Synthesis of Pt nanoparticle-loaded 1-aminopyrene functionalized reduced graphene oxide and its excellent electrocatalysis

Abstract

Pt nanoparticle (NP)-loaded 1-aminopyrene functionalized reduced graphene oxide composites (Pt/1-AP–rGO) were synthesized by a simple polyol process. The morphology and structure of the resulting composites were characterized by transmission electron microscopy, Raman spectroscopy and X-ray diffraction. In comparison with Pt/–rGO, the resulting Pt/1-AP–rGO exhibited much better dispersibility. Higher dispersion and smaller size of Pt NPs was also observed on 1-AP functionalized rGO. As a result, Pt/1-AP–rGO showed higher catalytic activity, better anti-poisoning ability and stability for methanol oxidation. Meanwhile, it also displayed better electrocatalytic performance toward H₂O₂. The strategy presented here offers an efficient way for the preparation of high-performance electrocatalysts, which will find promising applications in biosensors and fuel cells.

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Introduction

Due to their unique structure and composition, nanoparticles have shown excellent optical, electrical and catalytic properties in various fields.^1–5 For instance, Pt nanoparticle based catalysts are the most used electrocatalysts in fuel cells.^6–9 Previous investigations demonstrate that effectively controlling the size and dispersion of Pt NPs is a prerequisite for high catalytic performance. What's more, loading of Pt NPs on the surface of catalyst support is another way to improve the catalytic performance of Pt NPs.^10–12 In fact, finding suitable catalyst support is crucially important for the construction of biosensors and fuel cells.¹³ The desirable catalyst support should possess large specific surface area for the efficient dispersion of catalyst NPs, high electrical conductivity for promotion the fast electron transfer and excellent chemical stability for maintaining the stable catalyst structure.¹⁴

Recently, due to various intriguing properties, graphene has emerged as one of the most promising catalyst support.^15–17 In particular, graphene-supported Pt and PtM (M = Pd, Fe etc.) NPs composites have been extensively explored in the field of fuel cells and biosensors.^18–21 Various state-of-the-art approaches, such as in situ growth,²² self-assembly²³ and one-pot method,²⁴ have been developed for the preparation of graphene-supported NPs composites. Unfortunately, the easy aggregation of the graphene sheets leads to the decrease of the effective surface area and eventually detrimental to the electrochemical applications.²⁵ To address this issue, an effective strategy is by covalent or non-covalent attaching certain molecule onto graphene surface.²⁶ Even more important, the attached molecules with suitable groups, such as amino group, could also serve as nucleating sites of the NPs.²⁷ Non-covalent functionalization of graphene with aromatic molecule takes the advantages of simple decoration process and well-maintained intrinsic electronic properties of graphene. Pyrenecarboxylic acid, perylene tetracarboxylic acid and pyrenesulfonic acid sodium salt-functionalized graphene showed improved dispersibility and excellent electrochemical performance.^28–31

1-aminopyrene (1-AP) could be a suitable molecule for the functionalization of graphene which contains both benzene ring structure and amino group.^32,33 In this work, we utilized 1-AP as the non-covalent functional reagent to prepare Pt NPs-loaded reduced graphene oxide (rGO) composites. Results demonstrate that Pt NPs supported on 1-AP functionalized rGO showed better dispersity and smaller size than that on rGO. The resulting composites exhibited high electrocatalytic activities towards methanol oxidation and H₂O₂ reduction.

Experimental

Materials and apparatus

Graphite flake, Platinum tetrachloride (PtCl₄) and 1-aminopyrene (1-AP) were purchased from Aldrich. Methanol, ethylene glycol (EG), dimethyl formamide (DMF) and H₂O₂ (30%) were obtained from Beijing Co. (China). Chloroplatinic acid (H₂PtCl₆) solution was obtained from the PtCl₄ dissolved in hydrochloric acid. All chemicals were used as received.

UV-Vis analysis was performed with a UV-3000PC spectrophotometer. Particle size and morphology were obtained on transmission electron microscope (TEM, HITACHI H-8100 EM). X-ray diffraction (XRD) patterns were acquired using a D8 ADVANCE X-ray diffractometer (Bruker AXS, Germany) with Cu Kα radiation (λ = 1.54056 Å). Raman spectra were recorded on a Renishaw Raman microscope (model RM2000) with laser excitation wavelength of 514.5 nm. Pt-loading in the composites was determined by inductively coupled plasma optical emission spectroscopy (ICP-OES, Thermo Fisher).

Synthesis of graphite oxide

Graphite oxide was synthesized according to modified Hummer's method.³⁴ Briefly, graphite flakes (1 g) was added into a mixture of concentrated H₂SO₄ (15 mL), K₂S₂O₈ (5 g), and P₂O₅ (5 g). Then, the solution was kept at 80 °C for 6 hours. After cooling to room temperature, the mixture was diluted with double distilled water, filtered, and washed until the pH of the rinse water was about 6.5. Thereafter, the as-treated graphite was added to a mixture of cold (0 °C) concentrated H₂SO₄ (23 mL) and NaNO₃ (0.5 g). Subsequently, 3 g of KMnO₄ was added slowly with violent agitation. Then the solution was stirred at 40 °C for 2 h, the reaction was kept for 15 min after adding double distilled water (46 mL). Finally, 140 mL water and 10 mL 30% H₂O₂ was added to terminate the reaction, followed by washing and filtration.

Synthesis of Pt/1-AP–rGO

As a typical procedure to synthesize the Pt/1-AP–rGO, 5 mg graphite oxide was dispersed in 10 mL ethanol by ultrasonic treatment for 60 min (200 W). Subsequently, 5 mg 1-AP was added with stirring at room temperature for 12 h. The obtained solution was centrifuged and dried at 60 °C and denoted as 1-AP–GO. Then, 5 mg 1-AP–GO and 65 μL H₂PtCl₆ solution (100 mM) were dispersed in 10 mL ethylene glycol solution. The pH value of the solution was adjusted to 6.0 with 1.0 M KOH aqueous solution. Thereafter, the above dispersion was transferred into a 50 mL three-necked bottle and refluxed at 130 °C for 6 h. Finally, the blended solution was washed with ethanol and dried in vacuum at 80 °C overnight, obtaining Pt/1-AP–rGO. Similarly, Pt/rGO composite was prepared with the same procedure without the use of 1-AP.

Electrode modification and electrochemical measurements

Prior to the modification, glassy carbon electrode (GCE) was polished with alumina slurry and cleaned by ultrasonic. 2 μL well-dispersed Pt/1-AP–rGO suspensions (1.0 mg mL⁻¹) was dropped onto GCE and dried at room temperature. Then 1 μL 0.5% nafion solution was covered on the modified electrode. For comparison, Pt/rGO modified GCE (Pt/rGO–GCE) was prepared with the same procedure. Electrochemical measurements were performed on a CHI 832 electrochemical workstation (China). Ag/AgCl/(sat. KCl) and platinum wire were used as reference and auxiliary electrodes, respectively.

Results and discussion

The Pt/1-AP–rGO composite was synthesized via a simple polyol process (Scheme 1). Firstly, 1-AP was non-covalently assembled onto GO surface via π–π interaction between the pyrenyl groups of 1-AP and graphene sheets. The amino groups of 1-AP became weakly positive charged when the solution was adjusted to acid condition (pH = 6.0). Such an acid condition fairly facilitated the self-assembly of negatively charged PtCl₆²⁻, forming a uniform dispersion of Pt precursor on the surface of GO. Finally, 1-AP–GO and PtCl₆²⁻ were reduced simultaneously by ethylene glycol, obtaining the highly dispersed Pt nanoparticle-loaded rGO. The inset image of Fig. 1A shows the photographs of Pt/rGO and Pt/1-AP–rGO dispersed in ethylene glycol. Obviously, Pt/1-AP–rGO can be well dispersed in ethylene glycol, while Pt/rGO forms precipitate and floating particles. Thus, the presence of 1-AP could effectively avoid the aggregation of rGO.

Scheme 1 Schematic illustration of the synthetic process of Pt/1-AP–rGO.

Fig. 1 UV-visible spectra of (a) 1-AP, (b) Pt/1-AP–rGO and (c) Pt/rGO in ethanol. Inset: photographs of (1) Pt/rGO and (2) Pt/1-AP–rGO in ethylene glycol. Raman spectra of GO, Pt/rGO and Pt/1-AP–rGO.

UV/visible spectra were used to confirm the successful non-covalent binding of 1-AP. As shown in Fig. 1A, Pt/rGO shows a typical featureless spectrum (curve c). The two additional peaks around 355 and 281 nm observed in the spectrum of Pt/1-AP–rGO could be ascribed to the characteristic peaks of 1-AP at 360 and 285 nm (curve a), suggesting that rGO has been successfully functionalized by 1-AP.³²

Raman spectroscopy is an effective method to distinguish the ordered and disordered structure of carbon in carbonaceous materials. The Raman spectrum of GO, Pt/rGO and Pt/1-AP–rGO are shown in Fig. 1B. Two prominent scattering peaks located at 1350 and 1595 cm⁻¹ can be assigned to the E_2g phonon of sp² carbon atoms (D-band) and breathing mode of κ-point phonons of A_1g symmetry (G-band), respectively. The intensity ratio of D to G band (I_D/I_G) could be used to evaluate the degree of modification or defects of rGO. In this work, the I_D/I_G value of GO, Pt–rGO and Pt/1-AP–rGO is 0.97, 1.13 and 1.06. The increased I_D/I_G value when GO was reduced to rGO reveals a decrease in the average size of sp² domains in rGO compared with GO.³⁵ However, the I_D/I_G values decrease slightly for 1-AP–rGO compared with rGO, which could be attributed to the coverage of the defect sites on rGO by 1-AP.³³

Fig. 2A and B displays the TEM images of Pt/rGO and Pt/1-AP–rGO. Obviously, Pt NPs on 1-AP functioned rGO exhibit a better dispersion than that on pristine rGO. In fact, Pt/1-AP–rGO possessed smaller Pt size (3.3 nm) than Pt/rGO (4.8 nm) with a narrower size distribution. This result demonstrates that the presence of 1-AP on GO may provide more effective active sites for the nucleation of Pt NPs and benefit the achievement of Pt NPs with small size. Moreover, the aromatic group in 1-AP could also prevent the aggregation of rGO, leading to larger surface area and more active sites for the loading of Pt NPs.

Fig. 2 TEM images of Pt/rGO (A) and Pt/1-AP–rGO (B). Insets are the corresponding Pt NPs size distribution.

XRD patterns of GO, Pt/rGO and Pt/1-AP–rGO are shown in Fig. 3. The sharp diffraction peak at 2θ = 10.8° corresponds to the interlayer spacing of 0.87 nm for GO. However, a new wide diffraction peak appeared at 2θ = 25.1° for Pt/rGO and Pt/1-AP–rGO, confirming the successful reduction of GO to rGO. The three diffraction peaks located at 2θ = 39.5°, 45.8° and 67.4° could be indexed to the (111), (200) and (220) planes of face-centered cubic Pt crystals. Moreover, the broader diffraction peaks of Pt/1-AP–rGO suggest smaller size of Pt NPs on 1-AP–rGO than that on rGO, which is consistent with the TEM results.

Fig. 3 XRD patterns of Pt/rGO and Pt/1-AP–rGO. Inset image is the XRD pattern of GO.

Fig. 4A shows the cyclic voltammograms (CVs) of Pt/1-AP–rGO–GCE and Pt/rGO–GCE in 0.5 M H₂SO₄. Both electrodes exhibit the typical characteristics of hydrogen adsorption and desorption behavior on platinum, confirming the successful loading of Pt NPs on rGO. The electrochemical active surface area (ECSA) is an important data to evaluate the number of the available catalytic active sites, and are estimated from hydrogen desorption peaks from −0.2 to 0.2 V. In this work, ECSA for Pt/1-AP–rGO and Pt/rGO are calculated to be 35.1 and 25.2 m² g⁻¹, respectively. The increased ECSA of Pt/1-AP–rGO compared with Pt/rGO may originate from the smaller size of Pt NPs. We suppose that the improved dispersibility of Pt/1-AP–rGO also contribute to the increased ECSA.

Fig. 4 (A) CVs of Pt/rGO (black line) and Pt/1-AP–rGO (red line) modified GCE in 0.5 M H₂SO₄; (B) CVs and the modified electrodes in 1.0 M KOH containing 1.0 M CH₃OH. Scan rate: 50 mV s⁻¹.

The electrocatalytic activity of the as-prepared Pt/1-AP–rGO was firstly evaluated by investigating for methanol oxidation. Fig. 4B shows the typical CVs of methanol oxidation at Pt/1-AP–rGO–GCE and Pt/rGO–GCE, respectively. Both electrode present the typical CVs of methanol oxidation in alkaline condition, with a forward anodic peak current density (j_f) which is attributed to the oxidation of methanol to CO₂ and Pt adsorbed carbonaceous intermediates, as well as a back anodic peak current (j_b) originating from the additional oxidation of carbonaceous intermediates species to CO₂. The CVs of Pt/1-AP–rGO displays a j_f of 814 mA mg⁻¹ at −0.208 V in the forward scan, which is 2.1-fold higher than that of Pt/rGO (387 mA mg⁻¹), indicating a higher electrocatalytic activity of Pt/1-AP–rGO. The ratio of the j_f to j_b (j_f/j_b), is an important index of the catalyst tolerance to poisoning species generated during methanol oxidation. The calculated j_f/j_b of Pt/1-AP–rGO (3.39) is evidently larger than that of Pt/rGO (2.74), suggesting better tolerance to poisoning species. Furthermore, the j_f/j_b value for methanol oxidation on Pt/1-AP–rGO is also higher than that on Pt–GCFM³⁶ and Pt NPs@POM/CNT.³⁷ The long-time stability of Pt/1-AP–rGO towards methanol oxidation was examined by chronoamperometry at −0.2 V (not shown here). The results show that the current decreased rapidly at the first few minutes and then reached a steady state current. The current density of Pt/1-AP–rGO remains 37.1%, while 25.1% for Pt/rGO after 7200 s continuous testing. The results above demonstrate that Pt/1-AP–rGO possesses superior electrocatalytic activity, anti-poisoning ability and good stability for methanol oxidation in comparison with Pt/rGO.

We further studied the electrocatalytic activity of Pt/1-AP–rGO towards H₂O₂ reduction. Fig. 5A shows the CVs of Pt/1-AP–rGO–GCE and Pt/rGO–GCE in PBS (pH = 7.0) with 5 mM H₂O₂. Both electrodes exhibit remarkable reduction peak at about 0 V, while the reduction peak current at Pt/1-AP–rGO–GCE is about 2-fold as that at Pt/rGO–GCE. I–t curves of Pt/1-AP–rGO–GCE and Pt/rGO–GCE were obtained by successive injection of H₂O₂ at the potential of −0.1 V as shown in Fig. 5B. Pt/1-AP–rGO–GCE displays fast response to H₂O₂, and could reach 95% of the steady-state current in 2 s. The corresponding calibration curves (inset of Fig. 5B) reveal that Pt/1-AP–rGO exhibits higher sensitivity (3.47 μA mM⁻¹) than Pt/rGO (1.83 μA mM⁻¹). Although the Pt/1-AP–rGO–GCE does not exhibit wide linear range toward H₂O₂ as that for biosensors based on spherical porous Pd nanoparticle assemblies³⁸ and Au–graphene–HRP–chitosan biocomposites,³⁹ it shows relatively fast response to H₂O₂.

Fig. 5 (A) CVs of modified electrode in 0.2 mM PBS (pH = 7.0) with 5 mM H₂O₂ at a scan rate of 50 mV s⁻¹ (B) I–t curves of the modified electrode for successive injection of 1 mM H₂O₂ at −0.1 V, inset: calibration curves between amperometric current and concentration of H₂O₂ (black line: Pt/rGO–GCE, red line: Pt/1-AP–rGO–GCE).

The enhanced electrocatalytic performance can be ascribed to following factors: the larger ECSA of Pt/1-AP–rGO provide more active sites for the reduction of H₂O₂; Pt/1-AP–rGO can easily form a uniform film on the GCE surface, which could facilitate the diffusion of H₂O₂ and increase the response sensitivity, while Pt/rGO shows inhomogeneous aggregations on the GCE surface.

Conclusions

We developed a simple but efficient polyol process for the preparation of Pt-loaded 1-AP–rGO composites. Characterizations demonstrate that Pt NPs exhibited smaller size and better dispersion on the 1-AP functioned rGO in comparison with that on rGO. The as-prepared Pt/1-AP–rGO exhibited higher electrocatalytic activities towards methanol oxidation and H₂O₂ reduction than Pt/rGO. Our work provides a novel nanomaterial as high-performance platform for fuel cells and biosensor. This facile approach could be extended to the preparation of other NPs-loaded rGO composites.

Acknowledgements

We are grateful for the financial support from the National Natural Science Foundation of China (no. 21222505).

Notes and references

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