Phase composition, morphology, properties and improved catalytic activity of hydrothermally-derived manganese-doped ceria nanoparticles

Manganese-doped ceria nanoparticles were prepared by hydrothermal synthesis and the prepared samples were thermally treated at 500 °C for 2 hours. The samples were investigated using X-ray diffraction (XRD), high-resolution transmission electron microscopy (HR-TEM), energydispersive X-ray spectroscopy (EDS), N2 adsorption and X-ray photoelectron spectroscopy (XPS). XRD revealed that nanocrystalline ceria is the main phase in all samples, while a romanechite-like phase (Na2Mn O5 10) appears in the sample doped with 30% of Mn. TEM coupled with EDS exposed the presence of the same phase in the sample doped with 20% Mn. While ceria particles have spherical morphology and particle size ranging from 4.3 to 9.2 nm, the rare crystals of the romanechite-like phase adopt a tubular morphology with a length of at least 1 μm. However, the decrease in the ceria lattice constant and the EDS spectra of the ceria nanoparticles clearly indicate that a substantial amount of manganese entered the ceria crystal lattice. Manganese doping has a beneficial impact on the specific surface area of ceria. XPS measurements reveal a decrease in the Ce3+/Ce3++Ce4+ content in the doped samples which is replaced by Mn3+. Moreover, a drasticincrease in adsorbed oxygen is observed in the doped samples which is the consequence of the increase in Mn3+ species that promotes oxygen migrations to the surface of the sample. Compared to the pure sample, the doped samples showed significantly higher catalytic activity for the process of toluene oxidation.

Manganese doping for enhanced magnetic brightening and circular polarization control of dark excitons in paramagnetic layered hybrid metal-halide perovskites

Materials combining semiconductor functionalities with spin control are desired for the advancement of quantum technologies. Here, we study the magneto-optical properties of novel paramagnetic Ruddlesden-Popper hybrid perovskites Mn:(PEA)2PbI4 (PEA = phenethylammonium) and report magnetically brightened excitonic luminescence with strong circular polarization from the interaction with isolated Mn2+ ions. Using a combination of superconducting quantum interference device (SQUID) magnetometry, magneto-absorption and transient optical spectroscopy, we find that a dark exciton population is brightened by state mixing with the bright excitons in the presence of a magnetic field. Unexpectedly, the circular polarization of the dark exciton luminescence follows the Brillouin-shaped magnetization with a saturation polarization of 13% at 4 K and 6 T. From high-field transient magneto-luminescence we attribute our observations to spin-dependent exciton dynamics at early times after excitation, with first indications for a Mn-mediated spin-flip process. Our findings demonstrate manganese doping as a powerful approach to control excitonic spin physics in Ruddlesden-Popper perovskites, which will stimulate research on this highly tuneable material platform with promise for tailored interactions between magnetic moments and excitonic states.