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Titaniumdioxide chemical sensor working with AC voltage
Sensors and Actuators B 46 (1998) 114 – 119
Titaniumdioxide chemical sensor working with AC voltage Mohammed Rafiqul Islam a,*, Noriyuki Kumazawa b, Manabu Takeuchi a a
Department of Electrical and Electronic Engineering, Ibaraki Uni6ersity, 4 -12 -1 Nakanarusawa, Hitachi, 316, Japan b Faculty of Engineering (Mito branch), Ibaraki Uni6ersity, 2 -1 -1 Bunkyo, Mito, 310, Japan Received 21 May 1997; received in revised form 14 January 1998; accepted 16 January 1998
Abstract A novel method to distinguish chemical compounds is proposed, based on multidimensional information, derived from simple linear response. An ac voltage was applied to a titaniumdioxide (TiO2) semiconductor gas sensor. The resulting conductance, surface potential and phase difference of the input voltage-output current wave form were recorded. All the three parameters showed linear response to the concentration of the adsorbed chemical compounds in gaseous form and depended on their chemical structure. Three dimensional figure of these three linear parameters enabled us to distinguish alcohols and benzenes with a single detector. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Titaniumdioxide; AC voltage; Surface potential; Conductivity; Phase difference; Chemical sensor
1. Introduction The phenomenon of a conductive film sensitive to the presence of gases has been known and studied for many decades [1]. Commercial products relying on this reaction have been available for well over 25 years with the development of tin and zinc oxide sensors proposed by Seiyama et al. [2]. In conventional sensors, the parameters used to detect certain chemical species are limited. So, it is difficult to distinguish chemical species on the basis of static informations, like conductance and surface potential, used so far. This can be ascribed to the fact that the conventional gas sensors usually respond to interferants coexisting in a gas sample. To overcome this hurdle, the combination of different types of sensors (multiarray sensor) has been expected to be useful. Distinction among components and quantification of the components are considered to be possible, with this model, based on the assumption of a linear relationship between the output of the detectors and the concentration of individual species. But the model can not always be considered as a strong candidate, applicable for biosensors (like osmotic or taste sensors). The reason for this can be attributed to the nonlinear response of * Corresponding author. Tel.: +81 294 385091; fax: + 81 294 385275; e-mail: [email protected] 0925-4005/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S0925-4005(98)00096-3
these sensors because of the saturation on the surface adsorption, due to the concentration of the adsorbents. Nakata et al. [3] reported that discernment of gases can be possible in a gaseous mixture with a single detector using dynamic nonlinear response. They discussed about the responses in relation to the kinetics of the reaction at the sensor surface and the temperature-dependent barrier potential of the semiconductor. Recently, M.E. Hassan Armani et al. [4] reported that similar descripters can be generated by using ac current at suitable frequencies to follow the changes in sensor capacitance, conductance and dissipation factor. As mentioned at the beginning of this paper, it is unpracticable to detect individual gas molecule with a single dc type sensor, which has been used so far. So in this paper, the author report that application of ac voltage will enable us to add up the range of parameters, like capacitance or phase difference of the input voltage and output current, to the conventional values. Inclusion of this new linear parameter will take us a step forward to recognize the existence of different molecules, with a single sensor.
2. Experimental Binder resin, polyvinylidenfluoride (PVDF) was dissolved in an organic solvent N,N-dimethylacetamide
M. Rafiqul Islam et al. / Sensors and Actuators B 46 (1998) 114–119
(DMA). Titaniumdioxide (TiO2) powder (Wako chemicals, less than 1 mm in diameter) of Rutile structure was then dispersed in the attained solution to make the final slurry. Vacuum deposition of a pair of gold (Au) electrodes were conducted on Corning 7059 glass substrates. The area of each electrode and spacing between them were 1.3×0.3 cm2 and 0.1 cm, respectively. Films of around 10 mm in thickness were prepared with spin coating on substrates equipped with gold electrodes to fabricate a Corning 7059/ Au/TiO2 structure. Fig. 1 shows the structure of the device. Different types of alcohols (methanol, ethanol, npropanol, n-butanol, n-pentanol and n-hexanol) and aromatic compounds (benzene, monochlorobenzene, dichlorobenzene, nitrobenzene and aniline) were vaporized according to its individual saturated vapor pressure, at 20°C. To get equal concentration for each gas, the authors extracted different amounts from these vaporized gases depending on their vapor pressure at 20°C and then released it inside the cryostat for adsorption. All the chemicals were of special grade (Kanto Chemical, purity, by gas chromatography, 99.0%). To determine the response of the sensor to each compound, the authors exposed the sensor to the vapor of equal concentration. A function generator (Wavetek, Model 167) was used for applying sinusoidal voltage of various frequencies (1 40 Hz) to the sensor, set on a well-shielded cryostat. The input and output wave forms of the sensor were measured by an oscilloscope (Iwatsu, SS5802) and the values were stored on a digital recorder (Teac, DR-F1). The value of current was observed with an electrometer (Advantest, TR8652). All the measurements were conducted in dark, at room temperature.
3. Results and discussion
3.1. DC measurements In an effort to characterize the fundamental properties of the sensor, dc measurements were conducted.
Fig. 1. Structure of the surface type TiO2 sensor.
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Fig. 2. Normalized conductivity of a TiO2 gas sensor as a function of molecular weight for various alcohols and benzenes.
Fig. 2 shows that the change in normalized conductivity (conductivity of the sensor at adsorbed condition at a set concentration, divided by the value at nonadsorbed condition) for the alcohols are higher than that of the benzenes. A dc voltage of 2V was applied on the Au electrodes. The abscissa and ordinate represent the molecular weight and change of conductivity, respectively. For a nonadsorbed sensor the conductivity was around 2.6 × 1014 S/cm. This value increased linearly in the alcohol adsorbed sensors with the molecular weight. For comparison, the concentration of the adsorbed gas was the same, i.e. 500 ppm. Further rise in conductivity with the increase in concentration was observed. It is well known that in metal oxide semiconductors like, zinc oxide, titaniumdioxide etc. oxygen ions (O − ) tie up electrons at the surface. As a result the charge carrier density is reduced, which in turn increases the resistance [5]. At this stage when a reducing gas is adsorbed on the surface, carrier density gets an increment. So, it will be relevant to consider that due to this reason the conductivity increases in alcohol adsorbed sensor. To support the assumption, the authors examined the response of the adsorbed sensor, under the atmosphere of argon gas. At this condition, the conductivity showed an increase in value. But, when exposed to volatile chemicals no remarkable change in the value of conductivity was observed. Alcohols with increased -CH2- groups are more easily decomposed and oxidized than those with fewer one. Increase in the value of conductivity with the types of alcohols proves the fact. On the other hand, the conductivity of the aromatic gases varied from 1.5 to 2 times, where the value was in the vicinity of 7.6×1014 S/cm, depending on the acceptor (nitrobenzene, monochlorobenzene, dichlorobenzene) and donor (aniline) characteristics of the chemicals, used in the experiments. But the difference was not in a major degree, as the alcohols.
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Thus, from the above results the authors can say that the parameter of conductivity is useful to gather information about known compounds, but, difficult to distinguish the same when it remains unknown.
3.2. Application of the sinusoidal 6oltage
In the contrary, amino (-NH2) derivative is known as a donor that comparatively shows higher conductivity. The saturation frequency was in the range of 11 to 6 Hz, for benzene to dichlorobenzene, which means that it is easier for the aromatic compounds to adsorb than to desorb.
The relation between the input-output wave form is shown in Fig. 3 (alcohols) and Fig. 4 (benzenes). The frequency was 30 Hz. The authors found that the amplitude and shift in baseline of the output current not only changed with the type of alcohol, but also showed phase difference, which was typical for the individual chemical compound. These differences are well understood in Fig. 3, where the responses of methanol, ethanol, n-propanol, n-butanol, n-pentanol and n-hexanol are compared. The results are in good agreement with the dc measurement, because the increase in amplitude implies the decrease in resistivity. The shift in baseline can be considered to be due to the change in surface potential, that affects the work function (c) of the titaniumdioxide film, with chemical reaction at the surface. If the authors put the changed work function (or the effective work function) as, ceff, the authors can define that ceff =c − DV
(1)
where, DV, denote the surface potential of the bulk. This value ranged from 2.7 mV in methanol to 12.2 mV in n-hexanol gas adsorption. In benzene compounds it varied from 1.4 mV with dichlorobenzene to 2.9 mV with benzene. Difference in phase of the output current shows that a change in capacitance of the sensor also takes place with the adsorption of the gases. Fig. 5 shows the dependence of phase difference, on frequency (abscissa). It is notable that each gas compound shows a saturation value at a certain frequency, which the authors consider to be typical for the same. For the alcohols, the saturation frequency is directly proportional to the number of hydrocarbon contained in alcohol molecules. The reason of this saturation frequency can be ascribed to the fact that at this particular frequency it becomes difficult for the individual gas to polarize and create charge, because at this stage, the rate of reaction can not cope with the fast change of the applied electric field. The saturation frequency ranged from 26 to 11 Hz, for methanol to n-hexanol adsorption. Among the benzene derivatives, the characteristics of the substituents, as an electron withdrawing or donating group affect the overall properties of the chemical compounds. Chloro (-Cl) and nitro (-NO2) derivatives are known as acceptors, existence of which in benzene ring decrease the conductivity of the sensor.
Fig. 3. Relationship between input voltage and output current waveform for (a) unadsorbed and when exposed to (b) methanol, (c) ethanol, (d) n-propanol, (e) n-butanol, (f) n-pentanol, (g) n-hexanol gas. Applied ac voltage is 2V and 30 Hz. Gas concentration for each case is 500 ppm.
M. Rafiqul Islam et al. / Sensors and Actuators B 46 (1998) 114–119
Fig. 4. Relationship between input voltage and output current waveform for (a) unadsorbed and when exposed to (b) benzene, (c) monochlorobenze, (d) nitrobenzene and (e) aniline gas. Applied ac voltage is 2V and 30 Hz. Gas concentration for each case is 500 ppm.
3.3. Dependence of electrical properties on gas concentration The authors observed the response of the sensor, varying the concentration of the test gases from 200 to
Fig. 5. Phase difference as a function of applied voltage frequency, when exposed to 500 ppm of (a) methanol, (b) ethanol, (c) npropanol, (d) n-butanol, (e) n-pentanol and (f) n-hexanol gas.
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Fig. 6. A, Value of current as a function of (a) methanol, (b) ethanol, (c) n-propanol, (d) n-butanol, (e) n-pentanol and (f) n-hexanol gas concentration. Applied ac voltage is 2V and 30 Hz. B, Value of surface potential as a function of (a) methanol, (b) ethanol, (c) n-propanol, (d) n-butanol, (e) n-pentanol and (f) n-hexanol gas concentration. Applied ac voltage is 2V and 30 Hz.
1200 ppm. Dependencies of current and surface potential value on gas concentration are shown in Fig. 6a, b. All the parameters showed increase in values, with the increase in the concentration of the gas. For example, the conductivity in methanol gas adsorption increased to 3.2×1013 S/cm, which was almost four times larger than that of its initial value, i.e. at 200 ppm. For the sensor, conductivities for various gas adsorption shows a saturation at a concentration over 1000 ppm. Increase in the value of surface potential was also observed. The authors have not yet conducted any specific experiment to realize the exact mechanism for the same. But, as mentioned for Eq. (1) change in the work function of the film can be considered as a vital reason for the same. Further, it is seen that this parameter saturates over a gas concentration of 900 ppm. For, DV = (P× Cgas)/o
(2)
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where, P, o, Cgas, denote electric polarization, dielectric constant of the film and gas concentration [6], respectively. Since increase in gas concentration increases the number of electric dipoles, interaction of these decrease the number of charge that in consequence effect the surface potential. The difference in phase between input voltage and the output current intensified, which can be attributed to the change in charge concentration at the space-charge region on the surface. The authors conducted experiments with the same chemical several times for reconfirmation and found that this change in phase value was also typical for individual molecules. This explanation is applicable for all the gases. Fig. 7 shows the dependence of saturation frequency (in Fig. 5) on concentration of the gases for alcohols. The result is in good agreement with other values, as the saturation frequency increase linearly with gas concentration. It implies that increase in gas molecule increase the rate of reaction and as a result create a change in carrier concentration. The amount of this carrier concentration determines the overall electrical properties of the sensor. Alcohol molecules showed better and vivid response, comparing to that of the aromatic compounds.
4. Distinction of gas molecules Fig. 8 shows the three dimensional plot that represents the relationship among current-surface potentialphase difference, achieved from the experiment. It is note worthy that each group, containing three values represent individual gas molecule. Value of surface potential, phase difference and conductivity are shown in x, y and z-axis, respectively. The applied ac voltage was 2V, 30 Hz and for each case the adsorbed gas concentration was 500 ppm. As the response of the alcohols molecules are larger than those of the benzenes, it is easy to differentiate
Fig. 8. Relationship among surface potential, phase difference and conductivity in a TiO2 sensor, when exposed to (500 ppm) various alcohol and benzene compounds, in gaseous form. Applied ac voltage is 2V and 30 Hz.
these two different types of molecules, using the same parameters. Overall, the alcohol molecules show higher effect which enables us to envisage a border to differentiate the alcohol and aromatic compounds. In that respect the region of the alcohols stand over 0.12 nA and 1.67 mV, for the value of current and surface potential, respectively. As mentioned at the beginning of this paper, all the experiments are conducted at dark. So, if the authors consider about the response or sensitivity of the sensor, it is beyond any doubt that incidence of light or dye sensitization etc. can increase the responsivity of the sensor at a higher order. The authors have not discussed the response of mixed gases on the sensor in this paper and experiments for which is underway. In this context also, sensitivity of the sensor is considered to be an important factor for minute distinction of individual molecule, in a mixture. Increase in parameters, visual or numerical can also be pursued. For example, if the authors conduct the Fourier transformation of the output current value from the attained curves or measure the dissipation factor of the sensor impedence at various frequencies, it may enable the study to get additional information about individual molecules.
5. Conclusions
Fig. 7. Dependence of saturation frequency on gas concentration. Applied ac voltage is 2V and 30 Hz.
A new method to distinguish alcohol and benzene molecules is proposed by increasing the range of parameters with the application of ac voltage. Results showed that (1) the three parameters, i.e. conductivity, surface potential and phase difference show typical
M. Rafiqul Islam et al. / Sensors and Actuators B 46 (1998) 114–119
change in values with the type of the molecule, (2) the amount of adsorbed gas concentration and the frequency of the applied voltage affect the electrical properties of individual molecule, (3) these parameters can be considered as an important information to trace particular molecule. Use of multidimensional information availed in a linear response can be expected to give a boon to the development of artificial sensor mimicking olfaction which is capable of discriminating between mixed odors, easily.
References [1] P.T. Moseley, New trends and future prospects of thick and thin-film gas sensors, Sens. Actuators B 3 (1991) 167–174. [2] T. Seiyama, A. Kato, K. Fujiishi, M. Nagatani, A new detector for gaseous components using semiconducting thin films, Anal. Chem. 34 (1962) 1502–1503. [3] S. Nakata, S. Akabe, M. Nakasuji, K. Yoshikawa, Gas sensing based on a nonlinear response: discrimination between hydrocarbons and quantification of individual components in a gas mixture, Anal. Chem. 68 (1996) 2067–2072. [4] M.E. Hassan Amrani, P.A. Payne, K.C. Persaud, Multi-frequency measurements of organic conducting polymers for sensing of gases and vapours, Sens. Actuators B 33 (1996) 137 – 141. [5] W.M. Sears, K. Colbow, F. Consadori, General characteristics of thermally cycled tin oxide gas sensors, Semicond. Sci. Technol. 4 (1989) 351 – 355. [6] T. Ikoma, Applied Physics Handbook, 2, Maruzen, Tokyo, 1990, pp. 237 – 238.
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Biographies Mohammed Rafiqul Islam received his B.Sc. (1994) and M.Sc. (1996) degrees from Ibaraki University, Japan. Rafiq is now working for his Ph.D. in chemical sensors at the Department of Science and Technology, Ibaraki University. Noriyuki Kumazawa received his D.Sc. in applied chemistry in 1989 from Osaka city University, Osaka, Japan. He joined at Wakayama Medical College as an assistant professor in 1980. At present, he is an associate professor at the Department of Science and Technology, Ibaraki University, Japan. His research interests are in artificial lipid bilayer membrane and electrical properties of high order structure of synthetic polymers. Manabu Takeuchi received his B.Sc., M.Sc. and D.Sc. degrees from the Tokyo Institute of Technology, Tokyo, Japan, in 1966, 1968 and 1971, respectively. He is now a Professor of Department of Electrical and Electronic Engineering, Ibaraki University. His research interests include electronic and photoelectronic properties of organic and inorganic semiconductor layers. Dr. Takeuchi is a member of the Japan Society of Applied Physics and the Surface Finishing Society of Japan.
Titaniumdioxide chemical sensor working with AC voltage Mohammed Rafiqul Islam a,*, Noriyuki Kumazawa b, Manabu Takeuchi a a
Department of Electrical and Electronic Engineering, Ibaraki Uni6ersity, 4 -12 -1 Nakanarusawa, Hitachi, 316, Japan b Faculty of Engineering (Mito branch), Ibaraki Uni6ersity, 2 -1 -1 Bunkyo, Mito, 310, Japan Received 21 May 1997; received in revised form 14 January 1998; accepted 16 January 1998
Abstract A novel method to distinguish chemical compounds is proposed, based on multidimensional information, derived from simple linear response. An ac voltage was applied to a titaniumdioxide (TiO2) semiconductor gas sensor. The resulting conductance, surface potential and phase difference of the input voltage-output current wave form were recorded. All the three parameters showed linear response to the concentration of the adsorbed chemical compounds in gaseous form and depended on their chemical structure. Three dimensional figure of these three linear parameters enabled us to distinguish alcohols and benzenes with a single detector. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Titaniumdioxide; AC voltage; Surface potential; Conductivity; Phase difference; Chemical sensor
1. Introduction The phenomenon of a conductive film sensitive to the presence of gases has been known and studied for many decades [1]. Commercial products relying on this reaction have been available for well over 25 years with the development of tin and zinc oxide sensors proposed by Seiyama et al. [2]. In conventional sensors, the parameters used to detect certain chemical species are limited. So, it is difficult to distinguish chemical species on the basis of static informations, like conductance and surface potential, used so far. This can be ascribed to the fact that the conventional gas sensors usually respond to interferants coexisting in a gas sample. To overcome this hurdle, the combination of different types of sensors (multiarray sensor) has been expected to be useful. Distinction among components and quantification of the components are considered to be possible, with this model, based on the assumption of a linear relationship between the output of the detectors and the concentration of individual species. But the model can not always be considered as a strong candidate, applicable for biosensors (like osmotic or taste sensors). The reason for this can be attributed to the nonlinear response of * Corresponding author. Tel.: +81 294 385091; fax: + 81 294 385275; e-mail: [email protected] 0925-4005/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S0925-4005(98)00096-3
these sensors because of the saturation on the surface adsorption, due to the concentration of the adsorbents. Nakata et al. [3] reported that discernment of gases can be possible in a gaseous mixture with a single detector using dynamic nonlinear response. They discussed about the responses in relation to the kinetics of the reaction at the sensor surface and the temperature-dependent barrier potential of the semiconductor. Recently, M.E. Hassan Armani et al. [4] reported that similar descripters can be generated by using ac current at suitable frequencies to follow the changes in sensor capacitance, conductance and dissipation factor. As mentioned at the beginning of this paper, it is unpracticable to detect individual gas molecule with a single dc type sensor, which has been used so far. So in this paper, the author report that application of ac voltage will enable us to add up the range of parameters, like capacitance or phase difference of the input voltage and output current, to the conventional values. Inclusion of this new linear parameter will take us a step forward to recognize the existence of different molecules, with a single sensor.
2. Experimental Binder resin, polyvinylidenfluoride (PVDF) was dissolved in an organic solvent N,N-dimethylacetamide
M. Rafiqul Islam et al. / Sensors and Actuators B 46 (1998) 114–119
(DMA). Titaniumdioxide (TiO2) powder (Wako chemicals, less than 1 mm in diameter) of Rutile structure was then dispersed in the attained solution to make the final slurry. Vacuum deposition of a pair of gold (Au) electrodes were conducted on Corning 7059 glass substrates. The area of each electrode and spacing between them were 1.3×0.3 cm2 and 0.1 cm, respectively. Films of around 10 mm in thickness were prepared with spin coating on substrates equipped with gold electrodes to fabricate a Corning 7059/ Au/TiO2 structure. Fig. 1 shows the structure of the device. Different types of alcohols (methanol, ethanol, npropanol, n-butanol, n-pentanol and n-hexanol) and aromatic compounds (benzene, monochlorobenzene, dichlorobenzene, nitrobenzene and aniline) were vaporized according to its individual saturated vapor pressure, at 20°C. To get equal concentration for each gas, the authors extracted different amounts from these vaporized gases depending on their vapor pressure at 20°C and then released it inside the cryostat for adsorption. All the chemicals were of special grade (Kanto Chemical, purity, by gas chromatography, 99.0%). To determine the response of the sensor to each compound, the authors exposed the sensor to the vapor of equal concentration. A function generator (Wavetek, Model 167) was used for applying sinusoidal voltage of various frequencies (1 40 Hz) to the sensor, set on a well-shielded cryostat. The input and output wave forms of the sensor were measured by an oscilloscope (Iwatsu, SS5802) and the values were stored on a digital recorder (Teac, DR-F1). The value of current was observed with an electrometer (Advantest, TR8652). All the measurements were conducted in dark, at room temperature.
3. Results and discussion
3.1. DC measurements In an effort to characterize the fundamental properties of the sensor, dc measurements were conducted.
Fig. 1. Structure of the surface type TiO2 sensor.
115
Fig. 2. Normalized conductivity of a TiO2 gas sensor as a function of molecular weight for various alcohols and benzenes.
Fig. 2 shows that the change in normalized conductivity (conductivity of the sensor at adsorbed condition at a set concentration, divided by the value at nonadsorbed condition) for the alcohols are higher than that of the benzenes. A dc voltage of 2V was applied on the Au electrodes. The abscissa and ordinate represent the molecular weight and change of conductivity, respectively. For a nonadsorbed sensor the conductivity was around 2.6 × 1014 S/cm. This value increased linearly in the alcohol adsorbed sensors with the molecular weight. For comparison, the concentration of the adsorbed gas was the same, i.e. 500 ppm. Further rise in conductivity with the increase in concentration was observed. It is well known that in metal oxide semiconductors like, zinc oxide, titaniumdioxide etc. oxygen ions (O − ) tie up electrons at the surface. As a result the charge carrier density is reduced, which in turn increases the resistance [5]. At this stage when a reducing gas is adsorbed on the surface, carrier density gets an increment. So, it will be relevant to consider that due to this reason the conductivity increases in alcohol adsorbed sensor. To support the assumption, the authors examined the response of the adsorbed sensor, under the atmosphere of argon gas. At this condition, the conductivity showed an increase in value. But, when exposed to volatile chemicals no remarkable change in the value of conductivity was observed. Alcohols with increased -CH2- groups are more easily decomposed and oxidized than those with fewer one. Increase in the value of conductivity with the types of alcohols proves the fact. On the other hand, the conductivity of the aromatic gases varied from 1.5 to 2 times, where the value was in the vicinity of 7.6×1014 S/cm, depending on the acceptor (nitrobenzene, monochlorobenzene, dichlorobenzene) and donor (aniline) characteristics of the chemicals, used in the experiments. But the difference was not in a major degree, as the alcohols.
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Thus, from the above results the authors can say that the parameter of conductivity is useful to gather information about known compounds, but, difficult to distinguish the same when it remains unknown.
3.2. Application of the sinusoidal 6oltage
In the contrary, amino (-NH2) derivative is known as a donor that comparatively shows higher conductivity. The saturation frequency was in the range of 11 to 6 Hz, for benzene to dichlorobenzene, which means that it is easier for the aromatic compounds to adsorb than to desorb.
The relation between the input-output wave form is shown in Fig. 3 (alcohols) and Fig. 4 (benzenes). The frequency was 30 Hz. The authors found that the amplitude and shift in baseline of the output current not only changed with the type of alcohol, but also showed phase difference, which was typical for the individual chemical compound. These differences are well understood in Fig. 3, where the responses of methanol, ethanol, n-propanol, n-butanol, n-pentanol and n-hexanol are compared. The results are in good agreement with the dc measurement, because the increase in amplitude implies the decrease in resistivity. The shift in baseline can be considered to be due to the change in surface potential, that affects the work function (c) of the titaniumdioxide film, with chemical reaction at the surface. If the authors put the changed work function (or the effective work function) as, ceff, the authors can define that ceff =c − DV
(1)
where, DV, denote the surface potential of the bulk. This value ranged from 2.7 mV in methanol to 12.2 mV in n-hexanol gas adsorption. In benzene compounds it varied from 1.4 mV with dichlorobenzene to 2.9 mV with benzene. Difference in phase of the output current shows that a change in capacitance of the sensor also takes place with the adsorption of the gases. Fig. 5 shows the dependence of phase difference, on frequency (abscissa). It is notable that each gas compound shows a saturation value at a certain frequency, which the authors consider to be typical for the same. For the alcohols, the saturation frequency is directly proportional to the number of hydrocarbon contained in alcohol molecules. The reason of this saturation frequency can be ascribed to the fact that at this particular frequency it becomes difficult for the individual gas to polarize and create charge, because at this stage, the rate of reaction can not cope with the fast change of the applied electric field. The saturation frequency ranged from 26 to 11 Hz, for methanol to n-hexanol adsorption. Among the benzene derivatives, the characteristics of the substituents, as an electron withdrawing or donating group affect the overall properties of the chemical compounds. Chloro (-Cl) and nitro (-NO2) derivatives are known as acceptors, existence of which in benzene ring decrease the conductivity of the sensor.
Fig. 3. Relationship between input voltage and output current waveform for (a) unadsorbed and when exposed to (b) methanol, (c) ethanol, (d) n-propanol, (e) n-butanol, (f) n-pentanol, (g) n-hexanol gas. Applied ac voltage is 2V and 30 Hz. Gas concentration for each case is 500 ppm.
M. Rafiqul Islam et al. / Sensors and Actuators B 46 (1998) 114–119
Fig. 4. Relationship between input voltage and output current waveform for (a) unadsorbed and when exposed to (b) benzene, (c) monochlorobenze, (d) nitrobenzene and (e) aniline gas. Applied ac voltage is 2V and 30 Hz. Gas concentration for each case is 500 ppm.
3.3. Dependence of electrical properties on gas concentration The authors observed the response of the sensor, varying the concentration of the test gases from 200 to
Fig. 5. Phase difference as a function of applied voltage frequency, when exposed to 500 ppm of (a) methanol, (b) ethanol, (c) npropanol, (d) n-butanol, (e) n-pentanol and (f) n-hexanol gas.
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Fig. 6. A, Value of current as a function of (a) methanol, (b) ethanol, (c) n-propanol, (d) n-butanol, (e) n-pentanol and (f) n-hexanol gas concentration. Applied ac voltage is 2V and 30 Hz. B, Value of surface potential as a function of (a) methanol, (b) ethanol, (c) n-propanol, (d) n-butanol, (e) n-pentanol and (f) n-hexanol gas concentration. Applied ac voltage is 2V and 30 Hz.
1200 ppm. Dependencies of current and surface potential value on gas concentration are shown in Fig. 6a, b. All the parameters showed increase in values, with the increase in the concentration of the gas. For example, the conductivity in methanol gas adsorption increased to 3.2×1013 S/cm, which was almost four times larger than that of its initial value, i.e. at 200 ppm. For the sensor, conductivities for various gas adsorption shows a saturation at a concentration over 1000 ppm. Increase in the value of surface potential was also observed. The authors have not yet conducted any specific experiment to realize the exact mechanism for the same. But, as mentioned for Eq. (1) change in the work function of the film can be considered as a vital reason for the same. Further, it is seen that this parameter saturates over a gas concentration of 900 ppm. For, DV = (P× Cgas)/o
(2)
118
M. Rafiqul Islam et al. / Sensors and Actuators B 46 (1998) 114–119
where, P, o, Cgas, denote electric polarization, dielectric constant of the film and gas concentration [6], respectively. Since increase in gas concentration increases the number of electric dipoles, interaction of these decrease the number of charge that in consequence effect the surface potential. The difference in phase between input voltage and the output current intensified, which can be attributed to the change in charge concentration at the space-charge region on the surface. The authors conducted experiments with the same chemical several times for reconfirmation and found that this change in phase value was also typical for individual molecules. This explanation is applicable for all the gases. Fig. 7 shows the dependence of saturation frequency (in Fig. 5) on concentration of the gases for alcohols. The result is in good agreement with other values, as the saturation frequency increase linearly with gas concentration. It implies that increase in gas molecule increase the rate of reaction and as a result create a change in carrier concentration. The amount of this carrier concentration determines the overall electrical properties of the sensor. Alcohol molecules showed better and vivid response, comparing to that of the aromatic compounds.
4. Distinction of gas molecules Fig. 8 shows the three dimensional plot that represents the relationship among current-surface potentialphase difference, achieved from the experiment. It is note worthy that each group, containing three values represent individual gas molecule. Value of surface potential, phase difference and conductivity are shown in x, y and z-axis, respectively. The applied ac voltage was 2V, 30 Hz and for each case the adsorbed gas concentration was 500 ppm. As the response of the alcohols molecules are larger than those of the benzenes, it is easy to differentiate
Fig. 8. Relationship among surface potential, phase difference and conductivity in a TiO2 sensor, when exposed to (500 ppm) various alcohol and benzene compounds, in gaseous form. Applied ac voltage is 2V and 30 Hz.
these two different types of molecules, using the same parameters. Overall, the alcohol molecules show higher effect which enables us to envisage a border to differentiate the alcohol and aromatic compounds. In that respect the region of the alcohols stand over 0.12 nA and 1.67 mV, for the value of current and surface potential, respectively. As mentioned at the beginning of this paper, all the experiments are conducted at dark. So, if the authors consider about the response or sensitivity of the sensor, it is beyond any doubt that incidence of light or dye sensitization etc. can increase the responsivity of the sensor at a higher order. The authors have not discussed the response of mixed gases on the sensor in this paper and experiments for which is underway. In this context also, sensitivity of the sensor is considered to be an important factor for minute distinction of individual molecule, in a mixture. Increase in parameters, visual or numerical can also be pursued. For example, if the authors conduct the Fourier transformation of the output current value from the attained curves or measure the dissipation factor of the sensor impedence at various frequencies, it may enable the study to get additional information about individual molecules.
5. Conclusions
Fig. 7. Dependence of saturation frequency on gas concentration. Applied ac voltage is 2V and 30 Hz.
A new method to distinguish alcohol and benzene molecules is proposed by increasing the range of parameters with the application of ac voltage. Results showed that (1) the three parameters, i.e. conductivity, surface potential and phase difference show typical
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change in values with the type of the molecule, (2) the amount of adsorbed gas concentration and the frequency of the applied voltage affect the electrical properties of individual molecule, (3) these parameters can be considered as an important information to trace particular molecule. Use of multidimensional information availed in a linear response can be expected to give a boon to the development of artificial sensor mimicking olfaction which is capable of discriminating between mixed odors, easily.
References [1] P.T. Moseley, New trends and future prospects of thick and thin-film gas sensors, Sens. Actuators B 3 (1991) 167–174. [2] T. Seiyama, A. Kato, K. Fujiishi, M. Nagatani, A new detector for gaseous components using semiconducting thin films, Anal. Chem. 34 (1962) 1502–1503. [3] S. Nakata, S. Akabe, M. Nakasuji, K. Yoshikawa, Gas sensing based on a nonlinear response: discrimination between hydrocarbons and quantification of individual components in a gas mixture, Anal. Chem. 68 (1996) 2067–2072. [4] M.E. Hassan Amrani, P.A. Payne, K.C. Persaud, Multi-frequency measurements of organic conducting polymers for sensing of gases and vapours, Sens. Actuators B 33 (1996) 137 – 141. [5] W.M. Sears, K. Colbow, F. Consadori, General characteristics of thermally cycled tin oxide gas sensors, Semicond. Sci. Technol. 4 (1989) 351 – 355. [6] T. Ikoma, Applied Physics Handbook, 2, Maruzen, Tokyo, 1990, pp. 237 – 238.
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Biographies Mohammed Rafiqul Islam received his B.Sc. (1994) and M.Sc. (1996) degrees from Ibaraki University, Japan. Rafiq is now working for his Ph.D. in chemical sensors at the Department of Science and Technology, Ibaraki University. Noriyuki Kumazawa received his D.Sc. in applied chemistry in 1989 from Osaka city University, Osaka, Japan. He joined at Wakayama Medical College as an assistant professor in 1980. At present, he is an associate professor at the Department of Science and Technology, Ibaraki University, Japan. His research interests are in artificial lipid bilayer membrane and electrical properties of high order structure of synthetic polymers. Manabu Takeuchi received his B.Sc., M.Sc. and D.Sc. degrees from the Tokyo Institute of Technology, Tokyo, Japan, in 1966, 1968 and 1971, respectively. He is now a Professor of Department of Electrical and Electronic Engineering, Ibaraki University. His research interests include electronic and photoelectronic properties of organic and inorganic semiconductor layers. Dr. Takeuchi is a member of the Japan Society of Applied Physics and the Surface Finishing Society of Japan.