Synthesis of Titanium dioxide by ultrasound assisted sol-gel technique:

Effect of Calcination and Sonication Time.

D. V. Pinjari a, Krishnamurthy Prasad b, B. Pawar b, A. B. Pandit a and S. T.

Mhaske b*

a Chemical Engineering Division, Institute of Chemical Technology, Matunga,

Mumbai - 400 019. INDIA.

b Department of Polymer and Surface Engineering,

Institute of Chemical Technology, Matunga, Mumbai - 400 019. INDIA.

*Author to whom correspondence should be addressed

Email: stmhaske@gmail.com Tel: +91-22-33611111; Fax: +91-22-4145614

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Abstract:

Nanostructured Titanium dioxide was synthesized using both conventional and

ultrasound assisted sol-gel technique. All the specimens were subjected to a calcinations

temperature of 750°C. Calcination time was varied from 30 min to 3 hours to study the

effect of calcination time on the properties of the synthesized TiO2. The TiO2 specimens

were characterized using X- Ray Diffraction (XRD) and Scanning Electron Microscopy

(SEM). The sonication time was also varied. The influence of the calcination and

sonication time on the phase transformation process from anatase to rutile and its

consequent effect on the crystallite size and % crystallinity of the synthesized TiO2 was

studied and interpreted. It was found that 100% phase transformation was observed after

3 hours of calcinations for the ultrasound assisted sol-gel synthesized TiO2. The study on

the phase transformation via variation of sonication time yielded some interesting results.

It was seen as the time period of sonic impulse was increased an initial increase followed

by a decrease in rutile content was observed. In general the ultrasound assisted process

helped create TiO2 material with a higher % rutile than the conventional sol-gel process.

Keywords: Sol-Gel, Acoustic Cavitation (Ultrasound), Titanium dioxide (TiO2),

Calcination Time, Sonication Time.

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1. Introduction

The physical methods of nanoparticle synthesis have a few

inherent disadvantages which limits their usage. They cannot prepare

nanoparticles having controlled size distribution and also the sizes that

may be synthesized are often at the 100 nm and above level which is

not good enough for the newer demands of the nanoparticle industry.

On the other hand precipitating clusters of inorganic compounds from a solution

of chemical compounds has been an attractive proposition for researchers, primarily

because of the simplicity with which experiments can be conducted in a laboratory. This

is especially true if the goal is to just have a nanocrystalline powder, instead of a

“dispersible” nanoparticulate powder [1]. Solution processing can be classified into three

major categories. In addition there are various sub-categories like hydrothermal synthesis

and polyol techniques but the three most widely used techniques are:

1. Sol-Gel Processing.

2. Precipitation Method.

3. Water-Oil Micro-emulsions (Reverse Micelle) method.

The sol-gel process is a versatile solution process for making ceramic and glass

materials. The sol-gel process, as the name implies, involves the evolution of inorganic

networks through the formation of a colloidal suspension (sol) and gelation of the sol to

form a network in a continuous liquid phase (gel). A sol is a dispersion of the solid

particles, with diameter of 1-1000 nm [2], in a liquid where only the Brownian motions

suspend the particles.

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A gel is a state where both liquid and solid are dispersed in each other, which

presents a solid network containing liquid components. The precursors for synthesizing

these colloids consist of a metal or metalloid element surrounded by various reactive

ligands. Metal alkoxides are most popular because they react readily with water. The

most widely used metal alkoxides are the alkoxysilanes, such as tetramethoxysilane

(TMOS) and tetraethoxysilane (TEOS). However, other alkoxides such as aluminates,

titanates, and borates are also commonly used in the sol-gel process, often mixed with

TEOS.

Applying the sol-gel process, it is possible to fabricate ceramic or glass materials

in a wide variety of forms: ultra-fine or spherical shaped powders, thin film coatings,

ceramic fibers, microporous to nanoporous inorganic membranes, monolithic ceramics

and glasses, or extremely porous aerogel materials. 

The sol-gel process is widely used for manufacturing nanoparticles of various

metal oxide materials like TiO2 [3-5], ZnO [6, 7], SiO2 [8, 9], WO3 [10], etc. The sol–gel

route is very attractive because it is relatively easy to perform and allows us to tailor the

morphology of particles by relative rate of hydrolysis and condensation reactions [1]. The

sol-gel synthesis of metal oxides involves the following steps:

1. Sol formation: In this step, the metal alkoxide is dispersed in an acidic solution to

form a clear sol.

2. Gel formation: In this step, the sol is reacted with water forming a gel which is

nothing but the metal hydroxide complex formed by reaction of the precursor with

water.

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3. Dehydration: Generally a large excess of water is taken in the Gelation step to

ensure maximum conversion of the sol to gel. The excess water is removed in the

dehydration stage which may involve the use of high temperature operations like

azeotropic distillation.

4. Calcination: The fourth and final step of the sol-gel process is the Calcination

step. This is nothing but a high temperature assisted bond breakage and

reconfiguration process. Calcination is generally done at temperatures exceeding

300°C and can also be carried out temperatures as high as 1200°C. Calcination

deals with breaking of M-OH (where M represents metallic element like Si, Ti,

Al, etc.) bonds to form a metal oxide material MOx where x is the oxidation

number and generally x has the lowest permissible value.

Three main reactions occur during the sol-gel process: hydrolysis, alcohol

condensation, and water condensation. However, the characteristics and properties of a

particular sol-gel inorganic network are related to a number of factors that affect the rate

of hydrolysis and condensation reactions, such as, pH, temperature and time of reaction,

reagent concentrations, catalyst nature and concentration, H2O/Si molar ratio (R), aging

temperature and time, and drying. Of the factors listed above, pH, nature and

concentration of catalyst, H2O/Si molar ratio (R), and temperature have been identified as

most important [11].

The sol-gel technique requires annealing treatment of the amorphous precipitates,

to induce crystallization. The annealing process consists of two stages viz; Dehydration

and Calcination. Dehydration is carried out in an air-circulating oven at a suitable

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temperature while the calcination is done in a muffle furnace. The calcinations procedure

is used for conversion of the Titanyl hydroxide complex into Titanium dioxide. The

temperature to which the material is subjected to and the time frame of exposure to the

high temperatures used in the calcinations process is a huge influence on the crystalline

properties of the synthesized nanomaterial. The Calcination step perhaps has the most

influence on the properties of the synthesized material. The two factors that can control

the Calcination process are:

1. Calcination Temperature.

2. Calcination Time.

Selection of proper Calcination temperature is of extreme importance in the sol-

gel process. Metal oxide materials occur in various crystalline polymorphs and generally

these polymorphs are interconvertible polymorphs. Reversible and irreversible phase

transformations are a common feature in metal oxide chemistry. Some crystalline phases

have better packing and higher crystallinity in comparision to others but occur at only

higher temperature. A typical example being Titanium dioxide (TiO2) of which the more

crystalline rutile phase may be created at temperatures of 600°C and above. Titanium

dioxide occurs in mainly two crystalline polymorphs viz; anatase and rutile. The major

distinction between the two phases is in their differing behaviour under ultraviolet

irradiation. Anatase is photocatalytic in nature i.e. it produce free-radicals on account of

excitation by the UV radiation whereas rutile is a UV absorber [12]. Thus, for obtaining

phase purity ad selectivity in synthesis, selection of optimum calcination temperature is

essential.

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The effect of calcination temperature on the properties of TiO2 synthesized by

ultrasound assisted (US) and conventional sol-gel (NUS) synthesis was put forward in

our previous work [13]. As Calcination temperature was increased in both the US and

NUS synthesized TiO2 there was an increase in the % rutile content. The effect of

increased surface area and consequent increased rate of heat transfer to the bulk made it

possible to obtain 100% phase transformation in the US synthesized TiO2 at 750°C a full

100°C lesser than the temperature required for 100% phase transformation in the NUS

synthesized TiO2.

The effect of Calcination time on a particular material seems to be of particular

importance as the time period to which a material is subjected to high temperatures will

play a role in deciding what kind of crystalline microstructure will be developed in the

material. A fleeting Calcination procedure will not afford enough time for the lattice

structure to get developed completely thus creating a material with an inferior crystalline

structure. A sufficiently slow Calcination process will perhaps allow the lattices to get

oriented properly thus creating a more ordered microstructure thereby creating a more

crystalline material.

Acoustic cavitation technique is extremely useful for synthesizing nanomaterials.

Basically the extreme transient conditions generated in the vicinity and within the

collapsing cavitational bubbles have been used for the size reduction of the material to

the nano scale. In acoustic cavitation, it generates a transient localized hot zone with

extremely high temperature gradient and pressure. The sonochemical precursors used are

destructed due to sudden changes in temperature and pressure and are finally resulting in

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the formation of nanoparticles. The acoustic cavitation technique has a potential to

produce nanoparticles in large volume.

In this paper, the effect of calcination and sonication time on the properties of sol-

gel (US and NUS) synthesized TiO2 was studied and the trends in the properties of the

synthesized TiO2 were interpreted.

2. Materials and Methods

2.1 Materials

Titanium Tetraisopropoxide (TTIP) precursor was obtained from Spectrochem Pvt. Ltd.,

India. 2 –propanol (99.9% pure) was purchased from s. d. Fine Chemicals Ltd, Mumbai,

India. Glacial acetic acid solution was ordered from Merck Ltd, Mumbai, India.

2.2 Preparation of TiO2 by conventional (Non-Ultrasound assisted (NUS) Sol-Gel

Method.

5 mL of Titanium Tetraisopropoxide (TTIP) precursor was added dropwise to 30

mL of 2-propanol. The resulting clear solution was added dropwise under stirring to 5

mL of glacial acetic acid solution. The sol thus obtained was left at ambient temperature

for 24 hours. For getting nearly 100% conversion (i.e. for ensuring complete hydrolysis

of TTIP precursor), the solution needed to be kept as such for approximately 1 day.

The sol obtained after 24 hours was then converted into a gel by adding it to 30

mL of distilled water under stirring. The obtained gel was then dehydrated in an air

circulating oven at 110 oC for 3 hours. The dehydrated material was then calcined in a

muffle furnace (Expo Hi Tech Mumbai, India, Power input = 2 KW and Length x

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Breadth x Height = 30 cm x 10 cm x 10 cm) at 750oC for varying time periods from 30

min to 3 hours. The white powder obtained after calcinations was cooled, ground and

weighed to check for yield of the process.

2.3 Preparation of TiO2 by Ultrasound assisted (US) Sol-Gel Method

The mixture of TTIP precursor, 2-propanol and glacial acetic acid solution was

subjected to sonication using an Ultrasonic Horn (ACE 22 KHz) at 40% amplitude

actually delivering 29.2 watts of power for 10 minutes with a 5 second pulse and 5

second relaxation cycle. Thus, the sol obtained after keeping it for 24 hours was again

ultrasonically irradiated by keeping same parameters (output power and pulse &

relaxation cycle). The remaining procedure including the volume of the reactants was

kept the same (as reported in section 2.2). The purpose behind the dual sonication has

been reported earlier [13] After calcination the TiO2 powder (white in color) obtained

was cooled, ground, checked for yield and characterized using the same techniques as

that used for the NUS sol-gel synthesized specimens.

The calcinations times that the specimens were subjected to were 0.5, 1, 2 and 3

hours respectively. Accordingly, the NUS sol-gel synthesized TiO2 specimens were

labeled as TNUS 0.5, TNUS 1, TNUS 2, TNUS 2 and TNUS 3 while the US sol-gel

synthesized TiO2 specimens were labeled as TUS 0.5, TUS 1, TUS 2 and TUS 3.

Keeping in mind, heating rate for NUS and US sol gel specimens were kept same.

After the time period of calcination was optimized for both the NUS and US

specimens, a new set of US specimens were synthesized by varying the sonication time at

time t =0 h and t = 24 h. Initially, the US specimens were synthesized by using a

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sonication time of 10 minutes and t = 0 h and t = 24 h. two new sets of specimens were

synthesized using sonication time of 5 minutes and 15 minutes at t = 0 h and t = 24 h and

were labeled as TUS(5) and TUS(15) respectively.

3. Characterization

The TiO2 specimens were first characterized by studying their X-Ray Diffraction

patterns on a Rigaku Mini-Flex X-Ray Diffractometer. Crystallite sizes were determined

using the Debye-Scherrer equation. Scanning Electron Microscopy of the specimens was

carried out on a JEOL JSM 680LA 15 kV SEM to estimate the surface characteristics of

the specimen. Together the XRD and SEM methods provide exact knowledge regarding

the crystallite size and crystalline characteristics of the synthesized TiO2. 

4. Results and Discussions

Initially, the subject of the study was to optimize the Calcination time required for

attaining 100% phase transformation from anatase to rutile in the synthesized titanium

dioxide.

Table 1 shows the average yields obtained for the synthesis and it is observed

that the US synthesis has got a much higher % yield than the corresponding NUS process

and this is attributed to the ability of acoustic cavitation to improve the reaction kinetics

by improved mass transfer (on account of the micro-mixing caused by the acoustic shock

wave release on collapse of the cavitation bubble) and increase in reaction rates due to

radical formation due to the adverse conditions created by bubble collapse.

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4.1 X-ray Diffractometer (XRD) Study for Calcination Time optimization

The XRD patterns of the TiO2 specimens prepared by non ultrasound (NUS) sol-

gel method are shown in Figure 1. The anatase form of TiO2 shows a peak at 2θ = 25o

while the presence of rutile form is indicated by a peak at 2θ = 28o. % Rutile was

calculated by the following non-linear equation [14, 15]:

            % R = 100/[(A/R)*0.884 + 1] …………………………(1)

Where % R = Rutile content in percent, A = Peak Intensity of the peak at 2θ =

25°, R = Peak Intensity of the peak at 2θ = 28°

The crystallite sizes of the specimens were calculated by using the Debye-

Scherrer equation. The equation takes the form:

Dhkl = K/ (Bhkl x cos hkl)………………………….. (2)

Where B is the width of the peak at half maximum intensity of a specific phase

(hkl) in radians, K is a constant that varies with the method of taking the breadth (0.89 <

K < 1). Here, in this work, for calculation purpose K = 0.9, is the wavelength of

incident X-rays, = 1.54 Å for Cu Ka, is the center angle of the peak, D is the crystallite

length or primary crystallite size.

4.1.1 Phase transformation in TiO2 synthesized by Conventional (NUS) sol gel method

The X-Ray diffraction patterns of the specimens synthesized by the convention

i.e. Non-Ultrasound assisted sol-gel technique is shown in Figure 1. From the XRD

patterns it is clear that with an increase in calcination time there is an increase in the

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rutile content. This is proved by a continuous increase in the intensity of the peak at 2θ

= 28°.

The specimen calcined for 30 min i.e 0.5 hours (TNUS 0.5) shows the least rutile

content while the specimen calcined for 3 hours (TNUS 3) shows the maximum rutile

content amongst the NUS synthesized specimens (Figure 3). This is attributed to the fact

that when sufficiently large amount of time was provided for the calcination process the

heat supplied could permeate into the recesses of the specimen and an exposure to the

high temperature (750°C) brought about the phase transformation in the TiO2 specimens

from the meta-stable anatase phase to the more stable rutile phase.

The % crystallinity of the specimens was determined by the method described

previously [13] as expected, with an increase in rutile content, there is an increase in %

crystallinity (Figure 4). The rutile polymorph of TiO2 shows better packing density,

specific gravity and lower unit cell volume which results in it having higher %

crystallinity than anatase which on its part has lower packing density, specific gravity and

higher unit cell volume.

The trends in crystallite size and predominant crystalline phase are reported in

Table 2. Supporting the trend in % crystallinity, there is an increase in average crystallite

size as the calcination time is increased thereby serving as an indicator for increasing %

rutile content. Thus, TNUS 0.5 which has been calcined for the least amount of time

shows a lower crystallite size (24nm) than TNUS 3 which has been calcined for a longer

duration (35nm).

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4.1.2 Phase transformation in TiO2 synthesized by Ultrasound assisted (US) sol gel

method

The X-Ray diffraction patterns of the specimens synthesized by the ultrasound

assisted ie US sol-gel technique is shown in Figure 2. The major facet of the effect of

calcination time on the % rutile of the US synthesized specimens was the 100%

conversion from anatase to rutile observed in the specimen TUS 3.

Acoustic cavitation causes an increase in the effective surface area, thereby

increasing the rate of heat transfer to the bulk of the specimen. This rate is faster than that

for the NUS synthesized specimens, thus, 100% phase transformation occurs at a much

lower temperature for the US synthesized specimens than for the NUS synthesized

specimens.

As calcination time is increased from 0.5 h to 3 h as seen in the NUS specimens

there is an increase in the % rutile but what stands out is the fact that the increase in the

% rutile amongst the US specimens is much more pronounced than that observed

amongst the NUS specimens (Figure 3). This is attributed to the effect on the system of

the dual sonication process:

The pre-hydrolysis sonication was done to ensure an efficient micromixing of the

various reactants in the reaction system. This was done, therefore, to ensure that the

reactants were contacted properly in the highly adverse conditions exerted onto the

system during the 10 min of pre-hydrolysis sonication.

The post-hydrolysis sonication was intended to encourage formation of

nanoparticles (as nuclei) with smaller average particle sizes. This also ensured that during

calcination the heat was transferred more efficiently into the bulk of the material thereby

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ensuring that all of the particles in the bulk were subjected to the highest possible

temperature in the lowest possible time.

The combined effects of the dual sonication help in acceleration of phase

transformation from anatase to rutile, thereby, the US synthesized specimens show faster

progression in phase transformation from TUS 0.5 to TUS 3.

The crystallite size study in the case of the US synthesized specimens yielded

results similar to the NUS specimens in-so-far as the trends in values were concerned.

Thus, the two synthesis techniques were in-principle having a similar effect on the

synthesized material. The only difference was that the effect of phase transformation

observed in the US synthesized specimens was much more pronounced than that in the

case of the NUS synthesized specimens.

4.2 Scanning Electron Microscopy study

The SEM micrographs of the specimens are shown in Figure 5. As seen in our

previous work, the TiO2 synthesized using the US method and calcinated at 750°C was

shown to be having a block-type structure [16]. This is now observed in the micrograph

of specimen TUS 3 (Figure 5A). No such block-type structure is observed in the

corresponding NUS specimen TNUS 3 (Figure 5B). The reason for this can be explained

as follows, the specimen synthesized by the US method and calcinated for 3 hours at

750°C ie TUS 3 was shown to be a 100% rutile polymorph form of TiO2 and thus is more

crystalline than the anatase+rutile nature possessing TNUS 3. Therefore, the surface

micrograph of TUS 3 showed a more close packed and ordered appearance than that of

TNUS 3.

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From the study it was found that for the US synthesized specimens a calcination

time of 3 h was enough to produce 100% phase transformation from anatase to rutile in

the synthesized titanium dioxide. Therefore, using that calcination time, two new titanium

dioxide specimens were synthesized using the ultrasound assisted sol-gel method but

using a sonication time of 5 minutes and 15 minutes respectively. These specimens were

studied using the XRD and SEM methods also and their properties (morphology and

phase presence) were compared with the TNUS 3 and TUS 3 specimens.

4.3 XRD study of the specimens synthesized with varying sonication times

The X-ray diffractograms of the specimens synthesized using differing sonication

times are shown in Figure 6. What is obvious from the figure is that the titanium dioxide

synthesized by the US method shows the presence of 100% rutile (due to absence of any

peak at 2 = 25°) when calcinated at 750°C irrespective of the amount of sonication time

used during the synthesis. From the diffractograms we can get the data related to the %

Rutile, % crystallinity and crystallite sizes of the specimens and which is shown in Figure

7, Figure 8 and Table 3 respectively.

From Figure 7 it is seen that there is no difference in the % Rutile values (≈100%)

obtained for the three US synthesized specimens while the NUS specimen (TNUS 3)

shows a much lower % Rutile value and therefore we can conclude that the US process

indeed, hugely accelerates the phase transformation in titanium dioxide from anatase to

rutile.

The trend in % crystallinty (Figure 8) also reflects the trend in the rutile content.

The Rutile polymorph of TiO2 shows better packing and more stable crystalline structure

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than the meta-stable anatase polymorph and therefore the US specimens which show

rutile as the pre-dominant phase, therefore, show higher % crystallinity than the single

NUS specimen. However, it is seen that the specimen TUS 3 is showing the highest %

crystallinity amongst all. To explain this we have to study the trends in crystallite size

and also the yield of the reaction.

The values of crystallite size (Table 3) show that the specimen TUS 3 is having

the highest crystallite size. This further seems to indicate that TUS 3 is that specimen

which is having the most rutile character out of the rest. However, there is not much of a

significant difference in that aspect amongst the US specimens but they do show a

marked increase in rutile content and crystallinity from the lone NUS specimen.

4.4 % yield study of the specimen obtained using various sonication times

The % yields obtained for the various specimens are seen in Table 4. It is seen

again that the US specimens show a good improvement in the % yields as compared to

the NUS specimen which is attributed to the reasons extensively discussed in the

beginning of Section 4. It is seen that initially as the sonication time is increased we see

an increase in the yield from TUS(5) to TUS 3 and thereafter we see a decrease in %

yield from TUS 3 to TUS(15). This is attributed to the fact that as the sonication time is

increased, the extent of bubble collapse occurring in the system increases and an

increased exposure to such cavitating conditions results in improvement in mass transfer

and reaction rates and thus a consequent increase in yields. However, as the sonication

time the reaction system is subjected to increases beyond a certain margin, the high

instantaneous temperatures generated in the reaction system will result in partial

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vapourisation of the volatile 2-propanol used as the reaction medium. This results in a

shift of the reaction equilibrium to the left by Le-Chatelier’s principle and therefore we

see a reduction the yields. In addition, the high velocity acoustic streaming at higher

sonication times will result also in asymmetric bubble collapse in the fluid, thus totally

disrupting the formation of a close-packed crystal structure. Therefore, the TiO2

synthesized at higher sonic time input shows lower % crystallinity which is seen in the

values obtained in Figure 8. in the case of TUS 3 (ie a 10 minute sonication time ) the

likely scenario is that there would be an ideal balance between high temperature and

high energy collapse which would not cause local volatilization of 2-propanol but rather

the generated heat energy would be utilized solely for accelerating the phase

transformation process.

4.5 SEM study of the specimens synthesized at various sonication times

The SEM micrographs of the specimens synthesized at various sonication times

are shown in Figure 9. It is seen that with increase in sonication times we see a slow

development of a block type structure in the synthesized TiO2. The development of an

ordered surface morphology is hence, a characteristic of the US process. The structure

however, seems to vanish at the specimen TUS(15) which seems to suggest that the

higher sonication times are having a detrimental effect on the synthesized product. At

lower sonication times, the heat generated due to bubble collapse hastens the phase

transformation from a meta-stable anatase to more stable and crystalline rutile form

which thereby shows an increasingly ordered surface morphology but at higher

sonication times the integrity of the block type structure is lost. It may be due to the fact

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that the more sustained impact of the acoustic shock waves at higher sonication times, on

the surface of the TiO2 ruptures the formed block type structure.

5. Conclusion

The effect of calcination and sonication time on the phase transformation of sol-

gel synthesized (conventional and ultrasound-assisted) TiO2 was studied. It was found

that with increasing calcination time (at constant temperature of 750°C), there was an

increase in the rutile content of the specimens, signifying that at lower calcination times

the entire bulk of the specimen could not be exposed to the high temperature thereby the

meta-stable anatase phase was intact. The introduction of ultrasound and hence, acoustic

cavitation as a reaction aid helped produce specimens with a lesser degree of

agglomeration and thus helped the high temperature reach the inner recesses of the

specimen quicker, hastening the phase transformation and achieve 100% conversion from

the anatase phase (favoured by the conventional sol-gel process) to the rutile phase. After

the optimization of calcination time was set at 3 h, the sonication time was also vaied to

study the effect of sustained ultrasound contact on the reaction system. It was seen that as

the sonication time was increased from 0 (NUS) to 10 min, there is an increase in the

rutile content, crystallinity and % yields. Increase beyond that time resulted in a

detrimental effect on the system due to the cavity collapse energy being utilized for

purposes other than accelerating phase transformation. Thus, a calcination time of 3 h and

a sonication time of 10 min were determined as being the optimum parameters for

obtaining 100% rutile TiO2.

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6. References

1. G. Skandan, A. Singhal, Nanomaterials Handbook, Ed. Gugotsi, Y. Taylor

Francis, NY, 2006.

2. US PAT 6858666, 2005

3. K.M.S. Khalil and M.I. Zaki, “Preparation and characterization of sol–gel derived

mesoporous titania spheroids”, Powder Technology 120, (2001), 256–263.

4. A. Hosseinnia, M. Keyanpour-Rad, M. Kazemzad, M. Pazouki, “A novel

approach for preparation of highly crystalline anatase TiO2 nanopowder from the

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List of table and figures

Table

Table 1: Crystallite size (nm) and dominant crystalline phase of the TiO2 specimens

synthesized at different calcination times.

Table 2: Average % yields obtained for the NUS and US sol-gel processes

Table 3: Crystallite sizes of the specimens synthesized using varied sonication times

Table 4: % Yields of the specimens obtained using varied sonication times

Figures

Figure 1: X-ray diffraction patterns of conventional sol-gel synthesized nano-TiO2 at

various calcination times.

Figure 2: X-ray diffraction patterns of ultrasound assisted sol-gel synthesized nano-TiO2

at various calcination times.

Figure 3: Variation of % Rutile of the TiO2 specimens (NUS and US) with calcination

time.

Figure 4: Variation of % crystallinity of the TiO2 specimens (NUS and US) with

calcination time.

Figure 5: SEM Micrographs of Synthesized TiO2: (A) TNUS 3 at 10000X, (B) TUS 3 at

10000X.

Figure 6: X-Ray diffractograms of the specimens synthesized at various sonication times.

Figure 7: % Rutile of the specimens synthesized at various sonication times.

Figure 8: % Crystallinity of the specimens synthesized using various sonication times.

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Figure 9: SEM micrographs of the specimens synthesized using various sonication times.

A: TNUS 3, B: TUS(5), C: TUS 3, D: TUS(15).

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