Low-temperature preparation and characterization of iron-ion doped titania thin films.

Abstract

Iron-ion doped titania thin films with an anatase phase were successfully synthesized in this study using the high-pressure crystallization (HPC) process. The crystallization temperature of Fe(3+)-doped TiO(2) thin films was markedly reduced to be as low as 125 degrees C. The films prepared via the HPC process have a more uniform microstructure and smaller grain sizes than the films prepared via the atmospheric-pressure annealing process. The films prepared via both processes were found to have photocatalytic properties under visible light. The films prepared via the HPC process exhibited enhanced photocatalytic activities in comparison with the films annealed via the conventional process. Increasing the annealing temperature in the HPC process resulted in an improvement in the photocatalytic properties because of an increase in the crystallinity of the prepared films. The HPC process was demonstrated to be a potential method for synthesizing visible-light driven titania thin films with enhanced photocatalytic activities at low temperatures.

Full text

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Journal of Hazardous Materials 159 (2008) 636–639
Contents lists available at ScienceDirect
Journal of Hazardous Materials
journal homepage: www.elsevier.com/locate/jhazmat
Short communication
Low-temperature preparation and characterization
of iron-ion doped titania thin films
Chung-Hsin Lu∗, Chi-Yuan Hu, Chung-Hsien Wu
Department of Chemical Engineering, National Taiwan University, Taipei, Taiwan, ROC
a r t i c l e i n f o
Article history:
Received 28 September 2007
Received in revised form 12 February 2008
Accepted 16 February 2008
Available online 23 February 2008
Keywords:
Titania
Iron
Photocatalytic
Thin films
High pressure
a b s t r a c t
Iron-ion doped titania thin films with an anatase phase were successfully synthesized in this study using
the high-pressure crystallization (HPC) process. The crystallization temperature of Fe3+-doped TiO2thin
films was markedly reduced to be as low as 125◦C. The films prepared via the HPC process have a more
uniform microstructure and smaller grain sizes than the films prepared via the atmospheric-pressure
annealing process. The films prepared via both processes were found to have photocatalytic properties
under visible light. The films prepared via the HPC process exhibited enhanced photocatalytic activities
in comparison with the films annealed via the conventional process. Increasing the annealing temper-
ature in the HPC process resulted in an improvement in the photocatalytic properties because of an
increase in the crystallinity of the prepared films. The HPC process was demonstrated to be a potential
method for synthesizing visible-light driven titania thin films with enhanced photocatalytic activities at
low temperatures.
© 2008 Elsevier B.V. All rights reserved.
1. Introduction
Titanium dioxide has been intensively investigated as an impor-
tant material for degrading various chemical compounds under UV
light irradiation because of its strong photocatalytic activity [1–5].
Thepowder-typephotocatalystshavesomedisadvantagesinindus-
trial applications such as stirring problems during the reaction
processes and the requirement for separation after the operation
processes. Therefore, film-type photocatalysts were widely devel-
oped in order to extend the applicability of TiO2. Different methods
such as the sol–gel process, chemical vapor deposition (CVD) and
sputtering were employed to synthesize TiO2thin films [6–8].
Among these processes, the sol–gel method is the most commonly
used method because of its simple process and low cost materials.
TiO2only absorbs ultraviolet (UV) light because of its large band
gap (∼3.2eV) [9,10]. Various transition metal ions have been used
to dope into TiO2for shifting the absorption edge to visible light
region [11–14]. TiO2thin films doped with transition metal ions
havereceivedmuchattentioninthepastfewyears[15–20].Doping
Fe ions has been proven to be an effective approach in enhanc-
ing the photocatalytic activity of TiO2in visible light region [9].
Yu et al synthesized Fe-ion doped TiO2thin films via liquid phase
deposition and studied the effects of calcination temperatures on
photocatalyticactivities[18].Theyalsoadoptedthesol–gelprocess
∗Corresponding author. Tel.: +886 2 23651428.
E-mail address: chlu@ntu.edu.tw (C.-H. Lu).
to prepare Fe-ion doped TiO2films and investigated the influence
of Fe-ion doping on the hydrophilicity of TiO2films [19]. Sonawane
et al have studied the effects of Fe-ion concentration on the photo-
catalytic properties of TiO2thin films [21]. In the previous studies,
high-temperature annealing is usually required for obtaining crys-
tallized TiO2films. Annealing at high temperatures will cause the
species in the substrates to diffuse into the TiO2film. Therefore,
the photocatalytic activities of TiO2will be reduced due to the
inter-diffusionandinteractionbetweenthefilmandsubstrate[22].
To reduce the crystallization temperature of Fe3+-doped TiO2
thin films, a high-pressure crystallization (HPC) process was
employed to anneal the thin films at low temperatures. The HPC
process can be carried out at a relatively low temperature to pre-
vent diffusion between the film and the substrate [23–25]. In this
study, Fe3+-doped TiO2thin films were synthesized via the HPC
processes and the conventional annealing process. The phases in
the thin films prepared via these two processes were investigated.
The annealing temperature effects on the surface morphology of
the formed films were studied. The photocatalytic activities of the
filmsobtainedviatwodifferentprocesseswerealsoanalyzedunder
visible light.
2. Experimental
Iron-ion doped titanium dioxide films were prepared via a
metalorganic deposition method. Titanium tetraisopropoxide and
ferric nitrate were employed as the metalorganic solution source,
and2-methoxylethanolwasusedasthesolvent.Thissolutionhada
0304-3894/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jhazmat.2008.02.050

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C.-H. Lu et al. / Journal of Hazardous Materials 159 (2008) 636–639
637
nominal3%iron-ionconcentrationwithrespecttothetitaniamolar
percentage. The metalorganic solution was coated onto cleaned
glass substrates via a spin-coating process. The coated precursor
films were dried on a hot plate at 150◦C for 10min to remove the
organic solvent.
The as-pyrolyzed films were annealed via two different crys-
tallization processes: the atmospheric-pressure annealing process
and the high-pressure crystallization (HPC) process. The first pro-
cess heated the films at 300◦C for 30min to evaporate the residual
organics, and then annealed these films in a furnace under atmo-
spheric pressure (14.7psi) from 300 to 600◦C for 2h. The second
process annealed the films via a high-pressure (14.7–228.6psi)
crystallizationprocessat100–200◦Cfor2h.Theas-pyrolyzedfilms
were annealed in a sealed stainless-steel bomb to achieve crystal-
lization. The bottom of the bomb was filled with distilled water to
produce a high vapor-pressure environment at elevated tempera-
tures. The films were positioned above the water surface to avoid
direct contact with water during processing. The pressure in the
bomb was modulated by the saturated vapor pressure.
The formed phases were examined via X-ray diffraction (XRD)
using Cu K? radiation. The crystallization of the prepared films at
the different depths was investigated via grazing incident X-ray
diffraction (GIXD). The diffuse reflectance spectra were measured
using a UV–vis spectrophotometer. The morphology and the par-
ticle size of the prepared samples were investigated via scanning
electron microscope (SEM). Methylene blue solution was used to
determine the photocatalytic activities of the prepared films. A
30W FL lamp was employed as a light source and a 400nm glass
filter was utilized to cut off light with wavelength shorter than
400nm. The quantitative measurement of the methylene blue con-
centration was performed by measuring the absorbance peak of
methylene blue at 664nm using a UV–vis spectrophotometer. The
decolorization degree was calculated by calibration between the
measured absorbance and methylene blue solution concentration.
The decolorization value can be obtained using the following equa-
tion:
C
C0
where D represents the decolorization degree, C0is the beginning
methylene blue concentration, and C is the final methylene blue
concentration.
D = 1 −
(1)
3. Results and discussion
Fe3+-doped TiO2thin films were prepared via the atmospheric-
annealing and high-pressure crystallization processes. The XRD
patterns of the prepared films are illustrated in Fig. 1. As seen in
Fig. 1(a), when the film was annealed at 300◦C under atmospheric
pressure, only amorphous film was obtained. When the annealing
temperature increased to 325◦C, Fe3+-doped TiO2thin film with
an anatase phase was formed. After annealing at 400 and 600◦C,
the crystallinity of the thin films was further enhanced. During the
high-pressurecrystallization(HPC)process,whentheas-pyrolyzed
film was annealed at 100◦C under 14.7psi for 2h, the formed film
stillremainedamorphous.However,afterannealingat125◦Cunder
34.1psi, the amorphous film was converted into crystallized films
with an anatase phase, as seen in Fig. 1(b). In comparison with the
conventional annealing process under atmospheric pressure, the
HPC process significantly reduced the crystallization temperature
from 325 to 125◦C. When the as-pyrolyzed films were annealed
at 150, 175, and 200◦C under 70, 131.2, and 228.6psi, respectively,
thecrystallinityofthinfilmswasgraduallyincreased.TheHPCpro-
cess is considered to lead to a reduction in the critical free energy
required for the formation of stable nuclei, thereby facilitating the
nucleation process at low temperatures [26,27].
Fig. 1. X-ray diffraction patterns for Fe3+-doped TiO2 films prepared via (a) the
conventional atmospheric-pressure annealing process and (b) the high-pressure
crystallization process.
The surface morphology of Fe3+-doped TiO2thin films annealed
via the atmospheric pressure and high-pressure processes were
investigated. After annealing at 600◦C under atmospheric pres-
sure, the size of particles in the film was around 60nm as seen in
Fig.2(a).Thehigh-temperatureheatingpromotesthecoarseningof
the particles and form nonuniform surface. As seen in Fig. 2(b), the
film annealed at 150◦C under 70psi in the HPC process revealed a
uniform surface morphology and a reduced particle size of around
30nm. When the film was annealed at 200◦C under 228.6psi, the
average particle size of the thin film was slightly enlarged to 50nm
observed in Fig. 2(c). In comparison with the thin films prepared
under atmospheric pressure, the thin films annealed under high-
pressureprocessexhibitedamoreuniformanddensermorphology
with reduced grain size.
The photocatalytic activities of the prepared films under visi-
ble light (>400nm) were evaluated based on the decomposition
behavior of methylene blue solution. Fig. 3 illustrates the photo-
catalytic activities of the films prepared via the HPC process as a
function of annealing temperatures. After 19h-illumination under
visible light, the decolorization degree of the methylene blue solu-
tions were 74.4, 88.4, 91.9, and 94.4% for the films annealed at 125,
150,175,and200◦C,respectively.Itwasfoundthatthedecorloriza-
tion degree increased markedly with an increase in the annealing
temperatures under the HPC process, due to the enhancement of
crystallinity of TiO2anatase phase. Yu et al also reported that the
photocatalytic activity of TiO2significantly depends on its crys-
tallinity, microstructure, surface area and crystal structure [28].
The UV–vis spectrum of methylene blue degraded by the thin film
annealed via the conventional annealing process is also shown in
Fig. 3. The decolorization degree of methylene blue solution was
67.9% for the film annealed at 600◦C. The UV–vis absorption spec-
trumofthesamplesannealedat200◦CisillustratedinFig.4,which

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C.-H. Lu et al. / Journal of Hazardous Materials 159 (2008) 636–639
Fig. 2. Scanning electron micrographs of Fe3+-doped TiO2thin films annealed at
(a) 600◦C under atmospheric pressure, (b) 150◦C under 64psi and (c) 200◦C under
228.6psi.
Fig.3. UV–visspectraofmethylenebluedegradedbyFe3+-iondopedTiO2thinfilms
annealed at (i) 125◦C, (ii) 150◦C, (iii) 175◦C, (iv) 200◦C via the HPC process and (v)
600◦C under atmospheric pressure.
confirms visible light absorption in the Fe-ion doped samples. The
above results revealed that iron-ion doped titania thin films pre-
pared via the HPC process showed greater photocatalytic activities
than those obtained from the conventional annealing process. It
was concluded that the smaller sized particles and denser struc-
tures formed in the films were beneficial to the photocatalytic
activities of these films. The quantum yields of the prepared films
were calculated according to the method proposed in the literature
[29]. The obtained values for the films heated at 125, 150, 175 and
200◦C were 0.5, 0.8, 0.9 and 1.0%, respectively. The obtained values
arecomparabletothedatareportedinliteratureforthevisible-light
sensitized TiO2films [30,31]. The quantum yield of the films was
Fig. 4. UV–vis absorption spectra of non-doped and Fe3+-ion doped TiO2annealed
at 200◦C.

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639
Fig. 5. Grazing incident X-ray diffraction patterns of Fe3+-doped TiO2 thin film
annealed at 200◦C under 228.6psi.
found to increase with raising annealing temperatures as a result
of enhanced crystallization.
Grazing incident X-ray diffraction (GIXD) was employed for
investigating the crystallization state within the prepared films.
The GIXD patterns for the 200◦C-annealed film prepared via the
HPC process are illustrated in Fig. 5. The X-ray penetration depth
was calculated according to the following equation [32]:
D(ω) =sin(ω)
?
(2)
where ω is incident angle of the X-ray beam and ? is the linear
absorption coefficient. The penetration depths were calculated at
about0.141,0.282,and0.564?mforincidentangles0.5◦,1◦,and2◦,
respectively. The penetration depths of the X-ray into the thin films
became deeper with increasing X-ray beam incident angles. From
Fig.5,similarintensitiesintheFe3+-dopedTiO2thinfilmdiffraction
peakscanbeobserved.Thisimpliesthatthecrystallinitywithinthe
films is almost the same. It reveals that the crystallization process
took place within whole films.
The above results indicate that iron-ion doped titania thin
films annealed via the HPC process revealed better photocatalytic
properties under visible light illumination than the atmospheric-
annealed films. The prepared films exhibited more uniform
structure and had a smaller particle size than films annealed via
the conventional annealing process. The HPC process is considered
a potential method for preparing visible-light driven TiO2films at
low temperatures.
4. Conclusions
Iron-ion
atmospheric-pressure
lization (HPC) processes in this study. The HPC process successfully
reduced the crystallization temperature of TiO2as low as 125◦C. In
comparisontothinfilmspreparedunderatmosphericpressure,the
thin films prepared via the HPC process exhibited more uniform
microstructure with a smaller grain size. The films prepared via
both processes were found to have photocatalytic characteristics
undervisiblelight.However,thefilmspreparedviatheHPCprocess
exhibited better photocatalytic activities than the films prepared
via the atmospheric-pressure annealing process. When the heating
temperature was increased in the HPC process, the photocatalytic
properties of titania thin films were further improved due to the
increase in crystallinity. The HPC process was demonstrated an
approach with good potential for preparing visible-light driven
titania films at relatively low temperatures.
dopedtitania
annealing
thinfilms
and
were
high-pressure
prepared via
crystal-
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