A Study of Synthesis & [and] Performance of Flame-Made Semiconducting BiVO4 Nanoparticles for the Photocatalytic Degradation of Aqueous Organic Pollutants

Abstract

Semiconductor photocatalysis is an Advanced Oxidation Technology that uses light as a source of energy to accelerate the degradation of harmful organic pollutants. Current research addresses the development of photocatalysts capable of harvesting visible light (Chapter 1). Bismuth vanadate (BiVO4) is a semiconductor that can absorb visible light up to 500 nm. It has been successfully tested for the oxidation of water and some dyes. So far, this material has been synthesized with low specific surface areas (SSA). Flame Spray Synthesis is a cost-effective method for the synthesis of nanoparticles and can be easily scaled up. The Flame Spray Synthesis of BiVO4 nanoparticles is described in detail. Careful adjustment of the process parameters allowed to control particle size and crystallinity of the as-prepared BiVO4 powders. Temperature of the collection site was found to hold a critical role on the control of these two properties (Chapter 3). Photocatalytic activity of the as-synthesized powders was then evaluated for the degradation of the Methylene Blue dye under visible-light irradiation. Photocatalyst properties controlled by synthesis parameters were confronted with their photocatalytic activity. It was found that both crystalline and amorphous powders could carry out the N-demethylation of Methylene Blue. Higher SSA lead to higher activity. A crystalline sample with moderate SSA was also found to degrade phenol to some extent at acidic pH but was subject to deactivation. Addition of different electron acceptors improved its photocatalytic activity. In particular, hydrogen peroxide (H2O2) could be used as a powerful yet rapidly consumed electron scavenger that enabled the degradation of phenol and dichloroacetate at neutral pH (Chapter 4). Advanced electrochemical and photochemical methods gave evidence of the poor reducing power of conduction-band electrons of BiVO4. Besides, the presence of deep traps associated with the change of coordination geometry of vanadium surface atoms upon photoexcitation has been correlated with the apparent photocatalytic activity of flame-made BiVO4 at pH 2. Finally, a mechanism involving the regneration of MB after its reduction when used as an electron acceptor has been proposed (Chapter 5). The flame spray synthesis of BiVO4 might be developed to produce high-SSA amorphous photocatalysts for the selective N-demethylation of Methylene Blue. Regarding the development of novel photocatalysts, the understanding of the role of the atomic surface structure should become a priority to increase their activity.