Ultrathin ReS₂ synthesized in a test tube

June 01, 2021 - Ultrathin rhenium disulfide (ReS₂) may find future applications in nano-electrodes. A group of researchers from the universities of Nottingham (UK) and Ulm (Germany) has now for the first time synthesized ReS₂ nanoribbons and studied their stepwise formation at the single-atom level.

Rhenium(IV) disulfide, which has been tested as an anode material for Li-ion batteries and shown high capacitance and capacity retention after 100 cycles [1], has recently gained significant attention following the discovery that it, uniquely amongst the transition metal dichalcogenides (TMDs), possesses layerindependent optical, electrical, and vibrational properties, as well as in-plane anisotropicity [2, 3]. The layered ReS₂ material has a distorted 1T structure [4] which causes the in-plane anisotropicity of the layers, as demonstrated by Raman spectroscopy [5]. However, the synthesis of ultrathin monolayer ReS₂ nanoribbons could not be established so far.

Previously, Rhenium(IV) disulfide (ReS₂) was grown on the exterior of carbon nanotubes. Recently, single-walled carbon nanotubes (SWNTs) had been used as a test tube or nano-reactor for the synthesis of various ultrathin 1D or 2D materials such as 1D molybdenum and tungsten disulfides [6]. They have been shown to be suitable for the synthesis of a variety of chemical species, such as fullerenes [7-9], halides [10-12], and metal oxides [13, 14]. SWNTs offer strict control over the structure and composition of the material formed inside and have been applied as a template for a wide range of ultrathin species, such as, sulfur-terminated graphene nanoribbons [15], and metal sulfides [16, 17]. It has also been shown that SWNTs can act as a nanoreactor transferring electrons to and from guest species in order to stabilize them within the interior channel [18]. Inorganic reactions have also been conducted within SWNTs by encapsulating precursor molecules followed by a thermal activation of the system [19].

Now scientists using the Ulm dedicated CC/CS aberration corrected SALVE (sub-Ångström low voltage electron) transmission electron microscope (TEM) operating a 60 kV [20] are reporting the synthesis of ultrathin ReS₂ nanoribbons with diameters in the range of 1–2 nm inside SWNTs. The synthetic route starting from dirhenium decacarbonyl (Re₂(CO)₁₀) as the source of rhenium atoms enables the study of inorganic transformations in general. A combination of microscopy and spectroscopy was used to probe the structure and composition of the materials in SWNTs at each stage of the synthesis and could gain important insights for the future rational design of functional nanomaterials for a variety of applications. The stepwise synthesis route is based on the formation of rhenium iodide species, which have not been previously studied before at the nanoscale, and the reaction of rhenium iodide to form rhenium disulfide.

Stepwise synthesis
To prepare the ultrathin rhenium iodide and sulfide materials, a stepwise synthesis was utilised (Fig. 1). The scientists used dirhenium decacarbonyl Re₂(CO)₁₀ as the precursor due to its low sublimation temperature and labile carbonyl ligands; this precursor has been used before to prepare Re nanoparticles inside SWNTs by Cao et al. [21]. An iodide is used as the intermediate as it offers the opportunity to examine entrapped forms of rhenium iodide. The final step of the synthesis involves the reaction of the iodide species with hydrogen sulfide to form the ReS₂ nanoribbons.

Reaction to form the iodide species
The n[Re₂(CO)₁₀ + I_m^-)]@SWNT^mn+ system was subsequently heated under inert atmosphere. Aberration-corrected high-resolution TEM (AC-HRTEM) imaging at 60 kV after the thermally-activated reaction showed the presence of discrete tightly packed clusters of [Re₆I₁₄]²⁻ 0.7 nm in diameter within the SWNT channel only allowing minimal translation motion; however, the clusters appeared to rotate freely allowing various projections to be observed (Figs. 2(a)–(c)). EDX analysis identified the presence of both rhenium and iodine in an atomic ratio of approx. 6:14 (Fig. 2(d)). The stoichiometry observed combined with the geometries observed in the AC-HRTEM images identified the material as [Re₆I₁₄]²⁻ clusters which are structurally analogous to those previously discovered for Mo and W [18]. The structure of [Re₆I₁₄]²⁻ can therefore be described as an octahedron of rhenium atoms, with capping iodide ligands on each face, and terminal iodides at each vertex (Figs. 2(e) and 2(f)). Structural models of the clusters were constructed and TEM simulations were conducted, the resultant images matched the observed projections (Figs. 2(b) and 2(c)) such as a cluster having a projected octahedral geometry. Upon prolonged imaging of the rhenium iodide clusters, the 60 kV electron beam causes dimerisation and chain formation (Fig. 2(c)). This is due to the electron beam rupturing a terminal Re–I bond, forming an unsaturated Re cation which must bond with a μ3-I from an adjacent cluster, resulting in the formation of polymeric iodide with a repeat unit of [Re₆I>sub>12]^{n>sup/>. This is in contrast to [W₆I₁₄]2− that was reported to remain stable under the 80 kV e-beam irradiation [18], which considering that constituent atoms in [Re₆I₁₄]2− receive less energy from the e-beam than in [W₆I₁₄]2− molecules, indicates the Re–I bond to be weaker than W–I.}

Reaction to form rhenium disulfide
The reaction of the [Re₆I₁₄]²⁻ clusters with hydrogen sulfide gas produced nanoribbons of rhenium(IV) disulfide with lengths above 30 nm and width of approximately 1.3 nm. The term nanoribbon is defined as a continuous quasi-1D material that has a width greater than its height resulting in a strip of material, which in this case is a single-layer of ReS₂, with a width of four Re atoms. AC-HRTEM confirmed an ordered atomic structure and a twisted shape of the nanoribbon, whilst EDX analysis confirmed the presence of rhenium and sulfur in a 1:3 ratio (Figs. 3(a) and 3(e)).

Due to the large difference between the atomic numbers of rhenium (Z = 75) and sulfur (Z = 16), only rhenium atoms were observed in the bright-field AC-HRTEM image as the single-atom contrast is proportional to the square root of the atomic number of an element. However, the nanotube sidewall can also be observed, despite the low atomic number of carbon (Z = 6), due to the overlap of approximately four atoms of carbon in projection at the SWNT sidewall (a thickness contrast). On the left-hand side of the AC-HRTEM image of ReS₂@SWNT the nanoribbon is in an edge-on orientation in which the contrast is controlled by the thickness of the nanoribbon and the atoms overlapping in projection (Fig. 3(c)). The atoms in the edge-on projection are separated by a distance of 0.29 nm. On the right-hand side the image displays clearly the face-on projection, which consists of each Re atom forming three types of distinct Re…Re distances resulting in the establishment of Re4 parallelograms, a distinct feature of the distorted 1T structure (Fig. 3(d)). The nanoribbon is four rhenium atoms thick which equates to two rows of parallelograms. Each Re is bonded to six sulfur atoms in octahedral geometry and the sulfur edges are in contact with the nanotube sidewall. The Re4 parallelograms have a measured distance of 0.26 nm between adjacent rhenium atoms, whilst the two rows of parallelograms are separated by a distance of 0.40 nm. These values are in good agreement with those from the structural model of the nanoribbon derived from the bulk ReS₂ unit cell (Figs. 3(c) and 3(d)).

“AC-HRTEM identified not only the structure of the nanoribbon but also the striking elliptical distortion of the SWNT host as a result of both shrinkage and expansion in response to orientation of guest-nanoribbon” said Ute Kaiser, head of the group of electron microscopy at Ulm University.

The cross-section of the SWNT was measured to be a 1.1 nm × 1.8 nm ellipse, and the change in projected nanotube cross-section is perfectly matched with the twisting of the rhenium sulfide nanoribbon (Fig. 3(b)). This demonstrates the strong synergy between the nanoribbon and the nanotube, similar to previously observed for MoS2@SWNTs, as well as helical CoI2@ SWNT [22, 23].

The new ReS₂ nanoribbons are described in a paper co-authored by Ute Kaiser and Andrei Khlobystov, a professor of nanomaterials at Nottingham University as well as Luke T. Norman, a research fellow at Nottingham.

Rhenium(IV) disulfide is an extraordinary transition metal dichalcogenide that retains its bulk electronic properties when exfoliated to a monolayer. Moreover, previous computational studies have shown that ultrathin nanoribbons also retain these properties and could therefore lead to being used in nanoscale electronic devices.

“We’ve been using the extreme confinement within SWNT nano test tubes to study Re containing materials for a long time, but the search has been complicated by how many reaction steps that are needed to form ultrathin ReS₂ nanoribbons inside SWNT," the authors said. „In the end, a combination of aberration-corrected TEM and EDX together with Micro Raman spectroscopy could discover and characterize the various steps of their formation. It shows the power and importance of the combined capabilities of the different observational technologies and the use of nanotubes as test containers”

The synthesis within SWNTs was shown to be successful for the construction of ultrathin ReS₂ nanoribbons owing to the precise control of reactions within SWNTs. The insertion of the initial source of rhenium, dirhenium decacarbonyl, was shown to be aided by van der Waals interactions indicated by the G-band not shifting position in the corresponding Raman spectra. The second step involved the transformation of dirhenium decacarbonyl into rhenium iodide clusters, n[Re₆I₁₄²⁻]@SWNT²ⁿ⁺, which was now described for the first time. These clusters were isostructural with W analogues reported inside SWNTs previously; however, the Re–I bonds appear to be more reactive than the W–I under the electron beam which resulted in partial oligomerisation of rhenium iodide clusters to [Re₆I>sub>12]^{n>sup/>.The new hybrid material may find future applications as nano-electrodes.}

Resource: Norman, L. T., Biskupek, J., Rance, G. A., Stoppiello, C. T., Kaiser, U., & Khlobystov, A. N. (2022). Synthesis of ultrathin rhenium disulfide nanoribbons using nano test tubes. Nano Research, 15(2), 1282-1287, doi: 10.1007/s12274-021-3650-2, [PDF], see also the supporting information.