- Email: [email protected]
SnO2–ZnO hybrid nanofibers-based highly sensitive nitrogen dioxides sensor
Sensors and Actuators B 145 (2010) 592–595
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Short communication
SnO2 –ZnO hybrid nanofibers-based highly sensitive nitrogen dioxides sensor Jin-Ah Park ∗ , Jaehyun Moon, Su-Jae Lee, Seong Hyun Kim, Hye Yong Chu, Taehyoung Zyung Convergent Components & Materials Research Laboratory, Electronics and Telecommunications Research Institute, Daejeon 305-700, Republic of Korea
a r t i c l e
i n f o
Article history: Received 24 August 2009 Received in revised form 2 November 2009 Accepted 11 November 2009 Available online 17 November 2009 Keywords: Nanofiber Electrospinning Pulsed laser deposition ZnO SnO2 Gas sensing
a b s t r a c t SnO2 –ZnO hybrid nanofibers were fabricated by combining the electrospinning and the pulsed laser deposition methods. After calcining at 600 ◦ C, the nanocrystalline SnO2 coated ZnO nanofibers with a random network structure were obtained. The fiber diameter and the size of SnO2 deposit were 55–80 nm and 10–15 nm, respectively. SnO2 –ZnO hybrid nanofibers-based sensor exhibited very high gas response to NO2 concentration as low as 400 ppb level at 200 ◦ C. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Semiconducting metal oxides (SMOs) with nanostructures as gas sensing materials have been widely used in detection of toxic gases and vapors [1–3]. Typically, the gas sensing properties of the SMO-based sensors are highly dependent not only on their surface chemical reactivity and thermal stability, but also on the geometrical and compositional variations [4–7]. Recently, SMOs with one-dimensional (1D) nanostructures such as nanowires, nanorods, and nanofibers have been a subject of intensive research in gas sensing area because of their unique properties and interesting applications [8–10]. Many researches focus on the nanofibers which provide large surface-to-volume and length-to-diameter ratios, which contribute to the high sensitivity. Some techniques such as metal-organic chemical vapor deposition (MOCVD) [11], chemical vapor deposition (CVD) [12], carbothermal reduction synthesis [13], vapor–liquid–solid (VLS) [5], electrospinning (ES) [14] have been attempted to obtain 1D nanostructured SMOs. Some techniques have been attempted to obtain 1D nanostructured SMOs. In particular, the electrospinning (ES) method is a very attractive method for the fabrication of nanofibers, considering simplicity, reproducibility, flexibility and manipulation. Among available SMOs, the ZnO is one of the most well-defined gas sensing materials. The fabrication and characteristics of electrospun ZnO nanofibers has been reported [15–17]. However, the
∗ Corresponding author. Tel.: +82 42 860 1178; fax: +82 42 860 5202. E-mail address: [email protected] (J.-A. Park). 0925-4005/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2009.11.023
gas sensing properties of the ZnO nanofibers upon exposure to oxidizing and/or reducing gases of low concentration level still remain poor. It is worth mentioning at this point that the ZnO nanofibers can be further functionalized by surface modification with various catalysts, nanoparticles, and biological species, leading to achievement of exceptional gas response. In this study, we report on the fabrication of ZnO nanofibers using an ES method and the response of ZnO nanofibers upon NO2 gas that is the major cause of air pollution and acid rain. In order to improve the gas response of ZnO nanofibers to low NO2 concentration, we have coated a thin layer of nanocrystalline SnO2 on the ZnO nanofibers. SnO2 is an n-type SMO gas sensing material with excellent gas response toward exposure of environmental gases. SnO2 was deposited using a pulsed laser deposition (PLD) method. The NO2 sensing properties as well as the structure of SnO2 coated ZnO (SnO2 –ZnO) nanofibers were investigated.
2. Experimental To prepare the ES solution, 5 g of Zn acetate dihydrate (Zn(CH3 COO)·2H2 O, Inostek) with 0.8 mol was mixed with 3 g of PVP (poly(4-vinyl phenol), Aldrich, Mw = 20,000) and then stirred for 3 h at 60 ◦ C. Thereafter, 1 ml of ethanol was added to the mixture and then was sufficiently stirred to obtain a homogeneous solution. Finally, an ES solution with an optimized viscosity suitable for ES was obtained. To perform the ES process, the ES solution was loaded into a plastic syringe equipped with a needle. A needle of 250 m in diameter was connected to a high voltage power supply. The feeding rate was kept constant at 0.3 ml/h using a syringe
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pump. The SiO2 /Si substrate was placed 5 cm below the needle tip to collect the electrospun composite fibers. The applied voltage was maintained at 7 kV. The substrate temperature was maintained at 80 ◦ C during the ES process. The collecting time was about 5 min. In order to measure the gas sensing properties of the resultant nanofibers, the electrospun composite fibers were collected on platinum interdigital electrodes (Pt-IDEs) patterned SiO2 /Si substrate. The finger width, the finger gap and the overlap length of Pt-IDE were 15 m, 10 m and 500 m, respectively. The electrospun composite fibers were hot-pressed at 120 ◦ C to densify the non-woven mat of composite fibers. Subsequently, the composite fibers were baked on a hotplate at 250 ◦ C for 30 min to remove the solvent. Thereafter, the ZnO/PVP composite fibers were calcined at 600 ◦ C for 1 h in air to decompose the PVP and crystallize the ZnO. The SnO2 deposit was coated on the ZnO nanofibers using a PLD method with KrF excimer laser ( = 248 nm). The oxygen pressure, laser intensity and repetition rate were 26.66 Pa, 3 J/cm2 and 2 Hz, respectively. PLD process was carried out at room temperature for 1 min. Finally, to crystallize SnO2 deposit, the SnO2 coated ZnO nanofibers were annealed at 600 ◦ C for 30 m in air. In order to measure the gas sensing properties of the resultant nanofibers, the ZnO and SnO2 –ZnO nanofibers were collected on Pt-IDEs patterned SiO2 /Si substrates. The IDEs have dimensions of finger width of 15 m, the finger gap of 10 m and the overlap length of 500 m, respectively. For NO2 gas sensing test, the SnO2 –ZnO hybrid nanofibers-based sensor was placed in a sealed chamber equipped with heater and gas supplying units. The applied voltage was fixed at 100 mV. Various NO2 concentrations were obtained by diluting a premixed gas of NO2 20 ppm with dry air. The NO2 response was determined by comparing the resistance of SnO2 –ZnO hybrid nanofibers in dry air (Rair ) to that in NO2 (RNO2 ). The response is defined as RNO2 /Rair .
3. Results and discussion Fig. 1 shows the scanning electron microscopy (SEM) images of the electrospun ZnO/PVP composite fibers, the pristine ZnO nanofibers, and the SnO2 –ZnO hybrid nanofibers. The assembly of the ZnO/PVP composite fibers forms a multi-layered random network structure which is due to the instability of the spin jet, as shown in Fig. 1(a). The diameters of fibers were in a range of 250–350 nm. The surface morphology of the composite fibers appears to be smooth due to the polymeric feature. Fig. 1(b) shows the SEM images of pristine ZnO nanofibers obtained by calcining ZnO/PVP composite nanofibers. Although drastic shrinkage occurred after calcination, the ZnO nanofibers retained the network structure with the diameter of 55–80 nm. The pristine ZnO nanofibers have the one-dimensional nanostructures linked by the nanosized ZnO grains between 20 and 80 nm. Fig. 1(c) shows the SEM image of the SnO2 –ZnO hybrid nanofibers. Apparently, the ZnO nanofibers are covered with the SnO2 nanoparticles of 10 –15 nm. The bright-field scanning transmission electron microscope (STEM) images (Fig. 2(a)) indicates that a SnO2 –ZnO hybrid nanofiber of 72 nm in diameter in which polycrystalline SnO2 and ZnO phases coexist. Fig. 2(b) shows elemental mapping images of Zn (red) and Sn (green) obtained using an electron dispersive spectroscopy (EDS) embedded in the STEM of ZnO nanofiber. This result reveals that the SnO2 deposit (green) covers the ZnO nanofiber (red). The XRD pattern of Fig. 3 shows that the SnO2 –ZnO hybrid nanofibers is in the polycrystalline structure with two phases of rutile SnO2 (JCPDS 41-1445) and würtzite ZnO (JCPDS 36-1451) phases. This indicates that the annealing at 600 ◦ C is sufficient to not only to decompose the PVP but also crystallize both ZnO and
Fig. 1. SEM images of (a) the electrospun ZnO/PVP composite fibers, (b) the pristine ZnO nanofibers, and (c) the SnO2 –ZnO hybrid nanofibers.
SnO2 . No third phase such as Zn2 SnO4 was observed. Due to the 1D geometry of nanocrystalline oxides and the enhanced functionalizing due to SnO2 , the SnO2 –ZnO hybrid nanofibers are expected to have enhanced gas sensing properties. Fig. 4 shows that the responses of the SnO2 –ZnO hybrid nanofibers-based sensor are highly dependent on the working temperature (TW ) and the concentration of NO2 . The sensor exhibited relatively high response at the TW between 180 ◦ C and 200 ◦ C at a fixed NO2 concentration of 3.2 ppm (Fig. 4(a)). Thus, we choose 200 ◦ C as our TW in evaluating the SnO2 –ZnO hybrid nanofibersbased sensor. As the NO2 concentration increases from 0.4 ppm to 4 ppm, the response at 200 ◦ C increases from 6 to 105 as shown in Fig. 4(b). In contrast, the pristine ZnO nanofiber-based sensor showed poor response lower than 1.15. The increase in the resistance of the SnO2 –ZnO hybrid nanofibers upon exposure to NO2 can be described by the following surface electrochemical reaction processes [14,18]: NO2 (g) + e− ↔ NO2 − (ads) −
−
NO2 (g) + e ↔ O (ads) + NO(g)
(1) (2)
where “g” and “ads” refer to gas and adsorbate, respectively. Both reactions result in removal of free electrons (e− s). The
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J.-A. Park et al. / Sensors and Actuators B 145 (2010) 592–595
Fig. 2. The images of (a) bright-field STEM and (b) elemental mapping of Zn (red) and Sn (green) for a SnO2 –ZnO hybrid nanofiber of 72 nm in diameter.
thicknesses of the depleted layers of semiconducting oxides are approximately 3 nm [19]. Due to the prescribed reaction electron exchange take place at the surface of oxides particles, leading to change in the thicknesses of the depleted layers and electrical properties. Because of the removal of electrons, the depleted layer at the surface and/or the grain boundaries widens, leading to increase in the resistance [20]. The huge improvement in the response by SnO2 coating is due to, at least, two factors. One important factor determining the response is related to the surface morphology of the sensing material. In this context, the small SnO2 crystallites are thought to have an effect of facilitating the surface reactions by providing extra NO2 adsorption sites. Because the reaction is enhanced the change in resistance upon NO2 gas exposure is pronounced. Charge transfer process, which frequently appears in noble metal added SMOs-based sensors, can be an additional factor. Due to the close similarity in work function of SnO2 (3.6 eV) and ZnO (3.4 eV), the interface barrier between them is very low. Thus charge transfers can easily occur. As the surface reaction prescribes, the total numbers of individual reactions are proportional to the number of available e− s. Thus charge transfer contributes in
Fig. 4. (a) NO2 responses of the SnO2 –ZnO hybrid nanofibers-based sensor as a function of working temperature. (b) The comparison of NO2 responses for gas sensor based on the pristine ZnO nanofibers (blue) and the SnO2 –ZnO hybrid nanofibers (red) as a function of gas concentration.
the enhancement of reactions. In other words the facile electronic interaction between SnO2 and ZnO give rise to a synergetic effect in the NO2 sensing performance. In this scenario nanocrystalline SnO2 deposits can be the electron donor or the acceptor to or from ZnO nanofiber. 4. Conclusions In summary, we demonstrated a method to electrochemically functionalize pristine ZnO nanofibers by surface modification by coating nanocrystalline SnO2 . The enhanced gas sensing performances of resultant SnO2 –ZnO hybrid nanofibers are demonstrated by comparing the gas sensing performances of pristine ZnO nanofibers under various NO2 concentrations. The response of the SnO2 –ZnO hybrid nanofiber was highly sensitive at 200 ◦ C upon exposure to NO2 concentration as low as 0.4 ppm. A highly sensitive NO2 response of the SnO2 –ZnO hybrid nanofibers-based sensor is due to two factors; the extra adsorption due to nanocrystalline SnO2 coating and the charge transfer occurring between SnO2 and ZnO. The SnO2 –ZnO hybrid nanofibers are an attractive proposition for advanced NO2 gas sensor, detecting ppb level. Acknowledgement
Fig. 3. X-ray diffraction pattern of the SnO2 –ZnO hybrid nanofibers.
This work was supported by the basic R&D program of the Electronics and Telecommunication Research Institute [Project No.2200-S].
J.-A. Park et al. / Sensors and Actuators B 145 (2010) 592–595
References [1] E. Comini, G. Fagria, G. Sberveglieri, Z. Pan, Z.L. Wang, Local impedance imaging and spectroscopy of polycrystalline ZnO using contact atomic force microscopy, Appl. Phys. Lett. 81 (2002) 1869–1871. [2] I.D. Kim, A. Rothschild, T. Hyodo, H.L. Tuller, Microsphere templating as means of enhancing surface activity and gas sensitivity of CaCu3 Ti4 O12 thin films, Nano. Lett. 6 (2006) 193–198. [3] J. Tamaki, T. Hashishin, Y. Uno, D.V. Dao, S. Sugiyama, Ultrahigh-sensitive WO3 nanosensor with interdigitated Au nano-electrode for NO2 detection, Sens. Actuators B 132 (2008) 234–238. [4] S.J. Chang, T.J. Hsueh, I.C. Chen, B.R. Huang, Highly sensitive ZnO-nanowire CO sensors with the adsorption of Au nanoparticles, Nanotechnology 19 (2008) 175502–175503. [5] H. Xia, Y. Wang, F. Kong, S. Wang, B. Zhu, X. Guo, J. Zhang, Y. Wang, S. Wu, Au-doped WO3 -based sensor for NO2 detection at low operating temperature, Sens. Actuators B 134 (2008) 133–139. [6] C. Liangyuan, B. Shouli, Z. Guojun, L. Dianqing, C. Alfan, C.C. Liu, Synthesis of ZnO–SnO2 nanocomposites by microemulsion and sensing properties for NO2 , Sens. Actuators B 134 (2008) 360–366. [7] M. Yuasa, T. Masaki, T. Kida, K. Shimanoe, N. Yamazoe, Nano-sized PdO loaded SnO2 nanoparticles by reverse micelle method for highly sensitive CO gas sensor, Sens. Actuators B 136 (2009) 99–104. [8] T.J. Hsueh, S.J. Chang, Highly sensitive ZnO nanowire ethanol sensor with Pd adsorption, Appl. Phys. Lett. 91 (2007) 053111–53113. [9] Y.C. Lee, H. Huang, O.K. Tan, M.S. Tse, Semiconductor gas sensor based on Pddoped SnO2 nanorod thin films, Sens. Actuators B 132 (2008) 239–242. [10] I.D. Kim, A. Rothschild, B.H. Lee, D.Y. Kim, S.M. Jo, H.L. Tuller, Ultrasensitive chemiresistors based on electrospun TiO2 nanofibers, Nano Lett. 6 (2006) 2009–2013. [11] M.W. Ahn, K.S. Park, J.H. Heo, J.G. Park, D.W. Kim, K.J. Choi, J.H. Lee, S.H. Hong, Gas sensing properties of defect-controlled ZnO-nanowire gas sensor, Appl. Phys. Lett. 93 (2008) 263103–263113. [12] W. Wang, H. Huang, Z. Li, H. Zhang, Y. Wang, W. Zheng, C. Wang, Zinc oxide nanofiber gas sensors via electrospinning, J. Am. Ceram. Soc. 91 (2008) 3817–3819. [13] H. Wu, D. Lin, R. Zhang, W. Pan, ZnO nanofiber field-effect transistor assembled by electrospinning, J. Am. Ceram. Soc. 91 (2008) 656–659. [14] S.T. Shishiyanu, T.S. Shishiyanu, O.I. Lupan, Sensing characteristics of tin-doped ZnO thin films as NO2 gas sensor, Sens. Actuators B 107 (2005) 379–386. [15] S. Matsushima, Y. Teraoka, N. Miura, N. Yamazoe, Electronic interaction between metal additives and tin dioxides in tin dioxide-based gas sensors, Jpn. J. Appl. Phys. 27 (1988) 1798–1802. [16] J.A. Park, J. Moon, S.J. Lee, S.C. Lim, T. Zyung, Fabrication and characterization of ZnO nanofibers by electrospinning, Curr. Appl. Phys. 9 (2009) S210–S212. [17] J. Moon, J.A. Park, S.J. Lee, T. Zyung, Structure and electrical properties of ZnO–NiO mixed oxide nanofibers, Curr. Appl. Phys. 9 (2009) S213–S216. [18] N. Bârsan, U.J. Weimar, Conduction model of metal oxide gas sensors, Electroceramics 7 (2005) 143–167.
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Biographies Jin-Ah Park received the BS degree from Gyeongsang National University, Republic of Korea, in 1998, and PhD degree in Electrical Engineering from Dongeui University, Republic of Korea, in 2006, respectively. As a post-doctoral associate, he joined Electronics and Telecommunications Research Institute (ETRI) in 2007. His current research interests include oxide electronics, nanostructures, and their applications. Jaehyun Moon received the BS degree from Korea University, Seoul, Republic of Korea, in 1995, and PhD degree in Materials Science and Engineering from Carnegie Mellon University, Pittsburgh, USA, in 2003. From 2003 to 2004, he was a postdoctoral associate at Max-Planck Institute, Stuttgart, Germany. He joined Electronics and Telecommunications Research Institute (ETRI) in 2004. His current research interests include flexible display, low temperature Si processes, nanofibers and interface studies of materials. Su-Jae Lee received the BS degree in physics form Kyungsung University, Republic of Korea, in 1986 and the MS and PhD degrees in physics from Pusan National University, Busan, Republic of Korea, in 1988 and 1997, respectively. He joined Electronics and Telecommunications Research Institute (ETRI) in 1997. His research interests include the development of new multifunctional nanostructured oxides for the creation of nanooxide electronics devices and gas sensors. Seong Hyun Kim received the PhD degree in solid-state physics at Pusan National University, in 1998. He performed post-doctoral study at Electronics and Telecommunications Research Institute (ETRI), Daejeon, Korea, from 1998 to 1999. He joined ETRI in 1999 and worked on organic semiconductor devices. Currently, he is interested in making thin-film transistors and flexible electronics using organic semiconductors. Hye Yong Chu received the BS and MS degrees in physics from Kyung-Hee University in 1987 and 1989, respectively. She joined Electronics and Telecommunications Research Institute (ETRI) in 1989. She received PhD degree in Information Display from Kyung-Hee University in 2008. Her current research interests include novel device architectures in organic light emitting devices. Taehyoung Zyung graduated from Seoul National University, Republic of Korea, in 1977, and worked at KIST as a researcher for 3 and half years from 1978. He received PhD in Physical Chemistry at Texas Tech University in 1986. He performed postdoctoral study as a research associate at University of Illinois at Urbana-Champaign during 1986 to 1989. He joined Electronics and Telecommunications Research Institute (ETRI) in 1989 and worked on the organic semiconductor devices. He has published about 100 SCI journals and filed about 30 patents.
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Short communication
SnO2 –ZnO hybrid nanofibers-based highly sensitive nitrogen dioxides sensor Jin-Ah Park ∗ , Jaehyun Moon, Su-Jae Lee, Seong Hyun Kim, Hye Yong Chu, Taehyoung Zyung Convergent Components & Materials Research Laboratory, Electronics and Telecommunications Research Institute, Daejeon 305-700, Republic of Korea
a r t i c l e
i n f o
Article history: Received 24 August 2009 Received in revised form 2 November 2009 Accepted 11 November 2009 Available online 17 November 2009 Keywords: Nanofiber Electrospinning Pulsed laser deposition ZnO SnO2 Gas sensing
a b s t r a c t SnO2 –ZnO hybrid nanofibers were fabricated by combining the electrospinning and the pulsed laser deposition methods. After calcining at 600 ◦ C, the nanocrystalline SnO2 coated ZnO nanofibers with a random network structure were obtained. The fiber diameter and the size of SnO2 deposit were 55–80 nm and 10–15 nm, respectively. SnO2 –ZnO hybrid nanofibers-based sensor exhibited very high gas response to NO2 concentration as low as 400 ppb level at 200 ◦ C. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Semiconducting metal oxides (SMOs) with nanostructures as gas sensing materials have been widely used in detection of toxic gases and vapors [1–3]. Typically, the gas sensing properties of the SMO-based sensors are highly dependent not only on their surface chemical reactivity and thermal stability, but also on the geometrical and compositional variations [4–7]. Recently, SMOs with one-dimensional (1D) nanostructures such as nanowires, nanorods, and nanofibers have been a subject of intensive research in gas sensing area because of their unique properties and interesting applications [8–10]. Many researches focus on the nanofibers which provide large surface-to-volume and length-to-diameter ratios, which contribute to the high sensitivity. Some techniques such as metal-organic chemical vapor deposition (MOCVD) [11], chemical vapor deposition (CVD) [12], carbothermal reduction synthesis [13], vapor–liquid–solid (VLS) [5], electrospinning (ES) [14] have been attempted to obtain 1D nanostructured SMOs. Some techniques have been attempted to obtain 1D nanostructured SMOs. In particular, the electrospinning (ES) method is a very attractive method for the fabrication of nanofibers, considering simplicity, reproducibility, flexibility and manipulation. Among available SMOs, the ZnO is one of the most well-defined gas sensing materials. The fabrication and characteristics of electrospun ZnO nanofibers has been reported [15–17]. However, the
∗ Corresponding author. Tel.: +82 42 860 1178; fax: +82 42 860 5202. E-mail address: [email protected] (J.-A. Park). 0925-4005/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2009.11.023
gas sensing properties of the ZnO nanofibers upon exposure to oxidizing and/or reducing gases of low concentration level still remain poor. It is worth mentioning at this point that the ZnO nanofibers can be further functionalized by surface modification with various catalysts, nanoparticles, and biological species, leading to achievement of exceptional gas response. In this study, we report on the fabrication of ZnO nanofibers using an ES method and the response of ZnO nanofibers upon NO2 gas that is the major cause of air pollution and acid rain. In order to improve the gas response of ZnO nanofibers to low NO2 concentration, we have coated a thin layer of nanocrystalline SnO2 on the ZnO nanofibers. SnO2 is an n-type SMO gas sensing material with excellent gas response toward exposure of environmental gases. SnO2 was deposited using a pulsed laser deposition (PLD) method. The NO2 sensing properties as well as the structure of SnO2 coated ZnO (SnO2 –ZnO) nanofibers were investigated.
2. Experimental To prepare the ES solution, 5 g of Zn acetate dihydrate (Zn(CH3 COO)·2H2 O, Inostek) with 0.8 mol was mixed with 3 g of PVP (poly(4-vinyl phenol), Aldrich, Mw = 20,000) and then stirred for 3 h at 60 ◦ C. Thereafter, 1 ml of ethanol was added to the mixture and then was sufficiently stirred to obtain a homogeneous solution. Finally, an ES solution with an optimized viscosity suitable for ES was obtained. To perform the ES process, the ES solution was loaded into a plastic syringe equipped with a needle. A needle of 250 m in diameter was connected to a high voltage power supply. The feeding rate was kept constant at 0.3 ml/h using a syringe
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593
pump. The SiO2 /Si substrate was placed 5 cm below the needle tip to collect the electrospun composite fibers. The applied voltage was maintained at 7 kV. The substrate temperature was maintained at 80 ◦ C during the ES process. The collecting time was about 5 min. In order to measure the gas sensing properties of the resultant nanofibers, the electrospun composite fibers were collected on platinum interdigital electrodes (Pt-IDEs) patterned SiO2 /Si substrate. The finger width, the finger gap and the overlap length of Pt-IDE were 15 m, 10 m and 500 m, respectively. The electrospun composite fibers were hot-pressed at 120 ◦ C to densify the non-woven mat of composite fibers. Subsequently, the composite fibers were baked on a hotplate at 250 ◦ C for 30 min to remove the solvent. Thereafter, the ZnO/PVP composite fibers were calcined at 600 ◦ C for 1 h in air to decompose the PVP and crystallize the ZnO. The SnO2 deposit was coated on the ZnO nanofibers using a PLD method with KrF excimer laser ( = 248 nm). The oxygen pressure, laser intensity and repetition rate were 26.66 Pa, 3 J/cm2 and 2 Hz, respectively. PLD process was carried out at room temperature for 1 min. Finally, to crystallize SnO2 deposit, the SnO2 coated ZnO nanofibers were annealed at 600 ◦ C for 30 m in air. In order to measure the gas sensing properties of the resultant nanofibers, the ZnO and SnO2 –ZnO nanofibers were collected on Pt-IDEs patterned SiO2 /Si substrates. The IDEs have dimensions of finger width of 15 m, the finger gap of 10 m and the overlap length of 500 m, respectively. For NO2 gas sensing test, the SnO2 –ZnO hybrid nanofibers-based sensor was placed in a sealed chamber equipped with heater and gas supplying units. The applied voltage was fixed at 100 mV. Various NO2 concentrations were obtained by diluting a premixed gas of NO2 20 ppm with dry air. The NO2 response was determined by comparing the resistance of SnO2 –ZnO hybrid nanofibers in dry air (Rair ) to that in NO2 (RNO2 ). The response is defined as RNO2 /Rair .
3. Results and discussion Fig. 1 shows the scanning electron microscopy (SEM) images of the electrospun ZnO/PVP composite fibers, the pristine ZnO nanofibers, and the SnO2 –ZnO hybrid nanofibers. The assembly of the ZnO/PVP composite fibers forms a multi-layered random network structure which is due to the instability of the spin jet, as shown in Fig. 1(a). The diameters of fibers were in a range of 250–350 nm. The surface morphology of the composite fibers appears to be smooth due to the polymeric feature. Fig. 1(b) shows the SEM images of pristine ZnO nanofibers obtained by calcining ZnO/PVP composite nanofibers. Although drastic shrinkage occurred after calcination, the ZnO nanofibers retained the network structure with the diameter of 55–80 nm. The pristine ZnO nanofibers have the one-dimensional nanostructures linked by the nanosized ZnO grains between 20 and 80 nm. Fig. 1(c) shows the SEM image of the SnO2 –ZnO hybrid nanofibers. Apparently, the ZnO nanofibers are covered with the SnO2 nanoparticles of 10 –15 nm. The bright-field scanning transmission electron microscope (STEM) images (Fig. 2(a)) indicates that a SnO2 –ZnO hybrid nanofiber of 72 nm in diameter in which polycrystalline SnO2 and ZnO phases coexist. Fig. 2(b) shows elemental mapping images of Zn (red) and Sn (green) obtained using an electron dispersive spectroscopy (EDS) embedded in the STEM of ZnO nanofiber. This result reveals that the SnO2 deposit (green) covers the ZnO nanofiber (red). The XRD pattern of Fig. 3 shows that the SnO2 –ZnO hybrid nanofibers is in the polycrystalline structure with two phases of rutile SnO2 (JCPDS 41-1445) and würtzite ZnO (JCPDS 36-1451) phases. This indicates that the annealing at 600 ◦ C is sufficient to not only to decompose the PVP but also crystallize both ZnO and
Fig. 1. SEM images of (a) the electrospun ZnO/PVP composite fibers, (b) the pristine ZnO nanofibers, and (c) the SnO2 –ZnO hybrid nanofibers.
SnO2 . No third phase such as Zn2 SnO4 was observed. Due to the 1D geometry of nanocrystalline oxides and the enhanced functionalizing due to SnO2 , the SnO2 –ZnO hybrid nanofibers are expected to have enhanced gas sensing properties. Fig. 4 shows that the responses of the SnO2 –ZnO hybrid nanofibers-based sensor are highly dependent on the working temperature (TW ) and the concentration of NO2 . The sensor exhibited relatively high response at the TW between 180 ◦ C and 200 ◦ C at a fixed NO2 concentration of 3.2 ppm (Fig. 4(a)). Thus, we choose 200 ◦ C as our TW in evaluating the SnO2 –ZnO hybrid nanofibersbased sensor. As the NO2 concentration increases from 0.4 ppm to 4 ppm, the response at 200 ◦ C increases from 6 to 105 as shown in Fig. 4(b). In contrast, the pristine ZnO nanofiber-based sensor showed poor response lower than 1.15. The increase in the resistance of the SnO2 –ZnO hybrid nanofibers upon exposure to NO2 can be described by the following surface electrochemical reaction processes [14,18]: NO2 (g) + e− ↔ NO2 − (ads) −
−
NO2 (g) + e ↔ O (ads) + NO(g)
(1) (2)
where “g” and “ads” refer to gas and adsorbate, respectively. Both reactions result in removal of free electrons (e− s). The
594
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Fig. 2. The images of (a) bright-field STEM and (b) elemental mapping of Zn (red) and Sn (green) for a SnO2 –ZnO hybrid nanofiber of 72 nm in diameter.
thicknesses of the depleted layers of semiconducting oxides are approximately 3 nm [19]. Due to the prescribed reaction electron exchange take place at the surface of oxides particles, leading to change in the thicknesses of the depleted layers and electrical properties. Because of the removal of electrons, the depleted layer at the surface and/or the grain boundaries widens, leading to increase in the resistance [20]. The huge improvement in the response by SnO2 coating is due to, at least, two factors. One important factor determining the response is related to the surface morphology of the sensing material. In this context, the small SnO2 crystallites are thought to have an effect of facilitating the surface reactions by providing extra NO2 adsorption sites. Because the reaction is enhanced the change in resistance upon NO2 gas exposure is pronounced. Charge transfer process, which frequently appears in noble metal added SMOs-based sensors, can be an additional factor. Due to the close similarity in work function of SnO2 (3.6 eV) and ZnO (3.4 eV), the interface barrier between them is very low. Thus charge transfers can easily occur. As the surface reaction prescribes, the total numbers of individual reactions are proportional to the number of available e− s. Thus charge transfer contributes in
Fig. 4. (a) NO2 responses of the SnO2 –ZnO hybrid nanofibers-based sensor as a function of working temperature. (b) The comparison of NO2 responses for gas sensor based on the pristine ZnO nanofibers (blue) and the SnO2 –ZnO hybrid nanofibers (red) as a function of gas concentration.
the enhancement of reactions. In other words the facile electronic interaction between SnO2 and ZnO give rise to a synergetic effect in the NO2 sensing performance. In this scenario nanocrystalline SnO2 deposits can be the electron donor or the acceptor to or from ZnO nanofiber. 4. Conclusions In summary, we demonstrated a method to electrochemically functionalize pristine ZnO nanofibers by surface modification by coating nanocrystalline SnO2 . The enhanced gas sensing performances of resultant SnO2 –ZnO hybrid nanofibers are demonstrated by comparing the gas sensing performances of pristine ZnO nanofibers under various NO2 concentrations. The response of the SnO2 –ZnO hybrid nanofiber was highly sensitive at 200 ◦ C upon exposure to NO2 concentration as low as 0.4 ppm. A highly sensitive NO2 response of the SnO2 –ZnO hybrid nanofibers-based sensor is due to two factors; the extra adsorption due to nanocrystalline SnO2 coating and the charge transfer occurring between SnO2 and ZnO. The SnO2 –ZnO hybrid nanofibers are an attractive proposition for advanced NO2 gas sensor, detecting ppb level. Acknowledgement
Fig. 3. X-ray diffraction pattern of the SnO2 –ZnO hybrid nanofibers.
This work was supported by the basic R&D program of the Electronics and Telecommunication Research Institute [Project No.2200-S].
J.-A. Park et al. / Sensors and Actuators B 145 (2010) 592–595
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Biographies Jin-Ah Park received the BS degree from Gyeongsang National University, Republic of Korea, in 1998, and PhD degree in Electrical Engineering from Dongeui University, Republic of Korea, in 2006, respectively. As a post-doctoral associate, he joined Electronics and Telecommunications Research Institute (ETRI) in 2007. His current research interests include oxide electronics, nanostructures, and their applications. Jaehyun Moon received the BS degree from Korea University, Seoul, Republic of Korea, in 1995, and PhD degree in Materials Science and Engineering from Carnegie Mellon University, Pittsburgh, USA, in 2003. From 2003 to 2004, he was a postdoctoral associate at Max-Planck Institute, Stuttgart, Germany. He joined Electronics and Telecommunications Research Institute (ETRI) in 2004. His current research interests include flexible display, low temperature Si processes, nanofibers and interface studies of materials. Su-Jae Lee received the BS degree in physics form Kyungsung University, Republic of Korea, in 1986 and the MS and PhD degrees in physics from Pusan National University, Busan, Republic of Korea, in 1988 and 1997, respectively. He joined Electronics and Telecommunications Research Institute (ETRI) in 1997. His research interests include the development of new multifunctional nanostructured oxides for the creation of nanooxide electronics devices and gas sensors. Seong Hyun Kim received the PhD degree in solid-state physics at Pusan National University, in 1998. He performed post-doctoral study at Electronics and Telecommunications Research Institute (ETRI), Daejeon, Korea, from 1998 to 1999. He joined ETRI in 1999 and worked on organic semiconductor devices. Currently, he is interested in making thin-film transistors and flexible electronics using organic semiconductors. Hye Yong Chu received the BS and MS degrees in physics from Kyung-Hee University in 1987 and 1989, respectively. She joined Electronics and Telecommunications Research Institute (ETRI) in 1989. She received PhD degree in Information Display from Kyung-Hee University in 2008. Her current research interests include novel device architectures in organic light emitting devices. Taehyoung Zyung graduated from Seoul National University, Republic of Korea, in 1977, and worked at KIST as a researcher for 3 and half years from 1978. He received PhD in Physical Chemistry at Texas Tech University in 1986. He performed postdoctoral study as a research associate at University of Illinois at Urbana-Champaign during 1986 to 1989. He joined Electronics and Telecommunications Research Institute (ETRI) in 1989 and worked on the organic semiconductor devices. He has published about 100 SCI journals and filed about 30 patents.