Hierarchically structured 3D carbon nanotube electrodes for electrocatalytic applications

Hierarchically structured 3-dimensional electrodes based on branched carbon nanotubes (CNTs) are prepared on a glassy carbon (GC) substrate in a sequence of electrodeposition and chemical vapor deposition (CVD) steps as follows: Primary CNTs are grown over electrodeposited iron by CVD followed by a second Fe deposition and finally the CVD growth of secondary CNTs. The prepared 3-dimensional CNT structures (CNT/CNT/GC) exhibit enhanced double-layer capacitance and thus larger surface area compared to CNT/GC. Pt electrodeposition onto both types of electrodes yields a uniform and homogeneous Pt nanoparticle distribution. Each preparation step is followed by scanning electron microscopy, while the CNTs were additionally characterized by Raman spectroscopy. In this way it is demonstrated that by varying the parameters during the electrodeposition and CVD steps, a tuning of the structural parameters of the hierarchical electrodes is possible. The suitability of the hierarchical electrodes for electrocatalytic applications is demonstrated using the methanol electro-oxidation as a test reaction. The Pt mass specific activity towards methanol oxidation of Pt-CNT/CNT/GC is approximately 2.5 times higher than that of Pt-CNT/GC, and the hierarchical electrode exhibits a more negative onset potential. Both structures demonstrate an exceptionally high poisoning tolerance.

Highly Efficient Silver-Cobalt Composite Nanotube Electrocatalysts for Favorable Oxygen Reduction Reaction

This paper reports the synthesis and characterization of silver-cobalt (AgCo) bimetallic composite nanotubes. Cobalt oxide (Co₃O₄) nanotubes were fabricated by electrospinning and subsequent calcination in air and then reduced to cobalt (Co) metal nanotubes via further calcination under a H₂/Ar atmosphere. As-prepared Co nanotubes were then employed as templates for the following galvanic replacement reaction (GRR) with silver (Ag) precursor (AgNO₃), which produced AgCo composite nanotubes. Various AgCo nanotubes were readily synthesized with applying different reaction times for the reduction of Co₃O₄ nanotubes and GRR. One hour reduction was sufficiently long to convert Co₃O₄ to Co metal, and 3 h GRR was enough to deposit Ag layer on Co nanotubes. The tube morphology and copresence of Ag and Co in AgCo composite nanotubes were confirmed with SEM, HRTEM, XPS, and XRD analyses. Electroactivity of as-prepared AgCo composite nanotubes was characterized for ORR with rotating disk electrode (RDE) voltammetry. Among differently synthesized AgCo composite nanotubes, the one synthesized via 1 h reduction and 3 h GRR showed the best ORR activity (the most positive onset potential, greatest limiting current density, and highest number of electrons transferred). Furthermore, the ORR performance of the optimized AgCo composite nanotubes was superior compared to pure Co nanotubes, pure Ag nanowires, and bare platinum (Pt). High ethanol tolerance of AgCo composite nanotubes was also compared with the commercial Pt/C and then verified its excellent resistance to ethanol contamination.

Tailored Electrospinning of WO₃ Nanobelts as Efficient Ultraviolet Photodetectors with Photo-Dark Current Ratios up to 1000

In this work, polycrystalline WO3 nanobelts were fabricated via an electrospinning process combined with subsequent air calcination. The resultant products were characterized by X-ray diffraction, field-emission scanning electron microscopy, and high-resolution transmission electron microscopy in regard to the structures. It has been found that the applied voltage during the electrospinning process played the determined role in the formation of the WO3 nanobelts, allowing the controlled growth of the nanobelts. The ultraviolet (UV) photodetector assembled by an individual WO3 nanobelt exhibits a high sensitivity and a precise selectivity to the different wavelength lights, with a very low dark current and typical photo-dark current ratio up to 1000, which was the highest for any WO3 photodectectors ever reported. This work could not only push forward the facile preparation of WO3 nanobelts but also represent, for the first time, the possibility that the polycrystalline WO3 nanobelts could be a promising building block for the highly efficient UV photodetectors.

Core–shell hierarchical WO2/WO3 microspheres as an electrocatalyst support for methanol electrooxidation

Core–shell WO₂/WO₃ microspheres were used to prepare a Pt–WO₂/WO₃ catalyst with a mass specific current of 694 mA mg⁻¹ which is 1.4 and 3.0 times greater than those of commercial PtRu/C and Pt/WO₃.

Rational construction of strongly coupled metal-metal oxide-graphene nanostructure with excellent electrocatalytic activity and durability

The interaction within heterogeneous nanostructures can provide a great opportunity to radically enhance their electrocatalytic properties and increase their activity and durability. Here a rational, simple, and integrated strategy is reported to construct uniform and strongly coupled metal-metal oxide-graphene nanostructure as an electrocatalyst with high performance. We first simply synthesized the interacted SnO2-prGO (protected and reduced graphene oxide) hybrid with SnO2 nanoparticles (∼4 nm) selectively anchored on the oxygenated defects of rGO using an in situ redox and hydrolysis reaction. After the deposition of Pt, uniform Pt NPs are found to contact intimately and exclusively with the SnO2 phase in the SnO2-prGO hybrid. This constructed nanostructure (Pt-SnO2-prGO) exhibits significantly improved electrocatalytic activity (2.19-fold) and durability (2.08-fold) toward methanol oxidation over that of the state-of-the-art Pt/C catalyst. The detailed explanation of the strong coupling between SnO2 and graphene as well as between Pt and SnO2 is discussed, revealing that such a process can be used to immobilize various metal catalysts on metal-oxide-decorated catalysts for realizing advanced catalytic systems with enhanced performance.