Size-fractionation of molybdenum disulfide nanoparticles
Molybdenum disulfide is a narrow-band semiconductor that acquires promising photo catalytic properties when dispersed on the nanometer scale. This is due to the bandgap widening, which depends on the particle size. Many synthesis methods yield polydisperse nanoparticle samples. There is a need for size-fractionation of the MoS2 nanoparticles to target the specific size-dependent properties. MoS2 nanoparticles in this study were synthesized by decomposing Mo(CO)6 in solutions in the presence of dissolved sulfur. The particles obtained were in the 5-40 nm size range. Size-fractionation with an ultracentrifuge was attempted in different ways. Sedimentation centrifugation in pure cyclohexane for three minutes with increasing speeds involved repeated harvesting of the sedimenting portions of MoS2. Limited fractionation was achieved. For density gradient centrifugation, cyclohexane and bromoform were chosen to create a gradient because of the large difference in their densities. Centrifugation at 1000rpin for three minutes resulted in the particle distribution through the centrifuge tube. Harvested fractions were largely polydisperse with the particle size varying by a factor of 2-3 within each fraction. Some crude fractionation was achieved. Adding a viscous polymer, polystyrene, into the original gradient did not improve the separation drastically. Solid phase extraction with a silica media was attempted, but was unsuccessful because the particles would not elute from the column. Size exclusion chromatography was attempted using a poly (styrene¬co-divinylbenzene) polymer media of two different grain sizes (200-400 and 300-800 p.m) to fill the column. Several different parameters were used, but the method failed. The MoS2 particles did not pass through the column. It is hypothesized that the pore sizes were too small to allow any of the particles to pass through. In summary, sedimentation centrifugation and simple density gradient with no additives yielded the best results.