ABSTRACT
The electronic, magnetic and optical properties of YNiO3 were studied using the GGA+U method. This compound presents charge disproportionation with two nonequivalent Ni ion sites, namely, Ni1 with Ni(3-δ)+ and Ni2 with Ni(3+δ)+. A band-structure calculation was performed for the observed P21/n monoclinic phase; a 2 × 1 × 2 supercell with 80 atoms was used to reproduce the observed magnetic ordering. The density of states (DOS) calculation clearly showed the bonding differences between the Ni1 and the Ni2 sites; the Ni1 site has a more ionic character, whereas the Ni2 site presents a more covalent character. The band-structure results correspond to an insulator phase with a band gap of about 0.5 eV. The magnetic ordering is antiferromagnetic and the magnetic moments are about 1.24 µ B for Ni1 and around 0.00 µ B for Ni2. The calculated magnetic ordering results are in good agreement with neutron diffraction results. The calculated optical conductivity is similar to previous experimental data.
ABSTRACT
The discharge of synthetic dyes from different industrial sources has become a global issue of concern. Enormous amounts are released into wastewater each year, causing concerns due to the high toxic consequences. Photocatalytic semiconductors appear as a green and sustainable form of remediation. Among them, graphitic carbon nitride (g-C3N4) has been widely studied due to its low cost and ease of fabrication. In this work, the synthesis, characterization, and photocatalytic study over methylene blue of undoped, B/S-doped, and exfoliated heterojunctions of g-C3N4 are presented. The evaluation of the photocatalytic performance showed that exfoliated undoped/S-doped heterojunctions with 25, 50, and 75 mass % of S-doped (g-C3N4) present enhanced activity with an apparent reaction rate constant (kapp) of 1.92 × 10-2 min-1 for the 75% sample. These results are supported by photoluminescence (PL) experiments showing that this heterojunction presents the less probable electron-hole recombination. UV-vis diffuse reflectance and valence band-X-ray photoelectron spectroscopy (VB-XPS) allowed the calculation of the band-gap and the valence band positions, suggesting a band structure diagram describing a type I heterojunction. The photocatalytic activities calculated demonstrate that this property is related to the surface area and porosity of the samples, the semiconductor nature of the g-C3N4 structure, and, in this case, the heterojunction that modifies the band structure. These results are of great importance considering that scarce reports are found concerning exfoliated B/S-doped heterojunctions.
