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Gas sensing composites of metal oxides with vapor-deposited polypyrrole
Sensors and Actuators B 143 (2009) 6–11
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Gas sensing composites of metal oxides with vapor-deposited polypyrrole夽 N.O. Savage Department of Chemistry and Chemical Engineering, University of New Haven, West Haven, CT 06516, USA
a r t i c l e
i n f o
Article history: Received 14 August 2008 Received in revised form 28 August 2009 Accepted 31 August 2009 Available online 6 September 2009 Keywords: Molybdenum oxide Polypyrrole Composites Ethanol sensing
a b s t r a c t A composite of MoO3 and polypyrrole is prepared by a vaporization polymerization technique. Thick films of the sol–gel molybdenum oxide with added iron (III) chloride are deposited on sensing substrates and exposed to pyrrole vapors to initiate polymerization. Infrared analysis confirms that polypyrrole forms on the surface of the MoO3 . Sensing measurements are made between room temperature and 300 ◦ C to various concentrations of ethanol vapors. At room temperature the molybdenum oxide–polypyrrole composite responds to ethanol with improved sensing response over that of MoO3 or polypyrrole when tested separately. Additionally, as the concentration of ethanol is increased, there is a change in the direction of the response of the composite sensor, from an increase in resistance at low concentration to a decrease in resistance at higher concentrations. This change is attributed to a catalytic reaction of ethanol on the molybdenum oxide surface. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Composite materials for gas sensing applications have been an area of increasing research over the past several years. They range from mixed metal oxides and mixed polymers [1,2] to insulating materials blended with more conductive materials such as carbon black [3] to inorganic–organic hybrids [4–6]. These materials are of interest because they provide improved and/or unique responses to analyte gases, as compared to the materials of which they are comprised. A limited body of work exists which explores the sensing behavior of composites of metal oxides with polypyrrole [4–6]. Humidity sensors have been prepared from composites of polypyrrole with both TiO2 [4] and Fe3 O4 [5]. In the latter case, the material is also highly sensitive to N2 , O2 and CO2 concentration changes. WO3 –polypyrrole composites are sensitive to H2 S at 90 ◦ C, a response that far exceeds that of either polypyrrole or WO3 when tested independently [6]. The primary routes for preparing metal oxide/conducting polymer composites are either to mix particles of the two pure substances in the desired ratio [6], or to synthesize the polymer in the presence of metal oxide particles [4,5]. The second case is thought to result in a more uniform distribution of oxide particles in the polymer matrix [5]. This paper presents a third approach to preparing metal oxide/conducting polymer composites; a direct vaporization polymerization of a conducting polymer onto an oxide surface.
In this work, MoO3 is used as the inorganic component of the composite. MoO3 has been widely studied as a catalyst for ethanol oxidation. While studies on its gas sensing behavior are not uncommon, it has received far less attention than other oxides such as SnO2 and TiO2 . Molybdenum oxide sensors have been used to detect a variety of gases such as nitrogen oxides, carbon monoxide, ammonia and hydrocarbons [7–10] and studies show that MoO3 has excellent an response to ethanol in comparison to TiO2 and WO3 sensors [1]. Composites of polypyrrole with MoO3 have been prepared by Hosono et al. with oriented, chemical vapor-deposited MoO3 films and showed preferential gas sensing response to polar organic molecules [11]. Here, an approach used for preparing mixed polymer composites is applied to a MoO3 –polypyrrole system [2,12]. MoO3 films are exposed to pyrrole vapors, which polymerize on the surface of the MoO3 . This simple technique allows for direct deposition of polymer films on sensing substrates and other materials without specialized instrumentation. The resulting composite is tested for its gas sensing response to ethanol. Ethanol sensors are widely employed for the identification of intoxicated drivers, for monitoring of fermentation processes for food and beverage production and for leak detection. As the worldwide use of plant-derived ethanol fuel increases, there will be additional needs for detecting ethanol in automobiles and in fuel filling stations. 2. Experimental 2.1. Synthesis of composites
夽 Paper presented at the International Meeting of Chemical Sensors 2008 (IMCS12), July 13–16, 2008, Columbus, OH, USA. E-mail address: [email protected]. 0925-4005/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2009.08.052
MoO3 particles were synthesized by a sol–gel method starting with MoCl5 [13]. A 1:3 mixture of water and ethanol was added
N.O. Savage / Sensors and Actuators B 143 (2009) 6–11
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Fig. 1. Diagram of procedure for preparing the MoO3 –polypyrrole composite sensors.
dropwise to MoCl3 under N2 . The mixture was stirred for 1 h until a translucent green sol was formed. The MoO3 sol was dried in a 100 ◦ C oven and then heat-treated at 350 ◦ C for 3 h. FeCl3 , an oxidant used in the synthesis of polypyrrole, was added to the MoO3 by two different means. Method 1 involved adding a dilute solution of FeCl3 in place of water during the sol–gel synthesis of MoO3 , as described above. These samples are referred to as MolyFe1 in the text. The second procedure was to mix the heat-treated powders of MoO3 with a dilute solution of FeCl3 and allow them to air dry. These samples are referred to as MolyFe2 in the text. The process for preparing the composite sensors is shown in Fig. 1. The iron-doped molybdenum oxide powder (either MolyFe1 or MolyFe2) was deposited as a thick film onto alumina sensing substrates (15 mm × 10 mm alumina with gold interdigitated electrodes available from Electronics Design Center, Case Western Reserve University, Cleveland, OH) or onto glass slides. These MoO3 coated substrates were placed in a small jar with a small amount (less than 0.5 mL) of pyrrole. The jar was tightly sealed and left at room temperature for 24 h for polymerization to occur. For characterization purposes a few glass slides were coated only with polypyrrole. FeCl3 was deposited onto the slide from solution and the slide exposed to pyrrole vapor, as was done with the molybdenum oxide coated substrates. 2.2. Characterization of composites MoO3 samples were characterized by X-ray diffraction with a Phillips XRG 3100 diffractometer. MoO3 –polypyrrole composites were characterized using FTIR spectroscopy. An infrared microscope (IlluminatIR from SensIR, Danbury, CT) was used to compare samples on substrates before and after exposure to polypyrrole vapor in order to confirm that polymerization of the pyrrole vapor had occurred on the sensor surface. Cross-sectional images of the films were obtained with a FEG Scanning Electron Microscope (LEO 1550). D.C. resistance measurements of sensing films were made as a function of time as air or air contaminated with ethanol was passed over the sensor. The ethanol vapor was obtained by bubbling dry air through room temperature ethanol. The sensor was
Fig. 2. Infrared spectrum of polypyrrole film vapor-deposited on glass slide.
Fig. 3. Response of vapor-deposited polypyrrole to ethanol at (a) room temperature, (b) 50 ◦ C and (c) 100 ◦ C.
set in a tube furnace for temperature control. The 20 s of data prior to the sensor’s exposure to ethanol was averaged and taken as the resistance of the sensor in air, while the 20 s of data prior to turning off ethanol exposure was averaged and taken as the resistance of the sensor in ethanol. The sensors were recovered in air between each concentration of ethanol measured. Measurements were made between room temperature and 300 ◦ C. The results are reported as: Sensor response =
Resistanceethanol − Resistanceair Resistanceair
3. Results and discussion 3.1. Vapor-deposited polypyrrole films Polypyrrole films were directly deposited onto either glass slides or alumina sensing substrates that had been treated with FeCl3 . FeCl3 is a well-established oxidant used in the polymerization of pyrrole and also results in the incorporation of the chloride ion into the polymer as a counter ion.
Presence of FeCl3 on the surface of the glass slides or alumina substrates allows for the polymerization of the pyrrole to occur at the surface without the addition of light or heat. A similar approach was described by de Melo et al. [2] in preparing polymer blends and by Han and Shi [12], who use an organic ferric salt to form polypyrrole on metal surfaces. Fig. 2 shows the infrared spectrum of a polypyrrole film deposited directly onto a FeCl3 -coated glass slide. The expected stretching and bending vibrations of polypyrrole are observed in the spectrum. The peak at 1611 cm−1 is the bending vibration of the N–H bond in pyrrole. The peak at 1489 cm−1 is assigned to the C C and C–C vibration of the pyrrole ring. A second peak is expected at about 1550 cm−1 but is obscured by the strong N–H bending band. In plane bending vibrations due to C–H are found at 1380 cm−1 and 1108 cm−1 and the C–N stretch is found at 1231 cm−1 . Additional peaks at 977 cm−1 and 925 cm−1 can be assigned to C–C stretches of the disubstituted ring and C–H out of plane bend, respectively. The observed peaks for the vapor-deposited polypyrrole match those observed in other studies [14–16]. Polypyrrole films deposited on alumina sensing substrates were tested for their sensing response to ethanol. Fig. 3 presents the
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N.O. Savage / Sensors and Actuators B 143 (2009) 6–11
3.3. MoO3 –polypyrrole composites
Fig. 4. X-ray diffraction spectrum of molybdenum oxide cosynthesized with FeCl3 (Mo:Fe, 1:0.005) and heat-treated at 350 ◦ C for 3 h.
results of the vapor-deposited polypyrrole sensing behavior at room temperature, 50 ◦ C and 100 ◦ C at varying concentrations. The sensors’ response to ethanol is better at 50 ◦ C as compared to room temperature. At temperatures above 100 ◦ C, the response of the sensor is unstable; the sensor’s conductivity decreased continually during 24 h of observation. This is may be attributed to a degradation of the polymer at higher temperatures, which has been reported to begin by as low a temperature as 125 ◦ C [14,17,18]. The polypyrrole sensors prepared by this method showed slow response (often as long as 30 min to reach a stable response) and recovery times. The resistance of the vapor-deposited polypyrrole film increases upon exposure to ethanol, fitting the typical model for polypyrrole response to alcohols. Polypyrrole is a conductor due to the conjugation (alternating carbon single and double bonds) of the polymer chains. The observed resistance increase of conducting polymers upon exposure to organic vapors such as ethanol is attributed to polymer swelling. The organic solvent forms hydrogen bonds to the polymer, resulting in swelling of the polymer and distortion of the conductive pathways of the polymer chain [19,20]. An alternate description of conducting polymer behavior is that conduction occurs by the movement of the positive charge carriers along the polymer chain. An electron donating gas, such as ethanol, reduces the polymer, decreasing the number of positive charge carriers resulting in an increase in the polymer resistance [12,21].
MolyFe1 and MolyFe2 samples were deposited onto glass slides and then were exposed to pyrrole vapors at room temperature for 24 h to initiate the vaporization of polypyrrole. Prior to pyrrole exposure, the MolyFe1 sample is blue-gray in color, while the MolyFe2 sample is slightly orange due to the higher concentration of iron. Fig. 6 shows optical images of the samples before and after pyrrole exposure. The MolyFe1 shows a slight color change and the MolyFe2 shows a distinct color change to black. Polypyrrole films are typically black or greenish-black in color, so this color change suggests successful vapor deposition of the polymer film. SEM images of the MolyFe1 and MolyFe2 samples are presented in Fig. 7. In both cases, there is evidence of a film on the surface of the molybdenum oxide particles. The MolyFe1 sample has a film that is less than 20 nm thick, while the film on the MolyFe2 sample is on the order of 200 nm. Along with the color change, this suggests that increasing the amount of FeCl3 added to the MoO3 increases the thickness of the polypyrrole film that forms. Infrared spectroscopy was used to determine if the films observed with the optical and electron microscopes were polypyrrole. Infrared spectra of the MolyFe1 samples did not indicate the presence of polypyrrole because the film was to thin. However, the thicker films on the MolyFe2 samples are confirmed as polypyrrole. Fig. 8 shows the infrared spectra of MolyFe2 before and after pyrrole exposure. Peaks marked with an asterisk (*) are found in the infrared spectrum of polypyrrole (Fig. 2). The sensing behavior of the MolyFe1–polypyrrole and MolyFe2–polypyrrole composites was tested in response to ethanol. Unlike the sensors prepared without the polymer film these new materials were conductive at room temperature, allowing low temperature sensing behavior to be measured. Fig. 9 shows the sensor responses of the molybdenum oxide–polypyrrole composites, MolyFe1–polypyrrole and MolyFe2–polypyrrole, to ethanol at room temperature, 50 ◦ C and 100 ◦ C. The response at room temperature is generally greater
3.2. Sol–gel MoO3 –Fe samples Molybdenum oxide was prepared by the sol–gel procedure and doped with FeCl3 by two different methods. In the first method (sample MolyFe1) an aqueous sample of FeCl3 was added in place of water during the MoO3 sol–gel synthesis. The molar ratio of Mo:Fe in this sample is 1:0.005. X-ray diffraction data for MoFe1 is shown in Fig. 4. After a 3 h heat treatment at 350 ◦ C, the MoO3 is present in the expected orthorombic phase [13]. In the second method (sample MolyFe2), the dried powders were stirred in a solution of FeCl3 and dried. The MolyFe2 samples have a molar ratio of Mo:Fe of 1:0.05. Sensors were prepared from both of these materials and tested for a response to ethanol. As expected, neither MolyFe1 nor MolyFe2 was conductive at low temperatures. Typically, undoped molybdenum oxide is not conductive until temperatures greater than 200 ◦ C [7,8,21]. The results of sensing tests at 200 ◦ C and 300 ◦ C are shown in Fig. 5. Both sensors have a greater response to ethanol at 300 ◦ C as compared to 200 ◦ C, with response times averaging 5 min at both of these temperatures. The sensors also both responded to ethanol with a decrease in electrical resistance, as opposed to the polypyrrole, which had a resistance increase in response to ethanol. This n-type behavior is typical of many metal oxides, including MoO3 [7–10,22].
Fig. 5. Top: response of MolyFe1 sensor to ethanol vapor at 200 ◦ C and 300 ◦ C. Bottom: response of MolyFe2 sensor to ethanol vapor at 200 ◦ C and 300 ◦ C. Background gas for both sets of measurements is dry air.
N.O. Savage / Sensors and Actuators B 143 (2009) 6–11
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Fig. 6. Optical images of (a) MolyFe1; (b) MolyFe1 after exposure to pyrrole vapor; (c) MolyFe2; and (d) MolyFe2 after exposure to pyrrole vapor. Scale bar on images is equal to 1 mm.
than the response at 50 ◦ C for the composites as compared to polypyrrole alone. Additionally, as the concentration of ethanol increases, the response of the composite switches from a positive response (resistance increasing) to a negative response (resistance decreasing). At 0.3% ethanol, both composite materials have a positive response, similar to that of the vapor-deposited polypyrrole. At 1% ethanol, the MolyFe2–polypyrrole sensor starts
Fig. 7. SEM images of (a) MolyFe1 and (b) MolyFe2 after exposure to pyrrole vapor.
to respond with an increase in resistance but then switches to a resistance decrease after a few minutes. This is more readily understood from the resistance vs. time data given in Fig. 10. This pattern repeats itself as the concentration is increased again. For the MolyFe1–polypyrrole sensor, the switch between positive and negative sensing responses does not occur until the sample is exposed to 2% ethanol. The response times for these materials 4 min, much faster than the response times observed for the pure polypyrrole sensors. A resistance decrease in response to ethanol was not observed in other studies of polypyrrole–molybdenum oxide composites [11], which may be due to the higher concentrations of ethanol used in this study. It is not an entirely unusual response. Kriván et al. observed a decrease in the resistance of dodecylsulfate and chloride doped polypyrrole films in response to H2 S, while in work by Su and Huang, polypyrrole showed a resistance decrease in response to increased humidity [4]. Composites of polypyrrole with both ZnO and Fe2 O3 [22,23] have shown resistance decreases as well. A common theory to explain the resistance decrease observed in
Fig. 8. Infrared spectrum of polypyrrole film vapor deposited on MolyFe2. Asterisks indicate peaks due to polypyrrole. Inset is infrared spectrum of MolyFe2 before polypyrrole exposure.
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and propose that cation mobility may account for the resistance decrease. As temperature is increased to 100 ◦ C, MolyFe1–polypyrrole continues to respond to ethanol vapors (Fig. 9) but the response of the MolyFe2–polypyrrole starts to degrade, with conductivity steadily decreasing over time. This result is similar to that of the vapor-deposited polypyrrole indicating that at higher temperatures the polymer in the composite may be decomposing. When the temperature is raised above 200 ◦ C, the conductivity of the film in air is unstable for several hours as the polymer film decomposes, then both the MolyFe1–polypyrrole and MolyFe2–polypyrrole sensors respond in much the same way as the oxides without the polymer film. 4. Conclusion A composite of molybdenum oxide and polypyrrole was prepared by a vapor phase polymerization technique, a simple preparation technique that allows for polymer deposition directly onto sensing substrates. The composite material has a higher response to ethanol as compare to either polypyrrole or molybdenum oxide when tested at room temperature. Additionally, the composites show resistance increases at low concentrations of ethanol which changes to a resistance decrease as the concentration of the ethanol is increased. The composite material’s selectivity toward other vapors is currently under investigation. Fig. 9. Response of composite sensors to ethanol. Top figure shows the sensing response of MolyFe1–polypyrrole composite and the bottom figure shows the sensing response the MolyFe2–polypyrrole composite.
the composites is that it is the result of a competition between the swelling mechanism of the polypyrrole and n-type semiconducting behavior of the oxide. In this study, however, it was established that the MolyFe1 and MolyFe2 materials are not conductive (and therefore not sensors) at temperatures below 200 ◦ C. It is therefore unlikely that the electrical response of the MoO3 is influencing the observed resistance decrease. Instead, it is proposed that the MoO3 acts as a catalyst rather than a sensor at these lower temperatures, dissociating the ethanol into CH3 O− and H+ , which then adsorb on the oxide surface [24]. It is possible that migration of H+ from the MoO3 surface to the polypyrrole would result in a decrease in the resistance of the polymer film. In their study of iron oxide–polypyrrole composites, Brezoi and Ion describe a mechanism where H+ reacts with doubly ionized oxygen vacancies in the oxide to form the hydroxide ion, OH− and frees up an electron for conduction [25]. Kriván et al. attribute their observed increase in the conductivity of polypyrrole films upon exposure to H2 S and other acidic vapors to the doping of the polymer film with H+ [21]
Fig. 10. Resistance vs. time ethanol sensing data at room temperature for the MolyFe2–polypyrrole composite.
Acknowledgments This work has been funded by a grant from the Connecticut Space Grant Consortium. Special thanks goes to Professor Howard Harris, Department of Forensic Science at the University of New Haven, for use of the FTIR microscope and to Professor Perena Gouma at the State University of New York at Stony Brook and Jeff Gilarde at Wesleyan University for the electron microscopy. References [1] K. Galatisis, Y.X. Li, W. Wlodarski, E. Comini, G. Sberveglieri, C. Cantalini, S. Santucci, M. Passacantando, Comparison of single and binary oxide MoO3 , TiO2 and WO3 sol–gel gas sensors, Sens. Actuators B: Chem. 83 (2002) 276–280. [2] C.P. de Melo, B.B. Neto, E.G. de Lima, L.F.B. de Lira, J.E.G. de Souza, Use of conducting polypyrrole blends as gas sensors, Sens. Actuators B: Chem. 109 (2005) 348–354. [3] M.A. Ryan, A.V. Shevade, H. Zhou, M.L. Homer, Polymer-carbon black composite sensors in an electronic nose for air quality monitoring, Mater. Res. Soc. Bull. 29 (2004) 714–719. [4] P.-G. Su, L.-N. Huang, Humidity sensors based on TiO2 nanoparticles/polypyrrole composite thin films, Sens. Actuators B: Chem. 123 (2007) 501–507. [5] R.P. Tandon, M.R. Tripathy, A.K. Arora, S. Hotchandani, Gas and humidity response of iron oxide–polypyrrole nanocomposites, Sens. Actuators B: Chem. 114 (2006) 768–773. [6] L. Geng, X. Huang, Y. Zhao, P. Li, S. Wang, S. Zhang, S. Wu, H2 S sensitivity study of polypyrrole/WO3 materials, Solid-State Electron. 50 (2006) 723–726. [7] S. Barazook, R.P. Tandon, S. Hotchandani, MoO3 -based sensor for NO, NO2 and CH4 detection, Sens. Actuators B: Chem. 119 (2006) 691–694. [8] S.S. Sunu, E. Prabhu, V. Jayaraman, K.I. Gnanasekar, T.K. Seshagiri, T. Gnanasekaren, Electrical conductivity and gas sensing properties of MoO3 , Sens. Actuators B: Chem. 101 (2004) 161–174. [9] E. Comini, G. Faglia, G. Sberveglieri, C. Catalini, M. Passaantado, S. Santcci, Y. Li, W. Wlodarski, W. Qu, Carbon monoxide response of molybdenum oxide thin films deposited by different techniques, Sens. Actuators B: Chem. (2000) 168–174. [10] A.K. Prasad, D.J. Kubinski, P.I. Gouma, Comparison of sol–gel and ion beam deposited MoO3 thin film gas sensors for selective ammonia detection, Sens. Actuators B: Chem. 93 (2003) 25–30. [11] K. Hosono, I. Matsubara, N. Murayama, S. Woosuck, N. Izu, Synthesis of polypyrrole/MoO3 hybrid thin film and their volatile organic compound gassensing properties, Chem. Mater. 17 (2005) 349–354. [12] G. Han, G. Shi, Porous polypyrrole/polymethylmethacrylate film prepared by vapor deposition polymerization of pyrrole and its application for ammonia detection, Thin Solid Films 515 (2007) 6986–6991.
N.O. Savage / Sensors and Actuators B 143 (2009) 6–11 [13] M. Epifani, P. Imperatori, L. Mirenghi, M. Schioppa, P. Siciliano, Synthesis and characterization of MoO3 thin films and powders from a molybdenum chloromethoxide, Chem. Mater. 16 (2004) 5495–5501. [14] S.A. Waghuely, S.M. Yenorkar, S.S. Yawale, S.P. Yawale, Application of chemically synthesized conducting polymer–polypyrrole as a carbon dioxide sensor, Sens. Actuators B: Chem. 128 (2008) 366–373. [15] M.M. Chehimi, E. Abdeljalil, A study of the degradation and stability of polypyrrole by inverse gas chromatography, X-ray photoelectron spectroscopy, and conductivity measurements, Synth. Met. 145 (2004) 15–22. [16] Y.-C. Liu, K.-H. Yang, L.-H. Lin, J.-F. Tsai, Studies of thermal decay of electropolymerized polypyrrole using in situ surface-enhanced Raman spectroscopy, Electrochem. Commun. 10 (2008) 161–164. [17] T.K. Vishnuvardhan, V.R. Kulkarni, C. Basavaraja, S.C. Raghavendra, Synthesis, characterization and A.C. conductivity of polypyrrole/Y2 O3 composites, Bull. Mater. Sci. 29 (2006) 77–83. [18] J. Jang, J. Bae, Carbon nanofiber/polypyrrole nanocable as toxic gas sensor, Sens. Actuators B: Chem. 122 (2007) 7–13. [19] C.W. Lin, B.J. Hwang, C.R. Lee, Methanol sensors based on the conductive polymer composites from polypyrrole and poly(vinyl alcohol), Mater. Chem. Phys. 55 (1998) 139–144. [20] S. Hamilton, M.J. Hepher, J. Sommerville, Polypyrrole materials for detection and discrimination of volatile organic compounds, Sens. Actuators B: Chem. 107 (2005) 424–432.
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[21] E. Kriván, C. Visy, R. Dobay, G. Harsanyi, O. Berkesi, Irregular response of the polypyrrole film to H2 S, Electroanalysis 12 (2000) 1195–1200. [22] L. Geng, S. Wang, Y. Zhao, P. Li, S. Zhang, W. Huang, S. Wu, Study of the primary sensitivity of polypyrrole/r-Fe2 O3 to toxic gases, Mater. Chem. Phys. 99 (2006) 15–19. [23] L. Geng, Y. Zhao, X. Huang, S. Wang, S. Zhang, W. Huang, S. Wu, The preparation and gas sensitivity study of polypyrrole/zinc oxide, Synth. Met. 156 (2006) 1078–1082. [24] E. Comini, L. Yubao, Y. Brando, G. Sberveglieri, Gas sensing properties of MoO3 nanorods to CO and CH3 OH, Chem. Phys. Lett. 407 (2005) 368–371. [25] D.-V. Brezoi, R.M. Ion, Phase evolution induced by polypyrrole in ironoxide–polypyrrole composite, Sens. Actuators B: Chem. 109 (2005) 171–175.
Biography Nancy Ortins Savage is an assistant professor in the Department of Chemistry and Chemical Engineering at the University of New Haven. She received her PhD from The Ohio State University and was a National Research Council Postdoctoral Fellow at the National Institute of Standards and Technology in Gaithersburg, Maryland. Her research interest is in the use of inorganic–organic composites for chemical sensing.
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Gas sensing composites of metal oxides with vapor-deposited polypyrrole夽 N.O. Savage Department of Chemistry and Chemical Engineering, University of New Haven, West Haven, CT 06516, USA
a r t i c l e
i n f o
Article history: Received 14 August 2008 Received in revised form 28 August 2009 Accepted 31 August 2009 Available online 6 September 2009 Keywords: Molybdenum oxide Polypyrrole Composites Ethanol sensing
a b s t r a c t A composite of MoO3 and polypyrrole is prepared by a vaporization polymerization technique. Thick films of the sol–gel molybdenum oxide with added iron (III) chloride are deposited on sensing substrates and exposed to pyrrole vapors to initiate polymerization. Infrared analysis confirms that polypyrrole forms on the surface of the MoO3 . Sensing measurements are made between room temperature and 300 ◦ C to various concentrations of ethanol vapors. At room temperature the molybdenum oxide–polypyrrole composite responds to ethanol with improved sensing response over that of MoO3 or polypyrrole when tested separately. Additionally, as the concentration of ethanol is increased, there is a change in the direction of the response of the composite sensor, from an increase in resistance at low concentration to a decrease in resistance at higher concentrations. This change is attributed to a catalytic reaction of ethanol on the molybdenum oxide surface. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Composite materials for gas sensing applications have been an area of increasing research over the past several years. They range from mixed metal oxides and mixed polymers [1,2] to insulating materials blended with more conductive materials such as carbon black [3] to inorganic–organic hybrids [4–6]. These materials are of interest because they provide improved and/or unique responses to analyte gases, as compared to the materials of which they are comprised. A limited body of work exists which explores the sensing behavior of composites of metal oxides with polypyrrole [4–6]. Humidity sensors have been prepared from composites of polypyrrole with both TiO2 [4] and Fe3 O4 [5]. In the latter case, the material is also highly sensitive to N2 , O2 and CO2 concentration changes. WO3 –polypyrrole composites are sensitive to H2 S at 90 ◦ C, a response that far exceeds that of either polypyrrole or WO3 when tested independently [6]. The primary routes for preparing metal oxide/conducting polymer composites are either to mix particles of the two pure substances in the desired ratio [6], or to synthesize the polymer in the presence of metal oxide particles [4,5]. The second case is thought to result in a more uniform distribution of oxide particles in the polymer matrix [5]. This paper presents a third approach to preparing metal oxide/conducting polymer composites; a direct vaporization polymerization of a conducting polymer onto an oxide surface.
In this work, MoO3 is used as the inorganic component of the composite. MoO3 has been widely studied as a catalyst for ethanol oxidation. While studies on its gas sensing behavior are not uncommon, it has received far less attention than other oxides such as SnO2 and TiO2 . Molybdenum oxide sensors have been used to detect a variety of gases such as nitrogen oxides, carbon monoxide, ammonia and hydrocarbons [7–10] and studies show that MoO3 has excellent an response to ethanol in comparison to TiO2 and WO3 sensors [1]. Composites of polypyrrole with MoO3 have been prepared by Hosono et al. with oriented, chemical vapor-deposited MoO3 films and showed preferential gas sensing response to polar organic molecules [11]. Here, an approach used for preparing mixed polymer composites is applied to a MoO3 –polypyrrole system [2,12]. MoO3 films are exposed to pyrrole vapors, which polymerize on the surface of the MoO3 . This simple technique allows for direct deposition of polymer films on sensing substrates and other materials without specialized instrumentation. The resulting composite is tested for its gas sensing response to ethanol. Ethanol sensors are widely employed for the identification of intoxicated drivers, for monitoring of fermentation processes for food and beverage production and for leak detection. As the worldwide use of plant-derived ethanol fuel increases, there will be additional needs for detecting ethanol in automobiles and in fuel filling stations. 2. Experimental 2.1. Synthesis of composites
夽 Paper presented at the International Meeting of Chemical Sensors 2008 (IMCS12), July 13–16, 2008, Columbus, OH, USA. E-mail address: [email protected]. 0925-4005/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2009.08.052
MoO3 particles were synthesized by a sol–gel method starting with MoCl5 [13]. A 1:3 mixture of water and ethanol was added
N.O. Savage / Sensors and Actuators B 143 (2009) 6–11
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Fig. 1. Diagram of procedure for preparing the MoO3 –polypyrrole composite sensors.
dropwise to MoCl3 under N2 . The mixture was stirred for 1 h until a translucent green sol was formed. The MoO3 sol was dried in a 100 ◦ C oven and then heat-treated at 350 ◦ C for 3 h. FeCl3 , an oxidant used in the synthesis of polypyrrole, was added to the MoO3 by two different means. Method 1 involved adding a dilute solution of FeCl3 in place of water during the sol–gel synthesis of MoO3 , as described above. These samples are referred to as MolyFe1 in the text. The second procedure was to mix the heat-treated powders of MoO3 with a dilute solution of FeCl3 and allow them to air dry. These samples are referred to as MolyFe2 in the text. The process for preparing the composite sensors is shown in Fig. 1. The iron-doped molybdenum oxide powder (either MolyFe1 or MolyFe2) was deposited as a thick film onto alumina sensing substrates (15 mm × 10 mm alumina with gold interdigitated electrodes available from Electronics Design Center, Case Western Reserve University, Cleveland, OH) or onto glass slides. These MoO3 coated substrates were placed in a small jar with a small amount (less than 0.5 mL) of pyrrole. The jar was tightly sealed and left at room temperature for 24 h for polymerization to occur. For characterization purposes a few glass slides were coated only with polypyrrole. FeCl3 was deposited onto the slide from solution and the slide exposed to pyrrole vapor, as was done with the molybdenum oxide coated substrates. 2.2. Characterization of composites MoO3 samples were characterized by X-ray diffraction with a Phillips XRG 3100 diffractometer. MoO3 –polypyrrole composites were characterized using FTIR spectroscopy. An infrared microscope (IlluminatIR from SensIR, Danbury, CT) was used to compare samples on substrates before and after exposure to polypyrrole vapor in order to confirm that polymerization of the pyrrole vapor had occurred on the sensor surface. Cross-sectional images of the films were obtained with a FEG Scanning Electron Microscope (LEO 1550). D.C. resistance measurements of sensing films were made as a function of time as air or air contaminated with ethanol was passed over the sensor. The ethanol vapor was obtained by bubbling dry air through room temperature ethanol. The sensor was
Fig. 2. Infrared spectrum of polypyrrole film vapor-deposited on glass slide.
Fig. 3. Response of vapor-deposited polypyrrole to ethanol at (a) room temperature, (b) 50 ◦ C and (c) 100 ◦ C.
set in a tube furnace for temperature control. The 20 s of data prior to the sensor’s exposure to ethanol was averaged and taken as the resistance of the sensor in air, while the 20 s of data prior to turning off ethanol exposure was averaged and taken as the resistance of the sensor in ethanol. The sensors were recovered in air between each concentration of ethanol measured. Measurements were made between room temperature and 300 ◦ C. The results are reported as: Sensor response =
Resistanceethanol − Resistanceair Resistanceair
3. Results and discussion 3.1. Vapor-deposited polypyrrole films Polypyrrole films were directly deposited onto either glass slides or alumina sensing substrates that had been treated with FeCl3 . FeCl3 is a well-established oxidant used in the polymerization of pyrrole and also results in the incorporation of the chloride ion into the polymer as a counter ion.
Presence of FeCl3 on the surface of the glass slides or alumina substrates allows for the polymerization of the pyrrole to occur at the surface without the addition of light or heat. A similar approach was described by de Melo et al. [2] in preparing polymer blends and by Han and Shi [12], who use an organic ferric salt to form polypyrrole on metal surfaces. Fig. 2 shows the infrared spectrum of a polypyrrole film deposited directly onto a FeCl3 -coated glass slide. The expected stretching and bending vibrations of polypyrrole are observed in the spectrum. The peak at 1611 cm−1 is the bending vibration of the N–H bond in pyrrole. The peak at 1489 cm−1 is assigned to the C C and C–C vibration of the pyrrole ring. A second peak is expected at about 1550 cm−1 but is obscured by the strong N–H bending band. In plane bending vibrations due to C–H are found at 1380 cm−1 and 1108 cm−1 and the C–N stretch is found at 1231 cm−1 . Additional peaks at 977 cm−1 and 925 cm−1 can be assigned to C–C stretches of the disubstituted ring and C–H out of plane bend, respectively. The observed peaks for the vapor-deposited polypyrrole match those observed in other studies [14–16]. Polypyrrole films deposited on alumina sensing substrates were tested for their sensing response to ethanol. Fig. 3 presents the
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3.3. MoO3 –polypyrrole composites
Fig. 4. X-ray diffraction spectrum of molybdenum oxide cosynthesized with FeCl3 (Mo:Fe, 1:0.005) and heat-treated at 350 ◦ C for 3 h.
results of the vapor-deposited polypyrrole sensing behavior at room temperature, 50 ◦ C and 100 ◦ C at varying concentrations. The sensors’ response to ethanol is better at 50 ◦ C as compared to room temperature. At temperatures above 100 ◦ C, the response of the sensor is unstable; the sensor’s conductivity decreased continually during 24 h of observation. This is may be attributed to a degradation of the polymer at higher temperatures, which has been reported to begin by as low a temperature as 125 ◦ C [14,17,18]. The polypyrrole sensors prepared by this method showed slow response (often as long as 30 min to reach a stable response) and recovery times. The resistance of the vapor-deposited polypyrrole film increases upon exposure to ethanol, fitting the typical model for polypyrrole response to alcohols. Polypyrrole is a conductor due to the conjugation (alternating carbon single and double bonds) of the polymer chains. The observed resistance increase of conducting polymers upon exposure to organic vapors such as ethanol is attributed to polymer swelling. The organic solvent forms hydrogen bonds to the polymer, resulting in swelling of the polymer and distortion of the conductive pathways of the polymer chain [19,20]. An alternate description of conducting polymer behavior is that conduction occurs by the movement of the positive charge carriers along the polymer chain. An electron donating gas, such as ethanol, reduces the polymer, decreasing the number of positive charge carriers resulting in an increase in the polymer resistance [12,21].
MolyFe1 and MolyFe2 samples were deposited onto glass slides and then were exposed to pyrrole vapors at room temperature for 24 h to initiate the vaporization of polypyrrole. Prior to pyrrole exposure, the MolyFe1 sample is blue-gray in color, while the MolyFe2 sample is slightly orange due to the higher concentration of iron. Fig. 6 shows optical images of the samples before and after pyrrole exposure. The MolyFe1 shows a slight color change and the MolyFe2 shows a distinct color change to black. Polypyrrole films are typically black or greenish-black in color, so this color change suggests successful vapor deposition of the polymer film. SEM images of the MolyFe1 and MolyFe2 samples are presented in Fig. 7. In both cases, there is evidence of a film on the surface of the molybdenum oxide particles. The MolyFe1 sample has a film that is less than 20 nm thick, while the film on the MolyFe2 sample is on the order of 200 nm. Along with the color change, this suggests that increasing the amount of FeCl3 added to the MoO3 increases the thickness of the polypyrrole film that forms. Infrared spectroscopy was used to determine if the films observed with the optical and electron microscopes were polypyrrole. Infrared spectra of the MolyFe1 samples did not indicate the presence of polypyrrole because the film was to thin. However, the thicker films on the MolyFe2 samples are confirmed as polypyrrole. Fig. 8 shows the infrared spectra of MolyFe2 before and after pyrrole exposure. Peaks marked with an asterisk (*) are found in the infrared spectrum of polypyrrole (Fig. 2). The sensing behavior of the MolyFe1–polypyrrole and MolyFe2–polypyrrole composites was tested in response to ethanol. Unlike the sensors prepared without the polymer film these new materials were conductive at room temperature, allowing low temperature sensing behavior to be measured. Fig. 9 shows the sensor responses of the molybdenum oxide–polypyrrole composites, MolyFe1–polypyrrole and MolyFe2–polypyrrole, to ethanol at room temperature, 50 ◦ C and 100 ◦ C. The response at room temperature is generally greater
3.2. Sol–gel MoO3 –Fe samples Molybdenum oxide was prepared by the sol–gel procedure and doped with FeCl3 by two different methods. In the first method (sample MolyFe1) an aqueous sample of FeCl3 was added in place of water during the MoO3 sol–gel synthesis. The molar ratio of Mo:Fe in this sample is 1:0.005. X-ray diffraction data for MoFe1 is shown in Fig. 4. After a 3 h heat treatment at 350 ◦ C, the MoO3 is present in the expected orthorombic phase [13]. In the second method (sample MolyFe2), the dried powders were stirred in a solution of FeCl3 and dried. The MolyFe2 samples have a molar ratio of Mo:Fe of 1:0.05. Sensors were prepared from both of these materials and tested for a response to ethanol. As expected, neither MolyFe1 nor MolyFe2 was conductive at low temperatures. Typically, undoped molybdenum oxide is not conductive until temperatures greater than 200 ◦ C [7,8,21]. The results of sensing tests at 200 ◦ C and 300 ◦ C are shown in Fig. 5. Both sensors have a greater response to ethanol at 300 ◦ C as compared to 200 ◦ C, with response times averaging 5 min at both of these temperatures. The sensors also both responded to ethanol with a decrease in electrical resistance, as opposed to the polypyrrole, which had a resistance increase in response to ethanol. This n-type behavior is typical of many metal oxides, including MoO3 [7–10,22].
Fig. 5. Top: response of MolyFe1 sensor to ethanol vapor at 200 ◦ C and 300 ◦ C. Bottom: response of MolyFe2 sensor to ethanol vapor at 200 ◦ C and 300 ◦ C. Background gas for both sets of measurements is dry air.
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Fig. 6. Optical images of (a) MolyFe1; (b) MolyFe1 after exposure to pyrrole vapor; (c) MolyFe2; and (d) MolyFe2 after exposure to pyrrole vapor. Scale bar on images is equal to 1 mm.
than the response at 50 ◦ C for the composites as compared to polypyrrole alone. Additionally, as the concentration of ethanol increases, the response of the composite switches from a positive response (resistance increasing) to a negative response (resistance decreasing). At 0.3% ethanol, both composite materials have a positive response, similar to that of the vapor-deposited polypyrrole. At 1% ethanol, the MolyFe2–polypyrrole sensor starts
Fig. 7. SEM images of (a) MolyFe1 and (b) MolyFe2 after exposure to pyrrole vapor.
to respond with an increase in resistance but then switches to a resistance decrease after a few minutes. This is more readily understood from the resistance vs. time data given in Fig. 10. This pattern repeats itself as the concentration is increased again. For the MolyFe1–polypyrrole sensor, the switch between positive and negative sensing responses does not occur until the sample is exposed to 2% ethanol. The response times for these materials 4 min, much faster than the response times observed for the pure polypyrrole sensors. A resistance decrease in response to ethanol was not observed in other studies of polypyrrole–molybdenum oxide composites [11], which may be due to the higher concentrations of ethanol used in this study. It is not an entirely unusual response. Kriván et al. observed a decrease in the resistance of dodecylsulfate and chloride doped polypyrrole films in response to H2 S, while in work by Su and Huang, polypyrrole showed a resistance decrease in response to increased humidity [4]. Composites of polypyrrole with both ZnO and Fe2 O3 [22,23] have shown resistance decreases as well. A common theory to explain the resistance decrease observed in
Fig. 8. Infrared spectrum of polypyrrole film vapor deposited on MolyFe2. Asterisks indicate peaks due to polypyrrole. Inset is infrared spectrum of MolyFe2 before polypyrrole exposure.
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and propose that cation mobility may account for the resistance decrease. As temperature is increased to 100 ◦ C, MolyFe1–polypyrrole continues to respond to ethanol vapors (Fig. 9) but the response of the MolyFe2–polypyrrole starts to degrade, with conductivity steadily decreasing over time. This result is similar to that of the vapor-deposited polypyrrole indicating that at higher temperatures the polymer in the composite may be decomposing. When the temperature is raised above 200 ◦ C, the conductivity of the film in air is unstable for several hours as the polymer film decomposes, then both the MolyFe1–polypyrrole and MolyFe2–polypyrrole sensors respond in much the same way as the oxides without the polymer film. 4. Conclusion A composite of molybdenum oxide and polypyrrole was prepared by a vapor phase polymerization technique, a simple preparation technique that allows for polymer deposition directly onto sensing substrates. The composite material has a higher response to ethanol as compare to either polypyrrole or molybdenum oxide when tested at room temperature. Additionally, the composites show resistance increases at low concentrations of ethanol which changes to a resistance decrease as the concentration of the ethanol is increased. The composite material’s selectivity toward other vapors is currently under investigation. Fig. 9. Response of composite sensors to ethanol. Top figure shows the sensing response of MolyFe1–polypyrrole composite and the bottom figure shows the sensing response the MolyFe2–polypyrrole composite.
the composites is that it is the result of a competition between the swelling mechanism of the polypyrrole and n-type semiconducting behavior of the oxide. In this study, however, it was established that the MolyFe1 and MolyFe2 materials are not conductive (and therefore not sensors) at temperatures below 200 ◦ C. It is therefore unlikely that the electrical response of the MoO3 is influencing the observed resistance decrease. Instead, it is proposed that the MoO3 acts as a catalyst rather than a sensor at these lower temperatures, dissociating the ethanol into CH3 O− and H+ , which then adsorb on the oxide surface [24]. It is possible that migration of H+ from the MoO3 surface to the polypyrrole would result in a decrease in the resistance of the polymer film. In their study of iron oxide–polypyrrole composites, Brezoi and Ion describe a mechanism where H+ reacts with doubly ionized oxygen vacancies in the oxide to form the hydroxide ion, OH− and frees up an electron for conduction [25]. Kriván et al. attribute their observed increase in the conductivity of polypyrrole films upon exposure to H2 S and other acidic vapors to the doping of the polymer film with H+ [21]
Fig. 10. Resistance vs. time ethanol sensing data at room temperature for the MolyFe2–polypyrrole composite.
Acknowledgments This work has been funded by a grant from the Connecticut Space Grant Consortium. Special thanks goes to Professor Howard Harris, Department of Forensic Science at the University of New Haven, for use of the FTIR microscope and to Professor Perena Gouma at the State University of New York at Stony Brook and Jeff Gilarde at Wesleyan University for the electron microscopy. References [1] K. Galatisis, Y.X. Li, W. Wlodarski, E. Comini, G. Sberveglieri, C. Cantalini, S. Santucci, M. Passacantando, Comparison of single and binary oxide MoO3 , TiO2 and WO3 sol–gel gas sensors, Sens. Actuators B: Chem. 83 (2002) 276–280. [2] C.P. de Melo, B.B. Neto, E.G. de Lima, L.F.B. de Lira, J.E.G. de Souza, Use of conducting polypyrrole blends as gas sensors, Sens. Actuators B: Chem. 109 (2005) 348–354. [3] M.A. Ryan, A.V. Shevade, H. Zhou, M.L. Homer, Polymer-carbon black composite sensors in an electronic nose for air quality monitoring, Mater. Res. Soc. Bull. 29 (2004) 714–719. [4] P.-G. Su, L.-N. Huang, Humidity sensors based on TiO2 nanoparticles/polypyrrole composite thin films, Sens. Actuators B: Chem. 123 (2007) 501–507. [5] R.P. Tandon, M.R. Tripathy, A.K. Arora, S. Hotchandani, Gas and humidity response of iron oxide–polypyrrole nanocomposites, Sens. Actuators B: Chem. 114 (2006) 768–773. [6] L. Geng, X. Huang, Y. Zhao, P. Li, S. Wang, S. Zhang, S. Wu, H2 S sensitivity study of polypyrrole/WO3 materials, Solid-State Electron. 50 (2006) 723–726. [7] S. Barazook, R.P. Tandon, S. Hotchandani, MoO3 -based sensor for NO, NO2 and CH4 detection, Sens. Actuators B: Chem. 119 (2006) 691–694. [8] S.S. Sunu, E. Prabhu, V. Jayaraman, K.I. Gnanasekar, T.K. Seshagiri, T. Gnanasekaren, Electrical conductivity and gas sensing properties of MoO3 , Sens. Actuators B: Chem. 101 (2004) 161–174. [9] E. Comini, G. Faglia, G. Sberveglieri, C. Catalini, M. Passaantado, S. Santcci, Y. Li, W. Wlodarski, W. Qu, Carbon monoxide response of molybdenum oxide thin films deposited by different techniques, Sens. Actuators B: Chem. (2000) 168–174. [10] A.K. Prasad, D.J. Kubinski, P.I. Gouma, Comparison of sol–gel and ion beam deposited MoO3 thin film gas sensors for selective ammonia detection, Sens. Actuators B: Chem. 93 (2003) 25–30. [11] K. Hosono, I. Matsubara, N. Murayama, S. Woosuck, N. Izu, Synthesis of polypyrrole/MoO3 hybrid thin film and their volatile organic compound gassensing properties, Chem. Mater. 17 (2005) 349–354. [12] G. Han, G. Shi, Porous polypyrrole/polymethylmethacrylate film prepared by vapor deposition polymerization of pyrrole and its application for ammonia detection, Thin Solid Films 515 (2007) 6986–6991.
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Biography Nancy Ortins Savage is an assistant professor in the Department of Chemistry and Chemical Engineering at the University of New Haven. She received her PhD from The Ohio State University and was a National Research Council Postdoctoral Fellow at the National Institute of Standards and Technology in Gaithersburg, Maryland. Her research interest is in the use of inorganic–organic composites for chemical sensing.