Solvent effects on the heterogeneous growth of TiO2 nanostructure arrays by solvothermal synthesis

Abstract

One-dimensional titanium dioxide (TiO2) nanostructure arrays (nanoarrays) are important metal oxide nanomaterials that can be synthesized via facile solvothermal methods. The properties of the organic solvents can impose significant effects on the microstructures and properties of the final products. However, the discussions were limited to the homogeneously nucleated TiO2 nanomaterials in free-standing powder form, while the solvent effects are less understood during the heterogeneous growth of TiO2 nanoarrays on a substrate surface. In this work, six organic compounds, namely 2-butanone, n-decane, n-hexane, toluene, ethylene glycol and ethanol, were selected as the solvents for the solvothermal synthesis of TiO2 nanoarrays on the cordierite monolithic substrates. Special attentions are paid to the morphology, crystallinity, specific surface area, and porosity of the samples. The heterogeneous growth of TiO2 nanoarrays on substrate surfaces is found to favor the solvents with moderate dielectric constants, which can be partially dissolved in the aqueous solution and modulate the reaction rate during the solvothermal synthesis. Organic solvents with low dielectric constants may result in a complete separation between the precursors and aqueous solution, and therefore slow down the overall reaction, causing the insufficient growth of the nanoarrays. The TiO2 nanoarrays are obtained with optimum morphology from the combination of 2-butanone and titanium (IV) butoxide as solvent and precusor, respectively, with a high specific surface area up to 56 m2/g including cordierite substrate given a micron thickness. When loaded with Pt catalyst, the TiO2 nanoarray-based monolithic catalysts show excellent low-temperature catalytic activity and hydrothermal stability for the CO and hydrocarbon oxidation under the simulated exhausted conditions. This work shall shed light on a better understanding of the growth mechanism and rational design of TiO2 nanoarrays for high-performance catalytic converters.