TBAOH-mediated Synthesis Of SiO₂ Particles With Tunable Sizes And Shapes

Silica is a fundamental mineral commonly found in nature and has been widely used in industrial fields.Colloidal silica spheres uniform in size and shape have been applied as optical filters,photographic emulsions,chromatographic agents,sensing elements,catalysts,stabilizers,coating layers,and so on.The St???ber method is well-known to produce colloidal silica spheres by hydrolysis and condensation of tetraethyl orthosilicate?TEOS?in ethanol solvents in the presence of water and ammonium hydroxide?NH3·H2O?.Since then,this method has been becoming one of the mostly used routes for the syntheses of colloidal silica particles.In this approach,the size of silica particles can be simply tuned in a wide range from 10 to 500 nm by simply altering the concentration of ammonia used.Hydrolysis and condensation of TEOS are the basic reactions involved in St???ber method.A well understanding on them and their relationship with the nucleation and growth processes should be critical for precise control over the sizes,shapes and microstructures of the silica particles.In this work,tetrabutylammonium hydroxide?TBAOH?,an organic strong base,was introduced into the classical St???ber system as a co-catalyst and the effect of TBAOH on the hydrolysis/condensation process of the system was studied.The controllable preparation of subsize and non-spherical SiO₂ particles were carried out.The main work includes:1.TBAOH is an organic strong base,which was introduced into the St???ber system as a co-catalyst.The ionized OH-and TBA+ have impact on the hydrolysis/condensation process of TEOS.OH-promoted the hydrolysis of TEOS,increased the amount of "nucleus",and inhibited the growth of primary particles.The presence of TBA+ increased the ionic strength and promoted the growth of silicaparticles.In this chapter,subsize silica particles?6-35 nm?with tunable size and uniform distribution were prepared by adjusting the concentration of the co-catalyst TABOH,and its nucleation/growth process was investigated.The experiments were carried out under various cocentration of TBAOH in presence of NH3·H2O?0.3 M?At the low concentration of TBAOH?? 0.4 m M?,the size of particles increased with the increase of the TBAOH concentration.When TBAOH concentration is 0,0.2 and0.4 m M,the size of the prepared silica particles is 10,28 and 35 nm respectively.At the high concentration of TBAOH?> 0.4 m M?,the size of particles reduced with the increase of the TBAOH 0.4 m M,the Si OH concentration.When TBAOH concentration is 0.4,0.6,0.8,1.0 and 2.0 m M,the size of silica particles is 35,27,20,16 and 6 nm,respectively.Importantly,as prepared of silica particles show narrow size distribution,which are comparable to that prepared by the classical St???ber method.2.The non-spherical silica particles were prepared,where a prehydrolysis process was introduced through the catalysis by TBAOH for a period of time following by the addition of NH3·H2O.The size and morphology of silica particles was tunable by controlling the concentration of NH3·H2O and the prehydrolysis time.The particle size increased with the increase of NH3·H2O concentration.The aspect ratio of particles increased with the increase of the prehydrolysis time.When the prehydrolysis time is 0,20,40,60,90,120 and 150 min,the aspect ratio is 1.00,1.06,1.58,1.84,3.06,5.17 and 6.82 respectively.At the long prehydrolysis time,the rapid hydrolysis of TEOS results in the rapid formation of nuclei,which can undergo aggregation and growth of non-spherical silica particles.