1. KRISTA S. WALTON
M
aterials called metal–organic frame-
works (MOFs) have sparked intense
interest over the past few decades.
In particular, those that form permanently
porous architectures have tremendous poten-
tial for applications such as chemical sensing,
gas storage and catalysis. But techniques for
synthesizing these compounds are still often
developed through trial and error — in part
because the mechanisms that dictate the
self-assembly of MOF unit cells from their
constituent metal ions and ligands, and their
subsequent growth into nanoparticles, are
largely unknown and difficult to observe.
Writing in the Journal of the American Chemi-
cal Society, Patterson et al.1
help to solve this
problem by reporting the first observations of
the crystallization of MOFs made in real time,
using a technique called liquid-cell transmis-
sion electron microscopy.
Previous studies of MOF crystallization
have made use of various ex situ and in situ
analytical techniques, including high-
resolution transmission electron microscopy
(HRTEM) and energy-dispersive X-ray dif-
fraction (EDXRD). For example, HRTEM has
been used to examine the crystallization of
the well-characterized MOF-5, by analysing
samples taken at various time intervals early
in the compound’s synthesis2
. Time-resolved
in situ EDXRD has been used to determine the
kinetics of MOF crystallization as a function of
parameters that included pH, temperature and
ligand length3,4
. But the challenge of observing
Although Su and colleagues have made the
groundbreaking demonstration that high-
starch, low-methane rice plants can be gener-
ated, their study raises many issues. The most
obvious is that SUSIBA2 rice is a transgenic
plant, and thus raises biological and ethics
concerns. In addition to the general questions
surrounding the use of genetically modified
cropsforhumanconsumption,andhowaccess
to seed for such crops is controlled, we do not
yet have a clear picture of how this modifica-
tion affects rice plants’ survival and general
function.
Long-term and frequent measurements of
methane emissions from areas planted with
normal and transgenic rice are needed to
estimate what the annual global effect of the
widespread use of this crop would be, and how
it compares with that of other methane-miti-
gation strategies. Even more important will be
assessment of the long-term consequences of
lower carbon and oxygen input by the roots
of SUSIBA2 plants on soil processes and the
microbes that carry them out (Fig. 1). It has
recently been shown6
that highly specific
assemblages of microbial species occur in, on
and around rice roots, and that not all mem-
bers use plant-exuded carbon7
. Long-term
reduction of root-exuded carbon might alter
the composition of these communities, with
unknown consequences for microbes that are
plant pathogens or that benefit the plants, such
as the bacteria that decompose organic mate-
rial and deliver essential plant nutrients8
.
To compensate for the possible reduction
in plant nutrients, larger amounts of nitrogen
fertilizer would need to be applied. This can
affect both methane producers and consum-
ers9
and lead to undesirable environmental
effects, such as nitrate leaching to ground­
water and emission of the potent greenhouse
gas nitrous oxide. Also crucial for the amount
of methane emitted is the activity of methane-
consuming aerobic bacteria. The oxygen they
useflowsthoughtheplantstemsandrootsinto
the soil by the same route taken by methane
movingoutofthesoilintotheatmosphere,and
it is not known how the transport of gases is
affected in the transgenic rice.
Thus, translocating more carbon to the
stems and seeds of SUSIBA2 rice may bypass
methane cycling, but this activity has the
potential to affect a multitude of processes
involving soil carbon, nutrients and microbial
activity, with knock-on effects for the sustain-
ability of rice cultivation. However, Su and
colleagues have achieved the feat of making
high-starch rice available, and this will spur
scientists worldwide to conduct experiments
to verify whether this variety will enable more-
sustainable cultivation of the crop that feeds
half the human population. ■
Paul L. E. Bodelier is in the Department
of Microbial Ecology, Netherlands Institute
of Ecology (NIOO-KNAW), 6708 PB
INORGANIC CHEMISTRY
Movies of a growth
mechanism
A microscopy technique has been used to study the formation and growth of
crystals of porous solids known as metal–organic frameworks in real time.
The findings will aid the design of methods for making these useful compounds.
a b
Metal ion
Ligand
Wageningen, the Netherlands.
e-mail: p.bodelier@nioo.knaw.nl
1. IPCC Climate Change 2013: The Physical Science
Basis (eds Stocker, T. F. et al.) (Cambridge Univ.
Press, 2013).
2. Montzka, S. A., Dlugokencky, E. J. & Butler, J. H.
Nature 476, 43–50 (2011).
3. Su, J. et al. Nature 523, 602–606 (2015).
4. Hussain, S. et al. Environ. Sci. Pollut. Res. Int. 22,
3342–3360 (2015).
5. Denier van der Gon, H. A. C. et al. Proc. Natl Acad.
Figure 1 | Growth of a metal–organic framework. Patterson et al.1
have used liquid-cell transmission
electron microscopy to study the growth of the ZIF-8 metal–organic framework (MOF) from its
constituent metal ions and ligand molecules. They find evidence for a two-step process. a, First, the metal
ions and ligands diffuse towards a nascent ZIF-8 crystal. b, Second, the ions and ligands move to an edge
site, where they coordinate with each other, becoming part of the MOF lattice. This is the rate-limiting
step of the process. (Figure adapted from ref. 1.)
Sci. USA 99, 12021–12024 (2002).
6. Edwards, J. et al. Proc. Natl Acad. Sci. USA 112,
E911–E920 (2015).
7. Hernández, M., Dumont, M. G., Yuan, Q. & Conrad,
R. Appl. Environ. Microbiol. 81, 2244–2253
(2015).
8. Philippot, L., Raaijmakers, J. M., Lemanceau, P. &
Van der Putten, W. H. Nature Rev. Microbiol. 11,
789–799 (2013).
9. Bodelier, P. L. E. & Steenbergh, A. K. Curr. Opin. Env.
Sustain. 9–10, 26–36 (2014).
This article was published online on 22 July 2015.
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2. the crystallization process in real time persists.
Patterson and colleagues’ use of liquid-cell
transmission electron microscopy (LCTEM)
is a big step forward. This technique allows
dynamic processes that occur in liquids to be
imaged as they happen. It has been used to
observe systems such as biological structures5
and growing nanocrystals6
, but had not previ-
ously been applied to MOF syntheses.
Because analytical samples can be damaged
by the electron beam used in LCTEM, the
authors began by performing a series of con-
trol experiments using a zirconium-based
MOF called UiO-66, to decouple the effects
of beam irradiation on MOF synthesis from
the effects of the reaction mechanism. UiO-66
was a good choice for a control because it is
easy to synthesize and extremely stable, which
meant that it could be prepared ahead of the
LCTEM experiments without any risk of it
degrading before use. The authors observed
that the dissolution or growth of UiO-66 par-
ticles depends on the voltage of the electron
beam. A threshold dosage of 40,000 electrons
per square nanometre was also established —
as long as experiments were performed below
this limit, damage and particle motion during
crystallization were negligible.
Patterson et al. then chose another MOF,
ZIF-8, as the ideal candidate for demonstrat-
ing the LCTEM method. The growth mecha-
nisms of ZIF-8 have previously been studied7
using transmission electron microscopy on
samples removed from synthetic solutions of
the MOF, which provided a good comparison
with the LCTEM results. ZIF-8 can be synthe-
sized at room temperature in methanol, using
zinc nitrate as the metal source and 2-methyl­
imidazole molecules as the organic ligands.
The authors observed the growth of ZIF-8 in
real time over 11 minutes — the first particles
detected were 15 nm in diameter, with subse-
quent growth observed up to 50 nm.
The authors went on to record videos of the
particleformationandusedthemtodetermine
the growth kinetics of the MOF, uncovering
several important features of the crystalliza-
tion process. First, they proved by direct obser-
vation that ZIF-8 particles form through the
growth of smaller subunits, rather than by
particles coalescing. They also found that an
excess of ligand molecules leads to the forma-
tion of ZIF-8 particles that are smaller than
those formed when the metal-to-ligand ratio
is 1:1. The researchers had predicted this using
ex situ methods, but LCTEM enabled them to
observe the process as it occurred.
A series of careful growth experiments was
then performed under various accumulative
electron doses. The results convincingly show
that LCTEM can be applied effectively to study
nanoparticles that are easily damaged by elec-
tron beams. It remains to be seen whether the
technique will be effective at the higher tem-
peratures (typically greater than 100 °C) at
which most MOFs form.
A general conclusion from this work is that
MOF growth occurs through the transport of
metals and ligands to a nascent particle, fol-
lowed by their movement to an edge or surface
site, where bonding between the metal and
ligand finally occurs (Fig. 1). The attachment
of metal–ligand monomers to a surface site
is therefore the controlling factor in particle
growth,andtheprocessisnotdiffusion-limited.
The development of this ability to watch
particle formation in situ during MOF self-
assembly should enable a variety of compli-
cated synthetic questions to be answered. One
example is how the addition of ‘modulator’
compounds, which are sometimes used in
MOF syntheses to control the crystallinity
of the products, affects the growth kinetics.
For MOFs that can adopt different structures,
LCTEM could also shed light on what dictates
whether kinetic products — those that crys-
tallize most quickly — form during reactions,
ratherthanthemostthermodynamicallystable
products. LCTEM is a much-needed addition
to the MOF-characterization toolkit, and its
use in conjunction with other methods will
no doubt lead to the specific control of crystal
morphology, compositions and defects. ■
Krista S. Walton is at the School of Chemical
and Biomolecular Engineering, Georgia
Institute of Technology, Atlanta,
Georgia 30332, USA.
e-mail: krista.walton@chbe.gatech.edu
1. Patterson, J. P. et al. J. Am. Chem. Soc. 137,
7322–7328 (2015).
2. Zheng, C., Greer, H. F., Chianga, C.-Y. & Zhou, W.
CrystEngComm 16, 1064–1070 (2014).
3. Ragon, F., Chevreau, H., Devic, T., Serre, C. &
Horcajada, P. Chemistry 21, 7135–7143 (2015).
4. Ahnfeldt, T. et al. Chemistry 17, 6462–6468
(2011).
5. de Jonge, N., Peckys, D. B., Kremers, G. J. &
Piston, D. W. Proc. Natl Acad. Sci. USA 106,
2159–2164 (2009).
6. Liao, H.-G., Niu, K. & Zheng, H. Chem. Commun. 49,
11720–11727 (2013).
7. Venna, S. R., Jasinski, J. B. & Carreon, M. A. J. Am.
Chem. Soc. 132, 18030−18033 (2010).
MATERIALS SCIENCE
Composite for energy
storage takes the heat
A polymer-based material has been discovered that breaks the rules — it has
the right combination of properties for use in energy-storage devices called
dielectric capacitors, and can function at high temperatures. See Letter p.576
HARRY J. PLOEHN
D
evices known as dielectric capacitors
have a crucial role in applications that
require short, intense power pulses or
the conversion of direct current to alternating
current. These applications include electronic
systems for the integration of energy from
renewable sources into power grids1
, trans-
port2
and military weapon systems3
. They
depend on electrically insulating materials
known as dielectrics, which come in several
types. Polymeric dielectrics offer advantages
forlargecapacitors,butsufferfromlowoperat-
ing temperatures (usually well below 150 °C)
and low energy density (which means that
devices that use polymeric dielectrics occupy
large volumes). On page 576 of this issue, Li
et al.4
report that a composite of a polymer and
nanometre-scale sheets of boron nitride pro-
vides more than a 40% improvement in energy
density compared with the best-available poly-
mer dielectric, as well as remarkable stability at
temperatures up to 300 °C across a wide range
of electric-field frequencies.
Dielectric capacitors achieve the highest
rate of energy transfer (termed the power or
rate capability) of all capacitor types. They
store energy through a variety of molecular
and nanoscale electron-polarization mecha-
nisms5,6
that create oriented dipoles and
associated dipolar electric fields. For high
energy density, dielectric materials must
have a high density of dipoles that have large
induced dipole moments (which provide a
measure of a charged system’s polarity). A
dielectric’s rate capability depends on how fast
charges polarize and depolarize — how
fast the dipoles reorient — as an applied elec-
tric field varies. Invariably, not all of the energy
stored in dipolar electric fields is recovered
on depolarization; some is transferred into
molecular translation and vibration (thermal
energy) and is lost as heat, a process called
dielectric loss.
When and how polarized electrons begin to
‘leak’ (conduct) through a dielectric depends
on a property called the dielectric breakdown
field strength (Eb). Relatively small leakage
currents may occur at field strengths below Eb.
Once the field reaches Eb, it promotes a cas-
cadeofelectronsintothematerial’sconduction
band, resulting in catastrophic breakdown as
the dielectric is transformed from an insulator
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