Ruixia Shi

Growth of flower-like ZnO via surfactant-free hydrothermal synthesis on ITO substrate at low temperature

Without any surfactant, rod-like and three kinds of flower-like ZnO microstructures were synthesized on indium-doped tin oxide (ITO) glass substrates through a simple and environmentally-benign hydrothermal process at 70 °C. The result indicated that rod-like ZnO would be transformed into flower-like ZnO microstructures with decreasing the concentration of sodium hydroxide. The ends, numbers and diameters of the petals of flower-like ZnO varied greatly by modulating the concentration of sodium hydroxide. The secondary nucleation and growth phenomena of ZnO were observed. Time-dependent experiments results indicate that the flower-like ZnO formed in a short period of time. The evolution of morphology and size of ZnO microstructures depended on the reaction time. The amounts and diameters of the petals of flower-like ZnO changed with increasing reaction time. On the basis of our observations and the mechanism proposed previously, the possible growth mechanism for flower-like ZnO was proposed.

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.

Science as art: self-assembly of hybrid SiO2-coated nanocrystals

Hybrid SiO2-coated CdTe nanocrystals (NCs) show a drastic increase in fluorescence quantum yield with a significant red-shifted photoluminescence (PL) peak because of the hybrid shell containing CdS-like clusters which are very close to the CdTe core. With their hybrid SiO2 shell, CdTe NCs reveal self-assembly activity which creates one-dimensional nanostructured materials (fibers) with bright PL. Additionally, we experimentally observed the self-assembly of the hybrid SiO2-coated CdTe NCs into two-dimensional dendritic morphology and three-dimensional crystals through a droplet dewetting technique on a hydrophilic glass surface by using NaCl molecules as scaffolds. This phenomenon is ascribed to the domain growth of NaCl to form fractal structures through tip splitting and side branching dynamics. This is also due to a hydrodynamic mechanism through outward capiliary flow. The evaporation speed of solvent during droplet dewetting plays an important role in controlling the self-assembly of the hybrid SiO2-coated CdTe NCs. The experimental parameters such as the amount of sample on the hydrophilic glass surface and dewetting time are key for getting assemblies with tunable morphologies. The present strategy provides a new approach to study the self-assembly of a variety of NCs. This has a potential application for pattern manufacture in a natural way.

Photoluminescence Stability of Colloidal CdTe Quantum Dots in Various Buffer Solutions

Mercaptoacetic acid-capped CdTe quantum dots (QDs) are potential luminescent markers for biological analysis. The photoluminescence (PL) stability of the QDs in buffer solutions determines their practicability as markers in electrophoresis. The stability of the QDs was thus investigated in electrophoresis buffers including tris–borate–ethylenediaminetetraacetic acid (TBE) and tris–acetate–ethylenediaminetetraacetic acid (TAE). The QDs were completely unstable in high-concentrated buffers (≥0.1×). In the case of low concentrations (≤0.07× for TAE, ≤0.035× for TBE), the PL intensity of the QDs in two kinds of buffers decreased with increasing buffer concentrations. A red-shifted PL peak wavelength and PL intensity fluctuation were observed after dispersing the QDs in diluted TAE buffer solutions with concentrations of ≤0.07× for long time. According to the Stern–Volmer plots of PL degradation, the factors leading to the degradation were complicated, which was attributed to the actions of the components including tris, borate or acetic acid, and ethylenediaminetetraacetic acid as well as their mutual effects.

Fabrication of hollow coupled-layered ZnO microstructures using zinc glycerolate precursors

Hollow hamburger-like and other coupled-layered ZnO microstructures have been fabricated by a facile solvothermal process using the mixture of water and alcohol as solvents. The volume ratios of water/glycerol play an important role for the morphologies of the microstructures. In the case of a volume ratio of 1/2, the hamburger-like ZnO microstructures are created by employing zinc glycerolate (C3H6O3Zn) as a sacrificial template. A possible formation process from precursor C3H6O3Zn to ZnO is proposed by arresting a series of intermediate phases and the formation mechanism of the microstructures is proposed accordingly. The volume ratios of water/glycerol, such as 1/10, 1/6, 1/4, 1/2 and 1/1, were adjusted to create microstructures with various morphologies. The results indicated that the morphologies of the resulting samples changed from quasi-rhombic C3H6O3Zn plates to various coupled-layered ZnO microstructures, suggesting that water is responsible for the formation of coupled-layered structures. In addition, investigations into the formation of ZnO microstructures in mixtures of water and various alcohols, including water/ethanol, water/isopropyl alcohol, water/n-butyl alcohol, water/ethylene glycol and water/glycerol, reveal that alcohols are crucial for control of the morphologies of the microstructures. The multi-hydroxyl alcohols, e.g. ethylene glycol and glycerol, favour the formation of hollow structures compared with the case of single-hydroxyl alcohols.

Nanotubes with hybrid CdTe nanocrystals: Fabrication, property, and surface functional decoration for bioapplications

We have demonstrated a two-step synthesis for preparing morphology- and luminescence-tunable fibers that uses green-emitting CdTe nanocrystals (NCs), Cd2+, thioglycolic acid (TGA), and tetraethyl orthosilicate (TEOS). Hybrid CdTe NCs consisted of a CdTe core and CdS-like clusters in the fibers were created during the synthesis. The CdTe NCs were first coated with a very thin SiO2 shell containing Cd2+ and TGA molecules in an alkaline condition. The SiO2-coated CdTe NCs were assembled together with Cd–TGA clusters for the formation of the fibers. A subsequent reflux process enabled the creation of fibers and caused the formation of CdS-like clusters in the vicinity of CdTe NCs in the fibers. Because of these clusters closed to the CdTe NCs, their effective size increases to reduce the quantum size effect. This special core-shell structure was found to have extraordinary properties: a narrowed photoluminescence (PL) spectrum, a red-shifted PL peak, and a high PL efficiency. The morphology of the fibers (tubal and solid) depends on the concentrations of TGA, Cd2+ ions, and TEOS in the precursor solution together with the concentration of CdTe NCs in a subsequent reflux. These fibers, especially for nanotubes, exhibited not only a tunable morphology and emitting color but also a functional surface, making them well suited for a variety of applications, such as bio-probing and drug delivery. The nanotubes were conjugated with immunoglobulin G (IgG) by using a zero-length cross linker because of carboxyl groups on their surface. The SiO2 component in the nanotubes enables bio-conjugation with IgG through other functional groups, making these nanotubes amenable for bioapplications.