Formation of Na0.44MnO2 nanowires via stress-induced splitting of birnessite nanosheets

Abstract

High aspect ratio Na0.44MnO2 nanowires with a complex one-dimensional (1-D) tunnel structure have been synthesized. We found that the reaction went through High aspect ratio Na0.44MnO2 nanowires with a complex one-dimensional (1-D) tunnel structure have been synthesized. We found that the reaction went through
layered birnessite nanosheet intermediates, and that their conversion to the final product involved splitting of the nanosheets layered birnessite nanosheet intermediates, and that their conversion to the final product involved splitting of the nanosheets
into nanowires. Based on our observations, a stress-induced splitting mechanism for conversion of birnessite nanosheets to into nanowires. Based on our observations, a stress-induced splitting mechanism for conversion of birnessite nanosheets to
Na0.44MnO2 nanowires is proposed. The final and intermediate phases show topotaxy with 〈001〉f // 〈020〉b or 〈110〉b where f represents the final Na0.44MnO2 phase and b the intermediate birnessite phase. As a result of their high surface areas, the nanowires are efficient catalystse nanowires are efficient catalysts
for the oxidation of pinacyanol chloride dye. for the oxidation of pinacyanol chloride dye.

KeywordsBirnessite-manganese oxide-nanowire-nanosheet-stress-conversion KeywordsBirnessite-manganese oxide-nanowire-nanosheet-stress-conversion

Full text

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Formation of Na0.44MnO2 Nanowires via Stress-
Induced Splitting of Birnessite Nanosheets
Yanguang Li and Yiying Wu（
）
Department of Chemistry, The Ohio State University, 100 West 18th Avenue, Columbus, Ohio 43210, USA
Received: 1 September 2008/ Revised: 11 November 2008/Accepted: 12 November 2008
©Tsinghua Press and Springer-Verlag 2008. This article is published with open access at Springerlink.com
Address correspondence to wu@chemistry.ohio-state.edu
ABSTRACT
High aspect ratio Na0.44MnO2 nanowires with a complex one-dimensional (1 D) tunnel structure have been
synthesized. We found that the reaction went through layered birnessite nanosheet intermediates, and that their
conversion to the fi nal product involved splitting of the nanosheets into nanowires. Based on our observations,
a stress-induced splitting mechanism for conversion of birnessite nanosheets to Na0.44MnO2 nanowires is
proposed. The fi nal and intermediate phases show topotaxy with〈001〉f// 〈020〉b or〈110〉b where f represents
the fi nal Na0.44MnO2 phase and b the intermediate birnessite phase. As a result of their high surface areas, the
nanowires are effi cient catalysts for the oxidation of pinacyanol chloride dye.
KEYWORDS
Birnessite, manganese oxide, nanowire, nanosheet, stress, conversion
Manganese oxide nanowires with layered or
tunnel structures are attractive for applications
in batteries, separation and catalysis due to their
open-framework structures and interesting redox
properties [1 9]. There have been several studies of
their synthesis in the literature: for example, MnO2
nanorods were synthesized through a carefully
controlled hydrothermal reaction by oxidizing MnSO4
with (NH4)2S2O8 or KMnO4 [10, 11]. Nanowires with
cryptomelane [12], romanechite [6, 13], and RUB
7 [13 15] structures have also been produced in
aqueous solution. Most of these approaches involved
a birnessite phase observed as an intermediate [11]
or used as precursor [6, 13 15]. Birnessite possesses
a layered structure formed of edge-sharing MnO6
octahedra with Na+ cations and H2O molecules
filling the interlayer space (Fig. 1 (a)). Conversion
of birnessite has been used as a general strategy to
obtain a variety of tunnel structures [1]. However,
the underlying mechanism remains unclear. Recently,
the curling or rolling up of birnessite layers as a
result of weakened interlayer interactions during
hydrothermal treatment was proposed [11]. While
this mechanism can explain tube formation from
exfoliated molecular sheets, it cannot rationalize
the formation of non-hollow manganese oxide
nanowires.
Herein we report a stress-induced splitting
mechanism that transforms birnessite nanosheets
into Na0.44MnO2 nanowires. Na0.44MnO2 has a
complex tunnel structure. It consists of columns of
edge-sharing MnO5 square pyramids and sheets of
edge-sharing MnO6 octahedra extending parallel to
the c-axis (Fig. 1 (b)). They are connected by corner-
Nano Res (2009) 2: 54 60
DOI 10.1007/s12274-009-9003-1
Research Article

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Nano Res (2009) 2: 54 60
sharing to form two types of tunnels: large S-shaped,
and six-sided tunnels, both occupied by Na+ cations.
In our study, Na0.44MnO2 nanowires were
synthesized by a hydrothermal method. Their X-ray
diffraction (XRD) pattern shows diffraction peaks in
good agreement with the standard pattern (PDF No.
27 0750), except for some variation in peak intensities
due to nanowire orientation (Fig. 2(a)). An scanning
electron microscopy (SEM) image reveals that they are
pure nanowires with average length over 10 μm and
diameter smaller than 100 nm (Fig. 2(b)). Most of the
nanowires align with each other and form thicker
nanowire bundles.
Nanowire growth was tracked at different reaction
stages in order to probe their formation mechanism.
The birnessite phase appeared first shortly (~2 h)
after the start of the reaction as indicated by the XRD
pattern of Fig. 3(a)(A). An interlayer spacing of 0.72
nm is calculated from the diffraction peaks at 12.4° and
25.1°, in good agreement with Ref. [16]. The refl ection
at 15.4°, corresponding to an interlayer spacing of 0.56
nm comes from a dehydrated Na-birnessite phase [16].
The SEM image of the sample at this stage features
a large amount of thin nanosheets (see Fig. S-1(a) in
the Electronic Supplementary Material (ESM)). Their
flat geometry is the cause of the preferred orientation
indicated by the abnormally strong (00l) peaks of
birnessite in Fig. 3(a)(A). Figure 3(b) displays the
transmission electron microscopy (TEM) image of a
piece of Na-birnessite nanosheet collected at this stage
(2 h). The transparency indicates its small thickness.
Figure 1 Schematic illustrations of the crystal structures of
birnessite (a) and Na0.44MnO2 (b)
（a）
（b）
（a）（b）
Figure 2 (a) XRD pattern and (b) SEM image of annealed Na0.44MnO2 nanowires

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The corresponding selected area electron diffractions
(SAED) pattern shows diffraction spots with roughly
six-fold symmetry, and the zone axis is assignable to
the [001] axis (Fig. 3(c)). Na-birnessite was reported to
be triclinic by Lanson et al. [17, 18]. Its slight deviation
from the hexagonal symmetry originates from the
elongation along [100] of the Jahn-Teller distorted Mn3+
[17, 18]. Close examination of Fig. 3(b) reveals that some
nanowires split apart from
nanosheets near the edges.
When the reaction time
was extended to 2 days,
Na-birnessite was still the
main phase, together with
a significant amount of
unreacted Mn2O3 and some
diffuse peaks from the
final product Na0.44MnO2
in the XRD spectrum
(Fig. 3(a)(C)). Most of the
earlier nanosheets can be
seen to have transformed
into nanowires in the
corresponding SEM image
(Fig. S-1(c) in the ESM).
Figure 3(d) shows the TEM
image of one nanowire
collected at this stage. One
interesting feature in this
nanowire is the thinning
from the middle as if it
was about to split further
(marked by arrows). The
corresponding SAED
pattern displays elongated
lines perpendicular to the
nanowire growth direction,
in which the original six-
fold symmetric diffraction
spots are embedded but still
visible (marked by triangles
in Fig. 3(e)). The striped
line features here are a clear
indication of transverse
structural disorder of the
nanowires in the course of
conversion from the intermediate birnessite to the
fi nal Na0.44MnO2 phase. In contrast, the longitudinal
structural order was maintained.
To complete the conversion, reaction for a further
two days was found to be necessary. Broad but
pronounced peaks of Na0.44MnO2 appear in the XRD
pattern (Fig. 3(a)(D)). Figure 3(f) shows a TEM image
of the thin nanowires collected at this stage. They
Figure 3 Evolution of Na0.44MnO2 nanowires with time. (a) XRD patterns of the product isolated at
different reaction times of (A) 2 h, (B) 24 h, (C) 2 days, (D) 2×2 days, and (E) annealed at 600 °C. TEM
and SAED characterizations were also conducted on samples collected at (b), (c) 2 h, (d), (e) 2 days and (f),
(g) 2×2 days. The white arrows in (d) indicate nanowire thinning from the middle and the black triangles
in (e) mark the six-fold diffraction spots embedded in the elongated lines. In (e) and (g), the nanowires
are vertically oriented. See text for detailed discussion
（a）
（b）
（c）
（d）
（e）
（f）
（g）

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have diameters around 50 nm. A single-nanowire
SAED pattern shows diffraction spots of rectangular
symmetry (Fig. 3(g)), and nanowire growth direction
is determined to be oriented along [002]
direction of the 1-D tunnel. Similar elongated lines
are also present in the background, perpendicular
to the nanowire growth direction. We believe these
result from defects in the nanowires created during
the conversion process.
From the sequence of TEM images, it becomes
clear how the birnessite nanosheet intermediates
are converted into the Na0.44MnO2 nanowires. The
birnessite nanosheets were initially produced by
dissolution of Mn2O3 powder in concentrated NaOH
followed by recrystallization with concomitant
intercalation of Na+ and H2O. Conversion from the
birnessite to various 1-D tunnel structures has been
studied for a long time, and is believed to start with
a disproportionation reaction of neighboring Mn3+
ions in the manganese oxide layers into Mn4+ and
Mn2+ ions [15, 17, 19]. The Mn2+ ions migrate into
the interlayer galleries, undergo oxidation to Mn3+
by oxygen in the autoclave and assist the formation
of corner-sharing MnO6 octahedra. Vacancies left
behind weaken the birnessite layers. We believe a
the
similar process occurs in our system. This structural
reconstruction introduces a high density of defects,
causes stacking faults and in-plane stresses, and
eventually the nanosheets split into nanowires when
the stress cannot be accommodated any more. This
stress-induced splitting mechanism is supported
by our observations of the nanowire “brushes”
formed after reaction for 2 h. Their overall shape is
reminiscent of birnessite nanosheets, as shown by the
SEM images in Fig. 4.
Also, noteworthy in Fig. 4 is that the nanowire
brushes have two equivalent growth directions with
an angle of 60° or 120°. Taken in conjunction with
above TEM results, the SEM images suggest that
the conversion is topotactic with〈002〉
〈110〉b where f represents the final Na0.44MnO2
phase, and b the intermediate birnessite phase. This
crystallographic coherence can be understood by
comparing the crystal structures of birnessite and
the final product. Chains of edge-sharing MnO6
octahedra run along〈020〉 and〈110〉 in birnessite
(Fig. 4(c)). The same feature exists in Na0.44MnO2
along〈002〉 . Therefore,〈020〉 and〈110〉 directions
are the least stressed during the conversion, and thus
the nanosheets tend to split along these directions.
f //〈020〉
b or
Figure 4 SEM images of nanowire brushes captured at the intermediate stage of the reaction. Their growth is along two directions
simultaneously forming an angle of (a) 60° and (b) 120°. (c) Possible nanowire splitting directions in birnessite layers with the edge-sharing
MnO6 octahedra chains preserved. The dashed rectangle represents a 2-D unit cell in the ab plane
（a）（b）
（c）

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The proposal of a topotactic conversion is also
supported by the similar values of the d-spacings (d020
= 1.42 Å for birnessite, d002 = 1.41 Å for Na0.44MnO2).
To the best of our knowledge, this is the fi rst time
that such a mechanism has been proposed for this
type of conversion in manganese oxides. We believe
it could also apply to other nanowires with tunnel
structures synthesized from birnessite [6, 11 13, 15],
since it rationalizes some common features observed
in such reactions: for example, they all have their one-
dimensional tunnels oriented along the nanowires,
and they all have similarly small nanowire diameters
below 100 nm
possibly a result of reaching a stable
size in response to the transverse stress.
We also explored the effect of varying the
synthesis conditions. At lower temperatures (for
example, 120 °C, see Fig. S-2 in the ESM), birnessite
nanosheets are stable against conversion to
Na0.44MnO2 nanowires even after reaction for several
weeks. With lower NaOH solution concentrations (0.5
1 mol/L), another type of sodium manganese oxide
nanowire with a 2 × 4 tunnel structure is produced
(the Na RUB 7 phase, see Fig. S-3 in the ESM).
Na0.44MnO2 could also be ion-exchanged to Li0.44MnO2
by reaction with a molten salt mixture of 88:12 (molar
ratio) LiNO3: LiCl at 280 °C for 24 h (Fig. S-4 in the
ESM). The nanowire morphology remains intact, and
complete Na removal is indicated by the absence
of Na peaks in the energ dispersive X-ray (EDX)
spectrum.
The catalytic performance of Na0.44MnO2
nanowires for the oxidation of dyes was studied
in the presence of H2O2. Effluents from the textile
industry commonly contain high concentrations of
organic dyes, which are potential environmental and
health threats if not disposed of properly. An effective
and economic approach for their remediation is the
catalytic oxidation with H2O2 using abundant and
nontoxic oxide catalysts, like manganese oxide.
Here cationic pinacyanol chloride dye was selected
as the prototype to investigate the effectiveness of
the Na0.44MnO2 nanowires in the catalytic oxidation
of organic dyes [20]. Its UV vis spectrum shows
absorption peaks centered at 600 nm and 545 nm,
and a weak shoulder visible around 525 nm (Fig.
5). The peak at 600 nm corresponds to the transition
between the electronic ground state and the first
excited state of the monomer, while the two peaks
at shorter wavelength can be assigned to its dimeric
form [21]. Stirring a solution of dye and H2O2
solution with the nanowires resulted in a continuous
decrease in peak intensities. After reaction for 30 min,
the peaks at 600 nm and 545 nm were attenuated to
42% and 27% of their original levels, respectively. By
contrast, Na0.44MnO2 powder obtained from a solid
state reaction [4] showed much poorer reactivity
(dashed line). In a control experiment, when no
nanowire catalyst was added, the spectrum stayed
unchanged after the same reaction period. This
indicates the important role of the nanowire catalyst
in the oxidative decomposition of the dye, and that
any contribution from dye photo-bleaching can be
excluded.
It is believed that the oxidative decomposition
of the dye proceeds via an adsorption-oxidation-
desorption mechanism [20]. Exposed Mn3+ and
Mn4+ ions on the nanowire surface could serve as
the active sites for dye adsorption and catalytic
centers for the dissociation of H2O2 molecules to
free radicals such as HO·, HOO· or O2· [20, 22, 23],
which will immediately oxidize dye molecules
Figure 5 Changes in the visible absorbance spectra of pinacyanol
chloride dye during catalytic oxidation by nanowires with H2O2.
Spectra (solid lines from the top to bottom) were recorded at times
of 0, 10, 20, and 30 min, respectively. Na0.44MnO2 powder prepared
by a solid state reaction was also tested under the same conditions
for comparison; the absorbance spectrum of the dye solution after
treatment for 30 min is shown as a dashed line. The insert shows the
molecular structure of pinacyanol chloride

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in situ. The whole process is surface-limiting and
therefore making materials with high surface areas
like thin nanowires is benefi cial.
In summary, a successful synthesis of Na0.44MnO2
nanowires has been devised. Nanowire growth
was found to involve layered birnessite nanosheet
intermediates which undergo splitting into
nanowires. A stress-induced splitting mechanism is
proposed based on our observations. In addition,
catalytic oxidation of pinacyanol chloride dye by the
nanowires was demonstrated, and their effectiveness
comes from their high surface areas.
Methods
To synthesize Na0.44MnO2 nanowires, 0.5 mmol Mn2O3
(325 mesh, 99%, Sigma-Aldrich) powder was first
mixed with 10 mL of 5 mol/L NaOH (Mallinckrodt)
solution and magnetically stirred for 10 min.
The suspension was then transferred to a 23 mL
autoclave with a Tefl on liner (Parr 4749), and heated
at 200 °C for two days. The reaction was found to
be sensitive to autoclave size and solution filling
level, presumably because oxygen in the autoclave
participates in the reaction. After reaction for two
days, the raw product was repeatedly dispersed
in first water and then ethanol, sonicated and then
centrifuged to remove most of the NaOH. A second
hydrothermal reaction was found necessary to
consume all of the Mn2O3, and the raw product was
mixed again with 10 mL of 5 mol/L NaOH solution
and heated at 200 °C for another two days. The
resulting product was gel-like, and took the shape of
the container. Similar washing steps were adopted as
above. Finally, the product was annealed at 600 °C for
2 h with a ramping rate of 1 °C/min to obtain phase-
pure Na0.44MnO2 nanowires.
X-ray diffraction patterns were recorded on a
Riguka powder diffractometer operating at 40 kV and
25 mA, using Cu K α radiation. Data were collected
with a sampling interval of 0.01°/step and a counting
rate of 1 s/step. Scanning electron microscope images
and associated energy dispersive spectra were
obtained on a Sirion scanning electron microscope,
and transmission electron microscope images,
high-resolution transmission electron microscope
(HRTEM) images and associated selected area
electron diffractions (SAED) patterns were recorded
on a Tecnai TF-20 instrument.
Oxidative decomposition of pinacyanol chloride
dye was carried out at room temperature by
dispersing 25 mg nanowires in 50 mL of 10
L (40 ppm) dye and 5 × 10
The mixture was allowed to react for 30 min with
continuous stirring. UV vis measurements were
conducted on Perkin Elmer Lambda 950 UV vis NIR
spectrometer. Small aliquots of the mixture at various
time intervals were passed through Millipore 0.2 μm
syringe filters (Fisher brand) to remove dispersed
nanowires. The solution was diluted 5 times with
distilled water prior to the measurement.
4 mol/
4 mol/L H2O2 solution.
Acknowledgements
Yiying Wu acknowledges support from the U.S.
Department of Energy under Award No. DE-FG02-
07ER46427 and a Research Corporation Cottrell
Scholar Award.
Electronic Supplementary Material: SEM studies
of nanosheet-to-nanowire conversion, stable
Na-birnessite nanosheets synthesized at lower
temperature, Na-RUB-7 nanowires synthesized with
lower NaOH concentration, and ion-exchanged
Li0.44MnO2 nanowires are available in the online
version of this article at http://dx.doi.org/10.1007/
s12274-009-9003-1 and are accessible free of charge.
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