Synthesis of nanomaterials with controllable shapes and sizes has attracted a lot of interest in the last two decades owing to their shape- and size-dependent physical, chemical, electronic, optical, and catalytic properties. In particular, SnO2 nanomaterials have attracted considerable attention due to their wide applications in lithium ion batteries, gas sensors, sensitized solar cells, and catalysts. Various tin dioxide (SnO2) nanostructures, such as 0D nanoparticles, 1D nanorods, nanobelts, nanowires and nanotubes, 2D nanosheets, and 3D hierarchical architectures, have been prepared via a variety of methods. Among these different possible architectures, hierarchical nanostructures often exhibit novel physicochemical properties arising from their construction from low dimensional nano-building blocks. Besides, doping of nanomaterials has received much attention as a convenient way to tailor their electrical, optical and microstructural properties. Therefore, the design of hierarchical nanostructures with control on the morphology and/or electronic structures is very promising in exploring advanced functional nanomaterials. In this work, we design hierarchical SnO2 nanostructures, demonstrate possibilities of both their morphology control and doping, and address their potential application in lithium ion batteries and gas sensors. The main results are summarized as follows.
Hierarchical SnO micro/nanostructures composed of single crystalline SnO nanosheets with exposed {001} facets have been prepared by an ultrasonic aqueous synthesis in the presence of polyvinylpyrrolidone, which hinders the spontaneous formation of the truncated bipyramidal SnO microcrystals and exfoliate them into layered hierarchical structures and further into separate SnO nanosheets. The SnO nanosheets have been used as conformal sacrificial templates which have been converted into polycrystalline SnO2 as well as layered SnO/SnO2 nanostructures, by calcination in air. The concept of fabrication of two-dimensional tin oxide nanostructures demonstrated here may be relevant for the crystal design of layered materials, in general. The lithium storage properties of the SnO and corresponding SnO2 micro/nanostructures were compared, and the latter have been found to show improved properties as anode materials.
Hierarchical solid and hollow microspheres composed of oriented aligned cone-like SnO2 nanoparticles have been prepared by a hydrothermal route using either NH4F or NaF, as morphology controlling agents. Their structures and morphology evolution were comprehensively characterized by TEM, SEM, XRD, XPS and Brunauer-Emmett-Teller (BET), and a formation mechanism is proposed. Both solid and hollow SnO2 microspheres are formed via an Ostwald ripening process undergoing different reorganization paths in the presence of either NH4F or NaF. The solid spheres preferentially recrystallize starting from the cores and grow by consuming adjacent smaller particles, while the hollow spheres preferentially recrystallize starting from outer shells and grow by consuming the entrapped core materials via the mechanism of solid evacuation. As gas sensing materials, both solid and hollow SnO2 microspheres demonstrated sensitive and selective response to several hazardous gases, such as formaldehyde, ammonia, benzene, acetone, and methanol. As lithium storage materials, the hierarchical SnO2 hollow spheres showed higher charge/discharge capacity and better cyclic performance than the hierarchical SnO2 solid spheres. The discharge capacity of the hierarchical SnO2 hollow spheres was 187 mAh·g-1 higher than the solid spheres up to 50 cycles of discharge/charge.
Hierarchical SnO2 nanoflowers, which were assembled from single-crystalline SnO2 nanosheets with high-index (113) and (102) facets exposed, have been prepared via a hydrothermal method using sodium fluoride as the morphology controlling agent. Formation of the 3D hierarchical architecture comprising from SnO2 nanosheets takes place via Ostwald ripening mechanism, with the growth orientation regulated by the adsorbate fluorine species. The use of Sn(II) precursor resulted in simultaneous Sn2+ self-doping of SnO2 nanoflowers, leading to formation of tunable oxygen vacancies bandgap states and the corresponding shifting in the semiconductor Fermi levels, which contributed to the extended absorption in the visible spectral range. The increased density of states of Sn2+-doped SnO2 with selective facets gave rise to enhanced charge transfer, that is, high sensing response, and selectivity towards oxidizing gas NO2. The better gas sensing performance over (102) compared to (113) faceted SnO2 nanostructures was elucidated by surface energetic calculations and Bader analyses. These studies highlight the possibility of simultaneous engineering of surface energetics and electronic properties of SnO2 based materials.
Hierarchical assembly of Ti(IV)/Sn(II)-doped SnO2 nanosheets along titanate nanowires serving as both sacrificial templates and Ti(IV) source has been demonstrated, using SnCl2 as tin precursor and S(II) dopants and NaF as morphology controlling agent. Excess fluoride inhibited the hydrolysis of SnCl2, promoting heterogeneous nucleation of Sn(II)-doped SnO2 on the titanate nanowires due to the insufficient oxidization of Sn(II) into Sn(IV). Simultaneously, titanate nanowires were dissolved forming Ti4+ species under the etching effect of in situ generated HF resulting in spontaneous Ti4+ ion doping of SnO2 nanosheets formed under hydrothermal conditions. Compositional analysis indicates that Ti4+ ions are incorporated by substitution of Sn sites at a high level (16~18 at%), with uniform distribution and no phase separation. Mössbauer spectroscopy quantified the relative content of Sn(II) and Sn(IV) in both Sn(II)-doped and Ti(IV)/Sn(II) co-doped SnO2 samples. Electrochemical propeties were investigated as an anode material in lithium ions batteries, demonstrating that Ti-doped SnO2 nanosheets show improved cycle performance, which was attributed to the alleviation of inherent volume expansion of the SnO2-based anode materials by substituting part of Sn sites with Ti dopants. The developed strategy of the formation of 1D hierarchical structure with simultaneous doping is novel, and may be extended to generate other metal oxides with controlled morphology and simultaneous doping.
In summary, we demonstrated both the morphology control and the doping of the hierarchical SnO2 nanostructures, and showed their potential applications in gas sensors and lithium ion batteries. The hierarchical SnO2 nanostructures whose morphology is determined by a proper design of constituting building units, combined with advantages offered by doping possess a number of useful physical and chemical properties, and are important for a variety of energy and environment applications.
| Date of Award | 2 Oct 2013 |
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| Original language | English |
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| Awarding Institution | - City University of Hong Kong
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| Supervisor | Andrey ROGACH (Supervisor) |
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