(363d) Supercritical Carbon Dioxide: A Promising Green Solvent in One-Pot Synthesis of ZrO2-TiO2 Nanotubes

Recently, considerable effort has been devoted to synthesizing inorganic one-dimensional (1D) nanostructures (e.g. nanotubes, nanowires) because of their unique physical properties compared to their bulk counterparts. Many exciting potential applications are possible including catalysis, high efficiency solar cells, coatings, and sensors. Amongst the 1D oxide nanomaterials reported, titania (TiO₂) nanomaterials are receiving considerable attention due to titania's high activity, strong oxidation capability, and chemical stability. Modifying TiO₂ with ZrO₂ , one of the most suitable dopants, also enhances its performance by increasing thermal stability, surface area and reducing crystallite size as the performance of titania nanomaterials strongly relies on their crystallinity, crystallite size, crystal structure, and specific surface area.Several different processing techniques have been used for the preparation of TiO₂ based nanostructures, such as anodization, template techniques, hydrothermal processes, and soft chemical processes. However, each of these methods has limitations. Due to these limitations, alternative processes using a green solvent such as supercritical carbon dioxide (scCO₂) is attractive to synthesize TiO₂-base nanomaterials.

Here we present a very simple, surfactant free sol-gel synthesis route for producing high surface area, mesoporous, ZrO₂-TiO₂ binary nanotubes, under mild conditions. An acid modified sol-gel technique was employed to synthesize the one-dimensional bimetallic ZrO₂-TiO₂ materials studying several solvents of varying dielectric constants including supercritical carbon dioxide (scCO₂),hexane, isopropanal and ethanol. The synthesized materials were characterized by several physicochemical techniques (e.g. electron microscopy (SEM and TEM), BET, XRD, and FTIR) in order to assess the preparation method and the effects of the solvents on the morphology, porous structure, and thermal and crystallization behavior of the nanomaterials.

SEM and TEM analysis show that nanotubes having diameters of 55-140 nm were formed in scCO₂,whereas nanorods and/or tubes with diameter ranging from 40-250 nm were seen when the samples were prepared in organic solvent. The BET analysis also show that the surface area, and pore volume followed a linear relationship with dielectric constant of the utilized solvent. When low dielectric constant solvents such as scCO₂ were employed, synthesized materials had higher surface area. The presence of such solvents helps in the formation of porous structure with high surface area materials. However, when alcoholic solvents having high dielectric constants were used, dense materials having low surface areas were produced.

These high surface area, mesoporous, ZrO₂-TiO₂ binary nanotubes have lots of potential applications. In addition, this process is very easy to scale up for large scale industrial applications.