We implemented the popular Hubbard density-functional theory + U (DFT+U) method in its spherically averaged form in the all-electron, fullpotential DFT code FHI-aims. There, electronic states are expressed on a basis of highly localized numeric atomic orbitals (NAO), which straightforwardly lend themselves as projector functions for the DFT+U correction, yielding the necessary occupations of the correlated Hubbard subspace at no additional cost. We establish the efficacy of our implementation on the prototypical bulk NiO and obtain the well-known band gap opening effect of DFT+U. As a more stringent, real world test system, we then study polaron formation at the rutile TiO 2 (110) surface, where our results are in line with both experimental data as well as hybrid functional calculations. At this TiO 2 test system, yet in the bulk, we analyze some of the intricacies of using the DFT+U correction in a localized, numeric atomic orbital basis set. Specifically, we find that multiple localized radial basis functions of the same angular momentum can lead to highly erroneous predictions of ground-state properties. We also demonstrate a number of remedies to this problem. Finally, we highlight the critical influence of the exact choice of projector functions on DFT+U results using a number of projector functions of different spatial extent and composed of linear combinations of NAO basis functions. All of our efforts serve to highlight that, contrary to its deceptive ease of use, the DFT+U is far from the "black-box" approach it is sometimes made out to be.

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Reactivity at metal-oxide interfaces is of fundamental importance in heterogeneous catalysis. Herein, we report a thorough surface-science study on the growth and chemical activity of ultrathin ZnO films on Ag(111) by grazing-emission X-ray photoelectron spectroscopy and temperature-dependent infrared reflection−absorption spectroscopy using CO as a probe molecule. Compared to bilayer ZnO on Cu [Schott, V.; Angew. Chem. Int. Ed. 2013, 52, 11925−11929], we find a much decreased CO binding energy of 0.24 eV for bilayer ZnO on Ag. Furthermore, the anomalous, substantial red-shift of the CO stretch frequency with respect to the gas phase value identified for ZnO/Cu is absent in the ZnO/Ag system, where we instead report a slightly blue-shifted frequency at 2146 cm −1 for isolated CO molecules. In order to interpret these differences of ZnO thin layer supported on these two coinage metals, we carried out a thorough theoretical analysis using density functional theory calculations employing van der Waals-corrected generalized-gradient-approximation (GGA)-type and hybrid functionals. We show that bilayer ZnO forms a flat graphitic-like structure on Ag in contrast to the previously reported strongly corrugated ZnO film formed on Cu. While our results show that GGA-type functionals cannot in general be applied uncritically for CO adsorption on ZnO, we explicitly validate our results for the ZnO/Ag system by comparison to hybrid functional calculations for selected model systems.

Ionic liquids (ILs) are discussed in many current research papers extensively in terms of their potential use in the chemical industry, as process aids and novel materials. The long‐term stability of the IL is for industrial applications as important as to know which species arise during the degradation due to thermal, mechanical, chemical or electrochemical stress. The investigation of the long‐term stability of two selected ILs over several months under process‐like conditions is presented with a subsequent analysis by LC‐MS to identify the resulting decomposition products. Knowledge about the occurring species and their analytical quantification are basis for the selection of appropriate processes for the separation of the decomposition products and the development of recycling processes for ILs. Particularly melt crystallization processes are suitable for separating structurally similar decomposition products that typically occur in the IL degradation.

Lithium titanium oxide Li 4 Ti 5 O 12 (LTO) is an intriguing anode material promising particularly long lived batteries, due to its remarkable phase stability during (dis)charging of the cell. However, its usage is limited by its low intrinsic electronic conductivity. Introducing oxygen vacancies can be one method to overcome this drawback, possibly by altering the charge carrier transport mechanism. We use Hubbard corrected density-functional theory (DFT+U) to show that polaronic states in combination with a possible hopping mechanism can play a crucial role in the experimentally observed increase of electronic conductivity. To gauge polaronic charge mobility, we compute relative stabilities of different localization patterns and estimate polaron hopping barrier heights. With this we finally show how defect engineering can indeed raise the electronic conductivity of LTO up to the level of its ionic conductivity, thereby explaining first experimental results for reduced LTO.