Synthesis of Fullerene-like Cs2O Nanoparticles by Concentrated
Sunlight**
By Ana Albu-Yaron, Talmon Arad, Moshe Levy, Ronit Popovitz-Biro, Reshef Tenne,
Jeffrey M. Gordon,* Daniel Feuermann, Eugene A. Katz, Martin Jansen, and Claus Mühle
While the first reported fullerenes and nanotube structures
were composed of carbon, it was soon recognized that a
plethora of comparable inorganic candidates should also exist.[1–3] Because nanoparticles of compounds with a layered
(2D) structure are unstable against folding, they should be
able to form nanotubes and closed-cage inorganic fullerenelike (IF) structures. The first such nanostructures were identified in WS2[4] and MoS2.[5] A rich assortment of IF nanostructures have been synthesized, and are finding practical uses in
tribology, photonics, batteries, and catalysis.[6]
Among inorganic molecules that can achieve fullerene-like
nanostructures, cesium oxide (Cs2O)[7] occupies a doubly
prominent status. First, it is unique among known binary-alkali compounds in possessing a layered 3R-Cs2O anti-CdCl2
structure (powder diffraction file (PDF) no. 09-0104),[8] where
3R denotes a unit cell composed of three molecular layers
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[*] Prof. J. M. Gordon, Prof. D. Feuermann, Dr. E. A. Katz
Department of Solar Energy and Environmental Physics
Jacob Blaustein Institutes for Desert Research
Ben-Gurion University of the Negev
Sede Boqer Camps 84990 (Israel)
E-mail: [email protected]
Prof. J. M. Gordon
The Pearlstone Center for Aeronautical Engineering Studies
Department of Mechanical Engineering
Ben-Gurion University of the Negev
Beersheva 84105 (Israel)
Dr. A. Albu-Yaron, Prof. M. Levy, Dr. R. Popovitz-Biro,
Prof. R. Tenne
Department of Materials and Interfaces
Weizmann Institute of Science
Rehovot 76100 (Israel)
T. Arad
Electron Microscopy Unit
Weizmann Institute of Science
Rehovot 76100 (Israel)
Dr. E. A. Katz
The Ilse Katz Center for Meso and Nanoscale Science and
Technology
Ben-Gurion University of the Negev
Beersheva 84105 (Israel)
Prof. M. Jansen, C. Mühle
Max Planck Institute for Solid State Research
Heisenbergstrasse 1, 70569 Stuttgart (Germany)
[**] This work was supported in part (Rehovot and Stuttgart) by the
GMJ Schmidt Minerva Center, the Israel Science Foundation and
the German-Israel Foundation. EAK thanks the Israel Ministry of
Immigrant Absorption and the Deichmann Foundation for financial
support.
Adv. Mater. 2006, 18, 2993–2996
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DOI: 10.1002/adma.200600983
with rhombohedral symmetry. This suggests the likelihood of
forming closed polyhedra or tubular shapes. The affiliated absence of dangling bonds should result in an intrinsically low
reactivity of fullerene-like Cs2O nanoparticles. Second, overlayers of Cs2O reduce the work function of various photonic
devices. This property renders films of such materials particularly useful for a multitude of applications in photoemissive
systems, for example, photocathodes, negative-electron-affinity devices, image intensifiers, discharge lamps, television cameras, lasers, and catalytic converters.[9,10] Unfortunately, Cs2O
is extremely reactive in the ambient atmosphere, so its production and handling require high vacuum and very pure inert
conditions. The dearth of chemical stability of Cs2O translates
into problematic and expensive manufacturing and handling,
which limits its technological scope and device lifetime. These
realities motivate the quest for relatively uncomplicated highyield syntheses for chemically stable IF-Cs2O.
Recently, the first synthesis of IF-Cs2O nanoparticles
was described, based on continuous laser ablation of pure
3R-Cs2O powder in evacuated quartz ampoules.[11] The minute quantities generated encumbered both optimization of the
procedure as well as detailed studies of material structure and
properties. The expense of laser photons further militates
against economic large-scale viability for this process. These
considerations prompted the search for an alternative practical pathway to prepare IF-Cs2O. In the present work, experimental results for the exploitation of highly concentrated solar radiation (ultrabright incoherent light) toward that end
are described. The pyrolytic solar ablation of the 3R-Cs2O
precursor, in the absence of any other reagents, yielded considerable amounts of closed-cage (IF) nanoparticles as confirmed by transmission electron microscopy (TEM) and highresolution TEM (HRTEM).
The solar-driven synthesis of IF-Cs2O was performed directly in evacuated quartz ampoules that contained 3R-Cs2O
crystallites (see Experimental), under continuous irradiation
with a concentrated solar power of 2.0–7.7 W and periods
ranging from 30 to 840 s. A schematic of the solar ablation
system[12,13] and a photograph of the irradiation platform used
in this work are presented in Figure 1a and b respectively. A
transmissive optical fiber channeled high-flux sunlight from
an outdoor minidish concentrator to the indoor lab bench.
The fiber tip was held in close contact with the ampoule’s
quartz wall. The corresponding flux values at the distal fiber
tip were 2.5–9.8 W mm2. (Flux values incident on the Cs2O
© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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undergo decomposition induced by the solar flux. In contrast,
all IF-Cs2O nanoparticles emerged from localized intense irradiation restricted to the precursor 3R-Cs2O target material
(mode I). The vital role of annealing in markedly enhancing
the production of fullerene structures has been demonstrated
in the synthesis of both carbon[14] and inorganic[15] fullerenes.
The mode I configuration appears to produce the degree of
evaporation, annealing, and temperature gradient conducive
to the formation of IF-Cs2O material and its deposition in the
colder unilluminated regions of the ampoule, provided the input solar power is ≥ 6 W. There was no sizable difference in
the quality or quantity of the IF-Cs2O nanoparticles as a function of exposure time.
Samples were collected from the deposit that accumulated
on the quartz ampoule interior during solar irradiation for
TEM analysis (Fig. 2; see Experimental). The IF-Cs2O nanoparticles appeared in groups of large, dense agglomerates
500–2000 nm in size. Closer examination of the edges of the
agglomerates (Fig. 2) reveals a variety of closed-cage nested
(a)
(b)
Figure 1. a) Schematic of our solar fiber-optic minidish concentrator: a
mirrored paraboloidal dish 20 cm in diameter, and a small flat mirror
that images the sun into the upward-facing tip of an optical fiber. The fiber guides concentrated solar radiation to an indoor laboratory where direct irradiation of the Cs2O crystallites inside an evacuated quartz ampoule was performed. Input power is moderated by an iris that preserves
the broad angular distribution of delivered sunlight (half-angle of 41.3°).
b) Photograph of an ampoule irradiated with highly concentrated solar
radiation (prior to bright flashes of visible light—see text for details).
crystallites are lower since the power density of light emitted
over a large angular range is diluted in traversing the wall of
the quartz ampoule.) No damage to the ampoules could be
perceived. Flashes of orange-red light, clearly observable
through a neutral density filter, commenced after exposure
periods of tens of seconds at input solar power ≥ 4 W, even in
those exposures that did not yield IFs. The onset time of these
emissions occurred earlier for higher input power. Furthermore, IF-Cs2O was detected only at input solar power ≥ 6 W.
Two irradiation strategies were implemented. First, the
quartz ampoules containing the 3R-Cs2O precursor were illuminated at a fixed location (i.e., a given distance from the ampoule tip). In some instances the ampoule was static, and in
others the ampoule was manually rotated about its long axis
in order to expose as much material as possible at a set position to the concentrated sunlight (mode I). In the second
method (mode II), the ampoule was additionally moved along
its long axis, the aim being to irradiate all crystallites in the
ampoule, which also resulted in irradiation of the ablated
product, i.e., of the material deposited from the gas phase during the experiment.
Irrespective of the power level, no IF-Cs2O nanoparticles
were found in deposits obtained from experiments conducted
with mode II. Most likely, the IF nanoparticles that are produced during the ablation of the primary 3R-Cs2O crystallites
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Figure 2. TEM images of closed-cage nested IF-Cs2O nanostructures
achieved by intense solar irradiation of pure crystalline Cs2O in vacuum.
a) Quasispherical, 20–30 nm in diameter, composed of 14 layers.
b) Closed polyhedra, 25 nm in diameter, comprising 12 layers. Note the
hollow core in parts (a,b). c) Smaller spherical structure, 20 nm in diameter, constituting 12 layers and exhibiting a smaller core.
Cs2O structures of linear dimension 20–40 nm, with 10 to
15 layers, which were either faceted (Fig. 2b) or quasispherical (Fig. 2a and c). Larger nanoparticles, approximately
70–100 nm in size, that possessed at least 30 layers, were also
observed.
Each fringe within the nested structure represents a molecular sheet of Cs2O and reveals its intrinsic lamellar structure.
The measured interlayer fringe spacing is 0.635–0.645 nm
(Fig. 2a–c), in agreement with the established value of
0.638 nm for (0003) in Cs2O. The slightly larger values, occasionally observed mainly in the smaller particles, are probably
due to the strain that derives from the stronger curvature of
layers in smaller particles. Both intermediate incompletely
closed nanoparticles and closed-cage multiwalled structures
were apparent. Energy dispersive X-ray analysis of the individual closed nanoparticles revealed that only cesium and
oxygen were present, with an approximate ratio of 2:1.
The IF-Cs2O nanoparticles proved stable under the electron
beam of the TEM, in contrast to the surrounding amorphous and platelet 3R-Cs2O, which were also present on the
sampled grid. In fact, TEM analysis of the bulk (3R platelet)
material was exceedingly difficult due to its instability under
© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2006, 18, 2993–2996
Figure 3. a) HRTEM image of a faceted IF-Cs2O nanostructure, analyzed
following observation in the CM120 transmission electron microscope,
which resulted in a few minutes of exposure to the ambient atmosphere
for the transfer and mounting of the grid. Note amorphization of the outer molecular Cs2O sheets starting from the kinks between the facets, advancing inwards. b) FFT of the framed region in part (a). c) Line profile
of the framed area in part (a). The layer spacing obtained from parts
(b,c) is 0.644 nm. d) EELS spectrum, which exhibits only Cs and O in a
ratio of less than 2:1.
of Figure 3a, ascertained by both fast Fourier transform (FFT,
Fig. 3b) and line profile (Fig. 3c), indicates only minor damage
to the closed IF-Cs2O nanoparticles. Nevertheless, a small degree of oxidation and amorphization of the outermost layers,
starting at defects or kinks between the facets and advancing
inward, is discernible. In contrast, the surrounding material,
which did not contain IF-Cs2O, deteriorated rapidly and evaporated under the electron beam, most likely due to its high
content of adsorbed water. This observation is consistent with
the results of the previous study,[11] where expansion of the interlayer distance, which started from sharp cusps and defects,
was ascribed to water intercalation and subsequent exfoliation
and amorphization of the nested Cs2O nanostructure.
Adv. Mater. 2006, 18, 2993–2996
Electron energy loss spectroscopy (EELS) and imaging
with a Gatan imaging filter verified that only cesium and oxygen were present in the nanoparticles (with a small excess of
oxygen, Fig. 3d). Furthermore, the plasmonic part of the
EELS spectrum (< 30 eV) of the closed nanoparticles resembled that of the bulk (3R-Cs2O) material, which alludes to
the substantial similarity between the two materials.
Although the yield was determined qualitatively from an
overall impression of the TEM images only, it was found that
the amount of IF-Cs2O nanoparticles produced by the solar
ablation was five to seven times greater than the amount produced by laser ablation.
The reported melting point of Cs2O (in an inert gas environment) is 763 K.[16,17] Under evacuation, however, Cs2O decomposes rather than melts at 763 K.[16,17] The gas phase components readily form clusters with highly varying ratios of
cesium and oxygen, the most prevalent molecular species
being Cs2O.[18] Even a faint presence of oxygen (e.g., of the order 10–7 bar, 1 bar = 100 kPa) has been shown to dramatically
enhance cluster formation,[18] and the oxygen generated in the
solar-driven decomposition of Cs2O would appear to be more
than sufficient. Gas-phase reactions among the decomposed
cesium, oxygen, and cesium oxide clusters are undoubtedly responsible for the striking visible emissions during solar ablation.
The reaction conditions and irradiation strategies explored
thus far are limited, and need to be expanded in future studies. Molecular dynamic mechanisms for IF-Cs2O synthesis
have not yet been deciphered. Probing material evolution
in situ is problematic. Experimental studies of other fullerene
structures produced photothermally by laser ablation[14,15]
have demonstrated crucial roles for a minimum radiative flux,
adequate annealing at high temperature, long residence time
in the annealing zone (of the order of seconds to minutes),
and sharp temperature gradients. These characteristics would
appear to be present in the solar ablation experiments
in creating an environment far enough from equilibrium, yet
with sufficient annealing time, to complete the formation of
the (metastable) fullerenes.
In summary, a relatively simple, inexpensive, and reproducible photothermal procedure for synthesizing IF-Cs2O nanoparticles with immensely concentrated sunlight has been demonstrated and unambiguously confirmed with TEM and
HRTEM. The yields exceed those from the laser-ablation
method[11] that, to date, represents the sole experimental realization of IF-Cs2O. These findings intimate that other inorganic fullerenes, as well as nanotubes, could be candidates for
solar-assisted nanostructure synthesis.
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the intense electron beam. The interaction of the beam with
the material leads to thermal motion, charging, desorption of
trace quantities of H2O and O2, or possible chemical changes.
Figure 3a is an HRTEM image of part of a faceted closed
IF-Cs2O nanoparticle after exposure to air for approximately
20 min (during the transfer of the grid from the TEM into the
HRTEM). The 0.644 nm interlayer spacing in the framed area
Experimental
The 3R-Cs2O precursor in the present work was synthesized by reaction of measured amounts of liquid pure Cs metal [19] and a substoichiometric amount of dry O2 and heating at 200 °C for 3 days [20].
Then, the raw product was heated at 200 °C for 3 days under dry Ar
at 1 atm (1 atm = 101.3 kPa) and subsequently ground in a stream of
© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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dry Ar. This cycle was repeated two times. Finally, the excess Cs was
removed by sublimation twice in vacuum at 250 °C. The extremely airsensitive product, with a clear red-orange color and consisting of a single phase, analyzed by X-ray powder diffraction, was subsequently
sealed in evacuated quartz ampoules. The solar-driven synthesis of
IF-Cs2O nanoparticles is described in the text.
The fullerene content of the deposits collected on the inside of the
quartz ampoules was analyzed with a transmission electron microscope, including HRTEM, under inert conditions. All the manipulations were performed in an environmental cell attached to the CompuStage entry of the microscope, operating under a flow of dry Ar,
allowing safe preparation, mounting, handling, and introduction of
the specimen grid into the microscope without exposure to the ambient atmosphere [11]. The quartz ampoule was broken inside the
chamber and small amounts of the deposits were transferred onto a
gold grid for mounting into the microscope.
A TEM-Philips CM120 (120 kV) transmission electron microscope
with an EDS system (EDAX model Phoenix microanalyzer) and a
field-emission gun HRTEM model FEI Tecnai F-30 (300 kV) were
used. A Gatan imaging filter was used for EELS and for elemental
mapping. An FFT of the high-resolution images was obtained by Digital Micrograph software (Gatan).
Received: May 5, 2006
Revised: August 29, 2006
–
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© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2006, 18, 2993–2996