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Recently, considerable effort has been devoted to synthesizing materials with one-dimensional (1D) nanostructures due to their unique properties and potential applications in a diversity of applications including catalysis, high-efficiency solar cells, coatings, and sensors.[1] Particularly, titania (TiO 2 ) nanotubes are receiving considerable attention due to titania's high activity, strong oxidation capability, and chemical stability. In addition, being a low-cost material, TiO 2 is favorable for potential large-scale commercial applications. The performance of titania nanomaterials strongly relies on their crystallinity, crystallite size, crystal structure, specific surface area, thermal stability and quantum efficiency. [2,3] These properties highly depend on both the synthesis method, and the subsequent thermal-treatment technique -calcination. In some cases, doping with a second metal has been found very effective to improve the properties of TiO 2 . Zirconia has been reported as one of the most suitable dopants to enhance the thermal stability and activity of TiO 2 nanomaterials. [4][5][6] Conventionally, anodization, [7] template techniques, [8,9] hydrothermal processes, [10,11] and soft chemical processes [12] have been used to prepare TiO 2 nanotubes. However, each of these methods has limitations. The template technique requires high calcination temperatures to remove the template, resulting in a collapse of the tubular structure in the product.[12]Anodizing processes produce nanotubes with relatively-large diameters.[13] The multistep hydrothermal process requires a large amount of inorganic solvents, which may lead to environmental pollution. Due to these limitations, alternative processes using a green solvent such as supercritical carbon dioxide (scCO 2 ) are attractive to synthesize TiO 2 nanotubes. Previously, our group developed a direct sol-gel process to synthesize TiO 2 nanofibers, [14] where scCO 2 was used in place of an organic solvent. There are several favorable properties that make CO 2 an attractive solvent for the synthesis and processing of high-quality porous materials. Firstly, CO 2 is inexpensive, environmentally benign and non-flammable, with mild critical conditions (P c ¼ 73.8 bar; T c ¼ 31.1 8C), which makes this a green solvent that is suitable for both laboratory and commercial scale application. Furthermore, its physical properties such as density and solubility can be tuned by adjusting the operating temperature and pressure. Low viscosity, zero surface tension and high diffusivity of scCO 2 are also favorable in synthesizing superior ultrafine and uniform nanomaterials. In addition, scCO 2 can be easily and completely removed from products by venting; hence, no drying process is required and the porous structure can be maintained without collapsing the nanostructure. In addition, scCO 2 can be easily recycled after the pressure is diminished for potential scale-up applications. While exploring the synthesis of ZrO 2 -modified TiO 2 nanostructures by sol-gel methods in sc...

For the first time, high surface area nanofibers were synthesized using aluminum isopropoxide monomer with acetic acid as the polycondensation agent in the green solvent, supercritical carbon dioxide (scCO(2)). It was found that the synthesis temperature, pressure, concentration, and acid/alkoxide ratio had a large effect on fiber formation. By optimizing the experimental conditions at 80 degrees C and 6000 psi of scCO(2) using aluminum isopropoxide at a concentration of 0.3 mmol/mL and acid/alkoxide ratio of 10, alumina nanofibers were formed ranging from 11 to 22 nm in diameter and 500 to 1000 nm in length, and with surface areas up to 580 m(2)/g. Lower temperatures gave irregular shaped nanoparticles, while a lower acid/alkoxide ratio (5:1) resulted in the formation of low surface area alumina bars. Increasing pressure led to better separation of the nanofibers and higher surface areas. In addition to the synthesis conditions, the influence of calcination temperature on the structural, textural, and morphological properties of the materials was examined using various physicochemical techniques including electron microscopy, TGA/DTA, powder XRD, FTIR, XPS, and nitrogen adsorption/desorption analysis. The long fibers with high aspect ratios were found to be thermally stable even after calcining at up to 1050 degrees C. The mechanism of fiber formation in scCO(2) is proposed based on a [Al(OH)(CH(3)CO(2))(2)](n) polycondensate backbone.