Yongxiang Li

Physisorption-Based Charge Transfer in Two-Dimensional SnS2 for Selective and Reversible NO2 Gas Sensing

Nitrogen dioxide (NO2) is a gas species that plays an important role in certain industrial, farming, and healthcare sectors. However, there are still significant challenges for NO2 sensing at low detection limits, especially in the presence of other interfering gases. The NO2 selectivity of current gas-sensing technologies is significantly traded-off with their sensitivity and reversibility as well as fabrication and operating costs. In this work, we present an important progress for selective and reversible NO2 sensing by demonstrating an economical sensing platform based on the charge transfer between physisorbed NO2 gas molecules and two-dimensional (2D) tin disulfide (SnS2) flakes at low operating temperatures. The device shows high sensitivity and superior selectivity to NO2 at operating temperatures of less than 160 °C, which are well below those of chemisorptive and ion conductive NO2 sensors with much poorer selectivity. At the same time, excellent reversibility of the sensor is demonstrated, which has rarely been observed in other 2D material counterparts. Such impressive features originate from the planar morphology of 2D SnS2 as well as unique physical affinity and favorable electronic band positions of this material that facilitate the NO2 physisorption and charge transfer at parts per billion levels. The 2D SnS2-based sensor provides a real solution for low-cost and selective NO2 gas sensing.

Investigation of the oxygen gas sensing performance of Ga2O3 thin films with different dopants

Abstract The oxygen gas sensing performance of Ga2O3 semiconducting thin films doped with Ce, Sb, W and Zn have been investigated. These thin films have been prepared by the sol–gel process and were deposited on sapphire transducers with inter-digital electrodes and a platinum heater integrated. The sensors were exposed to various concentrations of oxygen gas in an ambient of nitrogen and the gas sensing performance has been examined. The responses of sensors doped with Ce, Sb, W and Zn were stable and reproducible at their respective operating temperatures. It was observed that Ga2O3 films doped with Ce, Zn and W are promising for oxygen gas sensing applications.

Concentration quenching of Eu2+ in SrO·Al2O3:Eu2+ phosphor

Abstract The luminescence of Eu2+ in alkaline earth aluminates of the type SrO·Al2O3 has been studied. In SrO·Al2O3:Eu2+ phosphor, green Eu2+ luminescence is observed from Eu2+ on the two different strontium sites present in the lattice. Their concentration quenching processes of the two inequivalent Eu2+ ions are investigated, respectively, and the corresponding concentration quenching mechanism is verified as dipole-dipole interaction. The value of the critical transfer distance is calculated.

Hydrothermal synthesis of bismuth oxide needles

Bismuth oxide (Bi2O3) needles were fabricated hydrothermally by altering the starting materials, mineralizers, reaction temperature and time. The non-agglomerated Bi2O3 needles with a monoclinic structure were prepared using Bi(OH)3 in an alkali solution. The hydrothermal reaction took place at 120 °C in 0.5 h. The length and aspect ratio (length over diameter) of the needles are in the ranges of 6–70 and 5–16 μm, respectively. With the rising reaction temperature and time, the diameter of Bi2O3 needles increased, while the aspect ratio decreased. The formation mechanism of Bi2O3 needles is discussed in the paper.

Hydrothermal synthesis of Na0.5Bi0.5TiO3 fine powders

The effects of processing parameters on the growth and morphology of hydrothermally derived Na0.5Bi0.5TiO3 (BNT) powders were investigated. The BNT powders were synthesized in alkaline solution at temperatures of 160-220 degreesC from Ti(OC4H9)(4) and Bi(NO3)(3).5H(2)O compounds. Alkaline concentration had a great effect on the phase composition and morphology of the resultant powders. BNT phase was not obtained with NaOH concentrations lower than 0.5 or higher than 14 M. The morphology of BNT powders changed from spherical agglomerates to uniform cubic particles with NaOH concentrations in the range 2-12 M. The sizes of the cubic particles were about 3.5-5.5 mum with a typical Na0.5Bi0.5TiO3 structure

Fabrication of textured bismuth titanate by templated grain growth using aqueous tape casting

Abstract Textured bismuth titanate (Bi 4 Ti 3 O 12 ) ceramics were fabricated by templated grain growth (TGG), using plate-like Bi 4 Ti 3 O 12 particles prepared by a molten salt method as the templates. The templates were aligned in the fine-grained matrix by aqueous tape casting with their major surface parallel to the casting plane. Effect of sintering conditions on the grain orientation in the material was investigated. It was found that the degree of grain orientation (Lotgering factor, f ) increased with the increase in sintering temperature, soaking time and heating rate. High Lotgering factor ( f ⩾0.92) can be obtained through careful control of the sintering parameters. The textured Bi 4 Ti 3 O 12 ceramics showed a high anisotropy in its dielectric properties in the directions parallel and perpendicular to the casting plane.

SrAl2O4: Eu2+, Dy3+ phosphors derived from a new sol-gel route

The SrAl 2 O 4 :Eu 2+ , Dy 3+ phosphor powders were prepared by a new sol-gel method using aluminum isopropoxide and strontium acetate as precursors. The sol-gel process and the structure of the phosphor powders were investigated by means of DSC-TG and XRD. It was found that the single-phase SrAl 2 O 4 was formed at 900 °C, which is 300 °C lower than that required for the conventional solid-state reaction. The particle morphology, photoluminescence and afterglow properties of the phosphors were studied in this article.

Low-temperature sintering of Bi4Ti3O12 derived from a co-precipitation method

Bi4Ti3O12 powder was prepared by a co-precipitation method and characterized with DTA, XRD, IR, BET and TEM. The results indicated that the as-prepared precursor consisted of well-dispersed particles of about 10 nm. Bi4Ti3O12 phase began to form at 470 jC, and pure Bi4Ti3O12was obtained after being calcined at 550 jC for 0.5 h. The powder obtained at 600 jC showed a tetragonal symmetry and a plate-like morphology with particle size of about 100 nm. Bi4Ti3O12 ceramics with 97.2% of theoretical density can be obtained by pressure-less sintering at 750 jC for 1 h, indicating the excellent sinterability of the prepared powder. D 2002 Elsevier Science B.V. All rights reserved.

Fabrication of Nanotube Thin Films and Their Gas Sensing Properties

The fabrication process and the growth mechanism of titanium/titania nanotubes prepared by anodization process is reviewed, and their applications in the fields of dye sensitized solar cells, photocatalysts, electrochromic devices, gas sensors, and biomaterials are presented. The anodization of Ti thin films on different substrates and the growth process of anodic titanium oxide are described using the current-time curves. Special attention is paid on the influences of the initial film smoothness on the resulted nanoporous morphologies. The “threshold barrier layer thickness model” is used to discuss the growth mechanism. As a case study for gas sensing, anodized highly ordered nanotube arrays and nanoporous thin films that show porous surface with an average diameter of 25 nm and interpore distance of 40 nm were prepared. Gas sensors based on such nanotube arrays and nanoporous thin films were fabricated, and their sensing properties were investigated. Excellent gas sensing properties were obtained for sensors prepared from these highly ordered nanotube arrays, which present stable response even at a low operating temperature of . Based on our experimental results, “H-induced desorption” mechanism was used for explaining the hydrogen gas sensing mechanism.

Investigation of sol-gel prepared Ga-Zn oxide thin films for oxygen gas sensing

Gallium oxide-zinc oxide (Ga 2 O 3 -ZnO) thin films have been prepared by the sol-gel process and their oxygen gas sensing performance has been investigated. These semiconducting films were deposited on alumina substrates with interdigital electrodes and single crystal silicon substrates for the electrical and microstructural characterization. X-ray photoelectron spectroscopy (XPS) showed that the actual concentrations of Ga and Zn thin films differ from the nominal values in the prepared solutions. Additionally, the concentration of ZnO decreases when the annealing temperature increases. Scanning electron microscopy (SEM) revealed that films with Ga/Zn atomic ratio 90:10 possess cracks and are inhomogeneous when compared to those with that of 50:50. The sensors with Zn 50 at.% had a much larger response at lower operating temperature (<430 C) compared to the Ga-dominated sensors, which operate above 450°C. Furthermore, these sensors showed greatest performance at temperatures in the range of 380-420 C. It was found that by increasing the amount of ZnO in the thin film sensors, the operating temperature decreased as well as the base resistance.