Abstract
The exfoliation ability of nitrate based Mg1−xAlx(OH)2(NO3)x·mH2O layered double hydroxides (Mg-Al LDH) in formamide into single or multilayer nanosheets depends strongly on nitrate anion orientation and layer charge. Our systematic studies used materials that were likely to disclose differences with respect to anion type and their concentrations in the interlayer gallery. We assured to avoid any carbonate incorporation into the galleries for nitrate, chloride, iodide, and sulfate based Mg-Al LDHs. Furthermore, the comparative exfoliation experiments were conducted for fully hydrated samples with as similar particle morphology as possible. The exfoliation of nitrate Mg-Al LDH is far superior to similar clays with carbonate, sulfate, chloride, or iodide as charge balancing anions. Quantitative analysis of exfoliation yield for pre-treated, fully hydrated samples, shows an optimum composition for exfoliation into single nanosheets of around x ≈ 0.25, while double or triple layered sheets are encountered for other x-values. We observe a clear correlation between the expansion of the interlayer gallery due to progressing tilts of nitrate anions and water molecules out of the horizontal interlayer plane, suspension turbidity, and degree of exfoliation. The established correlations extends to nitrate Ni-Al LDH materials. We finally claim that morphology is a dominating parameter, with house-of-card morphology particles exfoliation far less than platelet-like particles. Hence, hydrothermal treatment may be favorable to enhance exfoliation yields.
Highlights
Low dimensional solids find wide applications based on their chemical, electronic [1] photonic [2], magnetic [3], and mechanical properties [4]
We claim that morphology is a dominating parameter, with house-of-card morphology particles exfoliation far less than platelet-like particles
The thermodynamic stability and solubility of layered double hydroxides (LDH) based on different anions vary as NO3− < SO42− < CO32−; likewise the anion exchange ability NO3− < Cl− < SO42− < CO32− [6]
Summary
Low dimensional solids find wide applications based on their chemical, electronic [1] photonic [2], magnetic [3], and mechanical properties [4]. The layered double hydroxides (LDH), [MII1−xMIIIx(OH)2][An−]x/n·mH2O, are intensively studied as minerals and as synthesized compounds with highly tunable compositions. MIII is charge compensated by anions (An−) in the interlayer gallery along with water molecules. Their stability covers at least the range 0.20 ≤ x ≤ 0.33 [5]. A high surface area for the precipitated LDHs reflects a small particle size of platelet or house-of-card morphology. Their good ion exchange capacity makes LDHs applicable as, e.g., absorbents and additives. LDHs can be exfoliated, providing single 2D nanosheets of < 1 nm thickness that may serve as building bricks for functional nanocomposites with positively charged backbones, compared to silicates with negatively charged backbones [7,8,9,10]
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