Nanocomposites
In-Situ Confined Growth of Monodisperse Pt Nanoparticle@Graphene Nanobox Composites as Electrocatalytic Nanoreactors Yingying Lv, Yin Fang, Zhangxiong Wu, Xufang Qian, Yanfang Song, Renchao Che, Abdullah M. Asiri, Yongyao Xia, Bo Tu, and Dongyuan Zhao*
Monodisperse Pt nanoparticles (NPs) studded in a three-dimensional (3D) graphene nanobox are successfully synthesized through a simple in-situ confined growth route for the first time. The nano-zeolite A was used as a 3D substrate for in-situ growth of tri-layered graphenes on the crystal-surfaces, meanwhile, the inner micropores of which can also be utilized for the confined growth of Pt nanoparticles. The graphene sheets are curved on the edges to form a 3D hollow box morphology, where the monodisperse Pt nanoparticles are homogeneously studded on the inner surfaces. Moreover, the Pt content can be regulated from ∼8 to 50 wt%, and the particle size can be tuned from 2–5 nm by varying the pristine Pt-ion loading amount and CVD temperature. The Pt NP@graphene nanoboxes possess not only large pore volumes to effectively accommodate large amounts of oxygen, but also supply excellent electrical conductivity for the fast transfer of electrons (∼3.96 e−), resulting in a high efficiency (175 mA/mg Pt) and long-term stability (above 1000 cycles) for the oxygen reduction reaction.
1. Introduction As an environmentally benign source of electrical energy, fuel cells attract a great deal of research activities because of no emission of any hazardous molecules and the high Dr. Y. Y. Lv, Dr. Y. Fang, Dr. X. F. Qian, Y. F. Song, Prof. R. C. Che, Prof. Y. Y. Xia, Prof. B. Tu, Prof. D. Y. Zhao Department of Chemistry Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials Laboratory of Advanced Materials Fudan University Shanghai 200433, P. R. China E-mail: [email protected] Dr. Z. X. Wu, Prof. D. Y. Zhao Department of Chemical Engineering Monash University Clayton, VIC 3800, Australia Prof. A. M. Asiri Chemistry Department and The Center of Excellence for Advanced Materials Research King Abdulaziz University P.O. Box 80203 Jeddah 21589, Saudi Arabia DOI: 10.1002/smll.201402289 small 2014, DOI: 10.1002/smll.201402289
density of specific energy comparable to that of combustion engines.[1] Platinum (Pt) is the key catalyst for fuel cells, but much expensive. Nanosized Pt-based catalysts[2–8] with a high surface area are effective for catalytic activity and utilization efficiency. For the synthesis of Pt nanoparticles (NPs), complex procedures and expensive additional protective agents are usually needed.[4–7] Further annealing process at a high temperature can improve the crystallization degree of Pt nanoparticles, however, the aggregation of which cannot be avoided. Currently, the achievable size of Pt-catalysts is usually in the range of five to tens of nanometers,[4–7] although smaller particle size is preferred in catalysis. It is still quite challenging to synthesize monodisperse Pt NPs with a high crystallization degree and uniform particle size of ∼ 2 nm. For a typical catalyst of fuel cells, a carbon support is essential for the dispersion of Pt catalysts to improve electrical conductivity, which also helps to reduce the Pt usage in catalysts. Various carbon materials, such as carbon black, nanofibers and nanotubes, mesoporous carbons, have been explored as catalyst supports,[9–14] because their nanoporous structure prevents aggregation of active materials.[15–18] However, the nanoporous structure of the above carbon supports contradictorily results in poor conductivity, and the metal loading amount of the supported Pt catalysts is usually very
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low. In another aspect, graphene, a 2-D carbon material with graphic wall and excellent electric conductivity, is considered as an ideal carbon support for fuel cells.[19–23] Graphene can be prepared via exfoliation of graphite[24,25] or direct growth on substrates.[26,27] 3D graphene with nanoporous structure can effectively provide large pore volume for accommodation of catalysts and reactants, however, which is usually obtained through a complex templating method by using mono-layered graphene as precursor.[28,29] Generally, Pt-loading amount of commercial Pt/C catalysts is between 20 and 60 wt%.[8] The lower the loading is, the thicker the electrode catalyst layer has to be made for the same Pt usage on the electrode. In that case, mass transport of reactants and products in the electrode is limited. Unfortunately, the preparation of Pt/C catalysts with a high Pt-loading becomes much more difficult due to the agglomeration of Pt nanoparticles, which always makes it with a large size and wide size distribution.[8,9] Thus, synthesis of highly dispersed and crystallized Pt nanoparticles with uniform size through a simple method is highly desired but still remains a great challenge, especially for high loading. In another aspect, the degradation of fuel cell performance, which is mainly due to the metal nanoparticles dissolution/aggregation/Oswald ripening, cannot be avoided if the active metal nanoparticles are exposed in outer environment directly.[2] Hence, hybrid nanocomposites, if in which Pt nanoparticles are protected by carbon shells, excellent catalytic effect can be prospected. Rattle-type nanostructure is one of such typical hybrid nanostructure,[30–34] which has been demonstrated to be effective in long-term stability for electrochemical application. However, the large-size of the metal nanoparticles (about 20−300 nm) and the thick carbon walls, make poor effectiveness of these materials for electro-catalytic applications.[35,36] Hence, to design a simple novel hybrid nanostructure containing monodisperse Pt nanoparticles with a ultra-small particle size and three-dimensional (3D) carbon support with ultra-thin walls should be essential for highly stable fuel cells. In this work, for the first time, a novel in-situ confined growth route has been developed for constructing Pt NP@ graphene nanobox composites. Using methane as a carbon source and nano-zeolite as a 3D substrate, under a suitable CVD process, the acidity on the outer surfaces of the nanozeolite cube crystals leads to the growth of multi-layered graphenes to form a unique 3D nanostructure. Meanwhile, the Pt-ions adsorbed in the inner micropores of zeolites, can be reduced to Pt nanoparticles with ultra-fine and ultra-small size. Ultra-fine and ultra-small sized Pt NPs are homogeneously confined in the 3D graphene nanoboxes. Such unique nanocomposites can be utilized as an excellent catalytic nanoreactor for oxygen reduction reaction.
2. Results and Discussion 2.1. Graphene Nanobox The zeolite A nanocrystal substrates have a uniform and monodispersed cubic morphology and an average particle
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size of 120 nm (Figure 1A). Through a simple methane CVD process at 900 °C, graphene sheets can uniformly be grown around the each crystal-surfaces of the zeolite substrates. After removal of the zeolite substrates by HCl and HF, 3D graphene nanoboxes (GB) can be successfully obtained (Figure 1B). The nanoboxes have a uniform particle size of ∼120 nm, and basically smooth surfaces (Figure 1B–D). However, the graphene nanobox has deteriorated monodispersibility as a result of inter-particle carbon growth. The hollow interior can be observed directly in some cracked particles (Figure 1C). The TEM images (Figure 1E, F) confirm the hollow interiors of the nanoboxes with ultra-thin walls (the thickness of ∼1–2 nm). HRTEM images (Figure 1G) show that the hollow graphitic carbon nanoboxes have ultra-thin shells. From the curved plane of the cracked nanoboxes, the shells are composed of ∼3–5 layers of graphene sheets (Figure 1H, marked with arrow). Due to the ultra-thin walls, hexagonal structures of carbon atoms can be observed fuzzily (Figure 1H, marked with hexagon). The wide-angle XRD pattern of the pristine nano-zeolite A substrates (Figure S1A, line a) shows characteristic diffraction peaks assigned to the typical crystallographic LTA structure. Instead of signals from the zeolites, two diffraction peaks at around 2θ = 24.9, 43.2° are observed in XRD patterns of the graphene nanoboxes obtained after removal of the zeolite substrates (Figure S1A, line b), ascribing to the 002 and 101 reflections from graphitized carbon walls, respectively. Raman spectra (Figure S1B) of the graphene nanoboxes shows two sharp bands at ∼1335 cm−1 (D band) and ∼1574 cm−1 (G band) (ID/IG = 1.2), further confirming the existence of graphitic carbon. Compared with monolayered graphene, the existence of D band is probably due to structural defects arisen from porosity and curved edges, and the existence of oxygen-containing groups. The graphene nanobox has a specific surface area of ∼642 m2/g, which is similar with that of the 2D graphene sheets.[36] However, the graphene nanobox has a large pore volume of 3.82 cm3/g (Figure S2), which may be ascribing to its unique 3D hollow structure. The mesopores calculated from the pore size distribution are in the range of 5∼20 nm (Figure S2), which may be contributed by the interparticle pores.
2.2. Pt NP@Graphene Nanobox Using a Pt-ion-exchanged nano-crystal zeolite A as a substrate, through an in-situ confined CVD process, the novel 3D Pt NP@graphene nanobox (Pt@GB) can be successfully obtained. The typical Pt NP@graphene nanoboxes prepared at a temperature of 800 °C (Pt@GB-1) present the hollow cube morphology with a uniform particle size of approximate 120 nm (Figure 2A, B), which is identical to that of the graphene nanobox. The Pt@GB-1 sample (Figure 2B) still retains a relatively smooth surface. It should be noted that very limited amounts of Pt NPs are located on the external surfaces of the boxes. The SEM image of the cracked Pt NP@ graphene nanobox shows that the Pt nanoparticles are identified as brighter spots studded onto the internal surfaces of the hollow shells (Figure 2C), demonstrating that the Pt-NPs
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In-Situ Confined Growth of Monodisperse Pt Nanoparticle@Graphene Nanobox Composites
Figure 1. SEM images of (A) the pristine nano-zeolite A and (B) the 3D graphene nanoboxes. (C, D) HRSEM images of (C) the cracked and (D) individual graphene nanoboxes. (E) TEM image of the graphene nanoboxes in a large domain, and (F) enlarged TEM image of the graphene nanoboxes with ultra-thin walls, inset in (F) is the corresponding SAED patterns of the graphene nanoboxes. (G, H) HRTEM image of the edges in a broken graphene nanobox, showing that the each face of graphene nanoboxes is formed by a few layered graphene. (I) The hexagonally packed carbon atoms can also be approximately observed.
smaller than those in most previous reports.[19–21] The XRD pattern (Figure S3a) exhibits diffraction peaks at 39.7, 46.5, 67.4 and 81.4°, implies typical the 111, 200, and 220 reflections, corresponding to the face-centered cubic (fcc) metal Pt structure. The size of the Pt-NPs is calculated to be about 3 nm, in accordance with the TEM results. As the existence of the ultra-thin graphene walls, the Pt content in the composites is as high as 28 wt% (Figure S4a). When increasing the CVD temperature to 900 °C, another Pt NP@graphene nanobox with an enlarged particle size of about 4–5 nm (Pt@GB-2) can be obtained (Figure 2E, Figure S3b). Interestingly, the Pt content in Pt@GB-2 composites can be increased to ∼52 wt% (Figure S4b), probably ascribing to the accelerated reduction effect at a higher CVD temperature. Moreover, when the Pt loading amount in the original zeolite decreases, through a similar CVD procedure as the synthesis of the sample Pt@GB-1, the as-prepared Pt@ GB-1s sample possesses a lower Pt content of 11 wt% (Figure S4c). In this way, the Pt content can be conveniently tuned Figure 2. (A) SEM and (B) FESEM images of the Pt@GB-1 graphene nanobox composites with and the particle size can be retained at uniform cube morphology; (C) FESEM images of the cracked Pt@GB-1 graphene nanobox composites, showing hollow interior and ultra-thin wall thickness. TEM images (D-F) of about 2 nm. Taking the sample Pt@GB-1 as an (D) the sample Pt@GB-1, (E) Pt@GB-2 and (F) Pt@GB-1s nanoboxes, showing highly dispersed example, evidenced by the elemental Pt-NPs in the hollow graphene nanobox.
are almost fully confined into the internal surfaces of the nanoboxes. The TEM image (Figure 2D) of the sample Pt@ GB-1 confirms the hollow interior and the highly dispersed Pt nanoparticles. Notably, the Pt nanoparticles have ultrafine particle sizes of ca. 2–3 nm (Figure 2D), which are much
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The pristine Pt-ion exchanged zeolite A nanocrystals show a similar morphology as that of nano-zeolites (Figure S6A), suggesting that its morphology is unchanged during the ionexchange process. The Pt-ion clusters with a size of below 1 nm are well confined in the micropores of zeolites (Figure S6B). For comparison, the Pt-ion exchanged zeolites were treated in Ar atmosphere at 800 °C, some Pt-NPs with a size of ∼2 nm confined in the pores of zeolites could be clearly observed (Figure S6C). Through the in-situ confined CVD process by using methane as carbon precursor, Pt NP@zeolite@graphene composites with Pt-nanoparticles size at ∼2–3 nm (marked as the yellow circle in Figure S6D) can be obtained. And the Pt-nanoparticles are highly crystallized and well dispersed in the zeolites. On the external surfaces of the zeolites, 3–4 layered graphene (marked as the red arrow) can be observed clearly, which are grown tightly with the substrate. After etching the zeolite, the monodisperse Pt nanoparticles are encapsulated in the graphene boxes. Figure 3. (A) STEM image, (B) platinum and (C) carbon elemental maps of the Pt@GB-1 Based on the above results, we prographene nanobox sample, demonstrating that Pt-NPs are studded in the hollow graphene pose that the nano-zeolites are utilized as nanoboxes. (D) HRTEM image of the Pt@GB-1 graphene nanobox composites with a Pt particle a nanoreactor for not only the confined size of ∼2 nm. Inset in (D) is the corresponding SAED patterns of the sample Pt@GB-1. space for the growth of Pt-NPs but also the in-situ activity sites for the growth of graphene nanoboxes on the external surface mapping images, the Pt-NPs are homogeneously dispersed in (Scheme 1). During the high temperature CVD process, Ptthe graphitic carbon hollow box (Figure 3A-C). The Pt-NPs ions located in the micropore channels diffuse and aggregate are highly crystallized (Inset in Figure 3D) and spatially together (Scheme 1), and then are reduced to nanoparticles separated from each other (Figure 3D), indicating that this in methane atmosphere. Meanwhile, CH4 gas molecules are confined growth method can effectively prevent the agglom- primary adsorbed on the crystal surfaces of zeolite substrates, eration of Pt nanoparticles. Such 0D Pt-NPs and 3D graphene probably ascribing to the ultra-small pore size (4A) and nanoboxes integrate a high loading amount and high disper- the external acidic surfaces of the nano-zeolite crystals. Folsion of the ultra-fine nano-sized Pt-NPs in one confined nano- lowing an annealing process at high temperature, the surfaces structure, which is difficult to be approached in conventional of zeolites are almost fully decorated with carbon clusters supported Pt/carbon catalysts.[10–15] Moreover, the monodis- decomposed from CH4, and then leading to the formation of perse Pt nanoparticles may contact with the reactants freely few-layered graphene.[37–40] After etching off the zeolite suband the hollow box morphology would provide large volume strates, the Pt-nanoparticles are successfully situated inside for the accommodation of oxygen, when utilized as ORR the graphene hollow box (Scheme 1). In the novel Pt NP@ catalysts. Furthermore, Au-NPs@graphene nanobox com- graphene nanobox composites, ultra-fine and monodisperse posites with 20 wt% of Au can also be obtained (Figure S5) Pt-NPs are protected by hollow graphene nanoboxes. through this strategy by using the Au-ion-exchanged zeolites as the substrates, implying that the synthesis method can be extended to construct a variety of novel hybrid materials 2.4. Electrocatalytic Performance with similar structure arrangements. To gain insight into the oxygen reduction reaction (ORR) activity of Pt@grphene nanoboxes, we have examined the electrocatalytic properties of the typical sample Pt@GB-1 2.3. Understanding of the Growth Process in a N2- and O2-saturated aqueous HClO4 electrolyte soluIn order to understand the growth process of the Pt NP@ tion (0.1 M) by using cyclic voltammetry (CV) at a scan graphene nanobxes, a series of experiments were designed. rate of 5 mV/s (Figure 4A). In the case of the N2-saturated
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N2-saturated 0.1 M HClO4 electrolyte. The LSVs of the Pt@GB-1 composites exhibit a much higher ORR activity than that of the commercial catalyst (89 mA/mg Pt) (Figure 4D). The Pt@GB-1 catalysts also present a higher half-wave potential (0.83 V) than that of the commercial Pt/C catalyst (40% Pt) (0.79 V) (Figure 4D). Interestingly, the mass activity of the Pt@ GB catalysts can be further improved by the ADT tests (Figure 4D), with a tiny increase of ∼8% after 1000 cycles. In contrast, the commercial Pt/C electrode shows a much faster current decrease with only a ∼45% retention. The better durability of the composites Pt@GB-1 can be ascribed to the unique confined structure of 0D Pt-NPs within 3D graphene layers, which can enhance their interfacial contact, suppress the dissolution/agglomeration. Comparatively, the Pt@GB-1s composites with a similar Pt-nanoparticle size but a lower Pt content as that of the sample Pt@GB-1, performs comparable Pt mass Scheme 1. Schematic of the in-situ confined growth of the Pt-nanoparticle@graphene naobox activity (185 mA/mg Pt) (Figure S7). The composites. Using Pt ions exchanged nano-zeolite A crystals as a substrate, through a high low Pt-content of the sample Pt@GB-1s temperature CVD process, where the Pt-ions adsorbed in the micropores can diffuse and would be somehow a problem for the aggregate together. The Pt-ions are reduced to the ultra-small Pt nanoparticles in the methane industrial application. The sample Pt@ atmosphere. Meanwhile, the CH4 gas molecules adsorbed on the external surfaces of the GB-2 shows a lowest ORR mass activity zeolites can be decomposed, leading to the growth of few-layered graphene. After etching (134 mA/mg Pt), perhaps ascribing to its off the zeolite substrate, the Pt-nanoparticles are successfully situated inside a 3D graphene largest particle size of the Pt catalysts. hollow nanobox. Moreover, the Pt NP@graphene nanobox composites can also be served as excellent solution, the CV curves of the Pt@GB-1 composites exhibit cathodes for dye sensitized solar cells (Figure S8). strong peaks associated with hydrogen adsorption/desorption (E = 0.4 V) and Pt oxides formation/reduction (E > 0.6 V). When the CV curves were recorded in O2-saturated electro- 3. Conclusion lyte, a significant peak centered at 0.97 V was observed, indiNovel 3D graphene nanoboxes have been successfully syncating a high ORR activity. Subsequently, to examine the reaction kinetics for the thesized by in-situ growth on the crystal-surfaces of zeolites Pt@GB-1 electrodes, linear sweep voltammograms (LSVs) (such as zeolite A) though an ambient pressure CVD process. were recorded in an O2-saturated 0.1 M HClO4 electrolyte at Because of strong surface acidity and small-sized micropore, a scan rate of 10 mV/s by using a technique of rotating disk the methane molecules are predominated adsorbed on the electrode (RDE). The current density is obviously enhanced surfaces of zeolites. And as the catalytic effect of the outer with the increase of rotating speed from 400 to 2500 rpm, acidic surfaces of the zeolite nanocrystals, a few layers achieving a high limiting mass activity of ∼175 mA/mg Pt at of graphenes are formed on the surfaces. Inherited from 2500 rpm (Figure 4B). The corresponding Koutecky-Levich nanozeolite substrates, uniform cubic graphene nanoboxes plots (j−1 vs ω−1/2) at several potentials (Figure 4C) exhibit a with a particle size of about 120 nm are obtained after etching good linearity with the slopes remaining generally constant the zeolite substrates by HF and HCl acids. The nanoboxes over a potential range from 0.70 to 0.82 V, suggesting that are formed by ∼3–4 layered graphenes, which are curved on the electron transfer numbers of ORR at the different poten- the edge to form a 3D box morphology. The zeolite substrates tials are intrinsically similar. The electron transfer number can also offer microenvironment with uniform micropores as is calculated according to the Koutecky-Levich equation to confinement channels for the reduction of Pt ions to Pt nanobe ∼3.96, indicating a four-electron transfer mechanism in particles, the size of which can be tuned from 2 to 5 nm. In ORR over the Pt@GB-1 composites. Pt NP@graphene nanoboxes composites, the Pt nanopartiThe durability of the Pt@GB-1 composites with respect cles are studded in the “nanobox” to effectively contact with to a commercial catalyst 40 wt% Pt/C was assessed through reactant and the graphene nanoboxes with 3D morphology a standard accelerated durability test (ADT) in poten- facilitate to accommodate large volume of oxygen as well as tial cycling at 0–1.2 V with a scan rate of 50 mV/s under provide good conductivity. The resulting Pt NP@graphene small 2014, DOI: 10.1002/smll.201402289
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Figure 4. (A) CV curves of the Pt@GB-1 graphene nanobox composites recorded under N2 and O2-saturated 0.1 M HClO4 electrolyte at a scan rate of 5 mV/s, (B) polarization profiles of the Pt@GB-1 graphene nanobox composites recorded under O2-sparging electrolyte at 400–2500 rpm at a scan rate of 10 mV/s, and the corresponding (C) Koutecky-Levich plotsat various potentials of the Pt@GB-1 graphene nanobox composites; (D) the polarization profiles of the Pt@GB-1 graphene nanobox composites and commercial Pt/C catalysts before (dashed line) and after (solid line) the potential cycling for 1000 times respectively.
nanoboxes show excellent electrocatalytic activity for the ORR, including a higher current density (175 mA/mg Pt), higher electron transfer number (∼4 e−), and better durability (above 1000 cycles). We believe that our novel synthetic strategy can be further extended to develop other metal or metal oxide@3D graphene-based composite materials for various applications, such as sensors, batteries and supercapacitors.
4. Experimental Section Chemicals: High-purity tetramethylammonium (TMAOH), tetraethylorthosilicate (TEOS), aluminum isopropoxide [(iPro)3Al], sodium hydroxide (NaOH), hydrofluoric acid (HF) (40 wt%) and hydrochloric acid (HCl) were purchased from Shanghai Chemical Company. Pt(NH3)4Cl2 and HAuCl4 were purchased from Sigma Aldrich Company. High-purity methane gas (99.99%) was purchased from Sinopharm Chemical Reagent Co., Ltd. Commercial Pt/C catalysts were purchased from Johnson Mathey Company. All the chemicals were used as received without further purification. Synthesis of Nanozeolite A: Zeolite A nanocrystals with an average size of ca. 120 nm were synthesized by a hydrothermal method.[37]1 For a typical procedure, 0.75 g of (iPro)3Al was first dissolved in a mixture of distilled water (7.0 g) and 25% TMAOH (5.0 g) under vigorous stirring. Then, TEOS (3.83 g) and 1.0 M NaOH aqueous solution (0.60 g) were added in turn. After a hydrothermal treatment at 80 °C for 3 days, the zeolite products were collected by high-speed centrifugation (12 000 rpm)
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and thoroughly washed with copious amount of water. The dried product was then calcined at 550 °C in air for 2 h to obtain the final zeolite A nanocrystals, which was denoted as Nano-A. Pt ions were exchanged into the zeolite A nanocrystals by immersing 100 mg of Nano-A in 50 mL of 1.0 mM Pt(NH3)4Cl2 aqueous solution at room temperature for 48 h. Synthesis of Graphene Nanoboxes and Pt NP@graphene Nanoboxes: For the synthesis of graphene nanoboxes, the Nano-A zeolites were chosen as a 3D substrate. The growth of graphenes was limited onto the external crystal surfaces of Nano-A zeolites, which was achieved by a CVD process at 900 °C for 30 min with methane as a carbon precursor. For the synthesis of Pt NP@ graphene nanoboxes, the Pt-loaded Nano-A zeolites upon ionexchange were chosen as a substrate, which was achieved by a similar CVD process at 800 or 900 °C for 30 min. During all the CVD processes, argon was used as a protecting gas when heating to the target temperature and cooling to room temperature. The resultant composites in all the cases were washed with a 20% HF solution for several times, followed by refluxing at 60 °C in 2 M HCl for 3 h to completely remove the zeolite substrates. After filtered and washed by copious amount of water, the resultant carbon or Pt/carbon products were dried in an oven at 120 °C, which were denoted as GB, Pt@GB-1 and Pt@GB-2, respectively. For the synthesis of Pt@GB-1s with varying Pt content, another Pt-ion-exchanged zeolite A, which was obtained by immersing 100 mg of the Nano-A zeolite crystals into 50 mL of 0.2 mM Pt(NH3)4Cl2 aqueous solution, was utilized as a substrate. The other synthetic procedures were the same as those for the synthesis of the sample Pt@GB-1.
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Characterization: Powder X-ray diffraction (XRD) patterns were recorded by a Bruker D8 X-ray diffractometer (Germany) with Ni-filtered Cu Kα radiation (40 kV, 40 mA). Raman spectra were obtained with a Dilor LabRam-1B Raman spectrometer, using the He–Ne laser with the excitation wavelength of ∼632.8 nm. Thermogravimetric (TG) analyses were conducted by using a Mettler Toledo TGA-SDTA851 analyzer (Switzerland) from 30 to 900 °C under flowing air (40 mL/min). Scanning electron microscopy (SEM) images were recorded on a Hitachi S-4800 microscope (Japan) working at 1 kV acceleration voltage. Transmission electron microscopy (TEM) experiments were conducted on a JEOL 2011F microscope (Japan) operated at 200 kV. The samples for TEM measurements were suspended in ethanol and supported onto lacey carbon films on copper grids. Nitrogen sorption isotherms were measured on a Micromeritics ASAP 2420 analyzer at −196 °C. Before the measurements, the samples were outgassed at 180 °C in vacuum overnight. The Brumauer-Emmett-Teller (BET) method was utilized to calculate the specific surface areas. The pore size distributions derived from the adsorption branches of the isotherms were calculated by the NLDFT (Nonlocal Density Functional Theory) method, and the total pore volumes (Vt) were estimated from the adsorbed amounts at a relative pressure P/P0 of 0.995. Electrochemical Measurements: Electrochemical tests such as cyclic voltammetry (CV) curves were measured on a CHI 760 electrochemical analyzer system (Shanghai CHI Instruments Co.) under ambient conditions. A three-electrode glass cell was used, in which a Pt-wire was used as the counter electrode and a saturated calomel electrode (SCE, 0.2415 V vs the normal hydrogen electrode, RHE) as the reference electrode. The working electrode was made from the Pt@GB catalysts and Nafion as a binder. Typically, ∼1.0 mg of a specific catalyst was dispersed in 100 µL of a 0.5 wt% Nafion solution (in isopropanol) and ultra-sonicated at room temperature for 30 min. 10 µL of the above-prepared catalyst ink was injected onto the surface of a glassy carbon electrode and dried at ambient conditions for 15 min before electrochemical measurement. A HClO4 solution (0.1 M) was adopted as the electrolyte. The rotating disk electrode (RDE) technique was employed to study the ORR activity and kinetics with a rotating speed of 400∼2500 rpm and a typical scan rate of 10 mV/s. Durability of the Pt@GB catalysts was evaluated by a standard accelerated durability test (ADT) in potential cycling at 0–1.2 V with a scan rate of 50 mV/s under N2-saturated 0.1 M HClO4 electrolyte.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This work was supported by the State Key Basic Research Program of the PRC (2012CB224805, 2013CB934104), NSF of China (Grant No. 21210004), Shanghai Leading Academic Discipline Project, small 2014, DOI: 10.1002/smll.201402289
Project Number: B108, and King Abdulaziz University (KAU), under grant No. (32-3-1432/HiCi).
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Received: July 31, 2014 Revised: September 10, 2014 Published online:
small 2014, DOI: 10.1002/smll.201402289
In-Situ Confined Growth of Monodisperse Pt Nanoparticle@Graphene Nanobox Composites as Electrocatalytic Nanoreactors Yingying Lv, Yin Fang, Zhangxiong Wu, Xufang Qian, Yanfang Song, Renchao Che, Abdullah M. Asiri, Yongyao Xia, Bo Tu, and Dongyuan Zhao*
Monodisperse Pt nanoparticles (NPs) studded in a three-dimensional (3D) graphene nanobox are successfully synthesized through a simple in-situ confined growth route for the first time. The nano-zeolite A was used as a 3D substrate for in-situ growth of tri-layered graphenes on the crystal-surfaces, meanwhile, the inner micropores of which can also be utilized for the confined growth of Pt nanoparticles. The graphene sheets are curved on the edges to form a 3D hollow box morphology, where the monodisperse Pt nanoparticles are homogeneously studded on the inner surfaces. Moreover, the Pt content can be regulated from ∼8 to 50 wt%, and the particle size can be tuned from 2–5 nm by varying the pristine Pt-ion loading amount and CVD temperature. The Pt NP@graphene nanoboxes possess not only large pore volumes to effectively accommodate large amounts of oxygen, but also supply excellent electrical conductivity for the fast transfer of electrons (∼3.96 e−), resulting in a high efficiency (175 mA/mg Pt) and long-term stability (above 1000 cycles) for the oxygen reduction reaction.
1. Introduction As an environmentally benign source of electrical energy, fuel cells attract a great deal of research activities because of no emission of any hazardous molecules and the high Dr. Y. Y. Lv, Dr. Y. Fang, Dr. X. F. Qian, Y. F. Song, Prof. R. C. Che, Prof. Y. Y. Xia, Prof. B. Tu, Prof. D. Y. Zhao Department of Chemistry Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials Laboratory of Advanced Materials Fudan University Shanghai 200433, P. R. China E-mail: [email protected] Dr. Z. X. Wu, Prof. D. Y. Zhao Department of Chemical Engineering Monash University Clayton, VIC 3800, Australia Prof. A. M. Asiri Chemistry Department and The Center of Excellence for Advanced Materials Research King Abdulaziz University P.O. Box 80203 Jeddah 21589, Saudi Arabia DOI: 10.1002/smll.201402289 small 2014, DOI: 10.1002/smll.201402289
density of specific energy comparable to that of combustion engines.[1] Platinum (Pt) is the key catalyst for fuel cells, but much expensive. Nanosized Pt-based catalysts[2–8] with a high surface area are effective for catalytic activity and utilization efficiency. For the synthesis of Pt nanoparticles (NPs), complex procedures and expensive additional protective agents are usually needed.[4–7] Further annealing process at a high temperature can improve the crystallization degree of Pt nanoparticles, however, the aggregation of which cannot be avoided. Currently, the achievable size of Pt-catalysts is usually in the range of five to tens of nanometers,[4–7] although smaller particle size is preferred in catalysis. It is still quite challenging to synthesize monodisperse Pt NPs with a high crystallization degree and uniform particle size of ∼ 2 nm. For a typical catalyst of fuel cells, a carbon support is essential for the dispersion of Pt catalysts to improve electrical conductivity, which also helps to reduce the Pt usage in catalysts. Various carbon materials, such as carbon black, nanofibers and nanotubes, mesoporous carbons, have been explored as catalyst supports,[9–14] because their nanoporous structure prevents aggregation of active materials.[15–18] However, the nanoporous structure of the above carbon supports contradictorily results in poor conductivity, and the metal loading amount of the supported Pt catalysts is usually very
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low. In another aspect, graphene, a 2-D carbon material with graphic wall and excellent electric conductivity, is considered as an ideal carbon support for fuel cells.[19–23] Graphene can be prepared via exfoliation of graphite[24,25] or direct growth on substrates.[26,27] 3D graphene with nanoporous structure can effectively provide large pore volume for accommodation of catalysts and reactants, however, which is usually obtained through a complex templating method by using mono-layered graphene as precursor.[28,29] Generally, Pt-loading amount of commercial Pt/C catalysts is between 20 and 60 wt%.[8] The lower the loading is, the thicker the electrode catalyst layer has to be made for the same Pt usage on the electrode. In that case, mass transport of reactants and products in the electrode is limited. Unfortunately, the preparation of Pt/C catalysts with a high Pt-loading becomes much more difficult due to the agglomeration of Pt nanoparticles, which always makes it with a large size and wide size distribution.[8,9] Thus, synthesis of highly dispersed and crystallized Pt nanoparticles with uniform size through a simple method is highly desired but still remains a great challenge, especially for high loading. In another aspect, the degradation of fuel cell performance, which is mainly due to the metal nanoparticles dissolution/aggregation/Oswald ripening, cannot be avoided if the active metal nanoparticles are exposed in outer environment directly.[2] Hence, hybrid nanocomposites, if in which Pt nanoparticles are protected by carbon shells, excellent catalytic effect can be prospected. Rattle-type nanostructure is one of such typical hybrid nanostructure,[30–34] which has been demonstrated to be effective in long-term stability for electrochemical application. However, the large-size of the metal nanoparticles (about 20−300 nm) and the thick carbon walls, make poor effectiveness of these materials for electro-catalytic applications.[35,36] Hence, to design a simple novel hybrid nanostructure containing monodisperse Pt nanoparticles with a ultra-small particle size and three-dimensional (3D) carbon support with ultra-thin walls should be essential for highly stable fuel cells. In this work, for the first time, a novel in-situ confined growth route has been developed for constructing Pt NP@ graphene nanobox composites. Using methane as a carbon source and nano-zeolite as a 3D substrate, under a suitable CVD process, the acidity on the outer surfaces of the nanozeolite cube crystals leads to the growth of multi-layered graphenes to form a unique 3D nanostructure. Meanwhile, the Pt-ions adsorbed in the inner micropores of zeolites, can be reduced to Pt nanoparticles with ultra-fine and ultra-small size. Ultra-fine and ultra-small sized Pt NPs are homogeneously confined in the 3D graphene nanoboxes. Such unique nanocomposites can be utilized as an excellent catalytic nanoreactor for oxygen reduction reaction.
2. Results and Discussion 2.1. Graphene Nanobox The zeolite A nanocrystal substrates have a uniform and monodispersed cubic morphology and an average particle
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size of 120 nm (Figure 1A). Through a simple methane CVD process at 900 °C, graphene sheets can uniformly be grown around the each crystal-surfaces of the zeolite substrates. After removal of the zeolite substrates by HCl and HF, 3D graphene nanoboxes (GB) can be successfully obtained (Figure 1B). The nanoboxes have a uniform particle size of ∼120 nm, and basically smooth surfaces (Figure 1B–D). However, the graphene nanobox has deteriorated monodispersibility as a result of inter-particle carbon growth. The hollow interior can be observed directly in some cracked particles (Figure 1C). The TEM images (Figure 1E, F) confirm the hollow interiors of the nanoboxes with ultra-thin walls (the thickness of ∼1–2 nm). HRTEM images (Figure 1G) show that the hollow graphitic carbon nanoboxes have ultra-thin shells. From the curved plane of the cracked nanoboxes, the shells are composed of ∼3–5 layers of graphene sheets (Figure 1H, marked with arrow). Due to the ultra-thin walls, hexagonal structures of carbon atoms can be observed fuzzily (Figure 1H, marked with hexagon). The wide-angle XRD pattern of the pristine nano-zeolite A substrates (Figure S1A, line a) shows characteristic diffraction peaks assigned to the typical crystallographic LTA structure. Instead of signals from the zeolites, two diffraction peaks at around 2θ = 24.9, 43.2° are observed in XRD patterns of the graphene nanoboxes obtained after removal of the zeolite substrates (Figure S1A, line b), ascribing to the 002 and 101 reflections from graphitized carbon walls, respectively. Raman spectra (Figure S1B) of the graphene nanoboxes shows two sharp bands at ∼1335 cm−1 (D band) and ∼1574 cm−1 (G band) (ID/IG = 1.2), further confirming the existence of graphitic carbon. Compared with monolayered graphene, the existence of D band is probably due to structural defects arisen from porosity and curved edges, and the existence of oxygen-containing groups. The graphene nanobox has a specific surface area of ∼642 m2/g, which is similar with that of the 2D graphene sheets.[36] However, the graphene nanobox has a large pore volume of 3.82 cm3/g (Figure S2), which may be ascribing to its unique 3D hollow structure. The mesopores calculated from the pore size distribution are in the range of 5∼20 nm (Figure S2), which may be contributed by the interparticle pores.
2.2. Pt NP@Graphene Nanobox Using a Pt-ion-exchanged nano-crystal zeolite A as a substrate, through an in-situ confined CVD process, the novel 3D Pt NP@graphene nanobox (Pt@GB) can be successfully obtained. The typical Pt NP@graphene nanoboxes prepared at a temperature of 800 °C (Pt@GB-1) present the hollow cube morphology with a uniform particle size of approximate 120 nm (Figure 2A, B), which is identical to that of the graphene nanobox. The Pt@GB-1 sample (Figure 2B) still retains a relatively smooth surface. It should be noted that very limited amounts of Pt NPs are located on the external surfaces of the boxes. The SEM image of the cracked Pt NP@ graphene nanobox shows that the Pt nanoparticles are identified as brighter spots studded onto the internal surfaces of the hollow shells (Figure 2C), demonstrating that the Pt-NPs
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In-Situ Confined Growth of Monodisperse Pt Nanoparticle@Graphene Nanobox Composites
Figure 1. SEM images of (A) the pristine nano-zeolite A and (B) the 3D graphene nanoboxes. (C, D) HRSEM images of (C) the cracked and (D) individual graphene nanoboxes. (E) TEM image of the graphene nanoboxes in a large domain, and (F) enlarged TEM image of the graphene nanoboxes with ultra-thin walls, inset in (F) is the corresponding SAED patterns of the graphene nanoboxes. (G, H) HRTEM image of the edges in a broken graphene nanobox, showing that the each face of graphene nanoboxes is formed by a few layered graphene. (I) The hexagonally packed carbon atoms can also be approximately observed.
smaller than those in most previous reports.[19–21] The XRD pattern (Figure S3a) exhibits diffraction peaks at 39.7, 46.5, 67.4 and 81.4°, implies typical the 111, 200, and 220 reflections, corresponding to the face-centered cubic (fcc) metal Pt structure. The size of the Pt-NPs is calculated to be about 3 nm, in accordance with the TEM results. As the existence of the ultra-thin graphene walls, the Pt content in the composites is as high as 28 wt% (Figure S4a). When increasing the CVD temperature to 900 °C, another Pt NP@graphene nanobox with an enlarged particle size of about 4–5 nm (Pt@GB-2) can be obtained (Figure 2E, Figure S3b). Interestingly, the Pt content in Pt@GB-2 composites can be increased to ∼52 wt% (Figure S4b), probably ascribing to the accelerated reduction effect at a higher CVD temperature. Moreover, when the Pt loading amount in the original zeolite decreases, through a similar CVD procedure as the synthesis of the sample Pt@GB-1, the as-prepared Pt@ GB-1s sample possesses a lower Pt content of 11 wt% (Figure S4c). In this way, the Pt content can be conveniently tuned Figure 2. (A) SEM and (B) FESEM images of the Pt@GB-1 graphene nanobox composites with and the particle size can be retained at uniform cube morphology; (C) FESEM images of the cracked Pt@GB-1 graphene nanobox composites, showing hollow interior and ultra-thin wall thickness. TEM images (D-F) of about 2 nm. Taking the sample Pt@GB-1 as an (D) the sample Pt@GB-1, (E) Pt@GB-2 and (F) Pt@GB-1s nanoboxes, showing highly dispersed example, evidenced by the elemental Pt-NPs in the hollow graphene nanobox.
are almost fully confined into the internal surfaces of the nanoboxes. The TEM image (Figure 2D) of the sample Pt@ GB-1 confirms the hollow interior and the highly dispersed Pt nanoparticles. Notably, the Pt nanoparticles have ultrafine particle sizes of ca. 2–3 nm (Figure 2D), which are much
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The pristine Pt-ion exchanged zeolite A nanocrystals show a similar morphology as that of nano-zeolites (Figure S6A), suggesting that its morphology is unchanged during the ionexchange process. The Pt-ion clusters with a size of below 1 nm are well confined in the micropores of zeolites (Figure S6B). For comparison, the Pt-ion exchanged zeolites were treated in Ar atmosphere at 800 °C, some Pt-NPs with a size of ∼2 nm confined in the pores of zeolites could be clearly observed (Figure S6C). Through the in-situ confined CVD process by using methane as carbon precursor, Pt NP@zeolite@graphene composites with Pt-nanoparticles size at ∼2–3 nm (marked as the yellow circle in Figure S6D) can be obtained. And the Pt-nanoparticles are highly crystallized and well dispersed in the zeolites. On the external surfaces of the zeolites, 3–4 layered graphene (marked as the red arrow) can be observed clearly, which are grown tightly with the substrate. After etching the zeolite, the monodisperse Pt nanoparticles are encapsulated in the graphene boxes. Figure 3. (A) STEM image, (B) platinum and (C) carbon elemental maps of the Pt@GB-1 Based on the above results, we prographene nanobox sample, demonstrating that Pt-NPs are studded in the hollow graphene pose that the nano-zeolites are utilized as nanoboxes. (D) HRTEM image of the Pt@GB-1 graphene nanobox composites with a Pt particle a nanoreactor for not only the confined size of ∼2 nm. Inset in (D) is the corresponding SAED patterns of the sample Pt@GB-1. space for the growth of Pt-NPs but also the in-situ activity sites for the growth of graphene nanoboxes on the external surface mapping images, the Pt-NPs are homogeneously dispersed in (Scheme 1). During the high temperature CVD process, Ptthe graphitic carbon hollow box (Figure 3A-C). The Pt-NPs ions located in the micropore channels diffuse and aggregate are highly crystallized (Inset in Figure 3D) and spatially together (Scheme 1), and then are reduced to nanoparticles separated from each other (Figure 3D), indicating that this in methane atmosphere. Meanwhile, CH4 gas molecules are confined growth method can effectively prevent the agglom- primary adsorbed on the crystal surfaces of zeolite substrates, eration of Pt nanoparticles. Such 0D Pt-NPs and 3D graphene probably ascribing to the ultra-small pore size (4A) and nanoboxes integrate a high loading amount and high disper- the external acidic surfaces of the nano-zeolite crystals. Folsion of the ultra-fine nano-sized Pt-NPs in one confined nano- lowing an annealing process at high temperature, the surfaces structure, which is difficult to be approached in conventional of zeolites are almost fully decorated with carbon clusters supported Pt/carbon catalysts.[10–15] Moreover, the monodis- decomposed from CH4, and then leading to the formation of perse Pt nanoparticles may contact with the reactants freely few-layered graphene.[37–40] After etching off the zeolite suband the hollow box morphology would provide large volume strates, the Pt-nanoparticles are successfully situated inside for the accommodation of oxygen, when utilized as ORR the graphene hollow box (Scheme 1). In the novel Pt NP@ catalysts. Furthermore, Au-NPs@graphene nanobox com- graphene nanobox composites, ultra-fine and monodisperse posites with 20 wt% of Au can also be obtained (Figure S5) Pt-NPs are protected by hollow graphene nanoboxes. through this strategy by using the Au-ion-exchanged zeolites as the substrates, implying that the synthesis method can be extended to construct a variety of novel hybrid materials 2.4. Electrocatalytic Performance with similar structure arrangements. To gain insight into the oxygen reduction reaction (ORR) activity of Pt@grphene nanoboxes, we have examined the electrocatalytic properties of the typical sample Pt@GB-1 2.3. Understanding of the Growth Process in a N2- and O2-saturated aqueous HClO4 electrolyte soluIn order to understand the growth process of the Pt NP@ tion (0.1 M) by using cyclic voltammetry (CV) at a scan graphene nanobxes, a series of experiments were designed. rate of 5 mV/s (Figure 4A). In the case of the N2-saturated
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N2-saturated 0.1 M HClO4 electrolyte. The LSVs of the Pt@GB-1 composites exhibit a much higher ORR activity than that of the commercial catalyst (89 mA/mg Pt) (Figure 4D). The Pt@GB-1 catalysts also present a higher half-wave potential (0.83 V) than that of the commercial Pt/C catalyst (40% Pt) (0.79 V) (Figure 4D). Interestingly, the mass activity of the Pt@ GB catalysts can be further improved by the ADT tests (Figure 4D), with a tiny increase of ∼8% after 1000 cycles. In contrast, the commercial Pt/C electrode shows a much faster current decrease with only a ∼45% retention. The better durability of the composites Pt@GB-1 can be ascribed to the unique confined structure of 0D Pt-NPs within 3D graphene layers, which can enhance their interfacial contact, suppress the dissolution/agglomeration. Comparatively, the Pt@GB-1s composites with a similar Pt-nanoparticle size but a lower Pt content as that of the sample Pt@GB-1, performs comparable Pt mass Scheme 1. Schematic of the in-situ confined growth of the Pt-nanoparticle@graphene naobox activity (185 mA/mg Pt) (Figure S7). The composites. Using Pt ions exchanged nano-zeolite A crystals as a substrate, through a high low Pt-content of the sample Pt@GB-1s temperature CVD process, where the Pt-ions adsorbed in the micropores can diffuse and would be somehow a problem for the aggregate together. The Pt-ions are reduced to the ultra-small Pt nanoparticles in the methane industrial application. The sample Pt@ atmosphere. Meanwhile, the CH4 gas molecules adsorbed on the external surfaces of the GB-2 shows a lowest ORR mass activity zeolites can be decomposed, leading to the growth of few-layered graphene. After etching (134 mA/mg Pt), perhaps ascribing to its off the zeolite substrate, the Pt-nanoparticles are successfully situated inside a 3D graphene largest particle size of the Pt catalysts. hollow nanobox. Moreover, the Pt NP@graphene nanobox composites can also be served as excellent solution, the CV curves of the Pt@GB-1 composites exhibit cathodes for dye sensitized solar cells (Figure S8). strong peaks associated with hydrogen adsorption/desorption (E = 0.4 V) and Pt oxides formation/reduction (E > 0.6 V). When the CV curves were recorded in O2-saturated electro- 3. Conclusion lyte, a significant peak centered at 0.97 V was observed, indiNovel 3D graphene nanoboxes have been successfully syncating a high ORR activity. Subsequently, to examine the reaction kinetics for the thesized by in-situ growth on the crystal-surfaces of zeolites Pt@GB-1 electrodes, linear sweep voltammograms (LSVs) (such as zeolite A) though an ambient pressure CVD process. were recorded in an O2-saturated 0.1 M HClO4 electrolyte at Because of strong surface acidity and small-sized micropore, a scan rate of 10 mV/s by using a technique of rotating disk the methane molecules are predominated adsorbed on the electrode (RDE). The current density is obviously enhanced surfaces of zeolites. And as the catalytic effect of the outer with the increase of rotating speed from 400 to 2500 rpm, acidic surfaces of the zeolite nanocrystals, a few layers achieving a high limiting mass activity of ∼175 mA/mg Pt at of graphenes are formed on the surfaces. Inherited from 2500 rpm (Figure 4B). The corresponding Koutecky-Levich nanozeolite substrates, uniform cubic graphene nanoboxes plots (j−1 vs ω−1/2) at several potentials (Figure 4C) exhibit a with a particle size of about 120 nm are obtained after etching good linearity with the slopes remaining generally constant the zeolite substrates by HF and HCl acids. The nanoboxes over a potential range from 0.70 to 0.82 V, suggesting that are formed by ∼3–4 layered graphenes, which are curved on the electron transfer numbers of ORR at the different poten- the edge to form a 3D box morphology. The zeolite substrates tials are intrinsically similar. The electron transfer number can also offer microenvironment with uniform micropores as is calculated according to the Koutecky-Levich equation to confinement channels for the reduction of Pt ions to Pt nanobe ∼3.96, indicating a four-electron transfer mechanism in particles, the size of which can be tuned from 2 to 5 nm. In ORR over the Pt@GB-1 composites. Pt NP@graphene nanoboxes composites, the Pt nanopartiThe durability of the Pt@GB-1 composites with respect cles are studded in the “nanobox” to effectively contact with to a commercial catalyst 40 wt% Pt/C was assessed through reactant and the graphene nanoboxes with 3D morphology a standard accelerated durability test (ADT) in poten- facilitate to accommodate large volume of oxygen as well as tial cycling at 0–1.2 V with a scan rate of 50 mV/s under provide good conductivity. The resulting Pt NP@graphene small 2014, DOI: 10.1002/smll.201402289
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Figure 4. (A) CV curves of the Pt@GB-1 graphene nanobox composites recorded under N2 and O2-saturated 0.1 M HClO4 electrolyte at a scan rate of 5 mV/s, (B) polarization profiles of the Pt@GB-1 graphene nanobox composites recorded under O2-sparging electrolyte at 400–2500 rpm at a scan rate of 10 mV/s, and the corresponding (C) Koutecky-Levich plotsat various potentials of the Pt@GB-1 graphene nanobox composites; (D) the polarization profiles of the Pt@GB-1 graphene nanobox composites and commercial Pt/C catalysts before (dashed line) and after (solid line) the potential cycling for 1000 times respectively.
nanoboxes show excellent electrocatalytic activity for the ORR, including a higher current density (175 mA/mg Pt), higher electron transfer number (∼4 e−), and better durability (above 1000 cycles). We believe that our novel synthetic strategy can be further extended to develop other metal or metal oxide@3D graphene-based composite materials for various applications, such as sensors, batteries and supercapacitors.
4. Experimental Section Chemicals: High-purity tetramethylammonium (TMAOH), tetraethylorthosilicate (TEOS), aluminum isopropoxide [(iPro)3Al], sodium hydroxide (NaOH), hydrofluoric acid (HF) (40 wt%) and hydrochloric acid (HCl) were purchased from Shanghai Chemical Company. Pt(NH3)4Cl2 and HAuCl4 were purchased from Sigma Aldrich Company. High-purity methane gas (99.99%) was purchased from Sinopharm Chemical Reagent Co., Ltd. Commercial Pt/C catalysts were purchased from Johnson Mathey Company. All the chemicals were used as received without further purification. Synthesis of Nanozeolite A: Zeolite A nanocrystals with an average size of ca. 120 nm were synthesized by a hydrothermal method.[37]1 For a typical procedure, 0.75 g of (iPro)3Al was first dissolved in a mixture of distilled water (7.0 g) and 25% TMAOH (5.0 g) under vigorous stirring. Then, TEOS (3.83 g) and 1.0 M NaOH aqueous solution (0.60 g) were added in turn. After a hydrothermal treatment at 80 °C for 3 days, the zeolite products were collected by high-speed centrifugation (12 000 rpm)
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and thoroughly washed with copious amount of water. The dried product was then calcined at 550 °C in air for 2 h to obtain the final zeolite A nanocrystals, which was denoted as Nano-A. Pt ions were exchanged into the zeolite A nanocrystals by immersing 100 mg of Nano-A in 50 mL of 1.0 mM Pt(NH3)4Cl2 aqueous solution at room temperature for 48 h. Synthesis of Graphene Nanoboxes and Pt NP@graphene Nanoboxes: For the synthesis of graphene nanoboxes, the Nano-A zeolites were chosen as a 3D substrate. The growth of graphenes was limited onto the external crystal surfaces of Nano-A zeolites, which was achieved by a CVD process at 900 °C for 30 min with methane as a carbon precursor. For the synthesis of Pt NP@ graphene nanoboxes, the Pt-loaded Nano-A zeolites upon ionexchange were chosen as a substrate, which was achieved by a similar CVD process at 800 or 900 °C for 30 min. During all the CVD processes, argon was used as a protecting gas when heating to the target temperature and cooling to room temperature. The resultant composites in all the cases were washed with a 20% HF solution for several times, followed by refluxing at 60 °C in 2 M HCl for 3 h to completely remove the zeolite substrates. After filtered and washed by copious amount of water, the resultant carbon or Pt/carbon products were dried in an oven at 120 °C, which were denoted as GB, Pt@GB-1 and Pt@GB-2, respectively. For the synthesis of Pt@GB-1s with varying Pt content, another Pt-ion-exchanged zeolite A, which was obtained by immersing 100 mg of the Nano-A zeolite crystals into 50 mL of 0.2 mM Pt(NH3)4Cl2 aqueous solution, was utilized as a substrate. The other synthetic procedures were the same as those for the synthesis of the sample Pt@GB-1.
© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
small 2014, DOI: 10.1002/smll.201402289
In-Situ Confined Growth of Monodisperse Pt Nanoparticle@Graphene Nanobox Composites
Characterization: Powder X-ray diffraction (XRD) patterns were recorded by a Bruker D8 X-ray diffractometer (Germany) with Ni-filtered Cu Kα radiation (40 kV, 40 mA). Raman spectra were obtained with a Dilor LabRam-1B Raman spectrometer, using the He–Ne laser with the excitation wavelength of ∼632.8 nm. Thermogravimetric (TG) analyses were conducted by using a Mettler Toledo TGA-SDTA851 analyzer (Switzerland) from 30 to 900 °C under flowing air (40 mL/min). Scanning electron microscopy (SEM) images were recorded on a Hitachi S-4800 microscope (Japan) working at 1 kV acceleration voltage. Transmission electron microscopy (TEM) experiments were conducted on a JEOL 2011F microscope (Japan) operated at 200 kV. The samples for TEM measurements were suspended in ethanol and supported onto lacey carbon films on copper grids. Nitrogen sorption isotherms were measured on a Micromeritics ASAP 2420 analyzer at −196 °C. Before the measurements, the samples were outgassed at 180 °C in vacuum overnight. The Brumauer-Emmett-Teller (BET) method was utilized to calculate the specific surface areas. The pore size distributions derived from the adsorption branches of the isotherms were calculated by the NLDFT (Nonlocal Density Functional Theory) method, and the total pore volumes (Vt) were estimated from the adsorbed amounts at a relative pressure P/P0 of 0.995. Electrochemical Measurements: Electrochemical tests such as cyclic voltammetry (CV) curves were measured on a CHI 760 electrochemical analyzer system (Shanghai CHI Instruments Co.) under ambient conditions. A three-electrode glass cell was used, in which a Pt-wire was used as the counter electrode and a saturated calomel electrode (SCE, 0.2415 V vs the normal hydrogen electrode, RHE) as the reference electrode. The working electrode was made from the Pt@GB catalysts and Nafion as a binder. Typically, ∼1.0 mg of a specific catalyst was dispersed in 100 µL of a 0.5 wt% Nafion solution (in isopropanol) and ultra-sonicated at room temperature for 30 min. 10 µL of the above-prepared catalyst ink was injected onto the surface of a glassy carbon electrode and dried at ambient conditions for 15 min before electrochemical measurement. A HClO4 solution (0.1 M) was adopted as the electrolyte. The rotating disk electrode (RDE) technique was employed to study the ORR activity and kinetics with a rotating speed of 400∼2500 rpm and a typical scan rate of 10 mV/s. Durability of the Pt@GB catalysts was evaluated by a standard accelerated durability test (ADT) in potential cycling at 0–1.2 V with a scan rate of 50 mV/s under N2-saturated 0.1 M HClO4 electrolyte.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This work was supported by the State Key Basic Research Program of the PRC (2012CB224805, 2013CB934104), NSF of China (Grant No. 21210004), Shanghai Leading Academic Discipline Project, small 2014, DOI: 10.1002/smll.201402289
Project Number: B108, and King Abdulaziz University (KAU), under grant No. (32-3-1432/HiCi).
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Received: July 31, 2014 Revised: September 10, 2014 Published online:
small 2014, DOI: 10.1002/smll.201402289
