New Type Of BiPO₄Oxy-Acid Salt Photocatalyst Syntheses And Sdudy On Its Structrue-property Relationship

Our principal aim is to synthesis bismuth phosphate (BiPO₄), an oxy-acid saltcatalyst with high activity and stability. We controlled the crystal phase and particle sizeof BiPO4and introduced C₃N₄to decrease the band edge of BiPO4catalyst. The effectsof PO₄³-, defects, phases, crystal sizes and C₃N₄on the structure, optical properties,photocatalytic activity of BiPO₄catalyst were investigated in detail. This work mayprovide further understanding on the photocatalytic mechanisms for non-TiO₂catalysts.First, BiPO₄powder photocatalyst was prepared by hydrothermal method. BiPO4photocatalyst had an optical indirect band-gap of3.85eV. In a comparison, it was foundthat the photocatalytic activity of BiPO₄was as twice as that of TiO₂(P25, Degussa) forthe degradation of methylene blue (MB) dye. The inductive effect of PO₄³-helped thee-/h+separation, which played an important role in its excellent photocatalytic activity.It might extend to the synthesis of other inorganic nonmetal salts of oxy photocatalystswith suitable band gap and high activity for the environmental purification of organicpollutants in aqueous solution. Further results demonstrated the OH-related defectsinfluenced the band gap and the photocurrent, due to the trap of photogenerated carriersand forming recombination centers. These would further influence the activity andmineralization mechanism.Second, three kinds of crystal phase BiPO4(HBIP, nMBIP and mMBIP) werecontrolled synthesized by a hydrothermal methods. The governed factors for theformation of three crystal phase of BiPO4were depended both on the acidity of thesolution and reaction temperature. The BiPO4with nMBIP phase structure showedhigher activity for degradation of MB solution under UV irradiation than HBIP andmMBIP. The induce effect of the dipole moment derived from the distortion PO₄tetrahedron was demonstrated to promote the separation of e-/h+pairs in nMBIP. Thiscorrelation may help to design and develop other inorganic salt photocatalysts ofnonmetal oxoacid with high activity.Third, well dispersed BiPO₄nanocrystals with a diameter of⁹nm weresynthesized in oleic acid (OA) and bis(2-ethylhexyl) phosphate (BEHP) media by ahigh-temperature hydrolysis reaction. OA absorbed on the surface of the nanocrystals made the monodispersion. In this reaction, the diameters of the as prepared BiPO₄nanocrystals could be controlled from9nm to250nm with the OA/BEHP ratio (v/v)changing. The roles of BEHP and OA during the formation of the nanocrystals and fortuning the grain size of the particles had been discussed, and a possible formationmechanism of the nanocrystals was proposed. BiPO₄nanocrystals with a diameter of⁹nm after OA removal showed the highest activity for degradation of MB solution inthe UV region, which is also nearly as twice as the reported activity of BiPO₄,and2.6times higher than that of P25. The high BET surface, large band gap, surface effect, andshort mobility distance for carriers to the surface were recognized as the key factors forthe high photocatalytic activity of nanocrystals.Finally, C₃N₄hybridized BiPO4were obtained by a facile chemisorption approach.For C₃N₄/BiPO₄hybrid photocatalysts, they presented significant visible light activity,enhanced ultraviolet light activity and dramatic photoelectrical conversion efficiency.The C₃N₄/BiPO₄hybrid photocatalysts had dramatic visible light photocatalytic activityand enhanced ultraviolet photocatalytic performance about4.6times comparing withthat of P-25TiO₂photocatalyst. The HOMO and LUMO of C₃N₄were matched wellwith the valence band (VB) and conduction band (CB) of BiPO₄photocatalysts inenergy level. Therefore, the rapid transferring of photogenerated holes would lead toenhanced activity.