Thio sol-gel synthesis of titanium disulfide thin films and nanoparticles using titanium(IV) alkoxide precursors

Article abstract of DOI:10.1016/j.jpcs.2007.03.001

Titanium tetraisopropoxide reacts with hydrogen sulfide in butylamine solvent at room temperature to form an amorphous titanium alkoxy-sulfide which can be converted to crystalline titanium disulfide by heat treatment in a flowing hydrogen sulfide gas stream. The reaction has been studied using infrared and Raman spectroscopy, gas chromatography-mass spectrometry, X-ray diffractometry and energy-dispersive X-ray analysis measurements. Based on these studies, it is shown that a partially thiolysed alkoxide precursor forms through the replacement of a limited number of alkoxy groups by hydrosulfide moieties. This alkoxy-hydrosulfide is believed to form following a thiolysis-condensation mechanism similar to the hydrolysis-condensation process that occurs during the corresponding oxide sol-gel reaction. The alkoxy-hydrosulfide species can then be completely thiolysed at 800 °C in a stream of hydrogen sulfide to yield pure, hexagonal titanium disulfide in either film or particulate form.

Full text of DOI:10.1016/j.jpcs.2007.03.001

ARTICLE IN PRESS
Journal of Physics and Chemistry of Solids 68 (2007) 1428–1435
www.elsevier.com/locate/jpcs
Thio sol–gel synthesis of titanium disulfide thin films and nanoparticles
using titanium(IV) alkoxide precursors
Ã
Alexandru L. Let, David E. Mainwaring , Colin J. Rix, Pandiyan Murugaraj
School of Applied Sciences, Royal Melbourne Institute of Technology, G.P.O. Box 2476V, Melbourne 3001, Australia
Received 19 December 2006; received in revised form 5 March 2007; accepted 5 March 2007
Abstract
Titanium tetraisopropoxide reacts with hydrogen sulfide in butylamine solvent at room temperature to form an amorphous titanium
alkoxy-sulfide which can be converted to crystalline titanium disulfide by heat treatment in a flowing hydrogen sulfide gas stream. The
reaction has been studied using infrared and Raman spectroscopy, gas chromatography–mass spectrometry, X-ray diffractometry and
energy-dispersive X-ray analysis measurements. Based on these studies, it is shown that a partially thiolysed alkoxide precursor forms
through the replacement of a limited number of alkoxy groups by hydrosulfide moieties. This alkoxy-hydrosulfide is believed to form
following a thiolysis–condensation mechanism similar to the hydrolysis–condensation process that occurs during the corresponding
oxide sol–gel reaction. The alkoxy-hydrosulfide species can then be completely thiolysed at 800 1C in a stream of hydrogen sulfide to yield
pure, hexagonal titanium disulfide in either film or particulate form.
r 2007 Elsevier Ltd. All rights reserved.
PACS: 81.20.Fw
Keywords: A. Chalcogenide; A. Thin films; B. Sol–gel growth; C. X-ray diffraction
1. Introduction
This intercalate also possesses good electronic conductivity
and a high reversibility of the intercalation reaction. To
maximize lithium ion diffusion into titanium disulfide, a
large surface area is required such as in thin film or fine
particulate forms. In this type of application, it is highly
desirable that the film of titanium disulfide has a crystal-
lographic orientation such that the c-axis is parallel to the
plane of the substrate. In this arrangement, pores in the
crystals are perpendicular to the plane of the substrate and
are optimally oriented for the intercalation of lithium,
which constitutes the primary discharge reaction within a
lithium battery [4a]. Use of stoichiometric titanium
disulfide is imperative for battery use, as metal-rich
titanium sulfide has titanium located in the van der Waals
gap which can reduce the ability to intercalate [4b,4c]. All
these factors are strongly influenced by the chemical
methods and processing conditions used to synthesize the
material.
Transition-metal dichalcogenides have been studied
extensively as an important class of materials that exhibit
a wide range of electronic and optical properties [1]. For
example, titanium disulfide has been identified as an active
cathode material in lithium-based rechargeable batteries
[2]. This is mainly due to the facile reversibility of ion
intercalation, its high electronic conductivity and large
current capacity. The battery performance of titanium
disulfide as a cathode can be further improved if the
morphology and surface area can be optimized and the
material is synthesized in high purity [3]. As computer,
medical and other devices become smaller, there is a need
to produce smaller storage batteries to power such devices.
The layers of the titanium disulfide lattice exhibit a strong
tendency to intercalate lithium ions into the crystal matrix
up to a Li:Ti stoichiometry of 1:1, without phase changes.
Conventionally, titanium disulfide has been prepared by
the direct reaction of the elements at temperatures above
1000 1C [5]. This process requires a long processing time
Ã
Corresponding author. Tel.: +61 3 9925 4784; fax: +61 3 9925 4311.
E-mail address: david.mainwaring@rmit.edu (D.E. Mainwaring).
0022-3697/$ - see front matter r 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jpcs.2007.03.001

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and yields low surface area particles. Several vapor phase
reactions have also been investigated for the synthesis of
titanium disulfide powders and thin films, the most
common being the reaction of titanium tetrachloride with
hydrogen sulfide or other sulfur sources, including tert-
butylthiol and bis(trimethylsilyl) sulfide [6–9]. Titanium
disulfide thin films have also been fabricated by dual-source
chemical deposition techniques using tetrakismethylami-
dotitanium as precursor for titanium and hexamethyldisi-
lathiane, 2-methylpropanethiol and tertiary butyldisulfide
as sulfur precursors [9]. Recently, tetrakis(tert-butylthiolato)
titanium(IV) was used as a single-source precursor for
fabricating titanium disulfide thin films in a modified
chemical vapor deposition process which gave amorphous
films of titanium disulfide at low temperatures [10,11].
More recently, titanium-pyridine and pyridine-thiolates
have been used as precursors to synthesize titanium
disulfide [12]. Room-temperature chemical precipitation
routes, primarily using chlorides and various inorganic [13]
and organic [14] thiolysing agents have also been success-
fully implemented to synthesize fine particles of titanium
disulfide. These precipitates are often contaminated by
chloride ions as well as inorganic by-products of the
reaction. Mainwaring et al. [15] have recently employed an
emulsion-based titanium tetrachloride precipitation tech-
nique to prepare high-purity titanium disulfide nanosized
clusters that exhibited large blueshifts in their optical-
absorption features with decreasing quantum confining
cluster size.
Metal alkoxides are characterized by highly reactive
alkoxy (–OR, R ¼ alkyl) groups which make the metal
atoms susceptible to nucleophilic attack [16]. These
compounds undergo hydrolysis in the presence of water,
followed by condensation and polymerization reactions
which lead to the formation of the corresponding metal
oxide. The hydrolysis and condensation reactions in
solution can be controlled by the presence of an acid or a
base catalyst, leading to the formation of either a polymeric
gel or a fine powder [17]. Melling [18] has demonstrated the
applicability of alkoxides for the synthesis of sulfides by
reacting germanium alkoxide with hydrogen sulfide.
The sol–gel process is a promising method for the
synthesis of transition-metal dichalcogenides such as
titanium disulfide. One of the most important advantages
of the sol–gel process is that it provides an opportunity to
control the microstructure of sulfides for different applica-
tions. A thio ‘sol–gel’-type process has been reported for
the synthesis of titanium disulfide involving the reaction of
titanium alkoxide (e.g., titanium tetraisopropoxide [19,20]
or a titanium thiolate [21]) with hydrogen sulfide gas in
aromatic hydrocarbon solvents, such as toluene and
benzene. However, the reaction products were mixtures
of both metal oxides and metal sulfides, indicating further
precautions were needed to eliminate the presence of water.
In this paper, we present the synthesis of titanium disulfide
nanoparticles and films using a thio ‘sol–gel’ process
involving the reaction of titanium tetraisopropoxide with
hydrogen sulfide in an aliphatic amine solvent medium.
The reaction products were then characterized by various
techniques before and after heat treatment.
2. Experimental
Titanium tertaisopropoxide (Aldrich) was used as
received without further purification, butylamine (BuNH2;
Aldrich) was dried over potasium hydroxide pellets.
Hydrogen sulfide was used as the thiolysing agent,
obtained by reaction of solid ferrous sulfide (Rowe
Scientific) with hydrochloric acid (Rowe Scientific). All
reactions were conducted in glassware dried under vacuum,
and all manipulations of reactants and products were
accomplished under a nitrogen atmosphere.
In a typical preparation, titanium tetraisopropoxide was
dissolved in butylamine, under a dried nitrogen atmosphere
in a glove bag. The solvent:alkoxide ratio was 95:5 (v/v)
and hydrogen sulfide (H2S) gas, dried by passage through a
column of calcium sulfate desiccant, was bubbled through
the solution at room temperature. As soon as hydrogen
sulfide was introduced, the colorless titanium tetraisoprop-
oxide solution turned black, but the hydrogen sulfide flow
was continued for 3 h to ensure complete reaction.
Although the solution became viscous, no solid was
observed to form. Indeed, the solution was stable to
precipitation for at least 1 month. Solvent removal from
the black solution was achieved under vacuum yielding an
isolated solid which was pyrophoric and very air sensitive.
The parent precursor solution was used to deposit a thin
film by spin coating (3500 rpm and 30 s) onto silicon or
quartz substrates under a nitrogen atmosphere. After
drying in a stream of nitrogen, the films were heat-treated
in flowing hydrogen sulfide at temperatures from 600 to
800 1C for 6 h, using heating rates of 50 and 75 1C/h. The
flow chart in Fig. 1 provides a summary of the route used
to prepare thin-film titanium disulfide as well as its
characterization.
Elemental analysis of the product was performed with an
energy-dispersive X-ray analysis (EDAX) attachment on a
scanning electron microscope. The yellow oil trapped from
the reaction solution was analyzed by gas chromatogra-
phy–mass spectrometry (Hewlett Packard 5890). The
infrared spectra of particulate samples were obtained
between 4000 and 400 cmꢀ1 (potassium bromide pellet)
using a Perkin-Elmer System 2000 FT-IR spectrometer.
The Raman spectra of the as-prepared sample, and the
titanium disulfide obtained after heat treatment at 800 1C
for 6 h, were measured using a Reinshaw System 2000
equipped with a Peltier-cooled charge-coupled device
(CCD) for detection of Raman scattered light. X-ray
diffraction (XRD) patterns of the powders and thin films
were acquired from 2y ¼ 101 to 901 on a D8 Advance
X-ray diffractometer (Bruker) using Cu Ka radiation.
A Philips XL-30 scanning electron microscope was used to
examine the morphology of samples mounted on a graphite
surface. Transmission electron micrographs and electron

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Fig. 1. Schematic diagram showing the procedure for the preparation of titanium disulfide powders and thin films.
diffraction results were obtained on a Jeol 1010 transmis-
the average titanium disulfide nanocrystal domain-size in
terms of the Debeye–Scherer expression, D ¼ 0:9l=
ðb cos yÞ, where D is the average nanocrystal domain
diameter in angstrom, b is the corrected band broadening
(full-width at half-maximum (FWHM)), l is the X-ray
sion electron microscope (TEM). Samples of titanium
disulfide particulates for TEM examination were prepared
by ultrasonication in tetrahydrofuran for 5 min and placing
a few drops of the suspension onto a carbon grid.
˚
wavelength (1.5406 A) and y is the diffraction angle [22].
3. Results
The average diameter thus estimated for the titanium
disulfide nanoparticles, prepared in butylamine and annealed
at 800 1C for 6 h was found to be 27 nm and varied up
to about 32 nm when other amine solvents such as
di-butylamine were used in place of butylamine. All the
peaks in the titanium disulfide X-ray powder pattern could
be indexed to the hexagonal titanium disulfide phase, as
reported in the reference literature [23]. TEM studies
showed that these crystallites grew and agglomerated on
heating for periods of 6 h to yield aggregate particles that
ranged up to about 75 nm.
3.1. Microstructural studies
The as-prepared powder obtained at room temperature
was amorphous as indicated by the XRD pattern shown in
Fig. 2a. After heat-treatment and thiolysing at 600 1C,
hexagonal titanium disulfide containing a small amount of
titanium dioxide rutile and anatase phases was observed.
At 700 1C, the proportion of titanium disulfide increased at
the expense of the oxide phases, and at 800 1C phase-pure
titanium disulfide was formed (see Figs. 2b–d). The
broadening of the diffraction peaks was used to estimate
Fig. 3a shows the XRD pattern of a thin layer of
titanium disulfide prepared on a silicon substrate after

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1431
Fig. 3. XRD patterns of (a) thin film and (b) powder indicate the
hexagonal phase of titanium disulfide. (c) Literature stick pattern of
hexagonal titanium disulfide [23].
Fig. 2. XRD patterns of powder (a) as-prepared and (b) after heat
treatment at 600 1C (6 h), (c) 700 1C (6 h), and (d) 800 1C (6 h).
heating at 800 1C for 6 h, under a hydrogen sulfide
atmosphere. Three sharp peaks originating from the
hexagonal titanium disulfide thin films (reflections from
(0 0 1), (1 0 0) and (0 0 1)) were observed along with the
reported polycrystalline XRD data, indicative of the
preferential orientation of the thin films. This is in
agreement with earlier reports which indicate a variation
of intensity ratio of the XRD peaks with film thickness [24].
Thin films deposited on quartz substrates consisted of
slightly deformed plate-like crystallites that grew inclined
to the substrate similar to titanium disulfide films obtained
from CVD techniques [25], which may be attributed to
preferential orientation of crystal planes with growth of the
(0 0 1) plane more dominant than that of the corresponding
(1 0 1) and (1 1 0) planes. The films on glass substrates
showed a mosaic of crystallites as shown in Fig. 4, similar
to those obtained from the aerosol-assisted CVD deposi-
tion of tetrakis(tert-butylthiolato)titanium(IV) [26], the
form of which can be attributed to stronger cohesive
Fig. 4. Scanning electron micrograph of titanium disulfide thin films cast
on glass followed by heat treatment and thiolysed at 800 1C for 6 h.
interactions occurring between the deposited molecules
than between the film and substrate. Reported lattice
˚
parameters for bulk titanium disulfide are a0 ¼ 3.4049 A

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˚
and c0 ¼ 5.6912 A (c0/a0 ¼ 1.671) [27], whereas the lattice
parameters of the particulate titanium disulfide obtained
by the present thio sol–gel process were different with
isopropoxy groups, since the doublet seen at 1377 and
1360 cmꢀ1 is characteristic of the gem-dimethyl structure
present in the isopropoxy group. The samples also
exhibited weak absorption bands at 623 (very broad),
853, 1007 and 1072 cmꢀ1, which are assignable to the
isopropoxy group bonded to titanium [29]. The broad
absorption at 2200–3600 cmꢀ1 is consistent with the NH+3
group of amine salts [7]. Fig. 5b shows the IR spectrum of a
thin film of the precursor solution. For comparison, the IR
spectra of titanium tetraisopropoxide (Fig. 5c), butylamine
(Fig. 5d) and butylamine with hydrogen sulfide (Fig. 5e)
are also shown. Raman measurements (Fig. 6) on the
titanium sulfide powder show characteristic vibrations at
about 290–300 cmꢀ1 which can be attributed to a Ti–S
stretch [13,29] along with absorptions for the Eg and A1g
vibrational modes at 232 and 336 cmꢀ1 [19]. Fig. 6a shows
the Raman spectrum of the as-prepared powder obtained
from the reaction of titanium tetraisopropoxide with
hydrogen sulfide in butylamine, Fig. 6b shows the Raman
spectrum of the sample after heat treatment at 800 1C for
6 h, and Fig. 6c presents the spectrum of titanium
tetraisopropoxide for comparison. The EDAX data for
the as-prepared material gave a S/Ti ratio of 1.7 over a
number of sampling areas (see Fig. 7a) and a ratio of 2.0
for thin film (b) and powder sample (c) obtained after heat
treatment at 800 1C for 6 h.
˚
˚
a0 ¼ 3.4064 A and c0 ¼ 5.6892 A. Similar lattice dilation
has also been observed in nanoparticles of other materials
such as cadmium sulfide [28] and was suggested in 1930 by
Lennard-Jones, who predicted such an effect for very small
particles.
3.2. Spectral studies
Fig. 5 shows the FT-IR spectrum of the as-prepared
solid powder obtained from the reaction of titanium
tetraisopropoxide with hydrogen sulfide in butylamine as
solvent. The spectrum indicates that the powder contains
Fig. 5. Infrared spectrum of (a) the as-prepared powder, (b) thin film from
the reaction of titanium tetraisopropoxide with hydrogen sulfide in
butylamine, (c) titanium tetraisopropoxide, (d) butylamine and (e)
butylamine+hydrogen sulfide.
Fig. 6. Raman spectra of (a) as-prepared powder, (b) titanium disulfide
powder obtained after heat treatment and thiolysed at 800 1C (6 h) and
(c) titanium tetraisopropoxide.

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When H2O is replaced by hydrogen sulfide in Eq. (1),
then thiolysis can occur according to
nH2S þ TiðORÞ4 ! ðORÞ4ꢀnTiðSHÞn þ nROH
(4)
After the initial thiolysis reaction, condensation of the
alkoxy-hydrosulfides can occur, and may even lead to the
formation of M–S–M bonds due to the liberation of
hydrogen sulfide or ROH as shown below:
M ꢀ SH þ HS ꢀ M ! M ꢀ S ꢀ M þ H2S
(5)
(6)
M ꢀ OR þ HS ꢀ M ! M ꢀ S ꢀ M þ ROH
The overall reaction can thus be written as
2xðORÞ4ꢀnTiðSHÞn ! ½ðHSÞnꢀ1ðORÞ4ꢀnTi ꢀ S ꢀ TiðSHÞnꢀ1
ðORÞ4ꢀnꢁx þ xH2S
ð7Þ
½ðHSÞnꢀ1ðORÞ4ꢀnTi ꢀ S ꢀ TiðSHÞnꢀ1ðORÞ4ꢀn x
ꢁ
þ xH2S ! TiS2 þ 2xð4 ꢀ nÞROH
ð8Þ
The IR evidence indicates that the reaction in solution
results in only partial incorporation of sulfur in the ‘as-
prepared’ precursor powder. However, complete thiolysis
and condensation of alcohol groups is unlikely because of
the polar character of the Ti–O bonds (2.15 for Ti–O and
0.67 for Ti–S) [31]. Thus, the precursor is only partially
thiolysed and therefore requires thermal treatment with
hydrogen sulfide for the removal of the residual alkoxy
groups to produce pure titanium disulfide.
In order to confirm the reaction pathway, the solvent
was recovered after thiolysis by cold-trap distillation and
analyzed using GC/MS, which showed that isopropanol
was liberated during the reaction. This, in combination
with the IR results, suggests replacement of the isopropoxy
groups by hydrosulfide (ꢀSH) moieties derived from
hydrogen sulfide, resulting in the liberation of isopropanol
from the alkoxide. However, the replacement is not
complete, as indicated by the presence of isopropoxy
groups in the powder. Nevertheless, the significant
observation is that because the rate of thiolysis is slow,
the concentration of the condensing species remains below
the critical concentration necessary for homogeneous
nucleation and precipitation, therefore a solution of the
thiolysed precursor is obtained. In this context, it is
interesting to note that when similar reactions were
performed in the more basic secondary amine (di-butyla-
mine) and tertiary amine (tri-butylamine) solvents, black
precipitates formed within an hour.
Fig. 7. EDAX data for (a) as-prepared powder from the reaction
of titanium tetraisopropoxide with hydrogen sulfide in butylamine as
solvent, (b) thin film and (c) sample after heat treatment and thiolysed at
800 1C (6 h).
4. Discussion
The conventional water-promoted sol–gel process is
based on the hydrolysis and condensation of metal
alkoxide compounds [17]. Hydrolysis replaces the alkoxide
groups with hydroxyl groups, and subsequent condensa-
tion reactions involving the hydroxyl groups produce
M–O–M bonds according to
M ꢀ OR þ H2O ! M ꢀ OH þ ROH
(1)
(2)
(3)
The incomplete substitution of all the alkoxide groups by
hydrosulfide moieties can be considered on the basis of the
following points:
M ꢀ OH þ HO ꢀ M ! M ꢀ O ꢀ M þ H2O
M ꢀ OH þ OR ꢀ M ! M ꢀ O ꢀ M þ ROH
and we have used this chemistry previously to prepare
optical-grade thin films of titanium dioxide [30]. However,
the direct conversion of sol–gel derived titanium dioxide by
thiolysing under a hydrogen sulfide atmosphere is difficult
because the structure of the precursor already contains an
extensive network of Ti–O–Ti bonding [20].
(i) Metal alkoxides of general formula M(OR)n may be
considered as alcohol derivatives. They involve polar-
ized Md+–Odꢀ–R bonds whose degree of polarization
depends on the electronegativity of the metal atom
which can make the metal center very susceptible to
nucleophilic attack. For the same metal atom, the

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1434
covalent character (i.e., lower polarity) of the M–O
dies resulting from amine syntheses of titanium disulfide
will be reported subsequently.
bond parallels the increase in the inductive effect of the
alkyl group. Thus, the secondary alkoxide (isoprop-
oxide) has a higher covalent character compared to the
primary alkoxide (butoxide) [16].
5. Conclusions
(ii) Steric effects are also important, and alkoxides with
the smallest alkoxy group on the metal center and least
molecular complexity exhibit the lowest steric hin-
drance to nucleophilic attack. Such an alkoxide would
be most likely to react with hydrogen sulfide and
undergo thiolysis and condensation reactions to form
solid precursors useful for the synthesis of sulfides.
(iii) It is well known that the HSꢀ anion is a stronger
nucleophile than hydrogen sulfide, since S in hydrogen
sulfide has a partial charge, dS ¼ ꢀ0.13, while in HSꢀ,
dS ¼ ꢀ0.6, therefore, a larger concentration of HSꢀ
ions in solution would favor the thiolysis reaction. The
dissociation of hydrogen sulfide to H+ and HSꢀ is
more extensive in basic solvents, so n-alkylamines were
chosen as the solvent medium, since it is known that
there is no significant reaction between the alkoxide
and amine at room temperature [16].
A titanium alkoxy-sulfide precursor was successfully
synthesized by reacting titanium tetraisopropoxide with
hydrogen sulfide in butylamine medium. The reaction
product was characterized by various spectroscopic tech-
niques, which allowed a tentative reaction scheme to be
proposed. The precursor material was stable in solution for
long periods and is ideal for the fabrication of thin films
through spin coating. Upon controlled heat treatment in
hydrogen sulfide, the precursor material can be converted
into phase-pure titanium disulfide nanoparticles or a
uniform thin film coating on silicon or quartz wafers.
Acknowledgment
The Australian Cooperative Research Centre for Micro-
technology is acknowledged for the financial support of
this project.
The reactions in the n-alkylamine solvent can thus be
summarized as follows:
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R ꢀ NH2 þ H2S3R ꢀ NHþ3 HSꢀ
(9)
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ð10Þ
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