Synthesis of MoS₂/CNMs/TiO₂ Hybrid Nanostructures as Potential HER Catalysts

Molybdenum disulfide (MoS₂) is an emerging 2D nanomaterial with unique properties. It has been found as a very promising catalyst for several reactions, including hydrogen evolution reaction (HER).<br/>In this study, a novel, facile and low-cost method for preparing MoS₂-based materials is proposed. MoS₂ nanoparticles were obtained via wet chemical precipitation. The direct product of the synthesis was amorphous MoS₂. Further purification and annealing needed to be performed to get a high-quality product. This method allowed obtaining MoS₂ in an amorphous phase characterized by a high density of exposed sites ^1,2.<br/>Moreover, to activate the basal plane, promote its charge transport and catalytic activity, carbon nanomaterials (CNMs) were proposed. For the synthesis of hybrid nanostructures MoS₂/CNMs, various carbon nanomaterials, such as graphene oxide (GO), reduced graphene oxide (rGO), carbon nanotubes (CNTs) were used. The heterogeneous nucleation resulted in the production of carbon nanomaterials coated with heavily bonded MoS₂. The synthesis of hybrid nanostructures was conducted in the continuous flow reactor, i.e., impinging jet reactor. This type of reactor allows for continuous production and easy scalability. In addition, nanosized MoS2 is a p-type semiconductor with a bandgap in the range of 1.2–1.8 eV. To improve the light response-ability, an alternative semiconductor - titanium dioxide (TiO₂) was added to MoS₂/CNMs by mixing methods. TiO₂ nanoparticles were obtained via the hydrothermal method.<br/>A thorough analysis of obtained materials was conducted, using analytical techniques, such as thermogravimetric analysis (TGA), X-ray diffraction (XRD), particle size analysis by laser diffraction (PSD), Fourier-transform infrared spectroscopy (FT-IR), Raman spectroscopy, and scanning transmission electron microscopy (STEM). To check the catalytic potential towards HER, linear sweep voltammetry and cyclic voltammetry were conducted. The photocatalytic ability was measured using a photoelectrochemical cell.<br/>The molybdenum disulfide nanoparticles were deposited on the carbon nanomaterials surface with success. Annealing at a lower temperature of 550 °C allowed for obtaining the mostly amorphous form, which was confirmed by XRD analysis. FT-IR analysis of MoS₂/CNMs revealed characteristic absorption peaks for carbon nanomaterials. However, their reduction may have occurred during annealing. The hybrid nanostructures exhibit smaller particle sizes and narrower particle size distributions, which was confirmed by STEM and PSD analysis. The above results may have a significant influence on the improvement of the catalytic properties of MoS₂. Furthermore, the proposed method of producing hybrid nanostructures will allow their production on a larger scale, thus facilitating their commercialization. The obtained materials exhibited superior catalytic activity due to the exposed active sites of amorphous MoS₂, the enhanced photocatalytic response by TiO₂, nanometric sizes, good particle dispersion, and enhanced charge transport.<br/><br/>1. Wojtalik, M., Bojarska, Z. & Makowski, L. Experimental studies on the chemical wet synthesis for obtaining high-quality MoS2 nanoparticles using impinging jet reactor. <i>Journal of Solid State Chemistry</i> (2020). doi:10.1016/j.jssc.2020.121254<br/>2. Bojarska, Z., Mazurkiewicz-Pawlicka, M., Gierlotka, S. & Makowski, L. Production and properties of molybdenum disulfide/graphene oxide hybrid nanostructures for catalytic applications. <i>Nanomaterials</i> (2020). doi:10.3390/nano10091865