Facile fabrication of a multifunctional aramid nanofiber-based composite paper

Abstract

Nowadays, aramid nanofibers (ANFs), split from macroscopic Kevlar yarns in dimethyl sulfoxide (DMSO) and potassium hydroxide (KOH), can be used as versatile building blocks for macroscopic materials. Herein, novel Ag nanoparticles (NPs)/ANFs composite papers were facilely fabricated with a simple solution-blending and vacuum-filtration assembly. By adeptly exploiting the in situ reduction of dimethyl sulfoxide (DMSO), the Ag NPs with mean size of ∼10.2 nm can be well dispersed onto the surfaces of ANFs with electrostatic attraction between Ag⁺ ions and amide anions. The as-prepared Ag/ANFs composite papers exhibited flexibility and good mechanical and conductive properties. Moreover, the Ag/ANFs composite papers can act as high-performance catalysts and excellent surface enhanced Raman spectroscopy (SERS) substrates. When the feed weight ratios of ANFs and AgNO₃ achieved 1 : 10, the Ag/ANFs-1 : 10 composite papers showed outstanding catalytic performance for the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) in the presence of NaBH₄ with a high first-order rate constant of 0.33 min⁻¹. In addition, the Ag/ANFs-1 : 5 composite paper exhibited highly active and sensitive SERS enhancement for Rhodamine 6G (R6G) molecules at a detection limit of 10⁻¹² M.

---

1 Introduction

Increasingly, various nanosized materials such as carbon nanotubes,¹ cellulose,² metal nanowires,³ graphene⁴ and graphene-like materials⁵ have aroused great interest to serve as multi-functional building blocks for constructing macroscopic materials. Due to their unique structure and excellent performance, the assembled macroscopic materials are widely used in lithium-ion batteries, supercapacitors, water purification, catalysis, Raman enhancement and electron conduction.⁶

As is well-known, Kevlar (poly(paraphenylene terephthalamide)) fibers are extremely strong and stiff, with a tensile strength of ∼3.6 GPa and a modulus of ∼90 GPa.^7,8 The macroscopic status of Kevlar fibers restricts its potential applications in composite materials.⁹ Inspiringly, Kotov's group first reported that bulk macroscale fibers of Kevlar threads or fabrics can be split and formed into aramid nanofibers (ANFs) in dimethylsulfoxide (DMSO) by controlled deprotonation with potassium hydroxide (KOH).¹⁰ As one-dimensional nanofibers, Cao et al. first demonstrated that ANFs can be used as versatile nanometer-sized building blocks and prepared macroscopic thin films by vacuum-assisted filtration.¹¹ In addition, it was found that the mechanical properties of ANFs-based films could be tuned by varying the amounts of phosphoric acid (PA) and glutaraldehyde (GA) for hydrolysis and cross-linking. Afterward, based on the formation of ANFs, the different ANFs-based composites were developed by incorporating with graphene nanosheets^12,13 and multi-walled carbon nanotubes.¹⁴

As is well-known, nanosized fibrous materials are also considered as supporting materials for metal and metal oxide nanoparticles (NPs).^15–17 When the metal and metal oxide nanoparticles were decorated onto the surfaces of supporting fibrous materials, the derived multi-functional composites could be developed to take advantage of synergistic effects. To date, plenty of research has been conducted for developments in catalysis, sensing, optoelectronics and biological related regions, e.g., Ag/cellulose fibers,^18,19 Ag NPs/silk fibers,²⁰ Pd/CNT,²¹ Au/CNT hollow fibers,²² Pd–Au NPs/TiO₂ fibers²³ and Au/polypyrrole (PPy) nanofibers.^24,25 Similarly, when the metal or metal oxide NPs were modified onto the surfaces of ANFs, the multifunctional ANFs-based composites can be fabricated for expanding the application of ANFs. In fact, the ANFs were formed by abstraction of mobile hydrogen from amide groups and substantial reduction of the strength of hydrogen bonds between the poly(paraphenylene terephthalamide) (PPTA) chains in DMSO and KOH solvent system.^10,12 On the one hand, once the hydrogen of amide groups is ionized, the arisen negative charge induced the electrostatic repulsion between PPTA chains. On the other hand, it is worth noting that DMSO is known as an effective reductant for silver.²⁶ By this point, positive metal ions can be absorbed onto the surfaces of ANFs with electrostatic attraction with amide anions and reduced into metal NPs by in situ reduction by DMSO without any ionic and non-ionic surfactants.

In this report, novel silver NPs/ANFs composite papers were facilely fabricated with simple solution-blending and in situ reduction. Through vacuum filtration, the assembled Ag/ANFs composite papers exhibited flexibility and good mechanical and conductive properties. Above all, the Ag/ANFs composite papers can act as high-performance catalysts and excellent surface enhanced Raman spectroscopy (SERS) substrates.

2. Experimental

2.1 Materials

Kevlar 49 fibers were obtained from DuPont Company. Dimethylsulfoxide (DMSO), potassium hydroxide (KOH), silver nitrate (AgNO₃), ethanol, sodium borohydride (NaBH₄), 4-nitrophenol (4-NP) and Rhodamine 6G (R6G) were purchased from Sinopharm Chemical Reagent Co., Ltd. (SCRC) and used without further purification.

2.2 Preparation of aramid nanofibers/dimethylsulfoxide (ANFs/DMSO) dispersion

The ANFs/DMSO dispersion was prepared using the method first reported by Kotov's group.¹⁰ 1.0 g of Kevlar 49 fibers and 1.5 g of KOH were added into 500 mL of DMSO. After magnetic stirring for one week at room temperature, a dark red ANFs/DMSO dispersion (2 mg mL⁻¹) was finally obtained.

2.3 Fabrication of silver nanoparticles/ANFs (Ag/ANFs) composite paper

The Ag/ANFs composite paper was fabricated by simple solution-blending and in situ reduction. In brief, 25 mL of a certain concentration of AgNO₃ aqueous solution was slowly added into 25 mL of ANFs/DMSO dispersion (2 mg mL⁻¹) under magnetic stirring. Successively, the mixture solution was constantly magnetically stirred and heated at 80 °C for 3 h. The resulting slurry solution of Ag/ANFs was vacuum-assisted filtrated through PVDF filter membrane (47 mm in diameter and 0.22 μm pore in size) followed by extensive washing with deionized water. After peeling off from the filter membrane, the as-received Ag/ANFs composite paper was dried under vacuum at 80 °C for 12 h between two pieces of compacted glass. With different feed weight ratios of n ANFs and AgNO₃, the obtained Ag/ANFs composite papers can be expressed as Ag/ANFs-n, where n represents the relative weight ratio of the ANFs to AgNO₃. The thicknesses of ANFs, Ag/ANFs-1 : 1, Ag/ANFs-1 : 5 and Ag/ANFs-1 : 10 papers were about 0.035, 0.0041, 0.043 and 0.045 mm, respectively.

2.4 Characterization and instrumentation

Transmission electron microscopy (TEM) images were obtained on a JEOL JEM-2100F. Scanning electron microscopy (SEM) was conducted in a Hitachi S4800 cold field emission scanning electron microscope. Raman scattering spectra were measured using a confocal Raman microscope (LabRAM HR, HORIBA Jobin Yvon Inc.) with an excitation wavelength of 633 nm. X-ray diffraction (XRD) characterization was carried out on a Bruker D8 advance. X-ray photoelectron spectroscopy (XPS) measurements were performed on a Thermo Scientific ESCALAB 250Xi using Al Kα (1486.6 eV) excitation. TGA analyses were carried out on a Netzsch STA409PC apparatus. The mechanical properties of Ag/ANFs composite papers were measured with an MTS CMT-4203 instrument at room temperature with a humidity of about 25% at a crosshead speed of 2 mm min⁻¹ for PMMA composite films and their initial gage lengths were 10 mm. The electric conductivities of composite papers were determined by a four-probe method (RTS-8, Guangzhou 4probes Tech.) equipped with parallel probes (probe spacing: 1 mm).

2.4.1 Catalyzed reduction of 4-nitrophenol (4-NP). For the catalytic reduction of 4-NP, fresh aqueous solution of 4-NP (0.3 mM, 2.4 mL) were mixed with NaBH₄ (0.1 mM, 0.6 mL) as a reducing agent for 4-NP in the quartz cell (1.0 cm path length and 4 mL volume). The Ag/ANFs composite papers with different feed ratios were first cut into square pieces of (9 × 9 mm²) and then added into the above mixture solution of 4-NP and NaBH₄. The reaction was carried out at 25 °C at a constant pH value of 10. The process of reduction was monitored using a UV-vis spectrometer (Hitachi UV2550, Japan) in the range of 200–600 nm. The kinetic study was performed by measuring the change in intensity of the absorbance at 400 nm.

2.4.2 SERS experiment. The Ag/ANFs composite papers (9 × 9 mm²) and Rhodamine 6G (R6G) was used as SERS substrates and probe molecule, respectively. The soaking method was conducted to adsorb the R6G molecules onto the surfaces of the Ag/ANFs composite papers. The SERS substrates were respectively immersed into 20 mL of different concentrations of R6G ethanol solutions (10⁻⁶, 10⁻⁸, 10⁻¹⁰ and 10⁻¹² mol L⁻¹) for over 2 h, followed by rinsing with ethanol and drying at room temperature.

3. Results and discussion

As shown in Fig. 1a, the macroscale commercial Kevlar 49 yarns were split into aramid nanofibers and formed a dark red homogeneous dispersion by deprotonation of the amide groups (Fig. S1†). For fabrication of Ag/ANFs paper, a certain amount of AgNO₃ aqueous solution was slowly added into the ANFs/DMSO dispersion under magnetic stirring (Fig. 1b). Successively, after in situ reduction at 80 °C for 3 h, the Ag/ANFs dark brown flocculation suspension was obtained (Fig. 1c). It is generally recognized that the ANFs were formed by abstraction of mobile hydrogen from amide groups and a substantial reduction of the strength of hydrogen bonds between the PPTA chains. Once the hydrogen of the amide groups becomes ionized, the arisen negative charge induces the electrostatic repulsion between PPTA chains. From Fig. 1d, when the AgNO₃ aqueous solution was added, the silver ions can be attracted onto ANFs through the electrostatic attraction between ANFs and Ag⁺ ions. It's worth noting that DMSO is known as an effective reductant for silver.²⁶ The silver ions originating from silver nitrate can be spontaneously reduced in pure DMSO and DMSO/water solvent mixtures after reacting at 80 °C for 2 hours (Fig. S2†).²⁶ According to previous reports by Ahrland et al.,²⁷ the silver ions show a pronounced tendency to exhibit linear, 2-fold coordination. Ag(DMSO)NO₃ is first formed during the reduction process in the presence of DMSO (eqn (1)). After heating, the electrons of DMSO transferred to form a sulfoxide radical cation and accompany the reduction of silver ions (eqn (2) and (3)).²⁶ Finally, the cationic species is attacked by water and forms DMSO₂ (ref. 28) (eqn (4)) simultaneously making the solution more acidic (Fig. S3†).

AgNO₃ + (CH₃)₂SO: ⇋ Ag[(CH₃)₂SO:]NO₃ (1)

Ag[(CH₃)SO:]NO₃ → Ag⁰ + [(CH₃)SO˙]⁺ + NO₃⁻ (2)

AgNO₃ + [(CH₃)SO˙]⁺ → Ag⁰ + [(CH₃)SO]²⁺ + NO₃⁻ (3)

[(CH₃)SO]²⁺ + H₂O → DMSO₂ + 2H⁺ (4)

Fig. 1 (a) Schematic of the preparation of Ag/ANFs composites, (b) ANFs/DMSO dispersion (2 mg mL⁻¹), (c) silver nitrate aqueous solution (10 mg mL⁻¹), (d) Ag/ANFs flocculation suspension, (e) Ag/ANFs assembly composites, (f) as-prepared Ag/ANFs composite paper, (g) Ag/ANFs composite paper with a bended piece and (h) schematic diagram of the mechanism for Ag/ANFs composites formation.

Based on the reaction mechanism (eqn (5)), the Ag⁺ ions were reduced after reaction at 80 °C for 2 h. Successively, the Ag/ANFs were formed by in situ reduction in DMSO/H₂O mixed solvent with the assistance of DMSO. Ag/ANFs self-assembled composite papers were fabricated by vacuum-filtration of the Ag/ANFs slurry DMSO–water mixture dispersion followed by vacuum drying (Fig. 1e). As exhibited in Fig. 1f and g, the as-prepared Ag/ANFs composite paper possessed bright brown surfaces and exhibited good flexibility.

TEM images were first used to characterize the Ag/ANFs composite paper. As observed in Fig. 2a–c, the ANFs showed the obvious fiber morphology with widths ranging from 15 to 30 nm. This is consistent with previously reported results.²⁹ It is clearly seen that there are lots of Ag nanoparticles and they were well loaded and distributed onto the surfaces of the ANFs networks. From the inset histogram of Fig. 2a, the particle sizes of Ag nanoparticles mostly ranged from ∼4.48 to ∼22.14 nm and concentrated in ∼9.51 nm. The average sizes of Ag NPs were ∼10.2 nm. As can be seen from Fig. 2d and e, the Ag NPs are predominantly spherical in shape. The selected area electron diffraction image of a silver nanoparticle (inset in Fig. 2f) revealed that the Ag NPs had a single crystal nature with a cubic phase. In Fig. 2f, the crystal lattice of Ag NPs is continuous in the same direction with the interplanar crystal spacing of (0.24 nm) for (111) phase of Ag⁰.³⁰ Meanwhile, the HRTEM image clearly shows close contact between the Ag NPs and the ANFs surface. EDS mapping images of various elements in the Ag/ANFs composite were also obtained to examine the distribution of elements for Ag/ANFs. Homogeneous distributions of C are observed in Ag/ANFs. N, O and Ag were in compactly distributed in random positions on Ag/ANFs, which indicated that the surface of the ANFs was uniformly coated with Ag NPs in the active sites of ANFs amide groups. The content of Ag on the surface of Ag/ANFs was determined to be ∼19.2 wt% by EDS analysis, while C accounts for ∼72.7 wt%.

Fig. 2 (a)–(d) TEM images for Ag/ANFs composites with different magnification, (e) and (f) HR-TEM images for Ag/ANFs composites and (g)–(k) STEM and elemental mapping images of the Ag/ANFs composites. Inset of (a) distribution histograms of particle size for Ag NPs and inset of (f) selected area electron diffraction (SAED) pattern for an isolated section of Ag NPs.

XPS spectroscopy was further utilized to characterize the Ag/ANFs composite papers. As illustrated in Fig. 3a, the elements of C 1s, O 1s, N 1s and Ag 3d existed in the Ag/ANFs composite paper, while, C, O and N elements existed in bare ANFs, respectively. Fig. 3b shows the XPS high resolution spectra in the Ag 3d region of the Ag/ANFs composite paper. There are two peaks occurring at 369.5 and 375.6 eV that correspond with Ag 3d_5/2 and Ag 3d_3/2 binding energies, respectively.³¹ The splitting of the 3d doublet is 6.1 eV, indicating the metallic nature of Ag NPs.³² The XPS spectra for the C 1s region around 285 eV is shown in Fig. 3c. The binding energy peak at 284.6 eV was attributed to the C=C groups of PPTA chains, the peaks at 285.3 eV and 287.8 eV were assigned to C–C groups and C=O groups, respectively.³³ Moreover, the peak of C–N (originating from the amide groups) was located at 285.8 eV. Additionally, from Fig. 3d, the N 1s peak of ANFs was located at 400 eV, which originated from the O=C–NH₂ groups of nitrogen-containing aromatic PPTA polymers due to the weak π–π* satellite features.¹⁴ It is worth noting that the N 1s peak for Ag/ANFs composite paper was observed at 401.5 eV with an increase of 1.5 eV compared to the N 1s peak of pure ANFs. This may be due to the fact that the O=C–NH₂ acted as nucleation sites for Ag NPs on ANFs.³⁴

Fig. 3 XPS spectra of Ag/ANFs composite paper, (a) full survey scan spectrum, (b) N 1s, (c) Ag 3d and (d) C 1s peak.

Obviously, the Ag/ANFs composite can be fabricated by in situ reduction of DMSO. The XRD patterns were used for analyzing the Ag/ANFs composite papers with different feed mass ratios of ANFs and AgNO₃. In Fig. 4, pure ANFs paper showed the characteristic peaks of the 110 and 200 planes of Kevlar with 2θ values of 21.15° and 23.42°, respectively.^35,36 After incorporating with Ag NPs, the diffraction peaks of the Ag/ANFs composite papers with different feed mass ratios of AgNO₃ and ANF for 2θ values of 37.8, 44.1, 64.2 and 77.2° corresponded to the (111), (200), (220) and (311) crystal faces of the face-centered cubic (fcc) crystalline silver,³⁷ which was consistent with the values in the standard card (JCPDS 36-1451).

Fig. 4 XRD patterns of ANFs and Ag/ANFs papers with different feed weight ratios.

The surface morphologies of Ag/ANFs composite papers with different feed weight ratios of AgNO₃ and ANFs were characterized by SEM. In fact, ANFs exhibit high reactivity and can act as high-performance polymeric building blocks with π–π conjugation and strong hydrogen bonding between the polymer chains of ANFs. As shown in Fig. 5a, the surface of ANFs paper is smooth with distinct fibrous stripes. Toward to the Ag/ANFs composite papers, the surfaces become obviously rough and grainy, corresponding to the coating of Ag NPs onto the surfaces of ANFs. Increasing with the amount of AgNO₃, the grainy surfaces are more and more rough. For Ag/ANFs-1 : 1 paper, the Ag NPs are relatively uniform and loosely distributed on the surface together with the paper wrinkles. But for Ag/ANFs-1 : 5 and Ag/ANFs-1 : 10 papers, the wrinkles almost entirely disappeared and were replaced by coated Ag NPs. Nevertheless, the aggregates of Ag NPs began to turn up in large areas for Ag/ANFs-1 : 10 paper.

Fig. 5 SEM images for the surfaces of Ag/ANFs papers with different feed weight ratios: (a) ANFs, (b) Ag/ANFs-1 : 1, (c) Ag/ANFs-1 : 5 and (d) Ag/ANFs-1 : 10.

As building blocks, the ANFs can form continuous papers via vacuum-assisted filtration with strong hydrogen bonding and π–π conjugation between the ANFs. From Fig. 6a and b, the ultimate tensile strength of pure ANFs paper can achieve ∼139.8 MPa. However, the ultimate tensile strengths for Ag/ANFs composite papers obviously decreased. For Ag/ANFs-1 : 1, the ultimate tensile strength drops down to ∼129.1 MPa. Increasing the feed amount ratios of ANFs and AgNO₃, the mechanical properties for Ag/ANFs composite papers gradually decreases. When the feed ratio equals to 1 : 10, the ultimate tensile strength of Ag/ANFs-1 : 10 decreased to ∼93.9 MPa with ∼33% of reduction compared to pure ANFs paper. Similarly, the Young's modulus for Ag/ANFs composite papers with different feed ratios exhibited the same trending as the ultimate tensile strengths. The Young's modulus reduced from ∼3.82 GPa for pure ANFs paper to ∼3.18 GPa for Ag/ANFs-1 : 10 composite papers. It is clear that the mechanical properties of ANFs-based papers decreased after incorporating Ag NPs. This may be attributed to the decorated Ag NPs on the surfaces of ANFs loosening the compact structures of ANFs blocks. Nevertheless, with the introduction of Ag NPs, the electric conductivities of Ag/ANFs composite papers were significantly improved. As is well-known, Kevlar is a kind of high-insulating polymer. For pure ANFs paper, the volume resistivity was about 0.011 kΩ cm. In Fig. 6d, the electric conductivities of Ag/ANFs-1 : 1, Ag/ANFs-1 : 5 and Ag/ANFs-1 : 10 composite papers were ∼0.46, ∼0.53 and ∼0.62 S cm⁻¹, corresponding to ∼2.17, ∼1.89 and ∼1.62 Ω cm in volume resistivity, respectively.

Fig. 6 (a) Typical strain–stress curves for ANFs paper and Ag/ANFs composite papers with different feed weight ratios, (b) tensile strengths, (c) Young's modulus and (d) electric conductivities for ANFs and Ag/ANFs composite papers with different feed weight ratios.

SEM images for tensile fracture surfaces were further used to characterize the Ag/ANFs composite papers (Fig. 7). As observed in Fig. 7a and b, the tensile fracture surface of pure ANFs paper exhibited a distinct layered structure with interconnected fibrous networks. Compared to the pure ANFs paper, due to the intercalated Ag NPs, the layered structure with interconnected fibrous networks became indistinct for the Ag/ANFs composite papers. Obviously, there was delamination in the tensile fracture surface of Ag/ANFs-1 : 10 composite papers (Fig. 7g and h). As a matter of fact, the interchain bonds, especially the hydrogen bond between long molecular chains of PPTA, make the Kevlar extremely strong and stiff.³⁸ By abstraction of the mobile hydrogen from –NH groups and the substantial reduction of the strength of hydrogen bonds, the ANFs can be formed in DMSO in the presence of KOH.³⁹ For ANFs-assembled paper, the reconstruction of hydrogen bonding makes the ANFs paper have good mechanical properties with a relatively compact layered structure. In preparing of the Ag/ANFs composite paper, the –NH groups acted as active sites for adsorption of Ag ions. After reduction, the Ag NPs were formed and fixed onto the surfaces of ANFs. As a result, the compact structure of ANFs paper was evidently loosened, and reduced the mechanical performances. In addition, due to the excessive feed amounts of AgNO₃, there were evident agglomerates in the layered structure of Ag/ANFs composite paper, as indicated by red ellipses in Fig. 7h. Moreover, the agglomerations also caused the mechanical performances of Ag/ANFs composite paper to fall down.

Fig. 7 SEM images of tensile fracture surfaces for ANFs and Ag/ANFs composite papers with different feed weight ratios: (a and b) ANFs paper, (c and d) Ag/ANFs-1 : 1, (e and f) Ag/ANFs-1 : 5 paper and (g and h) Ag/ANFs-1 : 10.

For quantitative analyzing the Ag/ANFs composite paper, TGA curves for ANFs-based composite papers are illustrated in Fig. 8. The main weight loss of ANFs-based composite papers appeared ∼550 °C due to the thermal decomposition behavior of the polymer skeleton. Considering the Ag NPs had almost no weight loss under a nitrogen atmosphere, the percentage of loaded Ag NPs in Ag/ANFs composite papers with different feed ratios were calculated. By judging from the weight loss around 700 °C, the loaded weight amounts of Ag NPs accounted for ∼41.6, ∼51.7 and ∼57.5 wt% for Ag/ANFs-1 : 1, Ag/ANFs-1 : 5 and Ag/ANFs-1 : 10 composite papers, respectively.

Fig. 8 TGA curves for ANFs paper and Ag/ANFs composite papers with different feed weight ratios in nitrogen atmosphere.

As an important industrial intermediate, 4-aminophenol (4-AP) is widely used for drug, lubricants and hair-care products. Efficient and durable Ag-based catalytic system have been always a hot point for reduction of 4-nitrophenol (4-NP) to produce 4-AP. Therefore, the reduction of 4-NP to 4-AP by NaBH₄ in the aqueous phase was chosen as a model reaction to evaluate the catalytic activity of the Ag/ANFs composite papers. This reaction is known to be catalytically accelerated in the presence of noble metal NPs by facilitating electron relay from the donor (BH₄⁻) to the acceptor (4-NP) to overcome the kinetic barrier.⁴⁰ The conversion from 4-NP to 4-AP occurs via an intermediate 4-nitrophenolate ion formation. Thus, the progress of the reaction can be monitored by tracking the changes in the absorption spectra of the 4-nitrophenolate ion at 400 nm due to the formation of 4-nitrophenolate ion in the alkaline medium caused by NaBH₄.⁴¹ As shown in Fig. 9a, the absorption peak of 4-nitrophenolate ions for the mixture of 4-NP and NaBH₄ appeared at 400 nm. For understanding the catalytic performance, the composite papers were added into the mixture of 4-NP and NaBH₄. Obviously, towards to Ag/ANFs-1 : 1 composite papers, there is a rapid decrease in the intensity of the absorption peak at 400 nm in several minutes. Meanwhile, a new peak at ∼297 nm concomitantly appears. This indicated the reduction of 4-NP and the formation of 4-AP with the assistance of the Ag/ANFs composite paper. Afterward, the catalytic activity of Ag/ANFs composite papers with different feed weight ratios of ANFs and AgNO₃ were also studied. Since the concentration of NaBH₄ greatly exceeds that of 4-NP, the reduction can be assumed to be a pseudo first-order reaction based on the evaluation of the rate constant with regard to 4-NP only. Therefore, the reaction kinetics can be described as:

ln(C/C₀) = −kt,

where k is the apparent first-order rate constant (min⁻¹) and t is the reaction time.⁴² Here, C stands for the absorbance at any time t and C₀ for absorbance at time t₀. Fig. 9b shows the linear relationships between ln(C/C₀) and reaction time (t) in the reduction catalyzed by the Ag/ANFs composite papers with different feed weight ratios. The rate constants (k) can be obtained from the slopes of the linear sections of the plots.

Fig. 9 (a) Time-dependent UV-vis spectra for the catalytic reduction of 4-NP by NaBH₄ in the presence of Ag/ANFs-1 : 1 composite paper, (b) plots of ln(C_t/C₀) vs. time for the reduction of 4-NP by NaBH₄ for Ag/ANFs composite papers with different feed weight ratios and (c) the change of apparent first-order rate constant for Ag/ANFs-1 : 10 composite paper after recyclability tests.

The rate constants of Ag/ANFs-1 : 1, Ag/ANFs-1 : 5 and Ag/ANFs-1 : 10 composite papers were 0.087, 0.21 and 0.33 min⁻¹, respectively. Compared to the rate constant of ∼0.0128 min⁻¹ for only NaBH₄ and 4-NP, the Ag/ANFs composites significantly enhanced the catalytic rate (Fig. S4†). The rate constants were observed to increase with increasing feed weight ratios. The increasing catalytic rates could be attributed to the loading amounts of Ag NPs. The Ag NPs were formed by in situ reduction with DMSO without any ionic and non-ionic surfactants; as a result, the surfaces of Ag NPs can be totally bared to BH₄⁻ in the mixture of 4-NP and NaBH₄ for enhancing the catalytic activity. In addition, the Ag/ANFs composite papers also show a certain adsorption ability for 4-NP (Fig. S6†) due to the π–π stacking interactions with π-rich nature of ANFs. The adsorption of 4-NP can provide a high-concentration of 4-NP near the Ag NPs of Ag/ANFs composites, leading to an improvement of catalytic performance.^43,44 Because of the good flexibility and mechanical properties, the Ag/ANFs composite papers also exhibited stability and recyclability. From Fig. 9c, there is almost no change in the rate constants of Ag/ANFs-1 : 10 paper after six experiments under the same conditions.

SERS enables the detection of substances at low concentrations using silver or gold nanostructures.⁴⁵ The SERS technique has many applications such as environmental detection and biosensing. SERS is primarily due to highly concentrated electromagnetic (EM) fields.⁴⁶ The hot spots of EM are often associated with interstitial sites in nanostructures, consisting of two or more coupled nanoparticles, or otherwise nanostructured surfaces with closely spaced features.⁴⁷ On this basis, the Ag/ANFs composite papers with Ag NPs distributed on the surfaces of ANFs also can be used as SERS substrates. For evaluating the SERS activity of the Ag/ANFs composite paper, Rhodamine 6G (R6G) was used as the Raman probe molecule.

Fig. 10a shows the Raman spectra for 10⁻⁴ M of R6G adsorbed on the Ag/ANFs composite papers with different feed ratios. For the pure ANFs paper, there are no Raman features for R6G. Once Ag NPs were loaded in the ANFs paper, the characteristic bands began to appear. The Raman bands of 1183, 778 and 618 cm⁻¹ are attributed to the C–C stretching vibrations, out-of-plane bending motion of the hydrogen atoms of the xanthene skeleton and C–C–C ring in-plane bending, respectively, while 1508 and 1365 cm⁻¹ can be assigned to the aromatic C–C stretching vibrations of R6G molecules.^48,49 Clearly, the Ag/ANFs-1 : 5 composite paper displayed the best enhancement effect with the maximum SERS intensity. It is well-known that SERS is primarily due to highly concentrated electromagnetic (EM) fields. The hot spots of EM are often associated with interstitial sites in nanostructures, consisting of two or more coupled nanoparticles, or otherwise nanostructured surfaces with closely spaced features. The EM enhancement arises from the coupling of surface plasmons of Ag NPs constituents in the Ag/ANFs substrates. Consequently, the relatively better uniform and continuous distribution of Ag NPs in the Ag/ANFs-1 : 5 composite paper created higher local EM fields compared to the Ag/ANFs-1 : 1 and 1 : 10 papers.

Fig. 10 (a) SERS spectra of R6G (10⁻⁶ M) for ANFs paper and Ag/ANFs composite papers with different feed weight ratios. (b) SERS spectra of Ag/ANFs-1 : 5 composite paper under different concentration of R6G.

Next, the SERS spectra of Ag/ANFs-1 : 5 composite paper for different concentrations ranging from 10⁻⁸ down to 10⁻¹² M are shown in Fig. 10b. It is clear that the nature of the Raman profile for R6G is the same for all concentrations, even as low as 10⁻¹² M. From the inset of Fig. 10b, the normalized intensity bar profile for 618 and 1365 cm⁻¹ with different concentrations of R6G ethanol solution, the Ag/ANFs composite papers can acted as excellent substrates to detect R6G at a detection limit of 10⁻¹² M.

4. Conclusion

In conclusion, we reported a facile and novel approach for preparing Ag/ANFs composite paper through simple solution-blending and vacuum-filtration assembly. By adding the AgNO₃ aqueous solution into the ANFs/DMSO dispersion, Ag⁺ ions were first absorbed onto the surfaces of ANFs with electrostatic attraction, and then simultaneously reduced and coated onto the surfaces of ANFs in the presence of DMSO. The results demonstrated that the Ag NPs, with average sizes of ∼10.2 nm, were highly dispersed onto the surfaces of ANFs. The Ag/ANFs composite papers not only exhibited high mechanical properties, but also had good conductivities. As the feed amounts of AgNO₃ solution increases, the loading amount of Ag NPs also increases in the Ag/ANFs composite paper. The Ag/ANFs composite paper, with highly dispersed Ag NPs on the ANFs, showed high catalytic performance for the reduction of 4-NP to 4-AP in the presence of NaBH₄ and outstanding active/sensitive SERS responses towards R6G molecules, down to a concentration of 10⁻¹² M. The facile method for fabrication of Ag/ANFs composite paper can be extended to more preparations and the assembly of other composite materials.

Acknowledgements

This research was supported by Leading talent program of Shanghai, Sailing program of Shanghai science and technology commission (15YF1404700), Startup Fund for New Talent, Shanghai University of Electric Power (K-2014-046), National Nature Science Foundation of China (No. 21604051 and ​ 21671133) and Science and Technology Commission of Shanghai Municipality (No. 14JC1402500 and ​ 14DZ2261000). Additionally, the study was also sponsored by “Chenguang Program” supported by Shanghai Education Development Foundation and Shanghai Municipal Education Commission (14CG55).

References

1. M. L. Geier, J. J. McMorrow, W. Xu, J. Zhu, C. H. Kim, T. J. Marks and M. C. Hersam, Nat. Nanotechnol., 2015, 10, 944–948.
2. S. Saini, M. N. Belgacem, M.-C. B. Salon and J. Bras, Cellulose, 2016, 23, 795–810.
3. P. Lee, J. Lee, H. Lee, J. Yeo, S. Hong, K. H. Nam, D. Lee, S. S. Lee and S. H. Ko, Adv. Mater., 2012, 24, 3326–3332.
4. F. Bonaccorso, L. Colombo, G. Yu, M. Stoller, V. Tozzini, A. C. Ferrari, R. S. Ruoff and V. Pellegrini, Science, 2015, 347, 1246501.
5. M. Xu, T. Liang, M. Shi and H. Chen, Chem. Rev., 2013, 113, 3766–3798.
6. K. Ariga, J. Li, J. Fei, Q. Ji and J. P. Hill, Adv. Mater., 2016, 28, 1251–1286.
7. T. Lin, S. Wu, J. Lai and S. Shyu, Compos. Sci. Technol., 2000, 60, 1873–1878.
8. Q. Kuang, D. Zhang, J. C. Yu, Y.-W. Chang, M. Yue, Y. Hou and M. Yang, J. Phys. Chem. C, 2015, 119, 27467–27477.
9. L. Zhang, S. Bai, C. Su, Y. Zheng, Y. Qin, C. Xu and Z. L. Wang, Adv. Funct. Mater., 2015, 25, 5794–5798.
10. M. Yang, K. Cao, L. Sui, Y. Qi, J. Zhu, A. Waas, E. M. Arruda, J. Kieffer, M. Thouless and N. A. Kotov, ACS Nano, 2011, 5, 6945–6954.
11. K. Cao, C. P. Siepermann, M. Yang, A. M. Waas, N. A. Kotov, M. Thouless and E. M. Arruda, Adv. Funct. Mater., 2013, 23, 2072–2080.
12. J. Fan, Z. Shi, L. Zhang, J. Wang and J. Yin, Nanoscale, 2012, 4, 7046–7055.
13. J. Fan, Z. Shi, M. Tian and J. Yin, RSC Adv., 2013, 3, 17664–17667.
14. J. Zhu, W. Cao, M. Yue, Y. Hou, J. Han and M. Yang, ACS Nano, 2015, 9, 2489–2501.
15. R. J. Tseng, J. Huang, J. Ouyang, R. B. Kaner and Y. Yang, Nano Lett., 2005, 5, 1077–1080.
16. B. J. Gallon, R. W. Kojima, R. B. Kaner and P. L. Diaconescu, Angew. Chem., Int. Ed., 2007, 46, 7251–7254.
17. S. H. Nam, H.-S. Shim, Y.-S. Kim, M. A. Dar, J. G. Kim and W. B. Kim, ACS Appl. Mater. Interfaces, 2010, 2, 2046–2052.
18. J. He, T. Kunitake and A. Nakao, Chem. Mater., 2003, 15, 4401–4406.
19. J. Song, N. L. Birbach and J. P. Hinestroza, Cellulose, 2012, 19, 411–424.
20. W. Yu, T. Kuzuya, S. Hirai, Y. Tamada, K. Sawada and T. Iwasa, Appl. Surf. Sci., 2012, 262, 212–217.
21. W. Dong, P. Chen, W. Xia, P. Weide, H. Ruland, A. Kostka, K. Köhler and M. Muhler, ChemCatChem, 2016, 8, 1269–1273.
22. A. La Torre, M. d. C. Giménez-López, M. W. Fay, G. A. Rance, W. A. Solomonsz, T. W. Chamberlain, P. D. Brown and A. N. Khlobystov, ACS Nano, 2012, 6, 2000–2007.
23. Z. Lu, N. Liu, H.-W. Lee, J. Zhao, W. Li, Y. Li and Y. Cui, ACS Nano, 2015, 9, 2540–2547.
24. T. Yao, T. Cui, H. Wang, L. Xu, F. Cui and J. Wu, Nanoscale, 2014, 6, 7666–7674.
25. C. Li, Y. Su, X. Lv, H. Xia, H. Shi, X. Yang, J. Zhang and Y. Wang, Biosens. Bioelectron., 2012, 38, 402–406.
26. G. Rodriguez-Gattorno, D. Diaz, L. Rendon and G. Hernandez-Segura, J. Phys. Chem. B, 2002, 106, 2482–2487.
27. S. Ahrland, N. O. Björk and S. Quézel, Acta Chem. Scand., Ser. A, 1974, 28, 823–828.
28. M. Lin, Z. Wang, H. Fang, L. Liu, H. Yin, C. H. Yan and X. Fu, RSC Adv., 2016, 6, 10861–10864.
29. J. Fan, J. Wang, Z. Shi, S. Yu and J. Yin, Mater. Chem. Phys., 2013, 141, 861–868.
30. P. Zhang, C. Shao, Z. Zhang, M. Zhang, J. Mu, Z. Guo, Y. Sun and Y. Liu, J. Mater. Chem., 2011, 21, 17746–17753.
31. H. Cheng, B. Huang, P. Wang, Z. Wang, Z. Lou, J. Wang, X. Qin, X. Zhang and Y. Dai, Chem. Commun., 2011, 47, 7054–7056.
32. Y. Negishi, T. Iwai and M. Ide, Chem. Commun., 2010, 46, 4713–4715.
33. C. H. Schmitz, M. Schmid, S. Gärtner, H.-P. Steinrück, J. M. Gottfried and M. Sokolowski, J. Phys. Chem. C, 2011, 115, 18186–18194.
34. M. Iijima and H. Kamiya, Colloids Surf., A, 2015, 482, 195–202.
35. B. K. Little, Y. Li, V. Cammarata, R. Broughton and G. Mills, ACS Appl. Mater. Interfaces, 2011, 3, 1965–1973.
36. C. Arrieta, E. David, P. Dolez and T. Vu-Khanh, Polym. Compos., 2011, 32, 362–367.
37. Z. Zhang, F. Xu, W. Yang, M. Guo, X. Wang, B. Zhang and J. Tang, Chem. Commun., 2011, 47, 6440–6442.
38. F. Vollrath and D. P. Knight, Nature, 2001, 410, 541–548.
39. M. Yang, K. Cao, L. Sui, Y. Qi, J. Zhu, A. Waas, E. M. Arruda, J. Kieffer, M. D. Thouless and N. A. Kotov, ACS Nano, 2011, 5, 6945–6954.
40. Z. Zhang, C. Shao, P. Zou, P. Zhang, M. Zhang, J. Mu, Z. Guo, X. Li, C. Wang and Y. Liu, Chem. Commun., 2011, 47, 3906–3908.
41. S. Wunder, F. Polzer, Y. Lu, Y. Mei and M. Ballauff, J. Phys. Chem. C, 2010, 114, 8814–8820.
42. K. Krishnamoorthy, R. Mohan and S.-J. Kim, Appl. Phys. Lett., 2011, 98, 244101.
43. G. Chang, Y. Luo, W. Lu, X. Qin, A. M. Asiri, A. O. Al-Youbi and X. Sun, Catal. Sci. Technol., 2012, 2, 800–806.
44. Y. Zhang, S. Liu, W. Lu, L. Wang, J. Tian and X. Sun, Catal. Sci. Technol., 2011, 1, 1142–1144.
45. M. J. Mulvihill, X. Y. Ling, J. Henzie and P. Yang, J. Am. Chem. Soc., 2009, 132, 268–274.
46. Q. Min, M. J. L. Santos, E. M. Girotto, A. G. Brolo and R. Gordon, J. Phys. Chem. C, 2008, 112, 15098–15101.
47. S. J. Lee, J. M. Baik and M. Moskovits, Nano Lett., 2008, 8, 3244–3247.
48. S.-K. Li, Y.-X. Yan, J.-L. Wang and S.-H. Yu, Nanoscale, 2013, 5, 12616–12623.
49. R. Lu, A. Konzelmann, F. Xu, Y. Gong, J. Liu, Q. Liu, M. Xin, R. Hui and J. Z. Wu, Carbon, 2015, 86, 78–85.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra15895a

---