Yunshi Liu

Morphology adjustment of SnO2 and SnO2/CeO2 one dimensional nanostructures towards applications in gas sensing and CO oxidation

SnO2 and SnO2/CeO2 one dimensional (1D) nanostructures with various morphologies including solid nanofibers (NFs), nanobelts (NBs), nanotubes (NTs), and wire-in-tubes (WTs) were successfully prepared via a single-spinneret electrospinning process and a subsequent heat-treatment by adjusting the heating rate and the amount of CeO2. The interesting morphology evolution of samples from NTs to NBs was reported with increasing amounts of CeO2. In addition, SnO2 particles existed on the surface of the CeO2 matrix to form diverse structures in the 1D nanofibers. The band gap of the composite oxides decreased with the addition of CeO2. Compared with other nanostructures, SnO2/CeO2 NTs exhibited superior gas sensing properties, such as the highest response to ethanol. Due to the existence of Ce3+ and oxygen vacancies and the hollow structure, SnO2/CeO2 NTs revealed great CO oxidation performance, indicating the enhanced interactions between the catalyst and the target gas. These excellent properties were attributed to the prominent 1D hollow morphology, the good dispersion of SnO2 nanoparticles on the surface of the CeO2 matrix, the ideal crystallinity, and the composite interactions between the two oxides. In addition, the current method could be utilizable to fabricate other metal oxides with various morphologies for property controlling and important applications.

Morphology adjustment of one dimensional CeO2 nanostructures via calcination and their composite with Au nanoparticles towards enhanced catalysis

One dimensional (1D) CeO2 nanostructures have been successfully fabricated via a single nozzle electrospinning method and subsequent two-step calcination route by adjusting heating rates. The formation process of CeO2 nanotubes (NTs) and nanobelts (NBs) was investigated by observing the morphology evolution of fibers during calcination. Control of the heat-treatment procedure including solvent evaporation, the Kirkendall effect, polymer softening, and thermal decomposition of poly(vinylpyrrolidone) plays an important role in the morphologies of the samples. Well-dispersed and small Au nanoparticles (NPs) were deposited on 1D CeO2 nanostructures through a direct wet-chemical reaction or adding Au precursors during electrospinning. The results of the catalytic performance for CO oxidation suggest that CeO2 NTs display enhanced catalytic activities compared with CeO2 NBs. The loading of Au NPs significantly enhanced the catalytic properties of 1D CeO2 nanostructures. Because of the large surface areas, good crystallinity and the strong interfacial interactions between Au NPs and the CeO2 matrix, the 1% Au/CeO2 NTs exhibit superior catalytic activity with onset CO conversion at 65 °C and complete CO conversion at 142 °C.

Formation of SiO2@SnO2 core–shell nanofibers and their gas sensing properties

SiO2@SnO2 core–shell nanofibers (NFs) were successfully prepared by single-spinneret electrospinning and subsequent calcination process. The precursor solutions were prepared from poly(vinylpyrrolidone), SnO2 precursors, and tetraethylorthosilicate (TEOS) with prehydrolysis. The prehydrolysis of TEOS plays an important role for the formation of core–shell structure. After calcining, the resulting fiber sample had an amorphous SiO2 core and a shell consisted of SnO2 particles. The fibers with various morphologies were obtained through adjusting the molar ratio of Sn and Si and the possible formation mechanism of core–shell NFs was proposed. Both Kirkendall effect and grain growth played important roles for the formation of core–shell structure. Furthermore, SiO2 was used as support material to fix the SnO2 particles and avoid the collapse of the SnO2 structure. The amount of SnO2 precursors directly determined the compactness of the shell, resulting in the different gas sensing properties. The SiO2@SnO2 core–shell NF network sensor responds to ethanol, ammonia, benzene, toluene, chloroform, and hexane gases, but it exhibited enhanced gas response to ethanol with a short response time. Those SnO2 particles formed on the exterior of the fibers provided lots of contact area with the target gas to reduce resistance. In addition, the connectivity between particles also had certain influence on the electrical conductivity of the sample. The results demonstrate that single-spinneret electrospinning can also be used to prepare core–shell fibers with various applications.

Effect of Cd0.5Zn0.5S shells on temperature-dependent luminescence kinetics of CdSe quantum dots

CdSe/Cd0.5Zn0.5S core/shell quantum dots (QDs) with high photoluminescence (PL) efficiency up to 85% were fabricated in organic solutions at high temperature via an anisotropic shell growth on CdSe nanorods. The core/shell QDs with PL peak wavelengths from green to red were obtained by controlling the size of the cores and the thickness of the array Cd0.5Zn0.5S shells. Both the cores and the core/shell QDs revealed narrow size distributions which resulted in narrow PL spectra. Green-emitting CdSe cores with a Se-rich surface revealed a long average lifetime of ∼44 ns. After being coated with Cd0.5Zn0.5S shells, the average lifetime of QDs decreased drastically up to ∼23 ns. The average decay time of the core/shell QDs depended on their shell thickness. The temperature-dependent PL in a temperature range of 293 to 393 K was investigated for CdSe cores and highly luminescent CdSe/Cd0.5Zn0.5S core/shell QDs. Luminescent quenching occurred with increasing temperature for the cores even though the cores exhibited high crystallinity. In contrast, with increasing temperature, the emission PL peak wavelength of the core/shell QDs shifts towards lower energies, the PL bandwidth increases a little and the PL efficiencies decrease slightly. The red-shifted degree of the PL spectra with temperature is small (less than 10 nm).

Preparation and properties of highly stable quantum dot-based flexible silica films

Highly luminescent hydrophobic CdSe and CdSe/CdxZn1−xS quantum dots (QDs) were synthesized via an organic route. The phase transfer of the QDs was carried out through a ligand exchange from 3-aminopropyltrimethoxysilane (APS) instead of an organic capping agent to get aqueous QDs. A functional sol–gel SiO2 sol with a high QD concentration was obtained from the aqueous QD colloidal solution with APS through the hydrolysis and condensation which subsequently occurred. Flexible inorganic SiO2 films with QDs were fabricated via various methods. The photodegradation experiments of the resulting films were completed. It is surprising that the QDs in films were revealed to be highly stable. Especially, the PL intensity of the films increased dramatically after irradiation by 365 nm UV light. By integrating a thin CdSe QD–silica film on a solar cell, the enhanced current demonstrated that a thin film can facilitate the continuous development of solar cells. Because of their high PL brightness, multicolor emission, flexibility and stability, these films will have great potential applications.