Three-dimensional interpenetrating carbon pillars via template-free HFCVD: Phase evolution-guided growth mechanism and electron transport performance

Three-dimensional (3D) interpenetrating carbon architectures are essential for high-efficiency electron transport and electrochemical applications, yet their fabrication typically relies on sacrificial templates or complex substrate pre-patterning. In this work, we report a single-step, template-free synthesis of 3D interpenetrating carbon pillars via hot filament chemical vapor deposition (HFCVD) using NiFe₂O₄ as catalyst. By correlating the phase transitions (probed by XRD, Raman and XPS) with morphological evolution (visualized by SEM and TEM), we demonstrate that the carbothermal reduction of NiFe₂O₄ proceeds through metastable Fe–Ni phases, triggering progressive catalyst fragmentation. This continuous fragmentation orchestrates a five-stage growth mechanism: (I) catalyst encapsulation and initial fragmentation, (II) base-growth carbon nanotube (CNT) initiation, (III) pillar formation with secondary nucleation, (IV) pillar coalescence and divergence, and (V) the formation of a 3D interpenetrating network. A pivotal finding is that amorphous carbon (α-C), traditionally considered an unwanted byproduct, serves as a functional structural mediator. It facilitates catalyst migration and secondary nucleation while fusing adjacent and diverging CNT-cored pillars into a mechanically robust and electrically continuous framework. Compared to non-interpenetrating controls, the 3D interpenetrating carbon pillars exhibit a lower electron field emission turn-on field and significantly enhanced electrochemical performance, highlighted remarkably by a near two-orders-of-magnitude reduction in charge transfer resistance. These results establish the coupling of progressive catalyst evolution with functional carbon deposition as a versatile principle for designing advanced carbon scaffolds. © 2026 Elsevier Ltd.