Linjiang Wang

Anatase/rutile TiO2 nanocomposite microspheres with hierarchically porous structures for high-performance lithium-ion batteries

A new anatase/rutile TiO2 nanocomposite microspheres (ART) electrode with hierarchically porous structures was successfully synthesized by a one-step route under mild hydrothermal conditions. The morphology, crystal structure and phase composition, specific surface area and pore size distribution of the obtained nanocomposite were systematically investigated by X-ray diffraction (XRD), Raman spectroscopy, field-emission scanning electron microscopy (FESEM), high resolution transmission electron microscopy (HRTEM) and nitrogen adsorption–desorption measurements. The as-synthesized nanocomposite microspheres electrodes exhibited superior specific capacity and high-rate charge–discharge performance for lithium-ion batteries (LIBs) (∼103 mA h g−1 at 30 C after 100 charge–discharge cycles, 1 C = 170 mA g−1) as compared to commercial TiO2 nanoparticles (P25). The improvement is mainly attributed to enhanced Li-ion diffusion and efficient charge transport as evidenced from cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements. Most importantly, the materials architecture used in this study, comprising of dual-phase TiO2 nanocrystals with hierarchically porous structures would be a general concept that could be applicable in the development of other mixed-phase electrode materials for rechargeable lithium-ion batteries and supercapacitors.

Enhanced dye-sensitized solar cells performance using anatase TiO2 mesocrystals with the Wulff construction of nearly 100% exposed {101} facets as effective light scattering layer

Anatase TiO2 mesocrystals with a Wulff construction of nearly 100% exposed {101} facets were successfully synthesized by a facile, green solvothermal method. Their morphology, and crystal structure are characterized by powder X-ray diffraction (XRD), Field emission scanning electron microscopy (FESEM) and high-resolution transmission electron microscopy (HRTEM). Accordingly, a possible growth mechanism of anatase TiO2 mesocrystals is elucidated in this work. The as-prepared single anatase TiO2 mesocrystals mean center diameter is about 500 nm, and the length is about 1 μm. They exhibit high light adsorbance, high reflectance and low transmittance in the visible region due to the unique nearly 100% exposed {101} facets. When utilized as the scattering layer in dye-sensitized solar cells (DSSCs), such mesocrystals effectively enhanced light harvesting and led to an increase of the photocurrent of the DSSCs. As a result, by using an anatase TiO2 mesocrystal film as a scattering overlayer of a compact commercial P25 TiO2 nanoparticle film, the double layered DSSCs show a power conversion efficiency of 7.23%, indicating a great improvement compared to the DSSCs based on a P25 film (5.39%) and anatase TiO2 mesocrystal films, respectively. The synergetic effect of P25 and the mesocrystals as well as the latters unique feature of a Wulff construction of nearly 100% exposed (101) facets are probably responsible for the enhanced photoelectrical performance. In particular, we explore the possibility of the low surface area and exposed {101} facets as an efficient light scattering layer of DSSCs. Our work suggests that anatase TiO2 mesocrystals with the Wulff construction is a promising candidate as a superior scattering material for high-performance DSSCs.

Continuous hollow TiO2 structures with three-dimensional interconnected single crystals and large pore mesoporous shells for high-performance lithium-ion batteries

A continuous hollow TiO2 structure with three-dimensional (3D) interconnected anatase single crystals self-assembled into compact large pore mesoporous shells had been fabricated in this paper. The as-obtained hierarchically micro/nano-structured materials, denoted as CTAB–TiO2, are simply prepared via a hydrothermal method with the assistance of cetyltrimethylammonium bromide (CTAB) micelles. The results show that both CTAB and the thermal treatment played a key role in forming a continuous hollow micro/nano-structure with large mesopores. It has been found that the rate properties and cyclic performance of lithium-ion batteries based on annealed CTAB–TiO2, denoted as A-CTAB–TiO2, were greatly enhanced compared to the CTAB–TiO2 electrodes, this improvement was largely attributed to the reduction of the interfacial resistance due to nanocrystal–nanocrystal contact throughout the electrode films and the increased Li-ion diffusion properties of uniform large pore mesoporous shells (12.8 nm). The low-cost synthesis and excellent electrode properties of the continuous interconnected single crystals with a large pore mesoporous architecture could provide a new direction for developing higher-performance lithium storage electrodes.

The role of soft colloidal templates in the shape evolution of flower-like MgAl-LDH hierarchical microstructures

A flower-like MgAl layered double hydroxide (MgAl-LDH) hierarchical microstructure was synthesized by a self-assembly route, employing the anionic surfactant sodium dodecyl sulfate (SDS) as a soft template. X-ray diffraction, Fourier transform infrared spectroscopy, thermogravimetric analysis and Brunauer–Emmett–Teller analysis were employed to characterize the samples. Structural characterization revealed that the hierarchical microstructure materials are composed of nanosheets with a thickness of about 50 nm with a hierarchical porous structure with both large and small mesopores on the surface. The average diameter and specific surface area of the hierarchical porous microstructures were 1.544 nm and 41.20 m2 g−1, respectively. Furthermore, it was found that the diameter and density of the three-dimensional (3D) hierarchical microstructures could be varied by changing the growth parameters, such as growth time, growth temperature, initial reactant concentration, and surfactant type. Compared with two other surfactants, the anionic surfactant sodium lauryl sulfate (SLS) and the cationic surfactant cetyl trimethyl ammonium bromide (CTAB), the anionic surfactant SDS plays dual roles as both a capping agent and through its intercalation in controlling the shape of the MgAl-LDH hierarchical microstructure. Based on the analysis of the evolution of the microstructure and crystalline structure of the MgAl-LDHs, a corresponding formation mechanism of the three-dimensional hierarchical microstructures was proposed. In addition, the thermal stability of the as-prepared hierarchical microstructures of the MgAl-SDS-LDHs was further evaluated. The results show that MgAl-SDS-LDH can be preserved up to 500 °C and eventually be transformed into spinel MgAl2O4 with a similar hierarchical microstructure, indicating the high thermal stability of the hierarchical microstructure.

Fe2(MoO4)3 nanoparticle-anchored MoO3 nanowires: strong coupling via the reverse diffusion of heteroatoms and largely enhanced lithium storage properties

A novel hierarchical heterostructured Fe2(MoO4)3@MoO3 (FM) nanowire is developed by mild hydrothermal treatment of precursor MoO3 nanobelts with only the assistance of FeCl3·6H2O combined with the heat treatment. Subsequently, a model of reverse heteroatoms diffusion is proposed to describe the formation of FM nanowires. Further, it is found that the heteroatoms diffusion and facet-selective growth of Fe2(MoO4)3 on MoO3 nanobelts brought about the change of MoO3 morphology from nanobelts to nanowires, and Fe2(MoO4)3 nanoparticles are firmly anchored onto MoO3 nanowires. When tested as LIBs anodes, compared to pristine MoO3 nanobelts, high reversible lithium storage capacities of the anodes are obtained from novel FM nanowires. The FM nanowire electrode obtained at 500 °C (FM-500) exhibits superior reversible capacity of 585 mA h g−1 retained at a current density of 100 mA g−1 after 100 cycles, and 518 mA h g−1 retained after 100 cycles at 200 mA g−1. The distinct electrochemical activity of the Fe2(MoO4)3 nanoparticles probably activates the irreversible capacity of the MoO3 nanobelts. Moreover, the amorphous layer and structure stability of heterojunction electrodes are also beneficial for charge storage, electron transfer and lithium ion diffusion during the charge–discharge process.

Rutile TiO2 mesocrystallines with aggregated nanorod clusters: extremely rapid self-reaction of the single source and enhanced dye-sensitized solar cell performance

This paper reports a simple and environmentally friendly approach for the synthesis of rutile TiO2 mesocrystals (RTM) composed of single crystal aggregated nanorod clusters. It is a hydrothermal method involving titanium(III) chloride as the only reactant. The resulting one-dimensional rutile nanorods can easily assemble into three-dimensional hierarchical architectures without any surfactants or additives. By selecting appropriate experimental conditions, such as reaction time and ripening temperature, we can easily manipulate the RTM microstructure. Detailed experiments suggest that the growth of the RTM is spontaneous and an extremely rapid self-reaction of titanium(III) chloride, which is controlled by the thermodynamics process. Furthermore, the composites photoanode was fabricated by hybridizing RTM and TiO2 nanoparticles (Degussa-P25 powder) for use in dye-sensitized solar cells (DSSCs). The composite photoanode-based DSSC possesses superior performance to TiO2 nanoparticle cells. A high light-to-electricity conversion yield of 7.3% for composite photoanodes was achieved, significantly higher than that of TiO2 nanoparticle photoanodes with a similar thickness (5.45%). This result can be attributed to their synergistic effect of high crystallinity, light scattering and fast charge transfer capability.

Self-activated continuous pulverization film: an insight into the mechanism of the extraordinary long-life cyclability of hexagonal H4.5Mo5.25O18·(H2O)1.36 microrods

For large amounts of transition metal oxides, sulfides and carbon groups (IVA), the pulverization of electrode materials in lithium-ion batteries (LIBs) is always a serious and common problem due to volume expansion and stress accumulation resulting from phase transformation or alloying during the charge–discharge process, which leads to capacity fading and thus limits the cycling performance of LIBs. To solve these problems, conventionally, the rational design of electrode materials is needed. Here in this work, we report the synthesis of a novel anode material, hexagonal H4.5Mo5.25O18·(H2O)1.36 microrods (HMs) via a simple hydrothermal method. During the lithium-ion insertion/desertion process of the HMs, it was found that the HMs are first drastically transformed into Li2MoO4 nanotubes and then Li2MoO4 nanowire clusters embedded in amorphous sphere cages with a Li2O matrix, Mo metal and SEI thin film. Surprisingly, we discovered that the HMs exhibited extraordinary long-life cyclability with an unusual phenomenon: the specific capacity first decreased and then increased. The outstanding electrochemical performance could be explained by the formation of intermediate phase Li2MoO4 nanowires and amorphous sphere cages, which can maintain the lithium-ion paths and electronic transport, and prohibit the mechanical and chemical degradation of the electrode materials. The results show that the pulverization of the HM anode materials induced by lithium-ion insertion–extraction played a trigger role in the formation of a continuous pulverization film. Accordingly, a “damage-reconstruction” model based on ex situ XRD and FESEM analyses combined with ex situ XPS, FTIR and TEM characterizations of the charge–discharge process was proposed to explain such an unusual and intriguing finding. Compared with the conventional method for protecting electrode materials from pulverization, the robust continuous gel-like pulverization film containing a unique combination of intermediate phase and amorphous sphere cages provides a new insight into the mechanism for the extraordinary long-term cyclability of electrode materials.