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

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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