Probing the Surface of Oxide Nanoparticles Using DNP-Enhanced High-Resolution NMR of Quadrupolar Nuclei.

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 27Al and 17O spin-5/2 nuclei on the surface of γ-alumina. Moreover, we report high-resolution 17O 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 11B spin-3/2 nuclei on the surface of partially oxidized boron nitride supported on silica and to distinguish its different BO2OH active sites.

Key Ingredients for the Screening of Single Atom Catalysts for the Hydrogen Evolution Reaction: The Case of Titanium Nitride.

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 H2-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.

CO2 electroreduction on single atom catalysts: the role of the DFT functional.

One key process involving single atom catalysts (SACs) is the electroreduction of CO2 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 CO2. In this work, we performed a density functional theory (DFT) study of CO2 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.

Copper Single Atoms Chelated on Ligand-Modified Carbon for Ullmann-Type C-O Coupling.

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.

Pt Single Atoms Supported on Defect Ceria as an Active and Stable Dual-Site Catalyst for Alkaline Hydrogen Evolution.

This work evaluates the feasibility of alkaline hydrogen evolution reaction (HER) using Pt single-atoms (1.0 wt %) on defect-rich ceria (Pt1/CeOx) as an active and stable dual-site catalyst. The catalyst displayed a low overpotential and a small Tafel slope in an alkaline medium. Moreover, Pt1/CeOx presented a high mass activity and excellent durability, competing with those of the commercial Pt/C (20 wt %). In this picture, the defective CeOx 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 Pt2+ isolated atoms, the site where H2 is formed and released. DFT calculations were also performed to obtain mechanistic insight on the Pt1/CeOx 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 Pt2+ single atoms.

Copper Single Atoms Chelated on Ligand-Modified Carbon for Ullmann-type C-O Coupling.

Cross-coupling reactions are of great importance in chemistry due to their ability to facilitate the construction of complex organic molecules. Among these reactions, the Ullmann-type C-O coupling between phenols and aryl halides is particularly noteworthy and useful for preparing diarylethers. However, this reaction typically relies on homogeneous catalysts that rapidly deactivate under harsh reaction conditions. In this study, we introduce a novel heterogeneous catalyst for the Ullmann-type C-O coupling reaction, comprised of isolated Cu atoms chelated to a tetraethylenepentamine-pyrrole ligand that is immobilized on graphite nanoplatelets. The catalytic study reveals the recyclability of the material, and demonstrates the crucial role of the pyrrole linker in stabilizing the Cu sites. The work expands the potential of single-atom catalyst nanoarchitectures and underscores the significance of ligands in stabilizing metals in cationic forms, providing a novel, tailored catalyst for cross-coupling chemistries.

Modeling Single-Atom Catalysis.

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.

CO2 Activation on Cu/TiO2 Nanostructures: Importance of Dual Binding Site.

CO2 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 TiO2 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) CO2 activation on Cu/TiO2 nanostructures is surface sensitive, as the rutile and anatase surfaces can exhibit different behaviors; (2) the surface morphology is essential since CO2 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 TiO2 are in the +I oxidation state and can bind and activate CO2 via charge transfer from the oxide; (4) on supported Cu clusters CO2 activation occurs mostly at the metal/oxide interface; (5) the presence of O vacancy sites facilitates the spontaneous dissociation of CO2 to CO, or increases the electron density of the metal catalyst, two effects that can influence the mechanism of CO2 reduction to methanol or other chemicals.

pH- and Facet-Dependent Surface Chemistry of TiO2 in Aqueous Environment from First Principles.

TiO2 is a relevant catalytic material, and its chemistry in aqueous environment is a challenging aspect to address. Also, the morphology of TiO2 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 TiO2/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 H2O 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 TiO2 particles by considering the rutile (110) and (011), and anatase (101), (001), and (100) surfaces.

Identification of Intermediates in the Reaction Pathway of SO2 on the CaO Surface: From Physisorption to Sulfite to Sulfate.

The interaction of CaO and Ca(OH)2 with solvated or gaseous SO2 plays a crucial role in the corrosion of urban infrastructure by acid rain or in the removal of SO2 from flue gas. We carried out a combined spectroscopic and theoretical investigation on the interaction of SO2 with a CaO(001) single crystal. First, the surface chemistry of SO2 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 SO2 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.

Covalent Adsorption of N-Heterocyclic Carbenes on a Copper Oxide Surface.

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.

Probing the nature of Lewis acid sites on oxide surfaces with 31P(CH3)3 NMR: a theoretical analysis.

The characterization of catalytic oxide surfaces is often done by studying the properties of adsorbed probe molecules. The 31P NMR chemical shift of adsorbed trimethylphosphine, P(CH3)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 TiO2(101) and (001), rutile TiO2(110), tetragonal ZrO2(101), stepped ZrO2(134) and (145) surfaces, rutile SnO2(110), (101) and (100), wurtzite ZnO(101̄0), and cubic CeO2(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 31P 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 31P 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.

Quantum confinement in chalcogenides 2D nanostructures from first principles.

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: MoS2, WS2, SnS2, GaS, InSe, and HfS2and 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.

Magnetic nature and hyperfine interactions of transition metal atoms adsorbed on ultrathin insulating films: a challenge for DFT.

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.

Defect engineering of oxide surfaces: dream or reality?

In this brief perspective we analyze the present status of the field of defect engineering of oxide surfaces. In particular we discuss the tools and techniques available to generate, identify, quantify, and characterize point defects at oxide surfaces and the main areas where these centers play a role in practical applications.

Growth of N-Heterocyclic Carbene Assemblies on Cu(100) and Cu(111): From Single Molecules to Magic-Number Islands.

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.

Interfacing single-atom catalysis with continuous-flow organic electrosynthesis.

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.

Role of Dihydride and Dihydrogen Complexes in Hydrogen Evolution Reaction on Single-Atom Catalysts.

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.

Prediction of 2D ferromagnetism and monovalent europium ions in EuBr/graphene heterojunctions.

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 µB 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.

WO3/BiVO4 Photoanodes: Facets Matching at the Heterojunction and BiVO4 Layer Thickness Effects.

Photoelectrochemical solar energy conversion offers a way to directly store light into energy-rich chemicals. Photoanodes based on the WO3/BiVO4 heterojunction are most effective mainly thanks to the efficient separation of photogenerated charges. The WO3/BiVO4 interfacial space region in the heterojunction is investigated here with the increasing thickness of the BiVO4 layer over a WO3 scaffold. On the basis of X-ray diffraction analysis results, density functional theory simulations show a BiVO4 growth over the WO3 layer along the BiVO4 {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 WO3/BiVO4 photoanodes depends on both the irradiation wavelength and the thickness of the visible-light-absorbing BiVO4 layer, a 75 nm thick BiVO4 layer on WO3 being best performing.