Photoelectrochemical Designing of Nano-Porous Structures on TiO_2 Electrode and its Application to Gas Sensor

二氧化钛具有良好的化学、物理、光学和电学性能，是一种非常有吸引力的光催化剂、染料敏化太阳能电池和气敏传感器材料。在以往的研究中，我们已经发现，二氧化钛电极的光电化学刻蚀(光刻蚀)会产生独特的纳米多孔表面(称为纳米蜂窝结构)，这取决于每个颗粒的晶体取向。本文报道了二氧化钛电极的光电化学设计及其在气体传感器中的应用。由此得到的表面形态中一个有趣的特征是，当在一定条件下进行光刻时，特定的晶面，即金红石的(100)面，选择性地暴露出来。光刻二氧化钛表面具有较大的比表面积和较高的结晶度，是一种极具吸引力的传感器材料。通过控制二氧化钛的性质(载流子浓度、晶粒度等)改变了光刻图案。和光刻条件。当光刻量增加到1000C/cm~2时，在二氧化钛表面形成细长棒状表面结构(称为纳米棒结构)。通过监测检测氢气的电阻变化，测试了传感器的响应特性。在反向响应时间和灵敏度方面，TiO2光刻蚀改善了其气敏特性。颗粒体积中比表面积的增大似乎是导致感度较高的原因。光刻增加了空气中的电阻，这是因为O2在空气中的吸附量较高，但可以有效地将其解吸，从而将电阻降低到与未经处理的材料相同的水平。由于光刻不仅增加了比表面积，而且导致了允许快速吸附O2的(100)面的相对增加，因此由于它们在气敏过程中的份额增加，反向响应时间已经缩短

TiO_2 is a very attractive material for photocatalysts, dye-sensitized solar cells and gas sensors by its favorable chemical, physical, optical and electrical properties. In the previous study, we have found that the photoelectrochemical etching(photoetching) of TiO_2 electrode creates unique nano-porous surfaces (named nano-honeycomb structure), depending upon the crystallographic orientation of each grain.This work reports photoelectrochemical designing of TiO_2 electrode and its application to the gas sensor. One of the interesting features in thus obtained surface morphology is a selective exposure of the specific crystallographic faces, i.e., (100) surfaces of rutile, when photoetching is carried out under certain conditions. The large specific surface area and high crystallinity of photoetched TiO_2 surface makes it an attractive material for sensor device. The photoetching pattern has been changed by controlling the properties of TiO_2 (carrier density, grain size, etc.) and photoetching condition. When the photoetching quantity increase to 1,000 C/cm^2 thin and long rod-like surface structure (named nano-rod structure) is create on TiO_2 surface. The gas sensor response properties were tested by monitoring electric resistance changes for the detection of hydrogen gas. The photoetching of TiO_2 improve its gas sensor characteristics in terms of the reverse response time and sensitivity. The enlargement of the specific surface area in the grain bulk seems to be responsible to the higher sensitivity. The photoetching increases the resistance in air due to the adsorption of O_2 at a higher amount, which, however, can be efficiently desorbed to reduce resistance to the same level to that of the untreated material. As the photoetching not just increases the surface area but leads to a relative increase of the (100) faces which allow fast adsorption of O_2, the reverse response time has been shortened, due to their increased share in the gas sensing process