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

TiO_2具有良好的化学、物理、光学和电学性能，在光催化剂、染料敏化太阳能电池和气体传感器等方面具有广泛的应用前景。在前期的研究中，我们发现TiO_2电极的光电化学腐蚀（光刻）可以产生独特的纳米多孔表面（称为纳米蜂窝结构），这取决于每个晶粒的晶体取向，本文报道了TiO_2电极的光电化学设计及其在气体传感器中的应用。由此获得的表面形态中的一个有趣的特征是特定晶面的选择性暴露，即，(100)在一定条件下进行光刻时，光刻蚀后的TiO_2表面具有大的比表面积和高的结晶度，使其成为一种有吸引力的传感器材料。通过控制TiO_2的性质（载流子浓度、晶粒尺寸等），改变了光刻图形。和光刻条件。当光刻量增加到1,000 C/cm^2时，在TiO_2表面形成细长的棒状结构（称为纳米棒结构）。气体传感器的响应特性进行了测试，通过监测电阻的变化，用于检测氢气。光刻提高了TiO_2的气敏特性，改善了其反向响应时间和灵敏度。颗粒体积中比表面积的增大似乎是灵敏度较高的原因。在空气中，光刻由于O_2的吸附而使电阻增加，但O_2可以有效地脱附，使电阻降低到与未处理材料相同的水平。由于光刻不仅增加了表面积，而且导致允许O_2快速吸附的（100）面相对增加，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