High‑Performance Ag‑Nanohair‑Coated Activated Carbon Fibers via Self‑Assembly and Rapid Thermal Annealing

Background

Carbon fibers (CFs) are defined as fibers containing at least 92 % carbon by weight and are typically derived from polymeric precursors such as polyacrylonitrile (PAN), pitch, cellulose, lignin, and polyethylene[1,2]. PAN has long remained the most widely used precursor because of its excellent spinnability and high carbon yield, yet the high production cost still limits large‑scale deployment. Cellulose‑derived activated carbon fibers (ACFs) present a compelling, low‑cost alternative: their porous microstructure affords specific surface areas of 1 000–1 500 m² g⁻¹ and millions of 1–4 nm micropores, giving them superior adsorption performance compared with conventional activated carbon[3–5].

Nanotechnology has accelerated the integration of inorganic nanomaterials with carbon substrates, unlocking synergistic properties that surpass either component alone[6,7]. For example, Ag nanoparticle (AgNP)‑decorated CFs have demonstrated a four‑fold increase in photocatalytic CO₂ reduction to CH₃OH, attributed to enhanced CO₂ adsorption and electron transfer[1]. Likewise, CoSe₂ nanoparticles deposited on three‑dimensional CFs have yielded highly active, stable electrocatalysts for hydrogen evolution in acidic media[8]. However, most inorganic nanomaterials are spherical; anisotropic structures such as nanowires, nanosheets, and quantum dots are increasingly sought after because they can impart directional conductivity, surface plasmon resonance, and mechanical interlocking[9].

In this work, we designed ACFs “thickly overgrown” by Ag nanohair through a self‑assembly route followed by rapid thermal annealing. HBPAA‑modified AgNPs were synthesized via hydrothermal reduction on a HBPAA template. Acting as a molecular glue, HBPAA enabled positively charged AgNPs to uniformly self‑assemble onto viscose cellulose through electrostatic attraction and hydrogen bonding. The resulting AgNP‑coated VFs were then subjected to pre‑oxidation and carbonization in an open‑ended furnace sealed by high‑temperature flames at the inlet and outlet. Rapid cooling upon exit triggered capillary‑driven extrusion of molten Ag from ACF pores, which solidified into nanowires that protruded from the fiber surface.

Methods

Preparation of Ag Nanohair‑Grown ACFs

We first synthesized HBPAA‑capped AgNPs as described in our previous work[10]. Viscose fibers (2 g) were then immersed in a 4 000 mg L⁻¹ AgNP solution at 98 °C for 3 h to allow self‑assembly onto the fiber surface. After drying in an oven and storage in the dark, the coated fibers were subjected to a two‑step heat treatment: (i) oxidation at 350 °C in a water‑vapor atmosphere to form ladder‑type polymers, and (ii) carbonization up to 850 °C under inert gas to produce turbostatic carbon. The furnace was deliberately open‑ended and flame‑sealed at both ends, enabling the fibers to exit the chamber at ambient temperature, a key step for the rapid contraction that drives Ag liquid extrusion.

Measurements

Morphology and elemental distribution were examined by FESEM (S‑4200, Hitachi) equipped with EDS. Surface chemistry was probed by XPS (ESCALAB 250 XI, Thermo Scientific) and XRD (D8 ADVANCE, Bruker). Thermal behavior was assessed via TGA (TG 209 F3 Tarsus, Netzsch). Antimicrobial activity against E. coli and S. aureus was evaluated using the shake‑flask method (GB/T 20944.3‑2008).

Results and Discussion

The self‑assembly and rapid thermal annealing strategy yielded ACFs densely covered with Ag nanohair, as illustrated in Fig. 1. FESEM images (Fig. 2) revealed that pristine VFs possessed smooth, longitudinal grooves, whereas AgNP‑coated VFs displayed monodispersed bright white particles (3–80 nm) that retained the groove morphology. After carbonization, the fibers became fuzzy and bore irregularly shaped nanowires (~50 nm) protruding from the surface, confirming that AgNPs had transformed into nanowires.

Cross‑sectional FESEM (Fig. 3) showed that both pure ACFs and Ag‑nanohair‑grown ACFs contained abundant pores. In the latter, Ag nanowires appeared to grow from the pore mouths, with one end anchoring into the pore, suggesting a growth mechanism driven by capillary infiltration of molten Ag into pores and rapid solidification upon cooling. The melting point of 10–20 nm AgNPs (~129 °C) enables liquefaction at the carbonization temperature (850 °C), and the high‑temperature flame‑sealed furnace design ensures rapid quenching, which squeezes the molten Ag out of the pores as nanowires.

EDS analysis (Fig. 4) detected strong Ag signals in addition to C and O, confirming the presence of silver. XRD (Fig. 6) displayed characteristic peaks of face‑centred cubic Ag (111, 200, 220, 311) in the Ag‑nanohair‑grown ACFs, indicating that silver remained metallic after high‑temperature treatment. XPS (Fig. 7) further revealed Ag 3d binding energies of 373.9 eV and 367.9 eV, consistent with metallic Ag. The C/O ratio increased after carbonization, reflecting the removal of oxygen‑containing groups and the generation of CO/CH₄ reductants that likely reduced oxidized Ag to metallic form.

Thermogravimetric analysis (Fig. 8) demonstrated that the Ag‑nanohair‑grown ACFs retained excellent thermal stability, with only 8.4 % weight loss up to 1 000 °C. The presence of HBPAA on the fiber surface acted as a protective shell, delaying decomposition during oxidation. The thermal performance of Ag‑nanohair‑grown ACFs matched that of pristine ACFs, confirming that the Ag coating did not compromise structural integrity.

Antibacterial tests (Table 1) showed that the Ag‑nanohair‑grown ACFs achieved >99 % inhibition against E. coli and S. aureus, even after 30 ultrasonic washes. The durable activity is attributed to the strong mechanical interlock between the Ag nanohair and the porous ACF matrix, preventing leaching of Ag nanoparticles.

Preparation process of ACFs with dense Ag nanohair through self‑assembly and thermal expansion and contraction mechanisms

FESEM photographs of a ×3000 and b ×80000 pure VFs, c ×3000 and d ×40000 HBPAA/AgNP‑coated VFs, and e ×3000 and f ×80000 Ag nanohair‑grown ACFs

FESEM images of the cross‑section of the VF (a ×80k) and the surface of the pure ACF (b ×120k) and Ag nanohair‑grown ACF (c ×120k, d ×120k)

a SEM image and b EDS of the Ag nanohair‑grown ACF

Schematic diagram for fabrication of Ag nanohair‑grown ACFs

XRD patterns of (black) pure CFs, (red) AgNP‑coated CFs, (blue) Ag nanohair‑grown ACFs, and (purple) pure ACFs

XPS spectra: a wide scan, b C1s, and c Ag3d spectra of pure CFs, AgNP‑coated CFs, Ag nanohair‑grown ACFs, and pure ACFs. d, e are Ag3d spectra of AgNP‑coated VFs (d) and AgNP‑coated ACFs (e)

Thermogravimetric curves of (black) pure VFs, (red) AgNP‑coated VFs, (blue) pure ACFs, and (purple) Ag nanohair‑grown ACFs

Table 1 summarises antibacterial activity.

Conclusions

In summary, we have developed a scalable method to fabricate Ag‑nanohair‑coated activated carbon fibers by combining HBPAA‑mediated self‑assembly with an open‑ended rapid‑cooling furnace. The resulting fibers exhibit a dense network of metallic silver nanowires that are firmly embedded in the porous carbon matrix, endow the material with outstanding thermal stability and remarkable, long‑term antibacterial performance. This approach provides a versatile platform for integrating anisotropic metal nanostructures onto activated carbon fibers for applications ranging from filtration to antimicrobial textiles.