Guimei Gao

Preparation and luminescent properties of CaAl2O4:Eu3+,R+ (R=Li, Na, K) phosphors

CaAl2O4:Eu3+, R+(R=Li+ Na+, K+) red phosphors were synthesized by solid state reaction method. X-ray diffraction (XRD) and photoluminescence (PL) were employed to characterize their structural and luminescent properties. It was found that the optimal sintering temperature and sintering time were 1200 degrees C and 4 h, respectively. The optimal concentration of doped Eu3+ was 3 mol.%. Furthermore, under ultraviolet excitation with a wavelength of 254 nm, these samples showed red luminescence which were probably attributed to the transitions from D-5(0) excited state to F-7(J) (J=0-4) ground states of Eu3+ ions. The feature and the high intensity of hypersensitive transition from D-5(0)-> F-7(2) indicated that Eu3+ preferred to occupy a low symmetry site. The incorporation of alkali metal ions greatly enhanced the luminescence intensity probably due to the influence of charge compensation of alkali metal ions.

Removal of Cu2+ from Aqueous Solutions Using Na-A Zeolite from Oil Shale Ash

Abstract Na-A zeolite was synthesized using oil shale ash (OSA), which is a solid by-product of oil shale processing. The samples were characterized by various techniques, such as scanning electron microscopy, X-ray diffraction and Brunauer Emmet Teller method. The batch isothermal equilibrium adsorption experiments were performed to evaluate the ability of Na-A zeolite for removal of Cu (II) from aqueous solutions. The effects of operating parameters, such as concentration of copper solutions, adsorbent dosages, pH value of solutions and temperature, on the adsorption efficiency were investigated. The equilibrium adsorption data were fitted with Langmuir and Freundlich models. The maximum adsorption capacity of Na-A zeolite obtained from the Langmuir adsorption isotherm is 156.7 mg·g −1 of Cu (II). The increase of pH level in the adsorption process suggests that the uptake of heavy metals on the zeolite follows an ion exchange mechanism. The batch kinetic data fit the pseudo-second order equation well. The thermodynamic parameters, such as changes in Gibbs free energy (Δ G ), enthalpy (Δ H ) and entropy (Δ S ), are used to predict the nature of the adsorption process. The negative Δ G values at different temperatures confirm that the adsorption processes are spontaneous.

Synthesis and luminescence properties of a novel red-emitting phosphor SrCaSiO4: Eu3+ for ultraviolet white light-emitting diodes

Abstract A novel red phosphor based on Eu 3+ -activated SrCaSiO 4 was successfully synthesized by conventional solid state reaction method and the photoluminescence properties were investigated. X-ray diffraction (XRD) patterns indicated that SrCaSiO 4 : Eu 3+ phosphors belong to orthorhombic crystal system (space group= Pmnb ). The photoluminescence (PL) excitation spectrum showed broad-band absorption and the strongest excitation peak at 397 nm contributed to the 7 F 0 → 5 L 6 transition which matched well with the emission of a UV chip. The PL emission spectrum with the peaks at 577, 586, 592, 612, 650 and 685 nm, were probably attributed to the transitions from 5 D 0 to 7 F J ( J =0, 1, 2, 3, 4) of Eu 3+ ions. The quenching concentration of Eu 3+ was 10 mol.%, and the critical distance ( R c ) was about 1.2 nm. The fluorescence lifetime of Eu 3+ ions was found to be 3.2 ms. The results indicated that the SrCaSiO 4 : Eu 3+ might become an important red phosphor candidate for white-light-emitting diodes with near-UV LED chips.

Removal of Cu (II) from Aqueous Solution Using a Novel Crosslinked Alumina-Chitosan Hybrid Adsorbent

Abstract A novel biosorbent was developed by coating chitosan, a naturally and abundantly available biopolymer, on to activated alumina based on oil shale ash via crosslinking. The adsorbent was characterized by various techniques, such as Fourier transform infrared spectroscopy, scanning electron microscopy, thermogravimetric-differential thermal analysis, and X-ray photoelectron spectroscope. Batch isothermal equilibrium adsorption experiments were condcted to evaluate the adsorbent for the removal of Cu(II) from wastewater. The effect of pH and agitation time on the adsorption capacity was also investigated, indicating that the optimum pH was 6.0. The equilibrium adsorption data were correlated with Langmuir and Freundlich models. The maximum monolayer adsorption capacity of chitosan coated alumina sorbent as obtained from Langmuir adsorption isotherm was found to be 315.46 mg·g −1 for Cu(II). The adsorbent loaded with Cu(II) was readily regenerated using 0.1 mol·L −1 sodium hydroxide solution. All these indicated that chitosan coated alumina adsorbent not only have high adsorption activity, but also had good stability in the wastewater treatment process.

Controlled formation of a flower-like CdWO4–BiOCl–Bi2WO6 ternary hybrid photocatalyst with enhanced photocatalytic activity through one-pot hydrothermal reaction

In this study, we synthesized a flower-like CdWO4–BiOCl–Bi2WO6 ternary hybrid (CBB) and CdWO4–Bi2WO6 binary composite in the same system using the same raw materials with different molar ratios and compared their photocatalytic activities. The X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and transmission electron microscopy (TEM) results verify that the transformation of the CdWO4–BiOCl–Bi2WO6 ternary hybrid to the CdWO4–Bi2WO6 binary composite is a topological transformation. Their photocatalytic performances are evaluated by monitoring the degradation of Rhodamine B (RhB). The ternary hybrid (CBB-0.05) shows enhanced photocatalytic activity as compared to pure CdWO4, BiOCl, Bi2WO6, BiOCl–Bi2WO6, CdWO4–Bi2WO6 (CBB-0.10) and the other CBB composites. This remarkable enhancement is attributed to the fact that the suitable conduction band (CB) positions in the ternary hybrid can form a cascade structure and the two layered compounds BiOCl and Bi2WO6 can epitaxially grow from one phase to another with little strain at the interface; this increases the separation efficiency of charge carriers. In addition, the mechanism of the high photocatalytic activity is discussed based on the photoluminescence (PL) spectra and photoelectrochemical and active species trapping measurements.