**4. Conclusions**

The experimental work shows that MoO3*íx* (1) nanowhiskers and MoS2 (1) nanotubes synthesized using solar ablation system, are stable for more than a year in the ambient conditions. However, Pb atoms that were observed in these nanostructures, are not stable in high concentration and tend to diffuse out of the lattice. The Pb outdiffusion from these nanostructures did not influence their high-temperature stability and their conversion into MoS2 (3,4) nanotubes. In all cases MoS2 nanotubes were observed after the sulfidization and annealing of MoO3*íx* (2) nanowhiskers and MoS2 (2) nanotubes. According to the EDS measurements the initial Pb concentration in both nanostructures was reduced after one year in the drawer. While subsequent annealing of these nanostructures lead to additional reduction of their Pb concentration. It can be seen from the EDS spectra that in the case of molybdenum suboxide nanowhiskers MoO3*íx* (2) as a precursor material, the Pb peak disappears after two hours

(H2S) annealing, while the same peak disappears after 30 min annealing at 810 °C in the case of MoS2 (2) nanotubes.

DFT calculations have verified a very low affinity of Pb atoms as either substitutional or intercalating agents to the host MoS2 lattice. Both types of modifications should lead to the destabilization of the electronic structure of the pristine MoS2 and cause a weak chemical bonding between Pb and S atoms with the oxidation state of Pb2+. The calculations have also demonstrated that the interaction between Pb atoms as intercalates is slightly stronger, than the interaction between Pb atoms as substitutional dopants in the MoS2 lattice. Thus, Pb*y*/MoS2 intercalates might exist only at lower Pb content compared to a solid solution of Mo1*íx*Pb*x*S2. The latter phase seems to be a more favorite form of Pb within the fabricated MoS2 nanotubes. The estimations of the free energies for the phase separation of Mo1*íx*Pb*x*S2 solid solutions explain the experimentally observed time- and high-temperature reduction of the Pb content in the nanotube lattice. The experimentally reached Pb concentrations (*x* = 0.3) may be attributed to a thermodynamically unstable system which was obtained by a fast kinetic of the chemical reaction between lead atoms and the primary 2H-MoS2 lattice during the solar ablation experiments.

### **Acknowledgments**

We wish to thank Jeffrey M. Gordon and Daniel Feuerman from the Jacob Blaustein Institutes for Desert Research in Sede Boqer of the Ben-Gurion University for their assistance in the synthesis of the oxide nanowhisekrs and nanotubes. This research was supported by the ERC grant INTIF 226639; the EU ITN 317451 grant and a grant of the Israel Science Foundation. Reshef **Tenne** acknowledges the support of the Harold Perlman Foundation and the Irving and Azelle Waltcher Foundation in honor of Moshe Levy. We are grateful also to the Irving and Cherna Moskowitz Center for Nano and Bio-Nano Imaging. Reshef **Tenne** holds the Drake Family chair in Nanotechnology and is the director of the Helen and Martin Kimmel Center for Nanoscale Science.
