| 摘要 | 氢能已被广泛认为是最具有潜力解决能源危机和环境问题的理想替代能源之一，其开发备受各个领域科学家的重视，更是从根本上解决环境地球化学工作者的主要目标：对已被破坏的环境和生态进行修复以及对可能破坏环境与生态的人类活动进行干预和指导。在众多的氢能开发手段中，利用太阳能光催化分解水制取氢气是一种兼顾能耗、资源和环境的最为理想和最有前途的氢能开发手段之一。在光催化分解水过程中，最为首要的研究内容就是开发具有适宜能带结构能响应可见光，稳定地、高量子效率地光解水的固相半导体光催化剂。
本论文中，通过高温固相反应合成了呈四方晶系钨青铜结构的半导体光催化剂K4Ce2M10O30(M=Ta, Nb)，吸收边分别达到580 nm (M＝Ta) 和690 nm (M＝Nb)，对应带隙为2.2 eV和1.8 eV。可见光下（λ> 420 nm）光催化分解H2O产生H2和O2的活性表明它们不仅有适宜的带隙响应可见光，并且其价带和导带位置能满足完全分解水的电化学电位需要。在担载Pt、RuO2以及NiO（NiOx）等助催化剂对产氢性能有显著的提高。同时以乙醇钽和草酸铌可溶性前驱体，分别通过溶胶凝胶法（Sol-gel）和聚合物络合法(Polymerizable Complex)制备了K4Ce2Ta10O30和K4Ce2Nb10O30。通过湿法化学合成的光催化剂显示了更高的光催化活性，并且通过PC法制备的K4Ce2Nb10O30更是实现了在大于300 nm 的光辐射下完全分解纯水产生摩尔比为约2:1 的H2和O2。 通过高温固相反应得到Nb取代K4Ce2Ta10O30中部分晶格Ta形成的单相无限固溶体系列K4Ce2Ta10-xNbxO30（x=0~10）是结构一致的同系物，吸收边介于540 nm～710 nm 之间，并且随着x的增加，吸收边依次红移，光催化产氢活性依次降低，但是x=2，5，8时的产氧活性比x=0和10的高，光催化活性的差异主要源于它们光吸收特性和能带结构的差异。基于密度范函理论DFT的第一性原理计算结果表明，光催化剂K4Ce2M10O30(M=Ta, Nb)的能带结构为：导带主要由Ta 5d (Nb 4d)组成，处于高能级的电子未占据态的Ce 4f 与其有很明显的重迭，但由于其高度局域特性，不能很好地参与光生电子在导带的传导，从而其对光催化活性的贡献很小，而价带则由O 2p与Ta 5d (Nb 4d)以及电子占据态的Ce 4f杂化轨道组成。同时通过高温固相反应合成了系列含稀土元素的光催化剂K4Re2M10O30(Re=La, Ce, Nd, Sm, Y; M=Ta, Nb)，通过对它们及其前驱体氧化物的光吸收特性以及电子结构的第一性原理计算研究，合理的解释了只有当Ln＝Ce时才具有可见光响应特性的微观机理。 |
| 其他摘要 | Hydrogen energy is well recognized as one of the most ideal energy to solve the energy and environmental problems confronting the whole society currently, and the development of hydrogen energy has been attracted more attention from different field scientists. Especially, the development of hydrogen energy as the main candidate for future energy is the most crucial way to meet the objective of environmental geochemists, that is to treat those environment and ecosystem polluted and provide prevention measures to those human movements maybe resulting in destroys to environment and ecosystem. Among these ways and methods deployed to develop hydrogen, this using semiconductor photocatalysts transferring solar energy to hydrogen energy is one of the most promising and ideal ways considering energy cost, resource and environmental issues. In the course of photocatalysis, the most important and urgent work is to develop semiconductor photocatalysts possessing appropriate band structures responding to visible light to decomposing water with high quantum efficiency quantum and high stability.
In these paper, photocatalysts K4Ce2M10O30(M=Ta, Nb), being with tetragonal tungsten bronze structure, were prepared by conventional solid solution reaction. Uv-Vis diffuse reflectance spectra showed their absorption edges are 580 nm for M=Ta and 690 nm for M=Nb, corresponding to band gap ca. 1.8 eV and 2.3 eV respectively. Their photocatalytical activities for water decomposition under visible light (λ > 420 nm) demonstrates these photocatalysts possess appropriate chemical potential of band edges to meet the chemical level needed to oxidize and reduce water. Furthermore, the loading with noble metal such as Pt, RuO2 and NiO (NiOx) acting as co-catalyst have showed great promotion effect to the photocatalytic activities for H2 evolution. Additionally, photocatalysts K4Ce2M10O30(M=Ta, Nb) were synthesized successfully by Sol-gel and Polymerizable complex (PC) procedures using dissolvable precursors such as tantalum ethoxide and niobium oxalate. These photocatalysts prepared by above wet chemistry methods showed higher photocatalytic activities, compared with their counterparts from solid solution reaction process. Especially, K4Ce2Nb10O30 prepared by PC methods even realized the overall splitting pure water into H2 and O2 with molar ration of 2:1 under λ > 300 nm irradiation condition. The solid solution compounds K4Ce2Ta10-xNbxO30（x=0~10） prepared by the substitution of the part lattice Ta by Nb through conventional solid solution reaction process, showed absorption edges ranging from 540 nm to 690 nm and the shifting trend to long wavelength zone is consistent with the increase of the amount of Nb correspondingly. The activities for H2 evolution on K4Ce2Ta10-xNbxO30（x=0~10） decreased with the increase of x value, while these solid solution compounds with x=2, 5, 8 showed higher activities for photocatalytic O2 evolution. The first principle calculation based on density functional theory (DFT) within plane-wave pesudopotential (PWP) and Generalized Gradient Approximation (GGA) method were carried on to study the electronic structures of these semiconductor photocatalysts, the results indicated the conduction bands of these photocatalysts K4Ce2M10O30 (M=Ta, Nb) are mainly attributable to the Ta 5d (or Nb 4d) orbitals. Although the unoccupied Ce 4f orbitals have overlap in the bottom of conduction band, they are less effective in transferring electrons and photocatalytic activities for their high localized nature, while their valence bands are composed of hybridization with the bonding of O 2p + Ta 5d (or Nb 4d) and occupied Ce 4f orbitals. The first principle calculation was also carried on K4Re2M10O30(Re=La, Ce, Nd, Sm, Y; M=Ta, Nb) prepared by high temperature solid solution reaction. From the studies on the absorption properties and electronic structures of K4Re2M10O30(Re=La, Ce, Nd, Sm, Y; M=Ta, Nb), presented the reasonable propose about their structures and photocatalytic activities. |
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