Synthesis and gas sensing properties of poly [tetra( pyrrol-l-yl)~ilane]~ Phillip Evans,"Norman M. Ratcliffe,""James R.Smithband Sheelagh A. Campbellb"Department of Chemical and Physical Sciences, Faculty of Applied Sciences, University of the West of England (Bristol), Coldharbour Lane, Frenchay, Bristol, UK BS16 1Q Y Applied Electrochemistry Group, School of Chemistry, Physics and Radiography, Chemistry Division, University of Portsmouth, St. Michael's Building, White Swan Road, Portsmouth, UK PO1 2DT Conducting polymers such as polypyrrole and polythiophene offer a new approach to the design of modified electrodes and sensors. In the current work, the electrochemical and chemical polymerisation of tetra( pyrrol- 1-y1)silane is described. Resultant polymers with different anions have been characterised by electrochemical methods, XPS and microanalysis.Molecular geometry calculations suggest that both inter- and intra-molecular couplings are present in the film. Crosslinking of the polymeric matrix via P-linkages will result in a three-dimensional structure with a concomitant reduction in the degree of conjugation, accounting for the low film conductivity (CT ca. S cm-I). Preliminary results show that poly [tetra(pyrrol-1-yl)silane]is a promising material for the fabrication of gas sensors. It is unexpectedly sensitive to ammonia and trimethylamine gas when compared with polypyrrole and poly(N-methylpyrrole) prepared in a similar fashion. The properties of polypyrrole are being exploited for appli- cations in such diverse fields as cathode materials for rechargeable batteries,' selective membrane electrodes,2 elec- tromagnetic shielding material^,^ selective bio~ensors,~ ion-exchange chromatography resins,' biological markers6 and gas sensors.' There is a need for the syntheses of novel polypyrroles, polythiophenes and other conducting polymers for assess-ment as potential gas sensors for use in 'artificial noses'.Little information on the use of derivatised polypyrroles for gas sensor applications has appeared in the literature. Understanding these materials may offer new insights for molecular electronics and the fabrication of modified electrode^.^,^ Tetra(pyrro1- 1-y1)silane (1) consists of a central silicon atom, tetrahedrally coordinated to four pyrrole rings via the pyrrole nitrogen atoms (Fig.1). This compound was initially investigated as an intermediate for the chemical syntheses of 3-substituted pyrroles, as an extrapolation of the use of triisopropylsilyIpyrrole.'o,ll Electropolymerisation of 1 might be expected to occur predominantly through the a-positions, although some P,p'-couplings may also be present, causing a detrimental effect on cond~ctivity.l~*'~However, for many modified electrode appli- cations, electron transfer occurs at the underlying electrode surface rather than at the conducting polymer per se. Hence, the potentially higher porosity of poly(1) may offer greater scope for the physical entrapment of anions or other immobi- lised material for modified electrodes and sensor applications.In the current work, the electrochemical and chemical Fig. 1 Structure of tetra(pyrro1-1-y1)silane (1) ?Presented at the Second International Conference on Materials Chemistry, MC2, University of Kent at Canterbury, 17-21 July 1995. polymerisation of 1 is described and the application of the resultant polymer as a vapour sensor investigated. Experimenta1 Reagents Dichloromethane (Fisons, AR grade), tetrabutylammonium tetrafluoroborate (TBABF,, Fluka, Puris), lithium perchlorate (LiCIO,, BDH, ACS grade) and silver nitrate (Aldrich, AR grade) were used as received. Acetonitrile (Aldrich, HPLC grade) was distilled over P205and stored over alumina (Woelm N-Super 1).Pyrrole and N-methylpyrrole (Aldrich, AR grade) were redistilled immediately prior to use and stored under a nitrogen atmosphere at 0 "C.Light pttroleum (bp 40-60 "C; BDH, AR grade) was distilled over 4 A molecular sieves and stored over calcium chloride. Organic synthesis Preparation of tetra(pyrro1-I-y1)silane (I).14 Potassium (1.07 g, 0.027 mol) was cautiously added to a stirred cooled (0°C) solution of pyrrole (1.9 cm3, 0.027 mol) in light pet- roleum (bp 40-60°C) under a nitrogen blanket and left for 30 min. The mixture was slowly warmed to 65 "C to allow any residual potassium to react. The solution was then cooled to 0 "C and the white precipitate of potassium pyrrol-1-ide filtered off, washed with light petroleum (bp 40-60°C; 40 cm3) and dried in uucuo (40 "C, 1h, 0.95 mmHg) to yield potassium pyrrol-1-ide (2.23 g, 0.021 mol, 94%).All of this compound was suspended in light petroleum (bp 40-60 "C; 40 cm3) under nitrogen and cooled to 0 "C. Silicon tetrachloride (0.78 cm3, 6.8 x lod3mol) in light petroleum (7 cm3) was slowly added to the stirring mixture over a 20min period. Stirring was continued for a further 2 h and the product was recovered by Soxhlet extraction using light petroleum (bp 40-60 "C) as solvent. Tetra( pyrrol-1-y1)silane was recrystallised from light petroleum (bp 40-60 "C) to yield fine colourless needles (0.44g, 28%), mp 173 "C (uncorrected) (lit.,I4 173.4 "C); G,(CDC13) 6.32 (8 H, m, 8 x PH) 6.68 (8 H, m, 8 x aH); m/z 67 [(C,H,N)+], 80, 94 [(C4H4N)Si+], 106, 132, 146, 159 [(C4H4N)2Si+], 171, 199, 226 [(C4H4N)3Si+], 251, 265, 292, W+).J. Mater. Chem., 1996, 6(3), 295-299 295 Electrochemical studies Electrochemical studies were performed in a three-compart- ment divided cell l5 Platinum (disc area 0385 cm') and indium-tin oxide coated glass (IT0 glass) (thickness 100 nm, surface resistivity < 30 SZ m, Balzers High Vacuum Ltd , Milton Keynes, UK) were used as working electrodes The platinum electrode was polished prior to use with an alumina slurry (0 3 ym) Platinum gauze was used as the counter electrode All potentials were measured against an Ag/AgN03 reference electrode, consisting of a silver wire immersed in a solution of acetonitrile containing silver nitrate (0 01 mol dm-3) and TBABF, (0 1mol dmP3) This solution was separated from the surrounding electrolyte by a glass frit A Hi-Tek DT 2101 potentiostat coupled to a Hi-Tek PPRl wave form generator was used to generate the electrochemical signals Output was recorded on a Lloyd PL3 XYt recorder Charge passed during the experiments was measured with a Hi-Tek integrator Potentiodynamic and galvanostatic tech- niques were used to study electrochemical behaviour All solutions were freshly prepared and degassed with nitrogen for 15 min prior to each experiment For cyclic voltammetry experiments, the monomer together with the supporting elec- trolyte, TBABF, or LiClO,, were dissolved in either acetonitrile or dichloromethane (in the case of 1)and the potential cycled, typically from 0 to 2 V us Ag/Agf and the current monitored as a function of the applied potential For characterisation studies, films were grown from the same electrolyte composi- tions under constant current or potential conditions In the former, currents in the range 0 2-20 mA cm-2 were passed for between 10min to 5 h using a two electrode system and monitoring the potential as a function of time Potentiostatic growth was performed by stepping the potentials (El= 0, Ef= 200-5000 mV us Ag/Agf) Polymer redox behaviour was evaluated by cycling the films in the same electrolytes used for growth, but in the absence of monomer Polymers were grown on IT0 glass from a solution of the monomer (0 005 rnol dmP3) in dichloromethane [poly(l)] con- taining TBABF, (0 1mol dm-3) or acetonitrile (polypyrrole) containing LiClO, (0 1 rnol dmP3) by potential cycling between 0 and 2100 mV at a sweep rate of 5 mV s-l A total of 1 5 cycles were carried out, with termination of the final cycle at 2100 mV thus ensuring full doping of the film Excess supporting electro- lyte on film surfaces was removed by extensive rinsing with dichloromethane and the films dried under a stream of nitro- gen Samples were examined under a Vickers M41 Photoplan optical microscope and a JEOL JSM-35C scanning electron microscope (SEM) Films were gold sputtered for SEM obser- vation and film thickness measurements were made by viewing a cross-section of the film-substrate interface Conductivity measurements were performed by applying a known current through the film and measuring the resulting potential across the film In these experiments, a mercury drop of known area was used as a contact to the film surface Electrical conductivities of chemically synthesised polymers were measured on pressed pellets (5 tonnes) using the four- point-probe technique under dc conditions X-Ray photoelectron spectroscopy (XPS) studies were car- ried out using a VG Scientific ESCALAB Mk I1 instrument Lineshape analysis was performed on each peak in an attempt to resolve the broad signals A1-Ka radiation (1486 7 eV) was used as the X-ray source Binding energies were adjusted so that the main C( 1s) peak occurred at 285 00 eV and atomic percentages were calculated from the peak areas using standard atomic sensitivity factors l6 The atomic percentage for the C1 signal was calculated by overlapping the C1(2p3,,) and C1(2p1,,) peak areas, although binding energies refer to the C1( 2 ~ peak XPS studies of polypyrrole were also carried out for comparison purposes 296 J Muter Chem, 1996,6(3), 295-299 Molecular modelling studies Molecular geometry optimisation calculations were performed using the PM3 semi-empirical program Hyperchem@ Convergence was set to 001, iterations were limited to 50 and a PolakRibiere optimisation algorithm used Preparation of vapour sensors Tetra(pyrro1-1-y1)silane (1)(031 g, 0 001 mol) was dissolved in a few drops of dichloromethane and rapidly added to a stirred solution of copper(I1) bromide (0 91 g, 4 08 x mol) in acetonitrile-dichloromethane (1 1 v/v, 150cm3) then left for 70 min A black precipitate was formed which was collected and washed with copious amounts of acetonitrile until the eluent was clear The recovered polymer was dried zn vacuo (60 "C, 1h, 0 95 mmHg) and stored at 0 "C until required Polypyrrole and poly (N-methylpyrrole) were prepared in an identical fashion Yields poly( 1) 0 30 g, polypyrrole 0 07 g, poly(N-methylpyrrole) 0 08 g Sensor fabrication Sensors were constructed using gold on alumina interdigitated electrodes (GEC-Marconi, Wembley, UK) with a 125 pm gap between the electrode 'fingers' Poly( 1) (0 1 g) was ground in water (0 2 cm3) until a fine paste was obtained which was then transferred to the surface of the electrode and the water was allowed to evaporate in an oven regulated at 60 "C When dry, contacts to the interdigitated array were made using circuit board shell pins connected to lengths of wire Sensors with polypyrrole and poly(N-methylpyrrole) were fabricated as above Each sensor contained an approximate polymer loading of 10mg with a typical thickness of 67 ym (as determined by SEM) Sensing trials The sensing apparatus consisted of the coated electrode sus- pended in a flask of known volume into which a known concentration of analyte gas was introduced by means of a gas syringe Resistance change was monitored manually using a multimeter (Fluke73, Maplin Electronics Ltd ) Sensors were exposed to a range of ammonia and trimethyl- amine concentrations, ranging from 001 to 10% vapour by volume The maximum percentage change in resistance occur- ring during a 1min exposure was recorded The responses recorded were the average of three exposures at each gas concentration Recovery to the original baseline occurred within 20 min of removal from the vapour chamber Results and Discussion Electrochemical polymerisation The electrochemical behaviour of 1 in a TBABF, (0 1mol dm-3~dichloromethane electrolyte is shown in Fig 2 Monomer oxidation resulted in the formation of a smooth, blue-black homogeneous and very adherent film on the elec- trode surface The onset of pyrrole oxidation was observed at 450 mV On the return cycle, a nucleation loop at 745 mV, indicative of the formation of an electroactive film, was observed and a very broad reduction peak, corresponding to polymer dedoping, was present at 0 mV Increased currents for both processes were seen on the second scan, although by the third, these began to decrease The location of the polymer oxidation signal was not obvious but was thought to be under the broad monomer oxidation wave However, when the film was cycled in the same electrolyte in the absence of monomer, ~~)~only large capacitive currents were observed with no indication of redox behaviour The absence of a polymer oxidation peak has also been reported for 3-trimethyl~ilylthiophene~~~and In -0.5 I I I f -1000 -500 0 500 1000 E vs.AglAg' /mV Fig.2 Cyclic voltammogram of 1.Working electrode Pt disc (area 0.385 cm2); monomer conc.0.005 mol dm-3; v = 5 mV s-'; electrolyte 0.1 mol dm -3 TBABF,-dichloromethane; first scan (-), second scan (----), third scan (--. -.-)* this case was partly attributed to the high solubility of the polymer in its reduced form. Polymer characterisation Electrochemically polymerised 1 exhibited a conductivity of S cm-l with no enhancement in conductivity observed when the film was placed in an atmosphere of iodine for 24 h. The low conductivity observed together with the absence of redox behaviour suggests some disruption in the conjugation of the polymer which may be due to non-a,d-linkages between different pyrrole rings in adjacent monomers.The film could be removed from the electrode surface by abrasion, but was insoluble in acetone, ethanol, dichloromethane, THF, toluene and water. Fig. 3 shows an SEM image of poly(1) on IT0 glass. The film is essentially very smooth although higher regions appear to have a coral-like topography, quite different to that of p~lypyrrole.'**~~ Microanalysisof the chemically synthesised polymer Microanalysis data of chemically synthesised poly( 1) and polypyrrole are shown in Table 1. For polypyrrole, the C:N ratio was calculated to be 4 : 1, in agreement with the stoichio- metric ratio, with a Br :N ratio of 1:1, suggesting incorporation of one bromine for every monomer unit. In the case of poly( l), Fig. 3 SEM micrograph of the polymer formed from the electropolym- erisation of 1 on IT0 glass Table 1 Mi~roanalyslsdata of 1, poly( 1) and polypyrrole atom% compound element sample 1 sample 2 avcrage polypyrrole C H 28 55 120 28.25 1.16 28.40 1.18 8 29 8.26 8.28 51.17 50.64 50.91 0 22 0.22 0.22 41.53 41.72 4 1.62 3.03 3.05 3.04 11.05 11.16 11.11 19.14 18.97 1906 14.22 14.44 14 33 "Synthesised using copper(I1) bromide as oxidant.C :H and C :N ratios of 1.1:1 and 4.3 : 1 were obtained with a Br :N ratio of 1 :3.3, showing a similar dopant level to that measured for polypyrrole. X-Ray photoelectron spectroscopy studies of the electrochemically and chemically synthesised polymers The binding energies for carbon, boron (from the dopant tetrafluoroborate anion), nitrogen, oxygen and silicon observed in the XPS spectrum of the electropolymerised poly(1) film, together with atomic percentages, are summarised in Table 2. The degree of polymer doping can readily be obtained from the atomic percentage ratio of the N(1s) peak to that of the B( 1s) peak assuming four nitrogen atoms per monomer unit.Thus, it would appear that 2.1 BF,- anions are incorporated into the film for every monomer unit i.e. approximately one anion for every two pyrrole rings. The O(1s) signal at 532.50 eV can be attributed to the presence of carbonyl species, formed as a result of oxidation of the film.20.2' Two N(1s) signals were observed in the XPS spectrum of the electropolymerised film.These are significantly different from those previously reported for polypyrroles22 whereby a single signal may be resolved into a number of smaller overlap- ping signals. Since the atomic percentage ratios of the combined N( 1s) signal to the Si(2p) peak is 3.88, i.e. within experimental error of the theoretical value of 4.0 for that of the monomer, it was assumed that the nitrogen peaks are due to the pyrrole rings, not to tetrabutylammonium cation incorporation. The two nitrogen signals must therefore result from pyrrole rings in different chemical environments, although the exact nature of these is uncertain. One suggestion is the occurrence of inter- and intra-molecular couplings via the pyrrole rings, to form the polymer shown in Fig.4.The atomic percentage ratio of the two nitrogen peaks is close to 1 :1, in agreement with this structure. In addition, further oxidation reactions may then Table 2 XPS data of the film formed from the electropolymerisation of 1 and pyrrole ~~~~~ ~ ~ polyp yrrole POlY(1 ) signal binding energy/eV atom% binding energy/eV atom% --193.90 3.1 207.85 3.4 284.15 13.9 283.80 5.5 285.00 25.6 285.00 46.0 286.20 17.5 286.45 13.8 288.45 5.7 288.55 1.9 -291.75 0.9 400.30 4.7 399.95 2.8 401.10 4.2 402.40 3.2 403.30 1.8 101.75 1.6 532.80 19.9 532.50 7.2 -534.85 2.3 3. Mater. Chem., 1996, 6(3), 295-299 297 L -In Fig. 4 Proposed idealised structures for poly( l), polypyrrole and poly (N-methylpyrrole) occur via a-or P-linkages in the remaining uncoupled pyrrole rings leading to the formation of a three-dimensional structure.XPS of the chemically oxidised polymers was carried out to detect copper, the presence of which might affect sensor response. None was found in either poly( 1) or polypyrrole, whilst a small quantity, corresponding to 0.38 atom%, was detected in poly(N-methylpyrrole). This supports our suppo- sition that entrapped copper plays no part in the response of the polymers to volatile amines. Molecular modelling Molecular geometry optimisation calculations show four pyr- role units tetrahedrally positioned around the central silicon atom, with the plane of each neighbouring pyrrole ring oriented in such a way as to overcome steric hindrance between adjacent hydrogen atoms [Fig.5(a)]. This three-dimensional represen- tation of the molecule emphasises the availability of the pyrrole a-positions for monomer couplings. When two neighbouring pyrrole rings in 1 are intramolecularly coupled via their a-positions [Fig. 5(b)],no change in the enthalpy of formation (92 kJ mol-l) of this new structure results. This suggests that both intra- and inter-molecular couplings are energetically favourable in the polymerisation reaction. Thus, the polymer is likely to be highly crosslinked, a conclusion supported by electrochemical studies which show significantly reduced con- ductivity compared with polypyrrole. If the two remaining pyrrole rings are coupled in an intra- molecular fashion via the a&-positions, then a large increase in energy, to 167 kJmol-l, is observed due to strain on the tetrahedral geometry of the central silicon atom.Thus, we conclude that only one a,a'-intramolecular coupling may be present per monomer unit. Preliminary sensing trials Fig. 6(a) and (b)show response profiles of the three polymer sensors, polypyrrole, poly(N-methylpyrrole) and poly( l), to ammonia and trimethylamine vapours, respectively, in the concentration range of 0.01 to 10% vapour by volume. It can be seen that in both cases poly(1) yields a greater response than either of the other sensors. This is unexpected since our work and a previous report23 show that N-substituted pyrroles give a diminished response to ammonia vapour when compared with polypyrrole.A mechanism of H+ abstraction from the --NH of the pyrrole units by ammonia has been proposed as the basis for the reversible interaction between polypyrrole and ammonia.24 However, this mechanism cannot be the only process occurring and in the case of poly (N-methylpyrrole), the sensor response must be due to an interaction of the vapour with the charged polymeric backbone itself. From the XPS and microanalyses data we find no evidence to support any suggestion that entrapped copper ions may play any part in the responses observed. The reason for the high response of 298 J. Muter. Chem., 1996, 6(3),295-299 H P H Fig. 5 Molecular geometry of (a) 1 and (b) 1 containing one u,u'-pyrrole coupling poly( 1)to ammonia and trimethylamine is unclear at present.Steric arrangement of pyrrole rings in poly( 1) may permit greater interaction of the amine nitrogen electronlone pair with the charged polymeric conjugated backbone. This process may also be facilitated by the higher porosity of this polymer compared with polypyrrole, which would allow increased penetration of the analyte gas into the polymer matrix, thereby maximising its response. Conclusions Electropolymerisation of tetra( pyrrol-1-y1)silane results in the formation of a smooth and very adherent film on IT0 glass with a conductivity of S cm-l. These films appear to be highly doped with incorporation of approximately two BF4-anions per monomer unit. Molecular geometry calculations suggest that both inter- and intra-molecular couplings are present in the film.A three-dimensional structure is proposed in which fi-linkages are also present thus reducing the degree of conjugation and hence overall film conductivity. Preliminary results show that chemically prepared poly [tetra( pyrrol-1-yl)silane] is a promising material for the fabrication of gas sensing materials. It shows a superior response to ammonia and trimethylamine when compared with polypyrrole and poly(N-methylpyrrole) prepared in a similar manner. Such a sensor has potential application in a number of fields such as monitoring odours from agricultural buildings and activitie~,~~ food freshness monitoring26 and as a component of an electronic nose based sensor array system.27 !?! -t2 01 I I 0.1 1 2$ 100 0.01 K c-2 80 60 40 20 0 I I 0.1 1 10 vapour concentration (96) Fig.6 Response profiles of the three polymer sensors to a range of vapour concentrations of (a) ammonia and (b) trimethylamine.Poly( 1); 0poly(N-methylpyrrole); + polypyrrole. The authors thank Dr R. Ewen at the University of the West of England (Bristol) for the use of XPS facilities, Mr M. West (University of Bristol) for microanalyses and the EPSRC for financial support. References 1 B. 2. Lubentsov, G. I. Zvereva, Ya. H. Samovarov, S. M. Bystriak, 0.N. Timofeeva and M. L.Khidekel, Synth. Met., 1991,41, 1143. 2 P. C. T. Wong, B. Chambers, A, P, Anderson and P V. Wright, IEEE Con& Publ., Antennas and propagation, 1993,370,part 2,934.3 E. V. Thillo, G. 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