Zno Nanostructure Gas Sensors

One dimensional ZnO nanomaterials were grown by vapor phase method and solution method. Their morphologies and structures were further characterized by SEM, XRD, TEM and Raman spectrum. The results showed that the nanowires grown by CVD have a diameter of 100-200 nm and a length of over ten micrometers. The growth of this kind of nanomaterials followed VLS mechanism. In contrast, the growth of nanotetrahedron by CVD followed VS mechanism. The length of each whisker was about 3-5μm and the diameter decreased from 200 nm to 50 nm gradually from the root to the end; The hydrothermally grown ZnO nanorod array has a high density. The nanorods are vertical to the substrate. Their mean dianmeter was about 50 nm and length was about 5 5μm. Besides, all of the three kinds of nanomaterials are highly crystallized and have high purity and [0001] preferred orientation.Single ZnO nanowire Field Effect Transistor (FET) was assembled. Its electrical characteristic was tested and various parameters were evaluated. The charge carrier of this FET is electrons, with concentration and mobility of the 1.15×108 cm-1,20.5 cm2/(VS), respectively. The threshold voltage was about-16.7 V. The low-frequency transconductance were 34.0 nS,46.6 nS and 59.9 nS when the source-drain voltage were+1.5 V,+2V and+2.5V. This output characteristic curve of this FET will saturate when it has a minus gate voltage. Based on this FET, ZnO single-wire gas sensor was further fabricated and its gas sensing properties to O2 was tested. The results demonstrated that this sensor has high sensitivity, fast response and recovery process as well as good linearity. Self-heating effect plays a key role in enhancing its gas sensing performance. When the concentration of O2 was 100 ppm, the sensitivity increased as the increase of working current until it reached 2μA. If the current increased in further, the sensitivity will get saturated.Ti doped ZnO nanotetrahedron was prepared by co-annealing of ZnO nanotetrahedron and TiO2 powders. The morphology of the nanomaterials didn't change much after doping. The sample was pressured into pellet and assembled into sandwich-structured gas sensors. The testing results showed that its gas sensing performance was greatly improved by doping:The optimizing temperature decreased from 250℃to 240℃, the sensitivity was one order higher and the response and recovery time were both shortened. This is due to that Ti4+diffused into the crystal lattice of ZnO and substitutes part of the Zn2+, generating more Ti donors, which corresponds to more conductor electrons.Highly aligned ZnO nanorod array was in-situ prepared on the Au/Ni interdigitated electrodes via hydrothermal process.Its humidity sensing performance was characterized. When the relative humidity varies between 11%RH and 95%RH, both the capacitance and resistance response are over three orders. The resistance has higher sensitivity and faster response and recovery process while the capacitance has better linearity. Afterwards, TiO2 layer was coated on the surface of ZnO naorod to form ZnO/TiO2 core/shell nanomaterials. After coating, the surface became rougher and the diameter increased by 20 nm. Compared with pristine ZnO nanorod array, the sensitivity of the modified nanomaterials was greatly enhanced (pristine ZnO:103, ZnO/TiO2:105). Besides, humidity sensors based on this nanorod array has short response and recovery time as well as good reproducibility and hysteresis.The humidity sensing mechanism of the coated and uncoated samples were investigated through the analysis of complex impedance plots, showing that the samples worked in different mechanisms at different relative humidities:the sample conducts electricity by the jumping of charge carriers while at high relative humidities they worked by the diffusion of protons in water layers.The transition threshold of the two sensing mechanisms for the ZnO/TiO2 sample is about 33%RH at room temperature while the transition point was 75%RH for the uncoated sample. A straight line at full frequency range appeared at 55%RH for the ZnO/TiO2 sample, this, however, was not observed for ZnO nanorod array even at 95%RH. These two points indicate that the coating process enhanced the water adsorption process on the sensor surface. This is attributed to the large surface/volume ration of ZnO nanorod array and the super hydrophilicity of TiO2 layer on the surface of ZnO naorod array. Besides, capillary condensation is likely to happen on the interface of ZnO core and TiO2 shell as well as the grain boundary of TiO2 layer.