Adsorption of 5f-electron atoms (Th Cm) on graphene surface: An all-electron ZORA-DFT study

Excellent 5f-electron magnet of actinide atom decorated gh-C3N4 monolayer

Atomic functionality of two-dimensional (2D) materials, typically with a controllable doping route for offering regular atomic arrangement as well as excellent magnetism, is crucial for both fundamental studies and spintronic applications. Here, the adsorptions of the 5f-electron actinide series (An = Ac-Am) on porous graphene-like carbon-nitride (gh-C3N4) layers are explored to determine their structural stabilities, electronic nature and magnetic properties using the combination of density functional theory (DFT) calculations, ab initio molecular dynamics (AIMD), Monte Carlo (MC) simulations and chemical bonding analyses. Our investigations reveal that each An atom can be individually adsorbed at the vacancy site of gh-C3N4 sheet with high energetic, thermal and dynamical stabilities, which are rooted in the major interactions of ionic An-N bonding as well as the minor interactions of covalent bonding of An-5f6d states with N-2s2p states. The delocalization of a very few 5f electrons is dependent on whether they occupy the suborbitals that are matching and conducive to hybridize with the ligand orbitals forming the 5f-2s2p covalent bonds. We propose that the Ruderman-Kittel-Kasuya-Yosida (RKKY) mechanism plays a determining role for the inter-atomic 5f-5f magnetic exchange via the 6d electrons as the conduction electrons. Large magnetic moment and magnetic anisotropy energy (MAE) from the localized 5f electrons, together with the metallic characteristics owing to the delocalized 6d electrons, render these An-based 2D materials excellent metallic magnets, especially for the U@gh-C3N4 system with the modest magnetic moment of 0.6 μB, large MAE of 53 meV and high Curie temperature (TC) of 538 K.

Chemical bonding between thorium atoms and a carbon hexagon in carbon nanomaterials

We explore the unusual nature of chemical bonding of thorium atoms with a ring of six carbon atoms (hexagon) in novel carbon materials. Our ab initio calculations of Th-based metallofullerenes (Th@C60 and Th@C20) and Th bound to benzene or coronene at the Hartree-Fock level with the second order perturbation (MP2) correction accounting for the van der Waals interactions demonstrate that the optimal position of the thorium atom is where it faces the center of a hexagon and is located at a distance of 2.01-2.07 Å from the center. For Th encapsulated in C60 it is found at 2.01 Å, whereas the other local energy minima are shifted to larger energies (0.22 eV and higher). Inside C60 the highest local minimum at 1.17 eV is observed when Th faces the center of the five member carbon ring (pentagon). Based on our calculations for Th with benzene and coronene where the global minimum for Th corresponds to its position at 2.05 Å (benzene) or 2.02 Å (coronene) from the hexagon center, we conclude that a well pronounced minimum is likely to be present in graphene and in a single wall carbon nanotube. The ground state of Th is singlet, and other high spin states (triplet and quintet) lie higher in energy (>1.62 eV). We discuss a potential use of carbon nanomaterials with the 229Th isotope having its nuclear transition in the optical range, for metrological purposes.

Benzotriazole decorated graphene oxide for efficient removal of U(VI)

There is a need to develop highly efficient materials for capturing uranium from nuclear wastewater. Here, 5-methylbenzotriazole modified graphene oxide (MBTA-GO) was used to adsorb U(VI) from aqueous solution. By the trials of different conditions, we found that the removal of U(VI) from acidic solution was strongly dependent on pH but independent of ionic strength. The U(VI) adsorption was perfectly conformed to the pseudo-second-order kinetics and the adsorption isotherms were simulated by the Langmuir model well. A high removal capacity (q_max = 264 mg/g) for U(VI) at pH 3.5 was obtained. XPS, EXAFS analyses and DFT calculations revealed that the mechanism of uranium capture was ascribed to (i) the surface complexation by benzotriazole and carboxyl groups (providing lone pair electrons) on MBTA-GO and (ii) enhanced synergistic coordination ability of delocalized π-bond of triazole group toward U due to the transfer of electrons from graphene sheet to benzotriazole. DFT calculations further demonstrated that benzotriazole displayed stronger binding with U(VI) compared to carboxyl group due to higher binding energy of [Side/Surface-U-MBTA-GO] (79.745, 54.986 kcal/mol) than [MBTA-GO-COOH-U] (27.131 kcal/mol). This work will provide valuable insight into designing novel nitrogen-containing adsorbents for practical application in wastewater treatment.

Adsorption of Eu(III) and Th(IV) on three-dimensional graphene-based macrostructure studied by spectroscopic investigation

One of the most important reasons for the controversy over the development of nuclear energy is the proper disposal of spent fuel. Separation of actinide and lanthanide ions is an important part of safe long-term storage of radioactive waste. Herein, a three-dimensional (3D) graphene-based macrostructure (GOCS) was utilized to remove actinide thorium and lanthanide europium ions from aqueous solutions. The adsorption of Eu(III) and Th(IV) on the GOCS was evaluated as a function of adsorption time, solution pH, initial ion concentrations, and ionic strength. The experimentally determined maximum adsorption capacities of this GOCS for Eu(III) (pH 6.0) and Th(IV) (pH 3.0) are as high as 150 and 220 mg/g, respectively. By using Fourier transformation infrared (FT-IR), X-ray photoelectron (XPS), and extended X-ray absorption fine structure (EXAFS) spectroscopy, we concluded that the Eu(III) and Th(IV) adsorption was predominantly attributed to the inner-sphere coordination with various oxygen- and nitrogen-containing functional groups on GOCS surfaces. Our selective adsorption results demonstrate that the actinide and lanthanide ions can be effectively separated from transition metal ions. This study provides new clues to the overall recycling of actinide and lanthanide ions in radioactive environmental pollution treatments.