Structure electronique des oxydes de cobalt CoO et Co3O4

The total density of occupied states in the valence band of CoO and Co₃O₄ is determined by XPS and UPS. From variations of excitation probability of the bands, the 4 e V wide O2_p band is shown to be located around 5 eV for both oxides, while structures obtained from photoionisation of the localized 3d band spread over 10 eV range below the Fermi level overlapping with O2_p band. The 3d peaks located at binding energy <3 eV correspond to the calculated energy of the d_n ?1 manifold final state in the octahedral and tetrahedral crystal field of CoO and Co₃O₄. The 3d levels at higher binding energy are shown to occur from configuration interaction in both final and initial states. These last peaks are higher in intensity for CoO relative to Co₃O₄. A superior limit for the width of the 3d initial band in a one electron energy diagram is given to be <3 eV. This value associated to the Coulomb correlation energy measured equal to ~3 eV. This value associated to the Coulomb correlation energy measured equal to ~3 eV from shake-up and Auger energy confirms the Mott insulator nature of CoO.     

Facile synthesis and optical properties of Co3O4 nanostructures by the microwave route

Cobalt oxide (Co₃O₄) nanoplatelet shape like nanostructures have been successfully synthesized through a simple microwave route for the first time using cobalt acetate, NaOH and citric acid at 200 °C for 30 min. The structure and morphology of as-prepared Co₃O₄ nanoplatelets are characterized by means of powder X-ray diffraction (XRD), Fourier transform infrared spectrum (FTIR), and scanning electron microscope (SEM). XRD measurements indicate that the product has a perfect crystalline cubic phase of Co₃O₄ with a lattice constant a=8.082 Å. The SEM images show that the obtained Co₃O₄ nanopowder consists of nanoplatelets with diameter 125 nm and thickness 20 nm. Energy-dispersive X-ray spectroscopy (EDS) show that the composition of Co₃O₄ is stoichiometric. Room temperature photoluminescence measurement is exhibited by a strong UV emission and a suppressed green emission, confirming the good optical properties for the as-prepared Co₃O₄ nanoplatelets.     

Emission Mössbauer spectroscopy in the Co-O system under high oxygen pressure

Results obtained by means of the emission Mössbauer spectroscopy in CoO and Co₃O₄ systems kept under relatively high oxygen pressure (air close to the normal pressure) are reported. Broadening of the 14.4-keV ⁵⁷Fe Mössbauer line due to the diffusion of iron atoms in CoO has been investigated in the temperature range between 1275 and 1450 K. The excess of the linewidth caused by diffusive motions obeys the Arrhenius law with the activation energy of 1.86(8) eV. Mixed valence oxide Co₃O₄ was examined vs. temperature. Measurements were performed from the room temperature till 1073 K. Tetrahedrally coordinated iron impurities in the Co₃O₄ host lattice are in the high spin trivalent state despite that parent Co is divalent. The anharmonic contribution to the lattice vibrations at high temperatures was observed and analyzed for both CoO and Co₃O₄.     

Synthesis and characterization of Co3O4 nanorods by thermal decomposition of cobalt oxalate

Cobalt oxalate was used as a precursor to prepare Co₃O₄ nanorods by thermal decomposition. The combinations of triphenylphosphine and oleylamine were added as surfactants to control the morphology of the particles. The products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared (FT-IR) spectroscopy and X-ray photoelectron spectroscopy (XPS). The diameters of Co₃O₄ nanorods are 20 nm and the average lengths are around 500 nm. The hysteresis loops of the obtained samples reveal the ferromagnetic behaviors, the enhanced coercivity (H_c) and decreased saturation magnetization (M_s) in contrast to their respective bulk materials. The study provides a simple and efficient route to synthesize Co₃O₄ nanorods at low temperature.     

Cobalt terephthalate MOF-templated synthesis of porous nano-crystalline Co3O4 by the new indirect solid state thermolysis as cathode material of asymmetric supercapacitor

In this work, two different types of Co₃O₄ nano-crystals were synthesized by (i) conventional direct solid state thermolysis of cobalt terephthalate metal-organic framework (MOF-71) and (ii) new indirect solid state thermolysis of Co(OH)₂ derived by alkaline aqueous treatment of MOF-71. The products were then characterized by X-ray diffraction technique (XRD), Fourier transforms infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), Reflection electron energy loss spectroscopy (REELS), Brunauer, Emmett, and Teller (BET), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) techniques. By REELS analysis the energy band gap of MOF-71 was determined to be 3.7 eV. Further, electrochemical performance of each Co₃O₄ nanostructure was studied by the cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS) in a three-electrode system in KOH electrolyte. An asymmetric supercapacitor was fabricated using indirect Co₃O₄ nanoparticles as cathode and electrochemically reduced graphene oxide as anode, and the electrochemical properties were studied and showed a high energy density of 13.51 Wh kg⁻¹ along with a power density of 9775 W kg⁻¹ and good cycling stability with capacitance retention rate of 85% after 2000 cycles.     

Synthesis and magnetic properties of antiferromagnetic Co3O4 nanoparticles

Antiferromagnetic Co₃O₄ nanoparticles with diameter around 30 nm have been synthesized by a solution-based method. The phase identification by the wide-angle X-ray powder diffraction indicates that the Co₃O₄ nanoparticle has a cubic spinel structure with a lattice constant of 0.80843(2) nm. The image of field emission scanning electron microscope shows that the nanoparticles are assembled together to form nanorods. The magnetic properties of Co₃O₄ fine particles have been measured by a superconducting quantum interference device magnetometer. A deviation of the Néel temperature from the bulk is observed, which can be well described by the theory of finite-size scaling. An enhanced coercivity as well as a loop shift are observed in the field-cooled hysteresis loop. The exchange bias field decreases with increasing temperature and diminishes at the Néel temperature. The training effect and the opening of the loop reveal the existence of the spin-glass-like surface spins.     

Electronic structure studies of the spinel CoFe2O4 by X-ray photoelectron spectroscopy

The spinel CoFe₂O₄ has been synthesized by combustion reaction technique. X-ray photoelectron spectroscopy shows that samples are near-stoichiometric, and that the specimen surface both in the powder and bulk sample is most typically represented by the formula (Co_0.4Fe_0.6)[Co_0.6Fe_1.4]O₄, where cations in parentheses occupy tetrahedral sites and those within square brackets in octahedral sites. The results demonstrate that most of the iron ions are trivalent, but some Fe²⁺ may be present in the powder sample. The Co 2p_3/2 peak in powder sample composed three peaks with relative intensity of 45%, 40% and 15%, attributes to Co²⁺ in octahedral sites, tetrahedral sites and Co³⁺ in octahedral sites. The O 1s spectrum of the bulk sample is composed of two peaks: the main lattice peak at 529.90 eV, and a component at 531.53 eV, which is believed to be intrinsic to the sample surface. However, the vanishing of the O 1s shoulder peak of the powder specimen shows significant signs of decomposition.     

Microstructural properties and local atomic structures of cobalt oxide nanoparticles synthesised by mechanical ball-milling process

In this study, facile preparation of pure and nano-sized cobalt oxides particles was achieved using low-cost mechanical ball-milling synthesis route. Microstructural and morphological properties of synthesised products were characterised by X-ray diffraction (XRD) and transmission electron microscopy (TEM) techniques. XRD results indicated that the fabricated samples composed of cubic pure phase CoO and Co₃O₄ nanocrystalline particles with an average crystallite size of 37.2 and 31.8 nm, respectively. TEM images showed that the resulting samples consisted of agglomerates of particles with average diameter of about 37.6 nm for CoO and 31.9 nm for Co₃O₄. Phase purity of the prepared samples was further investigated due to their promising technological applications. Local atomic structure properties of the prepared nanoparticles were probed using synchrotron radiation-based X-ray absorption spectroscopy (XAS) including X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS). EXAFS data analysis further confirmed the formation of single-phase CoO and Co₃O₄ nanoparticles. In addition, structural properties of cobalt oxide nanoparticles were investigated by performing density functional theory calculations at B3LYP/TZVP level and Born–Oppenheimer molecular dynamics. Theoretical calculations for both prepared samples were found to be consistent with the experimental results derived from EXAFS analysis. Obtained results herein reveals that highly crystalline and pure phase CoO and Co₃O₄ nanoparticles can be synthesised using simple, inexpensive and eco-friendly ball-milling method for renewable energy applications involving fuel cells and water splitting devices.     

Synthesis and optical properties of single crystalline GaN nanorods with a rectangular cross-section

GaN nanorods were synthesized from the reaction of a Ga/Ga₂O₃ mixture with NH₃ on Si substrates by chemical vapor deposition. The synthesized products were characterized by scanning and transmission electron microscopy, X-ray diffraction, photoluminescence and Raman spectroscopy. The nanorods are highly single crystalline and possess uniform smooth surfaces. PL revealed only a strong emission at 3.268 eV, ascribed to free exciton (FX) transitions, at room temperature; while the well-known yellow luminescence band centered at 2.2-2.3 eV was not detected. Four first-order phonon modes, corresponding to the A₁(TO), E₁(TO), E₂(high), and A₁(LO) at ∼531, 554, 564, and 721 cm⁻¹, respectively, were observed by Raman backscattering. The red-shift of the FX emission peak and the down-shifts of the Raman modes by a few wave numbers are attributed to the presence of tensile strain inside GaN nanorods.     

Growth of β-Ga2O3 nanorods by ammoniating Ga2O3/V thin films on Si substrate

This paper reports that/3-Ga2O3 nanorods have been synthesized by ammoniating Ga2O3 films on a V middle layer deposited on Si（111） substrates. The synthesized nanorods were confirmed as monoclinic Ga2O3 by x-ray diffraction,Fourier transform infrared spectra. Scanning electron microscopy and transmission electron microscopy reveal that the grown β-Ga2O3 nanorods have a smooth and clean surface with diameters ranging from 100 nm to 200 nm and lengths typically up to 2μm. High resolution TEM and selected-area electron diffraction shows that the nanorods are pure monoclinic Ga2O3 single crystal. The photoluminescence spectrum indicates that the Ga2O3 nanorods have a good emission property. The growth mechanism is discussed briefly.     

Charge-discharge characteristics of nanocrystalline Co3O4 powders via aerosol flame synthesis

Nanocrystalline Co₃O₄ powders were synthesized by aerosol flame synthesis (AFS) method for the anode of lithium ion batteries and the basic electrochemical properties were investigated. The effects of synthesis conditions and heat-treatment temperature on the morphology, crystallite size and electrochemical properties were investigated. As-prepared soot contained Co₃O₄, CoO and Co(OH)₂, which were eventually converted into cubic spinel Co₃O₄ by post heat treatment. The as-prepared particle size was in the range of 10-30 nm and grew to 50-85 nm by the heat treatment. With growing particle size and improved crystallinity, charge-discharge capacity and cycle performance were improved and the discharge capacity of the powder heat-treated at 700 °C was 571 mAh/g after 30 cycles, which was better than Co₃O₄ powder reported in the previous literature.     

Electron energy loss spectra and X-ray photoelectron spectra of Ca2V2O7

We have measured the electron energy loss spectra of Ca₂V₂O₇ in the reflexion mode, at incident energies between 200 and 2400 eV, and the X-ray photoelectron spectra excited by Al K α radiation. The abundant loss structures observed can be correlated with the possible interband transitions, collective oscillations, and excitation of O2s and V3p electrons within the V₂O₇^4- ion. The gap width and molecular orbital (MO) spread (or splitting) is about l eV larger in the V₂O₇^4- ion than in its component VO₄^3- ion. Excitation of O2s states, which may occur together with some MO over-gap transitions, displaces the collective oscillations about 7 eV towards lower energies. Deeper V3p electrons are excited with a maximum energy loss some 7 eV above their binding energy. Cross transitions from Ca3p levels into some empty states of the V₂O₇^4- ion, or direct transitions to available states of the Ca²⁺ ion could not be unambiguously identified. The energy dependence of the excitation cross section and of the electron penetration depth results in a significant variation of the relative intensity of various losses over the investigated energy range.     

Electrochemical performances of nano-Co3O4 with different morphologies as anode materials for Li-ion batteries

Three kinds of Co₃O₄ nanomaterials with different morphologies were synthesized controllably by a post-anneal-assisted hydrothermal method in this study. X-ray diffraction and scanning electron microscopy indicated that all three kinds of samples were pure cubic phase of Co₃O₄ with morphologies of nanorods, nanoclusters, and nanoplates. Moreover, the transmission electron microscopy (TEM) and high-resolution TEM showed that the Co₃O₄ nanorods were bamboo-like and highly crystalline structures. When these materials were applied to the lithium-ion batteries (LIBs) as anode materials, the Co₃O₄ of nanorods demonstrated the best performance. It has a stable reversible capacity of 954 mAh g^?1 as the anode of a LIB, much higher than the other two kinds of Co₃O₄ of rod-like nanoclusters and nanoplates, even after 35 cycles. All results showed that the morphology and microstructure take very important roles in the performance of Co₃O₄ as the anode materials in LIBs.     

Optical spectroscopy of Ca3Sc2Si3O12, Ca3Y2Si3O12 and Ca3Lu2Si3O12 doped with Pr

The silicates Ca₃Sc₂Si₃O₁₂, Ca₃Y₂Si₃O₁₂ and Ca₃Lu₂Si₃O₁₂, both undoped and doped with Pr³⁺ ions, have been synthesized by solid-state reaction at high temperature. The luminescence spectroscopy and the excited state dynamics of the materials have been studied upon VUV and X-ray excitation using synchrotron radiation. All doped samples have shown efficient 5d-4f emission upon direct VUV excitation of 5d levels, but only Ca₃Sc₂Si₃O₁₂:Pr³⁺ shows luminescence upon interband VUV or X-ray excitation. The VUV excited emission spectra of Ca₃Y₂Si₃O₁₂:Pr³⁺ and Ca₃Lu₂Si₃O₁₂:Pr³⁺ show features attributed to emission from two distinct sites accommodating the Pr³⁺ dopant. The decay kinetics of the Pr³⁺ 5d-4f emission in Ca₃Sc₂Si₃O₁₂:Pr³⁺ upon VUV excitation across the band gap are characterized by decay times in the range 25-28 ns with no significant rise after the excitation pulse. They appear to be faster upon X-ray irradiation than for VUV excitation. Weak afterglow components are attributed to defect luminescence.     

Tm2O3相对于Si的能带偏移研究

利用分子束外延系统在Si (001) 衬底上制备了单晶Tm₂O₃薄膜, 利用X射线光电子能谱研究了Tm₂O₃相对于Si的能带偏移. 得出Tm₂O₃相对于Si的价带和导带偏移分别为3.1 eV± 0.2 eV和1.9 eV± 0.3 eV, 并得出了Tm₂O₃的禁带宽度为6.1 eV± 0.2 eV. 研究结果表明Tm₂O₃是一种很有前途的高k栅介质候选材料. 关键词： 2O₃')" href="#">Tm₂O₃ X射线光电子能谱 能带偏移     

Hydroxyethyl cellulose-assisted hydrothermal synthesis of Bi2S3 urchin-like colonies

Orthorhombic Bi₂S₃ with different morphologies was successfully synthesized by the acid-catalyst hydrothermal reactions of bismuth nitrate (Bi(NO₃)₃) and thiourea (NH₂CSNH₂) solutions containing different amounts of hydroxyethyl cellulose (HEC). Phase, morphologies, and optical properties were characterized by X-ray diffraction, selected area electron diffraction, scanning and transmission electron microscopy, and ultraviolet-visible spectroscopy. The products, hydrothermally synthesized in the HEC-free, 0.25 g HEC-added, 0.5 g HEC-added and 1.00 g HEC-added solutions, were respectively proved to be orthorhombic Bi₂S₃ irregular nanorods, complete urchin-like colonies of regular nanorods, incomplete urchin-like colonies of regular nanorods, and highly crystalline regular nanorods growing along the [001] direction. Tauc band gaps of the orthorhombic Bi₂S₃ nanorods, synthesized in the HEC-free, 0.25 g HEC-added, and 1.00 g HEC-added solutions were determined to be 3.0, 1.75 and 1.8 eV, respectively. Formation mechanism of orthorhombic Bi₂S₃ nanorods, synthesized in the HEC-free and HEC-added solutions, was also discussed at great detail.     

Fuchs-Kliewer phonon spectrum of Co3O4(110) single crystal surfaces by high resolution electron energy loss spectroscopy

The Fuchs-Kliewer phonon spectrum of single crystal Co₃O₄(110) has been analyzed by high resolution electron energy loss spectroscopy (HREELS) and the four fundamental phonon losses have been identified at 26.8, 47.5, 71.1 and 84.7 meV (216, 383, 573 and 683 cm⁻¹). This is the first HREELS study reported for an intrinsic spinel single-crystal surface with primary focus on the Fuchs-Kliewer phonon structure. The Co₃O₄ crystal is first characterized by Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS) and low-energy electron diffraction (LEED), which establish the composition, cleanliness, and order of the (110) surface. Electron scattering is then used to obtain a series of well-resolved Fuchs-Kliewer phonon spectra over 2.25-14.25 eV incident electron energy range. The variation in phonon intensity with primary beam energy is shown to agree with that predicted by dielectric theory.     

Cobalt ion as a structural probe in PbO–As2O3 glass ceramics

PbO–As₂O₃ glasses mixed with different concentrations of CoO (ranging from 0 to 1.0 mol%) were crystallized. The samples were characterized by X-ray diffraction, scanning electron microscopy, EDS and differential scanning calorimetric techniques. The X-ray diffraction studies have indicated the presence of Pb(As₂O₆), Pb₃(AsO₄)₂, Co₆As₂O₁₁, Co₃O₄ crystalline phases in these samples. Optical absorption, IR and photoluminescence studies of these samples have been carried out. The analysis of the results of these studies has indicated that the cobalt ions exist in Co²⁺ and Co³⁺ states in the glass matrix. The studies have further revealed that as the concentration of the CoO is increased, there is a gradual transformation of cobalt ions from tetrahedral to octahedral positions.     

VUV spectroscopic properties of Ce and Pr-doped AREP2O7-type alkali rare earth diphosphates (A=Na, K, Rb, Cs; RE=Y, Lu)

Spectroscopic properties of Ce³⁺ and Pr³⁺-doped AREP₂O₇-type alkali rare earth diphosphates (A=Na, K, Rb, Cs; RE=Y, Lu) have been investigated using VUV spectroscopy technique. Ce³⁺-doped samples show typical Ce³⁺ emission in the range of 325-450 nm. The strong host absorption band starting at around 160 nm indicates that the optical band gap of AREP₂O₇ hosts is at least 7.7 eV, and the host→Ce³⁺ energy transfer process is rather efficient. However, AREP₂O₇:Pr³⁺ samples show less efficient host→Pr³⁺ energy transfer. The direct Pr³⁺ 4f²→4f¹5d¹ excitation, which are 12160±640 cm⁻¹ higher respect to that of Ce³⁺, leads to strong 4f¹5d¹→4f² emission bands in the range of 230-325 nm but no obvious 4f²→4f² emission lines.     

Synthesis and characterization of monodisperse spherical core-shell structured SiO2@Y3Al5O12:Ce3+/Tb3+ phosphors for field emission displays

Nanocrystalline Y₃Al₅O₁₂: Ce³⁺/Tb³⁺ (average crystalline size 30 nm) phosphor layers were coated on non-aggregated, monodisperse and spherical SiO₂ particles by the sol-gel method, resulting in the formation of core-shell structured SiO₂@Y₃Al₅O₁₂:Ce³⁺/Tb³⁺ particles. X-ray diffraction, Fourier transform infrared spectroscopy, transmission electron microscopy, photoluminescence, cathodoluminescence spectra, as well as lifetimes were utilized to characterize the core-shell structured SiO₂@Y₃Al₅O₁₂:Ce³⁺/Tb³⁺ phosphor particles. The obtained core-shell structured phosphors consist of well-dispersed submicron spherical particles with a narrow size distribution. The thickness of the Y₃Al₅O₁₂:Ce³⁺/Tb³⁺ shells on the SiO₂ cores (average size about 500 nm, crystalline size about 30 nm) could be easily tailored by varying the number of deposition cycles (100 nm for four deposition cycles). Under the excitation of ultraviolet and low-voltage electron beams (1–3 kV), the core-shell SiO₂@Y₃Al₅O₁₂:Ce³⁺/Tb³⁺ particles show strong yellow-green and green emission corresponding to the 5d–4f emission of Ce³⁺ and ⁵D₄–⁷F _J (J = 6, 5, 4, 3) emission of Tb³⁺, respectively. These phosphors may have potential application in field emission displays.