Single Crystallization of Olivine Lithium Phosphate Nanowires using

Mar 12, 2014 - ABSTRACT: Electrospinning enables fabrication of nanowires (NWs) of various materials from a polymer solution. Nevertheless, few report...
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Single Crystallization of Olivine Lithium Phosphate Nanowires using Oriented Attachments Jun Kikkawa,*,† Eiji Hosono,*,‡ Masashi Okubo,‡ Koichi Kagesawa,‡ Haoshen Zhou,‡ Takuro Nagai,† and Koji Kimoto† †

National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan National Institute of Advanced Industrial Science and Technology, 1-1-1 Umezono, Tsukuba 305-8568, Japan

‡

S Supporting Information *

ABSTRACT: Electrospinning enables fabrication of nanowires (NWs) of various materials from a polymer solution. Nevertheless, few reports have described single crystallization of oxide and polyanion NWs. Its mechanism remains unknown. This report presents transmission electron microscopy observations of conversion from electrospun amorphous NWs to single-crystalline olivine lithium phosphate NWs. After nucleation and grain growth, single crystallization is achieved by the attachment of adjacent crystal grains with common crystallographic orientations in an amorphous phase confined to self-forming carbon shells. The present NW axes have no specific orientation. These results imply that self-forming shells play a key role in achieving single-crystalline NWs in electrospinning.

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INTRODUCTION Electrospinning is a versatile method to produce nanowires (NWs) of various materials such as polymers1,2 and oxides.3,4 In this method, a polymer solution is ejected as a jet from a needle tip by application of an electric field to form NWs on a target collector.3 Oxide and polyanion NWs are obtainable using a polymer solution, including metal salts and metal alkoxides with subsequent heating. However, most NWs fabricated through electrospinning are amorphous or polycrystalline.5,6 Few reports describe fabrication of singlecrystalline oxide and polyanion NWs through electrospinning.7−9 One example is single-crystalline LiFePO4 NWs with carbon shells.7 LiFePO4 is an important candidate for use as alternative positive electrodes of LiCoO2 for lithium ion batteries, enabling high safety, high cycling stability, high practical capacity, acceptable operating voltage, and low cost.10 In fact, NW geometry improves rate capabilities.11−13 Selfformed amorphous carbon shells coating LiFePO4 NWs play roles for increasing the effective electrical conduction,7,14,15 which is low (∼10−9 Ω−1·cm−1) for intrinsic LiFePO4,16,17 and for suppressing the oxidation of Fe2+ ion on the NW surface.15,18,19 Recently, more advanced single-crystalline NWs of an olivine-structured LiMn0.4Fe0.6PO4 covered with amorphous carbon shells were fabricated using electrospinning.20 Partial substitution of Fe2+ ions for Mn2+ ions increases the operation voltage because of the higher redox potential of Mn3+/Mn2+, 4.1 V, than that of Fe3+/Fe2+, 3.4 V. The singlecrystalline NW formation mechanism related to electrospinning remains unclear, but electrospinning is widely applicable for the fabrication of various single-crystalline NWs. For this study, we © 2014 American Chemical Society

used transmission electron microscopy (TEM) to investigate the formation mechanism of single-crystalline NWs from electrospun NWs by heating.

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EXPERIMENTAL METHODS

LiNO3 (0.2 mol dm−3), Mn(NO3)2·6H2O (0.08 mol dm−3), Fe(NO3)3·9H2O (0.12 mol dm−3), NH4H2PO4 (0.2 mol dm−3), and poly(acrylic acid) (0.8 g) were dissolved into a mixed solution (20 mL) of water, methanol, and nitric acid (10:9:1). This precursor solution was poured into a syringe connected to a metal needle. A direct current electric field of 25 kV was applied between the needle and an Al foil target to form NWs. The electrospun NWs were dried for an hour at 100 °C in vacuum to evaporate solvents such as methanol, nitric acid, and water. The dried polymeric precursor NWs were subsequently heated at 800 °C for 10 h in ambient Ar followed by natural cooling. Using X-ray diffraction (Figure S1, Supporting Information), we confirmed that NWs after heating have an orthorhombic olivine structure (lattice parameters: a = 1.03938 nm, b = 0.605311 nm, c = 0.472230 nm; space group: Pnm). To investigate the crystallization steps of NWs using ex situ TEM, we prepared NWs heated at 500 and 600 °C for 30 min in ambient Ar with subsequent natural cooling because a thermogravimetric analysis of the dried NWs showed weight reduction at 450−530 °C (Figure S2, Supporting Information), Received: January 8, 2014 Revised: March 9, 2014 Published: March 12, 2014 7678

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suggesting that key reactions occurred in that temperature range. Bright-field TEM observations were done using an electron microscope (HF-3000S; Hitachi Ltd.) operated at 300 kV. Dried and heated NWs were dispersed on copper grids covered with holey carbon films for ex situ TEM and molybdenum grids for in situ TEM. A double-tilt heating holder (652; Gatan Inc.) connected to a smartset hot stage controller (901; Gatan Inc.) was used for in situ TEM. Selected-area electron diffraction (SAED) patterns were recorded using an imaging plate system (FDL-5000; Fujifilm). The camera length was calibrated with crystal Si. SAED patterns in Figure 1a−c were acquired from ∼1.3 μmφ regions containing single NWs. SAED patterns in Figures 2a, e−g and 3c−f were acquired from the circled regions in Figures 2a and d and 3a.

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RESULTS AND DISCUSSION The dried NWs were amorphous, as shown in the inset SAED pattern (Figure 1a). After heating at 500 °C for 30 min in ambient Ar, we observed NWs of two types using ex situ TEM. For one type, many precipitates smaller than 50 nm were formed within the NW (Figure 1b). It is particularly interesting that the inset SAED pattern (Figure 1b) revealed that the NW remained amorphous: the precipitates were amorphous. Differences in averaged radial-intensity distributions of the inset SAED patterns between parts a and b of Figure 1 (Figure 1d) revealed that variation of chemical bond lengths occurred by heating, probably related with the appearance of the amorphous precipitates. The contrast between the amorphous precipitates and the matrix in the TEM image (Figure 1b) implies that the precipitates contain elements with larger atomic numbers such as Mn and Fe. For the other type, the NW core region was polycrystalline (Figure 1c). The amorphous precipitates were observed in the NW surface region. Peaks at 2.32, 3.31, 3.93, and 4.34 nm−1 in the radialintensity distribution (Figure 1d) were assigned to the 101, 211, 311, and 102 reflections of the olivine-structured LiMn0.4Fe0.6PO4 NWs (Figure S1, Supporting Information). The coexistence of amorphous (Figure 1b) and crystallized (Figure 1c) NWs and the reduction in weight of NWs at and around 500 °C (Figure S2, Supporting Information) imply that crystallization and the evaporation of gases by thermal decomposition of the polymeric precursor occurred at the same stage. The driving force of the crystallization is supersaturation, which occurs by decomposition of the polymeric precursor. Heating at 600 °C for 30 min in ambient Ar clarifies the core−shell boundary. The NW cores were composed of partly coalesced grains of 20−100 nm, whereas the NWs were covered with amorphous carbon shells (Figure 2a and b; also see Supporting Information for details of the shell). The inset SAED pattern acquired from the circled area revealed a polycrystalline NW core. However, particularly addressing the grains of numbers 1 and 2 in Figure 2a, both grains attach along a common crystallographic orientation, as shown in the highresolution TEM (HRTEM) image (Figure 2c) of the squared area C. The inset fast Fourier transform (FFT) pattern of the HRTEM image shows that these coalesced grains have an olivine structure observed along the [101] direction. The continuous (101)̅ lattice fringes demonstrated that an oriented attachment occurred.21,22 Differently from previous NWs crystallized by oriented attachments in a free solution,23−27 single crystallization is developed in a confined space of self-

Figure 1. Initial stage of crystallization of NWs. (a) Before heating, the dried NW was amorphous, as shown in the inset SAED pattern. (b, c) After heating at 500 °C for 30 min, the NW in (b) remained amorphous, although amorphous nanoparticles (indicated by arrows) were formed in the NW. The NW core in (c) was polycrystalline. (d) Averaged radial-intensity distributions of the SAED patterns in (a−c). Differences in this distribution especially at 2−4 1/nm between amorphous NWs in (a) and (b) are related with the formation of amorphous precipitates in (b). The peaks for NW in (c) are assigned to the reflection indices of the LiMn0.4Fe0.6PO4 NWs.

forming amorphous carbon shell in our case. After heating at 600 °C for 30 min in ambient Ar, other NWs showing further progressed crystallization were also observed (Figure 2d). The TEM image showed that most grains coalesced to form large crystals (Figure 2d). The grains were rather rounded and not faceted. It is noteworthy that the SAED patterns from areas E− G revealed single-crystalline characteristics of the olivine structure (Figure 2e−g), although the outlines of grains are discernible especially in area E, which indicates that the oriented attachment occurred considerably beyond the range of ∼1 μm. In Figure 2d−g, the electron-incident direction was set as [100] in area E for acquiring SAED patterns from areas E− G. The tilting of crystallographic orientation in areas F and G in 7679

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Figure 2. Crystallographic orientation alignment in NW. NWs after heating at 600 °C for 30 min. For the NW in (a), crystal grains grew and started to join within the NW, although the NW surface was covered with an amorphous shell. The inset SAED pattern in (a) shows that the NW core is polycrystalline. (b, c) HRTEM images in the squared areas of B and C in (a). The interface area between particle numbers 1 and 2 joined with a common crystallographic orientation in (c). The inset FFT pattern in (c) corresponds to the 101 diffractograms of the olivine structure. For the NW in (d), oriented attachments of crystalline grains are further progressed. SAED in (e−g) shows that the NWs incline to a single crystal. All SAED patterns were acquired from circled regions in (a) and (b).

Figure 3. Single-crystallized NW after heating at 800 °C for 10 h. (a, b) Domain features in the NW core were decreased in TEM images by the progression of oriented attachments and out-diffusion of carbon species in core regions. (c−f) SAED patterns acquired from circled regions along the NW in (a) show that the NW core is almost single crystalline beyond ∼7 μm.

After heating at 800 °C for 10 h in ambient Ar, further single crystallization progressed. Amorphous gaps left between grains in the NW core (presented in Figure 2a and d) disappeared, forming a complete crystal core (Figure 3a and b). SAED patterns acquired from areas C−F in Figure 3a showed the 011 SAED pattern of the olivine structure (Figure 3c−f), indicating

comparison to area E suggests the existence of strain and defects such as dislocations in the NW core, especially interfaces between coalesced grains. Extra spots in the 100 SAED pattern (Figure 2e) are attributed to the existence of crystallites rotated from the main crystallographic direction of crystal. 7680

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NW axis orientations. A complete core−shell NW is achieved after lengthy heating at 800 °C without breaking its geometry, irrespective of the crystallographic orientation of the NW axis, because the stable core−shell interface has lower interface energy as a result of the presence of chemical bonding of C−O, C−Fe, and C−Mn.29,30 For cases in which self-forming of the amorphous carbon shells does not occur, i.e., for cases in which the interface between amorphous carbon and LiMn0.4Fe0.6PO4 is extremely unstable, complete separation of amorphous carbon grains from LiMn0.4Fe0.6PO4 grains probably occurs at an early stage of crystallization. After lengthy heating at 800 °C, it is likely that LiMn0.4Fe0.6PO4 particles or polycrystalline products are formed rather than single-crystalline NWs. In this sense, self-forming amorphous carbon shells play a key role in confining grains and maintaining NW geometry to achieve single-crystalline NWs, in addition to the application benefit.

that single crystallization occurred almost completely beyond the range of ∼7 μm. The SAED pattern in Figure 3f shows perfect single crystallization, although diffused spots in Figure 3c denote the existence of rotation of crystal lattices around the [011] axis within ∼3° in area C. It is particularly interesting that the NW core axes have no specific crystallographic orientation (Figures 2d−g, 3, and S4 of Supporting Information), different from the [001] oriented LiFePO4 NWs covered with thinner (∼10 nm) amorphous carbon7 and other free-standing metaloxide NWs with bare facets of low surface energies.26,28 This lack of orientation implies that the existence of C−O, C−Fe, and C−Mn chemical bonds at the core−shell interfaces reduces their interface energies for any surface orientation of the core LiMn0.4Fe0.6PO4,29,30 enabling various orientations of NW axes. To observe the crystallization process directly, we also conducted in situ TEM using a heating holder under a vacuum of 2−4 × 10−6 Pa. Elevating the temperature from 24 °C, we observed the apparent decrease in NW diameter between 430 and 550 °C and the growth of grains in NWs (Figure S5, Supporting Information). Amorphous NWs including amorphous precipitates after heating at 500 °C for 30 min under an Ar gas atmosphere were also observed by elevating the temperature at a rate of 50 °C/min. For both cases, however, retention at 700−800 °C caused the formation of spherical grains. Their coalescence invariably caused low-density isolated crystalline grains in the amorphous carbon matrix (Figure S6, Supporting Information). The results demonstrated that the crystallization and shell-formation processes in the vacuum of reduction circumstance differ from those in ambient Ar and show that single crystallization is not achieved, irrespective of electron irradiation effects. The formation mechanism of the single-crystalline LiMn0.4Fe0.6PO4 NWs coated with amorphous carbon films in ambient Ar is explainable. First, the dried polymeric precursor NWs derived from LiNO3, Mn(NO3)2·6H2O, Fe(NO3)3·9H2O, NH4H2PO4, and poly(acrylic acid), (C3H4O2)n solutions, are amorphous at room temperature. At ∼400 °C, the original sources decomposed in core regions of NWs, whereas NW surface areas began to be covered with self-forming initial amorphous shells. At 450−530 °C, thermal decompositions of the original sources release gases, probably O2, H2O, and NOx, thereby reducing the NW diameter and engendering the formation of amorphous precipitates, i.e., precursor of LiMn0.4Fe0.6PO4 (Figure 1b). By further thermal decomposition, nucleation of LiMn0.4Fe0.6PO4 occurs at the precipitates in core regions. Then LiMn0.4Fe0.6PO4 grains grow independently. Simultaneously, the phase separation of LiMn0.4Fe0.6PO4 and amorphous carbon occurs, forming core and shell geometry by diffusion of carbon species toward the NW surfaces because the total interface energy between amorphous carbon and LiMn0.4Fe0.6PO4 interface decreases by the separation. In the NW core regions, coalescence of the grains progresses substantially according to the oriented attachments of adjacent grains with rotation and translation of grains in amorphous phases. In the case in which grains attach in misorientations, single crystallization is attainable by the subsequent rearrangement of atoms in smaller particles for eliminating grain boundaries and thereby decreasing the total free energy. In any case, the diffusion of constituent atoms of grains occurs after attachment because the NW cores after heating at 800 °C are truly solid, different from chain-like NWs.24−26 One large grain formed in a NW core region becomes a standard of crystallographic orientation of the NW, producing various

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CONCLUSION In summary, we clarified the single-crystallization mechanism of electrospun amorphous NWs converted into LiMn0.4Fe0.6PO4 NWs using TEM. Results show that single crystallization is achieved by oriented attachments of crystal LiMn0.4Fe0.6PO4 grains grown in an amorphous phase confined to self-forming carbon shells. Various NW axis orientations were attributed to the existence of stable core−shell interfaces. Self-forming carbon shells are a key for fabricating single-crystalline NWs. Optimizing the shell growth is expected to open the door to the fabrication of various single-crystalline NWs by electrospinning.

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ASSOCIATED CONTENT

S Supporting Information *

Details of methods and additional figures showing experimentally obtained results are given. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J. K.). *E-mail: [email protected] (E. H.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS J. K. appreciates fruitful comments offered by T. Hara, T. Aizawa, M. Mitome, M. Nagao (NIMS), and K. Ishizuka (HREM Research Inc.). This work was partly supported by “Nanotechnology Platform” (project no. A-13-NM-0060) of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.

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