temperature is sufficiently high. The results of
both our DFT-PBE and DFT-D3 calculations predict an energy barrier of about 129 kJ/mol for
this recombination step (Fig. 4). For desorption
prefactors of 1012 and 1013 s−1, we estimate that
the CH4 TPRS peak observed at 515 K corresponds
to activation energies of 130 and 140 kJ/mol,
The facile activation of CH4 on cus-Ir-O surface pairs may provide opportunities for developing catalysts and catalytic processes that can
promote efficient and selective methane function-alization. For example, certain coreactants may
directly react with CH4-derived fragments on
IrO2(110) to produce value-added compounds. It
may also be possible to modify the IrO2(110) surface to limit its oxidizing power and/or incorporate cus-Ir-O surface pairs into other materials
that promote more desirable methane chemis-tries, such as conversion to organic oxygenates
or higher hydrocarbons.
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We acknowledge financial support for this work provided by SABIC.
We also acknowledge the Ohio Supercomputing Center for
providing computational resources. All results are reported in the
main paper and supplementary materials.
Materials and Methods
Figs. S1 to S4
3 February 2017; accepted 24 March 2017
Quantitative 3D evolution of colloidal
nanoparticle oxidation in solution
Yugang Sun,1 Xiaobing Zuo,2 Subramanian K. R. S. Sankaranarayanan,3*
Sheng Peng,3 Badri Narayanan,3 Ganesh Kamath3
Real-time tracking of the three-dimensional (3D) evolution of colloidal nanoparticles in
solution is essential for understanding complex mechanisms involved in nanoparticle
growth and transformation. We used time-resolved small-angle and wide-angle x-ray
scattering simultaneously to monitor oxidation of highly uniform colloidal iron
nanoparticles, enabling the reconstruction of intermediate 3D morphologies of the
nanoparticles with a spatial resolution of ~5 angstroms. The in situ observations, combined
with large-scale reactive molecular dynamics simulations, reveal the details of the
transformation from solid metal nanoparticles to hollow metal oxide nanoshells via a
nanoscale Kirkendall process—for example, coalescence of voids as they grow and reversal
of mass diffusion direction depending on crystallinity. Our results highlight the complex
interplay between defect chemistry and defect dynamics in determining nanoparticle
transformation and formation.
Metal oxidation has been extensively stud- ied because of its broad relevance in areas uch as catalysis and corrosion, which can dramatically influence the properties and stability of materials, as well as for the
fabrication of functional metal oxides. For example,
forming iron oxide (FexOy) nanoparticles (NPs)
through the oxidation of iron NPs is an impor-
tant reaction that bears on many technological
areas, such as clean fuels (1), catalysis (2), and
electrochemical energy storage (3, 4). The Fe oxi-
dation state, geometry, crystallinity, and compo-
sition of the NPs strongly depend on the oxidation
process and play important roles in determining
their application performance. Therefore, real-
time monitoring of the oxidation reaction—in
particular, oxidation of colloidal NPs in solution—
is critical to track the detailed evolution of dif-
ferent parameters of NPs upon oxidation and
to synthesize oxide NPs that deliver appropriate
performance. In situ transmission electron micro-
scopy (TEM) has been developed to monitor
the growth (5–8), assembly (9), oxidation (10),
and ripening (11) of nanocrystals in thin liquid
TEM cells. However, it is still fundamentally dif-
ficult to study amorphous NPs in large-volume
solutions under ambient environmental and com-
plex reaction conditions and to obtain three-
dimensional (3D) structures with high spatial
resolution (12, 13). On the other hand, various in
situ x-ray techniques have been developed recently
to probe nanoparticle growth and transformation
in liquid environments in real time by taking ad-
vantage of the high penetration power of synchro-
tron x-rays (14–16).
We synthesized colloidal Fe NPs through thermal decomposition of iron pentacarbonyl [Fe(CO)5]
in 1-octadecene containing oleylamine as a stabilizer under N2 (17). Exposing the dispersion of Fe
NPs to the ambient environment oxidized surface
Fe atoms to quickly form an FexOy layer with a
thickness of 2 to 3 nm that prevented continuous
oxidation of the Fe NPs. The Fe core–FexOy shell
NPs (Fig. 1, A and B) were amorphous and highly
uniform in size and morphology (fig. S1). Further
oxidizing the NPs converted them to core-void-shell particles (Fig. 1, C and D) and shells with completely hollow interiors (Fig. 1, E and F) through
a nanoscale Kirkendall effect at elevated temperatures (100° to 180°C; Fig. 1H) (18). The initially
formed thin FexOy layer was necessary to provide
a matrix in which the diffusion of iron and oxygen could be differentiated to drive the nanoscale
Kirkendall process. The unbalanced diffusion of
iron and oxygen in the solid FexOy layer with outward iron diffusion faster than inward oxygen
diffusion led to the formation of iron oxide nanoparticles with hollow interiors. The amorphous
hollow nanoshells crystallized under incubation
at even higher temperatures. For example, FexOy
nanoshells that formed after annealing at 220°C
under N2 exhibited the crystalline lattices of Fe3O4
(Fig. 1G). The high uniformity of size and morphology was maintained during the multiple-step
oxidation process (figs. S2 to S5). Taking advantage of the high quality of the colloidal Fe NPs,
we applied the in situ small-angle x-ray scattering
(SAXS) technique—which is sensitive to NP size,
morphology, and electron density—to precisely track
the oxidation process of the colloidal Fe-FexOy
NPs in real time.
The SAXS pattern of these as-synthesized NPs
exhibits at least five well-defined Bessel oscillation
peaks and valleys (Fig. 2, bottom black curve),
confirming their spherical shape and uniform size
1Department of Chemistry, Temple University, 1901 North 13th
Street, Philadelphia, PA 19122, USA. 2X-ray Science Division,
Advanced Photon Source, Argonne National Laboratory, 9700
South Cass Avenue, Argonne, IL 60439, USA. 3Center for
Nanoscale Materials, Argonne National Laboratory, 9700 South
Cass Avenue, Argonne, IL 60439, USA.
*Corresponding author. Email: firstname.lastname@example.org (Y.S.);
email@example.com (X.Z.); firstname.lastname@example.org (S.K.R.S.S.)