Effect of coarsening of sonochemical synthesized anatase BET surface characteristics

7/30/2019 Effect of coarsening of sonochemical synthesized anatase BET surface characteristics

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Effect of coarsening of sonochemical synthesized anatase on BETsurface characteristics

Leonardo Gonzalez-Reyes a,b, Isaias Hernandez-Perez b,d, F.C. Robles Hernandez c,n

a Instituto de Ciencia y Tecnologa del Distrito Federal, ICyTDF. Republica de Chile 6, Centro 06010, Mexico D.F., Mexicob Universidad Autonoma Metropolitana-A, Departamento de Ciencias Basicas, Av. Sn. Pablo no. 180, Mexico 02200 D.F., Mexicoc University of Houston, College of Technology, Mechanical Engineering Technology, 304 Technology Building, Houston, TX, 77204-4020, USAd Universidad Autonoma Metropolitana-L, Division de Ciencias Basicas e Ingeniera, Lerma de Villada 52006 Edo. Mex. Mexico

a r t i c l e i n f o

Article history:

Received 3 May 2010

Received in revised form

16 November 2010

Accepted 17 November 2010Available online 26 November 2010

Keywords:

Anatase

Coarsening

Coalescence

Raman

Infrared

XRD

a b s t r a c t

In the present paper TiO2 (anatase) nanoparticles were synthesized by ultrasonic means proving the

potential of this method. The synthesized anatase is heat treated at a temperature of 500 1C in open air

atmosphere to coarse it. The heat treatment times went from 1 to 72 h, the temperature/time conditions

were selected to prevent phase transformation and to solely coarsen anatase from 6.2 to 28.3 nm. The

synthesized and heat treated anatase were characterized using Electron Microscopy (Transmission and

Scanning), X-ray diffraction (XRD), BrunauerEmmettTeller (BET) method, UVvis, Raman and Infrared

spectroscopy. In the present paper are proposed two algorithms that are capable of determining the BET

surface characteristics or the grain size based on the XRD or BET results, respectively.

Published by Elsevier Ltd.

1. Introduction

There are three allotropes of titanium dioxide (TiO2) in nature that

are mentioned in following along with their respective crystalline

structures. Rutile has a P42/mnm symmetry with a tetragonal crystal-

line structure; anatase is I41/amd and has a body centred tetragonal

crystalline structure and brookite is P/cab with an orthorhombic

structure. Rutile can be obtained from heat treated anatase under

different conditions (Henrich and Cox, 1994; Gouma and Mills, 2001).

Anatase is widely used for photo-catalysis, solar energy conversion,

protective surface coating, ceramics,pigments, biological, catalysis,as a

reductor, for photo-corrosion applications, etc. (Hoffmann et al., 1995;

Cai et al., 1992; Diebold, 2003; Gan et al., 1998; Fujishima et al., 2000;Braun, 1997; Al-Salim et al., 2000; Ito et al., 1999). The transformation

between anatase and rutile has been extensively studied suggesting

that this transformation is highly dependent on the conditions of the

synthesis (e.g. temperature, purity of the components, texture, grain

size, specific surface area, pore dimensions, etc; Kumar et al., 1992;

Reidy et al., 2006; Burns et al., 2004; Shannon, 1964; Gamboa and

Pasquevich, 1992). Many efforts have been directed to control the TiO2

nanostructure; however, several problems still remain unsolved. For

instance, annealing significantly affects microstructure, crystalline

structure, phase(s) and the grain size of anatase that might influence

its catalytic and photo-catalytic efficiency (Inagaki et al., 2001; Maira

et al., 2000; Chan et al., 1999). Unfortunately, these parameters cannot

be controlled independently making this a challenging topic.

Sonochemical treatment has been reported as a successful meth-

odology to produce nanostructured materials (Kenneth et al., 1999;

Gonzalez-Reyes et al., 2008; Suslick et al., 1999). The present work

proposes a method assisted by ultrasonic means to synthesize

nanostructured anatase. The nanostructured anatase is heat treated

at a temperature of 500 1C for different times to investigate the effects

of control coarsening and preventing phase transformation to rutile orany otherphase(s). The main goal ofthis work istoinvestigatethe effect

of the grain size of anatase on BET particle characteristics as well as

other effects (e.g. band gap) and how these changes can be predicted

using different characterization methods. In a parallel research

(Gonzalez-Reyes et al., 2010) demonstrated that the band gap of

anatase is affected by the grain size. This effect is directly related to the

quantum characteristics that evolve in grain smaller than 21 nm; such

grainshave a relatively large numberof brokenbonds and these effects

are minimized as the anatase grains coarsen. Based on the above

arguments the coarsening studies of anatase are of great importance

and are the main motivation of the present publication. In the present

work the anatase powders were characterized by means of: X-ray

Contents lists available at ScienceDirect

journal homepage: www.elsevier.com/locate/ces

Chemical Engineering Science

0009-2509/$- see front matter Published by Elsevier Ltd.

doi:10.1016/j.ces.2010.11.030

n Corresponding author. Fax: +1 505 213 7106.

E-mail addresses: [email protected] (L. Gonzalez-Reyes),

[email protected] (I. Hernandez-Perez),

[email protected] (F.C. Robles Hernandez).

Chemical Engineering Science 66 (2011) 721728
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diffraction (XRD), BrunauerEmmettTeller (BET) method, Electron

(Transmission and Scanning) Microscopy, UVvis, Infrared and Raman

Spectroscopy, the results are provided and discussed in the

present paper.

2. Experimental

2.1. Synthesis and materials

A mixture of 150 ml of titanium (IV) tetraisopropoxide

([(CH3)2CHO]4Ti) of commercial grade (97 wt% pure), acetone

(30 mL) and methanol (30 mL) are subjected to sonochemical

treatment. Methanol and acetone are used as pressure-transmit-

ting media. The mix of alcohol, acetone and [(CH3)2CHO]4Ti is

added into the ultrasonic bath andthe mix is ultrasonically treated

at 38 kHz for 50 min. The resultant colloid is dried out on a

magnetic mixer-heater set at 150 1C until the powders have a

dry appearance. No treatment above 150 1C is conducted to

preserve the crystalline structure and the grain size of the

synthesized anatase.

2.2. Heat treatment

The synthesized anatase was heat treated at 500 1C i n a

conventional electric resistance furnace in open air atmosphere

for times varying from 1 to 72 h. The heat treatment as a main

objective closely control the coarsening of anatase, but it does

prevent any phase transformation to rutile or any other phase.

Anatase obtained from sonochemical synthesis is identified in the

present paper as original anatase or original sample. Samples of

anatase heat treated at 500 1C are identified for their respective

heat treatment time (xh; where x denotes heat treatment time in

hours).

2.3. Characterization methods

The X-ray diffraction (XRD) was conducted on a Bruker D8

Discover apparatus that operates under y2y conditions. The

samples were scanned from 20 to 80, 2y degrees using a Cu Ka

radiation with a characteristic wavelength (l) of 0.15405 nm.

Lattice parameters (a and c) were determined using the

(1 0 1) and (2 0 0) reflections, respectively. Scherrer method was

used to determine the grain size (Cullity and Stock, 2001) based on

the (1 0 1) reflection, Scherrer equation follows:

D Kl

b1=2 Cosy1

where D is the average diameter of the calculated particles, Kis the

shape factor of the average grain size (the expected shape factor is

0.9), l is the wavelength characteristic in A (in this particular case

l1.5405 A), b1/2 is the width of the X-ray peak at half its high,based on the XRD tables the (1 0 1) reflection for anatase is

identified at yE12.651.

Transmission Electron Microscopy (TEM) was carried out on a

JEOL-2000FXII operated at 200 kV. Using TEM, phases, crystalline

structure and grain size were determined. Scanning Electron

Microscopy (SEM) was conducted on a Phillips XL-30 operated at

20 kV to determine the morphological changes of anatase for

different heat treatment times.

Raman spectroscopy was conducted on a Thermo Nicolet

apparatus model Almega, equipped with a laser with a wavelength

of 532 nm using medium intensity, a 1 cm1 shift and a resolution

of 0.5 cm1. UVvis was conducted on a Varian Cary I apparatus

using the diffuse reflectance method for powders in wavelengths

between 190900 nm. The band gap was determined with the

KulbekaMunk method (Zanjanchi et al., 2006). Infrared spectro-

scopy was carried out on a NicoletMagna 750 FTIR apparatus in

the region from 4000 to 400 cm1 with a scanning of 1 cm1. The

KBr disk method was used to prepare the anatase samples, no

mulling was required due to the size of the anatase powders; the

ratio KBr:TiO2 was 30:1.

The particle characteristics were determined using BET method

on a Micrometrics ASAP 2000 nitrogen adsorption apparatus. Prior

to the BET analysis, the samples were degassed and aged at 1001Cfor 24 h. The adsorption analysis was conducted using nitrogen

with relative pressures (P/Po) between 0.5 and 1.0. Pindicates the

equilibrium pressure among the gas and the solid and Po is the

pressure of thegas required forthe saturation at the temperature of

the experiment.

3. Results

Fig. 1(a) shows a SEM micrograph of original anatase particles

(as synthesized) and can be observed that the size of the particles

go fromnanometric to micrometric and have flaky appearance. The

large surface area exposed by the original anatase particles is

clearly shown in Fig. 1(a). Although, the anatase particles are

agglomerated they still have a large ratio surface area/particle size

that can represent advantages for catalysis and other applications.

Fig. 1(a) and (b) shows SEM micrographs anatase particles heat

treated at 500 1C for72 h. In the SEM micrograph it canbe observed

that the morphology of the anatase particles lose its flaky appear-

ance on the agglomerated particles. This is attributed to the

coarsening of anatase growing that is in preferential directions

resulting in anisotropic, thus polymorphic, growth (Fig. 1b).

The anatase particles observed in Fig. 1 are in fact composed of

agglomerations of nanometric crystals forming the observed flakes

(Fig. 2). The nanostructured nature of the above described anatase

(original) is clearly observed in the TEM dark fields ( Fig. 2(a)). An

anatase particle heat treated for 72 h at 500 1C is presented in

Fig. 2(b) showing that the grain size increases with heat treatment

time. Fig.2(c) compares the TEM-SelectedArea ElectronDiffractionPatters (SAEDP) for original anatase and anatase heat treated for

72 h. The comparison of the SAEDP demonstrates that after 72 h of

heat treatment at 500 1C anatase do not present phase transforma-

tions.The betterdefined rings in theheat treated anatase (Fig. 2(c))

arethe resultof coarsening;although,the nanometricnature of the

anatase is preserved.

Fig. 3 shows the XRD difractograms for the original and heat

treated samples from 1 to 72 h. The anatase heat treated for 72 h at

500 1C does notshowevidences of phasetransformation, whichis a

major objective in this research work. From Fig. 3 it is observed

there is an the increase in intensity of the reflections of anatase

peaks and the reduction in width as the heat treatment time

increases. This is translated in coarsening that results in higher

crystal quality (Cullity and Stock, 2001), hence more defined XRDreflections. This confirms the TEM-SAEDP results presented in

Fig. 2(c).

Fig. 4 shows the analysis of lattice volume for anatase as

determined from XRD and is compared with their respective band

gap as a function of heat treatment time. It can be observed that as

the grain size coarsens the lattice volume of anatase is unstable for

heat treatment times of less than 8 h corresponding to grain sizes

smaller than 17 nm and the changes in band gap are also observed

at similar times. The 17 nm is somehow in agreement with the

recently published work by (Gonzalez-Reyes et al., 2010) where

they reporta critical value of 21 nm. The average lattice volume, as

determined by XRD, for the original and heat treated anatase is

135.53 nm3 with a standard deviation of70.28 nm (0.21% differ-

ence). Such change in lattice volume can be considered negligible

L. Gonzalez-Reyes et al. / Chemical Engineering Science 66 (2011) 721728722


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that is consistent with the volume conservation (Callister, 2007).

Perhaps these changes seem insignificant the instability of the

volume can be related to residual stresses that evolve as a result of

the relative large number of broken bonds along the surface of the

anatase grains resulting in a quantum effects.

Fig. 5(a) shows the Raman spectrum of original anatase as

obtained by sonochemical synthesis. In Fig. 5(a) are observed

Raman bands at 143, 397, 515 and 637 cm1, the original sample

has all the Raman scattering bands observed in anatase previously

reported (Toshiaki et al., 1978; Balachandran and Eror, 1982).

ba

100 m200 m

hours

(110)

(101)(200)

(111)(210)(211)(002)

(310)

(112)

ReferencecHeat treated for 72

Fig. 2. TEM micrographs of the (a) as-synthesized anatase, (b) heat treated anatase for 72 h at 500 1C and (c) comparison of the Selected Area Electron Diffraction Patterns

(SAEDP) of anatase in the as synthesized and heat treated (72 h at 5001C) conditions. Note: the SAEDP shows no phase transformations even after 72 h of heat treatment.

a b

5 m 100 m

Fig. 1. SEM micrographs of original anatase (a) as obtained from the sonochemical synthesis and (b) heat treated for 72 h at 500 1C.

L. Gonzalez-Reyes et al. / Chemical Engineering Science 66 (2011) 721728 723


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No other band were detected or identified. A well-resolved Raman

peak is observed at 143 cm1 showing the highest intensity of all

the bands in the anatase phase. The Raman results further confirm

that theonlyphasepresentis anatase andthe intensity of the peaks

increases with heat treatment time.

Fig. 5(b) shows the Raman spectra of the original and heat

treated samples for different times. All bands show that Raman

scattering increases as the anatase grain size coarsen that isconsistent with the XRD and TEM results (Figs. 2 and 3). From

Fig. 5(b) is evident that the intensity of the Raman scattering

increases indicating that the number of atoms (Ti and O) forming

molecules of anatase also increases. This is consistent with the

coalescence and coarsening that is reflected in a surface area

reduction. In contrast, the width of the band decreases with heat

treatment time indicating the number of bonds forming anatase

increases with heat treatment time.

Fig. 6 shows selected IR spectroscopy results for the original and

heat treated anatase for 16 and 72 h at 500 1C. In the IR results the

following anatase bands are observed: 813.7, 1614.2, 1585.9,

2366.4 and 2328.2 cm1. It is importantto notice that the intensity

andlocation of thesepeaks change with heat treatment time that is

attributed to the reduction in surface area and the increase in

crystal quality of the anatase powders. The main band observed at813.7 cm1 corresponds to TiO vibration and the TiOTi torsion.

The identified bands between 1500 and 3500 cm1 for the original

samples correspond to the organic residue of carboxyl groups

(CO) and water (Mayo et al., 2004). Similar bands are observed in

the heat treated samples for up to 8 h (Figures not presented

herein). The CO residue was previously reported anddetected by

thermo gravimetric analysis (Gonzalez-Reyes et al., 2008). In the

present work neither water nor the CO groups have been

reported by any other characterization method but infrared

spectroscopy; although, this was previously reported by Mayo

et al. (2004). This water is probably absorbed by the sample during

its handling and exposure to the environment as previously

reported in reference Mayo et al. (2004).

In Fig. 6 it is observed that the infrared bands of anatase in theoriginal sample are weak, in fact, the band at 3387 cm1 from

water is more intense. This is attributed totwo main reasons: (i)the

size and number of pores that allow easy adsorption of water and

(ii) fine grain size of original anatase. After 16 h of heat treatment

the intensity of the OH symmetric and anti-symmetric stretches

(3380 cm1) is significantly reduced.The intensityof the scissoring

band (1624 cm1) is almost constant for heat treatment times as

long as 16 h and at this point is the only water band identified. This

is translated in a densification effect of anatase preventing the

excessive adsorption of water and limiting the interaction of water

to the surface of anatase.

Fig. 7(a) shows the coarsening path of the anatase particles as a

function of time. Except for the original anatase the coarsening path

occurs in a quasi-exponential fashion similarly to the behaviour

A(206)

A(202)

A(112)

A(103)

500C

A(220)

A(116)

A(215)

A(204)

A(211)

A(105)

A(200)

A(004)

72h

48h

2424h

16h

8h

4hIntensity(arb.unit)

Original

2h

1h

20 30 40 50 60 70 80

2 theta (degree)

A(101)

Fig. 3. Shows the XRD diffractograms for the original and the heat treated anatase

particles for various heat treatment times at 500 1C in ambient.

3.19

135 3.15

3.17

3.13

1343.09 B

andGap,(eV)

LatticeVolumeofAnatasa,(3)

3.07Lattice Volume Band Gap

0 10 20 30 40 50 60 70

Time, (h)

3.11

3.05

136

133

Fig. 4. Change in the lattice volume and band gap for anatase as a function of the

heat treatment time at 500 1C.

48h

72h

Int

ensity,(a.u.)

24h

16h

8h

Original1h

4h2h

Raman Shift, (cm-1

)

800700600500400300200100

Fig. 5. Raman spectrum of the (a) original sample synthesized by sonochemical

meansand (b)Ramanspectraof theheattreated anatase at 500 1C forvarioustimes.



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proposed by theLifshitz, Slyozov andWagner (LSW theory) (Hays et al.,

2005; Voorhees, 1992; Voorhees and Glicksman, 1984). The results

shown in Fig. 7(a) are in agreement with coarsening mechanisms

previously reported (Gonzalez-Reyes et al., 2008; Hays et al., 2005;

Voorhees,1992; Voorhees andGlicksman, 1984).In Fig. 7(b) and (c) the

comparison of heat, the effect of treatment time and grain size of

anatase vs. BET and XRD results are presented.

Table 1 shows the regression equations of the curves given in

Fig. 7(b) and (c) based on the two approaches (time and grain size,respectively). These regression equations can be used to predict the

surface characteristics of the heat treated anatase as a function of time

and grain size. All regression equations have R2 larger than 0.93 except

for the regression equation of the surface area as a function of time

(R20.81).Thehigh R2 valuesindicategood correlationamong theBET

results with XRD results and heat treatment time.

The first set of graphs in Fig. 7(b) and (c) show that the pore

diameter grows with heat treatment time that is the result of

coalescence of anatase particles. The second set of graphs in

Fig. 7(b) and (c) depict the reduction in pore volume with heat

treatment time as well as grain size. This effect indicates a

densification effect that is further confirmed with the infrared

results. Further, in order to express the phenomena presented in

the three graphs shown in Fig. 7(b) a more complex algorithms are

required (compare the equations given in Table 1). The algorithms

given in Table 1 for time ignore the original sample; it means, they

ignore the coalescence phenomenon. On the contrary the algo-

rithms for the curves presented in Fig. 7(c) are simpler and are a

better fit between the BET surface characteristics with grain sizes.

The third graph in Fig. 7(c) is potentially the most important

because it relates the grain size with surface area. The analysis of

Fig. 7 demonstrates that using the XRD results it is possible to

determine the surface area of anatase particles that is a key

parameter to estimate the potential of anatase for numerous

applications (e.g. catalytic, photo-catalytic, etc.).

Fig. 8 shows the coarsening evolution of the anatase particles.

The flaky appearance of the original anatase is again observed in

Fig. 8(a). It is important to notice that the original anatase is in the

form of micrometric and in some cases sub micrometric powders.

However, after 1 h of heat treatment an agglomeration effect is

observed and is associated with coalescence during the heat

treatment. Fig. 8(b)(i) show denser aggregates of nanostructuredanatase. The heat treated anatase from 1 to 16 h (Fig. 8cf) does not

show notorious differences. After this time the densification

becomes more apparent and at 72 h of heat treatment (Fig. 8(i))

the preferential growth of anatase particles is evident. Such

preferential growth is also identified by XRD, Raman and Infrared

and is associated tothe preferential growthof the planes(1 1 1) and

(1 1 2) and the 143 cm1 Raman band.

4. Discussions of the results

In the present work the heat treatment allowed the coarsening

of anatase from 6.2 nm (original anatase) to a size of 28.3 nm (heat

treated for 72 h). The grain sizes of anatase particles previously

99

101 Original

1432

1522

23553

387

87

90

93

96

%Transmittance

465

824

1646

2334

500100015002000250030003500

Wavenumbers (cm-1

)

99

101 16 h

90

93

96

%Transmittance

963

1370.47

1636

1702

87500100015002000250030003500

72h2342

2329

93

96

99

813

1614

87

90%Transmittance

500100015002000250030003500

Wave length (cm-1

)

101

Wavenumbers (cm-1

)

Fig. 6. Selected infrared spectroscopy results for original and heat treated anatase

for (b) 16 and (c) 72 h.

25

30

5

10

15

20

CrystalliteSize

(nm)

800

time (h)

3.75320

480

640

PoreDiameter

(nm)

2.25

3.00

TotalPore

Volume,(m3)

60

120

180

2403001.50

SurfaceArea

(m2g-1)

0

time (h)

0 10 20 30 40 50 60 70

0 10 20 30 40 50 60 70 5 10 15 20 25

0 10 20 30 40 50 60 705 10 15 20 25

Grain Size, (nm)

Fig. 7. (a)Coarsening path of anatase, BETcharacteristics of anatase as a function of

(b) heat treatment time and (c) grain size.



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reported in the literature are between 11.3 and 35 nm that are

comparable, in size, to the anatase studied in this research

(Ding et al., 1996; Reddy et al., 2003; Gribb and Banfield, 1997;

Zhu et al., 2005). Current findings at University of Houston show

that commercial anatase, with reported purity of499%, can have

up to 4% of rutile. Hence, anatase produce by sonochemical means

do not show any presence of rutile by any of the characterization

methods used in the present research work. In the present work

was determined that using the Spurr-Myers was not possible to

detectany rutilein the as synthesized or heat treated anatase forup

Table 1

Summary of the regression equations obtained of the BET and XRD characteristics of heat treated anatase in function of heat treatment time and grain size.

Particle characteristic (nm) Time (h) Grain size of anatase

Pore volume 106 (m3 g1) Pore volume 3563:6t7:38 Pore volume 9:3Size 39:4

R20.98 R20.95

Surface area (m2 g1) Surface area 1:19t123:3 Surface area 783:6Size0:85

R20.81 R20.99

Pore diameter (nm) Pore_diam: 0:455t308:47 Pore_diam: 0:86Size238:9Size 501:2

R20.97 R20.98

Grain Size as a function of the heat treatment time (h) Crystallite size 13t0:208

R20.97

ba c10 mm

10 m10 m

10 m10 m10 m

10 m

10 m

10 m

d e f

g h i

Fig. 8. Scanning Electron Micrographs of (a) original and (bi) heat treated anatase for 1, 2, 4, 8, 16, 24, 48, 72 h, respectively.



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to 72 h at 500 1C. This is further confirmed by Raman. IR, on the

other hand, shows the presence of water and traces of organic

residue in theas synthesizedanatase. The water is usually removed

during theheat treatment. And theorganic residue canbe removed

by further washing with deionised water.

Equations presented in Table 1 canbe used foranatase particles

with grain sizes of up to 28.3 nm covering most of the spectrum of

anatase produced by different methods (Ding et al., 1996; Reddy

et al., 2003; Gribb and Banfield, 1997; Zhu et al., 2005; Li et al.,2004; Banfield et al., 1993). Based on the high correlation of the

coarsening behaviour presented by anatase as a function of grain

size; it is possible to extrapolated these results to larger sizes

(35 nm) to cover the entire sizes where pure anatase coexist.

However, thispractice may notalways guarantee the highaccuracy

reported in the results presented in Table 1. The constants used in

the equations presented in Table 1 may vary for anatase produced

by other methods.However,once the constants are determined the

equations can be used as an alternative to predict BET results based

on XRD results, or vice versa, with good accuracy.

The coarsening of anatase particles follows an anisotropic

growth promoting the formation of polymorphic particles at long

heat treatment times. The coarsening of anatase particles is

confirmed by the better defined rings observed on the TEM-SAEDP

that is observed in the dark fields. This coarsening can certainly be

associated to an increase in crystal quality, but unfortunately,

reduces the surface area of the anatase particles as seen in the BET

results. Thecoarseningof anatase canbe detailed studied by means

of XRD and directly correlated to BET surface characteristics.

Similar approach is possible using the heat treatment time;

however, the heat treatment time approach is limited to heat

treated anatase ignoring the coalescense of anatase.

The use of grain size as an independent variable to determine

BET characteristics of anatase is better than the use of heat

treatment time, and most importantly this process considers the

phenomena of coalescence. Coalescence is time independent and

can be observed in Fig. 7 and further demonstrated in Fig. 8. The

algorithm proposed in this work can represent technological

advantages that can be translated in time savings, allowing aneasy prediction of BET characteristics using XRD results and vice

versa. It is expected that with the work recently published by

(Gonzalez-Reyes et al., 2010) this work can be used to further

determine other characteristics such as band gap and in the near

future, catalytic, electro-catalytic and photo-catalytic activities of

anatase.

In the literature reported different phase transformation tem-

peratures and heat treatmenttimesat whichanatase transforms to

rutile have been reported. But in general, these temperatures are

similar to the temperature used in the present work (500 1C)

(Reddy et al., 2003; Gribb and Banfield, 1997; Zhu et al., 2005 ; Li

et al., 2004). In addition to that, some reports indicate that anatase

can reach grain sizes of 60 nmor more, but in all of those cases this

anatase is reported as mix with rutile (Li et al., 2004).TheXRD results indicate that the change in lattice volume occur

only insamples heat treated forless than 8 h (Fig.4).Afterthe8 h of

heat treatment the lattice volume is almost constant and is

attributed to a higher crystal quality (less broken bonds at the

surface of the anatase grains) that is attributed to coalescence and

coarsening of anatase. During coalescence and coarsening of

anatase some of the atoms with broken bonds, at the grain

boundary, re-combine with the atoms of neighbouring grains

forming complete bonds and thus larger crystallographic planes.

This phenomenon is observed in the XRD, Raman and TEM results

presented herein.

The effect of coarsening relaxes or lowers the stress and strain

levels in anatase lattices reducingthe number of broken TiObonds

in the anatase that results in stable band-gaps. This implies that

band gap has a relation with the ratio among surface and bulk

atoms and when this ratio is relatively large quantum changes

occur. As the number of broken bonds reduces the band gap

becomes more stable. Similar effects were reported previously in

the literature for Co-doped SnO2 (Hays et al., 2005). Due to the

potential to change the band gap of anatase it is of interest to

explore different dopants fora wide variety of applications (Janisch

and Spalding, 2006;Tanget al., 1993). Modifying theanatases band

gap through doping and grain size can result in technologicaladvances increasing its the semi-conductor, catalytic and other

properties.

The infrared results show that the intensity of the anatase band

(813.7 cm1) increases with heat treatment time. Such band has

been previously associated with a texturing effect and preferential

growthof thin films alongthe (1 1 2) planepromoting grain growth

along the {1 0 1} planes (Ocama et al., 2006). The coarsening

mechanism is responsible for increasing the number of atoms

(Ti and O) forming anatase that results in an enlargement of the

planes that are capable to obey Braggs law. This is observed by the

increase in the XRD intensity and the reduction in the width of the

reflections. Similar effects are observed in Raman and TEM due to

theincreasein thenumberof atoms forming anatase molecules and

grains, respectively.

The infrared bands observed at approximately 3350, 2100 and

1650 cm1 correspond to water that is absorbed during sample

handling. The analysis of water using the infrared results is of great

importance in this work because the intensity of the water bands

decrease with heat treatment time (Fig. 8). The reduction in the

water content results in higher density and can be observed in the

SEM micrographs shown in Fig. 8. This effect has also influence in

other BET surface characteristics, such as pore volume and surface

area as observed in Fig. 7(b) and (c).

5. Conclusions

The characterization methods used herein are complementary

and allow a throughout analysis of anatase synthesized by sono-chemistry. Demonstrating that anatase produced by sonochemis-

try is nanostructured with untraceable amounts of rutile or any

other substance as indicated by XRD and Raman. Heat treatments

at 500 1C allow the coarsening of anatase from 6.2 to 28 nm and

hinder phase transformations. A correlation among bang-gap and

lattice volume is proposed; however, it is likely that the actual

mechanism affecting the band gap is the result of residual stresses

thatevolve by broken bonds (TiO) at anatasesgrain surface. There

is a good agreement among BET surface characteristics results and

XRD that permits proposing a new algorithm to predict BET results

based on XRD and vice versa.

Acknowledgements

LGR and IHP would like to thank CONACyT-Mexico, SEPI-IPN,

and the Instituto de Ciencia y Tecnologa del Distrito Federal,

Mexico (ICyTDF) Grant no. BI09-491 (LGR, IHP) for financial

support. FCRH wishes to express his appreciation to the University

of Houston andthe Government of Texas, fortheir support through

the Start Up, HEAFS and small grant programs.

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Effect of coarsening of sonochemical synthesized anatase BET surface characteristics