Fe
(green) and O (red) atoms.
(D) Simulated oxygen uptake rate
(black squares) and Fe/FexOy
interfacial area (red circles) in a NP as a
function of oxidation time.