Self-Assembly-Assisted Facile Synthesis of MoS2-Based Hybrid Tubular Nanostructures for Efficient Bifunctional Electrocatalysis.

In this work, MoS2-based hybrid tubular nanostructures are facilely synthesized via a self-assembly-assisted process and evaluated as a bifunctional electrocatalyst for hydrogen evolution reactions (HERs) and oxygen reduction reactions (ORRs). By simply mixing the reactants under ambient conditions, (NH4)2MoS4/polydopamine (PDA) hybrid nanospheres are formed. The protonated dopamine is linked to tetrahedral [MoS4]2- via weak N-H···S and O-H···S interactions, causing the PDA nanospheres merging together and forming nanorods under stirring-induced shear force. Moreover, the oxidative polymerization of dopamine proceeds on the surface of the nanorods, whereas it is prohibited inside the nanorods owing to lack of oxygen, leading to outward diffusion of dopamine and hence cavitation. After annealing, the tubular morphology is perfectly retained, while ultrafine MoS2 monolayers are formed due to the confinement of the framework. Benefiting from these unique structural features, the MoS2/C hybrid nanotubes possess abundant active sites and high surface area, as well as boost electronic and ionic transport, remarkably enhancing their electrocatalytic activities. The onset and half-wave potentials are 0.91 and 0.82 V, respectively, for ORR, close to those of Pt/C. Moreover, low onset potential and small Tafel slope are also observed for HER, demonstrating the potential of the hybrid nanotubes as a promising non-noble metal bifunctional electrocatalyst.

One-Pot Synthesis of Fe(III)-Polydopamine Complex Nanospheres: Morphological Evolution, Mechanism, and Application of the Carbonized Hybrid Nanospheres in Catalysis and Zn-Air Battery.

We report one-pot synthesis of Fe(III)-polydopamine (PDA) complex nanospheres, their structures, morphology evolution, and underlying mechanism. The complex nanospheres were synthesized by introducing ferric ions into the reaction mixture used for polymerization of dopamine. It is verified that both the oxidative polymerization of dopamine and Fe(III)-PDA complexation contribute to the "polymerization" process, in which the ferric ions form coordination bonds with both oxygen and nitrogen, as indicated by X-ray absorption fine-structure spectroscopy. In the "polymerization" process, the morphology of the complex nanostructures is gradually transformed from sheetlike to spherical at the feed Fe(III)/dopamine molar ratio of 1/3. The final size of the complex spheres is much smaller than its neat PDA counterpart. At higher feed Fe(III)/dopamine molar ratios, the final morphology of the "polymerization" products is sheetlike. The results suggest that the formation of spherical morphology is likely to be driven by covalent polymerization-induced decrease of hydrophilic functional groups, which causes reself-assembly of the PDA oligomers to reduce surface area. We also demonstrate that this one-pot synthesis route for hybrid nanospheres enables the facile construction of carbonized PDA (C-PDA) nanospheres uniformly embedded with Fe3O4 nanoparticles of only 3-5 nm in size. The C-PDA/Fe3O4 nanospheres exhibit catalytic activity toward oxygen reduction reaction and deliver a stable discharge voltage for over 200 h when utilized as the cathode in a primary Zn-air battery and are also good recyclable catalyst supports.

Self-Assembly-Induced Alternately Stacked Single-Layer MoS2 and N-doped Graphene: A Novel van der Waals Heterostructure for Lithium-Ion Batteries.

In this article, a simple self-assembly strategy for fabricating van der Waals heterostructures from isolated two-dimensional atomic crystals is presented. Specifically, dopamine (DOPA), an excellent self-assembly agent and carbon precursor, was adsorbed on exfoliated MoS2 monolayers through electrostatic interaction, and the surface-modified monolayers self-assembled spontaneously into DOPA-intercalated MoS2. The subsequent in situ conversion of DOPA to highly conductive nitrogen-doped graphene (NDG) in the interlayer space of MoS2 led to the formation of a novel NDG/MoS2 nanocomposite with well-defined alternating structure. The NDG/MoS2 was then studied as an anode for lithium-ion batteries (LIBs). The results show that alternating arrangement of NDG and MoS2 triggers synergistic effect between the two components. The kinetics and cycle life of the anode are greatly improved due to the enhanced electron and Li(+) transport as well as the effective immobilization of soluble polysulfide by NDG. A reversible capacity of more than 460 mAh/g could be delivered even at 5 A/g. Moreover, the abundant voids created at the MoS2-NDG interface also accommodate the volume change during cycling and provide additional active sites for Li(+) storage. These endow the NDG/MoS2 heterostructure with low charge-transfer resistance, high sulfur reservation, and structural robustness, rendering it an advanced anode material for LIBs.

Transition-metal-ion-mediated polymerization of dopamine: mussel-inspired approach for the facile synthesis of robust transition-metal nanoparticle-graphene hybrids.

Inspired by the high transition-metal-ion content in mussel glues, and the cross-linking and mechanical reinforcement effects of some transition-metal ions in mussel threads, high concentrations of nickel(II), cobalt(II), and manganese(II) ions have been purposely introduced into the reaction system for dopamine polymerization. Kinetics studies were conducted for the Ni(2+)-dopamine system to investigate the polymerization mechanism. The results show that the Ni(2+) ions could accelerate the assembly of dopamine oligomers in the polymerization process. Spectroscopic and electron microscopic studies reveal that the Ni(2+) ions are chelated with polydopamine (PDA) units, forming homogeneous Ni(2+)-PDA complexes. This facile one-pot approach is utilized to construct transition-metal-ion-PDA complex thin coatings on graphene oxide, which can be carbonized to produce robust hybrid nanosheets with well-dispersed metallic nickel/metallic cobalt/manganese(II) oxide nanoparticles embedded in PDA-derived thin graphitic carbon layers. The nickel-graphene hybrid prepared by using this approach shows good catalytic properties and recyclability for the reduction of p-nitrophenol.

Silicon nanoparticles encapsulated in hollow graphitized carbon nanofibers for lithium ion battery anodes.

Silicon (Si) is a promising material for lithium ion battery (LIB) anodes due to its high specific capacity. To overcome its shortcomings such as insulation property and large volume change during the charge-discharge process, a novel hybrid system, Si nanoparticles encapsulated in hollow graphitized carbon nanofibers, is studied. First, electrospun polyacrylonitrile (PAN)-Si hybrid nanofibers were obtained using water as the collector. The loose nanofiber lumps suspended in water have large inter-fiber distance, allowing in situ coating of a thin layer of polydopamine (PDA), the source for graphitized carbon, uniformly throughout the system. The designed morphology and structure were then realized by etching and calcination, and the morphology and structure were subsequently verified by various analytical techniques. Electrochemical measurements show that the resulting hollow hybrid nanofibers (C-PDA-Si NFs) exhibit much better cycling stability and rate capacity than conventional C/Si nanofibers derived by electrospinning of PAN-Si followed by calcination. For instance, the capacity of C-PDA-Si NFs is as high as 72.6% of the theoretical capacity after 50 cycles, and a high capacity of 500 mA h g(-1) can be delivered at a current density of 5 A g(-1). The significantly improved electrochemical properties of C-PDA-Si NFs are due to the excellent electrical conductivity of the carbonized PDA (C-PDA) shell that compensates for the insulation property of Si, the high electrochemical activity of C-PDA, which has a layered structure and is N-doped, the hollow nature of the nanofibers and small size of Si nanoparticles that ensure smooth insertion-extraction of lithium ions and more complete alloying with them, as well as the buffering effect of the remaining PAN-derived carbon around the Si nanoparticles, which stabilizes the structure.

Highly electrically conductive layered carbon derived from polydopamine and its functions in SnO2-based lithium ion battery anodes.

Thin carbonized polydopamine (C-PDA) coatings are found to have similar structures and electrical conductivities to those of multilayered graphene doped with heteroatoms. Greatly enhanced electrochemical properties are achieved with C-PDA-coated SnO(2) nanoparticles where the coating functions as a mechanical buffer layer and conducting bridge.