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Reduced graphene oxide supported platinum nanocubes composites: one-pot hydrothermal synthesis and enhanced catalytic activity

This content has been downloaded from IOPscience. Please scroll down to see the full text. 2015 Nanotechnology 26 065603 (http://iopscience.iop.org/0957-4484/26/6/065603) View the table of contents for this issue, or go to the journal homepage for more Download details: IP Address: 128.227.24.141 This content was downloaded on 07/05/2017 at 18:59 Please note that terms and conditions apply.

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Nanotechnology Nanotechnology 26 (2015) 065603 (8pp)

doi:10.1088/0957-4484/26/6/065603

Reduced graphene oxide supported platinum nanocubes composites: one-pot hydrothermal synthesis and enhanced catalytic activity Fumin Li1,3, Xueqing Gao1,3, Qi Xue1, Shuni Li1, Yu Chen1 and Jong-Min Lee2 1

School of Chemistry and Chemical Engineering, School of Materials Science and Engineering, Shaanxi Normal University, Xi’an 710062, People’s Republic of China 2 School of Chemical and Biomedical Engineering, Nanyang Technological University, Singapore 637459, Singapore E-mail: [email protected] and [email protected] Received 19 November 2014, revised 12 December 2014 Accepted for publication 15 December 2014 Published 22 January 2015 Abstract

Reduced graphene oxide (rGO) supported platinum nanocubes (Pt-NCs) composites (Pt-NCs/ rGO) were synthesized successfully by a water-based co-chemical reduction method, in which polyallylamine hydrochloride acted as a multi-functional molecule for the functionalization of graphene oxide, anchorage of PtII precursor, and control of Pt crystal facets. The morphology, structure, composition, and catalytic property of Pt-NCs/rGO composites were characterized in detail by various spectroscopic techniques. Transmission electron microscopy images showed well-defined Pt-NCs with an average size of 9 nm uniformly distributed on the rGO surface. The as-prepared Pt-NCs/rGO composites had excellent colloidal stability in the aqueous solution, and exhibited superior catalytic activity towards the hydrogenation reduction of nitro groups compared to commercial Pt black. The improved catalytic activity originated from the abundant exposed Pt{100} facets of Pt-NCs, excellent dispersion of Pt-NCs on the rGO surface, and synergistic effect between Pt-NCs and rGO. Keywords: reduced graphene oxide, platinum nanocubes, polyallylamine, catalysis, hydrogenation reduction (Some figures may appear in colour only in the online journal) 1. Introduction

example, the Pt{100} facets are more active than the Pt{111} facets towards the oxygen reduction reaction in H2SO4 electrolyte [5] and ammonia oxidation reaction in the NaOH electrolyte [6]. The Pt{100} facets also show high selectivity and activity towards the hydrogenation reaction of benzene to cyclohexane [7, 9] and the reduction of NO [8]. Thus, the shape-controlled synthesis of Pt nanocrystals becomes an effective strategy for improving their activity and selectivity. Since {100}-enclosed Pt nanocubes (Pt-NCs) were prepared by polyacrylate-assisted synthesis beginning in 1996 [10], many chemical reduction methods in organic solvents, including metal carbonyls compound-assisted synthesis [11–13], the

Platinum (Pt), as one kind of important noble metal, has been highly sought after as an industrial catalyst because of its excellent activity and durability [1–3]. However, its scarcity and high cost have limited its its widespread use. It is well known that the catalytic/electrocatalytic activity and selectivity of Pt nanocrystals are closely related to their surface structure because the surface atomic arrangement affects the configuration of the absorption and bonding of the reactants [4–8]. For 3

These authors contributed equally to this work.

0957-4484/15/065603+08$33.00

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© 2015 IOP Publishing Ltd Printed in the UK

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ethylene glycol method [14], N,N-dimethylformamide-assisted synthesis [15, 16], and chromium-assisted synthesis [17], have been successfully developed for the synthesis of high-quality Pt-NCs with controllable sizes. Meanwhile, these as-synthesized Pt-NCs exhibited significantly enhanced catalytic activity towards the hydrogenation reaction of pyrrole to n-butylamine [14], and oxygen reduction reaction [11, 12, 16, 17]. Unfortunately, these established methods are still limited to expensive organometallic precursors and noxious organic solvents and require extensive operator skills. To further reduce the overall utilization of scarce Pt, one of effective strategies is to load it onto the supporting nano materials that have a high surface area, good electrical conductivity, and low cost [18–22]. The supporting nanomaterials that have a large surface area provide abundant sites for anchoring metal nanocrystals, which efficiently decreases particle size [19, 23–25]. Meanwhile, the strong synergistic interaction between Pt nanocrystals and the support nanomaterials could further enhance catalytic activity and stability [20]. Recently, increasing attention has been given to the combination of graphene and Pt nanocrystals due to the outstanding physical and chemical properties of graphene, including large surface area, robust mechanical properties, excellent conductivity, and thermal stability [26–30]. Currently, various Pt nanocrystals/graphene composites have been successfully prepared by the one-step in situ growth route [31–34] or the two-step self-assembly method [35–38]. In comparison with the two-step self-assembly route, the one step in situ chemical-reduction growth route is highly useful for the large-scale synthesis of Pt nanocrystals/graphene composites because of its simplicity and low cost. However, such an in situ growth strategy generally lacks the morphology control of Pt nanocrystals due to the strong interaction between graphene and newly formed Pt nuclei [16, 26]. To the best of our knowledge, the reports on the one-pot preparation of Pt-NCs/graphene composites in aqueous solution and the investigation of their catalytic behavior towards the hydrogenation reduction of nitro functional groups is limited to date. Herein, we demonstrate a facile, one-pot water-based approach to achieve the formation of PtNCs and simultaneous deposition on reduced graphene oxides (rGO) by using polyallylamine hydrochloride (PAH) as a shape-selective agent and linker agent, and HCHO as a reducing agent. The morphology, structure, and composition of rGO supported Pt-NC (Pt-NCs/rGO) composites are characterized by various techniques. The resultant Pt-NCs/ rGO composites exhibited enhanced catalytic activity towards the hydrogenation reduction of the nitro functional groups because of the abundant exposed Pt{100} facets of Pt-NCs and their uniform distribution on the rGO surface.

Scheme 1. Molecular structures of (a) PAH and (b) 4-NP.

Materials TECH Co., Ltd. Potassium tetrachloroplatinate(II) (K2PtCl4), 4-nitrophenol (4-NP, scheme 1(b)), sodium borohydride (NaBH4), and formaldehyde solution (HCHO, 40%) were purchased from Sinopharm Chemical Reagent Co., Ltd Commercial Pt black was purchased from Johnson Matthey Corp. 2.2. Synthesis of Pt-NCs/rGO composites

A dose of 6.5 mg GO was dispersed into a solution of 10.0 mL of 0.05 M PAH (pH 4.3) by ultrasonic treatment for 60 min. Then, 1.0 mL of 0.05 M K2PtCl4 solution was added into the PAH-GO composites solution. After adding 0.3 mL of HCHO, the mixture was transferred to a 20 mL Teflonlined, stainless-steel autoclave, and was then heated at 140 °C for 6 h. After cooling, the obtained Pt-NCs/rGO composites were collected by centrifugation/washing cycles and dried in a vacuum dryer at 60 °C for 5 h. For comparison, Pt-graphene composites without the use of PAH (named as Pt/rGO-control) and unsupported Pt-NCs without the use of GO were also synthesized under the same experimental conditions. 2.3. Catalytic reduction of 4-NP

Typically, 2 mL of 2.0 × 10−4 M 4-NP and 1.0 mL of 0.1 M NaBH4 solutions were mixed in a quartzy cuvette. Then, 50 μL of Pt-NCs/rGO aqueous solution (1.0 g L–1) were added into the quartzy cuvette at room temperature (25 °C). The catalytic reduction of 4-NP was monitored by timedependent UV–vis spectra. For comparison, Pt/rGO-control, Pt-NCs, and commercial Pt black were also used as heterogeneous catalysts for the reduction of 4-NP. 2.4. Instruments

Transmission electron microscopy (TEM, EOL JEM-2100F) and x-ray powder diffraction (XRD, Model D/max-rC) were used to investigate product morphology and structure. The technology used to investigate bulk composition and surface composition of products includes the following: energy-dispersive spectrum (EDX), inductively coupled plasma atomic emission spectrum (ICP-AES), x-ray photoelectron spectroscopy (XPS, Thermo VG Scientific ESCALAB 250), zeta potential analyzer (Malvern Zetasizer Nano ZS90), Fourier transform infrared (FT-IR, Nicolet 520 SXFTIR) spectrometer. Ultraviolet–visible spectroscopy (UV–vis, UV3600) was used to investigate the catalytic activity of catalysts.

2. Experimental 2.1. Reagents and chemicals

PAH (MW: 150 000, scheme 1(a)) was supplied from Nitto Boseki Co., Ltd GO was purchased from Nanjing XFNANO 2

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Scheme 2. Schematic synthetic route of Pt-NCs/rGO composites.

−1

Figure 1. (A) FT-IR spectra of (a) GO, (b) PAH, and (c) PAH-GO composites. (B) Digital photographs of (a) 0.4 mg mL

3.5 mg mL

−1

GO and (b)

PAH-GO composites dispersed in water for 30 min.

3. Results and discussion

and alkoxy C–O stretches, respectively [40, 41]. For PAH, the absorption bands at 3450 and 1607 cm−1 correspond to stretching and in-plane bending vibrations of the N–H, while absorption bands at 2928 and 1500 cm–1 correspond to C–H stretching and bending of the alkyl group [40]. After treating GO with PAH via sonication, the changes in the C=O stretching, as well as the appearance of a small peak at 2928 cm–1 for the C–H stretch from PAH are observed. These FT-IR results verify the successful production of the PAHGO composites. Due to the bulky molecule size and excellent hydrophilic property of PAH, PAH functionalization of GO facilitates the solubility of GO. As expected, the 3.5 mg mL−1 PAH-GO composites are soluble in water [figure 1(B-b)]. No precipitation is observed even after three months (data not shown). For comparison, the dispersion of the GO in water (0.4 mg mL−1) is difficult [figure 1(B-a)]. Thus, the good solubility of PAH-GO composites facilitates the immobilization of Pt nanoparticles on the GO surface. The crystal structures of GO and Pt-NCs/rGO composites were investigated by XRD. A strong {002} peak at ∼11° is detected in the XRD pattern of GO [figure 2(A-a)],

3.1. Synthesis of Pt-NCs/rGO composites

The overall synthetic route of Pt-NCs/rGO composites is illustrated in scheme 2: (i) The oxygen-containing functional groups on the GO surface cause it to possess a number of negatively charged sites, which attracts positively charged PAH with a large number of primary amine groups through electrostatic interaction (step 1); (ii) PAH on PAH-GO composites effectively anchor PtII species through the strong coordination interaction between PAH and PtII species [39] (step 2); (iii) Both PtII species and GO are simultaneously reduced by HCHO to generate Pt-NCs/rGO composites under hydrothermal conditions (step 3). 3.2. Characterization of Pt-NCs/rGO composites

The interaction between PAH and GO was first confirmed by FT-IR [figure 1(A)]. For GO, the absorption bands at 1726, 1620, 1382, 1220, and 1055 cm−1 correspond to the stretching vibrations of C=O, aromatic C=C, carboxyl C–O, ether C–O, 3

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Figure 2. (A) XRD patterns of (a) GO and (b) Pt-NCs/rGO composites. (B) XPS spectra of (a) GO and (b) Pt-NCs/rGO composites in the C1s region. (C) EDX spectrum of Pt-NCs/rGO composites. (D) XPS spectrum of Pt-NCs/rGO composites in the Pt 4f region.

rGO surface [figure 2(D)]. The Pt4f signal of Pt-NCs/rGO composites is deconvoluted into two components (4f5/2 and 4f7/2). The first doublet Pt 4f peaks at 71.65 and 75.00 eV are assigned to metallic Pt whereas the second doublet Pt 4f peaks at 72.48 and 77.53 eV are assigned to Pt oxide. By measuring the relative peak areas, the percentage of Pt0 species is calculated to be 90.1%. The appearance of Pt oxide could arise from the partial oxidation of the Pt-NCs surface being exposed in air. The morphology and structural features of Pt-NCs/rGO composites were further investigated by TEM. The lowresolution TEM image clearly exhibits that many small Pt nanoparticles with well-monodispersity are successfully attached on the rGO surface [figure 3(A)]. The magnified TEM images show that monodisperse and cube-like Pt nanocrystals are the dominant products with a typical yield of 80% [figures 3(B)–(C)]. The average side length of the PtNCs is ca. 9.0 nm, which is consistent with the XRD result. The high-resolution TEM (HR-TEM) image shows wellresolved lattice fringes [figure 3(D)], indicating Pt-NCs have a single-crystalline structure. The interplanar spacing of the adjacent fringe is 0.198 nm [Insert in figure 3(D)], corresponding to the {100} lattice spacing of fcc Pt (0.196 nm). The corresponding fast Fourier transform (FFT) pattern shows diffraction spots with four-fold rotational symmetry [Insert in figure 3(D)], further confirming the Pt-NCs have a singlecrystalline nature, with a {100} lattice facet as the basal surface.

corresponding to its AB stacked structure. For Pt-NCs/rGO composites, this {002} peak disappears and a new, weak peak at ∼26° appears [figure 2(A-b)], indicating GO is reduced by HCHO [42, 43]. Meanwhile, the characteristic diffraction peaks of Pt{111}, Pt{200}, Pt{220}, and Pt{311} crystal facets are observed clearly, indicating Pt-NCs/rGO composites possess a face-centered cubic (fcc) crystal structure (JCPDS standard 04-0802 Pt). The intensity ratio between {200} and {111} peaks (1.00:1.84) is higher than the standard value (1.00:2.16) of JCPDS standard 04-0802 Pt, indicating that the Pt nanocrystals on rGO are mainly covered with {100} facets [16]. The average crystallite size of Pt-NCs/rGO composites is calculated to be 7.8 nm using the Scherrer equation from the peak {220}. The reduction of GO was further confirmed by XPS. The GO mainly shows a C–O peak at 286.6 eV and a C–C peak at 284.6 eV [figure 2(A-a)]. Compared with GO, Pt-NCs/rGO composites exhibit a decrease in C–O components and an increase in sp2-hybridized C–C components [figure 2(A-b)], indicating the effective removal of residual oxygen-containing groups in GO via the HCHO reduction. Additionally, a new component at 285.7 eV corresponding to the C–N bond is detected [figures 2(A)–(B)], implying the existence of PAH, as well. The composition of Pt-NCs/rGO composites was investigated by ICP-AES. ICP-AES analysis shows Pt loading in Pt-NCs/rGO composites is ca. 55.4 wt%, consistent with value (54.6 wt%) of EDX analysis [figure 2(C)]. XPS was used to analyze the surface composition of Pt-NCs on the 4

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Figure 3. Morphological and structural characterization of Pt-NCs/rGO: (A) low-resolution TEM; (B)–(C) magnified TEM; (D) HR-TEM

image of the single Pt-NCs. The inserts in (D) show the FFT pattern and magnified HR-TEM image of the single Pt-NCs.

Figure 4. TEM images of (A) the product prepared under the same condition as in figure 3 except for the exclusion of PAH and (B) the

product prepared under the same condition as in figure 3 except for the exclusion of GO.

For a practically catalytic reaction in a liquid phase system, the heterogeneous catalysts could offer advantages over their homogeneous counterparts, such as easy removal and recyclability of catalysts [44]. For Pt-NCs/rGO composites, the appearance of an N1s peak indicates the existence of PAH [figure 5(A)]. The binding of PAH on the rGO surface was visualized by EDX element mapping. Abundant C, N, and Pt elements were detected and all distributed evenly on the same region [figure 5(B)], indicating the uniform distribution of PAH on the rGO surface. The zeta potential of PtNCs/rGO composites was measured to be +49 ± 5 mV at pH 7.0. Thus, the strong electrostatic repulsion between composites and the excellent hydrophilic property of PAH molecules endow Pt-NCs/rGO composites with good watersolubility. As observed, no precipitation is observed after three months [figure 5(C)]. Thus, the as-prepared Pt-NCs/rGO composites could be highly attractive for a water-based heterogeneous catalysis system.

The TEM images show that Pt nanocrystals in the Pt/ rGO-control, without the use of PAH, are highly agglomerated and have an average size of around 40 nm [figure 4(A)]. This indicates that PAH acts as a stabilizing agent and facetselective agent to control the dispersivity and shape of Pt nanocrystals during the synthesis of the Pt-NCs/rGO composites. The bulky molecule size and excellent hydrophilic property of PAH molecules can effectively prevent the aggregation of Pt nanocrystals, whereas preferential adsorption of PAH on Pt{100} facets leads to the formation of PtNCs with six Pt{100} facets. In a controlled experiment without GO, well-defined 15 nm Pt-NCs are obtained [figure 4(B)], bigger than particle size (9 nm) of Pt-NCs/rGO composites. Since the particle size of nanocrystals closely relates to the amount of crystal nucleation [13, 39], the abundant nucleation sites afforded by GO with an ultra large surface area effectively decrease the particle size of Pt-NCs. 5

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Figure 5. (A) XPS spectrum of Pt-NCs/rGO composites in the N1s region. (B) Representative large-area SEM image of Pt-NCs/rGO

composites and corresponding EDX element mapping. (C) Digital photographs of 1.0 mg mL–1 Pt-NCs/rGO composites after (a) 2 min and (b) 3 months of storage at ambient temperature.

pseudo-first-order kinetics [inserts in figures 6(B)−(D)]. The reaction rate constant κ are calculated to be 0.0545, 0.0467, and 0.0364 min−1 for the reactions catalyzed by Pt-NCs/rGO composites, Pt-NCs, and commercial Pt black, respectively. The catalytic activity of Pt-NCs/rGO composites is superior to that of Pt-NCs and commercial Pt black under the same reaction conditions, indicating that Pt-NCs/rGO composites have enhanced catalytic activity. The improved catalytic activity of Pt-NCs/rGO composites might caused by the following reasons: (i) Pt-NCs/rGO composites have good water-solubility, inducing an efficient contact between Pt-NCs and 4-NP; (ii) rGO provides abundant adsorption sites due to π−π stacking between 4-NP and rGO, resulting in a high concentration of 4-NP near to Pt-NCs on rGO; (iii) Pt-NCs/rGO composites have a smaller particle size than unsupported Pt-NCs, providing more Pt active sites because of the bigger surface area; (iv) The Pt{100} facet is more reactive than the Pt{111} facet towards the hydrogenation reduction of nitro functional groups [49, 50], further contributing to enhanced catalytic activity. Additionally, recycling Pt-NCs/rGO composites was also investigated. After undergoing five catalysis cycles, no obvious loss of activity was observed, indicating the composites’ good durability.

3.3. Catalytic performance of Pt-NCs/rGO composites

The water-soluble Pt-NCs/rGO composites with small, uniformly distributed Pt-NCs are expected to offer excellent catalytic activity. Recently, the catalytic conversion of 4-NP to 4-aminophenol (4-AP) has been widely used as a model reaction to evaluate the catalytic activity of noble metal nanomaterials [45– 48]. During the reaction, the nucleophile BH−4 donates electrons to noble metal nanoparticles, whereas the electrophile 4-NP captures electrons from the noble metal nanoparticles. Therefore, noble metal nanoparticles can serve as catalytic electron relays for the 4-NP reduction in a NaBH4 solution. The initially light yellow 4-NP cannot change to colourless 4-AP in the absence of the catalyst even after 4 h (data not shown), while 4-NP is quantitatively converted to 4-AP within 2 h after the addition of the catalysts. The absorption peaks of 4-NP and 4-AP are centred at 311 and 400 nm, respectively. Thus, the reduction process of 4-NP can be monitored using UV–vis. For comparison, the catalytic activities of the rGO, unsupported Pt-NCs, and commercial Pt black were also investigated under the same experimental conditions. After adding rGO into the reaction solution, the absorbance bands are almost unchanged after 4 h [figure 6(A)], indicating the rGO has no catalytic activity for the A-NP reduction. In contrast, the peak of 4-NP at 400 nm decreases rapidly with time upon the addition of Pt-NCs/rGO composites, Pt-NCs, and commercial Pt black [figures 6(B)−(D)]. The reduction of 4-NP to 4-AP is finished within 56, 72, and 80 min by using Pt-NCs/rGO composites, Pt-NCs, and commercial Pt black as catalysts, respectively. The linear plots of ln (Ct/C0) versus time t indicate that the reactions are controlled by

4. Conclusion We developed a facile co-chemical approach to efficiently synthesize Pt-NCs/rGO composites with high shape selectivity. During the synthesis, PAH mainly acted as a stabilizing 6

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Figure 6. UV–vis spectra for successive reduction of of 4-NP with NaBH4 using (A) rGO, (B) Pt-NCs/rGO composites, (C) unsupported PtNCs, and (D) commercial Pt black as catalysts at 8 min intervals; Inserts in (B)–(D): the relationship between ln(Ct/C0) and reaction time (t), wherein the ratio of the 4-NP concentration (Ct at time t) to its initial value C0 was directly given by the relative intensity of the respective absorbance At/A0.

References

agent, shape-selective agent, and linker agent. Due to the strong coordination ability of PAH for various metal ions, including IrIII, PdII, CuII, NiII, CoII, and AgI, the proposed approach could be considered as a very general and powerful method for producing various rGO-supported metal nanoparticles composites. Meanwhile, Pt-NCs/rGO composites showed enhanced catalytic activity for 4-NP reduction because of complete Pt{100} facets, excellent water-solubility, and specific synergistic effects between Pt-NCs and rGO. Due to their high catalytic activity, the Pt-NCs/rGO composites might have wide potential applications in water-based heterogeneous catalysis.

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Acknowledgments This work was supported by the National Natural Science Foundation of China (21473111 and 21301114), Natural Science Foundation of Shaanxi Province (2013JQ2009), Fundamental Research Funds for the Central Universities (GK201402016), Starting Funds of Shaanxi Normal University, and the Academic Research Fund of the Ministry of Education in Singapore (RGT27/13). We thank Prof. Tianhong Lu and Prof. Yawen Tang at Nanjing normal university for their assistance in the sample characterization. 7

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Reduced graphene oxide supported platinum nanocubes composites: one-pot hydrothermal synthesis and enhanced catalytic activity.

Reduced graphene oxide (rGO) supported platinum nanocubes (Pt-NCs) composites (Pt-NCs/rGO) were synthesized successfully by a water-based co-chemical ...
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