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Material and NH3-sensing properties of polypyrrole-coated tungsten oxide nanofibers
Sensors and Actuators B 185 (2013) 523–529
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Material and NH3 -sensing properties of polypyrrole-coated tungsten oxide nanofibers Thi Anh Ho, Tae-Sun Jun, Yong Shin Kim ∗ Graduate School of Bio-Nano Engineering, Hanyang University, Ansan 426-791, South Korea
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Article history: Received 10 December 2012 Received in revised form 2 May 2013 Accepted 10 May 2013 Available online 18 May 2013 Keywords: Polypyrrole Coaxial nanofibers Gas sensor Vapor-phase polymerization Electrospinning
a b s t r a c t Nonwoven and coaxial polypyrrole (PPy)-coated tungsten oxide nanofibers were synthesized via electrospinning and vapor-phase polymerization, and their NH3 -sensing characteristics were investigated at various operation temperatures under 100 ◦ C. FT-IR and TEM results confirmed the growth of an ultrathin PPy layer on the WO3 surface. The core WO3 nanofiber formed by the axial agglomeration of polycrystalline WO3 nanoparticles had an average diameter of 102 nm, and the thickness of the sheath PPy layer was approximately 5 nm. Upon exposure to 1–20 ppm NH3 , the PPy-coated WO3 nanofiber mat exhibited sensitive and fast resistance-increasing (p-type) responses at an operating temperature of 100 ◦ C, due to the ultrathin PPy coating and a large surface area of the nanofiber mat. Furthermore, the NH3 detection characteristics revealed strong dependence on operation temperature, which may indicate the involvement of a p–n junction control mechanism in the core-sheath hetero-nanofiber structure. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Various types of conducting polymers have been employed as detection elements in chemical and bio-sensing devices due to their environmental stability, tunable conductivity, chemical diversity, operation at ambient temperature, and ease of processing [1,2]. Among these materials, polypyrrole (PPy) is one of the most studied conducting polymers because it is easily synthesized via chemical or electrochemical oxidation polymerization [3–6], and its material and sensing properties can be improved by combining PPy with additional organic or inorganic components. Doped PPy usually behaves like p-type semiconductors, and can give rise to a change in conductivity through interactions with gas or vapor. A number of PPy composite materials have been investigated in order to develop high performance gas sensors for a wide variety of analytes such as NH3 , NO2 , humidity, H2 S, and volatile organic compounds. This has included work with incorporation of metal oxides such as ZnO, WO3 , Fe2 O3 , and TiO2 /ZnO [7–10], insulating polymers [11,12], carbon [13,14] and metal [15] nanomaterials. Recently, one-dimensional nanofiber mats have attracted a great deal of attention as promising candidates for fabricating highly sensitive and rapid response chemical sensors due to their high surface-to-volume ratio and remarkable porosity [2,16,17]. An electrospinning technique provides a simple and versatile method for producing polymeric and inorganic nanofibers in large quantity.
∗ Corresponding author. Tel.: +82 31 400 5507; fax: +82 31 400 5457. E-mail address: [email protected] (Y.S. Kim). 0925-4005/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2013.05.039
However, because pure PPy is too stiff to be electrospun on its own, PPy composite nanofibers are usually prepared via electrospinning a mixture solution of PPy and another electrospinnable polymer [18,19] or vapor-phase polymerization of pyrrole monomers on electrospun nanofiber templates [20–22]. In our previous work, a very thin PPy layer was entirely coated on the surface of polyacrylonitrile (PAN) nanofibers to form a core-sheath-like structure via two-step vapor-phase polymerization: the successive supply of ferric tosylate (FeTos) oxidant and pyrrole molecules to the nanofiber template [23]. Such a coaxial PPy nanofiber mat with an ultrathin sheath sensing layer may be regarded as an optimum structure for achieving a highly sensitive, rapid-response sensor. Gas-sensing performance can be enhanced through the control of resistive p–n junction formation in metal-oxide semiconductor hetero-nanostructures (CuO–SnO2 , CuO–ZnO and Cr2 O3 –ZnO) [24–27]. In this work, we have prepared an ultrathin PPy-coated WO3 nanofiber mat and evaluated its sensing properties to NH3 gas at different operation temperatures in order to examine the p–n junction effect in conducting polymer sensor systems. The core-sheath nanofibers were synthesized via the two-step vapor-phase polymerization of p-type FeTos-doped PPy on n-type WO3 nanofibers produced with the electrospinning and sequential heat-treatment process [28]. Among various metal oxides, WO3 is chosen as the core material for low-temperature operation since it has a lower inter-grain energy barrier than other n-type metal oxides such as SnO2 and ZnO. The PPy-coated WO3 nanofiber sensor has demonstrated faster and more sensitive NH3 detection at an operating temperature of 100 ◦ C than at 20 and 70 ◦ C. This
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sensitivity enhancement has been discussed and explained with the variation of the p–n junction region in the one-dimensional core-sheath hetero-nanostructure.
2. Experiment A composite solution for electrospinning was prepared by dissolving 1.1 g tungstic acid (H2 WO4 , Alfa) in 10 mL H2 O2 (30%), and then adding a 10 mL ethanol solution containing 1.2 g polyvinylpyrrolidone (PVP, Mw = 1,300,000). The mixture was stirred for 2 days at room temperature to form a homogeneous solution. The electrospinning solution was loaded into a disposable plastic syringe with a 26-gauge metal needle at the tip. A voltage of 20 kV was applied to the needle, and a cylindrical ring collector was placed 15 cm from the tip. The electrically grounded collector was covered with aluminum foil and rotated at a speed of approximately 250 rpm. The solution was delivered by a syringe pump at a flow rate of 0.4 mL/h, and was stably pulled from the spinneret. During the electrospinning process, the relative humidity was held to a range of 20–30%. The electrospun composite nanofibers were collected on several pieces of Si wafer attached to the Al foil collector. A sensor substrate, containing a pair of comb-shaped Au electrodes formed on a SiO2 /Si substrate, was also positioned on the Al foil in order to fabricate a sensor. The composite nanofibers were thermally treated at 500 ◦ C for 3 h under ambient air conditions to obtain tungsten oxides. A slow heating rate of 1 ◦ C/min was used to calcine the composite nanofibers without destroying the fibrous morphology. After the calcination, the WO3 nanofiber mat was dipped into a butanol solution of 10 wt% FeTos for 10 min. Excess FeTos oxidants were washed away in ethanol, and the sample was dried in an oven at 60 ◦ C for 1 h. The FeTos-treated WO3 sample was placed in a reaction vessel connected to a reservoir containing a liquid pyrrole monomer. Vapor-phase PPy growth was performed with a reaction time of 15 min at ambient temperature through the pyrrole vapor delivery into the vessel. An average diameter and the morphology of the nanofibers were observed by using scanning electron microscopy (SEM; S-4800, Hitachi). More detailed microstructures were probed using transmission electron microscopy (TEM, JEM-2100F, JEOL). An energy dispersive X-ray (EDX) detector installed in the SEM equipment was used to probe the chemical compositions of a sample. X-ray diffraction (XRD) analysis was performed with a diffractometer (D/Max-2500, Rigaku) using Cu K˛ radiation. Fourier transform infrared spectroscopy (FT-IR; Scimitar 1000, Varian) was utilized to identify organic functional groups within the nanofibers. The nanofiber mat collected on a Si wafer was directly used for SEM, EDX, and XRD analyses, while nanofiber fragments detached from the Al foil were used to prepare a pellet for FT-IR, as well as a suspension solution for TEM. Chemoresistive response characteristics of the PPy-coated WO3 nanofiber sensor were evaluated using a home-made measurement system consisting of analyte-diluted gas delivery lines and a small detection chamber [29]. Gas-sensing measurements were carried out by placing a sensor in the chamber with electrical feedthroughs, and by blowing NH3 gas over the sensor at a flow rate of 1000 mL/min. Detection responses were recorded by a source meter (2400, Keithley) at three different temperatures of 20, 70, and 100 ◦ C. The operation temperature was regulated by positioning the chamber on a temperature-controllable hotplate. Ammonia concentrations in the range of 1–20 ppm were regulated by adjusting a relative flow rate ratio between sample and carrier gas. The sample flow was produced from a standard cylinder with a fixed NH3 concentration (Rigas, N2 dilution). The measurements were performed by using dry N2 as a carrier gas for the cases without
Fig. 1. (A) SEM image and (B) EDX spectrum of WO3 nanofibers formed after the thermal treatment at 500 ◦ C for 3 h. The white scale bars at the right-bottom corner correspond to 1 m. The number intensity and energy of X-rays emitted from the sample was depicted as y and x-axes, respectively, in EDX spectrum.
specific comment. The sensor response R was calculated by using the equation: R (%) =
(Rmax − R0 ) × 100 R0
(1)
where R0 is the initial resistance of a sensor and Rmax is the saturated maximum resistance upon exposure to the NH3 analyte. 3. Results and discussion 3.1. Material characterization As-electrospun composite mat was confirmed to consist of randomly oriented nanofibers with a smooth surface through SEM analysis. An average diameter of the nanofibers was found to be 178 nm, implying that nonwoven composite nanofibers were readily produced by electrospinning a mixture solution of tungstic acid and PVP. Fig. 1A displays a SEM image of the thermally-treated sample at 500 ◦ C. Although the one-dimensional fibrous structure is maintained, this image clearly exhibits a great enhancement in surface roughness and a considerable reduction in diameter. The formation of a rough surface indicates that amorphous tungsten oxides are converted into crystalline WO3 particles, agglomerated along the axial direction during the calcination process. An average diameter of the thermally-treated sample was 102 nm, corresponding to 43% shrinkage from the as-electrospun nanofibers. The diameter reduction could be interpreted with the pyrolysis of PVP polymer from the composite nanofibers. Furthermore, tungsten and oxygen elements were predominantly detected in an EDX
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Fig. 2. An XRD pattern of the thermally-treated WO3 nanofibers. The bottom bar plot displays a reference pattern of monoclinic WO3 crystal (JCPDS card no. 83-0950).
spectrum of the calcined sample, supporting the WO3 formation via selective PVP removal from the composite nanofibers (see Fig. 1B). Fig. 2 shows an XRD pattern of the thermally-treated nanofibers together with the bar plot of JCPDS card no. 83-0950. The observed peak positions were well matched to those of the reference plot, indicating the formation of polycrystalline WO3 nanofibers with a monoclinic crystal structure. The average crystal size of approximately 30 nm was calculated by the Debye–Scherrer equation: the average sizes of 27–33 nm were obtained from the four crystal planes of (0 0 2), (0 2 0), (2 0 0) and (1 2 0), observed at 2 positions of 23.2◦ , 23.6◦ , 24.4◦ , and 26.7◦ , respectively. The WO3 nanofibers changed color from light yellow to orange after the oxidant-dipping process, indicating the incorporation of FeTos molecules into the nanofiber mat. The sample color was further altered to gray after the subsequent vapor-phase polymerization. This observation could be interpreted as resulting from the formation of a small amount of PPy polymer, since PPy color was known to change from dark blue to black in a solution driven by the advancement in PPy polymerization. This type of vapor-phase PPy growth was reported to occur via oxidative polymerization mechanism even at ambient temperature when oxidant-containing polymer nanofibers were exposed to pyrrole vapor [20–23]. Fig. 3A shows a SEM image of PPy-WO3 nanofibers prepared using the dip-coating and vapor-phase polymerization process. The appearance of a PPy-WO3 mat, displaying nonwoven nanofibers with rough surface morphology, was very similar to that of the WO3 nanofibers shown in Fig. 1A. The PPy-WO3 nanofibers were found to have a slightly larger average diameter (ca. 5 nm) than the WO3 nanofibers. However, the XRD diffraction peak positions of PPyWO3 were observed to be identical to those of the monoclinic WO3 even though they displayed little variation in peak intensity. This observation also supports the formation of an ultrathin PPy layer on WO3 nanofibers. Furthermore, the EDX spectrum of the PPy-WO3 sample indicated the presence of C, N, Fe and S elements as well as W and O atoms resulting from WO3 (see Fig. 3B). The detection of the additional atoms suggests that PPy and FeTos compounds exist within the sample. FT-IR spectra of WO3 and PPy-WO3 nanofiber samples are shown in Fig. 4. The lower WO3 spectrum displays an intense, broad band in the region of 600–950 cm−1 with peaks at approximately 775 and 830 cm−1 . This band can be explained by the O W O stretching modes of monoclinic WO3 [30], thus supporting the formation of WO3 nanofibers with a monoclinic crystal structure. The PPy-WO3 sample (the upper spectrum) exhibits
Fig. 3. (A) SEM image and (B) EDX spectrum of the PPy-coated WO3 nanofibers prepared by the two-step vapor-phase polymerization process. The number intensity and energy of X-rays emitted from the sample was depicted as y and x-axes, respectively, in EDX spectrum.
noticeable additional peaks in the region of 1000–1200 cm−1 in addition to the strong O W O stretching and several weak side bands. Their peak positions are well assigned to the characteristic vibration modes of PPy molecules: N H bending at 1011 and 1040 cm−1 ; C O bending at 1125 cm−1 and C O bending at 1193 cm−1 [31]. These results provide direct evidence for the PPy growth through the two-step vapor-phase polymerization. Detailed nanostructure features of PPy-WO3 were further investigated by TEM. Fig. 5A shows a low-resolution TEM image, displaying the one-dimensional nanostructure assembled by
Fig. 4. FT-IR spectra of WO3 and PPy-coated WO3 samples.
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Fig. 5. (A) Low resolution and (B) high resolution TEM images of the PPy-coated WO3 nanofibers. The high resolution image displays lattice structures of WO3 crystals in the dotted ellipse regions.
irregular polygon nanoparticles. For the direct observation of WO3 lattice structures, the circled region, a, was further analyzed at high resolution, as shown in Fig. 5B. The region, b, designated by the dotted ellipse, exhibits a lattice structure with a inter-planar distance of 0.38 nm, assigned to the intensive (0 0 2), (0 2 0) or (2 0 0) deflection plane. In addition, the lattice structure of (0 2 2) plane is seen in the region c. The polygons with well-defined facets can be interpreted as monoclinic WO3 crystals. On the other hand, the surface region displays no lattice structure and smooth surface morphology, thus being regarded as an overlaid PPy layer with a thickness of several nm, as mentioned in the SEM observation. The PPy layer appears to have a substantial thickness variation. The larger thickness is found at the cavities between WO3 grains, probably due to the preferential FeTos adsorption at sites with a high negative curvature. Consequently, these TEM observations imply that the ultrathin PPy layer is well formed on the rough surface of WO3 nanofibers via two-step vapor-phase polymerization. 3.2. Ammonia-sensing properties Chemoresistive sensing performance was evaluated for the PPy-coated WO3 mat formed on the interdigitated Au substrate. Fig. 6A shows a response curve at an operation temperature of 100 ◦ C when the analyte was injected into the sensor for 2 min, and was then flushed by a N2 flow during a 2 min recovery period. The NH3 concentration is successively increased from 1, 5, 10, to 20 ppm, as displayed in the bottom plot of Fig. 6A. The responses display an increase in resistance upon exposure to the ammonia analyte. For example, the sensor resistance increased from 11.26 to 12.02 M for the 1 ppm NH3 exposure, which corresponds to
Fig. 6. (A) Time-profiled response curve and (B) detection response variation of the PPy-coated WO3 nanofiber sensor as a function of NH3 concentration at an operation temperature of 100 ◦ C. Ammonia analytes were supplied in the pulsed mode with gradually increasing concentration, as indicated in the bottom of (A).
a sensor response of R = 6.3%. Fig. 6B shows a gradual increase in response magnitude as a function of NH3 concentration. The detection sensitivity defined by the slope of the response curve was found to be lower at high concentrations: roughly 3.2%/ppm in the interval of 1–5 ppm, and 0.48%/ppm in the range of 5–20 ppm. Such a decrease in detection sensitivity at higher NH3 concentrations was previously reported in the PPy sensor systems [4,6,10,14], which might be explained by heterogeneousness in interaction strength between PPy and NH3 or by a partial saturation of PPy at a high concentration. The conductivity of Tos-doped PPy is readily altered by a chemical or electrochemical redox process when it is exposed to electron-donating ammonia. The redox reaction involves the transfer of electrons and the exchange of counter ions for the charge compensation on the polymer chain. This process may be represented by the following chemical reaction equation [5]: PPy+ /Tos− + NH3 (ı− ) ↔ PPy0 /NH3 (ı− )+ /Tos−
(2)
The positively-charged PPy backbone in an initial state is reversibly changed to a more neutral form due to the redox interactions between Tos-doped PPy and NH3 , thus leading to the p-type response of increasing resistance in the semiconducting sensor system. The interactions between WO3 and NH3 are expected to create a decrease in resistance, since WO3 is well known as an n-type semiconductor. The observed p-type response for the NH3
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Fig. 7. Time-profiled response curves of the PPy-coated WO3 nanofiber sensor at operation temperatures of 20 ◦ C (upper) and 100 ◦ C (lower) when 10 ppm NH3 was exposed.
detection at 100 ◦ C, therefore, indicates that the p-type PPy layer is primarily responsible for the chemoresistive response in the PPy-coated WO3 sensor. For a comparison study, PPy-coated PAN sensors were prepared with using a PAN nanofiber mat as a template instead of WO3 , and evaluated NH3 -sensing characteristics. The PPy-coated PAN sensor exhibited lower detection sensitivity and more irreversible baseline compared with the PPy-coated WO3 (data not shown). These results suggest that semiconducting WO3 is better core material in PPy-coated nanofibers for achieving a sensor of high performance than insulating PAN. Similar NH3 -sensing characteristics were also evaluated at various operation temperatures under 100 ◦ C, since PPy material can be degraded at high temperature. Fig. 7 displays typical response curves, measured at 20 and 100 ◦ C, for the sequential exposure of 10 ppm NH3 over two cycles. The response curve at 20 ◦ C exhibits inferior detection characteristics when compared with that at 100 ◦ C: long detection times, incomplete baseline recovery, and low response magnitude. The t90 detection times are 105 and 25 s at the operation temperatures of 20 and 100 ◦ C, respectively. The baseline resistance is changed from 44.68 to 46.22 M, corresponding to 3.5% increase, at 20 ◦ C after the measurement, while the resistance variation is less than 0.2% at 100 ◦ C. The slow detection and irreversible recovery at 20 ◦ C may be attributed to the existence of an energy barrier in the direction of a reaction coordination. In fact, a relatively long stabilization time of more than 1 h was necessary to reach a steady-state initial sensor resistance at ambient temperature, probably due to the slow desorption kinetics of adsorbed molecules. A great improvement in the response
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and recovery characteristics were observed when the operation temperature was increased to 70 ◦ C, while there was no noticeable change in response-curve shape in the temperature range of 70–100 ◦ C. The sensor responses for the detection of 10 ppm NH3 were 7.1% at room temperature and 6.1% at 70 ◦ C, being significantly lower than the magnitude of 22% obtained at 100 ◦ C. These results indicate that NH3 detection at 100 ◦ C results in a faster and higher response in the PPy-coated WO3 nanofiber sensor system than those at 20 and 70 ◦ C. High-performance detection at higher temperatures was not observed in a similar ultrathin PPy-coated PAN nanofiber sensor. The remarkable response enhancement at 100 ◦ C may be due to the formation of effective p–n junction in the PPy-coated WO3 nanofiber sensors, as previously reported in metal-oxide semiconductor hetero-nanostructures of CuO–SnO2 [24] and Cr2 O3 –ZnO [26]. Fig. 8 illustrates a schematic mechanism for the NH3 detection of the PPy-coated WO3 nanofiber sensor. A depletion layer is naturally formed at the interface between the p-type Tos-doped PPy and the n-type WO3 . Since WO3 has a very low conductivity at room temperature, and thus a small density of electron carriers, the positive space charges in the WO3 depletion region are distributed more sparsely than counterparts in the depletion region of Tos-doped PPy. The difference in space charge density can develop a wider depletion layer toward the core WO3 , when compared with the sheath PPy (see Fig. 8). When exposed to electron-donating NH3 , the positively-charged PPy backbone changes to a less conductive neutral form, as described in Eq. (2), thus displaying a resistanceincreasing p-type response. At higher temperature, tungsten oxides with a non-stoichiometric nature have been reported to produce larger amounts of free electrons from the oxygen vacancy donor levels through thermal activation [32]. The excess free electrons can diffuse into p-type PPy and then develop many space charge points in the p–n junction region through the hole-electron recombination process. If the WO3 space-charge density becomes comparable or larger than that of Tos-doped PPy at 100 ◦ C, the depletion layer can be considerably extended toward the p-type region. This space charge extension induces a reduction in the hole transfer crosssection area along the p-type PPy sheath layer. Considering the average PPy thickness of about 5 nm, the conduction pathway can be significantly modulated due to the variation in space charge depth. Upon exposure to reducing NH3 , electron injection into the positively-charged PPy will significantly reduce the conduction pathway to a great extent through the formation of nonconductive neutral form. Therefore, the enhanced response can be attributed to the high sensitivity of the active PPy layer having a thinner effective conduction thickness for the interactions between PPy and NH3 . This kind of sensitivity dependence on conduction channel width has been well demonstrated in single nanowire sensor systems [33]. The conduction variation along crystalline WO3 grains
Fig. 8. Proposed NH3 sensing mechanisms of the PPy-coated WO3 nanofiber sensor.
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Acknowledgement This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (NRF2012R1A1A2005473 and NRF-2012-0009838).
References
Fig. 9. Time-profiled H2 response curve of the PPy-coated WO3 sensor at 100 ◦ C in dry N2 environment.
can offer an additional possibility for the temperature-dependent response; however, the sensitive p-type response supports negligible direct interactions between WO3 and NH3 , as well as the formation of fully PPy-covered WO3 . NH3 -sensing measurements were conducted in dry air atmosphere instead of N2 for clarifying detection characteristics in more detail. They also exhibited a p-type NH3 response with comparable detection sensitivity. However, the sensor stabilization in resistance was very difficult to achieve in the ambient air probably due to the interactions between sensing materials and oxygen molecules. In addition, further experiments were performed to evaluate detection selectivity of the sensor for other analytes such as water, methanol and hydrogen molecules. Fig. 9 shows a timeprofiled H2 response curve of the PPy-coated WO3 sensor at 100 ◦ C in dry N2 environment. When compared to NH3 detection, the response magnitude corresponds to one fifth, and the response times are much longer. In cases of methanol and water, there was no noticeable response except for the significant fluctuation in background resistance. These results suggest that environmental durability of the PPy-coated WO3 nanofibers must be enhanced for a use in commercialized sensors, despite the marginal selectivity in NH3 detection. 4. Conclusion A coaxial conducting polymer nanofiber sensor has been fabricated by depositing an ultrathin p-type doped PPy layer onto n-type polycrystalline WO3 nanofibers. This core-sheath hetero-junction structure can provide an opportunity to enhance gas-sensing properties through the working of p–n junctions, together with the great surface area and high porosity resulting from a nanofiber mat. To the best of our knowledge, this study is the first experimental demonstration of the p–n junction influence on chemoresistive response in the conducting polymer system, although the effect was previously reported in metal-oxide semiconductor heteronanostructures. The NH3 -sensing measurements of our PPy-coated WO3 sensor were performed under concentrations of 1–20 ppm, and operating temperatures of 20, 70 and 100 ◦ C. A large sensor response and short detection times were observed at an operating temperature of 100 ◦ C. The unusual response enhancement at 100 ◦ C may be attributed to the highly sensitive modulation of a conductive PPy layer thickness for the NH3 exposure due to the extension of the p–n depletion region in the PPy-coated WO3 hetero-nanostructures.
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Biographies Thi Anh Ho received her BS degree from the Hanoi University of Technology, Vietnam in 2011. She is currently a graduate student in the Department of Bionano Engineering, Hanyang University, South Korea. Her research interest is primarily centered on the development of new sensing nanomaterials used in chemoresistive and electrochemiluminescence sensors. Tae-Sun Jun received his BS degree in Oriental Medical Food & Nutrition from the Semyung University in 2009. He is currently a graduate student in the Department of Bionano Engineering, Hanyang University, South Korea. His research activities are focused on novel sensing materials such as conducting polymers and core–shell nanofibers. Yong Shin Kim received a PhD in Chemistry from Korea Advanced Institute of Science and Technology (KAIST) in 1997. Following the completion of his degree, he worked as a senior research member at Electronics and Telecommunications Research Institute (ETRI) in South Korea, developing flat panel display devices and miniaturized electronic nose systems. Since 2007, he has been employed at Hanyang University in South Korea as an Associate Professor. His current research activities are focused on the development of smart chemical sensors and novel nanomaterials for sensor applications.
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Material and NH3 -sensing properties of polypyrrole-coated tungsten oxide nanofibers Thi Anh Ho, Tae-Sun Jun, Yong Shin Kim ∗ Graduate School of Bio-Nano Engineering, Hanyang University, Ansan 426-791, South Korea
a r t i c l e
i n f o
Article history: Received 10 December 2012 Received in revised form 2 May 2013 Accepted 10 May 2013 Available online 18 May 2013 Keywords: Polypyrrole Coaxial nanofibers Gas sensor Vapor-phase polymerization Electrospinning
a b s t r a c t Nonwoven and coaxial polypyrrole (PPy)-coated tungsten oxide nanofibers were synthesized via electrospinning and vapor-phase polymerization, and their NH3 -sensing characteristics were investigated at various operation temperatures under 100 ◦ C. FT-IR and TEM results confirmed the growth of an ultrathin PPy layer on the WO3 surface. The core WO3 nanofiber formed by the axial agglomeration of polycrystalline WO3 nanoparticles had an average diameter of 102 nm, and the thickness of the sheath PPy layer was approximately 5 nm. Upon exposure to 1–20 ppm NH3 , the PPy-coated WO3 nanofiber mat exhibited sensitive and fast resistance-increasing (p-type) responses at an operating temperature of 100 ◦ C, due to the ultrathin PPy coating and a large surface area of the nanofiber mat. Furthermore, the NH3 detection characteristics revealed strong dependence on operation temperature, which may indicate the involvement of a p–n junction control mechanism in the core-sheath hetero-nanofiber structure. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Various types of conducting polymers have been employed as detection elements in chemical and bio-sensing devices due to their environmental stability, tunable conductivity, chemical diversity, operation at ambient temperature, and ease of processing [1,2]. Among these materials, polypyrrole (PPy) is one of the most studied conducting polymers because it is easily synthesized via chemical or electrochemical oxidation polymerization [3–6], and its material and sensing properties can be improved by combining PPy with additional organic or inorganic components. Doped PPy usually behaves like p-type semiconductors, and can give rise to a change in conductivity through interactions with gas or vapor. A number of PPy composite materials have been investigated in order to develop high performance gas sensors for a wide variety of analytes such as NH3 , NO2 , humidity, H2 S, and volatile organic compounds. This has included work with incorporation of metal oxides such as ZnO, WO3 , Fe2 O3 , and TiO2 /ZnO [7–10], insulating polymers [11,12], carbon [13,14] and metal [15] nanomaterials. Recently, one-dimensional nanofiber mats have attracted a great deal of attention as promising candidates for fabricating highly sensitive and rapid response chemical sensors due to their high surface-to-volume ratio and remarkable porosity [2,16,17]. An electrospinning technique provides a simple and versatile method for producing polymeric and inorganic nanofibers in large quantity.
∗ Corresponding author. Tel.: +82 31 400 5507; fax: +82 31 400 5457. E-mail address: [email protected] (Y.S. Kim). 0925-4005/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2013.05.039
However, because pure PPy is too stiff to be electrospun on its own, PPy composite nanofibers are usually prepared via electrospinning a mixture solution of PPy and another electrospinnable polymer [18,19] or vapor-phase polymerization of pyrrole monomers on electrospun nanofiber templates [20–22]. In our previous work, a very thin PPy layer was entirely coated on the surface of polyacrylonitrile (PAN) nanofibers to form a core-sheath-like structure via two-step vapor-phase polymerization: the successive supply of ferric tosylate (FeTos) oxidant and pyrrole molecules to the nanofiber template [23]. Such a coaxial PPy nanofiber mat with an ultrathin sheath sensing layer may be regarded as an optimum structure for achieving a highly sensitive, rapid-response sensor. Gas-sensing performance can be enhanced through the control of resistive p–n junction formation in metal-oxide semiconductor hetero-nanostructures (CuO–SnO2 , CuO–ZnO and Cr2 O3 –ZnO) [24–27]. In this work, we have prepared an ultrathin PPy-coated WO3 nanofiber mat and evaluated its sensing properties to NH3 gas at different operation temperatures in order to examine the p–n junction effect in conducting polymer sensor systems. The core-sheath nanofibers were synthesized via the two-step vapor-phase polymerization of p-type FeTos-doped PPy on n-type WO3 nanofibers produced with the electrospinning and sequential heat-treatment process [28]. Among various metal oxides, WO3 is chosen as the core material for low-temperature operation since it has a lower inter-grain energy barrier than other n-type metal oxides such as SnO2 and ZnO. The PPy-coated WO3 nanofiber sensor has demonstrated faster and more sensitive NH3 detection at an operating temperature of 100 ◦ C than at 20 and 70 ◦ C. This
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sensitivity enhancement has been discussed and explained with the variation of the p–n junction region in the one-dimensional core-sheath hetero-nanostructure.
2. Experiment A composite solution for electrospinning was prepared by dissolving 1.1 g tungstic acid (H2 WO4 , Alfa) in 10 mL H2 O2 (30%), and then adding a 10 mL ethanol solution containing 1.2 g polyvinylpyrrolidone (PVP, Mw = 1,300,000). The mixture was stirred for 2 days at room temperature to form a homogeneous solution. The electrospinning solution was loaded into a disposable plastic syringe with a 26-gauge metal needle at the tip. A voltage of 20 kV was applied to the needle, and a cylindrical ring collector was placed 15 cm from the tip. The electrically grounded collector was covered with aluminum foil and rotated at a speed of approximately 250 rpm. The solution was delivered by a syringe pump at a flow rate of 0.4 mL/h, and was stably pulled from the spinneret. During the electrospinning process, the relative humidity was held to a range of 20–30%. The electrospun composite nanofibers were collected on several pieces of Si wafer attached to the Al foil collector. A sensor substrate, containing a pair of comb-shaped Au electrodes formed on a SiO2 /Si substrate, was also positioned on the Al foil in order to fabricate a sensor. The composite nanofibers were thermally treated at 500 ◦ C for 3 h under ambient air conditions to obtain tungsten oxides. A slow heating rate of 1 ◦ C/min was used to calcine the composite nanofibers without destroying the fibrous morphology. After the calcination, the WO3 nanofiber mat was dipped into a butanol solution of 10 wt% FeTos for 10 min. Excess FeTos oxidants were washed away in ethanol, and the sample was dried in an oven at 60 ◦ C for 1 h. The FeTos-treated WO3 sample was placed in a reaction vessel connected to a reservoir containing a liquid pyrrole monomer. Vapor-phase PPy growth was performed with a reaction time of 15 min at ambient temperature through the pyrrole vapor delivery into the vessel. An average diameter and the morphology of the nanofibers were observed by using scanning electron microscopy (SEM; S-4800, Hitachi). More detailed microstructures were probed using transmission electron microscopy (TEM, JEM-2100F, JEOL). An energy dispersive X-ray (EDX) detector installed in the SEM equipment was used to probe the chemical compositions of a sample. X-ray diffraction (XRD) analysis was performed with a diffractometer (D/Max-2500, Rigaku) using Cu K˛ radiation. Fourier transform infrared spectroscopy (FT-IR; Scimitar 1000, Varian) was utilized to identify organic functional groups within the nanofibers. The nanofiber mat collected on a Si wafer was directly used for SEM, EDX, and XRD analyses, while nanofiber fragments detached from the Al foil were used to prepare a pellet for FT-IR, as well as a suspension solution for TEM. Chemoresistive response characteristics of the PPy-coated WO3 nanofiber sensor were evaluated using a home-made measurement system consisting of analyte-diluted gas delivery lines and a small detection chamber [29]. Gas-sensing measurements were carried out by placing a sensor in the chamber with electrical feedthroughs, and by blowing NH3 gas over the sensor at a flow rate of 1000 mL/min. Detection responses were recorded by a source meter (2400, Keithley) at three different temperatures of 20, 70, and 100 ◦ C. The operation temperature was regulated by positioning the chamber on a temperature-controllable hotplate. Ammonia concentrations in the range of 1–20 ppm were regulated by adjusting a relative flow rate ratio between sample and carrier gas. The sample flow was produced from a standard cylinder with a fixed NH3 concentration (Rigas, N2 dilution). The measurements were performed by using dry N2 as a carrier gas for the cases without
Fig. 1. (A) SEM image and (B) EDX spectrum of WO3 nanofibers formed after the thermal treatment at 500 ◦ C for 3 h. The white scale bars at the right-bottom corner correspond to 1 m. The number intensity and energy of X-rays emitted from the sample was depicted as y and x-axes, respectively, in EDX spectrum.
specific comment. The sensor response R was calculated by using the equation: R (%) =
(Rmax − R0 ) × 100 R0
(1)
where R0 is the initial resistance of a sensor and Rmax is the saturated maximum resistance upon exposure to the NH3 analyte. 3. Results and discussion 3.1. Material characterization As-electrospun composite mat was confirmed to consist of randomly oriented nanofibers with a smooth surface through SEM analysis. An average diameter of the nanofibers was found to be 178 nm, implying that nonwoven composite nanofibers were readily produced by electrospinning a mixture solution of tungstic acid and PVP. Fig. 1A displays a SEM image of the thermally-treated sample at 500 ◦ C. Although the one-dimensional fibrous structure is maintained, this image clearly exhibits a great enhancement in surface roughness and a considerable reduction in diameter. The formation of a rough surface indicates that amorphous tungsten oxides are converted into crystalline WO3 particles, agglomerated along the axial direction during the calcination process. An average diameter of the thermally-treated sample was 102 nm, corresponding to 43% shrinkage from the as-electrospun nanofibers. The diameter reduction could be interpreted with the pyrolysis of PVP polymer from the composite nanofibers. Furthermore, tungsten and oxygen elements were predominantly detected in an EDX
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Fig. 2. An XRD pattern of the thermally-treated WO3 nanofibers. The bottom bar plot displays a reference pattern of monoclinic WO3 crystal (JCPDS card no. 83-0950).
spectrum of the calcined sample, supporting the WO3 formation via selective PVP removal from the composite nanofibers (see Fig. 1B). Fig. 2 shows an XRD pattern of the thermally-treated nanofibers together with the bar plot of JCPDS card no. 83-0950. The observed peak positions were well matched to those of the reference plot, indicating the formation of polycrystalline WO3 nanofibers with a monoclinic crystal structure. The average crystal size of approximately 30 nm was calculated by the Debye–Scherrer equation: the average sizes of 27–33 nm were obtained from the four crystal planes of (0 0 2), (0 2 0), (2 0 0) and (1 2 0), observed at 2 positions of 23.2◦ , 23.6◦ , 24.4◦ , and 26.7◦ , respectively. The WO3 nanofibers changed color from light yellow to orange after the oxidant-dipping process, indicating the incorporation of FeTos molecules into the nanofiber mat. The sample color was further altered to gray after the subsequent vapor-phase polymerization. This observation could be interpreted as resulting from the formation of a small amount of PPy polymer, since PPy color was known to change from dark blue to black in a solution driven by the advancement in PPy polymerization. This type of vapor-phase PPy growth was reported to occur via oxidative polymerization mechanism even at ambient temperature when oxidant-containing polymer nanofibers were exposed to pyrrole vapor [20–23]. Fig. 3A shows a SEM image of PPy-WO3 nanofibers prepared using the dip-coating and vapor-phase polymerization process. The appearance of a PPy-WO3 mat, displaying nonwoven nanofibers with rough surface morphology, was very similar to that of the WO3 nanofibers shown in Fig. 1A. The PPy-WO3 nanofibers were found to have a slightly larger average diameter (ca. 5 nm) than the WO3 nanofibers. However, the XRD diffraction peak positions of PPyWO3 were observed to be identical to those of the monoclinic WO3 even though they displayed little variation in peak intensity. This observation also supports the formation of an ultrathin PPy layer on WO3 nanofibers. Furthermore, the EDX spectrum of the PPy-WO3 sample indicated the presence of C, N, Fe and S elements as well as W and O atoms resulting from WO3 (see Fig. 3B). The detection of the additional atoms suggests that PPy and FeTos compounds exist within the sample. FT-IR spectra of WO3 and PPy-WO3 nanofiber samples are shown in Fig. 4. The lower WO3 spectrum displays an intense, broad band in the region of 600–950 cm−1 with peaks at approximately 775 and 830 cm−1 . This band can be explained by the O W O stretching modes of monoclinic WO3 [30], thus supporting the formation of WO3 nanofibers with a monoclinic crystal structure. The PPy-WO3 sample (the upper spectrum) exhibits
Fig. 3. (A) SEM image and (B) EDX spectrum of the PPy-coated WO3 nanofibers prepared by the two-step vapor-phase polymerization process. The number intensity and energy of X-rays emitted from the sample was depicted as y and x-axes, respectively, in EDX spectrum.
noticeable additional peaks in the region of 1000–1200 cm−1 in addition to the strong O W O stretching and several weak side bands. Their peak positions are well assigned to the characteristic vibration modes of PPy molecules: N H bending at 1011 and 1040 cm−1 ; C O bending at 1125 cm−1 and C O bending at 1193 cm−1 [31]. These results provide direct evidence for the PPy growth through the two-step vapor-phase polymerization. Detailed nanostructure features of PPy-WO3 were further investigated by TEM. Fig. 5A shows a low-resolution TEM image, displaying the one-dimensional nanostructure assembled by
Fig. 4. FT-IR spectra of WO3 and PPy-coated WO3 samples.
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Fig. 5. (A) Low resolution and (B) high resolution TEM images of the PPy-coated WO3 nanofibers. The high resolution image displays lattice structures of WO3 crystals in the dotted ellipse regions.
irregular polygon nanoparticles. For the direct observation of WO3 lattice structures, the circled region, a, was further analyzed at high resolution, as shown in Fig. 5B. The region, b, designated by the dotted ellipse, exhibits a lattice structure with a inter-planar distance of 0.38 nm, assigned to the intensive (0 0 2), (0 2 0) or (2 0 0) deflection plane. In addition, the lattice structure of (0 2 2) plane is seen in the region c. The polygons with well-defined facets can be interpreted as monoclinic WO3 crystals. On the other hand, the surface region displays no lattice structure and smooth surface morphology, thus being regarded as an overlaid PPy layer with a thickness of several nm, as mentioned in the SEM observation. The PPy layer appears to have a substantial thickness variation. The larger thickness is found at the cavities between WO3 grains, probably due to the preferential FeTos adsorption at sites with a high negative curvature. Consequently, these TEM observations imply that the ultrathin PPy layer is well formed on the rough surface of WO3 nanofibers via two-step vapor-phase polymerization. 3.2. Ammonia-sensing properties Chemoresistive sensing performance was evaluated for the PPy-coated WO3 mat formed on the interdigitated Au substrate. Fig. 6A shows a response curve at an operation temperature of 100 ◦ C when the analyte was injected into the sensor for 2 min, and was then flushed by a N2 flow during a 2 min recovery period. The NH3 concentration is successively increased from 1, 5, 10, to 20 ppm, as displayed in the bottom plot of Fig. 6A. The responses display an increase in resistance upon exposure to the ammonia analyte. For example, the sensor resistance increased from 11.26 to 12.02 M for the 1 ppm NH3 exposure, which corresponds to
Fig. 6. (A) Time-profiled response curve and (B) detection response variation of the PPy-coated WO3 nanofiber sensor as a function of NH3 concentration at an operation temperature of 100 ◦ C. Ammonia analytes were supplied in the pulsed mode with gradually increasing concentration, as indicated in the bottom of (A).
a sensor response of R = 6.3%. Fig. 6B shows a gradual increase in response magnitude as a function of NH3 concentration. The detection sensitivity defined by the slope of the response curve was found to be lower at high concentrations: roughly 3.2%/ppm in the interval of 1–5 ppm, and 0.48%/ppm in the range of 5–20 ppm. Such a decrease in detection sensitivity at higher NH3 concentrations was previously reported in the PPy sensor systems [4,6,10,14], which might be explained by heterogeneousness in interaction strength between PPy and NH3 or by a partial saturation of PPy at a high concentration. The conductivity of Tos-doped PPy is readily altered by a chemical or electrochemical redox process when it is exposed to electron-donating ammonia. The redox reaction involves the transfer of electrons and the exchange of counter ions for the charge compensation on the polymer chain. This process may be represented by the following chemical reaction equation [5]: PPy+ /Tos− + NH3 (ı− ) ↔ PPy0 /NH3 (ı− )+ /Tos−
(2)
The positively-charged PPy backbone in an initial state is reversibly changed to a more neutral form due to the redox interactions between Tos-doped PPy and NH3 , thus leading to the p-type response of increasing resistance in the semiconducting sensor system. The interactions between WO3 and NH3 are expected to create a decrease in resistance, since WO3 is well known as an n-type semiconductor. The observed p-type response for the NH3
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Fig. 7. Time-profiled response curves of the PPy-coated WO3 nanofiber sensor at operation temperatures of 20 ◦ C (upper) and 100 ◦ C (lower) when 10 ppm NH3 was exposed.
detection at 100 ◦ C, therefore, indicates that the p-type PPy layer is primarily responsible for the chemoresistive response in the PPy-coated WO3 sensor. For a comparison study, PPy-coated PAN sensors were prepared with using a PAN nanofiber mat as a template instead of WO3 , and evaluated NH3 -sensing characteristics. The PPy-coated PAN sensor exhibited lower detection sensitivity and more irreversible baseline compared with the PPy-coated WO3 (data not shown). These results suggest that semiconducting WO3 is better core material in PPy-coated nanofibers for achieving a sensor of high performance than insulating PAN. Similar NH3 -sensing characteristics were also evaluated at various operation temperatures under 100 ◦ C, since PPy material can be degraded at high temperature. Fig. 7 displays typical response curves, measured at 20 and 100 ◦ C, for the sequential exposure of 10 ppm NH3 over two cycles. The response curve at 20 ◦ C exhibits inferior detection characteristics when compared with that at 100 ◦ C: long detection times, incomplete baseline recovery, and low response magnitude. The t90 detection times are 105 and 25 s at the operation temperatures of 20 and 100 ◦ C, respectively. The baseline resistance is changed from 44.68 to 46.22 M, corresponding to 3.5% increase, at 20 ◦ C after the measurement, while the resistance variation is less than 0.2% at 100 ◦ C. The slow detection and irreversible recovery at 20 ◦ C may be attributed to the existence of an energy barrier in the direction of a reaction coordination. In fact, a relatively long stabilization time of more than 1 h was necessary to reach a steady-state initial sensor resistance at ambient temperature, probably due to the slow desorption kinetics of adsorbed molecules. A great improvement in the response
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and recovery characteristics were observed when the operation temperature was increased to 70 ◦ C, while there was no noticeable change in response-curve shape in the temperature range of 70–100 ◦ C. The sensor responses for the detection of 10 ppm NH3 were 7.1% at room temperature and 6.1% at 70 ◦ C, being significantly lower than the magnitude of 22% obtained at 100 ◦ C. These results indicate that NH3 detection at 100 ◦ C results in a faster and higher response in the PPy-coated WO3 nanofiber sensor system than those at 20 and 70 ◦ C. High-performance detection at higher temperatures was not observed in a similar ultrathin PPy-coated PAN nanofiber sensor. The remarkable response enhancement at 100 ◦ C may be due to the formation of effective p–n junction in the PPy-coated WO3 nanofiber sensors, as previously reported in metal-oxide semiconductor hetero-nanostructures of CuO–SnO2 [24] and Cr2 O3 –ZnO [26]. Fig. 8 illustrates a schematic mechanism for the NH3 detection of the PPy-coated WO3 nanofiber sensor. A depletion layer is naturally formed at the interface between the p-type Tos-doped PPy and the n-type WO3 . Since WO3 has a very low conductivity at room temperature, and thus a small density of electron carriers, the positive space charges in the WO3 depletion region are distributed more sparsely than counterparts in the depletion region of Tos-doped PPy. The difference in space charge density can develop a wider depletion layer toward the core WO3 , when compared with the sheath PPy (see Fig. 8). When exposed to electron-donating NH3 , the positively-charged PPy backbone changes to a less conductive neutral form, as described in Eq. (2), thus displaying a resistanceincreasing p-type response. At higher temperature, tungsten oxides with a non-stoichiometric nature have been reported to produce larger amounts of free electrons from the oxygen vacancy donor levels through thermal activation [32]. The excess free electrons can diffuse into p-type PPy and then develop many space charge points in the p–n junction region through the hole-electron recombination process. If the WO3 space-charge density becomes comparable or larger than that of Tos-doped PPy at 100 ◦ C, the depletion layer can be considerably extended toward the p-type region. This space charge extension induces a reduction in the hole transfer crosssection area along the p-type PPy sheath layer. Considering the average PPy thickness of about 5 nm, the conduction pathway can be significantly modulated due to the variation in space charge depth. Upon exposure to reducing NH3 , electron injection into the positively-charged PPy will significantly reduce the conduction pathway to a great extent through the formation of nonconductive neutral form. Therefore, the enhanced response can be attributed to the high sensitivity of the active PPy layer having a thinner effective conduction thickness for the interactions between PPy and NH3 . This kind of sensitivity dependence on conduction channel width has been well demonstrated in single nanowire sensor systems [33]. The conduction variation along crystalline WO3 grains
Fig. 8. Proposed NH3 sensing mechanisms of the PPy-coated WO3 nanofiber sensor.
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Acknowledgement This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (NRF2012R1A1A2005473 and NRF-2012-0009838).
References
Fig. 9. Time-profiled H2 response curve of the PPy-coated WO3 sensor at 100 ◦ C in dry N2 environment.
can offer an additional possibility for the temperature-dependent response; however, the sensitive p-type response supports negligible direct interactions between WO3 and NH3 , as well as the formation of fully PPy-covered WO3 . NH3 -sensing measurements were conducted in dry air atmosphere instead of N2 for clarifying detection characteristics in more detail. They also exhibited a p-type NH3 response with comparable detection sensitivity. However, the sensor stabilization in resistance was very difficult to achieve in the ambient air probably due to the interactions between sensing materials and oxygen molecules. In addition, further experiments were performed to evaluate detection selectivity of the sensor for other analytes such as water, methanol and hydrogen molecules. Fig. 9 shows a timeprofiled H2 response curve of the PPy-coated WO3 sensor at 100 ◦ C in dry N2 environment. When compared to NH3 detection, the response magnitude corresponds to one fifth, and the response times are much longer. In cases of methanol and water, there was no noticeable response except for the significant fluctuation in background resistance. These results suggest that environmental durability of the PPy-coated WO3 nanofibers must be enhanced for a use in commercialized sensors, despite the marginal selectivity in NH3 detection. 4. Conclusion A coaxial conducting polymer nanofiber sensor has been fabricated by depositing an ultrathin p-type doped PPy layer onto n-type polycrystalline WO3 nanofibers. This core-sheath hetero-junction structure can provide an opportunity to enhance gas-sensing properties through the working of p–n junctions, together with the great surface area and high porosity resulting from a nanofiber mat. To the best of our knowledge, this study is the first experimental demonstration of the p–n junction influence on chemoresistive response in the conducting polymer system, although the effect was previously reported in metal-oxide semiconductor heteronanostructures. The NH3 -sensing measurements of our PPy-coated WO3 sensor were performed under concentrations of 1–20 ppm, and operating temperatures of 20, 70 and 100 ◦ C. A large sensor response and short detection times were observed at an operating temperature of 100 ◦ C. The unusual response enhancement at 100 ◦ C may be attributed to the highly sensitive modulation of a conductive PPy layer thickness for the NH3 exposure due to the extension of the p–n depletion region in the PPy-coated WO3 hetero-nanostructures.
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Biographies Thi Anh Ho received her BS degree from the Hanoi University of Technology, Vietnam in 2011. She is currently a graduate student in the Department of Bionano Engineering, Hanyang University, South Korea. Her research interest is primarily centered on the development of new sensing nanomaterials used in chemoresistive and electrochemiluminescence sensors. Tae-Sun Jun received his BS degree in Oriental Medical Food & Nutrition from the Semyung University in 2009. He is currently a graduate student in the Department of Bionano Engineering, Hanyang University, South Korea. His research activities are focused on novel sensing materials such as conducting polymers and core–shell nanofibers. Yong Shin Kim received a PhD in Chemistry from Korea Advanced Institute of Science and Technology (KAIST) in 1997. Following the completion of his degree, he worked as a senior research member at Electronics and Telecommunications Research Institute (ETRI) in South Korea, developing flat panel display devices and miniaturized electronic nose systems. Since 2007, he has been employed at Hanyang University in South Korea as an Associate Professor. His current research activities are focused on the development of smart chemical sensors and novel nanomaterials for sensor applications.