张问问,陈东辉



片状Bi2O3的制备及其光催化性能

张问问,陈东辉*

(东华大学环境科学与工程,上海 201620)

利用化学沉淀法,分别以Bi(NO3)3×5H2O为铋源、稀HNO3为溶剂和HTMA为沉淀剂,制备出片状Bi2O3光催化剂.利用SEM、XRD和BET分别对制备的Bi2O3形貌、厚度、晶型和比表面积进行表征,并借助紫外可见漫反射和光化学反应仪对材料的光催化性能进行测试.结果表明:制备的片状Bi2O3光催化剂形貌较好,厚度均一,a-Bi2O3晶型,对可见光的吸收较强,表现出较好的光催化性能.在模拟可见光300W下,3.0h后Bi2O3光催化剂对四环素溶液的降解率可以达到82.6 %,远远高于市售纳米Bi2O3颗粒.

片状；Bi2O3；沉淀法；四环素

制药行业产生的废水因其成分复杂、有机物含量高、毒性大、色度深和含盐量高,特别是生化性差、间歇排放、废水的pH值和水质水量波动大等特点成为难处理的工业废水.近年来,随着我国医药工业的发展,制药废水已逐渐成为重要的水污染源之一,如何处理该类废水是当今环境保护需要解决的一个难题[1-3].根据近些年对废水处理技术的深入研究,高级氧化技术特别是光催化技术因绿色、高效、稳定性好、无二次污染等优点,在废水处理领域引起了极大的关注[4-7].但是该技术目前也存在一些问题,比如光催化剂的制备成本高、对自然光的利用率低和重复利用性差等问题.针对这些问题,进一步研发高催化活性的光催化材料是目前该领域最重要的研究课题之一.

无机纳米材料由于其独特的物理化学性,被广泛应用在药学、电学、光学、生物包埋和机械制造等领域[8-11],对人类生活水平的提高以及社会的发展做出重要贡献.在光催化领域,纳米材料,特别是纳米片状结构的材料因其粒径小,比表面积大,材料表面可提供的活性位点多,更有利于提高光催化性能[12-14],而受到越来越多研究者的青睐.Bi2O3材料具有较多的氧空位,较强的光生电子和空穴分离能力,较高的对自然光的利用率等特点而被认为是最具有发展前途的光催化剂之一[15-16].自1988年Harriman等[17]首次提出Bi2O3可以作为光催化剂开始,国内外关于Bi2O3作为光催化剂处理有机废水方面的研究就越来越多.虽然,国内外采用氧化、沉积沉降和溶剂热等方法已经成功制备了Bi2O3或者其复合物[18-21],基本建立了Bi2O3纤维状、颗粒状、球状、花状、网状等形貌[22-26]的制备方法.但是目前对片状Bi2O3的制备报道最多是模板法[27-28],对于液相沉淀法制备片状Bi2O3未见报道,更别提对其光催化性能研究的报道.

因此,本文拟通过一种简便、低成本的方法制备片状Bi2O3(SBO)光催化剂,研究SBO光催化剂的形貌、片状厚度、晶型和比表面积并在模拟可见光下对比市售纳米Bi2O3(CBO),以制药废水-四环素(Tetracycline,缩写:TT)为处理对象,探究其光催化性能.

1 材料与方法

1.1 材料

五水合硝酸铋(Bi(NO3)3•5H2O,AR)、硝酸(HNO3,AR,65-68%)、六亚甲基四胺(C6H12N4,RG, HTMA)、无水乙醇(C2H5OH、AR)、市售纳米氧化铋颗粒(CBO)、四环素(Tetracycline,TT),实验中所用的试剂均是购买后不作任何处理,直接使用,所用的去离子水(DT)是由本学校自供.

1.2 实验方法

SBO光催化材料的具体制备步骤如下:分别量取15.0mL DT和5.0mL HNO3溶液到称量瓶内,再称取0.002mol Bi(NO3)3·5H2O,溶解后加入装有50.0mL DT且搭建冷凝管的三口烧瓶中,搅拌加热至70℃, 30min后加入溶解有5.0g HTMA的20mL DT,继续加热搅拌1.0h后停止加热,再搅拌1.0h后离心洗涤三口烧瓶内的浑浊液(两次DT,一次C2H5OH)后在70℃烘箱中烘干,最后在550℃管式炉中煅烧2.0h后得到目标产物.

1.3 样品的光催化性能实验

称取60mg的SBO加入100mL 20mg/L的TT溶液,暗处理60min后开氙灯(功率调至300W,光源和反应器的距离保持在8.0cm)来模拟可见光,按照设定好的取样时间点进行取样,每次均取样4.5mL,在转速为6000r/min下离心3.0min后取上清液在TT溶液的最大吸收波长358nm处测量其吸光度,根据吸光度的变化来判定材料对TT溶液的降解率.CBO和空白对照组的实验步骤同上.

2 结果与讨论

2.1 TG分析

材料的煅烧温度对材料形貌和性能的影响不容忽视,因此利用热重分析仪研究材料在不同温度下的失重率,将有利于得到适宜的煅烧温度,图1是制备的SBO前驱体材料的热重曲线.从图中可以明显看出在50~500℃之间,SBO前躯体一直在慢慢失重,重量损失约7.0%,在500~545℃之间,失重速率较快,达到7.4%,此后,随着温度的升高,重量不再发生变化.分析失重的原因可能是沉淀法制备的SBO不是一开始就形成的,而是形成了前驱体.刚开始前驱体内的结晶水在慢慢减少,随着温度的升高,前驱体最终被氧化成Bi2O3.由此可知该方法制备的SBO的煅烧温度至少要达到545℃,方可将前驱体彻底氧化成SBO.本着绿色环保的理念,本文选择的煅烧温度是550℃.

图1 SBO前驱体的热重曲线

2.2 SEM-EDS分析

光催化剂的催化性能和材料形貌的相关性很大,因此研究光催化材料的形貌具有一定的意义.图2是CBO和SBO的SEM和EDS图,观察图2(A)可知:CBO呈无规则纳米颗粒状,且由大小不太均一的小颗粒堆积而成,分散性较差,颗粒的直径约在50~100nm之间;观察图2(B)可知:SBO呈片状,类似六边形,分散性较好,片状的厚度大约为100nm(图2(C)).该方法制备的SBO的形成机理可能是加入沉淀剂后,形成的纳米颗粒沉淀自发的聚集在一起来降低暴露面进而减少表面能,并进一步自组装成类六边形片状形貌,达到稳定[29].为了确定制备材料的元素组成,利用和SEM连用的EDS对材料进行扫描分析.如图2(D)和(E)所示,分别为CBO和SBO的元素谱图.由于制样中为了增加材料的导电性而用铝箔作为底层进行制样,所以会有Al元素出现,忽略其影响,主要元素就是Bi和O,因此CBO和SBO材料的组成就是Bi2O3.

2.3 XRD分析

图3 CBO和SBO的XRD图谱

氧化铋有着不同的晶体结构和化学性质,究其原因是由其具有4种不同的晶型(-Bi2O3、- Bi2O3、-Bi2O3和-Bi2O3)决定的,因此本文借助XRD来表征材料的晶型.如图3是CBO和SBO的XRD图谱,显示了CBO和SBO在2为10°~90°范围内的所有的峰.通过对比JCPDS 数据库(NO. 41–1449,NO.71-2274和NO.27-0050)可知:CBO的出峰较多,基本上涵盖了β-Bi2O3的特征峰(201,002, 220,222,400,203,421,402)[22];而SBO对应单斜结构-Bi2O3的特征衍射峰,结果和NO.71-2477相吻合[30],因此制备的SBO的晶型是-Bi2O3.此外,图中SBO的峰值都较强,意味着其结晶程度高,在光催化性能上可能会有所优势.

2.4 BET分析

为累积孔容;为孔径

在光催化领域,较大的比表面积有利于更多反应物在催化剂表面进行吸附,而更高的孔体积有利于各种反应物和产物在光催化反应过程中迅速扩散,从而提高光催化活性,使有机物更快的被降解[31],因此表征光催化剂的比表面积和孔分布有利于研究材料的光催化性能.图4是CBO和SBO的N2吸附-脱附等温线和孔大小分布曲线.图4(A)表明CBO和SBO属于介孔材料, CBO的比表面积是27.56m2/g,而制备的SBO可以达到33.21m2/g.从图4(B)中可以很明显地看出:CBO和SBO的孔径分布都主要集中在5~10nm,但是SBO材料含孔的比例较高于CBO.理论上来说,在光催化领域,SBO比CBO有优势,因为其比表面积稍大,孔较多.

2.5 DRUVS分析

图5 CBO和SBO的固体紫外漫反射图谱

2.6 材料光催化性能

利用TT溶液在可见光下的降解率来评价材料的光催化性能.图6是可见光下加入SBO和CBO以及空白对照组的TT溶液的降解率.光照之前,先暗处理1.0h使得光催化材料对TT吸附达到饱和.如图6所示:CBO和SBO对TT的吸附在40min内就基本上就达到了吸附平衡且SBO对TT的吸附能力较好,大约是CBO的3倍.在暗处理过程中,TT溶液没有发生自降解;但是当可见光照射时,TT溶液发生了自降解现象,3.0h后的自降解率达到27.0%,这归因于可见光下,TT可以吸收光子产生O2-·直接光解并同时进行自敏化光解[34-35].加入光催化材料后,加快了TT溶液的降解率;3.0h后加入CBO的TT溶液降解率是42.7%,相对空白组仅仅提高了15.7 %;而加入SBO的TT溶液的降解率达到了82.6%,降解率提高了55.6%,这可能归因于片状结构有助于提高材料对光的吸收强度和电子-空穴的分离,进而有利于材料进行光催化作用.根据方程分析ln(0/C)与光照时间()之间的关系,如图6(b)所示, ln(0/C)和的关系明显是线性的,表明光催化降解遵循伪一级的反应动力学.通过分析数据可知: SBO的光催化效率= 8.6´10-2是CBO的5.2倍(= 1.65´10-2),进一步表明所制备的SBO具有更强的光催化性能,在废水处理和保护领域具有广泛的应用前景.

2.7 材料稳定性

光催化剂的稳定性和再利用性对其实际应用具有重要的重用.图7显示了SBO在相同实验条件下的回收利用测试.SBO的回收是通过分别利用DT和C2H5OH离心洗涤3次,然后在80℃烘箱中烘干备用.从图7可以看出SBO在经过3次循环利用后,对TT溶液的降解率依然达到80%以上,表明SBO具有较好的稳定性和重复利用性,在废水处理领域具有应用的潜能.

图7 SBO的回用测试

3 结论

3.1 通过化学沉淀法,制备出的SBO材料不仅纯度比CBO高,而且对可见光表现出较强的吸收.

3.2 在模拟功率为300W的可见光下照射3.0h后,对TT的降解率达到82.6%,远远优先于CBO的42.7%.而且经过3次重复利用,对TT的降解率依然保持在80%以上.

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Preparation of Bi2O3with plate-like and its photocatalytic property.

ZHANG Wen-wen, CHEN Dong-hui*

(College of Environmental Science and Engineering, Donghua University, Shanghai 201620, China)., 2019,39(5)：1961~1966

Bi2O3photocatalyst with plate-like morphology was successfully prepared by the method of chemical precipitation which using Bi(NO3)3×5H2O as the source of bismuth, HNO3as solvent and HTMA as precipitants, respectively. The morphology, thickness, crystal form and specific surface area of the as-prepared Bi2O3were characterized by SEM, XRD and BETrespectively. Furthermore, the photocatalytic performance of the materials was tested by UV-Vis diffuse reflection and photochemical reaction apparatus. The results showed that the obtained Bi2O3photocatalyst with plate-like morphology has good morphology, uniform thickness, a-Bi2O3, stronger absorbent for visible-light and exhibits good photocatalytic property. Under the simulated visible light of 300W, the degradation rate of tetracycline wastewater can be reached to 82.6 % after 3.0h, which is much higher than that of commercially nanoparticles Bi2O3.

plate-like；Bi2O3；precipitation method；tetracycline

X703.5,O643

A

1000-6923(2019)05-1961-06

张问问(1989-),男,安徽亳州人,东华大学博士研究生,主要研究方向为环境污染控制.发表论文3篇.

2018-09-06

上海地方高校资助项目(50578020)

*责任作者, 教授, chendhisit@163.com