| In recent years,silver nanoparticles(Ag NPs,smaller than 10 nm)have received great attentions because of their unique photochemical,antibacterial and catalytic activities.The reported preparation method of Ag NPs is mainly the reduction method,by which the silver ions become Ag NPs.The reduction methods include chemical reduction and physical reduction.A variety of templates are used to precisely control the size,morphology and microstructure of Ag NPs during the reduction process.Based on our previous work about supramolecular gel,surface modified Ag NPs with small and uniform particle size have been prepared by using supramolecular gel as template.Due to their large specific surface area,however,the agglomeration problem of Ag NPs seriously restricts their applications.To overcome this problem,Ag NPs are usually loaded or dispersed in a solid matrix to avoid particle aggregation.Additionally,the solid matrix loading with Ag NPs can be easily separated from the reaction system for reuse when they are used as catalysts.Among the solid matrices,halloysite nanotubes(HNTs)play an important role due to their special nanotube morphology.HNTs are a kind of aluminosilicate clay with hollow cylinder structures.The inner and the outer diameter of HNTs range from 10 to 15 nm and 50 to 70 nm,respectively.Generally,the Ag/HNTs composites have better structural stability if the Ag NPs load into the lumen of HNTs.However,there are many problems for conventional way to load Ag NPs into the lumen of HNTs,such as complicated process,harsh conditions and so on.In addition,it is difficult for the conventional chemical reduction method to control the size of Ag NPs.Therefore,it remains a challenge to find a mild and simple loading method.Due to the different chemical structure,the outer wall of the HNTs is negatively charged while its inner wall is positively charged.Based on this unique character of HNTs,electrostatic interactions are used to selectively load Ag NPs or other chemicals.In this thesis,Ag NPs with different charge and particle size were prepared.As a reference,Au and Fe3O4 nanoparticles were also were prepared,too.Nanoparticles are easily loaded on halloysite by simple electrostatic interactions.The position of nanoparticles on HNTs can be controlled through the interactions between nanoparticles and HNTs.Furthermore,the Ag/HNTs composites were used as catalysts to catalyze the reduction of 4-nitrophenol(4-NP)by sodium borohydride,exhibiting good catalytic activities and good reusablities.The main content of this thesis is as follows:1.The research on the preparation of noble metal nanoparticles based on supermolecular gel is summarized.This method is also favorable to prepare noble metal nanoparticles with small and uniform size.Subsequently,as a kind of excellent material carrier,HNTs are introduced,including its unique structure,and its physical and chemical properties.At last,the surface modification technology of HNTs and the progress of the loading and application of the metal nanoparticles on HNTs are reviewed in detail.2.NAC-Ag supramolecular gel was prepared by the N-Acetyl-L-cysteine(NAC)and silver nitrate.NAC-modified Ag NPs were prepared by the UV-light reduction of NAC-Ag gel.The particle size of Ag NPs ranges from 1~5 nm and the average diameter is 2.7 nm.NAC-Ag NPs are negatively charged and have good dispersity in water.Compared with Ag NPs prepared by chemical reduction method,the NAC-Ag NPs obtained by UV-light reduction of supramolecular gel have small and uniform particle size,and possess good stability in water(stable storage for 6 months).3.Since the lumen of HNTs is positively charged,NAC-modified Ag NPs with negative charge may spontaneously enter into the lumen of HNT via the electrostatic interactions.Ag NPs are incompactly distributed in the lumen of halloysite without obvious aggregation.The catalytic activity of the obtained Ag-in-HNTs composites were evaluated by the reduction of 4-NP as a model reaction.When the molar ratio of Ag and 4-NP was set at 0.008,the rate constant of the reaction was found to be 0.91 min-1,indicating a good catalytic activity of Ag-in-HNTs composites.Since NAC-Ag NPs enter into the lumen of halloysite through electrostatic interactions,the loading process is irreversible so that the formed Ag-in-HNTs composites show a good structural stability.Additionally,no Ag NPs were found in the supernatant after the Ag NPs-in-HNTs suspension was stirred for 30 min,which suggests that the composites as a catalyst exhibit good reusability due to the stable loading of Ag NPs in the lumen of HNTs.4.Depending on the surface charge of the metal nanoparticles,their position on the HNTs can be controlled via electrostatic interactions,resulting in a particular structure of the composites.Noble metal nanoparticles(Au,Ag)and Fe3O4 nanoparticles with negative charge spontaneously enter into the lumen of HNT with positive charge via the electrostatic interactions,as long as the particle size is smaller than the inner diameter of HNTs.Metal nanoparticles with positive charge can only stay on the outer surface of HNTs.Even if the particle size is small enough,they can not yet enter the HNTs lumen due to the electrostatic repulsion.5.Metal nanoparticles can be immobilized on the outer surface of HNTs modified with silane coupling agent via the coordination interaction with the amino group on the surface.Meanwhile,metal nanoparticles with negative charge and small particle size are still possible to enter into the lumen of HNTs.It shows that there is a competition between coordination interactions and electrostatic interactions during the loading process.Due to the steric hindrance effect and strong force of the coordination bond,the coordination interactions play a leading role.Furthermore,HNTs modified by silane coupling agent did not change the positive charge state of the lumen of HNTs.If metal nanoparticles with negative charge are small enough,they can enter the lumen of HNTs by controlling the amount of nanoparticles dispersion.Taking full advantage of electrostatic interactions and coordination interactions,high density loading of metal nanoparticles inside and outside of HNTs can be achieved.6.HNTs was loaded with both magnetic Fe3O4 nanoparticles and Ag NPs,then the resulting Ag/Fe3O4/HNTs composites were used to catalyze the 4-NP reduction reaction.It shows that the composites not only maintain good catalytic activity,but also have good recycling ability.After seven recycles,the conversion rate of 4-NP reduction is still as high as 98.9%. |
