ZnO CoO Research Articles

Tailoring the dielectric and electrochemical properties of ZnO–CoO with Fe doping for energy storage devices

Tailoring the dielectric and electrochemical properties of ZnO–CoO with Fe doping for energy storage devices

Towards superior structural, optical and magnetic properties, and ligand field effects of $${\text{B}}_{2} {\text{O}}_{3}$$–ZnO–CoO glass doped with alkaline metal earth cations

Towards superior structural, optical and magnetic properties, and ligand field effects of $${\text{B}}_{2} {\text{O}}_{3}$$–ZnO–CoO glass doped with alkaline metal earth cations

Nano-Bricks Assembly Toward 1D Metal Oxide Nanorods.

The rational design of hybrid nanocrystals structures facilitates electronic and energetic communication between different component, which can optimize their specific performance. In this study, an efficient approach for building intricate ZnO@h-CoO nanocomposites and their derivatives is presented, based on a lattice-match/mismatch mechanism. Due to the ultra-low lattice mismatch between ZnO and hexagonal CoO (as low as 0.18%), the h-CoO layer enables epitaxial growth on the ZnO templates, and ZnO can also grow epitaxially outside the CoO layer with ease. Similarly, the thickness of the epitaxial layer and the number of alternating layers can be adjusted arbitrarily. In contrast to h-CoO, the growth of cubic crystalline oxides (such as MnO) on ZnO results in the formation of nanoparticles due to a large mismatch index (following the Volmer-Weber models). Interestingly, when h-CoO is introduced as a further component into the MnO/ZnO composite, the cubic crystalline particles on the surface of the ZnO do not disturb the epitaxial growth of the h-CoO, allowing for the formation of nanocomposites with more components. Furthermore, additional units can be added to the nanocomposite further based on the lattice-match/mismatch mechanism, which is analogous to the building nano-bricks.

Single-phase ZnCo2O4 derived ZnO–CoO mesoporous microspheres encapsulated by nitrogen-doped carbon shell as anode for high-performance lithium-ion batteries

The nanostructured hybrids composed of binary transition metal oxides (bi-TMOs) have emerged as promising anode materials in lithium ion batteries (LIBs) because of their unique physicochemical properties. Herein, we report ZnO/CoO mesoporous microspheres encapsulated by nitrogen-doped carbon shells (ZnO–CoO@NC) via self-templated solvothermal synthesis and subsequent in-situ pyrolysis of polypyrrole. The core of nanohybrid is derived from single-phase ZnCo2O4 microspheres and consists of uniform two-phase ZnO/CoO nanocrystals with interparticle mesopores. By virtue of the conductive layer of N-doped carbon and the interaction between mixed ZnO and CoO, the ZnO–CoO@NC mesoporous microspheres manifest the superior lithium storage properties as anode for LIBs, which exhibits the high Li+ storage capacity and the excellent cycling performance (1457 mAh g−1 at 0.5 A g−1 after 500 cycles), along with the competitive rate capability (381 mAh g−1 at 5 A g−1).

Fabrication of a novel ZnO–CoO/rGO nanocomposite for nonenzymatic detection of glucose and hydrogen peroxide

Herein, a novel nanocomposite of binary ZnO–CoO nanoparticles loaded on the graphene nanosheets (ZnO–CoO/rGO) has been successfully constructed via a facile, economical and two–step process. The obtained ZnO–CoO/rGO hybrids with high electrical conductivity and abundant active sites, could be modified on a glassy carbon electrode to detect glucose and H2O2 multi–functionally. The fabricated biosensor exhibits wide linear range for glucose (10μM to 11.205mM) and H2O2 (25μM to 11.1mM), and their corresponding sensitivity are 168.7μAmM−1cm−2 and 183.3μAmM−1cm−2. The limits of detection are 1.3μM and 0.44μM for the oxidation of glucose and the reduction of H2O2, respectively. Furthermore, remarkable selectivity, long–term stability and outstanding reproducibility of the non–enzyme biosensor prove that ZnO–CoO/rGO hybrids are the promising candidate in practical applications.

A facile, versatile approach to hydroxyl-anchored metal oxides with high Cr(VI) adsorption performance in water treatment.

In this study, a facile and versatile urea-assisted approach was proposed to synthesize Chinese rose-like NiO, pinecone-like ZnO and sponge-like CoO adsorbents. The presence of urea during syntheses endowed these adsorbents with high concentration of surface hydroxyl groups, which was estimated as 1.83, 1.32 and 4.19 mmol [OH−] g−1 for NiO, ZnO and CoO adsorbents, respectively. These surface hydroxyl groups would facilitate the adsorption of Cr(vi) species (e.g. HCrO4−, Cr2O72− and CrO42−) from wastewater by exchanging with hydroxyl protons or hydroxide ions, and hence result in extremely high maximum adsorbed amounts of Cr(vi), being 2974, 14 256 and 408 mg g−1 for NiO, ZnO and CoO adsorbents in the pH range of 5.02–5.66 at 298 K, respectively. More strikingly, the maximum adsorbed amounts of Cr(vi) would be greatly enhanced as the adsorbing temperature is increased, and even amount to 23 411 mg g−1 for ZnO adsorbents at 323 K. Based on the kinetics and equilibrium studies of adsorptive removal of Cr(vi) from wastewater, our synthetic route will greatly improve the adsorptivity of the as-synthesized metal-oxide adsorbents, and hence it will shed new light on the development of high-performance adsorbents.

Characterization and photocatalytic evaluation of ZnO–Co3O4 particles obtained by high energy milling. Part I: Processing, physicochemical and thermal characterization

In this paper the high energy milling was evaluated as a technique to obtain fine powders in the ZnO–Co3O4 system for utilization in photocatalytic processes. This study was divided in two parts named part I and II: the first part (present article, part I) describes the methodology of processing and the powders characterization before and after the milling; the second part (that will be present in a second article, part II) is related to the photocatalytic evaluation of the milled materials. At the first part, the raw materials (ZnO and Co3O4 powders) were characterized by Sedigraph technique, BET Analysis, Helium Pycnometry, X-Ray diffraction and Thermal Analysis (DSC/TGA) aiming to know their physicochemical, structural and thermal properties. The raw materials (95% ZnO, 5% Co3O4, molar ratio) were milled during four and ten hours (the dry and wet media milling were investigated). One sample was thermally treated in order to solubilize the cobalt in the zinc oxide lattice aiming the formation of the Zn0.95Co0.05O solid solution. The milled and heat-treated powders were characterized by X-ray diffraction and thermal analyses (DSC/TGA). The samples milled during ten hours also were analyzed by scanning electron microscopy. The results indicated that the starting materials were obtained in a micrometric scale and presented high purity. Moreover, there was observed any phase transformation or any detectable contamination of the materials during the milling. The powders showed nanocrystalline characteristics in all of conditions of milling; however, the comminution was more effective for powders milled in wet medium. The thermal treatment was efficient to obtain a ZnO–CoO solid solution.

Low temperature heat capacity of bulk and nanophase ZnO and Zn1−xCoxO wurtzite phases

The low temperature heat capacity of the ZnO–CoO solid solution system was measured from 2 to 300K using the heat capacity option of a Quantum Design Physical Property Measurement System (PPMS). The thermodynamic functions in this temperature range were derived by curve fitting. The standard entropies of bulk ZnO and bulk ZnO–CoO (wurtzite, 18mol% CoO) at T=298.15K were calculated to be (43.1±0.4)J·mol−1·K−1 and (45.2±0.5) J·mol−1·K−1, respectively. The surface entropy of ZnO was evaluated to be (0.02±0.01)mJ·K−1·m−2, which is essentially zero. No sharp magnetic transitions were observed in the solid solution samples. The nanophase solid solution, 12mol% CoO, appears to bind H2O on its surface more strongly than ZnO.

Fabrication and electrochemical behavior of flower-like ZnO–CoO–C nanowall arrays as anodes for lithium-ion batteries

This study reported the electrochemical performance of flower-like ZnO–CoO–C nanowall arrays as anodes of lithium-ion batteries. The arrays were fabricated through solution-immersion steps and subsequent calcination at 400 °C. At a rate of 0.5 C, the arrays exhibited a delithiation capacity of 438 mA h g −1 at the 50th cycle. The arrays still delivered a reversible capacity of 224 mA h g −1 at 2.0 C rate, much higher than those of the flower-like ZnO and ZnO–C nanowall arrays. The mechanism for the high capacity of flower-like ZnO–CoO–C nanowall arrays mainly resulted from the catalytic effect of Co phase on the decomposition of Li 2O and the conducting carbon layer formed on ZnO nanowalls. The present finding also provides a kind of nanostructured films that might be applied in solar cells and sensors, etc.

Phase diagram, crystal chemistry and thermoelectric properties of compounds in the Ca–Co–Zn–O system

In the Ca–Co–Zn–O system, we have determined the tie-line relationships and the thermoelectric properties, solid solution limits, and structures of two low-dimensional cobaltite series, Ca 3(Co, Zn) 4O 9− z and Ca 3(Co,Zn) 2O 6− z at 885 °C in air. In Ca 3(Co,Zn) 4O 9− z , which has a misfit layered structure, Zn was found to substitute in the Co site to a limit of Ca 3(Co 3.8Zn 0.2)O 9− z . The compound Ca 3(Co,Zn) 2O 6− z ( n=1 member of the homologous series, Ca n+2 (Co,Z n ) n (Co,Zn)′O 3 n+3− z ) consists of one-dimensional parallel (Co,Zn) 2O 6 6− chains that are built from successive alternating face-sharing (Co,Zn)O 6 trigonal prisms and ‘ n’ units of (Co,Zn)O 6 octahedra along the hexagonal c-axis. Zn substitutes in the Co site of Ca 3Co 2O 6 to a small amount of approximately Ca 3(Co 1.95Zn 0.05)O 6− z . In the ZnO–CoO z system, Zn substitutes in the tetrahedral Co site of Co 3O 4 to the maximum amount of (Co 2.49Zn 0.51)O 4− z and Co substitutes in the Zn site of ZnO to (Zn 0.94Co 0.06)O. The crystal structures of (Co 2.7Zn 0.3)O 4− z , (Zn 0.94Co 0.06)O, and Ca 3(Co 1.95 Zn 0.05)O 6− z are described. Despite the Ca 3(Co, Zn) 2O 6− z series having reasonably high Seebeck coefficients and relatively low thermal conductivity, the electrical resistivity values of its members are too high to achieve high figure of merit, ZT.

The structure, photoluminescence, optical and magnetic properties of ZnO films doped with ferromagnetic impurities

In this paper we report on ZnO–CoO thin films grown by sol–gel technology with using different solvent. As precursors zinc acetate dehydrate (Zn(CH3COO)2·2H2O), cobalt acetate tetrahydrate (Co(CH3COO)2·4H2O) as well as manganese chloride (MnCl2) were used. The films grown from solvent with methanol have a highly preferred orientation along the (1 0 0) plane. The films deposited from C=0.5 and 0.7 mol/l solvent with isopropanol and monoethanolamin and in smaller degree the C=0.3 mol/l film consist of disordered crystallites whereas the C=0.1 mol/l film has the intensive (0 0 2) peak, revealing a more obvious [0 0 2] preferred orientation. The all obtained films are highly transparent. The PL of ZnO–CoO films are more intensive in ultra-violet region in comparison the PL of ZnO film. The ESR spectra of ZnO–CoO multilayer films were investigated.

Room-temperature ferromagnetism in Co-doped ZnO fabricated by coprecipitation method

Nanocrystalline Zn1-xCoxO samples were prepared by coprecipitation and annealing at 300 ℃ for 3 h in 5vol.%H2/Ar flow. Scanning electron microprobe measurements indicate that the samples with nominal Co contents of 0.05, 0.10, and 0.15 have the actual Co contents of 0.054, 0.100, and 0.159 respectively. X-ray diffraction patterns show that the major phase of the samples is of wurtzite structure, while the minor phase of CoO is observed in samples with Co contents of x=0.100 and 0.159. X-ray photoelectron spectroscopy shows that Co has three states: Co incorporated into ZnO matrix, CoO and metallic Co. It was found through magnetic measurements, that room-temperature ferromagnetism exists in all the samples and it comes from both the weak ferromagnetism of Zn-doped CoO1-δ and the magnetism of metallic Co clusters.

Effects of cobalt doping on nonlinearity of zinc oxide

The microstructures and electrical properties of ZnO–Bi 2O 3–CoO (ZBC), ZnO–Bi 2O 3 (ZB) and ZnO–CoO (ZC) ceramics were investigated. Cobalt oxide addition could reduce bismuth loss at high sintering temperature. The same amount of cobalt within ZnO grains was found in both specimens, ZBC and ZC, suggesting that bismuth had no effect on the dissolution of cobalt in ZnO and cobalt substitution for zinc in the ZnO structure. The highest nonlinear coefficient of about 19 was found in the ZBC varistor sintered at 1000–1100 °C. For the ZC specimens, nonlinear properties could also be obtained in this sintering temperature range.

Extended solubility of CoO in ZnO and effects on magnetic properties

The metastable solid solubility extension of CoO in ZnO (wurtzite) was investigated in precursor-derived powders as well as in thin films grown on sapphire substrates by pulsed laser deposition. A maximum solubility of 30% Co2+ in ZnO was achieved in the powders. Transmission electron microscopy (TEM) of the films revealed them to have grown epitaxially and retained up to nearly 40% CoO in solid solution, but some Co2+ precipitated as rock-salt. The temperature dependence of the metastable solubility limit in the ZnO–CoO system was assessed and is discussed in terms of the relevant thermodynamic factors. The magnetic properties of n-type conductive Zn0.79Co0.2Al0.01Ofilms were studied, yielding evidence of a ferromagnetic phase with a TC of 25 K and a second, magnetically ordered, phase with positive exchange and arguably a TC of ∼250 K. Connections between the properties and microstructural observations in high resolution TEM are proposed.

Properties and intergranular phase analysis of a ZnO–CoO–Bi 2O 3 varistor

The electrical properties, microstructures and phase content of a ZnO–CoO–Bi 2O 3 varistor have been characterised. High-resolution TEM and selected area electron diffraction provided detailed information on the distribution and structure of the intergranular Bi-rich phase. Triple points between the ZnO matrix grains contained crystallites that could be indexed to tetragonal β-Bi 2O 3, cubic γ-Bi 2O 3 and δ-Bi 2O 3, or corresponding isostructural ZnO-Bi 2O 3 phases. Thin-film, 1–2 nm grain boundaries were found to be amorphous, but for the first time, crystalline precipitates (with the β-Bi 2O 3 type structure) were also detected in some thicker, 4–5 nm films.

Influence of cobalt oxide addition on electrical properties of ZnO-Pr6O11-based varistor ceramics

Influence of cobalt oxide addition on electrical properties of ZnO-Pr6O11-based varistor ceramics

Co/SiO 2 catalysts for selective hydrogenation of crotonaldehyde : III. Promoting effect of zinc

ZnO-Co/SiO 2 catalysts with various cobalt loadings and Co/Zn ratios were studied in terms of both their surface structure and catalytic hydrogenation of crotonaldehyde (CROALD), to investigate the influence of zinc on the properties of Co/SiO 2 catalysts. The ZnO–CoO x precursors were prepared in situ on the silica support by co-impregnation of the support with an aqueous solution of cobalt and zinc nitrates. The oxide precursors and catalysts were characterized by temperature-programmed reduction (TPR), temperature-programmed desorption of hydrogen (TPD-H 2), and diffuse reflectance FTIR spectroscopy (DRIFTS) of adsorbed CO. The TPD-H 2 and DRIFTS of CO indicated that the presence of Zn suppresses formation of the type α sites present in catalysts containing only Co and promotes surface-structure formation of ZnO x –Co sites obtained by reduction of the catalysts at high temperatures. The presence of ZnO x in the surface structure blocks both CO adsorption in bridge form and the formation of the hydrocarbonyl structure in the presence of H 2. For crotonaldehyde hydrogenation in the gas phase, selective hydrogenation of CC or CO groups was correlated with the relative proportion of species in the oxide precursors. The formation of ZnO x –Co at the surface strongly suppresses the CC hydrogenation activity. However, this surface structure is not stable during prolonged catalytic runs. After a time on stream, the catalytic properties revert to those of the unpromoted catalysts.

Spectroscopic properties of Co 2+: ZnAl 2O 4 nanocrystals in sol–gel derived glass–ceramics

Transparent glass–ceramics containing zinc–aluminum spinel (ZnAl 2O 4) nanocrystals doped with tetrahedrally coordinated Co 2+ ions were obtained by the sol–gel method for the first time. The gels of composition SiO 2–Al 2O 3–ZnO–CoO were prepared at room temperature and heat-treated at temperature ranging 800–950 °C. When the gel samples were heated up to 900 °C, ZnAl 2O 4 nanocrystals were precipitated. Co 2+ ions were located in tetrahedral sites in ZnAl 2O 4 nanocrystals. X-ray diffraction analysis shows that the crystallite sizes of ZnAl 2O 4 crystal become large with the heat-treatment temperature and time, and the crystallite diameter is in the range of 10–15 nm. The dependence of the absorption and emission spectra of the samples on heat-treatment temperature were presented. The difference in the luminescence between Co 2+ doped glass–ceramic and Co 2+ doped bulk crystal was analysed. The crystal field parameter D q of 423 cm −1 and the Racah parameters B of 773 cm −1 and C of 3478.5 cm −1 were calculated for tetrahedral Co 2+ ions.

Metastable solid solutions in the system ZnOCoO: synthesis by hydrolysis in polyol medium and study of the morphological characteristics

Metastable solid solutions in the system ZnOCoO have been synthesized by hydrolysis of ionic zinc and/or cobalt salts in polyol medium. The solubility of Co in the zincite oxide was significantly increased (at.% Co=65) compared to that obtained under thermodynamical equilibrium (at.% Co=6.5 at 800°C). CoO was also obtained via this route. The products are made up of sub-micrometer particles. A detailed study of their morphology along with pure ZnO particle morphology was conducted. Different growth mechanisms are evidenced.

Synthesis of Metastable, Wurzite‐Based Zinc Oxide‐Cobalt(II) Oxide Solid Solutions by Spray Pyrolysis

Aqueous solutions of acetates and nitrates of zinc and cobalt have been spray decomposed to study the production of extended solid solutions in the ZnO–CoO system. Examination of the products of a variety of synthesis conditions indicates that up to 70% CoO may be retained in the solid solution in the wurzite phase, even though a comparison of the equilibrium solubility in the phase diagram might be expected to favor the formation of a rock‐salt‐based solid solution.