Mussel-inspired chemistry for one-step synthesis of N-doped carbon–gold composites with morphology tailoring and their catalytic properties

Abstract

A coordination-assisted synthetic approach is reported for the synthesis of N-doped carbon supported highly active and stable gold catalysts starting with polydopamine-Au nanoparticles. The composites with tuning morphology via the control of the polymerization rate of dopamine could be used as efficient catalysts for reduction of 4-nitrophenol.

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Au nanoparticles (AuNPs) have received considerable attention for a wide variety of applications due to their attractive physico-chemical properties, especially in biosensors, optics and catalysis.¹ However, due to the high surface energy, they are prone to aggregating into bulk materials, and aggregation limits their use in above-mentioned applications.² In catalytic fields, AuNPs are usually reduced from precursor and stabilizing agents prevent their aggregation, they are then always loaded into carriers to enhance their stability and catalytic activity. As we all know, stabilizing agents can make the solution more complex and can be adherent to AuNPs' surface and is more difficult to remove. What's important, the AuNPs surfaces are passivity by the coverage of linker molecules through the strong bonding (i.e., S–Au covalent bonds).³ From the catalytic and environmental application considerations, aqueous phase synthesis of nano-crystals and the use of less surface-adsorbing capping agents should be promoted. For carrier systems, solid-supported noble metal catalysts have been found to offer an easy and effective way for the stabilization of AuNPs which is adopted to have better and broader applications.⁴ Impressive metal–solid substrate hybrid nanocomposite materials have been made to enhance the long-term stability of AuNPs and promote the compatibility and dispersity of AuNPs.⁵ Among these carriers, carbon-supported noble metal catalysts have been intensively studied, because carbon possess excellent physico-chemical properties, owing to their stability, sizes, ease of dispersion in liquid medium and facile formation of composites.⁶ However, the carbon carriers from the traditional carbon precursors barely contain N species. It has been demonstrated in the literature that nitrogen-doped carbon materials can improve electronic conductivity, charge transfer at the interface and stability of noble metal.⁷ Chemical vapour annealing and wet dipping process are general method to prepare nitrogen-doped carbon materials which show unique surface activity and electrochemistry performance. Although many successful examples have been reported to incorporate nitrogen into carbon, it is still a challenge to generate N-doped carbon-based AuNPs with facile production and excellent catalysis performance. So development of synthetic methods in aqueous solution using a nontoxic agent is quite attractive and endow the carbon with N species is important.

Dopamine, a bio-molecule that contains catechol and amine functional groups, can spontaneously deposit polydopamine (Pdop) layers on virtually any surface,⁸ which was proven to have the abilities to reduce metal ions to metal nanoparticles.⁹ Moreover, the Pdop has excellent thermal stability and could directly convert to N-rich carbon with high carbon yield.^10,11 Taking advantage of this new nitrogen-doped carbon precursor to AuNPs, it is possible to form a novel polydopamine-supported AuNPs (P-AuNPs), which can then be converted into N-doped carbon-based AuNPs (NC-AuNPs) under heat treatment. It is noted that the polymerization rate of dopamine could be tuned¹² and it is possible that the morphology of the mussel-inspired catalyst could be easily controlled by the dopamine concentration, reaction solvent and temperature. Thus, using dopamine as a precursor has a significant advantage in tuning the compositions and properties of the composites, which is essential for material design and optimization. To the best of our knowledge, polydopamine as the N-doped carbon precursor to prepare AuNPs-based catalyst in one-step has not been studied yet. Herein, we utilized dopamine as reducer, stabilizer and N-doped carbon precursor to generate P-AuNPs in one-step, and then carbonized into NC-AuNPs. Interesting, the composites with tuneable morphology and AuNPs size exhibited outstanding catalysis performance for the reduction of 4-nitrophenol by NaBH₄ in aqueous solution.

P-AuNPs were synthesized by reduction of HAuCl₄ in the presence of dopamine at alkaline solution (pH = 9). The mixture was subjected to be continuous magnetic stirring at room temperature for 5 h. During this process, the reduction of HAuCl₄ and self-oxidative polymerization of dopamine were conducted at the same time. The precipitates were collected by centrifugation, washed with water, and dried under vacuum. The detailed synthesis parameters of different P-AuNPs are listed in ESI†. NP-AuNPs were obtained by carbonizing the P-AuNPs at 500 °C for 3 h in nitrogen at a rate of 5 °C min⁻¹. The two resultant samples were black in colour, which could easily be dispersed in water by mild sonication. Photographs of the powder and their morphology in water are shown in Fig. S1A.† Powder XRD data presented in Fig. S1B† measured the crystalline nature of the two products, and indicating the presence of four planes of Au, namely, the diffraction peaks at 38°, 45°, 65°, and 78° correspond to the Au(111), Au(200), Au(220) and Au(311) crystal planes (JCPDS 4-0783), respectively. A broad peak at 22° is assigned to amorphous carbon, indicating some degree of ordered packing of Pdop molecules. After the carbonization, the diffraction peak of the N-doped carbon carrier is slightly shifted towards the higher angle side.¹³

First, the initial concentration of dopamine, determining the polymerization rate of dopamine, was used to fabricate P-AuNPs with tuning morphology. As soon as the HAuCl₄ solution (250 μL, 1 wt%) was dropwised into different concentration of dopamine solution (in deionized water), the colourless solution turned into a yellow and then gradually changed into a dark brown. Fig. 1A–D show the P-AuNPs synthesized at a dopamine concentration of 1.5, 1.0, 0.5, 0.3 mg mL⁻¹ (15 mL of water), products noted as P-AuNPs-1.5 to 0.3. Core–shell P-AuNPs spheres can be found in Fig. 1A and B, respectively. Not all P-AuNPs exhibit ideal sphere-like shape, few and large-size AuNPs are coated in Pdop layer, some of the P-AuNPs show shape-deformation and adhere with each other. When the dopamine concentration decreases to 0.5 mg mL⁻¹, no spheres can be obtained and the core is about 15 nm dispersed in carrier. With the further decrease of dopamine concentration to 0.3 mg mL⁻¹, plenty of monodisperse AuNPs with smaller size are embedded in Pdop carrier, the AuNPs with diameters around 8 nm. As noted above, this phenomenon may be related to the polymerization rate of dopamine as we mentioned before. At a high dosage of dopamine, the Au³⁺ ions were rapid reduced to Au⁰ by the superficial catechol groups and easy aggregate to AuNPs since oligodopamine used as stabilizer quick polymerized to bulk polydopamine. As the dosage of dopamine decrease, the mixed contain a higher level of oligodopamine, the oligo-polymer and the strong binding effect can undoubtedly suppress the growth of AuNPs, which results in the formation of smaller AuNPs uniformly dispersed in Pdop. Electron diffraction pattern of P-AuNPs-0.5 (Fig. 1E) exhibits four broad rings, which could be attributed to (111), (220), (422) and (440), respectively. The typical HRTEM image (Fig. 1F) with clear lattice fringes having a spacing of about 2.35 Å revealed that the growth of AuNPs occurred preferentially on the (111) plane. Energy dispersive X-ray spectroscopy (EDX) were used to analyse the composition of the prepared catalysts. Au, C, N and O are found within the solder, as shown in Fig. S2.†

Fig. 1 The TEM images of P-AuNPs-1.5 to 0.3 in different concentration of dopamine: (A) 1.5 mg mL⁻¹, (B) 1.0 mg mL⁻¹, (C) 0.5 mg mL⁻¹, (D) 0.3 mg mL⁻¹, (E) electron diffraction pattern of a nano-sized region containing AuNPs, (F) the HRTEM images of the P-AuNPs-0.5, the interplanar distance of the pdop-stabilized AuNP within the marked white frame is shown in the corresponding panel.

During the experiments, we noticed that the reaction takes place rapidly upon addition of HAuCl₄ into the previously prepared solution dopamine/water. As we know, reaction solvent^10,12 and temperature¹² undoubtedly could affect the polymerization of dopamine. Herein, isopropanol (ipa) and reaction temperature were employed to control the polymerization of dopamine to obtain P-AuNPs. Fig. 2A–C depicted the TEM images of P-AuNPs synthesized in water-isopropanol mixed solution (P-AuNPs-ipa) and water-isopropanol in ice bath (P-AuNPs-ice), respectively. Compared with P-AuNPs synthesized in water (Fig. 1A), the P-AuNPs exhibit well sphere-like shape and agglomeration of AuNPs core can be observed (Fig. 2A), while the volume ratio of water to isopropanol is fixed at 2 : 1. Interestingly, the strawberry-like P-AuNPs (Fig. 2B) with AuNPs immobilized on the Pdop substrate were fabricated by water–isopropanol (V_water:V_ipa = 2 : 1) in ice bath. Presumably these are caused by the hardening of Pdop at low-temperature, and such soft template could act as a universal substrate to grow metal. For P-AuNPs-0.3-ice, the AuNPs <5 nm and uniformly dispersed in the Pdop, as shown in Fig. 2C. The reduction of 4-nitrophenol by NaBH₄ was used as a model reaction to characterize the catalytic performance of P-AuNPs. First, 2.7 mL of 0.11 mM 4-nitrophenol solution and 0.1 mL of 0.3 M NaBH₄ solution were mixed in a 3 mL standard quartz cuvette. Then 0.2 mL of P-AuNPs with gold concentration of 0.25 mg mL⁻¹ which were determined with inductively coupled plasma emission spectrometry was added to catalyse the reduction of 4-nitrophenol at room temperature. The initial concentrations of 4-nitrophenol and NaBH₄ were kept to be 0.1 and 10 mM. The reduction of 4-nitrophenol into 4-aminophenol was completely finished about 4 min by NC-AuNPs-0.3-ice. The catalytic activity is higher than with other studies on the catalysis of this reaction by AuNPs. The plots of rate constants k vs. time of the AuNPs catalysed reduction of 4-nitrophenol by NaBH₄ in the presence of different P-AuNPs at room temperature are shown in Fig. 2D. It is interesting to note that the k obtained with different P-AuNPs exhibit the dependence on the size and the dispersity of AuNPs. Instead of a simple linear relationship between k and t⁻¹, the change of k with P-AuNPs can be divided into two regions. For core–shell P-AuNPs, the AuNPs were embedded in thick Pdop which act as a barrier, followed by a concomitant slowing down of the diffusion of reactants. However, P-AuNPs with uniformly dispersed AuNPs is more accessible to the reactants for the catalytic reduction. Clearly, the k shows the catalysis property of different P-AuNPs via the control of the polymerization rate of dopamine.

Fig. 2 The TEM images of P-AuNPs under various conditions: (A) P-AuNPs-1.5-ipa synthesized at 1.5 mg mL⁻¹ of dopamine with V_water:V_ipa = 2 : 1, (B) P-AuNPs-1.5-ice synthesized at 1.5 mg mL⁻¹ of dopamine with V_water:V_ipa = 2 : 1 in baths of ice, (C) P-AuNPs-0.3-ice synthesized at 0.3 mg mL⁻¹ of dopamine with V_water:V_ipa = 2 : 1 in baths of ice, (D) Plot of k versus t⁻¹ for 4-nitrophenol reduction by NaBH₄ in the presence of different P-AuNPs.

NC-AuNPs were synthesized by subsequent carbonization. The XRD pattern of NC-AuNPs indicates the N-doped carbon shows significantly increased structural order, compared with the Pdop, as indicated by the sharpening of the diffraction peak (Fig. S1B†). TEM images indicate that the NC-AuNPs have a consistent architecture, compared with corresponding P-AuNPs (Fig. 3). However, the NC-AuNPs catalysed reduction 4-nitrophenol became much faster (Fig. S3†). The explanation could be, that the N-doped carbon with a fairly ordered multi-layered structure during Pdop carbonization¹³ is helpful to reactant penetration and gold particles are much more catalytically active after calcination.

Fig. 3 The TEM images of NC-AuNPs under various conditions: (A) NC-AuNPs-0.3, (B) NC-AuNPs-0.3-ice.

In summary, the morphology of P-AuNPs could be tuned by controlling the polymerization of dopamine, such as the initial concentration of dopamine, reaction solvent and temperature. Pdop can act as reducer, stabilizer and N-doped carbon precursor. Results show these catalysts have high catalytic activity and stability. NC-AuNPs was synthesized followed by carbonization. This control over the morphology of P-AuNPs and the size of AuNPs core are of great importance in practical applications, particularly in the field of catalysis.

This work was supported by the National Basic Research Program of China (973 Program, 2012CB720300) and Natural Science Foundation of China (21264013, 21364010).

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Footnote

† Electronic supplementary information (ESI) available: Detailed synthesis parameters, photographs and XRD pattern of composites and their catalysis performance. See DOI: 10.1039/c3ra45566a

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