X-ray photoelectron spectroscopy of Co3O4, Fe3O4, Mn3O4, and related compounds

The metal 2p region spectra of the mixed valence spinels, Co₃O₄, Fe₃O₄, Mn₃O₄, and related compounds were studied. The satellite splittings of Co 2p_{$\text{3}\text{2}$} for the octahedrally coordinated cobaltous ions are 6.2 eV and those for the tetrahedrally coordinated ones are about 5.3 eV. The Co 2p spectrum for Co₃O₄ is considered to be the sum of spectra of magnetic cobaltous ions and low-spin cobaltic ions. In the cases of Fe₃O₄ and Mn₃O₄, the oxidation states were not clearly distinguished because both the divalent and trivalent ions of iron and manganese are high-spin.     

Fabrication and application of highly ordered mesoporous Co3O4, NiO, and their metals

Highly ordered mesoporous Co₃O₄, NiO, and their metals were synthesized by nanocasting method using there corresponding mesoporous SBA-15 silica as a template. The obtained porous metal oxides have high surface areas, large pore volume, and a narrow pore size distribution. The N₂-adsorption data for mesoporous metal oxides have provided the BET area of 257.7 m² g⁻¹ and the total pore volume of 0.46 cm³ g⁻¹. The mesoporous metals were employed as a catalyst in the synthesis of (S)-3-pyrrolidinol from chiral (S)-4-chloro-3-hydroxybutyronitrile, and a high yield to (S)-3-pyrrolidinol-salt was obtained on the mesoporous Co metal catalyst.     

Non-equilibrium effects in the magnetic behavior of Co3O4 nanoparticles

We report detailed studies of the non-equilibrium magnetic behavior of antiferromagnetic Co₃O₄ nanoparticles. The temperature and field dependence of magnetization, wait time dependence of magnetic relaxation (aging), memory effects, and temperature dependence of specific heat have been investigated to understand the magnetic behavior of these particles. We find that the system shows some features that are characteristic of nanoparticle magnetism such as bifurcation of field-cooled (FC) and zero-field-cooled (ZFC) susceptibilities and a slow relaxation of magnetization. However, strangely, the temperature at which the ZFC magnetization peaks coincides with the bifurcation temperature and does not shift on application of magnetic fields up to 1 kOe, unlike most other nanoparticle systems. Aging effects in these particles are negligible in both FC and ZFC protocols, and memory effects are present only in the FC protocol. We show that Co₃O₄ nanoparticles constitute a unique antiferromagnetic system which enters into a blocked state above the average Néel temperature.     

Optical investigation of charge-transfer transitions in spinel Co3O4

Optical transitions in normal-spinel Co₃O₄ have been identified by investigating the variation of its optical absorption spectrum with the replacement of Co by Zn. Three optical-transition structures were located at about 1.65, 2.4, and 2.8 eV from the measured dielectric function of Co₃O₄ by spectroscopic ellipsometry. The variation of the absorption structures with the Zn substitution (Zn_xCo_3−xO₄) can be explained in terms of charge-transfer transitions involving d states of Co ions. The 1.65 eV structure is assigned to a d-d charge-transfer transition between the t_2g states of octahedral Co³⁺ ion and t₂ states of tetrahedral Co²⁺ ion, t_2g(Co³⁺)→t₂(Co²⁺). The 2.4 and 2.8 eV structures are interpreted as due to charge-transfer transitions involving the p states of O²⁻ ion: p(O²⁻)→t₂(Co²⁺) for the 2.4 eV absorption and p(O²⁻)→e_g(Co³⁺) for the 2.8 eV absorption. The observed gradual reduction of the 1.65 and 2.4 eV absorption strength with the increase of the Zn composition for Zn_xCo_3−xO₄ can be explained in terms of the substitution of the tetrahedral Co²⁺ sites by Zn²⁺ ions. The crystal-field splitting Δ_Oh between the e_g and the t_2g states of the octahedral Co³⁺ ion is estimated to be 2 eV.     

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.     

径向生长的具有分级结构的四氧化三钴微晶及其在气体传感上的应用

通过在水热合成后追加退火处理,制备了径向生长的具有分级结构的树枝状三维Co₃O₄晶体,并用X射线衍射仪、扫描电子显微镜和透射电子显微镜对其结构和形貌进行了表征. 在110 oC对其气体探测性能的研究表明这种Co₃O₄分级结构对氨气有较高的探测灵敏度和响应速度(10 s),性能稳定并具有可重复性. 同时,还在较低的探测温度下对酒精、丙酮和苯进行了气敏探测.     

Influence of organic precursor on the structural and magnetic properties of Co3O4 nanoparticles

Antiferromagnetic Co₃O₄ nanoparticles were synthesized by the coprecipitation method. With the addition of the sucrose as chelating agent (sucrose) the size of the particles was reduced from 54 nm to 19 nm. The Co₃O₄ nanoparticles exhibit a cubic spinel structure identified for X-ray diffraction (XRD) and confirmed by Rietveld refinement. Scanning Electron Microscopy (SEM) images exhibit a spherical-like morphology and confirm the decrease of the particle size observed by XRD. The magnetic measurements as a function of temperature using a superconducting quantum interference device (SQUID) show a large surface anisotropy for samples obtained with the addition of sucrose accompanied by an exchange Bias effect indicating also the existence of a weak ferromagnetism. A decrease of the Néel temperature from the bulk (and other nanostructures-type) was observed, which can be associated with finite-size effect in the nanoparticles' shape.     

Neutron powder diffraction and magnetic studies of mesoporous Co3O4

Samples of mesoporous Co₃O₄, created by using mesoporous silicas KIT-6 and SBA-16 as hard templates to control the growth of Co₃O₄ have been investigated with SQUID magnetometry and neutron powder diffraction, to reveal the effects of high surface area on the magnetic and electronic properties. DC magnetic susceptibility measurements show lower Néel ordering temperatures and lower magnetic moments than in a “bulk” reference. A lower second transition temperature is also observed in the mesoporous samples, associated with the freezing of the surface (shell) magnetic moments. Measurements taken with increasing applied field at constant temperature show the materials to be antiferromagnetic as expected. Complementary parametric neutron powder diffraction studies show similar trends between the two mesoporous samples when looking at their Néel temperatures, and verify long range order within the samples.     

Reflux synthesis of Co3O4 nanoparticles and its magnetic characterization

Co₃O₄ nanoparticles have been prepared for the first time via reflux method, as an alternative low-temperature high-yield process, starting from one single precursor. A plausible mechanism is suggested for the synthetic process. XRD, TEM, FTIR and VSM were used for the structural, morphological, spectroscopic, and magnetic characterization of the product respectively. X-ray diffraction line profile fitting showed that average particle size of the sample is 28 nm. Morphology of the synthesized powder was observed to be thin nanosheets with a thickness of 2-3 nm based on SEM and TEM analyses. Magnetic measurements showed a deviation of the Neel temperature from the bulk value which is attributed to the finite size effects. A loop shift with an enhanced coercivity is observed in the field-cooled hysteresis loops. The opening of the hysteresis loop reveals the existence of the spin-glass like surface spins of the Co₃O₄ nanoparticles.     

Catalytic performance of Mn3O4 and Co3O4 nanocrystals prepared by sonochemical method in epoxidation of styrene and cyclooctene

A simple sonochemical method was developed to synthesis uniform sphere-like Co₃O₄ and Mn₃O₄ nanocrystals. Epoxidation of styrene and cyclooctene by anhydrous tert-butyl hydroperoxide over the prepared Co₃O₄ and Mn₃O₄ nanocatalysts was investigated. The results of conversion activity were compared with bulk Co₃O₄ and Mn₃O₄. Under optimized reaction conditions, the nanocatalysts showed a superior catalytic performance as compared to the bulk catalysts. Powder X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and BET surface area, were used to characterize and investigate the nanocatalysts.     

Comparison of magnetic properties in LixCoO2 and its decomposition products LiCo2O4 and Co3O4

Thermal stability of cathode material in the charged state is an important aspect for the safety of rechargeable batteries. It is well known that layered Li_xCoO₂ decomposes to a mixture of LiCoO₂ and Co₃O₄ at elevated temperatures. However, not many experimental evidences exist on intermediate phases those may form during the decomposition. Using magnetic measurements we show that it is possible to distinguish between the spinels LiCo₂O₄ and Co₃O₄ and thereby follow the decomposition of Li_xCoO₂. We characterize the magnetic behavior of thermally aged Li_xCoO₂ (x = 0.98, 0.76, 0.55) with increasing annealing time. Our results reveal the appearance of magnetic ordering in the thermally degraded products. The detailed analysis illustrates that the formation of Co₃O₄ is preceded by the formation of a meta stable LiCo₂O₄ phase.     

Preparation, Morphology Transformation and Magnetic Behavior of Co3O4 Nano-Leaves

A series of cubic phase Co3O4 nano-leaves were prepared via a combined approach of solution reaction and calcination. According to x-ray diffraction and electron microscopy, we find that the Co3 04 grain size increases with calcination temperature. This can induce many gaps in the products. M-T and M-H magnetization measurements reveal the typical antiferromagnetie behavior of nano-leaves. The effective moments of the samples prepared at 300, 400 and 500℃ are 5.6, 5.8 and 5.7μB per formula unit （FU）, respectively, larger than the bulk value of 4.14 μB/FU.     

A novel strategy to synthesize cobalt hydroxide and Co3O4 nanowires

Cobalt hydroxide ultra fine nanowires were prepared by a facile hydrothermal route using hydrogen peroxide. This method provides a simple, low cost, and large-scale route to produce β-cobalt hydroxide nanowires with an average diameter of 5 nm and a length of ca. 10 μm, which show a predominant well-crystalline hexagonal brucite-like phase. Their thermal decomposition produced highly uniform nanowires of cobalt oxide (Co₃O₄) under temperature 500 °C in the presence of oxygen gas. The produced cobalt oxide was characterized by X-ray diffraction, transmission electronic microscopy, and selected-area electron diffraction. The results indicated that cobalt oxide nanowires with an average diameter of 10 nm and a length of ca. 600 nm have been formed, which show a predominant well-crystalline cubic face-centered like phase.     

Magnetodielectric effect in Co3O4 nanoparticles grown within a silica glass

Co₃O₄ nanocrystals of sizes in the range 17.5-33.1 nm were grown within a sol-gel-derived silica glass matrix. Rietveld analysis of the X-ray diffraction data showed that the crystals had tetragonal symmetry. This was explained as arising due to a tensile strain induced at the glass-crystal interface as a result of thermal expansion mismatch between the phases. The crystals showed ferromagnetic behaviour with superparamagnetism setting-in at temperatures above 15 K. They also exhibited ferroelectric characteristics with large remanent polarization. Change in dielectric constant as a function of magnetic field was observed in these nanocomposites. This was discussed on the basis of magnetostriction effect.     

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.     

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.     

Bulk and surface properties of spinel Co3O4 by density functional calculations

DFT calculations are employed to bulk and surface properties of spinel oxide Co₃O₄. The bulk magnetic structure is calculated to be antiferromagnetic, with a Co²⁺ moment of 2.631 μ_B in the antiferromagnetic state. There are three predicted electron transitions O(2p) → Co²⁺(t_2g) of 2.2 eV, O(2p) → Co³⁺(e_g) of 2.9 eV and Co³⁺(t_2g) → Co²⁺(t_2g) of 3.3 eV, and the former two transitions are close to the corresponding experimental values 2.8 and 2.4 eV. The naturally occurring Co₃O₄ (1 1 0) and (1 1 1) surfaces were considered for surface calculations. For ideal Co₃O₄ (1 1 0) surfaces, the surface relaxations are not significant, while for ideal Co₃O₄ (1 1 1) surfaces the relaxation of Co²⁺ cations in the tetrahedral sites is drastic, which agrees with the experiment observation. The stability over different oxygen environments for possible ideal and defect surface terminations were explored.     

Synthesis of urchin-like Co3O4 hierarchical micro/nanostructures and their photocatalytic activity

Urchin-like Co₃O₄ hierarchical micro/nanostructures have been successfully synthesized by calcining urchin-like precursor CoCO₃, which are prepared by a facile hydrothermal route. The particle size of the urchin-like Co₃O₄ could be easily controlled by altering the calcination temperature. The morphology and structure of the as-prepared urchin-like products were characterized by XRD, FESEM and TEM. Photocatalytic measurement demonstrates that these urchin-like Co₃O₄ micro/nanostructures show good photocatalytic effect and their degradation efficiency is strongly dependent on their particle size. Furthermore, a plausible reaction mechanism is also proposed to illustrate the photocatalytic processes of Co₃O₄.     

三维海胆状Co3O4的微纳结构制备及锂电池性能

水热法制备的合成海胆状Co₃O₄前驱物在空气中退火得到三维海胆状Co₃O₄的微纳结构. 采用FESEM、TEM、HRTEM以及XRD对产物进行形貌和结构的表征. 结果表明,合成的海胆状结构Co₃O₄由许多粒径约为15 nm的颗粒串接形成. 锂电池测试性能表明,制备的海胆状Co₃O₄首次放电容量达到1.369 Ah/g,经过20次循环     

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.