Dopant-Dependent Structure–Property Relationships in Functionalized Graphene and MWCNTs for Sustainable Energy Applications

Tailoring the electronic and electrochemical properties of carbon nanomaterials through controlled chemical functionalization is critical for advancing next-generation energy and optoelectronic technologies. Herein, we present a systematic nanoscale engineering strategy based on halogen-functionalized graphene (fluorinated and brominated) integrated with lithium-modified multi-walled carbon nanotubes (MWCNTs). Graphene oxide (GO) was chemically modified using hydrofluoric acid (HF) and tetrabutylammonium bromide (TBAB) to produce fluorinated graphene (FG) and brominated graphene (G-TBAB), respectively, while MWCNTs were functionalized with lithium fluoride (LiF) to obtain Li-MWCNTs. This comparative platform establishes a unified dopant-dependent framework correlating electronegativity, bonding configuration, and interfacial nanoarchitecture with optoelectronic and electrochemical performance across distinct carbon allotropes. Comprehensive structural and chemical characterization (FTIR, XRD, XPS, SEM, TEM, AFM, UV–Vis, photoluminescence spectroscopy, and electrochemical impedance spectroscopy) reveals that covalent C–F bonding enhances charge transport and suppresses carrier recombination, yielding FG with the lowest charge-transfer resistance (2.54 Ω·cm⁻²) and improved conductivity. In contrast, bromine-mediated noncovalent functionalization via TBAB induces steric and electrostatic modulation of the graphene surface, enabling tunable photoluminescence while preserving the structural integrity of the carbon framework. Furthermore, lithium functionalization of MWCNTs promotes efficient ion diffusion and interfacial charge storage, thereby enhancing capacitance and exhibiting pronounced Warburg behavior characteristic of diffusion-controlled electrochemical processes. By directly linking dopant chemistry to nanoscale interfacial phenomena and functional performance, this work introduces a dopant-selective materials design strategy for tailoring carbon nanomaterials toward photovoltaic, optoelectronic, and energy storage applications.