ZnO纳米结构气敏传感器

【作者】 顾磊磊；

【导师】 陈国荣；

【作者基本信息】 复旦大学 ， 物理电子学， 2011， 硕士

【摘要】 本文采用气相法和水热法生长了一维ZnO纳米材料,采用多种手段(SEM、XRD、TEM、Raman等)对它们的结构进行了表征,表明CVD法生长ZnO纳米线符合VLS机制,纳米线直径在100-200 nm之间,长度为十几微米；CVD法生长四针状ZnO纳米材料属于VS机制,所得纳米材料呈正四面体型,每条晶须长度为3-5μm,从根部到尖端,直径由200 nm逐渐变为50 nm；水热法合成的ZnO纳米棒阵列,纳米棒排列紧密,垂直于基片生长,单根纳米棒的直径50 nm左右,长度约为5μm。这三种ZnO纳米结构都为六角纤锌矿结构,纯度极高且高度结晶。气相法和水热法生长的纳米线都具有择优取向,沿[0001]向择优生长。组装了单根ZnO纳米线场效应管(FET),测量了它的输出特性曲线和转移特性曲线,并估算了它的各项参数。该场效应管为n型,载流子浓度和迁移率分别为1.15×108cm-1,20.5 cm2/(VS)；阈值电压约为-16.7V；Vds=+1.5 V、+2 V和+2.5 V时的低频跨导gm分别为34.0 nS、46.6 nS和59.9 nS；此外,在Vgs<0时,该场效应管会出现饱和现象。制备了基于ZnO纳米线场效应管的气敏传感器。测试了它在室温下对O2的灵敏度。发现它灵敏度高(室温下对1000PPmO2灵敏度1.5,体材料和薄膜材料无响应)、响应迅速(室温下,500PPmO2,响应时间：10 s,恢复时间：50 s)；当O2浓度在1000 ppm以内时,灵敏度与O2浓度基本成线性；单根ZnO纳米线具有自加热效应,这对纳米线的气敏性能有影响,当O2浓度为100 ppm,工作电流较小时(I<2μA),灵敏度随工作电流增大而增大,工作电流继续增大(I>2μA),灵敏度达到饱和。将四针状ZnO纳米晶须与TiO2粉末共同退火生成了Ti掺杂四针状ZnO纳米结构。掺杂后,样品仍为正四面体型样品,形貌未发生明显变化。将样品压片,组装成三明治结构的气敏器件,测试了其对乙醇的气敏性能。表明掺杂后,样品的气敏性能得到明显提高：最佳工作温度稍有降低(250℃降至240℃)；样品电阻变小；灵敏度比掺杂前高出一个数量级；响应和恢复时间变短。这是由于Ti4+扩散入ZnO晶格中部分取代Zn2+后,产生更多的自由电子,增加了氧吸附位,使得物理吸附的氧分子更容易得到电子生成氧负离子造成的。采用水热法在Au/Ni叉指电极上原位生长了ZnO纳米棒阵列。分别测试了该纳米阵列的湿敏性能(不同相对湿度下的电阻响应和电容响应)。当相对湿度从11%变至95%时,电阻和电容变化都在三个数量级以上。相比之下,电阻响应具有更高的灵敏度,更小的响应和恢复时间,电容响应则具有更加良好的线性。采用溶胶凝胶法在ZnO纳米棒阵列表面涂覆了TiO2层,得到了ZnO/TiO2核壳结构纳米材料并分析了其结构,表明TiO2是涂覆在纳米棒表面,涂覆后表面变粗糙,平均直径增加了约20nm。测试了ZnO/TiO2核壳层结构纳米材料室温下对不同相对湿度的电容响应。较之纯ZnO纳米棒器件,灵敏度得到了显著提高(相对湿度11%变至95%时,灵敏度达五个数量级),响应和恢复时间缩短,重复性和迟滞特性良好。采用交流阻抗谱法分析了纯ZnO和ZnO/TiO2的湿敏机制。在不同相对湿度下,工作机制不同：低湿度时,材料靠电荷跳跃迁导电；高湿度时,靠质子在水膜中的扩散导电。TiO2层修饰后,该临界点由75%RH降至33%RH,而且ZnO/TiO2样品的交流阻抗谱在相对湿度为大于33%RH时只剩下一条斜率为1的直线而ZnO样品中在相对湿度为95%时也没有出现这种情况,说明TiO2修饰后,样品的吸水性大大提高。这主要是由于ZnO的大的比表面体和表面TiO2层的超亲水性造成的,同时水分子更容易在ZnO和TiO2界面及TiO2层的晶界中发生毛细凝结现象,这使ZnO/TiO2对水分子的吸附增大,提高灵敏度。更多还原

【Abstract】 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.更多还原