Synthesis and In Situ X-ray Diffraction Characterization of Two-Dimensional Perovskite-Type Oxide Colloids with a Controlled Molecular Thickness

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Abstract

A series of two-dimensional (2D) niobate nanosheets with a thickness controllable in increments of 0.4–0.5 nm were synthesized by chemically delaminating precursory layered perovskites into their unilamellar layers. The homologous layered compounds of KCa2Nan–3NbnO3n+1 for n = 4–6 were prepared as starting materials by repeated solid-state calcination of KCa2Nb3O10 (n = 3 analog) and NaNbO3 at 1573 K. These compounds were then converted into protonic forms and were further reacted with a tetrabutylammonium hydroxide solution to yield a translucent colloidal suspension. Nearly perfect delamination was confirmed by in situ X-ray diffraction (XRD) measurements on glue-like colloids centrifuged from the suspension, which showed total loss of basal diffraction series and an evolution of a broad, wavy pattern instead. The characteristic oscillating profile was dependent on n, or the layer thickness, and could be consistently reproduced by simulation in terms of X-ray scattering from the individual 2D fundamental unit of perovskite-type oxides or diffraction from ultrathin crystallites with a defined repeating (n times) unit composed of NbO6 octahedra and Ca/Na. Atomic force microscopy (AFM) observation of samples deposited on a Si substrate detected micrometer-sized sheets with a thickness of 2.7 nm (n = 4), 3.1 nm (n = 5), and 3.6 nm (n = 6), results that are compatible with the dimensions for n of corner-shared octahedra. In addition, in-plane XRD data showed sharp lines attributable to a 2D square lattice (a = 0.39 nm) of perovskite structures. These results along with chemical analysis data indicate the formation of a novel class of 2D perovskite oxides, Ca2−δ(Na,K)n−3+δNbnO3n+1– (n = 4–6), with a progressively increasing thickness by 0.4–0.5 nm. These nanosheets showed intense absorption of ultraviolet (UV) light due to their semiconducting nature. The bandgap energy was estimated as 3.96, 3.81, and 3.77 eV, being dependent on n, which may reflect the relaxing degree of size quantization with the nanosheet thickness. Aggregated nanosheets flocculated with a solution containing KCl and trace amounts of RuCl3 were heated at 773 K to produce photocatalytic materials loaded with RuO2 as a cocatalyst. The resulting samples underwent water splitting under UV irradiation. The evolution of H2 and O2 gas in a 2:1 ratio proceeded on the material from the n = 4 nanosheet, while deviation from stoichiometric decomposition as well as deterioration of activity were observed for the samples from the thicker nanosheets.

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