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The pertechnetate ion TcVIIO4− is a nuclear fission product whose major issue is the high mobility in the environment. Experimentally, it is well-known that Fe3O4 can reduce TcVIIO4− to TcIV species and retain such products quickly and completely, but the exact nature of the redox process and products is not completely understood. Therefore, we investigated the chemistry of TcVIIO4− and TcIV species at the Fe3O4(001) surface through a hybrid DFT functional (HSE06) method. We studied a possible initiation step of the TcVII reduction process. The interaction of the TcVIIO4− ion with magnetite surface leads to the formation of a reduced TcVI species without any change in the Tc coordination sphere, through an electron transfer that is favored by the magnetite surfaces with a higher FeII content. Furthermore, we explored various model structures for the immobilized TcIV final products. TcIV can be incorporated into a subsurface octahedral site or adsorbed on the surface in the form of TcIVO2·xH2O chains. We propose and discuss three model structures for the adsorbed TcIVO2·2H2O chains in terms of relative energies and simulated EXAFS spectra. Our results suggest that the periodicity of the Fe3O4(001) surface matches that of the TcO2·2H2O chains. The EXAFS analysis suggests that in experiments TcO2·xH2O chains were probably not formed as an inner-shell adsorption complex with the Fe3O4(001) surface.

The hydrogen evolution reaction (HER) is the process where protons from the electrolyte combine with the electrons from the metal electrode catalyst to produce atomic hydrogen adsorbed on the metal surface. The reaction ends when these hydrogen atoms then form molecular hydrogen and thereby desorbs from the surface of the catalyst. The bonding between the catalyst and the proton should be optimal such as to favour the formation of adsorbed H atom and at the same time not so strong as to hinder the H2 molecular desorption to occur. The main applications of HER are in the cathodic half cells of water splitting and fuel cells. The efficiency of such a cathodic material depends on how strongly or weakly the H atoms are adsorbed on the metal catalyst. We present an experimental and theoretical study of the state of the art HER metallic electrocatalyst platinum (Pt) by tuning its reactivity with graphene (Gr) and iron (Fe). Using density functional theory (DFT) the reactivity of hydrogen (H) in the two systems namely Gr/Pt(111) and Gr/Fe/Pt(111) is investigated. The presence of Gr is observed to increase the selectivity/permeability to protons [1] and at the same time weaken the H adsorption compared to bare Pt [2]. This should favour H diffusion at the interface and hence H2 molecular desorption. The H adsorption weakens further in the presence of Fe and even more with Gr and Fe. In addition to platinum the performance of gold (Au) in the presence of Fe and Gr has also been studied and compared in this work.

The presence of defects in the graphenic layers deposited on metal surfaces modifies the nature of the interaction. Unsaturated carbon atoms, due to vacancies in the lattice, form strong organometallic bonds with surface metal atoms that highly enhance the binding energy between the two materials. We investigate by means of a wide set of dispersion-corrected density functional theory calculations how such strong chemical bonds affect both the electronic properties of these hybrid interfaces and the chemical reactivity with water, which is commonly present in the working conditions. We compare different metal substrates (Cu vs Pt) that present a different type of interaction with graphene and with defective graphene. This comparative analysis allows us to unravel the controlling factors of water reactivity, the role played by the carbon vacancies and by the confinement or “graphene cover effect”. Water is capable of breaking the C–Cu bond by dissociating at the undercoordinated carbon atom of the vacancy, restoring the weak van der Waals type of interaction between the two materials that allows for an easy detachment of graphene from the metal, but the same is not true in the case of Pt, where C–Pt bonds are much stronger. These conclusions can be used to rationalize water reactivity at other defective graphene/metal interfaces.

Understanding the electronic structure of metal oxide semiconductors is crucial to their numerous technological applications, such as photoelectrochemical water splitting and solar cells. The needed experimental and theoretical knowledge goes beyond that of pristine bulk crystals, and must include the effects of surfaces and interfaces, as well as those due to the presence of intrinsic defects (e.g. oxygen vacancies), or dopants for band engineering. In this review, we present an account of the recent efforts in predicting and understanding the optoelectronic properties of oxides using ab initio theoretical methods. In particular, we discuss the performance of recently developed dielectric-dependent hybrid functionals, providing a comparison against the results of many-body GW calculations, including G 0 W 0 as well as more refined approaches, such as quasiparticle self-consistent GW. We summarize results in the recent literature for the band gap, the band level alignment at surfaces, and optical transition energies in defective oxides, including wide gap oxide semiconductors and transition metal oxides. Correlated transition metal oxides are also discussed. For each method, we describe successes and drawbacks, emphasizing the challenges faced by the development of improved theoretical approaches. The theoretical section is preceded by a critical overview of the main experimental techniques needed to characterize the optoelectronic properties of semiconductors, including absorption and reflection spectroscopy, photoemission, and scanning tunneling spectroscopy (STS).

The photoactivity of methanol adsorbed on the anatase TiO2 (101) surface was studied by a combination of scanning tunneling microscopy (STM), temperature-programmed desorption (TPD), X-ray photoemission spectroscopy (XPS), and density functional theory (DFT) calculations. Isolated methanol molecules adsorbed at the anatase (101) surface show a negligible photoactivity. Two ways of methanol activation were found. First, methoxy groups formed by reaction of methanol with coadsorbed O2 molecules or terminal OH groups are photoactive, and they turn into formaldehyde upon UV illumination. The methoxy species show an unusual C 1s core-level shift of 1.4 eV compared to methanol; their chemical assignment was verified by DFT calculations with inclusion of final-state effects. The second way of methanol activation opens at methanol coverages above 0.5 monolayer (ML), and methyl formate is produced in this reaction pathway. The adsorption of methanol in the coverage regime from 0 to 2 ML is described in detail; it is key for understanding the photocatalytic behavior at high coverages. There, a hydrogen-bonding network is established in the adsorbed methanol layer, and consequently, methanol dissociation becomes energetically more favorable. DFT calculations show that dissociation of the methanol molecule is always the key requirement for hole transfer from the substrate to the adsorbed methanol. We show that the hydrogen-bonding network established in the methanol layer dramatically changes the kinetics of proton transfer during the photoreaction.

The existence of alkali metal anions in interstitial cavities of crystalline silica and microporous materials (e.g., zeolites) has been suggested based on NMR and EPR measurements but has never been studied theoretically. In this work we report results of density functional theory calculations on the interaction of a Li- anion with cluster models of alpha quartz containing up to approximate to100 Si and O atoms. We also consider the interaction of Li- with orthosilicic acid, Si(OH)(4). The properties of Li- incorporated in a silica structure have been compared with those of the hydride anion, H-. We found that, in agreement with previous theoretical studies, H- forms stable structures where a Si atom becomes penta-coordinated with a bipyramidal trigonal structure. Li- behaves completely differently. Because of its larger size, Li- does not come close enough to the tetra-coordinated Si atom to induce the structural distortion observed for H-. On the contrary, the Li atom becomes incorporated into the silica structure, with formation of a =Si-Odelta--Lidelta+ linkage. The negative charge is transferred to the host silica structure where becomes trapped at pre-existing defects or induces additional structural changes in the network.

High-quality nitrogen-doped graphene on nickel is prepared by exploiting both the catalytic properties of nickel and the solubility of nitrogen atoms into its bulk. Following the standard chemical vapor deposition procedure, a previously nitrogen-doped nickel substrate is exposed to carbon-containing precursors so that nitrogen atoms, segregating to the surface, remain trapped in the growing graphene network. Morphological and chemical characterization by scanning tunneling microscopy and X-ray photoelectron spectroscopy demonstrates that the process yields a flat, wide, continuous nitrogen-doped graphene layer. Experimental results are combined with a thorough density functional theory investigation of possible structural models, to obtain a clear description at the atomic scale of the various configurations of the nitrogen atoms observed in the graphene mesh. This growth method is potentially scalable and suitable for the production of high-performance nano-devices with well-defined nitrogen centers, to be exploited as metal-free carbon-based catalysts in several applicative fields such as electrochemistry, energy storage, gas storage/sensing or wastewater treatment., 21 pages, including 4 figures, plus SI

Inorganic nanoparticles show promising properties that allow them to be efficiently used as drug carriers. The main limitation in this type of application is currently the drug loading capacity, which can be overcome with a proper functionalization of the nanoparticle surface. In this study, we present, for the first time, a computational approach based on metadynamics to estimate the binding free energy of the doxorubicin drug (DOX) to a functionalized TiO2 nanoparticle under different pH conditions. On a thermodynamic basis, we demonstrate the robustness of our approach to capture the overall mechanism behind the pH-triggered release of DOX due to environmental pH changes. Notably, binding free energy estimations align well with what is expected for a pH-sensitive drug delivery system. Based on our results, we envision the use of metadynamics as a promising computational tool for the rational design and in silico optimization of organic ligands with improved drug carrier properties.

Nonequilibrium molecular dynamics simulations were performed to study the electrokinetic properties of five mainstream TIPxP water models (namely, TIP3P-FB, TIP3Pm, TIP4P-FB, TIP4P-Ew, and TIP4P/2005) in NaCl aqueous solutions in the presence of a negatively charged TiO2 surface. The impact of solvent flexibility and system geometry on the electro-osmotic (EO) mobility and flow direction was systematically assessed and compared. We found that lack of water flexibility decelerates the forward EO flow of aqueous solutions at moderate (0.15 M) or high (0.30 M) NaCl concentrations, in some special cases to such an extent that EO flow reversal occurs. Zeta potential (ZP) values were then determined from the bulk EO mobilities using the Helmholtz-Smoluchowski formula. The straight comparison against available experimental data strongly suggests that water flexibility improves the ZP determination of NaCl solutions adjacent to a realistic TiO2 surface under neutral pH conditions.

Melem (2,6,10-triamino-s-heptazine) is the building block of melon, a carbon nitride (CN) polymer that is proven to produce H2 from water under visible illumination. With the aim of bringing additional insight into the electronic structure of CN materials, we performed a spectroscopic characterization of gas-phase melem and of a melem-based self-assembled 2D H-bonded layer on Au(111) by means of ultraviolet and X-ray photoemission spectroscopy (UPS, XPS) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. In parallel, we performed density functional theory (DFT) simulations of the same systems to unravel the molecular charge density redistribution caused by the in-plane H-bonds. Comparing the experimental results with the spectroscopic DFT simulations, we can correlate the induced charge accumulation on the Namino atoms to the red-shift of the corresponding N 1s binding energy (BE) and of the Namino 1s → LUMO+n transitions. Moreover, when introducing a supporting Au(111) surface in the computational simulations, we observe a molecule–substrate interaction that almost exclusively involves the out-of-plane molecular orbitals, leaving those engaged in the in-plane H-bonded network rather unperturbed.

In this paper we performed the deposition and self-assembly of a Pd-cyclometallated compound on Ag(1 1 0) surface for the first time. The system is investigated from the morphological and chemical point of view by scanning tunneling microscopy and x-ray photoemission spectroscopy, respectively, and the results are validated by ab-initio calculations. Our combined experimental and theoretical study aims at elucidating the atomistic details of the chemical steps following Pd cyclometallate deposition on the metallic substrate. To do that, we analyze the electronic and chemical properties of the species present on the surface at the end of the preparation process at room temperature and at 150 degrees C. We observe an unexpected complex chemistry: on one side, the organometallic molecules are found to dissociate into fragments, forming a well-ordered metal-carbon network; on the other side, Pd atoms become buried in the bulk of the metal substrate following metal exchange with surface Ag atoms. The details of this mechanistic study reveal the active role played by the metal substrate in promoting the chemistry of the deposited Pd cyclometallates and could open new perspectives for the application of this class of materials in heterogeneous catalysis.

The development of catalysts for the hydrogen evolution reaction is pivotal for the hydrogen economy. Thin iron films covered with monolayer graphene exhibit outstanding catalytic activity, surpassing even that of platinum, as demonstrated by a method based on evaluating the noise in the tunnelling current of electrochemical scanning tunnelling microscopy. Using this approach, we mapped with atomic-scale precision the electrochemical activity of the graphene–iron interface, and determined that single iron atoms trapped within carbon vacancies and curved graphene areas on step edges are exceptionally active. Density functional theory calculations confirmed the sequence of activity obtained experimentally. This work exemplifies the potential of electrochemical scanning tunnelling microscopy as the only technique able to determine both the atomic structure and relative catalytic performance of atomically well-defined sites in electrochemical operando conditions and provides a detailed rationale for the design of novel catalysts based on cheap and abundant metals such as iron. Establishing structure–activity relationships is crucial for the design of improved catalysts. Now, by developing a method based on electrochemical scanning tunnelling microscopy, the active sites of graphene/iron/platinum interfaces are visualized with atomic-scale precision in real time during the hydrogen evolution reaction.

We report the study of two-dimensional graphitic carbon nitride (GCN) functionalized with copper single atoms as a catalyst for the reduction of CO2 (CO2RR). The correct GCN structure, as well as the adsorption sites and the coordination of the Cu atoms, was carefully determined by combining experimental techniques, such as X-ray diffraction, transmission electron microscopy, X-ray absorption, and X-ray photoemission spectroscopy, with DFT theoretical calculations. The CO2RR products in KHCO3 and phosphate buffer solutions were determined by rotating ring disk electrode measurements and confirmed by 1H-NMR and gas chromatography. Formate was the only liquid product obtained in bicarbonate solution, whereas only hydrogen was obtained in phosphate solution. Finally, we demonstrated that GCN is a promising substrate able to stabilize metal atoms, since the characterization of the Cu-GCN system after the electrochemical work did not show the aggregation of the copper atoms.

Iron oxide magnetic nanoparticles (NPs) are excellent systems in catalysis and in nanomedicine, where they are mostly immersed in aqueous media. Even though the NP solvation by water is expected to play an active role, the detailed structural insight at the nanostructure oxide/water interface is still missing. Here, based on our previous efforts to obtain accurate models of dehydrated Fe3O4 NPs and of their magnetic properties and through multiscale molecular dynamics simulations combining the density functional tight binding method and force field, we unravel the atomistic details of the short range (chemical) and long range (physical) interfacial effects when magnetite nanoparticles are immersed in water. The influence of the first hydration shell on the structural, electronic and magnetic properties of Fe3O4 NPs is revealed by high-level hybrid density functional calculations. Hydrated Fe3O4 NPs possess larger magnetic moment than dehydrated ones. This work bridges the large gap between experimental studies on solvated Fe3O4 NPs and theoretical investigations on flat Fe3O4 surfaces covered with water and paves the way for further study of Fe3O4 NPs in biological environments. This journal is

The impact of iridium (Ir) doping on the oxygen vacancies, relative stability, crystallite size, surface area, and anatase-to-rutile transition of TiO2was comprehensively investigated in this study. Ir-doped TiO2(Ir-TiO2) was synthesized through a sol-gel technique, and the samples were annealed in the temperature range of 400-700 °C. Density functional theory calculations showed that the energy cost of an oxygen vacancy formation for Ir-TiO2was lower, as compared to that of the pristine TiO2, with the formation of Ir3+states in the band gap. Ir could provide more rutile nucleation sites and accelerate the rutile formation through the crystal strain relaxation. The entropy of mixing was reduced by the incorporation of Ir, which could induce the rutile formation with an increase in Gibbs free energy at temperatures below the normal phase transition temperature for pure TiO2. The rutile formation of Ir-TiO2could take place at a low annealing temperature (400 °C) compared to pristine TiO2(600 °C), indicating that the activation energy for phase transition could be decreased by incorporating Ir. XPS revealed the spin-orbit coupling of Ir 4f peaks, Ir 4f7/2(61.96 eV) and Ir 4f5/2(64.77 eV), due to the presence of Ir3+. Raman studies indicated the formation of charge-compensating oxygen vacancies and the presence of d states by Ir doping. It is concluded that the defects originated because the incorporation of Ir could facilitate rutile nucleation sites and thereby accelerate the phase transition through strain relaxation.

Gas sensors based on graphene are gaining more and more attention. The unique properties of the single-atom-thick, flexible, robust, and quanta-sensitive graphene make it an excellent candidate for sensors. However, its characteristics can be modified to suit specific applications by means of metal and nonmetal doping. In this systematic and detailed study, with the aid of density functional theory (DFT)-based simulations, we investigate the gas sensing capabilities of trapped transition-metal (TM) atoms at the graphene double vacancy (2VG). Eight different gas molecules, ranging from electron acceptors (Lewis acids) to donor species (Lewis bases), are investigated, namely, O2, NO2, NO, CO2, SO2, H2O, NH3, and CO. Four TM atoms Fe, Co, Ni, and Cu are considered as active sites for gas sensing. The effects of N-doping and the Ni(111) substrate are also examined. The performance of these systems in terms of stability, sensitivity, selectivity, and reusability is discussed for practical applications.

Molecules intercalate at the graphene/metal interface even though defect-free graphene is impermeable to any atomic and molecular species in the gas and liquid phase, except hydrogen. The mechanism of molecular intercalation is still a big open question. In this Letter, by means of a combined experimental (STM, XPS, and LEED) and theoretical (DFT) study, we present a proof of how CO molecules succeed in permeating the graphene layer and get into the confined zone between graphene and the Ni(111) surface. The presence of N-dopants in the graphene layer is found to highly facilitate the permeation process, reducing the CO threshold pressure by more than one order of magnitude, through the stabilization of multiatomic vacancy defects that are the open doors to the bidimensional nanospace, with crucial implications for the catalysis under cover and for the graphene-based electrochemistry.

Nanoparticle functionalization is a modern strategy in nanotechnology to build up devices for several applications. Modeling fully decorated metal oxide nanoparticles of realistic size (few nanometers) in an aqueous environment is a challenging task. In this work, we present a case study relevant for solar-light exploitation and for biomedical applications, i.e., a dopamine-functionalized TiO2 nanoparticle (1700 atoms) in bulk water, for which we have performed an extensive comparative investigation with both MM and QM/MM approaches of the structural properties and of the conformational dynamics. We have used a combined multiscale protocol for a more efficient exploration of the complex conformational space. On the basis of the results of this study and of some QM and experimental data, we have defined strengths and limitations of the existing force field parameters. Our findings will be useful for an improved modeling and simulation of many other similar hybrid bioinorganic nanosystems in an aqueous environment that are pivotal in a broad range of nanotechnological applications.

Metal-doped and metal-supported two-dimensional materials are attracting a lot of interest as potentially active electrocatalysts for reduction and oxidation processes. Previously, when a non-regular 2 D h-BN layer was grown on a Cu(111) surface, metal adatoms were found to spontaneously emerge from the bulk to fill the atomic holes in the structure and become available for surface catalysis. Herein, computational electrochemistry is used to investigate and compare the performance of Cu-doped and Cu-supported pristine and defective h-BN systems for the electrocatalytic water oxidation reaction. For the various model systems, the intermediate species of this multistep oxidation process are identified and the free-energy variations for each step of reaction are computed, even for those steps that do not involve an electron or a proton transfer. Both associative and O2 direct evolution mechanisms are considered. On this thermodynamic basis, the potential-determining step, the thermodynamic-determining step, and consequently the theoretical overpotential are determined for comparison with experiments. Small Cu clusters (tetramers) trapped in the h-BN defective lattice on a Cu(111) support are found to be very active for the water oxidation reaction since such systems are characterized by a low overpotential and by a small energy cost for O2 release from the catalyst, which is often observed to be a major limit for other potential electrocatalysts.

Sodium tantalate, NaTaO3, has attracted interest as one of the few photocatalysts capable to perform overall water splitting, that is, simultaneously produce oxygen and hydrogen from water. In particular, an interesting and not fully understood observation is that the efficiency of NaTaO3 increases dramatically in the presence of cation doping. To obtain better insight into the origin of this effect, we use first-principles calculations to investigate the fundamental structural, electronical, and chemical properties of pristine and Sr-doped NaTaO3, a system for which several experimental studies have recently become available. Our results show that Sr donor-acceptor codoping at Na and Ta sites significantly reduces the formation energy of the Sr dopants. Further study of the energetics of the oxygen evolution reaction (OER) shows a substantial reduction of the OER overpotential for the codoped material, consistent with recent suggestions that codoping is crucial for increasing NaTaO3's efficiency. The detailed insights provided by our work could benefit the design and preparation of new efficient catalysts based on NaTaO3

TiO2 nanoparticles (NPs) are attracting materials for biomedical applications, provided that they are coated with polymers to improve solubility, dispersion and biocompatibility. Conformation, coverage density and solvent effects largely influence their functionality and stability. In this work, we use atomistic molecular dynamics simulations to study polyethylene glycol (PEG) grafting to highly curved TiO2 NPs (2-3 nm) in different solvents. We compare the coating polymer conformations on NPs with those on (101) flat surfaces. In water, the transition from mushroom to brush conformation starts only at high density ({\sigma} = 2.25 chains/nm2). In dichloromethane (DCM), at low-medium coverage ({\sigma} < 1.35 chains/nm2), several interactions between the PEG chains backbone and undercoordinated Ti atoms are established, whereas at {\sigma} = 2.25 chains/nm2 the conformation clearly becomes brush-like. Finally, we demonstrate that these spherical brushes, when immersed in water, but not in DCM, follow the Daoud-Cotton (DC) classical scaling model for the polymer volume fraction dependence with the distance from the center of star-shaped systems.

Graphene-h-BN hybrid nanostructures are grown in one step on the Pt(111) surface by ultra-high vacuum chemical vapor deposition using a single precursor, the dimethylamino borane complex. By varying the deposition conditions, different nanostructures ranging from a fully continuous hybrid monolayer to well-separated Janus nanodots can be obtained. The growth starts with heterogeneous nucleation on morphological defects such as Pt step edges and proceeds by the addition of small clusters formed by the decomposition of the dimethylamino borane complex. Scanning tunneling microscopy measurements indicate that a sharp zigzag in-plane boundary is formed when graphene grows aligned with the Pt substrate and consequently with the h-BN layer as well. When graphene is rotated by 30°, the graphene armchair edges are seamlessly connected to h-BN zigzag edges. This is confirmed by a thorough density functional theory (DFT) study. Angle resolved photoemission spectroscopy (ARPES) data suggests that both h-BN and graphene present the typical electronic structure of self-standing non-interacting materials.[Figure not available: see fulltext.].

2D tungsten diselenide (2D-WSe2) is one of the most successful bidimensional materials for optoelectronic and photonic applications, thanks to its strong photoluminescence properties and to a characteristic large exciton binding energy. Although these optical properties are widely recognized by the scientific community, there is no general understanding of the atomistic details of the excitonic species giving rise to them. In this work, we present a density functional theory investigation of excitons in 2D-WSe2, where we compare results obtained by standard generalized gradient approximation (GGA) methods (including spin-orbit coupling) with those by hybrid density functionals. Our study provides information on the size of the self-trapped exciton, the number and type of atoms involved, the structural reorganization, the self-trapping energy, and the photoluminescence energy, whose computed value is in good agreement with experimental measurements in the literature. Moreover, based on the comparative analysis of the self-trapping energy for the exciton with that for isolated charge carriers (unbound electrons and holes), we also suggest a simplified approach for the theoretical estimation of the excitonic binding energy, which can be compared with previous estimates from different approaches or from experimental data.

The structure and electronic properties of carbon-based nanostructures obtained by metal surface assisted synthesis is highly dependent on the nature of the precursor molecule. Here, we report on a combined scanning tunneling microscopy, soft X-ray spectroscopy and density functional theory investigation on the surface assisted polymerization of Br-corannulene at Ag(110) and on the possibility of building a mesh of ?-conjugated polymers starting from buckyball shaped molecules. Indeed, the corannulene units form one-molecule-wide ribbons in which the natural concavity of the precursor molecule is maintained. These C-based nanostructures are corrugated and merge into a covalent network on the surface.

Computational modeling of titanium dioxide nanoparticles of realistic size is extremely relevant for the direct comparison with experiments but it is also a rather demanding task. We have recently worked on a multistep/scale procedure to obtain global optimized minimum structures for chemically stable spherical titania nanoparticles of increasing size, with diameter from 1.5 nm (~300 atoms) to 4.4 nm (~4000 atoms). We use first self-consistent-charge density functional tight-binding (SCC-DFTB) methodology to perform thermal annealing simulations to obtain globally optimized structures and then hybrid density functional theory (DFT) to refine them and to achieve high accuracy in the description of structural and electronic properties. This allows also to assess SCC-DFTB performance in comparison with DFT(B3LYP) results. As a further step, we investigate photoexcitation and photoemission processes involving electron/hole pair formation, separation, trapping and recombination in the nanosphere of medium size by hybrid DFT. Finally, we show how a recently defined new set of parameters for SCC-DFTB allows for a proper description of titania/water multilayers interface, which paves the way for modeling large realistic nanoparticles in aqueous environment.