Title of Invention

AN IMPROVED PROCESS FOR THE PREPARATION OF SURFACE MODIFIED ZINC OXIDE SENSOR MATERIAL USEFUL FOR SENSING AMMONIA GAS

Abstract A process for the preparation of surface modified zinc oxide sensor material useful for sensing ammonia gas, which comprises of thoroughly mixing zinc oxide with a binder such as herein described, in presence of solvent such as herein described, drying the resultant mixture for 1-2 hours, compacting the dried powder under pressure of 5 to 10 tons, to obtain zinc oxide pellets, calcining the pellets so obtained at a temperature ranging between 500°C to 1QOO°C for a period of 5 to 16 hours, gradually reducing the temperature at the rate of l°C/min. to 3°C/min. to room temperature, immersing the pellets in a dilute solutions of a ruthenium such as herein described, for a period[ranging between 10 minutes and 15 hours, drying the pellets to incorporate 0.1-3 them heating the resultant pellets at a temperature ranging between 200 and 500 C for a period of 0.5 to 16 hours, to obtain the surface modified zine oxide
Full Text This invention relates to an improved process for the preparation of surface modified zinc oxide sensor material useful for sensing ammonia. More particularly, it relates to surface modified/functionalized zinc oxide wherein, selectivity of the sensor is controlled by chemical self-organization of ruthenium cations on zinc oxide surface, which can be used to fabricate a device capable of sensing ammonia gas in the range of 100-1000 ppm level. This surface functionalized zinc oxide works on the principal of change in resistance or other electrical properties as a result of the adsorption of gas on the semiconducting material.
Ammonia can be sensed by different methods such as electrolytic method, where diaphragm electrodes are used for the detection of ammonia. However, this method has limitations due to its high cost as well as low sensitivity. Similarly palladium gate metal oxide semiconductor (MOS) senses ammonia gas but it has poor selectivity as it senses the hydrogen produced by the decomposition of ammonia. In addition drifts are common in these devices. Different semiconducting oxides such as SnO2, TiO2 and WO3 with various dopants and additives are known for ammonia sensing: However, inadequate sensitivity and selectivity still seems to be a major problem for application purposes. In addition, most of the semiconducting oxides used for these sensors get saturated easily and thermal cleaning at high temperatures is essential for their successful operation. In

the present invention we report a process wherein the presence of gas in ppm level gives a direct electrical output which is suitable for signal processing. The requirement for device fabrication such as flexibility for miniaturisation, compatibility with electrical circuitry for devices, minimum interference from other gases etc. give added advantages in comparison with the commercially employed ammonia sensors.
Zinc oxide is well known for its gas sensing properties. It has a wide band gap (3.2 eV) and is an n-type semiconductor. In zinc oxide, at elevated temperatures, chcmisorbcd oxygen reduces the number of electrons present in the conduction band thereby resulting in high resistance values. Adsorption of a reducing gas leads to a decrease in resistance, due to oxidation by the surface oxygen species, thereby resulting in an increase in the density of conduction band electrons . The major advantage of using these zinc oxide based materials are high sensitivity, low cost and fast response time.
However, there are also several limitations such as poor selectivity, ageing, humidity induced effects and Ostwald ripening, resulting in an increase in the grain size and hence decrease in the sensitivity with the materials used in the prior art. Attempts have been made to overcome some of these problems by using different methods of selectivity and sensitivity control such as using masks and filters temperature programming, doping with noble metals and using oxide

additives. However, the use of masks and filters increases the selectivity at the cost of loss of sensitivity. Doping with noble metals enhances the sensitivity but they are economically less viable as amount required is considerable to achieve the suitable sensing properties.
The main objective of the present invention ,therefore, is to provide a process for the preparation of surface modified/functionalized zinc oxide sensor material useful for sensing ammonia gas which overcomes most of the above mentioned drawbacks.
Another objective of the present invention is to provide a process for the preparation of surface modified/functionalized sensor materials with better selectivity and sensitivity for sensing ammonia gas.
It has been observed by the inventors that by surface modification/functionalization of zinc oxide with ruthenium species by allowing adatom formation from solution, a dramatic improvement in the sensitivity and selectivity towards ammonia takes place. The surface functionalized zinc oxide with ruthenium creates surface states (donor-acceptor levels) in the band gap giving rise to unusual physical and chemical properties.

The process for the preparation of a surface ruthenated zinc oxide consists of the following steps:
i) preparation of zinc oxide pellets from commercially available powder or by using conventional ceramic route of using zinc salts; and
ii) surface fictionalization of the above zinc oxide pellets prior to gas sensitivity measurements by using a very dilute solution of RuX3, where X represents iodide, bromide, chloride and fluoride as described below.
Accordingly, the present invention provides a process for the preparation of surface modified zinc oxide sensor material useful for sensing ammonia gas, which comprises of thoroughly mixing zinc oxide with a binder such as herein described, at a ratio in the range 2 to 5wt%, in presence of solvent such as herein described, drying the resultant mixture for 1-2 hours, compacting the dried powder under pressure of 5 to 10 tons, to obtain zinc oxide pellets, calcining the pellets so obtained at a temperature ranging between 500°C to 1000°C for a period of 5 to 16 hours, gradually reducing the temperature at the rate of l°C/min. to 3°C/min. to room temperature, immersing the pellets in a dilute solutions of a ruthenium halide such as herein described, for a period
ranging between 10 minutes and 15 hours, drying the pellets to incorporatred 0.1-3% in themheating the resultant pellets
at a temperature ranging between 200 and \j


500°C for a period of 0.5 to 16 hours, to obtain surface modified/ zinc oxide.
In an embodiment of the present invention the zinc oxide powder used is such as commercial variety or prepared by conventional methods using zinc salts.
In one of the embodiments of the present invention the binder used is such as Polyvinyl alcohol (PVA), ethyl cellulose, polythene, teflon and polyvinyl acetate or mixture thereof.
In another embodiment the ratio of concentration of binder to zinc oxide may be in the range of 2 to 5 wt%.
In still another embodiment, the mixture of zinc oxide and binder is prepared in the presence of solvent such as acetone, benzene, alcohols such as isopropanol, carbon tctrachloride, water.
In yet another embodiment, the solvent used for the preparation of the solution of ruthenium halide is selected from water and water miscible solvents such as acetone, alcohol, acetonitrile .

In still another embodiment the concentration of ruthenium chloride may range between 0.0001 and 1 molar.
In a feature of the present invention, the amount of ruthenium incorporated as a result of this procedure ranges between 0.1 and 3.0% by weight as estimated by EDAX analysis.
The surface functionalized material prepared as per the above process is especially suitable for selectively sensing ammonia gas in trace amounts. Besides, some of the major advantages of the above mentioned invention include minimal use of noble metal compounds as their incorporation is restricted only at the surface. Since the adsorption-desorption is a surface phenomenon, by surface functionalization, one can obtain all the advantages achieved by bulk doping with incorporation of minimum amount of noble metals. For example, the present process requires about 0.2 wt% of the ruthenium as compared to the prior art wherein, noble metals like Pt, Pd or Ru of about 4-8% are required to achieve better sensing properties.

The process of the present invention is described with reference to the following examples which are illustrative only and should not be construed to limit the scope of the invention in any manner.
EXAMPLE 1
3 wt% ethyl cellulose was added to 10 gms of commercially available zinc oxide powder and the mixture was thoroughly mixed using isopropanol. The resultant mixture was then dried under an IR lamp and several pellets ( 5 mm diameter and
^^
2 mm thickness ) were made by applying a compaction load of 5 tons. The pellets were then calcined at 500 °C for 6 hours in an open tubular furnace followed by slow cooling. After attaining room temperature the pellets were then dipped in a dilute aqueous solution (0.05M concentration) of ruthenium trichloride for 10 min followed by drying in an oven at 100 °C. These dried pellets were then subjected to heat treatment at 350 °C for 1 hours in an open tubular furnace. The resultant blackish-gray colored pellets of surface modified zinc oxide were provided with ohmic contacts on both sides with silver paste and kept in a chamber for sensing ammonia in trace amount with a continuous flow of air as described in our
copending patent application 572/2//99 Energy Dispersive X-ray Analysis
(ED AX) using SEM have indicated 0.3 wt% incorporation of ruthenium.

EXAMPLE- 2
10 gms of zinc oxide was prepared by the hydrolysis of zinc acetate and to this zinc oxide powder 5 wt % of polyvinyl alcohol was added. Subsequently this was mixed thoroughly using acetone and dried in an oven before pressing at 5 tons pressure into pellets of 10 mm diameter and 2 mm thickness. These pellets were then calcined at 600 °C for 10 hours followed by slow cooling. The pellets were then dipped in 0.1M ruthenium trichloride solution for 1 hour. They were dried at 100 °C heated at 300 °C for 12 hours. The amount of ruthenium as estimated by EDAX analysis was 2.5 wt%. These pellets were kept in a testing chamber at 300°C and ammonia gas sensitivity is determined using 2-probe resistivity measurement as described in copending application patent application NF-23/99.
EXAMPLE- 3
15 gms of zinc oxide was prepared by the hydrolysis of zinc nitrate and to this zinc oxide powder 2 wt % of teflon powder was added and mixed thoroughly

using benzene. The mixture was then dried under an IR lamp and then the powder was pressed at 7 tons pressure into pellets of 15 mm diameter and 2 mm thickness. These pellets were then calcined at 700 °C for 16 hours and then dipped in 0.01M ruthenium trichloride solution for 15 hours. The pellets were then dried and heated at 450°C for 12 hours. The obtained pellets were kept for testing its sensitivity toward ammonia.
The main advantages are:
1. It describes an easy route for preparation of surface modified zinc oxide material
useful for sensing ammonia gas, wherein, selectivity is controlled by chemical
self-organization of ruthenium cations on zinc oxide surface.
2. This method gives a sensor material which has low cost and better sensitivity
unlike other ammonia sensing materials like palladium gate metal oxide
semiconductor, electrolytic diaphragm electrode etc.
3. In this invention, the gas sensing in ppm level gives a direct electrical output
which ensures the ease for signal processing.







We Claim:

1. An improved process for the preparation of surface modified zinc oxide sensor material useful for sensing ammonia gas, which comprises of thoroughly mixing zinc oxide with a binder
such as herein described, at a ratio in the range 2 to 5wt%, in presence of solvent such as herein described, drying the resultant mixture for 1-2 hours, compacting the dried powder under pressure of 5 to 10 tons, to obtain zinc oxide pellets, calcining the pellets so obtained at a temperature ranging between 500°C to 1000°C for a period of 5 to 16 hours, gradually reducing the temperature at the rate of l°C/min. to 3°C/min. to room temperature, immersing the pellets in a dilute solutions of a ruthenium halide
such as herein described, for a period ranging. between 10 minutes and 15 hours, drying the pellets incorporated 0.1-3% heating the resultant pellets at a temperature ranging between 200
and 500°C for a period of 0.5 to 16 hours, to obtain the surface modified/ zinc oxide.
2. A process claimed in claim 1 wherein the binder used is selected from polyvinyl alcohol
(PVA), ethyl cellulose, polythene, Teflon or polyvinyl acetate or mixture thereof.
3. A process as claimed in claims 1 to 2 wherein, the solvents is selected from acetone,
benzene alcohol ,ethanol, isopropanol, carbon tetrachloride and water.
4. A process for the preparation of surface modified zinc oxide sensor material useful for
sensing ammonia gas, as substantially described herein before with reference to the
examples.

Documents:

1092-del-1999-abstract.pdf

1092-del-1999-claims.pdf

1092-del-1999-correspondence-others.pdf

1092-del-1999-correspondence-po.pdf

1092-del-1999-description (complete).pdf

1092-del-1999-form-1.pdf

1092-del-1999-form-19.pdf

1092-del-1999-form-2.pdf


Patent Number 221212
Indian Patent Application Number 1092/DEL/1999
PG Journal Number 31/2008
Publication Date 01-Aug-2008
Grant Date 19-Jun-2008
Date of Filing 10-Aug-1999
Name of Patentee COUNCIL OF SCIENTIFIC AND INDUSTRIAL RESEARCH
Applicant Address RAFI MARG, NEW DELHI-110001, INDIA.
Inventors:
# Inventor's Name Inventor's Address
1 VARSHA ASHISH CHAUDHARY NATIONAL CHEMICAL LABORATORY, PUNE-411 008, MAHARASHTR, INDIA.
2 IMTIAZ SIRAJUDDIN MULLA NATIONAL CHEMICAL LABORATORY, PUNE-411 008, MAHARASHTR, INDIA.
3 KUNJUKRISHNAPILLAI VIJAYAMOHANAN NATIONAL CHEMICAL LABORATORY, PUNE-411 008, MAHARASHTR, INDIA.
4 MOHAMMED ASLAM SHABBIR AHMED NATIONAL CHEMICAL LABORATORY, PUNE-411 008, MAHARASHTR, INDIA.
PCT International Classification Number G06K 7/00
PCT International Application Number N/A
PCT International Filing date
PCT Conventions:
# PCT Application Number Date of Convention Priority Country
1 NA