Self-assembly of amine-functionalized gold nanoparticles on phosphonate-functionalized graphene nanosheets: a highly active catalyst for the reduction of 4-nitrophenol

Abstract

Graphene nanosheet (GNS) supported gold nanoparticle (Au-NP) composites (Au-NP/GNS) have attracted a lot of attention due to their important applications in catalysis, sensing, optics, medicine, and fuel cells. In this study, based on the strong electrostatic and/or hydrogen bonding interactions between the amine-functionalized Au-NP (Au-NP@NH₂) and phosphonate-functionalized GNS (GNS-PO₃H₂), a self-assembly strategy was used to synthesize Au-NP/GNS composites. The physical and chemical properties of Au-NP@NH₂, GNS-PO₃H₂, and the Au-NP/GNS composites were fully investigated using various physical characterization, including X-ray powder diffraction, X-ray photoelectron spectroscopy, ultraviolet-visible spectroscopy, transmission electron microscopy, and zeta potential analysis. The experimental results demonstrate that the phosphonate-functionalization of GNS was critical for the generation of high-quality Au-NP/GNS composites. The as-prepared Au-NP/GNS composites show improved catalytic activity for the degradation of 4-nitrophenol compared to functionalized Au-NP, which was ascribed to the amine-functionalization of the Au-NP and the introduction of the GNS with high electrical conductivity and large surface area.

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Introduction

Chemically functionalized gold nanoparticles (Au-NP), consisting of a small Au-NP core and a monolayer of the functional molecule, have attracted a lot of attention in fields of catalysis, sensor and medicine due to the unique physical and chemical properties imparted by the functional molecule.^1–6 For example, the apo-glucose dehydrogenase enzyme can be reconstituted on pyrroloquinoline quinine functionalized Au-NP and displays excellent bioelectrocatalytic activity for glucose oxidation.² Bipyridine functionalized Au-NP can interact with Eu^III/Tb^III ions to yield phosphorescent nanomaterials.³ Polyvalent DNA-functionalized Au-NP selectively enhanced ribonuclease H activity, making the DNA-functionalized Au-NP ideal gene regulation agents.⁴ Glutathione functionalized Au-NP can serve as a carrier of a platinum(IV) drug used for the treatment of prostate cancer.⁵ Flavin-functionalized Au-NP can act as an efficient catalyst for aerobic organic transformations.⁶

It is well known that the support materials generally affect the catalytic activity and stability of metal NP.^7–13 Recent reports demonstrate that two-dimensional graphene nanosheets (GNS) have a high surface area as well as excellent mechanical and electronic properties, which make them a promising support material for the anchorage of metal NP.^14–19 During a catalytic reaction, the high electrical conductivity of the GNS facilitates electron transfer. Moreover, the large surface area of the GNS efficiently decreases the aggregation of the metal NP. These two favorable factors improve the catalytic activity and stability of metal NP.^15–19 Consequently, considerable efforts have been made to incorporate Au-NP into a GNS matrix and explore their potential applications in photothermal energy conversion,²⁰ plasmonic devices,²¹ photoacoustic imaging,²² photothermal therapy,²³ surface-enhanced Raman scattering,^24–29 organic photovoltaic devices,³⁰ fuel cells,^31–35 heterogeneous catalysis,^36,37 and electrochemical sensors.^38–43 At present, the self-assembly method is an efficient strategy for the synthesis of Au-NP/GNS composites.^{23,25,26,32,36,40} However, GNS tends to lead to irreversible aggregation in an aqueous solution due to strong π–π stacking interactions and the pristine surface of GNS lacking enough active sites for the anchorage of the metal NP.²⁷ Therefore, various aromatic molecules with amino groups,^44–46 sulfonic groups,^47–50 and carboxyl groups^51,52 have been widely used to functionalize GNS. Although the dibasic phosphonic acid groups (–PO₃H₂) have good biocompatibility, high hydrophilicity, adjustable charge density, and strong coordination ability,^53–56 little attention has been paid towards the phosphonate-functionalization of GNS to date.

4-Nitrophenol (4-NP), one of the most common nitroaromatic compounds, is widely used in industrial synthesis and exhibits great solubility and high chemical stability in water.^57–62 The ingestion of 4-NP has been linked to several public health problems such as fervescence, methemoglobinemia, and liver and kidney damage.⁵⁷ Thus, 4-NP is regarded as a highly hazardous and toxic pollutant by the US Environmental Protection Agency.⁵⁷ For remediating 4-NP contamination, the reductive transformation of 4-NP to less toxic 4-aminophenol (4-AP) is a simple and efficient method.^{58,59,63–66} To date, the Au-catalyzed reduction of 4-NP using NaBH₄ has been extensively investigated due to the high catalytic activity of Au-NP.^{57,58,60,62,67,68} However, the effect of surface groups in the Au-NP on catalytic activity of the Au-NP is rarely reported.

In our study, we have successfully synthesized amine-functionalized Au-NP composites (Au-NP@NH₂) using polyethyleneimine (PEI, Fig. 1) as both a reducing agent and stabilizing agent, which show remarkably improved catalytic activity for the reduction of 4-NP when compared to carboxylate-functionalized Au-NP composites (Au-NP@COOH). Moreover, phosphonate-functionalized GNS composites (GNS-PO₃H₂) were easily obtained based on the π–π stacking interactions between zoledronic acid (ZDA, Fig. 1) and the GNS. Furthermore, Au-NP@NH₂ can efficiently self-assemble on the GNS-PO₃H₂ surface due to strong electrostatic and/or hydrogen bonding interactions. The introduction of GNS further enhances the catalytic activity of the Au-NP@NH₂ in the reduction of 4-NP.

Fig. 1 A schematic of the synthetic route used to prepare the Au-NPs@NH₂/GNS-PO₃H₂ composites.

Experimental

Reagents and chemicals

PEI (M_w ≈ 600) was obtained from Aladdin Industrial Corporation. Graphene nanosheet (GNS) was purchased from Nanjing XFNANO Materials TECH Co., Ltd. Zoledronic acid (ZDA, Fig. 1), hydrogen tetrachloroaurate(III) tetrahydrate (HAuCl₄·4H₂O), 4-nitrophenol (4-NP), and sodium borohydride (NaBH₄) were purchased from Sinopharm Chemical Reagent Co., Ltd.

Synthesis of Au-NP@NH₂ and Au-NP@COOH

Au-NP@NH₂ composites were synthesized using PEI as both the reducing agent and stabilizing agent at room temperature (Fig. 1A). In a typical synthesis, 1.0 mL of 24.2 mM HAuCl₄ solution was added to 41.0 mL of 12.1 mM PEI aqueous solution. After adjusting the solution pH to 3.7, the mixture was allowed to stand for 2 h at room temperature until the color of the mixture changed to a red color.

For comparison, 13.0 nm Au-NP@COOH composites (ESI Fig. S1,† zeta potential: −42.9 mV at pH 4.8) were synthesized via the sodium citrate reduction method.⁶⁹ In a typical synthesis, 30.0 mL of 0.4 mM HAuCl₄ aqueous solution was charged into a three-neck flask equipped with a reflux condenser and heated at 100 °C. Then, 0.5 mL of 0.2 M sodium citrate solution was added rapidly into HAuCl₄ aqueous solution. The reaction was allowed to run until the solution turned wine red.

Synthesis of GNS-PO₃H₂

GNS-PO₃H₂ composites were synthesized using the π–π stacking interactions between GNS and ZDA (Fig. 1B). In a typical procedure, 10 mg of GNS was dispersed in 50 mL of 8.7 mM ZDA solution (pH 8.5) using ultrasonic treatment for 30 min. Then, the GNS-PO₃H₂ composites were separated by centrifugation.

Synthesis of Au-NPs@NH₂/GNS-PO₃H₂ composites

Au-NPs@NH₂/GNS-PO₃H₂ composites were obtained by the self-assembly of Au-NP@NH₂ on the surface of GNS-PO₃H₂ (Fig. 1C). In a typical procedure, 21.0 mL of 0.1 mg mL⁻¹ Au-NP@NH₂ solution was added into 10.0 mL of 1.0 mg mL⁻¹ GNS-PO₃H₂ suspension with ultrasonic treatment for 60 min at room temperature. After self-assembly, the Au-NPs@NH₂/GNS-PO₃H₂ composites were separated by centrifugation/washing cycles and then dried in a vacuum dryer. ICP-AES analysis shows that the Au-NPs@NH₂/GNS-PO₃H₂ composites contain 36.5 wt% of Au metal.

Catalytic reduction of 4-NP

For the catalytic reduction of 4-NP to 4-aminophenol (4-AP), 2.7 mL of 0.037 M NaBH₄ was added into 0.3 mL of 2.0 × 10⁻³ M 4-NP aqueous solution. Then, 15.0 μL of the Au-NPs@NH₂/GNS-PO₃H₂ composite (0.3 g L⁻¹) aqueous suspension was added to the abovementioned solution at room temperature. The reaction progress was monitored using time-dependent ultraviolet-visible spectroscopy (UV-vis).

For comparison, the Au-NP@NH₂ and Au-NP@COOH composites were also used as heterogeneous catalysts for the reduction of 4-NP using the same experimental procedure and reagent concentrations (including the concentration of Au-NP) as those employed for the Au-NPs@NH₂/GNS-PO₃H₂ composites.

Instrumentation

X-ray powder diffraction (XRD, Model D/max-rC), X-ray photoelectron spectroscopy (XPS, AXIS ULTRA), transmission electron microscopy (TEM, EOL JEM-2100F), and a zeta potential analyzer (Malvern Zetasizer Nano ZS90) were used to investigate the structure, surface composition, morphology and surface charge of the samples studied. Scanning electron microscopy-energy dispersive spectroscopy (SEM-EDX, FEI Quanta 200) and scanning transmission electron microscopy-energy dispersive spectroscopy (STEM-EDX, EOL JEM-2100F) were used to investigate the element distribution of the samples. The accurate Au loading in the Au-NPs@NH₂/GNS-PO₃H₂ composites was determined using inductively coupled plasma atomic emission spectrometry (ICP-AES, Prodigy 7). UV-vis spectrophotometer (UV2600) was used to monitor the real-time variation of the concentration of 4-NP.

Results and discussion

Characterization of Au-NPs@NH₂

The reduction of HAuCl₄ was first confirmed by a color change of the solution (Fig. 2A). After adjusting the solution to pH 3.7, a sequence of color changes was observed and a wine-red color solution was finally obtained at 2 h, indicating the formation of stable Au-NP. The interaction between HAuCl₄ and PEI was further investigated using UV-vis measurements (Fig. 2B). As the reaction progressed, the plasmon peak of the Au-NP around 528 nm gradually increased with time, which is characteristic for spherical Au-NP.⁷⁰ At 2 h, the plasmon peak value of the Au-NP reached a maximum value and the peak position was finally fixed at 526 nm, implying the HAuCl₄ was completely reduced by the PEI reagent. After storing for two months, the UV-vis spectrum of the Au-NP@NH₂ composite remained constant (data not shown), indicating the PEI also acts as an efficient stabilizing agent.

Fig. 2 (A) Images and (B) UV-vis spectra of the HAuCl₄ and PEI mixture at different reaction times. Inset: a plot of the absorbance intensity around 528 nm as a function of time.

The crystal structure and chemical composition of the Au-NP@NH₂ composite were investigated using XRD and XPS. The XRD pattern clearly shows the four diffraction peaks at 38.3°, 44.4°, 64.7° and 77.7° (Fig. 3A), which agrees well with standard diffraction data of face-centered cubic (fcc) crystal Au (JCPDS standard 04-0784). The binding energies of Au 4f_5/2 and Au 4f_7/2 were located at 86.8 and 83.1 eV (Fig. 3B), respectively, which are the typical values of metallic Au. This fact further confirms that HAuCl₄ was completely reduced by PEI. Importantly, the N 1s signal at 399.6 eV was detected in the Au-NP@NH₂ composite (Fig. 3B) and originated from the –NH₂ groups in the PEI. The appearance of the N 1s peak indicates the adsorption of PEI on the Au-NP surface. Furthermore, the zeta potential of the Au-NP@NH₂ composite was measured to be +49 mV at pH 5.0. It was clear that the –NH₂ group will be protonated due to its weak basic nature. Thus, the positive zeta potential value of the Au-NP@NH₂ composite confirms the PEI functionalization of the Au-NP.

Fig. 3 (A) XRD pattern of the Au-NPs@NH₂ and (B) XPS spectra of the Au-NPs@NH₂ in the N 1s and Au 4f regions.

The morphology and size distribution of the Au-NP@NH₂ composite were characterized using TEM. As observed, all particles are nearly spherical and well separated from each other (Fig. 4A). The selected area electron diffraction (SAED) image show an irregular spot pattern (inset in Fig. 4A), implying their polycrystalline properties. According to the size distribution histogram (Fig. 4B), the average size of the Au-NP@NH₂ was estimated to be 33 nm. The high resolution TEM (HRTEM) image of an individual Au-NP@NH₂ particle showed clear lattice fringes (Fig. 4C). In the further magnified HRTEM images, Au (111) planes with a d-spacing of 0.24 nm and Au (100) planes with a d-spacing of 0.21 nm were observed (Fig. 4D), which confirm their polycrystalline nature. EDX element mapping measurements show that the N element pattern was very similar to the Pt element pattern (Fig. 4F), demonstrating the uniform distribution of PEI on the surface of the Au-NP.

Fig. 4 (A) A typical TEM image of the Au-NPs@NH₂. Inset: the SAED pattern of the Au-NPs@NH₂. (B) The size distribution histogram of the Au-NPs@NH₂. (C) HRTEM image of an individual Au-NPs@NH₂. (D) Magnified HRTEM images obtained from the regions (1, 2, 3, and 4) marked by white squares in (C). (F) High-angle annular dark-field scanning TEM image of the Au-NPs@NH₂ and corresponding EDX element mapping patterns.

Characterization of GNS-PO₃H₂

The π–π stacking interactions between ZDA and the GNS were confirmed by UV-vis spectra (Fig. 5A). GNS has no obvious absorption peak in the range of 200–600 nm, whereas ZDA has a strong absorption peak at 209 nm. Moreover, the GNS-PO₃H₂ composites show an obvious peak at 211 nm, corresponding to ZDA with a red-shift (ca. 2 nm). The appearance of the characteristic peak of ZDA and its red-shift suggest the successful functionalization of GNS by ZDA.⁷¹ The phosphonate-functionalization of GNS was further chemically confirmed using XPS (Fig. 5B). The appearance of the characteristic N 1s peak at 401.2 eV and P 2p peak at 134.1 eV demonstrate the successful self-assembly of ZDA on the surface of the GNS surface via π–π stacking interactions. The distribution of ZDA on the surface of the GNS was investigated using EDX elemental mapping (Fig. 5C). As observed, C, N, and P elements are evenly distributed in the same region, demonstrating the uniform distribution of ZDA on the surface of the GNS. Moreover, the zeta potential of the GNS-PO₃H₂ composite was measured to be −18.6 mV at pH 5.0, indicating the negatively charged phosphonate groups were exposed on the outside of the GNS.

Fig. 5 (A) UV-vis spectra of (a) ZDA, (b) GNS and (c) GNS-PO₃H₂. (B) N 1s and P 2p XPS spectra of GNS-PO₃H₂. (C) A representative SEM image of GNS-PO₃H₂ and its corresponding EDX elemental mapping.

Characterization of the Au-NPs@NH₂/GNS-PO₃H₂ composites

Due to the strong electrostatic and/or hydrogen bonding interactions between the –PO₃H₂ groups and –NH₂ groups,^72–74 Au-NP@NH₂ can anchor easily on the surface of the GNS-PO₃H₂ via self-assembly. As observed, the wine-red color of the Au-NP@NH₂ and GNS-PO₃H₂ mixture completely disappears after sonication for 0.5 h (Fig. 6A), indicating the efficient anchorage of the Au-NP@NH₂ on the surface of the GNS-PO₃H₂. In contrast, the wine-red color of the Au-NP@NH₂ and pristine GNS mixture was retained after sonication for 0.5 h (Fig. 6B), indicating the Au-NP@NH₂ were not completely anchored on the pristine GNS surface due to the absence of interactions. The self-assembly of Au-NP@NH₂ on GNS-PO₃H₂ was confirmed by TEM. As observed, the spherical Au-NP@NH₂ particles were uniformly dispersed on the GNS-PO₃H₂ surface (Fig. 7A). In contrast, the few Au-NP@NH₂ anchored on the pristine GNS surface (Fig. 7B), was consistent with the abovementioned experimental observations. These experimental results confirm the phosphonate-functionalization of GNS facilitates the anchoring of the Au-NP@NH₂, which originates from the strong electrostatic and/or hydrogen bonding interactions present.

Fig. 6 (A) Images of the Au-NPs@NH₂ and GNS-PO₃H₂ mixture (a) before and (b) after sonication for 30 min. (B) Images of the Au-NPs@NH₂ and pristine GNS mixture (a) before and (b) after sonication for 30 min.

Fig. 7 Representative TEM images of the obtained Au-NPs@NH₂/GNS-PO₃H₂ composites using (A) GNS-PO₃H₂ and (B) pristine GNS as the support materials.

Catalytic activity of the Au-NPs@NH₂/GNS-PO₃H₂ composites

The catalytic activity of the Au-NPs@NH₂/GNS-PO₃H₂ composites for the reduction of 4-NP was investigated using time-dependent UV-vis spectra.^75–79 For comparison, the catalytic activities of the Au-NP@NH₂ and Au-NP@COOH composites were also evaluated. In the 4-NP/NaBH₄ reaction system, the absorbance peak at 400 nm was the characteristic absorbance peak of 4-nitrophenolate in an alkaline solution.^80,81 UV-vis spectra of the 4-NP/NaBH₄ mixture remained constant within 4 h, indicating the reduction of 4-NP was not complete in the absence of a catalyst (data not shown). Upon the introduction of the Au-NP@NH₂, Au-NP@COOH, and Au-NPs@NH₂/GNS-PO₃H₂ composites into the reaction solution; however, the absorbance peak of 4-nitrophenolate decreases gradually with time and finally disappears, indicating the complete conversion of 4-NP to 4-AP (Fig. 8A–C). The Au-NP@NH₂-catalyzed reduction of 4-NP was finished within 14 min (Fig. 8A), which was much shorter than that found for the Au-NP@COOH-catalyzed reduction of 4-NP (20 min) (Fig. 8B). Mainly, the positively charged Au-NP@NH₂ can sufficiently attract the negatively charged nitrophenolate and BH₄⁻ ions through electrostatic interactions, resulting in their enrichment on the Au-NP surface. Furthermore, it was observed that the Au-NPs@NH₂/GNS-PO₃H₂-catalytic reduction of 4-NP was finished within 7 min (Fig. 8C), showing an enhanced catalytic activity compared to Au-NP@NH₂ (Fig. 8A). According to the linear plots of ln(C_t/C₀) versus time t (insets in Fig. 8A–C),^80,81 the reaction rate constants were calculated to be 0.19, 0.126, and 0.325 min⁻¹ for the 4-NP reduction reactions catalyzed by Au-NP@NH₂, Au-NP@COOH and Au-NPs@NH₂/GNS-PO₃H₂, respectively.

Fig. 8 UV-vis spectra for the successive reduction of 4-NP using NaBH₄ in the presence of (A) Au-NPs@NH₂, (B) Au-NPs@COOH, (C) Au-NPs@NH₂/GNS-PO₃H₂, and (D) GNS-PO₃H₂ as catalysts.

Because GNS-PO₃H₂ do not show any obvious catalytic behaviour for the reduction of 4-NP within 5 h (Fig. 8D), the enhanced catalytic activity of the Au-NPs@NH₂/GNS-PO₃H₂ composites can be ascribed to the synergistic effect between the Au-NP@NH₂ and GNS-PO₃H₂. On the one hand, the highly conjugated sp² network of GNS provides excellent electrical conductivity for the composite, which accelerates electron transfer in the course of the 4-NP reduction. On the other hand, the introduction of GNS efficiently avoids the aggregation of Au-NP in the course of the 4-NP reduction, which also contributes to the improved catalytic activity of the Au-NPs@NH₂/GNS-PO₃H₂ composites. The adsorption of the substrate molecule on the GNS support with high surface area also likely facilitates the catalytic reduction of 4-NP.⁸² Furthermore, the recyclability of the Au-NPs@NH₂/GNS-PO₃H₂ composites for the reduction of 4-NP was also investigated by repeatedly adding a 4-NP solution. After undergoing six catalytic cycles, the Au-NPs@NH₂/GNS-PO₃H₂ composites exhibited an almost constant catalytic activity for the reduction of 4-NP within 7 min, indicating the Au-NPs@NH₂/GNS-PO₃H₂ composites possesses good stability.

Conclusions

We have successfully synthesized Au-NP@NH₂ and GNS-PO₃H₂ composites. By means of strong electrostatic and/or hydrogen bonding interactions between the –PO₃H₂ groups and –NH₂ groups, Au-NP@NH₂ can effectively self-assemble on the surface of GNS-PO₃H₂ to generate the high-quality Au-NP/GNS composites. On the one hand, the PEI-functionalization of Au-NP can enhance the catalytic activity of Au-NP for the reduction of 4-NP due to the enrichment of the reactant on the Au-NP surface. On the other hand, the introduction of GNS further improves the catalytic activity of Au-NP due to the high conductivity of GNS and good dispersibility of the Au-NP on the GNS surface. Due to the high catalytic activity of Au-NPs@NH₂/GNS-PO₃H₂ composites, we expect that the composites may have wide applications in the fields of catalysis and the environment.

Acknowledgements

This study was sponsored by the National Natural Science Foundation of China (21301114), the Natural Science Foundation of Shaanxi Province (2013JQ2009), and the Fundamental Research Funds for the Central Universities (GK201503036).

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra18228j

‡ These two authors made an equal contribution to this work.

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