The Synthesis Of Porous ZnO And Its Photocatalytic Propertiy

Zinc hydroxide carbonate with the smooth surface is synthesized through a facile hydrothermal process using CO(NH2)2and Zn(NO3)2·6H2O. XRD reveal its structure.Hierarchically snowflake-like ZnO with different pores sizes is obtained through calcinating zinc hydroxide carbonate at different temperature (450℃500℃550℃600℃750℃900℃). According to the XRD, ZnO samples got at different calcinating temperature are all hexagonal wurtzite structure. The raman patterns show that it is hexagonal wurtzite structure too. TEM and SEM reveal the existence of pores and the evolution of pores sizes with different calcinating temperature. ZnHC is calcinated at 400℃(Sample 1), the sizes of pores on the petal are mainly distributed in the range of 13-21 nm while calcinated at 450℃(Sample 2) the sizes of pores are mainly distributed in the range of 29-36nm. The pores sizes of ZnO calcinated at 450℃(Sample 2) change a little, comparing with the ZnO calcinated at 400℃(Sample 1). Compared to ZnO calcinated at 400℃and 450℃, the pores sizes of ZnO calcinated at 500℃(Sample 3) and 550℃(Sample 4) have increased dramatically. When ZnHC is calcinated at 500℃and 550℃,the sizes of pores on the petal are roughly estimated that they mainly distributed in the range of 30-50 nm and 60-80nm, respectively. Compared to the pores sizes of ZnO calcinated at 500℃, the pores sizes of ZnO calcinated at 550℃change a little. When ZnHC is calcinated at 600℃(Sample 5) and 750℃(Sample 6), the pores sizes change a little, but the distribution density of pores decreases apparently. When it is calcinated at 900℃(Sample 7), the pores disappear, and it becomes radial branches. The crystal defect is one of the powerful factors that determine the catalytic ability. During the photocatalytic process, defects can become centers that capture photo-induced electrons, the recombination of photo-induced electrons and holes can be effectively inhibited, enhancing the catalytic ability. Small pores come from the thermal decomposition. And large pores come from the recrystallisation. During the recrystallization process, the intervals between the pores collapse, leading to smaller pores interconnect that changes the smaller pores into the irregularly shaped larger pores. With the calcination temperature increasing, the recrystallisation becomes more and more dominant. When ZnHC is calcinated at 750℃, there are almost no pores on the petal of the structures. When ZnHC is calcinated at 900℃,the pores disappear and the hierarchical structures of ZnO turn into radial branches which are composed of nanorod. The intensity of defect photoluminescence peak is comparable to intrinsic photoluminescence peak. That shows the content of oxygen vacancies and defects is very large, that is very helpful to the catalysis. The catalytic ability of samples are tested through the degradation of Rhodamine B using the Xenon Lamp.The catalytic ability of different samples follow the sequence of : Sample 4 > Sample 3 > Sample 2 > Sample 6 > Sample 5 > Sample 1 > Sample 7. catalytic ability of different samples is mainly determined by the absorption intensity of peaks around 375nm, subsequently, the peaks around 273nm or 211nm nm. We think maybe it is mainly the existence of different pore sizes and the distribution intensity that make the absorption ability different then leading to the catalytic ability different. The catalytic ability of Sample 4 is tested. Methyl Orange and Rhodamine B are degraded using Sample 4 as the catalytist in the sunbeam in about 4h and 3.5h respectively.