Nitrogen-rich 1T'-MoS2 layered nanostructures using alkyl amines for high catalytic performance toward hydrogen evolution.

The imminent global energy crisis and current environmental issues have stimulated considerable research on high-performance catalysts for sustainable hydrogen energy generation. Two-dimensional layered MoS2 has recently drawn worldwide attention because of its excellent catalytic properties for the hydrogen evolution reaction (HER). In the present work, we prepared nitrogen (N)-rich 1T' (distorted 1T) phase MoS2 layered nanostructures using different alkyl amines with 1-4 nitrogen atoms (methylamine, ethylenediamine, diethylenetriamine, and triethylenetetramine) as intercalants. The amine molecules intercalate at 10 atomic%, and simultaneously supply the N atoms that substitute the S atoms to produce the N-doped MoS2, whose composition is MoS2(1-x)Nx, where x = 0.1-0.26. MoS2 prepared with amines having more N atoms has enhanced catalytic HER performance: a Tafel slope of 36 mV dec-1 and 10 mA cm-2 at -160 mV (vs. RHE). First-principles calculations showed that the amine intercalation and N doping increase the density of states near the Fermi level in a narrow range and bring about an effective overlap of the dz2(Mo), pz(S), and pz(N) states. These factors in turn increase the carrier (electron) concentration and mobility for improved HER. The calculation also predicted that the most active site is S vacancies. The present work illustrates how the HER catalytic performance of 1T' phase MoS2 can be effectively controlled by the amine molecules.

Intercalation of aromatic amine for the 2H-1T' phase transition of MoS2 by experiments and calculations.

The novel properties of two-dimensional materials have motivated extensive studies focused on transition metal dichalcogenides (TMDs), which led to many interesting findings in recent years. Further advances in this area would require the development of effective methods for producing nanostructured TMDs with a controlled structure. Herein, we report unique MoS2 layered nanostructures intercalated with dimethyl-p-phenylenediamine (DMPD) with various concentrations, synthesized by a one-step hydrothermal reaction. The MoS2 layers possess a significantly expanded interlayer spacing. Remarkably, as the concentration of DMPD increases, the MoS2 preferentially adopts a unique metallic 1T' (distorted 1T) phase. The intercalated MoS2 exhibits excellent catalytic performance in the hydrogen evolution reaction. First-principles calculations show that the phase transition from 2H to 1T' phase occurs with increasing concentrations of DMPD, which can be accelerated by the S vacancies. A significant charge transfer from the DMPD molecules to MoS2 stabilizes the 1T' over the 2H phase, driving the 2H-1T' phase conversion. The DMPD and the S vacancies increased the carrier concentration, which leads to the enhanced catalytic performance. The present work illustrates how the phase control of TMDs can be effectively achieved by the intercalation of electron-donating molecules.

Novel Method for Preparing Transmission Electron Microscopy Samples of Micrometer-Sized Powder Particles by Using Focused Ion Beam.

The preparation of transmission electron microscopy (TEM) samples from powders is quite difficult and challenging. For powders with particles in the 1-5 µm size range, it is especially difficult to select an adequate sample preparation technique. Epoxy is commonly used to bind powder, but drawbacks, such as differential milling originating from unequal milling rates between the epoxy and powder, remain. We propose a new, simple method for preparing TEM samples. This method is especially useful for powders with particles in the 1-5 µm size range that are vulnerable to oxidation. The method uses solder as an embedding agent together with focused ion beam (FIB) milling. The powder was embedded in low-temperature solder using a conventional hot-mounting instrument. Subsequently, FIB was used to fabricate thin TEM samples via the lift-out technique. The solder proved to be more effective than epoxy in producing thin TEM samples with large areas. The problem of differential milling was mitigated, and the solder binder was more stable than epoxy under an electron beam. This methodology can be applied for preparing TEM samples from various powders that are either vulnerable to oxidation or composed of high atomic number elements.