We employed atomic layer deposition (ALD) to deposit ultrathin SiO₂ layers on P25 TiO₂ nanoparticles to fabricate TiO₂/SiO₂ core/shell nanostructures. The ALD process was carried out in a fluidized bed reactor working at atmospheric pressure using SiCl₄ and H₂O as precursors, enabling the deposition of SiO₂ at 100 °C with the ability to control the thickness at the sub-nanometer level. By controlling the thickness of the SiO₂ in a very narrow range, i.e., below 2 nm, the photocatalytic activity of TiO₂ can be tuned. In particular, an enhancement was obtained for the SiO₂ layers with a thickness below 1.4 nm, in which the layer with a thickness of about 0.7 nm exhibited the highest photocatalytic activity; for SiO₂ layers thicker than 1.4 nm, the photocatalytic activity was strongly suppressed. The photocatalytic activity enhancement and the degradation mechanism of RhB by the TiO₂/SiO₂ photocatalysts were investigated by combining X-ray photoelectron spectroscopy, UV–Vis absorption spectroscopy, photoluminescence spectroscopy and the aid of charge carrier and radical scavengers. Our findings have revealed an improvement of photogenerated charge separation due to the SiO₂ coating and the dominating role of hydroxyl radicals in the degradation of RhB.

Experimental data from literature on the thermal quenching of the Eu²⁺ 5d-4f emission in the M_xSi_yN_z (M = alkali, alkaline earth or rare earth) nitridosilicates have been collected and evaluated. No clear correlation was observed between the activation energy for thermal quenching and the Stokes shift, suggesting that non-radiative relaxation via a thermally excited cross-over from the 5d excited state to the 4f ground state is not the main reason for thermal quenching in the nitridosilicates. Based on literature data on rare-earth charge transfer transitions, host-lattice bandgap and Eu²⁺ 5d-4f emission energy, the energy difference between the Eu²⁺ 5d level and the bottom of the host-lattice conduction band has been determined. This energy difference correlates fairly well with the quenching temperature, suggesting that thermal ionization of the 5d electron to the conduction band is responsible for the thermal quenching of the Eu²⁺ 5d-4f emission in the nitridosilicates. The energy difference between the lowest 5d level and the bottom of the conduction band, and consequently the quenching temperature, increases with increasing Si/N ratio of the nitridosilicates. From this, it is concluded that the combined effect of the larger Stokes shift and the raise in energy of the bottom of the host lattice conduction band with increasing Si/N ratio is stronger than the decrease of the centroid shift and crystal field splitting of the Eu²⁺ 5d level.

The red-emitting Sr ₂ Si ₅ N ₈ :Eu ²⁺ phosphor with a superior quantum efficiency and suitable emission spectrum has been widely used as a promising down-conversion material in white light-emitting diodes. However, its thermal degradation under high temperature handicaps its large scale application, and therefore must be reduced. Here, we proposed to increase the thermal stability of Sr ₂ Si ₅ N ₈ :Eu ²⁺ by coating a nanometer-order Al ₂ O ₃ film on each phosphor particle using an atomic layer deposition approach in a fluidized bed reactor. The deposited Al ₂ O ₃ layer was quite uniform and conformal when using O ₃ as the oxidizer, and its thickness could be controlled by the dosage type, deposition temperature and cycle numbers, which largely affects the photoluminescence properties and thermal degradation of the title phosphor. Thermal gravimetric analysis results showed that the oxidation temperature of the coated phosphor increased from 700 to 850 °C, suggesting that the coating layer has the function of anti-oxidation. Meanwhile, the coated phosphor particle surface became hydrophobic. Consequently, the thermal degradation of phosphor powders in air at 200 °C was greatly reduced and the stability of the fabricated LEDs with coated powders was also improved. Prospectively, the proposed approach provides a new strategy to improve the thermal stability of other phosphors.

Terbium and europium co-doped Sr ₈ ZnY(PO ₄ ) ₇ phosphors are successfully prepared through a high temperature solid-state reaction (SSR). The crystal structure of the as-prepared samples was identified to be Sr ₈ ZnY(PO ₄ ) ₇ (SZYP) pure phase by an X-ray powder diffraction technique. Under near-ultraviolet light excitation (378 nm), the SZYP:Tb ³⁺ and SZYP:Eu ³⁺ phosphors show green and red emission peaking at 545 and 618 nm, respectively. Moreover, an effective energy transfer process from Tb ³⁺ to Eu ³⁺ could be verified by the concentration dependence of emission intensity and lifetime. The energy transfer mechanism between Tb ³⁺ and Eu ³⁺ is determined to be governed by dipole-dipole interactions. The internal quantum efficiency (IQE) is evaluated to be as high as about 91%. The temperature-dependent spectra indicate that the SZYP:Tb ³⁺ ,Eu ³⁺ phosphor shows a high thermal stability. Furthermore, the as-fabricated white LED devices exhibit an excellent correlated color temperature (CCT) of 3223 K, a color rendering index (R _a ) of 85.8 and a luminance efficiency of 37.4 lm W ^-1 . All results imply that the SZYP:Tb ³⁺ ,Eu ³⁺ phosphors have a great potential for application in white LEDs.

The encapsulation conditions of Eu²⁺-doped calcium silicon nitride ((Ca_0.90Eu_0.10)₂Si₅N₈) and strontium silicon nitride ((Sr_0.98Eu_0.02)₂Si₅N₈) phosphors into phosphate glass ((50-x)Na₂O–xZnO–5B₂O₃–45P₂O₅; x = 10-40) were examined by pressureless sintering (normal or infrared firing) and subsequent oxygen-assisted hot isostatic pressing. The evaluation of water resistivity, as well as glass transition temperature (T_g), indicated that the excellent water resistivity and low T_g (313°C) could be achieved at x = 30. The colorless and clear glass body could be fabricated by infrared firing at 370°C for 20 minutes in air, followed by the hot isostatic pressing at 370°C for 24 hours (80%Ar + 20%O₂, pressure: 130 MPa). The encapsulations of 3.0 mass% (Ca_0.90Eu_0.10)₂Si₅N₈ and (Sr_0.98Eu_0.02)₂Si₅N₈ into the glass body produced reddish orange light emission (peak wavelength: 604 nm under excitation wavelength of 423 nm) and red-light emission (617 nm under 383 nm), respectively, showing higher chemical and thermal stability of nitride phosphors in the glass matrix.

For plant-growth lighting, novel deep-red emission phosphors with high brightness were obtained by co-doping Ti⁴⁺ and Mn⁴⁺ into a Ca₁₄Al₁₀Zn₆O₃₅ substrate through a conventional solid-state reaction strategy. The nominal Ca_14-(x+y)/2Al_10-x-yZn₆O₃₅:xTi⁴⁺,yMn⁴⁺ (CAZO:Ti⁴⁺,Mn⁴⁺) phosphors could be excited by both near-ultraviolet (NUV) and blue-light-emitting diode (LED) chips efficiently and exhibited a strong deep-red emission band ranging from 650 nm to 750 nm, which should be the result of the ²E → ⁴A₂ transition inside the [MnO₆]^8- octahedral. Multiple energy transfer from Ti⁴⁺ to Mn⁴⁺ was detected in this CAZO; whereby Ti⁴⁺ and Mn⁴⁺ phosphors were verified to be a result of the dipole-dipole interaction under excitation at 270 nm. LED plant-growth lights were fabricated using the as-prepared nominal Ca_13.825Al_9.65Zn₆O₃₅:0.15Mn⁴⁺,0.2Ti⁴⁺,0.005H₃BO₃ (CAZO:Mn⁴⁺,Ti⁴⁺,H₃BO₃) phosphors pumped by a 460 nm blue-chip; this luminaire could be used to greatly promote the cultivation of succulent plants. Combined with the attractive thermal stability as well as high quantum efficiency (QE) of this phosphor, it was demonstrated that these novel phosphors may be candidate deep-red luminescent materials for LED plant lighting.

Optical data of the Eu²⁺ doped nitridosilicates (M_xSi_yN_z) have been collected from the literature and have been analysed with regard to their dependence on structure and composition. Nitridosilicates with a higher degree of condensation, i.e. a higher Si/N ratio, generally have a higher Eu²⁺ 4f-5d absorption energy, a higher 5d-4f emission energy and a larger Stokes shift. The higher absorption and emission energies are due to the increase of the N by Si coordination number with increasing Si/N ratio. This results in more electrons on N that participate in the bonding with Si, and thus less electrons are available for Eu-N bonding, reducing the covalency of the Eu-N bonds. The lower covalency gives a weaker nephelauxetic effect, reducing the centroid shift of the 5d level. The lowest 4f-5d absorption energy further increases due to the reduction of the crystal field splitting of the 5d levels, as the Eu-N bonds become longer with increasing Si/N ratio. The Stokes shift increases with increasing degree of condensation despite an increase of lattice rigidity, ascribed to a decrease of local rigidity around the Eu²⁺ ion caused by the larger Eu-N bond lengths. Some nitridosilicates show deviations from the general trends attributed to peculiarities in their crystal structure and the way Eu²⁺ is substituted in the lattice. The relationships established in the present work will be helpful for the design and exploration of new Eu²⁺ doped nitride-based luminescent materials for practical applications.

To modify the luminescence properties of Ce³⁺-doped Y₃Al₅O₁₂ (YAG) phosphors, they have been coated with a carbon layer by chemical vapor deposition and subsequently heat-treated at high temperature under N₂ atmosphere. Luminescence of the carbon coated YAG:Ce³⁺ phosphors has been investigated as a function of heat-treatment at 1500 and 1650 °C. The 540 nm emission intensity of C@YAG:Ce³⁺ is the highest when heated at 1650 °C, while a blue emission at 400-420 nm is observed when heated at 1500 °C but not at 1650 °C. It is verified by X-ray diffraction (XRD) that the intriguing luminescence changes are induced by the formation of new phases in C@YAG:Ce³⁺-1500 °C, which disappear in C@YAG:Ce³⁺-1650 °C. In order to understand the mechanisms responsible for the enhancement of YAG:Ce³⁺ emission and the presence of the blue emission observed for C@YAG:Ce³⁺ -1500 °C, the samples have been investigated by a combination of several electron microscopy techniques, such as HRTEM, SEM-CL, and SEM-EDS. This local and cross-sectional analysis clearly reveals a gradual transformation of phase and morphology in heated C@YAG:Ce³⁺ phosphors, which is related to a reaction between C and YAG:Ce³⁺ in N₂ atmosphere. Through reaction between the carbon layer and YAG host materials, the emission colour of the phosphors can be modified from yellow, white, and then back to yellow under UV excitation as a function of heat-treatment in N₂ atmosphere.

One of the biggest problems in white light-emitting diodes (WLEDs) is the moisture-induced degradation of phosphors. This paper proposes a simple and feasible surface modification method to solve it, whereby a hydrophobic surface layer is developed on the surface of the phosphors. The particular case of orange-red-emitting Sr₂Si₅N₈:Eu²⁺ (SSN) phosphor was investigated. The mechanism to develop the hydrophobic layer involves hydrolysis and polymerization of tetraethylorthosilicate (TEOS) and polydimethylsiloxane (PDMS). The experimental results showed that the surface layer of SSN phosphor was successfully modified to a hydrophobic nanolayer (8 nm) of amorphous silicon dioxide that contains CH₃ groups in the surface. This hydrophobic surface layer gives the modified phosphor superior stability in high-pressure water steam conditions at 150°C.

RE³⁺ (RE = Pr, Sm, Tb, Er, Dy)-activated Y₄Si₂O₇N₂ samples were prepared by a solid-state reaction method at high temperature, and their photoluminescence properties were investigated. The absorption band located at about 250 nm is attributed to the host absorption. The 5d bands of Pr³⁺ and Tb³⁺ are at rather low energy in Y₄Si₂O₇N₂ compared to oxide. The direct Pr³⁺ 4f² → 4f¹5d¹ excitation at 275 nm leads to typical 4f² → 4f² line emissions (450–700 nm) and strong 4f¹5d¹ → 4f² broad band emission (300–450 nm), respectively. The charge transfer (N³⁻ → Sm³⁺) band of Sm³⁺ was observed at a somewhat lower energy of 4.68 eV compared to oxide, and Sm³⁺-activated sample shows a bright red emission originating from ⁴G_5/2 → ⁶H_J (J = 5/2, 7/2 and 9/2) transitions. For Tb³⁺-doped sample, the direct Tb³⁺ 4f⁸ → 4f⁷5d¹ excitation leads to ⁵D₃ → ⁷F_J (J = 6, 5, 4, 3) (blue) and ⁵D₄ → ⁷F_J (J = 6, 5, 4, 3) (green) line emissions, the cross-relaxation depended on Tb concentration has happened. The incorporation of Er³⁺ (or Dy³⁺) into Y₄Si₂O₇N₂ resulted in a typical Er³⁺ (or Dy³⁺) f-f line absorptions and emissions. Moreover, the energy transfer from the host lattice to the luminescent activators (Pr³⁺, Tb³⁺ and Sm³⁺) is observed. The energy level diagram containing the position of 4f and 5d energy levels of all Ln²⁺ and Ln³⁺ ions relative to the valence and conduction band of Y₄Si₂O₇N₂ has been established and studied based on the data presented in this work, and further provides a platform for studying the photoluminescence properties as well as the valence stability of the lanthanide ions.

To improve the thermal stability, Al₂O₃ has been successfully coated on a Y₃Al₅O₁₂:Ce³⁺ (YAG:Ce) phosphor powder host by using the Atomic Layer Deposition (ALD) approach in a fluidized bed reactor. Transmission Electron Microscopy (TEM) and Energy Dispersive X-ray spectroscopy (EDX) analysis indicate that coating an Al₂O₃ thin layer by ALD is highly feasible. The luminescence properties (such as excitation and emission as well as quantum efficiency and UV-absorption of the coated YAG:Ce phosphor) were systematically analysed, with the further examination of the thermal resistance characteristics. The Al₂O₃ thin layer coating with precisely controlled thickness by ALD can obviously improve the luminescence intensity and greatly enhances the thermal stability of the YAG:Ce phosphor. It is suggested that the alumina coating with tailoring thickness seems not only to act like a barrier to decrease the thermal quenching, but also as a great help to promote the light absorption and transfer.

Phosphor converted white light-emitting diodes (pc-WLEDs) are an alternative choice for general lighting due to their superior features such as high efficiency, durability and reliability. However, most pc-WLEDs in the market suffer from problems resulting from the lack of red emission, which can be resolved by adding red-emitting phosphors. The currently dominant red-emitting phosphors are Eu²⁺-doped nitrides, but the requirement of elevated temperature during synthesis makes them costly, and moreover the over broad emission may result in loss of lumen efficiency. Recently, transition metal Mn⁴⁺ doped materials showing very narrow red emission have attracted tremendous interest for compositions based on abundant resources and mild production processes, resulting in a highly efficient way to obtain phosphors with favorable luminescence properties. In this work, we describe the recent progress on transition metal Mn⁴⁺-doped inorganic luminescent materials, including oxides (subdivision into alkaline-earth germanates, arsenates, aluminates, titanates, pyrosilicates, phosphates, zirconates, gallates, simple oxides and others), fluorides and nitrides. More specifically, the review focuses on Mn⁴⁺ activated red-emitting phosphors that can be effectively excited by NUV and blue LED chips. The excitation and emission spectra as well as the preparation process of some representative phosphors are given and discussed. Meanwhile, the merits and drawbacks of several kinds of matrix materials in applications for white LEDs, as well as some problems, development trends and application prospects in this field of Mn⁴⁺ doped phosphors are summarized.

Y₄Si₂O₇N₂: Eu²⁺ phosphor has been prepared by a pretreatment method. Reduction in Eu³⁺ ions into Eu²⁺ by the use of hydrogen iodide (HI) is verified by X-ray absorption near-edge structure (XANES) and electrode potential analysis. Y₄Si₂O₇N₂: Eu²⁺ phosphor has a broad emission band in the range of 400-500 nm. Furthermore, the effect of Zr doping on the structure and luminescence properties of Y₄Si₂O₇N₂: Eu²⁺ phosphor is researched. It found that the Zr doping leads to an emission blueshift, and improves the luminescence intensity and thermal quenching behavior of Y₄Si₂O₇N₂: Eu²⁺ phosphors. Prospectively, the pretreatment approach could be extended to develop other Eu²⁺-doped compounds.

Many strategies have been adopted to improve thermal degradation of phosphors. Because of the stability and high transmittance of graphene, here we report a novel method of carbon coating on BaMgAl₁₀O₁₇:Eu²⁺ (BAM) phosphor particles through chemical vapor deposition. The chemical composition, microstructure, and luminescence performance of carbon-coated BAM were characterized carefully. This coating can be controlled within 3-10 atomic layers, depending on the reaction time. Because of the decrease of surface defects and the effective weakening effect of oxidizing Eu²⁺ to Eu³⁺ after carbon coating, different layer numbers showed an obvious effect on the optical properties of carbon-coated BAM. Carbon-coated BAM phosphors had higher emission intensity and better oxidation resistance at high temperature than uncoated BAM phosphors. These results indicate that the method of carbon coating on phosphor particles is a promising way to improve the luminescence properties of other phosphors used in lighting and display devices.

Synthesizing pure phase Ba₃Si₆O₉N₄ by the conventional solid-state reaction method is challenging because of easily formed secondary phase Ba₃Si₆O₁₂N₂ showing similar crystal structure. In this work, an alternative low temperature synthesis method is presented, and a series of green to blue emitting (Ba, Sr)₃Si₆O₉N₄: Eu²⁺ phosphors were prepared by a mechanochemical activation route. Variations in photoluminescence properties and crystal structure, as induced by the change in phosphor composition, were investigated. Under ultraviolet-light excitation, Ba₃Si₆O₉N₄: Eu²⁺ phosphor exhibited a strong narrow green emission at 518 nm and simultaneously a weak emission at 405 nm, which are ascribed to different Eu/Ba sites in Ba₃Si₆O₉N₄ lattice proved by Density Functional Theory (DFT) calculations. A continuous green to blue emission in (Ba, Sr)₃Si₆O₉N₄: Eu²⁺ phosphors could be achieved by tuning the crystal structure and local coordination environment acting on Eu²⁺ with Sr/Ba substitution. More Sr/Ba substitution improved thermal quenching and resulted in a different characteristic of emission peak shift upon increasing the temperature.