钨钼钒锰基氧化物微纳材料的设计合成及其在能源领域的应用

【作者】 周亮；

【导师】 赵东元； 余承忠；

【作者基本信息】 复旦大学 ， 无机化学， 2011， 博士

【摘要】 能源问题是当前全世界关注的焦点。太阳能是一种取之不尽用之不竭的清洁能源。利用太阳能,半导体光催化剂不仅可以催化降解水中、空气中有毒化合物,还可将太阳能直接转化为电能、化学能等。因此,光催化对解决能源短缺和环境污染等问题十分重要,而其核心问题则是高性能光催化材料的制备。此外,太阳能是一种间歇性能源,而化学电池是一种可以实现化学能与电能之间相互转化的储能装置,弥补太阳能间歇性的不足。目前发展最为迅猛的新型锂离子电池,其核心技术是高性能电极材料的制备。针对上述能源领域中存在的问题,本论文设计合成了一系列钨钼钒锰基新型氧化物材料,包括二元氧化钨、钼,三元钨钼氧化物固溶体、钒酸铜,四元钨酸钼酸铋固溶体,以及尖晶石型锰酸锂,对其形貌和结构进行了详细的表征,考察了其在光催化以及锂离子电池中的应用。在第三章中,以过氧聚钨酸为前驱物,通过一步水热反应合成了WO3·0.33H2O六方状单晶纳米片。正交相WO3·0.33H2O为介稳定相,经过350和550℃焙烧处理可分别转化为六方相h-WO3纳米片和单斜相m-WO3亚微米粒子。这些材料具有清洁的表面(没有表面活性剂的包裹),可望具有良好的光催化活性。在第四章中,以过氧钼酸为前驱物,通过一步水热反应合成了a-MoO3单晶纳米带。作为锂离子电池的正极材料,该a-MoO3单晶纳米带具有很大的放电容量(首次放电容量高达264 mAh/g),优异的倍率性能(5000mA/g的电流下仍具有176mAh/g的容量)和较好的循环性能(5000mAh/g电流下循环50次后仍具有114mAh/g的容量)。Mo6+的多电子还原反应为材料提供了巨大的放电容量,而材料的纳米带形貌保证了其倍率性能和循环性能。在第五章中,以过氧聚钨酸和过氧钼酸为前驱物,通过水热合成,制备了系列不同钼含量x的MoxW1-xO3·0.33H2O (x= 0,0.25,0.50,0.75)固溶体材料。随着固溶体材料中钼含量x的增加,材料中起到色心作用的M5+(M= Mo, W)含量也逐渐增加,导致材料的能带间隙由3.25 eV逐渐减小至2.77 eV。在第六章中,用水热合成的方法制备了一系列化学组成的Bi2MoxW1-xO6 (x= 0,0.25,0.50,0.75,1.00)固溶体材料。与目前研究最多的Bi2WO6光催化剂相比较,Bi2MoxW1-xO6 (x= 0.25,0.50,0.75)的价带顶得到了较大的升高,因此它们的能带间隙变窄,能吸收更多的可见光。以可见光(λ>400 nm)照射下亚甲基蓝的光降解反应为模型反应,研究了Bi2MoxW1-xO6固溶体材料的光催化活性,其中Bi2MO0.2W0.75O6材料具有最佳的光催化活性。材料的层状结构、纳米薄片形貌、能带结构、能带间隙和钨含量等均对其光催化活性有着重要的影响。该研究为设计合成能带间隙可调、形貌可控的高效光催化剂提供了新思路。在第七章中,以过氧钒酸为钒源,硝酸铜为铜源,用尿素原位地调节体系的pH值,通过简单的一步水热反应,制备了由Cu4V2.15O9.38纳米薄片和纳米厚片穿插而成的纳米／微米多级结构。以该CU4V2.15O9.38多级结构作为一次锂离子电池的正极材料,它在1.5 V以上显示出高达471mAh/g的首次放电容量。该容量远远超过了商品化的Ag2V4O11正极材料(理论容量为315 mAh/g)。因此,Cu4V2.15O9.38是一种非常有应用前景的一次锂离子电池正极材料,有望用于心率转复除颤器等医疗器件。在第八章中,我们首先用沉淀反应制备了MnCO3微米球,通过400℃C焙烧将其转化为多孔MnO2微米球,再通过浸渍锂盐和600℃高温焙烧,得到了LiMn204空心微米球材料。高温焙烧过程中介孔的融合以及"Kirkendall效应”是微米球中心空腔产生的原因。通过对MnCO3前驱物的“可控分解”和“选择性溶解”,可进一步控制产物LiMn2O4材料的墙壁厚度和空腔大小。拥有小空腔厚墙壁的LiMn2O4-A空心球比大空腔薄墙壁的LiMn2O4-B空心球具有更佳的电化学性能。在0.1和5 C (1 C= 148 mAh/g)电流下,LiMn2O4-A空心球分别具有120和106.7 mAh/g的容量,1 C电流下循环105次后其容量保持率为96.6%。与传统的研磨法相比较,本合成过程中用到的浸渍法能实现反应物在纳米级的均匀混合,降低锂化反应温度,提高产物的纯度和电化学性能。更多还原

【Abstract】 Energy has become a global social issue. The solar energy is an abundant and clean alternative energy. Photocatalysis is a technology utilizing the solar radiation to decompose the contaminants in water/air, and to convert the solar energy into electricity and chemical energy. Thus, photocatalysis plays an essential role for clean energy production and environmental remediation, where the preparation of high efficient photocatalysts is the key in this area. Because the solar energy is intermittent, energy storage devices such as batteries are required to provide back-up for intermittent solar energy. Lithium ion batteries (LIBs) are developing very fast and have dominated the markets of portable electronics in recent years. The key technology for LIBs is the fabrication of high performance electrode materials.This dissertation aims at the designed synthesis of novel materials with energy applications. A series of nano-and micro-structured W, Mo, V, and Mn based materials have been synthesized, including binary tungsten trioxide (WO3) and molybdenum trioxide (MoO3), ternary molybdenum-tungsten trioxide hydrate (MoxW1-xO3·0.33H2O) and copper vanadium oxide (Cu4V2.15O9.38), quaternary bismuth molybdenum tungsten oxide (Bi2MoxW1-xO6), and lithium manganese oxide spinel (LiMn2O4). The morphology and structure of these materials have been characterized in detail, their applications in LIBs and photocatalysis have also been studied.In Chapter 3, a facile route for the preparation of hexagonal shaped WO3·0.33H2O nanodiscs via hydrothermal treatment of peroxo-polytungstic acid solution has been developed. The orthorhombic WO3·0.33H2O is a metastable phase, calcination at 350 and 550℃leads to the formation of hexagonal h-WO3 nanodiscs and submicrometer sized monoclinic m-WO3 nanoparticles, respectively. The complete absence of protecting agents (e.g. surfactants) on the surface may make the products promising in photocatalysis.In Chapter 4, single-crystallineα-MoO3 nanobelts have been synthesized through a facile hydrothermal treatment method using peroxo-molybdic acid as the precursor. As a cathode material for LIBs, theα-MoO3 nanobelts exhibit high discharge capacity (264 mAh/g at 30 mA/g), excellent rate capability (176 mAh/g at 5000 mA/g), and good cycling performance (a capacity of 114 mAh/g was retained after 50 charge-discharge cycles at 5000 mAh/g). The multi-electron reduction process of Mo6+provides this material with high capacity, while the nanobelts morphology of the products ensures its rate capability and cycling performance.In Chapter 5, a series of MoxW1-xO3·/0.33H2O (x= 0,0.25,0.50,0.75) solid solutions have been synthesized through a facile hydrothermal treatment method, starting from a mixture of aqueous peroxo-polytungstic acid and peroxo-molybdic acid. With the increase of Mo content x, the band gap of MOxW1-xO3·0.33H2O narrowed from 3.25 to 2.77 eV. The increase M5+(M= Mo, W) fraction, which acts as the "color center" in the materials, is responsible for the narrowing of the band gap.In chapter 6, a series of homogeneous Bi2MoxW1-xO6 (x= 0,0.25,0.50,0.75,1.00) solid solutions have been synthesized by a hydrothermal crystallization method. When compared to the most studied Bi2WO6 photocatalyst, the valence band (VB) maximum of Bi2MoxW1-xO6 (x= 0.25,0.50,0.75) is elevated significantly, leading to a narrowed band gap and enhanced visible light harvesting ability. The photocatalytic activities of the Bi2MoxW1-xO6 solid solutions have been evaluated by utilizing the photodecomposition of methylene blue (MB) under visible light (λ> 400 nm) irradiation as a model reaction. Bi2Mo0.25W0.75O6 shows the highest photocatalytic activity for MB photodecomposition among the Bi2MoxW1-xO6 solid solutions. Our study sheds light on designing highly efficient visible-light-driven photocatalysts with controlled band gap and morphology.In Chapter 7, hierarchical Cu4V2.15O9.38 micro-/nanostructures composed of intergrown nanosheets and nanoplates have been synthesized via a facile hydrothermal treatment method. As a cathode material for primary LIBs, it delivers a high discharge capacity of 471 mAh/g above 1.5 V, which is much higher than the theoretical capacity of the commercial Ag2V4O11 cathode (315 mAh/g). The high discharge capacity makes the Cu4V2.15O9.38 a promising cathode material for implantable cardioverter defibrillators (ICDs).In Chapter 8, LiMn2O4 hollow spheres have been prepared using a simple solid state reaction between porous MnO2 microspheres and LiOH·H2O. The fusion of the initial mesopores and the "Kirkendall effect" during the lithiation process are responsible for the formation of the hollow interiors. Both the wall thickness and void size can be tuned by employing a "controlled decomposition" and "selective etching" process. The LiMn2O4-A hollow spheres with a thick wall and small void show much better electrochemical performances than that of LiMn2O4-B with a thin wall and large void. LiMn2O4-A hollow spheres deliver a discharge capacity of～120 mAh/g at 0.1 C,106.7 mAh/g at 5C, and retains 96.6% of the capacity after cycling at 1 C for 105 charge-discharge cycles. When compared to the traditional grounding method, the impregnation method we employed allows for a homogeneous contact of the reagents at the nanoscale, thus lowers the lithiation temperature, and leads to products with higher purity and better electrochemical performances.更多还原