Directly rolling nanosheets into nanotubes

Article abstract of DOI:10.1021/jp037200s

We report a process directly transforming 2D nanosheets into ID nanotubes. Using unilamellar nanosheets as starting materials, we succeeded in obtaining titanium oxide, manganese oxide, and niobate nanotubes (15 - 60 nm in diameter). The transformation was achieved in a simple procedure of an ion intercalation/deintercalation procedure at ambient temperature. The intercalation of Na ions leads to stacking up of the colloidal nanosheets. The gradual deintercalation/extraction of Na ions in water changes the inter-sheet bonding character. The peeled off nanosheets naturally roll up into nanotubes. It is most likely to be a universal process and has outstanding potential in customized syntheses of nanotubes from a wide range of lamellar solids.

Full text of DOI:10.1021/jp037200s

J. Phys. Chem. B 2004, 108, 2115-2119
2115
Directly Rolling Nanosheets into Nanotubes
Renzhi Ma,* Yoshio Bando, and Takayoshi Sasaki
AdVanced Materials Laboratory, National Institute for Materials Science,
Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan
ReceiVed: October 23, 2003; In Final Form: January 5, 2004
We report a process directly transforming 2D nanosheets into 1D nanotubes. Using unilamellar nanosheets as
starting materials, we succeeded in obtaining titanium oxide, manganese oxide, and niobate nanotubes (15-
60 nm in diameter). The transformation was achieved in a simple procedure of an ion intercalation/
deintercalation procedure at ambient temperature. The intercalation of Na ions leads to stacking up of the
colloidal nanosheets. The gradual deintercalation/extraction of Na ions in water changes the inter-sheet bonding
character. The peeled off nanosheets naturally roll up into nanotubes. It is most likely to be a universal
process and has outstanding potential in customized syntheses of nanotubes from a wide range of lamellar
solids.
Introduction
Nanoscale structures have attracted immense interest in recent
years.1 Thanks to the steadily increasing synthetic capabilities,
a so-called “nano-toolbox” filled with a variety of nanocrys-
tallites including nanoparticles/quantum dots (zero-dimension,
-4
0
D), nanotubes/nanorods/nanobelts (one-dimension, 1D), and
1
-6
nanosheets (two-dimension, 2D), have been explored.
Es-
pecially, 1D nanotubes are subjects of particular interest owing
to their unique atomic structures and great potential in electronic,
optical, mechanical, and bioscience applications. Diverse syn-
thetic routes have been directed to 1D nanotube growth. Most
of the synthesized nanotubes, represented by carbon, boron
nitride, oxides, and metal dichalcogenides, commonly possess
layered/lamellar structures.3 The formation of a hollow tube,
consisting of concentric cylinders or scrolls of molecular sheets,
may be modeled as the rolling and folding up of a sheet onto
itself, i.e., a 2D-to-1D structure transformation. It is thus a long
belief that all the lamellar solids could form tubular structures
only if proper conditions are offered.
,4
Figure 1. The architectures of 2D nanosheets of (A) Ti0.91
2
O , (B)
MnO , and (C) Ca Nb
2
2
3 10
O .
A variety of lamellar solids have been successfully exfoliated/
delaminated into single-layer nanosheets through soft chemical
Mallouk and co-workers observed spontaneous
nanotube formation in the exfoliated nanosheets of some
procedures.5
-11
a formation scenario can be really realized, it will be a signifi-
cant breakthrough for nanotube synthetic methods taking in
account the wide availability of a big family of exfoliated 2D
nanosheets.
particular hexaniobates and Ruddlesden-Popper phases of
layered perovskites.1
2,13
The driving force appears to be relief
Here we report a probable generalized method rolling 2D
nanosheets into 1D nanotubes. Three kinds of colloidal
of built-in strain due to noncentrosymmetry architecture in
individual sheets or the change of intrinsic bonding character
between neighboring layers. Some authors have also investigated
6
9
11
nanosheets, Ti0.91O2, MnO2, and Ca2Nb3O10 were used as
the starting materials. Titanium oxide and niobate are useful in
photochemical and photocatalytic applications owing to their
semiconducting nature. Layered manganese oxide with a feasible
redox capability holds promise for a range of applications
including energy storage and new solar cells. In our method, a
simple two-step procedure was explored based on ion intercala-
tion/deintercalation at ambient temperature. The intercalation
of Na ions leads to the stacking of the unilamellar nanosheets.
The gradual deintercalation/extraction of Na ions from the
restacked nanosheets in water induces driving force for nanotube
formation.
the formation of WS2 and VOx nanotubes in hydrothermal
procedures assisted by organic templates.1
4,15
They obtained
nanotubes together with some sheetlike crystallites with lateral
curling or bending, implying a possible “rolling mechanism”
from the sheets to nanotubes. Inspired by these intriguing
discoveries, is it possible to develop a generalized and controlled
procedure to intentionally roll 2D nanosheets into 1D nanotubes
1
6
directly, similar to the way of rolling up a thin film? If such
*
Author to whom correspondence should be addressed. Fax: 81-29-
8
51-6280. E-mail: MA.Renzhi@nims.go.jp.
1
0.1021/jp037200s CCC: $27.50 © 2004 American Chemical Society
Published on Web 01/24/2004

2116 J. Phys. Chem. B, Vol. 108, No. 7, 2004
Letters
Figure 2. The formation of titanium oxide nanotubes. (A) Starting 2D nanosheet (inset) ED pattern. (B) Atomic model of the nanosheet along
010] projection. (C) flocculated nanosheets using NaOH solution. (D,E) Nanosheets after 36 h water washing, note the formation of needle-shaped
[
+
crystallities. (F,G) TEM images of typical straight and conical nanotubes. (H) EDS measurement identifying there is no residual Na . (I) HRTEM
image showing the ∼8 Å interlayer distance in the tube walls. (J) ED pattern of the nanotube. The tube axis is depicted as shown.
Experimental Section
are illustrated in Figure 1. MO6 (M ) Ti, Mn, Nb) octahedra
are combined via edge or corner sharing to produce 2D arrays.
The lateral dimensions of these nanosheets generally range
Materials. All the nanosheets were prepared in a well-
established soft chemical procedure involving proton exchange,
0
.1-1 µm.
+
-
6-11
and subsequent TBA OH intercalation.
Starting lamellar
Transformation. The colloidal nanosheets underwent floc-
culation/restacking by changing ionic strength. Typically, 30
solids of lepidocrocite-type cesium titanate Cs0.7Ti1.82500.175O4
1
7
(
0 represents a vacancy), birnessite-type potassium manganese
3
3
18
cm of colloidal suspension was poured into 30 cm of NaOH
oxide K0.45MnO2, and perovskite-type niobate KCa2Nb3O10
-
3
known as Dion-Jacobson phase)19,20 were synthesized via
aqueous solution (1 mol dm ). Wool-like precipitate was
yielded and the mixture was maintained overnight under stirring.
After filtration, the resulting material was rinsed with copious
(
conventional solid-state calcination. Subsequent acid leaching
converted them into protonated forms of H0.7Ti1.82500.175O4‚H2O,
H0.13MnO2‚0.7H2O, and HCa2Nb3O10‚1.5H2O, respectively. The
resulting protonic product (0.4 g) was shaken vigorously in 100
3
distilled water, and then shaken in 100 cm distilled water. The
distilled water was changed to a fresh one after every 12 h. A
well-dispersed suspension was achieved generally after more
than 36 h shaking. The final product was collected from
centrifugation, and then air-dried at room temperature.
3
+
-
cm of an aqueous tetrabutylammonium hydroxide (TBA OH )
solution at ambient temperature for ∼2 weeks. The obtained
stable colloidal suspensions consist of exfoliated single-layer
nanosheets of hydrated Ti0.91O2, MnO2, and Ca2Nb3O10 with
Characterization. The products were characterized by a
number of different methodologies, including X-ray diffraction
(XRD, Cu KR), scanning electron microscopy (SEM), trans-
molecular thickness of approximately 0.75, 0.60, and 1.44 nm,
respectively.6
,9,11
The architectures of these single nanosheets

Letters
J. Phys. Chem. B, Vol. 108, No. 7, 2004 2117
Figure 3. The formation of manganese oxide nanotubes. (A,B) TEM images of typical straight and conical nanotubes. (C) EDS measurements
+
identifying the absence of residual Na . (D) Typical ED pattern of the MnO
2
nanotube showing the {000l} basal nodes and hexagonal arrangement
sheet along [0001] projection.
of {10 1h 0} spots. The tube axis is generally along [1 2h 10]. (E) Atomic model of the MnO
2
mission electron microscopy (TEM) combined with energy-
dispersive spectroscopy (EDS), and electron energy loss spec-
troscopy (EELS).
crystallites are nanotubes with apparent hollow core. The
nanotubes could be very straight (Figure 2F) or conical (Figure
2G). The diameter of a selected tube was monitored when it
was tilted along the tube axis. The diameter was found to remain
constant for different tilting angles (∼30°) about its axis,
identifying the true tubular nature and not wrinkles of the sheets.
EDS and EELS measurements conclude that there is no residual
Results and Discussion
Figure 2A depicts the typical TEM image of the starting
Ti0.91O2 nanosheets. The nanosheet exhibits a uniform and faint
contrast, reflecting unilamellar thickness. The ED pattern from
a single sheet (inset) indicates the face-centered rectangular unit
+
Na (detection limit 2%) in these nanotubes. Figure 2H gives
the typical EDS measurement, demonstrating the existence of
6
,21
only Ti and O (Ti/O ∼ 1:2; the Cu peaks are derived from the
cell of the single-crystal sheet (a ) 3.783 Å, c ) 2.978 Å).
Figure 2B illustrates the atomic model of the nanosheet along
+
copper grid). Taking into account the possible H , the nanotubes
are likely to be protonic titanium oxide.
[010] projection.
HRTEM images indicate that the tube walls are usually very
The flocculation of the colloidal nanosheets in NaOH solution
thin, only 3∼6 layers. The interlayer distance in the nanotubes
led to a porous aggregate (Figure 2C). The XRD patterns of
the flocculated products were basically comprised of {0k0} basal
series and {h0l} reflections, indicating the absence of a three-
dimension order. This suggests that the flocculation results in
turbostratic restacking of the single-layer nanosheets that
accommodate Na ions and water molecules between the
neighboring sheets.11 On the basis of the microscopic charac-
terization, turbostratic stacks of 5-20 nanosheets appear
dominant in the flocculated product. The morphology of the
product after filtering and shaking in water is shown in Figure
is ∼8.0 Å (Figure 2I), smaller than the (002) inter-planar spacing
6
(
9.4 Å, b ) 18.735 Å) in starting protonated titanate. This might
be due to shrinkage of interlayer distance as a direct consequence
of the loss of hydrated water in high vacuum during microscopic
observations. Figure 2J is the electron diffraction (ED) pattern
corresponding to the nanotube in Figure 2I. Some weak
reflection rings in the pattern are ascribed to the underlying
sheets beneath the tube. The primary diffraction spots with
relatively strong intensities are indexed to be two (020) nodes
at the edge of the zero spot and streaked {h0l} reflections, a
2D,E. The restacked nanosheets were partially delaminated again
2
2
and well dispersed. Most of the dispersed nanosheets exhibit
lateral curling. Some needle-shaped objects are also observed.
TEM observations demonstrate clearly that the needle-shaped
typical diffraction pattern of tubular structures. The tube axis
is perpendicular to the row of (020) reflections, as depicted in
Figure 2J, and the electron beam is incident at the central region

2118 J. Phys. Chem. B, Vol. 108, No. 7, 2004
Letters
Figure 4. The formation of niobate nanotubes. (A,B) TEM images of typical straight and conical nanotubes. (C) Typical EDS measurement. (D)
HRTEM image of a nanotube, the wall layers are separated by a 14.4 Å repeated distance. (E) ED pattern demonstrating that the nanotube may be
2 3 2 3
constructed by wrapping a sheet of Ca Nb O10 along [001] with the tube axis parallel to [100]. (F) Atomic model of Ca Nb O10 projected along
[001].
of the tube (the top and bottom sheets tangent to the tube and
normal to the incident beam) along [010]. It thus can be deduced
that this scroll-type tube may be constructed by wrapping up a
is perpendicular to the row of (000l) reflections. In this ED
pattern, the tube axis appears also generally perpendicular to
[10 1h 0]. We therefore can suggest that the tube axis might be
parallel to [1 2h 10]. A model MnO2 nanotube may be formed
via rolling up the sheet along [0001] normal in a tube axis
parallel to [1 2h 10].
2D sheet along [010] with the depicted tube axis.
Figure 3 shows the nanotubes obtained from MnO2 nanosheets.
There are also two types of nanotubes, straight and conical ones,
were attained (Figure 3A,B). The elemental composition by EDS
Figure 4 shows the TEM characterization of Ca2Nb3O10
nanotubes. Again, both straight and conical nanotubes were
observed (Figure 4A,B). There is also no detectable residual
+
and EELS confirmed the absence of Na in the nanotubes (see
a typical EDS in Figure 3C). The ED pattern in Figure 3D
recorded on an individual nanotube also clearly exhibits
reflection characteristic of a tubular structure. The interlayer
distance in the tube walls is 7.3 Å, consistent with (0001) inter-
planar spacing in layered protonic manganese oxide. The MnO2
nanosheet can be regarded as a 2D array with a hexagonal unit
+
Na (Figure 4C). The HRTEM image of a typical nanotube in
Figure 4D shows the well-resolved layer arrangement in the
tube walls. The interlayer distance is approximately 14.4 Å,
corresponds to the d(001) of protonic niobium oxide.23 The
Ca2Nb3O10 nanosheets are comprised of a square unit cell (a )
b ) 3.854 Å). On the basis of the ED pattern taken from an
individual nanotube (Figure 4E), it can be concluded that the
tube is rolled up from the 2D nanosheet (See atomic model in
Figure 4F) along [001] normal while the tube axis is parallel to
one of the unit cell axis (e.g., [100]). The diffraction spots are
also slightly elongated in the direction perpendicular to the tube
axis.
9
,18
cell (a ≈ 2.90Å).
The atomic model of the nanosheets
projected along [0001] is shown in Figure 3E. The arrangement
of the in-plane {10 1h 0} spots in Figure 3D also evidences the
hexagonal feature (There is an overlap between (0003) and
(10 1h 0) spots due to the close d values: d(0003) ) 2.43 Å,
d(10 1h 0) ) 2.51 Å). It is a common feature that the nodes
degenerate into streaks, indicative of the tubular morphology
and possible helicity.22 The incident electron beam is on the
central region of the nanotube along [0001] and the tube axis
All the three types of nanotubes have an outer diameter of
15∼60 nm and a wall thickness of 3∼6 layers. The dimensions

Letters
J. Phys. Chem. B, Vol. 108, No. 7, 2004 2119
correspond to a scroll from a single 2D sheet with 140∼1130
nm in lateral size, which is consistent with the starting
nanosheets (0.1-1 µm). This indicates that there is no fracture
or breakage of the starting nanosheets during the rolling up. As
a comparison, it is noteworthy that there are some previous
reports on the syntheses of titanium oxide or trititanate (H2Ti3O7)
nanotubes in hydrothermal or sonochemical treatments (110-
intercalation/deintercalation principle at ambient temperature.
The success was not only achieved in titanium oxide nanotubes,
but also in nanotubes of manganese oxides and niobate for the
first time. It directly verifies the assumption of rolling up a
molecular sheet into a cylindrical or conical nanotube. It is also
likely a universal process to yield 1D nanotubes from 2D
nanosheets and applicable to any other lamellar solids only if
the exfoliated nanosheets are available.
-
3
1
50 °C) of TiO2 nanoparticles in concentrated (10 mol dm )
NaOH solution (We recently discovered that the hydrothermal
2
4-27
nanotubes might be also of lepidocrocite nature).
The
Acknowledgment. R.M. acknowledges Drs. H. Tanaka, L.
Wang, and Y. Ebina for providing the colloidal nanosheets of
titanium oxide, manganese oxide, and niobate, respectively. This
work has been partly supported by CREST of JST (Japan
Science and Technology Agency).
diameters of those nanotubes are usually 5-10 nm, significantly
different from the dimensions in the present study.
The formation of the 1D nanotubes from 2D nanosheets may
be explained as follows. The starting unilamellar nanosheets
+
-
were stabilized in a suspension with TBA OH . During the
+
+
flocculation using NaOH solution, Na substituted for TBA .
The colloidal 2D nanosheets became unstable and got restacked
incorporating Na ions. The Na ions were intercalated into the
inter-sheet gallery and played a role in pinning the adjacent
sheets. This resulted in a porous aggregate of fine crystallites
of 5-20 turbostratic layered nanosheets. When shaking in water,
the inter-sheet Na ions were gradually deintercalated/extracted.
This deintercalation process was realized through the exchange
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2
In summary, we succeed in directly rolling 2D nanosheets
into 1D nanotubes. The transformation relies on a simple ion
(