Investigation of Local Structures in Layered Niobates by Solid-state NMR Spectroscopy.

摘要

Research on ion-exchangeable layered niobates has attracted great attention due to their unique structures and corresponding variations in properties and applications, such as ion conductors, solid acids, and water splitting catalysts. Families of layered niobates include double-layered or triple-layered Dion-Jacobson type perovskites (ALaNb2O7, A = Cs, Rb, K, H; AM2Nb3O10, A = Rb, K, H; M = Sr, Ca), layered niobates with both edge and corner sharing of NbO6 octahedra (KNb3O8, HNb3O6, Nb 6O17 and H4Nb6O17) and many others. Lately, more developments in the layered niobates through a variety of topochemical manipulations have been achieved. The topochemical reactions include ion exchange, exfoliation, substitution, and etc. As a result, many new materials have been successfully prepared, for example, solid solutions (ALa2NbTi2O10, ACaLaNb2TiO 10 and ACa2Nb3-xTaxO10, etc.), nanosheets (HNb3O8, H4Nb6O17, HLaNb2O7, HCa2Nb3O10, etc., to intercalate with organic molecules such as tetrabutylammonium hydroxide or n-butylamines), and nanoscrolls (from H2K2Nb 6O17). While these structural modifications often induce improvements in properties, the fundamental mechanisms of improvements in properties upon the modifications, especially local structural arrangements are poorly understood, which is often limited by structural characterizations. Particularly, the characterizations of the exfoliated nanosheets can be difficult by conventional X-ray diffraction (XRD) method due to disordered structures. Alternatively, solid-state nuclear magnetic resonance (NMR) spectroscopy is a useful tool to study local structures in solids. The structural information can be extracted by examining intrinsic interactions, such as quadrupolar, chemical shielding, and dipolar interactions, which are all associated with local environments surrounding a specific nucleus, 1H or 93Nb in layered niobates. The ultimate goal of this dissertation is to understand the relationships between local structures of layered niobates and their chemical or physical properties, and provide insights into further modifications and improvements. The primary objectives of this work are summarized below:;I. Synthesis of series of layered niobates (ALaNb2O7 , A = Cs, Rb, K; KNb3O8; K4Nb 6O17; RbLa2NbTi2O10 and RbCaLaNb2TiO10) by microwave heating or cation exchange methods, their protonated forms by acid exchange (HLaNb2O 7, H3ONb3O8 and HNb3O 8, H4Nb8O17, HLa2NbTi 2O10 and HCaLaNb2TiO10), and three nanosheet niobates by exfoliation (HNb3O8, H4Nb 6O17 and HLaNb2O7 nanosheets).;II. Structural characterizations of all niobates by powder XRD and solid-state NMR spectroscopy. Powder XRD is used to determine lattice constants and long-range structural ordering. Solid-state NMR is used to determine the electric field gradient parameters, chemical shift anisotropy parameters and dipolar coupling constants. Solid-state NMR techniques include 93Nb MQMAS, wide-line VOCS echo and WURST-echo; 1H{93Nb} CP, TRAPDOR, S-RESPDOR and iS-RESPDOR experiments.;III. Understanding the trends of changes in NMR parameters with respect to cation exchange, exfoliation and compositional alteration, and correlation of the NMR parameters with local environments and possible structural rearrangements.;IV. Identification of proton locations in the acid-exchanged niobates and surface acidity for the exfoliated nanosheets, based on 1H chemical shifts and dipolar coupling information from CP, S-RESPDOR and iS-RESPDOR experiments.