Effect of synthesis duration on the morphology and structure of carbon nanowalls on the surface of carbon paper

Abstract

This work investigates the evolution of the morphology and structural state of carbon nanowalls (CNWs) grown on carbon paper by capacitively coupled plasma-enhanced chemical vapor deposition (CCP-PECVD) as a function of synthesis duration (30-120 min) under fixed process parameters. The surface morphology of the coatings was examined by scanning electron microscopy (SEM) and atomic force microscopy (AFM), while structural changes were evaluated by Raman spectroscopy (λ = 473 nm). It is shown that at the early stage (30 min) CNWs form via island-like nucleation on defects of the carbon paper substrate, whereas increasing the deposition time to 60-90 min leads to intensive vertical growth and densification of the CNW array. AFM analysis reveals an increase in average and root-mean-square roughness (R_a: from 13.91 to 26.91 nm; R_ms: from 18.19 to 34.26 nm for 60-120 min), while the peak-to-valley roughness R_z increases up to 90 min (from 124.3 to 159.3 nm) and then reaches saturation (161.1 nm at 120 min). This behavior indicates a transition from predominantly vertical growth to a regime dominated by densification and secondary nucleation. Raman spectroscopy confirms a progressive increase in defect density and disorder of the sp² carbon network with increasing synthesis time: the I_D/I_G ratio rises from 0.61 to 1.84, the G-band full width at half maximum increases from 24.8 to 40.9 cm^-1, and the degree of graphitization decreases from 40.2 to 27.2 %. It is established that a synthesis duration of approximately 90 min provides an optimal balance between the development of vertical morphology and the preservation of structural ordering of the carbon phase, whereas 120 min results in maximum defect density and surface development, which is promising for applications requiring a high density of active sites.