Single Ti Atom Research Articles

Stable and 7.7 wt% hydrogen storage capacity of Ti decorated Irida-Graphene from first-principles calculations

Solid-state hydrogen storage is crucial for the widespread applications of hydrogen energy. It is a grand challenge to find appropriate materials that provide high hydrogen density and ambient temperature stability. Herein, we investigated the potential of Ti-decorated Irida-Graphene, a promising effective hydrogen storage system, as a novel hydrogen storage material using first-principles calculation. Irida-Graphene is a two-dimensional isomer of carbon consisting of tri-, hexa-, and octagon rings of carbon. Ti atoms are tightly bounded to the hexagonal rings. Binding energy analysis reveals that a single Ti atom in the primitive unit-cell of Ti-decorated Irida-Graphene is capable to bind up with 5H2 molecules and the average adsorption energy was −0.41 eV/H2. It indicates the gravimetric density of 7.7 wt%. The stability is attributed to Kubas-type interactions and ensured by a 5.0 eV diffusion energy barrier that prevents the Ti–Ti clustering. Further, ab initio molecular dynamics simulations results illustrate that the system remains stable at 600 K, higher than the desorption temperature of 524 K, implying the stability of the system during hydrogen recharge and discharge. The exceptional hydrogen storage performance suggests that Ti-decorated Irida-Graphene is an outstanding candidate for hydrogen storage materials.

Cation Vacancy Clusters in Ti3C2Tx MXene Induce Ultra‐Strong Interaction with Noble Metal Clusters for Efficient Electrocatalytic Hydrogen Evolution

AbstractMXenes are promising substrates for supported noble metal electrocatalysts. Yet, it is a significant challenge to modulate the metal–support interaction (MSI) for enhancing catalytic performance. Herein, employing a facile HF etching method, the cation vacancy structures in Ti3C2Tx MXenes are controllably tuned, producing nearly vacancy‐free (Ti3C2Tx‐V0), single Ti atom vacancy (Ti3C2Tx‐VS), or Ti vacancy cluster (Ti3C2Tx‐VC) engineered MXenes. Ruthenium atomic clusters, as a model catalyst, successfully anchor on all MXene substrates. Different from the terminal O/F coordination groups on routine Ti3C2Tx MXene surfaces, the Ti vacancy clusters in Ti3C2Tx‐VC create unique lattice carbon ligand environment toward Ru species, which induces ultra‐strong MSI. As a result, compared to Ti3C2Tx‐V0 and Ti3C2Tx‐VS, the Ti3C2Tx‐VC modulated Ru clusters (Ru@Ti3C2Tx‐VC) exhibit the optimized balance of H2O adsorption/dissociation and OH/H desorption, thereby delivering superior electrocatalytic performance in the alkaline hydrogen evolution reaction (HER). Within the wide range from laboratory‐level (90 mA cm−2) to industrial‐level (1.5 A cm−2) current density, Ru@Ti3C2Tx‐VC outperforms commercial Pt/C in terms of overpotential and mass activity. Moreover, as a universal substrate for noble metal catalysts, Ti3C2Tx‐VC can also anchor Ir/Pt/Rh atomic clusters and enable excellent HER catalytic activity. This work expands the scope of the MSI between MXene and noble metal catalysts.

Distribution Pattern of Metal Atoms in Bimetal-Doped Pyridinic-N4 Pores Determines Their Potential for Electrocatalytic N2 Reduction.

Doping two single transition-metal (TM) atoms on a substrate host opens numerous possibilities for catalyst design. However, what if the substrate contains more than one vacancy site? Then, the combination of two TMs along with their distribution patterns becomes a design parameter potentially complementary to the substrate itself and the bimetal composition. In this study, we investigate ammonia synthesis under mild electrocatalytic conditions on a transition-metal-doped porous C24N24 catalyst using density functional theory (DFT). The TMs studied include Ti, Mn, and Cu in a 2:4 dopant ratio (Ti2Mn4@C24N24 and Ti2Cu4@C24N24). Our computations show that a single Ti atom in both catalysts exhibits the highest selectivity for N2 fixation at ambient conditions. This work is a good theoretical model to establish the structure-activity relationship, and the knowledge earned from the metal-N4 moieties may help studies of related nanomaterials, especially those with curved structures.

Structural evolution, electronic properties and spectra of titanium clusters

The ground-state structures, electronic properties and spectra of Ti n c ( n = 3–16 and c = 0, ±1) clusters have been investigated based on density functional theory (DFT) and CALYPSO structure prediction. Geometry optimizations show that the neutral, cationic and anionic titanium clusters have the same growth pattern. The pentagonal bipyramid plays an important role in the growth process. Analysis of electronic properties shows that the thermal stability of cationic and anionic clusters is greater than that of neutral cluster. The pentagonal bipyramid and icosahedron clusters have relatively high stability. Chemical activity of titanium clusters decreases with the cluster size. The preferred dissociation pathway of Ti n + clusters is the loss of a single Ti atom to form Ti n − 1 + . Optical absorption of Ti n clusters and infrared and Raman spectra of Ti n + clusters have been simulated and can be used for their structural identification. The ground states of Ti n − clusters have been determined by comparing experimental and theoretical photoelectron spectra. • The pentagonal bipyramid plays an important role in the growth process of titanium clusters • The titanium cluster with pentagonal bipyramid or icosahedron geometry is more table than other clusters. • The preferred dissociation pathway of Ti n + clusters is the loss of a single Ti atom to form Ti n − 1 + . • The ground state of anionic Ti n − clusters are identified by photoelectron spectra.s

Layered TiCl2 Monolayer as an Antiferromagnetic Semiconductor

We report layered TiCl2 monolayer to be a robust antiferromagnetic (AFM) semiconductor by investigating its geometry, stability, electronic and magnetic properties using first-principles calculation and Monte Carlo (MC) simulation. The TiCl2 monolayer exfoliated from layered TiCl2 bulk is verified to be an AFM semiconductor with a relatively wide band gap by spin-polarized density of states and energy band structure calculations. Phonon dispersion and elastic modulus calculations show that the TiCl2 monolayer is dynamically and mechanically stable, respectively. The Néel temperature is evaluated to be 368[Formula: see text]K by MC simulation based on Ising model. Additionally, the band gap, the magnetic moment of a single Ti atom and the Néel temperature can be significantly tuned by applied biaxial strain, but the AFM semiconducting behavior is robust against the applied strains, indicating that the TiCl2 monolayer is a potential candidate for high-speed AFM spintronic applications.

Structure, stability, and optical absorption spectra of small TinCx clusters: a first-principles approach

ABSTRACT Titanium carbide molecular clusters are thought to form in the circumstellar envelopes of carbon-rich asymptotic giant branch (AGB) stars but, to date, their detection has remained elusive. To facilitate the astrophysical identification of those clusters in AGB and post-AGB environments, the molecular structures and optical absorption spectra of small TinCx clusters, with n = 1–4 and x = 1–4, and some selected larger clusters, Ti3C8, Ti4C8, Ti6C13, Ti7C13, Ti8C12, Ti9C15, and Ti13C22, have been calculated. The density functional formalism, within the B3LYP approximation for electronic exchange and correlation, was used to find the lowest energy structures. Except the clusters having a single Ti atom, the rest exhibit three-dimensional structures. Those are formed by a Ti fragment surrounded in general by carbon dimers. The optical spectra of TinCx, computed by time-dependent density functional theory, using the corrected CAM-B3LYP functional, show absorption features in the visible and near-infrared regions which may help in the identification of these clusters in space. In addition, most of the clusters have sizable electric dipole moments, allowing their detection by radioastronomical observations.

Dynamics of Photoexcited Small Polarons in Transition-Metal Oxides.

The dynamics of photoexcited polarons in transition-metal oxides (TMOs), including their formation, migration, and quenching, plays an important role in photocatalysis and photovoltaics. Taking rutile TiO2 as a prototypical system, we use ab initio nonadiabatic molecular dynamics simulation to investigate the dynamics of small polarons induced by photoexcitation at different temperatures. The photoexcited electron is trapped by the distortion of the surrounding lattice and forms a small polaron within tens of femtoseconds. Polaron migration among Ti atoms is strongly correlated with quenching through an electron-hole (e-h) recombination process. At low temperature, the polaron is localized on a single Ti atom and polaron quenching occurs within several nanoseconds. At increased temperature, as under solar cell operating conditions, thermal phonon excitation stimulates the hopping and delocalization of polarons, which induces fast polaron quenching through the e-h recombination within 200 ps. Our study proves that e-h recombination centers can be formed by photoexcited polarons, which provides new insights to understand the efficiency bottleneck of photocatalysis and photovoltaics in TMOs.

Ti- and Fe-related charge transition levels in β−Ga2O3

Deep-level transient spectroscopy measurements on β-Ga2O3 crystals reveal the presence of three defect signatures labeled E2a, E2b, and E3 with activation energies at around 0.66 eV, 0.73 eV, and 0.95 eV below the conduction band edge. Using secondary ion mass spectrometry, a correlation between the defect concentration associated with E3 and the Ti concentration present in the samples was found. Particularly, it is found that E3 is the dominant Ti-related defect in β-Ga2O3 and is associated with a single Ti atom. This finding is further corroborated by hybrid functional calculations that predict Ti substituting on an octahedral Ga site, denoted as TiGaII, to be a good candidate for E3. Moreover, the deep level transient spectroscopy results show that the level previously labeled E2 and attributed to Fe substituting on a gallium site (FeGa) consists of two overlapping signatures labeled E2a and E2b. We tentatively assign E2a and E2b to Fe substituting for Ga on a tetrahedral or an octahedral site, respectively.

A DFT study on the possibility of embedding a single Ti atom into the perfect stanene monolayer as a highly efficient gas sensor

In this work, we have surveyed, the interaction of various molecules (CO, NO, CO2, NO2, NH3 and SO2) with the pristine and Ti-embedded stanene monolayers employing the first-principles calculations. Firstly, the electronic properties of the pristine and Ti-embedded stanene were investigated. We found that after the adsorption of transition metal on stanene, the charge densities were remarkably accumulated on the embedded metal site. The most stable adsorption geometry, adsorption energies, charge density differences, charge transfer and electronic properties were thoroughly discussed. The results suggest that the Ti-embedded stanene can react with gas molecules more effectively as compared to the pristine one. All the studied gas molecules were strongly chemisorbed on the Ti-embedded stanene monolayer, whereas, on the pristine monolayer, these gas molecules show a weak physisorption. The considerable overlaps between the PDOS profiles of the Ti atom and different atoms of gas molecules indicate the formation of covalent bond between them. Our adsorption energy calculations indicate that the gas molecule interaction with Ti-embedded system gives rise to the most stable configuration as compared with that on the pristine stanene. Besides, the analysis of the charge difference plots represents the accumulation of charge densities on the adsorbed gas molecules. Our results thus suggest superior electronic properties for Ti-embedded stanene monolayers.

Evaluation of the Katsuki-Sharpless Epoxidation Precatalysts by ESI-FTMS, CID, and IRMPD Spectroscopy.

The Katsuki-Sharpless epoxidation reaction is one of the most recognized chiral catalytic reactions, allowing for chiral epoxides to be used as starting materials for a series of synthetic pathways. The complete understanding of this reaction mechanism depends on the identification and description of species formed from Ti(IV) alkoxides and alkyl tartrates precatalysts. Despite previous reports on the nature of a bimetallic catalyst based on IR spectroscopy and NMR analysis, some debate remains. Therefore, we carried out mass spectrometry analysis by direct extraction of the ions from the reaction media by ESI(-)-FT-ICR and evaluated the observed ions by CID and IRMPD experiments. These techniques allowed us to detect carboxylates that correlate to the species in the reaction media and to confirm the actual presence of the titanium-tartrate complexes in solution. Our IRMPD results suggest the carboxylate dimer as an asymmetric species with two tartrates coordinated to a single Ti atom, while the other Ti center is coordinated by labile alkoxydes that could be easily exchanged by the organic peroxide and the substrate, allowing the epoxidation reaction to take place.

CO Oxidation Catalyzed by a Single Ti Atom Supported on Divacancy Defective Graphene: A Dispersion‐Corrected DFT Study

AbstractWe report a detailed theoretical investigation on the possible reaction mechanisms of CO oxidation catalyzed by a single Ti atom supported on divacancy defective graphene (TiDVG). The reaction between the adsorbed O2 with CO molecules on TiDVG is compared via three different mechanisms, i. e. Eley‐Rideal (ER), Langmuir‐Hinshelwood (LH) and termolecular Eley‐Rideal (TER). According to our results, the energy barrier for the rate determining step of CO + O2 reaction in the TER mechanism is smaller than those of ER and LH ones. This can be attributed to the significant mixing of electronic states of adsorbed CO and O2 at the reaction site, as well as due to a sizable charge‐transfer from the surface to these molecules. On the other hand, the calculated adsorption energies indicate that the coadsorption of O2 and CO molecules is slightly more energetically favored with respect to the coadsorption of two CO molecules. This result together with the relatively small reaction barriers reveal that the LH mechanism is also possible to occur at low temperature. Our results reported here suggest that TiDVG can be regarded as a green, low‐cost and highly active catalyst for CO oxidation at room temperature.

Unravelling the electrochemical mechanisms for nitrogen fixation on single transition metal atoms embedded in defective graphitic carbon nitride

Single transition metal atoms supported by defective g-C3N4 are examined by DFT for electrochemical N2 fixation. The single Ti atom is the most promising candidate for its high activity and stability owing to the coordination number of the active center.

Theoretical study of hydrogen adsorption in Ti-decorated capped carbon nanotube

ABSTRACTWe present ab initio study using dispersion-corrected density functional theory calculations to investigate the hydrogen interaction with Ti-coated, one end closed, single-walled carbon nanotube (SWCNT). Our results demonstrate that a single Ti atom binds up to five hydrogen molecules on SWCNT cap top, whereas adsorption of four hydrogen molecules is energetically more favourable. The analyses from adsorption energy profile, highest occupied molecular orbital–lowest unoccupied molecular orbital gap and Mulliken charge distribution show contrast in first hydrogen molecule adsorption compared with the rest of four configurations. This is clearly due to the strongly different bonding nature of first hydrogen adsorption among others, between hydrogen molecules and Ti-coated SWCNT. These results not only support our understanding of adsorption nature of hydrogen in Ti-coated SWCNTs but also suggest new directions for smart storage techniques.

Localized states induced by an oxygen vacancy in rutile TiO2

Using density functional theory and model Hamiltonian analysis, we investigate the localized states induced by an oxygen vacancy in rutile TiO2. We identify two classes of localized states—the hybrid and the polaron. The hybrid state is caused by the orbital overlap between three Ti atoms next to a vacancy and is mainly derived from the Ti eg orbitals. The polaron state is caused by the local lattice distortion and is mainly composed of one particular t2g orbital from a single Ti atom. The first principles calculation shows that the polaron state is energetically favored, and the tight-binding analysis reveals the underlying connection between the bulk band structure and the orbital character of the polaron. The magnetic coupling between two nearby polaron states is found to be ferromagnetic. Using this picture, we analyze the results of recent theoretical calculations and experiments and discuss the connection to vacancies in SrTiO3.

Assessing the performance of screw deformed nanotube as potential hydrogen storage material

The hydrogen storage of screw deformed Ti-functionalized (5,2) single walled carbon nanotube is investigated by using the state of the art density functional theory calculations. The single Ti atom prefers to bind at the hollow site of the hexagonal ring with average adsorption energies per hydrogen molecule −0.56 and −0.52 eV for the un-deformed CNT-φ = 0 and deformed CNT-φ = 5 nanotubes, respectively. The hydrogen storage reactions 4H2 + Ti-CNT-φ = 0,5 are characterized in terms of projected densities of states and statistical thermodynamics. The free energies and enthalpies meet the ultimate targets of the department of energy for minimal and maximal temperatures and pressures. The closest reactions to zero free energy exhibit surface coverage values 0.951 and 0.816 as well as (direct/inverse) rate constant ratios 6.55 and 1.5. The translational term is found to exact a dominant effect on the total entropy change with temperature, and the more promising thermodynamics are assigned to the screw deformed nanotube.

Hydrogen storage reactions on titanium decorated carbon nanocones theoretical study

Hydrogen storage reactions on Ti decorated carbon nanocones (CNC) are investigated by using the state of the art density functional theory calculations. The single Ti atom prefers to bind at the bridge site between two hexagonal rings, and can bind up to 6 hydrogen molecules with average adsorption energies of −1.73, −0.74, −0.57, −0.45, −0.42, and −0.35 eV per hydrogen molecule. No evidence for metal clustering in the ideal circumstances, and the hydrogen storage capacity is expected to be as large as 14.34 wt%. Two types of interactions are recognized. While the interaction of 2H2 with Ti–CNC is irreversible at 532 K, the interaction of 3H2 with Ti–CNC is reversible at 392 K. Further characterizations of the former two reactions are considered in terms of projected densities of states, simulated infrared and proton magnetic resonance spectra, electrophilicity, and statistical thermodynamic stability. The free energy of the highest hydrogen storage capacity reaction between 6H2 and Ti–CNC meets the ultimate targets of department of energy at (233.15 K) and (11.843 atm) with surface coverage (0.941) and (direct/inverse) rate constants ratio (1.35).

Cooperatively enhanced catalytic properties of Ti@Al(100) near-surface alloy for aluminum hydrogenation

We have carried out detailed first-principles studies of the catalytic properties of Ti@Al(100) near-surface alloy. The single Ti atom, (0,2) Ti–Ti pair, and [0,2] Ti doping domain have better catalytic performances. These species doped in the top surface can develop back-bonding interaction with H2 to catalyze the splitting, which however on the other hand hinder the dissociated H atoms to diffuse. Doped in the subsurface, they can also enhance hydrogen interaction on aluminum to catalyze H2 splitting. The activation energies are 0.80, 0.68, and 0.48 eV for Ti atom, (0,2) pair, and [0,2] doping domain, respectively. Without Ti–H bond, the dissociated H atom could diffuse away with small energy cost. The structural expansion induced by titanium doping, the lower electronegativity of Ti, and the more valence electrons of Ti may cooperatively facilitate the charge transfer from the above Al atoms to H2 molecule, accounting for the enhanced splitting properties.

The Ti4 cluster activates water dissociation on defective graphene

Activation of the Ti4 cluster on defective graphene for water adsorption and dissociation was investigated via density functional theory. Both the vacancy and the Ti cluster can promote the water dissociation reaction. The vacancy can efficiently enhance the adsorption of Ti atoms to stabilize the cluster. However, compared to the role of the vacancy, the cluster plays a more important role in activating water dissociation. A single water molecule and a second one can almost freely dissociate with (or without) a low barrier on the Ti4 cluster, the barriers are substantially lower than that on a single Ti atom. The Ti4 cluster decorated graphene is a promising candidate for activation of the dissociation of water molecules.

Ligand field splittings in core level transitions for transition metal (TM) oxides: Tanabe-Sugano diagrams and (TM) dangling bonds in vacated O-atom defects

Defect states in the forbidden band-gap below the conduction band edge are active as electron traps in nano-grain high-) transition metal (TM) oxides with thickness >0.3 nm, e.g., ZrO2 and HfO2. These oxides have received considerable attention as gate-dielectrics in complementary metal oxide semiconductor (CMOS) devices, and more recently are emerging as candidates for charge storage and memory devices. To provide a theoretical basis for device functionality, ab-initio many-electron theory is combined with X-ray absorption spectroscopy (XAS) to study O K edge and TM core level transitions. These studies identify ligand field splittings (ΔLF) for defect state features,. When compared with those obtained from O-atom and TM-atom core spectroscopic transitions, this provides direct information about defect state sun-nm bonding arrangements. comparisons are made for (i) elemental TiO2 and Ti2O3 with different formal ionic charges, Ti4+ and Ti3+ and for (ii) Magneli Phase alloys, TinO2n-1, n is an integer 9≥n>3, and (TiO2)x(HfO2)1−x alloys. The alloys display multi-valent behavior from (i) different ionic-charge states, (ii} local bond-strain, and (iii) metallic hopping transport. The intrinsic bonding defects in TM oxides are identified as pairs of singly occupied dangling bonds. For 6-fold coordinated Ti-oxides defect excited states in 2nd derivative O K pre-edge spectra are essentially the same as single Ti-atom d2 transitions in Tanabe-Sugano (T-S) diagrams. O-vacated site defects in 8-fold coordinated ZrO2 and HfO2 are described by d8 T-S diagrams. T-S defect state ordering and splittings are functions of the coordination and symmetry of vacated site bordering TM atoms. ΔLF values from the analysis of T-S diagrams indicate medium range order (MRO) extending to 3rd and 4th nearest-neighbor (NN) TM-atoms. Values are different for 6-fold Ti, and 8-fold ZrO2 and HfO2, and scale inversely with differences in respective formal ionic radii. O-vacated site bonding defects in TM nano-grain oxides are qualitatively similar to vacant-site defects in non-crystalline SiO2 and GeO2 for ulta-thin films, < 0.2 nm thick, and yield similar performance in MOSCAPs on Ge substrates heralding applications in aggressively-scale CMOS devices.

Emergence of ferromagnetism at a vacancy on a non-magnetic ferroelectric PbTiO3 surface: A first-principles study

We investigated the origin of ferromagnetism due to a vacancy on a non-magnetic ferroelectric PbTiO3 (001) surface using first-principles spin-density-functional theory calculations, and successfully provided electronic-level insights into this phenomenon. The results demonstrate that an oxygen vacancy intrinsically induces ferromagnetism, which is remarkably enhanced and stabilized on the TiO2-terminated surface favorably rather than inside PbTiO3. Detailed electronic structure analysis revealed that the ferromagnetism originates from the spin-polarized dzx and dx2-y2 states of the two Ti atoms nearest to the oxygen vacancy on the surface. The origin of ferromagnetism for an oxygen vacancy inside PbTiO3 differs from that an oxygen vacancy on the surface; it is due to the eg (dx2-y2 and dz2) state of the single nearest Ti atom. These findings provide significant fundamental insights for designing multiferroics in conventional ferroelectrics.