Improvement in the conductivity ratio of ordered mesoporous Ag-TiO ₂ thin films for thermoelectric materials

High electrical conductivity, low thermal conductivity, and a high Seebeck coefficient are desired for thermoelectric materials. Therefore, knowledge of the relation between electrical conductivity and thermal conductivity is essential to improve thermoelectric properties. This work has focused on the synthesis of a mesoporous TiO₂ and the effect of impurity addition for applications in thermoelectric generation. Ordered mesoporous materials show properties including a large surface area and good pore size distribution, which makes them effective in thermoelectric generation by means of low thermal conductivity. The addition of impurities often changes the mesoporous structure and can be a useful way to enhance the thermoelectric properties by changing the carrier concentration. In this report, an ordered mesoporous TiO₂ film was synthesized with titanium tetraisopropoxide and triblock copolymer Pluronic P-123 (EO₂₀PO₇₀EO₂₀) as a surfactant. An acid catalyst (HCl) was added for the hydrolysis polycondensation reaction. In order to incorporate Ag into the TiO₂ matrix, AgNO₃ was added in amounts ranging from 0 to 3 at.%. After aging, the solution was spin-coated onto the substrate. The ordered mesoporous TiO₂ films were obtained by removing the Pluronic P-123 at various temperatures. X-ray diffraction, an ellipsometer, a 4-point probe, a scanning electron microscope and a thermal conductivity analyzer were utilized to elucidate the effects of Ag-additions and the mesoporous structure. Through the addition of 3 at.% Ag into the mesoporous TiO₂ film, the electrical to thermal conductivity ratio was increased from 113.14 K/V² to 1122.37 K/V².