Title of Invention

MONOCLINIC CeTi2O6 THIN FILM AND A SOL-GEL PROCESS FOR THE PREPARATION THEREOF

Abstract

A sol-gel process for the preparation of monoclinic CeTi206 thin film form A sol-gel process for the preparation of monoclinic CeTi206 thin film form, comprising the steps of preparing an alcoholic solution of about 0.20-0.25 M cerium chloride heptahydrate in absolute ethanol, adding the solution to titanium propoxide having concentration in the solution in the range of 0.331-0.624 M, stirring the mixture for a period of 4-10 min , after aging the solution for a period of about one week, spin coating the above said aged solution on electrically conducting substrate or micro slide glass, followed by air drying for a period of about 10-20 min. and annealing the afore said film in air, at a temperature in the range of 580-620°C for a period of 5-10 min. to obtain the desired monoclinic CeTi206 thin film.

Full Text

Field of the invention The present invention relates to a monoclinic CeTi206 thin film form and a sol-gel process for the preparation thereof. Background of the invention In the developmental process of W03 based transmissive ECDs, we are concentrating our efforts for developing ion storage counter electrode films with a high transmittance for visible light both in the charged and discharged state and an ion storage capacity exceeding 20 mCcm" 2 or comparable to W03 to provide sufficient number of ions for deep colouration. Therefore, work has been undertaken in this direction for the synthesis of Ce02 precursors doped with Ti02 via a wet chemistry route. Counter electrodes in transmissive electrochromic devices (ECDs) can be of two types. The first one involves an electrochromic (EC) layer, which is complementary with the chosen electrochromic material. The combination of W03 with NiOxHy is such typical example. The second possibility is an optically passive counter electrode, which remains colorless in both oxidized and reduced states. Whether active or passive the counter electrode has also to balance the charge shuttled from the active EC film through the ionic conductor - the electrolyte- layer. Thus the ion storage capacity of the counter electrode should equal the ion storage capacity of EC W03. Good cycling stability within the operational voltage and temperature range of the ECDs and high transmittance are other requirements of a counter electrode. Among optically passive counter electrode materials ln203: Sn has been reported as a candidate. Although it can be used also as a transparent conducting electrode its intercalation reaction is only partially reversible. Another material, which has been investigated extensively, is vanadium pentoxide (V205). It has high Li+ storage capacity, reversible intercalation kinetics for lithium, but the transmission in its bleached state is low. Tin oxide doped with different dopants Jike Mo and Sb has also been studied as a candidate for a passive counter electrode, but instability of Sn02 towards Li+ intercalation and the likely reaction leading to the formation of SnO, Sn and Li20 prompted exploratory work on other suitable materials for ECDs. In comparison with the above-mentioned materials, Ce02 appears to be more promising optically passive counter electrode material. The reversibility of the lithium ion intercalation reaction in Ce02 is reasonably good but the reaction kinetics is very slow as shown by D. Keomany, C. Poinsignon, D. Deroo. in Sol. Energy Mater. Sol. Cells 33 (1994) 429-44. Several attempts have been made in order to improve the reaction kinetics by way of mixing the oxide with other materials such as Ti, Zr, V, Sn, Mo and Si individually or with their mixtures. Films of pure and doped Ce02 have been made by different techniques. Sputtering technique has been adopted by M. Veszelei, L. Kullman, C. G. Granqvist. N. V. Rottkay, and M. Rubin in Appl. Opt, 37 (1998) 5993-6001 and the films thus obtained have been extensively studied. The potential of such films as passive counter electrodes has been reported. However, these authors have not obtained and reported the formation of CeTi206 compound in the thin film form. It is noteworthy that the application of Ce02-Ti02 mixed oxide films as passive counter electrodes has been shown by several authors. However, the potential of CeTi205 thin films as passive counter electrodes has never been reported earlier. Therefore, the present invention is the first report on the application of these films as passive counter electrodes for eiectrochromic devices. The widely used sol-gel process offers numerous advantages over the other conventional deposition techniques, which include tailor making of the film's properties, introduction of porosity in the films, low process cost and possible processing at low temperature. Preparation of Ce02 based films by sol-gel technique has been attempted following various routes. The use of alkoxides, the most popular precursor material in sol-gel processing has been reported by D. Keomany, C. Poinsignon, D. Deroo. in Sol. Energy Mater. Sol. Cells 33 (1994) 429-441. Alternately salts of cerium like CeCI3.7H20, [(NH4)2{Ce(N03)6}] in combination with Ti alkoxides have been shown by A. Makishima, M. Asami and K. Wada, in J. Non-Cryst. Solids 121 (1990) 310-314 as one of the routes to get Ce02-Ti02 films. Based on the earlier reports on these materials, A. Makishima, M. Asami and K. Wada. in J. Non-Cryst. Solids 121 (1990) 310-314 have performed a study in which the type of alkoxyl group of titanium alkoxide and the kind of catalyst have been varied in order to study their influence on the properties of the films. The films deposited by the authors have been annealed at 500°C and the XRD patterns of the films are characterized by the appearance of diffraction peaks of the Ce02 phase alone. Metal oxide semiconductors are used for gas sensing due to the dependence of their electrical conductivity on the ambient gas composition. They offer the possibility of "tailoring" the sensitivity and selectivity towards specific gas species. Gaining increased attention are mixed metal oxide compounds whereby varying the composition of the constituents, the sensor performance can be modified, i.e. improvement of sensitivity and selectivity, fabrication of n-and / or p-type semiconductor and modification of the sensor resistance for ease of electronic interface. Because of its chemical stability and high diffusion coefficient of oxygen vacancies, Ce02 is a promising material for fast oxygen gas sensing at high temperatures as reported by F. Millot, De Mierry in J. Phys. Chem. Solids 46 (1985) 797-801. Ti02 has been widely reported by A. Rothschild, F.Edelman, Y.Komen, F.Cosandey, in Sensors and Actuators B 67 (2000) 282-289, N.O. Savage, S.A. Akbar, P.K. Dutta, in Sensors and Actuators B 72 (2001) 239-248 and C. Garzella, E. Comini, E. Tempesti, C. Frigeri, G. Sberveglieri in Sensors and Actuators B 68 (2000) 189-196 for its gas sensing properties towards oxygen, carbon monoxide, methanol and ethanol and humidity. Mixed Ce02-Ti02 films deposited using a sol-gel process involving eerie ammonium nitrate and titanium butoxide have been reported to be oxygen gas sensors by A. Trinchi, Y.X. Li, W. Wlodarski, S. Kaciulis, L. Pandolfi, S. Viticoli, E. Comini, G. Sberveglieri in Sensors and Actuators B 95 (2003) 145-150. Photocatalytic reaction sensitized by Ti02 and other semiconducting materials has attracted extensive interest as a potential way of solving energy and environmental issues. Several cerium titanates in the powdered form have been investigated for photocatalytic activity. Yellow colored cerium titanate, CeTi206 with mainly Ce4+ state is known to cause photobleaching of methylene blue aqueous solution with irradiation of Xe discharge light as reported by S.O-Y-Matsuo, T.Omata, M.Yoshimura in J. Alloys and Compounds, 376 (2004) 262-267. Mixed Ce02-Ti02 films deposited using R.F and D.C sputtering are reported by Q.N.Zhao, C.L.Li, X.J.Zhao in Key Engineering Materials 249 (2003) 451-456 to decolorize methyl orange solutions upon irradiation of the UV light. Brannerite, UTi206 is an accessory phase in the titanate-based crystalline ceramics of synroc as reported by A.E. Ringwood, S.E. Kession, N.G. Ware, W. Hibberson, A. Major in Nature (London) 278 (1979) 219. The high U-content of brannerite (up to 62.8 wt.%) and its potential as a nuclear waste form for the immobilization of actinides have emphasized the importance of understanding radiation damage effects and their relation to composition and structure. The ideal formula of natural brannerite is (U,Th)1.xTi2+x06 with a deficiency in uranium and excess titanium. Many cation substitutions have been identified for both uranium (Pb,Ca,Th,Y and Ce) and titanium (Si, Al, Fe) in natural brannerite. Ideally, stoichiometric brannerite is monoclinic with space group C2/m. There are two different distorted octahedra in the structure. Distorted Ti06 octahedra form a zigzag sheet by sharing common edges, and each Ti octahedron shares three edges with titanium octahedral and three corners with uranium octahedra. The sheets of Ti06 octahedral are identical with those of the anatase structure parallel to (101) plane. The large cations (Th,U) are located at the interlayer sites and connect adjacent sheets. Each uranium octahedron shares two common edges with neighboring U06 octahedra and four corners with Ti06 octahedra. Oxygen atoms exist in a distorted HCP (hexagonal closely packed) array. Ce can substitute onto the U-site with little distortion of the octahedra. Ce is commonly used to estimate the properties of solids containing plutonium owing to their similar ionic radii (Ce(IV)=0.087 nm; Pu(IV=0.086 nm)). The compound CeTi206 is isostructural with PuTi206. As has been reported by K.B. Helean, A. Navrotsky, G.R. Lumpkin, M. Colella, J. Lian, R.C. Ewing, B. Ebbinghaus and J.G. Catalano in J. Nucl. Mater. 320 (2003) 231-244, CeTi206 in the powdered form prepared by sintering in air at 1350°C for > 100 h a pellet containing stoichiometric portions of the oxides, Ce02 and Ti02 is used to estimate the properties of PuTi206. In the present invention, the CeTi206 phase has been achieved in thin film by the sol-gel technique, which represents a reliable, low-cost chemical route. In comparison to the powdered CeTi206 material, which is formed at 1400-1500°C, the corresponding thin film is prepared by the sol-gel process at 600°C in the present invention. Our literature survey has shown that the preparation of CeTi206 in thin film form has never been carried out before. CeTi206 in powdered form has applications in areas of e.g. immobilization of nuclear waste form and photocatalytic activity. However, the same material in thin film form is useful in applications such as passive counter electrodes, sensors and photocatalytic activity. We have prepared CeTi206 films with a high transmittance for visible light both in the charged and discharged state and an ion storage capacity exceeding 20 mCcm"2. Using cerium chloride heptahydrate and titanium propoxide precursors, we have reported earlier in Sol. Ener. Mater. Sol. Cells 86 (2005) 85-103, the formation of a mixed compound of Ce02 and Ti02 i.e. Ce0162Ti02 at annealing temperature of 500°C from the Ce:Ti compositions, 4:1 and 2:1. It is evident from the chemical formula of the aforesaid compound that the oxygen content and thus the stoichiometric compositions of the two compounds i.e. CeO1.6.2Ti02 and CeTi206 are different. A comparative chart showing the results reported in the published papers and the present patent is given in Table I. Table I: A comparative chart showing the process steps, products obtained and the application of the films derived from different Ce:Ti compositions. (Table Removed) Objectives of the invention The main object of the present invention is to provide monoclinic CeTi206 phase in thin film form. Another object is to provide a sol-gel process for the preparation of monoclinic CeTi206 phase in thin film form, which obviates the drawbacks as detailed above. Yet another object of the present invention is to have a process, which will result in films with high chemical and mechanical stability. Yet another object of the present invention is to prepare films having pure CeTi206 as the only phase and without any other phase coexisting. Yet another object of the present invention is to deposit films with high ion storage capacity and transmittance characteristics. Still another object of the present invention is to use a process, which involves small number of steps. Brief description of drawings: Figure 1 represents the XRD pattern of the CeTi206 film. Figure 2 represents the bright field micrograph and electron diffraction pattern of CeTi206 film. Figure 3 represents the transmission profiles of CeTi206 film in the as-deposited, Li ion intercalated and Li ion deintercalated states. Figure 4 represents the SEM micrograph of CeTi206 film. Figure 5 represents the cyclic voltammograms of the CeTi206 film within the potential window of ± 1.0 V at a scan rate of 20 mVs'1. Summary of the invention Accordingly the present invention provides a monoclinic CeTi206 thin film having the following characteristics: a) pore size of the film in the range of 200-300 nm, b) crystalline size of 4.5-16 nm, c) transmission of 75-80% at 550 nm, d) transmission modulation of e) refractive index of about 1.99 f) indirect bandgap of about 3.25 eV and g) ion storage capacity of 19-23 mCcm"2, The present invention further provides a sol-gel process for the preparation of monoclinic CeTi206 thin film form, the said process comprising the steps of: a) preparing an alcoholic solution of about 0.20-0.25 M cerium chloride heptahydrate in absolute ethanol, b) adding the solution prepared in step a) to titanium propoxide having concentration in the solution in the range of 0.331-0.624 M, followed by stirring for a period of 4-10 min and aging at 20-30°C for a period of about one week, c) spin coating the above said aged solution on electrically conducting substrate or micro slide glass, followed by air drying for a period of about 10-20 min. and annealing the afore said film iii air, at a temperature in the range of 580-620°C for a period of 5-10 min. to obtain the desired monoclinic CeTi206 thin film. In an embodiment of the present invention the strength of the cerium chloride heptahydrate solution is 0.22 M. In yet another embodiment the concentration of titanium propoxide in the solution is in the range of 0.372-0.559 M. In yet another embodiment the Ce :Ti mole ratio used is in the range of 0.4:1 to 0.6:1. In yet another embodiment the solution obtained in step b) used for making film is in the state of just commencing to become a gel. In yet another embodiment the aged solution used obtained in step b) is spin coated at 3000 rpm for 35 s. In yet another embodiment the deposited films are dried in air for 15 min. In yet another embodiment the films are annealed at 600° for 5 min. In still another embodiment the conducting substrate used is fluorine doped tin oxide coated glass. Novelty In comparison to the powdered CeTi206 material reported earlier in the prior art, which is formed at 1400-1500°C, the corresponding thin film of CeTi206 material is prepared in the present invention by the sol-gel process at 600°C. Inventive steps: 1. To obtain CeTi206 compound in thin film form, the Ce:Ti mole ratio in the sol lies in the range of 0.4:1 to 0.6:1. 2. The other important parameter, which governs the formation of this compound in thin film form, is the temperature range. The optimum temperature to obtain this product is 600°C. The annealing is carried out in air at this temperature for 5 min. Below 580°C, the CeTi206 compound has not been obtained in thin film form. The main advantages of the present invention are: 1. Small number (four) of steps is required for the deposition of the films. 2. The preparation of sol used for the deposition of films takes very less time. 3. The formation of CeTi206 phase in thin film form makes its use as a photocatalytic agent much easier. Conventional powder catalysts suffer from disadvantages in stirring during the reaction and in separation after the reaction. Preparation of the catalysts coated as thin films makes it possible to overcome these disadvantages. 4. In contrast to the powdered CeTi206 material, which requires annealing of Ce02 and Ti02 in the range of 1400-1500°C, the CeTi206 film can be obtained at much lower temperature i.e. 600°C. 5. These films find applications in areas of e.g. passive counter electrodes, sensors, and photocatalytic activity. Detailed description of the invention The initial clear, colorless solution of cerium chloride heptahydrate is prepared by stirring the cerium salt in absolute ethanol until the salt dissolves completely. The solution is prepared at ambient temperature. Ti alkoxides readily hydrolyze in aqueous and alcoholic solutions. The addition of cerium based alcoholic solution to titanium propoxide such that Ce:Ti mole ratio is between 0.4:1 and 0.6:1 does not induce precipitation into the titanium propoxide. The cerium salt stabilizes the alkoxide solution and prevents the precipitation of the hydroxides. The gelation time of the solution varies depending on the content of Ti alkoxide in the solution. The gelation time for Ce:Ti mole ratio (0.6:1, 0.5:1 and 0.4:1) solutions is approximately one week depending on the ambient temperature and humidity conditions. For electrochromic applications, a high transparency in the films is a prerequisite. The films deposited from the solutions just reaching the state of gelation are highly transparent due to higher degree of porosity in them. The hydrolysis and condensation in the sols results in the elimination of small groups such as H20, C2H5OH etc, thereby leading to highly porous and thus transparent films. Therefore, after appropriate aging of the sol depending upon its ratio of precursor materials and annealing of the deposited films, high transmittance in the films can be obtained. The deposition sols after optimum aging have been spin coated onto electrically conducting (fluorine doped tin oxide, Sn02:F) and micro slide glass at 3000 rpm for 35 s. After intermittently drying the films for 15 min., the films have been thermally treated at 600°C for 5 min. in air in a furnace at the heating rate of 1-2°Cmin'1. Ce:Ti (0.6:1, 0.5:1 and 0.4:1) solutions based on cerium chloride heptahydrate and titanium propoxide have been prepared by dissolving the alkoxide in 0.22 M cerium chloride solution. The following examples illustrate the preparation of sol preferred for the deposition of CeTi206 films and should not be construed to limit the scope of the present invention. Example 1 0.22 M solution of cerium chloride heptahydrate (CeCI3.7H20, Merck) has been prepared in absolute ethanol (C2H5OH, Merck). The above clear, colorless solution has been added to titanium propoxide such that the Ce:Ti mole ratio in the solution is 0.5:1. The resultant bright yellow solution has been stirred for 5 min. After allowing the commencement of gelation in the resultant yellow solution, the sol has been spin coated on to fluorine doped tin oxide coated glass substrates and micro slide glass substrates at 3000 rpm for 35 s followed by drying in air for 15 min. Subsequently, the films have been thermally treated at 600°C for 5 min. in air at a heating rate of 1-2°Cmin"1. The films prepared as above with thickness 6800 A were tested for their optical passivity in a test cell containing a test electrolyte of 1 M lithium perchlorate in propylene carbonate and a platinum electrode. The XRD results (Figure 1) have shown the formation of pure CeTi206 phase in the films with an average crystallite size along (201) plane of 16 nm. The TEM studies (Figure 2) have also affirmed the formation of monoclinic CeTi206 structure in the films. The films have shown a transmission modulation of less than 1 % at 550 nm (Figure 3). The transmission of the films is nearly 80 % in the entire visible spectral region. The refractive index and indirect band gap of the film are 1.99 and 3.25 eV respectively. From Figure 4 showing the SEM micrograph of the CeTi206 film, a pore size of nearly 250 nm is determined. Ion storage capacity of the film calculated using the cyclic voltammogram illustrated in Figure 5 is 20.5 mCcm"2. Example 2 0.22 M solution of cerium chloride heptahydrate (CeCI3.7H20, Merck) has been prepared in absolute ethanol (C2H5OH, Merck). The above clear, colorless solution has been added to titanium propoxide such that the Ce:Ti mole ratio in the solution is 0.6:1. The resultant bright yellow solution has been stirred for 5 min. After allowing the commencement of gelation in the resultant yellow solution, the sol has been spin coated on to fluorine doped tin oxide coated glass substrates and micro slide glass substrates at 3000 rpm for 35 s followed by drying in air for 15 min. Subsequently, the films have been thermally treated at 600°C for 5 min. in air at a heating rate of 1-2°Cmin"1. The films prepared as above with thickness 6200 A were tested for their optical passivity in a test cell containing a test electrolyte of 1 M lithium perchlorate in propylene carbonate and a platinum electrode. The films have shown a transmission modulation of less than 1 % at 550 nm. The transmission of the films is nearly 80 % in the entire visible spectral region. The XRD results have shown the formation of pure CeTi206 phase in the films with an average crystallite size along (201) plane of 14.2 nm. The ion storage capacity of the film is 19.3 mCcm'2. Example 3 0.22 M solution of cerium chloride heptahydrate (CeCI3.7H20, Merck) has been prepared in absolute ethanol (C2H5OH, Merck). The above clear, colorless solution has been added to titanium propoxide such that the Ce:Ti mole ratio in the solution is 0.4:1. The resultant bright yellow solution has been stirred for 5 min. After allowing the commencement of gelation in the resultant yellow solution, the sol has been spin coated on to fluorine doped tin oxide coated glass substrates and micro slide glass substrates at 3000 rpm for 35 s followed by drying in air for 15 min. Subsequently, the films have been thermally treated at 600°C for 5 min. in air at a heating rate of 1-2°Cmin"1. The films prepared as above with thickness 10,000 A were tested for their optical passivity in a test cell containing a test electrolyte of 1 M lithium perchlorate in propylene carbonate and a platinum electrode. The films have shown a transmission modulation of less than 1 % at 550 nm. The transmission of the films is nearly 80 % in the entire visible spectral region. The XRD results have shown the formation of pure CeTi206 phase in the films with an average crystallite size along (201) plane of 4.7 nm. The ion storage capacity of the film is 23.0 mCcm-2. Table II shows the comparison of the parameters observed in the different films. Table II: Comparison of the parameters observed in the different films. (Table Removed) We claim 1. A sol-gel process for the preparation of monoclinic CeTi206 thin film form, the said process comprising the steps of: a) preparing an alcoholic solution of about 0.20-0.25 M cerium chloride heptahydrate in absolute ethanol, b) adding the solution prepared in step a) to titanium propoxide having concentration in the solution in the range of 0.331-0.624 M, c) stirring the mixture obtained in step b) for a period of 4-10 min d) after aging the solution as obtained in step c) for a period of about one week, spin coating the above said aged solution on electrically conducting substrate or micro slide glass, followed by air drying for a period of about 10-20 min. and annealing the afore said film in air, at a temperature in the range of 580-620°C for a period of 5-10 min. to obtain the desired monoclinic CeTi206 thin film. 2. A process as claimed in claim 1, wherein the strength of the cerium chloride heptahydrate solution is 0.22 M. 3. A process as claimed in claim 2, wherein the concentration of titanium propoxide in the solution is in the range of 0.372-0.559 M. 4. A process as claimed in claim 2, wherein the Ce :Ti mole ratio used is in the range of 0.4:1 to 0.6:1. 4. A process as claimed in claim 2 wherein the aged solution used obtained in step c) is spin coated at 3000 rpm for 35 s.. 5. A process as claimed in claim 2 wherein the films are annealed at 600° for 5 min.

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392-del-2006-Abstract-(22-05-2012).pdf

392-del-2006-abstract.pdf

392-del-2006-Claims-(22-05-2012).pdf

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392-del-2006-Correspondence Others-(22-05-2012).pdf

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392-del-2006-Petition-137-(22-05-2012).pdf