纳米氧化锌的形貌控制及性能研究
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摘要
纳米氧化锌(ZnO)作为一种新型多功能无机材料,在很多领域有着广阔的应用前景,尤其是在与人类生存和健康密切相关的光催化降解有机物污染和抗菌方面有着独特的优势。如何将光催化降解性能和抗菌性能结合起来是目前研究纳米氧化锌应用的一个重要分支,然而纳米氧化锌作为光催化材料和抗菌剂国内仍处于研究阶段。控制纳米氧化锌的形貌、在氧化锌表面吸附金属单质或晶格中掺入外来元素都会改变氧化锌本体的很多性能,如缺陷浓度、颗粒大小等,而这些因素会在一定程度上提高氧化锌的物理和化学性能。本文基于这一点,采用直接沉淀法和溶胶法制备特定形貌的纳米ZnO粉体和采用金属单质吸附到氧化锌表面形成金属-ZnO异质结粉体,拟通过控制形貌和形成异质结来提高纳米氧化锌的光催化和抗菌性能。
首先概述了ZnO在光催化和抗菌方面的研究进展以及纳米ZnO的制备方法,重点回顾了液相法制备特定形貌的纳米氧化锌的研究进展。然后采用直接沉淀法和溶胶法分别制备不同形貌纳米ZnO和纳米金属-ZnO异质结,研究了制备工艺参数和金属吸附对粉体的形貌、颗粒大小、结构和缺陷浓度的影响。最后,研究了不同形貌的纳米ZnO粉体和金属-ZnO异质结粉体的光催化和抗菌性。主要研究包括以下几个方面:
通过直接沉淀法,制备了三种形貌的纳米氧化锌粉体;并研究了反应温度、溶液的PH值、不同锌盐和表面活性剂对纳米氧化锌形貌的影响。
用NaOH作为沉淀剂,未加表面活性剂的条件下制备了柱状纳米氧化锌粉体,实验结果显示:纤锌矿结构的氧化锌晶体,长度方向上增长比直径方向上增长所需要能量少,生长更快。因此,反应温度从60℃升高到90℃,制备氧化锌的溶液中反应分子能量升高,使得生成氧化锌的趋势变大,氧化锌形貌从短柱状变为长柱状;溶液中阴离子离子半径大小顺序为:CH_3CO_2~-＞SO_4~(2-)＞NO_3~-＞Cl~-,离子半径越大,在氧化锌(0001)晶面上吸附量越少,对氧化锌(0001)晶面生长速度抑制越弱,其抑制顺序为:CH_3CO_2~-＜SO_4~(2-)＜NO_3~-＜Cl~-,选用的锌盐不同,粉体形貌从针状(以Zn(Ac)_2·2H_2O制备)变化到柱状(以ZnSO_4制备),然后到笋状(以Zn(NO_3)_2·6H_2O制备),最后为短柱状(以ZnCl_2制备),其中长径比分别为6.5:1—5:1—3.5:1—2:1。随着溶液PH值从酸性到碱性变化,粉体形貌从类球状变为柱状;溶液的强碱性进一步增加,粉体形貌从柱状转变成星状。
用氨水作为沉淀剂,未加表面活性剂的条件下制备了花状纳米氧化锌粉体,并用XRD和荧光光谱研究了煅烧前后的花状纳米氧化锌粉体性能变化,结果表明:煅烧后氧化锌结晶度提高,晶体缺陷和表面杂质减少,但氧化锌晶粒增大和比表面积减小,因此,如能结合其他工艺,在煅烧前后氧化锌晶粒大小和比表面积变化不大的情况下,煅烧工艺能改善纳米氧化锌粉体的光催化性能;煅烧工艺却大大降低了纳米氧化锌粉体的抗菌性能。为了制备出光催化性能和抗菌性能都较好的氧化锌粉体,以下在制备和改性氧化锌粉体时均不采用煅烧工艺。通过升高反应溶液的PH值,得到微米花状氧化锌粉体。
用NaOH作为沉淀剂,以柠檬酸钠(NaCA)为表面活性剂制备了片状纳米氧化锌。反应过程中,柠檬酸根和锌离子形成螯合物,抑制了纳米氧化锌的(0001)晶面生长,使得最终形貌为片状。升高温度,得到微米级片状氧化锌粉体。
在此基础上,研究了三种形貌(柱状、花状和片状)的纳米氧化锌粉体的可见荧光性能,结果显示:由于柠檬酸根包覆到片状氧化锌表面,补偿了ZnO粉体表面的一些悬键,减少了结构缺陷,片状氧化锌的可见荧光强度最低,样品的缺陷最少,但柠檬酸根包覆会引起氧化锌粉体比表面积减小。
为了制备颗粒尺寸更小的纳米氧化锌粉体,采用溶胶法制备出了不同溶剂中氧化锌溶胶,结果显示:水溶胶中颗粒尺寸较大,稳定性较差,表面存在表面活性剂的包覆,粉体的比表面积变小;醇溶液中溶胶颗粒尺寸较小,稳定性较好,无表面活性剂的包覆,粉体的比表面积变化不大,因此,改性制备和性能分析时均不采用加入表面活性剂工艺。对比醇溶剂(甲醇、乙醇和异丙醇)中氧化锌溶胶紫外吸收峰,乙醇中吸收峰最尖锐,半导体特征最明显,且乙醇毒性小,因此选用乙醇作为溶胶法制备氧化锌的溶剂。以下制备工艺中为了方便比较和讨论,在样品比表面积变化不大的基础上,依靠吸附、小尺寸效应和减少样品的缺陷浓度等手段对氧化锌粉体进行改性。
以甲基橙和大肠杆菌为模型来研究所制备粉体的光催化和抗菌性能。直接沉淀法制备出的三种形貌(柱状、花状和片状)纳米氧化锌粉体中,为了方便比较,研究了其中两种未加表面活性剂的纳米粉体(柱状、花状)光催化性能,结果显示:柱状氧化锌粉体颗粒尺寸较小,比表面积较大,表面的光催化活性点较多,光催化效率比花状粉体高。溶胶法制备出的纳米ZnO粉体晶粒(8.2 nm)比直接沉淀法(55 nm)制备出的粉体晶粒小,因此,溶胶法制备出的粉体光催化降解率较大,紫外光条件下粉体光催化降解率达到92%,而直接沉淀法制备出的柱状粉体光催化降解率为24%。溶胶法制备出的粉体比直接沉淀法制备出的粉体缺陷浓度大,两种方法制备出的粉体光催化实验结果说明,晶粒大小或颗粒大小以及比表面积大小比缺陷浓度多少对氧化锌粉体光催化性能的影响大。颗粒团聚对溶胶法制备出的纳米ZnO粉体光催化降解率影响较大,粉体在室温下水溶液中放置24 h,粉体产生团聚,紫外光条件下氧化锌粉体的光催化降解率从92%下降到56%。
对大肠杆菌实验表明,直接沉淀法制备出的两种形貌(柱状、花状)纳米氧化锌粉体中,柱状氧化锌粉体颗粒尺寸小,光催化性能好,很容易吸附沉积到细菌体内而更有效地杀灭细菌,因此柱状粉体比花状粉体抗菌效果较好,其MIC为50 ppm。溶胶法制备出的粉体晶粒(8.2 nm)比直接沉淀法制备出的晶粒(55 nm)小,光催化效率高,但粉体在抗菌测试过程中易团聚(光催化测试时,粉体在水中搅拌,相对于抗菌测试时团聚较少),而缺陷对抗菌性能影响不大,溶胶法制备出的粉体的抗菌性能和直接沉淀法差别不大,其MIC也为50 ppm。
为了提高氧化锌粉体的光催化效率和抗菌性能,以光催化性能和抗菌性能均较好的溶胶法工艺为基础,制备出掺铜纳米氧化锌粉体。由于低掺杂量(＜5.0%)下掺杂粉体的可见荧光强度增强,缺陷增多;掺杂使得粉体能带宽度变化较小,仍只能吸收紫外区光源,对光催化性能的改善贡献较小;而在高掺杂量(＞7.5%)下,粉体物相发生改变,粉体表面有氧化铜相出现,很难再通过该方法实现将铜掺杂进氧化锌晶格中,因此,采用本实验工艺条件下掺杂铜不适合提高溶胶法制备的氧化锌粉体的光催化性能。缺陷多少对掺杂氧化锌粉体抗菌性能影响不大,但缺陷增多,粉体的光催化途径杀菌效果会降低,而高掺杂量下吸附在氧化锌表面的氧化铜,在氧化物粉体中的抗菌性能最差,因此,采用本实验工艺条件下掺杂铜对抗菌性能的提高意义不大。
银吸附不仅可改善氧化锌表面的电荷分离效率、减少氧化锌粉体缺陷而且银本身就是极好的抗菌材料,因此为了改善氧化锌粉体的光催化和抗菌性能,选取直接沉淀法和溶胶法制备氧化锌中性能最佳的工艺为基础,分别制备出纳米Ag-ZnO异质结粉体。由于两种方法制备出的粉体颗粒大小、缺陷多少的不同,两种方法制备出的异质结粉体的光催化效率不同,实验显示:溶胶法制备出的异质结粉体光催化效率明显高于直接沉淀法制备出的粉体;溶胶法制备出粉体的光催化效率随着硝酸银加入量的增加而提高,自然光条件下光催化降解率最大达到85%,光催化降解率比溶胶法制备出的纯纳米氧化锌提高了20%;而直接沉淀法制备出的异质结粉体光催化降解率则随着硝酸银加入量的增加,先减小后增加,紫外光条件下光催化降解率最大达到30%。
用直接沉淀法和溶胶法两种方法制备出的异质结粉体抗菌实验结果均显示:由于异质结中银与氧化锌具有协同杀菌的作用,随着硝酸银加入量的增加,粉体的抗菌效果变好,样品的MIC从50 ppm下降到6.25 ppm;且溶胶法制备出异质结粉体的颗粒尺寸(～10 nm)较小,颗粒越小越容易吸附到细菌的细胞壁表面,通过吸附沉积杀菌,且颗粒越小,比表面积越大,越有利于光催化杀菌。因此,在硝酸银的加入量相同的条件下,溶胶法制备出的异质结粉体的比直接沉淀法制备出的粉体抗菌效果好。
直接沉淀法制备出的异质结粉体的光催化性能和抗菌性能随硝酸银加入量的增加,变化趋势不一致。这主要是直接沉淀法制备异质结粉体的过程中,银掺杂和银吸附是一对竞争过程同时存在。银掺杂随着硝酸银和还原剂加入量的增加,比重越来越小,但银掺杂导致粉体缺陷增多;而银吸附随着硝酸银和还原剂加入量的增加,比重越来越大(由于该实验过程是硝酸银溶液向还原剂溶液中滴加,反应前期随着还原剂的加入量增多,还原滴加的硝酸银概率越大,而掺杂概率越小;且硝酸银加入量越多,滴加时间越长,滴加到后期,溶液中氧化锌生成越多,银掺杂进氧化锌晶体概率更小),银吸附过程逐渐占主导,使得样品缺陷减小,因此,直接沉淀法制备过程中随着硝酸银加入量的增加,异质结粉体缺陷先增多后减少,光催化效率先减小后增大。虽然直接沉淀法制备出的异质结粉体光催化效率不高,异质结粉体通过光催化杀菌效果不佳,但银的抗菌效果优于氧化锌,银吸附能从根本上提高氧化锌粉体的抗菌性能,而银掺杂导致粉体缺陷增多对抗菌性能的影响不大,因此,随着硝酸银加入量的增加,直接沉淀法制备出的异质结粉体抗菌性能一直增强。
溶胶法制备出的纳米氧化锌粉体添加到塑料薄膜中,并研究其对大肠杆菌的杀菌性能,结果显示:在大肠杆菌浓度为10~5cfu/ml时,塑料薄膜对其24小时杀菌率达到99%以上,加工出的塑料薄膜对大肠杆菌生长有很强的抑制作用。
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 Zn~(2+) 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)_2·2H_2O) to 5:1(prepared from ZnSO_4·7H_2O) and then 3.5:1(prepared from Zn(NO)_3·6H_2O),finally to 2:1(prepared from ZnCl_2).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:CH_3CO_2~-＜SO_4~(2-)＜NO_3~-＜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 AgNO_3 concentration.On the contrary,their antibacterial properties increased with increasing of AgNO_3 concentration. However,the photocatalytic activity of nano-ZnO powders prepared by sol method increased with increasing of AgNO_3 concentration,while their antibacterial properties also increased with increasing of AgNO_3 concentration.The singular behavior should be caused by different AgNO_3 consumption process under different method.In direct precipitation method,when AgNO_3 adding into the Zn salts solution drop by drop,AgNO_3 has partly converted to Ag and the other part of Ag~+was introduced ZnO crystals.At low AgNO_3 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 AgNO_3 adding concentration,Ag~+ converting to Ag was the main process at reaction condition and the defect concentration decreased.While in sol method,AgNO_3 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 AgNO_3 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 10~5cfu/ml,the antibacterial rate of the antibacterial plastic film is 99%.It means that the as-synthsized plastic film has excellent antibacterial property.
首先概述了ZnO在光催化和抗菌方面的研究进展以及纳米ZnO的制备方法,重点回顾了液相法制备特定形貌的纳米氧化锌的研究进展。然后采用直接沉淀法和溶胶法分别制备不同形貌纳米ZnO和纳米金属-ZnO异质结,研究了制备工艺参数和金属吸附对粉体的形貌、颗粒大小、结构和缺陷浓度的影响。最后,研究了不同形貌的纳米ZnO粉体和金属-ZnO异质结粉体的光催化和抗菌性。主要研究包括以下几个方面:
通过直接沉淀法,制备了三种形貌的纳米氧化锌粉体;并研究了反应温度、溶液的PH值、不同锌盐和表面活性剂对纳米氧化锌形貌的影响。
用NaOH作为沉淀剂,未加表面活性剂的条件下制备了柱状纳米氧化锌粉体,实验结果显示:纤锌矿结构的氧化锌晶体,长度方向上增长比直径方向上增长所需要能量少,生长更快。因此,反应温度从60℃升高到90℃,制备氧化锌的溶液中反应分子能量升高,使得生成氧化锌的趋势变大,氧化锌形貌从短柱状变为长柱状;溶液中阴离子离子半径大小顺序为:CH_3CO_2~-＞SO_4~(2-)＞NO_3~-＞Cl~-,离子半径越大,在氧化锌(0001)晶面上吸附量越少,对氧化锌(0001)晶面生长速度抑制越弱,其抑制顺序为:CH_3CO_2~-＜SO_4~(2-)＜NO_3~-＜Cl~-,选用的锌盐不同,粉体形貌从针状(以Zn(Ac)_2·2H_2O制备)变化到柱状(以ZnSO_4制备),然后到笋状(以Zn(NO_3)_2·6H_2O制备),最后为短柱状(以ZnCl_2制备),其中长径比分别为6.5:1—5:1—3.5:1—2:1。随着溶液PH值从酸性到碱性变化,粉体形貌从类球状变为柱状;溶液的强碱性进一步增加,粉体形貌从柱状转变成星状。
用氨水作为沉淀剂,未加表面活性剂的条件下制备了花状纳米氧化锌粉体,并用XRD和荧光光谱研究了煅烧前后的花状纳米氧化锌粉体性能变化,结果表明:煅烧后氧化锌结晶度提高,晶体缺陷和表面杂质减少,但氧化锌晶粒增大和比表面积减小,因此,如能结合其他工艺,在煅烧前后氧化锌晶粒大小和比表面积变化不大的情况下,煅烧工艺能改善纳米氧化锌粉体的光催化性能;煅烧工艺却大大降低了纳米氧化锌粉体的抗菌性能。为了制备出光催化性能和抗菌性能都较好的氧化锌粉体,以下在制备和改性氧化锌粉体时均不采用煅烧工艺。通过升高反应溶液的PH值,得到微米花状氧化锌粉体。
用NaOH作为沉淀剂,以柠檬酸钠(NaCA)为表面活性剂制备了片状纳米氧化锌。反应过程中,柠檬酸根和锌离子形成螯合物,抑制了纳米氧化锌的(0001)晶面生长,使得最终形貌为片状。升高温度,得到微米级片状氧化锌粉体。
在此基础上,研究了三种形貌(柱状、花状和片状)的纳米氧化锌粉体的可见荧光性能,结果显示:由于柠檬酸根包覆到片状氧化锌表面,补偿了ZnO粉体表面的一些悬键,减少了结构缺陷,片状氧化锌的可见荧光强度最低,样品的缺陷最少,但柠檬酸根包覆会引起氧化锌粉体比表面积减小。
为了制备颗粒尺寸更小的纳米氧化锌粉体,采用溶胶法制备出了不同溶剂中氧化锌溶胶,结果显示:水溶胶中颗粒尺寸较大,稳定性较差,表面存在表面活性剂的包覆,粉体的比表面积变小;醇溶液中溶胶颗粒尺寸较小,稳定性较好,无表面活性剂的包覆,粉体的比表面积变化不大,因此,改性制备和性能分析时均不采用加入表面活性剂工艺。对比醇溶剂(甲醇、乙醇和异丙醇)中氧化锌溶胶紫外吸收峰,乙醇中吸收峰最尖锐,半导体特征最明显,且乙醇毒性小,因此选用乙醇作为溶胶法制备氧化锌的溶剂。以下制备工艺中为了方便比较和讨论,在样品比表面积变化不大的基础上,依靠吸附、小尺寸效应和减少样品的缺陷浓度等手段对氧化锌粉体进行改性。
以甲基橙和大肠杆菌为模型来研究所制备粉体的光催化和抗菌性能。直接沉淀法制备出的三种形貌(柱状、花状和片状)纳米氧化锌粉体中,为了方便比较,研究了其中两种未加表面活性剂的纳米粉体(柱状、花状)光催化性能,结果显示:柱状氧化锌粉体颗粒尺寸较小,比表面积较大,表面的光催化活性点较多,光催化效率比花状粉体高。溶胶法制备出的纳米ZnO粉体晶粒(8.2 nm)比直接沉淀法(55 nm)制备出的粉体晶粒小,因此,溶胶法制备出的粉体光催化降解率较大,紫外光条件下粉体光催化降解率达到92%,而直接沉淀法制备出的柱状粉体光催化降解率为24%。溶胶法制备出的粉体比直接沉淀法制备出的粉体缺陷浓度大,两种方法制备出的粉体光催化实验结果说明,晶粒大小或颗粒大小以及比表面积大小比缺陷浓度多少对氧化锌粉体光催化性能的影响大。颗粒团聚对溶胶法制备出的纳米ZnO粉体光催化降解率影响较大,粉体在室温下水溶液中放置24 h,粉体产生团聚,紫外光条件下氧化锌粉体的光催化降解率从92%下降到56%。
对大肠杆菌实验表明,直接沉淀法制备出的两种形貌(柱状、花状)纳米氧化锌粉体中,柱状氧化锌粉体颗粒尺寸小,光催化性能好,很容易吸附沉积到细菌体内而更有效地杀灭细菌,因此柱状粉体比花状粉体抗菌效果较好,其MIC为50 ppm。溶胶法制备出的粉体晶粒(8.2 nm)比直接沉淀法制备出的晶粒(55 nm)小,光催化效率高,但粉体在抗菌测试过程中易团聚(光催化测试时,粉体在水中搅拌,相对于抗菌测试时团聚较少),而缺陷对抗菌性能影响不大,溶胶法制备出的粉体的抗菌性能和直接沉淀法差别不大,其MIC也为50 ppm。
为了提高氧化锌粉体的光催化效率和抗菌性能,以光催化性能和抗菌性能均较好的溶胶法工艺为基础,制备出掺铜纳米氧化锌粉体。由于低掺杂量(＜5.0%)下掺杂粉体的可见荧光强度增强,缺陷增多;掺杂使得粉体能带宽度变化较小,仍只能吸收紫外区光源,对光催化性能的改善贡献较小;而在高掺杂量(＞7.5%)下,粉体物相发生改变,粉体表面有氧化铜相出现,很难再通过该方法实现将铜掺杂进氧化锌晶格中,因此,采用本实验工艺条件下掺杂铜不适合提高溶胶法制备的氧化锌粉体的光催化性能。缺陷多少对掺杂氧化锌粉体抗菌性能影响不大,但缺陷增多,粉体的光催化途径杀菌效果会降低,而高掺杂量下吸附在氧化锌表面的氧化铜,在氧化物粉体中的抗菌性能最差,因此,采用本实验工艺条件下掺杂铜对抗菌性能的提高意义不大。
银吸附不仅可改善氧化锌表面的电荷分离效率、减少氧化锌粉体缺陷而且银本身就是极好的抗菌材料,因此为了改善氧化锌粉体的光催化和抗菌性能,选取直接沉淀法和溶胶法制备氧化锌中性能最佳的工艺为基础,分别制备出纳米Ag-ZnO异质结粉体。由于两种方法制备出的粉体颗粒大小、缺陷多少的不同,两种方法制备出的异质结粉体的光催化效率不同,实验显示:溶胶法制备出的异质结粉体光催化效率明显高于直接沉淀法制备出的粉体;溶胶法制备出粉体的光催化效率随着硝酸银加入量的增加而提高,自然光条件下光催化降解率最大达到85%,光催化降解率比溶胶法制备出的纯纳米氧化锌提高了20%;而直接沉淀法制备出的异质结粉体光催化降解率则随着硝酸银加入量的增加,先减小后增加,紫外光条件下光催化降解率最大达到30%。
用直接沉淀法和溶胶法两种方法制备出的异质结粉体抗菌实验结果均显示:由于异质结中银与氧化锌具有协同杀菌的作用,随着硝酸银加入量的增加,粉体的抗菌效果变好,样品的MIC从50 ppm下降到6.25 ppm;且溶胶法制备出异质结粉体的颗粒尺寸(～10 nm)较小,颗粒越小越容易吸附到细菌的细胞壁表面,通过吸附沉积杀菌,且颗粒越小,比表面积越大,越有利于光催化杀菌。因此,在硝酸银的加入量相同的条件下,溶胶法制备出的异质结粉体的比直接沉淀法制备出的粉体抗菌效果好。
直接沉淀法制备出的异质结粉体的光催化性能和抗菌性能随硝酸银加入量的增加,变化趋势不一致。这主要是直接沉淀法制备异质结粉体的过程中,银掺杂和银吸附是一对竞争过程同时存在。银掺杂随着硝酸银和还原剂加入量的增加,比重越来越小,但银掺杂导致粉体缺陷增多;而银吸附随着硝酸银和还原剂加入量的增加,比重越来越大(由于该实验过程是硝酸银溶液向还原剂溶液中滴加,反应前期随着还原剂的加入量增多,还原滴加的硝酸银概率越大,而掺杂概率越小;且硝酸银加入量越多,滴加时间越长,滴加到后期,溶液中氧化锌生成越多,银掺杂进氧化锌晶体概率更小),银吸附过程逐渐占主导,使得样品缺陷减小,因此,直接沉淀法制备过程中随着硝酸银加入量的增加,异质结粉体缺陷先增多后减少,光催化效率先减小后增大。虽然直接沉淀法制备出的异质结粉体光催化效率不高,异质结粉体通过光催化杀菌效果不佳,但银的抗菌效果优于氧化锌,银吸附能从根本上提高氧化锌粉体的抗菌性能,而银掺杂导致粉体缺陷增多对抗菌性能的影响不大,因此,随着硝酸银加入量的增加,直接沉淀法制备出的异质结粉体抗菌性能一直增强。
溶胶法制备出的纳米氧化锌粉体添加到塑料薄膜中,并研究其对大肠杆菌的杀菌性能,结果显示:在大肠杆菌浓度为10~5cfu/ml时,塑料薄膜对其24小时杀菌率达到99%以上,加工出的塑料薄膜对大肠杆菌生长有很强的抑制作用。
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 Zn~(2+) 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)_2·2H_2O) to 5:1(prepared from ZnSO_4·7H_2O) and then 3.5:1(prepared from Zn(NO)_3·6H_2O),finally to 2:1(prepared from ZnCl_2).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:CH_3CO_2~-＜SO_4~(2-)＜NO_3~-＜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 AgNO_3 concentration.On the contrary,their antibacterial properties increased with increasing of AgNO_3 concentration. However,the photocatalytic activity of nano-ZnO powders prepared by sol method increased with increasing of AgNO_3 concentration,while their antibacterial properties also increased with increasing of AgNO_3 concentration.The singular behavior should be caused by different AgNO_3 consumption process under different method.In direct precipitation method,when AgNO_3 adding into the Zn salts solution drop by drop,AgNO_3 has partly converted to Ag and the other part of Ag~+was introduced ZnO crystals.At low AgNO_3 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 AgNO_3 adding concentration,Ag~+ converting to Ag was the main process at reaction condition and the defect concentration decreased.While in sol method,AgNO_3 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 AgNO_3 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 10~5cfu/ml,the antibacterial rate of the antibacterial plastic film is 99%.It means that the as-synthsized plastic film has excellent antibacterial property.
引文
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