Transition Metals Ti Research Articles

Molecular Beam Epitaxy of Transition Metal (Ti-, V-, and Cr-) Tellurides: From Monolayer Ditellurides to Multilayer Self-Intercalation Compounds.

Material growth by van der Waals epitaxy has the potential to isolate monolayer (ML) materials and synthesize ultrathin films not easily prepared by exfoliation or other growth methods. Here, the synthesis of the early transition metal (Ti, V, and Cr) tellurides by molecular beam epitaxy (MBE) in the mono- to few-layer regime is investigated. The layered ditellurides of these materials are known for their intriguing quantum- and layer dependent- properties. Here we show by a combination of in situ sample characterization and comparison with computational predictions that ML ditellurides with octahedral 1T structure are readily grown, but for multilayers, the transition metal dichalcogenide (TMDC) formation competes with self-intercalated compounds. CrTe2, a TMDC that is known to be metastable in bulk and easily decomposes into intercalation compounds, has been synthesized successfully in the ML regime at low growth temperatures. At elevated growth temperatures or for multilayers, only the intercalation compound, equivalent to a bulk Cr3Te4, could be obtained. ML VTe2 is more stable and can be synthesized at higher growth temperatures in the ML regime, but multilayers also convert to a bulk-equivalent V3Te4 compound. TiTe2 is the most stable of the TMDCs studied; nevertheless, a detailed analysis of multilayers also indicates the presence of intercalated metals. Computation suggests that the intercalation-induced distortion of the TMDC-layers is much reduced in Ti-telluride compared to V-, and Cr-telluride. This makes the identification of intercalated materials by scanning tunneling microscopy more challenging for Ti-telluride. The identification of self-intercalation compounds in MBE grown multilayer chalcogenides may explain observed lattice distortions in previously reported MBE grown early transition metal chalcogenides. On the other hand, these intercalation compounds in their ultrathin limit can be considered van der Waals materials in their own right. This class of materials is only accessible by direct growth methods but may be used as "building blocks" in MBE-grown van der Waals heterostructures. Controlling their growth is an important step for understanding and studying the properties of these materials.

Series of Near-IR-Absorbing Transition Metal Complexes with Redox Active Ligands.

New soluble and intensely near-IR-absorbing transition metal (Ti, Zr, V, Ni) complexes were synthesized using a redox non-innocent N,N’-bis(3,5-di-tertbutyl-2-hydroxy-phenyl) -1,2-phenylenediamine (H4L) as a ligand precursor. In all the complexes, ([Ti(Lox)2, [Zr(Lox)2], [V(Lsq1)(HLox)] and [Ni(HLox)2], two organic molecules coordinate to the metal center as tri- or tetradentate ligands. The solid-state structures of the complexes were determined using single crystal XRD, and the compounds were further characterized with Electrospray Ionisation Mass Spectrometry (ESI-MS). Thermoanalytical measurements indicated the thermal stabilities of the complexes. All compounds absorb strongly in the near-IR region and show very interesting magnetic and electrochemical properties. Moreover, it was shown that the V and Ni complexes can also convert absorbed near-IR photons to (un)paired electrons, which indicates great promise in photovoltaic applications.

Toxic and Physiological Metal Uptake and Release by Human Serum Transferrin

An atomistic understanding of metal transport in the human body is critical to anticipate the side effects of metal-based therapeutics and holds promise for new drugs and drug delivery designs. Human serum transferrin (hTF) is a central part of the transport processes because of its ubiquitous ferrying of physiological Fe(III) and other transition metals to tightly controlled parts of the body. There is an atomistic mechanism for the uptake process with Fe(III), but not for the release process, or for other metals. This study provides initial insight into these processes for a range of transition metals—Ti(IV), Co(III), Fe(III), Ga(III), Cr(III), Fe(II), Zn(II)—through fully atomistic, extensive quantum mechanical/discrete molecular dynamics sampling and provides, to our knowledge, a new technique we developed to calculate relative binding affinities between metal cations and the protein. It identifies protonation of Tyr188 as a trigger for metal release rather than protonation of Lys206 or Lys296. The study identifies the difficulty of metal release from hTF as potentially related to cytotoxicity. Simulations identify a few critical interactions that stabilize the metal binding site in a flexible, nuanced manner.

Development of ferromagnetism and formation energetics in 3d TM-doped SnO2: GGA and GGA + U calculations

Electronic and magnetic properties of several 3d transition metals (Ti, V, Cr, Mn, Fe, Co, Ni, and Cu) doped SnO2 by means of ab initio calculations. First, we successfully reproduced the experimental band gap (3.60 eV) of bulk SnO2 within GGA + U scheme. Our results show that calculated formation energies of doped systems strongly depend on the dopants atomic numbers (Z). It is found that Ti-doped SnO2 structure exhibits non-magnetic insulating behavior, while V-, Cr-, Mn-, Fe-, and eeCo-doped systems illustrate magnetic semiconducting states within both (GGA and GGA + U) schemes. Ni-doped SnO2 holds non-magnetic/magnetic semiconducting nature for GGA/GGA + U methods. Surprisingly, Cu-doped SnO2 displays magnetic metallic behavior having 85.3% spin-polarization within GGA scheme, while like half-metallic character is obtained for GGA + U. Hence, the present work indicates that Cu-doping is an effective way to induced interesting magnetism in SnO2, which have potential applications in spintronics.

Two-Dimensional Titanium Carbide (Ti3C2Tx) MXenes of Different Flake Sizes Studied by Scanning Electrochemical Microscopy in Different Electrolytes

Two-dimensional (2D) layered materials are studied in efforts to discover new compounds and for their fascinating properties engendered by their sheet-like structure and tunable surfaces. MXenes are an emergent class of layered, synthesized transition metal carbides and carbonitrides that are useful in addressing the formidable challenges of sensing at the energy-water nexus. This work reports systematic structural and electrochemical properties of titanium carbide (Ti3C2Tx) MXenes revealed by varying interlayer spacing, flake thickness and lateral size under different electrolytes. In addition to traditional electrode kinetics, we utilized surface sensitive scanning electrochemical microscopy (SECM) to gain a more complete understanding of rich MXene surface chemistry and corresponding knowledge about the physicochemical processes, including inherent electrochemistry and heterogeneous charge transfer characteristics, at electrode/electrolyte (solid/liquid) interfaces. We employed electron microscopy, ultraviolet–visible (UV–Vis) absorption spectroscopy, x-ray diffraction, and Raman spectroscopy to determine surface morphology, microscopic and electronic structure and lattice vibrational properties. It is shown that Ti3C2Tx or, specifically, transition metal Ti, undergoes irreversible oxidation and lithiation in a positive potential window, which strongly depends on the flake thickness and type (aqueous versus organic) of the electrolyte. Multi-layered and smaller Ti3C2Tx flakes exhibit faster electron transfer kinetics (kET = 1.2 cm s−1) with a potassium ferrocyanide [Fe(CN)6]4−/3− redox probe, compared to few-layered Ti3C2Tx (kET = 0.3 cm s−1) in aqueous and organic electrolyte (kET = 4.9 cm s−1) with a [Fe(CN)6]4−/3− redox probe, and compared to a few-layered Ti3C2Tx (kET = 0.9 cm s−1). In addition, the few-layered free standing Ti3C2Tx film electrode remains intact following irreversible oxidation. These properties help to establish structure–property-electroactivity relationships among different types of Ti3C2Tx MXenes.

Oxygen Activation on Four-Atom Metal Clusters and Alloys

The dissociation of oxygen molecules and generation of two separated oxygen atoms is an important step in fuel cells. The activation of the bond linking two oxygen atoms in the oxygen molecule usually needs large activation barriers on metals utilized as catalysts. In the present study, the characterization and catalytic performance of four-atom Al nanocluster (Al4) as well as XAl3 (X = Ti and Sc) nanoalloys in activation of the oxygen molecule are investigated using density functional theory. Our results show that the tetrahedral isomer of the Al4 nanocluster lies lower in energy (Ecoh = −33.35 kcal/mol) than does the corresponding rhombus isomer (Ecoh = −31.05 kcal/mol). Also, transition metal (Ti and Sc) doping reduces the reactivity of the Al4 nanocluster. In consistent with the suggestion of reactivity descriptors used in the present study, our results reveal that the maximum catalytic performance in activation of the oxygen-oxygen bond is related to the pristine Al4 clusters. The energy barrier of O2 activation on the Al4 system (21.26 kcal/mol) exhibits that the activation of the bond linking of two oxygen atoms on Al4 is kinetically preferable. In this respect, our study shows that the Al4 nanocluster can be considered as an excellent catalyst for activation the oxygen molecule.

Effect of transition metals (Sc, Ti, V, Cr and Mn) doping on electronic structure and optical properties of CdS

The influences of transition metals (Sc, Ti, V, Cr and Mn) doping at different distances on the stability, electronic structure, conductivity and optical properties of CdS are studied by using VASP code. The results show that the Cr-doped CdS is easy to form, followed by V, Ti, Sc and Mn. The next nearest-neighbor doping is the most stable in each transition metal-doped system. The band-gaps gradually decrease with the order of Sc, Ti, V, Cr, Mn-doping, all doped systems become the degenerate semiconductor. Doping not only enhances conductivity, but also improves photocatalytic performance, especially Mn-doped CdS has obvious improvement. The absorption intensity in the visible light range of the single-doping is more than that of two atoms doped system in different distances, of which Mn and Cr single-doping are the most prominent.

Influence of doping Mg2+ or Ti4+ captions on the microstructures, thermal radiation and thermal cycling behavior of plasma-sprayed Gd2Zr2O7 coatings

Gadolinium zirconate (Gd2Zr2O7) coatings doped by the transition metal Ti and the alkaline earth metal Mg were expected to have improved thermal radiation performance, which could be combined with their excellent thermal barrier properties to comprehensively improve the thermal insulating performance. The results show that the parent Gd2Zr2O7 powder as well as the Gd-site and Zr-site substituted powders crystallize as pyrochlore Gd2Zr2O7 in Fd-3m space group, while all the as-sprayed coatings have the combination of fluorite and a little part of pyrochlore phase. Gd2Zr2O7 ceramic has high mid-infrared emittance and the addition of Ti4+ into Gd2Zr2O7 can enhance the infrared absorption/emittance in a specific wavenumber range, dominantly in the near-infrared (0.75–2.5 μm) band due to the enhancement of electron transition induced by the impurity energy levels linked to the widening of the conduction band. The normal spectral infrared emissivity of Gd2Zr2O7-based coating was higher than 0.88 at 1073 K. The monolayered doped Gd2Zr2O7 coatings present very low thermal cycling lifetime, similar with the parent coating, mainly related with their low fracture toughness, despite (Gd1-xMgx)2Zr2O7 series display lower thermal conductivity than the parent one.

Nitride tuning of transition metal perovskites

Transition metal perovskite oxynitrides are emergent materials for applications as visible light-active photocatalysts for water splitting and CO2 reduction and as thermoelectric, dielectric, and magnetic materials. They have been reported for early transition metals Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, and W in the B sites and alkaline earth or rare earth metals in the A sites. Nitrogen is less electronegative and more polarizable than oxygen, and nitride is more charged than oxide. As a consequence, the introduction of nitride in an oxidic perovskite has important effects on the covalency of bonds, the energy of electronic levels, and the valence states of the cations. This work discusses fundamental and recent developments of perovskite oxynitrides of transition metals of groups 4, 5, and 6 as photocatalytic and electronic materials, focusing on the important aspects of synthetic methodologies, crystal structures, and anion ordering, in connection with the observed physical and chemical properties. Some examples of compounds with late transition metals and complex structures such as layered and double perovskites are also discussed.

Oxygen activation on four-atom metal clusters and alloys

The dissociation of oxygen molecules and generation of two separated oxygen atoms is an important step in fuel cells. The activation of the bond linking two oxygen atoms in the oxygen molecule usually needs large activation barriers on metals utilized as catalysts. In the present study, the characterization and catalytic performance of four-atom Al nanocluster (Al4) as well as XAl3 (X = Ti and Sc) nanoalloys in activation of the oxygen molecule are investigated using density functional theory. Our results show that the tetrahedral isomer of the Al4 nanocluster lies lower in energy (Ecoh = –33.35 kcal/mol) than does the corresponding rhombus isomer (Ecoh = –31.05 kcal/mol). Also, transition metal (Ti and Sc) doping reduces the reacti­vity of the Al4 nanocluster. In consistent with the suggestion of reactivity descriptors used in the present study, our results reveal that the maximum catalytic performance in activation of the oxygen-oxygen bond is related to the pristine Al4 clusters. The energy barrier of O2 activation on the Al4 system (21.26 kcal/mol) exhibits that the activation of the bond linking of two oxygen atoms on Al4 is kinetically preferable. In this respect, our study shows that the Al4 nanocluster can be considered as an excellent catalyst for activation the oxygen molecule.

Theoretical study of metal/silica interfaces: Ti, Fe, Cr and Ni on β-cristobalite.

The understanding of interfacial effects and adhesion at oxide-metal contacts is of key importance in modern technology. Metal-silica interfaces specifically are relevant in electronics, catalysis and nanotechnology. However, adhesion at these interfaces is hindered by a formation of siloxane rings on the silica surface which saturate the dangling bonds at stoichiometric terminations. In this context, we report a thorough density functional theory study of the interaction between β-cristobalite and selected 3d transition metals under different oxygen conditions. For any given interface stoichiometry, we find a progressive decrease of the metal/silica interaction along the series, following the increase of metal electronegativity. Crucially, in presence of early transition metals (Ti or Cr) the surface siloxane rings are spontaneously broken, allowing for strong adhesion. Late transition metals interact only weakly with reconstructed surfaces, similarly to what was found for zinc. In absence of reconstruction, stoichiometric silica/metal contacts behave similarly to alumina/metal contacts, but display larger interactions across the interface. Based on these results, we show that early transition metal or stainless steel buffers can significantly improve the weak adhesion between silica and zinc, responsible for a poor performance of anti-corrosive galvanic zinc coatings on modern advanced high strength steels.

Adsorption performance of M-doped (M = Ti and Cr) gallium nitride nanosheets towards SO2 and NO2: a DFT-D calculation.

The structure, adsorption characteristics, electronic properties, and charge transfer of SO2 and NO2 molecules on metal-doped gallium nitride nanosheets (M-GaNNSs; M = Ti and Cr) were scrutinized at the Grimme-corrected PBE/double numerical plus polarization (DNP) level of theory. Two types, MGa-GaNNSs and MN-GaNNSs, of doped nanostructures were found. The MGa sites are more stable than the MN sites. The results showed that adsorption of SO2 and NO2 molecules on TiGa,N-GaNNSs is energetically more favorable than the corresponding CrGa,N-GaNNSs. The stability order of complexes is energetically predicted to be as NO2–TiGa-GaNNS > NO2–TiN-GaNNS > SO2–TiGa-GaNNS > NO2–CrN-GaNNS > SO2–TiN-GaNNS > NO2–CrGa-GaNNS > SO2–CrN-GaNNS > SO2–CrGa-GaNNS. The electron population analysis shows that charge is transferred from MGa,N-GaNNSs to the adsorbed gases. The TiGa-GaNNS is more sensitive than the other doped nanostructures to NO2 and SO2 gases. It is estimated that the sensitivity of TiGa-GaNNS to NO2 gas is more than to SO2 gas.

Identification of Magic Numbers in Homonuclear Clusters: The ε3 Stability Ranking Function.

With the rise of cluster-assembled materials, an index that is able to rank and identify stable clusters or molecules is of great interest in materials sciences and engineering. In the present work, we applied a stability ranking function (ε3) in nanoclusters formed by simple metals (Na, Mg), main group elements (Al), or transition metals (Ti, Cu). The ε3 function parameters are molecular properties derived from the wave function. These parameters can be divided into kinetic and thermodynamic descriptors, in which the kinetic descriptors are the ionization potential and electronic excitation energy, while the atomization free Gibbs energy is the thermodynamic one. This simple ε3 function was able to identify the possible magic numbers of the studied clusters across the periodic table in a good agreement with previous experimental and theoretical works.

Synthesis and characterization of Ti-doped Bi4O4S3 superconductor

The substitution effects of 3d2 transition metal Ti on the Bi4O4S3 superconductor structure were studied. The presence of Ti in Bi2O2 layer spacer or superconducting BiS2 layers was investigated. For this study, the polycrystalline Bi4O4S3 superconductor was prepared following a two-step solid-state reaction. Grains consisted of lamellae, ~ 5–10 μm in the planar orientation and 100–500 nm thick, presenting a tetragonal crystalline structure, with space group I4/mmm. For the first time, Raman spectroscopy revealed three modes at room temperature, with no significant changes in the spectra with Ti doping. The superconductivity was observed by the Meissner effect from ZFC and FC magnetization measurements. For both samples, the critical temperature was around 3.5 K, showing that Ti doping did not change the TC value, probably due to fact that Ti is in the Bi2O2 layer spacer. Also, the doped sample presented a strong positive magnetic moment, probably due to the presence of ferromagnetic order.

Dopant screening of modified Fe2O3 oxygen carriers in chemical looping hydrogen production

Enhancing the deep reduction of Fe2O3 oxygen carriers (OCs) is a major challenge in the chemical looping hydrogen production (CLH) process. To solve this issue, the modification of Fe2O3 OCs by introducing foreign dopant is an effective strategy. Here, we propose a dopant screening scheme, where surface oxygen vacancy formation energy (Evac) is defined as a descriptor for the reducibility of modified Fe2O3 OCs. Using density functional theory (DFT) calculations, we evaluated the 18 potential dopants, including 3 alkali metals (Li, Na, and K), 2 alkaline earth metals (Mg and Ca), 11 transition metals (Sc, Ti, V, Cr, Mn, Co, Ni, Cu, Zn, Y, and Zr), and 2 rare earth metals (La and Ce). The results indicated that Li, Na, K, Mg, Ni, Cu, Zn, La, and Zr dopants, which not only significantly enhance the reduction reaction from Fe2O3 to Fe3O4 but improve the deep reduction from Fe3O4 to FeO, are screened as the promising candidates for modified Fe2O3 OCs in the CLH process. Our work provided an efficient approach for the initial screening of modified Fe2O3 OCs with enhanced deep reducibility.

Structures, electronic and magnetic properties of transition metal atoms encapsulated in B12N12 cage

The structures, electronic and magnetic properties of a transition metal (TM) atom encapsulated in a B12N12 cage have been investigated by PBE functional. The results show that 3d TM atoms are more suitable for the B12N12 cage. For 3d TM atoms, Ti and Ni are more suitable for the B12N12 cage than their neighbors. Ti@B12N12, Cr@B12N12, Fe@B12N12 and Ni@B12N12 clusters exhibit more kinetic stability. Only the Y@B12N12 clusters prefer to display ionic bond features. The maximum spins for the TM atoms of the TM@B12N12 clusters are occurred at Mn@B12N12, Fe@B12N12 and Co@B12N12.

Magnetic phase stability of transition metals doped (4,4) AlN nanosheet

Magnetic phase stability in transition metals doped (4,4) armchair AlN nanosheet were studied using first principle calculations for three different configurations and two magnetic states. Energy differences between ferromagnetic (FM) and antiferromagnetic (AFM) state suggest that the preferable magnetic state for the AlN nanosheet doped with a pair of Fe and Co atoms independent from doping site is AFM, while it is FM for Ni and Mn impurities. For the rest transition metals (Ti, V, Cr, and Cu) doped AlN nanosheet, magnetic state alters as the transition metals configuration varies. In order to identify the most stable configuration, the formation energy is calculated for all compounds. The results obtained from formation energy calculations revealed that the most stable configuration for a pair of (V, Mn, Fe, Co, and Cu) atoms doped AlN nanosheet is the configuration of I, while for Cr and Ni is the configuration of III. AlN nanosheets doped with (Ti, V, Fe, and Ni) atoms are more stable than nanosheets doped with Cr, Mn and Cu.

Prediction of improved magnetization and stability in Fe16N2 through alloying

We study the structural, energetic, electronic, and magnetic properties of Fe16 − xMxN2 alloys, where M represents 3d transition metals Ti, V, Cr, Mn, Co, and Ni, using special quasirandom structures and density functional theory calculations. We describe stabilization of Fe16N2 resulting from the enhanced occupation of bonding states relative to the corresponding antibonding states as observed from crystal orbital Hamiltonian population analysis. The hybrid HSE06 functional is employed to calculate a magnetic moment of 2.844 μB/Fe, agreeing with recent experimental work and suggesting the importance of electronic exchange effects. Upon alloying, magnetization is found to decrease with all transition metals excluding Mn, for which exceptionally strong ferromagnetic coupling is achieved via nitrogen-mediated exchange interactions. We identify a 1.41% magnetization increase at low Mn concentrations coupled with a decrease in formation energy, making Fe16 − xMnxN2 a suitable candidate for permanent magnet applications. Novel end-member systems of the form M16N2 are also investigated, with results implying stability and potential synthesizability of all compounds except Ti16N2 owing to weak metallic bonding among Ti atoms.

Adsorption of Urea over Transition Metal-Doped Graphene: A DFT Study

This work deploys the density functional theory to study the interactions between urea with pristine and transition metal (TM)-doped two-dimensional (2D) graphene structures. Four TM dopants, namely, V, Ti, Fe, and Cr, are considered in this study. Each of these doped structures has one TM atom replacing one carbon atom from 2D graphene sheets. Our results indicate that TM-doped graphene possesses strong chemical interactions with the urea molecule and, hence, has a better affinity towards urea as compared to pristine 2D graphene sheet. Significant charge transfer has also been observed between TM and O atoms. Moreover, among all the TM-doped structures, the V-doped structure is preferred in terms of adsorption energy. The electronic structure also changes after adsorption, which can be observed through the change in the density of states. Our study suggests that TM-doped graphene has the potential to be a sensing platform for urea.

Enhanced Cu/graphene adhesion by doping with Cr and Ti: A first principles prediction

Abstract We presented a density functional theory study on doping effects of transition metals (Cr and Ti) on the Cu/graphene interface adhesion. Various undoped Cu/graphene interface structures were constructed using both the sandwich and the surface models. Energetics calculations showed that the interface binding strength only weakly depends on interface coordination. Both interface models predicted the top-fcc coordination type as the most energy-favored, with a low binding energy value. Segregated Cr prefers to substituting for Cu, while Ti occupies a hollow site at the interface. Although the segregation tendencies are both very weak, once present on the interface, both dopants can greatly increase the interface binding energy and improve the adhesion.