pH-driven hydrothermal synthesis and formation mechanism of all BiPO4 polymorphs

Abstract

Understanding of formation mechanism of inorganic solids by solution chemistry is always disturbed by the interference effect of the impurities from starting materials. Herein, we exploited a new two-step route to the selective synthesis of BiPO₄ of different polymorphs with an aim to eliminate the impurity interference effect. The first step is the room-temperature solution synthesis of a hexagonal phase (HP), and the second step involves sufficient washing of HP and a subsequent hydrothermal treatment of HP under given conditions. The formation mechanism of BiPO₄ nanocrystals of different polymorphs was studied by monitoring the reaction parameters like pH, reaction temperature, time, and impurity ions as well as by sample characterizations using X-ray diffraction (XRD) and scanning electron microscope (SEM) techniques. It is found that the pH of the solution is the determinant parameter for the selective synthesis of BiPO₄ polymorphs. Namely, at 240 °C and under strong acidic conditions (pH < 1), HP underwent a phase transformation to a low-temperature monoclinic phase (LTMP), while under neutral or weakly acidic conditions (3 < pH < 7), HP transferred to a high-temperature monoclinic phase (HTMP). Further increasing the solution pH value up to 14 led to the formation of P-doped Bi₂O₃, a phase which is never accessible by conventional solution chemistry but firing in air at T > 750 °C. Based on these observations, two kinds of phase transition mechanisms were discussed.