Relationship between structure and ion intercalation properties in nickel hexacyanoferrate.

摘要

Electroactive nickel hexacyanoferrate (NiHCF) thin films are promising ion intercalation materials for the separation of radiocesium and other alkali cations. The functionality of NiHCF depends on the stoichiometry and structure of the material. Although very little is known about the structure of NiHCF as a thin film, bulk materials are known to have broadly tunable structures. This thesis aims to determine whether NiHCF thin films have tunability comparable to bulk materials.;The first step to characterizing these thin films is to quickly and accurately determine their oxidation state. Raman spectroscopy of cathodically deposited NiHCF films ∼200nm thick was used to probe C≡N stretching modes which are sensitive to the film's absolute oxidation state. A multivariate regression model was created using 143 Raman spectra from 9 different NiHCF thin films exposed to various 1M alkali cation solutions. Knowledge of the absolute oxidation state was combined with measurements of the film's cation content (found using energy dispersive x-ray spectroscopy, EDS) to estimate the stoichiometry as K+NiII4[Fe III(CN)6]3 for the oxidized material.;X-ray diffraction (XRD) data shows that these same films are polycrystalline with a cubic structure characteristic of those seen in bulk NiHCF. Lattice constants calculated from the XRD data were found to increase monotonically from 10.15A to 10.21A as the ratio of Cs+ versus K+ in the NiHCF matrix increased from 0 to 100%. These lattice constants were ∼0.1A shorter than those calculated using extended x-ray absorbance fine structure (EXAFS) data, a trend indicative of local disorder. Analysis of EXAFS data found the average coordination of nickel atoms to be between 4.4 and 5.1. Taken as a whole, cathodic deposition consistently produces a structure whose oxidized state is characteristic of the Pm-3m space group.;To impart greater structural variability, we used synthesis methods drawn from the literature of bulk NiHCF where variations in reagent concentrations and mixing order produces structural variants. A 26 member combinatorial library of electroactive NiHCF was made using microdispensing methods. Raman spectroscopy showed 15 members were electroactive. EDS analysis of the cation content indicated a wide variation in stoichiometry, verifying the tunability of electroactive NiHCF thin films.