Template Effect Of Bacterial Cellulose And Controlling Preparation Of ZnO Photocatalytic Nanocompsite

The research of immobilization technique of nano semiconductor photocatalyst powder such like ZnO shows high value in reality, for great advantages of high photocatalytic efficiency, stablility, no secondary pollution, recycle, recovery and free of agglomeration. One effective approach is the immobilization of photocatalyst through hybridization with an organic template, such as cellulosic fibers, to obtain great performance with good dispersion and stabilization.As biosynthesized cellulose, bacterial cellulose (BC) is ex-cellar product of the gram-negative bacteria Acetobacter xylinum. BC fiber has a nanoporous structure, certain nano-pore size distribution and large quantity of nanopores which could be used as "biotemplate" with hydroxyl groups along BC nanofibers as reactive sits. Therefore, BC template could be used as a matrix and combined with other polymers, organic or inorganic molecules to synthesize functional nano-composites with specific shape and size. Thus the research of surface structure properties and template effects of BC has significant meaning in broadening the application fields of functional nanocomposites.In this thesis, the influence of surface pretreatments and chemical structural modification of BC were studied. The surface properties and relationships between structure and performance of BC nanofibers were also investigated. Then BC and the amidoximated bacterial cellulose (AmBC) were chosen as templates for synthesizing zinc oxide nanocomposites through a facile in situ polyol reduction method. Moreover, the formation mechanism of nanoparticles and the template effects were studied. 1. Surface pretreatments, chemical modification and template effect of BCUltrasound irradiation, alkali swelling, and ethanol solvent exchange were applied to treat bacterial cellulose (BC) as surface pretreatments to modify the reactivity of hydroxyl groups. The results showed that compared with that BC hydrogel the pretreatments could improve its adsorption behavior of Zn2+ from 14.8 mg/g to 21.3,20.1 and 16.3 mg/g, respectively. The crystalline structure of BC was changed from cellulose I to cellulose II by alkali swelling according to X-ray diffraction and the as-obtained alkali cellulose was beneficial to the adsorption of metal ions. Ultrasound irradiation and ethanol solvent exchange didn't change the crystal form.BC was facilely amidoximated under moderate condition and the results revealed that amidoximation was an effective chemical modification of the BC hydroxyl groups to enhance the interactions between guest metal ions and host cellulose fiber while preserving the microfibrillar morphology. The absorption capacity for Zn2+ onto AmBC was improved to 71 mg/g at saturation. It is clear that amidoximation is a useful modification method which could serve as a better host matrix and shows better template effect than surface pretreatments. The as-prepared AmBC membrane exhibits better thermal stability and further investigation such as FTIR, XRD demonstrated the amidoxime group was successfully grafted onto BC hydrogel. The value of the degree of substitution and the nitrogen content of AmBC was 0.3403 and 5.78%.BC was used to fabricated BC/SiO2 nanocomposites through the in stiu hydrolysis by tetraethoxysilane(TEOS) and by removing the template, the SiO2 nanofibers could be obtained easily. The results revealed that the large quantity of reactive hydroxyl groups could control the growth of SiO2 particles along BC fiber by hydrogen bonding or condensation with TEOS. The concentration of TEOS was crucial to effectively control and change the morphology of the SiO2 nanofibers from granular packs, chain structure to nanofibers with smooth surface.2. Zinc oxide nanocomposites with a controlled shape and size have been successfully synthesized through a facile in situ polyol reduction method using BC and AmBC as templates. The structure and morphology of composites were characterized and compared by FESEM, XRD, FTIR, TGA, the BET surface area, the pore size, etc to evaluate different template effects and eventual changes.As for BC/ZnO composites, the size and morphology of ZnO particles are greatly affected by the hydrolytic time and concentration of zinc acetate, in which optimum ZnO nanoparticles with a wurtzite crystal structure and particle size of 55nm could be formed uniformly after 10 min hydrolysis of 0.5 wt% Zn(CH3COO)2. The BET surface area was found to be 101.34 m2/g and the maximum photocatalytic decolorization efficiency was 70% at 120min. Due to the reinforcement of template effect, the as-obtained AmBC/ZnO composites was proved to a larger amount of wurtzite ZnO nanoparticles and better photocatalytic activity. The nanocomposite synthesized at 0.05wt% Zn2+ displays the maximum decolorization efficiency of 91% at 120 min. The TG results indicated that the composite ZnO content had little relations with zinc acetate concentration and was about 21%.