ABSTRACT
Figure 7.1.Schematic.illustration.of.the.preparation.of.silica.hollow.spheres.by.the.template.method.The. principle. of. the. synthetic. strategy. is. straightforward,. involving.three. main. steps:. (1). preparing. the. original. template,. (2). depositing. the.target. shell. material. onto. the. surface. of. the. template,. and. (3). removing. the original template. In this case, the template simply serves as a scaffold around.which.a.desired.material. is.directed. to.grow. into.a.nanostructure,.with.its.morphology.complementary.to.that.of.the.template..Colloid.crystals.are.widely.used.hard-template.materials.because.of.easy.preparation.with. different particle sizes and easy functionalisation. The second step of shell formation.is.usually.the.most.challenging.part.of.the.process.because.of.the.unavoidable.and.usually.undesirable.homogeneous.nucleation.and.growth.of. the. shell.materials.. There. are. various.methods. of. coating.materials. on.these. hard. templates:. layer-by-layer. assembly. [29],. chemical. deposition.[30],.adsorption.[31],.casting.[32],.and.atomic.layer.deposition.[36]..Various.soft. templates. have. been. also. employed. for. the. synthesis. of. hollow. silica.nanostructures,. including.microemulsions. formed. in. a. two-phase. solution.[52-84],.micelles.[85-98],.vesicles.[99-111],.and.gas.bubbles.[112,.113].in. liquid, assembled by surfactants or supramolecules. Since the production cost of many well-defined sacrificial templates is usually high, large-scale synthesis.for.industrial.applications.using.these.methods.has.been.impractical.. Because of this, much effort has been devoted to establishing self-templating or.template-free.methods.of.synthesising.hollow.nanostructures,.where.no.additional.templates.are.needed. 7.2.1.1 Hard-template routesOver. the. past. several. decades,. hard-template-directed. approaches. have.been. the. most. commonly. used. methods. for. the. preparation. of. hollow.nanostructures.. By. coating. the. surface. of. the. template. particles. with. the.desired.materials.and.removing. the. template.by.a.post-treatment,.various.hollow.particles.can.be.easily.obtained..Till.now,.many.colloid.crystals.such.as.
