Morpology, Structure, Photocatalysis Of Hexagonal Column-like ZnO By Hydrothermal Method

ZnO semiconductor material shows great promising applications for well photocatalytic property. It has been demonstrated that the properties of ZnO crystal is related to its surface morphology, size and deficienly and so on. Therefore more attentions have been paid to the relationship between the morphologies of ZnO crystal and its photocatalytic activity (PA).Hydrothermal method was adopted to prepare ZnO crystallites with different Zinc salts and alkali liquor in this paper. The influence of reaction conditions on morphogies of ZnO crystallites and mechanism of ZnO crystal growth were researched. The photocatalytic properties of sample were researched through the photodegradation experiment of the methyl orange (MO), in order to discuss the relationship between the morphology, crystallinity, plane of ZnO crystallites and photocatalytic activity. RE-doped ZnO crystallites of well crystallized hexagonal column-like were prepared by hydrothermal method. The influence of doped ion and content of RE-doping on photocatalytic activity were researched. Meanwhile, ZnO micro-rods were systhesized onto slide glass by hydrothermal method. XRD, SEM, UV spectrophotometer and UV-Vis were used to analyze the crystal phase, microstructure, absorption specrum, photocatalytic property.The morphology change of ZnO prepared from Zn(AC)2·2H2O-NaOH reaction system at different ratios of OH-/Zn2+, reaction temperature and reaction time was discussed with the experiment of orthogonal design. The results showed that undecomposed zinc carbonate existed in the ZnO productors under low reaction temperature, when the temperature was raised to 160℃, zinc carbonate was decomposed. The effect of ratio of OH-/Zn+on the aspect ratio of ZnO crystallites was that when the ratio of OH-/Zn2+ was increased, the aspect ratio of ZnO was decreased. Column-like ZnO crystallites were prepared when the ratio of OH-/Zn2+ was 1:1, while flake-like ZnO crystallites were prepared when the ratio of OH-/Zn2+ was 3:1. The best crystal morphology of sample L10 was corresponding to the the ratio of OH-/Zn2+ was 1:1, the reation temperature was at 160℃and holding time was for 10h. The growth mechanism of ZnO crystallite in this reaction system may be explained according to the growth unit model of anion coordination polyhedra. Zn(OH)2 colloids are formed by hydrolysis of zinc acetate and sodium hydroxide. During the subsequent hydrothermal process, partial Zn(OH)2 colloids are transformed to ZnO crystal nucleis and [Zn(OH)4]2- growth units. Since the effect of hydroxyl ion on the structure of growth unites and boundary character of ZnO crystallite, growth units orientated grow on ZnO crystal nuclei with the conditions of different ratios of OH-/Zn2+, which results in forming different morphologies of ZnO crystallites.The influence factors on the crystallinity of ZnO crystallites were investigated with the experiment of orthogonal design in Zn(NO3)2·6H2O-KOH reaction system. The resulted show that the influence degree of crystallinity was as follows:the ratios of OH-/Zn2+>reation time> reation temperature. The best crystallinity of sample S10 was 91.2%, corresponding to the ratio of OH-/Zn2+ was 1:1, the holding time was for 15h, and the reation temperature was at 160℃. The result of SEM showed that the morphology of crystallites depended strongly on the ratio of OH-/Zn2+, the flake-like ZnO crystallized in strong basic solutions had samller size than those which were resulted from the process carried out in neutral or weak basic solution, and the shape of the later was column-like. The growth mechanism of ZnO crystallite in this reaction system can be explained according to the "growth condition-cystal structure-crystal morphology". Zn(OH)2 colloids are formed by hydrolysis of zinc acetate and potassium hydroxide. During the subsequent hydrothermal process, partial Zn(OH)2 colloids are transformed to ZnO crystal nuclei and [Zn(OH)4]2- growth units. Growth units orientated grow on ZnO crystal nuclei with different reaction conditions, which results in forming different morphologies of ZnO crystallites.In Zn(CHCOO)2-2H2O-HMTA reation system, the effects of the different reaction conditions to the morphology of ZnO crystallites and mechanism of ZnO crystal growth were discussed. The results showed that ZnO morphology and crystal quality were influenced by concentration and pH value of reaction liquid, the reaction temperature and time. The good crystallinity of ZnO corresponding to concentration and pH of solution were 0.57mol/L and 6.0, the temperature was at 97℃, and the holding time was for 16h. The growth mechanism of the hexagonal column-like ZnO crystallite is that the ZnO crystal nucleuses are formed by hydrolysis of zinc acetate and hexamethylenetetramine under appropriate hydrothermal condition. Since the (0001) face has faster growth rate than the other faces and the (0001) face can easily capture atoms, ZnO crystsl nucleus grow along the (0001) direction, which results in forming hexagonal column-like structure finally.The results of photodegradative experiments of methyl orange in different reation systems showed that the photodegradative rate of flake-like sample L2 was 84.27% which was higher than column-like ZnO samples, the first-order kinetic degradation rate (k1) of sample L2 was 8.1×10-3. The photodegration rate was increased when the crystallinity and roughness of samples increased, and when (0001) exposed, the photodegration rate of sample was higher, owing to the surface energy of the (0001) was higher than other planes. The mechanism is that the (0001) plane with Zn-Zn monotonous atomic outermost layer and the coordinatively unsaturated environment of Zn ions on the (0001) plane makes the exposed (0001) plane of ZnO crystallites have higher photocatalytic activity for the facile adsorption of oxygen molecules and hydroxyl ion on this plane as a result of the superoxide anion (·O2-) and hydroxyl radicals (·OH) generation.The photodegration of ZnO samples in Zn(CHCOO)2-2H2O-HMTA reation system were lower than other system, because the size of samples was bigger and the crystal surface was more smooth, the absorbability of MO was less than others, The results also indicatied that when the catalyst dose was 0.1g/100mL, the initial concentration of MO was 15mg/L, the pH value was 10.0, the efficiency of photocatalysis performance was the highest.The RE-doped ZnO crystallites were synthesized by hydrotherm method in order to increase the photocatalysis activity of sample H2 which was well-crystallized. The effect of contents of RE ions on the photocatalysis activity was that, when the dosage was 0.2mol%Yb3+, photocatalytic degradation rate achieved highest, and the k1 was 2.9×10-3, which the degradation rate raised 27.21%. The mechanism is that the Yb3+-doped sample D5 can widen the wavelength absorption range and decrease the band gap of ZnO crystallite, which leads to increase the amount of electron hole pair. In addition, the morphology of sample D5 is rougher than other samples which can degrade more methyl orange.ZnO hexagonal column-like crystallites were synthesized by hydrothermal method with zinc acetate and hexamethylenetetramine as raw materials. The growth mechanism showed that crystal morphologies resulted from ZnO in solution surrounding, which strongly depended on concentration and pH value of solution. The growth mechanism is that Zn+-aminocomplex [Zn(NH3)4]2+ ions which are formed by hydrolysis of hexamethylenetetramine can transform to ZnO during hydrothermal process. The aspect ratio of ZnO crystallites decreases in the higher concentration of reaction liquid because of the competition between nucleation and crystal growth, the acetate ions from the raw material can be absorbed on the (0001) plane, which reduces the growth velocity of (0001) plane. Ammonia can involve in the reaction to generate precursor Zn(NH3)4(OH)2 of ZnO crystallites when using Ammonia water as the pH adjustment, in addition, ammonium ions which are generated by hydrolysis of hexamethylenetetramine can absorb on the surface of ZnO crystal nucleus result in self-assembly and twinning ZnO crystallites.