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We report chemically exfoliated MoS2 nanosheets with a very high concentration of metallic 1T phase using a solvent free intercalation method. After removing the excess of negative charges from the surface of the nanosheets, highly conducting 1T phase MoS2 nanosheets exhibit excellent catalytic activity toward the evolution of hydrogen with a notably low Tafel slope of 40 mV/dec. By partially oxidizing MoS2, we found that the activity of 2H MoS2 is significantly reduced after oxidation, consistent with edge oxidation. On the other hand, 1T MoS2 remains unaffected after oxidation, suggesting that edges of the nanosheets are not the main active sites. The importance of electrical conductivity of the two phases on the hydrogen evolution reaction activity has been further confirmed by using carbon nanotubes to increase the conductivity of 2H MoS2.

A two-dimensional crystal of molybdenum disulfide (MoS2) monolayer is a photoluminescent direct gap semiconductor in striking contrast to its bulk counterpart. Exfoliation of bulk MoS2 via Li intercalation is an attractive route to large-scale synthesis of monolayer crystals. However, this method results in loss of pristine semiconducting properties of MoS2 due to structural changes that occur during Li intercalation. Here, we report structural and electronic properties of chemically exfoliated MoS2. The metastable metallic phase that emerges from Li intercalation was found to dominate the properties of as-exfoliated material, but mild annealing leads to gradual restoration of the semiconducting phase. Above an annealing temperature of 300 °C, chemically exfoliated MoS2 exhibit prominent band gap photoluminescence, similar to mechanically exfoliated monolayers, indicating that their semiconducting properties are largely restored.

The effect of the plasma on heating the growth substrate in plasma enhanced chemical vapor deposition (PECVD) of carbon nanotubes is characterized for the first time. This effect, which is commonly ignored in the nanotube/nanofiber literature, is the sole heating mechanism in this work for catalyst pretreatment and growth of straight and vertically aligned multiwalled carbon nanofibers. Significant temperatures, as high as 700 °C, are induced from a C2H2:NH3 direct current (dc) plasma with no other heat source present. To model the behavior of the plasma-heated substrate platform, we have developed a 1-D dc discharge model that incorporates a cathode platform energy balance, including ion bombardment, thermal radiation, and solid and gas conduction. The predicted gas-phase species present are correlated with the morphology of nanofibers grown by exclusive plasma heating as well as by heating from plasma in combination with a conventional resistive heater. The understanding of plasma heating and its accurate modeling are essential for reactor design for wafer scale production of vertically aligned nanofibers.

Molybdenum disulfide (MoS2) multilayers with functional polyelectrolyte nanospacing layers are presented. Taking advantage of the facile method of layer-by-layer (LbL) assembly, individual chemically exfoliated MoS2 layers are not only effectively isolated from interlayer coupling but also doped by functional polymeric layers. It is clearly demonstrated that MoS2 nanosheets separated by polymeric trilayers exhibit a much larger increase in photoluminescence (PL) as the number of layers is increased. The enhanced PL has been correlated to the ratio of excitons to trions with the type of polymeric spacers. Because uniform heterogeneous interfaces can be formed between various transition metal dichalcogenides and other soft materials, LbL assembly offers possibilities for further development in the solution-processable assemblies of two-dimensional materials.