Elsevier

Materials Letters

Volume 65, Issue 14, 31 July 2011, Pages 2136-2138
Materials Letters

Synthesis of titania–silica xerogels by co-solvent induced gelation at ambient temperature

https://doi.org/10.1016/j.matlet.2011.04.054Get rights and content

Abstract

A series of binary titania–silica mixed oxides was prepared by sol–gel synthesis at room temperature. The hydrolysis of titanium isopropoxide (Ti(iOPr)4) and tetraethylorthosilicate (TEOS) was facilitated by co-solvent induced gelation in acidic media. The resulting gels were dried, calcined, and then characterized by powder X-ray diffraction analysis, nitrogen sorption studies (at 77 K), diffuse reflectance spectroscopy, Raman microscopy, and transmission electron microscopy. The nitrogen sorption studies indicate that the porosities could be tuned when simple aromatic solvents such as toluene, p-xylene, or mesitylene were added as a co-solvent to the synthesis gel. The binary mixed metal oxide materials obtained in this study showed high activity towards the degradation of phenol, and possessed high surface areas, and large pore volumes with narrow pore size distribution without the need for additional hydrothermal synthesis or supercritical drying.

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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