ABSTRACT
Tin oxide hollow spheres (SnO2 HS) with high structural integrity were synthesized by using a one pot hydrothermal approach with organic moieties as structure controlling agents. By adjusting the proportion of acetylacetone (AcAc) in the precursor formulation, SnO2 HS of 200 and 350 nm dimensions, with a uniform shell thickness of about 50 nm, were prepared. Using the optimized solution composition with a Sn precursor, heating duration dependent structural evolution of SnO2 was performed at a fixed temperature of 160 °C, which revealed a transition from solid spheres (1 h) to aggregated spheres (4 h) to porous spheres (10 h) to optimized HS (13 h) and finally to broken enlarged HS (24 h). A heating temperature dependent study carried out with a constant heating span of 13 h showed a metamorphosis from spheres with solid cores (140 °C) to ones with hollow cores (160 °C), culminating with fragmented HS, expanded in dimensions (180 °C). A growth mechanism was proposed for the optimized SnO2 HS (2.5 or 5.0 mL of AcAc, 160 °C, 13 h) and the performance of these HS as anodes for Li ions batteries was evaluated by electrochemical studies. The 200 nm SnO2 HS demonstrated an initial lithium storage capacity of 1055 mA h g(-1) at a current density of 100 mA g(-1), and they retained a capacity of 540 mA h g(-1) after 50 charge-discharge cycles. The SnO2 HS also showed excellent rate capability as the electrode exhibited a capacity of 422 mA h g(-1) even at a high current density of 2000 mA g(-1). The notable capacity of SnO2 HS is a manifestation of the mono-disperse quality of the SnO2 HS coupled with the high number of electrochemically addressable sites, afforded by the large surface area of the HS and the striking cyclability is also attributed to the unique structure of HS, which is resistant to degradation upon repeated ion insertion/extraction. The SnO2 HS were also found to be luminescent, thus indicating their usefulness for not only energy storage but also for energy harvesting applications.
ABSTRACT
A molybdenum dioxide/multiwalled carbon nanotubes (MoO2/MWCNT) hybrid composed of spherical flowerlike nanostructures of MoO2, interconnected by MWCNTs has been prepared by a one-step hydrothermal route. The growth of MoO2 nanoparticles into spherical floral shapes with a monoclinic crystalline structure is steered by the dioctyl sulfosuccinate surfactant. The one-dimensional electron transport pathways provided by MWCNTs, which are in direct contact with MoO2 nanostructures, impart an enhanced reversible lithium storage capacity (1143 mA h g(-1) at a current density of 100 mA g(-1) after 200 cycles), high rate capability (408 mA h g(-1) at a high C-rate of 1000 mA g(-1)) and good cycling stability to the MoO2/MWCNT hybrid relative to neat MoO2. Surface potential mapping of the electrodes by Kelvin probe force microscopy, revealed a lower localized work function for the MoO2/MWCNT hybrid as compared to the neat oxide. This makes the MoO2/MWCNT hybrid more easily oxidizable than neat MoO2. Such a distinctive topology achieved for the MoO2/MWCNT hybrid, wherein the MWCNTs prevent the agglomeration of MoO2 nanostructures and thus preserve good electrical connectivities, makes it different in terms of both morphology and performance from all previously reported MoO2-based anode materials to date.
