Metal halide perovskites are promising materials for future optoelectronic applications. One intriguing property, important for many applications, is the tunability of the band gap via compositional engineering. While experimental reports on changes in absorption or photoluminescence show rather good agreement for different compounds, the physical origins of these changes, namely the variations in valence and conduction band positions, are not well characterized. Here, we determine ionization energy and electron affinity values of all primary tin- and lead-based perovskites using photoelectron spectroscopy data, supported by first-principles calculations and a tight-binding analysis. We demonstrate energy level variations are primarily determined by the relative positions of the atomic energy levels of metal cations and halide anions and secondarily influenced by the cation-anion interaction strength. These results mark a significant step towards understanding the electronic structure of this material class and provides the basis for rational design rules regarding the energetics in perovskite optoelectronics.

Zeolites are widely applied as solid acid catalysts in various technological processes. In this work we have computationally investigated how catalytic reactivity scales with acidity for a range of zeolites with different topologies and chemical compositions. We found that straightforward correlations are limited to zeolites with the same topology. The adsorption energies of bases such as carbon monoxide (CO), acetonitrile (CH3CN), ammonia (NH3), trimethylamine (N(CH3)3), and pyridine (C5H5N) give the same trend of acid strength for FAU zeolites with varying composition. Crystal orbital Hamilton populations (COHP) analysis provides a detailed molecular orbital picture of adsorbed base molecules on the Brønsted acid sites (BAS). Bonding is dominated by strong σ donation from guest molecules to the BAS for the adsorbed CO and CH3CN complexes. An electronic descriptor of acid strength is constructed based on the bond order calculations, which is an intrinsic parameter rather than adsorption energy that contains additional contributions due to secondary effects such as van der Waals interactions with the zeolite walls. The bond order parameter derived for the CH3CN adsorption complex represents a useful descriptor for the intrinsic acid strength of FAU zeolites. For FAU zeolites the activation energy for the conversion of π-adsorbed isobutene into alkoxy species correlates well with the acid strength determined by the NH3 adsorption energies. Other zeolites such as MFI and CHA do not follow the scaling relations obtained for FAU; we ascribe this to the different van der Waals interactions and steric effects induced by zeolite framework topology.

The one-pot Diels–Alder cycloaddition (DAC)/dehydration (D) tandem reaction between 2,5-dimethylfuran and ethylene is a potent pathway toward biomass-derived p-xylene. In this work, we present a cheap and active low-silica potassium-exchanged faujasite (KY, Si/Al = 2.6) catalyst. Catalyst optimization was guided by a computational study of the DAC/D reaction mechanism over different alkali-exchanged faujasites using periodic density functional theory calculations complemented by microkinetic modeling. Two types of faujasite models were compared, i.e., a high-silica alkali-exchanged faujasite model representing isolated active cation sites and a low-silica alkali-exchanged faujasite in which the reaction involves several cations in the proximity. The mechanistic study points to a significant synergetic cooperative effect of the ensemble of cations in the faujasite supercage on the DAC/D reaction. Alignment of the reactants by their interactions with the cationic sites and stabilization of reaction intermediates contribute to the high catalytic performance. Experiments confirmed the prediction that KY is the most active catalyst among low-silica alkali-exchanged faujasites. This work is an example of how the catalytic reactivity of zeolites depends on multiple interactions between the zeolite and reagents.

Overall photocatalytic water splitting is one of the most sought after processes for sustainable solar-to-chemical energy conversion. The efficiency of this process strongly depends on charge carrier recombination and interaction with surface adsorbates at different time scales. Here, we investigated how hydration of TiO2 P25 affects dynamics of photogenerated electrons at the millisecond to minute time scale characteristic for chemical reactions. We used rapid scan diffuse-reflectance infrared Fourier transform spectroscopy (DRIFTS). The decay of photogenerated electron absorption was substantially slower in the presence of associated water. For hydrated samples, the charge carrier recombination rates followed an Arrhenius-type behavior in the temperature range of 273–423 K; these became temperature-independent when the material was dehydrated at temperatures above 423 K or cooled below 273 K. A DFT+U analysis revealed that hydrogen bonding with adsorbed water stabilizes surface-trapped holes at anatase TiO2(101) facet and lowers the barriers for hole migration. Hence, hole mobility should be higher in the hydrated material than in the dehydrated system. This demonstrates that adsorbed associated water can efficiently stabilize photogenerated charge carriers in nanocrystalline TiO2 and suppress their recombination at the time scale up to minutes.

First-principles calculations have been performed to explore the charge transport process over defective CeO2(111). Charge transport can proceed either by direct migration of the oxygen anion (i.e., vacancy diffusion) or by a polaron-hopping-assisted mechanism. On the basis of DFT+U calculations, we found that the latter process is significantly more favorable than the former. The overall barrier for charge transport involving polaron migration, followed by oxygen diffusion, is determined by the barrier for polaron hopping, which amounts to 0.18 eV. This computed value is in good agreement with the experimental barrier for ceria with a low defect density. We have shown by a careful analysis of the magnetization density, the density of states, and the reaction pathway trajectory that this process is phonon induced. Our results provide valuable insights into carrier drift processes over defective metal oxide surfaces.

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Trends in adsorption energies as a function of transition metal differ for adsorbates that are attached atop a surface atom or are adsorbed onto a high coordination site. When adsorption onto early and late transition metals is compared variation in relative bond energies of adsorbates attached to different sites is large. A theoretical understanding is provided based on the analysis of the electronic structure of the respective chemical bonds. The electronic structure analysis is based on partial density of states (PDOS) and bond order overlap population densities from crystal orbital Hamiltonian population (COHP) calculations available from DFT electronic structure computations. This is complemented by calculations of Bader charge densities and electron density topology properties. Variation of the respective bond energies depends on the symmetry of the molecular orbitals that form the chemical bond. The key electronic structure parameters are the position of the Fermi level in the bonding or antibonding molecular orbital partial density of states region of the chemical bond and chemical bond polarity. These are very different for adsorbates adsorbed onto the same transition metal surface, but which have different coordination with surface metal atoms. The adsorption energies and the respective electronic structures of adatoms H, C and O and molecular fragments CHx (x = 1-3) are compared with those of the analogous molecules that contain a single transition metal atom. When adsorbed atop, trends in bond energies are remarkably similar to those of the corresponding molecules. The difference in bond energies of adsorbates and transition metal molecules, i.e. the embedding energy, is shown to consist of three contributions: quenching of the sometimes high molecular spin states, weakening of the adsorbate-surface interaction energy and weakening of the metal-metal atom bond energies next to the adsorbate. Conventional scaling rules of the interaction energies of adsorbed CHx (0 < x ≤ 3) fragments are satisfied only for adsorbates in high coordination sites. For the early transition metals a breaking of this rule is found for C and CH or N and NH when adsorbed atop a transition metal surface or when they are part of a transition metal molecule. The M-C bond energy is found to be only stronger than that of the M-CH bond as long as the Fermi level or the HOMO is located in the antibonding molecular orbital partial density of states of the chemical bond.

Hydrogen interaction with ruthenium is of particular importance for the ruthenium-capped multilayer reflectors used in extreme ultraviolet (EUV) lithography. Hydrogen causes blistering, which leads to a loss of reflectivity. This problem is aggravated by tin. This study aims to uncover the mechanism via which tin affects the hydrogen uptake, with a view to mitigation. We report here the results of a study of hydrogen interaction with the ruthenium surface in the presence of tin using Density Functional Theory and charge density analyses. Our calculations show a significant drop in the energy barrier to hydrogen penetration when a tin atom or a tin hydride molecule (SnHx) is adsorbed on the ruthenium surface; the barrier has been found to drop in all tested cases with tin, from 1.06 eV to as low as 0.28 eV in the case of stannane (SnH4). Analyses show that, due to charge transfer from the less electronegative tin to hydrogen and ruthenium, charge accumulates around the diffusing hydrogen atom and near the ruthenium surface atoms. The reduced atomic volume of hydrogen, together with the effect of electron–electron repulsion from the ruthenium surface charge, facilitates subsurface penetration. Understanding the nature of tin’s influence on hydrogen penetration will guide efforts to mitigate blistering damage of EUV optics. It also holds great interest for applications where hydrogen penetration is desirable, such as hydrogen storage.