Hui-Juan Yan

Highly Efficient Visible-Light-Driven Photocatalytic Hydrogen Production of CdS-Cluster-Decorated Graphene Nanosheets

The production of clean and renewable hydrogen through water splitting using photocatalysts has received much attention due to the increasing global energy crises. In this study, a high efficiency of the photocatalytic H(2) production was achieved using graphene nanosheets decorated with CdS clusters as visible-light-driven photocatalysts. The materials were prepared by a solvothermal method in which graphene oxide (GO) served as the support and cadmium acetate (Cd(Ac)(2)) as the CdS precursor. These nanosized composites reach a high H(2)-production rate of 1.12 mmol h(-1) (about 4.87 times higher than that of pure CdS nanoparticles) at graphene content of 1.0 wt % and Pt 0.5 wt % under visible-light irradiation and an apparent quantum efficiency (QE) of 22.5% at wavelength of 420 nm. This high photocatalytic H(2)-production activity is attributed predominantly to the presence of graphene, which serves as an electron collector and transporter to efficiently lengthen the lifetime of the photogenerated charge carriers from CdS nanoparticles. This work highlights the potential application of graphene-based materials in the field of energy conversion.

Mono dispersed SnO2 nanoparticles on both sides of single layer graphene sheets as anode materials in Li-ion batteries

A two step programmed method is developed to load mono dispersed SnO2 nanoparticles onto single layer graphene sheets. The SnO2-G composite has near mono dispersion of the SnO2 nanocrystals as well as a high SnO2 content of over 60 wt%. These outstanding features are desirable and enable the composite material to be an excellent anode material for Li-ion batteries.

Formation of Halogen Bond-Based 2D Supramolecular Assemblies by Electric Manipulation.

Halogen bonding has attracted much attention recently as an important driving force for supramolecular assembly and crystal engineering. Herein, we demonstrate for the first time the formation of a halogen bond-based open porous network on a graphite surface using ethynylpyridine and aryl-halide based building blocks. We found that the electrical stimuli of a scanning tunneling microscopy (STM) tip can induce the formation of a binary supramolecular structure on the basis of halogen bond formation between terminal pyridyl groups and perfluoro-iodobenzene. This electrical manipulation method can be applied to engineer a series of linear or porous structures by selecting halogen bond donor and acceptor fragments with different symmetries, as the directional interactions ultimately determine the structural outcome.

Scanning tunneling microscopy of the formation, transformation, and property of oligothiophene self-organizations on graphite and gold surfaces.

Two alkyl-substituted dual oligothiophenes, quarterthiophene (4T)-trimethylene (tm)-octithiophene (8T) and 4T-tm-4T, were used to fabricate molecular structures on highly oriented pyrolytic graphite and Au(111) surfaces. The resulted structures were investigated by scanning tunneling microscopy. The 4T-tm-8T and 4T-tm-4T molecules self-organize into long-range ordered structures with linear and/or quasi-hexagonal patterns on highly oriented pyrolytic graphite at ambient temperature. Thermal annealing induced a phase transformation from quasi-hexagonal to linear in 4T-tm-8T adlayer. The molecules adsorbed on Au(111) surface in randomly folded and linear conformation. Based on scanning tunneling microscopy results, the structural models for different self-organizations were proposed. Scanning tunneling spectroscopy measurement showed the electronic property of individual molecules in the patterns. These results are significant in understanding the chemistry of molecular structure, including its formation, transformation, and electronic properties. They also help to fabricate oligothiophene assemblies with desired structures for future molecular devices.

Graphene exfoliation in organic solvents and switching solubility in aqueous media with the aid of amphiphilic block copolymers

The successful exfoliation of graphite to graphene sheets in a liquid phase via tip sonication was achieved. A number of solvents were examined for several time periods and it was found that o-dichlorobenzene (o-DCB) and N-methyl-1,2-pyrolidone (NMP) are ideal solvents to exfoliate graphite and produce stable dispersions of graphene. The exfoliated graphene dispersions were characterized by complementary techniques including AFM, DLS, TGA and Raman. Furthermore, treatment of stable dispersions of exfoliated graphene sheets in NMP with poly[styrene-b-(2-vinylpyridine)] block copolymer, under acidic conditions, resulted in aqueous solubilization of graphene. Similar results were obtained, i.e. transfer of graphene from the organic to the aqueous phase, when the poly(isoprene-b-acrylic acid) block copolymer was added on exfoliated graphene in NMP.

Single nanowire electrode electrochemistry of silicon anode by in situ atomic force microscopy: solid electrolyte interphase growth and mechanical properties.

Silicon nanowires (SiNWs) have attracted great attention as promising anode materials for lithium ion batteries (LIBs) on account of their high capacity and improved cyclability compared with bulk silicon. The interface behavior, especially the solid electrolyte interphase (SEI), plays a significant role in the performance and stability of the electrodes. We report herein an in situ single nanowire atomic force microscopy (AFM) method to investigate the interface electrochemistry of silicon nanowire (SiNW) electrode. The morphology and Youngs modulus of the individual SiNW anode surface during the SEI growth were quantitatively tracked. Three distinct stages of the SEI formation on the SiNW anode were observed. On the basis of the potential-dependent morphology and Youngs modulus evolution of SEI, a mixture-packing structural model was proposed for the SEI film on SiNW anode.

Solvent-controlled 2D host-guest (2,7,12-trihexyloxytruxene/coronene) molecular nanostructures at organic liquid/solid interface investigated by scanning tunneling microscopy.

The two-dimensional (2D) self-assembled networks of 2,7,12-trihexyloxytruxene (Tr) are shown to accommodate coronene guest molecules on highly oriented pyrolytic graphite (HOPG) surfaces. The host-guest structures are revealed by scanning tunneling microscopy (STM) at liquid/solid interfaces. The effect of solvents on the host-guest structures is intensively investigated in different solvents such as 1,2,4-trichlorobenzene (TCB), 1-phenyloctane, 1-octanol, and tetradecane. In contrast to the similar 2D hexagonal self-assembly of Tr host template on HOPG in different solvents, the formation of host-guest nanostructures of coronene in Tr 2D network strongly depend on the polarity of the solvents. The thermodynamic equilibrium during the host-guest assembly process is discussed, and the solvent-guest interaction is proposed as a main contributor for the observed solvent effect in the 2D host-guest self-assembly process. The results are significant to surface host-guest chemistry and nanopatterning.

General Space-Confined On-Substrate Fabrication of Thickness-Adjustable Hybrid Perovskite Single-Crystalline Thin Films

Organic-inorganic hybrid perovskite single-crystalline thin films (SCTFs) are promising for enhancing photoelectric device performance due to high carrier mobility, long diffusion length, and carrier lifetime. However, bulk perovskite single crystals available today are not suitable for practical device application due to the unfavorable thickness. Herein, we report a facile space-confined solution-processed strategy to on-substrate grow various hybrid perovskite SCTFs in a size of submillimeter with adjustable thicknesses from nano- to micrometers. These SCTFs exhibit photoelectric properties comparable to bulk single crystals with low defect density and good air stability. The clear thickness-dependent colors allow fast visual selection of SCTFs with a suitable thickness for specific device application. The present substrate-independent growth of perovskite SCTFs opens up opportunities for on-chip fabrication of diverse high-performance devices.

Isomeric Routes to Schiff‐Base Single‐layered Covalent Organic Frameworks

With graphene-like topology and designable functional moieties, single-layered covalent organic frameworks (sCOFs) have attracted enormous interest for both fundamental research and application prospects. As the growth of sCOFs involves the assembly and reaction of precursors in a spatial defined manner, it is of great importance to understand the kinetics of sCOFs formation. Although several large families of sCOFs and bulk COF materials based on different coupling reactions have been reported, the synthesis of isomeric sCOFs by exchanging the coupling reaction moieties on precursors has been barely explored. Herein, a series of isomeric sCOFs based on Schiff-base reaction is designed to understand the effect of monomer structure on the growth kinetics of sCOFs. The distinctly different local packing motifs in the mixed assemblies for the two isomeric routes closely resemble to those in the assemblies of monomers, which affect the structural evolution process for highly ordered imine-linked sCOFs. In addition, surface diffusion of monomers and the molecule-substrate interaction, which is tunable by reaction temperature, also play an important role in structural evolutions. This study highlights the important roles of monomer structure and reaction temperature in the design and synthesis of covalent bond connected functional nanoporous networks.

Structural selection of graphene supramolecular assembly oriented by molecular conformation and alkyl chain

Graphene molecules, hexafluorotribenzo[a,g,m]coronene with n-carbon alkyl chains (FTBC-Cn, n = 4, 6, 8, 12) and Janus-type “double-concave” conformation, are used to fabricate self-assembly on highly oriented pyrolytic graphite surface. The structural dependence of the self-assemblies with molecular conformation and alkyl chain is investigated by scanning tunneling microscopy and density functional theory calculation. An interesting reverse face “up–down” way is observed in FTBC-C4 assembly due to the existence of hydrogen bonds. With the increase of the alkyl chain length and consequently stronger van der Waals interaction, the molecules no longer take alternating “up–down” orientation in their self-assembly and organize into various adlayers with lamellar, hexagonal honeycomb, and pseudohoneycomb structures based on the balance between intermolecular and molecule-substrate interactions. The results demonstrate that the featured “double-concave” molecules are available block for designing graphene nanopattern. From the results of scanning tunneling spectroscopy measurement, it is found that the electronic property of the featured graphene molecules is preserved when they are adsorbed on solid surface.