Titanium sulphide nanoclusters formed within inverse micelles

Article abstract of DOI:10.1016/j.ssc.2006.08.047

High-quality, nano-sized clusters of TiS₂ have been grown successfully inside inverse micellar cages and their optical properties studied. The clusters exhibit large blueshifts in the optical-absorption features with decreasing cluster size due to quantum confinement, affording control of the absorption thresholds and a demonstration of the crossover from band-like to molecule-like spectra as the size of the clusters becomes smaller than that of the exciton in the bulk. Crown Copyright

Full text of DOI:10.1016/j.ssc.2006.08.047

Solid State Communications 140 (2006) 355–358
www.elsevier.com/locate/ssc
Titanium sulphide nanoclusters formed within inverse micelles
David E. Mainwaring, Alexandru L. Let∗, Colin Rix, Pandiyan Murugaraj
School of Applied Sciences, Science, Engineering and Technology Portfolio, Royal Melbourne Institute of Technology, GPO Box 2476V, Melbourne,
VIC-3001, Australia
Received 2 May 2006; received in revised form 28 August 2006; accepted 31 August 2006 by V. Pellegrini
Available online 22 September 2006
Abstract
High-quality, nano-sized clusters of TiS have been grown successfully inside inverse micellar cages and their optical properties studied. The
2
clusters exhibit large blueshifts in the optical-absorption features with decreasing cluster size due to quantum confinement, affording control of
the absorption thresholds and a demonstration of the crossover from band-like to molecule-like spectra as the size of the clusters becomes smaller
than that of the exciton in the bulk.
c
Crown Copyright ꢀ 2006 Published by Elsevier Ltd. All rights reserved.
PACS: 73.61.Tm; 78.55.-m; 83.80.Pc; 61.46+w; 61.82.Rx
Keywords: A. Nanostructures; A. Quantum dots; B. Chemical synthesis
1. Introduction
van der Waals interactions between adjacent layers with S–Ti–S
˚
sandwiches stacked along the c axis (c = 5.6912 A).
Nano-sized clusters are a special class of material which
possesses a variety of unique electronic and chemical properties
as a consequence of quantum confinement, and their very large
surface-to-volume ratios. Such semiconductor nanoclusters are
often referred to as ‘quantum dots’ (QDs). The evolution of
cluster properties with size is an important phenomenon, since
the manifestation of electron and hole confinement gives way
to discrete molecule-like electronic states as the cluster size
decreases below the excitonic Bohr radius. These effects have
been studied experimentally and theoretically, especially in
group II–VI semiconductors (CdS, CdSe, ZnS, ZnSe) and,
to a much lesser extent, in group IV (Si, Ge) and III–V
semiconductors (GaAs) [1].
In its most common form, bulk titanium disulphide
(TiS2) crystallizes in a hexagonal layered structure (P3m¯ 1)
consisting of one hexagonally packed sheet of metal atoms
contained between two hexagonal sheets of chalcogens for
each layer, where atoms within a layer are bound by strong
covalent forces. This structural arrangement leads to metal
atoms surrounded by six chalcogen atoms in an octahedral
environment (Fig. 1), resulting in weak chalcogen–chalcogen
Titanium disulphide possesses one of the highest intercala-
tion energies when lithium diffuses into the van der Waals gap
between sulphur layers and most research has focused on its use
as the cathode material in lithium rechargeable batteries. This
has arisen from the need to produce portable power supplies,
since as computer, medical and other devices become smaller,
there is a need to produce smaller storage batteries to power
them. Other applications include its use as a solid state lubri-
cant in space-borne vehicles, as a hydrogenation catalyst [3],
and as a possible hydrogen storage material [4].
The optical and electronic properties of semiconductor
nanoclusters have recently been studied experimentally and
theoretically, especially in group II–VI semiconductors (CdS,
CdSe, ZnS, ZnSe) [5]. Much of this interest results from
the potential application of nanoscale electronic and optical
devices. Many of these applications exploit the fact that the
properties of these materials can be quite different from those
of bulk semiconductors.
Quantum confinement results in the dense continua of
valence and conduction band electronic states becoming
discrete, well separated states and causes an increase in the band
gap energy [6–8]. Thus, in nanometre-sized semiconductor
particles, the valence and conduction bands are replaced by
∗
Corresponding author. Tel.: +61 2 9428 0124; fax: +61 3 9639 1321.
E-mail address: alexandru@iprimus.com.au (A.L. Let).
c
0038-1098/$ - see front matter Crown Copyright ꢀ 2006 Published by Elsevier Ltd. All rights reserved.
doi:10.1016/j.ssc.2006.08.047

356
D.E. Mainwaring et al. / Solid State Communications 140 (2006) 355–358
by passage through two columns containing anhydrous CaSO4
desiccant (Rowe Scientific Pty Ltd, Australia). Precautions
were taken to avoid the escape of excess H2S into the
atmosphere by attaching an aqueous CuSO4 bubbler to the exit
of the reaction flask.
TiCl4 was dissolved in a degassed ternary tridodecyl-
methylammonium iodide (TDAI)/hexanol/octane (8/8/84% by
weight) inverse micelle solution, at concentrations between
8 × 10−4 to 1 × 10−3 M. Following H2S addition, the mixture
was allowed to stir for 1 h, then the inverse micelle solution
containing the TiS2 nanoclusters was extracted with an equal
volume of acetonitrile. This dissolved the inverse micelles, with
the hexanol and TDAI entering the acetonitrile phase, while the
nanoclusters partitioned between the nonpolar octane phase and
the polar acetonitrile/TDAI/hexanol phase.
Fig. 1. The structure of a TiS cluster [2].
2
manifolds of individual, delocalized electronic states. The
energy shift of the band edge, as well as the separations between
conduction and valence band states, can be larger than 1 eV. In
addition to these delocalized electronic states, semiconductor
nanoclusters can also have localized electron and hole traps
that often accumulate at the nanocluster surface, and the large
surface-to-volume ratio can result in a high density of trap
states. The presence of a high density of localized states along
with finite relaxation rates between localized and delocalized
states can affect the observed nanocluster optical properties
[9].
The preparation of such nanoparticles can be achieved using
micelle structures, which are able to create a reaction template
in the appropriate size range. It is well known that size control
during colloidal growth requires the distinct separation of
nucleation from growth. If nucleation occurs continuously, in
either space or time, a wide range of cluster sizes will inevitably
occur. However, if confinement of the reactant precursors can
be achieved to limit growth to a restricted region of space, a
narrow size distribution of clusters will occur.
2.2. Characterization
High-resolution transmission electron microscopy (HRTEM)
using Philips CM 200 FEG microscope has been used to study
the microstructure of particles. Fluorescence measurements
were carried out on a Perkin-Elmer LS-50B.
3. Results and discussion
According to electronic energy calculations, bulk TiS2 is a
semiconductor with a band gap of about 2 eV and a smaller
indirect gap of 1.4 eV [12]. In agreement with the predictions,
the experimentally measured electronic spectrum of bulk TiS2
exhibits two main absorption bands between 0 and 3.5 eV [13,
14]. The low energy band is centred around 2.3 eV (550 nm)
with a width of 1.5 eV and can be ascribed to electronic
transitions from the upper p-bands into the lower triplet of non-
bonding d-bands followed by a window due to the separation
between the lower and upper non-bonding d bands.
Comparison of the spectrum of the clusters to that of the
bulk crystal reveals a large blueshift for the clusters. The
absorption peak at 360 nm in the cluster spectrum is associated
with the excitonic peak at 550 nm of the bulk, corresponding
to a blueshift of 0.85 eV arising from the quantum-sized
confinement effect of the TiS2 clusters.
Surfactant molecules can form segregated droplet-like
aggregates called inverse micelles whose dimensions are
typically 1–10 nm. The modifier ‘inverse’ refers to the negative
curvature between the surfactant–oil interface (the oil is the
continuous medium). Nucleation only occurs in the micelle
interior because of the total lack of solubility of charged species
in the low dielectric constant inert oils used as the continuous
medium [10,11].
2. Experimental
2.1. Sample preparation
Owing to the presence of I− (from the TDAI) during
the synthesis, it was possible that the 360 nm peak was
due to molecular species rather than nanoclusters, specifically
I−3 /I−5 . Two results indicate that the peak is due to TiS2
nanoclusters rather than I−3 /I−5 . Firstly, the blank spectrum
(synthesis, except without the TiCl4) shows no absorption
at wavelengths >290 nm in either the acetonitrile or octane
phases. Secondly I−3 /I5− is insoluble in the octane phase [15].
These results indicate that while it is possible to incur spectral
contamination in the acetonitrile phase, this does not occur in
the octane phase and the absorption is due exclusively to TiS2
nanoclusters.
The following reagents were used as received: titanium
tetrachloride (TiCl4, Aldrich), iron(II) sulphide 99% (Sigma,
Australia), concentrated hydrochloric acid (32% w/w) (Asia
Pacific Speciality Chemicals Limited, Australia), tridodecyl-
methyl ammonium iodide (TDAI). Titanium tetrachloride and
TDAI were stored in a glovebox.
Typical sample volumes were 10–20 mL. High-purity
solvents and reagents are necessary and all solvents and
reagents were rigorously purified. Octane (Aldrich) was
distilled from sodium, acetonitrile (Aldrich) was distilled from
phosphorous pentoxide and hexanol was distilled from iodine-
activated magnesium.
Fig. 2 shows the optical-absorption spectra of TiS2
nanoclusters dispersed in octane, and the fluorescence emission
spectrum in both the octane and acetonitrile phases. The
fate of photogenerated electron–hole pairs in a semiconductor
Dry gaseous H2S was used as the sulphidizing agent: this
was obtained by reaction of solid FeS with HCl (32% w/w
conc.) and, to prevent water contamination, the gas was dried

D.E. Mainwaring et al. / Solid State Communications 140 (2006) 355–358
357
Fig. 2. Optical absorption spectrum of TiS nanoclusters and emission
2
spectra of these nanoclusters in octane and acetonitrile (excitation wave-
length = 312 nm).
Fig. 4. High resolution electron microscopic image of typical TiS nanocluster.
2
is determined by traps and recombination processes. In
nanoclusters, a large number of atoms are at, or near, the
surface, leading to a preponderance of dangling bonds and
defects which result in surface states. Electron- and hole-traps
may be described by a simple exciton-like model as discussed
for the case of MoS2 and WS2 in previous publications [16].
Fig. 3 shows the variation of the absorption spectrum with
particle size, where (a) is 2 nm, (b) 5 nm TiS2 nanoclusters
obtained by reverse micelle and (c) is TiS2 bulk [13].
While the absorption spectrum of these nanoclusters is the
same in the octane and acetonitrile environments, the emission
studies reveal some differences. The excitation wavelength for
the emission spectrum (Fig. 2) was chosen to be 312 nm. The
fluorescence spectrum of TiS2 nanoparticles in octane had a
peak centred at 450 nm, which appears at 416 nm in acetonitrile.
Since surface plasmon absorption is strongly dependent on the
dielectric constant of the medium, we could not confirm if this
shift is related to the polarity of solvents molecules.
Inset on the top left corner gives the electron diffraction pattern on the TiS
2
nanocluster typically of size ∼3 nm; inset on the top right corner gives the
electron diffraction pattern on a TiS nanocluster that is more crystalline.
2
Table 1
Calculated lattice parameters and particle sizes from the XRD, electron
diffraction and HRTEM data on TiS nanoparticles
2
XRD reflections
FWHM
Particle size
(nm)
Lattice parameters
(nm)
a
c
o
o
a
a
(1,0,1)
(0,0,1)
(1,0,1)
(0,0,1)
0.253
0.163
0.804
0.471
38.00
40.57
11.96
15.21
0.3405
0.3405
0.5612
0.5692
∗
∗
3–5 (HRTEM)
0.3408
0.5701
∗
indicates the parameters are obtained from electron diffraction.
a
The parameters of the TiS obtained by sol–gel [17].
2
Fig. 4 shows a TEM micrograph of a typical TiS2 cluster
deposited on a carbon grid. The indexed electron diffraction
pattern is shown in the inset of Fig. 4. X-ray diffraction showed
that nanoclusters in the size range 2–5 nm produced significant
line broadening, which decreased as the diameter approached
15 nm as shown in Table 1. Table 1 also indicates that
particle sizes calculated from the Debye–Scherrer expression
Fig. 3. Absorption spectrum of: (a) 2–3 nm, (b) 5 nm TiS nanoclusters in octane and (c) TiS bulk [13].
2
2

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D.E. Mainwaring et al. / Solid State Communications 140 (2006) 355–358
change with the h, k, l reflection used, indicating the lamella
morphology of the TiS2 nanoclusters. Generally, the lattice
parameters showed a gradual increase with decreasing particle
size as may be expected from lattice relaxation in nanosized
crystallites, with the co parameter showing significant size
dependence consistent with weak van der Waals forces along
the c axis.
In summary, we have successfully synthesized nano-sized
clusters of the layered semiconductor TiS2 at room temperature
and report their structural and optical characteristics. The large
blueshift in the optical absorption spectrum with decreasing
cluster size affords the potential to tailor band gaps. Significant
photoluminescence is observed at room temperature that is
absent in the bulk material, demonstrating the crossover from
solid-like to molecule-like optical absorption spectra as the size
of the clusters of these non-toxic materials becomes smaller
than that of the exciton in the bulk.
References
[1] A.D. Yoffe, Adv. Phys. 42 (1993) 173.
[2] Y.-S. Kim, M. Masataka, T. Isao, A. Hirohiko, Mater. Trans., JIM 39
(1988) 709.
[3] R.C. Bill, Wear 106 (1985) 283.
[4] J. Chen, S. Li, Z.L. Tao, Y.T. Shen, C.X. Cui, J. Amer. Chem. Soc. 125
(2003) 5284.
[5] L.E. Brus, J. Phys. Chem. 90 (1986) 2555.
[6] M.V. Rama, R.A. Friesner, J. Chem. Phys. 95 (1991) 8309.
[7] P.E. Lippens, M. Lannoo, Phys. Rev. B 39 (1989) 10935.
[8] J.M. Huang, D.F. Kelley, J. Chem. Phys. 113 (2000) 793.
[9] J.P. Wilcoxon, G. Samara, P. Newcomer, Mater. Res. Soc. Symp. Proc.
358 (1995) 277.
[10] J.S. Huang, J. Phys. Chem. B 82 (1985) 480.
[11] D.R. Larson, W.R. Zipfel, R.M. Williams, S.W. Clark, M.P. Bruchez,
F. Wise, W.W. Webb, Science 300 (2003) 1434.
[12] H.W. Myron, A.J. Freeman, Phys. Rev. B 9 (1974) 481.
[13] T.J. Wieting, M. Schlu˝ter, Electrons and Phonons in Layered Crystal
Structures, Reidel, Holland, 1979.
[14] A.R. Beal, J.C. Knights, W.Y. Liang, J. Phys. C: Solid State Phys. 5 (1972)
3540.
[15] R. Doolen, R. Laitinen, F. Parsapour, D.F. Kelley, J. Phys. Chem. B 102
(1998) 3906.
[16] J.M. Huang, R.A. Laitinen, D.F. Kelly, Phys. Rev. B 62 (2000) 10995.
[17] A. Let, Ph.D. Thesis, Royal Melbourne Institute of Technology,
Melbourne.
Acknowledgements
The CRC for Microtechnology is acknowledged for financial
support of the project, together with Ju Lin Peng for assistance
with the HRTEM.