Influence of VOC structures on sensing property of SmFeO3 semiconductive gas sensor

Sensors and Actuators B 202 (2014) 873–877

Contents lists available at ScienceDirect

Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb

Influence of VOC structures on sensing property of SmFeO3 semiconductive gas sensor Masami Mori a , Yoshiteru Itagaki a,∗ , Jun Iseda a , Yoshihiko Sadaoka b , Tsuyoshi Ueda c , Hirokazu Mitsuhashi c , Mikiya Nakatani c a

Graduate Schools of Science and Engineering, Ehime University, Matsuyama 890-8577, Ehime, Japan The Cooperative Center of Scientific and Industrial Research, Ehime University, Matsuyama 890-8577, Ehime, Japan c Engineering Department, Sensor Headquarters, New Cosmos Electric Co., Ltd., Yodogawa-ku, Osaka 532-0036, Japan b

a r t i c l e

i n f o

Article history: Received 8 March 2014 Received in revised form 26 May 2014 Accepted 11 June 2014 Available online 19 June 2014 Keywords: Perovskite-type oxide p-type semiconductor Competitive adsorption VOC ppb

a b s t r a c t A perovskite-type SmFeO3 oxide, which has a p-type semiconducting feature, was used as a chemoresistive sensor material for VOC detection. SmFeO3 particles were deposited on a comb-type Pt electrode printed on an alumina substrate. In this study, 15 kinds of VOCs (methanol, ethanol, propanol, butanol, acetic acid, propionic acid, methyl acetate, ethyl acetate, acetone, methylethylketone, benzene, toluene, xylene, ethyl benzene, and chlorobenzene) with concentrations below 10 ppm were examined in the range of 350 and 500 ◦ C. The resistance of the sensor increased when the referenced air was contaminated with the VOCs. The sensing behavior was characterized by the relationship, Rvoc /Rair = 1 + ˛C1/m . The observed results suggested the detecting ability of ppb-levels of several VOCs. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Recently, air quality contamination by many kinds of volatile organic compounds (VOCs) has become a serious problem for human life. Thus, environmental monitoring is now strongly demanded, especially in urban areas. Among the various sensing techniques used to detect such harmful gases, solid-state gas sensors are of practical interest due to their small size and ease of use. Above all, the chemo-resistive type sensors are the most widely investigated, because of their ease of use, low cost, etc. Many investigations have been made using several semiconducting gas sensors for the VOCs, especially, ethanol, benzene, toluene, xylene and so on. However, their gas sensing properties, such as sensitivity, selectivity and operating conditions related to the molecular structure of VOC are still unsatisfactory. So far, tin dioxide, which has an n-type semiconducting feature, has been the mainstream of investigations as a sensing material due to its good sensitivity, chemical stability and low cost. Meanwhile, perovskite-type oxides with an ABO3 structure have many application fields, such as electrode catalytic oxidation and gas sensitive characteristics. A number of perovskite type oxides prepared in air show p-type semiconducting properties, and their resistance decreases with the adsorption of oxidizing

gases such as ozone and NO2 and increased with the adsorption of oxidizable gases such as ethanol [1–12]. So far, it has been clarified that SmFeO3 semiconducting sensors show good responses especially to oxidative gases [1,2,5,7]. Furthermore, base conductivity of the sensor was adjustable with the partial substitution of Fe by Co [6,9]. Previously Mori et al. [13] reported a potentiometric VOC sensor where one of the Pt electrodes was covered with SmFeO3 catalytic layer. This study indicated that SmFeO3 was a good oxidation catalyst for VOCs. Although, it is generally known that the sensor response of p-type semiconductors is smaller than n-type semiconductors, SmFeO3 can be a candidate of VOC sensing material in a p-type semiconducting sensor. However, it is still lack of data on VOC sensing characteristics of a SmFeO3 semiconducting sensor. In order to develop a conventional VOC monitoring system for environmental air containing several VOCs in sub ppm levels, it is of intrinsic importance to know influence of VOC structure on the sensing property. Therefore in the present study, the sensing characteristics of SmFeO3 based p-type semiconducting gas sensor to the seventeen VOC molecules having different functional groups and molecular weights were examined. 2. Experimental 2.1. Preparation of SmFeO3 powder

∗ Corresponding author. Tel.: +81 89 927 9755. E-mail address: [email protected] (Y. Itagaki). http://dx.doi.org/10.1016/j.snb.2014.06.031 0925-4005/© 2014 Elsevier B.V. All rights reserved.

The SmFeO3 powders were obtained by the thermal decomposition of a Sm[Fe(CN)6 ]·4H2 O heteronuclear complex in air at 900 ◦ C

Resistance /Ohm

874

M. Mori et al. / Sensors and Actuators B 202 (2014) 873–877

10

9

10

8

o

350 C

107

10

6

10

5

o

400 C o

450 C o

500 C 104

20

Ethanol

Air

Ethanol

Air

40

60

Air 80

100

Time/min Fig. 1. Transient response curves of SmFeO3 sensor in 1 ppm ethanol for the temperature range between 350 and 500 ◦ C.

for 2 h. The complex was synthesized by mixing aqueous solutions of equimolar amounts of Sm(NO3 )3 ·6H2 O and K3 Fe(CN)6 with continuous stirring. The resulting precipitate was washed with water, ethanol and diethyl ether, before drying in air. The synthesized complex was decomposed to a perovskite-type SmFeO3 oxide at 900 ◦ C for 2 h. The obtained powders were bead-milled. The specific surface area was 21.6 m2 /g. 2.2. Preparation of sensing layer A 0.24 g sample of the powder was dispersed in 30 ml of acetylacetone and sonicated for 1 h at room temperature to obtain a suspension of the SmFeO3 particles. The obtained suspension was left for 1 h to precipitate the larger SmFeO3 particles. A SmFeO3 sensing layer was formed on comb-type Pt electrodes on an alumina substrate having 40 ␮m intervals. The sensing layer was formed by an electrophoretic deposition using the supernatant liquid of the above suspension. The amount of the SmFeO3 deposit was controlled by the deposition time at a constant applied dc voltage. After drying in ambient air, the film formed on the sensing substrate was heated at around 600 ◦ C. 2.3. Sensor examination The concentration of the VOC in the test air was controlled by changing the mixing ratio of the VOC vapor and dried air, and the flow rate of the mixed gas to the test chamber was 300 cm3 /min. The system consisted of a dried air generator and VOC generator equipped with flow controllers to accurately mix the sample gas concentration, and a digital electrometer to measure the electrical dc resistance. The temperature of the sensing element was controlled by applying a dc voltage to the Pt heater formed on the backside of the sensor element.

Fig. 2. Sensor responses, RVOC /Rair , for the series of VOCs.

are summarized in Fig. 2. Since the highest responses are confirmed at 400 ◦ C for each VOC, the sensing performance at 400 ◦ C is mainly discussed. Transient response curves for the contamination of 3 ppm VOCs are shown in Figs. 3–5. Common to all the VOCs, the sensor response to the VOC contaminations tends to be faster than the response in the recovery process. For the following discussion, the lowest resistance observed after 20 min under the uncontaminated air was defined as the resistance in air, Rair , in the series of the experimental measurements. It is generally recognized that in metal oxide samples, the resistance in air mainly represents the influence of surface reactions including electron exchange between oxygen and the oxide [14]. Adsorbed oxygen on the surfaces exists in the forms of O2 , O2 − , O− and O2− . As a result, the positive-hole concentration on the surface

3. Results and discussion Fig. 1 shows the resistance changes under flowing of dry air contaminated with 1 ppm of ethanol in the temperature range between 350 and 500 ◦ C. Since SmFeO3 has a p-type semiconducting feature, resistance of the sensor increased with the ethanol contamination. The sensor rapidly responded to the contaminatin; the 90% response time is about 0.5 s at each temperature. The recovery times are rather slower; the 90% recovery time is 3 s at 350 ◦ C and about 1 s at the higher temperatures. Herein, the sensor response is defined as the ratio of the resistances, Rvoc /Rair , where Rair and Rvoc are the resistance observed in air and the contaminated air with the VOC, respectively. The sensor responses for 3 ppm VOCs

Fig. 3. Transient response curves of SmFeO3 sensor in 3 ppm alcohols at 400 ◦ C.

M. Mori et al. / Sensors and Actuators B 202 (2014) 873–877

875

Process (3) is replacement of the adsorbed oxygen by the VOC. Processes (4) and (5) are known as the Eley–Rideal-type and Langmuir–Hinshelwood type reaction models, respectively. The electrical conductance of the senor is proportional to the concentration of the holen+ , and the surface concentrations of adsorbed oxygen and VOC are limited with the processes from (1) to (5). The number of adsorbed molecules is proportional to that of colliding molecules on the unoccupied adsorption sites, and the number of desorbing molecules is proportional to that of the adsorbed molecules. Therefore, the adsorption and desorption rates are given by the following relationships, respectively. (→) = kC (NS − NA ) (adsorption rate)

(6)



(←) = k NA (desorption rate)

(7)

where k and k are rate constants of the adsorption and desorption, respectively; NS and NA are the total number of adsorption sites and occupied sites, respectively; and C is the concentration of the molecules in the gas phase. Under the equilibrated state, i.e., v(→) = v(←), the coverage, NA /NS , could be expressed as Fig. 4. Transient response curves of SmFeO3 sensor in 3 ppm aromatic VOCs at 400 ◦ C.

NA /NS =

(k/k )C 1 + (k/k )C

(8)

Based on a competitive adsorption–desorption model for two components, the concentration of the adsorbed component is expressed as NA /NS =

KA CA 1 + KA CA + KB CB

(9)

or more generally, 1/n

NA /NS =

KA CA 1/n

1 + KA CA

(10)

1/m

+ KB CB

where KA and KB are adsorption–desorption equilibrium constants for components A and B, and n and m are constants. By applying this relationship to the mixture of oxygen and VOC, the oxygen coverage is given by 1/n

(O2 ) =

KO2 CO

2

1/n

(11)

1/m

1 + KO2 CO + KVOC CVOC 2

Fig. 5. Transient response curves of SmFeO3 sensor in 3 ppm acids, esters and ketones at 400 ◦ C.

of the oxide increases. Actually, the resistance of SmFeO3 decreases with the adsorption of oxidizing gases such as NO2 and ozone. On the other hand, the exposure to reducing gases, such as a VOC and water vapor, results in an increased resistance. The increase in the resistance with the exposure to the VOC-contaminated air represents a decrease in the concentration of the negatively charged oxygen species. The decrease in the charged oxygen on the surface of the metal oxide is due to the reaction and/or replacement with VOC molecules. The concentration of the charged oxygen on the surface is equilibrated with the adsorbed oxygen molecule. When O2 molecules are adsorbed in the form of O2 ␦- (ad), the possible reactions are as follows: O2 (g) ↔ O2 ␦- (ad) + hole␦+

(1)

VOC(g) + O2 ␦- (ad) ↔ VOC␦- (ad) + O2 (g)and/oradditive

(2)

VOC(g) + O2 ␦- (ad) + hole␦+ ↔ VOC(ad) + O2 (g)

(3)

VOC(g) + O2

␦-

(ad) + hole

␦+

→ oxidizedproducts

VOC(ad) + O2 ␦- (ad) + hole␦+ → oxidizedproducts

(4) (5)

and (O2 )air KVOC 1/m = 1+ C 1/n VOC (O2 )VOC 1 + KO2 CO

(12)

2

It has been widely known that the electric resistance of a semiconducting sensor is proportional to the power of gas concentration, that is so-called power law. Yamazoe and Shimanoe [15] theoretically explained the power law governing n-type semiconducting sensors by the combination of a depletion theory and surface adsorption or reaction of the associated gases. While, the electric resistance of a p-type semiconducting sensor has been also theoretically-modeled by the combination between the surface chemistry and the associated charge transfer processes [16–18]. If the concentration of the surface-adsorbed oxygen is a decisive factor in the sensor resistance, it is very striking that VOC molecules in just ppm-level affects the concentration of the adsorbed-oxygen. By assuming competitive adsorption between VOC and oxygen molecules, the VOC concentration dependence of the oxygen covKVOC and m. erage could now be simulated by the function of 1/n 1 + KO C

2 O2

In this study we tried to explain that the sensor resistance in a low concentration of VOC gas dominantly determined by the difference of the equilibrium constants of VOC and oxygen. In this assumption,

876

M. Mori et al. / Sensors and Actuators B 202 (2014) 873–877

Table 1 Characteristics ˛ and m for the VOCs at 400 ◦ C.

Alcohols Methanol Ethanol 1-Propanol 1-Butanol 2-Methyl-2-propanol Aromatics Benzene Toluene Chlorobenzene Ethylbenzene o-Xylene p-Xylene Carboxylic acids Acetic acid Propionic acid Esters Methyl acetate Ethyl acetate Ketones Acetone Methyl ethyl ketone

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

9

30

˛

m

R

18.11 21.33 23.88 28.30 10.08

2.825 3.884 3.719 4.994 2.456

0.905 0.999 0.914 0.875 0.994

1.816 4.991 2.098 10.79 15.66 15.38

1.323 1.199 2.124 5.547 4.243 4.426

2

0.984 0.949 0.840 0.950 0.968 0.837

17.02 23.84

2.891 3.197

0.917 0.960

14.33 15.88

3.436 2.818

0.986 0.970

27.88 31.92

6.604 5.112

0.919 0.928



(13)

RVOC /RO2 is expressed as RVOC Gair = = Rair GVOC



(O2 )air (O2 )VOC

3

20

1

10

15

7

17 16

6

10

15 5

5

14

13 12

0 20

40

60

80

100

120

Fig. 6. Correlation between characteristic value of ˛ (=

KVOC 1/n 2 O2

1 + KO C

) and molecular

weight. The VOCs in this figure is numbered as in Table 1.

Methanol Ethanol 1-Propanol 1-Butanol 2-Methyl-2-propanol Methyl acetate Ethyl acetate



Acetone (14)

Methyl ethyl ketone Acetic acid

When  = 1, Gair (O2 )air KVOC RVOC 1/m = = = 1+ C 1/n VOC Rair GVOC (O2 )VOC 1 + KO2 CO

Propionic acid (15)

Benzene

To confirm the sensor response law, the characteristics, ˛ (= ) and m, were estimated for the experimental results with 1/n

Chlorobenzene

2

KVOC

1 + KO C

2 O2

11

2

MW

the surface conductance, G, originating from the surface concentration of charged oxygen is proportional to the power of O2 coverage, that is G∞

25

Alcohols Aromatics Carboxylic acids Esters Ketones

4

8

α

VOC

35

the standard deviation, R2 and summarized in Table 1. The proposed sensor response law satisfies the observed sensor response of the 1/n VOC. It seems that the characteristic, KO2 CO , is hardly influenced 2 by the VOC gas concentration. Based on the adsorption models, a higher ˛-value suggests a stronger attractive interaction between the VOC and adsorption site and a higher reactivity of the VOC with adsorbed oxygen on the surface. The shift from physisorption to chemisorption results a higher ˛. Also, a stronger attractive interaction results in a higher m-value. A higher m-value suggests a lower VOC concentration dependency on the coverage of adsorbed oxygen molecules, i.e., chemisorption. When the VOC oxidative reactions with adsorbed oxygen occur in parallel, the KVOC value apparently increases, since the oxidative decomposition of VOC occurs with a decrease in the coverage of the oxygen molecules. Therefore, the characteristics, ˛ and m, are the preferable measures to realize the VOC effects on the coverage of the adsorbed oxygen molecules. The characteristic, ␣, is an equilibrium constant related to the adsorption, desorption rates and also to the decomposition/reaction rates of the adsorbed molecule. Fig. 6 shows the correlation between the characteristic, ˛, and molecular weight of the VOC. It seems that the characteristic, ␣, has a tendency to increase with an increase in the molecular weight and/or carbon number, except for the highly branched t-butanol and chlorobenzene, and the order of ˛ is as follows: aliphatic ketone > aliphatic alcohol > aliphatic acid > aliphatic acetate > aromatics. No correlation between the molecular structure and the characteristic, m,

Toluene 0.01ppm

Ethylbenzene

0.1ppm

o-Xylene p-Xylene 5

10

15 R

VOC

/R

20

25

air

Fig. 7. The estimated sensor responses of the series of VOCs.

could be determined. At this stage, experimental measurement of the response in a lower at VOC levels below sub ppm is very difficult, so the response in the lower concentration region less than ppm is interpreted with the aid of the confirmed response law and is shown in Fig. 7. The interpreted results suggest the possibility to detect VOC ppb-levels, excepted for benzene, toluene and chlorobenzene. 4. Conclusion In the present study, we proposed that the sensing behavior should be characterized by two factors, i.e., KVOC and m, to clarify the response and selectivity for VOCs. The concentration dependence of the response, Rvoc /Rair , could be expressed by a power law: Gair RVOC 1/m = = 1 + ˛ CVOC Rair GVOC

M. Mori et al. / Sensors and Actuators B 202 (2014) 873–877

The power law could be induced by the adsorption kinetics of oxygen molecules for the p-type semiconducting gas sensor assuming that the conductance is proportional to the surface concentration of the adsorbed oxygen. The observed higher values of ˛ and m suggest a lower VOC concentration dependency of the coverage of adsorbed oxygen molecules, i.e., chemisorption, especially for oxygen-containing VOCs. The characteristic, ˛, is strongly influenced by the side reactions of the VOC with adsorbed oxygen on the solid surface and is a preferable measure to discuss the interaction of the VOC with adsorbed oxygen and the effects of the VOC molecular structure. Additional experiments to determine the characteristics for the diverse number of VOCs are now in progress. At this stage, we will refrain from determining the effects of the molecular structure on the sensitivity and selectivity in detail since the sensing characteristics are strongly influenced by many factors, such as the catalytic activity, electrochemical process, molecular structure including steric hindrance, polarity and molecular volume, etc. References [1] M.C. Carotta, G. Martinelli, Y. Sadaoka, P. Nunziante, E. Traversa, Gas-sensitive electrical properties of perovskite-type SmFeO3 thick films, Sens. Actuators B 48 (1998) 270–276. [2] G. Martinelli, M.C. Carotta, M. Ferroni, Y. Sadaoka, E. Traversa, Screen-printed perovskite-type thick films as gas sensor for environmental monitoring, Sens. Actuators B 55 (1999) 99–110. [3] S. Matsushima, N. Sano, Y. Sadaoka, C3 H6 sensitive SmFeO3 prepared from thermal decomposition of heteronuclear complexes, {Ln[Fe(CN)6 ]·nH2 O}x (Ln = La, Sm, Dy), J. Ceram. Soc. Jpn. 108 (2000) 892–897. [4] E. Traversa, Y. Sadaoka, M. Carrota, G. Martinelli, Environmental monitoring field tests using screen-printed thick-film sensors based on semiconducting oxides, Sens. Actuators B 65 (2000) 181–185. [5] Y. Hosoya, Y. Itagaki, H. Aono, Y. Sadaoka, Ozone detection in air using SmFeO3 gas sensor, Sens. Actuators B 108 (2005) 198–201. [6] M. Mori, Y. Itagaki, Y. Sadaoka, Effect of VOC on ozone detection using semiconducting sensor with SmFe1−x Cox O3 perovskite-type oxides, Sens. Actuators B 163 (2012) 44–50. [7] M. Mori, J. Fujita, Y. Itagaki, Y. Sadaoka, Ozone detection in air using SmFeO3 gas sensor for air quality classification, J. Ceram. Soc. Jpn. 119 (12) (2011) 926–928. [8] X. Ge, Y. Liu, X. Liu, Preparation and gas-sensitive properties of LaFe1−y Coy O3 semiconducting materials, Sens. Actuators B 78 (2001) 171–174. [9] M. Zhao, H. Peng, J. Hu, Z. Han, Effect of cobalt doping on the microstructure, electrical and ethanol-sensing properties of SmFe1−x Cox O3 , Sens. Actuators B 129 (2008) 953–957. [10] K. Fan, H. Qin, Z. Zhang, L. Sun, L. Sun, J. Hu, Gas sensing properties of nanocrystalline La0.75 Ba0.25 FeO3 , Sens. Actuators B 171 (2012) 302–308. [11] X. Liu, B.C. Cheng, J. Hu, H. Qin, M. Jiang, Semiconducting gas sensor for ethanol based on LaMgx Fe1−x O3 nanocrystals, Sens. Actuators B 129 (2008) 50–58.

877

[12] P.-J. Yao, J. Wang, H.-Y. Du, J.-Q. Qi, Synthesis, characterization and formaldehyde gas sensitivity of La0.7 Sr0.3 FeO3 nanoparticles assembled nanowires, Mater. Chem. Phys. 134 (2012) 61–67. [13] M. Mori, H. Nishimura, Y. Itagaki, Y. Sadaoka, Potentiometric VOC detection in air using 8YSZ-based oxygen sensor modified with SmFeO3 catalytic layer, Sens. Actuators B 142 (2009) 141–146. [14] N. Barsan, U. Weimar, Conduction model of metal oxide gas sensors, J. Electroceram. 7 (2001) 143–167. [15] N. Yamazoe, K. Shimanoe, Theory of power laws for semiconductor gas sensors, Sens. Actuators B 128 (2008) 566–573. [16] N. Barsan, C. Simion, T. Heine, S. Pokhrel, U. Weimar, Modeling of sensing and transduction for p-type semiconducting metal oxide based gas sensors, J. Electroceram. 25 (2010) 11–19. [17] M. Hübner, C.E. Simion, A. Tomescu-Stânoiu, S. Pokhrel, N. Bârsan, U. Weimar, Influence of humidity on CO sensing with p-type CuO thick film gas sensors, Sens. Actuators B 153 (2011) 347–353. [18] K. Sahner, R. Moos, Modeling of hydrocarbon sensors based on p-type semiconducting perovskites, Phys. Chem. Chem. Phys. 9 (2007) 635–642.

Biographies Masami Mori received her B. Sci. degree in analytical chemistry from Ehime University in 2003. She obtained a Dr. Eng. degree from Ehime University in 2008. She is a research associate in the Department of Material Science and Engineering since 2003. Her main interest is fine-particle applications for chemical sensors and catalysts. Yoshiteru Itagaki received his M. Eng. in 1995 in Industrial Chemistry from Hiroshima University. He obtained a Dr. Eng. from Hiroshima University in 1998. He is Senior Assistant professor in Ehime University since 2011. His main interests are design and fabrication of functional ceramic membranes for chemical sensors and fuel cells. Yoshihiko Sadaoka received his M. Eng. degree in industrial chemistry from Ehime University in 1971. He has been on the Faculty of Engineering at Ehime University since 1971. He obtained a Dr. Eng. degree from Kyushu University in 1979. He is a Professor in the Department of Material Science and Engineering since 1996 and in the Cooperative Center of Scientific and Industrial Research since 2012. His main interests are inorganic and organic functional materials for chemical sensors and green materials. Jun Iseda received his BE degree in materials chemistry from Ehime University in 2011. He is currently a master student in the Department of Material Science and Engineering. His main interest is fine-particle applications for chemical sensors and catalysts. Tsuyoshi Ueda received his Ph.D. in engineering from Kumamoto University in 2008. His main interests are development of high sensitive semiconductor type gas sensors and gas chromatography embedded those sensors as the detector. Hirokazu Mitsuhashi received his BS degree from Kwansei Gakuin University in Japan in 1985. He has been engaged in the field of research, development, and application of gas sensors based on semiconductor. Mikiya Nakatani received his BS degree from Osaka Electro-Communication University in Japan in 1982. He has been engaged in the field of research, development, and application of gas sensors.