Solgel TiO2 in self-organization process: growth, ripening and sintering

Hsueh-Shih Chen* and Ramachandran Vasant Kumar

Received 23rd September 2011, Accepted 1st December 2011

DOI: 10.1039/c2ra00782g

TiO2 nanoparticles were synthesised by a self-organization method using a continuous water vapour

hydrolysis system without mixing. Formation of TiO2 particles was based on pure colloidal

interactions of hydrolyzed alkoxide molecules generated at the interface between titanium alkoxide

solution and water vapour. The effect of ethanol was proved to increase aggregation and packing of

primary particles, leading to a significant size enlargement of secondary particles in a post annealing

process above 500 uC. A new model to describe solgel TiO2 growth has been proposed, whichconsiders the formation of secondary particles to be the result of nucleation of oversaturated primary

particles, while the primary particles are the result of irreversible oligomerization of Ti oxo molecules.

Introduction

The solgel process offers many advantages for preparing oxide

materials such as low cost, high purity and a wide variety of

morphologies. The synthetic process of solgel materials in

general involves a wet-chemical process to form a precursor sol

or gel solution, which can be deposited on a substrate, followed

by a post heat treatment to convert the amorphous precursor

into crystalline particles or thin films.1 Typical solgel processing

of TiO2 nanocrystals includes the hydrolysis of titanium alkoxide

by water, which can react with alkoxyl groups, generating fully

or partially hydrolyzed (OR)42nTi(OH)n molecules. The (par-

tially) hydrolyzed molecules can react with each other or with

other alkoxides or hydroxides, forming an oxo bridge (O)

between Ti atoms (oxolation) together with an elimination of a

water molecule, as shown below. Briefly, the hydrolysis of Ti

alkoxides leads to hydroxyl groups, which is able to change

Ti(OR)4 to TiOTi via the condensation (i.e., olation or

oxolation process), and produces Ti-oxo-alkoxy or polyoxotita-

nate molecular clusters, for example, [Ti12O16](OPri)16,

[Ti11O13](OPri)18 and Ti16O16(OEt)32.

2 The Tioxoalkoxy clus-

ters can further grow to larger denser nanoparticles (NPs)

dispersing in a solution (i.e. sol). The sol particles can further

associate with each other via collisions after aging for a certain

period of time and thus form bigger isolate gel particles, gel

network, or porous films depending on experimental conditions.

For the hydrolysis of titanium alkoxides, it is known that the

amount of water strongly affects the morphology of final

products. According to the molar ratio of water to titanium

alkoxides (h), the solgel processing of TiO2 may be classified

into two regimes; low H2O/Ti molar ratio (h , 10) and high

H2O/Ti molar ratio (h . 10). At low h values, spherical

TiO2 particles with relatively uniform size about 0.51 mm are

obtained.3 At high h values, large aggregates of TiO2 rapidly

precipitate because fast hydrolysis leads to unstable colloids. The

aggregates can be separated to nano-sized particles (,100 nm) at

moderately elevated temperature by the so-called chemical

peptization process using acids such as nitric acid.4

Alcohol is often served as a solvent to slow down the

hydrolysis reaction of Ti alkoxides.5 However, effect of alcohols

on the morphology and microstructure is still an argument. Park

et al. reported that n-propanol acted as a dispersant for TiO2particles in the thermal hydrolysis of titanium tetrachloride in a

mixture of n-propanol and water.6 They found that without any

n-propanol TiO2 particles were small and agglomerated. With

addition of n-propanol (n-propanol/water = 3), TiO2 particles

became uniform in size and discrete. On the other hand,

Vorkapic et al. found that alcohols caused the aggregation of

TiO2 primary particles, which were synthesised via hydrolysis

condensation of alkoxides at a high h value.7 They found that

chemical factors relating to the hydrolysis and the condensation

(e.g. temperature, the length of alkoxyl of alkoxide) affected

the primary particle size rather than the final particle size. Also,

alcohols had a negative effect on the peptization. The smallest

secondary particles (about 20 nm) were obtained without any

alcohol modification. Thus, they suggested that formation of

TiO2 particles was controlled by the colloidal interactions

between primary particles. Moreover, it has been reported that

ethanol could cause the amorphization of TiO2 particles.8 Hague

et al. prepared TiO2 by hydrolyzing an isopropoxideethanol

mixture with a large excess of water (h = 165). Before

calcination, the samples were found to be the anatase. But the

samples became amorphous after they were rinsed twice by

ethanol. The authors suggested that this crystalline-to-

amorphous transformation was possibly due to the fact that

the hydrolysis reaction was slightly reversed by rinse of ethanol.

Although the solgel process is widely applied for preparing

TiO2, the growth mechanism of the solgel TiO2 is thought to be

complicated and experimental factors such as alcohol modifiers

Department of Materials Science and Metallurgy, University ofCambridge, Pembroke Street, Cambridge, CB2 3QZ, U.K.E-mail: sean.chen@cantab.net

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do not exhibit a similar effect on different synthetic systems. One

of the reasons is that hydrolysiscondensation of Ti alkoxide and

aggregation of monomers may occur simultaneously for a certain

period, which leads to different morphologies and sizes of TiO2particles, and therefore affects the crystal growth of TiO2 in

the post annealing process. Some growth models have been

proposed to describe the growth of solgel TiO2. A general

growth mechanism is that TiO2 primary particles generate from

the nucleation of supersaturated monomers, which are produced

by the hydrolysis of Ti alkoxides in the induction period.3 The

TiO2 primary particles can continue the growth via additions of

monomers and aggregation. This model assumes that monomers

can accumulate and form embryos in a reversible process

before they form stable nuclei when the critical concentration

is reached. On the other hand, the growth mode of TiO2 particles

is affected by the amount of water. For example, Oskam et al.

obtained nanocrystalline TiO2 NPs from the hydrolysis of Ti

alkoxide at a high water/Ti ratio, followed by peptization at

85 uC.9 The average radius of primary particles increased linearlywith time, which was consistent with the predication made by the

LifshitzSlyozovWagner (LSW) theory. They observed second-

ary particles formed by epitaxial self-assembly of the primary

particles with a size range 1.58 nm.

In this report, we synthesised TiO2 by employing a water

vapour hydrolysis system, in order to investigate the growth of

solgel TiO2 NPs and the influence of the initial processing

parameters such as alcohol on the morphology and microstruc-

ture of TiO2 in the sintering process. In this design, no mixing is

applied and the complexity of growth modes is simplified. The

hydrolysis is caused by condensed water from vapour occuring at

the interface between the alkoxide and water vapour phase,

which has very low water/alkoxide ratio (y 1/700 for one secondvapour input). The sol particles form at the interface region and

self-organise to gel particles via pure colloidal interactions. This

design allows us to examine pure interactions between solgel

particles excluding the effect of external mixing. Instead of

considering that primary particles are from the nucleation of

hydrolyzed monomers, a plausible model considering that TiO2secondary particles are from the nucleation of primary particles,

which are from irreversible coalescence of monomers, is

attempted to qualitatively describe to the growth of solgel TiO2.

Experimental

Materials

Titanium(IV) tetra-isopropoxide (TTIP, 97%) and absolute

ethanol (analytical reagent grade) were purchased form

Aldrich. All reagents were used without any further purification.

Doubly distilled water was used to initiate the hydrolysis and the

condensation process.

Synthesis of TiO2 by general approach

5 ml absolute ethanol was first used to modify TTIP (5 ml) using

magnetic stirring for 20 min in an argon atmosphere. Then, 5 ml

doubly distilled water was slowly added into the mixture of

ethanol and TTIP at room temperature to form a sol solution.

The molar ratio of water/TTIP is 15.4. Gel solution was obtained

by aging the sol for 18 h at room temperature. Annealing process

was carried out at 150 uC for 1 h, followed by 600 uC for 2 h witha heating rate of 5 uC min21. White crystalline TiO2 powder wereobtained after the annealing process.

Synthesis of TiO2 by vapour hydrolysis

An experimental system deigned to synthesise TiO2 via slow

hydrolysis of TTIP by water vapour is shown in Fig. 1(a). The

water vapour was generated by heating water at 70 uC (reservoirA) and carried by argon (99.9%) into reservoir B containing

TTIP kept at room temperature. The input rate of water vapour

was controlled by the flow rate of argon. The amount of water

input was estimated by the weight of water accumulated in

the reservoir C. Plastic pipes connecting each reservoir were

wrapped with aluminium foils to prevent water vapour from

condensation. In order to exclude the effects of stirring on the

formation and aggregation of particles, no stirring was applied

to the system throughout the process. As the TTIP was kept at

room temperature, the water vapour was expected to condense

to liquid as it contacted the TTIP surface. Thus, the hydrolysis

and the condensation would occur at the near surface region of

the TTIP. A typical input rate of water vapour was set at 5.21 61026 mol s21. The molar ratio of water/TTIP was about 1/700

for the vapour input lasting a second. The overall input time

(aging time) was eight hours. A white precipitate eventually

formed at the bottom of the reservoir. The product was then

milled to a powder for subsequent characterisation. Pure TTIP

and alcohol-modified TTIP were used as precursors. For

investigating the structure and morphology of samples in the

annealing process, pure TTIP or an ethanolTTIP mixture

(volume ratio = 0.1/1) was used. The overall reaction time was

8 h. Samples were placed into a furnace and taken out at a

desired temperature, as shown in Fig. 1(b).

Characterisation

The surface morphology of TiO2 was investigated by field

emission gun scanning electron microscopy (SEM). The average

size of particles was estimated according to SEM images for

more than 130 particles using computer software (ImageJ).

Powder X-ray diffraction (XRD) was employed to study the

crystallography of samples. The crystallite size of TiO2 is

estimated by FWHM according to Scherrers formula, D =

(0.9 l)/(b1/2 coshB), where D is the average grain size, l is the

Fig. 1 (a) Experimental design of vapour hydrolysis. (b) Evolution of

the microstructure and the morphology of samples in the annealing

process. Red dots indicate that samples were taken out from a furnace at

100, 200, 300, 400, 500, 600, and 600 uC (15 h).

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wavelength of Cu Ka (= 1.5405 A), b1/2 is the FWHM (full width

at half maximum), and hB is the diffraction angle.

Results and discussion

TiO2 synthesised by general approach

As-prepared TiO2 sol was a white suspension. After aging, larger

white gel particles formed at the bottom of the vessel if no

stirring was applied. Dried TiO2 gel samples have no crystal-

linity, showing that highly crystalline TiO2 would not form at

room temperature without heat treatment, as shown in the XRD

data in Fig. 2 (curve a). The TiO2 crystalline phase is generated

after a heat treatment is carried out, shown in Fig. 2 (curve b).

The XRD data indicate that annealed TiO2 powders have an

anatase phase with average grain size of 30.1 nm. It also shows

that a small amount of anatase TiO2 transforms into the rutile

phase. SEM images (Fig. 3(a)) show that the morphology of

TiO2 particles is oval-shaped and coalesced and has a mean size

about 1.02 mm 36%. A high magnification image in Fig. 3(b)

shows that the TiO2 is made up of secondary particles composed

of primary nanoparticles with diameters of 33.2 nm 25%.

Note that the primary particle size is close to the average grain

size estimated from the XRD data, inferring the majority of the

primary particles are nearly single crystalline. The above SEM

and XRD results suggest that amorphous primary particles first

generate in the solgel process and then aggregate into secondary

particles. Since there is no existence of TiO2 crystalline phases

before the annealing process, the formation of the primary

particles in this case may not be mixed with the conventional

nucleation-growth process of crystalline materials.

The growth of TiO2 from the general solgel process is

schematically shown in Fig. 4(a). When water is introduced into

the TTIP solution, Ti-oxo (or Ti-oxo-alkoxy) molecular clusters

are generated through the hydrolysiscondensation reaction of

the alkoxides and form particulate sol particles. The primary

particles may be viewed as the coalesced Tioxo molecular

clusters generated from hydrolyzed Ti alkoxides. Thus, the size

of the primary particle should be dependent on the hydrolysis

condensation and chemistry of the solution. For the secondary

particles, their average size is related to the dimension of the gel

particles that is determined by the interaction between colloidal

particles.7

TiO2 powders synthesised from the ordinary solgel process at

room temperature generally show a broad amorphous XRD

peak, which is actually contributed by numerous diffraction data

from Ti-oxo molecular clusters and domains in a microscopic

viewpoint. The Ti-oxo molecular clusters randomly connect with

each other and could not form periodic structures in a fast

hydrolysiscondenstation process. In some cases, crystalline

materials may be obtained by standing a solution of Tioxo

molecular clusters having a well-defined structure at room or

Fig. 2 XRD of TiO2 synthesised by a standard solgel method (volume

ratio TTIP/ethanol/water = 1/1/1). As-prepared sample (curves a) and

sample annealed at 600 uC for 2 h (curve b).

Fig. 3 SEM images of TiO2 particles synthesised by a normal solgel

method (volume ratio TTIP/ethanol/water = 1/1/1) at pH = 7. (a) 30006.(b) 70 0006.

Fig. 4 (a) Normal solgel process of TiO2 particles. (b) TiO2 prepared

by solgel process in the vapour hydrolysis design.

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elevated temperature for a longer time (e.g., couple of days). In

that case, stable Ti-oxo molecular clusters act as building blocks

being able to periodically assemble in a slow assembly process.

So the crystalline phase could be recorded by single crystal

XRD.2c,2d

TiO2 synthesised by vapour hydrolysis

Fig. 5 gives SEM images of a TiO2 sample prepared by

hydrolyzing pure TTIP in the vapour hydrolysis design. The

sample dried at room temperature contains secondary particles,

which are composed of primary particles. This result is similar to

those samples prepared by the standard solgel method. In the

experimental design of the vapour hydrolysis system, water

vapour is continuously transported into a vessel containing

TTIP. The vapour is condensed into liquid H2O at the interface

between the atmosphere and the TTIP surface. For a rate of

5.21 6 1026 mol s21, the molar ratio of water/TTIP is about1/700 for a second of the water vapour input. The water would

not largely accumulate since reaction between water and TTIP is

very fast. The characteristic time of the hydrolysiscondensation

of TTIP precursor reported is in the range of tens of

milliseconds.10,11 So it is expected that reaction between water

and TTIP mainly occurs at the interface region, where Ti-oxo-

alkoxy molecular clusters generate and migrate into the solution

due to the Brownian interaction and gravity, as shown in region I

Fig. 4(b). The generated Ti-oxo-alkoxy clusters would only

contain single or few Ti atoms due to a limited water input. Some

stable clusters TinOm(OiPr)4n-2m-l(OH)l, with n = 3, 11, 12, and

17, have been found in the solutions containing TTTIP at low

hydrolysis ratio h = [Water]/[Ti] , 1.12 The smallest stable Ti-

oxo-alkoxy cluster observed was Ti3-oxo clusters synthesised at

h y 0.05.13 Larger Ti-oxo clusters and nanoparticles wereobtained at higher h values (. 0.7).10

In the vapour hydrolysis system, formations of sol and gel

particles are based on self-organisation since there is no mixing

applied in this system. So the collisions among the particles

instead dominate the growth. The Ti-oxo-alkoxy molecular

clusters are first generated at the TTIP surface where the

hydrolysis and condensation take place. Ti-oxo-alkoxy clusters

could grow into larger sol particles via collisions. This event then

produces a colloidal solution (i.e. sol process, region I). If the sol

particles link to each other, large gel particles are produced and

fall down from near the surface region due to gravity. The

formation of secondary particles is caused by the aggregation

of the primary particles shown in region III in Fig. 4(b).

Consequently, the sol and gel particles are assigned to the

primary and the secondary particles respectively. In addition, the

definition of the term sol particle is somewhat confusing since

its scale (several to tens of nm) overlaps with those of molecular

clusters and of nanoparticles. Basically, sol particles may be

viewed as large molecular clusters (e.g., polymers) with denser

structures and equivalent to nanoparticles with either amor-

phous or crystalline structures. Gel particles may be viewed as

larger coalesced sol particles in the micron scale.

Evolution of the morphology of TiO2 obtained by annealing

samples at various temperatures is given in SEM images in

Fig. 5. It shows that the TiO2 secondary particles have irregular

shapes and no significant change in the morphology takes place

below 500 uC. For samples annealed at 600 uC and 600 uC/15 h,sintering of the particles is observed. The primary particles

coalesced and merged, as shown in Fig. 5 (images c and d). XRD

data show that there is no crystalline phase for an as-prepared

sample whereas the anatase appears at 100 uC (Fig. 6(a)). Theanatase is partly transformed into rutile after heating at 600 uCfor 15 h. The final product is a mixture of anatase and rutile, as

shown in Fig. 6(a). Variations of the crystallite size, the primary

particles, and the secondary particles, estimated from XRD data

and SEM images, are presented in Fig. 7 The as-prepared

primary and secondary particles at room temperature are 14 nm

14% and 1.3 mm 28% respectively. The size does not

significantly change for samples annealed at temperatures below

Fig. 5 SEM images of TiO2 dried at room temperature (a), 500 uC (b),600 uC (c), and 600 uC for 15 h (d). Samples were prepared by vapour-hydrolyzing pure TTIP.

Fig. 6 XRD of TiO2 annealed at temperatures from room temperature

(R.T.) to 600 uC and 600 uC for 15 h. Samples were prepared by vapour-hydrolyzing pure TTIP (a) and ethanolTTIP mixture (b).

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500 uC. The stability of the particles size below 500 uC isunderstandable; there is no significant sintering of TiO2 particles

below 500 uC observed (melting point of TiO2 y 1840 uC) sothey are able to keep their original size. But at 600 uC theprimary and the secondary particles readily enlarge since the

sintering causes grain growth. A slight decrease in size at 100 uCis ascribed to the removal of organic compounds that lead to

some shrinkage and/or cleavage of the large particles.

The grain size of TiO2, on the other hand, does not correspond

to the size of the primary particles. Initially the grain size is

4.5 nm and increases to 7.5 nm at 500 uC. Note that 7.5 nm is stillsmaller than the diameter of the primary particles. The grain

enlargement may be caused by the crystallisation in the primary

particles and/or sintering between the particles. In the present

case, the grain growth below 500 uC would be caused by thecrystallisation in the primary particles rather than the sintering

between the particles because the crystallisation of TiO2 is able to

take place below 500 uC, while the sintering requires a highertemperature to overcome the inter-particle diffusion barrier. The

sintering event could be observed when the temperature is equal

to 600 uC, as shown in Fig. 7. Both of the primary particles andthe secondary particles enlarge, indicating inter-particle grain

growth via necking occurs. Moreover, the grain size is nearly the

same as the primary particle size, indicating that the primary

particles are nearly single crystalline at 600 uC. A further heattreatment (600 uC for 15 h) for grain growth or coarseningoccurs via the sintering process between the primary particles, as

shown in Fig. 5(d). The primary particles disappear and larger

grains appear on the TiO2 secondary particles when heated at

600 uC for 15 h. It is noted that the sintering mainly occursamong primary particles, which transform to a certain layered

growth mode. The necking between the secondary particles is

hardly seen in the present study. Based on the above results,

crystal growth of TiO2 in the heat treatment process may be

illustrated in Fig. 8. TiO2 crystallites nucleate in the matrix of the

primary particles and further develop to larger grains with

increasing temperature. When the annealing temperature is high

enough, the primary particles become single crystalline. Further

grain growth can continue via inter-particle coalescence caused

by sintering of primary particles.

Influence of ethanol on the morphology of TiO2

Although alcohols, e.g. ethanol, have been widely used to dilute

TTIP for reducing the hydrolysis rate, however, alcohols affect

the solution chemistry and thus alter the self-organisation

process of TiO2 particles. For the addition of ethanol to TTIP

solution, a certain degree of the substitution for propoxy groups

by ethoxy ones is expected. The substitution is reversible. For

partially hydrolyzed polyoxotitanates, e.g. [Ti11O13](OPri)18,

[Ti11O13](OPri)13(OEt)5 could exist after reacting with ethanol.

2

On the other hand, it has been shown that alcohols destabilise

the colloidal solution and enhance the rate of re-aggregation

because alcohols decrease the dielectric constant of the solvent

that correlates with the zeta potential of TiO2.6,14 The degree of

aggregation of the primary particles was found to increases with

lengthening alkyl chain of alcohols when the TiO2 powders are

re-dispersed in various alcohols, for example, the degree of

aggregation: propanol . ethanol . methanol.

The effect of ethanol on the particle morphology in the vapour

hydrolysis system can be clearly examined. As shown in SEM

images in Fig. 9, the morphology of TiO2 particles is largely

Fig. 7 Size variations of TiO2 primary particles, secondary particles,

and crystallites prepared by vapour-hydrolyzing pure TTIP.

Fig. 8 Nucleation and growth of TiO2 in the solgel process.

Fig. 9 SEM images of TiO2 particles synthesised by pure TIP and

ethanolTTIP (volume ratio = 5/7). The samples were annealed at 150 uCfor an hour, followed by 400 uC for another hour with a heating rate of10 uC min21.

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changed by introducing ethanol to modify TTIP. Increasing the

volume ratio of ethanol (TTIP : ethanol from 5 : 0 to 5 : 7)

results in a denser packing for TiO2 secondary particles, implying

that ethanol is beneficial to the coalescence of the primary

particles. The secondary particles tend to be spherical in the

presence of ethanol. The ethanol increases the surface tension

of the secondary particles (if treating the primary particles as

atoms), and causes a reduction of the surface area of the

secondary particles. This inward force could be correlated to the

inter-particle interaction for the primary particles, which pull

inwards the secondary particles. The inter-particle interaction

may be related to the surface functional groups (e.g. hydroxyl

groups) and the zeta potential of TiO2 particles. Another

possibility is that ethanol stabilized the TTIP so that the

generation of the monomers and the primary particles are so

slow that the stacking of the particles is improved, which leads to

a dense structure.

The evolution of the crystallinity and morphology of the TiO2crystallite, the primary particles, and the secondary particles

from the hydrolysis of the ethanol-modified TTIP with respect to

annealing temperature are shown by XRD (Fig. 6(b)) and in

SEM images (Fig. 10). Variations of size of the primary particles

and the crystallites are shown in Fig. 11(a). The change in size of

the secondary particles is shown in Fig. 11(b). As-prepared TiO2from ethanolmodified TTIP is amorphous (curve R.T. in

Fig. 6(b)). The TiO2 crystallites form at 100 uC and have a sizeof about 2.6 nm, which is much smaller than those from pure

TTIP (about 4.5 nm), as shown in Fig. 11(a). This result shows

that the ethanol has a negative effect on the crystallisation of

TiO2 in the heating process. The ethanol molecules could offer

coordinative and hydrogen bonding to the anatase TiO2surface.15 This ethanol coordination to Ti would offer a chemical

stabilising effect on embryos and/or crystallites that delay the

crystallisation and crystal growth and thus the crystallite size

reduces. We have found that alcohols with longer alkyl groups

significantly suppressed the crystallization of TiO2 (results will

be published elsewhere).

The crystallite enlarges with increasing temperature from 2.6,

4.6, 6.0, 6.4, and 9.0 nm for samples annealed at 100, 200, 300,

400, and 500 uC, respectively, compared with those preparedwithout ethanol (4.4, 4.8, 6.0, 6.3, and 8.1 nm for 100, 200, 300,

400, and 500 uC respectively), implying that the ethanol effect isreduced at around 200 uC. For the primary particles preparedfrom ethanol-modified TTIP, similar to those from pure TTIP,

they become nearly single crystalline at around 600 uC and

Fig. 10 SEM images of TiO2 synthesised by vapour-hydrolyzing

ethanolTTIP mixture (TTIP/ethanol = 1/0.1) annealed at room

temperature (a), 500 uC (b), 600 uC (c), and 600 uC for 15 h (d).

Fig. 11 Evolution of sizes of the TiO2 crystallite, primary particle, and

secondary particle in the annealing process. (a) Variations of the

crystallite size and the primary particle size. (b) Variation of the

secondary particle size.

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enlarge readily after dwelling for 15 h. For the secondary

particles, their average size has no significant change at lower

annealing temperatures (100500 uC), but largely increases as theannealing temperature is above 500 uC. This event is similar tothose from pure TTIP; size of the secondary particles increases

due to the sintering of the primary particle. As the secondary

particles from ethanol-stabilised TTIP have densely packed

primary particles, the densification is more feasible and thus the

secondary particles enlarge faster than those from pure TTIP.

Growth model of TiO2 secondary nanoparticles

The general growth model of solgel TiO2 considers that primary

particles are the result of nucleation of supersaturated hydro-

lyzed monomers from hydrolysis of Ti alkoxides. In the

induction period, monomers (e.g., Ti-oxo-alkoxy) generate due

to the hydrolysis of alkoxides that increases the monomer

concentration.16 When the monomer concentration reaches a

critical level, nucleation occurs. The induction time is defined by

the visual turbidity of reaction liquid and reflects the rate at

which critical supersaturation is achieved.3 However, it was

found that nanoparticles already generate and grow during the

induction period in a dynamic light scattering study.17 So the

induction time may not precisely reflect the formation of

particles in the nanoscopic scale. On the other hand, accumula-

tion of monomers in the induction period is not considered in

some studies. As the Ti-O-Ti oxo bonds are very strong and

condensation of hydrolyzed clusters is irreversible, Ti-oxo

clusters enlarge through the reactions or additions with other

Ti-oxo clusters.18 In the present case, Ti-oxo-alkoxy clusters

generated at the TTIP surface are thought to connect with each

other via the condensation reaction between metalOH groups

and/or metalOR groups. The coalescence of Ti-oxo-alkoxy

clusters towards denser/bigger primary particles is considered an

irreversible process. Then, the primary particles drop down to

the lower pure TTIP area due to their increasing weight. The

surface of the primary particles could be deactivated because the

hydroxyl groups on the primary particles could be converted to

non-reactive alkyl chains by reacting with TTIP molecules. In

this pure TTIP region, there is no external water molecules

provided so the irreversible condensation between the primary

particles surfaces could be effectively suppressed. Instead, the

reversible coalescence process becomes dominant in the aggrega-

tion of the primary particles. The stability of coalesced primary

particles is determined by the GibbsThompson relationship, Cr= C0exp[(2cv)/(rkT)]. As there is no stirring in the present system,

the primary particles randomly collide with each other in the

solution due to Brownian motion and can result in a local

concentration fluctuation. In this situation, some primary

particles could coalesce to a meta-stable particle aggregate,

which could either grow larger (forward process), or dissociate

into individual primary particles (inverse process). Therefore, the

aggregate of the primary particles would have the critical size at

which the aggregate is stable. The process is somewhat like the

conventional homogeneous nucleation process of crystals, where

the number of the nuclei containing n monomers is given by the

Boltzmanns distribution, Nn = N(n)exp[2DGn/(kT)], where N(n)

is a function of n, k is Boltzmanns coefficient, T is temperature,

and DGn is the activation energy, which is a function of the

surface energy and the volume free energy. If the number of

monomers (n) is much more than embryos, N(n) can be assumed

a constant (N0), Nn = N0 exp[2DGn/(kT)]. The VolmerWeber

theory predicts the nucleation rate of oversaturated vapour

condensing to liquid.19 The nucleation rate is proportional to

number of the embryos at the critical size and the condensation

rate. The condensation rate of vapour is proportional to the

surface area of an embryo (Ac) and the probability of a vapour

atom liquidized on the embryo per unit area and time (P). I =

AcPNn = AcPN0 exp[2DGn/(kT)]. The VolmerWeber theory

assumes the process to be a quasi-steady state where the atoms

are constantly introduced to the condensation system. The model

also assumes that once the nuclei forms the particle will continue

to enlarge. The predication made by the theory has been found

to be consistent with the experimental results if the atoms are

much more than the nuclei. According to the theory, the

nucleation rate increases with increasing concentration of the

vapour atoms. In a colloidal system, the monomer concentration

strongly determines the nucleation and growth. Higher monomer

concentration allows generation of the smaller nuclei generated

according to the GibbsThompson relationship. Variation of

monomer concentration also affects the size and size distribution

of crystals.20 Higher monomer concentration prevents smaller

particles from dissolution in the solution and diminishes the size

broadening process (i.e. Ostwald ripening).

Fig. 12 shows that the secondary particle size decreases with

increasing vapour input rate. The average sizes of the secondary

particles are 0.83 mm and 0.42 mm for vapour rates of 3.47 61026, and 8.68 6 1026 mol s21, respectively. The particle sizedistribution of the samples also decreases from 73.9 to 42.3%

with increasing vapour input rate. Considering the smallest

stable aggregate of the primary particles to be the nuclei of the

secondary particles, the concentration of the primary particles

can alter the size of the secondary particles according the above

model. In the water vapour hydrolysis system, the vapour input

leads to the hydrolysis of TTIP at the interface that continuously

supplies the hydrolyzed molecules and the primary particles into

the TTIP solution, where the quasi-steady state is able to

establish if the generation and consumption of the primary

particles reach a dynamic equilibrium situation. This process

resembles the model of crystallisation. The addition of the

primary particles onto the secondary particles may be based on

Fig. 12 SEM images of TiO2 secondary particles produced by hydro-

lyzing pure TTIP with vapour input rates of 3.47 6 1026 mol s21 (a) and8.68 6 1026 mol s21 (b).

2300 | RSC Adv., 2012, 2, 22942301 This journal is The Royal Society of Chemistry 2012

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the reductions of the surface energy of the secondary particles.

The secondary particle size therefore decreases with increasing

the water vapour input rate.

Coalescence of secondary particles

Since the generation of the TiO2 secondary particles from the

primary particles is similar to the material crystallization process,

it is worth to check if there is a process similar to Ostwald

ripening occurring in the crystallization process. Fig. 13 shows

SEM images of the secondary particles (vapour rate = 5.21 61026 mol s21, TTIP/ethanol = 5/1) dried at 400 uC for 1 h, atwhich no sintering event occurs, proved in the above data.

The secondary particles appear to combine with each other

(Fig. 13(a)). The junction between the particles clearly shows

that the combination is not caused by the sintering (necking)

between the secondary particles (Fig. 13(b)) and appears to be

the coalescence of the secondary particles. The coalescence

would not be caused by the evaporation of solvents in the

separation process of products because the products formed in

the system were a solid precipitate in the bottom of the reaction

vessel rather than a colloidal solution. Presumably, the

combination of the secondary particles is caused by the

coalescence between secondary particles in the solution. This

event is similar to Ostwald ripening based on the difference in the

solubility of larger and smaller crystals. The driving force of

combination of the secondary particles in the present system

would be relating to the surface functional groups and the zeta-

potential of the secondary particles, which is under investigation.

Conclusion

TiO2 was prepared in a self-organization process with an ultraslow

hydrolysis rate via a vapour hydrolysis system without mixing and

in a normal solgel process for reference. Experimental results

from both methods suggest that solgel TiO2 were secondary

particles composed of primary particles, which were amorphous

before an annealing process was performed. The crystalline

anatase primary particles were generated at 100 uC and becamesingle crystalline at around 600 uC. The ethanol effect on solgelTiO2 was proved to aid aggregation and packing of TiO2 primary

particles. This phenomenon leads to a size enlargement of the

secondary particles when the post annealing temperature is above

500 uC. A plausible model considering that TiO2 secondaryparticles are the result of the nucleation of the primary particles is

attempted to qualitatively describe to the growth of solgel TiO2.

Experimental results show that the secondary particles size

reduces with increasing vapour input rate, which is consistent

with the prediction of the growth model.

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Fig. 13 Association of the secondary particles (Ostwald ripening)

before the coarsening of grains takes place. (a) A secondary particle

merged with two other smaller secondary particles. (b) The cross section

of the junction of a merged secondary particle.

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