From Coprecipitation to Nanostructure: Formation, Morphology, and Reactive-Site Accessibility in Fe-Containing Binary Layered Double Hydroxides

Binary Fe-containing LDHs and structurally related layered hydroxides are being widely investigated as adsorbents, catalysts, and redox-active nanomaterials. Yet, links among coprecipitation, phase formation, nanostructure, and reactive-site accessibility remain inconsistently interpreted. This critical review evaluates studies published mainly from 2015 to July 2026 on CoFe, NiFe, MgFe, ZnFe, MnFe, CuFe, and CaFe layered hydroxides and Fe(II)/Fe(III) green rust. Coprecipitation is not a single standardized route: local supersaturation, reagent delivery, mixing, complexation, atmosphere, interlayer chemistry, and aging can alter metal incorporation and phase development, while Fe-rich transient precursors may participate under system-specific conditions. The commonly cited Fe(III) fraction of x = 0.20–0.33 is an empirical guideline rather than a universal stability window, and conventional M(II)-Fe(III) LDHs, CaFe hydrocalumite-/AFm-related phases, and green rust require distinct crystal-chemical interpretations. Nominal composition and LDH-like diffraction cannot establish homogeneous cation incorporation or phase purity. In contrast, diffraction broadening, BET area, nanosheet dimensions, and XPS fitting do not provide stand-alone evidence of defects or accessible reactive sites. Progress toward predictive synthesis requires standardized reporting, system-specific synthesis–composition–phase maps, time-resolved studies, quantitative structure–accessibility relationships, and recognition of as-synthesized, working, recovered, and regenerated materials as potentially distinct structural states, supported by uncertainty analysis, negative outcomes, and validation across laboratories.