摘要
设计贵金属Au、Ag纳米结构高指数晶面及调制催化性质研究全球人口不断增加，能源需求日益扩大，气候变化和环境问题日趋严重，如何保护我们赖以生存的地球家园，可谓是迫在眉睫！催化降解污染物、发展可持续清洁能源是解决上述问题的有效手段。燃料电池及氢能源是目前最具应用前景的清洁能源，但无论是催化降解污染物，还是发展燃料电池或氢能源都需要高效的催化剂。因此，如何提高催化剂的催化性能和节约应用成本是技术应用的核心问题。也就是说设计优化催化剂的组成成分、纳米结构、形貌、裸露晶面及大小尺寸是提高催化剂催化性能的关键所在。
金Au、银Ag贵金属纳米催化剂，具有生物相容性、稳定性，是常用高效、持久的催化剂，在催化降解有机污染物、汽车尾气处理及CO2催化还原应用中都取得了出的效果。大量的实验证明，催化剂裸露的表面晶面直接影响着催化剂的催化反应速率及激活势垒。高指数晶面，相对于低指数的基础晶面，存有大量的台阶面、晶体缺陷及丰富的低配位原子，具有更好的催化活性。本文重点发展简单、绿、经济的水相合成方式，通过选择不同的还原剂、调控溶液的pH 环境、控制反应动力学、可控设计合成出富有高指数晶面的多孔Au、Ag纳米结构及多面体，优化并提升催化降解有机污染物（p-硝基苯酚等）、葡萄糖燃料电池及电解水析氢性能，揭示贵金属纳米催化剂的形貌、尺寸、结构与催化性能的构效关系。开展了如下的研究内容：
1.多孔Ag的可控制备及其催化性能研究。我们实现了对多孔Ag纳米晶的形
貌、尺寸、结晶性及孔径分布的可控制备，并优化提升了其催化性能。多孔Ag催化性能的优化主要是通过调控以下参数实现的：多孔Ag的比表面积及在多孔结构表面的台阶steps、突起ledges和缺陷kinks处的Ag原子数目。
这部分工作，我们主要是发展了一种简单的两步法合成体系。通过调节溶液的pH环境来调控多孔Ag的形貌及尺寸（形貌可实现梭形及四边形的可控合成，尺寸可实现2.5 μm到36 μm的调控）；通过调节热处理的温度、升温速率、加热时间调控多孔Ag纳米晶的结晶性、表面缺陷及孔径分布。随后，在p-硝基苯酚等有机污染物催化降解应用中，我们探究了多孔Ag的形貌与性能的构效关系。研究结果表明：多孔Ag在催化降解有机污染物时表现出优异的催化活性。其中，4 μm的四边形多孔Ag的催化活性最好，其反应速
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率可达到6.43*10-2 s-1，并且在多次循环利用后仍保持较高的催化活性。多孔Ag的高催化活性主要归结于：规则形貌（梭形或者四边形）、多孔结构、高比表面积、优异的渗透性、离子迁移率以及丰富的高指数晶面等高活性位点。
鉴于多孔Ag上述的结构优势，相信其在电化学催化、传感和驱动器等实际应用中存在潜在的应用前景。
2.多孔Au的可控制备及其GOR催化性能研究。我们创造性地发展了低温动
力学方式，成功合成出多孔Au纳米结构。此合成方法突破了经典图尔克维奇（Turkevich）利用柠檬酸钠热还原氯金酸制备Au的方式。其中，低温合成条件改变了柠檬酸根在Au表面的吸附方式，获得表面“清洁”的多孔Au 纳米晶。基于多孔Au的结构特点，我们探究了其表面结构与催化性能的关系。研究表明，表面“清洁”的多孔Au在葡萄糖电催化氧化中取得出的催化效果，其催化氧化电流密度可高达9 A cm-2 g-1，是Turkevich-Au（0.45 A cm-2 g-1）的20倍，也是当前所报道的最优值（2.65 A cm-2g-1）的3.4倍。多孔Au的高催化性能，主要归结于以下两方面：1）多孔Au具有清洁的表面、能够自由传输电子、最大化地暴露其活性位点；2）多孔Au具有大的电化学比表面积、多孔结构、良好的离子迁移率、丰富的高指能面及原子台阶位steps 等高活性位点。高性能多孔Au在葡萄糖电催化氧化中的成功应用，为葡萄糖燃料电池在未来生活中的实际应用提供了广阔的商业应用前景。
3.可控制备二十面体Au及其HER性能研究。我们继续发展了低温动力学的
方式，成功制备出表面“清洁”的多重孪晶二十面体Au。同时，通过调控反应时间，设计合成出截角二十面体Au。（截角）二十面体Au具有丰富的孪晶界及边角原子等高活性位点。将二十面体Au应用到电解水HER催化反应中，我们发现随着测试循环次数的增加，二十面体Au的催化活性随之增加（过电位、Tafel斜率、电阻都随之降低）。30 000次循环后，二十面体Au 的催化活性基本可以和商业Pt-C相媲美。初步的研究结果表明：催化剂的晶体缺陷能够显著增强其催化活性，多重孪晶中存在可调的应力应变；外加电位提供外部动力压缩了Au的晶格参数、使d带电子上移、优化了Au-H 键能、提高了电解水H
ER性能。
关键词：
贵金属纳米催化剂，高指数晶面，应力应变，多孔结构，清洁表面，葡萄糖氧化，析氢反应，电催化性能
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Abstract
To design synthesis of Au or Ag noble metal nanostructures with high-index facets and to modulate their catalytic activity
The increasingly serious energy crisis and environmental problems are the biggest obstacles for us to protecting our homes. Electrocatalytic energy-conversion processes are expected to be the most promising clean energy in the development of sustainable technologies that mitigate global warming and lower our dependence on fossil fuels. But, the highly efficient catalysts are very important for electrocatalytic energy-conversion processes. Therefore, it is critical to improve catalytic activity and generate cost-savings. That is to say, we need to optimize the compositions, nanostructures, shapes, l
attice and sizes of the catalysts.
Noble metal nanocrystals, including gold (Au) and silver (Ag), are the most efficient and stable catalysts owing to its relatively low cost compared with Pt, unique biocompatibility and high stability. They are commonly used to degrade contaminants, purify automotive exhaust and electrocatalytically reduce CO2. It is confirmed by experimental measurements and theoretical simulations that the reaction rate and activation energy depends on the crystal facet exposed on its surface. Compared with low-index facets, there are    a large amount of steps, crystal defects and low-coordinated atoms on the high-index facets. Therefore, nanocrystals with high-index facets have better catalytic performance. In this thesis, we focus on the controlled synthesis of noble metal nanocrystals with high-index facets (including Au and Ag) by a straightforward, environmental and economical means and further optimizing the catalytic degration of p-nitrophenol, glucose electrooxidation reaction (GOR) and hydrogen evolution reaction (HER). During the synthetic process, we select different reductant, modulate the solution pH environment, and design the reaction kinetics. And, we revealed structure-function relationship between catalytic performance and the shapes, sizes and nanostructures of nanocatalysts. The related research works are summarized as follows.
1.Design of porous Ag platelet structures with tunable porosity and high
catalytic activity. To optimize the catalytic performance for Ag nanocatalysts，
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we design shape-, size-, porosity-, and crystallinity-controlled and surface defect tailored porous Ag nanostructures. And, we implemented the following parameters to tune their catalytic activity: the surface-to-volume ratio, and the number of atomic steps, ledges and kinks on the porous Ag. In this work, a low cost and facile synthetic route is presented to produce regular porous Ag platelet structures with controlled size, porosity, crystallinity and surface defects by a two-step process. Initially, size-tailored regular Ag platelet precursors from 2.5 μm to 36 μm are obtained by merely adjusting the solution pH value; Simply by optimizing the annealing time, temperature and heating rate, porous Ag platelets with effectively tunable porosity, crystallinity and surface defects can be achieved.
Besides, the reduction of p-nitrophenol to p-aminophenol using NaBH4was chosen as a representative catalysis to evaluate the catalytic activity of the tunable porous Ag platelet structures. And, we study the relationship between catalytic performance and their shapes. The porous Ag platelet structures could catalyze the reduction of p-nitrophenol and dyes quite effectively at room temperature. It is noted that the QPP Ag(I) structure is the best one among the four porous Ag platelet structures, which is attri
buted to their regular morphologies, porosity, high surface-to-volume ratio, short diffusion length and good permeability.
Moreover, these porous Ag platelet structures because of their unique morphology and network characteristics will exhibit excellent electrochemical catalytic activity, or act as outstanding electrodes, sensors, actuators, etc.
2.Kinetically Controlled Synthesis of Nanoporous Au and their Enhanced
Electrocatalytic Activity for Glucose-based Biofuel Cells: we design surface-clean nanoporous gold (NPG) structures with high-index facets by a kinetically controlled self-assembly manner. This strategy breaks through the traditional trisodium citrate thermal-reducing chloroauric acid approach, which solutions are necessarily heated to a certain temperature for the reaction to initiate.
Meanwhile, water-ice bath leads to citrate ad-layer physically absorbing on Au, instead of chemisorbing by O-H bond cleavage. And, citrate ad-layer could be completely removed by a cryogenic washing technique. Finally, the Au nanocatalysts were evaluated as the anodes in glucose electrooxidation
IV
(GEO) to demonstrate their highly promising application. And, we investigated surface structure-activity relationship. As a result, NPG exhibits excellent GEO catalytic performance with current densities of 9 A cm-2 mg-1, which is 20 times higher than those of Turkevich-Au NPs (0.45 A cm-2 mg-1) at 1.2 V (vs. RHE).
The current density, j, of 9 A cm-2 mg-1 is 3.4 larger than the latest and best value
(2.65 A cm-2 mg-1). The remarkable catalytic performance of NPG can be ascribed
to the following aspects: i) the clean surfaces facilitate free access, charge transfer of glucose and maximize the exposure of active sites; ii) the porous structure possesses large electrochemically active surface areas, better permeability and more accessible high-index facets and active site. These results illustrate the promising prospect of NPG nanocatalyst in biofuel cell application.
3.Icosahedral Au nanoparticle catalysts with Pt-like activity for hydrogen
evolution reaction: we demonstrate a kinetic controlled means to prepare icosahedral multiply twinned Au nanoparticles with clean surface by reducing the concentration of reactant. At the same time, we design the truncated icosahedral Au by increasing the reaction time. There are a lot of crystal defects a
nd steps etc.active sites, which are beneficial to catalyze hydrogen evolution reaction (HER), on Au icosahedrons. Icosahedral Au serves as electrocatalysts for hydrogen generation and exhibits enhanced catalysis for HER with potential cycling increasing. The data showed a clear monotonic trend, the HER over-potentials, required to obtain a specified current density, shifts positively with the increased number of cyclic voltammogram (CV) cycles. After
30 000 CV cycles, icosahedral Au nanocatalysts exhibit excellent catalysis for
HER with Pt-like activity and much robust durability.Preliminary studies demonstrate that: i) Lattice strain, either compressive or tensile, can alter the surface electronic structure by modifying the distances between surface atoms and in turn catalytic activity; ii) Multiple twins in icodahedral Au nanocrystals can offer the necessary space for the change in the lattice; iii) Aqueous solution electrochemical means can compress the lattice in icosahedral Au nanocrystals, strengthen Au-H bonds and improve their catalytic performance for HER.
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