One-step hydrothermal synthesis of silver nanoparticles loaded on N-doped carbon and application for catalytic reduction of 4-nitrophenol

Abstract

In this work, we report a novel and facile one-step approach for the synthesis of silver nanoparticles (Ag NPs) loaded on N-doped carbon (NC) composites. Renewable natural chitosan not only acts as a reducer and stabilizer, but also serves as nitrogen and carbon source. After hydrothermal treatment, the spherical silver Ag NPs are obtained and loaded on the N-doped carbon. The composites were characterized by means of transmission electronic microscopy, scanning electronic microscopy, energy dispersive X-ray spectrum, X-ray diffraction and X-ray photoelectron spectroscopy, respectively. The catalytic activity of Ag-NC composites were measured for the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) with an excess amount of NaBH₄. The results showed that the composites displayed superior catalytic activity.

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Introduction

4-Nitrophenol (4-NP) is one of the most common organic pollutants, which widely exists in industrial and agricultural wastewaters.¹ To remove 4-NP, there are many methods of choice, such as adsorption, photocatalytic degradation, ozonation, biological, electrochemical treatment, and catalytic reduction.^1–3 Among these methods, the catalytic reduction of 4-nitrophenol to 4-aminophenol (4-AP) by borohydride in the present of noble metals has been considered an alternative effective and eco-friendly route. What’s more, the product of the catalytic reduction, 4-aminophenol (4-AP), is widely used in analgesic and antipyretic drugs, photographic developer, hair-dyeing agent, corrosion inhibitor, and so on.⁴

The noble metallic nanoparticles (Ag, Au, Pd, etc.), which are used in catalytic reduction of 4-NP to 4-AP, have received increasing attention due to their high catalytic performance as heterogeneous catalysts in many liquid-phase catalytic processes.^4–6 Because of its relatively low cost, Ag has attracted particular attention. However, pure Ag NPs of small size tend to easily aggregate to minimize their surface energy, leading to a remarkable reduction in their catalytic activities.⁷ To solve the problem of aggregation and enhance the catalytic activities, noble metallic nanoparticles have been immobilized on/into various support materials, such as carbon materials,^2,7–10 inorganic clays,^11,12 polymers,^13–15 metal oxides,^16,17 silica,¹⁸ and wastes.^19,20 Among these support materials, carbon materials such as carbon nanofibers,⁷ carbon nanotubes,²¹ graphene,² have been intensively studied due to their excellent properties, such as high mechanical strength, chemical stability, sizes, ease of dispersion in liquid medium and facile formation of composites.⁷ Furthermore, many efforts have demonstrated that nitrogen-doped carbon can effectively improve electronic conductivity, charge transfer at the interface and chemically active sites, which are beneficial to increase catalytic activity, improve dispersion of the noble metallic nanoparticles and a much higher resistance to nanoparticle agglomeration and coarsening.^22–26 However, the traditional methods, such as chemical vapour annealing and wet dipping process, to prepare N-doped carbon usually require very harsh and multistep processes. In addition, the nitrogen source (ammonia, amines, urea) is less sustainable and available.²⁷ So the development of a facile synthetic method and a stable nitrogen source to prepare N-doped carbon is significant and attractive.

As we know, chitosan is a product of the partial N-deacetylation of chitin which is the second most abundant natural biopolymer in the world after cellulose. It contains hydroxyl (–OH) and amino (–NH₂) groups that can serve as coordination sites to anchor metal ions and this can directly convert N-rich carbon materials.^15,27 It also possesses many fascinating properties like being biodegradable, hydrophilic, anti-bacterial, and non-toxic. Additionally, many reports in the literature found that chitosan was a very effective reducing and stabilizing agent for the preparation of noble metallic nanoparticles.²⁸ For example, Qian et al. found that chitosan as a mediator agent in the synthesis of crystalline Ag and Au nanoparticles under quiescent conditions.²⁹ Qian et al. studied a “green” route to form metal NPs–chitosan bioconjugates, where chitosan acts both as a reductant and scaffold for nanoparticles.³⁰ Hu et al. reported chitosan as supporting matrix, reductant and stabilizer for the synthesis of magnetic Au NPs/chitosan/Fe₃O₄ composites.¹⁵

To the best of our knowledge, chitosan as the N-doped carbon precursor to prepare a Ag NPs-based catalyst in one-step has not been studied yet. Traditionally, the preparation of Ag NPs incorporated N-doped carbon materials always needed a multi-step and time-consuming process. In this work, however, Ag NPs supported on N-doped carbon composites were obtained using a one-step hydrothermal process. Additionally, strong reducing or protecting agents, used in reducing silver and resisting nanoparticle agglomeration, were not needed, and an additional nitrogen source was not needed either. Chitosan not only served as a reducer and stabilizer, but also as nitrogen and carbon source to synthesise Ag-NC using a one-step facile hydrothermal treatment. The results indicate that the spheres of silver nanoparticles were loaded on N-doped carbon and the composites exhibited excellent catalytic performance towards the reduction of 4-nitrophenol by NaBH₄ in aqueous solution.

Experimental

Materials

Chitosan flakes (practical grade, >90.0% deacetylated; viscosity, <100 cps) were purchased from Shanghai Lanji technological development Co., Ltd. Silver nitrate was bought from Xi’an Chemical Reagent Factory. 4-Nitrophenol was obtained from Shanghai Chemical Reagent Factory. Sodium borohydride (NaBH₄) was purchased from Chengdu Kelong Chemical Reagent Company. Anhydrous ethanol was bought from Tianjin Fuyu Fine Chemical Reagent Company. All chemicals are analytical grade and without further purification.

Synthesis

Scheme 1 illustrates the procedure for the synthesis of Ag-NC composites. 500 mg chitosan flakes were dispersed in 50 mL AgNO₃ with concentration of 4, 8, 12 and 16 mmol L⁻¹. The mixture was sonicated for 10 min, and transferred into a Teflon-lined autoclave. The autoclave was sealed in a stainless steel tank and heated at 180 °C for 12 h. After the reaction, the reactor was allowed to cool naturally to room temperature. The black products were washed with distilled water and anhydrous ethanol for several times. The final products were dried overnight in a vacuum oven at 50 °C. The obtained samples were denoted as S1, S2, S3 and S4, respectively.

Scheme 1 Synthesis of Ag-NC composites.

Characterizations

The morphological features and composition of the as-prepared Ag-NC were obtained by energy dispersive X-ray spectroscopy (EDS), selected area electron diffraction (SAED) and transmission electron microscope (TEM, Tecnai G2 F20). The X-ray diffraction (XRD) patterns of the samples were collected in reflection mode (Cu-Kα radiation) on a Bruker D8 X-ray diffractometer oven with 2θ range of 10–80°. X-ray photoelectron spectroscopy (XPS) was performed using an Axis Ultra spectrometer with a monochromatized Al-Kα X-ray as excitation source (225 W). The Ag content of Ag-NC nanocomposite was also determined with inductively coupled plasma emission spectrometry (ICP).

Catalytic activity

To study the catalytic activity of the Ag-NC composites, the catalytic reduction of 4-nitrophenol by NaBH₄ was chosen as a model reaction.^7,9 In brief, 80 mL of 0.125 mM 4-nitrophenol solution and 20 mL of 0.1 M fresh prepared NaBH₄ solution were mixed at room temperature. Then, 25 mg of the Ag-NC composites were added to the above mixture. Parts of the mixture were filtered through a 0.22 μm membrane filter in 3 min intervals and the catalytic activity was tested by UV-vis absorption spectra (MAPADA UV-3200 spectrophotometer). To study reusability of the Ag-NC composites, the used composites were centrifugalized from the reaction solution and washed with water and ethanol, and then reused in the next cycle. The recycling experiments were performed 5 times.

Results and discussion

Fig. 1 shows the XRD patterns of NC and Ag-NC. Comparing the XRD pattern of Ag-NC (Fig. 1a) with that of NC (Fig. 1b), four new characteristic diffraction peaks at about 38.0°, 44.2°, 64.3° and 77.4° can be observed, which correspond to the (111), (200), (220) and (311) crystalline silver reflection of the face-centered lattice of silver, respectively (JCPDS no. 04-0783). Therefore, it confirms that silver ions were successfully reduced to Ag NPs and Ag NPs were immobilized on the NC.

Fig. 1 XRD patterns of NC, S1–S4.

In order to study the microstructure of the samples in more detail, TEM and HRTEM observations were carried out and images are shown in Fig. 2. The TEM images of S1–S4 are presented in Fig. 2a–d. In Fig. 2a we can see that a large number of small-sized spherical Ag NPs were successfully formed and loaded on NC without obvious aggregation. However, when more AgNO₃ was added to the synthetic system this reulted in aggregation of Ag NPs. Meanwhile, there is only a slight increased in average particle size. This phenomenon may be due to the carrier being insufficiently dense to prevent Ag NPs aggregation. The typical HRTEM image shown in Fig. 2e, with clear interlayer spacing of about 0.22 nm, which correspond to (111) phases of Ag. The electron diffraction pattern (SAED) of Ag-NC, displayed as inset of Fig. 2e, revealed that the Ag NPs had single crystal nature with cubic phase.⁷ Fig. 2f shows a typical energy-dispersive X-ray spectrum (EDS) taken from the Ag-NC. The result shows the peak of silver element along with the peaks of carbon, nitrogen and oxygen elements, which indicates Ag NPs were loaded on the NC. These observations provide supportive evidence that Ag-NC nanocomposite was successfully prepared.

Fig. 2 (a)–(d) TEM images of S1–S4; (e) HRTEM of the Ag-NC (inset: the selected area electron diffraction image of a silver nanoparticle); (f) EDX spectrum of Ag-NC.

More detailed information of the chemical and bonding environment of the Ag-NC was obtained by X-ray photoelectron spectroscopy (XPS). Fig. 3a shows the fully scanned spectra in the range of 0–1100 eV. The overview spectra demonstrate that the characteristic signals of carbon, nitrogen, and oxygen elements exist in pure chitosan, while, carbon, nitrogen, oxygen and silver exist in Ag-NC. Fig. 3b shows the high resolution XPS spectra for the C 1s, it can be perfectly deconvoluted into three peaks at 284.5, 285.9 and 287.3 eV, which match the reported values very well for graphitic carbon, secondary carbon and carbon bonded to oxygen or nitrogen.³¹ Fig. 3c showed N 1s XPS spectrum of the composite. The deconvolution yielded three kinds of nitrogen species, including pyridinic-type N (398.6 eV), amino-type N (399.6 eV) and pyridinium nitrogen of condensed polycycles (400.8 eV).^32,33 In Fig. 3d, it could be seen that two peaks occurred at 368.0 and 374.0 eV were well matched with Ag 3d_5/2 and Ag 3d_3/2 binding energies, respectively. The splitting of the 3d doublet is 6.0 eV, indicates the metallic nature of silver.¹⁴ What’s more, the different amount of Ag didn’t change the kinds of nitrogen species, as shown in Fig. S2.† These results further revealed that Ag NPs supported on N-doped carbon were successfully prepared.

Fig. 3 (a) XPS fully scanned spectra of the pure chitosan and Ag-NC; (b)–(d) high-resolution XPS spectra of C 1s, N 1s and Ag 3d for the Ag-NC.

The reduction of 4-NP to 4-AP by a large excess of NaBH₄ was chosen as a model reaction to characterize the catalytic performance of Ag-NC. The light yellow 4-NP solution shows a characteristic absorption peak at 317 nm, after addition of freshly NaBH₄ solution, the color of solution turns to yellow-green and the absorption peak shifts to 400 nm, which indicates the formation of 4-nitrophenolate anion (Fig. 4a).³⁴ And the color remains for a long time, indicating that the reduction did not occur without catalysts. After a small amount (25 mg) of sample was added, the absorption peak at 400 nm decreases quickly, meanwhile, a new peak appears at 295 nm and gradually increases with the reaction time, indicating conversion of 4-NP to 4-AP (Fig. 4b). After the reduction reaction finished, the absorption peak of the nitro compound had nearly disappeared, indicating that the catalysis reaction has proceeded successfully. Because of the presence of excess of NaBH₄ compared to that of 4-NP, the rate of reduction is independent of the concentration of NaBH₄, and therefore the reaction can be considered pseudo-first order kinetics. A linear relationship between ln(A_t/A₀) and reaction time is obtained in the reduction catalyzed by Ag-NC composites, where A_t and A₀ are the time-dependent and initial (t = 0) absorbance peak of 400 nm, respectively. As shown in Fig. 4c, the reaction rate constants (K) of the samples S1, S2, S3 and S4 (the content of Ag is 6.94, 14.36, 21.45 and 27.48%, which were determined with ICP) for 4-NP reduction, which are 0.111, 0.269, 0.350 and 0.156 min⁻¹, respectively. It is found that with increased AgNO₃ concentration, the catalytic activity of the samples S1, S2 and S3 increased. However, further increasing the AgNO₃ concentration to 16 mmol L⁻¹ (sample S4), the catalytic activity decreased efficiency. We consider that there are two reasons can explain this phenomenon. Firstly, as shown in Fig. 2a–d, in spite of Ag NPs showing a uniform distribution and without obvious aggregation when the AgNO₃ concentration is 4 mmol L⁻¹, the catalytic-activated sites are insufficient. While when the AgNO₃ concentration reaches 16 mmol L⁻¹, some Ag NPs aggregated, leading to a remarkable reduction in their catalytic activities.⁷ Secondly, too much silver loading can lead to the embedment inside the nitrogen-doped carbon, thereby blocking 4-NP contact with catalytic sites.¹⁵ In order to compare our catalytic activity with other matrix-supported Ag nanoparticles in the literature, the equation (k = K/m) was used to calculated the ratio of rate constant K over the total weight of the catalyst. The activity factor k of S3 was 14 min⁻¹ g⁻¹, which is higher than previous reported ratios for Ag nanoparticle-loaded glutaraldehyde-crosslinked calcium alginate (1.86 min⁻¹ g⁻¹),³⁵ Ag–Fe₂O₃–carbons nanocomposites (2.12 min⁻¹ g⁻¹),³⁶ and silver nanoparticles supported on surface-modified poly(N-vinylimidazale) (1.23 min⁻¹ g⁻¹).³⁷ Based on the weight of silver in S3, the activity factor k over the weight of silver is 65.28 min⁻¹ g⁻¹, larger than previously reported constants of many metallic silver catalysts.³⁸ In addition, the catalytic activity of NC was tested, as shown in Fig. S1,† it did not display any catalytic performance for the reduction of 4-NP. The highly catalytic performance of composites are attributed to two reasons: (i) The NC effectively prevents Ag NPs agglomeration. And thus Ag NPs possess a smaller particle size and larger special surface area, which provides more active sites to contact with 4-NP.⁵ (ii) If the band gap of the NC was opened, a space charge layer could form between the silver and NC, which is important for activating the Ag NPs to improve their electron density. And also, nitrogen doping can improve the binding between carbon and silver and increase the number of chemical active sites, which is beneficial to increase catalytic activity.²³ Furthermore, we investigated the reusability of S3 (Fig. 4d). With the same time (10 min) for reduction reaction, the conversion of 4-NP to 4-AP remained 78.4% after 5 cycles, indicating the excellent recyclability of the prepared Ag-NC composite.

Fig. 4 UV-vis spectra of (a) 4-NP before and after adding NaBH₄ solution, (b) the reduction of 4-NP in aqueous solution using S3 as catalyst, (c) ln(A_t/A₀) versus reaction time for the reduction of 4-NP, (d) the reusability of S3 as a catalyst for the reduction of 4-NP with NaBH₄.

Conclusions

In summary, we have adopted a relatively green and simple hydrothermal one-step synthesis of Ag-NC composites. Renewable natural chitosan not only act as a reducer and stabilizer, but also serves as nitrogen and carbon source. Results show the Ag NPs agglomeration was effectively prevented and the as-prepared catalysts exhibited superior catalytic activity towards the reduction of 4-nitrophenol in the presence of NaBH₄. The eco-friendly synthetic procedure complies with the requirements of green chemistry. In this work, a new facile avenue may be extended to prepare some other types of metal NPs and N-doped carbon composites, such as platinum or palladium.

Acknowledgements

This work was supported financially by funding from the National Natural Science Foundation of China (21367022).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra07589k

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