【作者】 李园园；

【导师】 黄新堂；

【作者基本信息】 华中师范大学 ， 凝聚态物理， 2009， 博士

【摘要】 开发可见光响应的新型半导体催化剂是环境污染控制和可再生能源开发利用的重要内容之一。作为一种优良的新型光催化剂,Bi₂WO₆在太阳能转换及环境污染治理方面都表现出优异的可见光催化活性,近年来引起人们的广泛关注。最近的报道显示Bi₂WO₆简单的纳米结构如纳米颗粒、纳米棒、纳米片等可以通过软化学法有效地制备。然而,Bi₂WO₆分等级结构特别是三维分等级结构的报道却很少。由于三维分等级结构的整体尺寸较大,但组成基元仍保持纳米尺度,既有利于催化剂的回收利用,又能保持纳米尺度带来的催化效率的提高。因此,制备分等级结构特别是三维分等级结构的光催化剂非常有现实意义。碳对电子/空穴具有十分优良的传输性能,已有报道显示碳的包覆能够显著提高紫外光响应的光催化剂的催化活性。但目前还未见碳对Bi₂WO₆催化剂改性研究的报道。本文采用水热法制备了不同形貌和结构的Bi₂WO₆催化剂,并对各种形貌的Bi₂WO₆产物在可见光下的催化活性进行了系统的研究。在Bi₂WO₆改性方面,我们利用葡萄糖在水热过程中碳化现象制备出碳修饰的Bi₂WO₆催化剂,研究了它们在可见光下对罗丹明B的降解行为,并探讨了葡萄糖添加量对煅烧产物光催化活性的影响。另外,我们还利用BiCl₃的水解现象采用室温浸泡法在ZnO阵列基底上制备出了Bi₃O₄Cl和BiOCl薄膜,初步探讨了薄膜的生长机制。具体工作如下:1、通过控制水热体系的温度、pH值以及表面活性剂的用量、起始原料的配比来控制产物的形貌,并制备出Bi₂WO₆简单的纳米片以及复杂的球状,八面体状和红细胞状的Bi₂WO₆分等级微米结构。（1）当水热体系中只有硝酸铋和钨酸钠时,180℃水热12h可以制备出分散性较好的二维Bi₂WO₆方形纳米片。这些方形纳米片是由更小的片以边-边连接的方式排列而成,是Bi₂WO₆晶体成熟过程中的各向异性导致的。（2）探讨了表面活性剂PVP对产物形貌的影响。当向反应体系中添加0.15g表面活性剂PVP时,由于PVP在Bi₂WO₆纳米片各个晶面的吸附不同,影响了初级纳米片的组装方式,产物为分等级Bi₂WO₆微米球。PVP的添加量对形貌的影响非常大,当不添加PVP时,产物为纯的方形纳米片;当PVP添加少量时,产物中绝大多数为分散的方形纳米片,少数为二维圆形平盘和三维结构;当PVP量增加时,产物中分散的层状纳米片显著减少,三维结构增多。但体系中PVP添加量过多时,产物中只出现二维圆形平盘。（3）调整反应体系的pH值,产物形貌也发生改变。当反应体系的pH值为7时,产物为八面体状的Bi₂WO₆。反应体系的pH值对Bi³⁺离子的水解速度有显著影响,当反应体系显中性时,Bi₂WO₆晶核和生长的速度比较合适。Bi₂WO₆初级纳米片向二维扩大面积长成更大薄片的过程实际上是沿着x,y,z三个方向等几率进行,使得面法线平行的薄片形成一组准方形片,面法线相互垂直的薄片形成八面体结构的骨架。（4）当反应体系中钨酸钠的摩尔质量与硝酸铋相等时,制备出的产物具有初级纳米片之间界面几乎消失的特点,表现为产物结构比较致密。这是由于钨酸钠的摩尔质量与硝酸铋相等时,反应体系中过量的WO₄^2-离子使Bi₂WO₆成核与生长的速度变快,初级纳米片在生长的同时就与邻近的其它纳米片以面-面相连的方式堆积,导致片与片之间界面有足够的机会溶解-重结晶。硝酸铋和钨酸钠都取0.5mmol时,产物的形貌与人体血液中的红细胞非常相似。2、以所合成的不同形貌的Bi₂WO₆为研究对象,探讨形貌、结构对Bi₂WO₆在可见光下催化活性的影响,研究发现,形貌对Bi₂WO₆的光催化活性有明显影响,分等级的球状和八面体状Bi₂WO₆表现出最强的催化活性,可见光照6h后可将罗丹明B基本降解完全。片状Bi₂WO₆的催化活性略低。相同形貌的Bi₂WO₆在不同pH值的催化体系中具有不同的催化活性,pH为7.5时,片状Bi₂WO₆和八面体状Bi₂WO₆分等级结构均表现出更高的催化活性,酸性下催化效果较差;我们认为这是由于Bi₂WO₆在酸性环境下不稳定,会部分转化为H₂WO₄,使得光催化剂的有效部分减少,因而催化活性较低。结晶度良好尺寸较小的Bi₂WO₆具有更高的催化活性。我们在水热180℃得到的各种形貌的Bi₂WO₆,除片状Bi₂WO₆为二维纳米结构外,其余三种都为整体尺寸较大的三维分等级结构。但是,由于这些Bi₂WO₆的组建基元为尺寸很小的纳米片,这些样品在相同条件下都表现出比固相法制备的Bi₂WO₆更高的催化活性。3、为了进一步提高Bi₂WO₆的催化活性,我们利用葡萄糖的碳化现象,在水热过程中添加不同量的葡萄糖,然后将水热产物进行煅烧,获得了碳修饰的Bi₂WO₆催化剂,并研究了其对罗丹明B的降解效率。与水热过程中未添加葡萄糖的煅烧产物相比,水热过程中添加适量葡萄糖能明显提高催化剂对可见光的吸收,增强催化剂对染料的吸附能力,加快光生电子从光催化剂材料向染料的传输速率,使得Bi₂WO₆的催化活性大大提高。但是过量的葡萄糖会使Bi₂WO₆表面大多被碳包覆,减少催化剂对光的本征吸收,使得样品的催化活性不如煅烧后纯的Bi₂WO₆。4、Bi氯氧化物作为铋系化合物中重要的的一类半导体,具有层状结构和高化学稳定性。铋氧层与氯层之间存在较强的内在电场,能够提高光生电子空穴对的有效分离,因此表现出较高的催化活性。我们用无水乙醇做溶剂,以BiCl₃为原料,以ZnO阵列为模板,来制备BiOCl阵列。XRD结果显示,浸泡时间较短时得到的产物为Bi₃O₄Cl,浸泡时间较长时得到的产物为BiOCl。SEM结果显示,两种产物均为很薄的纳米片组装的薄膜。TEM显示所得的Bi₃O₄Cl薄片为单晶结构。根据紫外可见漫反射吸收谱换算出两产物的禁带宽度分别为2.658eV和3.15eV。更多还原

【Abstract】 The development of new visible light-driven photocatalysts is essential to the control of environment pollution and the use of sustainable energies.Bi ₂WO₆ has shown visible-light photocatalytic property in solar energy conversion and the treatment of environment pollution.As an excellent visible-light photocatalyst,it has attracted much attention in recent years.Previous reports demonstrated that some simple nanostructures of Bi₂WO₆ such as nanoparticles,nanorods and nanosheets could be synthesized using soft chemical method.However,three-dimensional（3D） hierarchical structures of Bi₂WO₆ have rarely been reported.Since 3D structures are generally with larger size while their subunits are at the nanosize length scale,they possess the advantages of the subunits during photocatalytic reaction and can be easily recycled.Therefore,the fabrication of 3D Bi₂WO₆ hierarchical structures is highly desirable.On the other hand,carbon has very high conductivity;some reports showed that the carbon coating on UV-driven photocatalysts can significantly improve the photocatalytic performance.But there is no report on the modification of Bi₂WO₆ with carbon and exploitation of the photocatalytic activity of C-Bi₂WO₆ structures.This thesis first discusses the hydrothermal synthesis of Bi₂WO₆ hierarchical structures with different morphologies and investigates their visible light catalytic properties.With respect to the modification of Bi₂WO₆,C-Bi₂WO₆ nanostructures will be fabricated by the hydrothermal carbonization of glucose during the growth of Bi₂WO₆.The photocatalytic activity of C-Bi₂WO₆ nanostructures toward RhB degradation under visible light is investigated,and the effect of carbon content on the photocatalytic performance is discussed.In addition,Bi₃O₄Cl and BiOCl films are fabricated using ZnO nanorod array as template and BiCl₃ as the precursor.The film formation mechanism is primarily investigated.The main contents are as follows:1.The control of Bi₂WO₆ nanostructure morphology by varying the growth temperature,pH value,surfactant concentration,and the molar ratio of starting materials.Bi₂WO₆ dispersed nanosheets, hierarchical microspheres,octahedron- and red cell-like structures can be controllably synthesized.（1） When there are only Bi（NO₃）₃ and Na₂WO₄ in the reaction system,dispersed 2D Bi₂WO₆ square nanosheets can be obtained after 180℃hydrothermal synthesis for 12 h.These nanosheets actually consist of many smaller nanoplates assembled in an edge-by-edge fashion.The formation of 2D nanoplates is attributed to the specific crystal structure of Bi₂WO₆.（2） We discuss the effect of surfactant PVP on the morphology of products and find that PVP plays an important role in the structural formation.When 0.15g PVP is used,the product is Bi₂WO₆ hierarchical microsphere made up of numerous Bi₂WO₆ nanosheets.Without PVP,only dispersed square nanosheets can be attained.With the increase of PVP content,2D round disks and 3D structures emerge and the number of 2D sheets decreases,and finally we can get homogeneous 3D structures.However,with too much PVP added,only 2D round disks can be generated.（3） The morphology of Bi₂WO₆ can also be manipulated by changing the pH value of reaction system.When the pH is 7,the product is hierarchical octahedron-like structure.We propose that the pH can significantly affect the hydrolysis rate of Bi³⁺.At pH=7,the nucleation and growth rates of Bi₂WO₆ are appropriate,the initially formed small plates assemble in an edge-to-edge way with the gradual enlargement of the 2D surfaces.A subsequent layer-by-layer growth of the large nanoplates gives many parallel square plates.Since the formation of parallel plates occurs simultaneously in three dimensions（X,Y,Z）,three groups of parallel square plates finally cross-link with（and vertical to） each other,forming octahedron-like structures.（4） When the molar ratio of Na₂WO₄ to Bi₂WO₆ is 1,the structure of nanosheet is compact and the interfaces between primary nanoplates disappear.This is because under this condition the WO₄^2- is excessive,making the nucleation and growth of Bi₂WO₆ very quick.In this regard,primary nanoplates can simultaneously assemble in edge-to-edge and face-to-face fashions,so the interfaces between them have more opportunities to undergo the dissolution-recrystallization process.When Na₂WO₄ and Bi₂WO₆ are both 0.5 mmol,red-cell-like 3D hierarchical structures are attained.2.Based on the different structures obtained above,we study the effect of morphology and structure of Bi₂WO₆ on the visible-light photocatalytic activity.The results show that Bi₂WO₆ hierarchical microspheres and octahedra have the better performance than dispersed 2D nanosheets. The RhB can be thoroughly degraded after 6 h visible light irradiation.All the complex hierarchical structures of Bi₂WO₆ have superior performance over that fabricated by conventional solid-state reaction.In addition,Bi₂WO₆ with the same morphology has different photocatalytic activities in RhB solutions with different pH.When pH=7.5,2D sheets and 3D octahedra have better performance.The sensitivity of photoactivity to the pH value ascribes to the fact that Bi₂WO₆ is not stable under acid environment and can be transformed into H₂WO₄.3.In order to improve the photocatalytic property of Bi₂WO₆,we fabricate C-Bi₂WO₆ structures by the hydrothermal carbonization of glucose during the growth of Bi₂WO₆ followed by high-temperature calcination.The degradation of RhB under visible light using this photocatalyst is further investigated.We find that C-Bi₂WO₆ photocatalyst usually has better performance than pure Bi₂WO₆.The presence of carbon can significantly increase the visible light absorption and dye loading capability,and accelerate the electron transfer rate from photocatalyst to dye RhB.However, too much carbon on the Bi₂WO₆ surface makes the light all absorbed by carbon,the photocatalyst thus can not be utilized,leading to very poor performance,even worse than that of pure calcinated Bi₂WO₆.4.Bismuth oxychlorides are one kind of important semiconductors in Bi-system compounds and have layered Structure and high chemical stability.There are strong inner electric field between Bi-O layer and Cl layer,leading to very fast photo-generated electron-hole separation,so they exhibit good photocatalytic activities.We fabricate for the first time BLOCl film using ZnO nanorod array as the template,BiCl₃ as the precursor and ethanol as the solvent at room temperature.XRD pattern indicates that Bi₃O₄Cl is attained when the immersion time is short,and after long-time reaction, BiOCl is fabricated.SEM results demonstrate both Bi₃O₄Cl and BLOCl films are constructed by very thin nanosheets.TEM result reveals the single-crystal nature of the nanosheet.The bandgaps of Bi₃O₄Cl and BiOCl can be further determined as 2.658eV and 3.15eV respectively by using UV-vis diffuse reflectance spectroscopy.更多还原