Impact of Negative Thermal Expansion on Thermal Quenching of Luminescence of Sc₂Mo₃O₁₂:Eu³⁺

Thermal quenching (TQ) is a major challenge facing many phosphors, especially those used in the high-temperature range. To overcome this obstacle, here we present a new phenomenon of excitation-wavelength-dependent antithermal quenching (anti-TQ) luminescence over a broad temperature range. We have observed that the luminescence intensity of Eu3+-doped Sc2Mo3O12 almost triples at elevated temperatures along with intensified energy transfer from the host to the dopant upon excitation at the charge-transfer band of 277 nm. Specifically, the absolute emission intensity of Sc2Mo3O12:20%Eu3+ measured at 398 and 798 K reaches 292% and 97% of its initial intensity taken at 298 K, respectively. Similar to the photoluminescence emission intensity, the lifetime also elongates upon heating with a maximum value at 623 K. We have unveiled the mechanism of the excitation-wavelength-dependent anti-TQ luminescence, which is attributed to the promoted energy transfer induced by the negative thermal expansion (NTE) property of the Sc2Mo3O12 host. Moreover, we have demonstrated the efficient high-temperature luminescence thermometric performance of this anti-TQ luminescence phosphor using its temperature-dependent lifetime. The new phenomenon and experimental findings presented in this study provide inspiration for the future exploration of NTE-based phosphors with thermally enhanced luminescence performance via their intensified energy transfer for broad potential applications

General, Room-Temperature Method for the Synthesis of Isolated as Well as Arrays of Single-Crystalline ABO₄-Type Nanorods

Single-crystalline BaWO4 and BaCrO4 nanorods of reproducible shape and of varying sizes have been controllably prepared using a simple, room-temperature approach, based on the use of porous alumina template membranes. Aligned BaWO4 and BaCrO4 nanorod arrays can be obtained by dissolving the template. Our facile technique, which is analogous to biomineralization, offers a promising and generalized methodology to prepare other types of free-standing ABO4 nanorods and their corresponding nanorod arrays. Extensive characterization of these samples has been performed using scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), energy-dispersive X-ray spectroscopy (EDS), selected area electron diffraction (SAED), Raman spectroscopy, FT-infrared spectroscopy (FT-IR), and X-ray diffraction (XRD)

Size- and Shape-Dependent Transformation of Nanosized Titanate into Analogous Anatase Titania Nanostructures

A size- and shape-dependent morphological transformation was demonstrated during the hydrothermal soft chemical transformation, in neutral solution, of titanate nanostructures into their anatase titania counterparts. Specifically, lepidocrocite hydrogen titanate nanotubes with diameters of ∼10 nm were transformed into anatase nanoparticles with an average size of 12 nm. Lepidocrocite hydrogen titanate nanowires with relatively small diameters (average diameter range of ≤ 200 nm) were converted into single-crystalline anatase nanowires with relatively smooth surfaces. Larger diameter (>200 nm) titanate wires were transformed into analogous anatase submicron wire motifs, resembling clusters of adjoining anatase nanocrystals with perfectly parallel, oriented fringes. Our results indicate that as-synthesized TiO2 nanostructures possessed higher photocatalytic activity than the commercial titania precursors from whence they were derived

Systematic Studies on RE₂Hf₂O₇:5%Eu³⁺ (RE = Y, La, Pr, Gd, Er, and Lu) Nanoparticles: Effects of the A‑Site RE³⁺ Cation and Calcination on Structure and Photoluminescence

A series of 5 mol % Eu3+-doped rare earth (RE) hafnium oxide RE2Hf2O7 (RE = Y, La, Pr, Gd, Er, and Lu) nanoparticles (NPs) have been synthesized, calcinated, and systematically investigated using X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, Fourier transform infrared spectroscopy, and optically excited luminescence. Effects of the A-site RE3+ cation and calcination on the crystal structure of the RE2Hf2O7:5%Eu NPs were distinguished using XRD and Raman studies. Spectroscopic analysis showed that the La2Hf2O7:5%Eu3+ and Pr2Hf2O7:5%Eu3+ possessed ordered pyrochlore structures while the RE2Hf2O7:5%Eu3+ compositions (RE = Y, Er, and Lu) possessed disordered fluorite structure and were thermodynamically stable up to the highest calcination temperature employed in this study (1500 °C); however, a disordered–ordered transition observed in the Gd2Hf2O7:5%Eu3+ composition indicated that it was not thermodynamically stable. Detailed photoluminescence (PL) studies, including quantum yield and decay properties of each sample before and after calcination, were performed and correlated with their compositions and crystal structures. These results suggest that the A site RE3+ cations and calcination of these RE2Hf2O7:5%Eu NPs play important roles in their PL properties

Large-Scale Synthesis of Single-Crystalline Perovskite Nanostructures

Single-crystalline perovskite nanostructures with reproducible shape have been prepared using a simple, readily scaleable solid-state reaction in the presence of NaCl and a nonionic surfactant. Pristine BaTiO3 nanowires have diameters ranging from 50 to 80 nm with an aspect ratio larger than 25. Single-crystalline SrTiO3 nanocubes with a mean edge length of 80 nm have been produced using a similar procedure

Probing Structure−Parameter Correlations in the Molten Salt Synthesis of BaZrO₃ Perovskite Submicrometer-Sized Particles

Single-crystalline perovskite BaZrO3 submicrometer-sized particles were synthesized using a simple, scaleable molten salt method. In this paper, in addition to a time-dependent particle evolution study, we explored primarily the effects of different experimental processing parameters, such as the identity of the salt, annealing temperatures, overall reaction times, cooling rates, and the chemical nature of the precursor in determining their impact upon the purity, size, shape, and morphology of the as-obtained products. We also discuss the role of additional experimentally controllable factors such as the heating rate applied, the amount of salt used, the molar ratios of precursors involved, and the use of surfactant. By a judicious choice of experimental parameters and conditions, we describe herein a rational means of producing pure products with a reproducible composition and morphology

Near-Infrared-Emitting K₂CaP₂O₇:Cr³⁺ with Outstanding Thermal Stability

Near-infrared (NIR) phosphor-converted light-emitting diodes (pc-LEDs) have great potential as next-generation NIR light sources for a wide range of applications. However, the development of NIR-emitting phosphors that simultaneously produce long wavelength and broadband emission with high thermal stability continues to be a challenge. Here, we design a broadband NIR phosphor, Cr3+-activated K2CaP2O7. Under an optimal 460 nm excitation, it shows broadband NIR emission in the range of 650 to 1200 nm. More importantly, it demonstrates outstanding anti-thermal quenching (anti-TQ) performance, i.e., even enhanced luminescence intensity at 150 °C, which is 180% of that measured at room temperature. Using a combination of first-principles calculations and experimental analysis, the mechanism of the distinctive anti-TQ is clarified as the thermally induced energy transfer from energy levels of oxygen vacancy defects to Cr3+ 3d excited state centers. In addition, we have showcased the potential multifunctional applications of this tunable NIR-emitting phosphor in nondestructive NIR spectroscopy detection and night vision. It is expected that the exciting results from the Cr3+-activated K2CaP2O7 will contribute to a better understanding of how crystal defects affect luminous materials and encourage further research into defect control to create thermally stable phosphors with practical use

Type-II CuFe₂O₄/Graphitic Carbon Nitride Heterojunctions for High-Efficiency Photocatalytic and Electrocatalytic Hydrogen Generation

Solar water splitting has emerged as an urgent imperative as hydrogen emerges as an increasingly important form of energy storage. g-C3N4 is an ideal candidate for photocatalytic water splitting as a result of the excellent alignment of its band edges with water redox potentials. To mitigate electron–hole recombination that has limited the performance of g-C3N4, we have developed a semiconductor heterostructure of g-C3N4 with CuFe2O4 nanoparticles (NPs) as a highly efficient photocatalyst. Visible-light-driven photocatalytic properties of CuFe2O4/g-C3N4 heterostructures with different CuFe2O4 loadings have been examined with two sacrificial agents. An up to 2.5-fold enhancement in catalytic efficiency is observed for CuFe2O4/g-C3N4 heterostructures over g-C3N4 nanosheets alone with the apparent quantum yield of H2 production approaching 25%. The improved photocatalytic activity of the heterostructures suggests that introducing CuFe2O4 NPs provides more active sites and reduces electron–hole recombination. The g-C3N4/CuFe2O4 heterostructures furthermore show enhanced electrocatalytic HER activity as compared to the individual components as a result of which by making heterostructures g-C3N4 with CuFe2O4 increased the active catalytic surface for the electrocatalytic water splitting reaction. The enhanced faradaic efficiency of the prepared heterostructures makes it a potential candidate for efficient hydrogen generation. Nevertheless, the designed heterostructure materials exhibited significant photo- and electrocatalytic activity toward the HER, which demonstrates a method for methodically enhancing catalytic performance by creating heterostructures with the best energetic offsets

Pyrochlore Rare-Earth Hafnate RE₂Hf₂O₇ (RE = La and Pr) Nanoparticles Stabilized by Molten-Salt Synthesis at Low Temperature

Complex oxides of the RE2Hf2O7 series are functional materials that exist in the fluorite or pyrochlore phase depending on synthesis method and calcination temperature. In this study, we investigate the process of synthesis, crystal structure stabilization, and phase transition in a series of RE hafnate compounds, synthesized by the coprecipitation process of a single-source complex hydroxide precursor followed with direct calcination or molten-salt synthesis (MSS) method. Phase pure RE2Hf2O7 (RE = Y, La, Pr, Gd, Er, and Lu) ultrafine nanostructured powders were obtained after calcinating the precursor in a molten salt at 650 °C for 6 h. Moreover, we demonstrate that the MSS method can successfully stabilize ideal pyrochlore structures for La2Hf2O7 and Pr2Hf2O7 in the nanodomain, which is not possible to achieve by direct calcination of the coprecipitated precursor at 650 °C. We propose mechanisms to elucidate the differences in these two synthesis methods and highlight the superiority of the MSS method for the production of RE hafnate nanoparticles

Enhanced Photo/Electrocatalytic Hydrogen Evolution by Hydrothermally Derived Cu-Doped TiO₂ Solid Solution Nanostructures

Highly efficient nanocatalysts with a high specific surface area were successfully synthesized by a cost-effective and environmentally friendly hydrothermal method. Structural and elemental purity, size, morphology, specific surface area, and band gap of pristine and 1 to 5% Cu-doped TiO2 nanoparticles were characterized by powder X-ray diffraction (PXRD), X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR), energy dispersive X-ray analysis (EDAX), inductively coupled plasma mass spectrometry (ICP-MS), liquid chromatography-high resolution mass spectrometry (LC-HRMS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), BET surface area, Raman spectroscopy, photoluminescence spectroscopy (PL) and UV-visible diffused reflectance spectroscopy (UV-DRS) studies. The XPS and EPR findings indicated the successful integration of Cu ions into the TiO2 lattice. UV-DRS and BET surface area investigations revealed that with an increase in dopant concentration, Cu-doped TiO2 NPs show a decrease in band gap (3.19–3.08 eV) and an increase in specific surface area (169.9–188.2 m2/g). Among all compositions, 2.5% Cu-doped TiO2 has shown significant H2 evolution with an apparent quantum yield of 17.67%. Furthermore, the electrochemical water-splitting study shows that 5% Cu-doped TiO2 NPs have superiority over pristine TiO2 for H2 evolution reaction. It was thus revealed that the band gap tuning with the desired dopant concentration led to enhanced photo/electrocatalytic sustainable energy applications