Neural-network-backed evolutionary search for SrTiO3(110) surface reconstructions.

The determination of atomic structures in surface reconstructions has typically relied on structural models derived from intuition and domain knowledge. Evolutionary algorithms have emerged as powerful tools for such structure searches. However, when density functional theory is used to evaluate the energy the computational cost of a thorough exploration of the potential energy landscape is prohibitive. Here, we drive the exploration of the rich phase diagram of TiO x overlayer structures on SrTiO3(110) by combining the covariance matrix adaptation evolution strategy (CMA-ES) and a neural-network force field (NNFF) as a surrogate energy model. By training solely on SrTiO3(110) 4×1 overlayer structures and performing CMA-ES runs on 3×1, 4×1 and 5×1 overlayers, we verify the transferability of the NNFF. The speedup due to the surrogate model allows taking advantage of the stochastic nature of the CMA-ES to perform exhaustive sets of explorations and identify both known and new low-energy reconstructions.

Phosphate-Templated Encapsulation of a {CoII 4 O4 } Cubane in Germanotungstates as Carbon-Free Homogeneous Water Oxidation Photocatalysts.

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 PO4 3- templated synthesis of three abundant-metal-based germanotungstate (GT) clusters Na15 [Ge4 PCo4 (H2 O)2 W24 O94 ] ⋅ 38H2 O (Co4 ), Na2.5 K17.5 [Ge3 PCo9 (OH)5 (H2 O)4 W30 O115 ] ⋅ 45H2 O (Co9 ), Na6 K16 [Ge4 P4 Co20 (OH)14 (H2 O)18 W36 O150 ] ⋅ 61H2 O (Co20 ) with non-, quasi-, or full cubane motifs structurally strongly reminiscent of the naturally occurring {Mn4 Ca} 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 (H2 O)2 (PW9 O34 )2 ] {Co4 P2 W18 } 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 .

A comparative first-principles investigation on the defect chemistry of TiO2 anatase.

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.

First principles study of confinement effects for oxygen vacancies in BaZrO3 (001) ultra-thin films.

In this contribution, we study possible confinement effects on the atomic and electronic structure, and phonon properties of neutral (V(O)(x) and fully charged (V(O)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.

Thermodynamic properties of neutral and charged oxygen vacancies in BaZrO3 based on first principles phonon calculations.

The structural, electronic and thermodynamic properties of neutral and positively doubly charged oxygen vacancies in BaZrO3 are addressed by first principles phonon calculations. The calculations are performed using two complementary first principles approaches and functionals; the linear combination of atomic orbitals (LCAO) within the hybrid Hartree-Fock and density functional theory formalism (HF-DFT), and the projector augmented plane wave approach (PAW) within DFT. Phonons are shown to contribute significantly to the formation energy of the charged oxygen vacancy at high temperatures (∼1 eV at 1000 K), due to both its large distortion of the local structure, and its large negative formation volume. For the neutral vacancy, the resulting lattice distortions, and thus the contributions from phonons to the free formation energy, are significantly smaller. As a result, phonons affect the relative stability of the two defects at finite temperatures and the charge transition level for oxygen vacancies (+2/0) changes from 0.42 to 0.83 eV below the conduction band bottom from 0 K to 1000 K.