Physicochemical and functional peculiarities of metal oxide whiskers

Article abstract of DOI:10.1007/s11172-008-0133-5

Practical aspects of preparation and prospects for practical use of a series of the metal oxide whiskers were studied. The procedures for the synthesis were proposed, and the phase composition, micromorphology, and electrochemical and sensor characteristics of the macroscopic (up to 5-10 mm long) whiskers in the Ba-V-O, Ba-Mn-O, and Sn-O systems were analyzed. The electroconducting BaV₈O_21-δ whiskers were prepared by the hydrothermal treatment. These whiskers possess stable electrochemical characteristics appropriate for the development of novel secondary current sources. The protonated form of the Ba₆Mn₂₄O₄₈ whiskers produced by the isothermal vaporization of chloride fluxes is a mixed conductor with the proton and electron conductivity at a level of mS units at 25 °C. A new procedure by the thermal disproportionation of tin(ii) oxide under nitrogen was proposed for the growth of SnO₂ whiskers of various morphology. The produced whiskers have substantial sensor sensitivity toward a series of toxic components of the gaseous medium, such as nitrogen dioxide.

Full text of DOI:10.1007/s11172-008-0133-5

1042
Russian Chemical Bulletin, International Edition, Vol. 57, No. 5, pp. 1042—1053, May, 2008
Full Articles
Physicochemical and functional peculiarities of metal oxide whiskers
a,b
c
a
a
a
a
E. A. Goodilin,
E. A. Pomerantseva, D. A. Semenenko, P. B. Kocherginskaya, D. M. Itkis,
c
d
d
b
T. L. Kulova, A. M. Skundin, L. S. Leonova, Yu. A. Dobrovol´skii, M. N. Rumyantseva,
b
a
b
a,b
A. M. Gas´kov, S. B. Balakhonov, B. R. Churagulov, and Yu. D. Tretyakov
aMaterials Science Department, M. V. Lomonosov Moscow State University,
1
Leninskie Gory, 119992 Moscow, Russian Federation.
Fax: +7 (495) 939 0998. Eꢀmail: goodilin@inorg.chem.msu.ru
Chemical Department, M. V. Lomonosov Moscow State University,
b
1
Leninskie Gory, 119992 Moscow, Russian Federation.
Fax: +7 (495) 939 0998. Eꢀmail: goodilin@inorg.chem.msu.ru
A. N. Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences,
c
3
1 Leninsky prosp., 119991 Москва, Moscow, Russian Federation.
Fax: +7 (495) 952 5308. Eꢀmail: askundin@mail.ru
d
Institute of Problems of Chemical Physics, Russian Academy of Sciences,
1
prosp. Akad. Semenova, 142432 Chernogolovka, Moscow Region, Russian Federation.
Fax: +7 (496) 515 5420. Eꢀmail: dobr@icp.ac.ru
Practical aspects of preparation and prospects for practical use of a series of the metal oxide
whiskers were studied. The procedures for the synthesis were proposed, and the phase compoꢀ
sition, micromorphology, and electrochemical and sensor characteristics of the macroscopic
(
up to 5—10 mm long) whiskers in the Ba—V—O, Ba—Mn—O, and Sn—O systems were
analyzed. The electroconducting BaV O whiskers were prepared by the hydrothermal treatꢀ
8
21ꢀδ
ment. These whiskers possess stable electrochemical characteristics appropriate for the develꢀ
opment of novel secondary current sources. The protonated form of the Ba Mn O whiskers
6
24 48
produced by the isothermal vaporization of chloride fluxes is a mixed conductor with the
proton and electron conductivity at a level of mS units at 25 °C. A new procedure by the
thermal disproportionation of tin(II) oxide under nitrogen was proposed for the growth of SnO2
whiskers of various morphology. The produced whiskers have substantial sensor sensitivity
toward a series of toxic components of the gaseous medium, such as nitrogen dioxide.
Key words: whiskers, transition metal oxides, cathodic materials, sensors.
Achievements of the modern inorganic chemistry form
reliable basis for the creation of unique materials and
their further practical use. In therecent time, the developꢀ
ment of new procedures for the preparation of oneꢀdiꢀ
mensional systems, namely, nanotubulenes, nanoribbons,
and whiskers, becomes very significant. For example,
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 1023—1034, May, 2008.
066ꢀ5285/08/5705ꢀ1042 © 2008 Springer Science+Business Media, Inc.
1

Peculiarities of metal oxide whiskers
Russ.Chem.Bull., Int.Ed., Vol. 57, No. 5, May, 2008
1043
vanadium oxides and their derivatives attract attention of
researchers due to their special structure and properties.
Among these systems, great attention is given to nanoꢀ
were obtained. They were separated from the mother liquor,
dried in air, and used for electrochemical measurements.
The chloride flux KCl (Tm ≈ 770 °C) containing a specified
quantity of the oxide blend (up to 50 wt.%) was used for the
growth of whiskers in a Ba—Mn—O system. The mixture was
structured materials, including “nanowires,” “nanoribbons,”
,2
“
nanorods,” and “nanowhiskers,” 1 which are of doubtꢀ
heated to 800—1000 °С with a rate of 5 °С h–1 (р(О ) = 0.21 atm)
2
less practical interest for both fundamental research and
potential use as sensors, cathodic materials, and others.3
The crystal structure of the Ba Mn O phase has
in the regime of isothermal vaporization of the solvent, stored
for 2—10 days, and tempered in air. In order to prepare the
Hꢀform of whiskers, the whiskers were treated with 0.01, 0.1, 1,
10 M, and concentrated nitric acid for 1—10 days at room
temperature and for 5—7 h at 65—75 °C with vigorous stirring
on a magnetic stirrer containing a glassꢀceramic heater. Further
the whiskers were precipitated on a centrifuge, multiply washed
at room temperature with distilled water until neutral pH was
achieved, and dried in a drying oven at ~50 °C. The final product
was the Hꢀform of the considered phases of tunneling manganites.
,4
6
24 48
5
been found several years ago. This structure represents a
framework constructed due to the accretion of tunnels of
different shape and size in which positively charged ions
can be intercalated. Special interest to this compound is
caused by a possibility of synthesizing this phase as long,
6
flexible, and long crystalline fibers. The procedures for
the efficient insertion of protons and lithium into the
The SnO whiskers grew in a nitrogen flow containing the
2
whiskers and powdered samples of the Ba Mn O phase
6
24 48
products of the thermal decomposition of SnO. To prepare SnO,
have recently been proposed.7
—9
commercial SnCl •2 H O was dissolved in a minimum amount
2
2
Tin dioxide is one of the promising broadꢀband semiꢀ
conductors with unique electric and optical characterisꢀ
of hot HCl, and a saturated solution of Na CO was added to
2 3
pH > 7. The formed white precipitate of tin(II) oxohydrate was
heated for 2—3 h under the mother liquor (at 110 °С) for the
quantitative transformation of the oxohydrate into blue—black
tin(II) oxide with metallic luster. The resulting product was
thoroughly washed with distilled water and dried at 110 °С. An
alundum boat with the SnO powder was placed in the hot zone
of a tubular furnace (Nabertherm), which was purged with a
tics, whose studies remained prospective for several
decades.10 Thin films of alloyed tin dioxide are widely
1
1
used in transparent conducting electrodes and solar batꢀ
teries.12 The sensors, whose sensitive elements are based
on SnO , can find wide use as threshold sensors that react
2
to the presence in atmosphere of toxic for human organꢀ
ism or dangerous gases, including CO, NO, NO , H ,
–1
nitrogen flow (100 mL min ). The temperature regime was
experimentally optimized and consisted of two stages of
isothermal storage: for 1 h at 350 °С and for 1 h at 1050 °C.
Metallic plates of Sn and Pt were used in experiments. The
plates were placed in a cooler zone, and the products precipitated
on the boat walls and on the surface of the metallic plates.
The microstructure was studied and the chemical composition
of the samples was monitored on a LEO SUPRA 50VP scanning
electron microscope equipped with an autoemission source
and an Xꢀray spectrometer for microanalysis (Inca, Oxford
2
2
and others.13 It is shown in several works that an increase
in the ratio of the surface area to volume for the oneꢀ
dimensional SnO2 structures results in an appreciable
change in the sensor sensitivity.14 In the recent time, more
studies are directed to investigation of various oneꢀdiꢀ
mensional objects: nanoꢀ and microrods and whiskers of
tin dioxide. The SnO whiskers were prepared by sintering
2
1
5
16
with NaCl, by the hydrothermal method, thermal
decomposition,17 fast oxidation method, and thermal
vaporization in vacuo or at temperatures higher than
Instruments) at an accelerating voltage of 5—25 kV. Secondary
.
18
electron images were obtained with magnification up to 200 000.
The data of optical microscopy were obtained on a Nickon
Eclipse E600 POL metallographic microscope. A STOE
diffractometer (CuKα1 radiation (1.54183 Å), transmission
geometry, increment by 2θ 0.01—0.03°, 2θ range 5—70°) and a
Rigaku D/Maxꢀ2500 diffractometer with a rotating anode
1
300 °С (see Refs 19—23).
The purpose of this work is to prepare macroscopic
(
up to several millimeters long) filamentary crystals (whisꢀ
kers) based on vanadium, manganese, and tin oxides and
to study their morphological and functional characterisꢀ
tics promising for practical use.
(
Japan, CuKα1 radiation geometry (Bragg—Brentano geometry)
°
increment by 2θ 0.05°, 2θ range 10—80 , recording in quartz
cells without averaging rotation) were used for Xꢀray diffraction
analysis. An FRꢀ552 Guinier focusing camera was used to
determine the lattice parameters (CuKα1 radiation, recording
with a quartz crystal as a monochromator and germanium as
internal standard). The lattice parameters were calculated using
the standard program package for 10—15 reflections.
Experimental
Whiskers based on vanadium oxides were prepared by the
hydrothermal treatment of bariumꢀsubstituted xerogels of V O .
2
5
The V O •nH O gels were prepared by the interaction of
IR absorption spectra of the samples were recorded on a
Perkin Elmer LLC Spectrum One spectrometer (USA) in the
range from 400 to 8000 cm with a scan increment of 4 cm .
The samples were molded into pellets 13 mm in diameter with
spectrally pure KBr (molding force 4 t, pressure ~300 MPa)
based on 0.1—1 mg of the powder per 100 mg of KBr. The
spectra were analyzed according to literature data.
The lithium content in the samples was determined by atomic
emission spectroscopy with inductivelyꢀcoupled plasma (ICP)
2
5
2
crystalline vanadium(V) oxide with hydrogen peroxide according
2
4
–1
–1
to a standard procedure. The cherryꢀcolored product was dried
for ~6—7 h at ~50 °C. The xerogel plates were kept for 48 h in a
saturated solution of Ba(NO3)2 for ion exchange and placed
(
under the distilled water layer) into a sealed Teflon tube
mounted inside a steel autoclave, and the hydrothermal
treatment was carried out at 200—250 °C for 8—48 h. As a result
of the synthesis, yellow—green whiskers of various morphology

1044
Russ.Chem.Bull., Int.Ed., Vol. 57, No. 5, May, 2008
Goodilin et al.
on a Perkin Elmer Optima 5300 spectrometer (power of the
plasma burner 1350 V, argon (99.998%) pressure ~8.57 atm
ammonium heteropolytungstate (NH ) HPW O40•nH O
4
2
12
2
+
(n ≈ 12): Ptsponge/(NH ) HPW12O40•12 H O/H ꢀwhiskers/
4 2
2
–
1
(
120 psi), sample feed rate 2 mL s ). To prepare solutions, a
(NH ) HPW O •12 H O/Pt (Ptsponge is platinum sponge).
4
2
12 40
2
sponge
weighed sample was dissolved in nitric acid with the addition of
hydrogen peroxide. The nitric acid multielement standard
A ionic filter represented a protonic solid electrolyte (SEL)
–
2
–1
with the ion component of conductivity about 3•10 S cm
and the electron component less than 10– S cm . Exchange
8
–1
(
Perkin Elmer Optima Family) was used as the standard.
To evaluate the ability of the materials under study to
currents between the protonated whiskers and protonꢀconducting
electrochemical processes of lithium insertion and extraction,
electrodes were prepared and studied in traditional electroꢀ
chemical cells. The active mixture for the working electrodes
was prepared by mixing of 75% active material, 20% additive for
enhancing the electroconductivity (acetylene carbon black), and
electrolyte were calculated by an analysis of the impedance locus
+
of the cell: Pt
H ꢀwhiskers/Ptsponge.
/H ꢀwhiskers/(NH ) HPW O •12 H O/
sponge
4 2
12 40
2
+
Fourꢀelectrode electrochemical cells with ionic and
electronic potential probes were used for the quantitative
separation of the ion and electron components, respectively, of
the conductivity of the protonated Ba Mn O phase.
5
% binding (poly(vinylidene difluoride) dissolved in Nꢀmethylꢀ
pyrrolidone) followed by homogenation on an UZDNꢀ4D
ultrasonic disperser. The prepared active mixture was deposited
with a spatula on the both sides of the current feeder of a nickel
network with a thickness of 0.05 mm. To remove Nꢀmethylꢀ
pyrrolidone, the electrodes were dried for 5 h in a drying oven at
6
24 48
The sample of the protonated Ba Mn O phase was pressed
6
24 48
3
—2
under a pressure of 2•10 kg cm
into a cylindrical Teflon
ring; layers of the solid electrolyte and platinum sponge were
consequently pressed under the same pressure to the sample
from the both sides. Finally, the whole structure was clutched by
titanium punches, which were simultaneously used as current
9
5
0 °С. After this the electrodes were pressed under a pressure of
00 kg cm– for 30 s. Then the electrodes were repeatedly dried
2
+
in vacuo at 120 °С for 8 h to remove water traces. The amount of
the active substance on the electrodes 1.5Ѕ1.5 cm in size was, as
a rule, 7—10 mg. The counter and reference electrodes were
prepared by rolling thin strips of lithium (LEꢀ1 trade mark) of a
certain thickness on a nickel network with the welded current
feed of a nickel foil.
feeders: Ti/Ptsponge/(NH ) HPW12O40•nH O/H ꢀceramics/
4 2
2
(NH ) HPW O •nH O/Ptsponge/Ti.
4
2
12 40
2
To produce potential electrodes (probes) in the central
part of the Teflon ring, whose inner side contacts with the
ceramic sample under study, three holes 2 mm in diameter were
+
drilled by vertical, and first the H ꢀSEL layer and then the
The electrodes were tested (charge—discharge curves and
cyclic voltammograms were recorded) in sealed Teflon electroꢀ
chemical cells containing one working electrode, two counter
electrodes, and the reference electrode. All electrodes were
separated by separators of porous polyprolylene (trade mark
PORP (NPO Ufim, Moscow). Such cells rather well imitate the
conditions of functioning of galvanic cells. All procedures on
cell assembling were carried out in a dry box under an argon
atmosphere. The electrolyte used was the standard electrolyte
produced at the OAO Litiiꢀelement (Saratov, Russia), being a 1
platinum layer were pressed in them, which corresponded to
the Pt/H ꢀSEL/ceramics semielement. The distance between
+
the centers of these holes was taken as a distance between the
electrodes. Electrodes reversible by hydrogen were chosen as
reversible current electrodes. When studying the electron
conductivity, we measured the volt—ampere characteristic
between the potential Ni electrodes (probes) pressed uniformly
along the height of the studied ceramic samples in the cell:
Ti/ceramics under study/Ti, and titanium punches served as
current electrodes.
М solution of LiClO in a mixture of propylene carbonate (PC)
The sensor sensitivity was determined by the results of
measurements of the electroconductivity at a constant current
strength in artificial air and gas mixtures of specified composition.
The whiskers were deposited on a chip with a platinum heater
and gold contacts. The samples consisting of a whisker bundle
were fixed on the contacts with a conducting paste based
on metallic silver. The gas mixture with a specified NO2
concentration was created by a GDPꢀ102 dynamic flow generator
4
and dimethoxyethane (DME) (7 : 3). The water content in these
electrolytes measured by the Fischer method (684 KFꢀCoulometer,
Metrohm, Switzerland) was at most 50 ppm.
Galvanostatic charge—discharge curves were recorded using
a multichannel computerized setup for cycling produced at the
OAO Buster (St. Petersburg, Russia). The models were cycled in
the potential interval of the positive electrode 1.5—4.0 V at 20
°
С. The current density during cycling was 20 mA per 1 g of the
using a microflow source of the gas containing NO with the
calibrated bulk flow GH = 4.61 µg min . The change in the NO2
concentration in the gas mixture was achieved due to the
variation of the carrier gas (synthetic air) flow in the interval
100—1200 mL min–1. To create specified stable gas flows,
electronic blocks for gas flow formation (GFF) were used. The
2
–
–1
studied substance, which corresponds to 0.031—0.045 mA cm
2
.
Cyclic voltammograms (CV) were recorded on an ELꢀ2
potentiostat designed and produced at the A. N. Frumkin
Institute of Physical Chemistry and Electrochemistry (Russian
Academy of Sciences, Moscow, Russia). The potential sweep
rate was 0.13 mV s–1. For Xꢀray diffraction studies, the electrodes
after polarization were thoroughly washed with neat dimethoxyꢀ
ethane from electrolyte residues and dried over P O . All
NO concentrations were calculated by the formula
2
2
5
C – G /Q•103,
H
procedures, beginning from cell dismantling, were carried in
a dry box under an argon atmosphere.
where С is the mass concentration (mg m–3), G is the bulk flow
The electrochemical system consisting of a PSꢀ7 potentiostat
and a Zꢀ350M impedance meter (Elins) with automated computer
detection was used to study the proton conductivity. To reveal a
possibility of proton transfer, we carried out the study with
symmetric electrochemical cells with ionic filters based on
H
–
1
–1
(mg min ), and Q is the carrier gas consumption (mL min );
Q = 1200 mL min–1 corresponds to СNO2 = 2 ppm. The units
(milligram per cubic meter) were recalculated to other units
(ppm) by the Mendeleev—Clapeyron equation.

Peculiarities of metal oxide whiskers
Russ.Chem.Bull., Int.Ed., Vol. 57, No. 5, May, 2008
1045
Results and Discussion
a
3
2
Whiskers based on vanadium oxides
1
The synthesis of microcrystals and nanoribbons of vaꢀ
1
—4
nadium bronzes is widely discussed in the literature.
However, procedures for the formation of larger crystals
macroscopic “whiskers”) are absent. The morphology of
(
the whiskers prepared in this work is shown in Figs 1—5.
After the hydrothermal treatment, the crystal samples were
nanodimensional filaments and complex microstructures
formed from such filamentary crystals (see Fig. 1). Acꢀ
cording to the Xꢀray diffraction data, the resulting whisꢀ
kers up to 3 mm long and with the thickness much less
than 1 µm had the layered crystal structure (Fig. 6) and
predominant packing (texture), indicating that their morꢀ
phology is similar to that of the nanoribbons. According
to the energy dispersive Xꢀray (EDX) microanalysis data,
1
0 µm
b
the whisker composition is close to BaV O21ꢀδ
.
8
The charge—discharge curves of the BaV O
elecꢀ
21ꢀδ
8
trode are presented in Fig. 7. The initial discharge capacꢀ
ity of BaV O
was ~175 mA h g–1. In the second cycle,
8
21ꢀδ
–
1
the capacity decreased to 138 mA h g , which is characꢀ
teristic of the vanadium oxide electrodes and due to irreꢀ
versible processes that occur upon the primary electrode
polarization. Two areas corresponding to lithium inserꢀ
tion and a decrease in the oxidation state of vanadium are
well discernible in the cathodic part of the charge—disꢀ
charge curve. One prolonged area, reflecting lithium exꢀ
100 µm
c
traction from BaV O
and an increase in the oxidation
8
21ꢀδ
state of vanadium, is detected in the anodic part of the
charge—discharge curve. The cyclic voltammograms of
the BaV O
electrode show two peaks at potentials
8
21ꢀδ
of 2.77 and 2.44 V in the cathodic part of the curve. The
response peaks at potentials of 2.74 and 3.03 V are deꢀ
tected in the anodic part of the curve. The peaks in the CV
curve (as well as the areas in the charge—discharge curves)
reflect the insertion process (cathodic peaks) and lithium
extraction (anodic peaks). The difference between the
anodic peak potential and the potential of the correspondꢀ
ing cathodic peak is 0.25 V for the first peak and 0.30 V for
the second peak.
1
cm
Fig. 1. Microphotograph of the BaV O
whiskers prepared by
21ꢀδ
8
the hydrothermal synthesis: (a) scanning electron microscopy
data: 1, planar nanostructure transparent for electron beam; 2,
whiskers with the diameter less than 100 nm; 3, large accretions
of whiskers; (b) data of optical microscopy and (c) general view
of the conducting “paper” prepared by drying of the whisker
layer.
This difference between the potentials is characteristic
of intercalation materials. The discharge capacity calcuꢀ
lated from the cyclic voltammogram was 165 and 156
–
1
mA h g for the first and second cycles, respectively,
which agrees with the results of galvanostatic cycling. The
discharge capacity equal to 156 mA h g–1 corresponds to
the insertion, on the average, of 5.13 lithium atoms per
virtually unchanged from the second to 20th cycle, which
is especially important for practical use of these materials
in lithium accumulators. If the whiskers are dried under
the dynamic vacuum conditions (dehydrogenation) at
150—200 °C, then the capacity at the third cycle deꢀ
Li5.13BaV O
molecule (i.e., 0.64 lithium atom per vaꢀ
8
21ꢀδ
nadium atom), and the oxidation state of vanadium
changes from +5 to +4.36. The change in the discharge
capacity of the BaV O
cycling indicates that the capacity is stable and remains
electrode during galvanostatic
creases only by 10—15% and remains at a level of 500—
8
21ꢀδ
520 mA h g–
1
.

1
046
Russ.Chem.Bull., Int.Ed., Vol. 57, No. 5, May, 2008
a
Goodilin et al.
d
Cl
O
Mn
c
Mn
Ba
Ba
Mn
Cl
O
Mn
Ba Mn
Ba
b
Mn
1
µm
1 µm
e
f
2
µm
1 µm
Fig. 2. Morphology of growth of the Ba Mn24O48 whiskers: a, whisker with a drop of the solidified flux on the whisker top; b, EDX
6
spectrum obtained for the central part of the whisker (Ba : Mn = 8.2 : 28.1 ≈ 1 : 3.4, no potassium or chlorine was observed); c, EDX
spectrum obtained for a drop on the whisker top (Ba : Mn : Cl = 10.4 : 22.8 : 4.8); d, typical view of the medium part of the whisker
formed by the accretion of thinner whiskers; residues of the congealed flux are seen in the accretion sites; e, round bottom part (base)
of the whisker contacting with the initial BaMnO pellet; f, magnified image of the whisker base.
3
a
b
3
4
1
2
2
1
4
3
1
0 µm
1
mm
Fig. 3. Interaction of growing whiskers and chloride fluxes (optical microscopy): (a) general view of the whiskers growing in a
transparent melt/KCl crystal; (b) whiskers in a thin wedge of the solidified melt under large magnification: 1, layer of a solidified KCl
melt; 2, holes formed in the thin residual layer of a KCl melt due to vaporization; 3, whiskers; 4, colored (green—blue) crystallites in
the melt (magnified fragment is given in inset).

Peculiarities of metal oxide whiskers
Russ.Chem.Bull., Int.Ed., Vol. 57, No. 5, May, 2008
a
1047
a
2
µm
b
1
0 µm
b
2
00 nm
2
00 nm
Fig. 4. Morphology of the Hꢀform of whiskers of the Ba Mn O
6
24 48
phase prepared by the treatment with concentrated nitric acid at
6
5 °С for 5 h: (а) general view of the crystals and (b) formation
c
of nanowhiskers at the edge of a particular crystal.
Whiskers based on manganese oxides
Whiskers of the Ba Mn O phase, needleꢀlike crysꢀ
6
24 48
tals of the Ba2–xMn8ꢀyO16 hollandite, or their mixture were
obtained reproducibly and in high yield by the isothermal
vaporization of the chloride fluxes (KCl, NaCl, or their
eutectic mixture) in the Ba—Mn—O system, depending
on the temperature of synthesis (see Figs 2 and 3). The
both phases (hollandite Ba2–xMn8–yO16 and the
Ba Mn O phase) exist in the region enriched in
1
0 µm
Fig. 5. Morphology of the SnO whiskers: (а) general view of the
whiskers in the grow the boat and the optical microphotograph
2
6
24 48
manganese(IV) oxide and are characterized by relatively
small homogeneous regions by barium. However, the therꢀ
mal stabilities of these phases differ substantially. The
optimum temperature of crystal growth of the Ba Mn O
phase from KCl in air is 950—960 °C. An admixture of
hollandite needles was always observed at temperatures T
of the SnO whisker about 2 cm long against the background of a
2
fiveꢀkopeck coin; (b) microphotograph of the samples syntheꢀ
sized in experiments with the platinum support; inset, an image
of a whisker with the congealed drop on the end; (c) microphotoꢀ
graph of the samples synthesized in the presence of metallic tin.
6
24 48
<
900 °C, whereas hausmannite Mn O is formed at T >>
which is easily separated mechanically, and the unreacted
3
4
9
50 °C. The crystal growth of the Ba Mn O phase is
reactants and admixture phases (Mn O , Mn O , etc.)
6
24 48
2
3
3
4
favored by the use of the bariumꢀenriched blend, and the
crystals grow better when the oxide blend is used as densely
molded and preꢀannealed pellets placed in a melt of the
isothermally vaporizing chloride flux. The needles grow
on the external surface of the pellet forming a “felt,”
remain in the major bulk of the pellet. Flux vaporization
solves an important problem of separation of the prepared
crystals from the solvent, which is usual for the standard
method of crystal growth from solution in melt. Accordꢀ
ing to the EDX data, the whiskers are crystals with the

1
048
Russ.Chem.Bull., Int.Ed., Vol. 57, No. 5, May, 2008
Goodilin et al.
I/pulse
000
E/V (Li/Li)
a
0
03
a
0
01
7
6
5
4
3
2
1
4
000
000
000
000
000
000
1
2
3
3
2
005
5
0
100
150 Q/mAh g–1
2
4
6
8
10
d/Å
i/mA g–1
I/I0
530]
b
b
[
1
00
2
8
6
4
2
0
0
0
0
0
1
[
600]
5
0
0
[
411]
[
620]
[550]
211]
[321]
[
–
50
3
0
32
34
36
38
2θ/deg
I (arb. units)
с
–
100
1
4000
2000
0000
2
3
4 E/V
1
1
Q/mAh g–1
с
8
6
4
2
000
000
000
000
0
200
150
5
10
15
n
2
0
30
40
50
60
70
2θ/deg
Fig. 7. Electrochemical characteristics of the BaV O21ꢀδ whisꢀ
Fig. 6. Xꢀray diffraction data for whiskers: (а) BaV O21ꢀδ whisꢀ
kers with the designation of the main peaks composing the series
8
8
kers (electrolyte 1 М LiClO in a PC—DME (7 : 3) mixture;
4
1
, cycle 1; 2, cycle 2; 3, cycle 10: (а) charge—discharge curves
0
0l, (b) diffraction patterns of the samples of the initial (solid
of the BaV O
electrode, current density 20 mA g–1, (b) cyclic
line) and protonated (dotted line) Ba Mn O phase, and (c)
8
21ꢀδ
6
24 48
voltammograms of the BaV O electrode, potential sweep rate
Xꢀray pattern of the SnO whiskers.
8
21ꢀδ
2
0
.13 mV/s, and (c) change in the discharge capacity of the
BaV O
electrode during cycling, current density 20 mA g–1
.
8
21ꢀδ
chemically homogeneous composition in which the conꢀ
tent of potassium and chloride does not exceed 1 at.% (for
example, Ba : Mn : O : K : Cl = (7.4—7.6) : (25.0—27.5) :
The whiskers of the Ba Mn O phase can be divided
into two types: thicker freely standing whiskers growing
on the pellet surface (see Fig. 2) and thinner single whisꢀ
6
24 48
(
64.6—67.0) : (0—0.3) : (0—0.6) for a series of points
analyzed with an increment of 2—4 µm on the whisker
accretion with a thickness of 3—4 µm).
kers growing in the chloride flux layer (see Fig. 3). Whiskers
as a rule, form bundles consisting of thinner nanofibers
,

Peculiarities of metal oxide whiskers
Russ.Chem.Bull., Int.Ed., Vol. 57, No. 5, May, 2008
1049
oriented in one direction. In addition, rather bulky
characteristic of the initial structure are retained and the
material undergoes no appreciable amorphization. The unit
cell parameters for a sample of the Ba Mn O phase obꢀ
“
pseudosingle crystals,” as well as dendrites and “bushes,”
are met. One also can prepare a large amount of very thin,
long, and flexible fibers of the Ba Mn O phase with a
6
24 48
tained by storing in concentrated nitric acid at room temꢀ
perature for 7 days are a = 18.033(6) Å, and c = 2.833(1) Å,
i.e., the a parameter of the initial phase changes substanꢀ
tially (by ~0.15 Å), whereas the c parameter remains unꢀ
changed. Such a high value of changing the а parameter is
due, most likely, to the removal of the barium ions with
the large ionic radius from the structure. According to the
data of EDX microanalysis and atomic emission spectrosꢀ
copy with inductivelyꢀcoupled plasma, the barium conꢀ
tent in the Hꢀform of the Ba Mn O phase decreases by
6
24 48
diameter of 0.1—1 µm and a ratio of the length to diamꢀ
4
5
eter of 10 —10 and those reaching a length of several
millimeters. Such fibers are formed on the surface of the
chloride flux, which is transformed into the feltꢀlike layer
1
—2 mm thick consisting of the twisted bent Ba Mn O
6 24 48
whiskers (see Fig. 3). The latter can easily be separated
mechanically from a pellet.
Peculiarities of the growth of the Ba Mn O whisꢀ
6
24 48
kers are shown in Fig. 2. These data indicate that the
whiskers are formed via the mechanism relative to the
vapor—liquid—crystal (VLC) mechanism. The EDX data
6
24 48
~30 at.% compared to the starting phase. Second, the
decrease in the a parameter is a consequence of an inꢀ
crease in the average oxidation state of manganese from
3.5 to 3.8—3.9 (according to iodometric titration). Simiꢀ
lar phenomenon is observed for the extraction of alkaline
show that drops of nonvolatile BaCl are formed on the
2
top of the crystals. Nevertheless, these drops are not
strongly localized on the whisker tops and can flow down
or, on the contrary, rise by the capillary forces along the
lateral crystal surface, resulting in accretions of codirectly
growing whiskers. Probably, the nonvolatile components
2
5
metal ions from hollandite and lithium—manganese
2
6,27
spinels,
and the commonly accepted mechanism that
III
explains this process is the disproportionation of Mn to
IV
II
of the melt (e.g., BaCl ) can also be localized at the juncture
Mn remaining in the tunnel walls and Mn that transꢀ
fers into the solution. Indeed, an analysis of the chemical
composition of the solution obtained by the formation of
the Hꢀform of the Ba Mn O phase shows that the soluꢀ
2
site: in a region of the dihedral angle between the planar
surface of a thicker whisker and the top of a thinner whisꢀ
ker sliding along the planar surface during the growth time.
At the same time, several facts do not allow one to
explain completely the growth of the Ba Mn O whisꢀ
6
24 48
tion contains both the barium and manganese ions in the
ratio Ba : Mn ≈ 1 : 1.5, which excludes the congruent
dissolution of the phase in nitric acid and also indicates
that the protonation process can be restricted by a simple
ion exchange accompanied by washing out of the barium
ions to the solution. Thus, defects are formed in the tunꢀ
nel walls of the Ba Mn O phase. The defects represent
6
24 48
kers by the classical VLC mechanism. First, the BaCl2
drops should rapidly be consumed during the crystal
growth, because this is the only source of barium in the
system, so that the formation of long whiskers becomes
impossible. Second, the whiskers are well wet with a chloꢀ
ride melt; however, according to the classical VLC mechaꢀ
nism, the wetting should be sufficiently weak for a catalyst
drop to remain stable. Third, along with the freely standꢀ
ing whiskers, there is a fraction of “floating” whiskers that
form a layer on the chloride melt surface. They can also
be observed in the solidified layer of the chloride melt
even at a distance of 1 cm from the reactant pellet placed
in the flux (see Fig. 3). These whiskers are completely
immersed into the melt and, hence, we cannot believe
that they grow only due to the “catalyst” drops on their
tops. The studies show that the most part of whiskers grow
due to the extrusion of the crystals from their base (pedesꢀ
tal mechanism). Probably, three phases should be involved
in the formation of the Ba Mn O whiskers: gas (О that
6
24 48
the absent manganese atom and a portion of oxygen coorꢀ
dinately bound to it. In this case, the nearest coordination
environment of the manganese ions around the defect is
likely distorted.
According to the IR spectra data, the protons in the
structure are fixed due to the formation of the Mn—O—H
bonds. Vibrations at ~1100 cm– (Fig. 8), according to
published data, are most intense in the IR spectra of manꢀ
ganites and favor bending vibrations of the OH groups
1
2
8
bound to manganese. Independent structural studies
show that this is the hydroxyl that exists in the material
bulk. The IR spectrum (see Fig. 8) also exhibits a line at
3000—3500 cm– corresponding to stretching vibrations
of the OH groups bound to manganese in manganites.
The protons of the Mn—OH bond in tunneling and
layered manganese oxides are very labile, which results,
in some cases, in the appearance of a significant proton
conductivity (Figs 9 and 10). Investigation on the symꢀ
metric electrochemical cells was carried out to study a
possibility of proton transfer in the protonated Ba Mn O
1
6
24 48
2
oxidizes MnCl to the oxide phases), liquid (BaCl ), and
2
2
solid (whisker). At the same time, crystal growth can inꢀ
volve a single catalyst drop for each whisker and also due
to a localized center enriched with barium chloride, reꢀ
sulting in the growth of thick pseudosingleꢀcrystalline,
freely standing, and oriented in one direction whiskers or
a layer of whiskers floating on the chloride melt surface.
The Xꢀray diffraction analysis of the Hꢀform of the
Ba Mn O phase (see Fig. 6) shows that all major reflections
6
24 48
whiskers. The cyclic voltammograms (see Fig. 9) demonꢀ
strate the dependence of the current on both the potential
sweep rate and hydrogen content in the gas phase. In the
6
24 48

1
050
Russ.Chem.Bull., Int.Ed., Vol. 57, No. 5, May, 2008
Goodilin et al.
(ωC)–1/Ohm
0000
T
a
C
8
7
6
5
4
3
2
1
1
2
0000
0000
0000
0000
0000
0000
0000
0
KBr
2000
Fig. 8. IR spectrum of the Hꢀform of the Ba Mn O phase
A
MnO
B
1
000
3000
ν/cm–1
1
5000 30000 45000 60000
R/Ohm
6
24 48
prepared by keeping in concentrated HNO for 15 days. Arrows
3
–1
mark lines in a region of ~1100 cm corresponding to the bending
Cd1
b
–1
vibrations of the OH groups and lines at ~3500 cm belonging to
stretching vibrations of the OH groups. The vibrations of the lattice
Mn—O groups and residual line of the KBr matrix are also designated.
Rgb
Rb
Rf
Zf
i/µА
a
2
1
00
00
0
Cgb
Ra
Ca
Za
logi [A cm–1]
0
c
1
2
3
–
–
100
200
–
3.4
–
–
3.6
3.8
–
1200 –800 –400
0
400
800
ν/mV
i/µА
–4.0
b
1
–
4.2
2
1
00
–
2.6 –2.4 –2.2 –2.0
logH [mol L–1
]
2
00
0
2
3
Fig. 10. Impedance spectroscopy of the protonated magnetic
whiskers: (а) impedance spectra in various media (Т = 24 °С,
relative humidity 58%, spectra recording by the twoꢀelectrode
scheme between the current electrodes): 1, 100% air; 2, 50%
–
–
100
200
6
air—50% H mixture; А, 10 Hz; В and С, 0.1 Hz; (b) equivalent
2
scheme; (c) experimental dependence of the exchange currents
on the gas phase composition.
–
1200 –800 –400
0
400
800
ν/mV
both cases, the cyclograms demonstrate hysteresis pheꢀ
nomena, especially under an atmosphere with a high
hydrogen content. This type of cyclic volt—ampere
characteristics and the values of currents passing through
the cell indicate a considerable contribution of the proton
conductivity to the overall conductivity of the whiskers
complicated by the retarded diffusion. A substantial depenꢀ
Fig. 9. Plots of the voltammograms of the Pt
12H O/H ꢀwhiskers/(NH ) HPW O •12H O/Pt
trochemical cell (а) vs rate of potential application (50% H , air,
T = 24 °С, relative humidity 25%): 400 (1), 100 (2), and
0 mV s–1 (3); (b) vs gaseous medium composition at a fixed rate
of potential (400 mV s ) application: 1, 50% H ; 2, 5% H ;
, 100% air.
/(NH ) HPW O
12 40
sponge
4 2
+
•
elecꢀ
2
4 2
12 40
2
sponge
2
2
–
1
2
2
3

Peculiarities of metal oxide whiskers
Russ.Chem.Bull., Int.Ed., Vol. 57, No. 5, May, 2008
1051
dence of the volt—ampere characteristics on the hydroꢀ
gen concentration in the gas phase can also indicate that
the resistance at the electrode boundaries contributes to
the ion chains, which shunts the electron conductivity.
The impedance spectra obtained by the twoꢀelectrode
scheme are complicated. However, their shape (see Fig.
diffraction data (see Fig. 6), the SnO whiskers are classiꢀ
2
fied as the tetragonal modification of SnO (PDFꢀ2 card
2
no. 77ꢀ447). The isothermal vaporization of SnO in the
standard regime, which consists of heating to 1050 °С
with a rate of 10 °С min– followed by storing at this
temperature for 1 h in a nitrogen flow, results in the
precipitation of a white powder on the boat walls. Howꢀ
ever, the amount of the grown whiskers in these experiꢀ
ments is negligible. To enhance the whisker yield, the
temperature regime was optimized by the additional storꢀ
age at 250—400 °С. The optimum regime turned out to be
the crystal growth at 1050 °С with the preliminary isoꢀ
thermal storage of SnO at 350 °С. In this case, the whisker
yield increases by ~70 times.
1
1
0) depends on the hydrogen concentration in the gas
phase. The equivalent scheme describing the impedance
spectrum includes the resistance at the boundaries with
the electrodes (R ), contribution of the intercrystallite
b
boundaries (gb), and ion, electron, and diffusion compoꢀ
nents. The fourꢀprobe method was used for the quantitaꢀ
tive determination of the electron and ion components.
When measuring the ion component of the conductivity,
we measured the change in time of the potential between
the electrodes after galvanostatic pulse application beꢀ
tween the current electrodes. The specific conductance
2
9—31
It is known
that the growth of extended microꢀ
and nanocrystals of tin oxide can be initiated on the surꢀ
face of polycrystalline gold supports. We chose the tin and
platinum plates as the supports and obtained, as a result,
strongly branched, bent filamentary crystals with the
length from tens of microns to several millimeters and the
diameter from several nanometers to several microns, as
well as the unique whiskers, whose length reaches several
centimeters (see Fig. 5).
In the case of using the tin plates (see Fig. 5), the
vaporization of metallic Sn in the cold zone produces,
most likely, tin microdrops in the reaction space. They
act as the “catalyst” of future crystals. The growth comꢀ
ponent formed due to SnO disproportionation in the gas
phase is transferred by the nitrogen flow to a cooler zone,
(
conductivity) was determined after the potential reached
a constant value. The measurements were carried out at
different galvanostatic pulses. The use of this regime and
measurements on the samples of different thickness made
it possible to exclude the influence of the current of biꢀ
nary layer charging, which contributes to the initial peꢀ
riod after potential application. An analysis of the time
dependence between the potential electrodes after the
application of galvanostatic pulses of different value in air
and in an atmosphere containing hydrogen shows that a
constant emf value is established within ~30 s and the
resistance is proportional to the current and sample thickꢀ
ness, indicating no substantial contribution of the boundary
resistance. The composition of the environment (air and
air—oxygen mixture) exerts a weak effect on the measured
resistance value. The ion conductivity calculated by the
where the SnO crystals are formed via the VLC mechanism.
2
The growth of the branched crystals is due to the fact that
the tin vapors present in the gas phase are condensed on
the lateral walls of the boat and also on the surface of the
already grown whiskers, which results in the growth of
new crystals on the surface of the already formed crystals.
The use of the platinum support results in the formaꢀ
tion of smooth and lowly branched crystals (see Fig. 5).
According to published data, the Pt—Sn system contains
results of measurements is (9.9±0.5)•10–4 Ohm cm
–1
–1
.
When measuring the electron component of the conducꢀ
tivity, three potential electrodes were used to determine
the dependence of the resistance on the sample thickness.
The voltammograms obtained in the cell for measuring
the electron conductivity has a purely ohmic character,
and the determined resistance values are rigidly proporꢀ
tional to the ceramic thickness. The specific electron conꢀ
five intermediate phases, of which Pt Sn and PtSn melt
3
congruently at temperatures >1050 °С (synthesis temꢀ
perature), and the Pt Sn , PtSn , and PtSn compounds
2
3
2
4
–
4
ductance of the ceramics at 24 °C was (8.30±0.02)•10
are formed via the peritectic reactions at 840, 745, and
–
1
–1
Ohm cm . Thus, the Ba Mn O whiskers and their
522 °С, respectively. Therefore, PtSn drops can be formed
6
24 48
x
protonated Hꢀform can find use as electrochemically acꢀ
tive reinforcing filaments to improve the mechanical charꢀ
acteristics of flexible cathodic materials and protonꢀconꢀ
ducting polymers. Due to their “feltꢀlike” structure, the
Ba Mn O whiskers can act as efficient electrode mateꢀ
on the platinum plate surface after the condensation in
the cold zone of metallic tin formed due to SnO disproꢀ
portionation. It should be mentioned that the wettability
in a melt—metal system is considerably higher than that
at the melt—oxide boundary. Based on the EDX data for
the regions where the support is not covered by the whisꢀ
kers, we can conclude that a metallic tin layer is formed,
in fact, on the platinum plate surface. In this connection,
we can assume that during the experiment the drops are
preferentially condensed on the platinum support initiatꢀ
ing the crystal growth via the VLC mechanism. The conꢀ
gealed drops are seen on the ends of selected whiskers,
6
24 48
rials for gasꢀpermeable fuel cells or gas sensors.
Whiskers based on tin dioxide
The typical morphology of the tin dioxide whiskers
prepared in the present work by the growth in the optiꢀ
mized regime is shown in Fig. 5. According to the Xꢀray

1052
Russ.Chem.Bull., Int.Ed., Vol. 57, No. 5, May, 2008
Goodilin et al.
indicating in favor of this hypothesis. In addition, it has
been shown by the EDX method that the platinum is
virtually absent from the structure of these whiskers.
The preparation of filamentary semiconducting crysꢀ
R/kOhm
1
00 °C
1
1
20000
00000
0000
60000
tals of SnO makes it possible to use the whiskers as active
2
elements of future microsensor systems of different conꢀ
figuration. In the present work, the sensor properties of
the synthesized whiskers were characterized by model
changes in the resistance of their bundle introducing niꢀ
trogen dioxide vapors in a concentration of 0.8 ppm (Fig.
8
1
50 °C
4
2
0000
0000
0
1
1). The maximum sensor signal is observed at low temꢀ
peratures (~100 °C), although at other temperatures the
signal value is quite sufficient for the creation of highly
sensitive sensor devices to toxic and dangerous compoꢀ
nents of gaseous media.
200 °C
5
000 10000 15000 20000 25000
t/s
Fig. 11. Time dependence of the resistance of the SnO whiskers
2
From the viewpoint of fundamental research and pracꢀ
tical significance, whiskers are among the most promising
crystalline materials with the unique complex of properꢀ
ties. Whiskers, as a rule, have a perfect, almost ideal strucꢀ
ture, which excludes usual dislocation mechanisms of plasꢀ
tic deformation and approaches the strength of whiskers
to the theoretical limit. Whiskers are characterized by
unique flexibility, corrosion stability, and multiple enꢀ
hancement of the crystallographically induced anisotropy
of the properties. They can find wide use: from strengthꢀ
ening fibers to nanoelectronic devices. Although filamenꢀ
tary crystals have been known for more than fifty years,
the application of whiskers in the technology is rather
lopꢀsided: mainly as reinforcing filaments for the creation
of structure composite materials with improved mechaniꢀ
cal properties. At the same time, using the reproducible
and available experimental procedures described in this
work, one can synthesize macroscopic metal oxide whisꢀ
kers in a series of most important systems (Table 1). Their
functional characteristics allow these whiskers to be promꢀ
ising for the further practical use, including the creation
of new chemical current sources and sensor devices.
at the periodical introduction of 0.8 ppm NO at different temꢀ
peratures.
2
This work was financially supported by the Russian
Federation for Basic Research (Project Nos 07ꢀ03ꢀ00749ꢀ
a and 07ꢀ08ꢀ00639ꢀa), the Council on Grants at the Presiꢀ
dent of the Russian Federation (Program of State Support
for Young Russian Scientists, Grant MDꢀ1264.2007.3),
and the Federal Agency on Science and Innovations (Fedꢀ
eral Target Scientific Technical Program).
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motif
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2
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6
24 48
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7
Received January 10, 2008;
in revised form May 14, 2008