Aiyu Zhang

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 Mercaptocarboxylic Acids on Luminescent Properties of CdTe Quantum Dots

CdTe quantum dots (QDs) were prepared in an aqueous solution using various mercaptocarboxylic acids, such as 3-mercaptopropionic acid (MPA) and thioglycolic acid (TGA), as stabilizing agents. The experimental result indicated that these stabilizing agents played an important role for the properties of the QDs. Although both TGA and MPA-capped CdTe QDs exhibited the tunable photoluminescence (PL) from green to red color, the TGA-capped QDs revealed a higher PL quantum yield (QY) up to 60% than that of MPA-capped QDs (up to 50%) by using the optimum preparation conditions, such as a pH value of ~11.2 and a TGA/Cd molar ratio of 1.5. PL lifetime measurements indicate that the TGA-capped QDs exhibited a short average lifetime while the MPA-capped QDs revealed a long one. Furthermore, the average lifetime of the TGA-capped QDs increased with the increase of the QDs size, while a decreased lifetime for the MPA-capped QDs was obtained. This means that the PL lifetime depended strongly on the surface state of the CdTe QDs. These results should be utilized for the preparation and applications of QDs.

Highly luminescent quantum dots functionalized and their conjugation with IgG.

A sol-gel approach including partially removing capping agents, depositing SiO(2) monomers, and growing a SiO(2) shell was developed to generate a bio-compatible functionalization on CdTe quantum dots (QDs). The QDs retained their high photoluminescence (PL) efficiency after coating with a SiO(2) shell (22.5%) by controlling the surface state of the QDs to decrease PL degradation during a sol-gel preparation. Furthermore, the CdTe QDs with a SiO(2) shell were conjugated with IgG using 3-sulfo-N-hydroxysuccinimide or streptavidin-maleimide as linkers. The biotin-streptavidin linker resulted in a high PL efficiency retained (only ~23% lower than the initial value of the QDs), which is crucial for bio-applications.

Application of hybrid SiO2-coated CdTe nanocrystals for sensitive sensing of Cu2+ and Ag+ ions.

A new ion sensor based on hybrid SiO2 -coated CdTe nanocrystals (NCs) was prepared and applied for sensitive sensing of Cu(2+) and Ag(+) for the selective quenching of photoluminescence (PL) of NCs in the presence of ions. As shown by ion detection experiments conducted in pure water rather than buffer solution, PL responses of NCs were linearly proportional to concentrations of Cu(2+) and Ag(+) ions < 3 and 7 uM, respectively. Much lower detection limits of 42.37 nM for Cu(2+) and 39.40 nM for Ag(+) were also observed. In addition, the NC quenching mechanism was discussed in terms of the characterization of static and transient optical spectra. The transfer and trapping of photoinduced charges in NCs by surface energy levels of CuS and Ag2 S clusters as well as surface defects generated by the exchange of Cu(2+) and Ag(+) ions with Cd(2+) ion in NCs, resulted in PL quenching and other optical spectra changes, including steady-state absorption and transient PL spectra. It is our hope that these results will be helpful in the future preparation of new ion sensors.

Photoluminescence properties of aeschynite‐type LaNbTiO6:RE3+ (RE = Tb, Dy, Ho) down‐converting phosphors

In this study, a series of LaNbTiO6:RE(3+) (RE = Tb, Dy, Ho) down-converting phosphors were synthesized using a modified sol-gel combustion method, and their photoluminescence (PL) properties were investigated as a function of activator concentration and annealing temperature. The resultant particles were characterized using X-ray diffraction, transmission electron microscopy, scanning electron microscopy, UV/Vis diffuse reflectance spectroscopy and PL spectra. The highly crystalline LaNbTiO6:RE(3+) (RE = Tb, Dy, Ho) phosphors with an average size of 200-300 nm obtained at 1100°C have an orthorhombic aeschynite-type structure and exhibit the highest luminescent intensity in our study range. The emission spectra of LaNbTiO6:RE(3+) (RE = Tb, Dy, Ho) phosphors under excitations at UV/blue sources are mainly composed of characteristic peaks arising from the f-f transitions of RE(3+), including 489 nm ((5) D4 → (7) F6) and 545 nm ((5) D4 → (7) F5) for Tb(3+), 476 and 482 nm ((4) F9/2 → (6) H15/2) and 571 nm ((4) F9/2 → (6) H13/2) for Dy(3+), and 545 nm ((5) F4 + (5) S2 → (5) I8) for Ho(3+), respectively. The luminescent mechanisms were further investigated. It can be expected that these phosphors are of intense interest and potential importance for many optical applications.

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.

Morphology-tunable fibers with Fe3O4 nanocrystals fabricated through assembly

A facile method has been developed to encapsulate Fe(3)O(4) nanocrystals (NCs) in morphology-tunable fibers (belt-like, solid, and tubal) by using a sonochemistry driven synthesis and a subsequent reflux procedure. By adapting the use of tetraethyl orthosilicate, ammonia, Cd(2+), and thiolglycolic acid (TGA) to an ultrasound-driven synthesis, the Fe(3)O(4) NCs were coated with a thin composite shell. Supersonic treatment plays an important role to prevent the agglomeration of the Fe(3)O(4) NCs in an alkaline condition. The composite shell became thicker due to the deposition of SiO(2) monomers, Cd-TGA clusters, Cd(2+), and free TGA molecules during reflux. In addition, these composite shell-coated Fe(3)O(4) NCs were assembled in composite fibers which were created by the growth of Cd-TGA clusters and the deposition of SiO(2) monomers. The Fe(3)O(4) NCs mono-dispersed in fibers revealed superparamagnetic behavior. The magnetic saturation value of tubal fibers is lower than those of belt-like and solid fibers. These fibers with Fe(3)O(4) NCs would be utilizable for further application. The strategy described here should give a useful enlightenment for the design and fabrication of morphology-tunable fibers with functional NCs.

Micro- and nano-structures of iron oxide with tunable morphologies fabricated via solvothermal process

Micro- and nano-structures of iron oxide with tunable morphologies were fabricated through a solvothermal process by adjusting experimental parameters. In this route, the ethylene glycol (EG) solution of ferric chloride hexahydrate (FeCl3·6H2O), sodium bicarbonate (NaHCO3), and ethylenediamine (EDA) was heat-treated to create the micro- and nano-structures of Fe3O4, where EG acted as both solvent and reducing agent. NaHCO3 and EDA were crucial for the morphology and size of these micro- and nano-structures. Fe3O4 micro- and nano-structures revealed solid, porous and hollow morphologies by changing the reaction time, the type of solvent, and the amount of EDA and NaHCO3. Fe3O4 nanoparticles were created by controlling experimental parameters. As the reaction progressed, nanoparticles aggregated together to form solid Fe3O4 spheres, and then inner nanoparticles dissolved and recrystallized onto the outer shell of Fe3O4 spheres having been obtained through agglomerating together and then gradually developing into hollow micro- and nano-spheres due to an Ostwald ripening mechanism. Hematite (α-Fe2O3) was synthesized using ethyl alcohol instead of EG. These magnetic micro- and nano-structures revealed excellent magnetic properties.

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.