Unconfined flame synthesis of mono-layer graphene and other nanostructured materials
Creators: name: Hong, Hua
Publisher: name: No Publisher Supplied
Year: 2017

description: A modified multi-element inverse-diffusion flames (m-IDFs) burner setup is utilized to synthesize mono-layer graphene and carbon nanotubes (CNTs) on metal and non-metal substrates.  The growth mechanisms of mono-, bi-, and few-layer graphene (MLG, BLG, and FLG, respectively) and their defect level using unconfined flame synthesis is investigated, with systematic variation of parameters such as substrate material, temperature, growth time, carbon precursor, and hydrogen flow rate.  In-situ Raman measurement is employed to observe the evolution of the gas-phase precursor species in the synthesis flow.  The growth of graphene on copper is observed for a wide range of temperatures ranging from 850 ˚C to 1000 ˚C, with high-quality graphene produced at 1000 ˚C.  An effective etching phenomenon on graphene layers reducing the number of layers is uncovered in a post-growth hydrogen annealing process using the same setup, where the hydrocarbon precursor flow is turned off, but the hydrogen m-IDFs are maintained.  Such effect enables the growth of MLG in an open-atmosphere environment for the first time.  The effects of hydrogen annealing on graphene with different starting qualities and substrates are investigated.  The hydrogen annealing technique can also be utilized to create defects (depending on the critical initial defect level  ) such as nanoscale pores and vacancies in the graphene layer(s).  The critical D-peak-to-G-peak intensity (ID/IG) ratio found in this work is ~ 0.6.  The ID/IG ratio increases dramatically after hydrogen annealing when as-synthesized graphene on Cu exhibits an initial ratio of at least 0.6.  However, the ID/IG ratio does not change obviously after annealing if the initial ratio is lower than 0.6.  By controlling the annealing condition, highly-defective graphene films with tunable defects are directly synthesized using a two-step flame method.  Such defective graphene is important in its own right (compared to single-crystal graphene), as it has a myriad of applications, such as ultrafiltering membranes, gas sensors, and optoelectronics.  Here, graphene-based ion-selective membranes are fabricated and preliminarily tested for permeability and ion rejection rate.  Using the same setup, carbon nanotube (CNT) growth is examined on silicon wafers with pre-deposited catalytic nanoparticle seeds.  Different seeding recipes and processes are used to study the effects of catalytic nanoparticles on CNT growth on non-metal substrates using flame synthesis.  The transition from growing iron oxide nanocrystals to CNTs on stainless-steel substrates with different carbon content is studied.  At low temperature (e.g., 500 ˚C) the growth of uniform α-Fe2O3 nanoparticle films is found on alloys of 304, 304L, and 316L stainless steel.  On the other hand, at high temperature (e.g., 850 ˚C), the growth of CNTs are observed on 304 stainless steel because of the carbide-induced breakup of the surface, but not on 316L, whose carbon content is much lower. In addition, the growth of CNTs and γ-Fe2O3 hybrid materials is achieved by performing a two-step flame synthesis, where the temperature is initially set at 500 ˚C and then tuned to 850 ˚C.  Such hybrid materials afford applications in many areas, such as batteries and sensors.