Electrospun ZnO Nanowires as Gas Sensors for Ethanol Detection
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ZnO nanowires were produced using an electrospinning method and used in gas sensors for the detection of ethanol at 220 °C. This electrospinning technique allows the direct placement of ZnO nanowires during their synthesis to bridge the sensor electrodes. An excellent sensitivity of nearly 90% was obtained at a low ethanol concentration of 10 ppm, and the rest obtained at higher ethanol concentrations, up to 600 ppm, all equal to or greater than 90%.
KeywordsZinc oxide Electrospinning Ethanol sensor Sensitivity
ZnO is an interesting chemically and thermally stable n-type semiconductor with a large exciton binding energy (60 meV) and a large band gap (3.37 eV) energy. ZnO nanomaterial also appears to be the mostly studied one as it exhibits a wide variety of nanostructures such as nanowires , nanowalls [2–4], nanobelts , nanorods , nanosheets , and so on. Among its applications, ZnO nanowire is receiving greater interests for use in gas sensors for detecting, for example, ethanol. ZnO nanowires prepared by a reactive thermal deposition method were used for ethanol sensing . The sensitivity, i.e., (Ra − Rg)/Ra where Ra and Rg are, respectively, the resistance of the nanowires exposed to air without and with the detecting gas, increases from ~47% to ~98% while the ethanol concentration increases from 1 ppm to 200 ppm at a high temperature of 300 °C. However, at lower temperatures of 200 °C and 250 °C, the sensitivities drop significantly to ~67% and ~86%, respectively. The response time and recovery time are 10 s and 55 s, respectively, at a ethanol concentration of 200 ppm and a temperature of 300 °C. Ethanol sensors based on ZnO nanowires prepared using a self-catalyzed vapor–liquid–solid (VLS) method exhibit a sensitivity that increases from 18% to 61% while the ethanol concentration increases from 50 ppm to 1500 ppm at a high temperature of 300 °C . However, the response time of the ZnO nanowires to the ethanol is not clear. ZnO nanowires fabricated using a hydrothermal method show an ethanol sensitivity of 92% at an ethanol concentration of 100 ppm at 330 °C . ZnO nanobelts prepared by RF sputter-deposition method were also used for ethanol sensing . The sensitivity to ethanol increases from 86% to 96% while the ethanol concentration increases from 50 ppm to 1000 ppm at a temperature of 220 °C. However, the response–recovery characteristics of the ZnO nanobelt-based ethanol sensors were not reported. Common to these reports is that for the fabrication of gas sensors, the as-synthesized ZnO nanowires or nanorods must be removed from a substrate and/or be transferred into a solution, and then dispersed randomly onto the sensor devices. This “pick-and-place” technique raises a concern of incompatibility with the Si processing . However, by using the electrospuning process for the synthesis of nanowires, the “pick” step is eliminated as it allows a direct placement of nanowires onto a sensor chip.
In the past few decades, the electrospinning process was developed for the fabrication of nanowires. Electrospinning represents a very simple, versatile, and low cost method for large-scale fabrication of nanowires. This technique has been successfully used to produce various polymeric, inorganic, and hybrid nanowires or nanowires [12–14]. Considerable efforts have also been made to fabricate functional nanodevices using the electrospun nanowires [15, 16]. However, it is only recently that a few reports have shown the synthesis of ZnO nanowires using the electrospinning process [1, 17–19]. Moreover, there has been no report showing the use of electrospun ZnO nanowires as gas sensor, except a very recent one in which electrospun ZnO nanowires were used as a photoelectric gas sensor for the detection of oxygen under the illumination of a 500 W Xe lamp . However, the need of a Xe lamp limits its applications.
In this article, we demonstrate the use of electrospun ZnO nanowires in silicon-based gas sensors for the detection of ethanol with very high sensitivities. Gel of zinc acetate/polyvinyl alcohol (PVA) was used as the precursor. The precursor was loaded into a syringe, which was connected to a high-voltage power supply and served as the positive electrode. The negative electrode was an aluminum foil where chips having interdigitated electrodes were placed. The interdigitated electrode area is 1 mm × 1 mm and the distance between two adjacent interdigitated electrodes is 70 μm. Fixing chips on the surface of the aluminum foil allows the direct placement of electrospun polymeric nanowires onto its surface, leading to the bridging of two electrodes by the subsequently formed ZnO nanowires. For the electrospinning, an electric field of 15 kV was applied. The resulting electrospun polymeric nanowires were subjected to a calcination process without or with the chips at 600 °C to form inorganic ZnO nanowires. The calcination time ranged from 1 h to 7 h. The morphology of the obtained nanowires was examined using scanning electron microscopy (SEM). X-ray diffraction analysis was carried out to determine the crystalline structures of the nanowires. Cathodoluminescence (CL) spectroscopy analysis was also performed at the liquid nitrogen temperature. The gas sensing characteristics were measured in a cylindrical chamber. The chamber has an inlet port connected to a gas inlet valve and outlet port connected to an air pump. Sensors were connected to an outside multimeter to monitor the resistance changes.
We have fabricated electrospun polycrystalline ZnO nanowires directly placed onto interdigitated electrodes to form gas sensors for the detection of ethanol. Sensors exhibiting very high sensitivities, and fast response time and recovery time were demonstrated. The average nanowire diameter ranges from 220 ± 15 nm to 90 ± 10 nm and decreases with the calcination time. The ZnO nanowires exhibit a strong UV emission at 368 nm and a very insignificant emission or nearly none at 465 nm. For the detection of ethanol at 220 °C, an excellent sensitivity of nearly 90% was obtained at a low ethanol concentration of 10 ppm and the rest obtained at higher ethanol concentrations, up to 600 ppm, are all ≥90%. Fast response time and recovery times of 16 s and 25 s, were also obtained.
This work was supported by the National Science Council in Taiwan under grant No. 97-2120-M-006-001 and the Top University Program at the National Cheng Kung University in Taiwan under R048/D97-3360.