Study Of The BiPO₄ith Regular Morphology Prepared By Hydrothermal Method And Its Photocatalytic Activity

Many bismuth-based compounds can photocatalyze effectively the degradatione of organic pollutantsunder UV or visible light irradiation. Studies have found that BiPO4showed higher catalytic activity thanP25under UV light irradiation. The morphology, crystal phase and size of photocatalyst are the mainfactors affecting its catalytic activity, which are related with the synthesis method and preparationconditions. Hydrothermal method is one of the most economical method in many materials preparation.Addition of assistant reagent in the hydrothermal system is promising to obtain special morphology. Theassistant reagent acts as an growth modifier of crystal. In this study, a series of BiPO4catalysts have beensynthesized by hydrothermal method. The BiPO4catalysts were characterized by means of XRD, SEM, PLand UV–DRS. The influences of hydrothermal conditions on morphology, crystal phase and otherproperties was also discussed. The photocatalytic activitives of BiPO4catalysts were investigated bydegradating rhodamine B. The results are showed as follows:(1) BiPO4photocatalysts with different crystal structure and morphology were prepared bymannitol-assisted hydrothermal method and their photocatalytic acitivities for degradating rhodamine Bwere investigated. The result shows that the addition of mannitol changes the morphology rather than thecrystal structure of the product. Prismatic BiPO4was produced under high hydrothermal temperaturewithout mannitol addition. While polyhedral BiPO4was produced if mannitol was added. Moreover, thesize of polyhedron decreased with an increase in the addition amount of mannitol. A further increase mightresult in the hierarchical structure. The hydrothermal temperature and hydrothermal time had a greatinfluence on the phase structure and morphology. Lower hydrothermal temperature or shorter time makesfor hexagonal phase BiPO4in rod-like shape with protuberances on its cross sections. Higher temperatureor longer time makes for monoclinic phase. Nevertheless, the crystallinity degrees were both high.Monoclinic product was formed by hexagonal phase transition. It also suggests that monoclinic structure ismore stable than the hexagonal structure. The monoclinic BiPO4shows a higher photocatalytic activity thanthe hexagonal. The BiPO4obtained at170oC with mannitol assistance shows the higher photocatalytic activity than the one obtained without mannitol. Mannitol can improve the photocatalytic activity of BiPO4products in the process of preparation.(2) BiPO4photocatalysts with different crystal structure and morphology were prepared bygelatin-assisted hydrothermal method under different PH value and different amount gelatin addition,which were characterized by XRD, SEM, PL, UV-DRS and photocatalytic activity. The result shows thatwhen hydrothermal reaction takes place at170oC for24h. The PH value had a great influence on the phasestructure. Only prismatic BiPO4(space group P21/n, referred to as nMBIP) was obtained at PH=1,prismatic BiPO4(space group P21/m, referred to as mMBIP) and nMBIP obtained at PH=4and7, themixture of mMBIP and Bi3+3(PO4)2OH obtained at PH=9, respectively. The crystallinity degrees of allproducts are high. Different PH values of the hydrothermal system can result in the BiPO4with differentmorphology. The addition amount of gelatin changes the morphology rather than the crystal structure of theproduct. When the gelatin was added rod and prismatic BiPO4products eventually become irregularpolyhedron. Moreover, the size of polyhedron increased with an increase in the addition amount of gelatin.When the PH value improved or the addition amount of gelatin increased, the photocatalytic activity of theproduct reduced gradually. The nMBIP obtained at170oC with0.15g gelatin addition shows the highestphotocatalytic activity, followed by the product synthesized without gelatin. The photocatalytic activity ofBiPO4can be improved if appropriate gelatin is added in the process of preparation.(3) Snowflake-shaped BiPO4photocatalysts were prepared by EDTA-assisted hydrothermal method.The addition of EDTA changes the morphology rather than the crystal structure of the product. Theprismatic and irregular polyhedron BiPO4was obtained at180oC without EDTA addition. While polyhedralBiPO4was produced if a little EDTA was added. The hierarchical structure of beautiful snowflakes appearswith a further increase addition amount of EDTA. The hydrothermal temperature had a great influence onthe phase structure and morphology. Lower hydrothermal temperature makes for hexagonal phase BiPO4.Higher temperature makes for monoclinic phase. Nevertheless, the crystallinity degrees are all high. It alsosuggests that monoclinic structure is more stable than the hexagonal structure. Longer hydrothermal timeresults in BiPO4with higher photocatalytic activity. Monoclinic product transformed from hexagonal phasewith the hydrothermal time prolonging. The photocatalytic activity firstly increased and then decreasedwith the hydrothermal temperature rises. The monoclinic BiPO4shows higher photocatalytic activity than the hexagonal one. EDTA can obviously improve the photocatalytic activity of BiPO4products in theprocess of preparation. we currently proposed the possible growth mechanistic of BiPO4with EDTAassistance.