Efficient C-N coupling in the direct synthesis of urea from CO2 and N2 by amorphous SbxBi1-xOy clusters.

Although direct generation of high-value complex molecules and feedstock by coupling of ubiquitous small molecules such as CO2 and N2 holds great appeal as a potential alternative to current fossil-fuel technologies, suitable scalable and efficient catalysts to this end are not currently available as yet to be designed and developed. To this end, here we prepare and characterize SbxBi1-xOy clusters for direct urea synthesis from CO2 and N2 via C-N coupling. The introduction of Sb in the amorphous BiOx clusters changes the adsorption geometry of CO2 on the catalyst from O-connected to C-connected, creating the possibility for the formation of complex products such as urea. The modulated Bi(II) sites can effectively inject electrons into N2, promoting C-N coupling by advantageous modification of the symmetry for the frontier orbitals of CO2 and N2 involved in the rate-determining catalytic step. Compared with BiOx, SbxBi1-xOy clusters result in a lower reaction potential of only -0.3 V vs. RHE, an increased production yield of 307.97 µg h-1 mg-1cat, and a higher Faraday efficiency (10.9%), pointing to the present system as one of the best catalysts for urea synthesis in aqueous systems among those reported so far. Beyond the urea synthesis, the present results introduce and demonstrate unique strategies to modulate the electronic states of main group p-metals toward their use as effective catalysts for multistep electroreduction reactions requiring C-N coupling.

High-Valence Cu Induced by Photoelectric Reconstruction for Dynamically Stable Oxygen Evolution Sites.

Oxygen vacancies are generally considered to play a crucial role in the oxygen evolution reaction (OER). However, the generation of active sites created by oxygen vacancies is inevitably restricted by their condensation and elimination reactions. To overcome this limitation, here, we demonstrate a novel photoelectric reconstruction strategy to incorporate atomically dispersed Cu into ultrathin (about 2-3 molecular) amorphous oxyhydroxide (a-CuM, M = Co, Ni, Fe, or Zn), facilitating deprotonation of the reconstructed oxyhydroxide to generate high-valence Cu. The in situ XAFS results and first-principles calculations reveal that Cu atoms are stabilized at high valence during the OER process due to Jahn-Teller distortion, resulting in para-type double oxygen vacancies as dynamically stable catalytic sites. The optimal a-CuCo catalyst exhibits a record-high mass activity of 3404.7 A g-1 at an overpotential of 300 mV, superior to the benchmarking hydroxide and oxide catalysts. The developed photoelectric reconstruction strategy opens up a new pathway to construct in situ stable oxygen vacancies by high-valence Cu single sites, which extends the design rules for creating dynamically stable active sites.

Amorphous Bismuth-Tin Oxide Nanosheets with Optimized C-N Coupling for Efficient Urea Synthesis.

Closing the carbon and nitrogen cycles by electrochemical methods using renewable energy to convert abundant or harmful feedstocks into high-value C- or N-containing chemicals has the potential to transform the global energy landscape. However, efficient conversion avenues have to date been mostly realized for the independent reduction of CO2 or NO3-. The synthesis of more complex C-N compounds still suffers from low conversion efficiency due to the inability to find effective catalysts. To this end, here we present amorphous bismuth-tin oxide nanosheets, which effectively reduce the energy barrier of the catalytic reaction, facilitating efficient and highly selective urea production. With enhanced CO2 adsorption and activation on the catalyst, a C-N coupling pathway based on *CO2 rather than traditional *CO is realized. The optimized orbital symmetry of the C- (*CO2) and N-containing (*NO2) intermediates promotes a significant increase in the Faraday efficiency of urea production to an outstanding value of 78.36% at -0.4 V vs RHE. In parallel, the nitrogen and carbon selectivity for urea formation is also enhanced to 90.41% and 95.39%, respectively. The present results and insights provide a valuable reference for the further development of new catalysts for efficient synthesis of high-value C-N compounds from CO2.

Tuning Octahedral Tilting by Doping to Prevent Detrimental Phase Transition and Extend Carrier Lifetime in Organometallic Perovskites.

As one of the most promising materials for next-generation solar cells, organometallic perovskites have attracted substantial fundamental and applied interest. Using first-principles quantum dynamics calculations, we show that octahedral tilting plays an important role in stabilizing perovskite structures and extending carrier lifetimes. Doping the material with (K, Rb, Cs) ions at the A-site enhances octahedral tilting and the stability of the system relative to unfavorable phases. The stability of doped perovskites is maximized for uniform distribution of the dopants. Conversely, aggregation of dopants in the system inhibits octahedral tilting and the associated stabilization. The simulations also indicate that with enhanced octahedral tilting, the fundamental band gap increases, the coherence time and nonadiabatic coupling decrease, and the carrier lifetimes are thus extended. Our theoretical work uncovers and quantifies the heteroatom-doping stabilization mechanisms, opening up new avenues to enhancing the optical performance of organometallic perovskites.

Parallel Nanosheet Arrays for Industrial Oxygen Production.

According to the traditional nucleation theory, crystals in solution nucleate under thermal fluctuations with random crystal orientation. Thus, nanosheet arrays grown on a substrate always exhibit disordered arrangements, which impede mass transfer during catalysis. To overcome this limitation, here, we demonstrate stress-induced, oriented nucleation and growth of nanosheet arrays. A regularly self-growing parallel nanosheet array is realized on a curved growth substrate. During electrochemical oxygen production, the ordered array maintains a steady flow of liquids in the microchannels, suppressing the detrimental production of flow-blocking oxygen bubbles typical of randomly oriented nanosheet arrays. Controllable parallel arrays, fully covered fluffy-like ultrathin nanosheets, and amorphous disordered structures altogether enable full-scale design of hierarchical interfaces from the micro- to the atomic scale, significantly improving the otherwise sluggish kinetics of oxygen evolution toward industrial ultrafast production. Record-high ultrafast oxygen production of 135 L·min-1·m-2 with high working current of 4000 mA·cm-2 is steadily achieved at a competitively low cell voltage of 2.862 V. These results and related insights lay the basis for further developments in oriented nucleation and growth of crystals beyond classical nucleation approaches, with benefits for large-scale, industrial electrochemical processes as shown here for ultrafast oxygen production.

Activating Bi p-orbitals in Dispersed Clusters of Amorphous BiOx for Electrocatalytic Nitrogen Reduction.

Catalytic strategies based on main group metals are significantly less advanced than those of transition metal catalysis, leaving untapped areas of potentially fruitful research. We here demonstrate an effective approach for the modulation of Bi 6p energy levels during the construction of atomically dispersed clusters of amorphous BiOx . Bi oxidation state is proposed to strongly affects the nitrogen fixation activity, with the half-occupied pz orbitals of the Bi2+ ions being highly efficient toward electron injection into the inert N2 molecule. With sufficient catalytic sites to adsorb and activate N2 , the bonding between N2 and catalyst is able to be in situ identified. The catalyst shows an outstanding Faraday efficiency (≈30 %) and high yield (≈113 µg h-1 mg-1 cat ) in NH3 production, outperforming most of the existing catalysts in aqueous solution. These results lay the basis for developing the potential of p-block elements for catalysis of multi-electron reactions.

Kinetic Monte Carlo modeling of oxide thin film growth.

In spite of the increasing interest in and application of ultrathin film oxides in commercial devices, the understanding of the mechanisms that control the growth of these films at the atomic scale remains limited and scarce. This limited understanding prevents the rational design of novel solutions based on precise control of the structure and properties of ultrathin films. Such a limited understanding stems in no minor part from the fact that most of the available modeling methods are unable to access and robustly sample the nanosecond to second timescales required to simulate both atomic deposition and surface reorganization at ultrathin films. To contribute to this knowledge gap, here we have combined molecular dynamics and adaptive kinetic Monte Carlo simulations to study the deposition and growth of oxide materials over an extended timescale of up to ∼0.5 ms. In our pilot studies, we have examined the growth of binary oxide thin films on oxide substrates. We have investigated three scenarios: (i) the lattice parameter of both the substrate and thin film are identical, (ii) the lattice parameter of the thin film is smaller than the substrate, and (iii) the lattice parameter is greater than the substrate. Our calculations allow for the diffusion of ions between deposition events and the identification of growth mechanisms in oxide thin films. We make a detailed comparison with previous calculations. Our results are in good agreement with the available experimental results and demonstrate important limitations in former calculations, which fail to sample phase space correctly at the temperatures of interest (typically 300-1000 K) with self-evident limitations for the representative modeling of thin films growth. We believe that the present pilot study and proposed combined methodology open up for extended computational support in the understanding and design of ultrathin film growth conditions tailored to specific applications.

Beating the Stoner criterion using molecular interfaces.

Only three elements are ferromagnetic at room temperature: the transition metals iron, cobalt and nickel. The Stoner criterion explains why iron is ferromagnetic but manganese, for example, is not, even though both elements have an unfilled 3d shell and are adjacent in the periodic table: according to this criterion, the product of the density of states and the exchange integral must be greater than unity for spontaneous spin ordering to emerge. Here we demonstrate that it is possible to alter the electronic states of non-ferromagnetic materials, such as diamagnetic copper and paramagnetic manganese, to overcome the Stoner criterion and make them ferromagnetic at room temperature. This effect is achieved via interfaces between metallic thin films and C60 molecular layers. The emergent ferromagnetic state exists over several layers of the metal before being quenched at large sample thicknesses by the material's bulk properties. Although the induced magnetization is easily measurable by magnetometry, low-energy muon spin spectroscopy provides insight into its distribution by studying the depolarization process of low-energy muons implanted in the sample. This technique indicates localized spin-ordered states at, and close to, the metal-molecule interface. Density functional theory simulations suggest a mechanism based on magnetic hardening of the metal atoms, owing to electron transfer. This mechanism might allow for the exploitation of molecular coupling to design magnetic metamaterials using abundant, non-toxic components such as organic semiconductors. Charge transfer at molecular interfaces may thus be used to control spin polarization or magnetization, with consequences for the design of devices for electronic, power or computing applications (see, for example, refs 6 and 7).

The ONETEP linear-scaling density functional theory program.

We present an overview of the onetep program for linear-scaling density functional theory (DFT) calculations with large basis set (plane-wave) accuracy on parallel computers. The DFT energy is computed from the density matrix, which is constructed from spatially localized orbitals we call Non-orthogonal Generalized Wannier Functions (NGWFs), expressed in terms of periodic sinc (psinc) functions. During the calculation, both the density matrix and the NGWFs are optimized with localization constraints. By taking advantage of localization, onetep is able to perform calculations including thousands of atoms with computational effort, which scales linearly with the number or atoms. The code has a large and diverse range of capabilities, explored in this paper, including different boundary conditions, various exchange-correlation functionals (with and without exact exchange), finite electronic temperature methods for metallic systems, methods for strongly correlated systems, molecular dynamics, vibrational calculations, time-dependent DFT, electronic transport, core loss spectroscopy, implicit solvation, quantum mechanical (QM)/molecular mechanical and QM-in-QM embedding, density of states calculations, distributed multipole analysis, and methods for partitioning charges and interactions between fragments. Calculations with onetep provide unique insights into large and complex systems that require an accurate atomic-level description, ranging from biomolecular to chemical, to materials, and to physical problems, as we show with a small selection of illustrative examples. onetep has always aimed to be at the cutting edge of method and software developments, and it serves as a platform for developing new methods of electronic structure simulation. We therefore conclude by describing some of the challenges and directions for its future developments and applications.

Emergent magnetism at transition-metal-nanocarbon interfaces.

Charge transfer at metallo-molecular interfaces may be used to design multifunctional hybrids with an emergent magnetization that may offer an eco-friendly and tunable alternative to conventional magnets and devices. Here, we investigate the origin of the magnetism arising at these interfaces by using different techniques to probe 3d and 5d metal films such as Sc, Mn, Cu, and Pt in contact with fullerenes and rf-sputtered carbon layers. These systems exhibit small anisotropy and coercivity together with a high Curie point. Low-energy muon spin spectroscopy in Cu and Sc-C60 multilayers show a quick spin depolarization and oscillations attributed to nonuniform local magnetic fields close to the metallo-carbon interface. The hybridization state of the carbon layers plays a crucial role, and we observe an increased magnetization as sp3 orbitals are annealed into sp2-π graphitic states in sputtered carbon/copper multilayers. X-ray magnetic circular dichroism (XMCD) measurements at the carbon K edge of C60 layers in contact with Sc films show spin polarization in the lowest unoccupied molecular orbital (LUMO) and higher π*-molecular levels, whereas the dichroism in the σ*-resonances is small or nonexistent. These results support the idea of an interaction mediated via charge transfer from the metal and dz-π hybridization. Thin-film carbon-based magnets may allow for the manipulation of spin ordering at metallic surfaces using electrooptical signals, with potential applications in computing, sensors, and other multifunctional magnetic devices.

Imogolite Nanotubes and Their Permanently Polarized Bifunctional Surfaces for Photocatalytic Hydrogen Production.

To date, imogolite nanotubes (INTs) have been primarily used for environmental applications such as dye and pollutant degradation. However, imogolite's well-defined porous structure and distinctive electro-optical properties have prompted interest in the system's potential for energy-relevant chemical reactions. The imogolite structure leads to a permanent intrawall polarization arising from the presence of bifunctional surfaces at the inner and outer tube walls. Density functional theory simulations suggest such bifunctionality to encompass also spatially separated band edges. Altogether, these elements make INTs appealing candidates for facilitating chemical conversion reactions. Despite their potential, the exploitation of imogolite's features for photocatalysis is at its infancy, thence relatively unexplored. This perspective overviews the basic physical-chemical and optoelectronical properties of imogolite nanotubes, emphasizing their role as wide bandgap insulator. Imogolite nanotubes have multifaceted properties that could lead to beneficial outcomes in energy-related applications. This work illustrates two case studies demonstrating a step-forward on photocatalytic hydrogen production achieved through atomic doping or metal co-catalyst. INTs exhibit potential in energy conversion and storage, due to their ability to accommodate functions such as enhancing charge separation and influencing the chemical potentials of interacting species. Yet, tapping into potential for energy-relevant application needs further experimental research, computational, and theoretical analysis.

Circumventing the Theoretical Scaling Relation Limit for the Oxygen Evolution Reaction.

Transition metal hydr(oxy)oxides (TMHs) are considered efficient electrocatalysts for the oxygen evolution reaction (OER) under alkaline conditions. Toward identification of potential descriptors to circumvent the scaling relation limit for the OER, first-principles calculations were used to quantify the effects on the overpotential of different s (Mg), p (Al), and d (Ti, V, Cr, Fe, Co, Sc, and Zn) electron dopants in Ni-based TMHs. Both the adsorbate evolution mechanism (AEM) and the lattice oxygen-mediated mechanism (LOM) were examined. The results demonstrate that the formation energy of oxygen vacancies (EVO) is strongly affected by the chemical nature of the dopants. A linear relationship is identified between EVO and the free energy difference for the oxygen-oxygen coupling. A descriptor could be employed to discriminate whether the LOM is energetically favored over the AEM. These findings fill existing gaps in appropriate yet computationally light descriptors for direct identification between the AEM and LOM.

Electron traps and their effect on the surface chemistry of TiO2(110).

Oxygen vacancies on metal oxide surfaces have long been thought to play a key role in the surface chemistry. Such processes have been directly visualized in the case of the model photocatalyst surface TiO(2)(110) in reactions with water and molecular oxygen. These vacancies have been assumed to be neutral in calculations of the surface properties. However, by comparing experimental and simulated scanning tunneling microscopy images and spectra, we show that oxygen vacancies act as trapping centers and are negatively charged. We demonstrate that charging the defect significantly affects the reactivity by following the reaction of molecular oxygen with surface hydroxyl formed by water dissociation at the vacancies. Calculations with electronically charged hydroxyl favor a condensation reaction forming water and surface oxygen adatoms, in line with experimental observations. This contrasts with simulations using neutral hydroxyl where hydrogen peroxide is found to be the most stable product.

Dynamical Screening of Local Spin Moments at Metal-Molecule Interfaces.

Transition-metal phthalocyanine molecules have attracted considerable interest in the context of spintronics device development due to their amenability to diverse bonding regimes and their intrinsic magnetism. The latter is highly influenced by the quantum fluctuations that arise at the inevitable metal-molecule interface in a device architecture. In this study, we have systematically investigated the dynamical screening effects in phthalocyanine molecules hosting a series of transition-metal ions (Ti, V, Cr, Mn, Fe, Co, and Ni) in contact with the Cu(111) surface. Using comprehensive density functional theory plus Anderson's Impurity Model calculations, we show that the orbital-dependent hybridization and electron correlation together result in strong charge and spin fluctuations. While the instantaneous spin moments of the transition-metal ions are near atomic-like, we find that screening gives rise to considerable lowering or even quenching of these. Our results highlight the importance of quantum fluctuations in metal-contacted molecular devices, which may influence the results obtained from theoretical or experimental probes, depending on their possibly material-dependent characteristic sampling time-scales.

The Role of Thermal Fluctuations and Vibrational Entropy: A Theoretical Insight into the δ-to-α Transition of FAPbI3.

Formamidinium lead iodide as a typical organometal perovskite has attracted considerable interest due to its suitable electronic structure. However, the intrinsic mechanisms of its unwanted δ-to-α phase transition remain elusive. By combined first-principles calculations, lattice dynamics analysis, and molecular dynamics simulations, we assign the α phase to the highly dynamic tetragonal phase, with the high-symmetry cubic structure emerging as a dynamically unstable maximum in the system's potential energy landscape. Finite-temperature Gibbs free energy calculations confirm that the δ-to-α transition should be considered as a hexagonal-to-tetragonal transition in contrast to the previous hexagonal-to-cubic assignment. More importantly, phonon thermal property calculations indicate that the driving force of the process is the vibrational entropy difference. These results point out the dynamical nature of the α phase and the key role of the vibrational entropy in perovskite-related phase transitions, the harnessing of which is critical for the successful uptake of organometal perovskites in commercial applications.

Nonadiabatic Dynamics of Polaron Hopping and Coupling with Water on Reduced TiO2.

By interplay between first-principles molecular dynamics and nonadiabatic molecular dynamics simulations based on the decoherence-induced surface-hopping approach, we investigate and quantify the mechanisms through which different electron polaron hopping regimes in the reduced anatase TiO2(101) surface influence recombination of photogenerated charge carriers, also in the presence of adsorbed water (H2O) molecules. The simulations reveal that fast hopping regimes promote ultrafast recombination of photogenerated charge-carriers. Conversely, charge recombination is delayed in the presence of slower polaron hopping and even more so if the polaron is pinned at one Ti-site, as typical following adsorption of H2O on the anatase(101) surface. These trends are related to the observed enhancement of the space and energy overlap between conduction band minimum and polaron band gap states, and the ensuing nonadiabatic couplings (NAC) strengths, during a polaronic hop. We expect these insights on the beneficial role of polaron diffusion pinning for the extended lifetime of photoexcitations in TiO2 to sustain ongoing developments of photocatalytic strategies based on this substrate.

The role of permanent and induced electrostatic dipole moments for Schottky barriers in Janus MXY/graphene heterostructures: a first-principles study.

The Schottky barrier height (ESBH) is a crucial factor in determining the transport properties of semiconductor materials and it directly regulates the carrier mobility in opto-electronics devices. In principle, van der Waals (vdW) Janus heterostructures offer an appealing avenue for controlling the ESBH. However, the underlying atomistic mechanisms are far from understood conclusively, which prompts further research in the topic. To this end, here we carry out an extensive first-principles study of the electronic properties and ESHB of several vdW Janus MXY/graphene (M = Mo, W; X, Y = S, Se, Te) heterostructures. The results of the simulations show that by changing the composition and geometry of the heterostructure's interface, it is possible to control its electrical contact, and thence electron transport properties, from ohmic to Schottky with up to a factor seven variation in the ESBH. Detailed analysis of the simulations enables rationalization of this highly attractive property on the basis of the interplay between the permanent dipole moment of the Janus MXY sheet and the induced one due to interfacial charge redistribution at the MXY/Gr interface. Such an interplay is shown to be highly effective in altering the electrostatic potential difference across the vdW Janus heterostructure, determining its ESBH, and thence Schottky (ohmic) contact type. These computational findings contribute guidelines to control the electrical contacts in Janus heterostructures towards the rational design of electrical contacts in nanoscale devices.

Combined Role of Biaxial Strain and Nonstoichiometry for the Electronic, Magnetic, and Redox Properties of Lithiated Metal-Oxide Films: The LiMn2O4 Case.

Understanding the interplay between strain and nonstoichiometry for the electronic, magnetic, and redox properties of LiMn2O4 films is essential for their development as Li-ion battery (LIB) cathodes, photoelectrodes, and systems for sustainable spintronics applications as well as for emerging applications that combine these technologies. Here, density functional theory (DFT) simulations suggest that compressive strain increases the reduction drive of (111) LiMn2O4 films by inducing >1 eV upshift of the valence band edge. The DFT results indicate that, regardless of the crystallographic orientation for the LiMn2O4 film, biaxial expansion increases the magnetic moments of the Mn atoms. Conversely, biaxial compression reduces them. For ferromagnetic films, these changes can be substantial and as large as over 4 Bohr magnetons per unit cell over the simulated range of strain (from -6 to +3%). The DFT simulations also uncover a compensation mechanism whereby strain induces opposite changes in the magnetic moment of the Mn and O atoms, leading to an overall constant magnetic moment for the ferromagnetic films. The calculated strain-induced changes in atomic magnetic moments reflect modifications in the local electronic hybridization of both the Mn and O atoms, which in turn suggests strain-tunable, local chemical, and electrochemical reactivity. Several energy-favored (110) and (111) ferromagnetic surfaces turn out to be half-metallic with minority-spin band gaps as large as 3.2 eV and compatible with spin-dependent electron-transport and possible spin-dependent electrochemical and electrocatalytic properties. The resilience of the ferromagnetic, half-metallic states to surface nonstoichiometry and compositional changes invites exploration of the potential of LiMn2O4 thin films for sustainable spintronic applications beyond state-of-the-art, rare-earth metal-based, ferromagnetic half-metallic oxides.

The unique carrier mobility of monolayer Janus MoSSe nanoribbons: a first-principles study.

Charge-carrier mobility is a determining factor of the transport properties of semiconductor materials and is strongly related to the optoelectronic performance of nanoscale devices. Here, we investigate the electronic properties and charge carrier mobility of monolayer Janus MoSSe nanoribbons by means of first-principles simulations coupled with deformation potential theory. These simulations indicate that zigzag nanoribbons are metallic. Conversely, armchair nanoribbons are semiconducting and show oscillations in the calculated band gap as a function of edge-width according to the 3p < 3p + 1 < 3p + 2 rule, with p being the integer number of repeat units along the non-periodic direction of the nanoribbon. Although the charge-carrier mobility of armchair nanoribbons oscillates with the edge-width, its magnitude is comparable to its two-dimensional sheet counterpart. A robust room-temperature carrier mobility is calculated for 3.5 nm armchair nanoribbons with values ranging from 50 cm2 V-1 s-1 to 250 cm2 V-1 s-1 for electrons (e) and holes (h), respectively. A comparison of these values with the results for periodic flat sheet (e: 73.8 cm2 V-1 s-1; h: 157.2 cm2 V-1 s-1) reveals enhanced (suppressed) hole (electron) mobility in the Janus MoSSe nanoribbons. This is in contrast to what was previously found for MoS2 nanoribbons, namely larger mobility for electrons in comparison with holes. These differences are rationalized on the basis of the different structures, edge electronic states and deformation potentials present in the MoSSe nanoribbons. The present results provide the guidelines for the structural and electronic engineering of MoSSe nanoribbon edges towards tailored electron transport properties.

Amorphous Domains in Black Titanium Dioxide.

Although oxygen vacancies (Ov s) play a critical role for many applications of metal oxides, a controllable synthetic strategy for anisotropic Ov s remains a great challenge. Here, a novel strategy is proposed to achieve the regional dual structure with anisotropic Ov s at both the surface and in the interior of TiO2 by constructing amorphous domains. The as-prepared black TiO2 with amorphous domains exhibits superior activity in degrading rhodamine B (RhB) solutions, which can instantly decompose RhB with just a shake. First-principle simulations reveal that subsurface Ov s in TiO2 are energetically favored, resulting in the formation of amorphous domains in the interior region via diffusion of surface-formed Ov s into the subsurface. The stable Ov -induced amorphous domains in TiO2 with enhanced catalytic performances provide a scalable strategy to practical Ov engineering in functional metal oxides.