Polyol mediated synthesis of tungsten trioxide and Ti doped tungsten trioxide. Part 1: Synthesis and characterisation of the precursor material

Article abstract of DOI:10.1016/j.materresbull.2006.01.030

Polyol mediated synthesis for the preparation of tungsten trioxide and titanium doped tungsten trioxide has been reported. The reaction was carried out using chlorides of tungsten and titanium in diethylene glycol medium and water as the reagent for hydrolysis at 190 °C. Formation of a blue coloured dimensionally stable suspension of the precursor materials was observed during the course of the reaction. The particle sizes of the precursor materials were observed to be around 100 nm. The precursor materials were annealed to give tungsten trioxide and titanium doped tungsten trioxide. The precursor materials were characterised using TGA/DTA, FT-IR, optical spectra, SEM, TEM and powder XRD methods. It was observed that the doping of titanium could be effected at least up to 10% of Ti in WO₃. The TGA/DTA studies indicated that WO_3-x·H₂O is the dominant material that formed during the polyol mediated synthesis. The XRD data of the annealed samples revealed that the crystalline phase could be manipulated by varying the extent of titanium doping in the tungsten trioxide matrix.

Full text of DOI:10.1016/j.materresbull.2006.01.030

Materials Research Bulletin 41 (2006) 1476–1486
www.elsevier.com/locate/matresbu
Polyol mediated synthesis of tungsten trioxide and Ti doped
tungsten trioxide
Part 1: Synthesis and characterisation of the precursor material
*
P. Porkodi, V. Yegnaraman, D. Jeyakumar
Electrodics and Electrocatalysis Division, Central Electrochemical Research Institute, Karaikudi 630006, India
Received 25 July 2005; received in revised form 13 January 2006; accepted 23 January 2006
Available online 21 February 2006
Abstract
Polyol mediated synthesis for the preparation of tungsten trioxide and titanium doped tungsten trioxide has been reported. The
reaction was carried out using chlorides of tungsten and titanium in diethylene glycol medium and water as the reagent for
hydrolysis at 190 8C. Formation of a blue coloured dimensionally stable suspension of the precursor materials was observed during
the course of the reaction. The particle sizes of the precursor materials were observed to be around 100 nm. The precursor materials
were annealed to give tungsten trioxide and titanium doped tungsten trioxide. The precursor materials were characterised using
TGA/DTA, FT-IR, optical spectra, SEM, TEM and powder XRD methods. It was observed that the doping of titanium could be
effected at least up to 10% of Ti in WO3. The TGA/DTA studies indicated that WO3ꢀxꢁH2O is the dominant material that formed
during the polyol mediated synthesis. The XRD data of the annealed samples revealed that the crystalline phase could be
manipulated by varying the extent of titanium doping in the tungsten trioxide matrix.
# 2006 Published by Elsevier Ltd.
Keywords: A. Oxides; A. Semiconductors; C. Infrared spectroscopy; C. Thermogravimetric analysis; C. X-ray diffraction
1. Introduction
Tungsten trioxide has received much attention recently due to its potential in many technological applications,
viz., electrochromic devices [1–4], photochromic devices [5,6] and gas sensors [7–12]. WCl6 is a reactive
molecule and readily hydrolysed in the presence of water to form tungstic acid, which can be annealed at the
desired temperature to yield WO3. However, the morphology and particle size of the product cannot be controlled.
Hence, several synthetic routes have been adopted for the preparation of WO3 that include aqueous alcoholic
synthesis [9], pyrolysis [10], sol–co-precipitation [7,11], sol–gel method [9,12], emulsion method and ion
exchange method [13].
In aqueous alcoholic synthesis, a mixture of WCl6 and 2,4-pentadione in absolute ethanol was sonicated at 40 8C
for 2 h and allowed to stand at room temperature for 2 h. The precipitate formed was separated and annealed at 850 8C
* Corresponding author. Tel.: +91 4565 226204.
E-mail address: djkr@rediffmail.com (D. Jeyakumar).
0025-5408/$ – see front matter # 2006 Published by Elsevier Ltd.
doi:10.1016/j.materresbull.2006.01.030

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for 1 h to yield the yellow WO3. A similar procedure has been adopted in the sol–gel process, wherein WCl6 is
dissolved in ethanol to which 2,4-pentadione and water were added. The mixture was allowed to stand for 14 h to get
the sol. After evaporating the solvent, the powder is crushed and annealed at 850 8C for 1 h to get WO3. In yet another
variation of the synthetic procedure, WCl6 is mixed with a surfactant and ammonium hydroxide and the resultant
precipitate was separated and processed further to get WO3. In all these cases, though the reaction conditions are not
cumbersome, it is often essential to maintain the reaction conditions meticulously, especially if we desire to get
nanosized particles of WO3. Alternatively, ammonium paratungstate was pyrolysed at 500 8C for 5 h and the resultant
material was crushed in a planetary ball mill for 30 min to get WO3 powder in micrometer range. In emulsion-based
synthesis, Na2WO4 was passed through the cation exchange resin and the H2WO4 exiting from resin was dripped into a
solution containing either oxalate and acetate ion exchange ligand or water in oil-based emulsion. The solution stirred
for three days and further processed to get WO3. In addition to the above there are other preparative routes wherein
peroxotungstate was used as precursor [3,14,15] for the preparation of WO3. Komornicki et al. [15] have reported that
1.15 mol% of Ti could be doped in WO3 by this route. However, they have observed that rutile phase appeared
separately at higher Ti concentration.
Depero et al. has carried out the XRD simulation studies of WO3 and predicted that by suitably introducing static
disorder, either by oxygen deficiency in the matrix or by substituting the W by metal ions with lower oxidation states,
different crystalline phases of WO3 can be obtained. W(VI) and Ti(IV) have almost the same size (ionic radii
0.074 nm) and are known to occupy the same site. An attempt has been made to vary the level of doping of Ti in WO3
matrix and study the nature of the phase formation. For this purpose, it is felt that the preparative condition should
ensure atomic level mixing of Ti(IV) with W(VI) in solution.
Polyol mediated synthesis of materials is an attractive route, as the reaction conditions are simple and elegant that
yields nano sized particles in the form of dimensionally stable colloidal suspensions. This method employs high
boiling polyols, such as diethylene glycol, glycerol, glycol, etc., as the reaction medium [16–18] for the preparation of
oxide materials. Suitable reactants from homogenous solution (metal acetates/chlorides/alkoxides) are hydrolysed at
elevated temperature (ca. 190 8C) resulting in the formation of nano metal oxides/precursors as suspension in polyol. It
may be noted that the colloidally stable suspension can be used for coating the material onto desired substrates. More
importantly the reaction is carried out in homogenous solution which ensure atomic level mixing of reagents is ensured
and more apt to prepare Ti doped WO3 matrix.
Over and above, doping of Ti in WO3 matrix resulted in the increased sensitivity of NOX sensors. There is no
systematic study on the preparation of Ti doping in WO3 matrix and evaluating the material characteristics for gas
sensing. Hence, an attempt has been made to utilise the versatility of polyol-mediated synthetic method for the
preparation of nano sized WO3 and Ti doped WO3.
2. Experimental
2.1. Synthesis of WO3 and Ti doped WO3
WCl6 (14.4 g) was added to 200 ml of diethylene glycol (DEG) and the solution was heated in an oil bath at 80 8C to
give a pale yellow coloured solution and to this solution 40 ml of distilled water was added, the temperature was raised
to 190 8C and refluxed for 3 h. The yellow coloured solution immediately turned into pale green colour and gradually
changed to a deep blue coloured suspension. The precipitate was separated by centrifuging, boiled with ethanol and
washed thoroughly to remove DEG. The precipitate thus obtained was dried to form a fine powder by evaporating in a
Roto-evaporator system. The yield of the material was around 7.1 g. The blue coloured precursor was used for further
studies.
A similar preparative procedure was adopted for the preparation of titanium doped WO3 precursors, wherein
required amount of TiCl4 in propanol was added to the WCl6 solution. The level of doping was varied by using
1% or 10% (of TiCl4 solution) by weight of WCl6. Atomic level mixing is ensured as the reaction has been carried
out in homogeneous solution and hence, uniform doping of Ti in WO3 matrix is expected. For all the
compositions like in the case of WCl6 alone, blue coloured precursor materials were obtained. The materials were
centrifuged, boiled with ethanol and washed repeatedly with ethanol to give the precursor materials. The blue
precursor formed is annealed at various temperatures, ca., 150 8C, 300 8C, 400 8C, 500 8C, 600 8C, 700 8C and
800 8C.

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2.2. Characterization
The FT-IR spectra of the samples (precursor and the annealed materials) were recorded in KBr pellet using Nexus
FT-IR spectrometer 670 Model with DIGS detector. The optical spectra for the precursor samples were recorded using
Cary 500 scan UV–vis–NIR spectrophotometer. Optically clear glass plates were used as substrates for the
measurement of optical absorption spectra. The precursors/annealed samples at 700 8C were dispersed in absolute
ethanol and sonicated well. The solution was coated onto the glass substrate by spin coating (Milman spin coater). The
spin coated thin films were used for the absorption measurements. The XRD powder pattern of the precursor as well as
the samples annealed at various temperatures were measured using Cu Ka radiation (Philips PANalytical Model ‘X’
Pert PRO). TGA/DTA studies were carried out using (PL-STA 1000/1500) at a heating rate of 20 8C/min. The surface
morphology of the particles was studied using scanning electron microscope (JEOL JSM CF-35) and Transmission
Electron Microscope (CM200 SuperPower STEM). The cyclic voltammetric studies were carried out using a Wenking
Potentioscan Model POS 88 in conjunction with a Rikadenki x, y-t recorder. A three-electrode assembly used in this
study comprised a precursor coated Pt electrode (5 mm ꢂ 5 mm), a platinum wire counter electrode and Hg/Hg2SO4
(1.0 M H2SO4) reference electrode.
3. Results and discussion
3.1. Preparation of precursor material
WCl6 gave a yellow coloured solution in diethyleneglycol, which turned into a pale green colour solution. On
hydrolysis at 190 8C in the presence of water gave a deep blue coloured colloidal solution. If the hydrolysis of WCl6
alone is the anticipated reaction one may expect to get a yellow coloured WO3ꢁH2O as the product. However, this is not
the case when an alcohol was used as the reaction medium. This is due to the fact that W(VI) is a moderate oxidising
agent. Polyol has been reported to act as a reducing agent for noble metal ions [16]. Hence, W(VI) has been partly
reduced to W(V) in polyol to give a deep blue coloured non-stoichiometric precursor. It is established that the polyol-
mediated synthesis generally yields nanosized particles and in the present case a colloidally stable blue coloured
suspension. The colloidal solution was centrifuged and washed thoroughly with ethanol to yield the precursor
material. WO3 and the Ti doped WO3 materials were all deeply blue coloured immediately after isolation and were
used for the characterisation of the materials. It may be interesting to note that the precursor of tungsten trioxide
remained blue in colour on prolonged storage, wherein the 1% titanium doped material gradually changed to green
coloured sample and the 10% titanium doped sample turned into yellow coloured sample.
The precursor materials were annealed at high temperature to give light yellow to bright yellow coloured tungsten
oxide materials. This change in the colour may be due to the fact that the non-stoichiometric tungsten trioxide has been
oxidised to WO3 in the presence of air at high temperature. The same process would have been facilitated if titanium
was present in the matrix of the precursor.
3.2. TGA/DTA analysis
Fig. 1a depicts the TGA/DTA of the blue precursor obtained by the hydrolysis of WCl6 in polyol medium. It can be
seen from the figure that the precursor undergoes thermal decomposition by a number of exothermic processes with a
weight loss of 6.57%. This value corresponds to WO3ꢁH2O, wherein the loss of 7.2% is expected. Similar observations
have been made by Gotic et al. [21], wherein they have prepared WO3ꢁH2O by ion exchange method from Na2WO4
and the TG studies showed the weight loss of 6.6%. The first exothermic process was very broad around 150 8C under
which the weight loss was maximum. The second exothermic peak was also slightly broad (at 261 8C) with a minimum
weight loss and the third process took place at about 489 8C after which there was an increase in the weight of the
sample due to the oxidation of WO3ꢀx to WO3. These observations suggest that the precursor formed may be
WO3ꢀxꢁH2O. The TGA/DTA of the Ti doped precursors are presented in Fig. 1b and c. In both the cases it has been
observed that the nature of the processes were exothermic as in the case of WO3 precursor. The first peak appears
around 150 8C as a broad one and the second exothermic peak around 295 8C. The third exothermic process observed
in the case of WO3 precursor was not present, however, the trend of increase in the weight after 600 8C was observed.
In addition, the weight loss was increased from 6.56% to 9.7% and 13.3% for 1% and 10% Ti doped WO3 precursors,

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Fig. 1. TGA/DTA profiles of: (a) WO3-precursor, (b) WO3:1% Ti-precursor, (c) WO3:10% Ti-precursor.
respectively. This is due to the formation of Ti(OH)4 in the precursor. To confirm this TiCl4 was hydrolysed in polyol
medium by water wherein a white coloured material was obtained. The material was isolated and washed thoroughly
with alcohol to remove the diethylene glycol and then dried. The material thus obtained was subjected to TGA study
and 30% weight loss observed.
3.3. FT-IR spectra
Fig. 2 shows the FT-IR spectrum of WO3 precursor recorded in the region 400–4000 cmꢀ1. The peaks of the
precursor formed resemble to that of the tungstic acid. The very broad band from 1000 cmꢀ1 to 500 cmꢀ1 in WO3
precursor corresponds to the W–O vibrational mode. The peak at 806 cmꢀ1 of the precursor material is O–W–O
Fig. 2. FT-IR transmittance spectra of WO3-precursor.

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Fig. 3. Optical absorption spectrum of a thin film of the precursor on ITO glass electrodes obtained by spin coating: (a) WO3, (b) WO3:1% Ti.
Fig. 4. Optical absorption spectrum of the samples annealed at 700 8C: (a) WO3, (b) WO3:1% Ti.

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stretching frequency characteristics of WO3 framework. The band at low energy side of the spectrum i.e. below
500 cmꢀ1 is predominantly characterized by W–O asymmetric stretching frequency [19,20]. In the case of WO3ꢁH2O
the peak was observed at 652 cmꢀ1 and the peak at 800 cmꢀ1 appeared as shoulders wherein in the precursor the peak
was observed at 806 cmꢀ1. The other peaks also experience shifts in the frequency. The slight difference in the
spectrum from that of the tungstic acid may be due to the non-stoichiometric nature of the precursor and the blue
coloration is attributed to tungsten as an intervalent species, i.e. oxidation state less than W(VI) and greater than W(V).
The FT-IR spectrum of WO3 that was formed by annealing the precursor at 600 8C matches with the IR data of WO3 in
library (figure not shown). The band at 968 cmꢀ1 in WO3 corresponds to the W O terminal vibrational mode
stretching frequency [21]. The IR peaks due to H2O molecule in the precursor is absent in annealed sample.
3.4. Optical spectra
Figs. 3 and 4 show the optical spectra of the precursor samples and the samples annealed at 700 8C, respectively.
The precursor samples suspended in polyol and the samples heated at 700 8C were spin coated on a glass plate and the
optical spectrum was recorded in the wavelength region 300–2000 nm.
The optical spectrum of WO3 precursor shows a broad absorption spectrum around 1000 nm [22]. This is mainly
due to the oxygen non-stoichiometry [23] and the formation of intervalent compound of W, i.e. W(VI) and W(V). In
1% Ti doped WO3 there is a red shift of the fundamental absorption edge apart from the peak due to Ti. The red shift is
mainly attributed to the weakening of W–O interaction and the decrease in the oxygen non-stoichiometry.
Fig. 5. XRD patterns of: (a) precursor of WO3 as formed, (b) annealed at 300 8C, (c) annealed at 800 8C.

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WO3 annealed at 700 8C shows three absorption peaks due to intra band transition [24]. However, 1% and 10% Ti
doped WO3 shows red shift in the absorption peaks of WO3 and the absorption around 400 nm is due to Ti centres [25].
The doping of Ti in WO3 matrix decreases the bond length of W O/O–W–O and as a result the optical band gap
decreases. Hence the absorption takes place in the longer wavelength region leading to red shift.
3.5. XRD characterisation
The powder XRD pattern of the as formed precursor material is presented in Fig. 5a which exhibits three broad lines
centred around 258, 338 and 568. This observation suggests that the material obtained by the hydrolysis of WCl6 in
polyol medium is poorly crystalline in nature. Fig. 5b and c show the XRD patterns observed for the samples of WO3
obtained by annealing the precursor at 300 8C and 800 8C, respectively. It can be seen from the figure that there is no
significant difference between the samples as formed and dried and the sample heated at 300 8C. This indicates that up
to 300 8C the crystalline nature of the material did not change significantly. However, XRD pattern of the sample
annealed at 400 8C (figure not shown) shows three major peaks between 208 and 258. The peaks have high fullwidth at
half maximum values (FWHM). The sample annealed at 800 8C showed sharp lines pattern. The pattern is compared
with literature data reported by Depero et al. [26], Rao [27] and with the JCPDS data. The XRD pattern corresponds to
the monoclinic phase where all the three major peaks are of comparable intensity and the other minor diffraction peaks
also compare well with their simulated data. The same XRD pattern persists for the samples annealed at 600 8C and
700 8C as well (figures not shown) with the only difference being in the decrease in the peak height and increase in the
Fig. 6. XRD patterns of samples annealed at 700 8C: (a) WO3, (b) WO3:1% Ti, (c) WO3:10% Ti.

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FWHM values. This indicates that the crystallite size increases with the increase in annealing temperature. The
average crystallite size D, was determined by Scherer’s formula using the FWHM.
0:9l
D ¼
B cos u
The crystallite sizes of the undoped WO3 annealed at different temperatures ranging from 400 to 800 8C were
calculated and found to vary from 10 to 30.8 nm.
Fig. 6a–c depict the XRD pattern of the WO3:Ti (1% and 10%) annealed at 700 8C for 5 h along with that of
undoped WO3. The XRD patterns are significantly different from each other. The undoped WO3, as already discussed
exists in the monoclinic phase, whereas the Ti doped WO3 samples (both 1% and 10%) are not monoclinic. Depero
et al. [26] have simulated the XRD patterns for WO3 for various crystal system, viz, triclinic, monoclinic,
orthorhombic and tetragonal by introducing certain disorders. They have observed that in the case of WO3, symmetries
higher than monoclinic may result due to oxygen substoichiometry or doping with Ti, which has lower oxidation state
than W. The diffraction pattern of WO3:Ti(1%) closely resembles to tetragonal phase, wherein only two peaks are
observed at 23.1562 and 24.068 (intensity 60:100) unlike the three peaks occurring at 23.07, 23.52 and 24.35 (intensity
100:90:100) in the case of monoclinic phase (JCPDS Files 5-0363 and 5-0388). However, increase in the concentration
of Ti in the material, WO3:Ti(10%) results in the decrease in the symmetry when compared to 1% Ti doped WO3.
Three major peaks are observed, but on comparing the data it could be deduced that the pattern corresponds to
orthorhombic phase wherein the first peak is more intense unlike in the case of monoclinic phase.
From the XRD powder pattern we can clearly arrive at a conclusion that Ti can replace W from WO3 that lead to
corner sharing octahedra, resulting in the formation of tetragonal or orthorhombic phases. This is in accordance with
Fig. 7. TEM picture of WO3 precursor formed in the polyol mediated synthesis.

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the simulation studies of Depero et al. [26], wherein they have clearly showed that this type of corner sharing octahedra
in WO3 matrix leads to increase in the symmetry of the crystal lattice.
This is in contrast to the reports available in the recent literature. Komornicki et al. [15] have reported that when
1.15% Ti was doped in WO3 a weak (1 1 0) reflex of rutile phase appeared (2u = 27–288). The WO3 is also seen to have
the monoclinic phase intact. They have concluded that stability limit of Ti in WO3 is around 0.35 ꢃ 0.05 mol%. It may
be noted that both W(VI) and Ti(IV) have same ionic radii and hence Ti and W can occupy the same sites. The
observations made by Komornicki et al. may probably be due to the inhomogenity in the preparative method that has
been adopted. Alcoholysis of pure WCl6 [9], yields WO3 in orthorhombic phase at annealing temperature less than
550 8C and at temperatures over 700 8C it changes over to monoclinic phase.
3.6. SEM and TEM analysis
The TEM picture of the precursor material obtained by the hydrolysis of WCl6 alone is depicted in Fig. 7. It can be
seen from the figure that the particle size of precursor material is less than 100 nm. Similar trend has been observed for
Fig. 8. SEM pictures of: (a) precursor of WO3 formed in the polyol mediated synthesis, (b) WO3 precursor annealed at 700 8C.

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the titanium doped precursor materials as well. In Fig. 8a and b, the SEM pictures of the precursor material and the
sample annealed at 700 8C are presented. A perusal of the pictures suggests that the nanometer sized precursor
material grew into well-defined micron-sized crystals when annealed at 700 8C.
3.7. Electrochemical studies
The cyclic voltammetric studies of the precursor were carried using Pt electrodes in acidic media. The colloidal
suspension of the precursor in polyol was coated on the Pt and heated at 200 8C for 2 h, leaving a blue coloured coating
on the Pt electrode. This was used as working electrode and the cyclic votammogram (CV) was recorded in 0.1 M
H2SO4 using a Pt wire counter electrode and Hg/Hg2SO4 (1.0 M H2SO4) as the reference electrode. The CV’s of the
precursor materials obtained using WCl6 along with Ti doped ones are given in Fig. 9. It can be seen from Fig. 9a that
the WO3 precursor undergoes a redox process centred at ꢀ0.30 V wherein the reduction takes place at ꢀ0.420 Vand
the corresponding oxidation at ꢀ0.180 V [28]. During the recording of the CV it was noted that the film exhibits
electrochromism. The blue colour disappears during anodic sweep and the colour is restored during the reduction
cycle. The colouration can be attributed to the formation of F or F+ centres in oxygen ion vacancies [29]. Repetitive
cycling did not modify the voltammograms indicating the strong adherence of the precursor on Pt. This is attributed
mainly to the electrochemical process involved in the reaction [29] represented by
WO3ꢁH2O þ 2xHþ ! xOꢄꢄ ðin WO3ꢁH2OÞ þ xH2O
xOꢄꢄ ðin WO3ꢁH2OÞ þ 2xeꢀ ! WO3ꢀxꢁH2O
xOꢄꢄ is the oxygen ion vacancies where the electrons are trapped, thereby resulting in the formation of WV/WVI in the
matrix. The CV’s were recorded for different scan rates like 10 mV/s, 25 mV/s, 50 mV/s, 75 mV/s and 100 mV/s and
was noted that the peak current values varied linearly with the sweep rate indicating the redox process is surface bound.
Doping of Ti in WO3 has significantly changed the voltammograms. In the case of 1% Ti doped WO3 the CV shows
(Fig. 9b) that the reversibility of the redox process has enhanced dramatically. The redox process occurs around the
Fig. 9. Cyclic voltammograms recorded in 0.1 M H2SO4 for as formed: (a) WO3, (b) WO3:1% Ti, (c) WO3:10% Ti coated on Pt electrode. Scan rate:
50 mV/s.

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same potential window, viz., ꢀ0.33 V versus Hg/Hg2SO4(1.0 M H2SO4). Both the anodic and the cathodic peaks
appear at ꢀ0.33 V, with a peak separation of zero mV. In the case of 10% Ti doped WO3, the reduction process was
found to be more difficult. The redox peaks have shifted to more negative potentials, the reduction peak was found to
merge with the hydrogen evolution potential, wherein the anodic peak was broad and observed at around ꢀ0.5 V.
These differences may essentially arise due to the difference in the structure of the oxide phase with variation in the
doping level of Ti in the WO3 matrix.
4. Conclusion
Polyol mediated synthesis for the preparation of WO3 and WO3:Ti was optimised. The particle size of the precursor
was in the range of 100 nm. The precursor was annealed at a high temperature to yield WO3 and WO3:Ti. The
precursor as well as the WO3 and WO3:Ti materials were characterised using optical absorption, FT-IR, TG/DTA,
XRD and cyclic voltammetry. It was observed that doping of titanium in WO3 lattice has changed the phase from
monoclinic to tetragonal to orthorhombic depending on the percent of doping.
Acknowledgement
One of the authors P. Porkodi is thankful to Council of Scientific and Industrial Research, for the award of Research
internship.
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