Novel LaBO3 hollow nanospheres of size 34 ± 2 nm templated by polymeric micelles

Journal of Colloid and Interface Science 370 (2012) 51–57

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Novel LaBO3 hollow nanospheres of size 34 ± 2 nm templated by polymeric micelles Manickam Sasidharan a, Nanda Gunawardhana b, Hom Nath Luitel a, Toshiyuki Yokoi c, Masamichi Inoue d, Shin-ichi Yusa d, Takanori Watari a, Masaki Yoshio b, Takashi Tatsumi c, Kenichi Nakashima a,⇑ a

Department of Chemistry, Faculty of Science and Engineering, Saga University, 1 Honjo-machi, Saga 840-8502, Japan Advanced Research Center, Saga University, 1341 Yogamachi, Saga 840-0047, Japan c Division of Catalytic Chemistry, Chemical Resources Laboratory, Tokyo Institute of Technology, 4259-R1-9 Nagatsuta, Midori-ku, Yokohama 220-8503, Japan d Department of Materials Science and Chemistry, University of Hyogo, 2167 Shosha, Himeji 671-2280, Japan b

a r t i c l e

i n f o

Article history: Received 10 October 2011 Accepted 19 December 2011 Available online 28 December 2011 Keywords: LaBO3 Hollow nanospheres Triblock copolymer Micelles Lithium-ion battery

a b s t r a c t Novel lanthanum borate (LaBO3) hollow nanospheres of size 34 ± 2 nm have been reported for the first time by soft-template self-assembly process. Poly(styrene-b-acrylic acid-b-ethylene oxide) (PS-PAAPEO) micelle with core–shell–corona architecture serves as an efficient soft template for fabrication of LaBO3 hollow particles using sodium borohydride (NaBH4) and LaCl37H2O as the precursors. In this template, the PS block (core) acts as a template of the void space of hollow particle, the anionic PAA block (shell) serves as reaction field for metal ion interactions, and the PEO block (corona) stabilizes the polymer/lanthana composite particles. The PS-PAA-PEO micelles and the resulting LaBO3 hollow nanospheres were thoroughly characterized by dynamic light scattering (DLS), transmission electron microscope (TEM), X-ray diffraction, magic angle spinning-nuclear magnetic resonance (11B MAS NMR), energy dispersive X-ray analysis, thermal analyses, Fourier transform infra red spectroscopy, and nitrogen adsorption/desorption analyses. The nitrogen adsorption/desorption analyses and TEM observation of the hollow particles confirmed the presence of disordered mesopores in the LaBO3 shell domain. The solid state 11B MAS NMR spectra of LaBO3 hollow nanospheres revealed that the shell part contains both trigonal and tetrahedral boron species. The LaBO3 hollow particles were applied to anode materials in lithium-ion rechargeable batteries (LIBs). The hollow particles exhibited high coulombic efficiency and charge–discharge cycling capacities of up to 100 cycles in the LIBs. Ó 2012 Elsevier Inc. All rights reserved.

1. Introduction Novel shape-controlled nanocrystal synthesis could open a new domain of materials and technological interest. In order to develop the new generation of nanodevices and ‘‘smart’’ materials, ways to organize the nanoparticles into controlled architecture must be found and remains a challenging task. This conundrum has been addressed to some extent by the world of colloid and cluster science, which enables to organize particles on the micron and submicron scale [1]. Hollow spheres with controlled interior void coupled with hierarchical porous shell structure have meanwhile evoked manifold interest [2]. This addresses the large specific surface, the low specific weight, the potential high mechanical strength, which leads them to applications ranging from electrode materials to nanocontainer [3]. Therefore, various methods, such as templating, sonochemical, and hydrothermal methods, have been reported as the routes for the synthesis of inorganic hollow particles. In recent years, the use of sacrificial templates to fabricate such hollow nanoparticles has ⇑ Corresponding author. Fax: +81 952 28 8850. E-mail address: [email protected] (K. Nakashima). 0021-9797/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2011.12.050

proven successful [4]. Especially, there has been growing use of polymer nanoparticles such as latexes and polymeric micelles because it is easy to control the size, morphology, and surface functionality of the polymer nanoparticles. In general, latex particles are employed for fabricating hollow particles with relatively larger diameters ranging from submicrometers to micrometers, while polymeric micelles are used for smaller hollow particles with diameters of less than 100 nanometers [5]. In contrast to the many studies of latex templates [6], few studies have been reported for the use of polymeric micelle templates. One of the advantages of polymeric micelle templates is that the size and morphology of the micelles can be easily tuned by adjusting the block size, polymer combination, and solvent composition. Most of the polymeric micelles employed so far have a core–shell type architecture formed from AB diblock or ABA symmetric triblock copolymers [7]. In these systems, the corona of the micelles acts as a reservoir of the precursor of the inorganic material, and the core acts as a template of the hollow. The precursor of the inorganic material is sorbed into the corona of the micelles and forms the inorganic shell of the hollow particle after sol–gel or another reaction. The template polymer is removed by calcination or solvent extraction that leaves a void volume inside the inorganic shell.

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In this method, however, the template micelles become very unstable when the precursor is sorbed into the corona, leading to the formation of second-order aggregates. As an alternative route to avoid the problem associated with core–corona micelles, we proposed a novel method in which ABC triblock copolymer micelles with a core–shell–corona structure are used as a template [8]. In our system, the precursor of the inorganic material is designed to be selectively sorbed into the micelle shell in order that the corona is free from the precursor that keeps the micelles stable. We obtained silica nanospheres with an outer diameter of about 30 nm, which have homogeneous size distribution and show less aggregation [8]. After the successful synthesis of the silica nanospheres, we have applied this method to the syntheses of the hollow nanospheres of Nb2O5, CeO2, V2O5, CaCO3, and TiO2, MoO3 and WO3 [9–11] using different ABC triblock copolymers to demonstrate that our method is a versatile route for obtaining hollow nanospheres of various inorganic materials. Furthermore, we showed that the structure of the hollow nanospheres can be tuned by changing the chain length of each block of the template ABC terpolymers, that is, the size of the void space and the thickness of the inorganic shell, respectively, can be controlled by changing the chain lengths of the core-forming and shell-forming blocks of the template polymer [12]. In the present study, we have applied the method to the synthesis of LaBO3 hollow nanospheres that have never been reported so far despite of their potential usefulness. Rare earth (lanthanide series) compounds such as hydroxides, oxides, phosphates, and borates find potential applications in high performance luminescent and magnetoresistance devices, electrodes, and catalysts because of their unique electronic structures involving 4f electrons [13]. Advanced borate materials, ranging from low dimensional to three-dimensional frameworks, possess high vacuum ultraviolet (VUV) transparency, large electronic band gaps, chemical and environmental stability, and exceptionally large optical damage thresholds [14]. As rare earth borates serve as useful host-lattice for lanthanide ions, their photoluminescence properties has been studied extensively to the development of plasma display panel (PDPs) [15]. Lanthanum salts have been used as a biological tracer, because La3+ replaces Ca2+ in the conduction of nerve impulses along the axons of nerve cells, and also in structure promoting in cell membranes. Very recently, a new mesoporous iron(III)borate material has been synthesized hydrothermally using anionic and cationic surfactant as structure directing agents by cocondensation strategy and used as catalysts [16]. To date, to the best of our knowledge, hollow nanospheres of metal borates (including rare earth borates) have not been reported, although they are expected to have novel characteristics arising from the hollow architecture combined with the features of metal borates. Moreover, the well-known high-temperature process (above 1000 °C) involving conventional borate precursors is not suitable to obtain nanoparticles with few tens of nanometer. Herein, we report on the fabrication of LaBO3 hollow nanospheres and their applications to lithium-ion batteries (LIBs). The LaBO3 hollow nanospheres were synthesized with LaCl37H2O and NaBH4 using micelles of poly(styrene-b-acrylic acid-b-ethylene oxide) (PS-PAA-PEO) [17] as a soft template (Scheme 1). The soft templates are particularly attractive due to their modulability, flexibility, and simplicity. In the above polymer micelle with core–shell–corona type architecture, the core acts as a template for void space, the shell serves as a reaction medium, and the corona stabilizes the precursor-micelle composite [8–12].

H2O

LaCl3

pH 9

PS-PAA-PEO Core-Shell-Corona Micelle

NaBH4, pH 11 Calcination

LaBO3 Hollow Sphere Scheme 1. Schematic representation of hollow nanosphere.

PAA90-b-PS80 was synthesized via RAFT-controlled radical polymerization as shown in Scheme S1 (Supporting information). Poly(ethylene oxide) (PEO) based chain transfer agent (PEO47-CTA). 2,20 Azobis(isobutyronitrile) (AIBN) was crystallized from methanol. Styrene was washed with an aqueous alkaline solution and distilled from calcium hydride under reduced pressure. Acrylic acid, N,Ndimethylformamide (DMF), and dioxane were dried over 4 Å molecular sieves and distilled under reduced pressure. Na2CO3 (Wako), CaCl2 (Katayama), tris buffer (Katayama), and naproxen sodium (Sigma–Aldrich) were used without further purification. 2.2. Preparation of PEG47-b-PAAm Acrylic acid (2.18 g, 30.3 mmol), AIBN (12.5 mg, 0.08 mmol), and PEO47-CTA (475 mg, 0.20 mmol) were dissolved in dioxane (30 mL). The solution was degassed by purging with Ar gas for 30 min. Polymerization was carried out at 60 °C for 40 h. The polymerization mixture was dialyzed against pure water for 1 week. The diblock copolymer (PEO47-b-PAA90) was recovered by a freeze-drying technique (1.70 g, 64.0%). Number-average degree of polymerization (DP) of PAA block was estimated from 1H NMR spectrum in DMSOd6 to be 90. The number-average molecular weight, Mn(NMR) for the block copolymer estimated from 1H NMR is 8.85  103. Mn(GPC) and molecular weight distribution (Mw/Mn) were 1.53  104 and 1.31, respectively. 2.3. Preparation of PEO47-b-PAA90-b-PS80 Styrene (10.4 mg, 10 mmol), AIBN (8.23 mg, 0.05 mmol), and PEO47-b-PAA90 (1.11 g, 0.13 mmol) were dissolved in DMF (100 mL). The solution was degassed by purging with Ar gas for 30 min. The polymerization was carried out at 60 °C for 24 h. The polymerization mixture was dialyzed against acetone for 3 days and pure water for 1 day. The obtained triblock copolymer (PEO47b-PAA90-b-PS80) was recovered by a freeze-drying technique (2.09 g, 18.2%) and analyzed by 1H NMR (Fig. S2, Supporting information). DP of the PS block was 80 as estimated by 1H NMR in DMSO-d6. Mn(NMR) value for PEO47-b-PAA90-b-PS80 is 1.88  104. Mn(GPC) and Mw/Mn were 9.34  103 and 1.22, respectively.

2. Experimental 2.4. Preparation of micelles and LaBO3 hollow nanospheres 2.1. Chemicals Lanthanum(III)chloride heptahydrate (LaCl37H2O) and sodium borohydride (NaBH4) were obtained from Sigma–Aldrich. PEO47-b-

The triblock copolymer PS-PAA-PEO was dissolved in distilled water by stirring at room temperature for several days to obtain 0.5 g L1 micelle solutions. Then, the pH was adjusted to about

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nine using a diluted NaOH solution. All the hollow nanosphere syntheses were carried out at room temperature excluding the hollowing step that was performed at 600 °C. A desired amount of LaCl37H2O (0.00072 mol) was added to polymeric solution (10 mL) and stirred for a while before the addition of NaBH4 solution (0.00144 mol). Then the pH was further raised to about 11, and the clear solution slowly turns to a gelatinous precipitate. The composite particles were aged at room temperature for few days. The white precipitate was separated by centrifugation, washed thoroughly with distilled water, and dried at 60 °C. Finally, the core/shell particles were heated at 600 °C for simultaneous removal of the remaining templates as well as to crystallize the hollow particles. 2.5. Characterization

2.6. Electrochemical study For lithium insertion studies, the lanthanum borate hollow nanosphere (5 mg) was mixed mechanically with teflonized acetylene black (TAB-2, 5 mg) and then the mixture was pressed on a stainless steel mesh as the current collector under a pressure of 500 kg/ cm2 and dried at 160 °C for 4 h under vacuum. The electrochemical characterizations were carried out using CR-2032 coin type cells with lithium as an anode. The electrolyte used was 1 M LiPF6-EC: DMC (1:2 volume ratios, Ube Chemicals Co. Ltd.). The coin cell assembling was performed in a glove box filled with argon (dew point, lower than 80 °C). The galvanostatic charge–discharge tests of the coin cell were performed at the constant current density of 0.5 mA cm2. The cyclic voltammograms (CVs) were recorded with a Hokuto Denko HSV–100 in a beaker type cell containing a lanthanum borate working electrode and a lithium foil as a counter and reference electrode. 3. Results and discussion The PS-PAA-PEO triblock copolymer was dissolved in distilled water by stirring at room temperature for several days to obtain 0.5 g L1 micelle solutions, and the pH was adjusted to about nine (Scheme 1, step 1). The hydrodynamic diameter Dh (67 nm) and the f-potential (56 mV) of the micelles (pH 9) were obtained from dynamic light scattering (DLS) and electrophoretic light scattering (ELS) experiments, respectively. Nearly monodispersed spherical micelles with average diameter ca. 46 ± 1 nm were estimated from

Fig. 1. TEM image of PS-PAA-PEO micelles.

TEM image (Fig. 1). The difference in the micelle particle’s size between DLS and TEM is due to the fact that the latter accounts for only the core–shell part and excludes corona part of the micelle. In addition, the PAA block containing –COOH (pKa 4.6) exists in the deprotonated form (–COO) under the basic medium. On gradual addition of LaCl3 to the micelle solution (Scheme 1, step 2), the f-potential (56 mV) changes to zero indicating the effective interaction of –COO anions with La3+ cations (Fig. 2). The degree of neutralization in Fig. 2 is expressed as the ratio of amount of added La3+ ion precursor to amount of acrylic acid in the PAA block. The electrostatic interaction between La3+ and –COO ions is also evidenced from FTIR spectra of polymer before and after the addition of LaCl3. Due to the interaction with the metal ion, the carboxyl group stretching vibration (1735 cm1) is red shifted to 1560 cm1 (Fig. S4(B), Supporting information). Addition of NaBH4 followed by raising pH to about 11 using dilute NaOH produces white gelatinous precipitate (Fig. S5, Supporting information). The La3+ ions readily precipitate as La(OH)3 under alkaline conditions, which is the strongest base among the 0 lanthanide metal ions due to its largest ionic radius (1.03 Å A). On

10 0

Zeta-potential, mV

Hydrodynamic diameter (Dh) of the template micelles of PSPAA-PEO was measured with an Otsuka ELS-800 instrument. Small-angle X-ray diffraction patterns (SXRD) were collected on a Rigaku Rint-ultima diffractometer with CuKa radiation (40 kV, 20 mA) with 0.02° step size and 2 s step time over the range 0.7° 2h < 6°. Wide-angle X-ray diffraction (WXRD) patterns were obtained using a Shimadzu XRD-7000 diffractometer. The textural properties such as BET surface area and mesopore-size distribution were obtained using nitrogen adsorption/desorption isotherms with a Quantochrome, Autosorb 1C. The morphology of the samples was observed from JEOL TEM-1210 (acceleration voltage: 80 kV) and JEOL TEM-2100 electron microscopes (acceleration voltage: 200 kV). FTIR spectra were recorded on a Jasco FTIR7300 spectrometer. TG and DTA analyses were carried out using MAC Science TG-DTA 2100. 11B NMR spectra were recorded on a Bruker AMX-400 spectrometer equipped with a MAS unit. The spectra were recorded at 128.3 MHz referenced to an external borontrifluoro etherate with a contact time of 10 ls, a delay time of 10s and spinning rate of 14 kHz. Energy dispersed X-ray analysis was carried out with Hitachi S-3000N.

-10 -20 -30 -40 -50 -60 0

50

100

150

200

Degree of neutralization, % Fig. 2. Variation of zeta-potential due to charge neutralization of polyacrylic acid (COO) with La3+ ions.

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A

B 100 nm

100 nm

D 0.34 nm

C

2 nm

100 nm

E

La La

Fig. 3. TEM images of LaBO3 hollow nanospheres: (A) La3+/PAA = 5, (B) La3+/PAA = 10, (C) La3+/PAA = 15, (D) HRTEM image of B, and (E) EDX profile of sample B.

the other hand, after decomposition of NaBH4, boron(III) reacts with hydroxide ions to give boron hydroxide or orthoboric acid. The formation of La(OH)3 and B(OH)3 (ICDD-No.030-0620) is evidenced from wide-angle X-ray diffraction measurement and the La(OH)3 peaks are marked with asterisks (Fig. S6, Supporting information) [18]. After washing and air drying, the precursor-polymer composite particles were calcined at 600 °C for 4 h to accomplish dehydration and extensive cross-linking in the three-dimensional network between La(OH)3 and B(OH)3. Concurrently, the calcination also burns out remaining polymeric templates in the core domain of composite particles to produce the hollow nanospheres and leads to crystallization of shell part of LaBO3 hollow nanospheres.

Fig. 3A–C exhibit TEM images of lanthanum borate obtained with different La3+/PAA ratio from 5 to 15 while maintaining the amount of boron constant. The structural characteristics of the hollow particles are provided in Supporting information (Table S1). It is evident from the TEM pictures that the hollow particles show a uniform spherical shape but the shell surface structure becomes rather rough at high La3+ concentrations. At lower La3+/PAA ratio, the degree of aggregation of nanospheres is quite low (Fig. 3 A and B) compared with that of La3+/PAA = 20 (Fig. S7, Supporting information). The average particle diameter increase with La3+/ PAA ratio from 33 ± 2 to 36 ± 2 nm due to adsorption of a large amount of the precursors in the shell domain of the template micelles. As the La3+/PAA ratio increases from 5 to 20, the shell thick-

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10000 9000 8000 7000 6000 5000 4000 3000 2000 1000 0

4-coordinated species

1

(120)

0.5

2

2.5

3

3.5

4

(213)

(142)

(113)

(122) (140) (231) (311)

(022)

(002)

(200)

200 100

1.5

2 theta/degree

(211) (220)

300

3-coordinated species

B

Intensity, A.U

400

(011) (020)

Intensity/a.u

500

Intensity/a.u

(111)

600

A 0 10

20

30

40

50

60

70

2 theta/degree Fig. 4. Wide-angel XRD pattern of LaBO3 hollow nanospheres. Inset figure shows small-angle XRD pattern of the same sample.

ness also increased from 7 ± 2 to 10 ± 2 nm; however, the hollow void space remains nearly constant at 17 ± 2 nm. In addition, the PS block core size estimated from the TEM observation was found to be 27 ± 1 nm; but after calcinations, the void space diameter was reduced to 17 ± 2 nm (about 37%) due to shrinkage of hollow particles [8,19]. The high resolution transmission electron microscope (HRTEM, 3 D) allows the resolution of lattice fringes of the crystals to be correlated to the (1 1 1) planes of the lanthanum borate [20] and confirms the crystalline nature of the shell domain. Thermal analysis (TG/DTA) suggests that about 13 wt.% polymeric template (Fig. S8, Supporting information) is present in the composite particles after washing with water and that a portion of the polymer is already removed during separation and washing. The absence of C–H, –C = C–, and –COOH bond stretching vibrations of phenyl groups of polymer backbone in the FTIR spectra of the calcined sample (Fig. S4-C, Supporting information) is consistent with the thermal analyses. Furthermore, the bands at 1100–1450, 960, and 700–800 cm1 are ascribed to asymmetric, symmetric, and out-of-plane bending vibrations, respectively, of B–O bond of BO3 group [21]. This result confirms the presence of BO3 groups in the network structure. Fig. 4 exhibits the wide-angle X-ray diffraction pattern of LaBO3, which can be indexed as an orthorhombic structure [22] with the lattice parameters of a = 0.59 nm, b = 0.83 nm, and c = 0.51 nm, in agreement with the literature values (JCPDS 12-0762). Quite interestingly, small-angle XRD pattern reveals the existence of disordered mesopores in the shell domain (Fig. 4, inset) which is in consistence with the appearance of mesopores in the shell part of all the TEM pictures (Fig. 3A–C). An energy dispersive X-ray analysis (Fig. 3E) confirms the presence of lanthanum, boron, and oxygen in the material (Fig. S9, Supporting information). The peaks at 4.8, 5.1, 5.4, and 5.8 keV are assigned to L-electrons of La whereas the peak at 0.85 keV is due to K-electron of La. The K-shell electrons of boron and oxygen appear at 0.25 and 0.6 keV, respectively. The atomic ratio of La:B:O was found to be 16.5:54.3:29.2. In addition, as the energy dispersive X-ray analysis is a surface technique, the observed composition is slightly different from formula unit but it certainly reveals the presence of boron in the microstructure. Further concrete evidence for the presence of boron in the hollow nanospheres was obtained from 11 B MAS NMR spectroscopy. Boron is known to occupy three co-ordinate as well as four co-ordinate positions in any ceramic materials [23]. Fig. 5 exhibits 11B MAS NMR spectrum of LaBO3 calcined at two different temperatures 600 and 750 °C. The resonance peak around ‘0’ ppm is attributed to tetrahedral sites, and the peak at 13 ppm is assigned to trigonal borate species. At low temperature, the amount of tetrahedrally coordinated species was found to be

-50

-30

-10

10

30

50

PPM Fig. 5. 11B MAS NMR spectra of LaBO3 hollow nanospheres: (A) calcined at 600 °C and (B) calcined at 750 °C.

larger than the trigonally coordinated species mainly due to the reason that some of tetrahedral species undergo cleavage to give trigonally coordinated species. However, the hollow particles are quite stable even after heating at 750 °C despite the cleavage of some of tetrahedral species. Nitrogen sorption isotherm shows characteristics of disordered mesoporous materials (Fig. S10, Supporting information). The pore-size distribution curves obtained from the adsorption branches of the isotherms on the basis of nonlinear density functional theory (NLDFT) model show the existence of disordered mesopores consistent with the small-angle X-ray diffraction results. The mesopore size was found to be about 5.3 nm for major pores in the shell domain while it was found to be about 2.5 and 6.8 nm for minor pores. The hollow particles also exhibit high surface area ranging from 165 to 191 m2 g1 (Table S1, Supporting information) and the total pore volume was found to be 0.98– 1.20 cm3 g1 for LaBO3 hollow nanospheres synthesized with different La3+/PAA molar ratios. Thus, the combination of nitrogen sorption, low-angle XRD, and TEM data clearly confirms the existence of disordered mesopores in the shell domain of LaBO3 hollow nanospheres. To demonstrate usefulness of this material, we have applied the LaBO3 hollow nanospheres to lithium-ion batteries (LIBs). Cyclic voltammograms (CVs) of the LaBO3 in the potential window 0.005–2.5 V (vs. Li+/Li) at a scan rate of 20 mV/min are shown in Fig. 6A. During the first cycle, appearance of a strong reduction peak in the range of 0.9–0.5 V in the voltammogram suggests the formation of Li3BO3. The second reduction peak between 0.25 and 0.005 V corresponds to the formation of lithium-rich LixLa phases. Furthermore, for the first oxidation scanning, one broad peak appears in the voltage range of 0.005–0.3 V, which is virtually unchanged during the subsequent cycles; however, the shape of the reduction peak is changed in the second cycle and the broadness of the curve indicates lithium alloying with metallic lanthanum. In addition, the current of the oxidation peak in the second cycle is larger than that in the first cycle, indicating a possible activation process during the alloying/de-alloying process. The cycle performances and coulombic efficiency of LaBO3 cell obtained in this experiment are shown in Fig. 6B. The discharge capacity values for 1st, 2nd, 10th, 50th, and 100th cycles are 548, 250, 227, 215, and 213 mA h g1, respectively. The corresponding charge capacity values are 203, 202, 203, 207, and 212 mA h g1 for the 1st, 2nd, 10th, 50th, and 100th cycles, respectively. The large difference in discharge capacities on the first and subsequent cycles is attributed to the formation of Li3BO3 and metallic lanthanum according to

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5

600

Alloying De-alloying

Alloying De-alloying

Voltage / V

3

400 300 -1

4

Capacity / mAh.g

Capacity / mAh.g

-1

500

200 100 0

0

20

40

60

80

100

Cycle No

2

st

1

0

B 0

100

200

300

400

1 Cycle nd 2 Cycle th 10 Cycle th 50 Cycle th 100 Cycle 500

400

0.2 C 0.5 C 1.0 C 200

1.0 C 2.0 C 5.0 C 10.0 C

0

0

600

10

20

30

40

Cycle No

-1

Capacity / mAh.g

Fig. 7. Rate performance of LaBO3 hollow nanospheres in lithium-ion batteries.

0.5

nd

2 Cycle

capability as shown in Fig. 7. A total of 40 full cycles, with at least 5 cycles under each charge/discharge rate were performed. Similar to other nanostructured materials [27], the LaBO3 hollow particle also shows higher capacity (243 mA h g1) at slower rate (0.2 C) than that (79 mA h g1) at faster rate (10 C). However, the LaBO3 electrode regains its high-capacity when the rate was measured at 1 C after being exposed to high current loads, which indicates high stability of the electrode. Although, no data were available for strict comparison, the LaBO3 hollow particle shows excellent cycling performance and coulombic efficiency compared with LiMBO3 (M = Fe, Mn, Co) [28].

Current/ mA

0.0

-0.5 st

1 Cycle

-1.0

A

-1.5

-2.0 0.0

0.5

1.0

1.5

2.0

2.5

Voltage/ V Fig. 6. (A) Cyclic voltammograms of LaBO3 at a scan rate of 20 mV/min between 0.005 and 2.5 V vs. Li/Li+; (B) Galvanostatic charge/discharge curves of LaBO3 electrodes between 0.005 and 2.5 V (vs. Li/Li+). Inset figure shows cyclic performance of LaBO3 up to 100 cycles.

following electrochemical conversion reaction [24]. After conversion, the metallic lanthanum could alloy with lithium to make nanomatrix of LixLa alloy anode in the second step. þ

LaBO3 þ 3Li þ 3e La þ Li3 BO3

ð1Þ

La þ xLi þ xe Lix La

ð2Þ

The first reaction occurs in the crystalline shell domain of LaBO3 hollow nanospheres, whereas alloying of the metals predominantly takes place in the amorphous nanoparticle lying near or on the surface of shell structure. The cycle performance of this material is shown in Fig. 6 (B, inset); the coulombic efficiency is nearly 100% except first few cycles as evidenced from charge–discharge capacities. Generally, it is believed that alloying/de-alloying leads to a significant capacity fading with cycling due to large volume expansion [25]. The high coulombic efficiency in the present case is attributed to ultra small nanoparticles present near or on the shell surface of hollow nanospheres providing better electrical contact and shorter diffusion path during cycling [26]. More importantly, the void space could effectively buffers against charge storage and local volume change. At this juncture, though the role of much lighter Li3BO3 nanoparticles is uncertain, they can certainly act as a mechanical buffer to preserve the integrity of electrode structure. The LaBO3 hollow particles also exhibit good rate

4. Conclusions PS-PAA-PEO micelle with anionic PAA block acts as an efficient reaction center for La3+ interaction and subsequent self-assembly of LaBO3 hollow nanospheres in the presence of sodium borohydride under mild conditions. TEM observations after calcination confirmed the formation of LaBO3 hollow nanospheres with outer diameter of 34 ± 2 nm and smooth and crystalline shell structures. The hollow void space generated after removal of the PS core block was found to be 17 ± 2 nm which was unaltered with La3+/PAA ratios during the self-assembly process. The reduced size of the void space (17 ± 2 nm) compared with the initial size of the template PS core (27 ± 1 nm) is attributed to shrinkage of hollow particles during high-temperature treatment. The combined analyses by high resolution TEM, small-angle XRD, and nitrogen sorption suggest the presence of mesopores in the shell structure of the LaBO3 hollow nanospheres. The solid state 11B MAS NMR spectra of the LaBO3 hollow nanospheres suggest the presence of both trigonal and tetrahedral boron species. The LaBO3 hollow nanospheres as anode materials exhibited high cycling performance and coulombic efficiency in lithium-ion rechargeable batteries. Acknowledgments Mr. Toshimi Tabata, in medical school is thanked for technical assistance regarding TEM characterization. One of the authors (KN) thanks for a Grant-in-Aid for Scientific Research (20310054) from the Japan Society for the Promotion of Science (JSPS). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.jcis.2011.12.050.

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