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Notes Gas-Sensing Properties of Dilithium Octacyanophthalocyanine Langmuir-Blodgett Films Hong-Qi Xiang,† Keiji Tanaka, and Tisato Kajiyama* Department of Applied Chemistry, Faculty of Engineering, Kyushu University, Fukuoka 812-8581, Japan Received June 30, 2002. In Final Form: August 22, 2002
Introduction Semiconducting thin films of ligand-substituted phthalocyanines (Pc) have been extensively studied to design and construct gas-sensitive components for chemiresistor-type gas sensors because the family of Pc exhibits remarkable chemical and thermal stabilities to various environments in comparison with those of other organic semiconductors.1 Considering gas adsorption on Pc films is a surface phenomenon, ultrathin films with high surface uniformity should be prepared so that their functionality is promisingly manifested. The Langmuir-Blodgett (LB) technique enables one to gain direct access to precise thickness and orientation controls on the molecular level,2 and thus, this method seems to be most effective to realize well-defined Pc thin films. However, to apply this technique, the Pc molecules should possess good amphiphilicity. Otherwise, the Pc molecules would not form a stable monolayer at the air/water interface. Hence, much attention has been focused on LB films of Pc derivatives substituted peripherally with bulky alkyl and/or alkoxy groups,3-8 which are electron-donating groups, although the gas specificity would be controlled by changing substituents at the periphery of the Pc structure in addition to metals in the central cavity. Consequently, little has been studied for LB films of Pc derivatives with electron-withdrawing substituents such as cyano groups owing to their poor solubility in common organic solvents. * To whom correspondence should be addressed. Tel: +8192-642-3560. Fax: +81-92-651-5606. E-mail: kajiyama@cstf. kyushu-u.ac.jp. † Current address: Polymer Science & Engineering Department, University of Massachusetts, Amherst, MA 01003. (1) Snow, A. W.; Barger, W. R. Phthalocyanine Films in Chemical Sensors. In Phthalocyanines - Properties and Applications, Vol. 1; Lezonff, C. C., Lever, A. B. P., Eds.; VCH: New York, 1989. (2) Ulman, A. An Introduction to Ultrathin Organic Films from Langmuir-Blodgett to Self-Assembly; Academic Press: New York, 1991. (3) Pace, M. D.; Barger, W. R.; Snow, A. W. Langmuir 1989, 5, 973978. (4) Fujiki, M.; Tabei, H. Langmuir 1988, 4, 320-326. (5) Gorbunova, Y.; Rodriguez-Mendez, M. L.; Kalashnikova, I. P.; Tomilova, L. G.; de Saja, J. A. Langmuir 2001, 17, 5004-5010. Gutierrez, N.; Rodrı´guez-Me´ndez, M. L.; de Saja, J. A. Sens. Actuators, B 2001, 77, 437-442. (6) Lee, S.; Fukuda, K.; Anzai, J.-I. Mater. Sci. Eng., C 1998, 6, 4145. (7) Hu, W.-P.; Liu, Y.-Q.; Xu, Y.; Liu, S.-G.; Zhou, S.-Q.; Zeng, P.-J.; Zhu, D.-B. Sens. Actuators, B 1999, 56, 228-233. (b) Liu, Y.-Q.; Hu, W.-P.; Xu, Y.; Liu, S.-G.; Zhu, D.-B. J. Phys. Chem. B 2000, 104, 1185911863. (8) Rella, R.; Serra, A.; Siciliano, P.; Tepore, A.; Valli, L.; Zocco, A. Langmuir 1997, 13, 6562-6567. (b) Capone, S.; Mongelli, S.; Rella, R.; Siciliano, P.; Valli, L. Langmuir 1999, 15, 1748-1753.
This set a great obstacle to fully achieving the gas specificity of Pc LB films. In our previous work,9 we have succeeded in preparing LB films of a metallo Pc derivative with eight strong electron-withdrawing groups, dilithium octacyanophthalocyanine Li2Pc(CN)8 (Figure 1a). Voltammetry in conjunction with electron spectroscopic measurement revealed that the reduction reaction in the Li2Pc(CN)8 LB films took place at higher electric potentials compared with that of unsubstituted Pcs and ones with electrondonating substituents, and that the reducing behavior was closely related to the molecular aggregation states in the films. In this paper, we further explore the electric conducting behavior and its gas adsorption dependence for the Li2Pc(CN)8 films. Experimental Section Preparation of Li2Pc(CN)8 LB Films. LB films of Li2Pc(CN)8 were prepared on a computer-controlled Langmuir trough as described previously.9 The substrate used for the LB film deposition was a 13 × 10 × 1 mm3 ground glass, on which an interdigitated gold electrode pattern was lithographically printed. Gold was chosen for the electrode because it can form an ohmic contact with the LB film deposited on it.10 The electrode pattern consisted of sixty pairs of fingers, and each finger was 1 mm long and 10 µm wide with a gap of 5 µm between two fingers. Monolayer was transferred onto the substrate at a given surface pressure, lowering the subphase level which was attained by slowly sucking out the water through the area beyond a barrier. This procedure was repeated until LB films with various layer numbers were deposited onto the substrates. Electrical Conductivity Measurement. Figure 1b shows the experimental setup to examine electrical conductivity in the Li2Pc(CN)8 LB films. The film on interdigitated electrode was mounted into a 30-mL cylindrical glass bottle. The glass bottle was put on a heating stage to control the temperature of the film up to 573 K. A thermocouple was tightly fixed at the rear side of the electrode. Two holes were made on the bottle cap to flow gas in to and out of the bottle, respectively. Gases used in this study were hydrogen, hydrogen disulfide, oxygen, and nitrogen dioxide. The gases with different concentrations were prepared by diluting them with N2, and then fed into the system at a constant flow rate, which was attained by using a peristaltic pump. A direct-current (DC) voltage bias was applied, and the current in the LB film was monitored with a Keithley Sourcemeter interfaced to an IBM-compatible personal computer using LAB View software. Voltage was chosen to be low enough so that all current values were in Ohmic regime. The measurement was carried out by flowing nitrogen gas, which was inactive to what was measured, through the sample bottle until a steady current reading was obtained. Then the sample gas was pumped in and the electrical current was recorded. After the current reached its saturation value, the sample gas was turned off and the LB film was exposed to N2 again to restore its initial value of the current. Electronic Absorption Spectroscopic Characterization. Electronic absorption spectra were recorded using a Shimadzu UV-3100PC spectrometer with a slit width of 2.0 nm. In all spectroscopic measurements, the electronic absorption spectrum of a bare interdigitated electrode substrate itself was used as the reference for the measurements. (9) Xiang, H.-Q.; Tanaka, K.; Takahara, A.; Kajiyama, T. Langmuir 2002, 18, 2223-2228. (10) Aroca, R.; Bolourchi, H.; Battisti, D.; Najafi, K. Langmuir 1993, 9, 3138-3141.
10.1021/la026161t CCC: $22.00 © 2002 American Chemical Society Published on Web 10/19/2002
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Figure 1. Chemical structure of Li2Pc(CN)8 (a), and the experimental setup (b) to measure electrical conductivity and its gas dependence for Li2Pc(CN)8 LB films deposited on interdigital electrodes.
Figure 2. Electronic absorption spectra of Li2Pc(CN)8 LB films with different layers deposited on interdigital-electrode-coated glass substrates. From the lower to upper curves, the number of layers increases from 1 to 10. The inset shows the absorbance of aggregated Q-band at around 660 nm as a function of the number of layers deposited.
Results and Discussion LB Film Deposition. Each elemental monolayer for the LB Li2Pc(CN)8 was transferred at the surface pressure, π, of 11 mN m-1, at which it was in a solid state.9 Figure 2 shows the electronic absorption spectra of the Li2Pc(CN)8 LB films with different layers. The spectra in the Q-band region exhibited intense absorptions around 660 nm due to aggregated species, which were attributed to the development of a face-to-face stacking of the Pc molecules.11 Also, weak shoulders were observed at the lower energy side assigned to the monomeric ones. We have previously examined the average orientation of the (11) Cook, M. J. Optical and infrared spectroscopy of phthalocyanine molecular assemblies. In Spectroscopy of New Materials; Clark, R. J. H., Hester, R. E., Eds.; John Wiley & Sons: Chichester, U.K., 1993.
Figure 3. Electrical current change for a 10-layer Li2Pc(CN)8 LB film on exposure to (a) H2S and (b) NO2 gases. Both gases are of a concentration of 100 ppm in N2.
Pc stacking by using polarized electronic adsorption spectroscopy, and reported that the Pc molecules stood obliquely with an edge-on configuration on the substrate surface.9 In Figure 2, the aggregated Q-band absorption shifted from 656 nm for the monolayer LB film to 663 nm for the ten-layer one. This means that the order of molecular association slightly decreased by depositing 10 layers. In contrast, the absorption intensity of the aggregated Q-band linearly increased with the layer number, as shown in the inset of Figure 2. Thus, it seems reasonable to consider that the Li2Pc(CN)8 monolayers were successfully transferred from the water surface onto the substrate, although the molecular ordering in the LB multilayer films was somewhat inferior to its elemental layer. Gas-Sensing Measurement. Substitution of electronwithdrawing cyano groups for the hydrogen atoms on the periphery of Pc ring decreases the electron density on the conjugated macrocycle. Consequently, the Li2Pc(CN)8 is supposed to possess more positive redox potentials than the unsubstituted Pc. This was confirmed for the solutions and even in the films, and has been already reported in detail elsewhere.9,12,13 As the Li2Pc(CN)8 is applied as a gas-sensing element, it is inferred that its sensitivity to reducing gases is improved in comparison with before, namely, Li2Pc. Hence, in this study, the conducting behavior in the Li2Pc(CN)8 LB films before and after their exposure to a reducing gas of hydrogen disulfide was examined. Also, for a comparison, oxidizing gas of nitrogen dioxide was used as well. Figure 3 shows the current change as a function of time for 10-layer Li2Pc(CN)8 LB film exposed to H2S and NO2 with the concentration of 100 ppm. When the H2S gas was introduced into the gas chamber, the electrical current increased at first, and then the increment with time became small. The maximum current was approximately 500 times larger than its (12) Louati, A.; Elmeray, M.; Andre, J. J.; Simon, J.; Kadish, K. M.; Gross, M.; Giraudeau, A. Inorg. Chem. 1985, 24, 1175-1179. (13) Schumann, B.; Wohrle, D.; Jaeger, N. I. J. Electrochem. Soc. 1985, 132, 2144-2149.
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original value. This result makes it clear that the electric conductivity in the Li2Pc(CN)8 LB film is very sensitive to H2S gas in this low concentration region. Also, the current in the Li2Pc(CN)8 LB film decreased and nearly restored its initial value after exposing it to inert gas of nitrogen, meaning that the gas-sensing property is reversible. In contrast, the current response of the Li2Pc(CN)8 film was completely different, when NO2 gas was introduced into the chamber, as shown in the bottom part of Figure 3. The current was sharply decreased. However, the base current was recovered little after replacing the gas to N2, and this was the case even if the chamber was evacuated for 1 h at 373 K. A family of Pc with electron-donating substituents has been classified as p-type semiconductors.1 Their conductivities increase on exposure to oxidizing gases such as nitrogen dioxide, and decrease with exposure to reducing gases such as ammonia. On the other hand, in this experiment, the conductivity of the Li2Pc(CN)8 LB films changed in a completely opposite way when they were exposed to reducing and oxidizing gases. That is, the Li2Pc(CN)8 LB films behave as an n-type semiconductor. Thus, it can be claimed that the eight strong electronwithdrawing cyano substituents remarkably altered the type of charge carriers, from holes to electrons, in the Pc films. This observation for the Li2Pc(CN)8 LB films can be convincingly understood on the basis of the analogy of the gas-sensing behavior for general Pc films as follows.1,14 At first, reducing gas molecules are physically adsorbed onto the film surface, and then they diffuse into the interior region of the film. During this process, a charge-transfer interaction occurs between a Li2Pc(CN)8 and gas molecule being an analyte. In this interaction, the Li2Pc(CN)8 molecule acts as an electron acceptor, rather than a donator of general Pc molecules, in the redox couple. Finally, the negative charges (electron carriers) created are de-localized over the Pc ring, enhancing electric conductivity in the film. In the case of oxidizing gases, NO2 and O2 are of strong electrophilic character and the electron transfer from NO2 and O2 molecules to Li2Pc(CN)8 is impeded. On the contrary, their adsorption onto the film compensates or traps the initial electronic charge carriers in the film, thus lowering the concentration or the mobility of the delocalized electrons. And this causes a decrease in conductivity. An n-type semiconducting behavior has been also reported for a physical vapor deposition film of Pc, in which the periphery was substituted by 16 fluorines, ZnPcF16.15 When the ZnPcF16 film was exposed to NO2 gas, the electric conductivity first decreased and then increased. They interpreted the resumption of increasing conductivity by a notion that hole carriers are created in the film after the complete compensation or trapping of the initial delocalized electron carriers. However, such was not observed for the Li2Pc(CN)8 LB film exposed to NO2. The reason may lie in the more positive redox potentials of Li2Pc(CN)8 compared with that of ZnPcF16, and thus, NO2 is not strong enough to create hole carriers in the Li2Pc(CN)8 LB film as an oxidant. Figure 4 shows the electronic absorption spectra of a 10-layer Li2Pc(CN)8 LB film before and after exposing it to 100 ppm H2S gas for 40 min. In the Q-band region, an obvious change in the relative intensity of the aggregated and monomeric bands was observed. The portion of (14) Wang H.-Y.; Lando, J. B. Langmuir 1994, 10, 790-796. (15) Germain, J. P.; Pauly, A.; Maleysson, C.; Blanc, J. P.; Scho¨llhorn, B. Thin Solid Films 1998, 333, 235-239. (b) Scho¨llhorn, B.; Germain, J. P.; Pauly, A.; Maleysson, C.; Blanc, J. P. Thin Solid Films 1998, 326, 245-250.
Notes
Figure 4. Electronic absorption spectra of a 10-layer Li2Pc(CN)8 LB film (a) before and (b) after exposing it to 100 ppm H2S for 40 min.
Figure 5. Electrical current change on exposure to pure (a) H2 and (b) O2 for a 10-layer Li2Pc(CN)8 LB film.
monomeric Li2Pc(CN)8 species in the LB film increased after the H2S gas exposure. Such a phenomenon has been observed also in electrochemical reduction of the Li2Pc(CN)8 LB film.9 In that case, a strong electrostatic repulsion in the reduced aggregated species led to their partial disassociation, resulting in the formation of more monomeric species.9 Therefore, this spectroscopic variation seems to indicate that the Li2Pc(CN)8 molecules in the LB film first interacted with H2S molecules and then reduced. It is very interesting that the B-band variation after the H2S gas exposure is much more striking than the Q-band region. As shown in Figure 4, the LB film exhibited two overlapped absorptions at about 364 and 412 nm, and the peak intensity of the latter was greatly reduced after the film was exposed to H2S gas. In the case of electrochemical reduction of the Li2Pc(CN)8 LB films, a new absorption was observed in the wavelength range from 400 to 600 nm, which was assigned to the formation of π radical anions.16 Yet in Figure 4 there appeared no new absorption for the LB film exposed to H2S gas. The reason may be
Notes
ascribed to the heavy desorption of H2S molecules after taking the film out of the gas sensor chamber. Further work such as in situ electronic absorption analysis should be done to clarify this point of view. The gas-sensing specificity of the Li2Pc(CN)8 LB film was further examined with respect to the two other gases of hydrogen and oxygen. Figure 5 depicts the time-current relation for the LB film exposed to pure (a) H2 and (b) O2. In the experimental time scale, the current was almost invariant with respect to the time in which the film was exposed to the reducing gas of H2. This might be ascribed to the weak ligation ability of H2 molecules in comparison with that of H2S, and thus, no charge-transfer interaction took place between a Li2Pc(CN)8 molecule and a H2 one. The central metal ions of lithium may also account for this result, and its substitution by transitional metal ions with empty orbital for ligation may improve the sensitivity of the Pc LB film to H2 gas but reduce its gas-sensing specificity. For the similar reasons, the Li2Pc(CN)8 LB films showed no current response to O2 gas with the concentration of 100 ppm (not shown), and yet a smaller (16) Stillman, M. J. Absorption and Magnetic Circular Dichroism Spectral Properties of Phthalocyanines Part 2: Ring-Oxidized and RingReduced Complexes. In Phthalocyanines - Properties and Applications, Vol. 3; Lezonff, C. C., Lever, A. B. P., Eds.; VCH: New York, 1989.
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current decrease to pure O2 gas in comparison with that of 100 ppm NO2. Therefore, it seems reasonable to conclude that the Li2Pc(CN)8 LB film possesses a good selectivity to different gases such as H2S, H2, NO2, and O2 in terms of the current response. Conclusions Li2Pc(CN)8 LB films were successfully prepared and showed n-type semiconducting behavior rather than p-type. Owing to eight strong electron-withdrawing cyano groups on the periphery of the Pc structure, the Li2Pc(CN)8 LB film exhibited an excellent sensitivity to a reducing gas of H2S in comparison with that to the oxidizing gases of O2 and NO2. Also, a selective response of the LB film to the two reducing gases of H2S and H2 was observed. Acknowledgment. We are most grateful for helpful discussions with Prof. Atsushi Takahara, Kyushu University. This work was, in part, supported by a Grantin-Aid for Scientific Research (A) (13355034) and by a Grant-in-Aid for Scientific Research in the Priority Area of “Molecular Synchronization for Design of New Materials System” (404/13022253) from the Ministry of Education, Science, Sports, and Culture, Japan. LA026161T
