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Radiolysis route to Pt nanodendrites with enhanced comprehensive electrocatalytic performances for methanol oxidation
Catalysis Communications 62 (2015) 14–18
Contents lists available at ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short communication
Radiolysis route to Pt nanodendrites with enhanced comprehensive electrocatalytic performances for methanol oxidation Dengyu Pan a,⁎, Xueyuan Wang a, Jinghui Li a, Liang Wang a,⁎, Zhen Li b, Yuan Liu a, Haobo Liao a, Chuanqi Feng a, Jinkai Jiao a, Minghong Wu b a b
Institute of Nanochemistry and Nanobiology, Shanghai University, Shanghai 200444, PR China Shanghai Applied Radiation Institute, Shanghai University, Shanghai 200444, PR China
a r t i c l e
i n f o
Article history: Received 28 October 2014 Received in revised form 29 December 2014 Accepted 6 January 2015 Available online 7 January 2015
a b s t r a c t Unique Pt nanodendrites with controlled size were prepared by a rapid radiolysis route. The Pt nanodendrites display drastically enhanced electrochemical catalytic performances toward methanol oxidation. They exhibit a better CO tolerance for methanol oxidation compared with commercial catalysts and other nanodendritic Ptbased catalysts. © 2015 Elsevier B.V. All rights reserved.
Keywords: Pt nanodendrites Heterogenous catalysis Methanol oxidation Radiolysis
1. Introduction Direct methanol fuel cells (DMFCs) have been developed as attractive power sources for portable and automotive applications due to their high energy density, low pollutant emission, low operating temperature, and ease of handling liquid fuels [1]. To catalyze the oxidation of methanol for DMFCs, various catalysts such as Pt-based metals [1–7], metal oxides [8,9], and carbon materials [10,11] have been explored. Of them, Pt-based catalysts as the most active reported so far have currently attracted much attention [1–7]. Pt-based catalysts such as Pt black (PB), Pt/C, and PtCo/C have been commercialized, but their large-scale applications will be hampered by their relatively low activity, low poison tolerance, and high costs. In order to overcome these barriers, it is necessary to maximize the overall electrocatalytic performances of Pt-based catalysts by engineering their particle shape and size, surface structure, and chemical composition on the nanometer scale. Toward these targets, various nanostructures of Pt and its alloys have currently been explored, ranging from simple clusters [12,13], nanocrystals [14,15] and nanowires [16,17] to complex porous architectures [4,18–20]. Among them, porous nanostructures have been
⁎ Corresponding authors. E-mail addresses: [email protected] (D. Pan), [email protected] (L. Wang).
http://dx.doi.org/10.1016/j.catcom.2015.01.006 1566-7367/© 2015 Elsevier B.V. All rights reserved.
regarded as the most promising catalysts because of well-confined pore structures and better precious-metal utilization as well as much higher electrochemically active surface areas (ECSA) and specific catalytic activities. Nanodendrites are an interesting class of such porous nanostructures composed of open nanochannels and interconnected nanoarms with rich catalytically active edges, steps and corner atoms [7,21–24]. As expected, nanodendrites of Pt alloys showed remarkably enhanced poisoning tolerance, but nanodendrites of Pt alone exhibited a considerably low anti-poison ability [25,26]. The low poisoning tolerance indicates that methanol molecules are less effectively oxidized on catalysts during forward potential scan, and meanwhile more poisoning species, typically CO, are produced and lastingly occupy more active sites. Up to now, it is still challenging to develop simple and effective synthetic methods for preparing Pt-based catalysts with excellent comprehensive methanol electrooxidation performances without alloying or doping. Radiolysis reduction has been extensively used to synthesize mono- and multi-metallic clusters [27,28] and other nanostructures including porous Pt nanoballs and Pt–Ag nanowires [29,30]. Herein, we report for the first time the rapid, one-step synthesis of nearly monodisperse Pt nanodendrites with enhanced comprehensive methanol electrooxidation performances via electron beam radiolysis reduction in the presence of poly(vinyl pyrrolidone) (PVP). Different from common chemical reduction routes, radiolysis reduction in water is based upon the rapid radiolysis creation of high-
D. Pan et al. / Catalysis Communications 62 (2015) 14–18
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Fig. 1. TEM images of Pt nanodendrites grown in 0.04 M (a, b), 0.06 M (d, f), and 0.08 M (e) of H2PtCl6 aqueous solutions exposed to EB for 8 min at a dose of 140 kGy. (c) The size distribution of Pt nanodendrites obtained from the TEM image in (a).
energy reducing species (solvated electrons, H·radicals, and others) in a homogeneous distribution throughout the aqueous solution enabling the formation of nanodendrites with a narrow size distribution. Resultant Pt nanodendrites were found to exhibit not only remarkably increased specific and mass catalytic activities but also greatly enhanced poison tolerance, even higher than that of current state-of-the-art Pt-based alloy catalysts (PtCo/C).
2. Experimental and methods 2.1. Chemicals and materials All chemicals were of analytical grade and were used as received without further purification. All solutions were prepared with deionized water treated in a Millipore water purification system (Millipore Corp.).
Fig. 2. (a) HRTEM image of a single Pt nanodentrite; (b) HRTEM image of nanoarms constituting a Pt nanodentrite; (c) SAED pattern of Pt nanodendrites.
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2.2. Synthesis of Pt nanodendrites Pt nanodendrites were synthesized through electron beam radiolysis reduction of H2PtCl6·6H2O in a GJ-2-II electronic accelerator (Shanghai Xianfeng Co. Ltd.). Typically, 60 mL of H2PtCl6 aqueous solution with a concentration that varied from 0.04 to 0.08 M was mixed with 3.0 g of PVP and 3 mL of isopropanol (a scavenger of oxidative radicals) to form a micelle solution. The solution was then put into a sealed plastic bag and irradiated for 8 min under 2 MeV/10 mA conditions (140 kGy doses). Resulting Pt nanodendrites were isolated by centrifugation (16,000 rpm, 30 min), purified several times with water and absolute alcohol to remove excessive PVP, and then dried at 60 °C in vacuum for 10 h or redispersed in deionized water for further characterization. 2.3. Sample characterization Samples were characterized by transmission electron microscopy (TEM) on a JEOL JEM-2010F electron microscope operating at 200 kV, X-ray powder diffraction (XRD) on a Rigaku D/max-2500 using Cu Ka radiation, and FT-IR spectroscopy recorded on a Bio-Rad FTIR spectrometer FTS165. 2.4. Electrocatalytic measurements Measurements of cyclic voltammograms (CVs), linear-sweep voltammograms (LSVs), and chronoamperometric curves (CAs) were performed on a CHI660C electrochemical analyzer. A conventional threeelectrode cell was used, including a working electrode, a platinum wire as the counter electrode, and a saturated calomel electrode (SCE) as the reference electrode. The working electrode was fabricated by coating glassy carbon electrodes (GCE, 3 mm in diameter) with Pt nanodendrites or Pt black with a loading of 10 μg. Methanol electrooxidation measurements were carried out in a solution of 0.5 M of H2SO4 containing 1 M of CH3OH at the scan rate of 50 mV−1. 3. Results and discussion The Pt nanodendrites were synthesized based on the radiolysis reduction of H2PtCl6 dissolved in a PVP micelle solution exposed to high-energy electron beam (2 MeV) for merely a few minutes. Upon the irradiation, the color of the Pt complex solution changed from light brownish-yellow to opaque black, indicating of the production of Pt metal. After being isolated and washed several times to remove excessive PVP, resulting Pt nanodendrites were able to redisperse in water, owing to surface capping by PVP. The colloidal Pt nanodendrites show featureless UV–vis absorption from 300 to 800 nm (Figure S1). Their FT-IR spectrum exhibits the vibrations of C_O and N\C, ascribed to PVP molecules absorbed on the Pt surface (Figure S2). Their XRD pattern reveals their pure face-centered cubic (fcc) structure, whose (111), (200), (220), and (222) refraction peaks were observed (Figure S3). The size and shape of the Pt nanodendrites are characterized by TEM. Fig. 1 shows the TEM images of a series of samples prepared under different precursor concentrations (0.04, 0.06, 0.08 M). At 0.04 M, the lowmagnification view (Fig. 1a) shows that the sample consists of uniform spherical nanoparticles, whose diameters mainly distribute in the range of 12–28 nm (Fig. 1c), giving a mean diameter of 19 ± 7 nm. A higher magnification TEM image (Fig. 1b) reveals that these nanoparticles are actually nanodendrites composed of finer interconnected nanoarms (mean diameter: 2.5 nm) arranged in a 3D fractal shape. At higher concentrations, nanodendrites grow larger (Fig. 1d and e): their diameters range from 35 to 50 nm for 0.06 M and from 50 to 70 nm for 0.08 M, but the size of constituting nanoarms is nearly unvaried, irrespective of precursor concentrations. The high-resolution TEM (HRTEM) image of a single nanodendrite (Fig. 2a) further visualizes the interconnected nanoarms branching in various directions and thus forming open disordered nanochannels. The lattice fringes of {100}, {110}, and {111} were
Table 1 Electrocatalytic performance parameters of commercial and nanodendritic Pt-based catalysts toward methanol oxidation. Sample
Pt black Pt/C (E-TEK) PtCo/C (E-TEK) Nanodendritic Pt (Ref. [25]) Nanodendritic Pt (Ref. [31]) Nanodendritic Pt/Cu (Ref. [28]) Nanodendritic Pt (this paper)
ECSA (m2/g)
Peak current density Specific (mA/cm2)
Mass (A/mg)
27.2 53.3 48.7 14 42.6 – 39.5
0.5 1.26 1.70 1.25 1.09 0.96 1.29
0.14 0.52 0.83 – 0.46 – 0.52
Anti-poison If/Ib
0.91 0.76 1.32 b1 1.07 1.43 1.57
found to coherently extend across several nanoarms, as clearly shown in Fig. 2b. Most of the exposed facets of nanoarms were found to be {111}, but some {110} and high-index {311} facets could also be identified. This structural feature is also revealed by the selected area electron diffraction (SAED) pattern performed on several nanodendrites (Fig. 2c), where (111), (200), and (220) diffraction rings were observed along with some bright and isolated diffraction dots on these rings. The growth of Pt nanodendrites or porous nanostructures in solutions has been understood based upon heterogeneous nucleation and subsequent growth on Pt or Pd nanocrystals as seeds [31,32]. This growth image is also applicable to our case. H2PtCl6 was step-by-step reduced into zero-valence Pt by solvated electrons or other reducing species. At low concentrations of H2PtCl6, Pt nanodendrites were mass produced. However, under higher concentrations, besides nanodendrites, Pt nanocrystals with a diameter of 2–3 nm were also generated in the homogeneous solution (Fig. 1d, f) owing to the faster nucleation rate. Without the aid of the stabilizing agent, uncontrollable aggregation will occur, as shown in Figure S4. The metal nuclei must have a high autocatalytic activity that favors fast heterogeneous nucleation on some of their exposed facets and subsequent growth into nanoarms. These freshly formed nanoarms can also serve as new seeds inducing growth of nanodendrites. To examine this assumption, we performed the synthesis of Ag nanostructures under conditions similar to the case of Pt nanodendrites (Figure S5). Ag nanoparticles rather than nanodendrites were produced, possibly because Ag has a low catalytic activity than Pt, thus going against the nucleation and growth of nanoarms on Ag seeds. The electrochemical properties of uniform Pt nanodendrites prepared at 0.04 M of the precursor were explored. For comparison, four typical electrochemical parameters of our sample, commercial Ptbased catalysts (Pt black, Pt/C, and PtCo/C), and previously reported Pt-based nanodendrites [25,31] are listed in Table 1. As the most critical structural parameter for quantitative evaluation of the correlation of catalytic activities and structures of platinum-based catalysts, ECSAs
Fig. 3. Cyclic voltammograms (CVs) of Pt nanodendrites and commercial Pt black (for comparison) in Ar-saturated 0.5 M H2SO4 solution at room temperature. Scan rate: 50 V/s.
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Fig. 4. Methanol electrooxidation performances of Pt nanodendrites and commercial Pt black in 0.5 M of H2SO4 and 1 M of methanol solution: (a,b) CVs normalized to the Pt mass (mass activity) and the real Pt surface area (specific activity), respectively; (c) linear-sweep voltammograms (LSVs); (d) chronoamperometric (CA) curves recorded at 0.6 V (the inset shows the magnification within the time span from 1000–2000 s). Scan rate: 50 mV/s.
were measured by the electric charges of hydrogen adsorption and desorption by cyclic voltammetry (CV) in an aqueous solution of H2SO4 (0.5 M) (Fig. 3). The ECSA of our sample is determined to be 39.5 m2 g−1, much higher than that of the commercial Pt black catalyst (27.2 m2 g− 1) and that of reported nanodendritic Pt-based catalysts prepared by using a complicated organic-phase synthesis (14 m2 g−1) [28]. Given the high electrochemically active surface area, the Pt nanodendrites are expected to exhibit excellent electrocatalytic performances for the oxidation of methanol, which are evaluated by cyclic voltammetry (CV). CV curves were recorded in an aqueous solution of H2SO4 (0.5 M) and methanol (1 M) after 100 scan cycles, and normalized by both ECSA (specific activity) and Pt mass (mass activity). Notably, the mass peak current density of the Pt nanodendrites is 0.52 A mg− 1, 3.7 times higher than that of the PB (0.14 A mg−1) (Fig. 4a), and their specific peak current density is 1.28 mA cm−2, 2.5 times higher than of the PB (0.51 mA cm−2) (Fig. 4b). It is also noted that our sample shows the highest specific and mass activities among the Pt-based nanodendrites reported so far [25,31] (Table 1). Besides the three parameters of the ECSA, specific and mass activities, the fourth one, the peak current ratio of the forward to backward scans (If/Ib), is remarkable for our sample. This parameter is commonly used to evaluate the poisoning tolerance of catalysts in DMFCs [31–34]. Interestingly, we found that the If/Ib of our sample is as high as 1.57, even higher than that of Pt-based alloys (PtCo/C (E-TEK): 1.32; Pt/Cu: 1.43). These results indicate that the Pt nanodendrites synthesized by electron beam radiolysis show both high catalytic activities and better poison tolerance ability.
Linear-sweep voltammogram (LSV) measurements were performed on our sample and the PB. The onset potential (≈0.21 V) of our sample for methanol oxidation is much lower that of the PB (≈0.43 V), indicating that methanol oxidation is easier to occur on the surface of nanodendrites with richer catalytically active edges, steps and corner atoms (Fig. 4c). Moreover, at any given oxidation current density, the corresponding oxidation potential on the nanodendrites is distinctly lower than that on the PB, further displaying higher catalytic performance of the nanodendrites. In order to further evaluate the longterm electrocatalytic performance and the rate of surface poisoning, chronoamperometry (CA) curves of the Pt nanodendrites and PB were measured in 0.5 M H2SO4 with 1 M of methanol (Fig. 4d). The potential was held at 0.6 V and lasted for 2000 s during the measurements. The Pt nanodendrites exhibit a slower current decay over time in comparison with the PB, indicating a higher tolerance to the carbonaceous species generated during methanol oxidation. In addition, after quick decay at the beginning, the current density of the Pt nanodendrites reaches a steady state, with steady-state current density much higher than that of the PB, which further testifies that the Pt nanodendrites possess a higher catalytic efficiency for methanol electrooxidation than PB. 4. Conclusions In conclusion, we have successfully developed a rapid radiolysis route for fabrication of unique Pt nanodendrites with controlled size. The Pt nanodendrites display drastically enhanced electrochemical catalytic performances toward methanol oxidation, demonstrating their promising potential for use as a highly effective catalyst. Moreover,
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the Pt nanodendrites have better CO tolerance for methanol oxidation compared with commercial catalysts and other nanodendritic Ptbased catalysts. The Pt nanodendrites synthesized by electron beam irradiation have great potential in many fields beyond fuel cells, such as solar cells, hydrogen storage, and electrochemical sensors. Our studies will cast new light on the design and controllable fabrication of novel, efficient catalysts in electrochemical catalytic applications. Acknowledgments This work has been supported by the National Natural Science Foundation of China (No. 11174194, 91233102, 21371115), the Major State Basic Research Development Program of China (No. 2011CB933402), the Innovation Program of Shanghai Municipal Education Commission (No. 13YZ017), the Program for Changjiang Scholars and Innovative Research Team in University (No. IRT13078), the China Postdoctoral Science Foundation funded project (2012M520874), and the Shanghai Post-doctoral Scientific Program (13R21413100). Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.catcom.2015.01.006. References
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Contents lists available at ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short communication
Radiolysis route to Pt nanodendrites with enhanced comprehensive electrocatalytic performances for methanol oxidation Dengyu Pan a,⁎, Xueyuan Wang a, Jinghui Li a, Liang Wang a,⁎, Zhen Li b, Yuan Liu a, Haobo Liao a, Chuanqi Feng a, Jinkai Jiao a, Minghong Wu b a b
Institute of Nanochemistry and Nanobiology, Shanghai University, Shanghai 200444, PR China Shanghai Applied Radiation Institute, Shanghai University, Shanghai 200444, PR China
a r t i c l e
i n f o
Article history: Received 28 October 2014 Received in revised form 29 December 2014 Accepted 6 January 2015 Available online 7 January 2015
a b s t r a c t Unique Pt nanodendrites with controlled size were prepared by a rapid radiolysis route. The Pt nanodendrites display drastically enhanced electrochemical catalytic performances toward methanol oxidation. They exhibit a better CO tolerance for methanol oxidation compared with commercial catalysts and other nanodendritic Ptbased catalysts. © 2015 Elsevier B.V. All rights reserved.
Keywords: Pt nanodendrites Heterogenous catalysis Methanol oxidation Radiolysis
1. Introduction Direct methanol fuel cells (DMFCs) have been developed as attractive power sources for portable and automotive applications due to their high energy density, low pollutant emission, low operating temperature, and ease of handling liquid fuels [1]. To catalyze the oxidation of methanol for DMFCs, various catalysts such as Pt-based metals [1–7], metal oxides [8,9], and carbon materials [10,11] have been explored. Of them, Pt-based catalysts as the most active reported so far have currently attracted much attention [1–7]. Pt-based catalysts such as Pt black (PB), Pt/C, and PtCo/C have been commercialized, but their large-scale applications will be hampered by their relatively low activity, low poison tolerance, and high costs. In order to overcome these barriers, it is necessary to maximize the overall electrocatalytic performances of Pt-based catalysts by engineering their particle shape and size, surface structure, and chemical composition on the nanometer scale. Toward these targets, various nanostructures of Pt and its alloys have currently been explored, ranging from simple clusters [12,13], nanocrystals [14,15] and nanowires [16,17] to complex porous architectures [4,18–20]. Among them, porous nanostructures have been
⁎ Corresponding authors. E-mail addresses: [email protected] (D. Pan), [email protected] (L. Wang).
http://dx.doi.org/10.1016/j.catcom.2015.01.006 1566-7367/© 2015 Elsevier B.V. All rights reserved.
regarded as the most promising catalysts because of well-confined pore structures and better precious-metal utilization as well as much higher electrochemically active surface areas (ECSA) and specific catalytic activities. Nanodendrites are an interesting class of such porous nanostructures composed of open nanochannels and interconnected nanoarms with rich catalytically active edges, steps and corner atoms [7,21–24]. As expected, nanodendrites of Pt alloys showed remarkably enhanced poisoning tolerance, but nanodendrites of Pt alone exhibited a considerably low anti-poison ability [25,26]. The low poisoning tolerance indicates that methanol molecules are less effectively oxidized on catalysts during forward potential scan, and meanwhile more poisoning species, typically CO, are produced and lastingly occupy more active sites. Up to now, it is still challenging to develop simple and effective synthetic methods for preparing Pt-based catalysts with excellent comprehensive methanol electrooxidation performances without alloying or doping. Radiolysis reduction has been extensively used to synthesize mono- and multi-metallic clusters [27,28] and other nanostructures including porous Pt nanoballs and Pt–Ag nanowires [29,30]. Herein, we report for the first time the rapid, one-step synthesis of nearly monodisperse Pt nanodendrites with enhanced comprehensive methanol electrooxidation performances via electron beam radiolysis reduction in the presence of poly(vinyl pyrrolidone) (PVP). Different from common chemical reduction routes, radiolysis reduction in water is based upon the rapid radiolysis creation of high-
D. Pan et al. / Catalysis Communications 62 (2015) 14–18
15
Fig. 1. TEM images of Pt nanodendrites grown in 0.04 M (a, b), 0.06 M (d, f), and 0.08 M (e) of H2PtCl6 aqueous solutions exposed to EB for 8 min at a dose of 140 kGy. (c) The size distribution of Pt nanodendrites obtained from the TEM image in (a).
energy reducing species (solvated electrons, H·radicals, and others) in a homogeneous distribution throughout the aqueous solution enabling the formation of nanodendrites with a narrow size distribution. Resultant Pt nanodendrites were found to exhibit not only remarkably increased specific and mass catalytic activities but also greatly enhanced poison tolerance, even higher than that of current state-of-the-art Pt-based alloy catalysts (PtCo/C).
2. Experimental and methods 2.1. Chemicals and materials All chemicals were of analytical grade and were used as received without further purification. All solutions were prepared with deionized water treated in a Millipore water purification system (Millipore Corp.).
Fig. 2. (a) HRTEM image of a single Pt nanodentrite; (b) HRTEM image of nanoarms constituting a Pt nanodentrite; (c) SAED pattern of Pt nanodendrites.
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2.2. Synthesis of Pt nanodendrites Pt nanodendrites were synthesized through electron beam radiolysis reduction of H2PtCl6·6H2O in a GJ-2-II electronic accelerator (Shanghai Xianfeng Co. Ltd.). Typically, 60 mL of H2PtCl6 aqueous solution with a concentration that varied from 0.04 to 0.08 M was mixed with 3.0 g of PVP and 3 mL of isopropanol (a scavenger of oxidative radicals) to form a micelle solution. The solution was then put into a sealed plastic bag and irradiated for 8 min under 2 MeV/10 mA conditions (140 kGy doses). Resulting Pt nanodendrites were isolated by centrifugation (16,000 rpm, 30 min), purified several times with water and absolute alcohol to remove excessive PVP, and then dried at 60 °C in vacuum for 10 h or redispersed in deionized water for further characterization. 2.3. Sample characterization Samples were characterized by transmission electron microscopy (TEM) on a JEOL JEM-2010F electron microscope operating at 200 kV, X-ray powder diffraction (XRD) on a Rigaku D/max-2500 using Cu Ka radiation, and FT-IR spectroscopy recorded on a Bio-Rad FTIR spectrometer FTS165. 2.4. Electrocatalytic measurements Measurements of cyclic voltammograms (CVs), linear-sweep voltammograms (LSVs), and chronoamperometric curves (CAs) were performed on a CHI660C electrochemical analyzer. A conventional threeelectrode cell was used, including a working electrode, a platinum wire as the counter electrode, and a saturated calomel electrode (SCE) as the reference electrode. The working electrode was fabricated by coating glassy carbon electrodes (GCE, 3 mm in diameter) with Pt nanodendrites or Pt black with a loading of 10 μg. Methanol electrooxidation measurements were carried out in a solution of 0.5 M of H2SO4 containing 1 M of CH3OH at the scan rate of 50 mV−1. 3. Results and discussion The Pt nanodendrites were synthesized based on the radiolysis reduction of H2PtCl6 dissolved in a PVP micelle solution exposed to high-energy electron beam (2 MeV) for merely a few minutes. Upon the irradiation, the color of the Pt complex solution changed from light brownish-yellow to opaque black, indicating of the production of Pt metal. After being isolated and washed several times to remove excessive PVP, resulting Pt nanodendrites were able to redisperse in water, owing to surface capping by PVP. The colloidal Pt nanodendrites show featureless UV–vis absorption from 300 to 800 nm (Figure S1). Their FT-IR spectrum exhibits the vibrations of C_O and N\C, ascribed to PVP molecules absorbed on the Pt surface (Figure S2). Their XRD pattern reveals their pure face-centered cubic (fcc) structure, whose (111), (200), (220), and (222) refraction peaks were observed (Figure S3). The size and shape of the Pt nanodendrites are characterized by TEM. Fig. 1 shows the TEM images of a series of samples prepared under different precursor concentrations (0.04, 0.06, 0.08 M). At 0.04 M, the lowmagnification view (Fig. 1a) shows that the sample consists of uniform spherical nanoparticles, whose diameters mainly distribute in the range of 12–28 nm (Fig. 1c), giving a mean diameter of 19 ± 7 nm. A higher magnification TEM image (Fig. 1b) reveals that these nanoparticles are actually nanodendrites composed of finer interconnected nanoarms (mean diameter: 2.5 nm) arranged in a 3D fractal shape. At higher concentrations, nanodendrites grow larger (Fig. 1d and e): their diameters range from 35 to 50 nm for 0.06 M and from 50 to 70 nm for 0.08 M, but the size of constituting nanoarms is nearly unvaried, irrespective of precursor concentrations. The high-resolution TEM (HRTEM) image of a single nanodendrite (Fig. 2a) further visualizes the interconnected nanoarms branching in various directions and thus forming open disordered nanochannels. The lattice fringes of {100}, {110}, and {111} were
Table 1 Electrocatalytic performance parameters of commercial and nanodendritic Pt-based catalysts toward methanol oxidation. Sample
Pt black Pt/C (E-TEK) PtCo/C (E-TEK) Nanodendritic Pt (Ref. [25]) Nanodendritic Pt (Ref. [31]) Nanodendritic Pt/Cu (Ref. [28]) Nanodendritic Pt (this paper)
ECSA (m2/g)
Peak current density Specific (mA/cm2)
Mass (A/mg)
27.2 53.3 48.7 14 42.6 – 39.5
0.5 1.26 1.70 1.25 1.09 0.96 1.29
0.14 0.52 0.83 – 0.46 – 0.52
Anti-poison If/Ib
0.91 0.76 1.32 b1 1.07 1.43 1.57
found to coherently extend across several nanoarms, as clearly shown in Fig. 2b. Most of the exposed facets of nanoarms were found to be {111}, but some {110} and high-index {311} facets could also be identified. This structural feature is also revealed by the selected area electron diffraction (SAED) pattern performed on several nanodendrites (Fig. 2c), where (111), (200), and (220) diffraction rings were observed along with some bright and isolated diffraction dots on these rings. The growth of Pt nanodendrites or porous nanostructures in solutions has been understood based upon heterogeneous nucleation and subsequent growth on Pt or Pd nanocrystals as seeds [31,32]. This growth image is also applicable to our case. H2PtCl6 was step-by-step reduced into zero-valence Pt by solvated electrons or other reducing species. At low concentrations of H2PtCl6, Pt nanodendrites were mass produced. However, under higher concentrations, besides nanodendrites, Pt nanocrystals with a diameter of 2–3 nm were also generated in the homogeneous solution (Fig. 1d, f) owing to the faster nucleation rate. Without the aid of the stabilizing agent, uncontrollable aggregation will occur, as shown in Figure S4. The metal nuclei must have a high autocatalytic activity that favors fast heterogeneous nucleation on some of their exposed facets and subsequent growth into nanoarms. These freshly formed nanoarms can also serve as new seeds inducing growth of nanodendrites. To examine this assumption, we performed the synthesis of Ag nanostructures under conditions similar to the case of Pt nanodendrites (Figure S5). Ag nanoparticles rather than nanodendrites were produced, possibly because Ag has a low catalytic activity than Pt, thus going against the nucleation and growth of nanoarms on Ag seeds. The electrochemical properties of uniform Pt nanodendrites prepared at 0.04 M of the precursor were explored. For comparison, four typical electrochemical parameters of our sample, commercial Ptbased catalysts (Pt black, Pt/C, and PtCo/C), and previously reported Pt-based nanodendrites [25,31] are listed in Table 1. As the most critical structural parameter for quantitative evaluation of the correlation of catalytic activities and structures of platinum-based catalysts, ECSAs
Fig. 3. Cyclic voltammograms (CVs) of Pt nanodendrites and commercial Pt black (for comparison) in Ar-saturated 0.5 M H2SO4 solution at room temperature. Scan rate: 50 V/s.
D. Pan et al. / Catalysis Communications 62 (2015) 14–18
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Fig. 4. Methanol electrooxidation performances of Pt nanodendrites and commercial Pt black in 0.5 M of H2SO4 and 1 M of methanol solution: (a,b) CVs normalized to the Pt mass (mass activity) and the real Pt surface area (specific activity), respectively; (c) linear-sweep voltammograms (LSVs); (d) chronoamperometric (CA) curves recorded at 0.6 V (the inset shows the magnification within the time span from 1000–2000 s). Scan rate: 50 mV/s.
were measured by the electric charges of hydrogen adsorption and desorption by cyclic voltammetry (CV) in an aqueous solution of H2SO4 (0.5 M) (Fig. 3). The ECSA of our sample is determined to be 39.5 m2 g−1, much higher than that of the commercial Pt black catalyst (27.2 m2 g− 1) and that of reported nanodendritic Pt-based catalysts prepared by using a complicated organic-phase synthesis (14 m2 g−1) [28]. Given the high electrochemically active surface area, the Pt nanodendrites are expected to exhibit excellent electrocatalytic performances for the oxidation of methanol, which are evaluated by cyclic voltammetry (CV). CV curves were recorded in an aqueous solution of H2SO4 (0.5 M) and methanol (1 M) after 100 scan cycles, and normalized by both ECSA (specific activity) and Pt mass (mass activity). Notably, the mass peak current density of the Pt nanodendrites is 0.52 A mg− 1, 3.7 times higher than that of the PB (0.14 A mg−1) (Fig. 4a), and their specific peak current density is 1.28 mA cm−2, 2.5 times higher than of the PB (0.51 mA cm−2) (Fig. 4b). It is also noted that our sample shows the highest specific and mass activities among the Pt-based nanodendrites reported so far [25,31] (Table 1). Besides the three parameters of the ECSA, specific and mass activities, the fourth one, the peak current ratio of the forward to backward scans (If/Ib), is remarkable for our sample. This parameter is commonly used to evaluate the poisoning tolerance of catalysts in DMFCs [31–34]. Interestingly, we found that the If/Ib of our sample is as high as 1.57, even higher than that of Pt-based alloys (PtCo/C (E-TEK): 1.32; Pt/Cu: 1.43). These results indicate that the Pt nanodendrites synthesized by electron beam radiolysis show both high catalytic activities and better poison tolerance ability.
Linear-sweep voltammogram (LSV) measurements were performed on our sample and the PB. The onset potential (≈0.21 V) of our sample for methanol oxidation is much lower that of the PB (≈0.43 V), indicating that methanol oxidation is easier to occur on the surface of nanodendrites with richer catalytically active edges, steps and corner atoms (Fig. 4c). Moreover, at any given oxidation current density, the corresponding oxidation potential on the nanodendrites is distinctly lower than that on the PB, further displaying higher catalytic performance of the nanodendrites. In order to further evaluate the longterm electrocatalytic performance and the rate of surface poisoning, chronoamperometry (CA) curves of the Pt nanodendrites and PB were measured in 0.5 M H2SO4 with 1 M of methanol (Fig. 4d). The potential was held at 0.6 V and lasted for 2000 s during the measurements. The Pt nanodendrites exhibit a slower current decay over time in comparison with the PB, indicating a higher tolerance to the carbonaceous species generated during methanol oxidation. In addition, after quick decay at the beginning, the current density of the Pt nanodendrites reaches a steady state, with steady-state current density much higher than that of the PB, which further testifies that the Pt nanodendrites possess a higher catalytic efficiency for methanol electrooxidation than PB. 4. Conclusions In conclusion, we have successfully developed a rapid radiolysis route for fabrication of unique Pt nanodendrites with controlled size. The Pt nanodendrites display drastically enhanced electrochemical catalytic performances toward methanol oxidation, demonstrating their promising potential for use as a highly effective catalyst. Moreover,
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the Pt nanodendrites have better CO tolerance for methanol oxidation compared with commercial catalysts and other nanodendritic Ptbased catalysts. The Pt nanodendrites synthesized by electron beam irradiation have great potential in many fields beyond fuel cells, such as solar cells, hydrogen storage, and electrochemical sensors. Our studies will cast new light on the design and controllable fabrication of novel, efficient catalysts in electrochemical catalytic applications. Acknowledgments This work has been supported by the National Natural Science Foundation of China (No. 11174194, 91233102, 21371115), the Major State Basic Research Development Program of China (No. 2011CB933402), the Innovation Program of Shanghai Municipal Education Commission (No. 13YZ017), the Program for Changjiang Scholars and Innovative Research Team in University (No. IRT13078), the China Postdoctoral Science Foundation funded project (2012M520874), and the Shanghai Post-doctoral Scientific Program (13R21413100). Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.catcom.2015.01.006. References
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