Mn4+-, Tb3+,4+-, and Er3+-activated red phosphors in the MgAl2Si2O8 system

Mn⁴⁺ doped and Tb^3+,4+, Er³⁺ co-doped MgAl₂Si₂O₈-based phosphors were prepared by conventional solid-state synthesis at 1,300 °C. They were characterized by thermogravimetry, differential thermal analysis, X-ray powder diffraction, photoluminescence, and scanning electron microscopy. The luminescence mechanism of the phosphors, which showed broad red emission bands in the range of 600–715 nm and had different maximum intensities when activated by UV illumination, was discussed. Such a red emission can be attributed to the intrinsic ²E → ⁴A₂ transitions of Mn⁴⁺.     

Red phosphors in MgAl2Si2O8 doping with Mn4+, Gd3+ and Lu3+ and host-sensitized luminescence properties

Mn⁴⁺ doped and Gd³⁺, Lu³⁺ co-doped MgAl₂Si₂O₈-based phosphors were first of all synthesized by solid state reaction at about 1300.0 °C. They were characterized by thermogravimetry, differential thermal analysis, X-ray powder diffraction, photoluminescence, and scanning electron microscopy. The luminescence mechanism of the phosphors which showed broad red emission bands in the range of 610–715 nm and had a different maximum intensity when activated by UV illumination was discussed. Such a red emission can be attributed to the intrinsic ²E → ⁴A₂ transitions of Mn⁴⁺.     

Synthesis,characterization and photocatalytic properties of Mg1−xZnxAl2O4 spinel nanoparticles

Mg_1−xZn_xAl₂O₄ spinel nanoparticles with x = 0, 0.05, 0.10, 0.15 and 0.20 were prepared via the chemical coprecipitation method. The obtained samples were characterised by thermal gravimetric and differential scanning calorimetry, X-ray diffraction, Fourier transform infrared spectroscopy, UV–Vis diffuse reflection spectrum, transmission electron microscopy and ²⁷Al MAS-NMR spectroscopy. Mg_1−xZn_xAl₂O₄ spinel powders with the mean crystallite size of around 11 nm–14 nm were obtained. The crystallinity of the MgAl₂O₄ samples increases with the increase in the calcination temperature. At the same calcination temperature, higher amount of Zn²⁺ substitution leads to the higher level of crystallinity, but has no apparent influence on the mean crystallite size of the samples. The photocatalytic activity of the obtained Mg_1−xZn_xAl₂O₄ spinel nanoparticles was evaluated by monitoring the degradation of methylene blue under UV light. The degradation rates of methylene blue using the MgAl₂O₄ nanoparticles prepared at the calcination temperatures of 700 °C and 800 °C are much higher than those prepared at 900 °C and 1000 °C. The photocatalytic activities of the spinel powders with lower level of Zn²⁺ substitution such as Mg_0.95Zn_0.05Al₂O₄ are inferior to that of MgAl₂O₄. Results of ²⁷Al MAS-NMR spectroscopy analysis and the first principle total density of state calculations reveal that this is probably due to the substitutions of Zn²⁺ decreasing the degree of Al³⁺ ions inversion over the sites of tetrahedral and octahedral coordination. With the increase in the amounts of Zn²⁺ substitution, the effects of Zn²⁺ additions on the photocatalytic activities become gradually predominant, leading to the increases in the degradation rates. The methylene blue degraded by 99% within 4 h using the Mg_0.8Zn_0.2Al₂O₄ spinel powders.     

Photo and Thermo‐luminescence Properties of Mn4+, Ce4+ and Sm3+ in MgAl2Si2O8 Phosphor

Mn⁴⁺, Ce⁴⁺ and Sm³⁺ doped MgAl₂Si₂O₈‐based phosphors were synthesized at 1300 °C by solid state reaction and characterized by thermogravimetry (TG), differential thermal analysis (DTA), X‐ray powder diffraction (XRD), photoluminescence (PL), thermoluminescence (TL) and scanning electron microscopy (SEM). The phosphors showed broad red emission bands in the range of 610–715 nm and different maximum intensity when activated by UV illumination. Such a red emission can be attributed to the intrinsic ²E→⁴A₂ transitions of Mn⁴⁺.     

Rheological phase synthesis and electrochemical performance of Co3O4 for supercapacitors

Co₃O₄ was prepared by rheological phase reaction, and the effect of pyrolyzing temperature on the electrochemical performance of Co₃O₄ was investigated. XRD shows that higher temperature treatment results in sharper diffraction peaks, indicating an increase of particle size of Co₃O₄. The result of TEM shows that the particle sizes of Co₃O₄ are about 200 nm. The optimum pyrolyzing temperature is confirmed to be 600°C, and Co₃O₄ prepared at this temperature exhibits 390.8 F g^?1 of specific capacitance in 6 M KOH electrolyte at the scan rate of 5 mV s^?1. Co₃O₄ prepared at the temperature of 600°C shows an excellent cyclability.     

Preparation and spectroscopic properties of nanostructured glass-ceramics containing Yb3+, Er3+ ions and Co2+-doped spinel nanocrystals

Transparent glass-ceramics with Yb³⁺, Er³⁺ ions in glass matrix and tetrahedral Co²⁺-doped MgAl₂O₄ nanocrystals were synthesized. XRD patterns and FESEM micrograph of the glass-ceramics showed that MgAl₂O₄ nanocrystals (sizes of 10–20 nm) are uniformly dispersed in SiO₂ glass matrix. Absorption and emission spectra of the glass-ceramics indicated that Yb³⁺, Er³⁺ remain in SiO₂ glass matrix, while Co²⁺ occupied tetrahedral sites in MgAl₂O₄ nanocrystals, and can function as saturable absorber for Er³⁺. Transparent Co²⁺, Yb³⁺, Er³⁺ co-doped glass-ceramics possesses the spectral requirements and should be a potential laser material used for self-Q-switched microchip laser operating at 1.5–1.6 μm.     

Synthesis of Co3O4 nanoparticles with controllable size and their catalytic properity

Co₃O₄ nanoparticles of controllable size were synthesized via a variant microemulsion method combined with hydrothermal treatment. With this improved method, Co₃O₄ nanoparticles with uniform shape and narrow size distribution were controllably synthesized. By varying the surfactant and water ratio, particles of around 27.97 nm, 42.88 nm and 48.99 nm were synthesized. These nanoparticle catalysts behaved differently at low and high temperature. These samples showed high activity for CO oxidation and 100% CO conversion was achieved at 30 °C. However, the catalysts gradually lost their activity after about 100 min. Our research found that CO₂ had a huge influence on the stability of catalysts at low temperature. The relationship between stability at 30 °C and ratio of Co³⁺/Co²⁺, as well as particle size and desorption rate of CO₂, was also thoroughly investigated in this work.     

Host-sensitized phosphorescence of Mn4+, Eu3+, and Yb3+ in MgAl2Si2O8

Mn⁴⁺ doped and Eu³⁺, Yb³⁺ co-doped MgAl₂Si₂O₈-based phosphors were prepared by conventional solid state reaction at 1,300?°C. They were characterized by thermogravimetry, differential thermal analysis, X-ray powder diffraction, photoluminescence, and scanning electron microscopy. The luminescence mechanism of the phosphors, which showed broad red emission bands in the range of 600?C715?nm and had a different maximum intensity when activated by UV illumination, was discussed. Such a red emission can be attributed to the intrinsic ² E????⁴ A ₂ transitions of Mn⁴⁺.     

Thermal behaviour of cobalt oxides doped with ZrO2 and ThO2 and the possible cubic phase stabilization of zirconia

The effects of doping cobalt oxides with different amounts of ZrO₂ and ThO₂ (1.5–9 mol%) on the thermal stability of Co₃O₄ and the re-oxidation of CoO by O₂ to Co₃O₄ were investigated. The techniques employed were DTA, with a controlled rate of heating and cooling, X-ray diffraction, and IR spectrometry.The results obtained by DTA revealed that the addition of both Th⁴⁺ and Zr⁴⁺ (up to 6 mol%) exerted no appreciable effect on the thermal stability of Co₃O₄. Increasing the amount of the dopant ions to 9% resulted in no further change in the thermal stability of Co₃O₄ in the case of Th⁴⁺, and an increase of 16% in case of Zr⁴⁺-doping. However, ThO₂-doping of cobalt oxide was accompanied by an enhancement in the reactivity of CoO towards re-oxidation by O₂ to Co₃O₄ to an extent proportional to the amount of dopant oxide.The X-ray investigation of ZrO₂-doped cobalt oxides calcined in air at 1000°C revealed the presence of highly crystalline and stable zirconia in the cubic form. Such a stable phase could not be obtained at temperatures below 2370°C in the absence of stabilizing agents.X-ray and IR investigations of different solids showed the presence of free thoria and zirconia together with new thorium—cobalt and zirconium—cobalt compounds. However, the slow cooling of Zr-treated cobalt oxides from 1000°C to room temperature led to the decomposition of the newly formed compound. The d-spacings and absorption bands of the newly formed compounds were determined.     

Novel red-emitting phosphors, (Mg(1−x−y)MnxDyy)Al2Si2O8 and (Mg(1−x−y)MnxTmy)Al2Si2O8

Mn⁴⁺ doped and Dy³⁺, Tm³⁺ co-doped MgAl₂Si₂O₈-based phosphors were prepared by conventional solid state reaction at 1,300 °C. They were characterized by thermogravimetry, differential thermal analysis, X-ray powder diffraction, photoluminescence, and scanning electron microscopy. The luminescence mechanism of the phosphors, which showed broad red emission bands in the range of 600–715 nm and had a different maximum intensity when activated by UV illumination, was discussed. Such a red emission can be attributed to the ²E → ⁴A₂ transitions of Mn⁴⁺.     

Interaction between Co2P4O12 and alkali metal nitrate (chloride) melts

Reactions between Co₂P₄O₁₂ and alkali metal nitrate (chloride) melts was studied in the temperature range 350–400°C (800–850°C) at different phosphate/melt ratios. The effect of the nature of an alkali metal and the ratio between the components in the Co₂P₄O₁₂-M^INO₃ (M^ICl) system on composition of the reaction products was established. The resulting crystalline phases (NaCoPO₄, Na₄Co₃(PO₄)₂P₂O₇, Na₉Co₃(PO₄)₅ and KCoPO₄) were studied by X-ray powder diffraction, IR and electron spectroscopy, and scanning electron microscopy. The features of transformation of the Co₂P₄O₁₂ framework into KCoPO₄ under the effect of excessive alkali metal ions in the melt are discussed.     

Effect of Li2O-doping on the thermal stability of cobaltic oxide

The influence of lithium oxide-doping on the thermal stability of Co₃O₄ was studied using DTA, TG, DTG and X-ray diffraction techniques. Pure and doped cobaltic oxide specimens were prepared by thermal decomposition of pure basic cobalt carbonate and the basic carbonate mixed with different proportions of LiOH, in air, at different temperatures between 500 and 1100°C.Pure Co₃O₄ was found to start partial decomposition when heated in air at 830°C yielding the CoO phase. The complete decomposition was effected by heating at 1000°C.Doping of Co₃O₄ with different proportions of Li₂O was found to much increase its thermal stability. The temperatures at which the doped oxide samples started to undergo decomposition were increased to 865, 910 and 1050°C for 0.375, 0.75 and 3% Li₂O-doped solids, respectively. The DTA revealed that the 1.5% Li₂O-doped cobaltic oxide did not undergo any thermal decomposition till 1080°C. The X-ray investigation showed that the prolonged heating of 1.5 and 3% Li₂O-doped solids at 1100°C for 36 h effected only a partial decomposition of Co₃O₄ into CoO. Heating of these solids at temperatures varying between 900 and 1100°C led also to the formation of a new lithium oxide cobaltic oxide phase, the composition of which has not yet been identified.The role of Li₂O in increasing the thermal stability of Co₃O₄ was attributed to the substitution of some of its cobalt ions by Li⁺ ions, according to Verwey and De Boer's mechanism, leading to the transformation of some of the Co²⁺ into Co³⁺ ions thus increasing the oxidation state of the cobaltic oxide lattice.     

Highly dispersed Mn2O3−Co3O4 nanostructures on carbon matrix as heterogeneous Fenton-like catalyst

This work reports the synthesis of various carbon (Vulcan XC-72 R) supported metal oxide nanostructures, such as Mn₂O₃, Co₃O₄ and Mn₂O₃−Co₃O₄ as heterogeneous Fenton-like catalysts for the degradation of organic dye pollutants, namely Rhodamine B (RB) and Congo Red (CR) in wastewater. The activity results showed that the bimetallic Mn₂O₃−Co₃O₄/C catalyst exhibits much higher activity than the monometallic Mn₂O₃/C and Co₃O₄/C catalysts for the degradation of both RB and CR pollutants, due to the synergistic properties induced by the Mn−Co and/or Mn (Co)−support interactions. The degradation efficiency of RB and CR was considerably increased with an increase of reaction temperature from 25 to 45°C. Importantly, the bimetallic Mn₂O₃−Co₃O₄/C catalyst could maintain its catalytic activity up to five successive cycles, revealing its catalytic durability for wastewater purification. The structure–activity correlations demonstrated a probable mechanism for the degradation of organic dye pollutants in wastewater, involving •OH radical as well as Mn²⁺/Mn³⁺ or Co²⁺/Co³⁺ redox couple of the Mn₂O₃−Co₃O₄/C catalyst.     

The effect of different substrate temperatures on the structure and luminescence properties of Y2O3:Bi3+ thin films

Y₂O₃:Bi³⁺ phosphor thin films were prepared by pulsed laser deposition in the presence of oxygen (O₂) gas. The microstructure and photoluminescence (PL) of these films were found to be highly dependent on the substrate temperature. X-ray diffraction analysis showed that the Y₂O₃:Bi³⁺ films transformed from amorphous to cubic and monoclinic phases when the substrate temperature was increased up to 600 °C. At the higher substrate temperature of 600 °C, the cubic phase became dominant. The crystallinity of the thin films, therefore, increased with increasing substrate temperatures. Surface morphology results obtained by atomic force microscopy showed a decrease in the surface roughness with an increase in substrate temperature. The increase in the PL intensities was attributed to the crystallinity improvement and surface roughness decrease. The main PL emission peak position of the thin films prepared at substrate temperatures of 450 °C and 600 °C showed a shift to shorter wavelengths of 460 and 480 nm respectively, if compared to the main PL peak position of the powder at 495 nm. The shift was attributed to a different Bi³⁺ ion environment in the monoclinic and cubic phases.     

Rb3CoO2, a Novel Oxocobaltate(I) Synthesized via the Azide/Nitrate Route

Rb₃CoO₂ was prepared via the azide/nitrate route. Stoichiometric mixtures of the precursors (Co₃O₄, RbN₃ and RbNO₃) were heated in a special regime up to 500 °C and annealed at this temperature for 100 h in silver crucibles. The crystal structure of the obtained red product was solved and refined by powder methods (Pnma, Z = 4, 12.3489(2), 7.6648(1), 6.2251(1) Å). Rb₃CoO₂ is isostructural with K₃CoO₂ and contains Co¹⁺, which is coordinated by two oxygen atoms forming a slightly distorted dumb‐bell. Rb₃CoO₂ decomposes at 580 °C to Rb₂O, Co and CoO.     

Investigation of Luminescence Properties of Eu3+, Dy3+ and Gd3+ Doped MgAl2Si2O8 Red‐emitting Phosphors

Rare‐earth‐doped aluminosilicates of alkaline earth MgAl₂Si₂O₈: Eu³⁺, Dy³⁺ and MgAl₂Si₂O₈: Eu³⁺, Gd³⁺ were synthesized by the solid state reaction method at 1300 ^oC. The phosphors were characterized by X‐ray powder diffraction (XRD), photoluminescence (PL), thermoluminescence (TL) and scanning electron microscopy (SEM). X‐ray powder diffraction studies show that the phosphors were crystallized in the triclinic crystal system. The phosphors show characteristic broad band phosphorescence of Eu³⁺. This broad band phosphorescence has red emission bands in the range of 580–705 nm corresponding to ⁵D₀ → ⁷F_j (j:0,2,3,4) transitions of Eu³⁺.     

Synthesis,Crystal Structure,and Hydrogen Bonding of μ‐Hydroxo‐μ‐peroxo‐bis[bis(ethylenediamine)cobalt(III)] Squarate

Black‐brown needle‐shaped single crystals of [Co₂(en)₄(O₂)(OH)][C₄O₄]_1.5 · 4H₂O (en = ethylenediamine) were prepared in aqueous solution at room temperature [space group P$\bar{1}$ (no.2) with a = 800.20(8), b = 1225.48(7), c = 1403.84(9) pm, α = 100.282(5), β = 94.515(7), and γ = 95.596(6)°]. The Co³⁺ cations [Co(1), Co(2)] are coordinated in an octahedral manner by four nitrogen atoms stemming from the ethylenediamine molecules and two oxygen atoms each from a hydroxo group and a peroxo group, respectively. Both Co³⁺ coordination polyhedra are connected by a common corner and by the peroxo group leading to the dinuclear [(en)₂Co(O₂)(OH)Co(en)₂]³⁺ cation. The squarate dianions, not bonded to Co³⁺, and the [(en)₂Co(O₂)(OH)Co(en)₂]³⁺ cations are linked by hydrogen bonds forming a three‐dimensional supramolecular network containing water molecules. Magnetic measurements revealed a diamagnetic behavior indicating a low‐spin electron configuration of Co³⁺. The UV/Vis spectra show two LMCT bands [π*(O₂^2–) → d_σ*(Co³⁺)] at 274 and 368 nm and the d–d transition (¹A_1g → ¹T_1g) at 542 nm. Thermoanalytical investigations in air show that the compound is stable up to 120 °C. Subsequent decomposition processes to cobalt oxide are finished at 460 °C.     

Synthesis, characterization, photoluminescence and EPR investigations of Mn doped MgAl2O4 phosphors

MgAl₂O₄:Mn phosphors have been prepared at 500 °C by combustion route. Powder X-ray diffraction (XRD) indicated the presence of mono-MgAl₂O₄ phase. Scanning electron microscopy showed that the powder particle crystallites are mostly angular. Fourier transform infrared spectroscopy confirmed the presence of AlO₆ group which makes up the MgAl₂O₄ spinel. Photoluminescence studies showed green/red emission indicating that two independent luminescence channels in this phosphor. The green emission at 518 nm is due to ⁴T₁ → ⁶A₁ transition of Mn²⁺ ions. The emission at 650 nm is due to the charge-transfer deexcitation associated with the Mn ion. EPR spectrum exhibits allowed and forbidden hyperfine structure at g=2.003. The g≈2.00 is due to Mn²⁺ ion in an environment close to tetrahedral symmetry. It is observed that N and χ increase with decrease of temperature obeying the Boltzmann law. The variation of zero-field splitting parameter (D) with temperature is evaluated and discussed.     

Nanocrystals of Co2+‐Doped MgGa2O4: Preparation by a Low‐Temperature Combustion Method and Optical Properties

Co²⁺‐doped MgGa₂O₄ nanocrystals were prepared at 500°C by a low‐temperature combustion method without any further calcination. Powder X‐ray diffraction (XRD) indicated that the only crystalline phase in the product was MgGa₂O₄ with a grain size of 33–36 nm. Scanning electron microscopy (SEM) showed that the products contained pores formed by the gases evolved during the combustion reaction. The excitation and emission spectra of the nanocrystalline powders in the visible and near infrared regions were characteristic of tetrahedral Co²⁺ ions, suggesting that Co²⁺ ions replaced tetrahedral Mg²⁺ ions in the MgGa₂O₄ crystals. We assigned the visible and near infrared luminescent bands to the spin‐allowed ⁴T₁(⁴P) → and ⁴A₂(⁴F) and ⁴T₁(⁴P) → ⁴T₂(⁴F) transitions.