Formation of Silicon/Graphene Heterostructures through Co-Gas-Phase Synthesis

The deliberate assembly of heterostructures made up of different flakes in configurations such as 2D/2D or particle-flake (0D/2D) emerged as an important advancement for the energy storage application because it provides properties such as mechanical stability, large surface area and at the same time low material requirements. The wet-phase processing techniques for heterostructures, including polymer-facilitated charge-induced self-assembly and hydrothermal synthesis, have been effectively established. But there are some limitations associated with these processing techniques. Due to the additional dead weight and post processing steps such as annealing, both the capacity and the use of promising metastable phases suffer. Nonetheless, solvent-free and scalable gas-phase synthesis of 2D-materials containing heterostructures is an open area of research. In this regard, methods to create 2D/2D and 0D/2D heterostructures in the absence of linking materials at the interfaces (e.g., polymers, surfactants) are lacking. In this research work, a simple, competitive and scalable strategy to produce heterostructures by combining 0D nanoparticles and 2D graphene in the gas phase is reported. Gas-phase synthesis of graphene is known for more than a decade which is now further developed to give production rates of few hundred mg/h of few-layer graphene (FLG). By combining two independent gas phase reactors, this few-layer graphene is controllably mixed with other functional materials in the gas phase to obtain heterostructures without the use of structure directing agents. The reactor system was involving a microwave plasma and a hot wall reactor. Silicon nanoparticles were synthesized in the hot wall reactor, meanwhile graphene was produced in the MW plasma reactor. Both reactors were connected in such a way that the nanoparticles and graphene get in contact with each other directly in the gas phase after their inception. In this way the self-assembled heterostructures of graphene and nanoparticles are produced with the production rate reaching almost 800 mg/h. Heterostructures with varying concentrations of silicon (Si) and graphene were synthesized by adjusting the flow rate of the Si precursor. The weight percentages of Si and graphene were determined through thermogravimetric analysis (TGA). Advanced characterization techniques, including TEM, Raman spectroscopy, DSC, and XRD, were utilized to comprehensively analyze the morphology, composition, and structural properties. TEM images indicate silicon is present as almost spherical nanoparticles that form small aggregates. These particles are interconnected by graphene. EDX mapping showed silicon nanoparticles having graphene surrounding them. It is present between the aggregates of nanoparticles as well as connecting them to each other. Raman as well as XRD spectra confirmed the presence of silicon and graphene in heterostructures with extraordinary purity. The presence of silicon in crystalline as well as amorphous form was also confirmed from Raman spectra. The extent of amorphous behavior appeared to increasing with higher flow rates of the Si precursor. Overall, the characterization revealed that increasing the precursor flow rate can elevate the weight percentage of silicon (Si) up to a certain threshold, which is influenced by other synthesis parameters, such as temperature and pressure, that were kept constant during this process. The heterostructure have shown excellent long-term stability when used as anode material in Li ion batteries. Thus, the formation of these heterostructures in the gas phase offers the possibility to exploit the potential of silicon to significantly enhance the anode storage capacity and cyclic stability.