Gas Sensing Characteristics Of ZnO Nanorods

Various poisonous gases, flammable and explosive gases have an great effect onthe environment. The traditional gas sensor generally work in high temperature（morethan100centigrade）,and this is useless to flammable and explosive gases monitoring.Light activated is a promising means.In this paper, we fabricated ZnO nanorods using the hydrothermal method, andinvestigated their gas-sensing characters to ethanol, activated by light-emitting-diodes(LEDs) with various wavelengths. The results indicate that the gas-sensingcharacteristics of ZnO nanorods to ethanol strongly depend on the wavelength andintensity of the LEDs. The light-activated gas-sensing mechanism is also discussed.We observed ZnO nanorods by a scanning electron microscope, X-ray diffraction(XRD) patterns and photoluminescence spectra. LEDs with various wavelengths suchas370nm,408nm,456nm and518nm were used as a light source of activation, andthe distance between the LED and ZnO was about8mm. Ethanol was used as a targetgas and the sensing characteristics of the ZnO nanorods were investigated atroom-temperature with the exposure of ethanol both in dark and under lightillumination. The current variations at given bias20V between electrodes wererecorded with an Agilient4155C semiconductor parameter analyzer.The ZnO nanorods(grown at120℃) and Ag doping ZnO nanorods were slenderrod-like and there are also some small particles as shown in the SEM images. TheXRD pattern shows that the ZnO nanorods have a wurtzite structure. Thephotoluminescence (PL) spectrum of the samples which is excited by360nm lighthas two main bands:375-410nm band associated with inter-band emission and420-550nm band associated with defects. The defect-related emission is stronger thanthe inter-band emission. It indicates the ZnO nanorods would is n-type semiconductor.There are several other emission peaks, which may be due to the surface state of theZnO nanorods or small particles.We investigated the gas sensing characteristics of the ZnO nanorods activated byLEDs with various wavelengths at room temperature. The current variation is morepronounced when the ZnO nanorods are illuminated with shorter wavelength. Thelight-activated gas-sensing characteristics become significantly lower in the case of the excitation wavelength longer than456nm. This indicates that the sensingcharacteristics of the light-activated ZnO nanorods to ethanol are mainly related withthe inter-band excitation. The gas-sensing characteristics of the samples activated by a370nm LED under various intensities was investigated. The gas sensingcharacteristics strongly depended on the light intensity, the stronger the light, the moreobvious reduction the current. Gas sensitive properties of ZnO nanorods to ethanolunder ultraviolet excitation is a results of surface reaction process. The specificsurface reaction would is that the ethanol adsorbed on surface of ZnO nanorods obtainthe electrons and electrons are released after surface reaction with O2.As conclusions, at room temperature, the gas-sensing properties of ZnOnanorods activated by LED were investigated. Nano-zinc oxide showed goodsensitivity to ethanol under UV-LED activation. The light-induced carriers take adistinct role in the gas sensing characteristics.