# Synthesis of High-Thermal Stable Titanium  Dioxide Nanoparticles

Authors: M. Rashidzadeh
Journal: International Journal of Photoenergy
 (2008)

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## Abstract

Nanosized crystals of anatase phaseTiO2 with high-thermal stability was synthesized in the range of 15–17 nm via sol-gel method using titanium n-butoxide precursor. Synthesize was done at two different temperatures, that is, 200 and 300∘C and the final products were calcined at 400∘C. The final products were characterized by N2 adsorption, X-ray diffraction, FT-IR, and TEM and tested in a photoreactor using fluorescien as model molecule. For thermal stability investigation, the sample prepared at 300∘C was heated up to 800, 850, and 900∘C, the XRD results showed that the synthesized anatase phase was stable up to 850∘C. Photocatalytic activity of the prepared samples showed that the fluorescien degradation followed a pseudo-first-order kinetic.


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## Body

## 1. Introduction

TiO2 crystals exist
in three different  crystalline forms: anatase, brookite, and rutile. Anatase is
metastable and rutile is  considered to be the stable form of titania. Among
these polymorphs, anatse has attracted more attention for its use as
photocatalyst and catalyst support in industrial processes [1–5]. Generally,
large surface areas are required for catalyst supports to disperse a catalyst-active
material effectively to increase the number of active sites. Since the catalyst
is usually used at high temperature, high-thermal stability, as well as large
surface area is also important. Meanwhile, the synthesis of TiO2-photocatalyst
with high-thermal stable anatase phase is one of the key challenges in smart
coating for building materials application [6–8].
Photocatalytic activity and also catalytic properties of titania varies
depending on its crystallinity, particle size, crystal phase, and surface area
[2, 5, 9, 10]. Nanosized TiO2 powders are prepared by several
methods such as hydrothermal, sol-gel, microemulsion, and thermal decomposition
of alkoxides [11–15]. Hydrothermal
synthesis is a promising method to obtain nanocrystalline titania particles. The hydrothermal process in which the chemical reaction take place under autogenerated
pressure upon heating is sufficient to achieve the crystalline phase at
relatively low temperature [16]. In this work, titania photocatalyst with high
performance and high-thermal stability was prepared via sol-gel method followed
by hydrothermal process. The photocatalytic activity, thermal stability, and
degradation kinetics on a model dye, Fluorescein, of synthesized samples as
well as commercial sample were investigated.


## 2. Experimental

All reagents used in this study
were analytical grade. Degussa P25 sample,TiO2 (SBET:
54m2/gr, particle size: 29 nm) was used as reference. 21 mmol of titanium
n-butoxide (TNB) was dissolved in specified amount of toluene in a glass tube,
which was placed in a stainless Teflon-lined autoclave. Eight mL of deionized
water was added outside the glass tube in the autoclave. The autoclave was then
heated up the reaction temperature (i.e., 200 and 300°C) and held at that
temperature for 8 hours. During the heating, water was vaporized and TNB was
hydrolyzed. After the autoclave was cooled to ambient temperature, the
supernatant was decanted and the resulting precipitates were washed with
acetone and dried at room temperature. Finally, the dried samples calcined at
400°C for 3 hours. A part of samples were further heated at 800, 850, and 900°C
and held at these temperatures for 3 hours.


### 2.1. Characterization

The surface area of the samples was determined by nitrogen
adsorption using a quantasorb analyzer. Infrared spectra of samples were
recorded on a Bruker spectrometer IFS-88. The concentration of Fluorescein in
all aqueous solutions was analyzed using a UV-Vis. DU-500 spectrophotometer.
Morphology and the size ofTiO2 nanocrystallites were studied and
analyzed by TEM with a CM200, FEG-Philips. X-ray diffractometer type Philips PW
1840 (λ=1.54060Å′  at 40 kv and 20 mA) was used to identify the crystalline phase and also crystal size, using the
Scherrer method (4).Scherrer equation is as follows:(1)D=KλBcosθwhere K was taken as 0.9 and B is the full width of the
diffraction line at half of the maximum intensity.


### 2.2. Photocatalytic Activity

The photocatalytic activity of the samples was estimated by
decomposition of 90 ppm Fluorescein solution by using a 50–100 ppm of
photocatalyst. The photoreactor consisted of cylindrical glass reactor that a
light source from a 6 W Hg Philips lamp located axially at the center of the
vessel. A special glass atomizer as air diffuser was fixed at the bottom of the
reactor to uniformly disperse air into the above reaction mixture. A magnetic
stirrer was used to produce homogeneous reaction mixture. Degradation was
monitored by taking aliquots at different time intervals. These aliquots were
filtered and UV-vis  absorption spectra of the samples were recorded atλmax=490nm.


## 2.1. Characterization

The surface area of the samples was determined by nitrogen
adsorption using a quantasorb analyzer. Infrared spectra of samples were
recorded on a Bruker spectrometer IFS-88. The concentration of Fluorescein in
all aqueous solutions was analyzed using a UV-Vis. DU-500 spectrophotometer.
Morphology and the size ofTiO2 nanocrystallites were studied and
analyzed by TEM with a CM200, FEG-Philips. X-ray diffractometer type Philips PW
1840 (λ=1.54060Å′  at 40 kv and 20 mA) was used to identify the crystalline phase and also crystal size, using the
Scherrer method (4).Scherrer equation is as follows:(1)D=KλBcosθwhere K was taken as 0.9 and B is the full width of the
diffraction line at half of the maximum intensity.


## 2.2. Photocatalytic Activity

The photocatalytic activity of the samples was estimated by
decomposition of 90 ppm Fluorescein solution by using a 50–100 ppm of
photocatalyst. The photoreactor consisted of cylindrical glass reactor that a
light source from a 6 W Hg Philips lamp located axially at the center of the
vessel. A special glass atomizer as air diffuser was fixed at the bottom of the
reactor to uniformly disperse air into the above reaction mixture. A magnetic
stirrer was used to produce homogeneous reaction mixture. Degradation was
monitored by taking aliquots at different time intervals. These aliquots were
filtered and UV-vis  absorption spectra of the samples were recorded atλmax=490nm.


## 3. Results and Discussion

The FT-IR spectra of the titania samples hydrothermally
synthesized at various temperatures showed strong bands at 3403 and1631cm−1 which are related to the stretching mode of the OH group and the bending mode
of molecular water, respectively (Figures 1(a) and 1(b)).FT-IR spectra of titania powders prepared at (a)200°C and (b) 300°C.
(a)(b)The absorption of these bonds decreased with an increase in
the synthesize temperature, which indicates that surface hydroxyl groups enable
the condensation under hydrothermal condition. The broadband over the range of400–700cm−1,
related to bending and stretching mode of Ti–O–Ti and characteristic of well-ordered
TiO6 octahedrons [17]. There is no peak at 2900cm−1 regarding
to C–H stretching band, which means all organic compounds removed from the
samples after calcination at 400°C. The XRD patterns of prepared
samples and P25 obtained by calcination at different temperature are shown in
Figures 2–4. It can be seen
from Figure 2(a) that all diffraction peaks can be assigned to pure anatase
phase. Comparison of two XRD patterns in Figures 2(a) and 2(b)  shows
that  the  diffraction peaks sharpened with increasing
hydrothermal sharpened with increasing hydrothermal temperature, indicating
increasing crystallite size. The average crystal sizes of the samples
calculated using the Scherrer equation, where 15 nm, 17 nm, and 19 nm for
sample prepared at 200°C, 300°C, and P25, respectively,
whereas the  result showed that the crystal size  increased with increasing the
synthesize temperature. For thermal  stability investigation, the sample
synthesized at 300°C was calcined at 800, 850, and 900°C.
Their XRD patterns show that anatase phase was stable until 850°C
and only after this temperature, transformation to rutile had occurred (Figures
3 and 4).Figure 2
XRD patterns
of the samples prepared at (a)200°C  and (b) 300°C.Figure 3
XRD patterns
ofTiO2 prepared at 300°C and calcined at different temperature.Figure 4
XRD patterns
of (a) P-25 Degussa and (b) P-25 Degussa calcined at800°C.P25 sample received from Degussa had transformed to rutile
at800°C. It is reported that the transformation from anatase to
other crystalline phase, that is, rutile occurred at 600–700°C [18, 19].TEM image of the
titania sample hydrothermally synthesized at300°C is presented in
Figure 5. The average particle size of nanocrystallites was 15 nm, which agrees
with that obtained by the XRD analysis. BET surface area of samples prepared at
200 and 300°C were 114 and 78m2/gr, respectively. The
BET equivalent particle diameter was calculated as dBET=6/(ρTiO2×SBET),
where ρTiO2 is the weighted density of the particles (ρTiO2=3.84g/cm3).
It was 14 and 20 nm for samples prepared at 200 and 300°C,
respectively.Figure 5
TEM image of nanoparticles prepared at300°C.


### 3.1. Photocatalytic Studies

The photocatalytic activity of hydrothermally synthesized
nano-TiO2 at 300°C as well as commercial sample was
examined by degradation of Fluorescein dye. Concentrations change during
degradation at different irradiation  time with different amounts of photocatalyst is shown in Figure 6. The photoactivity was increased by
increasing the TiO2 amount in slurry. The conversion of Fluorescein
after 8 hours was 97% for sample prepared at 300°C (calcined at 400°C)
and 70.91% for same sample calcined at 800°C. The degradation
process, involving hydroxyl radical formation and subsequent degradation of the
dye by the hydroxyl radical, obeys Pseudo-first-order kinetics.Figure 6
Change in the
concentration of Fluorescein at different irradiation time with different
amount of nano-TiO2 (prepared at 300°C).The rate of degradation was assumed to obey
pseudo-first-order kinetics and hence the rate constant for degradation,K, was
obtained from  the first-order plot
according to the following equation:
(2)Ln(C0C)=Kt,
where C0 is the initial concentration, C is the
concentration at instant time (t), and K is the first-order rate constant.First-order degradation rate constants, obtained by
plotting the natural logarithm of the concentration against irradiation time, were0.2263h−1 for synthesized TiO2 and 0.0647h−1 for P25 (Figure 7). The prepared sample exhibits better activity than the
commercial type (P25).Figure 7
First-order
photodegradation kinetics of Fluorescein for P25 andTiO2 sample
(prepared at
300°C).


## 3.1. Photocatalytic Studies

The photocatalytic activity of hydrothermally synthesized
nano-TiO2 at 300°C as well as commercial sample was
examined by degradation of Fluorescein dye. Concentrations change during
degradation at different irradiation  time with different amounts of photocatalyst is shown in Figure 6. The photoactivity was increased by
increasing the TiO2 amount in slurry. The conversion of Fluorescein
after 8 hours was 97% for sample prepared at 300°C (calcined at 400°C)
and 70.91% for same sample calcined at 800°C. The degradation
process, involving hydroxyl radical formation and subsequent degradation of the
dye by the hydroxyl radical, obeys Pseudo-first-order kinetics.Figure 6
Change in the
concentration of Fluorescein at different irradiation time with different
amount of nano-TiO2 (prepared at 300°C).The rate of degradation was assumed to obey
pseudo-first-order kinetics and hence the rate constant for degradation,K, was
obtained from  the first-order plot
according to the following equation:
(2)Ln(C0C)=Kt,
where C0 is the initial concentration, C is the
concentration at instant time (t), and K is the first-order rate constant.First-order degradation rate constants, obtained by
plotting the natural logarithm of the concentration against irradiation time, were0.2263h−1 for synthesized TiO2 and 0.0647h−1 for P25 (Figure 7). The prepared sample exhibits better activity than the
commercial type (P25).Figure 7
First-order
photodegradation kinetics of Fluorescein for P25 andTiO2 sample
(prepared at
300°C).


## 4. Conclusion

A method for making a high-temperature stable
anatase phase without using any dopants was successfully achieved by
hydrothermal method. In prepared sample, the anatase phase was stable up to850°C.
The crystallinity of anatase phase increased with the hydrothermal temperature.
This method is, therefore, suitable to synthesize samples for the
high-temperature photocatalytic application in building materials. A
high-thermal stable anatase phase and simplicity of preparation technique are
the main advantages of this work.The photodegradation reaction of
Fluorescein followed pseudo-first-order reaction kinetics. The kinetic  data
showed that the prepared sample is somehow more active than the commercial one.



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*Source: 245981-2008-10-07.xml*