Thermodynamic Model for BiPO4(cr) and Bi(OH)3(am) Solubility in the Aqueous Na+–H+– $\mathrm{H}_{2}\mathrm{PO}_{4}^{-}$ – $\mathrm{HPO}_{4}^{2-}$ – $\mathrm{PO}_{4}^{3-}$ –OH−–Cl−–H2O System

Synthesis of Bi2S3/BiVO4 Heterojunction with a One-Step Hydrothermal Method Based on pH Control and the Evaluation of Visible-Light Photocatalytic Performance

“…The remaining Bi 3+ first reacted with OH − to form Bi(OH) 3 , as shown in (10). The reaction then continued, causing Bi(OH) 3 to dissolve, and forming Bi(OH) 4 − (11) [48]. ${\mathrm{Bi}}^{3+}+{\mathrm{OH}}^{-}\to \mathrm{Bi}{\left(\mathrm{OH}\right)}_3\downarrow$ $\mathrm{Bi}{\left(\mathrm{OH}\right)}_3+{\mathrm{OH}}^{-}\to \mathrm{Bi}{\left(\mathrm{OH}\right)}_4^{-}$…”

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

“…It has been reported that Bi 3+ and PO 4 32 ions exist in various phases, depending on solution pH. 22 As mentioned above, Bi 3+ and H 2 PO 4 2 were the main ionic species in strong acidic solution.…”

“…17,18 It is well documented that solubility is dependent on the polymorphic state, since different polymorphs have different energies and thus different solubilities. 19,20 According to conventional precipitation transformation theory, if there are two or more homogeneous polycrystallines in the solution system, a soluble phase always tends to transform into an insoluble phase. Therefore, the examination of the solubility distinction among these polymorphs in different pH value solutions may give some inspiration to help to understand the crystallization mechanism in solution.…”

“…In strong acidic solution, PO 4 32 ion reacted with H + to form several other species, such as HPO 4 22 , and H 2 PO 4 2 , and the number of PO 4 32 decreased. To keep K sp constant, the reaction will shift the equilibrium to the right side, which would dissolve more Bi 3+ and PO 4 32 ions, and thus more BiPO 4 crystal dissolved in acidic solution, as supported by the solubility results displayed in Table 2.…”

“…Considering that lattice water coordinated to the Bi atoms can stabilize the HP structure and suppress the phase transition to LTMP, 10 we proposed that the surface ionic layer adsorbed on BiPO 4 nanocrystals, which is directly coordinated to the Bi and P atoms, may account for polymorph selectivity in different pH solution under hydrothermal treatment. In basic solution, such as pH = 13, Bi(OH) 2 2 and HPO 4 22 adsorbed on BiPO 4 surface may stabilized HP in a way similar to the lattice water. In very acidic solution (pH = 0), the excess H 2 PO 4 2 and Bi 3+ might suppress the conversion from LTMP to HTMP by the adsorption on the surface of LTMP crystals.…”