International Journal of Photoenergy

Volume 2008, Article ID 267345, 7 pages

http://dx.doi.org/10.1155/2008/267345

## Thermal Behavior of Nanostructured Powder

^{1}Nuclear Research Institute Řež, plc., Řež 25068, Czech Republic^{2}Institute of Inorganic Chemistry, Academy of Sciences of the Czech Republic (AS CR), Řež 25068, Czech Republic^{3}Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan^{4}Research Center for Advanced Science and Technology, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8904, Japan

Received 3 September 2007; Accepted 7 December 2007

Academic Editor: M. Sabry A. Abdel-Mottaleb

Copyright © 2008 Vladimír Balek et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

Diffusion structural analysis (DSA), based on the measurement of the release of radon previously incorporated into the samples, was used to characterize thermal behavior of N-doped titania powder prepared by heat treatment of anatase in gaseous ammonia at and the reference powder prepared from the ST-01 anatase titania powder (Ishihara Ltd., Japan). The results of DSA, surface area and porosity measurements by nitrogen adsorption, SEM micrographs, XPS, and XRD are presented and discussed. The results of DSA are in agreement with the results of other methods and indicated the annealing of the subsurface structure irregularities of the samples. Transport properties of the samples were determined from the mobility of radon atoms released on sample heating in air. The decrease of radon permeability in the porous titania powders in the temperature range 850– due to annealing of the subsurface structure irregularities, that served as radon diffusion paths in the samples, was evaluated from the DSA results.

#### 1. Introduction

Titanium dioxide attracted a great
attention since Fujishima and Honda [1] discovered
in 1972 the photocatalytic splitting of water on TiO_{2} electrode.
Recently, the application of TiO_{2} focused on environmental
remediation, especially water detoxification and air purification [2–4]. TiO_{2} is an efficient photocatalyst, but UV light is necessary for its activation.
Solar energy contains only about 4% UV light and much of the rest is visible
light.

In order to utilize the visible (solar) light
efficiently for the photocatalytic reactions, titanium dioxide has to be
modified. There have been several approaches how to modify
TiO_{2}. Several authors [5–9] substituted
Ti^{4+} in TiO_{2} by Cr^{3+} or V^{3+} (V^{4+}) by metal ion implantation.
It was demonstrated that the absorption band of Cr^{3+}-doped TiO_{2} shifted and that NO_{x} decomposed giving rise to N_{2},
O_{2}, and N_{2}O as the result of the photocatalytic reaction
under visible light irradiation with wavelength larger than 450 nm. It was
reported [10–13] that it is possible to prepare the “oxygen-vacancy-based
visible-light-sensitive titania” by the treatment of anatase under reductive
hydrogen plasma.

N-doped
titania photocatalyst sensitive to visible light irradiation was described in 2001
by Asahi et al. [14]. Since then, various types of TiO_{2} doped with anions but for nitrogen such as sulfur and carbon have been widely
studied [15–23].

The purpose of this study is to characterize the thermal behavior of N-doped porous titania powder under in situ conditions of heating to 1050^{°}C in
air and to reveal differences in the annealing of porosity and subsurface
structure defects. The diffusion structural analysis (DSA) [24, 25] was used to this aim. The DSA is
based on the measurement of radon release rate from samples previously
labelled, where radon atoms serve as a microstructure probe. In our previous
studies, the DSA was already used to monitor the microstructure development of
titania powders as well as titania gel layers [25] during
heating of their precursors in various gas media.

#### 2. Experimental

##### 2.1. Preparation of Samples

Two samples were investigated in this study: a reference sample of nondoped titania powder
(called sample A) prepared by heating of anatase powder, type ST-01, Ishihara Sangyo
Kaisha, Ltd., Japan, (surface area 311 m^{2}/g) at 550^{°}C
in air for 3 hours, and N-doped titania (called sample B) prepared by heating of the anatase
powder (sample A) under NH_{3} gas flow at 575^{°}C for 3hours.

The chemical composition of the N-doped
titania estimated by Irie et al. [23] was TiO_{2-x}N_{x}, where *x* = 0.011. The value of *x* was determined as the ratio of the XPS peak areas corresponding to 396 eV and 531 eV.

##### 2.2. Methods

Scanning electron microscope (SEM) equipment by Philips, type XL 30CP, was used to characterize surface morphology.

Diffusion structural analysis (DSA)
measurements were carried out by using the modified equipment NETZSCH DTA model
409 (NETZSCH, Selb, Germany) [26]. The
radon-labelled titania powder (sample amount of 0.05 g) was situated in a
corundum crucible and heated in air in the temperature range 20–1050^{°}C at the
rate of 6^{°}C/min. The constant air flow (flow rate 50 mL/min) took the radon released from
the sample into the measuring chamber of radon radioactivity.

The DSA results are presented as
temperature dependencies of radon release rate *E* (in relative units); , where is *α*-radioactivity of radon released in
unit time from the labelled sample and is total *γ*-radioactivity of the labelled
sample. The value is proportional to the rate of radon
formation in the sample. Semiconductor and NaI (Tl) detectors were used for the
*α*- and *γ*-radioactivity measurements,
respectively.

##### 2.3. Labelling of Samples for DSA Measurements

Samples for DSA measurements were
labelled by ^{220}Rn using recoil energy 85 keV/atom during spontaneous *α*-decay of radionuclides ^{228}Th
and ^{224}Ra adsorbed as nitrates in trace amount on the sample surface from acetone
solution. The specific activity of the sample was 10^{5} Bq/gram. The
depth of ^{220}Rn ions implantation by the recoil energy of 85 keV into
the anatase was 60 nm from the surface of sample grains as calculated by means
of Monte Carlo method using TRIM code [27]. It has been
supposed that ^{220}Rn atoms formed by the spontaneous *α*-decay of ^{228}Th and ^{224}Ra
were trapped in the subsurface layers of the sample and that structural
irregularities served as diffusion paths for the radon atoms released by diffusion. The mechanisms of
radon diffusion in open pores, intergranular space, or interface boundaries,
respectively, were supposed to control the release of the radon from the
sample.

In general, the increase in the radon release rate, *E*, may characterize
an increase of the surface area of interfaces or porosity, whereas a decrease
in *E* may reflect processes like closing up structure irregularities that serve
as paths for radon migration, closing pores, and/or a decrease in the surface
area of the interfaces.

The advantage of the DSA application consists in the possibility to characterize the microstructure changes under in situ conditions of sample heating in a selected gas environment and to bring about information about transport properties of the samples.

#### 3. Results and Discussion

##### 3.1. Microstructure Characterization

From the XRD pattern in Figure 1 (curve
2), it followed that the N-doped sample investigated in this study was
homogeneous anatase, as no peaks of TiN were observed. It was found that values
of BET surface area and porosity of sample
B increased in comparison with sample A. The
etching of nondoped titania powder at the temperature of 575^{°}C with ammonia gas
caused an increase of its surface area from 63 to 151 m^{2}/g, and
thetotal porosity volume increased from 0.36 to 0.84 cm^{3}/g
(see Table 1).

SEM micrographs in Figure 2 characterized the surface morphology of samples A and B, respectively.

DSA results in Figure 3 were used to characterize the thermal behavior of the samples, their permeability for radon atoms, and microstructure changes on heating in air.

We assumed that the increase of the
radon release rate, *E*, observed in Figure 3 in the temperature range 50–800^{°}C was due to
the radon diffusion along structure irregularities in subsurface of the grains.
The random “single-jump” diffusion mechanism of radon was supposed to control
the release of radon in this temperature range.

From Figure 3, it followed that the
slope of the respective DSA curves slightly differed in this temperature range.
The increased slope of the radon release rate, *E*, was observed with N-doped
titania sample during air heating. It is in a good agreement with the higher
values of total porosity and surface area found for sample B in comparison with
reference sample A (see Table 1). From the observed decrease of the radon
release rate, *E*(*T*), it followed that annealing of
microstructure irregularities took place with both samples A and B on heating
in the temperature range 850–1000^{°}C
accompanied by a decrease of surface area and porosity. The surface area and
porosity values of sample B annealed to 950^{°}C decreased to 2.7 m^{2}/g
and 0.016 cm^{3}/g, respectively.

Growth of crystallites of the samples
on heating from 800 to 950^{°}C was confirmed by the SEM micrographs (see Figure 2). Moreover, from the
XRD pattern in Figure 1 (curve 3), it followed that in sample B
annealed to 950^{°}C the crystal transformation into rutile took place. In this respect, the DSA
results (see Figure 3) monitored the anatase-rutile transition and the
grain growth of the rutile particles during sample heating. The XRD and SEM
micrographs confirmed the interpretation of the DSA results. From the XRD
patterns presented in [28–30], it followed
that the anatase-rutile transition took place on heating to the temperatures
800–900^{°}C.

Figure 4 depicts XPS spectra of
the N-doped titania heated to 800^{°}C and 950^{°}C,
respectively. By means of X-ray photoelectron spectroscopy, it was demonstrated
that the presence of the nitrogen atoms was no more indicted in the sample
heated to 950^{°}C.

The microstructure development of the samples on heating was quantitatively described by the mathematical model [31, 32] supposing that pores and subsurface structure defects served as paths for radon diffusion. The application of the model enabled us to evaluate transport properties of the N-doped titania in comparison with the properties of the reference nondoped titania powder.

##### 3.2. Transport Properties of Porous Titania

It has been supposed that ^{220}Rn
atoms were incorporated to the depth of 60 nm from the surface and trapped in
the subsurface of the sample; the structural irregularities served as paths for
the radon atoms released by diffusion. We assumed that the increase of the
radon release rate, *E*, observed in the temperature range 50–800^{°}C (see Figure 3)
was due to the radon diffusion along structure irregularities in subsurface
layers of the grains. The random “single-jump” diffusion mechanism of radon was
supposed to control the release of radon in this temperature range.

The radon release rate, *E*(*T*), can be written as [31]

The term is radon release rate due to recoil and the
term is radon release rate
due to the diffusion, depending on the number of radon diffusion paths. The
term characterizes radon permeability along structure
irregularities serving as diffusion paths, and
Ψ(*T*) is the function characterizing the decrease
of the number of the radon diffusion paths. The advantage of the DSA
application consists in the possibility to characterize microstructure changes
under insitu conditions
of sample heating in a selected gas environment and to bring about information
about transport properties of the samples.

In the evaluation of the DSA results,
it was supposed that radon atoms can migrate along several independent paths,
such as micropores, intergranular space, as well as interface boundaries.
Consequently, the mechanism of radon diffusion along two independent paths of
the porous titania was considered. The following expression was used to
describe the temperature dependence of the radon release rate , due
to diffusion:
where *S/M* is surface area of open pores, intergranular
space, and interfaces serving as radon diffusion paths, is density of the sample, (*D*/λ)^{1/2} is average
radon diffusion length, *D* is radon diffusion coefficient, and is radon decay constant
(λ = 0.00127 s^{−1});
exp(), where is the
factor depending on the number of diffusion paths and their availability for
radon atoms migration, is activation energy of radon migration
involving both the activation energy of the escape of radon atoms from the
traps in the solid sample and that of the migration along diffusion paths in
the solid.

In the mathematical model used in this study for the evaluation of DSA results, it was supposed that the decrease in the temperature dependence of the radon release rate, , reflected a decrease of the number of radon diffusion paths.

The following expression for Ψ(*T*) functions was proposed [27] to describe the decrease of the
number of radon diffusion paths due to changes of the
microstructure:
where erf is the
sign for the integral Gaussian
function, is the temperature of maximal rate of the annealing of the defects that serve as radon
diffusion paths, and is the temperature interval of the respective
solid state process.

##### 3.3. Comparison of Experimental DSA Results with the Model Curves

It was demonstrated by DSA results (see
Figures 5(a) and 5(b)) that the etching nondoped titania powder at the
temperature of 575^{°}C with ammonia gas caused changes in radon permeability in
subsurface of samples. Radon diffusion parameters were calculated from the DSA
results (see Table 2).

From Figures 5(a) and 5(b), it followed that experimental DSA results and model curves of the temperature dependences of the radon release rate are in a good agreement. The DSA results characterized differences in the annealing of subsurface structure irregularities, serving as radon diffusion paths in titania powders.

The experimental DSA results were evaluated by means
of the mathematical model, supposing that nanopores and subsurface structure
irregularities served as paths for radon diffusion. The temperature dependences of radon
release rate, due to diffusion, were calculated using (2). The
temperature dependences
of Ψ(*T*) functions, that characterized the annealing
of surface and subsurface structure irregularities, were calculated using (3).

Figure 6 depicts temperature dependences of ln *D* versus
1/*T* calculated from the DSA results measured on heating of sample A and sample B in the temperature range 50–450^{°}C. The
determined values of the radon permeability characteristics (activation energy
of the radon diffusion) and the pre-exponential factor are presented in Table 2. Figure 7 depicts the model curves of the temperature
dependences of the radon release rate *E*(*T*) calculated from the DSA experimental
results in the range 500–1050^{°}C. The
functions Ψ(*T*) were used to characterize the annealing of
subsurface structure irregularities in the titania powders (samples A and B)
during heating in air. Figure 8 depicts the calculated temperature
dependences of the functions Ψ(*T*) for samples A and B. As it follows from Figure 8,
the maximum rate of the radon release rate decrease due to annealing of subsurface
structure irregularities was determined at 886^{°}C for sample A and 997^{°}C
for sample B. Radon atoms have been used in this study as a probe of the nanostructure
changes.

#### 4. Conclusions

Thermal behavior of N-doped titania
powders prepared by heat treatment of anatase in gaseous ammonia at 575^{°}C
was characterized by diffusion structural analysis (DSA). The radon
permeability in the N-doped titania as well as in the reference nondoped
titania powder was evaluated from the DSA results. The decrease of the radon
release rate observed in the temperature range 850–1000^{°}C made
it possible to characterize annealing of microstructure irregularities.

#### Acknowledgments

This work was supported in part by the Ministry of Education of the Czech Republic (Projects LA 292 and 1M4531433201) and Ministry for Science, Education, Sports and Culture of Japan.

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