化学溶液法制备BiVO4纳米晶及其结构、形貌及光谱性质研究

Structural, morphological and spectral properties of BiVO4 nanocrystals fabricated by a chemical protocol

摘要:

采用安全、绿色、节能的化学溶液法合成了BiVO4纳米晶. 通过改变反应体系的pH值，来调控纳米晶生长的动力学因素，探索不同结构、形貌和尺寸的纳米晶的形成机制及光谱性质的变化规律. 结果表明，在酸性条件，四方相BiVO4为热稳定相. 但在弱碱条件，由于热动力学，反应趋向于形成单斜相BiVO4. 在强碱条件，单斜相BiVO4的特征衍射峰强度降低. 在不同pH值条件下合成的BiVO4纳米晶的晶粒尺寸均处于纳米量级. 在化学合成过程中，体系的pH值对样品形貌的形成起着十分关键的作用. 由于Bi3+离子的水解作用，使其可以在不同的pH值条件下以不同的形式存在，导致在合成BiVO4过程中具有不同的合成路径，进而使得在不同pH值条件下合成的BiVO4样品具有不同的形貌特征. 采用Raman光谱研究了不同BiVO4纳米晶的局部结构，在326和363 cm-1的峰对应于VO43-四面体的反对称和对称弯曲振动，在712和828 cm-1的峰对应于V-O的对称和反对称拉伸振动. 不同BiVO4纳米晶在紫外和可见光区均有明显吸收，而其吸收边界的急剧下降推断是由于半导体的带间跃迁造成的. 不同晶相BiVO4纳米晶的吸收带边有明显的不同，表明随着晶相的改变，样品内部电子结构发生了变化. 在pH=3时合成的BiVO4纳米晶的发光峰强度最低，表明该复合催化剂的电子空穴复合效率最低，暗示其光催化活性较高.

Abstract:

BiVO4 nanocrystals were synthesized via a safe, green and energy-saving chemical protocol. The growth of the nanocrystals was regulated by changing the pH value of the reaction system. Furthermore, the formation mechanism and variation of spectral properties of the nanocrystals with different structures were explored. The results show that tetragonal BiVO4 is a thermally stable phase in acidic conditions. However, in weak alkaline conditions, the reaction tends to form monoclinic phase BiVO4 due to thermodynamics. In the strong alkaline conditions, the diffraction peak intensity of monoclinic BiVO4 decreases. All BiVO4 nanocrystals synthesized at different pH values are in the nanoscale. In the process of chemical synthesis, the pH value of the system plays a key role in the formed morphology of the sample. Bi3+ ions can exist in different forms at different pH values by hydrolysis, resulting in the synthesis routes of BiVO4 are different. And it makes BiVO4 samples synthesized at different pH conditions showing different structures. Raman spectroscopy was used to study the local structure of different BiVO4 nanocrystals. The peaks at 326 and 363 cm-1 are corresponded to the antisymmetric and symmetric bending vibrations of VO43- tetrahedron, while the peaks at 712 and 828 cm-1 are corresponded to the symmetric and antisymmetric tensile vibrations of V-O, respectively. Different BiVO4 nanocrystals have obvious absorption in the ultraviolet and visible regions, and the transition leads to the sharp decrease in the absorption boundary. The absorption band edge of BiVO4 nanocrystals with different crystal phases are obviously different, indicating that the internal electronic structure of the sample changes obviously. When the pH value is 3, the intensity of the luminescent peak of BiVO4 nanocrystal is the lowest, indicating that the material has the lowest electron-hole recombination efficiency, and suggesting that its photocatalytic activity is the highest.