Qingming Huang

A novel double-perovskite Gd2ZnTiO6:Mn4+ red phosphor for UV-based w-LEDs: structure and luminescence properties

Red-emitting Mn4+ activated oxide phosphors with a cheap price and excellent physical/chemical stability have become a hot research topic for their potential applications in white LEDs (w-LEDs). Herein, we report a novel double-perovskite Gd2ZnTiO6:Mn4+ (GZT:Mn4+) red phosphor. The material microstructures were characterized with the aid of XRD Rietveld refinement and HRTEM observations. The luminescence properties and dynamics of Mn4+ in GZT were studied in detail using low/room temperature steady/transient spectroscopic techniques. The crystal field strength and nephelauxetic effect influencing the Mn4+ emission energy were also analyzed. It is revealed that the special crystal structure of GZT featuring alternately slant-wise arranged [TiO6]/[ZnO6] octahedrons with the [–Mn4+–O2−–Zn2+–] bond angle deviating from 180° is beneficial to achieving efficient Mn4+: 2Eg → 4A2 transition in the deep-red region. After mixing the red-emitting GZT:Mn4+ with the commercial blue and green phosphors in various ratios, and then coupling the mixture with a 365 nm UV chip to build a w-LED, the white light was found to evolve from cool to warm with a tunable correlated color temperature (CCT) from 6977 K to 4742 K, a color rendering index (CRI) up to 82.9, and an improved R9 value to 43, which validates that GZT:Mn4+ is a promising red color converter for UV-based w-LEDs.

Bandgap Tailoring via Si Doping in Inverse-Garnet Mg3Y2Ge3O12:Ce3+ Persistent Phosphor Potentially Applicable in AC-LED

The state-of-the-art alternating-current light-emitting diode (AC-LED) technique suffers from adverse lighting flicker during each AC cycle. Aiming to compensate the dimming time of AC-LED, herein, we report a novel Mg3Y2(Ge1-xSix)3O12:Ce(3+) inverse-garnet persistent phosphor whose afterglow is efficiently activated by blue light with persistent luminescence in millisecond range. It is experimentally demonstrated that Si doping tailors the host bandgap, so that both the electron charging and detrapping in the persistent luminescence process are optimized. To explore the origin of the millisecond afterglow, we performed a series of thermoluminescence analyses, revealing three types of continuously distributed traps in the host. Finally, an AC-LED prototype device was fabricated, which exhibits the warm white emission with a reduced percent flicker of 71.7%. These results demonstrate that the newly developed persistent phosphor might be a promising candidate applicable in low flickering AC-LED which has advantages of cheaper price, longer lifetime, and higher energy utilization efficiency.

Lu2CaMg2(Si1−xGex)3O12:Ce3+ solid-solution phosphors: bandgap engineering for blue-light activated afterglow applicable to AC-LED

Persistent luminescence (PersL) has long commanded the curiosity of researchers owing to the complicated and profound physics behind it. In this work, the PersL mechanism in a new kind of persistent garnet phosphors, Lu2CaMg2(Si1−xGex)3O12:Ce3+, is studied from the new perspective of a “solid-solution” scheme. Different from the conventional study in pursuit of long PersL, we focus on manipulation of afterglow to the millisecond range and tentatively demonstrate its potential to compensate the flickering of the alternating current driven LED (AC-LED) in every AC cycle. Evidently, the tailored host bandgap favors efficient electron charging and facilitates electron detrapping, as well as redeploying trap distribution, which results in a blue light activated afterglow in the millisecond time range, and subsequently a reduced percent flicker of 64.1% for the AC-LED. This investigation is the first attempt to establish the design guidelines for new PersL materials with an adjustable millisecond ranged afterglow, and, hopefully, it paves a pathway to the development of burgeoning low-flickering AC-LED technology.

Structure and luminescence behavior of a single-ion activated single-phased Ba2Y3(SiO4)3F:Eu white-light phosphor

The development of a single-ion activated single-phased white-light phosphor is of great importance in the field of near ultraviolet based w-LED lighting. In this work, a Ba2Y3(SiO4)3F:Eu fluoride apatite, which shows a broad emission band covering the entire visible region with tunable color rendition, was successfully synthesized via a high-temperature solid-state route. The microstructure and composition of the phosphor were carefully examined with the aid of XRD Rietveld refinement, HRTEM and SEM analyses, as well as XPS measurement. Spectroscopic studies revealed the site occupancy conditions of Eu2+, i.e., the Eu2+(I) band centering at 470 nm originates from the [BaO9] 4f site, while the Eu2+(II) one peaking at 600 nm comes from the [BaO6F] 6h site. Interestingly, the obtained white light can be readily tuned from cool to warm just by varying the Eu doping content. The brightest luminescence was achieved when the Eu concentration reached 1 mol%, beyond which the d–d interaction-based energy transfer between Eu2+ ions would result in concentration quenching. The underlying mechanism of the incomplete conversion from Eu3+ to Eu2+ under a reducing atmosphere was explored, which was believed to be caused by the rigid framework of the crystal structure. After coupling Ba2Y3(SiO4)3F:Eu with a commercial 3W 370 nm UV chip, the constructed w-LED yielded warm white light with a CIE coordinate of (0.402, 0.371), CCT of 3530 K, and CRI of 83.5, upon being driven by a 350 mA forward-biased current.

Upconversion effective enhancement by producing various coordination surroundings of rare-Earth ions.

In this manuscript, we present a simple route to enhance upconversion (UC) emission by producing two different coordination sites of trivalent cations in a matrix material and adjusting crystal field asymmetry by Hf(4+) co-doping. A cubic phase, Y3.2Al0.32Yb0.4Er0.08F12, with these structural characteristics was synthesized successfully by introducing a small ion (Al(3+)) into YF3. X-ray diffraction (XRD), nuclear magnetic resonance (NMR), transmission electron microscopy (TEM), X-ray spectroscopy (XPS), and fluorescence spectrophotometry (FS) were employed for its crystalline structure and luminescent property analysis. As a result, the coordination environments of the rare-earth ions were varied more obviously than a hexagonal NaYF4 matrix with the same Hf(4+) co-doping concentration, with vertical comparison, UC luminescent intensities of cubic Y3.2Al0.32Yb0.4Er0.08F12 were largely enhanced (∼32-80 times greater than that of different band emissions), while the maximum enhancement of hexagonal NaYF4 was by a factor of ∼12. According to our experimental results, the mechanism has been demonstrated involving the crystalline structure, crystal field asymmetry, luminescence lifetime, hypersensitive transition, and so on. The study may be helpful for the design and fabrication of high-performance UC materials.

A highly-distorted octahedron with a C2v group symmetry inducing an ultra-intense zero phonon line in Mn4+-activated oxyfluoride Na2WO2F4

Owing to the magnetic dipole nature, the zero phonon line (ZPL) of Mn4+:2E → 4A2 transition is weak unless Mn4+ is situated at a site deviating from the centrosymmetric nature. Herein, we report a brand-new oxyfluoride, Na2WO2F4:Mn4+(NWOF:Mn4+), which shows unprecedented intense red ZPL at ∼620 nm along with relatively weak vibronic transitions under blue light excitation. This peculiar spectral feature is demonstrated to be originated from the highly distorted octahedral coordination environment in the C2v group symmetry surrounding Mn4+. High-resolution spectroscopic studies at 10 K disclose the fine structured electronic/vibronic transitions of Mn4+:2E, 2T1 → 4A2 and the weak electron–phonon interaction (Huang–Rhys factor S < 1) on the Mn4+ emissive state. Benefiting from the intense ZPL, an ultra-high color rendering index with Ra = 92.7 and R9 = 90.0 is achieved in the w-LED using YAG:Ce3+and NWOF:Mn4+ as color converters, and a wide color gamut of 107.1% NTSC in the w-LED using CsPbBr3 quantum dots and NWOF:Mn4+ is obtained. Herein, we first demonstrate that Mn4+-activated oxyfluorides have great potential in w-LED lighting and display applications. Our study can also enlighten researchers to design highly distorted octahedral sites for Mn4+ doping to achieve an ultra-intense ZPL.

Adsorption desulfurization performance and adsorption-diffusion study of B2O3 modified Ag-CeOx/TiO2-SiO2

In this work, the adsorption desulfurization performance and adsorption diffusion study of B2O3 modified Ag-CeOx/TiO2-SiO2 adsorbent were investigated. The adsorption desulfurization performance was studied by batch and fixed bed tests. The homogeneous surface diffusion model (HSDM) was employed to investigate the adsorption and diffusion behavior of 4,6-dimethyldibenzothiophene (4,6-DMDBT) in diesel. It was found that the addition of B2O3 promotes the dispersion of CeOx species and then further facilitates the dispersion and oxidation of Ag species resulting in higher adsorption desulfurization activity. Ag species are in state of Ag, Ag2O and Ag2O2, among which, Ag2O and Ag2O2 are found to be the active centers. The kinetics of adsorption desulfurization of model diesel fuel was investigated to provide guiding significance for the prediction of breakthrough curves of fixed-bed adsorption columns. The batch kinetic experiment modeled by HSDM model indicates that surface diffusion controls the main rate. The surface diffusion coefficient Ds determined by batch adsorption experiments is independent of operation conditions, which can be used to directly predict the breakthrough behavior in fixed bed adsorption. The modified HSDM model is proposed to describe the breakthrough behavior. Results indicate that the breakthrough time is affected by bed height, flowrate and influent concentration.

CsPbBr3/EuPO4 dual-phase devitrified glass for highly sensitive self-calibrating optical thermometry

Despite possessing admirable features for optical thermometry, a semiconductor (SC) nanocrystal based material has the innate shortcomings of poor stability towards environmental variation, complicated/unfriendly preparation procedures, low production, and the necessity of a delicate structural design to achieve dual-emission. To well address these issues, a kind of dual-phase devitrified glass (DG) strategy is proposed. In the case of dual-phase DG embedded with CsPbBr3 SC and EuPO4, fabricated via a glass self-crystallization route, it has been found that the spatial isolation of two emitting species enables non-interfering luminescence with different temperature-responsive behaviors. Utilizing the fluorescence intensity ratio (FIR) between the Eu3+: 4f → 4f emission from EuPO4 and the exciton emission from CsPbBr3, a self-calibrating thermometer with ultra-high sensitivity (Sa = 0.082 K−1 and Sr = 1.80% K−1, 303–483 K) is achieved. Importantly, the introduction of a CsPbBr3 perovskite SC into the inert glass matrix greatly improves its thermal stability, and so highly repeatable thermometry in the dual-phase DG is achieved (97.12% for 10 heating–cooling cycles). This work demonstrates an effective pathway for accurate, reliable and robust thermometry by integrating SC nanocrystals and rare-earth-ions into an inorganic glassy composite.

The surface construction of Mo–O–Bi coordination for high catalytic performance in gas-phase epoxidation of propylene with O2

To solve the issue of facile aggregation of active components and limited synergetic catalysis of MoO3/BixSi1–xO2 catalysts, we highlight an efficient strategy to achieve Mo–O–Bi coordination to fix MoO3 strongly on a surface and improve its dispersion. Incorporating the characterization methods of XRD, N2 adsorption/desorption, ICP, NH3-TPD, Raman spectroscopy, TEM and XPS, the surface chemical and structure properties of BixSi1–xO2 supports with and without modification and the supported Mo catalyst were systematically investigated. Both the dispersion of Mo and the synergetic catalysis of Bi/Mo were restrained by the self-passivated layer of the BixSi1–xO2 support, which led to limited catalytic performance. Mo–O–Bi coordination was achieved by modification of the support in which MoO3 was anchored on the support and monodispersed homogeneous MoO3 was obtained. Based on this approach, the epoxidation activity of the MoO3/BixSi1–xO2 catalyst was obviously enhanced; C3H6 conversion increased from 13.2% to 20.4%, and the selectivity of propylene oxide (PO) reached 73.2%. As such, this strategy may be useful for the preparation of other high performance supported catalysts.

Diatomite-Assisted Synthesis of Ordered Mesoporous Carbon and Its Application in Fuel Cells

Ordered mesoporous carbon has the promising development and potential in catalysis, because of its large amount of uniform pore size and high surface area. In this paper, ordered mesoporous carbon has been successfully synthesized via evaporation-induced self-assembly (EISA) method, using triblock copolymer Pluronic F127 as a soft template, resol as a carbon precursor and diatomaceous earth as the transient scaffolds. Diatomite is a kind of special biomaterial with 3D precision of tens of nanometers and of low price. Diatomite shells not only maintain the intact structure of the ordered mesoporous carbon, but also increase its aperture diameter and surface area to a certain degree, during the carbonization between 600 °C to 800 °C. When the samples were calcined in nitrogen, the features of diatomite could effectively reduce shrinkage ratio of sample structure, keeping the precursor fluid firmly adsorbed on the surface of diatom shells. The resulting materials were characterized by X-ray diffraction, transmission electron microscopy and N2 adsorption-desorption, etc. The results showed all the samples had highly ordered mesoporous structure. Under the support of diatomite, both of the specific surface area (717~773 m•g) and pore sizes (3.9~11.3 nm) of the mesoporous carbon have become much larger, which could be controlled by adjusting the ratio of diatomite to resol. The influence of the diatomite on the structure of the mesoporous carbon was discussed in the paper. Furthermore, we used the as-prepared ordered mesoporous carbon to load Pt nanoparticles. The Pt/carbon catalysts were prepared by microwave-assisted heating process with ethylene glycol as reductant. The electrocatalytic activity of carbon supported Pt nanoparticles was verified by cyclic voltammetry in CH3OH solution. We found the carbon materials with much larger specific surface areas had higher electrocatalytic activity. When the ratio of diatomite to resol was 0.5, the specific surface area of the sample was the largest, and the corresponding Pt/carbon showed the highest peak current of methanol oxidation.