The Role of Lead (Pb) in the High Temperature Formation of MoS2 Nanotubes

Recent studies have clearly indicated the favorable effect of lead as a growth promoter for MX2 (M = Mo, W; X = S, Se) nanotubes using MX2 powder as a precursor material. The experimental work indicated that the lead atoms are not stable in the molybdenum oxide lattice ion high concentration. The initial lead concentration in the oxide nanowhiskers (Pb:Mo ratio = 0.28) is reduced by one order of magnitude after one year in the drawer. The initial Pb concentration in the MoS2 nanotubes lattice (produced by solar ablation) is appreciably smaller (Pb:Mo ratio for the primary samples is 0.12) and is further reduced with time and annealing at 810 °C, without consuming the nanotubes. In order to elucidate the composition of these nanotubes in greater detail; the Pb-―modified‖ MX2 compounds were studied by means of DFT calculations and additional experimental work. The calculations indicate that Pb doping as well as Pb intercalation of MoS2 lead to the destabilization of the system; and therefore a high Pb content within the MoS2 lattice cannot be expected in the final products. Furthermore; substitutional doping (PbMo) leads to p-type semiconducting character; while intercalation of MoS2 by Pb atoms (Pby/MoS2) should cause n-type semiconducting behavior. This study not only sheds light on the role of added lead to the growth of the nanotubes and their role as electron donors; but furthermore could pave the way to a large scale synthesis of the MoS2 nanotubes.