Unpaired 3d Electrons Research Articles

Anomalous Effects of Lattice Strain and Ti3+ Interstitials on Room-Temperature Magnetic Ordering in Defect-Engineered Nano-TiO2

Defect engineering in n-type undoped metal oxides is a great challenge compared to the often-studied surface oxygen vacancies. The present investigation unravels new insights toward defect chemistry and defect engineering in anatase TiO2 nanoparticles. It is demonstrated that using the rapid cooling process, high concentration of Ti3+ interstitials, and lattice oxygen vacancies can be easily introduced in undoped metal oxides. The structural disorders in anatase TiO2 nanoparticles synthesized under two different argon-annealing processes have been comprehensively investigated using spectroscopy and electron microscopic analysis. Though excess of interstitial Ti3+ ions with one unpaired 3d electron in quenched TiO2 introduces local magnetic moments, they could be antiferromagnetically coupled via lattice Ti4+ ions, which limit the overall magnetic moment of the quenched materials. Lattice contraction was also found to enhance the ferromagnetic coupling between the defect complexes (Ti3+–F+ center) which he...

Ultrathin Cu-TCPP MOF nanosheets: a new theragnostic nanoplatform with magnetic resonance/near-infrared thermal imaging for synergistic phototherapy of cancers.

Nanostructures based on metal-organic frameworks (MOFs) have promising potential as theragnostic nanoplatforms for phototherapy of cancer cells. However, the MOFs alone are seldom reported to be used as photothermal agents mainly due to their poor near-infrared (NIR) light absorption.Methods: Ultrathin copper-tetrakis (4-carboxyphenyl) porphyrin (Cu-TCPP) MOF nanosheets were prepared by a facile solvothermal route. The photothermal therapy (PTT), photodynamic therapy (PDT), and T1-weighted magnetic resonance (MR) imaging capabilities and the high biocompatibility of these composite nanosheets were evaluated in vitro as well as in vivo in a mouse tumor model.Results: The ultrathin Cu-TCPP MOF nanosheets exhibited 1) strong NIR absorption because of the d-d energy band transition of Cu2+ and the ultrathin characteristic translating into excellent photothermal performance, 2) ability to produce singlet oxygen because of the inherent characteristic of TCPP, and 3) capability for MR imaging because of the unpaired 3d electrons of copper.Conclusion: Our study demonstrated that the Cu-TCPP MOF nanosheets are a promising phototherapy nanoplatform with the synergistic ability for PTT and PDT of cancer, guided by MR and infrared thermal imaging.

Origin of dielectric loss in Ba(Co1/3Nb2/3)O3 microwave ceramics

AbstractThe microwave dielectric loss of stoichiometric and non‐stoichiometric Ba(Co1/3Nb2/3)O3 ceramics have been measured from 2 to 300 K in magnetic fields ranging from 0 to 5 T using a dielectric resonator (DR) technique. The microwave absorption from spin excitations of unpaired d‐electrons in exchange coupled Co2+ ions dominate the loss of the Ba(Co1/3Nb2/3)O3 ceramics at cryogenic temperatures. Two peaks in the loss tangent (tan δ) vs temperature relation from a distinctly different origin occur at 25‐30 K and 90 K, which increase in magnitude with increasing Co content in the bulk dielectric samples. Evidence that these peaks result from polaron conduction from hopping between Co2+ and Co3+ ions includes (i) the peak's observed temperature range; (ii) the decrease in peak intensity of approximately a factor of two in a large applied magnetic fields (5 T); and (iii) a strong correlation between the peak's magnitude and both the fraction of the minority Co3+ in the dominant Co2+ matrix and D.C. conductivity at elevated temperatures. A magnetic‐field independent high temperature peak with a maximum at 250 K dominates the room temperature microwave loss whose magnitude correlates with those of the low temperature peaks. This suggests that the defects responsible for carrier conduction play an important role in establishing the loss tangent at room temperature.

Manganese mono-boride, an inexpensive room temperature ferromagnetic hard material

We synthesized orthorhombic FeB-type MnB (space group: Pnma) with high pressure and high temperature method. MnB is a promising soft magnetic material, which is ferromagnetic with Curie temperature as high as 546.3 K, and high magnetization value up to 155.5 emu/g, and comparatively low coercive field. The strong room temperature ferromagnetic properties stem from the positive exchange-correlation between manganese atoms and the large number of unpaired Mn 3d electrons. The asymptotic Vickers hardness (AVH) is 15.7 GPa which is far higher than that of traditional ferromagnetic materials. The high hardness is ascribed to the zigzag boron chains running through manganese lattice, as unraveled by X-ray photoelectron spectroscopy result and first principle calculations. This exploration opens a new class of materials with the integration of superior mechanical properties, lower cost, electrical conductivity, and fantastic soft magnetic properties which will be significant for scientific research and industrial application as advanced structural and functional materials.

Ultrasmall CuCo2S4 Nanocrystals: All‐in‐One Theragnosis Nanoplatform with Magnetic Resonance/Near‐Infrared Imaging for Efficiently Photothermal Therapy of Tumors

Copper‐based ternary bimetal chalcogenides have very promising potential as multifunctional theragnosis nanoplatform for photothermal treatment of tumors. However, the design and synthesis of such an effective platform remains challenging. In this study, hydrophilic CuCo2S4 nanocrystals (NCs) with a desirable size of ≈10 nm are synthesized by a simple one‐pot hydrothermal route. The as‐prepared ultrasmall CuCo2S4 NCs show: 1) intense near‐infrared absorption, which is attributed to 3d electronic transitions from the valence band to an intermediate band, as identified by density functional theory calculations; 2) high photothermal performance with a photothermal conversion efficiency up to 73.4%; and 3) capability for magnetic resonance (MR) imaging, as a result of the unpaired 3d electrons of cobalt. Finally, it is demonstrated that the CuCo2S4 NCs are a promising “all‐in‐one” photothermal theragnosis nanoplatform for photothermal cancer therapy under the irradiation of a 915 nm laser at a safe power density of 0.5 W cm−2, guided by MR and infrared thermal imaging. This work further promotes the potential applications of ternary bimetal chalcogenides for photothermal theragnosis therapy.

Computational design of ZnP(P)4 stacks: Three modes of binding

A large variety of metalloporphyrin arrays, including stacks, have been synthesized and extensively explored for their numerous applications in molecular devices. Motivated by the phenomenon of formation of stacks by regular metalloporphyrins, we performed the computational check of the stack formation between the MP(P)4 species without any linkers or substituents. For this we chose the ZnP(P)4 species as the simplest MP(P)4 compound. Three modes of binding or coordination were found to be possible between the monomeric ZnP(P)4 units. The “convexity-to-convexity” dimer I generally is the most stable compound with the highest binding energy. The dimers II and III are generally bound significantly weaker than the “convexity-to-convexity” dimer I. In the dimer I, the strongly convex shape of both monomer units was demonstrated. The Zn–Zn distances in the dimer I, ca. 3.5[Formula: see text]Å, were computed to be significantly shorter than in two other dimers. This could lead to additional interactions between the metal centers with unpaired d-electrons involved in the formation of such a dimer. In the dimer I significant decrease of the charge was found on the Zn-centers, along with slight decrease of the positive charge on the P-centers coordinated to the Zn-centers of another monomer, and slight buildup of the positive charge on the P-centers not coordinated to the Zn-centers of another monomer.

Origin of d‐π Interaction in Cobalt(II) Porphyrins under Synergistic Effects of Core Contraction and Axial Ligation: Implications for a Ligand Effect of Natural Distorted Tetrapyrrole

AbstractThe reactivity of the metalloporphyrins was closely related to their ligand effect at axial position. The electronic properties of six model Co(II) porphyrins are investigated by spectral and electrochemical methods. Structural parameters of the Co(II) complexes are directly obtained from their crystal structures. We demonstrate that the unpaired 3d electron of low‐spin Co(II) ions in nonplanar Co(II) porphyrin complexes activated by core contraction of porphyrin macrocycles can be further activated by the axial ligation of imidazole. The activated electron can combine with a π orbital of the porphyrin ring to form a new d‐π orbital, which can induce the Q‐band of Co(II) porphyrins to visibly split. Addition of imidazole causes the Co(II)/Co(III) and Co(II)/Co(I) reactions to shift to more negative potential. Our results indicate that strong axial ligation and core contraction both play important roles in electron transfer in redox catalysis involving Co(II) complexes.

Mechanistic insights into regioselective polymerization of 1,3-Dienes catalyzed by a bipyridine-ligated iron complex: A DFT study

The regioselective polymerizations of isoprene and 3-methyl-pentadiene catalyzed by a cationic iron (II) complex bearing bipyridine ligand have been computationally studied. Having achieved an agreement between calculation and experiment, it is found that the open-shell unpaired 3d-electrons localize on Fe center rather than partially distribute on the redox-active bipyridine ligand. The steric effect plays a more important role in controlling the regioselectivity in comparison with electronic factors. The deformation energy is mainly contributed by monomer and Fe-alkyl moieties rather than the bipyridine ligands themselves, although noncyclopentadienyl ancillary ligands are often deformed in most insertion transition states for selective polymerization of olefin. © 2016 Wiley Periodicals, Inc.

The new Zintl phases Eu21Cd4Sb18 and Eu21Mn4Sb18

Crystals of two new Zintl compounds, Eu21Mn4Sb18 and Eu21Cd4Sb18 have been synthesized using the molten metal flux method, and their structures have been established by single-crystal X-ray diffraction. Both compounds are isotypic and crystallize in the monoclinic space group C2/m (No. 12, Z=4). The structures are based on edge- and corner-shared MnSb4 or CdSb4 tetrahedra, which make octameric [Mn8Sb22] or [Cd8Sb22] polyanions. Homoatomic Sb–Sb bonds are present in both structures. The Eu atoms take the role of Eu2+cations with seven unpaired 4f electrons, as suggested by the temperature-dependent magnetization measurements. The magnetic susceptibilities of Eu21Mn4Sb18 and Eu21Cd4Sb18 indicate that both phases order anti-ferromagnetically with Néel temperatures of ca. 7K and ca. 10K, respectively. The unpaired 3d electrons of the Mn atoms in Eu21Mn4Sb18 do contribute to the magnetic response, however, the bulk magnetization measurements do not provide evidence for long-range ordering of the Mn spins down to 5K. Electrical resistivity measurements suggest that both compounds are narrow band gap semiconductors.

Optimization of the spin entropy by incorporating magnetic ion in a misfit-layered calcium cobaltite

Effects of (Lu, Ni) co-doping on spin entropy of Ca3Co4O9+δ have been investigated. Results of this study show that (Lu, Ni) co-doping can be more effective than single Lu doping for optimizing the spin entropy of Ca3Co4O9+δ. X-ray photo-emission spectroscopy confirms a reduction in Co4+ concentration, which leads to enhanced spin entropy from Co sites. The magnetic ions of Ni with unpaired 3d electrons can produce extra spin entropy through a new hopping model, thereby contributing to an increase in total spin entropy. Incorporating magnetic ions is an effective way to optimize the spin entropy of thermoelectric materials. This study opens a new way to broaden and design prospective thermoelectric materials.

Heavily Cr3+-modified Li4Ti5O12: An advanced anode material for rechargeable lithium-ion batteries

Heavily Cr[Formula: see text]-modified Li4Ti5O[Formula: see text] powders with a designed nominal composition of Li3Cr7Ti2O[Formula: see text] have been prepared by one-step solid-state reaction. X-ray diffraction (XRD) combined with Rietveld refinement indicates that these powders contain 96.5[Formula: see text]wt.% spinel Li[Formula: see text]Cr[Formula: see text]Ti[Formula: see text]O4 and 3.5[Formula: see text]wt.% Cr2O3. Due to the combination of Ti[Formula: see text]/Ti[Formula: see text] and Cr[Formula: see text]/Cr[Formula: see text] redox couples in Li[Formula: see text]Cr[Formula: see text]Ti[Formula: see text]O4 and the existence of Cr2O3, the composite exhibits a large first-cycle discharge capacity of 315[Formula: see text]mAh[Formula: see text] at a small current density of 62.5[Formula: see text]mA[Formula: see text]. Li[Formula: see text]Cr[Formula: see text]Ti[Formula: see text]O4 shows an improved Li[Formula: see text] ion diffusion coefficient and electronic conductivity, respectively arising from the small O[Formula: see text] ion fractional coefficient and unpaired 3d electrons in Cr[Formula: see text] ions. The majority of Cr2O3 is reduced to Cr after the first two lithiation processes, which benefits the electrical conduction between the Li[Formula: see text]Cr[Formula: see text]Ti[Formula: see text]O4 particles. Consequently, the composite exhibits a good rate performance and cyclability. Its capacity at 1000[Formula: see text]mA[Formula: see text] is as large as 141[Formula: see text]mAh[Formula: see text] with large retention of 90.1% after 100 cycles.

Mechanism of defect induced ferromagnetism in undoped and Cr doped TiO2 nanorods/nanoribbons

We have studied the effect of doping concentrations, growth temperature and calcinations on the structural, optical and magnetic properties of the undoped and Cr doped TiO2 nanorods/nanoribbons (NR/NRb), in order to develop an improved understanding on the mechanism of room temperature (RT) ferromagnetism (FM) in these nanostructures. Both undoped and doped TiO2 NR/NRb exhibit RT FM and a ∼2.6 fold enhancement in magnetization is observed in 0.3% Cr doped TiO2 NR/NRb as compared to the undoped NR/NRb, and the magnetization increases considerably after vacuum annealing. However, no measureable FM is observed for the precursor TiO2 powder, despite the presence of high concentration of oxygen vacancies in it. On the other hand, the magnetization decreases at higher doping concentration (0.7% Cr) as compared to 0.3% Cr doped sample. Thus, our studies revealed that a simple oxygen vacancy and/or Cr ions alone cannot yield the RT FM in TiO2 nanostructures. We argue that the oxygen vacancy with appropriate charge redistribution and their orbital overlapping, and exchange interaction with the nearby unpaired 3d electron of Ti3+ in undoped TiO2 and/or unpaired 3d electrons of Cr3+ dopant with proper charge distribution and occupation inside the host lattice in Cr doped TiO2 nanostructures play the pivotal role for the ferromagnetic ordering and observed RTFM. The presence of oxygen vacancy related F+-center and Cr3+ are confirmed from the electron spin resonance and x-ray photoelectron spectroscopy measurements.

Charge distribution and hyperfine interactions in the vicinity of impurity sites in In2O3 doped with Fe, Co, and Ni

In this paper, first-principles calculations based on density functional theory (DFT) were used to determine TM (TM=Fe, Ni, Co) and Cd impurity locations in the In2O3 host structure, their charge states, the electronic and structural relaxations induced in the host lattice as well as to interpret previous and supplementary experimental results of hyperfine interactions. Different techniques were carried out to characterize TM-doped In2O3 bulk samples prepared by the sol–gel method starting from very pure metals. Perturbed angular correlation (PAC) spectroscopy, a sensitive nuclear technique capable of measuring interactions from electronic charge and spins within an atomic distance, was used to experimentally determine hyperfine interactions at cation sites of In2O3 doped with Co and Ni using In111→Cd111 as probe nuclei. Room temperature results of magnetization measurements in In2O3 doped with Fe, Co and Ni show ferromagnetic ordering coexisting with a paramagnetic behavior for all samples. Results of PAC spectroscopy and DFT calculations show that TM atoms locate as second nearest neighbors of Cd probes preferentially occupy symmetric sites of the doped In2O3 crystal structure with lattice parameters slightly different from that of pure In2O3. Moreover, while a major population of 111Cd probes observes almost the same hyperfine interactions measured for pure In2O3, a small population detects magnetic dipole interactions with magnetic hyperfine field at Cd probes of 2.6T, 3.1T, and 4.6T, respectively for Ni, Co, and Fe doping presenting an almost linear dependence on the number of unpaired 3d electrons of the transition metal impurity.

Four- and five-coordinate high-spin iron(II) complexes bearing bidentate soft/hard SN ligands based on 2-aminopyridine

Several new coordinatively unsaturated iron(II) complexes of the types [Fe(SNR-Ph)X2] (X=Cl, Br; R=H, Me, Et) and the dimeric complex [Fe(SNH–Ph)(μ-Cl)Cl]2 containing bidentate N-(2-pyridyl)amino-diphenylphosphine sulfides were prepared and characterized by various methods including X-ray crystallography, 57Fe Mössbauer spectroscopy, SQUID magnetometry, ESI-MS and DFT calculations. All complexes exhibit magnetic moments very close to 4.9μB reflecting the four unpaired d-electrons. In solution, as revealed by ESI MS studies, all compounds appear to be labile and complexes [Fe(SNR-Ph)X2] seem to be in equilibrium with solvated FeX2, free ligand, and the five-coordinate cationic complexes [Fe(SNR-Ph)2X]+. The latter are the most prominent species in the ESI mass spectra. Despite their relatively low formal electron count all iron complexes were inert towards the addition of carbon monoxide.

Defect types and room temperature ferromagnetism in N-doped rutile TiO2 single crystals

The magnetic properties and defect types of virgin and N-doped TiO2 single crystals are probed by superconducting quantum interference device (SQUID), X-ray photoelectron spectroscopy (XPS), and positron annihilation analysis (PAS). Upon N doping, a twofold enhancement of the saturation magnetization is observed. Apparently, this enhancement is not related to an increase in oxygen vacancy, rather to unpaired 3d electrons in Ti3+, arising from titanium vacancies and the replacement of O with N atoms in the rutile structure. The production of titanium vacancies can enhance the room temperature ferromagnetism (RTFM), and substitution of O with N is the onset of ferromagnetism by inducing relatively strong ferromagnetic ordering.

Defects and hyperfine interactions in binary Fe-Al alloys studied by positron annihilation and Mössbauer spectroscopies

The defects, the behavior of 3d electrons and the hyperfine interactions in binary Fe-Al alloys with different Al contents have been studied by measurements of positron lifetime spectra, coincidence Doppler broadening spectra of positron annihilation radiation and Mössbauer spectra. The results show that on increasing the Al content in Fe-Al alloys, the mean positron lifetime of the alloys increase, while the mean electron density of the alloys decrease. The increase of Al content in binary Fe-Al alloys will decrease the amount of unpaired 3d electrons; as a consequence the probability of positron annihilation with 3d electrons and the hyperfine field decrease rapidly. Mössbauer spectra of binary Fe-Al alloys with Al content less than 25 at.% show discrete sextets and these alloys make a ferromagnetic contribution at room temperature. The Mössbauer spectrum of Fe70Al30 shows a broad singlet. As Al content higher than 40 at.%, the Mössbauer spectra of these alloys are singlet, that is, the alloys are paramagnetic. The behavior of a 3d electron and its effect on the hyperfine field of the binary Fe-Al alloy has been discussed.

Au as an efficient promoter for electrocatalytic oxidation of formic acid and carbon monoxide: a comparison between Pt-on-Au and PtAu alloy catalysts

The absence of unpaired d-electrons of gold leads to its lack of reactivity and paucity of catalytic activity. Synergistic activity of bimetallic PtAu has been proved, and its structure greatly influences on the electrocatalytic activity toward formic acid and carbon monoxide oxidation. Here, a comparison between Pt-modified Au (designated as Pt-on-Au) and PtAu alloy catalysts has been studied. The Pt-on-Au catalyst was prepared by electrodeposition of Pt on the pre-prepared Au, while PtAu alloy was obtained by co-electrodeposition. As a whole, both types of PtAu catalysts were found to be more active toward formic acid electrooxidation compared to pure Pt, exhibiting maximum activity on Pt-on-Au catalyst with Pt to Au atomic ratio of 1:10.22. Moreover, the Pt/Au atomic ratio directly relates to the oxidation pathway of formic acid and carbon monoxide oxidation. The results may be ascribed to much less COads on the surface than single Pt catalyst due to the effect of Au nanoparticles. CO stripping voltammograms present the obvious variation between Pt-on-Au and PtAu alloy catalysts. Meanwhile, the electrocatalytic activities of bimetallic PtAu are evaluated by electrochemical impedance spectroscopy and Tafel analysis.

Defect types and room-temperature ferromagnetism in undoped rutile TiO2 single crystals

Room-temperature ferromagnetism has been experimentally observed in annealed rutile TiO2 single crystals when a magnetic field is applied parallel to the sample plane. By combining X-ray absorption near the edge structure spectrum and positron annihilation lifetime spectroscopy, Ti3+—VO defect complexes (or clusters) have been identified in annealed crystals at a high vacuum. We elucidate that the unpaired 3d electrons in Ti3+ ions provide the observed room-temperature ferromagnetism. In addition, excess oxygen ions in the TiO2 lattice could induce a number of Ti vacancies which obviously increase magnetic moments.

Controlling magnetism of MoS2 sheets by embedding transition-metal atoms and applying strain

Prompted by recent experimental achievement of transition metal (TM) atoms substituted in MoS2 nanostructures during growth or saturating existing vacancies (Sun et al., ACS Nano, 2013, 7, 3506; Deepak et al., J. Am. Chem. Soc., 2007, 129, 12549), we explored, via density functional theory, the magnetic properties of a series of 3d TM atoms substituted in a MoS2 sheet, and found that Mn, Fe, Co, Ni, Cu and Zn substitutions can induce magnetism in the MoS2 sheet. The localizing unpaired 3d electrons of TM atoms respond to the introduction of a magnetic moment. Depending on the species of TM atoms, the substituted MoS2 sheet can be a metal, semiconductor or half-metal. Remarkably, the applied elastic strain can be used to control the strength of the spin-splitting of TM-3d orbitals, leading to an effective manipulation of the magnetism of the TM-substituted MoS2 sheet. We found that the magnetic moment of the Mn- and Fe-substituted MoS2 sheets can monotonously increase with the increase of tensile strain, while the magnetic moment of Co-, Ni-, Cu- and Zn-substituted MoS2 sheets initially increases and then decreases with the increase of tensile strain. An instructive mechanism was proposed to qualitatively explain the variation of magnetism with elastic strain. The finding of the magnetoelastic effect here is technologically important for the fabrication of strain-driven spin devices on MoS2 nanostructures, which allows us to go beyond the current scope limited to the spin devices within graphene and BN-based nanostructures.

The dominance of paramagnetic loss in microwave dielectric ceramics at cryogenic temperatures

Commercial manufacturers add transition metals and other impurities to high performance microwave ceramic compounds to improve their manufacturability. Measurements of microwave loss tangent were performed on Ba(Zn1/3Ta2/3)O3, ZrTiO4-ZnNb2O6, Ba(Zn1/3Nb2/3)O3, and BaTi4O9-BaZn2Ti4O11 ceramics. A marked increase in the loss at low temperatures is found in materials containing transition metal with unpaired d-electrons as a result of resonant spin excitations in isolated atoms (light doping) or exchange coupled clusters (moderate to high doping). The loss tangent can be drastically reduced by applying static magnetic fields. Our measurements also show that this mechanism significantly contributes to room temperature loss, but does not dominate.