Influences of Co doping on the structural,optical and magnetic properties of ZnO nanorods synthesized by hydrothermal route

Pure and Co-doped ZnO nanorods have been synthesized by a hydrothermal process. The structure, morphology and properties of as-prepared samples have been studied using X-ray diffraction (XRD), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), photoluminescence (PL) spectrometer as well as by superconducting quantum interference device (SQUID). The structure and morphology analyses show that Co doping can slightly impede the ZnO crystallinity, influence the nanorods morphology, but cannot change the preferred growth orientation of ZnO nanorods. The amount of Co doping contents is about 3.0 at% in ZnO nanorods and dopant Co²⁺ ions substitute Zn²⁺ ions sites in ZnO nanocrystal without forming any secondary phase. The optical measurements show that the Co doping can effectively tune energy band structure and enrich surface states in both UV and VL regions, which lead to novel PL properties of ZnO nanorods. In addition, ferromagnetic ordering of the as-synthesized Zn_1?xCo_xO nanorod arrays has been observed at room temperature, which should be ascribed to sp–d and d–d carrier exchange interactions and presence of abundant defects and oxygen vacancies.     

Facile synthesis of nanostructured Ni-Co/ZnO material: An efficient and inexpensive catalyst for Heck reactions under ligand-free conditions

A simple, efficient and economically viable method for the Heck reaction has been accomplished in the absence of phosphine ligand. The Heck reaction was performed using nanostructured Ni-Co/ZnO material as a heterogeneous catalyst in a DMF/H₂O solvent system and in the presence of K₂CO₃, at 120 °C. The Ni-Co/ZnO nanostructures were prepared by the facile reduction-impregnation method. The structural and morphological properties of Ni-Co/ZnO nanostructure were investigated using various physico-chemical characterization techniques. Structural studies displayed the formation of hexagonal (wurtzite) ZnO. Electron microscopy imaging showed the presence of agglomerated clusters of Ni-Co nanoparticles over the surfaces of elliptical, flower bud-like and irregularly shaped sub-micron sized particle bundles of ZnO. The elemental composition analysis (EDX) confirmed the loading of Ni and Co nanoparticles over the nanocrystalline ZnO. The surface chemical state analysis of Ni-Co/ZnO material validated that Ni nanostructure exists in Ni²⁺ and Ni³⁺ species, whereas, Co nanostructure exists in Co²⁺ and Co³⁺ species. UV–Vis diffuse reflectance spectroscopy displays red shift in the light absorption edge of Ni-Co/ZnO catalyst compared to pure ZnO. The as-prepared Ni-Co bimetallic supported ZnO nanostructure showed better catalytic activity and stability for the Heck reactions under phosphine ligand-free conditions. Ni-Co/ZnO catalyzed Heck reactions afforded the corresponding cross-coupled products with moderate to good yields (up to 92%). Ni-Co/ZnO catalyst could be reused for five successive runs without significant loss of catalytic activity.     

Synthesis of Multifunctional Mn‐Doped CdTe/ZnS Core/Shell Quantum Dots

The synthesis of a novel water‐soluble Mn‐doped CdTe/ZnS core‐shell quantum dots using a proposed ultrasonic assistant method and 3‐mercaptopropionic acid (MPA) as stabilizer is descried. To obtain a high luminescent intensity, post‐preparative treatments, including the pH value, reaction temperature, reflux time and atmosphere, have been investigated. For an excellent fluorescence of Mn‐doped CdTe/ZnS, the optimal conditions were pH 11, reflux temperature 100°C and reflux time 3 h under N₂ atmosphere. While for phosphorescent Mn‐doped CdTe/ZnS QDs, the synthesis at pH 11, reflux temperature 100°C and reflux time 3 h under air atmosphere gave the best strong phosphorescence. The characterizations of Mn‐doped CdTe/ZnS QDs were also identified using AFM, IR, powder XRD and thermogravimetric analysis. The data indicated that the photochemical stability and the photoluminescence of CdTe QDs are greatly enhanced by the outer inorganic ZnS shell, and the doping Mn²⁺ ions in the as‐prepared quantum dots contribute to strong luminescence. The strong luminescence of Mn‐doped CdTe/ZnS QDs reflected that Mn ions act as recombination centers for the excited electron‐hole pairs, attributing to the transition from the triplet state (⁴T₁) to the ground state (⁶A₁) of the Mn²⁺ ions. All the experiments demonstrated that the surface states played important roles in the optical properties of Mn‐doped CdTe/ZnS core‐shell quantum dots.     

Fabrication of Co + F Doped ZnO Films for Improved Visible Light Responsive Photocatalysis

The aim of this work is to investigate the efficacy of nebulizer sprayed ZnO–Co–F thin films for degrading the methylene blue (MB) aqueous solution under visible light irradiation. The physical properties of the prepared samples like structural, surface morphological and optical properties are studied using X-ray diffractometer (XRD), Scanning Electron Microscope (FESEM), UV-vis NIR spectrometer, spctroflurometer (PL) and Fourier Transform Infrared spectrometer (FTIR). The photocatalytic studies revealed that ZnO–Co–F film exhibited superior degradation efficiency over ZnO–Co and ZnO against MB as evinced by the degradation rate constants, 0.0782, 0.0475 and 0.0289 min^–1, respectively. This study showed that Co + F doping helps to remove the major limitations of ZnO and thereby leads towards better visible light activity. The structural and optical studies revealed the proper incorporation of Co²⁺ and F^– ions into the ZnO lattice and the reduction in band gap. PL emission confirmed that Co + F doping reduces the electronhole recombination rate and increases the availability of free carriers.     

Probing the Reactivity of ZnO and Au/ZnO Nanoparticles by Methanol Adsorption: A TPD and DRIFTS Study

The adsorption of methanol on pure ZnO and Au‐decorated ZnO nanoparticles and its thermal decomposition monitored by temperature‐programmed desorption (TPD) experiments and by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), both applied under continuous flow conditions in fixed bed reactors, is reported. Two distinguishable methoxy species are formed during methanol adsorption on ZnO differing in the C? O stretching bands. During the subsequent TPD experiments two different H₂ peaks are observed, indicating the conversion of methoxy into formate species. By applying different heating rates, activation energies of 109 kJ mol^?1 and 127 kJmol^?1 for the selective oxidation of the two methoxy species are derived. Correspondingly, the methoxy decomposition results in two distinguishable formate species, which are identified by the asymmetric and symmetric OCO stretching bands on pure ZnO and Au/ZnO. Based on the decreased intensities of the OH bands during methanol adsorption, which are specific for the various ZnO single crystal surfaces, on the different reactivities of these surfaces, and on the formate FTIR bands observed on ZnO single crystal surfaces, the two methoxy and the corresponding formate species are identified to be adsorbed on the exposed less reactive non‐polar ZnO(${10\bar 10}$ ) surface and on the highly reactive polar ZnO(${000\bar 1}$ ) surface. The simultaneous formation of H₂, CO, and CO₂ at about 550–600 K during the TPD experiments indicate the decomposition of adsorbed formate species. The CO/CO₂ ratio decreases with increasing Au loading, and a broad band due to electronic transitions from donor sites to the conduction band is observed in the DRIFT spectra for the Au‐decorated ZnO nanoparticles. Thus, the presence of the Au nanoparticles results in an enhanced reducibility of ZnO facilitating the generation of oxygen vacancies.     

Co doped ZnO nanowires as visible light photocatalysts

High aspect ratio cobalt doped ZnO nanowires showing strong photocatalytic activity and moderate ferromagnetic behaviour were successfully synthesized using a solvothermal method and characterized by scanning electron microscopy (SEM), X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), vibrating sample magnetometry (VSM) and UV–visible absorption spectroscopy. The photocatalytic activities evaluated for visible light driven degradation of an aqueous methylene orange (MO) solution were higher than for Co doped ZnO nanoparticles at the same doping level and synthesized by the same synthesis route. The rate constant for MO visible light photocatalytic degradation was 1.9·10⁻³ min⁻¹ in case of nanoparticles and 4.2·10⁻³ min⁻¹ in case of nanowires. We observe strongly enhanced visible light photocatalytic activity for moderate Co doping levels, with an optimum at a composition of Zn_0.95Co_0.05O. The enhanced photocatalytic activities of Co doped ZnO nanowires were attributed to the combined effects of enhanced visible light absorption at the Co sites in ZnO nanowires, and improved separation efficiency of photogenerated charge carriers at optimal Co doping.     

Interfacial Impregnation Chemistry in the Synthesis of Cobalt Catalysts Supported on Titania

The interfacial chemistry of the impregnation step involved in the synthesis of cobalt catalysts supported on titania was investigated with regard to the mode of interfacial deposition of the aqua complex [Co(H₂O)₆]²⁺ on the “titania/electrolyte solution” interface, the structure of the inner‐sphere complexes formed, and their relative interfacial concentrations. Several methodologies based on the application of deposition experiments and electrochemical techniques were used in conjunction with diffuse‐reflectance spectroscopy and EPR spectroscopy. These suggested the formation of mononuclear/oligonuclear inner‐sphere complexes on deposition of the [Co(H₂O)₆]²⁺ ions at the “titania/electrolyte solution” interface. The joint application of semiempirical quantum‐mechanical calculations, stereochemical considerations, and modeling of the deposition data revealed the exact structure of these complexes and allowed their relative concentrations at various Co^II surface concentrations to be determined. It was found that the interface speciation depends on the Co^II surface concentration. Mononuclear complexes are formed at the compact layer of the “titania/electrolyte solution” interface for low and medium Co^II surface concentrations. Formation of mono‐hydrolyzed Ti₂O–TiO and the dihydrolyzed TiO–TiO disubstituted configurations is very probable. In the first configuration one water ligand of the [Co(H₂O)₆]²⁺ ion is substituted by a bridging surface oxygen atom and another by a terminal surface oxygen atom. In the second configuration two water ligands of the [Co(H₂O)₆]²⁺ ion are substituted by two terminal surface oxygen atoms. Binuclear and trinuclear inner‐sphere complexes are formed, in addition to the mononuclear ones, at relatively high Co^II surface concentrations.     

Electronic absorption spectroscopy of cobalt ions in diluted magnetic semiconductor quantum dots: demonstration of an isocrystalline core/shell synthetic method.

This paper reports the application of ligand-field electronic absorption spectroscopy to probe Co(2+) dopant ions in diluted magnetic semiconductor quantum dots. It is found that standard inverted micelle coprecipitation methods for preparing Co(2+)-doped CdS (Co(2+):CdS) quantum dots yield dopant ions predominantly bound to the nanocrystal surfaces. These Co(2+):CdS nanocrystals are unstable with respect to solvation of surface-bound Co(2+), and time-dependent absorption measurements allow identification of two transient surface-bound intermediates involving solvent-cobalt coordination. Comparison with Co(2+):ZnS quantum dots prepared by the same methods, which show nearly isotropic dopant distribution, indicates that the large mismatch between the ionic radii of Co(2+) (0.74 A) and Cd(2+) (0.97 A) is responsible for exclusion of Co(2+) ions during CdS nanocrystal growth. An isocrystalline core/shell preparative method is developed that allows synthesis of internally doped Co(2+):CdS quantum dots through encapsulation of surface-bound ions beneath additional layers of CdS.     

ZnO/Co3O4 Porous Nanocomposites Derived from MOFs: Room‐Temperature Ferromagnetism and High Catalytic Oxidation of CO

ZnO/Co₃O₄ porous nanocomposites were successfully fabricated by the thermal decomposition of Prussian Blue analogue (PBA) Zn₃[Co(CN)₆]₂ nanospheres obtained at room temperature. Interestingly, ZnO/Co₃O₄ porous nanocomposites exhibit room‐temperature ferromagnetism. Moreover, the ZnO/Co₃O₄ porous nanocomposites show good catalytic activity for CO oxidation, and the CO conversion rate reaches 100 % at 250 °C. It is suggested that the synergistic effect of each component, relative high surface area (32 m² g^?1) and porous structure lead to the promising catalytic properties.     

In Situ Fabrication of Inorganic/Organic Hybrid Based on Cu/Co‐ZIF and Cu2O

We successfully fabricate a well‐defined inorganic/organic hybrid Cu₂O@Cu/Co‐ZIF (ZIF=zeolitic imidazolate frameworks) by use of growth of dual‐metal Cu/Co‐ZIF on the obtained Cu₂O hollow spheres. The key point of the strategy is coupling the in situ self‐sacrificing template. Cu₂O and the coordination of metal ions (Cu⁺ and Co²⁺) with 2‐methylimidazole. This new hybrid was characterized by powder X‐ray diffraction, (scanning) transmission electron microscopy, energy‐dispersive spectroscopy mapping, in situ FT‐IR spectroscopy, UV/Vis diffuse reflection spectroscopy, N₂ sorption measurements, and electron spin resonance. It was evidenced that Cu/Co‐ZIF nanocrystals have been assembled to continuous shells surrounding the Cu₂O cores as well as in the voids between layers and inner pores. Cu₂O@Cu/Co‐ZIF exhibits visible light responsiveness and holds potential as narrow band gap semiconductor and visible photocatalyst.     

Photochemical Synthesis and Spectral-Optical Characteristics of ZnO/Cu and ZnO/Ag/Cu Nanoheterostructures

The photoreduction of Cu²⁺ at the surface of ZnO nanoparticles and ZnO/Ag nanostructures was investigated. The spectral characteristics of the obtained ZnO/Cu and ZnO/Ag/Cu composites were studied in relation to the reaction conditions. It was shown that the ZnO/Ag nanoparticles have higher photocatalytic activity in the reduction of Cu²⁺ ions than the individual ZnO particles.     

Doping-induced emission of infrared light from Co2+-doped ZnSe quantum dots

ZnSe quantum dots doped with Co²⁺ have been prepared in aqueous solution by a one-pot method using thioglycolic acid as stabilizer. The quantum dots were characterized by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), UV?Cvisible spectrophotometry, and spectrofluorimetry. The results confirmed the quantum dots formed a single cubic phase with zinc blende structure. The average particle size of the quantum dots was approximately 5 nm. Co²⁺ ions were doped into ZnSe lattice sites by substitution. As a result, infrared (IR) emission of Co^{2+ 4}T₂(F) ?? ⁴A₂(F) at approximately 3.5 ??m was detected on excitation with 755 nm radiation.     

Copper‐Doped CdSe/ZnS Quantum Dots: Controllable Photoactivated Copper(I) Cation Storage and Release Vectors for Catalysis

The first photoactivated doped quantum dot vector for metal‐ion release has been developed. A facile method for doping copper(I) cations within ZnS quantum dot shells was achieved through the use of metal‐dithiocarbamates, with Cu⁺ ions elucidated by X‐ray photoelectron spectroscopy. Photoexcitation of the quantum dots has been shown to release Cu⁺ ions, which was employed as an effective catalyst for the Huisgen [3+2] cycloaddition reaction. The relationship between the extent of doping, catalytic activity, and the fluorescence quenching was also explored.     

Quenching of Photoluminescence in ZnO QDs Decorating Multiwalled Carbon Nanotubes

We report the controlled growth of ZnO quantum dots (QDs) on the sidewalls of multiwalled carbon nanotubes (MWCNTs) by a one‐step process and study the effect on the photoluminescence (PL) properties of the ZnO QDs‐MWCNT composite. The PL intensity of the composite is quenched and the lifetime is reduced compared to the only ZnO QDs. The origin of the PL quenching is discussed in terms of energy transfer, which is examined by varying the density and size of ZnO QDs by changing the molar concentration of the precursor solution for ZnO and the amount of MWCNT.     

Photoelectron Spectroscopy at the Graphene–Liquid Interface Reveals the Electronic Structure of an Electrodeposited Cobalt/Graphene Electrocatalyst

Electrochemically grown cobalt on graphene exhibits exceptional performance as a catalyst for the oxygen evolution reaction (OER) and provides the possibility of controlling the morphology and the chemical properties during deposition. However, the detailed atomic structure of this hybrid material is not well understood. To elucidate the Co/graphene electronic structure, we have developed a flow cell closed by a graphene membrane that provides electronic and chemical information on the active surfaces under atmospheric pressure and in the presence of liquids by means of X‐ray photoelectron spectroscopy (XPS). We found that cobalt is anchored on graphene via carbonyl‐like species, namely Co(CO)_x, promoting the reduction of Co³⁺ to Co²⁺, which is believed to be the active site of the catalyst.     

The Role of Reduced Graphite Oxide in Transition Metal Oxide Nanocomposites Used as Li Anode Material: An Operando Study on CoFe2O4/rGO

A composite consisting of CoFe₂O₄ spinel nanoparticles and reduced graphite oxide (rGO) is studied as an anode material during Li uptake and release by applying synchrotron operando X‐ray diffraction (XRD) and operando X‐ray absorption spectroscopy (XAS), yielding a comprehensive picture of the reaction mechanisms. In the early stages of Li uptake, a monoxide is formed as an intermediate phase containing Fe²⁺ and Co²⁺ ions; this observation is in contrast to reaction pathways proposed in the literature. In the fully discharged state, metallic Co and Fe nanoparticles are embedded in an amorphous Li₂O matrix. During charge, metallic Co and Fe are oxidized simultaneously to Co²⁺ and Fe³⁺, respectively, thus enabling a high and stable capacity to be achieved. Here, evidence is presented that the rGO acts as a support for the nanoparticles and prevents the particles from contact loss. The operando investigations are complemented by TEM, Raman spectroscopy, galvanostatic cycling, and cyclic voltammetry.     

Hydration of the Calcium Dication: Direct Evidence for Second Shell Formation from Infrared Spectroscopy

Infrared laser action spectroscopy in a Fourier‐transform ion cyclotron resonance mass spectrometer is used in conjunction with ab initio calculations to investigate doubly charged, hydrated clusters of calcium formed by electrospray ionization. Six water molecules coordinate directly to the calcium dication, whereas the seventh water molecule is incorporated into a second solvation shell. Spectral features indicate the presence of multiple structures of Ca(H₂O)₇²⁺ in which outer‐shell water molecules accept either one (single acceptor) or two (double acceptor) hydrogen bonds from inner‐shell water molecules. Double‐acceptor water molecules are predominately observed in the second solvent shells of clusters containing eight or nine water molecules. Increased hydration results in spectroscopic signatures consistent with additional second‐shell water molecules, particularly the appearance of inner‐shell water molecules that donate two hydrogen bonds (double donor) to the second solvent shell. This is the first reported use of infrared spectroscopy to investigate shell structure of a hydrated multiply charged cation in the gas phase and illustrates the effectiveness of this method to probe the structures of hydrated ions.     

Study of formation and thermal stability of the Fe/ZnO(000‐1) interface

Formation and thermal stability of the Fe/ZnO(000‐1) interface have been studied by means of X‐ray photoelectron spectroscopy and low energy electron diffraction. The results indicated a pseudo 2D growth mode for iron on ZnO. In addition, it could be shown that under ultra high vacuum conditions deposited Fe⁰ on a ZnO(000‐1) single crystal was partially oxidized by a small fraction of residual ? OH‐groups and ZnO to FeO. A strong temperature dependence of the interface reactivity was found upon annealing at temperatures up to 600 °C. Starting from 200 °C iron was first oxidized to bivalent iron oxide. After complete oxidation of Fe⁰ to Fe²⁺ at 375 °C, Fe²⁺ reacted to Fe³⁺. Above temperatures of 500 °C the deposited metallic iron was completely oxidized to trivalent iron. Further experiments with FeO on ZnO showed the oxidation state and the oxide film thickness of the deposited iron to be mainly dependent on the annealing temperature. Copyright © 2008 John Wiley & Sons, Ltd.     

Magnetic Anisotropy of Cyanide‐Bridged Core and Core–Shell Coordination Nanoparticles Probed by X‐ray Magnetic Circular Dichroism

The local symmetry and local magnetic properties of 6 nm‐sized, bimetallic, cyanide‐bridged CsNiCr(CN)₆ coordination nanoparticles 1 and 8 nm‐sized, trimetallic, CsNiCr(CN)₆@CsCoCr(CN)₆ core–shell nanoparticles 2 were studied by X‐ray absorption spectroscopy (XAS) and X‐ray magnetic circular dichroism (XMCD). The measurements were performed at the Ni^II, Co^II, and Cr^III L_2,3 edges. This study revealed the presence of distorted Ni^II sites located on the particle surface of 1 that account for the uniaxial magnetic anisotropy observed by SQUID measurements. For the core–shell particles, a combination of the exchange anisotropy between the core and the shell and the pronounced anisotropy of the Co^II ions is the origin of the large increase in coercive field from 120 to 890 Oe on going from 1 to 2 . In addition, XMCD allows the relative orientation of the magnetic moments throughout the core–shell particles to be determined. While for the bimetallic particles of 1 , alignment of the magnetic moments of Cr^III ions with those of Ni^II ions leads to uniform magnetization, in the core–shell particles 2 the magnetic moments of the isotropic Cr^III follow those of Co^II ions in the shell and those of Ni^II ions in the core, and this leads to nonuniform magnetization in the whole nanoobject, mainly due to the large difference in local anisotropy between the Co^II ions belonging to the surface and the Ni^II ions in the core.     

IR Spectroscopic Study of the Interaction of Transition Metal Ions with Hydroxyapatite Modified by Potassium Ferrocyanide

IR spectroscopy was used to study the modification of hydroxyapatite by potassium ferrocyanide and the interaction of transition metal ions with the modified adsorbent. The structural changes of hydroxyapatite upon the adsorption of Zn²⁺ ions were studied by X-ray phase analysis. Potassium ferrocyanide was found to interact with the surface hydroxyl groups of hydroxyapatite through the nitrogen unshared electron pair. Co²⁺, Ni²⁺, and Zn²⁺ ions were found to eliminate outer-sphere K⁺ ions of adsorbed ferrocyanide to give Fe²⁺— CN—M²⁺— NC—Fe²⁺ bridging structures. Zn²⁺ cations additionally eliminate a part of the Ca²⁺ ions from structural positions of hydroxyapatite, which leads to the appearance of a two-phase hydroxyapatite–sholzite system with heterogeneous distribution of the Ca²⁺ ions in the mot her mineral phase, while the Zr²⁺ ions are found in the new sholzite phase.