Introduction

Since their discovery in 1991 [1], carbon nanotubes (CNTs) have undergone significant advancements in research over the past three decades [2]. The exceptional mechanical, electrical, and thermal properties exhibited by CNTs present huge opportunities for the development of electric devices [3, 4], sensors [5], energy harvesting systems [6], and more. Concurrently, carbon microfibers (CMFs) represent a highly interesting one-dimensional material and serve as a crucial component in carbon microelectromechanical systems (C-MEMS) [7, 8], exhibiting unique transport properties. By adopting a hanging geometry, CMFs can fully utilize their functional potential without compromising performance, unlike fibers attached to substrates [9]. Consequently, the integration of CNTs and suspended CMFs within a unified architecture represents a significant advancement in the integration of micro and nano structures, holding the promise of significantly enhancing the performance and properties of C-MEMS-based devices [10, 11]. On the other hand, carbon fiber mats, known for their exceptional conductivity and surface area, are intentionally investigated for various electronic applications [32]. Additionally, the blue shift of the G-peak and the narrower peaks indicate a higher ordered structure, suggesting a higher tensile modulus [32]. Although we have not measured the mechanical properties in this study, the reinforcing effect of Fe–Co NPs on CMFs provides strong evidence of the quality of our samples in terms of mechanical properties. The existence of features in the RBM region of Raman spectra suggests that the CNTs exhibit metallic and conductive characteristics [33]. Moreover, the reported Young’s modulus and electrical conductivity of the host CFs fall within the ranges of 100–400 GPa and 103–107 S m−1, respectively [15, 34]. Therefore, we anticipate that the addition of CNT material will enhance the performance of existing CMFs. By comparing the Raman characteristics of the current hierarchical CMFs with other previously studied CFs, we can gain initial insights into their potential applications. The suspended hierarchical CMFs hold promise for sensor applications [35, 36] due to their ultra-sensitivity and fast response times [37]. With an ID/IG ratio around 1 and CNT diameters ranging from 16 to 245 nm, this material is suitable for fabricating lithium-ion electrodes [38]. Furthermore, the high aspect ratio CNTs, reaching up to 800, provides a large surface area that meets the demands of microfabrication in the C-MEMS system [39, 40].

The electrical improvement is attributed to the presence of CNTs, which enhance conductivity and reduce defects [41]. This outcome is further supported by the Raman results. Additionally, the electrical enhancement can be ascribed to the expanded surface area, facilitating more contact points between conductive carbon fibers [42]. In comparison to the force–distance curve in Fig. 6c, the force–distance relationship in Fig. 6d exhibits more peaks in both the approach and retract curves. These peaks seem to correlate with the surface roughness of the hierarchical structure observed in Fig. 6b. The variation in the shape of the force–distance curve reflects interactions between the tip and different regions of the sample surface [43]. Additionally, the additional peaks could be attributed to the unique conductivity properties of CNTs, leading to distinct electrostatic interactions between the AFM tip and the CNTs [44]. The diverse improvements observed in hierarchical carbon fiber structures, such as enhanced conductivity, reduced resistance, and unique surface properties, position them as versatile materials with applications in various electronic devices. The enhanced conductivity and reduced defects in the hierarchical structures address challenges related to conductivity and charge transport for electric applications [45]. Additionally, the expanded surface area and improved electrical characteristics can serve as components in microdevices and electronic circuits, contributing to the development of efficient and high-performance electronic systems [46]. The CMF structure significantly influences the growth and properties of the CNTs. Detailed investigations are described in Supplementary Sect. 4. This study primarily focuses on the preparation and characterization technique of an integrated micro/nano carbon structure using a novel method that combines subtractive (pyrolysis) and additive (deposition) processes. Based on these findings, a detailed investigation of the applications of the CNT forest on CMF platforms warrants a separate investigation in the future.

To assess the economic benefits of our integrated process combining CVD with pyrolysis, we conducted a preliminary comparison with the current technique used for decorating CNTs using embedded catalyst NPs within the CF matrix. The conventional CNT growing procedure, commonly found in literature [47, 48], involves multiple high-temperature thermal treatment stages in an inert or reductive atmosphere to reduce metallic sites and create particles. In the final step, CVD is performed to promote CNT growth on the CFs. The stringent treatment conditions can significantly diminish the mechanical and electrical performance of the resulting hierarchical fibers. Moreover, the catalyst precursors used in most cases are noble metals or high-cost materials such as Palladium (II) acetate (Pd(CH3CO2)2·xH2O) [49] or Nickel (II) acetate (Ni(CH3CO2)2·xH2O) [50]. Through screening calculations, our integrated process has been shown to reduce the operation time by 81% to 90%. The energy consumption and carrier gas usage are less than 3% and 1%, respectively. Further details of this calculation can be found in Supplementary Sect. 5. Thanks to the high activity of Fe–Co NPs, we eliminate the need for a reduction gas such as hydrogen, which requires stringent safety conditions. Additionally, our chemical compound cost is 1.7 USD/1 g, while other processes range from 1.2 to 7 USD/1 g (Supplementary Sect. 6). The preparation of Fe–Co NPs can be scaled up for industrial manufacturing using conventional equipment such as precipitation, stirring, and grinding devices. The raw materials, Fe and Co, are readily available as nitrate salts, and ethanol serves as the solvent. These chemicals are abundant and commonly used in the industry [51, 52]. The operating temperature range is not severe, typically between 70 and 200 °C at atmospheric pressure. Furthermore, we avoid the use of base or acid solutions, which can cause corrosion, thereby eliminating the need for resistance materials in the equipment. Importantly, our process does not generate harmful waste residues, and ethanol vapor can be easily recycled through a simple condensation process.

The characterization results suggest potential applications for hierarchical CMF structures with CNTs in sensors and lithium-ion electrodes, offering advantages such as ultra-sensitivity and fast response times. The hierarchical CMF structures also demonstrate suitability for microfabrication in C-MEMS systems. The enhanced electrical conductivity, reduced resistance, and distinctive surface properties position hierarchical carbon fiber structures favorably for a range of electronic applications. Adhesion force measurements, reflecting surface roughness and electrostatic interactions, further underscore these advantages. The integrated process, combining CVD with pyrolysis, demonstrates economic benefits over traditional multi-step processes, highlighting its potential for large-scale, cost-effective industrial manufacturing.

Conclusion

In conclusion, this study successfully demonstrates the fabrication of hierarchical CMF structures decorated with CNTs by simultaneously conducting pyrolysis and CVD. The local CVD of CNTs on suspended CMFs was achieved by utilizing Fe–Co NPs embedded in PAN fibers as catalysts. The SEM and elemental analysis revealed the formation of needle-like carbon structures on the pyrolyzed fiber surface, with carbon gases released from the pyrolyzing PAN fiber serving as the carbon source for localized CVD. The presence of an additional carbon source, such as camphor vapor, significantly enhanced the growth and density of CNTs on the CMF. XRD analysis confirmed the crystalline nature of the deposited CNTs and identified the phases of the catalyst powder. TEM characterization revealed entangled CNT mesh structures with varying diameters and lengths, indicating the successful growth of CNTs on CMFs. Raman analysis provided insights into the nanostructures and mechanical strengths of the hierarchical CMFs. The presence of CNTs resulted in sharper and higher G-band peaks, indicating an enhancement in structural order. The ID/IG ratio of the CNTs suggested higher-quality CNTs compared to other catalysts. The mechanical strength of CMFs improved with the addition of Fe–Co NPs, as evidenced by shifts in the D-peak and G-peak wavelengths and narrower peak widths. The RBM spectra of CNTs indicated metallic and conductive characteristics. The characterization results indicate potential applications in sensors and lithium-ion electrodes. The suspended configuration of the hierarchical CMFs provided advantages for ultra-sensitivity, fast response times, and microfabrication in C-MEMS systems. Additional enhancements, including improvements in conductivity, resistance reduction, and distinctive surface properties, have been thoroughly investigated to elucidate the advantages of hierarchical carbon fiber structures. Adhesion force measurements, revealing additional peaks in the force–distance relationship, indicate correlations with surface roughness and distinct electrostatic interactions. The significant reduction from 536 kΩ to 58 Ω upon incorporating CNTs underscores their positive impact on electrical conductivity and defect reduction. Collectively, the enhanced electrical conductivity and surface properties position hierarchical carbon fiber structures as promising candidates for a wide array of electronic applications. Furthermore, optimizing the process by integrating CVD with pyrolysis demonstrated economic benefits compared to traditional processes involving multiple intermediate steps. The reduction in operation time, energy consumption, and carrier gas usage, along with the elimination of costly catalyst precursors and the need for reduction gases, presented significant advantages. Overall, this study provides a promising approach for the efficient fabrication of hierarchical CMFs decorated with CNTs. The presented integrated process shows great potential for industrial manufacturing with its simplified and cost-effective methodology, making it a viable option for large-scale production.