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Ceramics International 40 (2014) 8659–8666www.elsevier.com/locate/ceramint

Structural characterization and optical property of TiO2 powders preparedby the sol–gel method

Y.F. Youa, C.H. Xua,n, S.S. Xub, S. Caoa, J.P. Wanga, Y.B. Huanga, S.Q. Shic

aSchool of Materials Science & Engineering, Henan University of Science & Technology, Luoyang, Henan, PR ChinabDepartment of Electronic and Information Engineering, The Hong Kong Polytechnic University, Hong Kong, PR China

cDepartment of Mechanical Engineering, The Hong Kong Polytechnic University, Hong Kong, PR China

Received 12 December 2013; received in revised form 16 January 2014; accepted 16 January 2014Available online 24 January 2014

Abstract

The structural characteristics and optical property of TiO2 powders with different phases are studied by various techniques in this paper. Butyltitanate and oxalic acid were used as Ti source and catalyst, respectively, for sol–gel synthesis of TiO2 powders. The synthesized products werecharacterized by X-ray diffraction, transmission electron microscopy, differential scanning calorimetry, thermogravimetric analysis andphotoluminescence techniques. X-ray diffraction patterns confirm TiO2 phase transformation from amorphous to anatase and rutile with theincrease in calcination temperatures from 200 1C to 700 1C. Two exothermic peaks at 275 1C and 485 1C on the differential scanning calorimetrycurve are responsible for the decomposition of the oxalate groups, which can enhance TiO2 phase transformation from amorphous to anatase andrutile. The crystalline sizes of the synthesized TiO2 powders are in the range of 20–65 nm. The coherent phase boundary between (200) crystalplane of anatase and (11�2) crystal plane of rutile is determined first time, based on the Fourier transform analysis. The near band edge emissionpeaks of anatase at 396 nm and rutile at 419 nm, and four peaks of the different defects in TiO2 crystals can be identified on thephotoluminescence spectra of the synthesized TiO2 powders. The activation energies for the grain growths of anatase and rutile were calculated,indicating that the surface chemistry may be a critical parameter to control grain growth. Based on the experimental results, the mechanism for theformation of anatase and rutile TiO2 is discussed.& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: A. Sol–gel processes; B. Grain boundaries; C. Optical properties; D. TiO2

1. Introduction

Nanocrystalline TiO2 is a well-known semiconductor materialwith photo-catalytic activities, and has a great potential forapplications in environmental purification, decomposition ofcarbonic acid gas, catalyst supports, fillers, pigments, photocon-ductors, gas sensors, biomaterials, dielectric materials, generationof hydrogen gas, solar cells, etc. [1]. TiO2 is also non-toxic,inexpensive, and stable in different chemical environments.

Several methods for the preparation of nanocrystalline TiO2

have been developed and they are electrochemical reaction [2],continuous reaction [3], supercritical carbon dioxide [4], precipita-tion [5], multi-gelation [6], chemical solvent and chemical vapor

e front matter & 2014 Elsevier Ltd and Techna Group S.r.l. All ri10.1016/j.ceramint.2014.01.083

nce to: P.O. Box 53, 263 Kaiyuan Avenue, Luoyang 471023,þ86 379 6423 1846; fax: þ86 379 6423 0597.ss: mmcjxu@haust.edu.cn (C.H. Xu).

decomposition [7,8], ultrasonic irradiation [9], RF sputtering [10],sol–gel [11], Aerogel and Xerogel [12,13].For the past two decades, the sol–gel route has become an

appropriate method for the preparation of nanocrystalline materials[14]. The benefits for preparing TiO2 by the sol–gel methodinclude the synthesis of nanosized crystalline powder with highpurity at a relatively low temperature, possibility of stoichiometrycontrolling process, and production of homogeneous materials.TiO2 has three different crystalline phases, rutile, anatase, andbrookite, among which rutile is in a thermodynamically stablestate while the latter two phases are in a metastable state.Generally, synthetic route of TiO2 production based on the sol–gel process usually results in amorphous TiO2 or anatase or rutiledepending on the preparation route and calcination temperature.The X-ray diffraction (XRD) technique is often used to identifythe existence of amorphous or anatase or rutile TiO2. However,the comprehensive characteristics of amorphous or anatase or

ghts reserved.

Fig. 1. XRD diffraction patterns of TiO2 powder samples calcined at varioustemperatures, showing the phase transformation from amorphous to anataseand rutile.

Y.F. You et al. / Ceramics International 40 (2014) 8659–86668660

rutile TiO2 under transmission electron microscopy (TEM),thermogravimetric analysis (TGA), differential scanning calorime-try (DSC) and photoluminescence (PL) have not been fullyunderstood. For example, the phase boundary between anataseand rutile during phase transformation, the effects of organiccomponents in sol on TiO2 phase transformation and the changeof near band edge emission of TiO2 with phase transformation stillremain unclear. In the present study, the structural characteristicsand optical property of amorphous or anatase or rutile TiO2

synthesized by a sol–gel method are studied by XRD, TEM,TGA-DSC and PL techniques. This research not only discoversthe characteristics relationship among different TiO2 phases butalso is beneficial to understand the mechanism of TiO2 phasetransformation.

2. Experiment

Butyl titanate (99.0%) was used as Ti source, oxalic acid(99.5%) as catalyst, ethanol (C2H5OH, 99.0%) and deionizedwater as dispersing media for sol–gel preparation of TiO2

powders. All reagents are analytical reagent grade withoutfurther treatment. Firstly, 20 ml butyl titanate was mixed with10 ml ethanol to form solution ‘A’ at room temperature. Inorder to prepare solution ‘B’, saturated oxalic acid solution hadbeen dropped in the mixture of 20 ml ethanol and 1 mldeionized water gradually until the pH value of the solution‘B’ was equal to 3. Then, solution ‘B’ was gradually droppedin the solution ‘A’ with stirring by a magnetic stirrer (78HW-1). After solution ‘B’ was added in solution ‘A’ totally, themixture of solutions ‘A’ and ‘B’ was stirred for 4 h during solpreparation. The sol was dried to form gel at room tempera-ture. The gel was heated in a tube furnace (SX2-4-10) at120 1C for 1 h to obtain fine particles. The fine particles wereground in an agate container by hands to form powders.Ground powders were calcined in the tube furnace in air attemperatures of 200 1C, 400 1C, 500 1C, 600 1C, and 700 1Cfor 2 h to obtain TiO2 powders.

For TGA and DSC measurements, sol sample of about20 mg was heated in flow N2/O2¼0.21 gas from 20 to 800 1Cwith a heating rate of 10 1C/min by a ThermogravimetricAnalyzer/Differential Scanning Calorimeter (Netzch).

Samples were examined directly using a D8 advanced X-raydiffractometer to identify the crystal structures. Cu Kα X-raywas used at 40 kV, 30 mA, and scanning range of 20–801.

To prepare samples for TEM examination, a bottle of theethanol solution with TiO2 powders calcined at 500 1C was putin an ultrasonic machine (SYS5200) for 10 min. Then, theresulting solution was dropped onto a carbon coated TEM grid.TEM images were performed using a JSM-2100 TEM.

PL spectra of the synthesized TiO2 powders were recordedat room temperature on a Fluorescence Spectrophotometer (F-280)using 300 nm excitation light.

3. Results

Fig. 1 shows the XRD patterns of TiO2 powder samplescalcined at various temperatures in air for 2 h. The peaks

marked with solid cycles and solid squares correspond toanatase and rutile phases, respectively. As shown on the top-right of Fig. 1, only a wide peak at 2θ¼281 is on the XRDpattern of the sample calcined at 200 1C, suggesting amor-phous structure. Calcination is a common treatment used toimprove the crystallinity of TiO2 powders. When groundpowders were calcined at 400–700 1C, the phase transforma-tion from amorphous to anatase and rutile occurred. The XRDpattern of the sample calcined at 400 1C exhibits anatase peaksat angles (2θ)¼24.81, 37.71, 48.01, 53.81, 54.81, 62.51, 68.21,70.01, and 74.81, corresponding to the reflections from {101},{004}, {200}, {105}, {211}, {204}, {116}, {220} and {215}crystal planes of anatase phase, respectively. All the diffractionpeaks agree with JCPDS card no. 21-1272 for anatase. Thisindicates that the amorphous TiO2 powders have crystallized inanatase phase at a temperature below 400 1C. The XRD patternof the TiO2 powders calcined at 500 1C reveals few rutilepeaks marked with solid squares in addition to the peakscorresponding to anatase phase, which indicates that rutileoccurred in the temperature range of 400–500 1C. The XRDpattern of the TiO2 sample calcined at 600 1C reveals morerutile peaks in addition to anatase peaks marked with solidcycles. For calcination at 700 1C, the strong intensity of rutilepeaks can be seen while the anatase peaks nearly vanish. Thecharacteristic peaks of rutile phase at 700 1C are at angles(2θ)¼27.51, 36.11, 39.21, 41.21, 44.01, 54.31, 56.61, 62.71,63.81, 69.01 and 69.81, corresponding to the reflections from{110}, {101}, {200}, {111}, {210}, {211}, {220}, {002},{310}, {301} and {112} crystal planes of rutile phase (JCPDScard no. 21-1276), respectively.The sol–gel method includes the hydrolysis and condensa-

tion reactions, which are catalyzed in the presence of acid. Thehydrolysis reaction of butyl titanate based on Eq. (1) leads tothe formation of original nuclei or basic units for titaniumdioxide while the condensation reaction based on Eq. (2) leadsto the growth of network system of the original basic units toform sol [15],

Ti OC4H9ð Þ4þ4H2O-Ti OHð Þ4þ4C4H9OH ð1Þ

RTi2OHþHO2TiR-RTi2O2TiRþH2O liquidð Þð2Þ

Y.F. You et al. / Ceramics International 40 (2014) 8659–8666 8661

where R is treble bond. Ti(OH)4 in Eq. (1) can be rewrittenas (OH)3RTi–OH, which is as written RTi–OH. The solsample was heated in a Thermogravimetric Analyzer/Differ-ential Scanning Calorimeter. The DSC (heat flow–tempera-ture)–TGA (weight loss–temperature) curves in Fig. 2 indicatethat endothermic and exothermic reactions and weight lossfrom the sol took place at temperatures of 20–800 1C. Weightloss rates are different with the rise in temperature. Thetemperatures for the ends of sharp weight loss are 140 1C,268 1C and 502 1C marked on the TGA curve in Fig. 2a. Anendothermic peak at 108 1C, two exothermic peaks at 275 1Cand 485 1C are marked on the DSC curve in Fig. 2b. Based onthe endothermic/exothermic peaks and weight loss rates, threeregions can be drawn during heating of the sol sample. Thefirst region is at the temperature range of 20–200 1C. In thefirst region, a weight loss of about 68% on the TGA curve inFig. 2a and the endothermic peak at 108 1C on the DSC curvein Fig. 2b are associated to the loss of water and ethanol in thesol sample. Water vapor is formed by condensation of Ti–OHgroups [16], according to the following equation:

RTi2OHþHO2TiR-RTi2O2TiRþH2O gasð Þ ð3Þ

Ethanol is eliminated mainly by physical evaporation. Thesecond region is at the temperature of 200–400 1C. The weightloss of about 11% between 240 and 270 1C on the TGA curvein Fig. 2a can be associated with decomposition of an oxalategroup [16]. The exothermic peak at 275 1C on the DSC curvein Fig. 2b can be associated with both decomposition of an

Fig. 2. Weight loss and heat flow of TiO2 gel (a) the TGA curve and (b) theDSC curve.

oxalate group and phase transformation of TiO2. The decom-position of the oxalate groups could be mainly bi-coordinatedor mono-coordinated to titanium atoms, by reaction with Ti–OHgroups, which results in the evolution of CO, CO2 and a minoramount of H2O, as proposed in the following reactions:

RTi2O2OCCO2OH-Ti2O2HþCOðgasÞþCO2 gasð Þð4Þ

RTi2O2OCCO2OHþHO2TiR-RTi2O2TiRþCO gasð ÞþCO2 gasð ÞþH2O gasð Þ ð5ÞThe third region is at the temperature of 400–600 1C. The

sharp weight loss of approximately 2% at 502 1C in Fig. 2a isattributed to the decomposition of an oxalate group [16]. Theexothermic peak at 485 1C in the DSC curve in Fig. 2b isattributed to both decomposition of an oxalate group and phasetransformation of TiO2. At high temperatures, the evolution ofCO2 and CO could be justified by the decomposition ofresidual oxalate anions which could be more strongly bridging-coordinated to titanium atoms, according to Eq. (4).Fig. 3 TEM images took from TiO2 powders calcined at

500 1C. TEM image on the top-left of Fig. 3a shows highdegree of aggregation of TiO2 grains to form a powder particle.The grain sizes of TiO2 are about 20–30 nm. The electricaldiffraction pattern (EDP) for the aggregation of TiO2 grains isshown on the right of Fig. 3a, suggesting rutile and anataseTiO2 phases. The polycrystalline diffraction rings from inner toouter on the right of Fig. 3a are responsible for the reflectionsfrom {101}, {004}, {200}, {105}, {211}, {204}, {116} and{220} crystal planes of anatase phase. The diffraction spots onthe right of Fig. 3a are responsible for the reflections from(211), (101), (110) and (111) crystal planes of rutile phase.From EDP, the fraction of anatase phase is significantly higherthan that of rutile phase, which is consistent with the XRDresults in Fig. 1. A high resolution image on the bottom-left ofFig. 3a shows the crystal lattice stripes of two TiO2 grainsmarked with ‘A’ for anatase and ‘R’ for rutile. The selectedzones from ‘A/B’ grains, ‘A’ grain and ‘R’ grain are shown onthe top of Fig. 3b. Phase transformation from ‘A’ grain to ‘R’grain was analyzed by the Fourier transform method. Fouriertransform for three selected zones on the top of Fig. 3b wasprocessed by the VEC (Visual computing in Electron Crystal-lography) software. After Fourier transform, the crystal latticestripes on the high resolution images of three selected zones onthe top of Fig. 3b became the diffraction spots on the bottom ofFig. 3b. The transformed spots for A(11�2) and R(200) on thebottom-left of Fig. 3b overlapped, indicating that A(11�2)and R(200) crystal planes are not only parallel each other butalso have coherent phase boundary. Both rutile and anatasehave tetragonal unit cells. Rutile phase contains two TiO2

molecules per unit cell having lattice constants a¼4.5937 Åand c¼2.9587 Å, and anatase contains four TiO2 moleculesper unit cell having lattice constants a¼3.7842 Å andc¼9.5146 Å. The mismatch of crystalline spaces betweend¼2.332 Å for A(11�2) and d¼2.297 Å for R(200) is only1.5%, which is beneficial to the formation of the coherentphase boundary between (200) crystal plane of anatase and

Fig. 3. TEM images for TiO2 powder calcined at 500 1C (a) the morphology on the top-left, EDP on the right, and a high resolution image on the bottom-leftand (b) three high resolution images (top) taken from the bottom-left of (a), the diffraction spots (bottom) obtained from the three high resolution images byFourier transform.

Fig. 4. Room-temperature PL spectra of TiO2 powders calcined at varioustemperatures.

Y.F. You et al. / Ceramics International 40 (2014) 8659–86668662

(11�2) crystal plane of rutile. The phase transformation fromanatase to rutile, based on the TEM technique, was alsoreported by other researchers. Commercial TiO2 powders withan initial particle size of 100 nm were heated and analyzedusing TEM. A lamellar region containing anatase and rutilewas observed [17]. The EDP pattern of the lamellar regionshowed an orientation relationship of A(112)//R(100), which isconsistent with the present research. However, A(112)//R(100)with coherent phase boundary has not been reported until thepresent research.

Fig. 4 shows the room-temperature PL spectra of TiO2

powders calcined at various temperatures. It is known that PLspectrum is the result of the recombination of excited electrons

0%

20%

40%

60%

80%

100%

Temperature (°C)

Rut

ile (%

)OA HA

0

20

40

60

80

Temperature (°C)

Gra

in S

ize

(nm

) OA-rutile

OA-anatase

HA-anatase

2

3

4

300 400 500 600 700 800

300 400 500 600 700 800

0.0008 0.001 0.0012 0.0014 0.0016

1/T

log(

D2 -

D02 )

OA-rutile

OA-anatase

HA-anatase

Fig. 5. Relation of phase transformations and calcination temperatures (a) themass fraction of the transformed rutile phase with calcination temperatures, (b)the grain sizes of anatase and rutile with calcination temperatures and (c)lnðDm

t �Dm0 Þ versus 1/T, showing activation energies for grain growths: OA for

oxalic acid and HA for hydrochloric acid.

Y.F. You et al. / Ceramics International 40 (2014) 8659–8666 8663

and holes, the lower PL intensity may indicate the lowerrecombination rate of electrons–holes under light irradiation [18].The PL intensities of TiO2 powders in the wavelength range of350–550 nm with calcination temperatures in Fig. 4 vary in thefollowing order: 700 1C4500 1C4600 1C4400 1C4200 1C.Calcination temperatures can improve the crystallinity of TiO2

powders in Fig. 1. The sample calcined at 200 1C has thelowest PL intensity, due to the amorphous structure. The XRDpatterns of samples calcined at 400 1C and at 500 1C exhibitanatase and almost anatase phases, respectively. However, thecrystallinity of TiO2 powders calcined at 500 1C is better thanthat calcined at 400 1C. Therefore, PL intensity of the samplecalcined at 500 1C is higher than that calcined at 400 1C. TheXRD patterns of samples calcined at 600 1C and at 700 1Cexhibit anatase/rutile and almost rutile, respectively. Moreover,the crystallinity of TiO2 powders calcined at 700 1C is betterthan that calcined at 600 1C. Therefore, PL intensity of thesample calcined at 700 1C is higher than that calcined at 600 1C.Similar phenomena also were observed by other research [10]although PL signals came from different measuring ranges in tworesearch. NIR photoluminescence properties of the Nd dopedTiO2 films in the wavelength range of 800–1200 nm wereinvestigated. The relative amounts and crystallinity of anataseand rutile could change remarkably with the Nd concentration.The sample with amorphous structure had the lowest PL intensityand the sample with better crystalline structures had higher PLintensity. From the above analysis, the good crystallinity of TiO2

powders shows that high PL intensity and pure phase (rutile oranatase) can also affect the intensity of PL signals.

The PL peaks centered at 396 nm and 419 nm, due to the nearband edge emission, are observed in the samples calcined attemperatures of 400–700 1C. However, the relative intensities oftwo peaks in the curves in Fig. 4 are different. With the increasein temperatures from 400 to 700 1C, the intensity of 396 nm peakdecreases while the intensity of 419 nm peak increases. Based onthe analysis of XRD result in Fig. 1, the sample calcined at400 1C contains anatase TiO2 and the sample calcined at 700 1Ccontains nearly total rutile TiO2, suggesting 396 nm peak fromanatase and 419 nm peak from rutile. The bandgap energies canbe calculated from the following equation:

Eg ¼hc

λð6Þ

where Eg is the bandgap energy (eV), h is Planck's constant(4.136� 10–15 eV s), c is the light velocity (2.998� 108 m/s),and λ is the wavelength (nm). Eg values for 396 nm peak fromanatase and for 418 nm peak from rutile are 3.12 eV and 2.96 eV,respectively, which are close to 3.2 eV for anatase and 3.0 eV forrutile [19].

Other four peaks at longer wavelength direction can be found inall curves in Fig. 4, which are attributed to different defects inTiO2 crystals. The blue emissions around 449 nm and 466 nm canbe assigned to intrinsic defects, particularly interstitial defects,which resulted from non-stoichiometric TiO2 [20]. The blue–greenemission at 481 nm can be attributed to the charge transfer fromTi3þ to oxygen anion in a TiO6

8� complex, and associated withoxygen vacancies at the surface [21]. The green emission around

490 nm can be attributed to the singly ionized oxygen vacancylevels, which developed during annealing of nanomaterials in thesol–gel process [22].

4. Discussions

4.1. The activation energy for TiO2 grain growth

From Fig. 1, the relative intensities of anatase and rutilediffraction peaks from the TiO2 powders calcined at 400–700 1C are different. The relative contents of the phases in asample can be calculated from the integrated intensities ofanatase {101} and rutile {110} peaks. If a sample containsonly anatase and rutile, the mass fraction of rutile (WR) can becalculated from the following equation [23]:

WR ¼ AR

0:866AAþARð7Þ

Y.F. You et al. / Ceramics International 40 (2014) 8659–86668664

where AA and AR represent the integrated intensities of the anatase{101} and rutile {110} peaks in Fig. 1, respectively. Through thecalculation based on Eq. (7), the mass fractions (percentagevalues) of rutile phase in the samples with the increase incalcination temperatures are estimated in Fig. 5a. The massfraction of the rutile phase marked on the OA curve (oxalic acidas catalyst) slightly increases in the calcination temperature rangesof 400–500 1C and 600–700 1C and significantly increases in thecalcination temperature range of 500–600 1C. In our previousreport, hydrochloric acid, other than oxalic acid, was used ascatalyst to synthesize TiO2 powders by the same process [24]. TheHA curve (hydrochloric acid as catalyst) in Fig. 5a shows themass fractions of rutile with the calcination temperatures whichwere obtained based on the experimental results in reference [24]to compare the functions of two types of acids as catalysts in thesol–gel process. The phase transformation temperature fromanatase to rutile in the case of hydrochloric acid is higher thanthat in the case of oxalic acid.

It is clearly observed from the XRD patterns in Fig. 1 that,with increasing temperature from 400 1C to 700 1C, thediffraction peaks become sharper, which suggests that thecrystalline qualities of the TiO2 powders are improved andthe crystalline sizes of the TiO2 powders increase. The meancrystalline size was calculated from the full-width at half-maximum (FWHM) of XRD lines in Fig. 1 by using theDebye–Scherrer formula [25],

D¼ kλ

β cos θð8Þ

where D is the average crystallite diameter in angstroms (Å), k isthe shape factor taken as 0.9, λ is the wavelength of X-rayradiation (Cu Kα¼1.5406 Å), β is the line width at half-maximumand θ is the Bragg angle. The average crystallite sizes of anataseand rutile TiO2 powders are determined from the broadening ofanatase {101} peak at (2θ¼24.81) and rutile {110} peak at(2θ¼27.51). Based on the calculation from Eq. (8), crystallitesizes of anatase and rutile TiO2 in the samples are shown as OA–anatase and OA–rutile curves in Fig. 5b. It can be seen that theaverage crystallite sizes of TiO2 increase as the calcinationtemperature increases. The crystallite sizes of anatase phase areless than those of rutile phase at the same calcination temperature.The HA–anatase curve in Fig. 5b shows the crystallite sizes ofanatase in the case of hydrochloric acid with the calcinationtemperatures which were obtained based on the experimentalresults in reference [24]. The grain sizes of anatase obtained byusing hydrochloric acid as catalyst is smaller than those by usingoxalic acid as catalyst.

Normally, grain growth kinetics is analyzed under isother-mal conditions in accordance with the well-known graingrowth kinetics equation [26],

Dmt �Dm

0 ¼K0texp � Qg

RT

� �ð9Þ

where Dt is the average grain size at time t, D0 is the initialgrain size, m is the kinetic grain growth exponent typicallybetween 2 and 4, K0 is a pre-exponential constant, Qg is theactivation energy for grain growth, R is the gas constant and T

is the absolute temperature. Eq. (9) can be rewritten as follows:

lnðDmt �Dm

0 Þ ¼ ln K0þ ln t� Qg

RTð10Þ

Fig. 5c shows the corresponding plots for the evaluation of theactivation energy for grain growth based on Eq. (10). Normalgrain growth in a pure, single phase should yield m¼2 [27],which is selected in this calculation. The grains sizes (D2h) atvarious temperatures for 2 h from Fig. 5b are used. D0 isexpressed as zero here. It is shown that data are well fitted bylinear function fitting and the correlation coefficients for allthe linear fitting in Fig. 5c are higher than 0.97. This suggeststhat the assumption of m¼2 is reasonable. The activation energiesfor the grain growths of anatase and rutile (oxalic acid as catalyst)are evaluated by the slopes to be 41 and 8 kJ mol�1, respectively.The activation energy for anatase grain growth (41 kJ mol�1) issignificantly higher than that for rutile grain growths (8 kJ mol�1).This is also observed by other researchers [27]. This fact suggeststhat the nucleation and growth processes of anatase and rutile donot share the same kinetic size dependence. The activation energyfor anatase grain growth in the case of hydrochloric acid is alsoobtained as 30 kJ mol�1. The activation energy for anatase graingrowth in the case of oxalic acid (41 kJ mol�1) is higher than thatin the case of hydrochloric acid (30 kJ mol�1). The calculatedactivation energies of all powders are lower than that of the bulk(350–700 kJ/mol). The activation energies for TiO2 grain growthsby a microemulsion synthetic route are in the range of 15–30 kJ mol�1 [27]. Compared with bulk TiO2, the low activationenergies for TiO2 crystalline growth would mean that thehydration of surface and near-surface layers (e.g., the nature acidand number of OH species) may be a critical parameter to controlgrowth size [28]. From Fig. 1, the phase transformation fromamorphous to anatase TiO2 occurs at the temperature between 200and 400 1C. The temperature for ending the sharp weight loss(268 1C) in the second region in Fig. 2a is lower than that forending the exothermic peak (323 1C) in Fig. 2b, indicating that theelimination of an oxalate group, based on Eqs. (4) and (5), canenhance the phase transformation from amorphous to anataseTiO2. From Fig. 1, the mass fraction of the rutile phasesignificantly increases at the calcination temperature of 500–600 1C. The temperature for ending the sharp weight loss(502 1C) in the third region in Fig. 2a is lower than that forending the exothermic peak (552 1C) in Fig. 2b, implying that theelimination of an oxalate group, according to the Eq. (4) can alsoenhance TiO2 phase transformation from anatase to rutile. Theactivation energies for TiO2 grain growths in the present experi-ment indicate that the surface chemistry might be a main factor toaffect phase transitions.

4.2. The possible mechanism for the growth of anatase andrutile TiO2

The product powders obtained in the present experimentinclude amorphous, anatase, and rutile TiO2, depending oncalcination temperatures. Generally, synthetic route of TiO2

production based on the sol–gel process usually results in

Y.F. You et al. / Ceramics International 40 (2014) 8659–8666 8665

amorphous TiO2, which will change to anatase and rutilefinally with the increase in calcination temperature.

Amorphous, anatase and rutile TiO2 can grow from TiO6

octahedra and the phase transition proceeds by the rearrange-ment of the octahedral. Hydrochloric acid (HCl) or oxalic acid(HOOCCOOH) should disperse the aggregated TiO6 octahedraand the related species in the amorphous phase during thehydrolysis and condensation reactions in the sol process. Thereare attraction forces between the Ti4þ ion of TiO6 octahedraand catalysts anions of Cl�1 or OCCOOH�1. Catalysts anionsof Cl1�, OCCOOH1� and OCCOO2� are mono-coordinatedand bi-coordinated to TiO6 octahedra as illustrated in Fig. 6a, band c, respectively, based on reference [16].

The phase transformation from amorphous to anatase TiO2

began to occur during calcination below the temperature of400 1C in Fig. 1. The protonation process followed by theface-sharing as shown in Fig. 6d [29] will result in thefavorable formation of anatase phase from the TiO6 octahedrain amorphous. The anions of Cl1�, OCCOOH1� andOCCOO2� exhibit different effects on the crystallization ofanatase TiO2 from the amorphous phase according to theexperimental results. From Fig. 5b, the grain sizes of anatase inthe case of HCl are smaller than those in the case ofHOOCCOOH. Yanagisawa and Overstone [30] confirm thatthe chloride anion enhances only the nucleation of anatase, notthe crystal growth. The larger grain size of anatase in the case

Fig. 6. Schematical illustration of (TiO62�) octahedra (a), (b), (c) the modes of

Cl1�, OCCOOH1� , and OCCOO2� bonded to octahedral, respectively, (d)forming face-sharing anatase phase and (e) edge-sharing rutile phase.

of HOOCCOOH may be ascribed to slow nucleation comparedto the case of HCl.The rutile occurs at about 500 1C from Fig. 1 and weight

loss of approximately 2% in Fig. 2a at about 500 1C due to thedecomposition of an oxalate group, implying that the phasetransformation may occur from amorphous to rutile TiO2. Onthe other hand, the phase transformation from anatase to rutilecould be observed on TEM results in Fig. 3b. It is well knownthat rutile TiO2 is in a thermodynamically stable state while theanatase TiO2 is in a metastable state. Therefore, the phasetransformation from anatase to rutile should be the mainmechanism for the formation of rutile in this experiment.From Fig. 5a, the mass fraction of rutile in the case of HCl isless than that in the case HOOCCOOH at the same calcinationtemperature, suggesting that the phase transformation fromanatase to rutile in the case of HCl is more difficult than that inthe case of HOOCCOOH. The bi-bonds of OCCOO2� onTiO6 octahedra in Fig. 6c occupy one full face of octahedraland inhibit the growing of chain along the opposite edges inFig. 6e, which inhibits the formation of rutile [31]. Thismechanism cannot explain the present experiment result inFig. 5a. One of the possible reasons is that the mechanism issuitable for the sol–gel process at a low temperature. Whensample was heated at a high temperature of 500 1C,OCCOO2� in TiO6 octahedra in Fig. 6c could decomposeinto CO and CO2, according to Eq. (4) while the structure inFig. 6a is more stable than those in Fig. 6b and c. Therefore,the phase transformation from anatase to rutile in the case ofHCl is more difficult than that in the case of HOOCCOOH inFig. 5a.

5. Conclusions

(1)

TiO2 powders were fabricated by oxalic acid (99.5%) ascatalyst in the sol–gel process. TiO2 powders calcined at200 1C have amorphous structure. TiO2 powders calcinedat 400 1C exhibit anatase phase. With the rise in calcina-tion temperature from 500 1C to 700 1C, the fraction ofrutile phase in the samples increases while the fraction ofanatase phase decreases.

(2)

The grain sizes of the prepared TiO2 powders are at therange of 20–65 nm. The coherent phase boundary between(200) crystal plane of anatase and (11�2) crystal plane ofrutile exists during the phase transformation from anataseto rutile at high temperature calcination. The activationenergy for anatase grain growth (41 kJ mol�1) is signifi-cantly higher than that for rutile grain growths(8 kJ mol�1).

(3)

The good crystallinity of TiO2 powders shows that highphotoluminescence intensity and pure phase of TiO2

powders can also affect photoluminescence intensity. Sixpeaks are on the photoluminescence spectra of TiO2

powders. Among them, the peaks at 396 nm and 419 nmare responsible for the near band edge emission of anataseand rutile, respectively. The blue emissions around 449 nmand 466 nm are assigned to intrinsic defects. The blue–green emission at 481 nm is attributed to the charge

Y.F. You et al. / Ceramics International 40 (2014) 8659–86668666

transfer from Ti3þ to oxygen anion in a TiO68� complex.

The green emission around 490 nm is due to the singlyionized oxygen vacancy levels.

Acknowledgments

This work was financially supported by the Henan Universityof Science and Technology through a grant of postgraduateinnovation fund (CXJJ-Z015), by NSRTP (201310464002) andby the Program for Changjiang Scholars and Innovative ResearchTeam in University (IRT1234).

References

[1] A. Mills, N. Elliott, G. Hill, D. Fallis, J.R. Durrant, R.L. Willis,Preparation and characterization of novel thick sol–gel titania filmphotocatalysts, Photochem. Photobiol. Sci. 5 (2003) 591–596.

[2] S. Wang, X.H. Wu, W. Qin, Z.H. Jiang, TiO2 films prepared by micro-plasma oxidation method for dye-sensitized solar cell, Electrochim. Acta53 (2007) 1883–1889.

[3] K.D. Kim, H.T. Kim, Synthesis of titanium dioxide nanoparticles using acontinuous reaction method, Colloids Surf. A: Physicochem. Eng. Asp.207 (2002) 263–269.

[4] C.I. Wu, J.W. Huang, Y.L. Wen, S.B. Wen, Y.H. Shen, M.Y. Yeh,Preparation of TiO2 nanoparticles by supercritical carbon dioxide, Mater.Lett. 62 (2008) 1923–1926.

[5] T. Ohno, M. Akiyoshi, T. Umebayashi, K. Asai, T. Mitsui,M. Matsumura, Preparation of S-doped TiO2 photocatalysts and theirphotocatalytic activities under visible light, Appl. Catal. A: Gen. 265(2004) 115–121.

[6] Y. Shiroto, T. Ono, S. Asaoka, M. Nakamura, Catalysts for hydrotreat-ment of heavy hydrocarbon oils containing asphaltenes, U.S. Patent4422960, 1983.

[7] T.K. Ghorai, D. Dhak, S.K. Biswas, S. Dalai, P. Pramanik, Photocatalyticoxidation of organic dyes by nano-sized metal molybdate incorporatedtitanium dioxide (MxMoxTi1�xO6) (M¼Ni, Cu, Zn) photocatalysts,J. Mol. Catal. A: Chem. 273 (2007) 224–229.

[8] B.H. Kim, J.Y. Lee, Y.H. Choa, M. Higuchi, N. Mizutani, Preparation ofTiO2 thin film by liquid sprayed mist CVD method, Mater. Sci. Eng. B107 (2004) 289–294.

[9] F. Peng, L. Cai, H. Yu, H. Wang, J. Yang, Synthesis and characterizationof substitutional and interstitial nitrogen-doped titanium dioxides withvisible light photocatalytic activity, J. Solid State Chem. 181 (2008)130–136.

[10] R. Pandiyan, V. Micheli, D. Ristic, R. Bartali, G. Pepponi, M. Barozzi,G. Gottardi, M. Ferrari, N. Laidani, Structural and near-infra redluminescence properties of Nd-doped TiO2 films deposited by RFsputtering, J. Mater. Chem. 22 (2012) 22424–22432.

[11] M. Crisan, A. Braileanu, M. Raileanu, M. Zaharescu, D. Crisan,N. Dragan, Sol–gel S-doped TiO2 materials for environmental protection,J. Non-Cryst. Solids 354 (2008) 705–711.

[12] T. Horikawa, M. Katoh, T. Tomida, Preparation and characterization ofnitrogen-doped mesoporous titania with high specific surface area,Microporous Mesoporous Mater. 110 (2008) 397–404.

[13] M. Zhou, J. Yu, Preparation and enhanced daylight-induced photocata-lytic activity of C, N, S-tridoped titanium dioxide powders, J. Hazard.Mater. 152 (2008) 1229–1236.

[14] K.N.P. Kumar, K. Keizer, A. Bruggraaf, Densification of nanostructuredtitania assisted by a phase transformation, Nature 358 (1992) 48–51.

[15] S.R. Kumar, C. Suresh, A.K. Vasudevan, N.R. Suja, P. Mukundan, Phasetransformation in sol–gel titania containing silica, Mater. Lett. 38 (1999) 161.

[16] R. Campostrini, M. Ischia, L. Palmisano, Pyrolysis study of sol–gelderived TiO2 powers, J. Therm. Anal. Calorim. 71 (2003) 1011–1021.

[17] P.I. Gouma, M.J. Mills, Anatase-to-rutile transformation in titaniapowders, J. Am. Ceram. Soc. 84 (2001) 619–622.

[18] X.Z. Li, F.B. Li, C.L. Yang, W.K. Ge, Photocatalytic activity of WOx–

TiO2 under visible light irradiation, J. Photochem. Photobiol. A 141(2001) 209–217.

[19] K.J. Xu, W.J. Yang, S.H. Dong, Effect of Ag/N codoping on photo-activity of TiO2 thin film, Surf. Eng. 27 (2011) 480–484.

[20] N. Bahadur, K. Jain, A.K. Srivastava, G.R. Gakhar, D. Haranath, Effectof nominal doping of Ag and Ni on the crystalline structure and photo-catalytic properties of mesoporous titania, Mater. Chem. Phys. 124 (2010)600–608.

[21] A.K. Tripathi, M.K. Singh, M.C. Mathpal, S.K. Mishra, A. Agarwal,Study of structural transformation in TiO2 nanoparticles and its opticalproperties, J. Alloys Compd. 549 (2013) 114–120.

[22] P.M. Kumar, S. Badrinarayanan, M. Sastry, Nanocrystalline TiO2 studiedby optical, FTIR and X-ray photoelectron spectroscopy: correlation topresence of surface states, Thin Solid Films 358 (2000) 122–130.

[23] H. Zhang, J.F. Banfield, Understanding polymorphic phase transforma-tion behavior during growth of nanocrystalline aggregates: insights fromTiO2, J. Phys. Chem. B 104 (2000) 3481–3487.

[24] Y.F. You, C.H. Xu, J.P. Wang, TiO2 nano-powders produced by sol–geland the study of transformation of crystal type, J. Henan Univ. Sci.Technol.: Nat. Sci. 34 (2013) 5–7.

[25] B.E. Warren, X-Ray Diffraction, Dover, New York, 1990, p. 251–259.[26] D.W. Ni, C.G. Schmidt, F. Teocoli, A. Kaiser, K.B. Andersen,

S. Ramousse, V. Esposito, Densification and grain growth duringsintering of porous Ce0.9Gd0.1O1.95 tape cast layers: a comprehensivestudy on heuristic methods, J. Eur. Ceram. Soc. 33 (2013) 2529–2537.

[27] M. Ferna´ndez-Garcı´a, X.Q. Wang, C. Belver, J.C. Hanson, J.A. Rodriguez, Anatase-TiO2 nanomaterials: morphological/size depen-dence of the crystallization and phase behavior phenomena, J. Phys.Chem. C 111 (2007) 674–682.

[28] G. Li, L. Li, J. Boerio-Goates, B.F. Woodfield, High purity anatase TiO2

nanocrystals: near room-temperature synthesis, grain growth kinetics, andsurface hydration chemistry, J. Am. Chem. Soc. 127 (2005) 8659.

[29] H.B. Yin, Y. Wada, T. Kitamura, S. Kambe, S. Murasawa, H. Mori,T. Sakatac, S. Yanagida, Hydrothermal synthesis of nanosized anataseand rutile TiO2 using amorphous phase TiO2, J. Mater. Chem. 11 (2001)1694–1703.

[30] K. Yanagisawa, J. Ovenstone, Crystallization of anatase from amorphoustitania using the hydrothermal technique: effects of starting material andtemperature, J. Phys. Chem. B 103 (1999) 7781.

[31] M. Kanna, S. Wongnawa, Mixed amorphous, nanocrystalline TiO2

powders prepared by sol–gel method: characterization and photocatalyticstudy, Mater. Chem. Phys. 110 (2008) 166–175.