In situ green synthesis of Au nanostructures on graphene oxide and their application for catalytic reduction of 4-nitrophenol

Abstract

In this communication, we develop a relatively green, and environment friendly route for the synthesis of Au nanostructures on tannic acid (TA)-functionalized graphene oxide (GO) using TA as a reducing and immobilizing agent. The morphologies of Au nanostructures can be controlled by the amount of HAuCl₄ used. The resultant Au nanostructures/GO nanocomposites exhibit good catalytic activity toward 4-nitrophenol (4-NP) reduction and the GO supports also enhance the catalytic activityvia a synergistic effect.

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During the past years, considerable attention from both fundamental and applied research has been paid to synthesizing and characterizing metal nanoparticles, particularly Au and Ag, due to their unique physical and chemical properties compared to bulk metals and their very important applications in catalysis.¹ Various methods for the preparation of such nanoparticles have been developed over the past years;² however, the chemical reduction of metal ions in a solution is still the most common preparative route. Unfortunately, unprotected metal colloids are susceptible to irreversible aggregation in solution due to their small size, resulting in a remarkable reduction in their catalytic activities. To address these disadvantages, one of the effective strategies is the introduction of metal nanoparticles on/into less expensive solid supports (such as polymers, carbon, metal oxides and so on) to form composite catalysts.³ Up to now, there have been many efforts toward assembling Au nanoparticles on/into many kinds of supports with different nanostructures.⁴ Among these nanostructures, two-dimensional (2D) plate-like structured graphene and its derivative, graphene oxide (GO), with high surface areas have been attractive choices as promising candidates for supports.⁵ Currently, graphene has been successfully utilized as a support to disperse and stabilize metal nanoparticles, while few reports of water-soluble GO used as supports are available.⁶ Compared to graphene, GO should be a better support for the fabrication of water-soluble nanoparticles/2D nanocarbons due to its good dispersion in aqueous solution for applications.

Tannic acid (TA; Fig. S1, ESI†) as a water-soluble, phenolic hydroxyl-rich compound is widely present in woods such as oak, walnut and mahogany and has been used as a common mordant in industry. Recently, the π–π interactions between the aromatic rings of TA and GO sheets have been used to disperse GO sheets in aqueous medium successfully.⁷ In this communication, we report on our recent finding that Au nanostructures can be in situ reduced and immobilized on TA-functionalized GO through a green, cost-effective strategy with the use of TA as a reducing and immobilizing agent. The morphologies of Au nanostructures thus formed can be controlled by the amount of gold salts used. It suggests that the resultant Au nanostructures/GO nanocomposites exhibit good activities in catalyzing the reduction of 4-nitrophenol (4-NP) by NaBH₄ and the GO support has a synergistic effect to enhance the catalytic activity of Au catalysts.

The successful preparation of Au nanoparticles/GO nanocomposites (see ESI† for preparation details) was confirmed by UV-vis absorption spectra, as shown in Fig. 1. The UV-vis spectrum of TA shows two peaks at 214 and 274 nm (curve a), which are assigned to π–π* and n–π* transition, respectively.⁷ The UV-vis spectrum of GO shows two peaks at 230 nm and 300 nm (curve b). The TA/GO shows the superimposed absorbance bands of TA and GO at 220 nm (curve c). After the addition of 30 μL HAuCl₄ into the suspension of TA/GO, a new absorbance band appears at 522 nm characteristic of the colloidal gold surface plasmon resonance band, indicating the formation of gold nanoparticles (curve d). Bayberry tannin as a phenolic hydroxyl-rich compound has recently been successfully used as a reducing and stabilizing agent for the synthesis of Au nanoparticles and Pd nanoparticles.⁸ The spontaneous formation of Au nanoparticles in our present study can also be attributed to the direct redox between TA and Au³⁺, where Au³⁺ is reduced to metallic Au by the phenolic hydroxyls of TA, which is evidenced by the blue shift of the superimposed absorbance bands of TA/GO from 220 nm to 223 nm due to the decreased absorbance of TA.

Fig. 1 UV-vis absorption spectra of aqueous dispersions of TA (a), GO (b), TA/GO (c), and Au nanoparticles/GO (d).

Fig. 2 shows the TEM images of the products thus formed (see ESI† for preparation details). It is seen that well-dispersed TA-functionalized GO sheets are observed in the suspension of TA/GO, as shown in Fig. 2a. After the addition of 30 μL of HAuCl₄ into the suspension of TA/GO, it is seen that the GO sheet has been decorated with many small Au nanoparticles about several nanometres to 20 nm in diameter (Fig. 2b). When 60 μL of HAuCl₄ was added into the suspension of TA/GO, Au nanoparticles with increased sizes of ca. 100 nm and a small amount of Au nanoplates are observed on the surface of GO (Fig. 2c). When the volume of HAuCl₄ was further increased up to 100 μL, more triangular and hexagonal Au nanoplates with increased edge sizes are obtained on the surface of GO and some small Au nanoparticles are also observed, as shown in Fig. 2d. Those above results indicate that with the increase in the amount of HAuCl₄, Au nanostructures with controllable morphologies from nanoparticles to nanoplates can be prepared on GO sheets with the use of TA as the reducing and immobilizing agent. Scheme 1 illustrates the noncovalent functionalization of GO by TA and the subsequent in situ synthesis of Au nanostructures with controllable morphologies on GO.

Fig. 2 TEM images of TA/GO (a), Au nanostructures/GO obtained with the use of 30 μL (b), 60 μL (c), and 100 μL (d) of HAuCl₄.

Scheme 1 A scheme (not to scale) to illustrate the noncovalent functionalization of GO by TA and the subsequent preparation of Au nanostructures/GO nanocomposites by in situ chemical reduction of gold salts.

It is well known that 4-aminophenol (4-AP) is very useful and important in many applications that include analgesic and antipyretic drugs, photographic developer, corrosion inhibitor, anticorrosion lubricant, and so on.⁹ The reduction of 4-NP by NaBH₄ in the presence of noble metal nanoparticles as catalysts has been intensively investigated for the efficient production of 4-AP.¹⁰ Therefore, the reduction of 4-NP to 4-AP with an excess amount of NaBH₄ was used as a model system to quantitatively evaluate the catalytic activities of the resultant Au nanostructures/GO nanocomposites. In the absence of our catalysts, the mixtures of 4-NP and NaBH₄ show a strong adsorption peak at 400 nm, corresponding to the 4-NP ions under alkaline conditions.¹¹ After the catalysts of Au nanostructures/GO nanocomposites were added into the reaction system, the reduction process was monitored by measuring the time-dependent adsorption spectra of the reaction mixture solution. As shown in Fig. 3, the absorption intensity of 4-NP at 400 nm decreases quickly with time, accompanied by the appearance of the new peaks of 4-AP at 230 nm and 300 nm, indicating conversion of 4-NP to 4-AP.¹² It should be noted that the reduction of 4-NP by NaBH₄ was finished within 18 minutes upon the addition of Au nanoparticles/GO (Fig. 3a), with the observation of a fading and ultimate bleaching of the yellow-green color of the reaction mixture in aqueous solution. However, a longer reaction time of 75 minutes was required to achieve the full reduction of 4-NP by NaBH₄ using Au nanoplates/GO as catalysts (Fig. 3b). We also investigated the effects of the GO support on the catalytic activity. A reaction time of 27 minutes was required to achieve the full reduction of 4-NP by NaBH₄ using Au nanoparticles alone (TA-reduced AuNPs, see ESI† for preparation details) as catalysts (Fig. 3c and Fig. S2, ESI†). Due to the presence of large excess of NaBH₄ compared to 4-NP, the rate of reduction is independent of the concentration of NaBH₄, and the reaction could be considered pseudo-first-order with respect to the concentration of 4-NP.¹³ Hence, ln(C_t/C₀) versus time can be obtained based on the absorbance as the function of time, and good linear correlations are observed, as shown in Fig. 3d, suggesting that the reactions follow pseudo-first-order kinetics. Then, the kinetic reaction rate constants (defined as k_app) are estimated from the slopes of the linear relationship to be 18.8 × 10⁻² min⁻¹ (Au nanoparticles/GO), 12.2 × 10⁻² min⁻¹ (Au nanoparticles) and 4.67 × 10⁻² min⁻¹ (Au nanoplates/GO), respectively. These results clearly indicate the Au nanoparticles/GO composites have higher catalytic activity for the reduction of 4-NP than the Au nanoplates/GO composites, due to the smaller sizes and larger active surface areas of Au nanoparticles compared with Au nanoplates. It is also important to point out that the Au nanoparticles/GO composites show relatively higher catalytic activity than Au nanoparticles alone. Such observation indicates that the GO support may play an active part in the catalysis, yielding a synergistic effect. The synergistically enhanced catalytic activity may be explained as follows: given that 4-NP is π-rich in nature, it is expected that 4-NP can be adsorbed onto GO via π–π stacking interactions. Such adsorption provides a high concentration of 4-NP near to the Au nanoparticles on GO, leading to highly efficient contact between them. In contrast, without a highly adsorbent GO support, 4-NP must collide with Au nanoparticles by chance, and remains in contact for the catalysis to proceed. When this is not achieved, 4-NP will pass back into solution and can only react further when it collides with Au nanoparticles again.¹⁴

Fig. 3 UV-vis adsorption spectra of the reduction of 4-NP by NaBH₄ in the presence of Au nanoparticles/GO (a), Au nanoplates/GO (b), and Au nanoparticles alone (c); (d) plot of ln(C_t/C₀) of 4-NP against time for the catalysts.

In summary, we have developed a relatively green and environment friendly route for the synthesis of Au nanostructures with controllable morphologies on TA-functionalized GO using TA as a reducing and stabilizing agent. These Au nanostructures/GO nanocomposites are found to exhibit good catalytic performance toward 4-NP reduction. The GO supports not only enhance the catalytic activity of Au nanostructuresvia a synergistic effect, but also lend the Au nanostructures/GO nanocomposites the easiness of separation and recovery for practical catalytic applications.

Acknowledgements

This work was supported by National Basic Research Program of China (No. 2011CB935800).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental section and supplementary figures. See DOI: 10.1039/c1cy00205h

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