Surface anchoring approach for growth of CeO2 nanocrystals on

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Surface anchoring approach for growth of CeO2 nanocrystals on Prussian blue capsules enable superior lithium storage Kai Wang, Fangzhou Zhang, Guanjia Zhu, Hui Zhang, Yuye Zhao, Lan She, and Jianping Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b11212 • Publication Date (Web): 16 Aug 2019 Downloaded from pubs.acs.org on August 16, 2019

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ACS Applied Materials & Interfaces

Surface Anchoring Approach for Growth of CeO2 Nanocrystals on Prussian Blue Capsules Enable Superior Lithium Storage Kai Wang,† Fangzhou Zhang,† Guanjia Zhu,† Hui Zhang,*,† Yuye Zhao,† Lan She,*,§ and Jianping Yang*,† † State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Donghua University, Shanghai 201620, China § Department of Inorganic Chemistry, School of Pharmacy, Second Military Medical University, 325 Guohe Road, Shanghai 200433, China

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Abstract: Prussian blue (PB) and its analogues (PBA) have been acknowledged as promising materials for the catalysis, energy storage, and bioapplications because of the varied constructions and tunable composition. The approaches for surface modification with metal oxide and boosting the performance, however, it is rarely reported. Herein, a facile surface anchoring strategy has been proposed to realize CeO2 nanocrystals uniformly depositing on the surface of PB. Besides, the size, thickness and depositing density of CeO2 nanocrystals can be regulated through adjusting the amount of precursor and the proportion of ethanol and deionized water. Furthermore, after a step of confined pyrolysis treatment under air atmosphere, CeO2 nanocrystals encapsulated iron oxide structure has been obtained. It shows remarkable cycling and rate performance when evaluated as an anode of lithium-ion battery. This CeO2 nanocrystals surface anchoring approach may not only promote the various applications of PB-based materials but also provide the opportunity for the architecture of other CeO2-based core-shell nanostructures.

Keywords: ceria anchoring, core-shell structures, confined pyrolysis, lithium-ion battery, Prussian blue

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1. Introduction Prussian blue (PB), comprising of ferrous and ferric ions coordinated by CN bridges, possesses a cubic face-centered structure and unique archetypal hexacyanometalate framework structure.1-3 In a typical framework of PB, ferrous and ferric ions can be easily replaced by other transition metal (such as Cd, Ni, Co, Mn, Cu) to form Prussian blue analogs (PBA).4-5 Moreover, PB and PBA are regarded as prospective precursors for the preparation of metal oxides,6-8 sulfides,9-11and phosphides12-14 by simple heat treatment with well-maintained structure.15 Owing to the diverse constructions and adjustable chemical components, PB and PBA materials have been employed in many areas, such as sensors,16-17 catalysis,18-19 photothermal-chemo therapy,20-21 and energy storage and conversion.22-24 For instance, Fe2O3 with hierarchical shell cube structure was attained via thermal transforming PB cubes for high capacity storage in lithiumion battery.25 A hollow interior cobalt hexacyanoferrate PBA was synthesized by cation exchange method to present a superior performance as Na-ion hybrid supercapacitors.26 Moreover, 3D structure NiCoFeP with hollow porous heterometallic phosphide could be fabricated by one step phosphatization of a NiCoFe PBA for promoting the OER and HER ability.27 Indeed, the clever combination of diverse components on the surface of PB can lead to the synergistic effort and further improve the performance. For this purpose, diverse materials including carbon,28 silica,29 noble metal nanoparticles30 have been reported for successful coating on the surface of PB and PBA materials. Representatively, Yang et al. fabricated Fe/C@mSiO2 with nano zero-valent Fe uniformly dispersing in the carbon framework by reduction of PB@mSiO2 under H2 atmosphere to 3



realize effectively removing Se (IV) in a low concentration condition.29 Such Fe/C@mSiO2 structure can hinder the aggregation of iron nanoparticles, thus providing more active sites and speeding up the reaction rate. 2D PB@MoS2 structure was fabricated by Coronado et al. to use as cathode of sodium-ion batteries, and it exhibited an exceptional performance with the capacity of 177 mA h g−1.31 After introducing MoS2, PB can fast nucleate on MoS2 to form a 2D structure with synergistic behavior which can accelerate Na+ diffusion causing a higher capacity than the theoretical value. Dou et al. in-suit synthesized PB@C as cathode of sodium-ion battery.32 The relation between PB cubes and carbon matrix can fast electron transfer efficiently and curtail the length of Na-ion diffusion, achieving excellent electrochemical performance. Although much efforts have been devoted to fabricating the core-shell structures, it is still difficult to develop other metal oxide shell and control the size and compactness of nanocrystals. In this work, a simple surface anchoring strategy has been proposed for the preparation of PB@CeO2 with controllable size distribution and shell thickness. CeO2 nanocrystals with different size, thickness and compactness have been anchored on the surface of PB by facilely adjusting the amount of the precursors and the ratio of ethanol and deionized water. Sintering the PB@CeO2 in air atmosphere leads to forming small Fe2O3 nanoparticles confined in CeO2 capsules. In this regard, the surface anchoring of CeO2 nanocrystals plays a critical role in the structural stability, realizing the spatially confined pyrolysis toward the formation of small Fe2O3 nanoparticles inside of capsules. As a nano-additive, CeO2 can not only improve the ionic conductivity and mechanical property of the polymer 4


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electrolytes for LIBs but also result in forming steady SEI film because of the reversible conversion between CeO2 and Ce2O3. Therefore, as-obtained capsules Fe2O3@CeO2 structure manifest a superior lithium storage performance when used as an anode material. 2. Experimental Section 2.1 Synthesis of PB cubes. In a typical synthetic procedure, K4Fe(CN)6·3H2O (1.1 g) and Polyvinepirrolydone (PVP, K30, MW: ~40000, 38g) were added into HCl solution (0.1 M, 500 mL) and then followed by magnetic stirring. 2 h later, a transparent and clear yellow solution was attained. The solution was then put into an electric oven and heated at 80 ℃ for 24 h. Afterward, the blue product was collected by centrifugation and washed three times with ethanol and distilled water and finally dried at 35 ℃ overnight in a vacuum oven. 2.2 Synthesis of PB@CeO2, CeO2, and Fe2O3@CeO2 capsules. PB cubes (50 mg) were homogeneously dissolved into a mixture of ethanol (20 ml) and water (20 ml) and settled with ultrasonication for 10 min. Subsequently, Ce(NO3)3·6H2O (17 mg) and hexamethylenetetramine (HMT) solution (15 mg) were added in sequence. Then, the mixed solution was heated at 70 ℃ and kept under reflux with magnetic stirring for 2 h before the solution is cooled to room temperature. The blue products were collected by centrifugation and washed once with water, then dried overnight at 60 ℃. Eventually, the resultant products were annealed at different temperature of 400 ℃, 500 ℃, 600 ℃ with a temperate ramp of 1 ℃ min-1 for 3 h in air. CeO2 is synthesized by the same method at the same condition without PB, followed by annealing at 400 ℃ in air.

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2.3 Materials characterization. The detailed morphologies and structures of these samples were investigated by field-emission scanning electron microscopy (FESEM, Hitachi SU8010) and transmission electron microscopy (TEM, JEM-2100 F microscope). Field emission transmission electron microscopy (FETEM) was conducted on a Talos F200S microscope operated at 200 kV. The phase and crystallinity were detected by wide-angle X-ray diffraction (XRD, Rigaku D/Max-2550 PC) for 2 ranging from 5 to 90 with Ni-filtered Cu Kα radiation (40 mA, 40 kV). Nitrogen absorption and desorption isotherms were characterized at 77 K (Micro ASAP2046). The samples were degassed at 180 ℃ for 6 h before measurements. The specific surface areas were calculated by Brunauer-EmmettTeller (BET) method using the adsorption data. The pore size distribution was calculated by the Barrett-JoynerHalenda (BJH) model from the adsorption branch. The total pore volume (Vtotal) was estimated from the adsorbed amount at P/P0 = 0.99. X-ray photoelectron spectroscopy (XPS) was carried out to detect the element valence of Ce, and Fe in the three samples with C 1s at 284.6 eV as reference. 2.4 Measurements of electrochemical performances. Coin-type half cells are employed to test electrochemical measurements. The as-synthesized active materials (70 wt%), PVDF (20 wt%), and super P (10 wt%) were mixed in N-methyl-2pyrrolidone (NMP) to obtain the electrode slurry. The resultant slurry was uniformed coated on copper foil and dried in a vacuum at 110 ℃ for 12 h. The diameter of the copper foil is 8 mm with a mass loading about 0.9-1.1 mg cm-2. To investigate the 6


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electrochemical characterizations, CR2032 coin cells were assembled in an argon-filled glove box with tiny (

Surface anchoring approach for growth of CeO2 nanocrystals on

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