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

BiWO 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 BiWO nanocrystals under hydrothermal conditions using X-ray total scattering (TS) and powder X-ray diffraction (PXRD) measurements. It is shown that the preferential growth of BiWO nanoplates is due to the presence of disordered layers of BiO molecular complexes in the precursor solution with an approximate length of 13 Å. These layers interact with tetrahedral WO molecular units and eventually form the disordered cubic (BiW)O) crystalline phase. When enough tungsten units are intertwined between BiO layers formation of BiWO pristine nanoplates takes place by necessary sideways addition of units in the 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 (BiW)O) impurity phase is always observed together with the main BiWO product. Excess tungsten is required in the precursor to form phase-pure BiWO material. Thus, the present study also reports a fast, scalable, and green method for production of this highly attractive photocatalyst.