BiVO4-based nanoparticles for visible light photocatalytic applications
Date of Issue2014
School of Electrical and Electronic Engineering
In recent years, clean renewable energy and sustainable environment are emerging as the top issues and challenges for humanity. Photocatalysis can be applied to convert solar energy into chemical energy, and is considered as an attractive strategy to tackle the environmental and energy challenges. Since the traditional photocatalyst TiO2 is limited in ultraviolet (UV) range applications, alternative materials have been widely explored. Among them, BiVO4 has shown great potential to extend beyond the UV region due to its suitable band-gap of 2.4 eV and favorable band edge alignment to water splitting. In this work, monoclinic m-BiVO4 nanoparticles were synthesized and surface-modified in order to improve the photocatalytic property. The present work started from the mechanism study of m-BiVO4 synthesis. It was found that a pH ≤ 9 was essential in the phase formation. Owing to the different reaction mechanisms involved under various pH conditions, the state and morphology of the Bi-precursor played a crucial role in determining the particle size and morphology of the synthesized product. In addition, the photocatalytic activity of these m-BiVO4 powders was found to be greatly influenced by both the specific surface area and local structure variation. Synthesis in acid solution was found to benefit complete crystallization with large distortion of the VO43- tetrahedron in the local structure, and the product exhibited the highest photocatalytic activity. In strong acid condition and with the presence of sodium dodecyl benzene sulfonate (SDBS), m-BiVO4 octahedral single crystals were successfully obtained with sizes adjustable in a broad range from 200 nm to 5 um. The influences of SDBS addition, reaction time, and acid concentration were studied while the products were extensively characterized to discover the crystal growth mechanism, which was proposed as a supersaturation process followed by the Ostwald ripening. Compared with irregular shaped particles, these octahedral crystals showed superior visible-light photocatalytic performance. Further, g-Bi2O3, the best photocatalytic polymorph of Bi2O3, was formed on the surface of m-BiVO4 octahedral crystals through an alkaline “etching” process. The product resulted in m-BiVO4@g-Bi2O3 core-shell heterostructure with p-n junction formation. It was found that such formation and yield of Bi2O3 was determined by both the alkaline concentration and reaction time. Moreover, the effect of Bi2O3 formation on the specific surface area of the composite particles was investigated. The RhB degradation test indicated that a 34% Bi2O3 composite phase provided the highest improvement of photocatalytic performance, which was attributed to a higher specific surface area and improved charge carrier transfer in the p-n heterojunction structure. In parallel, based on the photocatalytic capability of m-BiVO4, Au or Pt nanoparticles were successfully synthesized and loaded onto the surface of the m-BiVO4 nanocrystals, as evidenced by extensive characterization. Suitably loaded Au- and Pt-BiVO4 heterogeneous nanoparticles exhibited much higher visible-light photocatalytic activities than the pure m-BiVO4 crystals. Further study and comparison between Au and Pt loading revealed that the superior photocatalytic activity of Pt/m-BiVO4 composite nanoparticles lied in the better dispersion of Pt nanoparticles, while Au-loading resulted in severe particle agglomeration that decrease the specific surface area. Finally, as a combination of the achievements in the p-n composite formation and Pt nanoparticles loading, m-BiVO4/g-Bi2O3/Pt multi-composite nanoparticles were synthesized, which could further improve the photocatalytic performance.