Hongkang Wang

Arrays of ZnO/ZnxCd1–xSe Nanocables: Band Gap Engineering and Photovoltaic Applications

Arrays of ZnO/Zn(x)Cd(1-x)Se (0 ≤ x ≤ 1) core/shell nanocables with shells of tunable compositions have been synthesized on fluorine-doped tin oxide glass substrates via a simple ion-exchange approach. Through the effects of stoichiometry and type II heterojunction, optical absorptions of the nanocable arrays can be controllably tuned to cover almost the entire visible spectrum. Lattice parameters and band gaps of the ternary Zn(x)Cd(1-x)Se shells were found to have respectively linear and quadratic relationships with the Zn content (x). These ZnO/Zn(x)Cd(1-x)Se nanocable arrays are further demonstrated to be promising photoelectrodes for photoelectrochemical solar cells, giving a maximum power conversion efficiency up to 4.74%.

Hydrothermal synthesis of hierarchical SnO2 microspheres for gas sensing and lithium-ion batteries applications: Fluoride-mediated formation of solid and hollow structures

Hierarchical solid and hollow microspheres composed of oriented aligned cone-like SnO2 nanoparticles are prepared by a hydrothermal route using either NH4F or NaF, as morphology controlling agents. Their structures and morphology evolution are comprehensively characterized by TEM, SEM, XRD, XPS and the Brunauer–Emmett–Teller (BET) method, 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 demonstrate 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 show a higher charge/discharge capacity and better cyclic performance than the hierarchical SnO2 solid spheres. The discharge capacity of the hierarchical SnO2 hollow spheres is 187 mAh g−1 higher than the solid spheres for up to 50 discharge/charge cycles.

Synthesis of Anatase TiO2 Nanoshuttles by Self-Sacrificing of Titanate Nanowires

Anatase TiO(2) nanoshuttles have been successfully prepared via a hydrothermal method under alkaline conditions by employing titanate nanowires as the self-sacrificing precursors. The experimental results showed that a radical structural rearrangement took place from titanate wires to anatase TiO(2) shuttles during the hydrothermal reaction on the basis of a dissolution-recrystallization process. The surface of titanate nanowires plays a key role in the transformation process by providing both the structural units (e.g., TiO(6) octahedra) to realize anatase transformation and locations for the deposition and rearrangement of the dissolved structural units, while the formation of shuttle morphology is attributed to the minimization of surface energy with thermodynamically stable (101) facets of anatase TiO(2). The shape and phase transformation process were foundto be dependent on the hydrothermal reaction time. Raman and photoluminescence spectra confirmed the crystalline nature of the TiO(2) nanoshuttles.

Hierarchical growth of SnO2 nanostructured films on FTO substrates: structural defects induced by Sn(II) self-doping and their effects on optical and photoelectrochemical properties

Direct hydrothermal growth of Sn(II)-doped SnO2 films on fluorine-doped tin oxide (FTO) substrates results in the formation of upstanding SnO2 nanosheet arrays covered by hierarchical SnO2 nanoflowers. The n-type semiconductor films show extended photoresponse in the visible spectrum arising from the coexistence of Sn(II) dopant ions and oxygen vacancies in these hierarchical SnO2 nanostructures, which leads to a narrowed bandgap. Photoluminescence spectroscopy revealed that the emission in the UV, blue and red spectral ranges is related to the evolution of Sn(II) dopants and oxygen vacancies with annealing temperature, whereas oxygen vacancies are mostly responsible for visible emission. The Sn(II)-doped SnO2 films show higher photocurrent when sensitized with narrow bandgap CdS nanoparticles, serving as efficient electron acceptors.

Hierarchical assembly of Ti(IV)/Sn(II) co-doped SnO2 nanosheets along sacrificial titanate nanowires: synthesis, characterization and electrochemical properties

Hierarchical assembly of Ti(IV)/Sn(II)-doped SnO₂ nanosheets along titanate nanowires serving as both sacrificial templates and a Ti(IV) source is demonstrated, using SnCl2 as a tin precursor and Sn(II) dopants and NaF as the morphology controlling agent. Excess fluoride inhibits the hydrolysis of SnCl2, promoting heterogeneous nucleation of Sn(II)-doped SnO₂ on the titanate nanowires due to the insufficient oxidization of Sn(II) to Sn(IV). Simultaneously, titanate nanowires are dissolved forming Ti(4+) species under the etching effect of in situ generated HF resulting in spontaneous Ti(4+) ion doping of SnO₂ nanosheets formed under hydrothermal conditions. Compositional analysis indicates that Ti(4+) 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 properties were investigated as an anode material in lithium ion batteries, demonstrating that Ti-doped SnO₂ nanosheets show improved cycle performance, which is attributed to the alleviation of inherent volume expansion of the SnO₂-based anode materials by substituting part of Sn sites with Ti dopants.

Ternary Sn–Ti–O Based Nanostructures as Anodes for Lithium Ion Batteries

SnO(x) (x = 0, 1, 2) and TiO(2) are widely considered to be potential anode candidates for next generation lithium ion batteries. In terms of the lithium storage mechanisms, TiO(2) anodes operate on the base of the Li ion intercalation-deintercalation, and they typically display long cycling life and high rate capability, arising from the negligible cell volume change during the discharge-charge process, while their performance is limited by low specific capacity and low electronic conductivity. SnO(x) anodes rely on the alloying-dealloying reaction with Li ions, and typically exhibit large specific capacity but poor cycling performance, originating from the extremely large volume change and thus the resultant pulverization problems. Making use of their advantages and minimizing the disadvantages, numerous strategies have been developed in the recent years to design composite nanostructured Sn-Ti-O ternary systems. This Review aims to provide rational understanding on their design and the improvement of electrochemical properties of such systems, including SnO(x) -TiO(2) nanocomposites mixing at nanoscale and nanostructured Sn(x) Ti(1-x) O(2) solid solutions doped at the atomic level, as well as their combinations with carbon-based nanomaterials.

Fluoride-assisted coaxial growth of SnO2 over-layers on multiwall carbon nanotubes with controlled thickness for lithium ion batteries

We report a facile strategy to coaxially grow compact SnO2 over-layers on multiwall carbon nanotubes (MWCNTs) with hierarchical structures and controlled thickness via a hydrothermal method, using NaF as a morphology controlling and directing agent. The thickness of the SnO2 over-layers can be controlled from several tens of nanometers down to several nanometers by adjusting the ratio of carbon nanotubes to Sn precursor. When applied as anode materials for lithium ion batteries, carbon nanotubes with coaxially grown thin (~10 nm) SnO2 over-layers showed lithium storage performance with a reversible capacity of 431 mA h g−1 after 50 cycles, which is two times better than that of thick (~55 nm) SnO2 over-layers or mixtures of carbon nanotubes and separated SnO2 nanoparticles. The improved cyclic performance was attributed to reduced agglomeration, enhanced electronic conductivity and released internal strain during the lithium insertion/extraction.

Polyvinylpyrrolidone-assisted ultrasonic synthesis of Sno nanosheets and their use as conformal templates for tin dioxide nanostructures

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 layer-by-layer hierarchical structures and further into separate SnO nanosheets. The SnO nanosheets have been used as conformal sacrificial templates converted into polycrystalline SnO(2), as well as layered SnO/SnO(2) 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.

Honeycomb-like carbon nanoflakes as a host for SnO2 nanoparticles allowing enhanced lithium storage performance

While possessing potential advantages as electrodes for lithium-ion batteries, SnO2@carbon composites have been suffering from one common drawback – aggregation of Sn particles during the repeated alloying–dealloying cycles and the resulting pulverization issue. We combat this issue through the fabrication of honeycomb-like SnO2@carbon nanoflakes (SnO2@CNFs) that are able to confine SnO2 nanoparticles within well-separated carbon cavities, so that the Li–Sn alloying–dealloying reaction occurs in the independent microreactors thus avoiding aggregation of Sn metal particles formed. The SnO2 particle size, loading amount and the coverage density are controlled by adjusting the weight ratio between the tin precursor and the CNF. Transmission electron microscopy confirms that the highly graphitic honeycomb-like CNF matrix efficiently buffers and accommodates volume changes of the Li–Sn alloy. Used as anode materials for lithium-ion batteries, the SnO2@CNFs with 66.0 wt% SnO2 display the highest lithium storage capacity, delivering a discharge capacity of 940 mA h g−1 after 150 cycles at 200 mA g−1. For the long-term and high-rate applications, the SnO2@CNFs with 41.5 wt% SnO2 show the best electrochemical performance, delivering a discharge capacity of 400 mA h g−1 at 1 A g−1 after 500 cycles.

Hydrothermal synthesis and electrochemical properties of tin titanate nanowires coupled with SnO2 nanoparticles for Li-ion batteries

Tin titanate nanowires coupled with SnO2 nanoparticles have been prepared by combining hydrolysis of the Sn(II) precursor with tin-to-hydrogen ion exchange using layered hydrogen titanate nanowires as conformal templates under hydrothermal conditions. This synthetic strategy allows for incorporation of electrochemically active Sn into the layered titanate and simultaneous deposition of SnO2 nanoparticles on the as-prepared tin titanate nanowires. When used as anode materials in lithium ion batteries, the tin titanate nanowires coupled with SnO2 nanoparticles showed improved cycle performance and increased lithium storage capacity as compared with mesoporous SnO2 nanoparticle aggregates and hydrogen titanate nanowires. Electrochemical study indicated that introduction of SnO2 nanoparticles supported on tin titanate can buffer the large volume changes during the Li–Sn alloying and dealloying process in flexible layered titanate nanostructures with large interlayer distance. Besides, these composite structures exhibited remarkably low (<0.5 V) voltage for the Li insertion electrode in lithium ion batteries.