ISSN 10634576, Journal of Superhard Materials, 2007, Vol. 29, No. 3, pp. 162–165. © Allerton Press, Inc., 2007. Original Russian Text © L. Sedláková, M. Horáková, P. Hájková, A. Kolouch, J. Karásek, P. Špatenka, 2007, published in Sverkhtverdye Materialy, 2007, Vol. 29, No. 3, pp. 45–48.
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Photocatalytic Properties of Titanium OxideBased Films Deposited by PECVD L. Sedláková, M. Horáková, P. Hájková, A. Kolouch, J. Karásek, and P. Špatenka Technical University of Liberec, Faculty of Mechanical Engineering, Department of Material Science Technical University of Liberec, Faculty of Mechanical Engineering, Department of Machining and Assembly, Liberec, Czech Republic Abstract—Titanium oxidebased photocatalytic films and their application have been the subject of grow ing interest in recent years. Properties of these films are significantly influenced by applied deposition methods. This article is focused on varying parameters of the plasmaenhanced chemical vapor deposition method and their influence on resulting thin titanium oxide films. Depositions were carried out with vary ing bias, substrate temperature and substrate type. Resulting samples of titanium oxide films were tested for their photocatalytic properties. The test method was based on decomposition of model organic sub stance, acid orange II. The film thickness was measured by a mechanical profiler. DOI: 10.3103/S1063457607030094
1. INTRODUCTION Titanium oxide coatings are increasingly regarded material due to their very interesting properties and many laboratories all over the world conduct research on them. Numerous applications of this film involve, e. g., protective hard coatings on cutting tools, optical films for optical filters or photocatalytic layers. Partic ularly, the photocatalytic properties of titanium dioxide have been a subject of intensive studies in recent years. Basically, the photocatalytic process is initiated by the photogeneration of electronhole pairs in a semicon ductor by photon absorption of UV light. Energy required for initiation of this process is equal to or higher than the band gap of anatase Eg = 3.2 eV. The electrons and holes recombine dissipating their energy in the bulk material. In conductive materials i. e. metals, these pairs are immediately recombined. Titanium oxide semiconductors produce pairs with a longer persistence which are available for redox reactions with electron donor or acceptor species, adsorbed on the semiconductor surface or localized in the electrical double layer surrounding the particle [1–5]. Photocatalytic activity is usually closely connected with a lightinduced hydrophilicity. Due to photocatalytic activity and hydrophilicity, titanium oxide layers also exhibit selfclean ing and antibacterial properties after illumination. One of very promising applications is photoinduced removal of pollutants from air, water, or selfcleaning coatings for different substrates like walls, windows, tiles, etc. The photocatalytic properties depend on the crystalline structure and the chemical composition of the deposited films. Different methods of deposition of TiO2 coatings have been described in literature. Tech niques based on decomposition and chemical reactions with a Ticontaining compound give very promising results. The most frequently described deposition techniques are pyrolysis [6, 7], solgel deposition [8, 9], thermal and/or plasmaenhanced chemical vapour deposition (CVD or PECVD) [10–13]. The aim of this paper is the investigation of photocatalytic activity of a thin film prepared by the PlasmaEnhanced Chemical Vapor Deposition method. 2. EXPERIMENTAL The lowpressure PECVD device was an inhouse constructed RF planar reactor schematically shown in Fig. 1. The reactor consists of a highvoltage electrode (E) and a resistively heated substrate holder (T) placed in a grounded cylindrical vessel (V). The volume of the vessel was about 7 l. The powered electrode was capac itivecoupled to the RF generator (RF) via a matching unit (M). The applied power was up to 250 W with the frequency of 13.56 MHz. The vessel was evacuated by the pumping system consisted of a rotary oil pump (RoP) and a Root’s pump (RP). Ultimate pressure was below 1 Pa. The pumping system was protected by a cold trap (CT) cooled by liquid nitrogen. The butterfly valve (BV) was used to reduce the pumping speed and control the pressure. 162
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M
RF
F V E
NV S TTIP
SH BV RH CT RoP
G
RP Fig. 1. Schematic diagram of the vacuum PECVD deposition device.
Mixture of oxygen or argon with titanium IVisopropoxide (TTIP) as a precursor was used as the working gas. The reactive gas (G) was introduced into the reactor through a tube positioned on the top of the reactor. The precursor was introduced through the shower positioned above the substrate (S). The flow of the precursor was controlled by a needle valve (NV) and of the other gases by massflow meters (F). The precursor was evap orated in the evaporator at temperature of 65°C. Glass and Si wafers were typically used as substrates. Some depositions were held in the inverted reactor arrangement where the substrate was placed on the powered elec trode (E) positioned in the bottom. In this reactor layout the layer was subject to ion bombardment during the growth. The photocatalytic activity was evaluated from decomposition speed of aqueous solution of the orange II (sodium salt of sulphonated azo dye), exposed to UV light. The use of the orange II has several advantages. As an anion, it is not adsorbed on the negatively charged surface of titanium oxide in neutral aqueous solutions. Thus the oxidative degradation of orange II is dominantly mediated through primary photogenerated hydroxyl radicals. This is a general degradation way typical for the majority of organic structures. Orange II has an absorption band maximum at 483 nm and an absorption minimum at 350 nm. Therefore, orange II absorbs only a small part of the used radiation, while the majority is absorbed by titanium oxide. Moreover, none of the degradation intermediates of orange II absorbs at wavelength longer than 300 nm. The aqueous solution of orange II with concentration of 0.035 mM was used for testing. 25 ml of orange II solution was placed into crystallization dish together with a sample and covered by a silica glass cap to mini mize vaporization. Irradiation was provided by fluorescent black light tube (Philips 60 cm, 20 W, 365 nm). Solution was magnetically stirred during irradiation. UV/Vis spectroscopy was used to evaluate the concen tration changes. Photocatalytic efficiency was expressed by Photocatalytic degradation speed calculated by ∆cV r = , ∆t SP where r is the speed of the photocatalytic degradation, ∆c is the change in the orange II concentration, V is the solution volume, ∆t is the irradiation time, S is the sample surface and P is the UV intensity. Content and depth profile of titanium, oxygen and the amount of incorporated carbon in the coatings were determined by the Rutheford Backscatering Spectrometry (RBS) on selected samples. The thickness of the TiOx films was measured using the mechanical profiler Mitutoyo Ltd. 3. RESULTS AND DISCUSSION Figure 2 shows the photocatalytic degradation speed as a function of the deposition temperature for sam ples deposited by PECVD in the inverted arrangement. Films were deposited at 8 Pa with power 100 W using a mixture of oxygen and TTIP vapors. Substrates were placed on a high voltage electrode and were exposed to bias about –470 V. Deposition rate was similar for all samples and was about 1 µm/h. High ion bombardment also causes carbon incorporation into the films up to 10% as it was revealed from RBS. So high contamination can also increase structure disordering and promote growing of amorphous phase.
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0 100 200 300 400 100 200 300 400 500 Temperature, °C Temperature, °C Fig. 3. Fig. 2. Fig 2. Photocatalytic degradation speed as a function of deposition temperature: glass (♦), Si (䊏). Fig 3. Photocatalytic degradation speed as a function of deposition temperature: glass (♦), Si (䊏).
0
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Figure 3 shows the photocatalytic degradation speed as a function of the deposition temperature for sam ples deposited by PECVD in the arrangement shown in Fig. 1. Substrates were placed on the grounded sub strate holder. Higher photocatalytic activity was achieved by deposition on a crystalline substrate. Interfacial titanium silicide layer was created during deposition on Si wafer according to the investigation shown in [14]. This layer can play a role in the conversion of amorphous titanium dioxide to a crystalline one and can enhance the growth of crystal. The deposition rate of about 0.5 µm/h is more than 2 times lower than for samples deposited on the pow ered electrode. A low deposition rate results from a low precursor decomposition during deposition on grounded electrode due to a low plasma density in comparison to the films deposited in the region of a high plasma density. 4. CONCLUSIONS The TiOx layers deposited using vacuum PECVD and the influence of deposition parameters on photocat alytic activity were investigated. Two types of substrate were used. The presence of high bias during deposition lowered photocatalytic activity probably due to crystalline structure disordering by accelerated ions. The best photocatalytic activity was found for films deposited at temperatures above 450°C. Higher photocatalytic activity was achieved by deposition on crystalline substrate. ACKNOWLEDGMENT This work was supported by MSMT, project 1M4531433201. REFERENCE 1. Guillard, Ch., Debayle, D., Gagnaire, A., et al., Physical Properties and Photocatalytic Efficiencies of TiO2 Films Prepared by PECVD and Sol–Gel Methods, Materials Research Bulletin. , 2004, vol. 39, no. 10, pp. 1445–1458. 2. Bahnemann, D., Cunningham, J., Fox, M.A., et al., Photocatalytic Treatment of Waters, Aquatic and Surface Photo chemistry, Chap. 21, pp. 261–316, Helz, G.R., Zepp, R.G., Crosby, D.G., Eds., Boca Raton, FL: Lewis Publishers, 1994. 3. Mills, A., and Le Hunte, S. An Overview of Semiconductor Photocatalysis, J. Photochem. Photobiol. A: Chem., 1997, vol. 108, no. 1, pp. 1–35. 4. Herrmann, J.M., Heterogeneous Photocatalysis Fundamentals and Applications to the Removal of Various Types of Aqueous Pollutants, Catal. Today. , 1999, vol. 53, no. 1, pp. 115–129. 5. Herrmann, J.M., Environmental Catalysis, Catalytic Science Series, Chap. 9, pp. 171–194, F. Jansen, R. A. van Sau ten, Eds., London: Imperial College Press, 1999. 6. Zhang, S., Zhu Y. F., and Brodie D. E. Photoconducting TiO2 prepared by spray pyrolysis using TiCl4 // Thin Solid Films., 1992, vol. 213, no. 2, pp. 265–270. 7. Ianagi, H., Yohoka, Y., Hishiki T., et al., Characterization of Dyedoped TiO2 Films Prepared by Spray–Pyrolysis, Appl. Surf. Sci., 1997, vo1. 13/114, pp. 426–431. 8. Mills, A., Hill, G., Bhopal, S., et al., Thick Titanium Dioxide Films for Semiconductor Photocatalysis, J. Photochem. Photobiology A: Chem., 2003, vol. 160, no. 3, pp. 185–194. 9. Lee, Y.Ch., Hong, Y.P., Lee, H.Y., et al., Photocatalysis and Hydrophilicity of Doped TiO2 Thin Films, J. Colloid Interf. Sci., 2003, vol. 267, no. 1, pp. 127–131. JOURNAL OF SUPERHARD MATERIALS
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10. Leistner, T., Lehmbacher, K., Hearter, P., et al., MOCVD of Titanium Dioxide on the Basis of New Precursors, J. NonCrystalline Solids, 2002, vol. 303, no. 1, pp. 64–68. 11. Ahn, K.H., Park, Y.B., and Park, D.W., Kinetic and Mechanistic Study on the Chemical Vapor Deposition of Tita nium Dioxide Thin Films by in Situ FTIR Using TTIP, Surf. Coat. Technol. , 2003, vol. 171, no. 1–3, pp. 198–204. 12. Da Cruz, N.C., Rangel, E.C., Tabacknics, M.H., et al., The Effect of Ion Bombardment on the Properties of TiOx Films Deposited by a Modified Ionassisted PECVD Technique, Nuclear Inst. Methods Phys. Res. B., 2001, vols. 175–177, pp. 721–725. 13. Asari, E. and Souda, E., Atomic Structures of TiO2(110) Surface between p(1×1) and p(1×2) Studied by Scanning Tunneling Microscopy, Appl. Surf. Sci., 2002, vol. 193, no. 1–4, pp. 70–76. 14. Nishida, K., Morisawa, K., Hiraky, A., et al. Insitu Monitoring of PECVD Growth of TiO2 Films with Laser Raman Spectroscopy, Appl. Surf. Sci., 2000, vols. 159–160, no. 3–4, pp. 143–148.
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