Insights into the role of polyvinylpyrrolidone on the synthesis of

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Insights into the role of polyvinylpyrrolidone on the synthesis of palladium nanoparticles and their electrocatalytic properties Yanru Yin, Ning Ma, Jing Xue, Guoqiang Wang, Shuibo Liu, Hongliang Li, and Peizhi Guo Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b04032 • Publication Date (Web): 02 Jan 2019 Downloaded from http://pubs.acs.org on January 3, 2019

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Insights into the role of polyvinylpyrrolidone on the synthesis of palladium nanoparticles and their electrocatalytic properties Yanru Yin, Ning Ma, Jing Xue, Guoqiang Wang, Shuibo Liu, Hongliang Li, Peizhi Guo* 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, Qingdao, 266071, P.R.China *Corresponding

Author: [email protected], [email protected]

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ABSTRACT: Four types of palladium (Pd) nanoparticles were prepared from the systems containing PdCl2 or Na2PdCl4 with or without the assistance of polyvinylpyrrolidone (PVP). Two types of Pd nanoparticles obtained in the absence of PVP were obviously larger than those synthesized with the assistance of PVP. The former large Pd particles showed typical features in cycle voltammetry in H2SO4 solution, while two types of small Pd nanoparticles cannot. However, small nanoparticles treated firstly in electrochemical way in 0.5 M KOH solution displayed the adsorption and desorption peaks as typical Pd modified electrodes in H2SO4 solution. Large Pd nanoparticles from the PdCl2 synthesis system showed a catalytic specific current of 629 mA/mg in the electrocatalysis of ethanol while the current value of 262 mA/mg for large particles from the Na2PdCl4 system. The maximum catalytic currents of small Pd nanoparticles without surface cleaning treatment were 1382 and 1019 mA/mg for samples from the Na2PdCl4 and PdCl2 system, respectively, higher than that being treated in KOH solution first, and the electrocatalytic stability of the untreated two samples was better. However, small nanoparticles after the electrochemical treatment can reach the maximum catalytic current faster. The synthesis and their structure-property relation of four types of Pd nanoparticles have been discussed and analyzed based on systematically experimental data.

KEYWORDS: Palladium; Polyvinylpyrrolidone; Electrocatalysis; Hydrothermal synthesis.


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INTRODUCTION Noble metal Pd plays an important role in many scientific and industrial fields, such as catalysis, energy and environments due to its unique electronic structure, physical and chemical properties1-5. As for electrocatalysis, an important branch in catalysis and energy, Pd has attracted increasing interests during the past two decades owing to its lower price than platinum (Pt), moderate catalytic activity and unique catalytic process compared with Pt6-9. For example, electrocatalysis of formic acid on Pd electrocatalysts is proposed to undergo the electrochemical processes without the presence of CO intermediate10. Pd-based electrocatalysts have been explored and developed with diverse shape and structures11 by using various synthesis technologies including hydrothermal/solvethermal synthesis12, template method13-14, galvanic replacement15, reverse microemulsion method16, and electrochemical deposition6. There are also some new-released works observed the electrocatalytic activity and stability to ethanol with Pd network decorated in reduced graphene oxide17, 2-D Pd nanosheets18 and 3-D Pd nanostructures19. Aiming to improve electrocatalytic performance of Pd, one effective way is to decrease the size of Pd nanoparticles20. For example, ultrathin Pd nanosheets are obtained from the CO-assisted solution phase synthesis, which display high electrocatalytic activities towards ethanol9. Recently, we reported the synthesis of Pd nanoflowers with sharp thorns via a biomacromolecule-assisted low-temperature synthesis process21. For particles, the surface energy is greatly increased with the particle size down to nanometer scale. In fact, pure nanoparticles hardly existed separately well in solid form. Generally, small nanoparticles easily dispersed well in solution due to the particle charge


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nature or surfactant coverage on the particle surface. The well separated nanoparticles are usually covered by various types of surfactants or polymers, including polyvinylpyrrolidone (PVP)22, sodium dodecyl sulfate (SDS)23, hexadecyltrimethy ammonium bromide (CTAB)9, etc. PVP is particularly interesting due to its unique molecule structure and physicochemical properties. Various nanoparticles such as metal nanoparticles and inorganic functional compound nanoparticles have been obtained with the assistance of PVP24-25. For Pd catalysts, Pd nanoparticles, polyhedrons, nanorods/wires and hollow nanostructures have been prepared in the presence of PVP24. Actually, it is hard to remove PVP completely from the surface of small nanostructures26. This would affect the physicochemical properties of the targeted Pd nanostructures. At present, it is still necessary to clarify the role of PVP absorbed on the surface of Pd nanoparticles in their electrocatalytic performances. In this paper, the effect of PVP on the synthesis and properties of Pd nanoparticles were investigated by two systems. Large Pd nanoparticles are obtained directly by using PdCl2 and Na2PdCl4 as the precursor while small Pd nanoparticles are formed with the assistance of PVP in the synthesis system. Experimental data indicated that PVP molecules or residues absorbed onto the surface of small Pd nanoparticles can be effectively reduced during the initial treatment in potassium hydroxide solution through cyclic voltammetry (CV) technique although this treatment maybe slightly adjust the surface structure of Pd nanoparticles and lead to a decrease in the catalytic activity toward electrooxidation of ethanol27.

EXPERIMENTAL SECTION Materials and reagents


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Palladium (II) chloride, sodium chloride, potassium hydroxide, sulfuric acid, ethanol, ethylene glycol and acetone were purchased from Sinopharm Chemical Reagent Co.Ltd. Polyvinylpyrrolidone (PVP, M.W. =58,000) was purchased from Aladdin. All of the chemical reagents were analytical grade and used directly without further purification. Double distilled water was used in the experiments except ultrapure water (18.2 MΩ•cm) for electrochemical measurements. Synthesis of Pd nanoparticles In a typical route, PdCl2 (20 mg) were dissolved in ethylene glycol (30 mL) under stirring in the beaker. Then the mixture was transferred into a 50 mL teflon-lined autoclave, followed by the reaction in an oven at 150 °C for 5 hours. After cooled to room temperature, the as-formed products were washed thoroughly with the mixture of ethylene glycol and acetone and then dried at 60 °C for 6 hours in the oven. The final product was named as Pd-PC. The as-prepared product with the presence of NaCl (110.4 mg) in the initial solution was abbreviated as Pd-NPC. When PVP (66.7 mg) was introduced into the above two synthesis systems, the corresponding products were named as Pd-PC-PVP and Pd-NPC-PVP, respectively. Characterization The crystallization properties of the samples were characterized by Rigaku Ultima Ⅳ X-ray diffractometer (XRD, Cu-Kα radiation λ = 0.15418 nm). The morphologies structure and surface composition were examined by using JEOL JEM-2100 plus transmission electron microscope (TEM), high-resolution TEM (HRTEM), and X-Ray photoelectron spectroscopy (XPS, PHI 5000 VersaProbe Ⅲ).


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Electrochemical measurements CHI660D workstation was employed to conduct the electrochemical measurements at room temperature with a typical three-electrode cell in which the platinum plate was used as counter electrode, the saturated calomel electrode (SCE) in acidic solution or Hg/HgO electrode in alkaline media as the reference electrode. Glass carbon electrodes (GCEs) modified by Pd nanoparticles were used as working electrode which were prepared by casting 10 μL catalyst ink onto the bare GCE surface, and the ink were obtained by dispersing 1 mg sample into 1 mL ultrapure water. The used electrolyte solutions were 0.5 M H2SO4, 1 M KOH and 1 M KOH/1 M C2H5OH solutions for cyclic voltammetry measurements.

RESULTS AND DISCUSSION As shown in Figure 1, three diffraction peaks in the XRD patterns of Pd-PC, Pd-NPC, Pd-PC-PVP and Pd-NPC-PVP can match well with the index of a face-centered cubic (fcc) crystalline structure of Pd (JCPDS No. 46-1043). Diffraction peaks are observed at 2θ value of about 40.3°, 46.7° and 68.3° belong to (111), (220) and (200) planes of pure Pd, respectively28. Clearly, the XRD patterns can be divided into two types based on the synthesis systems with or without PVP. The diffraction peaks of Pd-PC and Pd-NPC in Figure 1A are much sharper, illustrating a large crystallite size than those of Pd-PC-PVP and Pd-NPC-PVP with broad diffraction peaks stated that the crystallized sizes are small.


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Figure 1 XRD patterns of Pd-NPC, Pd-PC, Pd-NPC-PVP, and Pd-PC-PVP. Figure 2 shows the TEM images of four types of Pd nanoparticles. Clearly, Pd-NPC and Pd-PC are large particles with aggregated structures, as shown in Figure 2A and B, respectively. The lattice spacing of Pd-NPC and Pd-PC are measured as about 2.26 and 2.23 Å attributed to the (111) planes of pure Pd. However, well dispersed small nanoparticles are observed in the TEM images of Pd-PC-PVP and Pd-NPC-PVP (Figure 2C and D). The sizes are about 5±0.5 nm and 7 ±0.6 nm for Pd-NPC-PVP and Pd-PC-PVP, respectively, which is based on the calculation of about 250 nanoparticles. It also can be concluded that Pd-NPC-PVP has a narrower size distribution than Pd-PC-PVP based on the TEM observations. Pd-NPC-PVP shows well crystalline nature, as evidenced in the inset in Figure 2C. The lattice spacing is measured to be around 2.25 Å and 2.11 Å ascribed to (111) and (200) planes of cubic Pd. The angle between these two planes in Pd-NPC-PVP is about 53°, in great accord to the theoretical value of pure Pd29. Similar spacing of about 2.24 Å is obtained for Pd-PC-PVP, as shown in the inset in Figure 2D. The sizes of four types of Pd nanoparticles are in good agreement with the XRD results. It is suggested that the presence of PVP can lead to the adsorption of the molecule onto the surface of Pd nuclei, and thus avoid the aggregation of nanoparticles. In the meantime, the formation of plenty of caped nanoparticles would limit the unlimited growth of nuclei to a large size.


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Figure 2 TEM and HRTEM images of Pd-NPC (A), Pd-PC (B), Pd-NPC-PVP (C), Pd-PC-PVP (D). Large particles have lower specific surface areas than small particles, leading to higher surface energies for the latter. Thereafter, small particles are apt to aggregate to a large size. The cover of particles with organic molecules is a typical approach to reduce the surface energy of particles and thus avoid their aggregation partially due to the existence of repulsive forces caused by the surface organic molecules. It is expected that large particles usually adsorb less organic molecules than small particles, and this would cause the potential change of physicochemical properties of small particles compared with large particles. As shown in Figure 3A, cyclic voltammetry (CV) of Pd-PC and Pd-NPC conducted in 0.5 M H2SO4 solutions shows exactly redox peaks which matches well with the characteristic peaks of Pd modified electrodes according to literatures7, 30-32. There is a sharp peak at about 0.44 V attributed to the reduction of produced Pd oxide33-34. Two peaks appear at about -0.18


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V and -0.02 V (vs SCE) in positive sweep considered as the adsorption of hydrogen occurs at the surface of Pd nanoparticles which can be used to evaluate the electrochemically active surface area (ECSA) of samples35-36. As calculated with the formula QHdes/(210 μC/cm2) in which Q is obtained by integrating desorption of hydrogen of the corresponding electrical quantity of the inflection point, the ECSAs of Pd-NPC and Pd-PC were 62 and 132 cm2/mg, respectively37-38. Figure 3B shows the CV curves of the GCEs modified by Pd-PC and Pd-NPC for electrooxidation of ethanol in alkaline media of 1 M KOH/1 M C2H5OH to evaluate their electrocatalytic activity. Pd-PC and Pd-NPC shows low catalytic mass specific current of about 629 and 262 mA/mg, respectively, due to the large crystallite size of both samples compared with the reported results39-42.

Figure 3 CV curves of Pd-NPC and Pd-PC in 0.5 M H2SO4 (A) and 1 M KOH/1 M C2H5OH (B) at the sweep rate of 50 mV/s. Figure 4 shows the CV curves of the GCEs modified by Pd-NPC-PVP in various conditions. It can be seen from Figure 4A that when scanning directly in 0.5 M H2SO4, the hydrogen adsorption peak and reduction peak of Pd oxide are not clear on the Pd-NPC-PVP/GCE, while the peaks appear (red curve in Figure 4B), for example, after reaching the highest catalytic activity (black curve in Figure 4C) during the process of


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electrocatalysis of ethanol. According to Figure 4C, the highest catalytic mass specific currents are respectively about 904 mA/mg and 1382 mA/mg for Pd-NPC-PVP/GCE with or without scanning in 0.5 M KOH first. The ECSA of Pd-NPC-PVP at the top activity without the treatment by cyclic voltammetry in KOH solution is about 24.6 cm2/mg with a current density of about 56.9 mA/cm2. It is proposed that the ECSA for Pd-NPC-PVP should not represent the nature of real active sites of the sample for electrocatalysis of ethanol31, 43-45. However, the mass activity provides an alternative way to evaluate the whole electrocatalytic activity of Pd nanoparticles.

Figure 4 (A) CV curves of sample Pd-NPC-PVP in 0.5 M H2SO4; (B) CV curves of Pd-NPC-PVP in 0.5 M H2SO4 after scanning in 0.5 M KOH and after reaching the top of ethanol electrooxidation; (C) CV curves of Pd-NPC-PVP for the electrocatalysis of ethanol in 1 M KOH/1 M C2H5OH. Figure 5 shows the CV curves of the Pd-PC-PVP modified GCEs under different conditions. As depicted in Figure 5A, there are no clear adsorption and reduction peaks for the initial Pd modified electrodes. After scanning in 0.5 M KOH and reaching the top of ethanol electrooxidation curves, the peaks can be easily observed (Figure 5B). Meanwhile, the catalytic current density shows the same rule as that of Pd-NPC-PVP/GCE. The highest mass specific current is around 1019 and 750 mA/mg for Pd-PC-PVP/GCE without and with


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scanning in 0.5 M KOH. The ECSA is about 97.6 cm2/mg based on the black curve in Figure 5B. The current density is about 10.25 mA/cm2 for Pd-PC-PVP, lower than Pd-NPC-PVP.

Figure 5 (A) CV curves of Pd-PC-PVP in 0.5 M H2SO4; (B) CV curves of Pd-PC-PVP in 0.5 M H2SO4 after scanning in 0.5 M KOH and after reaching the top of the electrocatalysis of ethanol; (C) CV curves of Pd-PC-PVP of the electrocatalysis of ethanol in 1 M KOH/1 M C2H5OH. According to the above data, it is clear that PVP can affect the electrocatalytic performance of the samples in a large extent. On the one hand, Pd nanoparticles prepared with PVP have a higher catalytic current than that obtained without PVP due to the size effect46-48, which is consistent with the results of XRD patterns. On the other hand, PVP or residue of PVP on the surface of Pd-PC-PVP and Pd-NPC-PVP has appreciable effects on their electrocatalytic performance49-50. No typical features in the CV curves of some Pd nanoparticles even after tens of potential cycling possibly due to the strongly absorbed surfactants in the surface of Pd, which can be solved by the formation of Pd hydrides treated with NaBH451. Synthesized Pt-Pd alloy networks displayed no clear hydrogen absorption and desorption peaks in 0.5 M H2SO4 solution which was ascribed to the synergistic effect and specific structure of Pt-Pd52. Similar phenomenon had also emerged in Pd-Ni alloy nanoparticles attributed to the prepared Ni occupied the active sites of Pd53. So the disappearance of typical peaks of Pd is not an


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accident phenomenon, it can be seen that Pd-PC-PVP and Pd-NPC-PVP treated by cyclic voltammetry in KOH solutions can show clear adsorption and reduction peaks while untreated Pd nanoparticles cannot. But there is a phenomenon that the highest current density of the treated nanoparticles is lower than those untreated. Figure 6A and C shows the variation of current density of ethanol electrooxidation with the cycle numbers for untreated Pd-PC-PVP and Pd-NPC-PVP. Both of the modified GCEs reach the maximum peak current density at about 70 cycles, possibly attributed to the existence of PVP or PVP residues on the particle surface which make it more difficult to activate the catalytic activity of the samples30. Meanwhile, Pd nanoparticles treated with KOH reach the maximum peak current density at about 30 cycles which are faster than the corresponding untreated samples, possibly due to the removal of some PVP or PVP residues from the surface of Pd nanoparticles. However, the electrochemical treatment could hardly remove all of the PVP because the maximum current density cannot be reached very quickly. And the cycle stability of Pd-NPC-PVP is much better than Pd-PC-PVP. The chronoamperometric curves in (B) and (D) of Figure 6 of samples prepared with PVP shows that the catalytic decreases rapidly within the first 20 seconds, and then decline slowly, followed by a proximate plateau especially for Pd-PC-PVP. These should be attributed to the competitive adsorption between ethanol and Pd-oxide on the catalytic surface in the process of ethanol electrooxidation that resulted in a decrease of the oxidation reaction rate54-56. During this process, some Pd nanoparticles maybe lost its activity because of corrosion in the alkaline media. It is obvious that both of Pd-PC-PVP/GCE and Pd-NPC-PVP/GCE treated with KOH display lower current densities than those without pretreatments and Pd-NPC-PVP/GCE


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shows a higher activity with the time up to 1000 s.

Figure 6 Chronoamperometric curves of as-prepared electrodes at -0.1 V (B, D), variation of current density along with the cycle number for ethanol electrooxidation in 1 M KOH/1 M C2H5OH (A, C). According to these results, it is confirmed that the variation of electrocatalytic activity of Pd nanoparticles should be related to the organic molecules on the particle surface. Based on the experimental data and reported literatures57, the possible mechanism of the removal of PVP during the cyclic voltammetry in KOH solutions is illustrated in Scheme 1. It is suggested that surface of Pd atoms should be bonded with the carbonyl group of PVP via chemisorptions bonds that may cover some active sites of Pd nanoparticles. When the sample is treated in KOH solutions by electrochemical methods, the OH- ion may attack the N-C bond in PVP and destroy the carbonyl group coordinated with Pd, the coordination of PVP and Pd atoms are then damaged, leading the active sites of Pd exposed, so it can be seen from Figure 4 and 5 that the desorption and adsorption peaks appear after electrochemical


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treatment in KOH solutions.

Scheme 1 The possible reaction mechanism of the removal of surface organic molecules of Pd nanoparticles by KOH. In order to better understand the structure of Pd nanoparticles, a study was performed by X-Ray photoelectron spectroscopy (XPS) as shown in Figure 7. It is found that the binding energy of Pd 3d5/2 of Pd-NPC and Pd-PC shows at 335.80 eV, while Pd 3d5/2 of Pd-NPC-PVP and Pd-PC-PVP decrease to a lower value at 334.10 eV attributed to the high charge density of Pd atoms. Meanwhile, the N 1s spectra of Pd-NPC-PVP and Pd-PC-PVP show peaks at 399.4 eV, illustrate the presence of PVP according to the reported literatures58-59. Figure 7B shows the presence of PdO 3d5/2 bonds at 355.9 eV for Pd-NPC-PVP and Pd-PC-PVP which is not shown in Figure 7A. At the same time, there is no corresponding bond for the PdO compound in the O 1s spectrum, therefore the presence of PdO bonds might be attributed to the combination of Pd and PVP. When treated in 0.5 M KOH, the PdO bonds would be destroyed to expose the active site. The results of XPS are consistent with the possible reaction mechanism of KOH with the PVP capped Pd nanoparticles, as shown in scheme 1.


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Figure 7 XPS Pd 3d spectra of Pd-NPC, Pd-NPC-PVP, Pd-PC, Pd -PC-PVP. As mentioned before, the electrochemical behavior of Pd nanoparticles definitely represents their surface structure obtained with or without the help of PVP. It is reported that the structure and morphology of nanoparticles may be altered after electrochemical measurements7. As depicted in Figure 8, the morphology of Pd-PC-PVP and Pd-NPC-PVP are slightly changed after the KOH treatment by using cyclic voltammetry. There have more large nanoparticles observed than before for Pd-NPC-PVP and possible aggregations appear for Pd-PC-PVP. However, the shape of both Pd-PC-PVP and Pd-NPC-PVP changed greatly after reaching to the state with the highest catalytic current density. Some irregular nanoparticles are observed in Pd-NPC-PVP at this condition. For Pd-PC-PVP, complicated wire-like inter-crossed structures are obtained. Even though the morphologies changed drastically, the crystalline natures shown in the HRTEM images are very similar in the insets in Figure 8, clearly indicating the nature of electrocatalysts Pd. In the meantime, the existence of PVP or PVP residue on the surface of Pd nanoparticles is suggested to play a positive role for the electrooxidation of ethanol for Pd-PC-PVP and Pd-NPC-PVP. In our recent reports, PVP also was used in the synthesis process21,50. As these Pd particles are very large compared with Pd-PC-PVP or Pd-NPC-PVP, we found that the CV curves of those Pd particles modified GCEs obtained from H2SO4 solutions are almost unaltered for those with or without


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pretreatment in KOH solutions by cyclic voltammetry.

Figure 8 TEM and HRTEM images of Pd-NPC-PVP (A, B) and Pd-PC-PVP (C, D) after treated directly in 0.5 M KOH in electrochemical method (A, C), and at the highest current density on the electrocatalysis of ethanol (B, D). Based on these observations, it may be clear that pretreatment of Pd-NPC-PVP and Pd-PC-PVP by using electrochemical method in KOH solutions would cause three changes. First, the removal of PVP or PVP residues from the nanoparticle surface has occurred. Second, the surface structure of Pd nanoparticles may be slight modified or rearranged during the removal process of PVP. This should be ascribed to a lower catalytic current density than that without undergoing the structural variation of Pd-PC-PVP or Pd-NPC-PVP. Consequently, the structural changes lead to the decrease in the catalytic activities of the electrocatalysts.


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CONCLUSION Large Pd particles have been synthesized from the synthesis systems containing PdCl2 and Na2PdCl4 while small nanoparticles are obtained with the addition of PVP into the start system. Well crystalline structure is formed in these four types of Pd particles. Large Pd particles show typical features of Pd modified electrodes in H2SO4 solution while small nanoparticles display similar phenomenon only after electrochemical treatment in KOH solution. Large Pd particles from the PdCl2 system show a mass specific current of 629 mA/mg, which is higher than that from Na2PdCl4. Small Pd nanoparticles can reach a higher catalytic activity of 1382 mA/mg toward electrocatalysis of ethanol than those which is treated by the cycle voltammetry in KOH solution although the latter can reach its highest catalytic activity quickly. It is suggested that the presence of PVP in the synthesis of Pd nanoparticles play a positive role both in the synthesis of small size and in the requirement of high catalytic performance. This should be helpful for the synthesis of novel nanostructures of electrocatalysts from the solution-based strategy.

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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Insights into the role of polyvinylpyrrolidone on the synthesis of

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