Synthesis of Stable Mesostructured Coupled Semiconductor Thin Films: meso-CdS-TiO₂ and meso-CdSe-TiO₂

Abstract

Cd(II) ions can be incorporated into the channels of mesostructured titania films, using the evaporation-induced self-assembly (EISA) approach, up to a record high Cd/Ti mole ratio of 25%. The film samples were obtained by spin or dip coating from a mixture of 1-butanol, [Cd(H(2)O)(4)](NO(3))(2), HNO(3), and Ti(OC(4)H(9))(4) and then aging the samples under 50% humidity at 30 degrees C (denoted as meso-xCd(II)-yTiO(2)). The nitrate ions, from nitric acid and cadmium nitrate, play important roles in the assembly process by coordinating as bidentate and bridged ligands to Cd(II) and Ti(IV) sites, respectively, in the mesostructured titania films. The film samples can be reacted under a H(2)S (or H(2)Se) gas atmosphere to produce CdS (or CdSe) on the channel surface and/or pore walls. However, the presence of such a large number of nitrate ions in the film samples also yields an extensive amount of nitric acid upon H(2)S (or H(2)Se) reaction, where the nanoparticles are not stable (they undergo decomposition back to metal ion and H(2)S or H(2)Se gas). However, this problem can be overcome by further aging the samples at 130 degrees C for a few hours before H(2)S (or H(2)Se) reaction. This step removes about 90% of the nitrate ions, eliminates the nitric acid production step, and stabilizes the CdS nanoparticles on the surface and/or walls of the pores of the coupled semiconductor films, denoted as meso-xCdS-yTiO(2). However, the H(2)Se reaction, additionally, needs to be carried at lower H(2)Se pressures in an N(2) atmosphere to produce stable CdSe nanoparticles on the surface and/or walls of the pores of the films, denoted as meso-xCdSe-yTiO(2). Otherwise, an excessive number of Se(8) particles form in the film samples.