Synthesis of Zn doped g-C3N4 in KCl-ZnCl2 molten salts: The temperature window for promoting the photocatalytic activity

Abstract The Zn doped g-C3N4 is successfully synthesized in eutectic KCl-ZnCl2 salt mixture with low melting temperature for the first time. More importantly, the significant improvement in the photocatalytic activity can only be achieved by locating the melting point of the salt mixture within the temperature window between dicyandiamide and melamine oligomer formation steps in the polycondensation process. The prepared g-C3N4 using dicyandiamide as the precursor (C3N4-D) shows ~9.3 times larger specific surface area (SSA), ~9.8 times larger photocurrent density and ~1.56 times longer photo-excited carrier lifetime than the bulk g-C3N4 synthesized via the conventional thermal polymerization method in air (C3N4-M-Air). In photocatalytic methyl orange (MO) degradation, the pseudo-first reaction rate constant of C3N4-D is ~4.15 times higher than the C3N4-M-Air control group. The significantly promoted MO photodegradation performance is attributed to the remarkably enlarged SSA, enhanced MO adsorption, the positively shifted valence band maximum (VBM) and suppressed electron-hole recombination due to Zn dopants.