Hsueh-Shih Chen

InGaN-CdSe-ZnSe quantum dots white LEDs

White light-emitting diodes (WLEDs) were fabricated by combining blue InGaN chips with luminescent colloidal core-shell CdSe-ZnSe quantum dots (QDs). The core-shell CdSe-ZnSe QDs synthesized by thermal deposition approach exhibited high photoluminescence efficiency with a quantum yield more than /spl sim/40%, and size-tunable emission wavelengths from 510 to 620 nm. Three-band red-green-blue WLED was successfully assembled by blue InGaN chips and green-emitting and red-emitting CdSe-ZnSe QDs. Based on QDs with flexibly selected color, the WLEDs exhibited white light with a CIE-1931 coordinate of (0.33, 0.33) and color rendering index R/sub a/ of 91.

White-light emission from organics-capped ZnSe quantum dots and application in white-light-emitting diodes

Organics-capped ZnSe quantum dots were synthesized by a colloidal chemical approach using ZnO and Se powder as precursors. The photoluminescence of the specimens showed strong white emission (∼200nm full width at half maximum) in the visible range under ambient conditions. The white emission was attributed to the mixing of blue emission of ZnSe nanocrystals exhibiting quantum confinement effect with green-red emission of radiative deep levels from ZnSe surface strained lattice. Based on organic-capped ZnSe quantum dots, the white-light-emitting diodes were fabricated using a near-UV InGaN chip as the excitation source. The diodes emitted white light with CIE chromaticity coordinates of (0.38 and 0.41) and show great potential for use in lighting applications.

A novel approach of chemical functionalization on nano-scaled silica particles

Chemical functionalization on nano-scaled silica particles was performed through the reaction between the silanol group of silica and the oxirane ring of epoxy compounds. The occurrence of the surface functionalization and the chemical structures of the resulting products were characterized with FTIR and 1H NMR. Energy dispersive x-ray spectrometry and transmission electronic micrography observation on the silica particles also confirmed the surface functionalization. This approach provides another method to organically functionalize nano-scaled silica particles and can be applied to the preparation of organic–inorganic nanocomposites.

Template-free synthesis of mesoporous anatase titania hollow spheres and their enhanced photocatalysis

Monodispersed mesoporous anatase titania (TiO2) hollow microspheres with controllable morphology and size are prepared via thioglycolic acid (TGA) chemically induced self-assembly for the first time. The formation process of TiO2 hollow spheres consists of three steps: hydrolysis of titanium butoxide (TBT) forming Ti-oxo clusters followed by connecting TGA via hydrogen bonding, assembly of the Ti-oxo clusters to solid spheres, and transformation of the TiO2 solid spheres into hollow spheres through the synergistic effect of thermodynamic (Ostwald ripening process) and kinetic (gas release) factors. TGA molecules firstly provide acidic conditions slowing down the hydrolysis rate of TBT, then control the formation of solid spheres, and finally create hollow structures. The prepared TiO2 hollow sphere with mesoporous structure has a large specific surface area (292.20 m2 g−1). Photocatalytic degradation of methyl orange (MO) by the TiO2 hollow spheres after reflux treatment improves significantly, owing to removal of TGA molecules from the TiO2 hollow spheres. Moreover, photocatalytic water splitting of highly crystalline TiO2 hollow spheres decorated with Pt nanoparticles exhibits an impressive performance for producing H2 (0.477 mmol h−1).

Toward highly efficient photocatalysis: a flow-through Pt@TiO2@AAO membrane nanoreactor prepared by atomic layer deposition

A Pt@TiO2@AAO membrane nanoreactor was fabricated by atomic layer deposition. The photodegradation test of methylene blue demonstrated that the nanoreactor shows efficient photocatalysis performance. It exhibited ~28% photodegradation of methylene blue after ten flow-through cycles, corresponding to about 2.7 × 10(-2) s of contact time of methylene blue with Pt@TiO2 nanotubes.

Sol–gel TiO2 in self-organization process: growth, ripening and sintering

TiO2 nanoparticles were synthesised by a self-organization method using a continuous water vapour hydrolysis system without mixing. Formation of TiO2 particles was based on pure colloidal interactions of hydrolyzed alkoxide molecules generated at the interface between titanium alkoxide solution and water vapour. The effect of ethanol was proved to increase aggregation and packing of primary particles, leading to a significant size enlargement of secondary particles in a post annealing process above 500 °C. A new model to describe sol–gel TiO2 growth has been proposed, which considers the formation of secondary particles to be the result of nucleation of oversaturated primary particles, while the primary particles are the result of irreversible oligomerization of Ti oxo molecules.

Effect of sequential morphology adjustment of hematite nanoplates to nanospindles on their properties and applications

We investigated the marked morphological changes of α-Fe2O3 nanocrystals from nanoplates to nanospindles via an environmentally friendly hydrothermal route. An aqueous Fe salt was used as the precursor and the volume ratio of ammonia to ethylene glycol was varied. The product was uniform and obtained at a high yield. By simply increasing the ratio of the aqueous phase, the morphology could be continuously tuned from nanoplates to nanospindles with the (001) facets gradually disappearing. A fundamental understanding of the shape evolution was obtained via a series of characterizations including X-ray powder diffraction, scanning electron microscopy and transmission electron microscopy. The NO3− ions and NH3 molecules played vital parts at increased temperatures, not only in the phase structure of the iron oxide, but also in the formation of different hematite morphologies with different properties. The decrease in the photocatalytic efficiency in the visible region with the change from nanoplates to nanospindles under the same conditions indicated that the increase in the number of exposed (001) facets promoted the photocatalytic performance. The magnetic and electrochemical properties of typical morphologies were investigated and showed the potential applications of the nanostructures in various fields.

Mesoporous TiO2/WO3 hollow fibers with interior interconnected nanotubes for photocatalytic application

Mesoporous TiO2/WO3 hollow fibers with interconnected nanotubes were prepared by a sol–gel method using polysulfone hollow fibers as a framework template and a block copolymer as an internal template. Polysulfone fibers were immersed in a sol–gel solution and the oxides were deposited on the fibers by vacuum impregnation. The polysulfone and copolymer were removed by thermal treatment at 500 °C for 3 h to form the mesoporous hollow fibers. They preserved the shape and morphology of the polysulfone template after heat treatment. The structure and optical properties of the fibers were characterized, and the performance of the fibers for photocatalytic reaction of methylene blue was investigated. The mesoporous TiO2/WO3 hollow fibers possessed higher surface area (∼130 m2 g−1) and lower band gap energy than pure TiO2 fibers. They exhibited better photodegradation efficiency of methylene blue than TiO2 and WO3 fibers and P25 powder.

Preparation and Characterization of Molecularly Homogeneous Silica–Titania Film by Sol–Gel Process with Different Synthetic Strategies

Three silica-titania thin films with various degrees of molecular homogeneity were synthesized by the sol-gel process with the same precursor formula but different reaction paths. The dried films prepared by a single spin-coating process have a thickness of 500-700 nm and displayed no cracks or pin holes. The transmittances and refractive indices of the samples are >97.8% in the range of 350-1800 nm and 1.62-1.65 at 500 nm, respectively. The in-plane and out-of-plane chemical homogeneities of the films were analyzed by X-ray photoelectron spectroscopy and Auger electron spectroscopy, respectively. For the film with the highest degree of homogeneity, the deviations of O, Si, and Ti atomic contents in both in-plane and out-of-plane directions are less than 1.5%, indicating that the film is highly molecularly homogeneous. It also possesses the highest transparency and the lowest refractive index among the three samples.

White light emission from semiconductor nanocrystals by in situ colour tuning in an alternating thermodynamic–kinetic fashion

White light emission from semiconductor nanocrystals was directly obtained by the in situ control of the size and size distribution of nanocrystals in a one-pot hot-injection synthesis. By directing the crystal growth from a thermodynamics-based regime into a kinetics-based one, nanocrystals with a multimodal size distribution showing two or three primary emission colours were simply obtained without any post-mixing processes. The colour axis and colour rendering index of the nanocrystals were flexibly adjusted by changing the experimental parameters of the synthetic process. The as-prepared nanocrystals showed a pure white light and a high colour rendering index so they could be directly used as phosphors for near-UV white light emitting diodes (WLEDs). With the capability of direct tuning the colour composition, the manufacturing process has been significantly simplified and is expected to be a general approach for nanocrystals-based WLEDs.