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One-step synthesis of hollow-like porous palladium sphere with enhanced electrocatalytic performance
Author’s Accepted Manuscript One-step synthesis of hollow-like porous palladium sphere with enhanced electrocatalytic performance Xiaoli Liu, Dongdong Xu, Jianchun Bao
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PII: DOI: Reference:
S0167-577X(16)31540-3 http://dx.doi.org/10.1016/j.matlet.2016.09.078 MLBLUE21516
To appear in: Materials Letters Received date: 16 August 2016 Revised date: 15 September 2016 Accepted date: 19 September 2016 Cite this article as: Xiaoli Liu, Dongdong Xu and Jianchun Bao, One-step synthesis of hollow-like porous palladium sphere with enhanced electrocatalytic performance, Materials Letters, http://dx.doi.org/10.1016/j.matlet.2016.09.078 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
One-step synthesis of hollow-like porous palladium sphere with enhanced electrocatalytic performance Xiaoli Liu, Dongdong Xu*, Jianchun Bao* Jiangsu Key Laboratory of Biofunctional Materials, School of Chemistry and Materials Science, Nanjing Normal University, Nanjing, 210023, P. R. China [email protected] [email protected] * Corresponding authors. Tel: +86-25-8589-1936. Abstract
Hollow-like porous Pd spheres were prepared by using dioctadecyldimethylammonium chloride and n-heptane in the aqueous solution. The Pd spheres possess both good mesopores and macropores. Due to the porous feature, they exhibited well electrocatalytic performance toward formic acid oxidation compared to the commercial Pd black. Keywords: Porous palladium; Self-assembly; Formic acid oxidation; Hollow sphere 1. Introduction Noble metal palladium, as a potential alternative of expensive platinum, has attracted intense interest as primary electrocatalyst.[1,2] Lots of Pd nanomaterials with various morphologies have been successfully prepared and also presented desired electrocatalytic performance in fuel cells.[3,4] Porous architectures, due to the large surface area and corresponding high mass transfer rate, are of particular interest when employed in catalysis.[5-7] However, unlike the synthesis system of porous amorphous silica,[8,9] the controlled construction of porous Pd is still a challenge due to the rigid crystal structure which would easily destroy the existed nanopores. Many innovative strategies have proved the capacity to produce nanopores inside Pd crystals, such as templating methods, galvanic replacements, and selective removal of other
1
components.[10-13] However, the obtained Pd nanostructures usually possess poor porosity. As well known, soft templates could direct fine mesopores inside the inorganic nanomaterials through the self-assembly process.
Huang reported the synthesis of porous Pd nanostructure by using
surfactant hexadecylpyridinium chloride (HPC) as shape-directing agent.[12] Mesoporous Pd films with perpendicular mesochannels were prepared via surfactant-directed synthesis and elaborated electrochemical deposition.[14] Porous single-crystalline palladium nanoparticles were produced via cetyltrimethylammonium chloride (CTAB).[15] We can find that all of above cases need the participation of certain surfactants. In this work, hollow-like porous Pd spheres were synthesized in the aqueous solution. H2O, Na2PdCl4, dioctadecyldimethylammonium chloride (DODAC) and n-heptane were simply mixed together and then L-ascorbic acid (AA) was added as the reducing agent. As DODAC possesses double hydrophobic carbon chains, it would be of great help for the hydrophobic interactions between alkyl chains and oil droplets (n-heptane). Pd spheres with both fine mesopores and macropores were obtained after the slow reduction process under 35 oC. Compared to commercial palladium black (PdB), porous Pd spheres exhibited good electrocatalytic activity toward formic acid oxidation reaction (FAOR). 2. Experimental section 2.1 Synthesis of hollow-like porous Pd sphere In a typical synthesis, 1.2 g n-heptane was added dropwise into 5 mL (0.01 M) DODAC aqueous solution. After 1 h, 0.4 mL (10 mM) aqueous solution of Na2PdCl4 was injected into the solution. Then, 0.5 mL (80 mM) fresh L-ascorbic acid solution was added quickly. The final mixture was kept under 35 oC for 5 h. All of the processes mentioned above were operated under continuous stirring. 2
The black products were collected by centrifugation and washed several times with ethanol. 2.2 Characterizations The X-ray diffraction (XRD) patterns were collected on a D/max 2500 VL/PC diffractometer equipped with graphite-monochromatized Cu Kα radiation (λ = 1.54060 Å) in 2θ ranging from 1o to 90o. The corresponding work voltage and current are 40 kV and 100 mA, respectively. The scanning electron microscope (SEM) images were obtained on a JSM-7600F apparatus at an accelerating voltage of 10 kV. The transmission electron microscope (TEM) observation was performed with a JEOL JEM-2100 microscope operated at 200 kV. 2.3 Electrochemical measurements Cyclic voltammograms (CVs) and chronoamperometric curves were achieved with a CHI 660D analyzer. A conventional three-electrode cell was used, consisting of Ag/AgCl electrode as a reference electrode, Pt foil as a counter electrode, and glassy carbon electrode as a working electrode. In the typical procedure, 4 mg catalysts were dispersed in 0.5 mL ethanol and 1.5 mL H2O and sonicated for 30 minutes to form a homogeneous ink. 6 μL ink was uniformly loaded on clean glassy carbon electrode surface. 5 μL of 0.1 wt % Nafion solution was then dropped onto the electrode surface. Electrochemical measurements were performed in a N2-saturated 0.5 M H2SO4 solution with or without 0.5 M HCOOH at a scan rate of 50 mV·s-1. All potential values were referenced to Ag/AgCl electrode. 3. Results and discussions The SEM image shows that the obtained Pd nanoparticles have a uniform sphere morphology (Fig. 1a). The size distribution indicates that the average diameter is about 156 nm. SEM image of a broken sphere proves that these Pd spheres do not possess the completely hollow structure. Instead, 3
some threadlike nanoparticles sprawl out from the centre and connect to the spherical shell (Fig. 1b). Therefore, hollow-like spheres were more appropriate to represent these Pd nanoparticles. TEM images in Figs. 1c and 1d also confirm the hollow-like structure because there are some dark parts in the centre of the spheres, unlike the TEM images of other hollow spheres as reported everywhere.[16] Abundant mesopores can be found throughout the Pd spheres and particularly along the spherical shell. Therefore, the Pd spheres have two types of pore channels, mesopores and macropores, respectively. From the low-angle XRD pattern in Fig. 2a, a broad peak around 1.4 degree further confirms the presence of abundant mesopores although the mesoporous structure is not ordered. High-angle XRD pattern (Fig. 2b) shows several well-resolved peaks, indexed as the (111), (200), (220), (311) and (222) reflections of Pd structure (JCPDS # 05-0681). Selected area electron diffraction of an individual Pd sphere presents five rings, belonging to the face-centered cubic (fcc) Pd phase (Fig. 2c). High resolution TEM image in Fig. 2d demonstrates the high-crystalline property with a lattice spacing of ~0.23 nm which can be ascribed to characteristic (111) planes of fcc Pd. In order to investigate the formation mechanism, the growth evolution process was carefully presented. Some representative TEM images observed at different time are given in Figs. 3a-d. At the initial period, some spheres (~50 nm) with many small Pd nanoparticles on the spherical shell were generated (Fig. 3a). Then, Pd nanoparticles locating in the junction grew into bigger ones as labeled in Fig. 3b. Finally, the adjacent spheres disappeared and turned into a bigger one (~150 nm) (Figs. 3c and d). Based on the above observations, a suggested formation mechanism was illustrated in Fig. 3e. Plenty of oil droplets (n-heptane) firstly form in the aqueous solution. The double carbon chains in DODAC embed into the oil droplets while hydrophilic quaternary ammonium jut into the 4
aqueous phase. The negatively-charged PdCl42- will interact with quaternary ammonium through electrostatic interaction. PdCl42- was reduced into elemental Pd nanoparticles (2~3 nm). The nanoparticles locating in the junction touched with each other and grew into bigger ones by Ostwald ripening. The crystalline growth sprawled out from the junctions and these adjacent spheres turn into a bigger sphere (~150 nm). The oil droplets produced the hollow-like structure (macropores) and the micelles directed the mesopores. As above statement, the use of DODAC with double hydrophobic chain plays the crucial role in the construction of this especial structure. By contrast, CTAB or HPC produced only solid porous Pd spheres (Fig. S1). It may be caused by the weak hydrophobic interactions between single carbon chain and oil droplets. The electrocatalytic activity of porous Pd was checked by FAOR. Fig. 4a displays the CV of porous Pd and PdB (Fig. S2) in the N2-saturated H2SO4 solution. The corresponding electrochemically active surface areas (ECSAs) were estimated to be nearly 13.4 and 5.0 m2·g-1 following the equation: ECSA = Q / (m·C).[17] The ECSA of porous Pd is 2.7 times than PdB which may attribute to the high specific surface area and more electrochemical active sites owning to the porous structure. The CV of porous Pd in H2SO4 and HCOOH solution exhibits a peak current density of 568 mA·mg-1, 3.5 times than PdB (161 mA·mg-1) (Fig. 4b). The electrocatalytic durability of porous Pd and PdB were checked through chronoamperometry tests (Fig. 4c). After 2500 s, the residual current of porous Pd is obviously higher than PdB catalysts. All of above results confirm that hollow-like porous Pd spheres have superior eletrocatalytic activity toward FAOR. 4. Conclusions Hollow-like porous Pd spheres could be synthesized by a simple method assisting by surfactant DODAC and n-heptane. The use of DODAC with double hydrophobic chain is the key point for the 5
construction of this porous structure. Owning to the porous feature, the obtained Pd spheres performed desired electrocatalytic activity toward FAOR. Acknowledgments This work was supported by National Natural Science Foundation of China (No. 21471081, 21533012 and 21501095), Natural Science Foundation of Jiangsu Province (No. BK20150969), and Natural Science Foundation of Jiangsu Higher Education Institutions of China (No. 15KJB150014). References [1] Bianchini C, Shen P K. Chem Rev 2009;109:4183-206. [2] Chen A, Ostrom C. Chem Rev 2015;115:11199-2044. [3] Chen J, Lim B, Lee E P, Xia Y. Nano Today 2009;4:81-95. [4] Kannan P, Maiyalagan T, Opallo M. Nano Energy 2013;2:677-87. [5] Feng J J, Zhou D L, Xi H X, Chen J R, Wang AJ. Nanoscale 2013;5:6754-7. [6] Yang L, Li Z, Lu X, Tong Y, Nie G, Wang C. ChemPlusChem 2013;78:522-7. [7] Zheng J N, Zhang M, Li F F, Li S S, Wang A J, Feng J J. Electrochim Acta 2014;130:446-52. [8] Wan Y, Zhao D. Chem Rev 2007;107:2821-60. [9] Davis M E. Nature 2002;417:813-21.`` [10] Fu G, Gong M, Tang Y, Xu L, Sun D, Lee J-M. J Mater Chem A 2015;3:21995-9. [11] González E, Arbiol J, Puntes V F. Science 2011;334:1377-80. [12] Huang X, Li Y, Chen Y, Zhou E, Xu Y, Zhou H, et al. Angew Chem Int Ed 2013;52:2520-4. [13] Kim S-W, Kim M, Lee W Y, Hyeon T. J Am Chem Soc 2002;124:7642-3. [14] Li C, Jiang B, Miyamoto N, Kim J H, Malgras V, Yamauchi Y. J Am Chem Soc 2015;137:11558-61. [15] Wang F, Li C, Sun L D, Xu C H, Wang J, Yu J C, et al. Angew Chem Int Ed Engl 2012;51:4872-6. 6
[16] Zhong Z, Yin Y, Gates B, Xia Y. Adv Mater 2000;12:206-9. [17] Biegler T, Rand D, Woods R. J Electroanal Chem 1971;29:269-77.
Figure Captions Fig. 1. SEM (a, b) and TEM (c, d) images of hollow-like porous Pd spheres. Insert of (a) is the diameter distribution. Fig. 2. Low-angle (a) and high-angle (b) XRD patterns of porous Pd spheres. (c) and (d) are highresolution TEM images of a Pd sphere. Insert of (c) is the corresponding selected area electron diffraction. Fig. 3. (a-d) TEM images of Pd nanoparticles captured during the evolution process of porous Pd sphere. (e) is the corresponding schematic diagram. Fig. 4. Cyclic voltammograms (CVs) of porous Pd sphere and PdB in the N2-saturated H2SO4 solution (a) without and (b) with 0.5 M HCOOH. (c) is the chronoamperometric curve in H2SO4 and HCOOH solution.
7
Fig. 1
Fig. 2
8
Fig. 3
Fig. 4
Hollow-like porous Pd spheres were prepared.
The Pd spheres possess both fine mesopores and macropores.
The double carbon chains in surfactant play the crucial role.
The obtained Pd nanosheets possess well electrocatalytic performance.
9
www.elsevier.com
PII: DOI: Reference:
S0167-577X(16)31540-3 http://dx.doi.org/10.1016/j.matlet.2016.09.078 MLBLUE21516
To appear in: Materials Letters Received date: 16 August 2016 Revised date: 15 September 2016 Accepted date: 19 September 2016 Cite this article as: Xiaoli Liu, Dongdong Xu and Jianchun Bao, One-step synthesis of hollow-like porous palladium sphere with enhanced electrocatalytic performance, Materials Letters, http://dx.doi.org/10.1016/j.matlet.2016.09.078 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
One-step synthesis of hollow-like porous palladium sphere with enhanced electrocatalytic performance Xiaoli Liu, Dongdong Xu*, Jianchun Bao* Jiangsu Key Laboratory of Biofunctional Materials, School of Chemistry and Materials Science, Nanjing Normal University, Nanjing, 210023, P. R. China [email protected] [email protected] * Corresponding authors. Tel: +86-25-8589-1936. Abstract
Hollow-like porous Pd spheres were prepared by using dioctadecyldimethylammonium chloride and n-heptane in the aqueous solution. The Pd spheres possess both good mesopores and macropores. Due to the porous feature, they exhibited well electrocatalytic performance toward formic acid oxidation compared to the commercial Pd black. Keywords: Porous palladium; Self-assembly; Formic acid oxidation; Hollow sphere 1. Introduction Noble metal palladium, as a potential alternative of expensive platinum, has attracted intense interest as primary electrocatalyst.[1,2] Lots of Pd nanomaterials with various morphologies have been successfully prepared and also presented desired electrocatalytic performance in fuel cells.[3,4] Porous architectures, due to the large surface area and corresponding high mass transfer rate, are of particular interest when employed in catalysis.[5-7] However, unlike the synthesis system of porous amorphous silica,[8,9] the controlled construction of porous Pd is still a challenge due to the rigid crystal structure which would easily destroy the existed nanopores. Many innovative strategies have proved the capacity to produce nanopores inside Pd crystals, such as templating methods, galvanic replacements, and selective removal of other
1
components.[10-13] However, the obtained Pd nanostructures usually possess poor porosity. As well known, soft templates could direct fine mesopores inside the inorganic nanomaterials through the self-assembly process.
Huang reported the synthesis of porous Pd nanostructure by using
surfactant hexadecylpyridinium chloride (HPC) as shape-directing agent.[12] Mesoporous Pd films with perpendicular mesochannels were prepared via surfactant-directed synthesis and elaborated electrochemical deposition.[14] Porous single-crystalline palladium nanoparticles were produced via cetyltrimethylammonium chloride (CTAB).[15] We can find that all of above cases need the participation of certain surfactants. In this work, hollow-like porous Pd spheres were synthesized in the aqueous solution. H2O, Na2PdCl4, dioctadecyldimethylammonium chloride (DODAC) and n-heptane were simply mixed together and then L-ascorbic acid (AA) was added as the reducing agent. As DODAC possesses double hydrophobic carbon chains, it would be of great help for the hydrophobic interactions between alkyl chains and oil droplets (n-heptane). Pd spheres with both fine mesopores and macropores were obtained after the slow reduction process under 35 oC. Compared to commercial palladium black (PdB), porous Pd spheres exhibited good electrocatalytic activity toward formic acid oxidation reaction (FAOR). 2. Experimental section 2.1 Synthesis of hollow-like porous Pd sphere In a typical synthesis, 1.2 g n-heptane was added dropwise into 5 mL (0.01 M) DODAC aqueous solution. After 1 h, 0.4 mL (10 mM) aqueous solution of Na2PdCl4 was injected into the solution. Then, 0.5 mL (80 mM) fresh L-ascorbic acid solution was added quickly. The final mixture was kept under 35 oC for 5 h. All of the processes mentioned above were operated under continuous stirring. 2
The black products were collected by centrifugation and washed several times with ethanol. 2.2 Characterizations The X-ray diffraction (XRD) patterns were collected on a D/max 2500 VL/PC diffractometer equipped with graphite-monochromatized Cu Kα radiation (λ = 1.54060 Å) in 2θ ranging from 1o to 90o. The corresponding work voltage and current are 40 kV and 100 mA, respectively. The scanning electron microscope (SEM) images were obtained on a JSM-7600F apparatus at an accelerating voltage of 10 kV. The transmission electron microscope (TEM) observation was performed with a JEOL JEM-2100 microscope operated at 200 kV. 2.3 Electrochemical measurements Cyclic voltammograms (CVs) and chronoamperometric curves were achieved with a CHI 660D analyzer. A conventional three-electrode cell was used, consisting of Ag/AgCl electrode as a reference electrode, Pt foil as a counter electrode, and glassy carbon electrode as a working electrode. In the typical procedure, 4 mg catalysts were dispersed in 0.5 mL ethanol and 1.5 mL H2O and sonicated for 30 minutes to form a homogeneous ink. 6 μL ink was uniformly loaded on clean glassy carbon electrode surface. 5 μL of 0.1 wt % Nafion solution was then dropped onto the electrode surface. Electrochemical measurements were performed in a N2-saturated 0.5 M H2SO4 solution with or without 0.5 M HCOOH at a scan rate of 50 mV·s-1. All potential values were referenced to Ag/AgCl electrode. 3. Results and discussions The SEM image shows that the obtained Pd nanoparticles have a uniform sphere morphology (Fig. 1a). The size distribution indicates that the average diameter is about 156 nm. SEM image of a broken sphere proves that these Pd spheres do not possess the completely hollow structure. Instead, 3
some threadlike nanoparticles sprawl out from the centre and connect to the spherical shell (Fig. 1b). Therefore, hollow-like spheres were more appropriate to represent these Pd nanoparticles. TEM images in Figs. 1c and 1d also confirm the hollow-like structure because there are some dark parts in the centre of the spheres, unlike the TEM images of other hollow spheres as reported everywhere.[16] Abundant mesopores can be found throughout the Pd spheres and particularly along the spherical shell. Therefore, the Pd spheres have two types of pore channels, mesopores and macropores, respectively. From the low-angle XRD pattern in Fig. 2a, a broad peak around 1.4 degree further confirms the presence of abundant mesopores although the mesoporous structure is not ordered. High-angle XRD pattern (Fig. 2b) shows several well-resolved peaks, indexed as the (111), (200), (220), (311) and (222) reflections of Pd structure (JCPDS # 05-0681). Selected area electron diffraction of an individual Pd sphere presents five rings, belonging to the face-centered cubic (fcc) Pd phase (Fig. 2c). High resolution TEM image in Fig. 2d demonstrates the high-crystalline property with a lattice spacing of ~0.23 nm which can be ascribed to characteristic (111) planes of fcc Pd. In order to investigate the formation mechanism, the growth evolution process was carefully presented. Some representative TEM images observed at different time are given in Figs. 3a-d. At the initial period, some spheres (~50 nm) with many small Pd nanoparticles on the spherical shell were generated (Fig. 3a). Then, Pd nanoparticles locating in the junction grew into bigger ones as labeled in Fig. 3b. Finally, the adjacent spheres disappeared and turned into a bigger one (~150 nm) (Figs. 3c and d). Based on the above observations, a suggested formation mechanism was illustrated in Fig. 3e. Plenty of oil droplets (n-heptane) firstly form in the aqueous solution. The double carbon chains in DODAC embed into the oil droplets while hydrophilic quaternary ammonium jut into the 4
aqueous phase. The negatively-charged PdCl42- will interact with quaternary ammonium through electrostatic interaction. PdCl42- was reduced into elemental Pd nanoparticles (2~3 nm). The nanoparticles locating in the junction touched with each other and grew into bigger ones by Ostwald ripening. The crystalline growth sprawled out from the junctions and these adjacent spheres turn into a bigger sphere (~150 nm). The oil droplets produced the hollow-like structure (macropores) and the micelles directed the mesopores. As above statement, the use of DODAC with double hydrophobic chain plays the crucial role in the construction of this especial structure. By contrast, CTAB or HPC produced only solid porous Pd spheres (Fig. S1). It may be caused by the weak hydrophobic interactions between single carbon chain and oil droplets. The electrocatalytic activity of porous Pd was checked by FAOR. Fig. 4a displays the CV of porous Pd and PdB (Fig. S2) in the N2-saturated H2SO4 solution. The corresponding electrochemically active surface areas (ECSAs) were estimated to be nearly 13.4 and 5.0 m2·g-1 following the equation: ECSA = Q / (m·C).[17] The ECSA of porous Pd is 2.7 times than PdB which may attribute to the high specific surface area and more electrochemical active sites owning to the porous structure. The CV of porous Pd in H2SO4 and HCOOH solution exhibits a peak current density of 568 mA·mg-1, 3.5 times than PdB (161 mA·mg-1) (Fig. 4b). The electrocatalytic durability of porous Pd and PdB were checked through chronoamperometry tests (Fig. 4c). After 2500 s, the residual current of porous Pd is obviously higher than PdB catalysts. All of above results confirm that hollow-like porous Pd spheres have superior eletrocatalytic activity toward FAOR. 4. Conclusions Hollow-like porous Pd spheres could be synthesized by a simple method assisting by surfactant DODAC and n-heptane. The use of DODAC with double hydrophobic chain is the key point for the 5
construction of this porous structure. Owning to the porous feature, the obtained Pd spheres performed desired electrocatalytic activity toward FAOR. Acknowledgments This work was supported by National Natural Science Foundation of China (No. 21471081, 21533012 and 21501095), Natural Science Foundation of Jiangsu Province (No. BK20150969), and Natural Science Foundation of Jiangsu Higher Education Institutions of China (No. 15KJB150014). References [1] Bianchini C, Shen P K. Chem Rev 2009;109:4183-206. [2] Chen A, Ostrom C. Chem Rev 2015;115:11199-2044. [3] Chen J, Lim B, Lee E P, Xia Y. Nano Today 2009;4:81-95. [4] Kannan P, Maiyalagan T, Opallo M. Nano Energy 2013;2:677-87. [5] Feng J J, Zhou D L, Xi H X, Chen J R, Wang AJ. Nanoscale 2013;5:6754-7. [6] Yang L, Li Z, Lu X, Tong Y, Nie G, Wang C. ChemPlusChem 2013;78:522-7. [7] Zheng J N, Zhang M, Li F F, Li S S, Wang A J, Feng J J. Electrochim Acta 2014;130:446-52. [8] Wan Y, Zhao D. Chem Rev 2007;107:2821-60. [9] Davis M E. Nature 2002;417:813-21.`` [10] Fu G, Gong M, Tang Y, Xu L, Sun D, Lee J-M. J Mater Chem A 2015;3:21995-9. [11] González E, Arbiol J, Puntes V F. Science 2011;334:1377-80. [12] Huang X, Li Y, Chen Y, Zhou E, Xu Y, Zhou H, et al. Angew Chem Int Ed 2013;52:2520-4. [13] Kim S-W, Kim M, Lee W Y, Hyeon T. J Am Chem Soc 2002;124:7642-3. [14] Li C, Jiang B, Miyamoto N, Kim J H, Malgras V, Yamauchi Y. J Am Chem Soc 2015;137:11558-61. [15] Wang F, Li C, Sun L D, Xu C H, Wang J, Yu J C, et al. Angew Chem Int Ed Engl 2012;51:4872-6. 6
[16] Zhong Z, Yin Y, Gates B, Xia Y. Adv Mater 2000;12:206-9. [17] Biegler T, Rand D, Woods R. J Electroanal Chem 1971;29:269-77.
Figure Captions Fig. 1. SEM (a, b) and TEM (c, d) images of hollow-like porous Pd spheres. Insert of (a) is the diameter distribution. Fig. 2. Low-angle (a) and high-angle (b) XRD patterns of porous Pd spheres. (c) and (d) are highresolution TEM images of a Pd sphere. Insert of (c) is the corresponding selected area electron diffraction. Fig. 3. (a-d) TEM images of Pd nanoparticles captured during the evolution process of porous Pd sphere. (e) is the corresponding schematic diagram. Fig. 4. Cyclic voltammograms (CVs) of porous Pd sphere and PdB in the N2-saturated H2SO4 solution (a) without and (b) with 0.5 M HCOOH. (c) is the chronoamperometric curve in H2SO4 and HCOOH solution.
7
Fig. 1
Fig. 2
8
Fig. 3
Fig. 4
Hollow-like porous Pd spheres were prepared.
The Pd spheres possess both fine mesopores and macropores.
The double carbon chains in surfactant play the crucial role.
The obtained Pd nanosheets possess well electrocatalytic performance.
9