Photocatalytic behavior of TiO2 thin films prepared by sol–gel process

Materials Chemistry and Physics 95 (2006) 79–83

Photocatalytic behavior of TiO2 thin films prepared by sol–gel process Ki Hyun Yoon a,b,c,∗ , Jung Sok Noh a , Chul Han Kwon a,b , Mamoun Muhammed c a

Department of Ceramic Engineering, Yonsei University, 134 Shinchon-dong, Seodaemoon-ku, Seoul 120-749, Republic of Korea b Research Center, TIOZ Co. Ltd., 775-4 Wonsi-dong, Ansan, Kyunggi-Do, Republic of Korea c Division of Materials Chemistry, Royal Institute of Technology, SE-10044 Stockholm, Sweden Received 24 December 2004; received in revised form 23 May 2005; accepted 1 June 2005

Abstract Titanium dioxide solution was prepared via sol–gel process with and without acetyl acetone (AcAc). The formation of the anatase phase in the thin films using AcAc was initiated at 300 ◦ C while the anatase phase in the thin films without using AcAc began to appear at 100 ◦ C. Surface roughness of the TiO2 films using and without using AcAc ranged from 0.414 to 3.08 nm and from 3.72 to 5.35 nm, respectively. Photocatalytic decomposition rate examined by methylene blue solution showed 80% at 400 ◦ C and 88% at 600 ◦ C for the TiO2 thin films using AcAc. On the other hand, the photocatalytic decomposition rate of the TiO2 thin films without using AcAc showed 80% at 100 ◦ C and 97% at 400 ◦ C, respectively. At the temperature above 400 ◦ C, the photocatalytic activity decreased due to appearance of the rutile phase. Comparison between the TiO2 thin films showing the maximum photocatalytic activity revealed that the TiO2 thin film without using AcAc showed 10% higher photocatalytic decomposition rate from the initial 10 to 120 min as compared with the TiO2 thin film using AcAc, which was closely related to the formation of the anatase phase and the surface roughness. © 2005 Elsevier B.V. All rights reserved. Keywords: Thin films; Sol–gel growth; Atomic force microscopy; Photoactivity

1. Introduction Titanium dioxide [1] is a wide bandgap material exhibiting photocatalytic decomposition and super-hydrophilicity. Recently, photocatalysis based on titanium dioxide has attracted much attention in terms of environmental applications. Titanium dioxide [2] possesses three different crystal structures: rutile (tetragonal), anatase (tetragonal) and brookite (orthorhombic). Among these, the anatase phase is known to exhibit better photocatalytic behavior, whereas rutile is the most stable phase. Even though the energy bandgap (3.23 eV) of the anatase phase is wider than that of the rutile (3.02 eV), recombination of electrons and holes occurs much faster on the surface of the rutile phase [3]. Since the photocatalytic activity is mostly confined to the surface of the photocatalytic material, its surface area must be increased to maximize the photocatalytic activity. One way to do this is ∗

Corresponding author. Tel.: +46 8 790 8157; fax: +46 8 20 7681. E-mail address: [email protected] (K.H. Yoon).

0254-0584/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2005.06.001

synthesis of nano-sized TiO2 particles to increase photocatalytic reaction sites on the surface. Also, the amount of the anatase phase must be maximized because the rutile phase shows less photocatalytic activity [4]. The sol–gel technique has emerged as a new processing route for nano-sized TiO2 thin films fabrication because of its simplicity. For many technological applications, low processing temperature is highly desirable because it enables the use of certain substrate materials and/or prevents harmful film–substrate interaction. Unfortunately, the sol–gel processing starting from metal alkoxide or some other metal–organic precursors still requires processing temperatures in excess of ∼400 ◦ C for the crystallization and removal of organics. The strong reactivity of the alkoxide towards H2 O often results in an uncontrolled precipitation and limiting the use of the sol–gel technology. These problems have been overcome with the aid of chelating agents, such as acetyl acetone (AcAc) [5] and acetic acid [6]. These chemical additives react with alkoxides and modify the ligand structure, enabling the hydrolysis and polycondensation reactions to be controlled.

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In this work, TiO2 solutions were prepared using alkoxide sol–gel process with and without AcAc and their physical and chemical properties were investigated. The thin films using these two different solutions were spin-coated on the glass substrates and their comparison was made in terms of the photocatalytic decomposition rate using methylene blue solution.

for photoactivity of the TiO2 thin films. UV irradiation was provided by two Germical lamps (253.7 nm, 20 W, Sankyo Denki, Japan). To examine the surface roughness of the TiO2 thin films, atomic force microscopy (AFM; Auto Probe Cp., Park Scientific Instrument, U.S.A.) was introduced.

3. Results and discussion 2. Experimental procedure Titanium isopropoxide (Ti[OCH(CH3 )2 ]4 ) and isopropyl alcohol were used as a precursor and a solvent, respectively, to prepare TiO2 solution via sol–gel process. After 10 min stirring of the mixture, water and nitric acid were added and stirred at 80 ◦ C for 8 h. In case of a solution with acetyl acetone, AcAc was added with isopropyl alcohol as a reaction controller and the mixture was stirred for 30 min. The final reaction took place with the aid of nitric acid at room temperature. In presence of AcAc, Ti(i-PrO)3 AcAc precursor was formed. Acetic acid displaced two isopropoxide ligands to form the alkoxide chelate Ti(i-PrO)2 (CH3 COO)2 . Schematic diagrams of preparation processes for both solutions are presented in Fig. 1. Both solutions with and without AcAc were coated on the glass substrates via spin coating at room temperature, being subject to post-deposition heat treatment at various temperatures. X-ray diffraction patterns were obtained with an X-ray diffractometer (Rigaku D/MAX-IIIC) using a Cu K␣ radiation. Fourier transform infrared (FT-IR) spectroscopy (FTIR300E, Jasco, Japan) was used to examine the chemical properties of the synthesized solutions and UV–vis spectrophotometer (Agilent 8453E, Agilent Tech., U.S.A.) was also used to determine the decomposition of methylene blue

Fig. 1. Schematic diagrams of preparation process for: (a) TiO2 solution with AcAc and (b) TiO2 solution without AcAc.

Fig. 2 shows FT-IR spectra of TiO2 solutions prepared by sol–gel process with and without AcAc. The IR band at 3400 cm−1 shows the presence of OH stretching vibrations, while the peak at 1640 cm−1 can be ascribed to the bending vibrations of adsorbed H2 O molecules. The IR bands at 500 cm−1 can be assigned to Ti O Ti bonds, which Lopez et al. [7] and Chhor et al. [8] also observed Ti O Ti band at 495–436 cm−1 . On the basis of the Ti O Ti frequency, the formation of the Ti O Ti bond by condensation reaction could be followed. The sharp peak at 1380 cm−1 is due to the presence of nitrates, which were added as HNO3 during the acidification in the sol–gel synthesis. With increasing heating temperature, the peak at 500 cm−1 was shifted to a lower wave number and its relative intensity slightly increased. Fig. 3 shows the XRD patterns of TiO2 powders obtained from the TiO2 solutions with and without AcAc. It can be seen that the peaks at 2θ of 25.28◦ , 38.08◦ , 47.92◦ , 53.32◦ and 62.66◦ are assigned to (1 0 1), (0 0 4), (2 0 0), (1 0 6) and (2 1 5) lattice planes of TiO2 , which are attributed to the anatase phase. All specimens exhibited characteristic diffraction peaks for the anatase phase until the temperature of the post-synthesis heat treatment exceeded 300 ◦ C. Even at room temperature, a trace of anatase phase was observed, which means the formation of the anatase phase already occurred during preparation of the solution. As the temperature reached 400 ◦ C, the rutile phase began to appear through the phase transition. At the same time, the intensity of the anatase phase began to decrease as the temperature exceeded

Fig. 2. FT-IR spectra of TiO2 solution with and without AcAc.

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Fig. 3. XRD patterns of TiO2 obtained from the solution without AcAc heat treated at various temperatures.

400 ◦ C and eventually almost disappeared at the temperature of 600 ◦ C. In case of the specimens using AcAc, the anatase phase existed at 300 ◦ C and persisted to post-heat treatment temperature of 400 ◦ C as shown in Fig. 4. The rutile phase began to appear at 500 ◦ C. In contrast to the specimens using AcAc, the peak intensity of the anatase phase was not reduced even with the appearance of the rutile phase. From the above observation, it can be concluded that the formation of the anatase phase in the specimens using AcAc continued during the heat treatment. When the Ti[OCH(CH3 )2 ]4 solution was heated, the reaction can be considered as two-step processes, hydrolysis reaction and condensation process. According to Gopal et al. [9], the formation of anatase and rutile is determined by the nucleation and the growth of TiO2 clusters. If the condensation starts before completion of hydrolysis, either amorphous or metastable anatase TiO2 is formed. In neutral and basic

Fig. 4. XRD patterns of TiO2 obtained from the solution with AcAc heat treated at various temperatures.

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conditions, condensation starts before complete hydrolysis and the formation of ordered structure is hindered, so the dried gel is amorphous. Acid environment promotes hydrolysis rate [10] and at the same time, the condensation rate decreases. Rutile [9] has two opposite edges of each octahedra which are shared forming a linear chain along the (0 0 1) direction. Chains are then linked to each other by sharing corner oxygen atoms. Anatase has no corner sharing, but has four edges shared per octahedron. The octahedra agglomerate through corner and edge sharing during the condensation reactions [11]. When the acid concentration is higher, the repulsion is larger and then the aggregation is smaller [12]. Therefore, higher acid concentrations promoted the formation of rutile, while lower acid concentrations promoted anatase formation. The anatase phase in Fig. 3 appeared at low temperatures compared with Fig. 4. The persistence of anatase phase throughout the heat treatment over the range of interest could be explained as follows. The addition of AcAc as a chelating agent to titanium isopropoxide leads to much less reactive titanium(acetylacetonate)triisopropoxide as an inorganic precursor and the peptization is retarded and then solution becomes stable. Therefore, during heat treatment, pyrolysis may induce the formation of the metastable anatase phase and converts the anatase phase to the rutile phase at 500 ◦ C. The effect of heat treatment of TiO2 thin films was examined in terms of the decomposition of methylene blue solution [13]. The photocatalytic decomposition rate was calculated using the following formula. Photocatalytic decomposition rate (%) =

C0 − C × 100, C0

where C0 is the initial concentration of the methylene blue solution and C is the final concentration after illumination of UV light. As shown in Fig. 5(a), the photocatalytic activity of the TiO2 thin films prepared without using AcAc increased to 80 and 97% at the heat treatment temperature of 100 and 400 ◦ C, respectively, and then decreased afterwards. This observation coincides with XRD data shown in Fig. 3. The decrease in the photocatalytic decomposition rate after the temperature reached 500 ◦ C is attributed to the appearance of the rutile phase and the decrease in anatase phase as shown in Fig. 3. In case of the TiO2 thin film with AcAc, the decomposition of methylene blue increased monotonically to the temperature of 600 ◦ C as shown in Fig. 5(b). The photocatalytic decomposition rate was 80 and 88% at the heat treatment of 400 and 600 ◦ C, respectively. This agrees well with the amount of the anatase phase formed upon heat treatment to 600 ◦ C as indicated in Fig. 4. Fig. 6 compares the photocatalytic decomposition of methylene blue for (a) the specimen using AcAc heated at 600 ◦ C for 1 h and (b) the specimen without using AcAc heated at 400 ◦ C for 1 h, respectively. Upon illumination of UV light for 120 min, the photocatalytic decomposition rate of the specimen without using AcAc exceeded that of

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Fig. 6. Comparison of photocatalytic decomposition rates between: (a) TiO2 thin film without using AcAc and (b) TiO2 thin film using AcAc at various illumination times.

Fig. 5. Photocatalytic decomposition rate of methylene blue (2 ppm) on: (a) TiO2 thin films using without AcAc and (b) TiO2 thin films using AcAc.

the specimen using AcAc by 10%. This difference can be explained in terms of the surface area. As shown in Fig. 7, the TiO2 thin film without using AcAc shows rougher surface than the TiO2 thin film using AcAc. Surface roughness ranged from 0.414 to 3.08 nm for the TiO2 thin film using AcAc and from 3.72 to 5.35 nm for the TiO2 thin film without using AcAc. The photocatalytic reaction is confined mostly to the surface under illumination. There-

fore, the more surface area, the more photocatalytic reaction occurs. According to Yu and Wang [14], when water to alkoxide molar ratio is less than the required stoichiometric ratio, the hydrolysis between alkoxides and H2 O is incomplete and the condensation occurs between the monomers of (OH)x -Ti(OR)4−x to form chain-like structures. Heat treatment will collapse the structures of the resultant cross-linked gels, resulting in the formation of irregular-shaped particles with a wider particle size distribution and low specific surface area. Increasing the water to alkoxide molar ratio to a higher level causes a strong nucleophilic reaction between H2 O and alkoxide molecules and more alkoxyl groups in the alkoxide are substituted by hydroxyl groups of H2 O. Therefore, the monomers, (OH)x -Ti(OR)4−x , due to complete hydrolysis, interact with each other to establish a threedimensional network structure via condensation and form solids, which give rise to a high specific surface areas even after the heat treatment, due to residual voids in the network structures.

Fig. 7. Surface roughness of: (a) TiO2 thin film using AcAc heated at 600 ◦ C for 1 h and (b) TiO2 thin film without using AcAc heated at 400 ◦ C for 1 h.

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In this study, the water to alkoxide molar ratio was 50 and 100 for the solutions with and without AcAc, respectively. Higher specific surface area for the TiO2 thin film without using AcAc resulted from either the formation of threedimensional network or combination of three-dimensional network and the formation of metal hydrate.

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Acknowledgment One of the authors (K.H. Yoon) would like to thank the Wenner-Gren Foundation for financial support.

References 4. Conclusion TiO2 thin films prepared using AcAc showed the anatase phase at 300 ◦ C while the anatase phase from the TiO2 thin films prepared without using AcAc began to form at 100 ◦ C. Surface roughness ranged from 0.414 to 3.08 nm and from 3.72 to 5.35 nm for the TiO2 thin films with and without using AcAc, respectively. Photocatalytic decomposition rate of the TiO2 thin films prepared using AcAc increased with heat treatment temperature. The TiO2 thin films prepared without using AcAc showed 80% of photocatalytic decomposition rate at 100 ◦ C and 97% at 400 ◦ C and the photocatalytic activity decreased at the temperatures above 400 ◦ C. The maximum photocatalytic decomposition rate of the TiO2 thin films prepared without using AcAc showed 10% higher for the initial 10–120 min as compared with the TiO2 thin films prepared using AcAc. The photocatalytic decomposition activity exhibited a close dependence on the formation of the anatase phase and the surface roughness.

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