Aerobic oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA) as a bioplastics monomer is efficiently promoted by a simple system based on a nonprecious-metal catalyst of MnO2 and NaHCO3. Kinetic studies indicate that the oxidation of 5-formyl-2-furancarboxylic acid (FFCA) to FDCA is the slowest step for the aerobic oxidation of HMF to FDCA over activated MnO2. We demonstrate through combined computational and experimental studies that HMF oxidation to FDCA is largely dependent on the MnO2 crystal structure. Density functional theory (DFT) calculations reveal that vacancy formation energies at the planar oxygen sites in α- and γ-MnO2 are higher than those at the bent oxygen sites. β- and λ-MnO2 consist of only planar and bent oxygen sites, respectively, with lower vacancy formation energies. Consequently, β- and λ-MnO2 are likely to be good candidates as oxidation catalysts. On the other hand, experimental studies reveal that the reaction rates per surface area for the slowest step (FFCA oxidation to FDCA) decrease in the order of β-MnO2 > λ-MnO2 > γ-MnO2 ≈ α-MnO2 > δ-MnO2 > ε-MnO2; the catalytic activity of β-MnO2 exceeds that of the previously reported activated MnO2 by three times. The order is in good agreement not only with the DFT calculation results, but also with the reduction rates per surface area determined by the H2-temperature-programmed reduction measurements for MnO2 catalysts. The successful synthesis of high-surface-area β-MnO2 significantly improves the catalytic activity for the aerobic oxidation of HMF to FDCA.

No abstract

Zinc tin nitride (ZnSnN 2 ) is attracting growing interest as a nontoxic and earth-abundant photoabsorber for thin-film photovoltaics. Carrier transport in ZnSnN 2 and consequently cell performance are strongly affected by point defects with deep levels acting as carrier recombination centers. In this study, the point defects in ZnSnN 2 are revisited by careful first-principles modeling based on recent experimental and theoretical results. It is shown that ZnSnN 2 does not have low-energy defects with deep levels, in contrast to previously reported results. Therefore, ZnSnN 2 is more promising as a photoabsorber material than formerly considered.

Small polarons and point defects in BaTiO 3 are investigated using hybrid functional calculations. Based on the experimentally-confirmed order-disorder-type phase transitions, Ti displacements along 111 directions are included in the cubic model. We reveal that the self-trapped electrons at Ti sites are stable in both rhombohedral and cubic BaTiO 3 and the Ti off-centering, which introduces antibonding hybridization between lowest-lying Ti-3d and O-2p orbitals at the conduction band minimum, is essential for stabilizing the self-trapped electrons. Our calculations are in contrast to previous theoretical studies, even qualitatively, but reasonably consistent with the long-standing experimentally-observed small polarons in BaTiO 3. This finding may explain why self-trapped electrons are not stable in SrTiO 3 but are in BaTiO 3 from the symmetry viewpoint.

p-Type doping in Cu­(I)-based semiconductors is pivotal for solar cell photoabsorbers and hole transport materials to improve the device performance. Impurity doping is a fundamental technology to overcome the intrinsic limits of hole concentration controlled by native defects. Here, we report that alkali metal impurities are prominent p-type dopants for the Cu­(I)-based cation-deficient hole conductors. When the size mismatch with Cu+ in the host lattice is increased, these isovalent impurities are preferentially located at interstitial positions to interact with the constituent Cu cations, forming stable impurity–defect complexes. We demonstrate that the Cs impurity in γ-CuI semiconductors enhances hole concentration controllability for single crystals and thin films in the range of 1013–1019 cm–3. First-principles calculations indicate that the Cs impurity forms impurity–defect complexes that act as shallow acceptors leading to the increased p-type conductivity. This isovalent doping provides an approach for controlled doping into cation-deficient semiconductors through an interaction of impurities with native defects.