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α-Alumina (α-Al 2 O 3), widely used as a raw material or production in advanced manufacturing industry, has attracted much attention in terms of its preparation temperature. A simple dissolution-evaporation-precipitation process was employed to obtain the amorphous Al-oxalate (AAO) precursor, serving as the starting material for the low temperature preparation of nano α-Al 2 O 3. The crystalline structure and thermal behavior of the AAO were compared with commercially available aluminum hydroxide (Al(OH) 3) powder using the X-ray powder diffraction (XRD) instrument, the thermal gravity analysis and differential scanning calorimetry (TG-DSC). The phase transition mechanism of AAO was investigated by the fourier transform infrared spectroscopy (FTIR), transmission electron microscope (TEM), scanning electron microscope (SEM), and 27Al magic angle spinning nuclear magnetic resonance (27Al-MAS-NMR). The DSC results indicated that AAO underwent a phase transformation at 982 °C. The α-alumina diffraction peaks appeared in AAO at 1000 °C, while α-Al 2 O 3 phase was observed for Al(OH) 3 at 1200 °C. Furthermore, an absorption peak at 452 cm−1, corresponding to Al–O bond in α-alumina was detected at 1000 °C. The proposed low-temperature preparation mechanism of α-alumina from AAO is as follows: four coordination Al-oxalate species entities coexist within the AAO sample; with increasing temperature, organic acid functional groups oxidize and volatilize, resulting in the formation of tetra-coordinate [AlO 4 ], penta-coordinate [AlO 5 ], and hexa-coordinate [AlO 6 ] atomic groups; these atomic clusters further polymerize to form a structurally dense α-alumina crystal at the nano-scale (∼50 nm) at around 1000 °C. The experimental findings present a novel approach and method for the low-temperature preparation of nano-sized alumina powder. Additionally, a feasible theoretical reference for the low-temperature phase transition mechanism of amorphous Al-oxalate was provided. [ABSTRACT FROM AUTHOR]

The lack of red light components in solid-state lighting technology urgently requires the development of new efficient red-emitting bulk phosphors. Herein, we successfully prepared a kind of red-emitting glass ceramics (GCs) (λ ex = 392 nm, λ em = 613 nm) containing Eu-doped strontium fluorophosphates (Sr 5 (PO 4) 3 F:Eu) nanocrystals via a melt-quenching method in SrF 2 –K 2 CO 3 –P 2 O 5 –H 3 BO 3 glass. The heat treatment temperature was found to significantly affect the grain size of the Sr 5 (PO 4) 3 F:Eu nanocrystals as well as the transmittance of the prepared GCs. After heat treatment at 600 °C for 2 h, the Sr 5 (PO 4) 3 F:Eu nanocrystals with grain size of about 18 nm were precipitated inside the GCs (GC600), which exhibited a high transmittance of 58–70 % in the range of 500–800 nm, with a stronger luminescence intensity than that of PG. Under excitation of 392 and 464 nm wavelengths, the CIE 1931 color coordinates of GC600 were around (0.626, 0.373) and (0.638, 0.362), respectively, both falling in red region. The present Sr 5 (PO 4) 3 F:Eu GCs are promising as red phosphors for UV-pumped white-light LEDs. [ABSTRACT FROM AUTHOR]

The marriage of traditional gas-sensing semiconductors with low dimensional nanostructures is expected to exhibit interesting properties in the detection of hazardous gases. Herein, novel hollow-spherical heterojunctions are assembled for thin film gas sensors by using p-type black phosphorus (BP) and n-type tin oxide (SnO 2). Green synthesis strategies are employed to prepare BP nanodots and SnO 2 quantum dots. The aerosol-assisted chemical vapor deposition is used to prepare thin films with hollow-spherical heterojunctions. The BP incorporation significantly improves the gas-sensing properties of SnO 2 -based thin films, which demonstrate excellent response and repeatability to H 2 S at room temperature. The BP-SnO 2 heterojunction demonstrates more than twice the response and sensitivity of pristine SnO 2 thin film. Moreover, the recovery ability is significantly improved and LOD is decreased to 50 ppb. The hollow-spherical heterojunction has a joint gas-sensing mechanism contributed by both of SnO 2 and BP. The SnO 2 provides fundamental receptor function for gas detection while incorporated BP facilitates the adsorption of target H 2 S and provides additional electrons through the p-n junction tunnel. In addition, a slight negative dependence on humidity is observed for the gas sensor. It is ascribed to the competitive adsorption of water molecules on the BP-SnO 2 heterojunction surface against H 2 S molecules and chemisorbed oxygen. This work not only proposes novel heterojunction nanostructures for room-temperature gas detection, but also contributes to the functional integration of traditional gas-sensitive materials and emerging 2D materials in sensor technology. [ABSTRACT FROM AUTHOR]

In recent years, lanthanide doped materials have been extensively studied in the field of fluorescence temperature sensing due to their abundant emission levels and sensitive thermal response. Temperature sensing based on fluorescence intensity ratio (FIR) of upconversion nanoparticles has the advantages of fast temperature response, non-aggressiveness, and high spatial resolution. However, the most reported FIR sensing has limited sensitivity, probably due to the use of thermal coupling levels. Herein, we report a novel FIR temperature measurement based on non-thermal coupling levels of NaGdF 4 :Yb3+/Er3+@NaGdF 4 @NaGdF 4 :Yb3+/Tm3+ core-shell-shell nanostructure, which has high sensitivity and robustness simultaneously. The relative sensitivity based on I 801 / I 654 and I 801 / I 841 of Tm3+ to Er3+ can reach up to 4.56 (303 K) and 3.82% K−1 (313 K), respectively. Between them, FIR of I 801 / I 841 is independent of excitation power and time. These results show the great potential of FIR based on non-thermal coupling levels in high-sensitive and robust temperature sensors. [ABSTRACT FROM AUTHOR]