Gas Sensitivity Measurements on NO2 Sensors Based on Copper(II

Feb 5, 1999 - Istituto per lo studio di nuovi Materiali per l'Elettronica (IME-CNR), Via Arnesano, 73100 Lecce, Italy, and Dipartimento di Scienza dei...
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Langmuir 1999, 15, 1748-1753

Gas Sensitivity Measurements on NO2 Sensors Based on Copper(II) Tetrakis(n-butylaminocarbonyl)phthalocyanine LB Films S. Capone,† S. Mongelli,‡ R. Rella,*,† P. Siciliano,† and L. Valli‡ Istituto per lo studio di nuovi Materiali per l’Elettronica (IME-CNR), Via Arnesano, 73100 Lecce, Italy, and Dipartimento di Scienza dei Materiali, INFM, Universita` degli Studi di Lecce, Via Monteroni, 73100 Lecce, Italy Received May 26, 1998. In Final Form: November 16, 1998 The NO2 gas-sensing characteristics of chemiresistors in the form of multilayered Langmuir-Blodgett films of a symmetrically substituted phthalocyanine, containing on the periphery four amidic groups -CONH-, have been studied. Floating layers were spread onto the water surface from a chloroform solution and were transferred onto both hydrophilic and hydrophobic quartz substrates using the vertical lifting method. Response and recovery times have been measured for different working temperatures at a fixed NO2 concentration. Dynamic response characteristics of the electrical conductance of the LB films to different NO2 concentrations, carried out in dry air, have shown a high sensitivity to concentrations of nitrogen dioxide smaller than 20 ppm at room temperature. All measurements have been carried out using coplanar configurations of the devices.

Introduction Recently, efforts toward the development of simple, inexpensive, and reliable devices have been increased with the aim to control air pollution and to detect toxic or badsmelling gases at low levels in the air, in the field of domestic and industry applications. One of the most simple and reliable methods to detect gases is by measuring the change in electric conductivity induced by the adsorption of gas molecules on the surface of organic or inorganic semiconductors. In the case of organic semiconductors the conductivity change is often caused by the change in charge-carrier concentration due to the donor-acceptor states arising from the gaseous adsorption. To this purpose, considerable attention has been paid by various authors to the study of the optical and electrical properties of metal-substituted phthalocyanines,1 typical chargetransfer complexes with semiconductor behavior.1-5 Therefore, the incorporation of the phthalocyanine macrocycle as the active layer in organic film gas sensors has became a matter for an enormous research effort. Phthalocyanine thin films can be produced by a variety of techniques, such as vacuum sublimation, electron-beam evaporation, solvent evaporation from a fine suspension, spin coating, and Langmuir-Blodgett (LB) deposition.6-9 The LB deposition technique plays a crucial role in providing * Corresponding author. E-mail: [email protected]. Fax +39 832 325299. † Istituto per lo studio di nuovi Materiali per l’Elettronica. ‡ Universita ` degli Studi di Lecce. (1) Phthalocyanines, Properties and Applications; Leznoff, C. C., Lever, A. B. P., Eds.; VCH: NewYork, 1989-1996; Vols. 1-4. (2) Rella, R.; Siciliano, P.; Manno, D.; Serra, A.; Taurino, A.; Tepore, A.; Valli, L.; Zocco, A. Sensors Actuators 1997, 44, 585. (3) Hua, Y. L.; Jiang, D. P.; Shu, Z. Y.; Petty, M. C.; Roberts, G. G.; Ahmad, M. M. Thin Solid Films 1990, 192, 383. (4) Nieuwenhuizen, M. S.; Nederlof, A. J.; Barendsz, A. W. Anal. Chem. 1988, 60, 230. (5) Bott, B.; Jones, T. A. Sens. Actuators 1984, 5, 43. (6) Cook, M. J. J. Mater. Chem. 1996, 6, 677. (7) Roberts, G. G. Langmuir-Blodgett Films; Plenum Press: New York, 1990. (8) Ulman, A.; An Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to Self-Assembly; Academic Press: Boston, MA, 1991.

molecular ultrathin films of phthalocyanine (Pc) derivatives which have been attracting intensive interest owing to their practical applicability for molecular devices. Among metal phthalocyanines, copper phthalocyanine and lead phthalocyanine have been investigated most extensively and are known to be useful in the detection of strongly electrophilic gases and, in particular, NO2. We have already studied the electrical, optical and structural properties of copper(II) tetrakis(alkoxycarbonyl)phthalocyanine LB films.10 The relative ease in changing the ordering and electrooptical properties of the films by the chemical synthesis of phthalocyanine derivatives with new lateral substituents has brought us to explore the possibility to use substituted phthalocyanines containing the amidic group (-CONH-) as LB materials; in previous works, phthalocyanines containing four pendant ester groups (-COO-) on the periphery proved excellent materials for the LB deposition.10-15 Moreover, to the best of our knowledge, in the field of thin films the class of symmetrically substituted metal tetrakis(alkylaminocarbonyl)phthalocyanines has been the subject of a few works.16-20 The molecular formula of the phthalocyanine dealt with in our study is illustrated in Figure 1. Copper(9) Petty, M. C.; Langmuir-Blodgett films, an introduction, Cambridge University Press: Cambridge, England, 1996. (10) Pasimeni, L.; Meneghetti, M.; Rella, R.; Valli, L.; Granito, C.; Troisi, L.; Thin Solid Films 1995, 265, 58. (11) Manno, D.; Rella, R.; Troisi, Valli, L. Thin Solid Films 1996, 280, 249. (12) Pasimeni, L.; Segre, U.; Toffoletti, A.; Valli, L.; Marigo, A. Thin Solid Films 1996, 284-285, 656. (13) Rella, R.; Serra, A.; Siciliano, P.; Tepore, A.; Valli, L.; Zocco, A. Supramol. Sci. 1997, 4, 461. (14) Wilde, J. N.; Petty, M. C.; Saffell, J.; Tepore, A.; Valli, L. Meas. Control, 1997, 30, 269. (15) Rella, R.; Serra, A.; Siciliano, P.; Tepore, A.; Valli, L.; Zocco, A. Sens. Actuators 1997, 42, 53. (16) Fujiki, M.; Tabei, H.; Imamura, S. Jpn. J. Appl. Phys. Part I 1985, 24, L685. (17) Fujiki, M.; Tabei, H.; Imamura S.; Jpn. J. Appl. Phys. Part I 1987, 26, 1224. (18) Fujiki, M.; Tabei, H.; Kurihara, T. Langmuir 1988, 4, 1123. (19) Fujiki, M.; Tabei, H.; Kurihara, T. J. Phys. Chem. 1988, 92, 1281. (20) Capone, S.; Rella, R.; Siciliano, P.; Vasanelli, L.; Valli, L.; Troisi, L. Thin Solid Films 1998, 327-329, 465.

10.1021/la980608+ CCC: $18.00 © 1999 American Chemical Society Published on Web 02/05/1999

Gas Sensitivity Measurements

Langmuir, Vol. 15, No. 5, 1999 1749

Figure 1. Molecular structure of Cu(tbac)Pc.

(II) tetrakis(n-butylaminocarbonyl)phthalocyanine will be labeled Cu(tbac)Pc from here on. In this work we have investigated a quantitative analysis about the variation of the electrical conductivity vs time due to successive exposures of the active layer to NO2 gas within the ppm range as well as sensor parameters such as response time and recovering time. Experimental Details Cu(tbac)Pc was synthesized following a four-steps procedure already reported in the literature.16 In the LB deposition a KSV5000 System 3 apparatus (subphase area of 850 cm2) was used. Ultrapure water (from a Millipore MilliRO-MilliQ system, resistivity 18.2 MΩ cm) was used as the subphase. The trough temperature was regulated at 20 °C by a Haake GH-D8 apparatus. The phthalocyanine derivative was dissolved in chloroform at a concentration of 4.6 × 10-4 M. Then 200 µL of the solution was spread by a gastight syringe onto the subphase. After the solvent evaporated off, the floating film at the airwater interface was compressed continuously at 2 Å2 molecule-1 s-1. The surface pressure was simultaneously monitored by a Wilhelmy plate while obtaining an isotherm of the sample. The surface layer was transferred onto both hydrophobic and hydrophilic quartzes by vertical dipping at a surface pressure of 25 mN/m and at a speed of 8 mm min-1 for the downstroke and 1-2 mm min-1 for the upstroke. Z-type deposition up to 50 layers was obtained onto both substrates. The deposition ratios for lowering the substrate through the floating layer onto water were close to zero and in the range 0.1-0.2 for the hydrophobic and hydrophilic substrates, respectively; for withdrawing the substrate the deposition ratios were in the range 0.8-0.95 for both kinds of quartzes. Electronic absorption spectra carried out on 30 layers thick Cu(tbac)Pc LB films were obtained by using a Varian Cary 5 double-beam spectrophotometer in the 300-800 nm spectral

range. Polarized UV-vis absorption measurements at normal incidence respect to the substrate were performed at room temperature in order to investigate the in-plane and out-of-plane anisotropy. For the electrical measurements, 30 layers thick LB films were reproducibly constructed. Gold was selected as the electrode material, deposited in the gap configuration onto the active layer, since it is well-known that it forms ohmic contact to the phthalocyanine. The geometry of the contacts gives an active surface of about 10 × 10 mm2. The dc resistance of the various samples was monitored by an electrometer, Keithley model 617. The gas effect on the electrical conductivity was measured in a dynamic pressure system implemented in our laboratory where dry air at ambient pressure was used as the carrier and reference gas. The gases employed in all experiments had purity level of 99.99%. The samples were placed in a chamber through which a gas could be passed. The gas concentration was varied by using a MKS Instruments mass flow controller, model 647. The working parameters, such as the operating temperature and gas flow rates and resistance of the samples, were controlled by a personal computer with an IEEE 488 board. In this way we obtained the electrical response of each sample in real time. The temperature control was carried out by a homemade thermocontroller while a platinum thermometer measured the temperature of the sample in the test chamber. The experimental procedure was performed by flowing dry air through the sample chamber until a steady resistance reading had been obtained. Then the active gas was admitted in its lowest concentration mixed with dry air, and the resistance was recorded during a period of time necessary to obtain a saturation current and calculate the response time of the sensor. The active gas was then turned off and the sample left to recover in dry air flux. The restore at the initial value of the resistance confirmed the reversibility of the process. At this point a higher concentration of the active gas was admitted to the chamber and the measurement procedure repeated in order

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Figure 2. Surface pressure vs area per molecule isotherm of Cu(tbac)Pc in chloroform (4.6 × 10-4 M) at 20 °C. to obtain the dynamic response of the sensor to different concentrations of the toxic gas and to evaluate the sensitivity of it.

Results and Discussion Isotherms and Depositions. The surface pressure vs area per molecule isotherm, using chloroform as the spreading solvent, is illustrated in Figure 2. The stability of the compressed surface film was tested at a constant surface pressure of 28 mN/m for 3 h after compression: there was no loss in area. The estimated value of the limiting area Aπf0 (obtained by extrapolating to zero the surface pressure from the steeply varying portion of the curve) is only 22 Å2/molecule. The calculated area per molecule obtained from a computer model (Cerius 2s Molecular Simulation Inc.) with molecular planes edged on the water surface (3.4 × 16.2 ) 55.1 Å2/molecule) appears not to be consistent with the formation of a homogeneous floating monolayer. A suggestion about the interpretation of such a small experimental and reproducible value could come from the solution spectra of Cu(tbac)Pc. Therefore, we examined as well the UV-vis spectra of Cu(tbac)Pc in chloroform with range of concentrations 5 × 10-6 M to 10-4 M. In these spectra, electron transitions are exhibited around 350 nm and in the range 600-700 nm. The latter spectral region evidences the presence of two main bands, known as QI (at about 680 nm) and QII (at about 610 nm), whose relative intensity depends on dilution.11 QI band was recognized as the Q-band of monomeric phthalocyanine and QII band as the one of aggregates.18,21-24 From Figure 3 it is possible to observe that at the concentration of 7.9 × 10-5 M (curve a), the QII band is much more intense than the one of the monomer. QI and QII bands have similar intensities around concentrations of 10-5 M (curves c and d), while monomer prevails for lower concentrations (curve e). This means that in the spreading solution (4.6 × 10-4 M) we have practically only aggregated species and that probably at the air-water interface stacks of aggregates are randomly arranged, thus resulting in an average limiting area of only 22 Å2/molecule. Additional evidence of sizable molecular association is obtained by observing the blue-shift of the QII band on increasing concentration.18,23,24 This can be interpreted by assuming the phthalocyanine (21) Barger, W. R.; Snow, A. W.; Wohltjen, H.; Jarvis, N. L. Thin Solid Films 1985, 133, 197. (22) Snow, A. W.; Jarvis, N. L. J. Am. Chem. Soc. 1984, 106, 4706. (23) Fujiki, M.; Tabei, H. Langmuir 1988, 4, 320. (24) Ouyang, J.; Lever, A. B. P. J. Phys. Chem. 1991, 95, 5272.

Capone et al.

Figure 3. A solution spectra of Cu(tbac)Pc in chloroform at 20 °C at various concentrations: (a) 7.9 × 10-5 M; (b) 4 × 10-5 M; (c) 1.3 × 10-5 M; (d) 9.9 × 10-6 M; (e) 5 × 10-6 M.

associated species in solution to be H-aggregates with a “playing cards” structure.25 Notwithstanding the probable unhomogeneous nature of the floating film at the air-water interface, Z-type transfer was performed onto both hydrophobic and hydrophilic glass and quartz substrates at 25 mN/m. The deposition ratio for the downstroke using hydrophobic substrates was close to zero, while that using hydrophilic substrates was in the range 0.1-0.2. This suggests the probable slightly wider uniformity of the films onto hydrophobic substrates than onto hydrophilic ones. Optical Characterization. Generally, the spectra of the LB multilayer films of pure phthalocyanines display a very strong QII absorption band due to aggregated species and only a weak shoulder in the region assigned to the monomeric absorption band. In particular, the QII band was attributed to the development of a one-dimensional linear stacking of the phthalocyanine molecules in the form of dimers, trimers, etc.10 The orientation of the Cu(tbac)Pc molecular macrocycles in linear stacking is induced by the LB deposition technique which produces the formation of ordered domains depending on the dipping direction. To this purpose we have already investigated the average orientation of this linear stacking by using polarized UV-vis adsorption spectroscopy at normal and oblique incidences and the statistical results reported in a previous paper 20 indicate that the macromolecules are on the average placed prone onto the substrate. For our analysis we have supposed the phthalocyanine ring as a flat circular plate. Further information about this molecular organization can be obtained by using the experimental procedure illustrated in Figure 4 where linearly polarized light was used. Figures 5 and 6 shown typical polarized electronic absorption spectra carried out at normal incidence on Cu(tbac)Pc LB films deposited onto hydrophobic and hydrophilic glass substrates respectively; they were both obtained by using light with electrical vectors perpendicular (⊥) and parallel (|) to the dipping direction. In the inset, the angular dependence of the main Q-bands at 610 nm is shown. These results were obtained by increasing the orientation of the electrical vector from 0° (parallel to the dipping direction) to 90° (perpendicular to the dipping direction). As one can see, the films possess weak dichroisms in which the main Q-bands show maxima at θ ) 90° and a minimum at θ ) 0° (θ being the orientation of the electric field of the polarized incident light with respect to the dipping direction). This happens because (25) Barber, D. C.; Freitag-Beeston, R. A.; Whitten, D. G. J. Phys. Chem. 1991, 95, 4074.

Gas Sensitivity Measurements

Figure 4. Experimental geometry used for optical measurements carried out onto LB multilayers by using polarized light at normal incidence respect to the substrate. Θ is the angle between the plane of polarization and the dipping direction.

Figure 5. Polarized electronic absorption spectra of an LB film of Cu(tbac)Pc 31 layers thick deposited on a hydrophobic glass substrate. In the inset, the angular dependence of the main Q-band intensity is shown (λ ) 610 nm).

Figure 6. A polarized electronic absorption spectra of an LB film of Cu(tbac)Pc 30 layers thick deposited on hydrophilic glass substrate. In the inset, the angular dependence of the main Q-band intensity is shown (λ ) 610 nm).

the degenerate M and L transitions of the Q-bands lie in the Pc ring plane; consequently, if the Pc ring planes are parallel to the substrate plane, dichroism does not occur.16 It is important to observe also that, for multilayers with the same number of deposited layers, the films deposited onto hydrophilic glass show a value of absorbance larger than that of the films deposited onto hydrophobic glass. This is probably connected to the transfer of the floating film from the water surface onto the glass substrates during the downstroke: the transfer ratio was close to zero for the hydrophobic substrates, and a bit larger (0.10.2) for the hydrophilic substrates. During the LB deposition it is necessary to consider the molecular arrangement in both the floating and the deposited films. This arrangement is determined by the microscopic intra- and interlayer molecular interactions. Generally, the behavior

Langmuir, Vol. 15, No. 5, 1999 1751

Figure 7. Response curves to 500 ppm NO2 gas of Cu(tbac)Pc LB films 31 and 30 layers thick, deposited on hydrophobic and hydrophilic glass substrates respectively and carried out a working temperature of 100 °C.

of the disk-shaped amphiphiles molecules at air/water interface depends on various types of intermolecular forces and, consequently, the hydrophobic-hydrophilic balance determines the orientation of the molecules as side-on or edge-on shape.26 In our case the optical experimental results indicate a side-on configuration where a columnar arrangement of the disks should be favored.27 A further support to this consideration comes from absorption measurements in the investigated spectral range. Values of 0.3-0.4 are justified by assuming a model in which the molecules form small “piles” on the water surface with the molecules lying flat on the surface. This configuration gives an apparent area per molecule of 16.2 × 16.2 Å2/number of molecules per “pile”. This permits to predict “piles” containing about 12 molecules and to justify the observed absorbances since in this way the floating film thickness of the monolayer would be 12 times greater than a simple monomolecular monolayer. This conclusion suggests the formation of a very poor ordered film. Electrical Response to NO2 Gas. All the electrical conductivity data on the Cu(tbac)Pc multilayers were obtained in a configuration parallel to the substrate. Figure 7 illustrates how NO2 gas in low concentrations in the test chamber produces a reversible decrease in the resistance of the Cu(tbac)Pc LB films at relatively low working temperatures. This behavior is evidenced by both kinds of active layers deposited onto hydrophilic and hydrophobic glass substrates. In both cases, the interaction between NO2 and copper phthalocyanine LB films can be explained by the following process: NO2 is a well-known oxidizing gas which, on contact with the π electron network of phthalocyanines, causes the transfer of an electron from the phthalocyanine ring to the gas. Consequently, the phthalocyanine ring becomes positively charged, and the charge carriers thus created justify the increased conductivity of the film. Practically, absorbed NO2 gas acts as an acceptor in the Pc lattice through the following reversible doping process:

Cu(tbac)Pc + NO2 T Cu(tbac)Pc+ + NO2- T h+ + Cu(tbac)Pc + NO2where h+ is the delocalized hole. These charge-transfer interactions with absorbed electron acceptor molecules are due to the low ionization energy and high polarization energies of phthalocyanines. The former reduced the (26) Aldrecht, O.; Cumming, W.; Krender, W.; Laschewsky, A.; Ringsdorf, H. Colloid Polym Sci. 1986, 264, 659. (27) Laschewsky, A.; Augew, B. Chem. Adv. Mater. 1989, 101, 1606.

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Figure 8. Variation of percentage change in resistance to 500 ppm NO2 gas vs work temperature of the Cu(tbac)Pc LB films. Key: (2) hydrophobic glass substrate (30 layers); (9) hydrophilic glass substrate (31 layers). In the inset, the typical response curve to 500 ppm NO2 gas obtained at room working temperature from Cu(tbac)Pc LB films 30 layers thick deposited onto glass substrate is shown.

energy required for the creation of a charge transferred state and the latter reduces the energy required to stabilize the products of such charge-transfer process. Furthermore, the use of large peripheral substituents, utilized to increase the solubility of this compound, produces a low polarizability environment. This is an advantage since the charged species created during the charge-transfer absorption are less stabilized. Of course, the charge transport process and the possibility to measure a macroscopic conductance depend on the relative orientation and separations of adjacent molecules and the variation in conductivity is controlled by the nature of the surface adsorption sites which is influenced by both the structure and morphology of the film. To this purpose, the LB technique allows us to realize highly uniform films for which all surface sites should ideally be identical, so that the kinetics of absorption and of displacement of absorbed gas are simplified. In reality, films of phthalocyanines obtained by LB technique are unespected to be entirely uniform, and structures containing a series of ordered domains in an amorphous matrix are more reasonable. Each domain is made by a series of columns placed side by side, and the structural interdomain correlation is determined by the nature of the substituent on the periphery of the macrocycle.28 As shown in Figure 8, the change in resistance evidenced from our samples in the presence of NO2 gas depends on the temperature of the active layer. In fact, the figure illustrates the response ∆R/R, to 500 ppm NO2 gas of the two different hydrophobic and hydrophilic systems as a function of the work temperature; here ∆R/R ) (Rair - Rgas)/Rgas, Rgas, and Rair being the resistance of the active layer in the presence of NO2 gas and in dry air, respectively. As one can see, taking into account the experimental errors, there is not substantial difference in the electrical response between the two different systems respect to the NO2 gas; in fact, the variation in the resistance starts from a value of about 3000% at room temperature and decreases at higher temperatures. Nevertheless, the inset of Figure 8 shows that when the samples are reported at room temperature, the response of the active layer due to the presence of NO2 gas in the test chamber is very slowly reversible. The decrease in ∆R/R with increasing temperature could be explained by considering the activation energy for semiconduction of these phthalocyanine multilayers. In a (28) Rella, R.; Serra, A.; Siciliano, P.; Tepore, A.; Valli, L.; Zocco, A. Langmuir 1997, 13, 6562.

Capone et al.

Figure 9. Electrical resistance changes in the presence of different NO2 concentration, for a typical Cu(tabc)Pc LB film 30 layers thick and deposited onto glass substrate, carried out at room working temperature. The arrows indicate gas injection.

previous paper,29 we showed how the activation energy for semiconduction is higher in dry air than in the presence of a NO2-air mixture, and this result has been reported also by other groups for sublimated CuPc film.30 Consequently, as temperature increases, the variation in the observed conductivity in dry air increases much faster than that in NO2-dry air mixture. Figure 9 shows a typical response curve of the 30 layers thick LB film as-deposited onto hydrophobic glass substrate to increasing NO2 concentrations obtained with the active layer working at room temperature (without a previous thermal treatment). It is clearly apparent that the injection of NO2 leads to a drop in the resistance, but the recovering in dry air condition, when NO2 gas is shut off, is very slow. The curves in Figures 8 and 9 have been obtained using a constant gas flow of about 6 L h-1; if the gas flow increases, the response time remains practically unchanged. Here the response time is defined as the time required by the resistance to undergo a 90% variation with respect to its equilibrium value following a step increase in the test gas concentration. In particular, at room temperature, the sensor exhibits a response time of about 300 s on exposure to a mixture of dry air and 20 ppm NO2. The response time decreases to about 100 s, when the working temperature of the active layer up increases to 100 °C, but at the same time the response in the presence of NO2 gas decreases. Consequently, it is necessary to make a compromise between sensitivity and response time according to the practical requirements. As observed in several works on organometallic semiconductors such as CuPc thin films exposed to NO2,31-33 the kinetics of the conductivity changes were found to follow the Elovich equation dθ/dt ) a exp(-bθ), where a and b are constants and θ is the covered surface fraction. Here, we assume that the conductivity increase is proportional to the number of adsorbed gas molecules on the phthalocyanine surface, following the Freundlich adsoption isotherm θ ) kPx where P is the pressure and k and x are constants.34 This consideration suggests that NO2 molecules are weakly chemisorbed by phthalocyanine (29) Rella, R.; Serra, A.; Siciliano, P.; Tepore, A.; Valli, L.; Zocco, A. Thin Solid Films 1996, 286, 256. (30) Sadaoka, Y.; Sakai, Y.; Jones, T. A.; Gopel, W. J. Mater. Sci. 1990, 25, 3024. (31) Honeybourne, Colin L.; Ewen, Richard J.; Callum, A. S.; Hill, J. Chem. Soc. Faraday Trans. 1 1984, 80, 851-863. (32) Archer, P. B. M.; Chadwick, A. V.; Miasik, J. J.; Tamizi, M.; Wright, J. D. Sens. Actuators 1989, 16, 379. (33) Blanc, J. P.; Deroniche, N.; El Hadri, A.; Germain, J. P.; Maleysson, C.; Robert, H. Sens. Actuators B1 1990, 130-133. (34) Elovich, S. Yu.; Zhalrova, G. M. Zh. Fiz. Khim. 1939, 13, 1761.

Gas Sensitivity Measurements

Figure 10. Elovich plots of relative resistance variation of Cu(tabc)Pc LB film 30 layers thick exposed to 500 ppm NO2 in dry air measured with the active layer working at room temperature and 100 °C.

molecules and that the activation energy for the adsorption process increases linearly with surface coverage. Obviously, it has been assumed that the number and the characteristics of the adsorption sites are constant and that the surface adsorption is not assisted, at the same time, by diffusion into the bulk of the film. In our case, the experimental variation in conductance ∆G vs t plot in the presence of 500 ppm NO2 can be fitted by the Elovich law ∆G ∝ ln(t + t0) with t0 ) 65 s and t0 ) 40 s for a Cu(tbac)Pc LB film at room temperature and 100 °C working temperature, respectively. Since the Elovich equation describes the gaseous sorption on the surface of the material, we stress that the linearity of ∆G vs ln(t + t0) for the region indicated in Figure 10, where a good agreement is obtained for short times, would imply that NO2 sorbs only at the surface of the active layer. This process starts with NO2 molecules that may replace previously sorbed O2 or N2 molecules onto the surface of the LB film or sorb on vacant sites. Moreover, this mechanism is limited by the rate of displacement onto the surface of the O2 or N2 molecules and does not contribute to the conductance variations.32 This justifies the slow electrical response during the first 30 or 40 s from the start of the NO2 admittance in the test chamber. At the same time, the displacement of the adsorbed oxygen by nitrogen dioxide is in fact favored by the presence of some chemisorbed nitrogendioxidemolecules producing Pc+NO2sites which exert electrostatic repulsions on the neighboring adsorbed and partially negative oxygen species. When the chemisorption process takes place, it causes electron transfer from the phthalocyanine molecules to the NO2 oxidizing gas molecules and consequently a delocalization of the hole, thus increasing the conductance of the layer. Figure 10 shows also that this process depends on the temperature of the active layer; practically, as temperature increases, the electrical response delay time is reduced but at the same time the ratio Rgas/Rair is reduced. This behavior surely will influence the sensitivity of the active layer. Naturally, this description does not consider the presence of ordered domains that constitute the LB film and their influence on electrical properties of the active layer as described in our previous paper.28 Measurements of the conductance change carried out onto our Cu(tbac)Pc LB multilayers at different NO2 concentrations, have shown that the conductance grows with increasing NO2 concentration. Figure 11 shows the normalized conductance vs gas concentration carried out

Langmuir, Vol. 15, No. 5, 1999 1753

Figure 11. -Relative conductance variation plotted against NO2 gas concentration for a typical Cu(tabc)Pc LB film 30 layers thick obtained with the active layer working at room temperature.

at room temperature. By considering the electrical conductance G directly related to the number of chemisorbed molecules on the active layer and taking into account that the Freundlich isotherm, at equilibrium, establishes that the concentration of molecular units of adsorbed NO2 is connected to the partial pressure P of the NO2 gas by the relation [NO2ads] ∝ PNO2n with 0 e n e 1, where P is in turn proportional to the concentration CNO2 of the NO2 gas in the mixture, we obtain that the dependence of the conductance upon gas concentration is well described by the relation σ ∝ CNO2R, where the number R depends on the stoichiometric coefficients of the equilibrium reaction between phthalocyanine macromolecules and NO2 chemisorbed gas.28 Thus, one should expect a linear plot of log(conductance variation) vs log(NO2 concentration) in the low concentration range as reported in Figure 11 according to G ∝ CNO21.3 relation. The slope of the curve reported in Figure 11 gives also information about the sensitivity of the active layer to NO2 gas. Moreover, the LB film of Cu(tbac)Pc shows a sensitivity threshold of about 5 ppm at room temperature. Conclusions Langmuir-Blodgett films of a copper substituted phthalocyanine containing amidic groups -(CONH)-[Cu(tbac)Pc] deposited on hydrophobic and hydrophilic substrates were prepared. Multilayers deposited onto hydrophobic substrates have shown better properties as gas sensors materials with a very low sensitivity threshold probably because of the formation of a more uniform multilayer as indicated by the deposition ratios. A direct correlation between the order and the sensitivity of the LB films of phthalocyanines has already been observed.13 We report also that oxidizing gases such as nitrogen dioxide interacting with multilayers of Cu(tbac)Pc increase the conductance of the active layer. This variation can be rapidly reversed when the LB film operates in dry air at about 100 °C. Acknowledgment. The authors wish to thank Mr. L. Dimo and Mr. F. Casino for technical assistance during the measurements. This work was partially supported by CEE-FESR subprogram II, project “Materiali, Processi e Dispositivi per sensoristica, optoelettronica ed elettronica di potenza”. LA980608+