Mechanical Exfoliation of 2D-Materials and Formation of Nanocomposites with Transition Metal Compounds by Vacuum Kinetic Spray Process for Energy Conversion and Sensing Applications

Abstract

The wide spreading of using two dimensional (2D) materials (i.e., graphene, MoS2, and Boron nitride) and their hybrid nanocomposites (NCs) with functional semiconductor compounds in electrochemical applications related to energy conversion, energy storage, and non-enzymatic sensing applications require a cost-effective technique that is amenable to large size production. This goal cannot be simply achieved because there is no single system for the fabrication of 2D materials and their hybrid NCs in one process. The top-down approaches based on the mechanical exfoliation of layered materials are commonly used for the mass production of pure 2D materials. The conventional methods for layered materials exfoliations through the mechanical approach are ball milling (wet or dry) and sonochemical techniques. In these techniques, the pure 2D materials were prepared in successive steps for a very long time. Also, these techniques require solvents with suitable surface energy that match with the type of the layered material to facilitate the stacked layers' exfoliation. The use of chemicals and solvents require further cleaning from waste product and the product only in the powder form. This requires hard labor work and increases the fabrication cost. Furthermore, the fabrication of 2D-materials hybrid NCs with a functional nanosized semiconductor cannot be achieved in one process, in which the pure 2D materials and the nanosized semiconductor materials are prepared individually. Then, both materials were mixed by chemical approach to form interfacial bonding between them. This would further increase the fabrication time consumption and reduce the overall cost efficiency. The dry spray coating based on the vacuum kinetic spray process was utilized for the direct deposition of nanosized thin films in one step from the corresponding micron powder at room temperature without any chemical treatment. There is various low-temperature dry manufacturing process such as cold spray, aerosol deposition (AD), as well as the nanoparticle deposition system (NPDS). The main difference between these techniques is the optimized pressure range for micron powder feeding as well as deposition pressure. The cold spray and AD techniques have similar system configurations of particle deposition but have some limitations on particle size range and type, which strongly affect the ability of deposition. As an example, the cold spray technique is mostly used for the deposition of metallic powder whereas the AD technique is frequently used for ceramic films. This indicated the limitation of deposition of metal/ceramics composites by these techniques. In contrast, the NPDS system has the optimum pressure range for deposition of either nano-sized ceramics or metallic as well as their hybrid NCs thin films at room temperature in one step. Furthermore, the graphene thin films were successfully deposited by the NPDS in one step on various substrate types. The kinetic induced transformation of graphite-staked layers to graphene nanosheets in one step by the NPDS at room temperature was explained in terms of the mechanical exfoliation of graphite stacked layers at impact velocity higher than a critical value. This would provide a new route for layered materials mechanical exfoliation based on the vacuum kinetic spray process that differs from the conventional technique that follows either sonochemical or ball milling approaches The ability of graphite stacked layers separation by the NPDS in one step motivated us to examine the ability of mechanical exfoliation of other graphene-like materials (i.e., layered materials) with a similar hexagonal structure such as molybdenum disulfide (MoS2) and boron nitride (BN) by the NPDS without the need of any chemical assistance. Hence, we could use the NPDS as a single system for the mechanical exfoliation for various layered materials. Moreover, the NPDS was used to fabricate graphene-based hybrid NCs with various functional nanosized semiconductors. So, we investigated the ability of hybrid NCs formation with another graphene-like material especially MoS2 to demonstrate the benefit of using the NPDS system as a single system for layered materials exfoliation and formation of hybrid NCs with other functional materials, which reduce the fabrication time and consequently enhance the fabrication cost efficiency. To improve the charge transfer kinetics at the electrode/electrolyte interface in the electrochemical energy applications the 2D-nanomaterials (NMs) like graphene and MoS2 nanosheets are commonly used. The hybridization between the 2D-NMs and the other functional nano-sized semiconductor would result in the improvement of overall activity toward various electrochemical applications. Optimization of modified electrode performance is strongly dependent on the composition constituent at the heterostructure interfaces. This indicates the need to study the effect of the composition ratio between the 2D NMs and the other functional materials on the electrochemical performance of the fabricated electrodes. The main objectives of this study are classified into three main parts: First, we want to provide a new mechanical approach for pure layered materials exfoliation using the NPDS according to the vacuum kinetic spray process. This was achieved by studying the mechanical exfoliation of various layered materials with a similar hexagonal phase such as graphite, MoS2, and BN using the same deposition condition by the NPDS. This indicated the applicability of NPDS as a single and solvent-free system for layered materials exfoliation. Second, we fabricated various graphene-based hybrid NCs with various transition metals (TMs) compounds (i.e., Ni(OH)2, Co3O4, Mn3O4, and ZnO) by the NPDS at room temperature in a one-step process on various substrate type (i.e., porous nickel foam and flat titanium sheet). All fabricated electrodes revealed the successful exfoliation of graphite stacked layers to graphene nanosheets in one step as well as the strong synergy improvement between the formed graphene nanosheets and the TMs species. The observed synergy improvement resulted in the activity enhancement toward water splitting-based energy conversion and non-enzymatic H2O2 detection applications. The catalytic activity of TMs-graphene hybrid NCs towards the water splitting through various routes including electrocatalytic, photocatalytic, and photo-electrocatalytic as well as and non-enzymatic H2O2 sensing was found to be strongly dependent on the initial ratio of TMs: graphite mixture, in which the optimum combination was estimated for each application. Third, we fabricated various MoS2-based hybrid NCs with spinel transition metals (TMs) compounds (i.e., Co3O4 and Mn3O4) by the NPDS at room temperature in a one-step process on various substrate type (i.e., porous nickel foam and flat titanium sheet). All fabricated electrodes revealed the successful exfoliation of MoS2 stacked layers to small MoS2 nanosheets and strong synergy improvement between the formed MoS2 nanosheets and the spinel TMs species. The observed synergy improvement resulted in the activity enhancement toward water splitting-based energy conversion and non-enzymatic H2O2 sensing applications. The catalytic activity of TMs-MoS2 hybrid NCs towards the electrocatalytic water splitting and non-enzymatic H2O2 sensing was found to be strongly dependent on the initial ratio of TMs: MoS2 mixture, in which the optimum combination was estimated for each application. The obtained results indicated that the NPDS technique is an efficient and eco-friendly technique for layered materials exfoliation and fabrication of hybrid NCs with high efficiency in various energy and sensing applications in one deposition step at room temperature in a short fabrication time. This indicated the improvement of fabrication cost efficiency compared with other conventional processes.