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Continuous wave (CW) and pulse electron paramagnetic resonance (EPR) experiments, in conjunction with density functional theory (DFT) calculations, provide a detailed description of defective centres produced upon nitrogen doping of polycrystalline ZnO. Two distinct paramagnetic species are formed upon annealing of ZnO nanoparticles in an NH3atmosphere, which are characterized by the interaction of the unpaired electron with one and two N nuclei. HYSCORE experiments provide the full hyperfine and quadrupole interaction tensors for the monomeric defect, which, on the basis of quantum chemical calculations, is assigned to a nitrogen ion substituting a lattice oxygen ion. [ABSTRACT FROM AUTHOR]

A computational screening of Single Atom Catalysts (SACs) bound to titanium nitride (TiN) is presented, for the Hydrogen Evolution Reaction (HER), based on density functional theory. The role of fundamental ingredients is explored to account for a reliable screening of SACs. Namely, the formation of H 2 -complexes besides the classical H * one impacts the predicted HER activity, in line with previous studies on other SACs. Also, the results indicate that one needs to adopt self-interaction-corrected functionals. Finally, predicting an active catalyst is of little help without an assessment of its stability. Thus, it is included in the theoretical framework the analysis of the stability of the SACs in working conditions of pH and voltage. Once unconventional intermediates and stability are considered in a self-interaction corrected scheme, the number of potential good catalysts for HER is strongly reduced since i) some potentially good catalysts are not stable against dissolution and ii) the formation of unconventional intermediates leads to thermodynamic barriers. This study highlights the importance of including ingredients for the prediction of new systems, such as the formation of unconventional intermediates, estimating the stability of SACs, and the adoption of self-interaction corrected functionals. Also, this study highlights some interesting candidates deserving of dedicated work., (© 2024 Wiley‐VCH GmbH.)

The surfaces of nanomaterials with applications in optoelectronics and catalysis control their physicochemical properties. NMR spectroscopy, enhanced by dynamic nuclear polarization (DNP), is a powerful approach to probe the local environment of spin-1/2 nuclei near surfaces. However, this technique often lacks robustness and resolution for half-integer quadrupolar nuclei, which represent more than 66% of the NMR-active isotopes. A novel pulse sequence is introduced here to circumvent these issues. This method is applied to observe with high-resolution 27 Al and 17 O spin-5/2 nuclei on the surface of γ-alumina. Moreover, we report high-resolution 17 O spectra of ZnO nanoparticles used in optoelectronics. Their assignment using DFT calculations allows the first NMR observation of vacancies near the surfaces. Finally, we employ the introduced NMR technique to observe 11 B spin-3/2 nuclei on the surface of partially oxidized boron nitride supported on silica and to distinguish its different BO 2 OH active sites.

One key process involving single atom catalysts (SACs) is the electroreduction of CO 2 to fuels. The chemistry of SACs differs largely from that of extended catalytic surfaces, presenting an opportunity to improve the ability to activate very stable molecules, such as CO 2 . In this work, we performed a density functional theory (DFT) study of CO 2 activation on a series of SACs, focusing on the role played by the adopted functional in activity predictions. The role of the exchange-correlation functional has been widely investigated in heterogenous catalysts, but it is less explored in SACs. We tested the widely used PBE and the PBE+ U corrected functionals against the more robust hybrid PBE0 functional. The results show that PBE is reliable if one is interested in qualitative predictions, but it leads to some inaccuracies in other cases. A possible way to attenuate this effect is by adopting the PBE+ U framework, as it gives results that are very similar to PBE0 at an acceptable computational cost. The results of this study further underline the importance of the computational framework adopted in predicting the activity of SACs. The work suggests that one needs to go beyond PBE for quantitative estimates, an important consideration when performing screening and high-throughput calculations.

Invited for this month's cover is the group of Gianvito Vilé at the Politecnico di Milano. The ChemSusChem cover image depicts in an artistic manner the concept of ligand entrapping of isolated metals to design single-atom catalysts. The Research Article itself is available at 10.1002/cssc.202301529., (© 2024 Wiley-VCH GmbH.)

This work evaluates the feasibility of alkaline hydrogen evolution reaction (HER) using Pt single-atoms (1.0 wt %) on defect-rich ceria (Pt 1 /CeO x ) as an active and stable dual-site catalyst. The catalyst displayed a low overpotential and a small Tafel slope in an alkaline medium. Moreover, Pt 1 /CeO x presented a high mass activity and excellent durability, competing with those of the commercial Pt/C (20 wt %). In this picture, the defective CeO x is active for water adsorption and dissociation to create H* intermediates, providing the first site where the reaction occurs. The H* intermediate species then migrate to adsorb and react on the Pt 2+ isolated atoms, the site where H 2 is formed and released. DFT calculations were also performed to obtain mechanistic insight on the Pt 1 /CeO x catalyst for the HER. The results indicate a new possibility to improve the state-of-the-art alkaline HER catalysts via a combined effect of the O vacancies on the ceria support and Pt 2+ single atoms.

Electronic structure calculations represent an essential complement of experiments to characterize single-atom catalysts (SACs), consisting of isolated metal atoms stabilized on a support, but also to predict new catalysts. However, simulating SACs with quantum chemistry approaches is not as simple as often assumed. In this work, the essential factors that characterize a reliable simulation of SACs activity are examined. The Perspective focuses on the importance of precise atomistic characterization of the active site, since even small changes in the metal atom's surroundings can result in large changes in reactivity. The dynamical behavior and stability of SACs under working conditions, as well as the importance of adopting appropriate methods to solve the Schrödinger equation for a quantitative evaluation of reaction energies are addressed. The Perspective also focuses on the relevance of the model adopted. For electrocatalysis this must include the effects of the solvent, the presence of electrolytes, the pH, and the external potential. Finally, it is discussed how the similarities between SACs and coordination compounds may result in reaction intermediates that usually are not observed on metal electrodes. When these aspects are not adequately considered, the predictive power of electronic structure calculations is quite limited., (© 2023 The Authors. Advanced Materials published by Wiley-VCH GmbH.)

CO 2 adsorption and activation on Cu single atom catalysts and Cu nanoclusters supported on the (110) surface of rutile and on the (101) surface of anatase TiO 2 have been investigated by means of first principles electronic structure calculations. The role of oxide reduction associated to the presence of oxygen vacancies has been considered. Five main messages emerge from this study. (1) CO 2 activation on Cu/TiO 2 nanostructures is surface sensitive, as the rutile and anatase surfaces can exhibit different behaviors; (2) the surface morphology is essential since CO 2 is activated only when the molecule can simultaneously bind to at least two active sites, such as a Cu atom on one side and an oxide ion on the other site; (3) Cu atoms on TiO 2 are in the +I oxidation state and can bind and activate CO 2 via charge transfer from the oxide; (4) on supported Cu clusters CO 2 activation occurs mostly at the metal/oxide interface; (5) the presence of O vacancy sites facilitates the spontaneous dissociation of CO 2 to CO, or increases the electron density of the metal catalyst, two effects that can influence the mechanism of CO 2 reduction to methanol or other chemicals., (© 2023 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)

The interaction of CaO and Ca(OH) 2 with solvated or gaseous SO 2 plays a crucial role in the corrosion of urban infrastructure by acid rain or in the removal of SO 2 from flue gas. We carried out a combined spectroscopic and theoretical investigation on the interaction of SO 2 with a CaO(001) single crystal. First, the surface chemistry of SO 2 was investigated at different temperatures using polarization-resolved IR reflection absorption spectroscopy. Three species were identified, and an in-depth density functional theory study was carried out, which allowed deriving a consistent picture. Unexpectedly, low temperature exposure to SO 2 solely yields a physisorbed species. Only above 100 K, the transformation of this weakly bound adsorbate first to a chemisorbed sulfite and then to a sulfate occurs, effectively passivatating the surface. Our results provide the basis for more efficient strategies in corrosion protection of urban infrastructure and in lime-based desulfurization of flue gas., (© 2023 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)

TiO 2 is a relevant catalytic material, and its chemistry in aqueous environment is a challenging aspect to address. Also, the morphology of TiO 2 particles at the nanoscale is often complex, spanning from faceted to spherical. In this work, we study the pH- and facet-dependent surface chemistry of TiO 2 /water interfaces by performing ab initio molecular dynamics simulations with the grand canonical formulation of species in solution. We first determined the acid-base equilibrium constants at the interface, which allows us to estimate the pH at the point of zero charge, an important experimental observable. Then, based on simulated equilibrium constants, we predict the amount of H + , OH - , and adsorbed H 2 O species present on the surfaces as a function of the pH, a relevant aspect for water splitting semi-reactions. We approximated the complex morphology of TiO 2 particles by considering the rutile (110) and (011), and anatase (101), (001), and (100) surfaces.

Tuning the properties of oxide surfaces through the adsorption of designed ligands is highly desirable for several applications, such as catalysis. N-Heterocyclic carbenes (NHCs) have been successfully employed as ligands for the modification of metallic surfaces. On the other hand, their potential as modifiers of ubiquitous oxide surfaces still needs to be developed. Here we show that a model NHC binds covalently to a copper oxide surface under UHV conditions. In particular, we report the first example of a covalent bond between NHCs and oxygen atoms from the oxide layer. This study demonstrates that NHC can also act as a strong anchor on oxide surfaces.

The characterization of catalytic oxide surfaces is often done by studying the properties of adsorbed probe molecules. The 31 P NMR chemical shift of adsorbed trimethylphosphine, P(CH 3 ) 3 or TMP, has been used to identify the presence of different facets in oxide nanocrystals and to study the acid-base properties of the adsorption sites. The NMR studies are often complemented by DFT calculations to provide additional information on TMP adsorption mode, bond strength, etc. So far, however, no systematic study has been undertaken in order to compare on the same footing the chemical shifts and the adsorption properties of TMP on different oxide surfaces. In this work we report the results of DFT+D (D = dispersion) calculations on the adsorption of TMP on the following oxide surfaces: anatase TiO 2 (101) and (001), rutile TiO 2 (110), tetragonal ZrO 2 (101), stepped ZrO 2 (134) and (145) surfaces, rutile SnO 2 (110), (101) and (100), wurtzite ZnO(101̄0), and cubic CeO 2 (111) and (110). Beside the stoichiometric surfaces, also reduced oxides have been considered creating O vacancies in various sites. TMP has been adsorbed on top of variously coordinated Lewis acid cation sites, with the aim to identify, also with the support of machine learning algorithms, trends or patterns that can help to correlate the 31 P chemical shift with physico-chemical properties of the oxide surfaces such as adsorption energy, Bader charges, cation-P distance, work function, etc. Some simple correlation can be found within the same oxide between the 31 P chemical shift and the adsorption energy, while when the full set of data is considered the only correlation found is with the net charge on the TMP molecule, a descriptor of the acid strength of the adsorption site.

We investigated the impact of quantum confinement on the band gap of chalcogenides 2D nanostructures by means of density functional theory. We studied six different systems: MoS 2 , WS 2 , SnS 2 , GaS, InSe, and HfS 2 and we simulated nanosheets of increasing thickness, ranging from ultrathin films to ∼10-13 nm thick slabs, a size where the properties converge to the bulk. In some cases, the convergence of the band gap with slab thickness is rather slow, and sizeable deviations from the bulk value are still present with few nm-thick sheets. The results of the simulations were compared with the available experimental data, finding a quantitative agreement. The impact of quantum confinement can be rationalized in terms of effective masses of electrons and holes and system's size. These results show the possibility of reliably describing quantum confinement effects on systems for which experimental data are not available., (© 2022 IOP Publishing Ltd.)

N-Heterocyclic carbenes (NHCs) have superior properties as building blocks of self-assembled monolayers (SAMs). Understanding the influence of the substrate in the molecular arrangement is a fundamental step before employing these ligands in technological applications. Herein, we study the molecular arrangement of a model NHC on Cu(100) and Cu(111). While mostly disordered phases appear on Cu(100), on Cu(111) well-defined structures are formed, evolving from magic-number islands to molecular ribbons with coverage. This work presents the first example of magic-number islands formed by NHC assemblies on flat surfaces. Diffusion and commensurability are key factors explaining the observed arrangements. These results shed light on the molecule-substrate interaction and open the possibility of tuning nanopatterned structures based on NHC assemblies., (© 2022 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

The magnetic ground state and the hyperfine coupling parameters of some first-row transition metal (TM) atoms (Ti, Cr, Mn, Fe, Co, and Ni) adsorbed on ultrathin insulating oxide films are studied by means of DFT calculations. The results obtained using GGA, screened hybrid, and GGA+ U functionals are compared for TMs adsorbed on free-standing MgO(100). Then, the case of adsorption on MgO mono- and bilayers supported on Ag(100) is studied using GGA+ U . Along with the problematic aspects inherent to the calculation of hyperfine coupling constants, a critical dependence on the magnetic state and electron configuration of the TM is reported, which implies a real challenge for the state-of-the-art DFT methods. In the cases where all functionals considered provide a coherent magnetic and electron configuration, however, the calculated hyperfine parameters do not depend significantly on the choice of the functional. In this respect, the role of the metal support in the hyperfine coupling constants is highly system-dependent and becomes crucial in all cases where the support modifies the oxidation state of the adatom, induces a change in the bonding site or simply induces a rearrangement of the orbital energy diagram. This has important implications for the modelling of single TM atoms deposited on insulating ultrathin films supported on metals for application in quantum technologies or as memory devices.

The global warming crisis has sparked a series of environmentally cautious trends in chemistry, allowing us to rethink the way we conduct our synthesis, and to incorporate more earth-abundant materials in our catalyst design. "Single-atom catalysis" has recently appeared on the catalytic spectrum, and has truly merged the benefits that homogeneous and heterogeneous analogues have to offer. Further still, the possibility to activate these catalysts by means of a suitable electric potential could pave the way for a true integration of diverse synthetic methodologies and renewable electricity. Despite their esteemed benefits, single-atom electrocatalysts are still limited to the energy sector (hydrogen evolution reaction, oxygen reduction, etc. ) and numerous examples in the literature still invoke the use of precious metals (Pd, Pt, Ir, etc. ). Additionally, batch electroreactors are employed, which limit the intensification of such processes. It is of paramount importance that the field continues to grow in a more sustainable direction, seeking new ventures into the space of organic electrosynthesis and flow electroreactor technologies. In this piece, we discuss some of the progress being made with earth abundant homogeneous and heterogeneous electrocatalysts and flow electrochemistry, within the context of organic electrosynthesis, and highlight the prospects of alternatively utilizing single-atom catalysts for such applications.

The hydrogen evolution reaction (HER) has a key role in electrochemical water splitting. Recently a lot of attention has been dedicated to HER from single atom catalysts (SACs). The activity of SACs in HER is usually rationalized or predicted using the original model proposed by Nørskov where the free energy of a H atom adsorbed on an extended metal surface M (formation of an MH intermediate) is used to explain the trends in the exchange current for HER. However, SACs differ substantially from metal surfaces and can be considered analogues of coordination compounds. In coordination chemistry, at variance with metal surfaces, stable dihydride or dihydrogen complexes (HMH) can form. We show that the same can occur on SACs and that the formation of stable HMH intermediates, in addition to the MH one, may change the kinetics of the process. Extending the original kinetic model to the case of two intermediates (MH and HMH), one obtains a three-dimensional volcano plot for the HER on SACs. DFT numerical simulations on 55 models demonstrate that the new kinetic model may lead to completely different conclusions about the activity of SACs in HER. The results are validated against selected experimental cases. The work provides an example of the important analogies between the chemistry of SACs and that of coordination compounds.

Europium, one of the rare-earth elements, exhibits +2 and +3 valence states and has been widely used for the magnetic modification of materials. Based on density functional theory calculations, we predicted 2D EuBr/graphene heterojunctions to exhibit metallicity, huge intrinsic-ferromagnetism nearly 7.0 μ B per Eu and the special monovalent Eu ions. Electron localization function (ELF), difference charge densities and Bader charge analyses demonstrated that there are cation-π interactions between the EuBr films and graphene. Graphene works as a substrate to enable the stability of EuBr monolayer crystals, where EuBr plays an important role to yield ferromagnetism and enhance metallicity in the heterojunctions. Monte Carlo simulations were used to estimate a Curie temperature of about 7 K, which, together with magnetic configurations, can be further modulated by external strains and charge-carrier doping. In general, our theoretical work predicts the properties of novel 2D ferromagnetic EuBr/graphene heterojunctions, suggesting the possibility of combining 2D intrinsic-ferromagnetic metal halide crystals and graphene, and opening up a new perspective in next-generation electronic, spintronic devices and high-performance sensors.

We studied the role of iso-valent heteroatoms replacing Ti 4+ cations in the lattice of two titania polymorphs, rutile (r-) and anatase (a-) by means of first principles calculations. The r-TiO 2 (110) and the a-TiO 2 (101) surfaces have been considered and Ti ions in the bulk, sub-surface, and surface sites have been replaced with Si, Ge, Sn, Pb, Zr, Hf, and Ce ions: surface or sub-surface sites are clearly preferred. Since the dopants have the same number of valence electrons as the replaced Ti atom, they can have only two effects: one is steric, related to the different size of the dopant compared to Ti 4+ ; the other is an orbital effect, due to the energy levels associated to the dopant not present on the pristine surface. Both these effects can modify locally the geometric and electronic structure of the surface, in particular by introducing new states in the band gap. To check the effect of the dopants on the surface reactivity we studied as an example the decomposition of HCOOH which can follow four different paths with desorption of (a) H 2, (b) CO, (c) H 2 O, or (d) CO 2 . The results show the very different behavior of the two titania polymorphs considered, rutile and anatase: rutile is more reactive and more easily reduced than anatase. For specular reasons, the presence of the dopants has in general more pronounced effects on anatase, as they can deeply modify the surface reactivity and the HCOOH decomposition path., (© 2021 IOP Publishing Ltd.)

Electronic structure calculations provide a useful complement to experimental characterization tools in the atomic-scale design of semiconductor heterojunctions for photocatalysis. The band alignment of the heterojunction is of fundamental importance to achieve an efficient charge carrier separation, so as to reduce electron/hole recombination and improve photoactivity. The accurate prediction of the offsets of valence and conduction bands in the constituent units is thus of key importance but poses several methodological and practical problems. In this Minireview we address some of these problems by considering selected examples of binary and ternary semiconductor heterojunctions and how these are determined at the level of density functional theory (DFT). The atomically precise description of the interface, the consequent charge polarization, the role of quantum confinement, the possibility to use facet engineering to determine a specific band alignment, are among the effects discussed, with particular attention to pros and cons of each one of these aspects. This analysis shows the increasingly important role of accurate electronic structure calculations to drive the design and the preparation of new interfaces with desired properties., (© 2021 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)

Understanding the mechanism behind the superior catalytic power of single- or few-atom heterogeneous catalysts has become an important topic in surface chemistry. This is particularly the case for gold, with TiO 2 being an efficient support. Here we use scanning tunneling microscopy/spectroscopy with theoretical calculations to investigate the adsorption geometry and local electronic structure of several-atom Au clusters on rutile TiO 2 (110), with the clusters fabricated by controlled manipulation of single atoms. Our study confirms that Au 1 and Au 2 clusters prefer adsorption at surface O vacancies. Au 3 clusters adsorb at O vacancies in a linear-chain configuration parallel to the surface; in the absence of O vacancies they adsorb at Ti 5c sites with a structure of a vertically pointing upright triangle. We find that both the electronic structure and cluster-substrate charge transfer depend critically on the cluster size, bonding configuration, and local environment. This suggests the possibility of engineering cluster selectivity for specific catalytic reactions.

Photoelectrochemical solar energy conversion offers a way to directly store light into energy-rich chemicals. Photoanodes based on the WO 3 /BiVO 4 heterojunction are most effective mainly thanks to the efficient separation of photogenerated charges. The WO 3 /BiVO 4 interfacial space region in the heterojunction is investigated here with the increasing thickness of the BiVO 4 layer over a WO 3 scaffold. On the basis of X-ray diffraction analysis results, density functional theory simulations show a BiVO 4 growth over the WO 3 layer along the BiVO 4 {010} face, driven by the formation of a stable interface with new covalent bonds, with a favorable band alignment and band bending between the two oxides. This crystal facet phase matching allows a smooth transition between the electronic states of the two oxides and may be a key factor ensuring the high efficiency attained with this heterojunction. The photoelectrochemical activity of the WO 3 /BiVO 4 photoanodes depends on both the irradiation wavelength and the thickness of the visible-light-absorbing BiVO 4 layer, a 75 nm thick BiVO 4 layer on WO 3 being best performing., Competing Interests: The authors declare no competing financial interest., (© 2021 American Chemical Society.)

Semiconductor heterojunctions are widely applied in solid-state device applications, including semiconductor lasers, solar cells, and transistors. In photocatalysis they are of interest due to their capability to hinder charge carriers' recombination. A key role in the performance of heterojunctions is that of the alignment of the band edges of the two units composing the junction. In this work, we compare the performances of three widely applied approaches for the simulation of semiconductors heterostructures, based on density functional theory calculations with hybrid functionals. We benchmark the band offsets of ten semiconductors heterostructures for which experimental values are available: AlP/GaP, AlP/Si, AlAs/GaAs, AlAs/Ge, GaAs/Ge, GaP/Si, ZnSe/Ge, ZnSe/AlAs, ZnSe/GaAs, and TiO 2 /SrTiO 3 . The methods considered are (i) the alternating slabs junction (ASJ), (ii) the surface terminated junction (STJ), and (iii) the independent units (IU) approach. Moreover, two different ways to determine a common reference have been considered, (i) the plane averaged electrostatic potential, and (ii) the energy of the core levels. Advantages, drawbacks and overall performances of each method are discussed. The results suggest that the accuracy in the estimation of the band offsets is ∼0.2 eV when the ASJ method is applied. The STJ approach provides a similar accuracy, while the neglection of any interface effect, as in the IU method, provides only a qualitative estimate of the band offset and can result in significant deviations from the experiment., (© 2021 IOP Publishing Ltd.)

The role of the support in tuning the properties of transition metal (TM) atoms is studied by means of density functional theory calculations. We have considered the adsorption of Cu, Ag, Au, Ni, Pd, and Pt atoms on crystalline silica bilayers, either free-standing or supported on Ru(0001) and Pt(111) metal surfaces. These systems have been compared with an hydroxylated SiO 2 /Si(100) film simulating the native oxide formed on a silicon wafer. The properties of the TM atoms change significantly on the various supports. While the unsupported silica bilayer weakly binds some of the TM atoms studied, the SiO 2 /Ru(0001) or SiO 2 /Pt(111) supports exhibit enhanced reactivity, sometimes resulting in a net electron transfer with the formation of charged species. Differences in the behavior of SiO 2 /Ru(0001) and SiO 2 /Pt(111) are rationalized in terms of different work functions and metal/oxide interfacial distances. No electron transfer is observed on the SiO 2 /Si(100) films. Here, the presence of hydroxyl groups on the surface provides relatively strong binding sites for the TM atoms that can be stabilized by the interaction with one or two OH groups. The final aspect that has been investigated is the porosity of the silica bilayer, at variance with the dense SiO 2 /Si(100) film. Depending on the atomic size, some TM atoms can penetrate spontaneously through the six-membered silica rings and become stabilized in the pores of the bilayer or at the SiO 2 /metal interface. This study shows how very different chemical properties can be obtained by depositing the same TM atom on different silica supports.

In this work we investigate TiO 2 and BiVO 4 nanostructures by means of density functional theory (DFT) calculations, to provide an estimate of the band alignment in TiO 2 /BiVO 4 interfaces, highly active in photo-electrochemistry and photocatalytic water splitting. Calculations were carried out with both DFT range separated and self-consistent dielectric dependent hybrid functionals (HSE06 and PBE0 DD ). The impact of systems' size has been investigated. The converged electronic levels of TiO 2 and BiVO 4 surfaces have been used to predict the band alignment in TiO 2 /BiVO 4 heterostructures. Results indicated that when TiO 2 (101) surface is matched with BiVO 4 (110), a type-II alignment is obtained, where the band edges of BiVO 4 are higher in energy that those of TiO 2 . This picture is favorable for charge-carriers separation upon photoexcitation, where electrons move toward TiO 2 and holes toward BiVO 4 . On the contrary, if TiO 2 (101) is interfaced to BiVO 4 (010) the offset between the band edges is negligible, thus reducing the driving force toward separation of charge carriers. These results rationalize the dependence on the facet's exposure of the observed photocatalytic performances of TiO 2 /BiVO 4 composites, where the TiO 2 (101)/BiVO 4 (110) interface outperforms the TiO 2 (101)/BiVO 4 (010) one.

α-CsPbX3 (X = Cl, Br, I) 2D nanostructures are widely used in solar cells, photocatalysis and photovoltaic applications, mainly because of their high efficiency in the conversion of solar energy. Based on hybrid Density Functional Theory (DFT) calculations we consider two aspects, (a) the role of surface termination, and (b) that of quantum size (thickness) of the 2D slabs. We show that the surface termination is a key aspect in determining the electronic properties. For the (001) surface of α-CsPbX3 perovskites there are two possible terminations, with similar stabilities but different positions of the band edges. In general, the band edges of the (110) surfaces, with the X-terminated surface being the most stable one, are lower in energy than the (001) ones. These conclusions are very important for the design of efficient heterostructures for solar light harvesting. Furthermore, the properties of α-CsPbX3 2D nanostructures can be tuned by varying the thickness. We present a general model to predict quantum size effects of α-CsPbX3 from ultrathin films (3-5 atomic layers) to the bulk. Finally, based on calculated electronic properties of CsPbX3 (and TiO2 surfaces), we estimate a type-II alignment in composites such as CsPbX3/TiO2, favourable for electron migration from the perovskite to TiO2. These results can help the rational design of halide perovskite nanostructures for solar energy harvesting, in particular by interfacing 2D materials with specific surfaces and terminations.

The present review reports on the preparation and atomic-scale characterization of the thinnest possible films of the glass-forming materials silica and germania. To this end state-of-the-art surface science techniques, in particular scanning probe microscopy, and density functional theory calculations have been employed. The investigated films range from monolayer to bilayer coverage where both, the crystalline and the amorphous films, contain characteristic XO 4 (X=Si,Ge) building blocks. A side-by-side comparison of silica and germania monolayer, zigzag phase and bilayer films supported on Mo(112), Ru(0001), Pt(111), and Au(111) leads to a more general comprehension of the network structure of glass former materials. This allows us to understand the crucial role of the metal support for the pathway from crystalline to amorphous ultrathin film growth., (© 2020 The Authors. Published by Wiley-VCH GmbH.)

The growth of ZnO clusters supported by ZnO-bilayers on Ag(111) and the interaction of these oxide nanostructures with water have been studied by a multi-technique approach combining temperature-dependent infrared reflection absorption spectroscopy (IRRAS), grazing-emission X-ray photoelectron spectroscopy, and density functional theory calculations. Our results reveal that the ZnO bilayers exhibiting graphite-like structure are chemically inactive for water dissociation, whereas small ZnO clusters formed on top of these well-defined, yet chemically passive supports show extremely high reactivity - water is dissociated without an apparent activation barrier. Systematic isotopic substitution experiments using H 2 16 O/D 2 16 O/D 2 18 O allow identification of various types of acidic hydroxyl groups. We demonstrate that a reliable characterization of these OH-species is possible via co-adsorption of CO, which leads to a red shift of the OD frequency due to the weak interaction via hydrogen bonding. The theoretical results provide atomic-level insight into the surface structure and chemical activity of the supported ZnO clusters and allow identification of the presence of under-coordinated Zn and O atoms at the edges and corners of the ZnO clusters as the active sites for H 2 O dissociation., (© 2020 The Authors. ChemPhysChem published by Wiley-VCH GmbH.)

In this work we investigate the role of quantum confinement in group III-V semiconductor thin films (2D nanostructures). To this end we have studied the electronic structure of nine materials (AlP, AlAs, AlSb, GaP, GaAs, GaSb, InP, InAs and InSb) by means of Density Functional Theory (DFT) calculations using a screened hybrid functional (HSE06). We focus on the structural and electronic properties of bulk and the (110) surfaces, for which we evaluate and rationalize the impact of system size to the band gap and band edge positions. Our results indicate that when the quantum confinement is strong, it mainly affects the position of the Conduction Band Minimum (CBM) of the semiconductor, while the Valence Band Maximum (VBM) is almost insensitive to the system size. The results can be rationalized in terms of electron and hole effective masses. Our conclusions, based on slabs, can be generalized to other cases of quantum confinement such as quantum dots, overcoming the need for an explicit consideration and calculation of the properties of semiconductor nanoparticles.

The theoretical description of the electronic structure of magnetic insulators and, in particular, of transition-metal oxides (TMOs), MnO, FeO, CoO, NiO, and CuO, poses several problems due to their highly correlated nature. Particularly challenging is the determination of the band gap. The most widely used approach is based on density functional theory (DFT) Kohn-Sham energy levels using self-interaction-corrected functionals (such as hybrid functionals). Here, we present a different approach based on the assumption that the band gap in some TMOs can have a partial Mott-Hubbard character and can be defined as the energy associated with the process M m + (3d n ) + M m + (3d n ) → M ( m +1)+ (3d n -1 ) + M ( m -1)+ (3d n +1 ). The band gap is thus associated with the removal (ionization potential, I ) and addition (electron affinity, A ) of one electron to an ion of the lattice. In fact, due to the hybridization of metal with ligand orbitals, these energy contributions are not purely atomic in nature. I and A can be computed accurately using the charge transition level (CTL) scheme. This procedure is based on the calculation of energy levels of charged states and goes beyond the approximations inherent to the Kohn-Sham (KS) approach. The novel and relevant aspect of this work is the extension of CTLs from the domain of point defects to a bulk property such as the band gap. The results show that the calculation based on CTLs provides band gaps in better agreement with experiments than the KS approach, with direct insight into the nature of the gap in these complex systems.

Adsorption, desorption and fragmentation of borazine on Pt(110) are studied by temperature-programmed desorption, ultraviolet photoemission spectroscopy, workfunction measurements and density functional theory. Borazine adsorbs in part dissociatively, forming an upright (B 3 N 3 H 5 ˙) ads adsorption complex. Radicals with a N-Pt bond are weakly bound and desorb recombinatively following second-order kinetics. Radicals with a B-Pt bond are similar in binding strength to the molecularly adsorbed species, which binds through dispersive forces to the (111) facets of the (1 × 2) reconstructed Pt(110). Both do not desorb but are dehydrogenated beyond T = 150 K. As T approaches 600 K the B-N ring progressively breaks down into its atomic constituents. The borazine ice multilayer is capable of trapping significant amounts of hydrogen. Previous studies of borazine adsorption on other transition metal surfaces yield a very similar pattern. Reported multiple molecular desorption peaks are artefacts. Implications for the nucleation and growth of h-BN monolayers at high temperatures are discussed.

In this work, we investigate the structural and electronic properties of the SrTiO 3 /TiO 2 (anatase) heterostructure by means of hybrid density functional theory calculations. The work is motivated by several experiments that pointed to SrTiO 3 /TiO 2 as a good system for photocatalytic applications, due to the small lattice mismatch between these two oxides and their favorable band alignment, leading to a type-II heterojunction, favoring the charge-carrier separation. The present results provide insights into the nature of the contact region and an estimation of the band offsets in the composite system. Our results are also compared with the available experimental values and with previous theoretical reports. The calculated offsets quantitatively agree with experimental measurements. In addition, we found significant interfacial effects that make the band offsets slightly increase with respect to those of the separated components. Last, we also discuss the role of point defects such as oxygen vacancies, finding that they do not remarkably affect the band alignment.

The free-standing Au 20 cluster has a unique tetrahedral shape and a large HOMO-LUMO (highest occupied molecular orbital-lowest unoccupied molecular orbital) gap of around 1.8 electron volts. The "magic" Au 20 has been intensively used as a model system for understanding the catalytic and optical properties of gold nanoclusters. However, direct real-space ground-state characterization at the atomic scale is still lacking, and obtaining fundamental information about the corresponding structural, electronic, and dynamical properties, is challenging. Here, using cluster-beam deposition and low-temperature scanning tunneling microscopy, atom-resolved topographic images and electronic spectra of supported Au 20 clusters are obtained. We demonstrate that individual size-selected Au 20 on ultrathin NaCl films maintains its pyramidal structure and large HOMO-LUMO gap. At higher cluster coverages, we find sintering of the clusters via Smoluchowski ripening to Au 20 n agglomerates. The evolution of the electron density of states deduced from the spectra reveals gap reduction with increasing agglomerate size., (Copyright © 2020 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works. Distributed under a Creative Commons Attribution NonCommercial License 4.0 (CC BY-NC).)

During the COVID-19 lockdown, the flourishing of online seminars is opening up the new frontiers of physics to an unprecedented number of scientists., (© Springer Nature Limited 2020.)

Calcium-molybdate ultrathin films were prepared on a Mo(001) crystal and characterized by means of scanning tunneling microscopy (STM), electron diffraction, photoelectron spectroscopy, and density functional theory (DFT). The films were grown via reactive Ca deposition, followed by a vacuum annealing step to trigger Mo diffusion from the support into the Ca-O ad-layer. A series of crystalline oxide configurations was revealed that evolves from a (3 × 3) to a (4 × 4) and (6 × 6) superstructure with increasing annealing temperature and finally decays to a binary MoO x phase. The stoichiometry of the initial (3 × 3) phase was estimated to CaMo 3 O 6 , yet with decreasing Ca concentration at higher temperature. In the search for a suitable structure model for DFT calculations, we have started with a bulk CaMo 5 O 8 configuration that was iteratively modified to match the experimental data. The optimized structure is made of regular sequences of flat-lying and upright standing Mo octahedrons, being separated from each other by Ca 2+ ion rows. With decreasing Ca content, the central Mo units grow in size, which explains the observed transition from (3 × 3) to (6 × 6) superstructures upon annealing. The proposed structure model rationalizes the periodicity and corrugation of the regular oxide surface as well as the characteristic domain patterns in the film. Its electronic properties, as deduced from STM conductance spectroscopy, can be correlated with an increasing metallicity of the ad-layer upon annealing. Our work presents a facile pathway to produce high-quality ternary oxide films via interdiffusion of atoms from a suitable metal support into a binary oxide layer.

The surface basicity of the alkaline-earth metal oxides has been investigated by studying the properties of 17 O nuclear magnetic resonance (NMR). To this end, we performed density functional theory calculations and determined the 17 O chemical shift and the quadrupolar coupling constants of the regular and stepped surfaces of MO (M = Mg, Ca, Sr, and Ba) oxides. The computed average chemical shift (δ iso av ) for 17 O NMR of bulk MgO, CaO, SrO, and BaO is 46, 301, 394, and 636 ppm, respectively, in excellent agreement with the experiment. The 17 O NMR chemical shifts correlate linearly with the Madelung potential in the four oxides. Next, we considered the changes in the 17 O chemical shift due to the adsorption of BR 3 (R = F and OCH 3 ) and pyrrole as probe molecules. We found that the 17 O NMR signal of the O ion directly bound to the probe molecule shifts considerably compared to the clean surface. This is due to a change in the polarization of the O charge distribution due to the molecular adsorption. This change is the largest for BaO, with the strongest bond and the shortest surface-adsorbate distance, and the smallest for MgO, thus showing a direct correlation between 17 O NMR and surface basicity. The 17 O chemical shift of the basic site correlates linearly also with several properties of the adsorbed molecules, providing a direct measure of the surface basicity.

Reproduction of the band gaps of semiconductors and insulators represents a well-known problem for standard DFT approaches based on semilocal functionals. The problem can be partly solved using hybrid functionals, in which a given portion of exact exchange is mixed with the DFT exchange. Recently, a new class of dielectric-dependent functionals has been introduced in which the amount of exact exchange is derived from the static dielectric function of a given compound. In this study we considered in a systematic way on an equal footing a set of 24 nonmagnetic three-dimensional (3D) bulk metal oxides and 24 quasi-two-dimensional (quasi-2D) semiconductors (oxides, hydroxides, chlorides, oxyhalides, nitrides, and transition metal dichalcogenides) and computed the corresponding Kohn-Sham band gaps with three global hybrid functionals and four range-separated hybrid functionals. These in turn were divided into standard (PBE0, B3LYP, HSE06, SC-BLYP) and dielectric-dependent (DD-B3LYP, DD-SC-BLYP, DD-CAM-B3LYP) functionals. We also performed a statistical analysis of the DFT data set along with structural parameters of these 2D and 3D materials. The surprising result is that overall there is no real improvement with the use of dielectric-dependent functionals compared to PBE0, HSE06, and B3LYP. Short-range DD-SC-BLYP gives a minor improvement in the band gaps for bulk metal oxides compared with standard SC-BLYP, but the mean absolute error is still 0.12 eV higher than with B3LYP. The use of dielectric-dependent standard or short-range functionals such as DD-B3LYP or DD-HSE06 worsens the situation. However, the dielectric-dependent version of the long-range-separated functional implemented with the Coulomb attenuating method (CAM), DD-CAM-B3LYP, leads to a clear improvement for band gaps of quasi-2D materials. On the basis of this analysis, the conclusion is that the use of a standard hybrid functional such as B3LYP or HSE06 is recommended for nonmagnetic bulk 3D metal oxides. On the other hand, the treatment of layered materials such as MoO 3 or V 2 O 5 benefits from the use of dielectric-dependent range-separated functionals.

The valence and conduction band (CB) alignments and the nature of the WO 3 /BiVO 4 heterojunction have been analysed in detail based on hybrid functionals density functional theory calculations. The WO 3 /BiVO 4 junction is widely studied in photocatalysis for its capability to reduce electron-hole recombination and to improve efficiency. This is assumed to be due to a favourable band alignment of the junction's components, which generates a flow of negative charge carriers towards WO 3 , and positive ones towards BiVO 4 . This conclusion is often based on the properties of the two isolated, non-interacting units. Here, we propose an explicit interface model where the (0 0 1) surface of WO 3 is put in contact with the (0 1 0) surface of BiVO 4 rotated by about 45°, which leads to a small strain and a favourable cation-anion matching. The interface displays a moderate charge transfer and a small interface dipole. This leads to only moderate effects on the band alignment, which remains qualitatively similar to that obtained from the two independent oxides. We also considered in detail the role of the amount of exact exchange used in the description of the heterojunction, and in particular of the BiVO 4 component, for which rather different hybrid functional approaches have been proposed in the literature.

In this work we study the effect of nitrogen (N)-doping on the structural and electronic properties of coexposed anatase (001)-(101) surfaces by means of Density Functional Theory (DFT). This work is motivated by recent experiments, showing that these materials are highly active for photocatalysis. The introduction of doping species such as N further improves their activity under visible light. Our results indicate a tendency toward dopant segregation at the interface at low N concentrations, while at larger concentrations also doping sites in the bulk regions are populated. N-Doping does not affect the reciprocal band alignment of TiO 2 (101) and (001), where the former stabilizes photogenerated electrons and the latter hosts the holes. However, N-doping enhances the visible light absorption of the composite material, due to the introduction of gap states. Moreover, N-doping strongly stabilizes oxygen vacancies, which in turn enhance the light absorption properties.

A new two-dimensional (2D) germanium dioxide film has been prepared. The film consists of interconnected germania tetrahedral units forming a bilayer structure, weakly coupled to the supporting Pt(111) metal-substrate. Density functional theory calculations predict a stable structure of 558-membered rings for germania films, while for silica films 6-membered rings are preferred. By varying the preparation conditions the degree of order in the germania films is tuned. Crystalline, intermediate ordered and purely amorphous film structures are resolved by analysing scanning tunnelling microscopy images., (© 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.)