In this study, we reported a facile approach for the creation of metal nanoparticles (metal NPs)/graphene nanocomposite. We used the mussel-inspired molecule dopamine to modify the surface of GO nanosheets and reduce metal ions for the in situ growth of metal NPs on the surface of GO sheets. -d) (c) Illustration of GO/PDDA/Ag NP fabrication and SERS detection procedure of folic acid using GO/PDDA/Ag NP as substrate and (d) TEM image of GO/PDDA/Ag NP.
Pathway for metal ion reduction by dopamine oxidation. a) Synthetic route to acetonide-protected dopamine (Dopa*). Representative AFM and TEM images of Ag/GO-Dopa; (a) GO, (b) GO-Dopa, (c-e) Ag/GO-Dopa with inset image of Ag NPs (f) Size distribution histogram of Ag NPs. Representative TEM images of Ag/GO-Dopa under different reaction conditions. a) controlled reaction time, (b) different AgNO3 precursor concentration and (c) solution pH. a) Time-dependent UV/vis absorption spectra for the reduction of 4-NP over Ag/GO-Dopa catalyst in aqueous medium at 298 K. Reaction conditions: 1.0 mol % catalyst and 800 equiv.
Representative TEM images of (a) Au/GO-Dopa and (b) Cu/GO-Dopa with a histogram of the corresponding nanoparticle size distribution. We also observed the catalytic activities of the Ag/GO-Dopa hybrid by monitoring the reduction of nitroarenes.
Graphene oxide as a promising material for functionalization of graphene
Methods to synthesize graphene oxide
Characterization of graphene oxide
Graphene oxide-based metal NPs composites
First, active surface area for the catalytic reactions increases, leading to the increased adsorption of reactant and faster reaction rate. As a result, the catalytic potential of metal graphene oxide composite is increased than that of metal NPs. This can be supported from our precious report on the catalytic activity of Au NPs/GO composite for the reduction of nitroarenes.44 Ag NPs/GO composite has excellent catalytic activity compared to Au NPs or GO.
Mulhaupt and co-workers have synthesized Pd NPs supported on GO used for the catalyst in the Suzuki-Miyaura coupling reaction.43 The Pd NPs/GO system has better performance than Pd on activated carbon because Pd NPs/GO has a high surface area with a good accessibility. In general, metal NP/GO composite is synthesized by in situ or ex situ method.15 One of the most commonly used in situ methods is solution-based deposition in which the metal salt is mixed with graphene in solution, followed by the addition of reduction. agent for reducing the metal salt.44, 48 In this case, toxic reagents such as hydrazine, sodium borohydride (NaBH4), etc. are heated or used. This method can control the size and distribution of metal NPs differently than the in situ method, but sometimes requires functionalization with other species that can interact with the metal NPs.
However, many previously introduced methods show that the harsh condition such as high temperature and pressure or harmful reducing agents are necessary for the formation of metal NPs in common. Recently, a facile and environmental method with a mild condition for the formation of metal NPs without the reducing agent and/or stabilizer has been attracted. But no studies have yet reported on the synthesis of metal NPs and deposition on the graphene in a mild state.
Mussel-inspired molecule: Dopamine
Ability of dopamine for the reduction of metal NPs
Experiment
- Preparation of acetonide protected dopamine (Dopa)*
- Phth-dopamine
- Phth-dopamine(acetonide)
- Dopamine(acetonide) (Dopa*)
- Preparation of GO solution
- Preparation of GO-Dopa solution
- Preparation of hybrid Ag/GO-Dopa
- Catalytic reduction of 4-nitrophenol by hybrid Ag/GO-Dopa catalyst
- Characterization of catalytic activity
Graphene oxide (GO) was obtained as a brown colored stable suspension (conc. 0.50 mg ml-1), followed by centrifugation at 4000 rpm for 10 min to remove any aggregates remaining in the GO suspension. The resulting suspension was dialyzed (MWCO Spectra/Por) for 3 days to remove any residues and byproduct. For the hydrolysis of the acetonide protecting group, 0.6 mL of TFA was added to 40 mL of GO-Dopa* solution and stirred for 3 h with subsequent dialysis for another day.
The protocol of the formation of Cu and Au nanoparticles has the same procedure as hybrid Ag/GO-Dopa. The mixture was immediately transferred into a quartz cuvette to be measured by UV/vis spectroscopy. Turnover frequency is defined as the number of molecules that can be converted to product per catalytic site per unit time.
Results and Discussion
- Synthesis of Ag/GO-Dopa
- Characterization of GO-Dopa and Ag/GO-Dopa
- Catalytic performance of Ag/GO-Dopa for nitroarenes reduction
- Versatility of mussel-inspired method in the fabrication of hybrid metal NPs/GO
The spectrum of GO-Dopa is very similar to that of the GO-Dopa* due to their analogous structures. The absorbed peaks of GO-Dopa and GO-Dopa* are blue-shifted from those of the UV light absorbed by GO at 230 nm. In the GO-Dopa*, the σ→σ* transition that required higher energy (or shorter wavelength) occurred due to the alkyl group of acetonide protecting dopamine on the GO.
GO-Dopa absorbed less energy (red shift) than the GO-Dopa*, due to the σ → π* transition of hydroxyl groups of dopamine. We confirmed the morphology of Ag/GO-Dopa nanocomposite (Hybrid Ag/GO-Dopa) by atomic force microscopy (AFM) and transmission electron microscopy (TEM). The GO-Dopa sheets are thicker than GO due to the functionality of the surface with dopamine.
Ag NPs decorated not only the edges of the GO-Dopa sheets but also the basal planes, although the densities are relatively higher at the edges due to the higher distribution of surface functional groups there. We confirmed that Ag NPs accounted for about 2.4% in the Ag/GO-Dopa suspension via ICP/MS measurement. Specifically, the catalytic reaction of nitroarenes has been demonstrated using various metal NPs with carbon materials such as Au/Graphene, 44 Au, Ag or Pt/CNT composites.72 We have studied the reduction of 4-nitrophenol (4-NP) in 4-aminophenol (4-AP) with Ag/GO-Dopa composite to evaluate the catalytic activity of Ag/GO-Dopa.
From the linear relationships of ln(Ct/C0), we found that the rate constant, k, for this reaction is 0.364 min-1 and the TOF is 14.6 min-1, which is a higher value than those previously reported ( Figure 16b).45 To clearly evaluate the catalytic activity of Ag/GO-Dopa, we performed control experiments using GO and GO-Dopa. GO and GO-Dopa showed negligible catalytic activity with a reaction rate constant, k, of 0.004 min-1 and 0.003 min-1, respectively (Figure 16c). We also observed that the catalytic activity of Ag/GO-Dopa is affected by the reaction temperature (Figure 16d).
Furthermore, we investigated the catalytic activity of Ag/GO-Dopa for the reduction of other nitroarenes. We found that Ag/GO-Dopa has superior catalytic activity in the reduction of a series of nitroarenes regardless of the different types and positions of the substituents (Figure 17). We predict that the reasons for the high catalytic activity of Ag/GO-Dopa are attributed to the high surface area and increased opportunities for reduction activity through increased affinity between nitroarenes and aromatic groups of GO.
To investigate the adaptability of mussel-inspired method to form hybrid metal NPs/GO, we synthesized Cu and Au NPs on the surface of the GO-Dopa sheet by following the protocol of Ag/GO-Dopa (Figure 18 ). Au or Cu/GO-Dopa also has good colloidal stability, as demonstrated through Ag/GO-Dopa suspension.
Conclusion
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