Investigations On Growth Mechanism And Controlled Syntheses Of Silica Particles In St(?)ber Method

Amorphous silica（SiO₂）is one of the most abundant material in nature,produced in many forms by sponges,diatoms,and plants,and is widely used in industrial applications.For example,in coatings,catalyst supports,drug delivery,energy storage,photonic crystal,etc.since silica has advantages as a shell material for its chemical inertness,optical transparency,porous structure and size-selective permeability.Up to now,extensive studies have been conducted on the fabrication of silica particles,of which the St?ber method is well-know to produce colloidal silica spheres by hydrolysis and condensation of tetraethyl orthosilicate（TEOS）in alcohol solvents in presence of water and ammonia catalyst.According to this method,first of all,the size of silica particles can be only tuned in a range from 10 to 500 nm;furthermore,this method hardly provides access to uniform-sized silica spheres with a size of below 200 nm;moreover,the direct analysis of the resulting nanoscale silica shells microstructure is difficult,and it is generally believed that the silica shells are expected to be uniform and robust.However,in this thesis,we investigate the variations in size,morphology and microstructure of silica particles with ammonia concentration in the classical St?ber method.By following the kinetics and identifying molecular structures of the products derived from TEOS hydrolysis/condensation under different ammonia concentrations,a unified model（in-situ seeded growth）is proposed to describe the relationship between hydrolysis/condensation balance of TEOS and nucleation/aggregation/growth processes of silica particles.Such a study is expected to provide theory and methodology for design and accurate syntheses of silica with tunable size,morphology and microstructure,which is vital important to promote their practical and broader applications.There are mainly four parts in this thesis:1.We systematically and quantitatively investigate the variations in sizes and size distribution（polydispersity）with ammonia concentration in the classical St?ber method.Understanding the effect of ammonia concentration on kinetics of hydrolysis/condensation of TEOS at molecular level,by carrying out quantitative and systematic analyses on hydrolysis/condensation rates and molecular structures of the hydrolysis/condensation products of TEOS.Investigating systematically the nucleation,aggregation,growth processes and microstructures of silica particles prepared under different ammonia concentrations.Proposing a unified model to describe the relation between hydrolysis/condensation balance of TEOS and nucleation/aggregation/growth processes of silica particles,and therefore unraveling the variations in sizes and size distributions of silica particles in the St?ber method.2.Based on the advanced understanding of the St?ber growth model,we have successfully produced monodisperse,spherical silica particles with sizes ranging from10 to 200 nm by using LiOH to facilitate the TEOS hydrolysis at low ammonia concentrations.It is worthy of noting that the induction time,T_i,is a highly sensitive to the efficiency of hydrolysis.Only when the TEOS hydrolysis is highly effective either at fairly high ammonia concentrations（≥0.95 M）or at low ammonia concentrations（<0.95 M）in the presence of Li OH,T_i can be very short,<20 min,over which rapid and substantial jump and fall in the conductivity of the reaction media are visible.That is a distinct indicator of clear temporal separation between Pathways I and II,which will be a fairly helpful guidance for the synthesis of monodisperse small silica particles via classic St?ber method.3.We demonstrated an alternative self-templated approach to prepare hollow silica structures by using tetra-methyl-ammonium hydroxide（TMAH）as catalysts.In this approach,first solid silica spheres were prepared by using TMAH instead of ammonia as catalyst to promote the hydrolysis and condensation of TEOS.The outer layer of such silica particles is more robust than the inner layer due to the special“aggregation-growth”mode.After being incubated in water,TMAH remained in the silica spheres acting as catalyst to promote the hydrolysis of silica species and the dissolution of the cores.At low TMAH concentrations,the primary silica particles（nuclei）underwent aggregation at the late stage of the reaction,resulting in the formation of small solid silica spheres loose in structure.It is readily to get small hollow silica spheres with thin shells after the incubation attributed to the relatively small difference in the interior and exterior layers of the solid spheres.At high TMAH concentration,the primary silica particles（nuclei）underwent aggregation at the early stage of the reaction due to the accelerated hydrolysis and condensation of TEOS,thus resulting in the formation of outer shells of solid silica particles compact in structure.Incubation of the solid spheres usually resulted in the formation of large hollow silica spheres with thick shells.4.We demonstrated an in-situ seeded growth approach for fabrication of multi-shell hollow silica spheres with controllable shell number and space between adjacent shells.In this approach,the catalyst,tetramethylammonium hydroxide（TMAH）,is injected in batches into the ethanol solution of tetraethyl orthosilicate（TEOS）to grow solid silica spheres,which are readily transferred into hollow ones after incubation in hot water.Number of the shells（1-8）in the hollow spheres well matches the times（1-8）of TMAH injected into the reaction solution and the void space between the two adjacent shells（5-25 nm）well depends on the relative molar ratio of TMAH used in the later and former injections.Such precise control should be helpful to promote their practical applications and inspire the design and syntheses of silica with complex structures.