Effect of Platelet-Shaped Surfaces and Silver-Cation Exchange on the Photocatalytic Hydrogen Production of RbLaNb₂O₇

Abstract

The layered Dion-Jacobsen RbLaNb2O7 photocatalyst was prepared in platelet-shaped morphologies using a RbCl flux and with a modulation of the particle morphologies using from 1:1–10:1 (RbCl:RbLaNb2O7) molar ratios and reaction times of 24 h–1 h at a temperature of 1100 °C. Further, the silver-ion exchanged AgLaNb2O7 product could be prepared by reaction of the RbLaNb2O7 particles within a AgNO3 flux at 250 °C for 24 h. These products were characterized by powder X-ray diffraction (e.g., Imma, a = 5.4763(8) Å, b = 22.4042(2) Å, c = 5.1576(3) Å, for RbLaNb2O7; I41/acd, a = 7.7803(2) Å, c = 42.5692(4) Å, for AgLaNb2O7). At a 10:1 flux ratio, rounded-platelet morphologies with smooth surfaces were observed by scanning electron microscopy (SEM) with thicknesses of ∼100–300 nm and lateral dimensions of ∼1.0–6.0 μm. These particle dimensions and morphologies were conserved during the silver-exchange reactions. Photocatalytic rates for hydrogen production were measured in aqueous methanol and yielded maximal rates for the AgLaNb2O7 particles of ∼1,457–2,102 μmol H2 g–1 h–1 under ultraviolet irradiation and ∼2–20 μmol H2 g–1 h–1 under visible irradiation. Photocatalytic rates were generally higher by an order of magnitude or more for the AgLaNb2O7 particles, with an apparent quantum yield of ∼0.94% at 350 nm. The photocatalytically active areas of the platelets were probed via the photoreduction of Pt islands onto their surfaces, and which revealed preferential deposition onto the particle edges as well as stepped “growth rings” across the particles’ faces. These features indicate the confinement of excited electrons to the lateral dimensions of the particles, and thus necessitating them to reach a particle edge or step for a reaction at the particle–solution interface. Electronic-structure calculations based on density functional theory (DFT) show the lowest-energy conduction band states arise from the Nb d-orbital and O p-orbital contributions that are confined to the two-dimensional (2D)-niobate sheets within the structure, and through which the excited-electrons can migrate. The optimum flux synthesis of photocatalyst particles (found at the intermediate 5:1 flux ratio) is found to be a balance between the amount of flux needed to achieve high particle crystallinity, but that does not also lead to well-faceted and smooth surfaces with large lateral dimensions.