Study Of Morphologically Controllable Synthesis And Their Properties Of ZnO Nanocrystals

As a new type multifunction inorganic material,nano-ZnO has wide application prospect in many fields,especially in photocatalytic and antibacterial materials which are closely correlated with the survival and health of human beings.How to combine the excellent photocatalytic activity with antibacterial performance of ZnO nanocrystals is still one of the hot-pursued topics in the field of its application.While the photocatalytic activity and antibacterial capability of ZnO has been explored in early work,its effectiveness in environmental purification and antibacterial materials is yet to be explored fully in our country.Morphologically controllable synthsizing,doping other ion into oxide matrix and adsorbing metal on the surface to form heterostructure,which play an important role in modifying the physical and chemical properties of mother oxide,for example defect concentration and particle size,at a certain extent.Based on this point,this paper aims to improve the photocatalytic and antibacterial properties of nano-ZnO through morphologically controllable synthesizing,doping with metal ion,which radius is close to Zn²⁺ ion and forming heterostructure with metal.Firstly,the research progress of ZnO in environment purification and antibacterial materials,and the studies of nano-ZnO preparation with special morphology were introduced in brief.Subsequently,ZnO nano-powders with special morphology,doped ZnO nano-powders and metal-ZnO(MZ) heterostructure were synthesized by direct precipitation and sol method.The influences of preparation technology,doping,adsorbing metal on the structure and morphology of as-synthesized nano-powders were also investigated.Finally,the photocatalytic and antibacterial properties of nano-ZnO powders with different morphologies and metal-ZnO (MZ) heterostructure were studied in detail.On the basis of experimental results,the mechanism for metal-ZnO(MZ) heterostructure to improve properties of nano-ZnO was primarily discussed.This work mainly includes the following aspects:ZnO nanocrystals were morphologically controllable synthesized through direct precipitation.It has been found that three different shapes of ZnO nanocrystals were easily obtained through direct precipitation meathod.Furthermore,the effect of reaction temperature,PH,the anion of reactant and dispering agent on their morphologies has also been discussed.By using the sodium hydroxide as precipitation agent,rod-like nano-ZnO powders can be obtained.Because(0001) plane of wurtzite ZnO crystal grows fast than other planes in ZnO crystal,with the increase of reaction temperature from 60℃to 90℃,the morphology of ZnO changed from short rod to long rod.It can also be found that the ratio of length to diameter varied from 6.5:1(prepared from Zn(Ac)₂·2H₂O) to 5:1(prepared from ZnSO₄·7H₂O) and then 3.5:1(prepared from Zn(NO)₃·6H₂O),finally to 2:1(prepared from ZnCl₂).It was supposed that the(0001) plane of ZnO crystal was restrained by anion of the zinc salt and the velocity of inhibition was as followed:CH₃CO₂^-＜SO₄^2-＜NO₃^-＜Cl^-. Accordingly,the morphology is changed from nanopricker to nanorod and then to bamboo shoot-like and finally to stubby nanocrystals.With the increase of PH value,the morphology of ZnO changed from ball-like to rod-like and then finally to star-like.By using the ammonia as precipitation agent,flower-like nano-ZnO powders can also be achieved.The influence of annealing process on the photoluminescence property was also studied.The result shows that photoluminescence peak was nearly quenched after annealing the flower-like sample at 600℃for 3 hours in air,which means the sample has little defects in their crystals after annealing them in air..Furthermore,the diffraction peak was much sharper,and the intensity of peak was much stronger after annealing the flower-like sample in air,which indicates the annealing process increase crystallization and decrease the specific surface area of the sample.Thus,if made the specific surface area of the sample unchangeable,the photocatalytic property of the sample can be improved by annealing process.However antibacterial property of the sample became weak after annealing process.Furthermore,with the increase PH value,flower-like micro-ZnO powders were obtained.The flake-like nano-ZnO powders can also be achieved by affiliating the citrate sodium(NaCA) into the solution and using the sodium hydroxide as precipitation agent. The result shows that citrate may be adsorbed preferably on the(0001) crystal plane of ZnO and the growth along this facet is therefore considerably restricted to produce flakes.With the increase of reaction temperature,the flake-like ZnO powders has grown from nanocrystals to microcrystals.The influence of three different morphologies on the photoluminescence properties was also studied.It was found that flake-like nano-ZnO powders have less defects among them.Because the flake-like ZnO powders was smallest among them,the photocatalytic activity or antibacterial properties of flake-like nano-ZnO powders might be better. However,adding the dispering agent(citrate sodium) into the reaction process will decrease the specific surface area of the flake-like ZnO sample,which was harmful to improve their photocatalytic and antibacterial properties.ZnO sol was synthesized in different solvent by sol way.It was found that the diameter of nanocrystals prepared in mellow solvents were samller and stablable than those prepared in water.In order to synthesize ZnO sol in water,the dispering agent was needed in the preparing process,which leads to decrease the specific surface area of the sample.Thus,in the following part,the preparing process without adding the dispering agent was chose to synthesize the ZnO sample.Furthermore,the absorption peak of ZnO sol in ethanol was more prominent than others made from other mellow solvents in UV curves.It means that the semiconducting character of the samples synthesized in ethanol was more obvious than others made from other mellow solvents.Furthermore the ethanol was nontoxic.Thus, ethanol will be chosed as the appropriate solvent to prepare the ZnO sol or their complex.Because of larger specific surface and smaller particle size,the photocatalytic activity of ZnO nanorods prepared was better than flower-like ZnO samples,which were prepared by direct precipitation method.However,the efficiency of degradation of ZnO nanorods prepared by direct precipitation method was dramatically lower than the powders prepared by sol method because ZnO nanorods were larger than the powders prepared by sol method. The results of the photocatalytic degradation test indicate that the ZnO powders,which prepared by sol method,have eximious photocatalytic activity.Under UV light irradiation for 5 h,92%of degradation of methyl orange aqueous solution can be obtained.However, the excellent performance cann't be lasted because of reuniting among the nano-ZnO powders prepared by sol method and the degradation rate has changed from 92%to 50% after drying the powders at room temperature for 24 h.MIC test shows that ZnO nanocrystals prepared by sol method and by direct direct precipitation method has the same MIC value for Escherichia,although the powders prepared by sol method is samller than that prepared by direct precipitation method. Because small ZnO particle is easy to growth without adding dispering agent,the antibacterial performance of ZnO powders synthesized by sol method is nearly same to that prepred by direct precipitation method.On the basis of sol method,Cu-doped nano-ZnO powders were also prepared to improve the photocatalytic activity and antibacterial properties of pure ZnO powders.It has been found that UV absorption curves from Cu-doped ZnO can be tunable in a range from 325 nm to 345 nm through Cu doping.However,only the sun-light in UV range can be absorbed by the Cu-doped ZnO nanocrystals.Thus,Cu-doping in ZnO mightn't be the appropriate candidate for improving photocatalytic activity of pure ZnO powders.In particular,at low concentration of Cu(＜5%),the intensity of the visible peak dramatically increases with increasing Cu concentration,which means Cu low-doping Cu ion can introduce defects in ZnO crystal;At high concentration of Cu(＞7.5%),the impurity peaks were observed and the main peaks of ZnO were nearly quenched in XRD curves,which means that high-doped Cu ion into ZnO crystal lattice using this method cann't be achieved. Furthermore,Cu_xO has poor antibacterial performance in oxide.Therefore,it can be deduced that this doping method wasn't suitable to improve their photocatalytic and antibacterial properties,which leaded to the increasing of defects in ZnO samples.In order to improve photocatalytic activity and antibacterial performance of the obtained nano-ZnO powders,nano-silver was absorbed on the surface of the powders to form Ag-ZnO nanoheterostructure through sol method and direct precipitation,respectively. The results of the photocatalytic degradation test of as-synthesized Ag-ZnO nanoheterostructure by sol method indicate that its efficiency can be greatly improved by depositing appropriate amount of silver.And when the percent of silver is about 0.4%, Ag-ZnO nanoheterostructure have the best photocatalytic property and its photocatalytic rate is about 85%under sunlight irradiation.Furthermore,all samples prepared by sol method have higher photocatalytic efficiency than P25 under the same condition.However, the result of the photocatalytic degradation rate of Ag-ZnO nanoheterostructure prepared by direct precipitation shows that the highest rate was 30%under UV irradiation condition. Furthermore,the photocatalytic degradation rate of Ag-ZnO nanoheterostructure prepared by direct precipitation decreased initially and then increased with increasing Ag content. The value of the MIC for Escherichia of Ag/ZnO nanoheterostructures synthesized by sol method and direct precipitation method confirms from 50 to 6.25 ppm.Because the Ag/ZnO nanoheterostructures synthesized by sol method is smaller and has less defects,its antibacterial property was better than the powders synthesized by direct precipitation under the same Ag concentration.It can be found that the photocatalytic activity of nano-ZnO powders prepared by direct precipitation method decreased with increasing of AgNO₃ concentration.On the contrary,their antibacterial properties increased with increasing of AgNO₃ concentration. However,the photocatalytic activity of nano-ZnO powders prepared by sol method increased with increasing of AgNO₃ concentration,while their antibacterial properties also increased with increasing of AgNO₃ concentration.The singular behavior should be caused by different AgNO₃ consumption process under different method.In direct precipitation method,when AgNO₃ adding into the Zn salts solution drop by drop,AgNO₃ has partly converted to Ag and the other part of Ag⁺was introduced ZnO crystals.At low AgNO₃ adding concentration,Ag⁺-doping was the main process at reaction condition and then the defect concentration increased with Ag⁺-doping in ZnO crystals.At high AgNO₃ adding concentration,Ag⁺ converting to Ag was the main process at reaction condition and the defect concentration decreased.While in sol method,AgNO₃ can completely convert to Ag if sunlight irradiation time is long enough.Because Ag has excellent antibacterial performance than ZnO and the defects of the as-synthsized samples has less influence on their antibacterial performance than Ag-absorbing,the photocatalytic activity of nano-ZnO powders prepared by direct precipitation method decreased and their antibacterial properties increased with increasing of AgNO₃ concentration.The nano-ZnO powders prepared by sol method were added to the plastic film and prepare the antibacterial plastic film.MIC test shows that:when the concentration of Escherichia is 10⁵cfu/ml,the antibacterial rate of the antibacterial plastic film is 99%.It means that the as-synthsized plastic film has excellent antibacterial property.