Abstract
Monoclinic ZrO2 has recently emerged as a new highly efficient material for the photovoltaic and photocatalytic applications. Herein, first-principles calculations were carried out to understand how Hydrogen doping can affect the electronic structure and optical properties of the material. The effects of Hydrogen interstitial and substitutional doping at different sites and concentrations in m-ZrO2 were examined by an extensive model study to predict the best structure with the optimal properties for use in solar energy conversion devices. Hydrogen interstitials (Hi) in pristine m-ZrO2 were found to lower the formation energy but without useful effects on the electronic or optical properties. Hydrogen mono- and co-occupying oxygen vacancy (Ov) were also investigated. At low concentration of Hydrogen mono-occupying oxygen vacancy (HOv), Hydrogen atoms introduced shallow states below the conduction band minimum (CBM) and increase the dielectric constant, which could be very useful for gate dielectric application. The number and position of such defect states strongly depend on the doping sites and concentration. At high oxygen vacancy concentration, the modeled HOv-Ov structure shows the formation of shallow and localized states that are only 1.1 eV below the CBM with significantly high dielectric constant and extended optical absorption to the infrared region. This strong absorption with the high permittivity and low exciton binding energies make the material an ideal candidate for use in solar energy harvesting devices. Finally, the band edge positions of pristine and doped structures with respect to the redox potentials of water splitting indicated that Hydrogen occupying oxygen vacancies can increase the photocatalytic activity of the material for hydrogen generation due the extremely improved optical absorption and the band gap states.
Highlights
As the formation energy of point defect strongly depends on the chemical potentials, the defect formation energy (Eform) was calculated as: Eform = Etotal(final) − Etotal(initial) − nxμXadded + nyμYremoved where Etotal is the total energy of the final and initial structures, nx and ny are the numbers of added and removed elements, respectively, and μXadded and μYremoved are the chemical potentials of the added and removed atoms that depend on the experimental growth conditions, which can be Zr-rich or O-rich for the extreme ones
Do the H atoms prefer to first co-occupy the vacancy or to mono-occupying all vacancies before co-occupying? To answer this question, we used a structure with two oxygen vacancies, a configuration where 2Hi Hov (2H) atoms co-occupying one vacancy ((2H)oxygen vacancy (Ov)-Ov) and the other configuration H atoms mono-occupying the vacancies (2HOv) to represent the two hypotheses, respectively
First-principles calculations using density functional theory (DFT) + U approach were carried out to study the effects of hydrogen doping on the electronic structure and optical properties of pristine and oxygen-deficient monoclinic Zirconia
Summary
Flat and half-field deep localized bandgap states act as recombination centers of light-generated electrons and holes in bulk materials To this end, Wang et al.[15] introduced H-doped black anatase (TiO2−xHx) and found that hydrogenation of TiO2 tends to produce strong band tailing near the conduction band minimum, resulting in bandgap narrowing with better solar absorption (≈83%), significant low recombination, and excellent photocatalytic activity. The article is organized as follows: (i) screening different doping strategies to identify the most stable and favorable defect structures, where the structural properties, formation energy, and thermodynamic stability were calculated, (ii) the bandgap, density of states, charge populations, and bond ionicity were investigated to study the atomic and the electronic structure of defected structures, (iii) the dielectric function and optical absorption were calculated to study the optical properties, and (iv) the band edge positions with respect to the redox potentials of water were calculated
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