Synthesis of titania–silica xerogels by co-solvent induced gelation at ambient temperature
Introduction
Titania is the most widely used material in energy and environmental research because of its myriad of potential applications [1]. However, its application in photocatalysis is limited by its low surface area, and hydrophilic nature, necessitating modification by the incorporation of a complementary oxide as a support. Of the various mixed oxides investigated, titania–silica (TiO2–SiO2) was found to be photocatalytically more active than other mixed oxide combinations due to enhanced adsorption and pre-concentration of reactants at the active centers [2]. The challenge in the synthesis procedure is the unequal hydrolysis, and condensation rates of silica and titania precursors. Different synthetic strategies have been adopted to overcome this phenomenon, and eventually form the catalytically active Si–O–Ti linkage [3], [4], [5]. TiO2·SiO2 “wet gels” are dried at ambient and supercritical drying conditions to produce xerogels and aerogels respectively. Synthesis of xerogels minimizes the need for energy intensive processes and obviates the use of conventional, tedious, energy intensive, and expensive instrumentation such as a high pressure reactor involved in supercritical drying. Another approach used is hydrothermal technique in which the gels are subjected to high temperatures and pressures in an autoclave to obtain high surface area and homogeneous mixed oxides. We have developed a simple and effective one pot synthetic procedure for the synthesis of TiO2–SiO2 xerogels by utilizing co-solvent induced gelation (CIG) at ambient conditions. This method produces materials whose textural properties are equivalent if not superior to hydrothermally synthesized materials, and more importantly enables tuning of porosities by the introduction of carefully selected co-solvent in appropriate amounts.
Section snippets
Preparation of TiO2–SiO2 nanomaterials
Tetraethylorthosilicate (TEOS) and titanium isopropoxide (Ti(iOPr)4) were used as the silica and titania precursors. Typically, ethanol was used as the primary solvent, and toluene, p-xylene, or mesitylene was chosen as one of the co-solvents. To 18 ml of the solvent, 1.65 ml (7.4 mmol) of TEOS, 1 ml (55.55 mmol) of water, and 0.25 ml (5.6 mmol) of conc. HNO3 was added. Finally, 2.2 ml (7.4 mmol) of (Ti(iOPr)4) was added drop wise with rapid stirring. The molar ratio of TiO2/SiO2 is 1:1. The reaction
Results and discussion
The textural properties of the titania–silica xerogels studied by nitrogen physisorption are listed in Table 1. In the preparation of these materials, we studied the effects of co-solvents on the gelation and the resultant textural properties. The time required for gelation in the absence of co-solvent is from several hours to weeks [6]. In this study, TS-01 prepared in the absence of co-solvent took 4 h for gelation. The presence of a co-solvent significantly reduced the gelation time to less
Conclusion
Titania–silica mixed oxide xerogels were prepared by CIG in a one pot synthesis with short gelation times without using any prehydrolysis procedures and stabilizing agents to control the hydrolysis and condensation rates. The porosities can be tuned and tailored by employing a proper co-solvent in suitable amounts. The xerogels studied exhibit mesoporosity and high surface areas irrespective of the co-solvent utilized. Toluene and mesitylene as co-solvents results in xerogels with very narrow
Acknowledgement
This work was supported by NSF–CHE 0722632, NSF-EPS-0903804, SD supported 2010 Center — CRDLM, DOE-DE-FG02-08ER64624, and DE-EE0000270. We are thankful to Dr. C. Jiang and Mr. A.T. Maingi for their assistance with the Raman studies and Dr. S.P. Ahrenkiel for the TEM measurements.
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