Enhancement Of The Photocatalytic Performance Of BiPO₄ And Its Degradation Mechanism For Organic Pollutants

As a stable, non-toxic, easy to separate and strongly oxidative new photocatalyst, BiPO₄ is promising in waste water treatment. However, the not-high-enough photocatalytic activity and a lack of visible light response restrict its practical application seriously. Therefore, this work focuses on improving the photocatalytic activity of BiPO₄ and extending the light absorption range. By combining with H₂O₂, doping with anion and coupling with narrow band gap semiconductor, the degradation efficiency of organic pollutants by BiPO₄ photocatalysis was improved. Besides, the mechanism for the enhanced photocatalytic performance and the degradation pathway of phenol were investigated. This work may provide theoretical basis and good ideas for constructing efficient photocatalytic systems.In this work, we first combined BiPO₄ photocatalysis with H₂O₂ photolysis to improve the removal efficiency of phenol. The degradation mechanism of phenol by BiPO₄ photocatalysis alone and by BiPO₄ photocatalysis assisted with H₂O₂ was proposed. The valance band maximum of BiPO₄ is rather low?ca. 3.5 V vs SCE?. Therefore, BiPO₄ can produce strongly oxidative holes under light irradiation, which can convert phenol to its intermediates and finally mineralize it to form non-toxic CO2 and H2 O. By comparison, phenol could be degraded quickly but could not be mineralized completely by H₂O₂ via hydroxyl radical mechanism. The addition of H₂O₂ could accelerate the conversion rate of phenol to its intermediates, but due to the consumption of the more oxidative holes by the adsorbed H₂O₂ on the surface of BiPO₄, low concentration of H₂O₂ would decrease the mineralization rate of phenol. When the concentration of H₂O₂ is higher than its adsorption saturation concentration on BiPO₄, most of them would remain freely in the solution, producing large amounts of hydroxyl radicals under light irradiation, which could significantly enhance the conversion rate of phenol, thus indirectly improving the mineralization rate of phenol by BiPO₄.Anioic doping is an effective method to improve the photocatalytic activity of photocatalysts. In this work, the fluorine doped BiPO₄?F-BiPO₄? was synthesized via an in-situ fluorination method. It was found that in a certain range, the photocatalytic activity of BiPO₄ for the degradation of methylene blue first increased then decreased with the increase of fluorine concentration.When the mole ratio of F:Bi was in the range of 0.02-0.1, the photocatalytic activities of F-BiPO₄ samples were all higher than that of pure BiPO₄. Among them, the F0.03-BiPO₄ possessed the highest photocatalytic activity, which was 30% higher than that of pure BiPO₄. The enhanced photocatalytic activity could be attributed to the stronger adsorption ability of the substrate, the larger number of active facets and higher separation ef?ciency of electron-hole pairs. Overdoping would decrease the photocatalytic activity due to the increased recombination rate of electron-hole pairs at high defect concentration.Finally, BiPO₄/BiOI visible-light responsive composite photocatalyst was synthesized via an anion exchange method. The existence of BiPO₄ enhanced the photocatalytic degradation and mineralization rate of phenol by BiOI under visible light?l>420 nm?. When the mole content of Bi PO4 was 7%, the degradation and mineralization rate of phenol by BiPO₄/BiOI were about 2 and 4 times as high as that of pure BiOI, respectively. The interaction between BiPO₄ and BiOI induced the charge transfer absorption, which produced holes with stronger oxidation ability than that produced by pure BiOI. The enhanced oxidation ability of the hole significantly improved the mineralization ability of BiPO₄/BiOI, which enhanced the degradation and mineralization rate of phenol. Besides, with a relatively large dipole moment, BiPO₄ could promote the separation and migration of electrons and holes, which would also be favorable for the enhancement of BiPO₄/BiOI's photocatalytic activity.