Tio2 Opals

ISSN 0020-1685, Inorganic Materials, 2009, Vol. 45, No. 11, pp. 12521262. Pleiades Publishing, Ltd., 2009.Original Russian Text S.N. Ivicheva, Yu.F. Kargin, O.A. Lyapina, G.Yu. Yurkov, S.V. Kutsev, L.I. Shvorneva, 2009, published in Neorganicheskie Materialy, 2009, Vol. 45,No. 11, pp. 13371348.

1252

INTRODUCTION

The preparation of finely dispersed oxides andrelated nanocomposites is an important area in moderninorganic synthesis and advanced materials engineer-ing. The properties of solids depend not only on theirchemical composition but also on their structure.Therefore, novel materials can be created by both uti-lizing various chemical compositions and devisinginnovative preparation processes that would allow oneto widely vary the structure-sensitive properties of thesystem [1].

A variety of approaches have been proposed for pro-ducing individual nanoparticles [13]. It has been shownthat nanoparticles can be stabilized on the surface of vari-ous materials: polystyrene and polyethylene microspheresand oxide supports, including SiO

2

. Nanocomposites canbe prepared using opal crystals [4, 5]. Considerableresearch effort has been concentrated on the synthesis ofnanocrystalline titania-based materials using hydrother-mal and supercritical solutions and ultrasonic and micro-wave processing [6, 7].

The high effective porosity of opal and the orderedarrangement of opal pores allow one to vary the refrac-tive-index difference by partially or fully filling thepores with high-index materials. Some oxides, such asTiO

2

in opal, can be used to fabricate composites withquantum-optical characteristics [8]. Titania coatings onporous oxide (SiO

2

) supports are used to enhance thespecific surface area, mechanical strength, thermal sta-bility, and selectivity of catalysts [9]. Xerogel- andTiO

2

-based composites find application in photocata-lytic processes. Note that only titania in a certain crys-talline form or a controlled mixture of crystallinephases exhibits activity for photostimulated catalyticand photoelectric reactions [10].

In nature, titania exists in three crystalline polymor-phs: rutile (

r

-

TiO

2

, sp. gr.

P

4

2

/

mnm

), anatase (

-

TiO

2

, sp.gr.

I

4

1

/

amd

), and brookite (

b

-

TiO

2

, sp. gr.

Pbca

) [11].Rutile and anatase have tetragonal structures, and broo-kite has an orthorhombic structure. Under ordinaryconditions, the thermodynamically stable crystallineform of TiO

2

is rutile, whereas anatase and brookite aremetastable phases and transform into rutile at tempera-tures of 4001000 and

750

, respectively [12, 13].The general mechanisms underlying the formation

of the metastable TiO

2

polymorphs at atmospheric pres-sure are now receiving particular attention. Severalmodels have been proposed to explain the formation ofthese thermodynamically metastable phases at lowtemperatures. One model relies on the assumption thatthe size factor (excess surface energy) plays a key rolein stabilizing the metastable titania polymorphs [14].Hengzholg and Banfield [15] showed using thermody-namic analysis of the phase stability of titania nanopar-ticles that, at a crystallite size under 14 nm, the thermo-dynamically stable phase is anatase. This finding corre-lates with the report by Zhang and Banfield [16] thatbrookite and anatase are more thermodynamically sta-ble than rutile at TiO

2

particle sizes below 35 nm.Another model relies on the assumption that the

thermodynamically metastable phases of TiO

2

are sta-bilized by ions present in the reaction system. Oneexample is the stabilization of anatase by the sulfate ionwhen solgel synthesis is conducted in the presence ofsmall amounts of sulfuric acid [17]. The anatase thusprepared remains stable up to

600700

and trans-forms into rutile at higher temperatures.

Solgel processing is widely used to synthesizeoxide nanopowders. The key step in this approach is theformation of a solid phase (hydroxide) via gelation in acolloid system [18], and typical precursors are metal

TiO

2

Nanoparticles in Opal Crystals

S. N. Ivicheva

a

, Yu. F. Kargin

a

, O. A. Lyapina

b

, G. Yu. Yurkov

a

, S. V. Kutsev

a

, and L. I. Shvorneva

a

a

Baikov Institute of Metallurgy and Materials Science, Russian Academy of Sciences,Leninskii pr. 49, Moscow, 119991 Russia

b

Moscow State University, Moscow, 119899 Russiae-mail: [email protected]

Received April 29, 2009

Abstract

We have studied the effect of the size factor on the phase composition of fine-particle TiO

2

usingthree-dimensional opal crystals as a system of nanoreactors for solgel synthesis of TiO

2

n

H

2

O. The resultsindicate that the stability region of the thermodynamically metastable TiO

2

polymorphs can be extended tohigher temperatures.

DOI:

10.1134/S0020168509110120

INORGANIC MATERIALS

Vol. 45

No. 11

2009

TiO

2

NANOPARTICLES IN OPAL CRYSTALS 1253

and nonmetal alkoxides. Hydrolytic decomposition ofalkoxides in water leads to sequential formation of a soland then a gel of the corresponding hydroxide. Subse-quent heat treatment leads to dehydration of the gel,and the hydroxide decomposes to form fine powder ofthe corresponding oxide.

In this work, a three-dimensional opal lattice is usedas a system of nanoreactors in order to study the effectof the size factor on the phase composition of the form-ing TiO

2

particles, with special attention to the presenceof stabilizing ions during solgel synthesis of TiO

2

n

H

2

O.

EXPERIMENTALThe synthesis process involved three steps.1. Synthesis of monodisperse silica spheres in a col-

loidal system by the StberFink method [19]. In thisstep, particular attention was paid to the preparation ofstarting reagents and optimization of the process condi-tions in order to ensure monodispersity and aggregationstability of the suspension and uniformity of the silicaspheres.

2. Preparation of an ordered structure and strength-ening of the opaline material using centrifugation, sed-imentation under conditions favorable for the transfor-mation of the hydrogel (wet residue) into xerogel (opalcrystal), and low- and high-temperature heat treatmentsto the desired drying and annealing schedule.

3. Preparation of opaltitania composites.The process proposed by Stber and Fink in 1968

involves as a key step the formation of spherical silicaparticles during tetraethyl orthosilicate (TEOS) hydrol-ysis in an organic solvent. The catalyst used was anammonia solution [20].

Opal crystals, consisting of ordered silica spheres,were prepared by two procedures: suspension centrifu-gation and natural sedimentation of particles, followedby heat treatment.

Opaltitania composites were prepared by solgelprocessing.

A solgel process involving tetraethyl orthotitanate(TEOT) or another titanium alkoxide, Ti

(

OR

)

4

(whereOR is an alkoxy group), may be regarded as a two-stepinorganic polymerization. The process is initiated byhydrolysis of the alkoxy group, which leads to the for-mation of an alcohol (ROH) and hydroxylated metalliccenters:

Ti

(

OR

)

4

+

H

2

O

[

Ti

(

OR

)

3

(

OH

)] +

ROH

,

where R is an alkyl group.This process then propagates in three dimensions to

give oxy polymers. The polycondensation leads to theformation of oxygen bridges:

(

OR

)

3

Ti

OH

+(

OR

)

3

Ti

O

(X = H or R).Because the Ti

(

OR

)

4

alkoxides offer good reactivityin hydrolysis processes, the polymerization rate is often

controlled by an acid catalyst, which slows down theformation of TiO

2

n

H

2

O.Nanocomposites of opal and ultrafine titanium

hydroxide particles were prepared by different solgelprocesses. One process involved, as a key step, rapidhydrolysis of a titanium alkoxide infiltrated into opal inpure form (procedure I, opal/TiO

2

-I). The other tookadvantage of slow hydrolysis: infiltration of stable acid(pH

2) titanium hydroxide sols into the ordered porestructure formed by monodisperse SiO

2

spheres (proce-dure II, opal/TiO

2

-II).Sols containing TiO

2

n

H

2

O particles were preparedby hydrolyzing Ti

(

OC

2

H

5

)

4

in ethanol with 30% hydro-chloric acid as a catalyst. The sols were stable for a timesufficient for complete infiltration of the opal samples(up to four days) and then converted to gels or suspen-sions, depending on composition. The white gel-likeprecipitates obtained in the solutions were processedand heat-treated in the same way as the opal-basedcomposites in order to compare their phase composi-tion with that of the TiO

2

particles produced in the opalcrystals.

The composite materials and hydrous TiO

2

powderswere air-dried for 10 h between 100 and 300

C at aheating rate of 10

C/min. High-temperature annealswere performed at 450, 550, 650, 750, and 1200

C for5 h.

The nanocomposites and powders were character-ized by thermogravimetry (TG), differential thermalanalysis (DTA), X-ray diffraction (XRD), X-raymicroanalysis, scanning electron microscopy (SEM),and transmission electron microscopy (TEM).

XRD examination was performed after each heat-treatment step on a Shimadzu XRD 6000 diffractome-ter (scan step of 0.02

(

2

), counting time of 2 s per datapoint) with CuK

radiation (

=

1.54184

). The phasespresent were identified using the WinXpow softwarepackage. The XRD data were used to determine thecrystallite size in the TiO

2

powders by the Scherrer for-mula.

The morphology and size of TiO

2

particles wereassessed by TEM on a JEOL JEM-100B. The compos-ite was dispersed in an alcohol-in-water solution bysonication, and the dispersion was applied to a carbon-coated copper grid.

SEM micrographs were obtained on a LEO-1420high-resolution microscope equipped with an X-raymicroanalysis system (accelerating voltage, 20 kV;magnifications from 20000

to 100000

).Thermal analysis was carried out with a Netzsch

STA 409 Luxx system in air between room temperatureand

1400

at a heating rate of

512

C/min. The sam-ple weight was

100

mg.The specific surface area and porosity were deter-

mined by gas sorption measurements using aMicromeritics TriStar 3000 analyzer.

1254

INORGANIC MATERIALS

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No. 11

2009

IVICHEVA et al.

RESULTS AND DISCUSSION

The synthesized opal had the form of a well-orderedthree-dimensional array of monodisperse amorphoussilica spheres about 300 nm in diameter. The specificsurface area of the opal was about 10 m2/g, and itseffective porosity was 45%. The electron micrographsin Fig. 1 illustrate the close-packed structure of the opalat different magnifications.

Opal/TiO2 nanocomposites were produced by a sin-gle infiltration with TEOT followed by hydrolysis (pro-

cedure I) and by a single infiltration with a sol contain-ing TiO2 nH2O particles in the system C2H5OH : H2O :HCl : (C2H5O)4Ti in the molar ratio 2 : 8 : 4 : 1 (proce-dure II). The charts in Fig. 2 represent the average ele-mental compositions of the composites as determinedby X-ray microanalysis. As seen, the composite pre-pared by procedure I (opal/TiO2-I) contains more TiO2.The specific surface area of the opal/TiO2-I compositeincreased markedly, to above 45 m2/g. Evaluation of themesopore size by the BarrettJoynerHalenda methodshowed that, in composite I, most of the pores were less

(b)

1 m

200 nm

Pa 1

Pa R1

Pa 1 = 972.2 nm

Pa R2

Pa 1 = 3.142 m

Pa R1

Pa 2

Pa 1

Pa 2 = 321.6 nm

()

Fig. 1. SEM micrographs of thin cleaved opal layers at different magnifications.

INORGANIC MATERIALS Vol. 45 No. 11 2009

TiO2 NANOPARTICLES IN OPAL CRYSTALS 1255

than 2 nm in size, which confirmed the formation ofultrafine TiO2 particles in the opal pores. The specificsurface area of the composite produced by procedure II(opal/TiO2-II) varied insignificantly because of thelower TiO2 concentration and partial clogging of openmesopores in the opal. At the same time, the TiO2 nH2O-II powders had a high specific surface area, inexcess of 90 m2/g.

The plain (uninfiltrated) opal was X-ray amorphous.XRD characterization showed that, after heat treatmentat 300C, the opal/TiO2-I composite consisted of twophases: amorphous silica (opal) and crystalline titaniawith the anatase structure. The opal/TiO2-II compositeheat-treated at 300C consisted of amorphous SiO2 andtwo crystalline forms of titania: anatase and rutile. Afterheat treatment at 300, the titania powders preparedby different procedures also differed in phase composi-tion: powder I was single-phase (anatase), whereaspowder II was two-phase and consisted of anatase andrutile in the ratio 10 : 1. The difference in phase compo-sition between the powders and composites can beunderstood in terms of TEOT hydrolysis conditions.The water : titanium alkoxide ratio in the hydrolyticpolycondensation reaction and the presence of the chlo-ride ion as a sol stabilizer influence the phase composi-tion of the reaction products, as supported by earlierresults [21, 22].

Figure 3 presents the TGA data and mass spectro-metric analyses of the released gas for the opal/ TiO2nanocomposites and TiO2 nH2O powders prepared bydifferent procedures. The shape of the TG and DSCcurves obtained during temperature-programmed heat-ing of the composites and powders is influenced by thepreparation procedure, the ratio of the components inthe solgel process, and the presence of the sol-stabiliz-ing ion.

The TG curves of the opal/ TiO2-I and opal/ TiO2-IIcomposites (Figs. 3a, 3b) show a multistep weight loss(total weight loss, 13%), which is largely due to waterremoval. Below 200, weight changes are due to therelease of physisorbed water from the mesopores of thecomposites and, in the case of composite II, hydrochlo-ric acid removal. Heating to 500600 removes boundwater and the TEOS hydrolysis products from themicropores in the silica spheres. Note the high C2H5 +background in the case of opal/ TiO2-II over the entiretemperature range studied, which is obviously relatedto the preparation conditions of the opal and composite.

The exothermic peaks at 450 and 500 in the DSCcurve of the opal/TiO2-I composite are due, accordingto XRD data, to the crystallization of amorphous TiO2 nH2O (anatase formation) and ordering of its crystalstructure during water removal. The broad exotherm inthe range 10001200 corresponds to the onset of thepolymorphic transformation of TiO2 into rutile or toopal crystallization, as suggested by XRD data. TheDSC curve of the opal/TiO2-II composite shows broad

exotherms and endotherms, attributable to crystalliza-tion of amorphous TiO2 in both the anatase and rutilephases and subsequent transformation of the metastableanatase phase into rutile.

Figure 4 illustrates the effect of heating and 5-h iso-thermal holds at each temperature on the phase compo-sition of the composites. It can be seen that, even afterlong-term annealing at 1200, the opal is amorphous,with trace levels of cristobalite, and that the predomi-nant crystalline phase in the composites is anatase.Under such conditions, uninfiltrated opal transformsinto cristobalite, as evidenced by TEM and XRDresults.

In the TiO2 nH2O-I powder (Fig. 3c), the totalweight loss exceeds 25%, which is due to the release ofH2O+, OH+-, and 2 -. The sharp exothermic peak at400 is due to the formation of anatase, which trans-forms into rutile above 500 (according to XRDresults, the rutile content increases). Figure 4c showsthe XRD patterns of the TiO2 nH2O-I powder afterholding at different temperatures. Rutile formation isseen to begin at a temperature as low as 550.

Heating the TiO2 nH2O-II powder leads to thesame weight loss (Fig. 3d), but without well-definedexotherms. This may be due to the formation of twocrystalline TiO2 polymorphs, anatase and rutile, even at

H5+

0 1 2 3 Energy, keV

Si

OTi

(c)

() (b)

60

50

40

30

20

10

0

60

50

40

30

20

10

0

wt %

Si SiTi TiO O

Fig. 2. X-ray microanalysis results for opal/TiO2 compos-ites: (a, b) elemental compositions of the composites pre-pared by procedures I and II, respectively; (c) X-ray emis-sion spectrum of the opal/TiO2 composites.

1256


IVICHEVA et al.

Fig. 3. TG and DSC curves and mass spectrometric analyses of the gas phase for the (a) opal/TiO2-I and (b) opal/TiO2-II nanocom-posites and (c) TiO2 nH2O-I and (d) TiO2 nH2O-II fine powders.

low temperatures and a subsequent phase transition, assupported by XRD data (Fig. 4d).

The total weight loss in the powders is influencedonly slightly by the composition of the starting solutionand the presence of the sol-stabilizing ion, which can beaccounted for by the small particle size of the powders

and, accordingly, by the high density of surface hydrox-yls. The specific surface area of the powders preparedby procedures I and II exceeded 90 m2/g. The TiO2-Iand TiO2-II powders heat-treated at 300 had almostthe same crystallite size (10 nm) of the anatase phasein the former case and the anatase and rutile phases in

G, %

88

200 400 600 800 1000Temperature, C

0.4

90

0.2

92

1.6

94

0.3

96

98

100

1200

0.1

0

0.1

0.2

0.3

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0

(b)

Weight change, 2.71%


Residual weight, 87.45%(1399.8C)

C2H5+

H2O+

OH+

HCl+

EXO

Ion current 109, ADSC, V/g

G, %

88200 400 600 800 1000

Temperature, C

0.4

90

0.2

92

2.5

94

0.3

96

98

100

0.1

0

0.1

0.2

0.3

2.0

1.5

1.0

0.5

(a)




H2O+

OH+

EXODSC, V/g

Ion current 109, A



the latter. According to XRD results, further annealingled to a systematic increase in crystallite size, whichdepended on the crystalline phase. Figure 5 shows thetemperature dependences of the crystallite size for ana-tase and rutile in the powders and composites. It fol-lows from the data for the powders (Fig. 5a) that ana-

tase is a stable phase at crystallite sizes of up to 35 nmwhen it gradually transforms into rutile (procedure I)and up to 20 nm when the as-prepared powder containsboth crystalline phases (procedure II). In the compos-ites (Fig. 5b), the crystallite size of the titania phasesformed in the opal pores is much smaller. This is attrib-

TG, %

75

200 400 600 800 1000Temperature, C

0.6

80

0.6

85

4.50.8

90

95

100

1200

0.4

0.2

0

0.2

0.4

4.0

3.5

3.0

2.5

2.0

1.5

0.5

0

(c)


C2H5+

H2O+

OH+

EXO

Ion curren 109, ADSC, V/g

420.0C

1.0

TG, %

75

200 400 600 800 1000Temperature, C

80

85

1.0

90

95

100

1200

0.2

0.1

0.2

0.3

0.4

0.8

0.6

0.2

0

(d)

Residual weight, 73.69%(1399.8C)C2H5

+

H2O+

OH+

EXO

Ion curren 108, ADSC, V/g

0.4

HCl+

0.1

0

Fig. 3. Contd.

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IVICHEVA et al.

15 20 25 30 35 40 452, deg

50

R-rutileA-anatase

A

R

AA

A

A

1200C

750C

650C

550C

450C

15 20 25 30 35 40 452, deg

50

A

R

A

A

300C

750C

650C

550C

450C

R

R

R

R

(a)

(b)

Inte

nsity

Fig. 4. XRD patterns of (a) opal/ TiO2-I, (b) TiO2 nH2O-I, (c) opal/ TiO2-II, and (d) TiO2 nH2O-II.

Inte

nsity

utable to the small size of the opal pores, which act asnanoreactors for TiO2 synthesis.

The detailed structure of the nanocomposites wasstudied by TEM and SEM using freshly cleaved sam-ples with various orientations. According to the TEM

and SEM results, the nanocomposites had a three-dimensional quasi-crystalline structure composed ofmonodisperse SiO2 spheres 300 nm in diameter, withTiO2 particles of various shapes in the opal pores and onthe surface of the spheres. The opal/ TiO2-I composite



contained rounded TiO2 nanoparticles on the order of10 nm in size, located predominantly in the opal pores(Fig. 6). The opal/TiO2-II composite contained accu-mulations of irregularly shaped TiO2 nanoparticles at

the contacts between SiO2 spheres and on their surface.There were also elongated and tubular particles about20 nm in width and over 50 nm in length, located in theopal pores (Fig. 7).

15 20 25 30 35 40 452, deg

50

A

R

AA

AA

1200C

750C

650C

550C

450C

15 20 25 30 35 40 452, deg

50

R

A

AA

750C

650C

550C

450C

300C

(c)

300C

55

55

R

RR

R

Fig. 4. Contd.

Inte

nsity

Inte

nsity

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IVICHEVA et al.

300 400 500 600 700 800

111098765432

D, nm

Temperature, C

R-II

A-II

A-I

(b)

706560555045403530252015105250 300 350 400 450 500 550 600 650 700 750 800

(a)

D, nm

Temperature, C

R-II

A-II

A-I

R-I

Fig. 5. Temperature dependences of the crystallite size for anatase (A) and rutile (R) in the (a) TiO2 powders and (b) opal/TiO2 com-posites prepared by procedures I and II.

20 nm(c) 0.02 m

10 nm 50 nm

(d)

() (b)

Fig. 6. TEM micrographs of the opal/TiO2-I nanocomposite.



CONCLUSIONS

A solgel process involving TEOT hydrolysis in analcohol-in-water solution was used to produce TiO2nanoparticles in three-dimensional opal crystals. In thepresence of the chloride ion as a stabilizer, we obtaineda mixture of two crystalline phases, anatase and rutile,in the form of elongated and tubular crystals about20 nm in width and over 50 nm in length. With no sta-bilizer ion, only anatase was formed: rounded particlesno greater than 10 nm in size. Independent of the syn-thesis conditions, the anatase in the opal pores remainsstable at annealing temperatures of up to 1200, with-out opal crystallization, which demonstrates that nano-structuring has an advantageous effect on the structuralperfection and properties of the forming phases.

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() (b)

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