# Analysis of Polymorphic Nanocrystals ofTiO2 by X-Ray Rietveld Refinement and High-Resolution Transmission Electron Microscopy: Acetaldehyde Decomposition

Authors: R. Carrera; N. Castillo; E. Arce; A. L. Vázquez; M. Moran-Pineda; J. A. Montoya; P. Del Ángel; S. Castillo
Journal: Research Letters in Nanotechnology
 (2008)

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## Abstract

In this work,TiO2 nanocrystals were synthesized by the sol-gel method. These materials were annealed at 200 and 500∘C; and characterized by the XRD-Rietveld refinement; and by BET and TEM. As for the low-temperature-treated sample (200∘C), nanocrystals with small crystallite sizes (7 nm) and high abundance of anatase, coexisting with the brookite phase, were obtained. Meanwhile, the sample annealed at 500∘C showed an increased crystallite size (22 nm) and an important polymorphic increment. The sample annealed at 200∘C showed a high activity in the photocatalytic decomposition of acetaldehyde.


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## Body

## 1. Introduction

TiO2 is a material that is widely used in electronics,
ceramics, catalysis, and pigment industries because of its optical and
photocatalytic properties, which stem from the
quantum size effect [1]. Likewise, TiO2 has become a very important material due to its applications in different
processes such as water purification; and more recently, in the control of air contaminant
gases present in both indoor and outdoor
environments, where the UV-light is the necessary energy source in the photocatalytic processes [2–4]. There are three types of TiO2 crystalline structures: rutile, anatase, and brookite. Rutile is the only
stable phase, whereas anatase and brookite are almost metastable at all
temperatures. Nowadays, the challenge for many researchers, in order to obtain
a photocatalytic material, is to control the following TiO2 properties:
the crystallite size, anatase-rutile transition, surface area, hydroxylation,
and thermal stability [5]. According to some
studies, the anatase phase is obtained at low temperatures, at around 350°C, which is useful for catalytic
and industrial applications [6]. Recently, the effect of the brookite phase on the anatase-rutile
transition in TiO2 nanoparticles has been studied, where the
proportion of brookite depends on both the method and conditions used [7]. For instance, by using either thermolysis or
hydrothermal synthesis, it is possible to obtain brookite at high temperature,
likewise, in some works, the role of brookite in the TiO2 crystal
size has been analyzed [8, 9]. Furthermore, both the TiO2 nanocrystals
and anatase-rutile transition phase have considerably attracted attention
because of their special physical and chemical characteristics in
photocatalytic applications; however, both characteristics depend on the
preparation methods [10–12]. Through the sol-gel method, it is possible
to obtain the smallest TiO2 crystal size, which is a fundamental
property to perform the-near-visible UV photocatalytic reactions; that is why TiO2 is
a very useful material in a variety of applications such as the decomposition
of both volatile organic compounds (VOCs) and gas-phase nitrogen oxides (NOx)
[13, 14].The aim of this work is the synthesis of TiO2 nanocrystals by the sol-gel method, where these materials were annealed at
200 and 500°C, and characterized by the XRD-Rietveld refinement, nitrogen adsorption (BET) and
high-resolution transmission electron microscopy (HRTEM) of polymorphic TiO2 for their
application as catalysts in the acetaldehyde
photodecomposition through in situ
microreactions photoassisted with UV light.


## 2. Experimental

The sol-gel TiO2 nanocrystal-catalysts were prepared as follows: 36.67 mL of
titanium (IV) isopropoxide (AldrichMo, USA, 99.9%) were dissolved in 60 mL of
2-propanol (Baker 99.9%). The solution was set under constant stirring; and
then, hydrochloric acid (Baker 36.5 vol.% in water) was added to adjust the
reaction medium at pH 3. The hydrolysis of the preparations (with 2-propanol as
solvent) was accomplished by adding 18 mL of bidistilled water (water/alkoxide
ratio of 1:2). The solutions were then
maintained under stirring and reflux until the gels were formed. Afterwards,
the gels were dried at 70°C for 12 hours; and then annealed at 200
and 500°C for 4 hours, respectively, with a heating rate of 20°C/min. The samples were
labeled as TiO2-P200 and TiO2-P500.In order to perform the XRD, a D500 Siemens
with a copper tube and Kα′ radiation of 1.5405, operating at 35 KeV and 15 mA, was used. The intensities were
determined in the 2Θ interval ranging from 20° to 80°. To refine each spectrum,
the Rietveld analysis was applied by using the full prof software by Rodríguez
Carbajal [15]. The crystal size was
determined by the Rietveld refinement and Scherrer equation [16]. The determination of the surface area was
performed by means of the nitrogen physisorption in an ASAP-2000 Micromeritics equipment. The high resolution transmission electron microscopy
(HRTEM) was performed in a JEOL JEM-2200FS microscope with a Schottky-type
field gun, working at 200 kV. The point resolution was of 0.19 nm; and the
information limit was better than 0.10 nm. The HRTEM
digital images were obtained using a CCD camera and the Digital Micrograph
Software from Gatan. In order to prepare
the materials for observation, the powdered samples were ultrasonically
dispersed in ethanol and supported on holey carbon-coated copper grids. From the obtained micrographs, the average
particle size was calculated by the surface/volume equation [17]. The photocatalytic
activity tests for the TiO2-P200, TiO2-P500 samples, and the witness (Degussa P25) were carried out in
experimental equipment at microreaction level. A quartz cell was used as a photoreactor
with a 365-UV lamp (UVP-light-sources) with an intensity of 100 μW/cm2. The tests were carried out by using acetaldehyde (CH3CHO) with a
concentration of 300 ppmv; and 2% of oxygen.


## 3. Results and Discussion

By the XRD and Rietveld refinement, the
phases and structures formed in each of the TiO2 samples were
determined using the unit cells and known space groups (Table 1) [18]. In the sol-gel
TiO2 catalysts, the three known titania phases, anatase
(tetragonal), rutile (monoclinic), and brookite (orthorhombic), were obtained
(Figure 1). The TiO2-P200 sample was less polymorphic
(anatase-brookite phases) than the TiO2-P500 sample
(anatase-bookite-rutile phases). The anatase-rutile transition was
determined as a function of the thermal treatment, where an appreciable
percentage of anatase was observed in the sample prepared at high temperature
(TiO2-P500); likewise, only in this sample appears the rutile phase.
With regard to the TiO2-P200 sample, anatase and brookite phases
with small crystal sizes were found, which could give specific photocatalytic
properties because of the nanometric-crystal size/phase ratio (Figure 1) [19].
The characterization parameters of each crystalline structure and their average
crystallite size were obtained from the corresponding Rietveld refinement. By the Rietveld
refinement, the TiO2-P200 sample showed the following phase
compositions: anatase (62.88%) and brookite (37.1%); whereas in the TiO2-P500
sample, its phase composition was anatase (82.67%), brookite (14.9%), and
rutile (2.43%) (Table 1). According to
these results, we can see that the handling of both the hydrolysis degree and pH in the sol-gel method enabled us to
synthesize TiO2 anatase at low temperature (200°C) since the anatase
phase transformation by other methods occurs at 450°C [5, 19].Table 1
DRX-Rietveld refinement, phase concentration, and crystal size of the sol-gel TiO2 samples.
SampleStructurePhase (%)Crystal size (nm)Scherrer crystal size (nm)Phasea (nm)b (nm)c (nm)TiO2-P200Tetragonal62.886.97.03Anatase0.37909260.37909260.9495732Orthorhombic37.126.018.26Brookite0.91676240.54164610.5210546TiO2-P500Tetragonal82.6720.522.04Anatase0.37861670.37861670.9506104Orthorhombic14.9013.134.02Brookite0.91425670.54420680.5191934Monoclinic2.4334.727.14Rutile0.45913370.45913370.2951845Figure 1
XRD-Rietveld refinement concentration for each structure in the sol-gel TiO2 samples.Both the anatase and brookite found in the TiO2-P200 sample showed a very
small crystallite size (≈6 nm); and on the
other hand, the TiO2-P500 sample showed a little anatase-rutile
transition due to the thermal treatment (anatase 82.67% and rutile 2.43%),
which suggests that the small crystallite size controls the anatase-rutile
transition and its stability; likewise, the synthesis method enabled us to
obtain brookite at low temperature [9]. According to Zhang and Banfield, the particle
size plays an important role in the phase stability; for instance, anatase is
more thermodynamically stable at sizes below 11 nm; and brookite is stable for
crystal sizes between 11 and 35 nm [20]. It is known that the brookite-rutile transformation is faster than the
anatase-rutile transformation, where there is an effect related with the
pressure on the small anatase crystallites; such a case could promote the
formation of a rutile nucleus at short transition-temperature periods; but in
this work, even at high temperatures, the anatase-rutile transition does not
occur; therefore, probably, the anatase-rutile transition could be modified
when the grain size was small enough (Table 1), (Figure 1) [21, 22].The structure of the policrystals, the
interplanar distances, and the TiO2-P200 and TiO2-P500
samples were determined by HRTEM. Figures 2 and 3 show the typical HRTEM
images for the TiO2-P200 sample with a morphology characteristic of
the tetragonal structure. The inset
corresponds to the fast Fourier transform (FFT) or digital diffractrogram. The diffraction spots correspond to the
interplanar distance d110=0.323 nm of the tetragonal TiO2 (anatase phase). The distribution of the
crystal size is shown in Figure 4; the average nanometric size of the crystals
is around 7 nm; and the standard deviation is 1.32 nm; these results confirm
the presence of nanostructured TiO2.Figure 2
Polymorphic nanocrystals of TiO2-P-200. (A) Diffraction.Figure 3
Details of TiO2-P-200
nanocrystals.Figure 4
Average crystal size (SD = 1.32 nm) of sol-gel TiO2 samples.Figure5
shows an image of the TiO2-P500 sample and the corresponding FFT. The
HRTEM image of the TiO2 particle was identified as the anatase phase
with zone axis [112]. The average
nanometric size of the crystals is 17 nm. Likewise, Figure 6 shows the image of a single crystal of the TiO2-P500
sample, with a morphology characteristic of the
TiO2 tetragonal structure, which corresponds to the anatase
phase. The morphology of the TiO2
nanostructured materials is equiaxial with a zone axis of [010]. The effect of the calcination temperature on the surface area in the samples is very
important; for instance, in the TiO2-P200 sample, this value is
tripled (189 m2/g) with respect to that in the TiO2-P500
sample (60 m2/g). There is also an effect on the TiO2 crystal size, which was more than doubled as a consequence of the sinterization process (Table 1).Figure 5
(A) Nanocrystals of TiO2-P-500. (B) Diffraction pattern of the TiO2.Figure 6
Details of nanocrystal of TiO2-P-500.The textural and morphological properties showed by the sol-gel TiO2 catalysts could be related to their activity in the acetaldehyde decomposition.
In the TiO2-P200 sample, a conversion higher than 95% was obtained
after 150 minutes, meanwhile the TiO2-P200 sample reached a
conversion near to 70% in the same period of time (Figure 7). It is important to note that the sol-gel
catalysts were more active than the P-25 commercial titania, which reaches only
30% of conversion in 150 minutes. In
our opinion, the high activity of the TiO2-P200 sample can be
attributed to (i) the presence of the anatase-brookite phase; (ii) the presence of an important abundance of
brookite; (iii) the small particle sizes
which were three times smaller than those obtained in the TiO2-P500
sample (Table 1). According to the CH3COH mineralization, assisted
with a lamp near the UV-vis (365 nm), showed by the sol-gel TiO2 catalyst, it could be considered as a good option to be applied in both indoor
and outdoor pollution control.Figure 7
Acetaldehyde conversions as a function of time for the TiO2 samples.


## 4. Conclusions

By varying the sol-gel parameters, it was
possible to obtain less polymorphic TiO2 at low temperature, since
the TiO2-P200 sample only showed two phases (anatase and brookite)
and a small crystal size (≈7 nm) whereas the
TiO2-P500 sample showed the three main structures (tetragonal, orthorhombic and monoclinic), likewise a bigger crystal size
(>22 nm). In the same way, by
handling the sol-gel method parameters, it was possible to increase the surface
area (189 m2/g). The TiO2 with less polymorphism and small crystal size showed high photoactivity in the
acetaldehyde decomposition; therefore, these two variables could play a major
role in photocatalysis.



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*Source: 138468-2008-08-03.xml*