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Characteristics of the substituted metal phthalocyanine NO2 sensor
Sensors and Actuators B, 13-14 (1993) 412-415
412
Characteristics of the substituted metal phthalocyanine NO2 sensor Koji Moriya, Hakaru Enomoto and Yuji Nakamura &sear&
and Development Centre, Osaka Gas Co., Ltd., 6-19-9 Torishima, Konohana-ku, Osaka 534 (Japan)
Abstract Substituted pbthalocyanines,metal tetra-tertiary-butylphthalocyanines,are gpplied to a thin-film nitrogen dioxide sensor, and response curves are measured. Although faster diEusion of the gas in the substituted phthakzyanene films is expected,the films are presupposed to be unstable. Some substituted phthalocyanines are decomposed at high temperature. Neither metal tetra-carboxyphthaloqacyanines nor metal tctra-aminophthalocyacyanines can be sublimated. Stable metal tetra-tertiary-butylphthalocyanines evaporate at about 400 “C. Copper tetra-tertiarybutylphthalocyanine is sublimated under vacuum on the quartz substrate, on which an interdigitated platinum electrode is formed. Vanadium-oxide-substituted phthalocyanines are also tested. Resistivity is measured in a heat-controlled quartz chamber under dry mixed gases. X-ray diffraction spectra are also measured. These films made from substituted phthalocyanines show higher resistivity and quicker recovery. SFM images show that the structure of these thin films is the same as that of 6hns from a simple metal phthalocyanine. Higher resistivity and higher recovery could originate from the wider stacking space and the weaker nitrogen dioxide affinity of those phthalocyanines.
1. Introduction Flue and waste gases from any combustion system contain NO,, and it causes many problems, such as photochemical smog and acid rain. NO, emission control has been the top issue of environmental problems. NO, sensors are a vital component of an NO, control system and an NO, sensor that can work in exhaust gas is needed. The detection of gases by measurement of electrical conductivity changes has been widely investigated [l, 21. Several studies proposed phthalocyanines (PCS)as the sensing material of an NO, sensor. Jones and Bott reported the characteristics of a plated thinfilm NO, sensor [3]. The sensitivity of the metal phthalocyanines and the other characteristics were strongly influenced by the crystal structure and/or crystallinity [4-6]. A substituted phthalocyanine using the LB film technique was introduced to make an NO, sensor [7]. Here we use a vacuum-sublimed film of substituted phthalocyanine and compare the response and recovery curves of the conductivity of the sensor with its film structure.
the different materials were deposited where possible by vacuum sublimation at 10m5 Torr, over an interdigitated platinum electrode array printed on 6 mm X 7 mm silicon substrates. The shutter of the sublimation apparatus was closed for a few minutes after reaching a certain source temperature to prevent contamination. Film thicknesses of the materials were controlled by the source temperature and the sublimation time. Ceramic heaters were put on the back side of the substrate with inorganic adhesive to heat the substrates. All measurements were conducted with the apparatus shown in Fig. 1. Both the temperature of the furnace and the voltage applied to the ceramic heater controlled the sensor temperature. The heater voltage (vh) was determined by a microcomputed function generator. The electrode of the sensor was connected with a 7
74
MFC
2. Experimental CuPc and VOPc were purchased from Kanto Chemical Ltd., PbPc from Tokyo Kasei Co., and Cu tetraR-PC, V tetra-R-PC, Pb tetra-R-PC (R: t-butyl, amino, carboxy) from Wako Pure Chemical Ind. Ltd. Films of
OT25-4005/93/$6.00
Standard Resislor
Recorder
Fig. 1. Schematic diagram of the measurement
apparatus.
0 1993 - Elswier Sequoia. All rights reserved
413
standard resistor in series. A standard voltage (V., 1.00 V) was supplied between the seosor and the resistor. The resistance of the sensor (RJ was calculated by R, =R,( I’, - v,)/V,
(1)
where V, was the readout of a recorder connected to the resistor through a voltage follower and R, was the resistance of the resistor. The sensors were exposed to a certain concentration of NO* in the air.
3. Results and discussion 3.1. Stability of the materials Vapourizatioo temperatures in vacuum and X-ray diffraction data are listed in Table 1. The films listed in Table 1 were sublimated under the following cooditions. The source temperature was 400 “C, the substrate temperature was 150 “C and the distance between the source and the substrate was 130 mm. Because of their planar molecular structure, metal phthalocyaoines are stable and some of them can be easily deposited by vacuum sublimation [3]. The vapourizatioo temperatures of the substituted phthalocyaoines were supposed to be high owing to the hydrogen bond of their substituent, and the substitueot-Pc bond of the substituted metal phthalocyaoine was supposed to be unstable. Actually, decomposition of M tetra-amino-PC and M tetra-carboxy-Pc at about 500 “C in vacuum prevented their being sublimated, but M tetra-t-butylPC could easily be deposited. The vapourization temperature of the Cu tetra-t-butyl-Pc was lower than that of CuPc; also that of VO tetra-t-butyl-Pc was lower than that of VPc. These facts oould be attributed to the characteristics of the t-butyl radical, which has no hydrogen bond site and occupies a vast space, and the C-C bond of which between MPcs is strong. A study
of many MPcs has pointed out that the difference of the coordination metal affected the characteristics of the MPc [3]. The vapourizatioo temperature difference among M tetra-t-butyl-Pcs (M- Cu, V, Pb) could be ascribed to the different metals. 3.2. S~ctures of the sublimated film Column structures of the sublimated fihn appeared in the SEM images, and the characteristics of the sensors were strongly related to these structures. Highly oriented films, containing a long column, showed high NO2 sensitivity [6]. The lattice spacing, d( - lOl), and half width of the peak, listed in Table 1, were obtained by the X-ray diffraction measurement. The index was followed by JCPDS-ICDD. The detailed structure of the phthalocyanine crystal has been reported by Robertson, who found that it is a monoclinic crystal belonging to the P2 l/a space group and that there are two phthalocyanioe molecules per unit cell [8]. The lattice spacing of the Cu tetra-t-butyl-Pc film was 17.17 8, and that of the CuPc film was 13.21 A. The Cu-Cu distance of the Cu tetra-t-butyl-Pc was 8.58 A, compared to 6.6 A in CuPc. The half width of the peak (- 101) of the Cu tetra-t-butyl-Pc was 0.391” compared to 0.335” in CuPc. This indicates that the distance between molecules of the Cu tetra-t-butyl-Pc was large compared to that of CuPc, although the grain size was same. The X-ray diffraction spectrum of the VOPc powder includes many peaks, but only a series of the peak attributed to (- 101) was observed in that of the VOPc film. This shows that the crystal structure of the sublimated film was highly oriented along the b axis. IO the case of the substituted VPc film, the (- 101) peak was broad and its half width was 3.71”. IO this sublimation of VO tetra-t-butyl-Pc, the grain size could be small. The lattice spacing of the PbPc film was small compared to that of the CuPc film and the half width of the
TABLE 1. Physical properties of the phthalocyanines Vaporization temp. (‘C)
Material
Cu tetra-t-butyi-Pc CuPc powder Cu tetra-t-but+-Pc CuPc film V tetra-t-butyl-Pc VPc powder V tetra-t-butyl-Pc VPc film Pb tetra-t-butyl-Pc PbPc powder Pb tetra-t-butyl-Pc PbPc film
powder
270 310
tilm powder
Lattice spacing (A)
Half width (‘)
17.17 13.21
0.391 0.335
11.71 12.35 11.25
0.210 3.714 0.456
12.72
0.165
11.93
0.220
280 380
fdm powder
X-ray diffraction
350
film
414 TABLE 2. Sensor characteristics of the phthalocyanines. Sublimation conditions: source temp., 400 “c, time, 5-30 min; substrate temp., 80-150 “C Material
Heater voltage (V)
CUPC CUPC Cu tetra-t-butyl-Pc Cu tetra-t-butyl-Pc VOPC V tetra-t-butyl-Pc V tetra-t-butyl-Pc
,!I
Resistance (ohm)
0.0 3.0 0.0 2.5 0.0 0.0 2.5
Air
NOz
4.0x 105 4.5 x ld 1.9x 10’ 1.0x 10’ 2.0x l@ 2.5x 10’ 1.5x 10’
4.6 x lti 9.0x 104 4.2~ 105 5.0x 10s 2.0x l(r 2.3x lb 1.3x ld
Response
30
60 90 time(min)
120
150
Fig. 2. Response and recovery curves of the copper phthalocyanines: 0, Cu tetra-t-butyl-Pc; A, CuPc; temperature 150 “C.
CuPc was also small compared to the CuPc. Though Pb is a heavy atom, the crystallinity of the PbPc film was high. 3.3. Charactekdics of the phthaloqanine NO, sensor Figure 2 shows typical response and recovery curves of the phthalocyanine NO2 sensors. The response and recovery of the Cu tetra-t-butyl-Pc were faster than that of the CuPc. The characteristics of sublimated films of the MPc as NO, sensors are listed in Table 2. The sublimated films of the Pb phthalocyanines were not stable in a mixed gas containing NO*, so their data were omitted. The resistance of the Cu tetra-t-butylPC in air was about 10 MfL, whereas that of the CuPc was about 500 l&. In a mixed gas containing 50 ppm of NO,, the resistance of Cu tetra-t-butyl-Pc was 500 kfi. As t-butyl is an electron-donating group, the number of holes in the substituted phthalocyanine seemed to be decreased. When the sensors were heated in the
Recovery (min)
10 15 10 15 25 20 30
I..
'0I...,
(min)
30
> 160 1 35 1 > 180 10 2
,.,...,
.
60
90
.
..I
120
,.
1.
time(min)
Fig. 3. Heat-cleaning effect on the recovery of the Cu tetra-tbutyl-Pc: A, with heat cleaning; 0, no heat cleaning; temperature 150 “C.
furnace, the 90% response times of those phthalocyanines had almost the same value of 10 min. The recovery time of the Cu tetra-t-butylPc was short compared to that of the CuPc. In the case of VOPc, the recovery time of the substituted phthalocyanine was short. These facts might be related to the diffusion rate and NOz affinity of the phthalocyanine crystal. The faster diffusion of NO2 in the substituted PC films and the weaker affinity of the film to NOz could quicken the recovery. The effect of heat cleaning on the phthalocyanines was also tested. When air was introduced into the chamber, the sensors were heated for a minute by the ceramic heater on the substrate. Figure 3 indicates the heat-cleaning effect on the recovery of the CuPc. The recovery was fast at suitable cleaning conditions. This indicated that the desorption rate of the NO2 was increased by heating.
415
4. Conclusions Metal tetra-butyl-Pc was stable enough to be sublimated. The lattice spacing of the substituted MPc was large, and NO, d8usion into the substituted MPc crystal was supposed to be fast. The fast recovery of the substituted MPc originated in the fast diffusion and weaker NO, affinity of the f&n. Heat cleaning could quicken the recovery times of the sensors.
Acknowledgement We wish to acknowledge the Osaka Gas Analysis Centre for the analysis of the X-ray diffraction results.
References S. R. Morrison, Semiconductor gas sensors, Sensors and Actuztors, 2 (1982) 329-342. G. Heihnd, Homogeneous semiconductinggassensors, Sensors and Actuators, 2 (1982) 343-362 T. A. Jones and B. Bott, Gas-induced electrical conductivity changes in metal phthalocyanines, Sensors and Actuators, 9 (1986) 27-37. Y. Sadaoka, Y. Sakai, N. Yamazoe and T. Seiyama, De&j Kogaky 50 (1982) 457-462. T. A. Jones and B. Bott, Rot. Int. Meet. Chemical Semorr, Bordeaux, Fmnce, hdy 7-10, 1984, p. 167. K. Moriya, H. Enomoto and Y. Nakamura, Tech. Dkest, 13th Chem. hnsor Sump.., Na8oya, Japan, 1991, p. 161. C. W. Fu, D. A. Batzel. S. E. Richer. W. H. Ko. C. D. Fune and M. E. Kenney, I+&. 4th Iti bonj Solid-&ate Se& andAchrators (Tmrwducem‘87), Tokya,Japn, June 2-51987, pp. 583-584. J. M. Robertson, J. Chem. Sot., 615 (1935); 2195 (1936); 219 (1937).
412
Characteristics of the substituted metal phthalocyanine NO2 sensor Koji Moriya, Hakaru Enomoto and Yuji Nakamura &sear&
and Development Centre, Osaka Gas Co., Ltd., 6-19-9 Torishima, Konohana-ku, Osaka 534 (Japan)
Abstract Substituted pbthalocyanines,metal tetra-tertiary-butylphthalocyanines,are gpplied to a thin-film nitrogen dioxide sensor, and response curves are measured. Although faster diEusion of the gas in the substituted phthakzyanene films is expected,the films are presupposed to be unstable. Some substituted phthalocyanines are decomposed at high temperature. Neither metal tetra-carboxyphthaloqacyanines nor metal tctra-aminophthalocyacyanines can be sublimated. Stable metal tetra-tertiary-butylphthalocyanines evaporate at about 400 “C. Copper tetra-tertiarybutylphthalocyanine is sublimated under vacuum on the quartz substrate, on which an interdigitated platinum electrode is formed. Vanadium-oxide-substituted phthalocyanines are also tested. Resistivity is measured in a heat-controlled quartz chamber under dry mixed gases. X-ray diffraction spectra are also measured. These films made from substituted phthalocyanines show higher resistivity and quicker recovery. SFM images show that the structure of these thin films is the same as that of 6hns from a simple metal phthalocyanine. Higher resistivity and higher recovery could originate from the wider stacking space and the weaker nitrogen dioxide affinity of those phthalocyanines.
1. Introduction Flue and waste gases from any combustion system contain NO,, and it causes many problems, such as photochemical smog and acid rain. NO, emission control has been the top issue of environmental problems. NO, sensors are a vital component of an NO, control system and an NO, sensor that can work in exhaust gas is needed. The detection of gases by measurement of electrical conductivity changes has been widely investigated [l, 21. Several studies proposed phthalocyanines (PCS)as the sensing material of an NO, sensor. Jones and Bott reported the characteristics of a plated thinfilm NO, sensor [3]. The sensitivity of the metal phthalocyanines and the other characteristics were strongly influenced by the crystal structure and/or crystallinity [4-6]. A substituted phthalocyanine using the LB film technique was introduced to make an NO, sensor [7]. Here we use a vacuum-sublimed film of substituted phthalocyanine and compare the response and recovery curves of the conductivity of the sensor with its film structure.
the different materials were deposited where possible by vacuum sublimation at 10m5 Torr, over an interdigitated platinum electrode array printed on 6 mm X 7 mm silicon substrates. The shutter of the sublimation apparatus was closed for a few minutes after reaching a certain source temperature to prevent contamination. Film thicknesses of the materials were controlled by the source temperature and the sublimation time. Ceramic heaters were put on the back side of the substrate with inorganic adhesive to heat the substrates. All measurements were conducted with the apparatus shown in Fig. 1. Both the temperature of the furnace and the voltage applied to the ceramic heater controlled the sensor temperature. The heater voltage (vh) was determined by a microcomputed function generator. The electrode of the sensor was connected with a 7
74
MFC
2. Experimental CuPc and VOPc were purchased from Kanto Chemical Ltd., PbPc from Tokyo Kasei Co., and Cu tetraR-PC, V tetra-R-PC, Pb tetra-R-PC (R: t-butyl, amino, carboxy) from Wako Pure Chemical Ind. Ltd. Films of
OT25-4005/93/$6.00
Standard Resislor
Recorder
Fig. 1. Schematic diagram of the measurement
apparatus.
0 1993 - Elswier Sequoia. All rights reserved
413
standard resistor in series. A standard voltage (V., 1.00 V) was supplied between the seosor and the resistor. The resistance of the sensor (RJ was calculated by R, =R,( I’, - v,)/V,
(1)
where V, was the readout of a recorder connected to the resistor through a voltage follower and R, was the resistance of the resistor. The sensors were exposed to a certain concentration of NO* in the air.
3. Results and discussion 3.1. Stability of the materials Vapourizatioo temperatures in vacuum and X-ray diffraction data are listed in Table 1. The films listed in Table 1 were sublimated under the following cooditions. The source temperature was 400 “C, the substrate temperature was 150 “C and the distance between the source and the substrate was 130 mm. Because of their planar molecular structure, metal phthalocyaoines are stable and some of them can be easily deposited by vacuum sublimation [3]. The vapourizatioo temperatures of the substituted phthalocyaoines were supposed to be high owing to the hydrogen bond of their substituent, and the substitueot-Pc bond of the substituted metal phthalocyaoine was supposed to be unstable. Actually, decomposition of M tetra-amino-PC and M tetra-carboxy-Pc at about 500 “C in vacuum prevented their being sublimated, but M tetra-t-butylPC could easily be deposited. The vapourization temperature of the Cu tetra-t-butyl-Pc was lower than that of CuPc; also that of VO tetra-t-butyl-Pc was lower than that of VPc. These facts oould be attributed to the characteristics of the t-butyl radical, which has no hydrogen bond site and occupies a vast space, and the C-C bond of which between MPcs is strong. A study
of many MPcs has pointed out that the difference of the coordination metal affected the characteristics of the MPc [3]. The vapourizatioo temperature difference among M tetra-t-butyl-Pcs (M- Cu, V, Pb) could be ascribed to the different metals. 3.2. S~ctures of the sublimated film Column structures of the sublimated fihn appeared in the SEM images, and the characteristics of the sensors were strongly related to these structures. Highly oriented films, containing a long column, showed high NO2 sensitivity [6]. The lattice spacing, d( - lOl), and half width of the peak, listed in Table 1, were obtained by the X-ray diffraction measurement. The index was followed by JCPDS-ICDD. The detailed structure of the phthalocyanine crystal has been reported by Robertson, who found that it is a monoclinic crystal belonging to the P2 l/a space group and that there are two phthalocyanioe molecules per unit cell [8]. The lattice spacing of the Cu tetra-t-butyl-Pc film was 17.17 8, and that of the CuPc film was 13.21 A. The Cu-Cu distance of the Cu tetra-t-butyl-Pc was 8.58 A, compared to 6.6 A in CuPc. The half width of the peak (- 101) of the Cu tetra-t-butyl-Pc was 0.391” compared to 0.335” in CuPc. This indicates that the distance between molecules of the Cu tetra-t-butyl-Pc was large compared to that of CuPc, although the grain size was same. The X-ray diffraction spectrum of the VOPc powder includes many peaks, but only a series of the peak attributed to (- 101) was observed in that of the VOPc film. This shows that the crystal structure of the sublimated film was highly oriented along the b axis. IO the case of the substituted VPc film, the (- 101) peak was broad and its half width was 3.71”. IO this sublimation of VO tetra-t-butyl-Pc, the grain size could be small. The lattice spacing of the PbPc film was small compared to that of the CuPc film and the half width of the
TABLE 1. Physical properties of the phthalocyanines Vaporization temp. (‘C)
Material
Cu tetra-t-butyi-Pc CuPc powder Cu tetra-t-but+-Pc CuPc film V tetra-t-butyl-Pc VPc powder V tetra-t-butyl-Pc VPc film Pb tetra-t-butyl-Pc PbPc powder Pb tetra-t-butyl-Pc PbPc film
powder
270 310
tilm powder
Lattice spacing (A)
Half width (‘)
17.17 13.21
0.391 0.335
11.71 12.35 11.25
0.210 3.714 0.456
12.72
0.165
11.93
0.220
280 380
fdm powder
X-ray diffraction
350
film
414 TABLE 2. Sensor characteristics of the phthalocyanines. Sublimation conditions: source temp., 400 “c, time, 5-30 min; substrate temp., 80-150 “C Material
Heater voltage (V)
CUPC CUPC Cu tetra-t-butyl-Pc Cu tetra-t-butyl-Pc VOPC V tetra-t-butyl-Pc V tetra-t-butyl-Pc
,!I
Resistance (ohm)
0.0 3.0 0.0 2.5 0.0 0.0 2.5
Air
NOz
4.0x 105 4.5 x ld 1.9x 10’ 1.0x 10’ 2.0x l@ 2.5x 10’ 1.5x 10’
4.6 x lti 9.0x 104 4.2~ 105 5.0x 10s 2.0x l(r 2.3x lb 1.3x ld
Response
30
60 90 time(min)
120
150
Fig. 2. Response and recovery curves of the copper phthalocyanines: 0, Cu tetra-t-butyl-Pc; A, CuPc; temperature 150 “C.
CuPc was also small compared to the CuPc. Though Pb is a heavy atom, the crystallinity of the PbPc film was high. 3.3. Charactekdics of the phthaloqanine NO, sensor Figure 2 shows typical response and recovery curves of the phthalocyanine NO2 sensors. The response and recovery of the Cu tetra-t-butyl-Pc were faster than that of the CuPc. The characteristics of sublimated films of the MPc as NO, sensors are listed in Table 2. The sublimated films of the Pb phthalocyanines were not stable in a mixed gas containing NO*, so their data were omitted. The resistance of the Cu tetra-t-butylPC in air was about 10 MfL, whereas that of the CuPc was about 500 l&. In a mixed gas containing 50 ppm of NO,, the resistance of Cu tetra-t-butyl-Pc was 500 kfi. As t-butyl is an electron-donating group, the number of holes in the substituted phthalocyanine seemed to be decreased. When the sensors were heated in the
Recovery (min)
10 15 10 15 25 20 30
I..
'0I...,
(min)
30
> 160 1 35 1 > 180 10 2
,.,...,
.
60
90
.
..I
120
,.
1.
time(min)
Fig. 3. Heat-cleaning effect on the recovery of the Cu tetra-tbutyl-Pc: A, with heat cleaning; 0, no heat cleaning; temperature 150 “C.
furnace, the 90% response times of those phthalocyanines had almost the same value of 10 min. The recovery time of the Cu tetra-t-butylPc was short compared to that of the CuPc. In the case of VOPc, the recovery time of the substituted phthalocyanine was short. These facts might be related to the diffusion rate and NOz affinity of the phthalocyanine crystal. The faster diffusion of NO2 in the substituted PC films and the weaker affinity of the film to NOz could quicken the recovery. The effect of heat cleaning on the phthalocyanines was also tested. When air was introduced into the chamber, the sensors were heated for a minute by the ceramic heater on the substrate. Figure 3 indicates the heat-cleaning effect on the recovery of the CuPc. The recovery was fast at suitable cleaning conditions. This indicated that the desorption rate of the NO2 was increased by heating.
415
4. Conclusions Metal tetra-butyl-Pc was stable enough to be sublimated. The lattice spacing of the substituted MPc was large, and NO, d8usion into the substituted MPc crystal was supposed to be fast. The fast recovery of the substituted MPc originated in the fast diffusion and weaker NO, affinity of the f&n. Heat cleaning could quicken the recovery times of the sensors.
Acknowledgement We wish to acknowledge the Osaka Gas Analysis Centre for the analysis of the X-ray diffraction results.
References S. R. Morrison, Semiconductor gas sensors, Sensors and Actuztors, 2 (1982) 329-342. G. Heihnd, Homogeneous semiconductinggassensors, Sensors and Actuators, 2 (1982) 343-362 T. A. Jones and B. Bott, Gas-induced electrical conductivity changes in metal phthalocyanines, Sensors and Actuators, 9 (1986) 27-37. Y. Sadaoka, Y. Sakai, N. Yamazoe and T. Seiyama, De&j Kogaky 50 (1982) 457-462. T. A. Jones and B. Bott, Rot. Int. Meet. Chemical Semorr, Bordeaux, Fmnce, hdy 7-10, 1984, p. 167. K. Moriya, H. Enomoto and Y. Nakamura, Tech. Dkest, 13th Chem. hnsor Sump.., Na8oya, Japan, 1991, p. 161. C. W. Fu, D. A. Batzel. S. E. Richer. W. H. Ko. C. D. Fune and M. E. Kenney, I+&. 4th Iti bonj Solid-&ate Se& andAchrators (Tmrwducem‘87), Tokya,Japn, June 2-51987, pp. 583-584. J. M. Robertson, J. Chem. Sot., 615 (1935); 2195 (1936); 219 (1937).