【作者】 李莉；

【导师】 陈志刚；

【作者基本信息】 江苏大学 ， 环境科学与工程， 2020， 博士

【摘要】 本课题为丰富光催化全分解水的深度理解,选择构建Ni基/2D-C₃N₄型和Co基/α-Fe₂O₃型非贵金属助催化剂/二维半导体吸光体系,分别从产氢半反应（HER）和产氧半反应（OER）角度进行科学研究。整体思路为根据目标反应的基本原理,针对性地选择适宜的助催化剂负载在光吸收单元表面构建研究平台,随后通过尺寸及形貌调控、组分及含量调节、本征性能改进、界面优化、晶相工程设计等策略提升反应活性,再结合各类谱学表征和理论计算方法对反应性能增强的机制展开深入研究,在实现高效催化剂系统化制备的同时,也为特定催化体系的设计策略提供可借鉴思路。本论文的主要研究内容及相关结论如下:（一）原位合成NiS_x/2D-C₃N₄复合催化剂及其光催化产氢性能研究。通过引入具备良好电荷输运性能和丰富质子还原催化位点的NiS_x助催化剂促进氮化碳产氢半反应效率。研究发现,表面负载助催化剂后,二维氮化碳作为吸光单元捕获光子产生电子-空穴对,而NiS_x作为催化单元高效抽取光生电荷,为质子还原提供更合适的催化位点,可以实现光生电荷分离效率和表面催化转化效率的同步提升。在可见光照射下,NiS_x/2D-C₃N₄的光催化体系产氢速率约4640μmol g^-1 h^-1,实现了超高效的析氢性能并在同等条件下超越了所有以2D-C₃N₄为载体的非Pt体系。研究结果说明NiS_x可以作为一种良好的非贵金属助剂促进二维氮化碳产氢性能的提升。（二）Ni₃N纳米点修饰2D-C₃N₄纳米片及其光催化产氢性能增强机制研究。以Ni₃N为助催化剂研究对象,研究氮化处理对其电子结构和氢吸附自由能的影响机制,并且对其光催化产氢效率进行评估。理论计算分析发现,在Ni金属的晶格里引入N原子,可以导致其电子结构改变,促进本征电荷输运性能的提升;同时,相比于Ni金属助催化剂,Ni₃N在理论上表现出了更合适的氢吸附自由能。通过在二维氮化碳光催化剂表面控制负载超小尺寸Ni₃N助剂,合理构建0D/2D高效反应界面,进一步研究助催化剂构效关系。经含量优化后的光催化剂（10%Ni₃N/2D-C₃N₄）析氢速率达到约1347.8μmol g^-1 h^-1,该性能在相同条件下合成的所有Ni基光催化剂中为最高值,且超过了大多数贵金属负载的光催化剂;表观量子效率在420 nm处达到了2.3%,性能达到所调研的非贵金属负载CN材料的光催化剂产氢活性的较高值。结合实验表征和理论计算分析可知,通过将金属Ni氮化处理可使其具有良好的电荷输运性质和理想的氢吸附自由能,Ni₃N可作为一种廉价、稳定、高效用于促进半导体光催化剂催化产氢性能的助催化剂,也同时验证了由理论指导实验这一催化体系设计策略的可行性。（三）CoO_x/α-Fe₂O₃产氧光催化剂的构筑及其催化活性增强机制研究。针对反应动力学速率迟缓的OER反应,首先可控化合成了二维形貌的α-Fe₂O₃纳米片,通过在其表面负载能够降低OER反应能垒的钴基助催化剂,实现了高效产氧催化体系的构建。α-Fe₂O₃本身并没有产氧活性,在负载钴基助催化剂成功组建CoO_x/α-Fe₂O₃的光催化体系后,光催化产氧活性相比于单体获得了显著提升,5%比例的复合物产氧量达到195.19μmol g^-1 h^-1。结合各类实验表征发现,体系活性的提升源于助催化剂CoO_x可以促进电荷分离效率的提高,输运更高浓度的空穴达到表面催化位点,同时CoO_x的引入可以有效降低反应能垒。本实验也证明构筑能级匹配的助催化剂/半导体体系用于提高OER性能,从而解决全分解水的瓶颈步骤是可行的。（四）Fe₂O₃纳米片表面负载不同晶相CoSe₂及其对催化产氧性能的影响机制研究。以CoSe₂为助催化剂研究对象,研究不同晶相的助催化剂负载对光催化OER活性的影响。在α-Fe₂O₃纳米片表面进行不同晶相的CoSe₂负载,活性探究实验表明,α-Fe₂O₃和c-CoSe₂/Fe₂O₃均没有明显光催化产氧活性,而o-CoSe₂/Fe₂O₃体现出了光催化OER性能的提升。结合理论模型分析和各项实验表征结果,虽然c-CoSe₂有潜在更好的电荷输运能力,但由于o-CoSe₂有比c-CoSe₂有更低的反应势垒和更优的分子活化能力,因而晶相的差异对OER表现出了一定的性能选择性,o-CoSe₂显现了更高效的催化动力学行为,最终决定其能突破反应决速限制并有效提升光催化OER性能。本部分工作可以为基于晶相工程调控的助催化剂/半导体体系的高效催化系统的设计提供可行性思路。综合系统化实验结果可推测,对于HER这类少电子参与、步骤简单的光催化反应,提高光吸收能力、优化电荷分离和输运效率以及增加催化反应效率均能使整体反应效率获得积极的提升;而对于OER这类多电子参与、步骤复杂的光催化反应,反应最终的顺利进行很大程度上取决于决速瓶颈是否突破,因此在这类反应中更优的分子活化能力和更低的反应势垒决定了其对反应是否表现有效的催化动力学行为,也即,光催化反应步骤中的催化转化步骤是决定反应的关键点,只有首先突破此步骤,其他步骤的优化才能进一步体现出对反应的积极推进作用。更多还原

【Abstract】 In order to enrich the deep understanding of photocatalytic water splitting,noble metal-free cocatalyst/two-dimensional photo harvester（Ni-based/2D-C₃N₄ and Co-based/α-Fe₂O₃,respectively）are constructed rationally.Scientific researchers are carried out comprehensively from the aspects of HER（half reaction of hydrogen production）and OER（half reaction of oxygen production）.The overall idea is to start with the feasible cocatalyst of each half reaction to have a basic understanding of the reaction process,then,the effects factors such as size,morphology,composition,synthesis process,electronic structure,interface construction and crystal phase on the reaction mechanism are further investigated.The main contents of this study are as follows:1.Studies on in-situ synthesis of NiS_x/2D-C₃N₄ photocatalyst and its photocatalytic HER performance.NiS_x cocatalyst with good charge transport property and rich proton reduction sites is introduced to promote the hydrogen evolution reaction efficiency of carbon nitride.It is found that,the two-dimensional carbon nitride is used as a light-absorbing unit to capture photons and produce electron-hole pairs,after the reasonable surface loading of cocatalyst,NiS_x then acts as a catalytic unit to extract photogenerated charges efficiently.As a result,the optimal NiS_x/2D-C₃N₄ achieve high photocatalytic hydrogen evolution rate of～4640μmol g^-1 h^-1 under visible light irradiation,which is almost the highest value among Pt-free 2D-C₃N₄ supported photocatalytic systems under the same reaction conditions.In all,the carries’separation efficiency and the surface catalytic conversion efficiency can be improved simultaneously by providing a more suitable catalytic site for proton reduction.The results show that NiS_x can be used as a good noble metal-free cocatalyst to promote the hydrogen production of two-dimensional carbon nitride.2.Studies on the enhancement mechanism of photocatalytic HER production for Ni₃N nanodots-modified 2D-C₃N₄ photocatalyst.During this section,the effect of nitridation on the electronic structure and hydrogen adsorption free energy of Ni₃N cocatalyst is studied,and the actual hydrogen production efficiency for the whole photocatalytic system is evaluated.Theoretical calculation and analysis show that the introduction of N atoms into the lattice of Ni atoms can change the molecule electronic structure and improve the intrinsic charge transport ability.That is,Ni₃N shows a more suitable hydrogen adsorption free energy in theory.The structure-activity relationship of the cocatalyst is further studied by controlling the size and morphology of Ni₃N on the surface of the two-dimensional carbon nitride photocatalyst and reasonably constructing the 0D/2D reaction interface.The well-assembled nanohybrids ultimately achieved the high efficiency catalysis of H₂ evolution rate reaching¹347.8μmol g^-1 h^-1（external quantum efficiency=2.3%at 420 nm）,ranking at the forefront among the 2D-C₃N₄supported Ni-based cocatalysts and noble metals.Based on the experimental characterization and theoretical calculation,it is considered that the Ni₃N prepared by this method has good charge transport ability and ideal hydrogen adsorption free energy.Indeed,it is a kind of cheap,stable,and efficient cocatalyst which can be used to promote the hydrogen production of semiconductor photocatalyst including two-dimensional carbon nitride.3.Studies on the photocatalytic OER performance enhancement of CoO_x loaded α-Fe₂O₃ photocatalytic system.In this part,two-dimensional morphology of hexagonalα-Fe₂O₃ is firstly controllable synthesized,and then cobalt-based cocatalyst which preferred for OER reaction is loaded to construct the photocatalytic system.Although the pure hexagonalα-Fe₂O₃ is inert for water oxidation,the photocatalytic O₂ evolution rate of 5%CoO_x/α-Fe₂O₃ is up to～195.19μmol g^-1 h^-1.The enhanced photocatalytic water oxidation performance can be ascribed to the effective separation of the photogenerated electron-hole pairs ofα-Fe₂O₃ and the high transfer kinetics of the holes.This experiment also proves that it is feasible to construct a co-catalyst/semiconductor system with matched energy levels to improve OER performance to solve the bottleneck of overall water splitting.4.Studies on the influence mechanism of CoSe₂ crystal phase engineering for photocatalytic OER reaction.The effect of crystal phase engineering for on oxygen evolution performance is based on different crystalline phases of CoSe₂ cocatalysts models.In this part,two phases of CoSe₂,that is o-CoSe₂ and c-CoSe₂,are firstly loaded on hexagonalα-Fe₂O₃.The activity test shows that bothα-Fe₂O₃ and c-CoSe₂/Fe₂O₃ have little oxygen evolution performance,but o-CoSe₂/Fe₂O₃ exhibits the improvement of photocatalytic OER.Combining experimental characterization and theoretical calculation analysis,we consider that although c-CoSe₂ has a better charge transport potentiality,o-CoSe₂ has a lower reaction potential barrier and better molecular activation capacity,which endows a more efficient catalytic kinetic behavior than c-CoSe₂,ultimately determining that it can break the reaction bottleneck and thus improving the performance of OER effectively and showing a better performance selectivity towards photocatalysis OER at the same time.This work can provide a corresponding idea for the design of high-efficiency catalyst system based on the crystal phase engineering of cocatalyst/semiconductor system.Based on the systematic experimental results,it is found that,as for the photocatalytic reaction with simple steps such as HER,the overall reaction efficiency can be positively improved by improving the light absorption capacity,optimizing the charge separation and transport efficiency,and increasing the catalytic reaction efficiency.While,as for the photocatalytic reaction with complex steps such as OER,the reaction depends largely on whether the bottleneck can be broken.Herein,the better molecular activation and lower barrier determine the catalytic kinetics of the overall reaction,namely,the catalytic conversion step is the key point,only after the breakthrough in this step,the optimization of other steps can further embody the promoting role of reaction.更多还原