**1. Introduction**

Inorganic nanotubes (INT) have become a prominent research subject in recent years. Their unique mechanical, optical and electrical properties [1–8] prompted extensive investigation. A number of synthetic routes have been developed, for example, chemical vapor transport using bromine for INT-MoS2 [9,10], bismuth catalyzed vapor-liquid-solid method for SnS2 nanotubes [11], misfit compounds superstructures of SnS-SnS2 and NbS-PbS2 nanotubes [12,13] and sulfidization of tungsten suboxide nanowhiskers leading to the formation of INT-WS2 [14].

Recent studies have shown a new strategy for successful synthesis of *MX*2 (*M* = Mo, W; *X* = S, Se) nanotubes using Pb as a growth promoter [15,16]. Figure 1 presents a SEM image of MoS2 nanotubes (Figure 1a) and high resolution TEM image of single MoS2 nanotube (Figure 1b) formed by irradiating MoS2 powder by a focused (×15,000) sunlight for 10 min [15]. The inset in Figure 1b shows the distance profile of nanotube layers. The interlayer spacing of 0.634 nm is somewhat larger than that of bulk 2H-MoS2 (0.62 nm). This 2% expansion of the lattice spacing in the nanotubes is well documented [17] and is attributed to strain relaxation. Figure 1c shows the EDS spectrum of single MoS2 nanotube after the irradiation, the table in the inset in Figure 1c shows the atomic % of the elements. In both cases the formation mechanism can be described as a Pb-promoted *MX*2 conversion

into Mo(Pb)O3í*x* nanowhiskers at high temperature (>2500 °C). Once formed, the nanowhiskers react back with the X-vapor which leads to the formation of *MX*2 nanotubes. Numerous attempts to synthesize MoS2 nanotubes in a conventional oven (up to 1000 °C) or induction furnace (up to 1600 °C), with the same precursor materials, or generation of Mo(Pb)O3í*x* nanowhiskers and their subsequent sulfidization did not succeed. It appears, therefore, that the high temperatures (>2500 °C) and the presence of lead are critical for the conversion of MoS2 powder into Mo(Pb)O3í*x* nanowhiskers which serve as template for the synthesis of *MX*2 nanotubes according to this process. In addition, the use of Pb as a growth promoter is environmentally unfavorable. Therefore, stabilization of molybdenum suboxide phases could be done using different metals, for example V, Ta or W [18–20].

**Figure 1.** MoS2 nanotubes formed using solar ablation system (**a**) SEM image of MoS2 nanotubes; (**b**) high resolution TEM image of a single MoS2 nanotube (adapted after [15]). The inset shows the profile of the interlayer distances of the nanotube wall. The interlayer spacing is somewhat larger than that of bulk 2H-MoS2 (0.62 nm); (**c**) EDS spectrum of a single MoS2 nanotube; the inset table shows atomic % of the nanotube's elements.

The main goal of the current work is to understand the role of lead (Pb) as a growth promoter of INT-MoS2; evaluate its solubility limits in the two lattices (oxide and sulfide) and their electronic state in the INT-MoS2 lattice. In particular, the stability of this heavy metal in the oxide precursor and the formed MoS2 nanotubes was investigated through both theory and experiment. Due to the fast kinetics of the process, the samples (oxide nanowhiskers and nanotubes) formed in such extreme conditions contain a certain amount of Pb. However, the content of Pb is found to be time-dependent and can considerably decrease upon thermal annealing. DFT calculations give preliminary information about the coordination and the electronic state of Pb atoms within the MoS2 lattice. They verify a low affinity of the Pb atoms as dopants or intercalants in the sulfide matrices and indicate that the experimentally observed high amount of lead (Pb) is too far from the thermodynamic equilibrium conditions.
