Synthesis and study of ammonium oxalate sol–gel templated silica gels

Abstract

Silica tubes and spheres were synthesised in the presence of oxalate ions by the sol–gel method from tetraethylorthosilicate (TEOS) as silica source. The syntheses were carried out in ethanol/water mixtures in the presence of oxalic acid and aqueous ammonia between 0 and 75 °C. TEM images show that the yield and morphology of the silica is influenced by synthesis conditions (temperature, oxalic acid and ammonia concentration, amount of water, length of ageing time, and length of time for TEOS hydrolysis). It was found that tubes, spheres and amorphous silica structures are formed with sizes (length, diameters) determined by the synthesis conditions. Further, once formed these materials can ‘dissolve’ under the synthesis conditions used.

Introduction

The shaping and synthesis of both inorganic and organic materials can be achieved at both the macroscopic and microscopic levels and indeed today the general principles to make ‘shaped’ materials are even to be found in the popular scientific literature [1]. Two generic procedures to make molecules with ‘shape’ represent the ‘top down’ and ‘bottom up’ approaches to materials synthesis, both with advantages and disadvantages determined by the final product composition, product properties, cost, etc. [2]. The recent advances in the ‘bottom up’ approach to molecular shape control, driven by self-assembly, templating and dendrimer synthesis has led to new strategies to materials synthesis where fine control of structure is possible. Thus, the synthesis of inorganic–organic hybrids in which the components are linked together in a precise relationship permits the synthesis of naturally occurring bio-minerals and hence a biomimetic approach to materials synthesis [3].

Recently it has been shown that templates can be used to transfer structural information, from the template to inorganic molecules/ions such that the shape (and even the chirality) of the newly formed material can now be predicted [4], [5]. An excellent review by Shinkai and co-workers has recently summarised the current status of templating procedures on inorganic materials [4].

One of the procedures used to transfer shape to inorganic molecules and ions is via the use of organic crystals as templates. This approach, although currently limited to the synthesis of spheres and rods/tubes/fibres, generally leads to facile template removal from the final structure (heating, solvent extraction). Indeed, Nakamura and Matsui [6] were the first to reveal the utility of this approach in the synthesis of silica tubes starting from TEOS (tetraethylorthosilicate) and using dl-tartaric acid as template precursor. Since this study, a number of other reports have appeared extending the approach to other similar template precursors, both organic (e.g. citric acid) [7], [8], [9] and inorganic ([Pt(NH₃)₄(HCO₃)₂]) [10], as well as to produce transition metal oxides (TiO₂, Nb₂O₅, ZrO₂ and Ta₂O₅ [11], V₃O₇ [12]).

The shape of the inorganic material formed in the synthesis has been shown to relate to the shape of the crystalline template with the diameter and length of tubular materials determined by the synthesis conditions [7], [8]. For example, transcription of shape by use of ammonium tartrate as template is proposed to occur via an interaction between the ${\mathrm{NH}}_4^{+}$ ions in the template and the silicate ions in basic solution [6], [8].

We have recently undertaken a study to explore both the range of templates that can be used to shape silica structures and also to evaluate the conditions under which silica tubes/spheres formation is favoured [9]. During our investigations on the synthesis of silica tubes/spheres in the presence of ammonium oxalate as template we have observed that synthesis conditions are important for structure synthesis, but also importantly that the silica structures formed can ‘dissolve’ under the synthesis conditions used. This second issue has only briefly been mentioned in the literature.

Herein we report on the study of the synthesis and dissolution of silicas formed from TEOS in the presence of oxalate ions. It is interesting to note that oxalic acid is often added to TEOS synthesis gels as a catalyst to assist with the silica hydrolysis reaction and that the oxalate ion is regarded as ‘neutral’ in the synthesis [13].