Mn3O4-reduced graphene oxide (RGO) nanocomposites were prepared by chemical decomposition of the manganese hexacyanoferrate (MnHCF) complex directly on the graphene surface. XRD studies revealed the formation of crystalline hausmannite Mn3O4 nanocubes in the as-prepared nanocomposites without any heat treatment. The FE-SEM images showed the formation of Mn3O4 nanocubes on the graphene surface in the as-prepared nanocomposites. HR-TEM studies confirmed the homogeneous dispersion of ∼25 nm Mn3O4 nanocubes on graphene nanosheets. The amount of Mn3O4 nanocubes and graphene in the nanocomposites was estimated using TGA analysis from room temperature to 800 °C in air. The FT-IR and Raman spectroscopic analysis confirmed the functional groups in the nanocomposites and defects in graphene nanosheets in the nanocomposites. Cyclic voltammetry and galvanostatic charge-discharge experiments demonstrated a high specific capacitance of 131 F g(-1) in 1 M Na2SO4 electrolyte at a current density of 0.5 A g(-1) for the RGM-0.5 nanocomposite. A capacitance retention of 99% was observed for 500 charge-discharge cycles at a current density of 5 A g(-1), which conformed the excellent stability of the RGM electrodes. The prepared Mn3O4-RGO nanocomposites are promising for electrochemical energy storage.
A graded nano-alloy of Au 100Àx Pt x (x ¼ 7, 15, 23, 32, 40, 51, 62, 73 and 86) nanoparticles (NPs) formed by co-reduction of HAuCl 4 and H 2 PtCl 6 and the details are presented in this work. Au 100Àx Pt x NPs were characterized using surface plasmon resonance (SPR) absorption spectroscopy and transmission electron microscopy (TEM). The NPs were dispersed in Vulcan carbon (Au 100Àx Pt x /C) and annealed at 250, 400, 600 and 800 C. The as-formed and annealed materials were characterized using TEM, high resolution transmission electron microscopy (HR-TEM), powder X-ray diffraction (XRD), cyclic voltammetry (CV) and X-ray photoelectron spectroscopy (XPS). The CV studies indicate excess Pt on the surface, which is corroborated by XPS and HR-TEM results. The XRD data show that Vegard's law is obeyed by the asformed material and the materials annealed at 250 and 400 C, indicating that these materials are not nano-alloys. The studies clearly indicate that the formation of Au 100Àx Pt x NPs is kinetically controlled rather than being controlled by the thermodynamic stability. The results demonstrate the formation of graded alloys of Au 100Àx Pt x NPs. Pt excess in the graded nano-alloy is reflected favourably in the electrochemical oxidation of small organics. In the methanol oxidation reaction (MOR), the peak current value per mg of Pt increases as a function of x, reaches a maximum value at x ¼ 23 and the ratio of forward current to reverse current for MOR reached an unprecedented value of 6.7, which shows the catalyst's stability against poisoning by carbonaceous intermediates.
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