Upload
alkimia
View
227
Download
0
Embed Size (px)
Citation preview
7/30/2019 Effect of coarsening of sonochemical synthesized anatase BET surface characteristics
1/8
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
http://-/?-http://www.elsevier.com/locate/ceshttp://localhost/var/www/apps/conversion/tmp/scratch_1/dx.doi.org/10.1016/j.ces.2010.11.030mailto:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]:[email protected]://localhost/var/www/apps/conversion/tmp/scratch_1/dx.doi.org/10.1016/j.ces.2010.11.030http://localhost/var/www/apps/conversion/tmp/scratch_1/dx.doi.org/10.1016/j.ces.2010.11.030mailto:[email protected]:[email protected]:[email protected]://localhost/var/www/apps/conversion/tmp/scratch_1/dx.doi.org/10.1016/j.ces.2010.11.030http://www.elsevier.com/locate/ceshttp://-/?-7/30/2019 Effect of coarsening of sonochemical synthesized anatase BET surface characteristics
2/8
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
7/30/2019 Effect of coarsening of sonochemical synthesized anatase BET surface characteristics
3/8
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
7/30/2019 Effect of coarsening of sonochemical synthesized anatase BET surface characteristics
4/8
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.
L. Gonzalez-Reyes et al. / Chemical Engineering Science 66 (2011) 721728724
7/30/2019 Effect of coarsening of sonochemical synthesized anatase BET surface characteristics
5/8
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.
L. Gonzalez-Reyes et al. / Chemical Engineering Science 66 (2011) 721728 725
7/30/2019 Effect of coarsening of sonochemical synthesized anatase BET surface characteristics
6/8
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.
L. Gonzalez-Reyes et al. / Chemical Engineering Science 66 (2011) 721728726
7/30/2019 Effect of coarsening of sonochemical synthesized anatase BET surface characteristics
7/8
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.
References
Al-Salim, N.I., et al., 2000. Characterisation and activity of solgel-prepared TiO2photocatalysts modified with Ca, Sr or Ba ion additives. J. Mater. Chem. 10,23582363.
Balachandran, U., Eror, N.G., 1982. Raman spectra of titanium dioxide. J. Sol. State
Chem. 42, 276282.
L. Gonzalez-Reyes et al. / Chemical Engineering Science 66 (2011) 721728 727
7/30/2019 Effect of coarsening of sonochemical synthesized anatase BET surface characteristics
8/8
Braun, J.H., 1997. Titanium dioxidea review. Coat J. Technol. 69, 5972.Burns,A.,et al., 2004.Neodymiumion dopanteffectson thephase transformationin
solgel derived titania nanostructures. Mater. Sci. Eng. B 111, 150155.Cai, R., Kubota, Y., Shuin, T., Hashimoto, K., Fujishima, A., 1992. Induction of
cytotoxicity by photoexcited TiO2 particles. Cancer Res. 52, 2346.Callister Jr., W.D., 2007. Materials science and engineering: an introduction. John
Willey & Sons, pp. 131173.Chan, C.K., et al., 1999. Effects of calcination on the microstructures and photo-
catalytic properties of nanosized titanium dioxide powders prepared by vaporhydrolysis. J. Am. Ceram. Soc. 82, 566572.
Cullity, B.D., Stock, S.R., 2001. Elements of X-ray diffraction. Prentice Hall, NJ USA,
pp. 385433.Diebold, U.,2003. Thesurfacescienceof titaniumdioxide.Surf. Sci.48 (58),53229.Ding, X.Z., Liu, X.H., He, Y.Z., 1996. Grain size dependence of anatase-to-rutile
structural transformation in gel-derived nanocrystalline titania powders.J. Mater. Sci. Lett. 1 5, 178 91891.
Fujishima, A., Rao, T.N., Tryk, D.A., 2000. Titanium dioxide photocatalysis.J. Photochem. Photobiol. C: Photochem. Rev. 1, 12 1.
Gamboa, J.A., Pasquevich, D.M., 1992. Reformulation of an aqueous alumina slipbased on modification of particle-size distribution and particle packing. J. Am.Ceram. Soc. 75, 290294.
Gan, J.Y., Chang, Y.C., Wu, T.B., 1998. Dielectric property of (TiO2)x(Ta2O5)1x thinfilms. Appl. Phys. Lett. 72, 332.
Gonzalez-Reyes, L., et al., 2008. Characterization of anatase synthesized by sonochem-ical means and effects of heat treatments. Proc. TMS Annu. Meeting 1, 497502.
Gonzalez-Reyes, L., et al., 2008. Sonochemical synthesis of nanostructured anataseand study of the kinetics among phase transformation and coarsening as afunction of heat treatment conditions. J. Eur. Ceram Soc. 28, 15851594.
Gonzalez-Reyes,L., Hernandez-Perez, I.,Daz-BarrigaArceo, L., Dorantes-Rosales, H.,
Arce-Estrada, E., Suarez-Parra, R., Cruz-Rivera, J.J., 2010. Temperature effectsduring Ostwald ripening on structural and bandgap properties of TiO2 nano-particles prepared by sonochemical synthesis. Mater. Sci. Eng. B 175 (1), 913.
Gouma, P.I., Mills, M.J., 2001. Anatase to rutile transformation in titania powders.J. Am. Ceram. Soc. 84, 6 19622.
Gribb, A.A., Banfield, J.F., 1997. Particle size effects on transformation kinetics andphase stability in nanocrystalline TiO2. Am. Mineralogist 82, 717728.
Hays,J., et al.,2005.Relationship between thestructuraland magnetic properties ofCo-doped SnO2 nanoparticle. Phys. Rev. B 72, 075203075210.
Henrich, V.E., Cox, P.A., 1994. The surface science of metal oxides. CambridgeUniversity Press, Cambridge, UK, 464 pp.
Hoffmann, M.R., Martin, S.T., Choi, W., Bahnemann, W.D., 1995. Environmentalapplications of semiconductor photocatalysis. Chem. Rev. 95, 6996.
Inagaki, M., et al., 2001. J. Inorg. Mater. 3, 809.Ito, S., Inoue, S., Kawada, H., Hara, M., Iwasaki, M., Tada, H., 1999. low-temperature
synthesis of nanometer-sized crystalline TiO2 particles and their photoinduced
decomposition of formic acid. J. Colloid Interface Sci. 216, 5964.Janisch, R., Spalding, N.A., 2006. Understanding ferromagnetism in Co-doped TiO2
anatase from first principles. Phys. Rev. B 73 035201035201.Kenneth, S., Suslick, J., Gareth, J., 1999. Applications of ultrasound to materials
chemistry. Annu. Rev. Mater. Sci. 29, 295326.Kumar, K.N.P., Keizer, K., Burggraaf,A., 1992. Densification of nanostructuredtitania
assisted by a phase transformation. Nature 358, 4851.Maira,A.J.,et al.,2000. Sizeeffectsin gas-phasephoto-oxidationof trichloroethylene
using nanometer-sized tio2 catalysts. J. Catal. 192, 185196.Mayo, D.W., Miller, F.A., Hannah, R.W., 2004. Course notes on the interpretation of
infrared and raman spectra, 1st ed. Wiley Inter-Science, New Jersey, USA.Toshiaki, O., Fujio, I., Yoshinori, F., 1978. Raman spectrum of anatase TiO2. Raman
Spectrosc. 7, 321324.Ocama, M., Pecharroman, C., Garcia, F., Holgado, J.P., Gonzalez-Elipe, A.R., 2006.
Analysis of texture and microstructure of anatase thin films by Fourier trans-
form infrared spectroscopy. Thin Solid Films 515, 15851591.Reddy, K.M., Manorama, S.V., Reddy, A.R., 2003. Bandgap studies on anatase
titanium dioxide nanoparticles. Mater. Chem. Phys. 78, 239.Reidy, D.J., Holmes, J.D., Morris, M.A., 2006. The critical size mechanism for the
anatase to rutile transformation in TiO2 and doped-TiO2. J. Eur. Ceram Soc. 26,
15271534.Shannon, R.D., 1964. Phase transformation studies in TiO2 supporting different
defect mechanisms in vacuum-reduced and hydrogen-reduced rutile. J. Appl.
Phys. 35, 3414.Suslick, K.S., et al., 1999. Acoustic cavitation and its chemical consequences. Phil.
Trans. R. Soc. London A 15 335335.
Tang,H., Berger, H.,Schmid,P.E.,Levy,E., Burry, G.,1993. Photoluminescence in TiO2anatase single crystals. Solid State Commun 87, 847850.
Voorhees, P.W., 1992. Ostwald ripening of two-phase mixtures. Annu. Rev. Mater.
Sci. 22, 197215.Voorhees, P.W., Glicksman, M.E., 1984. Solution to the multi-particle diffusion
problem with applications to Ostwald ripeningI. Theory. Acta Metall. 32,
20012011.Zanjanchi, M.A., Noei, H.,Moghimia, M.,2006.Rapid determination of aluminumby
UVvisdiffusereflectance spectroscopy withapplication of suitable adsorbents.
Talanta 70, 933939.Zhu, K.R., Zhang, M.S., Hong, J.M., Yin, Z., 2005. Size effect on phase transition
sequence of TiO2 nanocrystal. Mater. Sci. Eng. A 403 (403), 8793.
L. Gonzalez-Reyes et al. / Chemical Engineering Science 66 (2011) 721728728