Designing And Applications Of Multifunctional Materials With Core@shell-like Structure

In recent years, nanoparticles of noble metals are among the most promising catalysts because of their inherently robust physical and chemical properties. They exhibit remarkable catalytic activity and selectivity for many specific reactions in the liquid phase owing to their unique features, such as well-controlled small sizes, specific shapes, and high surface/volume ratio. Amongst them, Ag nanoparticles(Ag NPs) have been one of the most interesting topics in the consideration of its relatively cheap price. However, the common issue in the application of nanoparticles as a catalyst to a liquid-catalysis process, agglomeration and time-consuming separation become unavoidable owing to their small sizes. An effective strategy to prevent aggregation and facilitate handling of the metallic nanoparticles is to immobilize the nanoparticles on all kinds of supports with different shapes, sizes, well-controlled composition and thickness. In particular, emerging magnetic core-shell composite nanospheres, such as mechanically and chemically stable Fe3O4@SiO2 core-shell structure, which can disperse active materials on its surface and thereby yield large surface area of the catalyst and can also realize the fast and high-efficient separation and recovery, thus have been widely utilized as promising scaffolds of noble metal nanoparticles because of their large specific surface area, which is available for the dispersion of nanoparticles, and their controllable magnetic recyclability.In addition, the preparation procedure of nanoparticles always involved in various reagents such as sodium borohydride, hydrazine hydrate, which are hazardous, toxic to both environment and organism. Taking into consideration the development of green chemistry, more and more researchers begin to apply biocompatible polymers or the extract of plant root, stem, and leaf to green synthesize and efficiently stabilize nanoparticles. Noteworthily, chitosan(CS), a naturally occurring, biodegradable, biocompatible, and non-toxic polysaccharide, contains abundant hydroxyl, amino active functional groups, what make it an efficient adsorbent and stabilizer as well as reductant for noble metal ions and nanoparticals and is receiving increasing attention as matrix materials. Thence, we choose CS as the in situ green reductant and stabilizer of Ag NPs in aqueous solution without addition of any other toxic reducing agent or organic solvent.Nonetheless, the incorporation of Fe3O4@SiO2 and CS to form an intact whole is a big deal. Herein, in consideration of the positive electricity of amino-functionalized Fe3O4@SiO2(Fe3O4@SN) and CS, firstly we chose phosphotungstic acid(HPW), a typical polyanion, as an effective bridging agent for the immobilization process through strong electrostatic and hydrogen bonding interactions between Fe3O4@SN and CS. Furthermore, to increase mechanical strength of introduced CS layer, we prefer glutaraldehyde(GLA) as a more effective crosslinker. Then, we used the as-prepared robust support to in situ green reduce and stabilize Ag NPs. In the last, the catalytic performance of resulting magnetic core-shell catalyst, i.e. Fe3O4@SN/HPW@CG-Ag, was drastically investigated by the model reaction of catalytic reduction of 4-nitrophenol(4-NP) in the presence of NaBH4. Meaningfully, the multifunctional composite catalyst exhibited an exceptional catalytic performance(the reaction was accomplished within 7 min) and stability(could be reused at least 10 times) under the optimum preparation conditions such as the addition amount of HPW, GLA, as well as the initial concentration of AgNO3. What’s more, the catalyst can be easily separate and recover by an external magnet due to the existence of magnetic Fe3O4 core. In addition, products at different stage of preparation procedures were detailly characterized by different characterizations techniques.In view of the crosslinking mechanism of CS with GLA, i.e. the Schiff-base reaction between amino groups in CS and aldehyde groups in GLA, and of the amino groups on the surface of Fe3O4@SN, in this work, we attempted to use GLA instead of HPW as bridging agent to covalently accomplish the combination of Fe3O4@SN and CS. In comparison with the work discussed above, this preparation process and composition of supports were simpler, time-saving, and more cost-effective. We also used the new carrier Fe3O4@SN/GLA@CS as reductant and stabilizer to in situ preparate Ag-based nanocatalyst and on the basis of the work aforementioned, we further investigated the effect of reaction time and temperature on the reduction ability of CS and size, size distribution, as well as catalytic property of Ag NPs. Thereafter the optimum catalysts were applied to the catalytic reduction reaction of 4-NP, which demonstrated superior catalytic performance(the reduction reaction was finished within 3 min), reusability(reusable for at least six times), and facile recovery.Combined with the characterization results of these two experiments, we concluded that the reduction ability of CS for the generation of Ag NPs can be ascribable to amino functional groups. On the other hand, given the amino groups on the surface of Fe3O4@SN, we wanted to prove if they can directly reduce and generate Ag NPs under the optimum conditions of preparation process obtained above. Interestingly, as expected, there was more uniform, small size Ag NPs appearance on the surface of Fe3O4@SN. What’s more, the as-synthesized Ag-based core-shell catalyst showed much higher catalytic activity(the reaction was finished within 2 min) and more sensitive magnetic responsibility compared with the two catalysts prepared above owing to the lack of chitosan layer.Magnetic core-shell structure supports with different shells were successfully synthesized through diverse action mechanisms and methodologies. Hereafter, they were used as the green reductants and stabilizers to in situ generate corresponding Ag-based magnetic core-shell nanocatalysts with excellent catalytic performance, stability, reusability along with facile recovery. Therefore, the design philosophy of the multifunctional magnetic core-shell structure carriers and catalysts may pave the way for the synthesis of other cost-effective, green carrier and catalyst systems with long-term stability, easy separation and recovery.