GB2212270A

GB2212270A - Gas sensor using phthalocyanine

Info

Publication number: GB2212270A
Application number: GB8825692A
Authority: GB
Inventors: Thomas Alwyn Jones; Barry Bott
Original assignee: National Research Development Corp UK
Current assignee: National Research Development Corp UK
Priority date: 1987-11-06
Filing date: 1988-11-03
Publication date: 1989-07-19
Also published as: GB8825692D0; GB8726079D0; DE3836819A1

Description

1 ill t, 0 221227 0 131512 - 1 GAS SENSOR USING PHTHALOCYANINE This

invention relates to a gas sensor using metal phthalocyanine. Sensors for strongly electron accepting gases, in particular N02 and C12, are described in Sensors & Actuators 5098043-53. These sensors are based on the electrical conductivity changes effected by the adsorption of these gases on vacuum- sublimed films of lead phthalocyanine. While insensitive to most other gases, these sensors can detect 1 ppb N02 in air and 0.1 PPM C12 in air. Similar sensors using metal-free phthalocyanine instead of lead phthalocyanine responded similarly but because of the very low conductivity of that material are difficult to use at concentrations below 100 ppb N02 in air. Unfortunately the response time and sensitivity of these sensors are strongly temperature-dependent, the response time being also concentration-dependent. At 1700C, the sensor on exposure to a gas (< I ppm) reaches 90% of its steady-state response in 100 seconds, but at 1250C this takes over 600 seconds. At gas concentrations towards 100 ppm the response times are more than five times as long. Likewise, on stepchange removal of the < 1 ppm gas, at 1700C the sensor recovers 90% of 20 its change within 120 seconds, but at 12511C this takes 600 seconds, and again more than five times as long on removing a 100 ppm gas. The optimum operating temperature for such sensors is typically 1600C - 1700C. Better performance is possible at 2300C - 2400C but shortens the lifetime of the sensor. 25 It has been suggested to overcome this problem by using a metal phthalocyanine film which has been deposited by repeated dipping of a substrate to deposit a monolayer of aligned molecules each dip (i.e. a Langmuir-Blodgett film). While fast response times at room temperature can be achieved, the recovery time of the sensor is still unacceptably long.

According to the present invention, a gas sensor comprising a layer, exposable to gas, of a phthalocyanine on a substrate, and means for measuring the conductivity of the layer, is characterised in that the layer is continuous and is mostly crystalline and (where such exists) preferably crystallised as the -polymorph. Its thickness is in practice most conveniently 05 at least 2000 nm, preferably at least 3000 nm, for example 4000nm (and it may exceed 5000nm), but may be as thin as the substrate surface topography allows for continuity; Langmuir-Blodgett films, which are of molecular thickness only, are usable but depend on the substrate's being very smooth to maintain the 10 film's continuity and integrity. The layer when originally laid down (e.g. by vacuum sublimation) may be at least partially amorphous. Complete crystallinity may be achieved by annealing the at least partially amorphous layer, preferably for at least 15 (more preferably 20) e.g. 60 minutes, preferably at at least 15 2500C, more preferably at least 3200C, conveniently in air. The phthalocyanine may be, in ascending order of preference, magnesium, iron, cobalt, nickel, copper, zinc, hydrogen and lead. Hydrogen (metal-free) phthalocyanine can be used only if the substrate is relatively non conductive. 20 The substrate may be pressed sintered A1203 or, for thinner layers, may be glass, sapphire, silicon or pressed-disc KBr. In principle the substrate may be any material which has a surface conductance lower than the phthalocyanine film, does not chemically react with the phthalocyanine and is stable to temperatures above 3500C.

Since the sensor may be desired to operate at high temperature, some means of heating the substrate may be associated with it. Platinum-based heaters are preferable since their high temperature coefficient of resistance makes them suitable for measuring temperatures and the high stability of the metal allows high operating temperatures.

The invention will now be described by way of example. Fig. I shows schematically an A120, substrate 3 mm x 3 mm square and 0.4 mm thick, having on one surface 1 a printed platinum heater trimmed to a OOC resistance value of 10:t O-M, the obverse side 2 being an interdigitated platinum electrode system, V four fingers per electrode with a gap between the electrodes of 125pm Films of several of the phthalocyanines were vacuum sublimed over this electrode array in a conventional 'vacuum coating unit. In a preferred case, lead phthalocyanine was used, deposited to a thickness of 4000 nm.

Such substrates are not suitable for Langmuir-Blodgett films which may require film deposition on to smooth surfaces followed by deposition of the electrodes over the phthahlocyanine film. Although other means of film deposition such as from solution would in principle be usable, most of the phthalocyanines are not easily dissolved even in mixed organic solvents.

The deposited film was heated to 3400C for 20 minutes. (3200C at 20 minutes is of borderline reliability, while at 3000C heating must be continued for hours if not days to achieve the desired effect.) It is likely that the heating not only ensures crystallinity of the film and formation of the polymorph but also causes an interaction of the solid surface with atmospheric oxygen, so that the chemical nature of the solid surface is changed and the mechanism of adsorption of strong electron acceptor gases changed. A combination of this crystallisation and interaction and CL4 polymorphic transformation may occur so that the phthalocyanine material is changed both physically and chemically.

Following the heating, the films are lower in conductance and less sensitive, but faster responding to changes in concentration of electron acceptor gases and faster recovering when electron acceptor gases are removed. A gas sensor using these films can have a performance at room temperature better than that of a conventional gas sensor at 1701C. Thanks to this lower temperature of operation, less power is consumed by the gas sensor according to the invention. This makes it more suitable for portable and spot-readings equipment.

Sensors having these annealed films show the following properties:

room temperature ISOCIC minimum detectable > 10Oppb < 10Oppb concentration of NO response time to 90% < 60 sec of steady-state value < 20 sec recovery time to 90% < 60 sec < 20 sec selectivity sensitive to only no sensitivity strongly electron to electron donor accepting gases gases e.g. H2S.

e.g. N02, C12, 03.

effect of water vapour depresses sensitivity no effect The effect of water vapour at lower temperatures may be due to physisorption of water on the film.

4 is

Claims

I - A gas sensor comprising a layer, exposable to gas, of a phthalocyanine on a substrate, and means for measuring the conductivity of the layer, characterised in that the layer is continuous and mostly crystalline.
2. A gas sensor according to Claim 1, wherein the layer is crystallised as the -polymorph.
3. A gas sensor according to Claim I or 2, wherein the layer at least 2000 nm thick.
4. A gas sensor according to Claim 3, wherein the layer is at least 3000 nm thick.
5. A gas sensor according to any preceding claim wherein the phthalocyanine is any one of magnesium, iron, cobalt, nickel, copper, zinc, hydrogen (where the substrate has a high resistance), and lead.
6. A gas sensor according to any preceding claim, wherein the substrate is any one of alumina, glass, sapphire, silicon or pressed-disc KBr.
7. A gas sensor according to any preceding claim, wherein the crystallinity of the phthalocyanine is achieved by heating to a temperature of at least 2500C.
8. A gas sensor according to Claim 7, wherein said temperature is at least 3200C.
9. A gas sensor according to Claim 7 or 8, wherein the heating is in air.

Pnblinhed 1f389 at The Patent 0Mce. State House, 86 71 High Holborn, London WC1R 4TP. Further copies maybe obtained from The P tentomce- - - -- - __------ 11---