Hans Journal of Chemical Engineering and Technology
Vol.06 No.03(2016), Article ID:17637,7 pages
10.12677/HJCET.2016.63008

Preparation and Visible Light-Induced Photocatalytic Activity of BiVO4 with Olive-Like Morphology

Shuping Zhu, Yougen Tang*, Miaomiao Wei

College of Chemistry and Chemical Engineering, Central South University, Changsha Hunan

Received: May 3rd, 2016; accepted: May 22nd, 2016; published: May 25th, 2016

Copyright © 2016 by authors and Hans Publishers Inc.

This work is licensed under the Creative Commons Attribution International License (CC BY).

http://creativecommons.org/licenses/by/4.0/

ABSTRACT

Olive-like BiVO4 crystals were synthesized via a one-step hydrothermal method by using sodium citrate as the chelating agent and propane diacid as the shape control agent. The X-ray diffraction (XRD), scanning electron microscopy (SEM) and ultraviolet-visible diffuse reflectance spectrum (UV-Vis DRS) were employed to elucidate the structures, morphologies and the light absorption properties of the as-prepared BiVO4 samples. The photo-catalytic activities of BiVO4 crystals were evaluated by using rhodamine B (RhB) as simulative degradation pollutant. The results showed that all the samples possessed monoclinic structure and containing no any impurities. Both sodium citrate and propane acid have great influence on the morphologies of BiVO4 samples. The as-prepared olive-like BiVO4 samples had strong visible-light absorption abilities with the band gap value around 2.36 eV, and showed the highest photo-catalytic activities. The degradation rate of RhB could reach 94% after 2 h of irradiation with a 500 W Xe-illuminator. After three cycles of photo-catalytic degradation experiments, degradation rate still maintained 92%, which indicated that the olive-like samples had good resistant abilities to photocorrosion.

Keywords:Bismuth Vanadate, Photocatalytic, Propane Acid

橄榄状BiVO4的制备及其可见光催化性能研究

朱淑萍,唐有根*,魏淼淼

中南大学化学化工学院,湖南 长沙

收稿日期:2016年5月3日;录用日期:2016年5月22日;发布日期:2016年5月25日

摘 要

本文以柠檬酸钠为络合剂,丙二酸为形貌控制剂,通过一步水热法制备了橄榄状BiVO4光催化剂。采用X-射线衍射(XRD)、扫描电子显微镜(SEM)以及紫外–可见漫反射(UV-Vis DRS)对材料的晶型、形貌以及吸收光谱进行表征,并以罗丹明B(RhB)为模拟降解污染物,测试BiVO4的可见光催化活性。结果表明:所制备的样品纯度高,为单斜晶相BiVO4,柠檬酸钠及丙二酸都对材料的形貌产生重要的影响。合成出的橄榄状BiVO4具有很强的可见光吸收能力,禁带宽度为2.36 eV,表现出最强的光催化活性。在500 W的氙灯激发下,2 h后,对罗丹明的降解率达到94%,并且所制备的样品具有良好的抗光腐蚀性,三次循环降解后,仍保持了92%的降解率。

关键词 :BiVO4,光催化,丙二酸

1. 引言

随着科学技术的迅猛发展和人口的急剧增长,能源危机和环境污染成为制约人类文明进步的两大问题,近年来,半导体光催化技术为解决环境问题和能源问题提供了新的思路。自1972年,日本科学家Fijishima和Honda [1] 发现在紫外光的照射下,TiO2光电极能够分解水产生氢气和氧气以来,因无毒、来源充足、化学性质稳定(耐酸碱和光化学腐蚀)、光生电子的还原能力和空穴的氧化能力强等优点,TiO2成为公认的性能优良的光催化剂,得到了广泛的研究 [2] ,但因其禁带宽度较大(3.2 eV),只能吸收波长在400 nm以下的紫外光部分,对太阳能的利用率低,限制了其在实际中的应用 [3] 。

为有效利用太阳能,科学工作者对可见光响应光催化剂的研究有两个方向,一是对TiO2进行改进,使其光响应区域拓展到可见光范围,主要改性途径有金属和非金属掺杂 [4] 、贵金属沉积 [5] 、半导体复合 [6] 以及染料敏化 [7] 等,尽管这些方法在一定程度上能将TiO2光响应波长红移至可见光区,但依然存在许多的缺陷,例如:光响应拓展范围有限、材料稳定性降低、易发生光生电子–空穴复合等,从而限制了其进一步应用;二是开发新型的窄带隙半导体,主要包括Cu2O [8] 、CdS [9] 、BiVO4 [10] 、C3N4 [11] 等,其中,BiVO4因具有合适的禁带宽度和独特的层状结构而备受关注 [12] 。作为一种三元氧化物半导体,由于V3d、O2p、Bi6s轨道的杂化,在一定程度上降低了BiVO4导带位置,而Bi6s轨道参与价带的组成又导致价带顶升高,因此BiVO4具有较窄的带隙宽度,具有可见光响应性能 [13] 。BiVO4的光催化活性很大程度上受到其晶型和形貌的影响。在单斜白钨矿型、四方锆石型、四方白钨矿型三种晶型中,单斜白钨矿型BiVO4具有最窄的带隙和最佳的光催化活性 [14] 。科学家们探索了各种合成单斜白钨矿BiVO4的方法,如水热或溶剂热法 [15] 、溶液燃烧法 [16] 、超声辅助法 [17] 、共沉淀法 [18] 等,但合成出的晶体材料通常颗粒尺寸大、比表面积小,导致光催化活性不高。

本文通过简单的一步水热法,以柠檬酸钠作为络合剂,丙二酸为形貌控制剂,合成出了分散均匀并由纳米片堆叠而成的橄榄状单斜白钨矿相BiVO4半导体材料,在可见光下表现出良好的光催化降解罗丹明B效果,并进一步探讨了柠檬酸钠、丙二酸在该体系合成BiVO4中的作用。

2. 实验部分

2.1. 主要试剂

五水合硝酸铋(Bi(NO3)3·5H2O),偏钒酸铵(NH4VO3),氢氧化钠,浓硝酸(质量分数:65%~68%),丙二酸(C3H4O4),二水合柠檬酸三钠(C6H5Na3O7·2H2O),罗丹明B(RhB: C28H31ClN2O3),均为分析纯,使用过程中无需进一步纯化。

2.2. BiVO4的制备

将1 mmol Bi(NO3)3·5H2O和1 mmol NH4VO3分别溶解于30 mL 2 M HNO3和30 mL 2 M NaOH溶液中,形成无色透明溶液,分别记为A和B溶液,将B溶液缓慢地滴加到A溶液中,形成淡黄色悬浊液,在混合溶液中加入0.3 mmol二水合柠檬酸三钠和0.5 mmol丙二酸,磁力搅拌30 min后,用2 M NaOH溶液调节混合液的pH至5,继续搅拌30 min,转移到100 ml反应釜中,于180℃下反应12 h,自然冷却至室温后,将黄色沉淀用去离子水和无水乙醇分别洗涤数次,60℃下恒温干燥,备用,所得产物即为BiVO4,为方便表述,标记为BVO1。为研究柠檬酸钠及丙二酸对材料形貌的调控作用,进行了几组对比实验,未添加柠檬酸钠组标记为BVO2,既未添加柠檬酸钠同时又未添加丙二酸组标记为BVO3。

2.3. 材料的表征

采用Bruker(D8-ADVANCE)X—射线衍射仪分析样品的物相组成,检测条件:工作电压为40 KV,工作电流为300 mA,以10˚/min的速度从10˚扫描到80˚;利用NovaNanoSEM 230型扫描电子显微镜观察样品的表面形貌以及颗粒尺寸;采用日本岛津公司UV-2550型,固体BaSO4作参比,表征样品的带隙及光吸收能力;采用尤尼科仪器有限公司UV-3802型号的紫外可见光谱仪测试降解后,罗丹明溶液的吸光度曲线。

2.4. 催化活性评价

将100 mg所制备样品分散于200 ml(10 mg/L)罗丹明溶液中,黑暗条件下搅拌30 min以达到吸附平衡,以500 W氙灯为光源,在光源下放置400 nm滤光片,以保证光催化反应在可见光下进行,光源放置在溶液上方10 cm处。实验过程中不断进行磁力搅拌,每30 min取样约3 ml,在1000 r/min转速下离心,取上清液,测试其吸光度。

3. 结果与讨论

3.1. 样品的XRD分析

图1给出了BVO1的XRD图谱,从图中可以看出样品的衍射峰与单斜白钨矿相BiVO4的标准卡片(JCPDS No. 14-0688)相匹配,并不存在其他的杂质峰,说明所制备材料的纯度高,无其它杂质,所制备的样品为单斜白钨矿BiVO4

3.2. 样品的SEM分析

图2为BVO1,BVO2及BVO3的SEM图。从图中可以看出,同时添加柠檬酸钠和丙二酸时,得到分散性良好的橄榄状结构BiVO4,橄榄状结构由纳米片堆叠而成,长约为1 μm,宽约为0.5 μm,具有独特的层状结构;仅添加丙二酸时,得到片状拼接而成的饼干状结构的BiVO4;而加入任何添加剂时,产

Figure 1. The XRD pattern of BVO1

图1. BVO1的XRD图谱

Figure 2. SEM images of BVO1 (a) (b),BVO2 (c) (d),and BVO3 (e) (the scale bars in (a), (b), (c), (d) and (e) are 5 μm, 1 μm, 5 μm, 1 μm and 5 μm, respectively)

图2. BVO1的SEM图(a) (b);BVO2的SEM图(c) (d);BVO3的SEM图(e);(注:图(a),(b),(c),(d)和(e)中的标尺分别为5 μm,1 μm,5 μm,1 μm和5 μm)

物形貌不规则,团聚现象严重。

丙二酸具有两个羧基,能与多种金属络合,并且可以选择性地吸附在特定的晶面上控制晶体的生长 [19] ,而柠檬酸,也是一种极强的螯合剂,三个羧基和一个羟基的合适的分子结构使其在水热反应过程中起到交联的作用,使用柠檬酸或柠檬酸钠做添加剂通常得到球状或类球状结构的产物 [20] 。在pH = 5的弱酸性条件下,柠檬酸与Bi3+络合,形成Bi—柠檬酸络合物,进一步与VO3−反应,形成BiVO4,在最初形成的晶核中,丙二酸定向吸附在特定晶面上,控制晶体的生长方向,根据SEM结果推测,丙二酸的存在可能有利于片状结构的形成。未添加丙二酸和柠檬酸钠时,Bi3+迅速水解,并快速地与VO3−发生反应,使晶核快速生长,以至于产物形貌不规则。由此可见,柠檬酸钠和丙二酸在调节BiVO4形貌中起到了十分重要的作用。

3.3. 样品的UV-Vis DRS分析

图3给出了BVO1的紫外可见吸收光谱,从图中可以看出,该样品在紫外和可见光区域都显示出较

Figure 3. UV-Vis diffuse reflectance spectra of BVO1

图3. BVO1的紫外-可见漫反射光谱图

强的吸收能力,表明该样品是一种潜在的可见光响应型光催化剂,在可见光区域陡峭的吸收边归因于半

导体材料的带隙跃迁。从图中切线可以看出,BVO1的波长吸收阈值(λg)为525 nm,根据波长与能量的转换关系Eg = 1240/λg (eV),计算得到BVO1的禁带宽为2.36 eV,通过本实验方法合成出的BiVO4比普通方法合成出的BiVO4吸收边(2.4 eV)发生了少许红移。

3.4. 光催化活性分析

在低浓度下,RhB的浓度和吸光度遵循朗伯-比尔定律,因此可以用吸光度来衡量浓度的变化以及降解率:

式中η,,A分别表示降解率、RhB的初始浓度、降解后浓度、RhB的初始吸光度、降解后吸光度。

图4(a)和(b)为BVO1,BVO2及BVO3对RhB的降解曲线,从图中可以看出BVO1,BVO2及BVO3光催化性能依次降低,BVO1具有最佳的光催化效果,500 W可见光2 h后,BVO1,BVO2及BVO3对RhB的降解率依次是94%,82%和40%,结合SEM结果可以推测,BVO1良好的光催化效果,归因于其独特的层状结构,较高的电子-空穴分离效率,以及较大的比表面积。此外,根据图4(d) BVO1对RhB的循环降解曲线,三次降解循环之后,BVO1对RhB保持了90%以上的降解率,表明BVO1具有较好的抗光腐蚀性能,有望作为一种性能优良的光催化材料运用于实际的环境净化中。

4. 结论与展望

以柠檬酸为络合剂,丙二酸为形貌控制剂,成功制备了由纳米片堆叠而成的橄榄状单斜晶相BiVO4,显示出良好的光催化性能,并具有稳定的循环降解效果,仅添加丙二酸时得到片状拼接而成的饼干状

Figure 4. (a) The photo-catalytic performance (C/C0) of BVO1, BVO2 and BVO3 for the degradation of RhB versus visible light irradiation time; (b) the photo-catalytic degradation efficiency of RhB with BVO1, BVO2 and BVO3; (c) the temporal evolution of the absorption spectra of the RhB solution in the presence of BVO1 under exposure to visible light; (d) cycling runs in the photo-catalytic degradation of RhB in the presence of BVO1 photo-catalyst under visible-light irradiation

图4. (a) BVO1,BVO2及BVO3对罗丹明的降解率曲线;(b) BVO1,BVO2及BVO3对RhB的降解性能随时间的变化曲线;(c) BVO1光催化降解RhB随照射时间变化的紫外可见吸收光谱;(d) BVO1在可见光下对RhB的循环降解曲线

BiVO4,未添加柠檬酸钠和丙二酸时得到形貌不规则,团聚严重的BiVO4,表明柠檬酸钠和丙二酸在调控BiVO4形貌时起到了十分重要的作用。该方法合成出的纳米片层橄榄状分层结构BiVO4有望作为一种良好的可见光响应型催化剂应用于实际环境净化中。

文章引用

朱淑萍,唐有根,魏淼淼. 橄榄状BiVO4的制备及其可见光催化性能研究
Preparation and Visible Light-Induced Photocatalytic Activity of BiVO4 with Olive-Like Morphology[J]. 化学工程与技术, 2016, 06(03): 64-70. http://dx.doi.org/10.12677/HJCET.2016.63008

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