Transformative two-dimensional layered nanocrystals

Article abstract of DOI:10.1021/ja2049594

Regioselective chemical reactions and structural transformations of two-dimensional (2D) layered transition-metal chalcogenide (TMC) nanocrystals are described. Upon exposure of 2D TiS₂ nanodiscs to a chemical stimulus, such as Cu ion, selective chemical reaction begins to occur at the peripheral edges. This edge reaction is followed by ion diffusion, which is facilitated by interlayer nanochannels and leads to the formation of a heteroepitaxial TiS₂-Cu₂S intermediate. These processes eventually result in the generation of a single-crystalline, double-convex toroidal Cu₂S nanostructure. Such 2D regioselective chemical reactions also take place when other ionic reactants are used. The observations made and chemical principles uncovered in this effort indicate that a general approach exists for building various toroidal nanocrystals of substances such as Ag₂S, MnS, and CdS.

Full text of DOI:10.1021/ja2049594

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pubs.acs.org/JACS
Transformative Two-Dimensional Layered Nanocrystals
†
†
†
†
‡
,†
Sohee Jeong, Jae Hyo Han, Jung-tak Jang, Jung-wook Seo, Jin-Gyu Kim, and Jinwoo Cheon*
†
Department of Chemistry, Yonsei University, Seoul 120-749, Korea
‡
Division of Electron Microscopic Research, Korea Basic Science Institute, Daejeon 305-333, Korea
S Supporting Information
b
2
,3
theory to material synthesis and device fabrication. The
structural characteristics of 2D TMC nanomaterials, including
the anisotropy associated with the large aspect-ratio differences
between edges and planes and the weakly interacting layered
ABSTRACT: Regioselective chemical reactions and struc-
tural transformations of two-dimensional (2D) layered
transition-metal chalcogenide (TMC) nanocrystals are de-
3aꢀe
scribed. Upon exposure of 2D TiS nanodiscs to a chemical
structure that enables intercalation,
have been actively
2
stimulus, such as Cu ion, selective chemical reaction begins
to occur at the peripheral edges. This edge reaction is
followed by ion diffusion, which is facilitated by interlayer
nanochannels and leads to the formation of a heteroepitaxial
TiS ꢀCu S intermediate. These processes eventually result
investigated. However, the solid-state chemical reactivity of these
materials remains relatively unexplored.
4
In the study described herein, we have uncovered a unique
regioselective anisotropy of 2D layered TMCs that governs the
initial chemical reactivity and subsequent structural evolution
processes. As shown in Figure 1a, conventional shape-transforma-
tive reactions of spherical nanocrystals usually lead to the forma-
2
2
in the generation of a single-crystalline, double-convex
toroidal Cu S nanostructure. Such 2D regioselective chemi-
2
5
cal reactions also take place when other ionic reactants are
used. The observations made and chemical principles un-
covered in this effort indicate that a general approach exists
for building various toroidal nanocrystals of substances such
tion of hollow nanostructures having concavo-convex curvature.
In contrast, we have discovered that layered 2D structures behave
differently in transformative processes in that they give structurally
unique double-convex curvature (Figure 1b). This phenomenon is
driven by the existence of regioselective edge reactions and ion
migration through nanochannels between the layers.
as Ag S, MnS, and CdS.
2
The morphology resulting from these processes is a toroid
Figure 1c), a basic, highly symmetric, double-convex geo-
(
anoscale chemical reactivity forms an important basis for
metrical structure with a hole in its center like a doughnut.
N
many characteristics of nanomaterials ranging from catalytic
1
efficiency to structural transformations. Recently, 2D layered
nanomaterials, such as graphene analogues and transition-metal
chalcogenides (TMCs), have attracted significant interest from
Figure 2. TEM images of TiS discs and transformed Cu S toroids.
2
2
(a, b) Low-resolution and magnified images of TiS
2
disc nanocrystals (c)
2
Side view of TiS discs stacked together. (d) HRTEM image and (inset)
FFT pattern of TiS . (e, f) Low-resolution and magnified images of Cu S
2
2
toroids. (g) Side view of Cu
2
S toroids stacked together. (h) HRTEM
image and (inset) FFT pattern of Cu
2
S. (i) TEM image and schematic
Figure 1. Void curvatures in transformed nanocrystals. (a) Concavo-
convex curvature from a sphere to a hollow sphere. (b) Double-convex
curvature from a layer-structured disc to a toroid. (c) Schematic
illustration of 2D layered nanocrystals reacting with incoming ions
regioselectively to form a toroid as the final product.
drawing of a Cu S toroid exhibiting double-convex curvature.
2
Received: May 30, 2011
Published: August 29, 2011
r 2011 American Chemical Society
14500
dx.doi.org/10.1021/ja2049594
|J. Am. Chem. Soc. 2011, 133, 14500–14503

Journal of the American Chemical Society
COMMUNICATION
Figure 3. TEM images and XRD patterns of transformative nanostructures reflecting the effects of Cu ion on TiS . (a) TEM image and XRD pattern of a
2
pure TiS
CuCl /TiS
respectively. (e) Large-area image of the intermediate TiS
between TiS and Cu S showing that the (110) planes of Cu S with a 1.9 Å lattice fringe are oriented in the same direction as the (110) planes of TiS
2
nanodisc. (bꢀd) TEM images and XRD patterns from structures obtained by reactions carried out at different molar ratios of reactants: (b)
= 1; (c) CuCl /TiS = 2; (d) CuCl /TiS = 4. In the XRD patterns, the green squares and red stars represent peaks from TiS and Cu S,
ꢀCu S structure obtained with CuCl /TiS = 1. (f) High-magnification image at the interface
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
with a 1.7 Å lattice fringe. The FFT pattern (inset) shows green and red dots corresponding to TiS
2
and Cu
2
S, respectively. (gꢀi) Side-view TEM image
(
g) and EELS elemental analysis (h, i) of an intermediate TiS S structure. Ti is shown in green and Cu in red. (j) Schematic rotation of the
2
ꢀCu
2
intermediate structure by 90ꢀ, showing both planar and side-view images.
Because of their unique shape, toroids have been observed to
exhibit unusual optical and magnetic properties. Although tor-
crystalline toroid has lattice fringes of 3.3 and 1.9 Å (Figure 2h),
which are consistent with the (100) and (110) planes of Cu2S
and the hexagonal pattern of the FFT (Figure 2h inset). A side
6
7
oidal structures in the bulk or micrometer-size regime have been
8
9
fabricated, their nanoscale structures have been rarely observed.
view of the Cu S toroid indicates that it possesses double-convex
2
With the aim of exploring the synthetic accessibility of
nanoscale toroidal structures through the use of transformative
reactions, we prepared 2D disc-shaped TiS2 nanocrystals
consisting of SꢀTiꢀS layers stacked in the Æ001æ direction.
curvature (Figure 2i).
These chemical transformation processes are highly sensitive to
reaction conditions such as the temperature, time, and stoichiom-
etry of the reactants. For example, at lower temperature (100 ꢀC),
no significant chemical or morphological transformation occurs.
However, at 200 ꢀC, time-dependent morphological changes are
clearly observed (Figure S2). When the molar ratio of Cu ion to
TiS is 2:1, nucleation of Cu S was observed on the edges of the
The TiS nanodiscs were generated by adding carbon disulfide
2
(
CS ) to a mixture of titanium(IV) chloride and oleylamine
2
at 300 ꢀC. Transmission electron microscopy (TEM) analysis
(
Figure 2aꢀd) reveals that TiS produced in this manner is a
2
2
2
single crystalline disc having a diameter of 150 nm and a
thickness of 10 nm with lattice fringes of 2.9 Å for the (100)
planes and 1.7 Å for the (110) planes. Its single crystallinity
was further confirmed by the regular hexagonal patterns seen
in the fast Fourier transform (FFT) of a high-resolution TEM
discs within 30 min. After 1 h, small voids gradually appear in the
center of the discs, and within 2 h, enlarged voids are apparent and
the toroidal Cu S structure is produced. However, under these
2
conditions, the formation of a well-defined and single-crystalline
toroidal structure is not optimized.
(
HRTEM) lattice image (Figure 2d inset) and an electron
To achieve a completely transformative reaction, the amount
of reactant molecule relative to the template nanocrystal is
important. Using TEM and XRD, we monitored the dependence
of these transformative reactions on the amount of reactant at
200 ꢀC for 30 min (Figure 3aꢀd). When the molar ratio of Cu
ion is low (CuCl /TiS = 1), darkened regions indicating Cu S
diffraction (ED) pattern (Figure 4k).
The nanodiscs were used as a template for chemical transfor-
mations. CuCl was added to TiS nanodiscs in oleylamine, and
2
2
the mixture was heated at 200 ꢀC for 30 min. During this process,
the initial blue-black solution turned to a brown color, and the
product was then precipitated by adding a mixture of n-butanol
and hexane. TEM analysis (Figure 2eꢀh) indicates that the
product has a toroidal morphology. The exterior diameter of
the toroids is 160 nm, and the ring thickness is 31 nm
2
2
2
formation are confined primarily to the edges of the TiS (see
2
the TEM image in Figure 3b). Simultaneously, peaks from Cu2S
(red f in Figure 3b) are seen along with peaks of TiS
2
10
(green 9) in the XRD pattern. As the CuCl /TiS ratio is
2
2
(
Figure 2eꢀg). Elemental analysis using electron energy loss
increased to 2, the thickness of the dark-contrast edge region
increases in the TEM image and the intensities of the XRD peaks
spectroscopy (EELS) and X-ray diffraction (XRD) confirm that
the toroidal nanocrystals are comprised of pure Cu S (Figure S1
in the Supporting Information). The results show that the single-
associated with Cu S become larger (Figure 3c). Eventually,
2
2
when the CuCl /TiS ratio is increased to 4, the transformation
2
2
1
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dx.doi.org/10.1021/ja2049594 |J. Am. Chem. Soc. 2011, 133, 14500–14503

Journal of the American Chemical Society
COMMUNICATION
Figure 4. Side-view TEM images and structural changes from 2D discs to toroids. (aꢀe) TEM images of nanostructures obtained from reactions carried
out with different ratios of Cu ion (CuCl /TiS = 0, 0.5, 1, 2, and 4). The right-hand TEM images are false-colored as pink for Cu S and green for TiS for
clear visibility. The white dashed lines present the formation of the double-convex curvature of Cu S. (f) HRTEM image of layered TiS with lattice
fringes of 5.7 Å corresponding to the (001) plane. (g) HRTEM image of the structure obtained with CuCl /TiS = 0.5. The (002) planes of Cu S are
oriented parallel to the (001) planes of TiS . (h) HRTEM image of the structure obtained with CuCl /TiS = 1. The (001) planes of TiS , (002) planes
2
2
2
2
2
2
2
2
2
2
2
2
2
of Cu
of the structure obtained with CuCl
transformation of TiS discs to Cu S toroid structures via TiS
2 2
S, and Cu S layers elongated along the Æ102æ and Æ104æ directions with lattice fringes of 2.4 and 1.5 Å, respectively, are shown. (i) HRTEM image
0
0
2
/TiS
2
= 4, showing single-crystalline Cu
2
S with (002) lattices of 3.3 Å. (a ꢀe ) Schematic diagrams of the
00 00 0 0
2
2
2
2
ꢀCu S intermediates. (a ꢀe ) Cross-sectional views of (a ꢀe ). Green represents TiS
2
and red Cu S. (j) Plot showing the change in thickness of TiS and Cu S with increasing CuCl /TiS ratio. (k, l) ED patterns of TiS and Cu S,
2
2
2
2
2
2
2
respectively. Both ED patterns indicate single crystallinity.
to a toroidal shape becomes complete, as shown by the observation
that the peaks in the XRD pattern corresponded only to Cu2S
same time, the thickness of Cu S on the rim increases to 15 nm.
As the amount of Cu ion is increased further, continued decrease
2
(Figure 3d). The results of additional TEM analysis of intermediate
in both the thickness and diameter of the TiS template take
2
structures, in which Cu SandTiS coexist, indicate that Cu Sgrowth
place in concert with the growth of the Cu S rim (Figure 4d)
2
2
2
2
on the TiS template occurs in a heteroepitaxial manner (Figure 3e,
until a pure toroidal Cu S structure is formed (Figure 4e).
2
2
f). The interface between Cu S and TiS forms a well-aligned
During the formation of the toroidal Cu S structure, indicated by
2
2
2
hexagonal set of red and green spots for the (110) plane in the
FFT (Figure 3f inset) with lattice fringes of 1.9 and 1.7 Å (Figure 3f).
An EELS elemental scan analysis (Figure 3gꢀi) of a side view of the
structure displays a dark contrast in the center of the structure
white dashed lines in Figure 4cꢀe, the double-convex curvature
of Cu S becomes apparent. The size changes that occur under
2
the explored reaction conditions are summarized in Figure 4j.
During these processes, heteroepitaxial growth continues to take
place under the conditions described above, and well-defined
representing Ti (green) in the TiS layers and a surrounding lighter
2
region representing Cu (red) from the outer rims of Cu2S.
interfacial fringes, including (001) of TiS and (002), (102), and
2
To provide a better understanding of the chemical processes
of these morphological transformations, detailed TEM analyses
on side views of the products isolated under various reaction
conditions (CuCl /TiS ratios of 0.5, 1, 2, and 4) were carried
(104) of Cu S (Figure 4h), are clearly seen.
2
A side-view TEM image (Figure 4i) and the results of ED
measurements (Figure 4l) show that the toroidal Cu S generated
2
in the manner described above possesses single crystallinity with
a well-defined (002) lattice fringe. The observations suggest that
selective chemical exchange takes place on the edges of the 2D
nanodiscs and that corresponding anion migration via nano-
channels facilitates the shape transformation processes. Specifi-
2
2
out. Upon introduction of a relatively small amount of Cu ion,
(
CuCl /TiS ratio of 0.5), edge nucleation and growth of Cu S
2
2
2
appear in the TEM images as gray contrast (pink in the color-
coded image) (Figure 4b). The HRTEM view given in Figure 4g
shows that heteroepitaxial growth of Cu S takes place on the
cally, the reactive edges of the TiS template are first nucleated
2
2
0
edge of TiS , where a 3.3 Å (002) lattice fringe of Cu S is
with Cu S (Figure 4b ). Next, the interfacial exchange process
2
2
2
observed parallel to the 5.7 Å (001) fringe of the TiS template.
Clearly, these reactive peripheral positions of TiS2 serve
involving Cu and Ti ions proceeds, and sulfur ions migrate
through nanometer-sized channels. These events promote inward
2
as selective nucleation and growth sites because of the higher
Cu S growth, eventually producing the double-convex curvature
2
11
0
00
0
00 0 00
surface energies of the edge faces of TiS2. As the CuCl /TiS2
(Figure 4c ,c ,d ,d ,e ,e ). Importantly, the morphological
changes observed in this process are clearly different from those
involved in generating the concavo-convex hollow structures that
2
ratio is increased to 1, the thickness of TiS template decreases
2
from 10 to 6.5 nm, and the diameter decreases from 150 nm (the
5
original size of the TiS template) to 120 nm (Figure 4c). At the
have typically been observed in previous studies.
2
1
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Journal of the American Chemical Society
COMMUNICATION
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2
2
2
(b) TiS
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ꢀMnS; (c) CdS. The insets show higher-magnification images.
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2
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2
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2
of CdS, respectively (Figure 5).
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mechanisms in the structural transformation of 2D layered
nanocrystals are unprecedented. In addition, we have demon-
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synthesis of inorganic toroidal nanocrystals. This study has not
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ASSOCIATED CONTENT
9
3, 083904.
S
Supporting Information. Details of synthetic methods,
b
(7) (a) Ross, C. A.; Castano, F. J.; Morecroft, D.; Jung, W.; Smith,
elemental mapping, and XRD patterns. This material is available
free of charge via the Internet at http://pubs.acs.org.
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’
AUTHOR INFORMATION
(8) (a) Lu, G.; Li, W.; Yao, J.; Zhang, G.; Yang, B.; Shen, J. Adv.
Corresponding Author
jcheon@yonsei.ac.kr
Mater. 2002, 14, 1049. (b) Balzer, F.; Beermann, J.; Bozhevolnyi, S. I.;
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ACKNOWLEDGMENT
1
26, 10830. (b) Hobbs, K. L.; Larson, P. R.; Lian, G. D.; Keay, J. C.;
Johnson, M. B. Nano Lett. 2004, 4, 167. (c) Lee, E.; Kim, J.-K.; Lee, M.
This study was financially supported by the Creative Research
Initiative (2010-0018286), WCU (2008-9-1955), BK21, and
KBSIꢀHVEM (JEMꢀARM 1300S). We thank M.-C. Kim and
Professor E. Sim for surface energy calculations.
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(10) TiS , JCPDS no. 15-0853; Cu S, ICSD no. 16242.
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that of the planar face (001).
2
are ∼4.5 times larger than
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(
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