Development of deep red–emitting CaBiVO5:Pr3+ phosphor for multifunctional optoelectronic applications

Orthorhombic Pr³⁺-doped calcium bismuth vanadate (CBV: Pr³⁺) phosphors have been synthesized successfully via a citrate-gel method. The single-phase formation of CBV: Pr³⁺ phosphor has been endorsed by X-ray diffraction (XRD) analysis. The scanning electron microscopy (SEM) image reveals dense-particle packaging with the quasi-spherical shape for the prepared CBV: Pr³⁺ phosphors. Under blue light excitation, CBV: Pr³⁺ phosphors exhibit intense red emission bands located at 608 and 656 nm wavelengths, overlapping with the absorption spectrum of P_R phytochrome, which is present in plants. To achieve the maximum red intensity, the Pr³⁺ ion concentration is optimized to be 1.25 mol% in the CBV host, after which the emission intensity ceases due to concentration quenching. Dexter's theory disclosed the possibility of d-d multipolar interaction among Pr³⁺ ions at higher concentrations of Pr³⁺ ions in the CBV host. The CIE coordinates are found to be positioned in the pure red region for CBV: Pr³⁺ phosphor and in the proximity of red-emitting commercial phosphor. The temperature-dependent spectral studies manifest substantial thermal stability of the as-synthesized phosphor. All the studies mentioned above specify the tremendous potentiality of thermally stable CBV: Pr³⁺ phosphor in agricultural lighting and w-LED applications.     

Tunable photoluminescence of apatite phosphor Ca5.95−xSrxLa4(SiO4)2(PO4)4O2:0.05Eu2+ and its application in light-emitting diodes

A series of new apatite phosphors Ca_5.95−xSr_xLa₄(SiO₄)₂(PO₄)₄O₂:0.05Eu²⁺ (x = 0-5.95) were prepared with the solid-state method. The variations of the occupation rate and cell parameters were investigated in detail, demonstrating that the phosphors are pure phases and that the different occupation rates of La³⁺, Ca²⁺, and Sr²⁺ ions are due to the different electrostatic bond strengths. The reflectance and photoluminescence excitation spectra prove that the phosphors can be efficiently excited with near-ultraviolet (n-UV) light. The broad redshift (50 nm) in the photoluminescence spectra is attributed to the increase in the crystal field splitting when the Ca²⁺ ion is replaced by the larger Sr²⁺ ion. At 150°C, the obtained phosphors maintain an emission intensity of ~67%-77% of that at room temperature (25°C), which indicates relatively the high performance of apatite phosphors in the temperature-dependence experiment. Because of the substitution of the small Ca²⁺ ion by the large Sr²⁺ ion, the emission color changes from green to yellow. Finally, a series of self-made light emitting diodes lamps were fabricated by coating the Ca_5.95−xSr_xLa₄(SiO₄)₂(PO₄)₄O₂:0.05Eu²⁺ phosphors with commercial blue and red phosphors on an n-UV chip (λ_ex = 370 nm). The self-made white-emitting lamps display a continuous changing correlated color temperature (4053-9353 K) or commission international de L'eclairgae (from [0.29, 0.28] to [0.38, 0.37]), implying that the series apatite phosphors have great potential to meet the different requirements of applications.     

A single‐phase,thermally stable and color-tunable white light emitting Na2Ca1-x-yCexMnyP2O7 phosphors for white light emitting diodes via energy transfer

The solid-state reaction method was used to develop a series of Na₂Ca_1-x-yCe_xMn_yP₂O₇ phosphors in an H₂–N₂ environment. The crystal structure of the pyrophosphate host, valence state of dopants (Ce, Mn), emission behavior of dopants, energy transfer mechanism, and thermal quenching behavior were thoroughly examined. Doping with Ce³⁺ and Mn²⁺ ions enhanced the photoluminescence characteristics of Na₂Ca_1-x-yCe_xMn_yP₂O₇ while having negligible effect on the host's phase purity. Under 365 nm UV light irradiation, the addition of Ce³⁺ ion in the Na₂CaP₂O₇ host revealed an asymmetric band with the typical blue emission around 415 nm and a shoulder around 455 nm. To obtain white light, Mn²⁺ ion was supplementarily substituted to the present system. When the Mn²⁺ ions concentration was elevated in the Na₂CaP₂O₇ host, the emission intensity of 560 nm peak corresponding to Mn²⁺ transition enhanced significantly at the cost of Ce³⁺ emission of 415 nm. The systematic decrease of Ce³⁺ emission intensity and corresponding increase in the Mn²⁺ intensity with the increase in Mn²⁺ concentration indicated the possibility of effective energy transfer from Ce³⁺ to Mn²⁺ ions. The obtained results indicated that energy transfer from the Ce³⁺ to Mn²⁺ ions governed by dipole-quadrupole interaction. Because of the efficient energy transfer, the blue emission from Ce³⁺ and the orange red emission of Mn²⁺ provide white light from a single host along with high value of activation energy and low thermal quenching behaviour make the present phosphors to be suitable for high-power LEDs.     

Deep-red-emitting SrLaLiTeO6: Mn4+ double perovskites: Correlation between Mn4+-O2− bonding and photoluminescence

Mn⁴⁺-activated deep red-emitting SrLaLiTeO₆ phosphors are investigated for indoor plant growth LED applications for the first time. The phosphors crystallize in monoclinic (P2₁/n) symmetry is isostructural with SrLaLiTeO₆ host. B-site substitution of Mn⁴⁺ ions is confirmed from the redshift of high energy phonon modes in both Raman and IR spectra. The phosphor exhibited a far-red emission centered at 696 nm corresponding to the ²E_g → ⁴A_2g spin-forbidden transition of the Mn⁴⁺ ions. Approximate crystal field parameters depict the weak influence of neighboring ligand fields on Mn⁴⁺ ions and the least covalence of Mn⁴⁺-ligand bonding compared to other double perovskite phosphors. Moreover, the phosphors exhibit excellent thermal stability with an activation energy of 0.23 eV. Phosphor parameters including CCT, color purity, and quantum yield are evaluated and their values meet the requirements of a red-emitting phosphor for LED applications. Furthermore, the PL emission spectrum of SrLaLiTeO₆: Mn⁴⁺ matches with the absorption spectrum of plant phytochromes denoting the prospects of this phosphor for indoor plant growth LED applications.     

Al2O3–Ce:YAG composite phosphor ceramics for white laser lighting: Novel preparation and regulatable properties

In this work, a series of Al₂O₃–Ce:YAG phosphor powders were synthesized by regulating the excess Al³⁺ of (Y,Ce)₃Al₅O₁₂ via coprecipitation method for the first time, where Al³⁺, Ce³⁺, and Y³⁺ elements were uniformly distributed. With the increase of Al³⁺ content, the morphology of the powders changed from wormlike shapes to flaky shapes, and Y₃Al₅O₁₂ phases had a tendency to convert to YAlO₃ phases. The x wt.% Al₂O₃–(Y_0.999Ce_0.001)₃Al₅O₁₂ (x = 20, 30, 40, 50, 60, and 70) composite phosphor ceramics (CPCs) were obtained by vacuum sintering (1775°C × 10 h), where Al₂O₃ and Ce:YAG phases were also well-distributed. When the Al₂O₃ content was 30–40 wt.%, the average grain size of Al₂O₃ was close to that of Ce:YAG. A solid-state laser lighting device was constructed by a 450 nm laser source and CPCs in a reflection mode. By adjusting the laser power, the correlated color temperature (CCT) values of white laser diodes (LDs) were achieved close to the standard white light of 6500 K. Impressively, the white LDs equipped with the 40 wt.% Al₂O₃-containing CPCs showed the optimum CCT of 6498 K (color coordinates: 0.31 and 0.38), as well as a high luminous flux of 1169 lm and efficiency of 166 lm/W at the LD power of 7.05 W. This work has provided a potential idea to optimize the composition uniformity of Al₂O₃–Ce:YAG CPCs as also to explore their excellent performance in the application of white laser lighting.     

Near-ultraviolet light–induced dazzling red emission in CaGd2(MoO4)4:2xSm3+ compounds for phosphor-converted WLEDs

The Sm³⁺-activated CaGd₂(MoO₄)₄ phosphors were prepared through a sol-gel reaction route. From the results of excitation spectrum, three-dimensional emission spectra and contour lines, it was confirmed that the near-ultraviolet (NUV) light was the proper excitation light source for the synthesized phosphors. Under 405 nm irradiation, the luminescent behaviors of the studied samples were revealed to be dependent on the Sm³⁺ ion concentration and its optimal value of 0.03 mol was obtained. Through theoretical analysis, it is evident that the dipole-dipole interaction can be responsible for the involved concentration quenching mechanism in the final products and the critical distance was 39.7 Å. Moreover, the temperature-dependent emission spectra demonstrated that the studied samples had admirable thermal stability and the activation energy was decided to be 0.21 eV. Furthermore, the internal quantum efficiency of the Sm³⁺-activated CaGd₂(MoO₄)₄ phosphors was found to be 21.6%. Finally, to explore the practical applications of obtained compounds for indoor illumination, a white light-emitting diode (WLED) device which contained a NUV chip, prepared phosphors, and commercial blue-emitting and green-emitting phosphors was packaged. The packaged WLEDs device can emit dazzling white light with satisfied color coordinate of (0.305, 0.318), proper color rendering index (82.6), and correlated color temperature (7069 K).     

A single-phase white-emitting La10(SiO4)6O3：Eu2+/Eu3+ phosphor for near-UV LED-based application

A novel single-phase white-emitting phosphor La₁₀(SiO₄)₆O₃ (LSO): xEu has been synthesized by high-temperature solid-state reaction. Its crystal structure, luminescence properties, fluorescence decay time and oxygen vacancies have been characterized by X-ray diffraction (XRD) and photoluminescence (PL) spectra. XRD result shows a typical oxyapatite structure with the space group of P6₃/m. Characteristic excitation and emission peaks of Eu²⁺ and Eu³⁺ were observed from PL studies. The optimum doping concentration of Eu was found to be 7.5 mol% (x = 0.075). In this work, the lifetimes of Eu³⁺ and Eu²⁺ were considerably longer than those from some references. Under the excitation of different near ultraviolet (n-UV) longer wavelengths (λex = 360, 370, and 380 nm), the white light emission can be realized with the CIE chromaticity coordinates (0.3907, 0.3595), (0.3472, 0.3282), and (0.3504, 0.3062) for the phosphor LSO: 0.075Eu. The chromaticity coordinates of the phosphor were all located in the white region. Therefore, it is suggested that the explored LSO: 0.075Eu phosphor can be a good candidate for white light-emitting diodes (W-LEDs) application.     

Color tuning in (Ca1−xSrx)8MgLu(PO4)7:Eu2+,Mn2+ phosphor via host composition design and energy transfer

In this work, a series of Eu²⁺-doped (Ca_1−xSr_x)₈MgLu(PO₄)₇ and Eu²⁺/Mn²⁺-codoped Ca_6.5Sr_1.5MgLu(PO₄)₇ phosphors were prepared via the combustion-assisted solid-state reaction process. XRD patterns and Rietveld refinements were used to verify the incorporations of Sr into Ca₈MgLu(PO₄)₇:Eu²⁺. Upon the same excitation wavelength of 380 nm, the emission peaks of Eu²⁺-doped (Ca_1−xSr_x)₈MgLu(PO₄)₇ (0≤x≤1) phosphors red-shifted from 453 to 519 nm with increasing Sr/Ca ratio. The red-shift of the Eu²⁺ emission with increasing Sr/Ca ratio was ascribed to the change of Eu²⁺ emission at different lattice sites. With variation of the Mn²⁺ content, the emission color of Eu²⁺/Mn²⁺ codoped Ca_6.5Sr_1.5MgLu(PO₄)₇ phosphors exhibited the luminescence tunable from greenish blue to white and eventually to red. The energy transfer from Eu²⁺ to Mn²⁺ in Ca_6.5Sr_1.5MgLu(PO₄)₇ host matrix was demonstrated to be of a resonant type via a dipole- dipole mechanism with the critical distance of ∼16.7 Å. By the Sr substitution for Ca and properly tuning by the relative composition change of Eu²⁺/Mn²⁺, chromaticity coordinates of (0.329, 0.326) can be reached at near UV light excitation. The combination of host composition design and energy transfer may provide a novel strategy to obtain white light and tunable luminescence.     

Sb2O3–ZnO nanospindles: A potential material for photocatalytic and sensing applications

This paper reports the facile synthesis, characterization and applications of Sb₂O₃–ZnO nanospindles. The nanospindles were synthesized by facile diethanolammine assisted hydrothermal process and characterized in detail in terms of their morphological, structural, compositional and optical properties. The detailed characterizations revealed that the prepared nanoellipsoids are well-crystalline, grown in high density and possessing good optical properties. Further, the as-synthesized Sb₂O₃–ZnO nanospindles were found to be an efficient photocatalyst for the degradation of methylene blue (MB) dye under UV light. Sb₂O₃–ZnO nanospindles were also used as an efficient electron mediator to fabricate a robust, highly sensitive and reproducible chemical sensor for the detection of thiourea in aqueous medium. The fabricated chemical sensor possesses high sensitivity of 6.54 µA mmol L⁻¹ cm⁻². The sensing calibration plot was found to be linear (R²=0.91423) over the large concentration range from 1.56 mmol L⁻¹ to 100 mmol L⁻¹. The obtained results confirmed that the Sb₂O₃–ZnO nanospindles may hold great potential for the removal of organic pollutants and for monitoring of thiourea in aqueous solution.     

A high quantum yield red phosphor NaGdSiO4: Eu3+ with intense emissions from the 5D0→7F1,2 transition

Red phosphors with a high quantum yield and a lower thermal quenching are needed to improve the luminescence efficiency and the stability of phosphor-converted white light-emitting diodes (pc-WLEDs). We have designed a high quantum yield NaGdSiO₄ (NGSO) based phosphor with enhanced Eu³⁺ emissions of the ⁵D₀→⁷F₁ and ⁵D₀→⁷F₂ transitions. This design is based on the Eu³⁺ at both the inversion and non-inversion symmetry sites. In detail, we have studied the structure, morphology, and luminescence properties of NGSO: Eu³⁺ phosphors. Using a 394 nm UV excitation, a series of Eu³⁺ emissions of ⁵D₀→⁷F_J (0–4) transitions has been observed. The internal quantum efficiency (IQE) is 83.42% and the red color purity is 91.4%. These values are much higher than some reported results. The higher IQE and double intense ⁵D₀→⁷F₁ and ⁵D₀→⁷F₂ emissions might originate from an unusual structure disorder around Eu³⁺ ions in the NGSO lattice. The lifetime of the optimal phosphor NGSO: 0.5Eu³⁺ is about 2 ms, suitable for solid-state lighting. The intensities of the strong emissions at 595 and 624 nm of NGSO: 0.5Eu³⁺ at 150 °C is about 85% of that at 30 °C, demonstrating its excellent thermal stability. Furthermore, this red NGSO: 0.5Eu³⁺ phosphor was packaged into a warm pc-WLED, exhibiting a lower correlated color temperature (CCT) of 4222 K and a comparable color rendering index (CRI) of 86.7. These results show that this red phosphor could act as a red component of pc-WLEDs excited by the n-UV LED chip.     

Sintering behavior of ZrB2–SiC composites doped with Si3N4: A fractographical approach

ZrB₂–SiC composite ceramics were doped with 0, 1, 3 and 5 wt% Si₃N₄ plus 1.6 wt% carbon (pyrolized phenolic resin) as sintering aids and fabricated by hot pressing process under a relatively low pressure of 10 MPa at 1900 °C for 2 h. For a comparative study, similar ceramic compositions were also prepared by pressureless sintering route in the same processing conditions, with no applied external pressure. The effect of silicon nitride dopant on the microstructural evolution and sintering process of such ceramic composites was investigated by a fractographical approach as well as a thermodynamical analysis. The relative density increased by the addition of Si₃N₄ in hot pressed samples as a fully dense composite was achieved by adding 5 wt% silicon nitride. A reverse trend was observed in pressureless sintered composites and the relative density values decreased by further addition of Si₃N₄, due to the formation of gaseous products which resulted in the entrapment of more porosities in the final structure. The formation of ZrC phases in pressureless sintered samples and layered BN structures in hot pressed ceramics was detected by HRXRD method and discussed by fractographical SEM-EDS as well as thermodynamical analyses.     

Deep-red emission of Mn4+ and Cr3+ in (Li1−xAx)2MgTiO4 (A=Na and K) phosphor: Potential application as W-LED and compact spectrometer

Mn⁴⁺ doped and Mn⁴⁺/Cr³⁺ co-doped alkali metal titanate phosphors have been prepared by solid state reaction method. A part of Li⁺ ions in the Li₂MgTiO₄: Mn⁴⁺ are substituted with Na⁺ and K⁺ ions and consequently the intensity of Mn⁴⁺ emission at 678 nm is enhanced by 1.7 and 2.5 times, respectively. In the Mn⁴⁺/Cr³⁺ co-doped (Li_0.95K_0.05)₂MgTi_0.999O₄, both emission of Cr³⁺at 726 nm and emission of Mn⁴⁺ at 678 nm of Mn⁴⁺ are observed. It is interesting to find that the intensity ratio of 726–678 nm emissions in the Mn⁴⁺/Cr³⁺ phosphor continually increases with excitation wavelength increasing from 290 nm to 455 nm, which means that the intensity ratio in turn can be used to identify the excitation light wavelength. This refers a possible approach to design novel compact light-wavelength detector or spectrometer based on the phosphor. The mechanism of Na⁺ or K⁺ substitution induced luminescence enhancement in the Mn⁴⁺ phosphor and the competition between the Cr³⁺ and Mn⁴⁺ emissions in the Mn⁴⁺/Cr³⁺ co-doped has been discussed.     

Effect of codoping Ce3+ on the luminescence properties of Sr9Mg1.5(PO4)7:Eu2+ orange–yellow phosphor

Sr₉Mg_1.5(PO₄)₇:Eu²⁺ has recently been reported as a promising blue light-excited orange–yellow phosphor that can be used in white LED device. Here, Ce³⁺-codoping is found to be an effective strategy to improve the luminescence performance of Sr₉Mg_1.5(PO₄)₇:Eu²⁺ phosphor. The coexistence of Eu²⁺ and Eu³⁺ ions has been verified via photoluminescence spectral analysis. The reduction of Eu³⁺ to Eu²⁺ in Sr₉Mg_1.5(PO₄)₇ lattice cannot be completed in a reducing atmosphere, but can be promoted through codoping with Ce³⁺ ions to a great extent, which finally increase the effective concentration of Eu²⁺ in the crystal lattice. The Eu³⁺−Eu²⁺ reduction mechanism is analyzed using a charge compensation model. This work not only achieves enhanced luminescence of the Sr₉Mg_1.5(PO₄)₇:Eu²⁺ phosphor by codoping with Ce³⁺ ions, but also provides new insights into the design of Ce³⁺/Eu²⁺ codoped luminescent materials.     

Effect of Sr/Ca substitution on phase structure and photoluminescence properties of micro-SrxCa1−xAlSiN3: Eu2+ phosphor for high CRI white LEDs

Sr_xCa_1−xAlSiN₃: Eu²⁺ phosphors were prepared by using the high temperature solid state reaction in a 1.1 Mpa N₂ atmosphere. The phase structures, photoluminescence (PL) properties, and chromaticity properties of the phosphors affected by Sr/Ca Substitution have been investigated in detail. With increasing Sr content (x value), the crystal grain became bigger and the average grain size increased from 5 µm to 10 µm. PL emission bands of Sr_xCa_1−xAlSiN₃: Eu²⁺ showed a blue-shift from 660 (x=0) to 617 nm (x=0.8), the shoulder of the excitation spectra around 550 nm showed a slightly blue-shift and decay lifetime shortened from 776.96 (x=0.2) to 642.35 ns (x=0.8). Both the emission and excitation intensity of peak position increased with Sr content increased. The ideal white light with high CRI (Ra>88) can be obtained by mixing the Sr_xCa_1−xAlSiN₃: Eu²⁺ phosphors and commercial green phosphors with appropriate proportion of the components.     

Rh–Fe/Ca–Al2O3: A Unique Catalyst for CO-Free Hydrogen Production in Low Temperature Ethanol Steam Reforming

Low temperature ethanol steam reforming (ESR) was studied over a series of 1 wt% Rh–x % Fe catalysts with various Fe loading (x = 0–10 wt%) and on different supports (Ca–Al₂O₃, SiO₂ and ZrO₂). The results show that close interaction between Rh and Fe is required to reduce the CO selectivity to almost negligible values. In addition, Rh–Fe supported on Ca–Al₂O₃ exhibits the best performance in terms of CO selectivity and hydrogen yield as compared to other supports. Characterization by XPS and XANES indicates the presence of Fe_xO_y species upon reduction, resulting in the formation of coordinatively unsaturated ferrous (CUF) active sites along the Rh–Fe_xO_y interface. These CUF sites promote water–gas shift reaction during low temperature ESR. Temperature programmed oxidation and Raman spectroscopy of spent catalysts also indicate that the addition of iron oxide reduces coke deposition and forms more reactive coke. Hence, the catalyst lifespan is significantly extended.     

NiFe2O4@SiO2–PPA Nanoparticle: A Green Nanocatalyst for the Synthesis of β-Acetamido Ketones

ABSTRACT

In this work, silica coated magnetic nanoparticles of modified polyphosphoric acid (NiFe₂O₄@SiO₂–PPA deigned as NFS–PPA) represent as a reusable and green catalyst for one-pot four-component synthesis of β-acetamido ketones at room temperature under solvent-free conditions. This catalyst was synthesized and characterized by X-ray diffractions, transmission electron microscopy, scanning electron microscopy and vibrating sample magnetometry. The magnetic catalyst can be easily removed by a simple magnet and reused up to five times without any loss of its high catalytic activity. In addition to facility, this protocol enhances product purity and promises economic and also environmental profits.     

Regeneration behaviors of Fe/Si‐2 and Fe–Mn/Si‐2 catalysts for C2H6 dehydrogenation with CO2 to C2H4

The catalytic performance of Fe/Si‐2 and Fe–Mn/Si‐2 catalysts for conversion of C₂H₆ with CO₂ to C₂H₄ was examined in a continuous‐flow and fixed‐bed reactor. The results show that the Fe–Mn/Si‐2 catalyst exhibits much better reaction activity and selectivity to C₂H₄ than those of the Fe/Si‐2 catalyst. Furthermore, the coking–decoking behaviors of these catalysts were studied through TG. The catalytic performances of the catalysts after regeneration for conversion of C₂H₆ or dilute C₂H₆ in FCC off‐gas with CO₂ to C₂H₄ were also examined. The results show that both activity and selectivity of the Fe–Mn/Si‐2 catalyst after regeneration reached the same level as those of the fresh catalyst, whereas it is difficult for the Fe/Si‐2 catalyst to refresh its reaction behavior after regeneration. This revised version was published online in July 2006 with corrections to the Cover Date.     

The influence of Mo6+ doping on the luminescence properties of red-emitting phosphor Sr9Eu2W4−xMoxO24 (x = 0–4)

A series of red emitting phosphors Sr₉Eu₂W_4?xMo_xO₂₄ (x = 0–4) have been synthesized by solid-state reactions and their crystal structures, photoluminescence properties were studied. The excitation and emission spectra of Sr₉Eu₂W_4?xMo_xO₂₄ phosphors can be modified by Mo⁶⁺ doping. As the molybdate content increased, the Eu³⁺ emission intensity of Sr₉Eu₂W_4?xMo_xO₂₄ (x = 0–4) under 395 nm excitation was found to increase and reached a maximum at x = 2. The excitation spectra, the emission intensities and the chromaticity coordinates of Sr₉Eu₂W_4?xMo_xO₂₄ (x = 2) were compared to those of the conventional red phosphor Y₂O₂S: Eu³⁺. The intense red-emission under near-UV excitation suggests that Sr₉Eu₂W_4?xMo_xO₂₄ (x = 2) could be a potential candidate for white light generation by using near-UV LEDs. In this study, the effects of Mo⁶⁺ doping on the crystal structure and photoluminescence properties of Sr₉Eu₂W_4?xMo_xO₂₄ were discussed.