Evidence of Variations in Atomic Distribution in Disordered Mixed Metal Hydroxides

Abstract

Numerous fabrication protocols are known to yield transition metal oxides with structures related to layered double hydroxides, but the effect of fabrication protocol on the uniformity of mixed-metal compositions remain largely unexplored. We have analysed the apparent solubility limits and the structural implications of iron ions in nickel hydroxide lattices for materials prepared by four different fabrication protocols. Opposing shifts in the (100) and (001) reflection in powder X-ray diffraction results revealed a contraction of the nickel lattice upon successful incorporation of iron, with Ni-M distances exhibiting an apparently linear decrease with respect to iron content. This feature revealed the amount of iron incorporated into nickel-based materials to be dependent on fabrication protocol, varying from apparently negligible concentrations to over fifty atomic percent. The dependency of structure on fabrication protocols provides a handle to improve fundamental understanding of catalytically relevant coordination environments.

Linking Lattice Strain and Electron Transfer Kinetics in Crystalline Layered Double Hydroxides

“…Iron-doped nickel hydroxides rank as one of the best oxygen evolution reaction (OER) electrocatalysts in an alkaline environment, and a large range of synthetic protocols have been developed to prepare these materials . Materials exhibiting layered hydroxide-type structures can be prepared by pH-induced precipitation, , anodic electrodeposition, − cathodic electrodeposition, − photochemical deposition, − anodization of metallic electrodes, − hydrothermal techniques, − and precipitation in the presence of various small organic molecules. , Efforts at improving the catalytic properties of this family of materials have focused on tuning morphology, , adding dopants, introducing carbon additives, , acting as a support for single atoms, − or using alternative materials such as metal organic frameworks as synthetic precursors . Such a wide variety of fabrication and processing protocols yield variations in overall catalytic performance, but the incorporation of even trace amounts of Fe­(III) into Ni­(OH) 2 type materials consistently yields the same abrupt changes in electrocatalytic response.…”

“…Metal oxyhydroxides synthesized with a hydrothermal route yield more crystalline structures than fabrication techniques such as electrodeposition or co-precipitation. , Hydrothermally prepared β-Ni­(OH) 2 provides the highest quality crystals reported to date . Such highly crystalline forms of nickel hydroxide historically serve as alkaline battery cathode materials due to their exceptionally poor ability to catalyze OER; the electrocatalytic capabilities increase dramatically upon introduction of structural disorder. ,, This change in behavior marks hydrothermally synthesized samples of interest for studying critical structural features through structure–property analysis, particularly if the degree of structural disorder can be systematically affected.…”

Identification of non-traditional coordination environments for iron ions in nickel hydroxide lattices

“…Formamide is known to intercalate between the layers of layered double hydroxide lattices 62 and the observed behavior is reminiscent of previously reported interstratified stacking structures. 63,64 The (001) splitting is therefore assigned to presence of formamide between a fraction of layers in the material. Increasing the Fe-content results in broadening of all Bragg peaks for both sample series, particularly those involving the c-axis.…”

Analysis of Solid-State Reaction Mechanisms with Two-Dimensional Fourier Transform Infrared Correlation Spectroscopy

“…The behavior of vibrations from the organic ligands during photoinduced decomposition provides insights into the underlying structural evolution and reaction mechanism. ,, A continual decrease in the intensity of ν CH and δ CH bands with time that are exposed to UV irradiation signifies loss of ligands from the film (Figure B and Figure S1). This ligand loss process has been documented, with GC–MS studies indicating a mechanism whereby an LMCT reaction is followed by a series of radical reactions to liberate CO 2 and gaseous hydrocarbons. ,,,, Following an initial induction period, an exponential decay of C–H absorbance bands is observed with time, indicating ligands are lost through a first-order reaction mechanism for all samples (Figure C).…”

“…Photochemically induced decomposition of metal–organic complexes has been presented as a means of synthesizing compositionally uniform thin films of disordered metal hydroxides without requiring high temperatures. ,− Single transition-metal precursors or arbitrary mixtures and stoichiometries of transition metals have been used to synthesize a range of electrocatalysts. − The loss of ligand C–H vibrations in Fourier transform infrared (FTIR) spectroscopy is readily used to monitor the decomposition of metal–organic complexes, and the mechanism of photochemical decomposition has been studied from the perspective of ligand decomposition, but little is known regarding the structural evolution of the transition-metal centers. Mechanistic studies that analyzed gaseous products via gas chromatography–mass spectrometry (GC–MS) analysis indicate that photolysis initiates a ligand-to-metal charge transfer (LMCT) process that results in heterolytic cleavage of the carboxylate C–C bond, a subsequent release of CO 2 , and cross-coupling between radicals to yield volatile organic products and a metal film that reacts with oxygen to yield a metal oxide thin film. ,− Pump–probe spectroscopy has confirmed a similar mechanism underlying the UV-induced decomposition of transition-metal oxalate complexes . A detailed understanding from the transition-metal perspective is currently lacking.…”