Abstract
Mesoporous materials are a popular research topic. Mesoporous TiO2 has unique properties such as high specific surface areas, high porosities, and high thermal stability. They can be applied in dye sensitized solar cells, photocatalysis, phtocatalytic hydrogen production, to name just a few. In this study, we synthesized ordered mesoporous TiO2 based on an evaporation-induced self-assembly process by dip-coating and coating on Petri dish. The resulting thin films were aged at 40℃ and 50%RH, followed by calcination at 350℃ to afford the final product. The characteristics of mesoporous TiO2 were investigated by small-angle, wide-angle X-ray diffraction (SXRD, XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and N2 adsorption/desorption isotherms . This study introduced the concept of co-surfactant addition in this system. We added n-alkyl alcohols into the sol solution. The n-alkyl alcohol has both hydrophilic and hydrophobic groups, so it can situate at the hydrophilic-hydrophobic interface of the micelles to help stabilize ordered mesoporous structure, improving the regularity and increasing the volume of hydrophobic core to expand the pore. The pore structure obtained by dip-coating was 2D hexagonal structure and the pore size was about 10~14 nm. The pore structure by coating on Petri dish was also 2D hexagonal structure. The pore size was about 5~7 nm, and the specific surface area was about 155~190 m2/g. The result showed that adding n-alkyl alcohols significantly improved the regularity and expanded the pores. With increasing the amount of the same n-alkyl alcohol, the specific surface area and pore volume decreased, and pore size and pore wall thickness increased. As the n-alkyl chain length got longer, the specific surface area, pore size, and pore volume decreased. Among these n-alkyl alcohols, the best improvement came from C8H17OH. Because the boiling point of C8H17OH is near the decomposition temperature of the surfactant and the surfactant would stay in the structure when C8H17OH left. The boiling point of C16H33OH is near the sintering temperature of TiO2. When the co-surfactant molecules leave the structure, there is no surfactant left to support the structure.Furthermore, TiO2 crystals grew to destroy the structure. The pores obtained from addition of C8H17OH were larger than those from addition of C16H33OH. The n-alkyl chain of C16H33OH is longer than that of C8H17OH, and the chain may bend. There is also a steric effect in the hydrophobic core of the surfactant. The bending molecules may be pushed to the hydrophilic corona by the steric effect. It caused less increases in the hydrophobic core size, less expansion in pore sizes and more increases in pore wall thickness of C16H33OH than C8H17OH. The way of coating affected the thickness of the thin film, and the thickness would then affect the structure regularity. The thickness of thin films by dip-coating was about 1~2μm, and that by coating on Petri dish was about 1 mm. When the thickness was large, n-alkyl chains of C16H33OH would bend in random directions. It caused the molecules move away from the hydrophilic-hydrophobic interface. In the calcination process, bending molecules in random directions would damage the structure regularity significantly. C8H17OH did not bend because of its shorter n-alkyl chains, and so its existence showed improvements in structure regularity. The structure regularity ranking with the dip-coating process went as: C8OH-5.3:1 > C8OH-1:1 > C8OH-1:5.3 > C16OH-1:5.3 > C16OH-1:1 > C16OH-5.3:1 > C2OH。The structure regularity ranking with the coating on Petri dish process went as: C8OH-Petri dish-5.3:1 > C8OH-Petri dish-1:1 > C8OH-Petri dish-1:5.3 > C2OH-Petri dish > C16OH-Petri dish-1:5.3 > C16OH-Petri dish-1:1 > C16OH-Petri dish-5.3:1。