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Characterization of novel copper phthalocyanine Langmuir-Blodgett films for NO2 detection
ELSEVIER
Thin Solid Films 284-285
( 1996) 870-872
Characterization of novel copper phthalocyanine Langmuir-Blodgett films for NO2 detection R. Rella a, A. Serra ‘, P. Sicilian0 ‘, A. Tepore b, L. Troisi ‘, L. Valli ’ aCNR-IME. h Dipurtamento
Via Arnesano.
di Scienzu dei Materiali.
’ Dipartumento
di Biologia.
Univecritu
Universita
’ di
73100 Lecce. /t,l/y di Lecce. Viu Awesarro,
73100 kce.
Itul?
Lecce, Via Arnescmo, 73100 Lecce, It+
Abstract Novel Cu( II) [ tetrakis-( 3,3-dimethyl-l -butoxycarbonyl) ] phthalocyanine ( CuPcBC) thin films have been prepared by the LangmuirBlodgett (LB) technique from ethyl acetate and toluene solutions. Polarized light UV-Vis measurements show in-plane anisotropy of the orientation of molecules in the LB films. Dynamic response characteristics of the electrical conductance to different NO, concentrations in dry air are presented. The sensitivity as a function of temperature has been analysed; in particular. the CuPcBC LB film have shown a good sensitivity to NO? gas at an operating temperature of about 170 “C. Keyword.yt
Langmuir-Blodgen
films; Copper phthalocyanine;
Anisotropy;
1. Introduction In the last two decades considerable attention has been paid by various authors in the study of the optical and electrical properties of metal-substituted phthalocyanines with the goal of their possible applications in technology. In particular, these compounds have been proposed as new materials for gas sensors [ 1,2] More recently, a growing interest has been devoted to the deposition of these macrocycles by the Langmuir-Blodgett (LB) method, because of the enormous potentialities of this technique [ 3.41. In a previous work, we described the details of the synthesis of Cu( II) [ tetrakis- (3,3-dimethyl- 1-butoxycarbonyl) ] phthalocyanine (CuPcBC) and of the preparation and spectroscopic characterization of its LB films [ 51. In this paper, we present results on the electrical characterization of the LB films deposited using ethyl acetate as the solvent for the spreading solution. In particular, we show the reversible electrical conductivity changes in presence of different NO? concentrations in dry air.
2. Experimental
details
Two solutions of the phthalocyanine were prepared in ethyl acetate and toluene, both with a concentration of 4 X lop4 M. In the Langmuir trough experiments, a 200 ~1 portion of the solution was carefully spread onto the subphase. Ultrapure water (Millipore Milli-Q, resistivity 18.2 MR cm) was used 0040-6090/96/$15.00 6 1996 Elsevier Science S.A. SSD/0040-6090(9.5)08466-S
Nitrogen dioxide
as the subphase in a KSV5000 System 3 apparatus (850cm’). The trough temperature (20 “C) was regulated by a Haakc GH-D8 apparatus. The monolayer was then transferred onto alumina and hydrophobic quartz substrates for electrical and optical measurements, respectively, at a surface pressure of 15 mN- ’ in the case of solutions in the ethyl acetate and 18 mN_’ for toluene solutions. The speed of the dipper was maintained at 5 mm min- ’ In both cases. Optical absorption measurements at room temperature were carried out using a Varian Cary 5 double-beam spectrophotometer by polarized light at nearly normal incidence. Linear dichroism method performed for the LB films has allowed to get information on average molecular orientation of the molecules with respect to the substrate and dipping directions. In order to characterise electrically the multilayers, the phthalocyanine LB films were deposited onto alumina substrates fitted with interdigitated gold electrodes. The gas-sensing behaviour of the CuPcBC LB films was tested by a computer-controlled gas-flow system, which monitors the dilution of the oxidising gas in dry air at ambient pressure; programmable Keithley 617 electrometer connected to the computer records the conductivity data as a function of time, work temperature and gaseous treatment. 3. Results and discussion 3.1. Deposition and optical charucterizntiorr Isotherms were registered for both spreading solutions in ethyl acetate and toluene, as illustrated in Fig. 1(a) and 1(b),
R. Rella et al. /Thin Solid Films 284-285
871
(1996) NO-872
As already observed in a previous work, the average orientation of the molecular rings in CuPcBC LB films deposited from toluene solutions is different from the one in the LB films deposited from ethyl acetate solutions [ 61. 3.2. NO2 sensing characteristics
Area
per molecule
(A’lmolecule)
Fig. I. Surface pressure vs. area per molecule isotherms at 20 “C from ethyl acetate (a) and toluene (b) spreading solutions.
respectively. Both curves reported here represent at least duplicate runs that gave identical results. The limiting area per molecule is 43 A2 molecule-’ for ethyl acetate spreading solutions and 54 A’ molecule ’ for toluene. In the case of ethyl acetate as the spreading solvent, it is remarkable small with respect to the theoretical value of about 55 A’ molecule ’ for the molecules in an arrangement in which the planes of the phthalocyanines are in vertical position at the air-water interface. On the contrary, it is apparent an expansion of the curve for toluene solutions. In fact, the area per molecule is in good agreement with the calculated value. Whether toluene remained trapped in the spread layer at the air-water interface or altered the average degree of aggregation of monomers is unknown [ 71. Nevertheless, the deposition ratios in the case of ethyl acetate spreading solutions were always around 0.95, while for toluene solutions they ranged between 0.8 and 0.9. As already reported [ 5,6], the phthalocyanine compounds exhibit strong electronic transitions in the visible region (Qband at 600-800 nm) and in the near ultraviolet one (B or Soret band at 300-400 nm) due to r-r* transitions states of E, symmetry. Polarised UV-Vis spectra, carried out on our LB films, with the light beam perpendicular to the substrate plane and the electrical field vector parallel and perpendicular to the dipping direction indicate an in plane dichroism of the Q-band, whose transition moment lies in the plane of the phthalocyanine ring. In Table 1 the dichroic ratios A,,(620 nm) /A I (620 nm), calculated for the CuPcBC LB films deposited from ethyl acetate and toluene spreading solution respectively, are reported. The mean orientation of the molecules with respect to the substrate and the dipping direction, calculated by the method of Vandevyver et al. [ 81, is also reported in Table 1.
Dynamic variations in the electrical resistance arising from NO, chemisorption were analysed in a flowing gas system implemented in our laboratory, where dry air at ambient pressure was used as the carrier and the reference gas. It is well known that for every couple of material and gas there exists an operating temperature range in which the absorption process is sufficiently fast and the sensitivity is the most favourable. In this case, the gas sensitivity increases with increasing temperature in the range RT-170 “C and decreases for temperatures higher than 170 “C. As an example, Fig. 2 illustrates the dependence of the electrical resistance variation (AR) on the operating temperature at a concentration of 200 ppm NO? for a CuPcBC LB film. AR is defined as R, - R, R, being the electrical resistance in the reference gas and R the resistance in the test gas. It is clearly apparent that the maximum variation in the film conductivity, which is an index of the maximum sensitivity to the gas, is around a temperature of about 170 “C. In the inset of Fig. 2, we report the response curve of
100
150
200
250
300
Temperature@) Fig. 2. Gas sensitivity as a function of the temperature for a typical CuPcBC LB deposited by ethyl acetate spreading solution. The inset shows the response curve of a typical CuPcBC LB film deposited by ethyl acetate solution at its optimum operating temperature.
Table I Dichroic ratio and mean orientation of” axis values for films deposited onto quartz substrates Spreading solvent
Q,
D 45
Ethyl acetate Toluene
0.39 0.43
0.38 0.43
86 62
(deg.)
<+> 56 64
(deg.)
0
2000
4000
6000
6000
Time(s)
Fig. 3. Dynamic responses at different NO2 concentration ( loo-250 ppm) in dry air for a typical CuPcBC LB film deposited by ethyl acetate solution at a 170 “C work temperature.
872
R. Rella et al. /Thin Solid Films 284-285 (1996) 870-872
the same film following a step change in composition from air to 200 ppm NO,. The absorption of NO2 leads to a marked increase in the conductivity, which reaches its original value after NO2 flow is switched off. This is even more evident in Fig. 3, where the response curves obtained with different NO, concentrations are reported. The initial resistance is recovered when the test gas is shut off and this proves that the absorption process is reversible.
4. Conclusions
Thin LB films of Cu( II) [ tetrakis-( 3,3-dimethyl-l -butoxycarbonyl) ] phthalocyanine (CuPcBC) can be obtained on quartz or alumina substrates. In particular, we have observed that the arrangement of the molecules in the LB film depends on the spreading solvent used for the deposition. Finally, we show that the electrical conductivity of CuPcBC LB films changes considerably in the presence of little amounts of the highly toxic NOz gas in the atmosphere. These measurements show that our derivatized phthalocyanine is a promising material for NO* resistive chemical sensor.
Acknowledgements The authors wish to thank F. Casino, A.R. De Bartolomeo and L. Dimo for technical assistance during the measurements.
References [ 1 I CC. Leznoff and A.B.P. Lever (eds.), Phthalocyanines: Properties and Applications, VCH, New York, 1989. [2] H. Meier, Organic Semiconductors. Verlag, Weinheim, 1974. [ 31 G.G. Roberts, M.C. Petty, S. Baker, M.T. Fowlers and N.J. Thomas, Thin Solid Films. 132 (1985) 113; M.J. Daniel, R.C.O. Hart, R.M. Richardson and S.J. Roser, Thin Solid Films, 159 (1988) 395; S. Palacin, P. Lesieur. 1. Stefanelli and A. Barraud, Thin Solid Films, 159 (1988) 83; M.A. Mohammed, P. Ottenbreit. W. Prass, G. Schnurpfeil and D. Woehrle, Thin Solid Films. 213 ( 1992) 285. [4] H. Schultz, H. Lehmann, M. Rein and M. Hanack, in J.W. Buchler (ed.). Structure and Bonding, Vol. 74, Springer, Berlin, 1991. [ 51L. Pasimeni, M. Meneghetti, R. Rella, L. Valli, C. Granito and L. Troisi, Thin Mid Films, 265 ( 1995) 58-65. [6] L. Pasimeni, R. Rella, L. Troisi and L. Valli, Adv. Crystal Growth, 203 ( 1996) 297-302. [7] W.R. Barger, A.W. Snow, H. Wohltjen and N.L. Jarvis, Thin Solid Films, 133 (1985) 197. [8] M. Vandevyver, A. Barraud and A.A. Raudel-Teixier, J. Colloid Interjace Sci., R5 ( 1985) 571.
Thin Solid Films 284-285
( 1996) 870-872
Characterization of novel copper phthalocyanine Langmuir-Blodgett films for NO2 detection R. Rella a, A. Serra ‘, P. Sicilian0 ‘, A. Tepore b, L. Troisi ‘, L. Valli ’ aCNR-IME. h Dipurtamento
Via Arnesano.
di Scienzu dei Materiali.
’ Dipartumento
di Biologia.
Univecritu
Universita
’ di
73100 Lecce. /t,l/y di Lecce. Viu Awesarro,
73100 kce.
Itul?
Lecce, Via Arnescmo, 73100 Lecce, It+
Abstract Novel Cu( II) [ tetrakis-( 3,3-dimethyl-l -butoxycarbonyl) ] phthalocyanine ( CuPcBC) thin films have been prepared by the LangmuirBlodgett (LB) technique from ethyl acetate and toluene solutions. Polarized light UV-Vis measurements show in-plane anisotropy of the orientation of molecules in the LB films. Dynamic response characteristics of the electrical conductance to different NO, concentrations in dry air are presented. The sensitivity as a function of temperature has been analysed; in particular. the CuPcBC LB film have shown a good sensitivity to NO? gas at an operating temperature of about 170 “C. Keyword.yt
Langmuir-Blodgen
films; Copper phthalocyanine;
Anisotropy;
1. Introduction In the last two decades considerable attention has been paid by various authors in the study of the optical and electrical properties of metal-substituted phthalocyanines with the goal of their possible applications in technology. In particular, these compounds have been proposed as new materials for gas sensors [ 1,2] More recently, a growing interest has been devoted to the deposition of these macrocycles by the Langmuir-Blodgett (LB) method, because of the enormous potentialities of this technique [ 3.41. In a previous work, we described the details of the synthesis of Cu( II) [ tetrakis- (3,3-dimethyl- 1-butoxycarbonyl) ] phthalocyanine (CuPcBC) and of the preparation and spectroscopic characterization of its LB films [ 51. In this paper, we present results on the electrical characterization of the LB films deposited using ethyl acetate as the solvent for the spreading solution. In particular, we show the reversible electrical conductivity changes in presence of different NO? concentrations in dry air.
2. Experimental
details
Two solutions of the phthalocyanine were prepared in ethyl acetate and toluene, both with a concentration of 4 X lop4 M. In the Langmuir trough experiments, a 200 ~1 portion of the solution was carefully spread onto the subphase. Ultrapure water (Millipore Milli-Q, resistivity 18.2 MR cm) was used 0040-6090/96/$15.00 6 1996 Elsevier Science S.A. SSD/0040-6090(9.5)08466-S
Nitrogen dioxide
as the subphase in a KSV5000 System 3 apparatus (850cm’). The trough temperature (20 “C) was regulated by a Haakc GH-D8 apparatus. The monolayer was then transferred onto alumina and hydrophobic quartz substrates for electrical and optical measurements, respectively, at a surface pressure of 15 mN- ’ in the case of solutions in the ethyl acetate and 18 mN_’ for toluene solutions. The speed of the dipper was maintained at 5 mm min- ’ In both cases. Optical absorption measurements at room temperature were carried out using a Varian Cary 5 double-beam spectrophotometer by polarized light at nearly normal incidence. Linear dichroism method performed for the LB films has allowed to get information on average molecular orientation of the molecules with respect to the substrate and dipping directions. In order to characterise electrically the multilayers, the phthalocyanine LB films were deposited onto alumina substrates fitted with interdigitated gold electrodes. The gas-sensing behaviour of the CuPcBC LB films was tested by a computer-controlled gas-flow system, which monitors the dilution of the oxidising gas in dry air at ambient pressure; programmable Keithley 617 electrometer connected to the computer records the conductivity data as a function of time, work temperature and gaseous treatment. 3. Results and discussion 3.1. Deposition and optical charucterizntiorr Isotherms were registered for both spreading solutions in ethyl acetate and toluene, as illustrated in Fig. 1(a) and 1(b),
R. Rella et al. /Thin Solid Films 284-285
871
(1996) NO-872
As already observed in a previous work, the average orientation of the molecular rings in CuPcBC LB films deposited from toluene solutions is different from the one in the LB films deposited from ethyl acetate solutions [ 61. 3.2. NO2 sensing characteristics
Area
per molecule
(A’lmolecule)
Fig. I. Surface pressure vs. area per molecule isotherms at 20 “C from ethyl acetate (a) and toluene (b) spreading solutions.
respectively. Both curves reported here represent at least duplicate runs that gave identical results. The limiting area per molecule is 43 A2 molecule-’ for ethyl acetate spreading solutions and 54 A’ molecule ’ for toluene. In the case of ethyl acetate as the spreading solvent, it is remarkable small with respect to the theoretical value of about 55 A’ molecule ’ for the molecules in an arrangement in which the planes of the phthalocyanines are in vertical position at the air-water interface. On the contrary, it is apparent an expansion of the curve for toluene solutions. In fact, the area per molecule is in good agreement with the calculated value. Whether toluene remained trapped in the spread layer at the air-water interface or altered the average degree of aggregation of monomers is unknown [ 71. Nevertheless, the deposition ratios in the case of ethyl acetate spreading solutions were always around 0.95, while for toluene solutions they ranged between 0.8 and 0.9. As already reported [ 5,6], the phthalocyanine compounds exhibit strong electronic transitions in the visible region (Qband at 600-800 nm) and in the near ultraviolet one (B or Soret band at 300-400 nm) due to r-r* transitions states of E, symmetry. Polarised UV-Vis spectra, carried out on our LB films, with the light beam perpendicular to the substrate plane and the electrical field vector parallel and perpendicular to the dipping direction indicate an in plane dichroism of the Q-band, whose transition moment lies in the plane of the phthalocyanine ring. In Table 1 the dichroic ratios A,,(620 nm) /A I (620 nm), calculated for the CuPcBC LB films deposited from ethyl acetate and toluene spreading solution respectively, are reported. The mean orientation of the molecules with respect to the substrate and the dipping direction, calculated by the method of Vandevyver et al. [ 81, is also reported in Table 1.
Dynamic variations in the electrical resistance arising from NO, chemisorption were analysed in a flowing gas system implemented in our laboratory, where dry air at ambient pressure was used as the carrier and the reference gas. It is well known that for every couple of material and gas there exists an operating temperature range in which the absorption process is sufficiently fast and the sensitivity is the most favourable. In this case, the gas sensitivity increases with increasing temperature in the range RT-170 “C and decreases for temperatures higher than 170 “C. As an example, Fig. 2 illustrates the dependence of the electrical resistance variation (AR) on the operating temperature at a concentration of 200 ppm NO? for a CuPcBC LB film. AR is defined as R, - R, R, being the electrical resistance in the reference gas and R the resistance in the test gas. It is clearly apparent that the maximum variation in the film conductivity, which is an index of the maximum sensitivity to the gas, is around a temperature of about 170 “C. In the inset of Fig. 2, we report the response curve of
100
150
200
250
300
Temperature@) Fig. 2. Gas sensitivity as a function of the temperature for a typical CuPcBC LB deposited by ethyl acetate spreading solution. The inset shows the response curve of a typical CuPcBC LB film deposited by ethyl acetate solution at its optimum operating temperature.
Table I Dichroic ratio and mean orientation of” axis values for films deposited onto quartz substrates Spreading solvent
Q,
D 45
Ethyl acetate Toluene
0.39 0.43
0.38 0.43
86 62
(deg.)
<+> 56 64
(deg.)
0
2000
4000
6000
6000
Time(s)
Fig. 3. Dynamic responses at different NO2 concentration ( loo-250 ppm) in dry air for a typical CuPcBC LB film deposited by ethyl acetate solution at a 170 “C work temperature.
872
R. Rella et al. /Thin Solid Films 284-285 (1996) 870-872
the same film following a step change in composition from air to 200 ppm NO,. The absorption of NO2 leads to a marked increase in the conductivity, which reaches its original value after NO2 flow is switched off. This is even more evident in Fig. 3, where the response curves obtained with different NO, concentrations are reported. The initial resistance is recovered when the test gas is shut off and this proves that the absorption process is reversible.
4. Conclusions
Thin LB films of Cu( II) [ tetrakis-( 3,3-dimethyl-l -butoxycarbonyl) ] phthalocyanine (CuPcBC) can be obtained on quartz or alumina substrates. In particular, we have observed that the arrangement of the molecules in the LB film depends on the spreading solvent used for the deposition. Finally, we show that the electrical conductivity of CuPcBC LB films changes considerably in the presence of little amounts of the highly toxic NOz gas in the atmosphere. These measurements show that our derivatized phthalocyanine is a promising material for NO* resistive chemical sensor.
Acknowledgements The authors wish to thank F. Casino, A.R. De Bartolomeo and L. Dimo for technical assistance during the measurements.
References [ 1 I CC. Leznoff and A.B.P. Lever (eds.), Phthalocyanines: Properties and Applications, VCH, New York, 1989. [2] H. Meier, Organic Semiconductors. Verlag, Weinheim, 1974. [ 31 G.G. Roberts, M.C. Petty, S. Baker, M.T. Fowlers and N.J. Thomas, Thin Solid Films. 132 (1985) 113; M.J. Daniel, R.C.O. Hart, R.M. Richardson and S.J. Roser, Thin Solid Films, 159 (1988) 395; S. Palacin, P. Lesieur. 1. Stefanelli and A. Barraud, Thin Solid Films, 159 (1988) 83; M.A. Mohammed, P. Ottenbreit. W. Prass, G. Schnurpfeil and D. Woehrle, Thin Solid Films. 213 ( 1992) 285. [4] H. Schultz, H. Lehmann, M. Rein and M. Hanack, in J.W. Buchler (ed.). Structure and Bonding, Vol. 74, Springer, Berlin, 1991. [ 51L. Pasimeni, M. Meneghetti, R. Rella, L. Valli, C. Granito and L. Troisi, Thin Mid Films, 265 ( 1995) 58-65. [6] L. Pasimeni, R. Rella, L. Troisi and L. Valli, Adv. Crystal Growth, 203 ( 1996) 297-302. [7] W.R. Barger, A.W. Snow, H. Wohltjen and N.L. Jarvis, Thin Solid Films, 133 (1985) 197. [8] M. Vandevyver, A. Barraud and A.A. Raudel-Teixier, J. Colloid Interjace Sci., R5 ( 1985) 571.