Metal Adatoms Research Articles

Towards advanced complex quantum materials for spin-related applications and photo-induced heterogeneous catalysis: The case of (Fen)@g-CN1 (n = 2,3) and (Mn)@(g-CN1)2

Perspective heterostructures based on Fe and Mn adatoms on 2D g-C3N4 films of different morphology were designed and investigated by means of DFT GGA/PBE and GGA/HSE levels of theory. Spontaneous spin polarization of g-C3N4 and a variety of electronic properties were theoretically investigated. It was found that GGA HSE DFT potential is suitable to describe the main properties of g-C3N4-based heterostructures with transition metal adatoms. It was revealed that (Mn)@(g-CN1)2 heterostructure is a half-metal with 0.5 eV band gap in α-channel. (Fen)@g-CN1, n = 2,3) demonstrate different semiconducting properties which are determined by structure and number of Fe adatoms in the unit cells. The presence of several independent centers of Fe forming polyatomic clusters was stated as one of the interests in catalysis. It was found that all considered heterostructures may serve as perspective quantum materials in different spin-related applications.

Hydrogen storage on bidimensional GeC with transition metal adatoms

A density functional theory study is undertaken to explore H2 physisorption in bidimensional germanium carbide (GeC) decorated with transition metals (Cu, Ag and Au). Adsorption energy results show that the Au and Cu metal atoms are preferentially chemisorbed to a carbon atom, while Ag is adsorbed over a Ge–C bond. Hydrogen molecules are weakly adsorbed to the adatom, and the interaction mainly occurs between the charge density of the H–H bond and the slightly positively charged adatom. The Ag-decorated GeC monolayer possesses the maximum hydrogen storage capacity with seven molecules adsorbed on the adatom. We think this work could encourage theoretical and experimental studies of the GeC monolayer and related two-dimensional materials for green energy development applications.

Sn3C2 monolayer with transition metal adatom for gas sensing: a density functional theory studies

The gas sensing properties of pristine Sn3C2 monolayer and different transition metal adatom (TM-Sn3C2, where TM = Fe, Co, Ni, Cu, Ru, Rh, Pd and Ag) was investigated using van der Waals corrected density functional theory. The understanding and potential of use of Sn3C2 monolayers as sensors or adsorbent for CO, CO2, NO, NO2 and SO2 gaseous molecules is evaluated by calculating the adsorption and desorption energetics. From the calculated adsorption energies, we found that the pristine Sn3C2 monolayer and 3d TM has desirable properties for removal of the considered molecules based on their high adsorption energy, however the 4d TM is applicable as recoverable sensors. We applied an Arrhenius-type equation to evaluate the recovery time for the desorption of the molecules on the pristine and TM adatom on Sn3C2 monolayer. We found that the negative adsorption energies from −1 to −2 eV of the molecules resulted in easier recovery of the adsorbed gases at reasonable temperatures compared to adsorption energies in between 0 and −1 eV (weakly physiosorbed) and below −2 eV (strongly chemisorbed). Hence, we obtained that the Rh–Sn3C2, Ru–Sn3C2, Pd–Sn3C2, Pd–Sn3C2, and Rh–Sn3C2 monolayers are good recoverable scavengers for the CO, CO2, NO, NO2, and SO2 molecules. The current theoretical calculations provide new insight on the effect of TM adatoms on the structural, electronic, and magnetic properties of the Sn3C2 monolayer and different transition metal adatom as well as shed light on their application as gas sensors/scavengers.

Modulating oxygen electronic orbital occupancy of Cr-based MXenes via transition metal adsorbing for optimal HER activity

Modulating the surface electronic properties of the 2D MXenes is of significant importance to boost their hydrogen evolution reaction (HER) activity. Herein, a series of transition metal adatoms are employed to tune the surface electronic properties of Cr-based MXenes with oxygen function group for realizing impressive HER performance. The Results show that the charge of surface oxygen atoms, which is affected by both the host metal atoms and the adsorbed transition metal atoms, play critical role in the adsorption strength of hydrogen. The optimal performance is achieved by depositing Cr atom on the Cr2TiC2O2 MXene, which results in the adsorption free energy of hydrogen very close to zero (0.03 eV). Systematic electronic structure analyses confirm that the charge transfer from the adsorbed transition metals to the neighboring surface oxygen atoms could tune the orbital occupancy of oxygen and their adsorption strength to hydrogen atom and therefore the HER activity. These findings and concepts may be useful for the design of advanced MXene-based HER catalysts.

Ab initio study of hydrogen storage on metal-decorated GeC monolayers

Bidimensional nanostructures have been proposed as hydrogen-storage systems owing to their large surface-to-volume ratios. Germanium carbide monolayers (GeC-MLs) can offer attractive opportunities for H2 adsorption compared to graphene. However, this possibility has not been explored in detail. In this work, the adsorption of H2 molecules on GeC-MLs decorated with alkali metal (AM) and alkaline earth metal (AEM) adatoms was investigated using the density functional theory. Results showed that the AM adatoms were chemisorbed on the GeC-ML, whereas AEM adatoms were physisorbed. The H2 molecules presented negligible adsorption energies on the weakly adsorbed AEM adatoms. Conversely, the AM adatoms improved the H2 adsorption, possibly due to a large charge transfer from the adatoms to the GeC-ML. The potassium-decorated GeC-ML exhibited the most optimal H2 storage capacity, adsorbing up to six molecules and with a lower possibility of forming metal clusters than the other studied cases. These results may aid in the development of new efficient hydrogen-storage materials.

Theoretical investigation of quantum capacitance in the functionalized MoS2-monolayer

In this work, we investigated the electronic structure and the quantum capacitance of a set of functionalized MoS2 monolayers. The functionalizations have been done by using different ad-atom adsorption on MoS 2 monolayer. Density functional theory calculations are performed to obtain an accurate electronic structure of ad-atom doped MoS2 monolayer with a varying degree of doping concentration. Subsequently, the quantum capacitance in each functionalized system was estimated. A marked quantum capacitance above 200 μF cm−2 has been observed. Our calculations show that the quantum capacitance of MoS2 monolayer is significantly enhanced with substitutional doping of Mo with transition metal ad-atoms. The microscopic origin of such enhancement in quantum capacitance in this system has been analyzed. Our DFT-based calculation reveals that the generation of new electronic states at the proximity of the band-edge and the shift of Fermi level caused by the ad-atom adsorption results in a very high quantum capacitance in the system.

Effects of temperature on strain engineering and transition-metal adatom magnetization in phosphorene: Ab initio molecular dynamics studies

Using first-principles and ab initio molecular dynamics calculations, effects of temperatures on ferroelastic transition, band gap engineering, anisotropic carrier effective masses, and stability of transition metal adatom magnetization in phosphorene under tensile strains are investigated, which determine mainly its unique properties for designing novel low-dimensional electronic and spintronic devices. We find that (i) the ferroelastic transition of phosphorene under tensile strains predicted at 0 K is replaced by developments of a series of twinning structures above the room temperature (300 K), which will modify predictions on the band structure, carrier mobility, and thermal conductivity of phosphorene in strain engineering processes; (ii) the emerging twinning structure brings about strong ductility in phosphorene, which can sustain stretching up to a tensile strain of 73% at 300 K before the structural breakdown; (iii) the stable sites for the adsorbed TM atoms (V, Cr, and Mn) on phosphorene are higher in energy compared to those of the (interstitially) doped TM atoms in phosphorene. However, the potential barriers between the adsorbed and doped states are high enough to stabilize the adsorbed TM atoms on phosphorene even at high temperatures (450 K); (iv) under moderate tensile strains ($\ensuremath{\sim}10%$) at room temperature, the adsorbed TM atoms start to move into phosphorene as interstitial dopants. Such room or high temperature stretching may provide a possible way to prepare dilute magnetic semiconducting phosphorene using adsorbed TM atoms from simple chemical vapor depositions; (v) for some TM atoms (V, Cr), whose magnetic moments come from the $3d$ electron states near the Fermi level, their magnetic moments decrease substantially as they move into phosphorene, while for other TM atoms (Mn), whose magnetic moments come from the $3d$ electron states deep below the Fermi level, their magnetic moments are stable as their positions change. These results will deepen our understandings of phosphorene and broaden its technological applications as a versatile two-dimensional material.

Work function modulation of graphene with binary mixture of Cu and C60F36

Graphene is a promising candidate for flexible, transparent electrodes in organic electronic devices. In order to achieve enhanced device performance, work function (WF) modulation of graphene for better electric contact at the electrode/active materials interface is of significant importance. Herein, we have demonstrated the effective modulation of graphene WF with binary mixture of Cu and C60F36 and the highest WF of ∼6.04 eV is obtained after in situ thermal annealing at 150 °C. The ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS) measurement indicate that the growth of Cu on C60F36-modified graphene leads to C–F bonds breaking of C60F36 and the formation of Cu–F bonds on the basal plane of graphene, which is responsible for the large WF increase of graphene. The mechanism for the WF increase of binary mixture of Cu and C60F36 modified graphene (BMG) has been corroborated by density-functional theoretical (DFT) calculations, which shows that the C–F bond breaking of C60F36 is facilitated by Cu adatoms, and the WF increase can be attributed to the formation of surface dipole layer with the direction pointing from vacuum to graphene, which is consistent with UPS and XPS results. Moreover, our results indicate that other 3d transition metal adatoms (Ti, V, Fe, Co, and Ni) on graphene can also facilitate the C–F bond breaking of C60F36 and the formation of surface dipole layer, resulting in the WF increase of graphene.

Mechanism of transition metal interaction with graphene sheet reflected in its plasmonic excitations: Effect of gas adsorption phenomena studied by a combination of solid state and molecular orbital approaches

In this study, plasmonic properties of transition metal atoms (Sc-Zn, Pd, Pt) adsorbed on various sites of graphene sheets were investigated before and after CO adsorption by density functional theory (DFT). To this aim, Electron Energy Loss Spectroscopy (EELS) of these systems were obtained by periodic DFT. The results indicated that the highest in-plane plasmon peaks undergo blue shifts after CO adsorption, in such a way that Co–graphene system demonstrates the highest blue shift of 2.42 eV from Visible to UV region. Also, out-of-plane plasmons do not exhibit significant sensitivity to CO adsorption. For deeper understanding of electronic structure of metal-graphene systems, appropriate molecular models were considered, and natural bond orbital (NBO) analysis applied. Comparison of solid-state net charges with the NBO charges showed that CO adsorption has more non-local characteristics, while the metal atom adsorption on bare graphene surface has more local nature. Moreover, Jahn–Teller distortion of metal adatom strongly depends on the net charges of nearest-neighbor C atoms on the surface. In addition, spin distribution on metal 3d orbitals plays a role to some extent. These results can be useful for experimental researchers who want to construct the sensors and optical devices based on metal-graphene interactions.

Magnetic 3d adatoms on free-standing and Ni(111)-supported graphene

We present an extensive density functional theory analysis of the structural, electronic, and magnetic properties of isolated $3d$ transition metal adatoms (from Ti to Co) adsorbed on free-standing and Ni(111)-supported graphene. We discuss how the energetics of different adsorption sites is influenced by the filling of $d$-orbital filling across the $3d$ series and identify a direct correlation between the adatom-graphene distance and the degree of charge transfer. The presence of the Ni substrate is found to have stronger impact on the adatoms at the end of the series, leading to modifications of the preferred adsorption site, charge transfer, and spin properties. The magnetic exchange coupling between the spin of the adatom and the Ni magnetization changes as a function of the adatom both in sign (preferred antiferromagnetic exchange for Ti, V, and Cr, and ferromagnetic alignment for the other elements) and in magnitude (from 90 meV for Mn to $\ensuremath{\approx}10$ meV for Fe an Co).

Density Functional Theory Study of the Adsorption of Metal Adatoms on Graphene

In this study, we investigated the influence of the interaction between graphene and other materials as a basis for controlling the electronic structure of nanocarbon materials. First-principles calculations based on density functional theory (DFT) were performed on the optimized structure, adsorption energies and electronic states when copper and aluminum atoms were placed on graphene. As a result, we found that copper and aluminum are stable at the bridge and the hollow site, respectively. It was found that the adsorption energy of aluminum atom on graphene is larger than that of copper atom. It is considered that the difference in adsorption energy is caused by the difference in the dominant electron orbitals of the copper atom and the aluminum atom.

Selective Construction of Magic Hierarchical Metal–Organic Clusters on Surfaces

Magic organic clusters, representing well-defined zero-dimensional organic clusters with identical sizes and configurations, have received increased interests in recent years. Previously, the magic clusters were mainly stabilized through van der Waals force, C–H...π interaction, hydrogen bonding, dipole interaction, etc., which yet lack thermal stability and tunable electronic transport properties for potential applications. The introduction of metal adatoms into the organic systems would be an excellent choice for facilitating more stable magic clusters as the metal adatoms could serve as a nucleation center and help for clustering of organic ligands with increased stabilities. Considering the limited coordination number of metal species, it would be of great interest to introduce multilevel interactions besides metal–organic bonding, which may provide new avenues for controllable fabrication of more complicated and larger magic clusters. Herein, we have achieved the controllable fabrication of three distinct magic metal–organic clusters, especially two hierarchical ones with different sizes on the reconstructed Au(111) and unreconstructed Ag(111) surface. The key for various unprecedented magic hierarchical clusters here is the selection of the organic ligands with only one active carboxyl (−COOH) group which possesses bonding flexibility and diversity features after dehydrogenation but avoids the usual two-dimensional network of those containing more −COOH groups.

Theoretical studies on the electronic and optoelectronic properties of DNA/RNA hybrid-metal complexes

The electronic and optoelectronic properties of DNA/RNA functionalized conductive metal adatoms were studied using DFT and TDDFT methods. Four hybrid structures were selected from the experimental work on RNA microchips. The occupied number of states by the system at each energy level were studied by Density of States (DOS). Atom-in Molecules (AIM) analysis and Noncovalent Interaction (NCI) PLOT were used to study the interaction in these complexes. Results indicated that HOMO-LUMO (HL) Gap is larger for C1–C4 complexes as compared to the HL gap for DNA/RNA hybrid-metal interacted complexes. This clearly indicates strong DNA/RNA-metal interaction, which is also observed in NCIPLOT analysis. Al and Au are chemically bonded to DNA/RNA hybrids, while the adsorption of Cu is between weak chemisorption and strong physisorption. The interacted bond distance of DNA/RNA-(Al, Cu) complexes are smaller than the bond distance of DNA/RNA-Au complexes. AIM analysis predicts noncovalent interaction for the studied complexes. TDDFT results indicate that few complexes can be used as fluorescent biomarkers as the absorption and emission wavelength lies in the visible region.

Single Atom Catalysts (SAC) trapped in defective and nitrogen-doped graphene supported on metal substrates

Single Atom Catalysts (SAC) in graphene have been recently gaining more and more attention. They are usually non-noble transition metal (TM) adatoms getting trapped at the carbon vacancies during the fabrication of the graphene layer, which then act as active centers for catalysis and adsorption. In this work we present a systematic and comparative investigation, by means of dispersion–corrected density functional theory (DFT) calculations, of Fe, Co, Ni, and Cu as possible SACs when they become trapped at graphene C vacancies. The stability of these TM atoms is further increased by introducing pyridinic nitrogen (N) atoms and transforming graphene into a giant porphyrin-like macrocyclic ligand. The structural, electronic and energetics properties of these systems, even under the effect of a metal substrate (weakly interacting Cu (111) or strongly interacting Ni (111)), are comparatively examined in great detail by means of crystal/ligand field theories and through ad-hoc energy decomposition analysis to highlight trends and peculiar behaviors. The position of the TM d-orbitals with respect to the Fermi level of the whole system is of considerable importance for designing prospective device applications in catalysis, electrocatalysis and sensors. To this purpose, we also examine how the reactivity of the SACs in graphene towards the hydrogen evolution reaction (HER) can be tuned with N-doping and with different substrates.

Vibrational Frequencies of Cerium-Oxide-Bound CO: A Challenge for Conventional DFT Methods.

In ceria-based catalysis, the shape of the catalyst particle, which determines the exposed crystal facets, profoundly affects its reactivity. The vibrational frequency of adsorbed carbon monoxide (CO) can be used as a sensitive probe to identify the exposed surface facets, provided reference data on well-defined single crystal surfaces together with a definitive theoretical assignment exist. We investigate the adsorption of CO on the CeO_{2}(110) and (111) surfaces and show that the commonly applied DFT(PBE)+U method does not provide reliable CO vibrational frequencies by comparing with state-of-the-art infrared spectroscopy experiments for monocrystalline CeO_{2} surfaces. Good agreement requires the hybrid DFT approach with the HSE06 functional. The failure of conventional density-functional theory (DFT) is explained in terms of its inability to accurately describe the facet- and configuration-specific donation and backdonation effects that control the changes in the C─O bond length upon CO adsorption and the CO force constant. Our findings thus provide a theoretical basis for the detailed interpretation of experiments and open up the path to characterize more complex scenarios, including oxygen vacancies and metal adatoms.

Adatom Surface Modification of Metal Nanoparticle Electrodes Though Underpotential Deposition for CO2 Electroreduction Catalysis

The electrocatalytic CO2 reduction reaction to form higher order carbon-coupled products is a complex interplay of CO binding energies, kinetic rate-constant branching ratios, CO2 solubility, and pH gradients. The result of this wide, and often uncontrolled parameter space is a distribution of CO2RR products, especially in carbon-coupled reactions. Designing electrocatalysts that promote any of these parameters individually is an attractive route to not only build selective CO2RR catalysts, but also to provide the framework of design principals for selective electrocatalysis. To this end, systematically varying the electrode kinetics of CO2 reduction reactions on model system substrates through minor surface modifications is an informative approach for understanding this complicated reaction and how certain products can be expressed or suppressed.Metal nanoparticle electrocatalysts are an interesting model system as recent advances in synthetic methods enable a large degree of size, shape, and facet control which allow systematic investigations of several well controlled parameters. Palladium nanoparticles are particularly interesting as Pd alloys with hydrogen prior to electroreduction catalysis in aqueous media. This alloying provides a continuous supply of reducing hydrogen equivalents for electrocatalysis. Pd surfaces can also be modified with submonolayer amounts of metal adatoms through underpotential electrodeposition. This surface modification can, in principal, tune both the binding energies of CO and the kinetic rate constants of the hydrogen evolution reaction, both critical parameters for CO2RR. This wide parameter space presents opportunities for applied and fundamental research. I will discuss recent results aimed at modifying the electrode kinetics of palladium nanocubes with 100 surface termination by modifying the surface with various metal adatoms in different amounts. I will show how Pd nanocube surface modification influences not only the surface, but bulk, hydrogen alloying properties of the nanocube as well.

Structural characterisation of molecular conformation and the incorporation of adatoms in an on-surface Ullmann-type reaction

The on-surface synthesis of covalently bonded materials differs from solution-phase synthesis in several respects. The transition from a three-dimensional reaction volume to quasi-two-dimensional confinement, as is the case for on-surface synthesis, has the potential to facilitate alternative reaction pathways to those available in solution. Ullmann-type reactions, where the surface plays a role in the coupling of aryl-halide functionalised species, has been shown to facilitate extended one- and two-dimensional structures. Here we employ a combination of scanning tunnelling microscopy (STM), X-ray photoelectron spectroscopy (XPS) and X-ray standing wave (XSW) analysis to perform a chemical and structural characterisation of the Ullmann-type coupling of two iodine functionalised species on a Ag(111) surface held under ultra-high vacuum (UHV) conditions. Our results allow characterisation of molecular conformations and adsorption geometries within an on-surface reaction and provide insight into the incorporation of metal adatoms within the intermediate structures of the reaction.

Stability of heterogeneous single-atom catalysts: a scaling law mapping thermodynamics to kinetics

Heterogeneous single-atom catalysts (SACs) hold the promise of combining high catalytic performance with maximum utilization of often precious metals. We extend the current thermodynamic view of SAC stability in terms of the binding energy (Ebind) of single-metal atoms on a support to a kinetic (transport) one by considering the activation barrier for metal atom diffusion. A rapid computational screening approach allows predicting diffusion barriers for metal–support pairs based on Ebind of a metal atom to the support and the cohesive energy of the bulk metal (Ec). Metal–support combinations relevant to contemporary catalysis are explored by density functional theory. Assisted by machine-learning methods, we find that the diffusion activation barrier correlates with (Ebind)2/Ec in the physical descriptor space. This diffusion scaling-law provides a simple model for screening thermodynamics to kinetics of metal adatom on a support.

Theoretical modelling of porous silicon decorated with metal atoms for hydrogen storage

There is experimental evidence suggesting that metal adatoms enhance the physisorption of hydrogen molecules in porous silicon. However, theoretical reports about the physical properties for this material to be suitable for hydrogen storage are scarce. Thus, in this work we employ Density Functional Theory to study the effects of decoration with metals on the hydrogen-adsorption properties on hydrogen-passivated porous silicon. The results indicate that lithium and palladium decorating atoms are strongly bonded to the porous silicon—preventing the adverse effects of clusterization—while beryllium is not. Lithium and palladium exhibit physisorption capacity up to 5 and 4 hydrogen molecules per adatom, respectively. In contrast, adsorption of hydrogen molecules in beryllium is too weak as the adatom is not chemisorbed on the surface of the pore. The hydrogen passivation of the pore surface proves to be beneficial for a strong chemisorption of the decorating atoms.

Unveiling the adsorption properties of 3d, 4d, and 5d metal adatoms on the MoS2 monolayer: A DFT-D3 investigation

The adsorption of metal (M) adatoms on two-dimensional (2D) transition-metal dichalcogenides (TMDs) monolayers have been used to tune the physical-chemical properties of 2D TMDs such as MoS2, however, our atomistic understanding of the binding mechanism, site preference, charge transfer, diffusion barriers, etc., of metal adatoms on MoS2 is far from satisfactory. Thus, we report spin-polarized density functional theory calculations within the D3 van der Waals (vdW) correction to characterize the adsorption properties of 15 metal adatoms (3d, 4d, 5d) on MoS2 at low-coverage limit. All adatoms can be separated into two groups, namely, Fe, Co, Ni, Ru, Rh, Os, Ir, and Pt bind covalently to the MoS2 monolayer on the topMo site, where the binding mechanism can be explained by the combination of two factors, namely, (i) distortions within the MoS2 monolayers, which breaks the degeneracy of the electronic states and reduces their magnitude near the valence band maximum, and (ii) strong hybridization between the partially unoccupied d- and Mo dz2-states. In contrast, Pd has a strong preference for the hollow site compared with the topMo site, however, Cu, Zn, Ag, Cd, and Hg adatoms bind to the topMo and hollow sites with nearly the same energy (differences less than 0.10eV) due to the weak hybridization of the fully occupied metal d-states, while Au binds preferentially on the topS site. Thus, the M-MoS2 interactions are dominated by covalent and van der Waals interactions, which shows strong and weak dependence on the adsorption site preference, respectively.