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Trimetallic PtAgCu@PtCu core@shell concave nanooctahedrons with enhanced activity for formic acid oxidation reaction
Nano Energy (]]]]) ], ]]]–]]]
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RAPID COMMUNICATION
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Trimetallic PtAgCu@PtCu core@shell concave nanooctahedrons with enhanced activity for formic acid oxidation reaction
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Geng-Tao Fu1,b, Bao-Yu Xiac,1, Ru-Guang Mac, Yu Chena,c,n, Ya-Wen Tangb, Jong-Min Leec,nn a
School of Materials Science and Engineering, Shaanxi Normal University, Xi’an 710062, PR China Jiangsu Key Laboratory of New Power Batteries, School of Chemistry and Materials Science, Nanjing Normal University, Nanjing 210023, PR China c School of Chemical and Biomedical Engineering, Nanyang Technological University, Singapore 637459, Singapore b
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Received 1 December 2014; received in revised form 23 January 2015; accepted 26 January 2015
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KEYWORDS
Abstract
Core@shell nanostructures; High-index facets; Formic acid oxidation; Reaction pathway; Electrocatalytic activity
Trimetallic PtAgCu@PtCu core@shell concave nanooctahedrons (CSCNOs) enclosed by highindex facets are synthesized by a simple one-pot hydrothermal reduction method. The catalytic reduction growth of both AgI and CuII on preformed Pt crystal nuclei is proposed for the formation of core@shell nanostructures. Moreover, the oxidative etching is also critical in the final morphology of concave nanocrystals. The as-prepared PtAgCu@PtCu CSCNOs demonstrate excellent activity and durability for the formic acid oxidation reaction (FAOR) with an unusual reaction pathway due to the geometric effect, electronic effect, and bi-functional mechanism. Our work provides some clues for the future design of nanocatalysts for a specific fuel molecular electrocatalytic oxidation, with the aim of the improvement of activity and durability. & 2015 Published by Elsevier Ltd.
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Corresponding author at: School of Chemical and Biomedical Engineering, Nanyang Technological University, Singapore 637459, Singapore. nn Q3 Corresponding author. E-mail addresses: [email protected] (Y. Chen), [email protected] (J.-M. Lee). 1 Dr. G.T. Fu and Dr. B.Y. Xia contributed equally to this work. Q2
Introduction Direct formic acid fuel cells (DFAFCs) have been widely considered as a promising green energy conversion technique due to their high electromotive force, reasonable power density, low fuel crossover, and facile power-system integration [1–4]. Pd-based electrocatalysts are highly active for the formic
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http://dx.doi.org/10.1016/j.nanoen.2015.01.041 2211-2855/& 2015 Published by Elsevier Ltd.
Please cite this article as: G.-T. Fu, et al., Trimetallic PtAgCu@PtCu core@shell concave nanooctahedrons with enhanced activity for formic acid oxidation reaction, Nano Energy (2015), http://dx.doi.org/10.1016/j.nanoen.2015.01.041
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acid oxidation reaction (FAOR) [2,5–7]. However, their poor stability due to the Pd dissolution in acidic solution and slow adsorption of CO-like intermediates limits their practical applications [2,6]. Pt-based nanocatalysts also exhibit excellent activity for the FAOR, but the dehydration pathway at lower potential is predominant on the Pt surface. Thus the produced CO-like intermediates would absorb and poison the surface of Pt catalysts. Incorporation of other transition metals with Pt to form bimetallic alloys, such as PtPd, PtAu, PtCu, PtCo, PtMn, PtNi etc., has been considered as an effective method to facilitate the dehydrogenation of the FAOR [8–18], and enhance the activity and durability consequently for the possible geometry and electronic effects. Besides the chemical composition, the morphologies and structures of Pt-based nanocatalysts also determine the catalytic performance. Especially, concave Pt-based nanocrystals with high-index facets have received unprecedented interests in electrocatalysts due to the unique surface structure which contains high density of atomic steps and kinks [19–32]. Although remarkable progress has been achieved in controllable synthesis of Pt-based nanocatalysts with high-index facets [23,33,34], trimetallic Pt-based nanocrystals with high-index facets has been rarely reported yet. In addition, the underlying mechanism of FAOR has not been fully understood for Pt-based multimetallic nanostructures. Therefore, it is challenging and also significant to develop an effective and simple method to prepare trimetallic Pt-based nanocrystals and also investigate their electrochemical properties for the FAOR. Herein, we exploit an effective one-pot method to synthesize PtAgCu@PtCu core@shell concave nanooctahedrons (CSCNOs) with high-index facets. The catalytic reduction growth of both Ag and Cu on preformed Pt crystal nuclei is proposed for the formation of concave PtAgCu@PtCu core@shell nanocrystals. Moreover, the oxidative etching is also critical in determining the final morphology of resultant concave nanocrystals. Because of the unique surface structure and possible synergistic effect, the asprepared PtAgCu@PtCu CSCNOs demonstrate excellent activity and stability for the FAOR. Importantly, the enhanced direct reaction pathway and the suppressive dehydration pathway of the FAOR are also proposed as the reasons for the enhanced electrochemical performance.
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Experimental Reagents and chemicals
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Polyallylamine hydrochloride (PAH, Scheme S1, weightaverage molecular weight 150,000) was supplied from Nitto Boseki Co., Ltd. (Tokyo, Japan). Potassium tetrachloroplatinate (II) (K2PtCl4), cupric chloride (CuCl2), silver nitrate (AgNO3), formaldehyde solution (HCHO, 40%), sulfuric acid (H2SO4), and formic acid (HCOOH) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Commercial Pt black were purchased from Johnson Matthey Corporation.
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Synthesis of PtAgCu@PtCu CSCNOs
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In a typical synthesis, 0.5 mL K2PtCl4 (0.05 M), 0.5 mL CuCl2 (0.05 M), 0.1 mL AgNO3 (0.05 M) and 1.1 mL PAH (0.50 M,
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molarity of PAH given with respect to the repeating unit) were added into 7.3 mL of H2O with continued stirring. After adding a 0.5 mL HCHO solution (40%), the resultant mixture (pH 3.0) was transferred to a 20-mL Teflon-lined stainless-steel autoclave, and was then heated at 180 1C for 4 h. After being cooled to room temperature, the obtained PtAgCu@PtCu CSCNOs were separated by centrifugation at 15,000 rpm for 15 min, washed with acetic acid for 12 h, and then dried at 60 1C for 5 h in a vacuum dryer. The cyclic voltammetry (CV) tests and X-ray photoelectron spectroscopy (XPS) demonstrated that a majority of PAH molecules on PtAgCu@PtCu CSCNOs surface could be removed via acetic acid washing (Figure S1). Under the same conditions, a series of controlled experiments were also conducted to investigate the formation/growth mechanisms of the PtAgCu@PtCu CSCNOs.
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Electrochemical measurements
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All electrochemical experiments were performed by using a CHI 760 D electrochemical analyzer at 30 71 1C. A standard three-electrode system was used for all electrochemical experiments, which consisted of a platinum wire as the auxiliary electrode, a saturated calomel reference electrode protected by Luggin capillary with KCl solution as the reference electrode, and a catalyst modified glassy carbon electrode as the working electrode. Potentials in this study were reported with respect to the SCE. An evenly distributed suspension of catalyst was prepared by ultrasonic the mixture of 10 mg catalyst and 5 mL H2O for 30 min, and 6 μL of the resulting suspension was drop-cast onto the surface of the glassy carbon electrode. After drying at room temperature, 3 μL of Nafion solution (5 wt%) was covered on the modified electrode surface and allowed drying again. Thus, the working electrode was obtained, and the specific loading of metal on the electrode surface was about 170 μg cm 2. Electrochemical measurements were conducted in N2-saturated 0.5 M H2SO4 solution or N2-saturated 0.5 M H2SO4 solution with 0.5 M HCOOH.
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Characterization
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Transmission electron microscopy (TEM) images were taken using a JEOL JEM-2100F transmission electron microscopy operated at 200 kV. High-resolution TEM (HRTEM), highangle annular dark-field scanning transmission electron microscopy (HAADF-STEM), and energy dispersive X-ray (EDX) elemental mapping measurements were performed on FEI Tecnai G2 F20 microscope equipped with an EDAX X-ray detector. Scanning electron microscopy (SEM) images were captured on a JSM-2010 microscope at an accelerating voltage of 20 kV. X-ray diffraction (XRD) patterns were obtained with a Model D/max-rC X-ray diffractometer using Cu Ka radiation source (λ = 1.5406 Å) and operating at 40 kV and 100 mA. XPS measurements were carried out on a Thermo VG Scientific ESCALAB 250 spectrometer. The binding energy was calibrated by means of the C 1s peak energy of 284.6 eV. The composition of samples was investigated by EDX technique and Leeman inductively coupled plasma atomic emission spectrometry (ICP-AES).
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Please cite this article as: G.-T. Fu, et al., Trimetallic PtAgCu@PtCu core@shell concave nanooctahedrons with enhanced activity for formic acid oxidation reaction, Nano Energy (2015), http://dx.doi.org/10.1016/j.nanoen.2015.01.041
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Results and discussion
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The PtAgCu@PtCu CSCNOs are obtained after hydrothermal treatment of K2PtCl4, AgNO3 and CuCl2 in formaldehyde solution at 180 1C for 4 h in the presence of PAH. The morphology and structure of the PtAgCu@PtCu CSCNOs are characterized by TEM. The PtAgCu@PtCu CSCNOs have a uniform size of ca. 40 nm and exhibit a mostly star-like morphology with six arms (Figure 1A). The darker contrast in the center than the sharp horns confirms the formation of a concave structure (inset of Figure 1A). The concave octahedral structure is also demonstrated by high-angle annular dark-field scanning TEM (HAADFSTEM) images (Figure 1B) and SEM (Figure S2). To identify this morphology more directly, TEM and selected-area electron diffraction (SAED) along the three low-index zone axes are performed (Figure 1C). Both TEM and SAED patterns clearly demonstrate the single-crystalline nature of the PtAgCu@PtCu CSCNOs (Figure 1C). Specially, an individual PtAgCu@PtCu CSCNO with a cubic shape is projected from the [001] direction. The apex angles are measured to be 65.81 for the crystals
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3 oriented along [001] zone axes with geometrical model view from top (Figure 1C-c). According to the apex angle of arms, we can infer that the exposed facets of the PtAgCu@PtCu CSCNOs are {310} facets. The direction identification of negative curvature by high-resolution TEM (HR-TEM) image clearly shows the atomic steps at the edge of a PtAgCu@PtCu CSCNO (Figure 1D-a), matching the two dimensional (2D) atomic model of {310} facets. Other stereoscopic modes viewed from different directions in Figure 1C also demonstrate the concave structure with six prominent arms. The crystal structure and composition of the trimetallic PtAgCu@PtCu CSCNOs are characterized by XRD, EDX, ICP-AES, and XPS (Figure 2). Four typical diffraction peaks are observed clearly, indicating that the trimetallic PtAgCu nanocrystals have the face-centered cubic (fcc) structure (Figure 2A). No diffraction peaks for single-component Pt, Ag and Cu are observed in XRD pattern and all diffraction peaks of the PtAgCu@PtCu CSCNOs shift to a higher angle than pure fcc Pt (JCPDS no. 04-0802), which is indicative of uniform alloy formation. The Pt:Ag:Cu atomic ratio is estimated to be
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Figure 1 (A) TEM images of the PtAgCu@PtCu CSCNOs. (Inset a) TEM image of an individual nanocrystals; (inset b) a concave octahedron model. (B) HAADF-STEM image of the PtAgCu@PtCu CSCNOs. (Inset) Magnified HAADF-STEM image. (C) (a, a0 , a″) SAED patterns, (b, b0 , b″) TEM images, and (c, c0 , c″) geometric models of an individual PtAgCu@PtCu CSCNO along the (a, b) 〈100〉, (a0 , b0 ) 〈110〉, and (a″, b″) 〈111〉 zone axes directions. Eye and arrow in c, c0 , and c″ patterns indicate the projection direction of a concave octahedron model. (D) (a) The magnified HR-TEM image and (b) corresponding atomic model of the region indicated by the white lines in (a). Please cite this article as: G.-T. Fu, et al., Trimetallic PtAgCu@PtCu core@shell concave nanooctahedrons with enhanced activity for formic acid oxidation reaction, Nano Energy (2015), http://dx.doi.org/10.1016/j.nanoen.2015.01.041
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91 Figure 2 (A) XRD pattern of the PtAgCu@PtCu CSCNOs. (B) XPS survey scan spectrum of the PtAgCu@PtCu CSCNOs. (C) EDX mapping and (D) line scanning profiles of the PtAgCu@PtCu CSCNOs.
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45.5:9.3:45.2 by EDX (Figure S3), in good agreement with the result from ICP-AES analysis (45.8:8.5:45.7). XPS data reveals the surface atomic ratio of Pt:Ag:Cu is 48.7:2.3:49.0 (Figure 2B). Compared to EDX and ICP-AES results, the lower Ag content measured by XPS hints a PtCu-rich shell. The EDX elemental mapping images and EDX line scanning profiles are performed to get more insight into the distribution of each element in the PtAgCu@PtCu CSCNOs. The core@shell structure of a PtAgCu core and a PtCu shell are clearly revealed by EDX elemental images and line scanning profiles (Figure 2C and D) [35,36]. We first investigated the effects of involved reaction precursors on the final morphologies and compositions of the PtAgCu@PtCu CSCNOs. Only irregular and randomly aggregated nanocrystals were obtained in the absence of PAH (Figure S4), indicating that PAH serves as effective capping agent to ensure the dispersion of nanocrystals. Single-component CuII or AgI metal precursor cannot be reduced under the present experimental conditions. Monodisperse Pt nanocubes with six {100} facets are obtained when only PtII is used as metal precursor (Figure 3A). In the absence of Ag precursor, bimetallic PtCu alloy nanoflowers are generated (Figures 3B and S5), while bimetallic PtAg alloy nanocuboctahedra are obtained in the absence of Cu precursor (Figures 3C and S6). Therefore, the presence of Ag and Cu ions plays an important role on the formation of concave PtAgCu@PtCu nanocrystals. Considering the strong adsorption and interaction of AgI and CuII ions on Pt nuclei,
the autocatalytic reduction growth of Ag and Cu ions on preformed Pt nuclei is accounted for the formation of trimetallic nanocrystals. Taking into account the facts of the lower amount of AgI precursor than that of CuII precursor in reaction system and the higher reduction potential of AgI/Ag0 redox couple ( + 0.7996 V) than that of CuII/Cu0 redox couple (+ 0.340 V), the formation of PtAgCu@PtCu core@shell structure originates from the faster consumption of AgI precursor than that of CuII precursor during the autocatalytic reduction growth of both CuII and AgI on the preformed Pt crystal nuclei, as shown in XPS and EDX analysis (Figures 2 and 3). In addition, it is worth noting that well-defined concave nanooctahedrons cannot be obtained by bubbling N2 gas to eliminate typical etchant O2 (Figure 3D). To better understand the formation/growth mechanism of PtAgCu@PtCu CSCNOs, time dependent morphologies evolution process is also investigated by TEM. At the early stage, some octahedral nanocrystals of 15 nm are obtained (Figure 4A). With increasing reaction times, the diameter of nanocrystals gradually increases to 40 nm, accompanied with gradual formation of well-defined concave nanooctahedrons (Figure 4B–D). Recent works show nanocuboctahedron is an intermediate during the evolution from nanocubes to nanooctahedrons via the oxidative etching [37,38]. And, nanooctahedrons can transform into concave nanooctahedrons via the oxidative etching [39]. Considering the three facts: (i) the addition of AgI results in the formation of bimetallic PtAg alloy nanocuboctahedra
Please cite this article as: G.-T. Fu, et al., Trimetallic PtAgCu@PtCu core@shell concave nanooctahedrons with enhanced activity for formic acid oxidation reaction, Nano Energy (2015), http://dx.doi.org/10.1016/j.nanoen.2015.01.041
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Figure 3 TEM images of the Pt nanocubes (A), PtCu nanoflowers (B), PtAg nanocuboctahedra (C) and PtAgCu nanocrystals prepared by displacing the air with N2 to eliminate typical etchant O2 (D). Insets in (B) and (C) are EDX elemental mapping patterns and line scanning profiles.
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Figure 4 TEM images of the PtAgCu@PtCu intermediates collected at different growth stages: (A) 30 min, (B) 1 h, (C) 2 h, and (D) 4 h. (Inset a) TEM image of an individual nanooctahedron in red circle.
(Figure 3C); (ii) some octahedral nanocrystals are clearly observed in at early reaction stage (Figure 4A); (iii) O2 has important effect in determining the morphology of the products in our synthesis (Figure 3D), we infer that the formation of concave PtAgCu nanocrystals originates from oxidative etching. In a recent work, PtNi concave nanooctahedrons from PtNi nanooctahedrons via coordinating complex (i.e., dimethylglyoxime) assisted oxidative etching in the presence of air [39]. The dimethylglyoxime selectively coordinates to NiII species generated on nanooctahedrons via O2
oxidation, resulting in the higher susceptibility and dissolution rate of Ni species compared to Pt. The chemical etching process played a crucial role in the transformation of PtNi nanooctahedrons in to PtNi concave nanooctahedrons. In our synthesis, PAH has also excellent coordination capability for CuII ion and can generate water-soluble PAH-CuII complex (Figure S7). During the reaction, the Cu atoms on the corners and edges on preformed PtAgCu@PtCu nanooctahedrons are preferably removed via PAH-assisted oxidative etching, followed by the deposition and segregation of the remaining Pt
Please cite this article as: G.-T. Fu, et al., Trimetallic PtAgCu@PtCu core@shell concave nanooctahedrons with enhanced activity for formic acid oxidation reaction, Nano Energy (2015), http://dx.doi.org/10.1016/j.nanoen.2015.01.041
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atoms due to HCHO reduction. The continuous remove– deposition process contributes to the formation of PtAgCu@PtCu nanooctahedrons, similar to the case of PtNi concave nanooctahedrons. In the presence of dissolved oxygen, continuous Cu oxidation and reduction cycles occur during growth. The oxidative etching effectively eliminate twinned structures and affect atomic addition on preformed Pt nanocrystals surface, which facilitate the emergence of concave structure with high-index facets [40]. The electrochemical properties of the PtAgCu@PtCu CSCNOs are next evaluated by cyclic voltammetry (CV). Asprepared PtCu alloy nanoflowers and commercial Pt black are also compared. Prior to electrochemical tests for the FAOR, the working electrode was pretreated by cycling the potential between 0.242 and 0.958 V for 10 cycles (Figure S8). As observed, a weak and broad peak of Cu dissolution is observed at the first potential cycling. After 10 cycles, the cyclic voltammograms become stable, indicating that Cu dissolution has either ceased or dropped to undetectable level. Compared to the previous reports [41,42], the oxidation peak of Cu dissolution at the PtAgCu@PtCu CSCNOs is very small. After electrochemical pretreatment, XPS measurements show the surface chemical composition of the PtAgCu@PtCu CSCNOs is Pt:Ag:Cu=51.1:2.5:46.4 (Figure S9A), which is very close to the proportion of un-treated PtAgCu@PtCu CSCNOs (48.7:2.3:49.0). These results indicate only a small amount of Cu atoms at PtAgCu@PtCu CSCNOs is dissolved, which may be attributed to their high alloying degree. In addition, the morphology and electronic structure of the PtAgCu@PtCu CSCNOs almost remain after electrochemical pretreatment (Figures S9B and S10). The electrochemically surface areas (ECSA) of the PtAgCu@PtCu CSCNOs, PtCu alloy nanoflowers, and Pt black are determined to be 19.0, 23.5, and 17.8 m2 g 1, respectively, by the hydrogen adsorption–desorption method in conjunction with CV in 0.5 M H2SO4 solution (Figure S11). Figure 5A shows ECSA-normalized cyclic voltammograms of different electrocatalysts for the FAOR. Obviously, PtAgCu@PtCu CSCNOs (1.52 mA cm 2) demonstrate remarkably enhanced activity compared with PtCu alloy nanoflowers (0.97 mA cm 2) and commercial Pt black (0.16 mA cm 2) at 0.3 V. Moreover, peak I at 0.30 V and peak II at 0.67 V for commercial Pt black correspond to the oxidation of formic acid via the dehydrogenation pathway and oxidation of COads formed via the dehydration pathway, respectively. The ratio R between peak I and peak II is used to determine the pathway of FAOR on electrocatalysts. For the Pt black (Figure 5A), peak I current is 6.25 times lower than peak II current (R=0.16), indicating dehydration pathway is predominant. For PtCu alloy nanoflowers, peak I current is close to peak II current (R=1.05), indicating that alloying Pt with Cu facilitates the dehydrogenation pathway of the FAOR relative to pure Pt. Nevertheless, the PtAgCu@PtCu CSCNOs exhibit the highest R (1.30), illustrating that the FAOR on the PtAgCu@PtCu CSCNOs accomplishes mainly through the dehydrogenation pathway. To support this claim, the chronoamperometry at 0.3 V for 200 s is performed to allow the formation (if any) of COads poisoning intermediates on electrocatalysts surface, after which the accumulated COads intermediates are electrochemically stripped off in 0.5 M H2SO4 solution. As observed, the oxidation charge of accumulated COads on the PtAgCu@PtCu CSCNOs is 2.14 and 2.51 times smaller than that on the PtCu
alloy nanoflowers and commercial Pt black, respectively (Figure 5B), further confirming that dehydrogenation pathway of FAOR on the PtAgCu@PtCu CSCNOs is predominant. Meanwhile, it is worth noting that the R value (1.30) is also higher than that on previous reported Pt-based nanocrystals (such as Pt–Co [43], Pt–Mn [16], Pt–Cu [17,18], Pt–Au [15], and Pt–Ag [9,44]), confirming the superior electrocatalytic activity of the PtAgCu@PtCu CSCNOs for the FAOR. The mass activity (the current densities are normalized with respect to metal mass) of the PtAgCu@PtCu CSCNOs for the FAOR has been also remarkably enhanced compared to those of PtCu alloy nanoflowers and commercial Pt black (Figure S12). The enhanced activity of concave PtAgCu@PtCu core@shell nanocrystals for the FAOR is mainly attributed to their unique concave surface and trimetallic composition. The chronoamperometry results also demonstrate the superior electrocatalytic stability of the PtAgCu@PtCu CSCNOs partly due to their good crystallinity and alloy degree (Figure 5C). Recent results indicate that reaction pathway of the FAOR is sensitive to the actual surface structure and arrangement of Pt atoms. Three neighboring Pt atoms are required for the dehydration pathway but one isolated Pt atom is enough for the dehydrogenation pathway of FAOR (i.e., the ensemble effect) [45]. The ensemble effect can explain the observed results here because alloying Pt with Cu can effectively separate the surface Pt atoms and high-index facets have the more abundant low-coordinated atoms (i.e., isolated atoms) relative to low-index facets, thus making the dehydrogenation pathway dominant on the PtAgCu@PtCu CSCNOs. The electrocatalytic activity of Pt-based nanocrystals for the FAOR also depends the electronic structure of Pt atoms. The binding energies of Pt 4f, Cu 2p, and Ag 3d in the PtAgCu@PtCu CSCNOs exhibit a negative shift compared to the standard values of bulk Pt, Cu and Ag, respectively (Figure S13), attributing to the interaction between residual PAH and metal atoms (Figures S1 and S14). Due to the formation of N– Pt bond, the lone pair electrons of –NH2 groups in PAH effectively donate electrons to Pt, resulting in the shift of elemental binding energy [46,47]. The increased electron density in Pt facilitates the activation of formic acid molecules due to preferred electron back-donation from Pt to the 2πn orbital of the adsorbate, which improves the electrocatalytic activity of the PtAgCu@PtCu CSCNOs [48]. In addition, it is observed that the onset oxidation potential of the PtAgCu@PtCu CSCNOs with pre-absorbed CO negatively shift ca. 14 and 62 mV compared to those of PtCu alloy nanoflowers and Pt black (Figure 5D), indicating that the PtAgCu@PtCu CSCNOs have the priority for CO oxidation. Similar to the cases for Pt–Cu and Pt–Ag nanocrystals [18,49], the Cu and Ag component incorporated into Pt-based nanocrystals can facilitate CO oxidation with a unique ability to generate hydroxyl species from water at low potential (i.e., bi-functional mechanism [50]). Thus, the less accumulation and the more easy oxidation of COads on the PtAgCu@PtCu CSCNOs surface contribute to a large enhancement in durability.
Conclusions
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In summary, we present a facile one-pot hydrothermal method to prepare concave PtAgCu@PtCu core@shell nanooctahedrons. The catalytic reduction of both Ag and
Please cite this article as: G.-T. Fu, et al., Trimetallic PtAgCu@PtCu core@shell concave nanooctahedrons with enhanced activity for formic acid oxidation reaction, Nano Energy (2015), http://dx.doi.org/10.1016/j.nanoen.2015.01.041
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Figure 5 (A) The ESCA-normalized cyclic voltammograms for the PtAgCu@PtCu CSCNOs, PtCu alloy nanoflowers, and commercial Pt black in N2-saturated 0.5 M HCOOH +0.5 M H2SO4 solution at the scan rate of 50 mV s 1. (B) The PtAgCu@PtCu CSCNOs, PtCu alloy nanoflowers, and commercial Pt black were exposed to N2-saturated 0.5 M HCOOH +0.5 M H2SO4 solution for 200 s at 0.35 V potential, rinsed in water, and then placed in a 0.5 M H2SO4 solution for CV measurements. (C) Chronoamperometry curves for the PtAgCu@PtCu CSCNOs, PtCu alloy nanoflowers, and commercial Pt black in N2-saturated 0.5 M HCOOH+ 0.5 M H2SO4 solution for 3000 s at 0.35 V potential. (D) Pre-absorbed CO-stripping voltammograms for the PtAgCu@PtCu CSCNOs, PtCu alloy nanoflowers, and commercial Pt black in N2-saturated 0.5 M H2SO4 solution at a scan rate of 50 mV s 1.
Cu on preformed Pt crystal nuclei is responsible for the formation of the trimetallic alloy. The formation of the core@shell structure originates from the faster consumption of the AgI precursor than that of the CuII precursor during the autocatalytic reduction growth of both CuII and AgI. The oxidative etching contributes to the transformation of Pt nanocubes into PtAgCu@PtCu concave nanooctahedrons. The as-prepared PtAgCu@PtCu CSCNOs display superior electrocatalytic activity and stability for the FAOR relative to the commercial Pt black. The concave surface incorporated with high index facets {310} in this trimetallic nanocrystals induce geometry and alloy effects, and thus the dehydrogenation pathway of the FAOR is strengthened. Our work would provide some valuable clues to design and prepare enhanced electrocatalysts specifically for the FAOR by tailoring their surface structure and surface composition.
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The authors are grateful for the financial support of NSFC (21473111, 21376122 and 21273116), Natural Science Foundation of Jiangsu Province (BK20131395), United Fund of NSFC and Yunnan Province (U1137602), Industry-Academia Cooperation Innovation Fund Project of Jiangsu Province (BY2012001), Fundamental Research Funds for the Central Universities (GK201402016), and the Academic Research Fund of the Ministry of Education in Singapore (RGT27/13).
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Appendix A.
Supporting information
Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/ j.nanoen.2015.01.041.
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Geng-Tao Fu received his B.S. degree from the Department of Chemistry and Chemical Engineering, Binzhou University in 2011, and his M. S. degree from the College of Chemistry and Materials Science, Nanjing Normal University in 2014. He is currently pursuing his Ph.D. degree at Nanjing Normal University. His research interests focus on the advanced nanomaterials for electrochemical applications.
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Bao-Yu Xia received his Ph.D. degree in Materials Science and Engineering from Shanghai Jiao Tong University in 2010. He then moved to Nanyang Technological Univerisity, where he is a research fellow under the supervision of Prof. X. Wang and Prof. X.W. Lou. His research interests are in the areas of nanostructured electrocatalysts and their application in energy conversion.
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Ru-Guang Ma received his Ph.D. in materials science from City University of Hong Kong in 2013. Then he worked at Nanyang Technological University as a post-doctoral researcher. In July 2014, he joined Shanghai Institute of Ceramics, Chinese Academy of Sciences (SICCAS). He is currently an assistant professor in the State Key Lab of High Performance Ceramics and Superfine Microstructure, SICCAS. His research interests include design and synthesis of new nanostructured electrode materials for supercapacitors, lithium–oxygen batteries and novel non-precious metal catalysts.
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Yu Chen received his Ph.D. from School of Chemistry & Chemical Engineering in Nanjing University in 2009. Then he worked at School of Chemistry and Materials Science, Nanjing Normal University as an associate professor. In January 2014, he joined Shaanxi Normal University as an associate professor at School of Materials Science and Engineering. He worked at Nanyang Technological University as a post-doctoral researcher in the same year. His research interests include the design and synthesis of precious metal electrocatalysts, the fabrication and application of functionalized interface, and the electrochemical biosensor.
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Ya-Wen Tang received his B.S. degree and M.S. degree from Nanjing Normal University in 1992 and 2002, respectively, and his Ph.D. from Nanjing University of Science & Technology in 2011. He is currently a professor at College of Chemistry and Materials Science at Nanjing Normal University. His main research interests are the synthesis and assembly of nanomaterials, and their applications in batteries, fuel cells and photocatalysis.
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Jong-Min Lee received his Ph.D. degree from the Department of Chemical Engineering, Columbia University in 2003. He worked in Chemical Science Division, Lawrence Berkeley National Laboratory and in Department of Chemical Engineering, University of California at Berkeley as a postdoctoral fellow in 2006–2008. Since 2008, he has been appointed as an assistant professor in School of Chemical and Biomedical Engineering at Nanyang Technological University. His research interests are in development of ionic liquids for various applications and in development of mesoporous materials for electrochemical energy systems.
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RAPID COMMUNICATION
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Trimetallic PtAgCu@PtCu core@shell concave nanooctahedrons with enhanced activity for formic acid oxidation reaction
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Geng-Tao Fu1,b, Bao-Yu Xiac,1, Ru-Guang Mac, Yu Chena,c,n, Ya-Wen Tangb, Jong-Min Leec,nn a
School of Materials Science and Engineering, Shaanxi Normal University, Xi’an 710062, PR China Jiangsu Key Laboratory of New Power Batteries, School of Chemistry and Materials Science, Nanjing Normal University, Nanjing 210023, PR China c School of Chemical and Biomedical Engineering, Nanyang Technological University, Singapore 637459, Singapore b
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Received 1 December 2014; received in revised form 23 January 2015; accepted 26 January 2015
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KEYWORDS
Abstract
Core@shell nanostructures; High-index facets; Formic acid oxidation; Reaction pathway; Electrocatalytic activity
Trimetallic PtAgCu@PtCu core@shell concave nanooctahedrons (CSCNOs) enclosed by highindex facets are synthesized by a simple one-pot hydrothermal reduction method. The catalytic reduction growth of both AgI and CuII on preformed Pt crystal nuclei is proposed for the formation of core@shell nanostructures. Moreover, the oxidative etching is also critical in the final morphology of concave nanocrystals. The as-prepared PtAgCu@PtCu CSCNOs demonstrate excellent activity and durability for the formic acid oxidation reaction (FAOR) with an unusual reaction pathway due to the geometric effect, electronic effect, and bi-functional mechanism. Our work provides some clues for the future design of nanocatalysts for a specific fuel molecular electrocatalytic oxidation, with the aim of the improvement of activity and durability. & 2015 Published by Elsevier Ltd.
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Corresponding author at: School of Chemical and Biomedical Engineering, Nanyang Technological University, Singapore 637459, Singapore. nn Q3 Corresponding author. E-mail addresses: [email protected] (Y. Chen), [email protected] (J.-M. Lee). 1 Dr. G.T. Fu and Dr. B.Y. Xia contributed equally to this work. Q2
Introduction Direct formic acid fuel cells (DFAFCs) have been widely considered as a promising green energy conversion technique due to their high electromotive force, reasonable power density, low fuel crossover, and facile power-system integration [1–4]. Pd-based electrocatalysts are highly active for the formic
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http://dx.doi.org/10.1016/j.nanoen.2015.01.041 2211-2855/& 2015 Published by Elsevier Ltd.
Please cite this article as: G.-T. Fu, et al., Trimetallic PtAgCu@PtCu core@shell concave nanooctahedrons with enhanced activity for formic acid oxidation reaction, Nano Energy (2015), http://dx.doi.org/10.1016/j.nanoen.2015.01.041
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acid oxidation reaction (FAOR) [2,5–7]. However, their poor stability due to the Pd dissolution in acidic solution and slow adsorption of CO-like intermediates limits their practical applications [2,6]. Pt-based nanocatalysts also exhibit excellent activity for the FAOR, but the dehydration pathway at lower potential is predominant on the Pt surface. Thus the produced CO-like intermediates would absorb and poison the surface of Pt catalysts. Incorporation of other transition metals with Pt to form bimetallic alloys, such as PtPd, PtAu, PtCu, PtCo, PtMn, PtNi etc., has been considered as an effective method to facilitate the dehydrogenation of the FAOR [8–18], and enhance the activity and durability consequently for the possible geometry and electronic effects. Besides the chemical composition, the morphologies and structures of Pt-based nanocatalysts also determine the catalytic performance. Especially, concave Pt-based nanocrystals with high-index facets have received unprecedented interests in electrocatalysts due to the unique surface structure which contains high density of atomic steps and kinks [19–32]. Although remarkable progress has been achieved in controllable synthesis of Pt-based nanocatalysts with high-index facets [23,33,34], trimetallic Pt-based nanocrystals with high-index facets has been rarely reported yet. In addition, the underlying mechanism of FAOR has not been fully understood for Pt-based multimetallic nanostructures. Therefore, it is challenging and also significant to develop an effective and simple method to prepare trimetallic Pt-based nanocrystals and also investigate their electrochemical properties for the FAOR. Herein, we exploit an effective one-pot method to synthesize PtAgCu@PtCu core@shell concave nanooctahedrons (CSCNOs) with high-index facets. The catalytic reduction growth of both Ag and Cu on preformed Pt crystal nuclei is proposed for the formation of concave PtAgCu@PtCu core@shell nanocrystals. Moreover, the oxidative etching is also critical in determining the final morphology of resultant concave nanocrystals. Because of the unique surface structure and possible synergistic effect, the asprepared PtAgCu@PtCu CSCNOs demonstrate excellent activity and stability for the FAOR. Importantly, the enhanced direct reaction pathway and the suppressive dehydration pathway of the FAOR are also proposed as the reasons for the enhanced electrochemical performance.
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Experimental Reagents and chemicals
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Polyallylamine hydrochloride (PAH, Scheme S1, weightaverage molecular weight 150,000) was supplied from Nitto Boseki Co., Ltd. (Tokyo, Japan). Potassium tetrachloroplatinate (II) (K2PtCl4), cupric chloride (CuCl2), silver nitrate (AgNO3), formaldehyde solution (HCHO, 40%), sulfuric acid (H2SO4), and formic acid (HCOOH) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Commercial Pt black were purchased from Johnson Matthey Corporation.
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Synthesis of PtAgCu@PtCu CSCNOs
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In a typical synthesis, 0.5 mL K2PtCl4 (0.05 M), 0.5 mL CuCl2 (0.05 M), 0.1 mL AgNO3 (0.05 M) and 1.1 mL PAH (0.50 M,
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molarity of PAH given with respect to the repeating unit) were added into 7.3 mL of H2O with continued stirring. After adding a 0.5 mL HCHO solution (40%), the resultant mixture (pH 3.0) was transferred to a 20-mL Teflon-lined stainless-steel autoclave, and was then heated at 180 1C for 4 h. After being cooled to room temperature, the obtained PtAgCu@PtCu CSCNOs were separated by centrifugation at 15,000 rpm for 15 min, washed with acetic acid for 12 h, and then dried at 60 1C for 5 h in a vacuum dryer. The cyclic voltammetry (CV) tests and X-ray photoelectron spectroscopy (XPS) demonstrated that a majority of PAH molecules on PtAgCu@PtCu CSCNOs surface could be removed via acetic acid washing (Figure S1). Under the same conditions, a series of controlled experiments were also conducted to investigate the formation/growth mechanisms of the PtAgCu@PtCu CSCNOs.
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Electrochemical measurements
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All electrochemical experiments were performed by using a CHI 760 D electrochemical analyzer at 30 71 1C. A standard three-electrode system was used for all electrochemical experiments, which consisted of a platinum wire as the auxiliary electrode, a saturated calomel reference electrode protected by Luggin capillary with KCl solution as the reference electrode, and a catalyst modified glassy carbon electrode as the working electrode. Potentials in this study were reported with respect to the SCE. An evenly distributed suspension of catalyst was prepared by ultrasonic the mixture of 10 mg catalyst and 5 mL H2O for 30 min, and 6 μL of the resulting suspension was drop-cast onto the surface of the glassy carbon electrode. After drying at room temperature, 3 μL of Nafion solution (5 wt%) was covered on the modified electrode surface and allowed drying again. Thus, the working electrode was obtained, and the specific loading of metal on the electrode surface was about 170 μg cm 2. Electrochemical measurements were conducted in N2-saturated 0.5 M H2SO4 solution or N2-saturated 0.5 M H2SO4 solution with 0.5 M HCOOH.
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Characterization
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Transmission electron microscopy (TEM) images were taken using a JEOL JEM-2100F transmission electron microscopy operated at 200 kV. High-resolution TEM (HRTEM), highangle annular dark-field scanning transmission electron microscopy (HAADF-STEM), and energy dispersive X-ray (EDX) elemental mapping measurements were performed on FEI Tecnai G2 F20 microscope equipped with an EDAX X-ray detector. Scanning electron microscopy (SEM) images were captured on a JSM-2010 microscope at an accelerating voltage of 20 kV. X-ray diffraction (XRD) patterns were obtained with a Model D/max-rC X-ray diffractometer using Cu Ka radiation source (λ = 1.5406 Å) and operating at 40 kV and 100 mA. XPS measurements were carried out on a Thermo VG Scientific ESCALAB 250 spectrometer. The binding energy was calibrated by means of the C 1s peak energy of 284.6 eV. The composition of samples was investigated by EDX technique and Leeman inductively coupled plasma atomic emission spectrometry (ICP-AES).
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Results and discussion
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The PtAgCu@PtCu CSCNOs are obtained after hydrothermal treatment of K2PtCl4, AgNO3 and CuCl2 in formaldehyde solution at 180 1C for 4 h in the presence of PAH. The morphology and structure of the PtAgCu@PtCu CSCNOs are characterized by TEM. The PtAgCu@PtCu CSCNOs have a uniform size of ca. 40 nm and exhibit a mostly star-like morphology with six arms (Figure 1A). The darker contrast in the center than the sharp horns confirms the formation of a concave structure (inset of Figure 1A). The concave octahedral structure is also demonstrated by high-angle annular dark-field scanning TEM (HAADFSTEM) images (Figure 1B) and SEM (Figure S2). To identify this morphology more directly, TEM and selected-area electron diffraction (SAED) along the three low-index zone axes are performed (Figure 1C). Both TEM and SAED patterns clearly demonstrate the single-crystalline nature of the PtAgCu@PtCu CSCNOs (Figure 1C). Specially, an individual PtAgCu@PtCu CSCNO with a cubic shape is projected from the [001] direction. The apex angles are measured to be 65.81 for the crystals
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3 oriented along [001] zone axes with geometrical model view from top (Figure 1C-c). According to the apex angle of arms, we can infer that the exposed facets of the PtAgCu@PtCu CSCNOs are {310} facets. The direction identification of negative curvature by high-resolution TEM (HR-TEM) image clearly shows the atomic steps at the edge of a PtAgCu@PtCu CSCNO (Figure 1D-a), matching the two dimensional (2D) atomic model of {310} facets. Other stereoscopic modes viewed from different directions in Figure 1C also demonstrate the concave structure with six prominent arms. The crystal structure and composition of the trimetallic PtAgCu@PtCu CSCNOs are characterized by XRD, EDX, ICP-AES, and XPS (Figure 2). Four typical diffraction peaks are observed clearly, indicating that the trimetallic PtAgCu nanocrystals have the face-centered cubic (fcc) structure (Figure 2A). No diffraction peaks for single-component Pt, Ag and Cu are observed in XRD pattern and all diffraction peaks of the PtAgCu@PtCu CSCNOs shift to a higher angle than pure fcc Pt (JCPDS no. 04-0802), which is indicative of uniform alloy formation. The Pt:Ag:Cu atomic ratio is estimated to be
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Figure 1 (A) TEM images of the PtAgCu@PtCu CSCNOs. (Inset a) TEM image of an individual nanocrystals; (inset b) a concave octahedron model. (B) HAADF-STEM image of the PtAgCu@PtCu CSCNOs. (Inset) Magnified HAADF-STEM image. (C) (a, a0 , a″) SAED patterns, (b, b0 , b″) TEM images, and (c, c0 , c″) geometric models of an individual PtAgCu@PtCu CSCNO along the (a, b) 〈100〉, (a0 , b0 ) 〈110〉, and (a″, b″) 〈111〉 zone axes directions. Eye and arrow in c, c0 , and c″ patterns indicate the projection direction of a concave octahedron model. (D) (a) The magnified HR-TEM image and (b) corresponding atomic model of the region indicated by the white lines in (a). Please cite this article as: G.-T. Fu, et al., Trimetallic PtAgCu@PtCu core@shell concave nanooctahedrons with enhanced activity for formic acid oxidation reaction, Nano Energy (2015), http://dx.doi.org/10.1016/j.nanoen.2015.01.041
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91 Figure 2 (A) XRD pattern of the PtAgCu@PtCu CSCNOs. (B) XPS survey scan spectrum of the PtAgCu@PtCu CSCNOs. (C) EDX mapping and (D) line scanning profiles of the PtAgCu@PtCu CSCNOs.
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45.5:9.3:45.2 by EDX (Figure S3), in good agreement with the result from ICP-AES analysis (45.8:8.5:45.7). XPS data reveals the surface atomic ratio of Pt:Ag:Cu is 48.7:2.3:49.0 (Figure 2B). Compared to EDX and ICP-AES results, the lower Ag content measured by XPS hints a PtCu-rich shell. The EDX elemental mapping images and EDX line scanning profiles are performed to get more insight into the distribution of each element in the PtAgCu@PtCu CSCNOs. The core@shell structure of a PtAgCu core and a PtCu shell are clearly revealed by EDX elemental images and line scanning profiles (Figure 2C and D) [35,36]. We first investigated the effects of involved reaction precursors on the final morphologies and compositions of the PtAgCu@PtCu CSCNOs. Only irregular and randomly aggregated nanocrystals were obtained in the absence of PAH (Figure S4), indicating that PAH serves as effective capping agent to ensure the dispersion of nanocrystals. Single-component CuII or AgI metal precursor cannot be reduced under the present experimental conditions. Monodisperse Pt nanocubes with six {100} facets are obtained when only PtII is used as metal precursor (Figure 3A). In the absence of Ag precursor, bimetallic PtCu alloy nanoflowers are generated (Figures 3B and S5), while bimetallic PtAg alloy nanocuboctahedra are obtained in the absence of Cu precursor (Figures 3C and S6). Therefore, the presence of Ag and Cu ions plays an important role on the formation of concave PtAgCu@PtCu nanocrystals. Considering the strong adsorption and interaction of AgI and CuII ions on Pt nuclei,
the autocatalytic reduction growth of Ag and Cu ions on preformed Pt nuclei is accounted for the formation of trimetallic nanocrystals. Taking into account the facts of the lower amount of AgI precursor than that of CuII precursor in reaction system and the higher reduction potential of AgI/Ag0 redox couple ( + 0.7996 V) than that of CuII/Cu0 redox couple (+ 0.340 V), the formation of PtAgCu@PtCu core@shell structure originates from the faster consumption of AgI precursor than that of CuII precursor during the autocatalytic reduction growth of both CuII and AgI on the preformed Pt crystal nuclei, as shown in XPS and EDX analysis (Figures 2 and 3). In addition, it is worth noting that well-defined concave nanooctahedrons cannot be obtained by bubbling N2 gas to eliminate typical etchant O2 (Figure 3D). To better understand the formation/growth mechanism of PtAgCu@PtCu CSCNOs, time dependent morphologies evolution process is also investigated by TEM. At the early stage, some octahedral nanocrystals of 15 nm are obtained (Figure 4A). With increasing reaction times, the diameter of nanocrystals gradually increases to 40 nm, accompanied with gradual formation of well-defined concave nanooctahedrons (Figure 4B–D). Recent works show nanocuboctahedron is an intermediate during the evolution from nanocubes to nanooctahedrons via the oxidative etching [37,38]. And, nanooctahedrons can transform into concave nanooctahedrons via the oxidative etching [39]. Considering the three facts: (i) the addition of AgI results in the formation of bimetallic PtAg alloy nanocuboctahedra
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Figure 3 TEM images of the Pt nanocubes (A), PtCu nanoflowers (B), PtAg nanocuboctahedra (C) and PtAgCu nanocrystals prepared by displacing the air with N2 to eliminate typical etchant O2 (D). Insets in (B) and (C) are EDX elemental mapping patterns and line scanning profiles.
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Figure 4 TEM images of the PtAgCu@PtCu intermediates collected at different growth stages: (A) 30 min, (B) 1 h, (C) 2 h, and (D) 4 h. (Inset a) TEM image of an individual nanooctahedron in red circle.
(Figure 3C); (ii) some octahedral nanocrystals are clearly observed in at early reaction stage (Figure 4A); (iii) O2 has important effect in determining the morphology of the products in our synthesis (Figure 3D), we infer that the formation of concave PtAgCu nanocrystals originates from oxidative etching. In a recent work, PtNi concave nanooctahedrons from PtNi nanooctahedrons via coordinating complex (i.e., dimethylglyoxime) assisted oxidative etching in the presence of air [39]. The dimethylglyoxime selectively coordinates to NiII species generated on nanooctahedrons via O2
oxidation, resulting in the higher susceptibility and dissolution rate of Ni species compared to Pt. The chemical etching process played a crucial role in the transformation of PtNi nanooctahedrons in to PtNi concave nanooctahedrons. In our synthesis, PAH has also excellent coordination capability for CuII ion and can generate water-soluble PAH-CuII complex (Figure S7). During the reaction, the Cu atoms on the corners and edges on preformed PtAgCu@PtCu nanooctahedrons are preferably removed via PAH-assisted oxidative etching, followed by the deposition and segregation of the remaining Pt
Please cite this article as: G.-T. Fu, et al., Trimetallic PtAgCu@PtCu core@shell concave nanooctahedrons with enhanced activity for formic acid oxidation reaction, Nano Energy (2015), http://dx.doi.org/10.1016/j.nanoen.2015.01.041
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atoms due to HCHO reduction. The continuous remove– deposition process contributes to the formation of PtAgCu@PtCu nanooctahedrons, similar to the case of PtNi concave nanooctahedrons. In the presence of dissolved oxygen, continuous Cu oxidation and reduction cycles occur during growth. The oxidative etching effectively eliminate twinned structures and affect atomic addition on preformed Pt nanocrystals surface, which facilitate the emergence of concave structure with high-index facets [40]. The electrochemical properties of the PtAgCu@PtCu CSCNOs are next evaluated by cyclic voltammetry (CV). Asprepared PtCu alloy nanoflowers and commercial Pt black are also compared. Prior to electrochemical tests for the FAOR, the working electrode was pretreated by cycling the potential between 0.242 and 0.958 V for 10 cycles (Figure S8). As observed, a weak and broad peak of Cu dissolution is observed at the first potential cycling. After 10 cycles, the cyclic voltammograms become stable, indicating that Cu dissolution has either ceased or dropped to undetectable level. Compared to the previous reports [41,42], the oxidation peak of Cu dissolution at the PtAgCu@PtCu CSCNOs is very small. After electrochemical pretreatment, XPS measurements show the surface chemical composition of the PtAgCu@PtCu CSCNOs is Pt:Ag:Cu=51.1:2.5:46.4 (Figure S9A), which is very close to the proportion of un-treated PtAgCu@PtCu CSCNOs (48.7:2.3:49.0). These results indicate only a small amount of Cu atoms at PtAgCu@PtCu CSCNOs is dissolved, which may be attributed to their high alloying degree. In addition, the morphology and electronic structure of the PtAgCu@PtCu CSCNOs almost remain after electrochemical pretreatment (Figures S9B and S10). The electrochemically surface areas (ECSA) of the PtAgCu@PtCu CSCNOs, PtCu alloy nanoflowers, and Pt black are determined to be 19.0, 23.5, and 17.8 m2 g 1, respectively, by the hydrogen adsorption–desorption method in conjunction with CV in 0.5 M H2SO4 solution (Figure S11). Figure 5A shows ECSA-normalized cyclic voltammograms of different electrocatalysts for the FAOR. Obviously, PtAgCu@PtCu CSCNOs (1.52 mA cm 2) demonstrate remarkably enhanced activity compared with PtCu alloy nanoflowers (0.97 mA cm 2) and commercial Pt black (0.16 mA cm 2) at 0.3 V. Moreover, peak I at 0.30 V and peak II at 0.67 V for commercial Pt black correspond to the oxidation of formic acid via the dehydrogenation pathway and oxidation of COads formed via the dehydration pathway, respectively. The ratio R between peak I and peak II is used to determine the pathway of FAOR on electrocatalysts. For the Pt black (Figure 5A), peak I current is 6.25 times lower than peak II current (R=0.16), indicating dehydration pathway is predominant. For PtCu alloy nanoflowers, peak I current is close to peak II current (R=1.05), indicating that alloying Pt with Cu facilitates the dehydrogenation pathway of the FAOR relative to pure Pt. Nevertheless, the PtAgCu@PtCu CSCNOs exhibit the highest R (1.30), illustrating that the FAOR on the PtAgCu@PtCu CSCNOs accomplishes mainly through the dehydrogenation pathway. To support this claim, the chronoamperometry at 0.3 V for 200 s is performed to allow the formation (if any) of COads poisoning intermediates on electrocatalysts surface, after which the accumulated COads intermediates are electrochemically stripped off in 0.5 M H2SO4 solution. As observed, the oxidation charge of accumulated COads on the PtAgCu@PtCu CSCNOs is 2.14 and 2.51 times smaller than that on the PtCu
alloy nanoflowers and commercial Pt black, respectively (Figure 5B), further confirming that dehydrogenation pathway of FAOR on the PtAgCu@PtCu CSCNOs is predominant. Meanwhile, it is worth noting that the R value (1.30) is also higher than that on previous reported Pt-based nanocrystals (such as Pt–Co [43], Pt–Mn [16], Pt–Cu [17,18], Pt–Au [15], and Pt–Ag [9,44]), confirming the superior electrocatalytic activity of the PtAgCu@PtCu CSCNOs for the FAOR. The mass activity (the current densities are normalized with respect to metal mass) of the PtAgCu@PtCu CSCNOs for the FAOR has been also remarkably enhanced compared to those of PtCu alloy nanoflowers and commercial Pt black (Figure S12). The enhanced activity of concave PtAgCu@PtCu core@shell nanocrystals for the FAOR is mainly attributed to their unique concave surface and trimetallic composition. The chronoamperometry results also demonstrate the superior electrocatalytic stability of the PtAgCu@PtCu CSCNOs partly due to their good crystallinity and alloy degree (Figure 5C). Recent results indicate that reaction pathway of the FAOR is sensitive to the actual surface structure and arrangement of Pt atoms. Three neighboring Pt atoms are required for the dehydration pathway but one isolated Pt atom is enough for the dehydrogenation pathway of FAOR (i.e., the ensemble effect) [45]. The ensemble effect can explain the observed results here because alloying Pt with Cu can effectively separate the surface Pt atoms and high-index facets have the more abundant low-coordinated atoms (i.e., isolated atoms) relative to low-index facets, thus making the dehydrogenation pathway dominant on the PtAgCu@PtCu CSCNOs. The electrocatalytic activity of Pt-based nanocrystals for the FAOR also depends the electronic structure of Pt atoms. The binding energies of Pt 4f, Cu 2p, and Ag 3d in the PtAgCu@PtCu CSCNOs exhibit a negative shift compared to the standard values of bulk Pt, Cu and Ag, respectively (Figure S13), attributing to the interaction between residual PAH and metal atoms (Figures S1 and S14). Due to the formation of N– Pt bond, the lone pair electrons of –NH2 groups in PAH effectively donate electrons to Pt, resulting in the shift of elemental binding energy [46,47]. The increased electron density in Pt facilitates the activation of formic acid molecules due to preferred electron back-donation from Pt to the 2πn orbital of the adsorbate, which improves the electrocatalytic activity of the PtAgCu@PtCu CSCNOs [48]. In addition, it is observed that the onset oxidation potential of the PtAgCu@PtCu CSCNOs with pre-absorbed CO negatively shift ca. 14 and 62 mV compared to those of PtCu alloy nanoflowers and Pt black (Figure 5D), indicating that the PtAgCu@PtCu CSCNOs have the priority for CO oxidation. Similar to the cases for Pt–Cu and Pt–Ag nanocrystals [18,49], the Cu and Ag component incorporated into Pt-based nanocrystals can facilitate CO oxidation with a unique ability to generate hydroxyl species from water at low potential (i.e., bi-functional mechanism [50]). Thus, the less accumulation and the more easy oxidation of COads on the PtAgCu@PtCu CSCNOs surface contribute to a large enhancement in durability.
Conclusions
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In summary, we present a facile one-pot hydrothermal method to prepare concave PtAgCu@PtCu core@shell nanooctahedrons. The catalytic reduction of both Ag and
Please cite this article as: G.-T. Fu, et al., Trimetallic PtAgCu@PtCu core@shell concave nanooctahedrons with enhanced activity for formic acid oxidation reaction, Nano Energy (2015), http://dx.doi.org/10.1016/j.nanoen.2015.01.041
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Figure 5 (A) The ESCA-normalized cyclic voltammograms for the PtAgCu@PtCu CSCNOs, PtCu alloy nanoflowers, and commercial Pt black in N2-saturated 0.5 M HCOOH +0.5 M H2SO4 solution at the scan rate of 50 mV s 1. (B) The PtAgCu@PtCu CSCNOs, PtCu alloy nanoflowers, and commercial Pt black were exposed to N2-saturated 0.5 M HCOOH +0.5 M H2SO4 solution for 200 s at 0.35 V potential, rinsed in water, and then placed in a 0.5 M H2SO4 solution for CV measurements. (C) Chronoamperometry curves for the PtAgCu@PtCu CSCNOs, PtCu alloy nanoflowers, and commercial Pt black in N2-saturated 0.5 M HCOOH+ 0.5 M H2SO4 solution for 3000 s at 0.35 V potential. (D) Pre-absorbed CO-stripping voltammograms for the PtAgCu@PtCu CSCNOs, PtCu alloy nanoflowers, and commercial Pt black in N2-saturated 0.5 M H2SO4 solution at a scan rate of 50 mV s 1.
Cu on preformed Pt crystal nuclei is responsible for the formation of the trimetallic alloy. The formation of the core@shell structure originates from the faster consumption of the AgI precursor than that of the CuII precursor during the autocatalytic reduction growth of both CuII and AgI. The oxidative etching contributes to the transformation of Pt nanocubes into PtAgCu@PtCu concave nanooctahedrons. The as-prepared PtAgCu@PtCu CSCNOs display superior electrocatalytic activity and stability for the FAOR relative to the commercial Pt black. The concave surface incorporated with high index facets {310} in this trimetallic nanocrystals induce geometry and alloy effects, and thus the dehydrogenation pathway of the FAOR is strengthened. Our work would provide some valuable clues to design and prepare enhanced electrocatalysts specifically for the FAOR by tailoring their surface structure and surface composition.
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The authors are grateful for the financial support of NSFC (21473111, 21376122 and 21273116), Natural Science Foundation of Jiangsu Province (BK20131395), United Fund of NSFC and Yunnan Province (U1137602), Industry-Academia Cooperation Innovation Fund Project of Jiangsu Province (BY2012001), Fundamental Research Funds for the Central Universities (GK201402016), and the Academic Research Fund of the Ministry of Education in Singapore (RGT27/13).
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Appendix A.
Supporting information
Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/ j.nanoen.2015.01.041.
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Geng-Tao Fu received his B.S. degree from the Department of Chemistry and Chemical Engineering, Binzhou University in 2011, and his M. S. degree from the College of Chemistry and Materials Science, Nanjing Normal University in 2014. He is currently pursuing his Ph.D. degree at Nanjing Normal University. His research interests focus on the advanced nanomaterials for electrochemical applications.
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Bao-Yu Xia received his Ph.D. degree in Materials Science and Engineering from Shanghai Jiao Tong University in 2010. He then moved to Nanyang Technological Univerisity, where he is a research fellow under the supervision of Prof. X. Wang and Prof. X.W. Lou. His research interests are in the areas of nanostructured electrocatalysts and their application in energy conversion.
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Ru-Guang Ma received his Ph.D. in materials science from City University of Hong Kong in 2013. Then he worked at Nanyang Technological University as a post-doctoral researcher. In July 2014, he joined Shanghai Institute of Ceramics, Chinese Academy of Sciences (SICCAS). He is currently an assistant professor in the State Key Lab of High Performance Ceramics and Superfine Microstructure, SICCAS. His research interests include design and synthesis of new nanostructured electrode materials for supercapacitors, lithium–oxygen batteries and novel non-precious metal catalysts.
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Yu Chen received his Ph.D. from School of Chemistry & Chemical Engineering in Nanjing University in 2009. Then he worked at School of Chemistry and Materials Science, Nanjing Normal University as an associate professor. In January 2014, he joined Shaanxi Normal University as an associate professor at School of Materials Science and Engineering. He worked at Nanyang Technological University as a post-doctoral researcher in the same year. His research interests include the design and synthesis of precious metal electrocatalysts, the fabrication and application of functionalized interface, and the electrochemical biosensor.
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Ya-Wen Tang received his B.S. degree and M.S. degree from Nanjing Normal University in 1992 and 2002, respectively, and his Ph.D. from Nanjing University of Science & Technology in 2011. He is currently a professor at College of Chemistry and Materials Science at Nanjing Normal University. His main research interests are the synthesis and assembly of nanomaterials, and their applications in batteries, fuel cells and photocatalysis.
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Jong-Min Lee received his Ph.D. degree from the Department of Chemical Engineering, Columbia University in 2003. He worked in Chemical Science Division, Lawrence Berkeley National Laboratory and in Department of Chemical Engineering, University of California at Berkeley as a postdoctoral fellow in 2006–2008. Since 2008, he has been appointed as an assistant professor in School of Chemical and Biomedical Engineering at Nanyang Technological University. His research interests are in development of ionic liquids for various applications and in development of mesoporous materials for electrochemical energy systems.
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