1376
IEEE ELECTRON DEVICE LETTERS, VOL. 36, NO. 12, DECEMBER 2015
ZnO Nanowire-Reduced Graphene Oxide Hybrid Based Portable NH3 Gas Sensing Electron Device Zhen Sun, Da Huang, Zhi Yang, Xiaolin Li, Nantao Hu, Chao Yang, Hao Wei, Guilin Yin, Dannong He, and Yafei Zhang Abstract— A ZnO nanowire-reduced graphene oxide (ZnOrGO) based portable ammonia (NH3 ) gas sensing electron device working at room temperature has been demonstrated for the first time. The sensor is developed on a microelectrode of microelectromechanical systems and supported by peripheral circuits and a hosting computer, which enables the real-time detection of NH3 at room temperature. In contrast to the traditional sensors based on pure graphene or ZnO nanowires alone, the ZnO-rGO based gas sensing electron device can detect low-concentration (1 ppm) NH3 with higher sensitivity (∼7.2%). Besides, this sensor exhibits satisfying properties at sensing NH3 with the concentration as low as 500 ppb at room temperature. Index Terms— Gas sensing electron device, ZnO nanowire, reduced graphene oxide, ammonia.
I. I NTRODUCTION
I
N RECENT years, the world has witnessed emerging uses of gas sensors in diversified fields including industrial manufacture, national defense, environmental protection, gas monitoring, and so on [1]–[7]. Ammonia (NH3 ) is a common type of caustic and hazardous gas that emitted during industrial or agricultural chemical procedures. It is highly dangerous for people to be exposed in a concentration of 350 ppm for over 15 min [8]. Therefore, it is necessary to design a sensor that is able to detect NH3 within a desired spectrum of concentration. Graphene has enjoyed significant attentions for its ideal two dimensional (2D) structure, excellent electrical, optical and thermal properties [9]–[12]. It can be obtained by mechanical exfoliation of graphite, chemical vapor deposition or Manuscript received October 9, 2015; revised October 25, 2015; accepted October 26, 2015. Date of publication October 29, 2015; date of current version November 20, 2015. This work was supported in part by the Program for New Century Excellent Talents in University under Grant NCET-12-0356, in part by the National Basic Research Program of China under Grant 2013CB932500, in part by the Shanghai Jiao Tong University Agri-X Funding under Grant Agri-X2015007, in part by the National Natural Science Foundation of China under Grant 21171117, Grant 61376003, and Grant 61574091, in part by the Program of Shanghai Academic/Technology Research Leader under Grant 15XD1525200, and in part by the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning. The review of this letter was arranged by Editor E. A. Gutiérrez-D. The authors are with the Key Laboratory for Thin Film and Microfabrication, Department of Micro/Nano Electronics, School of Electronic Information and Electrical Engineering, Ministry of Education, Shanghai Jiao Tong University, Shanghai 200240, China, and also with the National Engineering Research Center for Nanotechnology, Shanghai 200241, China (e-mail:
[email protected];
[email protected];
[email protected]). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LED.2015.2496177
reduction of graphene oxide (GO). The reduced graphene oxide (rGO) is usually considered as chemically derived graphene [13]. RGO shows potential in the detection of various gases such as NH3 , DMMP, H2 , and so on [14]. However, it is still a challenge to develop the sensing properties based on rGO with stable and highly sensitive characteristics. ZnO nanowire is an important kind of n-type semiconductor material with considerably large surface areas. However, several studies are reported that ZnO nanowire is not good enough to fabricate gas sensors since its unsatisfied electronic properties at room temperature [15]. Herein, we use ZnO nanowire-reduced graphene oxide (ZnO-rGO) hybrids as the sensing material, which will not only feature the intrinsic properties of rGO but also improve the conductivity of ZnO nanowire, thus increases the reaction sites and enhances the NH3 gas sensing behavior of their composites. Additionally, the design of a portable sensing electron device based on the ZnO-rGO hybrids is carried out. It enables this sensor to detect a very low level (500 ppb) of NH3 at room temperature, which is highly important to its commercial application. II. E XPERIMENTS ZnO nanowires were prepared by modified carbothermal reduction which has been reported in our previous work [16]. GO was obtained by a modified Hummers method. ZnO-GO hybrids were synthesized by stirring 0.5 g ZnO nanowires, 0.5 g GO and 0.1 g PVP in 100 mL of deionized (DI) water for 1 h and then washed with DI water for several times. Afterwards, the resulting solution was dried overnight and thermally reduced at 300 °C to form ZnO-rGO nanocomposites. The as-prepared nanocomposites were dispersed in 50 mL of DI water and ultrasonicated for 15 min to form ZnO-rGO solution. To obtain the electrodes, a standard microfabrication process of micro-electromechanical systems (MEMS) as shown in Fig. 1 was used, which has been illustrated in our previous work [17]. The interdigitated electrode fingers were fabricated by sputtering 2∼3 μm Au on a patterned photoresist mold on a Si/SiO2 wafer and then removing the photoresist by a lift-off process. Next, the electrodes were sonicated in ethanol for several minutes, washed with DI water thoroughly, and dried by nitrogen flow. Subsquently, 0.l μL of the ZnO-rGO solution was extracted and deposited on the electrode using a microsyringe. Finally, networks of the
0741-3106 © 2015 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.
SUN et al.: ZnO-rGO HYBRID-BASED PORTABLE NH3 GAS SENSING ELECTRON DEVICE
Fig. 1.
1377
MEMS fabrication process of the electrodes.
Fig. 3. SEM images of the ZnO-rGO hybrids with (a) low magnification and (b) high magnification; (c) Photo of the gas sensors and schematic diagram of the ZnO-rGO gas sensing electron device.
Fig. 2.
System design of portable gas sensing electron device.
ZnO-rGO hybrids that bridged the neighboring electrode fingers could be formed after dried in an oven at 60 °C for 1 h. As shown in Fig. 2, the portable ZnO-rGO gas sensing electron device system was designed and mainly consists of five parts, including power supplies, signal modulating circuits, analog to digital (A/D) conversion circuits, microprogrammed control unit (MCU), and hosting computer. The battery power supply was stabilized to 5 V as the analog power supplied by MAX666. The signal modulating circuits were based on AD620, a high accuracy instrumentation amplifier and A/D conversion circuit, based on ADS1210, converted the analog voltage signal from AD620 into digital records. The reference voltages were stabilized at 1.25 and 2.50 V supplied by REF1112 and REF1004, respectively. MCU, which based on STC10F08XE, was in charge of operating working procedures of the A/D converter, dealing with the converted data and communicating with the hosting computer using a pair of bluetooth. III. R ESULTS AND D ISCUSSION Scanning electron microscopy (SEM, JEOL JSM-7800F Prime, Thermo Scientific, America) images of the ZnO-rGO hybrids are shown in Fig. 3(a) and (b). It can be observed that the nanowires arrange uniformly on the rGO sheets and contact with each other closely, which will benefit the electron transfer between ZnO nanowires and rGO. Fig. 3(c) shows the photo of the gas sensors and the schematic diagram of structure of the ZnO-rGO based gas sensing electron device. The ZnO-rGO thin films are tightly adhered to the electrodes. Fig. 4 shows the schematic diagram of the peripheral circuits for the sensing electron device. Power supply for analog
Fig. 4.
Schematic diagram of the peripheral circuits.
circuits should be high quality and low noisy due to the existence of micro signal in signal modulating circuit. Hence, analog circuits and digital circuits were separated to use different power supplies, so that analog circuits would not be interfered by noises from digital circuits. The shutdown input of battery power supply allows the regulator to be turned on or off with a CMOS logic level signal below 0.3 V or above 1.4 V. In signal modulating circuit, a Wheatstone bridge was adopted to convert the measured signal to a micro voltage signal and then modulated to match the input range of AD620. I-V curves of the ZnO-rGO based gas sensing electron device are shown in Fig. 5. The values of the current output were recorded when the voltages on the sensor changed. It can be inferred from Fig. 5(a) that the ZnO-rGO hybrids show semiconductor properties when the voltage ranges from -5 to 5 V. Fig. 5(b) shows that when the voltage ranges from -0.5 to 0.5 V, the resistance remains stable with the increase of the voltage and the resistance comes out to be
1378
IEEE ELECTRON DEVICE LETTERS, VOL. 36, NO. 12, DECEMBER 2015
TABLE I C OMPARISON OF D IFFERENT NH 3 S ENSORS BASED ON G RAPHENE OR rGO
Fig. 5. I-V curves of the ZnO-rGO based gas sensing electron device when the voltage ranges from (a) -5 to 5 V and (b) -0.5 to 0.5 V.
Fig. 6. (a) Response curve of the ZnO-rGO based gas sensing electron device when exposed to 1 ppm NH3 at room temperature; (b) Gas sensing properties for the ZnO-rGO based gas sensing electron device upon different concentrations of NH3 at room temperature.
about 4 k, thus the hybrids show the conductor properties. Since it is important to stabilize the output currents, voltage on the sensor should be in the range of 0 to 0.5V. Herein, we define the sensor response (R) toward NH3 according to the following equation: R ×100 = | RGAS − R0 | × 100 R (%) = R R0 Where R0 is the resistance of sensing electron device before exposure to the gas vapors and RGAS is the resistance of sensing electron device after being exposed to gas for enough time. We define the response time as the time for achieving 90% of the entire sensing response and the recovery time as the time for recovering 90% of the sensing response. Fig. 6(a) shows the response curve of the sensing electron device towards 1 ppm NH3 at room temperature. The voltage on the device is calculated to be 100 mV according to the following equation: V =
R0 × Vbridge R0 + R8
Where R0 is the resistance of sensing electron device and R8 is the resistance cascaded to the sensor in the Wheatstone bridge. Vbridge is the voltage on this series circuit. It can be inferred from the curve that when under the background gas, R0 is about 4175 . After NH3 purged into the test chamber for less than 50 s, RG AS reaches up to about 4475 , indicating that the sensor response (R) is about 7.2% and the response time is less than 50 s. Besides, it is shown that the recovery time is less than 200 s. Compared with the previous reports on NH3 sensors [14], [18]–[20] which are shown in Table I, our sensing electron device exhibits high response and short response/recovery time.
Fig. 6(b) shows the gas sensing properties of our sensing electron device exposed to different concentrations of NH3 from 500 ppb to 5000 ppm at room temperature. The results indicate that the sensing electron device exhibits excellent and high reversible responses to different concentrations of NH3 gas. In response to as low as 500 ppb NH3 , about 3% of the sensor response can be observed and the variation increases with the increasing NH3 concentration. The response tends to saturate at a higher concentration. It can be explained by the saturated adsorption of NH3 molecules on the surface of ZnOrGO hybrids, which causes the saturated response. The enhancement of ZnO nanowires on rGO to detect NH3 might be explained as follows. During the sensing process, electrons transfer on the surfaces of the ZnO-rGO hybrids following the equations below: O2 +e− → O− 2 − 4NH3 +3O− 2 → 6H2 O + 2N2 +3e ZnO nanowires network on the surface of rGO provide more channels for electron transfers, so more O2 are ionized to O− 2 on the hybrid surfaces, and more NH3 molecules can react with O− 2 , which enhances the sensing performance of the hybrids. IV. C ONCLUSION In conclusion, a functional portable ZnO-rGO based gas sensing electron device was built and the sensing properties were tested. This device exhibited outstanding performances to detect NH3 with the concentration as low as 500 ppb at room temperature. Therefore, our ZnO-rGO based gas sensing electron device could offer a universal platform for the realtime, hypersensitive and accurate detections of harmful gases at room temperature. R EFERENCES [1] C. S. Rout, M. Hegde, A. Govindaraj, and C. N. R. Rao, “Ammonia sensors based on metal oxide nanostructures,” Nanotechnology, vol. 18, no. 20, pp. 205504-1–205504-9, Apr. 2007. DOI: 10.1088/0957-4484/18/20/205504 [2] J. Wang, L. Wei, L. Zhang, C. Jiang, E. S.-W. Kong, and Y. Zhang, “Preparation of high aspect ratio nickel oxide nanowires and their gas sensing devices with fast response and high sensitivity,” J. Mater. Chem., vol. 22, no. 17, pp. 8327–8335, May 2012. DOI: 10.1039/c2jm169348 [3] T.-Y. Chen, H.-L. Chen, C.-S. Hsu, C.-C. Huang, C.-F. Chang, P.-C. Chou, and W.-C. Liu, “On an ammonia gas sensor based on a Pt/AlGaN heterostructure field-effect transistor,” IEEE Electron Device Lett., vol. 33, no. 4, pp. 612–614, Apr. 2012. DOI: 10.1109/LED.2012.2184832
SUN et al.: ZnO-rGO HYBRID-BASED PORTABLE NH3 GAS SENSING ELECTRON DEVICE
[4] X. Huang, N. Hu, R. Gao, Y. Yu, Y. Wang, Z. Yang, E. S.-W. Kong, H. Wei, and Y. Zhang, “Reduced graphene oxide–polyaniline hybrid: Preparation, characterization and its applications for ammonia gas sensing,” J. Mater. Chem., vol. 22, no. 42, pp. 22488–22495, Nov. 2012. DOI: 10.1039/c2jm34340a [5] N. Hu, Y. Wang, J. Chai, R. Gao, Z. Yang, E. S.-W. Kong, and Y. Zhang, “Gas sensor based on p-phenylenediamine reduced graphene oxide,” Sens. Actuators B, Chem., vol. 163, no. 1, pp. 107–114, Mar. 2012. DOI: 10.1016/j.snb.2012.01.016 [6] C.-H. Kim, S.-W. Yoo, D.-W. Nam, S. Seo, and J.-H. Lee, “Effect of temperature and humidity on NO2 and NH3 gas sensitivity of bottom-gate graphene FETs prepared by ICP-CVD,” IEEE Electron Device Lett., vol. 33, no. 7, pp. 1084–1086, Jul. 2007. DOI: 10.1109/LED.2012.2193867 [7] H. Wang, Z. Sun, Q. Lu, F. Zeng, and D. Su, “One-pot synthesis of (Au nanorod)–(metal sulfide) core–shell nanostructures with enhanced gas-sensing property,” Small, vol. 8, no. 8, pp. 1167–1172, Feb. 2012. DOI: 10.1002/smll.201102287 [8] T. Becker, S. Mühlberger, C. B.-V. Braunmühl, G. Müller, T. Ziemann, and K. V. Hechtenberg, “Air pollution monitoring using tin-oxide-based microreactor systems,” Sens. Actuators B, Chem., vol. 69, nos. 1–2, pp. 108–119, Sep. 2000. DOI: 10.1016/S0925-4005(00)00516-5 [9] X. Mao, C. Cheng, B. Huang, Z. Zhang, S. Gan, H. Chen, and H. Chen, “Optoelectronic mixer based on graphene FET,” IEEE Electron Device Lett., vol. 36, no. 3, pp. 253–255, Mar. 2015. DOI: 10.1109/LED.2015.2396038 [10] Z. Yang, R. Gao, N. Hu, J. Chai, Y. Cheng, L. Zhang, H. Wei, E. S.-W. Kong, and Y. Zhang, “The prospective two-dimensional graphene nanosheets: Preparation, functionalization and applications,” Nano-Micro Lett., vol. 4, no. 1, pp. 1–9, Jan. 2012. DOI: 10.3786/nml.v4i1.p1-9 [11] N. Hu, Z. Yang, Y. Wang, L. Zhang, Y. Wang, X. Huang, H. Wei, L. Wei, and Y. Zhang, “Ultrafast and sensitive room temperature NH3 gas sensors based on chemically reduced graphene oxide,” Nanotechnology, vol. 25, no. 2, pp. 025502-1–025502-9, Jan. 2014. DOI: 10.1088/0957-4484/25/2/025502
1379
[12] F. Yavari, Z. Chen, A. V. Thomas, W. Ren, H.-M. Cheng, and N. Koratkar, “High sensitivity gas detection using a macroscopic threedimensional graphene foam network,” Sci. Rep., vol. 23, no. 166, pp. 166-1–166-5, Nov. 2011. DOI: 10.1038/srep00166 [13] W. Yuan and G. Shi, “Graphene-based gas sensors,” J. Mater. Chem. A, vol. 1, no. 35, pp. 10078–10091, Jun. 2013. DOI: 10.1039/c3ta11774j [14] S. S. Varghese, S. Lonkar, K. K. Singh, S. Swaminathan, and A. Abdala, “Recent advances in graphene based gas sensors,” Sens. Actuators B, Chem., vol. 218, pp. 160–183, Oct. 2015. DOI: 10.1016/j.snb.2015.04.062 [15] J. L. Pau, J. Waters, E. Rivera, S. M. Kim, and P. Kung, “Low leakage current ZnO nanowire Schottky photodiodes built by dielectrophoretic contact,” IEEE Electron Device Lett., vol. 36, no. 8, pp. 814–816, Aug. 2015. DOI: 10.1109/LED.2015.2442678 [16] Z. Zhou, C. Zhan, Y. Wang, Y. Su, Z. Yang, and Y. Zhang, “Rapid mass production of ZnO nanowires by a modified carbothermal reduction method,” Mater. Lett., vol. 65, no. 5, pp. 832–835, Mar. 2011. DOI: 10.1016/j.matlet.2010.12.032 [17] Y. Wang, Z. Zhou, Z. Yang, X. Chen, D. Xu, and Y. Zhang, “Gas sensors based on deposited single-walled carbon nanotube networks for DMMP detection,” Nanotechnology, vol. 20, no. 34, pp. 345502-1–345502-8, Aug. 2009. DOI: 10.1088/0957-4484/20/34/345502 [18] M. Gautam and A. H. Jayatissa, “Gas sensing properties of graphene synthesized by chemical vapor deposition,” Mater. Sci. Eng., C, vol. 31, no. 7, pp. 1405–1411, Oct. 2011. DOI: 10.1016/j.msec.2011.05.008 [19] Y. Wang, L. Zhang, N. Hu, Y. Wang, Y. Zhang, Z. Zhou, Y. Liu, S. Shen, and C. Peng, “Ammonia gas sensors based on chemically reduced graphene oxide sheets self-assembled on Au electrodes,” Nanosc. Res. Lett., vol. 9, pp. 1–12, May 2014. DOI: 10.1186/1556-276X-9-251 [20] X. Zhou, X. Wang, B. Wang, Z. Chen, C. He, and Y. Wu, “Preparation, characterization and NH3 sensing properties of reduced graphene oxide/copper phthalocyanine hybrid material,” Sens. Actuators B, Chem., vol. 193, pp. 340–348, Mar. 2014. DOI: 10.1016/j.snb.2013.11.090
