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To develop novel luminescent materials for optical temperature measurement, a series of Yb3+- and Er3+-doped Ca3Sc2Si3O12 (CSS) upconversion (UC) phosphors were synthesized by the sol– gel combustion method. The crystal structure, phase purity, and element distribution of the samples were characterized by powder X-ray diffraction and a transmission electron microscope (TEM). The detailed study of the photoluminescence emission spectra of the samples shows that the addition of Yb3+ can greatly enhance the emission of Er3+ by effective energy transfer. The prepared Yb3+ and Er3+ co-doped CSS phosphors exhibit green emission bands near 522 and 555 nm and red emission bands near 658 nm, which correspond to the 2H11/2→4 I15/2, 4S3/2→4 I15/2, and 4F9/2→4 I15/2 transitions of Er3+, respectively. The temperature-dependent behavior of the CSS:0.2Yb3+,0.02Er3+ sample was carefully studied by the fluorescence intensity ratio (FIR) technique. The results indicate the excellent sensitivity of the sample, with a maximum absolute sensitivity of 0.67% K−1 at 500 K and a relative sensitivity of 1.34% K−1 at 300 K. We demonstrate here that the temperature measurement performance of FIR technology using the CSS:Yb3+,Er3+ phosphor is not inferior to that of infrared thermal imaging thermometers. Therefore, CSS:Yb3+,Er3+ phosphors have great potential applications in the field of optical thermometry.

We report an organic solvent–assisted (OSA) co-precipitation strategy for the production of Mn4+-activated K2TiF6 phosphor. The phosphor particle size was controlled through the selection of organic solvents with an alcohol functional group and different carbon chain lengths used in the synthesis. The synergistic effect of the organic solvent and hydrofluoric acid results in large smoothed hexagonal-shaped crystal sheets of particles that become larger as the carbon chain length of the organic solvent increases. The photoluminescence (PL) properties of K2TiF6:Mn powders strongly depend on the size and thickness of the particles. The addition of n-butanol during the synthesis increases the emission intensity of K2TiF6:Mn by 208%. The PL quantum efficiency of phosphors prepared using the n-butanol-assisted strategy is much higher (98.2%) than that of conventionally prepared phosphors (89.9%). Our findings demonstrate a way to prepare the K2TiF6:Mn phosphor with targeted morphology and very high quantum efficiency and also provide the route for the optimization of all Mn4+-activated fluoride phosphors used in white light-emitting diodes.

Mn4+-activated fluorides rank among the most significant red phosphors for white light emitting diodes (LEDs) with enhanced color rendition, while their poor moisture resistance hinders further application. Surface coating with a protective layer is general approach to improving moisture resistance. However, these methods usually suffer from complex post-processing and poor shell stability. Herein, the introduction of passivation ions (Nb5+) may suppress the hydrolysis of [MnF6]2–, which provides a facile approach to obtaining moisture-resistant fluoride phosphors. Compared to K2TiF6:5.0%Mn4+, the moisture resistance and luminescence intensity of K2Ti0.949Nb0.051F6.051:5.0%Mn4+ improved significantly following the addition of Nb5+. In addition, the combination of as-prepared K2Ti0.949Nb0.051F6.051:5.0%Mn4+ red phosphor, YAG:Ce yellow phosphor and blue-chip enables the production of warm white LEDs with excellent color rendition (Ra = 90.2), low correlated color temperature (CCT = 3816 K), and high luminous efficacy (132.2 lm/W). © 2021

The practical applications of non-aqueous lithium-oxygen batteries are impeded by large overpotentials and unsatisfactory cycling durability. Reported here is that commonly encountered fatal problems can be efficiently solved by using a carbon- and binder-free electrode of titanium coated with TiO2 nanotube arrays (TNAs) and gold nanoparticles (AuNPs). Ultraviolet irradiation of the TNAs generates positively charged holes, which efficiently decompose Li2O2 and Li2CO3 during recharging, thereby reducing the overpotential to one that is near the equilibrium potential for Li2O2 formation. The AuNPs promote Li2O2 formation, resulting in a large discharge capacity. The electrode exhibits excellent stability with about 100 % coulombic efficiency during continuous cycling of up to 200 cycles, which is due to the carbon- and binder-free composition. This work reveals a new strategy towards the development of highly efficient oxygen electrode materials for lithium-oxygen batteries.

Modulation of structural order-disorder transition and structural oxygen defects can provide fundamental insights in the optimization of luminescence performances of phosphors. In this study, the luminescence of Ba3ZnTa2O9(BZT) was systematically elucidated, including the emission arising from the charge transfer (Nb5+/Ta5+ O2-) and anti-site oxygen defect emission due to the B-site disordering (defect type: [2ZnNb3-+ 3Vo2+]), as corroborated by the electronic structural calculations and detailed experiments. We present a two-step design for the optimization of luminescence properties of Ba3ZnTa2O9phosphor. In the first step, the B-site equivalent doping (Ta5+substituted by Nb5+) was used to modulate the B-site atomic arrangement, which induced the formation of the cubic phase with B-site full disordering. In the second step, the A-site nonequivalent doping (Ba2+substituted by Eu3+) was used to decrease the oxygen defect concentration. After the two-step optimization, the photoluminescence excitation spectrum of Ba3-yEuyZnNb2O9(y= 0.1) shows a broad band excitation (300-400 nm), which is a good match with the near-UV LED chip emission. Even more importantly, the emission spectrum covers the entire visible spectral region and exhibits a remarkably enhanced emission intensity (a 40 times enhancement when compared to that of the intrinsic BZT). The fabricated LED device comprising an n-UV chip (= 370 nm) and a single-component Ba3-yEuyZnNb2O9(y= 0.1) phosphor coating emits a warm white light with a low correlated color temperature (CCT = 4813 K) and a good color rendering index (Ra= 82.36).

Crystal phosphors have many unique advantages compared with powdery ones. Herein, room-temperature-grown millimeter-sized single-crystal phosphors of Cs2XF6:Mn4+ (X = Ge, Si, and Ti) with remarkably higher external quantum efficiency than the corresponding powdery samples are reported. In addition, as compared with the powdery ones, the crystal samples exhibit much better stability toward water under different pH conditions. The red light-emitting diodes (LEDs) based on Cs2XF6:Mn4+ crystals show significantly improved luminous efficiency than those based on their corresponding powders. An assembled white LED device composed of the two layers of phosphors, i.e., the crystal (such as Cs2GeF6:Mn4+) and commercial Y3Al5O12:Ce3+, on a blue chip exhibits intense warm white light with high luminous efficiency (up to 193 lm W−1), high color rendering indexes (88), and low correlated color temperatures (3107 K). Hence, these crystals with greatly improved efficiency and stability can be potentially applied in high-quality LED backlighting display and white LED lighting, especially inside the micro-LED devices. © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Lithium-oxygen (Li–O2) batteries can exhibit high theoretical energy density and be surely suitable for potential energy storage. However, they suffer from early discharge termination and consequently low practical capacity, which has been regarded as the blockage of the diffusion of oxygen and the electron transfer on cathode surface. Herein, based on experimental results and theoretical simulation, it is confirmed that the discharge termination is largely caused by the surface adsorption of oxygen and the corresponding reaction intermediates. A TiO2-coated binder-free carbon paper has been prepared and used as cathode for Li–O2 battery. During the first discharge, the discharge plateau at ca. 2.55 V has not been observed due to the weak adsorption of oxygen on TiO2 surface, indicative of an early discharge termination of Li–O2 battery. It is further identified that the formed vacancies on TiO2 surface during lithiation/delithiation prevents the early discharge termination. Therefore, the interaction between oxygen and electrode surface plays a key role in discharge termination mechanism of Li–O2 batteries.

Luminescence of monoclinic lithium metatitanate (Li2TiO3) powders activated with different quantities of Mn4+ is studied in detail. Its strong deep-red emission arising from the Mn4+ 2Eg → 4A2g spin forbidden transition is centered at around 688 nm and is suitable for luminescence thermometry. Structural and electron paramagnetic resonance analyses show that Mn4+ ions are equally distributed in two almost identical Ti4+ sites in which they are octahedrally coordinated by six oxygen ions. Calculations based on the exchange charge model of the crystal field provided values of Racah parameters (B=760 cm−1, C= 2993 cm−1), crystal-field splitting Dq= 2043 cm−1, and the nephelauxetic parameter β1=0.9775. The maximal quantum efficiency of 24.1% at room temperature is found for 0.126% Mn4+ concentration. Temperature quenching of emission occurs by a cross-over via 4T2 excited state of the Mn4+ ions with T1/2=262 K and is quite favorable for the application in the lifetime-based luminescence thermometry since relative changes in emission decay values are exceptionally-large (around 3.21% at room temperature). We derived theoretical expressions for the temperature dependence of the absolute and relative sensitivities and discuss the influence of host material properties on lifetime sensitivities. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Single-component white light phosphors with a broad and full color spectrum are urgently required to overcome residual problems with commercial phosphors. In this paper, we describe how Eu3+, as the dopant of Sr3ZnTa2O9 (SZT), plays an important role in structural modulation (including structural order-disorder and intrinsic oxygen defects), as demonstrated by electronic structural calculations and systematic experiments. Furthermore, D-5(0) F-7(2) emission of Eu3+ provides the red component of the emission spectrum and increases the color rendering index of SZT:Eu3+ phosphors. Consequently, the resulting phosphor SZT:10%Eu3+ shows significantly enhanced emission intensity, and its broadband emission covers the entire visible region from 400 nm to 720 nm. A fabricated LED device using a near-ultraviolet 370 nm chip coated with a single-component, the SZT:10%Eu3+ phosphor, shows warm white emission with a high color rendering index (R-a = 82).

One of prospective ways for boosting efficiency of luminescent materials is their combination with noble metal nanoparticles. Collective, so-called plasmon, oscillations of surface electrons in a nanoparticle can resonantly interact with incident or fluorescent light and cause an increase in the light absorption cross section or radiative rate for an adjacent emitter. Plasmonic inorganic phosphors require gentle host crystallization at which added noble nanoparticles will not suffer from aggregation or oxidation. The prospective plasmonic Mg2TiO4:Mn4+ phosphor containing core@shell Ag@SiO2 nanoparticles is prepared here by spare low-temperature annealing of a sol-gel host precursor. It is revealed that Mn4+ luminescence nonmonotonously depends on the size and concentration of 40 and 70 nm silver nanoparticles. It is demonstrated that luminescence of the Mg2TiO4:Mn4+ phosphor can be up to a 1.5 times increase when Mn4+ excitation is supported by localized surface plasmon resonance in Ag@SiO2 nanoparticles. © 2019 American Chemical Society.

The local crystal structure engineering becomes an important strategy to design new phosphors with enhanced optical and thermal performance of white light-emitting diodes. Herein, a series of Na3Sc2(PO4)3: Eu2+ and KyNa2.97-ySc2(PO4)3: 0.03Eu2+ phosphors were synthesized via traditional high temperature solid-state reaction method. X-ray powder diffraction analysis and Rietveld refinement provide insight in the detailed crystal structure. Furthermore, Eu2+ doped Na3Sc2(PO4)3 exhibits bright blue emission in 400–540 nm spectral range with a maximum value at ~ 460 nm under n-UV light excitation. The concentration quenching mechanism of Eu2+ in Na3Sc2(PO4)3 is certified to be a dipole-dipole interaction. Additionally, crystal structure tailoring is a potential strategy to design new phosphors for particular applications. Therefore, the effects of K+ substitution on the structure and photoluminescence of Eu2+ activated Na3Sc2(PO4)3 is presented in detail. Rietveld refinement data revealed that unit cell volume and Na/K–O band length increase when K+ occupy the Na+ sites. This sensitive local structure resulted in a considerable enhancement of the photoluminescence intensity of Eu2+. Incorporation of K+ in the crystal structure is a feasible route to realize fine-tuning of emission color and broaden the color gamut. In the meantime, Na2.7K0.27Sc2(PO4)3: 0.03Eu2+ phosphor exhibits excellent thermal stability at high temperature over a significant radiative recombination of energy transfer from traps to Eu2+. These results confirm that Na2.7K0.27Sc2(PO4)3: 0.03Eu2+ phosphor might be used as a blue component in n-UV chip activated white light-emitting diodes for the next-generation of indoor solid-state lighting applications. © 2019 Elsevier B.V.