Nanoscale oxygen generators: MgO2-based fillings of BN nanotubes

Article abstract of DOI:10.1021/jp030383y

Boron nitride (BN) nanotubes having only tens of nanometers in diameter and open tip-ends were uniformly filled with a MgO₂-based material. The crystal structures and chemical compositions of the tubes and filling media were analyzed using high-resolution and energy-filtered electron microscopy, electron diffraction, electron energy loss, and energy dispersion X-ray spectroscopy. A low melting point (～88°C) and easy decomposition of the filling inside the tubes under moderate heating, e.g., during in-situ electron irradiation in an electron microscope, and/or just during room temperature aging, may generate a spatially localized oxygen outflow from the nanotubes. The MgO₂-filled material is uniquely characterized by a stable oxygen release rate with long release term. Thus the first nanotube-based oxygen generator has been realized.

Full text of DOI:10.1021/jp030383y

8726
J. Phys. Chem. B 2003, 107, 8726-8729
Nanoscale Oxygen Generators: MgO2-Based Fillings of BN Nanotubes
Dmitri Golberg,* Yoshio Bando, Keita Fushimi, Masanori Mitome, Laure Bourgeois,† and
Cheng-Chun Tang
AdVanced Materials Laboratory and Nanomaterials Laboratory, National Institute for Materials Science,
Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan
ReceiVed: March 28, 2003; In Final Form: June 23, 2003
Boron nitride (BN) nanotubes having only tens of nanometers in diameter and open tip-ends were uniformly
filled with a MgO2-based material. The crystal structures and chemical compositions of the tubes and filling
media were analyzed using high-resolution and energy-filtered electron microscopy, electron diffraction, electron
energy loss, and energy dispersion X-ray spectroscopy. A low melting point (∼88 °C) and easy decomposition
of the filling inside the tubes under moderate heating, e.g., during in-situ electron irradiation in an electron
microscope, and/or just during room temperature aging, may generate a spatially localized oxygen outflow
from the nanotubes. The MgO2-filled material is uniquely characterized by a stable oxygen release rate with
long release term. Thus the first nanotube-based oxygen generator has been realized.
Utilizing nanotubes1 as ultimate ultralight gas containers has
attracted major interest with respect to practical applications.2-5
Hydrogen and nitrogen were observed to penetrate, remain
inside, and be easily released from C nanotubes.2-5 However,
the possibility of using nanotubes as localized oxygen burners
and/or generators has not yet been investigated. In addition,
gaseous oxygen is expected to quickly damage the interior of a
C nanotubular structure via fast oxidation at relatively moderate
temperatures (∼500-600 °C), making a C nanotube-based
oxygen container probably unfeasible. A boron nitride (BN)
nanotube6,7 is the counterpart of a C nanotube in which
alternating B and N atoms entirely substitute for C atoms in a
honeycomb network. The BN tubes have been found to be much
more stable to oxidation in air by revealing an excellent thermal
and chemical stability.8
Magnesium forms several oxides, e.g., MgO, MgO2, and
MgO4.9,10 The last oxide is highly unstable at atmospheric
pressure and temperatures just above ∼30 °C. A hypothetical
Mg2O phase has also been considered.10 MgO is one of the most
stable inorganic compounds with a melting point of ∼2800 °C.
By contrast, MgO2 is an unstable material exhibiting melting
at just ∼88 °C, followed by gradual decomposition, which, in
turn, generates oxygen. An ab initio quantum mechanical study
has displayed that peroxide MgO2 is energetically unstable,
having a decomposition reaction energy of -26.8 kJ/mol, and
that the presence of water and/or hydrogen peroxide may be
necessary for stabilization of the structure.11 In fact, pure MgO2
was shown to be prone to self-ignition in air.12
suggested as the best possible candidate for use in saturated
surface soil applications due to its lowest oxygen release rate
among other soil chemical amendments.16 Moreover, an oxygen
release compound based on MgO2 was found to be efficient in
the decontamination of ground soils from petroleum traces17
and restoration of normal glucose and cholesterol levels in a
human body.18
The present Letter describes the synthesis and analysis of
pure open-ended BN nanotubes, only tens of nanometers in
diameter, filled with a MgO2-based material that may easily
decompose inside the tubes and thus locally generate oxygen
when subjected to marginal heating, as for example, by an
electron beam, and/or just during room-temperature aging. Thus
the first nanoscale oxygen burner or generator has been created.
A mixture of B and MgO (with molar ratio of 1:1) was placed
in a BN crucible and heated to 1300 °C using a high-frequency
(21.5 MHz) induction furnace.19 At this temperature B reacted
with MgO to form B2O2 and Mg vapors.20 The vapors were
transported with the aid of an Ar flow into a reaction chamber
whose temperature was preliminarily set to ∼1550 °C, with
subsequent introduction of a flow of NH3. A BN material
crystallized on a Mo substrate located in the upper zone of the
chamber through a simple chemical reaction between B2O2 and
NH3. The involved chemical reactions are written as follows:19
2B(s) + 2MgO(s) f B2O2(g) + 2Mg(g)
(1)
B2O2(g) + 2NH3(g) f 2BN(s) + 2H2O(g) + H2(g) (2)
In practice, it has been observed that MgO2 films may form
on conventional MgO materials during heating.13 It has been
found that, compared to other peroxides, MgO2 can generate
O2 gas during a relatively long period of time.14,15 For example,
concrete briquets used in a permeable barrier system for the
groundwater flow and containing 21 wt % MgO2, released
oxygen at measurable rates for up to 300 days.15 MgO2 was
After complete evaporation of the reagents and subsequent
cooling to room temperature over 2 h, a bright-grey product
was obtained at the Mo substrate.
The resultant product was analyzed using high-resolution
transmission electron microscopes JEOL-3000F and JEOL-
3100FEF, both operated at 300 kV. The former microscope was
equipped with a Gatan 766 electron energy loss (EELS)
spectrometer and an X-ray energy dispersion (EDS) detector,
whereas the latter microscope was equipped with an energy filter
(Omega-type), making possible elemental mappings along with
* To whom correspondence should be addressed. E-mail: golberg.
dmitri@nims.go.jp.
† Permanent address: School of Physics and Materials Engineering, P.O.
Box 69M, Monash University, Victoria 3800, Australia.
10.1021/jp030383y CCC: $25.00 © 2003 American Chemical Society
Published on Web 08/02/2003

Letters
J. Phys. Chem. B, Vol. 107, No. 34, 2003 8727
Figure 1. Representative transmission electron microscope (TEM)
images of the filled nanotubes (a-d) found in the resultant synthesis
product. Low-magnification TEM image (a) and an image clearly
displaying a filled tube/empty tube boundary (b). Electron beam
irradiation-induced changes within the filling [(c) and (d)], beam
intensity ∼60 pA/cm2: initial irradiation, e.g., 1-2 min (c); longer
irradiation, e.g., 3-5 min (d).
Figure 2. Results of filled nanotube chemical characterization. TEM
image of a half-filled nanotube with two encircled domains; in black
(hollow tube) and in red (filled tube) (a). The inset highlights a typically
open-tube tip-end. (b) Comparative EEL spectra recorded from a hollow
tube part (black line) and a filled tube part (red line); the inset depicts
an enlarged portion of a O core-loss K-edge located at 532 eV. Position
of the B (188 eV) and N (401 eV) core-loss K-edges and a calculated
B/N atomic ratio are also shown. (c), (d) Comparative ED spectra taken
from a hollow tube part [peaks marked in black in (c)] and a filled
tube part [peaks marked in red in (d)]. A calculated atomic O/Mg ratio
of ∼2.0, consistent with a MgO2 formula, is marked in the lower ED
spectrum. A Cu signal in the upper ED spectrum in (c) originates from
the supporting Cu TEM grid, whereas all other reflections come from
either exclusively BN shielding layers (c) or superimposed from a BN
tubular shield and a MgO2-based core filling (d).
structural and electron diffraction studies. A three window
procedure was utilized for elemental map recording (two preedge
and one postedge energy-filtered images were taken). The
exposure time was set at 10, 12, 15, 18, and 20 s for B, C, N,
O, and Mg map acquisitions, respectively. The microscope
magnification was adjusted to 15 × 104 during mapping. EEL
spectra quantification was performed using hydrogenic cross-
sections. Absolute edge energies were calibrated using the
graphitic π* B peak at 188 eV.
Figure 1a-d depicts representative filled nanotubes found
in the product. Nanotubes were approximately 20-100 nm in
diameter and several micrometers in length. They displayed
solely open tip-ends. The yield of filled nanotubes, as compared
to that of unfilled ones, was estimated as ∼50 vol %. It is
striking that the present utilization of a refractory metal template
(Mo) for the nanotube crystallization and increase in the furnace
temperature from 1300 °C (used in the previous synthesis of
unfilled pure BN nanotubes using the similar experimental
setup)19 to 1550 °C was found to be crucial for obtaining filled
tubes, albeit the reason behind this phenomenon has not been
well-understood.
During an initial microscopic study, we immediately noted
that the filling material was easily melted and evaporated from
inside the tubes during conventional TEM operations. Parts c
and d of Figure 1 display two consecutive HRTEM images of
a filled tube subjected to irradiation with a ∼60 pA/cm2 electron
beam current density over several minutes. Evaporation and
fragmentation of the filling is obvious in both figures.
The chemical composition of the nanotubes and their fillings
was analyzed using EELS and EDS. The data are respectively
shown in Figure 2a-c. Only the B and N K-edges are detected
in the EEL spectrum, Figure 2b (black), taken on the hollow
nanotube part encircled in Figure 2a. Quantification of the EEL
spectrum gives a B/N ratio of ∼1.08 ( 0.15 (the error arises
due to insurmountable uncertainty in background subtraction).
Thus the tube is made of pure BN tubular shells. By contrast,
an EEL spectrum, Figure 2b (red), taken on a filled tube part,
also encircled in Figure 2a, while still exhibiting analogous B
and N edges, in addition, displays a notable O peak, obviously
originating from the tube filling. Quantification of the “red”
EEL spectrum, Figure 2b, again gives a B/N ratio of ∼1.09 (
0.15, but an O/B ratio of 0.46 ( 0.07. This implies (i) the B
and N signals solely originate from the shielding BN tubular
walls rather than being overlapped from the tube and the filling
and (ii) O is present in the filling. An ED spectrum, Figure 2c,
taken on the unfilled nanotube part reveals only B, N, and Cu
signals. The Cu one originates from the supporting TEM grid.
No other notable peaks are seen in the spectrum. However,
strong Mg and O X-ray peaks, along with the familiar B and N
peaks (originating from the shielding stoichiometric BN tubular
layers), appeared in the ED spectrum, Figure 2d, taken from
the filled nanotube part using a ∼5.7 pA/cm2 electron beam
current density over a limited exposure time (3 s). Quantification
of this ED spectrum gives a O/Mg ratio of ∼2.0, i.e., the MgO2
formula. Thus, generally, the chemical composition of the filling
might be suggested to be close to a stoichiometric MgO2. We
would not fully rule out the presence of doping species, i.e., B,
C, and N, readily available during the high-temperature syn-
thesis, in the filled MgO2 phase at a level not detectable by our
analysis means. It is also noted that ∼0-12 at. % of a Si
impurity, originating from the quartz tube and crucible used,

8728 J. Phys. Chem. B, Vol. 107, No. 34, 2003
Letters
Figure 3. Transmission electron microscopy (TEM) images (a-d) and energy-filtered elemental maps (e-h) of a representative MgO2-based-
filled BN nanotube. Electron beam irradiation heating with a beam intensity of ∼18 pA/cm2 over 10 min was performed from (a) to (c). A fragment
of the filling close to a drilled hole (depicted with an arrow) in (c) is enlarged in (d) to clearly display the nature of the numerous dark-contrast
areas appearing under heating. This was assigned to precipitates of a stable Mg-containing phase (e.g., Mg-based solid solution), marked with the
arrows in (d). The B (e), N (f), Mg (g), and O (h) elemental maps were constructed at some intermediate irradiation stage well before the irradiation-
induced hole appearance in (c). Each element composition profile across the filled tube (along the cross-sectional lines marked in white) is attached
to the corresponding elemental map in (e-h) as an inset.
was sometimes observed within the MgO2-based fillings during
numerous EDS runs undertaken. We also suggest that the H2O
and H2 molecules forming via the reaction 2 may stabilize a
MgO2-based lattice during cooling, in line with the theoretical
and experimental facts available in the literature.11,12
magnified in Figure 3d, where clear atomic fringes (or stacking
faults) assigned to appearing second phase domains (most likely
Mg-based solid solution in the present case) become visible.
Such domains look relatively dark on a highly inhomogeneous
spatially resolved O map, Figure 3h, in accord with the lack of
oxygen in them, as compared to the O-rich MgO2-based matrix.
We also note the marked brightness variations of the O map
along the filling in Figure 3h. Significant variations in the O
content in irradiated MgO2-based fillings are also effectively
highlighted through taking O composition profiles across the
filling (Figure 3h, inset). Conversely, the fully correlating B
and N maps, and anti-correlating Mg map peculiar to the tube
walls and the filled matter, respectively, are rather uniform,
Figure 3e-g. Again no significant amounts of B and N are seen
in the filling (the B and N maps are not sufficiently bright in
the central part; rather they show the highest brightness in the
tube walls); the results are in a good agreement with those
obtained with the EELS and EDS techniques. Therefore all the
experimental facts may be reasonably well interpreted in the
sense that oxygen may be released from the MgO2-based filling
upon moderate heating; thus an O-poor Mg-containing phase,
e.g., a stable MgO or Mg-based solid solution, should appear
as a result of the oxygen outflow. It is also natural to suggest
that the escape of O primarily takes place through the open
tube ends, Figure 2a (inset).
No detailed diagram for the Mg-O system has been
published. The schematically assessed diagram is available for
the range 0-50 at. % O only.10 At low temperatures Mg at its
O-rich limit is considered to be in equilibrium with MgO2, which
is stable in a range of O2 fugacities greater than those at which
MgO is stable.10 Most likely, the present MgO2-based fillings
are formed during cooling Mg- and/or MgO-based-filled BN
nanotubes (frequently observed by TEM and EDS in the
specimens) to room temperature in an oxygen-containing
environment with a high partial oxygen pressure (probably
experienced by some Mg-containing nanotube cores). For
instance, Mg may be oxidized to MgO2 below ∼230 °C (∆H°f298
) 623 kJ/mol).9
Figure 3a-h depicts a sequence of MgO2-based-filled open-
ended BN tube transformations under in situ electron irradiation
with a beam current density ∼18 pA/cm2 and representative B,
N, Mg, and O elemental maps taken during an intermediate
irradiation stage. Electron irradiation may lead to moderate tube
heating to an estimated temperature of 100-150 °C. First, we
note that some dark-contrast precipitates appear during the initial
heating stage, Figure 3b (some of them are barely visible in
Figure 1a, albeit with bright-gray contrast, likely due to
pretransformed domains; in fact, low dose irradiation is needed
for an original HRTEM photo to be taken). Importantly, such
dark-contrast regions were not peculiar to fresh, nonirradiated
filling areas. Finally, a focused electron probe drills a hole in
the filling, Figure 3c, due to complete melting and evaporation
of the filled matter. More intense further irradiation heating leads
to fragmentation of the filling and appearance of numerous voids
in it, e.g., Figure 1c,d. The dark-contrast precipitate area is
Figure 4a-d shows the results of a detailed study of a
regarded oxygen release from the MgO2-based fillings. Parts a
and b of Figure 4 compare two representative ED spectra (out
of ∼30 studied) taken on the as-prepared specimen composed
of MgO2-based filled BN nanotubes and the same specimen
aged at room temperature over 2 months. The oxygen content
dramatically drops from O/Mg ∼ 2.0 (Figure 4a) to O/Mg ∼
1.0 (Figure 4b) atomic ratios. The latter composition matches a
stable stoichiometric MgO oxide. Oxygen outflow was also
observed on an individual BN-shielded MgO2-based filling

Letters
J. Phys. Chem. B, Vol. 107, No. 34, 2003 8729
decompose, leaving stable Mg-containing phases inside the
tubes, while the oxygen may be locally generated from the
nanotubes. Thus the first nanoscale oxygen burner/generator has
become available.
Finally, let us suggest several intriguing practical applications
of the observed phenomenon. Localized (several tens of
nanometer pipe cross-section) oxygen outflow may selectively
and effectively produce nanosize metallic domain oxidation.
This may be applied in quantum dots preparation or local
insulation of parts of a given conducting network composed of
inorganic conducting nanowires. Study on various biological
cell, bacteria, and/or individual molecule interaction with a point
(nanometer size) oxygen source may shed light on their living
and deteriorating conditions. This may be of high value in
biological and medical science.
Acknowledgment. The present work was carried out in line
with the “Nanoscale Materials” Project tenable at the National
Institute for Materials Science, Tsukuba, Japan. We are grateful
to Drs. R. Ma, F. Xu, Y. Li, and E. Kopnin for valuable
discussions.
Figure 4. (a), (b) Representative comparative ED spectra consecutively
taken on an as-prepared BN-shielded MgO2-based-filled nanotube
specimen and the same specimen aged over 2 months at room
temperature, respectively. The O-peaks of interest are marked in black
in the spectra. Note that the latter filling displays stable MgO
composition. Cu peaks in the spectra solely originate from the TEM
grid used. (c) A decay in the O-content in a given oxygen-rich filling
(i.e., unstable MgO2, displayed in the inset) as a function of the
irradiation time with a ∼5.7 pA/cm2 current density; the O/Mg ratio
quickly drops to ∼1.0 (i.e., stable MgO) just within several tens of
seconds and, then, does not notably change. (d) High-resolution TEM
image of a filled BN nanotube aged at room temperature and viewed
along the [001] zone axis of a fcc lattice peculiar to a filling matter. A
measured lattice parameter of ∼4.2 Å perfectly matches that of MgO.
A 3.33 Å fringe separation natural for the (0002) shielding BN tubular
layers is also shown.
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during its electron beam heating, Figure 4c. A decay in oxygen
content quickly proceeds just within several tens of seconds,
again from O/Mg ∼ 2.0 to O/Mg ∼ 1.0 atomic ratios, and then,
does not notably change. Finally, the sol presence of stable
MgO-based fillings, rather than unstable MgO2-based fillings
in the specimens aged at room temperature was confirmed
through high-resolution and electron diffraction work, Figure
4d. MgO is known to possess a face-centered-cubic (fcc) NaCl-
type lattice with a lattice parameter of 4.2 Å.21 A HRTEM image
of the fragment of an aged BN tube-shielded filling is shown
in Figure 4d. The image may be easily indexed as the NaCl
lattice viewed along the [100] zone axis and exhibiting the (020)
and (002) lattice fringes consistent with a 4.2 Å lattice parameter
of the fcc structure.
To sum up, we were able to create MgO2-based fillings inside
chemically and thermally stable BN nanotubes. Under moderate
heating and/or room temperature aging, the fillings easily