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Understanding native point defects is fundamental in order to comprehend the properties of TiO2 anatase in technological applications. The previous first-principles reports of defect-relevant quantities, such as formation energies and charge transition levels, are, however, scattered over a wide range. We perform a comparative study employing different approaches based on semilocal with Hubbard correction (DFT+U) and screened hybrid functionals in order to investigate the dependence defect properties on the employed computational method. While the defects in TiO2 anatase, as in most transition-metal oxides, generally induce the localization of electrons or holes on atomic sites, we notice that, provided an alignment of the valence bands has been performed, the calculated defect formation energies and transition levels using semilocal functionals are in a fair agreement with those obtained using hybrid functionals. A similar conclusion can be reached for the thermochemistry of the Ti–O system and the limit values of the elemental chemical potentials. We interpret this as a cancellation of error between the self-interaction error and the overbinding of the O2 molecule in semilocal functionals. Inclusion of a U term in the electron Hamiltonian offers a convenient way for obtaining more precise geometric and electronic configurations of the defective systems. [ABSTRACT FROM AUTHOR]

The ever‐growing interest in sustainable energy sources leads to a search for an efficient, stable, and inexpensive homogeneous water oxidation catalyst (WOC). Herein, the PO43− templated synthesis of three abundant‐metal‐based germanotungstate (GT) clusters Na15[Ge4PCo4(H2O)2W24O94] ⋅ 38H2O (Co4), Na2.5K17.5[Ge3PCo9(OH)5(H2O)4W30O115] ⋅ 45H2O (Co9), Na6K16[Ge4P4Co20(OH)14(H2O)18W36O150] ⋅ 61H2O (Co20) with non‐, quasi‐, or full cubane motifs structurally strongly reminiscent of the naturally occurring {Mn4Ca} oxygen evolving complex (OEC) in photosystem II was achieved. Under the conditions tested, all three GT‐scaffolds were active molecular WOCs, with Co9 and Co20 outperforming the well‐known Na10[Co4(H2O)2(PW9O34)2] {Co4P2W18} by a factor of 2 as shown by a direct comparison of their turnover numbers (TONs). With TONs up to 159.9 and a turnover frequency of 0.608 s−1 Co9 currently represents the fastest Co‐GT‐based WOC, and photoluminescence emission spectroscopy provided insights into its photocatalytic WOC mechanism. Cyclic voltammetry, dynamic light scattering, UV/Vis and IR spectroscopy showed recyclability and integrity of the catalysts under the applied conditions. The experimental results were supported by computational studies, which highlighted that the facilitated oxidation of Co9 was due to the higher energy of its highest occupied molecular orbital electrons as compared to Co4. [ABSTRACT FROM AUTHOR]

In this first-principles study we investigate the atomic, electronic, and vibrational structure of BaZrO3(001) ultrathin films and surfaces, using a hybrid functional and a local Gaussian-like basis set. The low-index nonpolar (001) surface is known to be the most stable. We considered both possible kinds of nonpolar terminations (BaO and ZrO2) for the (001) surface. The systems were studied using a slab model. Ultrathin films were modeled using slabs with the number of atomic planes ranging from three to seven, whereas surfaces were modeled with much thicker slabs composed of 15 atomic planes. In order to estimate the Gibbs free energy at finite temperatures, lattice vibrational frequencies were also calculated. We found that phonons noticeably affect the relative thermodynamic stability of the two termination layers: while at room temperature the BaO termination has the lowest energy, at intermediate temperatures (500 K) both terminations can coexist, and at higher temperatures (900 K) the ZrO2-terminated surface becomes the most stable. We considered the effect of two-dimensional confinement on the structural, electronic, and vibrational properties of these ultrathin films. We found these confinement effects to be short ranged, with the properties of three-plane films to be the only ones that noticeably differ from the bulk material. Finally, we briefly consider confinement effects in such ultrathin films containing neutral and fully charged oxygen vacancies (charge states 0 and +2). We show, in particular, how lattice vibrations affect the Gibbs formation energy of a neutral oxygen vacancy making it completely independent of the film thickness at high temperatures (1000 K), due to cancellation of enthalpy and entropy contributions. [ABSTRACT FROM AUTHOR]

In this contribution, we study possible confinement effects on the atomic and electronic structure, and phonon properties of neutral UGRAPHIC DISPLAY="INLINE" ID="UGT3" SRC="UGT3"/ and fully charged UGRAPHIC DISPLAY="INLINE" ID="UGT4" SRC="UGT4"/ oxygen vacancies in BaZrO3 (001) ultra-thin films. First principles phonon calculations were performed as a function of film thickness (from 3 to 7 atomic planes) with two complementary DFT methods. The calculations reveal that for both types of vacancies the confinement effect is very short-range; for films containing 5 planes or more, the oxygen vacancy properties are predicted to be similar to those observed in the bulk material. [ABSTRACT FROM AUTHOR]

The perovskite BaZrO3 has attracted considerable attention in the recent decade due to its high temperature proton conducting properties, and possible application as electrolyte in intermediate temperature fuel cells and electrolyzers. In this contribution, we performed, for the first time, first-principles calculations of the phonon contribution to the defect thermodynamics of the ZrO2 terminated (001) surface of BaZrO3. The approach allows us to determine both the segregation enthalpy and entropy of defects, which we apply to two fundamental defects in BaZrO3; fully charged oxygen vacancies (vO••) and protonic defects (OHO•). The calculations show that both defects exhibit favorable segregation enthalpies of -65 and -125 kJ/mol, respectively. Further, the vibrational formation entropy of the surface vO•• is significantly higher than that of the bulk vO••, due to smaller local structural relaxations of the surface defect, leading to a finite surface segregation entropy of 53 J/mol K. OHO•, on the other hand, displays nearly identical vibrational spectra at the surface and in the bulk, and the segregation entropy is therefore negligible. Hence, phonons not only stabilize the surface vO•• compared to the bulk defect thermodynamically at high temperatures, but also affect the relative stability of vO•• and OHO• at the surface. Finally, we apply a simplified space charge model to the (001) surface, and show that neglect of phonons results in strongly underestimated surface concentrations of vO••. [ABSTRACT FROM AUTHOR]

First principles calculations of the atomic, electronic structure, and phonons frequencies were performed to investigate confinement effects on the oxygen vacancy (the F color center) in BaZrO3 ultrathin films. The defect was considered both inside the films and on the (001) surface with BaO and ZrO2 terminations. We consider the effects of the one-dimensional confinement through varying the number of planes in the films from 3 to 9 and comparing the results with those obtained for the bulk material. We have studied how the one-dimensional confinement affects the electronic properties (band structure, Mulliken atomic charges) and the phonon frequencies of the defective systems, and thus how the defect formation energy in thin films at finite temperatures is affected as compared with the bulk material. [ABSTRACT FROM AUTHOR]