Swelling and Gel/Sol Formation of Perchlorate-Type Layered Double Hydroxides in Concentrated Aqueous Solutions of Amino Acid-Related Zwitterionic Compounds

ClO₄^–MgAl-LDH3, a MgAl (Mg/Al = 3) layered double hydroxide (LDH) containing perchlorate, swells and forms colloidal suspensions (sols) via the gel state in concentrated aqueous solutions of zwitterionic compounds related to amino acids. In total, 36 zwitterionic compounds with different molecular structures and additional functional groups were examined at various concentrations, and the sol-formation ability was judged by the transmittance (at λ = 589 nm) of the resulting suspensions. At low concentration, the obtained suspensions were turbid, with transmittances of ∼0%. However, above the threshold concentration (0.3–1.0 M), osmotic swelling occurred and the transmittances of the suspensions increased sharply with increases in concentration to reach maximum values of 70–95%. The threshold concentration and maximum transmittance value depended on the structure and the location of the functional groups. The enhancement of the permittivity of water by the zwitterions and the formation of H-bond networks were assumed to be the reasons for the swelling phenomenon. Similar gel/sol formation was observed for ClO₄^–LDHs with Mg/Al = 2, Ni/Al = 2, 3, and Co/Al = 2 and some NO₃^–LDHs. Large ClO₄^–LDH films could be prepared by filtration of the colloidal suspensions followed by washing and drying processes

Bulk Functional Materials Design Using Oxide Nanosheets as Building Blocks: A New Upconversion Material Fabricated by Flocculation of Ca₂Nb₃O₁₀^– Nanosheets with Rare-Earth Ions

A new upconversion (UC) material was designed by flocculating a Ca₂Nb₃O₁₀^– nanosheet, which acts as thermal and structural stabilizer, with Ho³⁺ photoactivator, Yb³⁺ sensitizer, and Y³⁺ space filler. The flocculated product consists of the restacked nanosheets and the rare-earth ions in the internanosheet gallery. The restacked sheet faces of the Ca₂Nb₃O₁₀^– nanosheet building blocks are self-organized in a parallel manner, and their crystallographic coherency extends to three layers on average. On the other hand, the nanosheet building blocks are randomly staggered along the in-layer direction. Chemical composition of the flocculated product was estimated as (Ho_0.096Yb_0.23Y_0.164)­Ca_1.76□_0.24Nb₃O₁₀·1.4H₂O. Heat treatment of the flocculated product at 500 °C was necessary in order to suppress nonradiative energy loss via OH vibration and to induce UC emission. Even after the heat treatment, perovskite-type atomic arrangement of the Ca₂Nb₃O₁₀^– nanosheet building block was retained. Upon laser irradiation at 980 nm, two UC emission bands around 550 and 660 nm were observed, and the emission was visible to the eye. The result from this study suggests that flocculation of nanosheets, as building blocks, with counterions is a promising way to design bulk functional materials that are rather difficult or impossible to prepare by conventional synthetic approaches

Dynamic Breathing of CO₂ by Hydrotalcite

The carbon cycle of carbonate solids (e.g., limestone) involves weathering and metamorphic events, which usually occur over millions of years. Here we show that carbonate anion intercalated layered double hydroxide (LDH), a class of hydrotalcite, undergoes an ultrarapid carbon cycle with uptake of atmospheric CO₂ under ambient conditions. The use of ¹³C-labeling enabled monitoring by IR spectroscopy of the dynamic exchange between initially intercalated ¹³C-labeled carbonate anions and carbonate anions derived from atmospheric CO₂. Exchange is promoted by conditions of low humidity with a half-life of exchange of ∼24 h. Since hydrotalcite-like clay minerals exist in Nature, our finding implies that the global carbon cycle involving exchange between lithosphere and atmosphere is much more dynamic than previously thought

Naked-Eye Discrimination of Methanol from Ethanol Using Composite Film of Oxoporphyrinogen and Layered Double Hydroxide

Methanol is a highly toxic substance, but it is unfortunately very difficult to differentiate from other alcohols (especially ethanol) without performing chemical analyses. Here we report that a composite film prepared from oxoporphyrinogen (OxP) and a layered double hydroxide (LDH) undergoes a visible color change (from magenta to purple) when exposed to methanol, a change that does not occur upon exposure to ethanol. Interestingly, methanol-induced color variation of the OxP-LDH composite film is retained even after removal of methanol under reduced pressure, a condition that does not occur in the case of conventional solvatochromic dyes. The original state of the OxP-LDH composite film could be recovered by rinsing it with tetrahydrofuran (THF), enabling repeated usage of the composite film. The mechanism of color variation, based on solid-state ¹³C–CP/MAS NMR and solution-state ¹³C NMR studies, is proposed to be anion transfer from LDH to OxP triggered by methanol exposure