Why Does Bi2WO6 Visible-Light Photocatalyst Always Form as Nanoplatelets?

Bi2WO6 nanocrystals exhibit excellent photocatalytic properties in the visible range of the solar spectrum, and intense efforts are directed at designing effective synthesis processes with control of size, morphology, and hierarchical structure. All known hydrothermal syntheses produce either nanoplatelet morphology or hierarchical structures based on such primary entities. Here we investigate the nucleation and growth of Bi2WO6 nanocrystals under hydrothermal conditions using in situ X-ray total scattering (TS) and powder X-ray diffraction (PXRD) measurements. It is shown that the preferential growth of Bi2WO6 nanoplates is due to the presence of disordered layers of Bi2O22+ molecular complexes in the precursor solution with an approximate length of 13 Å. These layers interact with tetrahedral WO42- molecular units and eventually form the disordered cubic (Bi0.933W0.067)O1.6) crystalline phase. When enough tungsten units are intertwined between Bi2O22+ layers formation of Bi2WO6 pristine nanoplates takes place by necessary sideways addition of units in the ac plane. The experimentally observed formation mechanism suggests that the Bi/W atomic ratio must play a central role in the nucleation (assembly of initial crystal layers). Indeed, it is observed in separate continuous flow supercritical synthesis that for a stoichiometric (Bi/W = 2:1) precursor, a (Bi0.933W0.067)O1.6) impurity phase is always observed together with the main Bi2WO6 product. Excess tungsten is required in the precursor to form phase-pure Bi2WO6 material. Thus, the present study also reports a fast, scalable, and green method for production of this highly attractive photocatalyst.