Facet-Dependent Selective Adsorption of Mn-Doped α-Fe2O3 Nanocrystals toward Heavy-Metal Ions

Engineering exposed active facets by doping impurities can dramatically modify the morphology and physicochemical properties of nanocrystalline hematite. In the work described in this paper, through the combination of laser ablation in liquid and hydrothermal treatment techniques, faceted Mn-doped alpha-Fe2O3 nanocrystals (NCs) were prepared by adjusting the doping level of elemental Mn. With the increase of Mn doping level, the hematite crystal evolved sequentially from isotropic polyhedral nanoparticles (NPs) to {116}-faceted saucer-shaped nanosheets (NSs), and then to {001}-faceted hexagonal NSs. Electrochemical stripping tests revealed that the Mn-doped alpha-Fe2O3 NCs show a facet-dependent adsorption ability toward Pb(II), Cd(II), and Hg(II) heavy-metal ions; that is, {001}-faceted hexagonal NSs exhibit high and selective adsorption toward Pb2+ ions, while {116}-faceted saucer-shaped NSs present strong and selective adsorption toward Cd2+ and Hg2+ ions. Density functional theory (DFT) calculations found that Pb2+ and Cd2+ ions have the highest adsorption energy on the {001} and {116} facet, respectively, while Hg2+ ions have the largest adsorption energy on the {110} facet. Carved with a variety of facets, such as {012} and {104} facets with weak and even positive adsorption energy, polyhedral NPs show poor adsorption ability and nonselective adsorption toward the three heavy-metal ions selected. Our experimental and computational results not only provide new insights into the facet-related properties, but also guide the design of crystals with exposed active facets for specific applications.