Title of Invention

"AN IMPROVED PROCESS FOR THE PREPARATION OF SURFACE MODIFIED THIN FILM"

Abstract

An improved process for the preparation of surface modified thin film which comprises preparing a solution of tin source by conventional method coating of a substrate with this solution of tin source at a temperature of 200 to 700°C to obtain the substrate with a coated thickness of 0.1 micron to 10 microns, annealing the substrate at the temperature of their coating for 0.1 to 10 hours, allowing then to attain the room temperature, spinning the coated substrate with the solution of capped copper nanoclusters prepared by known method for 10 seconds to 5 minutes or dipping the coated substrates in the said solution of copper nanocluster for a period of 10 minutes to 12 hours, removing and drying the substrates to obtain the desired substrate modified thin film sensor material.

Full Text

The present invention relates to an improved process for the preparation of surface modified thin film. More particularly it relates to surface modified tin oxide film which can be used to fabricate a device capable of sensing H2S at room temperature. This surface modified ceramic material can be used to make H2S sensors working on the principal of change in resistance or other electrical properties as a result of the adsorption of H2S on the sensor element. Hydrogen sulfide is a poisonous gas and its detection in trace levels is important since it affects different human organs. For example, H2S has the ability to temporarily deaden the human sense of smell. Therefore it is considered an insidious poison since it fails to provide a good warning of gas concentration. The safety concentration of gas in air under long exposure is 100-150 ppm. In addition to health concern, the presence of sulfide during production of photographic products may directly affect the quality of the product. Also, H2S is a byproduct of many industrial-manufacturing processes such as petrochemical industry, wastewater and wastewater treatment plants, dyes and synthetic petroleum production plants, and hence there is a need to detect H2S in ppm level. Hydrogen sulfide is a colorless, poinsonous gas having a characteristic rotten egg odor. Detection of this gas in low concentrations is important due to environmental, safety as well as health hazard affecting different human organs. H2S has the ability to temporarily deaden the human sense of smell. The permissible exposure limit set by the U.S. Occupational Safety and Health Administration for H2S is 10 ppm, which is 30 times lower than that immediately dangerous to life, whereas that set by the U.S National Institute for Occupational Safety and Health is 300 ppm. Despite a low human odor threshold for the gas in air ( rapidly fatigues, and therefore, fails to provide a good warning of gas concentration. The permissible concentration of gas in air for human beings under longexposure is 100-150 ppm. Hence, its detection has been demanded for safety as well as for auto-ventilation and oral odor checking. In addition to health concern, the presence of sulfide during production of photographic products may directly affect the quality of the product. Also, H2S is a byproduct of many industrial-manufacturing processes such as petrochemical industry, wastewater and wastewater treatment plants, dyes and synthetic petroleum production plants, and hence detection of this gas in trace levels is important. Although different types of materials such as, tin oxide, tungsten oxide and zinc oxide are known for sensing H2S, their response is not sufficiently accurate for low level detection ranging from 10 to 100 ppm by volume, especially at room temperature. In addition, most of the semiconducting oxides used for these sensors get saturated easily and thermal cleaning at high temperatures is essential for their operation. However, sensors made by using surface modified semiconducting oxides, prepared by the process described in the present invention are more convenient, as their direct electrical output is suitable for signal processing at room temperature. More significantly, flexibility for miniaturization, compatibility with electrical circuitry for devices and minimum interference from thermal and vibrational noise give other advantages. The commonly employed semiconducting oxides include tin oxide and zinc oxide with different dopants for appropriate conductivity changes due to gas adsorption. Tin oxide is a well known representative example for this type of sensor materials. They are large band gap (3.8 eV) n-type semiconductors. In tin oxide the chemisorbed oxygen can reduce the number of electrons present in the conduction band , thereby resulting in high resistance values for the oxide. Adsorption of reducing gases decreases the resistance as these molecules can be oxidized by the surface oxygen species leading to an increase in the density of conduction band electron. The main advantage of using these tin oxide based materials are their high sensitivity towards detection of gas, low cost and fast response time. Reference may be made to numerous attempts made to enhance their selectivity for a particular gas by using filters or doping with noble metals. However, they show large cross-sensitivity and their lack of specificity cannot be completely compensated by the use of dopants and additives. For example, FeNbO4 wherein mixed valance states of Fe2+/Fe3+ has been observed, it shows good sensitivity towards Hz and H2S with moderate sensitivity for LPG ( K.I. Gnanasekar et. al. Sensor & Actuators, B55, 1999, 170). Similarly, CuO/SnOj thin film heterostructures detect H2S at low concentrations (100 ppm) at 200°C (G Sarla Devi et. al. J. Electrochem. Soc. 142, 1995, 2745). In addition SnO2 doped with Pt/Pd (M. Gaidi et.al, Sens. & Actuators, B62, 2000,43) and ZnO doped with Pd (A. P. Chatterjee et. al,, J. Mats. Science Letters, 34, 1999, 217) are also known to detect H2S in low concentrations. US patent 6046054 discusses the detection of H2S employing Cri.gTo.aOj+x. It has been observed that by prior treatment of the sensor to low concentrations of H2S at 200 - 600°C, the sensitivity and response time are considerably enhanced. Further due to such conditioning the effect of humidity is almost negligible. The drawbacks such as long term drift, poisoning by humidity, temperature effects and high temperature operation with the materials used in the prior art. Another problem with these sensor materials is their lack of selectivity since they simultaneously sense most of the oxidizable gases such as carbon monoxide, hydrogen, alcohol, H2S, LPG with comparable sensitivity values. In addition, though the metal oxides are made from naturally available mineral resources and are more economical most of the noble metals used as additives are not economically viable. Some of these sensors have drawback as these sensors are operated at high temperatures (200°C - 400°C) thereby increasing the power consumption. In addition tin oxide doped with La and Pb also give good selectivity to HjS but again at higher operating temperatures (JP 5087758A2). Further H2S sensors in US patent no. 3479257 have a drawback as they are not at all selective since they detect various gases such as H2, H2S, propane, butane, ammonia and alcohol. The lack of selectivity can be a major problem for detection of H2S since it is generally in very trace amounts in the atmosphere and HZ is more predominant. The main object of the present invention is to provide an improved process for the preparation of surface modified HaS sensor materials which obviates the drawbacks as detailed above. Another object of the present invention is to provide a process for the preparation of the said sensor materials with better selectivity, sensitivity and ability for sensing H2S gas at room temperature. Yet another object of the present invention is the modification of the tin oxide surface with Cu clusters capped with different functional groups like thiol, carboxyl, amino etc. to produce better sensing materials removing the drawbacks in the prior art materials. Still another object of the present invention is to surface modify the tin oxide with capped Cu clusters so as to allows selective detection of H2S at room temperature with very high sensitivity. This is especially significant since most of the H2S sensors work at higher temperatures. Yet another object of the present invention is to make room temperature operating sensing material so as to reduce the cost dramatically due to reduced power consumption. Besides this the fabrication of a device also becomes easy since a heater is not required. The improved process for the preparation of surface modified thin film sensor material which comprises of deposition of thin films of tin oxide, synthesizing Cu clusters capped with different functional groups in different non aqueous solvent and then modifying the surface of the films of tin oxide by either spin coating or dipping consists of three steps: i)Preparation of thin amorphous films of tin oxide by various methods such as spray pyrolysis, spin coating, chemical vapor deposition, sol-gel processes and vacuum deposition using salts of tin such as tin tetrachloride, tin dichloride or organometallic compounds such as tetramethyltin or from water soluble tin salts on different substrates such as glass, alumina and silica at temperatures ranging from 200 to 700°C followed by annealing between 200 to 500°C and ii)preparation of Cu nanoclusters capped with organic molecules with different functional groups and varying chain lengths (dithiol, tridecylamine, lauric acid, octadecaenthiol, dodecanethiol, octanethiol, hexanethiol) by Brust synthesis [J.Chem.Soc.Chem.Commun,801, 1994]. Preparation of metal nanocluster typically involves a 500 ml KB flask having three openings, was securely held with a clamp. The flask is connected to a Argon cylinder for the inert atmosphere. The second opening is connected to an oil bubbler to maintain the argon flow. Third is kept for the insertion of solvent and compound and then tightly closed. The flask is then immersed in a Dewar containing ice. The synthesis followed a standard procedure within which three conditions are strictly followed: (1) The molar ratio of Cu salts: Capping agent is kept constant (2) The experiments are performed in ice bathe. (3) The rate of addition of the reducing agent is kept constant. In one of the experiments 60 ml of 30 mM aqueous copper chloride solution, 200 ml of toluene containing 1 mM concentration of R-X where R is the alkyl group with different number of C (C5-C18) atoms and X is the functional group like carboxylic, amino, thio etc. is added by vigorous stirring. 50 ml of 0.4 M NaBH4 aqueous solution is added to the vigorously stirred solution of copper chloride and R-X containing toluene. NaBH4 solution is added slowly, with constant rigorous stirring, into the homogeneous mixture of CuCl2 and toluene containing the capping agent. A black color is formed during the addition of reducing agent, which initially disappears for sometime and then reappears. The stirring is continued upto 8 hrs. The black aqueous solution gets cleared after sometime and all the Cu ions are transferred into the toluene layer. Thoroughly stirred reaction mixture is then transferred into a dry clean separating funnel. The black organic layer is then collected and heated very slowly at round 80°C under Ar atmosphere. A very slow heating gives the dark brown color powder. The powder obtained is then preserved in the dry oxygen free atomosphere. lii) surface modification of the above thin films of tin oxide prior to gas sensitivity measurements by using a very dilute solutions (0.0001M to 0.1 M) of Cu clusters capped with organic molecules with different functional groups and varying chain lengths by dipping and spin coating techniques for a sufficient time. Accordingly, the present invention provides an improved process for the preparation of surface modified thin film which comprises preparing a solution of tin source by conventional method coating of a substrate with this solution of tin source at a temperature of 200 to 700°C to obtain the substrate with a coated thickness of 0.1 micron to 10 microns, annealing the substrate at the temperature of their coating for 0.1 to 10 hours, allowing then to attain the room temperature, spinning the coated substrate with the solution of capped copper nanoclusters prepared by known method for 10 seconds to 5 minutes or dipping the coated substrates in the said solution of copper nanocluster for a period of 10 minutes to 12 hours, removing and drying the substrates to obtain the desired substrate modified thin film sensor material. In an embodiment of the present invention the substrate used is selected from glass, alumina and silica. In an another embodiment of the present invention the solvent used for preparing the solution of tin source is selected from the group consisting of acetone, benzene, isopropanol, ethanol and water. In an another embodiment of the present invention the solvent used for preparing the solution of Cu clusters is selected form are toluene, hexane and ethanol. In an another embodiment of the present invention the capping agent used is selected form the group consisting of dithiol, tridecylamine, lauric acid, dodecanethiol, octanethiol and hexanethiol. In an another embodiment of the present invention the reducing agents used are sodium borohydride, hydrazine hydride and lithium ammonium hydride. In an another embodiment of the present invention the tin oxide film is modified by Cu capped clusters using spin coating and dip technique. The materials prepared as per the process described in the present invention are especially suitable for sensing HjS in trace amounts and the main advantages of this invention include minimum use of Cu clusters since the incorporation occurs only on the surface. Since the adsorption-desorption phenomenon is restricted only to the surface and does not go to the bulk there is no need for any bulk doping. For example, this uses typically 0.05% of the Cu clusters while the prior art employs Cu catalysts of about 4-8% and this reduction will lead to substantial economic gain. The following examples are given by way of illustration and therefore should not be construed to limit the scope of the present invention. EXAMPLE -1 1 ml of SnCl4 was added to 10 ml ethanol. The solution is transferred in a spray gun with nozzle diameter of 1mm and held at a distance of 25cms from the substrates on to which the films are to be deposited. The glass substrates are thoroughly cleaned by boiling in dilute acid then cleaning by using soap solution followed by ultrasonic frequency treatment. The glass substrates are heated to 500°C and held at that temperature for spraying. The solution is atomized on the glass substrates at a rate of 71b/min using compressed air. The resultant films are annealed at 500°C for 12 hrs. For preparation of Cu clusters, a 10 mM copper acetate solution is prepared in water. A 1 mM solution of lauric acid is made in toluene and added to the aqueous copper chloride solution. A 0.1 M solution of sodium borohydride in water is added to the mixture slowly, stirring continuously. The solution is stirred continuously for 4 hours till the Cu ions are transferred to the non-aqueous phase. The nonaqueous phase is separated using a seperating funnel. 0.1 ml of this cluster solution was transferred on to the thin film of tin oxide, which was spinned at a speed of 1700 rpm for 30 sec. The procedure is repeated twice. ED AX measurements indicate that there is 0.02 wt% of monolayer protected Cu clusters on the tin oxide film. EXAMPLE- 2 A gel of tin chloride is prepared by using precursors. 1 ml of this gel is placed on a glass substrate, which is cleaned as described in Example 1. The substrate is spinned at a rate of 2000 rpm for 30 s. This procedure is repeated once more. The film is then heated at 470°C for 10 hrs. For preparation of Cu clusters, a 20 mM copper sulphate solution is prepared in water. A 2.5 mM solution of tridecylamine is made in hexane and added to the aqueous copper sulphate solution. A 0.1 M solution of hydrazine hydride in water is added to the mixture slowly, stirring continuously. The solution is stirred continuously for 2 hours till the Cu ions are transferred to the non-aqueous phase. The nonaqueous phase is separated using a seperating funnel. The tin oxide film is dipped in the Cu cluster solution for 1hr and then removed and dried in air. The amount of Cu clusters as obtained from EDAX measurements is 0.05 wt%. EXAMPLE- 3 Tin oxide thin films are prepared by using MO-CVD technique. For MO-CVD tetramethyltin (TMT) is used. TMT is stable in air and moisture and has relatively higher vapour pressure at room temperature. TMT vapor is transported to the hot substrate by bubbling an inert gas such as nitrogen through the liquid. The gas is bubbled at the rate of 2.0 lit/min through the TMT for the vapour transport. The experimental set up consists of an open tube reactor in which substrate is maintained at the temperature of 480°C. For preparation of Cu clusters, a 30 mM copper nitrate solution is prepared in water. A 5mM solution of dodecanethiol is made in ethanol and added to the aqueous copper nitrate solution. A 0.25 M solution of sodium borohydride in water is added to the mixture slowly, stirring continuously. The solution is stirred continuously for 4 hours till the Cu ions are transferred to the non-aqueous phase. The nonaqueous phase is separated using a seperating funnel. The films of thin oxide are modified by Cu clusters by spin coating these Cu clusters at a rate of 1000 rpm. The amount of Cu clusters determined by EDAX analysis 0.1 wt%. The main advantages of the present invention are: 1. A material with high selectivity towards hydrogen sulphide gas. 2. Low cost gas sensing and substrate materials. 3. Low temperature preparation reduces the production cost. We claim : 1 An improved process for the preparation of surface modified thin film which comprises preparing a solution of tin source by conventional method coating of a substrate with this solution of tin source at a temperature of 200 to 700°C to obtain the substrate with a coated thickness of 0.1 micron to 10 microns, annealing the substrate at the temperature of their coating for 0.1 to 10 hours, allowing then to attain the room temperature, spinning the coated substrate with the solution of capped copper nanoclusters prepared by known method for 10 seconds to 5 minutes or dipping the coated substrates in the said solution of copper nanocluster for a period of 10 minutes to 12 hours, removing and drying the substrates to obtain the desired substrate modified thin film sensor material. 2 An improved process as claimed in claim 1 wherein the substrate is selected from glass, alumina and silica. 3 An improved process as claimed in claims 1 & 2 wherein the solvent used for preparing the solution of tin source is selected from the group consisting of acetone, benzene, isopropanol, ethanol and water. 4 An improved process as claimed in claims 1 to 3 wherein the solvent used for preparing the solution of Cu clusters is selected form are toluene, hexane and ethanol. 5 An improved process as claimed in claims 1 to 4 wherein the capping agent used is selected form the group consisting of dithiol, tridecylamine, lauric acid, dodecanethiol, octanethiol and hexanethiol. 6 An improved process for the preparation of surface modified thin film substantially as herein described with reference to examples accompanying this specification.

Documents:

482-del-2001-abstract.pdf

482-del-2001-claims.pdf

482-del-2001-correspondence-others.pdf

482-del-2001-correspondence-po.pdf

482-del-2001-description (complete).pdf

482-del-2001-form-1.pdf

482-del-2001-form-18.pdf

482-del-2001-form-2.pdf