Synthesis And Characterization Of Highly Organized Nanoporous Materials For Special Applications

Since the first reports of mesostructured and nanoporous silica thin films by evaporation-induces self-assembly (EISA) this kind of materials has gained significant attention. The structure directors that induce the formation of the mesostructure and upon thermal removal generate the nanopores are usually commercially available surfactants, e.g. neutral block-copolymers (Pluronics), poly(oxyethylene)alkyl ethers (Brij), or alkyltrimethylammonium halides (CTAB, CTAC). During the spin or dip coating they allow the inorganic precursor to arrange and condense around them replicating in a cooperative mechanism their structure.While the first synthetic attempts were focused on silica materials the procedures were successfully transferred to metal oxides to generate mesostructured metal oxide thin films. However, due to the flexibility of the amorphous pore walls in the silica phases the transformation was not completely successful. Most metal oxides crystallize during the thermal removal of the template or degradation upon extraction procedures, but the structure can not be well kept then after. The reason is the lacking support of the structure director after its removal. The strong forces that occur during the nucleation and crystal growth in the pore walls are not hold and lead to a complete loss of structure except a few cases.In our report, highly ordered and homogeneous nanoporous thin films of palladium doped tin dioxide have been synthesized from tin chloride precursors at low pH value. In comparison to the materials reported so far in the literature our thin films depict highly ordered mesostructures maintained at temperatures up to 400℃in combination with highly crystalline tin oxide of cassiterite structure. By varying the synthesis parameters and the conditions of the post-synthetic treatments, materials differing in their physicochemical characteristics and sensing behavior were obtained. The best results with respect to mesopore ordering, structure stability and sensing behavior show the SnO2 thin film doped with 4 wt%Pd prepared with 12 mol/1 HCl, spin-coated at 10%RH, treated with post-synthetic procedure and calcined with a gradual increase in temperature. In general the synthesis route described in the paper has the advantage of being easier, less costly, and highly reproducible. Due to their crystallinity and doping, these films show high sensitivity towards hydrogen gas at room temperature. We also introduce a solution to maintain the advantages of both conventional methods by the development of a post-synthetic hydrothermal treatment that allows us to crystallize or mesostructure various inorganic precursors around commercially available templates at relatively low temperatures. The thin film is amorphous before the treatment and consists of crystalline inorganic nanoparticles arranged around ordered nanoporous after it as shown by XRD. These results are confirmed by HRTEM investigations which demonstrate that the crystallization of tin dioxide is complete at 100℃PSHT. As a main benefit, the films can be heated up to continue crystallite growth without disrupting the mesoscopic order, when the complete removal of template is finished at 350℃. The presented strategy allows the preparation of mesoporous doped tin dioxide thin films with a well-defined Fmmm nanostructure and nice mesoporosity even up to 600℃.The small size of the single nanocrystals and their three-dimensional continuous architecture after the post synthetic hydrothermal treament are crucial for the electrochemical properties of the metal oxide thin films. Due to the crystallinity and doping the film shows high sensitivity towards hydrogen gas at room temperature despite the presence of the template inside the mesopores. After template removal the sensor is able to maintain a sufficiently ordered and stable nanoporous network allowing fast gas diffusion through the whole structure and texture of the thin film.we show a new synthetic approach that overcomes the problems of the so far established methods. It allows the synthesis of metal oxide thin films with ordered nanopores at low temperatures. Furthermore, in case of titania and tin dioxide crystalline pore walls can be achieved at unprecedented low temperatures allowing an easy thermal removal of the template by calcination without loss of mesostructure or significant shrinkage of the thin film normal to the substrate. This process will allow to prepare new transparent thin film materials for semi-conducting sensors and dye-sensitized solar cells and significantly enhance their desired properties.