Flux Synthesis of Na_2Ca_2Nb_4O_(13): The Influence of Particle Shapes, Surface Features, and Surface Areas on Photocatalytic Hydrogen Production

摘要

The layered perovskite (n = 4) Ruddlesden-Popper phase Na_2Ca_2Nb_4O_(13) was prepared within molten NaCl and Na_2SO_4 fluxes, yielding either rod-shaped or platelet-shaped particles, respectively. The flux-to-reactant molar ratios of 5:1 or 20:1 were found to significantly influence particle sizes and surface areas, while still maintaining the overall particle shapes. Measured surface areas of flux-prepared Na_2Ca_2Nb_4O_(13) particles ranged from ~0.36 to 4.6 m /g, with the highest surface areas obtained using a 5:1 (NaCl-to-Na_2Ca_2Nb_4O_(13)) molar ratio. All samples exhibited a bandgap size of ~3.3 eV, as determined by UV-Vis diffuse reflectance measurements. Photocatalytic rates for hydrogen production under ultraviolet light for platinized Na_2Ca_2Nb_4O_(13) particles in an aqueous methanol solution ranged from ~230 to 1355 umol H_2 g~(-1) h~(-1) when using the photochemical deposition (PCD) method of platiniza-tion, and ~ 113-1099 μmol H_2 g~(-1) h~(-1) when using the incipient wetness impregnation (IWI) method of platinization. The higher photocatalytic rates were obtained for the rod-shaped particles with the highest surface areas, with an apparent quantum yield (AQY) measured at ~6.5% at 350 nm. For the platelet-shaped particles, the higher photocatalytic rates were observed for the sample with the lowest surface area but the largest concentration of stepped edges and grooves observed at the particle surfaces. The latter origin of the photocatalytic activity is confirmed by the significant enhancement of the photocatalytic rates by the PCD method that allows for the preferential deposition of the surface Pt cocatalyst islands at the stepped edges and grooves, while the photocatalytic enhancement is much smaller when using the more general IWI platinization method.