The Effects of Exfoliation, Organic Solvents and Anodic Activation on Catalytic Hydrogen Evolution Reaction of Tungsten Disulfide

International audienceThe rational design of transition metal dichalcogenide electrocatalysts for efficiently catalyzing hydrogen evolution reaction (HER) is believed to lead to the generation of a renewable energy carrier. To this end our work has made three main contributions. At first, we have demonstrated that exfoliation via ionic liquid assisted grinding combined with gradient centrifugation is an efficient method to exfoliate bulk WS2 to nanosheets with a thickness of a few atomic layers and lateral size dimensions in the range of 100 nm to 2 nm. These WS2 nanosheets decorated with scattered nanodots exhibited highly enhanced catalytic performance for HER with an onset potential of-130 mV vs. RHE, an overpotential of 337 mV at 10 mA cm-2 and a Tafel slope of 80 mV dec-1 in 0.5 M H2SO4. Secondly, we found a strong aging effect on the electrocatalytic performance of WS2 stored in high boiling point organic solvents such as dimethylformamide (DMF). Importantly, the HER ability could be recovered by removing the organic (DMF) residues, which obstructed the electron transport, with acetone. Thirdly, we established that the HER performance of WS2 nanosheets/nanodots could be significantly enhanced, by activating the electrode surface at a positive voltage for a very short time (60 s), decreasing the kinetic overpotential by more than 80 mV at 10 mA cm-2. The performance enhancement was found to arise primarily from the ability of a formed proton-intercalated amorphous tungsten trioxide (a-WO3) to provide additional active sites and favourably modify the immediate chemical environment of the WS2 catalyst, rendering it more favorable for local proton delivery and/or transport to the active edge site of WS2. Our results provide new insights into the effects of organic solvents and electrochemical activation on the catalytic performance of two-dimensional WS2 for HER

Quantitative determination of the spatial orientation of graphene by polarized Raman spectroscopy

Polarized Raman spectroscopy has been employed to characterize transverse sections of graphene monolayers upon both a copper substrate and a polyester film. Well-defined Raman spectra can be obtained from the one atom thick transverse sections of graphene because of the strong resonance Raman scattering. The intensity of Raman 2D band (I2D) is independent of the axis of laser polarization when the laser beam is perpendicular to the surface of the graphene monolayer but I2D is found to vary as approximately the 4th power of the cosine of the angle between the axis of laser polarization and the plane of graphene when the direction of laser propagation is parallel to the graphene sheet. It is demonstrated that a generalized spherical expanded harmonics orientation distribution function (ODF) can be used to quantify the spatial orientation of the graphene. The roughness of the graphene, evaluated using atomic force microscopy, shows a good correlation with the ODF determined using polarized Raman spectroscopy, showing how the Raman technique may be employed to quantify the spatial orientation of graphene. It is also demonstrated how the technique can be used to quantify the orientation of graphene in high-ordered pyrolytic graphite and graphene paper

Dispersal of pristine graphene for biological studies

Herein, we address the conflicting behaviour of different pristine graphene dispersions through their careful preparation and characterization in aqueous media.</p

Determining the Level and Location of Functional Groups on Few-Layer Graphene and Their Effect on the Mechanical Properties of Nanocomposites

Graphene is a highly desirable material for a variety of applications; in the case of nanocomposites, it can be functionalized and added as a nanofiller to alter the ultimate product properties, such as tensile strength. However, often the material properties of the functionalized graphene and the location of any chemical species, attached via different functionalization processes, are not known. Thus, it is not necessarily understood why improvements in product performance are achieved, which hinders the rate of product development. Here, a commercially available powder containing few-layer graphene (FLG) flakes is characterized before and after plasma or chemical functionalization with either nitrogen or oxygen species. A range of measurement techniques, including tip-enhanced Raman spectroscopy (TERS), time-of-flight secondary ion mass spectrometry (ToF-SIMS), and NanoSIMS, were used to examine the physical and chemical changes in the FLG material at both the micro- and nanoscale. This is the first reported TERS imaging of commercially available FLG flakes of submicron lateral size, revealing the location of the defects (edge versus basal plane) and variations in the level of functionalization. Graphene-polymer composites were then produced, and the dispersion of the graphitic material in the matrix was visualized using ToF-SIMS. Finally, mechanical testing of the composites demonstrated that the final product performance could be enhanced but differed depending on the properties of the original graphitic material