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Ultrasensitive NH3 Gas Sensor from Polyaniline Nanograin Enchased TiO2 Fibers Jian Gong,†,‡ Yinhua Li,† Zeshan Hu,† Zhengzhi Zhou,† and Yulin Deng*,† School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0620, and Key Laboratory of Polyoxometalate Science of Ministry of Education, Department of Chemistry, Northeast Normal UniVersity, Changchun, Jilin 130024, People’s Republic of China ReceiVed: January 24, 2010; ReVised Manuscript ReceiVed: April 22, 2010
In this communication, we reported for the first time an ultrasensitive nanostructrued sensor that can detect 50 ppt of NH3 gas in air. Specifically, nanograins of a p-type conductive polymer, polyaniline (PANI), are enchased on an electrospun n-type semiconductive TiO2 fiber surface. The resistance of the p-n heterojunctions combining with the bulk resistance of PANI nanograins can function as electric current switches when NH3 gas is absorbed by PANI nanoparticles. As a result, the sensor sensitivity can be significantly improved. The sensor fabricated in this work is 1000 times more sensitive than the best PANI sensor reported in the literature. 1. Introduction In the recent years, significant progress has been achieved in developing highly sensitive sensors.1-5 Among many different strategies for producing such sensors, the combination of nanostructured inorganic materials with conducting polymers has been particularly interesting.3,6-8 The sensitivity enhancement of such type sensors is due to a synergic effect of organic and inorganic materials. Polyaniline (PANI) is one of the most extensively studied conductive polymers due to its stability and tunable conductivity from metallic form (H+ doped) to nonconductive form (H+ dedoped). The unique conductivity of PANI as a function of H+ doping degree has been used for sensing NH3 gas.9 Various composites of PANI with inorganic and polymeric counterparts have been synthesized and used for improving the sensor sensitivity in recent years. Jiang and Yu et al. prepared a PANI/TiO2 nanocomposite thin film with an in situ chemical oxidation polymerization.10 The PANI/TiO2 nanocomposite thin film sensor can detect NH3 gas at concentrations of ∼20 ppm. Gong and co-workers prepared PANI/Ag composite nanotubes by using template and self-assembly polymerization process, respectively.11,12 These PANI/Ag composite nanotubes can detect about 5 ppm of NH3 gas. Liu and Craighead used a nanolithographic deposition process to create an individual PANI/poly(ethylene oxide) nanowire sensor.9 This device shows a rapid and reversible resistance change upon exposure to NH3 gas at 500 ppb concentrations. To the best of our knowledge, the highest sensitivity reported in literature for NH3 gas detection is 50 ppb, which was fabricated with PANI functionalized single-walled carbon nanotubes.13 In the current study, we reported a unique nanostructured sensor that can detect 50 ppt NH3 gas in air, which is one thousand times more sensitive than the best PANI related sensor reported in literature for NH3 detection. The PN junction materials consisting of semiconductive inorganic oxides and conductive polymers have been widely used to enhance the sensitivity of nanostructured sensors.14-19 It has been well-known that, when a p-type conductive polymer and a n-type inorganic semiconductive oxide are in direct * To whom the correspondence should be addressed, yulin.deng@ chbe.gatech.edu (Y. Deng). † Georgia Institute of Technology. ‡ Northeast Normal University.
contact, a p-n junction can form at the interface of these two materials.20,21 Although sensors based on nanostructured p-n junction have been reported, there are still many challenges in making a ultrasensitive sensor. To the best of our knowledge, PANI-based sensors reported in literature have only been able to measure NH3 in low parts per billion level to date. A field-effect transistor is the most common technique that uses p-n junctions to enhance nanosensor sensitivity and selectivity. In this communication, we present an alternative way to take the advantages of the p-n junctions at the interface of metal oxide and conductive polymer for fabricating ultrasensitive gas sensors. Specifically, nanoparticles of a p-type conductive polymer, PANI, are enchased on the surface of an electrospun semiconductive TiO2 microfiber. The PN heterojunctions formed between TiO2 microfibers and PANI nanoparticles function as electric current switches when NH3 gas is absorbed by PANI nanoparticles. With this unique construction, the sensor can detect as low as 50 ppt of NH3 gas in air, which is 1000 times more sensitive than the best PANI related NH3 gas sensor reported in literature. 2. Experimental Details Materials. All chemicals were analytical grade. Aniline was distilled twice under vacuum before use. Ammonium persulfate (APS), HCl, and NH3 · H2O were purchased from Sigma-Aldrich and used as the oxidant, doping, and dedoping reagent, respectively, as received. Device Fabrication. TiO2 microfibers were prepared first by calcining a precursor of electrospun polyvinylpyrrolidone/ Ti(IV)-isopropoxide microfibers at 600 °C for 4 h. For doing this, 1.5 mL of Ti(IV)-isopropoxide and 3 mL of acetic acid were added to 10 mL of ethanol containing 0.45 g of polyvinylpyrrolidone (PVP) (Mw ) 1300000). With vigorous stirring for 4 h, a viscous solution was obtained. Then the solution was contained in a plastic capillary tube. The negative terminal was attached to the aluminum foil target electrode. A metal pin connected to a high-voltage generator was placed in the solution, and the solution was kept in the capillary by adjusting the angle between the capillary and the aluminum foil. The distance from tip to collector was 12 cm. A voltage of 20 kV was applied to the solution. The precursor of electrospinning microfibers was
10.1021/jp100685r 2010 American Chemical Society Published on Web 05/10/2010
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Figure 1. A optical microscope image of the sensor (left) and high-magnification scanning electron microscopy images of TiO2 microfibers (middle) and TiO2 microfibers enchased with PANI nanograins (right).
then calcined at 600 °C for 4 h, and the heating rate was 10 °C/min. A TiO2 microfiber web was obtained. The TiO2 microfiber web was then immersed in a solution of aniline for polymerization for 48 h with ammonium persulfate as the initiator. Typically, 0.02 mL of aniline was added in 22 mL of HCl (1 mol/L) at room temperature. TiO2 microfibers were then dispersed in the above solution for 15 min. At the end, 2 mL of APS (0.055 g) was dropwised into the mixed solution. The final solution was immobilized for 48 h at 0-5 °C. The microfibers were washed with deionized water and ethanol, respectively, and then dried at room temperature. TiO2 microfibers enchased with PANI nanograins were obtained after polymerization. The sensor device was fabricated by the following process. A web of TiO2 microfibers enchased with PANI nanograins was shredded and dispersed in ethanol solution that was gently stirred for 5 min. A glass plate (1 cm ×1.5 cm) was washed with water and ethanol several times before coated with Au. The thickness of coated Au layer is 80-100 nm. The gilded glass plate was cut into two isolated pieces with a sharp knife, and the gap between the two gold pieces (no gold was coated on the gap) was ca. 80 µm. The two parts of gilded glass on the glass separated by the gap served as two electrodes. The gilded glass plate was immersed in the fiber suspension, and then picked up. TiO2 fibers with nanograins enchased on the surface were collected by the gilded glass plate and allowed to dry. The image of the device is shown in Figure 1 (left). To test the TiO2-PANI p-n junction, a device was prepared. Three drops of Ti(IV)-isopropoxide were first dripped on a ITO glass (1 cm ×1.5 cm). The ITO glass with Ti(IV)-isopropoxide was then calcined at 600 °C for 1 h, and the heating rate was 10 °C/min. In addition, another ITO glass was immersed in a solution of aniline polymerization. Typically, 0.02 mL of aniline was added in 22 mL of HCl (1 mol/L) at room temperature. The ITO glass was then dispersed in the above solution for 15 min. At the end, 2 mL of APS (0.055 g) was dropwised into the mixed solution. The final solution was immobilized for 48 h at 0-5 °C. The ITO glass was washed with deionized water and ethanol, respectively, and then dried at room temperature. In the end, the two ITO glasses were in face-to-face contact and formed an ordinary device. Characterization. The morphology of the samples was observed using a LEO 1530 thermally assisted field emission scanning electron microscope (SEM). A Fourier transform infrared (FT-IR) spectrum was measured on a Magna-IR 550 spectrometer with direct TiO2/PANI composite microfiber mats. The frequency range was 2000-400 cm-1. The X-ray diffraction (XRD) was measured with X’Pert X-ray diffractometer using Cu KR source. Scans were made from 5 to 45° (2θ) at the speed of 2° min-1.
SCHEME 1: Schematic of Nanosized p-n Heterojunction as a Switch to Control the Electric Current Flow in TiO2 Microfibers
Device Testing. The sensor made of TiO2 microfibers enchased with PANI nanograins was placed in an airproof test box (26 L) with a small electric fan inside. The box and the device were flushed with air continuously until the current of the sensor reached a steady value. A round-bottomed flask (500 mL) containing 2 µL of NH3 · H2O (28% ammonium hydroxide water solution) was heated until NH3 · H2O completely gasified. Then a required volume of the NH3 · H2O vapor was taken with a micrometer sized syringe and injected into the test box. The change in resistance was monitored and recorded automatically with a computer. The current-voltage (I-V) characteristics of the devices were measured with a DS345 30 MHz (Stanford Research Systems) as a voltage source and a SR 570 (Stanford Research Systems) as a monitor for the electric current signal at room temperature (22 °C) with a relative humidity of 47 ( 2%. 3. Results and Discussion Scheme 1a shows a simple case where an open voltage, V, is applied to a semiconductive TiO2 microfiber. Because of the high resistance of the TiO2 microfiber, the current flowing through the fiber is very low. However, if a conductive metal particle is attached to a TiO2 microfiber, the current of the system increases because the electrons preferably flow through the low resistant metal particle to reduce the total resistance. The more conductive particles enchased on the surface of TiO2 microfibers, the higher the conductivity of the microfibers. However, if a conductive PANI nanoparticle is attached to a TiO2 microfiber, as shown in Scheme 1b, the situation is complicated due to the formation of a p-n junction between a
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Figure 2. The frequency diameter distribution of TiO2 microfibers (left) and the frequency size distribution of the PANI particles enchased on the surface of the TiO2 microfibers (right).
Figure 3. FT-IR spectrum (left) and XRD pattern (right) of the TiO2 microfibers enchased with PANI nanograins.
p-type PANI and an n-type TiO2 fiber.15,19 On one hand, the current flows preferably through the conductive PANI particle due to the fact that PANI is much more conductive than the TiO2 microfiber. On the other hand, the current flowing through the PANI nanoparticle may be not favorable because it must overcome the reverse-bias resistance of the p-n junction diode between the p-type PANI particle and n-type TiO2 microfiber (the reverse-bias resistance exists when the current flows from p-type PANI nanoparticle to n-type TiO2 semicondutive nanifiber). The equivalent current circuit is shown on the bottom of Scheme 1b, where R1 is the resistance of the forward bias of the p-n junction (the current flows from the TiO2 microfiber to the PANI nanoparticle), and R3 is the resistance of reverse bias of the p-n junction (the current flows from PANI nanoparticle to TiO2 microfiber). R2 and R4 are the bulk resistances of PANI nanoparticle and TiO2 microfiber, respectively. Obviously, because of the bulk resistance of TiO2 (R4), the majority of the current will flow through the PANI nanoparticle if the electric field applied to the reverse bias of the p-n junction is higher than the breakdown voltage of the diode (the depletion region of the p-n junction will be broken down and R3 is almost zero in this case). At a constant V (during the sensor test experiments, V was kept as a constant), the electric field applied to the system must follow the following expression
V ) I1(R1 + R2 + R3) ) I2R4
(1)
Because H+-doped PANI is a conductive polymer, both R1 (forward-bias resistance) and R2 (bulk PANI resistance) are
much smaller than R3 (reverse-bias resistance), eq 1 can then be simplified as
V ) I1R3 ) I2R4
(2)
If V is just above the breakdown voltage, the depletion layer in the p-n junction will be broken down, resulting in a sudden and dramatic decrease in R3. As a result, I1 must be much greater than I2, and the conductivity of the entire system is high in this case, as shown in Scheme 1b (left). However, when the device is in contact with NH3 gas, the PANI nanoparticles on the TiO2 surface will be dedoped, so the bulk resistance of PANI, R2, will increase dramatically. At a certain point, the resistance (R2 + R3) is high enough that we can almost completely turn off the route containing the PANI nanoparticle (see Scheme 1b (right). As a result, the current can only flow through a pure TiO2 fiber, resulting in a large drop in the entire current when NH3 gas absorbs on PANI nanograins. Therefore, the combination of the p-n depletion layer and the bulk resistance change of PANI nanoparticles can function as a current switch, which turns on the current circuit by absorbing H+, and turns off the current circuit by adsorbing NH3 gas. According to the above mechanism, TiO2 microfibers with enchased PANI nanograins on their surface were prepared. As shown in Figure 1 (left), the device is made of the microfiber mats which are tiled on a gilded glass plate with an uncoated gap of about 80 µm. The middle image of Figure 1 shows a SEM image of TiO2 microfibers obtained using electrospinning combined with calcining. It is easy seen that the surface of TiO2 microfibers is smooth. The SEM image on the right in Figure
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Figure 4. Schematic of the ITO-PANI-TiO2-ITO device with sandwich films (left) and its I-V curve (right).
Figure 5. I-V characteristic of a sensor made of the TiO2 microfibers enchased with PANI nanograins in different concentrations of NH3 gas.
1 shows there are many PANI nanograins enchased on the surface of the TiO2 microfibers. Figure 2 shows the frequency diameter distribution of TiO2 microfibers and the frequency size distribution of the PANI particles enchased on the surface of the TiO2 microfibers, which is 600 and 40 nm, respectively. The TiO2 microfibers with enchased PANI nanograins are characterized by FT-IR spectroscopy as shown in Figure 3 (left). The characteristic peaks at 1570 and 1490 cm-1 are due to the CdC stretching vibration of quinoid and benzenoid rings, respectively. The peaks near 1300 cm-1 are correlated with the C-N stretching vibration with aromatic conjugation. A wide peak at 1140 cm-1 assigned to the QdNH+sB absorption peaks (where Q and B denote quinoid ring and benzene ring, respectively) is also observed.22,23 The characteristic peak of TiO2 is found at 446 cm-1. In addition, the wide peak around 700 cm-1 may be assigned to Ti-O-Ti and C-C and C-H
for the benzenoid unit.24 Figure 3 (right) shows the XRD pattern of the TiO2 microfibers enchased with PANI nanograins. The XRD pattern exhibits sharp peaks corresponding to 110, 101, 111, and 211 Bragg reflections of TiO2, which indicates that the TiO2 microfibers consist of only rutile crystals (JCPDS 211276). In addition, a broad peak centered at 2θ ) 20.9° may be ascribed to the periodicity parallel to the PANI chains.11,12 In order to investigate the existence of p-n junction between PANI nanograins and TiO2 microfibers, a device with sandwich films made of ITO-PANI-TiO2-ITO (where ITO is a glass slide coated with indium-doped tin oxide) is designed as shown in Figure 4 (left). The I-V curve between the p-type PANI film and n-type TiO2 films is shown in Figure 4 (right), which indicates a rectifying contact with high current rectification ratio at 2 V. This result suggests that a p-n heterojunction on the surface between PANI and TiO2 can be created. Figure 5 shows the I-V characteristic of the sensor made of the TiO2 microfibers enchased with PANI nanograins before and after exposure to different concentrations of NH3 gas. The I-V characteristic before the sensor was exposed to NH3 gas clearly demonstrates rectifying behavior of the p-n heterojunctions. It is interested to note that the I-V curve is symmetric for 0 ppm NH3 with a turn-on voltage of ∼0.6 V at both positive and negative voltages. However, after the sensor was exposed to different concentrations of NH3 gas, the I-V curve changed from non-Ohmic to Ohmic with increasing the concentration of NH3 gas from 0 to 1 ppm, which indicates the disappearance and turnoff of the p-n junction diodes because of the dedoping of the PANI, and the current flows solely through the TiO2 microfibers rather than PANI nanograins, as shown in the right picture of Scheme 1b. The I-V curve shown in Figure 5 agrees well with the theoretical expectation. It should be noted that there are many PANI nanograins along the TiO2 microfibers. The current flows
Figure 6. Current responses of a sensor made of TiO2 microfibers enchased with PANI nanograins to different concentrations of NH3 gas as a function of time (left) and reproducibility of the sensor exposed to 10 ppb NH3 gas (right).
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through the PANI nanograins therefore can be treated as a series connection of n-p-n junctions. Figure 6 shows the dynamic behaviors of the sensor exposed to different concentrations of NH3 gas. As shown in Figure 6 (left), the current of the sensor decreases dramatically when it was exposed to NH3 gas. Even though the NH3 concentration in air is as low as 50 ppt, the change in current is still very obvious, which indicates that the sensor prepared by enchased p-type PANI nanograins on n-type TiO2 semiconductive microfibers has superior gas-sensing performance. The sensitivity of sensor (S) is defined as S ) (IM - IS)/IM, where IM is the maximum sensor current before NH3 gas injection and IS is the smallest sensor current after addition of NH3 gas. As shown in Figure 6 (left), with the increase of NH3 concentration, the sensitivity greatly increases. According to above equation, the sensitivities are about 0.018, 0.009, and 0.004 for 200, 100, and 50 ppt of NH3 gas, respectively. It should be noted that a sensor that can detect tens of parts-per-trillion levels of NH3 gas has never been reported before. This is almost 1000 times higher than the best PANI-based sensor reported in literature.13 The results also show that a sensor made from TiO2 microfibers enchased with PANI nanograins exhibited an excellent reversibility to NH3 gas. A determination curve of the response from the first to fifth exposures to NH3 gas is shown in Figure 6 (right). It is very easy for the sensor to recover by flushing air through the test chamber at room temperature after the sensor has been exposed to NH3 gas. 4. Conclusions In conclusion, the enchased PANI nanograins on the surface of the TiO2 microfibers can function as nanoswitches to turn off the current circuit when they are in contact with NH3 gas. A huge increase in the sensitivity to NH3 gas is due to the quick increase in the device resistance consisting of the bulk resistance of PANI nanograins and the PN heterojuction depletion layer between PANI and TiO2 fiber. A high response to 50 ppt of NH3 gas has been achieved.
Gong et al. Acknowledgment. Gong and Li thank the China Scholarship Council (CSC) for providing the scholarships. References and Notes (1) Comini, E. Anal. Chim. Acta 2006, 568, 28. (2) Lee, D. D.; Lee, D. K. IEEE Sens. J. 2001, 1, 214. (3) Ehrmann, S.; Jungst, J.; Goschnick, J.; Everhard, D. Sens. Actuators, B 2000, 65, 247. (4) Gong, J.; Li, Y.; Chai, Y. X.; Hu, Z.; Deng, Y. J. Phys. Chem. C 2010, 114 (2), 1293. (5) Lao, C.; Kuang, Q.; Wang, Z.; Park, M.; Deng, Y. Appl. Phys. Lett. 2007, 90, 262107. (6) Virji, S.; Huang, J. X.; Kaner, R. B.; Weiller, B. H. Nano Lett. 2004, 4, 491. (7) Sun, Q.; Wu, B.; Fuller, T.; Ding, Y.; Deng, Y. Fabrication of Aligned Polyaniline Nanofiber Array via a Facile Wet Chemical Process. Macromol. Rapid Commun. 2009, 30 (12), 1027. (8) Nogueira, A. F.; Micaromi, L.; Gazotti, W. A.; De-Paoli, M. A. Electrochem. Commun. 1999, 1, 262. (9) Liu, H.; Kameoka, J.; Cazplewski, D. A.; Craighead, H. G. Nano Lett. 2004, 4, 671. (10) Tai, H.; Jiang, Y.; Xie, G.; Yu, J.; Chen, X. Sens. Actuators, B 2007, 125, 644. (11) Li, X.; Gao, Y.; Gong, J.; Zhang, L.; Qu, L. Y. J. Phys. Chem. C 2009, 113, 69. (12) Gao, Y.; Shan, D. C.; Cao, F.; Gong, J.; Li, X.; Ma, H. Y.; Su, Z. M.; Qu, L. Y. J. Phys. Chem. C 2009, 113, 15175. (13) Zhang, T.; Nix, M. B.; Yoo, B. Y.; Deshusses, M. A.; Myung, N. V. Electroanalysis 2006, 18, 1153. (14) Khang, D. Y.; Jiang, H.; Huang, Y.; Rogers, J. A. Science 2006, 311, 208. (15) Liu, Z.; Guo, W.; Fu, D.; Chen, W. Synth. Met. 2006, 156, 414. (16) Wollenstein, J.; Ihlenfeld, F.; Jaegle, M.; Kuhner, G.; Bottner, H.; Becker, W. J. Sens. Actuators, B 2000, 68, 22. (17) Li, Y.; Gong, J.; McCune, M.; He, G.; Deng, Y. Synth. Met. 2010, 160 (5-6), 499. (18) Calvin, H. W.; Mark, C. L. J. Am. Chem. Soc. 2004, 126, 10536. (19) Dhawale, D. S.; Salunkhe, R. R.; Patil, U. M.; Gurav, K. V.; More, A. M.; Lokhande, C. D. Sens. Actuators, B 2008, 134, 988. (20) Cheng, C. H. W.; Lonergan, M. C. J. Am. Chem. Soc. 2004, 126, 10536. (21) He, J.; Lin, H. Y. H.; McConney, M. E.; Tsukruk, V. V.; Wang, Z. L. J. Appl. Phys. 2007, 102, 084303. (22) Zhang, Z.; Wan, M. X. AdV. Mater. 2002, 14, 1314. (23) Zhang, Z.; Wan, M. X.; Wei, Y. AdV. Funct. Mater. 2006, 16, 1100. Khan, R.; Dhayal, M. Electrochem. Commun. 2008, 10, 492.
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