КОЛЛОИДНЫЙ ЖУРНАЛ, 2007, том 69, № 6, с. 742-746
УДК 539.216.2:546.826-31:541.145
Ce(4+) DOPED TiO2 THIN FILMS: CHARACTERIZATION AND PHOTOCATALYSIS © 2007 r. Yukou Du*1, Mingchun Du*, Yan Qiao*, Jingtao Dai*, Jingkun Xu**, Ping Yang*
*Department of Chemistry and Chemical Engineering, Suzhou University Suzhou 215123, P. R. China **Jiangxi Key Laboratory of Organic Chemistry, Jiangxi Science and Technology Normal University
Nanchang 330013, P. R. China Поступила в редакцию 09.01.2007 г.
Undoped and Ce(4+) doped TiO2 thin films were prepared by sol-gel method. The samples were characterized using scanning electron microscopy, the photocatalytic reactivity was evaluated by degradation of methylene blue in the aqueous solution. 0.5 wt. % Ce(4+) doped TiO2 thin films calcinated at 400°C show the highest photocatalytic activity.
INTRODUCTION
EXPERIMENTAL
The TiO2 photocatalysis has extensive applications in many fields [1-3], such as pollutant decomposing [4], photoelectricity generation [5], and material cleansing [6]. But the band gap of TiO2 is too wide (~3.2 eV) to provide the effective photocatalytic processes during the irradiation with the sunlight since only approximately 4% of the total radiation of the solar spectrum is in the ultraviolet region. The TiO2 modification by noble metal deposition [7-8], rare earth metal ion doping [9-10], and semiconductor compounding [11] can provide the photoinduced electron and holes efficient separation. As a result the UV-vis absorption enhances the photocatalytic activity of TiO2.
Rare earth metals have incompletely occupied 4f and empty 5d orbitals that provides the high metal valency. Their oxides have selective absorption, high electrical conductivity, and thermal stability; as catalysts or assistant catalysts, they may achieve the same effect as the noble and transition metals co-deposited onto TiO2 [12].
Francisco et al. [13] found that the CuO/TiO2 system modified by CeO2 displayed a higher activity for methanol dehydrogenation then the copper catalyst supported only on TiO2 or CeO2. Zhu et al. [14] studied Pd/CeO2-TiO2 catalyst for CO oxidation at low temperature by a temperature-programmed reduction (TPR) with H2 and CO as reducing agents, indicated that the special Pd-Ce-Ti interaction in Pd/CeO2-TiO2 is favorable for the reduction by PdO and interfacial CeO2 species. In our work, Ce(4+) doped TiO2 thin films were characterized by scanning electron microscopy (SEM), and their photocatalytic activity was evaluated by degradation of methylene blue in aqueous solution.
1 Corresponding author: Yukou Du, E-mail: duyk@suda.edu.cn.
Materials
[(C4H9O)4Ti] (C.P.), [(NH4)2Ce(NO3)6] (A.R.), anhydrous isopropanol (A.R.), PVP (K-30, MW = 40000, C.P.), and methylene blue (C.P.) were obtained from Si-nopharm Chemical Reagent Co., Ltd. Doubly distilled water was used in all the experiments.
Microscopy
SEM experiments were performed with a S-4700 Scaning Electron Microscope at 15 kV.
Preparation of TiO2 thin films
Colloidal titanium dioxide was prepared via controlled hydrolysis of tetrabutyl titanate. In a typical experiment, 1.63 g PVP was added to a 70 mL of double-distilled water and the pH of the solution was adjusted to 1.3 with nitric acid. Then, a desired volume (e.g., 0.25 mL for 0.5 wt. % Ce(4+) doping) of 0.015 g/mL [(NH4)2Ce(NO3)6] solution was added in the mixture. 1 mL of [(QHO^Ti] dissolved in 19 mL of anhydrous isopropanol was added dropwise to the obtained mixture under vigorous stiring at room temperature for about 6 h, resulting in a transparent colloidal solution. Likewise, a set of transparent TiO2 colloids with different amounts of Ce(4+) were prepared by varying the volume of [(NH4)2Ce(NO3)6] solution added.
A cleaned substrate (soda lime glass) was dipped into a transparent colloid prepared for 10 min. Following that, the substrate was withdrawn at a constant rate of 1.5 mm s-1 and dried in air. If necessary, these procedures were repeated. Finally, it was calcinated at desired temperature for 1 h.
(a)
15.0kV 11.9 mm X100 k
Fig. 1. SEM images of different content Ce(4+) doped TiO2 thin films calcined at 400°C: blank substrate (a), undoped (b), 0.5 wt. % doped (c), 1.0 wt. % doped (d), and 2.0 wt. % doped (e).
Photocatalytic activity evaluation
The photodegradation reaction was carried out in a 100 mL Pyrex glass vessel. A TiO2 thin film glass substrate was supported by the glass bracket, 19 mL reaction solution immersed the substrate completely with constant magnetic stiring. The UV light beam was directed perpendicular to the surface of the solution. The distance between the top surface of the solution and the light source was 8 cm. Reactions were always performed with the same initial concentration of methylene blue, 5 mg/L and a constant system temperature of 20°C. The values of absorption at 662 nm of reaction solution were measured at different reaction time: 0, 10, 20, 30, 40, and 50 min.
Degradation rate was calculated using the relation
(A - A)
—--X 100%, where A0 is the initial absorption and A
A0
is the absorption at a given moment.
RESULTS AND DISCUSSION
SEM
Figure 1 shows SEM images of different content Ce(4+) doped TiO2 thin films calcinated at 400°C. Figure 1 shows that as the Ce(4+) doping content increases, particles on the surface of glass substrates undergo an aggregation process. The particle diameter of the undoped sample is approximately 37.9 nm, while it is of 27.8 nm for 0.5 wt. % Ce(4+) doped TiO2 thin film, which has more uniform particle distribution. Porous structure is observed in the samples of 1.0 wt. % doped and 2.0 wt. % doped. CeO2 scattered in TiO2 thin films prohibits particles growth making them smaller and more uniform than in pure TiO2 film. Further increase in CeO2 content results in bigger particles.
Figure 2 shows SEM images of 0.5 wt. % Ce(4+) doped TiO2 thin films calcinated at different temperatures. At 300°C the PVP is not burnt completely PVP and parti-
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Fig. 2. SEM images of 0.5 wt. % Ce(4+) doped TiO2 thin films calcined at different temperatures: 300 (a), 400 (b), and 500°C (c).
Fig. 3. SEM images of 0.5 wt. % Ce(4+) doped TiO2 thin films calcined at 400°C with different coating layers: one (a), two (b), and three (c).
cles are not separate. At 400°C, particles are separated and uniformly distributed. At 500°C, the distance between particles becomes smaller and the interaction between particles becomes stronger that results in their agglomeration.
Figure 3 shows SEM images of 0.5 wt. % Ce(4+) doped TiO2 thin films calcined at 400°C with different coating layers. As the number of coating layers increase, the degree of particle agglomeration increases.
Degradation rate (%)
Degradation rate (%)
Fig. 4. Photocatalysis kinetics of different content Ce(4+) doped TiO2 thin films calcined at 400°C: undoped (1), 0.5 wt. % Ce(4+) (2), 1.0 wt. % Ce(4+) (3), and 2.0 wt. % Ce(4+) (3).
Fig. 5. Photocatalysis kinetics of 0.5 wt. % Ce(4+) doped TiO2 thin films calcined at different temperatures: 300 (1), 400 (2), 500°C (3).
Photocatalytic activity evaluation
Figure 4 shows the kinetics of photocatalysis for different content Ce(4+) doped TiO2 thin films calcinated at 400°C. The calcined at 400°C, 0.5 wt. % Ce(4+) doped TiO2 films exhibits the highest photocatalytic activity, that is in accordance with SEM image shown in Fig. 1. In 0.5 wt. % Ce(4+) doped TiO2 thin film calcinated at 400°C, particles are distributed uniformly, the diameter of particles is about 27.8 nm, they have stronger quantum size effect. As rare earth metal, Ce has the selective absorption resulting in high photoatalytic reaction rate. The radius of Ce4+ (0.102 nm) is much bigger than the radius of Ti4+ (0.068 nm), thus Ce4+ ions create more new crystal defects, such as crystal dislocation. During the calcination at certain temperature, these crystal defects destroy the order of nanocrystalline TiO2, forming the centers of capturing the photoinduced electrons.
Otherwise, the absorption of Ce4+ is not selective, and Ce4+ has no catalytic activity. As Ce(4+) doping content increases, CeO2 does not incorporate into the TiO2 crystal and exist at the surface, blocking the surface active centers. Excessive CeO2 covers the TiO2 surface, hinders photoinduced electrons and holes to transfer to the surface, and reduces the photocatalytic activity of TiO2 thin films.
Figure 5 shows the kinetics of photocatalysis of 0.5 wt. % Ce(4+) doped TiO2 thin films calcinated at different temperatures. The sample calcined at 400°C exhibits the best photocatalytic activity, that is in accordance with SEM images in Fig. 2.
At low calcination temperature, anatase TiO2 does not grow enough, and amorphous TiO2 has low photocataly-sis. But at high calcination temperature, particles agglomerate and the diameter of particles increase, specific sur-
face area diminishes, and rutile TiO2 grows more, that decreases photocatalytic activity of the films.
Figure 6 shows the kinetics of photocatalysis of 0.5 wt. % Ce(4+) doped TiO2 thin films calcinated at 400°C with different coating layers. As the number of coating layers increases, photocatalytic activity of Ce(4+) doped TiO2 films decreases because mutual overlapping of particles, the diameter of particles increases, the traps are eliminated, and the probability of photoreaction decreases. Moreover, multiple coating results in dense layers, reduces surface area and photocatalytic activity.
Irradiation time (min)
Fig. 6. Photocatalysis kinetics of 0.5 wt % Ce(4+) doped TiO2 thin films calcined at 400°C with different coating layers: one (1), two (2), and three layers (3).
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DU h flp.
CONCLUSIONS
Ce(4+) doping hinders the TiO2 nanoparticles growth, create defects in crystals which increase the efficiency of separation of photoinduced electrons and holes. The rate of degradation of methylene blue in the aqueous solution is highest for 0.5 wt. % Ce(4+) doped TiO2 one laye
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