Yurong Shi

Highly dispersive {001} facets-exposed nanocrystalline TiO2 on high quality graphene as a high performance photocatalyst

A series of composites of nanocrystalline TiO2 with exposed {001} facets and high quality graphene sheets (GS) were synthesized via a one-step hydrothermal reaction in an ethanol–water solvent. The obtained {001} facets-exposed TiO2/GS photocatalysts were characterized by Raman spectroscopy (RS), X-ray photoelectron spectroscopy (XPS), X-ray diffraction pattern (XRD), field emission scanning electron microscopy (SEM), transmission electron microscopy (TEM), ultraviolet visible (UV-vis) diffuse reflectance spectroscopy (DRS) and Electrical Impedance Spectroscopy (EIS). They showed a better photocatalyst properties than P25 and other normal TiO2/GS composites, which could be explained on the basis of the formation of chemical Ti–O–C bond and the formation of nano-sized Schottky interfaces at the contacts between TiO2 and GS. The influence of the exposed {001} facets on the photocatalytic activity was investigated. The result showed that positively charged dye molecules are preferentially adsorbed onto the TiO2/GS composites due to the photogenerated charge gathered on GS. Overall, this work could provide new insights into the fabrication of TiO2–carbon composites as high performance photocatalysts and facilitate their application in addressing environmental protection issues.

Synthesis, crystal structure and luminescence characteristics of a novel red phosphor Ca19Mg2(PO4)14:Eu3+ for light emitting diodes and field emission displays

In order to explore a new family of phosphate phosphors, the synthesis and crystal structure of a novel phosphate family, Ca19M2(PO4)14 (M = Mg, Zn, Mn), were investigated. Eu3+ doped Ca19Mg2(PO4)14 red phosphor was also successfully synthesized with the objective of application in ultraviolet-based light-emitting diodes (LEDs) and field emission displays (FEDs). The characteristic photoluminescence properties were studied in detail by photoluminescence excitation (PLE), emission (PL) spectra and decay times. The Ca19Mg2(PO4)14:0.06Eu3+ phosphor offers higher brightness and thermal stability than the commercial Y2O3:Eu3+. The unexpected temperature-dependent luminescence from higher 5D1 states was observed and was explained via the configuration coordinate diagram. The cathodoluminescence (CL) spectra as a function of accelerating voltage and probe current were also measured. Excellent degradation properties with good color stability were obtained by continuous low-voltage electron-beam excitation of the phosphor. The results indicate that the phosphor Ca19Mg2(PO4)14:0.06Eu3+ can be a suitable red-emitting phosphor candidate for LEDs and FEDs.

Tunable luminescence Y 3 Al 5 O 12 :0.06Ce 3+ , xMn 2+ phosphors with different charge compensators for warm white light emitting diodes

Y₃Al₅O₁₂:0.06Ce³⁺, xMn²⁺ (YAG:0.06Ce,xMn) phosphors have been synthesized and the effect of different charge compensators on the color adjustment has been investigated for the first time. The luminescence properties of Mn2+ singly doped and co-doped with Ce3+ into YAG host have been discussed. It is observed that in singly doped sample, Mn2+ ions not only occupy two kinds of Al3+ sites to generate a yellow and a deep red emission bands, but also occupy Y3+ sites to obtain a green emission band in YAG host. Considering Mn2+ substitution for Al3+, quadrivalence ions including Zr4+, Ge4+ and Si4+ ions are introduced to balance the charge difference. The results show that Si4+ as charge compensator exhibits the best tunable effect on controlling the Mn2+ emissions in YAG:0.06Ce, xMn. In Si4+-Mn2+ co-doped samples, the emission color can be tuned from greenish-yellow to red with increasing the content of Mn2+. The Commission International de L’Eclairage (CIE) chromaticity coordinates are also investigated.

Red-emitting oxonitridosilicate phosphors Sr2SiNzO4−1.5z:Eu2+ for white light-emitting diodes: structure and luminescence properties

A novel red-emitting oxonitridosilicate phosphors, Sr2SiNzO4−1.5z:Eu2+ (0.7 < z < 1.2), was prepared by solid state reaction in NH3–N2 atmosphere. The crystal structure was determined by Rietveld analysis on powder X-ray data. Sr2SiNzO4−1.5z (0.7 < z < 1.2) crystallizes in an orthorhombic structure with the space group of Pmnb: ba−c (no. 62), and cell parameter a = 5.67366(5) A, b = 7.09777(4) A, c = 9.75112(1) A. Sr2SiNzO4−1.5z:Eu2+ (0.7 < z < 1.2) exhibited broad-band red emission centred at ∼620 nm (FWHM ≈ 95 nm) under blue light irradiation with a high QE value of 78.0% and good thermal stability, its emission intensity remains 87% at 150 °C of that measured at room temperature. The outstanding luminescent properties allow it to be an attractive red luminescent material for white LEDs.

Enhanced Photoluminescence and Thermal Properties of Size Mismatch in Sr2.97-x-yEu0.03MgxBaySiO5 for High-Power White Light-Emitting Diodes

In this Study, Mg(2+) and Ba(2+) act to enhance the maximum emission of Sr2.97SiO5:0.03Eu(2+) significantly and redshift the emission band to the orange-red region in Sr(2.97-x-y)Mg(x)Ba(y)SiO5:0.03Eu(2+). Size mismatch between the host and the doped cations tunes the photoluminescence spectra shift systematically. A slight blue shift when increasing the amount of Mg(2+) occurs in the Sr(2.97-x)Eu0.03Mg(x)SiO5 lattices, and a rapid red shift occurs when Ba(2+) is codoped in the Sr(2.57-y)Eu0.03Mg0.4Ba(y)SiO5 lattices. The emission spectra were tuned from 585 to 601 nm by changing the concentration of Ba(2+). Accordingly, we propose the underlying mechanisms of the changes in the photoluminescence properties by adjusting the cation composition of phosphors. The influence of the size mismatch on the thermal quenching is also observed. This mechanism could be widely applied to oxide materials and could be useful in tuning the photoluminescence properties, which are sensitive to local coordination environment. The emission bands of Sr(2.97-x-y)Eu0.03Mg(x)Ba(y)SiO5 show the blue shift with increasing temperature, which could be described in terms of back tunneling of the excited electrons from the low-energy excited state to the high-energy excited state. Thus, the Sr(2.97-x-y)Eu0.03Mg(x)Ba(y)SiO5 phosphors could have potential applications in the daylight LEDs or warm white LEDs.

A Single-Component White-Emitting CaSr2Al2O6:Ce3+, Li+, Mn2+ Phosphor via Energy Transfer

A series of single-component Ce(3+), Li(+), Mn(2+) ions codoped color-tunable CaSr2Al2O6 phosphors were synthesized by a high-temperature solid-state reaction, and the photoluminescence properties as well as the energy transfer mechanism from Ce(3+) to Mn(2+) ions have been investigated in detail. The Ce(3+) activated phosphors have strong absorption in the range of 250-420 nm and can give a blue emission centered at about 460 nm. When Mn(2+) ions are codoped, the emission of CaSr2Al2O6:Ce(3+), Li(+), Mn(2+) phosphors can be tuned from blue to red through adjusting the doping concentration of the Mn(2+) ions, under the irradiation of 358 nm. When the concentration of Mn(2+) is increased to 0.02, a warm-white light can be obtained with good CIE coordinates of (0.388, 0.323) and a low CCT of 3284 K. The energy transfer mechanism from the Ce(3+) to Mn(2+) ions is demonstrated to be a quadrupole-quadrupole interaction based on the analysis of the decay curves of the phosphors. The thermal quenching stability was also investigated. The results indicate that CaSr2Al2O6:Ce(3+), Li(+), Mn(2+) samples might have potential applications in w-LEDs.

Electronic structure and photo/cathodoluminescence properties investigation of green emission phosphor NaBaScSi2O7:Eu2+ with high thermal stability

As new light sources for next-generation illumination, white light-emitting diodes (LEDs) have been developed extensively and are commercially available due to their excellent advantages. However, the current white LEDs present in the market based on the combination of a blue chip and a yellow phosphor cannot satisfy the need for indoor illumination or some other colourful fields due to the lack of a sufficient red spectral component. Here we report a green phosphor, NaBaScSi2O7:Eu2+, which can be effectively excited using a near-ultraviolet chip and emit bright green light with extremely excellent thermal stability. The electronic structure and characteristic photoluminescence and cathodoluminescence properties as well as the thermal quenching properties were investigated in detail. The origin of the desired green luminescence was also determined by analyzing the crystal structure and measuring fluorescence lifetimes and the site-selective excitation and emission spectra. In addition, to investigate its application in field emission displays, the cathodoluminescence (CL) spectra of NaBaScSi2O7:Eu2+ as a function of the accelerating voltage, probe current and the electron radiation time were also measured and discussed in detail. The current results indicate that NaBaScSi2O7:Eu2+ can serve as a potential green phosphor for application in high-power white LEDs and field emission displays.

A new type of color tunable composite phosphor Y2SiO5:Ce/Y3Al5O12:Ce for field emission displays

A simple but effective strategy was introduced to realize color tunability of a composite phosphor Y2SiO5:Ce/Y3Al5O12:Ce (YSO:Ce/YAG:Ce). The main idea was to use the cathodoluminescence of the YSO:Ce phosphor to additionally pump the photoluminescence of the YAG:Ce phosphor based on radiative energy transfer. Morphology as well as the cathodoluminescence properties of the YSO:Ce/YAG:Ce phosphors were investigated in detail. According to radiative energy transfer, an intense yellow emission with excellent cathodoluminescence properties can be obtained from YSO:Ce/YAG:Ce phosphors under low voltage electron beam excitation. Moreover, the emission color of the composite phosphors can be tuned from yellow to blue through adjusting the YSO:Ce content. The mechanism for the enhanced yellow emission and the color tunability were also discussed. It was experimentally proved that the color gamut and display hue could be greatly enriched and enhanced when employing the YSO:Ce/YAG:Ce composite phosphor as an additional phosphor for the typical tricolor FED phosphors.

Crystal structure and luminescence properties of a cyan emitting Ca10(SiO4)3(SO4)3F2:Eu2+ phosphor

Europium-doped apatite Ca10(SiO4)3(SO4)3F2 (CSSF) has been successfully synthesized by solid state reaction. The crystal structure of CSSF:0.006Eu2+ is refined by the Maud refinement method. Optical properties of the prepared samples are found to depend on the rare-earth metal–oxygen distances and lattice iconicity. The excitation spectra of the CSSF:Eu2+ phosphors centered at 350 nm and covered the range from 250 to 450 nm. Under 350 nm excitation, the emission spectra of CSSF:Eu2+ phosphors show a blue (centered at 420 nm) and a green (centered at 525 nm) emission band, respectively. Meanwhile, the concentration quenching and energy transfer mechanism have been investigated via the configuration coordinate diagram. The key parameters, such as the temperature-dependent photoluminescence and CIE values of the CSSF:Eu2+ phosphor have also been studied.

Full-color emission generation from single phased phosphor Sr 10 [(PO 4 ) 5.5 (BO 4 ) 0.5 ](BO 2 ): Ce 3+ , Mn 2+ , Tb 3+ for white light emitting diodes

A single-phase full-color emitting Sr10[(PO4)5.5(BO4)0.5](BO2): Ce3+, Mn2+, Tb3+ phosphor was synthesized by solid-state reaction for the first time. The characteristic luminescence property of Ce3+ is investigated by Gauss fitting. Energy transfer from Ce3+ to Mn2+ and Ce3+ to Tb3+ in Sr10[(PO4)5.5(BO4)0.5](BO2) is detailedly studied by luminescence spectra, energy-transfer efficiency and lifetimes. Through effective energy transfer, the wavelength-tunable warm white light can be realized with superior chromaticity coordinates of (0.35, 0.32), high color rendering index (Ra = 89) and low correlated color temperature (CCT = 4373K) by coupling the emission bands centered at 441, 542 and 649 nm attributed to the contribution from Ce3+, Mn2+ and Tb3+, respectively. The results indicate the white Sr10[(PO4)5.5(BO4)0.5](BO2):Ce3+, Mn2+, Tb3+ phosphor can serve as a promising candidate for phosphor-converted white-light UV-LEDs.