形貌可控的“有序结”光催化剂和金属助剂的合成研究

发布时间：2018-03-14 09:34

本文选题：光催化　切入点：制氢　出处：《西安交通大学》2017年博士论文　论文类型：学位论文

【摘要】：面对化石能源的日益枯竭及由此带来的能源和环境危机,寻找新型可再生、无污染的替代能源已经刻不容缓。利用太阳能光催化分解水制氢,将太阳能转换为可存储、高能量密度且清洁无污染的氢能,是解决这些危机的有效途径之一。然而制约这一技术产业化的瓶颈在于高效可见光光催化剂的开发。随着对半导体光催化过程的研究,人们发现,光生电荷的分离及其在晶体表面发生有效氧化还原反应是提高太阳能氢能转化效率并最终工业化的关键。本文首先从提高光生电子和空穴分离出发,以具有暴露{101}和{001}晶面的Cu_2WS_4单晶十面体和孪晶Cd_(1-x)Zn_x S固溶体纳米球和一维纳米棒为模型光催化剂,提出通过有序“结”结构设计,实现光催化产氢反应中的物理过程强化,并构建了这些有序“结”与产氢性能的定性和定量的关系。其次,从强化界面化学反应和揭示金属助催化剂促进光催化反应的作用机制的角度着手,通过对金属助催化剂生长动力学的表面扩散和刻蚀再机理的系统研究,完成了不同形貌金属助催化剂生长过程的动力学定量理论体系的构建,为后面深入研究其作用机理打下基础。概括起来,主要分为以下内容和结论。首先,采用一步水热方法,以Cu Cl、Na_2WO_4为前驱体,硫代乙酰胺为硫源成功合成具有{001}和{101}两种晶面且比例可控的十面体形貌Cu_2WS_4单晶半导体。通过原位光还原贵金属和创新的氧化刻蚀实验发现这两种晶面能有效分离光生电子和空穴,这种类似于半导体中“结”的结构,可被称之为“类同质结”。该催化剂在负载1.5 wt%Ru金属助催化剂后,效率可以提高30倍左右,425 nm处表观量子效率也达到11%,为国际同期报道光催化剂的较高水平。基于对该体系下的光致氧化和还原反应的动力学的定性认识(还原反应为光反应中的限速步骤),通过对和{101}晶面比例的控制,实现了对氧化还原反应活性位面积的有效调控,并在高{001}/{001}晶面比例的Cu_2WS_4十面体上获得了更高的产氢活性。这种借助有单晶晶面设计所形成的类同质结,分布有序、结构可调,可以方便地用于光催化反应动力学的定量研究,也为获得更高活性的光催化剂提供了新的思路。晶面类同质结虽然有序,但其受晶面个数的限制而数量有限,急需找到一种既能保持有序性、还能高密度获得的结构。基于晶面面结构差异而产生“结”效果的思想,很容易让人联想到晶体中的缺陷。缺陷处也会发晶体结构畸变,可联想到面心立方晶体中的平行共格孪晶面。以Cd_(1-x)Zn_x S固溶体为模型,利用水热方法合成了包含大量贯穿整个晶体的平行共格孪晶面的闪锌矿(Zinc blende,ZB)结构球形Cd_(1-x)Zn_x S半导体。在结构上,每一个这种共格孪晶面实际上在ZB Cd_(1-x)Zn_x S晶体中引入了一个具有3层双原子层的纤锌矿(Wurtzite,WZ)结构片段。通过计算发现,Cd_(1-x)Zn_x S晶体中ZB/WZ界面可以形成II型交错式能带排列结构,并能迅速发生光生电荷分离,因此也是“结”的作用方式,可称之为“孪晶同质结”。在电势差的作用下,电子能够定向移动到两个孪晶面中间的ZB区域发生还原反应,空穴被吸收到孪晶面上发生氧化反应。对比相同成分非孪晶固溶体,发现孪晶Cd_(1-x)Zn_x S固溶体纳米球具有更高的产氢活性,其最高产氢活性出现在x=0.5处,即Cd_(0.5)Zn_(0.5)S,为1.79 mmol h~(-1),425 nm处量子效率为43%,是国际同期报道的无负载光催化剂中最高值。进一步研究发现,这些结平行、有序地分布于晶体中,因此,如果能在孪晶面垂直方向获得一维结构,应该能够更好地发挥平行孪晶同质结的协同效应。利用动力学控制,制备了具有长径比约为1.9和4.0的纳米孪晶棒,而基于这些光催化剂的光催化分解水产氢结果也证明了上述想法。具有最高长径比的Cd_(1-x)Zn_x S纳米棒,其可见光产氢速率达到了2.58 mmol h~(-1),其425 nm处的量子效率为62%,为目前国际报道无负载光催化剂的最高效率。研究结果直接证明了以高密度、有次序的平行共晶格孪晶面为结构基础的一维同质结对光催化的效率的提高作用。通过单晶晶面和孪晶构建的(类)同质结,如同异质结光催化剂一样可以广泛应用于其他半导体光催化剂的设计,开创了强化电荷分离物理过程的新方法。光催化是界面上的光化学反应,需要基于化学反应的协同强化才能进一步提高光催化效率。金属助催化剂在产氢过程中起着至关重要的作用,而其在光催化中的作用机制与其微观结构息息相关。基于此,本文最后以金属Pd为模型,使用种子生长的方法,对金属生长过程中表面扩散、刻蚀再生长过程进行了系统的定量研究,实现了对单金属、双金属(包括合金)纳米立方体、纳米八面体和各类高指数面纳米晶体等的定量动力学控制。在含有助催化剂参与的光催化界面化学反应过程中,助催化剂分布状态(如大小、形貌等)的定量控制是实现助催化剂作用机理认识及效用最大化的有效途径。
[Abstract]:With the increasing depletion of fossil fuels and the resulting energy and environmental crisis, looking for new renewable, non polluting alternative energy sources is urgent. The photocatalytic decomposition of water to hydrogen using solar energy and convert solar energy into storage, high energy density and clean energy, is one of the effective ways to solve the crisis. However, the bottleneck the technology industry is to develop efficient visible light catalyst. With the research of semiconductor photocatalytic process it was found that the separation of photogenerated charge and effective redox reaction is to improve the conversion efficiency of solar hydrogen and ultimately the key to industrialization on the crystal surface. This paper improve the separation of the photogenerated electrons and holes starting with the exposure of {101} and {001} surface of single crystal Cu_2WS_4 ten surface and twin Cd_ (1-x) Zn_x S solid solution nanoparticles and a rod for Wiener meters Model of photocatalyst, put forward through the orderly "knot" structure design, physical implementation process of photocatalytic hydrogen production in strengthening, and the construction of the relationship between these ordered "knot" and hydrogen production performance of qualitative and quantitative. Secondly, starting from the mechanism of enhancement of interfacial chemical reaction and reveal the metal catalytic agent to promote the photocatalytic reaction the angle of the system, through the research of surface diffusion on metal catalyst growth kinetics and etching mechanism, completes the construction of quantitative kinetic theory system with different morphologies of metal catalyst of the growth process, for the following deep to lay the foundation for the study of its mechanism. To sum up, mainly divided into the following contents and conclusions. First of all, a one-step hydrothermal method with Cu, Cl, Na_2WO_4 as precursor and thioacetamide were successfully synthesized with {001} and {101} two kinds of crystal surface and the proportion of the ten controllable shape surface Cu_2WS_4 The single crystal semiconductor etching. Oxidation reduction of noble metal and innovation by in situ light found that two kinds of crystal surface can effectively separate the photogenerated electrons and holes, the similar structure on the semiconductor in the "knot", can be called the "homojunctions." the catalyst at a load of 1.5 wt%Ru metal catalyst, efficiency can be improved by about 30 times, 425 nm apparent quantum efficiency reached 11%, as reported in the same period of photocatalyst international high level. Qualitative understanding of the oxidation and reduction reaction induced by dynamics in the system of light (based on the reduction reaction is light reaction in the limit of Su Buzhou), by controlling the crystal surface and {101} ratio the realization of the effective regulation of the redox reaction activity area, and in the high {001}/{001} surface ratio of Cu_2WS_4 ten surface was obtained on hydrogen production with higher activity. This is formed by single crystal homogeneous design Node distribution, orderly, adjustable structure, can be used for the quantitative study of photocatalytic reaction kinetics, to light the more active catalysts provide a new way of thinking. Crystal class homojunctions although orderly, but affected by the limited number of planes and a limited number of urgent need to find a can keep order, can obtain high density structure. Have a "knot" effect of crystal plane structure based on the difference of the thought, it is reminiscent of the defects in the crystal. The defects will be made of crystal structure distortion, Lenovo to F.C.C parallel coherent twin plane (1-x to Cd_. Zn_x) S solid solution as a model, a large number of runs through the whole crystal parallel coherent twin surface of sphalerite was synthesized by hydrothermal method including (Zinc blende, ZB) Cd_ (1-x) Zn_x spherical structure. The structure of S semiconductor, ZB Cd_ in each of the coherent twin surface reality (1-x) Zn_ The introduction of X S crystal in a double atomic layer has 3 layers of wurtzite structure (Wurtzite, WZ) fragment. It was found that the Cd_ (1-x) ZB/WZ interface Zn_x S crystal can be formed in a staggered type II band alignment structure, and can quickly place photoinduced charge separation, mode of action and is therefore "knot", which can be called "twin homojunctions." in the potential function, the electron can move to two ZB regional directional twin plane in the middle of the reduction reaction, the hole is absorbed into the oxidation reaction on the twin plane with the same components. Untwinned solid solution, the results show that the twins Cd_ (1-x) Zn_x S solid nanospheres with hydrogen producing high activity, the highest hydrogen production activity appeared at x=0.5, Cd_ (0.5) Zn_ (0.5) S, 1.79 mmol h~ (-1), 425 nm quantum efficiency is 43%, no photocatalyst maximum in international reporting period further study found that these Parallel, orderly distribution in the crystal, therefore, if we can get one-dimensional structure in the twin plane vertical direction, should be able to play a better parallel twin homojunctions synergy. Controlled by kinetics, were prepared with the length diameter ratio is 1.9 and 4 nano twin rods, and the photocatalytic decomposition of these catalysts hydrogen results also proved that the idea has the highest aspect ratio. Based on Cd_ (1-x) Zn_x S nanorods, the visible light hydrogen production rate of 2.58 mmol h~ (-1), the quantum efficiency at 425 nm was 62%, the highest efficiency of current international reported photocatalysts. The results directly proved to high density, increase efficiency and order of the co twin plane parallel lattice structure based on one-dimensional homogeneous photocatalysis. Constructed by single crystal and twin (class) homojunction, heterojunction photocatalysts can be the same as Widely applied to other semiconductor photocatalyst design, created a new method to enhance the charge separation process. Physical photocatalysis is a photochemical reaction on the interface, need chemical reaction and enhanced in order to further improve the photocatalytic efficiency. Based on the metal catalyst plays a crucial role in the hydrogen production process, and its photocatalytic the mechanism is closely related to its microstructure. Based on this, this paper finally using metal Pd as a model, using the method of seed growth, the metal surface diffusion growth process, etching regrowth process was studied systematically, the realization of single metal, double metal (including alloy) nanocubes, quantitative dynamics control nano eight surface and all kinds of high index surface nano crystal etc.. The photocatalytic interfacial chemical reaction with the catalyst involved in the process of the catalyst distribution (such as large The quantitative control of small, morphologies, etc. is an effective way to realize the mechanism understanding and utility maximization of cocatalyst.

【学位授予单位】：西安交通大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：O643.36;TQ116.2

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