High-Sensitivity Humidity Sensor Based on a Single SnO₂ Nanowire

Abstract

Nanowires (NWs) and nanobelts (NBs) are considered as ideal building blocks for constructing nanosized devices due to high surface to volume ratio and their special physical and chemical properties resulting from the reduced sizes. Until now, many semiconductor NWs/NBs have been successfully applied in nanodevices, including nanolasers, 1 nanogenerators, 2 and various chemical and biological nanosensors. 3,4 Well-known humidity control is very important for many fields in technology and our daily life. In the past years, many detection techniques have been explored from old wet and dry bulb thermometry to modern capacitive, resistive, and thermal conductive moisture detectors. In order to further promote the sensitivity, selectivity, chemical and thermal stability, intensive efforts have been put in the exploration of a humidity sensor based on nanostructured materials such as carbon nanotubes, 5 metal oxide nanoparticles, 6 and NW films. 7 Being an important n-type semiconductor with a wide band gap (E g ) 3.6 eV at 300 K), SnO 2 possesses many unique optical and electrical properties: remarkable receptivity variation in gaseous environment, high optical transparency in the visible range (up to 97%), low resistivity (10 -4 to 10 6 Ω‚cm -1 ), and excellent chemical stability. These properties make SnO 2 NWs/NBs well suited for chemical sensors and transparent conducting electrodes. To date, many nanodevices based on SnO 2 NWs/NBs have been fabricated, including field effect transistors (FET), 8 field emissions, 9 UV sensors, 10 and gas sensors. 3a,11 In this communication, we present a new type of SnO 2 nanodevice, a humidity detector using a single SnO 2 NW as the sensing unit. This new type of SnO 2 NW-based sensor has fast and sensitive response to relative humidity (RH) in air from a wide range of environments at room temperature (30°C). In addition, it has relatively good reproducibility, and its linear response to RH makes it to calibrate.Single-crystalline SnO 2 NWs to be used as humidity sensors were synthesized by chemical vapor deposition (CVD) using Au nanoparticles as catalysts in a homemade synthetic apparatus. 12 Experimental details are available in Supporting Information. Figure 1a is a typical SEM image of as-synthesized NWs with high yield. The XRD pattern indicates that the NWs are rutile structured SnO 2 with a good crystallinity ( Figure S1). The diameter of SnO 2 NWs ranges from 50 to 300 nm, and the length of SnO 2 NWs is up to tens of micrometers. Low-magnification TEM image (upper inset of Figure 1a) and corresponding EDS analysis ( Figure S2) show that a Au nanoparticle exists at the tip of the SnO 2 NW, which is the representative characteristic of the vapor-liquid-solid (VLS) growth mechanism. 13 The selective area electron diffraction (SAED, lower inset of Figure 1a) pattern taken from the body of the SnO 2 NW reveals that the as-synthesized SnO 2 NW is single crystalline and grows along the [001] direction.Cathodoluminescence (CL) analysis is a suitable technique to determine th...

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