Polydopamine-assisted formation of Co3O4-nanocube-anchored reduced graphene oxide composite for high-performance supercapacitors

Tailoring transition-metal oxide nanoparticles with two-dimensional carbon has become a favorite way to improve their electrochemical performance. In this study, a composite of reduced graphene oxide was anchored by Co₃O₄ nanocubes and easily prepared with the assistance of polydopamine (PDA), using a combination of hydrothermal reaction and pyrolysis (Co₃O₄@PDA-rGO). Polydopamine, which possesses abundant catechol and amine groups, could be easily grafted onto graphene oxide to reduce the aggregation of graphene particles. Furthermore, PDA provided active sites, i.e., catechol and amine groups, which coordinated with Co²⁺, enabling enrichment of metal ions on the surface of graphene. After the pyrolysis of Co²⁺-containing PDA-grafted graphene at 400 °C, the Co²⁺ ions were converted into Co₃O₄ nanocubes, while the PDA carbonized to form N-doped porous carbon on the surface of graphene. The resulting product, Co₃O₄@PDA-rGO, demonstrated extraordinary supercapacitive behavior with good cycling stability owing to its unique porous structure as well as the intimate contact between Co₃O₄ and the carbon matrix.     

Morphology control of the hematite photoanodes for photoelectrochemical water splitting

The morphology of hematite photoanode is a significant relevant factor in its photoelectrochemical (PEC) performance. Hematite nanowires and nanocubes as well as nanorods with intentional Sn doping were prepared by hydrothermal processes containing disparate additives. The band-gap decreases in the sequence of nanowires, nanorods and nanocubes. Compared with nanorods, nanowires show higher carrier density but a lower light absorbance. With both inhibited bulk and surface charge recombination, nanowires achieve an enhanced photocurrent. Meanwhile, it is more complicated for the charge conversion in the hematite nanocubes. Light absorption is limited due to the compact arrangement of nanocubes. Besides, nanocubes show a highly oriented (104) plane which is unfavorable to the charge conductivity. Despite the negative factors hindering its PEC performance, the extremely high carrier density in the nanocubes benefits to the distinctly enhanced photocurrent collected from the hematite samples annealed at 550 and 650 °C respectively. However, the superiority of hematite nanocubes annealed under 800 °C is restricted by the high onset potential. Still, attributed to the high surface charge transfer efficiency, the hematite nanocubes achieve the highest photocurrent among the samples at biases above 1.3 V. Electrodes made of hematite nanorods, nanowires and nanocubes annealed at 800 °C achieve a photocurrent of 1.01, 1.30 and 1.40 mA cm⁻² at 1.6 V vs. RHE, respectively.     

Tuning the morphology of SrTiO3 nanocubes and their enhanced electrical conductivity

Strontium titanate (SrTiO₃) nanocubes were prepared via the solvothermal method in an isopropanol-water system, using cetyltrimethylammonium bromide (CTAB) as a capping agent and tert-butylamine (TBA) as a mineralizer. We found that the presence of TBA produced a synergistic effect in the nucleation and growth of SrTiO₃ in the solution, which enabled control over the formation of well-defined cubic particles. X-ray diffraction patterns of SrTiO₃, synthesized with the addition of 1 mol of TBA, showed a perovskite structure with high crystallinity. The presence of specific absorption peaks in the fourier-transform infrared (FTIR) spectrum confirmed the existence of CTAB and TBA on the surface of the particles. Transmission electron microscope (TEM) images showed that the sharp-edged cubic particles had narrow size distributions, and selected area electron diffraction (SAED) patterns indicated a high crystallinity. An increase in electrical conductivity values was found in the bulk samples with regular nanocube particles compared to those of non-regular nanocubes.     

Design and facile one-pot synthesis of uniform PdAg cubic nanocages as efficient electrocatalyst for the oxygen reduction reaction

Shape-controlled synthesis of multicomponent metallic nano-alloy materials is of great significance for the design and preparation of high-efficiency electrocatalysts. In this work, a simplified one-pot synthetic strategy for the preparation of uniform three-dimensional (3D) PdAg cubic nanocages (PdAg–CNC) was developed. The morphology and structure of PdAg–CNC were characterized by transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). We elucidate a series of reaction pathways for the formation of various morphologies (sphere, cubic nanocage and nanowire) of PdAg alloys by tuning the amount of glycine and controlling the Pd/Ag molar ratio. The PdAg–CNC shows enhanced electrocatalytic activity, good durability and excellent methanol tolerance in comparison with commercial Pd–C and Pt–C catalysts for the oxygen reduction reaction (ORR) in alkaline medium. The excellent ORR performance of the PdAg–CNC can be correlated to the Pd–Ag alloy formation and unique mesoporous nanocage structure. This work demonstrates a facile strategy for shape-controlled synthesis of multicomponent metallic nano-alloys materials and their catalytic application.     

Nano-sized cube-shaped single crystalline oxides and their potentials; composition,assembly and functions

Nanocrystals, especially with an anisotropic shape such as cubic, are candidates as building blocks for new bottom-up approaches to materials assembly, yielding a functional architecture. Such materials also receive attention because of their intrinsic size-dependent properties and resulting applications. Here, we report synthesis and characteristics of CeO₂, BaTiO₃ and SrTiO₃ nanocubes and the ordered assemblies. The importance of shape as well as size distribution is clarified for the bottom-up development of 3D fine structures. CeO₂ nanocube assemblies with enhanced surface of specified crystal planes such as (1 0 0) and (1 1 1) would open new surface-dominant devices. BaTiO₃ and SrTiO₃ nanocubes with narrow size distributions and robust diversity in compositions were obtained. BaTiO₃ films made up of ordered nanocube assemblies were fabricated on various substrates by evaporation-induced self-assembly method. Regardless of the substrate, the nanocubes exhibited {1 0 0} orientations and a high degree of face-to-face ordering, which remained even after heat treatment at 850 °C. The supracrystal films exhibited distinct ferroelectric behaviors.