Carrageenan Asissted Synthesis of Palladium Nanoflowers and Their

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Carrageenan assisted synthesis of palladium nanoflowers and their electrocatalytic activity toward ethanol Ning Ma, Xuehua Liu, Ziqi Yang, Guojun Tai, Yanru Yin, Shuibo Liu, Hongliang Li, Peizhi Guo, and Xiu Song Zhao ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03425 • Publication Date (Web): 06 Dec 2017 Downloaded from http://pubs.acs.org on December 7, 2017

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ACS Sustainable Chemistry & Engineering

Carrageenan assisted synthesis of palladium nanoflowers and their electrocatalytic activity toward ethanol Ning Ma, Xuehua Liu, Ziqi Yang, Guojun Tai, Yanru Yin, Shuibo Liu, Hongliang Li, Peizhi Guo* and X. S. Zhao

Institute of Materials for Energy and Environment, State Key Laboratory Breeding Based of New Fiber Materials and Modern Textile, School of Materials Science and Engineering, Qingdao University, 308# Ningxia Road, Qingdao, 266071, P. R. China. *Corresponding Author: [email protected]; [email protected]

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ABSTRACT: Palladium (Pd) nanoflowers with tunable thorns were prepared in a facile and rapid route with the assistance of carrageenan. The superior nature of well dispersed Pd nanoflowers has been disclosed by X-ray diffraction, scanning electron microscope and transmission electron microscope measurements. The growth of thorns of Pd nanostructures was first stimulated with the continuous introduction of L-ascorbic acid in the synthesis system and then reached a maximum length of about 132 nm, followed by a slight decrease in concentration. Electrochemical data showed that Pd nanoflowers with the longest thorns exhibited the highest catalytic current density of 1160 mA/mg, optimized tolerance to the poisoning and the lowest onset potential while others showed a better cycle stability with a catalytic activity maintenance of above 96% after 300 cycles. It is suggested that the structure of thin thorns of Pd nanoflowers should be easier to be damaged than thick thorns during the electrocatalysis of ethanol. The relationship between Pd nanoflowers and properties as well as the key factors to form nanoflower structure were discussed based on the experimental data.

KEYWORDS: Pd nanoflowers; Biomacromolecule; Carrageenan; Assembly; Electrocatalysis

INTRODUCTION

In response to the energy crisis, clean and efficient fuel cells have aroused a heated discussion.1,2 Among all types of fuel cells, direct ethanol fuel cells (DEFCs) are regarded as a suitable portable power source because of their high energy density and convert efficiency.3 As


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the fuel of the DEFCs, ethanol can be obtained from renewable source with advantages of low cost and low toxicity.4,5 However, designing high-efficiency catalysts and reducing the cost of production become the key problems to solve. Palladium (Pd) nanoparticles have attracted extensive interests because they exhibit high catalytic activity for alcohols and acids,6-9 good poisoning tolerance and lower price than Pt.7,10,11 The size and structures of noble metal catalysts play key roles in determining their catalytic performance.12 Hence, great efforts have been devoted to synthesize the Pd nanoparticles in a controlled manner.6,13,14 Pd nanoparticles with various shapes and structure, such as nanosheets,15,16,17 polyhedrons,18-22 nanorods23,24 and concave structure,25 have been synthesized with liquid phase synthetic methods by using polyvinylpyrrolidone (PVP), cetyltrimethyl ammonium bromide (CTAB) or citric acid as capping agents in usual.26 For example, singlecrystalline Pd tetrapod nanocrystals were prepared in the copresence of CO and H2.27 Concave Pd nanocubes were obtained by using the seed growth route under the control of the reduction kinetics.25 However, in response to the development of the green chemistry28,29, it is critical to find environment-friendly and renewable capping agents to replace the traditional agents. Recently, the study of biomacromolecules has been attracting more and more attention.30,31 Carrageenan is a natural macromolecule from ocean with diverse groups and various applications, such as drug carriers,32,33 food additives34 and stabilizing agents35 and its structure is similar to that of PVP, CTAB and other capping agents, which may affect the growing direction and rate of different crystal faces.36,37


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Herein, we report the synthesis of Pd nanoflowers (NFs) by using carrageenan as the capping agent and L-ascorbic acid as the reducing agent. Experimental data showed that carrageenan was the key to synthesize the Pd NFs and the concentration of the L-ascorbic acid also made a difference to the structure. The electrocatalytic activity of Pd NFs was greatly affected by the structure of the nanoparticles. The cycle stabilities of Pd NFs were found to be closely related to their inherent structures. EXPERIMENTAL SECTION

Materials and reagents

Palladium(II) chloride, potassium bromide, L-ascorbic acid (AA), sulfuric acid and ethanol were purchased from Sinopharm Chemical Reagent Co. Ltd. Carrageenan was purchased from Aladdin. Pd/C (5%) was purchased from Shaanxi Rock New Materials Co. Ltd. All the chemical reagents were analytical grade and used without further purification. Double distilled water was used in the experiments except ultrapure water (18.2 MΩ•cm) for electrochemical measurements. Synthesis of Pd Nanoflowers

In a typical synthesis, PdCl2 (20 mg), KBr (330 mg), carrageenan (30 mg) and double distilled water (26 mL) were mixed in a 50 mL beaker. The mixture was stirred for 2 hours to ensure complete dissolution. Then, aqueous L-ascorbic acid solution was quickly added to the above solution to form a homogenous mixture. After that, the homogeneous mixture was heated at 50 °C for 30 min in a water bath with magnetic stirring. In the heating process, the color of the


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solution gradually changed from orange to black. Finally, the black products were collected by centrifugation and thoroughly washed with distilled water and ethanol before being dried in an oven at 60 °C for 6 hours. The quality of L-ascorbic acid in the synthetic solutions was set to be 20, 40, 60, 120 and 180 mg, and the corresponding Pd products were named as Pd-AA20, PdAA40, Pd-AA60, Pd-AA120 and Pd-AA180, respectively. Sample Pd-AA60(0) was obtained from the same synthetic system as Pd-AA60 just without carrageenan. 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). Electrochemical measurements

All the electrochemical measurements were conducted on a CHI660D workstation with a typical three-electrode cell at room temperature. A platinum foil was used as the counter electrode and a saturated calomel electrode (SCE) in acidic solution or an Hg/HgO electrode in alkaline media as the reference electrode. Pd-modified glass carbon electrodes (GCEs) were employed as the working electrodes, which were prepared by dropping 10 µL catalyst ink on the surface of bare GCEs. The ink was obtained by dispersing 1.0 mg sample in 1 mL ultrapure water through ultrasonication for 1 hour. Cyclic voltammetry (CV) measurements were taken in


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0.5 M H2SO4 solutions at a sweep rate of 50 mV/s at room temperature. CV curves of electrooxidation of ethanol were recorded in aqueous solutions of 1 M KOH containing 1 M C2H5OH at a sweep rate of 50 mV/s. RESULTS AND DISCUSSION

Figure 1 shows the representative SEM images of four types of Pd NFs. It is obvious that all the products have a definitely flower-like morphology except Pd-AA60(0), which was endowed with an irregular shape and a rough surface. Sample Pd-AA60 has a relatively homogeneous particle size distribution with the average size of about 280 ± 30 nm, which is attributed to its sharp thorns. As for Pd-AA20 and Pd-AA180, their size distribution is not uniform with the particles size of about 200 ± 80 nm. They both have a flower-like morphology with obtuse thorns. Pd-AA60 possesses the longest and the thinnest thorns about 132 ± 15 nm for length and 25 ± 7 nm for thickness while 77 ± 7 nm, 46 ± 6 nm for Pd-AA20 and 70 ± 12 nm, 43 ± 11 nm for Pd-AA180, respectively, as shown in Figure S7 E. These data are measured from the SEM images of 200 corresponding samples. It is obvious that Pd-AA60(0) without the assistance of carrageenan doesn’t show well dispersibility (Figure 1D and H). The irregular particles connect each other to form a chain-like structure. In contrast, the nanoparticles of other samples show a well boundary and the presence of thorns of Pd-AA20, Pd-AA60 and Pd-AA180 may provide more active sites to enhance electrocatalytic activity toward ethanol. The carrageenan in the synthesis process can be considered as a kind of capping agents and surfactants which is


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supposed to play a key role in determining the morphology of these Pd nanoflowers.14,21,38 As confirmed by XRD results (Figure S1), only three sharp diffraction peaks are observed, indicating the formation of pure face-centered cubic (fcc) Pd (JCPDS PDF No. 46-1043). The average crystalline dimensions of Pd-AA20, Pd-AA60, Pd-AA180 and Pd-AA60(0) were about 23.9, 20.6, 21.1 and 21.2 nm, respectively, as calculated from the (111) peak according to the Scherrer equation.39

Figure 1 SEM images of Pd-AA20 (A, E), Pd-AA60 (B, F), Pd-AA180 (C, G) and Pd-AA60(0) (D, H).

Figure 2 and Figure S2 show the TEM and HRTEM images of samples Pd-AA20, Pd-AA60, Pd-AA180 and Pd-AA60(0). The TEM images at low magnification (Figure S2) show the individual particles of the corresponding samples, of which particle sizes match well with the observation of SEM images. The bright diffraction spots in the selected area electron diffraction (SAED) patterns indicated the existence of well crystalline nanoparticles in each sample (Figure


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S2).40 It can be easily seen that the thorns growing on the surface of Pd-AA60 are thinner and longer than any other samples shown in Figure 2, as evidenced by the SEM measurements.

Figure 2 TEM images of Pd-AA20 (A), Pd-AA60 (B), Pd-AA180 (C) and Pd-AA60(0) (D). The inserts in each TEM image are the HRTEM image and FFT pattern of the corresponding sample.

The high-resolution TEM (HRTEM) images of four samples, as shown the insets of Figure 2, are used to illustrate the detailed features. The lattice spacing of all the samples was measured at 2.27~2.30 Å, indicating that the Pd particles grow along the direction.41 In addition, the corresponding fast fourier transformation (FFT) pattern (insets of Figure 2) of the thorns reveals that the whole thorn is a single crystal.42 Such thorns are assembled together to form nanoflower structure which can be considered as the reason for the grain size inhomogeneity between SEM observation and XRD experiment results.43,44 In order to find the key factors in the process of forming Pd NFs, different control experiments were performed by changing the capping agents and reducing agents. Aggregated nanoparticles


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were obtained at a higher temperature when hydroxylamine hydrochloride (Figure S3 A) or formic acid (Figure S3 B) was used as the reducing agent instead of L-ascorbic acid. In addition, the Pd NFs disappeared when employed citric acid (Figure S4 A) or CTAB (Figure S4 B) as capping agent in the starting systems. However, the concentration of carrageenan (Figure S5) only has a slight effect on the morphology of Pd NFs. The results above show that the Pd NFs are obtained only at the appropriate reduction rate while using carrageenan as capping agent. In comparison, the structure of carrageenan is more complex with more functional groups than citric acid and CTAB. Owing to the abundant hydroxyls (OH-) and sulfate groups (OSO3-) on the carrageenan,34 they may be absorbed onto the surface of Pd nanoparticles to form the flower-like structure. In order to obtain the formation process of the Pd NFs when used carrageenan as capping agent, the intermediary products were collected at 5, 10 and 30 min and examined by SEM and XRD as shown in Figure 3 and Figure S6, respectively. There were a large number of Pd seeds at the beginning. Then block-like Pd nanoparticles were obtained at t = 10 min. As the reaction proceeded, the reduced Pd atoms tended to deposit on the edges and corners of the seeds with the carrageenan as capping agent. Otherwise, only irregular nanoparticles were obtained without carrageenan (Pd-AA60(0)). The schematic illustration of the growth mechanism of Pd NFs has been shown in Figure 4. All intermediates are pure Pd nanoparticles demonstrated by the XRD patterns (Figure S6).


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Figure 3 SEM images of intermediates from the synthesis system for Pd-AA60 (A, B, C) and Pd-AA60(0) (D, E, F) with different reaction time: 5 (A, D), 10 (B, E) and 30 min (C, F).

Figures 4 The schematic illustration of formation mechanism of Pd nanoflowers.

Electrochemical measurements were carried out to report the electrocatalytic performance of the as-prepared samples. Figure 5A shows the typical CV curves of the Pd NFs modified GCEs in sulfuric acid solutions at the scan rate of 50 mV s-1. The redox peaks displayed on the CV curves matched well with the characteristic peaks of Pd NFs modified electrodes reported in the literatures.9,45,46


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The sharp peak at 0.44V was attributed to the reduction peak of produced Pd oxide, and peaks observed at -1.7V and -0.5V (vs SCE) corresponded to the adsorption/desorption of hydrogen at the surface of Pd NFs, which were used to estimate the electrochemically active surface areas (ECSAs). The ECSAs of Pd-AA20, Pd-AA60, Pd-AA180 and Pd-AA60(0) were calculated as 143, 180, 169 and 144 cm2/mg, respectively, based on the formula QHdes /210 µC cm-2, among which Q stands for the corresponding electrical quantity of the inflection point according to the integral of the desorption of hydrogen.47 In the meantime, the reduction region of palladium oxide, performed in 1M KOH at a scan rate of 20 mV s-1, was also used to calculate the ECSAs with the formula QPdOX/405 µC cm-2 (Figure 5B).45 The ECSAs of Pd-AA20, Pd-AA60, PdAA180 and Pd-AA60(0) were 151, 223, 202 and 90 cm2/mg, respectively, which showed a slightly higher ECSAs value than the above ones except Pd-AA60(0).

Figure 5 (A) CV curves of the Pd NFs modified GCEs in aqueous H2SO4 (0.5 M) solutions at a scan rate of 50 mV/s; (B) CV curves of the Pd NFs modified GCEs in aqueous KOH (1 M) solutions at a scan rate of 20 mV/s.


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The electrocatalytic activity of Pd NFs for ethanol oxidation was investigated in alkaline medium. The mass activity of sample was used to evaluate the catalytic performance. As depicted in Figure 6A and S7 G, Pd-AA60 exhibits the highest catalytic current density of 1160 mA mg-1 which is obviously higher than commercial Pd/C (Figure S8) and the reported Pd/C samples,17,42 while 738, 840 and 542 mA mg-1 are corresponding to Pd-AA20, Pd-AA180 and Pd-AA60(0), respectively. The catalytic activity for four Pd NFs electrocatalysts is in the order of Pd-AA60 > Pd-AA180 > Pd-AA20 > Pd-AA60(0). Moreover, as shown in a magnified profile (insert in Figure 6A), the onset potential of the Pd-AA60 modified GCE shows the most negative shift among all the modified electrodes, suggesting the significant enhancement of Pd-AA60 in the kinetics of the ethanol oxidation reaction. For a better understanding of the catalytic activity, the current density normalized to the ECSAs are displayed in Figure 6B. Pd-AA60 also shows the highest current density value of about 6.27 mA cm-2 and the Pd-AA60(0) shows the lowest of about 3.77 mA cm-2, while Pd-AA20 and Pd-AA180 exchange position compared with the mass activity. This indicates that the ECSAs of the samples also contribute to the electrocatalytic performance of Pd NFs except the morphology and structure.


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Figure 6 CV curves of the Pd NFs modified GCEs in 1 M KOH/1 M C2H5OH at 50 mV s-1 normalized by mass (A) and surface area (B). The inset curves in A are the magnification of the onset potential for ethanol oxidation in A.

The Pd-AA60 modified GCE exhibited an excellent catalytic activity toward the electrooxidation of ethanol. The CV curves of Pd-AA60 at different cycles are shown in Figure 7A. The current density value increases sharply in the first 7 cycles and arrives at the highest value of 1160 mA mg-1 around 26 cycles. With the increasing of cycle times, the anodic peaks display a positive shift and the catalytic activity gradually declined. Noting that approximate 95.8% of the catalytic activity maintained well through 100 cycles, which shows a great catalytic performance and the final current density after 300 cycles decreased to 74.1 % of the highest value. Figure 7B shows the CVs of Pd-AA60 at various sweeping rates. The peak current density enhanced with the increasing of the scan rate. With a detailed analysis, it was evidence that the peak current density showed a linear relationship with the square root of the sweeping rate (V1/2)


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as shown in Figure 7C. It is suggested that the kinetics of the overall process for electrooxidation of ethanol on the modified electrode followed a diffusion-controlled process.48

Figure 7 (A) CV curves of the Pd NFs modified GCEs at different cycles with a scanning rate of 50 mV s-1; (B) CV curves of the Pd NFs modified GCEs at different scan rates; (C) The corresponding plot of forward peak current versus the square root of the scan rate.

Figure 8A demonstrates the cycle stability curves of the Pd NFs modified GCEs for the electrooxidation of ethanol. It can be seen that all the Pd NFs reach the maximum catalytic current density at 20-30 cycles. The catalytic current density of Pd-AA60 after 300 cycles declines greater than these of Pd-AA20, Pd-AA180 and Pd-AA60(0), of which the current density maintained above 96%. Transient current densities of ethanol oxidation of the Pd NFs modified GCEs was monitored by the chronoamperometry technique at -0.1 V (Figure 8B). All the polarization currents show a rapid decay in the initial period for the four catalysts, probably due to the generation of poisonous intermediate species during the ethanol electrooxidation process in alkaline media at the beginning of the test.49 Then the electrocatalytic activity gradually decreases under the subsequent measurement. All the four samples show clear current density platforms that change


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from 82, 208, 61 and 49 mA mg-1 at around 100 s to 15, 29, 12 and 8 mA mg-1 at the end of 1000 s, respectively. It is confirmed that Pd-AA60 possesses the highest electrocatalytic performance and optimized tolerance to the poisoning during the ethanol oxidation process among all the samples.50

Figure 8 (A) The variation of current density along with the cycle number for ethanol oxidation in 1 M KOH/1 M C2H5OH; (B) Chronoamperometric curves of as-prepared electrodes at -0.1 V.

This indicated that the sharp thorns of Pd-AA60 are apt to change during the electrocatalysis while other structures may be maintained as depicted in the Figure 9 and S9. The morphology and structure of the Pd-AA60 were no longer complete but into debris after 300 cycles in 1 M C2H5OH + 1 M KOH media. However, there was no obvious damage on the other three samples after the same electrochemical test (Figure 9B and S9). Therefore, this destruction of morphology and structure might be attributed to the fact that the thorns of Pd-AA60 were unstable to keep steady state during the cycles as evidenced by the presence of lattice defects which were observed from the HRTEM images of Pd-AA60 (Figure S10). Thereafter, more and


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more unstable debris produced may be easily dropped during the testing process. Finally, the catalytic activity toward ethanol shows a significant attenuation after 100 cycles as shown in Figure 8A. As for Pd-AA60(0), the catalytic activity curve is still very smooth even tested after 300 cycles ascribed to the short and thick thorns formed by many nanoparticles that could not be broken completely during the electrochemical measurements.44

Figure 9 TEM images of Pd-AA60 (A) and Pd-AA60(0) (B) after electrochemical cycle test. The insert in each TEM image is the single nanoparticle of the corresponding sample.

CONCLUSION

Pd nanoflowers with controlled structure have been synthesized by using a novel capping agent of carrageenan which is crucial to the formation of nanoflower structures. The length of thorns in Pd nanoflowers can be easily adjusted through changing the concentration of Lascorbic acid in the synthesis system. It is found that Pd nanoflowers with the longest thorns showed the highest electrocatalytic current density toward oxidation of ethanol while Pd nanoflowers with short thorns displayed a better cycle stability. These results would be helpful


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for the design and synthesis of novel palladium-based nanostructures with excellent electrocatalytic performance.

ASSOCIATED CONTENT

Supporting Information.

Additional supporting figures (Figure S1-S10) for the morphology of Pd NPs as noted in the text, TEM and SAED images of Pd nanoflowers, XRD patterns and SEM images of the Pd nanoflowers and intermediate products as well as CV curves of Pd nanoflowers and commercial Pd/C. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENT This work is financially supported by the National Natural Science Foundation of China (No. U1232104 and 21773133), National college students innovation and entrepreneurship training program and the Taishan Scholars Advantageous and Distinctive Discipline Program for supporting the research team of energy storage materials of Shandong Province, P. R. China.

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Table of Contents

Diverse Pd nanoflowers are synthesized by using biomacromolecule as capping agent. Electrocatalytic activity and stability of Pd nanoflowers are obviously affected by their structures.


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Carrageenan Asissted Synthesis of Palladium Nanoflowers and Their

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