Surface modification of carbon fibers with hydrophilic Fe3O4 nanoparticles for nickel-based multifunctional composites

Journal Pre-proofs Full Length Article Surface modification of carbon fibers with hydrophilic Fe3O4 nanoparticles for nickel-based multifunctional composites Min Zhang, Lei Ding, Jing Zheng, Libin Liu, Hamed Alsulami, Marwan Amin Kutbi, Jingli Xu PII: DOI: Reference:

S0169-4332(20)30104-5 https://doi.org/10.1016/j.apsusc.2020.145348 APSUSC 145348

To appear in:

Applied Surface Science

Received Date: Revised Date: Accepted Date:

3 November 2019 2 January 2020 9 January 2020

Please cite this article as: M. Zhang, L. Ding, J. Zheng, L. Liu, H. Alsulami, M. Amin Kutbi, J. Xu, Surface modification of carbon fibers with hydrophilic Fe3O4 nanoparticles for nickel-based multifunctional composites, Applied Surface Science (2020), doi: https://doi.org/10.1016/j.apsusc.2020.145348

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Surface modification of carbon fibers with hydrophilic Fe3O4 nanoparticles for nickel-based multifunctional composites

Min Zhanga,*, Lei Dinga, Jing Zhenga, Libin Liub*, Hamed Alsulamic, Marwan Amin Kutbic, Jingli Xua a

College of Chemistry and Chemical Engineering, Shanghai University of Engineering Science, Shanghai

201620, China. Email: [email protected]

b

School of Chemistry and Pharmaceutical Engineering, Qilu University of Technology (Shandong

Academy of Sciences), Jinan 250353, China. Email: [email protected].

c

Department of Mathematics, Faculty of Science, King Abdulaziz University, P.O. Box 80203, Jeddah

21589, Saudi Arabia.

*Corresponding Author: E-mail: [email protected], [email protected].

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ABSTRACT Grafting functional components onto carbon fibers is important for designing many multifunctional composites. However, this process is often hindered by the low reactivity of carbon fibers. Hence, developing innovative grafting strategies with functional groups or nanomaterials on carbon fibers (CFs) is highly desired. Herein, we proposed a facile method to grow hydrophilic Fe3O4 NPs on non-pretreated CFs using a polyol-assisted hydrothermal method, changing hydrophobic CFs to hydrophilic. Benefiting from the hydrophilic property of CFs@Fe3O4, the resultant CFs@Fe3O4 can be successfully coated by SiO2 layer to form CFs@Fe3O4@SiO2 composites. After a hydrothermal reaction with nickel ions, the CFs@Fe3O4@nickel

silicate

composites

were

fabricated.

Followed

by

coating

polydopamine(PDA) layer and then carbonizing in N2 atmosphere, CFs@Fe3O4@SiO2-C/Ni hybrids can be finally obtained. Notably, the hydrophilic Fe3O4 NPs play a crucial role in the fabrication process, not only improving the interfacial adhesion of carbon fibers for further modification but also providing the main magnetism resource. Moreover, the density and size of Ni NPs can be tailored by tuning pyrolysis temperature. Experimental results confirm that CFs@Fe3O4@SiO2-C/Ni hybrids synthesized at 700 °C gave the best activity on the reduction in 4-nitrophenol, which are easy to collect and recycle due to their unique structure and good magnetic property. Keywords: Magnetic carbon fibers, polyol-assisted hydrothermal method, nickel nanoparticles, 4-nitrophenol

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1. Introduction Carbon fibers (CFs) have been considered as significant structural materials with wide applications in catalysis [1, 2], batteries [3, 4], supercapacitors [5, 6] etc. due to their inherent advantages such as strong strength, outstanding thermal stability, and excellent electronic conductivity and thermal stability [7-10]. Moreover, carbon fibers can well carry nano-active materials and effectively prevent their aggregation. Additionally, carbon fibers have been considered as a good way to bridge the world of the nano and macro length scales, which does two things: one is constructing one-dimensional (1D) macrostructures with nanoscopic architectures; the other is maintaining the original 1D structure [11]. Recently, extensive carbon fiber-based materials have been reported in various fields [12, 13]. Nevertheless, it is not easy to grow active materials on carbon fibers because of the weak interfacial interaction between carbon fibers and the surrounding matrix, attributing to the smooth surface and the low surface energy of pristine carbon fibers. To improve the carbon fiber-active materials interfacial adhesion to enhance mechanical interlocking, intensive strategies have been explored such as chemical treatment [14, 15], discharge plasma treatments [16], and electrochemical oxidation [17, 18]. However, these approaches may cause a decrease in the ultimate strength of CFs although they can introduce functional groups on the surface of carbon fibers to strengthen the adhesion at the interface. An alternative interphase method is to coat chemicals that can introduce some hydroxyl groups to invert carbon fibers from amphiphobic to hydrophilic for further modification. For example, Chen et al. have applied polydopamine (PDA) as a bridge to adjust the surface properties of CFs to fabricate

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polymer-based composites such as the PDA-functionalized SCFs (C18-PDA-SCFs) [19]. More recently, Xie et al. adopted PDA to pretreat pristine CFs before the growth of ZnO nanorods to introduce the oxygen-containing functional groups, contributing to the enhanced interface strength of CFs [20]. So far, few chemicals that may activate hydroxyl groups on carbon fibers are reported. Hence, it is necessary to explore more functional materials that can introduce active groups to enhance the interfacial strength of fibers. Recently, the polyol-assisted fabrication of magnetic nanoparticles has been developed to functionalize carbon nanotubes with Fe3O4 NPs and the carbon nanotubes are not pretreated by concentrated nitric acid or other reagents. Inspired on this finding as well as the similarity between CNTs and CFs, herein, a facile strategy of functionalizing the CFs with Fe3O4 NPs using polyol assisted methods has been proposed. Moreover, the successful integration of the individual Fe3O4 NPs with carbon fibers could enable the CFs composites to gain enhanced performance [21, 22]. Therefore, the coating Fe3O4 NPs on CFs could solve three issues simultaneously; the low reactivity, hydrophobic surface of CFs and the separation efficiency, typical of “kill three birds with one stone”. This greatly encourages us to design Fe3O4/CFs composites to investigate the synergistic effect of the binary composition and nanostructures’ unique properties, contributing to the synthesis of advanced hybrids for enhanced performance. Metallic nickel is one of the most popular catalysts and protein adsorbent due to its advantages of magnetic property, cost-effectiveness, and high catalytic activity. However, these single nickel NPs tend to aggregate together during the application due to the large -4-

surface area and good activities, resulting in poor performance and thus decrease cycling lifetime. To solve these problems, carbon nanofibers have been used as supports to disperse Ni NPs to improve the stability and leaching-resistance capability of these nickel NPs [23]. However, the low loading amount of metallic Ni NPs on 1D carbon fibers and poor magnetic response, greatly limiting their practical application. To improve the loading amount of nickel NPs and the separation efficiency, the hierarchical nickel silicate, as well as the Fe3O4 NPs, were all integrated with CFs to achieve this target. Nickel silicates have been extensively studied due to the enhanced performance and prospective new applications [24-26]. Moreover, the uniform dispersion of the metallic Ni species would be well maintained during the high-temperature pyrolysis process for the good hierarchical structures of nickel silicates [27, 28]. Herein, the Fe3O4 NPs functionalized the surface of 1D carbon fibers can be an ideal substrate for the in situ formation of nickel NPs. The Fe3O4 NPs generated on carbon fibers as a new magnetic material platform can form additional roughness, enhance the surface energy, and increase the functionality, contributing to high-performance hybrids by simple surface/interface manipulation. As a proof of concept, we grow Fe3O4 NPs on 1D carbon fibers (CFs) by polyol hydrothermal method to offer nucleation sites for the in situ generations of a Fe3O4 NPs on the fibers. Applying this facile approach, we find that SiO2 could be applied as an inorganic layer to enhance the surface/interface properties of fibers for further functionality, which can be easily converted to nickel silicate by hydrothermal reaction with nickel ions. After coated by PDA and subsequent thermal treatment in N2, the

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hierarchical CFs@Fe3O4@SiO2-C/Ni composites were finally fabricated. This material exhibits prominent tensile properties, enhanced catalysis property, excellent cycling stability and easy separation of magnetic property and unique cloth structure. The advantages of the designed CFs@Fe3O4@SiO2-C/Ni composites are as follows: (1) The high-density coating of Fe3O4 spheres not only provides carbon fibers with magnetism but also successfully improves the interfacial adhesion of carbon fibers, contributing to the further modification. (2) The magnetic CFs@Fe3O4@SiO2-C/Ni hybrids can be easily generated from PDA coated CFs@Fe3O4@NiSiO3 hybrid via a pyrolysis process in N2 without any other extra reductants. (3) It is easy to collect and recycle CFs@Fe3O4@SiO2-C/Ni composites with little loss during the application owing to its unique cloth structure as well as high magnetic response. Furthermore, the synergistic effect from both coated Fe3O4 NPs and the newly generated Ni NPs enhanced the magnetism of the carbon fibers, which contributes to the easy collection of composites with an external magnet. (4) Owing to the high loading amount of Ni NPs on CFs, the resultant CFs@Fe3O4@SiO2-C/Ni composites display improved catalytic activity on the reduction of 4-NP.

2. Experimental 2.1. Materials Carbon fibers were offered by the Carbon Fiber Lab from Donghua University. Tetraethoxysilane (TEOS, 95%) was purchased from the Alfa Aesar Chemical Company. Nickel chloride hexahydrate (NiCl2·6H2O), NH3·H2O (28-30%), ethanol and deionized water (DI water) were used for analytical grade experiments. Other chemicals were obtained from -6-

Shanghai chemical reagent company. 2.2 Synthesis of CFs@Fe3O4@SiO2 Magnetic carbon fibers (CFs@Fe3O4) was fabricated via a facile hydrothermal approach reported before [29]. Typically, pieces of carbon fibers (2cm×2cm) were put into a 50 mL Teflon-lined autoclave, which contains a homogenous solution of 25 mL ethylene glycol, 1 g FeCl3·6H2O, 0.25 g polyethylene glycol, and 0.9 g sodium acetate. Subsequently, the autoclave was sealed and heated to 200 °C for 16 h, the resultant magnetic carbon fibers (noted as CFs@Fe3O4-1) was collected, rinsed with DI water and ethanol, dried at 60 °C for 2 h. For comparison, magnetic carbon fibers with low coverage of Fe3O4 NPs was also prepared by decreasing the amount of FeCl3·6H2O (0.1 g or 0.5 g). And then, the synthesized hybrid CFs@Fe3O4 was dipped into a mixture solution of 2 mL ammonia solution, 30 mL ethanol and 3 mL H2O by ultrasonication, stirring for 5 min, followed by the addition of 0.3 mL TEOS. The reaction system was stirred for 15 h at room temperature to fabricate CFs@Fe3O4@SiO2-1. The final composites were collected, rinsed with DI water and ethanol, dried at 60 °C for 2 h. 2.3 Synthesis of CFs@Fe3O4@SiO2-C/Ni The obtained CFs@Fe3O4@SiO2-1 composites were put into a 50 mL Teflon-lined autoclave with deionized water (40 mL) solution containing NiCl2·6H2O (0.238 g), ammonia chloride (0.535 g), and ammonia solution(1 mL). And then, the autoclave was sealed and heated to 140 °C for 12 h, the prepared composites were collected, rinsed with DI water and ethanol for several times, then dried at 60 °C for 2 h to result in CFs@Fe3O4@NiSiO3-1. And -7-

then, the synthesized CFs@Fe3O4@NiSiO3-1 composites were dipped into a mixture solution containing 1 mL ammonia solution, 16 mL ethanol, and 12 mL H2O by ultrasonication for 5 min, followed by adding 50 mg dopamine, stirring for 24 h. The obtained products CFs@Fe3O4@NiSiO3@PDA-1 were collected, rinsed with DI water and ethanol, dried at 60 °C for 2 h. After annealing in N2 at 500 °C for 5 h, CFs@Fe3O4@SiO2-C/Ni-1 composites were fabricated (also denoted as C-Ni/500). Meanwhile, CFs@Fe3O4@NiSiO3-1 composites were also carbonized at 700 °C, and 900 °C to obtain hybrids C-Ni/700 and C-Ni/900, respectively. Finally, all the annealed products were used to the reduction of 4-nitrophenol.

2.4 Catalyzed reduction of 4-nitrophenol The catalysis of 4-nitrophenol (4-NP) was selected as a typical reduction model to access the catalytic performance of CFs@Fe3O4@SiO2-C/Ni-1. Typically, mixing 4-NP (0.1 mmol, 5 mL) and NaBH4 (1 mg) in a tube, and then 5 mg C/Ni-500 (C/Ni-400 or C/Ni-700) was mixed with the system to induce the reduction reaction. Subsequently, the concentration of 4-NP was recorded by UV-vis absorption in the range of 250-500 nm.

3. Instrumentation The morphology and structure of the materials were investigated by scanning electron microscopy (SEM, JEOL-4800) and transmission electron microscopy (TEM, JEOL-1011). X-ray diffraction (XRD) was used to determine the crystal structure of the materials. X-ray photoelectron (XPS) spectra were tested on a Thermo ESCALAB250 X-ray photoelectron spectrometer. And the solution concentration was recorded by UV-2450 spectrophotometer -8-

(Shimadzu, Japan).

4. Results and discussion 4.1 Fabrication and characterization of CFs@Fe3O4@SiO2-C/Ni

Scheme

1.

The

Schematic

illustrating

the

synthesis

procedure

and

catalytic

test

of

CFs@Fe3O4@SiO2-C/Ni.

In Scheme 1, the synthesis procedure of CFs@Fe3O4@SiO2-C/Ni composites is -9-

illustrated. The one-pot reaction does not employ pre-oxidation to grow functional groups before modification, which is used to maintain the tensile strength and morphology of the carbon fibers. This functionalization opens up new opportunities as a precursor reaction for further grafting reactions without sacrificing fiber strength. Moreover, the application of a magnetic field also provides a magnetic force to promote the separation of CFs from the reaction system. We believe that the Fe3O4 NPs functionalized with 1D carbon fibers is an ideal substrate for the in situ generation of nickel NPs due to its functional component of hydrophilic Fe3O4 NPs. As a proof of concept, we modify Fe3O4 NPs on 1D CFs by a polyol-assisted hydrothermal method to offer nucleation sites for the in-situ generations of a Fe3O4 NPs on the CFs. The Fe3O4 NPs generated onto 1D carbon fibers as a new magnetic material “armor” can grow extra roughness, enhance the specific surface area, contributing to high-performance hybrids via simple surface/interface manipulation. The bare carbon fibers with an average diameter of about 9.5 m (Fig. S1a, Table S1) show smooth surfaces. While CFs@Fe3O4 composites with high-density Fe3O4 NPs display a diameter of about 11 m and rather rough surfaces (Fig. 1a, b, Table S1) due to the success coverage of magnetic Fe3O4 NPs. As presented in Fig. 1a and b, the Fe3O4 NPs prepared by the polyol assisted hydrothermal method are uniformly dispersed onto the CFs and the average size of the particle lies in the range of 100-200 nm. The high coverage of loading Fe3O4 NPs reveals that there is a strong interaction between hydrophobic CFs and hydrophilic Fe3O4 NPs nanoparticle, which may be ascribed to the high surface energy of carbon nanofibers. The crystalline structure of CFs@Fe3O4 was further characterized through XRD and several diffraction peaks displayed in the CFs@Fe3O4 hybrids (Fig. 2 Aa), which can be assigned to the face-centered cubic (fcc) Fe3O4 phase (JCPDS 19-0629). While as for the naked CFs, a strong absorption band at about 25 ° ascribed to the C (002) plane from the carbon fibers was

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observed (Fig. S2). To investigate the influence of the loaded Fe3O4 NPs on the interfacial adhesion, structure, and morphology of CFs composite, the amount of FeCl3·6H2O was adjusted to 0.1 g, 0.5 g, and 1.5 g, respectively while keeping the other parameters fixed. When decreasing the amount of FeCl3·6H2O to 0.1 g, low coverage of Fe3O4 NPs was immobilized on the surface of CFs (Fig. S3a, b). As the amount of FeCl3·6H2O was increased to 0.5 g, 1.5 g, the obtained CFs@Fe3O4 held higher density of Fe3O4 NPs compared to that of [email protected] (Fig. S4 and S5). It is worth noting that there are no obvious differences in covering density of Fe3O4 between

[email protected]

and

[email protected]

(Fig.

S5).

Hence,

combining

the

above-mentioned discussion, magnetic carbon fibers based on 1 g FeCl3·6H2O was chosen to prepare the samples for catalysis unless otherwise specified. Notably, the loading of Fe3O4 NPs on CFs greatly improved the wettability of the CFs@Fe3O4 in water. Contact angle (CA) measurement (Fig. S6) showed that the sample of CFs@Fe3O4 displayed superhydrophilicity with a CA close to 0°, while the original CFs showed hydrophobicity with a CA of about 180°. The water droplet was promptly absorbed by the hydrophilic CFs@Fe3O4 when touching the magnetic fiber surface during the contact angle measurement, demonstrating the superhydrophilic property of the carbon fiber composites. Notably, the structural integrity of the magnetic CFs composite can be well maintained, which will exhibit excellent performance in various application such as energy storage, catalysis, and adsorption, etc. Despite these potential merits, to apply magnetic CFs in further applications, more efforts should be done to functionalize magnetic CFs surface with multi-components to improve compatibility, stability, and functionality. To realize this target, a silica layer was coated onto the CFs@Fe3O4 via a stöber approach to obtain CFs@Fe3O4@SiO2 composites with an average diameter of about 11.3 m (Table - 11 -

S1) [30]. As illustrated in Fig. 1c, the SiO2 layer was coated on the surface of CFs@Fe3O4, forming a core-shell heterostructure of CFs@Fe3O4@SiO2. As a comparison, without modification of hydrophilic Fe3O4 particles, the SiO2 layer cannot be deposited on the surface of CFs [31] (Fig. S1b). This further proves the dominant role of anchoring Fe3O4 particles. Compared to the one for CF@Fe3O4, a similar XRD pattern was observed for CFs@Fe3O4@SiO2 composites (Fig. 2 Ab), implying that the crystalline structure of the material was not affected by the coating of a thin silica layer. To convert the SiO2 layer into nickel silicate, a hydrothermal reaction with nickel ions in alkaline solution was needed. In this hydrothermal process, the silica layer was dissolved to form silicate anions in NH3-NH4Cl buffer and reacted with the Ni2+ cations to obtain nickel silicate. The fabricated nickel silicate preferentially deposited on the surface of CFs@Fe3O4 composites. Owing to the formation of hierarchical nickel silicate nanostructures, the average diameter of CFs@Fe3O4@NiSiO3 is increased to about 14 m (Table S1). It could be found that the surface of CFs@Fe3O4@NiSiO3 became rougher and the decorated hierarchical Fe3O4@NiSiO3 are connected (Fig. 1e). In this condition, the CFs@Fe3O4@SiO2 not only acts as a skeleton but also be self-template to generate the hierarchical Fe3O4@NiSiO3 attached on CFs. This unique structure provides a large surface area as well as rich nickel ions content. While the XRD pattern of hybrid CFs@Fe3O4@NiSiO3 (Fig. 2 Ac) displays several new diffraction peaks, which can be assigned to Ni3Si2O5(OH)4 (JCPDS card no. 49-1859) [28]. Also, the separation efficiency of carbon fibers can be effectively improved with the magnetic property of Ni, but the low magnetic response of Ni NPs greatly limit their real application. Thus, based on the above observation, developing a mild strategy of fabricating the Ni NPs with high coverage as well as keeping a good magnetic response is highly needed for practical application. Subsequently, a thin shell of PDA was finely covered on the surface - 12 -

of CFs@Fe3O4@NiSiO3 via the modified stöber method [32]. As shown in Fig. 1h, hybrid CFs@Fe3O4@NiSiO3@PDA (average diameter: 14.2 m) (Table S1) still maintains good hierarchical nanostructures, which reveals that the thickness of coated PDA is thin. It can be seen that the further modification of PDA film on CFs@Fe3O4@NiSiO3 does not affect the XRD pattern due to the amorphous of PDA in CFs@Fe3O4@NiSiO3@PDA product (Fig. 2 Ad). It is well demonstrated that the carbonized PDA (or other polymers) can reduce transition metal ions with a reduction potential of −0.27 volts or higher in situ (Ag+, Co2+, Ni2+) to the corresponding metal (Zn, Co, Ni) by carbonization treatment in an inert atmosphere [33-35]. Followed by a pyrolysis process at 500 °C under a nitrogen atmosphere, high coverage of well-dispersed Ni NPs anchored on CFs@Fe3O4@SiO2-C/Ni hybrids (average diameter: 12 m) (Table S1) were successfully synthesized (Fig. 3 B and inset and Fig. 4 b, c) [36-38]. The mapping result (Fig. 3 C-F) combined with the related the energy dispersive spectroscopy (EDS) (Fig. S7) indicates the uniform distribution of C, O, Si, Fe, Ni elements over the entire structure. Also, the related ratios of weight and atom are displayed in Table S2. As shown in Fig. 2 Ae, three diffractions at 2θ=44.2°, 54.3°, and 74.6° are observed in the XRD pattern for the CFs@Fe3O4@SiO2-C/Ni composites, demonstrating the generation of

metallic

nickel

(JCPDS

No.45

04–0850).

Furthermore,

the

structure

of

CFs@Fe3O4@SiO2-C/Ni is further confirmed by Raman spectroscopy, which is a nondestructive and powerful tool to explore carbonaceous materials. As illustrated in Fig. 2 B, the CFs@Fe3O4@SiO2-C/Ni composites exhibit two characteristic peaks at 1355 and 1585 cm−1 ascribed to the D band and G band, respectively. Hence, the intensity ratio between G and D band (IG/ID) is extensively applied to evaluate graphitic degree or the density of defects of carbon materials [39-42]. For

CFs@Fe3O4@SiO2-C/Ni hybrid, its value of ID/IG

is estimated to be 0.88, demonstrating a good graphitic carbon layer formed when annealed at

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500 °C.

Fig. 1. SEM images of CFs@Fe3O4-1 (a, b), CFs@Fe3O4@SiO2-1 (c, d), CFs@Fe3O4@NiSiO3-1 (e, f), and

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CFs@Fe3O4@NiSiO3@PDA-1 (g, h).

Fig. 2. (A) XRD pattern of CFs@Fe3O4 (a), CFs@Fe3O4@SiO2 (b), CFs@Fe3O4@NiSiO3 (c), CFs@Fe3O4@NiSiO3@PDA

(d),

and

CFs@Fe3O4@SiO2-C/Ni

(e);

(B)

Raman

spectra

of

CFs@Fe3O4@SiO2-C/Ni.

Fig. 3. SEM (A) and TEM (B) images CFs@Fe3O4@SiO2-C/Ni-1, element mappings of CFs@Fe3O4@SiO2-C/Ni-1 (C-F).

Apart from the polyol-assisted hydrothermal method, other synthetic strategies including precipitation method [43] and thermal decomposition of Fe(acac)3 [44] have also been - 15 -

explored

to

fabricate

magnetic

NPs

decorated

CFs.

For

comparison,

CFs@Fe3O4@SiO2-C/Ni-2 composites based on CF@Fe3O4-2 which are prepared by thermal decomposition were also fabricated and the related SEM images are revealed in Fig. 4c, d and Fig. S8. It can be seen that the average size of the loaded Fe3O4 spheres for CF@Fe3O4-1 are much bigger (~185 nm) than the one for CFs@Fe3O4-2 (~8 nm), which may account for the reason why the magnetism for CFs@Fe3O4-1 is stronger than that of the CFs@Fe3O4-2. As shown in Fig. 4e, f, a piece of CFs@Fe3O4-1 membrane can be rapidly (about 30 s) attached to the magnet while it takes about 80 s for hybrids CFs@Fe3O4-2. Additionally, a piece of magnetic carbon fiber based on 0.1 g FeCl3·6H2O takes about 110 s to be attached to one magnet (Fig. S9), indicating a good magnetism can be achieved via a hydrothermal method even though the coverage of Fe3O4 is low. Moreover, the morphologies of the fabricated products CFs@Fe3O4@NiSiO3@PDA-2 and CFs@Fe3O4@SiO2-C/Ni-2 are not as good as those for the composites based on hydrothermal condition (Fig. 1 and Fig. S8). Thus, CFs@Fe3O4-1hybrids from hydrothermal method were chosen to prepare the following samples for catalysis unless otherwise specified.

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Fig. 4. SEM images of CFs@Fe3O4-1 (a, b) and CFs@Fe3O4-2 (c, d); Photographs for illustrating CFs@Fe3O4-1 (e) and CFs@Fe3O4-2 (f) attached to one magnet.

Furthermore, the effect of the pyrolysis temperature on the structure of the composites is evaluated and the related SEM and TEM images are displayed in Fig. 5. Annealing hybrid CFs@Fe3O4@NiSiO3@PDA at different temperatures (500 °C, 700 °C, and 900 °C) to obtain C-Ni/500, C-Ni/700, and C-Ni/900, respectively. As revealed in Fig. 5, numerous Ni NPs well immobilized on the surface of carbon fiber composites which possess fine hierarchical structure. For the hybrid pyrolyzed at 500 °C, high-loading of nickel NPs with a small average size (~10 nm) well dispersed on magnetic carbon fibers (Fig. 5b, c). Increasing the - 17 -

annealing temperature to 700 °C, the average diameter of the immobilized metallic nickel NPs increased to 23 nm while their density maintains almost the same as the one for sample C-Ni/500 (Fig. 5e, f). The size of Ni NPs sharply increased (~56 nm) and serious sintering occurred when the carbonization temperature further increased to 900 °C (Fig. 5i, j). The phenomenon mentioned above suggests that along with the increase of thermal temperature, obvious aggregation of Ni NPs formed, leading to the formation of a larger size of Ni NPs, which matches well with the XRD pattern (Fig. S10). The intensities of the characteristic peaks of metallic Ni become stronger as the increase of the pyrolysis temperature, demonstrating nickel nanoparticles with better crystallinity and larger size generated. Additionally, the average size of nickel particles and the size of the crystal particle (estimated by the Scherrer’s equation) of different composites resulted from various temperatures are shown in Table S3. The average size of nickel nanoparticles dramatically increased from 10 to 56 nm in the range of 500-900 °C and the whole trend of the size of the crystal particle is following our experimental results. Moreover, it is worth noting that a clear peak appeared at around 24.5° ascribing to graphitic carbon for all hybrids and its intensity became much stronger when the temperature increased to 900 °C, demonstrating that high temperature is beneficial for the formation of good graphitic carbon nanostructures [45]. Also, the separation efficiency of magnetic carbon fibers can be effectively improved with the magnetic property of Ni.

- 18 -

Fig. 5. SEM and TEM images of CFs@Fe3O4@SiO2-C/Ni composites which were obtained by annealing CFs@Fe3O4@NiSiO3@PDA under an N2 atmosphere at 500 °C (a, b, c), 700 °C (d, e, f), and 900 °C (h, i, j) respectively.

More detailed information about CFs@Fe3O4@SiO2-C/Ni was tested by using X-ray photoelectron spectroscopy (XPS) and the results are displayed in Fig. 6. From the survey spectrum (Fig. 6A), the existence of elements Si, N, C, O, Fe, and Ni is confirmed, which matches well with the XRD results discussed above. The high-resolution XPS spectra of Ni 2p (Fig. 6B) exhibit two peaks at about 855.6 eV and 874.1 eV, attributed to Ni 2p3/2 and Ni 2p1/2 of the oxidized nickel state, respectively [46]. Furthermore, the spectra at the binding energy of about 853.0 eV and 873.1 eV are attributed to metallic nickel with the zero-valent state [47, 48]. Notably, it is difficult to avoid the formation of NiO, resulting from the

- 19 -

oxidation of metallic nickel in the air. However, most of the Ni species are stable as the metallic state in CFs@Fe3O4@SiO2-C/Ni composites. The reasons can be listed as follows: Firstly, the formation of nickel oxide on the surface can be a good protective layer to suppress the further oxidation of metallic Ni. Secondly, XPS only distinguishes the valence state and composition of the sample’s surface (~10 nm), indicating that the Ni (zero) is the majority of nickel species. For the XPS spectra of Si element, the peaks of Si 2s and Si 2p are presented in Fig. 6A and the binding energy of Si 2p was about 103.2 eV (Fig. 6C), higher than the 102.8 eV for silicates [49]. The above results indicated that silica was formed under an annealing condition and maintained the original hierarchical structure. As shown in Fig. 6D, the Binding Energies (BE) of 710.5 eV and 724.0 eV are not observed in Fe 2p spectrum, which is attributed to Fe2p1/2 and Fe2p3/2, respectively [50, 51]. This is due to that the Fe3O4 spheres are coated with a layer thickness of composites SiO2-C/Ni shell above 10 nm (the limit depth of XPS).

- 20 -

Fig. 6. XPS spectra of CFs@Fe3O4@SiO2-C/Ni: (A) full scan, (B) Ni2p, (C) C1s, (D) N1s.

Furthermore, to testify the versatility of this strategy, another metal silicates cobalt silicate was also fabricated (Fig. S11). With the presence of Co2+ in the hydrothermal process of fabricating silicate, the high-temperature hydrothermal reaction resulted in CoSiO3 with good hierarchical structures coating on the surface of CFs@Fe3O4 to obtain hybrid CFs@Fe3O4@CoSiO3,

followed

CFs@Fe3O4@CoSiO3@PDA

and

by

the the

CFs@Fe3O4@SiO2-C/Co.

- 21 -

modification final

of

carbonization

PDA gave

to

form sample

5. Catalytic investigation

Fig. 7. (A) Time-dependent UV-vis spectra of the reduction of 4-NP by C/Ni-400 catalyst; (B) C/C0 and ln(C/C0) vs. t for reducing 4-NP by C/Ni-400 composites; (C) Time-dependent UV-vis spectra of the reduction of 4-NP by C/Ni-500 catalyst; (D) C/C0 and ln(C/C0) vs. t for reducing 4-NP by C/Ni-500 composites; (E) Time-dependent UV-vis spectra of the reduction of 4-NP by C/Ni-700 catalyst; (F) C/C0

- 22 -

and ln(C/C0) vs. t for reducing 4-NP by C/Ni-700 composites; (H) Plot of ln(Ct/C0) vs. t for different catalysts; (I) The reusability of the catalyst.

The catalytic activities of CFs@Fe3O4@SiO2-C/Ni composites were preliminarily examined by the reduction model of 4-nitrophenol (4-NP). Because of the easy record of 4-NP and 4-AP by UV-vis absorption and without by-production generated, the reduction of 4-nitrophenol to 4-aminophenol (4-AP) is normally applied to investigate the catalytic performance of metal NPs [52]. In the absence of CFs@Fe3O4@SiO2-C/Ni catalysts, the adsorption peak at 400 nm maintained almost unchanged even for hours [53]. By contrast, after the addition of a suitable amount of catalysts, the reduction reactions were initiated immediately with a decolorization of the 4-nitrophenolate solution. Additionally, the peak at 400 nm rapidly decreased along with the newly generated adsorption peaks at 235 and 295 nm, demonstrating the generation of 4-AP [54]. As illustrated in Fig. 7A, C, and E, the reduction was finished within 4 min for C-Ni/500, 16 min for C-Ni/700, and 40 min for C-Ni/900, respectively. Considering the high concentration of NaBH4, the pseudo-first-order kinetics can be applied to assess the reduction rate. The reaction kinetics can be written as ln(Ct/C0) = -kt, where k is the rate constant, C0 and Ct are the 4-NP concentration at the beginning and at time t, respectively. As shown in Fig. 7B, D, and F, the plot of ln(Ct/C0) against t exhibited a good straight line with the slope k, and the k values for C-Ni/500, C-Ni/700, and C-Ni/900 are 18.6×10-3 s-1, 2.5×10-3 s-1, and 1.01×10-3 s-1 (Fig. 7H), respectively, suggesting that with the increase of nickel nanoparticles’ size, the catalytic - 23 -

activity and efficiency of catalysts decreased. Notably, because of the different loading amounts of nickel in various composites, activity parameter K is more reasonable to compare different catalysts, where K = k/m [55, 56]. The ICP data of different samples were displayed in Table 1, based on these data, the activity parameter K was estimated to be 49.93× 10−3, 6.10× 10−3, and 2.08 × 10−3 mg−1 s−1 for C-Ni/500, C-Ni/700, and C-Ni/900, respectively. It can be found that the catalyst C-Ni/500 exhibited outstanding catalytic performance even after 6 cycles (Fig. 7I). The excellent stability can be contributed to the in suit reaction and unique structure of the hybrids, which enables the tiny nickel nanoparticles anchor on the substrates very firmly. Moreover, the SEM (Fig. S12) images display that the morphology of the spent catalyst C-Ni/500 and the anchored Ni NPs do not change much even after six catalysis cycles when compared to the fresh catalyst. Table 1 compared the catalytic performance in our report and other previous works, C-Ni/500 catalyst displays the highest activity compared to C-Ni/700 and C-Ni/900 because of the better morphology of nickel NPs. Notably, it can be easily found that the catalytic performance of C-Ni/500 is much higher than most of the Ni-based catalysts [57-61] and lower than some of the noble catalyst [62, 63].

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Table 1. Comparison of the catalytic performance of Ni-based catalysts and noble catalysts for the reduction of 4-NP.

Samples

k (×10-3 s-1)

K (×10-3 mg-1s-1)

References

C-Ni/500

18.6

49.93

This work

C-Ni/700

2.5

6.10

This work

C-Ni/900

1.01

2.08

This work

230 nm Ni/SiO2 MHMs

4.5

1.5

57

320 nm Ni/SiO2 MHMs

3.8

1.3

57

Ni/p (AMPS)

0.9

0.15

58

Ni@SiO2

2.8

0.94

59

Ni (modified)

2.4

0.80

60

RANEY®Ni

0.32

0.11

60

RGO-Ni

0.25

0.04

61

Au-Fe3O4

10.5

27.6

62

Fe3O4@SiO2-Au@mSiO2

7

105

63

6. Conclusion Herein, magnetic Fe3O4 NPs have been grown on carbon fibers through a facile polyol assisted hydrothermal reaction to successfully invert hydrophobic carbon fibers to hydrophilic magnetic carbon fibers, which plays a dual role in both providing magnetism as well as changing water wettability of carbon fibers: Firstly, because of the enhanced interfacial adhesion attributing to the modifying of Fe3O4 NPs, active Fe3O4 NPs could be easily grown on carbon fibers. Secondly, the high-density of magnetic spheres endow the CFs with a good - 25 -

magnetic response. To the best of our knowledge, this is the first report on applying Fe3O4 NPs to improve the surface property of carbon fibers via a simple polyol-assisted hydrothermal process. With the novel magnetic NPs-based functionalization approach to improving the interfacial adhesion force of CFs, a silica layer can be easily modified on magnetic CFs to prepare nickel silicate subsequently. Combining a coating of polydopamine (PDA) with following annealing process, numerous nickel NPs were anchored on magnetic CFs to result in final product composites CFs@Fe3O4@SiO2-C/Ni. Moreover, the loading amount and size of the Ni NPs can be effectively tuned by changing the pyrolysis temperature, leading to multifunctional magnetic hybrids with different catalytic activities. Notably, it is convenient to collect and recycle hybrids CFs@Fe3O4@SiO2-C/Ni with little loss in the application process because of their unique cloth structure as well as the enhanced magnetism derived from both the coated Fe3O4 spheres and the newly generated Ni NPs. Moreover, the magnetic CFs@Fe3O4@SiO2-C/Ni composites can be easily generated from hybrid CFs@Fe3O4@NiSiO3@PDA via a carbonization process in N2 without any other extra reductants. Furthermore, due to the high loading amount of Ni NPs on CFs, the resultant CFs@Fe3O4@SiO2-C/Ni hybrids revealed enhanced catalytic activity on the reduction of 4-NP.

Acknowledgements The authors gratefully acknowledge the financial support provided by National Natural Science Foundation of China (grant number 21305086) and Natural Science Foundation of - 26 -

Shanghai (18ZR1416400).

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Graphical abstract

Herein, we proposed a facile method to grow hydrophilic Fe3O4 NPs on non-pretreated CFs using a polyol-assisted hydrothermal method, which played a vital role in constructing CFs@Fe3O4@SiO2-C/Ni composites. Benefiting from high coverage of Ni NPs, the unique cloth structure, and magnetic property, they exhibited excellent performance in catalysis on 4-nitrophenol.

- 37 -

Highlights 1. Hydrophobic CFs can be converted to hydrophilic magnetic CFs by a simple polyol-assisted hydrothermal method. 2. A silica layer can be easily coated on magnetic CFs owing to the hydrophilic property of the magnetic CFs. 3. CFs@Fe3O4@SiO2-C/Ni hybrids are easy to collect and recycle due to their unique structure and good magnetic property.

- 38 -

Author contributions Lei Ding: Data curation, Resources, Formal analysis, Writing. Min Zhang: Conceptualization, Funding acquisition, Investigation, Methodology, Supervision, Validation, Writing-original draft, Writing-review & editing. Jing Zheng, Jingli Xu: Investigation. Libin Liu: Data curation and Project administration. Hamed Alsulami, Marwan Amin Kutbi: Writing-review & editing.

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