The Hubbard-U correction and optical properties of d0 metal oxide photocatalysts

Insights into the electronic structure, optical properties, and photocatalytic potential of Gd2CoCrO6 perovskite: a comprehensive theoretical and experimental investigation

In this study, we present a comprehensive theoretical and experimental investigation into the electronic structure, optical properties, and photocatalytic potential of Gd2CoCrO6 (GCCO) double perovskite. Using first-principles calculations with the generalized-gradient-approximation plus Hubbard U (GGA + U) method, we explored the effects of Coulomb interactions on the electronic properties. Our calculations revealed that GCCO exhibits a half-metallic nature, displaying metallic behavior for up-spin and semiconducting behavior for down-spin states. The optimized U eff value of 4.2 eV accurately reproduces the direct bandgap of 2.25 eV, which aligns closely with experimental results obtained through UV-visible absorption spectroscopy and photoluminescence analysis. Additionally, time-resolved photoluminescence (TRPL) measurements indicate a mean charge carrier lifetime of 2.37 ns, suggesting effective charge separation. Mott-Schottky analysis and valence band X-ray photoelectron spectroscopy (XPS) confirm the n-type semiconducting nature of GCCO with favorable band edge positions for redox reactions. The combination of theoretical insights and experimental characterization indicates that GCCO holds significant promise as a photocatalyst for applications in renewable energy production and environmental remediation, particularly in solar-driven water splitting and pollutant degradation. Our study provides crucial insights into the electronic structure and optical properties of double perovskites like GCCO, highlighting their suitability for photocatalytic applications. Furthermore, the research paves the way for future work in the compositional engineering and defect modulation of double perovskites to optimize their photocatalytic efficiency.

Thermally Methanol Oxidation via the Mn1@Co3O4(111) Facet: Non-CO Reaction Pathway

Co3O4, as the support of single-atom catalysts, is effective in electron-structure modulation to get distinct methanol adsorption behaviors and adjustable reaction pathways for the methanol oxidation reaction. Herein, we considered the facets that constitute a Co vacancy of the Co3O4(111) facet and a foreign metal atom M (M = Fe, Ni, Cu, Ru, Rh, Pd, Ag, Os, Ir, Pt, Au, Mn) leading to single-atom catalysts. The Mn1@Co3O4(111) facet is the facet considered the most favorable among all of the possible terminations. Oxygen adsorption, decomposition, and its co-adsorption with methanol are the vital steps of methanol oxidation at the exposed Mn1@Co3O4(111) facet, giving rise to the stable configuration: two O* and one CH3OH* adsorbates. Then, the Mn1@Co3O4(111) facet activates the O-H and C-H bonds within CH3OH*, advances CH3O* → H2CO* → HCOO* → COO*, and releases the products H2, H2O, and CO2 consecutively.

Eliminating Delocalization Error to Improve Heterogeneous Catalysis Predictions with Molecular DFT + U

Approximate semilocal density functional theory (DFT) is known to underestimate surface formation energies yet paradoxically overbind adsorbates on catalytic transition-metal oxide surfaces due to delocalization error. The low-cost DFT + U approach only improves surface formation energies for early transition-metal oxides or adsorption energies for late transition-metal oxides. In this work, we demonstrate that this inefficacy arises due to the conventional usage of metal-centered atomic orbitals as projectors within DFT + U. We analyze electron density rearrangement during surface formation and O atom adsorption on rutile transition-metal oxides to highlight that a standard DFT + U correction fails to tune properties when the corresponding density rearrangement is highly delocalized across both metal and oxygen sites. To improve both surface properties simultaneously while retaining the simplicity of a single-site DFT + U correction, we systematically construct multi-atom-centered molecular-orbital-like projectors for DFT + U. We demonstrate this molecular DFT + U approach for tuning adsorption energies and surface formation energies of minimal two-dimensional models of representative early (i.e., TiO₂) and late (i.e., PtO₂) transition-metal oxides. Molecular DFT + U simultaneously corrects adsorption energies and surface formation energies of multilayer models of rutile TiO₂(110) and PtO₂(110) to resolve the paradoxical description of surface stability and surface reactivity of semilocal DFT.

Phase Properties of Different HfO2 Polymorphs: A DFT-Based Study

Background: Hafnium Dioxide (HfO2) represents a hopeful material for gate dielectric thin films in the field of semiconductor integrated circuits. For HfO2, several crystal structures are possible, with different properties which can be difficult to describe in detail from an experimental point of view. In this study, a detailed computational approach has been shown to present a complete analysis of four HfO2 polymorphs, outlining the intrinsic properties of each phase on the basis of atomistic displacements. Methods: Density functional theory (DFT) based methods have been used to accurately describe the chemical physical properties of the polymorphs. Corrective Hubbard (U) semi-empirical terms have been added to exchange correlation energy in order to better reproduce the excited-state properties of HfO2 polymorphs. Results: the monoclinic phase resulted in the lowest cohesive energy, while the orthorhombic showed peculiar properties due to its intrinsic ferroelectric behavior. DFT + U methods showed the different responses of the four polymorphs to an applied field, and the orthorhombic phase was the least likely to undergo point defects as oxygen vacancies. Conclusions: The obtained results give a deeper insight into the differences in excited states phenomena in relation to each specific HfO2 polymorph.

Reaction pathways in the solid state and the Hubbard U correction

We investigate how the Hubbard U correction influences vacancy defect migration barriers in transition metal oxide semiconductors. We show that, depending on the occupation of the transition metal d orbitals, the Hubbard U correction can cause severe instabilities in the migration barrier energies predicted using generalized gradient approximation density functional theory (GGA DFT). For the d⁰ oxide SrTiO₃, applying a Hubbard correction to the Ti⁴⁺ 3d orbitals below 4-5 eV yields a migration barrier of ∼0.4 eV. However, above this threshold, the barrier increases suddenly to ∼2 eV. This sudden increase in the transition state barrier arises from the Hubbard U correction changing the Ti⁴⁺ t_2g/e_g orbital occupation, and hence electron density localization, along the migration pathway. Similar results are observed in the d¹⁰ oxide ZnO; however, significantly larger Hubbard U corrections must be applied to the Zn²⁺ 3d orbitals for the same instability to be observed. These results highlight important limitations to the application of the Hubbard U correction when modeling reactive pathways in solid state materials using GGA DFT.