Nov 9 2010
R.Vijayalakshmi and K. V. Rajendran
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1833-122X) Volume 6 November 2010
Topics CoveredAbstract KeywordsIntroductionExperimental ProcedureResults And
DiscussionEffect Mechanism Of Doping K+ On
TiO2 Phase TransformationConclusionAcknowledgmentReferenceContact Details
Pure and K doped TiO2 nanoparticles were prepared by the sol-gel
process. The effect of K+ doping on TiO2 anatase to rutile
phase transformation was investigated. It is found that the K doping shifted the
phase transformation and has a stabilization effect on the anatase grain growth.
With a suitable amount (ca. 1&3mol %) the K dopant reduces anatase grain
size and increases the specific surface area of TiO2 powder.
Nanoparticles, Sol-Gel, Doping, Grain Size,
Surface Area, Powder
Semiconductor photocatalysis has been investigated extensively for light-
stimulated degradation of pollutants, particularly for complete destruction of
toxic and non-biodegradable compounds to carbon dioxide and inorganic
constituents. Several semiconductors exhibit band-gap energies suitable for
photocatalytic degradation of contaminants. Among the photo catalysts applied,
TiO2 is one of the most widely employed photocatalytic semiconducting
materials because of the peculiarities of chemical inertness, non-photo
corrosion, low cost and non-toxicity. TiO2 has three kinds of
different crystal structure; anatase, brookite and rutile. Among them, anatase
and brookite are meta-stable phases and irreversibly transform to the thermally
stable rutile phase upon heat treatment in the temperature range from 450 to
900°C , in which better crystallinity and larger crystallite size can be
obtained at the same time. Many studies have clarified that anatase exhibited
greater photocatalytic property than rutile because the highly hydroxylate
surface responsible for photocatalytic reaction is readily formed in the anatase
structure. Therefore, the stabilization of anatase phase becomes a subject of
interest to be studied [2, 3].
Doping the impurity ion is known as one of the effective ways to manipulate
the internal properties of TiO2 such as crystalline structure and
crystallite size [4, 5]. Research of the effect of impurity doping on the phase
transition has already begun during 1960s. The similarities between dopant and
host ions; electron valence, ionic radius and chemical property have been
reported to significantly effect on the ion interchanging during crystal
nucleation process, in which the dopant ion is allowed to enter in the host
lattice as either substitutional or interstitial site [6-10]. It is therefore
interesting and necessary to examine the effect of metal additives on the
TiO2 phase transformation and grain growth.
In our work, using the sol-gel method, we prepared pure and K+
doped TiO2 powders and characterized by means of X-ray Diffraction
(XRD), Transmission Electron Microscope (TEM), Scanning Electron Microscope
(SEM), Energy Dispersive Spectroscope (EDS) and Ultra Violet – Visible (UV-Vis)
absorption and also investigated the effect of K+ doped on the
TiO2 anatase to rutile phase transformation and anatase grain growth
The sol-gel synthesized TiO2 was obtained from Titanium (IV)
isopropoxide (TTIP) was dissolved in absolute ethanol and distilled water was
added to the solution in terms of a molar ratio of Ti: H2O=1:4.
Nitric acid was used to adjust the pH and for restrain the hydrolysis process of
the solution. The solution was vigorously stirred for 30 min in order to form
Sols. After aging for 24 hrs, the Sols were transformed into gels. In order to
obtain nanoparticles, the gels were dried under 120°C for 2 hr to evaporate
water and organic material to the maximum extent. Sintering processes at the
various temperatures from 450 – 750°C were subsequently carried out to obtain
desired TiO2 crystalline.
The alkaline ion doped TiO2 nanoparticles were synthesized with
the same method mentioned above, except for the addition of the corresponding
alkaline dopant (KNO3) in ethanol. The doping concentration was
varied for the mol fraction; 1& 3mol % and it was designated as
K1 and K3 respectively. The crystalline phases of
TiO2 were determined using X-ray diffractometer Schimadzu model: XRD
6000 with CuKa radiation in the range 20-80° (λ=0.154nm). UV-Vis absorption
spectra were recorded on a Varian Cary 5E spectrophotometer at room temperature
in the range between 200 to 1000nm.The microstructure and grain size were
analysed through a Scanning Electron Microscope Hitachi S-4500 and Transmission
Electron Microscope (TEM) using a model JEOL-2010 microscope.
Results And Discussion
The sol samples synthesized by sol-gel method are amorphous, and gradually
transform to crystal state during the sintering process. In order to analyze the
relative ratio in the anatase-rutile mixed phases, XRD is a very effective
procedure because each of the components in the mixture gives their
characteristic pattern, independently . The powder XRD clarified that the
anatase phase was stabilized and the transition to rutile phase was suppressed
with increasing the doping concentrations. The specific surface area &
particle size of pure, K1 and K3- TiO2 samples
calcined at 450, 550, 650 and 750°C are summarized in Table 1. It was observed
from the table that the surface areas are in the decreasing order of
TiO2>K1>K3 when the samples are calcined
at 450, 550, 650 and 750°C respectively. Dopant K+ may disperse on or
compound with TiO2, which prevents TiO2 agglomeration and
reduces the diminishing rates of surface area with increasing calcination
temperature, rendering K+ doped TiO2 more porous than
plain TiO2 .
Table1. The specific surface area and particle size of
pure, K1 and K3- TiO2 nanoparticles
Surface Area (m2/g)|
|450°C 550°C 650°C 750°
||450°C 550°C 650°C 750°
||14 20 28 -
||94.4 66.1 50.65 -|
|1% K doped TiO2
||10 15 19 25
||132.2 88.1 69.55 52.8|
|3% K doped TiO2
||7 13 17 22
||188.8 101.66 77.7 60.1|
Fig.1 (a, b and c) showed the XRD patterns of pure and K doped
TiO2 nanoparticles calcined at different temperature. The material
shows a high degree of crystallinity and existence of fully anatase phase at
450°C. We choose 450°C as calcination temperature, as this temperature was found
to have highest activity among samples calcined at different temperature. It
could be seen that the rutile began to appear for pure TiO2 samples
when the calcining temperature was 550°C, and the anatase phase disappeared when
calcining temperature was 650°C, while a little rutile phase appeared for
K-doped TiO2 samples when the calcining temperature was 550°C, and
even most was still anatase phase at the calcining temperature of 650°C. These
demonstrated that K+ dopant could greatly inhibit the phase
transformation from anatase to rutile, and enhance the beginning temperature of
phase transformation obviously . The crystal sizes of anatase and rutile
phases increases with increasing calcination temperature, and those in
K+ doped TiO2 are smaller than those of pure
TiO2 since particle agglomeration is retarded by doping
Figure 1. XRD patterns of the as-prepared samples (a)
pure TiO2 (b) 1% K-doped TiO2 (c) 3% K-doped
The surface morphologies of the pure & K+ doped
TiO2 sample were evaluated by SEM analysis which clearly indicated a
significant change in crystalline growth that effectively leads the reduction of
crystalline size, as shown in Fig. (2 a, b and c). In addition, more uniform and
homogeneous distribution of nanoparticles was obtained by doping K+
ion into the TiO2 nanoparticles. Although it is clear that alkaline
ions could successfully suppress the nanoparticle size as well as stabilize the
anatase phase of TiO2.
Figure 2. SEM micrographs of pure and doped
TiO2 calcined at 450°C (a) SEM of pure TiO2 (b) SEM of 1%
K in TiO2 (c) SEM of 3% K in TiO2
Effect Mechanism Of Doping K+ On TiO2
In general, the ionic radius and calcining temperature are two of the most
important conditions, which can strongly influence the ability of the dopant to
enter into TiO2 crystal lattice to form stable solid solution. If the
ionic radius of the dopant is much bigger or smaller than that of Ti4+, the
dopant substituting for TiO2 crystal lattice ions must result into
Crystal Lattice Distortion (CLD). Thus, certain amount of energies can be
accumulated so that the substitution process can be suppressed . It could be
seen from Fig.1 that the XRD peaks of pure and K-doped TiO2
nanoparticles had the same positions mostly demonstrating that K+ did
not enter into TiO2 crystal lattice to substitute for Ti4+. This was
because the radius of K+ (1.51Å) was much bigger than that of Ti4+
(0.64Å). In addition, the phase about K+ element could not be found
in Fig.1, possibly demonstrating that K+ was dispersed uniformly onto
TiO2 nanoparticle. Thus, the chemical bonds of Ti-O-K three elements
around the anatase crystallites could easily occur during the process of thermal
treatment, which possibly inhibited producing and growing of the crystal nucleus
For semiconductor nanoparticles, the quantum confinement effect is expected,
and the absorption edge will be shifted to a higher energy when the particle
size decreases. The K+ doped TiO2 exhibited an absorption
edge at 345 and 364 nm which is blue shift considerably compared with the pure
TiO2 (376 nm) (shown in Fig.3). The absorption edge of doped
TiO2 is stronger than that of the pure TiO2. Figure
reveals that absorption intensities and the threshold wavelength decrease in
order of TiO2 >K1 > K3. Due to a larger
particle, size for samples treated at 550 and 650°C the absorption edge appears
red shift to some extent . Considering the blue shift of the absorption
positions from the bulk TiO2, the absorption onsets of the present
samples can be assigned to the direct transition of electron in the
Figure 3. DRS patterns of pure and doped TiO2
calcined at 450°C
Fig.4a showed the TEM photographs of pure and 3mol% K doped TiO2
nanoparticles calcined at 450°C. It could be found that the pure and K doped
TiO2 nanoparticles both appeared similar sphere, with the average
particle size of about 7 and 14 nm respectively, demonstrating that K dopant
could inhibit the increase of TiO2 particle size. In fig. 5 the
Selected Area Electron Diffraction (SAED) pattern of K+ doped
TiO2 at 450°C is shown. The first four rings are assigned to the
(101), (004), (200), (005) reflections of the anatase phase. The Selected Area
Electron Diffraction (SAED) studies are in good agreement with the XRD
measurements. Energy dispersive Spectrum (EDS) displayed in Fig. 6 and table 2
furnish the composition of various elements in the prepared sample.
Figure 4. TEM photomicrographs of pure
and doped TiO2 calcined at 450°C (i) TEM of pure TiO2 (ii)
TEM of 3% K in TiO2
Figure 5. SAED pattern of the K-doped
Figure 6. EDS spectrum of the K-doped
Table 2. The composition of various elements presents in
K3-doped TiO2 nanoparticles.
Investigation in the present study has revealed that nanoparticles of
TiO2 doped with K+ prepared using the sol-gel method shows
a synergistic effect, which shifted the transformation anatase-rutile to higher
temperature. According to the XRD analysis, the K+ did not enter the
TiO2 crystal lattice to substitute for Ti4+. Indeed the radius of
K+ (1.51Å) is much larger than that of Ti4+ (0.64Å). They were
probably dispersed uniformly onto TiO2 nanoparticles. The particle
size of pure TiO2 (14nm) is larger than that of K1 &
K3 doped TiO2 nanoparticles (10nm & 7nm), revealing
that the introduction of K can effectively prevent TiO2 from further
growing up in the process of calcination. The K doped TiO2
nanoparticles showed a concomitant blue shift in the absorption spectrum with a
decrease in the particle size. The absorption edge of the K doped
TiO2 nanoparticles calculated to be 345 and 364nm are slightly
smaller than the value of 376nm for the pure TiO2.
The authors are grateful to the University Grant Commission, for extending
financial assistance to carry out this work.
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R.Vijayalakshmi and K. V. Rajendran
Department of Physics,
Chennai, TamilNadu, India
E-mail : [email protected]
This paper was also published in print form in "Advances in
Technology of Materials and Materials Processing", 12 (2010)