Wan Lijun

Morphology control and shape evolution in 3D hierarchical superstructures

Hierarchical structures, in which structure is generated and controlled simultaneously at different size scales, have attracted increasing attention due to their potentials in both theoretical research and practical applications. In this review, a “non-classical crystallization” mechanism is discussed for their possibilities in morphology control of hierarchically-structured materials. Differently, this crystallization route is not based on the attaching and detaching of monomers as happened in the classical case, but through the self-organization of preformed building blocks as nanosized subunits, whose oriented attachment leads to mesocrystals with favorable morphology and texture. Representative materials including both inorganic and organic crystals are reported with possible mechanisms proposed. Synthetic protocols based on this mechanism provide unique inspirations for materials design and could be applied to morphological and structural control of new materials with optimized functions.

Morphology and modulus evolution of graphite anode in lithium ion battery: An in situ AFM investigation

We investigated the interfacial electrochemical processes on graphite anode of lithium ion battery by using highly oriented pyrolytic graphite (HOPG) as a model system. In situ electrochemical atomic force microscopy experiments were performed in 1 M lithium bis(trifluoromethanesulfonyl)imide/ethylene carbonate/diethyl carbonate to reveal the formation process of solid electrolyte interphase (SEI) on HOPG basal plane during potential variation. At 1.45 V, the initial deposition of SEI began at the defects of HOPG surface. After that, direct solvent decomposition took place at about 1.3 V, and the whole surface was covered with SEI. The thickness of SEI was 10.4 ± 0.2 nm after one cycle, and increased to 13.8 ± 0.2 nm in the second cycle, which is due to the insufficient electron blocking ability of the surface film. The Young’s modulus of SEI was measured by a peak force quantitative nanomechanical mapping (QNM). The Young’s modulus of SEI is inhomogeneous. The statistic value is 45 ± 22 MPa, which is in agreement with the organic property of SEI on basal plane of HOPG.

SnO 2 hollow spheres: Polymer bead-templated hydrothermal synthesis and their electrochemical properties for lithium storage

SnO2 hollow spheres have been synthesized via a facile hydrothermal method using sulfonated polystyrene beads as a template followed by a calcination process in air. X-ray diffraction, scanning electron microscopy, and transmission electron microscopy show that the as-obtained SnO2 hollow spheres have a wall thickness of about 50 nm, and consist of nanosized SnO2 particles with a mean diameter of about 15 nm. Electrochemical measurements indicate that the SnO2 hollow spheres exhibit improved electrochemical performance in terms of specific capacity and rate capability in comparison with commercial SnO2 when used as anode materials for lithium-ion batteries. The enhanced performance may be attributed to the spherical and hollow structure, as well as the building blocks of SnO2 nanoparticles.

Electrode materials for lithium secondary batteries with high energy densities

Lithium-ion battery is a widely-used secondary battery with high energy density. However, with a fast development in its field of application, an urgent requirement is raised in the further improving its energy density. This article summarizes the recent research progressions in the cathode and anode materials of lithium-ion batteries with high energy densities as well as some novel lithium secondary batteries based on lithium metal, highlights the selections of cathode and anode materials with high capacities, the designs of micro-nano structures of them, and the strategies toward surface coating and synthesis of these materials with some recent works made in this field by our group. This review also gives a discussion on the future development directions of several lithium metal secondary batteries with high energy densities such as lithium-sulfur battery and lithium-air battery.

Carbon-free Cu2ZnSn(S,Se)4 film prepared via a non-hydrazine route

The kesterite Cu2ZnSn(S,Se)4 (CZTSSe) is an ideal candidate for light harvesting materials in earth-abundant low-cost thin-film solar cells (TFSC). Although the solution-based processing is a most promising approach to achieve low-cost solar cells with high power conversion efficiency, the issues of poor crystallinity and carbon residue in CZTSSe thin films are still challenging. Herein, a non-hydrazine solution-based method was reported to fabricate highly crystallized and carbon-free kesterite CZTSSe thin films. Interestingly, it was found that the synthetic atmosphere of metal organic precursors have a dramatic impact on the morphology and crystallinity of CZTSSe films. By optimizing the processing parameters, we were able to obtain a kesterite CZTSSe film composed of compact large crystal grains with trace carbon residues. Also, a viable reactive ion etching (RIE) processing with optimized etching conditions was then developed to successfully eliminate trace carbon residues on the surface of the CZTSSe film.

Topography and functional information of plasma membrane

By using atomic force microscope (AFM), the topography and function of the plasmalemma surface of the isolated protoplasts from winter wheat mesophyll cells were observed, and compared with dead protoplasts induced by dehydrating stress. The observational results revealed that the plasma membrane of living protoplasts was in a state of polarization. Lipid layers of different cells and membrane areas exhibited distinct active states. The surfaces of plasma membranes were unequal, and were characterized of regionalisation. In addition, lattice structures were visualized in some regions of the membrane surface. These typical structures were assumed to be lipid molecular complexes, which were measured to be 15.8±0.09 nm in diameter and 1.9±0.3 nm in height. Both two-dimensional and three-dimensional imaging showed that the plasmalemma surfaces of winter wheat protoplasts were covered with numerous protruding particles. In order to determine the chemical nature of the protruding particles, living protoplasts were treated by proteolytic enzyme. Under the effect of enzyme, large particles became relatively looser, resulting that their width was increased and their height decreased. The results demonstrated that these particles were likely to be of protein nature. These protein particles at plasmalemma surface were different in size and unequal in distribution. The diameter of large protein particles ranged from 200 to 440 nm, with a central micropore, and the apparent height of them was found to vary from 12 to 40 nm. The diameter of mid-sized protein particles was between 40–60 nm, and a range of 1.8–5 nm was given for the apparent height of them. As for small protein particles, obtained values were 12–40 nm for their diameter and 0.7–2.2 nm for height. Some invaginated pits were also observed at the plasma membrane. They were formed by the endocytosis of protoplast. Distribution density of them at plasmalemma was about 16 pits per 15 μm2. According to their size, we classified the invaginated pits into two types-larger pits measuring 139 nm in diameter and 7.2 nm in depth, and smaller pits measuring 96 nm in diameter and 2.3 nm in depth. On dehydration-induced dead protoplasts, the degree of polarization of plasma membranes decreased. Lipid molecular layers appeared relatively smooth, and the quantity of integral proteins reduced a lot. Invaginated pits were still detectable at the membrane surface, but due to dehydration-induced protoplast contraction, the orifice diameter of pits reduced, and their depth increased. Larger pits averagely measuring 47.4 nm in diameter and 31.9 nm in depth, and smaller pits measuring 26.5 nm in diameter and 43 nm in depth at average. The measured thickness of plasma membranes of mesophyll cells from winter wheat examined by AFM was 6.6–9.8 nm, thicker in regions covered with proteins.

The Self-assembly Structure of Pyrazine Derivative on Highly Oriented Pyrolytic Graphite

The nanostructure studies have been performed to the fluorescent and liquid-crystal molecule(2, 5-bis-[2-(3,4-bis-dodecyloxy-phenyl)-vinyl]-3,6-dimethyl-pyrazine(BPDP12)) on highly oriented pyrolytic graphite(HOPG) using scanning tunneling microscopy (STM). The well-organized 2-D monolayer images reveal two kindsof configurations. The structure (I) is a stable and closely packed arrangement, in which all the conjugated cores areparallel to each other and the alkyl chains of the molecules interdigitate over the full length;while the structure (II)is an unstable and unclosely packed arrangement, in which all the conjugated cores are separated by the alkylchains. The formation of above two structures owes to the competition between the stronger intermolecular π-πinteractions and the van der Waals forces of alkyl chains during the self-assembly.

STM investigation of surfactant molecules

Adsorption and self-organization of sodium alkyl sulfonates (STS and SHS) have been studied on HOPG by using the in situ scanning tunneling microscopy (STM). Both SHS and STS molecules adsorb on the HOPG surface and form long-range well-ordered monolayers. The neighboring molecules in different rows form a “head to head” configuration. In the high-resolution images of STS and SHS molecules, one end of the molecules shows bright spots which are attributed to the SO3−groups.

Electric-field-controlled interfacial assembly structures investigated by scanning tunneling microscopy

Self-assembly plays an important role in the construction of nano-structures and nano-materials. The controlled-assembly of functional molecules on solid surfaces is one active research field in nanofabrication. Various external sources, such as light, temperature, electric field, magnetic field, can be applied to switch interfacial supramolecular architectures. This review summarizes recent STM studies in electric-field-controlled interfacial assembly structures such as disorder-order transformation, molecular orientation transformation, 2D/3D phase transformation and chemical reactions. The prospects of future development in this field are outlined.

3D graphene-based electrode materials foradvanced electrochemical energy storage

Designing specific structured electrode materials is essential for efficient electrochemical energy storage. Graphene owns unique 2D structure and possesses excellent electrical conductivity, ultrahigh specific surface areas, as well as colloidal self-assembling behavior, which make it an ideal candidate for constructing hybrid electrode materials with remarkably enhanced electrochemical performances. This article reviews the recent research progress made on various 3D graphene-based electrode materials and their applications in electrochemical energy storage such as Li-ion and Li-S batteries. Based on the recent works made by our group, we highlight the principles of designing graphene-based materials, and discuss various advanced structures of graphene-based electrodes. Furthermore, future research directions for the development of novel and more efficient graphene-based electrode materials are proposed.