Showing papers by "Xiaobo Chen published in 2004"

Highly Efficient Formation of Visible Light Tunable TiO2-xNx Photocatalysts and Their Transformation at the Nanoscale

Abstract: Using a simple nanoscale exclusive synthesis route, TiO2-xNx photocatalysts that can be tuned to absorb across the visible region are produced in seconds at room temperature. The photocatalysts are formed by employing the direct nitridation of anatase TiO2 nanostructures with alkylammonium salts. Depending on the degree of TiO2 nanoparticle agglomeration, catalytically active TiO2-xNx anatase structured particles are obtained whose absorption onset extends well into the visible region at λ ∼ 550 nm. The introduction of a small quantity of palladium in the form of the chloride or nitrate facilitates further nitrogen uptake, appears to lead to a partial phase transformation, displays a counterion effect when compared also to the acetate, and produces a material absorbing well into the near-infrared. The introduction of palladium via the chloride also facilitates the formation of small tetrahedral and octahedral palladium-based crystallites throughout the TiO2-xNx lattice. Surprisingly, no organics appear to...

Photoelectron Spectroscopic Investigation of Nitrogen-Doped Titania Nanoparticles

Abstract: A series of titania (TiO2)-based nanometer-sized photocatalysts, including nitrogen-doped TiO2 nanoparticles (NPs), are investigated using X-ray photoelectron spectroscopy (XPS). Conclusive evidence is obtained for O−Ti−N bond formation during the doping process. Therefore, this substitutional doping is held accountable for the significant increase in photocatalytic activity in nitrogen-doped TiO2 NPs.

Electron antineutrino search at the Sudbury Neutrino Observatory

Abstract: Upper limits on the (ν) over bar (e) flux at the Sudbury Neutrino Observatory have been set based on the (ν) over bar (e) charged-current reaction on deuterium. The reaction produces a positron and two neutrons in coincidence. This distinctive signature allows a search with very low background for (ν) over bar (e)'s from the Sun and other potential sources. Both differential and integral limits on the (ν) over bar (e) flux have been placed in the energy range from 4-14.8 MeV. For an energy-independent nu(e)-->(ν) over bar (e) conversion mechanism, the integral limit on the flux of solar (ν) over bar (e)'s in the energy range from 4-14.8 MeV is found to be Phi( nu e)less than or equal to3.4x10(4) cm(-2) s(-1) (90% C.L.), which corresponds to 0.81% of the standard solar model B-8 nu(e) flux of 5.05x10(6) cm(-2) s(-1), and is consistent with the more sensitive limit from KamLAND in the 8.3-14.8 MeV range of 3.7x10(2) cm(-2) s(-1) (90% C.L.). In the energy range from 4-8 MeV, a search for (ν) over bar (e)'s is conducted using coincidences in which only the two neutrons are detected. Assuming a (ν) over bar (e) spectrum for the neutron induced fission of naturally occurring elements, a flux limit of Phi((ν) over bare)less than or equal to2.0x10(6) cm(-2) s(-1) (90% C.L.) is obtained.

Characterization of the formation of NaA zeolite membrane under microwave radiation

Abstract: Zeolite membranes have gained much attention in recent years, due to their potential application in various fields such as catalysis, gas separation, pervaporation, etc. [1–4]. There are many reports on the synthesis, characterization and applications of zeolite membranes [1–4], but literatures about the formation mechanism of zeolite membrane are fewer [5–7]. To prepare zeolite membranes, many methods have been developed, including hydrothermal crystallization [8], vapor phase synthesis [9], and microwave heating [10, 11]. It was reported that microwave heating and crystal seeding could help the formation and improve the performance of zeolite membranes [10]. It was proposed that the formation of zeolite membranes included the following three stages: sol or gel adhesion on the substrate to form the gel layer, the nucleation and crystallization stage, and the dissolving stage [10, 12, 13]. However, there are few reports on the direct observation of these stages, especially under microwave radiation. In this report, by using scanning electron microscopy (SEM), the formation stages of zeolite NaA membrane in microwave radiation environment were observed and confirmed with the results from X-ray diffraction (XRD) and gas permeation measurements. The disk shaped substrates (30 mm in diameter, 3 mm in thickness) used were porous α-Al2O3 made by casting, with pore radius of 0.3–0.5 μm and ca. 50% porosity. The precursor of the gel was prepared by mixing sodium hydroxide, sodium aluminate, water glass and water, and aging for 24 h with vigorous stirring. The final molar composition of the gel was 3Na2O: Al2O3: 2SiO2: 200H2O. After one face of the substrates was rubbed with zeolite NaA crystals as seeds, the substrates were put vertically in Teflon vessels containing 120 ml gel. Then the vessels were transferred into a microwave oven, heated up to 90 ◦C within 1 min, and reacted for a range of specified times. After the reaction, the membranes were vigorously ultrasonically vibrated in water for 10 min to remove the species physically absorbed on the substrates, washed by de-ionized water to pH = 7.0 and dried at 150 ◦C for 3 h. The membranes were characterized by scanning electron microscopy (SEM) on a JEM-1200E scanning electron microscope and Xray diffraction (XRD) using Cu Kα (λ = 0.154 nm) radiation operating at 40 kV and 100 mA on a Rigaku D max /b powder diffractometer. The integrity of the membrane was evaluated with gas permeation tests.

Spectroscopic investigation of II–VI core-shell nanoparticles: CdSe/CdS

Abstract: CdSe and CdSe/CdS core/shell nanoparticles were prepared by a one-pot synthesis process. The CdSe/CdS core/shell nanoparticles showed red-shift absorption and emission spectra compared to the CdSe core. An optimal reaction time for CdS to form on the CdSe nanoparticles was evaluated and the highest photoluminescence yield of approximately equal to 20% was observed for the core/shell nanoparticles. Using nanosecond laser spectroscopy, the photoluminescence of CdSe nanoparticle was investigated and found to exhibit a monoexponential decay with a lifetime of 52.2 ns, while the photoluminescence of CdSe/CdS core/shell nanoparticles demonstrated biexponential decay characterised by a fast component around 25 ns and a slow component around 100 ns. The Stokes shift, photoluminescence yield, and lifetimes were correlated and attributed to the stress in the interface induced by the lattice mismatch of CdSe and CdS. The red-shift of the absorption and emission spectra of CdSe/CdS nanoparticles was discussed with the concepts of the quantum confinement and electronic confinement effects.

Highly Efficient Formation of Visible Light Tunable TiO2‐xNx Photocatalysts and Their Transformation at the Nanoscale.

Abstract: Using a simple nanoscale exclusive synthesis route, TiO2-xNx photocatalysts that can be tuned to absorb across the visible region are produced in seconds at room temperature. The photocatalysts are formed by employing the direct nitridation of anatase TiO2 nanostructures with alkylammonium salts. Depending on the degree of TiO2 nanoparticle agglomeration, catalytically active TiO2-xNx anatase structured particles are obtained whose absorption onset extends well into the visible region at λ ∼ 550 nm. The introduction of a small quantity of palladium in the form of the chloride or nitrate facilitates further nitrogen uptake, appears to lead to a partial phase transformation, displays a counterion effect when compared also to the acetate, and produces a material absorbing well into the near-infrared. The introduction of palladium via the chloride also facilitates the formation of small tetrahedral and octahedral palladium-based crystallites throughout the TiO2-xNx lattice. Surprisingly, no organics appear to...