Salt‐Effect‐Based Synthesis and Anomalous Magnetic Properties of Rare‐Earth Oxide Nanosheets with Sub‐1 nm Thickness

Abstract

The combination of the salt effect and organic solutionphase methods has been realized to synthesize ultrathin rare-earth oxide (RE2O3, RE=Gd, Ho, Y) nanosheets with controllable thickness in sub-1 nm regime. The mechanism of salt effect is demonstrated. Enhanced magnetization intensity has been observed at low temperature for colloidal Ho2O3 nanosheets, compared with bulk Gd2O3. In particular, for colloidal Ho2O3 nanosheets anomalous magnetization under high magnetic field was observed at 60 K; such magnetization is not present in the bulk phase or in Gd2O3 nanosheets. The synthesis of anisotropic two-dimensional (2D) nanocrystals, such as nanosheets, nanodisks and nanoplates, is of both fundamental and technological importance, not only for nanocrystallography, but also for their sizeand shapedependent physical and chemical properties and their potential use for device applications. Different strategies, such as exfoliation, templates, reconstruction from primary nanocrystals, and colloidal synthesis are among the most common methods used to synthesize 2D nanostructures of various nanomaterials, such as graphene, metals, oxides, and chalcogenides. Nanoplates and nanodisks of rare earth oxides and rare-earth-related inorganic compounds have been successfully synthesized with one-pot colloidal synthesis methods in high-temperature organic solution phase and intriguing optical and magnetic properties have been found. The thickness and width of the rareearth-containing 2D nanostructures obtained by one-pot liquid-phase methods are between 1 and 50 nm. These facts could indicate that standard ligands, such as oleic acid (OA) and oleylamine (OM), cannot stabilize such nanocrystals with sub-1 nm thickness. However, it is a great challenge to synthesize ultrathin rare earth oxide nanosheets with sub1 nm thickness, which owing to their extremely small size could give an opportunity to investigate special optical and magnetic properties originating from the 4f electronic configurations. Generally speaking, a smaller number of nuclei in the nucleation stage and a relatively high chemical potential of the monomer(s) or active species in the subsequent growth stage are two rational preconditions for one-pot colloidal synthesis of highly anisotropic 2D nanosheets. However, to some extent, the above two preconditions are in conflict with each other: relatively high chemical potential of the monomer(s) usually results in a larger number of nuclei. This would explain why the colloidal synthesis of 2D nanosheets with width greater than 100 nm has met with highly limited success. At the same time, the salt effect, an example of Le Chatelier s principle in chemistry has not been systematically investigated in well-established high-temperature organic solution-phase systems to synthesize colloidal nanocrystals. The main result of the salt effect is the stabilization of the monomer(s) in an ionic environment and serving as the reservoir for the monomer(s) during the growth. Furthermore, the cations and anions disassociated from organic salt would stabilize special nuclei and certain facets of nanocrystals for anisotropic growth, along with OA and OM. The combination of the salt effect and a standard organic solution-phase method would open a new window for controlled synthesis of colloidal nanocrystals. In this communication, focusing on rare earth oxides as proofs of concept, we harness the salt effect to synthesize rare earth oxide nanosheets with controllable and sub-1 nm thickness in the presence of organic salt (tetrabutylammonium bromide, TBAB). The colloidal Gd2O3 nanosheets with sub-1 nm thickness and width up to 200 nm exhibit a higher Weiss constant and magnetization intensity compared with bulk counterparts under low temperature. For Ho2O3 nanosheets, anomalous magnetization under high magnetic field was also observed at 60 K; such magnetization is not present in bulk phase Ho2O3 or in Gd2O3 nanosheets. XRD patterns indicate that the nanosheets are of Gd2O3 (the results for two nanosheets (NS) samples denoted Gd2O3NS1 and Gd2O3NS2 are reported) with a bixbyite crystal structure with a space group Ia3 and a lattice constant of 1.08 nm (Figure 1A, B). The small-angle XRD (SAXRD) patterns show the lamellar superlattices (SL) of the nanosheets. The series of SAXRD peaks can be assigned to {00l} with a period of 3.75 and 3.45 nm, for Gd2O3NS1 and Gd2O3NS2, respectively, and these d spacing values correspond with the long chain molecular bilayer plus the thickness of inorganic part of Gd2O3 nanosheets. By subtracting the organic molecular bilayer thickness of about [a] Dr. Q. Zhang, Prof. Dr. B. Yan Department of Chemistry, Tongji University Siping Road 1239, Shanghai 200092 (P. R. China) Fax: (+86)21-65982287 E-mail : byan@tongji.edu.cn Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201103596.