The Synthesis Of ZnO Nanoparticles And BG/ZnO Composites And Their Photocatalytic Properties

With the development of economy,the environment condition is becoming worse and worse.The treatment of waste water attracts more and more interest.Photocatalytic activity is widely used due to its low cost,non-toxic and environment friendly.When the semiconductor is illustrated by the light can form the photo-generated electron/hole pairs.The photo-generated electron shows reducibility and the photo-generated holes shows oxidability,the holes and electrons can form free radical by combining with other ions or molecules.The free radicals can oxidate organic molecules to non-pollution material.However,in the process of the electron transition,some electrons can recombine with the photo-generated holes,it will decrease the photocatalytic activity.In hence,it’s important to find appreciate photocatalyst,semiconductor ZnO is widely used in this field due to its better environmental friendly and strong photo-stability compared with other materials.However,a major limitation of achieving high photocatalytic efficiency is the quick recombination of photogenerated electron/hole pairs which can greatly reduce the efficiency of photocatalysis.In this thesis ZnO nanoparticles were synthesized by one-pot hydrothermal method and the as-prepared samples were annealed in different atmosphere to regulate the oxygen vacancy.the p-n hetero-junction composites were synthesized by hydrothermal method.The results of this work are summarized as follow:（1）Rod-like ZnO nanoparticles（NPs）had been synthesized by a simple one-step and environment friendly hydrothermal method.Then,the as-prepared samples were annealed in N2,O2 and air at 550 ℃ for 2 hours to regulate the oxygen vacancy.In the process of electronic transition,oxygen vacancy provide trapping sites to significantly increase the separation of photo-generated charge and inhibit the recombination of photo-induced electron/hole pairs for the degradation of organic dyes.Whereas,too much oxygen defects will trap photo-generated electrons,which can combine with the VB holes and decrease the photocatalytic activity.The right ratio of oxygen vacancies can improve the photocatalytic activity.The X-ray diffraction（XRD）,scanning electron microscopy（SEM）,X-ray photoelectron spectroscopy（XPS）and ultraviolet-visible（UV-vis）were used to characterized the crystallinity,morphology and the correlation between oxygen vacancy content and the photocatalytic activity.These characterization results reveal that anneal way will adjust absorption range and oxygen vacancy quantity to improve photocatalytic activity.When the illumination time was extended to 40 min,the efficiency of photodegradation reached up to 95.6%for ZnO NPs annealed in air,89.3%for non-annealed ZnO NPs,62.9%for ZnO NPs annealed in O2,and 61.0%for ZnO NPs annealed in N2.The difference of photodegradation efficiency might be due to the different amount of oxygen vacancies in the ZnO samples.Therefore,enhanced degradation efficiency arises from a modest content of oxygen vacancy.（2）Graphene oxide is synthesized by advanced Hummer’s method and the r-GO/ZnO composites were synthesized by hydrothermal method at 2-6 hours and 100^-180 ℃,the as prepared samples were used as photocatalyst to degrade MB organic dye.The results showed that the sample synthesized at 4 hours,120℃ shows better degradation efficiency,when illustrated with 60 minutes,the efficiency is up to 95%.In hence,the best condition is at at 4hours,120℃.（3）In this text,we report the synthesis of p-type boron-doped graphene（BG）by reduction of graphene oxide（GO）with borane-tetrahydrofuran（BH3-THF）adduct under reflux.To investigate the effect of different boron percentage of boron-doped graphene,the Fourier-transform infrared（FT-IR）and X-ray photoelectron spectroscopy（XPS）were applied.The XPS results of boron-doped graphene confirm the existence of boron in doped graphene,which indicates the boron atoms doped in the graphene lattice.Under the condition,the BG/ZnO p-n hetero-junction composites were synthesized and the p-n hetero-junction can increase the separation of photo-generated charge and inhibit the recombination of photo-induced electron/hole pairs to improve degrade efficiency.Then the composites were used as photocatalyst.The results showed that the p-n hetero-junction composites showed superior degradation efficiency(k=76.2×10^-3 min^-1)than the r-GO/ZnO(k=24.3×10^-3 min^-1)composites and pure ZnO(k=17.1 × 10^-3 min^-1).