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

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Journal of Hazardous Materials 159 (2008) 636–639

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Journal of Hazardous Materials

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / j h a z m a t

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 TiO}

2thin

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.

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]. The powder-type photocatalysts have some disadvantages in indus-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 TiO2 thin 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 (∼3.2 eV)[9,10]. Various transition metal ions have been used to dope into TiO2for shifting the absorption edge to visible light

region[11–14]. TiO2 thin ﬁlms doped with transition metal ions

have received much attention in the past few years[15–20]. Doping Fe ions has been proven to be an effective approach in enhanc-ing the photocatalytic activity of TiO2 in 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 photocatalytic activities[18]. They also adopted the sol–gel process

∗ Corresponding author. Tel.: +886 2 23651428.

E-mail address:[email protected](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 TiO2 ﬁlm. Therefore,

the photocatalytic activities of TiO2 will be reduced due to the

inter-diffusion and interaction between the ﬁlm and substrate[22]. To reduce the crystallization temperature of Fe3+_{-doped TiO}

2

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 TiO}

2 thin 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 films obtained via two different processes were also analyzed under 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, and 2-methoxylethanol was used as the solvent. This solution had a

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C.-H. Lu et al. / Journal of Hazardous Materials 159 (2008) 636–639 637

nominal 3% iron-ion concentration with respect to the titania molar 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 10 min 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 30 min to evaporate the residual organics, and then annealed these films in a furnace under atmo-spheric pressure (14.7 psi) from 300 to 600◦C for 2 h. The second process annealed the films via a high-pressure (14.7–228.6 psi) crystallization process at 100–200◦C for 2 h. The as-pyrolyzed films 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 30 W FL lamp was employed as a light source and a 400 nm glass filter was utilized to cut off light with wavelength shorter than 400 nm. The quantitative measurement of the methylene blue con-centration was performed by measuring the absorbance peak of methylene blue at 664 nm 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:

D = 1 − C C0

(1) where D represents the decolorization degree, C0is the beginning

methylene blue concentration, and C is the ﬁnal methylene blue concentration.

3. Results and discussion Fe3+_{-doped TiO}

2thin ﬁlms were prepared via the

atmospheric-annealing and high-pressure crystallization processes. The XRD patterns of the prepared films are illustrated inFig. 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 TiO}

2 thin 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-pressure crystallization (HPC) process, when the as-pyrolyzed film was annealed at 100◦C under 14.7 psi for 2 h, the formed film still remained amorphous. However, after annealing at 125◦C under 34.1 psi, the amorphous film was converted into crystallized films with an anatase phase, as seen inFig. 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 228.6 psi, respectively, the crystallinity of thin films was gradually increased. The HPC pro-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 TiO}

2ﬁlms prepared via (a) the conventional atmospheric-pressure annealing process and (b) the high-pressure crystallization process.

The surface morphology of Fe3+_{-doped TiO}

2thin 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 60 nm as seen in Fig. 2(a). The high-temperature heating promotes the coarsening of the particles and form nonuniform surface. As seen inFig. 2(b), the film annealed at 150◦C under 70 psi in the HPC process revealed a uniform surface morphology and a reduced particle size of around 30 nm. When the film was annealed at 200◦C under 228.6 psi, the average particle size of the thin film was slightly enlarged to 50 nm observed inFig. 2(c). In comparison with the thin films prepared under atmospheric pressure, the thin films annealed under high-pressure process exhibited a more uniform and denser morphology with reduced grain size.

The photocatalytic activities of the prepared films under visi-ble light (>400 nm) were evaluated based on the decomposition behavior of methylene blue solution.Fig. 3illustrates the photo-catalytic activities of the films prepared via the HPC process as a function of annealing temperatures. After 19 h-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, and 200◦C, respectively. It was found that the decorloriza-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 TiO2 signiﬁcantly 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-trum of the samples annealed at 200◦C is illustrated inFig. 4, which

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638 C.-H. Lu et al. / Journal of Hazardous Materials 159 (2008) 636–639

Fig. 2. Scanning electron micrographs of Fe3+_{-doped TiO}

2thin ﬁlms annealed at (a) 600◦_{C under atmospheric pressure, (b) 150}◦_{C under 64 psi and (c) 200}◦_{C under} 228.6 psi.

Fig. 3. UV–vis spectra of methylene blue degraded by Fe3+_{-ion doped TiO} 2thin ﬁlms 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 are comparable to the data reported in literature for the visible-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 TiO} 2annealed at 200◦C.

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C.-H. Lu et al. / Journal of Hazardous Materials 159 (2008) 636–639 639

Fig. 5. Grazing incident X-ray diffraction patterns of Fe3+_{-doped TiO}

2thin ﬁlm annealed at 200◦_{C under 228.6 psi.}

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 inFig. 5. The X-ray penetration depth was calculated according to the following equation[32]:

D(ω) =sin(ω) (2)

whereω is incident angle of the X-ray beam and is the linear absorption coefﬁcient. The penetration depths were calculated at about 0.141, 0.282, and 0.564_{␮m for incident angles 0.5}◦, 1◦, and 2◦, respectively. The penetration depths of the X-ray into the thin ﬁlms became deeper with increasing X-ray beam incident angles. From Fig. 5, similar intensities in the Fe3+_{-doped TiO}

2thin ﬁlm diffraction

peaks can be observed. This implies that the crystallinity within the 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 doped titania thin ﬁlms were prepared via

atmospheric-pressure annealing and high-pressure crystal-lization (HPC) processes in this study. The HPC process successfully reduced the crystallization temperature of TiO2as low as 125◦C. In

comparison to thin films prepared under atmospheric pressure, 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 under visible light. However, the films prepared via the HPC process 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.

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