J. Chem. Soc. Faraday Trans. I 1986,82 11 17-1 126 Conducting Polymer Gas Sensors Jan J. Miasik,? Alan Hooper and Bruce C. Tofield* Materials Development Division AERE Harwell Oxfordshire OX1 1 ORA Recent results with solid-state semiconductor gas sensors based on organic sensor elements are reviewed. Devices based on metal phthalocyanines show useful responses to NO,. Lead phthalocyanine combines the highest conductivity with the maximum sensitivity to NO,. A thin-film lead phthalocyanine sensor has successfully been used to monitor NO produced by shot-firing in coal mines. To obtain reasonable conductance and speed of response and recovery phthalocyanine sensors have been operated at 170 "C. Conducting polymer materials and particularly chemically doped polypyrrole show responses to toxic gases at ambient temperature.Initial work using polypyrrole black impregnated filter paper showed a response to ammonia. More recently using polypyrrole films electrochemically deposited over electrode arrays responses to nitrogen dioxide and hydrogen sulphide have also been obtained. Organic-semiconductor gas sensors may have advantages compared to metal-oxide devices in their sensitivity to toxic gases and in their ability to operate at or near room temperature. However the mechanisms of device function are not yet well understood. Increasing concern with environmental and personal protection together with widespread requirements for more accurate process control has created a need for new or improved sensors for measuring both physical and chemical parameters.This need for better sensors is strongly influenced by the increasing use of intelligent microelectronics for monitoring and control. Prominent among the requirements are sensors for a wide range of gases from organic and inorganic pollutants which must be measured at the parts per million level or lower through to flammable gases such as methane and hydrogen or gases such as water vapour and CO, monitored for control of environmental comfort which must be measured at the level of 1 % or more. While many methods of gas sensing have been developed in the laboratory environment and some have found commercial application there remains over a wide range of both applications and gases to be measured a continuing requirement for specific and sensitive gas sensors which combine durability and stability of response with cheapness.From life-support systems through food processing industrial drying and monitoring of chemical plant to vehicle exhaust pollution control the frequent lack of cheap and reliable gas sensors inhibits the installation and use of the most cost-effective control and monitoring systems. The semiconducting gas sensor where the resistance of a porous pellet or thin film of a metal oxide such as SnO or ZnO can be sensitive to the presence of combustible or toxic gases in air has emerged in recent years as a particularly economical solution to certain gas monitoring app1ications.l While some applications for sensing of toxic gases have been proposed for metal oxide semiconducting gas sensors the principal applications at present are in the detection of flammable gases such as H, CO and CH, at relatively high concentrations to protect against fire or explosion.A large market has t Also at Department of Chemistry University of Kent Canterbury. 1117 1118 Conducting Polymer Gas Sensors been developed in Japan in particular where a high proportion of homes are equipped with flammable gas detectors based on SnO,. The principles and properties of such devices have recently been reviewed. Although such devices are cheap and straightforward in application current designs which are based mainly on SnO, suffer from certain shortcomings which militate against their wider application. Prominent among these are their relative lack of sensitivity to toxic gases at low concentrations their sensitivity to ambient moisture and their inherent lack of specificity; this last disadvantage can be overcome to some extent by careful control of microstructure and use of incorporated catalysts.Finally a serious dis- advantage is the requirement in most applications for operation at 300 "C or above. The power drain required entails that battery operation is only practical for intermittent use. However because of the sensor drift which is observed in practice on heating and cooling such on-off operation is not generally practicable. In the majority of applications therefore it is necessary to power devices continuously from the mains. A wide range of applications including portable personal monitors and all uses where mains power is not available are therefore inaccessible with present technology.The development of semiconducting gas-sensing materials which could be sensitive to a wider range of gases and particularly to toxic gases which could show significant sensitivity at low concentrations and could also operate closer to ambient temperature would permit consideration of this type of device for a much wider range of applications than is satisfied at present. In this paper we describe results on semiconducting polymeric organic materials which show some promise for the detection of toxic gases close to room temperature. Phthalocyanine Sensors Several organic materials have been shown to exhibit resistivity changes when exposed to various gases.The class which has been investigated most fully in recent years is the metal phthalocyanines. Although conducting polymers of the type discussed below can be synthesised from phthalocyanine (Pc) prec~rsors,~ gas-sensing studies have concen- trated mainly on unpolymerised metal phthalocyanines which can be vacuum sublimed on to substrates to form thick-film sensor structures. In air hydrogen and metal phthalocyanines are poorly conducting p-type semiconductors. On exposure to highly electrophilic gases such as NO, significant conductance increases are observed for devices operated above 100 "C. Hydrogen and several metal phthalocyanines are sensitive to NO, but lead phthalo- cyanine combines the highest sensitivity with the highest conductance permitting measurement of NO down to 1 ppb in air.4 H,Pc shows a similar sensitivity to NO changes but has an order of magnitude lower conductance making measurements at low NO partial pressures more difficult.PbPc gas sensor construction has been described4 and the response of different metal phthalocyanines has been detem~ined.~ The responses of several different metal phthalocyanines to changes in NO partial pressures at 170 "C are shown in fig. 1. PbPc and ZnPc are more sensitive than the transition-metal phthalocyanines tested. MgPc (not shown) is very insensitive. Neither the mechanism of gas response nor the understanding of such variations in sensitivity in terms of the electronic structure of the solid and the gas-solid interactions are presently well understood.It is believed that the NO molecules replace at least some of the adsorbed 0 molecules present on the Pc surface in air. Some of the complexities of the absorption process revealed through conductivity changes have been reported.6 While it seems fairly clear that the effects result from surface absorption rather than bulk equilibration with NO, much more must be done before there is a clear understanding of the nature of the active sites and the effect of surface structure on the response. Nevertheless while a lack of such detailed understanding of sensor response is clearly 1119 J. J. Miasik A . Hooper and B. C. Tofield 3 x 10-9 COPC A NiPc 100 / 10 concent rat ion NO2 / ppb Fig. 1. The variation of conductance with NO concentration for metal phthalocyanines at 170 "C [after ref.(5)]. a barrier to ultimate device optimisation it has never been a bar to the development of gas-sensor technology and an industrial application of PbPc NO sensors has recently been described. In mines it is necessary to identify fires or indicators of fires at the earliest opportunity. This can be achieved by using a products of combustion (POC) sensor based for example on a metal-oxide semiconductor. However there is a possibility of interference from gases produced either from explosives used in shot-firing or from diesel engine exhausts. The higher temperatures characterising the latter events however result in enhanced NO production compared to that in fires and an NO sensor in combination with a POC sensor may be used to distinguish the routine event from the fire.Such an application has been demonstrated' in a U.K. coal mine using a tube bundle system and an array of sensors to sense inflammables POC NO and NO,. While the NO sensor was not sensitive to shot-firing events because of the strong absorption of NO on the walls of the tube bundle system the NO sensor which consisted of a PbPc sensor immediately behind a CrO oxidiser (which converted NO to NO immediately in front of the device) was clearly sensitive to shot-firing operations while being totally unaffected by the presence of products from a fire. The PbPc sensors were reported to retain both stability and sensitivity over a period of more than six months of continuous operation.Phthalocyanine gas detectors can indeed monitor inorganic pollutants such as NO,. They are sensitive only to highly electrophilic gases such as Cl, F and BF,. NO sensors are therefore insensitive to the other gases which may be present in a typical sensing environment for example flammable gases products of combustion or molecules such as HCl SO and H,S. Other advantages of phthalocyanine sensors are their ease of preparation and their fairly long-term stability. Disadvantages are their low conductance long recovery times even for operation around 170 "C and the memory and history effects which can be observed. The latter may be ameliorated if a better understanding of the device function is obtained. High-temperature operation is necessary both to obtain sufficient conductance and acceptable speed of response and recovery for devices as conventionally prepared.Although phthalocyanines are stable to temperatures well above 170 "C the appreciable volatility observed above this temperature limits their use at higher temperatures. In any case as observed above operation above 100 "C is undesirable in many applications in 1120 polyacetylene (PA) Conducting Polymer Gas Sensors polyphenylene sulphide ( P W poly t hiop hene (PT) H Fig. 2. Backbone repeat units of some bond-alternant polymers. High conductivity may be induce by negative-ion doping to create positive defects on the polymer backbone. Some polymers ca also be doped by positive ions. view of the power-drain required for sensor heating and the inability to operatt continuously via battery operation.More rapid response and recovery has been demonstrated8 for NO sensing at roon temperature using asymmetrically substituted CuPc films laid down by Langmuir- Blodgett techniques. No demonstrations of the viability of such very thin film device$ over extended periods of operation have however yet been reported. Conducting Polymer Sensors Since the discovery of a relatively straightforward synthetic route towards thin films of polyacetyleneg and the discoverylO that semiconducting polyacetylene could be converted into a very highly conducting form by chemical doping with iodine or other ionic dopants there has been considerable and growing interest in such doped conducting organic polymers.While original interest was directed particularly at the possible application of such materials as reversible electrodes in alkali-metal batteries,ll several other potential applications may be envisaged. These include electromagnetic interference shielding gas sensors displays conducting plastics capacitors junction devices and metals replacement. The lack of processability of most of these materials in contrast to conventional plastics is presently a significant barrier to exploitation. Many organic monomers may be polymerised to yield a linear carbon-carbon bond backbone with alternating single and double bonds. Many such materials have now been demonstrated to show large conductivity enhancements on chemical doping although most work to date has been carried out with relatively simple repeat units based on easily available monomer precursors (fig.2). The work on gas sensing reported in this paper has been conducted using polypyrrole. Doped conducting polypyrrole is one of the most stable conducting polymers in ambient environments. The semiconducting un-doped polymers can be synthesised in a number of ways for example by catalytic or chemical oxidation of the monomer or by elimination from a polymeric precursor. Doped material may also be prepared directly in the conducting state for example by reaction of the monomer with NOPF or analogous salts (and elimination of NO). Plasma polymerisation techniques may also be used. The chemical doping of the polymers may be achieved by direct reaction with oxidising species to produce an insertion material containing negative ions.Charge compensation is achieved Plate 1 J . Chem. Soc. Faraday Trans. 1 Vol. 82 part 4 Plate 1. Sensor structure consisting of doped polypyrrole electrochemically synthesised on to interdigitated gold electrodes screen-printed on an alumina substrate. J J. MIASIK A. HOOPER AND B. C. TOFIELD (Facing p . 1 12 1) 1121 doped copper aluminium graphite trans PA J . J . Miasik A . Hooper and B. C . Tofield si I icon conductive resins (Al-flake-filled resins) semi-insulating resins e.g. carbon-fi lled plastics) I comparison materials ’filled‘ conductive polymers PPP pps undoped conducting polymers Fig. 3.Comparison of the resistivities of doped and undoped conducting polymers with reference materials and with ‘filled ’ conductive polymers such as Al-flake-filled resins and carbon-loaded plastics. by the creation of positively charged defects on the polymer backbone leading to a large increase in electrical conductivity. Examples of such reactions are iAsF + PPS -+ [PPS+AsF,] + AsF,. Some materials e.g. polyacetylene or polyparaphenylene may also be negatively doped to a highly conducting state by insertion of positive ions such as Li+ or Na+. The doping levels generally achieved are much higher than those used in inorganic semiconductors such as silicon. Dopant ion to monomer repeat ratios can vary typically from 1 100 to 1 3 depending on the material under study and the preparative conditions.Study of the details of the mechanisms of conductivity enhancement is still a topic of active research.12 Note that while high values of conductivity up to lo3 S cm-l have been observed in doped polyacetylene typical values observed in materials such as doped polypyrrole often fall in the range 1-100 S cm-l. The conductivities observed are therefore more comparable to those of ‘ filled’ conductive polymers such as Al-flake-filled resins rather than the much higher values of simple metals such as copper and aluminium (fig. 3). It is unlikely that doped conducting polymers which may be characterised as being very good semiconductors have sufficiently high conductivities to find many applications as direct replacements for metals.For their evaluation as gas sensing elements it is convenient to study materials in thin-film form laid down over interdigitated electrodes. In this case direct electrochemical synthesis of the doped conducting polymer is most convenient. Electrochemical oxidation of the polymer directly on to the electrode substrate permits good control of growth rate and film thickness facile incorporation of a range of counter ions and permits the stoichiometry of the doped material to be modified in a relatively straightforward fashion. We have generally synthesised polypyrrole from aqueous solution although other materials such as polythiophene must be synthesised from organic solvents such as acetonitrile. In sensor work BF; introduced from LiBF in solution during synthesis is a suitable counter ion.A black polypyrrole layer laid down on an interdigitated gold electrode screen-printed on an alumina substrate is shown in plate 1. The fairly close electrode spacing permits growth of the polymer film across the inter-electrode gap so producing a sensor 1122 Conducting Polymer Gas Sensors structure. The conductance of materials such as BFT-doped polypyrrole in thin-film sensor form is sufficient at ambient temperature to make conductance monitoring of gas sensor response quite straightforward. While the general principles covering the formation and properties of conducting polymers are understood the materials as synthesised are often rather poorly character- ised. Chain lengths and molecular weights are often quite difficult to establish as is the degree of cross-linking which is generally thought to be present in almost all materials.Most materials particularly in the doped form are amorphous so that characterisation of the molecular structure is difficult. Many materials and particularly those synthesised electrochemically or catalytically are fairly porous but the nature and extent of the porosity and microstructure is generally not well known neither is the homogeneity of materials at the macromolecular level. With such poor understanding of the chemical compositional and structural characteristics of most materials it is not surprising to find that understanding of the surface chemistry particularly in terms of conductance mechanisms and gas absorption sites is more or less non-existent.The gas-sensing properties of polypyrrole were demonstrated by Nylander et al. l3 using a sensing element consisting of pyrrole black impregnated filter paper. Pyrrole is readily oxidised even in air at ambient temperature and a conducting polymeric form may be readily prepared by chemical oxidation. Nylander et al. used acidified hydrogen peroxide to precipitate pyrrole black onto the filter paper substrate. Although known for many years pyrrole black is poorly characterised but in preparations from HC1 presumably contains an oxidised polypyrrole backbone with Cl- counter ions. It was discovered that the resistance of the pyrrole black containing filter paper was sensitive to the presence of ammonia either in argon or in oxygen-argon mixtures.Sensor operation could be obtained at room temperature and at high ammonia concentrations (0.5-5% ) an almost linear response was obtained with typically 30% resistance change per one percent of ammonia concentration. Response times of < 1 min were observed at these highconcentrations but this increased significantly at lower ammoniaconcentrations in the ppm range although the sensitivity was higher at the lower concentration range. Several other gases including H, CO CO, CH and several aromatics yielded no response. The device was sensitive to other amines giving rise to large reversible responses as with ammonia. A certain sensitivity to moisture possibly resulting from NH,OH formation from reaction between NH and the N-hydrogen was observed.Both pyrrole black and the metal phthalocyanines are p-type semiconductors. The exposure of electrophilic gases such as NO to the phthalocyanines acts to withdraw electron density from the Pc network so increasing the conductance. Electron-donating gases such as ammonia if adsorbed have an opposite effect in that the carrier density will be reduced. Hence there is a resistance increase of conducting polypyrrole on exposure to ammonia. Such an explanation of the response is necessarily highly qualitative at the present stage of understanding. The work with pyrrole black and in particular the interesting responses which could be achieved at room temperature have encouraged us to pursue the characterisation of polypyrrole gas sensor response using electrochemically prepared films on interdigitated electrodes (plate 1).Preliminary results are reported here. We show that such sensors are indeed responsive to ammonia in line with the earlier work but that responses to other gases can also be obtained under suitable conditions. We demonstrate response both to NO and H,S. All work so far has been conducted at ambient temperature using films prepared from aqueous solution. The qualitative response of a device sensitive both to ammonia and NO is shown in fig. 4 where it will be seen that the resistance changes are as expected increasing with exposure to ammonia and decreasing in NO,. Depending on the preparative conditions devices may be made sensitive to a particular gas and insensitive to others although a detailed understanding of the factors affecting selectivity is not yet to hand.1123 5 Fig. 4. Typical qualitative response characteristic at room temperature of a conducting polypyrrole sensor to pulses of (a) NH and (b) NO in air. J . J. Miasik A . Hooper and B. C. Tofield 4 NH3 on 60 75 105 30 15 90 15 120 0 135 45 10 timelmin time/min Fig. 5. Resistance changes at room temperature for a conducting polypyrrole sensor to 15 min pulses of 0.1 % NH in air. For the screening of gas responses we have found it convenient to operate in a mode where gas pulses are supplied at the 0.1 % level in air for periods of 15 min on and 15 min off. Gas mixing and data collection are under computer control. Examples of responses obtained for one particular sensor treated to be sensitive to ammonia H,S or NO are shown in fig.5-7. Response is very rapid in all cases although equilibrium values seem to be achieved only in the case of H,S within this time period. Although there is some evolution in baseline resistance experiments over longer periods of up to a day show good reversibility of sensor response (fig. 8). 1124 620 61 0 g 600 \ 5 590 Y 1- 580 570 1 90 560 I 0 Conducting Polymer Gas Sensors I I 60 time/min 30 Fig. 6. Resistance changes at room temperature for a conducting polypyrrole sensor to 15 min pulses of 0.1 % NO in air. Fig. 7. Resistance changes at room temperature for a conducting polypyrrole sensor to 15 min pulses of 0.1 % H,S in air.The response curve as a function of gas concentration has not yet been completely determined for any of the gases studied but responses at three different ammonia concentrations (fig. 9) show as expected an increasing response as the concentration is raised from 0.01 % through 0.1 to 1 % in air. 1125 1.5 I I I I 20 0 ' 0 2 4 18 22 J. J. Miasik A . Hooper and B. C . Tofield I I I I I 6 16 14 12 1 8 10 time/h Fig. 8. Resistance changes at room temperature for a conducting polypyrrole sensor to 15 min pulses of 0.1 % NH in air over a period of 20 h. Conclusions The recent work at Harwell outlined in this paper confirms the sensitivity of conducting polypyrrole to ammonia and demonstrates also that the material can be sensitive to other gases.Results have been presented for NO and H,S. A thin-film device structure on a conventional interdigitated electrode array printed on a ceramic substrate has been demonstrated. 1126 The sign of the resistance change observed in the presence of ammonia and NO is consistent with the p-type behaviour of polypyrrole. The response observed with H,S is the same sign as that with NO,. Although the resistance of polypyrrole as currently prepared does increase somewhat over extended periods in air initial results to date indicate good reproducibility of sensor results over periods of several hours. The material is also very stable against chemical attack and sensor response is maintained after immersion for example in strong acid or strong alkali.References Conducting Polymer Gas Sensors Along with phthalocyanine detectors for NO sensing the results to date on polypyrrole clearly indicate that doped conducting polymers have interesting potential for sensing toxic gases such as ammonia NO and H,S. Of particular attraction is the lack of interference by flammable gases such as H, CO and CH, or by CO, and the fact that devices can be operated at room temperature. Nevertheless our understanding of device function is extremely primitive compared to the situation with metal-oxide devices such as those based on SnO,. To provide a better understanding which may be necessary to assist development of commercial devices it will be necessary to characterise more fully not only device structure but also the properties of the polypyrrole which may affect its gas sensitivity.These include most of the areas of generaluncertainty in our understanding of conducting polymers mentioned above as well as the nature of the gas-polymer interaction. The effect of moisture mentioned in the earlier work must also be clarified. The analogous requirements for phthalocyanine sensors have been discussed above. If progress in these areas can be made there is no doubt that both phthalocyanine and conducting polymer based devices offer much promise as resistance modulating gas sensing elements for toxic gases. They are thus very complementary to metal oxide devices which are most useful in the detection of flammable gases. Conducting polymer devices have the added attraction of being able to operate at or close to ambient temperature.We gratefully acknowledge the support received for work on conducting polymers from the Department of Trade and Industry. The assistance of P. Taylor with the experimental work on conducting polymer gas sensors is also gratefully acknowledged. 1 J. Watson and R. A. Yates Electron. Eng. 1985 May 47. 2 P. T. Moseley and B. C. Tofield Muter. Sci. Technol. 1985 1 505. 3 T. J. Marks Science 1985 227 881. 4 B. Bott and T. A. Jones Sensors and Actuators 1984 5 43. 5 T. A. Jones and B. Bott in Transducers '85 (1985 International Conference on Solid-state Sensors and Actuators) Digest of Technical Papers (IEEE 1985) pp. 414-417 6 J. D. Wright A. T. Chadwick B. Meadows and J. J. Miasik Mol. Cryst. Liq. Cryst. 1983 93 315. 7 B. Bott and T. A. Jones ref. (9 pp. 128-131. 8 S. Baker G. G. Roberts and M. C. Petty ZEE P r x . 1983,130,260. 9 H. Shirakawa and S. Ikeda Polym. J. 1971 2 231. 10 P. J. Nigrey A. G. MacDiarmid and A. J. Heeger J. Chem. Soc. Chem. Commun. 1979 594. 11 D. MacInnes Jr M. A. Druy P. J. Nigrey D. P. Nairns A. G. MacDiarmid and A. J. Heeger J. Chem. Soc. Chem. Commun. 1981 317. 12 A. J. Heeger Philos. Trans. R. Soc. London Ser. A 1985,314 17. 13 C. Nylander M. Armgarth and I. Lundstrom Proc. Zntl. Meeting on Chemical Sensors Fukuoka (1983) ed. T. Seiyama K. Fueki J. Shiokawa and S. Suzuki (Elsevier Amsterdam 1983) pp. 203-207. Paper 5 / 1884; Received 2 1st October 1985