Yolk-shell nanostructures/yolk-shell nanoparticles are defined as a hybrid structure, a mixture of core/shell and hollow particles, where a core particle is encapsulated inside the hollow shell and may move freely inside the shell. Of the various classifications of yolk-shell nanostructures, a structure with an inorganic core and inorganic shell (inorganic/inorganic) has been studied widely due to their unique optical, magnetic, electrical, mechanical, and catalytic properties. In the work presented here, among the different inorganic/inorganic yolk-shell nanostructures noble metal@silica yolk-shell nanostructures has been chosen as the topic of interest. Silica shell possesses many advantages such as chemical inertness, tunable pore sizes, diverse surface morphologies, increasing suspension stability, no reduction in LSPR properties of noble metal nanoparticles when used as a coating for such particles. Noble metal nanoparticles such as AgNPs and AuNPs, on the other hand, possess unique structural, optical, catalytic, and quantum properties. Hence yolk-shell nanostructures with a combination of Ag or Au core and a silica shell (Ag@SiO2 and Au@SiO2) would open to endless possibilities.
In this study, four areas were mainly explored: mechanism of silica shell formation over a given template, the synthetic modifications of Ag@SiO2 and Au@SiO2 yolk-shell nanostructures, their application as a potential catalyst, and devising of a flow type catalytic reactor. Despite the growing number of contributions on the topic of yolk-shell nanostructures, particularly Au@SiO2 and Ag@SiO2 yolk-shell nanostructures, a potential for improvement lies in all four aforementioned areas.
As an initial study, the effect of different processing conditions as well as the mechanism of silica shell formation over reactive block copolymer templates was investigated. An asymmetric PS-b-P4VP block copolymer was chosen as a structure directing component to deposit silica shell. In order to deposit silica shell, PS-b-P4VP micelles with a collapsed PS core and a swollen P4VP corona was prepared via a solvent exchange method. The growth of silica shell over the PS-b-P4VP micelles (reactive template) was done using in-situ DLS and TEM. The experimental data obtained revealed the 4 distinct stages involved in the silica shell formation over the reactive BCP micellar template starting from the accumulation of silica precursor around the P4VP corona followed by a reactive template mediated hydrolysis-condensation reaction of the silica precursor which eventually lead to the shell densification and shell growth around the micelles. An understanding of the mechanism of silica shell formation over reactive templates provides a direct way to encapsulate various active species such as metal nanoparticles and quantum dots and paves the way for the template mediated synthesis of hybrid nanostructures such as yolk-shell nanoparticles. These studies also serve as a platform to fine-tune the properties of such hybrid nanostructures by varying the reaction parameters during silica shell deposition and reaction time.
The next part of the work focused mainly on the synthesis, process optimisation and characterization of Ag@SiO2 and Au@SiO2 yolk-shell nanostructures, and their potential use as a nanocatalyst. A well-known soft template mediated synthesis of the yolk-shell nanostructure was adopted for the present work. For this PS-b-P4VP micelle was used as a dual template for both encapsulation of nanoparticle and the deposition of silica shell. The nanoparticles were entrapped selectively to the BCP micellar core and silica deposition was done by reacting the nanoparticle-loaded micelles with an acidic silica sol which lead to the formation of Ag@PS-b-P4VP@SiO2 or Au@PS-b-P4VP@SiO2 particles with respect to the nanoparticle used. In the case of Ag@PS-b-P4VP particles, upon silica deposition, a partial dissolution of AgNPs was observed whereas AuNPs were stable against dissolution. Hence yolk-shell nanostructures with AuNPs were studied further. As-prepared Au@PS-b-P4VP@SiO2 particles were then subjected to pyrolysis to remove the BCP template. The resulting yolk-shell nanostructures comprised of an AuNP core and a hollow mesoporous silica shell. Upon removal of the BCP template, the Au@SiO2 particles fused together and formed large aggregates. The catalytic properties of Au@SiO2 yolk-shell nanoparticles were explored using a model reaction of reduction of 4-nitrophenol and proved to have good catalytic activity and efficient recyclability. It was observed that catalytic efficiency was hindered by the particle aggregates formed after pyrolysis by creating an inhomogeneity in the system and inaccessibility of the catalytic surface for the reactants. Hence synthetic modifications were needed to overcome such drawbacks.
Next part of the work deals with the synthetic modification of Au@SiO2 yolk-shell nanoparticles done by embedding them in a porous silica structure (PSS). Such structural morphology was attained by gelating the excess silica precursor while synthesising the Au@PS-b-P4VP@SiO2 particles. The pyrolytic removal of block copolymer results in the formation of Au@SiO2@PSS catalyst and the porous nature of both the shell and the silica structure provides an easy access for the reactants to the nanocatalyst surface located inside. The catalytic properties of Au@SiO2@PSS were studied using a model reaction of catalytic reduction of 4-nitrophenol (4-NP) and reductive degradation of different dyes. Kinetic studies show that Au@SiO2@PSS catalyst possesses enhanced catalytic activity as compared to other analogous systems reported in the literature so far. Furthermore, catalytic experiments on the reductive degradation of different dyes show that Au@SiO2@PSS catalyst can be considered as a very promising candidate for wastewater treatment.
Another proposed direction of applying the Au@SiO2 yolk-shells is by devising a continuous flow catalytic system composed of Au@SiO2 yolk-shell nanoparticles for the effective degradation of azo dyes as a promising candidate for wastewater treatment. This was done by infiltrating the Au@PS-b-P4VP@SiO2 particles inside a porous glass substrate (frits) and the subsequent pyrolytic removal of the BCP template resulting in the formation of Au@SiO2 yolk-shell nanostructures sintered inside the frit pores. The flow catalytic reactor was exploited in terms of studying its catalytic activity in the degradation of azo dyes and 4-nitrophenol and proved to have a catalytic efficiency of ca. 99% in terms of reagent conversion and has a long-term stability under flow. Thus, with a few modifications, these flow type systems can open the doors to a very promising continuous flow catalytic reactor in the future.
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa.de:bsz:14-qucosa-232735 |
| Date | 30 January 2018 |
| Creators | Shajkumar, Aruni |
| Contributors | Technische Universität Dresden, Fakultät Mathematik und Naturwissenschaften, Prof. Dr. Manfred Stamm, Prof. Dr. Alexander Eychmüller, Prof. Dr. Manfred Stamm |
| Publisher | Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | English |
| Detected Language | English |
| Type | doc-type:doctoralThesis |
| Format | application/pdf |
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