Studies On The Activities Of Antibacterium And Organic Pollutant Decomposition Of Zinc Oxides

Low-dimensional structure of ZnO is a hot research topic in the field of functional materials in recent years. Because of the good biocompatibility and environmental safety, ZnO-based long-term antibacterial and environmental purification materials have attracted lots of attentions of academia and industry. Antibacterial activity and photocatalytical activity of ZnO are closely related to the production of reactive oxygen species, such as hydroxyl radical (·OH), superoxide anion (·O2-) and hydroxyl peroxide (H2O2). However, the mechanism and affecting factors of generation of the reactive oxygen species in ZnO, which are distinctly the bases for adjusting its functional activity, are still lacking of systematically study. In this dissertation, we comparatively studied the productions and affecting mechanisms of reactive oxygen species, and related antibacterial and/or photocatalytical properties of selected three types of ZnO, and then proposed a facile approach to improve the antibacterial activity.Firstly, specific reactions of the reactive oxygen species were reviewed and comparatively analyzed. Considering the requirements in specificity, sensitivity and operability of the detecting methods in this study, electron spin resonance was chosen for qualitatively study of the free radicals such as·OH and·O2-. Then, specific methods for quantificationally detecting the reactive oxygen species in ZnO suspensions were developed respectively. The typical measurements included terephthalic acid hydroxylation methods combining with fluorescence analysis (TAH-F) for·OH, nitro blue tetrazolium reduction method collaborating with spectrophotometry analysis (NBTR-S) for·O2-, and starch iodometric method incorporating with spectrophotometry analysis (SIM-S) for H2O2. The results indicated that, without light irradiation, there were no·OH or·O2-but H2O2generation in ZnO suspension; while under simulated sunlight,·OH,·O2-and H2O2all generated in ZnO suspension. The cumulative productions of·OH and·O2-both increased with time under simulated sunlight. The cumulative production of H2O2increased along with the extension of time either in dark or under simulated sunlight, and the yield of H2O2under simulated sunlight was higher than that in in dark. The production of·OH,·O2-and H2O2of the three kinds of ZnO, which are tetrapod-like ZnO whisker (T-ZnO), nano-sized ZnO (n-ZnO) and micro-sized ZnO (m-ZnO), were comparatively tested in this work. The results showed that the activity forH production were T-ZnO, m-ZnO and n-ZnO in descending order, the activity for·O2-production were m-ZnO, T-ZnO and n-ZnO in descending order, and the activity for H2O2production were T-ZnO, n-ZnO and m-ZnO in descending order. By analyzing the material characteristics of the three typs of ZnO, it was found that the specific surface area was not the main factor on the production of·OH,·O2-and H2O2, and lattice distortion had a negative effect on the production of·OH and·O2-. Further investigations showed that oxygen vacancies in skin layers of the crystal played an important role in the formation of H2O2, and having a positive effect on the production of·OH and negative effect on·O2-.The antibacterial properties of ZnO with different morphologies under simulated sunlight and in dark were respectively evaluated with Escherichia coli (E. coli) by zone of inhibition and bactericidal rate. The results indicated that T-ZnO, n-ZnO and m-ZnO all exhibited antibacterial effect against E. coli both under simulated sunlight and in dark, and the antibacterial activity under simulated sunlight condition was higher than that in dark. In the same condition, the antibacterial activity displayed differences among the three types of ZnO, which was sorted in descending order as T-ZnO, n-ZnO and m-ZnO either under simulated sunlight or in dark. Results of Zn2+dissolution and reactive oxygen species productions indicated that the H2O2generating was the main mechanism for antibacterial activity of ZnO. Furthermore, the effects of O2in atmosphere and oxygen vacancy in the skin layer of ZnO crystal on the production of H2O2were studied, and the results indicated that they both had significant role on H2O2formation. Based on the experimental studies, a method for adjusting the antibacterial activity by controlling oxygen vacancies concentration was preliminarily advised.The activity of ZnO for decomposing organic contamination was evaluated using methyl orange as the substrate. The results showed that the concentration of methyl orange in ZnO suspensions decreased with time under simulated sunlight condition. The degradation rate of methyl orange in the aqueous suspensions displayed differences among the three types of ZnO, which was sorted in descending order as m-ZnO, T-ZnO, n-ZnO. Comparing the production of reactive oxygen species with the degradation rate of methyl orange, we concluded that the degradation of methyl orange in ZnO suspension was not consistent with the production of-OH, but showed the same order as the production of·O2-, which indicating that the degradation of methyl orange was not based on the·OH oxidation mechanism, but most likely based on the direct photo-generated hole oxidation mechanism.Copper nanoparticles were uniformly deposited onto the surface of T-ZnO by in-situ decomposition of cupric tartrate. The productions of·OH, O2-and H2O2were determined. The results showed that the production of-OH did not change significantly. But, the production of·O2-decreased obviously, which indicated that Schottky barrier was formed and effective in trapping electrons. The production of H2O2in dark did obviously changed, but it decreased under simulated sunlight. The antibacterial activity in dark of the modified T-ZnO increased with the increase of loading amount of copper nanoparticles. The value of minimum inhibitory concentration (MIC) of the deposit-modified sample with1.0mol%of copper nanoparticles was125mg/L, which was significantly lower than the origianl T-ZnO of500mg/L. Further studies indicated that the improvement of antibacterial activity of T-ZnO after deposition of copper nanoparticles was attributed neither to increasing H2O2production nor to releasing Cu2+, but may be derived from the synergistic antibacterial effect between Cu and ZnO. Afte deposited with Cu nanoparticles, the activity of T-ZnO for decomposing methyl orange did not change significantly.Cu2+doped ZnO was prepared by means of solid solution treatment. The change of lattice constants, the redshift of light absorption threshold and the offset of binding energy between Zn and O of the doped ZnO were investigated, and the results indicated that Cu2+had been successfully doped into the lattice of ZnO and the impurity level had been successfully introduced into ZnO. The productions of·OH,·O2-and H2O2all significantly decreased after doping. The similar level of decrease of·OH and·O2-illustrated that the doped Cu2+did not play the role in separating photogenerated electrons and holes in ZnO, but acted as recombination centers of the photo-induced electrons and holes. The activity of ZnO for decomposing methyl orange also decreased after Cu2+-doping, which was consistent with the measurements of reactive oxygen species. The antibacterial activity of ZnO decreased after doping, however, for the doped samples, the antibacterial activity increased with increase of doping concentration of Cu2+.Finally, the application of ZnO as antibacterial materials for manned spacecraft was pre-explored. Antibacterial coatings were prepared and coated on the surfaces of aluminum alloys, bulkhead cloth and curtain cloth by spraying polyurethane (PU) varnish mixed with modified T-ZnO. Scanning electron microscopy (SEM) observations showed that T-ZnO was uniformly distributed on the surfaces, and the tetrapod-like structure was preserved. Polypropylene (PP) composites filled with modified T-ZnO were prepared by twin-screw extrusion and molding method. SEM analyses demonstrated that the tetrapod-like structure of T-ZnO was destroyed and mainly instead by single spicules or fragments existing in PP. A lot of ZnO needles exposed on the surfaces of the mentioned four kinds of materials, which could ensure contacting with bacteria and playing activity. The antibacteriral rates of PU coating and PP composite with4wt%T-ZnO against E. coli were all higher than98%. The effect of adding amount of T-ZnO onantibacteriral rate was studied with PP, and the result indicated that antibacteriral rates increased with the loading of T-ZnO increasing. The antibacteriral rate could get96%when T-ZnO was2wt%in the composites, which met the requirement of use as antimicrobial products.