In the current work, heteroatom-doped graphene materials containing different atomic ratios of nitrogen and sulphur were employed as electrocatalysts for the oxygen reduction reaction (ORR) in acidic and alkaline media. To this end, the hydrothermal route and different chemical reducing agents were employed to synthesize the catalytic materials. The physicochemical characterization of the catalysts was performed by several techniques, such as X-ray diffraction, Raman spectroscopy and elemental analysis; meanwhile, the electrochemical performance of the materials toward the ORR was analyzed by linear sweep voltammetry (LSV), rotating disk electrode (RDE) and rotating ring-disk electrode (RRDE) techniques. The main results indicate that the ORR using heteroatom-doped graphene is a direct four-electron pathway, for which the catalytic activity is higher in alkaline than in acidic media. Indeed, a change of the reaction mechanism was observed with the insertion of N into the graphenic network, by the rate determining step changes from the first electrochemical step (formation of adsorbed OOH) on glassy carbon to the removal of adsorbed O (O ad ) from the N-graphene surface. Moreover, the addition of sulphur atoms into the N-graphene structure increases the catalytic activity toward the ORR, as the desorption of O ad is accelerated.
A photocatalytic method based on layer-by-layer photocatalyst films made of titanium dioxide nanoparticles and graphene oxide, TiO 2 NPs/GO, is proposed for performing the synthesis and immobilization of spherical AgNPs simultaneously using diluted AgNO 3 solutions and UV irradiation (254 nm). The novelty provided by this method is that the amount and aggregation extent of AgNPs can be controlled by the number of TiO 2 NP/GO bilayers and the composition of the outermost layer in the photocatalyst film. When the outermost layer is made of GO, more AgNPs are deposited because GO serves as an anchoring harbor for Ag + ions and a venue to the transportation of photoexcited electrons for subsequent photo reduction to Ag 0 . The photodeposition follows a first-order kinetics, whereas in the equilibrium it can be fitted by the Langmuir isotherm model for which surface coverage as high as 60% is attained after 70 min of UV irradiation. The TiO 2 NP/GO/AgNP substrates show surface-enhanced resonant Raman scattering (SERRS) activity for nickel(II) phthalocyanine-tetrasulfonic acid tetrasodium salt (NiTsPc). The SERRS activity is ascribed to a combination of local electromagnetic field enhancement because of the plasmon resonance of AgNPs and proper excitation of samples in resonance with the Q-band of NiTsPc.
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