Lefu Mei

Crystal structure and Temperature-Dependent Luminescence Characteristics of KMg4(PO4)3:Eu2+ phosphor for White Light-emitting diodes

The KMg4(PO4)3:Eu2+ phosphor was prepared by the conventional high temperature solid-state reaction. The crystal structure, luminescence and reflectance spectra, thermal stability, quantum efficiency and the application for N-UV LED were studied respectively. The phase formation and crystal structure of KMg4(PO4)3:Eu2+ were confirmed from the powder X-ray diffraction and the Rietveld refinement. The concentration quenching of Eu2+ in the KMg4(PO4)3 host was determined to be 1mol% and the quenching mechanism was certified to be the dipole–dipole interaction. The energy transfer critical distance of as-prepared phosphor was calculated to be about 35.84Å. Furthermore, the phosphor exhibited good thermal stability and the corresponding activation energy ΔE was reckoned to be 0.24eV. Upon excitation at 365nm, the internal quantum efficiency of the optimized KMg4(PO4)3:Eu2+ was estimated to be 50.44%. The white N-UV LEDs was fabricated via KMg4(PO4)3:Eu2+, green-emitting (Ba,Sr)2SiO4:Eu2+, and red-emitting CaAlSiN3:Eu2+ phosphors with a near-UV chip. The excellent color rendering index (Ra = 96) at a correlated color temperature (5227.08K) with CIE coordinates of x = 0.34, y = 0.35 of the WLED device indicates that KMg4(PO4)3:Eu2+ is a promising blue-emitting phosphor for white N-UV light emitting diodes (LEDs).

Emission red shift and energy transfer behavior of color-tunable KMg4(PO4)3:Eu2+,Mn2+ phosphors

Eu2+- and Mn2+-co-doped KMg4(PO4)3 phosphors were prepared via conventional high temperature solid-state reactions. Their crystal structures, luminescence properties, emission red shifts, and energy transfer between Eu2+ and Mn2+ were investigated systematically. Under excitation at 365 nm, KMg4(PO4)3:Eu2+,Mn2+ phosphors exhibited a broad excitation band ranging from 250 to 425 nm and two broad emission bands that peaked at 450 nm and 625 nm, which were ascribed to the 4f–5d transition of Eu2+ and the 4T1 → 6A1 transition of Mn2+ ions, respectively. Three emission bands of Mn2+ were observed in KMg4(PO4)3: Eu2+,Mn2+, which can be attributed to the disordering of Mn2+ in the Mg2+ sites to form different luminescence centers. The energy transfer between the Eu2+ and Mn2+ ions is of a resonant type via a dipole–quadrupole mechanism. The emission red shift that takes place with increasing Mn2+ concentration and operating temperature are discussed in relation to the crystal structure and energy transfer in KMg4(PO4)3:Eu2+,Mn2+. Utilizing the redshift and the energy transfer from Eu2+ to Mn2+, KMg4(PO4)3:Eu2+,Mn2+ phosphors can be tuned from blue to pink by appropriate adjustment of the Mn2+ content and may have potential application for white light-emitting diodes and plantlet culturing.

Photocatalytic Degradation of Methylene Blue Using TiO2 Impregnated Diatomite

Nano-TiO2 showed a good catalytic activity, but it is easy to agglomerate, resulting in the reduction or even complete loss of photocatalytic activity. The dispersion of TiO2 particles on porous materials was a potential solution to this problem. Diatomite has high specific surface and absorbability because of its particular shell structure. Thus, TiO2/diatomite composite, prepared by loading TiO2 on the surface of diatomite, was a good photocatalyst, through absorbing organic compounds with diatomite and degrading them with TiO2. Scanning electron microscopy (SEM), energy dispersive spectrum (EDS), X-ray diffraction (XRD), chemical analysis, and Fourier transform infrared spectrometry (FTIR) indicated that TiO2 was impregnated well on the surface of diatomite. Furthermore, TiO2/diatomite was more active than nano-TiO2 for the degradation of methylene blue (MB) in solution. MB at concentrations of 15 and 35 ppm can be completely degraded in 20 and 40 min, respectively.

Ca6La4(SiO4)2(PO4)4O2:Eu2+: a novel apatite green-emitting phosphor for near-ultraviolet excited w-LEDs

A novel apatite phosphor Ca6La4(SiO4)2(PO4)4O2:Eu2+ was prepared by conventional high-temperature solid-state reaction. Phase purity was examined by XRD and XPS analysis. The crystal structure information, such as space group, cell parameters and atomic coordinates, were refined by the Rietveld method, revealing that Eu2+ occupied the sites of Ca2+ ions. Moreover, low-temperature experiments, including low-temperature PL spectra and low-temperature decay curve, were used to prove the existence of two luminescence centers in Ca6La4(SiO4)2(PO4)4O2:Eu2+. With the increase in doping concentration of Eu2+, the emission wavelength shows a red shift from 498 nm to 510 nm, which is mainly caused by the increase in crystal-field splitting by Eu2+. The optimized concentration of Eu2+ was confirmed to be 0.01, the Rc was calculated to be 20.09 A and the energy transfer between Eu2+ was demonstrated to be by exchange interaction. Moreover, good thermal stability has been proved by a temperature-dependence experiment; it shows that the phosphor can maintain 55% of emitting intensity at 150 °C compared to that at room temperature. Finally, the Ca6La4(SiO4)2(PO4)4O2:Eu2+ phosphor was fabricated with commercial red (CaAlSiN3:Eu2+) and blue (BAM:Eu2+) phosphor coating on a n-UV chip. This proves that this green phosphor has the potential to be used in a w-LED lamp.

A novel single-phase white light emitting phosphor Ca9La(PO4)5(SiO4)F2:Dy3+: synthesis, crystal structure and luminescence properties

A novel single-phase white light emitting phosphor Ca9La(PO4)5(SiO4)F2:Dy3+ was prepared through traditional high-temperature solid state technology. The crystal structures of Ca9La(PO4)5(SiO4)F2 with or without Dy3+ ions were refined by the Rietveld method. The diffuse reflection spectra, excitation spectra, emission spectra, and decay times were characterized to investigate the photoluminescence properties for application in white light-emitting diodes. The results showed that the Ca9La(PO4)5(SiO4)F2:Dy3+ phosphor could efficiently assimilate n-UV light and emit blue (∼485 nm) and yellow light (∼580 nm), originating from the f–f transitions of Dy3+. The critical Dy3+ quenching concentration (QC) was determined to be about 15 mol%, and the corresponding QC mechanism was verified to be the dipole–dipole interaction. Additionally, the emission colors of all samples were located close to the ideal white light region, and the optimal chromaticity coordinates and correlated color temperature (CCT) were determined to be (x = 0.338, y = 0.336) and 5262 K. All the above results indicate that the as-prepared Ca9La(PO4)5(SiO4)F2:Dy3+ phosphor could serve as a promising candidate for white-light n-UV-LEDs.

Luminescence and energy transfer of a color tunable phosphor: Tb3+ and Eu3+ co-doped ScPO4

A series of novel emission-tunable ScPO4:xTb3+, yEu3+ phosphors were prepared by a high temperature solid-state reaction. The phase purity was examined using X-ray diffraction refinement. X-ray photoelectron spectroscopy (XPS) and the crystal information, luminescence properties and energy transfer between Tb3+ and Eu3+ are analyzed systematically. The cross relaxation from 5D3 to 5D4 of single doped Tb3+ in the host is investigated. The energy transfer between Tb3+ and Eu3+ has been demonstrated by the decay times, which are ascribed to the dipole–dipole (d–d) mechanism, and the ηT reaches 54.4%. Additionally, the energy transfer critical distance between Tb3+ and Eu3+ was calculated to be about 12.95 A. The emission color can be adjusted from green to yellow to orange-red by tuning the ratio of Tb3+/Eu3+. The ScPO4:0.03Tb3+, 0.025Eu3+ exhibits good thermal stability, indicating its great potential in w-LED applications.

Design of a Yellow-Emitting Phosphor with Enhanced Red Emission via Valence State-control for Warm White LEDs Application.

The phosphor-converted warm W-LED have being rapidly developed due to the stringent requirements of general illumination. Here, we utilized a strategy to synergistically enhance the red region and emission intensity of novel Eu-activated yellow-emitting LaSiO2N phosphors. This was realized by predicting optimum crystal structure, and governing the concentration of doping ions as well as preparation temperature. By using these straight-forward methods, we were able to vary the valence to enhance the red region and improve the quantum efficiency of LaSiO2N phosphor. The warm W-LED lamp fabricated with this red region enhanced LaSiO2N:Eu phosphor exhibited high CRI (Ra = 86), suitable CCT (5783 K) and CIE chromaticity (0.33, 0.36), indicating this synergistically enhanced strategy could be used for design of yellow-emitting phosphor materials to obtain warm W-LEDs.

Preparation, crystal structure and up-conversion luminescence of Er3+, Yb3+ co-doped Gd2(WO4)3

Up-conversion (UC) phosphors Gd2(WO4)3:Er3+/Yb3+ were synthesized by a high temperature solid-state reaction method. The crystal structure of Gd2(WO4)3:3% Er3+/10% Yb3+ was refined by Rietveld method and it was showed that Er3+/Yb3+ were successfully doped into the host lattice replacing Gd3+. Under 980 nm laser excitation, intense green and weak red emissions centered at around 532 nm, 553 nm, and 669 nm were observed, which were assigned to the Er3+ ion transitions of 4H11/2 → 4I15/2, 4S3/2 → 4I15/2 and 4F9/2 → 4I15/2, respectively. The optimum Er3+ doping concentration was determined as 3 mol% when the Yb3+ concentration was fixed at 10 mol%. The pump power study indicated that the energy transfer from Yb3+ to Er3+ in Er3+, Yb3+ co-doped Gd2(WO4)3 was a two-photon process, and the related UC mechanism of energy transfer was discussed in detail.

Ca/Sr ratio dependent structure and up-conversion luminescence of (Ca1-xSrx)In2O4: Yb3+/Ho3+ phosphors

Up-conversion (UC) phosphors of (Ca1−xSrx)In2O4 : Yb3+/Ho3+ (x = 0, 0.1, 0.3, 0.5, 0.7, 0.9, 1.0) were prepared. Based on the crystal structure evolution of these series solid solution samples, which were characterized by Rietveld refinement, the variation of UC luminescent properties was discussed in detail. Sr and Ca occupied one position and Yb/Ho dissolved in the In ion site in the (Ca1−xSrx)In2O4 lattice. With increasing Sr substituting Ca atoms, the cell parameters and cell volumes of these samples increased linearly, and distortions of (Ca/Sr)O8 polyhedron were formed. The distortions on crystal structures showed a negative relation with UC luminescent intensities in these series phosphors.

A novel luminescence probe based on layered double hydroxides loaded with quantum dots for simultaneous detection of heavy metal ions in water

As most heavy metals are highly toxic upon accumulation in the human body, it is urgent to develop accurate, low-cost, and on-site methods to detect multiple heavy metal ions in real water samples. Quantum dots (QDs) are an approved choice for use in sensors and exhibit favorable luminescence in aqueous solution, but they often become quenched when isolated from their suspensions due to agglomeration. Therefore, QDs must exist in a solid state in order to be successfully applied to luminescence detection. This work reports the fabrication of a novel luminescence composite based on glutathione-capped Mn-doped ZnS quantum dots (GSH-Mn-ZnS QDs) and layered double hydroxides (LDH). The composite is solid and exhibits enhanced luminescence intensity, as the structure of LDH prevents the aggregation of QDs. Most importantly, it exhibits a similar response when used as a sensor for detecting Pb2+, Cr3+ and Hg2+ with a linear range of 1 × 10−6 M to 1 × 10−3 M for each heavy metal, and a detection limit for the mixed metal ions of 9.3 × 10−7 M. In addition, the composite was successfully applied for detection in lake water with low interference. Therefore, a practical method is presented for the design and fabrication of a QD–LDH composite that can be used for qualitative and quantitative testing of mixed heavy metal ions simultaneously in real water samples.