Activated Carbon Fibers “Thickly Overgrown” by Ag Nanohair Through Self-Assembly and Rapid Thermal Annealing
© The Author(s). 2017
Received: 23 July 2017
Accepted: 17 October 2017
Published: 9 November 2017
Anisotropic nanomaterial-modified carbon fibers attract increasing attention because of their superior properties over traditional ones. In this study, activated carbon fibers (ACFs) “thickly overgrown” by Ag nanohair were prepared through self-assembly and rapid thermal annealing. Viscose fibers with well-dispersed silver nanoparticles (AgNPs) on surfaces were first prepared through self-assembly of hyperbranched poly(amino-amine) (HBPAA)-capped AgNPs on viscose surfaces. HBPAA endowed the AgNP surfaces with negative charges and abundant amino groups, allowing AgNPs to monodispersively self-assemble to fiber surfaces. Ag nanohair-grown ACFs were prepared by sequential pre-oxidation and carbonization. Because the carbonization furnace was open-ended, ACFs are immediately transferrable to the outside of the furnace. Therefore, the Ag liquid adsorbed by ACF pores squeezed out to form Ag nanowires through thermal contraction. FESEM characterization indicated that Ag nanohairs stood on ACF surface and grew from ACF caps. XPS and XRD characterization showed that Ag successfully assembled to fiber surfaces and retained its metallic state even after high-temperature carbonization. TG analysis suggested that Ag nanohair-grown ACFs maintained their excellent thermal stabilities. Finally, the fabricated ACFs showed excellent and durable antibacterial activities, and the developed method may provide a potential strategy for preparing metal nanowire-grown ACFs.
Carbon fibers (CFs) can be defined as fibers that consist of at least 92% carbon by weight and are prepared from polymeric precursors, such as polyacrylonitrile (PAN), pitch, cellulose, lignin, and polyethylene [1, 2]. PAN was first used as a precursor for the preparation of CFs and still remains as an important starting material. With the development of the manufacturing industry, the demand for CFs has greatly increased because of their outstanding performances such as high tensile strengths, low densities, high moduli, excellent chemical and thermal stability, and/or strong adsorption ability for various inorganic and organic materials. However, the production cost of CFs is one major obstacle in large-scale applications. Biological materials such as biopolymers or polymers from biogenic sources are especially interesting sources for CFs and are inexpensive .
Viscose fibers (VFs) are typical regenerated cellulose fibers frequently used for the preparation of active carbon fibers (ACFs). Cellulose-based ACFs possess much weaker mechanical properties than CFs although the adsorption ability of the former is much stronger than the latter . The specific surface area of ACFs is up to 1000–1500 m2/g, with millions of 1–4-nm micropores dispersed on the fiber surface. Therefore, ACFs show superior adsorption ability to activated carbon, making them potentially applicable in wastewater treatment, air purification, individual protection, and so on [4, 5]. Nowadays, nanoscience and technology has made a remarkable headway. The integration of nanomaterials and carbon materials has become a popular research topic because of their outstanding properties. The fabricated composites not only inherit their respective advantages but also obtain new advanced functions under synergistic effects [6, 7]. For example, Ding et al. prepared Ag nanoparticle (AgNP)-decorated CFs by simple dipping, and the composite CFs showed fourfold higher photocatalytic activity than pure AgNPs during conversion of CO2 to CH3OH, which primarily resulted from higher CO2 adsorption and more efficient electron transfer from AgNPs to CO2 . Wan et al. synthesized highly dispersed CoSe2 nanoparticles on three-dimensional nanonet-like CFs by electrostatic spinning, and the electrocatalyst product possessed highly active, efficient, and stable properties for hydrogen evolution in acidic media . However, current nanomaterials, especially inorganic nanomaterials, are usually spherical. With the increasingly high requirement on the performances of nanomaterial/CF composites, modification of CFs with anisotropic nanomaterials such as nanowires, nanosheets, and nano-quantum dots have become a focus because of their certain superior properties over nanoparticles .
In this study, we designed ACFs “thickly overgrown” by Ag nanohair through self-assembly and rapid thermal annealing. Hyperbranched poly(amino-amine) (HBPAA)-modified AgNPs were synthesized by hydrothermal reduction on the HBPAA template. With HBPAA serving as a “molecular glue,” positively charged AgNPs uniformly self-assembled to the fiber surfaces through intermolecular electrostatic and hydrogen-bonding interactions between HBPAA and viscose cellulose. Ag nanohair-grown ACFs were prepared by pre-oxidation and carbonization of AgNP-coated VFs. To successfully grow Ag nanohairs on ACFs, an open-ended carbonization furnace sealed by high-temperature flames in the entrance and exit was chosen. Therefore, ACFs could rapidly cool down upon leaving the furnace, triggering the fast cold contraction of pores. Ag liquid would be squeezed out and cooled down to form Ag nanowires.
Preparation of Ag Nanohair-Grown ACFs
Molecular-mediated self-assembly technology was applied to guide the AgNPs into the VF surface, forming a monodispersive coating. Briefly, HBPAA-capped AgNPs were firstly synthesized as described in our previous study . Then, self-assembly of AgNPs on VFs were conducted by impregnation with 2 g VFs in a solution of HBPAA-capped AgNPs (4000 mg/L) at 98 °C for 3 h. AgNP-coated VFs were dried in an oven and stored in the dark.
Samples were characterized by FESEM (S-4200; Hitachi, Japan) equipped with an energy-dispersive X-ray spectroscopy (EDS), XPS (ESCALAB 250 XI; Thermos Scientific, USA), XRD (D8 ADVANCE, Bruker, Germany), and TG (TG 209 F3 Tarsus; Germany Netzsch Instruments, Inc., Germany). Antimicrobial activities of fiber samples were measured against Escherichia coli and Staphylococcus aureus using a shake-flask method (GB/T 20944.3-2008 [China]).
Results and Discussion
In addition, XRD pattern of AgNP-coated VFs showed one additional diffraction peak at around 38.3°, which can be indexed to the (111) plane of the face-centered-cubic phase of metallic Ag (JCPDS No. 04-0783) . By contrast, the XRD pattern of Ag nanohair-grown ACFs showed four clear diffraction peaks at around 38.3°, 44.3°, 64.4°, and 74.5°, which can be indexed to (111), (200), (220), and (311) planes of the face-centered-cubic phase of metallic Ag (JCPDS No. 04-0783), respectively, suggesting the metal valence of AgNPs . A stronger signal strength arose from the mass loss of VFs during carbonization, which also suggested that AgNPs underwent reduction during carbonization, mainly owing to the CO gas reductant generated through VF pyrolysis. In addition, the crystal structure of Ag nanohair-grown ACFs was similar to pure ACFs, indicating that Ag did not change the crystal structure.
Possible chemical change in the surface was evaluated by XPS (Fig. 7). All wide-scan XPS spectra (Fig. 7a) showed two ultra-strong peaks located at around 284 and 532 eV, corresponding to C1s and O1s, respectively [16, 17]. These peaks mainly derived from VFs or ACFs. However, we found that the C/O ratio decreased after AgNP self-assembly, suggesting the attachment of carbonyl-containing HBPAA on the VF surface. Notably, pure ACFs and Ag hair-grown ACFs showed much higher C/O ratio, indicating the removal of most oxygen-containing groups from ACFs. Such decomposition groups probably transformed into gaseous reductants, such as CO and CH4, which had an ability to reduce the oxidized AgNPs to metallic AgNPs.
HBPAA was very important for self-assembly of AgNPs on VFs because it endowed AgNP surfaces with positive charges and abundant amino groups, making AgNPs compatible to negatively charged hydroxyl-containing viscose cellulose . The attachment of HBPAA on VFs could be verified by analysis of C1s XPS spectra as shown Fig. 7b. The C1s peaks of four samples could be classified into four categories: carbon without oxygen bonds (C–C/C–H x ) (284.5 eV), carbon single bond to oxygen or nitrogen (C–O/C–N) (286.4 eV), carbon with two oxygen and/or nitrogen bond (O–C–O/N–C=O) (287.8 eV), and carboxyl (O–C=O) (289.0 eV), attributed by VFs, ACFs, and/or HBPAA [18, 19]. Compared with VFs, ACFs, and AgNP-coated ACFs, AgNP-coated VFs showed much higher content of C–O/C–N and O–C–O/N–C=O. The enhanced peaks were owing to superposition of VFs and HBPAA.
The Ag3d deconvolution analysis shown in Fig. 7d demonstrated that the fitted Ag3d3/2 and Ag3d5/2 peaks were 373.77 and 367.77 eV for AgNP-coated VFs, agreeing with the standard values of metallic Ag (373.9 and 367.9 eV) . This indicated AgNPs maintained their metallic nature when AgNPs were adsorbed to the viscose surface. Similarly, the deconvolved Ag3d3/2 and Ag3d5/2 peaks of AgNP-coated ACFs were 373.97 and 367.97 eV, suggesting the metallic state of AgNPs after the carbonization treatment (Fig. 7e). Note that the relative Ag3d intensity of Ag hair-grown ACFs was much higher than that of AgNP-coated VFs, agreeing with the above-discussed XRD analysis (Fig. 7a).
Antibacterial activities of ACFs against E. coli and S. aureus
Surviving cells (cfu/mL)
Surviving cells (cfu/mL)
3.57 × 104
5.4 × 103
Ag nanohair-grown ACFs
Ag nanohair-grown ACFs with ultrasonic washing 30 times
Ag nanohair-grown ACFs were prepared through self-assembly of AgNPs on VF surfaces and subsequent pre-oxidation and carbonization. HBPAA served as a “molecular glue” in adhering AgNPs to VF surfaces and in forming a monodispersive AgNP coating. The Ag nanohair-grown ACFs were prepared by sequential pre-oxidation and carbonization. The growth mechanism for Ag nanohair boils down to capillary and thermal expansion effects. To instantly reduce the temperature of ACFs, we designed an open-ended carbide furnace. ACFs are immediately transferrable to the outside of the furnace after completion of carbonization. Through thermal contraction, the Ag liquid squeezed out to form Ag nanowires. Ag nanohair stood on the ACF surface and grew from the ACF pores, as shown by FESEM. XPS and XRD characterization showed that Ag had successfully self-assembled to fiber surfaces and retained their metallic state even after high-temperature carbonization, owing to the gaseous reductants generated during carbonization. TG analysis suggested that Ag nanohair-grown ACFs maintained their excellent thermal stability. Finally, the fabricated ACFs showed excellent and durable antibacterial activities as a result of their strong binding.
The work is supported by the Production and Research Prospective Joint Research Project of Jiangsu Province under grant no. BY2015047-13.
XF conceived the project and wrote the manuscript. XY, SX, and QW designed and conducted the experiments. All authors discussed the results and commented on the manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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