Epitaxial growth and cationic exchange properties of layered KNb3O8 thin films

Abstract

The KNb₃O₈ potassium triniobate phase is a layered compound that shows excellent photocatalytic activity, intercalation properties and electrochemical performances. In this study, we report the synthesis of this niobate in thin film form. Thin films of layered KNb₃O₈ phase were grown by pulsed laser deposition on (100)-oriented strontium titanate SrTiO₃ substrates using targets with different compositions and various deposition parameters. Samples were analyzed by X-ray diffraction, scanning and transmission electron microscopy, X-ray energy dispersive and Rutherford backscattering spectroscopies. This work shows that KNb₃O₈ films are obtained in a narrow composition range, close to K_1.13Nb₃O_8.7 that can be achieved with several combinations of the elaboration parameters (target composition, deposition temperature, target–substrate distance, laser fluence). Slight potassium amount variations in the film composition lead to the growth of other niobates (i.e. K₂Nb₈O₂₁, K₆Nb_10.88O₃₀, K₃Nb₇O₁₉, K₄Nb₆O₁₇). The KNb₃O₈ films are either made of elongated crystals (500 nm long, 50–75 nm wide, and 40 nm thick) or of flat lamellas. In both cases, they are (010)-preferentially oriented with epitaxial relationships with the substrate, namely (010)KNb₃O₈//(100)SrTiO₃, [100]KNb₃O₈//[010]SrTiO₃ and [001]KNb₃O₈//[001]SrTiO₃. Protonation of the KNb₃O₈ films leads to a complete exchange of K⁺ ions by H₃O⁺ ions that are themselves exchanged by Sn²⁺ ions after immersion in a tin chloride solution confirming the exchange reaction capabilities also in thin films. Exchange of K⁺ by Sn²⁺ leads to the band gap reduction of 0.33 eV.