Molten salt assisted self-assembly process: synthesis of mesoporous transition metal oxide thin films and CdSe sensitized TiO2 photoanodes

Abstract

Fabrication of mesoporous transition metal oxide thin films are important for the development of many energy related technologies. Molten salt-assisted-self-assembly (MASA) method, that has simple stages such as preparation of a clear solution of the precursors in water or ethanol, coating of this solution on a substrate as a thin film, and calcination of the film at an elevated temperature is a simple and useful method to fabricate mesoporous thin films. In this thesis, mesoporous transition metal oxides thin films and CdSe sensitized TiO2 photoanodes have been fabricated using MASA approach and characterized using multi-analytical techniques. In the first part of the thesis, CdSe/TiO2 thin films were synthesized by reacting mesoporous CdTiO3 thin film under a H2Se gas atmosphere. Many synthesis parameters were optimized in the synthesis of mesoporous CdTiO3 thin films. The film thickness, calcination temperature, and H2Se reaction condition have been changed to determine the optimum synthesis conditions for an efficient photoanode of a quantum dot sensitized solar cell (QDSSC). However, the reactivity of CdTiO3 towards to H2Se gas is low and selenium forms as a side product upon H2Se reaction. For the reactivity problem, mesoporous CdO-SiO2 (denoted as meso-CdO-SiO2) thin films were synthesized to increase the CdSe nanoparticle population, as the sensitizer in the CdSe sensitized TiO2 photoanode. However, The meso-CdSe-SiO2 thin films are not active in solar cells because of an insulating nature of silica. Second part of the thesis involves infiltration of MASA solution (precursors of salt-surfactant and a polymerizing agent, such as Si(OCH3)4 or Ti(OC4H9)4) into the pore of prefabricated films (using P25) of mesoporous titania (denoted as meso-P25). In latter steps, the above films were calcined and then reacted under H2Se to obtain the photoanodes (denoted meso-CdSe-SiO2-P25 and meso-CdSe-TiO2-P25). The synthesis conditions were optimized by changing the synthesis parameters (such as precursor concentrations, calcination, and H2Se reaction temperatures) and using XRD, FTIR, Raman, 29Si-MAS-NMR, EXAFS, XANES, SEM, TEM, N2-sorption techniques. However, formation of a meso-CdO-SiO2 thin film, on top of the meso-P25 film, was observed upon using a concentrated MASA solution in the infiltration step. Therefore, multiple loading method has been established to increase the CdSe-SiO2 layer in the pores of meso-P25 using diluter MASA solutions. Also, the H2Se reaction conditions were optimized by controlling the reaction atmosphere and temperature. Effects of silica amount in the CdO-SiO2 system on the photoanode has been examined by measuring the I-V curves, of the solar cells fabricated using our photoanodes. In the last part of the thesis, the MASA method has been adopted for the synthesis of mesoporous transition metal oxides thin films. Firstly, mesoporous iron oxide film has been synthesized using MASA approach and characterized using above analytical tools. The thermal and structural properties of the [Fe(H2O)6](NO3)3/surfactants (10-lauryl ether and CTAB) mesophases have been investigated for the synthesis of a well-ordered iron oxide films. Effects of calcination temperature, on the crystallinity, and porosity of mesoporous Fe2O3, have been demonstrated by using TEM, SEM, XRD, and N2 sorption techniques. Later, other mesoporous transition metal oxides (such as ZnO, CuO, NiO, Co3O4 and Mn2O3) have been synthesized using the MASA approach. The transition metal salts ([Zn(H2O)6](NO3)2, [Cu(H2O)6](NO3)2, [Ni(H2O)6](NO3)2, [Co(H2O)6](NO3)2, [Mn(H2O)6](NO3)2)-surfactant mesophases have been used as the starting liquid crystalline materials that can be calcined at high temperatures (above 300 °C) to obtain the thin films. The synthesized mesoporous metal oxide thin films were characterized by using above analytical tools.