Dynamics and Mechanism of Intercalation/De-Intercalation of Rhodamine B during the Polymorphic Transformation of the CdAl Layered Double Hydroxide to the Brucite-like Cadmium Hydroxide

Abstract

We studied the kinetics of intercalation of a fluorescent probe (rhodamine B (RhB)) during the formation of hierarchal microspheres of cadmium-aluminum layered double hydroxide (CdAlA LDH) and its de-intercalation upon transformation from the LDH phase into the cadmium hydroxide β phase (Cd(OH)2) using a reaction-diffusion framework (RDF) where the hydroxide anions diffuse into an agar gel matrix containing the proper salts-fluorescent probe mixture. In this framework, we achieved the stabilization of the CdAlA LDH, which is known to be thermodynamically unstable and transforms into Cd(OH)2 and Al(OH)3 in a short period. RDF is advantageous as it allows with ease the extraction of the cosynthesized polymorphs and their characterization using X-ray diffraction (XRD), differential scanning calorimetry (DSC), thermal gravimetric analysis (TGA), solid-state nuclear magnetic resonance (SSNMR), Fourier transform infrared (FT-IR), and energy dispersive X-ray (EDX). The kinetics of inter/de-intercalation is studied using in situ steady-state fluorescence measurements. The existence of RhB between the LDH layers and its expel during the transition into the β phase are examined via fluorescence microscopy, XRD, and SSNMR. The activation energies of intercalation and de-intercalation of RhB are determined and show dependence on the cationic ratio of the corresponding LDH. We find that the energies of de-intercalation are systematically higher than those of intercalation, indicating that the dyes are stabilized due to the probe-brucite sheets interactions. SSNMR is used to shed light on the mechanism of intercalation and stabilization of RhB inside the layers of the LDH.