Synthesis of Pd3Pb colloidal nanocrystal assembly and their electrocatalytic activity toward ethanol oxidation

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Synthesis of Pd3Pb colloidal nanocrystal assembly and their electrocatalytic activity toward ethanol oxidation Jing Sun, Min Yang, Yuxin Gong, Hongliang Li, Peizhi Guo* Institute of Materials for Energy and Environment, State Key Laboratory of Bio-Fibers and Eco-Textiles, School of Materials Science and Engineering, Qingdao University, Qingdao 266071, PR China

G R A P H I C A L A B S T R A C T

A R T I C LE I N FO

A B S T R A C T

Keywords: Assembly Palladium Alloy Electrocatalysis

Palladium-based electrocatalysts have become an important research field in energy generation. Introducing another metal element in palladium-based catalysts may improve the electrocatalytic performance while reducing costs. In this work, colloidal nanocrystal assembly (CNA) of Pd3Pb alloy has been synthesized via an aqueous-solution synthesis method. The content of Pb in Pd3Pb CNAs can be adjusted to obtain an optimal value with the highest catalytic current density of 3543 mA/mgPd. Meantime, the morphology of the Pd3Pb catalysts was found to form the well-separated spherical structures with the size range from 120 nm to 140 nm. The variation of the molar ratio of elements Pd and Pb in Pd3Pb CNAs leads to form mainly spherical assemblies with a minor part of nanoparticles. Combined the XRD, TEM and electrochemical results, the high catalytic performance towards electrooxidation of ethanol for Pd3Pb CNAs was suggested to be ascribed to the nature of small crystallite sizes and the assembly structure along with the formation of bimetallic alloy.

1. Introduction Concurrent with the development of society and the advancement of science and technology, energy demand is growing [1–5]. High-efficient, portable electrochemical devices are crucial to meet the grim facts of rapid growth [6–9]. Therefore, how to realize the energy conversions including the transition between chemical energy and ⁎

electrical energy have received great attention [10–19]. Ethanol is one type of the most valuable renewable resources, and has been selected as an important small molecule for fuel cells [20–28]. Indeed, the electrocatalysis of alcohols, including methanol and ethanol, becomes a research hotspot and the development of electrocatalysts for fuel oxidation has made great progress in recent years [20,29–35]. As the electrocatalyst palladium (Pd) is usually more affordable

Corresponding author. E-mail addresses: [email protected], [email protected] (P. Guo).

https://doi.org/10.1016/j.colsurfa.2019.124224 Received 29 September 2019; Received in revised form 10 November 2019; Accepted 12 November 2019 Available online 13 November 2019 0927-7757/ © 2019 Elsevier B.V. All rights reserved.

Please cite this article as: Jing Sun, et al., Colloids and Surfaces A, https://doi.org/10.1016/j.colsurfa.2019.124224

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Five products were prepared from the systems with the Pb(NO3)2 content of 0 mg/5 mg/15 mg/25 mg/45 mg named as Pd/Pd3Pb-5/ Pd3Pb-15/Pd3Pb-25/Pd3Pb-45 CNAs respectively.

than platinum (Pt) and the latter is easily adsorbed by toxic intermediates like CO [36–38]. A variety of Pd nanostructures such as cubes, octahedrons, dodecahedrons and nanosheets have been explored and synthesized which showed high electrocatalytic activity [3,11,31,39–42]. In order to reduce the cost and improve electrocatalytic activity, transition metal elements are selectively incorporated to prepare diverse palladium-based alloy nanocatalysts [21,32,43–58], such as PdCu [3,24,55], PdFe [51–53], PdAg [59,60] and PdPb [1,57,58] nanoparticles as well as other unique nanostructures [21,23]. Accordingly, better electrocatalytic activity can be usually achieved due to the synergy effect between different metals for alloy catalysts, thus simultaneously affecting the adsorption of reactants on the catalyst surface. For instance, the content of Pd on the surface of PdCu nanoparticles has determined the electrocatalytic activity of bimetallic catalysts toward oxidation of ethanol [24]. PdAg hollow nanodendrites can be synthesized from Ag nanoparticles serve as the self-template, which exhibited excellent electrocatalytic performance under the alkaline condition [60]. Core/shell Pd-Pb/Pd nanosheets and nanocubes have homogeneous tensile strain along (001) planes on both the top-Pd and edge-Pd surfaces, which exhibit superior activity for oxygen reduction reaction at very high tensile strain [58]. Generally, synthesis of inorganic nanostructures needs surfactants [56], including polyvinyl pyrrolidone (PVP), to reduce the surface energy and stabilize the dispersed nanoparticles. Unlike the commonly used PVP, amphiphilic triblock poly (ethylene oxide)-poly (propylene oxide)-poly (ethylene oxide) [Pluronic F127 (PEO-PPO-PEO)] copolymer is found to be easier to be washed away from metal surfaces. Thereafter, F127 is also used as a structure directing agent and as a capping agent for alloy synthesis [61]. For instance, Au@Pt core shell bimetallic colloids were synthesized with the assistance by block copolymer [62]. Pt-on-Au bimetallic dendritic nanoparticles were obtained by an one-step method with pluronic F127 [63]. In this work, colloidal nanocrystal assemblies (CNAs) of Pd3Pb alloy were synthesized by an aqueous-phase reaction strategy at room temperature. Experimental results showed that the concentration of Pb precursor in the synthesis systems could affect the composition of the alloy and finally adjusted the electrocatalytic activity of the catalysts. The usage of F127 leads to the formation of spherical alloy assembly. It is found that the highest catalytic current density of the alloy CNAs can reach as high as 3543 mA/mgPd.

2.3. Characterization The crystallographic information and composition were investigated using a Rigaku Ultima IV X-ray diffractometer (XRD, Cu Kα radiation λ = 0.15418 nm). The morphology and structure of the samples were examined by a JEOL JSM-7800F scanning electron microscope (SEM) and a JEOL JEM-2100 plus transmission electron microscope (TEM). X-ray photoelectron spectroscopy (XPS) was performed with an ESCALab220i-XL electronic spectrometer from VG scientific using 300 W A kα radiation. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) coupled with energy-dispersive X-ray (EDX) elemental mappings were measured with a JEOL JEM-2100F transmission electron microscope at an accelerating voltage of 200 kV. 2.4. Electrochemical measurements Electrochemical measurements were carried out on a CHI610E workstation at room temperature. During the measurements of cyclic voltammetry (CV) and chronoamperometric tests were conducted with a standard three-electrode system. The detailed description was shown in our recent reports [64]. 3. Results and discussion 3.1. Morphology and structure The XRD patterns of all samples are shown in Fig. 1. The wide diffraction peaks observed indicated that small particles were formed in these samples. All the diffraction peaks of Pd CNAs were consistent with pure Pd (JCPDS No.46-1043) with a face-center cubic (fcc) structure. The four peaks at 2θ values of 39.9°, 46.5°, 68.0° and 81.6° were corresponding to the (111), (200), (220), and (311) crystalline planes of cubic Pd, respectively. Obviously, the peak position of the bimetallic catalysts showed a slight difference compared with those of pure Pd. They corresponded to the standard peaks of the (111), (200), (220) and (311) planes of Pd3Pb (JCPDS No.20-0827) phase without obvious impurity peaks, indicating that Pd3Pb intermetallic compound were synthesized. Furthermore, the diffraction peaks of Pd3Pb CNAs were shifted to smaller 2θ values compared with Pd caused by the

2. Experimental section 2.1. Materials and reagents Palladium (II) nitrate dihydrate (Pd(NO3)2·2H2O), lead(II)nitrate (Pb(NO3)2), L-ascorbic acid (AA), sulfuric acid and ethanol were purchased from Sinopharm Chemical Reagent Co. Ltd. Pluronic F127 [(PEO-PPO-PEO)(Item NO.P2443)] was obtained from Aldrich. All the chemical reagents were analytical grade and used without further purification. Ultrapure water (18.2 MΩ cm) was used directly throughout the synthetic experiments and electrochemical measurements. 2.2. Synthesis of PdPb CNAs catalysts In a typical synthesis, 20 mg of Pd(NO3)2·2H2O and a certain weight of Pb(NO3)2 were added to a bottle (30 mL) containing 20 mL ultrapure water and ultrasound it for 5 min to completely dissolve afterward. Then, 100 mg of F127 as capping agents were added to the above solution with ultrasound until evenly dispersed. After adding aqueous AA solution (5 mL, 11.36 mM), the mixed solution has reacted under continuous stirring for three hours. The precipitates were collected by centrifugation and washed five times with ultrapure water. Finally, all the products were obtained after vacuum drying at 60 °C for six hours before use.

Fig. 1. XRD patterns of (a) Pd3Pb-45, (b) Pd3Pb-25, (c) Pd3Pb-15 and (d) Pd3Pb-5 CNAs as well as (e) Pd CNAs. 2

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most of Pd3Pb-15 CNAs are dispersed with an average particle size in the range of 120−140 nm. The rough surface seen from the TEM and SEM images of all the CNAs would be helpful for the increase of active sites for the electrocatalysts. XPS measurement was used to further investigate the composition of bimetallic CNAs, as shown in Fig. 4. The binding energies of Pd 3d5/2 and Pd 3d3/2 of all the CNAs are given as the two peaks in Fig. 4A respectively. Clearly, the values of Pd 3d3/2 are varied obviously. In addition, the binding energies of Pd 3d3/2 are around 342.9, 342.8, 342.7, 342.8 and 342.8 eV for Pd CNAs, Pd3Pb-5, Pd3Pb-15, Pd3Pb-25 and Pd3Pb-45 CNAs, respectively. Compared with Pd CNAs, the peak positions of Pd based CNAs moved slightly to a lower value of binding energy, especially for Pd3Pb-15 CNAs. Element Pd was mainly in the form of Pd(II) state, particularly obvious in the binding energy data of Pd 3d5/2. Fig. 4B shows the binding energy profile of Pb4f of the bimetallic CNAs. Pb also exists in two valence states, namely Pb(0) and Pb(II). Obviously, the peaks of Pb(0) in Pb 4f7/2 were stronger than that in the Pb 4f5/2. In the meantime, the gaps between two peaks either for Pb4f5/ 2 or for Pb4f7/2 are 1.9 eV for Pd3Pb-5, 1.2 eV for Pd3Pb-15, 1.4 eV for Pd3Pb-25 and 1.6 eV for Pd3Pb-45. As derived from Fig. 4A and B, Pd3Pb-15 CNAs shows a unique composition features, which affect its structure. This was led by the control of the content of Pb precursor in starting system, where in the concentration of Pd precursor for the synthesis of Pd3Pb-15 CNAs is only in the middle range. The occurrence of the shifts for elements Pd and Pb should be attributed to the strong electron interaction between these two elements, further confirming the formation of Pd3Pb-based alloy. The formation of Pd3Pb CNAs would be in favor of electrocatalysis of alcohols inspired by the reported bimetallic catalysts [57,58]. To explore the synthesis mechanism of Pd3Pb-15 CNAs, intermediates collected from different reaction durations were characterized by XRD (Fig. S4) and SEM (Fig. S3) techniques. It is seen from Fig. S4 that bimetallic structures were formed even in a short reaction time. However, the strongest (111) peak is obviously shifted to a lower 2θ position, indicating that Pd is easier to reduce than Pb in the synthesis system at the early stage and Pb are gradually formed under an extended reaction time. In the meantime, the size of the intermediates was also enlarged with the increase of reaction time (Fig. S3).

introduction of Pb in Pd samples [58]. It is reasonable that Pd3Pb CNAs have a larger lattice constant than pure Pd due to the large atomic radius of Pb. The crystallite sizes were calculated to be around 20.6 nm, 19.0 nm, 13.9 nm, 11.7 nm and 12.8 nm for Pd CNAs, Pd3Pb-5, Pd3Pb15, Pd3Pb-25 and Pd3Pb-45 CNAs, respectively, based on the (111) planes according to Scherrer formula. Figs. 2 and 3a shows the TEM images of Pd CNAs and all the Pd3Pb CNAs. It is seen that all the samples mainly show spherical structures with the size range of 50 nm–200 nm, similar to the corresponding SEM images (Figs. S1 and S2). Fig. 2a shows the TEM image of pure Pd samples. Pd nanospheres were observed to be not well separated with a lot of irregular nanoparticles existed. However, the borders of bimetallic Pd3Pb-5 (Fig. 2b), Pd3Pb-15 (Fig. 3a), Pd3Pb-25 (Fig. 2c) and Pd3Pb-45 CNAs (Fig. 2d) were more distinct. It is observed that the Pb content of assemblies in bimetallic catalysts is increased firstly with the increase of the Pb precursor in the raw system, but slightly decreases at a high concentration of Pb(NO3)2 in the solution(Table S1). As depicted Fig. S1, the particle size distributions of the four types of Pd3Pb CNAs were not well uniform. Based on the XRD, SEM and TEM results, it is clear that assembly structures for these bimetallic structures were formed because the crystallite sizes of primary nanoparticles in the assemblies were much less than those of the CNAs [65,66]. Combined with the SEM (Figs. S1 and S2) and TEM (Figs. 2 and 3) observations, Pd3Pb-15 CNAs shows almost singular submirospheres, superior to any of other bimetallic CNAs. As depicted in Fig. 2e, the lattice spacing in the HRTEM images of Pd CNAs was calculated to be about 2.31 Å, ascribed to the (111) crystalline plane of Pd. The crystalline nature could be further verified by the selected area electron diffraction (SAED) patterns in Fig. 2f, corresponding to the (111), (200), (220) and (311) planes from inner to outer circles in good agreement with the XRD results. In the meantime, the SAED patterns of all the Pd based bimetallic CNAs also have significant bright spots, showing that the catalysts have good crystallinity as shown in Figs. 2h, j, l and 3 c. It is seen from Fig. 2g, i and k that the lattice spacing of the (111) plane of the Pd3Pb CNAs are usually larger than that of Pd CNAs, indicating the increase of the lattice spacing after introducing element Pb in Pd catalysts due to the large atomic radius of Pb (1.75 Å) than that of Pd (1.37 Å). Based on the energy dispersive system (EDS) by SEM equipment (Table S1), the atomic ratios of Pd/Pb are measured to be about 2.49, 2.19, 2.17 and 2.45 for Pd3Pb-5, Pd3Pb15, Pd3Pb-25 and Pd3Pb-45 CNAs, respectively. To understand the elements Pd and Pb in the bimetallic CNAs better, the element distributions of singular CNA (Fig. 3d and e) and a group of CNAs (Fig. 3f and g) of Pd3Pb-15 were presented in the STEM measurements. The images depicted in the second row of Fig. 3 denote that elements Pd and Pb were evenly distributed in the CNAs, especially for Pd. Fig. 3f and g which gives a clear description that all the Pd3Pb-15 CNAs are composed of Pd3Pb alloy. Based on the SEM (Fig. S2) and TEM (Fig. 3) results, it is seen that

3.2. Electrochemical property The electrooxidation of ethanol was used to measure the electrocatalytic properties of the as-prepared Pd CNAs and palladium-based bimetallic CNAs. Fig. 5A displayed the cyclic voltammograms of all the Pd-based catalysts in 1 M KOH at a scan rate of 50 mV/s. The peaks, appeared at around −0.2 V attributed to the reduction of Pb-OHads or PdOx [24], are at about −0.27 V, −0.22 V, −0.21 V, −0.28 V and −0.22 V for Pd CNAs, Pd3Pb-5, Pd3Pb-15, Pd3Pb-25 and Pd3Pb-45 CNAs, respectively. The positive shift of Pd3Pb CNAs compared with Pd

Fig. 2. (a–d) TEM images, (e, g, i and k) HRTEM images, and (f, h, and j and l) SAED patterns of (a, e and f) Pd CNAs, (b, g and h) Pd3Pb-5, (c, i, and j) Pd3Pb-25 and (d, k and l) Pd3Pb-45 CNAs. 3

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Fig. 3. Morphological and structural analysis of Pd3Pb-15 CNAs. (a) TEM image, (b) HRTEM image, (c) SAED pattern, (d) STEM image of singular nanosphere, (e) EDX element distribution maps of Pd + Pb, Pd and Pb in singular nanosphere. (f) STEM image of multiple nanospheres and corresponding elemental mappings in (g).

Fig. 4. XPS analysis of Pd 3d (A) and Pb 4f (B): (a) Pd3Pb-45, (b) Pd3Pb-25, (c) Pd3Pb-15, (d) Pd3Pb-5 CNAs as well as (e) Pd CNAs.

solution of 1 M KOH + 1 M C2H5OH. The oxidation currents were uniform to the weight of element Pd denoted as the mass actively with the unit of mA/mgPd. As shown in Fig. 5B, the catalytic current densities toward ethanol oxidation were about 1010, 3341, 3543, 3252 and 2075 mA/mgPd for Pd CNAs, Pd3Pb-5, Pd3Pb-15, Pd3Pb-25 and Pd3Pb45 CNAs, respectively, with the on-set potentials were the order of Pd3Pb-15 CNAs > Pd3Pb-5 CNAs > Pd3Pb-25 CNAs > Pd3Pb-45 CNAs > Pd CNAs. These denoted that the bimetallic structures are superior for the electrocatalysis of ethanol compared with pure Pd catalyst. Clearly, Pd3Pb-15 CNAs displays the highest electrocatalytic

CNAs suggests the reduction of PdOx should be easier because of the formation of Pd3Pb alloy structures, which is helpful for the transfer of the −OHads between Pd and Pb active sites, leading to the maintenance of active −OHads species or clear the catalyst surface [24,67]. The electrochemical active surface areas (ECSAs) were calculated to be about 24.0, 179, 425, 170 and 111 cm2 mg−1, respectively, for Pd CNAs, Pd3Pb-5, Pd3Pb-15, Pd3Pb-25 and Pd3Pb-45 CNAs. Pd3Pb-15 CNAs had the largest ECSA possibly ascribed to its well-separated nature with a rough surface. Fig. 5B showed typical CV curves of the electrodes modified by Pd and all the Pd3Pb CNAs electrocatalysts in a 4

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Fig. 5. CV curves of the modified electrodes in aqueous 1 M KOH solution (A) and in aqueous 1 M KOH/1 M C2H5OH solution (B). Sweep rate: 50 mV/s.

CNAs with sweep speed, ranging from 10 mV/s to 100 mV/s. As the sweep rate enhanced, the current density gradually increased from 2691 mA/mgPd to 3846 mA/mgPd. Correspondingly, the peak current potential moved to a positive direction with the scan rate increasing. Fig. 6B reveals that there is a clear linear relationship between the square root of the forward peak current density of ethanol oxidation and the root of scan rate (V1/2), indicating the exitance of a diffusioncontrolled processed during the ethanol oxidation reaction. These results were in good accord with pure Pd nanostructures and alloy catalysts. The stability of the catalysts during ethanol oxidation was measured by two techniques: cyclic voltammetry and linear sweep voltammetry. As depicted in Fig. 7A, within ten circles of CV measurement, Pd3Pb-5, Pd3Pb-15, Pd3Pb-25, and Pd3Pb-45 CNAs reached the corresponding maximum catalytic current density quickly. After that, the current densities of the four alloys were obviously attenuated. For Pd3Pb-5, the activity was drastically decreased after 130 cycles, while Pd3Pb-15, Pd3Pb-25 and Pd3Pb-45 maintain about 56.8 %, 46.9 % and 53.1 % of the corresponding highest values after 200 cycles, respectively. In contrast, the performance of Pd was more stable but showed a much lower activity. Fig. 7B was the chronoamperometry curves of all samples at -0.1 V (vs. Hg/HgO). In the beginning, the current densities of all catalysts dropped dramatically [24]. In alkaline media, ethanol was rapidly dehydrogenated to produce toxic intermediates, adsorbing on the surface of the catalysts instead of being removed. These toxic intermediates occupied a large amount of surface area and reduced the effective active sites of the catalysts, which resulted in a greatly reduced reaction rate. After about 25 s of reaction, the chronoamperometry curves were

activity. It is evident that the current density of forward scan peak of Pd CNAs was lower than that of backward scan peaks while all the Pd3Pb CNAs display the opposite order. As derived from the SEM-EDS results (Table S1), the Pd contents of Pd3Pb-5 and Pd3Pb-45 are similar and slightly higher than those of Pd3Pb-15 and Pd3Pb-25 CNAs. These indicate that the formation of the alloy structure based on Pd3Pb intermetallic compounds can enhance the catalytic activity compared with pure Pd CNAs. However, a lower Pd content in Pd3Pb-15 shows the highest activity which should also be ascribed to the unique alloy feature and assembly structure [57]. It is also clear that the concentration of Pb precursor in the synthetic systems is varied in a large range. However, four types of Pd3Pb CNAs were formed. The electrocatalytic activity of Pd3Pb-15 CNAs is higher than the other catalysts in the forward scan with the catalytic current density up to 3543 mA/mgPd. Peak current of Pd CNAs appeared at -0.20 V for the ethanol oxidation with an onset potential at −0.5 V. As shown in the forward sweep curve, the initial potentials of Pd3Pb CNAs for the ethanol oxidation were less than that of Pd CNAs. Moreover, the catalytic peaks of Pd3Pb alloys move significantly to the right side, which suggests that the formation of Pd3Pb CNAs should be significantly important for the kinetics of ethanol oxidation reaction. The current intensities of forward peaks were larger than backward peaks similar to reported Pd nanostructures [57,58]. The peak currents of Pd3Pb CNAs was significantly higher than some reported Pd or Pt bimetallic catalysts and commercial Pd/C [68], which indicated that the incorporation of Pb in Pd3Pb CNAs greatly improved the catalytic performance of electrooxidation of ethanol. In order to study the controlling factors of the oxidation process of ethanol, Fig. 6A shows the variation of current density of Pd3Pb-15

Fig. 6. (A) CV curves of the Pd3Pb-15 CNAs modified GCEs at different scan rates. (B) Corresponding plot of forward peak current versus the square root of the scan rate. 5

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Fig. 7. (A) Variation of current density along with the cycle number for ethanol oxidation in 1 M KOH/1 M C2H5OH. (B) Chronoamperometry curves of the asprepared electrodes at −0.1 V.

Fig. 8. TEM images of Pd3Pb-15 CNAs after electrochemical cycle test.

the ration design of novel assembly structure with high electrochemical performance.

smoother than at the beginning. At the end of 1000s, Pd3Pb-15 CNAs still showed the highest catalytic activity about 780 mA/mg. The catalytic performance of Pd CNAs was the lowest during the whole test, which further conformed that the incorporation of element Pb greatly improved the stability and tolerability of bimetallic catalysts. Fig. 8 displays the TEM images of Pd3Pb-15 CNASs after ethanol oxidation of 200 cycles. By comparing of the morphology of the catalyst before and after the electrocatalysis, the bimetallic electrocatalyst kept the spheric assembly structure within large nanoparticles formed after the test. At the same time, the catalytic current density can only maintain 56.8 % of the highest electrocatalytic activity. This indicated that small nanoparticles were more possible to display an enhanced catalytic activity compared with large particles. Even though the decrease in electrochemical activity is also observed in Pd3Pb CNAs, the maintenance of the morphology still gives a reasonable clue due to rational design new Pd-based nanostructures with high electrocatalytic performance [57,58].

Author contributions J.S. and P.G. conceived and designed the experiments. J.S. and M.Y performed the experiments. J.S. and P.G. analysed the data and wrote the paper. All authors discussed and commented on the manuscript. Declaration of Competing Interest There are no conflicts of interest. Acknowledgements This work is financially supported by the National Natural Science Foundation of China (No. 21773133), the Double First Class University Construction of Shandong Province and the Taishan Scholars Advantageous and Distinctive Discipline Program for supporting the research team of energy storage materials of Shandong Province, P. R. China.

4. Conclusion Pd3Pb CNAs with the content of Pb adjusted were synthesized by the continuous introducing Pb(NO3)2 in the synthesis system at room temperature. It is observed that the Pb content of assemblies in bimetallic catalysts is increased firstly with the increase of the Pb precursor in the raw system, but decreases slightly at a high concentration of Pb (NO3)2 in the solution. Among them, the catalytic performance of the sample reached a maximum once the 15 mg Pb(NO3)2 was added in the synthetic system, that is, 3543 mA/mgPd. It is also found that the formation of Pd phase was easier than Pb due to the high reduction potential of the latter. Meantime, the unique assembly feature combined a small crystallite size of primary nanoparticles would be favorable for

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