Magnetic properties of nitrogen-doped ZrO2: Theoretical evidence of absence of room temperature ferromagnetism

Synthesis and characterization of a heterojunction rGO/ZrO2/Ag3PO4 nanocomposite for degradation of organic contaminants

Synergy between surface adsorption and photocatalysis is key for effective contaminant degradation in the liquid phase. Herein, we report a heterojunction photocatalyst of reduced graphene oxide (rGO)/zirconium dioxide (ZrO₂)/silver phosphate (Ag₃PO₄) that incorporates this synergy for 4-nitrophenol (PNP) removal. Compared with other photocatalyst combinations, ZrO₂ and Ag₃PO₄ coupling generates reactive species with greater degradation potential. ZrO₂ and rGO were synthesized by a green approach using a one-step hydrothermal reaction in ethanol-water. The growth of rGO/ZrO₂ and Ag₃PO₄ were accomplished and the functions of each part were well developed together. The rGO/ZrO₂/Ag₃PO₄ composite exhibited enhanced light absorption and a low band gap energy (2.3 eV) owing to rGO and Ag₃PO₄ integration. The composite's photocatalytic activity was much higher than that of ZrO₂, Ag₃PO₄, or ZrO₂/Ag₃PO₄. The maximal adsorption of PNP was 26.88 mg/g, and a pseudo-first-order model described the PNP degradation kinetics (k = 0.034 min^-1). Synergy between the three components resulted in 97% PNP removal in 90 min, and even after five cycles, 94% PNP removal was obtained. The quantum yield of the system (7.31 × 10^-5 molecules/photon) was compared with those in previous reports to assess the photocatalytic performance and energy requirements.

Theory of Ferromagnetism in Reduced ZrO2-x Nanoparticles

Bulk ZrO2 is both nonreducible and nonmagnetic. Recent experimental results show that dopant-free, oxygen-deficient ZrO2-x nanostructures exhibit a ferromagnetic behavior at room temperature (RT). Here, we provide a comprehensive theoretical foundation for the observed RT ferromagnetism of zirconia nanostructures. ZrO2 nanoparticles containing up to 700 atoms (3 nm) have been studied with the help of density functional theory. Oxygen vacancies in ZrO2 nanoparticles form more easily than in bulk zirconia and result in electrons trapped in 4d levels of low-coordinated Zr ions. Provided the number of these sites exceeds that of excess electrons, the resulting ground state is high spin and the ordering is ferromagnetic. The work provides a general basis to explain magnetism in intrinsically nonmagnetic oxides without the help of dopants.

Ferromagnetism in nitrogen-doped BaO: a self-interaction corrected DFT study

The nature of isolated nitrogen-dopants in bulk BaO (N-BaO) and their magnetic interaction have been investigated by means of density functional theory calculations based on hybrid, self-interaction corrected functionals. A thermodynamic analysis shows that N is preferably incorporated into interstitial sites. Only at very high temperatures and under oxygen poor-conditions the formation of N dopants substitutional to O is preferred. The spin density is rather localized in both cases, as proven by the calculation of the hyperfine coupling constants of the electron spin with the nuclear spin of nitrogen. The magnetic interaction of two N defects in various configurations has been considered as a function of their distance. Different behaviors have been observed, depending on the position of the N defects and on the nature of the dominating magnetic mechanism. In any case, the computed Curie temperature is below room temperature, suggesting that RT ferromagnetism cannot be attained for this kind of doping in BaO. Finally, the interplay of oxygen vacancies with the N dopants has been studied. The inclusion of nitrogen has the effect of drastically reducing the cost to create oxygen vacancies. These in turn quench the magnetic moment of N-dopants, contributing to reducing the concentration of magnetic impurities.