A facile hydrothermal strategy to synthesize sulfur-doped reduced graphene oxide (S-RGO) sheets with good conductivity is proposed by using only graphene oxide (GO) sheets and sodium sulphide (Na2S) as precursors through a hydrothermal reaction process at 200 °C in one pot. The introduced Na2S can act as not only a sulfur dopant, but also as a highly efficient reducing agent in the formation of S-RGO sheets, which dramatically improves the electrical conductivities of the resulting S-RGO sheets compared with previous reports. The current reaches about 50.0 mA at an applied bias of 2.0 V for the optimized sample with 2.22 at% sulfur doping. This current value is much higher than that of RGO sheets (∼1.2 mA) annealed at 200 °C, and very close to that of single-layer graphene sheets (∼68.0 mA) prepared using chemical vapor deposition under the same test conditions. The resulting highly conductive S-RGO sheets offer many promising technological applications such as efficient metal-free electrocatalysts in oxygen reduction reactions in fuel cells and as supercapacitor electrode materials for high-performance Li-ion batteries.
Four configurations of whispering-gallery-mode (WGM) microcavities were designed and fabricated to modulate the optically pumped lasing characteristics by polymer modification on hexagonal ZnO microrod surfaces. On the basis of the total internal reflection (TIR) at the boundary of microcavities, the lasing characteristics were improved by raising the relative refractive index. Considering the different reflective conditions at various side surfaces, the typical lasing mode equation for whispering-gallery microcavity was modified to adapt for general situation even with unsymmetrical structure, and then employed to discuss the observed lasing behaviors, in the polyvinylcarbazole (PVK) modified ZnO microrods, such as mode position, mode numbers and quality factor. The optical field distributions for TE modes of the four configurations were also simulated by 2-dimensional finite difference time-domain (FDTD) method. The simulation agreed well with the experimental results to support the resonance mechanism.
It is a great challenge to directly assemble two-dimensional (2D) graphene oxide (GO) sheets into 1D fibers without any polymer or surfactant for their promising multifunctional applications. Herein, a facile self-assembly strategy is proposed to fabricate neat GO fibers from cost-efficient, aqueous GO suspension at a liquid/air interface based on the repulsive electrostatic forces, attractive van der Waals forces, and π-π stacking. During the self-assembly process and ultrasonic cleaning, the morphology variated from the source graphite powder through GO sheets to GO fibers and finally to neat GO fiber films. It is interesting to note that the electrical property of the GO fiber films was improved dramatically after subsequent low-temperature thermal annealing. The morphological evolution process and formation mechanism were analyzed on the basis of optical microscopy, scanning electron microscopy, and transmission electron microscopy observation, and the electrical characteristics was also discussion.
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