A transparent, high purity titanium dioxide thin film composed of densely packed nanometer sized grains has been successfully deposited on a glass substrate at 30°C from an aqueous solution of TiO2-HF with the addition of boric acid as a scavenger by liquid phase deposition technique. From X-ray diffraction measurement, the deposited film was found to be amorphous and turns crystalline at 500°C. The deposited film showed excellent adherence to the substrate and was characterized by homogeneous flat surface. TiO2 thin films can be used as a photocatalyst to clean up organohalides, a class of compound in pesticides that pollute the ground water. Photocatalytic degradation experiments show that indanthrene golden orange dye undergoes degradation efficiently in presence of TiO2 thin films by exposing its aqueous solution to ultraviolet light. The suitable surface structure and porosity increases the photocatalytic activity. It was also observed that hemin doped TiO2 thin films break up organohalides at a surprisingly high rate under visible light

Begum, N.S.

Farveez Ahmed, H.M.

Bulletin of Materials Science

ePrints@Bangalore University

Bull.  Mater.  Sci.,  Vol.  31,  No.  1,  February  2008,  pp.  43–48.  ©  Indian  Academy  of  Sciences. 43Synthesis of nanocrystalline TiO2 thin films by liquid phase deposition technique and its application for photocatalytic degradation studies NOOR  SHAHINA  BEGUM*  and  H  M  FARVEEZ  AHMED Department of Post-Graduate Studies and Research in Chemistry, Central College Campus, Bangalore University, Bangalore 560 001, India MS received 27 June 2007; revised 18 November 2007 Abstract. A transparent, high purity titanium dioxide thin film composed of densely packed nanometer sized grains has been successfully deposited on a glass substrate at 30°C from an aqueous solution of TiO2–HF with the addition of boric acid as a scavenger by liquid phase deposition technique. From X-ray diffraction measure-ment, the deposited film was found to be amorphous and turns crystalline at 500°C. The deposited film showed excellent adherence to the substrate and was characterized by homogeneous flat surface. TiO2thin films can be used as a photocatalyst to clean up organohalides, a class of compound in pesticides that pollute the ground water. Photocatalytic degradation experiments show that indanthrene golden orange dye under-goes degradation efficiently in presence of TiO2thin films by exposing its aqueous solution to ultraviolet light. The suitable surface structure and porosity increases the photocatalytic activity. It was also observed that hemin doped TiO2 thin films break up organohalides at a surprisingly high rate under visible light. Keywords. TiO2 thin film; LPD technique; photodegradation. 1. Introduction Considerable interest has been generated in recent years to develop materials, which can provide sufficient photo-sensitivity in UV/visible region. TiO2 is well known as an efficient photo catalyst, active only under UV light and has attracted a great deal of attention with the increase in number of recent environmental problems. However, the use of powder catalyst results in disadvantages of stirring during the reaction and separation after the reaction (Negi-shi et al 1995). Preparation of TiO2 catalyst as a thin  film will make it possible to overcome these disadvan-tages and to extend it to industrial applications. Generally, the preparation methods of metal oxide thin film are broadly classified as dry and wet processes. Dry processes are vacuum evaporation, sputtering, chemical vapour deposition, etc (Meng and Dos Santos 1993; Battiston et al 1994; Lobl et al 1994) and the wet processes are sol–gel, spray pyrolysis, liquid phase deposition, etc (Nagayama et al 1988; Richardson and Rubin 2000; Yu et al 2000; Deki et al 2001). However, there are several engineering problems associated with sol–gel and spray pyrolysis such as the requirement of special and costly apparatus for the deposition of films and these methods are not suitable for coating the substrate with a large surface area or complex morphology. Hence, in our present studies, a latest and simple novel wet process known as liquid phase deposi-tion (Nagayama et al 1988; Richardson and Rubin 2000; Deki et al 2001) has been adopted to develop thin films of bare TiO2and hemin doped TiO2 to overcome some of these problems. The resultant films have been characterized structurally and optically. The morphology and composition of the films were ascertained by SEM and EDAX. The TiO2 thin film is applied as a photocatalyst for the photo-degradation of indanthrene golden orange dye (IGOR). TiO2 thin film is doped with hemin, a naturally occurring substance. The hemin doped TiO2 thin film was found to increase the rate of degradation under visible light. 2. Experimental 2.1 Preparation of TiO2 thin films TiO2 powder was supersaturated in 5% aqueous HF. The concentration of titanium ion was adjusted to 0⋅384 mol/ dm3. This solution was then diluted to 0⋅15 mol/dm3 of titanium ion with distilled water and used as a precursor solution. The sodium silicate glass after degreasing and washing with double distilled water was immersed into the treatment solution and suspended vertically. The boric acid solution, ~ 0⋅05 mol/dm3, was added, which acts as a free F– scavenger and shifts the equilibrium to one side. The solution was maintained at 30° for 40 h. After 40 h, the sample was taken out from the treatment solution, washed with distilled water and dried at room tempera-ture. Calcinations of deposited film were carried out at *Author for correspondence (noorsb@rediffmail.com) Noor  Shahina  Begum  and  H  M  Farveez  Ahmed 44various temperatures. The hemin doped TiO2 was also prepared by mixing the powder hemin in different pro-portions to the above precursor solution. 2.2 Photocatalytic degradation TiO2 films were settled in aqueous IGOR solution of con-centration, 100 mg/dm3, in a beaker. A pressure mercury lamp of about 200 W was used as light source. The solu-tion was bubbled with air during irradiation. The concen-tration of aqueous IGOR was determined with a UV-visible spectrophotometer by measuring the absorbance between 200 and 700 nm. IGOR showed intense band at about 470 nm. 2.3 Characterization of deposited films X-ray diffraction spectra were obtained from an X-ray diffractometer (type PW 3710 based tube anode: CuKα1:1⋅54056 and CuKα2: 1⋅54439). Optical properties were studied by using UV-vis-NIR, scanning spectrophotome-ter (type, Shimadzu-2003, Japan) in the wavelength range 300–1400 nm. The structural properties and chemical composition of coating were studied using scanning elec-tron microscope and EDAX (type: Leica Cambridge Ltd./ Leica S440i for SEM and Oxford link software for EDAX). 3. Results and discussion The surface morphology of the TiO2 film and hemin doped TiO2 films were recorded by scanning electron microscopy (figure 1a) which indicates that the films are homogeneous with amorphous background. The film an-nealed above 400°C exhibited a uniform distribution of grains with an average grain size of about 50–100 nm (figure 1b). The SEM of hemin-doped TiO2 thin film shows the compact structure with the segregation of hemin atoms (figures 1c and d).  The chemical composition of the TiO2 and hemin doped TiO2 films were also analysed by using EDAX spectra (figures 2a and b). The EDAX data confirmed thatFigure 1. Scanning electron micrographs of a. TiO2 thin films annealed at 300°C. b. TiO2 thin films annealed at 400°C, c. hemin-doped TiO2 thin films annealed at 300°C and d. hemin-doped TiO2 thin films annealed at 400°C show-ing the segregation of hemin atoms in TiO2 matrix. Synthesis  of  nanocrystalline  TiO2  thin  films  by  LPD  technique 45the as deposited film and the film annealed at 500°C are nearly stoichiometric and indicates the peaks for TiO2and Fe atoms. This confirmed the formation of hemin-doped TiO2 thin film.  The XRD pattern of TiO2 thin films annealed at diffe-rent temperatures is shown in figure 3. The broad and di-ffused X-ray diffraction patterns of as deposited film and film annealed up to 200°C indicate that the films are amorphous in nature. However, the films calcined at 400°C and above are polycrystalline and exhibited diffra-ction peaks with predominantly [101] orientation at 2θequal to 25°. At 500°C, a very sharp peak is observed which can be attributed to the formation of anatase phase.  In the present investigation, the UV-visible spectra of TiO2 thin films were recorded in the wavelength range 300–1400 nm. The optical absorption and transmittance spectra of TiO2 thin films annealed at different tempera-tures are shown in figures 4 and 5. The as deposited film shows minimum absorption, but the absorption decreases Figure 2. a. EDAX spectra of pure TiO2 thin film and b. TiO2thin film doped with hemin. with increasing annealing temperature of the films (figure 4). Similarly the as deposited film exhibited high trans- Figure 3. XRD pattern of TiO2 thin films at different tempe-ratures. Figure 4. UV absorption spectra of TiO2 thin films annealed at different temperatures (curves (a) absorption spectra of TiO2thin film annealed at 500°C, (b) absorption spectra of the film annealed at 300°C and (c) absorption spectra of the film an-nealed at 200°C). Noor  Shahina  Begum  and  H  M  Farveez  Ahmed 46mittance in the visible region with a sharp absorption edge between 350 and 400 nm (figure 5). However, trans-mittance decreases with increase in temperature. The high transmittances indicate smooth surfaces and relatively good film homogeneity. Absorption coefficient of the films can be calculated from transmittance and reflec-tance data. By determining the absorption coefficient, α,the optical band gap, Eg, can be calculated using the rela-tion (Ramana et al 1997) αhν = (hν – Eg)n,where hν is the photon energy, n the exponent that deter-mines the type of electronic transition causing the absorp-tion and can take the values 1/2, 2/3, 2 and 3/2. The optical band gap for direct allowed transition was found to be around 3⋅2 eV and this is in good agreement with the theoretically reported values. 3.1 Photocatalytic degradation of indanthrene  orange dye (IGOR) Titanium dioxide is a well known photocatalyst in pre-sence of UV light. However, thin films of TiO2 also exhi-bit photocatalytic activity under UV light and degrade organic pollutants very effectively and overcome the di- Figure 5. UV transmittance spectra of TiO2 thin film anne-aled at 400°C. fficulties of powder catalyst (Negishi et al 1995; Kishi-moto et al 1998). In addition, the same catalyst can be used repeatedly, which is not possible in case of powder catalyst. The rate of degradation increases with increas-ing number of OH–, the amount and life time of holes, the surface area which the reactant can contact, transmittance and crystallinity of TiO2 thin films etc. Hence, thin films of TiO2 annealed at 500°C degraded organic dye effec-tively. The photodegradation of indanthrene orange dye will occur when TiO2 thin film is illuminated by light of wavelength, 388 nm, in presence of water containing dis-solved oxygen and organic dye. Many organic compounds are known to degrade to CO2 and water under these con-ditions (Yuan and Zhang 1991; Andrew Burns et al2002). The primary step in photodegradation is certainly the generation of electrons and holes within TiO2 particles. 3.2 Mechanism TiO2hν > 388 nm TiO2(e–+ h+);h+ + OH–→ OH•;h+ + H2O → OH• + H+;e–+ O2→ O–2•;IGOR + OH•→ CO2 + H2O + NO–3.The photogenerated conduction band electron (e–) gets trapped by the dissolved oxygen to form superoxide ion, O–2. IGOR is attacked by hydroxyl radicals formed in the above equations and generates organic radicals or some other intermediates. Eventually all the parent compounds and intermediates are oxidized into CO2 and NO–3 (Yu et al 2000; Mayer and Obare 2004).  Figure 6a shows a decrease in the concentration of IGOR with time. After 70 min, the concentration almost reduces to 0⋅02 g. Unfortunately, the large bandgap of TiO2 lies in UV range, so that only 5–8% of sunlight photons have the requisite energy to activate the catalyst. A visible light (λ = 400–800 nm) catalyst would be much more effective and efficient in degradation process.  It is evident from figure 6b that hemin, when anchored with TiO2 thin film, shows excellent photocatalytic ac-tivities for the visible light photodegradation of IGOR compared with non-doped TiO2 thin films. In fact, TiO2doped with 0⋅1 mg hemin degraded about 80% of the dye within a short reaction time. Whereas 0⋅15 mg of hemin doped TiO2 thin film reduces the concentration of the dye to almost zero. This is attributed to the fact that the Fe3+ ion present in hemin is activated to its reduced form, Fe2+, which traps the electron and increases the lifetime of holes. Electron trappings reduce surface recombination and facilitate the holes to diffuse to the particle surface and participate in the oxidation reaction and hence photo-degradation is predominant. Synthesis  of  nanocrystalline  TiO2  thin  films  by  LPD  technique 47Figure 6. Photodegradation of IGOR organic dye by a. bare TiO2 thin film and b.hemin-doped TiO2 thin films (curves (a) TiO2 thin film without hemin, (b) with 0⋅1 mg hemin and (c) with 0⋅15 mg hemin). Figure 7. UV absorption spectra recorded for IGOR aqueous solution during photodegradation recorded at every 10 min interval using bare TiO2 thin film. Figure 8. UV absorption spectra of IGOR aqueous solution during photodegradation recorded by using hemin-doped TiO2thin film with different concentrations of hemin (curves (a) with-out hemin, (b) with 0⋅1 mg hemin and (c) with 0⋅15 mg hemin).  Similarly, figures 7 and 8 show the UV-vis spectra of IGOR aqueous solution before and after degradation with bare TiO2 thin film and with hemin-doped TiO2 thin film, respectively. The absorption of light gradually decreases with time. 4. Conclusions Transparent and homogeneous TiO2 thin films have been prepared by a highly economical technique known as liquid phase deposition technique (LPD) on soda lime glass. The UV-vis spectrum shows the optical band gap of TiO2 thin films to be 3⋅2 eV which is in good agreement with theoretical values. The XRD pattern of thin films shows a single anatase phase at 500°C with some orienta-tion effect in [101] peak. The photocatalytic activity of TiO2 thin film has been investigated. The film annealed at 500°C degraded the organic dye effectively in presence of UV light. However, the hemin-doped TiO2thin films degraded the dye at a surprisingly high rate under visible light. The rate of degradation increases with increasing concentration of hemin. Acknowledgements The authors are grateful to Dr K R Gunasekar, Department of Instrumentation, Indian Institute of Science, Bangalore, for the XRD, SEM and EDAX data. The authors would also like to thank Dr L Gomathi Devi and Dr B Geetha, Department of Chemistry, Bangalore University, Banga-lore, for their help in photodegradation studies. One of the authors (NSB) thanks DST–FIST for the grant. References Andrew Burns, Li W, Baker C and Shah S I 2002 Material Res-earch Society Symp. Proc. Vol. 703Battiston G A, Garbasi R, Porchia M and Marigo A 1994 Thin Solid Films 239 186 Noor  Shahina  Begum  and  H  M  Farveez  Ahmed 48Deki S, Henin Yu Yu Ko, Fujita T, Akamatsu K, Mizuhata X and Kajinami A 2001 Eur. Phys. J. D16 325 Kishimoto Hirotsagu, Takahama Koichi, Hashimoto Noboru, Aoi Yushifumi and Deki Shigehito 1998 J. Mater. Chem. 82019 Lobl P, Huppertz M and Mergel D 1994 Thin Solid Films 251 72Mayer Gerald and Obare Sherine 2004 Electrochemical Society. Inc, 206th meeting, Abs 1533Nagayama H, Honda H and Kawahara H 1988 J. Electrochem. Soc. 135 2013 Negishi N, Iyoda T, Hashimoto K and Fujishima A 1995 Chem. Lett. 841 Meng L-J and Dos Santos M P 1993 Thin Solid Films 226 22 Ramana C V and Mohammed Hussain O 1997 Adv. Mater. Opt. Electron. 7 225 Richardson T J and Rubin M D 2000 4th Int. conf. on electro-chromism, Uppsala, SwedenYu Jaiguo, Zhao Xiujian and Zhao Qingnan 2000 Thin Solid Films 379 7 Yuan Z and Zhang L 2001 J. Mater. Chem. 11 1265 

Synthesis of nanocrystalline TiO2 thin films by liquid phase deposition technique and its application for photocatalytic degradation studies

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Synthesis of nanocrystalline TiO2 thin films by liquid phase deposition technique and its application for photocatalytic degradation studies

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